Doc. no. | D???? |
Date: | 2025-05-29 |
Project: | Programming Language C++ |
Reply to: | Jonathan Wakely <lwgchair@gmail.com> |
Revised 2025-05-29 at 16:58:28 UTC
Reference ISO/IEC IS 14882:2020(E)
Also see:
This document contains only library issues which have been closed by the Library Working Group (LWG) after being found to be defects in the standard. That is, issues which have a status of DR, TC1, C++11, C++14, C++17, or Resolved. See the Library Closed Issues List for issues closed as non-defects. See the Library Active Issues List for active issues and more information. The introductory material in that document also applies to this document.
Section: 16.4.3.3 [using.linkage] Status: TC1 Submitter: Beman Dawes Opened: 1997-11-16 Last modified: 2016-08-09
Priority: Not Prioritized
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Discussion:
The change specified in the proposed resolution below did not make it into the Standard. This change was accepted in principle at the London meeting, and the exact wording below was accepted at the Morristown meeting.
Proposed resolution:
Change 16.4.3.3 [using.linkage] paragraph 2 from:
It is unspecified whether a name from the Standard C library declared with external linkage has either extern "C" or extern "C++" linkage.
to:
Whether a name from the Standard C library declared with external linkage has extern "C" or extern "C++" linkage is implementation defined. It is recommended that an implementation use extern "C++" linkage for this purpose.
Section: 17.5 [support.start.term] Status: TC1 Submitter: Steve Clamage Opened: 1997-12-12 Last modified: 2016-08-09
Priority: Not Prioritized
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Discussion:
We appear not to have covered all the possibilities of
exit processing with respect to
atexit registration.
Example 1: (C and C++)
#include <stdlib.h> void f1() { } void f2() { atexit(f1); } int main() { atexit(f2); // the only use of f2 return 0; // for C compatibility }
At program exit, f2 gets called due to its registration in main. Running f2 causes f1 to be newly registered during the exit processing. Is this a valid program? If so, what are its semantics?
Interestingly, neither the C standard, nor the C++ draft standard nor the forthcoming C9X Committee Draft says directly whether you can register a function with atexit during exit processing.
All 3 standards say that functions are run in reverse order of their registration. Since f1 is registered last, it ought to be run first, but by the time it is registered, it is too late to be first.
If the program is valid, the standards are self-contradictory about its semantics.
Example 2: (C++ only)
void F() { static T t; } // type T has a destructor int main() { atexit(F); // the only use of F }
Function F registered with atexit has a local static variable t, and F is called for the first time during exit processing. A local static object is initialized the first time control flow passes through its definition, and all static objects are destroyed during exit processing. Is the code valid? If so, what are its semantics?
Section 18.3 "Start and termination" says that if a function F is registered with atexit before a static object t is initialized, F will not be called until after t's destructor completes.
In example 2, function F is registered with atexit before its local static object O could possibly be initialized. On that basis, it must not be called by exit processing until after O's destructor completes. But the destructor cannot be run until after F is called, since otherwise the object could not be constructed in the first place.
If the program is valid, the standard is self-contradictory about its semantics.
I plan to submit Example 1 as a public comment on the C9X CD, with a recommendation that the results be undefined. (Alternative: make it unspecified. I don't think it is worthwhile to specify the case where f1 itself registers additional functions, each of which registers still more functions.)
I think we should resolve the situation in the whatever way the C committee decides.
For Example 2, I recommend we declare the results undefined.
[See reflector message lib-6500 for further discussion.]
Proposed resolution:
Change section 18.3/8 from:
First, objects with static storage duration are destroyed and functions registered by calling atexit are called. Objects with static storage duration are destroyed in the reverse order of the completion of their constructor. (Automatic objects are not destroyed as a result of calling exit().) Functions registered with atexit are called in the reverse order of their registration. A function registered with atexit before an object obj1 of static storage duration is initialized will not be called until obj1's destruction has completed. A function registered with atexit after an object obj2 of static storage duration is initialized will be called before obj2's destruction starts.
to:
First, objects with static storage duration are destroyed and functions registered by calling atexit are called. Non-local objects with static storage duration are destroyed in the reverse order of the completion of their constructor. (Automatic objects are not destroyed as a result of calling exit().) Functions registered with atexit are called in the reverse order of their registration, except that a function is called after any previously registered functions that had already been called at the time it was registered. A function registered with atexit before a non-local object obj1 of static storage duration is initialized will not be called until obj1's destruction has completed. A function registered with atexit after a non-local object obj2 of static storage duration is initialized will be called before obj2's destruction starts. A local static object obj3 is destroyed at the same time it would be if a function calling the obj3 destructor were registered with atexit at the completion of the obj3 constructor.
Rationale:
See 99-0039/N1215, October 22, 1999, by Stephen D. Clamage for the analysis supporting to the proposed resolution.
Section: 27.4.3.7.8 [string.swap] Status: TC1 Submitter: Jack Reeves Opened: 1997-12-11 Last modified: 2016-11-12
Priority: Not Prioritized
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Duplicate of: 87
Discussion:
At the very end of the basic_string class definition is the signature: int compare(size_type pos1, size_type n1, const charT* s, size_type n2 = npos) const; In the following text this is defined as: returns basic_string<charT,traits,Allocator>(*this,pos1,n1).compare( basic_string<charT,traits,Allocator>(s,n2);
Since the constructor basic_string(const charT* s, size_type n, const Allocator& a = Allocator()) clearly requires that s != NULL and n < npos and further states that it throws length_error if n == npos, it appears the compare() signature above should always throw length error if invoked like so: str.compare(1, str.size()-1, s); where 's' is some null terminated character array.
This appears to be a typo since the obvious intent is to allow either the call above or something like: str.compare(1, str.size()-1, s, strlen(s)-1);
This would imply that what was really intended was two signatures int compare(size_type pos1, size_type n1, const charT* s) const int compare(size_type pos1, size_type n1, const charT* s, size_type n2) const; each defined in terms of the corresponding constructor.
Proposed resolution:
Replace the compare signature in 27.4.3 [basic.string] (at the very end of the basic_string synopsis) which reads:
int compare(size_type pos1, size_type n1,
const charT* s, size_type n2 = npos) const;
with:
int compare(size_type pos1, size_type n1,
const charT* s) const;
int compare(size_type pos1, size_type n1,
const charT* s, size_type n2) const;
Replace the portion of 27.4.3.7.8 [string.swap] paragraphs 5 and 6 which read:
int compare(size_type pos, size_type n1,
Returns:
charT * s, size_type n2 = npos) const;
basic_string<charT,traits,Allocator>(*this, pos, n1).compare(
basic_string<charT,traits,Allocator>( s, n2))
with:
int compare(size_type pos, size_type n1,
Returns:
const charT * s) const;
Returns:
basic_string<charT,traits,Allocator>(*this, pos, n1).compare(
basic_string<charT,traits,Allocator>( s ))
int compare(size_type pos, size_type n1,
const charT * s, size_type n2) const;
basic_string<charT,traits,Allocator>(*this, pos, n1).compare(
basic_string<charT,traits,Allocator>( s, n2))
Editors please note that in addition to splitting the signature, the third argument becomes const, matching the existing synopsis.
Rationale:
While the LWG dislikes adding signatures, this is a clear defect in the Standard which must be fixed. The same problem was also identified in issues 7 (item 5) and 87.
Section: 27 [strings] Status: TC1 Submitter: Matt Austern Opened: 1997-12-15 Last modified: 2016-11-12
Priority: Not Prioritized
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Discussion:
(1) In 27.4.3.7.4 [string.insert], the description of template <class InputIterator> insert(iterator, InputIterator, InputIterator) makes no sense. It refers to a member function that doesn't exist. It also talks about the return value of a void function.
(2) Several versions of basic_string::replace don't appear in the class synopsis.
(3) basic_string::push_back appears in the synopsis, but is never described elsewhere. In the synopsis its argument is const charT, which doesn't makes much sense; it should probably be charT, or possible const charT&.
(4) basic_string::pop_back is missing.
(5) int compare(size_type pos, size_type n1, charT* s, size_type n2 = npos) make no sense. First, it's const charT* in the synopsis and charT* in the description. Second, given what it says in RETURNS, leaving out the final argument will always result in an exception getting thrown. This is paragraphs 5 and 6 of 27.4.3.7.8 [string.swap]
(6) In table 37, in section 27.2.2 [char.traits.require], there's a note for X::move(s, p, n). It says "Copies correctly even where p is in [s, s+n)". This is correct as far as it goes, but it doesn't go far enough; it should also guarantee that the copy is correct even where s in in [p, p+n). These are two orthogonal guarantees, and neither one follows from the other. Both guarantees are necessary if X::move is supposed to have the same sort of semantics as memmove (which was clearly the intent), and both guarantees are necessary if X::move is actually supposed to be useful.
Proposed resolution:
ITEM 1: In 21.3.5.4 [lib.string::insert], change paragraph 16 to
EFFECTS: Equivalent to insert(p - begin(), basic_string(first, last)).
ITEM 2: Not a defect; the Standard is clear.. There are ten versions of replace() in
the synopsis, and ten versions in 21.3.5.6 [lib.string::replace].
ITEM 3: Change the declaration of push_back in the string synopsis (21.3,
[lib.basic.string]) from:
void push_back(const charT)
to
void push_back(charT)
Add the following text immediately after 21.3.5.2 [lib.string::append], paragraph 10.
void basic_string::push_back(charT c);
EFFECTS: Equivalent to append(static_cast<size_type>(1), c);
ITEM 4: Not a defect. The omission appears to have been deliberate.
ITEM 5: Duplicate; see issue 5 (and 87).
ITEM 6: In table 37, Replace:
"Copies correctly even where p is in [s, s+n)."
with:
"Copies correctly even where the ranges [p, p+n) and [s,
s+n) overlap."
Section: 28.3.3.1.6 [locale.statics] Status: TC1 Submitter: Matt Austern Opened: 1997-12-24 Last modified: 2016-08-09
Priority: Not Prioritized
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Discussion:
It appears there's an important guarantee missing from clause 22. We're told that invoking locale::global(L) sets the C locale if L has a name. However, we're not told whether or not invoking setlocale(s) sets the global C++ locale.
The intent, I think, is that it should not, but I can't find any such words anywhere.
Proposed resolution:
Add a sentence at the end of 28.3.3.1.6 [locale.statics], paragraph 2:
No library function other than
locale::global()
shall affect the value returned bylocale()
.
Section: 17.6.3 [new.delete] Status: TC1 Submitter: Steve Clamage Opened: 1998-01-04 Last modified: 2016-08-09
Priority: Not Prioritized
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Discussion:
Scott Meyers, in a comp.std.c++ posting: I just noticed that section 3.7.3.1 of CD2 seems to allow for the possibility that all calls to operator new(0) yield the same pointer, an implementation technique specifically prohibited by ARM 5.3.3.Was this prohibition really lifted? Does the FDIS agree with CD2 in the regard? [Issues list maintainer's note: the IS is the same.]
Proposed resolution:
Change the last paragraph of 3.7.3 from:
Any allocation and/or deallocation functions defined in a C++ program shall conform to the semantics specified in 3.7.3.1 and 3.7.3.2.
to:
Any allocation and/or deallocation functions defined in a C++ program, including the default versions in the library, shall conform to the semantics specified in 3.7.3.1 and 3.7.3.2.
Change 3.7.3.1/2, next-to-last sentence, from :
If the size of the space requested is zero, the value returned shall not be a null pointer value (4.10).
to:
Even if the size of the space requested is zero, the request can fail. If the request succeeds, the value returned shall be a non-null pointer value (4.10) p0 different from any previously returned value p1, unless that value p1 was since passed to an operator delete.
5.3.4/7 currently reads:
When the value of the expression in a direct-new-declarator is zero, the allocation function is called to allocate an array with no elements. The pointer returned by the new-expression is non-null. [Note: If the library allocation function is called, the pointer returned is distinct from the pointer to any other object.]
Retain the first sentence, and delete the remainder.
18.5.1 currently has no text. Add the following:
Except where otherwise specified, the provisions of 3.7.3 apply to the library versions of operator new and operator delete.
To 18.5.1.3, add the following text:
The provisions of 3.7.3 do not apply to these reserved placement forms of operator new and operator delete.
Rationale:
See 99-0040/N1216, October 22, 1999, by Stephen D. Clamage for the analysis supporting to the proposed resolution.
Section: 22.9.2 [template.bitset] Status: TC1 Submitter: Matt Austern Opened: 1998-01-22 Last modified: 2016-08-09
Priority: Not Prioritized
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Discussion:
(1) bitset<>::operator[] is mentioned in the class synopsis (23.3.5), but it is not documented in 23.3.5.2.
(2) The class synopsis only gives a single signature for bitset<>::operator[], reference operator[](size_t pos). This doesn't make much sense. It ought to be overloaded on const. reference operator[](size_t pos); bool operator[](size_t pos) const.
(3) Bitset's stream input function (23.3.5.3) ought to skip all whitespace before trying to extract 0s and 1s. The standard doesn't explicitly say that, though. This should go in the Effects clause.
Proposed resolution:
ITEMS 1 AND 2:
In the bitset synopsis (22.9.2 [template.bitset]),
replace the member function
reference operator[](size_t pos);
with the two member functions
bool operator[](size_t pos) const;
reference operator[](size_t pos);
Add the following text at the end of 22.9.2.3 [bitset.members],
immediately after paragraph 45:
bool operator[](size_t pos) const;
Requires: pos is valid
Throws: nothing
Returns:test(pos)
bitset<N>::reference operator[](size_t pos);
Requires: pos is valid
Throws: nothing
Returns: An object of typebitset<N>::reference
such that(*this)[pos] == this->test(pos)
, and such that(*this)[pos] = val
is equivalent tothis->set(pos, val);
Rationale:
The LWG believes Item 3 is not a defect. "Formatted input" implies the desired semantics. See 31.7.5.3 [istream.formatted].
Section: 31.7.5.3.3 [istream.extractors] Status: TC1 Submitter: William M. Miller Opened: 1998-03-03 Last modified: 2017-04-22
Priority: Not Prioritized
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Discussion:
In 27.6.1.2.3, there is a reference to "eos", which is the only one in the whole draft (at least using Acrobat search), so it's undefined.
Proposed resolution:
In [istream::extractors], replace "eos" with "charT()"
Section: 28.3.3.1.4 [locale.members] Status: TC1 Submitter: Nathan Myers Opened: 1998-08-06 Last modified: 2016-08-09
Priority: Not Prioritized
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Discussion:
locale::combine is the only member function of locale (other than constructors and destructor) that is not const. There is no reason for it not to be const, and good reasons why it should have been const. Furthermore, leaving it non-const conflicts with 22.1.1 paragraph 6: "An instance of a locale is immutable."
History: this member function originally was a constructor. it happened that the interface it specified had no corresponding language syntax, so it was changed to a member function. As constructors are never const, there was no "const" in the interface which was transformed into member "combine". It should have been added at that time, but the omission was not noticed.
Proposed resolution:
In 28.3.3.1 [locale] and also in 28.3.3.1.4 [locale.members], add "const" to the declaration of member combine:
template <class Facet> locale combine(const locale& other) const;
Section: 28.3.3.1.4 [locale.members] Status: TC1 Submitter: Nathan Myers Opened: 1998-08-06 Last modified: 2016-08-09
Priority: Not Prioritized
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Discussion:
locale::name() is described as returning a string that can be passed to a locale constructor, but there is no matching constructor.
Proposed resolution:
In 28.3.3.1.4 [locale.members], paragraph 5, replace
"locale(name())
" with
"locale(name().c_str())
".
Section: 28.3.4.2.5 [locale.codecvt] Status: TC1 Submitter: Nathan Myers Opened: 1998-08-06 Last modified: 2016-08-09
Priority: Not Prioritized
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Discussion:
The new virtual members ctype_byname<char>::do_widen and do_narrow did not get edited in properly. Instead, the member do_widen appears four times, with wrong argument lists.
Proposed resolution:
The correct declarations for the overloaded members
do_narrow
and do_widen
should be copied
from 28.3.4.2.4 [facet.ctype.special].
Section: 28.3.4.3.2.3 [facet.num.get.virtuals] Status: TC1 Submitter: Nathan Myers Opened: 1998-08-06 Last modified: 2016-08-09
Priority: Not Prioritized
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Discussion:
This section describes the process of parsing a text boolean value from the input
stream. It does not say it recognizes either of the sequences "true" or
"false" and returns the corresponding bool value; instead, it says it recognizes
only one of those sequences, and chooses which according to the received value of a
reference argument intended for returning the result, and reports an error if the other
sequence is found. (!) Furthermore, it claims to get the names from the ctype<>
facet rather than the numpunct<> facet, and it examines the "boolalpha"
flag wrongly; it doesn't define the value "loc"; and finally, it computes
wrongly whether to use numeric or "alpha" parsing.
I believe the correct algorithm is "as if":
// in, err, val, and str are arguments. err = 0; const numpunct<charT>& np = use_facet<numpunct<charT> >(str.getloc()); const string_type t = np.truename(), f = np.falsename(); bool tm = true, fm = true; size_t pos = 0; while (tm && pos < t.size() || fm && pos < f.size()) { if (in == end) { err = str.eofbit; } bool matched = false; if (tm && pos < t.size()) { if (!err && t[pos] == *in) matched = true; else tm = false; } if (fm && pos < f.size()) { if (!err && f[pos] == *in) matched = true; else fm = false; } if (matched) { ++in; ++pos; } if (pos > t.size()) tm = false; if (pos > f.size()) fm = false; } if (tm == fm || pos == 0) { err |= str.failbit; } else { val = tm; } return in;
Notice this works reasonably when the candidate strings are both empty, or equal, or when one is a substring of the other. The proposed text below captures the logic of the code above.
Proposed resolution:
In 28.3.4.3.2.3 [facet.num.get.virtuals], in the first line of paragraph 14, change "&&" to "&".
Then, replace paragraphs 15 and 16 as follows:
Otherwise target sequences are determined "as if" by calling the members
falsename()
andtruename()
of the facet obtained byuse_facet<numpunct<charT> >(str.getloc())
. Successive characters in the range[in,end)
(see [lib.sequence.reqmts]) are obtained and matched against corresponding positions in the target sequences only as necessary to identify a unique match. The input iteratorin
is compared toend
only when necessary to obtain a character. If and only if a target sequence is uniquely matched,val
is set to the corresponding value.
The
in
iterator is always left pointing one position beyond the last character successfully matched. Ifval
is set, then err is set tostr.goodbit
; or tostr.eofbit
if, when seeking another character to match, it is found that(in==end)
. Ifval
is not set, then err is set tostr.failbit
; or to(str.failbit|str.eofbit)
if the reason for the failure was that(in==end)
. [Example: for targetstrue
:"a" andfalse
:"abb", the input sequence "a" yieldsval==true
anderr==str.eofbit
; the input sequence "abc" yieldserr=str.failbit
, within
ending at the 'c' element. For targetstrue
:"1" andfalse
:"0", the input sequence "1" yieldsval==true
anderr=str.goodbit
. For empty targets (""), any input sequence yieldserr==str.failbit
. --end example]
Section: 28.3.4.3.2.2 [facet.num.get.members] Status: TC1 Submitter: Nathan Myers Opened: 1998-08-06 Last modified: 2016-08-09
Priority: Not Prioritized
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Discussion:
In the list of num_get<> non-virtual members on page 22-23, the member that parses bool values was omitted from the list of definitions of non-virtual members, though it is listed in the class definition and the corresponding virtual is listed everywhere appropriate.
Proposed resolution:
Add at the beginning of 28.3.4.3.2.2 [facet.num.get.members] another get member for bool&, copied from the entry in 28.3.4.3.2 [locale.num.get].
Section: 28.3.4.2.5 [locale.codecvt] Status: TC1 Submitter: Nathan Myers Opened: 1998-08-06 Last modified: 2016-08-09
Priority: Not Prioritized
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Duplicate of: 10
Discussion:
In the definitions of codecvt<>::do_out and do_in, they are specified to return noconv if "no conversion is needed". This definition is too vague, and does not say normatively what is done with the buffers.
Proposed resolution:
Change the entry for noconv in the table under paragraph 4 in section 28.3.4.2.5.3 [locale.codecvt.virtuals] to read:
noconv
:internT
andexternT
are the same type, and input sequence is identical to converted sequence.
Change the Note in paragraph 2 to normative text as follows:
If returns
noconv
,internT
andexternT
are the same type and the converted sequence is identical to the input sequence[from,from_next)
.to_next
is set equal toto
, the value ofstate
is unchanged, and there are no changes to the values in[to, to_limit)
.
Section: 28.3.4.4.1.3 [facet.numpunct.virtuals] Status: TC1 Submitter: Nathan Myers Opened: 1998-08-06 Last modified: 2016-08-09
Priority: Not Prioritized
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Discussion:
The synopsis for numpunct<>::do_thousands_sep, and the definition of numpunct<>::thousands_sep which calls it, specify that it returns a value of type char_type. Here it is erroneously described as returning a "string_type".
Proposed resolution:
In 28.3.4.4.1.3 [facet.numpunct.virtuals], above paragraph 2, change "string_type" to "char_type".
Section: 28.3.3.1.2.1 [locale.category] Status: TC1 Submitter: Nathan Myers Opened: 1998-08-06 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
In the second table in the section, captioned "Required instantiations", the instantiations for codecvt_byname<> have been omitted. These are necessary to allow users to construct a locale by name from facets.
Proposed resolution:
Add in 28.3.3.1.2.1 [locale.category] to the table captioned "Required instantiations", in the category "ctype" the lines
codecvt_byname<char,char,mbstate_t>, codecvt_byname<wchar_t,char,mbstate_t>
Section: 31.10.4.4 [ifstream.members] Status: TC1 Submitter: Nathan Myers Opened: 1998-08-06 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
The description of basic_istream<>::open leaves unanswered questions about how it responds to or changes flags in the error status for the stream. A strict reading indicates that it ignores the bits and does not change them, which confuses users who do not expect eofbit and failbit to remain set after a successful open. There are three reasonable resolutions: 1) status quo 2) fail if fail(), ignore eofbit 3) clear failbit and eofbit on call to open().
Proposed resolution:
In 31.10.4.4 [ifstream.members] paragraph 3, and in 31.10.5.4 [ofstream.members] paragraph 3, under open() effects, add a footnote:
A successful open does not change the error state.
Rationale:
This may seem surprising to some users, but it's just an instance of a general rule: error flags are never cleared by the implementation. The only way error flags are are ever cleared is if the user explicitly clears them by hand.
The LWG believed that preserving this general rule was important enough so that an exception shouldn't be made just for this one case. The resolution of this issue clarifies what the LWG believes to have been the original intent.
Section: 28.3.4.3.2.3 [facet.num.get.virtuals] Status: CD1 Submitter: Nathan Myers Opened: 1998-08-06 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
The current description of numeric input does not account for the possibility of overflow. This is an implicit result of changing the description to rely on the definition of scanf() (which fails to report overflow), and conflicts with the documented behavior of traditional and current implementations.
Users expect, when reading a character sequence that results in a value unrepresentable in the specified type, to have an error reported. The standard as written does not permit this.
Further comments from Dietmar:
I don't feel comfortable with the proposed resolution to issue 23: It kind of simplifies the issue to much. Here is what is going on:
Currently, the behavior of numeric overflow is rather counter intuitive and hard to trace, so I will describe it briefly:
failbit
is set if scanf()
would
return an input error; otherwise a value is converted to the rules
of scanf
.
scanf()
is defined in terms of fscanf()
.
fscanf()
returns an input failure if during conversion no
character matching the conversion specification could be extracted
before reaching EOF. This is the only reason for fscanf()
to fail due to an input error and clearly does not apply to the case
of overflow.
fscanf()
which basically says that strtod
,
strtol()
, etc. are to be used for the conversion.
strtod()
, strtol()
, etc. functions consume as
many matching characters as there are and on overflow continue to
consume matching characters but also return a value identical to
the maximum (or minimum for signed types if there was a leading minus)
value of the corresponding type and set errno
to ERANGE
.
errno
after reading an element and, of course, clearing errno
before trying a conversion. With the current wording, it can be
detected whether the overflow was due to a positive or negative
number for signed types.
Further discussion from Redmond:
The basic problem is that we've defined our behavior,
including our error-reporting behavior, in terms of C90. However,
C90's method of reporting overflow in scanf is not technically an
"input error". The strto_*
functions are more precise.
There was general consensus that failbit
should be set
upon overflow. We considered three options based on this:
errno
to
indicated the precise nature of the error.Straw poll: (1) 5; (2) 0; (3) 8.
Discussed at Lillehammer. General outline of what we want the solution to look like: we want to say that overflow is an error, and provide a way to distinguish overflow from other kinds of errors. Choose candidate field the same way scanf does, but don't describe the rest of the process in terms of format. If a finite input field is too large (positive or negative) to be represented as a finite value, then set failbit and assign the nearest representable value. Bill will provide wording.
Discussed at Toronto: N2327 is in alignment with the direction we wanted to go with in Lillehammer. Bill to work on.
Proposed resolution:
Change 28.3.4.3.2.3 [facet.num.get.virtuals], end of p3:
Stage 3:
The result of stage 2 processing can be one ofThe sequence ofchar
s accumulated in stage 2 (the field) is converted to a numeric value by the rules of one of the functions declared in the header<cstdlib>
:
A sequence ofFor a signed integer value, the functionchar
s has been accumulated in stage 2 that is converted (according to the rules ofscanf
) to a value of the type of val. This value is stored in val andios_base::goodbit
is stored in err.strtoll
.The sequence ofFor an unsigned integer value, the functionchar
s accumulated in stage 2 would have causedscanf
to report an input failure.ios_base::failbit
is assigned to err.strtoull
.- For a floating-point value, the function
strtold
.The numeric value to be stored can be one of:
- zero, if the conversion function fails to convert the entire field.
ios_base::failbit
is assigned to err.- the most positive representable value, if the field represents a value too large positive to be represented in val.
ios_base::failbit
is assigned to err.- the most negative representable value (zero for unsigned integer), if the field represents a value too large negative to be represented in val.
ios_base::failbit
is assigned to err.- the converted value, otherwise.
The resultant numeric value is stored in val.
Change 28.3.4.3.2.3 [facet.num.get.virtuals], p6-p7:
iter_type do_get(iter_type in, iter_type end, ios_base& str, ios_base::iostate& err, bool& val) const;-6- Effects: If
(str.flags()&ios_base::boolalpha)==0
then input proceeds as it would for along
except that if a value is being stored into val, the value is determined according to the following: If the value to be stored is 0 thenfalse
is stored. If the value is 1 thentrue
is stored. Otherwiseerr|=ios_base::failbit
is performed and no valuetrue
is stored.andios_base::failbit
is assigned to err.-7- Otherwise target sequences are determined "as if" by calling the members
falsename()
andtruename()
of the facet obtained byuse_facet<numpunct<charT> >(str.getloc())
. Successive characters in the range[in,end)
(see 23.1.1) are obtained and matched against corresponding positions in the target sequences only as necessary to identify a unique match. The input iterator in is compared to end only when necessary to obtain a character. Ifand only ifa target sequence is uniquely matched, val is set to the corresponding value. Otherwisefalse
is stored andios_base::failbit
is assigned to err.
Section: 28.3.4.2.5 [locale.codecvt] Status: TC1 Submitter: Nathan Myers Opened: 1998-08-06 Last modified: 2016-01-28
Priority: Not Prioritized
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Duplicate of: 72
Discussion:
The description of codecvt<>::do_out and do_in mentions a symbol "do_convert" which is not defined in the standard. This is a leftover from an edit, and should be "do_in and do_out".
Proposed resolution:
In 28.3.4.2.5 [locale.codecvt], paragraph 3, change "do_convert" to "do_in or do_out". Also, in 28.3.4.2.5.3 [locale.codecvt.virtuals], change "do_convert()" to "do_in or do_out".
Section: 27.4.4.4 [string.io] Status: TC1 Submitter: Nathan Myers Opened: 1998-08-06 Last modified: 2016-01-28
Priority: Not Prioritized
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Duplicate of: 67
Discussion:
In the description of operator<< applied to strings, the standard says that uses the smaller of os.width() and str.size(), to pad "as described in stage 3" elsewhere; but this is inconsistent, as this allows no possibility of space for padding.
Proposed resolution:
Change 27.4.4.4 [string.io] paragraph 4 from:
"... where n
is the smaller of os.width()
and str.size()
;
..."
to:
"... where n
is the larger of os.width()
and str.size()
;
..."
Section: 31.7.5.2.4 [istream.sentry] Status: TC1 Submitter: Nathan Myers Opened: 1998-08-06 Last modified: 2021-06-06
Priority: Not Prioritized
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Discussion:
In paragraph 6, the code in the example:
template <class charT, class traits = char_traits<charT> > basic_istream<charT,traits>::sentry( basic_istream<charT,traits>& is, bool noskipws = false) { ... int_type c; typedef ctype<charT> ctype_type; const ctype_type& ctype = use_facet<ctype_type>(is.getloc()); while ((c = is.rdbuf()->snextc()) != traits::eof()) { if (ctype.is(ctype.space,c)==0) { is.rdbuf()->sputbackc (c); break; } } ... }
fails to demonstrate correct use of the facilities described. In particular, it fails to use traits operators, and specifies incorrect semantics. (E.g. it specifies skipping over the first character in the sequence without examining it.)
Proposed resolution:
Remove the example above from [istream::sentry] paragraph 6.
Rationale:
The originally proposed replacement code for the example was not correct. The LWG tried in Kona and again in Tokyo to correct it without success. In Tokyo, an implementor reported that actual working code ran over one page in length and was quite complicated. The LWG decided that it would be counter-productive to include such a lengthy example, which might well still contain errors.
Section: 27.4.3.7.5 [string.erase] Status: TC1 Submitter: Nathan Myers Opened: 1998-08-06 Last modified: 2016-11-12
Priority: Not Prioritized
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Discussion:
The string::erase(iterator first, iterator last) is specified to return an element one place beyond the next element after the last one erased. E.g. for the string "abcde", erasing the range ['b'..'d') would yield an iterator for element 'e', while 'd' has not been erased.
Proposed resolution:
In 27.4.3.7.5 [string.erase], paragraph 10, change:
Returns: an iterator which points to the element immediately following _last_ prior to the element being erased.
to read
Returns: an iterator which points to the element pointed to by _last_ prior to the other elements being erased.
Section: 28.3.4.2.4.3 [facet.ctype.char.members] Status: TC1 Submitter: Nathan Myers Opened: 1998-08-06 Last modified: 2016-01-28
Priority: Not Prioritized
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Duplicate of: 236
Discussion:
The description of the vector form of ctype<char>::is can be interpreted to mean something very different from what was intended. Paragraph 4 says
Effects: The second form, for all *p in the range [low, high), assigns vec[p-low] to table()[(unsigned char)*p].
This is intended to copy the value indexed from table()[] into the place identified in vec[].
Proposed resolution:
Change 28.3.4.2.4.3 [facet.ctype.char.members], paragraph 4, to read
Effects: The second form, for all *p in the range [low, high), assigns into vec[p-low] the value table()[(unsigned char)*p].
Section: 31.4.3 [narrow.stream.objects] Status: TC1 Submitter: Nathan Myers Opened: 1998-08-06 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
Sections 31.4.3 [narrow.stream.objects] and 31.4.4 [wide.stream.objects] mention a function ios_base::init, which is not defined. Probably they mean basic_ios<>::init, defined in 31.5.4.2 [basic.ios.cons], paragraph 3.
Proposed resolution:
[R12: modified to include paragraph 5.]
In 31.4.3 [narrow.stream.objects] paragraph 2 and 5, change
ios_base::init
to
basic_ios<char>::init
Also, make a similar change in 31.4.4 [wide.stream.objects] except it should read
basic_ios<wchar_t>::init
Section: 28.3.3.1.2.1 [locale.category] Status: TC1 Submitter: Nathan Myers Opened: 1998-08-06 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
Paragraph 2 implies that the C macros LC_CTYPE etc. are defined in <cctype>, where they are in fact defined elsewhere to appear in <clocale>.
Proposed resolution:
In 28.3.3.1.2.1 [locale.category], paragraph 2, change "<cctype>" to read "<clocale>".
Section: 28.3.3.1 [locale] Status: TC1 Submitter: Nathan Myers Opened: 1998-08-06 Last modified: 2016-01-28
Priority: Not Prioritized
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Duplicate of: 378
Discussion:
Paragraph 6, says "An instance of locale
is
immutable; once a facet reference is obtained from it,
...". This has caused some confusion, because locale variables
are manifestly assignable.
Proposed resolution:
In 28.3.3.1 [locale] replace paragraph 6
An instance of
locale
is immutable; once a facet reference is obtained from it, that reference remains usable as long as the locale value itself exists.
with
Once a facet reference is obtained from a locale object by calling use_facet<>, that reference remains usable, and the results from member functions of it may be cached and re-used, as long as some locale object refers to that facet.
Section: 31.6.3.5.4 [streambuf.virt.pback] Status: TC1 Submitter: Nathan Myers Opened: 1998-08-06 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
The description of the required state before calling virtual member basic_streambuf<>::pbackfail requirements is inconsistent with the conditions described in 27.5.2.2.4 [lib.streambuf.pub.pback] where member sputbackc calls it. Specifically, the latter says it calls pbackfail if:
traits::eq(c,gptr()[-1]) is false
where pbackfail claims to require:
traits::eq(*gptr(),traits::to_char_type(c)) returns false
It appears that the pbackfail description is wrong.
Proposed resolution:
In 31.6.3.5.4 [streambuf.virt.pback], paragraph 1, change:
"
traits::eq(*gptr(),traits::to_char_type( c))
"
to
"
traits::eq(traits::to_char_type(c),gptr()[-1])
"
Rationale:
Note deliberate reordering of arguments for clarity in addition to the correction of the argument value.
Section: 28.3.4.2.5 [locale.codecvt] Status: TC1 Submitter: Nathan Myers Opened: 1998-08-06 Last modified: 2016-01-28
Priority: Not Prioritized
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Duplicate of: 43
Discussion:
In the table defining the results from do_out and do_in, the specification for the result error says
encountered a from_type character it could not convert
but from_type is not defined. This clearly is intended to be an externT for do_in, or an internT for do_out.
Proposed resolution:
In 28.3.4.2.5.3 [locale.codecvt.virtuals] paragraph 4, replace the definition in the table for the case of _error_ with
encountered a character in
[from,from_end)
that it could not convert.
Section: 28.3.4.3.3.3 [facet.num.put.virtuals] Status: TC1 Submitter: Nathan Myers Opened: 1998-08-06 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
In paragraph 19, Effects:, members truename() and falsename are used from facet ctype<charT>, but it has no such members. Note that this is also a problem in 22.2.2.1.2, addressed in (4).
Proposed resolution:
In 28.3.4.3.3.3 [facet.num.put.virtuals], paragraph 19, in the Effects: clause for member put(...., bool), replace the initialization of the string_type value s as follows:
const numpunct& np = use_facet<numpunct<charT> >(loc); string_type s = val ? np.truename() : np.falsename();
Section: 31.5 [iostreams.base] Status: TC1 Submitter: Nathan Myers Opened: 1998-08-06 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
In 31.5.5.1 [fmtflags.manip], we have a definition for a manipulator named "unitbuf". Unlike other manipulators, it's not listed in synopsis. Similarly for "nounitbuf".
Proposed resolution:
Add to the synopsis for <ios> in 31.5 [iostreams.base], after the entry for "nouppercase", the prototypes:
ios_base& unitbuf(ios_base& str); ios_base& nounitbuf(ios_base& str);
Section: 31.5.2.6 [ios.base.storage] Status: TC1 Submitter: Nathan Myers Opened: 1998-08-06 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
In the definitions for ios_base::iword and pword, the lifetime of the storage is specified badly, so that an implementation which only keeps the last value stored appears to conform. In particular, it says:
The reference returned may become invalid after another call to the object's iword member with a different index ...
This is not idle speculation; at least one implementation was done this way.
Proposed resolution:
Add in 31.5.2.6 [ios.base.storage], in both paragraph 2 and also in paragraph 4, replace the sentence:
The reference returned may become invalid after another call to the object's iword [pword] member with a different index, after a call to its copyfmt member, or when the object is destroyed.
with:
The reference returned is invalid after any other operations on the object. However, the value of the storage referred to is retained, so that until the next call to copyfmt, calling iword [pword] with the same index yields another reference to the same value.
substituting "iword" or "pword" as appropriate.
Section: 28.3.3.1 [locale] Status: TC1 Submitter: Nathan Myers Opened: 1998-08-06 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
In the overview of locale semantics, paragraph 4, is the sentence
If Facet is not present in a locale (or, failing that, in the global locale), it throws the standard exception bad_cast.
This is not supported by the definition of use_facet<>, and represents semantics from an old draft.
Proposed resolution:
In 28.3.3.1 [locale], paragraph 4, delete the parenthesized expression
(or, failing that, in the global locale)
Section: 28.3.3.2 [locale.global.templates] Status: TC1 Submitter: Nathan Myers Opened: 1998-08-06 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
It has been noticed by Esa Pulkkinen that the definition of "facet" is incomplete. In particular, a class derived from another facet, but which does not define a member id, cannot safely serve as the argument F to use_facet<F>(loc), because there is no guarantee that a reference to the facet instance stored in loc is safely convertible to F.
Proposed resolution:
In the definition of std::use_facet<>(), replace the text in paragraph 1 which reads:
Get a reference to a facet of a locale.
with:
Requires:
Facet
is a facet class whose definition contains the public static memberid
as defined in 28.3.3.1.2.2 [locale.facet].
[
Kona: strike as overspecification the text "(not inherits)"
from the original resolution, which read "... whose definition
contains (not inherits) the public static member
id
..."
]
Section: 24.6.4.4 [istreambuf.iterator.ops] Status: TC1 Submitter: Nathan Myers Opened: 1998-08-06 Last modified: 2017-11-29
Priority: Not Prioritized
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Discussion:
Following the definition of istreambuf_iterator<>::operator++(int) in paragraph 3, the standard contains three lines of garbage text left over from a previous edit.
istreambuf_iterator<charT,traits> tmp = *this; sbuf_->sbumpc(); return(tmp);
Proposed resolution:
In [istreambuf.iterator::op++], delete the three lines of code at the end of paragraph 3.
Section: 99 [facets.examples] Status: TC1 Submitter: Nathan Myers Opened: 1998-08-06 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
Paragraph 3 of the locale examples is a description of part of an implementation technique that has lost its referent, and doesn't mean anything.
Proposed resolution:
Delete 99 [facets.examples] paragraph 3 which begins "This initialization/identification system depends...", or (at the editor's option) replace it with a place-holder to keep the paragraph numbering the same.
Section: 31.5.2 [ios.base] Status: TC1 Submitter: Nathan Myers Opened: 1998-08-06 Last modified: 2016-01-28
Priority: Not Prioritized
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Duplicate of: 157
Discussion:
The description of ios_base::iword() and pword() in 31.5.2.5 [ios.members.static], say that if they fail, they "set badbit, which may throw an exception". However, ios_base offers no interface to set or to test badbit; those interfaces are defined in basic_ios<>.
Proposed resolution:
Change the description in 31.5.2.6 [ios.base.storage] in paragraph 2, and also in paragraph 4, as follows. Replace
If the function fails it sets badbit, which may throw an exception.
with
If the function fails, and
*this
is a base sub-object of abasic_ios<>
object or sub-object, the effect is equivalent to callingbasic_ios<>::setstate(badbit)
on the derived object (which may throwfailure
).
[Kona: LWG reviewed wording; setstate(failbit) changed to setstate(badbit).]
Section: 27.4.3 [basic.string] Status: TC1 Submitter: Nathan Myers Opened: 1998-08-06 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
The basic_string<> copy constructor:
basic_string(const basic_string& str, size_type pos = 0, size_type n = npos, const Allocator& a = Allocator());
specifies an Allocator argument default value that is counter-intuitive. The natural choice for a the allocator to copy from is str.get_allocator(). Though this cannot be expressed in default-argument notation, overloading suffices.
Alternatively, the other containers in Clause 23 (deque, list, vector) do not have this form of constructor, so it is inconsistent, and an evident source of confusion, for basic_string<> to have it, so it might better be removed.
Proposed resolution:
In 27.4.3 [basic.string], replace the declaration of the copy constructor as follows:
basic_string(const basic_string& str); basic_string(const basic_string& str, size_type pos, size_type n = npos, const Allocator& a = Allocator());
In 27.4.3.2 [string.require], replace the copy constructor declaration as above. Add to paragraph 5, Effects:
In the first form, the Allocator value used is copied from
str.get_allocator()
.
Rationale:
The LWG believes the constructor is actually broken, rather than just an unfortunate design choice.
The LWG considered two other possible resolutions:
A. In 27.4.3 [basic.string], replace the declaration of the copy constructor as follows:
basic_string(const basic_string& str, size_type pos = 0, size_type n = npos); basic_string(const basic_string& str, size_type pos, size_type n, const Allocator& a);
In 27.4.3.2 [string.require], replace the copy constructor declaration as above. Add to paragraph 5, Effects:
When no
Allocator
argument is provided, the string is constructed using the valuestr.get_allocator()
.
B. In 27.4.3 [basic.string], and also in 27.4.3.2 [string.require], replace the declaration of the copy constructor as follows:
basic_string(const basic_string& str, size_type pos = 0, size_type n = npos);
The proposed resolution reflects the original intent of the LWG. It was also noted by Pete Becker that this fix "will cause a small amount of existing code to now work correctly."
[ Kona: issue editing snafu fixed - the proposed resolution now correctly reflects the LWG consensus. ]
Section: 31 [input.output] Status: CD1 Submitter: Nathan Myers Opened: 1998-08-06 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
Many of the specifications for iostreams specify that character values or their int_type equivalents are compared using operators == or !=, though in other places traits::eq() or traits::eq_int_type is specified to be used throughout. This is an inconsistency; we should change uses of == and != to use the traits members instead.
Proposed resolution:
[Pre-Kona: Dietmar supplied wording]
List of changes to clause 27:
tofillch == fill()
traits::eq(fillch, fill())
toc == delim for the next available input character c
traits::eq(c, delim) for the next available input character c
toc == delim for the next available input character c
traits::eq(c, delim) for the next available input character c
toc == delim for the next available input character c
traits::eq(c, delim) for the next available input character c
toc == delim for the next available input character c
traits::eq_int_type(c, delim) for the next available input character c
toThe last condition will never occur if delim == traits::eof()
The last condition will never occur if traits::eq_int_type(delim, traits::eof()).
towhile ((c = is.rdbuf()->snextc()) != traits::eof()) {
while (!traits::eq_int_type(c = is.rdbuf()->snextc(), traits::eof())) {
List of changes to Chapter 21:
toat(xpos+I) == str.at(I) for all elements ...
traits::eq(at(xpos+I), str.at(I)) for all elements ...
toat(xpos+I) == str.at(I) for all elements ...
traits::eq(at(xpos+I), str.at(I)) for all elements ...
toat(xpos+I) == str.at(I) for all elements ...
traits::eq(at(xpos+I), str.at(I)) for all elements ...
toat(xpos+I) == str.at(I) for all elements ...
traits::eq(at(xpos+I), str.at(I)) for all elements ...
toat(xpos+I) == str.at(I) for all elements ...
traits::eq(at(xpos+I), str.at(I)) for all elements ...
toat(xpos+I) == str.at(I) for all elements ...
traits::eq(at(xpos+I), str.at(I)) for all elements ...
toc == delim for the next available input character c
traits::eq(c, delim) for the next available input character c
Notes:
Section: 99 [depr.str.strstreams] Status: TC1 Submitter: Brendan Kehoe Opened: 1998-06-01 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
See lib-6522 and edit-814.
Proposed resolution:
Change 99 [depr.strstreambuf] (since streambuf is a typedef of basic_streambuf<char>) from:
virtual streambuf<char>* setbuf(char* s, streamsize n);
to:
virtual streambuf* setbuf(char* s, streamsize n);
In [depr.strstream] insert the semicolon now missing after int_type:
namespace std { class strstream : public basic_iostream<char> { public: // Types typedef char char_type; typedef typename char_traits<char>::int_type int_type typedef typename char_traits<char>::pos_type pos_type;
Section: 31.5.2.4 [ios.base.locales] Status: TC1 Submitter: Matt Austern Opened: 1998-06-21 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
Section 27.4.2.3 specifies how imbue() and getloc() work. That section has two RETURNS clauses, and they make no sense as stated. They make perfect sense, though, if you swap them. Am I correct in thinking that paragraphs 2 and 4 just got mixed up by accident?
Proposed resolution:
In 31.5.2.4 [ios.base.locales] swap paragraphs 2 and 4.
Section: 31.5.2.2.1 [ios.failure] Status: TC1 Submitter: Matt Austern Opened: 1998-06-21 Last modified: 2021-06-06
Priority: Not Prioritized
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Discussion:
27.4.2.1.1, paragraph 2, says that class failure initializes the base class, exception, with exception(msg). Class exception (see 18.6.1) has no such constructor.
Proposed resolution:
Replace [ios::failure], paragraph 2, with
EFFECTS: Constructs an object of class
failure
.
Section: 31.5.2.5 [ios.members.static] Status: CD1 Submitter: Matt Austern Opened: 1998-06-21 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
Two problems
(1) 27.4.2.4 doesn't say what ios_base::sync_with_stdio(f) returns. Does it return f, or does it return the previous synchronization state? My guess is the latter, but the standard doesn't say so.
(2) 27.4.2.4 doesn't say what it means for streams to be synchronized with stdio. Again, of course, I can make some guesses. (And I'm unhappy about the performance implications of those guesses, but that's another matter.)
Proposed resolution:
Change the following sentence in 31.5.2.5 [ios.members.static] returns clause from:
true
if the standard iostream objects (27.3) are synchronized and otherwise returnsfalse
.
to:
true
if the previous state of the standard iostream objects (27.3) was synchronized and otherwise returnsfalse
.
Add the following immediately after 31.5.2.5 [ios.members.static], paragraph 2:
When a standard iostream object str is synchronized with a standard stdio stream f, the effect of inserting a character c by
fputc(f, c);is the same as the effect of
str.rdbuf()->sputc(c);for any sequence of characters; the effect of extracting a character c by
c = fgetc(f);is the same as the effect of:
c = str.rdbuf()->sbumpc(c);for any sequences of characters; and the effect of pushing back a character c by
ungetc(c, f);is the same as the effect of
str.rdbuf()->sputbackc(c);for any sequence of characters. [Footnote: This implies that operations on a standard iostream object can be mixed arbitrarily with operations on the corresponding stdio stream. In practical terms, synchronization usually means that a standard iostream object and a standard stdio object share a buffer. --End Footnote]
[pre-Copenhagen: PJP and Matt contributed the definition of "synchronization"]
[post-Copenhagen: proposed resolution was revised slightly: text was added in the non-normative footnote to say that operations on the two streams can be mixed arbitrarily.]
Section: 31.5.2 [ios.base] Status: TC1 Submitter: Matt Austern Opened: 1998-06-21 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
As written, ios_base has a copy constructor and an assignment operator. (Nothing in the standard says it doesn't have one, and all classes have copy constructors and assignment operators unless you take specific steps to avoid them.) However, nothing in 27.4.2 says what the copy constructor and assignment operator do.
My guess is that this was an oversight, that ios_base is, like basic_ios, not supposed to have a copy constructor or an assignment operator.
Jerry Schwarz comments: Yes, its an oversight, but in the opposite sense to what you're suggesting. At one point there was a definite intention that you could copy ios_base. It's an easy way to save the entire state of a stream for future use. As you note, to carry out that intention would have required a explicit description of the semantics (e.g. what happens to the iarray and parray stuff).
Proposed resolution:
In 31.5.2 [ios.base], class ios_base, specify the copy constructor and operator= members as being private.
Rationale:
The LWG believes the difficulty of specifying correct semantics outweighs any benefit of allowing ios_base objects to be copyable.
Section: 23.2 [container.requirements] Status: TC1 Submitter: David Vandevoorde Opened: 1998-06-23 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
The std::sort algorithm can in general only sort a given sequence by moving around values. The list<>::sort() member on the other hand could move around values or just update internal pointers. Either method can leave iterators into the list<> dereferencable, but they would point to different things.
Does the FDIS mandate anywhere which method should be used for list<>::sort()?
Matt Austern comments:
I think you've found an omission in the standard.
The library working group discussed this point, and there was supposed to be a general requirement saying that list, set, map, multiset, and multimap may not invalidate iterators, or change the values that iterators point to, except when an operation does it explicitly. So, for example, insert() doesn't invalidate any iterators and erase() and remove() only invalidate iterators pointing to the elements that are being erased.
I looked for that general requirement in the FDIS, and, while I found a limited form of it for the sorted associative containers, I didn't find it for list. It looks like it just got omitted.
The intention, though, is that list<>::sort does not invalidate any iterators and does not change the values that any iterator points to. There would be no reason to have the member function otherwise.
Proposed resolution:
Add a new paragraph at the end of 23.1:
Unless otherwise specified (either explicitly or by defining a function in terms of other functions), invoking a container member function or passing a container as an argument to a library function shall not invalidate iterators to, or change the values of, objects within that container.
Rationale:
This was US issue CD2-23-011; it was accepted in London but the change was not made due to an editing oversight. The wording in the proposed resolution below is somewhat updated from CD2-23-011, particularly the addition of the phrase "or change the values of"
Section: 31.5.3.3 [fpos.operations] Status: TC1 Submitter: Matt Austern Opened: 1998-06-23 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
First, 31.5.4.2 [basic.ios.cons], table 89. This is pretty obvious: it should be titled "basic_ios<>() effects", not "ios_base() effects".
[The second item is a duplicate; see issue 6(i) for resolution.]
Second, 31.5.3.3 [fpos.operations] table 88 . There are a couple different things wrong with it, some of which I've already discussed with Jerry, but the most obvious mechanical sort of error is that it uses expressions like P(i) and p(i), without ever defining what sort of thing "i" is.
(The other problem is that it requires support for streampos arithmetic. This is impossible on some systems, i.e. ones where file position is a complicated structure rather than just a number. Jerry tells me that the intention was to require syntactic support for streampos arithmetic, but that it wasn't actually supposed to do anything meaningful except on platforms, like Unix, where genuine arithmetic is possible.)
Proposed resolution:
Change 31.5.4.2 [basic.ios.cons] table 89 title from "ios_base() effects" to "basic_ios<>() effects".
Section: 31.5.4.2 [basic.ios.cons] Status: TC1 Submitter: Matt Austern Opened: 1998-06-23 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
There's nothing in 27.4.4 saying what basic_ios's destructor does. The important question is whether basic_ios::~basic_ios() destroys rdbuf().
Proposed resolution:
Add after 31.5.4.2 [basic.ios.cons] paragraph 2:
virtual ~basic_ios();
Notes: The destructor does not destroy
rdbuf()
.
Rationale:
The LWG reviewed the additional question of whether or not
rdbuf(0)
may set badbit
. The answer is
clearly yes; it may be set via clear()
. See 31.5.4.3 [basic.ios.members], paragraph 6. This issue was reviewed at length
by the LWG, which removed from the original proposed resolution a
footnote which incorrectly said "rdbuf(0)
does not set
badbit
".
Section: 31.6.3.2 [streambuf.cons] Status: TC1 Submitter: Matt Austern Opened: 1998-06-25 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
The class synopsis for basic_streambuf shows a (virtual) destructor, but the standard doesn't say what that destructor does. My assumption is that it does nothing, but the standard should say so explicitly.
Proposed resolution:
Add after 31.6.3.2 [streambuf.cons] paragraph 2:
virtual ~basic_streambuf();
Effects: None.
Section: 31 [input.output] Status: TC1 Submitter: Matt Austern Opened: 1998-06-26 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
Several member functions in clause 27 are defined in certain circumstances to return an "invalid stream position", a term that is defined nowhere in the standard. Two places (27.5.2.4.2, paragraph 4, and 27.8.1.4, paragraph 15) contain a cross-reference to a definition in _lib.iostreams.definitions_, a nonexistent section.
I suspect that the invalid stream position is just supposed to be pos_type(-1). Probably best to say explicitly in (for example) 27.5.2.4.2 that the return value is pos_type(-1), rather than to use the term "invalid stream position", define that term somewhere, and then put in a cross-reference.
The phrase "invalid stream position" appears ten times in the C++ Standard. In seven places it refers to a return value, and it should be changed. In three places it refers to an argument, and it should not be changed. Here are the three places where "invalid stream position" should not be changed:
31.8.2.5 [stringbuf.virtuals], paragraph 14
31.10.3.5 [filebuf.virtuals], paragraph 14
99 [depr.strstreambuf.virtuals], paragraph 17
Proposed resolution:
In 31.6.3.5.2 [streambuf.virt.buffer], paragraph 4, change "Returns an
object of class pos_type that stores an invalid stream position
(_lib.iostreams.definitions_)" to "Returns
pos_type(off_type(-1))
".
In 31.6.3.5.2 [streambuf.virt.buffer], paragraph 6, change "Returns
an object of class pos_type that stores an invalid stream
position" to "Returns pos_type(off_type(-1))
".
In 31.8.2.5 [stringbuf.virtuals], paragraph 13, change "the object
stores an invalid stream position" to "the return value is
pos_type(off_type(-1))
".
In 31.10.3.5 [filebuf.virtuals], paragraph 13, change "returns an
invalid stream position (27.4.3)" to "returns
pos_type(off_type(-1))
"
In 31.10.3.5 [filebuf.virtuals], paragraph 15, change "Otherwise
returns an invalid stream position (_lib.iostreams.definitions_)"
to "Otherwise returns pos_type(off_type(-1))
"
In 99 [depr.strstreambuf.virtuals], paragraph 15, change "the object
stores an invalid stream position" to "the return value is
pos_type(off_type(-1))
"
In 99 [depr.strstreambuf.virtuals], paragraph 18, change "the object
stores an invalid stream position" to "the return value is
pos_type(off_type(-1))
"
Section: 31.6.3 [streambuf] Status: TC1 Submitter: Matt Austern Opened: 1998-06-29 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
The class summary for basic_streambuf<>, in 27.5.2, says that showmanyc has return type int. However, 27.5.2.4.3 says that its return type is streamsize.
Proposed resolution:
Change showmanyc
's return type in the
31.6.3 [streambuf] class summary to streamsize
.
Section: 27.2.4.6 [char.traits.specializations.wchar.t] Status: TC1 Submitter: Matt Austern Opened: 1998-07-01 Last modified: 2016-01-28
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Discussion:
21.1.3.2, paragraph 3, says "The types streampos and wstreampos may be different if the implementation supports no shift encoding in narrow-oriented iostreams but supports one or more shift encodings in wide-oriented streams".
That's wrong: the two are the same type. The <iosfwd> summary in 27.2 says that streampos and wstreampos are, respectively, synonyms for fpos<char_traits<char>::state_type> and fpos<char_traits<wchar_t>::state_type>, and, flipping back to clause 21, we see in 21.1.3.1 and 21.1.3.2 that char_traits<char>::state_type and char_traits<wchar_t>::state_type must both be mbstate_t.
Proposed resolution:
Remove the sentence in 27.2.4.6 [char.traits.specializations.wchar.t] paragraph 3 which begins "The types streampos and wstreampos may be different..." .
Section: 31.6.3.4.2 [streambuf.get.area] Status: TC1 Submitter: Matt Austern Opened: 1998-07-28 Last modified: 2016-01-28
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Discussion:
27.5.2.3.1 says that basic_streambuf::gbump() "Advances the next pointer for the input sequence by n."
The straightforward interpretation is that it is just gptr() += n. An alternative interpretation, though, is that it behaves as if it calls sbumpc n times. (The issue, of course, is whether it might ever call underflow.) There is a similar ambiguity in the case of pbump.
(The "classic" AT&T implementation used the former interpretation.)
Proposed resolution:
Change 31.6.3.4.2 [streambuf.get.area] paragraph 4 gbump effects from:
Effects: Advances the next pointer for the input sequence by n.
to:
Effects: Adds
n
to the next pointer for the input sequence.
Make the same change to 31.6.3.4.3 [streambuf.put.area] paragraph 4 pbump effects.
Section: 31.7.5.3.1 [istream.formatted.reqmts] Status: TC1 Submitter: Matt Austern Opened: 1998-08-03 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
Paragraph 1 of 27.6.1.2.1 contains general requirements for all formatted input functions. Some of the functions defined in section 27.6.1.2 explicitly say that those requirements apply ("Behaves like a formatted input member (as described in 27.6.1.2.1)"), but others don't. The question: is 27.6.1.2.1 supposed to apply to everything in 27.6.1.2, or only to those member functions that explicitly say "behaves like a formatted input member"? Or to put it differently: are we to assume that everything that appears in a section called "Formatted input functions" really is a formatted input function? I assume that 27.6.1.2.1 is intended to apply to the arithmetic extractors (27.6.1.2.2), but I assume that it is not intended to apply to extractors like
basic_istream& operator>>(basic_istream& (*pf)(basic_istream&));
and
basic_istream& operator>>(basic_streammbuf*);
There is a similar ambiguity for unformatted input, formatted output, and unformatted output.
Comments from Judy Ward: It seems like the problem is that the basic_istream and basic_ostream operator <<()'s that are used for the manipulators and streambuf* are in the wrong section and should have their own separate section or be modified to make it clear that the "Common requirements" listed in section 27.6.1.2.1 (for basic_istream) and section 27.6.2.5.1 (for basic_ostream) do not apply to them.
Additional comments from Dietmar Kühl: It appears to be somewhat
nonsensical to consider the functions defined in [istream::extractors] paragraphs 1 to 5 to be "Formatted input
function" but since these functions are defined in a section
labeled "Formatted input functions" it is unclear to me
whether these operators are considered formatted input functions which
have to conform to the "common requirements" from 31.7.5.3.1 [istream.formatted.reqmts]: If this is the case, all manipulators, not
just ws
, would skip whitespace unless noskipws
is
set (... but setting noskipws
using the manipulator syntax
would also skip whitespace :-)
It is not clear which functions
are to be considered unformatted input functions. As written, it seems
that all functions in 31.7.5.4 [istream.unformatted] are unformatted input
functions. However, it does not really make much sense to construct a
sentry object for gcount()
, sync()
, ... Also it is
unclear what happens to the gcount()
if
eg. gcount()
, putback()
, unget()
, or
sync()
is called: These functions don't extract characters,
some of them even "unextract" a character. Should this still
be reflected in gcount()
? Of course, it could be read as if
after a call to gcount()
gcount()
return 0
(the last unformatted input function, gcount()
, didn't
extract any character) and after a call to putback()
gcount()
returns -1
(the last unformatted input
function putback()
did "extract" back into the
stream). Correspondingly for unget()
. Is this what is
intended? If so, this should be clarified. Otherwise, a corresponding
clarification should be used.
Proposed resolution:
In 27.6.1.2.2 [lib.istream.formatted.arithmetic], paragraph 1. Change the beginning of the second sentence from "The conversion occurs" to "These extractors behave as formatted input functions (as described in 27.6.1.2.1). After a sentry object is constructed, the conversion occurs"
In 27.6.1.2.3, [lib.istream::extractors], before paragraph 1. Add an effects clause. "Effects: None. This extractor does not behave as a formatted input function (as described in 27.6.1.2.1).
In 27.6.1.2.3, [lib.istream::extractors], paragraph 2. Change the effects clause to "Effects: Calls pf(*this). This extractor does not behave as a formatted input function (as described in 27.6.1.2.1).
In 27.6.1.2.3, [lib.istream::extractors], paragraph 4. Change the effects clause to "Effects: Calls pf(*this). This extractor does not behave as a formatted input function (as described in 27.6.1.2.1).
In 27.6.1.2.3, [lib.istream::extractors], paragraph 12. Change the first two sentences from "If sb is null, calls setstate(failbit), which may throw ios_base::failure (27.4.4.3). Extracts characters from *this..." to "Behaves as a formatted input function (as described in 27.6.1.2.1). If sb is null, calls setstate(failbit), which may throw ios_base::failure (27.4.4.3). After a sentry object is constructed, extracts characters from *this...".
In 27.6.1.3, [lib.istream.unformatted], before paragraph 2. Add an effects clause. "Effects: none. This member function does not behave as an unformatted input function (as described in 27.6.1.3, paragraph 1)."
In 27.6.1.3, [lib.istream.unformatted], paragraph 3. Change the beginning of the first sentence of the effects clause from "Extracts a character" to "Behaves as an unformatted input function (as described in 27.6.1.3, paragraph 1). After constructing a sentry object, extracts a character"
In 27.6.1.3, [lib.istream.unformatted], paragraph 5. Change the beginning of the first sentence of the effects clause from "Extracts a character" to "Behaves as an unformatted input function (as described in 27.6.1.3, paragraph 1). After constructing a sentry object, extracts a character"
In 27.6.1.3, [lib.istream.unformatted], paragraph 7. Change the beginning of the first sentence of the effects clause from "Extracts characters" to "Behaves as an unformatted input function (as described in 27.6.1.3, paragraph 1). After constructing a sentry object, extracts characters"
[No change needed in paragraph 10, because it refers to paragraph 7.]
In 27.6.1.3, [lib.istream.unformatted], paragraph 12. Change the beginning of the first sentence of the effects clause from "Extracts characters" to "Behaves as an unformatted input function (as described in 27.6.1.3, paragraph 1). After constructing a sentry object, extracts characters"
[No change needed in paragraph 15.]
In 27.6.1.3, [lib.istream.unformatted], paragraph 17. Change the beginning of the first sentence of the effects clause from "Extracts characters" to "Behaves as an unformatted input function (as described in 27.6.1.3, paragraph 1). After constructing a sentry object, extracts characters"
[No change needed in paragraph 23.]
In 27.6.1.3, [lib.istream.unformatted], paragraph 24. Change the beginning of the first sentence of the effects clause from "Extracts characters" to "Behaves as an unformatted input function (as described in 27.6.1.3, paragraph 1). After constructing a sentry object, extracts characters"
In 27.6.1.3, [lib.istream.unformatted], before paragraph 27. Add an Effects clause: "Effects: Behaves as an unformatted input function (as described in 27.6.1.3, paragraph 1). After constructing a sentry object, reads but does not extract the current input character."
In 27.6.1.3, [lib.istream.unformatted], paragraph 28. Change the first sentence of the Effects clause from "If !good() calls" to Behaves as an unformatted input function (as described in 27.6.1.3, paragraph 1). After constructing a sentry object, if !good() calls"
In 27.6.1.3, [lib.istream.unformatted], paragraph 30. Change the first sentence of the Effects clause from "If !good() calls" to "Behaves as an unformatted input function (as described in 27.6.1.3, paragraph 1). After constructing a sentry object, if !good() calls"
In 27.6.1.3, [lib.istream.unformatted], paragraph 32. Change the first sentence of the Effects clause from "If !good() calls..." to "Behaves as an unformatted input function (as described in 27.6.1.3, paragraph 1). After constructing a sentry object, if !good() calls..." Add a new sentence to the end of the Effects clause: "[Note: this function extracts no characters, so the value returned by the next call to gcount() is 0.]"
In 27.6.1.3, [lib.istream.unformatted], paragraph 34. Change the first sentence of the Effects clause from "If !good() calls" to "Behaves as an unformatted input function (as described in 27.6.1.3, paragraph 1). After constructing a sentry object, if !good() calls". Add a new sentence to the end of the Effects clause: "[Note: this function extracts no characters, so the value returned by the next call to gcount() is 0.]"
In 27.6.1.3, [lib.istream.unformatted], paragraph 36. Change the first sentence of the Effects clause from "If !rdbuf() is" to "Behaves as an unformatted input function (as described in 27.6.1.3, paragraph 1), except that it does not count the number of characters extracted and does not affect the value returned by subsequent calls to gcount(). After constructing a sentry object, if rdbuf() is"
In 27.6.1.3, [lib.istream.unformatted], before paragraph 37. Add an Effects clause: "Effects: Behaves as an unformatted input function (as described in 27.6.1.3, paragraph 1), except that it does not count the number of characters extracted and does not affect the value returned by subsequent calls to gcount()." Change the first sentence of paragraph 37 from "if fail()" to "after constructing a sentry object, if fail()".
In 27.6.1.3, [lib.istream.unformatted], paragraph 38. Change the first sentence of the Effects clause from "If fail()" to "Behaves as an unformatted input function (as described in 27.6.1.3, paragraph 1), except that it does not count the number of characters extracted and does not affect the value returned by subsequent calls to gcount(). After constructing a sentry object, if fail()
In 27.6.1.3, [lib.istream.unformatted], paragraph 40. Change the first sentence of the Effects clause from "If fail()" to "Behaves as an unformatted input function (as described in 27.6.1.3, paragraph 1), except that it does not count the number of characters extracted and does not affect the value returned by subsequent calls to gcount(). After constructing a sentry object, if fail()
In 27.6.2.5.2 [lib.ostream.inserters.arithmetic], paragraph 1. Change the beginning of the third sentence from "The formatting conversion" to "These extractors behave as formatted output functions (as described in 27.6.2.5.1). After the sentry object is constructed, the conversion occurs".
In 27.6.2.5.3 [lib.ostream.inserters], before paragraph 1. Add an effects clause: "Effects: None. Does not behave as a formatted output function (as described in 27.6.2.5.1).".
In 27.6.2.5.3 [lib.ostream.inserters], paragraph 2. Change the effects clause to "Effects: calls pf(*this). This extractor does not behave as a formatted output function (as described in 27.6.2.5.1).".
In 27.6.2.5.3 [lib.ostream.inserters], paragraph 4. Change the effects clause to "Effects: calls pf(*this). This extractor does not behave as a formatted output function (as described in 27.6.2.5.1).".
In 27.6.2.5.3 [lib.ostream.inserters], paragraph 6. Change the first sentence from "If sb" to "Behaves as a formatted output function (as described in 27.6.2.5.1). After the sentry object is constructed, if sb".
In 27.6.2.6 [lib.ostream.unformatted], paragraph 2. Change the first sentence from "Inserts the character" to "Behaves as an unformatted output function (as described in 27.6.2.6, paragraph 1). After constructing a sentry object, inserts the character".
In 27.6.2.6 [lib.ostream.unformatted], paragraph 5. Change the first sentence from "Obtains characters" to "Behaves as an unformatted output function (as described in 27.6.2.6, paragraph 1). After constructing a sentry object, obtains characters".
In 27.6.2.6 [lib.ostream.unformatted], paragraph 7. Add a new sentence at the end of the paragraph: "Does not behave as an unformatted output function (as described in 27.6.2.6, paragraph 1)."
Rationale:
See J16/99-0043==WG21/N1219, Proposed Resolution to Library Issue 60, by Judy Ward and Matt Austern. This proposed resolution is section VI of that paper.
Section: 31.7.5.4 [istream.unformatted] Status: TC1 Submitter: Matt Austern Opened: 1998-08-06 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
The introduction to the section on unformatted input (27.6.1.3) says that every unformatted input function catches all exceptions that were thrown during input, sets badbit, and then conditionally rethrows the exception. That seems clear enough. Several of the specific functions, however, such as get() and read(), are documented in some circumstances as setting eofbit and/or failbit. (The standard notes, correctly, that setting eofbit or failbit can sometimes result in an exception being thrown.) The question: if one of these functions throws an exception triggered by setting failbit, is this an exception "thrown during input" and hence covered by 27.6.1.3, or does 27.6.1.3 only refer to a limited class of exceptions? Just to make this concrete, suppose you have the following snippet.
char buffer[N]; istream is; ... is.exceptions(istream::failbit); // Throw on failbit but not on badbit. is.read(buffer, N);
Now suppose we reach EOF before we've read N characters. What iostate bits can we expect to be set, and what exception (if any) will be thrown?
Proposed resolution:
In 27.6.1.3, paragraph 1, after the sentence that begins
"If an exception is thrown...", add the following
parenthetical comment: "(Exceptions thrown from
basic_ios<>::clear()
are not caught or rethrown.)"
Rationale:
The LWG looked to two alternative wordings, and choose the proposed resolution as better standardese.
Sync
's return valueSection: 31.7.5.4 [istream.unformatted] Status: TC1 Submitter: Matt Austern Opened: 1998-08-06 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
The Effects clause for sync() (27.6.1.3, paragraph 36) says that it "calls rdbuf()->pubsync() and, if that function returns -1 ... returns traits::eof()."
That looks suspicious, because traits::eof() is of type traits::int_type while the return type of sync() is int.
Proposed resolution:
In 31.7.5.4 [istream.unformatted], paragraph 36, change "returns
traits::eof()
" to "returns -1
".
Section: 31.7.6.4 [ostream.unformatted] Status: TC1 Submitter: Matt Austern Opened: 1998-08-11 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
Clause 27 details an exception-handling policy for formatted input, unformatted input, and formatted output. It says nothing for unformatted output (27.6.2.6). 27.6.2.6 should either include the same kind of exception-handling policy as in the other three places, or else it should have a footnote saying that the omission is deliberate.
Proposed resolution:
In 27.6.2.6, paragraph 1, replace the last sentence ("In any case, the unformatted output function ends by destroying the sentry object, then returning the value specified for the formatted output function.") with the following text:
If an exception is thrown during output, then
ios::badbit
is turned on [Footnote: without causing anios::failure
to be thrown.] in*this
's error state. If(exceptions() & badbit) != 0
then the exception is rethrown. In any case, the unformatted output function ends by destroying the sentry object, then, if no exception was thrown, returning the value specified for the formatted output function.
Rationale:
This exception-handling policy is consistent with that of formatted input, unformatted input, and formatted output.
basic_istream::operator>>(basic_streambuf*)
Section: 31.7.5.3.3 [istream.extractors] Status: TC1 Submitter: Matt Austern Opened: 1998-08-11 Last modified: 2017-04-22
Priority: Not Prioritized
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Discussion:
27.6.1.2.3, paragraph 13, is ambiguous. It can be interpreted two different ways, depending on whether the second sentence is read as an elaboration of the first.
Proposed resolution:
Replace [istream::extractors], paragraph 13, which begins "If the function inserts no characters ..." with:
If the function inserts no characters, it calls
setstate(failbit)
, which may throwios_base::failure
(27.4.4.3). If it inserted no characters because it caught an exception thrown while extracting characters fromsb
andfailbit
is on inexceptions()
(27.4.4.3), then the caught exception is rethrown.
Section: 99 [depr.strstreambuf.virtuals] Status: TC1 Submitter: Matt Austern Opened: 1998-08-18 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
D.7.1.3, paragraph 19, says that strstreambuf::setbuf "Performs an operation that is defined separately for each class derived from strstreambuf". This is obviously an incorrect cut-and-paste from basic_streambuf. There are no classes derived from strstreambuf.
Proposed resolution:
99 [depr.strstreambuf.virtuals], paragraph 19, replace the setbuf effects clause which currently says "Performs an operation that is defined separately for each class derived from strstreambuf" with:
Effects: implementation defined, except that
setbuf(0,0)
has no effect.
Section: 31.7.5.3.3 [istream.extractors] Status: TC1 Submitter: Angelika Langer Opened: 1998-07-14 Last modified: 2017-04-22
Priority: Not Prioritized
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Discussion:
Extractors for char* (27.6.1.2.3) do not store a null character after the extracted character sequence whereas the unformatted functions like get() do. Why is this?
Comment from Jerry Schwarz: There is apparently an editing glitch. You'll notice that the last item of the list of what stops extraction doesn't make any sense. It was supposed to be the line that said a null is stored.
Proposed resolution:
[istream::extractors], paragraph 7, change the last list item from:
A null byte (
charT()
) in the next position, which may be the first position if no characters were extracted.
to become a new paragraph which reads:
Operator>> then stores a null byte (
charT()
) in the next position, which may be the first position if no characters were extracted.
Section: 23.3.13 [vector] Status: TC1 Submitter: Andrew Koenig Opened: 1998-07-29 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
The issue is this: Must the elements of a vector be in contiguous memory?
(Please note that this is entirely separate from the question of whether a vector iterator is required to be a pointer; the answer to that question is clearly "no," as it would rule out debugging implementations)
Proposed resolution:
Add the following text to the end of 23.3.13 [vector], paragraph 1.
The elements of a vector are stored contiguously, meaning that if v is a
vector<T, Allocator>
where T is some type other thanbool
, then it obeys the identity&v[n] == &v[0] + n
for all0 <= n < v.size()
.
Rationale:
The LWG feels that as a practical matter the answer is clearly "yes". There was considerable discussion as to the best way to express the concept of "contiguous", which is not directly defined in the standard. Discussion included:
Section: 17.9 [support.exception], 99 [uncaught] Status: TC1 Submitter: Steve Clamage Opened: 1998-08-03 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
In article 3E04@pratique.fr, Valentin Bonnard writes:
uncaught_exception() doesn't have a throw specification.
It is intentional ? Does it means that one should be prepared to handle exceptions thrown from uncaught_exception() ?
uncaught_exception() is called in exception handling contexts where exception safety is very important.
Proposed resolution:
In [except.uncaught], paragraph 1, 17.9 [support.exception], and 99 [uncaught], add "throw()" to uncaught_exception().
Section: 28.3.4.6.2 [locale.time.get] Status: TC1 Submitter: Nathan Myers Opened: 1998-08-13 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
The locale facet member time_get<>::do_get_monthname
is described in 28.3.4.6.2.3 [locale.time.get.virtuals] with five arguments,
consistent with do_get_weekday and with its specified use by member
get_monthname. However, in the synopsis, it is specified instead with
four arguments. The missing argument is the "end" iterator
value.
Proposed resolution:
In 28.3.4.6.2 [locale.time.get], add an "end" argument to the declaration of member do_monthname as follows:
virtual iter_type do_get_monthname(iter_type s, iter_type end, ios_base&, ios_base::iostate& err, tm* t) const;
codecvt::do_max_length
Section: 28.3.4.2.5 [locale.codecvt] Status: TC1 Submitter: Matt Austern Opened: 1998-09-08 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
The text of codecvt::do_max_length
's "Returns"
clause (22.2.1.5.2, paragraph 11) is garbled. It has unbalanced
parentheses and a spurious n.
Proposed resolution:
Replace 28.3.4.2.5.3 [locale.codecvt.virtuals] paragraph 11 with the following:
Returns: The maximum value that
do_length(state, from, from_end, 1)
can return for any valid range[from, from_end)
andstateT
valuestate
. The specializationcodecvt<char, char, mbstate_t>::do_max_length()
returns 1.
codecvt::length
's argument typesSection: 28.3.4.2.5 [locale.codecvt] Status: TC1 Submitter: Matt Austern Opened: 1998-09-18 Last modified: 2023-03-29
Priority: Not Prioritized
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Discussion:
The class synopses for classes codecvt<>
(22.2.1.5)
and codecvt_byname<>
(22.2.1.6) say that the first
parameter of the member functions length
and
do_length
is of type const stateT&
. The member
function descriptions, however (22.2.1.5.1, paragraph 6; 22.2.1.5.2,
paragraph 9) say that the type is stateT&
. Either the
synopsis or the summary must be changed.
If (as I believe) the member function descriptions are correct,
then we must also add text saying how do_length
changes its
stateT
argument.
Proposed resolution:
In 28.3.4.2.5 [locale.codecvt], and also in 28.3.4.2.6 [locale.codecvt.byname],
change the stateT
argument type on both member
length()
and member do_length()
from
const stateT&
to
stateT&
In 28.3.4.2.5.3 [locale.codecvt.virtuals], add to the definition for member
do_length
a paragraph:
Effects: The effect on the
state
argument is ``as if'' it calleddo_in(state, from, from_end, from, to, to+max, to)
forto
pointing to a buffer of at leastmax
elements.
codecvt
facet always convert one internal character at a time?Section: 28.3.4.2.5 [locale.codecvt] Status: CD1 Submitter: Matt Austern Opened: 1998-09-25 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
This issue concerns the requirements on classes derived from
codecvt
, including user-defined classes. What are the
restrictions on the conversion from external characters
(e.g. char
) to internal characters (e.g. wchar_t
)?
Or, alternatively, what assumptions about codecvt
facets can
the I/O library make?
The question is whether it's possible to convert from internal characters to external characters one internal character at a time, and whether, given a valid sequence of external characters, it's possible to pick off internal characters one at a time. Or, to put it differently: given a sequence of external characters and the corresponding sequence of internal characters, does a position in the internal sequence correspond to some position in the external sequence?
To make this concrete, suppose that [first, last)
is a
sequence of M external characters and that [ifirst,
ilast)
is the corresponding sequence of N internal
characters, where N > 1. That is, my_encoding.in()
,
applied to [first, last)
, yields [ifirst,
ilast)
. Now the question: does there necessarily exist a
subsequence of external characters, [first, last_1)
, such
that the corresponding sequence of internal characters is the single
character *ifirst
?
(What a "no" answer would mean is that
my_encoding
translates sequences only as blocks. There's a
sequence of M external characters that maps to a sequence of
N internal characters, but that external sequence has no
subsequence that maps to N-1 internal characters.)
Some of the wording in the standard, such as the description of
codecvt::do_max_length
(28.3.4.2.5.3 [locale.codecvt.virtuals],
paragraph 11) and basic_filebuf::underflow
(31.10.3.5 [filebuf.virtuals], paragraph 3) suggests that it must always be
possible to pick off internal characters one at a time from a sequence
of external characters. However, this is never explicitly stated one
way or the other.
This issue seems (and is) quite technical, but it is important if
we expect users to provide their own encoding facets. This is an area
where the standard library calls user-supplied code, so a well-defined
set of requirements for the user-supplied code is crucial. Users must
be aware of the assumptions that the library makes. This issue affects
positioning operations on basic_filebuf
, unbuffered input,
and several of codecvt
's member functions.
Proposed resolution:
Add the following text as a new paragraph, following 28.3.4.2.5.3 [locale.codecvt.virtuals] paragraph 2:
A
codecvt
facet that is used bybasic_filebuf
(31.10 [file.streams]) must have the property that ifdo_out(state, from, from_end, from_next, to, to_lim, to_next)would return
ok
, wherefrom != from_end
, thendo_out(state, from, from + 1, from_next, to, to_end, to_next)must also return
ok
, and that ifdo_in(state, from, from_end, from_next, to, to_lim, to_next)would return
ok
, whereto != to_lim
, thendo_in(state, from, from_end, from_next, to, to + 1, to_next)must also return
ok
. [Footnote: Informally, this means thatbasic_filebuf
assumes that the mapping from internal to external characters is 1 to N: acodecvt
that is used bybasic_filebuf
must be able to translate characters one internal character at a time. --End Footnote]
[Redmond: Minor change in proposed resolution. Original
proposed resolution talked about "success", with a parenthetical
comment that success meant returning ok
. New wording
removes all talk about "success", and just talks about the
return value.]
Rationale:
The proposed resoluion says that conversions can be performed one internal character at a time. This rules out some encodings that would otherwise be legal. The alternative answer would mean there would be some internal positions that do not correspond to any external file position.
An example of an encoding that this rules out is one where the
internT
and externT
are of the same type, and
where the internal sequence c1 c2
corresponds to the
external sequence c2 c1
.
It was generally agreed that basic_filebuf
relies
on this property: it was designed under the assumption that
the external-to-internal mapping is N-to-1, and it is not clear
that basic_filebuf
is implementable without that
restriction.
The proposed resolution is expressed as a restriction on
codecvt
when used by basic_filebuf
, rather
than a blanket restriction on all codecvt
facets,
because basic_filebuf
is the only other part of the
library that uses codecvt
. If a user wants to define
a codecvt
facet that implements a more general N-to-M
mapping, there is no reason to prohibit it, so long as the user
does not expect basic_filebuf
to be able to use it.
Section: 31.5.2 [ios.base] Status: TC1 Submitter: Nico Josuttis Opened: 1998-09-29 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
typo: event_call_back should be event_callback
Proposed resolution:
In the 31.5.2 [ios.base] synopsis change "event_call_back" to "event_callback".
Section: 29.4.2 [complex.syn], 29.4.7 [complex.value.ops] Status: TC1 Submitter: Nico Josuttis Opened: 1998-09-29 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
In 29.4.2 [complex.syn] polar is declared as follows:
template<class T> complex<T> polar(const T&, const T&);
In 29.4.7 [complex.value.ops] it is declared as follows:
template<class T> complex<T> polar(const T& rho, const T& theta = 0);
Thus whether the second parameter is optional is not clear.
Proposed resolution:
In 29.4.2 [complex.syn] change:
template<class T> complex<T> polar(const T&, const T&);
to:
template<class T> complex<T> polar(const T& rho, const T& theta = 0);
Section: 29.4.2 [complex.syn], 29.4.3 [complex] Status: TC1 Submitter: Nico Josuttis Opened: 1998-09-29 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
Both 26.2.1 and 26.2.2 contain declarations of global operators for class complex. This redundancy should be removed.
Proposed resolution:
Reduce redundancy according to the general style of the standard.
Section: 27.4.3 [basic.string] Status: TC1 Submitter: Nico Josuttis Opened: 1998-09-29 Last modified: 2016-01-28
Priority: Not Prioritized
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Duplicate of: 89
Discussion:
Many string member functions throw if size is getting or exceeding npos. However, I wonder why they don't throw if size is getting or exceeding max_size() instead of npos. May be npos is known at compile time, while max_size() is known at runtime. However, what happens if size exceeds max_size() but not npos, then? It seems the standard lacks some clarifications here.
Proposed resolution:
After 27.4.3 [basic.string] paragraph 4 ("The functions described in this clause...") add a new paragraph:
For any string operation, if as a result of the operation,
size()
would exceedmax_size()
then the operation throwslength_error
.
Rationale:
The LWG believes length_error is the correct exception to throw.
Section: 27.4.3.2 [string.require] Status: TC1 Submitter: Nico Josuttis Opened: 1998-09-29 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
The constructor from a range:
template<class InputIterator> basic_string(InputIterator begin, InputIterator end, const Allocator& a = Allocator());
lacks a throws clause. However, I would expect that it throws according to the other constructors if the numbers of characters in the range equals npos (or exceeds max_size(), see above).
Proposed resolution:
In 27.4.3.2 [string.require], Strike throws paragraphs for constructors which say "Throws: length_error if n == npos."
Rationale:
Throws clauses for length_error if n == npos are no longer needed because they are subsumed by the general wording added by the resolution for issue 83(i).
Section: 27.4.4.4 [string.io] Status: TC1 Submitter: Nico Josuttis Opened: 1998-09-29 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
The effect of operator >> for strings contain the following item:
isspace(c,getloc())
is true for the next available input
character c.
Here getloc()
has to be replaced by is.getloc()
.
Proposed resolution:
In 27.4.4.4 [string.io] paragraph 1 Effects clause replace:
isspace(c,getloc())
is true for the next available input character c.
with:
isspace(c,is.getloc())
is true for the next available input character c.
Section: 27.4.4.4 [string.io] Status: CD1 Submitter: Nico Josuttis Opened: 1998-09-29 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
Operator >> and getline() for strings read until eof() in the input stream is true. However, this might never happen, if the stream can't read anymore without reaching EOF. So shouldn't it be changed into that it reads until !good() ?
Proposed resolution:
In 27.4.4.4 [string.io], paragraph 1, replace:
Effects: Begins by constructing a sentry object k as if k were constructed by typename basic_istream<charT,traits>::sentry k( is). If bool( k) is true, it calls str.erase() and then extracts characters from is and appends them to str as if by calling str.append(1, c). If is.width() is greater than zero, the maximum number n of characters appended is is.width(); otherwise n is str.max_size(). Characters are extracted and appended until any of the following occurs:
with:
Effects: Behaves as a formatted input function (31.7.5.3.1 [istream.formatted.reqmts]). After constructing a sentry object, if the sentry converts to true, calls str.erase() and then extracts characters from is and appends them to str as if by calling str.append(1,c). If is.width() is greater than zero, the maximum number n of characters appended is is.width(); otherwise n is str.max_size(). Characters are extracted and appended until any of the following occurs:
In 27.4.4.4 [string.io], paragraph 6, replace
Effects: Begins by constructing a sentry object k as if by typename basic_istream<charT,traits>::sentry k( is, true). If bool( k) is true, it calls str.erase() and then extracts characters from is and appends them to str as if by calling str.append(1, c) until any of the following occurs:
with:
Effects: Behaves as an unformatted input function (31.7.5.4 [istream.unformatted]), except that it does not affect the value returned by subsequent calls to basic_istream<>::gcount(). After constructing a sentry object, if the sentry converts to true, calls str.erase() and then extracts characters from is and appends them to str as if by calling str.append(1,c) until any of the following occurs:
[Redmond: Made changes in proposed resolution. operator>>
should be a formatted input function, not an unformatted input function.
getline
should not be required to set gcount
, since
there is no mechanism for gcount
to be set except by one of
basic_istream
's member functions.]
[Curaçao: Nico agrees with proposed resolution.]
Rationale:
The real issue here is whether or not these string input functions get their characters from a streambuf, rather than by calling an istream's member functions, a streambuf signals failure either by returning eof or by throwing an exception; there are no other possibilities. The proposed resolution makes it clear that these two functions do get characters from a streambuf.
Section: 26 [algorithms] Status: CD1 Submitter: Nico Josuttis Opened: 1998-09-29 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
The standard does not state, how often a function object is copied, called, or the order of calls inside an algorithm. This may lead to surprising/buggy behavior. Consider the following example:
class Nth { // function object that returns true for the nth element private: int nth; // element to return true for int count; // element counter public: Nth (int n) : nth(n), count(0) { } bool operator() (int) { return ++count == nth; } }; .... // remove third element list<int>::iterator pos; pos = remove_if(coll.begin(),coll.end(), // range Nth(3)), // remove criterion coll.erase(pos,coll.end());
This call, in fact removes the 3rd AND the 6th element. This happens because the usual implementation of the algorithm copies the function object internally:
template <class ForwIter, class Predicate> ForwIter std::remove_if(ForwIter beg, ForwIter end, Predicate op) { beg = find_if(beg, end, op); if (beg == end) { return beg; } else { ForwIter next = beg; return remove_copy_if(++next, end, beg, op); } }
The algorithm uses find_if() to find the first element that should be removed. However, it then uses a copy of the passed function object to process the resulting elements (if any). Here, Nth is used again and removes also the sixth element. This behavior compromises the advantage of function objects being able to have a state. Without any cost it could be avoided (just implement it directly instead of calling find_if()).
Proposed resolution:
Add a new paragraph following 26 [algorithms] paragraph 8:
[Note: Unless otherwise specified, algorithms that take function objects as arguments are permitted to copy those function objects freely. Programmers for whom object identity is important should consider using a wrapper class that points to a noncopied implementation object, or some equivalent solution.]
[Dublin: Pete Becker felt that this may not be a defect, but rather something that programmers need to be educated about. There was discussion of adding wording to the effect that the number and order of calls to function objects, including predicates, not affect the behavior of the function object.]
[Pre-Kona: Nico comments: It seems the problem is that we don't have a clear statement of "predicate" in the standard. People including me seemed to think "a function returning a Boolean value and being able to be called by an STL algorithm or be used as sorting criterion or ... is a predicate". But a predicate has more requirements: It should never change its behavior due to a call or being copied. IMHO we have to state this in the standard. If you like, see section 8.1.4 of my library book for a detailed discussion.]
[Kona: Nico will provide wording to the effect that "unless otherwise specified, the number of copies of and calls to function objects by algorithms is unspecified". Consider placing in 26 [algorithms] after paragraph 9.]
[Santa Cruz: The standard doesn't currently guarantee that
functions object won't be copied, and what isn't forbidden is
allowed. It is believed (especially since implementations that were
written in concert with the standard do make copies of function
objects) that this was intentional. Thus, no normative change is
needed. What we should put in is a non-normative note suggesting to
programmers that if they want to guarantee the lack of copying they
should use something like the ref
wrapper.]
[Oxford: Matt provided wording.]
Section: 24.3.5.3 [input.iterators] Status: CD1 Submitter: AFNOR Opened: 1998-10-07 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
Table 72 in 24.3.5.3 [input.iterators] specifies semantics for
*r++
of:
{ T tmp = *r; ++r; return tmp; }
There are two problems with this. First, the return type is specified to be "T", as opposed to something like "convertible to T". This is too specific: we want to allow *r++ to return an lvalue.
Second, writing the semantics in terms of code misleadingly suggests that the effects *r++ should precisely replicate the behavior of this code, including side effects. (Does this mean that *r++ should invoke the copy constructor exactly as many times as the sample code above would?) See issue 334(i) for a similar problem.
Proposed resolution:
In Table 72 in 24.3.5.3 [input.iterators], change the return type
for *r++
from T
to "convertible to T".
Rationale:
This issue has two parts: the return type, and the number of times the copy constructor is invoked.
The LWG believes the the first part is a real issue. It's
inappropriate for the return type to be specified so much more
precisely for *r++ than it is for *r. In particular, if r is of
(say) type int*
, then *r++ isn't int
,
but int&
.
The LWG does not believe that the number of times the copy constructor is invoked is a real issue. This can vary in any case, because of language rules on copy constructor elision. That's too much to read into these semantics clauses.
Additionally, as Dave Abrahams pointed out (c++std-lib-13703): since we're told (24.1/3) that forward iterators satisfy all the requirements of input iterators, we can't impose any requirements in the Input Iterator requirements table that forward iterators don't satisfy.
Section: 23.2.7 [associative.reqmts] Status: CD1 Submitter: AFNOR Opened: 1998-10-07 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
Set::iterator is described as implementation-defined with a reference to the container requirement; the container requirement says that const_iterator is an iterator pointing to const T and iterator an iterator pointing to T.
23.1.2 paragraph 2 implies that the keys should not be modified to break the ordering of elements. But that is not clearly specified. Especially considering that the current standard requires that iterator for associative containers be different from const_iterator. Set, for example, has the following:
typedef implementation defined iterator;
// See _lib.container.requirements_
23.2 [container.requirements] actually requires that iterator type pointing to T (table 65). Disallowing user modification of keys by changing the standard to require an iterator for associative container to be the same as const_iterator would be overkill since that will unnecessarily significantly restrict the usage of associative container. A class to be used as elements of set, for example, can no longer be modified easily without either redesigning the class (using mutable on fields that have nothing to do with ordering), or using const_cast, which defeats requiring iterator to be const_iterator. The proposed solution goes in line with trusting user knows what he is doing.
Other Options Evaluated:
Option A. In 23.2.7 [associative.reqmts], paragraph 2, after first sentence, and before "In addition,...", add one line:
Modification of keys shall not change their strict weak ordering.
Option B. Add three new sentences to 23.2.7 [associative.reqmts]:
At the end of paragraph 5: "Keys in an associative container are immutable." At the end of paragraph 6: "For associative containers where the value type is the same as the key type, both
iterator
andconst_iterator
are constant iterators. It is unspecified whether or notiterator
andconst_iterator
are the same type."
Option C. To 23.2.7 [associative.reqmts], paragraph 3, which currently reads:
The phrase ``equivalence of keys'' means the equivalence relation imposed by the comparison and not the operator== on keys. That is, two keys k1 and k2 in the same container are considered to be equivalent if for the comparison object comp, comp(k1, k2) == false && comp(k2, k1) == false.
add the following:
For any two keys k1 and k2 in the same container, comp(k1, k2) shall return the same value whenever it is evaluated. [Note: If k2 is removed from the container and later reinserted, comp(k1, k2) must still return a consistent value but this value may be different than it was the first time k1 and k2 were in the same container. This is intended to allow usage like a string key that contains a filename, where comp compares file contents; if k2 is removed, the file is changed, and the same k2 (filename) is reinserted, comp(k1, k2) must again return a consistent value but this value may be different than it was the previous time k2 was in the container.]
Proposed resolution:
Add the following to 23.2.7 [associative.reqmts] at the indicated location:
At the end of paragraph 3: "For any two keys k1 and k2 in the same container, calling comp(k1, k2) shall always return the same value."
At the end of paragraph 5: "Keys in an associative container are immutable."
At the end of paragraph 6: "For associative containers where the value type is the same as the key type, both
iterator
andconst_iterator
are constant iterators. It is unspecified whether or notiterator
andconst_iterator
are the same type."
Rationale:
Several arguments were advanced for and against allowing set elements to be mutable as long as the ordering was not effected. The argument which swayed the LWG was one of safety; if elements were mutable, there would be no compile-time way to detect of a simple user oversight which caused ordering to be modified. There was a report that this had actually happened in practice, and had been painful to diagnose. If users need to modify elements, it is possible to use mutable members or const_cast.
Simply requiring that keys be immutable is not sufficient, because the comparison object may indirectly (via pointers) operate on values outside of the keys.
The types iterator
and const_iterator
are permitted
to be different types to allow for potential future work in which some
member functions might be overloaded between the two types. No such
member functions exist now, and the LWG believes that user functionality
will not be impaired by permitting the two types to be the same. A
function that operates on both iterator types can be defined for
const_iterator
alone, and can rely on the automatic
conversion from iterator
to const_iterator
.
[Tokyo: The LWG crafted the proposed resolution and rationale.]
Section: 29.6.5 [template.slice.array] Status: TC1 Submitter: AFNOR Opened: 1998-10-07 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
This is the only place in the whole standard where the implementation has to document something private.
Proposed resolution:
Remove the comment which says "// remainder implementation defined" from:
Section: 17.7.3 [type.info] Status: TC1 Submitter: AFNOR Opened: 1998-10-07 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
In 18.6.1, paragraphs 8-9, the lifetime of the return value of exception::what() is left unspecified. This issue has implications with exception safety of exception handling: some exceptions should not throw bad_alloc.
Proposed resolution:
Add to 17.7.3 [type.info] paragraph 9 (exception::what notes clause) the sentence:
The return value remains valid until the exception object from which it is obtained is destroyed or a non-const member function of the exception object is called.
Rationale:
If an exception object has non-const members, they may be used
to set internal state that should affect the contents of the string
returned by what()
.
Section: 99 [depr.lib.binders] Status: CD1 Submitter: Bjarne Stroustrup Opened: 1998-10-07 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
There are no versions of binders that apply to non-const elements of a sequence. This makes examples like for_each() using bind2nd() on page 521 of "The C++ Programming Language (3rd)" non-conforming. Suitable versions of the binders need to be added.
Further discussion from Nico:
What is probably meant here is shown in the following example:
class Elem { public: void print (int i) const { } void modify (int i) { } };
int main() { vector<Elem> coll(2); for_each (coll.begin(), coll.end(), bind2nd(mem_fun_ref(&Elem::print),42)); // OK for_each (coll.begin(), coll.end(), bind2nd(mem_fun_ref(&Elem::modify),42)); // ERROR }
The error results from the fact that bind2nd() passes its first argument (the argument of the sequence) as constant reference. See the following typical implementation:
template <class Operation> class binder2nd : public unary_function<typename Operation::first_argument_type, typename Operation::result_type> { protected: Operation op; typename Operation::second_argument_type value; public: binder2nd(const Operation& o, const typename Operation::second_argument_type& v) : op(o), value(v) {}typename Operation::result_type operator()(const typename Operation::first_argument_type& x) const { return op(x, value); } };
The solution is to overload operator () of bind2nd for non-constant arguments:
template <class Operation> class binder2nd : public unary_function<typename Operation::first_argument_type, typename Operation::result_type> { protected: Operation op; typename Operation::second_argument_type value; public: binder2nd(const Operation& o, const typename Operation::second_argument_type& v) : op(o), value(v) {}typename Operation::result_type operator()(const typename Operation::first_argument_type& x) const { return op(x, value); } typename Operation::result_type operator()(typename Operation::first_argument_type& x) const { return op(x, value); } };
Proposed resolution:
Howard believes there is a flaw in this resolution. See c++std-lib-9127. We may need to reopen this issue.
In 99 [depr.lib.binders] in the declaration of binder1st after:
typename Operation::result_type
operator()(const typename Operation::second_argument_type& x) const;
insert:
typename Operation::result_type
operator()(typename Operation::second_argument_type& x) const;
In 99 [depr.lib.binders] in the declaration of binder2nd after:
typename Operation::result_type
operator()(const typename Operation::first_argument_type& x) const;
insert:
typename Operation::result_type
operator()(typename Operation::first_argument_type& x) const;
[Kona: The LWG discussed this at some length.It was agreed that this is a mistake in the design, but there was no consensus on whether it was a defect in the Standard. Straw vote: NAD - 5. Accept proposed resolution - 3. Leave open - 6.]
[Copenhagen: It was generally agreed that this was a defect. Strap poll: NAD - 0. Accept proposed resolution - 10. Leave open - 1.]
Section: 24.6.4 [istreambuf.iterator], 24.6.4.4 [istreambuf.iterator.ops] Status: TC1 Submitter: Nathan Myers Opened: 1998-10-15 Last modified: 2023-02-07
Priority: Not Prioritized
View other active issues in [istreambuf.iterator].
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Discussion:
Member istreambuf_iterator<>::equal is not declared "const", yet [istreambuf.iterator::op==] says that operator==, which is const, calls it. This is contradictory.
Proposed resolution:
In 24.6.4 [istreambuf.iterator] and also in [istreambuf.iterator::equal], replace:
bool equal(istreambuf_iterator& b);
with:
bool equal(const istreambuf_iterator& b) const;
ostreambuf_iterator
constructorSection: 24.6.5.2 [ostreambuf.iter.cons] Status: TC1 Submitter: Matt Austern Opened: 1998-10-20 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
The requires clause for ostreambuf_iterator
's
constructor from an ostream_type
(24.5.4.1, paragraph 1)
reads "s is not null". However, s is a
reference, and references can't be null.
Proposed resolution:
In 24.6.5.2 [ostreambuf.iter.cons]:
Move the current paragraph 1, which reads "Requires: s is not null.", from the first constructor to the second constructor.
Insert a new paragraph 1 Requires clause for the first constructor reading:
Requires:
s.rdbuf()
is not null.
Section: 17.6.3.4 [new.delete.placement] Status: TC1 Submitter: Steve Clamage Opened: 1998-10-28 Last modified: 2016-01-28
Priority: Not Prioritized
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Duplicate of: 196
Discussion:
Section 18.5.1.3 contains the following example:
[Example: This can be useful for constructing an object at a known address: char place[sizeof(Something)]; Something* p = new (place) Something(); -end example]
First code line: "place" need not have any special alignment, and the following constructor could fail due to misaligned data.
Second code line: Aren't the parens on Something() incorrect? [Dublin: the LWG believes the () are correct.]
Examples are not normative, but nevertheless should not show code that is invalid or likely to fail.
Proposed resolution:
Replace the first line of code in the example in 17.6.3.4 [new.delete.placement] with:
void* place = operator new(sizeof(Something));
Section: 99 [depr.strstream.cons] Status: TC1 Submitter: Steve Clamage Opened: 1998-11-02 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
D.7.4.1 strstream constructors paragraph 2 says:
Effects: Constructs an object of class strstream, initializing the base class with iostream(& sb) and initializing sb with one of the two constructors:
- If mode&app==0, then s shall designate the first element of an array of n elements. The constructor is strstreambuf(s, n, s).
- If mode&app==0, then s shall designate the first element of an array of n elements that contains an NTBS whose first element is designated by s. The constructor is strstreambuf(s, n, s+std::strlen(s)).
Notice the second condition is the same as the first. I think the second condition should be "If mode&app==app", or "mode&app!=0", meaning that the append bit is set.
Proposed resolution:
In [depr.ostrstream.cons] paragraph 2 and 99 [depr.strstream.cons]
paragraph 2, change the first condition to (mode&app)==0
and the second condition to (mode&app)!=0
.
basic_ostream
uses nonexistent num_put
member functionsSection: 31.7.6.3.2 [ostream.inserters.arithmetic] Status: CD1 Submitter: Matt Austern Opened: 1998-11-20 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
The effects clause for numeric inserters says that
insertion of a value x
, whose type is either bool
,
short
, unsigned short
, int
, unsigned
int
, long
, unsigned long
, float
,
double
, long double
, or const void*
, is
delegated to num_put
, and that insertion is performed as if
through the following code fragment:
bool failed = use_facet< num_put<charT,ostreambuf_iterator<charT,traits> > >(getloc()).put(*this, *this, fill(), val). failed();
This doesn't work, because num_put<>
::put is only
overloaded for the types bool
, long
, unsigned
long
, double
, long double
, and const
void*
. That is, the code fragment in the standard is incorrect
(it is diagnosed as ambiguous at compile time) for the types
short
, unsigned short
, int
, unsigned
int
, and float
.
We must either add new member functions to num_put
, or
else change the description in ostream
so that it only calls
functions that are actually there. I prefer the latter.
Proposed resolution:
Replace 27.6.2.5.2, paragraph 1 with the following:
The classes num_get<> and num_put<> handle locale-dependent numeric formatting and parsing. These inserter functions use the imbued locale value to perform numeric formatting. When val is of type bool, long, unsigned long, double, long double, or const void*, the formatting conversion occurs as if it performed the following code fragment:
bool failed = use_facet< num_put<charT,ostreambuf_iterator<charT,traits> > >(getloc()).put(*this, *this, fill(), val). failed();When val is of type short the formatting conversion occurs as if it performed the following code fragment:
ios_base::fmtflags baseflags = ios_base::flags() & ios_base::basefield; bool failed = use_facet< num_put<charT,ostreambuf_iterator<charT,traits> > >(getloc()).put(*this, *this, fill(), baseflags == ios_base::oct || baseflags == ios_base::hex ? static_cast<long>(static_cast<unsigned short>(val)) : static_cast<long>(val)). failed();When val is of type int the formatting conversion occurs as if it performed the following code fragment:
ios_base::fmtflags baseflags = ios_base::flags() & ios_base::basefield; bool failed = use_facet< num_put<charT,ostreambuf_iterator<charT,traits> > >(getloc()).put(*this, *this, fill(), baseflags == ios_base::oct || baseflags == ios_base::hex ? static_cast<long>(static_cast<unsigned int>(val)) : static_cast<long>(val)). failed();When val is of type unsigned short or unsigned int the formatting conversion occurs as if it performed the following code fragment:
bool failed = use_facet< num_put<charT,ostreambuf_iterator<charT,traits> > >(getloc()).put(*this, *this, fill(), static_cast<unsigned long>(val)). failed();When val is of type float the formatting conversion occurs as if it performed the following code fragment:
bool failed = use_facet< num_put<charT,ostreambuf_iterator<charT,traits> > >(getloc()).put(*this, *this, fill(), static_cast<double>(val)). failed();
[post-Toronto: This differs from the previous proposed resolution; PJP provided the new wording. The differences are in signed short and int output.]
Rationale:
The original proposed resolution was to cast int and short to long, unsigned int and unsigned short to unsigned long, and float to double, thus ensuring that we don't try to use nonexistent num_put<> member functions. The current proposed resolution is more complicated, but gives more expected results for hex and octal output of signed short and signed int. (On a system with 16-bit short, for example, printing short(-1) in hex format should yield 0xffff.)
basic_istream
uses nonexistent num_get
member functionsSection: 31.7.5.3.2 [istream.formatted.arithmetic] Status: CD1 Submitter: Matt Austern Opened: 1998-11-20 Last modified: 2016-01-28
Priority: Not Prioritized
View all other issues in [istream.formatted.arithmetic].
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Discussion:
Formatted input is defined for the types short
, unsigned short
, int
,
unsigned int
, long
, unsigned long
, float
, double
,
long double
, bool
, and void*
. According to section 27.6.1.2.2,
formatted input of a value x
is done as if by the following code fragment:
typedef num_get< charT,istreambuf_iterator<charT,traits> > numget; iostate err = 0; use_facet< numget >(loc).get(*this, 0, *this, err, val); setstate(err);
According to section 28.3.4.3.2.2 [facet.num.get.members], however,
num_get<>::get()
is only overloaded for the types
bool
, long
, unsigned short
, unsigned
int
, unsigned long
, unsigned long
,
float
, double
, long double
, and
void*
. Comparing the lists from the two sections, we find
that 27.6.1.2.2 is using a nonexistent function for types
short
and int
.
Proposed resolution:
In 31.7.5.3.2 [istream.formatted.arithmetic] Arithmetic Extractors, remove the two lines (1st and 3rd) which read:
operator>>(short& val); ... operator>>(int& val);
And add the following at the end of that section (27.6.1.2.2) :
operator>>(short& val);The conversion occurs as if performed by the following code fragment (using the same notation as for the preceding code fragment):
typedef num_get< charT,istreambuf_iterator<charT,traits> > numget; iostate err = 0; long lval; use_facet< numget >(loc).get(*this, 0, *this, err, lval); if (err == 0 && (lval < numeric_limits<short>::min() || numeric_limits<short>::max() < lval)) err = ios_base::failbit; setstate(err);operator>>(int& val);The conversion occurs as if performed by the following code fragment (using the same notation as for the preceding code fragment):
typedef num_get< charT,istreambuf_iterator<charT,traits> > numget; iostate err = 0; long lval; use_facet< numget >(loc).get(*this, 0, *this, err, lval); if (err == 0 && (lval < numeric_limits<int>::min() || numeric_limits<int>::max() < lval)) err = ios_base::failbit; setstate(err);
[Post-Tokyo: PJP provided the above wording.]
Section: 16.4.6.14 [res.on.exception.handling] Status: TC1 Submitter: Judy Ward Opened: 1998-12-15 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
Section 16.4.6.14 [res.on.exception.handling] states:
"An implementation may strengthen the exception-specification for a function by removing listed exceptions."
The problem is that if an implementation is allowed to do this for virtual functions, then a library user cannot write a class that portably derives from that class.
For example, this would not compile if ios_base::failure::~failure had an empty exception specification:
#include <ios> #include <string> class D : public std::ios_base::failure { public: D(const std::string&); ~D(); // error - exception specification must be compatible with // overridden virtual function ios_base::failure::~failure() };
Proposed resolution:
Change Section 16.4.6.14 [res.on.exception.handling] from:
"may strengthen the exception-specification for a function"
to:
"may strengthen the exception-specification for a non-virtual function".
Section: 16.4.5.3 [reserved.names] Status: CD1 Submitter: Judy Ward Opened: 1998-12-15 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
The original issue asked whether a library implementor could specialize standard library templates for built-in types. (This was an issue because users are permitted to explicitly instantiate standard library templates.)
Specializations are no longer a problem, because of the resolution to core issue 259. Under the proposed resolution, it will be legal for a translation unit to contain both a specialization and an explicit instantiation of the same template, provided that the specialization comes first. In such a case, the explicit instantiation will be ignored. Further discussion of library issue 120 assumes that the core 259 resolution will be adopted.
However, as noted in lib-7047, one piece of this issue still remains: what happens if a standard library implementor explicitly instantiates a standard library templates? It's illegal for a program to contain two different explicit instantiations of the same template for the same type in two different translation units (ODR violation), and the core working group doesn't believe it is practical to relax that restriction.
The issue, then, is: are users allowed to explicitly instantiate standard library templates for non-user defined types? The status quo answer is 'yes'. Changing it to 'no' would give library implementors more freedom.
This is an issue because, for performance reasons, library implementors often need to explicitly instantiate standard library templates. (for example, std::basic_string<char>) Does giving users freedom to explicitly instantiate standard library templates for non-user defined types make it impossible or painfully difficult for library implementors to do this?
John Spicer suggests, in lib-8957, that library implementors have a mechanism they can use for explicit instantiations that doesn't prevent users from performing their own explicit instantiations: put each explicit instantiation in its own object file. (Different solutions might be necessary for Unix DSOs or MS-Windows DLLs.) On some platforms, library implementors might not need to do anything special: the "undefined behavior" that results from having two different explicit instantiations might be harmless.
Proposed resolution:
Append to 16.4.5.3 [reserved.names] paragraph 1:
A program may explicitly instantiate any templates in the standard library only if the declaration depends on the name of a user-defined type of external linkage and the instantiation meets the standard library requirements for the original template.
[Kona: changed the wording from "a user-defined name" to "the name of a user-defined type"]
Rationale:
The LWG considered another possible resolution:
In light of the resolution to core issue 259, no normative changes in the library clauses are necessary. Add the following non-normative note to the end of 16.4.5.3 [reserved.names] paragraph 1:
[Note: A program may explicitly instantiate standard library templates, even when an explicit instantiation does not depend on a user-defined name. --end note]
The LWG rejected this because it was believed that it would make it unnecessarily difficult for library implementors to write high-quality implementations. A program may not include an explicit instantiation of the same template, for the same template arguments, in two different translation units. If users are allowed to provide explicit instantiations of Standard Library templates for built-in types, then library implementors aren't, at least not without nonportable tricks.
The most serious problem is a class template that has writeable
static member variables. Unfortunately, such class templates are
important and, in existing Standard Library implementations, are
often explicitly specialized by library implementors: locale facets,
which have a writeable static member variable id
. If a
user's explicit instantiation collided with the implementations
explicit instantiation, iostream initialization could cause locales
to be constructed in an inconsistent state.
One proposed implementation technique was for Standard Library implementors to provide explicit instantiations in separate object files, so that they would not be picked up by the linker when the user also provides an explicit instantiation. However, this technique only applies for Standard Library implementations that are packaged as static archives. Most Standard Library implementations nowadays are packaged as dynamic libraries, so this technique would not apply.
The Committee is now considering standardization of dynamic linking. If there are such changes in the future, it may be appropriate to revisit this issue later.
Section: 31.6.3 [streambuf] Status: TC1 Submitter: Judy Ward Opened: 1998-12-15 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
Section 27.5.2 describes the streambuf classes this way:
The class streambuf is a specialization of the template class basic_streambuf specialized for the type char.
The class wstreambuf is a specialization of the template class basic_streambuf specialized for the type wchar_t.
This implies that these classes must be template specializations, not typedefs.
It doesn't seem this was intended, since Section 27.5 has them declared as typedefs.
Proposed resolution:
Remove 31.6.3 [streambuf] paragraphs 2 and 3 (the above two sentences).
Rationale:
The streambuf
synopsis already has a declaration for the
typedefs and that is sufficient.
Section: 29.6.5.4 [slice.arr.fill], 29.6.7.4 [gslice.array.fill], 29.6.8.4 [mask.array.fill], 29.6.9.4 [indirect.array.fill] Status: CD1 Submitter: Judy Ward Opened: 1998-12-15 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
One of the operator= in the valarray helper arrays is const and one is not. For example, look at slice_array. This operator= in Section 29.6.5.2 [slice.arr.assign] is const:
void operator=(const valarray<T>&) const;
but this one in Section 29.6.5.4 [slice.arr.fill] is not:
void operator=(const T&);
The description of the semantics for these two functions is similar.
Proposed resolution:
29.6.5 [template.slice.array] Template class slice_array
In the class template definition for slice_array, replace the member function declaration
void operator=(const T&);with
void operator=(const T&) const;
29.6.5.4 [slice.arr.fill] slice_array fill function
Change the function declaration
void operator=(const T&);to
void operator=(const T&) const;
29.6.7 [template.gslice.array] Template class gslice_array
In the class template definition for gslice_array, replace the member function declaration
void operator=(const T&);with
void operator=(const T&) const;
29.6.7.4 [gslice.array.fill] gslice_array fill function
Change the function declaration
void operator=(const T&);to
void operator=(const T&) const;
29.6.8 [template.mask.array] Template class mask_array
In the class template definition for mask_array, replace the member function declaration
void operator=(const T&);with
void operator=(const T&) const;
29.6.8.4 [mask.array.fill] mask_array fill function
Change the function declaration
void operator=(const T&);to
void operator=(const T&) const;
29.6.9 [template.indirect.array] Template class indirect_array
In the class template definition for indirect_array, replace the member function declaration
void operator=(const T&);with
void operator=(const T&) const;
29.6.9.4 [indirect.array.fill] indirect_array fill function
Change the function declaration
void operator=(const T&);to
void operator=(const T&) const;
[Redmond: Robert provided wording.]
Rationale:
There's no good reason for one version of operator= being const and another one not. Because of issue 253(i), this now matters: these functions are now callable in more circumstances. In many existing implementations, both versions are already const.
Section: 28.3.4.2.3 [locale.ctype.byname] Status: TC1 Submitter: Judy Ward Opened: 1998-12-15 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
In Section 28.3.4.2.3 [locale.ctype.byname] ctype_byname<charT>::do_scan_is() and do_scan_not() are declared to return a const char* not a const charT*.
Proposed resolution:
Change Section 28.3.4.2.3 [locale.ctype.byname] do_scan_is()
and
do_scan_not()
to return a const
charT*
.
Section: 29.6.2 [template.valarray] Status: TC1 Submitter: Judy Ward Opened: 1998-12-15 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
In Section 29.6.2 [template.valarray] valarray<T>::operator!() is declared to return a valarray<T>, but in Section 29.6.2.6 [valarray.unary] it is declared to return a valarray<bool>. The latter appears to be correct.
Proposed resolution:
Change in Section 29.6.2 [template.valarray] the declaration of
operator!()
so that the return type is
valarray<bool>
.
Section: 28.3.4.2.2.3 [locale.ctype.virtuals] Status: TC1 Submitter: Judy Ward Opened: 1998-12-15 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
Typos in 22.2.1.1.2 need to be fixed.
Proposed resolution:
In Section 28.3.4.2.2.3 [locale.ctype.virtuals] change:
do_widen(do_narrow(c),0) == c
to:
do_widen(do_narrow(c,0)) == c
and change:
(is(M,c) || !ctc.is(M, do_narrow(c),dfault) )
to:
(is(M,c) || !ctc.is(M, do_narrow(c,dfault)) )
Section: 99 [auto.ptr] Status: TC1 Submitter: Greg Colvin Opened: 1999-02-17 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
There are two problems with the current auto_ptr
wording
in the standard:
First, the auto_ptr_ref
definition cannot be nested
because auto_ptr<Derived>::auto_ptr_ref
is unrelated to
auto_ptr<Base>::auto_ptr_ref
. Also submitted by
Nathan Myers, with the same proposed resolution.
Second, there is no auto_ptr
assignment operator taking an
auto_ptr_ref
argument.
I have discussed these problems with my proposal coauthor, Bill Gibbons, and with some compiler and library implementors, and we believe that these problems are not desired or desirable implications of the standard.
25 Aug 1999: The proposed resolution now reflects changes suggested by Dave Abrahams, with Greg Colvin's concurrence; 1) changed "assignment operator" to "public assignment operator", 2) changed effects to specify use of release(), 3) made the conversion to auto_ptr_ref const.
2 Feb 2000: Lisa Lippincott comments: [The resolution of] this issue states that the conversion from auto_ptr to auto_ptr_ref should be const. This is not acceptable, because it would allow initialization and assignment from _any_ const auto_ptr! It also introduces an implementation difficulty in writing this conversion function -- namely, somewhere along the line, a const_cast will be necessary to remove that const so that release() may be called. This may result in undefined behavior [7.1.5.1/4]. The conversion operator does not have to be const, because a non-const implicit object parameter may be bound to an rvalue [13.3.3.1.4/3] [13.3.1/5].
Tokyo: The LWG removed the following from the proposed resolution:
In 21.3.5 [meta.unary], paragraph 2, and 21.3.5.4 [meta.unary.prop], paragraph 2, make the conversion to auto_ptr_ref const:
template<class Y> operator auto_ptr_ref<Y>() const throw();
Proposed resolution:
In 21.3.5 [meta.unary], paragraph 2, move
the auto_ptr_ref
definition to namespace scope.
In 21.3.5 [meta.unary], paragraph 2, add
a public assignment operator to the auto_ptr
definition:
auto_ptr& operator=(auto_ptr_ref<X> r) throw();
Also add the assignment operator to 21.3.5.4 [meta.unary.prop]:
auto_ptr& operator=(auto_ptr_ref<X> r) throw()Effects: Calls
reset(p.release())
for theauto_ptr p
thatr
holds a reference to.
Returns:*this
.
Section: 31.7.5.4 [istream.unformatted], 31.7.6.2.5 [ostream.seeks] Status: TC1 Submitter: Angelika Langer Opened: 1999-02-22 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
Currently, the standard does not specify how seekg() and seekp() indicate failure. They are not required to set failbit, and they can't return an error indication because they must return *this, i.e. the stream. Hence, it is undefined what happens if they fail. And they can fail, for instance, when a file stream is disconnected from the underlying file (is_open()==false) or when a wide character file stream must perform a state-dependent code conversion, etc.
The stream functions seekg() and seekp() should set failbit in the stream state in case of failure.
Proposed resolution:
Add to the Effects: clause of seekg() in 31.7.5.4 [istream.unformatted] and to the Effects: clause of seekp() in 31.7.6.2.5 [ostream.seeks]:
In case of failure, the function calls
setstate(failbit)
(which may throwios_base::failure
).
Rationale:
Setting failbit is the usual error reporting mechanism for streams
Section: 23.2.7 [associative.reqmts], 23.2.4 [sequence.reqmts] Status: CD1 Submitter: Andrew Koenig Opened: 1999-03-02 Last modified: 2016-01-28
Priority: Not Prioritized
View other active issues in [associative.reqmts].
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Duplicate of: 451
Discussion:
Table 67 (23.1.1) says that container::erase(iterator) returns an iterator. Table 69 (23.1.2) says that in addition to this requirement, associative containers also say that container::erase(iterator) returns void. That's not an addition; it's a change to the requirements, which has the effect of making associative containers fail to meet the requirements for containers.
Proposed resolution:
In 23.2.7 [associative.reqmts], in Table 69 Associative container
requirements, change the return type of a.erase(q)
from
void
to iterator
. Change the
assertion/not/pre/post-condition from "erases the element pointed to
by q
" to "erases the element pointed to by q
.
Returns an iterator pointing to the element immediately following q
prior to the element being erased. If no such element exists, a.end()
is returned."
In 23.2.7 [associative.reqmts], in Table 69 Associative container
requirements, change the return type of a.erase(q1, q2)
from void
to iterator
. Change the
assertion/not/pre/post-condition from "erases the elements in the
range [q1, q2)
" to "erases the elements in the range [q1,
q2)
. Returns q2."
In 23.4.3 [map], in the map
class synopsis; and
in 23.4.4 [multimap], in the multimap
class synopsis; and
in 23.4.6 [set], in the set
class synopsis; and
in 23.4.7 [multiset], in the multiset
class synopsis:
change the signature of the first erase
overload to
iterator erase(iterator position);
and change the signature of the third erase
overload to
iterator erase(iterator first, iterator last);
[Pre-Kona: reopened at the request of Howard Hinnant]
[Post-Kona: the LWG agrees the return type should be
iterator
, not void
. (Alex Stepanov agrees too.)
Matt provided wording.]
[ Sydney: the proposed wording went in the right direction, but it wasn't good enough. We want to return an iterator from the range form of erase as well as the single-iterator form. Also, the wording is slightly different from the wording we have for sequences; there's no good reason for having a difference. Matt provided new wording, (reflected above) which we will review at the next meeting. ]
[ Redmond: formally voted into WP. ]
Section: 23.3.11.3 [list.capacity] Status: TC1 Submitter: Howard Hinnant Opened: 1999-03-06 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
The description reads:
-1- Effects:
if (sz > size()) insert(end(), sz-size(), c); else if (sz < size()) erase(begin()+sz, end()); else ; // do nothing
Obviously list::resize should not be specified in terms of random access iterators.
Proposed resolution:
Change 23.3.11.3 [list.capacity] paragraph 1 to:
Effects:
if (sz > size()) insert(end(), sz-size(), c); else if (sz < size()) { iterator i = begin(); advance(i, sz); erase(i, end()); }
[Dublin: The LWG asked Howard to discuss exception safety offline with David Abrahams. They had a discussion and believe there is no issue of exception safety with the proposed resolution.]
Section: 23.4.3 [map] Status: TC1 Submitter: Howard Hinnant Opened: 1999-03-06 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
The title says it all.
Proposed resolution:
Insert in 23.4.3 [map], paragraph 2, after operator= in the map declaration:
allocator_type get_allocator() const;
Section: 23.3.13.2 [vector.cons] Status: TC1 Submitter: Howard Hinnant Opened: 1999-03-06 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
The complexity description says: "It does at most 2N calls to the copy constructor of T and logN reallocations if they are just input iterators ...".
This appears to be overly restrictive, dictating the precise memory/performance tradeoff for the implementor.
Proposed resolution:
Change 23.3.13.2 [vector.cons], paragraph 1 to:
-1- Complexity: The constructor template <class InputIterator> vector(InputIterator first, InputIterator last) makes only N calls to the copy constructor of T (where N is the distance between first and last) and no reallocations if iterators first and last are of forward, bidirectional, or random access categories. It makes order N calls to the copy constructor of T and order logN reallocations if they are just input iterators.
Rationale:
"at most 2N calls" is correct only if the growth factor is greater than or equal to 2.
Section: 31.7.5.4 [istream.unformatted] Status: CD1 Submitter: Howard Hinnant Opened: 1999-03-06 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
I may be misunderstanding the intent, but should not seekg set only the input stream and seekp set only the output stream? The description seems to say that each should set both input and output streams. If that's really the intent, I withdraw this proposal.
Proposed resolution:
In section 27.6.1.3 change:
basic_istream<charT,traits>& seekg(pos_type pos); Effects: If fail() != true, executes rdbuf()->pubseekpos(pos).
To:
basic_istream<charT,traits>& seekg(pos_type pos); Effects: If fail() != true, executes rdbuf()->pubseekpos(pos, ios_base::in).
In section 27.6.1.3 change:
basic_istream<charT,traits>& seekg(off_type& off, ios_base::seekdir dir); Effects: If fail() != true, executes rdbuf()->pubseekoff(off, dir).
To:
basic_istream<charT,traits>& seekg(off_type& off, ios_base::seekdir dir); Effects: If fail() != true, executes rdbuf()->pubseekoff(off, dir, ios_base::in).
In section 27.6.2.4, paragraph 2 change:
-2- Effects: If fail() != true, executes rdbuf()->pubseekpos(pos).
To:
-2- Effects: If fail() != true, executes rdbuf()->pubseekpos(pos, ios_base::out).
In section 27.6.2.4, paragraph 4 change:
-4- Effects: If fail() != true, executes rdbuf()->pubseekoff(off, dir).
To:
-4- Effects: If fail() != true, executes rdbuf()->pubseekoff(off, dir, ios_base::out).
[Dublin: Dietmar Kühl thinks this is probably correct, but would like the opinion of more iostream experts before taking action.]
[Tokyo: Reviewed by the LWG. PJP noted that although his docs are incorrect, his implementation already implements the Proposed Resolution.]
[Post-Tokyo: Matt Austern comments:
Is it a problem with basic_istream and basic_ostream, or is it a problem
with basic_stringbuf?
We could resolve the issue either by changing basic_istream and
basic_ostream, or by changing basic_stringbuf. I prefer the latter
change (or maybe both changes): I don't see any reason for the standard to
require that std::stringbuf s(std::string("foo"), std::ios_base::in);
s.pubseekoff(0, std::ios_base::beg); must fail.
This requirement is a bit weird. There's no similar requirement
for basic_streambuf<>::seekpos, or for basic_filebuf<>::seekoff or
basic_filebuf<>::seekpos.]
Section: 28.3.3.1 [locale] Status: TC1 Submitter: Angelika Langer Opened: 1999-03-17 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
Section 28.3.3.1 [locale] says:
-4- In the call to use_facet<Facet>(loc), the type argument chooses a facet, making available all members of the named type. If Facet is not present in a locale (or, failing that, in the global locale), it throws the standard exception bad_cast. A C++ program can check if a locale implements a particular facet with the template function has_facet<Facet>().
This contradicts the specification given in section
28.3.3.2 [locale.global.templates]:
template <class Facet> const Facet& use_facet(const
locale& loc);
-1- Get a reference to a facet of a locale.
-2- Returns: a reference to the corresponding facet of loc, if present.
-3- Throws: bad_cast if has_facet<Facet>(loc) is false.
-4- Notes: The reference returned remains valid at least as long as any copy of loc exists
Proposed resolution:
Remove the phrase "(or, failing that, in the global locale)" from section 22.1.1.
Rationale:
Needed for consistency with the way locales are handled elsewhere in the standard.
Section: 23.2.4 [sequence.reqmts] Status: TC1 Submitter: Andrew Koenig Opened: 1999-03-30 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
The sentence introducing the Optional sequence operation table (23.1.1 paragraph 12) has two problems:
A. It says ``The operations in table 68 are provided only for the containers for which
they take constant time.''
That could be interpreted in two ways, one of them being ``Even though table 68 shows
particular operations as being provided, implementations are free to omit them if they
cannot implement them in constant time.''
B. That paragraph says nothing about amortized constant time, and it should.
Proposed resolution:
Replace the wording in 23.1.1 paragraph 12 which begins ``The operations in table 68 are provided only..." with:
Table 68 lists sequence operations that are provided for some types of sequential containers but not others. An implementation shall provide these operations for all container types shown in the ``container'' column, and shall implement them so as to take amortized constant time.
Section: 27.4.3.7.4 [string.insert], 27.4.3.7.6 [string.replace] Status: TC1 Submitter: Arch Robison Opened: 1999-04-28 Last modified: 2016-11-12
Priority: Not Prioritized
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Discussion:
Sections 21.3.6.4 paragraph 1 and 21.3.6.6 paragraph 1 surely have misprints where they
say:
xpos <= pos
and pos < size();
Surely the document meant to say ``xpos < size()
'' in both places.
[Judy Ward also sent in this issue for 21.3.6.4 with the same proposed resolution.]
Proposed resolution:
Change Sections 21.3.6.4 paragraph 1 and 21.3.6.6 paragraph 1, the line which says:
xpos <= pos
and pos < size();
to:
xpos <= pos
and xpos < size();
Section: 26.8.11 [alg.lex.comparison] Status: TC1 Submitter: Howard Hinnant Opened: 1999-06-20 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
The lexicographical_compare complexity is specified as:
"At most min((last1 - first1), (last2 - first2))
applications of the corresponding comparison."
The best I can do is twice that expensive.
Nicolai Josuttis comments in lib-6862: You mean, to check for equality you have to check both < and >? Yes, IMO you are right! (and Matt states this complexity in his book)
Proposed resolution:
Change 26.8.11 [alg.lex.comparison] complexity to:
At most
2*min((last1 - first1), (last2 - first2))
applications of the corresponding comparison.
Change the example at the end of paragraph 3 to read:
[Example:
for ( ; first1 != last1 && first2 != last2 ; ++first1, ++first2) {
if (*first1 < *first2) return true;
if (*first2 < *first1) return false;
}
return first1 == last1 && first2 != last2;
--end example]
Section: 23.3.5.2 [deque.cons] Status: TC1 Submitter: Herb Sutter Opened: 1999-05-09 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
In 23.3.5.2 [deque.cons] paragraph 6, the deque ctor that takes an iterator range appears to have complexity requirements which are incorrect, and which contradict the complexity requirements for insert(). I suspect that the text in question, below, was taken from vector:
Complexity: If the iterators first and last are forward iterators, bidirectional iterators, or random access iterators the constructor makes only N calls to the copy constructor, and performs no reallocations, where N is last - first.
The word "reallocations" does not really apply to deque. Further, all of the following appears to be spurious:
It makes at most 2N calls to the copy constructor of T and log N reallocations if they are input iterators.1)
1) The complexity is greater in the case of input iterators because each element must be added individually: it is impossible to determine the distance between first abd last before doing the copying.
This makes perfect sense for vector, but not for deque. Why should deque gain an efficiency advantage from knowing in advance the number of elements to insert?
Proposed resolution:
In 23.3.5.2 [deque.cons] paragraph 6, replace the Complexity description, including the footnote, with the following text (which also corrects the "abd" typo):
Complexity: Makes last - first calls to the copy constructor of T.
Section: 29.4.6 [complex.ops] Status: TC1 Submitter: Angelika Langer Opened: 1999-05-12 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
The extractor for complex numbers is specified as:
template<class T, class charT, class traits>
basic_istream<charT, traits>&
operator>>(basic_istream<charT, traits>& is, complex<T>& x);
Effects: Extracts a complex number x of the form: u, (u), or (u,v), where u is the real part and v is the imaginary part (lib.istream.formatted).
Requires: The input values be convertible to T. If bad input is encountered, calls is.setstate(ios::failbit) (which may throw ios::failure (lib.iostate.flags).
Returns: is .
Is it intended that the extractor for complex numbers does not skip
whitespace, unlike all other extractors in the standard library do?
Shouldn't a sentry be used?
The inserter for complex numbers is specified as:
template<class T, class charT, class traits>
basic_ostream<charT, traits>&
operator<<(basic_ostream<charT, traits>& o, const complex<T>& x);
Effects: inserts the complex number x onto the stream o as if it were implemented as follows:
template<class T, class charT, class traits>
basic_ostream<charT, traits>&
operator<<(basic_ostream<charT, traits>& o, const complex<T>& x)
{
basic_ostringstream<charT, traits> s;
s.flags(o.flags());
s.imbue(o.getloc());
s.precision(o.precision());
s << '(' << x.real() << "," << x.imag() << ')';
return o << s.str();
}
Is it intended that the inserter for complex numbers ignores the field width and does not do any padding? If, with the suggested implementation above, the field width were set in the stream then the opening parentheses would be adjusted, but the rest not, because the field width is reset to zero after each insertion.
I think that both operations should use sentries, for sake of consistency with the other inserters and extractors in the library. Regarding the issue of padding in the inserter, I don't know what the intent was.
Proposed resolution:
After 29.4.6 [complex.ops] paragraph 14 (operator>>), add a Notes clause:
Notes: This extraction is performed as a series of simpler extractions. Therefore, the skipping of whitespace is specified to be the same for each of the simpler extractions.
Rationale:
For extractors, the note is added to make it clear that skipping whitespace follows an "all-or-none" rule.
For inserters, the LWG believes there is no defect; the standard is correct as written.
Section: 16.4.6.4 [global.functions] Status: TC1 Submitter: Lois Goldthwaite Opened: 1999-06-04 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
The library had many global functions until 17.4.1.1 [lib.contents] paragraph 2 was added:
All library entities except macros, operator new and operator delete are defined within the namespace std or namespaces nested within namespace std.
It appears "global function" was never updated in the following:
17.4.4.3 - Global functions [lib.global.functions]
-1- It is unspecified whether any global functions in the C++ Standard Library are defined as inline (dcl.fct.spec).
-2- A call to a global function signature described in Clauses lib.language.support through lib.input.output behaves the same as if the implementation declares no additional global function signatures.*
[Footnote: A valid C++ program always calls the expected library global function. An implementation may also define additional global functions that would otherwise not be called by a valid C++ program. --- end footnote]
-3- A global function cannot be declared by the implementation as taking additional default arguments.
17.4.4.4 - Member functions [lib.member.functions]
-2- An implementation can declare additional non-virtual member function signatures within a class:-- by adding arguments with default values to a member function signature; The same latitude does not extend to the implementation of virtual or global functions, however.
Proposed resolution:
Change "global" to "global or non-member" in:
17.4.4.3 [lib.global.functions] section title,
17.4.4.3 [lib.global.functions] para 1,
17.4.4.3 [lib.global.functions] para 2 in 2 places plus 2 places in the footnote,
17.4.4.3 [lib.global.functions] para 3,
17.4.4.4 [lib.member.functions] para 2
Rationale:
Because operator new and delete are global, the proposed resolution was changed from "non-member" to "global or non-member.
Section: 99 [facets.examples] Status: TC1 Submitter: Jeremy Siek Opened: 1999-06-03 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
In 99 [facets.examples] paragraph 13, the do_truename() and do_falsename() functions in the example facet BoolNames should be const. The functions they are overriding in numpunct_byname<char> are const.
Proposed resolution:
In 99 [facets.examples] paragraph 13, insert "const" in two places:
string do_truename() const { return "Oui Oui!"; }
string do_falsename() const { return "Mais Non!"; }
Section: 23.2.4 [sequence.reqmts] Status: C++11 Submitter: Andrew Koenig Opened: 1999-06-28 Last modified: 2016-11-12
Priority: Not Prioritized
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Discussion:
Suppose that c and c1 are sequential containers and i is an iterator that refers to an element of c. Then I can insert a copy of c1's elements into c ahead of element i by executing
c.insert(i, c1.begin(), c1.end());
If c is a vector, it is fairly easy for me to find out where the newly inserted elements are, even though i is now invalid:
size_t i_loc = i - c.begin(); c.insert(i, c1.begin(), c1.end());
and now the first inserted element is at c.begin()+i_loc and one
past the last is at c.begin()+i_loc+c1.size().
But what if c is a list? I can still find the location of one
past the last inserted element, because i is still valid.
To find the location of the first inserted element, though,
I must execute something like
for (size_t n = c1.size(); n; --n) --i;
because i is now no longer a random-access iterator.
Alternatively, I might write something like
bool first = i == c.begin(); list<T>::iterator j = i; if (!first) --j; c.insert(i, c1.begin(), c1.end()); if (first) j = c.begin(); else ++j;
which, although wretched, requires less overhead.
But I think the right solution is to change the definition of insert
so that instead of returning void, it returns an iterator that refers
to the first element inserted, if any, and otherwise is a copy of its
first argument.
[ Summit: ]
Reopened by Alisdair.
[ Post Summit Alisdair adds: ]
In addition to the original rationale for C++03, this change also gives a consistent interface for all container insert operations i.e. they all return an iterator to the (first) inserted item.
Proposed wording provided.
[ 2009-07 Frankfurt ]
Q: why isn't this change also proposed for associative containers?
A: The returned iterator wouldn't necessarily point to a contiguous range.
Moved to Ready.
Proposed resolution:
23.2.4 [sequence.reqmts] Table 83
change return type from void
to iterator
for the following rows:
Table 83 — Sequence container requirements (in addition to container) Expression Return type Assertion/note pre-/post-condition a.insert(p,n,t)
voiditeratorInserts n
copies oft
beforep
.a.insert(p,i,j)
voiditeratorEach iterator in the range [i,j)
shall be dereferenced exactly once. pre:i
andj
are not iterators intoa
. Inserts copies of elements in[i, j)
beforep
a.insert(p,il)
voiditeratora.insert(p, il.begin(), il.end())
.
Add after p6 23.2.4 [sequence.reqmts]:
-6- ...
The iterator returned from
a.insert(p,n,t)
points to the copy of the first element inserted intoa
, orp
ifn == 0
.The iterator returned from
a.insert(p,i,j)
points to the copy of the first element inserted intoa
, orp
ifi == j
.The iterator returned from
a.insert(p,il)
points to the copy of the first element inserted intoa
, orp
ifil
is empty.
p2 23.3.5 [deque] Update class definition, change return type
from void
to iterator
:
voiditerator insert(const_iterator position, size_type n, const T& x); template <class InputIterator>voiditerator insert(const_iterator position, InputIterator first, InputIterator last);voiditerator insert(const_iterator position, initializer_list<T>);
23.3.5.4 [deque.modifiers] change return type from void
to iterator
on following declarations:
voiditerator insert(const_iterator position, size_type n, const T& x); template <class InputIterator>voiditerator insert(const_iterator position, InputIterator first, InputIterator last);
Add the following (missing) declaration
iterator insert(const_iterator position, initializer_list<T>);
[forwardlist] Update class definition, change return type
from void
to iterator
:
voiditerator insert_after(const_iterator position, initializer_list<T> il);voiditerator insert_after(const_iterator position, size_type n, const T& x); template <class InputIterator>voiditerator insert_after(const_iterator position, InputIterator first, InputIterator last);
p8 [forwardlist.modifiers] change return type from void
to iterator
:
voiditerator insert_after(const_iterator position, size_type n, const T& x);
Add paragraph:
Returns: position.
p10 [forwardlist.modifiers] change return type from void
to iterator
:
template <class InputIterator>voiditerator insert_after(const_iterator position, InputIterator first, InputIterator last);
Add paragraph:
Returns: position.
p12 [forwardlist.modifiers] change return type from void
to iterator
on following declarations:
voiditerator insert_after(const_iterator position, initializer_list<T> il);
change return type from void
to iterator
on following declarations:
p2 23.3.11 [list] Update class definition, change return type from void
to iterator
:
voiditerator insert(const_iterator position, size_type n, const T& x); template <class InputIterator>voiditerator insert(const_iterator position, InputIterator first, InputIterator last);voiditerator insert(const_iterator position, initializer_list<T>);
23.3.11.4 [list.modifiers] change return type from void
to iterator
on following declarations:
voiditerator insert(const_iterator position, size_type n, const T& x); template <class InputIterator>voiditerator insert(const_iterator position, InputIterator first, InputIterator last);
Add the following (missing) declaration
iterator insert(const_iterator position, initializer_list<T>);
p2 23.3.13 [vector]
Update class definition, change return type from void
to iterator
:
voiditerator insert(const_iterator position, T&& x);voiditerator insert(const_iterator position, size_type n, const T& x); template <class InputIterator>voiditerator insert(const_iterator position, InputIterator first, InputIterator last);voiditerator insert(const_iterator position, initializer_list<T>);
23.3.13.5 [vector.modifiers] change return type from void
to iterator
on following declarations:
voiditerator insert(const_iterator position, size_type n, const T& x); template <class InputIterator>voiditerator insert(const_iterator position, InputIterator first, InputIterator last);
Add the following (missing) declaration
iterator insert(const_iterator position, initializer_list<T>);
p1 23.3.14 [vector.bool] Update class definition, change return type from void
to iterator
:
voiditerator insert (const_iterator position, size_type n, const bool& x); template <class InputIterator>voiditerator insert(const_iterator position, InputIterator first, InputIterator last);voiditerator insert(const_iterator position, initializer_list<bool> il);
p5 27.4.3 [basic.string] Update class definition, change return type from void
to iterator
:
voiditerator insert(const_iterator p, size_type n, charT c); template<class InputIterator>voiditerator insert(const_iterator p, InputIterator first, InputIterator last);voiditerator insert(const_iterator p, initializer_list<charT>);
p13 27.4.3.7.4 [string.insert] change return type from void
to iterator
:
voiditerator insert(const_iterator p, size_type n, charT c);
Add paragraph:
Returns: an iterator which refers to the copy of the first inserted character, or
p
ifn == 0
.
p15 27.4.3.7.4 [string.insert] change return type from void
to iterator
:
template<class InputIterator>voiditerator insert(const_iterator p, InputIterator first, InputIterator last);
Add paragraph:
Returns: an iterator which refers to the copy of the first inserted character, or
p
iffirst == last
.
p17 27.4.3.7.4 [string.insert] change return type from void
to iterator
:
voiditerator insert(const_iterator p, initializer_list<charT> il);
Add paragraph:
Returns: an iterator which refers to the copy of the first inserted character, or
p
ifil
is empty.
Rationale:
[ The following was the C++98/03 rationale and does not necessarily apply to the proposed resolution in the C++0X time frame: ]
The LWG believes this was an intentional design decision and so is not a defect. It may be worth revisiting for the next standard.
Section: 26.6.9 [alg.find.first.of] Status: TC1 Submitter: Matt McClure Opened: 1999-06-30 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
Proposed resolution:
Change 26.6.9 [alg.find.first.of] paragraph 2 from:
Returns: The first iterator i in the range [first1, last1) such that for some integer j in the range [first2, last2) ...
to:
Returns: The first iterator i in the range [first1, last1) such that for some iterator j in the range [first2, last2) ...
Section: 23.2.4 [sequence.reqmts] Status: TC1 Submitter: Ed Brey Opened: 1999-06-21 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
For both sequences and associative containers, a.clear() has the
semantics of erase(a.begin(),a.end()), which is undefined for an empty
container since erase(q1,q2) requires that q1 be dereferenceable
(23.1.1,3 and 23.1.2,7). When the container is empty, a.begin() is
not dereferenceable.
The requirement that q1 be unconditionally dereferenceable causes many
operations to be intuitively undefined, of which clearing an empty
container is probably the most dire.
Since q1 and q2 are only referenced in the range [q1, q2), and [q1,
q2) is required to be a valid range, stating that q1 and q2 must be
iterators or certain kinds of iterators is unnecessary.
Proposed resolution:
In 23.1.1, paragraph 3, change:
p and q2 denote valid iterators to a, q and q1 denote valid dereferenceable iterators to a, [q1, q2) denotes a valid range
to:
p denotes a valid iterator to a, q denotes a valid dereferenceable iterator to a, [q1, q2) denotes a valid range in a
In 23.1.2, paragraph 7, change:
p and q2 are valid iterators to a, q and q1 are valid dereferenceable iterators to a, [q1, q2) is a valid range
to
p is a valid iterator to a, q is a valid dereferenceable iterator to a, [q1, q2) is a valid range into a
scan_is()
semanticsSection: 28.3.4.2.2.3 [locale.ctype.virtuals] Status: TC1 Submitter: Dietmar Kühl Opened: 1999-07-20 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
The semantics of scan_is()
(paragraphs 4 and 6) is not exactly described
because there is no function is()
which only takes a character as
argument. Also, in the effects clause (paragraph 3), the semantic is also kept
vague.
Proposed resolution:
In 28.3.4.2.2.3 [locale.ctype.virtuals] paragraphs 4 and 6, change the returns clause from:
"... such that
is(*p)
would..."
to: "... such that is(m, *p)
would...."
narrow()
semanticsSection: 28.3.4.2.4.3 [facet.ctype.char.members] Status: CD1 Submitter: Dietmar Kühl Opened: 1999-07-20 Last modified: 2016-01-28
Priority: Not Prioritized
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Duplicate of: 207
Discussion:
The description of the array version of narrow()
(in
paragraph 11) is flawed: There is no member do_narrow()
which
takes only three arguments because in addition to the range a default
character is needed.
Additionally, for both widen
and narrow
we have
two signatures followed by a Returns clause that only addresses
one of them.
Proposed resolution:
Change the returns clause in 28.3.4.2.4.3 [facet.ctype.char.members] paragraph 10 from:
Returns: do_widen(low, high, to).
to:
Returns: do_widen(c) or do_widen(low, high, to), respectively.
Change 28.3.4.2.4.3 [facet.ctype.char.members] paragraph 10 and 11 from:
char narrow(char c, char /*dfault*/) const; const char* narrow(const char* low, const char* high, char /*dfault*/, char* to) const;
Returns: do_narrow(low, high, to).
to:
char narrow(char c, char dfault) const; const char* narrow(const char* low, const char* high, char dfault, char* to) const;
Returns: do_narrow(c, dfault) or do_narrow(low, high, dfault, to), respectively.
[Kona: 1) the problem occurs in additional places, 2) a user defined version could be different.]
[Post-Tokyo: Dietmar provided the above wording at the request of the LWG. He could find no other places the problem occurred. He asks for clarification of the Kona "a user defined version..." comment above. Perhaps it was a circuitous way of saying "dfault" needed to be uncommented?]
[Post-Toronto: the issues list maintainer has merged in the proposed resolution from issue 207(i), which addresses the same paragraphs.]
double
specifier for do_get()
Section: 28.3.4.3.2.3 [facet.num.get.virtuals] Status: TC1 Submitter: Dietmar Kühl Opened: 1999-07-20 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
The table in paragraph 7 for the length modifier does not list the length
modifier l
to be applied if the type is double
. Thus, the
standard asks the implementation to do undefined things when using scanf()
(the missing length modifier for scanf()
when scanning double
s
is actually a problem I found quite often in production code, too).
Proposed resolution:
In 28.3.4.3.2.3 [facet.num.get.virtuals], paragraph 7, add a row in the Length
Modifier table to say that for double
a length modifier
l
is to be used.
Rationale:
The standard makes an embarrassing beginner's mistake.
Init
Section: 31.4 [iostream.objects] Status: TC1 Submitter: Dietmar Kühl Opened: 1999-07-20 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
There are conflicting statements about where the class
Init
is defined. According to 31.4 [iostream.objects] paragraph 2
it is defined as basic_ios::Init
, according to 31.5.2 [ios.base] it is defined as ios_base::Init
.
Proposed resolution:
Change 31.4 [iostream.objects] paragraph 2 from
"basic_ios::Init"
to
"ios_base::Init"
.
Rationale:
Although not strictly wrong, the standard was misleading enough to warrant the change.
imbue()
descriptionSection: 31.5.2.4 [ios.base.locales] Status: TC1 Submitter: Dietmar Kühl Opened: 1999-07-20 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
There is a small discrepancy between the declarations of
imbue()
: in 31.5.2 [ios.base] the argument is passed as
locale const&
(correct), in 31.5.2.4 [ios.base.locales] it
is passed as locale const
(wrong).
Proposed resolution:
In 31.5.2.4 [ios.base.locales] change the imbue
argument
from "locale const" to "locale
const&".
setbuf()
Section: 31.6.3.5.2 [streambuf.virt.buffer] Status: TC1 Submitter: Dietmar Kühl Opened: 1999-07-20 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
The default behavior of setbuf()
is described only for the
situation that gptr() != 0 && gptr() != egptr()
:
namely to do nothing. What has to be done in other situations
is not described although there is actually only one reasonable
approach, namely to do nothing, too.
Since changing the buffer would almost certainly mess up most
buffer management of derived classes unless these classes do it
themselves, the default behavior of setbuf()
should always be
to do nothing.
Proposed resolution:
Change 31.6.3.5.2 [streambuf.virt.buffer], paragraph 3, Default behavior, to: "Default behavior: Does nothing. Returns this."
underflow()
Section: 31.6.3.5.3 [streambuf.virt.get] Status: TC1 Submitter: Dietmar Kühl Opened: 1999-07-20 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
The description of the meaning of the result of
showmanyc()
seems to be rather strange: It uses calls to
underflow()
. Using underflow()
is strange because
this function only reads the current character but does not extract
it, uflow()
would extract the current character. This should
be fixed to use sbumpc()
instead.
Proposed resolution:
Change 31.6.3.5.3 [streambuf.virt.get] paragraph 1,
showmanyc()
returns clause, by replacing the word
"supplied" with the words "extracted from the
stream".
exception()
Section: 31.7.5.2 [istream] Status: TC1 Submitter: Dietmar Kühl Opened: 1999-07-20 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
The paragraph 4 refers to the function exception()
which
is not defined. Probably, the referred function is
basic_ios<>::exceptions()
.
Proposed resolution:
In 31.7.5.2 [istream], 31.7.5.4 [istream.unformatted], paragraph 1,
31.7.6.2 [ostream], paragraph 3, and 31.7.6.3.1 [ostream.formatted.reqmts],
paragraph 1, change "exception()" to
"exceptions()"
.
[Note to Editor: "exceptions" with an "s" is the correct spelling.]
istream_iterator
vs. istreambuf_iterator
Section: 31.7.5.3.2 [istream.formatted.arithmetic] Status: TC1 Submitter: Dietmar Kühl Opened: 1999-07-20 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
The note in the second paragraph pretends that the first argument
is an object of type istream_iterator
. This is wrong: It is
an object of type istreambuf_iterator
.
Proposed resolution:
Change 31.7.5.3.2 [istream.formatted.arithmetic] from:
The first argument provides an object of the istream_iterator class...
to
The first argument provides an object of the istreambuf_iterator class...
Section: 28.3.4.6.4.3 [locale.time.put.virtuals] Status: TC1 Submitter: Angelika Langer Opened: 1999-07-23 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
In 28.3.4.6.4.3 [locale.time.put.virtuals] the do_put() function is specified
as taking a fill character as an argument, but the description of the
function does not say whether the character is used at all and, if so,
in which way. The same holds for any format control parameters that
are accessible through the ios_base& argument, such as the
adjustment or the field width. Is strftime() supposed to use the fill
character in any way? In any case, the specification of
time_put.do_put() looks inconsistent to me.
Is the
signature of do_put() wrong, or is the effects clause incomplete?
Proposed resolution:
Add the following note after 28.3.4.6.4.3 [locale.time.put.virtuals] paragraph 2:
[Note: the
fill
argument may be used in the implementation-defined formats, or by derivations. A space character is a reasonable default for this argument. --end Note]
Rationale:
The LWG felt that while the normative text was correct,
users need some guidance on what to pass for the fill
argument since the standard doesn't say how it's used.
xsputn()
, pubsync()
never called by basic_ostream
members?Section: 31.7.6.2 [ostream] Status: CD1 Submitter: Dietmar Kühl Opened: 1999-07-20 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
Paragraph 2 explicitly states that none of the basic_ostream
functions falling into one of the groups "formatted output functions"
and "unformatted output functions" calls any stream buffer function
which might call a virtual function other than overflow()
. Basically
this is fine but this implies that sputn()
(this function would call
the virtual function xsputn()
) is never called by any of the standard
output functions. Is this really intended? At minimum it would be convenient to
call xsputn()
for strings... Also, the statement that overflow()
is the only virtual member of basic_streambuf
called is in conflict
with the definition of flush()
which calls rdbuf()->pubsync()
and thereby the virtual function sync()
(flush()
is listed
under "unformatted output functions").
In addition, I guess that the sentence starting with "They may use other
public members of basic_ostream
..." probably was intended to
start with "They may use other public members of basic_streamuf
..."
although the problem with the virtual members exists in both cases.
I see two obvious resolutions:
xsputn()
will never be
called by any ostream member and that this is intended.overflow()
and xsputn()
.
Of course, the problem with flush()
has to be resolved in some way.Proposed resolution:
Change the last sentence of 27.6.2.1 (lib.ostream) paragraph 2 from:
They may use other public members of basic_ostream except that they do not invoke any virtual members of rdbuf() except overflow().
to:
They may use other public members of basic_ostream except that they shall not invoke any virtual members of rdbuf() except overflow(), xsputn(), and sync().
[Kona: the LWG believes this is a problem. Wish to ask Jerry or PJP why the standard is written this way.]
[Post-Tokyo: Dietmar supplied wording at the request of the LWG. He comments: The rules can be made a little bit more specific if necessary be explicitly spelling out what virtuals are allowed to be called from what functions and eg to state specifically that flush() is allowed to call sync() while other functions are not.]
traits_type::length()
Section: 31.7.6.3.4 [ostream.inserters.character] Status: CD1 Submitter: Dietmar Kühl Opened: 1999-07-20 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
Paragraph 4 states that the length is determined using
traits::length(s)
. Unfortunately, this function is not
defined for example if the character type is wchar_t
and the
type of s
is char const*
. Similar problems exist if
the character type is char
and the type of s
is
either signed char const*
or unsigned char
const*
.
Proposed resolution:
Change 31.7.6.3.4 [ostream.inserters.character] paragraph 4 from:
Effects: Behaves like an formatted inserter (as described in lib.ostream.formatted.reqmts) of out. After a sentry object is constructed it inserts characters. The number of characters starting at s to be inserted is traits::length(s). Padding is determined as described in lib.facet.num.put.virtuals. The traits::length(s) characters starting at s are widened using out.widen (lib.basic.ios.members). The widened characters and any required padding are inserted into out. Calls width(0).
to:
Effects: Behaves like a formatted inserter (as described in lib.ostream.formatted.reqmts) of out. After a sentry object is constructed it inserts n characters starting at s, where n is the number that would be computed as if by:
- traits::length(s) for the overload where the first argument is of type basic_ostream<charT, traits>& and the second is of type const charT*, and also for the overload where the first argument is of type basic_ostream<char, traits>& and the second is of type const char*.
- std::char_traits<char>::length(s) for the overload where the first argument is of type basic_ostream<charT, traits>& and the second is of type const char*.
- traits::length(reinterpret_cast<const char*>(s)) for the other two overloads.
Padding is determined as described in lib.facet.num.put.virtuals. The n characters starting at s are widened using out.widen (lib.basic.ios.members). The widened characters and any required padding are inserted into out. Calls width(0).
[Santa Cruz: Matt supplied new wording]
[Kona: changed "where n is" to " where n is the number that would be computed as if by"]
Rationale:
We have five separate cases. In two of them we can use the user-supplied traits class without any fuss. In the other three we try to use something as close to that user-supplied class as possible. In two cases we've got a traits class that's appropriate for char and what we've got is a const signed char* or a const unsigned char*; that's close enough so we can just use a reinterpret cast, and continue to use the user-supplied traits class. Finally, there's one case where we just have to give up: where we've got a traits class for some arbitrary charT type, and we somehow have to deal with a const char*. There's nothing better to do but fall back to char_traits<char>
Section: 31.7.6.4 [ostream.unformatted] Status: TC1 Submitter: Dietmar Kühl Opened: 1999-07-20 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
The first paragraph begins with a descriptions what has to be done in formatted output functions. Probably this is a typo and the paragraph really want to describe unformatted output functions...
Proposed resolution:
In 31.7.6.4 [ostream.unformatted] paragraph 1, the first and last sentences, change the word "formatted" to "unformatted":
"Each unformatted output function begins ..."
"... value specified for the unformatted output function."
overflow()
mandatedSection: 31.8.2.5 [stringbuf.virtuals] Status: TC1 Submitter: Dietmar Kühl Opened: 1999-07-20 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
Paragraph 8, Notes, of this section seems to mandate an extremely
inefficient way of buffer handling for basic_stringbuf
,
especially in view of the restriction that basic_ostream
member functions are not allowed to use xsputn()
(see 31.7.6.2 [ostream]): For each character to be inserted, a new buffer
is to be created.
Of course, the resolution below requires some handling of
simultaneous input and output since it is no longer possible to update
egptr()
whenever epptr()
is changed. A possible
solution is to handle this in underflow()
.
Proposed resolution:
In 31.8.2.5 [stringbuf.virtuals] paragraph 8, Notes, insert the words "at least" as in the following:
To make a write position available, the function reallocates (or initially allocates) an array object with a sufficient number of elements to hold the current array object (if any), plus at least one additional write position.
traits_type
Section: 31.8.5 [stringstream] Status: TC1 Submitter: Dietmar Kühl Opened: 1999-07-20 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
The classes basic_stringstream
(31.8.5 [stringstream]),
basic_istringstream
(31.8.3 [istringstream]), and
basic_ostringstream
(31.8.4 [ostringstream]) are inconsistent
in their definition of the type traits_type
: For
istringstream
, this type is defined, for the other two it is
not. This should be consistent.
Proposed resolution:
Proposed resolution:
To the declarations of
basic_ostringstream
(31.8.4 [ostringstream]) and
basic_stringstream
(31.8.5 [stringstream]) add:
typedef traits traits_type;
seekpos()
semantics due to joint positionSection: 31.10.3.5 [filebuf.virtuals] Status: CD1 Submitter: Dietmar Kühl Opened: 1999-07-20 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
Overridden virtual functions, seekpos()
In 31.10.3 [filebuf] paragraph 3, it is stated that a joint input and
output position is maintained by basic_filebuf
. Still, the
description of seekpos()
seems to talk about different file
positions. In particular, it is unclear (at least to me) what is
supposed to happen to the output buffer (if there is one) if only the
input position is changed. The standard seems to mandate that the
output buffer is kept and processed as if there was no positioning of
the output position (by changing the input position). Of course, this
can be exactly what you want if the flag ios_base::ate
is
set. However, I think, the standard should say something like
this:
(which & mode) == 0
neither read nor write position is
changed and the call fails. Otherwise, the joint read and write position is
altered to correspond to sp
.Plus the appropriate error handling, that is...
Proposed resolution:
Change the unnumbered paragraph in 27.8.1.4 (lib.filebuf.virtuals) before paragraph 14 from:
pos_type seekpos(pos_type sp, ios_base::openmode = ios_base::in | ios_base::out);
Alters the file position, if possible, to correspond to the position stored in sp (as described below).
- if (which&ios_base::in)!=0, set the file position to sp, then update the input sequence
- if (which&ios_base::out)!=0, then update the output sequence, write any unshift sequence, and set the file position to sp.
to:
pos_type seekpos(pos_type sp, ios_base::openmode = ios_base::in | ios_base::out);
Alters the file position, if possible, to correspond to the position stored in sp (as described below). Altering the file position performs as follows:
1. if (om & ios_base::out)!=0, then update the output sequence and write any unshift sequence;
2. set the file position to sp;
3. if (om & ios_base::in)!=0, then update the input sequence;
where om is the open mode passed to the last call to open(). The operation fails if is_open() returns false.
[Kona: Dietmar is working on a proposed resolution.]
[Post-Tokyo: Dietmar supplied the above wording.]
basic_istream::ignore()
Section: 31.7.5.4 [istream.unformatted] Status: TC1 Submitter: Greg Comeau, Dietmar Kühl Opened: 1999-07-23 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
In 31.7.5.2 [istream] the function
ignore()
gets an object of type streamsize
as first
argument. However, in 31.7.5.4 [istream.unformatted]
paragraph 23 the first argument is of type int.
As far as I can see this is not really a contradiction because
everything is consistent if streamsize
is typedef to be
int
. However, this is almost certainly not what was
intended. The same thing happened to basic_filebuf::setbuf()
,
as described in issue 173(i).
Darin Adler also submitted this issue, commenting: Either 27.6.1.1 should be modified to show a first parameter of type int, or 27.6.1.3 should be modified to show a first parameter of type streamsize and use numeric_limits<streamsize>::max.
Proposed resolution:
In 31.7.5.4 [istream.unformatted] paragraph 23 and 24, change both uses
of int
in the description of ignore()
to
streamsize
.
basic_filebuf::setbuf()
Section: 31.10.3.5 [filebuf.virtuals] Status: TC1 Submitter: Greg Comeau, Dietmar Kühl Opened: 1999-07-23 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
In 31.10.3 [filebuf] the function setbuf()
gets an
object of type streamsize
as second argument. However, in
31.10.3.5 [filebuf.virtuals] paragraph 9 the second argument is of type
int
.
As far as I can see this is not really a contradiction because
everything is consistent if streamsize
is typedef to be
int
. However, this is almost certainly not what was
intended. The same thing happened to basic_istream::ignore()
,
as described in issue 172(i).
Proposed resolution:
In 31.10.3.5 [filebuf.virtuals] paragraph 9, change all uses of
int
in the description of setbuf()
to
streamsize
.
OFF_T
vs. POS_T
Section: 99 [depr.ios.members] Status: TC1 Submitter: Dietmar Kühl Opened: 1999-07-23 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
According to paragraph 1 of this section, streampos
is the
type OFF_T
, the same type as streamoff
. However, in
paragraph 6 the streampos
gets the type POS_T
Proposed resolution:
Change 99 [depr.ios.members] paragraph 1 from "typedef
OFF_T streampos;
" to "typedef POS_T
streampos;
"
basic_streambuf::pubseekpos()
and a few other functions.Section: 99 [depr.ios.members] Status: TC1 Submitter: Dietmar Kühl Opened: 1999-07-23 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
According to paragraph 8 of this section, the methods
basic_streambuf::pubseekpos()
,
basic_ifstream::open()
, and basic_ofstream::open
"may" be overloaded by a version of this function taking the
type ios_base::open_mode
as last argument argument instead of
ios_base::openmode
(ios_base::open_mode
is defined
in this section to be an alias for one of the integral types). The
clause specifies, that the last argument has a default argument in
three cases. However, this generates an ambiguity with the overloaded
version because now the arguments are absolutely identical if the last
argument is not specified.
Proposed resolution:
In 99 [depr.ios.members] paragraph 8, remove the default arguments for
basic_streambuf::pubseekpos()
,
basic_ifstream::open()
, and
basic_ofstream::open().
exceptions()
in ios_base
...?Section: 99 [depr.ios.members] Status: TC1 Submitter: Dietmar Kühl Opened: 1999-07-23 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
The "overload" for the function exceptions()
in
paragraph 8 gives the impression that there is another function of
this function defined in class ios_base
. However, this is not
the case. Thus, it is hard to tell how the semantics (paragraph 9) can
be implemented: "Call the corresponding member function specified
in clause 31 [input.output]."
Proposed resolution:
In 99 [depr.ios.members] paragraph 8, move the declaration of the
function exceptions()
into class basic_ios
.
Section: 23.2 [container.requirements] Status: CD1 Submitter: Judy Ward Opened: 1998-07-02 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
Currently the following will not compile on two well-known standard library implementations:
#include <set> using namespace std; void f(const set<int> &s) { set<int>::iterator i; if (i==s.end()); // s.end() returns a const_iterator }
The reason this doesn't compile is because operator== was implemented as a member function of the nested classes set:iterator and set::const_iterator, and there is no conversion from const_iterator to iterator. Surprisingly, (s.end() == i) does work, though, because of the conversion from iterator to const_iterator.
I don't see a requirement anywhere in the standard that this must work. Should there be one? If so, I think the requirement would need to be added to the tables in section 24.1.1. I'm not sure about the wording. If this requirement existed in the standard, I would think that implementors would have to make the comparison operators non-member functions.
This issues was also raised on comp.std.c++ by Darin Adler. The example given was:
bool check_equal(std::deque<int>::iterator i, std::deque<int>::const_iterator ci) { return i == ci; }
Comment from John Potter:
In case nobody has noticed, accepting it will break reverse_iterator.
The fix is to make the comparison operators templated on two types.
template <class Iterator1, class Iterator2> bool operator== (reverse_iterator<Iterator1> const& x, reverse_iterator<Iterator2> const& y);Obviously: return x.base() == y.base();
Currently, no reverse_iterator to const_reverse_iterator compares are valid.
BTW, I think the issue is in support of bad code. Compares should be between two iterators of the same type. All std::algorithms require the begin and end iterators to be of the same type.
Proposed resolution:
Insert this paragraph after 23.2 [container.requirements] paragraph 7:
In the expressions
i == j i != j i < j i <= j i >= j i > j i - jWhere i and j denote objects of a container's iterator type, either or both may be replaced by an object of the container's const_iterator type referring to the same element with no change in semantics.
[post-Toronto: Judy supplied a proposed resolution saying that
iterator
and const_iterator
could be freely mixed in
iterator comparison and difference operations.]
[Redmond: Dave and Howard supplied a new proposed resolution which explicitly listed expressions; there was concern that the previous proposed resolution was too informal.]
Rationale:
The LWG believes it is clear that the above wording applies only to
the nested types X::iterator
and X::const_iterator
,
where X
is a container. There is no requirement that
X::reverse_iterator
and X::const_reverse_iterator
can be mixed. If mixing them is considered important, that's a
separate issue. (Issue 280(i).)
Section: 27.4.3 [basic.string] Status: CD1 Submitter: Dave Abrahams Opened: 1999-07-01 Last modified: 2016-11-12
Priority: Not Prioritized
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Discussion:
It is the constness of the container which should control whether it can be modified through a member function such as erase(), not the constness of the iterators. The iterators only serve to give positioning information.
Here's a simple and typical example problem which is currently very difficult or impossible to solve without the change proposed below.
Wrap a standard container C in a class W which allows clients to find and read (but not modify) a subrange of (C.begin(), C.end()]. The only modification clients are allowed to make to elements in this subrange is to erase them from C through the use of a member function of W.
[ post Bellevue, Alisdair adds: ]
This issue was implemented by N2350 for everything but
basic_string
.Note that the specific example in this issue (
basic_string
) is the one place we forgot to amend in N2350, so we might open this issue for that single container?
[ Sophia Antipolis: ]
This was a fix that was intended for all standard library containers, and has been done for other containers, but string was missed.
The wording updated.
We did not make the change in
replace
, because this change would affect the implementation because the string may be written into. This is an issue that should be taken up by concepts.We note that the supplied wording addresses the initializer list provided in N2679.
Proposed resolution:
Update the following signature in the basic_string
class template definition in
27.4.3 [basic.string], p5:
namespace std { template<class charT, class traits = char_traits<charT>, class Allocator = allocator<charT> > class basic_string { ... iterator insert(const_iterator p, charT c); void insert(const_iterator p, size_type n, charT c); template<class InputIterator> void insert(const_iterator p, InputIterator first, InputIterator last); void insert(const_iterator p, initializer_list<charT>); ... iterator erase(const_iterator const_position); iterator erase(const_iterator first, const_iterator last); ... }; }
Update the following signatures in 27.4.3.7.4 [string.insert]:
iterator insert(const_iterator p, charT c); void insert(const_iterator p, size_type n, charT c); template<class InputIterator> void insert(const_iterator p, InputIterator first, InputIterator last); void insert(const_iterator p, initializer_list<charT>);
Update the following signatures in 27.4.3.7.5 [string.erase]:
iterator erase(const_iterator const_position); iterator erase(const_iterator first, const_iterator last);
Rationale:
The issue was discussed at length. It was generally agreed that 1) There is no major technical argument against the change (although there is a minor argument that some obscure programs may break), and 2) Such a change would not break const correctness. The concerns about making the change were 1) it is user detectable (although only in boundary cases), 2) it changes a large number of signatures, and 3) it seems more of a design issue that an out-and-out defect.
The LWG believes that this issue should be considered as part of a general review of const issues for the next revision of the standard. Also see issue 200(i).
Section: 22.3 [pairs] Status: TC1 Submitter: Andrew Koenig Opened: 1999-08-03 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
The claim has surfaced in Usenet that expressions such as
make_pair("abc", 3)
are illegal, notwithstanding their use in examples, because template instantiation tries to bind the first template
parameter to const char (&)[4]
, which type is uncopyable.
I doubt anyone intended that behavior...
Proposed resolution:
In 22.2 [utility], paragraph 1 change the following declaration of make_pair():
template <class T1, class T2> pair<T1,T2> make_pair(const T1&, const T2&);
to:
template <class T1, class T2> pair<T1,T2> make_pair(T1, T2);
In 22.3 [pairs] paragraph 7 and the line before, change:
template <class T1, class T2> pair<T1, T2> make_pair(const T1& x, const T2& y);
to:
template <class T1, class T2> pair<T1, T2> make_pair(T1 x, T2 y);
and add the following footnote to the effects clause:
According to 12.8 [class.copy], an implementation is permitted to not perform a copy of an argument, thus avoiding unnecessary copies.
Rationale:
Two potential fixes were suggested by Matt Austern and Dietmar Kühl, respectively, 1) overloading with array arguments, and 2) use of a reference_traits class with a specialization for arrays. Andy Koenig suggested changing to pass by value. In discussion, it appeared that this was a much smaller change to the standard that the other two suggestions, and any efficiency concerns were more than offset by the advantages of the solution. Two implementors reported that the proposed resolution passed their test suites.
Section: 16 [library] Status: CD1 Submitter: Al Stevens Opened: 1999-08-15 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
Many references to size_t
throughout the document
omit the std::
namespace qualification.
For example, 16.4.5.6 [replacement.functions] paragraph 2:
operator new(size_t) operator new(size_t, const std::nothrow_t&) operator new[](size_t) operator new[](size_t, const std::nothrow_t&)
Proposed resolution:
In 16.4.5.6 [replacement.functions] paragraph 2: replace:
- operator new(size_t)
- operator new(size_t, const std::nothrow_t&)
- operator new[](size_t)
- operator new[](size_t, const std::nothrow_t&)
by:
- operator new(std::size_t) - operator new(std::size_t, const std::nothrow_t&) - operator new[](std::size_t) - operator new[](std::size_t, const std::nothrow_t&)
In [lib.allocator.requirements] 20.1.5, paragraph 4: replace:
The typedef members pointer, const_pointer, size_type, and difference_type are required to be T*, T const*, size_t, and ptrdiff_t, respectively.
by:
The typedef members pointer, const_pointer, size_type, and difference_type are required to be T*, T const*, std::size_t, and std::ptrdiff_t, respectively.
In [lib.allocator.members] 20.4.1.1, paragraphs 3 and 6: replace:
3 Notes: Uses ::operator new(size_t) (18.4.1).
6 Note: the storage is obtained by calling ::operator new(size_t), but it is unspecified when or how often this function is called. The use of hint is unspecified, but intended as an aid to locality if an implementation so desires.
by:
3 Notes: Uses ::operator new(std::size_t) (18.4.1).
6 Note: the storage is obtained by calling ::operator new(std::size_t), but it is unspecified when or how often this function is called. The use of hint is unspecified, but intended as an aid to locality if an implementation so desires.
In [lib.char.traits.require] 21.1.1, paragraph 1: replace:
In Table 37, X denotes a Traits class defining types and functions for the character container type CharT; c and d denote values of type CharT; p and q denote values of type const CharT*; s denotes a value of type CharT*; n, i and j denote values of type size_t; e and f denote values of type X::int_type; pos denotes a value of type X::pos_type; and state denotes a value of type X::state_type.
by:
In Table 37, X denotes a Traits class defining types and functions for the character container type CharT; c and d denote values of type CharT; p and q denote values of type const CharT*; s denotes a value of type CharT*; n, i and j denote values of type std::size_t; e and f denote values of type X::int_type; pos denotes a value of type X::pos_type; and state denotes a value of type X::state_type.
In [lib.char.traits.require] 21.1.1, table 37: replace the return type of X::length(p): "size_t" by "std::size_t".
In [lib.std.iterator.tags] 24.3.3, paragraph 2: replace:
typedef ptrdiff_t difference_type;
by:
typedef std::ptrdiff_t difference_type;
In [lib.locale.ctype] 22.2.1.1 put namespace std { ...} around the declaration of template <class charT> class ctype.
In [lib.iterator.traits] 24.3.1, paragraph 2 put namespace std { ...} around the declaration of:
template<class Iterator> struct iterator_traits
template<class T> struct iterator_traits<T*>
template<class T> struct iterator_traits<const T*>
Rationale:
The LWG believes correcting names like size_t
and
ptrdiff_t
to std::size_t
and std::ptrdiff_t
to be essentially editorial. There there can't be another size_t or
ptrdiff_t meant anyway because, according to 16.4.5.3.5 [extern.types],
For each type T from the Standard C library, the types ::T and std::T are reserved to the implementation and, when defined, ::T shall be identical to std::T.
The issue is treated as a Defect Report to make explicit the Project Editor's authority to make this change.
[Post-Tokyo: Nico Josuttis provided the above wording at the request of the LWG.]
[Toronto: This is tangentially related to issue 229(i), but only tangentially: the intent of this issue is to
address use of the name size_t
in contexts outside of
namespace std, such as in the description of ::operator new
.
The proposed changes should be reviewed to make sure they are
correct.]
[pre-Copenhagen: Nico has reviewed the changes and believes them to be correct.]
Section: 31.7.7 [std.manip] Status: CD1 Submitter: Andy Sawyer Opened: 1999-07-07 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
31.7.7 [std.manip] paragraph 3 says (clause numbering added for exposition):
Returns: An object s of unspecified type such that if [1] out is an (instance of) basic_ostream then the expression out<<s behaves as if f(s) were called, and if [2] in is an (instance of) basic_istream then the expression in>>s behaves as if f(s) were called. Where f can be defined as: ios_base& f(ios_base& str, ios_base::fmtflags mask) { // reset specified flags str.setf(ios_base::fmtflags(0), mask); return str; } [3] The expression out<<s has type ostream& and value out. [4] The expression in>>s has type istream& and value in.
Given the definitions [1] and [2] for out and in, surely [3] should read: "The expression out << s has type basic_ostream& ..." and [4] should read: "The expression in >> s has type basic_istream& ..."
If the wording in the standard is correct, I can see no way of implementing any of the manipulators so that they will work with wide character streams.
e.g. wcout << setbase( 16 );
Must have value 'wcout' (which makes sense) and type 'ostream&' (which doesn't).
The same "cut'n'paste" type also seems to occur in Paras 4,5,7 and 8. In addition, Para 6 [setfill] has a similar error, but relates only to ostreams.
I'd be happier if there was a better way of saying this, to make it clear that the value of the expression is "the same specialization of basic_ostream as out"&
Proposed resolution:
Replace section 31.7.7 [std.manip] except paragraph 1 with the following:
2- The type designated smanip in each of the following function descriptions is implementation-specified and may be different for each function.
smanip resetiosflags(ios_base::fmtflags mask);
-3- Returns: An object s of unspecified type such that if out is an instance of basic_ostream<charT,traits> then the expression out<<s behaves as if f(s, mask) were called, or if in is an instance of basic_istream<charT,traits> then the expression in>>s behaves as if f(s, mask) were called. The function f can be defined as:*
[Footnote: The expression cin >> resetiosflags(ios_base::skipws) clears ios_base::skipws in the format flags stored in the basic_istream<charT,traits> object cin (the same as cin >> noskipws), and the expression cout << resetiosflags(ios_base::showbase) clears ios_base::showbase in the format flags stored in the basic_ostream<charT,traits> object cout (the same as cout << noshowbase). --- end footnote]
ios_base& f(ios_base& str, ios_base::fmtflags mask)
{
// reset specified flags
str.setf(ios_base::fmtflags(0), mask);
return str;
}
The expression out<<s has type basic_ostream<charT,traits>& and value out. The expression in>>s has type basic_istream<charT,traits>& and value in.
smanip setiosflags(ios_base::fmtflags mask);
-4- Returns: An object s of unspecified type such that if out is an instance of basic_ostream<charT,traits> then the expression out<<s behaves as if f(s, mask) were called, or if in is an instance of basic_istream<charT,traits> then the expression in>>s behaves as if f(s, mask) were called. The function f can be defined as:
ios_base& f(ios_base& str, ios_base::fmtflags mask)
{
// set specified flags
str.setf(mask);
return str;
}
The expression out<<s has type basic_ostream<charT,traits>& and value out. The expression in>>s has type basic_istream<charT,traits>& and value in.
smanip setbase(int base);
-5- Returns: An object s of unspecified type such that if out is an instance of basic_ostream<charT,traits> then the expression out<<s behaves as if f(s, base) were called, or if in is an instance of basic_istream<charT,traits> then the expression in>>s behaves as if f(s, base) were called. The function f can be defined as:
ios_base& f(ios_base& str, int base)
{
// set basefield
str.setf(base == 8 ? ios_base::oct :
base == 10 ? ios_base::dec :
base == 16 ? ios_base::hex :
ios_base::fmtflags(0), ios_base::basefield);
return str;
}
The expression out<<s has type basic_ostream<charT,traits>& and value out. The expression in>>s has type basic_istream<charT,traits>& and value in.
smanip setfill(char_type c);
-6- Returns: An object s of unspecified type such that if out is (or is derived from) basic_ostream<charT,traits> and c has type charT then the expression out<<s behaves as if f(s, c) were called. The function f can be defined as:
template<class charT, class traits>
basic_ios<charT,traits>& f(basic_ios<charT,traits>& str, charT c)
{
// set fill character
str.fill(c);
return str;
}
The expression out<<s has type basic_ostream<charT,traits>& and value out.
smanip setprecision(int n);
-7- Returns: An object s of unspecified type such that if out is an instance of basic_ostream<charT,traits> then the expression out<<s behaves as if f(s, n) were called, or if in is an instance of basic_istream<charT,traits> then the expression in>>s behaves as if f(s, n) were called. The function f can be defined as:
ios_base& f(ios_base& str, int n)
{
// set precision
str.precision(n);
return str;
}
The expression out<<s has type basic_ostream<charT,traits>& and value out. The expression in>>s has type basic_istream<charT,traits>& and value in
.
smanip setw(int n);
-8- Returns: An object s of unspecified type such that if out is an instance of basic_ostream<charT,traits> then the expression out<<s behaves as if f(s, n) were called, or if in is an instance of basic_istream<charT,traits> then the expression in>>s behaves as if f(s, n) were called. The function f can be defined as:
ios_base& f(ios_base& str, int n)
{
// set width
str.width(n);
return str;
}
The expression out<<s has type basic_ostream<charT,traits>& and value out. The expression in>>s has type basic_istream<charT,traits>& and value in.
[Kona: Andy Sawyer and Beman Dawes will work to improve the wording of the proposed resolution.]
[Tokyo - The LWG noted that issue 216(i) involves the same paragraphs.]
[Post-Tokyo: The issues list maintainer combined the proposed resolution of this issue with the proposed resolution for issue 216(i) as they both involved the same paragraphs, and were so intertwined that dealing with them separately appear fraught with error. The full text was supplied by Bill Plauger; it was cross checked against changes supplied by Andy Sawyer. It should be further checked by the LWG.]
Section: 17.3.5.3 [numeric.special] Status: CD1 Submitter: Gabriel Dos Reis Opened: 1999-07-21 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
bools are defined by the standard to be of integer types, as per 6.8.2 [basic.fundamental] paragraph 7. However "integer types" seems to have a special meaning for the author of 18.2. The net effect is an unclear and confusing specification for numeric_limits<bool> as evidenced below.
18.2.1.2/7 says numeric_limits<>::digits is, for built-in integer types, the number of non-sign bits in the representation.
4.5/4 states that a bool promotes to int ; whereas 4.12/1 says any non zero arithmetical value converts to true.
I don't think it makes sense at all to require numeric_limits<bool>::digits and numeric_limits<bool>::digits10 to be meaningful.
The standard defines what constitutes a signed (resp. unsigned) integer types. It doesn't categorize bool as being signed or unsigned. And the set of values of bool type has only two elements.
I don't think it makes sense to require numeric_limits<bool>::is_signed to be meaningful.
18.2.1.2/18 for numeric_limits<integer_type>::radix says:
For integer types, specifies the base of the representation.186)
This disposition is at best misleading and confusing for the standard requires a "pure binary numeration system" for integer types as per 3.9.1/7
The footnote 186) says: "Distinguishes types with base other than 2 (e.g BCD)." This also erroneous as the standard never defines any integer types with base representation other than 2.
Furthermore, numeric_limits<bool>::is_modulo and numeric_limits<bool>::is_signed have similar problems.
Proposed resolution:
Append to the end of 17.3.5.3 [numeric.special]:
The specialization for bool shall be provided as follows:
namespace std { template<> class numeric_limits<bool> { public: static const bool is_specialized = true; static bool min() throw() { return false; } static bool max() throw() { return true; } static const int digits = 1; static const int digits10 = 0; static const bool is_signed = false; static const bool is_integer = true; static const bool is_exact = true; static const int radix = 2; static bool epsilon() throw() { return 0; } static bool round_error() throw() { return 0; } static const int min_exponent = 0; static const int min_exponent10 = 0; static const int max_exponent = 0; static const int max_exponent10 = 0; static const bool has_infinity = false; static const bool has_quiet_NaN = false; static const bool has_signaling_NaN = false; static const float_denorm_style has_denorm = denorm_absent; static const bool has_denorm_loss = false; static bool infinity() throw() { return 0; } static bool quiet_NaN() throw() { return 0; } static bool signaling_NaN() throw() { return 0; } static bool denorm_min() throw() { return 0; } static const bool is_iec559 = false; static const bool is_bounded = true; static const bool is_modulo = false; static const bool traps = false; static const bool tinyness_before = false; static const float_round_style round_style = round_toward_zero; }; }
[Tokyo: The LWG desires wording that specifies exact values rather than more general wording in the original proposed resolution.]
[Post-Tokyo: At the request of the LWG in Tokyo, Nico Josuttis provided the above wording.]
Section: 22.10 [function.objects] Status: CD1 Submitter: UK Panel Opened: 1999-07-26 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
Paragraph 4 of 22.10 [function.objects] says:
[Example: To negate every element of a: transform(a.begin(), a.end(), a.begin(), negate<double>()); The corresponding functions will inline the addition and the negation. end example]
(Note: The "addition" referred to in the above is in para 3) we can find no other wording, except this (non-normative) example which suggests that any "inlining" will take place in this case.
Indeed both:
17.4.4.3 Global Functions [lib.global.functions] 1 It is unspecified whether any global functions in the C++ Standard Library are defined as inline (7.1.2).
and
17.4.4.4 Member Functions [lib.member.functions] 1 It is unspecified whether any member functions in the C++ Standard Library are defined as inline (7.1.2).
take care to state that this may indeed NOT be the case.
Thus the example "mandates" behavior that is explicitly not required elsewhere.
Proposed resolution:
In 22.10 [function.objects] paragraph 1, remove the sentence:
They are important for the effective use of the library.
Remove 22.10 [function.objects] paragraph 2, which reads:
Using function objects together with function templates increases the expressive power of the library as well as making the resulting code much more efficient.
In 22.10 [function.objects] paragraph 4, remove the sentence:
The corresponding functions will inline the addition and the negation.
[Kona: The LWG agreed there was a defect.]
[Tokyo: The LWG crafted the proposed resolution.]
Section: 22.9.2.3 [bitset.members] Status: CD1 Submitter: Darin Adler Opened: 1999-08-13 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
In section 22.9.2.3 [bitset.members], paragraph 13 defines the bitset::set operation to take a second parameter of type int. The function tests whether this value is non-zero to determine whether to set the bit to true or false. The type of this second parameter should be bool. For one thing, the intent is to specify a Boolean value. For another, the result type from test() is bool. In addition, it's possible to slice an integer that's larger than an int. This can't happen with bool, since conversion to bool has the semantic of translating 0 to false and any non-zero value to true.
Proposed resolution:
In 22.9.2 [template.bitset] Para 1 Replace:
bitset<N>& set(size_t pos, int val = true );
With:
bitset<N>& set(size_t pos, bool val = true );
In 22.9.2.3 [bitset.members] Para 12(.5) Replace:
bitset<N>& set(size_t pos, int val = 1 );
With:
bitset<N>& set(size_t pos, bool val = true );
[Kona: The LWG agrees with the description. Andy Sawyer will work on better P/R wording.]
[Post-Tokyo: Andy provided the above wording.]
Rationale:
bool
is a better choice. It is believed that binary
compatibility is not an issue, because this member function is
usually implemented as inline
, and because it is already
the case that users cannot rely on the type of a pointer to a
nonvirtual member of a standard library class.
Section: 26.7.3 [alg.swap] Status: CD1 Submitter: Andrew Koenig Opened: 1999-08-14 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
The description of iter_swap in 25.2.2 paragraph 7,says that it
``exchanges the values'' of the objects to which two iterators
refer.
What it doesn't say is whether it does so using swap
or using the assignment operator and copy constructor.
This
question is an important one to answer, because swap is specialized to
work efficiently for standard containers.
For example:
vector<int> v1, v2; iter_swap(&v1, &v2);
Is this call to iter_swap equivalent to calling swap(v1, v2)? Or is it equivalent to
{ vector<int> temp = v1; v1 = v2; v2 = temp; }
The first alternative is O(1); the second is O(n).
A LWG member, Dave Abrahams, comments:
Not an objection necessarily, but I want to point out the cost of that requirement:
iter_swap(list<T>::iterator, list<T>::iterator)
can currently be specialized to be more efficient than iter_swap(T*,T*) for many T (by using splicing). Your proposal would make that optimization illegal.
[Kona: The LWG notes the original need for iter_swap was proxy iterators which are no longer permitted.]
Proposed resolution:
Change the effect clause of iter_swap in 25.2.2 paragraph 7 from:
Exchanges the values pointed to by the two iterators a and b.
to
swap(*a, *b)
.
Rationale:
It's useful to say just what iter_swap
does. There may be
some iterators for which we want to specialize iter_swap
,
but the fully general version should have a general specification.
Note that in the specific case of list<T>::iterator
,
iter_swap should not be specialized as suggested above. That would do
much more than exchanging the two iterators' values: it would change
predecessor/successor relationships, possibly moving the iterator from
one list to another. That would surely be inappropriate.
Section: 31.5.2.3 [fmtflags.state] Status: TC1 Submitter: Andrew Koenig Opened: 1999-08-25 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
27.4.2.2 paragraph 9 claims that setprecision() sets the precision,
and includes a parenthetical note saying that it is the number of
digits after the decimal point.
This claim is not strictly correct. For example, in the default
floating-point output format, setprecision sets the number of
significant digits printed, not the number of digits after the decimal
point.
I would like the committee to look at the definition carefully and
correct the statement in 27.4.2.2
Proposed resolution:
Remove from 31.5.2.3 [fmtflags.state], paragraph 9, the text "(number of digits after the decimal point)".
Section: 26.8.8 [alg.heap.operations] Status: TC1 Submitter: Markus Mauhart Opened: 1999-09-24 Last modified: 2024-01-25
Priority: Not Prioritized
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Duplicate of: 216
Discussion:
25.3.6 [lib.alg.heap.operations] states two key properties of a heap [a,b), the first of them
is
"(1) *a is the largest element"
I think this is incorrect and should be changed to the wording in the proposed
resolution.
Actually there are two independent changes:
A-"part of largest equivalence class" instead of "largest", cause 25.3 [lib.alg.sorting] asserts "strict weak ordering" for all its sub clauses.
B-Take 'an oldest' from that equivalence class, otherwise the heap functions could not be used for a priority queue as explained in 23.2.3.2.2 [lib.priqueue.members] (where I assume that a "priority queue" respects priority AND time).
Proposed resolution:
Change 26.8.8 [alg.heap.operations] property (1) from:
(1) *a is the largest element
to:
(1) There is no element greater than
*a
basic_istream::sentry
's constructor ever set eofbit?Section: 31.7.5.2.4 [istream.sentry] Status: TC1 Submitter: Matt Austern Opened: 1999-10-13 Last modified: 2021-06-06
Priority: Not Prioritized
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Discussion:
Suppose that is.flags() & ios_base::skipws
is nonzero.
What should basic_istream<>::sentry
's constructor do if it
reaches eof while skipping whitespace? 27.6.1.1.2/5 suggests it
should set failbit. Should it set eofbit as well? The standard
doesn't seem to answer that question.
On the one hand, nothing in [istream::sentry] says that
basic_istream<>::sentry
should ever set eofbit. On the
other hand, 31.7.5.2 [istream] paragraph 4 says that if
extraction from a streambuf
"returns
traits::eof()
, then the input function, except as explicitly
noted otherwise, completes its actions and does
setstate(eofbit)"
. So the question comes down to
whether basic_istream<>::sentry
's constructor is an
input function.
Comments from Jerry Schwarz:
It was always my intention that eofbit should be set any time that a virtual returned something to indicate eof, no matter what reason iostream code had for calling the virtual.
The motivation for this is that I did not want to require streambufs to behave consistently if their virtuals are called after they have signaled eof.
The classic case is a streambuf reading from a UNIX file. EOF isn't really a state for UNIX file descriptors. The convention is that a read on UNIX returns 0 bytes to indicate "EOF", but the file descriptor isn't shut down in any way and future reads do not necessarily also return 0 bytes. In particular, you can read from tty's on UNIX even after they have signaled "EOF". (It isn't always understood that a ^D on UNIX is not an EOF indicator, but an EOL indicator. By typing a "line" consisting solely of ^D you cause a read to return 0 bytes, and by convention this is interpreted as end of file.)
Proposed resolution:
Add a sentence to the end of 27.6.1.1.2 paragraph 2:
If
is.rdbuf()->sbumpc()
oris.rdbuf()->sgetc()
returnstraits::eof()
, the function callssetstate(failbit | eofbit)
(which may throwios_base::failure
).
Section: 24.3.4 [iterator.concepts] Status: CD1 Submitter: Beman Dawes Opened: 1999-11-03 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
Is a pointer or reference obtained from an iterator still valid after destruction of the iterator?
Is a pointer or reference obtained from an iterator still valid after the value of the iterator changes?
#include <iostream> #include <vector> #include <iterator> int main() { typedef std::vector<int> vec_t; vec_t v; v.push_back( 1 ); // Is a pointer or reference obtained from an iterator still // valid after destruction of the iterator? int * p = &*v.begin(); std::cout << *p << '\n'; // OK? // Is a pointer or reference obtained from an iterator still // valid after the value of the iterator changes? vec_t::iterator iter( v.begin() ); p = &*iter++; std::cout << *p << '\n'; // OK? return 0; }
The standard doesn't appear to directly address these questions. The standard needs to be clarified. At least two real-world cases have been reported where library implementors wasted considerable effort because of the lack of clarity in the standard. The question is important because requiring pointers and references to remain valid has the effect for practical purposes of prohibiting iterators from pointing to cached rather than actual elements of containers.
The standard itself assumes that pointers and references obtained from an iterator are still valid after iterator destruction or change. The definition of reverse_iterator::operator*(), 24.5.1.5 [reverse.iter.conv], which returns a reference, defines effects:
Iterator tmp = current; return *--tmp;
The definition of reverse_iterator::operator->(), [reverse.iter.op.star], which returns a pointer, defines effects:
return &(operator*());
Because the standard itself assumes pointers and references remain valid after iterator destruction or change, the standard should say so explicitly. This will also reduce the chance of user code breaking unexpectedly when porting to a different standard library implementation.
Proposed resolution:
Add a new paragraph to 24.3.4 [iterator.concepts]:
Destruction of an iterator may invalidate pointers and references previously obtained from that iterator.
Replace paragraph 1 of 24.5.1.5 [reverse.iter.conv] with:
Effects:
this->tmp = current; --this->tmp; return *this->tmp;[Note: This operation must use an auxiliary member variable, rather than a temporary variable, to avoid returning a reference that persists beyond the lifetime of its associated iterator. (See 24.3.4 [iterator.concepts].) The name of this member variable is shown for exposition only. --end note]
[Post-Tokyo: The issue has been reformulated purely in terms of iterators.]
[Pre-Toronto: Steve Cleary pointed out the no-invalidation assumption by reverse_iterator. The issue and proposed resolution was reformulated yet again to reflect this reality.]
[Copenhagen: Steve Cleary pointed out that reverse_iterator
assumes its underlying iterator has persistent pointers and
references. Andy Koenig pointed out that it is possible to rewrite
reverse_iterator so that it no longer makes such an assupmption.
However, this issue is related to issue 299(i). If we
decide it is intentional that p[n]
may return by value
instead of reference when p
is a Random Access Iterator,
other changes in reverse_iterator will be necessary.]
Rationale:
This issue has been discussed extensively. Note that it is
not an issue about the behavior of predefined iterators. It is
asking whether or not user-defined iterators are permitted to have
transient pointers and references. Several people presented examples
of useful user-defined iterators that have such a property; examples
include a B-tree iterator, and an "iota iterator" that doesn't point
to memory. Library implementors already seem to be able to cope with
such iterators: they take pains to avoid forming references to memory
that gets iterated past. The only place where this is a problem is
reverse_iterator
, so this issue changes
reverse_iterator
to make it work.
This resolution does not weaken any guarantees provided by
predefined iterators like list<int>::iterator
.
Clause 23 should be reviewed to make sure that guarantees for
predefined iterators are as strong as users expect.
allocate(0)
return?Section: 16.4.4.6 [allocator.requirements] Status: TC1 Submitter: Matt Austern Opened: 1999-11-19 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
Suppose that A
is a class that conforms to the
Allocator requirements of Table 32, and a
is an
object of class A
What should be the return
value of a.allocate(0)
? Three reasonable
possibilities: forbid the argument 0
, return
a null pointer, or require that the return value be a
unique non-null pointer.
Proposed resolution:
Add a note to the allocate
row of Table 32:
"[Note: If n == 0
, the return value is unspecified. --end note]"
Rationale:
A key to understanding this issue is that the ultimate use of allocate() is to construct an iterator, and that iterator for zero length sequences must be the container's past-the-end representation. Since this already implies special case code, it would be over-specification to mandate the return value.
Section: 24.3.5.5 [forward.iterators] Status: CD1 Submitter: Matt Austern Opened: 1999-11-19 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
In table 74, the return type of the expression *a
is given
as T&
, where T
is the iterator's value type.
For constant iterators, however, this is wrong. ("Value type"
is never defined very precisely, but it is clear that the value type
of, say, std::list<int>::const_iterator
is supposed to be
int
, not const int
.)
Proposed resolution:
In table 74, in the *a
and *r++
rows, change the
return type from "T&
" to "T&
if X
is mutable, otherwise const T&
".
In the a->m
row, change the return type from
"U&
" to "U&
if X
is mutable,
otherwise const U&
".
[Tokyo: The LWG believes this is the tip of a larger iceberg; there are multiple const problems with the STL portion of the library and that these should be addressed as a single package. Note that issue 180(i) has already been declared NAD Future for that very reason.]
[Redmond: the LWG thinks this is separable from other constness issues. This issue is just cleanup; it clarifies language that was written before we had iterator_traits. Proposed resolution was modified: the original version only discussed *a. It was pointed out that we also need to worry about *r++ and a->m.]
Section: 17.3 [support.limits] Status: CD1 Submitter: Stephen Cleary Opened: 1999-12-21 Last modified: 2017-06-15
Priority: Not Prioritized
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Discussion:
In some places in this section, the terms "fundamental types" and "scalar types" are used when the term "arithmetic types" is intended. The current usage is incorrect because void is a fundamental type and pointers are scalar types, neither of which should have specializations of numeric_limits.
[Lillehammer: it remains true that numeric_limits is using imprecise language. However, none of the proposals for changed wording are clearer. A redesign of numeric_limits is needed, but this is more a task than an open issue.]
Proposed resolution:
Change 17.3 [support.limits] to:
-1- The headers
<limits>
,<climits>
,<cfloat>
, and<cinttypes>
supply characteristics of implementation-dependentfundamentalarithmetic types (3.9.1).
Change [limits] to:
-1- The
numeric_limits
component provides a C++ program with information about various properties of the implementation's representation of thefundamentalarithmetic types.-2- Specializations shall be provided for each
fundamentalarithmetic type, both floating point and integer, includingbool
. The memberis_specialized
shall betrue
for all such specializations ofnumeric_limits
.-4- Non-
fundamentalarithmetic standard types, such ascomplex<T>
(26.3.2), shall not have specializations.
Change 17.3.5 [numeric.limits] to:
-1- The memberis_specialized
makes it possible to distinguish between fundamental types, which have specializations, and non-scalar types, which do not.
Section: 26.7.9 [alg.unique] Status: CD1 Submitter: Andrew Koenig Opened: 2000-01-13 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
What should unique() do if you give it a predicate that is not an equivalence relation? There are at least two plausible answers:
1. You can't, because 25.2.8 says that it it "eliminates all but the first element from every consecutive group of equal elements..." and it wouldn't make sense to interpret "equal" as meaning anything but an equivalence relation. [It also doesn't make sense to interpret "equal" as meaning ==, because then there would never be any sense in giving a predicate as an argument at all.]
2. The word "equal" should be interpreted to mean whatever the predicate says, even if it is not an equivalence relation (and in particular, even if it is not transitive).
The example that raised this question is from Usenet:
int f[] = { 1, 3, 7, 1, 2 }; int* z = unique(f, f+5, greater<int>());
If one blindly applies the definition using the predicate greater<int>, and ignore the word "equal", you get:
Eliminates all but the first element from every consecutive group of elements referred to by the iterator i in the range [first, last) for which *i > *(i - 1).
The first surprise is the order of the comparison. If we wanted to
allow for the predicate not being an equivalence relation, then we
should surely compare elements the other way: pred(*(i - 1), *i). If
we do that, then the description would seem to say: "Break the
sequence into subsequences whose elements are in strictly increasing
order, and keep only the first element of each subsequence". So the
result would be 1, 1, 2. If we take the description at its word, it
would seem to call for strictly DEcreasing order, in which case the
result should be 1, 3, 7, 2.
In fact, the SGI implementation of unique() does neither: It yields 1,
3, 7.
Proposed resolution:
Change 26.7.9 [alg.unique] paragraph 1 to:
For a nonempty range, eliminates all but the first element from every consecutive group of equivalent elements referred to by the iterator
i
in the range [first+1, last) for which the following conditions hold:*(i-1) == *i
orpred(*(i-1), *i) != false
.
Also insert a new paragraph, paragraph 2a, that reads: "Requires: The comparison function must be an equivalence relation."
[Redmond: discussed arguments for and against requiring the comparison function to be an equivalence relation. Straw poll: 14-2-5. First number is to require that it be an equivalence relation, second number is to explicitly not require that it be an equivalence relation, third number is people who believe they need more time to consider the issue. A separate issue: Andy Sawyer pointed out that "i-1" is incorrect, since "i" can refer to the first iterator in the range. Matt provided wording to address this problem.]
[Curaçao: The LWG changed "... the range (first, last)..." to "... the range [first+1, last)..." for clarity. They considered this change close enough to editorial to not require another round of review.]
Rationale:
The LWG also considered an alternative resolution: change 26.7.9 [alg.unique] paragraph 1 to:
For a nonempty range, eliminates all but the first element from every consecutive group of elements referred to by the iterator
i
in the range (first, last) for which the following conditions hold:*(i-1) == *i
orpred(*(i-1), *i) != false
.
Also insert a new paragraph, paragraph 1a, that reads: "Notes: The comparison function need not be an equivalence relation."
Informally: the proposed resolution imposes an explicit requirement that the comparison function be an equivalence relation. The alternative resolution does not, and it gives enough information so that the behavior of unique() for a non-equivalence relation is specified. Both resolutions are consistent with the behavior of existing implementations.
Section: 17.6.3.2 [new.delete.single] Status: CD1 Submitter: Howard Hinnant Opened: 1999-08-29 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
As specified, the implementation of the nothrow version of operator new does not necessarily call the ordinary operator new, but may instead simply call the same underlying allocator and return a null pointer instead of throwing an exception in case of failure.
Such an implementation breaks code that replaces the ordinary version of new, but not the nothrow version. If the ordinary version of new/delete is replaced, and if the replaced delete is not compatible with pointers returned from the library versions of new, then when the replaced delete receives a pointer allocated by the library new(nothrow), crash follows.
The fix appears to be that the lib version of new(nothrow) must call the ordinary new. Thus when the ordinary new gets replaced, the lib version will call the replaced ordinary new and things will continue to work.
An alternative would be to have the ordinary new call new(nothrow). This seems sub-optimal to me as the ordinary version of new is the version most commonly replaced in practice. So one would still need to replace both ordinary and nothrow versions if one wanted to replace the ordinary version.
Another alternative is to put in clear text that if one version is replaced, then the other must also be replaced to maintain compatibility. Then the proposed resolution below would just be a quality of implementation issue. There is already such text in paragraph 7 (under the new(nothrow) version). But this nuance is easily missed if one reads only the paragraphs relating to the ordinary new.
N2158 has been written explaining the rationale for the proposed resolution below.
Proposed resolution:
Change 18.5.1.1 [new.delete.single]:
void* operator new(std::size_t size, const std::nothrow_t&) throw();-5- Effects: Same as above, except that it is called by a placement version of a new-expression when a C++ program prefers a null pointer result as an error indication, instead of a
bad_alloc
exception.-6- Replaceable: a C++ program may define a function with this function signature that displaces the default version defined by the C++ Standard library.
-7- Required behavior: Return a non-null pointer to suitably aligned storage (3.7.4), or else return a null pointer. This nothrow version of operator new returns a pointer obtained as if acquired from the (possibly replaced) ordinary version. This requirement is binding on a replacement version of this function.
-8- Default behavior:
- Calls
operator new(size)
.- If the call to
operator new(size)
returns normally, returns the result of that call, else- if the call to
operator new(size)
throws an exception, returns a null pointer.Executes a loop: Within the loop, the function first attempts to allocate the requested storage. Whether the attempt involves a call to the Standard C library functionmalloc
is unspecified.Returns a pointer to the allocated storage if the attempt is successful. Otherwise, if the last argument toset_new_handler()
was a null pointer, return a null pointer.Otherwise, the function calls the current new_handler (18.5.2.2). If the called function returns, the loop repeats.The loop terminates when an attempt to allocate the requested storage is successful or when a called new_handler function does not return. If the called new_handler function terminates by throwing abad_alloc exception
, the function returns a null pointer.-9- [Example:
T* p1 = new T; // throws bad_alloc if it fails T* p2 = new(nothrow) T; // returns 0 if it fails--end example]
void operator delete(void* ptr) throw();void operator delete(void* ptr, const std::nothrow_t&) throw();-10- Effects: The deallocation function (3.7.4.2) called by a delete-expression to render the value of
ptr
invalid.-11- Replaceable: a C++ program may define a function with this function signature that displaces the default version defined by the C++ Standard library.
-12- Requires: the value of
ptr
is null or the value returned by an earlier call to thedefault(possibly replaced)operator new(std::size_t)
oroperator new(std::size_t, const std::nothrow_t&)
.-13- Default behavior:
- For a null value of
ptr
, do nothing.- Any other value of
ptr
shall be a value returned earlier by a call to the defaultoperator new
, which was not invalidated by an intervening call tooperator delete(void*)
(17.4.3.7). For such a non-null value ofptr
, reclaims storage allocated by the earlier call to the defaultoperator new
.-14- Remarks: It is unspecified under what conditions part or all of such reclaimed storage is allocated by a subsequent call to
operator new
or any ofcalloc
,malloc
, orrealloc
, declared in<cstdlib>
.void operator delete(void* ptr, const std::nothrow_t&) throw();-15- Effects: Same as above, except that it is called by the implementation when an exception propagates from a nothrow placement version of the new-expression (i.e. when the constructor throws an exception).
-16- Replaceable: a C++ program may define a function with this function signature that displaces the default version defined by the C++ Standard library.
-17- Requires: the value of
ptr
is null or the value returned by an earlier call to the (possibly replaced)operator new(std::size_t)
oroperator new(std::size_t, const std::nothrow_t&)
.-18- Default behavior: Calls
operator delete(ptr)
.
Change 18.5.1.2 [new.delete.array]
void* operator new[](std::size_t size, const std::nothrow_t&) throw();-5- Effects: Same as above, except that it is called by a placement version of a new-expression when a C++ program prefers a null pointer result as an error indication, instead of a
bad_alloc
exception.-6- Replaceable: a C++ program can define a function with this function signature that displaces the default version defined by the C++ Standard library.
-7- Required behavior:
Same as for operatorReturn a non-null pointer to suitably aligned storage (3.7.4), or else return a null pointer. This nothrow version of operator new returns a pointer obtained as if acquired from the (possibly replaced)new(std::size_t, const std::nothrow_t&)
. This nothrow version of operatornew[]
returns a pointer obtained as if acquired from the ordinary version.operator new[](std::size_t size)
. This requirement is binding on a replacement version of this function.-8- Default behavior:
Returnsoperator new(size, nothrow)
.
- Calls
operator new[](size)
.- If the call to
operator new[](size)
returns normally, returns the result of that call, else- if the call to
operator new[](size)
throws an exception, returns a null pointer.void operator delete[](void* ptr) throw(); void operator delete[](void* ptr, const std::nothrow_t&) throw();-9- Effects: The deallocation function (3.7.4.2) called by the array form of a delete-expression to render the value of
ptr
invalid.-10- Replaceable: a C++ program can define a function with this function signature that displaces the default version defined by the C++ Standard library.
-11- Requires: the value of
ptr
is null or the value returned by an earlier call tooperator new[](std::size_t)
oroperator new[](std::size_t, const std::nothrow_t&)
.-12- Default behavior: Calls
operator delete(ptr)
oroperator delete[](ptr
respectively., std::nothrow)
Rationale:
Yes, they may become unlinked, and that is by design. If a user replaces one, the user should also replace the other.
[ Reopened due to a gcc conversation between Howard, Martin and Gaby. Forwarding or not is visible behavior to the client and it would be useful for the client to know which behavior it could depend on. ]
[ Batavia: Robert voiced serious reservations about backwards compatibility for his customers. ]
Section: 24.3.4 [iterator.concepts] Status: TC1 Submitter: Stephen Cleary Opened: 2000-02-02 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
In 24.1 paragraph 5, it is stated ". . . Dereferenceable and past-the-end values are always non-singular."
This places an unnecessary restriction on past-the-end iterators for containers with forward iterators (for example, a singly-linked list). If the past-the-end value on such a container was a well-known singular value, it would still satisfy all forward iterator requirements.
Removing this restriction would allow, for example, a singly-linked list without a "footer" node.
This would have an impact on existing code that expects past-the-end iterators obtained from different (generic) containers being not equal.
Proposed resolution:
Change 24.3.4 [iterator.concepts] paragraph 5, the last sentence, from:
Dereferenceable and past-the-end values are always non-singular.
to:
Dereferenceable values are always non-singular.
Rationale:
For some kinds of containers, including singly linked lists and zero-length vectors, null pointers are perfectly reasonable past-the-end iterators. Null pointers are singular.
Section: 27.4.3 [basic.string] Status: TC1 Submitter: Igor Stauder Opened: 2000-02-11 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
In Section 27.4.3 [basic.string] the basic_string member function declarations use a consistent style except for the following functions:
void push_back(const charT); basic_string& assign(const basic_string&); void swap(basic_string<charT,traits,Allocator>&);
- push_back, assign, swap: missing argument name
- push_back: use of const with charT (i.e. POD type passed by value
not by reference - should be charT or const charT& )
- swap: redundant use of template parameters in argument
basic_string<charT,traits,Allocator>&
Proposed resolution:
In Section 27.4.3 [basic.string] change the basic_string member function declarations push_back, assign, and swap to:
void push_back(charT c); basic_string& assign(const basic_string& str); void swap(basic_string& str);
Rationale:
Although the standard is in general not consistent in declaration style, the basic_string declarations are consistent other than the above. The LWG felt that this was sufficient reason to merit the change.
Section: 26 [algorithms] Status: TC1 Submitter: Lisa Lippincott Opened: 2000-02-15 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
In paragraph 9 of section 26 [algorithms], it is written:
In the description of the algorithms operators + and - are used for some of the iterator categories for which they do not have to be defined. In these cases the semantics of [...] a-b is the same as of
return distance(a, b);
Proposed resolution:
On the last line of paragraph 9 of section 26 [algorithms] change
"a-b"
to "b-a".
Rationale:
There are two ways to fix the defect; change the description to b-a or change the return to distance(b,a). The LWG preferred the former for consistency.
Section: 27.4.4.4 [string.io] Status: TC1 Submitter: Scott Snyder Opened: 2000-02-04 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
The description of the stream extraction operator for std::string (section 21.3.7.9 [lib.string.io]) does not contain a requirement that failbit be set in the case that the operator fails to extract any characters from the input stream.
This implies that the typical construction
std::istream is; std::string str; ... while (is >> str) ... ;
(which tests failbit) is not required to terminate at EOF.
Furthermore, this is inconsistent with other extraction operators, which do include this requirement. (See sections 31.7.5.3 [istream.formatted] and 31.7.5.4 [istream.unformatted]), where this requirement is present, either explicitly or implicitly, for the extraction operators. It is also present explicitly in the description of getline (istream&, string&, charT) in section 27.4.4.4 [string.io] paragraph 8.)
Proposed resolution:
Insert new paragraph after paragraph 2 in section 27.4.4.4 [string.io]:
If the function extracts no characters, it calls is.setstate(ios::failbit) which may throw ios_base::failure (27.4.4.3).
Section: 26.8.9 [alg.min.max] Status: TC1 Submitter: Nico Josuttis Opened: 2000-02-26 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
The standard doesn't specify what min_element() and max_element() shall return if the range is empty (first equals last). The usual implementations return last. This problem seems also apply to partition(), stable_partition(), next_permutation(), and prev_permutation().
Proposed resolution:
In 26.8.9 [alg.min.max] - Minimum and maximum, paragraphs 7 and 9, append: Returns last if first==last.
Rationale:
The LWG looked in some detail at all of the above mentioned algorithms, but believes that except for min_element() and max_element() it is already clear that last is returned if first == last.
Section: 23.4.6 [set], 23.4.7 [multiset] Status: CD1 Submitter: Judy Ward Opened: 2000-02-28 Last modified: 2016-01-28
Priority: Not Prioritized
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Duplicate of: 450
Discussion:
The specification for the associative container requirements in Table 69 state that the find member function should "return iterator; const_iterator for constant a". The map and multimap container descriptions have two overloaded versions of find, but set and multiset do not, all they have is:
iterator find(const key_type & x) const;
Proposed resolution:
Change the prototypes for find(), lower_bound(), upper_bound(), and equal_range() in section 23.4.6 [set] and section 23.4.7 [multiset] to each have two overloads:
iterator find(const key_type & x); const_iterator find(const key_type & x) const;iterator lower_bound(const key_type & x); const_iterator lower_bound(const key_type & x) const;iterator upper_bound(const key_type & x); const_iterator upper_bound(const key_type & x) const;pair<iterator, iterator> equal_range(const key_type & x); pair<const_iterator, const_iterator> equal_range(const key_type & x) const;
[Tokyo: At the request of the LWG, Judy Ward provided wording extending the proposed resolution to lower_bound, upper_bound, and equal_range.]
Section: 99 [facets.examples] Status: TC1 Submitter: Martin Sebor Opened: 2000-02-29 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
The example in 22.2.8, paragraph 11 contains the following errors:
1) The member function `My::JCtype::is_kanji()' is non-const; the function must be const in order for it to be callable on a const object (a reference to which which is what std::use_facet<>() returns).
2) In file filt.C, the definition of `JCtype::id' must be qualified with the name of the namespace `My'.
3) In the definition of `loc' and subsequently in the call to use_facet<>() in main(), the name of the facet is misspelled: it should read `My::JCtype' rather than `My::JCType'.
Proposed resolution:
Replace the "Classifying Japanese characters" example in 22.2.8, paragraph 11 with the following:
#include <locale>
namespace My { using namespace std; class JCtype : public locale::facet { public: static locale::id id; // required for use as a new locale facet bool is_kanji (wchar_t c) const; JCtype() {} protected: ~JCtype() {} }; }
// file: filt.C #include <iostream> #include <locale> #include "jctype" // above std::locale::id My::JCtype::id; // the static JCtype member declared above.
int main() { using namespace std; typedef ctype<wchar_t> wctype; locale loc(locale(""), // the user's preferred locale... new My::JCtype); // and a new feature ... wchar_t c = use_facet<wctype>(loc).widen('!'); if (!use_facet<My::JCtype>(loc).is_kanji(c)) cout << "no it isn't!" << endl; return 0; }
Section: 31.5.2.8 [ios.base.cons] Status: TC1 Submitter: Jonathan Schilling, Howard Hinnant Opened: 2000-03-13 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
The pre-conditions for the ios_base destructor are described in 27.4.2.7 paragraph 2:
Effects: Destroys an object of class ios_base. Calls each registered callback pair (fn,index) (27.4.2.6) as (*fn)(erase_event,*this,index) at such time that any ios_base member function called from within fn has well defined results.
But what is not clear is: If no callback functions were ever registered, does it matter whether the ios_base members were ever initialized?
For instance, does this program have defined behavior:
#include <ios>class D : public std::ios_base { };int main() { D d; }
It seems that registration of a callback function would surely affect the state of an ios_base. That is, when you register a callback function with an ios_base, the ios_base must record that fact somehow.
But if after construction the ios_base is in an indeterminate state, and that state is not made determinate before the destructor is called, then how would the destructor know if any callbacks had indeed been registered? And if the number of callbacks that had been registered is indeterminate, then is not the behavior of the destructor undefined?
By comparison, the basic_ios class description in 27.4.4.1 paragraph 2 makes it explicit that destruction before initialization results in undefined behavior.
Proposed resolution:
Modify 27.4.2.7 paragraph 1 from
Effects: Each ios_base member has an indeterminate value after construction.
to
Effects: Each ios_base member has an indeterminate value after construction. These members must be initialized by calling basic_ios::init. If an ios_base object is destroyed before these initializations have taken place, the behavior is undefined.
Section: 28.3.4.3.2.3 [facet.num.get.virtuals] Status: CD1 Submitter: Matt Austern Opened: 2000-03-14 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
Stage 2 processing of numeric conversion is broken.
Table 55 in 22.2.2.1.2 says that when basefield is 0 the integral conversion specifier is %i. A %i specifier determines a number's base by its prefix (0 for octal, 0x for hex), so the intention is clearly that a 0x prefix is allowed. Paragraph 8 in the same section, however, describes very precisely how characters are processed. (It must be done "as if" by a specified code fragment.) That description does not allow a 0x prefix to be recognized.
Very roughly, stage 2 processing reads a char_type ct. It converts ct to a char, not by using narrow but by looking it up in a translation table that was created by widening the string literal "0123456789abcdefABCDEF+-". The character "x" is not found in that table, so it can't be recognized by stage 2 processing.
Proposed resolution:
In 22.2.2.1.2 paragraph 8, replace the line:
static const char src[] = "0123456789abcdefABCDEF+-";
with the line:
static const char src[] = "0123456789abcdefxABCDEFX+-";
Rationale:
If we're using the technique of widening a string literal, the string literal must contain every character we wish to recognize. This technique has the consequence that alternate representations of digits will not be recognized. This design decision was made deliberately, with full knowledge of that limitation.
Section: 16.3.2.4 [structure.specifications] Status: TC1 Submitter: Judy Ward Opened: 2000-03-17 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
Section 21.3.6.8 describes the basic_string::compare function this way:
21.3.6.8 - basic_string::compare [lib.string::compare] int compare(size_type pos1, size_type n1, const basic_string<charT,traits,Allocator>& str , size_type pos2 , size_type n2 ) const; -4- Returns: basic_string<charT,traits,Allocator>(*this,pos1,n1).compare( basic_string<charT,traits,Allocator>(str,pos2,n2)) .
and the constructor that's implicitly called by the above is defined to throw an out-of-range exception if pos > str.size(). See section 27.4.3.2 [string.require] paragraph 4.
On the other hand, the compare function descriptions themselves don't have "Throws: " clauses and according to 17.3.1.3, paragraph 3, elements that do not apply to a function are omitted.
So it seems there is an inconsistency in the standard -- are the "Effects" clauses correct, or are the "Throws" clauses missing?
Proposed resolution:
In 16.3.2.4 [structure.specifications] paragraph 3, the footnote 148 attached to the sentence "Descriptions of function semantics contain the following elements (as appropriate):", insert the word "further" so that the foot note reads:
To save space, items that do not apply to a function are omitted. For example, if a function does not specify any further preconditions, there will be no "Requires" paragraph.
Rationale:
The standard is somewhat inconsistent, but a failure to note a throw condition in a throws clause does not grant permission not to throw. The inconsistent wording is in a footnote, and thus non-normative. The proposed resolution from the LWG clarifies the footnote.
Section: 26.7.10 [alg.reverse] Status: TC1 Submitter: Dave Abrahams Opened: 2000-03-21 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
Shouldn't the effects say "applies iter_swap to all pairs..."?
Proposed resolution:
In 26.7.10 [alg.reverse], replace:
Effects: For each non-negative integer i <= (last - first)/2, applies swap to all pairs of iterators first + i, (last - i) - 1.
with:
Effects: For each non-negative integer i <= (last - first)/2, applies iter_swap to all pairs of iterators first + i, (last - i) - 1.
Section: 23.2.7 [associative.reqmts] Status: TC1 Submitter: Ed Brey Opened: 2000-03-23 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
In the associative container requirements table in 23.1.2 paragraph 7, a.clear() has complexity "log(size()) + N". However, the meaning of N is not defined.
Proposed resolution:
In the associative container requirements table in 23.1.2 paragraph
7, the complexity of a.clear(), change "log(size()) + N" to
"linear in size()
".
Rationale:
It's the "log(size())", not the "N", that is in
error: there's no difference between O(N) and O(N +
log(N)). The text in the standard is probably an incorrect
cut-and-paste from the range version of erase
.
Section: 16.4.6.4 [global.functions] Status: CD1 Submitter: Dave Abrahams Opened: 2000-04-01 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
Are algorithms in std:: allowed to use other algorithms without qualification, so functions in user namespaces might be found through Koenig lookup?
For example, a popular standard library implementation includes this implementation of std::unique:
namespace std { template <class _ForwardIter> _ForwardIter unique(_ForwardIter __first, _ForwardIter __last) { __first = adjacent_find(__first, __last); return unique_copy(__first, __last, __first); } }
Imagine two users on opposite sides of town, each using unique on his own sequences bounded by my_iterators . User1 looks at his standard library implementation and says, "I know how to implement a more efficient unique_copy for my_iterators", and writes:
namespace user1 { class my_iterator; // faster version for my_iterator my_iterator unique_copy(my_iterator, my_iterator, my_iterator); }
user1::unique_copy() is selected by Koenig lookup, as he intended.
User2 has other needs, and writes:
namespace user2 { class my_iterator; // Returns true iff *c is a unique copy of *a and *b. bool unique_copy(my_iterator a, my_iterator b, my_iterator c); }
User2 is shocked to find later that his fully-qualified use of std::unique(user2::my_iterator, user2::my_iterator, user2::my_iterator) fails to compile (if he's lucky). Looking in the standard, he sees the following Effects clause for unique():
Effects: Eliminates all but the first element from every consecutive group of equal elements referred to by the iterator i in the range [first, last) for which the following corresponding conditions hold: *i == *(i - 1) or pred(*i, *(i - 1)) != false
The standard gives user2 absolutely no reason to think he can interfere with std::unique by defining names in namespace user2. His standard library has been built with the template export feature, so he is unable to inspect the implementation. User1 eventually compiles his code with another compiler, and his version of unique_copy silently stops being called. Eventually, he realizes that he was depending on an implementation detail of his library and had no right to expect his unique_copy() to be called portably.
On the face of it, and given above scenario, it may seem obvious that the implementation of unique() shown is non-conforming because it uses unique_copy() rather than ::std::unique_copy(). Most standard library implementations, however, seem to disagree with this notion.
[Tokyo: Steve Adamczyk from the core working group indicates that "std::" is sufficient; leading "::" qualification is not required because any namespace qualification is sufficient to suppress Koenig lookup.]
Proposed resolution:
Add a paragraph and a note at the end of 16.4.6.4 [global.functions]:
Unless otherwise specified, no global or non-member function in the standard library shall use a function from another namespace which is found through argument-dependent name lookup (6.5.4 [basic.lookup.argdep]).
[Note: the phrase "unless otherwise specified" is intended to allow Koenig lookup in cases like that of ostream_iterators:
Effects:*out_stream << value;
if(delim != 0) *out_stream << delim;
return (*this);--end note]
[Tokyo: The LWG agrees that this is a defect in the standard, but is as yet unsure if the proposed resolution is the best solution. Furthermore, the LWG believes that the same problem of unqualified library names applies to wording in the standard itself, and has opened issue 229(i) accordingly. Any resolution of issue 225(i) should be coordinated with the resolution of issue 229(i).]
[Toronto: The LWG is not sure if this is a defect in the
standard. Most LWG members believe that an implementation of
std::unique
like the one quoted in this issue is already
illegal, since, under certain circumstances, its semantics are not
those specified in the standard. The standard's description of
unique
does not say that overloading adjacent_find
should have any effect.]
[Curaçao: An LWG-subgroup spent an afternoon working on issues 225, 226, and 229. Their conclusion was that the issues should be separated into an LWG portion (Howard's paper, N1387=02-0045), and a EWG portion (Dave will write a proposal). The LWG and EWG had (separate) discussions of this plan the next day. The proposed resolution for this issue is in accordance with Howard's paper.]
Rationale:
It could be argued that this proposed isn't strictly necessary, that the Standard doesn't grant implementors license to write a standard function that behaves differently than specified in the Standard just because of an unrelated user-defined name in some other namespace. However, this is at worst a clarification. It is surely right that algorithsm shouldn't pick up random names, that user-defined names should have no effect unless otherwise specified. Issue 226(i) deals with the question of when it is appropriate for the standard to explicitly specify otherwise.
Section: 16.4.5.3 [reserved.names] Status: CD1 Submitter: Dave Abrahams Opened: 2000-04-01 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
The issues are:
1. How can a 3rd party library implementor (lib1) write a version of a standard algorithm which is specialized to work with his own class template?
2. How can another library implementor (lib2) write a generic algorithm which will take advantage of the specialized algorithm in lib1?
This appears to be the only viable answer under current language rules:
namespace lib1 { // arbitrary-precision numbers using T as a basic unit template <class T> class big_num { //... };// defining this in namespace std is illegal (it would be an // overload), so we hope users will rely on Koenig lookup template <class T> void swap(big_int<T>&, big_int<T>&); }#include <algorithm> namespace lib2 { template <class T> void generic_sort(T* start, T* end) { ... // using-declaration required so we can work on built-in types using std::swap; // use Koenig lookup to find specialized algorithm if available swap(*x, *y); } }
This answer has some drawbacks. First of all, it makes writing lib2 difficult and somewhat slippery. The implementor needs to remember to write the using-declaration, or generic_sort will fail to compile when T is a built-in type. The second drawback is that the use of this style in lib2 effectively "reserves" names in any namespace which defines types which may eventually be used with lib2. This may seem innocuous at first when applied to names like swap, but consider more ambiguous names like unique_copy() instead. It is easy to imagine the user wanting to define these names differently in his own namespace. A definition with semantics incompatible with the standard library could cause serious problems (see issue 225(i)).
Why, you may ask, can't we just partially specialize std::swap()? It's because the language doesn't allow for partial specialization of function templates. If you write:
namespace std { template <class T> void swap(lib1::big_int<T>&, lib1::big_int<T>&); }
You have just overloaded std::swap, which is illegal under the current language rules. On the other hand, the following full specialization is legal:
namespace std { template <> void swap(lib1::other_type&, lib1::other_type&); }
This issue reflects concerns raised by the "Namespace issue with specialized swap" thread on comp.lang.c++.moderated. A similar set of concerns was earlier raised on the boost.org mailing list and the ACCU-general mailing list. Also see library reflector message c++std-lib-7354.
J. C. van Winkel points out (in c++std-lib-9565) another unexpected fact: it's impossible to output a container of std::pair's using copy and an ostream_iterator, as long as both pair-members are built-in or std:: types. That's because a user-defined operator<< for (for example) std::pair<const std::string, int> will not be found: lookup for operator<< will be performed only in namespace std. Opinions differed on whether or not this was a defect, and, if so, whether the defect is that something is wrong with user-defined functionality and std, or whether it's that the standard library does not provide an operator<< for std::pair<>.
Proposed resolution:
Adopt the wording proposed in Howard Hinnant's paper N1523=03-0106, "Proposed Resolution To LWG issues 225, 226, 229".
[Tokyo: Summary, "There is no conforming way to extend std::swap for user defined templates." The LWG agrees that there is a problem. Would like more information before proceeding. This may be a core issue. Core issue 229 has been opened to discuss the core aspects of this problem. It was also noted that submissions regarding this issue have been received from several sources, but too late to be integrated into the issues list. ]
[Post-Tokyo: A paper with several proposed resolutions, J16/00-0029==WG21/N1252, "Shades of namespace std functions " by Alan Griffiths, is in the Post-Tokyo mailing. It should be considered a part of this issue.]
[Toronto: Dave Abrahams and Peter Dimov have proposed a
resolution that involves core changes: it would add partial
specialization of function template. The Core Working Group is
reluctant to add partial specialization of function templates. It is
viewed as a large change, CWG believes that proposal presented leaves
some syntactic issues unanswered; if the CWG does add partial
specialization of function templates, it wishes to develop its own
proposal. The LWG continues to believe that there is a serious
problem: there is no good way for users to force the library to use
user specializations of generic standard library functions, and in
certain cases (e.g. transcendental functions called by
valarray
and complex
) this is important. Koenig
lookup isn't adequate, since names within the library must be
qualified with std
(see issue 225), specialization doesn't
work (we don't have partial specialization of function templates), and
users aren't permitted to add overloads within namespace std.
]
[Copenhagen: Discussed at length, with no consensus. Relevant
papers in the pre-Copenhagen mailing: N1289, N1295, N1296. Discussion
focused on four options. (1) Relax restrictions on overloads within
namespace std. (2) Mandate that the standard library use unqualified
calls for swap
and possibly other functions. (3) Introduce
helper class templates for swap
and possibly other functions.
(4) Introduce partial specialization of function templates. Every
option had both support and opposition. Straw poll (first number is
support, second is strongly opposed): (1) 6, 4; (2) 6, 7; (3) 3, 8;
(4) 4, 4.]
[Redmond: Discussed, again no consensus. Herb presented an
argument that a user who is defining a type T
with an
associated swap
should not be expected to put that
swap
in namespace std, either by overloading or by partial
specialization. The argument is that swap
is part of
T
's interface, and thus should to in the same namespace as
T
and only in that namespace. If we accept this argument,
the consequence is that standard library functions should use
unqualified call of swap
. (And which other functions? Any?)
A small group (Nathan, Howard, Jeremy, Dave, Matt, Walter, Marc) will
try to put together a proposal before the next meeting.]
[Curaçao: An LWG-subgroup spent an afternoon working on issues 225, 226, and 229. Their conclusion was that the issues should be separated into an LWG portion (Howard's paper, N1387=02-0045), and a EWG portion (Dave will write a proposal). The LWG and EWG had (separate) discussions of this plan the next day. The proposed resolution is the one proposed by Howard.]
[Santa Cruz: the LWG agreed with the general direction of Howard's paper, N1387. (Roughly: Koenig lookup is disabled unless we say otherwise; this issue is about when we do say otherwise.) However, there were concerns about wording. Howard will provide new wording. Bill and Jeremy will review it.]
[Kona: Howard proposed the new wording. The LWG accepted his proposed resolution.]
Rationale:
Informally: introduce a Swappable concept, and specify that the
value types of the iterators passed to certain standard algorithms
(such as iter_swap, swap_ranges, reverse, rotate, and sort) conform
to that concept. The Swappable concept will make it clear that
these algorithms use unqualified lookup for the calls
to swap
. Also, in 29.6.3.3 [valarray.transcend] paragraph 1,
state that the valarray transcendentals use unqualified lookup.
Section: 26.7.3 [alg.swap] Status: TC1 Submitter: Dave Abrahams Opened: 2000-04-09 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
25.2.2 reads:
template<class T> void swap(T& a, T& b);
Requires: Type T is Assignable (_lib.container.requirements_).
Effects: Exchanges values stored in two locations.
The only reasonable** generic implementation of swap requires construction of a new temporary copy of one of its arguments:
template<class T> void swap(T& a, T& b); { T tmp(a); a = b; b = tmp; }
But a type which is only Assignable cannot be swapped by this implementation.
**Yes, there's also an unreasonable implementation which would require T to be DefaultConstructible instead of CopyConstructible. I don't think this is worthy of consideration:
template<class T> void swap(T& a, T& b); { T tmp; tmp = a; a = b; b = tmp; }
Proposed resolution:
Change 25.2.2 paragraph 1 from:
Requires: Type T is Assignable (23.1).
to:
Requires: Type T is CopyConstructible (20.1.3) and Assignable (23.1)
Section: 28.3.4 [locale.categories] Status: CD1 Submitter: Dietmar Kühl Opened: 2000-04-20 Last modified: 2018-08-10
Priority: Not Prioritized
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Discussion:
The sections 28.3.4.2.3 [locale.ctype.byname], 28.3.4.2.6 [locale.codecvt.byname], 28.3.4.4.2 [locale.numpunct.byname], 28.3.4.5.2 [locale.collate.byname], 28.3.4.6.5 [locale.time.put.byname], 28.3.4.7.5 [locale.moneypunct.byname], and 28.3.4.8.3 [locale.messages.byname] overspecify the definitions of the "..._byname" classes by listing a bunch of virtual functions. At the same time, no semantics of these functions are defined. Real implementations do not define these functions because the functional part of the facets is actually implemented in the corresponding base classes and the constructor of the "..._byname" version just provides suitable date used by these implementations. For example, the 'numpunct' methods just return values from a struct. The base class uses a statically initialized struct while the derived version reads the contents of this struct from a table. However, no virtual function is defined in 'numpunct_byname'.
For most classes this does not impose a problem but specifically for 'ctype' it does: The specialization for 'ctype_byname<char>' is required because otherwise the semantics would change due to the virtual functions defined in the general version for 'ctype_byname': In 'ctype<char>' the method 'do_is()' is not virtual but it is made virtual in both 'ctype<cT>' and 'ctype_byname<cT>'. Thus, a class derived from 'ctype_byname<char>' can tell whether this class is specialized or not under the current specification: Without the specialization, 'do_is()' is virtual while with specialization it is not virtual.
Proposed resolution:
Change section 22.2.1.2 (lib.locale.ctype.byname) to become:
namespace std { template <class charT> class ctype_byname : public ctype<charT> { public: typedef ctype<charT>::mask mask; explicit ctype_byname(const char*, size_t refs = 0); protected: ~ctype_byname(); // virtual }; }
Change section 22.2.1.6 (lib.locale.codecvt.byname) to become:
namespace std { template <class internT, class externT, class stateT> class codecvt_byname : public codecvt<internT, externT, stateT> { public: explicit codecvt_byname(const char*, size_t refs = 0); protected: ~codecvt_byname(); // virtual }; }
Change section 22.2.3.2 (lib.locale.numpunct.byname) to become:
namespace std { template <class charT> class numpunct_byname : public numpunct<charT> { // this class is specialized for char and wchar_t. public: typedef charT char_type; typedef basic_string<charT> string_type; explicit numpunct_byname(const char*, size_t refs = 0); protected: ~numpunct_byname(); // virtual }; }
Change section 22.2.4.2 (lib.locale.collate.byname) to become:
namespace std { template <class charT> class collate_byname : public collate<charT> { public: typedef basic_string<charT> string_type; explicit collate_byname(const char*, size_t refs = 0); protected: ~collate_byname(); // virtual }; }
Change section 22.2.5.2 (lib.locale.time.get.byname) to become:
namespace std { template <class charT, class InputIterator = istreambuf_iterator<charT> > class time_get_byname : public time_get<charT, InputIterator> { public: typedef time_base::dateorder dateorder; typedef InputIterator iter_type
explicit time_get_byname(const char*, size_t refs = 0); protected: ~time_get_byname(); // virtual }; }
Change section 22.2.5.4 (lib.locale.time.put.byname) to become:
namespace std { template <class charT, class OutputIterator = ostreambuf_iterator<charT> > class time_put_byname : public time_put<charT, OutputIterator> { public: typedef charT char_type; typedef OutputIterator iter_type;
explicit time_put_byname(const char*, size_t refs = 0); protected: ~time_put_byname(); // virtual }; }"
Change section 22.2.6.4 (lib.locale.moneypunct.byname) to become:
namespace std { template <class charT, bool Intl = false> class moneypunct_byname : public moneypunct<charT, Intl> { public: typedef money_base::pattern pattern; typedef basic_string<charT> string_type;
explicit moneypunct_byname(const char*, size_t refs = 0); protected: ~moneypunct_byname(); // virtual }; }
Change section 22.2.7.2 (lib.locale.messages.byname) to become:
namespace std { template <class charT> class messages_byname : public messages<charT> { public: typedef messages_base::catalog catalog; typedef basic_string<charT> string_type;
explicit messages_byname(const char*, size_t refs = 0); protected: ~messages_byname(); // virtual }; }
Remove section 28.3.4.2.5 [locale.codecvt] completely (because in this case only those members are defined to be virtual which are defined to be virtual in 'ctype<cT>'.)
[Post-Tokyo: Dietmar Kühl submitted this issue at the request of the LWG to solve the underlying problems raised by issue 138(i).]
[Copenhagen: proposed resolution was revised slightly, to remove
three last virtual functions from messages_byname
.]
Section: 16.4.2.2 [contents] Status: CD1 Submitter: Steve Clamage Opened: 2000-04-19 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
Throughout the library chapters, the descriptions of library entities refer to other library entities without necessarily qualifying the names.
For example, section 25.2.2 "Swap" describes the effect of swap_ranges in terms of the unqualified name "swap". This section could reasonably be interpreted to mean that the library must be implemented so as to do a lookup of the unqualified name "swap", allowing users to override any ::std::swap function when Koenig lookup applies.
Although it would have been best to use explicit qualification with "::std::" throughout, too many lines in the standard would have to be adjusted to make that change in a Technical Corrigendum.
Issue 182(i), which addresses qualification of
size_t
, is a special case of this.
Proposed resolution:
To section 17.4.1.1 "Library contents" Add the following paragraph:
Whenever a name x defined in the standard library is mentioned, the name x is assumed to be fully qualified as ::std::x, unless explicitly described otherwise. For example, if the Effects section for library function F is described as calling library function G, the function ::std::G is meant.
[Post-Tokyo: Steve Clamage submitted this issue at the request of the LWG to solve a problem in the standard itself similar to the problem within implementations of library identified by issue 225(i). Any resolution of issue 225(i) should be coordinated with the resolution of this issue.]
[post-Toronto: Howard is undecided about whether it is
appropriate for all standard library function names referred to in
other standard library functions to be explicitly qualified by
std
: it is common advice that users should define global
functions that operate on their class in the same namespace as the
class, and this requires argument-dependent lookup if those functions
are intended to be called by library code. Several LWG members are
concerned that valarray appears to require argument-dependent lookup,
but that the wording may not be clear enough to fall under
"unless explicitly described otherwise".]
[Curaçao: An LWG-subgroup spent an afternoon working on issues 225, 226, and 229. Their conclusion was that the issues should be separated into an LWG portion (Howard's paper, N1387=02-0045), and a EWG portion (Dave will write a proposal). The LWG and EWG had (separate) discussions of this plan the next day. This paper resolves issues 225 and 226. In light of that resolution, the proposed resolution for the current issue makes sense.]
Section: 16 [library] Status: CD1 Submitter: Beman Dawes Opened: 2000-04-26 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
Issue 227(i) identified an instance (std::swap) where Assignable was specified without also specifying CopyConstructible. The LWG asked that the standard be searched to determine if the same defect existed elsewhere.
There are a number of places (see proposed resolution below) where Assignable is specified without also specifying CopyConstructible. There are also several cases where both are specified. For example, 29.5.3 [rand.req].
Proposed resolution:
In 23.2 [container.requirements] table 65 for value_type: change "T is Assignable" to "T is CopyConstructible and Assignable"
In 23.2.7 [associative.reqmts] table 69 X::key_type; change
"Key is Assignable" to "Key is
CopyConstructible and Assignable"
In 24.3.5.4 [output.iterators] paragraph 1, change:
A class or a built-in type X satisfies the requirements of an output iterator if X is an Assignable type (23.1) and also the following expressions are valid, as shown in Table 73:
to:
A class or a built-in type X satisfies the requirements of an output iterator if X is a CopyConstructible (20.1.3) and Assignable type (23.1) and also the following expressions are valid, as shown in Table 73:
[Post-Tokyo: Beman Dawes submitted this issue at the request of the LWG. He asks that the 26.7.5 [alg.replace] and 26.7.6 [alg.fill] changes be studied carefully, as it is not clear that CopyConstructible is really a requirement and may be overspecification.]
[Portions of the resolution for issue 230 have been superceded by the resolution of issue 276(i).]
Rationale:
The original proposed resolution also included changes to input iterator, fill, and replace. The LWG believes that those changes are not necessary. The LWG considered some blanket statement, where an Assignable type was also required to be Copy Constructible, but decided against this because fill and replace really don't require the Copy Constructible property.
Section: 28.3.4.3.3.3 [facet.num.put.virtuals] Status: CD1 Submitter: James Kanze, Stephen Clamage Opened: 2000-04-25 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
What is the following program supposed to output?
#include <iostream> int main() { std::cout.setf( std::ios::scientific , std::ios::floatfield ) ; std::cout.precision( 0 ) ; std::cout << 1.00 << '\n' ; return 0 ; }
From my C experience, I would expect "1e+00"; this is what
printf("%.0e" , 1.00 );
does. G++ outputs
"1.000000e+00".
The only indication I can find in the standard is 22.2.2.2.2/11, where it says "For conversion from a floating-point type, if (flags & fixed) != 0 or if str.precision() > 0, then str.precision() is specified in the conversion specification." This is an obvious error, however, fixed is not a mask for a field, but a value that a multi-bit field may take -- the results of and'ing fmtflags with ios::fixed are not defined, at least not if ios::scientific has been set. G++'s behavior corresponds to what might happen if you do use (flags & fixed) != 0 with a typical implementation (floatfield == 3 << something, fixed == 1 << something, and scientific == 2 << something).
Presumably, the intent is either (flags & floatfield) != 0, or (flags & floatfield) == fixed; the first gives something more or less like the effect of precision in a printf floating point conversion. Only more or less, of course. In order to implement printf formatting correctly, you must know whether the precision was explicitly set or not. Say by initializing it to -1, instead of 6, and stating that for floating point conversions, if precision < -1, 6 will be used, for fixed point, if precision < -1, 1 will be used, etc. Plus, of course, if precision == 0 and flags & floatfield == 0, 1 should be = used. But it probably isn't necessary to emulate all of the anomalies of printf:-).
Proposed resolution:
Replace 28.3.4.3.3.3 [facet.num.put.virtuals], paragraph 11, with the following sentence:
For conversion from a floating-point type,
str.precision()
is specified in the conversion specification.
Rationale:
The floatfield determines whether numbers are formatted as if
with %f, %e, or %g. If the fixed
bit is set, it's %f,
if scientific
it's %e, and if both bits are set, or
neither, it's %g.
Turning to the C standard, a precision of 0 is meaningful for %f and %e. For %g, precision 0 is taken to be the same as precision 1.
The proposed resolution has the effect that if neither
fixed
nor scientific
is set we'll be
specifying a precision of 0, which will be internally
turned into 1. There's no need to call it out as a special
case.
The output of the above program will be "1e+00".
[Post-Curaçao: Howard provided improved wording covering the case where precision is 0 and mode is %g.]
Section: 16.4.5.3 [reserved.names] Status: CD1 Submitter: Peter Dimov Opened: 2000-04-18 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
17.4.3.1/1 uses the term "depends" to limit the set of allowed specializations of standard templates to those that "depend on a user-defined name of external linkage."
This term, however, is not adequately defined, making it possible to construct a specialization that is, I believe, technically legal according to 17.4.3.1/1, but that specializes a standard template for a built-in type such as 'int'.
The following code demonstrates the problem:
#include <algorithm>template<class T> struct X { typedef T type; };namespace std { template<> void swap(::X<int>::type& i, ::X<int>::type& j); }
Proposed resolution:
Change "user-defined name" to "user-defined type".
Rationale:
This terminology is used in section 2.5.2 and 4.1.1 of The C++ Programming Language. It disallows the example in the issue, since the underlying type itself is not user-defined. The only possible problem I can see is for non-type templates, but there's no possible way for a user to come up with a specialization for bitset, for example, that might not have already been specialized by the implementor?
[Toronto: this may be related to issue 120(i).]
[post-Toronto: Judy provided the above proposed resolution and rationale.]
Section: 23.2.7 [associative.reqmts] Status: CD1 Submitter: Andrew Koenig Opened: 2000-04-30 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
If mm
is a multimap and p
is an iterator
into the multimap, then mm.insert(p, x)
inserts
x
into mm
with p
as a hint as
to where it should go. Table 69 claims that the execution time is
amortized constant if the insert winds up taking place adjacent to
p
, but does not say when, if ever, this is guaranteed to
happen. All it says it that p
is a hint as to where to
insert.
The question is whether there is any guarantee about the relationship
between p
and the insertion point, and, if so, what it
is.
I believe the present state is that there is no guarantee: The user
can supply p
, and the implementation is allowed to
disregard it entirely.
Additional comments from Nathan:
The vote [in Redmond] was on whether to elaborately specify the use of
the hint, or to require behavior only if the value could be inserted
adjacent to the hint. I would like to ensure that we have a chance to
vote for a deterministic treatment: "before, if possible, otherwise
after, otherwise anywhere appropriate", as an alternative to the
proposed "before or after, if possible, otherwise [...]".
[Toronto: there was general agreement that this is a real defect: when inserting an element x into a multiset that already contains several copies of x, there is no way to know whether the hint will be used. The proposed resolution was that the new element should always be inserted as close to the hint as possible. So, for example, if there is a subsequence of equivalent values, then providing a.begin() as the hint means that the new element should be inserted before the subsequence even if a.begin() is far away. JC van Winkel supplied precise wording for this proposed resolution, and also for an alternative resolution in which hints are only used when they are adjacent to the insertion point.]
[Copenhagen: the LWG agreed to the original proposed resolution, in which an insertion hint would be used even when it is far from the insertion point. This was contingent on seeing a example implementation showing that it is possible to implement this requirement without loss of efficiency. John Potter provided such a example implementation.]
[Redmond: The LWG was reluctant to adopt the proposal that emerged from Copenhagen: it seemed excessively complicated, and went beyond fixing the defect that we identified in Toronto. PJP provided the new wording described in this issue. Nathan agrees that we shouldn't adopt the more detailed semantics, and notes: "we know that you can do it efficiently enough with a red-black tree, but there are other (perhaps better) balanced tree techniques that might differ enough to make the detailed semantics hard to satisfy."]
[Curaçao: Nathan should give us the alternative wording he suggests so the LWG can decide between the two options.]
[Lillehammer: The LWG previously rejected the more detailed semantics, because it seemed more loike a new feature than like defect fixing. We're now more sympathetic to it, but we (especially Bill) are still worried about performance. N1780 describes a naive algorithm, but it's not clear whether there is a non-naive implementation. Is it possible to implement this as efficently as the current version of insert?]
[Post Lillehammer: N1780 updated in post meeting mailing with feedback from Lillehammer with more information regarding performance. ]
[ Batavia: 1780 accepted with minor wording changes in the proposed wording (reflected in the proposed resolution below). Concerns about the performance of the algorithm were satisfactorily met by 1780. 371(i) already handles the stability of equal ranges and so that part of the resolution from 1780 is no longer needed (or reflected in the proposed wording below). ]
Proposed resolution:
Change the indicated rows of the "Associative container requirements" Table in 23.2.7 [associative.reqmts] to:
expression | return type | assertion/note pre/post-condition |
complexity |
---|---|---|---|
a_eq.insert(t) |
iterator |
inserts t and returns the iterator pointing to the newly inserted
element. If a range containing elements equivalent to t exists in
a_eq , t is inserted at the end of that range.
|
logarithmic |
a.insert(p,t) |
iterator |
inserts t if and only if there is no element with key equivalent to the
key of t in containers with unique keys; always inserts t in containers
with equivalent keys. always returns the iterator pointing to the element with key
equivalent to the key of t . p is a hint pointing to where
the insert should start to search.t is inserted as close as possible
to the position just prior to p .
|
logarithmic in general, but amortized constant if t is inserted right p .
|
Section: 20.2.10.2 [allocator.members] Status: CD1 Submitter: Dietmar Kühl Opened: 2000-04-24 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
In paragraphs 12 and 13 the effects of construct()
and
destruct()
are described as returns but the functions actually
return void
.
Proposed resolution:
Substitute "Returns" by "Effect".
Section: 24.5.1.2 [reverse.iterator] Status: CD1 Submitter: Dietmar Kühl Opened: 2000-04-24 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
The declaration of reverse_iterator
lists a default
constructor. However, no specification is given what this constructor
should do.
Proposed resolution:
In section 24.5.1.4 [reverse.iter.cons] add the following paragraph:
reverse_iterator()
Default initializes
current
. Iterator operations applied to the resulting iterator have defined behavior if and only if the corresponding operations are defined on a default constructed iterator of typeIterator
.
[pre-Copenhagen: Dietmar provide wording for proposed resolution.]
Section: 23.3.5.2 [deque.cons] Status: CD1 Submitter: Dietmar Kühl Opened: 2000-04-24 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
The complexity specification in paragraph 6 says that the complexity
is linear in first - last
. Even if operator-()
is
defined on iterators this term is in general undefined because it
would have to be last - first
.
Proposed resolution:
Change paragraph 6 from
Linear in first - last.
to become
Linear in distance(first, last).
Section: 31.8.2.2 [stringbuf.cons] Status: CD1 Submitter: Dietmar Kühl Opened: 2000-05-11 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
In 27.7.1.1 paragraph 4 the results of calling the constructor of
'basic_stringbuf' are said to be str() == str
. This is fine
that far but consider this code:
std::basic_stringbuf<char> sbuf("hello, world", std::ios_base::openmode(0)); std::cout << "'" << sbuf.str() << "'\n";
Paragraph 3 of 27.7.1.1 basically says that in this case neither
the output sequence nor the input sequence is initialized and
paragraph 2 of 27.7.1.2 basically says that str()
either
returns the input or the output sequence. None of them is initialized,
ie. both are empty, in which case the return from str()
is
defined to be basic_string<cT>()
.
However, probably only test cases in some testsuites will detect this "problem"...
Proposed resolution:
Remove 27.7.1.1 paragraph 4.
Rationale:
We could fix 27.7.1.1 paragraph 4, but there would be no point. If we fixed it, it would say just the same thing as text that's already in the standard.
Section: 26.7.9 [alg.unique] Status: CD1 Submitter: Angelika Langer Opened: 2000-05-15 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
The complexity of unique and unique_copy are inconsistent with each other and inconsistent with the implementations. The standard specifies:
for unique():
-3- Complexity: If the range (last - first) is not empty, exactly (last - first) - 1 applications of the corresponding predicate, otherwise no applications of the predicate.
for unique_copy():
-7- Complexity: Exactly last - first applications of the corresponding predicate.
The implementations do it the other way round: unique() applies the predicate last-first times and unique_copy() applies it last-first-1 times.
As both algorithms use the predicate for pair-wise comparison of sequence elements I don't see a justification for unique_copy() applying the predicate last-first times, especially since it is not specified to which pair in the sequence the predicate is applied twice.
Proposed resolution:
Change both complexity sections in 26.7.9 [alg.unique] to:
Complexity: For nonempty ranges, exactly last - first - 1 applications of the corresponding predicate.
Section: 26.6.10 [alg.adjacent.find] Status: CD1 Submitter: Angelika Langer Opened: 2000-05-15 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
The complexity section of adjacent_find is defective:
template <class ForwardIterator> ForwardIterator adjacent_find(ForwardIterator first, ForwardIterator last BinaryPredicate pred);-1- Returns: The first iterator i such that both i and i + 1 are in the range [first, last) for which the following corresponding conditions hold: *i == *(i + 1), pred(*i, *(i + 1)) != false. Returns last if no such iterator is found.
-2- Complexity: Exactly find(first, last, value) - first applications of the corresponding predicate.
In the Complexity section, it is not defined what "value" is supposed to mean. My best guess is that "value" means an object for which one of the conditions pred(*i,value) or pred(value,*i) is true, where i is the iterator defined in the Returns section. However, the value type of the input sequence need not be equality-comparable and for this reason the term find(first, last, value) - first is meaningless.
A term such as find_if(first, last, bind2nd(pred,*i)) - first or find_if(first, last, bind1st(pred,*i)) - first might come closer to the intended specification. Binders can only be applied to function objects that have the function call operator declared const, which is not required of predicates because they can have non-const data members. For this reason, a specification using a binder could only be an "as-if" specification.
Proposed resolution:
Change the complexity section in 26.6.10 [alg.adjacent.find] to:
For a nonempty range, exactly
min((i - first) + 1, (last - first) - 1)
applications of the corresponding predicate, where i isadjacent_find
's return value.
[Copenhagen: the original resolution specified an upper bound. The LWG preferred an exact count.]
Section: 26.7.9 [alg.unique] Status: CD1 Submitter: Angelika Langer Opened: 2000-05-15 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
Some popular implementations of unique_copy() create temporary copies of values in the input sequence, at least if the input iterator is a pointer. Such an implementation is built on the assumption that the value type is CopyConstructible and Assignable.
It is common practice in the standard that algorithms explicitly specify any additional requirements that they impose on any of the types used by the algorithm. An example of an algorithm that creates temporary copies and correctly specifies the additional requirements is accumulate(), 29.5.3 [rand.req].
Since the specifications of unique() and unique_copy() do not require CopyConstructible and Assignable of the InputIterator's value type the above mentioned implementations are not standard-compliant. I cannot judge whether this is a defect in the standard or a defect in the implementations.
Proposed resolution:
In 25.2.8 change:
-4- Requires: The ranges [first, last) and [result, result+(last-first)) shall not overlap.
to:
-4- Requires: The ranges [first, last) and [result, result+(last-first)) shall not overlap. The expression *result = *first must be valid. If neither InputIterator nor OutputIterator meets the requirements of forward iterator then the value type of InputIterator must be copy constructible. Otherwise copy constructible is not required.
[Redmond: the original proposed resolution didn't impose an
explicit requirement that the iterator's value type must be copy
constructible, on the grounds that an input iterator's value type must
always be copy constructible. Not everyone in the LWG thought that
this requirement was clear from table 72. It has been suggested that
it might be possible to implement unique_copy
without
requiring assignability, although current implementations do impose
that requirement. Howard provided new wording.]
[ Curaçao: The LWG changed the PR editorially to specify "neither...nor...meet..." as clearer than "both...and...do not meet...". Change believed to be so minor as not to require re-review. ]
Section: 26.7.4 [alg.transform], 29.5 [rand] Status: CD1 Submitter: Angelika Langer Opened: 2000-05-15 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
The algorithms transform(), accumulate(), inner_product(), partial_sum(), and adjacent_difference() require that the function object supplied to them shall not have any side effects.
The standard defines a side effect in 6.9.1 [intro.execution] as:
-7- Accessing an object designated by a volatile lvalue (basic.lval), modifying an object, calling a library I/O function, or calling a function that does any of those operations are all side effects, which are changes in the state of the execution environment.
As a consequence, the function call operator of a function object supplied to any of the algorithms listed above cannot modify data members, cannot invoke any function that has a side effect, and cannot even create and modify temporary objects. It is difficult to imagine a function object that is still useful under these severe limitations. For instance, any non-trivial transformator supplied to transform() might involve creation and modification of temporaries, which is prohibited according to the current wording of the standard.
On the other hand, popular implementations of these algorithms exhibit uniform and predictable behavior when invoked with a side-effect-producing function objects. It looks like the strong requirement is not needed for efficient implementation of these algorithms.
The requirement of side-effect-free function objects could be replaced by a more relaxed basic requirement (which would hold for all function objects supplied to any algorithm in the standard library):
A function objects supplied to an algorithm shall not invalidate any iterator or sequence that is used by the algorithm. Invalidation of the sequence includes destruction of the sorting order if the algorithm relies on the sorting order (see section 25.3 - Sorting and related operations [lib.alg.sorting]).
I can't judge whether it is intended that the function objects supplied to transform(), accumulate(), inner_product(), partial_sum(), or adjacent_difference() shall not modify sequence elements through dereferenced iterators.
It is debatable whether this issue is a defect or a change request. Since the consequences for user-supplied function objects are drastic and limit the usefulness of the algorithms significantly I would consider it a defect.
Proposed resolution:
Things to notice about these changes:
Change 25.2.3/2 from:
-2- Requires: op and binary_op shall not have any side effects.
to:
-2- Requires: in the ranges [first1, last1], [first2, first2 + (last1 - first1)] and [result, result + (last1- first1)], op and binary_op shall neither modify elements nor invalidate iterators or subranges. [Footnote: The use of fully closed ranges is intentional --end footnote]
Change 25.2.3/2 from:
-2- Requires: op and binary_op shall not have any side effects.
to:
-2- Requires: op and binary_op shall not invalidate iterators or subranges, or modify elements in the ranges [first1, last1], [first2, first2 + (last1 - first1)], and [result, result + (last1 - first1)]. [Footnote: The use of fully closed ranges is intentional --end footnote]
Change 26.4.1/2 from:
-2- Requires: T must meet the requirements of CopyConstructible (lib.copyconstructible) and Assignable (lib.container.requirements) types. binary_op shall not cause side effects.
to:
-2- Requires: T must meet the requirements of CopyConstructible (lib.copyconstructible) and Assignable (lib.container.requirements) types. In the range [first, last], binary_op shall neither modify elements nor invalidate iterators or subranges. [Footnote: The use of a fully closed range is intentional --end footnote]
Change 26.4.2/2 from:
-2- Requires: T must meet the requirements of CopyConstructible (lib.copyconstructible) and Assignable (lib.container.requirements) types. binary_op1 and binary_op2 shall not cause side effects.
to:
-2- Requires: T must meet the requirements of CopyConstructible (lib.copyconstructible) and Assignable (lib.container.requirements) types. In the ranges [first, last] and [first2, first2 + (last - first)], binary_op1 and binary_op2 shall neither modify elements nor invalidate iterators or subranges. [Footnote: The use of fully closed ranges is intentional --end footnote]
Change 26.4.3/4 from:
-4- Requires: binary_op is expected not to have any side effects.
to:
-4- Requires: In the ranges [first, last] and [result, result + (last - first)], binary_op shall neither modify elements nor invalidate iterators or subranges. [Footnote: The use of fully closed ranges is intentional --end footnote]
Change 26.4.4/2 from:
-2- Requires: binary_op shall not have any side effects.
to:
-2- Requires: In the ranges [first, last] and [result, result + (last - first)], binary_op shall neither modify elements nor invalidate iterators or subranges. [Footnote: The use of fully closed ranges is intentional --end footnote]
[Toronto: Dave Abrahams supplied wording.]
[Copenhagen: Proposed resolution was modified slightly. Matt added footnotes pointing out that the use of closed ranges was intentional.]
get
and getline
when sentry reports failureSection: 31.7.5.4 [istream.unformatted] Status: CD1 Submitter: Martin Sebor Opened: 2000-05-15 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
basic_istream<>::get(), and basic_istream<>::getline(), are unclear with respect to the behavior and side-effects of the named functions in case of an error.
27.6.1.3, p1 states that "... If the sentry object returns true, when converted to a value of type bool, the function endeavors to obtain the requested input..." It is not clear from this (or the rest of the paragraph) what precisely the behavior should be when the sentry ctor exits by throwing an exception or when the sentry object returns false. In particular, what is the number of characters extracted that gcount() returns supposed to be?
27.6.1.3 p8 and p19 say about the effects of get() and getline(): "... In any case, it then stores a null character (using charT()) into the next successive location of the array." Is not clear whether this sentence applies if either of the conditions above holds (i.e., when sentry fails).
Proposed resolution:
Add to 27.6.1.3, p1 after the sentence
"... If the sentry object returns true, when converted to a value of type bool, the function endeavors to obtain the requested input."
the following
"Otherwise, if the sentry constructor exits by throwing an exception or if the sentry object returns false, when converted to a value of type bool, the function returns without attempting to obtain any input. In either case the number of extracted characters is set to 0; unformatted input functions taking a character array of non-zero size as an argument shall also store a null character (using charT()) in the first location of the array."
Rationale:
Although the general philosophy of the input functions is that the
argument should not be modified upon failure, getline
historically added a terminating null unconditionally. Most
implementations still do that. Earlier versions of the draft standard
had language that made this an unambiguous requirement; those words
were moved to a place where their context made them less clear. See
Jerry Schwarz's message c++std-lib-7618.
vector
, deque::insert
complexitySection: 23.3.13.5 [vector.modifiers] Status: CD1 Submitter: Lisa Lippincott Opened: 2000-06-06 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
Paragraph 2 of 23.3.13.5 [vector.modifiers] describes the complexity
of vector::insert
:
Complexity: If first and last are forward iterators, bidirectional iterators, or random access iterators, the complexity is linear in the number of elements in the range [first, last) plus the distance to the end of the vector. If they are input iterators, the complexity is proportional to the number of elements in the range [first, last) times the distance to the end of the vector.
First, this fails to address the non-iterator forms of
insert
.
Second, the complexity for input iterators misses an edge case --
it requires that an arbitrary number of elements can be added at
the end of a vector
in constant time.
I looked to see if deque
had a similar problem, and was
surprised to find that deque
places no requirement on the
complexity of inserting multiple elements (23.3.5.4 [deque.modifiers],
paragraph 3):
Complexity: In the worst case, inserting a single element into a deque takes time linear in the minimum of the distance from the insertion point to the beginning of the deque and the distance from the insertion point to the end of the deque. Inserting a single element either at the beginning or end of a deque always takes constant time and causes a single call to the copy constructor of T.
Proposed resolution:
Change Paragraph 2 of 23.3.13.5 [vector.modifiers] to
Complexity: The complexity is linear in the number of elements inserted plus the distance to the end of the vector.
[For input iterators, one may achieve this complexity by first
inserting at the end of the vector
, and then using
rotate
.]
Change 23.3.5.4 [deque.modifiers], paragraph 3, to:
Complexity: The complexity is linear in the number of elements inserted plus the shorter of the distances to the beginning and end of the deque. Inserting a single element at either the beginning or the end of a deque causes a single call to the copy constructor of T.
Rationale:
This is a real defect, and proposed resolution fixes it: some complexities aren't specified that should be. This proposed resolution does constrain deque implementations (it rules out the most naive possible implementations), but the LWG doesn't see a reason to permit that implementation.
Section: 28.3.4.6 [category.time] Status: CD1 Submitter: Martin Sebor Opened: 2000-06-22 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
There is no requirement that any of time_get member functions set ios::eofbit when they reach the end iterator while parsing their input. Since members of both the num_get and money_get facets are required to do so (22.2.2.1.2, and 22.2.6.1.2, respectively), time_get members should follow the same requirement for consistency.
Proposed resolution:
Add paragraph 2 to section 22.2.5.1 with the following text:
If the end iterator is reached during parsing by any of the get() member functions, the member sets ios_base::eofbit in err.
Rationale:
Two alternative resolutions were proposed. The LWG chose this one because it was more consistent with the way eof is described for other input facets.
Section: 23.3.11.5 [list.ops] Status: CD1 Submitter: Brian Parker Opened: 2000-07-14 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
Section 23.3.11.5 [list.ops] states that
void splice(iterator position, list<T, Allocator>& x);
invalidates all iterators and references to list x
.
This is unnecessary and defeats an important feature of splice. In
fact, the SGI STL guarantees that iterators to x
remain valid
after splice
.
Proposed resolution:
Add a footnote to 23.3.11.5 [list.ops], paragraph 1:
[Footnote: As specified in [default.con.req], paragraphs 4-5, the semantics described in this clause applies only to the case where allocators compare equal. --end footnote]
In 23.3.11.5 [list.ops], replace paragraph 4 with:
Effects: Inserts the contents of x before position and x becomes empty. Pointers and references to the moved elements of x now refer to those same elements but as members of *this. Iterators referring to the moved elements will continue to refer to their elements, but they now behave as iterators into *this, not into x.
In 23.3.11.5 [list.ops], replace paragraph 7 with:
Effects: Inserts an element pointed to by i from list x before position and removes the element from x. The result is unchanged if position == i or position == ++i. Pointers and references to *i continue to refer to this same element but as a member of *this. Iterators to *i (including i itself) continue to refer to the same element, but now behave as iterators into *this, not into x.
In 23.3.11.5 [list.ops], replace paragraph 12 with:
Requires: [first, last) is a valid range in x. The result is undefined if position is an iterator in the range [first, last). Pointers and references to the moved elements of x now refer to those same elements but as members of *this. Iterators referring to the moved elements will continue to refer to their elements, but they now behave as iterators into *this, not into x.
[pre-Copenhagen: Howard provided wording.]
Rationale:
The original proposed resolution said that iterators and references would remain "valid". The new proposed resolution clarifies what that means. Note that this only applies to the case of equal allocators. From [default.con.req] paragraph 4, the behavior of list when allocators compare nonequal is outside the scope of the standard.
Section: 31.8.2 [stringbuf] Status: CD1 Submitter: Martin Sebor Opened: 2000-07-28 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
The synopsis for the template class basic_stringbuf
doesn't list a typedef for the template parameter
Allocator
. This makes it impossible to determine the type of
the allocator at compile time. It's also inconsistent with all other
template classes in the library that do provide a typedef for the
Allocator
parameter.
Proposed resolution:
Add to the synopses of the class templates basic_stringbuf (27.7.1), basic_istringstream (27.7.2), basic_ostringstream (27.7.3), and basic_stringstream (27.7.4) the typedef:
typedef Allocator allocator_type;
Section: 31.8 [string.streams] Status: CD1 Submitter: Martin Sebor Opened: 2000-07-28 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
27.7.2.2, p1 uses a C-style cast rather than the more appropriate const_cast<> in the Returns clause for basic_istringstream<>::rdbuf(). The same C-style cast is being used in 27.7.3.2, p1, D.7.2.2, p1, and D.7.3.2, p1, and perhaps elsewhere. 27.7.6, p1 and D.7.2.2, p1 are missing the cast altogether.
C-style casts have not been deprecated, so the first part of this issue is stylistic rather than a matter of correctness.
Proposed resolution:
In 27.7.2.2, p1 replace
-1- Returns: (basic_stringbuf<charT,traits,Allocator>*)&sb.
with
-1- Returns: const_cast<basic_stringbuf<charT,traits,Allocator>*>(&sb).
In 27.7.3.2, p1 replace
-1- Returns: (basic_stringbuf<charT,traits,Allocator>*)&sb.
with
-1- Returns: const_cast<basic_stringbuf<charT,traits,Allocator>*>(&sb).
In 27.7.6, p1, replace
-1- Returns: &sb
with
-1- Returns: const_cast<basic_stringbuf<charT,traits,Allocator>*>(&sb).
In D.7.2.2, p1 replace
-2- Returns: &sb.
with
-2- Returns: const_cast<strstreambuf*>(&sb).
Section: 29.6.2.2 [valarray.cons], 29.6.2.3 [valarray.assign] Status: CD1 Submitter: Robert Klarer Opened: 2000-07-31 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
This discussion is adapted from message c++std-lib-7056 posted November 11, 1999. I don't think that anyone can reasonably claim that the problem described below is NAD.
These valarray constructors can never be called:
template <class T> valarray<T>::valarray(const slice_array<T> &); template <class T> valarray<T>::valarray(const gslice_array<T> &); template <class T> valarray<T>::valarray(const mask_array<T> &); template <class T> valarray<T>::valarray(const indirect_array<T> &);
Similarly, these valarray assignment operators cannot be called:
template <class T> valarray<T> valarray<T>::operator=(const slice_array<T> &); template <class T> valarray<T> valarray<T>::operator=(const gslice_array<T> &); template <class T> valarray<T> valarray<T>::operator=(const mask_array<T> &); template <class T> valarray<T> valarray<T>::operator=(const indirect_array<T> &);
Please consider the following example:
#include <valarray> using namespace std; int main() { valarray<double> va1(12); valarray<double> va2(va1[slice(1,4,3)]); // line 1 }
Since the valarray va1 is non-const, the result of the sub-expression va1[slice(1,4,3)] at line 1 is an rvalue of type const std::slice_array<double>. This slice_array rvalue is then used to construct va2. The constructor that is used to construct va2 is declared like this:
template <class T> valarray<T>::valarray(const slice_array<T> &);
Notice the constructor's const reference parameter. When the constructor is called, a slice_array must be bound to this reference. The rules for binding an rvalue to a const reference are in 8.5.3, paragraph 5 (see also 13.3.3.1.4). Specifically, paragraph 5 indicates that a second slice_array rvalue is constructed (in this case copy-constructed) from the first one; it is this second rvalue that is bound to the reference parameter. Paragraph 5 also requires that the constructor that is used for this purpose be callable, regardless of whether the second rvalue is elided. The copy-constructor in this case is not callable, however, because it is private. Therefore, the compiler should report an error.
Since slice_arrays are always rvalues, the valarray constructor that has a parameter of type const slice_array<T> & can never be called. The same reasoning applies to the three other constructors and the four assignment operators that are listed at the beginning of this post. Furthermore, since these functions cannot be called, the valarray helper classes are almost entirely useless.
Proposed resolution:
slice_array:
gslice_array:
mask_array:
indirect_array:
[Proposed resolution was modified in Santa Cruz: explicitly make copy constructor and copy assignment operators public, instead of removing them.]
Rationale:
Keeping the valarray constructors private is untenable. Merely making valarray a friend of the helper classes isn't good enough, because access to the copy constructor is checked in the user's environment.
Making the assignment operator public is not strictly necessary to solve this problem. A majority of the LWG (straw poll: 13-4) believed we should make the assignment operators public, in addition to the copy constructors, for reasons of symmetry and user expectation.
std::string
Section: 19.2 [std.exceptions], 31.5.2.2.1 [ios.failure] Status: CD1 Submitter: Dave Abrahams Opened: 2000-08-01 Last modified: 2023-02-07
Priority: Not Prioritized
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Discussion:
Many of the standard exception types which implementations are required to throw are constructed with a const std::string& parameter. For example:
19.1.5 Class out_of_range [lib.out.of.range] namespace std { class out_of_range : public logic_error { public: explicit out_of_range(const string& what_arg); }; } 1 The class out_of_range defines the type of objects thrown as excep- tions to report an argument value not in its expected range. out_of_range(const string& what_arg); Effects: Constructs an object of class out_of_range. Postcondition: strcmp(what(), what_arg.c_str()) == 0.
There are at least two problems with this:
There may be no cure for (1) other than changing the interface to out_of_range, though one could reasonably argue that (1) is not a defect. Personally I don't care that much if out-of-memory is reported when I only have 20 bytes left, in the case when out_of_range would have been reported. People who use exception-specifications might care a lot, though.
There is a cure for (2), but it isn't completely obvious. I think a note for implementors should be made in the standard. Avoiding possible termination in this case shouldn't be left up to chance. The cure is to use a reference-counted "string" implementation in the exception object. I am not necessarily referring to a std::string here; any simple reference-counting scheme for a NTBS would do.
Further discussion, in email:
...I'm not so concerned about (1). After all, a library implementation can add const char* constructors as an extension, and users don't need to avail themselves of the standard exceptions, though this is a lame position to be forced into. FWIW, std::exception and std::bad_alloc don't require a temporary basic_string.
...I don't think the fixed-size buffer is a solution to the problem,
strictly speaking, because you can't satisfy the postcondition
strcmp(what(), what_arg.c_str()) == 0
For all values of what_arg (i.e. very long values). That means that
the only truly conforming solution requires a dynamic allocation.
Further discussion, from Redmond:
The most important progress we made at the Redmond meeting was
realizing that there are two separable issues here: the const
string& constructor, and the copy constructor. If a user writes
something like throw std::out_of_range("foo")
, the const
string& constructor is invoked before anything gets thrown. The
copy constructor is potentially invoked during stack unwinding.
The copy constructor is a more serious problem, becuase failure
during stack unwinding invokes terminate
. The copy
constructor must be nothrow. Curaçao: Howard thinks this
requirement may already be present.
The fundamental problem is that it's difficult to get the nothrow requirement to work well with the requirement that the exception objects store a string of unbounded size, particularly if you also try to make the const string& constructor nothrow. Options discussed include:
(Not all of these options are mutually exclusive.)
Proposed resolution:
Change 19.2.3 [logic.error]
namespace std { class logic_error : public exception { public: explicit logic_error(const string& what_arg); explicit logic_error(const char* what_arg); }; }...
logic_error(const char* what_arg);
-4- Effects: Constructs an object of class
logic_error
.-5- Postcondition:
strcmp(what(), what_arg) == 0
.
Change 19.2.4 [domain.error]
namespace std { class domain_error : public logic_error { public: explicit domain_error(const string& what_arg); explicit domain_error(const char* what_arg); }; }...
domain_error(const char* what_arg);
-4- Effects: Constructs an object of class
domain_error
.-5- Postcondition:
strcmp(what(), what_arg) == 0
.
Change 19.2.5 [invalid.argument]
namespace std { class invalid_argument : public logic_error { public: explicit invalid_argument(const string& what_arg); explicit invalid_argument(const char* what_arg); }; }...
invalid_argument(const char* what_arg);
-4- Effects: Constructs an object of class
invalid_argument
.-5- Postcondition:
strcmp(what(), what_arg) == 0
.
Change 19.2.6 [length.error]
namespace std { class length_error : public logic_error { public: explicit length_error(const string& what_arg); explicit length_error(const char* what_arg); }; }...
length_error(const char* what_arg);
-4- Effects: Constructs an object of class
length_error
.-5- Postcondition:
strcmp(what(), what_arg) == 0
.
Change 19.2.7 [out.of.range]
namespace std { class out_of_range : public logic_error { public: explicit out_of_range(const string& what_arg); explicit out_of_range(const char* what_arg); }; }...
out_of_range(const char* what_arg);
-4- Effects: Constructs an object of class
out_of_range
.-5- Postcondition:
strcmp(what(), what_arg) == 0
.
Change 19.2.8 [runtime.error]
namespace std { class runtime_error : public exception { public: explicit runtime_error(const string& what_arg); explicit runtime_error(const char* what_arg); }; }...
runtime_error(const char* what_arg);
-4- Effects: Constructs an object of class
runtime_error
.-5- Postcondition:
strcmp(what(), what_arg) == 0
.
Change 19.2.9 [range.error]
namespace std { class range_error : public runtime_error { public: explicit range_error(const string& what_arg); explicit range_error(const char* what_arg); }; }...
range_error(const char* what_arg);
-4- Effects: Constructs an object of class
range_error
.-5- Postcondition:
strcmp(what(), what_arg) == 0
.
Change 19.2.10 [overflow.error]
namespace std { class overflow_error : public runtime_error { public: explicit overflow_error(const string& what_arg); explicit overflow_error(const char* what_arg); }; }...
overflow_error(const char* what_arg);
-4- Effects: Constructs an object of class
overflow_error
.-5- Postcondition:
strcmp(what(), what_arg) == 0
.
Change 19.2.11 [underflow.error]
namespace std { class underflow_error : public runtime_error { public: explicit underflow_error(const string& what_arg); explicit underflow_error(const char* what_arg); }; }...
underflow_error(const char* what_arg);
-4- Effects: Constructs an object of class
underflow_error
.-5- Postcondition:
strcmp(what(), what_arg) == 0
.
Change [ios::failure]
namespace std { class ios_base::failure : public exception { public: explicit failure(const string& msg); explicit failure(const char* msg); virtual const char* what() const throw(); }; }...
failure(const char* msg);
-4- Effects: Constructs an object of class
failure
.-5- Postcondition:
strcmp(what(), msg) == 0
.
Rationale:
Throwing a bad_alloc while trying to construct a message for another exception-derived class is not necessarily a bad thing. And the bad_alloc constructor already has a no throw spec on it (18.4.2.1).
Future:
All involved would like to see const char* constructors added, but this should probably be done for C++0X as opposed to a DR.
I believe the no throw specs currently decorating these functions could be improved by some kind of static no throw spec checking mechanism (in a future C++ language). As they stand, the copy constructors might fail via a call to unexpected. I think what is intended here is that the copy constructors can't fail.
[Pre-Sydney: reopened at the request of Howard Hinnant. Post-Redmond: James Kanze noticed that the copy constructors of exception-derived classes do not have nothrow clauses. Those classes have no copy constructors declared, meaning the compiler-generated implicit copy constructors are used, and those compiler-generated constructors might in principle throw anything.]
[
Batavia: Merged copy constructor and assignment operator spec into exception
and added ios::failure
into the proposed resolution.
]
[
Oxford: The proposed resolution simply addresses the issue of constructing
the exception objects with const char*
and string literals without
the need to explicit include or construct a std::string
.
]
Section: 31.5.4.3 [basic.ios.members] Status: CD1 Submitter: Martin Sebor Opened: 2000-08-21 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
27.4.4.2, p17 says
-17- Before copying any parts of rhs, calls each registered callback pair (fn,index) as (*fn)(erase_event,*this,index). After all parts but exceptions() have been replaced, calls each callback pair that was copied from rhs as (*fn)(copy_event,*this,index).
The name copy_event isn't defined anywhere. The intended name was copyfmt_event.
Proposed resolution:
Replace copy_event with copyfmt_event in the named paragraph.
Section: 16.4.4.6 [allocator.requirements] Status: CD1 Submitter: Matt Austern Opened: 2000-08-22 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
From lib-7752:
I've been assuming (and probably everyone else has been assuming) that allocator instances have a particular property, and I don't think that property can be deduced from anything in Table 32.
I think we have to assume that allocator type conversion is a homomorphism. That is, if x1 and x2 are of type X, where X::value_type is T, and if type Y is X::template rebind<U>::other, then Y(x1) == Y(x2) if and only if x1 == x2.
Further discussion: Howard Hinnant writes, in lib-7757:
I think I can prove that this is not provable by Table 32. And I agree it needs to be true except for the "and only if". If x1 != x2, I see no reason why it can't be true that Y(x1) == Y(x2). Admittedly I can't think of a practical instance where this would happen, or be valuable. But I also don't see a need to add that extra restriction. I think we only need:
if (x1 == x2) then Y(x1) == Y(x2)
If we decide that == on allocators is transitive, then I think I can prove the above. But I don't think == is necessarily transitive on allocators. That is:
Given x1 == x2 and x2 == x3, this does not mean x1 == x3.
Example:
x1 can deallocate pointers from: x1, x2, x3
x2 can deallocate pointers from: x1, x2, x4
x3 can deallocate pointers from: x1, x3
x4 can deallocate pointers from: x2, x4x1 == x2, and x2 == x4, but x1 != x4
[Toronto: LWG members offered multiple opinions. One
opinion is that it should not be required that x1 == x2
implies Y(x1) == Y(x2)
, and that it should not even be
required that X(x1) == x1
. Another opinion is that
the second line from the bottom in table 32 already implies the
desired property. This issue should be considered in light of
other issues related to allocator instances.]
Proposed resolution:
Accept proposed wording from N2436 part 3.
[Lillehammer: Same conclusion as before: this should be considered as part of an allocator redesign, not solved on its own.]
[ Batavia: An allocator redesign is not forthcoming and thus we fixed this one issue. ]
[ Toronto: Reopened at the request of the project editor (Pete) because the proposed wording did not fit within the indicated table. The intent of the resolution remains unchanged. Pablo to work with Pete on improved wording. ]
[ Kona (2007): The LWG adopted the proposed resolution of N2387 for this issue which was subsequently split out into a separate paper N2436 for the purposes of voting. The resolution in N2436 addresses this issue. The LWG voted to accelerate this issue to Ready status to be voted into the WP at Kona. ]
basic_string::operator[]
and const correctnessSection: 27.4.3.5 [string.capacity] Status: CD1 Submitter: Chris Newton Opened: 2000-08-27 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
Paraphrased from a message that Chris Newton posted to comp.std.c++:
The standard's description of basic_string<>::operator[]
seems to violate const correctness.
The standard (21.3.4/1) says that "If pos < size()
,
returns data()[pos]
." The types don't work. The
return value of data()
is const charT*
, but
operator[]
has a non-const version whose return type is reference
.
Proposed resolution:
In section 21.3.4, paragraph 1, change
"data()[pos]
" to "*(begin() +
pos)
".
istream_iterator::operator++(int)
Section: 24.6.2.3 [istream.iterator.ops] Status: CD1 Submitter: Martin Sebor Opened: 2000-08-27 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
The synopsis of istream_iterator::operator++(int) in 24.5.1 shows it as returning the iterator by value. 24.5.1.2, p5 shows the same operator as returning the iterator by reference. That's incorrect given the Effects clause below (since a temporary is returned). The `&' is probably just a typo.
Proposed resolution:
Change the declaration in 24.5.1.2, p5 from
istream_iterator<T,charT,traits,Distance>& operator++(int);
to
istream_iterator<T,charT,traits,Distance> operator++(int);
(that is, remove the `&').
istream_iterator::operator!=
Section: 24.6.2.3 [istream.iterator.ops] Status: CD1 Submitter: Martin Sebor Opened: 2000-08-27 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
24.5.1, p3 lists the synopsis for
template <class T, class charT, class traits, class Distance> bool operator!=(const istream_iterator<T,charT,traits,Distance>& x, const istream_iterator<T,charT,traits,Distance>& y);
but there is no description of what the operator does (i.e., no Effects or Returns clause) in 24.5.1.2.
Proposed resolution:
Add paragraph 7 to the end of section 24.5.1.2 with the following text:
template <class T, class charT, class traits, class Distance> bool operator!=(const istream_iterator<T,charT,traits,Distance>& x, const istream_iterator<T,charT,traits,Distance>& y);
-7- Returns: !(x == y).
Section: 16.3.3.3.3 [bitmask.types] Status: CD1 Submitter: Beman Dawes Opened: 2000-09-03 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
The ~ operation should be applied after the cast to int_type.
Proposed resolution:
Change 17.3.2.1.2 [lib.bitmask.types] operator~ from:
bitmask operator~ ( bitmask X ) { return static_cast< bitmask>(static_cast<int_type>(~ X)); }
to:
bitmask operator~ ( bitmask X ) { return static_cast< bitmask>(~static_cast<int_type>(X)); }
basic_string
reference countingSection: 27.4.3 [basic.string] Status: CD1 Submitter: Kevlin Henney Opened: 2000-09-04 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
The note in paragraph 6 suggests that the invalidation rules for references, pointers, and iterators in paragraph 5 permit a reference- counted implementation (actually, according to paragraph 6, they permit a "reference counted implementation", but this is a minor editorial fix).
However, the last sub-bullet is so worded as to make a reference-counted implementation unviable. In the following example none of the conditions for iterator invalidation are satisfied:
// first example: "*******************" should be printed twice string original = "some arbitrary text", copy = original; const string & alias = original; string::const_iterator i = alias.begin(), e = alias.end(); for(string::iterator j = original.begin(); j != original.end(); ++j) *j = '*'; while(i != e) cout << *i++; cout << endl; cout << original << endl;
Similarly, in the following example:
// second example: "some arbitrary text" should be printed out string original = "some arbitrary text", copy = original; const string & alias = original; string::const_iterator i = alias.begin(); original.begin(); while(i != alias.end()) cout << *i++;
I have tested this on three string implementations, two of which were reference counted. The reference-counted implementations gave "surprising behavior" because they invalidated iterators on the first call to non-const begin since construction. The current wording does not permit such invalidation because it does not take into account the first call since construction, only the first call since various member and non-member function calls.
Proposed resolution:
Change the following sentence in 21.3 paragraph 5 from
Subsequent to any of the above uses except the forms of insert() and erase() which return iterators, the first call to non-const member functions operator[](), at(), begin(), rbegin(), end(), or rend().
to
Following construction or any of the above uses, except the forms of insert() and erase() that return iterators, the first call to non- const member functions operator[](), at(), begin(), rbegin(), end(), or rend().
insert(i, j)
complexity requirements are not feasible.Section: 23.2.7 [associative.reqmts] Status: CD1 Submitter: John Potter Opened: 2000-09-07 Last modified: 2016-01-28
Priority: Not Prioritized
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Duplicate of: 102
Discussion:
Table 69 requires linear time if [i, j) is sorted. Sorted is necessary but not sufficient. Consider inserting a sorted range of even integers into a set<int> containing the odd integers in the same range.
Proposed resolution:
In Table 69, in section 23.1.2, change the complexity clause for insertion of a range from "N log(size() + N) (N is the distance from i to j) in general; linear if [i, j) is sorted according to value_comp()" to "N log(size() + N), where N is the distance from i to j".
[Copenhagen: Minor fix in proposed resolution: fixed unbalanced parens in the revised wording.]
Rationale:
Testing for valid insertions could be less efficient than simply inserting the elements when the range is not both sorted and between two adjacent existing elements; this could be a QOI issue.
The LWG considered two other options: (a) specifying that the complexity was linear if [i, j) is sorted according to value_comp() and between two adjacent existing elements; or (b) changing to Klog(size() + N) + (N - K) (N is the distance from i to j and K is the number of elements which do not insert immediately after the previous element from [i, j) including the first). The LWG felt that, since we can't guarantee linear time complexity whenever the range to be inserted is sorted, it's more trouble than it's worth to say that it's linear in some special cases.
Section: 22.3 [pairs] Status: CD1 Submitter: Martin Sebor Opened: 2000-09-11 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
I don't see any requirements on the types of the elements of the std::pair container in 20.2.2. From the descriptions of the member functions it appears that they must at least satisfy the requirements of 20.1.3 [lib.copyconstructible] and 20.1.4 [lib.default.con.req], and in the case of the [in]equality operators also the requirements of 20.1.1 [lib.equalitycomparable] and 20.1.2 [lib.lessthancomparable].
I believe that the the CopyConstructible requirement is unnecessary in the case of 20.2.2, p2.
Proposed resolution:
Change the Effects clause in 20.2.2, p2 from
-2- Effects: Initializes its members as if implemented:
pair() : first(T1()), second(T2()) {}
to
-2- Effects: Initializes its members as if implemented:
pair() : first(), second() {}
Rationale:
The existing specification of pair's constructor appears to be a historical artifact: there was concern that pair's members be properly zero-initialized when they are built-in types. At one time there was uncertainty about whether they would be zero-initialized if the default constructor was written the obvious way. This has been clarified by core issue 178, and there is no longer any doubt that the straightforward implementation is correct.
Section: 17.9.4 [bad.exception] Status: CD1 Submitter: Martin Sebor Opened: 2000-09-24 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
The synopsis for std::bad_exception lists the function ~bad_exception() but there is no description of what the function does (the Effects clause is missing).
Proposed resolution:
Remove the destructor from the class synopses of
bad_alloc
(17.6.4.1 [bad.alloc]),
bad_cast
(17.7.4 [bad.cast]),
bad_typeid
(17.7.5 [bad.typeid]),
and bad_exception
(17.9.4 [bad.exception]).
Rationale:
This is a general problem with the exception classes in clause 18. The proposed resolution is to remove the destructors from the class synopses, rather than to document the destructors' behavior, because removing them is more consistent with how exception classes are described in clause 19.
Section: 28.3.3.1 [locale] Status: CD1 Submitter: Martin Sebor Opened: 2000-10-05 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
The synopsis of the class std::locale in 22.1.1 contains two typos: the semicolons after the declarations of the default ctor locale::locale() and the copy ctor locale::locale(const locale&) are missing.
Proposed resolution:
Add the missing semicolons, i.e., change
// construct/copy/destroy: locale() throw() locale(const locale& other) throw()
in the synopsis in 22.1.1 to
// construct/copy/destroy: locale() throw(); locale(const locale& other) throw();
Section: 26.8.4 [alg.binary.search] Status: CD1 Submitter: Matt Austern Opened: 2000-10-18 Last modified: 2016-01-28
Priority: Not Prioritized
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Duplicate of: 472
Discussion:
Each of the four binary search algorithms (lower_bound, upper_bound, equal_range, binary_search) has a form that allows the user to pass a comparison function object. According to 25.3, paragraph 2, that comparison function object has to be a strict weak ordering.
This requirement is slightly too strict. Suppose we are searching through a sequence containing objects of type X, where X is some large record with an integer key. We might reasonably want to look up a record by key, in which case we would want to write something like this:
struct key_comp { bool operator()(const X& x, int n) const { return x.key() < n; } } std::lower_bound(first, last, 47, key_comp());
key_comp is not a strict weak ordering, but there is no reason to prohibit its use in lower_bound.
There's no difficulty in implementing lower_bound so that it allows the use of something like key_comp. (It will probably work unless an implementor takes special pains to forbid it.) What's difficult is formulating language in the standard to specify what kind of comparison function is acceptable. We need a notion that's slightly more general than that of a strict weak ordering, one that can encompass a comparison function that involves different types. Expressing that notion may be complicated.
Additional questions raised at the Toronto meeting:
equal_range
.operator()
?Additional discussion from Copenhagen:
Proposed resolution:
Change 25.3 [lib.alg.sorting] paragraph 3 from:
3 For all algorithms that take Compare, there is a version that uses operator< instead. That is, comp(*i, *j) != false defaults to *i < *j != false. For the algorithms to work correctly, comp has to induce a strict weak ordering on the values.
to:
3 For all algorithms that take Compare, there is a version that uses operator< instead. That is, comp(*i, *j) != false defaults to *i < *j != false. For algorithms other than those described in lib.alg.binary.search (25.3.3) to work correctly, comp has to induce a strict weak ordering on the values.
Add the following paragraph after 25.3 [lib.alg.sorting] paragraph 5:
-6- A sequence [start, finish) is partitioned with respect to an expression f(e) if there exists an integer n such that for all 0 <= i < distance(start, finish), f(*(begin+i)) is true if and only if i < n.
Change 25.3.3 [lib.alg.binary.search] paragraph 1 from:
-1- All of the algorithms in this section are versions of binary search and assume that the sequence being searched is in order according to the implied or explicit comparison function. They work on non-random access iterators minimizing the number of comparisons, which will be logarithmic for all types of iterators. They are especially appropriate for random access iterators, because these algorithms do a logarithmic number of steps through the data structure. For non-random access iterators they execute a linear number of steps.
to:
-1- All of the algorithms in this section are versions of binary search and assume that the sequence being searched is partitioned with respect to an expression formed by binding the search key to an argument of the implied or explicit comparison function. They work on non-random access iterators minimizing the number of comparisons, which will be logarithmic for all types of iterators. They are especially appropriate for random access iterators, because these algorithms do a logarithmic number of steps through the data structure. For non-random access iterators they execute a linear number of steps.
Change 25.3.3.1 [lib.lower.bound] paragraph 1 from:
-1- Requires: Type T is LessThanComparable (lib.lessthancomparable).
to:
-1- Requires: The elements e of [first, last) are partitioned with respect to the expression e < value or comp(e, value)
Remove 25.3.3.1 [lib.lower.bound] paragraph 2:
-2- Effects: Finds the first position into which value can be inserted without violating the ordering.
Change 25.3.3.2 [lib.upper.bound] paragraph 1 from:
-1- Requires: Type T is LessThanComparable (lib.lessthancomparable).
to:
-1- Requires: The elements e of [first, last) are partitioned with respect to the expression !(value < e) or !comp(value, e)
Remove 25.3.3.2 [lib.upper.bound] paragraph 2:
-2- Effects: Finds the furthermost position into which value can be inserted without violating the ordering.
Change 25.3.3.3 [lib.equal.range] paragraph 1 from:
-1- Requires: Type T is LessThanComparable (lib.lessthancomparable).
to:
-1- Requires: The elements e of [first, last) are partitioned with respect to the expressions e < value and !(value < e) or comp(e, value) and !comp(value, e). Also, for all elements e of [first, last), e < value implies !(value < e) or comp(e, value) implies !comp(value, e)
Change 25.3.3.3 [lib.equal.range] paragraph 2 from:
-2- Effects: Finds the largest subrange [i, j) such that the value can be inserted at any iterator k in it without violating the ordering. k satisfies the corresponding conditions: !(*k < value) && !(value < *k) or comp(*k, value) == false && comp(value, *k) == false.
to:
-2- Returns: make_pair(lower_bound(first, last, value), upper_bound(first, last, value)) or make_pair(lower_bound(first, last, value, comp), upper_bound(first, last, value, comp))
Change 25.3.3.3 [lib.binary.search] paragraph 1 from:
-1- Requires: Type T is LessThanComparable (lib.lessthancomparable).
to:
-1- Requires: The elements e of [first, last) are partitioned with respect to the expressions e < value and !(value < e) or comp(e, value) and !comp(value, e). Also, for all elements e of [first, last), e < value implies !(value < e) or comp(e, value) implies !comp(value, e)
[Copenhagen: Dave Abrahams provided this wording]
[Redmond: Minor changes in wording. (Removed "non-negative", and changed the "other than those described in" wording.) Also, the LWG decided to accept the "optional" part.]
Rationale:
The proposed resolution reinterprets binary search. Instead of thinking about searching for a value in a sorted range, we view that as an important special case of a more general algorithm: searching for the partition point in a partitioned range.
We also add a guarantee that the old wording did not: we ensure that the upper bound is no earlier than the lower bound, that the pair returned by equal_range is a valid range, and that the first part of that pair is the lower bound.
Section: 31.7.5.7 [iostreamclass] Status: CD1 Submitter: Martin Sebor Opened: 2000-11-02 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
Class template basic_iostream has no typedefs. The typedefs it inherits from its base classes can't be used, since (for example) basic_iostream<T>::traits_type is ambiguous.
Proposed resolution:
Add the following to basic_iostream's class synopsis in
31.7.5.7 [iostreamclass], immediately after public
:
// types: typedef charT char_type; typedef typename traits::int_type int_type; typedef typename traits::pos_type pos_type; typedef typename traits::off_type off_type; typedef traits traits_type;
Section: 31.5.4.4 [iostate.flags] Status: CD1 Submitter: Martin Sebor Opened: 2000-11-02 Last modified: 2016-01-28
Priority: Not Prioritized
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Duplicate of: 569
Discussion:
27.4.4.3, p4 says about the postcondition of the function: If rdbuf()!=0 then state == rdstate(); otherwise rdstate()==state|ios_base::badbit.
The expression on the right-hand-side of the operator==() needs to be parenthesized in order for the whole expression to ever evaluate to anything but non-zero.
Proposed resolution:
Add parentheses like so: rdstate()==(state|ios_base::badbit).
Section: 31 [input.output] Status: CD1 Submitter: Martin Sebor Opened: 2000-11-02 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
27.5.2.4.2, p4, and 27.8.1.6, p2, 27.8.1.7, p3, 27.8.1.9, p2, 27.8.1.10, p3 refer to in and/or out w/o ios_base:: qualification. That's incorrect since the names are members of a dependent base class (14.6.2 [temp.dep]) and thus not visible.
Proposed resolution:
Qualify the names with the name of the class of which they are members, i.e., ios_base.
Section: 16.4.4.6 [allocator.requirements] Status: CD1 Submitter: Martin Sebor Opened: 2000-11-02 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
I see that table 31 in 20.1.5, p3 allows T in std::allocator<T> to be of any type. But the synopsis in 20.4.1 calls for allocator<>::address() to be overloaded on reference and const_reference, which is ill-formed for all T = const U. In other words, this won't work:
template class std::allocator<const int>;
The obvious solution is to disallow specializations of allocators on const types. However, while containers' elements are required to be assignable (which rules out specializations on const T's), I think that allocators might perhaps be potentially useful for const values in other contexts. So if allocators are to allow const types a partial specialization of std::allocator<const T> would probably have to be provided.
Proposed resolution:
Change the text in row 1, column 2 of table 32 in 20.1.5, p3 from
any type
to
any non-const, non-reference type
[Redmond: previous proposed resolution was "any non-const, non-volatile, non-reference type". Got rid of the "non-volatile".]
Rationale:
Two resolutions were originally proposed: one that partially specialized std::allocator for const types, and one that said an allocator's value type may not be const. The LWG chose the second. The first wouldn't be appropriate, because allocators are intended for use by containers, and const value types don't work in containers. Encouraging the use of allocators with const value types would only lead to unsafe code.
The original text for proposed resolution 2 was modified so that it also forbids volatile types and reference types.
[Curaçao: LWG double checked and believes volatile is correctly excluded from the PR.]
Section: 28.3.4.3.2.2 [facet.num.get.members] Status: CD1 Submitter: Matt Austern Opened: 2000-11-02 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
In 22.2.2.1.1, we have a list of overloads for num_get<>::get(). There are eight overloads, all of which are identical except for the last parameter. The overloads are:
There is a similar list, in 22.2.2.1.2, of overloads for num_get<>::do_get(). In this list, the last parameter has the types:
These two lists are not identical. They should be, since
get
is supposed to call do_get
with exactly
the arguments it was given.
Proposed resolution:
In 28.3.4.3.2.2 [facet.num.get.members], change
iter_type get(iter_type in, iter_type end, ios_base& str, ios_base::iostate& err, short& val) const;
to
iter_type get(iter_type in, iter_type end, ios_base& str, ios_base::iostate& err, float& val) const;
Section: 23.2 [container.requirements] Status: CD1 Submitter: Peter Dimov Opened: 2000-11-07 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
23.1/3 states that the objects stored in a container must be Assignable. 23.4.3 [map], paragraph 2, states that map satisfies all requirements for a container, while in the same time defining value_type as pair<const Key, T> - a type that is not Assignable.
It should be noted that there exists a valid and non-contradictory interpretation of the current text. The wording in 23.1/3 avoids mentioning value_type, referring instead to "objects stored in a container." One might argue that map does not store objects of type map::value_type, but of map::mapped_type instead, and that the Assignable requirement applies to map::mapped_type, not map::value_type.
However, this makes map a special case (other containers store objects of type value_type) and the Assignable requirement is needlessly restrictive in general.
For example, the proposed resolution of active library issue 103(i) is to make set::iterator a constant iterator; this means that no set operations can exploit the fact that the stored objects are Assignable.
This is related to, but slightly broader than, closed issue 140(i).
Proposed resolution:
23.1/3: Strike the trailing part of the sentence:
, and the additional requirements of Assignable types from 23.1/3
so that it reads:
-3- The type of objects stored in these components must meet the requirements of CopyConstructible types (lib.copyconstructible).
23.1/4: Modify to make clear that this requirement is not for all containers. Change to:
-4- Table 64 defines the Assignable requirement. Some containers require this property of the types to be stored in the container. T is the type used to instantiate the container. t is a value of T, and u is a value of (possibly const) T.
23.1, Table 65: in the first row, change "T is Assignable" to "T is CopyConstructible".
23.2.1/2: Add sentence for Assignable requirement. Change to:
-2- A deque satisfies all of the requirements of a container and of a reversible container (given in tables in lib.container.requirements) and of a sequence, including the optional sequence requirements (lib.sequence.reqmts). In addition to the requirements on the stored object described in 23.1[lib.container.requirements], the stored object must also meet the requirements of Assignable. Descriptions are provided here only for operations on deque that are not described in one of these tables or for operations where there is additional semantic information.
23.2.2/2: Add Assignable requirement to specific methods of list. Change to:
-2- A list satisfies all of the requirements of a container and of a reversible container (given in two tables in lib.container.requirements) and of a sequence, including most of the the optional sequence requirements (lib.sequence.reqmts). The exceptions are the operator[] and at member functions, which are not provided. [Footnote: These member functions are only provided by containers whose iterators are random access iterators. --- end foonote]
list does not require the stored type T to be Assignable unless the following methods are instantiated: [Footnote: Implementors are permitted but not required to take advantage of T's Assignable properties for these methods. -- end foonote]
list<T,Allocator>& operator=(const list<T,Allocator>& x ); template <class InputIterator> void assign(InputIterator first, InputIterator last); void assign(size_type n, const T& t);Descriptions are provided here only for operations on list that are not described in one of these tables or for operations where there is additional semantic information.
23.2.4/2: Add sentence for Assignable requirement. Change to:
-2- A vector satisfies all of the requirements of a container and of a reversible container (given in two tables in lib.container.requirements) and of a sequence, including most of the optional sequence requirements (lib.sequence.reqmts). The exceptions are the push_front and pop_front member functions, which are not provided. In addition to the requirements on the stored object described in 23.1[lib.container.requirements], the stored object must also meet the requirements of Assignable. Descriptions are provided here only for operations on vector that are not described in one of these tables or for operations where there is additional semantic information.
Rationale:
list, set, multiset, map, multimap are able to store non-Assignables.
However, there is some concern about list<T>
:
although in general there's no reason for T to be Assignable, some
implementations of the member functions operator=
and
assign
do rely on that requirement. The LWG does not want
to forbid such implementations.
Note that the type stored in a standard container must still satisfy the requirements of the container's allocator; this rules out, for example, such types as "const int". See issue 274(i) for more details.
In principle we could also relax the "Assignable" requirement for
individual vector
member functions, such as
push_back
. However, the LWG did not see great value in such
selective relaxation. Doing so would remove implementors' freedom to
implement vector::push_back
in terms of
vector::insert
.
Section: 23.3.11.5 [list.ops] Status: CD1 Submitter: P.J. Plauger Opened: 2000-11-27 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
Section 23.3.11.5 [list.ops] states that
void splice(iterator position, list<T, Allocator>& x);
invalidates all iterators and references to list x
.
But what does the C++ Standard mean by "invalidate"? You can still dereference the iterator to a spliced list element, but you'd better not use it to delimit a range within the original list. For the latter operation, it has definitely lost some of its validity.
If we accept the proposed resolution to issue 250(i), then we'd better clarify that a "valid" iterator need no longer designate an element within the same container as it once did. We then have to clarify what we mean by invalidating a past-the-end iterator, as when a vector or string grows by reallocation. Clearly, such an iterator has a different kind of validity. Perhaps we should introduce separate terms for the two kinds of "validity."
Proposed resolution:
Add the following text to the end of section 24.3.4 [iterator.concepts], after paragraph 5:
An invalid iterator is an iterator that may be singular. [Footnote: This definition applies to pointers, since pointers are iterators. The effect of dereferencing an iterator that has been invalidated is undefined.]
[post-Copenhagen: Matt provided wording.]
[Redmond: General agreement with the intent, some objections to the wording. Dave provided new wording.]
Rationale:
This resolution simply defines a term that the Standard uses but never defines, "invalid", in terms of a term that is defined, "singular".
Why do we say "may be singular", instead of "is singular"? That's becuase a valid iterator is one that is known to be nonsingular. Invalidating an iterator means changing it in such a way that it's no longer known to be nonsingular. An example: inserting an element into the middle of a vector is correctly said to invalidate all iterators pointing into the vector. That doesn't necessarily mean they all become singular.
Section: 24.5.1 [reverse.iterators] Status: CD1 Submitter: Steve Cleary Opened: 2000-11-27 Last modified: 2020-03-29
Priority: Not Prioritized
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Discussion:
This came from an email from Steve Cleary to Fergus in reference to issue 179(i). The library working group briefly discussed this in Toronto and believed it should be a separate issue. There was also some reservations about whether this was a worthwhile problem to fix.
Steve said: "Fixing reverse_iterator
. std::reverse_iterator
can
(and should) be changed to preserve these additional
requirements." He also said in email that it can be done without
breaking user's code: "If you take a look at my suggested
solution, reverse_iterator doesn't have to take two parameters; there
is no danger of breaking existing code, except someone taking the
address of one of the reverse_iterator global operator functions, and
I have to doubt if anyone has ever done that. . . But, just in
case they have, you can leave the old global functions in as well --
they won't interfere with the two-template-argument functions. With
that, I don't see how any user code could break."
Proposed resolution:
Section: 24.5.1.2 [reverse.iterator] add/change the following declarations:
A) Add a templated assignment operator, after the same manner as the templated copy constructor, i.e.: template < class U > reverse_iterator < Iterator >& operator=(const reverse_iterator< U >& u); B) Make all global functions (except the operator+) have two template parameters instead of one, that is, for operator ==, !=, <, >, <=, >=, - replace: template < class Iterator > typename reverse_iterator< Iterator >::difference_type operator-( const reverse_iterator< Iterator >& x, const reverse_iterator< Iterator >& y); with: template < class Iterator1, class Iterator2 > typename reverse_iterator < Iterator1 >::difference_type operator-( const reverse_iterator < Iterator1 > & x, const reverse_iterator < Iterator2 > & y);
Also make the addition/changes for these signatures in [reverse.iter.ops].
[ Copenhagen: The LWG is concerned that the proposed resolution introduces new overloads. Experience shows that introducing overloads is always risky, and that it would be inappropriate to make this change without implementation experience. It may be desirable to provide this feature in a different way. ]
[ Lillehammer: We now have implementation experience, and agree that this solution is safe and correct. ]
[2020-03-29; Jonathan Wakely comments]
The issue title is misleading, it is not about comparing to const
-qualified
reverse_iterator
s, but comparing to reverse_iterator<const-iterator>
.
Section: 26.8.9 [alg.min.max] Status: CD1 Submitter: Martin Sebor Opened: 2000-12-02 Last modified: 2016-01-28
Priority: Not Prioritized
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Duplicate of: 486
Discussion:
The requirements in 25.3.7, p1 and 4 call for T to satisfy the
requirements of LessThanComparable
( [lessthancomparable])
and CopyConstructible
(16.4.4.2 [utility.arg.requirements]).
Since the functions take and return their arguments and result by
const reference, I believe the CopyConstructible
requirement
is unnecessary.
Proposed resolution:
Remove the CopyConstructible
requirement. Specifically, replace
25.3.7, p1 with
-1- Requires: Type T is LessThanComparable
( [lessthancomparable]).
and replace 25.3.7, p4 with
-4- Requires: Type T is LessThanComparable
( [lessthancomparable]).
Section: 28.3.4.3.3.3 [facet.num.put.virtuals] Status: CD1 Submitter: Howard Hinnant Opened: 2000-12-05 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
Paragraph 16 mistakenly singles out integral types for inserting thousands_sep() characters. This conflicts with the syntax for floating point numbers described under 22.2.3.1/2.
Proposed resolution:
Change paragraph 16 from:
For integral types, punct.thousands_sep() characters are inserted into the sequence as determined by the value returned by punct.do_grouping() using the method described in 28.3.4.4.1.3 [facet.numpunct.virtuals].
To:
For arithmetic types, punct.thousands_sep() characters are inserted into the sequence as determined by the value returned by punct.do_grouping() using the method described in 28.3.4.4.1.3 [facet.numpunct.virtuals].
[ Copenhagen: Opinions were divided about whether this is actually an inconsistency, but at best it seems to have been unintentional. This is only an issue for floating-point output: The standard is unambiguous that implementations must parse thousands_sep characters when performing floating-point. The standard is also unambiguous that this requirement does not apply to the "C" locale. ]
[ A survey of existing practice is needed; it is believed that some implementations do insert thousands_sep characters for floating-point output and others fail to insert thousands_sep characters for floating-point input even though this is unambiguously required by the standard. ]
[Post-Curaçao: the above proposed resolution is the consensus of Howard, Bill, Pete, Benjamin, Nathan, Dietmar, Boris, and Martin.]
Section: 26.7.5 [alg.replace] Status: CD1 Submitter: Martin Sebor Opened: 2000-12-15 Last modified: 2016-01-28
Priority: Not Prioritized
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Duplicate of: 483
Discussion:
(revision of the further discussion) There are a number of problems with the requires clauses for the algorithms in 25.1 and 25.2. The requires clause of each algorithm should describe the necessary and sufficient requirements on the inputs to the algorithm such that the algorithm compiles and runs properly. Many of the requires clauses fail to do this. Here is a summary of the kinds of mistakes:
Here is the list of algorithms that contain mistakes:
Also, in the requirements for EqualityComparable, the requirement that the operator be defined for const objects is lacking.
Proposed resolution:
20.1.1 Change p1 from
In Table 28, T
is a type to be supplied by a C++ program
instantiating a template, a
, b
, and c
are
values of type T
.
to
In Table 28, T
is a type to be supplied by a C++ program
instantiating a template, a
, b
, and c
are
values of type const T
.
25 Between p8 and p9
Add the following sentence:
When the description of an algorithm gives an expression such as
*first == value
for a condition, it is required that the expression
evaluate to either true or false in boolean contexts.
25.1.2 Change p1 by deleting the requires clause.
25.1.6 Change p1 by deleting the requires clause.
25.1.9
Change p4 from
-4- Requires: Type T
is EqualityComparable
(20.1.1), type Size is convertible to integral type (4.7.12.3).
to
-4- Requires: The type Size
is convertible to integral
type (4.7.12.3).
25.2.4 Change p1 from
-1- Requires: Type T
is Assignable
(23.1 ) (and, for replace()
, EqualityComparable
(20.1.1 )).
to
-1- Requires: The expression *first = new_value
must be valid.
and change p4 from
-4- Requires: Type T
is Assignable
(23.1) (and,
for replace_copy()
, EqualityComparable
(20.1.1)). The ranges [first, last)
and [result, result +
(last - first))
shall not overlap.
to
-4- Requires: The results of the expressions *first
and
new_value
must be writable to the result output iterator. The
ranges [first, last)
and [result, result + (last -
first))
shall not overlap.
25.2.5 Change p1 from
-1- Requires: Type T
is Assignable
(23.1). The
type Size
is convertible to an integral type (4.7.12.3).
to
-1- Requires: The expression value
must be is writable to
the output iterator. The type Size
is convertible to an
integral type (4.7.12.3).
25.2.7 Change p1 from
-1- Requires: Type T
is EqualityComparable
(20.1.1).
to
-1- Requires: The value type of the iterator must be
Assignable
(23.1).
Rationale:
The general idea of the proposed solution is to remove the faulty requires clauses and let the returns and effects clauses speak for themselves. That is, the returns clauses contain expressions that must be valid, and therefore already imply the correct requirements. In addition, a sentence is added at the beginning of chapter 25 saying that expressions given as conditions must evaluate to true or false in a boolean context. An alternative would be to say that the type of these condition expressions must be literally bool, but that would be imposing a greater restriction that what the standard currently says (which is convertible to bool).
Section: 22.10.8 [comparisons] Status: CD1 Submitter: Martin Sebor Opened: 2000-12-26 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
The example in 22.10.8 [comparisons], p6 shows how to use the C
library function strcmp()
with the function pointer adapter
ptr_fun()
. But since it's unspecified whether the C library
functions have extern "C"
or extern
"C++"
linkage [16.4.3.3 [using.linkage]], and since
function pointers with different the language linkage specifications
(9.12 [dcl.link]) are incompatible, whether this example is
well-formed is unspecified.
Proposed resolution:
Change 22.10.8 [comparisons] paragraph 6 from:
[Example:
replace_if(v.begin(), v.end(), not1(bind2nd(ptr_fun(strcmp), "C")), "C++");replaces each
C
withC++
in sequencev
.
to:
[Example:
int compare(const char*, const char*); replace_if(v.begin(), v.end(), not1(bind2nd(ptr_fun(compare), "abc")), "def");replaces each
abc
withdef
in sequencev
.
Also, remove footnote 215 in that same paragraph.
[Copenhagen: Minor change in the proposed resolution. Since this issue deals in part with C and C++ linkage, it was believed to be too confusing for the strings in the example to be "C" and "C++". ]
[Redmond: More minor changes. Got rid of the footnote (which seems to make a sweeping normative requirement, even though footnotes aren't normative), and changed the sentence after the footnote so that it corresponds to the new code fragment.]
Section: 31.10.4.2 [ifstream.cons] Status: CD1 Submitter: Martin Sebor Opened: 2000-12-31 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
31.10.4.2 [ifstream.cons], p2, 31.10.5.2 [ofstream.cons], p2, and 31.10.6.2 [fstream.cons], p2 say about the effects of each constructor:
... If that function returns a null pointer, calls
setstate(failbit)
(which may throw ios_base::failure
).
The parenthetical note doesn't apply since the ctors cannot throw an
exception due to the requirement in 31.5.4.2 [basic.ios.cons], p3
that exceptions()
be initialized to ios_base::goodbit
.
Proposed resolution:
Strike the parenthetical note from the Effects clause in each of the paragraphs mentioned above.
Section: 26.13 [alg.c.library] Status: CD1 Submitter: Judy Ward Opened: 2000-12-30 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
The <cstdlib> header file contains prototypes for bsearch and qsort (C++ Standard section 25.4 paragraphs 3 and 4) and other prototypes (C++ Standard section 21.4 paragraph 1 table 49) that require the typedef size_t. Yet size_t is not listed in the <cstdlib> synopsis table 78 in section 25.4.
Proposed resolution:
Add the type size_t to Table 78 (section 25.4) and add the type size_t <cstdlib> to Table 97 (section C.2).
Rationale:
Since size_t is in <stdlib.h>, it must also be in <cstdlib>.
Section: 19.4 [errno] Status: CD1 Submitter: Judy Ward Opened: 2000-12-30 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
ISO/IEC 9899:1990/Amendment1:1994 Section 4.3 States: "The list of macros defined in <errno.h> is adjusted to include a new macro, EILSEQ"
ISO/IEC 14882:1998(E) section 19.3 does not refer to the above amendment.
Proposed resolution:
Update Table 26 (section 19.3) "Header <cerrno> synopsis" and Table 95 (section C.2) "Standard Macros" to include EILSEQ.
Section: 26.8.7 [alg.set.operations] Status: CD1 Submitter: Matt Austern Opened: 2001-01-03 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
The standard library contains four algorithms that compute set
operations on sorted ranges: set_union
, set_intersection
,
set_difference
, and set_symmetric_difference
. Each
of these algorithms takes two sorted ranges as inputs, and writes the
output of the appropriate set operation to an output range. The elements
in the output range are sorted.
The ordinary mathematical definitions are generalized so that they
apply to ranges containing multiple copies of a given element. Two
elements are considered to be "the same" if, according to an
ordering relation provided by the user, neither one is less than the
other. So, for example, if one input range contains five copies of an
element and another contains three, the output range of set_union
will contain five copies, the output range of
set_intersection
will contain three, the output range of
set_difference
will contain two, and the output range of
set_symmetric_difference
will contain two.
Because two elements can be "the same" for the purposes of these set algorithms, without being identical in other respects (consider, for example, strings under case-insensitive comparison), this raises a number of unanswered questions:
The standard should either answer these questions, or explicitly say that the answers are unspecified. I prefer the former option, since, as far as I know, all existing implementations behave the same way.
Proposed resolution:
Add the following to the end of 26.8.7.3 [set.union] paragraph 5:
If [first1, last1) contains m elements that are equivalent to each other and [first2, last2) contains n elements that are equivalent to them, then max(m, n) of these elements will be copied to the output range: all m of these elements from [first1, last1), and the last max(n-m, 0) of them from [first2, last2), in that order.
Add the following to the end of 26.8.7.4 [set.intersection] paragraph 5:
If [first1, last1) contains m elements that are equivalent to each other and [first2, last2) contains n elements that are equivalent to them, the first min(m, n) of those elements from [first1, last1) are copied to the output range.
Add a new paragraph, Notes, after 26.8.7.5 [set.difference] paragraph 4:
If [first1, last1) contains m elements that are equivalent to each other and [first2, last2) contains n elements that are equivalent to them, the last max(m-n, 0) elements from [first1, last1) are copied to the output range.
Add a new paragraph, Notes, after 26.8.7.6 [set.symmetric.difference] paragraph 4:
If [first1, last1) contains m elements that are equivalent to each other and [first2, last2) contains n elements that are equivalent to them, then |m - n| of those elements will be copied to the output range: the last m - n of these elements from [first1, last1) if m > n, and the last n - m of these elements from [first2, last2) if m < n.
[Santa Cruz: it's believed that this language is clearer than what's in the Standard. However, it's also believed that the Standard may already make these guarantees (although not quite in these words). Bill and Howard will check and see whether they think that some or all of these changes may be redundant. If so, we may close this issue as NAD.]
Rationale:
For simple cases, these descriptions are equivalent to what's already in the Standard. For more complicated cases, they describe the behavior of existing implementations.
Section: 31.5.4.3 [basic.ios.members] Status: CD1 Submitter: Martin Sebor Opened: 2001-01-05 Last modified: 2016-01-28
Priority: Not Prioritized
View all other issues in [basic.ios.members].
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Discussion:
The Effects clause of the member function copyfmt()
in
27.4.4.2, p15 doesn't consider the case where the left-hand side
argument is identical to the argument on the right-hand side, that is
(this == &rhs)
. If the two arguments are identical there
is no need to copy any of the data members or call any callbacks
registered with register_callback()
. Also, as Howard Hinnant
points out in message c++std-lib-8149 it appears to be incorrect to
allow the object to fire erase_event
followed by
copyfmt_event
since the callback handling the latter event
may inadvertently attempt to access memory freed by the former.
Proposed resolution:
Change the Effects clause in 27.4.4.2, p15 from
-15- Effects:Assigns to the member objects of
*this
the corresponding member objects ofrhs
, except that...
to
-15- Effects:If
(this == &rhs)
does nothing. Otherwise assigns to the member objects of*this
the corresponding member objects ofrhs
, except that...
Section: 16.4.5.3.3 [macro.names] Status: CD1 Submitter: James Kanze Opened: 2001-01-11 Last modified: 2016-01-28
Priority: Not Prioritized
View other active issues in [macro.names].
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Discussion:
Paragraph 2 of 16.4.5.3.3 [macro.names] reads: "A translation unit that includes a header shall not contain any macros that define names declared in that header." As I read this, it would mean that the following program is legal:
#define npos 3.14 #include <sstream>
since npos is not defined in <sstream>. It is, however, defined in <string>, and it is hard to imagine an implementation in which <sstream> didn't include <string>.
I think that this phrase was probably formulated before it was decided that a standard header may freely include other standard headers. The phrase would be perfectly appropriate for C, for example. In light of 16.4.6.2 [res.on.headers] paragraph 1, however, it isn't stringent enough.
Proposed resolution:
For 16.4.5.3.3 [macro.names], replace the current wording, which reads:
Each name defined as a macro in a header is reserved to the implementation for any use if the translation unit includes the header.168)
A translation unit that includes a header shall not contain any macros that define names declared or defined in that header. Nor shall such a translation unit define macros for names lexically identical to keywords.
168) It is not permissible to remove a library macro definition by using the #undef directive.
with the wording:
A translation unit that includes a standard library header shall not #define or #undef names declared in any standard library header.
A translation unit shall not #define or #undef names lexically identical to keywords.
[Lillehammer: Beman provided new wording]
Section: 29.7 [c.math] Status: CD1 Submitter: Jens Maurer Opened: 2001-01-12 Last modified: 2016-01-28
Priority: Not Prioritized
View all other issues in [c.math].
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Discussion:
Table 80 lists the contents of the <cmath> header. It does not
list abs()
. However, 26.5, paragraph 6, which lists added
signatures present in <cmath>, does say that several overloads
of abs()
should be defined in <cmath>.
Proposed resolution:
Add abs
to Table 80. Also, remove the parenthetical list
of functions "(abs(), div(), rand(), srand())" from 29.6 [numarray],
paragraph 1.
[Copenhagen: Modified proposed resolution so that it also gets rid of that vestigial list of functions in paragraph 1.]
Rationale:
All this DR does is fix a typo; it's uncontroversial. A separate question is whether we're doing the right thing in putting some overloads in <cmath> that we aren't also putting in <cstdlib>. That's issue 323(i).
Section: 22.3 [pairs] Status: C++11 Submitter: Martin Sebor Opened: 2001-01-14 Last modified: 2016-01-28
Priority: Not Prioritized
View all other issues in [pairs].
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Discussion:
The synopsis of the header <utility>
in 22.2 [utility]
lists the complete set of equality and relational operators for pair
but the section describing the template and the operators only describes
operator==()
and operator<()
, and it fails to mention
any requirements on the template arguments. The remaining operators are
not mentioned at all.
[ 2009-09-27 Alisdair reopens. ]
The issue is a lack of wording specifying the semantics of
std::pair
relational operators. The rationale is that this is covered by catch-all wording in the relops component, and that as relops directly precedespair
in the document this is an easy connection to make.Reading the current working paper I make two observations:
- relops no longer immediately precedes
pair
in the order of specification. However, even if it did, there is a lot ofpair
specification itself between the (apparently) unrelated relops and the relational operators forpair
. (The catch-all still requiresoperator==
andoperator<
to be specified explicitly)- No other library component relies on the catch-all clause. The following all explicitly document all six relational operators, usually in a manner that could have deferred to the relops clause.
tuple unique_ptr duration time_point basic_string queue stack move_iterator reverse_iterator regex submatch thread::idThe container components provide their own (equivalent) definition in 23.2.2 [container.requirements.general] Table 90 -- Container requirements and do so do not defer to relops.
Shared_ptr
explicitly documentsoperator!=
and does not supply the other 3 missing operators (>
,>=
,<=
) so does not meet the reqirements of the relops clause.
Weak_ptr
only supportsoperator<
so would not be covered by relops.At the very least I would request a note pointing to the relops clause we rely on to provide this definition. If this route is taken, I would recommend reducing many of the above listed clauses to a similar note rather than providing redundant specification.
My preference would be to supply the 4 missing specifications consistent with the rest of the library.
[
2009-10-11 Daniel opens 1233(i) which deals with the same issue as
it pertains to unique_ptr
.
]
[ 2009-10 Santa Cruz: ]
Move to Ready
Proposed resolution:
After p20 22.3 [pairs] add:
template <class T1, class T2> bool operator!=(const pair<T1,T2>& x, const pair<T1,T2>& y);Returns:
!(x==y)
template <class T1, class T2> bool operator> (const pair<T1,T2>& x, const pair<T1,T2>& y);Returns:
y < x
template <class T1, class T2> bool operator>=(const pair<T1,T2>& x, const pair<T1,T2>& y);Returns:
!(x < y)
template <class T1, class T2> bool operator<=(const pair<T1,T2>& x, const pair<T1,T2>& y);Returns:
!(y < x)
Rationale:
[operators] paragraph 10 already specifies the semantics.
That paragraph says that, if declarations of operator!=, operator>,
operator<=, and operator>= appear without definitions, they are
defined as specified in [operators]. There should be no user
confusion, since that paragraph happens to immediately precede the
specification of pair
.
Section: 22.10.10 [logical.operations] Status: CD1 Submitter: Martin Sebor Opened: 2001-01-06 Last modified: 2016-01-28
Priority: Not Prioritized
View all issues with CD1 status.
Discussion:
The class templates const_mem_fun_t
in 20.5.8, p8 and
const_mem_fun1_t
in 20.5.8, p9 derive from unary_function<T*, S>
, and
binary_function<T*,
A, S>
, respectively. Consequently, their argument_type
, and
first_argument_type
members, respectively, are both defined to be T*
(non-const).
However, their function call member operator takes a const T*
argument. It is my opinion that argument_type
should be const
T*
instead, so that one can easily refer to it in generic code. The
example below derived from existing code fails to compile due to the
discrepancy:
template <class T>
void foo (typename T::argument_type arg) // #1
{
typename T::result_type (T::*pf) (typename
T::argument_type)
const = // #2
&T::operator();
}
struct X { /* ... */ };
int main ()
{
const X x;
foo<std::const_mem_fun_t<void, X>
>(&x);
// #3
}
#1 foo()
takes a plain unqualified X*
as an argument
#2 the type of the pointer is incompatible with the type of the member
function
#3 the address of a constant being passed to a function taking a non-const
X*
Proposed resolution:
Replace the top portion of the definition of the class template const_mem_fun_t in 20.5.8, p8
template <class S, class T> class const_mem_fun_t
: public
unary_function<T*, S> {
with
template <class S, class T> class const_mem_fun_t
: public
unary_function<const T*, S> {
Also replace the top portion of the definition of the class template const_mem_fun1_t in 20.5.8, p9
template <class S, class T, class A> class const_mem_fun1_t
: public
binary_function<T*, A, S> {
with
template <class S, class T, class A> class const_mem_fun1_t
: public
binary_function<const T*, A, S> {
Rationale:
This is simply a contradiction: the argument_type
typedef,
and the argument type itself, are not the same.
Section: 17.6.3.3 [new.delete.array] Status: CD1 Submitter: John A. Pedretti Opened: 2001-01-10 Last modified: 2016-01-28
Priority: Not Prioritized
View all other issues in [new.delete.array].
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Discussion:
The default behavior of operator delete[]
described in 18.5.1.2, p12 -
namely that for non-null value of ptr, the operator reclaims storage
allocated by the earlier call to the default operator new[]
- is not
correct in all cases. Since the specified operator new[]
default
behavior is to call operator new
(18.5.1.2, p4, p8), which can be
replaced, along with operator delete
, by the user, to implement their
own memory management, the specified default behavior of operator
delete[]
must be to call operator delete
.
Proposed resolution:
Change 18.5.1.2, p12 from
-12- Default behavior:
- For a null value of
ptr
, does nothing.- Any other value of
ptr
shall be a value returned earlier by a call to the defaultoperator new[](std::size_t)
. [Footnote: The value must not have been invalidated by an intervening call tooperator delete[](void*)
(16.4.5.9 [res.on.arguments]). --- end footnote] For such a non-null value ofptr
, reclaims storage allocated by the earlier call to the defaultoperator new[]
.
to
-12- Default behavior: Calls
operator delete(
ptr) oroperator delete(ptr, std::nothrow)
respectively.
and expunge paragraph 13.
Section: 23.3.11.5 [list.ops] Status: CD1 Submitter: John Pedretti Opened: 2001-01-23 Last modified: 2016-01-28
Priority: Not Prioritized
View all other issues in [list.ops].
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Discussion:
The "Effects" clause for list::merge() (23.3.11.5 [list.ops], p23) appears to be incomplete: it doesn't cover the case where the argument list is identical to *this (i.e., this == &x). The requirement in the note in p24 (below) is that x be empty after the merge which is surely unintended in this case.
Proposed resolution:
In 23.3.11.5 [list.ops], replace paragraps 23-25 with:
23 Effects: if (&x == this) does nothing; otherwise, merges the two sorted ranges [begin(), end()) and [x.begin(), x.end()). The result is a range in which the elements will be sorted in non-decreasing order according to the ordering defined by comp; that is, for every iterator i in the range other than the first, the condition comp(*i, *(i - 1)) will be false.
24 Notes: Stable: if (&x != this), then for equivalent elements in the two original ranges, the elements from the original range [begin(), end()) always precede the elements from the original range [x.begin(), x.end()). If (&x != this) the range [x.begin(), x.end()) is empty after the merge.
25 Complexity: At most size() + x.size() - 1 applications of comp if (&x ! = this); otherwise, no applications of comp are performed. If an exception is thrown other than by a comparison there are no effects.
[Copenhagen: The original proposed resolution did not fix all of
the problems in 23.3.11.5 [list.ops], p22-25. Three different
paragraphs (23, 24, 25) describe the effects of merge
.
Changing p23, without changing the other two, appears to introduce
contradictions. Additionally, "merges the argument list into the
list" is excessively vague.]
[Post-Curaçao: Robert Klarer provided new wording.]
Section: 27.4.3.2 [string.require] Status: CD1 Submitter: Martin Sebor Opened: 2001-01-27 Last modified: 2016-01-28
Priority: Not Prioritized
View all other issues in [string.require].
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Discussion:
The effects clause for the basic_string template ctor in 21.3.1, p15 leaves out the third argument of type Allocator. I believe this to be a mistake.
Proposed resolution:
Replace
-15- Effects: If
InputIterator
is an integral type, equivalent to
basic_string(static_cast<size_type>(begin), static_cast<value_type>(end))
with
-15- Effects: If
InputIterator
is an integral type, equivalent to
basic_string(static_cast<size_type>(begin), static_cast<value_type>(end), a)
Section: 22.9.4 [bitset.operators] Status: CD1 Submitter: Matt Austern Opened: 2001-02-05 Last modified: 2016-01-28
Priority: Not Prioritized
View all other issues in [bitset.operators].
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Discussion:
In 23.3.5.3, we are told that bitset
's input operator
"Extracts up to N (single-byte) characters from
is.", where is is a stream of type
basic_istream<charT, traits>
.
The standard does not say what it means to extract single byte
characters from a stream whose character type, charT
, is in
general not a single-byte character type. Existing implementations
differ.
A reasonable solution will probably involve widen()
and/or
narrow()
, since they are the supplied mechanism for
converting a single character between char
and
arbitrary charT
.
Narrowing the input characters is not the same as widening the
literals '0'
and '1'
, because there may be some
locales in which more than one wide character maps to the narrow
character '0'
. Narrowing means that alternate
representations may be used for bitset input, widening means that
they may not be.
Note that for numeric input, num_get<>
(22.2.2.1.2/8) compares input characters to widened version of narrow
character literals.
From Pete Becker, in c++std-lib-8224:
Different writing systems can have different representations for the digits that represent 0 and 1. For example, in the Unicode representation of the Devanagari script (used in many of the Indic languages) the digit 0 is 0x0966, and the digit 1 is 0x0967. Calling narrow would translate those into '0' and '1'. But Unicode also provides the ASCII values 0x0030 and 0x0031 for for the Latin representations of '0' and '1', as well as code points for the same numeric values in several other scripts (Tamil has no character for 0, but does have the digits 1-9), and any of these values would also be narrowed to '0' and '1'.
...
It's fairly common to intermix both native and Latin representations of numbers in a document. So I think the rule has to be that if a wide character represents a digit whose value is 0 then the bit should be cleared; if it represents a digit whose value is 1 then the bit should be set; otherwise throw an exception. So in a Devanagari locale, both 0x0966 and 0x0030 would clear the bit, and both 0x0967 and 0x0031 would set it. Widen can't do that. It would pick one of those two values, and exclude the other one.
From Jens Maurer, in c++std-lib-8233:
Whatever we decide, I would find it most surprising if bitset conversion worked differently from int conversion with regard to alternate local representations of numbers.
Thus, I think the options are:
- Have a new defect issue for 22.2.2.1.2/8 so that it will require the use of narrow().
- Have a defect issue for bitset() which describes clearly that widen() is to be used.
Proposed resolution:
Replace the first two sentences of paragraph 5 with:
Extracts up to N characters from is. Stores these characters in a temporary object str of type
basic_string<charT, traits>
, then evaluates the expressionx = bitset<N>(str)
.
Replace the third bullet item in paragraph 5 with:
is.widen(0)
nor is.widen(1)
(in which case the input character
is not extracted).
Rationale:
Input for bitset
should work the same way as numeric
input. Using widen
does mean that alternative digit
representations will not be recognized, but this was a known
consequence of the design choice.
Section: 28.3.4.2.6 [locale.codecvt.byname] Status: CD1 Submitter: Howard Hinnant Opened: 2001-01-24 Last modified: 2016-01-28
Priority: Not Prioritized
View all other issues in [locale.codecvt.byname].
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Discussion:
22.2.1.5/3 introduces codecvt in part with:
codecvt<wchar_t,char,mbstate_t> converts between the native character sets for tiny and wide characters. Instantiations on mbstate_t perform conversion between encodings known to the library implementor.
But 22.2.1.5.2/10 describes do_length in part with:
... codecvt<wchar_t, char, mbstate_t> ... return(s) the lesser of max and (from_end-from).
The semantics of do_in and do_length are linked. What one does must be consistent with what the other does. 22.2.1.5/3 leads me to believe that the vendor is allowed to choose the algorithm that codecvt<wchar_t,char,mbstate_t>::do_in performs so that it makes his customers happy on a given platform. But 22.2.1.5.2/10 explicitly says what codecvt<wchar_t,char,mbstate_t>::do_length must return. And thus indirectly specifies the algorithm that codecvt<wchar_t,char,mbstate_t>::do_in must perform. I believe that this is not what was intended and is a defect.
Discussion from the -lib reflector:
This proposal would have the effect of making the semantics of
all of the virtual functions in codecvt<wchar_t, char,
mbstate_t>
implementation specified. Is that what we want, or
do we want to mandate specific behavior for the base class virtuals
and leave the implementation specified behavior for the codecvt_byname
derived class? The tradeoff is that former allows implementors to
write a base class that actually does something useful, while the
latter gives users a way to get known and specified---albeit
useless---behavior, and is consistent with the way the standard
handles other facets. It is not clear what the original intention
was.
Nathan has suggest a compromise: a character that is a widened version of the characters in the basic execution character set must be converted to a one-byte sequence, but there is no such requirement for characters that are not part of the basic execution character set.
Proposed resolution:
Change 22.2.1.5.2/5 from:
The instantiations required in Table 51 (lib.locale.category), namely codecvt<wchar_t,char,mbstate_t> and codecvt<char,char,mbstate_t>, store no characters. Stores no more than (to_limit-to) destination elements. It always leaves the to_next pointer pointing one beyond the last element successfully stored.
to:
Stores no more than (to_limit-to) destination elements, and leaves the to_next pointer pointing one beyond the last element successfully stored. codecvt<char,char,mbstate_t> stores no characters.
Change 22.2.1.5.2/10 from:
-10- Returns: (from_next-from) where from_next is the largest value in the range [from,from_end] such that the sequence of values in the range [from,from_next) represents max or fewer valid complete characters of type internT. The instantiations required in Table 51 (21.1.1.1.1), namely codecvt<wchar_t, char, mbstate_t> and codecvt<char, char, mbstate_t>, return the lesser of max and (from_end-from).
to:
-10- Returns: (from_next-from) where from_next is the largest value in the range [from,from_end] such that the sequence of values in the range [from,from_next) represents max or fewer valid complete characters of type internT. The instantiation codecvt<char, char, mbstate_t> returns the lesser of max and (from_end-from).
[Redmond: Nathan suggested an alternative resolution: same as above, but require that, in the default encoding, a character from the basic execution character set would map to a single external character. The straw poll was 8-1 in favor of the proposed resolution.]
Rationale:
The default encoding should be whatever users of a given platform would expect to be the most natural. This varies from platform to platform. In many cases there is a preexisting C library, and users would expect the default encoding to be whatever C uses in the default "C" locale. We could impose a guarantee like the one Nathan suggested (a character from the basic execution character set must map to a single external character), but this would rule out important encodings that are in common use: it would rule out JIS, for example, and it would rule out a fixed-width encoding of UCS-4.
[Curaçao: fixed rationale typo at the request of Ichiro Koshida; "shift-JIS" changed to "JIS".]
Section: 17.2 [support.types] Status: CD1 Submitter: Steve Clamage Opened: 2001-02-21 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
Spliced together from reflector messages c++std-lib-8294 and -8295:
18.1, paragraph 5, reads: "The macro offsetof
accepts a restricted set of type arguments in this
International Standard. type shall be a POD structure or a POD
union (clause 9). The result of applying the offsetof macro to a field
that is a static data member or a function member is
undefined."
For the POD requirement, it doesn't say "no diagnostic required" or "undefined behavior". I read 4.1 [intro.compliance], paragraph 1, to mean that a diagnostic is required. It's not clear whether this requirement was intended. While it's possible to provide such a diagnostic, the extra complication doesn't seem to add any value.
Proposed resolution:
Change 18.1, paragraph 5, to "If type is not a POD structure or a POD union the results are undefined."
[Copenhagen: straw poll was 7-4 in favor. It was generally agreed that requiring a diagnostic was inadvertent, but some LWG members thought that diagnostics should be required whenever possible.]
Section: 23.3.11 [list] Status: CD1 Submitter: Howard Hinnant Opened: 2001-03-13 Last modified: 2016-01-28
Priority: Not Prioritized
View all issues with CD1 status.
Discussion:
From reflector message c++std-lib-8330. See also lib-8317.
The standard is currently inconsistent in 23.3.11.3 [list.capacity] paragraph 1 and 23.3.11.4 [list.modifiers] paragraph 1. 23.2.3.3/1, for example, says:
-1- Any sequence supporting operations back(), push_back() and pop_back() can be used to instantiate stack. In particular, vector (lib.vector), list (lib.list) and deque (lib.deque) can be used.
But this is false: vector<bool> can not be used, because the container adaptors return a T& rather than using the underlying container's reference type.
This is a contradiction that can be fixed by:
I propose 3. This does not preclude option 2 if we choose to do it later (see issue 96(i)); the issues are independent. Option 3 offers a small step towards support for proxied containers. This small step fixes a current contradiction, is easy for vendors to implement, is already implemented in at least one popular lib, and does not break any code.
Proposed resolution:
Summary: Add reference and const_reference typedefs to queue, priority_queue and stack. Change return types of "value_type&" to "reference". Change return types of "const value_type&" to "const_reference". Details:
Change 23.2.3.1/1 from:
namespace std { template <class T, class Container = deque<T> > class queue { public: typedef typename Container::value_type value_type; typedef typename Container::size_type size_type; typedef Container container_type; protected: Container c; public: explicit queue(const Container& = Container()); bool empty() const { return c.empty(); } size_type size() const { return c.size(); } value_type& front() { return c.front(); } const value_type& front() const { return c.front(); } value_type& back() { return c.back(); } const value_type& back() const { return c.back(); } void push(const value_type& x) { c.push_back(x); } void pop() { c.pop_front(); } };
to:
namespace std { template <class T, class Container = deque<T> > class queue { public: typedef typename Container::value_type value_type; typedef typename Container::reference reference; typedef typename Container::const_reference const_reference; typedef typename Container::value_type value_type; typedef typename Container::size_type size_type; typedef Container container_type; protected: Container c; public: explicit queue(const Container& = Container()); bool empty() const { return c.empty(); } size_type size() const { return c.size(); } reference front() { return c.front(); } const_reference front() const { return c.front(); } reference back() { return c.back(); } const_reference back() const { return c.back(); } void push(const value_type& x) { c.push_back(x); } void pop() { c.pop_front(); } };
Change 23.2.3.2/1 from:
namespace std { template <class T, class Container = vector<T>, class Compare = less<typename Container::value_type> > class priority_queue { public: typedef typename Container::value_type value_type; typedef typename Container::size_type size_type; typedef Container container_type; protected: Container c; Compare comp; public: explicit priority_queue(const Compare& x = Compare(), const Container& = Container()); template <class InputIterator> priority_queue(InputIterator first, InputIterator last, const Compare& x = Compare(), const Container& = Container()); bool empty() const { return c.empty(); } size_type size() const { return c.size(); } const value_type& top() const { return c.front(); } void push(const value_type& x); void pop(); }; // no equality is provided }
to:
namespace std { template <class T, class Container = vector<T>, class Compare = less<typename Container::value_type> > class priority_queue { public: typedef typename Container::value_type value_type; typedef typename Container::reference reference; typedef typename Container::const_reference const_reference; typedef typename Container::size_type size_type; typedef Container container_type; protected: Container c; Compare comp; public: explicit priority_queue(const Compare& x = Compare(), const Container& = Container()); template <class InputIterator> priority_queue(InputIterator first, InputIterator last, const Compare& x = Compare(), const Container& = Container()); bool empty() const { return c.empty(); } size_type size() const { return c.size(); } const_reference top() const { return c.front(); } void push(const value_type& x); void pop(); }; // no equality is provided }
And change 23.2.3.3/1 from:
namespace std { template <class T, class Container = deque<T> > class stack { public: typedef typename Container::value_type value_type; typedef typename Container::size_type size_type; typedef Container container_type; protected: Container c; public: explicit stack(const Container& = Container()); bool empty() const { return c.empty(); } size_type size() const { return c.size(); } value_type& top() { return c.back(); } const value_type& top() const { return c.back(); } void push(const value_type& x) { c.push_back(x); } void pop() { c.pop_back(); } }; template <class T, class Container> bool operator==(const stack<T, Container>& x, const stack<T, Container>& y); template <class T, class Container> bool operator< (const stack<T, Container>& x, const stack<T, Container>& y); template <class T, class Container> bool operator!=(const stack<T, Container>& x, const stack<T, Container>& y); template <class T, class Container> bool operator> (const stack<T, Container>& x, const stack<T, Container>& y); template <class T, class Container> bool operator>=(const stack<T, Container>& x, const stack<T, Container>& y); template <class T, class Container> bool operator<=(const stack<T, Container>& x, const stack<T, Container>& y); }
to:
namespace std { template <class T, class Container = deque<T> > class stack { public: typedef typename Container::value_type value_type; typedef typename Container::reference reference; typedef typename Container::const_reference const_reference; typedef typename Container::size_type size_type; typedef Container container_type; protected: Container c; public: explicit stack(const Container& = Container()); bool empty() const { return c.empty(); } size_type size() const { return c.size(); } reference top() { return c.back(); } const_reference top() const { return c.back(); } void push(const value_type& x) { c.push_back(x); } void pop() { c.pop_back(); } }; template <class T, class Container> bool operator==(const stack<T, Container>& x, const stack<T, Container>& y); template <class T, class Container> bool operator< (const stack<T, Container>& x, const stack<T, Container>& y); template <class T, class Container> bool operator!=(const stack<T, Container>& x, const stack<T, Container>& y); template <class T, class Container> bool operator> (const stack<T, Container>& x, const stack<T, Container>& y); template <class T, class Container> bool operator>=(const stack<T, Container>& x, const stack<T, Container>& y); template <class T, class Container> bool operator<=(const stack<T, Container>& x, const stack<T, Container>& y); }
[Copenhagen: This change was discussed before the IS was released and it was deliberately not adopted. Nevertheless, the LWG believes (straw poll: 10-2) that it is a genuine defect.]
Section: 31 [input.output] Status: CD1 Submitter: Martin Sebor Opened: 2001-03-15 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
Table 82 in section 27 mentions the header <cstdlib> for String streams (31.8 [string.streams]) and the headers <cstdio> and <cwchar> for File streams (31.10 [file.streams]). It's not clear why these headers are mentioned in this context since they do not define any of the library entities described by the subclauses. According to 16.4.2.2 [contents], only such headers are to be listed in the summary.
Proposed resolution:
Remove <cstdlib> and <cwchar> from Table 82.
[Copenhagen: changed the proposed resolution slightly. The original proposed resolution also said to remove <cstdio> from Table 82. However, <cstdio> is mentioned several times within section 31.10 [file.streams], including 31.13 [c.files].]
Section: 16.4.2.3 [headers], 19.4 [errno] Status: CD1 Submitter: Steve Clamage Opened: 2001-03-21 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
Exactly how should errno be declared in a conforming C++ header?
The C standard says in 7.1.4 that it is unspecified whether errno is a macro or an identifier with external linkage. In some implementations it can be either, depending on compile-time options. (E.g., on Solaris in multi-threading mode, errno is a macro that expands to a function call, but is an extern int otherwise. "Unspecified" allows such variability.)
The C++ standard:
I find no other references to errno.
We should either explicitly say that errno must be a macro, even
though it need not be a macro in C, or else explicitly leave it
unspecified. We also need to say something about namespace std.
A user who includes <cerrno> needs to know whether to write
errno
, or ::errno
, or std::errno
, or
else <cerrno> is useless.
Two acceptable fixes:
errno must be a macro. This is trivially satisfied by adding
#define errno (::std::errno)
to the headers if errno is not already a macro. You then always
write errno without any scope qualification, and it always expands
to a correct reference. Since it is always a macro, you know to
avoid using errno as a local identifer.
errno is in the global namespace. This fix is inferior, because ::errno is not guaranteed to be well-formed.
[ This issue was first raised in 1999, but it slipped through the cracks. ]
Proposed resolution:
Change the Note in section 17.4.1.2p5 from
Note: the names defined as macros in C include the following: assert, errno, offsetof, setjmp, va_arg, va_end, and va_start.
to
Note: the names defined as macros in C include the following: assert, offsetof, setjmp, va_arg, va_end, and va_start.
In section 19.3, change paragraph 2 from
The contents are the same as the Standard C library header <errno.h>.
to
The contents are the same as the Standard C library header <errno.h>, except that errno shall be defined as a macro.
Rationale:
C++ must not leave it up to the implementation to decide whether or not a name is a macro; it must explicitly specify exactly which names are required to be macros. The only one that really works is for it to be a macro.
[Curaçao: additional rationale added.]
Section: 31.7.6.2 [ostream] Status: CD1 Submitter: Andy Sawyer Opened: 2001-03-21 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
In 31.7.6.2 [ostream], the synopsis of class basic_ostream says:
// partial specializationss template<class traits> basic_ostream<char,traits>& operator<<( basic_ostream<char,traits>&, const char * );
Problems:
Proposed resolution:
In the synopsis in 31.7.6.2 [ostream], remove the // partial specializationss comment. Also remove the same comment (correctly spelled, but still incorrect) from the synopsis in 31.7.6.3.4 [ostream.inserters.character].
[ Pre-Redmond: added 31.7.6.3.4 [ostream.inserters.character] because of Martin's comment in c++std-lib-8939. ]
Section: 22 [utilities] Status: CD1 Submitter: Martin Sebor Opened: 2001-03-29 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
Table 27 in section 20 lists the header <memory> (only) for Memory (lib.memory) but neglects to mention the headers <cstdlib> and <cstring> that are discussed in 21.3.7 [meta.rel].
Proposed resolution:
Add <cstdlib> and <cstring> to Table 27, in the same row as <memory>.
Section: 23.3.11.5 [list.ops] Status: CD1 Submitter: Andy Sawyer Opened: 2001-05-01 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
23.3.11.5 [list.ops], Para 21 describes the complexity of list::unique as: "If the range (last - first) is not empty, exactly (last - first) -1 applications of the corresponding predicate, otherwise no applications of the predicate)".
"(last - first)" is not a range.
Proposed resolution:
Change the "range" from (last - first) to [first, last).
Section: 23.2.7 [associative.reqmts] Status: CD1 Submitter: Martin Sebor Opened: 2001-05-04 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
Table 69 says this about a_uniq.insert(t):
inserts t if and only if there is no element in the container with key equivalent to the key of t. The bool component of the returned pair indicates whether the insertion takes place and the iterator component of the pair points to the element with key equivalent to the key of t.
The description should be more specific about exactly how the bool component indicates whether the insertion takes place.
Proposed resolution:
Change the text in question to
...The bool component of the returned pair is true if and only if the insertion takes place...
Section: 28.3 [localization] Status: CD1 Submitter: Martin Sebor Opened: 2001-05-04 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
The localization section of the standard refers to specializations of the facet templates as instantiations even though the required facets are typically specialized rather than explicitly (or implicitly) instantiated. In the case of ctype<char> and ctype_byname<char> (and the wchar_t versions), these facets are actually required to be specialized. The terminology should be corrected to make it clear that the standard doesn't mandate explicit instantiation (the term specialization encompasses both explicit instantiations and specializations).
Proposed resolution:
In the following paragraphs, replace all occurrences of the word instantiation or instantiations with specialization or specializations, respectively:
22.1.1.1.1, p4, Table 52, 22.2.1.1, p2, 22.2.1.5, p3, 22.2.1.5.1, p5, 22.2.1.5.2, p10, 22.2.2, p2, 22.2.3.1, p1, 22.2.3.1.2, p1, p2 and p3, 22.2.4.1, p1, 22.2.4.1.2, p1, 22,2,5, p1, 22,2,6, p2, 22.2.6.3.2, p7, and Footnote 242.
And change the text in 22.1.1.1.1, p4 from
An implementation is required to provide those instantiations for facet templates identified as members of a category, and for those shown in Table 52:
to
An implementation is required to provide those specializations...
[Nathan will review these changes, and will look for places where explicit specialization is necessary.]
Rationale:
This is a simple matter of outdated language. The language to describe templates was clarified during the standardization process, but the wording in clause 22 was never updated to reflect that change.
Section: 28.3.4.4.2 [locale.numpunct.byname] Status: CD1 Submitter: Martin Sebor Opened: 2001-05-12 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
The definition of the numpunct_byname template contains the following comment:
namespace std { template <class charT> class numpunct_byname : public numpunct<charT> { // this class is specialized for char and wchar_t. ...
There is no documentation of the specializations and it seems conceivable that an implementation will not explicitly specialize the template at all, but simply provide the primary template.
Proposed resolution:
Remove the comment from the text in 22.2.3.2 and from the proposed resolution of library issue 228(i).
Section: 17.6.3.2 [new.delete.single], 17.6.3.3 [new.delete.array] Status: CD1 Submitter: Beman Dawes Opened: 2001-05-15 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
The standard specifies 16.3.2.4 [structure.specifications] that "Required behavior" elements describe "the semantics of a function definition provided by either the implementation or a C++ program."
The standard specifies 16.3.2.4 [structure.specifications] that "Requires" elements describe "the preconditions for calling the function."
In the sections noted below, the current wording specifies "Required Behavior" for what are actually preconditions, and thus should be specified as "Requires".
Proposed resolution:
In 17.6.3.2 [new.delete.single] Para 12 Change:
Required behavior: accept a value of ptr that is null or that was returned by an earlier call ...
to:
Requires: the value of ptr is null or the value returned by an earlier call ...
In 17.6.3.3 [new.delete.array] Para 11 Change:
Required behavior: accept a value of ptr that is null or that was returned by an earlier call ...
to:
Requires: the value of ptr is null or the value returned by an earlier call ...
Section: 23.3.11.2 [list.cons] Status: CD1 Submitter: Howard Hinnant Opened: 2001-05-17 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
Section 23.3.11.2 [list.cons], paragraphs 6-8 specify that list assign (both forms) have the "effects" of a call to erase followed by a call to insert.
I would like to document that implementers have the freedom to implement assign by other methods, as long as the end result is the same and the exception guarantee is as good or better than the basic guarantee.
The motivation for this is to use T's assignment operator to recycle existing nodes in the list instead of erasing them and reallocating them with new values. It is also worth noting that, with careful coding, most common cases of assign (everything but assignment with true input iterators) can elevate the exception safety to strong if T's assignment has a nothrow guarantee (with no extra memory cost). Metrowerks does this. However I do not propose that this subtlety be standardized. It is a QoI issue.
Existing practise: Metrowerks and SGI recycle nodes, Dinkumware and Rogue Wave don't.
Proposed resolution:
Change 23.3.11.2 [list.cons]/7 from:
Effects:
erase(begin(), end()); insert(begin(), first, last);
to:
Effects: Replaces the contents of the list with the range [first, last).
In 23.2.4 [sequence.reqmts], in Table 67 (sequence requirements), add two new rows:
a.assign(i,j) void pre: i,j are not iterators into a. Replaces elements in a with a copy of [i, j). a.assign(n,t) void pre: t is not a reference into a. Replaces elements in a with n copies of t.
Change 23.3.11.2 [list.cons]/8 from:
Effects:
erase(begin(), end()); insert(begin(), n, t);
to:
Effects: Replaces the contents of the list with n copies of t.
[Redmond: Proposed resolution was changed slightly. Previous version made explicit statement about exception safety, which wasn't consistent with the way exception safety is expressed elsewhere. Also, the change in the sequence requirements is new. Without that change, the proposed resolution would have required that assignment of a subrange would have to work. That too would have been overspecification; it would effectively mandate that assignment use a temporary. Howard provided wording. ]
[Curaçao: Made editorial improvement in wording; changed "Replaces elements in a with copies of elements in [i, j)." with "Replaces the elements of a with a copy of [i, j)." Changes not deemed serious enough to requre rereview.]
Section: 28.3.4.3.2.3 [facet.num.get.virtuals] Status: CD1 Submitter: Kevin Djang Opened: 2001-05-17 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
Section 22.2.2.1.2 at p7 states that "A length specifier is added to the conversion function, if needed, as indicated in Table 56." However, Table 56 uses the term "length modifier", not "length specifier".
Proposed resolution:
In 22.2.2.1.2 at p7, change the text "A length specifier is added ..." to be "A length modifier is added ..."
Rationale:
C uses the term "length modifier". We should be consistent.
Section: 23.2 [container.requirements] Status: CD1 Submitter: Matt Austern Opened: 2001-05-17 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
It's widely assumed that, if X is a container, iterator_traits<X::iterator>::value_type and iterator_traits<X::const_iterator>::value_type should both be X::value_type. However, this is nowhere stated. The language in Table 65 is not precise about the iterators' value types (it predates iterator_traits), and could even be interpreted as saying that iterator_traits<X::const_iterator>::value_type should be "const X::value_type".
Proposed resolution:
In Table 65 ("Container Requirements"), change the return type for X::iterator to "iterator type whose value type is T". Change the return type for X::const_iterator to "constant iterator type whose value type is T".
Rationale:
This belongs as a container requirement, rather than an iterator requirement, because the whole notion of iterator/const_iterator pairs is specific to containers' iterator.
It is existing practice that (for example) iterator_traits<list<int>::const_iterator>::value_type is "int", rather than "const int". This is consistent with the way that const pointers are handled: the standard already requires that iterator_traits<const int*>::value_type is int.
Section: 24.3.5.4 [output.iterators] Status: CD1 Submitter: Dave Abrahams Opened: 2001-06-07 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
Table 73 suggests that output iterators have value types. It requires the expression "*a = t". Additionally, although Table 73 never lists "a = t" or "X(a) = t" in the "expressions" column, it contains a note saying that "a = t" and "X(a) = t" have equivalent (but nowhere specified!) semantics.
According to 24.1/9, t is supposed to be "a value of value type T":
In the following sections, a and b denote values of X, n denotes a value of the difference type Distance, u, tmp, and m denote identifiers, r denotes a value of X&, t denotes a value of value type T.
Two other parts of the standard that are relevant to whether output iterators have value types:
The first of these passages suggests that "*i" is supposed to return a useful value, which contradicts the note in 24.1.2/2 saying that the only valid use of "*i" for output iterators is in an expression of the form "*i = t". The second of these passages appears to contradict Table 73, because it suggests that "*i"'s return value should be void. The second passage is also broken in the case of a an iterator type, like non-const pointers, that satisfies both the output iterator requirements and the forward iterator requirements.
What should the standard say about *i
's return value when
i is an output iterator, and what should it say about that t is in the
expression "*i = t"? Finally, should the standard say anything about
output iterators' pointer and reference types?
Proposed resolution:
24.1 p1, change
All iterators
i
support the expression*i
, resulting in a value of some class, enumeration, or built-in typeT
, called the value type of the iterator.
to
All input iterators
i
support the expression*i
, resulting in a value of some class, enumeration, or built-in typeT
, called the value type of the iterator. All output iterators support the expression*i = o
whereo
is a value of some type that is in the set of types that are writable to the particular iterator type ofi
.
24.1 p9, add
o
denotes a value of some type that is writable to the output iterator.
Table 73, change
*a = t
to
*r = o
and change
*r++ = t
to
*r++ = o
[post-Redmond: Jeremy provided wording]
Rationale:
The LWG considered two options: change all of the language that seems to imply that output iterators have value types, thus making it clear that output iterators have no value types, or else define value types for output iterator consistently. The LWG chose the former option, because it seems clear that output iterators were never intended to have value types. This was a deliberate design decision, and any language suggesting otherwise is simply a mistake.
A future revision of the standard may wish to revisit this design decision.
Section: 28.3.4.7.4.3 [locale.moneypunct.virtuals] Status: CD1 Submitter: Martin Sebor Opened: 2001-07-02 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
The Returns clause in 22.2.6.3.2, p3 says about moneypunct<charT>::do_grouping()
Returns: A pattern defined identically as the result of numpunct<charT>::do_grouping().241)
Footnote 241 then reads
This is most commonly the value "\003" (not "3").
The returns clause seems to imply that the two member functions must return an identical value which in reality may or may not be true, since the facets are usually implemented in terms of struct std::lconv and return the value of the grouping and mon_grouping, respectively. The footnote also implies that the member function of the moneypunct facet (rather than the overridden virtual functions in moneypunct_byname) most commonly return "\003", which contradicts the C standard which specifies the value of "" for the (most common) C locale.
Proposed resolution:
Replace the text in Returns clause in 22.2.6.3.2, p3 with the following:
Returns: A pattern defined identically as, but not necessarily equal to, the result of numpunct<charT>::do_grouping().241)
and replace the text in Footnote 241 with the following:
To specify grouping by 3s the value is "\003", not "3".
Rationale:
The fundamental problem is that the description of the locale facet virtuals serves two purposes: describing the behavior of the base class, and describing the meaning of and constraints on the behavior in arbitrary derived classes. The new wording makes that separation a little bit clearer. The footnote (which is nonnormative) is not supposed to say what the grouping is in the "C" locale or in any other locale. It is just a reminder that the values are interpreted as small integers, not ASCII characters.
Section: 28.3.3.1.2.1 [locale.category] Status: CD1 Submitter: Tiki Wan Opened: 2001-07-06 Last modified: 2016-01-28
Priority: Not Prioritized
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Duplicate of: 447
Discussion:
The wchar_t
versions of time_get
and
time_get_byname
are listed incorrectly in table 52,
required instantiations. In both cases the second template
parameter is given as OutputIterator. It should instead be
InputIterator, since these are input facets.
Proposed resolution:
In table 52, required instantiations, in 28.3.3.1.2.1 [locale.category], change
time_get<wchar_t, OutputIterator> time_get_byname<wchar_t, OutputIterator>
to
time_get<wchar_t, InputIterator> time_get_byname<wchar_t, InputIterator>
[Redmond: Very minor change in proposed resolution. Original had a typo, wchart instead of wchar_t.]
Section: 28.3.4.7.3.3 [locale.money.put.virtuals] Status: CD1 Submitter: Martin Sebor Opened: 2001-07-07 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
The sprintf format string , "%.01f" (that's the digit one), in the description of the do_put() member functions of the money_put facet in 22.2.6.2.2, p1 is incorrect. First, the f format specifier is wrong for values of type long double, and second, the precision of 01 doesn't seem to make sense. What was most likely intended was "%.0Lf"., that is a precision of zero followed by the L length modifier.
Proposed resolution:
Change the format string to "%.0Lf".
Rationale:
Fixes an obvious typo
Section: 23.3.13.3 [vector.capacity], 23.3.13.5 [vector.modifiers] Status: CD1 Submitter: Anthony Williams Opened: 2001-07-13 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
There is an apparent contradiction about which circumstances can cause a reallocation of a vector in Section 23.3.13.3 [vector.capacity] and section 23.3.13.5 [vector.modifiers].
23.3.13.3 [vector.capacity],p5 says:
Notes: Reallocation invalidates all the references, pointers, and iterators referring to the elements in the sequence. It is guaranteed that no reallocation takes place during insertions that happen after a call to reserve() until the time when an insertion would make the size of the vector greater than the size specified in the most recent call to reserve().
Which implies if I do
std::vector<int> vec; vec.reserve(23); vec.reserve(0); vec.insert(vec.end(),1);
then the implementation may reallocate the vector for the insert, as the size specified in the previous call to reserve was zero.
However, the previous paragraphs (23.3.13.3 [vector.capacity], p1-2) state:
(capacity) Returns: The total number of elements the vector can hold without requiring reallocation
...After reserve(), capacity() is greater or equal to the argument of reserve if reallocation happens; and equal to the previous value of capacity() otherwise...
This implies that vec.capacity() is still 23, and so the insert() should not require a reallocation, as vec.size() is 0. This is backed up by 23.3.13.5 [vector.modifiers], p1:
(insert) Notes: Causes reallocation if the new size is greater than the old capacity.
Though this doesn't rule out reallocation if the new size is less than the old capacity, I think the intent is clear.
Proposed resolution:
Change the wording of 23.3.13.3 [vector.capacity] paragraph 5 to:
Notes: Reallocation invalidates all the references, pointers, and iterators referring to the elements in the sequence. It is guaranteed that no reallocation takes place during insertions that happen after a call to reserve() until the time when an insertion would make the size of the vector greater than the value of capacity().
[Redmond: original proposed resolution was modified slightly. In the original, the guarantee was that there would be no reallocation until the size would be greater than the value of capacity() after the most recent call to reserve(). The LWG did not believe that the "after the most recent call to reserve()" added any useful information.]
Rationale:
There was general agreement that, when reserve() is called twice in succession and the argument to the second invocation is smaller than the argument to the first, the intent was for the second invocation to have no effect. Wording implying that such cases have an effect on reallocation guarantees was inadvertant.
Section: 31.5.2.2.1 [ios.failure] Status: CD1 Submitter: PremAnand M. Rao Opened: 2001-08-23 Last modified: 2021-06-06
Priority: Not Prioritized
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Discussion:
With the change in 16.4.6.14 [res.on.exception.handling] to state "An implementation may strengthen the exception-specification for a non-virtual function by removing listed exceptions." (issue 119(i)) and the following declaration of ~failure() in ios_base::failure
namespace std { class ios_base::failure : public exception { public: ... virtual ~failure(); ... }; }
the class failure cannot be implemented since in 17.7.3 [type.info] the destructor of class exception has an empty exception specification:
namespace std { class exception { public: ... virtual ~exception() throw(); ... }; }
Proposed resolution:
Remove the declaration of ~failure().
Rationale:
The proposed resolution is consistent with the way that destructors
of other classes derived from exception
are handled.
Section: 31.7.6.5 [ostream.manip] Status: CD1 Submitter: PremAnand M. Rao Opened: 2001-08-27 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
A footnote in 31.7.6.5 [ostream.manip] states:
[Footnote: The effect of executing cout << endl is to insert a newline character in the output sequence controlled by cout, then synchronize it with any external file with which it might be associated. --- end foonote]
Does the term "file" here refer to the external device? This leads to some implementation ambiguity on systems with fully buffered files where a newline does not cause a flush to the device.
Choosing to sync with the device leads to significant performance penalties for each call to endl, while not sync-ing leads to errors under special circumstances.
I could not find any other statement that explicitly defined the behavior one way or the other.
Proposed resolution:
Remove footnote 300 from section 31.7.6.5 [ostream.manip].
Rationale:
We already have normative text saying what endl
does: it
inserts a newline character and calls flush
. This footnote
is at best redundant, at worst (as this issue says) misleading,
because it appears to make promises about what flush
does.
Section: 23.4.3.3 [map.access] Status: CD1 Submitter: Andrea Griffini Opened: 2001-09-02 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
The current standard describes map::operator[] using a code example. That code example is however quite inefficient because it requires several useless copies of both the passed key_type value and of default constructed mapped_type instances. My opinion is that was not meant by the comitee to require all those temporary copies.
Currently map::operator[] behaviour is specified as:
Returns: (*((insert(make_pair(x, T()))).first)).second.
This specification however uses make_pair that is a template function of which parameters in this case will be deduced being of type const key_type& and const T&. This will create a pair<key_type,T> that isn't the correct type expected by map::insert so another copy will be required using the template conversion constructor available in pair to build the required pair<const key_type,T> instance.
If we consider calling of key_type copy constructor and mapped_type default constructor and copy constructor as observable behaviour (as I think we should) then the standard is in this place requiring two copies of a key_type element plus a default construction and two copy construction of a mapped_type (supposing the addressed element is already present in the map; otherwise at least another copy construction for each type).
A simple (half) solution would be replacing the description with:
Returns: (*((insert(value_type(x, T()))).first)).second.
This will remove the wrong typed pair construction that requires one extra copy of both key and value.
However still the using of map::insert requires temporary objects while the operation, from a logical point of view, doesn't require any.
I think that a better solution would be leaving free an implementer to use a different approach than map::insert that, because of its interface, forces default constructed temporaries and copies in this case. The best solution in my opinion would be just requiring map::operator[] to return a reference to the mapped_type part of the contained element creating a default element with the specified key if no such an element is already present in the container. Also a logarithmic complexity requirement should be specified for the operation.
This would allow library implementers to write alternative implementations not using map::insert and reaching optimal performance in both cases of the addressed element being present or absent from the map (no temporaries at all and just the creation of a new pair inside the container if the element isn't present). Some implementer has already taken this option but I think that the current wording of the standard rules that as non-conforming.
Proposed resolution:
Replace 23.4.3.3 [map.access] paragraph 1 with
-1- Effects: If there is no key equivalent to x in the map, inserts value_type(x, T()) into the map.
-2- Returns: A reference to the mapped_type corresponding to x in *this.
-3- Complexity: logarithmic.
[This is the second option mentioned above. Howard provided wording. We may also wish to have a blanket statement somewhere in clause 17 saying that we do not intend the semantics of sample code fragments to be interpreted as specifing exactly how many copies are made. See issue 98(i) for a similar problem.]
Rationale:
This is the second solution described above; as noted, it is consistent with existing practice.
Note that we now need to specify the complexity explicitly, because
we are no longer defining operator[]
in terms of
insert
.
Section: 27.2.2 [char.traits.require] Status: CD1 Submitter: Andy Sawyer Opened: 2001-09-06 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
Table 37, in 27.2.2 [char.traits.require], descibes char_traits::assign as:
X::assign(c,d) assigns c = d.
And para 1 says:
[...] c and d denote values of type CharT [...]
Naturally, if c and d are values, then the assignment is (effectively) meaningless. It's clearly intended that (in the case of assign, at least), 'c' is intended to be a reference type.
I did a quick survey of the four implementations I happened to have lying around, and sure enough they all have signatures:
assign( charT&, const charT& );
(or the equivalent). It's also described this way in Nico's book. (Not to mention the synopses of char_traits<char> in 21.1.3.1 and char_traits<wchar_t> in 21.1.3.2...)
Proposed resolution:
Add the following to 21.1.1 para 1:
r denotes an lvalue of CharT
and change the description of assign in the table to:
X::assign(r,d) assigns r = d
Section: 16 [library] Status: CD1 Submitter: Detlef Vollmann Opened: 2001-09-05 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
From c++std-edit-873:
16.4.2.3 [headers], Table 11. In this table, the header <strstream> is missing.
This shows a general problem: The whole clause 17 refers quite often to clauses 18 through 27, but D.7 is also a part of the standard library (though a deprecated one).
Proposed resolution:
To 16.4.2.3 [headers] Table 11, C++ Library Headers, add "<strstream>".
In the following places, change "clauses 17 through 27" to "clauses 17 through 27 and Annex D":
Section: 26.7.5 [alg.replace] Status: CD1 Submitter: Detlef Vollmann Opened: 2001-09-07 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
From c++std-edit-876:
In section 26.7.5 [alg.replace] before p4: The name of the first parameter of template replace_copy_if should be "InputIterator" instead of "Iterator". According to 16.3.3.3 [type.descriptions] p1 the parameter name conveys real normative meaning.
Proposed resolution:
Change Iterator
to InputIterator
.
Section: 28.3.4 [locale.categories] Status: CD1 Submitter: Martin Sebor Opened: 2001-09-17 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
From Stage 2 processing in 28.3.4.3.2.3 [facet.num.get.virtuals], p8 and 9 (the original text or the text corrected by the proposed resolution of issue 221(i)) it seems clear that no whitespace is allowed within a number, but 28.3.4.4.1 [locale.numpunct], p2, which gives the format for integer and floating point values, says that whitespace is optional between a plusminus and a sign.
The text needs to be clarified to either consistently allow or disallow whitespace between a plusminus and a sign. It might be worthwhile to consider the fact that the C library stdio facility does not permit whitespace embedded in numbers and neither does the C or C++ core language (the syntax of integer-literals is given in 5.13.2 [lex.icon], that of floating-point-literals in 5.13.4 [lex.fcon] of the C++ standard).
Proposed resolution:
Change the first part of 28.3.4.4.1 [locale.numpunct] paragraph 2 from:
The syntax for number formats is as follows, where
digit
represents the radix set specified by thefmtflags
argument value,whitespace
is as determined by the facetctype<charT>
(22.2.1.1), andthousands-sep
anddecimal-point
are the results of correspondingnumpunct<charT>
members. Integer values have the format:integer ::= [sign] units sign ::= plusminus [whitespace] plusminus ::= '+' | '-' units ::= digits [thousands-sep units] digits ::= digit [digits]
to:
The syntax for number formats is as follows, where
digit
represents the radix set specified by thefmtflags
argument value, andthousands-sep
anddecimal-point
are the results of correspondingnumpunct<charT>
members. Integer values have the format:integer ::= [sign] units sign ::= plusminus plusminus ::= '+' | '-' units ::= digits [thousands-sep units] digits ::= digit [digits]
Rationale:
It's not clear whether the format described in 28.3.4.4.1 [locale.numpunct] paragraph 2 has any normative weight: nothing in the standard says how, or whether, it's used. However, there's no reason for it to differ gratuitously from the very specific description of numeric processing in 28.3.4.3.2.3 [facet.num.get.virtuals]. The proposed resolution removes all mention of "whitespace" from that format.
Section: 28.3.4.2 [category.ctype], 16.3.3.3.3 [bitmask.types] Status: CD1 Submitter: Martin Sebor Opened: 2001-09-17 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
The ctype_category::mask type is declared to be an enum in 28.3.4.2 [category.ctype] with p1 then stating that it is a bitmask type, most likely referring to the definition of bitmask type in 16.3.3.3.3 [bitmask.types], p1. However, the said definition only applies to clause 27, making the reference in 22.2.1 somewhat dubious.
Proposed resolution:
Clarify 17.3.2.1.2, p1 by changing the current text from
Several types defined in clause 27 are bitmask types. Each bitmask type can be implemented as an enumerated type that overloads certain operators, as an integer type, or as a bitset (22.9.2 [template.bitset]).
to read
Several types defined in clauses lib.language.support through lib.input.output and Annex D are bitmask types. Each bitmask type can be implemented as an enumerated type that overloads certain operators, as an integer type, or as a bitset (lib.template.bitset).
Additionally, change the definition in 22.2.1 to adopt the same convention as in clause 27 by replacing the existing text with the following (note, in particluar, the cross-reference to 17.3.2.1.2 in 22.2.1, p1):
22.2.1 The ctype category [lib.category.ctype]
namespace std { class ctype_base { public: typedef T mask; // numeric values are for exposition only. static const mask space = 1 << 0; static const mask print = 1 << 1; static const mask cntrl = 1 << 2; static const mask upper = 1 << 3; static const mask lower = 1 << 4; static const mask alpha = 1 << 5; static const mask digit = 1 << 6; static const mask punct = 1 << 7; static const mask xdigit = 1 << 8; static const mask alnum = alpha | digit; static const mask graph = alnum | punct; }; }The type
mask
is a bitmask type (16.3.3.3.3 [bitmask.types]).
[Curaçao: The LWG notes that T above should be bold-italics to be consistent with the rest of the standard.]
has_facet<Facet>(loc)
Section: 28.3.3.1.2.1 [locale.category] Status: CD1 Submitter: Martin Sebor Opened: 2001-09-18 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
It's unclear whether 22.1.1.1.1, p3 says that
has_facet<Facet>(loc)
returns true for any Facet
from Table 51 or whether it includes Table 52 as well:
For any locale
loc
either constructed, or returned by locale::classic(), and any facetFacet
that is a member of a standard category,has_facet<Facet>(loc)
is true. Each locale member function which takes alocale::category
argument operates on the corresponding set of facets.
It seems that it comes down to which facets are considered to be members of a standard category. Intuitively, I would classify all the facets in Table 52 as members of their respective standard categories, but there are an unbounded set of them...
The paragraph implies that, for instance, has_facet<num_put<C,
OutputIterator> >(loc)
must always return true. I don't think that's
possible. If it were, then use_facet<num_put<C, OutputIterator>
>(loc)
would have to return a reference to a distinct object for each
valid specialization of num_put<C, OutputIteratory>
, which is
clearly impossible.
On the other hand, if none of the facets in Table 52 is a member of a standard category then none of the locale member functions that operate on entire categories of facets will work properly.
It seems that what p3 should mention that it's required (permitted?)
to hold only for specializations of Facet
from Table 52 on
C
from the set { char
, wchar_t
}, and
InputIterator
and OutputIterator
from the set of
{
{i,o}streambuf_iterator
<{char
,wchar_t
}>
}.
Proposed resolution:
In 28.3.3.1.2.1 [locale.category], paragraph 3, change "that is a member of a standard category" to "shown in Table 51".
Rationale:
The facets in Table 52 are an unbounded set. Locales should not be required to contain an infinite number of facets.
It's not necessary to talk about which values of InputIterator and OutputIterator must be supported. Table 51 already contains a complete list of the ones we need.
Section: 23.3.13.3 [vector.capacity] Status: CD1 Submitter: Anthony Williams Opened: 2001-09-27 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
It is a common idiom to reduce the capacity of a vector by swapping it with an empty one:
std::vector<SomeType> vec; // fill vec with data std::vector<SomeType>().swap(vec); // vec is now empty, with minimal capacity
However, the wording of 23.3.13.3 [vector.capacity]paragraph 5 prevents the capacity of a vector being reduced, following a call to reserve(). This invalidates the idiom, as swap() is thus prevented from reducing the capacity. The proposed wording for issue 329(i) does not affect this. Consequently, the example above requires the temporary to be expanded to cater for the contents of vec, and the contents be copied across. This is a linear-time operation.
However, the container requirements state that swap must have constant complexity (23.2 [container.requirements] note to table 65).
This is an important issue, as reallocation affects the validity of references and iterators.
If the wording of 23.2.4.2p5 is taken to be the desired intent, then references and iterators remain valid after a call to swap, if they refer to an element before the new end() of the vector into which they originally pointed, in which case they refer to the element at the same index position. Iterators and references that referred to an element whose index position was beyond the new end of the vector are invalidated.
If the note to table 65 is taken as the desired intent, then there are two possibilities with regard to iterators and references:
Proposed resolution:
Add a new paragraph after 23.3.13.3 [vector.capacity] paragraph 5:
void swap(vector<T,Allocator>& x);Effects: Exchanges the contents and capacity() of
*this
with that ofx
.Complexity: Constant time.
[This solves the problem reported for this issue. We may also have a problem with a circular definition of swap() for other containers.]
Rationale:
swap should be constant time. The clear intent is that it should just do pointer twiddling, and that it should exchange all properties of the two vectors, including their reallocation guarantees.
Section: 16 [library] Status: Resolved Submitter: Martin Sebor Opened: 2001-10-09 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
The synopses of the C++ library headers clearly show which names are required to be defined in each header. Since in order to implement the classes and templates defined in these headers declarations of other templates (but not necessarily their definitions) are typically necessary the standard in 17.4.4, p1 permits library implementers to include any headers needed to implement the definitions in each header.
For instance, although it is not explicitly specified in the synopsis of
<string>
, at the point of definition of the std::basic_string
template
the declaration of the std::allocator
template must be in scope. All
current implementations simply include <memory>
from within <string>
,
either directly or indirectly, to bring the declaration of
std::allocator
into scope.
Additionally, however, some implementation also include <istream>
and
<ostream>
at the top of <string>
to bring the declarations of
std::basic_istream
and std::basic_ostream
into scope (which are needed
in order to implement the string inserter and extractor operators
(21.3.7.9 [lib.string.io])). Other implementations only include
<iosfwd>
, since strictly speaking, only the declarations and not the
full definitions are necessary.
Obviously, it is possible to implement <string>
without actually
providing the full definitions of all the templates std::basic_string
uses (std::allocator
, std::basic_istream
, and std::basic_ostream
).
Furthermore, not only is it possible, doing so is likely to have a
positive effect on compile-time efficiency.
But while it may seem perfectly reasonable to expect a program that uses
the std::basic_string
insertion and extraction operators to also
explicitly include <istream>
or <ostream>
, respectively, it doesn't seem
reasonable to also expect it to explicitly include <memory>
. Since
what's reasonable and what isn't is highly subjective one would expect
the standard to specify what can and what cannot be assumed.
Unfortunately, that isn't the case.
The examples below demonstrate the issue.
Example 1:
It is not clear whether the following program is complete:
#include <string> extern std::basic_ostream<char> &strm; int main () { strm << std::string ("Hello, World!\n"); }
or whether one must explicitly include <memory>
or
<ostream>
(or both) in addition to <string>
in order for
the program to compile.
Example 2:
Similarly, it is unclear whether the following program is complete:
#include <istream> extern std::basic_iostream<char> &strm; int main () { strm << "Hello, World!\n"; }
or whether one needs to explicitly include <ostream>
, and
perhaps even other headers containing the definitions of other
required templates:
#include <ios> #include <istream> #include <ostream> #include <streambuf> extern std::basic_iostream<char> &strm; int main () { strm << "Hello, World!\n"; }
Example 3:
Likewise, it seems unclear whether the program below is complete:
#include <iterator> bool foo (std::istream_iterator<int> a, std::istream_iterator<int> b) { return a == b; } int main () { }
or whether one should be required to include <istream>
.
There are many more examples that demonstrate this lack of a requirement. I believe that in a good number of cases it would be unreasonable to require that a program explicitly include all the headers necessary for a particular template to be specialized, but I think that there are cases such as some of those above where it would be desirable to allow implementations to include only as much as necessary and not more.
[ post Bellevue: ]
Position taken in prior reviews is that the idea of a table of header dependencies is a good one. Our view is that a full paper is needed to do justice to this, and we've made that recommendation to the issue author.
[ 2009-07 Frankfurt ]
Proposed resolution:
For every C++ library header, supply a minimum set of other C++ library headers that are required to be included by that header. The proposed list is below (C++ headers for C Library Facilities, table 12 in 17.4.1.2, p3, are omitted):
+------------+--------------------+ | C++ header |required to include | +============+====================+ |<algorithm> | | +------------+--------------------+ |<bitset> | | +------------+--------------------+ |<complex> | | +------------+--------------------+ |<deque> |<memory> | +------------+--------------------+ |<exception> | | +------------+--------------------+ |<fstream> |<ios> | +------------+--------------------+ |<functional>| | +------------+--------------------+ |<iomanip> |<ios> | +------------+--------------------+ |<ios> |<streambuf> | +------------+--------------------+ |<iosfwd> | | +------------+--------------------+ |<iostream> |<istream>, <ostream>| +------------+--------------------+ |<istream> |<ios> | +------------+--------------------+ |<iterator> | | +------------+--------------------+ |<limits> | | +------------+--------------------+ |<list> |<memory> | +------------+--------------------+ |<locale> | | +------------+--------------------+ |<map> |<memory> | +------------+--------------------+ |<memory> | | +------------+--------------------+ |<new> |<exception> | +------------+--------------------+ |<numeric> | | +------------+--------------------+ |<ostream> |<ios> | +------------+--------------------+ |<queue> |<deque> | +------------+--------------------+ |<set> |<memory> | +------------+--------------------+ |<sstream> |<ios>, <string> | +------------+--------------------+ |<stack> |<deque> | +------------+--------------------+ |<stdexcept> | | +------------+--------------------+ |<streambuf> |<ios> | +------------+--------------------+ |<string> |<memory> | +------------+--------------------+ |<strstream> | | +------------+--------------------+ |<typeinfo> |<exception> | +------------+--------------------+ |<utility> | | +------------+--------------------+ |<valarray> | | +------------+--------------------+ |<vector> |<memory> | +------------+--------------------+
Rationale:
The portability problem is real. A program that works correctly on one implementation might fail on another, because of different header dependencies. This problem was understood before the standard was completed, and it was a conscious design choice.
One possible way to deal with this, as a library extension, would
be an <all>
header.
Hinnant: It's time we dealt with this issue for C++0X. Reopened.
Section: 27.5 [c.strings] Status: CD1 Submitter: Clark Nelson Opened: 2001-10-19 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
C99, and presumably amendment 1 to C90, specify that <wchar.h> declares struct tm as an incomplete type. However, table 48 in 27.5 [c.strings] does not mention the type tm as being declared in <cwchar>. Is this omission intentional or accidental?
Proposed resolution:
In section 27.5 [c.strings], add "tm" to table 48.
Section: 24.3.4 [iterator.concepts] Status: CD1 Submitter: Jeremy Siek Opened: 2001-10-20 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
Iterator member functions and operators that do not change the state of the iterator should be defined as const member functions or as functions that take iterators either by const reference or by value. The standard does not explicitly state which functions should be const. Since this a fairly common mistake, the following changes are suggested to make this explicit.
The tables almost indicate constness properly through naming: r for non-const and a,b for const iterators. The following changes make this more explicit and also fix a couple problems.
Proposed resolution:
In 24.3.4 [iterator.concepts] Change the first section of p9 from "In the following sections, a and b denote values of X..." to "In the following sections, a and b denote values of type const X...".
In Table 73, change
a->m U& ...
to
a->m const U& ... r->m U& ...
In Table 73 expression column, change
*a = t
to
*r = t
[Redmond: The container requirements should be reviewed to see if the same problem appears there.]
Section: 28.3.3.1.2.1 [locale.category] Status: CD1 Submitter: P.J. Plauger, Nathan Myers Opened: 2001-10-23 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
In 28.3.3.1.2.1 [locale.category] paragraph 1, the category members
are described as bitmask elements. In fact, the bitmask requirements
in 16.3.3.3.3 [bitmask.types] don't seem quite right: none
and all
are bitmask constants, not bitmask elements.
In particular, the requirements for none
interact poorly
with the requirement that the LC_* constants from the C library must
be recognizable as C++ locale category constants. LC_* values should
not be mixed with these values to make category values.
We have two options for the proposed resolution. Informally:
option 1 removes the requirement that LC_* values be recognized as
category arguments. Option 2 changes the category type so that this
requirement is implementable, by allowing none
to be some
value such as 0x1000 instead of 0.
Nathan writes: "I believe my proposed resolution [Option 2] merely re-expresses the status quo more clearly, without introducing any changes beyond resolving the DR.
Proposed resolution:
Replace the first two paragraphs of 28.3.3.1.2 [locale.types] with:
typedef int category;Valid category values include the
locale
member bitmask elementscollate
,ctype
,monetary
,numeric
,time
, andmessages
, each of which represents a single locale category. In addition,locale
member bitmask constantnone
is defined as zero and represents no category. And locale member bitmask constantall
is defined such that the expression(collate | ctype | monetary | numeric | time | messages | all) == allis
true
, and represents the union of all categories. Further the expression(X | Y)
, whereX
andY
each represent a single category, represents the union of the two categories.
locale
member functions expecting acategory
argument require one of thecategory
values defined above, or the union of two or more such values. Such acategory
argument identifies a set of locale categories. Each locale category, in turn, identifies a set of locale facets, including at least those shown in Table 51:
[Curaçao: need input from locale experts.]
Rationale:
The LWG considered, and rejected, an alternate proposal (described as "Option 2" in the discussion). The main reason for rejecting it was that library implementors were concerened about implementation difficult, given that getting a C++ library to work smoothly with a separately written C library is already a delicate business. Some library implementers were also concerned about the issue of adding extra locale categories.
Option 2:
Replace the first paragraph of 28.3.3.1.2 [locale.types] with:Valid category values include the enumerated values. In addition, the result of applying commutative operators | and & to any two valid values is valid, and results in the setwise union and intersection, respectively, of the argument categories. The values
all
andnone
are defined such that for any valid valuecat
, the expressions(cat | all == all)
,(cat & all == cat)
,(cat | none == cat)
and(cat & none == none)
are true. For non-equal valuescat1
andcat2
of the remaining enumerated values,(cat1 & cat2 == none)
is true. For any valid categoriescat1
andcat2
, the result of(cat1 & ~cat2)
is valid, and equals the setwise union of those categories found incat1
but not found incat2
. [Footnote: it is not required thatall
equal the setwise union of the other enumerated values; implementations may add extra categories.]
Section: 24.6.3 [ostream.iterator] Status: CD1 Submitter: Andy Sawyer Opened: 2001-10-24 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
24.5.2 [lib.ostream.iterator] states:
[...] private: // basic_ostream<charT,traits>* out_stream; exposition only // const char* delim; exposition only
Whilst it's clearly marked "exposition only", I suspect 'delim' should be of type 'const charT*'.
Proposed resolution:
In 24.6.3 [ostream.iterator], replace const char* delim
with
const charT* delim
.
Section: 27.2.3 [char.traits.typedefs] Status: CD1 Submitter: Martin Sebor Opened: 2001-12-02 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
(1)
There are no requirements on the stateT
template parameter of
fpos
listed in 27.4.3. The interface appears to require that
the type be at least Assignable and CopyConstructible (27.4.3.1, p1),
and I think also DefaultConstructible (to implement the operations in
Table 88).
21.1.2, p3, however, only requires that
char_traits<charT>::state_type
meet the requirements of
CopyConstructible types.
(2)
Additionally, the stateT
template argument has no
corresponding typedef in fpos which might make it difficult to use in
generic code.
Proposed resolution:
Modify 21.1.2, p4 from
Requires: state_type
shall meet the requirements of
CopyConstructible types (20.1.3).
Requires: state_type shall meet the requirements of Assignable (23.1, p4), CopyConstructible (20.1.3), and DefaultConstructible (20.1.4) types.
Rationale:
The LWG feels this is two issues, as indicated above. The first is a defect---std::basic_fstream is unimplementable without these additional requirements---and the proposed resolution fixes it. The second is questionable; who would use that typedef? The class template fpos is used only in a very few places, all of which know the state type already. Unless motivation is provided, the second should be considered NAD.
std::pair
missing template assignmentSection: 22.3 [pairs] Status: Resolved Submitter: Martin Sebor Opened: 2001-12-02 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
The class template std::pair
defines a template ctor (20.2.2, p4) but
no template assignment operator. This may lead to inefficient code since
assigning an object of pair<C, D>
to pair<A, B>
where the types C
and D
are distinct from but convertible to
A
and B
, respectively, results in a call to the template copy
ctor to construct an unnamed temporary of type pair<A, B>
followed by an ordinary (perhaps implicitly defined) assignment operator,
instead of just a straight assignment.
Proposed resolution:
Add the following declaration to the definition of std::pair
:
template<class U, class V> pair& operator=(const pair<U, V> &p);
And also add a paragraph describing the effects of the function template to the end of 20.2.2:
template<class U, class V> pair& operator=(const pair<U, V> &p);
Effects: first = p.first;
second = p.second;
Returns: *this
[Curaçao: There is no indication this is was anything other than a design decision, and thus NAD. May be appropriate for a future standard.]
[
Pre Bellevue: It was recognized that this was taken care of by
N1856,
and thus moved from NAD Future to NAD EditorialResolved.
]
Section: 23.2.7 [associative.reqmts] Status: CD1 Submitter: Hans Aberg Opened: 2001-12-17 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
Discussions in the thread "Associative container lower/upper bound requirements" on comp.std.c++ suggests that there is a defect in the C++ standard, Table 69 of section 23.1.2, "Associative containers", [lib.associative.reqmts]. It currently says:
a.find(k): returns an iterator pointing to an element with the key equivalent to k, or a.end() if such an element is not found.
a.lower_bound(k): returns an iterator pointing to the first element with key not less than k.
a.upper_bound(k): returns an iterator pointing to the first element with key greater than k.
We have "or a.end() if such an element is not found" for
find
, but not for upper_bound
or
lower_bound
. As the text stands, one would be forced to
insert a new element into the container and return an iterator to that
in case the sought iterator does not exist, which does not seem to be
the intention (and not possible with the "const" versions).
Proposed resolution:
Change Table 69 of section 23.2.7 [associative.reqmts] indicated entries to:
a.lower_bound(k): returns an iterator pointing to the first element with key not less than k, or a.end() if such an element is not found.
a.upper_bound(k): returns an iterator pointing to the first element with key greater than k, or a.end() if such an element is not found.
[Curaçao: LWG reviewed PR.]
Section: 23.2.4 [sequence.reqmts] Status: CD1 Submitter: Yaroslav Mironov Opened: 2002-01-23 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
Table 68 "Optional Sequence Operations" in 23.1.1/12 specifies operational semantics for "a.back()" as "*--a.end()", which may be ill-formed [because calling operator-- on a temporary (the return) of a built-in type is ill-formed], provided a.end() returns a simple pointer rvalue (this is almost always the case for std::vector::end(), for example). Thus, the specification is not only incorrect, it demonstrates a dangerous construct: "--a.end()" may successfully compile and run as intended, but after changing the type of the container or the mode of compilation it may produce compile-time error.
Proposed resolution:
Change the specification in table 68 "Optional Sequence Operations" in 23.1.1/12 for "a.back()" from
*--a.end()
to
{ iterator tmp = a.end(); --tmp; return *tmp; }
and the specification for "a.pop_back()" from
a.erase(--a.end())
to
{ iterator tmp = a.end(); --tmp; a.erase(tmp); }
[Curaçao: LWG changed PR from "{ X::iterator tmp = a.end(); return *--tmp; }" to "*a.rbegin()", and from "{ X::iterator tmp = a.end(); a.erase(--tmp); }" to "a.erase(rbegin())".]
[There is a second possible defect; table 68 "Optional Sequence Operations" in the "Operational Semantics" column uses operations present only in the "Reversible Container" requirements, yet there is no stated dependency between these separate requirements tables. Ask in Santa Cruz if the LWG would like a new issue opened.]
[Santa Cruz: the proposed resolution is even worse than what's in the current standard: erase is undefined for reverse iterator. If we're going to make the change, we need to define a temporary and use operator--. Additionally, we don't know how prevalent this is: do we need to make this change in more than one place? Martin has volunteered to review the standard and see if this problem occurs elsewhere.]
[Oxford: Matt provided new wording to address the concerns raised in Santa Cruz. It does not appear that this problem appears anywhere else in clauses 23 or 24.]
[Kona: In definition of operational semantics of back(), change "*tmp" to "return *tmp;"]
thousands_sep
after a decimal_point
Section: 28.3.4.3.2.3 [facet.num.get.virtuals] Status: CD1 Submitter: Martin Sebor Opened: 2002-03-12 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
I don't think thousands_sep
is being treated correctly after
decimal_point has been seen. Since grouping applies only to the
integral part of the number, the first such occurrence should, IMO,
terminate Stage 2. (If it does not terminate it, then 22.2.2.1.2, p12
and 22.2.3.1.2, p3 need to explain how thousands_sep
is to be
interpreted in the fractional part of a number.)
The easiest change I can think of that resolves this issue would be something like below.
Proposed resolution:
Change the first sentence of 22.2.2.1.2, p9 from
If discard is true then the position of the character is remembered, but the character is otherwise ignored. If it is not discarded, then a check is made to determine if c is allowed as the next character of an input field of the conversion specifier returned by stage 1. If so it is accumulated.
to
If
discard
is true, then if'.'
has not yet been accumulated, then the position of the character is remembered, but the character is otherwise ignored. Otherwise, if'.'
has already been accumulated, the character is discarded and Stage 2 terminates. ...
Rationale:
We believe this reflects the intent of the Standard. Thousands sep characters after the decimal point are not useful in any locale. Some formatting conventions do group digits that follow the decimal point, but they usually introduce a different grouping character instead of reusing the thousand sep character. If we want to add support for such conventions, we need to do so explicitly.
Section: 28.3.4.3.3.2 [facet.num.put.members] Status: CD1 Submitter: Martin Sebor Opened: 2002-03-12 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
22.2.2.2.1, p1:
iter_type put (iter_type out, ios_base& str, char_type fill, bool val) const; ... 1 Returns: do_put (out, str, fill, val).
AFAICS, the behavior of do_put (..., bool) is not documented anywhere, however, 22.2.2.2.2, p23:
iter_type put (iter_type out, ios_base& str, char_type fill, bool val) const;Effects: If (str.flags() & ios_base::boolalpha) == 0 then do out = do_put(out, str, fill, (int)val) Otherwise do
string_type s = val ? use_facet<ctype<charT> >(loc).truename() : use_facet<ctype<charT> >(loc).falsename();and then insert the characters of s into out. out.
This means that the bool overload of do_put()
will never be called,
which contradicts the first paragraph. Perhaps the declaration
should read do_put()
, and not put()
?
Note also that there is no Returns clause for this function, which should probably be corrected, just as should the second occurrence of "out." in the text.
I think the least invasive change to fix it would be something like the following:
Proposed resolution:
In 28.3.4.3.3.3 [facet.num.put.virtuals], just above paragraph 1, remove
the bool
overload.
In 28.3.4.3.3.3 [facet.num.put.virtuals], p23, make the following changes
Replace
put()
withdo_put()
in the declaration of the member function.
Change the Effects clause to a Returns clause (to avoid the requirement to call
do_put(..., int)
fromdo_put (..., bool))
like so:
23 Returns: If
(str.flags() & ios_base::boolalpha) == 0
thendo_put (out, str, fill, (long)val)
Otherwise the function obtains a strings
as if bystring_type s = val ? use_facet<ctype<charT> >(loc).truename() : use_facet<ctype<charT> >(loc).falsename();and then inserts each character
c
of s into out via*out++ = c
and returnsout
.
Rationale:
This fixes a couple of obvious typos, and also fixes what appears to be a requirement of gratuitous inefficiency.
Section: 28.3.3.1 [locale] Status: CD1 Submitter: Martin Sebor Opened: 2002-03-12 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
22.1.1, p7 (copied below) allows iostream formatters and extractors
to make assumptions about the values returned from facet members.
However, such assumptions are apparently not guaranteed to hold
in other cases (e.g., when the facet members are being called directly
rather than as a result of iostream calls, or between successive
calls to the same iostream functions with no interevening calls to
imbue()
, or even when the facet member functions are called
from other member functions of other facets). This restriction
prevents locale from being implemented efficiently.
Proposed resolution:
Change the first sentence in 22.1.1, p7 from
In successive calls to a locale facet member function during a call to an iostream inserter or extractor or a streambuf member function, the returned result shall be identical. [Note: This implies that such results may safely be reused without calling the locale facet member function again, and that member functions of iostream classes cannot safely call
imbue()
themselves, except as specified elsewhere. --end note]
to
In successive calls to a locale facet member function on a facet object installed in the same locale, the returned result shall be identical. ...
Rationale:
This change is reasonable becuase it clarifies the intent of this part of the standard.
Section: 99 [depr.lib.binders] Status: CD1 Submitter: Andrew Demkin Opened: 2002-04-26 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
The definition of bind1st() (99 [depr.lib.binders]) can result in the construction of an unsafe binding between incompatible pointer types. For example, given a function whose first parameter type is 'pointer to T', it's possible without error to bind an argument of type 'pointer to U' when U does not derive from T:
foo(T*, int); struct T {}; struct U {}; U u; int* p; int* q; for_each(p, q, bind1st(ptr_fun(foo), &u)); // unsafe binding
The definition of bind1st() includes a functional-style conversion to map its argument to the expected argument type of the bound function (see below):
typename Operation::first_argument_type(x)
A functional-style conversion (99 [depr.lib.binders]) is defined to be semantically equivalent to an explicit cast expression (99 [depr.lib.binders]), which may (according to 5.4, paragraph 5) be interpreted as a reinterpret_cast, thus masking the error.
The problem and proposed change also apply to 99 [depr.lib.binders].
Proposed resolution:
Add this sentence to the end of 99 [depr.lib.binders]/1:
"Binders bind1st
and bind2nd
are deprecated in
favor of std::tr1::bind
."
(Notes to editor: (1) when and if tr1::bind is incorporated into the standard, "std::tr1::bind" should be changed to "std::bind". (2) 20.5.6 should probably be moved to Annex D.
Rationale:
There is no point in fixing bind1st and bind2nd. tr1::bind is a superior solution. It solves this problem and others.
Section: 31.5.2.2.1 [ios.failure] Status: CD1 Submitter: Walter Brown and Marc Paterno Opened: 2002-05-20 Last modified: 2021-06-06
Priority: Not Prioritized
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Discussion:
The destructor of ios_base::failure should have an empty throw specification, because the destructor of its base class, exception, is declared in this way.
Proposed resolution:
Change the destructor to
virtual ~failure() throw();
Rationale:
Fixes an obvious glitch. This is almost editorial.
Section: 31.6.3.5.2 [streambuf.virt.buffer] Status: CD1 Submitter: Walter Brown, Marc Paterno Opened: 2002-05-10 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
31.6.3.5.2 [streambuf.virt.buffer] paragraph 1 is inconsistent with the Effects clause for seekoff.
Proposed resolution:
Make this paragraph, the Effects clause for setbuf, consistent in wording with the Effects clause for seekoff in paragraph 3 by amending paragraph 1 to indicate the purpose of setbuf:
Original text:
1 Effects: Performs an operation that is defined separately for each class derived from basic_streambuf in this clause (27.7.1.3, 27.8.1.4).
Proposed text:
1 Effects: Influences stream buffering in a way that is defined separately for each class derived from basic_streambuf in this clause (27.7.1.3, 27.8.1.4).
Rationale:
The LWG doesn't believe there is any normative difference between the existing wording and what's in the proposed resolution, but the change may make the intent clearer.
Section: 31 [input.output] Status: CD1 Submitter: Walter Brown, Marc Paterno Opened: 2002-05-10 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
Some stream and streambuf member functions are declared non-const, even thought they appear only to report information rather than to change an object's logical state. They should be declared const. See document N1360 for details and rationale.
The list of member functions under discussion: in_avail
,
showmanyc
, tellg
, tellp
, is_open
.
Proposed resolution:
In 27.8.1.5, 27.8.1.7, 27.8.1.8, 27.8.1.10, 27.8.1.11, and 27.8.1.13
Replace
bool is_open();
with
bool is_open() const;
Rationale:
Of the changes proposed in N1360, the only one that is safe is changing the filestreams' is_open to const. The LWG believed that this was NAD the first time it considered this issue (issue 73(i)), but now thinks otherwise. The corresponding streambuf member function, after all,is already const.
The other proposed changes are less safe, because some streambuf functions that appear merely to report a value do actually perform mutating operations. It's not even clear that they should be considered "logically const", because streambuf has two interfaces, a public one and a protected one. These functions may, and often do, change the state as exposed by the protected interface, even if the state exposed by the public interface is unchanged.
Note that implementers can make this change in a binary compatible way by providing both overloads; this would be a conforming extension.
Section: 31.4 [iostream.objects] Status: CD1 Submitter: Ruslan Abdikeev Opened: 2002-07-08 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
Is it safe to use standard iostream objects from constructors of static objects? Are standard iostream objects constructed and are their associations established at that time?
Surpisingly enough, Standard does NOT require that.
27.3/2 [lib.iostream.objects] guarantees that standard iostream objects are constructed and their associations are established before the body of main() begins execution. It also refers to ios_base::Init class as the panacea for constructors of static objects.
However, there's nothing in 27.3 [lib.iostream.objects], in 27.4.2 [lib.ios.base], and in 27.4.2.1.6 [lib.ios::Init], that would require implementations to allow access to standard iostream objects from constructors of static objects.
Details:
Core text refers to some magic object ios_base::Init, which will be discussed below:
"The [standard iostream] objects are constructed, and their associations are established at some time prior to or during first time an object of class basic_ios<charT,traits>::Init is constructed, and in any case before the body of main begins execution." (27.3/2 [lib.iostream.objects])
The first non-normative footnote encourages implementations to initialize standard iostream objects earlier than required.
However, the second non-normative footnote makes an explicit and unsupported claim:
"Constructors and destructors for static objects can access these [standard iostream] objects to read input from stdin or write output to stdout or stderr." (27.3/2 footnote 265 [lib.iostream.objects])
The only bit of magic is related to that ios_base::Init class. AFAIK, the rationale behind ios_base::Init was to bring an instance of this class to each translation unit which #included <iostream> or related header. Such an inclusion would support the claim of footnote quoted above, because in order to use some standard iostream object it is necessary to #include <iostream>.
However, while Standard explicitly describes ios_base::Init as an appropriate class for doing the trick, I failed to found a mention of an _instance_ of ios_base::Init in Standard.
Proposed resolution:
Add to 31.4 [iostream.objects], p2, immediately before the last sentence of the paragraph, the following two sentences:
If a translation unit includes <iostream>, or explicitly constructs an ios_base::Init object, these stream objects shall be constructed before dynamic initialization of non-local objects defined later in that translation unit, and these stream objects shall be destroyed after the destruction of dynamically initialized non-local objects defined later in that translation unit.
[Lillehammer: Matt provided wording.]
[Mont Tremblant: Matt provided revised wording.]
Rationale:
The original proposed resolution unconditionally required implementations to define an ios_base::Init object of some implementation-defined name in the header <iostream>. That's an overspecification. First, defining the object may be unnecessary and even detrimental to performance if an implementation can guarantee that the 8 standard iostream objects will be initialized before any other user-defined object in a program. Second, there is no need to require implementations to document the name of the object.
The new proposed resolution gives users guidance on what they need to do to ensure that stream objects are constructed during startup.
Section: 31.7.5.4 [istream.unformatted] Status: CD1 Submitter: Ray Lischner Opened: 2002-07-15 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
Defect report for description of basic_istream::get (section 31.7.5.4 [istream.unformatted]), paragraph 15. The description for the get function with the following signature:
basic_istream<charT,traits>& get(basic_streambuf<char_type,traits>& sb);
is incorrect. It reads
Effects: Calls get(s,n,widen('\n'))
which I believe should be:
Effects: Calls get(sb,widen('\n'))
Proposed resolution:
Change the Effects paragraph to:
Effects: Calls get(sb,this->widen('\n'))
[Pre-Oxford: Minor correction from Howard: replaced 'widen' with 'this->widen'.]
Rationale:
Fixes an obvious typo.
Section: 23.2 [container.requirements] Status: CD1 Submitter: Frank Compagner Opened: 2002-07-20 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
The requirements for multiset and multimap containers (23.1 [lib.containers.requirements], 23.1.2 [lib.associative.reqmnts], 23.3.2 [lib.multimap] and 23.3.4 [lib.multiset]) make no mention of the stability of the required (mutating) member functions. It appears the standard allows these functions to reorder equivalent elements of the container at will, yet the pervasive red-black tree implementation appears to provide stable behaviour.
This is of most concern when considering the behaviour of erase(). A stability requirement would guarantee the correct working of the following 'idiom' that removes elements based on a certain predicate function.
multimap<int, int> m; multimap<int, int>::iterator i = m.begin(); while (i != m.end()) { if (pred(i)) m.erase (i++); else ++i; }
Although clause 23.1.2/8 guarantees that i remains a valid iterator througout this loop, absence of the stability requirement could potentially result in elements being skipped. This would make this code incorrect, and, furthermore, means that there is no way of erasing these elements without iterating first over the entire container, and second over the elements to be erased. This would be unfortunate, and have a negative impact on both performance and code simplicity.
If the stability requirement is intended, it should be made explicit (probably through an extra paragraph in clause 23.1.2).
If it turns out stability cannot be guaranteed, i'd argue that a remark or footnote is called for (also somewhere in clause 23.1.2) to warn against relying on stable behaviour (as demonstrated by the code above). If most implementations will display stable behaviour, any problems emerging on an implementation without stable behaviour will be hard to track down by users. This would also make the need for an erase_if() member function that much greater.
This issue is somewhat related to LWG issue 130(i).
Proposed resolution:
Add the following to the end of 23.2.7 [associative.reqmts] paragraph 4:
"For multiset
and multimap
, insert
and erase
are stable: they preserve the relative ordering of equivalent
elements.
[Lillehammer: Matt provided wording]
[Joe Gottman points out that the provided wording does not address multimap and multiset. N1780 also addresses this issue and suggests wording.]
[Mont Tremblant: Changed set and map to multiset and multimap.]
Rationale:
The LWG agrees that this guarantee is necessary for common user idioms to work, and that all existing implementations provide this property. Note that this resolution guarantees stability for multimap and multiset, not for all associative containers in general.
Section: 31.7.5.3.1 [istream.formatted.reqmts], 31.7.6.3.1 [ostream.formatted.reqmts] Status: CD1 Submitter: Keith Baker Opened: 2002-07-23 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
In 31.7.5.3.1 [istream.formatted.reqmts] and 31.7.6.3.1 [ostream.formatted.reqmts] (exception()&badbit) != 0 is used in testing for rethrow, yet exception() is the constructor to class std::exception in 17.7.3 [type.info] that has no return type. Should member function exceptions() found in 31.5.4 [ios] be used instead?
Proposed resolution:
In 31.7.5.3.1 [istream.formatted.reqmts] and 31.7.6.3.1 [ostream.formatted.reqmts], change "(exception()&badbit) != 0" to "(exceptions()&badbit) != 0".
Rationale:
Fixes an obvious typo.
Section: 31.8.2.5 [stringbuf.virtuals] Status: CD1 Submitter: Ray Lischner Opened: 2002-08-14 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
In Section 31.8.2.5 [stringbuf.virtuals]: Table 90, Table 91, and paragraph 14 all contain references to "basic_ios::" which should be "ios_base::".
Proposed resolution:
Change all references to "basic_ios" in Table 90, Table 91, and paragraph 14 to "ios_base".
Rationale:
Fixes an obvious typo.
Section: 31.8.2.5 [stringbuf.virtuals] Status: CD1 Submitter: Ray Lischner Opened: 2002-08-14 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
In Section 31.8.2.5 [stringbuf.virtuals], Table 90, the implication is that the four conditions should be mutually exclusive, but they are not. The first two cases, as written, are subcases of the third.
As written, it is unclear what should be the result if cases 1 and 2 are both true, but case 3 is false.
Proposed resolution:
Rewrite these conditions as:
(which & (ios_base::in|ios_base::out)) == ios_base::in
(which & (ios_base::in|ios_base::out)) == ios_base::out
(which & (ios_base::in|ios_base::out)) == (ios_base::in|ios_base::out) and way == either ios_base::beg or ios_base::end
Otherwise
Rationale:
It's clear what we wanted to say, we just failed to say it. This fixes it.
Section: 28.3.4.2.2.3 [locale.ctype.virtuals] Status: CD1 Submitter: Martin Sebor Opened: 2002-09-06 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
The last sentence in 22.2.1.1.2, p11 below doesn't seem to make sense.
charT do_widen (char c) const; -11- Effects: Applies the simplest reasonable transformation from a char value or sequence of char values to the corresponding charT value or values. The only characters for which unique transformations are required are those in the basic source character set (2.2). For any named ctype category with a ctype<charT> facet ctw and valid ctype_base::mask value M (is(M, c) || !ctw.is(M, do_widen(c))) is true.
Shouldn't the last sentence instead read
For any named ctype category with a ctype<char> facet ctc and valid ctype_base::mask value M (ctc.is(M, c) || !is(M, do_widen(c))) is true.
I.e., if the narrow character c is not a member of a class of characters then neither is the widened form of c. (To paraphrase footnote 224.)
Proposed resolution:
Replace the last sentence of 28.3.4.2.2.3 [locale.ctype.virtuals], p11 with the following text:
For any named ctype category with a ctype<char> facet ctc and valid ctype_base::mask value M, (ctc.is(M, c) || !is(M, do_widen(c))) is true.
[Kona: Minor edit. Added a comma after the M for clarity.]
Rationale:
The LWG believes this is just a typo, and that this is the correct fix.
Section: 28.3.4.2.6 [locale.codecvt.byname] Status: CD1 Submitter: Martin Sebor Opened: 2002-09-06 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
Tables 53 and 54 in 28.3.4.2.6 [locale.codecvt.byname] are both titled "convert result values," when surely "do_in/do_out result values" must have been intended for Table 53 and "do_unshift result values" for Table 54.
Table 54, row 3 says that the meaning of partial is "more characters needed to be supplied to complete termination." The function is not supplied any characters, it is given a buffer which it fills with characters or, more precisely, destination elements (i.e., an escape sequence). So partial means that space for more than (to_limit - to) destination elements was needed to terminate a sequence given the value of state.
Proposed resolution:
Change the title of Table 53 to "do_in/do_out result values" and the title of Table 54 to "do_unshift result values."
Change the text in Table 54, row 3 (the partial row), under the heading Meaning, to "space for more than (to_limit - to) destination elements was needed to terminate a sequence given the value of state."
Section: 28.3.4.2.6 [locale.codecvt.byname] Status: CD1 Submitter: Martin Sebor Opened: 2002-09-06 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
All but one codecvt member functions that take a state_type argument list as one of their preconditions that the state_type argument have a valid value. However, according to 22.2.1.5.2, p6, codecvt::do_unshift() is the only codecvt member that is supposed to return error if the state_type object is invalid.
It seems to me that the treatment of state_type by all codecvt member functions should be the same and the current requirements should be changed. Since the detection of invalid state_type values may be difficult in general or computationally expensive in some specific cases, I propose the following:
Proposed resolution:
Add a new paragraph before 22.2.1.5.2, p5, and after the function declaration below
result do_unshift(stateT& state, externT* to, externT* to_limit, externT*& to_next) const;
as follows:
Requires: (to <= to_end) well defined and true; state initialized, if at the beginning of a sequence, or else equal to the result of converting the preceding characters in the sequence.
and change the text in Table 54, row 4, the error row, under the heading Meaning, from
state has invalid value
to
an unspecified error has occurred
Rationale:
The intent is that implementations should not be required to detect invalid state values; such a requirement appears nowhere else. An invalid state value is a precondition violation, i.e. undefined behavior. Implementations that do choose to detect invalid state values, or that choose to detect any other kind of error, may return error as an indication.
Section: 24.3.5.6 [bidirectional.iterators] Status: CD1 Submitter: ysapir (submitted via comp.std.c++) Opened: 2002-10-17 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
Following a discussion on the boost list regarding end iterators and the possibility of performing operator--() on them, it seems to me that there is a typo in the standard. This typo has nothing to do with that discussion.
I have checked this newsgroup, as well as attempted a search of the Active/Defect/Closed Issues List on the site for the words "s is derefer" so I believe this has not been proposed before. Furthermore, the "Lists by Index" mentions only DR 299(i) on section 24.1.4, and DR 299(i) is not related to this issue.
The standard makes the following assertion on bidirectional iterators, in section 24.1.4 [lib.bidirectional.iterators], Table 75:
operational assertion/note expression return type semantics pre/post-condition --r X& pre: there exists s such that r == ++s. post: s is dereferenceable. --(++r) == r. --r == --s implies r == s. &r == &--r.
(See http://lists.boost.org/Archives/boost/2002/10/37636.php.)
In particular, "s is dereferenceable" seems to be in error. It seems that the intention was to say "r is dereferenceable".
If it were to say "r is dereferenceable" it would make perfect sense. Since s must be dereferenceable prior to operator++, then the natural result of operator-- (to undo operator++) would be to make r dereferenceable. Furthermore, without other assertions, and basing only on precondition and postconditions, we could not otherwise know this. So it is also interesting information.
Proposed resolution:
Change the guarantee to "postcondition: r is dereferenceable."
Rationale:
Fixes an obvious typo
Section: 26.8.4.4 [equal.range] Status: CD1 Submitter: Hans Bos Opened: 2002-10-18 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
Section 26.8.4.4 [equal.range] states that at most 2 * log(last - first) + 1 comparisons are allowed for equal_range.
It is not possible to implement equal_range with these constraints.
In a range of one element as in:
int x = 1; equal_range(&x, &x + 1, 1)
it is easy to see that at least 2 comparison operations are needed.
For this case at most 2 * log(1) + 1 = 1 comparison is allowed.
I have checked a few libraries and they all use the same (nonconforming) algorithm for equal_range that has a complexity of
2* log(distance(first, last)) + 2.
I guess this is the algorithm that the standard assumes for equal_range.
It is easy to see that 2 * log(distance) + 2 comparisons are enough since equal range can be implemented with lower_bound and upper_bound (both log(distance) + 1).
I think it is better to require something like 2log(distance) + O(1) (or even logarithmic as multiset::equal_range). Then an implementation has more room to optimize for certain cases (e.g. have log(distance) characteristics when at most match is found in the range but 2log(distance) + 4 for the worst case).
Proposed resolution:
In 26.8.4.2 [lower.bound]/4, change log(last - first) + 1
to log2(last - first) + O(1)
.
In 26.8.4.3 [upper.bound]/4, change log(last - first) + 1
to log2(last - first) + O(1)
.
In 26.8.4.4 [equal.range]/4, change 2*log(last - first) + 1
to 2*log2(last - first) + O(1)
.
[Matt provided wording]
Rationale:
The LWG considered just saying O(log n) for all three, but
decided that threw away too much valuable information. The fact
that lower_bound is twice as fast as equal_range is important.
However, it's better to allow an arbitrary additive constant than to
specify an exact count. An exact count would have to
involve floor
or ceil
. It would be too easy to
get this wrong, and don't provide any substantial value for users.
Section: 24.5.1.6 [reverse.iter.elem] Status: CD1 Submitter: Matt Austern Opened: 2002-10-23 Last modified: 2021-06-06
Priority: Not Prioritized
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Discussion:
In [reverse.iter.op-=], reverse_iterator<>::operator[]
is specified as having a return type of reverse_iterator::reference
,
which is the same as iterator_traits<Iterator>::reference
.
(Where Iterator
is the underlying iterator type.)
The trouble is that Iterator
's own operator[] doesn't
necessarily have a return type
of iterator_traits<Iterator>::reference
. Its
return type is merely required to be convertible
to Iterator
's value type. The return type specified for
reverse_iterator's operator[] would thus appear to be impossible.
With the resolution of issue 299(i), the type of
a[n]
will continue to be required (for random access
iterators) to be convertible to the value type, and also a[n] =
t
will be a valid expression. Implementations of
reverse_iterator
will likely need to return a proxy from
operator[]
to meet these requirements. As mentioned in the
comment from Dave Abrahams, the simplest way to specify that
reverse_iterator
meet this requirement to just mandate
it and leave the return type of operator[]
unspecified.
Proposed resolution:
In 24.5.1.3 [reverse.iter.requirements] change:
reference operator[](difference_type n) const;
to:
unspecified operator[](difference_type n) const; // see 24.3.5.7 [random.access.iterators]
[ Comments from Dave Abrahams: IMO we should resolve 386 by just saying that the return type of reverse_iterator's operator[] is unspecified, allowing the random access iterator requirements to impose an appropriate return type. If we accept 299's proposed resolution (and I think we should), the return type will be readable and writable, which is about as good as we can do. ]
std::complex
over-encapsulatedSection: 29.4 [complex.numbers] Status: CD1 Submitter: Gabriel Dos Reis Opened: 2002-11-08 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
The absence of explicit description of std::complex<T>
layout
makes it imposible to reuse existing software developed in traditional
languages like Fortran or C with unambigous and commonly accepted
layout assumptions. There ought to be a way for practitioners to
predict with confidence the layout of std::complex<T>
whenever T
is a numerical datatype. The absence of ways to access individual
parts of a std::complex<T>
object as lvalues unduly promotes
severe pessimizations. For example, the only way to change,
independently, the real and imaginary parts is to write something like
complex<T> z; // ... // set the real part to r z = complex<T>(r, z.imag()); // ... // set the imaginary part to i z = complex<T>(z.real(), i);
At this point, it seems appropriate to recall that a complex number
is, in effect, just a pair of numbers with no particular invariant to
maintain. Existing practice in numerical computations has it that a
complex number datatype is usually represented by Cartesian
coordinates. Therefore the over-encapsulation put in the specification
of std::complex<>
is not justified.
Proposed resolution:
Add the following requirements to 29.4 [complex.numbers] as 26.3/4:
If
z
is an lvalue expression of type cvstd::complex<T>
then
- the expression
reinterpret_cast<cv T(&)[2]>(z)
is well-formed; andreinterpret_cast<cv T(&)[2]>(z)[0]
designates the real part ofz
; andreinterpret_cast<cv T(&)[2]>(z)[1]
designates the imaginary part ofz
.Moreover, if
a
is an expression of pointer type cvcomplex<T>*
and the expressiona[i]
is well-defined for an integer expressioni
then:
reinterpret_cast<cv T*>(a)[2*i]
designates the real part ofa[i]
; andreinterpret_cast<cv T*>(a)[2*i+1]
designates the imaginary part ofa[i]
.
In 29.4.3 [complex] and [complex.special] add the following member functions
(changing T
to concrete types as appropriate for the specializations).
void real(T); void imag(T);
Add to 29.4.4 [complex.members]
T real() const;Returns: the value of the real component
void real(T val);Assigns
val
to the real component.T imag() const;Returns: the value of the imaginary component
void imag(T val);Assigns
val
to the imaginary component.
[Kona: The layout guarantee is absolutely necessary for C
compatibility. However, there was disagreement about the other part
of this proposal: retrieving elements of the complex number as
lvalues. An alternative: continue to have real()
and imag()
return
rvalues, but add set_real()
and set_imag()
. Straw poll: return
lvalues - 2, add setter functions - 5. Related issue: do we want
reinterpret_cast as the interface for converting a complex to an
array of two reals, or do we want to provide a more explicit way of
doing it? Howard will try to resolve this issue for the next
meeting.]
[pre-Sydney: Howard summarized the options in n1589.]
[ Bellevue: ]
Second half of proposed wording replaced and moved to Ready.
[ Pre-Sophia Antipolis, Howard adds: ]
Added the members to [complex.special] and changed from Ready to Review.
[ Post-Sophia Antipolis: ]
Moved from WP back to Ready so that the "and [complex.special]" in the proposed resolution can be officially applied.
Rationale:
The LWG believes that C99 compatibility would be enough justification for this change even without other considerations. All existing implementations already have the layout proposed here.
Section: 29.6.2.4 [valarray.access] Status: CD1 Submitter: Gabriel Dos Reis Opened: 2002-11-08 Last modified: 2016-01-28
Priority: Not Prioritized
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Duplicate of: 77
Discussion:
Consider the following program:
#include <iostream> #include <ostream> #include <vector> #include <valarray> #include <algorithm> #include <iterator> template<typename Array> void print(const Array& a) { using namespace std; typedef typename Array::value_type T; copy(&a[0], &a[0] + a.size(), ostream_iterator<T>(std::cout, " ")); } template<typename T, unsigned N> unsigned size(T(&)[N]) { return N; } int main() { double array[] = { 0.89, 9.3, 7, 6.23 }; std::vector<double> v(array, array + size(array)); std::valarray<double> w(array, size(array)); print(v); // #1 std::cout << std::endl; print(w); // #2 std::cout << std::endl; }
While the call numbered #1 succeeds, the call numbered #2 fails because the const version of the member function valarray<T>::operator[](size_t) returns a value instead of a const-reference. That seems to be so for no apparent reason, no benefit. Not only does that defeats users' expectation but it also does hinder existing software (written either in C or Fortran) integration within programs written in C++. There is no reason why subscripting an expression of type valarray<T> that is const-qualified should not return a const T&.
Proposed resolution:
In the class synopsis in 29.6.2 [template.valarray], and in 29.6.2.4 [valarray.access] just above paragraph 1, change
T operator[](size_t const);
to
const T& operator[](size_t const);
[Kona: fixed a minor typo: put semicolon at the end of the line wehre it belongs.]
Rationale:
Return by value seems to serve no purpose. Valaray was explicitly designed to have a specified layout so that it could easily be integrated with libraries in other languages, and return by value defeats that purpose. It is believed that this change will have no impact on allowable optimizations.
Section: 28.3.3.3.2 [conversions.character] Status: CD1 Submitter: James Kanze Opened: 2002-12-10 Last modified: 2021-06-06
Priority: Not Prioritized
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Discussion:
The specifications of toupper and tolower both specify the functions as const, althought they are not member functions, and are not specified as const in the header file synopsis in section 28.3.3 [locales].
Proposed resolution:
In [conversions], remove const
from the function
declarations of std::toupper and std::tolower
Rationale:
Fixes an obvious typo
Section: 29.7 [c.math] Status: CD1 Submitter: James Kanze Opened: 2003-01-03 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
In 29.7 [c.math], the C++ standard refers to the C standard for the definition of rand(); in the C standard, it is written that "The implementation shall behave as if no library function calls the rand function."
In 26.7.13 [alg.random.shuffle], there is no specification as to how the two parameter version of the function generates its random value. I believe that all current implementations in fact call rand() (in contradiction with the requirement avove); if an implementation does not call rand(), there is the question of how whatever random generator it does use is seeded. Something is missing.
Proposed resolution:
In [lib.c.math], add a paragraph specifying that the C definition of rand shal be modified to say that "Unless otherwise specified, the implementation shall behave as if no library function calls the rand function."
In [lib.alg.random.shuffle], add a sentence to the effect that "In
the two argument form of the function, the underlying source of
random numbers is implementation defined. [Note: in particular, an
implementation is permitted to use rand
.]
Rationale:
The original proposed resolution proposed requiring the
two-argument from of random_shuffle
to
use rand
. We don't want to do that, because some existing
implementations already use something else: gcc
uses lrand48
, for example. Using rand
presents a
problem if the number of elements in the sequence is greater than
RAND_MAX.
Section: 22.9.2.2 [bitset.cons] Status: CD1 Submitter: Martin Sebor Opened: 2003-01-05 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
23.3.5.1, p6 [lib.bitset.cons] talks about a generic character having the value of 0 or 1 but there is no definition of what that means for charT other than char and wchar_t. And even for those two types, the values 0 and 1 are not actually what is intended -- the values '0' and '1' are. This, along with the converse problem in the description of to_string() in 23.3.5.2, p33, looks like a defect remotely related to DR 303(i).
23.3.5.1: -6- An element of the constructed string has value zero if the corresponding character in str, beginning at position pos, is 0. Otherwise, the element has the value one.
23.3.5.2: -33- Effects: Constructs a string object of the appropriate type and initializes it to a string of length N characters. Each character is determined by the value of its corresponding bit position in *this. Character position N ?- 1 corresponds to bit position zero. Subsequent decreasing character positions correspond to increasing bit positions. Bit value zero becomes the character 0, bit value one becomes the character 1.
Also note the typo in 23.3.5.1, p6: the object under construction is a bitset, not a string.
[ Sophia Antipolis: ]
We note that
bitset
has been moved from section 23 to section 20, by another issue (842(i)) previously resolved at this meeting.Disposition: move to ready.
We request that Howard submit a separate issue regarding the three
to_string
overloads.
[ post Bellevue: ]
We are happy with the resolution as proposed, and we move this to Ready.
[ Howard adds: ]
The proposed wording neglects the 3 newer
to_string
overloads.
Proposed resolution:
Change the constructor's function declaration immediately before 22.9.2.2 [bitset.cons] p3 to:
template <class charT, class traits, class Allocator> explicit bitset(const basic_string<charT, traits, Allocator>& str, typename basic_string<charT, traits, Allocator>::size_type pos = 0, typename basic_string<charT, traits, Allocator>::size_type n = basic_string<charT, traits, Allocator>::npos, charT zero = charT('0'), charT one = charT('1'))
Change the first two sentences of 22.9.2.2 [bitset.cons] p6 to: "An element of the constructed string has value 0 if the corresponding character in str, beginning at position pos, is zero. Otherwise, the element has the value 1.
Change the text of the second sentence in 23.3.5.1, p5 to read: "The function then throws invalid_argument if any of the rlen characters in str beginning at position pos is other than zero or one. The function uses traits::eq() to compare the character values."
Change the declaration of the to_string
member function
immediately before 22.9.2.3 [bitset.members] p33 to:
template <class charT, class traits, class Allocator> basic_string<charT, traits, Allocator> to_string(charT zero = charT('0'), charT one = charT('1')) const;
Change the last sentence of 22.9.2.3 [bitset.members] p33 to: "Bit
value 0 becomes the character zero
, bit value 1 becomes the
character one
.
Change 22.9.4 [bitset.operators] p8 to:
Returns:
os << x.template to_string<charT,traits,allocator<charT> >( use_facet<ctype<charT> >(os.getloc()).widen('0'), use_facet<ctype<charT> >(os.getloc()).widen('1'));
Rationale:
There is a real problem here: we need the character values of '0' and '1', and we have no way to get them since strings don't have imbued locales. In principle the "right" solution would be to provide an extra object, either a ctype facet or a full locale, which would be used to widen '0' and '1'. However, there was some discomfort about using such a heavyweight mechanism. The proposed resolution allows those users who care about this issue to get it right.
We fix the inserter to use the new arguments. Note that we already fixed the analogous problem with the extractor in issue 303(i).
Section: 20.2.10.2 [allocator.members] Status: CD1 Submitter: Markus Mauhart Opened: 2003-02-27 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
20.2.10.2 [allocator.members] allocator members, contains the following 3 lines:
12 Returns: new((void *) p) T( val) void destroy(pointer p); 13 Returns: ((T*) p)->~T()
The type cast "(T*) p" in the last line is redundant cause we know that std::allocator<T>::pointer is a typedef for T*.
Proposed resolution:
Replace "((T*) p)" with "p".
Rationale:
Just a typo, this is really editorial.
Section: 16.4.4.6 [allocator.requirements] Status: CD1 Submitter: Markus Mauhart Opened: 2003-02-27 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
I think that in par2 of [default.con.req] the last two lines of table 32 contain two incorrect type casts. The lines are ...
a.construct(p,t) Effect: new((void*)p) T(t) a.destroy(p) Effect: ((T*)p)?->~T()
.... with the prerequisits coming from the preceding two paragraphs, especially from table 31:
alloc<T> a ;// an allocator for T alloc<T>::pointer p ;// random access iterator // (may be different from T*) alloc<T>::reference r = *p;// T& T const& t ;
For that two type casts ("(void*)p" and "(T*)p") to be well-formed this would require then conversions to T* and void* for all alloc<T>::pointer, so it would implicitely introduce extra requirements for alloc<T>::pointer, additionally to the only current requirement (being a random access iterator).
Proposed resolution:
Accept proposed wording from N2436 part 1.
Note: Actually I would prefer to replace "((T*)p)?->dtor_name" with "p?->dtor_name", but AFAICS this is not possible cause of an omission in 12.4.6 [over.ref] (for which I have filed another DR on 29.11.2002).
[Kona: The LWG thinks this is somewhere on the border between
Open and NAD. The intend is clear: construct
constructs an
object at the location p. It's reading too much into the
description to think that literally calling new
is
required. Tweaking this description is low priority until we can do
a thorough review of allocators, and, in particular, allocators with
non-default pointer types.]
[ Batavia: Proposed resolution changed to less code and more description. ]
[ post Oxford: This would be rendered NAD Editorial by acceptance of N2257. ]
[ Kona (2007): The LWG adopted the proposed resolution of N2387 for this issue which was subsequently split out into a separate paper N2436 for the purposes of voting. The resolution in N2436 addresses this issue. The LWG voted to accelerate this issue to Ready status to be voted into the WP at Kona. ]
Section: 16.4.4.6 [allocator.requirements], 20.2.10.2 [allocator.members] Status: CD1 Submitter: Markus Mauhart Opened: 2003-02-27 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
This applies to the new expression that is contained in both par12 of 20.2.10.2 [allocator.members] and in par2 (table 32) of [default.con.req]. I think this new expression is wrong, involving unintended side effects.
20.2.10.2 [allocator.members] contains the following 3 lines:
11 Returns: the largest value N for which the call allocate(N,0) might succeed. void construct(pointer p, const_reference val); 12 Returns: new((void *) p) T( val)
[default.con.req] in table 32 has the following line:
a.construct(p,t) Effect: new((void*)p) T(t)
.... with the prerequisits coming from the preceding two paragraphs, especially from table 31:
alloc<T> a ;// an allocator for T alloc<T>::pointer p ;// random access iterator // (may be different from T*) alloc<T>::reference r = *p;// T& T const& t ;
Cause of using "new" but not "::new", any existing "T::operator new" function will hide the global placement new function. When there is no "T::operator new" with adequate signature, every_alloc<T>::construct(..) is ill-formed, and most std::container<T,every_alloc<T>> use it; a workaround would be adding placement new and delete functions with adequate signature and semantic to class T, but class T might come from another party. Maybe even worse is the case when T has placement new and delete functions with adequate signature but with "unknown" semantic: I dont like to speculate about it, but whoever implements any_container<T,any_alloc> and wants to use construct(..) probably must think about it.
Proposed resolution:
Replace "new" with "::new" in both cases.
Section: 27.4.3.7.8 [string.swap] Status: CD1 Submitter: Beman Dawes Opened: 2003-03-25 Last modified: 2016-11-12
Priority: Not Prioritized
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Discussion:
std::basic_string, 27.4.3 [basic.string] paragraph 2 says that basic_string "conforms to the requirements of a Sequence, as specified in (23.1.1)." The sequence requirements specified in (23.1.1) to not include any prohibition on swap members throwing exceptions.
Section 23.2 [container.requirements] paragraph 10 does limit conditions under which exceptions may be thrown, but applies only to "all container types defined in this clause" and so excludes basic_string::swap because it is defined elsewhere.
Eric Niebler points out that 27.4.3 [basic.string] paragraph 5 explicitly permits basic_string::swap to invalidates iterators, which is disallowed by 23.2 [container.requirements] paragraph 10. Thus the standard would be contradictory if it were read or extended to read as having basic_string meet 23.2 [container.requirements] paragraph 10 requirements.
Yet several LWG members have expressed the belief that the original intent was that basic_string::swap should not throw exceptions as specified by 23.2 [container.requirements] paragraph 10, and that the standard is unclear on this issue. The complexity of basic_string::swap is specified as "constant time", indicating the intent was to avoid copying (which could cause a bad_alloc or other exception). An important use of swap is to ensure that exceptions are not thrown in exception-safe code.
Note: There remains long standing concern over whether or not it is possible to reasonably meet the 23.2 [container.requirements] paragraph 10 swap requirements when allocators are unequal. The specification of basic_string::swap exception requirements is in no way intended to address, prejudice, or otherwise impact that concern.
Proposed resolution:
In 27.4.3.7.8 [string.swap], add a throws clause:
Throws: Shall not throw exceptions.
Section: 16.4.5.6 [replacement.functions], 17.6.3 [new.delete] Status: CD1 Submitter: Matt Austern Opened: 2003-04-24 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
The eight basic dynamic memory allocation functions (single-object and array versions of ::operator new and ::operator delete, in the ordinary and nothrow forms) are replaceable. A C++ program may provide an alternative definition for any of them, which will be used in preference to the implementation's definition.
Three different parts of the standard mention requirements on replacement functions: 16.4.5.6 [replacement.functions], 17.6.3.2 [new.delete.single] and 17.6.3.3 [new.delete.array], and 6.7.6.4 [basic.stc.auto].
None of these three places say whether a replacement function may
be declared inline. 17.6.3.2 [new.delete.single] paragraph 2 specifies a
signature for the replacement function, but that's not enough:
the inline
specifier is not part of a function's signature.
One might also reason from 9.2.3 [dcl.fct.spec] paragraph 2, which
requires that "an inline function shall be defined in every
translation unit in which it is used," but this may not be quite
specific enough either. We should either explicitly allow or
explicitly forbid inline replacement memory allocation
functions.
Proposed resolution:
Add a new sentence to the end of 16.4.5.6 [replacement.functions] paragraph 3:
"The program's definitions shall not be specified as inline
.
No diagnostic is required."
[Kona: added "no diagnostic is required"]
Rationale:
The fact that inline
isn't mentioned appears to have been
nothing more than an oversight. Existing implementations do not
permit inline functions as replacement memory allocation functions.
Providing this functionality would be difficult in some cases, and is
believed to be of limited value.
Section: 26.13 [alg.c.library] Status: CD1 Submitter: Ray Lischner Opened: 2003-04-08 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
Section 26.13 [alg.c.library] describes bsearch and qsort, from the C standard library. Paragraph 4 does not list any restrictions on qsort, but it should limit the base parameter to point to POD. Presumably, qsort sorts the array by copying bytes, which requires POD.
Proposed resolution:
In 26.13 [alg.c.library] paragraph 4, just after the declarations and before the nonnormative note, add these words: "both of which have the same behavior as the original declaration. The behavior is undefined unless the objects in the array pointed to by base are of POD type."
[Something along these lines is clearly necessary. Matt provided wording.]
Section: 23.3.13.5 [vector.modifiers] Status: CD1 Submitter: Dave Abrahams Opened: 2003-04-27 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
There is a possible defect in the standard: the standard text was never intended to prevent arbitrary ForwardIterators, whose operations may throw exceptions, from being passed, and it also wasn't intended to require a temporary buffer in the case where ForwardIterators were passed (and I think most implementations don't use one). As is, the standard appears to impose requirements that aren't met by any existing implementation.
Proposed resolution:
Replace 23.3.13.5 [vector.modifiers] paragraph 1 with:
1- Notes: Causes reallocation if the new size is greater than the old capacity. If no reallocation happens, all the iterators and references before the insertion point remain valid. If an exception is thrown other than by the copy constructor or assignment operator of T or by any InputIterator operation there are no effects.
[We probably need to say something similar for deque.]
Section: 24.3.4 [iterator.concepts] Status: CD1 Submitter: Nathan Myers Opened: 2003-06-03 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
Clause 24.3.4 [iterator.concepts], paragraph 5, says that the only expression that is defined for a singular iterator is "an assignment of a non-singular value to an iterator that holds a singular value". This means that destroying a singular iterator (e.g. letting an automatic variable go out of scope) is technically undefined behavior. This seems overly strict, and probably unintentional.
Proposed resolution:
Change the sentence in question to "... the only exceptions are destroying an iterator that holds a singular value, or the assignment of a non-singular value to an iterator that holds a singular value."
Section: 31.10.4.4 [ifstream.members], 31.10.5.4 [ofstream.members] Status: CD1 Submitter: Nathan Myers Opened: 2003-06-03 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
A strict reading of [fstreams] shows that opening or closing a basic_[io]fstream does not affect the error bits. This means, for example, that if you read through a file up to EOF, and then close the stream and reopen it at the beginning of the file, the EOF bit in the stream's error state is still set. This is counterintuitive.
The LWG considered this issue once before, as issue 22(i), and put in a footnote to clarify that the strict reading was indeed correct. We did that because we believed the standard was unambiguous and consistent, and that we should not make architectural changes in a TC. Now that we're working on a new revision of the language, those considerations no longer apply.
Proposed resolution:
Change 31.10.4.4 [ifstream.members], para. 3 from:
Calls rdbuf()->open(s,mode|in). If that function returns a null pointer, calls setstate(failbit) (which may throw ios_base::failure [Footnote: (lib.iostate.flags)].
to:
Calls rdbuf()->open(s,mode|in). If that function returns a null pointer, calls setstate(failbit) (which may throw ios_base::failure [Footnote: (lib.iostate.flags)), else calls clear().
Change 31.10.5.4 [ofstream.members], para. 3 from:
Calls rdbuf()->open(s,mode|out). If that function returns a null pointer, calls setstate(failbit) (which may throw ios_base::failure [Footnote: (lib.iostate.flags)).
to:
Calls rdbuf()->open(s,mode|out). If that function returns a null pointer, calls setstate(failbit) (which may throw ios_base::failure [Footnote: (lib.iostate.flags)), else calls clear().
Change 31.10.6.4 [fstream.members], para. 3 from:
Calls rdbuf()->open(s,mode), If that function returns a null pointer, calls setstate(failbit), (which may throw ios_base::failure). (lib.iostate.flags) )
to:
Calls rdbuf()->open(s,mode), If that function returns a null pointer, calls setstate(failbit), (which may throw ios_base::failure). (lib.iostate.flags) ), else calls clear().
[Kona: the LWG agrees this is a good idea. Post-Kona: Bill provided wording. He suggests having open, not close, clear the error flags.]
[Post-Sydney: Howard provided a new proposed resolution. The old one didn't make sense because it proposed to fix this at the level of basic_filebuf, which doesn't have access to the stream's error state. Howard's proposed resolution fixes this at the level of the three fstream class template instead.]
Section: 23.3.11.2 [list.cons], 23.3.11.4 [list.modifiers] Status: CD1 Submitter: Hans Bos Opened: 2003-06-07 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
Sections 23.3.11.2 [list.cons] and 23.3.11.4 [list.modifiers] list comparison operators (==, !=, <, <=, >, =>) for queue and stack. Only the semantics for queue::operator== (23.3.11.2 [list.cons] par2) and queue::operator< (23.3.11.2 [list.cons] par3) are defined.
Proposed resolution:
Add the following new paragraphs after 23.3.11.2 [list.cons] paragraph 3:
operator!=Returns:
x.c != y.c
operator>Returns:
x.c > y.c
operator<=Returns:
x.c <= y.c
operator>=Returns:
x.c >= y.c
Add the following paragraphs at the end of 23.3.11.4 [list.modifiers]:
operator==Returns:
x.c == y.c
operator<Returns:
x.c < y.c
operator!=Returns:
x.c != y.c
operator>Returns:
x.c > y.c
operator<=Returns:
x.c <= y.c
operator>=Returns:
x.c >= y.c
[Kona: Matt provided wording.]
Rationale:
There isn't any real doubt about what these operators are supposed to do, but we ought to spell it out.
Section: 26.8.7 [alg.set.operations] Status: CD1 Submitter: Daniel Frey Opened: 2003-07-09 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
26.8.7 [alg.set.operations] paragraph 1 reads: "The semantics of the set operations are generalized to multisets in a standard way by defining union() to contain the maximum number of occurrences of every element, intersection() to contain the minimum, and so on."
This is wrong. The name of the functions are set_union() and set_intersection(), not union() and intersection().
Proposed resolution:
Change that sentence to use the correct names.
Section: 31.5.4.4 [iostate.flags] Status: CD1 Submitter: Martin Sebor Opened: 2003-07-10 Last modified: 2016-01-28
Priority: Not Prioritized
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Duplicate of: 429
Discussion:
The Effects clause in 31.5.4.4 [iostate.flags] paragraph 5 says that the function only throws if the respective bits are already set prior to the function call. That's obviously not the intent. The typo ought to be corrected and the text reworded as: "If (state & exceptions()) == 0, returns. ..."
Proposed resolution:
In 31.5.4.4 [iostate.flags] paragraph 5, replace "If (rdstate() & exceptions()) == 0" with "If ((state | (rdbuf() ? goodbit : badbit)) & exceptions()) == 0".
[Kona: the original proposed resolution wasn't quite right. We really do mean rdstate(); the ambiguity is that the wording in the standard doesn't make it clear whether we mean rdstate() before setting the new state, or rdsate() after setting it. We intend the latter, of course. Post-Kona: Martin provided wording.]
Section: 31.7.5.3.3 [istream.extractors] Status: CD1 Submitter: Bo Persson Opened: 2003-07-13 Last modified: 2017-04-22
Priority: Not Prioritized
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Discussion:
The second sentence of the proposed resolution says:
"If it inserted no characters because it caught an exception thrown while extracting characters from sb and ..."
However, we are not extracting from sb, but extracting from the basic_istream (*this) and inserting into sb. I can't really tell if "extracting" or "sb" is a typo.
[ Sydney: Definitely a real issue. We are, indeed, extracting characters from an istream and not from sb. The problem was there in the FDIS and wasn't fixed by issue 64(i). Probably what was intended was to have *this instead of sb. We're talking about the exception flag state of a basic_istream object, and there's only one basic_istream object in this discussion, so that would be a consistent interpretation. (But we need to be careful: the exception policy of this member function must be consistent with that of other extractors.) PJP will provide wording. ]
Proposed resolution:
Change the sentence from:
If it inserted no characters because it caught an exception thrown while extracting characters from sb and failbit is on in exceptions(), then the caught exception is rethrown.
to:
If it inserted no characters because it caught an exception thrown while extracting characters from *this and failbit is on in exceptions(), then the caught exception is rethrown.
Section: 23.3.13.5 [vector.modifiers] Status: CD1 Submitter: Matt Austern Opened: 2003-08-19 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
Consider the following code fragment:
int A[8] = { 1,3,5,7,9,8,4,2 }; std::vector<int> v(A, A+8); std::vector<int>::iterator i1 = v.begin() + 3; std::vector<int>::iterator i2 = v.begin() + 4; v.erase(i1);
Which iterators are invalidated by v.erase(i1)
: i1, i2,
both, or neither?
On all existing implementations that I know of, the status of i1 and i2 is the same: both of them will be iterators that point to some elements of the vector (albeit not the same elements they did before). You won't get a crash if you use them. Depending on exactly what you mean by "invalidate", you might say that neither one has been invalidated because they still point to something, or you might say that both have been invalidated because in both cases the elements they point to have been changed out from under the iterator.
The standard doesn't say either of those things. It says that erase invalidates all iterators and references "after the point of the erase". This doesn't include i1, since it's at the point of the erase instead of after it. I can't think of any sensible definition of invalidation by which one can say that i2 is invalidated but i1 isn't.
(This issue is important if you try to reason about iterator validity based only on the guarantees in the standard, rather than reasoning from typical implementation techniques. Strict debugging modes, which some programmers find useful, do not use typical implementation techniques.)
Proposed resolution:
In 23.3.13.5 [vector.modifiers] paragraph 3, change "Invalidates all the iterators and references after the point of the erase" to "Invalidates iterators and references at or after the point of the erase".
Rationale:
I believe this was essentially a typographical error, and that it was taken for granted that erasing an element invalidates iterators that point to it. The effects clause in question treats iterators and references in parallel, and it would seem counterintuitive to say that a reference to an erased value remains valid.
Section: 31.7.5.5 [istream.manip] Status: CD1 Submitter: Martin Sebor Opened: 2003-09-18 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
According to 27.6.1.4, the ws() manipulator is not required to construct the sentry object. The manipulator is also not a member function so the text in 27.6.1, p1 through 4 that describes the exception policy for istream member functions does not apply. That seems inconsistent with the rest of extractors and all the other input functions (i.e., ws will not cause a tied stream to be flushed before extraction, it doesn't check the stream's exceptions or catch exceptions thrown during input, and it doesn't affect the stream's gcount).
Proposed resolution:
Add to 31.7.5.5 [istream.manip], immediately before the first sentence of paragraph 1, the following text:
Behaves as an unformatted input function (as described in 27.6.1.3, paragraph 1), except that it does not count the number of characters extracted and does not affect the value returned by subsequent calls to is.gcount(). After constructing a sentry object...
[Post-Kona: Martin provided wording]
Section: 17.3.6 [climits.syn] Status: CD1 Submitter: Martin Sebor Opened: 2003-09-18 Last modified: 2017-06-15
Priority: Not Prioritized
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Discussion:
Given two overloads of the function foo(), one taking an argument of type
int and the other taking a long, which one will the call foo(LONG_MAX)
resolve to? The expected answer should be foo(long), but whether that
is true depends on the #defintion of the LONG_MAX macro, specifically
its type. This issue is about the fact that the type of these macros
is not actually required to be the same as the the type each respective
limit.
Section 18.2.2 of the C++ Standard does not specify the exact types of
the XXX_MIN and XXX_MAX macros #defined in the <climits> and <limits.h>
headers such as INT_MAX and LONG_MAX and instead defers to the C standard.
Section 5.2.4.2.1, p1 of the C standard specifies that "The values [of
these constants] shall be replaced by constant expressions suitable for use
in #if preprocessing directives. Moreover, except for CHAR_BIT and MB_LEN_MAX,
the following shall be replaced by expressions that have the same type as
would an expression that is an object of the corresponding type converted
according to the integer promotions."
The "corresponding type converted according to the integer promotions" for
LONG_MAX is, according to 6.4.4.1, p5 of the C standard, the type of long
converted to the first of the following set of types that can represent it:
int, long int, long long int. So on an implementation where (sizeof(long)
== sizeof(int)) this type is actually int, while on an implementation where
(sizeof(long) > sizeof(int)) holds this type will be long.
This is not an issue in C since the type of the macro cannot be detected
by any conforming C program, but it presents a portability problem in C++
where the actual type is easily detectable by overload resolution.
[Kona: the LWG does not believe this is a defect. The C macro
definitions are what they are; we've got a better
mechanism, std::numeric_limits
, that is specified more
precisely than the C limit macros. At most we should add a
nonnormative note recommending that users who care about the exact
types of limit quantities should use <limits> instead of
<climits>.]
Proposed resolution:
Change [c.limits], paragraph 2:
-2- The contents are the same as the Standard C library header
<limits.h>
. [Note: The types of the macros in<climits>
are not guaranteed to match the type to which they refer.--end note]
failbit
if eofbit
is already setSection: 31.7.5.2.4 [istream.sentry] Status: C++11 Submitter: Martin Sebor Opened: 2003-09-18 Last modified: 2021-06-06
Priority: Not Prioritized
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Discussion:
[istream::sentry], p2 says that istream::sentry
ctor prepares for input if is.good()
is true. p4 then goes on to say that the ctor sets the sentry::ok_
member to
true if the stream state is good after any preparation. 31.7.5.3.1 [istream.formatted.reqmts], p1 then
says that a formatted input function endeavors to obtain the requested input
if the sentry's operator bool()
returns true.
Given these requirements, no formatted extractor should ever set failbit
if
the initial stream rdstate() == eofbit
. That is contrary to the behavior of
all implementations I tested. The program below prints out
eof = 1, fail = 0 eof = 1, fail = 1
on all of them.
#include <sstream> #include <cstdio> int main() { std::istringstream strm ("1"); int i = 0; strm >> i; std::printf ("eof = %d, fail = %d\n", !!strm.eof (), !!strm.fail ()); strm >> i; std::printf ("eof = %d, fail = %d\n", !!strm.eof (), !!strm.fail ()); }
Comments from Jerry Schwarz (c++std-lib-11373):
Jerry Schwarz wrote:
I don't know where (if anywhere) it says it in the standard, but the
formatted extractors are supposed to set failbit
if they don't extract
any characters. If they didn't then simple loops like
while (cin >> x);
would loop forever.
Further comments from Martin Sebor:
The question is which part of the extraction should prevent this from happening
by setting failbit
when eofbit
is already set. It could either be the sentry
object or the extractor. It seems that most implementations have chosen to
set failbit
in the sentry [...] so that's the text that will need to be
corrected.
Pre Berlin: This issue is related to 342(i). If the sentry
sets failbit
when it finds eofbit
already set, then
you can never seek away from the end of stream.
Kona: Possibly NAD. If eofbit
is set then good()
will return false. We
then set ok to false. We believe that the sentry's
constructor should always set failbit
when ok is false, and
we also think the standard already says that. Possibly it could be
clearer.
[ 2009-07 Frankfurt ]
Moved to Ready.
Proposed resolution:
Change [istream::sentry], p2 to:
explicit sentry(basic_istream<charT,traits>& is , bool noskipws = false);-2- Effects: If
is.good()
istrue
false
, callsis.setstate(failbit)
. Otherwise prepares for formatted or unformatted input. ...
Section: 31.10 [file.streams] Status: CD1 Submitter: Martin Sebor Opened: 2003-09-18 Last modified: 2017-06-15
Priority: Not Prioritized
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Discussion:
7.19.1, p2, of C99 requires that the FILE type only be declared in <stdio.h>. None of the (implementation-defined) members of the struct is mentioned anywhere for obvious reasons.
C++ says in 27.8.1, p2 that FILE is a type that's defined in <cstdio>. Is it really the intent that FILE be a complete type or is an implementation allowed to just declare it without providing a full definition?
Proposed resolution:
In the first sentence of [fstreams] paragraph 2, change "defined" to "declared".
Rationale:
We don't want to impose any restrictions beyond what the C standard already says. We don't want to make anything implementation defined, because that imposes new requirements in implementations.
Section: 16.4.5.3 [reserved.names] Status: CD1 Submitter: Martin Sebor Opened: 2003-09-18 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
It has been suggested that 17.4.3.1, p1 may or may not allow programs to explicitly specialize members of standard templates on user-defined types. The answer to the question might have an impact where library requirements are given using the "as if" rule. I.e., if programs are allowed to specialize member functions they will be able to detect an implementation's strict conformance to Effects clauses that describe the behavior of the function in terms of the other member function (the one explicitly specialized by the program) by relying on the "as if" rule.
Proposed resolution:
Add the following sentence to 16.4.5.3 [reserved.names], p1:
It is undefined for a C++ program to add declarations or definitions to namespace std or namespaces within namespace
std
unless otherwise specified. A program may add template specializations for any standard library template to namespacestd
. Such a specialization (complete or partial) of a standard library template results in undefined behavior unless the declaration depends on a user-defined type of external linkage and unless the specialization meets the standard library requirements for the original template.168) A program has undefined behavior if it declares
- an explicit specialization of any member function of a standard library class template, or
- an explicit specialization of any member function template of a standard library class or class template, or
- an explicit or partial specialization of any member class template of a standard library class or class template.
A program may explicitly instantiate any templates in the standard library only if the declaration depends on the name of a user-defined type of external linkage and the instantiation meets the standard library requirements for the original template.
[Kona: straw poll was 6-1 that user programs should not be allowed to specialize individual member functions of standard library class templates, and that doing so invokes undefined behavior. Post-Kona: Martin provided wording.]
[Sydney: The LWG agrees that the standard shouldn't permit users to specialize individual member functions unless they specialize the whole class, but we're not sure these words say what we want them to; they could be read as prohibiting the specialization of any standard library class templates. We need to consult with CWG to make sure we use the right wording.]
Section: 99 [depr.temporary.buffer] Status: CD1 Submitter: Martin Sebor Opened: 2003-09-18 Last modified: 2017-06-15
Priority: Not Prioritized
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Discussion:
The standard is not clear about the requirements on the value returned from a call to get_temporary_buffer(0). In particular, it fails to specify whether the call should return a distinct pointer each time it is called (like operator new), or whether the value is unspecified (as if returned by malloc). The standard also fails to mention what the required behavior is when the argument is less than 0.
Proposed resolution:
Change 21.3.4 [meta.help] paragraph 2 from "...or a pair of 0 values if no storage can be obtained" to "...or a pair of 0 values if no storage can be obtained or if n <= 0."
[Kona: Matt provided wording]
Section: 26.6.15 [alg.search], 26.7.6 [alg.fill], 26.7.7 [alg.generate] Status: CD1 Submitter: Martin Sebor Opened: 2003-09-18 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
The complexity requirements for these function templates are incorrect (or don't even make sense) for negative n:
25.1.9, p7 (search_n):
Complexity: At most (last1 - first1) * count applications
of the corresponding predicate.
25.2.5, p3 (fill_n):
Complexity: Exactly last - first (or n) assignments.
25.2.6, p3 (generate_n):
Complexity: Exactly last - first (or n) assignments.
In addition, the Requirements or the Effects clauses for the latter two templates don't say anything about the behavior when n is negative.
Proposed resolution:
Change 25.1.9, p7 to
Complexity: At most (last1 - first1) * count applications of the corresponding predicate if count is positive, or 0 otherwise.
Change 25.2.5, p2 to
Effects: Assigns value through all the iterators in the range [first, last), or [first, first + n) if n is positive, none otherwise.
Change 25.2.5, p3 to:
Complexity: Exactly last - first (or n if n is positive, or 0 otherwise) assignments.
Change 25.2.6, p1 to (notice the correction for the misspelled "through"):
Effects: Invokes the function object genand assigns the return value of gen through all the iterators in the range [first, last), or [first, first + n) if n is positive, or [first, first) otherwise.
Change 25.2.6, p3 to:
Complexity: Exactly last - first (or n if n is positive, or 0 otherwise) assignments.
Rationale:
Informally, we want to say that whenever we see a negative number we treat it the same as if it were zero. We believe the above changes do that (although they may not be the minimal way of saying so). The LWG considered and rejected the alternative of saying that negative numbers are undefined behavior.
Section: 28.3.4.3.2.3 [facet.num.get.virtuals] Status: C++11 Submitter: Martin Sebor Opened: 2003-09-18 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
The requirements specified in Stage 2 and reiterated in the rationale of DR 221 (and echoed again in DR 303) specify that num_get<charT>:: do_get() compares characters on the stream against the widened elements of "012...abc...ABCX+-"
An implementation is required to allow programs to instantiate the num_get template on any charT that satisfies the requirements on a user-defined character type. These requirements do not include the ability of the character type to be equality comparable (the char_traits template must be used to perform tests for equality). Hence, the num_get template cannot be implemented to support any arbitrary character type. The num_get template must either make the assumption that the character type is equality-comparable (as some popular implementations do), or it may use char_traits<charT> to do the comparisons (some other popular implementations do that). This diversity of approaches makes it difficult to write portable programs that attempt to instantiate the num_get template on user-defined types.
[Kona: the heart of the problem is that we're theoretically
supposed to use traits classes for all fundamental character
operations like assignment and comparison, but facets don't have
traits parameters. This is a fundamental design flaw and it
appears all over the place, not just in this one place. It's not
clear what the correct solution is, but a thorough review of facets
and traits is in order. The LWG considered and rejected the
possibility of changing numeric facets to use narrowing instead of
widening. This may be a good idea for other reasons (see issue
459(i)), but it doesn't solve the problem raised by this
issue. Whether we use widen or narrow the num_get
facet
still has no idea which traits class the user wants to use for
the comparison, because only streams, not facets, are passed traits
classes. The standard does not require that two different
traits classes with the same char_type
must necessarily
have the same behavior.]
Informally, one possibility: require that some of the basic
character operations, such as eq
, lt
,
and assign
, must behave the same way for all traits classes
with the same char_type
. If we accept that limitation on
traits classes, then the facet could reasonably be required to
use char_traits<charT>
.
[ 2009-07 Frankfurt ]
There was general agreement that the standard only needs to specify the behavior when the character type is char or wchar_t.
Beman: we don't need to worry about C++1x because there is a non-zero possibility that we would have a replacement facility for iostreams that would solve these problems.
We need to change the following sentence in [locale.category], paragraph 6 to specify that C is char and wchar_t:
"A template formal parameter with name C represents the set of all possible specializations on a parameter that satisfies the requirements for a character on which any member of the iostream components can be instantiated."
We also need to specify in 27 that the basic character operations, such as eq, lt, and assign use std::char_traits.
Daniel volunteered to provide wording.
[ 2009-09-19 Daniel provided wording. ]
[ 2009-10 Santa Cruz: ]
Leave as Open. Alisdair and/or Tom will provide wording based on discussions. We want to clearly state that streams and locales work just on
char
andwchar_t
(except where otherwise specified).
[ 2010-02-06 Tom updated the proposed wording. ]
[ The original proposed wording is preserved here: ]
Change 28.3.3.1.2.1 [locale.category]/6:
[..] A template formal parameter with name
C
represents the set of all possible specializations on achar
orwchar_t
parameterthat satisfies the requirements for a character on which any of the iostream components can be instantiated. [..]Add the following sentence to the end of 28.3.4.3 [category.numeric]/2:
[..] These specializations refer to [..], and also for the
ctype<>
facet to perform character classification. Implementations are encouraged but not required to use thechar_traits<charT>
functions for all comparisons and assignments of characters of typecharT
that do not belong to the set of required specializations.Change 28.3.4.3.2.3 [facet.num.get.virtuals]/3:
Stage 2: If
in==end
then stage 2 terminates. Otherwise acharT
is taken fromin
and local variables are initialized as if bychar_type ct = *in; using tr = char_traits<char_type>; const char_type* pos = tr::find(atoms, sizeof(src) - 1, ct); char c = src[find(atoms, atoms + sizeof(src) - 1, ct) - atomspos ? pos - atoms : sizeof(src) - 1]; if (tr::eq(ct,ct ==use_facet<numpunct<charT>(loc).decimal_point())) c = '.'; bool discard = tr::eq(ct,ct ==use_facet<numpunct<charT>(loc).thousands_sep()) && use_facet<numpunct<charT> >(loc).grouping().length() != 0;where the values
src
andatoms
are defined as if by: [..][Remark of the author: I considered to replace the initialization "
char_type ct = *in;
" by the sequence "char_type ct; tr::assign(ct, *in);
", but decided against it, because it is a copy-initialization context, not an assignment]Add the following sentence to the end of 28.3.4.6 [category.time]/1:
[..] Their members use [..] , to determine formatting details. Implementations are encouraged but not required to use the
char_traits<charT>
functions for all comparisons and assignments of characters of typecharT
that do not belong to the set of required specializations.Change 28.3.4.6.2.2 [locale.time.get.members]/8 bullet 4:
The next element ofFor the next elementfmt
is equal to'%'
c
offmt char_traits<char_type>::eq(c, use_facet<ctype<char_type>>(f.getloc()).widen('%')) == true
, [..]Add the following sentence to the end of 28.3.4.7 [category.monetary]/2:
Their members use [..] to determine formatting details. Implementations are encouraged but not required to use the
char_traits<charT>
functions for all comparisons and assignments of characters of typecharT
that do not belong to the set of required specializations.Change 28.3.4.7.2.3 [locale.money.get.virtuals]/4:
[..] The value
units
is produced as if by:for (int i = 0; i < n; ++i) buf2[i] = src[char_traits<charT>::find(atoms,atoms+sizeof(src), buf1[i]) - atoms]; buf2[n] = 0; sscanf(buf2, "%Lf", &units);Change 28.3.4.7.3.3 [locale.money.put.virtuals]/1:
[..] for character buffers
buf1
andbuf2
. If for the first characterc
indigits
orbuf2
is equal toct.widen('-')
char_traits<charT>::eq(c, ct.widen('-')) == true
, [..]Add a footnote to the first sentence of 31.7.5.3.2 [istream.formatted.arithmetic]/1:
As in the case of the inserters, these extractors depend on the locale's
num_get<>
(22.4.2.1) object to perform parsing the input stream data.(footnote) [..]footnote) If the traits of the input stream has different semantics for
lt()
,eq()
, andassign()
thanchar_traits<char_type>
, this may give surprising results.Add a footnote to the second sentence of 31.7.6.3.2 [ostream.inserters.arithmetic]/1:
Effects: The classes
num_get<>
andnum_put<>
handle locale-dependent numeric formatting and parsing. These inserter functions use the imbued locale value to perform numeric formatting.(footnote) [..]footnote) If the traits of the output stream has different semantics for
lt()
,eq()
, andassign()
thanchar_traits<char_type>
, this may give surprising results.Add a footnote after the first sentence of 31.7.8 [ext.manip]/4:
Returns: An object of unspecified type such that if in is an object of type
basic_istream<charT, traits>
then the expressionin >> get_money(mon, intl)
behaves as if it calledf(in, mon, intl)
, where the functionf
is defined as:(footnote) [..]footnote) If the traits of the input stream has different semantics for
lt()
,eq()
, andassign()
thanchar_traits<char_type>
, this may give surprising results.Add a footnote after the first sentence of 31.7.8 [ext.manip]/5:
Returns: An object of unspecified type such that if
out
is an object of typebasic_ostream<charT, traits>
then the expressionout << put_money(mon, intl)
behaves as a formatted input function that callsf(out, mon, intl)
, where the functionf
is defined as:(footnote) [..]footnote) If the traits of the output stream has different semantics for
lt()
,eq()
, andassign()
thanchar_traits<char_type>
, this may give surprising results.13) Add a footnote after the first sentence of 31.7.8 [ext.manip]/8:
Returns: An object of unspecified type such that if
in
is an object of type basic_istream<charT, traits>
then the expressionin >>get_time(tmb, fmt)
behaves as if it calledf(in, tmb, fmt)
, where the functionf
is defined as:(footnote) [..]footnote) If the traits of the input stream has different semantics for
lt()
,eq()
, andassign()
thanchar_traits<char_type>
, this may give surprising results.Add a footnote after the first sentence of 31.7.8 [ext.manip]/10:
Returns: An object of unspecified type such that if
out
is an object of typebasic_ostream<charT, traits>
then the expressionout <<put_time(tmb, fmt)
behaves as if it calledf(out, tmb, fmt)
, where the functionf
is defined as:(footnote) [..]footnote) If the traits of the output stream has different semantics for
lt()
,eq()
, andassign()
thanchar_traits<char_type>
, this may give surprising results.
[ 2010 Pittsburgh: ]
Moved to Ready with only two of the bullets. The original wording is preserved here:
Change 28.3.3.1.2.1 [locale.category]/6:
[..] A template formal parameter with name
C
represents the setof all possible specializations on aof types containingchar
,wchar_t
, and any other implementation-defined character typeparameterthat satisfies the requirements for a character on which any of the iostream components can be instantiated. [..]Add the following sentence to the end of 28.3.4.3 [category.numeric]/2:
[..] These specializations refer to [..], and also for the
ctype<>
facet to perform character classification. [Note: Implementations are encouraged but not required to use thechar_traits<charT>
functions for all comparisons and assignments of characters of typecharT
that do not belong to the set of required specializations - end note].Change 28.3.4.3.2.3 [facet.num.get.virtuals]/3:
Stage 2: If
in==end
then stage 2 terminates. Otherwise acharT
is taken fromin
and local variables are initialized as if bychar_type ct = *in; using tr = char_traits<char_type>; const char_type* pos = tr::find(atoms, sizeof(src) - 1, ct); char c = src[find(atoms, atoms + sizeof(src) - 1, ct) - atomspos ? pos - atoms : sizeof(src) - 1]; if (tr::eq(ct,ct ==use_facet<numpunct<charT>(loc).decimal_point())) c = '.'; bool discard = tr::eq(ct,ct ==use_facet<numpunct<charT>(loc).thousands_sep()) && use_facet<numpunct<charT> >(loc).grouping().length() != 0;where the values
src
andatoms
are defined as if by: [..][Remark of the author: I considered to replace the initialization "
char_type ct = *in;
" by the sequence "char_type ct; tr::assign(ct, *in);
", but decided against it, because it is a copy-initialization context, not an assignment]Add the following sentence to the end of 28.3.4.6 [category.time]/1:
[..] Their members use [..] , to determine formatting details. [Note: Implementations are encouraged but not required to use the
char_traits<charT>
functions for all comparisons and assignments of characters of typecharT
that do not belong to the set of required specializations - end note].Change 28.3.4.6.2.2 [locale.time.get.members]/8 bullet 4:
The next element ofFor the next elementfmt
is equal to'%'
c
offmt char_traits<char_type>::eq(c, use_facet<ctype<char_type>>(f.getloc()).widen('%')) == true
, [..]Add the following sentence to the end of 28.3.4.7 [category.monetary]/2:
Their members use [..] to determine formatting details. [Note: Implementations are encouraged but not required to use the
char_traits<charT>
functions for all comparisons and assignments of characters of typecharT
that do not belong to the set of required specializations - end note].Change 28.3.4.7.2.3 [locale.money.get.virtuals]/4:
[..] The value
units
is produced as if by:for (int i = 0; i < n; ++i) buf2[i] = src[char_traits<charT>::find(atoms,atoms+sizeof(src), buf1[i]) - atoms]; buf2[n] = 0; sscanf(buf2, "%Lf", &units);Change 28.3.4.7.3.3 [locale.money.put.virtuals]/1:
[..] for character buffers
buf1
andbuf2
. If for the first characterc
indigits
orbuf2
is equal toct.widen('-')
char_traits<charT>::eq(c, ct.widen('-')) == true
, [..]Add a new paragraph after the first paragraph of 31.2.3 [iostreams.limits.pos]/1:
In the classes of clause 27, a template formal parameter with name
charT
represents one of the set of types containingchar
,wchar_t
, and any other implementation-defined character type that satisfies the requirements for a character on which any of the iostream components can be instantiated.Add a footnote to the first sentence of 31.7.5.3.2 [istream.formatted.arithmetic]/1:
As in the case of the inserters, these extractors depend on the locale's
num_get<>
(22.4.2.1) object to perform parsing the input stream data.(footnote) [..]footnote) If the traits of the input stream has different semantics for
lt()
,eq()
, andassign()
thanchar_traits<char_type>
, this may give surprising results.Add a footnote to the second sentence of 31.7.6.3.2 [ostream.inserters.arithmetic]/1:
Effects: The classes
num_get<>
andnum_put<>
handle locale-dependent numeric formatting and parsing. These inserter functions use the imbued locale value to perform numeric formatting.(footnote) [..]footnote) If the traits of the output stream has different semantics for
lt()
,eq()
, andassign()
thanchar_traits<char_type>
, this may give surprising results.Add a footnote after the first sentence of 31.7.8 [ext.manip]/4:
Returns: An object of unspecified type such that if in is an object of type
basic_istream<charT, traits>
then the expressionin >> get_money(mon, intl)
behaves as if it calledf(in, mon, intl)
, where the functionf
is defined as:(footnote) [..]footnote) If the traits of the input stream has different semantics for
lt()
,eq()
, andassign()
thanchar_traits<char_type>
, this may give surprising results.Add a footnote after the first sentence of 31.7.8 [ext.manip]/5:
Returns: An object of unspecified type such that if
out
is an object of typebasic_ostream<charT, traits>
then the expressionout << put_money(mon, intl)
behaves as a formatted input function that callsf(out, mon, intl)
, where the functionf
is defined as:(footnote) [..]footnote) If the traits of the output stream has different semantics for
lt()
,eq()
, andassign()
thanchar_traits<char_type>
, this may give surprising results.Add a footnote after the first sentence of 31.7.8 [ext.manip]/8:
Returns: An object of unspecified type such that if
in
is an object of type basic_istream<charT, traits>
then the expressionin >>get_time(tmb, fmt)
behaves as if it calledf(in, tmb, fmt)
, where the functionf
is defined as:(footnote) [..]footnote) If the traits of the input stream has different semantics for
lt()
,eq()
, andassign()
thanchar_traits<char_type>
, this may give surprising results.Add a footnote after the first sentence of 31.7.8 [ext.manip]/10:
Returns: An object of unspecified type such that if
out
is an object of typebasic_ostream<charT, traits>
then the expressionout <<put_time(tmb, fmt)
behaves as if it calledf(out, tmb, fmt)
, where the functionf
is defined as:(footnote) [..]footnote) If the traits of the output stream has different semantics for
lt()
,eq()
, andassign()
thanchar_traits<char_type>
, this may give surprising results.
Proposed resolution:
Change 28.3.3.1.2.1 [locale.category]/6:
[..] A template formal parameter with name
C
represents the setof all possible specializations on aof types containingchar
,wchar_t
, and any other implementation-defined character typeparameterthat satisfies the requirements for a character on which any of the iostream components can be instantiated. [..]
Add a new paragraph after the first paragraph of 31.2.3 [iostreams.limits.pos]/1:
In the classes of clause 27, a template formal parameter with name
charT
represents one of the set of types containingchar
,wchar_t
, and any other implementation-defined character type that satisfies the requirements for a character on which any of the iostream components can be instantiated.
Section: 27.4.3.7.5 [string.erase] Status: CD1 Submitter: Martin Sebor Opened: 2003-09-18 Last modified: 2016-11-12
Priority: Not Prioritized
View all other issues in [string.erase].
View all issues with CD1 status.
Discussion:
23.1.1, p3 along with Table 67 specify as a prerequisite for a.erase(q) that q must be a valid dereferenceable iterator into the sequence a.
However, 21.3.5.5, p5 describing string::erase(p) only requires that p be a valid iterator.
This may be interepreted as a relaxation of the general requirement, which is most likely not the intent.
Proposed resolution:
Remove 27.4.3.7.5 [string.erase] paragraph 5.
Rationale:
The LWG considered two options: changing the string requirements to match the general container requirements, or just removing the erroneous string requirements altogether. The LWG chose the latter option, on the grounds that duplicating text always risks the possibility that it might be duplicated incorrectly.
valarray
subset operationsSection: 29.6.2.5 [valarray.sub] Status: C++11 Submitter: Martin Sebor Opened: 2003-09-18 Last modified: 2016-01-28
Priority: Not Prioritized
View all issues with C++11 status.
Discussion:
The standard fails to specify the behavior of valarray::operator[](slice) and other valarray subset operations when they are passed an "invalid" slice object, i.e., either a slice that doesn't make sense at all (e.g., slice (0, 1, 0) or one that doesn't specify a valid subset of the valarray object (e.g., slice (2, 1, 1) for a valarray of size 1).
[Kona: the LWG believes that invalid slices should invoke undefined behavior. Valarrays are supposed to be designed for high performance, so we don't want to require specific checking. We need wording to express this decision.]
[ Bellevue: ]
Please note that the standard also fails to specify the behavior of slice_array and gslice_array in the valid case. Bill Plauger will endeavor to provide revised wording for
slice_array
andgslice_array
.
[ post-Bellevue: Bill provided wording. ]
[ 2009-07 Frankfurt ]
Move to Ready.
[ 2009-11-04 Pete opens: ]
The resolution to LWG issue 430(i) has not been applied — there have been changes to the underlying text, and the resolution needs to be reworked.
[ 2010-03-09 Matt updated wording. ]
[ 2010 Pittsburgh: Moved to Ready for Pittsburgh. ]
Proposed resolution:
Replace 29.6.2.5 [valarray.sub], with the following:
The member operator is overloaded to provide several ways to select sequences of elements from among those controlled by
*this
. Each of these operations returns a subset of the array. The const-qualified versions return this subset as a newvalarray
. The non-const versions return a class template object which has reference semantics to the original array, working in conjunction with various overloads ofoperator=
(and other assigning operators) to allow selective replacement (slicing) of the controlled sequence. In each case the selected element(s) must exist.valarray<T> operator[](slice slicearr) const;This function returns an object of class
valarray<T>
containing those elements of the controlled sequence designated byslicearr
. [Example:valarray<char> v0("abcdefghijklmnop", 16); valarray<char> v1("ABCDE", 5); v0[slice(2, 5, 3)] = v1; // v0 == valarray<char>("abAdeBghCjkDmnEp", 16)end example]
valarray<T> operator[](slice slicearr);This function selects those elements of the controlled sequence designated by
slicearr
. [Example:valarray<char> v0("abcdefghijklmnop", 16); valarray<char> v1("ABCDE", 5); v0[slice(2, 5, 3)] = v1; // v0 == valarray<char>("abAdeBghCjkDmnEp", 16)end example]
valarray<T> operator[](const gslice& gslicearr) const;This function returns an object of class
valarray<T>
containing those elements of the controlled sequence designated bygslicearr
. [Example:valarray<char> v0("abcdefghijklmnop", 16); const size_t lv[] = {2, 3}; const size_t dv[] = {7, 2}; const valarray<size_t> len(lv, 2), str(dv, 2); // v0[gslice(3, len, str)] returns // valarray<char>("dfhkmo", 6)end example]
gslice_array<T> operator[](const gslice& gslicearr);This function selects those elements of the controlled sequence designated by
gslicearr
. [Example:valarray<char> v0("abcdefghijklmnop", 16); valarray<char> v1("ABCDEF", 6); const size_t lv[] = {2, 3}; const size_t dv[] = {7, 2}; const valarray<size_t> len(lv, 2), str(dv, 2); v0[gslice(3, len, str)] = v1; // v0 == valarray<char>("abcAeBgCijDlEnFp", 16)end example]
valarray<T> operator[](const valarray<bool>& boolarr) const;This function returns an object of class
valarray<T>
containing those elements of the controlled sequence designated byboolarr
. [Example:valarray<char> v0("abcdefghijklmnop", 16); const bool vb[] = {false, false, true, true, false, true}; // v0[valarray<bool>(vb, 6)] returns // valarray<char>("cdf", 3)end example]
mask_array<T> operator[](const valarray<bool>& boolarr);This function selects those elements of the controlled sequence designated by
boolarr
. [Example:valarray<char> v0("abcdefghijklmnop", 16); valarray<char> v1("ABC", 3); const bool vb[] = {false, false, true, true, false, true}; v0[valarray<bool>(vb, 6)] = v1; // v0 == valarray<char>("abABeCghijklmnop", 16)end example]
valarray<T> operator[](const valarray<size_t>& indarr) const;This function returns an object of class
valarray<T>
containing those elements of the controlled sequence designated byindarr
. [Example:valarray<char> v0("abcdefghijklmnop", 16); const size_t vi[] = {7, 5, 2, 3, 8}; // v0[valarray<size_t>(vi, 5)] returns // valarray<char>("hfcdi", 5)end example]
indirect_array<T> operator[](const valarray<size_t>& indarr);This function selects those elements of the controlled sequence designated by
indarr
. [Example:valarray<char> v0("abcdefghijklmnop", 16); valarray<char> v1("ABCDE", 5); const size_t vi[] = {7, 5, 2, 3, 8}; v0[valarray<size_t>(vi, 5)] = v1; // v0 == valarray<char>("abCDeBgAEjklmnop", 16)end example]
Section: 16.4.4.6 [allocator.requirements], 26 [algorithms] Status: Resolved Submitter: Matt Austern Opened: 2003-09-20 Last modified: 2016-01-28
Priority: Not Prioritized
View other active issues in [allocator.requirements].
View all other issues in [allocator.requirements].
View all issues with Resolved status.
Discussion:
Clause 16.4.4.6 [allocator.requirements] paragraph 4 says that implementations are permitted to supply containers that are unable to cope with allocator instances and that container implementations may assume that all instances of an allocator type compare equal. We gave implementers this latitude as a temporary hack, and eventually we want to get rid of it. What happens when we're dealing with allocators that don't compare equal?
In particular: suppose that v1
and v2
are both
objects of type vector<int, my_alloc>
and that
v1.get_allocator() != v2.get_allocator()
. What happens if
we write v1.swap(v2)
? Informally, three possibilities:
1. This operation is illegal. Perhaps we could say that an implementation is required to check and to throw an exception, or perhaps we could say it's undefined behavior.
2. The operation performs a slow swap (i.e. using three
invocations of operator=
, leaving each allocator with its
original container. This would be an O(N) operation.
3. The operation swaps both the vectors' contents and their allocators. This would be an O(1) operation. That is:
my_alloc a1(...); my_alloc a2(...); assert(a1 != a2); vector<int, my_alloc> v1(a1); vector<int, my_alloc> v2(a2); assert(a1 == v1.get_allocator()); assert(a2 == v2.get_allocator()); v1.swap(v2); assert(a1 == v2.get_allocator()); assert(a2 == v1.get_allocator());
[Kona: This is part of a general problem. We need a paper saying how to deal with unequal allocators in general.]
[pre-Sydney: Howard argues for option 3 in N1599. ]
[ 2007-01-12, Howard: This issue will now tend to come up more often with move constructors and move assignment operators. For containers, these members transfer resources (i.e. the allocated memory) just like swap. ]
[
Batavia: There is agreement to overload the container swap
on the allocator's Swappable
requirement using concepts. If the allocator supports Swappable, then container's swap will
swap allocators, else it will perform a "slow swap" using copy construction and copy assignment.
]
[ 2009-04-28 Pablo adds: ]
Fixed in N2525. I argued for marking this Tentatively-Ready right after Bellevue, but there was a concern that N2525 would break in the presence of the RVO. (That breakage had nothing to do with swap, but never-the-less). I addressed that breakage in in N2840 (Summit) by means of a non-normative reference:
[Note: in situations where the copy constructor for a container is elided, this function is not called. The behavior in these cases is as if
select_on_container_copy_construction
returnedx
— end note]
[ 2009-10 Santa Cruz: ]
NAD EditorialResolved. Addressed by N2982.
Proposed resolution:
Section: 31.8.2.5 [stringbuf.virtuals] Status: CD1 Submitter: Christian W Brock Opened: 2003-09-24 Last modified: 2016-01-28
Priority: Not Prioritized
View other active issues in [stringbuf.virtuals].
View all other issues in [stringbuf.virtuals].
View all issues with CD1 status.
Discussion:
27.7.1.3 par 8 says:
Notes: The function can make a write position available only if ( mode & ios_base::out) != 0. To make a write position available, the function reallocates (or initially allocates) an array object with a sufficient number of elements to hold the current array object (if any), plus one additional write position. If ( mode & ios_base::in) != 0, the function alters the read end pointer egptr() to point just past the new write position (as does the write end pointer epptr()).
The sentences "plus one additional write position." and especially "(as does the write end pointer epptr())" COULD by interpreted (and is interpreted by at least my library vendor) as:
post-condition: epptr() == pptr()+1
This WOULD force sputc() to call the virtual overflow() each time.
The proposed change also affects Defect Report 169.
Proposed resolution:
27.7.1.1/2 Change:
2- Notes: The function allocates no array object.
to:
2- Postcondition: str() == "".
27.7.1.1/3 Change:
-3- Effects: Constructs an object of class basic_stringbuf, initializing the base class with basic_streambuf() (lib.streambuf.cons), and initializing mode with which . Then copies the content of str into the basic_stringbuf underlying character sequence and initializes the input and output sequences according to which. If which & ios_base::out is true, initializes the output sequence with the underlying sequence. If which & ios_base::in is true, initializes the input sequence with the underlying sequence.
to:
-3- Effects: Constructs an object of class basic_stringbuf, initializing the base class with basic_streambuf() (lib.streambuf.cons), and initializing mode with which. Then copies the content of str into the basic_stringbuf underlying character sequence. If which & ios_base::out is true, initializes the output sequence such that pbase() points to the first underlying character, epptr() points one past the last underlying character, and if (which & ios_base::ate) is true, pptr() is set equal to epptr() else pptr() is set equal to pbase(). If which & ios_base::in is true, initializes the input sequence such that eback() and gptr() point to the first underlying character and egptr() points one past the last underlying character.
27.7.1.2/1 Change:
-1- Returns: A basic_string object whose content is equal to the basic_stringbuf underlying character sequence. If the buffer is only created in input mode, the underlying character sequence is equal to the input sequence; otherwise, it is equal to the output sequence. In case of an empty underlying character sequence, the function returns basic_string<charT,traits,Allocator>().
to:
-1- Returns: A basic_string object whose content is equal to the basic_stringbuf underlying character sequence. If the basic_stringbuf was created only in input mode, the resultant basic_string contains the character sequence in the range [eback(), egptr()). If the basic_stringbuf was created with (which & ios_base::out) being true then the resultant basic_string contains the character sequence in the range [pbase(), high_mark) where high_mark represents the position one past the highest initialized character in the buffer. Characters can be initialized either through writing to the stream, or by constructing the basic_stringbuf with a basic_string, or by calling the str(basic_string) member function. In the case of calling the str(basic_string) member function, all characters initialized prior to the call are now considered uninitialized (except for those characters re-initialized by the new basic_string). Otherwise the basic_stringbuf has been created in neither input nor output mode and a zero length basic_string is returned.
27.7.1.2/2 Change:
-2- Effects: If the basic_stringbuf's underlying character sequence is not empty, deallocates it. Then copies the content of s into the basic_stringbuf underlying character sequence and initializes the input and output sequences according to the mode stored when creating the basic_stringbuf object. If (mode&ios_base::out) is true, then initializes the output sequence with the underlying sequence. If (mode&ios_base::in) is true, then initializes the input sequence with the underlying sequence.
to:
-2- Effects: Copies the content of s into the basic_stringbuf underlying character sequence. If mode & ios_base::out is true, initializes the output sequence such that pbase() points to the first underlying character, epptr() points one past the last underlying character, and if (mode & ios_base::ate) is true, pptr() is set equal to epptr() else pptr() is set equal to pbase(). If mode & ios_base::in is true, initializes the input sequence such that eback() and gptr() point to the first underlying character and egptr() points one past the last underlying character.
Remove 27.2.1.2/3. (Same rationale as issue 238: incorrect and unnecessary.)
27.7.1.3/1 Change:
1- Returns: If the input sequence has a read position available, returns traits::to_int_type(*gptr()). Otherwise, returns traits::eof().
to:
1- Returns: If the input sequence has a read position available, returns traits::to_int_type(*gptr()). Otherwise, returns traits::eof(). Any character in the underlying buffer which has been initialized is considered to be part of the input sequence.
27.7.1.3/9 Change:
-9- Notes: The function can make a write position available only if ( mode & ios_base::out) != 0. To make a write position available, the function reallocates (or initially allocates) an array object with a sufficient number of elements to hold the current array object (if any), plus one additional write position. If ( mode & ios_base::in) != 0, the function alters the read end pointer egptr() to point just past the new write position (as does the write end pointer epptr()).
to:
-9- The function can make a write position available only if ( mode & ios_base::out) != 0. To make a write position available, the function reallocates (or initially allocates) an array object with a sufficient number of elements to hold the current array object (if any), plus one additional write position. If ( mode & ios_base::in) != 0, the function alters the read end pointer egptr() to point just past the new write position.
27.7.1.3/12 Change:
-12- _ If (newoff + off) < 0, or (xend - xbeg) < (newoff + off), the positioning operation fails. Otherwise, the function assigns xbeg + newoff + off to the next pointer xnext .
to:
-12- _ If (newoff + off) < 0, or if (newoff + off) refers to an uninitialized character (as defined in 31.8.2.4 [stringbuf.members] paragraph 1), the positioning operation fails. Otherwise, the function assigns xbeg + newoff + off to the next pointer xnext .
[post-Kona: Howard provided wording. At Kona the LWG agreed that something along these lines was a good idea, but the original proposed resolution didn't say enough about the effect of various member functions on the underlying character sequences.]
Rationale:
The current basic_stringbuf description is over-constrained in such a way as to prohibit vendors from making this the high-performance in-memory stream it was meant to be. The fundamental problem is that the pointers: eback(), gptr(), egptr(), pbase(), pptr(), epptr() are observable from a derived client, and the current description restricts the range [pbase(), epptr()) from being grown geometrically. This change allows, but does not require, geometric growth of this range.
Backwards compatibility issues: These changes will break code that derives from basic_stringbuf, observes epptr(), and depends upon [pbase(), epptr()) growing by one character on each call to overflow() (i.e. test suites). Otherwise there are no backwards compatibility issues.
27.7.1.1/2: The non-normative note is non-binding, and if it were binding, would be over specification. The recommended change focuses on the important observable fact.
27.7.1.1/3: This change does two things: 1. It describes exactly what must happen in terms of the sequences. The terms "input sequence" and "output sequence" are not well defined. 2. It introduces a common extension: open with app or ate mode. I concur with issue 238 that paragraph 4 is both wrong and unnecessary.
27.7.1.2/1: This change is the crux of the efficiency issue. The resultant basic_string is not dependent upon epptr(), and thus implementors are free to grow the underlying buffer geometrically during overflow() *and* place epptr() at the end of that buffer.
27.7.1.2/2: Made consistent with the proposed 27.7.1.1/3.
27.7.1.3/1: Clarifies that characters written to the stream beyond the initially specified string are available for reading in an i/o basic_streambuf.
27.7.1.3/9: Made normative by removing "Notes:", and removed the trailing parenthetical comment concerning epptr().
27.7.1.3/12: Restricting the positioning to [xbeg, xend) is no longer allowable since [pbase(), epptr()) may now contain uninitialized characters. Positioning is only allowable over the initialized range.
Section: 22.9.2.3 [bitset.members] Status: CD1 Submitter: Martin Sebor Opened: 2003-10-15 Last modified: 2016-01-28
Priority: Not Prioritized
View all other issues in [bitset.members].
View all issues with CD1 status.
Discussion:
It has been pointed out a number of times that the bitset to_string() member function template is tedious to use since callers must explicitly specify the entire template argument list (3 arguments). At least two implementations provide a number of overloads of this template to make it easier to use.
Proposed resolution:
In order to allow callers to specify no template arguments at all, just the first one (charT), or the first 2 (charT and traits), in addition to all three template arguments, add the following three overloads to both the interface (declarations only) of the class template bitset as well as to section 23.3.5.2, immediately after p34, the Returns clause of the existing to_string() member function template:
template <class charT, class traits> basic_string<charT, traits, allocator<charT> > to_string () const; -34.1- Returns: to_string<charT, traits, allocator<charT> >(). template <class charT> basic_string<charT, char_traits<charT>, allocator<charT> > to_string () const; -34.2- Returns: to_string<charT, char_traits<charT>, allocator<charT> >(). basic_string<char, char_traits<char>, allocator<char> > to_string () const; -34.3- Returns: to_string<char, char_traits<char>, allocator<char> >().
[Kona: the LWG agrees that this is an improvement over the status quo. Dietmar thought about an alternative using a proxy object but now believes that the proposed resolution above is the right choice. ]
Section: 27.4.4.4 [string.io] Status: CD1 Submitter: Martin Sebor Opened: 2003-10-15 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
It has been pointed out that the proposed resolution in DR 25(i) may not be
quite up to snuff:
http://gcc.gnu.org/ml/libstdc++/2003-09/msg00147.html
It looks like Petur is right. The complete corrected text is copied below. I think we may have have been confused by the reference to 22.2.2.2.2 and the subsequent description of `n' which actually talks about the second argument to sputn(), not about the number of fill characters to pad with.
So the question is: was the original text correct? If the intent was to follow classic iostreams then it most likely wasn't, since setting width() to less than the length of the string doesn't truncate it on output. This is also the behavior of most implementations (except for SGI's standard iostreams where the operator does truncate).
Proposed resolution:
Change the text in 21.3.7.9, p4 from
If bool(k) is true, inserts characters as if by calling os.rdbuf()->sputn(str.data(), n), padding as described in stage 3 of lib.facet.num.put.virtuals, where n is the larger of os.width() and str.size();
to
If bool(k) is true, determines padding as described in lib.facet.num.put.virtuals, and then inserts the resulting sequence of characters
seq
as if by callingos.rdbuf()->sputn(seq, n)
, wheren
is the larger ofos.width()
andstr.size()
;
[Kona: it appears that neither the original wording, DR25, nor the proposed resolution, is quite what we want. We want to say that the string will be output, padded to os.width() if necessary. We don't want to duplicate the padding rules in clause 22, because they're complicated, but we need to be careful because they weren't quite written with quite this case in mind. We need to say what the character sequence is, and then defer to clause 22. Post-Kona: Benjamin provided wording.]
Section: 28.3.3.1.2.2 [locale.facet] Status: CD1 Submitter: Martin Sebor Opened: 2003-10-15 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
Is "const std::ctype<char>" a valid template argument to has_facet, use_facet, and the locale template ctor? And if so, does it designate the same Facet as the non-const "std::ctype<char>?" What about "volatile std::ctype<char>?" Different implementations behave differently: some fail to compile, others accept such types but behave inconsistently.
Proposed resolution:
Change 22.1.1.1.2, p1 to read:
Template parameters in this clause which are required to be facets are those named Facet in declarations. A program that passes a type that is not a facet, or a type that refers to volatile-qualified facet, as an (explicit or deduced) template parameter to a locale function expecting a facet, is ill-formed. A const-qualified facet is a valid template argument to any locale function that expects a Facet template parameter.
[Kona: changed the last sentence from a footnote to normative text.]
Section: 23.2.4 [sequence.reqmts] Status: CD1 Submitter: Howard Hinnant Opened: 2003-10-20 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
Section 23.2.4 [sequence.reqmts], paragraphs 9-11, fixed up the problem noticed with statements like:
vector<int> v(10, 1);
The intent of the above statement was to construct with:
vector(size_type, const value_type&);
but early implementations failed to compile as they bound to:
template <class InputIterator> vector(InputIterator f, InputIterator l);
instead.
Paragraphs 9-11 say that if InputIterator is an integral type, then the member template constructor will have the same effect as:
vector<static_cast<size_type>(f), static_cast<value_type>(l));
(and similarly for the other member template functions of sequences).
There is also a note that describes one implementation technique:
One way that sequence implementors can satisfy this requirement is to specialize the member template for every integral type.
This might look something like:
template <class T> struct vector { typedef unsigned size_type; explicit vector(size_type) {} vector(size_type, const T&) {} template <class I> vector(I, I); // ... }; template <class T> template <class I> vector<T>::vector(I, I) { ... } template <> template <> vector<int>::vector(int, int) { ... } template <> template <> vector<int>::vector(unsigned, unsigned) { ... } // ...
Label this solution 'A'.
The standard also says:
Less cumbersome implementation techniques also exist.
A popular technique is to not specialize as above, but instead catch every call with the member template, detect the type of InputIterator, and then redirect to the correct logic. Something like:
template <class T> template <class I> vector<T>::vector(I f, I l) { choose_init(f, l, int2type<is_integral<I>::value>()); } template <class T> template <class I> vector<T>::choose_init(I f, I l, int2type<false>) { // construct with iterators } template <class T> template <class I> vector<T>::choose_init(I f, I l, int2type<true>) { size_type sz = static_cast<size_type>(f); value_type v = static_cast<value_type>(l); // construct with sz,v }
Label this solution 'B'.
Both of these solutions solve the case the standard specifically mentions:
vector<int> v(10, 1); // ok, vector size 10, initialized to 1
However, (and here is the problem), the two solutions have different behavior in some cases where the value_type of the sequence is not an integral type. For example consider:
pair<char, char> p('a', 'b'); vector<vector<pair<char, char> > > d('a', 'b');
The second line of this snippet is likely an error. Solution A catches the error and refuses to compile. The reason is that there is no specialization of the member template constructor that looks like:
template <> template <> vector<vector<pair<char, char> > >::vector(char, char) { ... }
So the expression binds to the unspecialized member template constructor, and then fails (compile time) because char is not an InputIterator.
Solution B compiles the above example though. 'a' is casted to an unsigned integral type and used to size the outer vector. 'b' is static casted to the inner vector using it's explicit constructor:
explicit vector(size_type n);
and so you end up with a static_cast<size_type>('a') by static_cast<size_type>('b') matrix.
It is certainly possible that this is what the coder intended. But the explicit qualifier on the inner vector has been thwarted at any rate.
The standard is not clear whether the expression:
vector<vector<pair<char, char> > > d('a', 'b');
(and similar expressions) are:
My preference is listed in the order presented.
There are still other techniques for implementing the requirements of paragraphs 9-11, namely the "restricted template technique" (e.g. enable_if). This technique is the most compact and easy way of coding the requirements, and has the behavior of #2 (rejects the above expression).
Choosing 1 would allow all implementation techniques I'm aware of. Choosing 2 would allow only solution 'A' and the enable_if technique. Choosing 3 would allow only solution 'B'.
Possible wording for a future standard if we wanted to actively reject the expression above would be to change "static_cast" in paragraphs 9-11 to "implicit_cast" where that is defined by:
template <class T, class U> inline T implicit_cast(const U& u) { return u; }
Proposed resolution:
Replace 23.2.4 [sequence.reqmts] paragraphs 9 - 11 with:
For every sequence defined in this clause and in clause lib.strings:
If the constructor
template <class InputIterator> X(InputIterator f, InputIterator l, const allocator_type& a = allocator_type())
is called with a type InputIterator that does not qualify as an input iterator, then the constructor will behave as if the overloaded constructor:
X(size_type, const value_type& = value_type(), const allocator_type& = allocator_type())
were called instead, with the arguments static_cast<size_type>(f), l and a, respectively.
If the member functions of the forms:
template <class InputIterator> // such as insert() rt fx1(iterator p, InputIterator f, InputIterator l); template <class InputIterator> // such as append(), assign() rt fx2(InputIterator f, InputIterator l); template <class InputIterator> // such as replace() rt fx3(iterator i1, iterator i2, InputIterator f, InputIterator l);
are called with a type InputIterator that does not qualify as an input iterator, then these functions will behave as if the overloaded member functions:
rt fx1(iterator, size_type, const value_type&); rt fx2(size_type, const value_type&); rt fx3(iterator, iterator, size_type, const value_type&);
were called instead, with the same arguments.
In the previous paragraph the alternative binding will fail if f is not implicitly convertible to X::size_type or if l is not implicitly convertible to X::value_type.
The extent to which an implementation determines that a type cannot be an input iterator is unspecified, except that as a minimum integral types shall not qualify as input iterators.
[
Kona: agreed that the current standard requires v('a', 'b')
to be accepted, and also agreed that this is surprising behavior. The
LWG considered several options, including something like
implicit_cast, which doesn't appear to be quite what we want. We
considered Howards three options: allow acceptance or rejection,
require rejection as a compile time error, and require acceptance. By
straw poll (1-6-1), we chose to require a compile time error.
Post-Kona: Howard provided wording.
]
[ Sydney: The LWG agreed with this general direction, but there was some discomfort with the wording in the original proposed resolution. Howard submitted new wording, and we will review this again in Redmond. ]
[Redmond: one very small change in wording: the first argument
is cast to size_t. This fixes the problem of something like
vector<vector<int> >(5, 5)
, where int is not
implicitly convertible to the value type.]
Rationale:
The proposed resolution fixes:
vector<int> v(10, 1);
since as integral types 10 and 1 must be disqualified as input iterators and therefore the (size,value) constructor is called (as if).
The proposed resolution breaks:
vector<vector<T> > v(10, 1);
because the integral type 1 is not *implicitly* convertible to vector<T>. The wording above requires a diagnostic.
The proposed resolution leaves the behavior of the following code unspecified.
struct A { operator int () const {return 10;} }; struct B { B(A) {} }; vector<B> v(A(), A());
The implementation may or may not detect that A is not an input iterator and employee the (size,value) constructor. Note though that in the above example if the B(A) constructor is qualified explicit, then the implementation must reject the constructor as A is no longer implicitly convertible to B.
Section: 31.5.3 [fpos] Status: CD1 Submitter: Vincent Leloup Opened: 2003-11-17 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
In section 31.5.3.2 [fpos.members] fpos<stateT>::state() is declared non const, but in section 31.5.3 [fpos] it is declared const.
Proposed resolution:
In section 31.5.3.2 [fpos.members], change the declaration of
fpos<stateT>::state()
to const.
Section: 31.7.6.2.4 [ostream.sentry] Status: CD1 Submitter: Vincent Leloup Opened: 2003-11-18 Last modified: 2021-06-06
Priority: Not Prioritized
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Discussion:
In section [ostream::sentry] paragraph 4, in description part basic_ostream<charT, traits>::sentry::operator bool() is declared as non const, but in section 27.6.2.3, in synopsis it is declared const.
Proposed resolution:
In section [ostream::sentry] paragraph 4, change the declaration
of sentry::operator bool()
to const.
Section: 31.10.3.4 [filebuf.members] Status: CD1 Submitter: Vincent Leloup Opened: 2003-11-20 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
In section 31.10.3.4 [filebuf.members] par6, in effects description of basic_filebuf<charT, traits>::close(), overflow(EOF) is used twice; should be overflow(traits::eof()).
Proposed resolution:
Change overflow(EOF) to overflow(traits::eof()).
Section: 31.10 [file.streams] Status: CD1 Submitter: Vincent Leloup Opened: 2003-11-20 Last modified: 2017-06-15
Priority: Not Prioritized
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Discussion:
31.10.4.4 [ifstream.members] p1, 31.10.5.4 [ofstream.members] p1, 31.10.6.4 [fstream.members] p1 seems have same problem as exposed in LWG issue 252(i).
Proposed resolution:
[Sydney: Genuine defect. 27.8.1.13 needs a cast to cast away constness. The other two places are stylistic: we could change the C-style casts to const_cast. Post-Sydney: Howard provided wording. ]
Change 27.8.1.7/1 from:
Returns: (basic_filebuf<charT,traits>*)&sb.
to:
Returns: const_cast<basic_filebuf<charT,traits>*>(&sb).
Change 27.8.1.10/1 from:
Returns: (basic_filebuf<charT,traits>*)&sb.
to:
Returns: const_cast<basic_filebuf<charT,traits>*>(&sb).
Change 27.8.1.13/1 from:
Returns: &sb.
to:
Returns: const_cast<basic_filebuf<charT,traits>*>(&sb).
Section: 24.3.2.3 [iterator.traits] Status: CD1 Submitter: Dave Abrahams Opened: 2003-12-09 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
The standard places no restrictions at all on the reference type of input, output, or forward iterators (for forward iterators it only specifies that *x must be value_type& and doesn't mention the reference type). Bidirectional iterators' reference type is restricted only by implication, since the base iterator's reference type is used as the return type of reverse_iterator's operator*, which must be T& in order to be a conforming forward iterator.
Here's what I think we ought to be able to expect from an input or forward iterator's reference type R, where a is an iterator and V is its value_type
A mutable forward iterator ought to satisfy, for x of type V:
{ R r = *a; r = x; } is equivalent to *a = x;
I think these requirements capture existing container iterators (including vector<bool>'s), but render istream_iterator invalid; its reference type would have to be changed to a constant reference.
(Jeremy Siek) During the discussion in Sydney, it was felt that a
simpler long term solution for this was needed. The solution proposed
was to require reference
to be the same type as *a
and pointer
to be the same type as a->
. Most
iterators in the Standard Library already meet this requirement. Some
iterators are output iterators, and do not need to meet the
requirement, and others are only specified through the general
iterator requirements (which will change with this resolution). The
sole case where there is an explicit definition of the reference type
that will need to change is istreambuf_iterator
which returns
charT
from operator*
but has a reference type of
charT&
. We propose changing the reference type of
istreambuf_iterator
to charT
.
The other option for resolving the issue with pointer
,
mentioned in the note below, is to remove pointer
altogether. I prefer placing requirements on pointer
to
removing it for two reasons. First, pointer
will become
useful for implementing iterator adaptors and in particular,
reverse_iterator
will become more well defined. Second,
removing pointer
is a rather drastic and publicly-visible
action to take.
The proposed resolution technically enlarges the requirements for
iterators, which means there are existing iterators (such as
istreambuf_iterator
, and potentially some programmer-defined
iterators) that will no longer meet the requirements. Will this break
existing code? The scenario in which it would is if an algorithm
implementation (say in the Standard Library) is changed to rely on
iterator_traits::reference
, and then is used with one of the
iterators that do not have an appropriately defined
iterator_traits::reference
.
The proposed resolution makes one other subtle change. Previously,
it was required that output iterators have a difference_type
and value_type
of void
, which means that a forward
iterator could not be an output iterator. This is clearly a mistake,
so I've changed the wording to say that those types may be
void
.
Proposed resolution:
In 24.3.2.3 [iterator.traits], after:
be defined as the iterator's difference type, value type and iterator category, respectively.
add
In addition, the types
iterator_traits<Iterator>::reference iterator_traits<Iterator>::pointermust be defined as the iterator's reference and pointer types, that is, the same type as the type of
*a
anda->
, respectively.
In 24.3.2.3 [iterator.traits], change:
In the case of an output iterator, the types
iterator_traits<Iterator>::difference_type iterator_traits<Iterator>::value_typeare both defined as
void
.
to:
In the case of an output iterator, the types
iterator_traits<Iterator>::difference_type iterator_traits<Iterator>::value_type iterator_traits<Iterator>::reference iterator_traits<Iterator>::pointermay be defined as
void
.
In 24.6.4 [istreambuf.iterator], change:
typename traits::off_type, charT*, charT&>
to:
typename traits::off_type, charT*, charT>
[ Redmond: there was concern in Sydney that this might not be the only place where things were underspecified and needed to be changed. Jeremy reviewed iterators in the standard and confirmed that nothing else needed to be changed. ]
Section: 24.3.5.7 [random.access.iterators] Status: CD1 Submitter: Dave Abrahams Opened: 2004-01-07 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
Table 76, the random access iterator requirement table, says that the return type of a[n] must be "convertible to T". When an iterator's value_type T is an abstract class, nothing is convertible to T. Surely this isn't an intended restriction?
Proposed resolution:
Change the return type to "convertible to T const&".
Section: 17.2 [support.types] Status: CD1 Submitter: Pete Becker Opened: 2004-01-15 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
Original text:
The macro offsetof accepts a restricted set of type arguments in this International Standard. type shall be a POD structure or a POD union (clause 9). The result of applying the offsetof macro to a field that is a static data member or a function member is undefined."
Revised text:
"If type is not a POD structure or a POD union the results are undefined."
Looks to me like the revised text should have replaced only the second sentence. It doesn't make sense standing alone.
Proposed resolution:
Change 18.1, paragraph 5, to:
The macro offsetof accepts a restricted set of type arguments in this International Standard. If type is not a POD structure or a POD union the results are undefined. The result of applying the offsetof macro to a field that is a static data member or a function member is undefined."
Section: 31.8.2.5 [stringbuf.virtuals] Status: CD1 Submitter: Bill Plauger Opened: 2004-01-30 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
pos_type basic_stringbuf::seekoff(off_type, ios_base::seekdir, ios_base::openmode);
is obliged to fail if nothing has been inserted into the stream. This is unnecessary and undesirable. It should be permissible to seek to an effective offset of zero.
[ Sydney: Agreed that this is an annoying problem: seeking to zero should be legal. Bill will provide wording. ]
Proposed resolution:
Change the sentence from:
For a sequence to be positioned, if its next pointer (either gptr() or pptr()) is a null pointer, the positioning operation fails.
to:
For a sequence to be positioned, if its next pointer (either gptr() or pptr()) is a null pointer and the new offset newoff is nonzero, the positioning operation fails.
Section: 31.4 [iostream.objects] Status: CD1 Submitter: Bill Plauger Opened: 2004-01-30 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
Both cerr::tie() and wcerr::tie() are obliged to be null at program startup. This is overspecification and overkill. It is both traditional and useful to tie cerr to cout, to ensure that standard output is drained whenever an error message is written. This behavior should at least be permitted if not required. Same for wcerr::tie().
Proposed resolution:
Add to the description of cerr:
After the object cerr is initialized, cerr.tie() returns &cout. Its state is otherwise the same as required for basic_ios<char>::init (lib.basic.ios.cons).
Add to the description of wcerr:
After the object wcerr is initialized, wcerr.tie() returns &wcout. Its state is otherwise the same as required for basic_ios<wchar_t>::init (lib.basic.ios.cons).
[Sydney: straw poll (3-1): we should require, not just permit, cout and cerr to be tied on startup. Pre-Redmond: Bill will provide wording.]
Section: 16.4.2.3 [headers] Status: CD1 Submitter: Bill Plauger Opened: 2004-01-30 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
The C++ Standard effectively requires that the traditional C headers (of the form <xxx.h>) be defined in terms of the newer C++ headers (of the form <cxxx>). Clauses 17.4.1.2/4 and D.5 combine to require that:
The rules were left in this form despited repeated and heated objections from several compiler vendors. The C headers are often beyond the direct control of C++ implementors. In some organizations, it's all they can do to get a few #ifdef __cplusplus tests added. Third-party library vendors can perhaps wrap the C headers. But neither of these approaches supports the drastic restructuring required by the C++ Standard. As a result, it is still widespread practice to ignore this conformance requirement, nearly seven years after the committee last debated this topic. Instead, what is often implemented is:
The practical benefit for implementors with the second approach is that they can use existing C library headers, as they are pretty much obliged to do. The practical cost for programmers facing a mix of implementations is that they have to assume weaker rules:
There also exists the possibility of subtle differences due to Koenig lookup, but there are so few non-builtin types defined in the C headers that I've yet to see an example of any real problems in this area.
It is worth observing that the rate at which programmers fall afoul of these differences has remained small, at least as measured by newsgroup postings and our own bug reports. (By an overwhelming margin, the commonest problem is still that programmers include <string> and can't understand why the typename string isn't defined -- this a decade after the committee invented namespace std, nominally for the benefit of all programmers.)
We should accept the fact that we made a serious mistake and rectify it, however belatedly, by explicitly allowing either of the two schemes for declaring C names in headers.
[Sydney: This issue has been debated many times, and will
certainly have to be discussed in full committee before any action
can be taken. However, the preliminary sentiment of the LWG was in
favor of the change. (6 yes, 0 no, 2 abstain) Robert Klarer
suggests that we might also want to undeprecate the
C-style .h
headers.]
Proposed resolution:
Add to 16.4.2.3 [headers], para. 4:
Except as noted in clauses 18 through 27 and Annex D, the contents of each header cname shall be the same as that of the corresponding header name.h, as specified in ISO/IEC 9899:1990 Programming Languages C (Clause 7), or ISO/IEC:1990 Programming Languages-C AMENDMENT 1: C Integrity, (Clause 7), as appropriate, as if by inclusion. In the C++ Standard Library, however, the declarations
and definitions(except for names which are defined as macros in C) are within namespace scope (3.3.5) of the namespace std. It is unspecified whether these names are first declared within the global namespace scope and are then injected into namespace std by explicit using-declarations (9.10 [namespace.udecl]).
Change [depr.c.headers], para. 2-3:
-2- Every C header, each of which has a name of the form name.h, behaves as if each name placed in the Standard library namespace by the corresponding cname header is
alsoplaced within the global namespace scope.of the namespaceIt is unspecified whether these names are first declared or defined within namespace scope (6.4.6 [basic.scope.namespace]) of the namespacestd
and is followed by an explicit using-declaration (9.10 [namespace.udecl]).std
and are then injected into the global namespace scope by explicit using-declarations (9.10 [namespace.udecl]).-3- [Example: The header
<cstdlib>
assuredly provides its declarations and definitions within the namespacestd
. It may also provide these names within the global namespace. The header<stdlib.h>
makes these available also inassuredly provides the same declarations and definitions within the global namespace, much as in the C Standard. It may also provide these names within the namespacestd
. -- end example]
Section: 22.9.2.2 [bitset.cons] Status: CD1 Submitter: Dag Henriksson Opened: 2004-01-30 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
The constructor from unsigned long says it initializes "the first M bit positions to the corresponding bit values in val. M is the smaller of N and the value CHAR_BIT * sizeof(unsigned long)."
Object-representation vs. value-representation strikes again. CHAR_BIT * sizeof (unsigned long) does not give us the number of bits an unsigned long uses to hold the value. Thus, the first M bit position above is not guaranteed to have any corresponding bit values in val.
Proposed resolution:
In 22.9.2.2 [bitset.cons] paragraph 2, change "M is the smaller of
N and the value CHAR_BIT * sizeof (unsigned long). (249)" to
"M
is the smaller of N
and the number of bits in
the value representation (section 6.8 [basic.types]) of unsigned
long
."
Section: 31.10 [file.streams] Status: CD1 Submitter: Ben Hutchings Opened: 2004-04-01 Last modified: 2017-06-15
Priority: Not Prioritized
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Discussion:
The second parameters of the non-default constructor and of the open member function for basic_fstream, named "mode", are optional according to the class declaration in 27.8.1.11 [lib.fstream]. The specifications of these members in 27.8.1.12 [lib.fstream.cons] and 27.8.1.13 lib.fstream.members] disagree with this, though the constructor declaration has the "explicit" function-specifier implying that it is intended to be callable with one argument.
Proposed resolution:
In 31.10.6.2 [fstream.cons], change
explicit basic_fstream(const char* s, ios_base::openmode mode);
to
explicit basic_fstream(const char* s, ios_base::openmode mode = ios_base::in|ios_base::out);
In 31.10.6.4 [fstream.members], change
void open(const char*s, ios_base::openmode mode);
to
void open(const char*s, ios_base::openmode mode = ios_base::in|ios_base::out);
Section: 28.3.4.6.2.3 [locale.time.get.virtuals] Status: CD1 Submitter: Bill Plauger Opened: 2004-03-23 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
Template time_get currently contains difficult, if not impossible, requirements for do_date_order, do_get_time, and do_get_date. All require the implementation to scan a field generated by the %x or %X conversion specifier in strftime. Yes, do_date_order can always return no_order, but that doesn't help the other functions. The problem is that %x can be nearly anything, and it can vary widely with locales. It's horribly onerous to have to parse "third sunday after Michaelmas in the year of our Lord two thousand and three," but that's what we currently ask of do_get_date. More practically, it leads some people to think that if %x produces 10.2.04, we should know to look for dots as separators. Still not easy.
Note that this is the opposite effect from the intent stated in the footnote earlier in this subclause:
"In other words, user confirmation is required for reliable parsing of user-entered dates and times, but machine-generated formats can be parsed reliably. This allows parsers to be aggressive about interpreting user variations on standard formats."
We should give both implementers and users an easier and more reliable alternative: provide a (short) list of alternative delimiters and say what the default date order is for no_order. For backward compatibility, and maximum latitude, we can permit an implementation to parse whatever %x or %X generates, but we shouldn't require it.
Proposed resolution:
In the description:
iter_type do_get_time(iter_type s, iter_type end, ios_base& str, ios_base::iostate& err, tm* t) const;
2 Effects: Reads characters starting at suntil it has extracted those struct tm members, and remaining format characters, used by time_put<>::put to produce the format specified by 'X', or until it encounters an error or end of sequence.
change: 'X'
to: "%H:%M:%S"
Change
iter_type do_get_date(iter_type s, iter_type end, ios_base& str, ios_base::iostate& err, tm* t) const; 4 Effects: Reads characters starting at s until it has extracted those struct tm members, and remaining format characters, used by time_put<>::put to produce the format specified by 'x', or until it encounters an error.
to
iter_type do_get_date(iter_type s, iter_type end, ios_base& str, ios_base::iostate& err, tm* t) const;
4 Effects: Reads characters starting at s until it has extracted those struct tm members, and remaining format characters, used by time_put<>::put to produce one of the following formats, or until it encounters an error. The format depends on the value returned by date_order() as follows:
date_order() format no_order "%m/%d/%y" dmy "%d/%m/%y" mdy "%m/%d/%y" ymd "%y/%m/%d" ydm "%y/%d/%m"
An implementation may also accept additional implementation-defined formats.
[Redmond: agreed that this is a real problem. The solution is probably to match C99's parsing rules. Bill provided wording. ]
Section: 23.3.13 [vector], 23.4.3 [map] Status: CD1 Submitter: Thorsten Ottosen Opened: 2004-05-12 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
To add slightly more convenience to vector<T> and map<Key,T> we should consider to add
Rationale:
Proposed resolution:
In 23.3.13 [vector], add the following to the vector
synopsis after "element access" and before "modifiers":
// [lib.vector.data] data access pointer data(); const_pointer data() const;
Add a new subsection of 23.3.13 [vector]:
23.2.4.x
vector
data accesspointer data(); const_pointer data() const;Returns: A pointer such that [data(), data() + size()) is a valid range. For a non-empty vector, data() == &front().
Complexity: Constant time.
Throws: Nothing.
In 23.4.3 [map], add the following to the map
synopsis immediately after the line for operator[]:
T& at(const key_type& x); const T& at(const key_type& x) const;
Add the following to 23.4.3.3 [map.access]:
T& at(const key_type& x); const T& at(const key_type& x) const;Returns: A reference to the element whose key is equivalent to x, if such an element is present in the map.
Throws:
out_of_range
if no such element is present.
Rationale:
Neither of these additions provides any new functionality but the
LWG agreed that they are convenient, especially for novices. The
exception type chosen for at
, std::out_of_range
,
was chosen to match vector::at
.
Section: 16.4.2.3 [headers] Status: CD1 Submitter: Steve Clamage Opened: 2004-06-03 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
C header <iso646.h> defines macros for some operators, such as not_eq for !=.
Section 16.4.2.3 [headers] "Headers" says that except as noted in clauses 18 through 27, the <cname> C++ header contents are the same as the C header <name.h>. In particular, table 12 lists <ciso646> as a C++ header.
I don't find any other mention of <ciso646>, or any mention of <iso646.h>, in clauses 17 thorough 27. That implies that the contents of <ciso646> are the same as C header <iso646.h>.
Annex C (informative, not normative) in [diff.header.iso646.h] C.2.2.2 "Header <iso646.h>" says that the alternative tokens are not defined as macros in <ciso646>, but does not mention the contents of <iso646.h>.
I don't find any normative text to support C.2.2.2.
Proposed resolution:
Add to section 17.4.1.2 Headers [lib.headers] a new paragraph after paragraph 6 (the one about functions must be functions):
Identifiers that are keywords or operators in C++ shall not be defined as macros in C++ standard library headers. [Footnote:In particular, including the standard header <iso646.h> or <ciso646> has no effect.
[post-Redmond: Steve provided wording.]
Section: 27.2.4.2 [char.traits.specializations.char] Status: CD1 Submitter: Martin Sebor Opened: 2004-06-28 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
Table 37 describes the requirements on Traits::compare() in terms of those on Traits::lt(). 21.1.3.1, p6 requires char_traits<char>::lt() to yield the same result as operator<(char, char).
Most, if not all, implementations of char_traits<char>::compare() call memcmp() for efficiency. However, the C standard requires both memcmp() and strcmp() to interpret characters under comparison as unsigned, regardless of the signedness of char. As a result, all these char_traits implementations fail to meet the requirement imposed by Table 37 on compare() when char is signed.
Read email thread starting with c++std-lib-13499 for more.
Proposed resolution:
Change 21.1.3.1, p6 from
The two-argument members assign, eq, and lt are defined identically to the built-in operators =, ==, and < respectively.
to
The two-argument member assign is defined identically to the built-in operator =. The two argument members eq and lt are defined identically to the built-in operators == and < for type unsigned char.
[Redmond: The LWG agreed with this general direction, but we
also need to change eq
to be consistent with this change.
Post-Redmond: Martin provided wording.]
Section: 31.5.4.4 [iostate.flags] Status: CD1 Submitter: Martin Sebor Opened: 2004-06-28 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
The program below is required to compile but when run it typically produces unexpected results due to the user-defined conversion from std::cout or any object derived from basic_ios to void*.
#include <cassert> #include <iostream> int main () { assert (std::cin.tie () == std::cout); // calls std::cout.ios::operator void*() }
Proposed resolution:
Replace std::basic_ios<charT, traits>::operator void*() with another conversion operator to some unspecified type that is guaranteed not to be convertible to any other type except for bool (a pointer-to-member might be one such suitable type). In addition, make it clear that the pointer type need not be a pointer to a complete type and when non-null, the value need not be valid.
Specifically, change in [lib.ios] the signature of
operator void*() const;
to
operator unspecified-bool-type() const;
and change [lib.iostate.flags], p1 from
operator void*() const;
to
operator unspecified-bool-type() const; -1- Returns: if fail() then a value that will evaluate false in a boolean context; otherwise a value that will evaluate true in a boolean context. The value type returned shall not be convertible to int. -2- [Note: This conversion can be used in contexts where a bool is expected (e.g., an if condition); however, implicit conversions (e.g., to int) that can occur with bool are not allowed, eliminating some sources of user error. One possible implementation choice for this type is pointer-to-member. - end note]
[Redmond: 5-4 straw poll in favor of doing this.]
[Lillehammer: Doug provided revised wording for "unspecified-bool-type".]
Section: 23.3.13 [vector] Status: CD1 Submitter: Martin Sebor Opened: 2004-06-28 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
The overloads of relational operators for vector<bool> specified in [lib.vector.bool] are redundant (they are semantically identical to those provided for the vector primary template) and may even be diagnosed as ill-formed (refer to Daveed Vandevoorde's explanation in c++std-lib-13647).
Proposed resolution:
Remove all overloads of overloads of relational operators for vector<bool> from [lib.vector.bool].
what()
implementation-definedSection: 17.9.3 [exception] Status: C++11 Submitter: Martin Sebor Opened: 2004-06-28 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
[lib.exception] specifies the following:
exception (const exception&) throw(); exception& operator= (const exception&) throw(); -4- Effects: Copies an exception object. -5- Notes: The effects of calling what() after assignment are implementation-defined.
First, does the Note only apply to the assignment operator? If so, what are the effects of calling what() on a copy of an object? Is the returned pointer supposed to point to an identical copy of the NTBS returned by what() called on the original object or not?
Second, is this Note intended to extend to all the derived classes in section 19? I.e., does the standard provide any guarantee for the effects of what() called on a copy of any of the derived class described in section 19?
Finally, if the answer to the first question is no, I believe it constitutes a defect since throwing an exception object typically implies invoking the copy ctor on the object. If the answer is yes, then I believe the standard ought to be clarified to spell out exactly what the effects are on the copy (i.e., after the copy ctor was called).
[Redmond: Yes, this is fuzzy. The issue of derived classes is fuzzy too.]
[ Batavia: Howard provided wording. ]
[ Bellevue: ]
Eric concerned this is unimplementable, due to nothrow guarantees. Suggested implementation would involve reference counting.
Is the implied reference counting subtle enough to call out a note on implementation? Probably not.
If reference counting required, could we tighten specification further to require same pointer value? Probably an overspecification, especially if exception classes defer evalutation of final string to calls to what().
Remember issue moved open and not resolved at Batavia, but cannot remember who objected to canvas a disenting opinion - please speak up if you disagree while reading these minutes!
Move to Ready as we are accepting words unmodified.
[ Sophia Antipolis: ]
The issue was pulled from Ready. It needs to make clear that only homogenous copying is intended to be supported, not coping from a derived to a base.
[ Batavia (2009-05): ]
Howard supplied the following replacement wording for paragraph 7 of the proposed resolution:
-7- Postcondition:
what()
shall return the same NTBS as would be obtained by usingstatic_cast
to cast the rhs to the same types as the lhs and then callingwhat()
on that possibly sliced object.Pete asks what "the same NTBS" means.
[ 2009-07-30 Niels adds: ]
Further discussion in the thread starting with c++std-lib-24512.
[ 2009-09-24 Niels provided updated wording: ]
I think the resolution should at least guarantee that the result of
what()
is independent of whether the compiler does copy-elision. And for any class derived fromstd::excepion
that has a constructor that allows specifying awhat_arg
, it should make sure that the text of a user-providedwhat_arg
is preserved, when the object is copied. Note that all the implementations I've tested already appear to satisfy the proposed resolution, including MSVC 2008 SP1, Apache stdcxx-4.2.1, GCC 4.1.2, GCC 4.3.2, and CodeGear C++ 6.13.The proposed resolution was updated with help from Daniel Krügler; the update aims to clarify that the proposed postcondition only applies to homogeneous copying.
[ 2009-10 Santa Cruz: ]
Moved to Ready after inserting "publicly accessible" in two places.
Proposed resolution:
Change 17.9.3 [exception] to:
-1- The class
exception
defines the base class for the types of objects thrown as exceptions by C++ standard library components, and certain expressions, to report errors detected during program execution.Each standard library class
T
that derives from classexception
shall have a publicly accessible copy constructor and a publicly accessible copy assignment operator that do not exit with an exception. These member functions shall preserve the following postcondition: If two objects lhs and rhs both have dynamic typeT
, and lhs is a copy of rhs, thenstrcmp(lhs.what(), rhs.what()) == 0
....
exception(const exception& rhs) throw(); exception& operator=(const exception& rhs) throw();-4- Effects: Copies an exception object.
-5- Remarks: The effects of callingwhat()
after assignment are implementation-defined.-5- Postcondition: If
*this
and rhs both have dynamic typeexception
thenstrcmp(what(), rhs.what()) == 0
.
ctype
callsSection: 28.3.4.2.2 [locale.ctype] Status: C++11 Submitter: Martin Sebor Opened: 2004-07-01 Last modified: 2016-01-28
Priority: Not Prioritized
View all issues with C++11 status.
Discussion:
Most ctype
member functions come in two forms: one that operates
on a single character at a time and another form that operates
on a range of characters. Both forms are typically described by
a single Effects and/or Returns clause.
The Returns clause of each of the single-character non-virtual forms suggests that the function calls the corresponding single character virtual function, and that the array form calls the corresponding virtual array form. Neither of the two forms of each virtual member function is required to be implemented in terms of the other.
There are three problems:
1. One is that while the standard does suggest that each non-virtual member function calls the corresponding form of the virtual function, it doesn't actually explicitly require it.
Implementations that cache results from some of the virtual member functions for some or all values of their arguments might want to call the array form from the non-array form the first time to fill the cache and avoid any or most subsequent virtual calls. Programs that rely on each form of the virtual function being called from the corresponding non-virtual function will see unexpected behavior when using such implementations.
2. The second problem is that either form of each of the virtual functions can be overridden by a user-defined function in a derived class to return a value that is different from the one produced by the virtual function of the alternate form that has not been overriden.
Thus, it might be possible for, say, ctype::widen(c)
to return one
value, while for ctype::widen(&c, &c + 1, &wc)
to set
wc
to another value. This is almost certainly not intended. Both
forms of every function should be required to return the same result
for the same character, otherwise the same program using an
implementation that calls one form of the functions will behave
differently than when using another implementation that calls the
other form of the function "under the hood."
3. The last problem is that the standard text fails to specify whether one form of any of the virtual functions is permitted to be implemented in terms of the other form or not, and if so, whether it is required or permitted to call the overridden virtual function or not.
Thus, a program that overrides one of the virtual functions so that it calls the other form which then calls the base member might end up in an infinite loop if the called form of the base implementation of the function in turn calls the other form.
Lillehammer: Part of this isn't a real problem. We already talk about
caching. 22.1.1/6 But part is a real problem. ctype
virtuals may call
each other, so users don't know which ones to override to avoid avoid
infinite loops.
This is a problem for all facet virtuals, not just ctype
virtuals,
so we probably want a blanket statement in clause 22 for all
facets. The LWG is leaning toward a blanket prohibition, that a
facet's virtuals may never call each other. We might want to do that
in clause 27 too, for that matter. A review is necessary. Bill will
provide wording.
[ 2009-07 Frankfurt, Howard provided wording directed by consensus. ]
[ 2009-10 Santa Cruz: ]
Move to Ready.
Proposed resolution:
Add paragraph 3 to 28.3.4 [locale.categories]:
-3- Within this clause it is unspecified if one virtual function calls another virtual function.
Rationale:
We are explicitly not addressing bullet item #2, thus giving implementors more latitude. Users will have to override both virtual functions, not just one.
Section: 31.7.6.3.4 [ostream.inserters.character] Status: CD1 Submitter: Martin Sebor Opened: 2004-07-01 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
I think Footnote 297 is confused. The paragraph it applies to seems quite clear in that widen() is only called if the object is not a char stream (i.e., not basic_ostream<char>), so it's irrelevant what the value of widen(c) is otherwise.
Proposed resolution:
I propose to strike the Footnote.
Section: 26.6.5 [alg.foreach] Status: CD1 Submitter: Stephan T. Lavavej, Jaakko Jarvi Opened: 2004-07-09 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
It is not clear whether the function object passed to for_each is allowed to modify the elements of the sequence being iterated over.
for_each is classified without explanation in [lib.alg.nonmodifying], "25.1 Non-modifying sequence operations". 'Non-modifying sequence operation' is never defined.
25(5) says: "If an algorithm's Effects section says that a value pointed to by any iterator passed as an argument is modified, then that algorithm has an additional type requirement: The type of that argument shall satisfy the requirements of a mutable iterator (24.1)."
for_each's Effects section does not mention whether arguments can be modified:
"Effects: Applies f to the result of dereferencing every iterator in the range [first, last), starting from first and proceeding to last - 1."
Every other algorithm in [lib.alg.nonmodifying] is "really" non-modifying in the sense that neither the algorithms themselves nor the function objects passed to the algorithms may modify the sequences or elements in any way. This DR affects only for_each.
We suspect that for_each's classification in "non-modifying sequence operations" means that the algorithm itself does not inherently modify the sequence or the elements in the sequence, but that the function object passed to it may modify the elements it operates on.
The original STL document by Stepanov and Lee explicitly prohibited the function object from modifying its argument. The "obvious" implementation of for_each found in several standard library implementations, however, does not impose this restriction. As a result, we suspect that the use of for_each with function objects that modify their arguments is wide-spread. If the restriction was reinstated, all such code would become non-conforming. Further, none of the other algorithms in the Standard could serve the purpose of for_each (transform does not guarantee the order in which its function object is called).
We suggest that the standard be clarified to explicitly allow the function object passed to for_each modify its argument.
Proposed resolution:
Add a nonnormative note to the Effects in 26.6.5 [alg.foreach]: If the type of 'first' satisfies the requirements of a mutable iterator, 'f' may apply nonconstant functions through the dereferenced iterators passed to it.
Rationale:
The LWG believes that nothing in the standard prohibits function objects that modify the sequence elements. The problem is that for_each is in a secion entitled "nonmutating algorithms", and the title may be confusing. A nonnormative note should clarify that.
Section: 24.3.5.5 [forward.iterators] Status: CD1 Submitter: Dave Abrahams Opened: 2004-07-11 Last modified: 2016-01-28
Priority: Not Prioritized
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Duplicate of: 477
Discussion:
The Forward Iterator requirements table contains the following:
expression return type operational precondition semantics ========== ================== =========== ========================== a->m U& if X is mutable, (*a).m pre: (*a).m is well-defined. otherwise const U& r->m U& (*r).m pre: (*r).m is well-defined.
The second line may be unnecessary. Paragraph 11 of [lib.iterator.requirements] says:
In the following sections, a and b denote values of type const X, n denotes a value of the difference type Distance, u, tmp, and m denote identifiers, r denotes a value of X&, t denotes a value of value type T, o denotes a value of some type that is writable to the output iterator.
Because operators can be overloaded on an iterator's const-ness, the current requirements allow iterators to make many of the operations specified using the identifiers a and b invalid for non-const iterators.
Proposed resolution:
Remove the "r->m" line from the Forward Iterator requirements table. Change
"const X"
to
"X or const X"
in paragraph 11 of [lib.iterator.requirements].
Rationale:
This is a defect because it constrains an lvalue to returning a modifiable lvalue.
Section: 22.3 [pairs], 22.4 [tuple] Status: Resolved Submitter: Andrew Koenig Opened: 2004-09-14 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
(Based on recent comp.std.c++ discussion)
Pair (and tuple) should specialize std::swap to work in terms of std::swap on their components. For example, there's no obvious reason why swapping two objects of type pair<vector<int>, list<double> > should not take O(1).
[Lillehammer: We agree it should be swappable. Howard will provide wording.]
[
Post Oxford: We got swap
for pair
but accidently
missed tuple
. tuple::swap
is being tracked by 522(i).
]
Proposed resolution:
Wording provided in N1856.
Rationale:
Recommend NADResolved, fixed by
N1856.
Section: 24.3.5.4 [output.iterators] Status: Resolved Submitter: Chris Jefferson Opened: 2004-10-13 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
The note on 24.1.2 Output iterators insufficiently limits what can be performed on output iterators. While it requires that each iterator is progressed through only once and that each iterator is written to only once, it does not require the following things:
Note: Here it is assumed that x
is an output iterator of type X
which
has not yet been assigned to.
a) That each value of the output iterator is written to:
The standard allows:
++x; ++x; ++x;
b) That assignments to the output iterator are made in order
X a(x); ++a; *a=1; *x=2;
is allowed
c) Chains of output iterators cannot be constructed:
X a(x); ++a; X b(a); ++b; X c(b); ++c;
is allowed, and under the current
wording (I believe) x,a,b,c
could be written to in any order.
I do not believe this was the intension of the standard?
[Lillehammer: Real issue. There are lots of constraints we intended but didn't specify. Should be solved as part of iterator redesign.]
[ 2009-07 Frankfurt ]
Bill provided wording according to consensus.
[ 2009-07-21 Alisdair requests change from Review to Open. See thread starting with c++std-lib-24459 for discussion. ]
[ 2009-10 Santa Cruz: ]
Modified wording. Set to Review.
[ 2009-10 Santa Cruz: ]
Move to Ready after looking at again in a larger group in Santa Cruz.
[ 2010 Pittsburgh: ]
Moved to
NAD EditorialResolved. Rationale added below.
Rationale:
Solved by N3066.
Proposed resolution:
Change Table 101 — Output iterator requirements in 24.3.5.4 [output.iterators]:
Table 101 — Output iterator requirements Expression Return type Operational semantics Assertion/note pre-/post-condition X(a)
a = t
is equivalent toX(a) = t
. note: a destructor is assumed.X u(a);
X u = a;
*r = o
result is not used Post: r
is not required to be dereferenceable.r
is incrementable.++r
X&
&r == &++r
Post:r
is dereferenceable, unless otherwise specified.r
is not required to be incrementable.r++
convertible to const X&
{X tmp = r;
++r;
return tmp;}Post: r
is dereferenceable, unless otherwise specified.r
is not required to be incrementable.*r++ = o;
result is not used
Section: 26.7.11 [alg.rotate] Status: CD1 Submitter: Howard Hinnant Opened: 2004-11-22 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
rotate takes 3 iterators: first, middle and last which point into a sequence, and rearranges the sequence such that the subrange [middle, last) is now at the beginning of the sequence and the subrange [first, middle) follows. The return type is void.
In many use cases of rotate, the client needs to know where the subrange [first, middle) starts after the rotate is performed. This might look like:
rotate(first, middle, last); Iterator i = advance(first, distance(middle, last));
Unless the iterators are random access, the computation to find the start of the subrange [first, middle) has linear complexity. However, it is not difficult for rotate to return this information with negligible additional computation expense. So the client could code:
Iterator i = rotate(first, middle, last);
and the resulting program becomes significantly more efficient.
While the backwards compatibility hit with this change is not zero, it is very small (similar to that of lwg 130(i)), and there is a significant benefit to the change.
Proposed resolution:
In 26 [algorithms] p2, change:
template<class ForwardIterator>voidForwardIterator rotate(ForwardIterator first, ForwardIterator middle, ForwardIterator last);
In 26.7.11 [alg.rotate], change:
template<class ForwardIterator>voidForwardIterator rotate(ForwardIterator first, ForwardIterator middle, ForwardIterator last);
In 26.7.11 [alg.rotate] insert a new paragraph after p1:
Returns:
first + (last - middle)
.
[ The LWG agrees with this idea, but has one quibble: we want to make sure not to give the impression that the function "advance" is actually called, just that the nth iterator is returned. (Calling advance is observable behavior, since users can specialize it for their own iterators.) Howard will provide wording. ]
[Howard provided wording for mid-meeting-mailing Jun. 2005.]
[ Toronto: moved to Ready. ]
Section: 28.3 [localization] Status: CD1 Submitter: Beman Dawes Opened: 2005-01-10 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
It appears that there are no requirements specified for many of the template parameters in clause 22. It looks like this issue has never come up, except perhaps for Facet.
Clause 22 isn't even listed in 17.3.2.1 [lib.type.descriptions], either, which is the wording that allows requirements on template parameters to be identified by name.
So one issue is that 17.3.2.1 [lib.type.descriptions] Should be changed to cover clause 22. A better change, which will cover us in the future, would be to say that it applies to all the library clauses. Then if a template gets added to any library clause we are covered.
charT, InputIterator, and other names with requirements defined elsewhere are fine, assuming the 17.3.2.1 [lib.type.descriptions] fix. But there are a few template arguments names which I don't think have requirements given elsewhere:
Proposed resolution:
Change 16.3.3.3 [type.descriptions], paragraph 1, from:
The Requirements subclauses may describe names that are used to specify constraints on template arguments.153) These names are used in clauses 20, 23, 25, and 26 to describe the types that may be supplied as arguments by a C++ program when instantiating template components from the library.
to:
The Requirements subclauses may describe names that are used to specify constraints on template arguments.153) These names are used in library clauses to describe the types that may be supplied as arguments by a C++ program when instantiating template components from the library.
In the front matter of class 22, locales, add:
Template parameter types internT and externT shall meet the requirements of charT (described in 27 [strings]).
Rationale:
Again, a blanket clause isn't blanket enough. Also, we've got a couple of names that we don't have blanket requirement statements for. The only issue is what to do about stateT. This wording is thin, but probably adequate.
Section: 23.3.13 [vector] Status: CD1 Submitter: richard@ex-parrot.com Opened: 2005-02-10 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
In the synopsis of the std::vector<bool> specialisation in 23.3.13 [vector], the non-template assign() function has the signature
void assign( size_type n, const T& t );
The type, T, is not defined in this context.
Proposed resolution:
Replace "T" with "value_type".
Section: 17.3.5.2 [numeric.limits.members] Status: CD1 Submitter: Martin Sebor Opened: 2005-03-02 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
18.2.1.2, p59 says this much about the traps member of numeric_limits:
static const bool traps;
-59- true if trapping is implemented for the type.204)
Footnote 204: Required by LIA-1.
It's not clear what is meant by "is implemented" here.
In the context of floating point numbers it seems reasonable to expect to be able to use traps to determine whether a program can "safely" use infinity(), quiet_NaN(), etc., in arithmetic expressions, that is without causing a trap (i.e., on UNIX without having to worry about getting a signal). When traps is true, I would expect any of the operations in section 7 of IEEE 754 to cause a trap (and my program to get a SIGFPE). So, for example, on Alpha, I would expect traps to be true by default (unless I compiled my program with the -ieee option), false by default on most other popular architectures, including IA64, MIPS, PA-RISC, PPC, SPARC, and x86 which require traps to be explicitly enabled by the program.
Another possible interpretation of p59 is that traps should be true on any implementation that supports traps regardless of whether they are enabled by default or not. I don't think such an interpretation makes the traps member very useful, even though that is how traps is implemented on several platforms. It is also the only way to implement traps on platforms that allow programs to enable and disable trapping at runtime.
Proposed resolution:
Change p59 to read:
True if, at program startup, there exists a value of the type that would cause an arithmetic operation using that value to trap.
Rationale:
Real issue, since trapping can be turned on and off. Unclear what a static query can say about a dynamic issue. The real advice we should give users is to use cfenv for these sorts of queries. But this new proposed resolution is at least consistent and slightly better than nothing.
Section: 26.8.5 [alg.partitions] Status: C++11 Submitter: Sean Parent, Joe Gottman Opened: 2005-05-04 Last modified: 2025-03-13
Priority: Not Prioritized
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Discussion:
Problem: The iterator requirements for partition() and stable_partition() [25.2.12] are listed as BidirectionalIterator, however, there are efficient algorithms for these functions that only require ForwardIterator that have been known since before the standard existed. The SGI implementation includes these (see https://www.boost.org/sgi/stl/partition.html and https://www.boost.org/sgi/stl/stable_partition.html).
[ 2009-04-30 Alisdair adds: ]
Now we have concepts this is easier to express!
Proposed resolution:
Add the following signature to:
Header
<algorithm>
synopsis [algorithms.syn]
p3 Partitions 26.8.5 [alg.partitions]template<ForwardIterator Iter, Predicate<auto, Iter::value_type> Pred> requires ShuffleIterator<Iter> && CopyConstructible<Pred> Iter partition(Iter first, Iter last, Pred pred);Update p3 Partitions 26.8.5 [alg.partitions]:
Complexity:
At mostIf(last - first)/2
swaps. Exactlylast - first
applications of the predicate are done.Iter
satisfiesBidirectionalIterator
, at most(last - first)/2
swaps. Exactlylast - first
applications of the predicate are done.If
Iter
merely satisfiedForwardIterator
at most(last - first)
swaps are done. Exactly(last - first)
applications of the predicate are done.[Editorial note: I looked for existing precedent in how we might call out distinct overloads overloads from a set of constrained templates, but there is not much existing practice to lean on. advance/distance were the only algorithms I could find, and that wording is no clearer.]
[ 2009-07 Frankfurt ]
Hinnant: if you want to partition your std::forward_list, you'll need partition() to accept ForwardIterators.
No objection to Ready.
Move to Ready.
Proposed resolution:
Change 25.2.12 from
template<class BidirectionalIterator, class Predicate> BidirectionalIterator partition(BidirectionalIterato r first, BidirectionalIterator last, Predicate pred);
to
template<class ForwardIterator, class Predicate> ForwardIterator partition(ForwardIterator first, ForwardIterator last, Predicate pred);
Change the complexity from
At most (last - first)/2 swaps are done. Exactly (last - first) applications of the predicate are done.
to
If ForwardIterator is a bidirectional_iterator, at most (last - first)/2 swaps are done; otherwise at most (last - first) swaps are done. Exactly (last - first) applications of the predicate are done.
Rationale:
Partition is a "foundation" algorithm useful in many contexts (like sorting as just one example) - my motivation for extending it to include forward iterators is foward_list - without this extension you can't partition an foward_list (without writing your own partition). Holes like this in the standard library weaken the argument for generic programming (ideally I'd be able to provide a library that would refine std::partition() to other concepts without fear of conflicting with other libraries doing the same - but that is a digression). I consider the fact that partition isn't defined to work for ForwardIterator a minor embarrassment.
[Mont Tremblant: Moved to Open, request motivation and use cases by next meeting. Sean provided further rationale by post-meeting mailing.]
Section: 29.5.3 [rand.req], 99 [tr.rand.req] Status: CD1 Submitter: Walter Brown Opened: 2005-07-03 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
Table 17: Random distribution requirements
Row 1 requires that each random distribution provide a nested type "input_type"; this type denotes the type of the values that the distribution consumes.
Inspection of all distributions in [tr.rand.dist] reveals that each distribution provides a second typedef ("result_type") that denotes the type of the values the distribution produces when called.
Proposed resolution:
It seems to me that this is also a requirement for all distributions and should therefore be indicated as such via a new second row to this table 17:
X::result_type | T | --- | compile-time |
[ Berlin: Voted to WP. N1932 adopts the proposed resolution: see Table 5 row 1. ]
Section: 29.5 [rand], 99 [tr.rand.var] Status: CD1 Submitter: Walter Brown Opened: 2005-07-03 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
Paragraph 11 of [tr.rand.var] equires that the member template
template<class T> result_type operator() (T value);
return
distribution()(e, value)
However, not all distributions have an operator() with a corresponding signature.
[ Berlin: As a working group we voted in favor of N1932 which makes this moot: variate_generator has been eliminated. Then in full committee we voted to give this issue WP status (mistakenly). ]
Proposed resolution:
We therefore recommend that we insert the following precondition before paragraph 11:
Precondition:
distribution().operator()(e,value)
is well-formed.
Section: 29.5.6 [rand.predef], 99 [tr.rand.predef] Status: CD1 Submitter: Walter Brown Opened: 2005-07-03 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
The fifth of these engines with predefined parameters, ranlux64_base_01, appears to have an unintentional error for which there is a simple correction. The two pre-defined subtract_with_carry_01 engines are given as:
typedef subtract_with_carry_01<float, 24, 10, 24> ranlux_base_01; typedef subtract_with_carry_01<double, 48, 10, 24> ranlux64_base_01;
We demonstrate below that ranlux64_base_01 fails to meet the intent of the random number generation proposal, but that the simple correction to
typedef subtract_with_carry_01<double, 48, 5, 12> ranlux64_base_01;
does meet the intent of defining well-known good parameterizations.
The ranlux64_base_01 engine as presented fails to meet the intent for predefined engines, stated in proposal N1398 (section E):
In order to make good random numbers available to a large number of library users, this proposal not only defines generic random-number engines, but also provides a number of predefined well-known good parameterizations for those.
The predefined ranlux_base_01 engine has been proven [1,2,3] to have a very long period and so meets this criterion. This property makes it suitable for use in the excellent discard_block engines defined subsequently. The proof of long period relies on the fact (proven in [1]) that 2**(w*r) - 2**(w*s) + 1 is prime (w, r, and s are template parameters to subtract_with_carry_01, as defined in [tr.rand.eng.sub1]).
The ranlux64_base_01 engine as presented in [tr.rand.predef] uses w=48, r=24, s=10. For these numbers, the combination 2**(w*r)-2**(w*s)+1 is non-prime (though explicit factorization would be a challenge). In consequence, while it is certainly possible for some seeding states that this engine would have a very long period, it is not at all "well-known" that this is the case. The intent in the N1398 proposal involved the base of the ranlux64 engine, which finds heavy use in the physics community. This is isomorphic to the predefined ranlux_base_01, but exploits the ability of double variables to hold (at least) 48 bits of mantissa, to deliver 48 random bits at a time rather than 24.
Proposed resolution:
To achieve this intended behavior, the correct template parameteriztion would be:
typedef subtract_with_carry_01<double, 48, 5, 12> ranlux64_base_01;
The sequence of mantissa bits delivered by this is isomorphic (treating each double as having the bits of two floats) to that delivered by ranlux_base_01.
References:
[ Berlin: Voted to WP. N1932 adopts the proposed resolution in 26.3.5, just above paragraph 5. ]
Section: 23.2.8 [unord.req], 99 [tr.unord.req] Status: CD1 Submitter: Matt Austern Opened: 2005-07-03 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
Issue 371 deals with stability of multiset/multimap under insert and erase (i.e. do they preserve the relative order in ranges of equal elements). The same issue applies to unordered_multiset and unordered_multimap.
[ Moved to open (from review): There is no resolution. ]
[ Toronto: We have a resolution now. Moved to Review. Some concern was noted as to whether this conflicted with existing practice or not. An additional concern was in specifying (partial) ordering for an unordered container. ]
Proposed resolution:
Wording for the proposed resolution is taken from the equivalent text for associative containers.
Change 23.2.8 [unord.req], Unordered associative containers, paragraph 6 to:
An unordered associative container supports unique keys if it may contain at most one element for each key. Otherwise, it supports equivalent keys.
unordered_set
andunordered_map
support unique keys.unordered_multiset
andunordered_multimap
support equivalent keys. In containers that support equivalent keys, elements with equivalent keys are adjacent to each other. Forunordered_multiset
andunordered_multimap
,insert
anderase
preserve the relative ordering of equivalent elements.
Change 23.2.8 [unord.req], Unordered associative containers, paragraph 8 to:
The elements of an unordered associative container are organized into buckets. Keys with the same hash code appear in the same bucket. The number of buckets is automatically increased as elements are added to an unordered associative container, so that the average number of elements per bucket is kept below a bound. Rehashing invalidates iterators, changes ordering between elements, and changes which buckets elements appear in, but does not invalidate pointers or references to elements. For
unordered_multiset
andunordered_multimap
, rehashing preserves the relative ordering of equivalent elements.
Section: 23.3.3 [array], 99 [tr.array.array] Status: CD1 Submitter: Pete Becker Opened: 2005-07-03 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
array<>::data()
is present in the class synopsis, but not documented.
Proposed resolution:
Add a new section, after 6.2.2.3:
T* data() const T* data() const;
Returns: elems
.
Change 6.2.2.4/2 to:
In the case where
N == 0
,begin() == end()
. The return value ofdata()
is unspecified.
Section: 22.10.15 [func.bind], 99 [tr.func.bind] Status: CD1 Submitter: Pete Becker Opened: 2005-07-03 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
In the original proposal for binders, the return type of bind() when called with a pointer to member data as it's callable object was defined to be mem_fn(ptr); when Peter Dimov and I unified the descriptions of the TR1 function objects we hoisted the descriptions of return types into the INVOKE pseudo-function and into result_of. Unfortunately, we left pointer to member data out of result_of, so bind doesn't have any specified behavior when called with a pointer to member data.
Proposed resolution:
[ Pete and Peter will provide wording. ]
In 20.5.4 [lib.func.ret] ([tr.func.ret]) p3 add the following bullet after bullet 2:
F
is a member data pointer type R T::*
, type
shall be cv R&
when T1
is cv U1&
,
R
otherwise.[ Peter provided wording. ]
Section: 22.10.6 [refwrap], 99 [tr.util.refwrp.refwrp] Status: CD1 Submitter: Pete Becker Opened: 2005-07-03 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
2.1.2/3, second bullet item currently says that reference_wrapper<T> is derived from unary_function<T, R> if T is:
a pointer to member function type with cv-qualifier cv and no arguments; the type T1 is cv T* and R is the return type of the pointer to member function;
The type of T1 can't be cv T*, 'cause that's a pointer to a pointer to member function. It should be a pointer to the class that T is a pointer to member of. Like this:
a pointer to a member function R T0::f() cv (where cv represents the member function's cv-qualifiers); the type T1 is cv T0*
Similarly, bullet item 2 in 2.1.2/4 should be:
a pointer to a member function R T0::f(T2) cv (where cv represents the member function's cv-qualifiers); the type T1 is cv T0*
Proposed resolution:
Change bullet item 2 in 2.1.2/3:
- a pointer to member function
type with cv-qualifiercv
and no arguments; the typeT1
iscv T*
andR
is the return type of the pointer to member functionR T0::f() cv
(wherecv
represents the member function's cv-qualifiers); the typeT1
iscv T0*
Change bullet item 2 in 2.1.2/4:
- a pointer to member function
with cv-qualifiercv
and taking one argument of typeT2
; the typeT1
iscv T*
andR
is the return type of the pointer to member functionR T0::f(T2) cv
(wherecv
represents the member function's cv-qualifiers); the typeT1
iscv T0*
Section: 22.4 [tuple], 99 [tr.tuple] Status: CD1 Submitter: Andy Koenig Opened: 2005-07-03 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
Tuple doesn't define swap(). It should.
[ Berlin: Doug to provide wording. ]
[ Batavia: Howard to provide wording. ]
[ Toronto: Howard to provide wording (really this time). ]
[ Bellevue: Alisdair provided wording. ]
Proposed resolution:
Add these signatures to 22.4 [tuple]
template <class... Types> void swap(tuple<Types...>& x, tuple<Types...>& y); template <class... Types> void swap(tuple<Types...>&& x, tuple<Types...>& y); template <class... Types> void swap(tuple<Types...>& x, tuple<Types...>&& y);
Add this signature to 22.4.4 [tuple.tuple]
void swap(tuple&&);
Add the following two sections to the end of the tuple clauses
20.3.1.7 tuple swap [tuple.swap]
void swap(tuple&& rhs);Requires: Each type in
Types
shall beSwappable
.Effects: Calls
swap
for each element in*this
and its corresponding element inrhs
.Throws: Nothing, unless one of the element-wise
swap
calls throw an exception.20.3.1.8 tuple specialized algorithms [tuple.special]
template <class... Types> void swap(tuple<Types...>& x, tuple<Types...>& y); template <class... Types> void swap(tuple<Types...>&& x, tuple<Types...>& y); template <class... Types> void swap(tuple<Types...>& x, tuple<Types...>&& y);Effects: x.swap(y)
Section: 28.6 [re] Status: CD1 Submitter: Eric Niebler Opened: 2005-07-01 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
This defect is also being discussed on the Boost developers list. The full discussion can be found here: http://lists.boost.org/boost/2005/07/29546.php
-- Begin original message --
Also, I may have found another issue, closely related to the one under discussion. It regards case-insensitive matching of named character classes. The regex_traits<> provides two functions for working with named char classes: lookup_classname and isctype. To match a char class such as [[:alpha:]], you pass "alpha" to lookup_classname and get a bitmask. Later, you pass a char and the bitmask to isctype and get a bool yes/no answer.
But how does case-insensitivity work in this scenario? Suppose we're doing a case-insensitive match on [[:lower:]]. It should behave as if it were [[:lower:][:upper:]], right? But there doesn't seem to be enough smarts in the regex_traits interface to do this.
Imagine I write a traits class which recognizes [[:fubar:]], and the "fubar" char class happens to be case-sensitive. How is the regex engine to know that? And how should it do a case-insensitive match of a character against the [[:fubar:]] char class? John, can you confirm this is a legitimate problem?
I see two options:
1) Add a bool icase parameter to lookup_classname. Then, lookup_classname( "upper", true ) will know to return lower|upper instead of just upper.
2) Add a isctype_nocase function
I prefer (1) because the extra computation happens at the time the pattern is compiled rather than when it is executed.
-- End original message --
For what it's worth, John has also expressed his preference for option (1) above.
Proposed resolution:
Adopt the proposed resolution in N2409.
[ Kona (2007): The LWG adopted the proposed resolution of N2409 for this issue. The LWG voted to accelerate this issue to Ready status to be voted into the WP at Kona. ]
Section: 21.3.5 [meta.unary], 99 [tr.meta.unary] Status: Resolved Submitter: Robert Klarer Opened: 2005-07-11 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
It is not completely clear how the primary type traits deal with cv-qualified types. And several of the secondary type traits seem to be lacking a definition.
[ Berlin: Howard to provide wording. ]
Proposed resolution:
Wording provided in N2028. A revision (N2157) provides more detail for motivation.
Rationale:
Solved by revision (N2157) in the WP.
tr1::bind
has lost its Throws clauseSection: 22.10.15.4 [func.bind.bind], 99 [tr.func.bind.bind] Status: CD1 Submitter: Peter Dimov Opened: 2005-10-01 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
The original bind proposal gives the guarantee that tr1::bind(f, t1, ..., tN)
does not throw when the copy constructors of f, t1, ..., tN
don't.
This guarantee is not present in the final version of TR1.
I'm pretty certain that we never removed it on purpose. Editorial omission? :-)
[ Berlin: not quite editorial, needs proposed wording. ]
[ Batavia: Doug to translate wording to variadic templates. ]
[ Toronto: We agree but aren't quite happy with the wording. The "t"'s no longer refer to anything. Alan to provide improved wording. ]
[ Pre-Bellevue: Alisdair provided wording. ]
TR1 proposed resolution:
In 99 [tr.func.bind.bind], add a new paragraph after p2:
Throws: Nothing unless one of the copy constructors of
f, t1, t2, ..., tN
throws an exception.Add a new paragraph after p4:
Throws: nothing unless one of the copy constructors of
f, t1, t2, ..., tN
throws an exception.
Proposed resolution:
In 22.10.15.4 [func.bind.bind], add a new paragraph after p2:
Throws: Nothing unless the copy constructor of
F
or of one of the types in theBoundArgs...
pack expansion throws an exception.
In 22.10.15.4 [func.bind.bind], add a new paragraph after p4:
Throws: Nothing unless the copy constructor of
F
or of one of the types in theBoundArgs...
pack expansion throws an exception.
Section: 27.4.3 [basic.string] Status: CD1 Submitter: Matt Austern Opened: 2005-11-15 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
Issue 69(i), which was incorporated into C++03, mandated
that the elements of a vector must be stored in contiguous memory.
Should the same also apply to basic_string
?
We almost require contiguity already. Clause 23.4.7 [multiset]
defines operator[]
as data()[pos]
. What's missing
is a similar guarantee if we access the string's elements via the
iterator interface.
Given the existence of data()
, and the definition of
operator[]
and at
in terms of data
,
I don't believe it's possible to write a useful and standard-
conforming basic_string
that isn't contiguous. I'm not
aware of any non-contiguous implementation. We should just require
it.
Proposed resolution:
Add the following text to the end of 27.4.3 [basic.string], paragraph 2.
The characters in a string are stored contiguously, meaning that if
s
is abasic_string<charT, Allocator>
, then it obeys the identity&*(s.begin() + n) == &*s.begin() + n
for all0 <= n < s.size()
.
Rationale:
Not standardizing this existing practice does not give implementors more freedom. We thought it might a decade ago. But the vendors have spoken both with their implementations, and with their voice at the LWG meetings. The implementations are going to be contiguous no matter what the standard says. So the standard might as well give string clients more design choices.
Section: 31.7.5.4 [istream.unformatted] Status: CD1 Submitter: Martin Sebor Opened: 2005-11-23 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
The array forms of unformatted input functions don't seem to have well-defined
semantics for zero-element arrays in a couple of cases. The affected ones
(istream::get()
and istream::getline()
) are supposed to
terminate when (n - 1)
characters are stored, which obviously can
never be true when (n == 0)
holds to start with. See
c++std-lib-16071.
Proposed resolution:
I suggest changing 27.6.1.3, p7 (istream::get()
), bullet 1 to read:
(n < 1)
is true or (n - 1)
characters
are stored;
Change 27.6.1.3, p9:
If the function stores no characters, it calls
setstate(failbit)
(which may throwios_base::failure
(27.4.4.3)). In any case, if(n > 0)
is true it then stores a null character into the next successive location of the array.
and similarly p17 (istream::getline()
), bullet 3 to:
(n < 1)
is true or (n - 1)
characters
are stored (in which case the function calls
setstate(failbit)
).
In addition, to clarify that istream::getline()
must not store the
terminating NUL character unless the the array has non-zero size, Robert
Klarer suggests in c++std-lib-16082 to change 27.6.1.3, p20 to read:
In any case, provided
(n > 0)
is true, it then stores a null character (using charT()) into the next successive location of the array.
[
post-Redmond: Pete noticed that the current resolution for get
requires
writing to out of bounds memory when n == 0
. Martin provided fix.
]
Section: 20.3.2.2.11 [util.smartptr.getdeleter], 99 [tr.util.smartptr.getdeleter] Status: CD1 Submitter: Paolo Carlini Opened: 2005-11-09 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
I'm seeing something that looks like a typo. The Return of get_deleter
says:
If
*this
owns a deleterd
...
but get_deleter
is a free function!
Proposed resolution:
Therefore, I think should be:
If
owns a deleter
*thispd
...
Section: 27.4.3 [basic.string] Status: CD1 Submitter: Alisdair Meredith Opened: 2005-11-16 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
OK, we all know std::basic_string is bloated and already has way too many members. However, I propose it is missing 3 useful members that are often expected by users believing it is a close approximation of the container concept. All 3 are listed in table 71 as 'optional'
i/ pop_back.
This is the one I feel most strongly about, as I only just discovered it was missing as we are switching to a more conforming standard library <g>
I find it particularly inconsistent to support push_back, but not pop_back.
ii/ back.
There are certainly cases where I want to examine the last character of a string before deciding to append, or to trim trailing path separators from directory names etc. *rbegin() somehow feels inelegant.
iii/ front
This one I don't feel strongly about, but if I can get the first two, this one feels that it should be added as a 'me too' for consistency.
I believe this would be similarly useful to the data() member recently added to vector, or at() member added to the maps.
Proposed resolution:
Add the following members to definition of class template basic_string, 21.3p7
void pop_back () const charT & front() const charT & front() const charT & back() const charT & back()
Add the following paragraphs to basic_string description
21.3.4p5
const charT & front() const charT & front()Precondition:
!empty()
Effects: Equivalent to
operator[](0)
.
21.3.4p6
const charT & back() const charT & back()Precondition:
!empty()
Effects: Equivalent to
operator[]( size() - 1)
.
21.3.5.5p10
void pop_back ()Precondition:
!empty()
Effects: Equivalent to
erase( size() - 1, 1 )
.
Update Table 71: (optional sequence operations) Add basic_string to the list of containers for the following operations.
a.front() a.back() a.push_back() a.pop_back() a[n]
[ Berlin: Has support. Alisdair provided wording. ]
Section: 27.4.3.7.8 [string.swap] Status: CD1 Submitter: Beman Dawes Opened: 2005-12-14 Last modified: 2016-11-12
Priority: Not Prioritized
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Discussion:
std::string::swap currently says for effects and postcondition:
Effects: Swaps the contents of the two strings.
Postcondition:
*this
contains the characters that were ins
,s
contains the characters that were in*this
.
Specifying both Effects and Postcondition seems redundant, and the postcondition needs to be made stronger. Users would be unhappy if the characters were not in the same order after the swap.
Proposed resolution:
Effects: Swaps the contents of the two strings.Postcondition:
*this
contains the same sequence of characters thatwerewas ins
,s
contains the same sequence of characters thatwerewas in*this
.
Section: 31.7.5.4 [istream.unformatted] Status: CD1 Submitter: Paolo Carlini Opened: 2006-02-12 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
In the most recent working draft, I'm still seeing:
seekg(off_type& off, ios_base::seekdir dir)
and
seekp(pos_type& pos) seekp(off_type& off, ios_base::seekdir dir)
that is, by reference off and pos arguments.
Proposed resolution:
After 27.6.1.3p42 change:
basic_istream<charT,traits>& seekg(off_type&off, ios_base::seekdir dir);
After 27.6.2.4p1 change:
basic_ostream<charT,traits>& seekp(pos_type&pos);
After 27.6.2.4p3 change:
basic_ostream<charT,traits>& seekp(off_type&off, ios_base::seekdir dir);
Section: 26.7.9 [alg.unique] Status: CD1 Submitter: Howard Hinnant Opened: 2006-02-09 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
I believe I botched the resolution of 241(i) "Does unique_copy() require CopyConstructible and Assignable?" which now has WP status.
This talks about unique_copy
requirements and currently reads:
-5- Requires: The ranges
[first, last)
and[result, result+(last-first))
shall not overlap. The expression*result = *first
shall be valid. If neitherInputIterator
norOutputIterator
meets the requirements of forward iterator then the value type ofInputIterator
must be CopyConstructible (20.1.3). Otherwise CopyConstructible is not required.
The problem (which Paolo discovered) is that when the iterators are at their
most restrictive (InputIterator
, OutputIterator
), then we want
InputIterator::value_type
to be both CopyConstructible
and
CopyAssignable
(for the most efficient implementation). However this
proposed resolution only makes it clear that it is CopyConstructible
,
and that one can assign from *first
to *result
.
This latter requirement does not necessarily imply that you can:
*first = *first;
Proposed resolution:
-5- Requires: The ranges
[first, last)
and[result, result+(last-first))
shall not overlap. The expression*result = *first
shall be valid. If neitherInputIterator
norOutputIterator
meets the requirements of forward iterator then thevalue typevalue_type
ofInputIterator
must be CopyConstructible (20.1.3) and Assignable. Otherwise CopyConstructible is not required.
partial_sum
and adjacent_difference
should mention requirementsSection: 26.10.7 [partial.sum] Status: C++11 Submitter: Marc Schoolderman Opened: 2006-02-06 Last modified: 2016-01-28
Priority: Not Prioritized
View all issues with C++11 status.
Discussion:
There are some problems in the definition of partial_sum
and
adjacent_difference
in 26.4 [lib.numeric.ops]
Unlike accumulate
and inner_product
, these functions are not
parametrized on a "type T", instead, 26.4.3 [lib.partial.sum] simply
specifies the effects clause as;
Assigns to every element referred to by iterator
i
in the range[result,result + (last - first))
a value correspondingly equal to((...(* first + *( first + 1)) + ...) + *( first + ( i - result )))
And similarly for BinaryOperation. Using just this definition, it seems logical to expect that:
char i_array[4] = { 100, 100, 100, 100 }; int o_array[4]; std::partial_sum(i_array, i_array+4, o_array);
Is equivalent to
int o_array[4] = { 100, 100+100, 100+100+100, 100+100+100+100 };
i.e. 100, 200, 300, 400, with addition happening in the result type
,
int
.
Yet all implementations I have tested produce 100, -56, 44, -112,
because they are using an accumulator of the InputIterator
's
value_type
, which in this case is char
, not int
.
The issue becomes more noticeable when the result of the expression *i +
*(i+1)
or binary_op(*i, *i-1)
can't be converted to the
value_type
. In a contrived example:
enum not_int { x = 1, y = 2 }; ... not_int e_array[4] = { x, x, y, y }; std::partial_sum(e_array, e_array+4, o_array);
Is it the intent that the operations happen in the input type
, or in
the result type
?
If the intent is that operations happen in the result type
, something
like this should be added to the "Requires" clause of 26.4.3/4
[lib.partial.sum]:
The type of
*i + *(i+1)
orbinary_op(*i, *(i+1))
shall meet the requirements ofCopyConstructible
(20.1.3) andAssignable
(23.1) types.
(As also required for T
in 26.4.1 [lib.accumulate] and 26.4.2
[lib.inner.product].)
The "auto initializer" feature proposed in
N1894
is not required to
implement partial_sum
this way. The 'narrowing' behaviour can still be
obtained by using the std::plus<>
function object.
If the intent is that operations happen in the input type
, then
something like this should be added instead;
The type of *first shall meet the requirements of
CopyConstructible
(20.1.3) andAssignable
(23.1) types. The result of*i + *(i+1)
orbinary_op(*i, *(i+1))
shall be convertible to this type.
The 'widening' behaviour can then be obtained by writing a custom proxy iterator, which is somewhat involved.
In both cases, the semantics should probably be clarified.
26.4.4 [lib.adjacent.difference] is similarly underspecified, although all implementations seem to perform operations in the 'result' type:
unsigned char i_array[4] = { 4, 3, 2, 1 }; int o_array[4]; std::adjacent_difference(i_array, i_array+4, o_array);
o_array is 4, -1, -1, -1 as expected, not 4, 255, 255, 255.
In any case, adjacent_difference
doesn't mention the requirements on the
value_type
; it can be brought in line with the rest of 26.4
[lib.numeric.ops] by adding the following to 26.4.4/2
[lib.adjacent.difference]:
The type of
*first
shall meet the requirements ofCopyConstructible
(20.1.3) andAssignable
(23.1) types."
[ Berlin: Giving output iterator's value_types very controversial. Suggestion of adding signatures to allow user to specify "accumulator". ]
[ Bellevue: ]
The intent of the algorithms is to perform their calculations using the type of the input iterator. Proposed wording provided.
[ Sophia Antipolis: ]
We did not agree that the proposed resolution was correct. For example, when the arguments are types
(float*, float*, double*)
, the highest-quality solution would use double as the type of the accumulator. If the intent of the wording is to require that the type of the accumulator must be theinput_iterator
'svalue_type
, the wording should specify it.
[ 2009-05-09 Alisdair adds: ]
Now that we have the facility, the 'best' accumulator type could probably be deduced as:
std::common_type<InIter::value_type, OutIter::reference>::typeThis type would then have additional requirements of constructability and incrementability/assignability.
If this extracting an accumulator type from a pair/set of iterators (with additional requirements on that type) is a problem for multiple functions, it might be worth extracting into a SharedAccumulator concept or similar.
I'll go no further in writing up wording now, until the group gives a clearer indication of preferred direction.
[ 2009-07 Frankfurt ]
The proposed resolution isn't quite right. For example, "the type of
*first
" should be changed to "iterator::value_type
" or similar. Daniel volunteered to correct the wording.
[ 2009-07-29 Daniel corrected wording. ]
[ 2009-10 Santa Cruz: ]
Move to Ready.
Proposed resolution:
Change 26.10.7 [partial.sum]/1 as indicated:
Effects: Let
VT
beInputIterator
's value type. For a nonempty range, initializes an accumulatoracc
of typeVT
with*first
and performs*result = acc
. For every iteratori
in[first + 1, last)
in order,acc
is then modified byacc = acc + *i
oracc = binary_op(acc, *i)
and is assigned to*(result + (i - first))
.Assigns to every element referred to by iteratori
in the range[result,result + (last - first))
a value correspondingly equal to((...(*first + *(first + 1)) + ...) + *(first + (i - result)))
orbinary_op(binary_op(..., binary_op(*first, *(first + 1)),...), *(first + (i - result)))
Change 26.10.7 [partial.sum]/3 as indicated:
Complexity: Exactly
max((last - first) - 1, 0)
applications ofthe binary operation.
binary_op
Change 26.10.7 [partial.sum]/4 as indicated:
Requires:
VT
shall be constructible from the type of*first
, the result ofacc + *i
orbinary_op(acc, *i)
shall be implicitly convertible toVT
, and the result of the expressionacc
shall be writable to theresult
output iterator. In the ranges[first,last]
and[result,result + (last - first)]
[..]
Change 26.10.12 [adjacent.difference]/1 as indicated:
Effects: Let
VT
beInputIterator
's value type. For a nonempty range, initializes an accumulatoracc
of typeVT
with*first
and performs*result = acc
. For every iteratori
in[first + 1, last)
in order, initializes a valueval
of typeVT
with*i
, assigns the result ofval - acc
orbinary_op(val, acc)
to*(result + (i - first))
and modifiesacc = std::move(val)
.Assigns to every element referred to by iteratori
in the range[result + 1, result + (last - first))
a value correspondingly equal to*(first + (i - result)) - *(first + (i - result) - 1)
orbinary_op(*(first + (i - result)), *(first + (i - result) - 1)).
result gets the value of *first.
Change 26.10.12 [adjacent.difference]/2 as indicated:
Requires:
VT
shall beMoveAssignable
([moveassignable]) and shall be constructible from the type of*first
. The result of the expressionacc
and the result of the expressionval - acc
orbinary_op(val, acc)
shall be writable to theresult
output iterator. In the ranges[first,last]
[..]
Change 26.10.12 [adjacent.difference]/5 as indicated:
Complexity: Exactly
max((last - first) - 1, 0)
applications ofthe binary operation.binary_op
Section: 20.3.2.2.6 [util.smartptr.shared.obs], 99 [tr.util.smartptr.shared.obs] Status: CD1 Submitter: Martin Sebor Opened: 2005-10-15 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
I'm trying to reconcile the note in tr.util.smartptr.shared.obs, p6 that talks about the operator*() member function of shared_ptr:
Notes: When T is void, attempting to instantiate this member function renders the program ill-formed. [Note: Instantiating shared_ptr<void> does not necessarily result in instantiating this member function. --end note]
with the requirement in temp.inst, p1:
The implicit instantiation of a class template specialization causes the implicit instantiation of the declarations, but not of the definitions...
I assume that what the note is really trying to say is that "instantiating shared_ptr<void> *must not* result in instantiating this member function." That is, that this function must not be declared a member of shared_ptr<void>. Is my interpretation correct?
Proposed resolution:
Change 2.2.3.5p6
-6-
Notes:WhenT
isvoid
,attempting to instantiate this member function renders the program ill-formed. [Note: Instantiatingit is unspecified whether this member function is declared or not, and if so, what its return type is, except that the declaration (although not necessarily the definition) of the function shall be well-formed.shared_ptr<void>
does not necessarily result in instantiating this member function. --end note]
Section: 20.3.2.2 [util.smartptr.shared], 99 [tr.util.smartptr.shared] Status: CD1 Submitter: Martin Sebor Opened: 2005-10-16 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
Is the void specialization of the template assignment operator taking a shared_ptr<void> as an argument supposed be well-formed?
I.e., is this snippet well-formed:
shared_ptr<void> p; p.operator=<void>(p);
Gcc complains about auto_ptr<void>::operator*() returning a reference to void. I suspect it's because shared_ptr has two template assignment operators, one of which takes auto_ptr, and the auto_ptr template gets implicitly instantiated in the process of overload resolution.
The only way I see around it is to do the same trick with auto_ptr<void> operator*() as with the same operator in shared_ptr<void>.
PS Strangely enough, the EDG front end doesn't mind the code, even though in a small test case (below) I can reproduce the error with it as well.
template <class T> struct A { T& operator*() { return *(T*)0; } }; template <class T> struct B { void operator= (const B&) { } template <class U> void operator= (const B<U>&) { } template <class U> void operator= (const A<U>&) { } }; int main () { B<void> b; b.operator=<void>(b); }
Proposed resolution:
In [lib.memory] change:
template<class X> class auto_ptr; template<> class auto_ptr<void>;
In [lib.auto.ptr]/2 add the following before the last closing brace:
template<> class auto_ptr<void> { public: typedef void element_type; };
Section: 20.3.2.2.6 [util.smartptr.shared.obs], 99 [tr.util.smartptr.shared.obs] Status: CD1 Submitter: Martin Sebor Opened: 2005-10-18 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
Peter Dimov wrote: To: C++ libraries mailing list Message c++std-lib-15614 [...] The intent is for both use_count() and unique() to work in a threaded environment. They are intrinsically prone to race conditions, but they never return garbage.
This is a crucial piece of information that I really wish were captured in the text. Having this in a non-normative note would have made everything crystal clear to me and probably stopped me from ever starting this discussion :) Instead, the sentence in p12 "use only for debugging and testing purposes, not for production code" very strongly suggests that implementations can and even are encouraged to return garbage (when threads are involved) for performance reasons.
How about adding an informative note along these lines:
Note: Implementations are encouraged to provide well-defined behavior for use_count() and unique() even in the presence of multiple threads.
I don't necessarily insist on the exact wording, just that we capture the intent.
Proposed resolution:
Change 20.3.2.2.6 [util.smartptr.shared.obs] p12:
[Note:
use_count()
is not necessarily efficient.Use only for debugging and testing purposes, not for production code.--end note]
Change 20.3.2.3.6 [util.smartptr.weak.obs] p3:
[Note:
use_count()
is not necessarily efficient.Use only for debugging and testing purposes, not for production code.--end note]
Section: 29.6.4 [class.slice] Status: CD1 Submitter: Howard Hinnant Opened: 2005-11-03 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
If one explicitly constructs a slice or glice with the default constructor, does the standard require this slice to have any usable state? It says "creates a slice which specifies no elements", which could be interpreted two ways:
Here is a bit of code to illustrate:
#include <iostream> #include <valarray> int main() { std::valarray<int> v(10); std::valarray<int> v2 = v[std::slice()]; std::cout << "v[slice()].size() = " << v2.size() << '\n'; }
Is the behavior undefined? Or should the output be:
v[slice()].size() = 0
There is a similar question and wording for gslice at 26.3.6.1p1.
Proposed resolution:
[Martin suggests removing the second sentence in 29.6.4.2 [cons.slice] as well.]
Change 29.6.4.2 [cons.slice]:
1 -
The default constructor forThe default constructor is equivalent toslice
creates aslice
which specifies no elements.slice(0, 0, 0)
. A default constructor is provided only to permit the declaration of arrays of slices. The constructor with arguments for a slice takes a start, length, and stride parameter.
Change 29.6.6.2 [gslice.cons]:
1 -
The default constructor creates aThe default constructor is equivalent togslice
which specifies no elements.gslice(0, valarray<size_t>(), valarray<size_t>())
. The constructor with arguments builds agslice
based on a specification of start, lengths, and strides, as explained in the previous section.
Section: 20.3.2.2.11 [util.smartptr.getdeleter], 99 [tr.util.smartptr.shared.dest] Status: CD1 Submitter: Matt Austern Opened: 2006-01-10 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
The description of ~shared_ptr doesn't say when the shared_ptr's deleter, if any, is destroyed. In principle there are two possibilities: it is destroyed unconditionally whenever ~shared_ptr is executed (which, from an implementation standpoint, means that the deleter is copied whenever the shared_ptr is copied), or it is destroyed immediately after the owned pointer is destroyed (which, from an implementation standpoint, means that the deleter object is shared between instances). We should say which it is.
Proposed resolution:
Add after the first sentence of 20.3.2.2.11 [util.smartptr.getdeleter]/1:
The returned pointer remains valid as long as there exists a
shared_ptr
instance that ownsd
.[Note: it is unspecified whether the pointer remains valid longer than that. This can happen if the implementation doesn't destroy the deleter until all
weak_ptr
instances in the ownership group are destroyed. -- end note]
pow(float,int)
be?Section: 29.7 [c.math] Status: CD1 Submitter: Howard Hinnant Opened: 2006-01-12 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
Assuming we adopt the C compatibility package from C99 what should be the return type of the following signature be:
? pow(float, int);
C++03 says that the return type should be float
.
TR1 and C90/99 say the return type should be double
. This can put
clients into a situation where C++03 provides answers that are not as high
quality as C90/C99/TR1. For example:
#include <math.h> int main() { float x = 2080703.375F; double y = pow(x, 2); }
Assuming an IEEE 32 bit float and IEEE 64 bit double, C90/C99/TR1 all suggest:
y = 4329326534736.390625
which is exactly right. While C++98/C++03 demands:
y = 4329326510080.
which is only approximately right.
I recommend that C++0X adopt the mixed mode arithmetic already adopted by
Fortran, C and TR1 and make the return type of pow(float,int)
be
double
.
[
Kona (2007): Other functions that are affected by this issue include
ldexp
, scalbln
, and scalbn
. We also believe that there is a typo in
26.7/10: float nexttoward(float, long double);
[sic] should be float
nexttoward(float, float);
Proposed Disposition: Review (the proposed
resolution appears above, rather than below, the heading "Proposed
resolution")
]
[Howard, post Kona:]
Unfortunately I strongly disagree with a part of the resolution from Kona. I am moving from New to Open instead of to Review because I do not believe we have consensus on the intent of the resolution.
This issue does not include
ldexp
,scalbln
, andscalbn
because the second integral parameter in each of these signatures (from C99) is not a generic parameter according to C99 7.22p2. The corresponding C++ overloads are intended (as far as I know) to correspond directly to C99's definition of generic parameter.For similar reasons, I do not believe that the second
long double
parameter ofnexttoward
, nor the return type of this function, is in error. I believe the correct signature is:float nexttoward(float, long double);which is what both the C++0X working paper and C99 state (as far as I currently understand).
This is really only about
pow(float, int)
. And this is because C++98 took one route (withpow
only) and C99 took another (with many math functions in<tgmath.h>
. The proposed resolution basically says: C++98 got it wrong and C99 got it right; let's go with C99.
[ Bellevue: ]
This signature was not picked up from C99. Instead, if one types
pow(2.0f,2)
, the promotion rules will invoke "double pow(double, double)", which generally gives special treatment for integral exponents, preserving full accuracy of the result. New proposed wording provided.
Proposed resolution:
Change 29.7 [c.math] p10:
The added signatures are:
...float pow(float, int);...double pow(double, int);...long double pow(long double, int);
Section: 17.15 [support.c.headers] Status: CD1 Submitter: Howard Hinnant Opened: 2006-01-23 Last modified: 2023-02-07
Priority: Not Prioritized
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Discussion:
Previously xxx.h was parsable by C++. But in the case of C99's <complex.h> it isn't. Otherwise we could model it just like <string.h>, <cstring>, <string>:
In the case of C's complex, the C API won't compile in C++. So we have:
The ? can't refer to the C API. TR1 currently says:
Proposed resolution:
Change 26.3.11 [cmplxh]:
The header behaves as if it includes the header
<ccomplex>
., and provides sufficient using declarations to declare in the global namespace all function and type names declared or defined in the neader[Note:<complex>
.<complex.h>
does not promote any interface into the global namespace as there is no C interface to promote. --end note]
Section: 26.7.13 [alg.random.shuffle] Status: CD1 Submitter: Martin Sebor Opened: 2006-01-25 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
...is specified to shuffle its range by calling swap but not how (or even that) it's supposed to use the RandomNumberGenerator argument passed to it.
Shouldn't we require that the generator object actually be used by the algorithm to obtain a series of random numbers and specify how many times its operator() should be invoked by the algorithm?
See N2391 and N2423 for some further discussion.
Proposed resolution:
Adopt the proposed resolution in N2423.
[ Kona (2007): The LWG adopted the proposed resolution of N2423 for this issue. The LWG voted to accelerate this issue to Ready status to be voted into the WP at Kona. ]
Compare
a BinaryPredicate
?Section: 26.8 [alg.sorting] Status: C++11 Submitter: Martin Sebor Opened: 2006-02-05 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
In 25, p8 we allow BinaryPredicates to return a type that's convertible to bool but need not actually be bool. That allows predicates to return things like proxies and requires that implementations be careful about what kinds of expressions they use the result of the predicate in (e.g., the expression in if (!pred(a, b)) need not be well-formed since the negation operator may be inaccessible or return a type that's not convertible to bool).
Here's the text for reference:
...if an algorithm takes BinaryPredicate binary_pred as its argument and first1 and first2 as its iterator arguments, it should work correctly in the construct if (binary_pred(*first1, first2)){...}.
In 25.3, p2 we require that the Compare function object return true of false, which would seem to preclude such proxies. The relevant text is here:
Compare is used as a function object which returns true if the first argument is less than the second, and false otherwise...
[ Portland: Jack to define "convertible to bool" such that short circuiting isn't destroyed. ]
[ 2009-07-28 Reopened by Alisdair. No longer solved by concepts. ]
[ 2009-10 Santa Cruz: ]
Move to Review once wording received. Stefanus to send proposed wording.
[ 2009-10 Santa Cruz: ]
Move to Review once wording received. Stefanus to send proposed wording.
[ 2009-10-24 Stefanus supplied wording. ]
Move to Review once wording received. Stefanus to send proposed wording. Old proposed wording here:
I think we could fix this by rewording 25.3, p2 to read somthing like:
-2-
Compare
isused as a function object which returnsatrue
if the first argumentBinaryPredicate
. The return value of the function call operator applied to an object of typeCompare
, when converted to typebool
, yieldstrue
if the first argument of the call is less than the second, andfalse
otherwise.Compare comp
is used throughout for algorithms assuming an ordering relation. It is assumed thatcomp
will not apply any non-constant function through the dereferenced iterator.
[ 2010-01-17: ]
Howard expresses concern that the current direction of the proposed wording outlaws expressions such as:
if (!comp(x, y))Daniel provides wording which addresses that concern.
The previous wording is saved here:
Change 26.8 [alg.sorting] p2:
Compare
is used as a function object. The return value of the function call operator applied to an object of type Compare, when converted to type bool, yields true if the first argument of the callwhich returnsis less than the second, andtrue
if the first argumentfalse
otherwise.Compare comp
is used throughout for algorithms assuming an ordering relation. It is assumed thatcomp
will not apply any non-constant function through the dereferenced iterator.
[ 2010-01-22 Moved to Tentatively Ready after 5 positive votes on c++std-lib. ]
Proposed resolution:
Change 26.1 [algorithms.general]/7+8 as indicated. [This change is
recommended to bring the return value requirements of BinaryPredicate
and Compare
in sync.]
7 The
Predicate
parameter is used whenever an algorithm expects a function object that when applied to the result of dereferencing the corresponding iterator returns a value testable astrue
. In other words, if an algorithm takesPredicate pred
as its argument andfirst
as its iterator argument, it should work correctly in the constructif(pred(*first)){...}
pred(*first)
contextually converted tobool
(7.3 [conv]). The function objectpred
shall not apply any nonconstant function through the dereferenced iterator. This function object may be a pointer to function, or an object of a type with an appropriate function call operator.8 The
BinaryPredicate
parameter is used whenever an algorithm expects a function object that when applied to the result of dereferencing two corresponding iterators or to dereferencing an iterator and typeT
whenT
is part of the signature returns a value testable astrue
. In other words, if an algorithm takesBinaryPredicate
binary_pred
as its argument andfirst1
andfirst2
as its iterator arguments, it should work correctly in the constructif (binary_pred(*first1, *first2)){...}
binary_pred(*first1, *first2)
contextually converted tobool
(7.3 [conv]).BinaryPredicate
always takes the first iterator type as its first argument, that is, in those cases whenT value
is part of the signature, it should work correctly in thecontext ofconstructif (binary_pred(*first1, value)){...}
binary_pred(*first1, value)
contextually converted tobool
(7.3 [conv]).binary_pred
shall not apply any non-constant function through the dereferenced iterators.
Change 26.8 [alg.sorting]/2 as indicated:
2
Compare
isused asa function object type (22.10 [function.objects]). The return value of the function call operation applied to an object of typeCompare
, when contextually converted to typebool
(7.3 [conv]), yieldstrue
if the first argument of the callwhich returnsis less than the second, andtrue
if the first argumentfalse
otherwise.Compare comp
is used throughout for algorithms assuming an ordering relation. It is assumed thatcomp
will not apply any non-constant function through the dereferenced iterator.
Section: 17.3.5 [numeric.limits] Status: CD1 Submitter: Martin Sebor Opened: 2006-02-19 Last modified: 2017-06-15
Priority: Not Prioritized
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Discussion:
[limits], p2 requires implementations to provide specializations of the
numeric_limits
template for each scalar type. While this
could be interepreted to include cv-qualified forms of such types such
an interepretation is not reflected in the synopsis of the
<limits>
header.
The absence of specializations of the template on cv-qualified forms
of fundamental types makes numeric_limits
difficult to
use in generic code where the constness (or volatility) of a type is
not always immediately apparent. In such contexts, the primary
template ends up being instantiated instead of the provided
specialization, typically yielding unexpected behavior.
Require that specializations of numeric_limits
on
cv-qualified fundamental types have the same semantics as those on the
unqualifed forms of the same types.
Proposed resolution:
Add to the synopsis of the <limits>
header,
immediately below the declaration of the primary template, the
following:
template <class T> class numeric_limits<const T>; template <class T> class numeric_limits<volatile T>; template <class T> class numeric_limits<const volatile T>;
Add a new paragraph to the end of 17.3.5 [numeric.limits], with the following text:
-new-para- The value of each member of a numeric_limits
specialization on a cv-qualified T is equal to the value of the same
member of numeric_limits<T>
.
[ Portland: Martin will clarify that user-defined types get cv-specializations automatically. ]
Section: 24.5.2.4.3 [inserter] Status: CD1 Submitter: Howard Hinnant Opened: 2006-02-21 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
The declaration of std::inserter
is:
template <class Container, class Iterator> insert_iterator<Container> inserter(Container& x, Iterator i);
The template parameter Iterator
in this function is completely unrelated
to the template parameter Container
when it doesn't need to be. This
causes the code to be overly generic. That is, any type at all can be deduced
as Iterator
, whether or not it makes sense. Now the same is true of
Container
. However, for every free (unconstrained) template parameter
one has in a signature, the opportunity for a mistaken binding grows geometrically.
It would be much better if inserter
had the following signature instead:
template <class Container> insert_iterator<Container> inserter(Container& x, typename Container::iterator i);
Now there is only one free template parameter. And the second argument to
inserter
must be implicitly convertible to the container's iterator,
else the call will not be a viable overload (allowing other functions in the
overload set to take precedence). Furthermore, the first parameter must have a
nested type named iterator
, or again the binding to std::inserter
is not viable. Contrast this with the current situation
where any type can bind to Container
or Iterator
and those
types need not be anything closely related to containers or iterators.
This can adversely impact well written code. Consider:
#include <iterator> #include <string> namespace my { template <class String> struct my_type {}; struct my_container { template <class String> void push_back(const my_type<String>&); }; template <class String> void inserter(const my_type<String>& m, my_container& c) {c.push_back(m);} } // my int main() { my::my_container c; my::my_type<std::string> m; inserter(m, c); }
Today this code fails because the call to inserter
binds to
std::inserter
instead of to my::inserter
. However with the
proposed change std::inserter
will no longer be a viable function which
leaves only my::inserter
in the overload resolution set. Everything
works as the client intends.
To make matters a little more insidious, the above example works today if you simply change the first argument to an rvalue:
inserter(my::my_type(), c);
It will also work if instantiated with some string type other than
std::string
(or any other std
type). It will also work if
<iterator>
happens to not get included.
And it will fail again for such inocuous reaons as my_type
or
my_container
privately deriving from any std
type.
It seems unfortunate that such simple changes in the client's code can result in such radically differing behavior.
Proposed resolution:
Change 24.2:
24.2 Header
<iterator>
synopsis... template <class Container, class Iterator> insert_iterator<Container> inserter(Container& x,Iteratortypename Container::iterator i); ...
Change 24.4.2.5:
24.4.2.5 Class template
insert_iterator
... template <class Container, class Iterator> insert_iterator<Container> inserter(Container& x,Iteratortypename Container::iterator i); ...
Change 24.4.2.6.5:
24.4.2.6.5
inserter
template <class Container, class Inserter> insert_iterator<Container> inserter(Container& x,Insertertypename Container::iterator i);-1- Returns:
insert_iterator<Container>(x,
.typename Container::iterator(i))
[ Kona (2007): This issue will probably be addressed as a part of the concepts overhaul of the library anyway, but the proposed resolution is correct in the absence of concepts. Proposed Disposition: Ready ]
Section: 31.8 [string.streams] Status: CD1 Submitter: Martin Sebor Opened: 2006-02-23 Last modified: 2016-01-28
Priority: Not Prioritized
View all other issues in [string.streams].
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Discussion:
For better efficiency, the requirement on the stringbuf ctor that
takes a string argument should be loosened up to let it set
epptr()
beyond just one past the last initialized
character just like overflow()
has been changed to be
allowed to do (see issue 432). That way the first call to
sputc()
on an object won't necessarily cause a call to
overflow
. The corresponding change should be made to the
string overload of the str()
member function.
Proposed resolution:
Change 27.7.1.1, p3 of the Working Draft, N1804, as follows:
explicit basic_stringbuf(const basic_string<charT,traits,Allocator>& str, ios_base::openmode which = ios_base::in | ios_base::out);-3- Effects: Constructs an object of class
basic_stringbuf
, initializing the base class withbasic_streambuf()
(27.5.2.1), and initializingmode
withwhich
. Then callsstr(s)
.copies the content of str into thebasic_stringbuf
underlying character sequence. Ifwhich & ios_base::out
is true, initializes the output sequence such thatpbase()
points to the first underlying character,epptr()
points one past the last underlying character, andpptr()
is equal toepptr()
ifwhich & ios_base::ate
is true, otherwisepptr()
is equal topbase()
. Ifwhich & ios_base::in
is true, initializes the input sequence such thateback()
andgptr()
point to the first underlying character andegptr()
points one past the last underlying character.
Change the Effects clause of the str()
in 27.7.1.2, p2 to
read:
-2- Effects: Copies the contents of
s
into thebasic_stringbuf
underlying character sequence and initializes the input and output sequences according tomode
.Ifmode & ios_base::out
is true, initializes the output sequence such thatpbase()
points to the first underlying character,epptr()
points one past the last underlying character, andpptr()
is equal toepptr()
ifmode & ios_base::in
is true, otherwisepptr()
is equal topbase()
. Ifmode & ios_base::in
is true, initializes the input sequence such thateback()
andgptr()
point to the first underlying character andegptr()
points one past the last underlying character.-3- Postconditions: If
mode & ios_base::out
is true,pbase()
points to the first underlying character and(epptr() >= pbase() + s.size())
holds; in addition, ifmode & ios_base::in
is true,(pptr() == pbase() + s.data())
holds, otherwise(pptr() == pbase())
is true. Ifmode & ios_base::in
is true,eback()
points to the first underlying character, and(gptr() == eback())
and(egptr() == eback() + s.size())
hold.
[ Kona (2007) Moved to Ready. ]
Section: 31.8.2.5 [stringbuf.virtuals] Status: CD1 Submitter: Martin Sebor Opened: 2006-02-23 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
According to Table 92 (unchanged by issue 432), when (way ==
end)
the newoff
value in out mode is computed as
the difference between epptr()
and pbase()
.
This value isn't meaningful unless the value of epptr()
can be precisely controlled by a program. That used to be possible
until we accepted the resolution of issue 432, but since then the
requirements on overflow()
have been relaxed to allow it
to make more than 1 write position available (i.e., by setting
epptr()
to some unspecified value past
pptr()
). So after the first call to
overflow()
positioning the output sequence relative to
end will have unspecified results.
In addition, in in|out
mode, since (egptr() ==
epptr())
need not hold, there are two different possible values
for newoff
: epptr() - pbase()
and
egptr() - eback()
.
Proposed resolution:
Change the newoff
column in the last row of Table 94 to
read:
the
endhigh mark pointer minus the beginning pointer ().
xendhigh_mark - xbeg
[ Kona (2007) Moved to Ready. ]
stringbuf
seekpos underspecifiedSection: 31.8.2.5 [stringbuf.virtuals] Status: C++11 Submitter: Martin Sebor Opened: 2006-02-23 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
The effects of the seekpos()
member function of
basic_stringbuf
simply say that the function positions
the input and/or output sequences but fail to spell out exactly
how. This is in contrast to the detail in which seekoff()
is described.
[ 2009-07 Frankfurt ]
Move to Ready.
Proposed resolution:
Change 27.7.1.3, p13 to read:
-13- Effects: Equivalent to
seekoff(off_type(sp), ios_base::beg, which)
.Alters the stream position within the controlled sequences, if possible, to correspond to the stream position stored insp
(as described below).
If(which & ios_base::in) != 0
, positions the input sequence.If(which & ios_base::out) != 0
, positions the output sequence.Ifsp
is an invalid stream position, or if the function positions neither sequence, the positioning operation fails. Ifsp
has not been obtained by a previous successful call to one of the positioning functions (seekoff
,seekpos
,tellg
,tellp
) the effect is undefined.
[
Kona (2007): A pos_type
is a position in a stream by
definition, so there is no ambiguity as to what it means. Proposed
Disposition: NAD
]
[
Post-Kona Martin adds:
I'm afraid I disagree
with the Kona '07 rationale for marking it NAD. The only text
that describes precisely what it means to position the input
or output sequence is in seekoff()
. The seekpos()
Effects
clause is inadequate in comparison and the proposed resolution
plugs the hole by specifying seekpos()
in terms of seekoff()
.
]
xsputn
inefficientSection: 31.6.3.5.5 [streambuf.virt.put] Status: C++11 Submitter: Martin Sebor Opened: 2006-02-23 Last modified: 2016-01-28
Priority: Not Prioritized
View all issues with C++11 status.
Discussion:
streambuf::xsputn()
is specified to have the effect of
"writing up to n
characters to the output sequence as if by
repeated calls to sputc(c)
."
Since sputc()
is required to call overflow()
when
(pptr() == epptr())
is true, strictly speaking
xsputn()
should do the same. However, doing so would be
suboptimal in some interesting cases, such as in unbuffered mode or
when the buffer is basic_stringbuf
.
Assuming calling overflow()
is not really intended to be
required and the wording is simply meant to describe the general
effect of appending to the end of the sequence it would be worthwhile
to mention in xsputn()
that the function is not actually
required to cause a call to overflow()
.
[ 2009-07 Frankfurt ]
Move to Ready.
Proposed resolution:
Add the following sentence to the xsputn()
Effects clause in
27.5.2.4.5, p1 (N1804):
-1- Effects: Writes up to
n
characters to the output sequence as if by repeated calls tosputc(c)
. The characters written are obtained from successive elements of the array whose first element is designated bys
. Writing stops when eithern
characters have been written or a call tosputc(c)
would returntraits::eof()
. It is uspecified whether the function callsoverflow()
when(pptr() == epptr())
becomes true or whether it achieves the same effects by other means.
In addition, I suggest to add a footnote to this function with the
same text as Footnote 292 to make it extra clear that derived classes
are permitted to override xsputn()
for efficiency.
[
Kona (2007): We want to permit a streambuf
that streams output directly
to a device without making calls to sputc
or overflow
. We believe that
has always been the intention of the committee. We believe that the
proposed wording doesn't accomplish that. Proposed Disposition: Open
]
Section: 31.7.5.4 [istream.unformatted] Status: CD1 Submitter: Martin Sebor Opened: 2006-02-23 Last modified: 2016-01-28
Priority: Not Prioritized
View all other issues in [istream.unformatted].
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Discussion:
The array forms of unformatted input functions don't have well-defined
semantics for zero-element arrays in a couple of cases. The affected
ones (istream::get()
and getline()
) are supposed to
terminate when (n - 1)
characters are stored, which obviously
can never be true when (n == 0)
to start with.
Proposed resolution:
I propose the following changes (references are relative to the Working Draft (document N1804).
Change 27.6.1.3, p8 (istream::get()
), bullet 1 as follows:
if
(n < 1)
is true or(n - 1)
characters are stored;
Similarly, change 27.6.1.3, p18 (istream::getline()
), bullet
3 as follows:
(n < 1)
is true or(n - 1)
characters are stored (in which case the function callssetstate(failbit)
).
Finally, change p21 as follows:
In any case, provided
(n > 0)
is true, it then stores a null character (using charT()) into the next successive location of the array.
Section: 31.7 [iostream.format] Status: CD1 Submitter: Martin Sebor Opened: 2006-02-25 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
Issue 60 explicitly made the extractor and inserter operators that
take a basic_streambuf*
argument formatted input and output
functions, respectively. I believe that's wrong, certainly in the
case of the extractor, since formatted functions begin by extracting
and discarding whitespace. The extractor should not discard any
characters.
Proposed resolution:
I propose to change each operator to behave as unformatted input and output function, respectively. The changes below are relative to the working draft document number N1804.
Specifically, change 27.6.1.2.3, p14 as follows:
Effects: Behaves as an unformatted input function (as described in
27.6.1.2.127.6.1.3, paragraph 1).
And change 27.6.2.5.3, p7 as follows:
Effects: Behaves as an unformatted output function (as described in
27.6.2.5.127.6.2.6, paragraph 1).
[ Kona (2007): Proposed Disposition: Ready ]
Section: 31.4 [iostream.objects] Status: CD1 Submitter: Pete Becker Opened: 2006-04-18 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
lib.iostream.objects requires that the standard stream objects are never destroyed, and it requires that they be destroyed.
DR 369 adds words to say that we really mean for ios_base::Init
objects to force
construction of standard stream objects. It ends, though, with the phrase "these
stream objects shall be destroyed after the destruction of dynamically ...".
However, the rule for destruction is stated in the standard: "The objects are
not destroyed during program execution."
Proposed resolution:
Change 31.4 [iostream.objects]/1:
-2- The objects are constructed and the associations are established at some time prior to or during the first time an object of class
ios_base::Init
is constructed, and in any case before the body of main begins execution.290) The objects are not destroyed during program execution.291) If a translation unit includes<iostream>
or explicitly constructs anios_base::Init
object, these stream objects shall be constructed before dynamic initialization of non-local objects defined later in that translation unit, and these stream objects shall be destroyed after the destruction of dynamically initialized non-local objects defined later in that translation unit.
[ Kona (2007): From 31.4 [iostream.objects]/2, strike the words "...and these stream objects shall be destroyed after the destruction of dynamically initialized non-local objects defined later in that translation unit." Proposed Disposition: Review ]
Section: 20.3.2.2.3 [util.smartptr.shared.dest], 99 [tr.util.smartptr.shared.dest] Status: CD1 Submitter: Peter Dimov Opened: 2006-04-23 Last modified: 2016-01-28
Priority: Not Prioritized
View all other issues in [util.smartptr.shared.dest].
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Discussion:
[tr.util.smartptr.shared.dest] says in its second bullet:
"If *this shares ownership with another shared_ptr instance (use_count() > 1), decrements that instance's use count."
The problem with this formulation is that it presupposes the existence of an "use count" variable that can be decremented and that is part of the state of a shared_ptr instance (because of the "that instance's use count".)
This is contrary to the spirit of the rest of the specification that carefully avoids to require an use count variable. Instead, use_count() is specified to return a value, a number of instances.
In multithreaded code, the usual implicit assumption is that a shared variable should not be accessed by more than one thread without explicit synchronization, and by introducing the concept of an "use count" variable, the current wording implies that two shared_ptr instances that share ownership cannot be destroyed simultaneously.
In addition, if we allow the interpretation that an use count variable is part of shared_ptr's state, this would lead to other undesirable consequences WRT multiple threads. For example,
p1 = p2;
would now visibly modify the state of p2, a "write" operation, requiring a lock.
Proposed resolution:
Change the first two bullets of [lib.util.smartptr.shared.dest]/1 to:
- If
*this
is empty or shares ownership with anothershared_ptr
instance (use_count() > 1
), there are no side effects.If*this
shares ownership with anothershared_ptr
instance (use_count() > 1
), decrements that instance's use count.
Add the following paragraph after [lib.util.smartptr.shared.dest]/1:
[Note: since the destruction of
*this
decreases the number of instances in*this
's ownership group by one, allshared_ptr
instances that share ownership with*this
will report anuse_count()
that is one lower than its previous value after*this
is destroyed. --end note]
Section: 26.6.9 [alg.find.first.of] Status: CD1 Submitter: Doug Gregor Opened: 2006-04-25 Last modified: 2016-01-28
Priority: Not Prioritized
View all other issues in [alg.find.first.of].
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Discussion:
In 25.1.4 Find First [lib.alg.find.first], the two iterator type parameters to find_first_of are specified to require Forward Iterators, as follows:
template<class ForwardIterator1, class ForwardIterator2> ForwardIterator1 find_first_of(ForwardIterator1 first1, ForwardIterator1 last1, ForwardIterator2 first2, ForwardIterator2 last2); template<class ForwardIterator1, class ForwardIterator2, class BinaryPredicate> ForwardIterator1 find_first_of(ForwardIterator1 first1, ForwardIterator1 last1, ForwardIterator2 first2, ForwardIterator2 last2, BinaryPredicate pred);
However, ForwardIterator1 need not actually be a Forward Iterator; an Input Iterator suffices, because we do not need the multi-pass property of the Forward Iterator or a true reference.
Proposed resolution:
Change the declarations of find_first_of
to:
template<classForwardIterator1InputIterator1, class ForwardIterator2>ForwardIterator1InputIterator1 find_first_of(ForwardIterator1InputIterator1 first1,ForwardIterator1InputIterator1 last1, ForwardIterator2 first2, ForwardIterator2 last2); template<classForwardIterator1InputIterator1, class ForwardIterator2, class BinaryPredicate>ForwardIterator1InputIterator1 find_first_of(ForwardIterator1InputIterator1 first1,ForwardIterator1InputIterator1 last1, ForwardIterator2 first2, ForwardIterator2 last2, BinaryPredicate pred);
Section: 26.8.4.3 [upper.bound] Status: CD1 Submitter: Seungbeom Kim Opened: 2006-05-03 Last modified: 2016-01-28
Priority: Not Prioritized
View all issues with CD1 status.
Discussion:
ISO/IEC 14882:2003 says:
25.3.3.2 upper_bound
Returns: The furthermost iterator
i
in the range[first, last)
such that for any iteratorj
in the range[first, i)
the following corresponding conditions hold:!(value < *j)
orcomp(value, *j) == false
.
From the description above, upper_bound cannot return last, since it's not in the interval [first, last). This seems to be a typo, because if value is greater than or equal to any other values in the range, or if the range is empty, returning last seems to be the intended behaviour. The corresponding interval for lower_bound is also [first, last].
Proposed resolution:
Change [lib.upper.bound]:
Returns: The furthermost iterator
i
in the range[first, last
such that for any iterator)]j
in the range[first, i)
the following corresponding conditions hold:!(value < *j)
orcomp(value, *j) == false
.
Section: 20.2.10.2 [allocator.members] Status: CD1 Submitter: Martin Sebor Opened: 2006-05-17 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
The description of the allocator member function
allocate()
requires that the hint argument be
either 0 or a value previously returned from allocate()
.
Footnote 227 further suggests that containers may pass the address of
an adjacent element as this argument.
I believe that either the footnote is wrong or the normative
requirement that the argument be a value previously returned from a
call to allocate()
is wrong. The latter is supported by
the resolution to issue 20-004 proposed in c++std-lib-3736 by Nathan
Myers. In addition, the hint is an ordinary void* and not the
pointer
type returned by allocate()
, with
the two types potentially being incompatible and the requirement
impossible to satisfy.
See also c++std-lib-14323 for some more context on where this came up (again).
Proposed resolution:
Remove the requirement in 20.6.1.1, p4 that the hint be a value
previously returned from allocate()
. Specifically, change
the paragraph as follows:
Requires: hint either 0 or previously obtained from member
[Note: The value hint may be used by an
implementation to help improve performance. -- end note]
allocate
and not yet passed to member deallocate
.
The value hint may be used by an implementation to help improve performance
223).
[Footnote: 223)In a container member function, the address of an adjacent element is often a good choice to pass for this argument.
flush()
not unformatted functionSection: 31.7.6.4 [ostream.unformatted] Status: CD1 Submitter: Martin Sebor Opened: 2006-06-14 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
The resolution of issue 60 changed basic_ostream::flush()
so as not to require it to behave as an unformatted output function.
That has at least two in my opinion problematic consequences:
First, flush()
now calls rdbuf()->pubsync()
unconditionally, without regard to the state of the stream. I can't
think of any reason why flush()
should behave differently
from the vast majority of stream functions in this respect.
Second, flush()
is not required to catch exceptions from
pubsync()
or set badbit
in response to such
events. That doesn't seem right either, as most other stream functions
do so.
Proposed resolution:
I propose to revert the resolution of issue 60 with respect to
flush()
. Specifically, I propose to change 27.6.2.6, p7
as follows:
Effects: Behaves as an unformatted output function (as described
in 27.6.2.6, paragraph 1). If rdbuf()
is not a null
pointer, constructs a sentry object. If this object returns
true
when converted to a value of type bool the function
calls rdbuf()->pubsync()
. If that function returns
-1 calls setstate(badbit)
(which may throw
ios_base::failure
(27.4.4.3)). Otherwise, if the
sentry object returns false
, does nothing.Does
not behave as an unformatted output function (as described in
27.6.2.6, paragraph 1).
[ Kona (2007): Proposed Disposition: Ready ]
Section: 27.4.4.4 [string.io] Status: CD1 Submitter: Martin Sebor Opened: 2006-06-22 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
Section and paragraph numbers in this paper are relative to the working draft document number N2009 from 4/21/2006.
The basic_string
extractor in 21.3.7.9, p1 is clearly
required to behave as a formatted input function, as is the
std::getline()
overload for string described in p7.
However, the basic_string
inserter described in p5 of the
same section has no such requirement. This has implications on how the
operator responds to exceptions thrown from xsputn()
(formatted output functions are required to set badbit
and swallow the exception unless badbit
is also set in
exceptions()
; the string inserter doesn't have any such
requirement).
I don't see anything in the spec for the string inserter that would justify requiring it to treat exceptions differently from all other similar operators. (If it did, I think it should be made this explicit by saying that the operator "does not behave as a formatted output function" as has been made customary by the adoption of the resolution of issue 60).
Proposed resolution:
I propose to change the Effects clause in 21.3.7.9, p5, as follows:
Effects:
Begins by constructing a sentry object k as if k were constructed by typenameBehaves as a formatted output function (27.6.2.5.1). After constructing abasic_ostream<charT, traits>::sentry k (os)
. Ifbool(k)
istrue
,sentry
object, if this object returnstrue
when converted to a value of typebool
, determines padding as described in 22.2.2.2.2, then inserts the resulting sequence of charactersseq
as if by callingos.rdbuf()->sputn(seq , n)
, wheren
is the larger ofos.width()
andstr.size()
; then callsos.width(0)
.If the call to sputn fails, callsos.setstate(ios_base::failbit)
.
This proposed resilution assumes the resolution of issue 394 (i.e.,
that all formatted output functions are required to set
ios_base::badbit
in response to any kind of streambuf
failure), and implicitly assumes that a return value of
sputn(seq, n)
other than n
indicates a failure.
Section: 23.2 [container.requirements] Status: CD1 Submitter: Peter Dimov Opened: 2006-08-02 Last modified: 2016-01-28
Priority: Not Prioritized
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Duplicate of: 536
Discussion:
There appears to be no requirements on the InputIterators used in sequences in 23.1.1 in terms of their value_type, and the requirements in 23.1.2 appear to be overly strict (requires InputIterator::value_type be the same type as the container's value_type).
Proposed resolution:
Change 23.1.1 p3:
In Tables 82 and 83,
X
denotes a sequence class,a
denotes a value ofX
,i
andj
denote iterators satisfying input iterator requirements and refer to elements implicitly convertible tovalue_type
,[i, j)
denotes a valid range,n
denotes a value ofX::size_type
,p
denotes a valid iterator toa
,q
denotes a valid dereferenceable iterator toa
,[q1, q2)
denotes a valid range ina
, andt
denotes a value ofX::value_type
.
Change 23.1.2 p7:
In Table 84,
X
is an associative container class,a
is a value ofX
,a_uniq
is a value ofX
whenX
supports unique keys, anda_eq
is a value ofX
whenX
supports multiple keys,i
andj
satisfy input iterator requirements and refer to elementsofimplicitly convertible tovalue_type
,[i, j)
is a valid range,p
is a valid iterator toa
,q
is a valid dereferenceable iterator toa
,[q1, q2)
is a valid range ina
,t
is a value ofX::value_type
,k
is a value ofX::key_type
andc
is a value of typeX::key_compare
.
Rationale:
Concepts will probably come in and rewrite this section anyway. But just in case it is easy to fix this up as a safety net and as a clear statement of intent.
Section: 17.4.1 [cstdint.syn], 99 [tr.c99.cstdint] Status: CD1 Submitter: Walter Brown Opened: 2006-08-28 Last modified: 2023-02-07
Priority: Not Prioritized
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Discussion:
Clause 18.3 of the current Working Paper (N2009) deals with the new C++ headers <cstdint> and <stdint.h>. These are of course based on the C99 header <stdint.h>, and were part of TR1.
Per 18.3.1/1, these headers define a number of macros and function macros. While the WP does not mention __STDC_CONSTANT_MACROS in this context, C99 footnotes do mention __STDC_CONSTANT_MACROS. Further, 18.3.1/2 states that "The header defines all ... macros the same as C99 subclause 7.18."
Therefore, if I wish to have the above-referenced macros and function macros defined, must I #define __STDC_CONSTANT_MACROS before I #include <cstdint>, or does the C++ header define these macros/function macros unconditionally?
Proposed resolution:
To put this issue to rest for C++0X, I propose the following addition to 18.3.1/2 of the Working Paper N2009:
[Note: The macros defined by <cstdint> are provided unconditionally: in particular, the symbols __STDC_LIMIT_MACROS and __STDC_CONSTANT_MACROS (mentioned in C99 footnotes 219, 220, and 222) play no role in C++. --end note]
Swappable
in terms of CopyConstructible
and Assignable
Section: 16.4.4.2 [utility.arg.requirements] Status: Resolved Submitter: Niels Dekker Opened: 2006-11-02 Last modified: 2016-01-28
Priority: Not Prioritized
View all other issues in [utility.arg.requirements].
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Discussion:
It seems undesirable to define the Swappable
requirement in terms of
CopyConstructible
and Assignable
requirements. And likewise, once the
MoveConstructible
and MoveAssignable
requirements (N1860) have made it
into the Working Draft, it seems undesirable to define the Swappable
requirement in terms of those requirements. Instead, it appears
preferable to have the Swappable
requirement defined exclusively in
terms of the existence of an appropriate swap function.
Section 20.1.4 [lib.swappable] of the current Working Draft (N2009) says:
The Swappable requirement is met by satisfying one or more of the following conditions:
- T is Swappable if T satisfies the CopyConstructible requirements (20.1.3) and the Assignable requirements (23.1);
- T is Swappable if a namespace scope function named swap exists in the same namespace as the definition of T, such that the expression swap(t,u) is valid and has the semantics described in Table 33.
I can think of three disadvantages of this definition:
If a client's type T satisfies the first condition (T is both CopyConstructible and Assignable), the client cannot stop T from satisfying the Swappable requirement without stopping T from satisfying the first condition.
A client might want to stop T from satisfying the Swappable requirement, because swapping by means of copy construction and assignment might throw an exception, and she might find a throwing swap unacceptable for her type. On the other hand, she might not feel the need to fully implement her own swap function for this type. In this case she would want to be able to simply prevent algorithms that would swap objects of type T from being used, e.g., by declaring a swap function for T, and leaving this function purposely undefined. This would trigger a link error, if an attempt would be made to use such an algorithm for this type. For most standard library implementations, this practice would indeed have the effect of stopping T from satisfying the Swappable requirement.
A client's type T that does not satisfy the first condition can not be made Swappable by providing a specialization of std::swap for T.
While I'm aware about the fact that people have mixed feelings about providing a specialization of std::swap, it is well-defined to do so. It sounds rather counter-intuitive to say that T is not Swappable, if it has a valid and semantically correct specialization of std::swap. Also in practice, providing such a specialization will have the same effect as satisfying the Swappable requirement.
For a client's type T that satisfies both conditions of the Swappable requirement, it is not specified which of the two conditions prevails. After reading section 20.1.4 [lib.swappable], one might wonder whether objects of T will be swapped by doing copy construction and assignments, or by calling the swap function of T.
I'm aware that the intention of the Draft is to prefer calling the swap function of T over doing copy construction and assignments. Still in my opinion, it would be better to make this clear in the wording of the definition of Swappable.
I would like to have the Swappable requirement defined in such a way that the following code fragment will correctly swap two objects of a type T, if and only if T is Swappable:
using std::swap; swap(t, u); // t and u are of type T.
This is also the way Scott Meyers recommends calling a swap function, in Effective C++, Third Edition, item 25.
Most aspects of this issue have been dealt with in a discussion on comp.std.c++ about the Swappable requirement, from 13 September to 4 October 2006, including valuable input by David Abrahams, Pete Becker, Greg Herlihy, Howard Hinnant and others.
[ San Francisco: ]
Recommend
NADResolved. Solved by N2774.
[ 2009-07 Frankfurt ]
Moved to Open. Waiting for non-concepts draft.
[ 2009-11-08 Howard adds: ]
[ 2010-02-03 Sean Hunt adds: ]
While reading N3000, I independently came across Issue 594. Having seen that it's an issue under discussion, I think the proposed wording needs fixing to something more like "...function call swap(t,u) that includes std::swap in its overload set is valid...", because "...is valid within the namespace std..." does not allow other libraries to simply use the Swappable requirement by referring to the standard's definition, since they cannot actually perform any calls within std.
This wording I suggested would also make overloads visible in the same scope as the
using std::swap
valid for Swappable requirements; a more complex wording limiting the non-ADL overload set to std::swap might be required.
[ 2010 Pittsburgh: ]
Moved to NAD Editorial. Rationale added.
Rationale:
Solved by N3048.
Proposed resolution:
Change section 20.1.4 [lib.swappable] as follows:
The Swappable requirement is met by satisfying
one or more of the following conditions:the following condition:
T is Swappable if T satisfies the CopyConstructible requirements (20.1.3) and the Assignable requirements (23.1);T is Swappable if a namespace scope function named swap exists in the same namespace as the definition of T, such that the expression swap(t,u) is valid and has the semantics described in Table 33.T is Swappable if an unqualified function call swap(t,u) is valid within the namespace std, and has the semantics described in Table 33.
fabs(complex<T>)
redundant / wrongly specifiedSection: 29.4.7 [complex.value.ops] Status: CD1 Submitter: Stefan Große Pawig Opened: 2006-09-24 Last modified: 2016-01-28
Priority: Not Prioritized
View all other issues in [complex.value.ops].
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Discussion:
TR1 introduced, in the C compatibility chapter, the function
fabs(complex<T>)
:
----- SNIP ----- 8.1.1 Synopsis [tr.c99.cmplx.syn] namespace std { namespace tr1 { [...] template<class T> complex<T> fabs(const complex<T>& x); } // namespace tr1 } // namespace std [...] 8.1.8 Function fabs [tr.c99.cmplx.fabs] 1 Effects: Behaves the same as C99 function cabs, defined in subclause 7.3.8.1. ----- SNIP -----
The current C++0X draft document (n2009.pdf) adopted this definition in chapter 26.3.1 (under the comment // 26.3.7 values) and 26.3.7/7.
But in C99 (ISO/IEC 9899:1999 as well as the 9899:TC2 draft document n1124), the referenced subclause reads
----- SNIP ----- 7.3.8.1 The cabs functions Synopsis 1 #include <complex.h> double cabs(double complex z); float cabsf(float complex z); long double cabsl(long double z); Description 2 The cabs functions compute the complex absolute value (also called norm, modulus, or magnitude) of z. Returns 3 The cabs functions return the complex absolute value. ----- SNIP -----
Note that the return type of the cabs*() functions is not a complex type. Thus, they are equivalent to the already well established template<class T> T abs(const complex<T>& x); (26.2.7/2 in ISO/IEC 14882:1998, 26.3.7/2 in the current draft document n2009.pdf).
So either the return value of fabs() is specified wrongly, or fabs() does not behave the same as C99's cabs*().
Possible Resolutions
This depends on the intention behind the introduction of fabs().
If the intention was to provide a /complex/ valued function that calculates the magnitude of its argument, this should be explicitly specified. In TR1, the categorization under "C compatibility" is definitely wrong, since C99 does not provide such a complex valued function.
Also, it remains questionable if such a complex valued function is really needed, since complex<T> supports construction and assignment from real valued arguments. There is no difference in observable behaviour between
complex<double> x, y; y = fabs(x); complex<double> z(fabs(x));
and
complex<double> x, y; y = abs(x); complex<double> z(abs(x));
If on the other hand the intention was to provide the intended functionality of C99, fabs() should be either declared deprecated or (for C++0X) removed from the standard, since the functionality is already provided by the corresponding overloads of abs().
[ Bellevue: ]
Bill believes that
abs()
is a suitable overload. We should removefabs()
.
Proposed resolution:
Change the synopsis in 29.4.2 [complex.syn]:
template<class T> complex<T> fabs(const complex<T>&);
Remove 29.4.7 [complex.value.ops], p7:
template<class T> complex<T> fabs(const complex<T>& x);
-7- Effects: Behaves the same as C99 functioncabs
, defined in subclause 7.3.8.1.
[
Kona (2007): Change the return type of fabs(complex)
to T
.
Proposed Disposition: Ready
]
Section: 31.10.3.4 [filebuf.members] Status: CD1 Submitter: Thomas Plum Opened: 2006-09-26 Last modified: 2016-01-28
Priority: Not Prioritized
View all other issues in [filebuf.members].
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Discussion:
In testing 31.10.3.4 [filebuf.members], Table 112 (in the latest N2009 draft), we invoke
ostr.open("somename", ios_base::out | ios_base::in | ios_base::app)
and we expect the open to fail, because out|in|app is not listed in Table 92, and just before the table we see very specific words:
If mode is not some combination of flags shown in the table then the open fails.
But the corresponding table in the C standard, 7.19.5.3, provides two modes "a+" and "a+b", to which the C++ modes out|in|app and out|in|app|binary would presumably apply.
We would like to argue that the intent of Table 112 was to match the semantics of 7.19.5.3 and that the omission of "a+" and "a+b" was unintentional. (Otherwise there would be valid and useful behaviors available in C file I/O which are unavailable using C++, for no valid functional reason.)
We further request that the missing modes be explicitly restored to the WP, for inclusion in C++0x.
[ Martin adds: ]
...besides "a+" and "a+b" the C++ table is also missing a row for a lone app bit which in at least two current implementation as well as in Classic Iostreams corresponds to the C stdio "a" mode and has been traditionally documented as implying ios::out. Which means the table should also have a row for in|app meaning the same thing as "a+" already proposed in the issue.
Proposed resolution:
Add to the table "File open modes" in 31.10.3.4 [filebuf.members]:
File open modes ios_base
Flag combinationstdio
equivalentbinary
in
out
trunc
app
+
"w"
+
+
"a"
+
"a"
+
+
"w"
+
"r"
+
+
"r+"
+
+
+
"w+"
+
+
+
"a+"
+
+
"a+"
+
+
"wb"
+
+
+
"ab"
+
+
"ab"
+
+
+
"wb"
+
+
"rb"
+
+
+
"r+b"
+
+
+
+
"w+b"
+
+
+
+
"a+b"
+
+
+
"a+b"
[ Kona (2007) Added proposed wording and moved to Review. ]
Section: 3.2 [dec.tr::trdec.types.types] Status: TRDec Submitter: Daniel Krugler Opened: 2006-05-28 Last modified: 2016-01-31
Priority: Not Prioritized
View all other issues in [dec.tr::trdec.types.types].
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Discussion:
In a private email, Daniel writes:
I would like to ask, what where the reason for the decision to define the semantics of the integral conversion of the decimal types, namely
"operator long long() const; Returns: Returns the result of the conversion of *this to the type long long, as if performed by the expression llrounddXX(*this)."where XX stands for either 32, 64, or 128, corresponding to the proper decimal type. The exact meaning of llrounddXX is not given in that paper, so I compared it to the corresponding definition given in C99, 2nd edition (ISO 9899), which says in 7.12.9.7 p. 2:
"The lround and llround functions round their argument to the nearest integer value, rounding halfway cases away from zero, regardless of the current rounding direction. [..]"
Now considering the fact that integral conversion of the usual floating-point types ("4.9 Floating-integral conversions") has truncation semantic I wonder why this conversion behaviour has not been transferred for the decimal types.
Robert comments:
Also, there is a further error in the Returns: clause for converting decimal::decimal128
to long long
. It currently calls llroundd64
, not llroundd128
.
Proposed resolution:
Change the Returns: clause in 3.2.2.4 to:
Returns: Returns the result of the conversion of
*this
to the typelong long
, as if performed by the expressionllroundd32(*this)
while the decimal rounding direction mode [3.5.2]FE_DEC_TOWARD_ZERO
is in effect.
Change the Returns: clause in 3.2.3.4 to:
Returns: Returns the result of the conversion of
*this
to the typelong long
, as if performed by the expressionllroundd64(*this)
while the decimal rounding direction mode [3.5.2]FE_DEC_TOWARD_ZERO
is in effect.
Change the Returns: clause in 3.2.4.4 to:
Returns: Returns the result of the conversion of
*this
to the typelong long
, as if performed by the expressionllroundd64(*this)
llroundd128(*this)
while the decimal rounding direction mode [3.5.2]FE_DEC_TOWARD_ZERO
is in effect.
Section: 3.1 [dec.tr::trdec.types.encodings] Status: TRDec Submitter: Daniel Krugler Opened: 2006-05-28 Last modified: 2016-01-31
Priority: Not Prioritized
View all issues with TRDec status.
Discussion:
Daniel writes in a private email:
- 3.1 'Decimal type encodings' says in its note:
"this implies that sizeof(std::decimal::decimal32) == 4, sizeof(std::decimal::decimal64) == 8, and sizeof(std::decimal::decimal128) == 16."This is a wrong assertion, because the definition of 'byte' in 1.7 'The C+ + memory model' of ISO 14882 (2nd edition) does not specify that a byte must be necessarily 8 bits large, which would be necessary to compare with the specified bit sizes of the types decimal32, decimal64, and decimal128.
Proposed resolution:
Change 3.1 as follows:
The three decimal encoding formats defined in IEEE-754R correspond to the three decimal floating types as follows:
- decimal32 is a decimal32 number, which is encoded in four consecutive
bytesoctets (32 bits)- decimal64 is a decimal64 number, which is encoded in eight consecutive
bytesoctets (64 bits)- decimal128 is a decimal128 number, which is encoded in 16 consecutive
bytesoctets (128 bits)
[Note: this implies thatsizeof(std::decimal::decimal32) == 4
,sizeof(std::decimal::decimal64) == 8
, andsizeof(std::decimal::decimal128) == 16
. --end note]
Section: 3.9 [dec.tr::trdec.types.cwchar] Status: TRDec Submitter: Daniel Krugler Opened: 2006-05-28 Last modified: 2016-01-31
Priority: Not Prioritized
View all issues with TRDec status.
Discussion:
Daniel writes:
- 3.9.1 'Additions to <cwchar>' provides wrong signatures to the wcstod32, wcstod64, and wcstod128 functions ([the parameters have type pointer-to-] char instead of wchar_t).
Proposed resolution:
Change "3.9.1 Additions to <cwchar>
synopsis" to:
namespace std { namespace decimal { // 3.9.2 wcstod functions: decimal32 wcstod32 (constcharwchar_t * nptr,charwchar_t ** endptr); decimal64 wcstod64 (constcharwchar_t * nptr,charwchar_t ** endptr); decimal128 wcstod128 (constcharwchar_t * nptr,charwchar_t ** endptr); } }
Section: 3.3 [dec.tr::trdec.types.limits] Status: TRDec Submitter: Daniel Krugler Opened: 2006-05-28 Last modified: 2016-01-31
Priority: Not Prioritized
View all issues with TRDec status.
Discussion:
Daniel writes in a private email:
- 3.3 'Additions to header <limits>' contains two errors in the specialisation of numeric_limits<decimal::decimal128>:
- The static member max() returns DEC128_MIN, this should be DEC128_MAX.
- The static member digits is assigned to 384, this should be 34 (Probably mixed up with the max. exponent for decimal::decimal64).
Proposed resolution:
In "3.3 Additions to header <limits>
" change numeric_limits<decimal::decimal128> as follows:
template<> class numeric_limits<decimal::decimal128> { public: static const bool is_specialized = true; static decimal::decimal128 min() throw() { return DEC128_MIN; } static decimal::decimal128 max() throw() { returnDEC128_MIN;DEC128_MAX; } static const int digits =38434; /* ... */
Section: 3 [dec.tr::trdec.types] Status: TRDec Submitter: Daniel Krügler Opened: 2006-05-28 Last modified: 2016-01-31
Priority: Not Prioritized
View all other issues in [dec.tr::trdec.types].
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Discussion:
The document uses the term "generic floating types," but defines it nowhere.
Proposed resolution:
Change the first paragraph of "3 Decimal floating-point types" as follows:
This Technical Report introduces three decimal floating-point types, named decimal32, decimal64, and decimal128. The set of values of type decimal32 is a subset of the set of values of type decimal64; the set of values of the type decimal64 is a subset of the set of values of the type decimal128. Support for decimal128 is optional. These types supplement the Standard C++ types
float
,double
, andlong double
, which are collectively described as the basic floating types.
Section: 3 [dec.tr::trdec.types] Status: TRDec Submitter: Martin Sebor Opened: 2006-05-28 Last modified: 2016-01-31
Priority: Not Prioritized
View all other issues in [dec.tr::trdec.types].
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Discussion:
In c++std-lib-17198, Martin writes:
Each of the three classes proposed in the paper (decimal32, decimal64, and decimal128) explicitly declares and specifies the semantics of its copy constructor, copy assignment operator, and destructor. Since the semantics of all three functions are identical to the trivial versions implicitly generated by the compiler in the absence of any declarations it is safe to drop them from the spec. This change would make the proposed classes consistent with other similar classes already in the standard (e.g., std::complex).
Proposed resolution:
Change "3.2.2 Class decimal32
" as follows:
namespace std { namespace decimal { class decimal32 { public: // 3.2.2.1 construct/copy/destroy: decimal32();decimal32(const decimal32 & d32);decimal32 & operator=(const decimal32 & d32);~decimal32();/* ... */
Change "3.2.2.1 construct/copy/destroy" as follows:
decimal32(); Effects: Constructs an object of type decimal32 with the value 0;decimal32(const decimal32 & d32);decimal32 & operator=(const decimal32 & d32);Effects: Copies an object of type decimal32.~decimal32();Effects: Destroys an object of type decimal32.
Change "3.2.3 Class decimal64
" as follows:
namespace std { namespace decimal { class decimal64 { public: // 3.2.3.1 construct/copy/destroy: decimal64();decimal64(const decimal64 & d64);decimal64 & operator=(const decimal64 & d64);~decimal64();/* ... */
Change "3.2.3.1 construct/copy/destroy" as follows:
decimal64(); Effects: Constructs an object of type decimal64 with the value 0;decimal64(const decimal64 & d64);decimal64 & operator=(const decimal64 & d64);Effects: Copies an object of type decimal64.~decimal64();Effects: Destroys an object of type decimal64.
Change "3.2.4 Class decimal128
" as follows:
namespace std { namespace decimal { class decimal128 { public: // 3.2.4.1 construct/copy/destroy: decimal128();decimal128(const decimal128 & d128);decimal128 & operator=(const decimal128 & d128);~decimal128();/* ... */
Change "3.2.4.1 construct/copy/destroy" as follows:
decimal128(); Effects: Constructs an object of type decimal128 with the value 0;decimal128(const decimal128 & d128);decimal128 & operator=(const decimal128 & d128);Effects: Copies an object of type decimal128.~decimal128();Effects: Destroys an object of type decimal128.
Section: 3 [dec.tr::trdec.types] Status: TRDec Submitter: Martin Sebor Opened: 2006-05-28 Last modified: 2016-01-31
Priority: Not Prioritized
View all other issues in [dec.tr::trdec.types].
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Discussion:
In c++std-lib-17197, Martin writes:
The extended_num_get and extended_num_put facets are designed to store a reference to a num_get or num_put facet which the extended facets delegate the parsing and formatting of types other than decimal. One form of the extended facet's ctor (the default ctor and the size_t overload) obtains the reference from the global C++ locale while the other form takes this reference as an argument.
The problem with storing a reference to a facet in another object (as opposed to storing the locale object in which the facet is installed) is that doing so bypasses the reference counting mechanism designed to prevent a facet that is still being referenced (i.e., one that is still installed in some locale) from being destroyed when another locale that contains it is destroyed. Separating a facet reference from the locale it comes from van make it cumbersome (and in some cases might even make it impossible) for programs to prevent invalidating the reference. (The danger of this design is highlighted in the paper.)
This problem could be easily avoided by having the extended facets store a copy of the locale from which they would extract the base facet either at construction time or when needed. To make it possible, the forms of ctors of the extended facets that take a reference to the base facet would need to be changed to take a locale argument instead.
Proposed resolution:
1. Change the extended_num_get
synopsis in 3.10.2 as follows:
extended_num_get(conststd::num_get<charT, InputIterator>std::locale & b, size_t refs = 0); /* ... */// const std::num_get<charT, InputIterator> & base; exposition only// std::locale baseloc; exposition only
2. Change the description of the above constructor in 3.10.2.1:
extended_num_get(conststd::num_get<charT, InputIterator>std::locale & b, size_t refs = 0);
Effects: Constructs an
extended_num_get
facet as if by:extended_num_get(conststd::num_get<charT, InputIterator>std::locale & b, size_t refs = 0) : facet(refs), baseloc(b) { /* ... */ }
Notes: Care must be taken by the implementation to ensure that the lifetime of the facet referenced by base exceeds that of the resultingextended_num_get
facet.
3. Change the Returns: clause for do_get(iter_type, iter_type, ios_base &, ios_base::iostate &, bool &) const
, et al to
Returns:
.
basestd::use_facet<std::num_get<charT, InputIterator> >(baseloc).get(in, end, str, err, val)
4. Change the extended_num_put
synopsis in 3.10.3 as follows:
extended_num_put(conststd::num_put<charT, OutputIterator>std::locale & b, size_t refs = 0); /* ... */// const std::num_put<charT, OutputIterator> & base; exposition only// std::locale baseloc; exposition only
5. Change the description of the above constructor in 3.10.3.1:
extended_num_put(conststd::num_put<charT, OutputIterator>std::locale & b, size_t refs = 0);
Effects: Constructs an
extended_num_put
facet as if by:extended_num_put(conststd::num_put<charT, OutputIterator>std::locale & b, size_t refs = 0) : facet(refs), baseloc(b) { /* ... */ }
Notes: Care must be taken by the implementation to ensure that the lifetime of the facet referenced by base exceeds that of the resultingextended_num_put
facet.
6. Change the Returns: clause for do_put(iter_type, ios_base &, char_type, bool &) const
, et al to
Returns:
.
basestd::use_facet<std::num_put<charT, OutputIterator> >(baseloc).put(s, f, fill, val)
[ Redmond: We would prefer to rename "extended" to "decimal". ]
Section: 3.4 [dec.tr::trdec.types.cdecfloat] Status: TRDec Submitter: Robert Klarer Opened: 2006-10-17 Last modified: 2016-01-31
Priority: Not Prioritized
View all issues with TRDec status.
Discussion:
In Berlin, WG14 decided to drop the <decfloat.h> header. The contents of that header have been moved into <float.h>. For the sake of C compatibility, we should make corresponding changes.
Proposed resolution:
1. Change the heading of subclause 3.4, "Headers <cdecfloat>
and <decfloat.h>
" to "Additions to headers <cfloat>
and <float.h>
."
2. Change the text of subclause 3.4 as follows:
The standard C++ headers<cfloat>
and<float.h>
define characteristics of the floating-point typesfloat
,double
, andlong double
. Their contents remain unchanged by this Technical Report.
Headers<cdecfloat>
and<decfloat.h>
define characteristics of the decimal floating-point typesdecimal32
,decimal64
, anddecimal128
. As well,<decfloat.h>
defines the convenience typedefs_Decimal32
,_Decimal64
, and_Decimal128
, for compatibilty with the C programming language.The header
<cfloat>
is described in [tr.c99.cfloat]. The header<float.h>
is described in [tr.c99.floath]. These headers are extended by this Technical Report to define characteristics of the decimal floating-point typesdecimal32
,decimal64
, anddecimal128
. As well,<float.h>
is extended to define the convenience typedefs_Decimal32
,_Decimal64
, and_Decimal128
for compatibility with the C programming language.
3. Change the heading of subclause 3.4.1, "Header <cdecfloat>
synopsis" to "Additions to header <cfloat>
synopsis."
4. Change the heading of subclause 3.4.2, "Header <decfloat.h>
synopsis" to "Additions to header <float.h>
synopsis."
5. Change the contents of 3.4.2 as follows:
#include <cdecfloat>// C-compatibility convenience typedefs: typedef std::decimal::decimal32 _Decimal32; typedef std::decimal::decimal64 _Decimal64; typedef std::decimal::decimal128 _Decimal128;
Section: 29.5.8.1 [rand.util.seedseq] Status: CD1 Submitter: Charles Karney Opened: 2006-10-26 Last modified: 2016-01-28
Priority: Not Prioritized
View all other issues in [rand.util.seedseq].
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Discussion:
Short seed vectors of 32-bit quantities all result in different states. However this is not true of seed vectors of 16-bit (or smaller) quantities. For example these two seeds
unsigned short seed = {1, 2, 3}; unsigned short seed = {1, 2, 3, 0};
both pack to
unsigned seed = {0x20001, 0x3};
yielding the same state.
See N2391 and N2423 for some further discussion.
Proposed resolution:
Adopt the proposed resolution in N2423.
[ Kona (2007): The LWG adopted the proposed resolution of N2423 for this issue. The LWG voted to accelerate this issue to Ready status to be voted into the WP at Kona. ]
Section: 29.5.8.1 [rand.util.seedseq] Status: CD1 Submitter: Charles Karney Opened: 2006-10-26 Last modified: 2016-01-28
Priority: Not Prioritized
View all other issues in [rand.util.seedseq].
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Discussion:
In 26.4.7.1 [rand.util.seedseq] /6, the order of packing the inputs into b and the treatment of signed quantities is unclear. Better to spell it out.
See N2391 and N2423 for some further discussion.
Proposed resolution:
Adopt the proposed resolution in N2423.
[ Kona (2007): The LWG adopted the proposed resolution of N2423 for this issue. The LWG voted to accelerate this issue to Ready status to be voted into the WP at Kona. ]
Section: 29.5.5.3 [rand.adapt.ibits], 99 [tr.rand] Status: CD1 Submitter: Walter E. Brown Opened: 2006-11-02 Last modified: 2016-01-28
Priority: Not Prioritized
View all issues with CD1 status.
Discussion:
In preparing N2111, an error on my part resulted in the omission of the following line from the template synopsis in the cited section:
static const size_t word_size = w;
(This same constant is found, for example, in 26.4.3.3 [rand.eng.sub].)
Proposed resolution:
Add the above declaration as the first line after the comment in [rand.adapt.ibits] p4:
// engine characteristics static const size_t word_size = w;
and accept my apologies for the oversight.
Section: 22.10.17.3.2 [func.wrap.func.con], 99 [tr.func.wrap.func.con] Status: CD1 Submitter: Scott Meyers Opened: 2006-11-02 Last modified: 2016-01-28
Priority: Not Prioritized
View all other issues in [func.wrap.func.con].
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Discussion:
My suggestion is that implementers of both tr1::function and its official C++0x successor be explicitly encouraged (but not required) to optimize for the cases mentioned above, i.e., function pointers and small function objects. They could do this by using a small internal buffer akin to the buffer used by implementations of the small string optimization. (That would make this the small functor optimization -- SFO :-}) The form of this encouragement could be a note in the standard akin to footnote 214 of the current standard.
Dave Abrahams notes:
"shall not throw exceptions" should really be "nothing," both to be more grammatical and to be consistent with existing wording in the standard.
Doug Gregor comments: I think this is a good idea. Currently, implementations of tr1::function are required to have non-throwing constructors and assignment operators when the target function object is a function pointer or a reference_wrapper. The common case, however, is for a tr1::function to store either an empty function object or a member pointer + an object pointer.
The function implementation in the upcoming Boost 1.34.0 uses the "SFO", so that the function objects for typical bind expressions like
bind(&X::f, this, _1, _2, _3)
do not require heap allocation when stored in a boost::function. I believe Dinkumware's implementation also performs this optimization.
Proposed resolution:
Revise 20.5.14.2.1 p6 [func.wrap.func.con] to add a note as follows:
Throws: shall not throw exceptions if
f
's target is a function pointer or a function object passed viareference_wrapper
. Otherwise, may throwbad_alloc
or any exception thrown by the copy constructor of the stored function object.Note: Implementations are encouraged to avoid the use of dynamically allocated memory for "small" function objects, e.g., where
f
's target is an object holding only a pointer or reference to an object and a member function pointer (a "bound member function").
Section: 16.4.5.8 [res.on.functions] Status: CD1 Submitter: Nicola Musatti Opened: 2006-11-13 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
In the latest available draft standard (N2134) § 17.4.3.6 [res.on.functions] states:
-1- In certain cases (replacement functions, handler functions, operations on types used to instantiate standard library template components), the C++ Standard Library depends on components supplied by a C++ program. If these components do not meet their requirements, the Standard places no requirements on the implementation.
-2- In particular, the effects are undefined in the following cases:
[...]
- if an incomplete type (3.9) is used as a template argument when instantiating a template component.
This is contradicted by § 20.6.6.2/2 [util.smartptr.shared] which states:
[...]
The template parameter
T
ofshared_ptr
may be an incomplete type.
Proposed resolution:
Modify the last bullet of § 17.4.3.6/2 [res.on.functions] to allow for exceptions:
- if an incomplete type (3.9) is used as a template argument when instantiating a template component, unless specifically allowed for the component.
Section: 17.3.5.2 [numeric.limits.members] Status: CD1 Submitter: Chris Jefferson Opened: 2006-11-10 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
18.2.1.2 55 states that "A type is modulo if it is possible to add two positive numbers together and have a result that wraps around to a third number that is less". This seems insufficient for the following reasons:
[ Batavia: Related to N2144. Pete: is there an ISO definition of modulo? Underflow on signed behavior is undefined. ]
[ Bellevue: accept resolution, move to ready status. Does this mandate that is_modulo be true on platforms for which int happens to b modulo? A: the standard already seems to require that. ]
Proposed resolution:
Suggest 17.3.5.2 [numeric.limits.members], paragraph 57 is amended to:
A type is modulo if,
it is possible to add two positive numbers and have a result that wraps around to a third number that is less.given any operation involving +,- or * on values of that type whose value would fall outside the range[min(), max()]
, then the value returned differs from the true value by an integer multiple of(max() - min() + 1)
.
Section: 17.3.5.3 [numeric.special] Status: CD1 Submitter: Bo Persson Opened: 2006-11-20 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
Section 17.3.5.3 [numeric.special] starts out by saying that "All members shall be provided for all specializations."
Then it goes on to show specializations for float and bool, where one member is missing (max_digits10).
Maarten Kronenburg adds:
I agree, just adding the comment that the exact number of decimal digits is digits * ln(radix) / ln(10), where probably this real number is rounded downward for digits10, and rounded upward for max_digits10 (when radix=10, then digits10=max_digits10). Why not add this exact definition also to the standard, so the user knows what these numbers exactly mean.
Howard adds:
For reference, here are the correct formulas from N1822:
digits10 = floor((digits-1) * log10(2)) max_digits10 = ceil((1 + digits) * log10(2))
We are also missing a statement regarding for what specializations this member has meaning.
Proposed resolution:
Change and add after 17.3.5.2 [numeric.limits.members], p11:
static const int max_digits10;-11- Number of base 10 digits required to ensure that values which differ
by only one epsilonare always differentiated.-12- Meaningful for all floating point types.
Change 17.3.5.3 [numeric.special], p2:
template<> class numeric_limits<float> { public: static const bool is_specialized = true; ... static const int digits10 = 6; static const int max_digits10 = 9; ...
Change 17.3.5.3 [numeric.special], p3:
template<> class numeric_limits<bool> { public: static const bool is_specialized = true; ... static const int digits10 = 0; static const int max_digits10 = 0; ...
Section: 28.3.4.2.3 [locale.ctype.byname] Status: CD1 Submitter: Bo Persson Opened: 2006-12-16 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
Section 22.2.1.2 defines the ctype_byname class template. It contains the line
typedef ctype<charT>::mask mask;
Proposed resolution:
as this is a dependent type, it should obviously be
typedef typename ctype<charT>::mask mask;
Section: 29.6.2.8 [valarray.members] Status: CD1 Submitter: Gabriel Dos Reis Opened: 2007-01-10 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
I would respectfully request an issue be opened with the intention to
clarify the wording for size() == 0
for cshift
.
Proposed resolution:
Change 29.6.2.8 [valarray.members], paragraph 10:
valarray<T> cshift(int n) const;This function returns an object of class
valarray<T>
, of lengthsize()
,each of whose elementsthat is a circular shift ofI
is(*this)[(I + n ) % size()]
. Thus, if element zero is taken as the leftmost element, a positive value of n shifts the elements circularly left n places.*this
. If element zero is taken as the leftmost element, a non-negative value of n shifts the elements circularly left n places and a negative value of n shifts the elements circularly right -n places.
Rationale:
We do not believe that there is any real ambiguity about what happens
when size() == 0
, but we do believe that spelling this out as a C++
expression causes more trouble that it solves. The expression is
certainly wrong when n < 0
, since the sign of % with negative arguments
is implementation defined.
[ Kona (2007) Changed proposed wording, added rationale and set to Review. ]
Section: 17.14 [support.runtime] Status: CD1 Submitter: Lawrence Crowl Opened: 2007-01-12 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
The wording for longjmp
is confusing.
17.14 [support.runtime] -4- Other runtime support
The function signature
longjmp(jmp_buf jbuf, int val)
has more restricted behavior in this International Standard. If any automatic objects would be destroyed by a thrown exception transferring control to another (destination) point in the program, then a call tolongjmp(jbuf, val)
that the throw point that transfers control to the same (destination) point has undefined behavior.
Someone at Google thinks that should say "then a call to longjmp(jbuf, val)
*at* the throw point that transfers control".
Bill Gibbons thinks it should say something like "If any automatic objects would be destroyed by an exception thrown at the point of the longjmp and caught only at the point of the setjmp, the behavior is undefined."
Proposed resolution:
In general, accept Bill Gibbons' recommendation, but add "call" to indicate that the undefined behavior comes from the dynamic call, not from its presence in the code. In 17.14 [support.runtime] paragraph 4, change
The function signature
longjmp(jmp_buf jbuf, int val)
has more restricted behavior in this International Standard.If any automatic objects would be destroyed by a thrown exception transferring control to another (destination) point in the program, then a call toAlongjmp(jbuf, val)
that the throw point that transfers control to the same (destination) point has undefined behavior.setjmp
/longjmp
call pair has undefined behavior if replacing thesetjmp
andlongjmp
bycatch
andthrow
would destroy any automatic objects.
Section: 29.6.2.2 [valarray.cons] Status: CD1 Submitter: Martin Sebor Opened: 2007-01-20 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
The Effects clause for the default valarray
ctor
suggests that it is possible to increase the size of an empty
valarray
object by calling other non-const member
functions of the class besides resize()
. However, such an
interpretation would be contradicted by the requirement on the copy
assignment operator (and apparently also that on the computed
assignments) that the assigned arrays be the same size. See the
reflector discussion starting with c++std-lib-17871.
In addition, Footnote 280 uses some questionable normative language.
Proposed resolution:
Reword the Effects clause and Footnote 280 as follows (29.6.2.2 [valarray.cons]):
valarray();
Effects: Constructs an object of class
valarray<T>
,279) which has zero lengthuntil it is passed into a library function as a modifiable lvalue or through a non-constant this pointer.280)Postcondition:
size() == 0
.Footnote 280: This default constructor is essential, since arrays of
valarray
are likely to prove useful. There shall also be a way to change the size of an array after initialization; this is supplied by the semanticsmay be useful. The length of an empty array can be increased after initialization by means of theresize()
member function.
Section: 29.6 [numarray] Status: CD1 Submitter: Martin Sebor Opened: 2007-01-20 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
The computed and "fill" assignment operators of valarray
helper array class templates (slice_array
,
gslice_array
, mask_array
, and
indirect_array
) are const member functions of each class
template (the latter by the resolution of 123(i)
since they have reference semantics and thus do not affect
the state of the object on which they are called. However, the copy
assignment operators of these class templates, which also have
reference semantics, are non-const. The absence of constness opens
the door to speculation about whether they really are intended to have
reference semantics (existing implementations vary widely).
Pre-Kona, Martin adds:
I realized that adding the const qualifier to the functions as I suggested would break the const correctness of the classes. A few possible solutions come to mind:
Proposed resolution:
Declare the copy assignment operators of all four helper array class templates const.
Specifically, make the following edits:
Change the signature in 29.6.5 [template.slice.array] and 29.6.5.2 [slice.arr.assign] as follows:
const slice_array& operator= (const slice_array&) const;
Change the signature in 29.6.7 [template.gslice.array] and 29.6.7.2 [gslice.array.assign] as follows:
const gslice_array& operator= (const gslice_array&) const;
Change the signature in 29.6.8 [template.mask.array] and 29.6.8.2 [mask.array.assign] as follows:
const mask_array& operator= (const mask_array&) const;
Change the signature in 29.6.9 [template.indirect.array] and 29.6.9.2 [indirect.array.assign] as follows:
const indirect_array& operator= (const indirect_array&) const;
[ Kona (2007) Added const qualification to the return types and set to Ready. ]
filebuf
dtor and close
on errorSection: 31.10.6.4 [fstream.members] Status: CD1 Submitter: Martin Sebor Opened: 2007-01-20 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
basic_filebuf
dtor is specified to have the following
straightforward effects:
Effects: Destroys an object of class
basic_filebuf
. Callsclose()
.
close()
does a lot of potentially complicated processing,
including calling overflow()
to write out the termination
sequence (to bring the output sequence to its initial shift
state). Since any of the functions called during the processing can
throw an exception, what should the effects of an exception be on the
dtor? Should the dtor catch and swallow it or should it propagate it
to the caller? The text doesn't seem to provide any guidance in this
regard other than the general restriction on throwing (but not
propagating) exceptions from destructors of library classes in
16.4.6.14 [res.on.exception.handling].
Further, the last thing close()
is specified to do is
call fclose()
to close the FILE
pointer. The
last sentence of the Effects clause reads:
... If any of the calls to
overflow
orstd::fclose
fails thenclose
fails.
This suggests that close()
might be required to call
fclose()
if and only if none of the calls to
overflow()
fails, and avoid closing the FILE
otherwise. This way, if overflow()
failed to flush out
the data, the caller would have the opportunity to try to flush it
again (perhaps after trying to deal with whatever problem may have
caused the failure), rather than losing it outright.
On the other hand, the function's Postcondition specifies that
is_open() == false
, which suggests that it should call
fclose()
unconditionally. However, since
Postcondition clauses are specified for many functions in the
standard, including constructors where they obviously cannot apply
after an exception, it's not clear whether this Postcondition
clause is intended to apply even after an exception.
It might be worth noting that the traditional behavior (Classic
Iostreams fstream::close()
and C fclose()
)
is to close the FILE
unconditionally, regardless of
errors.
[ See 397(i) and 418(i) for related issues. ]
Proposed resolution:
After discussing this on the reflector (see the thread starting with
c++std-lib-17650) we propose that close()
be clarified to
match the traditional behavior, that is to close the FILE
unconditionally, even after errors or exceptions. In addition, we
propose the dtor description be amended so as to explicitly require it
to catch and swallow any exceptions thrown by close()
.
Specifically, we propose to make the following edits in 31.10.3.4 [filebuf.members]:
basic_filebuf<charT,traits>* close();
Effects: If
is_open() == false
, returns a null pointer. If a put area exists, callsoverflow(traits::eof())
to flush characters. If the last virtual member function called on*this
(betweenunderflow
,overflow
,seekoff
, andseekpos
) wasoverflow
then callsa_codecvt.unshift
(possibly several times) to determine a termination sequence, inserts those characters and callsoverflow(traits::eof())
again. Finally, regardless of whether any of the preceding calls fails or throws an exception, the functionitcloses the file ("as if" by callingstd::fclose(file)
).334) If any of the calls made by the functionto, includingoverflow
orstd::fclose
, fails thenclose
fails by returning a null pointer. If one of these calls throws an exception, the exception is caught and rethrown after closing the file.
And to make the following edits in 31.10.3.2 [filebuf.cons].
virtual ~basic_filebuf();
Effects: Destroys an object of class
basic_filebuf<charT,traits>
. Callsclose()
. If an exception occurs during the destruction of the object, including the call toclose()
, the exception is caught but not rethrown (see 16.4.6.14 [res.on.exception.handling]).
pubimbue
forbidden to call imbue
Section: 31.2.1 [iostream.limits.imbue] Status: CD1 Submitter: Martin Sebor Opened: 2007-01-20 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
31.2.1 [iostream.limits.imbue] specifies that "no function described in
clause 27 except for ios_base::imbue
causes any instance
of basic_ios::imbue
or
basic_streambuf::imbue
to be called."
That contradicts the Effects clause for
basic_streambuf::pubimbue()
which requires the function
to do just that: call basic_streambuf::imbue()
.
Proposed resolution:
To fix this, rephrase the sentence above to allow
pubimbue
to do what it was designed to do. Specifically.
change 31.2.1 [iostream.limits.imbue], p1 to read:
No function described in clause 27 except for
ios_base::imbue
andbasic_filebuf::pubimbue
causes any instance ofbasic_ios::imbue
orbasic_streambuf::imbue
to be called. ...
valarray
assignment and arrays of unequal lengthSection: 29.6.2.3 [valarray.assign] Status: CD1 Submitter: Martin Sebor Opened: 2007-01-20 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
The behavior of the valarray
copy assignment operator is
defined only when both sides have the same number of elements and the
spec is explicit about assignments of arrays of unequal lengths having
undefined behavior.
However, the generalized subscripting assignment operators overloaded
on slice_array
et al (29.6.2.3 [valarray.assign]) don't have any
such restriction, leading the reader to believe that the behavior of
these overloads is well defined regardless of the lengths of the
arguments.
For example, based on the reading of the spec the behavior of the snippet below can be expected to be well-defined:
const std::slice from_0_to_3 (0, 3, 1); // refers to elements 0, 1, 2 const std::valarray<int> a (1, 3); // a = { 1, 1, 1 } std::valarray<int> b (2, 4); // b = { 2, 2, 2, 2 } b = a [from_0_to_3];
In practice, b
may end up being { 1, 1, 1 }
,
{ 1, 1, 1, 2 }
, or anything else, indicating that
existing implementations vary.
Quoting from Section 3.4, Assignment operators, of Al Vermeulen's Proposal for Standard C++ Array Classes (see c++std-lib-704; N0308):
...if the size of the array on the right hand side of the equal sign differs from the size of the array on the left, a run time error occurs. How this error is handled is implementation dependent; for compilers which support it, throwing an exception would be reasonable.
And see more history in N0280.
It has been argued in discussions on the committee's reflector that
the semantics of all valarray
assignment operators should
be permitted to be undefined unless the length of the arrays being
assigned is the same as the length of the one being assigned from. See
the thread starting at c++std-lib-17786.
In order to reflect such views, the standard must specify that the size of the array referred to by the argument of the assignment must match the size of the array under assignment, for example by adding a Requires clause to 29.6.2.3 [valarray.assign] as follows:
Requires: The length of the array to which the argument refers equals
size()
.
Note that it's far from clear that such leeway is necessary in order
to implement valarray
efficiently.
Proposed resolution:
Insert new paragraph into 29.6.2.3 [valarray.assign]:
valarray<T>& operator=(const slice_array<T>&); valarray<T>& operator=(const gslice_array<T>&); valarray<T>& operator=(const mask_array<T>&); valarray<T>& operator=(const indirect_array<T>&);Requires: The length of the array to which the argument refers equals
size()
.These operators allow the results of a generalized subscripting operation to be assigned directly to a
valarray
.
Section: 16 [library] Status: Resolved Submitter: Martin Sebor Opened: 2007-01-20 Last modified: 2016-01-28
Priority: Not Prioritized
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Duplicate of: 895
Discussion:
Many member functions of basic_string
are overloaded,
with some of the overloads taking a string
argument,
others value_type*
, others size_type
, and
others still iterators
. Often, the requirements on one of
the overloads are expressed in the form of Effects,
Throws, and in the Working Paper
(N2134)
also Remark clauses, while those on the rest of the overloads
via a reference to this overload and using a Returns clause.
The difference between the two forms of specification is that per 16.3.2.4 [structure.specifications], p3, an Effects clause specifies "actions performed by the functions," i.e., its observable effects, while a Returns clause is "a description of the return value(s) of a function" that does not impose any requirements on the function's observable effects.
Since only Notes are explicitly defined to be informative and all other paragraphs are explicitly defined to be normative, like Effects and Returns, the new Remark clauses also impose normative requirements.
So by this strict reading of the standard there are some member
functions of basic_string
that are required to throw an
exception under some conditions or use specific traits members while
many other otherwise equivalent overloads, while obliged to return the
same values, aren't required to follow the exact same requirements
with regards to the observable effects.
Here's an example of this problem that was precipitated by the change from informative Notes to normative Remarks (presumably made to address 424(i)):
In the Working Paper, find(string, size_type)
contains a
Remark clause (which is just a Note in the current
standard) requiring it to use traits::eq()
.
find(const charT *s, size_type pos)
is specified to
return find(string(s), pos)
by a Returns clause
and so it is not required to use traits::eq()
. However,
the Working Paper has replaced the original informative Note
about the function using traits::length()
with a
normative requirement in the form of a Remark. Calling
traits::length()
may be suboptimal, for example when the
argument is a very long array whose initial substring doesn't appear
anywhere in *this
.
Here's another similar example, one that existed even prior to the introduction of Remarks:
insert(size_type pos, string, size_type, size_type)
is
required to throw out_of_range
if pos >
size()
.
insert(size_type pos, string str)
is specified to return
insert(pos, str, 0, npos)
by a Returns clause and
so its effects when pos > size()
are strictly speaking
unspecified.
I believe a careful review of the current Effects and Returns clauses is needed in order to identify all such problematic cases. In addition, a review of the Working Paper should be done to make sure that the newly introduced normative Remark clauses do not impose any undesirable normative requirements in place of the original informative Notes.
[ Batavia: Alan and Pete to work. ]
[ Bellevue: Marked as NAD Editorial. ]
[ Post-Sophia Antipolis: Martin indicates there is still work to be done on this issue. Reopened. ]
[ Batavia (2009-05): ]
Tom proposes we say that, unless specified otherwise, it is always the caller's responsibility to verify that supplied arguments meet the called function's requirements. If further semantics are specified (e.g., that the function throws under certain conditions), then it is up to the implementer to check those conditions. Alan feels strongly that our current use of Requires in this context is confusing, especially now that
requires
is a new keyword.
[ 2009-07 Frankfurt ]
Move to Tentatively NAD.
[ 2009 Santa Cruz: ]
Move to Open. Martin will work on proposed wording.
[ 2010 Pittsburgh: ]
Moved to NAD Editorial, solved by revision to N3021.
Rationale:
Solved by revision to N3021.
Proposed resolution:
Section: 28.6.7 [re.regex] Status: CD1 Submitter: Bo Persson Opened: 2007-01-23 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
Section 28.6.7 [re.regex] lists a constructor
template<class InputIterator> basic_regex(InputIterator first, InputIterator last, flag_type f = regex_constants::ECMAScript);
However, in section 28.6.7.2 [re.regex.construct], this constructor takes a
pair of ForwardIterator
.
Proposed resolution:
Change 28.6.7.2 [re.regex.construct]:
template <classForwardIteratorInputIterator> basic_regex(ForwardIteratorInputIterator first,ForwardIteratorInputIterator last, flag_type f = regex_constants::ECMAScript);
complex<T>
insertion and locale dependenceSection: 29.4.6 [complex.ops] Status: CD1 Submitter: Gabriel Dos Reis Opened: 2007-01-28 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
is there an issue opened for (0,3) as complex number with the French local? With the English local, the above parses as an imaginery complex number. With the French locale it parses as a real complex number.
Further notes/ideas from the lib-reflector, messages 17982-17984:
Add additional entries in
num_punct
to cover the complex separator (French would be ';').Insert a space before the comma, which should eliminate the ambiguity.
Solve the problem for ordered sequences in general, perhaps with a dedicated facet. Then complex should use that solution.
[ Bellevue: ]
After much discussion, we agreed on the following: Add a footnote:
[In a locale in which comma is being used as a decimal point character, inserting "showbase" into the output stream forces all outputs to show an explicit decimal point character; then all inserted complex sequences will extract unambiguously.]
And move this to READY status.
[ Pre-Sophia Antipolis, Howard adds: ]
Changed "showbase" to "showpoint" and changed from Ready to Review.
[ Post-Sophia Antipolis: ]
I neglected to pull this issue from the formal motions page after the "showbase" to "showpoint" change. In Sophia Antipolis this change was reviewed by the LWG and the issue was set to Ready. We subsequently voted the footnote into the WP with "showbase".
I'm changing from WP back to Ready to pick up the "showbase" to "showpoint" change.
Proposed resolution:
Add a footnote to 29.4.6 [complex.ops] p16:
[In a locale in which comma is being used as a decimal point character, inserting
showpoint
into the output stream forces all outputs to show an explicit decimal point character; then all inserted complex sequences will extract unambiguously.]
valarray
Section: 29.6.2.2 [valarray.cons] Status: C++11 Submitter: Martin Sebor Opened: 2007-01-28 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
Section 29.2 [numeric.requirements], p1 suggests that a
valarray
specialization on a type T
that
satisfies the requirements enumerated in the paragraph is itself a
valid type on which valarray
may be instantiated
(Footnote 269 makes this clear). I.e.,
valarray<valarray<T> >
is valid as long as
T
is valid. However, since implementations of
valarray
are permitted to initialize storage allocated by
the class by invoking the default ctor of T
followed by
the copy assignment operator, such implementations of
valarray
wouldn't work with (perhaps user-defined)
specializations of valarray
whose assignment operator had
undefined behavior when the size of its argument didn't match the size
of *this
. By "wouldn't work" I mean that it would
be impossible to resize such an array of arrays by calling the
resize()
member function on it if the function used the
copy assignment operator after constructing all elements using the
default ctor (e.g., by invoking new value_type[N]
) to
obtain default-initialized storage) as it's permitted to do.
Stated more generally, the problem is that
valarray<valarray<T> >::resize(size_t)
isn't
required or guaranteed to have well-defined semantics for every type
T
that satisfies all requirements in
29.2 [numeric.requirements].
I believe this problem was introduced by the adoption of the
resolution outlined in N0857,
Assignment of valarrays, from 1996. The copy assignment
operator of the original numerical array classes proposed in N0280,
as well as the one proposed in N0308
(both from 1993), had well-defined semantics for arrays of unequal
size (the latter explicitly only when *this
was empty;
assignment of non empty arrays of unequal size was a runtime error).
The justification for the change given in N0857 was the "loss of performance [deemed] only significant for very simple operations on small arrays or for architectures with very few registers."
Since tiny arrays on a limited subset of hardware architectures are
likely to be an exceedingly rare case (despite the continued
popularity of x86) I propose to revert the resolution and make the
behavior of all valarray
assignment operators
well-defined even for non-conformal arrays (i.e., arrays of unequal
size). I have implemented this change and measured no significant
degradation in performance in the common case (non-empty arrays of
equal size). I have measured a 50% (and in some cases even greater)
speedup in the case of assignments to empty arrays versus calling
resize()
first followed by an invocation of the copy
assignment operator.
[ Bellevue: ]
If no proposed wording by June meeting, this issue should be closed NAD.
[ 2009-07 Frankfurt ]
Move resolution 1 to Ready.
Howard: second resolution has been commented out (made invisible). Can be brought back on demand.
Proposed resolution:
Change 29.6.2.3 [valarray.assign], p1 as follows:
valarray<T>& operator=(const valarray<T>& x);
-1- Each element of the
*this
array is assigned the value of the corresponding element of the argument array.The resulting behavior is undefined ifWhen the length of the argument array is not equal to the length of the *this array.resizes*this
to make the two arrays the same length, as if by callingresize(x.size())
, before performing the assignment.
And add a new paragraph just below paragraph 1 with the following text:
-2- Postcondition:
size() == x.size()
.
Also add the following paragraph to 29.6.2.3 [valarray.assign], immediately after p4:
-?- When the length,
N
of the array referred to by the argument is not equal to the length of*this
, the operator resizes*this
to make the two arrays the same length, as if by callingresize(N)
, before performing the assignment.
[ pre-Sophia Antipolis, Martin adds the following compromise wording, but prefers the original proposed resolution: ]
[
Kona (2007): Gaby to propose wording for an alternative resolution in
which you can assign to a valarray
of size 0, but not to any other
valarray
whose size is unequal to the right hand side of the assignment.
]
allocator.address()
doesn't work for types overloading operator&
Section: 20.2.10.2 [allocator.members] Status: CD1 Submitter: Howard Hinnant Opened: 2007-02-07 Last modified: 2016-01-28
Priority: Not Prioritized
View all other issues in [allocator.members].
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Duplicate of: 350
Discussion:
20.2.10.2 [allocator.members] says:
pointer address(reference x) const;-1- Returns:
&x
.
20.2.10.2 [allocator.members] defines CopyConstructible
which currently not
only defines the semantics of copy construction, but also restricts what an overloaded
operator&
may do. I believe proposals are in the works (such as concepts
and rvalue reference) to decouple these two requirements. Indeed it is not evident
that we should disallow overloading operator&
to return something other
than the address of *this
.
An example of when you want to overload operator&
to return something
other than the object's address is proxy references such as vector<bool>
(or its replacement, currently code-named bit_vector
). Taking the address of
such a proxy reference should logically yield a proxy pointer, which when dereferenced,
yields a copy of the original proxy reference again.
On the other hand, some code truly needs the address of an object, and not a proxy
(typically for determining the identity of an object compared to a reference object).
boost has long recognized this dilemma and solved it with
boost::addressof
.
It appears to me that this would be useful functionality for the default allocator. Adopting
this definition for allocator::address
would free the standard of requiring
anything special from types which overload operator&
. Issue 580(i)
is expected to make use of allocator::address
mandatory for containers.
Proposed resolution:
Change 20.2.10.2 [allocator.members]:
pointer address(reference x) const;-1- Returns:
The actual address of object referenced by x, even in the presence of an overloaded&x
.operator&
.const_pointer address(address(const_reference x) const;-2- Returns:
The actual address of object referenced by x, even in the presence of an overloaded&x
.operator&
.
[ post Oxford: This would be rendered NAD Editorial by acceptance of N2257. ]
[ Kona (2007): The LWG adopted the proposed resolution of N2387 for this issue which was subsequently split out into a separate paper N2436 for the purposes of voting. The resolution in N2436 addresses this issue. The LWG voted to accelerate this issue to Ready status to be voted into the WP at Kona. ]
allocator::address
Section: 16.4.4.6 [allocator.requirements] Status: Resolved Submitter: Howard Hinnant Opened: 2007-02-08 Last modified: 2016-01-28
Priority: Not Prioritized
View other active issues in [allocator.requirements].
View all other issues in [allocator.requirements].
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Discussion:
The table of allocator requirements in 16.4.4.6 [allocator.requirements] describes
allocator::address
as:
a.address(r) a.address(s)
where r
and s
are described as:
a value of type
X::reference
obtained by the expression*p
.
and p
is
a value of type
X::pointer
, obtained by callinga1.allocate
, wherea1 == a
This all implies that to get the address of some value of type T
that
value must have been allocated by this allocator or a copy of it.
However sometimes container code needs to compare the address of an external value of
type T
with an internal value. For example list::remove(const T& t)
may want to compare the address of the external value t
with that of a value
stored within the list. Similarly vector
or deque insert
may
want to make similar comparisons (to check for self-referencing calls).
Mandating that allocator::address
can only be called for values which the
allocator allocated seems overly restrictive.
[ post San Francisco: ]
Pablo recommends NAD Editorial, solved by N2768.
[ 2009-04-28 Pablo adds: ]
Tentatively-ready NAD Editorial as fixed by N2768.
[ 2009-07 Frankfurt ]
Fixed by N2768.
[ 2009-07-28 Reopened by Alisdair. No longer solved by concepts. ]
[ 2009-10 Santa Cruz: ]
NAD EditorialResolved. Addressed by N2982.
Proposed resolution:
Change 16.4.4.6 [allocator.requirements]:
r
: a value of typeX::reference
obtained by the expression *p.
s
: a value of typeX::const_reference
obtained by the expression.*q
or by conversion from a valuer
[ post Oxford: This would be rendered NAD Editorial by acceptance of N2257. ]
[ Kona (2007): This issue is section 8 of N2387. There was some discussion of it but no resolution to this issue was recorded. Moved to Open. ]
deque
end invalidation during eraseSection: 23.3.5.4 [deque.modifiers] Status: CD1 Submitter: Steve LoBasso Opened: 2007-02-17 Last modified: 2016-01-28
Priority: Not Prioritized
View other active issues in [deque.modifiers].
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Discussion:
The standard states at 23.3.5.4 [deque.modifiers]/4:
deque erase(...)Effects: ... An erase at either end of the deque invalidates only the iterators and the references to the erased elements.
This does not state that iterators to end will be invalidated. It needs to be amended in such a way as to account for end invalidation.
Something like:
Any time the last element is erased, iterators to end are invalidated.
This would handle situations like:
erase(begin(), end()) erase(end() - 1) pop_back() resize(n, ...) where n < size() pop_front() with size() == 1
[ Post Kona, Steve LoBasso notes: ]
My only issue with the proposed resolution is that it might not be clear that
pop_front()
[wheresize() == 1
] can invalidate past-the-end iterators.
[ Kona (2007): Proposed wording added and moved to Review. ]
[ Bellevue: ]
Note that there is existing code that relies on iterators not being invalidated, but there are also existing implementations that do invalidate iterators. Thus, such code is not portable in any case. There is a
pop_front()
note, which should possibly be a separate issue. Mike Spertus to evaluate and, if need be, file an issue.
Proposed resolution:
Change 23.3.5.4 [deque.modifiers], p4:
iterator erase(const_iterator position); iterator erase(const_iterator first, const_iterator last);-4- Effects: An erase in the middle of the
deque
invalidates all the iterators and references to elements of thedeque
and the past-the-end iterator. An erase at either end of thedeque
invalidates only the iterators and the references to the erased elements, except that erasing at the end also invalidates the past-the-end iterator.
(unsigned) long long
Section: 31.7.6.3.2 [ostream.inserters.arithmetic] Status: CD1 Submitter: Daniel Krügler Opened: 2007-02-17 Last modified: 2016-01-28
Priority: Not Prioritized
View all other issues in [ostream.inserters.arithmetic].
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Discussion:
The arithmetic inserters are described in 31.7.6.3.2 [ostream.inserters.arithmetic]. Although the section starts with a listing of the inserters including the new ones:
operator<<(long long val ); operator<<(unsigned long long val );
the text in paragraph 1, which describes the corresponding effects
of the inserters, depending on the actual type of val, does not
handle the types long long
and unsigned long long
.
[ Alisdair: In addition to the (unsigned) long long problem, that whole paragraph misses any reference to extended integral types supplied by the implementation - one of the additions by core a couple of working papers back. ]
Proposed resolution:
In 31.7.6.3.2 [ostream.inserters.arithmetic]/1 change the third sentence
When val is of type
bool
,long
,unsigned long
, long long, unsigned long long,double
,long double
, orconst void*
, the formatting conversion occurs as if it performed the following code fragment:
Section: 31.10.3 [filebuf], 28.3.4.3.3.3 [facet.num.put.virtuals] Status: CD1 Submitter: Daniel Krügler Opened: 2007-02-20 Last modified: 2016-01-28
Priority: Not Prioritized
View all issues with CD1 status.
Discussion:
The current standard 14882:2003(E) as well as N2134 have the following defects:
31.10.3 [filebuf]/5 says:
In order to support file I/O and multibyte/wide character conversion, conversions are performed using members of a facet, referred to as
a_codecvt
in following sections, obtained "as if" bycodecvt<charT,char,typename traits::state_type> a_codecvt = use_facet<codecvt<charT,char,typename traits::state_type> >(getloc());
use_facet
returns a const facet
reference and no facet is
copyconstructible, so the codecvt construction should fail to compile.
A similar issue arises in 28.3.4.3.3.3 [facet.num.put.virtuals]/15 for num_punct
.
Proposed resolution:
In 31.10.3 [filebuf]/5 change the "as if" code
const codecvt<charT,char,typename traits::state_type>& a_codecvt = use_facet<codecvt<charT,char,typename traits::state_type> >(getloc());
In 28.3.4.3.3.3 [facet.num.put.virtuals]/15 (This is para 5 in N2134) change
A local variable
punct
is initialized viaconst numpunct<charT>& punct = use_facet< numpunct<charT> >(str.getloc() );
(Please note also the additional provided trailing semicolon)
Section: 28.6.9.6 [re.results.form] Status: CD1 Submitter: Daniel Krügler Opened: 2007-02-26 Last modified: 2016-01-28
Priority: Not Prioritized
View all issues with CD1 status.
Discussion:
28.6.9.6 [re.results.form] (root and para 3) in N2134 defines the two function template members format as non-const functions, although they are declared as const in 28.6.9 [re.results]/3.
Proposed resolution:
Add the missing const
specifier to both format
overloads described
in section 28.6.9.6 [re.results.form].
Section: 28.6.11.2 [re.tokiter] Status: CD1 Submitter: Daniel Krügler Opened: 2007-03-05 Last modified: 2016-01-28
Priority: Not Prioritized
View all other issues in [re.tokiter].
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Discussion:
Both the class definition of regex_token_iterator (28.6.11.2 [re.tokiter]/6) and the latter member specifications (28.6.11.2.3 [re.tokiter.comp]/1+2) declare both comparison operators as non-const functions. Furtheron, both dereference operators are unexpectedly also declared as non-const in 28.6.11.2 [re.tokiter]/6 as well as in (28.6.11.2.4 [re.tokiter.deref]/1+2).
Proposed resolution:
1) In (28.6.11.2 [re.tokiter]/6) change the current declarations
bool operator==(const regex_token_iterator&) const; bool operator!=(const regex_token_iterator&) const; const value_type& operator*() const; const value_type* operator->() const;
2) In 28.6.11.2.3 [re.tokiter.comp] change the following declarations
bool operator==(const regex_token_iterator& right) const; bool operator!=(const regex_token_iterator& right) const;
3) In 28.6.11.2.4 [re.tokiter.deref] change the following declarations
const value_type& operator*() const; const value_type* operator->() const;
[ Kona (2007): The LWG adopted the proposed resolution of N2409 for this issue (which is to adopt the proposed wording in this issue). The LWG voted to accelerate this issue to Ready status to be voted into the WP at Kona. ]
Section: 28.6.11.2.2 [re.tokiter.cnstr] Status: CD1 Submitter: Daniel Krügler Opened: 2007-03-05 Last modified: 2016-01-28
Priority: Not Prioritized
View all other issues in [re.tokiter.cnstr].
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Discussion:
The text provided in 28.6.11.2.2 [re.tokiter.cnstr]/2+3 describes the effects of the three non-default constructors of class template regex_token_iterator but is does not clarify which values are legal values for submatch/submatches. This becomes an issue, if one takes 28.6.11.2 [re.tokiter]/9 into account, which explains the notion of a "current match" by saying:
The current match is
(*position).prefix()
ifsubs[N] == -1
, or(*position)[subs[N]]
for any other value ofsubs[N]
.
It's not clear to me, whether other negative values except -1 are legal arguments or not - it seems they are not.
Proposed resolution:
Add the following precondition paragraph just before the current 28.6.11.2.2 [re.tokiter.cnstr]/2:
Requires: Each of the initialization values of
subs
must be>= -1
.
[ Kona (2007): The LWG adopted the proposed resolution of N2409 for this issue (which is to adopt the proposed wording in this issue). The LWG voted to accelerate this issue to Ready status to be voted into the WP at Kona. ]
Section: 28.6.11.1 [re.regiter] Status: CD1 Submitter: Daniel Krügler Opened: 2007-03-05 Last modified: 2016-01-28
Priority: Not Prioritized
View all issues with CD1 status.
Discussion:
Both the class definition of regex_iterator (28.6.11.1 [re.regiter]/1) and the latter member specification (28.6.11.1.3 [re.regiter.comp]/1+2) declare both comparison operators as non-const functions. Furtheron, both dereference operators are unexpectedly also declared as non-const in 28.6.11.1 [re.regiter]/1 as well as in (28.6.11.1.4 [re.regiter.deref]/1+2).
Proposed resolution:
1) In (28.6.11.1 [re.regiter]/1) change the current declarations
bool operator==(const regex_iterator&) const; bool operator!=(const regex_iterator&) const; const value_type& operator*() const; const value_type* operator->() const;
2) In 28.6.11.1.4 [re.regiter.deref] change the following declarations
const value_type& operator*() const; const value_type* operator->() const;
3) In 28.6.11.1.3 [re.regiter.comp] change the following declarations
bool operator==(const regex_iterator& right) const; bool operator!=(const regex_iterator& right) const;
[ Kona (2007): The LWG adopted the proposed resolution of N2409 for this issue (which is to adopt the proposed wording in this issue). The LWG voted to accelerate this issue to Ready status to be voted into the WP at Kona. ]
Section: 29.5.3.4 [rand.req.eng] Status: CD1 Submitter: Daniel Krügler Opened: 2007-03-08 Last modified: 2016-01-28
Priority: Not Prioritized
View all other issues in [rand.req.eng].
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Discussion:
Table 98 and para 5 in 29.5.3.4 [rand.req.eng] specify the IO insertion and extraction semantic of random number engines. It can be shown, v.i., that the specification of the extractor cannot guarantee to fulfill the requirement from para 5:
If a textual representation written via os << x was subsequently read via is >> v, then x == v provided that there have been no intervening invocations of x or of v.
The problem is, that the extraction process described in table 98 misses to specify that it will initially set the if.fmtflags to ios_base::dec, see table 104:
dec: converts integer input or generates integer output in decimal base
Proof: The following small program demonstrates the violation of requirements (exception safety not fulfilled):
#include <cassert> #include <ostream> #include <iostream> #include <iomanip> #include <sstream> class RanNumEngine { int state; public: RanNumEngine() : state(42) {} bool operator==(RanNumEngine other) const { return state == other.state; } template <typename Ch, typename Tr> friend std::basic_ostream<Ch, Tr>& operator<<(std::basic_ostream<Ch, Tr>& os, RanNumEngine engine) { Ch old = os.fill(os.widen(' ')); // Sets space character std::ios_base::fmtflags f = os.flags(); os << std::dec << std::left << engine.state; // Adds ios_base::dec|ios_base::left os.fill(old); // Undo os.flags(f); return os; } template <typename Ch, typename Tr> friend std::basic_istream<Ch, Tr>& operator>>(std::basic_istream<Ch, Tr>& is, RanNumEngine& engine) { // Uncomment only for the fix. //std::ios_base::fmtflags f = is.flags(); //is >> std::dec; is >> engine.state; //is.flags(f); return is; } }; int main() { std::stringstream s; s << std::setfill('#'); // No problem s << std::oct; // Yikes! // Here starts para 5 requirements: RanNumEngine x; s << x; RanNumEngine v; s >> v; assert(x == v); // Fails: 42 == 34 }
A second, minor issue seems to be, that the insertion description from table 98 unnecessarily requires the addition of ios_base::fixed (which only influences floating-point numbers). Its not entirely clear to me whether the proposed standard does require that the state of random number engines is stored in integral types or not, but I have the impression that this is the indent, see e.g. p. 3
The specification of each random number engine defines the size of its state in multiples of the size of its result_type.
If other types than integrals are supported, then I wonder why no requirements are specified for the precision of the stream.
See N2391 and N2423 for some further discussion.
Proposed resolution:
Adopt the proposed resolution in N2423.
[ Kona (2007): The LWG adopted the proposed resolution of N2423 for this issue. The LWG voted to accelerate this issue to Ready status to be voted into the WP at Kona. ]
Section: 29.5.8.2 [rand.util.canonical] Status: CD1 Submitter: Daniel Krügler Opened: 2007-03-08 Last modified: 2016-01-28
Priority: Not Prioritized
View all other issues in [rand.util.canonical].
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Discussion:
In 29.5.2 [rand.synopsis] we have the declaration
template<class RealType, class UniformRandomNumberGenerator, size_t bits> result_type generate_canonical(UniformRandomNumberGenerator& g);
Besides the "result_type" issue (already recognized by Bo Persson at Sun, 11 Feb 2007 05:26:47 GMT in this group) it's clear, that the template parameter order is not reasonably choosen: Obviously one always needs to specify all three parameters, although usually only two are required, namely the result type RealType and the wanted bits, because UniformRandomNumberGenerator can usually be deduced.
See N2391 and N2423 for some further discussion.
Proposed resolution:
Adopt the proposed resolution in N2423.
[ Kona (2007): The LWG adopted the proposed resolution of N2423 for this issue. The LWG voted to accelerate this issue to Ready status to be voted into the WP at Kona. ]
Section: 22.10 [function.objects] Status: Resolved Submitter: Daniel Krügler Opened: 2007-03-19 Last modified: 2016-01-28
Priority: Not Prioritized
View all other issues in [function.objects].
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Discussion:
The header <functional>
synopsis in 22.10 [function.objects]
contains the following two free comparison operator templates
for the function
class template
template<class Function1, class Function2> void operator==(const function<Function1>&, const function<Function2>&); template<class Function1, class Function2> void operator!=(const function<Function1>&, const function<Function2>&);
which are nowhere described. I assume that they are relicts before the corresponding two private and undefined member templates in the function template (see 22.10.17.3 [func.wrap.func] and [func.wrap.func.undef]) have been introduced. The original free function templates should be removed, because using an undefined entity would lead to an ODR violation of the user.
Proposed resolution:
Remove the above mentioned two function templates from
the header <functional>
synopsis (22.10 [function.objects])
template<class Function1, class Function2> void operator==(const function<Function1>&, const function<Function2>&); template<class Function1, class Function2> void operator!=(const function<Function1>&, const function<Function2>&);
Rationale:
Fixed by N2292 Standard Library Applications for Deleted Functions.
istreambuf_iterator
should have an operator->()
Section: 24.6.4 [istreambuf.iterator] Status: C++11 Submitter: Niels Dekker Opened: 2007-03-25 Last modified: 2016-01-28
Priority: Not Prioritized
View other active issues in [istreambuf.iterator].
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Discussion:
Greg Herlihy has clearly demonstrated that a user defined input
iterator should have an operator->()
, even if its
value type is a built-in type (comp.std.c++, "Re: Should any iterator
have an operator->()
in C++0x?", March 2007). And as Howard
Hinnant remarked in the same thread that the input iterator
istreambuf_iterator
doesn't have one, this must be a
defect!
Based on Greg's example, the following code demonstrates the issue:
#include <iostream> #include <fstream> #include <streambuf> typedef char C; int main () { std::ifstream s("filename", std::ios::in); std::istreambuf_iterator<char> i(s); (*i).~C(); // This is well-formed... i->~C(); // ... so this should be supported! }
Of course, operator->
is also needed when the value_type
of
istreambuf_iterator
is a class.
The operator->
could be implemented in various ways. For instance,
by storing the current value inside the iterator, and returning its
address. Or by returning a proxy, like operator_arrow_proxy
, from
http://www.boost.org/boost/iterator/iterator_facade.hpp
I hope that the resolution of this issue will contribute to getting a clear and consistent definition of iterator concepts.
[
Kona (2007): The proposed resolution is inconsistent because the return
type of istreambuf_iterator::operator->()
is specified to be pointer
,
but the proposed text also states that "operator->
may return a proxy."
]
[ Niels Dekker (mailed to Howard Hinnant): ]
The proposed resolution does not seem inconsistent to me.
istreambuf_iterator::operator->()
should haveistreambuf_iterator::pointer
as return type, and this return type may in fact be a proxy.AFAIK, the resolution of 445(i) ("
iterator_traits::reference
unspecified for some iterator categories") implies that for any iterator classIter
, the return type ofoperator->()
isIter::pointer
, by definition. I don't thinkIter::pointer
needs to be a raw pointer.Still I wouldn't mind if the text "
operator->
may return a proxy" would be removed from the resolution. I think it's up to the library implementation, how to implementistreambuf_iterator::operator->()
. As longs as it behaves as expected:i->m
should have the same effect as(*i).m
. Even for an explicit destructor call,i->~C()
. The main issue is just:istreambuf_iterator
should have anoperator->()
!
[ 2009-04-30 Alisdair adds: ]
Note that
operator->
is now a requirement in theInputIterator
concept, so this issue cannot be ignored or existing valid programs will break when compiled with an 0x library.
[ 2009-05-29 Alisdair adds: ]
I agree with the observation that in principle the type 'pointer' may be a proxy, and the words highlighting this are redundant.
However, in the current draught
pointer
is required to be exactly 'charT *
' by the derivation fromstd::iterator
. At a minimum, the 4th parameter of this base class template should become unspecified. That permits the introduction of a proxy as a nested class in some further undocumented (not even exposition-only) base.It also permits the
istream_iterator
approach where the cached value is stored in the iterator itself, and the iterator serves as its own proxy for post-incrementoperator++
- removing the need for the existing exposition-only nested classproxy
.Note that the current
proxy
class also has exactly the right properties to serve as the pointerproxy
too. This is likely to be a common case where anInputIterator
does not hold internal state but delegates to another class.Proposed Resolution:
In addition to the current proposal:
24.6.4 [istreambuf.iterator]
template<class charT, class traits = char_traits<charT> > class istreambuf_iterator : public iterator<input_iterator_tag, charT, typename traits::off_type,charT*unspecified, charT> {
[ 2009-07 Frankfurt ]
Move the additional part into the proposed resolution, and wrap the descriptive text in a Note.
[Howard: done.]
Move to Ready.
Proposed resolution:
Add to the synopsis in 24.6.4 [istreambuf.iterator]:
charT operator*() const; pointer operator->() const; istreambuf_iterator<charT,traits>& operator++();
24.6.4 [istreambuf.iterator]
template<class charT, class traits = char_traits<charT> > class istreambuf_iterator : public iterator<input_iterator_tag, charT, typename traits::off_type,charT*unspecified, charT> {
Change 24.6.4 [istreambuf.iterator], p1:
The class template
istreambuf_iterator
reads successive characters from thestreambuf
for which it was constructed.operator*
provides access to the current input character, if any. [Note:operator->
may return a proxy. — end note] Each timeoperator++
is evaluated, the iterator advances to the next input character. If the end of stream is reached (streambuf_type::sgetc()
returnstraits::eof()
), the iterator becomes equal to the end of stream iterator value. The default constructoristreambuf_iterator()
and the constructoristreambuf_iterator(0)
both construct an end of stream iterator object suitable for use as an end-of-range.
Section: 22.10 [function.objects] Status: CD1 Submitter: Beman Dawes Opened: 2007-04-02 Last modified: 2016-01-28
Priority: Not Prioritized
View all other issues in [function.objects].
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Discussion:
Section 22.10 [function.objects] provides function objects for some unary and binary operations, but others are missing. In a LWG reflector discussion, beginning with c++std-lib-18078, pros and cons of adding some of the missing operations were discussed. Bjarne Stroustrup commented "Why standardize what isn't used? Yes, I see the chicken and egg problems here, but it would be nice to see a couple of genuine uses before making additions."
A number of libraries, including Rogue Wave, GNU, Adobe ASL, and Boost, have already added these functions, either publicly or for internal use. For example, Doug Gregor commented: "Boost will also add ... (|, &, ^) in 1.35.0, because we need those function objects to represent various parallel collective operations (reductions, prefix reductions, etc.) in the new Message Passing Interface (MPI) library."
Because the bitwise operators have the strongest use cases, the proposed resolution is limited to them.
Proposed resolution:
To 22.10 [function.objects], Function objects, paragraph 2, add to the header <functional> synopsis:
template <class T> struct bit_and; template <class T> struct bit_or; template <class T> struct bit_xor;
At a location in clause 20 to be determined by the Project Editor, add:
The library provides basic function object classes for all of the bitwise operators in the language ([expr.bit.and], [expr.or], [exp.xor]).
template <class T> struct bit_and : binary_function<T,T,T> { T operator()(const T& x , const T& y ) const; };
operator()
returnsx & y
.template <class T> struct bit_or : binary_function<T,T,T> { T operator()(const T& x , const T& y ) const; };
operator()
returnsx | y
.template <class T> struct bit_xor : binary_function<T,T,T> { T operator()(const T& x , const T& y ) const; };
operator()
returnsx ^ y
.
Section: 31.7.5.3.2 [istream.formatted.arithmetic] Status: CD1 Submitter: Daniel Krügler Opened: 2007-04-01 Last modified: 2016-01-28
Priority: Not Prioritized
View all other issues in [istream.formatted.arithmetic].
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Discussion:
To the more drastic changes of 31.7.5.3.2 [istream.formatted.arithmetic] in the current draft N2134 belong the explicit description of the extraction of the types short and int in terms of as-if code fragments.
Proposed resolution:
In 31.7.5.3.2 [istream.formatted.arithmetic]/2 change the current as-if code fragment
typedef num_get<charT,istreambuf_iterator<charT,traits> > numget; iostate err = 0; long lval; use_facet<numget>(loc).get(*this, 0, *this, err, lval ); if (err == 0) {&&if (lval < numeric_limits<short>::min() || numeric_limits<short>::max() < lval))err = ios_base::failbit; else val = static_cast<short>(lval); } setstate(err);
Similarily in 31.7.5.3.2 [istream.formatted.arithmetic]/3 change the current as-if fragment
typedef num_get<charT,istreambuf_iterator<charT,traits> > numget; iostate err = 0; long lval; use_facet<numget>(loc).get(*this, 0, *this, err, lval ); if (err == 0) {&&if (lval < numeric_limits<int>::min() || numeric_limits<int>::max() < lval))err = ios_base::failbit; else val = static_cast<int>(lval); } setstate(err);
[
Kona (2007): Note to the editor: the name lval in the call to use_facet
is incorrectly italicized in the code fragments corresponding to
operator>>(short &)
and operator >>(int &)
. Also, val -- which appears
twice on the line with the static_cast
in the proposed resolution --
should be italicized. Also, in response to part two of the issue: this
is deliberate.
]
do_unshift
for codecvt<char, char, mbstate_t>
Section: 28.3.4.2.5.3 [locale.codecvt.virtuals] Status: CD1 Submitter: Thomas Plum Opened: 2007-04-16 Last modified: 2016-01-28
Priority: Not Prioritized
View all other issues in [locale.codecvt.virtuals].
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Discussion:
28.3.4.2.5.3 [locale.codecvt.virtuals], para 7 says (regarding do_unshift
):
Effects: Places characters starting at to that should be appended to terminate a sequence when the current
stateT
is given bystate
.237) Stores no more than(to_limit - to)
destination elements, and leaves theto_next
pointer pointing one beyond the last element successfully stored.codecvt<char, char, mbstate_t>
stores no characters.
The following objection has been raised:
Since the C++ Standard permits a nontrivial conversion for the required instantiations of
codecvt
, it is overly restrictive to say thatdo_unshift
must store no characters and returnnoconv
.
[Plum ref _222152Y50]
Proposed resolution:
Change 28.3.4.2.5.3 [locale.codecvt.virtuals], p7:
Effects: Places characters starting at to that should be appended to terminate a sequence when the current
stateT
is given by state.237) Stores no more than (to_limit -to) destination elements, and leaves the to_next pointer pointing one beyond the last element successfully stored.codecvt<char, char, mbstate_t>
stores no characters.
do_unshift
return valueSection: 28.3.4.2.5.3 [locale.codecvt.virtuals] Status: CD1 Submitter: Thomas Plum Opened: 2007-04-16 Last modified: 2016-01-28
Priority: Not Prioritized
View all other issues in [locale.codecvt.virtuals].
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Discussion:
28.3.4.2.5.3 [locale.codecvt.virtuals], para 8 says:
codecvt<char,char,mbstate_t>
, returnsnoconv
.
The following objection has been raised:
Despite what the C++ Standard says,
unshift
can't always returnnoconv
for the default facets, since they can be nontrivial. At least one implementation does whatever the C functions do.
[Plum ref _222152Y62]
Proposed resolution:
Change 28.3.4.2.5.3 [locale.codecvt.virtuals], p8:
Returns: An enumeration value, as summarized in Table 76:
...
codecvt<char,char,mbstate_t>
, returnsnoconv
.
moneypunct::do_curr_symbol()
Section: 28.3.4.7.4.3 [locale.moneypunct.virtuals] Status: CD1 Submitter: Thomas Plum Opened: 2007-04-16 Last modified: 2016-01-28
Priority: Not Prioritized
View all other issues in [locale.moneypunct.virtuals].
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Discussion:
28.3.4.7.4.3 [locale.moneypunct.virtuals], para 4 footnote 257 says
257) For international specializations (second template parameter
true
) this is always four characters long, usually three letters and a space.
The following objection has been raised:
The international currency symbol is whatever the underlying locale says it is, not necessarily four characters long.
[Plum ref _222632Y41]
Proposed resolution:
Change footnote 253 in 28.3.4.7.4.3 [locale.moneypunct.virtuals]:
253) For international specializations (second template parameter
true
) this isalwaystypically four characters long, usually three letters and a space.
hexfloat
Section: 28.3.4.3.3.3 [facet.num.put.virtuals] Status: C++11 Submitter: John Salmon Opened: 2007-04-20 Last modified: 2016-01-28
Priority: Not Prioritized
View other active issues in [facet.num.put.virtuals].
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View all issues with C++11 status.
Discussion:
I am trying to understand how TR1 supports hex float (%a) output.
As far as I can tell, it does so via the following:
8.15 Additions to header <locale>
[tr.c99.locale]
In subclause 28.3.4.3.3.3 [facet.num.put.virtuals], Table 58 Floating-point conversions, after
the line:
floatfield == ios_base::scientific %E
add the two lines:
floatfield == ios_base::fixed | ios_base::scientific && !uppercase %a floatfield == ios_base::fixed | ios_base::scientific %A 2
[Note: The additional requirements on print and scan functions, later in this clause, ensure that the print functions generate hexadecimal floating-point fields with a %a or %A conversion specifier, and that the scan functions match hexadecimal floating-point fields with a %g conversion specifier. end note]
Following the thread, in 28.3.4.3.3.3 [facet.num.put.virtuals], we find:
For conversion from a floating-point type, if (flags & fixed) != 0
or
if str.precision() > 0
, then str.precision()
is specified in the
conversion specification.
This would seem to imply that when floatfield == fixed|scientific
, the
precision of the conversion specifier is to be taken from
str.precision()
. Is this really what's intended? I sincerely hope
that I'm either missing something or this is an oversight. Please
tell me that the committee did not intend to mandate that hex floats
(and doubles) should by default be printed as if by %.6a.
[ Howard: I think the fundamental issue we overlooked was that with %f, %e, %g, the default precision was always 6. With %a the default precision is not 6, it is infinity. So for the first time, we need to distinguish between the default value of precision, and the precision value 6. ]
[ 2009-07 Frankfurt ]
Leave this open for Robert and Daniel to work on.
Straw poll: Disposition?
- Default is %.6a (i.e. NAD): 2
- Always %a (no precision): 6
- precision(-1) == %a: 3
Daniel and Robert have direction to write up wording for the "always %a" solution.
[ 2009-07-15 Robert provided wording. ]
[ 2009-10 Santa Cruz: ]
Move to Ready.
Proposed resolution:
Change 28.3.4.3.3.3 [facet.num.put.virtuals], Stage 1, under p5 (near the end of Stage 1):
For conversion from a floating-point type,
str.precision()
is specified as precision in the conversion specification iffloatfield != (ios_base::fixed | ios_base::scientific)
, else no precision is specified.
[ Kona (2007): Robert volunteers to propose wording. ]
Section: 16.4.4.2 [utility.arg.requirements] Status: CD1 Submitter: Howard Hinnant Opened: 2007-05-04 Last modified: 2016-01-28
Priority: Not Prioritized
View all other issues in [utility.arg.requirements].
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Discussion:
The current Swappable
is:
Table 37: Swappable
requirements [swappable]expression return type post-condition swap(s,t)
void
t
has the value originally held byu
, andu
has the value originally held byt
The Swappable requirement is met by satisfying one or more of the following conditions:
T
is Swappable ifT
satisfies theCopyConstructible
requirements (Table 34) and theCopyAssignable
requirements (Table 36);T
is Swappable if a namespace scope function namedswap
exists in the same namespace as the definition ofT
, such that the expressionswap(t,u)
is valid and has the semantics described in this table.
With the passage of rvalue reference into the language, Swappable
needs to be updated to
require only MoveConstructible
and MoveAssignable
. This is a minimum.
Additionally we may want to support proxy references such that the following code is acceptable:
namespace Mine { template <class T> struct proxy {...}; template <class T> struct proxied_iterator { typedef T value_type; typedef proxy<T> reference; reference operator*() const; ... }; struct A { // heavy type, has an optimized swap, maybe isn't even copyable or movable, just swappable void swap(A&); ... }; void swap(A&, A&); void swap(proxy<A>, A&); void swap(A&, proxy<A>); void swap(proxy<A>, proxy<A>); } // Mine ... Mine::proxied_iterator<Mine::A> i(...) Mine::A a; swap(*i1, a);
I.e. here is a call to swap
which the user enables swapping between a proxy to a class and the class
itself. We do not need to anything in terms of implementation except not block their way with overly
constrained concepts. That is, the Swappable
concept should be expanded to allow swapping
between two different types for the case that one is binding to a user-defined swap
.
Proposed resolution:
Change 16.4.4.2 [utility.arg.requirements]:
-1- The template definitions in the C++ Standard Library refer to various named requirements whose details are set out in tables 31-38. In these tables,
T
is a type to be supplied by a C++ program instantiating a template;a
,b
, andc
are values of typeconst T
;s
andt
are modifiable lvalues of typeT
;u
is a value of type (possiblyconst
)T
; andrv
is a non-const
rvalue of typeT
.
Table 37: Swappable
requirements [swappable]expression return type post-condition swap(s,t)
void
t
has the value originally held byu
, andu
has the value originally held byt
The
Swappable
requirement is met by satisfying one or more of the following conditions:
T
isSwappable
ifT
satisfies theMoveConstructible requirements (TableCopyConstructible
3433) and theMoveAssignable requirements (TableCopyAssignable
3635);T
isSwappable
if a namespace scope function namedswap
exists in the same namespace as the definition ofT
, such that the expressionswap(t,u)
is valid and has the semantics described in this table.
[
Kona (2007): We like the change to the Swappable
requirements to use
move semantics. The issue relating to the support of proxies is
separable from the one relating to move semantics, and it's bigger than
just swap. We'd like to address only the move semantics changes under
this issue, and open a separated issue (742(i)) to handle proxies. Also, there
may be a third issue, in that the current definition of Swappable
does
not permit rvalues to be operands to a swap operation, and Howard's
proposed resolution would allow the right-most operand to be an rvalue,
but it would not allow the left-most operand to be an rvalue (some swap
functions in the library have been overloaded to permit left operands to
swap to be rvalues).
]
unique_ptr
updateSection: 20.3.1 [unique.ptr] Status: CD1 Submitter: Howard Hinnant Opened: 2007-05-04 Last modified: 2016-01-28
Priority: Not Prioritized
View all other issues in [unique.ptr].
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Discussion:
Since the publication of
N1856
there have been a few small but significant advances which should be included into
unique_ptr
. There exists a
example implementation
for all of these changes.
Even though unique_ptr<void>
is not a valid use case (unlike for shared_ptr<void>
),
unexpected cases to crop up which require the instantiation of the interface of unique_ptr<void>
even if it is never used. For example see 541(i) for how this accidently happened to auto_ptr
.
I believe the most robust way to protect unique_ptr
against this
type of failure is to augment the return type of unique_ptr<T>:operator*()
with
add_lvalue_reference<T>::type
. This means that given an instantiated unique_ptr<void>
the act of dereferencing it will simply return void
instead of causing a compile time failure.
This is simpler than creating a unique_ptr<void>
specialization which isn't robust in the
face of cv-
qualified void
types.
This resolution also supports instantiations such as unique_ptr<void, free_deleter>
which could be very useful to the client.
Efforts have been made to better support containers and smart pointers in shared
memory contexts. One of the key hurdles in such support is not assuming that a
pointer type is actually a T*
. This can easily be accomplished
for unique_ptr
by having the deleter define the pointer type:
D::pointer
. Furthermore this type can easily be defaulted to
T*
should the deleter D
choose not to define a pointer
type (example implementation
here).
This change has no run time overhead. It has no interface overhead on
authors of custom delter types. It simply allows (but not requires)
authors of custom deleter types to define a smart pointer for the
storage type of unique_ptr
if they find such functionality
useful. std::default_delete
is an example of a deleter which
defaults pointer
to T*
by simply ignoring this issue
and not including a pointer typedef
.
When the deleter type is a function pointer then it is unsafe to construct
a unique_ptr
without specifying the function pointer in the constructor.
This case is easy to check for with a static_assert
assuring that the
deleter is not a pointer type in those constructors which do not accept deleters.
unique_ptr<A, void(*)(void*)> p(new A); // error, no function given to delete the pointer!
[ Kona (2007): We don't like the solution given to the first bullet in light of concepts. The second bullet solves the problem of supporting fancy pointers for one library component only. The full LWG needs to decide whether to solve the problem of supporting fancy pointers piecemeal, or whether a paper addressing the whole library is needed. We think that the third bullet is correct. ]
[ Post Kona: Howard adds example user code related to the first bullet: ]
void legacy_code(void*, std::size_t); void foo(std::size_t N) { std::unique_ptr<void, void(*)(void*)> ptr(std::malloc(N), std::free); legacy_code(ptr.get(), N); } // unique_ptr used for exception safety purposes
I.e. unique_ptr<void>
is a useful tool that we don't want
to disable with concepts. The only part of unique_ptr<void>
we
want to disable (with concepts or by other means) are the two member functions:
T& operator*() const; T* operator->() const;
Proposed resolution:
[ I am grateful for the generous aid of Peter Dimov and Ion Gaztañaga in helping formulate and review the proposed resolutions below. ]
Change 20.3.1.3 [unique.ptr.single]:
template <class T, class D = default_delete<T>> class unique_ptr { ...T&typename add_lvalue_reference<T>::type operator*() const; ... };
Change 20.3.1.3.5 [unique.ptr.single.observers]:
T&typename add_lvalue_reference<T>::type operator*() const;
Change 20.3.1.3 [unique.ptr.single]:
template <class T, class D = default_delete<T>> class unique_ptr { public: typedef implementation (see description below) pointer; ... explicit unique_ptr(T*pointer p); ... unique_ptr(T*pointer p, implementation defined (see description below) d); unique_ptr(T*pointer p, implementation defined (see description below) d); ...T*pointer operator->() const;T*pointer get() const; ...T*pointer release(); void reset(T*pointer p =0pointer()); };
-3- If the type remove_reference<D>::type::pointer
exists, then unique_ptr<T, D>::pointer
is a typedef to
remove_reference<D>::type::pointer
. Otherwise
unique_ptr<T, D>::pointer
is a typedef to T*
.
The type unique_ptr<T, D>::pointer
shall be CopyConstructible
and CopyAssignable
.
Change 20.3.1.3.2 [unique.ptr.single.ctor]:
unique_ptr(T*pointer p); ... unique_ptr(T*pointer p, implementation defined d); unique_ptr(T*pointer p, implementation defined d); ... unique_ptr(T*pointer p, const A& d); unique_ptr(T*pointer p, A&& d); ... unique_ptr(T*pointer p, A& d); unique_ptr(T*pointer p, A&& d); ... unique_ptr(T*pointer p, const A& d); unique_ptr(T*pointer p, const A&& d); ...
-23- Requires: If D
is not a reference type,
construction of the deleter D
from an rvalue of type E
must shall be well formed and not throw an exception. If D
is a
reference type, then E
must shall be the same type as D
(diagnostic required). U*
unique_ptr<U,E>::pointer
must shall be implicitly convertible to
pointer.
T*
-25- Postconditions: get() == value u.get()
had before
the construction, modulo any required offset adjustments resulting from
the cast from U*
unique_ptr<U,E>::pointer
to
pointer. T*
get_deleter()
returns a reference to the
internally stored deleter which was constructed from
u.get_deleter()
.
Change 20.3.1.3.4 [unique.ptr.single.asgn]:
-8- Requires: Assignment of the deleter
D
from an rvalueD
mustshall not throw an exception.U*
unique_ptr<U,E>::pointer
mustshall be implicitly convertible topointer.T*
Change 20.3.1.3.5 [unique.ptr.single.observers]:
T*pointer operator->() const;...
T*pointer get() const;
Change 20.3.1.3.6 [unique.ptr.single.modifiers]:
T*pointer release();...
void reset(T*pointer p =0pointer());
Change 20.3.1.4 [unique.ptr.runtime]:
template <class T, class D> class unique_ptr<T[], D> { public: typedef implementation pointer; ... explicit unique_ptr(T*pointer p); ... unique_ptr(T*pointer p, implementation defined d); unique_ptr(T*pointer p, implementation defined d); ...T*pointer get() const; ...T*pointer release(); void reset(T*pointer p =0pointer()); };
Change 20.3.1.4.2 [unique.ptr.runtime.ctor]:
unique_ptr(T*pointer p); unique_ptr(T*pointer p, implementation defined d); unique_ptr(T*pointer p, implementation defined d);These constructors behave the same as in the primary template except that they do not accept pointer types which are convertible to
T*
pointer
. [Note: One implementation technique is to create private templated overloads of these members. -- end note]
Change 20.3.1.4.5 [unique.ptr.runtime.modifiers]:
void reset(T*pointer p =0pointer());-1- Requires: Does not accept pointer types which are convertible to
T*
pointer
(diagnostic required). [Note: One implementation technique is to create a private templated overload. -- end note]
Change 20.3.1.3.2 [unique.ptr.single.ctor]:
unique_ptr();Requires:
D
mustshall be default constructible, and that constructionmustshall not throw an exception.D
mustshall not be a reference type or pointer type (diagnostic required).unique_ptr(T*pointer p);Requires: The expression
D()(p)
mustshall be well formed. The default constructor ofD
mustshall not throw an exception.D
mustshall not be a reference type or pointer type (diagnostic required).
shared_ptr
interface changes for consistency with N1856Section: 20.3.2.2 [util.smartptr.shared] Status: CD1 Submitter: Peter Dimov Opened: 2007-05-05 Last modified: 2016-01-28
Priority: Not Prioritized
View all other issues in [util.smartptr.shared].
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Discussion:
N1856 does not propose
any changes to shared_ptr
. It needs to be updated to use a rvalue reference where appropriate
and to interoperate with unique_ptr
as it does with auto_ptr
.
Proposed resolution:
Change 20.3.2.2 [util.smartptr.shared] as follows:
template<class Y> explicit shared_ptr(auto_ptr<Y>&&& r); template<class Y, class D> explicit shared_ptr(const unique_ptr<Y,D>& r) = delete; template<class Y, class D> explicit shared_ptr(unique_ptr<Y,D>&& r); ... template<class Y> shared_ptr& operator=(auto_ptr<Y>&&& r); template<class Y, class D> shared_ptr& operator=(const unique_ptr<Y,D>& r) = delete; template<class Y, class D> shared_ptr& operator=(unique_ptr<Y,D>&& r);
Change 20.3.2.2.2 [util.smartptr.shared.const] as follows:
template<class Y> shared_ptr(auto_ptr<Y>&&& r);
Add to 20.3.2.2.2 [util.smartptr.shared.const]:
template<class Y, class D> shared_ptr(unique_ptr<Y, D>&& r);Effects: Equivalent to
shared_ptr( r.release(), r.get_deleter() )
whenD
is not a reference type,shared_ptr( r.release(), ref( r.get_deleter() ) )
otherwise.Exception safety: If an exception is thrown, the constructor has no effect.
Change 20.3.2.2.4 [util.smartptr.shared.assign] as follows:
template<class Y> shared_ptr& operator=(auto_ptr<Y>&&& r);
Add to 20.3.2.2.4 [util.smartptr.shared.assign]:
template<class Y, class D> shared_ptr& operator=(unique_ptr<Y,D>&& r);-4- Effects: Equivalent to
shared_ptr(std::move(r)).swap(*this)
.-5- Returns:
*this
.
[
Kona (2007): We may need to open an issue (743(i)) to deal with the question of
whether shared_ptr
needs an rvalue swap
.
]
Section: 23.2 [container.requirements] Status: CD1 Submitter: Howard Hinnant Opened: 2007-05-05 Last modified: 2016-01-28
Priority: Not Prioritized
View all other issues in [container.requirements].
View all issues with CD1 status.
Discussion:
James Hopkin pointed out to me that if vector<T>
move assignment is O(1)
(just a swap
) then containers such as vector<shared_ptr<ostream>>
might have
the wrong semantics under move assignment when the source is not truly an rvalue, but a
moved-from lvalue (destructors could run late).
vector<shared_ptr<ostream>>
v1;vector<shared_ptr<ostream>>
v2; ... v1 = v2; // #1 v1 = std::move(v2); // #2
Move semantics means not caring what happens to the source (v2
in this example).
It doesn't mean not caring what happens to the target (v1
). In the above example
both assignments should have the same effect on v1
. Any non-shared ostream
's
v1
owns before the assignment should be closed, whether v1
is undergoing
copy assignment or move assignment.
This implies that the semantics of move assignment of a generic container should be
clear, swap
instead of just swap. An alternative which could achieve the same
effect would be to move assign each element. In either case, the complexity of move
assignment needs to be relaxed to O(v1.size())
.
The performance hit of this change is not nearly as drastic as it sounds.
In practice, the target of a move assignment has always just been move constructed
or move assigned from. Therefore under clear, swap
semantics (in
this common use case) we are still achieving O(1) complexity.
Proposed resolution:
Change 23.2 [container.requirements]:
Table 89: Container requirements expression return type operational semantics assertion/note pre/post-condition complexity a = rv;
X&
All existing elements of a
are either move assigned or destructeda
shall be equal to the value thatrv
had before this construction(Note C)linearNotes: the algorithms
swap()
,equal()
andlexicographical_compare()
are defined in clause 25. Those entries marked "(Note A)" should have constant complexity. Those entries marked "(Note B)" have constant complexity unlessallocator_propagate_never<X::allocator_type>::value
istrue
, in which case they have linear complexity.Those entries marked "(Note C)" have constant complexity ifa.get_allocator() == rv.get_allocator()
or if eitherallocator_propagate_on_move_assignment<X::allocator_type>::value
istrue
orallocator_propagate_on_copy_assignment<X::allocator_type>::value
istrue
and linear complexity otherwise.
[ post Bellevue Howard adds: ]
This issue was voted to WP in Bellevue, but accidently got stepped on by N2525 which was voted to WP simulataneously. Moving back to Open for the purpose of getting the wording right. The intent of this issue and N2525 are not in conflict.
[ post Sophia Antipolis Howard updated proposed wording: ]
Section: 23.5 [unord] Status: C++11 Submitter: Howard Hinnant Opened: 2007-05-05 Last modified: 2016-01-28
Priority: Not Prioritized
View all other issues in [unord].
View all issues with C++11 status.
Discussion:
Move semantics are missing from the unordered
containers. The proposed
resolution below adds move-support consistent with
N1858
and the current working draft.
The current proposed resolution simply lists the requirements for each function. These might better be hoisted into the requirements table for unordered associative containers. Futhermore a mild reorganization of the container requirements could well be in order. This defect report is purposefully ignoring these larger issues and just focusing on getting the unordered containers "moved".
[ 2009-07-28 Reopened by Alisdair. No longer solved by concepts. ]
[ 2009-10-17 Removed rvalue-swaps from wording. ]
[ 2009-10 Santa Cruz: ]
Move to Review. Alisdair will review proposed wording.
[ 2009-10-29 Daniel updates wording. ]
[ 2010-01-26 Alisdair updates wording. ]
[ 2010-02-10 Howard updates wording to reference the unordered container requirements table (modified by 704(i)) as much as possible. ]
[ Voted to WP in Bellevue. ]
[ post Bellevue, Pete notes: ]
Please remind people who are reviewing issues to check that the text modifications match the current draft. Issue 676, for example, adds two overloads for unordered_map::insert taking a hint. One takes a const_iterator and returns a const_iterator, and the other takes an iterator and returns an iterator. This was correct at the time the issue was written, but was changed in Toronto so there is only one hint overload, taking a const_iterator and returning an iterator.
This issue is not ready. In addition to the relatively minor signature problem I mentioned earlier, it puts requirements in the wrong places. Instead of duplicating requirements throughout the template specifications, it should put them in the front matter that talks about requirements for unordered containers in general. This presentation problem is editorial, but I'm not willing to do the extensive rewrite that it requires. Please put it back into Open status.
[ 2010-02-11 Moved to Tentatively Ready after 5 positive votes on c++std-lib. ]
[ 2010-02-24 Pete moved to Open: ]
The descriptions of the semantics of the added
insert
functions belong in the requirements table. That's where the rest of theinsert
functions are.
[ 2010 Pittsburgh: ]
Move issue 676 to Ready for Pittsburgh. Nico to send Howard an issue for the broader problem.
Rationale:
[ San Francisco: ]
Solved by N2776.
[ Rationale is obsolete. ]
Proposed resolution:
unordered_map
Change 23.5.3 [unord.map]:
class unordered_map { ... unordered_map(const unordered_map&); unordered_map(unordered_map&&); unordered_map(const Allocator&); unordered_map(const unordered_map&, const Allocator&); unordered_map(unordered_map&&, const Allocator&); ... unordered_map& operator=(const unordered_map&); unordered_map& operator=(unordered_map&&); ... // modifiers ...std::pair<iterator, bool> insert(const value_type& obj); template <class P> pair<iterator, bool> insert(P&& obj); iterator insert(const_iterator hint, const value_type& obj); template <class P> iterator insert(const_iterator hint, P&& obj); ... mapped_type& operator[](const key_type& k); mapped_type& operator[](key_type&& k); ... };
Add to 23.5.3.3 [unord.map.elem]:
mapped_type& operator[](const key_type& k);...
Requires:
key_type
shall beCopyConstructible
andmapped_type
shall beDefaultConstructible
.Complexity: Average case
O(1)
, worst caseO(size())
.mapped_type& operator[](key_type&& k);Requires:
key_type
shall beMoveConstructible
andmapped_type
shall beDefaultConstructible
.Effects: If the
unordered_map
does not already contain an element whose key is equivalent tok
, inserts the valuevalue_type(std::move(k), mapped_type())
.Returns: A reference to
x.second
, wherex
is the (unique) element whose key is equivalent tok
.Complexity: Average case
O(1)
, worst caseO(size())
.
Add new section [unord.map.modifiers]:
template <class P> pair<iterator, bool> insert(P&& x);Requires:
value_type
is constructible fromstd::forward<P>(x)
.Effects: Inserts
x
converted tovalue_type
if and only if there is no element in the container with key equivalent to the key ofvalue_type(x)
.Returns: The
bool
component of the returnedpair
indicates whether the insertion takes place, and the iterator component points to the element with key equivalent to the key ofvalue_type(x)
.Complexity: Average case
O(1)
, worst caseO(size())
.Remarks:
P
shall be implicitly convertible tovalue_type
, else this signature shall not participate in overload resolution.template <class P> iterator insert(const_iterator hint, P&& x);Requires:
value_type
is constructible fromstd::forward<P>(x)
.Effects: Inserts
x
converted tovalue_type
if and only if there is no element in the container with key equivalent to the key ofvalue_type(x)
. The iteratorhint
is a hint pointing to where the search should start. Implementations are permitted to ignore the hint.Returns: An iterator pointing to the element with key equivalent to the key of
value_type(x)
.Complexity: Average case
O(1)
, worst caseO(size())
.Remarks:
P
shall be implicitly convertible tovalue_type
, else this signature shall not participate in overload resolution.
unordered_multimap
Change 23.5.4 [unord.multimap]:
class unordered_multimap { ... unordered_multimap(const unordered_multimap&); unordered_multimap(unordered_multimap&&); unordered_multimap(const Allocator&); unordered_multimap(const unordered_multimap&, const Allocator&); unordered_multimap(unordered_multimap&&, const Allocator&); ... unordered_multimap& operator=(const unordered_multimap&); unordered_multimap& operator=(unordered_multimap&&); ... // modifiers ... iterator insert(const value_type& obj); template <class P> iterator insert(P&& obj); iterator insert(const_iterator hint, const value_type& obj); template <class P> iterator insert(const_iterator hint, P&& obj); ... };
Add new section [unord.multimap.modifiers]:
template <class P> iterator insert(P&& x);Requires:
value_type
is constructible fromstd::forward<P>(x)
.Effects: Inserts
x
converted tovalue_type
.Returns: An iterator pointing to the element with key equivalent to the key of
value_type(x)
.Complexity: Average case
O(1)
, worst caseO(size())
.Remarks:
P
shall be implicitly convertible tovalue_type
, else this signature shall not participate in overload resolution.template <class P> iterator insert(const_iterator hint, P&& x);Requires:
value_type
is constructible fromstd::forward<P>(x)
.Effects: Inserts
x
converted tovalue_type
if and only if there is no element in the container with key equivalent to the key ofvalue_type(x)
. The iteratorhint
is a hint pointing to where the search should start. Implementations are permitted to ignore the hint.Returns: An iterator pointing to the element with key equivalent to the key of
value_type(x)
.Complexity: Average case
O(1)
, worst caseO(size())
.Remarks:
P
shall be implicitly convertible tovalue_type
, else this signature shall not participate in overload resolution.
unordered_set
Change 23.5.6 [unord.set]:
class unordered_set { ... unordered_set(const unordered_set&); unordered_set(unordered_set&&); unordered_set(const Allocator&); unordered_set(const unordered_set&, const Allocator&); unordered_set(unordered_set&&, const Allocator&); ... unordered_set& operator=(const unordered_set&); unordered_set& operator=(unordered_set&&); ... // modifiers ...std::pair<iterator, bool> insert(const value_type& obj); pair<iterator, bool> insert(value_type&& obj); iterator insert(const_iterator hint, const value_type& obj); iterator insert(const_iterator hint, value_type&& obj); ... };
unordered_multiset
Change 23.5.7 [unord.multiset]:
class unordered_multiset { ... unordered_multiset(const unordered_multiset&); unordered_multiset(unordered_multiset&&); unordered_multiset(const Allocator&); unordered_multiset(const unordered_multiset&, const Allocator&); unordered_multiset(unordered_multiset&&, const Allocator&); ... unordered_multiset& operator=(const unordered_multiset&); unordered_multiset& operator=(unordered_multiset&&); ... // modifiers ... iterator insert(const value_type& obj); iterator insert(value_type&& obj); iterator insert(const_iterator hint, const value_type& obj); iterator insert(const_iterator hint, value_type&& obj); ... };
Section: 29.5.8.1 [rand.util.seedseq] Status: CD1 Submitter: Charles Karney Opened: 2007-05-15 Last modified: 2016-01-28
Priority: Not Prioritized
View all other issues in [rand.util.seedseq].
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Discussion:
seed_seq::randomize
provides a mechanism for initializing random number
engines which ideally would yield "distant" states when given "close"
seeds. The algorithm for seed_seq::randomize
given in the current
Working Draft for C++,
N2284
(2007-05-08), has 3 weaknesses
Collisions in state. Because of the way the state is initialized, seeds of different lengths may result in the same state. The current version of seed_seq has the following properties:
s <= n
, each of the 2^(32s) seed vectors results in a
distinct state.The proposed algorithm (below) has the considerably stronger properties:
(2^(32n)-1)/(2^32-1)
seed vectors of lengths s < n
result in distinct states.
2^(32n)
seed vectors of length s == n
result in
distinct states.
Poor mixing of v'
s entropy into the state. Consider v.size() == n
and hold v[n/2]
thru v[n-1]
fixed while varying v[0]
thru v[n/2-1]
,
a total of 2^(16n)
possibilities. Because of the simple recursion
used in seed_seq
, begin[n/2]
thru begin[n-1]
can take on only 2^64
possible states.
The proposed algorithm uses a more complex recursion which results in much better mixing.
seed_seq::randomize
is undefined for v.size() == 0
. The proposed
algorithm remedies this.
The current algorithm for seed_seq::randomize
is adapted by me from the
initialization procedure for the Mersenne Twister by Makoto Matsumoto
and Takuji Nishimura. The weakness (2) given above was communicated to
me by Matsumoto last year.
The proposed replacement for seed_seq::randomize
is due to Mutsuo Saito,
a student of Matsumoto, and is given in the implementation of the
SIMD-oriented Fast Mersenne Twister random number generator SFMT.
http://www.math.sci.hiroshima-u.ac.jp/~m-mat/MT/SFMT/index.html
http://www.math.sci.hiroshima-u.ac.jp/~m-mat/MT/SFMT/SFMT-src-1.2.tar.gz
See Mutsuo Saito, An Application of Finite Field: Design and Implementation of 128-bit Instruction-Based Fast Pseudorandom Number Generator, Master's Thesis, Dept. of Math., Hiroshima University (Feb. 2007) http://www.math.sci.hiroshima-u.ac.jp/~m-mat/MT/SFMT/M062821.pdf
One change has been made here, namely to treat the case of small n
(setting t = (n-1)/2
for n < 7
).
Since seed_seq
was introduced relatively recently there is little cost
in making this incompatible improvement to it.
See N2391 and N2423 for some further discussion.
Proposed resolution:
Adopt the proposed resolution in N2423.
[ Kona (2007): The LWG adopted the proposed resolution of N2423 for this issue. The LWG voted to accelerate this issue to Ready status to be voted into the WP at Kona. ]
Section: 29.5.3.4 [rand.req.eng] Status: CD1 Submitter: Charles Karney Opened: 2007-05-15 Last modified: 2016-01-28
Priority: Not Prioritized
View all other issues in [rand.req.eng].
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Discussion:
Section 29.5.3.4 [rand.req.eng] Random number engine requirements:
This change follows naturally from the proposed change to
seed_seq::randomize
in 677(i).
In table 104 the description of X(q)
contains a special treatment of
the case q.size() == 0
. This is undesirable for 4 reasons:
X()
.X(q)
with q.size() > 0
.X(q)
in
paragraphs 29.5.4.2 [rand.eng.lcong] p5, 29.5.4.3 [rand.eng.mers] p8, and 29.5.4.4 [rand.eng.sub] p10 where
there is no special treatment of q.size() == 0
.seed_seq::randomize
given above
allows for the case q.size() == 0
.See N2391 and N2423 for some further discussion.
Proposed resolution:
Adopt the proposed resolution in N2423.
[ Kona (2007): The LWG adopted the proposed resolution of N2423 for this issue. The LWG voted to accelerate this issue to Ready status to be voted into the WP at Kona. ]
Section: 23.3 [sequences] Status: CD1 Submitter: Howard Hinnant Opened: 2007-06-11 Last modified: 2016-01-28
Priority: Not Prioritized
View all other issues in [sequences].
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Discussion:
The C++98 standard specifies that one member function alone of the containers
passes its parameter (T
) by value instead of by const reference:
void resize(size_type sz, T c = T());
This fact has been discussed / debated repeatedly over the years, the first time being even before C++98 was ratified. The rationale for passing this parameter by value has been:
So that self referencing statements are guaranteed to work, for example:
v.resize(v.size() + 1, v[0]);
However this rationale is not convincing as the signature for push_back
is:
void push_back(const T& x);
And push_back
has similar semantics to resize
(append).
And push_back
must also work in the self referencing case:
v.push_back(v[0]); // must work
The problem with passing T
by value is that it can be significantly more
expensive than passing by reference. The converse is also true, however when it is
true it is usually far less dramatic (e.g. for scalar types).
Even with move semantics available, passing this parameter by value can be expensive.
Consider for example vector<vector<int>>
:
std::vector<int> x(1000); std::vector<std::vector<int>> v; ... v.resize(v.size()+1, x);
In the pass-by-value case, x
is copied once to the parameter of
resize
. And then internally, since the code can not know at compile
time by how much resize
is growing the vector
, x
is
usually copied (not moved) a second time from resize
's parameter into its proper place
within the vector
.
With pass-by-const-reference, the x
in the above example need be copied
only once. In this case, x
has an expensive copy constructor and so any
copies that can be saved represents a significant savings.
If we can be efficient for push_back
, we should be efficient for resize
as well. The resize taking a reference parameter has been coded and shipped in the
CodeWarrior library with no reports of problems which I am aware of.
Proposed resolution:
Change 23.3.5 [deque], p2:
class deque { ... void resize(size_type sz, const T& c);
Change 23.3.5.3 [deque.capacity], p3:
void resize(size_type sz, const T& c);
Change 23.3.11 [list], p2:
class list { ... void resize(size_type sz, const T& c);
Change 23.3.11.3 [list.capacity], p3:
void resize(size_type sz, const T& c);
Change 23.3.13 [vector], p2:
class vector { ... void resize(size_type sz, const T& c);
Change 23.3.13.3 [vector.capacity], p11:
void resize(size_type sz, const T& c);
operator->
returnSection: 24.5.4.2 [move.iterator] Status: CD1 Submitter: Howard Hinnant Opened: 2007-06-11 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
move_iterator
's operator->
return type pointer
does not consistently match the type which is returned in the description
in [move.iter.op.ref].
template <class Iterator> class move_iterator { public: ... typedef typename iterator_traits<Iterator>::pointer pointer; ... pointer operator->() const {return current;} ... private: Iterator current; // exposition only };
There are two possible fixes.
pointer operator->() const {return &*current;}
typedef Iterator pointer;
The first solution is the one chosen by reverse_iterator
. A potential
disadvantage of this is it may not work well with iterators which return a
proxy on dereference and that proxy has overloaded operator&()
. Proxy
references often need to overloaad operator&()
to return a proxy
pointer. That proxy pointer may or may not be the same type as the iterator's
pointer
type.
By simply returning the Iterator
and taking advantage of the fact that
the language forwards calls to operator->
automatically until it
finds a non-class type, the second solution avoids the issue of an overloaded
operator&()
entirely.
Proposed resolution:
Change the synopsis in 24.5.4.2 [move.iterator]:
typedeftypename iterator_traits<Iterator>::pointerpointer;
Section: 28.6.8.3 [re.submatch.op] Status: CD1 Submitter: Nozomu Katoo Opened: 2007-05-27 Last modified: 2016-01-28
Priority: Not Prioritized
View all other issues in [re.submatch.op].
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Discussion:
In 28.6.8.3 [re.submatch.op] of N2284, operator functions numbered 31-42 seem impossible to compare. E.g.:
template <class BiIter> bool operator==(typename iterator_traits<BiIter>::value_type const& lhs, const sub_match<BiIter>& rhs);-31- Returns:
lhs == rhs.str()
.
When char*
is used as BiIter
, iterator_traits<BiIter>::value_type
would be
char
, so that lhs == rhs.str()
ends up comparing a char
value and an object
of std::basic_string<char>
. However, the behaviour of comparison between
these two types is not defined in 27.4.4 [string.nonmembers] of N2284.
This applies when wchar_t*
is used as BiIter
.
Proposed resolution:
Adopt the proposed resolution in N2409.
[ Kona (2007): The LWG adopted the proposed resolution of N2409 for this issue. The LWG voted to accelerate this issue to Ready status to be voted into the WP at Kona. ]
Section: 28.6.7.2 [re.regex.construct] Status: CD1 Submitter: Eric Niebler Opened: 2007-06-03 Last modified: 2016-01-28
Priority: Not Prioritized
View other active issues in [re.regex.construct].
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Discussion:
Looking at N2284, 28.6.7 [re.regex], p3 basic_regex
class template synopsis shows this
constructor:
template <class InputIterator> basic_regex(InputIterator first, InputIterator last, flag_type f = regex_constants::ECMAScript);
In 28.6.7.2 [re.regex.construct], p15, the constructor appears with this signature:
template <class ForwardIterator> basic_regex(ForwardIterator first, ForwardIterator last, flag_type f = regex_constants::ECMAScript);
ForwardIterator
is probably correct, so the synopsis is wrong.
[ John adds: ]
I think either could be implemented? Although an input iterator would probably require an internal copy of the string being made.
I have no strong feelings either way, although I think my original intent was
InputIterator
.
Proposed resolution:
Adopt the proposed resolution in N2409.
[ Kona (2007): The LWG adopted the proposed resolution of N2409 for this issue. The LWG voted to accelerate this issue to Ready status to be voted into the WP at Kona. ]
Section: 24.5.1.9 [reverse.iter.nonmember], 24.5.4.9 [move.iter.nonmember] Status: CD1 Submitter: Bo Persson Opened: 2007-06-10 Last modified: 2021-06-06
Priority: Not Prioritized
View all issues with CD1 status.
Discussion:
In C++03 the difference between two reverse_iterators
ri1 - ri2
is possible to compute only if both iterators have the same base
iterator. The result type is the difference_type
of the base iterator.
In the current draft, the operator is defined as [reverse.iter.opdiff]
template<class Iterator1, class Iterator2> typename reverse_iterator<Iterator>::difference_type operator-(const reverse_iterator<Iterator1>& x, const reverse_iterator<Iterator2>& y);
The return type is the same as the C++03 one, based on the no longer
present Iterator
template parameter.
Besides being slightly invalid, should this operator work only when
Iterator1
and Iterator2
has the same difference_type
? Or should the
implementation choose one of them? Which one?
The same problem now also appears in operator-()
for move_iterator
24.5.4.9 [move.iter.nonmember].
Proposed resolution:
Change the synopsis in 24.5.1.2 [reverse.iterator]:
template <class Iterator1, class Iterator2>typename reverse_iterator<Iterator>::difference_typeauto operator-( const reverse_iterator<Iterator1>& x, const reverse_iterator<Iterator2>& y) -> decltype(y.current - x.current);
Change [reverse.iter.opdiff]:
template <class Iterator1, class Iterator2>typename reverse_iterator<Iterator>::difference_typeauto operator-( const reverse_iterator<Iterator1>& x, const reverse_iterator<Iterator2>& y) -> decltype(y.current - x.current);Returns:
y.current - x.current
.
Change the synopsis in 24.5.4.2 [move.iterator]:
template <class Iterator1, class Iterator2>typename move_iterator<Iterator>::difference_typeauto operator-( const move_iterator<Iterator1>& x, const move_iterator<Iterator2>& y) -> decltype(x.base() - y.base());
Change 24.5.4.9 [move.iter.nonmember]:
template <class Iterator1, class Iterator2>typename move_iterator<Iterator>::difference_typeauto operator-( const move_iterator<Iterator1>& x, const move_iterator<Iterator2>& y) -> decltype(x.base() - y.base());Returns:
x.base() - y.base()
.
[
Pre Bellevue: This issue needs to wait until the auto -> return
language feature
goes in.
]
Section: 20.3.2.2.2 [util.smartptr.shared.const], 20.3.2.3.2 [util.smartptr.weak.const] Status: CD1 Submitter: Peter Dimov Opened: 2007-05-10 Last modified: 2016-01-28
Priority: Not Prioritized
View other active issues in [util.smartptr.shared.const].
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Discussion:
Since all conversions from shared_ptr<T>
to shared_ptr<U>
have the same
rank regardless of the relationship between T
and U
, reasonable user
code that works with raw pointers fails with shared_ptr
:
void f( shared_ptr<void> ); void f( shared_ptr<int> ); int main() { f( shared_ptr<double>() ); // ambiguous }
Now that we officially have enable_if
, we can constrain the constructor
and the corresponding assignment operator to only participate in the
overload resolution when the pointer types are compatible.
Proposed resolution:
In 20.3.2.2.2 [util.smartptr.shared.const], change:
-14- Requires:
For the second constructorThe second constructor shall not participate in the overload resolution unlessY*
shall beis implicitly convertible toT*
.
In 20.3.2.3.2 [util.smartptr.weak.const], change:
template<class Y> weak_ptr(shared_ptr<Y> const& r);weak_ptr(weak_ptr const& r);template<class Y> weak_ptr(weak_ptr<Y> const& r);weak_ptr(weak_ptr const& r); template<class Y> weak_ptr(weak_ptr<Y> const& r); template<class Y> weak_ptr(shared_ptr<Y> const& r);-4- Requires:
FortThe second and third constructors,shall not participate in the overload resolution unlessY*
shall beis implicitly convertible toT*
.
Section: 22.10.6.2 [refwrap.const] Status: C++11 Submitter: Peter Dimov Opened: 2007-05-10 Last modified: 2016-01-28
Priority: Not Prioritized
View all other issues in [refwrap.const].
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Discussion:
A reference_wrapper
can be constructed from an rvalue, either by using
the constructor, or via cref
(and ref
in some corner cases). This leads
to a dangling reference being stored into the reference_wrapper
object.
Now that we have a mechanism to detect an rvalue, we can fix them to
disallow this source of undefined behavior.
Also please see the thread starting at c++std-lib-17398 for some good discussion on this subject.
[ 2009-05-09 Alisdair adds: ]
Now that
ref/cref
are constained thatT
must be anObjectType
, I do not believe there is any risk of bindingref
to a temporary (which would rely on deducingT
to be an rvalue reference type)However, the problem for
cref
remains, so I recommend retaining that deleted overload.
[ 2009-05-10 Howard adds: ]
Without:
template <class T> void ref(const T&& t) = delete;I believe this program will compile:
#include <functional> struct A {}; const A source() {return A();} int main() { std::reference_wrapper<const A> r = std::ref(source()); }I.e. in:
template <ObjectType T> reference_wrapper<T> ref(T& t);this:
ref(source())deduces
T
asconst A
, and so:ref(const A& t)will bind to a temporary (tested with a pre-concepts rvalue-ref enabled compiler).
Therefore I think we still need the ref-protection. I respectfully disagree with Alisdair's comment and am in favor of the proposed wording as it stands. Also, CWG 606 (noted below) has now been "favorably" resolved.
[ Batavia (2009-05): ]
We agree with the proposed resolution. Move to Tentatively Ready.
Proposed resolution:
In 22.10 [function.objects], add the following two signatures to the synopsis:
template <class T> void ref(const T&& t) = delete; template <class T> void cref(const T&& t) = delete;
[ N2292 addresses the first part of the resolution but not the second. ]
[ Bellevue: Doug noticed problems with the current wording. ]
[ post Bellevue: Howard and Peter provided revised wording. ]
[ This resolution depends on a "favorable" resolution of CWG 606: that is, the "special deduction rule" is disabled with the const T&& pattern. ]
Section: 22.10.6.2 [refwrap.const] Status: CD1 Submitter: Peter Dimov Opened: 2007-05-10 Last modified: 2016-01-28
Priority: Not Prioritized
View all other issues in [refwrap.const].
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Discussion:
The constructor of reference_wrapper
is currently explicit
. The primary
motivation behind this is the safety problem with respect to rvalues,
which is addressed by the proposed resolution of the previous issue.
Therefore we should consider relaxing the requirements on the
constructor since requests for the implicit conversion keep resurfacing.
Also please see the thread starting at c++std-lib-17398 for some good discussion on this subject.
Proposed resolution:
Remove the explicit
from the constructor of reference_wrapper
. If the
proposed resolution of the previous issue is accepted, remove the
explicit
from the T&&
constructor as well to keep them in sync.
Section: 23.5 [unord], 99 [tr.hash] Status: CD1 Submitter: Joaquín M López Muñoz Opened: 2007-06-14 Last modified: 2016-01-28
Priority: Not Prioritized
View all other issues in [unord].
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Discussion:
The last version of TR1 does not include the following member functions for unordered containers:
const_local_iterator cbegin(size_type n) const; const_local_iterator cend(size_type n) const;
which looks like an oversight to me. I've checked th TR1 issues lists and the latest working draft of the C++0x std (N2284) and haven't found any mention to these menfuns or to their absence.
Is this really an oversight, or am I missing something?
Proposed resolution:
Add the following two rows to table 93 (unordered associative container requirements) in section 23.2.8 [unord.req]:
Unordered associative container requirements (in addition to container) expression return type assertion/note pre/post-condition complexity b.cbegin(n)
const_local_iterator
n
shall be in the range[0, bucket_count())
. Note:[b.cbegin(n), b.cend(n))
is a valid range containing all of the elements in then
th bucket.Constant b.cend(n)
const_local_iterator
n
shall be in the range[0, bucket_count())
.Constant
Add to the synopsis in 23.5.3 [unord.map]:
const_local_iterator cbegin(size_type n) const; const_local_iterator cend(size_type n) const;
Add to the synopsis in 23.5.4 [unord.multimap]:
const_local_iterator cbegin(size_type n) const; const_local_iterator cend(size_type n) const;
Add to the synopsis in 23.5.6 [unord.set]:
const_local_iterator cbegin(size_type n) const; const_local_iterator cend(size_type n) const;
Add to the synopsis in 23.5.7 [unord.multiset]:
const_local_iterator cbegin(size_type n) const; const_local_iterator cend(size_type n) const;
get_money
and put_money
should be formatted I/O functionsSection: 31.7.8 [ext.manip] Status: CD1 Submitter: Martin Sebor Opened: 2007-06-22 Last modified: 2016-01-28
Priority: Not Prioritized
View all other issues in [ext.manip].
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Discussion:
In a private email Bill Plauger notes:
I believe that the function that implements
get_money
[from N2072] should behave as a formatted input function, and the function that implementsput_money
should behave as a formatted output function. This has implications regarding the skipping of whitespace and the handling of errors, among other things.The words don't say that right now and I'm far from convinced that such a change is editorial.
Martin's response:
I agree that the manipulators should handle exceptions the same way as formatted I/O functions do. The text in N2072 assumes so but the Returns clause explicitly omits exception handling for the sake of brevity. The spec should be clarified to that effect.
As for dealing with whitespace, I also agree it would make sense for the extractors and inserters involving the new manipulators to treat it the same way as formatted I/O.
Proposed resolution:
Add a new paragraph immediately above p4 of 31.7.8 [ext.manip] with the following text:
Effects: The expression
in >> get_money(mon, intl)
described below behaves as a formatted input function (as described in 31.7.5.3.1 [istream.formatted.reqmts]).
Also change p4 of 31.7.8 [ext.manip] as follows:
Returns: An object
s
of unspecified type such that ifin
is an object of typebasic_istream<charT, traits>
then the expressionin >> get_money(mon, intl)
behaves as a formatted input function that callsf(in, mon, intl)
were called. The functionf
can be defined as...
[ post Bellevue: ]
We recommend moving immediately to Review. We've looked at the issue and have a consensus that the proposed resolution is correct, but want an iostream expert to sign off. Alisdair has taken the action item to putt this up on the reflector for possible movement by Howard to Tenatively Ready.
std::bitset::all()
missingSection: 22.9.2 [template.bitset] Status: CD1 Submitter: Martin Sebor Opened: 2007-06-22 Last modified: 2016-01-28
Priority: Not Prioritized
View other active issues in [template.bitset].
View all other issues in [template.bitset].
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Discussion:
The bitset
class template provides the member function
any()
to determine whether an object of the type has any
bits set, and the member function none()
to determine
whether all of an object's bits are clear. However, the template does
not provide a corresponding function to discover whether a
bitset
object has all its bits set. While it is
possible, even easy, to obtain this information by comparing the
result of count()
with the result of size()
for equality (i.e., via b.count() == b.size()
) the
operation is less efficient than a member function designed
specifically for that purpose could be. (count()
must
count all non-zero bits in a bitset
a word at a time
while all()
could stop counting as soon as it encountered
the first word with a zero bit).
Proposed resolution:
Add a declaration of the new member function all()
to the
defintion of the bitset
template in 22.9.2 [template.bitset], p1,
right above the declaration of any()
as shown below:
bool operator!=(const bitset<N>& rhs) const; bool test(size_t pos) const; bool all() const; bool any() const; bool none() const;
Add a description of the new member function to the end of 22.9.2.3 [bitset.members] with the following text:
bool all() const;
Returns:
count() == size()
.
In addition, change the description of any()
and
none()
for consistency with all()
as
follows:
bool any() const;
Returns:
true
if any bit in*this
is onecount() != 0
.
bool none() const;
Returns:
true
if no bit in*this
is onecount() == 0
.
std::bitset
and long long
Section: 22.9.2 [template.bitset] Status: CD1 Submitter: Martin Sebor Opened: 2007-06-22 Last modified: 2016-01-28
Priority: Not Prioritized
View other active issues in [template.bitset].
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Discussion:
Objects of the bitset
class template specializations can
be constructed from and explicitly converted to values of the widest
C++ integer type, unsigned long
. With the introduction
of long long
into the language the template should be
enhanced to make it possible to interoperate with values of this type
as well, or perhaps uintmax_t
. See c++std-lib-18274 for
a brief discussion in support of this change.
Proposed resolution:
For simplicity, instead of adding overloads for unsigned long
long
and dealing with possible ambiguities in the spec, replace
the bitset
ctor that takes an unsigned long
argument with one taking unsigned long long
in the
definition of the template as shown below. (The standard permits
implementations to add overloads on other integer types or employ
template tricks to achieve the same effect provided they don't cause
ambiguities or changes in behavior.)
// [bitset.cons] constructors: bitset(); bitset(unsigned long long val); template<class charT, class traits, class Allocator> explicit bitset( const basic_string<charT,traits,Allocator>& str, typename basic_string<charT,traits,Allocator>::size_type pos = 0, typename basic_string<charT,traits,Allocator>::size_type n = basic_string<charT,traits,Allocator>::npos);
Make a corresponding change in 22.9.2.2 [bitset.cons], p2:
bitset(unsigned long long val);
Effects: Constructs an object of class bitset<N>, initializing the first
M
bit positions to the corresponding bit values inval
.M
is the smaller ofN
and the number of bits in the value representation (section [basic.types]) ofunsigned long long
. IfM < N
istrue
, the remaining bit positions are initialized to zero.
Additionally, introduce a new member function to_ullong()
to make it possible to convert bitset
to values of the
new type. Add the following declaration to the definition of the
template, immediate after the declaration of to_ulong()
in 22.9.2 [template.bitset], p1, as shown below:
// element access: bool operator[](size_t pos) const; // for b[i]; reference operator[](size_t pos); // for b[i]; unsigned long to_ulong() const; unsigned long long to_ullong() const; template <class charT, class traits, class Allocator> basic_string<charT, traits, Allocator> to_string() const;
And add a description of the new member function to 22.9.2.3 [bitset.members],
below the description of the existing to_ulong()
(if
possible), with the following text:
unsigned long long to_ullong() const;
Throws:
overflow_error
if the integral valuex
corresponding to the bits in*this
cannot be represented as typeunsigned long long
.Returns:
x
.
Section: 28.3.4.2.4 [facet.ctype.special] Status: CD1 Submitter: Martin Sebor Opened: 2007-06-22 Last modified: 2016-01-28
Priority: Not Prioritized
View all issues with CD1 status.
Discussion:
The ctype<char>::classic_table()
static member
function returns a pointer to an array of const
ctype_base::mask
objects (enums) that contains
ctype<char>::table_size
elements. The table
describes the properties of the character set in the "C" locale (i.e.,
whether a character at an index given by its value is alpha, digit,
punct, etc.), and is typically used to initialize the
ctype<char>
facet in the classic "C" locale (the
protected ctype<char>
member function
table()
then returns the same value as
classic_table()
).
However, while ctype<char>::table_size
(the size of
the table) is a public static const member of the
ctype<char>
specialization, the
classic_table()
static member function is protected. That
makes getting at the classic data less than convenient (i.e., one has
to create a whole derived class just to get at the masks array). It
makes little sense to expose the size of the table in the public
interface while making the table itself protected, especially when the
table is a constant object.
The same argument can be made for the non-static protected member
function table()
.
Proposed resolution:
Make the ctype<char>::classic_table()
and
ctype<char>::table()
member functions public by
moving their declarations into the public section of the definition of
specialization in 28.3.4.2.4 [facet.ctype.special] as shown below:
static locale::id id; static const size_t table_size = IMPLEMENTATION_DEFINED;protected:const mask* table() const throw(); static const mask* classic_table() throw(); protected: ~ctype(); // virtual virtual char do_toupper(char c) const;
istream::operator>>(int&)
brokenSection: 31.7.5.3.2 [istream.formatted.arithmetic] Status: C++11 Submitter: Martin Sebor Opened: 2007-06-23 Last modified: 2016-01-28
Priority: Not Prioritized
View all other issues in [istream.formatted.arithmetic].
View all issues with C++11 status.
Discussion:
From message c++std-lib-17897:
The code shown in 31.7.5.3.2 [istream.formatted.arithmetic] as the "as if"
implementation of the two arithmetic extractors that don't have a
corresponding num_get
interface (i.e., the
short
and int
overloads) is subtly buggy in
how it deals with EOF
, overflow, and other similar
conditions (in addition to containing a few typos).
One problem is that if num_get::get()
reaches the EOF
after reading in an otherwise valid value that exceeds the limits of
the narrower type (but not LONG_MIN
or
LONG_MAX
), it will set err
to
eofbit
. Because of the if condition testing for
(err == 0)
, the extractor won't set
failbit
(and presumably, return a bogus value to the
caller).
Another problem with the code is that it never actually sets the
argument to the extracted value. It can't happen after the call to
setstate()
since the function may throw, so we need to
show when and how it's done (we can't just punt as say: "it happens
afterwards"). However, it turns out that showing how it's done isn't
quite so easy since the argument is normally left unchanged by the
facet on error except when the error is due to a misplaced thousands
separator, which causes failbit
to be set but doesn't
prevent the facet from storing the value.
[ Batavia (2009-05): ]
We believe this part of the Standard has been recently adjusted and that this issue was addressed during that rewrite.
Move to NAD.
[ 2009-05-28 Howard adds: ]
I've moved this issue from Tentatively NAD to Open.
The current wording of N2857 in 28.3.4.3.2.3 [facet.num.get.virtuals] p3, stage 3 appears to indicate that in parsing arithmetic types, the value is always set, but sometimes in addition to setting
failbit
.
- If there is a range error, the value is set to min or max, else
- if there is a conversion error, the value is set to 0, else
- if there is a grouping error, the value is set to whatever it would be if grouping were ignored, else
- the value is set to its error-free result.
However there is a contradictory sentence in 28.3.4.3.2.3 [facet.num.get.virtuals] p1.
31.7.5.3.2 [istream.formatted.arithmetic] should mimic the behavior of 28.3.4.3.2.3 [facet.num.get.virtuals] (whatever we decide that behavior is) for
int
andshort
, and currently does not. I believe that the correct code fragment should look like:typedef num_get<charT,istreambuf_iterator<charT,traits> > numget; iostate err = ios_base::goodbit; long lval; use_facet<numget>(loc).get(*this, 0, *this, err, lval); if (lval < numeric_limits<int>::min()) { err |= ios_base::failbit; val = numeric_limits<int>::min(); } else if (lval > numeric_limits<int>::max()) { err |= ios_base::failbit; val = numeric_limits<int>::max(); } else val = static_cast<int>(lval); setstate(err);
[ 2009-07 Frankfurt ]
Move to Ready.
Proposed resolution:
Change 28.3.4.3.2.3 [facet.num.get.virtuals], p1:
-1- Effects: Reads characters from
in
, interpreting them according tostr.flags()
,use_facet<ctype<charT> >(loc)
, anduse_facet< numpunct<charT> >(loc)
, whereloc
isstr.getloc()
.If an error occurs,val
is unchanged; otherwise it is set to the resulting value.
Change 31.7.5.3.2 [istream.formatted.arithmetic], p2 and p3:
operator>>(short& val);-2- The conversion occurs as if performed by the following code fragment (using the same notation as for the preceding code fragment):
typedef num_get<charT,istreambuf_iterator<charT,traits> > numget; iostate err = iostate_base::goodbit; long lval; use_facet<numget>(loc).get(*this, 0, *this, err, lval);if (err != 0) ; else if (lval < numeric_limits<short>::min() || numeric_limits<short>::max() < lval) err = ios_base::failbit;if (lval < numeric_limits<short>::min()) { err |= ios_base::failbit; val = numeric_limits<short>::min(); } else if (lval > numeric_limits<short>::max()) { err |= ios_base::failbit; val = numeric_limits<short>::max(); } else val = static_cast<short>(lval); setstate(err);operator>>(int& val);-3- The conversion occurs as if performed by the following code fragment (using the same notation as for the preceding code fragment):
typedef num_get<charT,istreambuf_iterator<charT,traits> > numget; iostate err = iostate_base::goodbit; long lval; use_facet<numget>(loc).get(*this, 0, *this, err, lval);if (err != 0) ; else if (lval < numeric_limits<int>::min() || numeric_limits<int>::max() < lval) err = ios_base::failbit;if (lval < numeric_limits<int>::min()) { err |= ios_base::failbit; val = numeric_limits<int>::min(); } else if (lval > numeric_limits<int>::max()) { err |= ios_base::failbit; val = numeric_limits<int>::max(); } else val = static_cast<int>(lval); setstate(err);
<system_error>
header leads to name clashesSection: 19.5 [syserr] Status: Resolved Submitter: Daniel Krügler Opened: 2007-06-24 Last modified: 2016-01-28
Priority: Not Prioritized
View all other issues in [syserr].
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Discussion:
The most recent state of
N2241
as well as the current draft
N2284
(section 19.5 [syserr], p.2) proposes a
new
enumeration type posix_errno
immediatly in the namespace std
. One of
the enumerators has the name invalid_argument
, or fully qualified:
std::invalid_argument
. This name clashes with the exception type
std::invalid_argument
, see 19.2 [std.exceptions]/p.3. This clash makes
e.g. the following snippet invalid:
#include <system_error> #include <stdexcept> void foo() { throw std::invalid_argument("Don't call us - we call you!"); }
I propose that this enumeration type (and probably the remaining parts
of
<system_error>
as well) should be moved into one additional inner
namespace, e.g. sys
or system
to reduce foreseeable future clashes
due
to the great number of members that std::posix_errno
already contains
(Btw.: Why has the already proposed std::sys
sub-namespace from
N2066
been rejected?). A further clash candidate seems to be
std::protocol_error
(a reasonable name for an exception related to a std network library,
I guess).
Another possible resolution would rely on the proposed strongly typed enums, as described in N2213. But maybe the forbidden implicit conversion to integral types would make these enumerators less attractive in this special case?
Proposed resolution:
Fixed by issue 7 of N2422.
system_error
needs const char*
constructorsSection: 19.5.8.1 [syserr.syserr.overview] Status: CD1 Submitter: Daniel Krügler Opened: 2007-06-24 Last modified: 2016-01-28
Priority: Not Prioritized
View all issues with CD1 status.
Discussion:
In 19.5.8.1 [syserr.syserr.overview] we have the class definition of
std::system_error
. In contrast to all exception classes, which
are constructible with a what_arg string
(see 19.2 [std.exceptions],
or ios_base::failure
in [ios::failure]), only overloads with with
const string&
are possible. For consistency with the re-designed
remaining exception classes this class should also provide
c'tors which accept a const char* what_arg
string.
Please note that this proposed addition makes sense even
considering the given implementation hint for what()
, because
what_arg
is required to be set as what_arg
of the base class
runtime_error
, which now has the additional c'tor overload
accepting a const char*
.
Proposed resolution:
This proposed wording assumes issue 832(i) has been accepted and applied to the working paper.
Change 19.5.8.1 [syserr.syserr.overview] Class system_error overview, as indicated:
public: system_error(error_code ec, const string& what_arg); system_error(error_code ec, const char* what_arg); system_error(error_code ec); system_error(int ev, const error_category* ecat, const string& what_arg); system_error(int ev, const error_category* ecat, const char* what_arg); system_error(int ev, const error_category* ecat);
To 19.5.8.2 [syserr.syserr.members] Class system_error members add:
system_error(error_code ec, const char* what_arg);Effects: Constructs an object of class
system_error
.Postconditions:
code() == ec
andstrcmp(runtime_error::what(), what_arg) == 0
.system_error(int ev, const error_category* ecat, const char* what_arg);Effects: Constructs an object of class
system_error
.Postconditions:
code() == error_code(ev, ecat)
andstrcmp(runtime_error::what(), what_arg) == 0
.
Section: 29.5 [rand] Status: CD1 Submitter: P.J. Plauger Opened: 2007-07-01 Last modified: 2016-01-28
Priority: Not Prioritized
View all other issues in [rand].
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Discussion:
N2111
changes min/max
in several places in random from member
functions to static data members. I believe this introduces
a needless backward compatibility problem between C++0X and
TR1. I'd like us to find new names for the static data members,
or perhaps change min/max
to constexpr
s in C++0X.
See N2391 and N2423 for some further discussion.
Proposed resolution:
Adopt the proposed resolution in N2423.
[ Kona (2007): The LWG adopted the proposed resolution of N2423 for this issue. The LWG voted to accelerate this issue to Ready status to be voted into the WP at Kona. ]
identity
Section: 22.2.4 [forward] Status: CD1 Submitter: P.J. Plauger Opened: 2007-07-01 Last modified: 2016-01-28
Priority: Not Prioritized
View all other issues in [forward].
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Discussion:
N1856
defines struct identity
in <utility>
which clashes with
the traditional definition of struct identity
in <functional>
(not standard, but a common extension from old STL). Be nice
if we could avoid this name clash for backward compatibility.
Proposed resolution:
Change 22.2.4 [forward]:
template <class T> struct identity { typedef T type; const T& operator()(const T& x) const; };const T& operator()(const T& x) const;Returns:
x
.
map::at()
need a complexity specificationSection: 23.4.3.3 [map.access] Status: CD1 Submitter: Joe Gottman Opened: 2007-07-03 Last modified: 2016-01-28
Priority: Not Prioritized
View all other issues in [map.access].
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Discussion:
map::at()
need a complexity specification.
Proposed resolution:
Add the following to the specification of map::at()
, 23.4.3.3 [map.access]:
Complexity: logarithmic.
MoveAssignable
requirement for container value type overly strictSection: 23.2 [container.requirements] Status: C++11 Submitter: Howard Hinnant Opened: 2007-05-20 Last modified: 2016-01-28
Priority: Not Prioritized
View all other issues in [container.requirements].
View all issues with C++11 status.
Discussion:
The move-related changes inadvertently overwrote the intent of 276(i).
Issue 276(i) removed the requirement of CopyAssignable
from
most of the member functions of node-based containers. But the move-related changes
unnecessarily introduced the MoveAssignable
requirement for those members which used to
require CopyAssignable
.
We also discussed (c++std-lib-18722) the possibility of dropping MoveAssignable
from some of the sequence requirements. Additionally the in-place construction
work may further reduce requirements. For purposes of an easy reference, here are the
minimum sequence requirements as I currently understand them. Those items in requirements
table in the working draft which do not appear below have been purposefully omitted for
brevity as they do not have any requirements of this nature. Some items which do not
have any requirements of this nature are included below just to confirm that they were
not omitted by mistake.
X u(a) | value_type must be CopyConstructible |
X u(rv) | array requires value_type to be CopyConstructible |
a = u | Sequences require value_type to be CopyConstructible and CopyAssignable .
Associative containers require value_type to be CopyConstructible . |
a = rv | array requires value_type to be CopyAssignable .
Sequences containers with propagate_on_container_move_assignment == false allocators require value_type to be MoveConstructible and MoveAssignable .
Associative containers with propagate_on_container_move_assignment == false allocators require value_type to be MoveConstructible . |
swap(a,u) | array requires value_type to be Swappable . |
X(n) | value_type must be DefaultConstructible |
X(n, t) | value_type must be CopyConstructible |
X(i, j) | Sequences require value_type to be constructible from *i . Additionally if input_iterators
are used, vector and deque require MoveContructible and MoveAssignable . |
a.insert(p, t) | The value_type must be CopyConstructible .
The sequences vector and deque also require the value_type to be CopyAssignable . |
a.insert(p, rv) | The value_type must be MoveConstructible .
The sequences vector and deque also require the value_type to be MoveAssignable . |
a.insert(p, n, t) | The value_type must be CopyConstructible .
The sequences vector and deque also require the value_type to be CopyAssignable . |
a.insert(p, i, j) | If the iterators return an lvalue the value_type must be CopyConstructible .
The sequences vector and deque also require the value_type to be CopyAssignable when the iterators return an lvalue.
If the iterators return an rvalue the value_type must be MoveConstructible .
The sequences vector and deque also require the value_type to be MoveAssignable when the iterators return an rvalue. |
a.erase(p) | The sequences vector and deque require the value_type to be MoveAssignable . |
a.erase(q1, q2) | The sequences vector and deque require the value_type to be MoveAssignable . |
a.clear() | |
a.assign(i, j) | If the iterators return an lvalue the value_type must be CopyConstructible and CopyAssignable .
If the iterators return an rvalue the value_type must be MoveConstructible and MoveAssignable . |
a.assign(n, t) | The value_type must be CopyConstructible and CopyAssignable . |
a.resize(n) | The value_type must be DefaultConstructible .
The sequence vector also requires the value_type to be MoveConstructible . |
a.resize(n, t) | The value_type must be CopyConstructible . |
a.front() | |
a.back() | |
a.push_front(t) | The value_type must be CopyConstructible . |
a.push_front(rv) | The value_type must be MoveConstructible . |
a.push_back(t) | The value_type must be CopyConstructible . |
a.push_back(rv) | The value_type must be MoveConstructible . |
a.pop_front() | |
a.pop_back() | |
a[n] | |
a.at[n] |
X(i, j) | If the iterators return an lvalue the value_type must be CopyConstructible .
If the iterators return an rvalue the value_type must be MoveConstructible . |
a_uniq.insert(t) | The value_type must be CopyConstructible . |
a_uniq.insert(rv) | The key_type and the mapped_type (if it exists) must be MoveConstructible . |
a_eq.insert(t) | The value_type must be CopyConstructible . |
a_eq.insert(rv) | The key_type and the mapped_type (if it exists) must be MoveConstructible . |
a.insert(p, t) | The value_type must be CopyConstructible . |
a.insert(p, rv) | The key_type and the mapped_type (if it exists) must be MoveConstructible . |
a.insert(i, j) | If the iterators return an lvalue the value_type must be CopyConstructible .
If the iterators return an rvalue the key_type and the mapped_type (if it exists) must be MoveConstructible .. |
X(i, j, n, hf, eq) | If the iterators return an lvalue the value_type must be CopyConstructible .
If the iterators return an rvalue the value_type must be MoveConstructible . |
a_uniq.insert(t) | The value_type must be CopyConstructible . |
a_uniq.insert(rv) | The key_type and the mapped_type (if it exists) must be MoveConstructible . |
a_eq.insert(t) | The value_type must be CopyConstructible . |
a_eq.insert(rv) | The key_type and the mapped_type (if it exists) must be MoveConstructible . |
a.insert(p, t) | The value_type must be CopyConstructible . |
a.insert(p, rv) | The key_type and the mapped_type (if it exists) must be MoveConstructible . |
a.insert(i, j) | If the iterators return an lvalue the value_type must be CopyConstructible .
If the iterators return an rvalue the key_type and the mapped_type (if it exists) must be MoveConstructible .. |
map[lvalue-key] | The key_type must be CopyConstructible .
The mapped_type must be DefaultConstructible and MoveConstructible . |
map[rvalue-key] | The key_type must be MoveConstructible .
The mapped_type must be DefaultConstructible and MoveConstructible . |
[ Kona (2007): Howard and Alan to update requirements table in issue with emplace signatures. ]
[ Bellevue: This should be handled as part of the concepts work. ]
[ 2009-07-20 Reopened by Howard: ]
This is one of the issues that was "solved by concepts" and is now no longer solved.
In a nutshell, concepts adopted the "minimum requirements" philosophy outlined in the discussion of this issue, and enforced it. My strong suggestion is that we translate the concepts specification into documentation for the containers.
What this means for vendors is that they will have to implement container members being careful to only use those characteristics of a type that the concepts specification formally allowed. Note that I am not talking about
enable_if
'ing everything. I am simply suggesting that (for example) we tell the vendor he can't callT's
copy constructor or move constructor within theemplace
member function, etc.What this means for customers is that they will be able to use types within C++03 containers which are sometimes not CopyConstructible, and sometimes not even MoveConstructible, etc.
[ 2009-10 Santa Cruz: ]
Leave open. Howard to provide wording.
[ 2010-02-06 Howard provides wording. ]
[ 2010-02-08 Moved to Tentatively Ready after 5 positive votes on c++std-lib. ]
[ 2010-02-10 Howard opened. I neglected to reduce the requirements on value_type for the insert function of the ordered and unordered associative containers when the argument is an rvalue. Fixed it. ]
[ 2010-02-11 Moved to Tentatively Ready after 5 positive votes on c++std-lib. ]
[ 2010-03-08 Nico opens: ]
I took the task to see whether 868(i) is covered by 704 already. However, by doing that I have the impression that 704 is a big mistake.
Take e.g. the second change of 868(i):
Change 23.3.5.2 [deque.cons] para 5:
Effects: Constructs a
deque
withn
default constructed elements.where "default constructed" should be replaced by "value-initialized". This is the constructor out of a number of elements:
ContType c(num)704 says:
Remove the entire section 23.3.5.2 [deque.cons].
[ This section is already specified by the requirements tables. ]
BUT, there is no requirement table that lists this constructor at all, which means that we would lose the entire specification of this function !!!
In fact, I found with further investigation, if we follow 704 to remove 23.3.2.1 we
- have no semantics for
ContType c(num)
- have no complexity and no allocator specification for
ContType c(num,val)
- have no semantics for
ContType c(num,val,alloc)
- - have no complexity and no allocator specification for
ContType c(beg,end)
- - have no semantics for
ContType c(beg,end,alloc)
- - have different wording (which might or might not give the same guarantees) for the
assign
functionsbecause all these guarantees are given in the removed section but nowhere else (as far as I saw).
Looks to me that 704 need a significant review before we take that change, because chances are high that there are similar flaws in other proposed changes there (provided I am not missing anything).
[ 2010 Pittsburgh: ]
Removed the parts from the proposed wording that removed existing sections, and set to Ready for Pittsburgh.
Rationale:
[ post San Francisco: ]
Solved by N2776.
This rationale is obsolete.
Proposed resolution:
Change 23.2.2 [container.requirements.general]/4:
4 In Tables 91 and 92,
X
denotes a container class containing objects of typeT
,a
andb
denote values of typeX
,u
denotes an identifier,r
denotesan lvalue or a const rvaluea non-const value of typeX
, andrv
denotes a non-const rvalue of typeX
.
Change the following rows in Table 91 — Container requirements 23.2.2 [container.requirements.general]:
Table 91 — Container requirements Expression Return type Assertion/note
pre-/post-conditionComplexity X::value_type
T
Requires: T
isDestructible
.compile time
Change 23.2.2 [container.requirements.general]/10:
Unless otherwise specified (see 23.2.4.1, 23.2.5.1, 23.3.2.3, and 23.3.6.4) all container types defined in this Clause meet the following additional requirements:
…
no
erase()
,clear()
,pop_back()
orpop_front()
function throws an exception.…
Insert a new paragraph prior to 23.2.2 [container.requirements.general]/14:
The descriptions of the requirements of the type
T
in this section use the termsCopyConstructible
,MoveConstructible
, constructible from*i
, and constructible fromargs
. These terms are equivalent to the following expression using the appropriate arguments:allocator_traits<allocator_type>::construct(x.get_allocator(), q, args...);where
x
is a non-const lvalue of some container typeX
andq
has typeX::value_type*
.[Example: The container is going to move construct a
T
, so will call:allocator_traits<allocator_type>::construct(get_allocator(), q, std::move(t));The default implementation of construct will call:
::new (q) T(std::forward<T>(t)); // where forward is the same as move here, cast to rvalueBut the allocator author may override the above definition of
construct
and do the construction ofT
by some other means. — end example]14 ...
Add to 23.2.2 [container.requirements.general]/14:
14 In Table 93,
X
denotes an allocator-aware container class with avalue_type
ofT
using allocator of typeA
,u
denotes a variable,a
andb
denote non-const lvalues of typeX
,t
denotes an lvalue or a const rvalue of typeX
,rv
denotes a non-const rvalue of typeX
,m
is a value of typeA
, andQ
is an allocator type.
Change or add the following rows in Table 93 — Allocator-aware container requirements in 23.2.2 [container.requirements.general]:
Table 93 — Allocator-aware container requirements Expression Return type Assertion/note
pre-/post-conditionComplexity X(t, m)
X u(t, m);Requires: T
isCopyConstructible
.
post:u == t
,
get_allocator() == m
linear X(rv, m)
X u(rv, m);Requires: T
isMoveConstructible
.
post:u
shall have the same elements, or copies of the elements, thatrv
had before this construction,
get_allocator() == m
constant if m == rv.get_allocator()
, otherwise lineara = t
X&
Requires: T
isCopyConstructible
andCopyAssignable
post:a == t
.linear a = rv
X&
Requires: If allocator_traits< allocator_type > ::propagate_on_container_move_assignment ::value
isfalse
,T
isMoveConstructible
andMoveAssignable
.
All existing elements ofa
are either move assigned to or destroyed.
a
shall be equal to the value thatrv
had before this assignmentlinear a.swap(b);
void
exchanges the contents of a
andb
constant
Change the following rows in Table 94 — Sequence container requirements (in addition to container) in 23.2.4 [sequence.reqmts]:
Table 94 — Sequence container requirements (in addition to container) Expression Return type Assertion/note
pre-/post-conditionX(i, j)
X a(i, j)Requires: If the iterator's dereference operation returns an lvalue or a const rvalue,T
shall beCopyConstructible
.T
shall be constructible from*i
.
If the iterator does not meet the forward iterator requirements (24.3.5.5 [forward.iterators]), thenvector
also requiresT
to beMoveConstructible
.
Each iterator in the range[i,j)
shall be dereferenced exactly once.
post:size() ==
distance betweeni
andj
Constructs a sequence container equal to the range[i, j)
a = il;
X&
Requires: T
isCopyConstructible
andCopyAssignable
.
a = X(il);
Assigns the range[il.begin(), il.end())
intoa
. All existing elements ofa
are either assigned or destroyed.
rReturns*this;
a.emplace(p, args);
iterator
Requires: ConstructibleAsElement<A, T, Args>
.T
is constructible fromargs
.vector
anddeque
also requireT
to beMoveConstructible
andMoveAssignable
. Inserts an object of typeT
constructed withstd::forward<Args>(args)...
beforep
.a.insert(p, t);
iterator
Requires: ConstructibleAsElement<A, T, Args>
andT
shall beCopyAssignable
.T
shall beCopyConstructible
.vector
anddeque
also requireT
to beCopyAssignable
. Inserts a copyt
beforep
.a.insert(p, rv);
iterator
Requires: ConstructibleAsElement<A, T, T&&>
andT
shall beMoveAssignable
.T
shall beMoveConstructible
.vector
anddeque
also requireT
to beMoveAssignable
. Inserts a copyrv
beforep
.a.insert(p, i, j)
iterator
Requires: If the iterator's dereference operation returns an lvalue or a const rvalue,T
shall beCopyConstructible
.T
shall be constructible from*i
.
If the iterator does not meet the forward iterator requirements (24.3.5.5 [forward.iterators]), thenvector
also requiresT
to beMoveConstructible
andMoveAssignable
.
Each iterator in the range[i,j)
shall be dereferenced exactly once.
pre:i
andj
are not iterators intoa
.
Inserts copies of elements in[i, j)
beforep
a.erase(q);
iterator
Requires: T
andT
shall beMoveAssignable
.vector
anddeque
requireT
to beMoveAssignable
. Erases the element pointed to byq
.a.erase(q1, q2);
iterator
Requires: T
andT
shall beMoveAssignable
.vector
anddeque
requireT
to beMoveAssignable
. Erases the elements in the range[q1, q2)
.a.clear();
void
erase(begin(), end())
Destroys all elements ina
. Invalidates all references, pointers, and iterators referring to the elements ofa
and may invalidate the past-the-end iterator.
post:
size() == 0a.empty() == truea.assign(i, j)
void
Requires: If the iterator's dereference operation returns an lvalue or a const rvalue,T
shall beCopyConstructible
andCopyAssignable
.T
shall be constructible and assignable from*i
. If the iterator does not meet the forward iterator requirements (24.3.5.5 [forward.iterators]), thenvector
also requiresT
to beMoveConstructible
.
Each iterator in the range[i,j)
shall be dereferenced exactly once.
pre:i
,j
are not iterators intoa
.
Replaces elements ina
with a copy of[i, j)
.
Change the following rows in Table 95 — Optional sequence container operations in 23.2.4 [sequence.reqmts]:
Table 95 — Optional sequence container operations Expression Return type Operational semantics Container a.emplace_front(args)
void
a.emplace(a.begin(), std::forward<Args>(args)...)
Prepends an object of typeT
constructed withstd::forward<Args>(args)...
.
Requires:ConstructibleAsElement<A, T, Args>
T
shall be constructible fromargs
.list
,deque
,forward_list
a.emplace_back(args)
void
a.emplace(a.end(), std::forward<Args>(args)...)
Appends an object of typeT
constructed withstd::forward<Args>(args)...
.
Requires:ConstructibleAsElement<A, T, Args>
T
shall be constructible fromargs
.vector
also requiresT
to beMoveConstructible
.list
,deque
,vector
a.push_front(t)
void
a.insert(a.begin(), t)
Prepends a copy oft
.
Requires:ConstructibleAsElement<A, T, T>
andT
shall beCopyAssignable
.T
shall beCopyConstructible
.list
,deque
,forward_list
a.push_front(rv)
void
a.insert(a.begin(), t)
Prepends a copy ofrv
.
Requires:ConstructibleAsElement<A, T, T&&>
andT
shall beMoveAssignable
.T
shall beMoveConstructible
.list
,deque
,forward_list
a.push_back(t)
void
a.insert(a.end(), t)
Appends a copy oft
.
Requires:ConstructibleAsElement<A, T, T>
andT
shall beCopyAssignable
.T
shall beCopyConstructible
.vector
,list
,deque
,basic_string
a.push_back(rv)
void
a.insert(a.end(), t)
Appends a copy ofrv
.
Requires:ConstructibleAsElement<A, T, T&&>
andT
shall beMoveAssignable
.T
shall beMoveConstructible
.vector
,list
,deque
,basic_string
a.pop_front()
void
a.erase(a.begin())
Destroys the first element.
Requires:a.empty()
shall befalse
.list
,deque
,forward_list
a.pop_back()
void
{ iterator tmp = a.end();
--tmp;
a.erase(tmp); }
Destroys the last element.
Requires:a.empty()
shall befalse
.vector
,list
,deque
,basic_string
Insert a new paragraph prior to 23.2.7 [associative.reqmts]/7, and edit paragraph 7:
The associative containers meet all of the requirements of Allocator-aware containers (23.2.2 [container.requirements.general]), except for the containers
map
andmultimap
, the requirements placed onvalue_type
in Table 93 apply instead directly tokey_type
andmapped_type
. [Note: For examplekey_type
andmapped_type
are sometimes required to beCopyAssignable
even though thevalue_type
(pair<const key_type, mapped_type>
) is notCopyAssignable
. — end note]7 In Table 96,
X
denotes an associative container class, a denotes a value ofX
,a_uniq
denotes a value ofX
whenX
supports unique keys,a_eq
denotes a value ofX
whenX
supports multiple keys,u
denotes an identifier,r
denotes an lvalue or a const rvalue of typeX
,rv
denotes a non-const rvalue of typeX
,i
andj
satisfy input iterator requirements and refer to elements implicitly convertible tovalue_type
,[i,j)
denotes a valid range,p
denotes a valid const iterator toa
,q
denotes a valid dereferenceable const iterator toa
,[q1, q2)
denotes a valid range of const iterators ina
,il
designates an object of typeinitializer_list<value_type>
,t
denotes a value ofX::value_type
,k
denotes a value ofX::key_type
andc
denotes a value of typeX::key_compare
.A
denotes the storage allocator used byX
, if any, orstd::allocator<X::value_type>
otherwise, andm
denotes an allocator of a type convertible toA
.
Change or add the following rows in Table 96 — Associative container requirements (in addition to container) in 23.2.7 [associative.reqmts]:
Table 96 — Associative container requirements (in addition to container) Expression Return type Assertion/note
pre-/post-conditionComplexity X::key_type
Key
Requires: Key
isCopyConstructible
andCopyAssignable
Destructible
compile time X::mapped_type
(map
andmultimap
only)T
Requires: T
isDestructible
compile time X(c)
X a(c);Requires: .ConstructibleAsElement<A, key_compare, key_compare>
key_compare
isCopyConstructible
.
Constructs an empty container.
Uses a copy ofc
as a comparison object.constant X()
X a;Requires: .ConstructibleAsElement<A, key_compare, key_compare>
key_compare
isDefaultConstructible
.
Constructs an empty container.
UsesCompare()
as a comparison object.constant X(i, j, c)
X a(i, j, c);Requires: .ConstructibleAsElement<A, key_compare, key_compare>
key_compare
isCopyConstructible
.value_type
shall be constructible from*i
.
Constructs an empty container ans inserts elements from the range[i, j)
into it; usesc
as a comparison object.N
logN
in general (N
is the distance fromi
toj
); linear if[i, j)
is sorted withvalue_comp()
X(i, j)
X a(i, j);Requires: .ConstructibleAsElement<A, key_compare, key_compare>
value_type
shall be constructible from*i
.key_compare
isDefaultConstructible
.
Same as above, but usesCompare()
as a comparison object.same as above a = il
X&
a = X(il);
return *this;
Requires:T
isCopyConstructible
andCopyAssignable
.
Assigns the range[il.begin(), il.end())
intoa
. All existing elements ofa
are either assigned or destroyed.Same as.
a = X(il)
N
logN
in general (N
isil.size()
added to the existing size ofa
); linear if[il.begin(), il.end())
is sorted withvalue_comp()
a_uniq.emplace(args)
pair<iterator, bool>
Requires: T
shall be constructible fromargs
inserts aT
objectt
constructed withstd::forward<Args>(args)...
if and only if there is no element in the container with key equivalent to the key oft
. Thebool
component of the returned pair is true if and only if the insertion takes place, and the iterator component of the pair points to the element with key equivalent to the key oft
.logarithmic a_eq.emplace(args)
iterator
Requires: T
shall be constructible fromargs
inserts aT
objectt
constructed withstd::forward<Args>(args)...
and returns the iterator pointing to the newly inserted element.logarithmic a_uniq.insert(t)
pair<iterator, bool>
Requires: T
shall beMoveConstructible
ift
is a non-const rvalue expression, elseT
shall beCopyConstructible
.
insertst
if and only if there is no element in the container with key equivalent to the key oft
. Thebool
component of the returned pair is true if and only if the insertion takes place, and the iterator component of the pair points to the element with key equivalent to the key oft
.logarithmic a_eq.insert(t)
iterator
Requires: T
shall beMoveConstructible
ift
is a non-const rvalue expression, elseT
shall beCopyConstructible
.
insertst
and returns the iterator pointing to the newly inserted element. If a range containing elements equivalent tot
exists ina_eq
,t
is inserted at the end of that range.logarithmic a.insert(p, t)
iterator
Requires: T
shall beMoveConstructible
ift
is a non-const rvalue expression, elseT
shall beCopyConstructible
.
insertst
if and only if there is no element with key equivalent to the key oft
in containers with unique keys; always insertst
in containers with equivalent keys; always returns the iterator pointing to the element with key equivalent to the key oft
.t
is inserted as close as possible to the position just prior top
.logarithmic in general, but amortized constant if t
is inserted right beforep
.a.insert(i, j)
void
Requires: T
shall be constructible from*i
.
pre:i
,j
are not iterators intoa
. inserts each element from the range[i,j)
if and only if there is no element with key equivalent to the key of that element in containers with unique keys; always inserts that element in containers with equivalent keys.N log(size() + N ) (N is the distance from i to j)
Insert a new paragraph prior to 23.2.8 [unord.req]/9:
The unordered associative containers meet all of the requirements of Allocator-aware containers (23.2.2 [container.requirements.general]), except for the containers
unordered_map
andunordered_multimap
, the requirements placed onvalue_type
in Table 93 apply instead directly tokey_type
andmapped_type
. [Note: For examplekey_type
andmapped_type
are sometimes required to beCopyAssignable
even though thevalue_type
(pair<const key_type, mapped_type>
) is notCopyAssignable
. — end note]9 ...
Change or add the following rows in Table 98 — Unordered associative container requirements (in addition to container) in 23.2.8 [unord.req]:
Table 98 — Unordered associative container requirements (in addition to container) Expression Return type Assertion/note
pre-/post-conditionComplexity X::key_type
Key
Requires: Key
shall beCopyAssignable
andCopyConstructible
Destructible
compile time X::mapped_type
(unordered_map
andunordered_multimap
only)T
Requires: T
isDestructible
compile time X(n, hf, eq)
X a(n, hf, eq)X
Requires: hasher
andkey_equal
areCopyConstructible
. Constructs an empty container with at leastn
buckets, usinghf
as the hash function andeq
as the key equality predicate.O(N)
X(n, hf)
X a(n, hf)X
Requires: hasher
isCopyConstructible
andkey_equal
isDefaultConstructible
. Constructs an empty container with at leastn
buckets, usinghf
as the hash function andkey_equal()
as the key equality predicate.O(N)
X(n)
X a(n)X
Requires: hasher
andkey_equal
areDefaultConstructible
. Constructs an empty container with at leastn
buckets, usinghasher()
as the hash function andkey_equal()
as the key equality predicate.O(N)
X()
X aX
Requires: hasher
andkey_equal
areDefaultConstructible
. Constructs an empty container an unspecified number of buckets, usinghasher()
as the hash function andkey_equal()
as the key equality predicate.constant X(i, j, n, hf, eq)
X a(i, j, n, hf, eq)X
Requires: value_type
is constructible from*i
.hasher
andkey_equal
areCopyConstructible
.
Constructs an empty container with at leastn
buckets, usinghf
as the hash function andeq
as the key equality predicate, and inserts elements from[i, j)
into it.Average case O(N)
(N
isdistance(i, j)
), worst caseO(N2)
X(i, j, n, hf)
X a(i, j, n, hf)X
Requires: value_type
is constructible from*i
.hasher
isCopyConstructible
andkey_equal
isDefaultConstructible
.
Constructs an empty container with at leastn
buckets, usinghf
as the hash function andkey_equal()
as the key equality predicate, and inserts elements from[i, j)
into it.Average case O(N)
(N
isdistance(i, j)
), worst caseO(N2)
X(i, j, n)
X a(i, j, n)X
Requires: value_type
is constructible from*i
.hasher
andkey_equal
areDefaultConstructible
.
Constructs an empty container with at leastn
buckets, usinghasher()
as the hash function andkey_equal()
as the key equality predicate, and inserts elements from[i, j)
into it.Average case O(N)
(N
isdistance(i, j)
), worst caseO(N2)
X(i, j)
X a(i, j)X
Requires: value_type
is constructible from*i
.hasher
andkey_equal
areDefaultConstructible
.
Constructs an empty container with an unspecified number of buckets, usinghasher()
as the hash function andkey_equal()
as the key equality predicate, and inserts elements from[i, j)
into it.Average case O(N)
(N
isdistance(i, j)
), worst caseO(N2)
X(b)
X a(b)X
Copy constructor. In addition to the contained elementsrequirements of Table 93 (23.2.2 [container.requirements.general]), copies the hash function, predicate, and maximum load factor.Average case linear in b.size()
, worst case quadratic.a = b
X&
Copy assignment operator. In addition to the contained elementsrequirements of Table 93 (23.2.2 [container.requirements.general]), copies the hash function, predicate, and maximum load factor.Average case linear in b.size()
, worst case quadratic.a = il
X&
a = X(il); return *this;
Requires:T
isCopyConstructible
andCopyAssignable
.
Assigns the range[il.begin(), il.end())
intoa
. All existing elements ofa
are either assigned or destroyed.Average case linear in il.size()
, worst case quadratic.a_uniq.emplace(args)
pair<iterator, bool>
Requires: T
shall be constructible fromargs
inserts aT
objectt
constructed withstd::forward<Args>(args)...
if and only if there is no element in the container with key equivalent to the key oft
. Thebool
component of the returned pair is true if and only if the insertion takes place, and the iterator component of the pair points to the element with key equivalent to the key oft
.Average case O(1), worst case O( a_uniq.size()
).a_eq.emplace(args)
iterator
Requires: T
shall be constructible fromargs
inserts aT
objectt
constructed withstd::forward<Args>(args)...
and returns the iterator pointing to the newly inserted element.Average case O(1), worst case O( a_eq.size()
).a.emplace_hint(p, args)
iterator
Requires: T
shall be constructible fromargs
equivalent toa.emplace( std::forward<Args>(args)...)
. Return value is an iterator pointing to the element with the key equivalent to the newly inserted element. Theconst_iterator p
is a hint pointing to where the search should start. Implementations are permitted to ignore the hint.Average case O(1), worst case O( a.size()
).a_uniq.insert(t)
pair<iterator, bool>
Requires: T
shall beMoveConstructible
ift
is a non-const rvalue expression, elseT
shall beCopyConstructible
.
Insertst
if and only if there is no element in the container with key equivalent to the key oft
. Thebool
component of the returned pair indicates whether the insertion takes place, and the iterator component points to the element with key equivalent to the key oft
.Average case O(1), worst case O( a_uniq.size()
).a_eq.insert(t)
iterator
Requires: T
shall beMoveConstructible
ift
is a non-const rvalue expression, elseT
shall beCopyConstructible
.
Insertst
, and returns an iterator pointing to the newly inserted element.Average case O(1), worst case O( a_uniq.size()
).a.insert(q, t)
iterator
Requires: T
shall beMoveConstructible
ift
is a non-const rvalue expression, elseT
shall beCopyConstructible
.
Equivalent toa.insert(t)
. Return value is an iterator pointing to the element with the key equivalent to that oft
. The iteratorq
is a hint pointing to where the search should start. Implementations are permitted to ignore the hint.Average case O(1), worst case O( a_uniq.size()
).a.insert(i, j)
void
Requires: T
shall be constructible from*i
.
Pre:i
andj
are not iterators ina
. Equivalent toa.insert(t)
for each element in[i,j)
.Average case O( N
), whereN
isdistance(i, j)
. Worst case O(N * a.size()
).
Change [forwardlist]/2:
2 A
forward_list
satisfies all of the requirements of a container (table 91), except that thesize()
member function is not provided. Aforward_list
also satisfies all of the requirements of an allocator-aware container (table 93). Andforward_list
provides theassign
member functions as specified in Table 94, Sequence container requirements, and several of the optional sequence container requirements (Table 95). Descriptions are provided here only for operations onforward_list
that are not described in that table or for operations where there is additional semantic information.
Add a new paragraph after [forwardlist.modifiers]/23:
void clear();23 Effects: Erases all elements in the range
[begin(),end())
.Remarks: Does not invalidate past-the-end iterators.
Change 23.3.13.3 [vector.capacity]/13:
void resize(size_type sz, const T& c);13 Requires:
T
shall beCopyConstructible
. Ifvalue_type
has a move constructor, that constructor shall not throw any exceptions.
In 23.5.6 [unord.set] and 23.5.7 [unord.multiset] substitute
"Key
" for "Value
".
[ The above substitution is normative as it ties into the requirements table. ]
decay
incompletely specifiedSection: 21.3.8.7 [meta.trans.other] Status: CD1 Submitter: Thorsten Ottosen Opened: 2007-07-08 Last modified: 2016-01-28
Priority: Not Prioritized
View all other issues in [meta.trans.other].
View all issues with CD1 status.
Discussion:
The current working draft has a type-trait decay
in 21.3.8.7 [meta.trans.other].
Its use is to turn C++03 pass-by-value parameters into efficient C++0x pass-by-rvalue-reference parameters. However, the current definition introduces an incompatible change where the cv-qualification of the parameter type is retained. The deduced type should loose such cv-qualification, as pass-by-value does.
Proposed resolution:
In 21.3.8.7 [meta.trans.other] change the last sentence:
Otherwise the member typedef
type
equalsremove_cv<U>::type
.
In 22.4.5 [tuple.creation]/1 change:
where eachLetVi
inVTypes
isX&
if, for the corresponding typeTi
inTypes
,remove_cv<remove_reference<Ti>::type>::type
equalsreference_wrapper<X>
, otherwiseVi
isdecay<Ti>::type
.Ui
bedecay<Ti>::type
for eachTi
inTypes
. Then eachVi
inVTypes
isX&
ifUi
equalsreference_wrapper<X>
, otherwiseVi
isUi
.
make_pair()
should behave as make_tuple()
wrt. reference_wrapper()
Section: 22.3 [pairs] Status: CD1 Submitter: Thorsten Ottosen Opened: 2007-07-08 Last modified: 2016-01-28
Priority: Not Prioritized
View all other issues in [pairs].
View all issues with CD1 status.
Discussion:
The current draft has make_pair()
in 22.3 [pairs]/16
and make_tuple()
in 22.4.5 [tuple.creation].
make_tuple()
detects the presence of
reference_wrapper<X>
arguments and "unwraps" the reference in
such cases. make_pair()
would OTOH create a
reference_wrapper<X>
member. I suggest that the two
functions are made to behave similar in this respect to minimize
confusion.
Proposed resolution:
In 22.2 [utility] change the synopsis for make_pair() to read
template <class T1, class T2> pair<typename decay<T1>::typeV1,typename decay<T2>::typeV2> make_pair(T1&&, T2&&);
In 22.3 [pairs]/16 change the declaration to match the above synopsis. Then change the 22.3 [pairs]/17 to:
Returns:
pair<
wheretypename decay<T1>::typeV1,typename decay<T2>::typeV2>(forward<T1>(x),forward<T2>(y))V1
andV2
are determined as follows: LetUi
bedecay<Ti>::type
for eachTi
. Then eachVi
isX&
ifUi
equalsreference_wrapper<X>
, otherwiseVi
isUi
.
char_traits::not_eof
has wrong signatureSection: 27.2.4 [char.traits.specializations] Status: CD1 Submitter: Bo Persson Opened: 2007-08-13 Last modified: 2016-01-28
Priority: Not Prioritized
View all other issues in [char.traits.specializations].
View all issues with CD1 status.
Discussion:
The changes made for constexpr
in 27.2.4 [char.traits.specializations] have
not only changed the not_eof
function from pass by const reference to
pass by value, it has also changed the parameter type from int_type
to
char_type
.
This doesn't work for type char
, and is inconsistent with the
requirements in Table 56, Traits requirements, 27.2.2 [char.traits.require].
Pete adds:
For what it's worth, that may not have been an intentional change. N2349, which detailed the changes for adding constant expressions to the library, has strikeout bars through the
const
and the&
that surround thechar_type
argument, but none throughchar_type
itself. So the intention may have been just to change to pass by value, with text incorrectly copied from the standard.
Proposed resolution:
Change the signature in 27.2.4.2 [char.traits.specializations.char], [char.traits.specializations.char16_t], [char.traits.specializations.char32_t], and 27.2.4.6 [char.traits.specializations.wchar.t] to
static constexpr int_type not_eof(char_typeint_type c);
[ Bellevue: ]
Resolution: NAD editorial - up to Pete's judgment
[ Post Sophia Antipolis ]
Moved from Pending NAD Editorial to Review. The proposed wording appears to be correct but non-editorial.
Section: 20.3.2.2 [util.smartptr.shared] Status: CD1 Submitter: Peter Dimov Opened: 2007-08-24 Last modified: 2016-01-28
Priority: Not Prioritized
View all other issues in [util.smartptr.shared].
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Discussion:
A discussion on
comp.std.c++
has identified a contradiction in the shared_ptr
specification.
The shared_ptr
move constructor and the cast functions are
missing postconditions for the get()
accessor.
[ Bellevue: ]
Move to "ready", adopting the first (Peter's) proposed resolution.
Note to the project editor: there is an editorial issue here. The wording for the postconditions of the casts is slightly awkward, and the editor should consider rewording "If w is the return value...", e. g. as "For a return value w...".
Proposed resolution:
Add to 20.3.2.2.2 [util.smartptr.shared.const]:
shared_ptr(shared_ptr&& r); template<class Y> shared_ptr(shared_ptr<Y>&& r);Postconditions:
*this
shall contain the old value ofr
.r
shall be empty.r.get() == 0
.
Add to 20.3.2.2.10 [util.smartptr.shared.cast]:
template<class T, class U> shared_ptr<T> static_pointer_cast(shared_ptr<U> const& r);Postconditions: If
w
is the return value,w.get() == static_cast<T*>(r.get()) && w.use_count() == r.use_count()
.
template<class T, class U> shared_ptr<T> dynamic_pointer_cast(shared_ptr<U> const& r);Postconditions: If
w
is the return value,w.get() == dynamic_cast<T*>(r.get())
.
template<class T, class U> shared_ptr<T> const_pointer_cast(shared_ptr<U> const& r);Postconditions: If
w
is the return value,w.get() == const_cast<T*>(r.get()) && w.use_count() == r.use_count()
.
Alberto Ganesh Barbati has written an alternative proposal where he suggests (among other things) that the casts be respecified in terms of the aliasing constructor as follows:
Change 20.3.2.2.10 [util.smartptr.shared.cast]:
-2- Returns:
Ifr
is empty, anempty shared_ptr<T>;
otherwise, ashared_ptr<T>
object that storesstatic_cast<T*>(r.get())
and shares ownership withr
.shared_ptr<T>(r, static_cast<T*>(r.get())
.
-6- Returns:
Whendynamic_cast<T*>(r.get())
returns a nonzero value, ashared_ptr<T>
object that stores a copy of it and shares ownership withr
;Otherwise, an emptyshared_ptr<T>
object.- If
p = dynamic_cast<T*>(r.get())
is a non-null pointer,shared_ptr<T>(r, p);
- Otherwise,
shared_ptr<T>()
.
-10- Returns:
Ifr
is empty, anempty shared_ptr<T>;
otherwise, ashared_ptr<T>
object that storesconst_cast<T*>(r.get())
and shares ownership withr
.shared_ptr<T>(r, const_cast<T*>(r.get())
.
This takes care of the missing postconditions for the casts by bringing in the aliasing constructor postcondition "by reference".
shared_ptr
Section: 20.3.2.2.6 [util.smartptr.shared.obs] Status: C++11 Submitter: Peter Dimov Opened: 2007-08-24 Last modified: 2016-01-28
Priority: Not Prioritized
View all other issues in [util.smartptr.shared.obs].
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Discussion:
A discussion on
comp.std.c++
has identified a contradiction in the shared_ptr
specification.
The note:
[ Note: this constructor allows creation of an empty shared_ptr instance with a non-NULL stored pointer. -end note ]
after the aliasing constructor
template<class Y> shared_ptr(shared_ptr<Y> const& r, T *p);
reflects the intent of
N2351
to, well, allow the creation of an empty shared_ptr
with a non-NULL stored pointer.
This is contradicted by the second sentence in the Returns clause of 20.3.2.2.6 [util.smartptr.shared.obs]:
T* get() const;Returns: the stored pointer. Returns a null pointer if
*this
is empty.
[ Bellevue: ]
Adopt option 1 and move to review, not ready.
There was a lot of confusion about what an empty
shared_ptr
is (the term isn't defined anywhere), and whether we have a good mental model for how one behaves. We think it might be possible to deduce what the definition should be, but the words just aren't there. We need to open an issue on the use of this undefined term. (The resolution of that issue might affect the resolution of issue 711(i).)The LWG is getting more uncomfortable with the aliasing proposal (N2351) now that we realize some of its implications, and we need to keep an eye on it, but there isn't support for removing this feature at this time.
[ Sophia Antipolis: ]
We heard from Peter Dimov, who explained his reason for preferring solution 1.
Because it doesn't seem to add anything. It simply makes the behavior for p = 0 undefined. For programmers who don't create empty pointers with p = 0, there is no difference. Those who do insist on creating them presumably have a good reason, and it costs nothing for us to define the behavior in this case.
The aliasing constructor is sharp enough as it is, so "protecting" users doesn't make much sense in this particular case.
> Do you have a use case for r being empty and r being non-null?
I have received a few requests for it from "performance-conscious" people (you should be familiar with this mindset) who don't like the overhead of allocating and maintaining a control block when a null deleter is used to approximate a raw pointer. It is obviously an "at your own risk", low-level feature; essentially a raw pointer behind a shared_ptr facade.
We could not agree upon a resolution to the issue; some of us thought that Peter's description above is supporting an undesirable behavior.
[ 2009-07 Frankfurt: ]
We favor option 1, move to Ready.
[ Howard: Option 2 commented out for clarity, and can be brought back. ]
Proposed resolution:
In keeping the N2351 spirit and obviously my preference, change 20.3.2.2.6 [util.smartptr.shared.obs]:
T* get() const;Returns: the stored pointer.
Returns a null pointer if*this
is empty.
seed_seq::size
no longer usefulSection: 29.5.8.1 [rand.util.seedseq] Status: CD1 Submitter: Marc Paterno Opened: 2007-08-25 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
One of the motivations for incorporating seed_seq::size()
was to simplify the wording
in other parts of 29.5 [rand].
As a side effect of resolving related issues,
all such references
to seed_seq::size()
will have been excised.
More importantly,
the present specification is contradictory,
as "The number of 32-bit units the object can deliver"
is not the same as "the result of v.size()
."
See N2391 and N2423 for some further discussion.
Proposed resolution:
Adopt the proposed resolution in N2423.
[ Kona (2007): The LWG adopted the proposed resolution of N2423 for this issue. The LWG voted to accelerate this issue to Ready status to be voted into the WP at Kona. ]
sort()
complexity is too laxSection: 26.8.2.1 [sort] Status: CD1 Submitter: Matt Austern Opened: 2007-08-30 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
The complexity of sort()
is specified as "Approximately N
log(N)
(where N == last - first
) comparisons on the
average", with no worst case complicity specified. The intention was to
allow a median-of-three quicksort implementation, which is usually O(N
log N)
but can be quadratic for pathological inputs. However, there is
no longer any reason to allow implementers the freedom to have a
worst-cast-quadratic sort algorithm. Implementers who want to use
quicksort can use a variant like David Musser's "Introsort" (Software
Practice and Experience 27:983-993, 1997), which is guaranteed to be O(N
log N)
in the worst case without incurring additional overhead in the
average case. Most C++ library implementers already do this, and there
is no reason not to guarantee it in the standard.
Proposed resolution:
In 26.8.2.1 [sort], change the complexity to "O(N log N)", and remove footnote 266:
Complexity:
ApproximatelyO(N log(N)) (where N == last - first ) comparisonson the average.266)
266) If the worst case behavior is importantstable_sort()
(25.3.1.2) orpartial_sort()
(25.3.1.3) should be used.
search_n
complexity is too laxSection: 26.6.15 [alg.search] Status: CD1 Submitter: Matt Austern Opened: 2007-08-30 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
The complexity for search_n
(26.6.15 [alg.search] par 7) is specified as "At most
(last - first ) * count applications of the corresponding predicate if
count is positive, or 0 otherwise." This is unnecessarily pessimistic.
Regardless of the value of count, there is no reason to examine any
element in the range more than once.
Proposed resolution:
Change the complexity to "At most (last - first) applications of the corresponding predicate".
template<class ForwardIterator, class Size, class T> ForwardIterator search_n(ForwardIterator first , ForwardIterator last , Size count , const T& value ); template<class ForwardIterator, class Size, class T, class BinaryPredicate> ForwardIterator search_n(ForwardIterator first , ForwardIterator last , Size count , const T& value , BinaryPredicate pred );Complexity: At most
(last - first )
applications of the corresponding predicate* countif.count
is positive, or 0 otherwise
minmax_element
complexity is too laxSection: 26.8.9 [alg.min.max] Status: CD1 Submitter: Matt Austern Opened: 2007-08-30 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
The complexity for minmax_element
(26.8.9 [alg.min.max] par 16) says "At most max(2 *
(last - first ) - 2, 0)
applications of the corresponding comparisons",
i.e. the worst case complexity is no better than calling min_element
and
max_element
separately. This is gratuitously inefficient. There is a
well known technique that does better: see section 9.1 of CLRS
(Introduction to Algorithms, by Cormen, Leiserson, Rivest, and Stein).
Proposed resolution:
Change 26.8.9 [alg.min.max] to:
template<class ForwardIterator> pair<ForwardIterator, ForwardIterator> minmax_element(ForwardIterator first , ForwardIterator last); template<class ForwardIterator, class Compare> pair<ForwardIterator, ForwardIterator> minmax_element(ForwardIterator first , ForwardIterator last , Compare comp);Returns:
make_pair(m, M)
, wherem
isthe first iterator inmin_element(first, last)
ormin_element(first, last, comp)
[first, last)
such that no iterator in the range refers to a smaller element, and whereM
isthe last iterator inmax_element(first, last)
ormax_element(first, last, comp)
[first, last)
such that no iterator in the range refers to a larger element.Complexity: At most
max(2 * (last - first ) - 2, 0)
max(⌊(3/2) (N-1)⌋, 0)
applications of the correspondingcomparisonspredicate, whereN
isdistance(first, last)
.
Section: 28.6.12 [re.grammar] Status: C++11 Submitter: Stephan T. Lavavej Opened: 2007-08-31 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
[tr.re.grammar]/3 and C++0x WP 28.6.12 [re.grammar]/3 say:
The following productions within the ECMAScript grammar are modified as follows:
CharacterClass :: [ [lookahead ∉ {^}] ClassRanges ] [ ^ ClassRanges ]
This definition for CharacterClass
appears to be exactly identical to that in ECMA-262.
Was an actual modification intended here and accidentally omitted, or was this production accidentally included?
[ Batavia (2009-05): ]
We agree that what is specified is identical to what ECMA-262 specifies. Pete would like to take a bit of time to assess whether we had intended, but failed, to make a change. It would also be useful to hear from John Maddock on the issue.
Move to Open.
[ 2009-07 Frankfurt: ]
Move to Ready.
Proposed resolution:
Remove this mention of the CharacterClass production.
CharacterClass :: [ [lookahead ∉ {^}] ClassRanges ] [ ^ ClassRanges ]
std::is_literal
type traits should be providedSection: 21 [meta] Status: Resolved Submitter: Daniel Krügler Opened: 2007-08-25 Last modified: 2016-01-28
Priority: Not Prioritized
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Duplicate of: 750
Discussion:
Since the inclusion of constexpr
in the standard draft N2369 we have
a new type category "literal", which is defined in 6.8 [basic.types]/p.11:
-11- A type is a literal type if it is:
- a scalar type; or
a class type (clause 9) with
- a trivial copy constructor,
- a trivial destructor,
- at least one constexpr constructor other than the copy constructor,
- no virtual base classes, and
- all non-static data members and base classes of literal types; or
- an array of literal type.
I strongly suggest that the standard provides a type traits for literal types in 21.3.5.4 [meta.unary.prop] for several reasons:
The special problem of reason (c) is that I don't see currently a way to portably test the condition for literal class types:
- at least one constexpr constructor other than the copy constructor,
[ Alisdair is considering preparing a paper listing a number of missing type traits, and feels that it might be useful to handle them all together rather than piecemeal. This would affect issue 719 and 750. These two issues should move to OPEN pending AM paper on type traits. ]
[ 2009-07 Frankfurt: ]
Beman, Daniel, and Alisdair will work on a paper proposing new type traits.
[ Addressed in N2947. ]
[ 2009-10 Santa Cruz: ]
NAD EditorialResolved. Solved by N2984.
Proposed resolution:
In 21.3.3 [meta.type.synop] in the group "type properties", just below the line
template <class T> struct is_pod;
add a new one:
template <class T> struct is_literal;
In 21.3.5.4 [meta.unary.prop], table Type Property Predicates, just
below the line for the is_pod
property add a new line:
Template | Condition | Preconditions |
---|---|---|
template <class T> struct is_literal; |
T is a literal type (3.9) |
T shall be a complete type, an
array of unknown bound, or
(possibly cv-qualified) void . |
Section: 23.3.3 [array], 22.9.2 [template.bitset] Status: CD1 Submitter: Daniel Krügler Opened: 2007-08-25 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
bool array<T,N>::empty() const
should be a
constexpr
because this is easily to proof and to implement following it's operational
semantics defined by Table 87 (Container requirements) which says: a.size() == 0
.
bool bitset<N>::test() const
must be a
constexpr
(otherwise it would violate the specification of constexpr
bitset<N>::operator[](size_t) const
, because it's return clause delegates to test()
).
bitset<N>::bitset(unsigned long)
can
be declared as a constexpr
. Current implementations usually have no such bitset
c'tor which would fulfill the requirements of a constexpr
c'tor because they have a
non-empty c'tor body that typically contains for-loops or memcpy
to compute the
initialisation. What have I overlooked here?
[ Sophia Antipolis: ]
We handle this as two parts
- The proposed resolution is correct; move to ready.
- The issue points out a real problem, but the issue is larger than just this solution. We believe a paper is needed, applying the full new features of C++ (including extensible literals) to update
std::bitset
. We note that we do not consider this new work, and that is should be handled by the Library Working Group.In order to have a consistent working paper, Alisdair and Daniel produced a new wording for the resolution.
Proposed resolution:
In the class template definition of 23.3.3 [array]/p. 3 change
constexpr bool empty() const;
In the class template definition of 22.9.2 [template.bitset]/p. 1 change
constexpr bool test(size_t pos ) const;
and in 22.9.2.3 [bitset.members] change
constexpr bool test(size_t pos ) const;
nanf
and nanl
Section: 29.7 [c.math] Status: CD1 Submitter: Daniel Krügler Opened: 2007-08-27 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
In the listing of 29.7 [c.math], table 108: Header <cmath>
synopsis I miss
the following C99 functions (from 7.12.11.2):
float nanf(const char *tagp); long double nanl(const char *tagp);
(Note: These functions cannot be overloaded and they are also not listed anywhere else)
Proposed resolution:
In 29.7 [c.math], table 108, section "Functions", add nanf
and nanl
just after the existing entry nan
.
basic_regex
should be moveableSection: 28.6.7 [re.regex] Status: C++11 Submitter: Daniel Krügler Opened: 2007-08-29 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
Addresses UK 316
According to the current state of the standard draft, the class
template basic_regex
, as described in 28.6.7 [re.regex]/3, is
neither MoveConstructible
nor MoveAssignable
.
IMO it should be, because typical regex state machines tend
to have a rather large data quantum and I have seen several
use cases, where a factory function returns regex values,
which would take advantage of moveabilities.
[ Sophia Antipolis: ]
Needs wording for the semantics, the idea is agreed upon.
[ Post Summit Daniel updated wording to reflect new "swap rules". ]
[ 2009-07 Frankfurt: ]
Move to Ready.
Proposed resolution:
In the class definition of basic_regex
, just below 28.6.7 [re.regex]/3,
perform the following changes:
Just after basic_regex(const basic_regex&);
insert:
basic_regex(basic_regex&&);
Just after basic_regex& operator=(const basic_regex&);
insert:
basic_regex& operator=(basic_regex&&);
Just after basic_regex& assign(const basic_regex& that);
insert:
basic_regex& assign(basic_regex&& that);
In 28.6.7.2 [re.regex.construct], just after p.11 add the following new member definition:
basic_regex(basic_regex&& e);Effects: Move-constructs a
basic_regex
instance frome
.Postconditions:
flags()
andmark_count()
returne.flags()
ande.mark_count()
, respectively, thate
had before construction, leavinge
in a valid state with an unspecified value.Throws: nothing.
Also in 28.6.7.2 [re.regex.construct], just after p.18 add the following new member definition:
basic_regex& operator=(basic_regex&& e);Effects: Returns the result of
assign(std::move(e))
.
In 28.6.7.3 [re.regex.assign], just after p. 2 add the following new member definition:
basic_regex& assign(basic_regex&& rhs);Effects: Move-assigns a
basic_regex
instance fromrhs
and returns*this
.Postconditions:
flags()
andmark_count()
returnrhs.flags()
andrhs.mark_count()
, respectively, thatrhs
had before assignment, leavingrhs
in a valid state with an unspecified value.Throws: nothing.
DefaultConstructible
is not definedSection: 16.4.4.2 [utility.arg.requirements] Status: C++11 Submitter: Pablo Halpern Opened: 2007-09-12 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
The DefaultConstructible
requirement is referenced in
several places in the August 2007 working draft
N2369,
but is not defined anywhere.
[ Bellevue: ]
Walking into the default/value-initialization mess...
Why two lines? Because we need both expressions to be valid.
AJM not sure what the phrase "default constructed" means. This is unfortunate, as the phrase is already used 24 times in the library!
Example:
const int
would not accept first line, but will accept the second.This is an issue that must be solved by concepts, but we might need to solve it independantly first.
It seems that the requirements are the syntax in the proposed first column is valid, but not clear what semantics we need.
A table where there is no post-condition seems odd, but appears to sum up our position best.
At a minimum an object is declared and is destructible.
Move to open, as no-one happy to produce wording on the fly.
[ 2009-07-28 Reopened by Alisdair. No longer solved by concepts. ]
[ 2009-08-17 Daniel adds "[defaultconstructible]" to table title. 408(i) depends upon this issue. ]
[ 2009-08-18 Alisdair adds: ]
Looking at the proposed table in this issue, it really needs two rows:
Table 33: DefaultConstructible
requirements [defaultconstructible]expression post-condition T t;
t
is default-initialized.T{}
Object of type T
is value-initialized.Note I am using the new brace-initialization syntax that is unambiguous in all use cases (no most vexing parse.)
[ 2009-10-03 Daniel adds: ]
The suggested definition
T{}
describing it as value-initialization is wrong, because it belongs to list-initialization which would - as the current rules are - always prefer a initializer-list constructor over a default-constructor. I don't consider this as an appropriate definition ofDefaultConstructible
. My primary suggestion is to ask core, whether the special caseT{}
(which also easily leads to ambiguity situations for more than one initializer-list in a class) would always prefer a default-constructor - if any - before considering an initializer-list constructor or to provide another syntax form to prefer value-initialization over list-initialization. If that fails I would fall back to suggest to use the expressionT()
instead ofT{}
with all it's disadvantages for the meaning of the expressionT t();
[ 2009-10 Santa Cruz: ]
Leave Open. Core is looking to make Alisdair's proposed resolution correct.
[ 2010-01-24 At Alisdair's request, moved his proposal into the proposed wording section. The old wording is preserved here: ]
In section 16.4.4.2 [utility.arg.requirements], before table 33, add the following table:
Table 33:
DefaultConstructible
requirements [defaultconstructible]
expression
post-condition
T t;
T()
T
is default constructed.
[ 2010-02-04: Moved to Tentatively Ready after 5 positive votes on c++std-lib. ]
Rationale:
[ San Francisco: ]
We believe concepts will solve this problem (N2774).
[ Rationale is obsolete. ]
Proposed resolution:
In section 16.4.4.2 [utility.arg.requirements], before table 33, add the following table:
Table 33: DefaultConstructible
requirements [defaultconstructible]expression post-condition T t;
Object t
is default-initialized.T u{};
Object u
is value-initialized.T()
T{}A temporary object of type T
is value-initialized.
regex_replace()
doesn't accept basic_string
s with custom traits and allocatorsSection: 28.6.10.4 [re.alg.replace] Status: C++11 Submitter: Stephan T. Lavavej Opened: 2007-09-22 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
regex_match()
and regex_search()
take const basic_string<charT, ST,
SA>&
. regex_replace()
takes const basic_string<charT>&
. This prevents
regex_replace()
from accepting basic_string
s with custom traits and
allocators.
Overloads of regex_replace()
taking basic_string
should be additionally
templated on class ST, class SA
and take const basic_string<charT, ST,
SA>&
. Consistency with regex_match()
and regex_search()
would place
class ST, class SA
as the first template arguments; compatibility with
existing code using TR1 and giving explicit template arguments to
regex_replace()
would place class ST, class SA
as the last template
arguments.
[ Batavia (2009-05): ]
Bill comments, "We need to look at the depth of this change."
Pete remarks that we are here dealing with a convenience function that saves a user from calling the iterato-based overload.
Move to Open.
[ 2009-07 Frankfurt: ]
Howard to ask Stephan Lavavej to provide wording.
[ 2009-07-17 Stephan provided wording. ]
[ 2009-07-25 Daniel tweaks both this issue and 726(i). ]
One relevant part of the proposed resolution below suggests to add a new overload of the format member function in the
match_results
class template that accepts two character pointers defining thebegin
andend
of a format range. A more general approach could have proposed a pair of iterators instead, but the used pair of char pointers reflects existing practice. If the committee strongly favors an iterator-based signature, this could be simply changed. I think that the minimum requirement should be aBidirectionalIterator
, but current implementations take advantage (at least partially) of theRandomAccessIterator
sub interface of the char pointers.Suggested Resolution:
[Moved into the proposed resloution]
[ 2009-07-30 Stephan agrees with Daniel's wording. Howard places Daniel's wording in the Proposed Resolution. ]
[ 2009-10 Santa Cruz: ]
Move to Review. Chair is anxious to move this to Ready in Pittsburgh.
[ 2010-01-27 Moved to Tentatively Ready after 5 positive votes on c++std-lib. ]
Proposed resolution:
Change 28.6.3 [re.syn] as indicated:
// 28.11.4, function template regex_replace: template <class OutputIterator, class BidirectionalIterator, class traits, class charT, class ST, class SA> OutputIterator regex_replace(OutputIterator out, BidirectionalIterator first, BidirectionalIterator last, const basic_regex<charT, traits>& e, const basic_string<charT, ST, SA>& fmt, regex_constants::match_flag_type flags = regex_constants::match_default); template <class OutputIterator, class BidirectionalIterator, class traits, class charT> OutputIterator regex_replace(OutputIterator out, BidirectionalIterator first, BidirectionalIterator last, const basic_regex<charT, traits>& e, const charT* fmt, regex_constants::match_flag_type flags = regex_constants::match_default); template <class traits, class charT, class ST, class SA, class FST, class FSA> basic_string<charT, ST, SA> regex_replace(const basic_string<charT, ST, SA>& s, const basic_regex<charT, traits>& e, const basic_string<charT, FST, FSA>& fmt, regex_constants::match_flag_type flags = regex_constants::match_default); template <class traits, class charT, class ST, class SA> basic_string<charT, ST, SA> regex_replace(const basic_string<charT, ST, SA>& s, const basic_regex<charT, traits>& e, const charT* fmt, regex_constants::match_flag_type flags = regex_constants::match_default); template <class traits, class charT, class ST, class SA> basic_string<charT> regex_replace(const charT* s, const basic_regex<charT, traits>& e, const basic_string<charT, ST, SA>& fmt, regex_constants::match_flag_type flags = regex_constants::match_default); template <class traits, class charT> basic_string<charT> regex_replace(const charT* s, const basic_regex<charT, traits>& e, const charT* fmt, regex_constants::match_flag_type flags = regex_constants::match_default);
Change 28.6.9 [re.results]/3, class template match_results
as
indicated:
template <class OutputIter> OutputIter format(OutputIter out, const char_type* fmt_first, const char_type* fmt_last, regex_constants::match_flag_type flags = regex_constants::format_default) const; template <class OutputIter, class ST, class SA> OutputIter format(OutputIter out, conststring_typebasic_string<char_type, ST, SA>& fmt, regex_constants::match_flag_type flags = regex_constants::format_default) const; template <class ST, class SA>string_typebasic_string<char_type, ST, SA> format(conststring_typebasic_string<char_type, ST, SA>& fmt, regex_constants::match_flag_type flags = regex_constants::format_default) const; string_type format(const char_type* fmt, regex_constants::match_flag_type flags = regex_constants::format_default) const;
Insert at the very beginning of 28.6.9.6 [re.results.form] the following:
template <class OutputIter> OutputIter format(OutputIter out, const char_type* fmt_first, const char_type* fmt_last, regex_constants::match_flag_type flags = regex_constants::format_default) const;1 Requires: The type
OutputIter
shall satisfy the requirements for an Output Iterator (24.3.5.4 [output.iterators]).2 Effects: Copies the character sequence
[fmt_first,fmt_last)
toOutputIter out
. Replaces each format specifier or escape sequence in the copied range with either the character(s) it represents or the sequence of characters within*this
to which it refers. The bitmasks specified inflags
determine which format specifiers and escape sequences are recognized.3 Returns:
out
.
Change 28.6.9.6 [re.results.form], before p. 1 until p. 3 as indicated:
template <class OutputIter, class ST, class SA> OutputIter format(OutputIter out, conststring_typebasic_string<char_type, ST, SA>& fmt, regex_constants::match_flag_type flags = regex_constants::format_default) const;
1 Requires: The typeOutputIter
shall satisfy the requirements for an Output Iterator (24.2.3).2 Effects:
Copies the character sequenceEquivalent to[fmt.begin(),fmt.end())
toOutputIter out
. Replaces each format specifier or escape sequence infmt
with either the character(s) it represents or the sequence of characters within*this
to which it refers. The bitmasks specified inflags
determines what format specifiers and escape sequences are recognizedreturn format(out, fmt.data(), fmt.data() + fmt.size(), flags)
.
3 Returns:out
.
Change 28.6.9.6 [re.results.form], before p. 4 until p. 4 as indicated:
template <class ST, class SA>string_typebasic_string<char_type, ST, SA> format(conststring_typebasic_string<char_type, ST, SA>& fmt, regex_constants::match_flag_type flags = regex_constants::format_default) const;Effects:
Returns a copy of the stringConstructs an empty stringfmt
. Replaces each format specifier or escape sequence infmt
with either the character(s) it represents or the sequence of characters within*this
to which it refers. The bitmasks specified in flags determines what format specifiers and escape sequences are recognized.result
of typebasic_string<char_type, ST, SA>
, and callsformat(back_inserter(result), fmt, flags)
.Returns:
result
At the end of 28.6.9.6 [re.results.form] insert as indicated:
string_type format(const char_type* fmt, regex_constants::match_flag_type flags = regex_constants::format_default) const;Effects: Constructs an empty string
result
of typestring_type
, and callsformat(back_inserter(result), fmt, fmt + char_traits<char_type>::length(fmt), flags)
.Returns:
result
Change 28.6.10.4 [re.alg.replace] before p. 1 as indicated:
template <class OutputIterator, class BidirectionalIterator, class traits, class charT, class ST, class SA> OutputIterator regex_replace(OutputIterator out, BidirectionalIterator first, BidirectionalIterator last, const basic_regex<charT, traits>& e, const basic_string<charT, ST, SA>& fmt, regex_constants::match_flag_type flags = regex_constants::match_default); template <class OutputIterator, class BidirectionalIterator, class traits, class charT> OutputIterator regex_replace(OutputIterator out, BidirectionalIterator first, BidirectionalIterator last, const basic_regex<charT, traits>& e, const charT* fmt, regex_constants::match_flag_type flags = regex_constants::match_default);Effects: [..]. If any matches are found then, for each such match, if
!(flags & regex_constants::format_no_copy)
callsstd::copy(m.prefix().first, m.prefix().second, out)
, and then callsm.format(out, fmt, flags)
for the first form of the function andm.format(out, fmt, fmt + char_traits<charT>::length(fmt), flags)
for the second form. [..].
Change 28.6.10.4 [re.alg.replace] before p. 3 as indicated:
template <class traits, class charT, class ST, class SA, class FST, class FSA> basic_string<charT, ST, SA> regex_replace(const basic_string<charT, ST, SA>& s, const basic_regex<charT, traits>& e, const basic_string<charT, FST, FSA>& fmt, regex_constants::match_flag_type flags = regex_constants::match_default); template <class traits, class charT, class ST, class SA> basic_string<charT, ST, SA> regex_replace(const basic_string<charT, ST, SA>& s, const basic_regex<charT, traits>& e, const charT* fmt, regex_constants::match_flag_type flags = regex_constants::match_default);Effects: Constructs an empty string
result
of typebasic_string<charT, ST, SA>
, callsregex_replace(back_inserter(result), s.begin(), s.end(), e, fmt, flags)
, and then returnsresult
.
At the end of 28.6.10.4 [re.alg.replace] add the following new prototype description:
template <class traits, class charT, class ST, class SA> basic_string<charT> regex_replace(const charT* s, const basic_regex<charT, traits>& e, const basic_string<charT, ST, SA>& fmt, regex_constants::match_flag_type flags = regex_constants::match_default); template <class traits, class charT> basic_string<charT> regex_replace(const charT* s, const basic_regex<charT, traits>& e, const charT* fmt, regex_constants::match_flag_type flags = regex_constants::match_default);Effects: Constructs an empty string
result
of typebasic_string<charT>
, callsregex_replace(back_inserter(result), s, s + char_traits<charT>::length(s), e, fmt, flags)
, and then returnsresult
.
Section: 29.5.4.3 [rand.eng.mers] Status: CD1 Submitter: Stephan Tolksdorf Opened: 2007-09-21 Last modified: 2016-01-28
Priority: Not Prioritized
View all other issues in [rand.eng.mers].
View all issues with CD1 status.
Discussion:
The mersenne_twister_engine
is required to use a seeding method that is given
as an algorithm parameterized over the number of bits W
. I doubt whether the given generalization
of an algorithm that was originally developed only for unsigned 32-bit integers is appropriate
for other bit widths. For instance, W
could be theoretically 16 and UIntType
a 16-bit integer, in
which case the given multiplier would not fit into the UIntType
. Moreover, T. Nishimura and M.
Matsumoto have chosen a dif ferent multiplier for their 64 bit Mersenne Twister
[reference].
I see two possible resolutions:
W
of the mersenne_twister_template
to values of 32 or 64 and use the
multiplier from [the above reference] for the 64-bit case (my preference)W
as a 32-bit array of appropriate length (and a specified byte
order) and always employ the 32-bit algorithm for seeding
See N2424 for further discussion.
[ Bellevue: ]
Stephan Tolksdorf has additional comments on N2424. He comments: "there is a typo in the required behaviour for mt19937_64: It should be the 10000th (not 100000th) invocation whose value is given, and the value should be 9981545732273789042 (not 14002232017267485025)." These values need checking.
Take the proposed recommendation in N2424 and move to REVIEW.
Proposed resolution:
See N2424 for the proposed resolution.
[ Stephan Tolksdorf adds pre-Bellevue: ]
I support the proposed resolution in N2424, but there is a typo in the required behaviour for
mt19937_64
: It should be the 10000th (not 100000th) invocation whose value is given, and the value should be 9981545732273789042 (not 14002232017267485025). The change to para. 8 proposed by Charles Karney should also be included in the proposed wording.
[ Sophia Antipolis: ]
Note the main part of the issue is resolved by N2424.
Section: 99 [rand.dist.samp.genpdf] Status: Resolved Submitter: Stephan Tolksdorf Opened: 2007-09-21 Last modified: 2016-01-28
Priority: Not Prioritized
View all other issues in [rand.dist.samp.genpdf].
View all issues with Resolved status.
Duplicate of: 795
Discussion:
99 [rand.dist.samp.genpdf] describes the interface for a distribution template that is meant to simulate random numbers from any general distribution given only the density and the support of the distribution. I'm not aware of any general purpose algorithm that would be capable of correctly and efficiently implementing the described functionality. From what I know, this is essentially an unsolved research problem. Existing algorithms either require more knowledge about the distribution and the problem domain or work only under very limited circumstances. Even the state of the art special purpose library UNU.RAN does not solve the problem in full generality, and in any case, testing and customer support for such a library feature would be a nightmare.
Possible resolution: For these reasons, I propose to delete section 99 [rand.dist.samp.genpdf].
[ Bellevue: ]
Disagreement persists.
Objection to this issue is that this function takes a general functor. The general approach would be to normalize this function, integrate it, and take the inverse of the integral, which is not possible in general. An example function is sin(1+n*x) — for any spatial frequency that the implementor chooses, there is a value of n that renders that choice arbitrarily erroneous.
Correction: The formula above should instead read 1+sin(n*x).
Objector proposes the following possible compromise positions:
- rand.dist.samp.genpdf takes an number of points so that implementor need not guess.
- replace rand.disk.samp.genpdf with an extension to either or both of the discrete functions to take arguments that take a functor and number of points in place of the list of probabilities. Reference issues 793 and 794.
Proposed resolution:
See N2813 for the proposed resolution.
Rationale:
Addressed by N2836 "Wording Tweaks for Concept-enabled Random Number Generation in C++0X".
Section: 29.5.9.5.3 [rand.dist.norm.chisq] Status: CD1 Submitter: Stephan Tolksdorf Opened: 2007-09-21 Last modified: 2016-01-28
Priority: Not Prioritized
View all issues with CD1 status.
Discussion:
chi_squared_distribution
, fisher_f_distribution
and student_t_distribution
have parameters for the "degrees of freedom" n
and m
that are specified as integers. For the
following two reasons this is an unnecessary restriction: First, in many applications such as
Bayesian inference or Monte Carlo simulations it is more convenient to treat the respective param-
eters as continuous variables. Second, the standard non-naive algorithms (i.e.
O(1) algorithms) for simulating from these distributions work with floating-point parameters anyway (all
three distributions could be easily implemented using the Gamma distribution, for instance).
Similar arguments could in principle be made for the parameters t
and k
of the discrete
binomial_distribution
and negative_binomial_distribution
, though in both cases continuous
parameters are less frequently used in practice and in case of the binomial_distribution
the implementation would be significantly complicated by a non-discrete parameter (in most
implementations one would need an approximation of the log-gamma function instead of just the
log-factorial function).
Possible resolution: For these reasons, I propose to change the type of the respective parameters to double.
[ Bellevue: ]
In N2424. Not wildly enthusiastic, not really felt necessary. Less frequently used in practice. Not terribly bad either. Move to OPEN.
[ Sophia Antipolis: ]
Marc Paterno: The generalizations were explicitly left out when designing the facility. It's harder to test.
Marc Paterno: Ask implementers whether floating-point is a significant burden.
Alisdair: It's neater to do it now, do ask Bill Plauger.
Disposition: move to review with the option for "NAD" if it's not straightforward to implement; unanimous consent.
Proposed resolution:
See N2424 for the proposed resolution.
[ Stephan Tolksdorf adds pre-Bellevue: ]
In 29.5.9.5.3 [rand.dist.norm.chisq]:
Delete ", where
n
is a positive integer" in the first paragraph.Replace both occurrences of "
explicit chi_squared_distribution(int n = 1);
" with "explicit chi_squared_distribution(RealType n = 1);
".Replace both occurrences of "
int n() const;
" with "RealType n() const;
".In 29.5.9.5.5 [rand.dist.norm.f]:
Delete ", where
m
andn
are positive integers" in the first paragraph.Replace both occurrences of
explicit fisher_f_distribution(int m = 1, int n = 1);with
explicit fisher_f_distribution(RealType m = 1, RealType n = 1);Replace both occurrences of "
int m() const;" with "RealType m() const;
".Replace both occurrences of "
int n() const;" with "RealType n() const;
".In 29.5.9.5.6 [rand.dist.norm.t]:
Delete ", where
n
is a positive integer" in the first paragraph.Replace both occurrences of "
explicit student_t_distribution(int n = 1);
" with "explicit student_t_distribution(RealType n = 1);
".Replace both occurrences of "
int n() const;
" with "RealType n() const;
".
*_ptr<T[N]>
Section: 20.3.1 [unique.ptr] Status: CD1 Submitter: Herb Sutter Opened: 2007-10-04 Last modified: 2016-01-28
Priority: Not Prioritized
View all other issues in [unique.ptr].
View all issues with CD1 status.
Discussion:
Please don't provide *_ptr<T[N]>
. It doesn't enable any useful
bounds-checking (e.g., you could imagine that doing op++
on a
shared_ptr<T[N]>
yields a shared_ptr<T[N-1]>
, but that promising path
immediately falters on op--
which can't reliably dereference because we
don't know the lower bound). Also, most buffers you'd want to point to
don't have a compile-time known size.
To enable any bounds-checking would require run-time information, with
the usual triplet: base (lower bound), current offset, and max offset
(upper bound). And I can sympathize with the point of view that you
wouldn't want to require this on *_ptr
itself. But please let's not
follow the <T[N]>
path, especially not with additional functions to
query the bounds etc., because this sets wrong user expectations by
embarking on a path that doesn't go all the way to bounds checking as it
seems to imply.
If bounds checking is desired, consider a checked_*_ptr
instead (e.g.,
checked_shared_ptr
). And make the interfaces otherwise identical so that
user code could easily #define/typedef
between prepending checked_
on
debug builds and not doing so on release builds (for example).
Note that some may object that checked_*_ptr
may seem to make the smart
pointer more like vector
, and we don't want two ways to spell vector
. I
don't agree, but if that were true that would be another reason to
remove *_ptr<T[N]>
which equally makes the smart pointer more like
std::array.
:-)
[ Bellevue: ]
Suggestion that fixed-size array instantiations are going to fail at compile time anyway (if we remove specialization) due to pointer decay, at least that appears to be result from available compilers.
So concerns about about requiring static_assert seem unfounded.
After a little more experimentation with compiler, it appears that fixed size arrays would only work at all if we supply these explicit specialization. So removing them appears less breaking than originally thought.
straw poll unanimous move to Ready.
Proposed resolution:
Change the synopsis under 20.3.1 [unique.ptr] p2:
... template<class T> struct default_delete; template<class T> struct default_delete<T[]>;template<class T, size_t N> struct default_delete<T[N]>;template<class T, class D = default_delete<T>> class unique_ptr; template<class T, class D> class unique_ptr<T[], D>;template<class T, class D, size_t N> class unique_ptr<T[N], D>;...
Remove the entire section [unique.ptr.dltr.dflt2] default_delete<T[N]>
.
Remove the entire section [unique.ptr.compiletime] unique_ptr
for array objects with a compile time length
and its subsections: [unique.ptr.compiletime.dtor], [unique.ptr.compiletime.observers],
[unique.ptr.compiletime.modifiers].
swap
for proxy iteratorsSection: 16.4.4.2 [utility.arg.requirements] Status: Resolved Submitter: Howard Hinnant Opened: 2007-10-10 Last modified: 2016-01-28
Priority: Not Prioritized
View all other issues in [utility.arg.requirements].
View all issues with Resolved status.
Discussion:
This issue was split from 672(i). 672(i) now just
deals with changing the requirements of T
in the Swappable
requirement from CopyConstructible
and CopyAssignable
to
MoveConstructible
and MoveAssignable
.
This issue seeks to widen the Swappable
requirement to support proxy iterators. Here
is example code:
namespace Mine { template <class T> struct proxy {...}; template <class T> struct proxied_iterator { typedef T value_type; typedef proxy<T> reference; reference operator*() const; ... }; struct A { // heavy type, has an optimized swap, maybe isn't even copyable or movable, just swappable void swap(A&); ... }; void swap(A&, A&); void swap(proxy<A>, A&); void swap(A&, proxy<A>); void swap(proxy<A>, proxy<A>); } // Mine ... Mine::proxied_iterator<Mine::A> i(...) Mine::A a; swap(*i1, a);
The key point to note in the above code is that in the call to swap
, *i1
and a
are different types (currently types can only be Swappable
with the
same type). A secondary point is that to support proxies, one must be able to pass rvalues
to swap
. But note that I am not stating that the general purpose std::swap
should accept rvalues! Only that overloaded swap
s, as in the example above, be allowed
to take rvalues.
That is, no standard library code needs to change. We simply need to have a more flexible
definition of Swappable
.
[ Bellevue: ]
While we believe Concepts work will define a swappable concept, we should still resolve this issue if possible to give guidance to the Concepts work.
Would an ambiguous swap function in two namespaces found by ADL break this wording? Suggest that the phrase "valid expression" means such a pair of types would still not be swappable.
Motivation is proxy-iterators, but facility is considerably more general. Are we happy going so far?
We think this wording is probably correct and probably an improvement on what's there in the WP. On the other hand, what's already there in the WP is awfully complicated. Why do we need the two bullet points? They're too implementation-centric. They don't add anything to the semantics of what swap() means, which is there in the post-condition. What's wrong with saying that types are swappable if you can call swap() and it satisfies the semantics of swapping?
[ 2009-07-28 Reopened by Alisdair. No longer solved by concepts. ]
[ 2009-10 Santa Cruz: ]
Leave as Open. Dave to provide wording.
[ 2009-11-08 Howard adds: ]
Updated wording to sync with N3000. Also this issue is very closely related to 594(i).
[ 2010 Pittsburgh: ]
Moved to
NAD EditorialResolved. Rationale added.
Rationale:
Solved by N3048.
Proposed resolution:
Change 16.4.4.2 [utility.arg.requirements]:
-1- The template definitions in the C++ Standard Library refer to various named requirements whose details are set out in tables 31-38. In these tables,
T
andV
areis atypes to be supplied by a C++ program instantiating a template;a
,b
, andc
are values of typeconst T
;s
andt
are modifiable lvalues of typeT
;u
is a value of type (possiblyconst
)T
;andrv
is a non-const
rvalue of typeT
;w
is a value of typeT
; andv
is a value of typeV
.
Table 37: Swappable
requirements [swappable]expression Return type Post-condition swap(
sw,tv)void
t
w
has the value originally held byu
v
, andu
v
has the value originally held byt
w
The
Swappable
requirement is met by satisfying one or more of the following conditions:
T
isSwappable
ifT
andV
are the same type andT
satisfies theMoveConstructible
requirements (Table 33) and theMoveAssignable
requirements (Table 35);T
isSwappable
withV
if a namespace scope function namedswap
exists in the same namespace as the definition ofT
orV
, such that the expressionswap(
is valid and has the semantics described in this table.sw,tv)T
isSwappable
ifT
is an array type whose element type isSwappable
.
Rationale:
[ post San Francisco: ]
Solved by N2758.
swap
for shared_ptr
Section: 20.3.2.2.9 [util.smartptr.shared.spec] Status: CD1 Submitter: Howard Hinnant Opened: 2007-10-10 Last modified: 2016-01-28
Priority: Not Prioritized
View all issues with CD1 status.
Discussion:
When the LWG looked at 674(i) in Kona the following note was made:
We may need to open an issue to deal with the question of whether
shared_ptr
needs an rvalueswap
.
This issue was opened in response to that note.
I believe allowing rvalue shared_ptr
s to swap
is both
appropriate, and consistent with how other library components are currently specified.
[ Bellevue: ]
Concern that the three signatures for swap is needlessly complicated, but this issue merely brings shared_ptr into equal complexity with the rest of the library. Will open a new issue for concern about triplicate signatures.
Adopt issue as written.
Proposed resolution:
Change the synopsis in 20.3.2.2 [util.smartptr.shared]:
void swap(shared_ptr&& r); ... template<class T> void swap(shared_ptr<T>& a, shared_ptr<T>& b); template<class T> void swap(shared_ptr<T>&& a, shared_ptr<T>& b); template<class T> void swap(shared_ptr<T>& a, shared_ptr<T>&& b);
Change 20.3.2.2.5 [util.smartptr.shared.mod]:
void swap(shared_ptr&& r);
Change 20.3.2.2.9 [util.smartptr.shared.spec]:
template<class T> void swap(shared_ptr<T>& a, shared_ptr<T>& b); template<class T> void swap(shared_ptr<T>&& a, shared_ptr<T>& b); template<class T> void swap(shared_ptr<T>& a, shared_ptr<T>&& b);
exception_ptr
?Section: 17.9.7 [propagation] Status: CD1 Submitter: Alisdair Meredith Opened: 2007-10-10 Last modified: 2016-01-28
Priority: Not Prioritized
View all other issues in [propagation].
View all issues with CD1 status.
Discussion:
Without some lifetime guarantee, it is hard to know how this type can be
used. Very specifically, I don't see how the current wording would
guarantee and exception_ptr
caught at the end of one thread could be safely
stored and rethrown in another thread - the original motivation for this
API.
(Peter Dimov agreed it should be clearer, maybe a non-normative note to explain?)
[ Bellevue: ]
Agree the issue is real.
Intent is lifetime is similar to a shared_ptr (and we might even want to consider explicitly saying that it is a shared_ptr< unspecified type >).
We expect that most implementations will use shared_ptr, and the standard should be clear that the exception_ptr type is intended to be something whose semantics are smart-pointer-like so that the user does not need to worry about lifetime management. We still need someone to draught those words - suggest emailing Peter Dimov.
Move to Open.
Proposed resolution:
Change 17.9.7 [propagation]/7:
-7- Returns: An
exception_ptr
object that refers to the currently handled exception or a copy of the currently handled exception, or a nullexception_ptr
object if no exception is being handled. The referenced object remains valid at least as long as there is anexception_ptr
that refers to it. If the function needs to allocate memory and the attempt fails, it returns anexception_ptr
object that refers to an instance ofbad_alloc
. It is unspecified whether the return values of two successive calls tocurrent_exception
refer to the same exception object. [Note: that is, it is unspecified whethercurrent_exception
creates a new copy each time it is called. --end note]
current_exception
may fail with bad_alloc
Section: 17.9.7 [propagation] Status: CD1 Submitter: Alisdair Meredith Opened: 2007-10-10 Last modified: 2016-01-28
Priority: Not Prioritized
View all other issues in [propagation].
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Discussion:
I understand that the attempt to copy an exception may run out of memory,
but I believe this is the only part of the standard that mandates failure
with specifically bad_alloc
, as opposed to allowing an
implementation-defined type derived from bad_alloc
. For instance, the Core
language for a failed new expression is:
Any other allocation function that fails to allocate storage shall indicate failure only by throwing an exception of a type that would match a handler (15.3) of type
std::bad_alloc
(18.5.2.1).
I think we should allow similar freedom here (or add a blanket compatible-exception freedom paragraph in 17)
I prefer the clause 17 approach myself, and maybe clean up any outstanding wording that could also rely on it.
Although filed against a specific case, this issue is a problem throughout the library.
[ Bellevue: ]
Is issue bigger than library?
No - Core are already very clear about their wording, which is inspiration for the issue.
While not sold on the original 18.7.5 use case, the generalised 17.4.4.8 wording is the real issue.
Accept the broad view and move to ready
Proposed resolution:
Add the following exemption clause to 16.4.6.14 [res.on.exception.handling]:
A function may throw a type not listed in its Throws clause so long as it is derived from a class named in the Throws clause, and would be caught by an exception handler for the base type.
has_nothrow_copy_constructor<T>::value
is true if T
has 'a' nothrow copy constructor.Section: 21.3.5.4 [meta.unary.prop] Status: CD1 Submitter: Alisdair Meredith Opened: 2007-10-10 Last modified: 2016-01-28
Priority: Not Prioritized
View other active issues in [meta.unary.prop].
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Discussion:
Unfortunately a class can have multiple copy constructors, and I believe to be useful this trait should only return true is ALL copy constructors are no-throw.
For instance:
struct awkward { awkward( const awkward & ) throw() {} awkward( awkward & ) { throw "oops"; } };
Proposed resolution:
Change 21.3.5.4 [meta.unary.prop]:
has_trivial_copy_constructor
T
is a trivial type (3.9) or a reference type or a class typewith a trivial copy constructorwhere all copy constructors are trivial (12.8).
has_trivial_assign
T
is neitherconst
nor a reference type, andT
is a trivial type (3.9) or a class typewith a trivial copy assignment operatorwhere all copy assignment operators are trivial (12.8).
has_nothrow_copy_constructor
has_trivial_copy_constructor<T>::value
istrue
orT
is a class typewith awhere all copy constructorsthat isare known not to throw any exceptions orT
is an array of such a class type
has_nothrow_assign
T
is neitherconst
nor a reference type, andhas_trivial_assign<T>::value
istrue
orT
is a class typewith awhere all copy assignment operators takeingan lvalue of typeT
that is known not to throw any exceptions orT
is an array of such a class type.
Section: 16.4.4.6 [allocator.requirements] Status: C++11 Submitter: Hans Boehm Opened: 2007-10-11 Last modified: 2016-01-28
Priority: Not Prioritized
View other active issues in [allocator.requirements].
View all other issues in [allocator.requirements].
View all issues with C++11 status.
Discussion:
Did LWG recently discuss 16.4.4.6 [allocator.requirements]-2, which states that "All the operations on the allocators are expected to be amortized constant time."?
As I think I pointed out earlier, this is currently fiction for
allocate()
if it has to obtain memory from the OS, and it's unclear to
me how to interpret this for construct()
and destroy()
if they deal with
large objects. Would it be controversial to officially let these take
time linear in the size of the object, as they already do in real life?
Allocate()
more blatantly takes time proportional to the size of the
object if you mix in GC. But it's not really a new problem, and I think
we'd be confusing things by leaving the bogus requirements there. The
current requirement on allocate()
is generally not important anyway,
since it takes O(size) to construct objects in the resulting space.
There are real performance issues here, but they're all concerned with
the constants, not the asymptotic complexity.
Proposed resolution:
Change 16.4.4.6 [allocator.requirements]/2:
-2- Table 39 describes the requirements on types manipulated through allocators.
All the operations on the allocators are expected to be amortized constant time.Table 40 describes the requirements on allocator types.
Section: 16.4.4.2 [utility.arg.requirements] Status: C++11 Submitter: Yechezkel Mett Opened: 2007-10-14 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
The draft standard n2369 uses the term move constructor in a few places, but doesn't seem to define it.
MoveConstructible
requirements are defined in Table 33 in 16.4.4.2 [utility.arg.requirements] as
follows:
MoveConstructible
requirementsexpression post-condition T t = rv
t
is equivalent to the value ofrv
before the construction[Note: There is no requirement on the value of rv
after the construction. -- end note]
(where rv
is a non-const rvalue of type T
).
So I assume the move constructor is the constructor that would be used in filling the above requirement.
For vector::reserve
, vector::resize
and the vector
modifiers given in
23.3.13.5 [vector.modifiers] we have
Requires: If
value_type
has a move constructor, that constructor shall not throw any exceptions.
Firstly "If value_type
has a move constructor" is superfluous; every
type which can be put into a vector
has a move constructor (a copy
constructor is also a move constructor). Secondly it means that for
any value_type
which has a throwing copy constructor and no other move
constructor these functions cannot be used -- which I think will come
as a shock to people who have been using such types in vector
until
now!
I can see two ways to correct this. The simpler, which is presumably
what was intended, is to say "If value_type
has a move constructor and
no copy constructor, the move constructor shall not throw any
exceptions" or "If value_type
has a move constructor which changes the
value of its parameter,".
The other alternative is add to MoveConstructible
the requirement that
the expression does not throw. This would mean that not every type
that satisfies the CopyConstructible
requirements also satisfies the
MoveConstructible
requirements. It would mean changing requirements in
various places in the draft to allow either MoveConstructible
or
CopyConstructible
, but I think the result would be clearer and
possibly more concise too.
Proposed resolution:
Add new defintions to [definitions]:
move constructor
a constructor which accepts only rvalue arguments of that type, and modifies the rvalue as a side effect during the construction.
move assignment operator
an assignment operator which accepts only rvalue arguments of that type, and modifies the rvalue as a side effect during the assignment.
move assignment
use of the move assignment operator.
[ Howard adds post-Bellevue: ]
Unfortunately I believe the wording recommended by the LWG in Bellevue is incorrect.
reserve
et. al. will use a move constructor if one is available, else it will use a copy constructor. A type may have both. If the move constructor is used, it must not throw. If the copy constructor is used, it can throw. The sentence in the proposed wording is correct without the recommended insertion. The Bellevue LWG recommended moving this issue to Ready. I am unfortunately pulling it back to Open. But I'm drafting wording to atone for this egregious action. :-)
std::vector
and std:string
lack explicit shrink-to-fit operationsSection: 23.3.13.3 [vector.capacity], 27.4.3.5 [string.capacity] Status: CD1 Submitter: Beman Dawes Opened: 2007-10-31 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
A std::vector
can be shrunk-to-fit via the swap idiom:
vector<int> v; ... v.swap(vector<int>(v)); // shrink to fitor:
vector<int>(v).swap(v); // shrink to fitor:
swap(v, vector<int>(v)); // shrink to fit
A non-binding request for shrink-to-fit can be made to a std::string
via:
string s; ... s.reserve(0);
Neither of these is at all obvious to beginners, and even some experienced C++ programmers are not aware that shrink-to-fit is trivially available.
Lack of explicit functions to perform these commonly requested operations makes vector and string less usable for non-experts. Because the idioms are somewhat obscure, code readability is impaired. It is also unfortunate that two similar vector-like containers use different syntax for the same operation.
The proposed resolution addresses these concerns. The proposed function takes no arguments to keep the solution simple and focused.
Proposed resolution:
To Class template basic_string 27.4.3 [basic.string] synopsis, Class template vector 23.3.13 [vector] synopsis, and Class vector<bool> 23.3.14 [vector.bool] synopsis, add:
void shrink_to_fit();
To basic_string capacity 27.4.3.5 [string.capacity] and vector capacity 23.3.13.3 [vector.capacity], add:
void shrink_to_fit();Remarks:
shrink_to_fit
is a non-binding request to reducecapacity()
tosize()
. [Note: The request is non-binding to allow latitude for implementation-specific optimizations. — end note]
[
850(i) has been added to deal with this issue with respect to deque
.
]
Section: 23.6 [container.adaptors] Status: Resolved Submitter: Paolo Carlini Opened: 2007-10-31 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
After n2369 we have a single push_back
overload in the sequence containers,
of the "emplace" type. At variance with that, still in n2461, we have
two separate overloads, the C++03 one + one taking an rvalue reference
in the container adaptors. Therefore, simply from a consistency point of
view, I was wondering whether the container adaptors should be aligned
with the specifications of the sequence container themselves: thus have
a single push
along the lines:
template<typename... _Args> void push(_Args&&... __args) { c.push_back(std::forward<_Args>(__args)...); }
Proposed resolution:
Change 23.6.3.1 [queue.defn]:
void push(const value_type& x) { c.push_back(x); }void push(value_type&& x) { c.push_back(std::move(x)); }template<class... Args> void push(Args&&... args) { c.push_back(std::forward<Args>(args)...); }
Change 23.6.4 [priority.queue]:
void push(const value_type& x) { c.push_back(x); }void push(value_type&& x) { c.push_back(std::move(x)); }template<class... Args> void push(Args&&... args) { c.push_back(std::forward<Args>(args)...); }
Change 23.6.4.4 [priqueue.members]:
void push(const value_type& x);
Effects:c.push_back(x);push_heap(c.begin(), c.end(), comp);template<class... Args> void push(value_typeArgs&&...xargs);Effects:
c.push_back(std::moveforward<Args>(xargs)...); push_heap(c.begin(), c.end(), comp);
Change 23.6.6.2 [stack.defn]:
void push(const value_type& x) { c.push_back(x); }void push(value_type&& x) { c.push_back(std::move(x)); }template<class... Args> void push(Args&&... args) { c.push_back(std::forward<Args>(args)...); }
Rationale:
Addressed by N2680 Proposed Wording for Placement Insert (Revision 1).
shared_ptr
and nullptr
Section: 20.3.2.2 [util.smartptr.shared] Status: C++11 Submitter: Joe Gottman Opened: 2007-10-31 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
Consider the following program:
int main() { shared_ptr<int> p(nullptr); return 0; }
This program will fail to compile because shared_ptr
uses the following
template constructor to construct itself from pointers:
template <class Y> shared_ptr(Y *);
According
to N2431,
the conversion from nullptr_t
to Y *
is not
deducible, so the above constructor will not be found. There are similar problems with the
constructors that take a pointer and a deleter
or a
pointer, a deleter
and an allocator, as well as the
corresponding forms of reset()
. Note that N2435
will solve this problem for constructing from just nullptr
, but not for constructors that use
deleters
or allocators or for reset()
.
In the case of the functions that take deleters, there is the additional
question of what argument should be passed to the deleter when it is
eventually called. There are two reasonable possibilities: nullptr
or
static_cast<T *>(0)
, where T
is the template argument of the
shared_ptr
. It is not immediately clear which of these is better. If
D::operator()
is a template function similar to shared_ptr
's
constructor, then d(static_cast<T*>(0))
will compile and d(nullptr)
will not. On the other hand, if D::operator()()
takes a parameter that
is a pointer to some type other that T
(for instance U*
where U
derives
from T
) then d(nullptr)
will compile and d(static_cast<T *>(0))
may not.
[ Bellevue: ]
The general idea is right, we need to be able to pass a nullptr to a shared_ptr, but there are a few borderline editorial issues here. (For example, the single-argument nullptr_t constructor in the class synopsis isn't marked explicit, but it is marked explicit in the proposed wording for 20.6.6.2.1. There is a missing empty parenthesis in the form that takes a nullptr_t, a deleter, and an allocator.)
More seriously: this issue says that a shared_ptr constructed from a nullptr is empty. Since "empty" is undefined, it's hard to know whether that's right. This issue is pending on handling that term better.
Peter suggests definition of empty should be "does not own anything"
Is there an editorial issue that post-conditions should refer to get() = nullptr, rather than get() = 0?
No strong feeling towards accept or NAD, but prefer to make a decision than leave it open.
Seems there are no technical merits between NAD and Ready, comes down to "Do we intentially want to allow/disallow null pointers with these functions". Staw Poll - support null pointers 5 - No null pointers 0
Move to Ready, modulo editorial comments
[ post Bellevue Peter adds: ]
The following wording changes are less intrusive:
In 20.3.2.2.2 [util.smartptr.shared.const], add:
shared_ptr(nullptr_t);after:
shared_ptr();(Absence of explicit intentional.)
px.reset( nullptr )
seems a somewhat contrived way to writepx.reset()
, so I'm not convinced of its utility.It's similarly not clear to me whether the deleter constructors need to be extended to take
nullptr
, but if they need to:Add
template<class D> shared_ptr(nullptr_t p, D d); template<class D, class A> shared_ptr(nullptr_t p, D d, A a);after
template<class Y, class D> shared_ptr(Y* p, D d); template<class Y, class D, class A> shared_ptr(Y* p, D d, A a);Note that this changes the semantics of the new constructors such that they consistently call
d(p)
instead ofd((T*)0)
whenp
isnullptr
.The ability to be able to pass
0/NULL
to a function that takes ashared_ptr
has repeatedly been requested by users, but the other additions that the proposed resolution makes are not supported by real world demand or motivating examples.It might be useful to split the obvious and non-controversial
nullptr_t
constructor into a separate issue. Waiting for "empty" to be clarified is unnecessary; this is effectively an alias for the default constructor.
[ Sophia Antipolis: ]
We want to remove the reset functions from the proposed resolution.
The remaining proposed resolution text (addressing the constructors) are wanted.
Disposition: move to review. The review should check the wording in the then-current working draft.
Proposed resolution:
In 20.3.2.2 [util.smartptr.shared] p4, add to the definition/synopsis
of shared_ptr
:
template<class D> shared_ptr(nullptr_t p, D d); template<class D, class A> shared_ptr(nullptr_t p, D d, A a);
after
template<class Y, class D> shared_ptr(Y* p, D d); template<class Y, class D, class A> shared_ptr(Y* p, D d, A a);
In 20.3.2.2.2 [util.smartptr.shared.const] add:
template<class D> shared_ptr(nullptr_t p, D d); template<class D, class A> shared_ptr(nullptr_t p, D d, A a);
after
template<class Y, class D> shared_ptr(Y* p, D d); template<class Y, class D, class A> shared_ptr(Y* p, D d, A a);
(reusing the following paragraphs 20.3.2.2.2 [util.smartptr.shared.const]/9-13 that speak of p.)
In 20.3.2.2.2 [util.smartptr.shared.const]/10, change
Effects: Constructs a
shared_ptr
object that owns thepointerobjectp
and the deleterd
. The second constructor shall use a copy ofa
to allocate memory for internal use.
Rationale:
[ San Francisco: ]
"pointer" is changed to "object" to handle the fact that
nullptr_t
isn't a pointer.
Section: 23.2 [container.requirements] Status: CD1 Submitter: Jens Maurer Opened: 2007-11-06 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
23.2 [container.requirements] says:
-12- Objects passed to member functions of a container as rvalue references shall not be elements of that container. No diagnostic required.
A reference is not an object, but this sentence appears to claim so.
What is probably meant here:
An object bound to an rvalue reference parameter of a member function of a container shall not be an element of that container; no diagnostic required.
Proposed resolution:
Change 23.2 [container.requirements]:
-12-
Objects passed to member functions of a container as rvalue references shall not be elementsAn object bound to an rvalue reference parameter of a member function of a container shall not be an element of that container.;Nno diagnostic required.
unordered_map
needs an at()
member functionSection: 23.5.3.3 [unord.map.elem] Status: CD1 Submitter: Joe Gottman Opened: 2007-11-15 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
The new member function at()
was recently added to std::map()
. It acts
like operator[]()
, except it throws an exception when the input key is
not found. It is useful when the map
is const
, the value_type
of the
key doesn't have a default constructor, it is an error if the key is
not found, or the user wants to avoid accidentally adding an element to
the map. For exactly these same reasons, at()
would be equally useful
in std::unordered_map
.
Proposed resolution:
Add the following functions to the definition of unordered_map
under "lookup" (23.5.3 [unord.map]):
mapped_type& at(const key_type& k); const mapped_type &at(const key_type &k) const;
Add the following definitions to 23.5.3.3 [unord.map.elem]:
mapped_type& at(const key_type& k); const mapped_type &at(const key_type &k) const;Returns: A reference to
x.second
, wherex
is the (unique) element whose key is equivalent tok
.Throws: An exception object of type
out_of_range
if no such element is present.
[ Bellevue: Editorial note: the "(unique)" differs from map. ]
std::unique_ptr
requires complete type?Section: 20.3.1 [unique.ptr] Status: CD1 Submitter: Daniel Krügler Opened: 2007-11-30 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
In contrast to the proposed std::shared_ptr
, std::unique_ptr
does currently not support incomplete types, because it
gives no explicit grant - thus instantiating unique_ptr
with
an incomplete pointee type T
automatically belongs to
undefined behaviour according to 16.4.5.8 [res.on.functions]/2, last
bullet. This is an unnecessary restriction and prevents
many well-established patterns - like the bridge pattern - for std::unique_ptr
.
[ Bellevue: ]
Move to open. The LWG is comfortable with the intent of allowing incomplete types and making
unique_ptr
more likeshared_ptr
, but we are not comfortable with the wording. The specification forunique_ptr
should be more like that ofshared_ptr
. We need to know, for individual member functions, which ones require their types to be complete. Theshared_ptr
specification is careful to say that for each function, and we need the same level of care here. We also aren't comfortable with the "part of the operational semantic" language; it's not used elsewhere in the standard, and it's not clear what it means. We need a volunteer to produce new wording.
Proposed resolution:
The proposed changes in the following revision refers to the current state of N2521 including the assumption that [unique.ptr.compiletime] will be removed according to the current state of 740(i).
The specialization unique_ptr<T[]>
has some more restrictive constraints on
type-completeness on T
than unique_ptr<T>
. The following proposed wordings
try to cope with that. If the committee sees less usefulness on relaxed
constraints on unique_ptr<T[]>
, the alternative would be to stop this relaxation
e.g. by adding one further bullet to 20.3.1.4 [unique.ptr.runtime]/1:
"T
shall be a complete type, if used as template argument of
unique_ptr<T[], D>
This issue has some overlap with 673(i), but it seems not to cause any
problems with this one,
because 673(i) adds only optional requirements on D
that do not conflict
with the here discussed
ones, provided that D::pointer
's operations (including default
construction, copy construction/assignment,
and pointer conversion) are specified not to throw, otherwise this
would have impact on the
current specification of unique_ptr
.
In 20.3.1 [unique.ptr]/2 add as the last sentence to the existing para:
The
unique_ptr
provides a semantics of strict ownership. Aunique_ptr
owns the object it holds a pointer to. Aunique_ptr
is notCopyConstructible
, norCopyAssignable
, however it isMoveConstructible
andMoveAssignable
. The template parameterT
ofunique_ptr
may be an incomplete type. [ Note: The uses ofunique_ptr
include providing exception safety for dynamically allcoated memory, passing ownership of dynamically allocated memory to a function, and returning dynamically allocated memory from a function. -- end note ]
20.3.1.3.2 [unique.ptr.single.ctor]/1: No changes necessary.
[ N.B.: We only need the requirement that
D
isDefaultConstructible
. The current wording says just this. ]
In 20.3.1.3.2 [unique.ptr.single.ctor]/5 change the requires clause to say:
Requires:
The expressionD()(p)
shall be well formed. The default constructor ofD
shall not throw an exception.D
must not be a reference type.D
shall be default constructible, and that construction shall not throw an exception.[ N.B.: There is no need that the expression
D()(p)
is well-formed at this point. I assume that the current wording is based on the correspondingshared_ptr
wording. In case ofshared_ptr
this requirement is necessary, because the corresponding c'tor *can* fail and must invoke deletep/d(p)
in this case.Unique_ptr
is simpler in this regard. The *only* functions that must insist on well-formedness and well-definedness of the expressionget_deleter()(get())
are (1) the destructor and (2)reset
. The reasoning for the wording change to explicitly requireDefaultConstructible
ofD
is to guarantee that invocation ofD
's default c'tor is both well-formed and well-defined. Note also that we do *not* need the requirement thatT
must be complete, also in contrast toshared_ptr
.Shared_ptr
needs this, because it's c'tor is a template c'tor which potentially requiresConvertible<Y*, X*>
, which again requires Completeness ofY
, if!SameType<X, Y>
]
Merge 20.3.1.3.2 [unique.ptr.single.ctor]/12+13 thereby removing the sentence of 12, but transferring the "requires" to 13:
Requires: If
D
is not an lvalue-reference type then[..][ N.B.: For the same reasons as for (3), there is no need that
d(p)
is well-formed/well-defined at this point. The current wording guarantees all what we need, namely that the initialization of both theT*
pointer and theD
deleter are well-formed and well-defined. ]
20.3.1.3.2 [unique.ptr.single.ctor]/21:
Requires: If
D
is not a reference type, construction of the deleterD
from an rvalue of typeE
shall be well formed and shall not throw an exception. IfD
is a reference type, thenE
shall be the same type asD
(diagnostic required).U*
shall be implicitly convertible toT*
. [Note: These requirements imply thatT
andU
be complete types. -- end note]
[
N.B.: The current wording of 21 already implicitly guarantees that U
is completely defined, because it requires that Convertible<U*, T*>
is
true. If the committee wishes this explicit requirement can be added,
e.g. "U
shall be a complete type."
]
20.3.1.3.3 [unique.ptr.single.dtor]: Just before p1 add a new paragraph:
Requires: The expression
get_deleter()(get())
shall be well-formed, shall have well-defined behavior, and shall not throw exceptions. [Note: The use ofdefault_delete
requiresT
to be a complete type. -- end note][ N.B.: This requirement ensures that the whole responsibility on type-completeness of
T
is delegated to this expression. ]
20.3.1.3.4 [unique.ptr.single.asgn]/1: No changes necessary, except the current editorial issue, that "must shall" has to be changed to "shall", but this change is not a special part of this resolution.
[ N.B. The current wording is sufficient, because we can delegate all further requirements on the requirements of the effects clause ]
20.3.1.3.4 [unique.ptr.single.asgn]/6:
Requires: Assignment of the deleter
D
from an rvalueD
shall not throw an exception.U*
shall be implicitly convertible toT*
. [Note: These requirements imply thatT
andU
be complete types. -- end note]
[
N.B.: The current wording of p. 6 already implicitly guarantees that
U
is completely defined, because it requires that Convertible<U*, T*>
is true, see (6)+(8).
]
20.3.1.3.4 [unique.ptr.single.asgn]/11: No changes necessary.
[ N.B.: Delegation to requirements of effects clause is sufficient. ]
20.3.1.3.5 [unique.ptr.single.observers]/1+4+7+9+11:
T* operator->() const;Note: Use typically requires
T
shall be complete. — end note]
20.3.1.3.6 [unique.ptr.single.modifiers]/4: Just before p. 4 add a new paragraph:
Requires: The expression
get_deleter()(get())
shall be well-formed, shall have well-defined behavior, and shall not throw exceptions.
20.3.1.4 [unique.ptr.runtime]: Add one additional bullet on paragraph 1:
A specialization for array types is provided with a slightly altered interface.
- ...
T
shall be a complete type.
[ post Bellevue: Daniel provided revised wording. ]
Section: 24.3.4 [iterator.concepts] Status: C++11 Submitter: Martin Sebor Opened: 2007-12-14 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
Issue 278(i) defines the meaning of the term "invalid iterator" as one that may be singular.
Consider the following code:
std::deque<int> x, y; std::deque<int>::iterator i = x.end(), j = y.end(); x.swap(y);
Given that swap()
is required not to invalidate iterators
and using the definition above, what should be the expected result of
comparing i
and j
to x.end()
and y.end()
, respectively, after the swap()
?
I.e., is the expression below required to evaluate
to true
?
i == y.end() && j == x.end()
(There are at least two implementations where the expression
returns false
.)
More generally, is the definition introduced in issue 278(i) meant to make any guarantees about whether iterators actually point to the same elements or be associated with the same containers after a non-invalidating operation as they did before?
Here's a motivating example intended to demonstrate the importance of the question:
Container x, y ({ 1, 2}); // pseudocode to initialize y with { 1, 2 } Container::iterator i = y.begin() + 1; Container::iterator j = y.end(); std::swap(x, y); std::find(i, j, 3);
swap()
guarantees that i
and j
continue to be valid. Unless the spec says that even though they are
valid they may no longer denote a valid range the code above must be
well-defined. Expert opinions on this differ as does the behavior of
popular implementations for some standard Containers
.
[ San Francisco: ]
Pablo: add a note to the last bullet of paragraph 11 of 23.1.1 clarifying that the
end()
iterator doesn't refer to an element and that it can therefore be invalidated.Proposed wording:
[Note: The
end()
iterator does not refer to any element and can therefore be invalidated. -- end note]Howard will add this proposed wording to the issue and then move it to Review.
[ Post Summit: ]
Lawrence: suggestion: "Note: The
end()
iterator does not refer to any element"Walter: "Note: The
end()
iterator can nevertheless be invalidated, because it does not refer to any element."Nick: "The
end()
iterator does not refer to any element. It is therefore subject to being invalidated."Consensus: go with Nick
With that update, Recommend Tentatively Ready.
Proposed resolution:
Add to 23.2.2 [container.requirements.general], p11:
Unless otherwise specified (see 23.1.4.1, 23.1.5.1, 23.2.2.3, and 23.2.6.4) all container types defined in this Clause meet the following additional requirements:
- ...
- no
swap()
function invalidates any references, pointers, or iterators referring to the elements of the containers being swapped. [Note: Theend()
iterator does not refer to any element. It is therefore subject to being invalidated. — end note]
Section: 23.2 [container.requirements], 23.2.8.2 [unord.req.except] Status: CD1 Submitter: Ion Gaztañaga Opened: 2007-12-22 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
23.2 [container.requirements]p10 states:
Unless otherwise specified (see 23.2.2.3 and 23.2.5.4) all container types defined in this clause meet the following additional requirements:
- [...]
- no
erase()
,pop_back()
orpop_front()
function throws an exception.
23.3.5.4 [deque.modifiers] and 23.3.13.5 [vector.modifiers] offer
additional guarantees for deque
/vector insert()
and
erase()
members. However, 23.2 [container.requirements] p10
does not mention 23.2.8.2 [unord.req.except] that specifies exception
safety guarantees for unordered containers. In addition,
23.2.8.2 [unord.req.except] p1 offers the following guaratee for
erase()
:
No
erase()
function throws an exception unless that exception is thrown by the container's Hash or Pred object (if any).
Summary:
According to 23.2 [container.requirements] p10 no
erase()
function should throw an exception unless otherwise
specified. Although does not explicitly mention 23.2.8.2 [unord.req.except], this section offers additional guarantees
for unordered containers, allowing erase()
to throw if
predicate or hash function throws.
In contrast, associative containers have no exception safety guarantees
section so no erase()
function should throw, including
erase(k)
that needs to use the predicate function to
perform its work. This means that the predicate of an associative
container is not allowed to throw.
So:
erase(k)
for associative containers is not allowed to throw. On
the other hand, erase(k)
for unordered associative containers
is allowed to throw.
erase(q)
for associative containers is not allowed to throw. On
the other hand, erase(q)
for unordered associative containers
is allowed to throw if it uses the hash or predicate.
erase(q)
is
allowed to throw, the destructor of the object would need to rethrow the
exception or swallow it, leaving the object registered.
Proposed resolution:
Create a new sub-section of 23.2.7 [associative.reqmts] (perhaps [associative.req.except]) titled "Exception safety guarantees".
1 For associative containers, no
clear()
function throws an exception.erase(k)
does not throw an exception unless that exception is thrown by the container's Pred object (if any).2 For associative containers, if an exception is thrown by any operation from within an
insert()
function inserting a single element, theinsert()
function has no effect.3 For associative containers, no
swap
function throws an exception unless that exception is thrown by the copy constructor or copy assignment operator of the container's Pred object (if any).
Change 23.2.8.2 [unord.req.except]p1:
For unordered associative containers, no
clear()
function throws an exception.Noerase(k)
functiondoes not throwsan exception unless that exception is thrown by the container's Hash or Pred object (if any).
Change 23.2 [container.requirements] p10 to add references to new sections:
Unless otherwise specified (see [deque.modifiers],
and[vector.modifiers], [associative.req.except], [unord.req.except]) all container types defined in this clause meet the following additional requirements:
Change 23.2 [container.requirements] p10 referring to swap
:
- no
swap()
function throws an exceptionunless that exception is thrown by the copy constructor or assignment operator of the container's Compare object (if any; see [associative.reqmts]).
Section: 23 [containers] Status: Resolved Submitter: Sylvain Pion Opened: 2007-12-28 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
Playing with g++'s C++0X mode, I noticed that the following code, which used to compile:
#include <vector> int main() { std::vector<char *> v; v.push_back(0); }
now fails with the following error message:
.../include/c++/4.3.0/ext/new_allocator.h: In member function 'void __gnu_cxx::new_allocator<_Tp>::construct(_Tp*, _Args&& ...) [with _Args = int, _Tp = char*]': .../include/c++/4.3.0/bits/stl_vector.h:707: instantiated from 'void std::vector<_Tp, _Alloc>::push_back(_Args&& ...) [with _Args = int, _Tp = char*, _Alloc = std::allocator<char*>]' test.cpp:6: instantiated from here .../include/c++/4.3.0/ext/new_allocator.h:114: error: invalid conversion from 'int' to 'char*'
As far as I know, g++ follows the current draft here.
Does the committee really intend to break compatibility for such cases?
[ Sylvain adds: ]
I just noticed that
std::pair
has the same issue. The following now fails with GCC's -std=c++0x mode:#include <utility> int main() { std::pair<char *, char *> p (0,0); }I have not made any general audit for such problems elsewhere.
[ Bellevue: ]
Motivation is to handle the old-style int-zero-valued
NULL
pointers. Problem: this solution requires concepts in some cases, which some users will be slow to adopt. Some discussion of alternatives involving prohibiting variadic forms and additional library-implementation complexity.Discussion of "perfect world" solutions, the only such solution put forward being to retroactively prohibit use of the integer zero for a
NULL
pointer. This approach was deemed unacceptable given the large bodies of pre-existing code that do use integer zero for aNULL
pointer.Another approach is to change the member names. Yet another approach is to forbid the extension in absence of concepts.
Resolution: These issues (756(i), 767(i), 760(i), 763(i)) will be subsumed into a paper to be produced by Alan Talbot in time for review at the 2008 meeting in France. Once this paper is produced, these issues will be moved to
NADResolved.
Proposed resolution:
Add the following rows to Table 90 "Optional sequence container operations", 23.2.4 [sequence.reqmts]:
expression return type assertion/note
pre-/post-conditioncontainer a.push_front(t)
void
a.insert(a.begin(), t)
Requires:T
shall beCopyConstructible
.list, deque
a.push_front(rv)
void
a.insert(a.begin(), rv)
Requires:T
shall beMoveConstructible
.list, deque
a.push_back(t)
void
a.insert(a.end(), t)
Requires:T
shall beCopyConstructible
.list, deque, vector, basic_string
a.push_back(rv)
void
a.insert(a.end(), rv)
Requires:T
shall beMoveConstructible
.list, deque, vector, basic_string
Change the synopsis in 23.3.5 [deque]:
void push_front(const T& x); void push_front(T&& x); void push_back(const T& x); void push_back(T&& x); template <class... Args> requires Constructible<T, Args&&...> void push_front(Args&&... args); template <class... Args> requires Constructible<T, Args&&...> void push_back(Args&&... args);
Change 23.3.5.4 [deque.modifiers]:
void push_front(const T& x); void push_front(T&& x); void push_back(const T& x); void push_back(T&& x); template <class... Args> requires Constructible<T, Args&&...> void push_front(Args&&... args); template <class... Args> requires Constructible<T, Args&&...> void push_back(Args&&... args);
Change the synopsis in 23.3.11 [list]:
void push_front(const T& x); void push_front(T&& x); void push_back(const T& x); void push_back(T&& x); template <class... Args> requires Constructible<T, Args&&...> void push_front(Args&&... args); template <class... Args> requires Constructible<T, Args&&...> void push_back(Args&&... args);
Change 23.3.11.4 [list.modifiers]:
void push_front(const T& x); void push_front(T&& x); void push_back(const T& x); void push_back(T&& x); template <class... Args> requires Constructible<T, Args&&...> void push_front(Args&&... args); template <class... Args> requires Constructible<T, Args&&...> void push_back(Args&&... args);
Change the synopsis in 23.3.13 [vector]:
void push_back(const T& x); void push_back(T&& x); template <class... Args> requires Constructible<T, Args&&...> void push_back(Args&&... args);
Change 23.3.13.5 [vector.modifiers]:
void push_back(const T& x); void push_back(T&& x); template <class... Args> requires Constructible<T, Args&&...> void push_back(Args&&... args);
Rationale:
Addressed by N2680 Proposed Wording for Placement Insert (Revision 1).
If there is still an issue with pair, Howard should submit another issue.
Section: 32.5.8 [atomics.types.generic] Status: CD1 Submitter: Alberto Ganesh Barbati Opened: 2007-12-28 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
in the latest publicly available draft, paper
N2641,
in section 32.5.8 [atomics.types.generic], the following specialization of the template
atomic<>
is provided for pointers:
template <class T> struct atomic<T*> : atomic_address { T* fetch_add(ptrdiff_t, memory_order = memory_order_seq_cst) volatile; T* fetch_sub(ptrdiff_t, memory_order = memory_order_seq_cst) volatile; atomic() = default; constexpr explicit atomic(T); atomic(const atomic&) = delete; atomic& operator=(const atomic&) = delete; T* operator=(T*) volatile; T* operator++(int) volatile; T* operator--(int) volatile; T* operator++() volatile; T* operator--() volatile; T* operator+=(ptrdiff_t) volatile; T* operator-=(ptrdiff_t) volatile; };
First of all, there is a typo in the non-default constructor which
should take a T*
rather than a T
.
As you can see, the specialization redefine and therefore hide a few
methods from the base class atomic_address
, namely fetch_add
, fetch_sub
,
operator=
, operator+=
and operator-=
. That's good, but... what happened
to the other methods, in particular these ones:
void store(T*, memory_order = memory_order_seq_cst) volatile; T* load( memory_order = memory_order_seq_cst ) volatile; T* swap( T*, memory_order = memory_order_seq_cst ) volatile; bool compare_swap( T*&, T*, memory_order, memory_order ) volatile; bool compare_swap( T*&, T*, memory_order = memory_order_seq_cst ) volatile;
By reading paper
N2427 "C++ Atomic Types and Operations",
I see that the
definition of the specialization atomic<T*>
matches the one in the
draft, but in the example implementation the methods load()
, swap()
and compare_swap()
are indeed present.
Strangely, the example implementation does not redefine the method
store()
. It's true that a T*
is always convertible to void*
, but not
hiding the void*
signature from the base class makes the class
error-prone to say the least: it lets you assign pointers of any type to
a T*
, without any hint from the compiler.
Is there a true intent to remove them from the specialization or are they just missing from the definition because of a mistake?
[ Bellevue: ]
The proposed revisions are accepted.
Further discussion: why is the ctor labeled "constexpr"? Lawrence said this permits the object to be statically initialized, and that's important because otherwise there would be a race condition on initialization.
Proposed resolution:
Change the synopsis in 32.5.8 [atomics.types.generic]:
template <class T> struct atomic<T*> : atomic_address { void store(T*, memory_order = memory_order_seq_cst) volatile; T* load( memory_order = memory_order_seq_cst ) volatile; T* swap( T*, memory_order = memory_order_seq_cst ) volatile; bool compare_swap( T*&, T*, memory_order, memory_order ) volatile; bool compare_swap( T*&, T*, memory_order = memory_order_seq_cst ) volatile; T* fetch_add(ptrdiff_t, memory_order = memory_order_seq_cst) volatile; T* fetch_sub(ptrdiff_t, memory_order = memory_order_seq_cst) volatile; atomic() = default; constexpr explicit atomic(T*); atomic(const atomic&) = delete; atomic& operator=(const atomic&) = delete; T* operator=(T*) volatile; T* operator++(int) volatile; T* operator--(int) volatile; T* operator++() volatile; T* operator--() volatile; T* operator+=(ptrdiff_t) volatile; T* operator-=(ptrdiff_t) volatile; };
Section: 22.10.17.3 [func.wrap.func] Status: CD1 Submitter: Daniel Krügler Opened: 2008-01-10 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
N2461 already replaced in 22.10.17.3 [func.wrap.func] it's originally proposed
(implicit) conversion operator to "unspecified-bool-type" by the new
explicit bool conversion, but the inverse conversion should also
use the new std::nullptr_t
type instead of "unspecified-null-pointer-
type".
Proposed resolution:
In 22.10 [function.objects], header <functional>
synopsis replace:
template<class R, class... ArgTypes> bool operator==(const function<R(ArgTypes...)>&,unspecified-null-pointer-typenullptr_t); template<class R, class... ArgTypes> bool operator==(unspecified-null-pointer-typenullptr_t , const function<R(ArgTypes...)>&); template<class R, class... ArgTypes> bool operator!=(const function<R(ArgTypes...)>&,unspecified-null-pointer-typenullptr_t); template<class R, class... ArgTypes> bool operator!=(unspecified-null-pointer-typenullptr_t , const function<R(ArgTypes...)>&);
In the class function synopsis of 22.10.17.3 [func.wrap.func] replace
function(unspecified-null-pointer-typenullptr_t); ... function& operator=(unspecified-null-pointer-typenullptr_t);
In 22.10.17.3 [func.wrap.func], "Null pointer comparisons" replace:
template <class R, class... ArgTypes> bool operator==(const function<R(ArgTypes...)>&,unspecified-null-pointer-typenullptr_t); template <class R, class... ArgTypes> bool operator==(unspecified-null-pointer-typenullptr_t , const function<R(ArgTypes...)>&); template <class R, class... ArgTypes> bool operator!=(const function<R(ArgTypes...)>&,unspecified-null-pointer-typenullptr_t); template <class R, class... ArgTypes> bool operator!=(unspecified-null-pointer-typenullptr_t , const function<R(ArgTypes...)>&);
In 22.10.17.3.2 [func.wrap.func.con], replace
function(unspecified-null-pointer-typenullptr_t); ... function& operator=(unspecified-null-pointer-typenullptr_t);
In 22.10.17.3.7 [func.wrap.func.nullptr], replace
template <class R, class... ArgTypes> bool operator==(const function<R(ArgTypes...)>& f,unspecified-null-pointer-typenullptr_t); template <class R, class... ArgTypes> bool operator==(unspecified-null-pointer-typenullptr_t , const function<R(ArgTypes...)>& f);
and replace
template <class R, class... ArgTypes> bool operator!=(const function<R(ArgTypes...)>& f,unspecified-null-pointer-typenullptr_t); template <class R, class... ArgTypes> bool operator!=(unspecified-null-pointer-typenullptr_t , const function<R(ArgTypes...)>& f);
Section: 22.10.17 [func.wrap] Status: CD1 Submitter: Daniel Krügler Opened: 2008-01-10 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
It is expected that typical implementations of std::function
will
use dynamic memory allocations at least under given conditions,
so it seems appropriate to change the current lvalue swappabilty of
this class to rvalue swappability.
Proposed resolution:
In 22.10 [function.objects], header <functional>
synopsis, just below of
template<class R, class... ArgTypes> void swap(function<R(ArgTypes...)>&, function<R(ArgTypes...)>&); template<class R, class... ArgTypes> void swap(function<R(ArgTypes...)>&&, function<R(ArgTypes...)>&); template<class R, class... ArgTypes> void swap(function<R(ArgTypes...)>&, function<R(ArgTypes...)>&&);
In 22.10.17.3 [func.wrap.func] class function
definition, change
void swap(function&&);
In 22.10.17.3 [func.wrap.func], just below of
template <class R, class... ArgTypes> void swap(function<R(ArgTypes...)>&, function<R(ArgTypes...)>&); template <class R, class... ArgTypes> void swap(function<R(ArgTypes...)>&&, function<R(ArgTypes...)>&); template <class R, class... ArgTypes> void swap(function<R(ArgTypes...)>&, function<R(ArgTypes...)>&&);
In 22.10.17.3.3 [func.wrap.func.mod] change
void swap(function&& other);
In 22.10.17.3.8 [func.wrap.func.alg] add the two overloads
template<class R, class... ArgTypes> void swap(function<R(ArgTypes...)>&& f1, function<R(ArgTypes...)>& f2); template<class R, class... ArgTypes> void swap(function<R(ArgTypes...)>& f1, function<R(ArgTypes...)>&& f2);
Section: 27.4.5 [string.conversions] Status: CD1 Submitter: Daniel Krügler Opened: 2008-01-13 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
The new to_string
and to_wstring
functions described in 27.4.5 [string.conversions]
have throws clauses (paragraphs 8 and 16) which say:
Throws: nothing
Since all overloads return either a std::string
or a std::wstring
by value
this throws clause is impossible to realize in general, since the basic_string
constructors can fail due to out-of-memory conditions. Either these throws
clauses should be removed or should be more detailled like:
Throws: Nothing if the string construction throws nothing
Further there is an editorial issue in p. 14: All three to_wstring
overloads return a string
, which should be wstring
instead (The
header <string>
synopsis of 27.4 [string.classes] is correct in this
regard).
Proposed resolution:
In 27.4.5 [string.conversions], remove the paragraphs 8 and 16.
string to_string(long long val); string to_string(unsigned long long val); string to_string(long double val);
Throws: nothing
wstring to_wstring(long long val); wstring to_wstring(unsigned long long val); wstring to_wstring(long double val);
Throws: nothing
Section: 27.4.5 [string.conversions] Status: CD1 Submitter: Daniel Krügler Opened: 2008-01-13 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
The return clause 27.4.5 [string.conversions] paragraph 15 of the new to_wstring
overloads says:
Returns: each function returns a
wstring
object holding the character representation of the value of its argument that would be generated by callingwsprintf(buf, fmt, val)
with a format specifier ofL"%lld"
,L"%ulld"
, orL"%f"
, respectively.
Problem is: There does not exist any wsprintf
function in C99 (I checked
the 2nd edition of ISO 9899, and the first and the second corrigenda from
2001-09-01 and 2004-11-15). What probably meant here is the function
swprintf
from <wchar.h>/<cwchar>
, but this has the non-equivalent
declaration:
int swprintf(wchar_t * restrict s, size_t n, const wchar_t * restrict format, ...);
therefore the paragraph needs to mention the size_t
parameter n
.
Proposed resolution:
Change the current wording of 27.4.5 [string.conversions] p. 15 to:
Returns:
eEach function returns awstring
object holding the character representation of the value of its argument that would be generated by callingwith a format specifier
wsswprintf(buf, bufsz, fmt, val)fmt
ofL"%lld"
,L"%ulld"
, orL"%f"
, respectively, wherebuf
designates an internal character buffer of sufficient sizebufsz
.
[Hint to the editor: The resolution also adds to mention the name of the format specifier "fmt"]
I also would like to remark that the current wording of it's equivalent
paragraph 7 should also mention the meaning of buf
and fmt
.
Change the current wording of 27.4.5 [string.conversions] p. 7 to:
Returns:
eEach function returns a string object holding the character representation of the value of its argument that would be generated by callingsprintf(buf, fmt, val)
with a format specifierfmt
of"%lld"
,"%ulld"
, or"%f"
, respectively, wherebuf
designates an internal character buffer of sufficient size.
swap
undefined for most containersSection: 23 [containers] Status: C++11 Submitter: Alisdair Meredith Opened: 2008-01-14 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
It appears most containers declare but do not define a member-swap function.
This is unfortunate, as all overload the swap
algorithm to call the
member-swap function!
(required for swappable
guarantees [Table 37] and Container Requirements
[Table 87])
Note in particular that Table 87 gives semantics of a.swap(b)
as swap(a,b)
,
yet for all containers we define swap(a,b)
to call a.swap(b)
- a circular
definition.
A quick survey of clause 23 shows that the following containers provide a definition for member-swap:
array queue stack vector
Whereas the following declare it, but do not define the semantics:
deque list map multimap multiset priority_queue set unordered_map unordered_multi_map unordered_multi_set unordered_set
Suggested resolution:
Provide a definition for each of the affected containers...
[ Bellevue: ]
Move to Open and ask Alisdair to provide wording.
[ 2009-07 Frankfurt: ]
Daniel to provide wording. N2590 is no longer applicable.
[ 2009-07-28 Daniel provided wording. ]
- It assumes that the proposed resolution for 883(i) is applied, which breaks the circularity of definition between member
swap
and freeswap
.- It uses the notation of the pre-concept allocator trait
allocator_propagation_map
, which might be renamed after the next refactoring phase of generalized allocators.- It requires that compare objects, key equal functions and hash functions in containers are swapped via unqualified free
swap
according to 594(i).
[ 2009-09-30 Daniel adds: ]
The outcome of this issue should be considered with the outcome of 1198(i) both in style and in content (e.g. bullet 9 suggests to define the semantic of
void priority_queue::swap(priority_queue&)
in terms of the memberswap
of the container).
[ 2009-10 Santa Cruz: ]
Looked at, but took no action on as it overlaps too much with N2982. Waiting for a new draft WP.
[ 2009-10 Santa Cruz: ]
Leave as open. Pablo to provide wording.
[ 2009-10-26 Pablo updated wording. Here is the wording he replaced: ]
Add a new Throws clause just after 99 [allocator.propagation.map]/5:
static void swap(Alloc& a, Alloc& b);Effects: [..]
Throws: Nothing.
[ This exception requirement is added, such that it's combination with the general container requirements of N2723 [container.requirements.general]/9 make it unambiguously clear that the following descriptions of "swaps the allocators" have the following meaning: (a) This swap is done by calling
allocator_propagation_map<allocator_type>::swap
and (b) This allocator swap does never propagate an exception ]Change 23.2.7.2 [associative.reqmts.except]/3 as indicated:
For associative containers, no
swap
function throws an exception unless that exception is thrown by thecopy constructor or copy assignment operatorswap
of the container'sPred
objects(if any).Change 23.2.8.2 [unord.req.except]/3 as indicated:
For unordered associative containers, no
swap
function throws an exception unless that exception is thrown by thecopy constructor or copy assignment operatorswap
of the container'sHash
orPred
objects, respectively(if any).Insert a new paragraph just after 23.3 [sequences]/1:
In addition to being available via inclusion of the
<algorithm>
header, theswap
function templates in 26.7.3 [alg.swap] are also available when the header<queue>
is included.[ There is a new issue in process that will suggest a minimum header for
swap
andmove
. If this one is provided, this text can be removed and the header dependency should be added to<queue>
]Add one further clause at the end of 23.3.3.4 [array.special]:
[This part is added, because otherwise
array::swap
would otherwise contradict the general contract of 23.2.2 [container.requirements.general] p. 10 b. 5]Throws: Nothing, unless one of the element-wise
swap
calls throws an exception.
In 23.3.5 [deque], class template
deque
synopsis change as indicated:void swap(deque<T,Alloc>&);At the end of 23.3.5.4 [deque.modifiers] add as indicated:
void swap(deque& x);Effects: Exchanges the contents and swaps the allocators of
*this
with that ofx
.Complexity: Constant time.
In [forwardlist], class template
forward_list
synopsis change as indicated:void swap(forward_list<T,Allocator>&);At the end of [forwardlist.modifiers] add as indicated:
void swap(forward_list& x);Effects: Exchanges the contents and swaps the allocators of
*this
with that ofx
.Complexity: Constant time.
In 23.3.11 [list], class template
list
synopsis change as indicated:void swap(list<T,Allocator>&);At the end of 23.3.11.4 [list.modifiers] add as indicated:
void swap(list& x);Effects: Exchanges the contents and swaps the allocators of
*this
with that ofx
.Complexity: Constant time.
At the end of 23.6.4.4 [priqueue.members] add a new prototype description:
void swap(priority_queue& q);Requires:
Compare
shall satisfy theSwappable
requirements ( [swappable]).[ This requirement is added to ensure that even a user defined
swap
which is found by ADL forCompare
satisfies theSwappable
requirements ]Effects:
this->c.swap(q.c); swap(this->comp, q.comp);
Throws: What and if
c.swap(q.c)
andswap(comp, q.comp)
throws.[ This part is added, because otherwise
priority_queue::swap
would otherwise contradict the general contract of 23.2.2 [container.requirements.general] p. 10 b. 5 ]
In 23.3.13 [vector], class template
vector
synopsis change as indicated:void swap(vector<T,Allocator>&);Change 23.3.13.3 [vector.capacity] p. 8 as indicated:
void swap(vector<T,Allocator>& x);Effects: Exchanges the contents and
capacity()
and swaps the allocators of*this
with that ofx
.Insert a new paragraph just before 23.4 [associative]/1:
In addition to being available via inclusion of the
<algorithm>
header, theswap
function templates in 26.7.3 [alg.swap] are also available when any of the headers<map>
or<set>
are included.
In 23.4.3 [map], class template
map
synopsis change as indicated:void swap(map<Key,T,Compare,Allocator>&);At the end of 23.4.3.4 [map.modifiers] add as indicated:
void swap(map& x);Requires: Compare shall satisfy the
Swappable
requirements ( [swappable]).[ This requirement is added to ensure that even a user defined
swap
which is found by ADL forCompare
satisfies theSwappable
requirements ]Effects: Exchanges the contents and swaps the allocators of
*this
with that ofx
, followed by an unqualifiedswap
of the comparison objects of*this
andx
.Complexity: Constant time
In 23.4.4 [multimap], class template
multimap
synopsis change as indicated:void swap(multimap<Key,T,Compare,Allocator>&);At the end of 23.4.4.3 [multimap.modifiers] add as indicated:
void swap(multimap& x);Requires: Compare shall satisfy the
Swappable
requirements ( [swappable]).Effects: Exchanges the contents and swaps the allocators of
*this
with that ofx
, followed by an unqualifiedswap
of the comparison objects of*this
andx
.Complexity: Constant time
In 23.4.6 [set], class template
set
synopsis change as indicated:void swap(set<Key,Compare,Allocator>&);After section 23.4.6.2 [set.cons] add a new section
set
modifiers 23.4.6.4 [set.modifiers] and add the following paragraphs:void swap(set& x);Requires: Compare shall satisfy the
Swappable
requirements ( [swappable]).Effects: Exchanges the contents and swaps the allocators of
*this
with that ofx
, followed by an unqualifiedswap
of the comparison objects of*this
andx
.Complexity: Constant time
In 23.4.7 [multiset], class template
multiset
synosis, change as indicated:void swap(multiset<Key,Compare,Allocator>&);After section 23.4.7.2 [multiset.cons] add a new section
multiset
modifiers [multiset.modifiers] and add the following paragraphs:void swap(multiset& x);Requires: Compare shall satisfy the
Swappable
requirements ( [swappable]).Effects: Exchanges the contents and swaps the allocators of
*this
with that ofx
, followed by an unqualifiedswap
of the comparison objects of*this
andx
.Complexity: Constant time
Insert a new paragraph just before 23.5 [unord] p. 1:
In addition to being available via inclusion of the
<algorithm>
header, theswap
function templates in 26.7.3 [alg.swap] are also available when any of the headers<unordered_map>
or<unordered_set>
are included.After section 23.5.3.3 [unord.map.elem] add a new section unordered_map modifiers 23.5.3.4 [unord.map.modifiers] and add the following paragraphs:
void swap(unordered_map& x);Requires:
Hash
andPred
shall satisfy theSwappable
requirements ( [swappable]).[ This requirement is added to ensure that even a user defined
swap
which is found by ADL forHash
andPred
satisfies theSwappable
requirements ]Effects: Exchanges the contents and hash policy and swaps the allocators of
*this
with that ofx
, followed by an unqualifiedswap
of thePred
objects and an unqualifiedswap
of theHash
objects of*this
andx
.Complexity: Constant time
After section 23.5.4.2 [unord.multimap.cnstr] add a new section unordered_multimap modifiers 23.5.4.3 [unord.multimap.modifiers] and add the following paragraphs:
void swap(unordered_multimap& x);Requires:
Hash
andPred
shall satisfy theSwappable
requirements ( [swappable]).Effects: Exchanges the contents and hash policy and swaps the allocators of
*this
with that ofx
, followed by an unqualifiedswap
of thePred
objects and an unqualifiedswap
of theHash
objects of*this
andx
Complexity: Constant time
After section 23.5.6.2 [unord.set.cnstr] add a new section unordered_set modifiers 23.5.6.4 [unord.set.modifiers] and add the following paragraphs:
void swap(unordered_set& x);Requires:
Hash
andPred
shall satisfy theSwappable
requirements ( [swappable]).Effects: Exchanges the contents and hash policy and swaps the allocators of
*this
with that ofx
, followed by an unqualifiedswap
of thePred
objects and an unqualifiedswap
of theHash
objects of*this
andx
Complexity: Constant time
After section 23.5.7.2 [unord.multiset.cnstr] add a new section unordered_multiset modifiers [unord.multiset.modifiers] and add the following paragraphs:
void swap(unordered_multiset& x);Requires:
Hash
andPred
shall satisfy theSwappable
requirements ( [swappable]).Effects: Exchanges the contents and hash policy and swaps the allocators of
*this
with that ofx
, followed by an unqualifiedswap
of thePred
objects and an unqualifiedswap
of theHash
objects of*this
andx
Complexity: Constant time
[ 2009-10-30 Pablo and Daniel updated wording. ]
[ 2010 Pittsburgh: Ready for Pittsburgh. ]
Proposed resolution:
[ This resolution is based on the September 2009 WP, N2960, except that it assumes that N2982 and issues 883(i) and 1232(i) have already been applied. Note in particular that Table 91 in N2960 is refered to as Table 90 because N2982 removed the old Table 90. This resolution also addresses issue 431(i). ]
In 23.2.2 [container.requirements.general], replace the a.swap(b) row in table 90, "container requirements" (was table 91 before the application of N2982 to the WP):
a.swap(b)
void
swap(a,b)Exchange the contents ofa
andb
.(Note A) swap(a,b)
void
a.swap(b)
(Note A)
Modify the notes immediately following Table 90 in 23.2.2 [container.requirements.general] as follows (The wording below is after the application of N2982 to N2960. The editor might also want to combine Notes A and B into one.):
Notes: the algorithms
swap(),equal() and lexicographical_compare() are defined in Clause 25. Those entries marked "(Note A)" or "(Note B)"shouldhave linear complexity for array and constant complexity for all other standard containers.
In 23.2.2 [container.requirements.general], before paragraph 8, add:
The expression
a.swap(b)
, for containersa
andb
of a standard container type other thanarray
, exchanges the values ofa
andb
without invoking any move, copy, or swap operations on the individual container elements. AnyCompare
,Pred
, orHash
function objects belonging toa
andb
shall beswappable
and are exchanged by unqualified calls to non-memberswap
. Ifallocator_traits<allocator_type>::propagate_on_container_swap::value == true
, then the allocators ofa
andb
are also exchanged using an unqualified call to non-memberswap
. Otherwise, the behavior is undefined unlessa.get_allocator() == b.get_allocator()
. Each iterator refering to an element in one container before the swap shall refer to the same element in the other container after the swap. It is unspecified whether an iterator with valuea.end()
before the swap will have valueb.end()
after the swap. In addition to being available via inclusion of the<utility>
header, theswap
function template in 26.7.3 [alg.swap] is also available within the definition of every standard container'sswap
function.
[ Note to the editor: Paragraph 2 starts with a sentence fragment, clearly from an editing or source-control error. ]
Modify 23.2.7.2 [associative.reqmts.except] as follows:
23.2.4.1 Exception safety guarantees 23.2.7.2 [associative.reqmts.except]
For associative containers, no
clear()
function throws an exception.erase(k)
does not throw an exception unless that exception is thrown by the container'sobject (if any).
PredCompareFor associative containers, if an exception is thrown by any operation from within an
insert()
function inserting a single element, theinsert()
function has no effect.For associative containers, no
swap
function throws an exception unless that exception is thrown by thecopy constructor or copy assignment operatorswap of the container'sobject (if any).
PredCompare
Modify 23.2.8.2 [unord.req.except], paragraph 3 as follows:
For unordered associative containers, no
swap
function throws an exception unless that exception is thrown by thecopy constructor or copy assignment operatorswap of the container'sHash
orPred
object (if any).
Modify section 23.3.3.4 [array.special]:
array specialized algorithms 23.3.3.4 [array.special]
template <class T, size_t N> void swap(array<T,N>& x,array<T,N>& y);
Effects:
swap_ranges(x.begin(), x.end(), y.begin() );x.swap(y);
Add a new section after [array.fill] (Note to the editor: array::fill make use of a concept requirement that must be removed or changed to text.):
array::swap [array.swap]
void swap(array& y);
Effects:
swap_ranges(this->begin(), this->end(), y.begin() );
Throws: Nothing unless one of the element-wise swap calls throws an exception.
[Note: Unlike other containers'
swap
functions,array::swap
takes linear, not constant, time, may exit via an exception, and does not cause iterators to become associated with the other container. — end note]
Insert a new paragraph just after 23.6 [container.adaptors]/1:
For container adaptors, no
swap
function throws an exception unless that exception is thrown by the swap of the adaptor'sContainer
orCompare
object (if any).
Section: 22.4.7 [tuple.helper] Status: CD1 Submitter: Alisdair Meredith Opened: 2008-01-16 Last modified: 2016-01-28
Priority: Not Prioritized
View all other issues in [tuple.helper].
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Discussion:
The tuple element access API identifies the element in the sequence
using signed integers, and then goes on to enforce the requirement that
I be >= 0. There is a much easier way to do this - declare I as
unsigned
.
In fact the proposal is to use std::size_t
, matching the
type used in the tuple_size
API.
A second suggestion is that it is hard to imagine an API that deduces and index at compile time and returns a reference throwing an exception. Add a specific Throws: Nothing paragraph to each element access API.
In addition to tuple
, update the API applies to
pair
and array
, and should be updated
accordingly.
A third observation is that the return type of the get
functions for std::pair
is pseudo-code, but it is not
clearly marked as such. There is actually no need for pseudo-code as
the return type can be specified precisely with a call to
tuple_element
. This is already done for
std::tuple
, and std::array
does not have a
problem as all elements are of type T
.
Proposed resolution:
Update header <utility> synopsis in 22.2 [utility]
// 20.2.3, tuple-like access to pair: template <class T> class tuple_size; template <intsize_t I, class T> class tuple_element; template <class T1, class T2> struct tuple_size<std::pair<T1, T2> >; template <class T1, class T2> struct tuple_element<0, std::pair<T1, T2> >; template <class T1, class T2> struct tuple_element<1, std::pair<T1, T2> >; template<intsize_t I, class T1, class T2>Ptypename tuple_element<I, std::pair<T1, T2> >::type & get(std::pair<T1, T2>&); template<intsize_t I, class T1, class T2> constPtypename tuple_element<I, std::pair<T1, T2> >::type & get(const std::pair<T1, T2>&);
Update 22.3 [pairs] Pairs
template<intsize_t I, class T1, class T2>Ptypename tuple_element<I, std::pair<T1, T2> >::type & get(pair<T1, T2>&); template<intsize_t I, class T1, class T2> constPtypename tuple_element<I, std::pair<T1, T2> >::type & get(const pair<T1, T2>&);
24 Return type: If
I == 0
then P
is T1
, if I == 1
then P
is T2
, and otherwise the program is ill-formed.
25 Returns: If I == 0
returns p.first
, otherwise if I == 1
returns p.second
, and otherwise the program is ill-formed.
Throws: Nothing.
Update header <tuple> synopsis in 22.4 [tuple] with a APIs as below:
template <intsize_t I, class T> class tuple_element; // undefined template <intsize_t I, class... Types> class tuple_element<I, tuple<Types...> >; // 20.3.1.4, element access: template <intsize_t I, class... Types> typename tuple_element<I, tuple<Types...> >::type& get(tuple<Types...>&); template <intsize_t I, class ... types> typename tuple_element<I, tuple<Types...> >::type const& get(const tuple<Types...>&);
Update 22.4.7 [tuple.helper] Tuple helper classes
template <intsize_t I, class... Types> class tuple_element<I, tuple<Types...> > { public: typedef TI type; };
1 Requires:
. The program is ill-formed if 0 <= I and I < sizeof...(Types)I
is out of bounds.
2 Type: TI
is the type of the I
th element of Types
, where indexing is zero-based.
Update 22.4.8 [tuple.elem] Element access
template <intsize_t I, class... types > typename tuple_element<I, tuple<Types...> >::type& get(tuple<Types...>& t);
1 Requires:
. The program is ill-formed if 0 <= I and I < sizeof...(Types)I
is out of bounds.
I
th element of t
, where indexing is zero-based.
Throws: Nothing.
template <intsize_t I, class... types> typename tuple_element<I, tuple<Types...> >::type const& get(const tuple<Types...>& t);
3 Requires:
. The program is ill-formed if 0 <= I and I < sizeof...(Types)I
is out of bounds.
4 Returns: A const reference to the I
th element of t
, where indexing is zero-based.
Throws: Nothing.
Update header <array> synopsis in 22.2 [utility]
template <class T> class tuple_size; // forward declaration template <intsize_t I, class T> class tuple_element; // forward declaration template <class T, size_t N> struct tuple_size<array<T, N> >; template <intsize_t I, class T, size_t N> struct tuple_element<I, array<T, N> >; template <intsize_t I, class T, size_t N> T& get(array<T, N>&); template <intsize_t I, class T, size_t N> const T& get(const array<T, N>&);
Update 23.3.3.7 [array.tuple] Tuple interface to class template array
tuple_element<size_t I, array<T, N> >::type
3 Requires:
The program is ill-formed if 0 <= I < N.I
is out of bounds.
4 Value: The type T
.
template <intsize_t I, class T, size_t N> T& get(array<T, N>& a);
5 Requires:
. The program is ill-formed if 0 <= I < NI
is out of bounds.
Returns: A reference to the I
th element of a
, where indexing is zero-based.
Throws: Nothing.
template <intsize_t I, class T, size_t N> const T& get(const array<T, N>& a);
6 Requires:
. The program is ill-formed if 0 <= I < NI
is out of bounds.
7 Returns: A const reference to the I
th element of a
, where indexing is zero-based.
Throws: Nothing.
[ Bellevue: Note also that the phrase "The program is ill-formed if I is out of bounds" in the requires clauses are probably unnecessary, and could be removed at the editor's discretion. Also std:: qualification for pair is also unnecessary. ]
assign
function of std::array
Section: 23.3.3 [array] Status: CD1 Submitter: Daniel Krügler Opened: 2008-01-20 Last modified: 2016-01-28
Priority: Not Prioritized
View all other issues in [array].
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Discussion:
The class template array synopsis in 23.3.3 [array] p. 3 declares a member function
void assign(const T& u);
which's semantic is no-where described. Since this signature is not part of the container requirements, such a semantic cannot be derived by those.
I found only one reference to this function in the issue list, 588(i) where the question is raised:
what's the effect of calling
assign(T&)
on a zero-sized array?
which does not answer the basic question of this issue.
If this function shall be part of the std::array
, it's probable
semantic should correspond to that of boost::array
, but of
course such wording must be added.
Proposed resolution:
Just after the section [array.data] add the following new section:
23.2.1.5 array::fill [array.fill]
void fill(const T& u);1: Effects:
fill_n(begin(), N, u)
[N.B: I wonder, why class array
does not have a "modifiers"
section. If it had, then assign
would naturally belong to it]
Change the synopsis in 23.3.3 [array]/3:
template <class T, size_t N> struct array { ... voidassignfill(const T& u); ...
[ Bellevue: ]
Suggest substituting "fill" instead of "assign".
Set state to Review given substitution of "fill" for "assign".
Section: 32.5.8.2 [atomics.types.operations] Status: CD1 Submitter: Lawrence Crowl Opened: 2008-01-21 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
The load functions are defined as
C atomic_load(volatile A* object); C atomic_load_explicit(volatile A* object, memory_order); C A::load(memory_order order = memory_order_seq_cst) volatile;
which prevents their use in const
contexts.
[ post Bellevue Peter adds: ]
Issue 777(i) suggests making
atomic_load
operate onconst
objects. There is a subtle point here. Atomic loads do not generally write to the object, except potentially for thememory_order_seq_cst
constraint. Depending on the architecture, a dummy write with the same value may be required to be issued by the atomic load to maintain sequential consistency. This, in turn, may make the following code:const atomic_int x{}; int main() { x.load(); }dump core under a straightforward implementation that puts const objects in a read-only section.
There are ways to sidestep the problem, but it needs to be considered.
The tradeoff is between making the data member of the atomic types mutable and requiring the user to explicitly mark atomic members as mutable, as is already the case with mutexes.
Proposed resolution:
Add the const
qualifier to *object
and *this
.
C atomic_load(const volatile A* object); C atomic_load_explicit(const volatile A* object, memory_order); C A::load(memory_order order = memory_order_seq_cst) const volatile;
Section: 22.9.2.2 [bitset.cons] Status: CD1 Submitter: Thorsten Ottosen Opened: 2008-01-24 Last modified: 2016-01-28
Priority: Not Prioritized
View all other issues in [bitset.cons].
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Duplicate of: 116
Discussion:
A small issue with std::bitset
: it does not have any constructor
taking a string literal, which is clumsy and looks like an oversigt when
we tried to enable uniform use of string
and const char*
in the library.
Suggestion: Add
explicit bitset( const char* str );
to std::bitset.
Proposed resolution:
Add to synopsis in 22.9.2 [template.bitset]
explicit bitset( const char* str );
Add to synopsis in 22.9.2.2 [bitset.cons]
explicit bitset( const char* str );Effects: Constructs a
bitset
as ifbitset(string(str))
.
Section: 26.7.8 [alg.remove] Status: CD1 Submitter: Daniel Krügler Opened: 2008-01-25 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
The resolution of 283(i) did not resolve similar necessary changes for algorithm
remove_copy[_if]
, which seems to be an oversight.
Proposed resolution:
In 26.7.8 [alg.remove] p.6, replace the N2461 requires clause with:
Requires:
TypeThe rangesT
isEqualityComparable
(31).[first,last)
and[result,result + (last - first))
shall not overlap. The expression*result = *first
shall be valid.
std::merge()
specification incorrect/insufficientSection: 26.8.6 [alg.merge] Status: C++11 Submitter: Daniel Krügler Opened: 2008-01-25 Last modified: 2016-01-28
Priority: Not Prioritized
View all other issues in [alg.merge].
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Discussion:
Though issue 283(i) has fixed many open issues, it seems that some are still open:
Both 25.3.4 [lib.alg.merge] in 14882:2003 and 26.8.6 [alg.merge] in N2461 have no Requires element and the Effects element contains some requirements, which is probably editorial. Worse is that:
[first, last)
, which is not defined by the
function arguments or otherwise.
[ Post Summit Alisdair adds: ]
Suggest:
(where
last
is equal tonext(result, distance(first1, last1) + distance(first2, last2))
, such that resulting range will be sorted in non-decreasing order; that is, for every iteratori
in[result,last)
other thanresult
, the condition*i < *prev(i)
or, respectively,comp(*i, *prev(i))
will befalse
.Note that this might still not be technically accurate in the case of
InputIterators
, depending on other resolutions working their way through the system (1011(i)).
[ Post Summit Daniel adds: ]
If we want to use
prev
andnext
here (Note:merge
is sufficiently satisfied withInputIterator
) we should instead add more to 26 [algorithms] p. 6, but I can currently not propose any good wording for this.
[ Batavia (2009-05): ]
Pete points out the existing wording in [algorithms] p. 4 that permits the use of + in algorithm specifications.
Alisdair points out that that wording may not apply to input iterators.
Move to Review.
[ 2009-07 Frankfurt: ]
Move to Ready.
[ 2009-08-23 Daniel reopens: ]
The proposed wording must be rephrased, because the part
for every iterator
i
in[result,last)
other thanresult
, the condition*i < *(i - 1)
or, respectively,comp(*i, *(i - 1))
will befalse
"isn't meaningful, because the range
[result,last)
is that of a pureOutputIterator
, which is not readable in general.[Howard: Proposed wording updated by Daniel, status moved from Ready to Review.]
[ 2009-10 Santa Cruz: ]
Matt has some different words to propose. Those words have been moved into the proposed wording section, and the original proposed wording now appears here:
In 26.8.6 [alg.merge] replace p.1+ 2:
Effects:
MergesCopies all the elements of the two sorted ranges[first1,last1)
and[first2,last2)
into the range[result,result + (last1 - first1) + (last2 - first2))
, such that resulting range will be sorted in non-decreasing order; that is for every pair of iteratorsi
andj
of either input ranges, where*i
was copied to the output range before*j
was copied to the output range, the condition*j < *i
or, respectively,comp(*j, *i)
will befalse
.Requires:The resulting range shall not overlap with either of the original ranges.
The list will be sorted in non-decreasing order according to the ordering defined bycomp
; that is, for every iteratori
in[first,last)
other thanfirst
, the condition*i < *(i - 1)
orcomp(*i, *(i - 1))
will befalse
.
[ 2010-02-10 Moved to Tentatively Ready after 5 positive votes on c++std-lib. ]
Proposed resolution:
Change 26.8.6 [alg.merge] 1 and 2:
1
Effects: Merges two sorted ranges[first1,last1)
and[first2,last2)
into the range[result, result + (last1 - first1) + (last2 - first2))
.Effects: Copies all the elements of the two ranges
[first1,last1)
and[first2,last2)
into the range[result, result_last)
, whereresult_last
isresult + (last1 - first1) + (last2 - first2)
, such that the resulting range satisfiesis_sorted(result, result_last)
oris_sorted(result, result_last, comp)
, respectively.2 Requires: The ranges
[first1,last1)
and[first2,last2)
shall be sorted with respect tooperator<
orcomp
. The resulting range shall not overlap with either of the original ranges.The list will be sorted in non-decreasing order according to the ordering defined bycomp
; that is, for every iteratori
in[first,last)
other thanfirst
, the condition*i < *(i - 1)
orcomp(*i, *(i - 1))
will befalse
.
Change 26.8.6 [alg.merge] p. 6+7 as indicated [This ensures harmonization
between inplace_merge
and merge
]
6 Effects: Merges two
sortedconsecutive ranges[first,middle)
and[middle,last)
, putting the result of the merge into the range[first,last)
. The resulting range will be in non-decreasing order; that is, for every iteratori
in[first,last)
other thanfirst
, the condition*i < *(i - 1)
or, respectively,comp(*i, *(i - 1))
will be false.7 Requires: The ranges
[first,middle)
and[middle,last)
shall be sorted with respect tooperator<
orcomp
. The type of*first
shall satisfy theSwappable
requirements (37), theMoveConstructible
requirements (Table 33), and the theMoveAssignable
requirements (Table 35).
std::complex
should add missing C99 functionsSection: 29.4.7 [complex.value.ops] Status: CD1 Submitter: Daniel Krügler Opened: 2008-01-26 Last modified: 2016-01-28
Priority: Not Prioritized
View all other issues in [complex.value.ops].
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Discussion:
A comparision of the N2461 header <complex>
synopsis ([complex.syn])
with the C99 standard (ISO 9899, 2nd edition and the two corrigenda) show
some complex functions that are missing in C++. These are:
cproj
functions
cerf cerfc cexp2 cexpm1 clog10 clog1p clog2 clgamma ctgamma
I propose that at least the required cproj
overloads are provided as equivalent
C++ functions. This addition is easy to do in one sentence (delegation to C99
function).
Please note also that the current entry polar
in 29.4.10 [cmplx.over] p. 1
should be removed from the mentioned overload list. It does not make sense to require that a
function already expecting scalar arguments
should cast these arguments into corresponding
complex<T>
arguments, which are not accepted by
this function.
Proposed resolution:
In 29.4.2 [complex.syn] add just between the declaration of conj
and fabs
:
template<class T> complex<T> conj(const complex<T>&); template<class T> complex<T> proj(const complex<T>&); template<class T> complex<T> fabs(const complex<T>&);
In 29.4.7 [complex.value.ops] just after p.6 (return clause of conj
) add:
template<class T> complex<T> proj(const complex<T>& x);Effects: Behaves the same as C99 function
cproj
, defined in subclause 7.3.9.4."
In 29.4.10 [cmplx.over] p. 1, add one further entry proj
to
the overload list.
The following function templates shall have additional overloads:
arg norm conjpolarproj imag real
seed_seq
constructor is uselessSection: 29.5.8.1 [rand.util.seedseq] Status: CD1 Submitter: Daniel Krügler Opened: 2008-01-27 Last modified: 2015-10-20
Priority: Not Prioritized
View all other issues in [rand.util.seedseq].
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Discussion:
Part of the resolution of n2423, issue 8 was the proposal to
extend the seed_seq
constructor accepting an input range
as follows (which is now part of N2461):
template<class InputIterator, size_t u = numeric_limits<iterator_traits<InputIterator>::value_type>::digits> seed_seq(InputIterator begin, InputIterator end);
First, the expression iterator_traits<InputIterator>::value_type
is invalid due to missing typename
keyword, which is easy to
fix.
Second (and worse), while the language now supports default
template arguments of function templates, this customization
point via the second size_t
template parameter is of no advantage,
because u
can never be deduced, and worse - because it is a
constructor function template - it can also never be explicitly
provided (13.10.2 [temp.arg.explicit]/7).
The question arises, which advantages result from a compile-time
knowledge of u
versus a run time knowledge? If run time knowledge
suffices, this parameter should be provided as normal function
default argument [Resolution marked (A)], if compile-time knowledge
is important, this could be done via a tagging template or more
user-friendly via a standardized helper generator function
(make_seed_seq
), which allows this [Resolution marked (B)].
[ Bellevue: ]
Fermilab does not have a strong opinion. Would prefer to go with solution A. Bill agrees that solution A is a lot simpler and does the job.
Proposed Resolution: Accept Solution A.
Issue 803(i) claims to make this issue moot.
Proposed resolution:
In 29.5.8.1 [rand.util.seedseq]/2, class seed_seq
synopsis replace:
class seed_seq { public: ... template<class InputIterator, size_t u = numeric_limits<iterator_traits<InputIterator>::value_type>::digits> seed_seq(InputIterator begin, InputIterator end, size_t u = numeric_limits<typename iterator_traits<InputIterator>::value_type>::digits); ... };
and do a similar replacement in the member description between p.3 and p.4.
In 29.5.8.1 [rand.util.seedseq]/2, class seed_seq
synopsis and in the
member description between p.3 and p.4 replace:
template<class InputIterator, size_t u = numeric_limits<iterator_traits<InputIterator>::value_type>::digits> seed_seq(InputIterator begin, InputIterator end); template<class InputIterator, size_t u> seed_seq(InputIterator begin, InputIterator end, implementation-defined s);
In 29.5.2 [rand.synopsis], header <random>
synopsis, immediately after the
class seed_seq
declaration and in 29.5.8.1 [rand.util.seedseq]/2, immediately
after the class seed_seq
definition add:
template<size_t u, class InputIterator> seed_seq make_seed_seq(InputIterator begin, InputIterator end);
In 29.5.8.1 [rand.util.seedseq], just before p.5 insert two paragraphs:
The first constructor behaves as if it would provide an integral constant expression
u
of typesize_t
of valuenumeric_limits<typename iterator_traits<InputIterator>::value_type>::digits
.The second constructor uses an implementation-defined mechanism to provide an integral constant expression
u
of typesize_t
and is called by the functionmake_seed_seq
.
In 29.5.8.1 [rand.util.seedseq], just after the last paragraph add:
template<size_t u, class InputIterator> seed_seq make_seed_seq(InputIterator begin, InputIterator end);where
u
is used to construct an objects
of implementation-defined type.Returns:
seed_seq(begin, end, s)
;
thread::id
reuseSection: 32.4.3.2 [thread.thread.id] Status: CD1 Submitter: Hans Boehm Opened: 2008-02-01 Last modified: 2016-01-28
Priority: Not Prioritized
View all other issues in [thread.thread.id].
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Discussion:
The current working paper
(N2497,
integrated just before Bellevue) is
not completely clear whether a given thread::id
value may be reused once
a thread has exited and has been joined or detached. Posix allows
thread ids (pthread_t
values) to be reused in this case. Although it is
not completely clear whether this originally was the right decision, it
is clearly the established practice, and we believe it was always the
intent of the C++ threads API to follow Posix and allow this. Howard
Hinnant's example implementation implicitly relies on allowing reuse
of ids, since it uses Posix thread ids directly.
It is important to be clear on this point, since it the reuse of thread ids often requires extra care in client code, which would not be necessary if thread ids were unique across all time. For example, a hash table indexed by thread id may have to be careful not to associate data values from an old thread with a new one that happens to reuse the id. Simply removing the old entry after joining a thread may not be sufficient, if it creates a visible window between the join and removal during which a new thread with the same id could have been created and added to the table.
[ post Bellevue Peter adds: ]
There is a real issue with
thread::id
reuse, but I urge the LWG to reconsider fixing this by disallowing reuse, rather than explicitly allowing it. Dealing with thread id reuse is an incredibly painful exercise that would just force the world to reimplement a non-conflictingthread::id
over and over.In addition, it would be nice if a
thread::id
could be manipulated atomically in a lock-free manner, as motivated by the recursive lock example:http://www.decadent.org.uk/pipermail/cpp-threads/2006-August/001091.html
Proposed resolution:
Add a sentence to 32.4.3.2 [thread.thread.id]/p1:
An object of type
thread::id
provides a unique identifier for each thread of execution and a single distinct value for all thread objects that do not represent a thread of execution ([thread.threads.class]). Each thread of execution has athread::id
that is not equal to thethread::id
of other threads of execution and that is not equal to thethread::id
ofstd::thread
objects that do not represent threads of execution. The library may reuse the value of athread::id
of a terminated thread that can no longer be joined.
Section: 30 [time] Status: Resolved Submitter: Christopher Kohlhoff, Jeff Garland Opened: 2008-02-03 Last modified: 2016-01-28
Priority: Not Prioritized
View all other issues in [time].
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Discussion:
The draft C++0x thread library requires that the time points of type
system_time
and returned by get_system_time()
represent Coordinated
Universal Time (UTC) (section [datetime.system]). This can lead to
surprising behavior when a library user performs a duration-based wait,
such as condition_variable::timed_wait()
. A complete explanation of the
problem may be found in the
Rationale for the Monotonic Clock
section in POSIX, but in summary:
condition_variable::timed_wait()
(and its POSIX
equivalent, pthread_cond_timedwait()
) are specified using absolute times
to address the problem of spurious wakeups.
condition_variable::timed_wait()
that behave as if by calling the
corresponding absolute time overload with a time point value of
get_system_time() + rel_time
.
POSIX solves the problem by introducing a new monotonic clock, which is
unaffected by changes to the system time. When a condition variable is
initialized, the user may specify whether the monotonic clock is to be
used. (It is worth noting that on POSIX systems it is not possible to
use condition_variable::native_handle()
to access this facility, since
the desired clock type must be specified during construction of the
condition variable object.)
In the context of the C++0x thread library, there are added dimensions to the problem due to the need to support platforms other than POSIX:
One possible minimal solution:
system_time
and get_system_time()
implementation-defined (i.e standard library implementors may choose the
appropriate underlying clock based on the capabilities of the target
platform).
system_time::seconds_since_epoch()
.
explicit system_time(time_t secs, nanoseconds ns
= 0)
to explicit system_time(nanoseconds ns)
.
Proposed resolution:
Rationale:
Addressed by N2661: A Foundation to Sleep On.
binary_search
Section: 26.8.4.5 [binary.search] Status: CD1 Submitter: Daniel Krügler Opened: 2007-09-08 Last modified: 2016-01-28
Priority: Not Prioritized
View all issues with CD1 status.
Discussion:
In 26.8.4.5 [binary.search] p. 3 the complexity of binary_search
is described as
At most
log(last - first) + 2
comparisons.
This should be precised and brought in line with the nomenclature used for
lower_bound
, upper_bound
, and equal_range
.
All existing libraries I'm aware of, delegate to
lower_bound
(+ one further comparison). Since
issue 384(i) has now WP status, the resolution of #787 should
be brought in-line with 384(i) by changing the + 2
to + O(1)
.
[ Sophia Antipolis: ]
Alisdair prefers to apply an upper bound instead of O(1), but that would require fixing for
lower_bound
,upper_bound
etc. as well. If he really cares about it, he'll send an issue to Howard.
Proposed resolution:
Change 26.8.4.5 [binary.search]/3
Complexity: At most
log2(last - first) +
comparisons.2O(1)
Section: 24.6.2 [istream.iterator] Status: C++11 Submitter: Martin Sebor Opened: 2008-02-06 Last modified: 2016-01-28
Priority: Not Prioritized
View all other issues in [istream.iterator].
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Discussion:
Addresses UK 287
It is not clear what the initial state of an
istream_iterator
should be. Is _value_ initialized by reading the stream, or default/value initialized? If it is initialized by reading the stream, what happens if the initialization is deferred until first dereference, when ideally the iterator value should have been that of an end-of-stream iterator which is not safely dereferencable?Recommendation: Specify _value_ is initialized by reading the stream, or the iterator takes on the end-of-stream value if the stream is empty.
The description of how an istream_iterator object becomes an end-of-stream iterator is a) ambiguous and b) out of date WRT issue 468(i):
istream_iterator
reads (usingoperator>>
) successive elements from the input stream for which it was constructed. After it is constructed, and every time++
is used, the iterator reads and stores a value ofT
. If the end of stream is reached (operator void*()
on the stream returnsfalse
), the iterator becomes equal to the end-of-stream iterator value. The constructor with no argumentsistream_iterator()
always constructs an end of stream input iterator object, which is the only legitimate iterator to be used for the end condition. The result ofoperator*
on an end of stream is not defined. For any other iterator value aconst T&
is returned. The result ofoperator->
on an end of stream is not defined. For any other iterator value aconst T*
is returned. It is impossible to store things into istream iterators. The main peculiarity of the istream iterators is the fact that++
operators are not equality preserving, that is,i == j
does not guarantee at all that++i == ++j
. Every time++
is used a new value is read.
istream::operator void*()
returns null if istream::fail()
is true
,
otherwise non-null. istream::fail()
returns true
if failbit
or
badbit
is set in rdstate()
. Reaching the end of stream doesn't
necessarily imply that failbit
or badbit
is set (e.g., after
extracting an int
from stringstream("123")
the stream object will
have reached the end of stream but fail()
is false
and operator
void*()
will return a non-null value).
Also I would prefer to be explicit about calling fail()
here
(there is no operator void*()
anymore.)
[ Summit: ]
Moved from Ready to Open for the purposes of using this issue to address NB UK 287. Martin to handle.
[ 2009-07 Frankfurt: ]
This improves the wording.
Move to Ready.
Proposed resolution:
Change 24.6.2 [istream.iterator]/1:
istream_iterator
reads (usingoperator>>
) successive elements from the input stream for which it was constructed. After it is constructed, and every time++
is used, the iterator reads and stores a value ofT
. Ifthe end of stream is reachedthe iterator fails to read and store a value ofT
(on the stream returns
operator void*()fail()), the iterator becomes equal to the end-of-stream iterator value. The constructor with no arguments
falsetrueistream_iterator()
always constructs an end of stream input iterator object, which is the only legitimate iterator to be used for the end condition. The result ofoperator*
on an end of stream is not defined. For any other iterator value aconst T&
is returned. The result ofoperator->
on an end of stream is not defined. For any other iterator value aconst T*
is returned. It is impossible to store things into istream iterators. The main peculiarity of the istream iterators is the fact that++
operators are not equality preserving, that is,i == j
does not guarantee at all that++i == ++j
. Every time++
is used a new value is read.
xor_combine_engine(result_type)
should be explicitSection: 99 [rand.adapt.xor] Status: CD1 Submitter: P.J. Plauger Opened: 2008-02-09 Last modified: 2016-01-28
Priority: Not Prioritized
View all other issues in [rand.adapt.xor].
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Discussion:
xor_combine_engine(result_type)
should be explicit
. (Obvious oversight.)
[ Bellevue: ]
Non-controversial. Bill is right, but Fermilab believes that this is easy to use badly and hard to use right, and so it should be removed entirely. Got into TR1 by well defined route, do we have permission to remove stuff? Should probably check with Jens, as it is believed he is the originator. Broad consensus that this is not a robust engine adapter.
Proposed resolution:
Remove xor_combine_engine from synopsis of 29.5.2 [rand.synopsis].
Remove 99 [rand.adapt.xor] xor_combine_engine
.
piecewise_constant_distribution
is undefined for a range with just one endpointSection: 29.5.9.6.2 [rand.dist.samp.pconst] Status: CD1 Submitter: P.J. Plauger Opened: 2008-02-09 Last modified: 2016-01-28
Priority: Not Prioritized
View all other issues in [rand.dist.samp.pconst].
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Discussion:
piecewise_constant_distribution
is undefined for a range with just one
endpoint. (Probably should be the same as an empty range.)
Proposed resolution:
Change 29.5.9.6.2 [rand.dist.samp.pconst] paragraph 3b:
b) If
firstB == lastB
or the sequencew
has the length zero,
discrete_distribution
missing constructorSection: 29.5.9.6.1 [rand.dist.samp.discrete] Status: Resolved Submitter: P.J. Plauger Opened: 2008-02-09 Last modified: 2016-01-28
Priority: Not Prioritized
View all other issues in [rand.dist.samp.discrete].
View all issues with Resolved status.
Discussion:
discrete_distribution
should have a constructor like:
template<class _Fn> discrete_distribution(result_type _Count, double _Low, double _High, _Fn& _Func);
(Makes it easier to fill a histogram with function values over a range.)
[ Bellevue: ]
How do you specify the function so that it does not return negative values? If you do it is a bad construction. This requirement is already there. Where in each bin does one evaluate the function? In the middle. Need to revisit tomorrow.
[ Sophia Antipolis: ]
Bill is not requesting this.
Marc Paterno:
_Fn
cannot return negative values at the points where the function is sampled. It is sampled in the middle of each bin._Fn
cannot return 0 everywhere it is sampled.Jens: lambda expressions are rvalues
Add a library issue to provide an
initializer_list<double>
constructor fordiscrete_distribution
.Marc Paterno: dislikes reference for
_Fn
parameter. Make it pass-by-value (to use lambda), usestd::ref
to wrap giant-state function objects.Daniel: See
random_shuffle
, pass-by-rvalue-reference.Daniel to draft wording.
[ Pre San Francisco, Daniel provided wording: ]
The here proposed changes of the WP refer to the current state of N2691. During the Sophia Antipolis meeting two different proposals came up regarding the functor argument type, either by value or by rvalue-reference. For consistence with existing conventions (state-free algorithms and the
general_pdf_distribution
c'tor signature) the author decided to propose a function argument that is provided by value. If severe concerns exists that stateful functions would be of dominant relevance, it should be possible to replace the two occurrences ofFunc
byFunc&&
in this proposal as part of an editorial process.
Proposed resolution:
Non-concept version of the proposed resolution
In 29.5.9.6.1 [rand.dist.samp.discrete]/1, class discrete_distribution
, just
before the member declaration
explicit discrete_distribution(const param_type& parm);
insert:
template<typename Func> discrete_distribution(result_type nf, double xmin, double xmax, Func fw);
Between p.4 and p.5 insert a series of new paragraphs as part of the new member description::
template<typename Func> discrete_distribution(result_type nf, double xmin, double xmax, Func fw);Complexity: Exactly nf invocations of fw.
Requires:
- fw shall be callable with one argument of type double, and shall return values of a type convertible to double;
- If nf > 0, the relation
xmin
<xmax
shall hold, and for all sample valuesxk
, fw(xk
) shall return a weight valuewk
that is non-negative, non-NaN, and non-infinity;- The following relations shall hold: nf ≥ 0, and 0 < S =
w0
+. . .+wn-1
.Effects:
- If nf == 0, sets n = 1 and lets the sequence w have length n = 1 and consist of the single value
w0
= 1.Otherwise, sets n = nf, deltax = (
xmax
-xmin
)/n andxcent
=xmin
+ 0.5 * deltax.For each k = 0, . . . ,n-1, calculates:
xk
=xcent
+ k * deltaxwk
= fw(xk
)Constructs a discrete_distribution object with probabilities:
pk
=wk
/S for k = 0, . . . , n-1.
Concept version of the proposed resolution
In 29.5.9.6.1 [rand.dist.samp.discrete]/1, class discrete_distribution
, just
before the member declaration
explicit discrete_distribution(const param_type& parm);
insert:
template<Callable<auto, double> Func> requires Convertible<Func::result_type, double> discrete_distribution(result_type nf, double xmin, double xmax, Func fw);
Between p.4 and p.5 insert a series of new paragraphs as part of the new member description::
template<Callable<auto, double> Func> requires Convertible<Func::result_type, double> discrete_distribution(result_type nf, double xmin, double xmax, Func fw);Complexity: Exactly nf invocations of fw.
Requires:
- If nf > 0, the relation
xmin
<xmax
shall hold, and for all sample valuesxk
, fw(xk
) shall return a weight valuewk
that is non-negative, non-NaN, and non-infinity;- The following relations shall hold: nf ≥ 0, and 0 < S =
w0
+. . .+wn-1
.Effects:
- If nf == 0, sets n = 1 and lets the sequence w have length n = 1 and consist of the single value
w0
= 1.Otherwise, sets n = nf, deltax = (
For each k = 0, . . . ,n-1, calculates:xmax
-xmin
)/n andxcent
=xmin
+ 0.5 * deltax.xk
=xcent
+ k * deltaxwk
= fw(xk
)Constructs a discrete_distribution object with probabilities:
pk
=wk
/S for k = 0, . . . , n-1.
Rationale:
Addressed by N2836 "Wording Tweaks for Concept-enabled Random Number Generation in C++0X".
piecewise_constant_distribution
missing constructorSection: 29.5.9.6.2 [rand.dist.samp.pconst] Status: Resolved Submitter: P.J. Plauger Opened: 2008-02-09 Last modified: 2016-01-28
Priority: Not Prioritized
View all other issues in [rand.dist.samp.pconst].
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Discussion:
piecewise_constant_distribution
should have a constructor like:
template<class _Fn> piecewise_constant_distribution(size_t _Count, _Ty _Low, _Ty _High, _Fn& _Func);
(Makes it easier to fill a histogram with function values over a range.
The two (reference 793(i)) make a sensible replacement for
general_pdf_distribution
.)
[ Sophia Antipolis: ]
Marc: uses variable width of bins and weight for each bin. This is not giving enough flexibility to control both variables.
Add a library issue to provide an constructor taking an
initializer_list<double>
and_Fn
forpiecewise_constant_distribution
.Daniel to draft wording.
[ Pre San Francisco, Daniel provided wording. ]
The here proposed changes of the WP refer to the current state of N2691. For reasons explained in 793(i), the author decided to propose a function argument that is provided by value. The issue proposes a c'tor signature, that does not take advantage of the full flexibility of
piecewise_constant_distribution
, because it restricts on a constant bin width, but the use-case seems to be popular enough to justify it's introduction.
Proposed resolution:
Non-concept version of the proposed resolution
In 29.5.9.6.2 [rand.dist.samp.pconst]/1, class piecewise_constant_distribution
,
just before the member declaration
explicit piecewise_constant_distribution(const param_type& parm);
insert:
template<typename Func> piecewise_constant_distribution(size_t nf, RealType xmin, RealType xmax, Func fw);
Between p.4 and p.5 insert a new sequence of paragraphs nominated below as [p5_1], [p5_2], [p5_3], and [p5_4] as part of the new member description:
template<typename Func> piecewise_constant_distribution(size_t nf, RealType xmin, RealType xmax, Func fw);[p5_1] Complexity: Exactly
nf
invocations offw
.[p5_2] Requires:
fw
shall be callable with one argument of typeRealType
, and shall return values of a type convertible to double;- For all sample values
xk
defined below, fw(xk
) shall return a weight valuewk
that is non-negative, non-NaN, and non-infinity;- The following relations shall hold:
xmin
<xmax
, and 0 < S =w0
+. . .+wn-1
.[p5_3] Effects:
If nf == 0,
- sets deltax =
xmax
-xmin
, and- lets the sequence
w
have lengthn = 1
and consist of the single valuew0
= 1, and- lets the sequence
b
have lengthn+1
withb0
=xmin
andb1
=xmax
Otherwise,
- sets
n = nf
,deltax =
(xmax
-xmin
)/n,xcent
=xmin
+ 0.5 * deltax, andlets the sequences
for each k = 0, . . . ,n-1, calculates:w
andb
have lengthn
andn+1
, resp. anddxk
= k * deltaxbk
=xmin
+dxk
xk
=xcent
+dxk
wk
= fw(xk
),and
- sets
bn
=xmax
Constructs a
piecewise_constant_distribution
object with the above computed sequenceb
as the interval boundaries and with the probability densities:
ρk
=wk
/(S * deltax) for k = 0, . . . , n-1.[p5_4] [Note: In this context, the subintervals [
bk
,bk+1
) are commonly known as the bins of a histogram. -- end note]
Concept version of the proposed resolution
In 29.5.9.6.2 [rand.dist.samp.pconst]/1, class piecewise_constant_distribution
,
just before the member declaration
explicit piecewise_constant_distribution(const param_type& parm);
insert:
template<Callable<auto, RealType> Func> requires Convertible<Func::result_type, double> piecewise_constant_distribution(size_t nf, RealType xmin, RealType xmax, Func fw);
Between p.4 and p.5 insert a new sequence of paragraphs nominated below as [p5_1], [p5_2], [p5_3], and [p5_4] as part of the new member description:
template<Callable<auto, RealType> Func> requires Convertible<Func::result_type, double> piecewise_constant_distribution(size_t nf, RealType xmin, RealType xmax, Func fw);[p5_1] Complexity: Exactly
nf
invocations offw
.[p5_2] Requires:
- For all sample values
xk
defined below, fw(xk
) shall return a weight valuewk
that is non-negative, non-NaN, and non-infinity;- The following relations shall hold:
xmin
<xmax
, and 0 < S =w0
+. . .+wn-1
.[p5_3] Effects:
If nf == 0,
- sets deltax =
xmax
-xmin
, and- lets the sequence
w
have lengthn = 1
and consist of the single valuew0
= 1, and- lets the sequence
b
have lengthn+1
withb0
=xmin
andb1
=xmax
Otherwise,
- sets
n = nf
,deltax =
(xmax
-xmin
)/n,xcent
=xmin
+ 0.5 * deltax, andlets the sequences
for each k = 0, . . . ,n-1, calculates:w
andb
have lengthn
andn+1
, resp. anddxk
= k * deltaxbk
=xmin
+dxk
xk
=xcent
+dxk
wk
= fw(xk
),and
- sets
bn
=xmax
Constructs a
piecewise_constant_distribution
object with the above computed sequenceb
as the interval boundaries and with the probability densities:
ρk
=wk
/(S * deltax) for k = 0, . . . , n-1.[p5_4] [Note: In this context, the subintervals [
bk
,bk+1
) are commonly known as the bins of a histogram. -- end note]
Rationale:
Addressed by N2836 "Wording Tweaks for Concept-enabled Random Number Generation in C++0X".
Section: 99 [depr.lib.binders] Status: CD1 Submitter: Daniel Krügler Opened: 2008-02-14 Last modified: 2016-01-28
Priority: Not Prioritized
View all other issues in [depr.lib.binders].
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Discussion:
N2521
and its earlier predecessors have moved the old binders from
[lib.binders] to 99 [depr.lib.binders] thereby introducing some renaming
of the template parameter names (Operation -> Fn
). During this
renaming process the protected data member op
was also renamed to
fn
, which seems as an unnecessary interface breakage to me - even if
this user access point is probably rarely used.
Proposed resolution:
Change [depr.lib.binder.1st]:
template <class Fn> class binder1st : public unary_function<typename Fn::second_argument_type, typename Fn::result_type> { protected: Fnfnop; typename Fn::first_argument_type value; public: binder1st(const Fn& x, const typename Fn::first_argument_type& y); typename Fn::result_type operator()(const typename Fn::second_argument_type& x) const; typename Fn::result_type operator()(typename Fn::second_argument_type& x) const; };-1- The constructor initializes
fn
op
withx
andvalue
withy
.-2-
operator()
returns.
fnop(value,x)
Change [depr.lib.binder.2nd]:
template <class Fn> class binder2nd : public unary_function<typename Fn::first_argument_type, typename Fn::result_type> { protected: Fnfnop; typename Fn::second_argument_type value; public: binder2nd(const Fn& x, const typename Fn::second_argument_type& y); typename Fn::result_type operator()(const typename Fn::first_argument_type& x) const; typename Fn::result_type operator()(typename Fn::first_argument_type& x) const; };-1- The constructor initializes
fn
op
withx
andvalue
withy
.-2-
operator()
returns.
fnop(value,x)
Section: 29.5.8.1 [rand.util.seedseq] Status: Resolved Submitter: Stephan Tolksdorf Opened: 2008-02-18 Last modified: 2016-01-28
Priority: Not Prioritized
View all other issues in [rand.util.seedseq].
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Discussion:
The for-loop in the algorithm specification has n
iterations, where n
is
defined to be end - begin
, i.e. the number of supplied w-bit quantities.
Previous versions of this algorithm and the general logic behind it
suggest that this is an oversight and that in the context of the
for-loop n
should be the number of full 32-bit quantities in b
(rounded
upwards). If w
is 64, the current algorithm throws away half of all bits
in b
. If w
is 16, the current algorithm sets half of all elements in v
to 0.
There are two more minor issues:
end - begin
is not defined since
InputIterator
is not required to be a random access iterator.
seed_seq
constructor, including bool
. IMHO allowing bool
s unnecessarily
complicates the implementation without any real benefit to the user.
I'd suggest to exclude bool
s as input.
[ Bellevue: ]
Move to Open: Bill will try to propose a resolution by the next meeting.
[ post Bellevue: Bill provided wording. ]
This issue is made moot if 803(i) is accepted.
Proposed resolution:
Replace 29.5.8.1 [rand.util.seedseq] paragraph 6 with:
Effects: Constructs a
seed_seq
object by effectively concatenating the low-orderu
bits of each of the elements of the supplied sequence[begin, end)
in ascending order of significance to make a (possibly very large) unsigned binary numberb
having a total ofn
bits, and then carrying out the following algorithm:for( v.clear(); n > 0; n -= 32 ) v.push_back(b mod 232), b /= 232;
Rationale:
Addressed by N2836 "Wording Tweaks for Concept-enabled Random Number Generation in C++0X".
tuple
and pair
trivial membersSection: 22.4 [tuple] Status: Resolved Submitter: Lawrence Crowl Opened: 2008-02-18 Last modified: 2016-01-28
Priority: Not Prioritized
View all other issues in [tuple].
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Discussion:
Classes with trivial special member functions are inherently more
efficient than classes without such functions. This efficiency is
particularly pronounced on modern ABIs that can pass small classes
in registers. Examples include value classes such as complex numbers
and floating-point intervals. Perhaps more important, though, are
classes that are simple collections, like pair
and tuple
. When the
parameter types of these classes are trivial, the pair
s and tuple
s
themselves can be trivial, leading to substantial performance wins.
The current working draft make specification of trivial functions
(where possible) much easer through default
ed and delete
d functions.
As long as the semantics of defaulted and deleted functions match
the intended semantics, specification of defaulted and deleted
functions will yield more efficient programs.
There are at least two cases where specification of an explicitly defaulted function may be desirable.
First, the std::pair
template has a non-trivial default constructor,
which prevents static initialization of the pair even when the
types are statically initializable. Changing the definition to
pair() = default;
would enable such initialization. Unfortunately, the change is not semantically neutral in that the current definition effectively forces value initialization whereas the change would not value initialize in some contexts.
** Does the committee confirm that forced value initialization
was the intent? If not, does the committee wish to change the
behavior of std::pair
in C++0x?
Second, the same default constructor issue applies to std::tuple
.
Furthermore, the tuple
copy constructor is current non-trivial,
which effectively prevents passing it in registers. To enable
passing tuples
in registers, the copy constructor should be
make explicitly default
ed. The new declarations are:
tuple() = default; tuple(const tuple&) = default;
This changes is not implementation neutral. In particular, it prevents implementations based on pointers to the parameter types. It does however, permit implementations using the parameter types as bases.
** How does the committee wish to trade implementation efficiency versus implementation flexibility?
[ Bellevue: ]
General agreement; the first half of the issue is NAD.
Before voting on the second half, it was agreed that a "Strongly Favor" vote meant support for trivial tuples (assuming usual requirements met), even at the expense of other desired qualities. A "Weakly Favor" vote meant support only if not at the expense of other desired qualities.
Concensus: Go forward, but not at expense of other desired qualities.
It was agreed to Alisdair should fold this work in with his other pair/tuple action items, above, and that issue 801 should be "open", but tabled until Alisdair's proposals are disposed of.
[ 2009-05-27 Daniel adds: ]
[ 2009-07 Frankfurt: ]
Wait for dust to settle from fixing exception safety problem with rvalue refs.
[ 2009-07-20 Alisdair adds: ]
Basically, this issue is what should we do with the default constructor for pairs and tuples of trivial types. The motivation of the issue was to force static initialization rather than dynamic initialization, and was rejected in the case of pair as it would change the meaning of existing programs. The advice was "do the best we can" for tuple without changing existing meaning.
Frankfurt seems to simply wait and see the resolution on no-throw move constructors, which (I believe) is only tangentially related to this issue, but as good as any to defer until Santa Cruz.
Looking again now, I think constant (static) initialization for pair can be salvaged by making the default construct constexpr. I have a clarification from Core that this is intended to work, even if the constructor is not trivial/constexpr, so long as no temporaries are implied in the process (even if elided).
[ 2009-10 Santa Cruz: ]
Leave as open. Alisdair to provide wording.
[ 2010 Pittsburgh: ]
We believe this may be NAD Editorial since both pair and tuple now have constexpr default constructors, but we're not sure.
[ 2010 Rapperswil: ]
Daniel believes his pair/tuple paper will resolve this issue.
constexpr
will allow static initialization, and he is already changing the move and copy constructors to be defaulted.
[ 2010-10-24 Daniel adds: ]
The proposed resolution of n3140 should resolve this issue.
Proposed resolution:
See n3140.
seed_seq::seq_seq
Section: 29.5.8.1 [rand.util.seedseq] Status: Resolved Submitter: Charles Karney Opened: 2008-02-22 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
seed_seq(InputIterator begin, InputIterator end);
constructs a seed_seq
object repacking the bits of supplied sequence [begin, end)
into a
32-bit vector.
This repacking triggers several problems:
seed_seq::generate
required the
introduction of the initial "if (w < 32) v.push_back(n);
" (Otherwise
the unsigned short vectors [1, 0] and [1] generate the same sequence.)
u
.
(Otherwise some sequences could not be obtained on computers where no
integer types are exactly 32-bits wide.)
I propose simplifying this seed_seq
constructor to be "32-bit only".
Despite it's being simpler, there is NO loss of functionality (see
below).
Here's how the description would read
29.5.8.1 [rand.util.seedseq] Class
seed_seq
template<class InputIterator> seed_seq(InputIterator begin, InputIterator end);5 Requires: NO CHANGE
6 Effects: Constructs a
seed_seq
object byfor (InputIterator s = begin; s != end; ++s) v.push_back((*s) mod 232);
Discussion:
The chief virtues here are simplicity, portability, and generality.
iterator_traits<InputIterator>::value_type =
uint_least32_t
the user is guaranteed to get the same behavior across
platforms.
Arguments (and counter-arguments) against making this change (and retaining the n2461 behavior) are:
The user can pass an array of unsigned char
and seed_seq
will nicely
repack it.
Response: So what? Consider the seed string "ABC". The n2461 proposal results in
v = { 0x3, 0x434241 };
while the simplified proposal yields
v = { 0x41, 0x42, 0x43 };
The results produced by seed_seq::generate
with the two inputs are
different but nevertheless equivalently "mixed up" and this remains
true even if the seed string is long.
With long strings (e.g., with bit-length comparable to the number of
bits in the state), v
is longer (by a factor of 4) with the simplified
proposal and seed_seq::generate
will be slower.
Response: It's unlikely that the efficiency of seed_seq::generate
will
be a big issue. If it is, the user is free to repack the seed vector
before constructing seed_seq
.
A user can pass an array of 64-bit integers and all the bits will be used.
Response: Indeed. However, there are many instances in the n2461 where integers are silently coerced to a narrower width and this should just be a case of the user needing to read the documentation. The user can of course get equivalent behavior by repacking his seed into 32-bit pieces. Furthermore, the unportability of the n2461 proposal with
unsigned long s[] = {1, 2, 3, 4}; seed_seq q(s, s+4);
which typically results in v = {1, 2, 3, 4}
on 32-bit machines and in
v = {1, 0, 2, 0, 3, 0, 4, 0}
on 64-bit machines is a major pitfall for
unsuspecting users.
Note: this proposal renders moot issues 782(i) and 800(i).
[ Bellevue: ]
Walter needs to ask Fermilab for guidance. Defer till tomorrow. Bill likes the proposed resolution.
[ Sophia Antipolis: ]
Marc Paterno wants portable behavior between 32bit and 64bit machines; we've gone to significant trouble to support portability of engines and their values.
Jens: the new algorithm looks perfectly portable
Marc Paterno to review off-line.
Modify the proposed resolution to read "Constructs a seed_seq object by the following algorithm ..."
Disposition: move to review; unanimous consent.
Proposed resolution:
Change 29.5.8.1 [rand.util.seedseq]:
template<class InputIterator, size_t u = numeric_limits<iterator_traits<InputIterator>::value_type>::digits> seed_seq(InputIterator begin, InputIterator end);-5- Requires:
InputIterator
shall satisfy the requirements of an input iterator (24.1.1) such thatiterator_traits<InputIterator>::value_type
shall denote an integral type.-6- Constructs a
seed_seq
object by the following algorithmrearranging some or all of the bits of the supplied sequence[begin,end)
of w-bit quantities into 32-bit units, as if by the following:
First extract the rightmostu
bits from each of then = end - begin
elements of the supplied sequence and concatenate all the extracted bits to initialize a single (possibly very large) unsigned binary number,b = ∑n-1i=0 (begin[i] mod 2u) · 2w·i
(in which the bits of eachbegin[i]
are treated as denoting an unsigned quantity). Then carry out the following algorithm:
v.clear(); if ($w$ < 32) v.push_back($n$); for( ; $n$ > 0; --$n$) v.push_back(b mod 232), b /= 232;for (InputIterator s = begin; s != end; ++s) v.push_back((*s) mod 232);
Rationale:
Addressed by N2836 "Wording Tweaks for Concept-enabled Random Number Generation in C++0X".
error_code
/error_condition
Section: 19.5 [syserr] Status: CD1 Submitter: Daniel Krügler Opened: 2008-02-24 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
19.5.4.1 [syserr.errcode.overview]/1, class error_code
and
19.5.5.1 [syserr.errcondition.overview]/, class error_condition
synopses
declare an expository data member cat_
:
const error_category& cat_; // exposition only
which is used to define the semantics of several members. The decision to use a member of reference type lead to several problems:
(Copy)Assignable
, which is probably not the intent.
The simple fix would be to replace the reference by a pointer member.
operator=
overload (template with ErrorCodeEnum
argument) makes in invalid
usage of std::enable_if
: By using the default value for the second enable_if
parameter the return type would be defined to be void&
even in otherwise
valid circumstances - this return type must be explicitly provided (In
error_condition
the first declaration uses an explicit value, but of wrong
type).
message
throws clauses (
19.5.3.2 [syserr.errcat.virtuals]/10, 19.5.4.4 [syserr.errcode.observers]/8, and
19.5.5.4 [syserr.errcondition.observers]/6) guarantee "throws nothing",
although
they return a std::string
by value, which might throw in out-of-memory
conditions (see related issue 771(i)).
[ Sophia Antipolis: ]
Part A: NAD (editorial), cleared by the resolution of issue 832(i).
Part B: Technically correct, save for typo. Rendered moot by the concept proposal (N2620) NAD (editorial).
Part C: We agree; this is consistent with the resolution of issue 721(i).
Howard: please ping Beman, asking him to clear away parts A and B from the wording in the proposed resolution, so it is clear to the editor what needs to be applied to the working paper.
Beman provided updated wording. Since issue 832(i) is not going forward, the provided wording includes resolution of part A.
Proposed resolution:
Resolution of part A:
Change 19.5.4.1 [syserr.errcode.overview] Class error_code overview synopsis as indicated:
private: int val_; // exposition only const error_category&* cat_; // exposition onlyChange 19.5.4.2 [syserr.errcode.constructors] Class error_code constructors as indicated:
error_code();Effects: Constructs an object of type
error_code
.Postconditions:
val_ == 0
andcat_ == &system_category
.Throws: Nothing.
error_code(int val, const error_category& cat);Effects: Constructs an object of type
error_code
.Postconditions:
val_ == val
andcat_ == &cat
.Throws: Nothing.
Change 19.5.4.3 [syserr.errcode.modifiers] Class error_code modifiers as indicated:
void assign(int val, const error_category& cat);Postconditions:
val_ == val
andcat_ == &cat
.Throws: Nothing.
Change 19.5.4.4 [syserr.errcode.observers] Class error_code observers as indicated:
const error_category& category() const;Returns:
*cat_
.Throws: Nothing.
Change 19.5.5.1 [syserr.errcondition.overview] Class error_condition overview synopsis as indicated:
private: int val_; // exposition only const error_category&* cat_; // exposition onlyChange 19.5.5.2 [syserr.errcondition.constructors] Class error_condition constructors as indicated:
[ (If the proposed resolution of issue 805(i) has already been applied, the name
posix_category
will have been changed togeneric_category
. That has no effect on this resolution.) ]error_condition();Effects: Constructs an object of type
error_condition
.Postconditions:
val_ == 0
andcat_ == &posix_category
.Throws: Nothing.
error_condition(int val, const error_category& cat);Effects: Constructs an object of type
error_condition
.Postconditions:
val_ == val
andcat_ == &cat
.Throws: Nothing.
Change 19.5.5.3 [syserr.errcondition.modifiers] Class error_condition modifiers as indicated:
void assign(int val, const error_category& cat);Postconditions:
val_ == val
andcat_ == &cat
.Throws: Nothing.
Change 19.5.5.4 [syserr.errcondition.observers] Class error_condition observers as indicated:
const error_category& category() const;Returns:
*cat_
.Throws: Nothing.
Resolution of part C:
In 19.5.3.2 [syserr.errcat.virtuals], remove the throws clause p. 10.
virtual string message(int ev) const = 0;Returns: A string that describes the error condition denoted by
ev
.
Throws: Nothing.In 19.5.4.4 [syserr.errcode.observers], remove the throws clause p. 8.
string message() const;Returns:
category().message(value())
.
Throws: Nothing.In 19.5.5.4 [syserr.errcondition.observers], remove the throws clause p. 6.
string message() const;Returns:
category().message(value())
.
Throws: Nothing.
posix_error::posix_errno
concernsSection: 19.5 [syserr] Status: CD1 Submitter: Jens Maurer Opened: 2008-02-24 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
19.5 [syserr]
namespace posix_error { enum posix_errno { address_family_not_supported, // EAFNOSUPPORT ...
should rather use the new scoped-enum facility (9.8.1 [dcl.enum]),
which would avoid the necessity for a new posix_error
namespace, if I understand correctly.
[ Further discussion: ]
See N2347, Strongly Typed Enums, since renamed Scoped Enums.
Alberto Ganesh Barbati also raised this issue in private email, and also proposed the scoped-enum solution.
Nick Stoughton asked in Bellevue that
posix_error
andposix_errno
not be used as names. The LWG agreed.The wording for the Proposed resolution was provided by Beman Dawes.
Proposed resolution:
Change System error support 19.5 [syserr] as indicated:
namespace posix_error {enumposix_errnoclass errc { address_family_not_supported, // EAFNOSUPPORT ... wrong_protocol_type, // EPROTOTYPE };} // namespace posix_errortemplate <> struct is_error_condition_enum<posix_error::posix_errnoerrc> : public true_type {}namespace posix_error {error_code make_error_code(posix_errnoerrc e); error_condition make_error_condition(posix_errnoerrc e);} // namespace posix_error
Change System error support 19.5 [syserr] :
Theis_error_code_enum
andis_error_condition_enum
templates may be specialized for user-defined types to indicate that such a type is eligible for classerror_code
and classerror_condition
automatic conversions, respectively.
Change System error support 19.5 [syserr] and its subsections:
- remove all occurrences of
posix_error::
- change all instances of
posix_errno
toerrc
- change all instances of
posix_category
togeneric_category
- change all instances of
get_posix_category
toget_generic_category
Change Error category objects 19.5.3.5 [syserr.errcat.objects], paragraph 2:
Remarks: The object's
default_error_condition
and equivalent virtual functions shall behave as specified for the classerror_category
. The object's name virtual function shall return a pointer to the string"POSIX""generic".
Change 19.5.4.5 [syserr.errcode.nonmembers] Class error_code
non-member functions as indicated:
error_code make_error_code(posix_errnoerrc e);Returns:
error_code(static_cast<int>(e),
.posixgeneric_category)
Change 19.5.5.5 [syserr.errcondition.nonmembers] Class error_condition
non-member functions as indicated:
error_condition make_error_condition(posix_errnoerrc e);Returns:
error_condition(static_cast<int>(e),
.posixgeneric_category)
Rationale:
Names Considered | |
---|---|
portable |
Too non-specific. Did not wish to reserve such a common word in namespace std. Not quite the right meaning, either. |
portable_error |
Too long. Explicit qualification is always required for scoped enums, so
a short name is desirable. Not quite the right meaning, either. May be
misleading because *_error in the std lib is usually an exception class
name.
|
std_error |
Fairly short, yet explicit. But in fully qualified names like
std::std_error::not_enough_memory , the std_ would be unfortunate. Not
quite the right meaning, either. May be misleading because *_error in
the std lib is usually an exception class name.
|
generic |
Short enough. The category could be generic_category . Fully qualified
names like std::generic::not_enough_memory read well. Reserving in
namespace std seems dicey.
|
generic_error |
Longish. The category could be generic_category . Fully qualified names
like std::generic_error::not_enough_memory read well. Misleading because
*_error in the std lib is usually an exception class name.
|
generic_err |
A bit less longish. The category could be generic_category . Fully
qualified names like std::generic_err::not_enough_memory read well.
|
gen_err |
Shorter still. The category could be generic_category . Fully qualified
names like std::gen_err::not_enough_memory read well.
|
generr |
Shorter still. The category could be generic_category . Fully qualified
names like std::generr::not_enough_memory read well.
|
error |
Shorter still. The category could be generic_category . Fully qualified
names like std::error::not_enough_memory read well. Do we want to use
this general a name?
|
err |
Shorter still. The category could be generic_category . Fully qualified
names like std::err::not_enough_memory read well. Although alone it
looks odd as a name, given the existing errno and namespace std names,
it seems fairly intuitive.
Problem: err is used throughout the standard library as an argument name
and in examples as a variable name; it seems too confusing to add yet
another use of the name.
|
errc |
Short enough. The "c" stands for "constant". The category could be
generic_category . Fully qualified names like
std::errc::not_enough_memory read well. Although alone it looks odd as a
name, given the existing errno and namespace std names, it seems fairly
intuitive. There are no uses of errc in the current C++ standard.
|
unique_ptr::reset
effects incorrect, too permissiveSection: 20.3.1.3.6 [unique.ptr.single.modifiers] Status: CD1 Submitter: Peter Dimov Opened: 2008-03-13 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
void unique_ptr::reset(T* p = 0)
is currently specified as:
Effects: If
p == get()
there are no effects. Otherwiseget_deleter()(get())
.
There are two problems with this. One, if get() == 0
and p != 0
, the
deleter is called with a NULL
pointer, and this is probably not what's
intended (the destructor avoids calling the deleter with 0.)
Two, the special check for get() == p
is generally not needed and such a
situation usually indicates an error in the client code, which is being
masked. As a data point, boost::shared_ptr
was changed to assert on such
self-resets in 2001 and there were no complaints.
One might think that self-resets are necessary for operator=
to work; it's specified to perform
reset( u.release() );
and the self-assignment
p = move(p);
might appear to result in a self-reset. But it doesn't; the release()
is
performed first, zeroing the stored pointer. In other words, p.reset(
q.release() )
works even when p
and q
are the same unique_ptr
, and there
is no need to special-case p.reset( q.get() )
to work in a similar
scenario, as it definitely doesn't when p
and q
are separate.
Proposed resolution:
Change 20.3.1.3.6 [unique.ptr.single.modifiers]:
void reset(T* p = 0);-4- Effects: If
there are no effects. Otherwise
p ==get() == 0get_deleter()(get())
.
Change 20.3.1.4.5 [unique.ptr.runtime.modifiers]:
void reset(T* p = 0);...
-2- Effects: If
there are no effects. Otherwise
p ==get() == 0get_deleter()(get())
.
tuple
construction should not fail unless its element's construction failsSection: 22.4.4.2 [tuple.cnstr] Status: CD1 Submitter: Howard Hinnant Opened: 2008-03-13 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
527(i) Added a throws clause to bind
constructors. I believe the same throws clause
should be added to tuple
except it ought to take into account move constructors as well.
Proposed resolution:
Add to 22.4.4.2 [tuple.cnstr]:
For each
tuple
constructor and assignment operator, an exception is thrown only if the construction or assignment of one of the types inTypes
throws an exception.
Section: 22.2.4 [forward] Status: CD1 Submitter: Jens Maurer Opened: 2008-03-13 Last modified: 2016-02-01
Priority: Not Prioritized
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Discussion:
p4 (forward) says:
Return type: If
T
is an lvalue-reference type, an lvalue; otherwise, an rvalue.
First of all, lvalue-ness and rvalue-ness are properties of an expression, not of a type (see 7.2.1 [basic.lval]). Thus, the phrasing "Return type" is wrong. Second, the phrase says exactly what the core language wording says for folding references in 13.4.2 [temp.arg.type]/p4 and for function return values in 7.6.1.3 [expr.call]/p10. (If we feel the wording should be retained, it should at most be a note with cross-references to those sections.)
The prose after the example talks about "forwarding as an int&
(an lvalue)" etc.
In my opinion, this is a category error: "int&
" is a type, "lvalue" is a
property of an expression, orthogonal to its type. (Btw, expressions cannot
have reference type, ever.)
Similar with move:
Return type: an rvalue.
is just wrong and also redundant.
Proposed resolution:
Change 22.2.4 [forward] as indicated:
template <class T> T&& forward(typename identity<T>::type&& t);...
Return type: IfT
is an lvalue-reference type, an lvalue; otherwise, an rvalue....
-7- In the first call to
factory
,A1
is deduced asint
, so 2 is forwarded toA
's constructor asanan rvalueint&&
(). In the second call to factory,A1
is deduced asint&
, soi
is forwarded toA
's constructor asanan lvalueint&
(). In both cases,A2
is deduced as double, so 1.414 is forwarded toA
's constructor asan rvaluedouble&&
().template <class T> typename remove_reference<T>::type&& move(T&& t);...
Return type: an rvalue.
std::swap
should be overloaded for array typesSection: 26.7.3 [alg.swap] Status: CD1 Submitter: Niels Dekker Opened: 2008-02-28 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
For the sake of generic programming, the header <algorithm>
should provide an
overload of std::swap
for array types:
template<class T, size_t N> void swap(T (&a)[N], T (&b)[N]);
It became apparent to me that this overload is missing, when I considered how to write a swap
function for a generic wrapper class template.
(Actually I was thinking of Boost's value_initialized.)
Please look at the following template, W
, and suppose that is intended to be a very
generic wrapper:
template<class T> class W { public: T data; };
Clearly W<T>
is CopyConstructible and CopyAssignable, and therefore
Swappable, whenever T
is CopyConstructible and CopyAssignable.
Moreover, W<T>
is also Swappable when T
is an array type
whose element type is CopyConstructible and CopyAssignable.
Still it is recommended to add a custom swap function template to such a class template,
for the sake of efficiency and exception safety.
(E.g., Scott Meyers, Effective C++, Third Edition, item 25: Consider support for a non-throwing
swap.)
This function template is typically written as follows:
template<class T> void swap(W<T>& x, W<T>& y) { using std::swap; swap(x.data, y.data); }
Unfortunately, this will introduce an undesirable inconsistency, when T
is an array.
For instance, W<std::string[8]>
is Swappable, but the current Standard does not
allow calling the custom swap function that was especially written for W
!
W<std::string[8]> w1, w2; // Two objects of a Swappable type. std::swap(w1, w2); // Well-defined, but inefficient. using std::swap; swap(w1, w2); // Ill-formed, just because ADL finds W's swap function!!!
W
's swap
function would try to call std::swap
for an array,
std::string[8]
, which is not supported by the Standard Library.
This issue is easily solved by providing an overload of std::swap
for array types.
This swap function should be implemented in terms of swapping the elements of the arrays, so that
it would be non-throwing for arrays whose element types have a non-throwing swap.
Note that such an overload of std::swap
should also support multi-dimensional
arrays. Fortunately that isn't really an issue, because it would do so automatically, by
means of recursion.
For your information, there was a discussion on this issue at comp.lang.c++.moderated: [Standard Library] Shouldn't std::swap be overloaded for C-style arrays?
Proposed resolution:
Add an extra condition to the definition of Swappable requirements [swappable] in 16.4.4.2 [utility.arg.requirements]:
-
T
isSwappable
ifT
is an array type whose element type isSwappable
.
Add the following to 26.7.3 [alg.swap]:
template<class T, size_t N> void swap(T (&a)[N], T (&b)[N]);Requires: Type
T
shall beSwappable
.Effects:
swap_ranges(a, a + N, b);
Section: 31.7.8 [ext.manip] Status: C++11 Submitter: Daniel Krügler Opened: 2008-03-01 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
The recent draft (as well as the original proposal n2072) uses an
operational semantic
for get_money
([ext.manip]/4) and put_money
([ext.manip]/6), which uses
istreambuf_iterator<charT>
and
ostreambuf_iterator<charT>
resp, instead of the iterator instances, with explicitly provided
traits argument (The operational semantic defined by f
is also traits
dependent). This is an obvious oversight because both *stream_buf
c'tors expect a basic_streambuf<charT,traits>
as argument.
The same problem occurs within the get_time
and put_time
semantic
where additional to the problem we
have an editorial issue in get_time
(streambuf_iterator
instead of
istreambuf_iterator
).
[ Batavia (2009-05): ]
This appears to be an issue of presentation.
We agree with the proposed resolution. Move to Tentatively Ready.
Proposed resolution:
In 31.7.8 [ext.manip]/4 within function f
replace the first line
template <class charT, class traits, class moneyT> void f(basic_ios<charT, traits>& str, moneyT& mon, bool intl) { typedef istreambuf_iterator<charT, traits> Iter; ...
In 31.7.8 [ext.manip]/5 remove the first template charT
parameter:
template <class charT,class moneyT> unspecified put_money(const moneyT& mon, bool intl = false);
In 31.7.8 [ext.manip]/6 within function f
replace the first line
template <class charT, class traits, class moneyT> void f(basic_ios<charT, traits>& str, const moneyT& mon, bool intl) { typedef ostreambuf_iterator<charT, traits> Iter; ...
In 31.7.8 [ext.manip]/8 within function f
replace the first line
template <class charT, class traits> void f(basic_ios<charT, traits>& str, struct tm *tmb, const charT *fmt) { typedef istreambuf_iterator<charT, traits> Iter; ...
In 31.7.8 [ext.manip]/10 within function f
replace the first line
template <class charT, class traits> void f(basic_ios<charT, traits>& str, const struct tm *tmb, const charT *fmt) { typedef ostreambuf_iterator<charT, traits> Iter; ...
In 31.7 [iostream.format], Header <iomanip>
synopsis change:
template <class charT,class moneyT> T8 put_money(const moneyT& mon, bool intl = false);
pair
of pointers no longer works with literal 0Section: 22.3 [pairs] Status: C++11 Submitter: Doug Gregor Opened: 2008-03-14 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
#include <utility> int main() { std::pair<char *, char *> p (0,0); }
I just got a bug report about that, because it's valid C++03, but not
C++0x. The important realization, for me, is that the emplace
proposal---which made push_back
variadic, causing the push_back(0)
issue---didn't cause this break in backward compatibility. The break
actually happened when we added this pair constructor as part of adding
rvalue references into the language, long before variadic templates or
emplace came along:
template<class U, class V> pair(U&& x, V&& y);
Now, concepts will address this issue by constraining that pair
constructor to only U
's and V
's that can properly construct "first" and
"second", e.g. (from
N2322):
template<class U , class V > requires Constructible<T1, U&&> && Constructible<T2, V&&> pair(U&& x , V&& y );
[ San Francisco: ]
Suggested to resolve using pass-by-value for that case.
Side question: Should pair interoperate with tuples? Can construct a tuple of a pair, but not a pair from a two-element tuple.
[ 2009-07-28 Reopened by Alisdair. No longer solved by concepts. ]
[ 2009-10 Santa Cruz: ]
Leave as open. Howard to provide wording.
[ 2010-02-06 Howard provided wording. ]
[ 2010-02-09 Moved to Tentatively Ready after 6 positive votes on c++std-lib. ]
Rationale:
[ San Francisco: ]
Solved by N2770.
[ The rationale is obsolete. ]
Proposed resolution:
Add a paragraph to 22.3 [pairs]:
template<class U, class V> pair(U&& x, V&& y);6 Effects: The constructor initializes
first
withstd::forward<U>(x)
and second withstd::forward<V>(y)
.Remarks:
U
shall be implicitly convertible tofirst_type
andV
shall be implicitly convertible tosecond_type
, else this constructor shall not participate in overload resolution.
shared_ptr
Section: 20.3.2.2 [util.smartptr.shared] Status: CD1 Submitter: Matt Austern Opened: 2008-02-26 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
Several places in 20.3.2.2 [util.smartptr.shared] refer to an "empty" shared_ptr
.
However, that term is nowhere defined. The closest thing we have to a
definition is that the default constructor creates an empty shared_ptr
and that a copy of a default-constructed shared_ptr
is empty. Are any
other shared_ptr
s empty? For example, is shared_ptr((T*) 0)
empty? What
are the properties of an empty shared_ptr
? We should either clarify this
term or stop using it.
One reason it's not good enough to leave this term up to the reader's
intuition is that, in light of
N2351
and issue 711(i), most readers'
intuitive understanding is likely to be wrong. Intuitively one might
expect that an empty shared_ptr
is one that doesn't store a pointer,
but, whatever the definition is, that isn't it.
[ Peter adds: ]
Or, what is an "empty"
shared_ptr
?
Are any other
shared_ptrs
empty?Yes. Whether a given
shared_ptr
instance is empty or not is (*) completely specified by the last mutating operation on that instance. Give me an example and I'll tell you whether theshared_ptr
is empty or not.(*) If it isn't, this is a legitimate defect.
For example, is
shared_ptr((T*) 0)
empty?No. If it were empty, it would have a
use_count()
of 0, whereas it is specified to have anuse_count()
of 1.What are the properties of an empty
shared_ptr
?The properties of an empty
shared_ptr
can be derived from the specification. One example is that its destructor is a no-op. Another is that itsuse_count()
returns 0. I can enumerate the full list if you really like.We should either clarify this term or stop using it.
I don't agree with the imperative tone
A clarification would be either a no-op - if it doesn't contradict the existing wording - or a big mistake if it does.
I agree that a clarification that is formally a no-op may add value.
However, that term is nowhere defined.
Terms can be useful without a definition. Consider the following simplistic example. We have a type
X
with the following operations defined:X x; X x2(x); X f(X x); X g(X x1, X x2);A default-constructed value is green.
A copy has the same color as the original.
f(x)
returns a red value if the argument is green, a green value otherwise.
g(x1,x2)
returns a green value if the arguments are of the same color, a red value otherwise.Given these definitions, you can determine the color of every instance of type
X
, even if you have absolutely no idea what green and red mean.Green and red are "nowhere defined" and completely defined at the same time.
Alisdair's wording is fine.
Proposed resolution:
Append the following sentance to 20.3.2.2 [util.smartptr.shared]
The
shared_ptr
class template stores a pointer, usually obtained vianew
.shared_ptr
implements semantics of shared ownership; the last remaining owner of the pointer is responsible for destroying the object, or otherwise releasing the resources associated with the stored pointer. Ashared_ptr
object that does not own a pointer is said to be empty.
vector<bool>::swap(reference, reference)
not definedSection: 23.3.14 [vector.bool] Status: C++11 Submitter: Alisdair Meredith Opened: 2008-03-17 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
vector<bool>::swap(reference, reference)
has no definition.
[ San Francisco: ]
Move to Open. Alisdair to provide a resolution.
[ Post Summit Daniel provided wording. ]
[ Batavia (2009-05): ]
We agree with the proposed resolution. Move to Tentatively Ready.
Proposed resolution:
Just after 23.3.14 [vector.bool]/5 add the following prototype and description:
static void swap(reference x, reference y);
-6- Effects: Exchanges the contents of
x
andy
as-if by:bool b = x; x = y; y = b;
std::function
and reference_closure
do not use perfect forwardingSection: 22.10.17.3.5 [func.wrap.func.inv] Status: Resolved Submitter: Alisdair Meredith Opened: 2008-03-16 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
std::function
and reference_closure
should use "perfect forwarding" as
described in the rvalue core proposal.
[ Sophia Antipolis: ]
According to Doug Gregor, as far as
std::function
is concerned, perfect forwarding can not be obtained because of type erasure. Not everyone agreed with this diagnosis of forwarding.
[ 2009-05-01 Howard adds: ]
Sebastian Gesemann brought to my attention that the
CopyConstructible
requirement onfunction
'sArgTypes...
is an unnecessary restriction.template<Returnable R, CopyConstructible... ArgTypes> class function<R(ArgTypes...)> ...On further investigation, this complaint seemed to be the same issue as this one. I believe the reason
CopyConstructible
was put onArgTypes
in the first place was because of the nature of the invoke member:template<class R, class ...ArgTypes> R function<R(ArgTypes...)>::operator()(ArgTypes... arg) const { if (f_ == 0) throw bad_function_call(); return (*f_)(arg...); }However now with rvalue-refs, "by value" no longer implies
CopyConstructible
(as Sebastian correctly points out). If rvalue arguments are supplied,MoveConstructible
is sufficient. Furthermore, the constraint need not be applied infunction
if I understand correctly. Rather the client must apply the proper constraints at the call site. Therefore, at the very least, I recommend thatCopyConstructible
be removed from the template classfunction
.Furthermore we need to mandate that the invoker is coded as:
template<class R, class ...ArgTypes> R function<R(ArgTypes...)>::operator()(ArgTypes... arg) const { if (f_ == 0) throw bad_function_call(); return (*f_)(std::forward<ArgTypes>(arg)...); }Note that
ArgTypes&&
(the "perfect forwarding signature") is not appropriate here as this is not a deduced context forArgTypes
. Instead the client's arguments must implicitly convert to the non-deducedArgType
type. Catching these arguments by value makes sense to enable decay.Next
forward
is used to move theArgTypes
as efficiently as possible, and also with minimum requirements (notCopyConstructible
) to the type-erased functor. For object types, this will be amove
. For reference typeArgTypes
, this will be a copy. The end result must be that the following is a valid program:#include <functional> #include <memory> #include <cassert> std::unique_ptr<int> f(std::unique_ptr<int> p, int& i) { ++i; return std::move(p); } int main() { int i = 2; std::function<std::unique_ptr<int>(std::unique_ptr<int>, int&> g(f); std::unique_ptr<int> p = g(std::unique_ptr<int>(new int(1)), i); assert(*p == 1); assert(i == 3); }[ Tested in pre-concepts rvalue-ref-enabled compiler. ]
In the example above, the first
ArgType
isunique_ptr<int>
and the secondArgType
isint&
. Both must work!
[ 2009-05-27 Daniel adds: ]
in the 2009-05-01 comment of above mentioned issue Howard
- Recommends to replace the
CopyConstructible
requirement by aMoveConstructible
requirement- Says: "Furthermore, the constraint need not be applied in
function
if I understand correctly. Rather the client must apply the proper constraints at the call site"I'm fine with (a), but I think comment (b) is incorrect, at least in the sense I read these sentences. Let's look at Howard's example code:
function<R(ArgTypes...)>::operator()(ArgTypes... arg) const { if (f_ == 0) throw bad_function_call(); return (*f_)(std::forward<ArgTypes>(arg)...); }In the constrained scope of this
operator()
overload the expression "(*f_)(std::forward<ArgTypes>(arg)...)
" must be valid. How can it do so, ifArgTypes
aren't at leastMoveConstructible
?
[ 2009-07 Frankfurt: ]
Leave this open and wait until concepts are removed from the Working Draft so that we know how to write the proposed resolution in terms of diffs to otherwise stable text.
[ 2009-10 Santa Cruz: ]
Leave as open. Howard to provide wording. Howard welcomes any help.
[ 2009-12-12 Jonathan Wakely adds: ]
22.10.17.3 [func.wrap.func] says
2 A function object
f
of typeF
is Callable for argument typesT1, T2, ..., TN
inArgTypes
and a return typeR
, if, given lvaluest1, t2, ..., tN
of typesT1, T2, ..., TN
, respectively,INVOKE (f, t1, t2, ..., tN)
is well formed (20.7.2) and, ifR
is notvoid
, convertible toR
.N.B. lvalues, which means you can't use
function<R(T&&)>
orfunction<R(unique_ptr<T>)>
I recently implemented rvalue arguments in GCC's
std::function
, all that was needed was to usestd::forward<ArgTypes>
in a few places. The example in issue 815 works.I think 815 could be resolved by removing the requirement that the target function be callable with lvalues. Saying
ArgTypes
need to beCopyConstructible
is wrong, and IMHO sayingMoveConstructible
is unnecessary, since the by-value signature implies that already, but if it is needed it should only be onoperator()
, not the whole class (you could in theory instantiatestd::function<R(noncopyable)>
as long as you don't invoke the call operator.)I think defining invocation in terms of
INVOKE
already implies perfect forwarding, so we don't need to say explicitly thatstd::forward
should be used (N.B. the types that are forwarded are those inArgTypes
, which can differ from the actual parameter types of the target function. The actual parameter types have gone via type erasure, but that's not a problem - IMHO forwarding the arguments asArgTypes
is the right thing to do anyway.)Is it sufficient to simply replace "lvalues" with "values"? or do we need to say something like "lvalues when
Ti
is an lvalue-reference and rvalues otherwise"? I prefer the former, so I propose the following resolution for 815:Edit 22.10.17.3 [func.wrap.func] paragraph 2:
2 A function object
f
of typeF
is Callable for argument typesT1, T2, ..., TN
inArgTypes
and a return typeR
, if, givenlvaluest1, t2, ..., tN
of typesT1, T2, ..., TN
, respectively,INVOKE (f, t1, t2, ..., tN)
is well formed (20.7.2) and, ifR
is notvoid
, convertible toR
.
[ 2009-12-12 Daniel adds: ]
I don't like the reduction to "values" and prefer the alternative solution suggested using "lvalues when Ti is an lvalue-reference and rvalues otherwise". The reason why I dislike the shorter version is based on different usages of "values" as part of defining the semantics of requirement tables via expressions. E.g. 16.4.4.2 [utility.arg.requirements]/1 says "
a
,b
, andc
are values of typeconst T;
" or similar in 23.2.2 [container.requirements.general]/4 or /14 etc. My current reading of all these parts is that both rvalues and lvalues are required to be supported, but this interpretation would violate the intention of the suggested fix of #815, if I correctly understand Jonathan's rationale.
[ 2009-12-12 Howard adds: ]
"lvalues when Ti is an lvalue-reference and rvalues otherwise"
doesn't quite work here because the
Ti
aren't deduced. They are specified by thefunction
type.Ti
might beconst int&
(an lvalue reference) and a validti
might be2
(a non-const rvalue). I've taken another stab at the wording using "expressions" and "bindable to".
[ 2010-02-09 Wording updated by Jonathan, Ganesh and Daniel. ]
[ 2010-02-09 Moved to Tentatively Ready after 5 positive votes on c++std-lib. ]
[ 2010-02-10 Daniel opens to improve wording. ]
[ 2010-02-11 This issue is now addressed by 870(i). ]
[ 2010-02-12 Moved to Tentatively NAD Editorial after 5 positive votes on c++std-lib. Rationale added below. ]
Rationale:
Proposed resolution:
Edit 22.10.17.3 [func.wrap.func] paragraph 2:
2 A function object
f
of typeF
is Callable for argument typesT1, T2, ..., TN
inArgTypes
andareturn typeR
,if, given lvaluesthe expressiont1, t2, ..., tN
of typesT1, T2, ..., TN
, respectively,INVOKE(f, declval<ArgTypes>()..., R
, considered as an unevaluated operand (7 [expr]), is well formed (20.7.2)t1, t2, ..., tN)and, if.R
is notvoid
, convertible toR
bind()
's returned functor have a nofail copy ctor when bind()
is nofail?Section: 22.10.15.4 [func.bind.bind] Status: Resolved Submitter: Stephan T. Lavavej Opened: 2008-02-08 Last modified: 2016-01-28
Priority: Not Prioritized
View all other issues in [func.bind.bind].
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Discussion:
Library Issue 527(i) notes that bind(f, t1, ..., tN)
should be nofail when f, t1, ..., tN
have nofail copy ctors.
However, no guarantees are provided for the copy ctor of the functor
returned by bind()
. (It's guaranteed to have a copy ctor, which can
throw implementation-defined exceptions: bind()
returns a forwarding
call wrapper, TR1 3.6.3/2. A forwarding call wrapper is a call wrapper,
TR1 3.3/4. Every call wrapper shall be CopyConstructible, TR1 3.3/4.
Everything without an exception-specification may throw
implementation-defined exceptions unless otherwise specified, C++03
17.4.4.8/3.)
Should the nofail guarantee requested by Library Issue 527(i) be extended
to cover both calling bind()
and copying the returned functor?
[ Howard adds: ]
tuple
construction should probably have a similar guarantee.
[ San Francisco: ]
Howard to provide wording.
[ Post Summit, Anthony provided wording. ]
[ Batavia (2009-05): ]
Part of all of this issue appears to be rendered moot by the proposed resolution to issue 817(i) (q.v.). We recommend the issues be considered simultaneously (or possibly even merged) to ensure there is no overlap. Move to Open, and likewise for issue 817(i).
[ 2009-07 Frankfurt: ]
[ 2009-10 Santa Cruz: ]
[ 2010-02-11 Moved from Ready to Tentatively NAD Editorial, rationale added below. ]
Rationale:
This issue is solved as proposed by 817(i).
Proposed resolution:
Add a new sentence to the end of paragraphs 2 and 4 of 22.10.15.4 [func.bind.bind]:
-2- Returns: A forwarding call wrapper
g
with a weak result type (20.6.2). The effect ofg(u1, u2, ..., uM)
shall beINVOKE(f, v1, v2, ..., vN, Callable<F cv,V1, V2, ..., VN>::result_type)
, where cv represents the cv-qualifiers ofg
and the values and types of the bound argumentsv1, v2, ..., vN
are determined as specified below. The copy constructor and move constructor of the forwarding call wrapper shall throw an exception if and only if the corresponding constructor ofF
or any of the types inBoundArgs...
throw an exception....
-5- Returns: A forwarding call wrapper
g
with a nested typeresult_type
defined as a synonym forR
. The effect ofg(u1, u2, ..., uM)
shall beINVOKE(f, v1, v2, ..., vN, R)
, where the values and types of the bound argumentsv1, v2, ..., vN
are determined as specified below. The copy constructor and move constructor of the forwarding call wrapper shall throw an exception if and only if the corresponding constructor ofF
or any of the types inBoundArgs...
throw an exception.
bind
needs to be movedSection: 22.10.15.4 [func.bind.bind] Status: C++11 Submitter: Howard Hinnant Opened: 2008-03-17 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
Addresses US 72, JP 38 and DE 21
The functor returned by bind()
should have a move constructor that
requires only move construction of its contained functor and bound arguments.
That way move-only functors can be passed to objects such as thread
.
This issue is related to issue 816(i).
US 72:
bind
should support move-only functors and bound arguments.
JP 38:
add the move requirement for bind's return type.
For example, assume following
th1
andth2
,void f(vector<int> v) { } vector<int> v{ ... }; thread th1([v]{ f(v); }); thread th2(bind(f, v));When function object are set to thread,
v
is moved toth1
's lambda expression in a Move Constructor of lambda expression becauseth1
's lambda expression has a Move Constructor. Butbind
ofth2
's return type doesn't have the requirement of Move, so it may not moved but copied.Add the requirement of move to get rid of this useless copy.
And also, add the
MoveConstructible
as well asCopyConstructible
.
DE 21
The specification for bind claims twice that "the values and types for the bound arguments v1, v2, ..., vN are determined as specified below". No such specification appears to exist.
[ San Francisco: ]
Howard to provide wording.
[ Post Summit Alisdair and Howard provided wording. ]
Several issues are being combined in this resolution. They are all touching the same words so this is an attempt to keep one issue from stepping on another, and a place to see the complete solution in one place.
bind
needs to be "moved".- 22.10.15.4 [func.bind.bind]/p3, p6 and p7 were accidently removed from N2798.
- Issue 929(i) argues for a way to pass by && for efficiency but retain the decaying behavior of pass by value for the
thread
constructor. That same solution is applicable here.
[ Batavia (2009-05): ]
We were going to recommend moving this issue to Tentatively Ready until we noticed potential overlap with issue 816 (q.v.).
Move to Open, and recommend both issues be considered together (and possibly merged).
[ 2009-07 Frankfurt: ]
The proposed resolution uses concepts. Leave Open.
[ 2009-10 Santa Cruz: ]
Leave as Open. Howard to provide deconceptified wording.
[ 2009-11-07 Howard updates wording. ]
[ 2009-11-15 Further updates by Peter, Chris and Daniel. ]
[ Moved to Tentatively Ready after 6 positive votes on c++std-lib. ]
Proposed resolution:
Change 22.10 [function.objects] p2:
template<class Fn, class...TypesBoundArgs> unspecified bind(Fn&&,TypesBoundArgs&&...); template<class R, class Fn, class...TypesBoundArgs> unspecified bind(Fn&&,TypesBoundArgs&&...);
Change 22.10.4 [func.require]:
4 Every call wrapper (22.10.3 [func.def]) shall be
. A simple call wrapper is a call wrapper that is
CopyMoveConstructibleCopyConstructible
andCopyAssignable
and whose copy constructor, move constructor and assignment operator do not throw exceptions. A forwarding call wrapper is a call wrapper that can be called with an argument list. [Note: in a typical implementation forwarding call wrappers have an overloaded function call operator of the formtemplate<class...ArgTypesUnBoundsArgs> R operator()(ArgTypesUnBoundsArgs&&... unbound_args) cv-qual;— end note]
Change 22.10.15.4 [func.bind.bind]:
Within this clause:
- Let
FD
be a synonym for the typedecay<F>::type
.- Let
fd
be an lvalue of typeFD
constructed fromstd::forward<F>(f)
.- Let
Ti
be a synonym for the ith type in the template parameter packBoundArgs
.- Let
TiD
be a synonym for the typedecay<Ti>::type
.- Let
ti
be the ith argument in the function parameter packbound_args
.- Let
tid
be an lvalue of typeTiD
constructed fromstd::forward<Ti>(ti)
.- Let
Uj
be the jth deduced type of theUnBoundArgs&&...
parameter of theoperator()
of the forwarding call wrapper.- Let
uj
be the jth argument associated withUj
.template<class F, class... BoundArgs> unspecified bind(F&& f, BoundArgs&&... bound_args);-1- Requires:
is_constructible<FD, F>::value
shall betrue
. For eachTi
inBoundArgs
,is_constructible<TiD, Ti>::value
shall betrue
.F
and eachTi
inBoundArgs
shall be CopyConstructible.INVOKE(fd, w1, w2, ..., wN)
(22.10.4 [func.require]) shall be a valid expression for some values w1, w2, ..., wN, whereN == sizeof...(bound_args)
.-2- Returns: A forwarding call wrapper
g
with a weak result type (22.10.4 [func.require]). The effect ofg(u1, u2, ..., uM)
shall beINVOKE(fd, v1, v2, ..., vN, result_of<FD cv (V1, V2, ..., VN)>::type)
, where cv represents the cv-qualifiers ofg
and the values and types of the bound argumentsv1, v2, ..., vN
are determined as specified below. The copy constructor and move constructor of the forwarding call wrapper shall throw an exception if and only if the corresponding constructor ofFD
or of any of the typesTiD
throws an exception.-3- Throws: Nothing unless the
copyconstructionorofor of one of the values
Ffdtid
types in thethrows an exception.BoundArgs...
pack expansionRemarks: The unspecified return type shall satisfy the requirements of
MoveConstructible
. If all ofFD
andTiD
satisfy the requirements ofCopyConstructible
then the unspecified return type shall satisfy the requirements ofCopyConstructible
. [Note: This implies that all ofFD
andTiD
shall beMoveConstructible
— end note]template<class R, class F, class... BoundArgs> unspecified bind(F&& f, BoundArgs&&... bound_args);-4- Requires:
is_constructible<FD, F>::value
shall betrue
. For eachTi
inBoundArgs
,is_constructible<TiD, Ti>::value
shall betrue
.F
and eachTi
inBoundArgs
shall be CopyConstructible.INVOKE(fd, w1, w2, ..., wN)
shall be a valid expression for some values w1, w2, ..., wN, whereN == sizeof...(bound_args)
.-5- Returns: A forwarding call wrapper
g
with a nested typeresult_type
defined as a synonym forR
. The effect ofg(u1, u2, ..., uM)
shall beINVOKE(fd, v1, v2, ..., vN, R)
, where the values and types of the bound argumentsv1, v2, ..., vN
are determined as specified below. The copy constructor and move constructor of the forwarding call wrapper shall throw an exception if and only if the corresponding constructor ofFD
or of any of the typesTiD
throws an exception.-6- Throws: Nothing unless the
copyconstructionorofor of one of the values
Ffdtid
types in thethrows an exception.BoundArgs...
pack expansionRemarks: The unspecified return type shall satisfy the requirements of
MoveConstructible
. If all ofFD
andTiD
satisfy the requirements ofCopyConstructible
then the unspecified return type shall satisfy the requirements ofCopyConstructible
. [Note: This implies that all ofFD
andTiD
shall beMoveConstructible
— end note]-7- The values of the bound arguments
v1, v2, ..., vN
and their corresponding typesV1, V2, ..., VN
depend on the typesTiD
derived fromof the corresponding argumentthe call toti
inbound_args
of typeTi
inBoundArgs
inbind
and the cv-qualifiers cv of the call wrapperg
as follows:
- if
is
tiTiDof typereference_wrapper<T>
the argument istid.get()
and its typeVi
isT&
;- if the value of
is
std::is_bind_expression<TiD>::valuetrue
the argument istid(std::forward<Uj>(uj)...
and its typeu1, u2, ..., uM)Vi
isresult_of<TiD cv (Uj...
;U1&, U2&, ..., UM&)>::type- if the value
j
ofis not zero the argument is
std::is_placeholder<TiD>::valuestd::forward<Uj>(uj)
and its typeVi
isUj&&
;- otherwise the value is
tid
and its typeVi
isTiD cv &
.
Section: 32.5.4 [atomics.order] Status: CD1 Submitter: Jens Maurer Opened: 2008-03-22 Last modified: 2016-01-28
Priority: Not Prioritized
View other active issues in [atomics.order].
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Discussion:
32.5.4 [atomics.order] p1 says in the table that
Element Meaning memory_order_acq_rel
the operation has both acquire and release semantics
To my naked eye, that seems to imply that even an atomic read has both acquire and release semantics.
Then, p1 says in the table:
Element Meaning memory_order_seq_cst
the operation has both acquire and release semantics, and, in addition, has sequentially-consistent operation ordering
So that seems to be "the same thing" as memory_order_acq_rel
, with additional
constraints.
I'm then reading p2, where it says:
The
memory_order_seq_cst
operations that load a value are acquire operations on the affected locations. Thememory_order_seq_cst
operations that store a value are release operations on the affected locations.
That seems to imply that atomic reads only have acquire semantics. If that
is intended, does this also apply to memory_order_acq_rel
and the individual
load/store operations as well?
Also, the table in p1 contains phrases with "thus" that seem to indicate consequences of normative wording in 6.9.2 [intro.multithread]. That shouldn't be in normative text, for the fear of redundant or inconsistent specification with the other normative text.
Double-check 32.5.8.2 [atomics.types.operations] that each operation clearly says whether it's a load or a store operation, or both. (It could be clearer, IMO. Solution not in current proposed resolution.)
32.5.4 [atomics.order] p2: What's a "consistent execution"? It's not defined in 6.9.2 [intro.multithread], it's just used in notes there.
And why does 32.5.8.2 [atomics.types.operations] p9 for "load" say:
Requires: The order argument shall not be
memory_order_acquire
normemory_order_acq_rel
.
(Since this is exactly the same restriction as for "store", it seems to be a typo.)
And then: 32.5.8.2 [atomics.types.operations] p12:
These operations are read-modify-write operations in the sense of the "synchronizes with" definition (6.9.2 [intro.multithread]), so both such an operation and the evaluation that produced the input value synchronize with any evaluation that reads the updated value.
This is redundant with 6.9.2 [intro.multithread], see above for the reasoning.
[ San Francisco: ]
Boehm: "I don't think that this changes anything terribly substantive, but it improves the text."
Note that "Rephrase the table in as [sic] follows..." should read "Replace the table in [atomics.order] with the following...."
The proposed resolution needs more work. Crowl volunteered to address all of the atomics issues in one paper.
This issue is addressed in N2783.
Proposed resolution:
edit 32.5.4 [atomics.order], paragraph 1 as follows.
The enumeration
memory_order
specifies the detailed regular (non-atomic) memory synchronization order as defined inClause 1.7section 1.10 and may provide for operation ordering. Its enumerated values and their meanings are as follows:
- For
memory_order_relaxed
,- no operation orders memory.
- For
memory_order_release
,memory_order_acq_rel
, andmemory_order_seq_cst
,- a store operation performs a release operation on the affected memory location.
- For
memory_order_consume
,- a load operation performs a consume operation on the affected memory location.
- For
memory_order_acquire
,memory_order_acq_rel
, andmemory_order_seq_cst
,- a load operation performs an acquire operation on the affected memory location.
remove table 136 in 32.5.4 [atomics.order].
Table 136 — memory_order effectsElementMeaningmemory_order_relaxed
the operation does not order memorymemory_order_release
the operation performs a release operation on the affected memory location, thus making regular memory writes visible to other threads through the atomic variable to which it is appliedmemory_order_acquire
the operation performs an acquire operation on the affected memory location, thus making regular memory writes in other threads released through the atomic variable to which it is applied visible to the current threadmemory_order_consume
the operation performs a consume operation on the affected memory location, thus making regular memory writes in other threads released through the atomic variable to which it is applied visible to the regular memory reads that are dependencies of this consume operation.memory_order_acq_rel
the operation has both acquire and release semanticsmemory_order_seq_cst
the operation has both acquire and release semantics, and, in addition, has sequentially-consistent operation ordering
edit 32.5.4 [atomics.order], paragraph 2 as follows.
TheTherememory_order_seq_cst
operations that load a value are acquire operations on the affected locations. Thememory_order_seq_cst
operations that store a value are release operations on the affected locations. In addition, in a consistent execution, theremust beis a single total order S on allmemory_order_seq_cst
operations, consistent with the happens before order and modification orders for all affected locations, such that eachmemory_order_seq_cst
operation observes either the last preceding modification according to this order S, or the result of an operation that is notmemory_order_seq_cst
. [Note: Although it is not explicitly required that S include locks, it can always be extended to an order that does include lock and unlock operations, since the ordering between those is already included in the happens before ordering. —end note]
Section: 17.9.8 [except.nested] Status: C++11 Submitter: Alisdair Meredith Opened: 2008-03-25 Last modified: 2016-01-28
Priority: Not Prioritized
View all other issues in [except.nested].
View all issues with C++11 status.
Discussion:
Looking at the wording I submitted for rethrow_if_nested
, I don't think I
got it quite right.
The current wording says:
template <class E> void rethrow_if_nested(const E& e);Effects: Calls
e.rethrow_nested()
only ife
is publicly derived fromnested_exception
.
This is trying to be a bit subtle, by requiring e
(not E
) to be publicly
derived from nested_exception
the idea is that a dynamic_cast
would be
required to be sure. Unfortunately, if e
is dynamically but not statically
derived from nested_exception
, e.rethrow_nested()
is ill-formed.
[ San Francisco: ]
Alisdair was volunteered to provide wording.
[ 2009-10 Santa Cruz: ]
Leave as Open. Alisdair to provide wording.
[ 2009-11-09 Alisdair provided wording. ]
[ 2010-03-10 Dietmar updated wording. ]
[ 2010 Pittsburgh: ]
Moved to Ready for Pittsburgh.
Proposed resolution:
Change 17.9.8 [except.nested], p8:
template <class E> void rethrow_if_nested(const E& e);-8- Effects:
CallsOnly if the dynamic type ofe.rethrow_nested()
oe
is publicly and unambiguously derived fromnested_exception
this callsdynamic_cast<const nested_exception&>(e).rethrow_nested()
.
current_exception()
's interaction with throwing copy ctorsSection: 17.9.7 [propagation] Status: CD1 Submitter: Stephan T. Lavavej Opened: 2008-03-26 Last modified: 2016-01-28
Priority: Not Prioritized
View all other issues in [propagation].
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Discussion:
As of N2521, the Working Paper appears to be silent about what
current_exception()
should do if it tries to copy the currently handled
exception and its copy constructor throws. 17.9.7 [propagation]/7 says "If the
function needs to allocate memory and the attempt fails, it returns an
exception_ptr
object that refers to an instance of bad_alloc
.", but
doesn't say anything about what should happen if memory allocation
succeeds but the actual copying fails.
I see three alternatives: (1) return an exception_ptr
object that refers
to an instance of some fixed exception type, (2) return an exception_ptr
object that refers to an instance of the copy ctor's thrown exception
(but if that has a throwing copy ctor, an infinite loop can occur), or
(3) call terminate()
.
I believe that terminate()
is the most reasonable course of action, but
before we go implement that, I wanted to raise this issue.
[ Peter's summary: ]
The current practice is to not have throwing copy constructors in exception classes, because this can lead to
terminate()
as described in 14.6.2 [except.terminate]. Thus callingterminate()
in this situation seems consistent and does not introduce any new problems.However, the resolution of core issue 475 may relax this requirement:
The CWG agreed with the position that
std::uncaught_exception()
should returnfalse
during the copy to the exception object and thatstd::terminate()
should not be called if that constructor exits with an exception.Since throwing copy constructors will no longer call
terminate()
, option (3) doesn't seem reasonable as it is deemed too drastic a response in a recoverable situation.Option (2) cannot be adopted by itself, because a potential infinite recursion will need to be terminated by one of the other options.
Proposed resolution:
Add the following paragraph after 17.9.7 [propagation]/7:
Returns (continued): If the attempt to copy the current exception object throws an exception, the function returns an
exception_ptr
that refers to the thrown exception or, if this is not possible, to an instance ofbad_exception
.[Note: The copy constructor of the thrown exception may also fail, so the implementation is allowed to substitute a
bad_exception
to avoid infinite recursion. -- end note.]
Rationale:
[ San Francisco: ]
Pete: there may be an implied assumption in the proposed wording that current_exception() copies the existing exception object; the implementation may not actually do that.
Pete will make the required editorial tweaks to rectify this.
unique_ptr
Section: 20.3.1.4.5 [unique.ptr.runtime.modifiers] Status: C++11 Submitter: Alisdair Meredith Opened: 2008-03-30 Last modified: 2016-01-28
Priority: Not Prioritized
View all other issues in [unique.ptr.runtime.modifiers].
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Discussion:
Reading resolution of LWG issue 673(i) I noticed the following:
void reset(T*pointer p =0pointer());-1- Requires: Does not accept pointer types which are convertible to
T*
pointer
(diagnostic required). [Note: One implementation technique is to create a private templated overload. -- end note]
This could be cleaned up by mandating the overload as a public deleted
function. In addition, we should probably overload reset
on nullptr_t
to be a stronger match than the deleted overload. Words...
Proposed resolution:
Add to class template definition in 20.3.1.4 [unique.ptr.runtime]
// modifiers pointer release(); void reset(pointer p = pointer()); void reset( nullptr_t ); template< typename U > void reset( U ) = delete; void swap(unique_ptr&& u);
Update 20.3.1.4.5 [unique.ptr.runtime.modifiers]
void reset(pointer p = pointer()); void reset(nullptr_t);
-1- Requires: Does not accept pointer types which are convertible topointer
(diagnostic required). [Note: One implementation technique is to create a private templated overload. -- end note]Effects: If
get() == nullptr
there are no effects. Otherwiseget_deleter()(get())
....
[ Note this wording incorporates resolutions for 806(i) (New) and 673(i) (Ready). ]
identity<void>
seems brokenSection: 22.2.4 [forward] Status: Resolved Submitter: Walter Brown Opened: 2008-04-09 Last modified: 2016-01-28
Priority: Not Prioritized
View all other issues in [forward].
View all issues with Resolved status.
Discussion:
N2588 seems to have added an operator()
member function to the
identity<>
helper in 22.2.4 [forward]. I believe this change makes it no
longer possible to instantiate identity<void>
, as it would require
forming a reference-to-void
type as this operator()
's parameter type.
Suggested resolution: Specialize identity<void>
so as not to require
the member function's presence.
[ Sophia Antipolis: ]
Jens: suggests to add a requires clause to avoid specializing on
void
.Alisdair: also consider cv-qualified
void
.Alberto provided proposed wording.
[ 2009-07-30 Daniel reopens: ]
This issue became closed, because the
ReferentType
requirement fixed the problem - this is no longer the case. In retrospective it seems to be that the root of current issues aroundstd::identity
(823, 700(i), 939(i)) is that it was standardized as something very different (an unconditional type mapper) than traditional usage indicated (a function object that should derive fromstd::unary_function)
, as the SGI definition does. This issue could be solved, ifstd::identity
is removed (one proposal of 939(i)), but until this has been decided, this issue should remain open. An alternative for removing it, would be, to do the following:
Let
identity
stay as a real function object, which would now properly derive fromunary_function
:template <class T> struct identity : unary_function<T, T> { const T& operator()(const T&) const; };Invent (if needed) a generic type wrapper (corresponding to concept
IdentityOf
), e.g.identity_of
, and move it's prototype description back to 22.2.4 [forward]:template <class T> struct identity_of { typedef T type; };and adapt the
std::forward
signature to useidentity_of
instead ofidentity
.
[ 2009-10 Santa Cruz: ]
Proposed resolution:
Change definition of identity
in 22.2.4 [forward], paragraph 2, to:
template <class T> struct identity { typedef T type; requires ReferentType<T> const T& operator()(const T& x) const; };
...
requires ReferentType<T> const T& operator()(const T& x) const;
Rationale:
The point here is to able to write T&
given T
and ReferentType
is
precisely the concept that guarantees so, according to N2677
(Foundational concepts). Because of this, it seems preferable than an
explicit check for cv void
using SameType/remove_cv
as it was suggested
in Sophia. In particular, Daniel remarked that there may be types other
than cv void
which aren't referent types (int[]
, perhaps?).
basic_string
inserterSection: 27.4.4.4 [string.io] Status: CD1 Submitter: Alisdair Meredith Opened: 2008-04-10 Last modified: 2016-01-28
Priority: Not Prioritized
View all other issues in [string.io].
View all issues with CD1 status.
Discussion:
In the current working paper, the <string>
header synopsis at the end of
27.4 [string.classes] lists a single operator<<
overload
for basic_string
.
template<class charT, class traits, class Allocator> basic_ostream<charT, traits>& operator<<(basic_ostream<charT, traits>&& os, const basic_string<charT,traits,Allocator>& str);
The definition in 27.4.4.4 [string.io] lists two:
template<class charT, class traits, class Allocator> basic_ostream<charT, traits>& operator<<(basic_ostream<charT, traits>& os, const basic_string<charT,traits,Allocator>& str); template<class charT, class traits, class Allocator> basic_ostream<charT, traits>& operator<<(basic_ostream<charT, traits>&& os, const basic_string<charT,traits,Allocator>& str);
I believe the synopsis in 27.4 [string.classes] is correct, and the first of the two signatures in 27.4.4.4 [string.io] should be deleted.
Proposed resolution:
Delete the first of the two signatures in 27.4.4.4 [string.io]:
template<class charT, class traits, class Allocator> basic_ostream<charT, traits>& operator<<(basic_ostream<charT, traits>& os, const basic_string<charT,traits,Allocator>& str);template<class charT, class traits, class Allocator> basic_ostream<charT, traits>& operator<<(basic_ostream<charT, traits>&& os, const basic_string<charT,traits,Allocator>& str);
Section: 19.5.4.1 [syserr.errcode.overview], 20.3.2.2.12 [util.smartptr.shared.io], 99 [facets.examples], 22.9.4 [bitset.operators], 29.4.6 [complex.ops], 31.6 [stream.buffers], 28.6.8 [re.submatch] Status: Resolved Submitter: Alisdair Meredith Opened: 2008-04-10 Last modified: 2017-03-21
Priority: Not Prioritized
View all other issues in [syserr.errcode.overview].
View all issues with Resolved status.
Discussion:
Addresses UK 220
Should the following use rvalues references to stream in insert/extract operators?
[ Sophia Antipolis ]
Agree with the idea in the issue, Alisdair to provide wording.
[ Daniel adds 2009-02-14: ]
The proposal given in the paper N2831 apparently resolves this issue.
[ Batavia (2009-05): ]
The cited paper is an earlier version of N2844, which changed the rvalue reference binding rules. That paper includes generic templates
operator<<
andoperator>>
that adapt rvalue streams.We therefore agree with Daniel's observation. Move to
NAD EditorialResolved.
Proposed resolution:
constexpr shared_ptr::shared_ptr()?
Section: 20.3.2.2.2 [util.smartptr.shared.const] Status: Resolved Submitter: Peter Dimov Opened: 2008-04-11 Last modified: 2016-01-28
Priority: Not Prioritized
View other active issues in [util.smartptr.shared.const].
View all other issues in [util.smartptr.shared.const].
View all issues with Resolved status.
Discussion:
Would anyone object to making the default constructor of shared_ptr
(and
weak_ptr
and enable_shared_from_this
) constexpr
? This would enable
static initialization for shared_ptr
variables, eliminating another
unfair advantage of raw pointers.
[ San Francisco: ]
It's not clear to us that you can initialize a pointer with the literal 0 in a constant expression. We need to ask CWG to make sure this works. Bjarne has been appointed to do this.
Core got back to us and assured as that
nullptr
would do the job nicely here.
[ 2009-05-01 Alisdair adds: ]
I don't believe that
constexpr
will buy anything in this case.shared_ptr/weak_ptr/enable_shared_from_this
cannot be literal types as they have a non-trivial copy constructor. As they do not produce literal types, then theconstexpr
default constructor will not guarantee constant initialization, and so not buy the hoped for optimization.I recommend referring this back to Core to see if we can get static initialization for types with
constexpr
constructors, even if they are not literal types. Otherwise this should be closed as NAD.
[ 2009-05-26 Daniel adds: ]
If Alisdair's 2009-05-01 comment is correct, wouldn't that also make
constexpr mutex()
useless, because this class has a non-trivial destructor? (828(i))
[ 2009-07-21 Alisdair adds: ]
The feedback from core is that this and similar uses of
constexpr
constructors to force static initialization should be supported. If there are any problems with this in the working draught, we should file core issues.Recommend we declare the default constructor
constexpr
as the issue suggests (proposed wording added).
[ 2009-10 Santa Cruz: ]
NAD EditorialResolved. Solved by N2994.
Proposed resolution:
Change 20.3.2.2 [util.smartptr.shared] and 20.3.2.2.2 [util.smartptr.shared.const]:
constexpr shared_ptr();
Change 20.3.2.3 [util.smartptr.weak] and 20.3.2.3.2 [util.smartptr.weak.const]:
constexpr weak_ptr();
Change 20.3.2.7 [util.smartptr.enab] (2 places):
constexpr enable_shared_from_this();
std::mutex
?Section: 32.6.4.2.2 [thread.mutex.class] Status: Resolved Submitter: Peter Dimov Opened: 2008-04-18 Last modified: 2016-01-28
Priority: Not Prioritized
View all other issues in [thread.mutex.class].
View all issues with Resolved status.
Discussion:
[Note: I'm assuming here that [basic.start.init]/1 will be fixed.]
Currently std::mutex
doesn't support static initialization. This is a
regression with respect to pthread_mutex_t
, which does. I believe that
we should strive to eliminate such regressions in expressive power where
possible, both to ease migration and to not provide incentives to (or
force) people to forego the C++ primitives in favor of pthreads.
[ Sophia Antipolis: ]
We believe this is implementable on POSIX, because the initializer-list feature and the constexpr feature make this work. Double-check core language about static initialization for this case. Ask core for a core issue about order of destruction of statically-initialized objects wrt. dynamically-initialized objects (should come afterwards). Check non-POSIX systems for implementability.
If ubiquitous implementability cannot be assured, plan B is to introduce another constructor, make this constexpr, which is conditionally-supported. To avoid ambiguities, this new constructor needs to have an additional parameter.
[ Post Summit: ]
Jens: constant initialization seems to be ok core-language wise
Consensus: Defer to threading experts, in particular a Microsoft platform expert.
Lawrence to send e-mail to Herb Sutter, Jonathan Caves, Anthony Wiliams, Paul McKenney, Martin Tasker, Hans Boehm, Bill Plauger, Pete Becker, Peter Dimov to alert them of this issue.
Lawrence: What about header file shared with C? The initialization syntax is different in C and C++.
Recommend Keep in Review
[ Batavia (2009-05): ]
Keep in Review status pending feedback from members of the Concurrency subgroup.
[ See related comments from Alisdair and Daniel in 827(i). ]
[ 2009-10 Santa Cruz: ]
NAD EditorialResolved. Addressed by N2994.
Proposed resolution:
Change 32.6.4.2.2 [thread.mutex.class]:
class mutex { public: constexpr mutex(); ...
Section: 17.9.7 [propagation] Status: CD1 Submitter: Beman Dawes Opened: 2008-04-20 Last modified: 2016-01-28
Priority: Not Prioritized
View all other issues in [propagation].
View all issues with CD1 status.
Discussion:
Consider this code:
exception_ptr xp;try {do_something(); } catch (const runtime_error& ) {xp = current_exception();} ... rethrow_exception(xp);
Say do_something()
throws an exception object of type
range_error
. What is the type of the exception object thrown by
rethrow_exception(xp)
above? It must have a type of range_error
;
if it were of type runtime_error
it still isn't possible to
propagate an exception and the exception_ptr/current_exception/rethrow_exception
machinery serves no useful purpose.
Unfortunately, the current wording does not explicitly say that. Different people read the current wording and come to different conclusions. While it may be possible to deduce the correct type from the current wording, it would be much clearer to come right out and explicitly say what the type of the referred to exception is.
[ Peter adds: ]
I don't like the proposed resolution of 829. The normative text is unambiguous that the
exception_ptr
refers to the currently handled exception. This term has a standard meaning, see 14.4 [except.handle]/8; this is the exception thatthrow;
would rethrow, see 14.2 [except.throw]/7.A better way to address this is to simply add the non-normative example in question as a clarification. The term currently handled exception should be italicized and cross-referenced. A [Note: the currently handled exception is the exception that a throw expression without an operand (14.2 [except.throw]/7) would rethrow. --end note] is also an option.
Proposed resolution:
After 17.9.7 [propagation] , paragraph 7, add the indicated text:
exception_ptr current_exception();Returns:
exception_ptr
object that refers to the currently handled exception (14.4 [except.handle]) or a copy of the currently handled exception, or a nullexception_ptr
object if no exception is being handled. If the function needs to allocate memory and the attempt fails, it returns anexception_ptr
object that refers to an instance ofbad_alloc
. It is unspecified whether the return values of two successive calls tocurrent_exception
refer to the same exception object. [Note: that is, it is unspecified whethercurrent_exception
creates a new copy each time it is called. -- end note]Throws: nothing.
unique_ptr::pointer
requirements underspecifiedSection: 20.3.1.3 [unique.ptr.single] Status: Resolved Submitter: Daniel Krügler Opened: 2008-05-14 Last modified: 2016-01-28
Priority: Not Prioritized
View other active issues in [unique.ptr.single].
View all other issues in [unique.ptr.single].
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Discussion:
Issue 673(i) (including recent updates by 821(i)) proposes a useful
extension point for unique_ptr
by granting support for an optional
deleter_type::pointer
to act as pointer-like replacement for element_type*
(In the following: pointer
).
Unfortunately no requirements are specified for the type pointer
which has
impact on at least two key features of unique_ptr
:
The unique_ptr
specification makes great efforts to require that essentially all
operations cannot throw and therefore adds proper wording to the affected
operations of the deleter as well. If user-provided pointer
-emulating types
("smart pointers") will be allowed, either all throw-nothing clauses have to
be replaced by weaker "An exception is thrown only if pointer
's {op} throws
an exception"-clauses or it has to be said explicitly that all used
operations of pointer
are required not to throw. I understand the main
focus of unique_ptr
to be as near as possible to the advantages of native pointers which cannot
fail and thus strongly favor the second choice. Also, the alternative position
would make it much harder to write safe and simple template code for
unique_ptr
. Additionally, I assume that a general statement need to be given
that all of the expressions of pointer
used to define semantics are required to
be well-formed and well-defined (also as back-end for 762(i)).
[ Sophia Antipolis: ]
Howard: We maybe need a core concept
PointerLike
, but we don't need the arithmetic (seeshared_ptr
vs.vector<T>::iterator
.Howard will go through and enumerate the individual requirements wrt.
pointer
for each member function.
[ 2009-07 Frankfurt: ]
Move to Ready.
[ 2009-10-15 Alisdair pulls from Ready: ]
I hate to pull an issue out of Ready status, but I don't think 834 is fully baked yet.
For reference the proposed resolution is to add the following words:
unique_ptr<T, D>::pointer
's operations shall be well-formed, shall have well defined behavior, and shall not throw exceptions.This leaves me with a big question : which operations?
Are all pointer operations required to be nothrow, including operations that have nothing to do with interactions with
unique_ptr
? This was much simpler with concepts where we could point to operations within a certain concept, and so nail down the interactions.
[ 2009-10-15 Daniel adds: ]
I volunteer to prepare a more fine-grained solution, but I would like to ask for feedback that helps me doing so. If this question is asked early in the meeting I might be able to fix it within the week, but I cannot promise that now.
[ 2009-10 Santa Cruz: ]
Leave in open. Daniel to provide wording as already suggested.
[ 2009-12-22 Daniel provided wording and rationale. ]
[ 2010 Pittsburgh: Moved to NAD Editorial. Rationale added below. ]
Rationale:
The here proposed resolution has considerable overlap with the requirements that are used in the allocator requirements.
This might be a convincing argument to isolate the common subset into one requirement. The reason I did not do that is basically because we might find out that they are either over-constraining or under-constraining at this late point of specification. Note also that as a result of the idea of a general requirement set I added the requirement:
A default-initialized object may have a singular value
even though this does not play a relevant role for unique_ptr
.
One further characteristics of the resolution is that availability of relational
operators of unique_ptr<T, D>::pointer
is not part of the basic
requirements, which is in sync with the allocator requirements on pointer-like
(this means that unique_ptr
can hold a void_pointer
or
const_void_pointer
).
Solved by N3073.
Proposed resolution:
Change 20.3.1.3 [unique.ptr.single] p. 1 as indicated: [The intent is to
replace the coupling between T*
and the deleter's operator()
by a coupling between unique_ptr<T, D>::pointer
and this
operator()
]
1 - The default type for the template parameter
D
isdefault_delete
. A client-supplied template argumentD
shall be a function pointer or functor for which, given a valued
of typeD
and apointervalueptr
of type, the expression
T*unique_ptr<T, D>::pointerd(ptr)
is valid and has the effect of deallocating the pointer as appropriate for that deleter.D
may also be an lvalue-reference to a deleter.
Change 20.3.1.3 [unique.ptr.single] p. 3 as indicated:
3 - If the type
remove_reference<D>::type::pointer
exists, thenunique_ptr<T, D>::pointer
shall be a synonym forremove_reference<D>::type::pointer
. Otherwiseunique_ptr<T, D>::pointer
shall be a synonym forT*
. The typeunique_ptr<T, D>::pointer
shallbesatisfy the requirements ofEqualityComparable
,DefaultConstructible
,CopyConstructible
(Table 34) and,CopyAssignable
(Table 36),Swappable
, andDestructible
(16.4.4.2 [utility.arg.requirements]). A default-initialized object may have a singular value. A value-initialized object produces the null value of the type. The null value shall be equivalent only to itself. An object of this type can be copy-initialized with a value of typenullptr_t
, compared for equality with a value of typenullptr_t
, and assigned a value of typenullptr_t
. The effect shall be as if a value-initialized object had been used in place of the null pointer constant. An objectp
of this type can be contextually converted tobool
. The effect shall be as ifp != nullptr
had been evaluated in place ofp
. No operation on this type which is part of the above mentioned requirements shall exit via an exception.[Note: Given an allocator type
X
(16.4.4.6 [allocator.requirements]), the typesX::pointer
,X::const_pointer
,X::void_pointer
, andX::const_void_pointer
may be used asunique_ptr<T, D>::pointer
— end note]In addition to being available via inclusion of the
<utility>
header, theswap
function template in 22.2.2 [utility.swap] is also available within the definition ofunique_ptr
'sswap
function.
Change 20.3.1.3.2 [unique.ptr.single.ctor] p. 2+3 as indicated: [The first
change ensures that we explicitly say, how the stored pointer is initialized.
This is important for a constexpr
function, because this may make a
difference for user-defined pointer-like types]
constexpr unique_ptr();...
2 - Effects: Constructs a
unique_ptr
which owns nothing, value-initializing the stored pointer.3 - Postconditions:
get() ==
.0nullptr
Change 20.3.1.3.2 [unique.ptr.single.ctor] p. 6+7 as indicated: [This is a step-by-fix to ensure consistency to the changes of N2976]
unique_ptr(pointer p);...
6 - Effects: Constructs a
unique_ptr
which ownsp
, initializing the stored pointer withp
.7 - Postconditions:
get() == p
.get_deleter()
returns a reference to adefault constructedvalue-initialized deleterD
.
Insert a new effects clause in 20.3.1.3.2 [unique.ptr.single.ctor] just before p. 14: [The intent is to fix the current lack of specification in which way the stored pointer is initialized]
unique_ptr(pointer p,implementation-definedsee below d1); unique_ptr(pointer p,implementation-definedsee below d2);...
Effects: Constructs a
unique_ptr
which ownsp
, initializing the stored pointer withp
and the initializing the deleter as described above.14 - Postconditions:
get() == p
.get_deleter()
returns a reference to the internally stored deleter. IfD
is a reference type thenget_deleter()
returns a reference to the lvalued
.
Change 20.3.1.3.2 [unique.ptr.single.ctor] p. 18+22 as indicated: [The intent is to clarify that the moved-from source must contain a null pointer, there is no other choice left]
unique_ptr(unique_ptr&& u);[..]
18 - Postconditions:
get() == value u.get()
had before the construction andu.get() == nullptr
.get_deleter()
returns a reference to the internally stored deleter which was constructed fromu.get_deleter()
. IfD
is a reference type thenget_deleter()
andu.get_deleter()
both reference the same lvalue deleter.template <class U, class E> unique_ptr(unique_ptr<U, E>&& u);[..]
22 - Postconditions:
get() == value u.get()
had before the construction, modulo any required offset adjustments resulting from the cast fromunique_ptr<U, E>::pointer
topointer
andu.get() == nullptr
.get_deleter()
returns a reference to the internally stored deleter which was constructed fromu.get_deleter()
.
Change 20.3.1.3.2 [unique.ptr.single.ctor] p. 20 as indicated: [With the possibility of user-defined pointer-like types the implication does only exist, if those are built-in pointers. Note that this change should also be applied with the acceptance of 950(i)]
template <class U, class E> unique_ptr(unique_ptr<U, E>&& u);20 - Requires: If
D
is not a reference type, construction of the deleterD
from an rvalue of typeE
shall be well formed and shall not throw an exception. IfD
is a reference type, thenE
shall be the same type asD
(diagnostic required).unique_ptr<U, E>::pointer
shall be implicitly convertible topointer
.[Note: These requirements imply thatT
andU
are complete types. — end note]
Change 20.3.1.3.3 [unique.ptr.single.dtor] p. 2 as indicated:
~unique_ptr();...
2 - Effects: If
get() ==
there are no effects. Otherwise0nullptrget_deleter()(get())
.
Change 20.3.1.3.4 [unique.ptr.single.asgn] p. 3+8 as indicated: [The intent is to clarify that the moved-from source must contain a null pointer, there is no other choice left]
unique_ptr& operator=(unique_ptr&& u);[..]
3 - Postconditions: This
unique_ptr
now owns the pointer whichu
owned, andu
no longer owns it,u.get() == nullptr
. [Note: IfD
is a reference type, then the referenced lvalue deleters are move assigned. — end note]template <class U, class E> unique_ptr& operator=(unique_ptr<U, E>&& u);[..]
8 - Postconditions: This
unique_ptr
now owns the pointer whichu
owned, andu
no longer owns it,u.get() == nullptr
.
Change 20.3.1.3.4 [unique.ptr.single.asgn] p. 6 as indicated: [With the possibility of user-defined pointer-like types the implication does only exist, if those are built-in pointers. Note that this change should also be applied with the acceptance of 950(i)]
template <class U, class E> unique_ptr& operator=(unique_ptr<U, E>&& u);[..]
6 - Requires: Assignment of the deleter
D
from an rvalueD
shall not throw an exception.unique_ptr<U, E>::pointer
shall be implicitly convertible topointer
.[Note: These requirements imply thatT
andU
are complete types. — end note]
Change 20.3.1.3.4 [unique.ptr.single.asgn] before p. 11 and p. 12 as indicated: [The first change is a simple typo fix]
unique_ptr& operator=(nullptr_t});11 - Effects:
reset()
.12 - Postcondition:
get() ==
0nullptr
Change 20.3.1.3.5 [unique.ptr.single.observers] p. 1+4+12 as indicated:
typename add_lvalue_reference<T>::type operator*() const;1 - Requires:
get() !=
. The variable definition0nullptradd_lvalue_reference<T>::type t = *get()
shall be well formed, shall have well-defined behavior, and shall not exit via an exception.[..]
pointer operator->() const;4 - Requires:
get() !=
.0nullptr[..]
explicit operator bool() const;12 - Returns:
get() !=
.0nullptr
Change 20.3.1.3.6 [unique.ptr.single.modifiers] p. 1 as indicated:
pointer release();1 - Postcondition:
get() ==
.0nullptr
Change 20.3.1.3.6 [unique.ptr.single.modifiers] p. 9 as indicated: [The
intent is to ensure that potentially user-defined swaps are used. A side-step
fix and harmonization with the specification of the the deleter is realized.
Please note the additional requirement in bullet 2 of this proposed resolution
regarding the availability of the generic swap
templates within the
member swap
function.]
void swap(unique_ptr& u);8 - Requires: The deleter
D
shall beSwappable
and shall not throw an exception underswap
.9 - Effects: The stored pointers of
*this
andu
are exchanged by an unqualified call to non-memberswap
. The stored deleters areexchanged by an unqualified call to non-memberswap
'd (unqualified)swap
.
Change 20.3.1.4.4 [unique.ptr.runtime.observers] p. 1 as indicated:
T& operator[](size_t i) const;Requires:
i <
the size of the array to which the stored pointer points. The variable definitionT& t = get()[i]
shall be well formed, shall have well-defined behavior, and shall not exit via an exception.
Change 20.3.1.4.5 [unique.ptr.runtime.modifiers] p. 1 as indicated:
void reset(pointer p = pointer()); void reset(nullptr_t p);1 - Effects: If
get() ==
there are no effects. Otherwise0nullptrget_deleter()(get())
.
Change 20.3.1.6 [unique.ptr.special] as indicated: [We don't add the relational operators to the basic requirement set, therefore we need special handling here]
template <class T1, class D1, class T2, class D2> bool operator==(const unique_ptr<T1, D1>& x, const unique_ptr<T2, D2>& y);Requires: The variable definition
bool b = x.get() == y.get();
shall be well formed, shall have well-defined behavior, and shall not exit via an exception.2 - Returns:
x.get() == y.get()
.Throws: nothing.
template <class T1, class D1, class T2, class D2> bool operator!=(const unique_ptr<T1, D1>& x, const unique_ptr<T2, D2>& y);Requires: The variable definition
bool b = x.get() != y.get();
shall be well formed, shall have well-defined behavior, and shall not exit via an exception.3 - Returns:
x.get() != y.get()
.Throws: nothing.
template <class T1, class D1, class T2, class D2> bool operator<(const unique_ptr<T1, D1>& x, const unique_ptr<T2, D2>& y);Requires: The variable definition
bool b = x.get() < y.get()
; shall be well formed, shall have well-defined behavior, and shall not exit via an exception.4 - Returns:
x.get() < y.get()
.Throws: nothing.
template <class T1, class D1, class T2, class D2> bool operator<=(const unique_ptr<T1, D1>& x, const unique_ptr<T2, D2>& y);Requires: The variable definition
bool b = x.get() <= y.get();
shall be well formed, shall have well-defined behavior, and shall not exit via an exception.5 - Returns:
x.get() <= y.get()
.Throws: nothing.
template <class T1, class D1, class T2, class D2> bool operator>(const unique_ptr<T1, D1>& x, const unique_ptr<T2, D2>& y);Requires: The variable definition
bool b = x.get() > y.get();
shall be well formed, shall have well-defined behavior, and shall not exit via an exception.6 - Returns:
x.get() > y.get()
.Throws: nothing.
template <class T1, class D1, class T2, class D2> bool operator>=(const unique_ptr<T1, D1>& x, const unique_ptr<T2, D2>& y);Requires: The variable definition
bool b = x.get() >= y.get();
shall be well formed, shall have well-defined behavior, and shall not exit via an exception.7 - Returns:
x.get() >= y.get()
.Throws: nothing.
Section: 31.5.4.3 [basic.ios.members] Status: C++11 Submitter: Martin Sebor Opened: 2008-05-17 Last modified: 2016-01-28
Priority: Not Prioritized
View all other issues in [basic.ios.members].
View all issues with C++11 status.
Discussion:
The fix for issue 581(i), now integrated into the working paper, overlooks a couple of minor problems.
First, being an unformatted function once again, flush()
is required to create a sentry object whose constructor must, among
other things, flush the tied stream. When two streams are tied
together, either directly or through another intermediate stream
object, flushing one will also cause a call to flush()
on
the other tied stream(s) and vice versa, ad infinitum. The program
below demonstrates the problem.
Second, as Bo Persson notes in his
comp.lang.c++.moderated post,
for streams with the unitbuf
flag set such
as std::stderr
, the destructor of the sentry object will
again call flush()
. This seems to create an infinite
recursion for std::cerr << std::flush;
#include <iostream> int main () { std::cout.tie (&std::cerr); std::cerr.tie (&std::cout); std::cout << "cout\n"; std::cerr << "cerr\n"; }
[ Batavia (2009-05): ]
We agree with the proposed resolution. Move to Review.
[ 2009-05-26 Daniel adds: ]
I think that the most recently suggested change in [ostream::sentry] need some further word-smithing. As written, it would make the behavior undefined, if under conditions when
pubsync()
should be called, but when in this scenarioos.rdbuf()
returns 0.This case is explicitly handled in
flush()
and needs to be taken care of. My suggested fix is:If
((os.flags() & ios_base::unitbuf) && !uncaught_exception()
&& os.rdbuf() != 0
) is true, callsos.flush()
os.rdbuf()->pubsync()
.Two secondary questions are:
- Should
pubsync()
be invoked in any case or shouldn't a base requirement for this trial be thatos.good() == true
as required in the originalflush()
case?- Since
uncaught_exception()
is explicitly tested, shouldn't a return value of -1 ofpubsync()
producesetstate(badbit)
(which may throwios_base::failure
)?
[ 2009-07 Frankfurt: ]
Daniel volunteered to modify the proposed resolution to address his two questions.
Move back to Open.
[ 2009-07-26 Daniel provided wording. Moved to Review. ]
[ 2009-10-13 Daniel adds: ]
This proposed wording is written to match the outcome of 397(i).
[ 2009 Santa Cruz: ]
Move to Open. Martin to propose updated wording that will also resolve issue 397(i) consistently.
[ 2010-02-15 Martin provided wording. ]
[ 2010 Pittsburgh: ]
Moved to Ready for Pittsburgh.
Proposed resolution:
Just before 31.5.4.3 [basic.ios.members] p. 2 insert a new paragraph:
Requires: If
(tiestr != 0)
istrue
,tiestr
must not be reachable by traversing the linked list of tied stream objects starting fromtiestr->tie()
.
Change [ostream::sentry] p. 4 as indicated:
If
((os.flags() & ios_base::unitbuf) && !uncaught_exception() && os.good())
istrue
, callsos.flush()
os.rdbuf()->pubsync()
. If that function returns -1 setsbadbit
inos.rdstate()
without propagating an exception.
Add after [ostream::sentry] p17, the following paragraph:
Throws: Nothing.
money_base::space
and
money_base::none
on money_get
Section: 28.3.4.7.2.3 [locale.money.get.virtuals] Status: C++11 Submitter: Martin Sebor Opened: 2008-05-17 Last modified: 2016-01-28
Priority: Not Prioritized
View all other issues in [locale.money.get.virtuals].
View all issues with C++11 status.
Duplicate of: 670
Discussion:
In paragraph 2, 28.3.4.7.2.3 [locale.money.get.virtuals] specifies the following:
Where
space
ornone
appears in the format pattern, except at the end, optional white space (as recognized byct.is
) is consumed after any required space.
This requirement can be (and has been) interpreted two mutually exclusive ways by different readers. One possible interpretation is that:
- where
money_base::space
appears in the format, at least one space is required, and- where
money_base::none
appears in the format, space is allowed but not required.
The other is that:
where either
money_base::space
ormoney_base::none
appears in the format, white space is optional.
[ San Francisco: ]
Martin will revise the proposed resolution.
[ 2009-07 Frankfurt: ]
There is a noun missing from the proposed resolution. It's not clear that the last sentence would be helpful, even if the word were not missing:
In either case, any required MISSINGWORD followed by all optional whitespace (as recognized by
ct.is()
) is consumed.Strike this sentence and move to Review.
[ Howard: done. ]
[ 2009-10 Santa Cruz: ]
Move to Ready.
Proposed resolution:
I propose to change the text to make it clear that the first interpretation is intended, that is, to make following change to 28.3.4.7.2.3 [locale.money.get.virtuals], p. 2:
When
money_base::space
ormoney_base::none
appears as the last element in the format pattern,except at the end, optional white space (as recognized byno white space is consumed. Otherwise, wherect.is
) is consumed after any required space.money_base::space
appears in any of the initial elements of the format pattern, at least one white space character is required. Wheremoney_base::none
appears in any of the initial elements of the format pattern, white space is allowed but not required. If(str.flags() & str.showbase)
isfalse
, ...
Section: 24.6.2 [istream.iterator] Status: C++11 Submitter: Martin Sebor Opened: 2008-05-17 Last modified: 2016-01-28
Priority: Not Prioritized
View all other issues in [istream.iterator].
View all issues with C++11 status.
Discussion:
From message c++std-lib-20003...
The description of istream_iterator
in
24.6.2 [istream.iterator], p. 1 specifies that objects of the
class become the end-of-stream (EOS) iterators under the
following condition (see also issue 788(i) another problem
with this paragraph):
If the end of stream is reached (
operator void*()
on the stream returnsfalse
), the iterator becomes equal to the end-of-stream iterator value.
One possible implementation approach that has been used in practice is
for the iterator to set its in_stream
pointer to 0 when
it reaches the end of the stream, just like the default ctor does on
initialization. The problem with this approach is that
the Effects clause for operator++()
says the
iterator unconditionally extracts the next value from the stream by
evaluating *in_stream >> value
, without checking
for (in_stream == 0)
.
Conformance to the requirement outlined in the Effects clause
can easily be verified in programs by setting eofbit
or failbit
in exceptions()
of the associated
stream and attempting to iterate past the end of the stream: each
past-the-end access should trigger an exception. This suggests that
some other, more elaborate technique might be intended.
Another approach, one that allows operator++()
to attempt
to extract the value even for EOS iterators (just as long
as in_stream
is non-0) is for the iterator to maintain a
flag indicating whether it has reached the end of the stream. This
technique would satisfy the presumed requirement implied by
the Effects clause mentioned above, but it isn't supported by
the exposition-only members of the class (no such flag is shown). This
approach is also found in existing practice.
The inconsistency between existing implementations raises the question
of whether the intent of the specification is that a non-EOS iterator
that has reached the EOS become a non-EOS one again after the
stream's eofbit
flag has been cleared? That is, are the
assertions in the program below expected to pass?
sstream strm ("1 "); istream_iterator eos; istream_iterator it (strm); int i; i = *it++ assert (it == eos); strm.clear (); strm << "2 3 "; assert (it != eos); i = *++it; assert (3 == i);
Or is it intended that once an iterator becomes EOS it stays EOS until the end of its lifetime?
[ San Francisco: ]
We like the direction of the proposed resolution. We're not sure about the wording, and we need more time to reflect on it,
Move to Open. Detlef to rewrite the proposed resolution in such a way that no reference is made to exposition only members of
istream_iterator
.
[ 2009-07 Frankfurt: ]
Move to Ready.
Proposed resolution:
The discussion of this issue on the reflector suggests that the intent
of the standard is for an istreambuf_iterator
that has
reached the EOS to remain in the EOS state until the end of its
lifetime. Implementations that permit EOS iterators to return to a
non-EOS state may only do so as an extension, and only as a result of
calling istream_iterator
member functions on EOS
iterators whose behavior is in this case undefined.
To this end we propose to change 24.6.2 [istream.iterator], p1, as follows:
The result of operator-> on an end-of-stream is not defined. For any other iterator value a
const T*
is returned. Invokingoperator++()
on an end-of-stream iterator is undefined. It is impossible to store things into istream iterators...
Add pre/postconditions to the member function descriptions of istream_iterator
like so:
istream_iterator();Effects: Constructs the end-of-stream iterator.
Postcondition:in_stream == 0
.istream_iterator(istream_type &s);Effects: Initializes
in_stream
with &s. value may be initialized during construction or the first time it is referenced.
Postcondition:in_stream == &s
.istream_iterator(const istream_iterator &x);Effects: Constructs a copy of
x
.
Postcondition:in_stream == x.in_stream
.istream_iterator& operator++();Requires:
in_stream != 0
.
Effects:*in_stream >> value
.istream_iterator& operator++(int);Requires:
in_stream != 0
.
Effects:istream_iterator tmp (*this); *in_stream >> value; return tmp;
Section: 23.4 [associative], 23.5 [unord] Status: Resolved Submitter: Alan Talbot Opened: 2008-05-18 Last modified: 2020-09-06
Priority: Not Prioritized
View all other issues in [associative].
View all issues with Resolved status.
Discussion:
Splice is a very useful feature of list
. This functionality is also very
useful for any other node based container, and I frequently wish it were
available for maps and sets. It seems like an omission that these
containers lack this capability. Although the complexity for a splice is
the same as for an insert, the actual time can be much less since the
objects need not be reallocated and copied. When the element objects are
heavy and the compare operations are fast (say a map<int, huge_thingy>
)
this can be a big win.
Suggested resolution:
Add the following signatures to map, set, multimap, multiset, and the unordered associative containers:
void splice(list<T,Allocator>&& x); void splice(list<T,Allocator>&& x, const_iterator i); void splice(list<T,Allocator>&& x, const_iterator first, const_iterator last);
Hint versions of these are also useful to the extent hint is useful. (I'm looking for guidance about whether hints are in fact useful.)
void splice(const_iterator position, list<T,Allocator>&& x); void splice(const_iterator position, list<T,Allocator>&& x, const_iterator i); void splice(const_iterator position, list<T,Allocator>&& x, const_iterator first, const_iterator last);
[ Sophia Antipolis: ]
Don't try to
splice "list"
into the other containers, it should be container-type.
forward_list
already hassplice_after
.Would "
splice
" make sense for anunordered_map
?Jens, Robert: "
splice
" is not the right term, it implies maintaining ordering inlist
s.Howard:
adopt
?Jens:
absorb
?Alan:
subsume
?Robert:
recycle
?Howard:
transfer
? (but no direction)Jens:
transfer_from
. No.Alisdair: Can we give a nothrow guarantee? If your
compare()
andhash()
doesn't throw, yes.Daniel: For
unordered_map
, we can't guarantee nothrow.
[ San Francisco: ]
Martin: this would possibly outlaw an implementation technique that is currently in use; caching nodes in containers.
Alan: if you cache in the allocator, rather than the individual container, this proposal doesn't interfere with that.
Martin: I'm not opposed to this, but I'd like to see an implementation that demonstrates that it works.
[ 2009-07 Frankfurt: ]
NAD Future.
[ 2009-09-19 Howard adds: ]
I'm not disagreeing with the NAD Future resolution. But when the future gets here, here is a possibility worth exploring:
Add to the "unique" associative containers:
typedef details node_ptr; node_ptr remove(const_iterator p); pair<iterator, bool> insert(node_ptr&& nd); iterator insert(const_iterator p, node_ptr&& nd);And add to the "multi" associative containers:
typedef details node_ptr; node_ptr remove(const_iterator p); iterator insert(node_ptr&& nd); iterator insert(const_iterator p, node_ptr&& nd);
Container::node_ptr
is a smart pointer much likeunique_ptr
. It owns a node obtained from the container it was removed from. It maintains a reference to the allocator in the container so that it can properly deallocate the node if asked to, even if the allocator is stateful. This being said, thenode_ptr
can not outlive the container for this reason.The
node_ptr
offers "const
-free" access to the node'svalue_type
.With this interface, clients have a great deal of flexibility:
- A client can remove a node from one container, and insert it into another (without any heap allocation). This is the splice functionality this issue asks for.
- A client can remove a node from a container, change its key or value, and insert it back into the same container, or another container, all without the cost of allocating a node.
- If the Compare function is nothrow (which is very common), then this functionality is nothrow unless modifying the value throws. And if this does throw, it does so outside of the containers involved.
- If the Compare function does throw, the
insert
function will have the argumentnd
retain ownership of the node.- The
node_ptr
should be independent of theCompare
parameter so that a node can be transferred fromset<T, C1, A>
toset<T, C2, A>
(for example).Here is how the customer might use this functionality:
Splice a node from one container to another:
m2.insert(m1.remove(i));Change the "key" in a
std::map
without the cost of node reallocation:auto p = m.remove(i); p->first = new_key; m.insert(std::move(p));Change the "value" in a
std::set
without the cost of node reallocation:auto p = s.remove(i); *p = new_value; s.insert(std::move(p));Move a move-only or heavy object out of an associative container (as opposed to the proposal in 1041(i)):
MoveOnly x = std::move(*s.remove(i));
remove(i)
transfers ownership of the node from the set to a temporarynode_ptr
.- The
node_ptr
is dereferenced, and that non-const reference is sent tomove
to cast it to an rvalue.- The rvalue
MoveOnly
is move constructed intox
from thenode_ptr
.~node_ptr()
destructs the moved-fromMoveOnly
and deallocates the node.Contrast this with the 1041(i) solution:
MoveOnly x = std::move(s.extract(i).first);The former requires one move construction for
x
while the latter requires two (one into thepair
and then one intox
). Either of these constructions can throw (say if there is only a copy constructor forx
). With the former, the point of throw is outside of the containers
, after the element has been removed from the container. With the latter, one throwing construction takes place prior to the removal of the element, and the second takes place after the element is removed.The "node insertion" API maintains the API associated with inserting
value_type
s so the customer can use familiar techniques for getting an iterator to the inserted node, or finding out whether it was inserted or not for the "unique" containers.Lightly prototyped. No implementation problems. Appears to work great for the client.
[08-2016, Post-Chicago]
Move to Tentatively Resolved
Proposed resolution:
This functionality is provided by P0083R3
ConstructibleAsElement
and bit containersSection: 23.2 [container.requirements], 23.3.14 [vector.bool], 22.9.2 [template.bitset] Status: CD1 Submitter: Howard Hinnant Opened: 2008-06-03 Last modified: 2016-01-28
Priority: Not Prioritized
View all other issues in [container.requirements].
View all issues with CD1 status.
Discussion:
23.2 [container.requirements] p. 3 says:
Objects stored in these components shall be constructed using
construct_element
(20.6.9). For each operation that inserts an element of typeT
into a container (insert
,push_back
,push_front
,emplace
, etc.) with argumentsargs... T
shall beConstructibleAsElement
, as described in table 88. [Note: If the component is instantiated with a scoped allocator of typeA
(i.e., an allocator for whichis_scoped_allocator<A>::value
istrue
), thenconstruct_element
may pass an inner allocator argument toT
's constructor. -- end note]
However vector<bool, A>
(23.3.14 [vector.bool]) and bitset<N>
(22.9.2 [template.bitset]) store bits, not bool
s, and bitset<N>
does not even have an allocator. But these containers are governed by this clause. Clearly this
is not implementable.
Proposed resolution:
Change 23.2 [container.requirements] p. 3:
Objects stored in these components shall be constructed using
construct_element
(20.6.9), unless otherwise specified. For each operation that inserts an element of typeT
into a container (insert
,push_back
,push_front
,emplace
, etc.) with argumentsargs... T
shall beConstructibleAsElement
, as described in table 88. [Note: If the component is instantiated with a scoped allocator of typeA
(i.e., an allocator for whichis_scoped_allocator<A>::value
istrue
), thenconstruct_element
may pass an inner allocator argument toT
's constructor. -- end note]
Change 23.3.14 [vector.bool]/p2:
Unless described below, all operations have the same requirements and semantics as the primary
vector
template, except that operations dealing with thebool
value type map to bit values in the container storage, andconstruct_element
(23.2 [container.requirements]) is not used to construct these values.
Move 22.9.2 [template.bitset] to clause 20.
Section: 99 [func.referenceclosure.cons] Status: CD1 Submitter: Lawrence Crowl Opened: 2008-06-02 Last modified: 2016-01-28
Priority: Not Prioritized
View all issues with CD1 status.
Discussion:
The std::reference_closure
type has a deleted copy assignment operator
under the theory that references cannot be assigned, and hence the
assignment of its reference member must necessarily be ill-formed.
However, other types, notably std::reference_wrapper
and std::function
provide for the "copying of references", and thus the current definition
of std::reference_closure
seems unnecessarily restrictive. In particular,
it should be possible to write generic functions using both std::function
and std::reference_closure
, but this generality is much harder when
one such type does not support assignment.
The definition of reference_closure
does not necessarily imply direct
implementation via reference types. Indeed, the reference_closure
is
best implemented via a frame pointer, for which there is no standard
type.
The semantics of assignment are effectively obtained by use of the default destructor and default copy assignment operator via
x.~reference_closure(); new (x) reference_closure(y);
So the copy assignment operator generates no significant real burden to the implementation.
Proposed resolution:
In [func.referenceclosure] Class template reference_closure,
replace the =delete
in the copy assignment operator in the synopsis
with =default
.
template<class R , class... ArgTypes > class reference_closure<R (ArgTypes...)> { public: ... reference_closure& operator=(const reference_closure&) =deletedefault; ...
In 99 [func.referenceclosure.cons] Construct, copy, destroy, add the member function description
reference_closure& operator=(const reference_closure& f)Postcondition:
*this
is a copy off
.Returns:
*this
.
complex pow
return type is ambiguousSection: 29.4.10 [cmplx.over] Status: CD1 Submitter: Howard Hinnant Opened: 2008-06-03 Last modified: 2016-01-28
Priority: Not Prioritized
View all other issues in [cmplx.over].
View all issues with CD1 status.
Discussion:
The current working draft is in an inconsistent state.
29.4.8 [complex.transcendentals] says that:
pow(complex<float>(), int())
returns acomplex<float>
.
29.4.10 [cmplx.over] says that:
pow(complex<float>(), int())
returns acomplex<double>
.
[ Sophia Antipolis: ]
Since
int
promotes todouble
, and C99 doesn't have anint
-based overload forpow
, the C99 result iscomplex<double>
, see also C99 7.22, see also library issue 550(i).Special note: ask P.J. Plauger.
Looks fine.
Proposed resolution:
Strike this pow
overload in 29.4.2 [complex.syn] and in 29.4.8 [complex.transcendentals]:
template<class T> complex<T> pow(const complex<T>& x, int y);
Section: 32.5.8 [atomics.types.generic] Status: CD1 Submitter: Alisdair Meredith Opened: 2008-06-03 Last modified: 2016-01-28
Priority: Not Prioritized
View all other issues in [atomics.types.generic].
View all issues with CD1 status.
Discussion:
The atomic classes (and class templates) are required to support aggregate initialization (99 [atomics.types.integral] p. 2 / 99 [atomics.types.address] p. 1) yet also have user declared constructors, so cannot be aggregates.
This problem might be solved with the introduction of the proposed initialization syntax at Antipolis, but the wording above should be altered. Either strike the sentence as redundant with new syntax, or refer to 'brace initialization'.
[ Jens adds: ]
Note that
atomic_itype a1 = { 5 };would be aggregate-initialization syntax (now coming under the disguise of brace initialization), but would be ill-formed, because the corresponding constructor for atomic_itype is explicit. This works, though:
atomic_itype a2 { 6 };
[ San Francisco: ]
The preferred approach to resolving this is to remove the explicit specifiers from the atomic integral type constructors.
Lawrence will provide wording.
This issue is addressed in N2783.
Proposed resolution:
within the synopsis in 99 [atomics.types.integral] edit as follows.
.... typedef struct atomic_bool { .... constexpr
explicitatomic_bool(bool); .... typedef struct atomic_itype { .... constexprexplicitatomic_itype(integral); ....
edit 99 [atomics.types.integral] paragraph 2 as follows.
The atomic integral types shall have standard layout. They shall each have a trivial default constructor, a constexpr
explicitvalue constructor, a deleted copy constructor, a deleted copy assignment operator, and a trivial destructor. They shall each support aggregate initialization syntax.
within the synopsis of 99 [atomics.types.address] edit as follows.
.... typedef struct atomic_address { .... constexpr
explicitatomic_address(void*); ....
edit 99 [atomics.types.address] paragraph 1 as follows.
The type
atomic_address
shall have standard layout. It shall have a trivial default constructor, a constexprexplicitvalue constructor, a deleted copy constructor, a deleted copy assignment operator, and a trivial destructor. It shall support aggregate initialization syntax.
within the synopsis of 32.5.8 [atomics.types.generic] edit as follows.
.... template <class T> struct atomic { .... constexpr
explicitatomic(T); .... template <> struct atomic<integral> : atomic_itype { .... constexprexplicitatomic(integral); .... template <> struct atomic<T*> : atomic_address { .... constexprexplicitatomic(T*); ....
edit 32.5.8 [atomics.types.generic] paragraph 2 as follows.
Specializations of the
atomic
template shall have a deleted copy constructor, a deleted copy assignment operator, and a constexprexplicitvalue constructor.
Section: 32.5.8.2 [atomics.types.operations] Status: CD1 Submitter: Alisdair Meredith Opened: 2008-06-03 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
The atomic classes and class templates (99 [atomics.types.integral] / 99 [atomics.types.address]) have a constexpr constructor taking a value of the appropriate type for that atomic. However, neither clause provides semantics or a definition for this constructor. I'm not sure if the initialization is implied by use of constexpr keyword (which restricts the form of a constructor) but even if that is the case, I think it is worth spelling out explicitly as the inference would be far too subtle in that case.
[ San Francisco: ]
Lawrence will provide wording.
This issue is addressed in N2783.
Proposed resolution:
before the description of ...is_lock_free
,
that is before 32.5.8.2 [atomics.types.operations] paragraph 4,
add the following description.
constexpr A::A(C desired);
- Effects:
- Initializes the object with the value
desired
. [Note: Construction is not atomic. —end note]
Section: 27.4.3.2 [string.require] Status: C++11 Submitter: Hervé Brönnimann Opened: 2008-06-05 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
In March, on comp.lang.c++.moderated, I asked what were the string exception safety guarantees are, because I cannot see *any* in the working paper, and any implementation I know offers the strong exception safety guarantee (string unchanged if a member throws exception). The closest the current draft comes to offering any guarantees is 27.4.3 [basic.string], para 3:
The class template
basic_string
conforms to the requirements for a Sequence Container (23.1.1), for a Reversible Container (23.1), and for an Allocator-aware container (91). The iterators supported bybasic_string
are random access iterators (24.1.5).
However, the chapter 23 only says, on the topic of exceptions: 23.2 [container.requirements], para 10:
Unless otherwise specified (see 23.2.2.3 and 23.2.6.4) all container types defined in this clause meet the following additional requirements:
- if an exception is thrown by...
I take it as saying that this paragraph has *no* implication on
std::basic_string
, as basic_string
isn't defined in Clause 23 and
this paragraph does not define a *requirement* of Sequence
nor Reversible Container, just of the models defined in Clause 23.
In addition, LWG Issue 718(i) proposes to remove 23.2 [container.requirements], para 3.
Finally, the fact that no operation on Traits should throw
exceptions has no bearing, except to suggest (since the only
other throws should be allocation, out_of_range
, or length_error
)
that the strong exception guarantee can be achieved.
The reaction in that group by Niels Dekker, Martin Sebor, and Bo Persson, was all that this would be worth an LWG issue.
A related issue is that erase()
does not throw. This should be
stated somewhere (and again, I don't think that the 23.2 [container.requirements], para 1
applies here).
[ San Francisco: ]
Implementors will study this to confirm that it is actually possible.
[ Daniel adds 2009-02-14: ]
The proposed resolution of paper N2815 interacts with this issue (the paper does not refer to this issue).
[ 2009-07 Frankfurt: ]
Move to Ready.
Proposed resolution:
Add a blanket statement in 27.4.3.2 [string.require]:
- if any member function or operator of
basic_string<charT, traits, Allocator>
throws, that function or operator has no effect.- no
erase()
orpop_back()
function throws.
As far as I can tell, this is achieved by any implementation. If I made a
mistake and it is not possible to offer this guarantee, then
either state all the functions for which this is possible
(certainly at least operator+=
, append
, assign
, and insert
),
or add paragraphs to Effects clauses wherever appropriate.
std::hash
specializations for std::bitset/std::vector<bool>
Section: 22.10.19 [unord.hash] Status: CD1 Submitter: Thorsten Ottosen Opened: 2008-06-05 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
In the current working draft, std::hash<T>
is specialized for builtin
types and a few other types. Bitsets seems like one that is missing from
the list, not because it cannot not be done by the user, but because it
is hard or impossible to write an efficient implementation that works on
32bit/64bit chunks at a time. For example, std::bitset
is too much
encapsulated in this respect.
Proposed resolution:
Add the following to the synopsis in 22.10 [function.objects]/2:
template<class Allocator> struct hash<std::vector<bool,Allocator>>; template<size_t N> struct hash<std::bitset<N>>;
Modify the last sentence of 22.10.19 [unord.hash]/1 to end with:
... and
std::string
,std::u16string
,std::u32string
,std::wstring
,std::error_code
,std::thread::id
,std::bitset
,and std::vector<bool>
.
shrink_to_fit
apply to std::deque
?Section: 23.3.5.3 [deque.capacity] Status: CD1 Submitter: Niels Dekker Opened: 2008-06-05 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
Issue 755(i) added a shrink_to_fit
function to std::vector
and std::string
.
It did not yet deal with std::deque
, because of the fundamental
difference between std::deque
and the other two container types. The
need for std::deque
may seem less evident, because one might think that
for this container, the overhead is a small map, and some number of
blocks that's bounded by a small constant.
The container overhead can in fact be arbitrarily large (i.e. is not
necessarily O(N) where N is the number of elements currently held by the
deque
). As Bill Plauger noted in a reflector message, unless the map of
block pointers is shrunk, it must hold at least maxN⁄B pointers where
maxN is the maximum of N over the lifetime of the deque
since its
creation. This is independent of how the map is implemented
(vector
-like circular buffer and all), and maxN bears no relation to N,
the number of elements it currently holds.
Hervé Brönnimann reports a situation where a deque
of requests grew very
large due to some temporary backup (the front request hanging), and the
map of the deque
grew quite large before getting back to normal. Just
to put some color on it, assuming a deque
with 1K pointer elements in
steady regime, that held, at some point in its lifetime, maxN=10M
pointers, with one block holding 128 elements, the spine must be at
least (maxN ⁄ 128), in that case 100K. In that case, shrink-to-fit
would allow to reuse about 100K which would otherwise never be reclaimed
in the lifetime of the deque
.
An added bonus would be that it allows implementations to hang on to
empty blocks at the end (but does not care if they do or not). A
shrink_to_fit
would take care of both shrinks, and guarantee that at
most O(B) space is used in addition to the storage to hold the N
elements and the N⁄B block pointers.
Proposed resolution:
To class template deque
23.3.5 [deque] synopsis, add:
void shrink_to_fit();
To deque capacity 23.3.5.3 [deque.capacity], add:
void shrink_to_fit();Remarks:
shrink_to_fit
is a non-binding request to reduce memory use. [Note: The request is non-binding to allow latitude for implementation-specific optimizations. — end note]
begin(n)
mistakenly const
Section: 23.5 [unord] Status: CD1 Submitter: Robert Klarer Opened: 2008-06-12 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
In 3 of the four unordered containers the local begin
member is mistakenly declared const
:
local_iterator begin(size_type n) const;
Proposed resolution:
Change the synopsis in 23.5.3 [unord.map], 23.5.4 [unord.multimap], and 23.5.7 [unord.multiset]:
local_iterator begin(size_type n)const;
to_string
needs updating with zero
and one
Section: 22.9.2 [template.bitset] Status: C++11 Submitter: Howard Hinnant Opened: 2008-06-18 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
Issue 396(i) adds defaulted arguments to the to_string
member, but neglects to update
the three newer to_string
overloads.
[ post San Francisco: ]
Daniel found problems with the wording and provided fixes. Moved from Ready to Review.
[ Post Summit: ]
Alisdair: suggest to not repeat the default arguments in B, C, D (definition of to_string members)
Walter: This is not really a definition.
Consensus: Add note to the editor: Please apply editor's judgement whether default arguments should be repeated for B, C, D changes.
Recommend Tentatively Ready.
[ 2009-05-09: See alternative solution in issue 1113(i). ]
Proposed resolution:
replace in 22.9.2 [template.bitset]/1 (class bitset
)
template <class charT, class traits> basic_string<charT, traits, allocator<charT> > to_string(charT zero = charT('0'), charT one = charT('1')) const; template <class charT> basic_string<charT, char_traits<charT>, allocator<charT> > to_string(charT zero = charT('0'), charT one = charT('1')) const; basic_string<char, char_traits<char>, allocator<char> > to_string(char zero = '0', char one = '1') const;
replace in 22.9.2.3 [bitset.members]/37
template <class charT, class traits> basic_string<charT, traits, allocator<charT> > to_string(charT zero = charT('0'), charT one = charT('1')) const;37 Returns:
to_string<charT, traits, allocator<charT> >(zero, one)
.
replace in 22.9.2.3 [bitset.members]/38
template <class charT> basic_string<charT, char_traits<charT>, allocator<charT> > to_string(charT zero = charT('0'), charT one = charT('1')) const;38 Returns:
to_string<charT, char_traits<charT>, allocator<charT> >(zero, one)
.
replace in 22.9.2.3 [bitset.members]/39
basic_string<char, char_traits<char>, allocator<char> > to_string(char zero = '0', char one = '1') const;39 Returns:
to_string<char, char_traits<char>, allocator<char> >(zero, one)
.
default_delete
converting constructor underspecifiedSection: 20.3.1.2.2 [unique.ptr.dltr.dflt] Status: C++11 Submitter: Howard Hinnant Opened: 2008-06-18 Last modified: 2016-01-28
Priority: Not Prioritized
View all other issues in [unique.ptr.dltr.dflt].
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Discussion:
No relationship between U
and T
in the converting constructor for default_delete
template.
Requirements: U*
is convertible to T*
and has_virtual_destructor<T>
;
the latter should also become a concept.
Rules out cross-casting.
The requirements for unique_ptr
conversions should be the same as those on the deleter.
[ Howard adds 2008-11-26: ]
I believe we need to be careful to not outlaw the following use case, and I believe the current proposed wording (
requires Convertible<U*, T*> && HasVirtualDestructor<T>
) does so:#include <memory> int main() { std::unique_ptr<int> p1(new int(1)); std::unique_ptr<const int> p2(move(p1)); int i = *p2; // *p2 = i; // should not compile }
I've removed "
&& HasVirtualDestructor<T>
" from therequires
clause in the proposed wording.
[ Post Summit: ]
Alisdair: This issue has to stay in review pending a paper constraining
unique_ptr
.Consensus: We agree with the resolution, but
unique_ptr
needs to be constrained, too.Recommend Keep in Review.
[ Batavia (2009-05): ]
Keep in Review status for the reasons cited.
[ 2009-07 Frankfurt: ]
The proposed resolution uses concepts. Howard needs to rewrite the proposed resolution.
Move back to Open.
[ 2009-07-26 Howard provided rewritten proposed wording and moved to Review. ]
[ 2009-10 Santa Cruz: ]
Move to Ready.
Proposed resolution:
Add after 20.3.1.2.2 [unique.ptr.dltr.dflt], p1:
template <class U> default_delete(const default_delete<U>& other);-1- Effects: ...
Remarks: This constructor shall participate in overload resolution if and only if
U*
is implicitly convertible toT*
.
aligned_union
Section: 21.3.8.7 [meta.trans.other] Status: CD1 Submitter: Jens Maurer Opened: 2008-06-12 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
With the arrival of extended unions
(N2544),
there is no
known use of aligned_union
that couldn't be handled by
the "extended unions" core-language facility.
Proposed resolution:
Remove the following signature from 21.3.3 [meta.type.synop]:
template <std::size_t Len, class... Types> struct aligned_union;
Remove the second row from table 51 in 21.3.8.7 [meta.trans.other], starting with:
template <std::size_t Len, class... Types> struct aligned_union;
condition_variable::time_wait
return bool
error proneSection: 32.7.4 [thread.condition.condvar] Status: C++11 Submitter: Beman Dawes Opened: 2008-06-13 Last modified: 2016-01-28
Priority: Not Prioritized
View all other issues in [thread.condition.condvar].
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Discussion:
The meaning of the bool
returned by condition_variable::timed_wait
is so
obscure that even the class' designer can't deduce it correctly. Several
people have independently stumbled on this issue.
It might be simpler to change the return type to a scoped enum:
enum class timeout { not_reached, reached };
That's the same cost as returning a bool
, but not subject to mistakes. Your example below would be:
if (cv.wait_until(lk, time_limit) == timeout::reached ) throw time_out();
[ Beman to supply exact wording. ]
[ San Francisco: ]
There is concern that the enumeration names are just as confusing, if not more so, as the bool. You might have awoken because of a signal or a spurious wakeup, for example.
Group feels that this is a defect that needs fixing.
Group prefers returning an enum over a void return.
Howard to provide wording.
[ 2009-06-14 Beman provided wording. ]
[ 2009-07 Frankfurt: ]
Move to Ready.
Proposed resolution:
Change Condition variables 32.7 [thread.condition], Header condition_variable synopsis, as indicated:
namespace std { class condition_variable; class condition_variable_any; enum class cv_status { no_timeout, timeout }; }
Change class condition_variable
32.7.4 [thread.condition.condvar] as indicated:
class condition_variable { public: ... template <class Clock, class Duration>boolcv_status wait_until(unique_lock<mutex>& lock, const chrono::time_point<Clock, Duration>& abs_time); template <class Clock, class Duration, class Predicate> bool wait_until(unique_lock<mutex>& lock, const chrono::time_point<Clock, Duration>& abs_time, Predicate pred); template <class Rep, class Period>boolcv_status wait_for(unique_lock<mutex>& lock, const chrono::duration<Rep, Period>& rel_time); template <class Rep, class Period, class Predicate> bool wait_for(unique_lock<mutex>& lock, const chrono::duration<Rep, Period>& rel_time, Predicate pred); ... }; ... template <class Clock, class Duration>boolcv_status wait_until(unique_lock<mutex>& lock, const chrono::time_point<Clock, Duration>& abs_time);-15- Precondition:
lock
is locked by the calling thread, and either
- no other thread is waiting on this
condition_variable
object orlock.mutex()
returns the same value for each of thelock
arguments supplied by all concurrently waiting threads (viawait
,wait_for
orwait_until
.).-16- Effects:
- Atomically calls
lock.unlock()
and blocks on*this
.- When unblocked, calls
lock.lock()
(possibly blocking on the lock) and returns.- The function will unblock when signaled by a call to
notify_one()
, a call tonotify_all()
,by the current time exceedingifabs_time
Clock::now() >= abs_time
, or spuriously.- If the function exits via an exception,
lock.unlock()
shall be called prior to exiting the function scope.-17- Postcondition:
lock
is locked by the calling thread.-18- Returns:
Clock::now() < abs_time
cv_status::timeout
if the function unblocked becauseabs_time
was reached, otherwisecv_status::no_timeout
.-19- Throws:
std::system_error
when the effects or postcondition cannot be achieved.-20- Error conditions:
operation_not_permitted
— if the thread does not own the lock.- equivalent error condition from
lock.lock()
orlock.unlock()
.template <class Rep, class Period>boolcv_status wait_for(unique_lock<mutex>& lock, const chrono::duration<Rep, Period>& rel_time);-21-
EffectsReturns:wait_until(lock, chrono::monotonic_clock::now() + rel_time)
-22- Returns:false
if the call is returning because the time duration specified byrel_time
has elapsed, otherwisetrue
.[ This part of the wording may conflict with 859(i) in detail, but does not do so in spirit. If both issues are accepted, there is a logical merge. ]
template <class Clock, class Duration, class Predicate> bool wait_until(unique_lock<mutex>& lock, const chrono::time_point<Clock, Duration>& abs_time, Predicate pred);-23- Effects:
while (!pred()) if (!wait_until(lock, abs_time) == cv_status::timeout) return pred(); return true;-24- Returns:
pred()
.-25- [Note: The returned value indicates whether the predicate evaluates to
true
regardless of whether the timeout was triggered. — end note].
Change Class condition_variable_any 32.7.5 [thread.condition.condvarany] as indicated:
class condition_variable_any { public: ... template <class Lock, class Clock, class Duration>boolcv_status wait_until(Lock& lock, const chrono::time_point<Clock, Duration>& abs_time); template <class Lock, class Clock, class Duration, class Predicate> bool wait_until(Lock& lock, const chrono::time_point<Clock, Duration>& abs_time, Predicate pred); template <class Lock, class Rep, class Period>boolcv_status wait_for(Lock& lock, const chrono::duration<Rep, Period>& rel_time); template <class Lock, class Rep, class Period, class Predicate> bool wait_for(Lock& lock, const chrono::duration<Rep, Period>& rel_time, Predicate pred); ... }; ... template <class Lock, class Clock, class Duration>boolcv_status wait_until(Lock& lock, const chrono::time_point<Clock, Duration>& abs_time);-13- Effects:
- Atomically calls
lock.unlock()
and blocks on*this
.- When unblocked, calls
lock.lock()
(possibly blocking on the lock) and returns.- The function will unblock when signaled by a call to
notify_one()
, a call tonotify_all()
,by the current time exceedingifabs_time
Clock::now() >= abs_time
, or spuriously.- If the function exits via an exception,
lock.unlock()
shall be called prior to exiting the function scope.-14- Postcondition:
lock
is locked by the calling thread.-15- Returns:
Clock::now() < abs_time
cv_status::timeout
if the function unblocked becauseabs_time
was reached, otherwisecv_status::no_timeout
.-16- Throws:
std::system_error
when the effects or postcondition cannot be achieved.-17- Error conditions:
- equivalent error condition from
lock.lock()
orlock.unlock()
.template <class Lock, class Rep, class Period>boolcv_status wait_for(Lock& lock, const chrono::duration<Rep, Period>& rel_time);-18-
EffectsReturns:wait_until(lock, chrono::monotonic_clock::now() + rel_time)
-19- Returns:false
if the call is returning because the time duration specified byrel_time
has elapsed, otherwisetrue
.[ This part of the wording may conflict with 859(i) in detail, but does not do so in spirit. If both issues are accepted, there is a logical merge. ]
template <class Lock, class Clock, class Duration, class Predicate> bool wait_until(Lock& lock, const chrono::time_point<Clock, Duration>&rel_timeabs_time, Predicate pred);-20- Effects:
while (!pred()) if (!wait_until(lock, abs_time) == cv_status::timeout) return pred(); return true;-21- Returns:
pred()
.-22- [Note: The returned value indicates whether the predicate evaluates to
true
regardless of whether the timeout was triggered. — end note].
Section: 99 [util.dynamic.safety] Status: CD1 Submitter: Pete Becker Opened: 2008-06-21 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
The first sentence of the Effects clause for undeclare_reachable
seems
to be missing some words. I can't parse
... for all non-null
p
referencing the argument is no longer declared reachable...
I take it the intent is that undeclare_reachable
should be called only
when there has been a corresponding call to declare_reachable
. In
particular, although the wording seems to allow it, I assume that code
shouldn't call declare_reachable
once then call undeclare_reachable
twice.
I don't know what "shall be live" in the Requires clause means.
In the final Note for undeclare_reachable
, what does "cannot be
deallocated" mean? Is this different from "will not be able to collect"?
For the wording on nesting of declare_reachable
and
undeclare_reachable
, the words for locking and unlocking recursive
mutexes probably are a good model.
[ San Francisco: ]
Nick: what does "shall be live" mean?
Hans: I can provide an appropriate cross-reference to the Project Editor to clarify the intent.
Proposed resolution:
In 99 [util.dynamic.safety] (N2670, Minimal Support for Garbage Collection)
Add at the beginning, before paragraph 39
A complete object is declared reachable while the number of calls to
declare_reachable
with an argument referencing the object exceeds the number ofundeclare_reachable
calls with pointers to the same complete object.
Change paragraph 42 (Requires clause for undeclare_reachable
)
If
p
is not null,the complete object referenced bydeclare_reachable(p)
was previously calledp
shall have been previously declared reachable, and shall be live (6.7.4 [basic.life]) from the time of the call until the lastundeclare_reachable(p)
call on the object.
Change the first sentence in paragraph 44 (Effects clause for
undeclare_reachable
):
Effects:
Once the number of calls toAfter a call toundeclare_reachable(p)
equals the number of calls todeclare_reachable(p)
for all non-nullp
referencing the argument is no longer declared reachable. When this happens, pointers to the object referenced by p may not be subsequently dereferenced.undeclare_reachable(p)
, ifp
is not null and the objectq
referenced byp
is no longer declared reachable, then dereferencing any pointer toq
that is not safely derived results in undefined behavior. ...
Change the final note:
[Note: It is expected that calls to declare_reachable(p)
will consume a small amount of memory, in addition to that occupied
by the referenced object, until the matching call to
undeclare_reachable(p)
is encountered. In addition, the
referenced object cannot be deallocated during this period, and garbage
collecting implementations will not be able to collect the object while
it is declared reachable. Long running programs should arrange
that calls for short-lived objects are matched. --end
note]
Section: 32.7 [thread.condition] Status: C++11 Submitter: Pete Becker Opened: 2008-06-23 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
N2661
says that there is a class named monotonic_clock
. It also says that this
name may be a synonym for system_clock
, and that it's conditionally
supported. So the actual requirement is that it can be monotonic or not,
and you can tell by looking at is_monotonic
, or it might not exist at
all (since it's conditionally supported). Okay, maybe too much
flexibility, but so be it.
A problem comes up in the threading specification, where several
variants of wait_for
explicitly use monotonic_clock::now()
. What is the
meaning of an effects clause that says
wait_until(lock, chrono::monotonic_clock::now() + rel_time)
when monotonic_clock
is not required to exist?
[ San Francisco: ]
Nick: maybe instead of saying that
chrono::monotonic_clock
is conditionally supported, we could say that it's always there, but not necessarily supported..Beman: I'd prefer a typedef that identifies the best clock to use for
wait_for
locks.Tom: combine the two concepts; create a duration clock type, but keep the is_monotonic test.
Howard: if we create a
duration_clock
type, is it a typedef or an entirely true type?There was broad preference for a typedef.
Move to Open. Howard to provide wording to add a typedef for duration_clock and to replace all uses of monotonic_clock in function calls and signatures with duration_clock.
[ Howard notes post-San Francisco: ]
After further thought I do not believe that creating a
duration_clock typedef
is the best way to proceed. An implementation may not need to use atime_point
to implement thewait_for
functions.For example, on POSIX systems
sleep_for
can be implemented in terms ofnanosleep
which takes only a duration in terms of nanoseconds. The current working paper does not describesleep_for
in terms ofsleep_until
. And paragraph 2 of 32.2.4 [thread.req.timing] has the words strongly encouraging implementations to use monotonic clocks forsleep_for
:2 The member functions whose names end in
_for
take an argument that specifies a relative time. Implementations should use a monotonic clock to measure time for these functions.I believe the approach taken in describing the effects of
sleep_for
andtry_lock_for
is also appropriate forwait_for
. I.e. these are not described in terms of their_until
variants.
[ 2009-07 Frankfurt: ]
Beman will send some suggested wording changes to Howard.
Move to Ready.
[ 2009-07-21 Beman added the requested wording changes to 962(i). ]
Proposed resolution:
Change 32.7.4 [thread.condition.condvar], p21-22:
template <class Rep, class Period> bool wait_for(unique_lock<mutex>& lock, const chrono::duration<Rep, Period>& rel_time);Precondition:
lock
is locked by the calling thread, and either
- no other thread is waiting on this
condition_variable
object orlock.mutex()
returns the same value for each of thelock
arguments supplied by all concurrently waiting threads (viawait
,wait_for
orwait_until
).21 Effects:
wait_until(lock, chrono::monotonic_clock::now() + rel_time)
- Atomically calls
lock.unlock()
and blocks on*this
.- When unblocked, calls
lock.lock()
(possibly blocking on the lock) and returns.- The function will unblock when signaled by a call to
notify_one()
, a call tonotify_all()
, by the elapsed timerel_time
passing (32.2.4 [thread.req.timing]), or spuriously.- If the function exits via an exception,
lock.unlock()
shall be called prior to exiting the function scope.Postcondition:
lock
is locked by the calling thread.22 Returns:
false
if the call is returning because the time duration specified byrel_time
has elapsed, otherwisetrue
.[ This part of the wording may conflict with 857(i) in detail, but does not do so in spirit. If both issues are accepted, there is a logical merge. ]
Throws:
std::system_error
when the effects or postcondition cannot be achieved.Error conditions:
operation_not_permitted
-- if the thread does not own the lock.- equivalent error condition from
lock.lock()
orlock.unlock()
.
Change 32.7.4 [thread.condition.condvar], p26-p29:
template <class Rep, class Period, class Predicate> bool wait_for(unique_lock<mutex>& lock, const chrono::duration<Rep, Period>& rel_time, Predicate pred);Precondition:
lock
is locked by the calling thread, and either
- no other thread is waiting on this
condition_variable
object orlock.mutex()
returns the same value for each of thelock
arguments supplied by all concurrently waiting threads (viawait
,wait_for
orwait_until
).26 Effects:
wait_until(lock, chrono::monotonic_clock::now() + rel_time, std::move(pred))
- Executes a loop: Within the loop the function first evaluates
pred()
and exits the loop if the result ofpred()
istrue
.- Atomically calls
lock.unlock()
and blocks on*this
.- When unblocked, calls
lock.lock()
(possibly blocking on the lock).- The function will unblock when signaled by a call to
notify_one()
, a call tonotify_all()
, by the elapsed timerel_time
passing (30.1.4 [thread.req.timing]), or spuriously.- If the function exits via an exception,
lock.unlock()
shall be called prior to exiting the function scope.- The loop terminates when
pred()
returnstrue
or when the time duration specified byrel_time
has elapsed.27 [Note: There is no blocking if
pred()
is initiallytrue
, even if the timeout has already expired. -- end note]Postcondition:
lock
is locked by the calling thread.28 Returns:
pred()
29 [Note: The returned value indicates whether the predicate evaluates to
true
regardless of whether the timeout was triggered. -- end note]Throws:
std::system_error
when the effects or postcondition cannot be achieved.Error conditions:
operation_not_permitted
-- if the thread does not own the lock.- equivalent error condition from
lock.lock()
orlock.unlock()
.
Change 32.7.5 [thread.condition.condvarany], p18-19:
template <class Lock, class Rep, class Period> bool wait_for(Lock& lock, const chrono::duration<Rep, Period>& rel_time);18 Effects:
wait_until(lock, chrono::monotonic_clock::now() + rel_time)
- Atomically calls
lock.unlock()
and blocks on*this
.- When unblocked, calls
lock.lock()
(possibly blocking on the lock) and returns.- The function will unblock when signaled by a call to
notify_one()
, a call tonotify_all()
, by the elapsed timerel_time
passing (32.2.4 [thread.req.timing]), or spuriously.- If the function exits via an exception,
lock.unlock()
shall be called prior to exiting the function scope.Postcondition:
lock
is locked by the calling thread.19 Returns:
false
if the call is returning because the time duration specified byrel_time
has elapsed, otherwisetrue
.Throws:
std::system_error
when the returned value, effects, or postcondition cannot be achieved.Error conditions:
- equivalent error condition from
lock.lock()
orlock.unlock()
.
Change 32.7.5 [thread.condition.condvarany], p23-p26:
template <class Lock, class Rep, class Period, class Predicate> bool wait_for(Lock& lock, const chrono::duration<Rep, Period>& rel_time, Predicate pred);Precondition:
lock
is locked by the calling thread, and either
- no other thread is waiting on this
condition_variable
object orlock.mutex()
returns the same value for each of thelock
arguments supplied by all concurrently waiting threads (viawait
,wait_for
orwait_until
).23 Effects:
wait_until(lock, chrono::monotonic_clock::now() + rel_time, std::move(pred))
- Executes a loop: Within the loop the function first evaluates
pred()
and exits the loop if the result ofpred()
istrue
.- Atomically calls
lock.unlock()
and blocks on*this
.- When unblocked, calls
lock.lock()
(possibly blocking on the lock).- The function will unblock when signaled by a call to
notify_one()
, a call tonotify_all()
, by the elapsed timerel_time
passing (30.1.4 [thread.req.timing]), or spuriously.- If the function exits via an exception,
lock.unlock()
shall be called prior to exiting the function scope.- The loop terminates when
pred()
returnstrue
or when the time duration specified byrel_time
has elapsed.24 [Note: There is no blocking if
pred()
is initiallytrue
, even if the timeout has already expired. -- end note]Postcondition:
lock
is locked by the calling thread.25 Returns:
pred()
26 [Note: The returned value indicates whether the predicate evaluates to
true
regardless of whether the timeout was triggered. -- end note]Throws:
std::system_error
when the effects or postcondition cannot be achieved.Error conditions:
operation_not_permitted
-- if the thread does not own the lock.- equivalent error condition from
lock.lock()
orlock.unlock()
.
Section: 29 [numerics] Status: C++11 Submitter: Lawrence Crowl Opened: 2008-06-23 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
There are a number of functions that affect the floating point state. These function need to be thread-safe, but I'm unsure of the right approach in the standard, as we inherit them from C.
[ San Francisco: ]
Nick: I think we already say that these functions do not introduce data races; see 17.6.5.6/20
Pete: there's more to it than not introducing data races; are these states maintained per thread?
Howard: 21.5/14 says that strtok and strerror are not required to avoid data races, and 20.9/2 says the same about asctime, gmtime, ctime, and gmtime.
Nick: POSIX has a list of not-safe functions. All other functions are implicitly thread safe.
Lawrence is to form a group between meetings to attack this issue. Nick and Tom volunteered to work with Lawrence.
Move to Open.
[ Post Summit: ]
Hans: Sane oses seem ok. Sensible thing is implementable and makes sense.
Nick: Default wording seems to cover this? Hole in POSIX, these functions need to be added to list of thread-unsafe functions.
Lawrence: Not sufficient, not "thread-safe" per our definition, but think of state as a thread-local variable. Need something like "these functions only affect state in the current thread."
Hans: Suggest the following wording: "The floating point environment is maintained per-thread."
Walter: Any other examples of state being thread safe that are not already covered elsewhere?
Have thread unsafe functions paper which needs to be updated. Should just fold in 29.3 [cfenv] functions.
Recommend Open. Lawrence instead suggests leaving it open until we have suitable wording that may or may not include the thread local commentary.
[ 2009-09-23 Hans provided wording. ]
If I understand the history correctly, Nick, as the Posix liaison, should probably get a veto on this, since I think it came from Posix (?) via WG14 and should probably really be addressed there (?). But I think we are basically in agreement that there is no other sane way to do this, and hence we don't have to worry too much about stepping on toes. As far as I can tell, this same issue also exists in the latest Posix standard (?).
[ 2009-10 Santa Cruz: ]
Moved to Ready.
Proposed resolution:
Add at the end of 29.3.1 [cfenv.syn]:
2 The header defines all functions, types, and macros the same as C99 7.6.
A separate floating point environment shall be maintained for each thread. Each function accesses the environment corresponding to its calling thread.
EqualityComparable
for std::forward_list
Section: 23.2 [container.requirements] Status: C++11 Submitter: Daniel Krügler Opened: 2008-06-24 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
Table 89, Container requirements, defines operator==
in terms of the container
member function size()
and the algorithm std::equal
:
==
is an equivalence relation.a.size() == b.size() && equal(a.begin(), a.end(), b.begin()
The new container forward_list
does not provide a size
member function
by design but does provide operator==
and operator!=
without specifying it's semantic.
Other parts of the (sequence) container requirements do also depend on
size()
, e.g. empty()
or clear()
, but this issue explicitly attempts to solve the missing
EqualityComparable
specification,
because of the special design choices of forward_list
.
I propose to apply one of the following resolutions, which are described as:
O(N)
calls of std::distance()
with the corresponding container ranges and instead uses a special
equals
implementation which takes two container ranges instead of 1 1/2.
size()
is replaced
by distance
with corresponding performance disadvantages.
Both proposal choices are discussed, the preferred choice of the author is to apply (A).
[ San Francisco: ]
There's an Option C: change the requirements table to use distance().
LWG found Option C acceptable.
Martin will draft the wording for Option C.
[ post San Francisco: ]
Martin provided wording for Option C.
[ 2009-07 Frankfurt ]
Other operational semantics (see, for example, Tables 82 and 83) are written in terms of a container's size() member. Daniel to update proposed resolution C.
[ Howard: Commented out options A and B. ]
[ 2009-07-26 Daniel updated proposed resolution C. ]
[ 2009-10 Santa Cruz: ]
Mark NAD Editorial. Addressed by N2986.
[ 2009-10 Santa Cruz: ]
Reopened. N2986 was rejected in full committee on procedural grounds.
[ 2010-01-30 Howard updated Table numbers. ]
[ 2010 Pittsburgh: ]
Moved to Ready for Pittsburgh.
Proposed resolution:
Option (C):
In 23.2.2 [container.requirements.general] change Table 90 -- Container requirements as indicated:
Change the text in the Assertion/note column in the row for "
X u
;" as follows:post:
u.
size() == 0empty() == trueChange the text in the Assertion/note column in the row for "
X();
" as follows:
X().
size() == 0empty() == trueChange the text in the Operational Semantics column in the row for "
a == b
" as follows:
==
is an equivalence relation.
a.size()distance(a.begin(), a.end()) ==b.size()distance(b.begin(), b.end()) && equal(a.begin(), a.end(), b.begin())Add text in the Ass./Note/pre-/post-condition column in the row for "
a == b
" as follows:Requires:
T
isEqualityComparable
Change the text in the Operational Semantics column in the row for "
a.size()
" as follows:
a.end() - a.begin()distance(a.begin(), a.end())Change the text in the Operational Semantics column in the row for "
a.max_size()
" as follows:
of the largest possible container
size()distance(begin(), end())Change the text in the Operational Semantics column in the row for "
a.empty()
" as follows:
a.size() == 0a.begin() == a.end()In 23.2.2 [container.requirements.general] change Table 93 — Allocator-aware container requirements as indicated:
Change the text in the Assertion/note column in the row for "
X() / X u;
" as follows:Requires:
A
isDefaultConstructible
post:,
u.size() == 0u.empty() == trueget_allocator() == A()
Change the text in the Assertion/note column in the row for "
X(m) / X u(m);
" as follows:post:
u.size() == 0u.empty() == true, get_allocator() == mIn 23.2.4 [sequence.reqmts] change Table 94 — Sequence container requirements as indicated:
Change the text in the Assertion/note column in the row for "
X(n, t) / X a(n, t)
" as follows:post:
size()distance(begin(), end()) == n [..]Change the text in the Assertion/note column in the row for "
X(i, j) / X a(i, j)
" as follows:[..] post:
size() ==
distance betweeni
andj
distance(begin(), end()) == distance(i, j)
[..]Change the text in the Assertion/note column in the row for "
a.clear()
" as follows:
a.erase(a.begin(), a.end())
post:
size() == 0a.empty() == trueIn 23.2.7 [associative.reqmts] change Table 96 — Associative container requirements as indicated:
[ Not every occurrence of
size()
was replaced, because all current associative containers have asize
. The following changes ensure consistency regarding the semantics of "erase
" for all tables and adds some missing objects ]
Change the text in the Complexity column in the row for
X(i,j,c)/Xa(i,j,c);
as follows:
N
logN
in general (N
== distance(i, j)
is the distance from); ...i
toj
Change the text in the Complexity column in the row for "
a.insert(i, j)
" as follows:
N log(a.size() + N)
(whereN
is the distance fromi
toj
)N == distance(i, j)
Change the text in the Complexity column in the row for "
a.erase(k)
" as follows:
log(a.size()) + a.count(k)
Change the text in the Complexity column in the row for "
a.erase(q1, q2)
" as follows:
log(a.size()) + N
whereN
is the distance fromq1
toq2
== distance(q1, q2)
.Change the text in the Assertion/note column in the row for "
a.clear()
" as follows:
a.erase(a.begin(),a.end())
post:
size() == 0a.empty() == trueChange the text in the Complexity column in the row for "
a.clear()
" as follows:linear in
a.size()
Change the text in the Complexity column in the row for "
a.count(k)
" as follows:
log(a.size()) + a.count(k)
In 23.2.8 [unord.req] change Table 98 — Unordered associative container requirements as indicated:
[ The same rational as for Table 96 applies here ]
Change the text in the Assertion/note column in the row for "
a.clear()
" as follows:[..] Post:
a.
size() == 0empty() == true
Section: 26.7.6 [alg.fill], 26.7.7 [alg.generate] Status: C++11 Submitter: Daniel Krügler Opened: 2008-07-13 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
In regard to library defect 488(i) I found some more algorithms which
unnecessarily throw away information. These are typically algorithms,
which sequentially write into an OutputIterator
, but do not return the
final value of this output iterator. These cases are:
template<class OutputIterator, class Size, class T> void fill_n(OutputIterator first, Size n, const T& value);
template<class OutputIterator, class Size, class Generator> void generate_n(OutputIterator first, Size n, Generator gen);
In both cases the minimum requirements on the iterator are
OutputIterator
, which means according to the requirements of
24.3.5.4 [output.iterators] p. 2 that only single-pass iterations are guaranteed.
So, if users of fill_n
and generate_n
have only an OutputIterator
available, they have no chance to continue pushing further values
into it, which seems to be a severe limitation to me.
[ Post Summit Daniel "conceptualized" the wording. ]
[ Batavia (2009-05): ]
Alisdair likes the idea, but has concerns about the specific wording about the returns clauses.
Alan notes this is a feature request.
Bill notes we have made similar changes to other algorithms.
Move to Open.
[ 2009-07 Frankfurt ]
We have a consensus for moving forward on this issue, but Daniel needs to deconceptify it.
[ 2009-07-25 Daniel provided non-concepts wording. ]
[ 2009-10 Santa Cruz: ]
Moved to Ready.
Proposed resolution:
Replace the current declaration of fill_n
in 26 [algorithms]/2, header
<algorithm>
synopsis and in 26.7.6 [alg.fill] by
template<class OutputIterator, class Size, class T>voidOutputIterator fill_n(OutputIterator first, Size n, const T& value);
Just after the effects clause add a new returns clause saying:
Returns: For
fill_n
and positiven
, returnsfirst + n
. Otherwise returnsfirst
forfill_n
.
Replace the current declaration of generate_n
in 26 [algorithms]/2,
header <algorithm>
synopsis and in 26.7.7 [alg.generate] by
template<class OutputIterator, class Size, class Generator>voidOutputIterator generate_n(OutputIterator first, Size n, Generator gen);
Just after the effects clause add a new returns clause saying:
For
generate_n
and positiven
, returnsfirst + n
. Otherwise returnsfirst
forgenerate_n
.
Section: 26.11 [specialized.algorithms], 20.3.2.2.7 [util.smartptr.shared.create] Status: C++11 Submitter: Alberto Ganesh Barbati Opened: 2008-07-14 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
LWG issue 402(i) replaced "new
" with "::new
" in the placement
new-expression in 20.2.10.2 [allocator.members]. I believe the rationale
given in 402(i) applies also to the following other contexts:
in 26.11 [specialized.algorithms], all four algorithms unitialized_copy
,
unitialized_copy_n
, unitialized_fill
and unitialized_fill_n
use
the unqualified placement new-expression in some variation of the form:
new (static_cast<void*>(&*result)) typename iterator_traits<ForwardIterator>::value_type(*first);
in 20.3.2.2.7 [util.smartptr.shared.create] there is a reference to the unqualified placement new-expression:
new (pv) T(std::forward<Args>(args)...),
I suggest to add qualification in all those places. As far as I know, these are all the remaining places in the whole library that explicitly use a placement new-expression. Should other uses come out, they should be qualified as well.
As an aside, a qualified placement new-expression does not need
additional requirements to be compiled in a constrained context. By
adding qualification, the HasPlacementNew
concept introduced recently in
N2677 (Foundational Concepts)
would no longer be needed by library and
should therefore be removed.
[ San Francisco: ]
Detlef: If we move this to Ready, it's likely that we'll forget about the side comment about the
HasPlacementNew
concept.
[ post San Francisco: ]
Daniel:
HasPlacementNew
has been removed from N2774 (Foundational Concepts).
Proposed resolution:
Replace "new
" with "::new
" in:
Section: 23 [containers] Status: C++11 Submitter: Alberto Ganesh Barbati Opened: 2008-07-22 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
The term "default constructed" is often used in wording that predates
the introduction of the concept of value-initialization. In a few such
places the concept of value-initialization is more correct than the
current wording (for example when the type involved can be a built-in)
so a replacement is in order. Two of such places are already covered by
issue 867(i). This issue deliberately addresses the hopefully
non-controversial changes in the attempt of being approved more quickly.
A few other occurrences (for example in std::tuple
,
std::reverse_iterator
and std::move_iterator
) are left to separate
issues. For std::reverse_iterator
, see also issue 408(i). This issue is
related with issue 724(i).
[ San Francisco: ]
The list provided in the proposed resolution is not complete. James Dennett will review the library and provide a complete list and will double-check the vocabulary.
[ 2009-07 Frankfurt ]
The proposed resolution is incomplete.
Move to Tentatively NAD Future. Howard will contact Ganesh for wording. If wording is forthcoming, Howard will move it back to Review.
[ 2009-07-18 Ganesh updated the proposed wording. ]
Howard: Moved back to Review. Note that 16.4.4.2 [utility.arg.requirements] refers to a section that is not in the current working paper, but does refer to a section that we expect to reappear after the de-concepts merge. This was a point of confusion we did not recognize when we reviewed this issue in Frankfurt.
Howard: Ganesh also includes a survey of places in the WP surveyed for changes of this nature and purposefully not treated:
Places where changes are not being proposed
In the following paragraphs, we are not proposing changes because it's not clear whether we actually prefer value-initialization over default-initialization (now partially covered by 1012(i)):
20.3.1.3.2 [unique.ptr.single.ctor] para 3 e 7
24.5.1.4 [reverse.iter.cons] para 1
[move.iter.op.const] para 1
In the following paragraphs, the expression "default constructed" need not be changed, because the relevant type does not depend on a template parameter and has a user-provided constructor:
[func.referenceclosure.invoke] para 12, type: reference_closure
32.4.3.3 [thread.thread.constr] para 30, type: thread
32.4.3.6 [thread.thread.member] para 52, type: thread_id
32.4.5 [thread.thread.this], para 1, type: thread_id
[ 2009-08-18 Daniel adds: ]
I have no objections against the currently suggested changes, but I also cross-checked with the list regarding intentionally excluded changes, and from this I miss the discussion of
27.4.3.2 [string.require] p. 2:
"[..] The
Allocator
object used shall be a copy of theAllocator>
object passed to thebasic_string
object's constructor or, if the constructor does not take anAllocator
argument, a copy of a default-constructedAllocator
object."N2723, 29.5.3.4 [rand.req.eng], Table 109, expression "
T()
":Pre-/post-condition: "Creates an engine with the same initial state as all other default-constructed engines of type
X
."as well as in 29.5.6 [rand.predef]/1-9 (N2914), 29.5.8.1 [rand.util.seedseq]/3, 31.7.5.2.2 [istream.cons]/3, 31.7.6.2.2 [ostream.cons]/9 (N2914), 28.6.12 [re.grammar]/2, 32.4.3.5 [thread.thread.assign]/1 (N2914),
[ Candidates for the "the expression "default constructed" need not be changed" list ]
I'm fine, if these would be added to the intentionally exclusion list, but mentioning them makes it easier for other potential reviewers to decide on the relevance or not-relevance of them for this issue.
I suggest to remove the reference of [func.referenceclosure.invoke] in the "it's not clear" list, because this component does no longer exist.
I also suggest to add a short comment that all paragraphs in the resolution whether they refer to N2723 or to N2914 numbering, because e.g. "Change 23.3.5.2 [deque.cons] para 5" is an N2723 coordinate, while "Change 23.3.5.3 [deque.capacity] para 1" is an N2914 coordinate. Even better would be to use one default document for the numbering (probably N2914) and mention special cases (e.g. "Change 16.4.4.2 [utility.arg.requirements] para 2" as referring to N2723 numbering).
[ 2009-08-18 Alisdair adds: ]
I strongly believe the term "default constructed" should not appear in the library clauses unless we very clearly define a meaning for it, and I am not sure what that would be.
In those cases where we do not want to replace "default constructed" with "vale initialized" we should be using "default initialized". If we have a term that could mean either, we reduce portability of programs.
I have not done an exhaustive review to clarify if that is a vendor freedom we have reason to support (e.g. value-init in debug, default-init in release) so I may yet be convinced that LWG has reason to define this new term of art, but generally C++ initialization is confusing enough without supporting further ill-defined terms.
[ 2009-10 Santa Cruz: ]
Move to Ready.
[ 2010 Pittsburgh: ]
Moved to review in order to enable conflict resolution with 704(i).
[ 2010-03-26 Daniel harmonized the wording with the upcoming FCD. ]
[ 2010 Rapperswil: ]
Move to Ready.
[ Adopted at 2010-11 Batavia ]
Proposed resolution:
Change 16.4.4.2 [utility.arg.requirements] para 2:
2 In general, a default constructor is not required. Certain container class member function signatures specify
the default constructorT()
as a default argument.T()
shall be a well-defined expression (8.5) if one of those signatures is called using the default argument (8.3.6).
Change 23.3.5.2 [deque.cons] para 3:
3 Effects: Constructs a deque with
n
default constructedvalue-initialized elements.
Change 23.3.5.3 [deque.capacity] para 1:
1 Effects: If
sz < size()
, equivalent toerase(begin() + sz, end());
. Ifsize() < sz
, appendssz - size()
default constructedvalue-initialized elements to the sequence.
Change [forwardlist.cons] para 3:
3 Effects: Constructs a
forward_list
object withn
default constructedvalue-initialized elements.
Change [forwardlist.modifiers] para 22:
22 Effects: [...] For the first signature the inserted elements are
default constructedvalue-initialized, and for the second signature they are copies ofc
.
Change 23.3.11.2 [list.cons] para 3:
3 Effects: Constructs a
list
withn
default constructedvalue-initialized elements.
Change 23.3.11.3 [list.capacity] para 1:
1 Effects: If
sz < size()
, equivalent tolist<T>::iterator it = begin(); advance(it, sz); erase(it, end());
. Ifsize() < sz
,appends sz - size()
default constructedvalue-initialized elements to the sequence.
Change 23.3.13.2 [vector.cons] para 3:
3 Effects: Constructs a
vector
withn
default constructedvalue-initialized elements.
Change 23.3.13.3 [vector.capacity] para 9:
9 Effects: If
sz < size()
, equivalent toerase(begin() + sz, end());
. Ifsize() < sz
, appendssz - size()
default constructedvalue-initialized elements to the sequence.
Section: 23.2.8 [unord.req] Status: C++11 Submitter: Sohail Somani Opened: 2008-07-22 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
Is there any language in the current draft specifying the behaviour of the following snippet?
unordered_set<int> s; unordered_set<int>::local_iterator it = s.end(0); // Iterate past end - the unspecified part it++;
I don't think there is anything about s.end(n)
being considered an
iterator for the past-the-end value though (I think) it should be.
[ San Francisco: ]
We believe that this is not a substantive change, but the proposed change to the wording is clearer than what we have now.
[ Post Summit: ]
Recommend Tentatively Ready.
Proposed resolution:
Change Table 97 "Unordered associative container requirements" in 23.2.8 [unord.req]:
Table 97: Unordered associative container requirements expression return type assertion/note pre/post-condition complexity b.begin(n)
local_iterator
const_local_iterator
for constb
.Pre: n shall be in the range [0,b.bucket_count()). Note: [b.begin(n), b.end(n)) is a valid range containing all of the elements in the nth bucket.b.begin(n)
returns an iterator referring to the first element in the bucket. If the bucket is empty, thenb.begin(n) == b.end(n)
.Constant b.end(n)
local_iterator
const_local_iterator
for constb
.Pre: n shall be in the range [0, b.bucket_count())
.b.end(n)
returns an iterator which is the past-the-end value for the bucket.Constant
Section: 23.2.8 [unord.req] Status: C++11 Submitter: Daniel Krügler Opened: 2008-08-17 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
Good ol' associative containers allow both function pointers and function objects as feasible comparators, as described in 23.2.7 [associative.reqmts]/2:
Each associative container is parameterized on
Key
and an ordering relationCompare
that induces a strict weak ordering (25.3) on elements of Key. [..]. The object of typeCompare
is called the comparison object of a container. This comparison object may be a pointer to function or an object of a type with an appropriate function call operator.[..]
The corresponding wording for unordered containers is not so clear, but I read it to disallow function pointers for the hasher and I miss a clear statement for the equality predicate, see 23.2.8 [unord.req]/3+4+5:
Each unordered associative container is parameterized by
Key
, by a function objectHash
that acts as a hash function for values of typeKey
, and by a binary predicatePred
that induces an equivalence relation on values of typeKey
.[..]A hash function is a function object that takes a single argument of type
Key
and returns a value of typestd::size_t
.Two values
k1
andk2
of typeKey
are considered equal if the container's equality function object returnstrue
when passed those values.[..]
and table 97 says in the column "assertion...post-condition" for the
expression X::hasher
:
Hash
shall be a unary function object type such that the expressionhf(k)
has typestd::size_t
.
Note that 22.10 [function.objects]/1 defines as "Function objects are
objects with an operator()
defined.[..]"
Does this restriction exist by design or is it an oversight? If an oversight, I suggest that to apply the following
[ 2009-07-28 Reopened by Alisdair. No longer solved by concepts. ]
[ 2009-10 Santa Cruz: ]
Ask Daniel to provide proposed wording that: makes it explicit that function pointers are function objects at the beginning of 22.10 [function.objects]; fixes the "requirements" for typedefs in 22.10.6 [refwrap] to instead state that the function objects defined in that clause have these typedefs, but not that these typedefs are requirements on function objects; remove the wording that explicitly calls out that associative container comparators may be function pointers.
[ 2009-12-19 Daniel updates wording and rationale. ]
[ 2010-02-11 Moved to Tentatively Ready after 5 positive votes on c++std-lib. ]
Rationale:
The below provided wording also affects some part of the library which is involved with callable types (22.10.3 [func.def]/3). Reason for this is that callable objects do have a lot in common with function objects. A simple formula seems to be:
callable objects = function objects + pointers to member
The latter group is excluded from function objects because of the expression-based usage of function objects in the algorithm clause, which is incompatible with the notation to dereference pointers to member without a concept map available in the language.
This analysis showed some currently existing normative definition differences between the above subset of callable objects and function objects which seem to be unintended: Backed by the Santa Cruz outcome function objects should include both function pointers and "object[s] with an operator() defined". This clearly excludes class types with a conversion function to a function pointer or all similar conversion function situations described in 12.2 [over.match]/2 b. 2. In contrast to this, the wording for callable types seems to be less constrained (22.10.3 [func.def]/3):
A callable type is a [..] class type whose objects can appear immediately to the left of a function call operator.
The rationale given in N1673 and a recent private communication with Peter Dimov revealed that the intention of this wording was to cover the above mentioned class types with conversion functions as well. To me the current wording of callable types can be read either way and I suggest to make the intention more explicit by replacing
[..] class type whose objects can appear immediately to the left of a function call operator
by
[..] class type whose objects can appear as the leftmost subexpression of a function call expression 7.6.1.3 [expr.call].
and to use the same definition for the class type part of function objects, because there is no reason to exclude class types with a conversion function to e.g. pointer to function from being used in algorithms.
Now this last term "function objects" itself brings us to a third unsatisfactory
state: The term is used both for objects (e.g. "Function objects are
objects[..]" in 22.10 [function.objects]/1) and for types (e.g. "Each
unordered associative container is parameterized [..] by a function object Hash
that acts as a hash function [..]" in 23.2.8 [unord.req]/3). This
impreciseness should be fixed and I suggest to introduce the term function
object type as the counter part to callable type. This word seems
to be a quite natural choice, because the library already uses it here and there
(e.g. "Hash shall be a unary function object type such that the expression
hf(k)
has type std::size_t
." in Table 98, "X::hasher
"
or "Requires: T
shall be a function object type [..]" in
22.10.17.3.6 [func.wrap.func.targ]/3).
Finally I would like to add that part of the issue 870 discussion related to the requirements for typedefs in 22.10.6 [refwrap] during the Santa Cruz meeting is now handled by the new issue 1290(i).
Obsolete rationale:
[ San Francisco: ]
This is fixed by N2776.
Proposed resolution:
Change 22.10 [function.objects]/1 as indicated:
1
Function objects are objects with anAn object type (6.8 [basic.types]) that can be the type of the postfix-expression in a function call (7.6.1.3 [expr.call], 12.2.2.2 [over.match.call]) is called a function object type*. A function object is an object of a function object type. In the places where one would expect to pass a pointer to a function to an algorithmic template (Clause 26 [algorithms]), the interface is specified to acceptoperator()
defined.an object with ana function object. This not only makes algorithmic templates work with pointers to functions, but also enables them to work with arbitrary function objects.operator()
defined* Such a type is either a function pointer or a class type which often has a member
operator()
, but in some cases it can omit that member and provide a conversion to a pointer to function.
Change 22.10.3 [func.def]/3 as indicated: [The intent is to make the commonality of callable types and function object types more explicit and to get rid of wording redundancies]
3 A callable type is
a pointer to function,a pointer to memberfunction, a pointer to member data,or aclass type whose objects can appear immediately to the left of a function call operatorfunction object type (22.10 [function.objects]).
Change [bind]/1 as indicated:
1 The function template
bind
returns an object that binds afunctioncallable object passed as an argument to additional arguments.
Change 22.10.15 [func.bind]/1 as indicated:
1 This subclause describes a uniform mechanism for binding arguments of
functioncallable objects.
Change 22.10.17 [func.wrap]/1 as indicated:
1 This subclause describes a polymorphic wrapper class that encapsulates arbitrary
functioncallable objects.
Change 22.10.17.3 [func.wrap.func]/2 as indicated [The reason for this
change is that 22.10.17.3 [func.wrap.func]/1 clearly says that all callable
types may be wrapped by std::function
and current implementations
indeed do provide support for pointer to members as well. One further suggested
improvement is to set the below definition of Callable in italics]:
2 A
functioncallable objectf
of typeF
isCallableCallable for argument typesT1, T2, ..., TN
inArgTypes
andareturn typeR
,if, given lvaluesthe expressiont1, t2, ..., tN
of typesT1, T2, ..., TN
, respectively,INVOKE(f, declval<ArgTypes>()..., R
, considered as an unevaluated operand (7 [expr]), is well formed (20.7.2)t1, t2, ..., tN)and, if.R
is notvoid
, convertible toR
Change 22.10.17.3.2 [func.wrap.func.con]/7 as indicated:
function(const function& f); template <class A> function(allocator_arg_t, const A& a, const function& f);...
7 Throws: shall not throw exceptions if
f
's target is a function pointer or afunctioncallable object passed viareference_wrapper
. Otherwise, may throwbad_alloc
or any exception thrown by the copy constructor of the storedfunctioncallable object. [Note: Implementations are encouraged to avoid the use of dynamically allocated memory for smallfunctioncallable objects, e.g., wheref
's target is an object holding only a pointer or reference to an object and a member function pointer. — end note]
Change 22.10.17.3.2 [func.wrap.func.con]/11 as indicated:
template<class F> function(F f); template <class F, class A> function(allocator_arg_t, const A& a, F f);...
11 [..] [Note: implementations are encouraged to avoid the use of dynamically allocated memory for small
functioncallable objects, for example, wheref
's target is an object holding only a pointer or reference to an object and a member function pointer. — end note]
Change 22.10.17.3.5 [func.wrap.func.inv]/3 as indicated:
R operator()(ArgTypes... args) const...
3 Throws:
bad_function_call
if!*this
; otherwise, any exception thrown by the wrappedfunctioncallable object.
Change 22.10.17.3.6 [func.wrap.func.targ]/3 as indicated:
template<typename T> T* target(); template<typename T> const T* target() const;...
3 Requires:
T
shall be afunction objecttype that is Callable (22.10.17.3 [func.wrap.func]) for parameter typesArgTypes
and return typeR
.
Change 23.2.7 [associative.reqmts]/2 as indicated: [The suggested
removal seems harmless, because 26.8 [alg.sorting]1 already clarifies
that Compare
is a function object type. Nevertheless it is recommended,
because the explicit naming of "pointer to function" is misleading]
2 Each associative container is parameterized on
Key
and an ordering relationCompare
that induces a strict weak ordering (26.8 [alg.sorting]) on elements ofKey
. In addition,map
andmultimap
associate an arbitrary typeT
with theKey
. The object of typeCompare
is called the comparison object of a container.This comparison object may be a pointer to function or an object of a type with an appropriate function call operator.
Change 23.2.8 [unord.req]/3 as indicated:
3 Each unordered associative container is parameterized by
Key
, by a function object typeHash
that acts as a hash function for values of typeKey
, and by a binary predicatePred
that induces an equivalence relation on values of type Key. [..]
Change 26.1 [algorithms.general]/7 as indicated: [The intent is to bring this part in sync with 22.10 [function.objects]]
7 The
Predicate
parameter is used whenever an algorithm expects a function object (22.10 [function.objects]) that when applied to the result of dereferencing the corresponding iterator returns a value testable astrue
. In other words, if an algorithm takesPredicate pred
as its argument andfirst
as its iterator argument, it should work correctly in the constructif (pred(*first)){...}
. The function objectpred
shall not apply any nonconstant function through the dereferenced iterator.This function object may be a pointer to function, or an object of a type with an appropriate function call operator.
Change 20.3.1.3 [unique.ptr.single]/1 as indicated:
1 The default type for the template parameter
D
isdefault_delete
. A client-supplied template argumentD
shall be a functionpointer or functorobject type for which, given a valued
of typeD
and a pointerptr
of typeT*
, the expressiond(ptr)
is valid and has the effect of deallocating the pointer as appropriate for that deleter.D
may also be an lvalue-reference to a deleter.
T
are too strongSection: 26.10.13 [numeric.iota] Status: C++11 Submitter: Daniel Krügler Opened: 2008-08-20 Last modified: 2016-01-28
Priority: Not Prioritized
View all issues with C++11 status.
Discussion:
According to the recent WP
N2691,
26.10.13 [numeric.iota]/1, the requires clause
of std::iota
says:
T
shall meet the requirements ofCopyConstructible
andAssignable
types, and shall be convertible toForwardIterator
's value type.[..]
Neither CopyConstructible
nor Assignable
is needed, instead MoveConstructible
seems to be the correct choice. I guess the current wording resulted as an
artifact from comparing it with similar numerical algorithms like accumulate
.
Note: If this function will be conceptualized, the here proposed
MoveConstructible
requirement can be removed, because this
is an implied requirement of function arguments, see
N2710/[temp.req.impl]/3, last bullet.
[ post San Francisco: ]
Issue pulled by author prior to review.
[ 2009-07-30 Daniel reopened: ]
with the absence of concepts, this issue (closed) is valid again and I suggest to reopen it. I also revised by proposed resolution based on N2723 wording:
[ 2009-10 Santa Cruz: ]
Change 'convertible' to 'assignable', Move To Ready.
Proposed resolution:
Change the first sentence of 26.10.13 [numeric.iota]/1:
Requires:
T
shallmeet the requirements ofbe assignable toCopyConstructible
andAssignable
types, and shallForwardIterator
's value type. [..]
move_iterator::operator[]
has wrong return typeSection: 24.5.4.6 [move.iter.elem] Status: C++11 Submitter: Doug Gregor Opened: 2008-08-21 Last modified: 2021-06-06
Priority: Not Prioritized
View all other issues in [move.iter.elem].
View all issues with C++11 status.
Discussion:
move_iterator
's operator[]
is declared as:
reference operator[](difference_type n) const;
This has the same problem that reverse_iterator
's operator[]
used to
have: if the underlying iterator's operator[]
returns a proxy, the
implicit conversion to value_type&&
could end up referencing a temporary
that has already been destroyed. This is essentially the same issue that
we dealt with for reverse_iterator
in DR 386(i).
[ 2009-07-28 Reopened by Alisdair. No longer solved by concepts. ]
[ 2009-08-15 Howard adds: ]
I recommend closing this as a duplicate of 1051(i) which addresses this issue for both
move_iterator
andreverse_iterator
.
[ 2009-10 Santa Cruz: ]
Move to Ready. Note that if 1051(i) is reopened, it may yield a better resolution, but 1051(i) is currently marked NAD.
Proposed resolution:
In 24.5.4.2 [move.iterator] and [move.iter.op.index], change the declaration of
move_iterator
's operator[]
to:
referenceunspecified operator[](difference_type n) const;
Rationale:
[ San Francisco: ]
NAD Editorial, see N2777.
initializer_list
constructor for discrete_distribution
Section: 29.5.9.6.1 [rand.dist.samp.discrete] Status: Resolved Submitter: Daniel Krügler Opened: 2008-08-22 Last modified: 2016-01-28
Priority: Not Prioritized
View all other issues in [rand.dist.samp.discrete].
View all issues with Resolved status.
Discussion:
During the Sophia Antipolis meeting it was decided to separate from 793(i) a
subrequest that adds initializer list support to discrete_distribution
, specifically,
the issue proposed to add a c'tor taking a initializer_list<double>
.
Proposed resolution:
In 29.5.9.6.1 [rand.dist.samp.discrete] p. 1, class discrete_distribution
,
just before the member declaration
explicit discrete_distribution(const param_type& parm);
insert
discrete_distribution(initializer_list<double> wl);
Between p.4 and p.5 of the same section insert a new paragraph as part of the new member description:
discrete_distribution(initializer_list<double> wl);Effects: Same as
discrete_distribution(wl.begin(), wl.end())
.
Rationale:
Addressed by N2836 "Wording Tweaks for Concept-enabled Random Number Generation in C++0X".
initializer_list
constructor for piecewise_constant_distribution
Section: 29.5.9.6.2 [rand.dist.samp.pconst] Status: Resolved Submitter: Daniel Krügler Opened: 2008-08-22 Last modified: 2016-01-28
Priority: Not Prioritized
View all other issues in [rand.dist.samp.pconst].
View all issues with Resolved status.
Discussion:
During the Sophia Antipolis meeting it was decided to separate from
794(i) a subrequest that adds initializer list support to
piecewise_constant_distribution
, specifically, the issue proposed
to add a c'tor taking a initializer_list<double>
and a Callable
to evaluate
weight values. For consistency with the remainder of this class and
the remainder of the initializer_list
-aware library the author decided to
change the list argument type to the template parameter RealType
instead. For the reasoning to use Func
instead of Func&&
as c'tor
function argument see issue 793(i).
Proposed resolution:
Non-concept version of the proposed resolution
In 29.5.9.6.2 [rand.dist.samp.pconst]/1, class piecewise_constant_distribution
,
just before the member declaration
explicit piecewise_constant_distribution(const param_type& parm);
insert
template<typename Func> piecewise_constant_distribution(initializer_list<RealType> bl, Func fw);
Between p.4 and p.5 of the same section insert a series of new paragraphs nominated below as [p5_1], [p5_2], and [p5_3] as part of the new member description:
template<typename Func> piecewise_constant_distribution(initializer_list<RealType> bl, Func fw);[p5_1] Complexity: Exactly
nf = max(bl.size(), 1) - 1
invocations offw
.[p5_2] Requires:
fw
shall be callable with one argument of typeRealType
, and shall return values of a type convertible todouble
;- The relation
0 < S = w0+. . .+wn-1
shall hold. For all sampled valuesxk
defined below,fw(xk)
shall return a weight valuewk
that is non-negative, non-NaN, and non-infinity;- If
nf > 0
letbk = *(bl.begin() + k), k = 0, . . . , bl.size()-1
and the following relations shall hold fork = 0, . . . , nf-1: bk < bk+1
.[p5_3] Effects:
If
nf == 0
,
- lets the sequence
w
have lengthn = 1
and consist of the single valuew0 = 1
, and- lets the sequence
b
have lengthn+1
withb0 = 0
andb1 = 1
.Otherwise,
- sets
n = nf
, and[bl.begin(), bl.end())
shall form the sequenceb
of lengthn+1
, andlets the sequences
w
have lengthn
and for eachk = 0, . . . ,n-1
, calculates:xk = 0.5*(bk+1 + bk) wk = fw(xk)
Constructs a
piecewise_constant_distribution
object with the above computed sequenceb
as the interval boundaries and with the probability densities:ρk = wk/(S * (bk+1 - bk)) for k = 0, . . . , n-1.
Concept version of the proposed resolution
In 29.5.9.6.2 [rand.dist.samp.pconst]/1, class piecewise_constant_distribution
,
just before the member declaration
explicit piecewise_constant_distribution(const param_type& parm);
insert
template<Callable<auto, RealType> Func> requires Convertible<Func::result_type, double> piecewise_constant_distribution(initializer_list<RealType> bl, Func fw);
Between p.4 and p.5 of the same section insert a series of new paragraphs nominated below as [p5_1], [p5_2], and [p5_3] as part of the new member description:
template<Callable<auto, RealType> Func> requires Convertible<Func::result_type, double> piecewise_constant_distribution(initializer_list<RealType> bl, Func fw);[p5_1] Complexity: Exactly
nf = max(bl.size(), 1) - 1
invocations offw
.[p5_2] Requires:
- The relation
0 < S = w0+. . .+wn-1
shall hold. For all sampled valuesxk
defined below,fw(xk)
shall return a weight valuewk
that is non-negative, non-NaN, and non-infinity;- If
nf > 0
letbk = *(bl.begin() + k), k = 0, . . . , bl.size()-1
and the following relations shall hold fork = 0, . . . , nf-1: bk < bk+1
.[p5_3] Effects:
If
nf == 0
,
- lets the sequence
w
have lengthn = 1
and consist of the single valuew0 = 1
, and- lets the sequence
b
have lengthn+1
withb0 = 0
andb1 = 1
.Otherwise,
- sets
n = nf
, and[bl.begin(), bl.end())
shall form the sequenceb
of lengthn+1
, andlets the sequences
w
have lengthn
and for eachk = 0, . . . ,n-1
, calculates:xk = 0.5*(bk+1 + bk) wk = fw(xk)
Constructs a
piecewise_constant_distribution
object with the above computed sequenceb
as the interval boundaries and with the probability densities:ρk = wk/(S * (bk+1 - bk)) for k = 0, . . . , n-1.
Rationale:
Addressed by N2836 "Wording Tweaks for Concept-enabled Random Number Generation in C++0X".
basic_string
access operations should give stronger guaranteesSection: 27.4.3 [basic.string] Status: C++11 Submitter: Daniel Krügler Opened: 2008-08-22 Last modified: 2016-01-28
Priority: Not Prioritized
View other active issues in [basic.string].
View all other issues in [basic.string].
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Discussion:
During the Sophia Antipolis meeting it was decided to split-off some
parts of the
n2647
("Concurrency modifications for basic_string
")
proposal into a separate issue, because these weren't actually
concurrency-related. The here proposed changes refer to the recent
update document
n2668
and attempt to take advantage of the
stricter structural requirements.
Indeed there exists some leeway for more guarantees that would be very useful for programmers, especially if interaction with transactionary or exception-unaware C API code is important. This would also allow compilers to take advantage of more performance optimizations, because more functions can have throw() specifications. This proposal uses the form of "Throws: Nothing" clauses to reach the same effect, because there already exists a different issue in progress to clean-up the current existing "schizophrenia" of the standard in this regard.
Due to earlier support for copy-on-write, we find the following unnecessary limitations for C++0x:
data()
and c_str()
simply return
a pointer to their guts, which is a non-failure operation. This should
be spelled out. It is also noteworthy to mention that the same
guarantees should also be given by the size query functions,
because the combination of pointer to content and the length is
typically needed during interaction with low-level API.
data()
and c_str()
simply return
a pointer to their guts, which is guaranteed O(1). This should be
spelled out.
operator[]
allows reading access to the terminator
char. For more intuitive usage of strings, reading access to this
position should be extended to the non-const case. In contrast
to C++03 this reading access should now be homogeneously
an lvalue access.
The proposed resolution is split into a main part (A) and a secondary part (B) (earlier called "Adjunct Adjunct Proposal"). (B) extends (A) by also making access to index position size() of the at() overloads a no-throw operation. This was separated, because this part is theoretically observable in specifically designed test programs.
[ San Francisco: ]
We oppose part 1 of the issue but hope to address
size()
in issue 877(i).We do not support part B. 4 of the issue because of the breaking API change.
We support part A. 2 of the issue.
On support part A. 3 of the issue:
Pete's broader comment: now that we know that
basic_string
will be a block of contiguous memory, we should just rewrite its specification with that in mind. The expression of the specification will be simpler and probably more correct as a result.
[ 2009-07 Frankfurt ]
Move proposed resolution A to Ready.
[ Howard: Commented out part B. ]
Proposed resolution:
In 27.4.3.5 [string.capacity], just after p. 1 add a new paragraph:
Throws: Nothing.
In 27.4.3.6 [string.access] replace p. 1 by the following 4 paragraghs:
Requires:
pos ≤ size()
.Returns: If
pos < size()
, returns*(begin() + pos)
. Otherwise, returns a reference to acharT()
that shall not be modified.Throws: Nothing.
Complexity: Constant time.
In 27.4.3.8.1 [string.accessors] replace the now common returns
clause of c_str()
and data()
by the following three paragraphs:
Returns: A pointer
p
such thatp+i == &operator[](i)
for eachi
in[0, size()]
.Throws: Nothing.
Complexity: Constant time.
forward_list
preconditionsSection: 23.3.7 [forward.list] Status: C++11 Submitter: Martin Sebor Opened: 2008-08-23 Last modified: 2023-02-07
Priority: Not Prioritized
View all other issues in [forward.list].
View all issues with C++11 status.
Discussion:
forward_list
member functions that take
a forward_list::iterator
(denoted position
in the
function signatures) argument have the following precondition:
Requires:
position
is dereferenceable or equal tobefore_begin()
.
I believe what's actually intended is this:
Requires:
position
is in the range [before_begin()
,end()
).
That is, when it's dereferenceable, position
must point
into *this
, not just any forward_list
object.
[ San Francisco: ]
Robert suggested alternate proposed wording which had large support.
[ Post Summit: ]
Walter: "position is before_begin() or a dereferenceable": add "is" after the "or"
With that minor update, Recommend Tentatively Ready.
Proposed resolution:
Change the Requires clauses [forwardlist], p21, p24, p26, p29, and, [forwardlist.ops], p39, p43, p47 as follows:
Requires:
position
isbefore_begin()
or is a dereferenceable iterator in the range[begin(), end())
or equal to. ...before_begin()
Section: 32.5 [atomics] Status: Resolved Submitter: Lawrence Crowl Opened: 2008-08-24 Last modified: 2016-01-28
Priority: Not Prioritized
View all other issues in [atomics].
View all issues with Resolved status.
Duplicate of: 942
Discussion:
The atomic_exchange
and atomic_exchange_explicit
functions seem to
be inconsistently missing parameters.
[ Post Summit: ]
Lawrence: Need to write up a list for Pete with details.
Detlef: Should not be New, we already talked about in Concurrency group.
Recommend Open.
[ 2009-07 Frankfurt ]
Lawrence will handle all issues relating to atomics in a single paper.
LWG will defer discussion on atomics until that paper appears.
Move to Open.
[ 2009-08-17 Handled by N2925. ]
[ 2009-10 Santa Cruz: ]
NAD EditorialResolved. Solved by N2992.
Proposed resolution:
Add the appropriate parameters. For example,
bool atomic_exchange(volatile atomic_bool*, bool); bool atomic_exchange_explicit(volatile atomic_bool*, bool, memory_order);
shared_ptr
conversion issueSection: 20.3.2.2.2 [util.smartptr.shared.const] Status: C++11 Submitter: Peter Dimov Opened: 2008-08-30 Last modified: 2016-01-28
Priority: Not Prioritized
View other active issues in [util.smartptr.shared.const].
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Discussion:
We've changed shared_ptr<Y>
to not convert to shared_ptr<T>
when Y*
doesn't convert to T*
by resolving issue 687(i). This only fixed the
converting copy constructor though.
N2351
later added move support, and
the converting move constructor is not constrained.
[ San Francisco: ]
We might be able to move this to NAD, Editorial once shared_ptr is conceptualized, but we want to revisit this issue to make sure.
[ 2009-07 Frankfurt ]
Moved to Ready.
This issue now represents the favored format for specifying constrained templates.
Proposed resolution:
We need to change the Requires clause of the move constructor:
shared_ptr(shared_ptr&& r); template<class Y> shared_ptr(shared_ptr<Y>&& r);
RequiresRemarks:For the second constructorThe second constructor shall not participate in overload resolution unlessY*
shall be convertible toT*
.Y*
is convertible toT*
.
in order to actually make the example in 687(i) compile (it now resolves to the move constructor).
duration
non-member arithmetic requirementsSection: 30.5.6 [time.duration.nonmember] Status: CD1 Submitter: Howard Hinnant Opened: 2008-09-08 Last modified: 2016-01-28
Priority: Not Prioritized
View all other issues in [time.duration.nonmember].
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Discussion:
N2661
specified the following requirements for the non-member duration
arithmetic:
template <class Rep1, class Period, class Rep2> duration<typename common_type<Rep1, Rep2>::type, Period> operator*(const duration<Rep1, Period>& d, const Rep2& s);Requires: Let
CR
represent thecommon_type
ofRep1
andRep2
. BothRep1
andRep2
shall be implicitly convertible to CR, diagnostic required.template <class Rep1, class Period, class Rep2> duration<typename common_type<Rep1, Rep2>::type, Period> operator*(const Rep1& s, const duration<Rep2, Period>& d);Requires: Let
CR
represent thecommon_type
ofRep1
andRep2
. BothRep1
andRep2
shall be implicitly convertible toCR
, diagnostic required.template <class Rep1, class Period, class Rep2> duration<typename common_type<Rep1, Rep2>::type, Period> operator/(const duration<Rep1, Period>& d, const Rep2& s);Requires: Let
CR
represent thecommon_type
ofRep1
andRep2
. BothRep1
andRep2
shall be implicitly convertible toCR
, andRep2
shall not be an instantiation ofduration
, diagnostic required.
During transcription into the working paper, the requirements clauses on these three functions was changed to:
Requires:
CR(Rep1, Rep2)
shall exist. Diagnostic required.
This is a non editorial change with respect to
N2661
as user written representations which are used in duration
need not be
implicitly convertible to or from arithmetic types in order to interoperate with
duration
s based on arithmetic types. An explicit conversion will do
fine for most expressions as long as there exists a common_type
specialization
relating the user written representation and the arithmetic type. For example:
class saturate { public: explicit saturate(long long i); ... }; namespace std { template <> struct common_type<saturate, long long> { typedef saturate type; }; template <> struct common_type<long long, saturate> { typedef saturate type; }; } // std millisecond ms(3); // integral-based duration duration<saturate, milli> my_ms = ms; // ok, even with explicit conversions my_ms = my_ms + ms; // ok, even with explicit conversions
However, when dealing with multiplication of a duration and its representation,
implicit convertibility is required between the rhs and the lhs's representation
for the member *=
operator:
template <class Rep, class Period = ratio<1>> class duration { public: ... duration& operator*=(const rep& rhs); ... }; ... ms *= 2; // ok, 2 is implicitly convertible to long long my_ms *= saturate(2); // ok, rhs is lhs's representation my_ms *= 2; // error, 2 is not implicitly convertible to saturate
The last line does not (and should not) compile. And we want non-member multiplication to have the same behavior as member arithmetic:
my_ms = my_ms * saturate(2); // ok, rhs is lhs's representation my_ms = my_ms * 2; // should be error, 2 is not implicitly convertible to saturate
The requirements clauses of N2661 make the last line an error as expected. However the latest working draft at this time (N2723) allows the last line to compile.
All that being said, there does appear to be an error in these requirements clauses as specified by N2661.
Requires: ... Both
Rep1
andRep2
shall be implicitly convertible to CR, diagnostic required.
It is not necessary for both Rep
s to be implicitly convertible to
the CR
. It is only necessary for the rhs Rep
to be implicitly
convertible to the CR
. The Rep
within the duration
should be allowed to only be explicitly convertible to the CR
. The
explicit-conversion-requirement is covered under 30.5.8 [time.duration.cast].
Proposed resolution:
Change the requirements clauses under 30.5.6 [time.duration.nonmember]:
template <class Rep1, class Period, class Rep2> duration<typename common_type<Rep1, Rep2>::type, Period> operator*(const duration<Rep1, Period>& d, const Rep2& s);Requires:
CR(Rep1, Rep2)
shall exist.Rep2
shall be implicitly convertible toCR(Rep1, Rep2)
. Diagnostic required.template <class Rep1, class Period, class Rep2> duration<typename common_type<Rep1, Rep2>::type, Period> operator*(const Rep1& s, const duration<Rep2, Period>& d);Requires
d behavior:CR(Rep1, Rep2)
shall exist.Rep1
shall be implicitly convertible toCR(Rep1, Rep2)
. Diagnostic required.template <class Rep1, class Period, class Rep2> duration<typename common_type<Rep1, Rep2>::type, Period> operator/(const duration<Rep1, Period>& d, const Rep2& s);Requires:
CR(Rep1, Rep2)
shall existRep2
shall be implicitly convertible toCR(Rep1, Rep2)
andRep2
shall not be an instantiation ofduration
. Diagnostic required.
Section: 23 [containers] Status: C++11 Submitter: Alisdair Meredith Opened: 2008-09-10 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
Note in particular that Table 90 "Container Requirements" gives
semantics of a.swap(b)
as swap(a,b)
, yet for all
containers we define swap(a,b)
to call a.swap(b)
- a
circular definition.
[ San Francisco: ]
Robert to propose a resolution along the lines of "Postcondition: "a = b, b = a" This will be a little tricky for the hash containers, since they don't have
operator==
.
[ Post Summit Anthony Williams provided proposed wording. ]
[ 2009-07 Frankfurt ]
Moved to Ready with minor edits (which have been made).
Proposed resolution:
In table 80 in section 23.2.2 [container.requirements.general],
replace the postcondition of a.swap(b)
with the following:
Table 80 -- Container requirements Expression Return type Operational semantics Assertion/note pre-/post-conidtion Complexity ... ... ... ... ... a.swap(b);
void
Exchange the contents ofswap(a,b)
a
andb
.(Note A)
Remove the reference to swap from the paragraph following the table.
Notes: the algorithms
swap()
,equal()
andlexicographical_compare()
are defined in Clause 25. ...
shared_ptr
swapSection: 20.3.2.2.5 [util.smartptr.shared.mod] Status: Resolved Submitter: Jonathan Wakely Opened: 2008-09-15 Last modified: 2016-01-28
Priority: Not Prioritized
View all issues with Resolved status.
Discussion:
#include <memory> #include <cassert> struct A { }; struct B : A { }; int main() { std::shared_ptr<A> pa(new A); std::shared_ptr<B> pb(new B); std::swap<A>(pa, pb); // N.B. no argument deduction assert( pa.get() == pb.get() ); return 0; }
Is this behaviour correct (I believe it is) and if so, is it unavoidable, or not worth worrying about?
This calls the lvalue/rvalue swap overload for shared_ptr
:
template<class T> void swap( shared_ptr<T> & a, shared_ptr<T> && b );
silently converting the second argument from shared_ptr<B>
to
shared_ptr<A>
and binding the rvalue ref to the produced temporary.
This is not, in my opinion, a shared_ptr
problem; it is a general issue
with the rvalue swap overloads. Do we want to prevent this code from
compiling? If so, how?
Perhaps we should limit rvalue args to swap to those types that would
benefit from the "swap trick". Or, since we now have shrink_to_fit()
, just
eliminate the rvalue swap overloads altogether. The original motivation
was:
vector<A> v = ...; ... swap(v, vector<A>(v));
[ Batavia (2009-05): ]
We agree with the proposed resolution. Move to
NAD EditorialResolved.
Proposed resolution:
Recommend NAD EditorialResolved, fixed by
N2844.
pair
assignmentSection: 22.3 [pairs] Status: C++11 Submitter: Alisdair Meredith Opened: 2008-09-15 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
20.2.3 pairs Missing assignemnt operator: template<class U , class V> requires CopyAssignable<T1, U> && CopyAssignable<T2, V> pair& operator=(pair<U , V> const & p );
Well, that's interesting. This assignment operator isn't in the
current working paper, either. Perhaps we deemed it acceptable to
build a temporary of type pair
from pair<U, V>
, then move-assign
from that temporary?
It sounds more like an issue waiting to be opened, unless you want to plug it now. As written we risk moving from lvalues.
[ San Francisco: ]
Would be NAD if better ctors fixed it.
[ post San Francisco: ]
Possibly NAD Editorial, solved by N2770.
[ 2009-05-25 Alisdair adds: ]
Issue 885(i) was something I reported while reviewing the library concepts documents ahead of San Francisco. The missing operator was added as part of the paper adopted at that meeting (N2770) and I can confirm this operator is present in the current working paper. I recommend NAD.
[ 2009-07 Frankfurt ]
We agree with the intent, but we need to wait for the dust to settle on concepts.
[ 2010-03-11 Stefanus provided wording. ]
[ 2010 Pittsburgh: Moved to Ready for Pittsburgh. ]
Proposed resolution:
Add the following declaration 22.3.2 [pairs.pair], before the
declaration of pair& operator=(pair&& p);
:
template<class U, class V> pair& operator=(const pair<U, V>& p);
Add the following description to 22.3.2 [pairs.pair] after paragraph 11 (before
the description of pair& operator=(pair&& p);)
:
template<class U, class V> pair& operator=(const pair<U, V>& p);Requires:
T1
shall satisfy the requirements ofCopyAssignable
fromU
.T2
shall satisfy the requirements ofCopyAssignable
fromV
.Effects: Assigns
p.first
tofirst
andp.second
tosecond
.Returns:
*this
.
tuple
constructionSection: 22.4.4.2 [tuple.cnstr] Status: C++11 Submitter: Alisdair Meredith Opened: 2008-09-15 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
22.4.4.2 [tuple.cnstr]:
Effects: Default initializes each element.
Could be clarified to state each "non-trivial" element. Otherwise we have a conflict with Core deinfition of default initialization - trivial types do not get initialized (rather than initialization having no effect)
I'm going to punt on this one, because it's not an issue that's related to concepts. I suggest bringing it to Howard's attention on the reflector.
[ San Francisco: ]
Text in draft doesn't mean anything, changing to "non-trivial" makes it meaningful.
We prefer "value initializes". Present implementations use value-initialization. Users who don't want value initialization have alternatives.
Request resolution text from Alisdair.
This issue relates to Issue 868(i) default construction and value-initialization.
[ 2009-05-04 Alisdair provided wording and adds: ]
Note: This IS a change of semantic from TR1, although one the room agreed with during the discussion. To preserve TR1 semantics, this would have been worded:
requires DefaultConstructible<Types>... tuple();-2- Effects: Default-initializes each non-trivial element.
[ 2009-07 Frankfurt ]
Move to Ready.
Proposed resolution:
Change p2 in Construction 22.4.4.2 [tuple.cnstr]:
requires DefaultConstructible<Types>... tuple();-2- Effects:
DefaultValue-initializes each element.
this_thread::yield
too strongSection: 32.4.5 [thread.thread.this] Status: C++11 Submitter: Lawrence Crowl Opened: 2008-09-15 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
I never thought I'd say this, but this_thread::yield
seems to be too
strong in specification. The issue is that some systems distinguish
between yielding to another thread in the same process and yielding
to another process. Given that the C++ standard only talks about
a single program, one can infer that the specification allows yielding
only to another thread within the same program. Posix has no
facility for that behavior. Can you please file an issue to weaken
the wording. Perhaps "Offers the operating system the opportunity
to reschedule."
[ Post Summit: ]
Recommend move to Tentatively Ready.
Proposed resolution:
Change 32.4.5 [thread.thread.this]/3:
void this_thread::yield();Effects: Offers the
operating systemimplementation the opportunity to reschedule.another thread.
thread::id
comparisonsSection: 32.4.3.2 [thread.thread.id] Status: Resolved Submitter: Lawrence Crowl Opened: 2008-09-15 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
Addresses UK 324
The thread::id
type supports the full set of comparison operators. This
is substantially more than is required for the associative containers that
justified them. Please place an issue against the threads library.
[ San Francisco: ]
Would depend on proposed extension to POSIX, or non-standard extension. What about hash? POSIX discussing op. POSIX not known to be considering support needed for hash, op.
Group expresses support for putting ids in both unordered and ordered containers.
[ post San Francisco: ]
Howard: It turns out the current working paper N2723 already has
hash<thread::id>
(22.10 [function.objects], 22.10.19 [unord.hash]). We simply overlooked it in the meeting. It is a good thing we voted in favor of it (again). :-)Recommend NAD.
[ Post Summit: ]
Recommend to close as NAD. For POSIX, see if we need to add a function to convert
pthread_t
to integer.
[ Post Summit, Alisdair adds: ]
The recommendation for LWG-889/UK-324 is NAD, already specified.
It is not clear to me that the specification is complete.
In particular, the synopsis of
<functional>
in 22.10 [function.objects] does not mentionhash< thread::id>
norhash< error_code >
, although their existence is implied by 22.10.19 [unord.hash], p1.I am fairly uncomfortable putting the declaration for the
thread_id
specialization into<functional>
asid
is a nested class insidestd::thread
, so it implies that<functional>
would require the definition of thethread
class template in order to forward declaredthread::id
and form this specialization.It seems better to me that the dependency goes the other way around (
<thread>
will more typically make use of<functional>
than vice-versa) and thehash<thread::id>
specialization be declared in the<thread>
header.I think
hash<error_code>
could go into either<system_error>
or<functional>
and have no immediate preference either way. However, it should clearly appear in the synopsis of one of these two.Recommend moving 889 back to open, and tying in a reference to UK-324.
[ Batavia (2009-05): ]
Howard observes that
thread::id
need not be a nested class; it could be atypedef
for a more visible type.
[ 2009-05-24 Alisdair adds: ]
I do not believe this is correct.
thread::id
is explicitly documents as a nested class, rather than as an unspecified typedef analogous to an iterator. If the intent is that this is not implemented as a nested class (under the as-if freedoms) then this is a novel form of standardese.
[ 2009-07 Frankfurt ]
Decided we want to move hash specialization for thread_id to the thread header. Alisdair to provide wording.
[ 2009-07-28 Alisdair provided wording, moved to Review. ]
[ 2009-10 Santa Cruz: ]
Add a strike for
hash<thread::id>
. Move to Ready
[ 2009-11-13 The proposed wording of 1182(i) is a superset of the wording in this issue. ]
[ 2010-02-09 Moved from Ready to Open: ]
Issue 1182(i) is not quite a superset of this issue and it is controversial whether or not the note:
hash template specialization allows
thread::id
objects to be used as keys in unordered containers.should be added to the WP.
[
2010-02-09 Objections to moving this to NAD EditorialResolved, addressed by 1182(i) have been removed. Set to Tentatively NAD EditorialResolved.
]
Rationale:
Proposed resolution:
Remove the following prototype from the synopsis in 22.10 [function.objects]:
template <> struct hash<std::thread::id>;
Add to 32.4 [thread.threads], p1 Header <thread>
synopsis:
template <class T> struct hash; template <> struct hash<thread::id>;
Add template specialization below class definition in 32.4.3.2 [thread.thread.id]
template <> struct hash<thread::id> : public unary_function<thread::id, size_t> { size_t operator()(thread::id val) const; };
Extend note in p2 32.4.3.2 [thread.thread.id] with second sentence:
[Note: Relational operators allow
thread::id
objects to be used as keys in associative containers.hash
template specialization allowsthread::id
objects to be used as keys in unordered containers. — end note]
Add new paragraph to end of 32.4.3.2 [thread.thread.id]
template <> struct hash<thread::id>;An explicit specialization of the class template hash (22.10.19 [unord.hash]) shall be provided for the type
thread::id
.
<system_error>
initializationSection: 19.5.3 [syserr.errcat] Status: C++11 Submitter: Beman Dawes Opened: 2008-09-14 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
The static const error_category
objects generic_category
and
system_category
in header <system_error>
are currently declared:
const error_category& get_generic_category(); const error_category& get_system_category(); static const error_category& generic_category = get_generic_category(); static const error_category& system_category = get_system_category();
This formulation has several problems:
IO streams uses a somewhat different formulation for iostream_category, but still suffer much the same problems.
The original plan was to eliminate these problems by applying the C++0x
constexpr
feature. See LWG issue 832(i). However, that approach turned out
to be unimplementable, since it would require a constexpr
object of a
class with virtual functions, and that is not allowed by the core
language.
The proposed resolution was developed as an alternative. It mitigates the above problems by removing initialization from the visible interface, allowing implementations flexibility.
Implementation experience:
Prototype implementations of the current WP interface and proposed
resolution interface were tested with recent Codegear, GCC, Intel, and Microsoft
compilers on Windows. The code generated by the Microsoft compiler was studied
at length; the WP and proposal versions generated very similar code. For both versions
the compiler did make use of static
initialization; apparently the compiler applied an implicit constexpr
where useful, even in cases where constexpr
would not be permitted by
the language!
Acknowledgements:
Martin Sebor, Chris Kohlhoff, and John Lakos provided useful ideas and comments on initialization issues.
[ San Francisco: ]
Martin: prefers not to create more file-scope static objects, and would like to see
get_*
functions instead.
[Pre-Summit:]
Beman: The proposed resolution has been reworked to remove the file-scope static objects, per Martin's suggestions. The
get_
prefix has been eliminated from the function names as no longer necessary and to conform with standard library naming practice.
[ Post Summit: ]
Agreement that this is wise and essential, text provided works and has been implemented. Seems to be widespread consensus. Move to Tentative Ready.
Proposed resolution:
Change 16.4.6.16 [value.error.codes] Value of error codes as indicated:
Certain functions in the C++ standard library report errors via a
std::error_code
(19.4.2.2) object. That object'scategory()
member shall returna reference tostd::system_category
for errors originating from the operating system, or a reference to an implementation-defined error_category object for errors originating elsewhere. The implementation shall define the possible values of value() for each of these error categories. [Example: For operating systems that are based on POSIX, implementations are encouraged to define the
()
std::system_category
values as identical to the POSIX
()
errno
values, with additional values as defined by the operating system's documentation. Implementations for operating systems that are not based on POSIX are encouraged to define values identical to the operating system's values. For errors that do not originate from the operating system, the implementation may provide enums for the associated values --end example]
Change 19.5.3.1 [syserr.errcat.overview] Class error_category
overview
error_category
synopsis as indicated:
const error_category&get_generic_category(); const error_category&get_system_category();static storage-class-specifier const error_category& generic_category = get_generic_category(); static storage-class-specifier const error_category& system_category = get_system_category();
Change 19.5.3.5 [syserr.errcat.objects] Error category objects as indicated:
const error_category&get_generic_category();Returns: A reference to an object of a type derived from class
error_category
.Remarks: The object's
default_error_condition
andequivalent
virtual functions shall behave as specified for the classerror_category
. The object'sname
virtual function shall return a pointer to the string"GENERIC"
.const error_category&get_system_category();Returns: A reference to an object of a type derived from class
error_category
.Remarks: The object's
equivalent
virtual functions shall behave as specified for classerror_category
. The object'sname
virtual function shall return a pointer to the string"system"
. The object'sdefault_error_condition
virtual function shall behave as follows:If the argument
ev
corresponds to a POSIXerrno
valueposv
, the function shall returnerror_condition(posv, generic_category())
. Otherwise, the function shall returnerror_condition(ev, system_category())
. What constitutes correspondence for any given operating system is unspecified. [Note: The number of potential system error codes is large and unbounded, and some may not correspond to any POSIXerrno
value. Thus implementations are given latitude in determining correspondence. — end note]
Change 19.5.4.2 [syserr.errcode.constructors] Class error_code
constructors
as indicated:
error_code();Effects: Constructs an object of type error_code.
Postconditions:
val_ == 0
andcat_ == &system_category
()
.
Change 19.5.4.3 [syserr.errcode.modifiers] Class error_code
modifiers as
indicated:
void clear();Postconditions:
value() == 0
andcategory() == system_category
()
.
Change 19.5.4.5 [syserr.errcode.nonmembers] Class error_code
non-member
functions as indicated:
error_code make_error_code(errc e);Returns:
error_code(static_cast<int>(e), generic_category
()
)
.
Change 19.5.5.2 [syserr.errcondition.constructors] Class error_condition
constructors as indicated:
error_condition();Effects: Constructs an object of type
error_condition
.Postconditions:
val_ == 0
andcat_ == &generic_category
()
.
Change 19.5.5.3 [syserr.errcondition.modifiers] Class error_condition
modifiers as indicated:
void clear();Postconditions:
value() == 0
andcategory() == generic_category
()
.
Change 19.5.5.5 [syserr.errcondition.nonmembers] Class error_condition
non-member functions as indicated:
error_condition make_error_condition(errc e);Returns:
error_condition(static_cast<int>(e), generic_category())
.
Change 31.5 [iostreams.base] Iostreams base classes, Header <ios>
synopsis as indicated:
concept_map ErrorCodeEnum<io_errc> { }; error_code make_error_code(io_errc e); error_condition make_error_condition(io_errc e);storage-class-specifierconst error_category& iostream_category();
Change [ios::failure] Class ios_base::failure, paragraph 2 as indicated:
When throwing
ios_base::failure
exceptions, implementations should provide values ofec
that identify the specific reason for the failure. [ Note: Errors arising from the operating system would typically be reported assystem_category
()
errors with an error value of the error number reported by the operating system. Errors arising from within the stream library would typically be reported aserror_code(io_errc::stream, iostream_category())
. — end note ]
Change 31.5.6 [error.reporting] Error reporting as indicated:
error_code make_error_code(io_errc e);Returns:
error_code(static_cast<int>(e), iostream_category
())
.error_condition make_error_condition(io_errc e);Returns:
error_condition(static_cast<int>(e), iostream_category
())
.storage-class-specifierconst error_category& iostream_category();The implementation shall initialize iostream_category. Its storage-class-specifier may be static or extern. It is unspecified whether initialization is static or dynamic (3.6.2). If initialization is dynamic, it shall occur before completion of the dynamic initialization of the first translation unit dynamically initialized that includes header <system_error>.
Returns: A reference to an object of a type derived from class
error_category
.Remarks: The object's default_error_condition and equivalent virtual functions shall behave as specified for the class error_category. The object's name virtual function shall return a pointer to the string "iostream".
std::thread
, std::call_once
issueSection: 32.4.3.3 [thread.thread.constr], 32.6.7.2 [thread.once.callonce] Status: C++11 Submitter: Peter Dimov Opened: 2008-09-15 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
I notice that the vararg overloads of std::thread
and std::call_once
(N2723 32.4.3.3 [thread.thread.constr] and 32.6.7.2 [thread.once.callonce]) are no longer specified in terms of
std::bind
; instead, some of the std::bind
wording has been inlined into
the specification.
There are two problems with this.
First, the specification (and implementation) in terms of std::bind
allows, for example:
std::thread th( f, 1, std::bind( g ) );
which executes f( 1, g() )
in a thread. This can be useful. The
"inlined" formulation changes it to execute f( 1, bind(g) )
in a thread.
Second, assuming that we don't want the above, the specification has copied the wording
INVOKE(func, w1, w2, ..., wN)
(20.6.2) shall be a valid expression for some valuesw1, w2, ..., wN
but this is not needed since we know that our argument list is args; it should simply be
INVOKE(func, args...)
(20.6.2) shall be a valid expression
[ Summit: ]
Move to open.
[ Post Summit Anthony provided proposed wording. ]
[ 2009-07 Frankfurt ]
Leave Open. Await decision for thread variadic constructor.
[ 2009-10 Santa Cruz: ]
See proposed wording for 929(i) for this, for the formulation on how to solve this. 929(i) modifies the thread constructor to have "pass by value" behavior with pass by reference efficiency through the use of the
decay
trait. This same formula would be useful forcall_once
.
[ 2010-02-11 Anthony updates wording. ]
[ 2010-02-12 Moved to Tentatively Ready after 5 postive votes on c++std-lib. ]
Proposed resolution:
Modify 32.6.7.2 [thread.once.callonce] p1-p2 with the following:
template<class Callable, class ...Args> void call_once(once_flag& flag, Callable&& func, Args&&... args);Given a function as follows:
template<typename T> typename decay<T>::type decay_copy(T&& v) { return std::forward<T>(v); }1 Requires:
The template parametersCallable
and eachTi
inArgs
shallbesatisfy theCopyConstructible
if an lvalue and otherwiseMoveConstructible
requirements.INVOKE(decay_copy(std::forward<Callable>(func),
(22.10.4 [func.require]) shall be a valid expressionw1, w2, ..., wNdecay_copy(std::forward<Args>(args))...)for some values.w1, w2, ..., wN
, whereN == sizeof...(Args)
2 Effects: Calls to
call_once
on the sameonce_flag
object are serialized. If there has been a prior effective call tocall_once
on the sameonce_flag
object, the call tocall_once
returns without invokingfunc
. If there has been no prior effective call tocall_once
on the sameonce_flag
object,the argumentfunc
(or a copy thereof) is called as if by invokingfunc(args)
INVOKE(decay_copy(std::forward<Callable>(func)), decay_copy(std::forward<Args>(args))...)
is executed. The call tocall_once
is effective if and only iffunc(args)
INVOKE(decay_copy(std::forward<Callable>(func)), decay_copy(std::forward<Args>(args))...)
returns without throwing an exception. If an exception is thrown it is propagated to the caller.
std::mutex
issueSection: 32.6.4.2.2 [thread.mutex.class] Status: C++11 Submitter: Peter Dimov Opened: 2008-09-15 Last modified: 2016-01-28
Priority: Not Prioritized
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Duplicate of: 905
Discussion:
32.6.4.2.2 [thread.mutex.class]/27 (in N2723) says that the behavior is undefined if:
mutex
object calls lock()
or
try_lock()
on that object
I don't believe that this is right. Calling lock()
or try_lock()
on a
locked mutex
is well defined in the general case. try_lock()
is required
to fail and return false
. lock()
is required to either throw an
exception (and is allowed to do so if it detects deadlock) or to block
until the mutex
is free. These general requirements apply regardless of
the current owner of the mutex
; they should apply even if it's owned by
the current thread.
Making double lock()
undefined behavior probably can be justified (even
though I'd still disagree with the justification), but try_lock()
on a
locked mutex
must fail.
[ Summit: ]
Move to open. Proposed resolution:
- In 32.6.4 [thread.mutex.requirements] paragraph 12, change the error condition for
resource_deadlock_would_occur
to: "if the implementation detects that a deadlock would occur"- Strike 32.6.4.2.2 [thread.mutex.class] paragraph 3 bullet 2 "a thread that owns a mutex object calls
lock()
ortry_lock()
on that object, or"
[ 2009-07 Frankfurt ]
Move to Review. Alisdair to provide note.
[ 2009-07-31 Alisdair provided note. ]
[ 2009-10 Santa Cruz: ]
Moved to Ready.
[ 2009-11-18 Peter Opens: ]
I don't believe that the proposed note:
[Note: a program may deadlock if the thread that owns a
mutex
object callslock()
ortry_lock()
on that object. If the program can detect the deadlock, aresource_deadlock_would_occur
error condition may be observed. — end note]is entirely correct. "or
try_lock()
" should be removed, becausetry_lock
is non-blocking and doesn't deadlock; it just returnsfalse
when it fails to lock the mutex.[ Howard: I've set to Open and updated the wording per Peter's suggestion. ]
[ 2009-11-18 Moved to Tentatively Ready after 5 positive votes on c++std-lib. ]
Proposed resolution:
In 32.6.4 [thread.mutex.requirements] paragraph 12 change:
- ...
resource_deadlock_would_occur
-- if thecurrent thread already owns the mutex and is able to detect itimplementation detects that a deadlock would occur.- ...
Strike 32.6.4.2.2 [thread.mutex.class] paragraph 3 bullet 2:
-3- The behavior of a program is undefined if:
- ...
a thread that owns amutex
object callslock()
ortry_lock()
on that object, or- ...
Add the following note after p3 32.6.4.2.2 [thread.mutex.class]
[Note: a program may deadlock if the thread that owns a
mutex
object callslock()
on that object. If the implementation can detect the deadlock, aresource_deadlock_would_occur
error condition may be observed. — end note]
Section: 17.14 [support.runtime] Status: C++11 Submitter: Lawrence Crowl, Alisdair Meredith Opened: 2008-09-17 Last modified: 2016-01-28
Priority: Not Prioritized
View all other issues in [support.runtime].
View all issues with C++11 status.
Discussion:
The interaction between longjmp
and exceptions seems unnecessarily
restrictive and not in keeping with existing practice.
Proposed resolution:
Edit paragraph 4 of 17.14 [support.runtime] as follows:
The function signature
longjmp(jmp_buf jbuf, int val)
has more restricted behavior in this International Standard. Asetjmp/longjmp
call pair has undefined behavior if replacing thesetjmp
andlongjmp
bycatch
andthrow
woulddestroyinvoke any non-trivial destructors for any automatic objects.
Section: 20.3.2.2 [util.smartptr.shared] Status: C++11 Submitter: Hans Boehm Opened: 2008-09-16 Last modified: 2016-01-28
Priority: Not Prioritized
View all other issues in [util.smartptr.shared].
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Discussion:
It is unclear whether shared_ptr
is thread-safe in the sense that
multiple threads may simultaneously copy a shared_ptr
. However this
is a critical piece of information for the client, and it has significant
impact on usability for many applications. (Detlef Vollman thinks it
is currently clear that it is not thread-safe. Hans Boehm thinks
it currently requires thread safety, since the use_count
is not an
explicit field, and constructors and assignment take a const reference
to an existing shared_ptr
.)
Pro thread-safety:
Many multi-threaded usages are impossible. A thread-safe version can be used to destroy an object when the last thread drops it, something that is often required, and for which we have no other easy mechanism.
Against thread-safety:
The thread-safe version is well-known to be far more expensive, even if used by a single thread. Many applications, including all single-threaded ones, do not care.
[ San Francisco: ]
Beman: this is a complicated issue, and would like to move this to Open and await comment from Peter Dimov; we need very careful and complete rationale for any decision we make; let's go slow
Detlef: I think that
shared_ptr
should not be thread-safe.Hans: When you create a thread with a lambda, it in some cases makes it very difficult for the lambda to reference anything in the heap. It's currently ambiguous as to whether you can use a
shared_ptr
to get at an object.Leave in Open. Detlef will submit an alternative proposed resolution that makes
shared_ptr
explicitly unsafe.A third option is to support both threadsafe and non-safe share_ptrs, and to let the programmer decide which behavior they want.
Beman: Peter, do you support the PR?
Peter:
Yes, I support the proposed resolution, and I certainly oppose any attempts to
make shared_ptr
thread-unsafe.I'd mildly prefer if
[Note: This is true in spite of that fact that such functions often modify
use_count()
--end note]is changed to
[Note: This is true in spite of that fact that such functions often cause a change in
use_count()
--end note](or something along these lines) to emphasise that
use_count()
is not, conceptually, a variable, but a return value.
[ 2009-07 Frankfurt ]
Vote: Do we want one thread-safe shared pointer or two? If two, one would allow concurrent construction and destruction of shared pointers, and one would not be thread-safe. If one, then it would be thread-safe.
No concensus on that vote.
Hans to improve wording in consultation with Pete. Leave Open.
[ 2009-10 Santa Cruz: ]
Move to Ready. Ask Editor to clear up wording a little when integrating to make it clear that the portion after the first comma only applies for the presence of data races.
[ 2009-10-24 Hans adds: ]
I think we need to pull 896 back from ready, unfortunately. My wording doesn't say the right thing.
I suspect we really want to say something along the lines of:
For purposes of determining the presence of a data race, member functions access and modify only the
shared_ptr
andweak_ptr
objects themselves and not objects they refer to. Changes inuse_count()
do not reflect modifications that can introduce data races.But I think this needs further discussion by experts to make sure this is right.
Detlef and I agree continue to disagree on the resolution, but I think we agree that it would be good to try to expedite this so that it can be in CD2, since it's likely to generate NB comments no matter what we do. And lack of clarity of intent is probably the worst option. I think it would be good to look at this between meetings.
[ 2010-01-20 Howard: ]
I've moved Hans' suggested wording above into the proposed resolution section and preserved the previous wording here:
Make it explicitly thread-safe, in this weak sense, as I believe was intended:
Insert in 20.3.2.2 [util.smartptr.shared], before p5:
For purposes of determining the presence of a data race, member functions do not modify
const shared_ptr
and constweak_ptr
arguments, nor any objects they refer to. [Note: This is true in spite of that fact that such functions often cause a change inuse_count()
--end note]On looking at the text, I'm not sure we need a similar disclaimer anywhere else, since nothing else has the problem with the modified
use_count()
. I think Howard arrived at a similar conclusion.
[ 2010 Pittsburgh: Moved to Ready for Pittsburgh ]
Proposed resolution:
Insert a new paragraph at the end of 20.3.2.2 [util.smartptr.shared]:
For purposes of determining the presence of a data race, member functions access and modify only the
shared_ptr
andweak_ptr
objects themselves and not objects they refer to. Changes inuse_count()
do not reflect modifications that can introduce data races.
Section: 23.3.7.5 [forward.list.modifiers] Status: Resolved Submitter: Howard Hinnant Opened: 2008-09-22 Last modified: 2023-02-07
Priority: Not Prioritized
View all other issues in [forward.list.modifiers].
View all issues with Resolved status.
Discussion:
This issue was split off from 892(i) at the request of the LWG.
[ San Francisco: ]
This issue is more complicated than it looks.
paragraph 47: replace each
(first, last) with (first, last]
add a statement after paragraph 48 that complexity is O(1)
remove the complexity statement from the first overload of splice_after
We may have the same problems with other modifiers, like erase_after. Should it require that all iterators in the range (position, last] be dereferenceable?
There are actually 3 issues here:
What value should erase_after
return? With list
, code often
looks like:
for (auto i = l.begin(); i != l.end();) { // inspect *i and decide if you want to erase it // ... if (I want to erase *i) i = l.erase(i); else ++i; }
I.e. the iterator returned from erase
is useful for setting up the
logic for operating on the next element. For forward_list
this might
look something like:
auto i = fl.before_begin(); auto ip1 = i; for (++ip1; ip1 != fl.end(); ++ip1) { // inspect *(i+1) and decide if you want to erase it // ... if (I want to erase *(i+1)) i = fl.erase_after(i); else ++i; ip1 = i; }
In the above example code, it is convenient if erase_after
returns
the element prior to the erased element (range) instead of the element
after the erase element (range).
Existing practice:
There is not a strong technical argument for either solution over the other.
With all other containers, operations always work on the range
[first, last)
and/or prior to the given position
.
With forward_list
, operations sometimes work on the range
(first, last]
and/or after the given position
.
This is simply due to the fact that in order to operate on
*first
(with forward_list
) one needs access to
*(first-1)
. And that's not practical with
forward_list
. So the operating range needs to start with (first
,
not [first
(as the current working paper says).
Additionally, if one is interested in splicing the range (first, last)
,
then (with forward_list
), one needs practical (constant time) access to
*(last-1)
so that one can set the next field in this node to
the proper value. As this is not possible with forward_list
, one must
specify the last element of interest instead of one past the last element of
interest. The syntax for doing this is to pass (first, last]
instead
of (first, last)
.
With erase_after
we have a choice of either erasing the range
(first, last]
or (first, last)
. Choosing the latter
enables:
x.erase_after(pos, x.end());
With the former, the above statement is inconvenient or expensive due to the lack
of constant time access to x.end()-1
. However we could introduce:
iterator erase_to_end(const_iterator position);
to compensate.
The advantage of the former ((first, last]
) for erase_after
is a consistency with splice_after
which uses (first, last]
as the specified range. But this either requires the addition of erase_to_end
or giving up such functionality.
splice_after
should work on the source range (first, last]
if the operation is to be Ο(1). When splicing an entire list x
the
algorithm needs (x.before_begin(), x.end()-1]
. Unfortunately x.end()-1
is not available in constant time unless we specify that it must be. In order to
make x.end()-1
available in constant time, the implementation would have
to dedicate a pointer to it. I believe the design of
N2543
intended a nominal overhead of foward_list
of 1 pointer. Thus splicing
one entire forward_list
into another can not be Ο(1).
[ Batavia (2009-05): ]
We agree with the proposed resolution.
Move to Review.
[ 2009-07 Frankfurt ]
We may need a new issue to correct splice_after, because it may no longer be correct to accept an rvalues as an argument. Merge may be affected, too. This might be issue 1133(i). (Howard: confirmed)
Move this to Ready, but the Requires clause of the second form of splice_after should say "(first, last)," not "(first, last]" (there are three occurrences). There was considerable discussion on this. (Howard: fixed)
Alan suggested removing the "foward_last<T. Alloc>&& x" parameter from the second form of splice_after, because it is redundant. PJP wanted to keep it, because it allows him to check for bad ranges (i.e. "Granny knots").
We prefer to keep
x
.Beman. Whenever we deviate from the customary half-open range in the specification, we should add a non-normative comment to the standard explaining the deviation. This clarifies the intention and spares the committee much confusion in the future.
Alan to write a non-normative comment to explain the use of fully-closed ranges.
Move to Ready, with the changes described above. (Howard: awaiting note from Alan)
[ 2009-10 Santa Cruz: ]
NAD EditorialResolved, addressed by N2988.
Proposed resolution:
Wording below assumes issue 878(i) is accepted, but this issue is independent of that issue.
Change [forwardlist.modifiers]:
iterator erase_after(const_iterator position);Requires: The iterator following
position
is dereferenceable.Effects: Erases the element pointed to by the iterator following
position
.Returns:
An iterator pointing to the element following the one that was erased, orAn iterator equal toend()
if no such element existsposition
.iterator erase_after(const_iterator position, const_iterator last);Requires: All iterators in the range
are dereferenceable.
[(position,last)Effects: Erases the elements in the range
.
[(position,last)Returns: An iterator equal to
position
last
Change [forwardlist.ops]:
void splice_after(const_iterator position, forward_list<T,Allocator>&& x);Requires:
position
isbefore_begin()
or a dereferenceable iterator in the range[begin(), end))
.&x != this
.Effects: Inserts the contents of
x
afterposition
, andx
becomes empty. Pointers and references to the moved elements ofx
now refer to those same elements but as members of*this
. Iterators referring to the moved elements will continue to refer to their elements, but they now behave as iterators into*this
, not intox
.Throws: Nothing.
Complexity:
Ο(1)Ο(distance(x.begin(), x.end())
)...
void splice_after(const_iterator position, forward_list<T,Allocator>&& x, const_iterator first, const_iterator last);Requires:
position
isbefore_begin()
or a dereferenceable iterator in the range[begin(), end))
.(first,last)
is a valid range inx
, and all iterators in the range(first,last)
are dereferenceable.position
is not an iterator in the range(first,last)
.Effects: Inserts elements in the range
(first,last)
afterposition
and removes the elements fromx
. Pointers and references to the moved elements ofx
now refer to those same elements but as members of*this
. Iterators referring to the moved elements will continue to refer to their elements, but they now behave as iterators into*this
, not intox
.Complexity: Ο(1).
Section: 23.3.7.6 [forward.list.ops] Status: C++11 Submitter: Arch Robison Opened: 2008-09-08 Last modified: 2023-02-07
Priority: Not Prioritized
View all other issues in [forward.list.ops].
View all issues with C++11 status.
Discussion:
I ran across a small contradiction in working draft n2723.
[forwardlist]p2: A
forward_list
satisfies all of the requirements of a container (table 90), except that thesize()
member function is not provided.[forwardlist.ops]p57: Complexity: At most
size() + x.size() - 1
comparisons.
Presumably [forwardlist.ops]p57 needs to be rephrased to not use
size()
, or note that it is used there only for sake of notational convenience.
[ 2009-03-29 Beman provided proposed wording. ]
[ Batavia (2009-05): ]
We agree with the proposed resolution.
Move to Tentatively Ready.
Proposed resolution:
Change [forwardlist.ops], forward_list operations, paragraph 19, merge complexity as indicated:
Complexity: At most
comparisons.
size() + x.size()distance(begin(), end()) + distance(x.begin(), x.end()) - 1
shared_ptr
for nullptr_t
Section: 20.3.2.2.3 [util.smartptr.shared.dest] Status: C++11 Submitter: Peter Dimov Opened: 2008-09-18 Last modified: 2016-01-28
Priority: Not Prioritized
View all other issues in [util.smartptr.shared.dest].
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Discussion:
James Dennett, message c++std-lib-22442:
The wording below addresses one case of this, but opening an issue to address the need to sanity check uses of the term "pointer" in 20.3.2.2 [util.smartptr.shared] would be a good thing.
There's one more reference, in ~shared_ptr;
we can apply your suggested change to it, too. That is:
Change 20.3.2.2.3 [util.smartptr.shared.dest]/1 second bullet from:
Otherwise, if *this owns a pointer p and a deleter d, d(p) is called.
to:
Otherwise, if *this owns an object p and a deleter d, d(p) is called.
[ Post Summit: ]
Recommend Review.
[ Batavia (2009-05): ]
Peter Dimov notes the analogous change has already been made to "the new
nullptr_t
taking constructors in 20.3.2.2.2 [util.smartptr.shared.const] p9-13."We agree with the proposed resolution. Move to Tentatively Ready.
Proposed resolution:
Change 20.3.2.2.3 [util.smartptr.shared.dest] p1 second bullet:
- ...
- Otherwise, if
*this
ownsa pointeran objectp
and a deleterd
,d(p)
is called.
Section: 31.10.4 [ifstream] Status: C++11 Submitter: Niels Dekker Opened: 2008-09-20 Last modified: 2023-02-07
Priority: Not Prioritized
View all issues with C++11 status.
Discussion:
It appears that we have an issue similar to issue 675(i) regarding the move-assignment of
stream types. For example, when assigning to an std::ifstream
, ifstream1
, it
seems preferable to close the file originally held by ifstream1
:
ifstream1 = std::move(ifstream2);
The current Draft
(N2723)
specifies that the move-assignment of stream types like ifstream
has the
same effect as a swap:
Assign and swap [ifstream.assign]
basic_ifstream& operator=(basic_ifstream&& rhs);Effects:
swap(rhs)
.
[ Batavia (2009-05): ]
Howard agrees with the analysis and the direction proposed.
Move to Open pending specific wording to be supplied by Howard.
[ 2009-07 Frankfurt: ]
Howard is going to write wording.
[ 2009-07-26 Howard provided wording. ]
[ 2009-09-13 Niels adds: ]
Note: The proposed change of 31.10.3.3 [filebuf.assign] p1 depends on the resolution of LWG 1204(i), which allows implementations to assume that
*this
andrhs
refer to different objects.
[ 2009 Santa Cruz: ]
Leave as Open. Too closely related to 911(i) to move on at this time.
[ 2010 Pittsburgh: ]
Moved to Ready for Pittsburgh.
Proposed resolution:
Change 31.8.2.3 [stringbuf.assign]/1:
basic_stringbuf& operator=(basic_stringbuf&& rhs);-1- Effects:
After the move assignmentswap(rhs)
.*this
reflects the same observable state it would have if it had been move constructed fromrhs
(31.8.2.2 [stringbuf.cons]).
Change [istringstream.assign]/1:
basic_istringstream& operator=(basic_istringstream&& rhs);-1- Effects:
Move assigns the base and members ofswap(rhs)
.*this
with the respective base and members ofrhs
.
Change [ostringstream.assign]/1:
basic_ostringstream& operator=(basic_ostringstream&& rhs);-1- Effects:
Move assigns the base and members ofswap(rhs)
.*this
with the respective base and members ofrhs
.
Change [stringstream.assign]/1:
basic_stringstream& operator=(basic_stringstream&& rhs);-1- Effects:
Move assigns the base and members ofswap(rhs)
.*this
with the respective base and members ofrhs
.
Change 31.10.3.3 [filebuf.assign]/1:
basic_filebuf& operator=(basic_filebuf&& rhs);-1- Effects:
Begins by callingswap(rhs)
.this->close()
. After the move assignment*this
reflects the same observable state it would have if it had been move constructed fromrhs
(31.10.3.2 [filebuf.cons]).
Change [ifstream.assign]/1:
basic_ifstream& operator=(basic_ifstream&& rhs);-1- Effects:
Move assigns the base and members ofswap(rhs)
.*this
with the respective base and members ofrhs
.
Change [ofstream.assign]/1:
basic_ofstream& operator=(basic_ofstream&& rhs);-1- Effects:
Move assigns the base and members ofswap(rhs)
.*this
with the respective base and members ofrhs
.
Change [fstream.assign]/1:
basic_fstream& operator=(basic_fstream&& rhs);-1- Effects:
Move assigns the base and members ofswap(rhs)
.*this
with the respective base and members ofrhs
.
result_of
argument typesSection: 99 [func.ret] Status: C++11 Submitter: Jonathan Wakely Opened: 2008-09-10 Last modified: 2016-01-28
Priority: Not Prioritized
View all other issues in [func.ret].
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Discussion:
The WP and TR1 have the same text regarding the argument types of a
result_of
expression:
The values
ti
are lvalues when the corresponding typeTi
is a reference type, and rvalues otherwise.
I read this to mean that this compiles:
typedef int (*func)(int&); result_of<func(int&&)>::type i = 0;
even though this doesn't:
int f(int&); f( std::move(0) );
Should the text be updated to say "when Ti
is an lvalue-reference
type" or am I missing something?
I later came up with this self-contained example which won't compile, but I think it should:
struct X { void operator()(int&); int operator()(int&&); } x; std::result_of< X(int&&) >::type i = x(std::move(0));
[ Post Summit: ]
Recommend Tentatively Ready.
Proposed resolution:
Change 99 [func.ret], p1:
... The values
ti
are lvalues when the corresponding typeTi
is an lvalue-reference type, and rvalues otherwise.
Section: 22.9.2.3 [bitset.members] Status: C++11 Submitter: Daniel Krügler Opened: 2008-09-26 Last modified: 2016-01-28
Priority: Not Prioritized
View all other issues in [bitset.members].
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Discussion:
The current standard 14882::2003(E) as well as the current draft
N2723
have in common a contradiction of the operational semantics of member function
test
22.9.2.3 [bitset.members] p.56-58 and the immutable
member operator[]
overload 22.9.2.3 [bitset.members] p.64-66 (all references
are defined in terms of
N2723):
bool test(size_t pos) const;
Requires:
pos
is validThrows:
out_of_range
ifpos
does not correspond to a valid bit position.Returns:
true
if the bit at positionpos
in*this
has the value one.
constexpr bool operator[](size_t pos) const;
Requires:
pos
shall be valid.Throws: nothing.
Returns:
test(pos)
.
Three interpretations:
operator[]
overload is indeed allowed to throw an exception
(via test()
, if pos
corresponds to an invalid bit position) which does
not leave the call frame. In this case this function cannot be a
constexpr
function, because test()
is not, due to
7.7 [expr.const]/2, last bullet.
test
in case of an
invalid bit position. There is only little evidence for this interpretation.
operator[]
should not throw any exception,
but that test
has the contract to do so, if the provided bit position
is invalid.
The problem became worse, because issue 720(i)
recently voted into WP argued that member test
logically must be
a constexpr
function, because it was used to define the semantics
of another constexpr
function (the operator[]
overload).
Three alternatives are proposed, corresponding to the three bullets (A), (B), and (C), the author suggests to follow proposal (C).
Proposed alternatives:
Remove the constexpr
specifier in front of operator[]
overload and
undo that of member test
(assuming 720(i) is accepted) in both the
class declaration 22.9.2 [template.bitset]/1 and in the member description
before 22.9.2.3 [bitset.members]/56 and before /64 to read:
constexprbool test(size_t pos) const; ..constexprbool operator[](size_t pos) const;
Change the throws clause of p. 65 to read:
Throws:
nothingout_of_range
ifpos
does not correspond to a valid bit position.
Replace the throws clause p. 57 to read:
Throws:
nothing.out_of_range
ifpos
does not correspond to a valid bit position
Undo the addition of the constexpr
specifier to the test
member
function in both class declaration 22.9.2 [template.bitset]/1 and in the
member description before 22.9.2.3 [bitset.members]/56, assuming that 720(i)
was applied.
constexprbool test(size_t pos) const;
Change the returns clause p. 66 to read:
Returns:
test(pos)
true
if the bit at positionpos
in*this
has the value one, otherwisefalse
.
[ Post Summit: ]
Lawrence: proposed resolutions A, B, C are mutually exclusive.
Recommend Review with option C.
[ Batavia (2009-05): ]
We agree with the proposed resolution. Move to Tentatively Ready.
Proposed resolution:
Undo the addition of the constexpr
specifier to the test
member
function in both class declaration 22.9.2 [template.bitset] p.1 and in the
member description before 22.9.2.3 [bitset.members] p.56, assuming that 720(i)
was applied.
constexprbool test(size_t pos) const;
Change the returns clause p. 66 to read:
Returns:
test(pos)
true
if the bit at positionpos
in*this
has the value one, otherwisefalse
.
Section: 32.5.8 [atomics.types.generic] Status: Resolved Submitter: Anthony Williams Opened: 2008-09-26 Last modified: 2017-11-29
Priority: Not Prioritized
View all other issues in [atomics.types.generic].
View all issues with Resolved status.
Discussion:
Addresses US 90
The deleted copy-assignment operators for the atomic types are not marked as volatile in N2723, whereas the assignment operators from the associated non-atomic types are. e.g.
atomic_bool& operator=(atomic_bool const&) = delete; atomic_bool& operator=(bool) volatile;
This leads to ambiguity when assigning a non-atomic value to a non-volatile instance of an atomic type:
atomic_bool b; b=false;
Both assignment operators require a standard conversions: the
copy-assignment operator can use the implicit atomic_bool(bool)
conversion constructor to convert false
to an instance of
atomic_bool
, or b
can undergo a qualification conversion in order to
use the assignment from a plain bool
.
This is only a problem once issue 845(i) is applied.
[ Summit: ]
Move to open. Assign to Lawrence. Related to US 90 comment.
[ 2009-08-17 Handled by N2925. ]
[ 2009-10 Santa Cruz: ]
NAD EditorialResolved. Addressed by N2992.
Proposed resolution:
Add volatile qualification to the deleted copy-assignment operator of all the atomic types:
atomic_bool& operator=(atomic_bool const&) volatile = delete; atomic_itype& operator=(atomic_itype const&) volatile = delete;
etc.
This will mean that the deleted copy-assignment operator will require
two conversions in the above example, and thus be a worse match than
the assignment from plain bool
.
regex_token_iterator
should use initializer_list
Section: 28.6.11.2 [re.tokiter] Status: C++11 Submitter: Daniel Krügler Opened: 2008-09-26 Last modified: 2016-01-28
Priority: Not Prioritized
View all other issues in [re.tokiter].
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Discussion:
Addresses UK 319
Construction of a regex_token_iterator
(28.6.11.2 [re.tokiter]/6+) usually
requires the provision of a sequence of integer values, which
can currently be done via a std::vector<int>
or
a C array of int
. Since the introduction of initializer_list
in the
standard it seems much more reasonable to provide a
corresponding constructor that accepts an initializer_list<int>
instead. This could be done as a pure addition or one could
even consider replacement. The author suggests the
replacement strategy (A), but provides an alternative additive
proposal (B) as a fall-back, because of the handiness of this
range type:
[ Batavia (2009-05): ]
We strongly recommend alternative B of the proposed resolution in order that existing code not be broken. With that understanding, move to Tentatively Ready.
Original proposed wording:
In 28.6.11.2 [re.tokiter]/6 and the list 28.6.11.2.2 [re.tokiter.cnstr]/10-11 change the constructor declaration:
template <std::size_t N>regex_token_iterator(BidirectionalIterator a, BidirectionalIterator b, const regex_type& re,const int (&submatches)[N]initializer_list<int> submatches, regex_constants::match_flag_type m = regex_constants::match_default);
In 28.6.11.2.2 [re.tokiter.cnstr]/12 change the last sentence
The third constructor initializes the member
subs
to hold a copy of the sequence of integer values pointed to by the iterator range[
.&submatches.begin(),&submatches.end()+ N)
In 28.6.11.2 [re.tokiter]/6 and the list 28.6.11.2.2 [re.tokiter.cnstr]/10-11 insert the
following constructor declaration between the already existing ones
accepting a std::vector
and a C array of int
, resp.:
regex_token_iterator(BidirectionalIterator a, BidirectionalIterator b, const regex_type& re, initializer_list<int> submatches, regex_constants::match_flag_type m = regex_constants::match_default);
In 28.6.11.2.2 [re.tokiter.cnstr]/12 change the last sentence
The third and fourth constructor initialize
sthe membersubs
to hold a copy of the sequence of integer values pointed to by the iterator range[&submatches,&submatches + N)
and[submatches.begin(),submatches.end())
, respectively.
Proposed resolution:
In 28.6.11.2 [re.tokiter]/6 and the list 28.6.11.2.2 [re.tokiter.cnstr]/10-11 insert the
following constructor declaration between the already existing ones
accepting a std::vector
and a C array of int
, resp.:
regex_token_iterator(BidirectionalIterator a, BidirectionalIterator b, const regex_type& re, initializer_list<int> submatches, regex_constants::match_flag_type m = regex_constants::match_default);
In 28.6.11.2.2 [re.tokiter.cnstr]/12 change the last sentence
The third and fourth constructor initialize
sthe membersubs
to hold a copy of the sequence of integer values pointed to by the iterator range[&submatches,&submatches + N)
and[submatches.begin(),submatches.end())
, respectively.
move/swap
semanticSection: 31.7.5 [input.streams], 31.7.6 [output.streams] Status: C++11 Submitter: Alberto Ganesh Barbati Opened: 2008-09-29 Last modified: 2016-01-28
Priority: Not Prioritized
View all issues with C++11 status.
Discussion:
Class template basic_istream
, basic_ostream
and basic_iostream
implements public move constructors, move assignment operators and swap
method and free functions. This might induce both the user and the
compiler to think that those types are MoveConstructible
, MoveAssignable
and Swappable
. However, those class templates fail to fulfill the user
expectations. For example:
std::ostream os(std::ofstream("file.txt")); assert(os.rdbuf() == 0); // buffer object is not moved to os, file.txt has been closed std::vector<std::ostream> v; v.push_back(std::ofstream("file.txt")); v.reserve(100); // causes reallocation assert(v[0].rdbuf() == 0); // file.txt has been closed! std::ostream&& os1 = std::ofstream("file1.txt"); os1 = std::ofstream("file2.txt"); os1 << "hello, world"; // still writes to file1.txt, not to file2.txt! std::ostream&& os1 = std::ofstream("file1.txt"); std::ostream&& os2 = std::ofstream("file2.txt"); std::swap(os1, os2); os1 << "hello, world"; // writes to file1.txt, not to file2.txt!
This is because the move constructor, the move assignment operator and
swap
are all implemented through calls to std::basic_ios
member
functions move()
and swap()
that do not move nor swap the controlled
stream buffers. That can't happen because the stream buffers may have
different types.
Notice that for basic_streambuf
, the member function swap()
is
protected. I believe that is correct and all of basic_istream
,
basic_ostream
, basic_iostream
should do the same as the move ctor, move
assignment operator and swap member function are needed by the derived
fstream
s and stringstream
s template. The free swap functions for
basic_(i|o|io)stream
templates should be removed for the same reason.
[ Batavia (2009-05): ]
We note that the rvalue swap functions have already been removed.
Bill is unsure about making the affected functions protected; he believes they may need to be public.
We are also unsure about removing the lvalue swap functions as proposed.
Move to Open.
[ 2009-07 Frankfurt: ]
It's not clear that the use case is compelling.
Howard: This needs to be implemented and tested.
[ 2009-07-26 Howard adds: ]
I started out thinking I would recommend NAD for this one. I've turned around to agree with the proposed resolution (which I've updated to the current draft). I did not fully understand Ganesh's rationale, and attempt to describe my improved understanding below.
The move constructor, move assignment operator, and swap function are different for
basic_istream
,basic_ostream
andbasic_iostream
than other classes. A timely conversation with Daniel reminded me of this long forgotten fact. These members are sufficiently different that they would be extremely confusing to use in general, but they are very much needed for derived clients.
- The move constructor moves everything but the
rdbuf
pointer.- The move assignment operator moves everything but the
rdbuf
pointer.- The swap function swaps everything but the
rdbuf
pointer.The reason for this behavior is that for the std-derived classes (stringstreams, filestreams), the
rdbuf
pointer points back into the class itself (self referencing). It can't be swapped or moved. But this fact isn't born out at thestream
level. Rather it is born out at thefstream
/sstream
level. And the lower levels just need to deal with that fact by not messing around with therdbuf
pointer which is stored down at the lower levels.In a nutshell, it is very confusing for all of those who are not so intimately related with streams that they've implemented them. And it is even fairly confusing for some of those who have (including myself). I do not think it is safe to swap or move
istreams
orostreams
because this will (by necessary design) separate stream state from streambuffer state. Derived classes (such asfstream
andstringstream
must be used to keep the stream state and stream buffer consistently packaged as one unit during a move or swap.I've implemented this proposal and am living with it day to day.
[ 2009 Santa Cruz: ]
Leave Open. Pablo expected to propose alternative wording which would rename move construction, move assignment and swap, and may or may not make them protected. This will impact issue 900(i).
[ 2010 Pittsburgh: ]
Moved to Ready for Pittsburgh.
Proposed resolution:
31.7.5.2 [istream]: make the following member functions protected:
basic_istream(basic_istream&& rhs); basic_istream& operator=(basic_istream&& rhs); void swap(basic_istream& rhs);
Ditto: remove the swap free function signature
// swap: template <class charT, class traits> void swap(basic_istream<charT, traits>& x, basic_istream<charT, traits>& y);
31.7.5.2.3 [istream.assign]: remove paragraph 4
template <class charT, class traits> void swap(basic_istream<charT, traits>& x, basic_istream<charT, traits>& y);
Effects:x.swap(y)
.
31.7.5.7 [iostreamclass]: make the following member function protected:
basic_iostream(basic_iostream&& rhs); basic_iostream& operator=(basic_iostream&& rhs); void swap(basic_iostream& rhs);
Ditto: remove the swap free function signature
template <class charT, class traits> void swap(basic_iostream<charT, traits>& x, basic_iostream<charT, traits>& y);
31.7.5.7.4 [iostream.assign]: remove paragraph 3
template <class charT, class traits> void swap(basic_iostream<charT, traits>& x, basic_iostream<charT, traits>& y);
Effects:x.swap(y)
.
31.7.6.2 [ostream]: make the following member function protected:
basic_ostream(basic_ostream&& rhs); basic_ostream& operator=(basic_ostream&& rhs); void swap(basic_ostream& rhs);
Ditto: remove the swap free function signature
// swap: template <class charT, class traits> void swap(basic_ostream<charT, traits>& x, basic_ostream<charT, traits>& y);
31.7.6.2.3 [ostream.assign]: remove paragraph 4
template <class charT, class traits> void swap(basic_ostream<charT, traits>& x, basic_ostream<charT, traits>& y);
Effects:x.swap(y)
.
Section: 22.10.16 [func.memfn] Status: C++11 Submitter: Bronek Kozicki Opened: 2008-10-06 Last modified: 2016-01-28
Priority: Not Prioritized
View all other issues in [func.memfn].
View all issues with C++11 status.
Duplicate of: 1230
Discussion:
Daniel Krügler wrote:
Shouldn't above list be completed for &- and &&-qualified member functions This would cause to add:
template<Returnable R, class T, CopyConstructible... Args> unspecified mem_fn(R (T::* pm)(Args...) &); template<Returnable R, class T, CopyConstructible... Args> unspecified mem_fn(R (T::* pm)(Args...) const &); template<Returnable R, class T, CopyConstructible... Args> unspecified mem_fn(R (T::* pm)(Args...) volatile &); template<Returnable R, class T, CopyConstructible... Args> unspecified mem_fn(R (T::* pm)(Args...) const volatile &); template<Returnable R, class T, CopyConstructible... Args> unspecified mem_fn(R (T::* pm)(Args...) &&); template<Returnable R, class T, CopyConstructible... Args> unspecified mem_fn(R (T::* pm)(Args...) const &&); template<Returnable R, class T, CopyConstructible... Args> unspecified mem_fn(R (T::* pm)(Args...) volatile &&); template<Returnable R, class T, CopyConstructible... Args> unspecified mem_fn(R (T::* pm)(Args...) const volatile &&);
yes, absolutely. Thanks for spotting this. Without this change mem_fn
cannot be initialized from pointer to ref-qualified member function. I
believe semantics of such function pointer is well defined.
[ Post Summit Daniel provided wording. ]
[ Batavia (2009-05): ]
We need to think about whether we really want to go down the proposed path of combinatorial explosion. Perhaps a Note would suffice.
We would really like to have an implementation before proceeding.
Move to Open, and recommend this be deferred until after the next Committee Draft has been issued.
[ 2009-10-10 Daniel updated wording to post-concepts. ]
[ 2009-10 Santa Cruz: ]
Move to Ready.
Proposed resolution:
Change 22.10 [function.objects]/2, header
<functional>
synopsis as follows:
// 20.7.14, member function adaptors: template<class R, class T> unspecified mem_fn(R T::*); template<class R, class T, class ...Args> unspecified mem_fn(R (T::*)(Args...)); template<class R, class T, class ...Args> unspecified mem_fn(R (T::*)(Args...) const); template<class R, class T, class ...Args> unspecified mem_fn(R (T::*)(Args...) volatile); template<class R, class T, class ...Args> unspecified mem_fn(R (T::*)(Args...) const volatile); template<class R, class T, class ...Args> unspecified mem_fn(R (T::*)(Args...) &); template<class R, class T, class ...Args> unspecified mem_fn(R (T::*)(Args...) const &); template<class R, class T, class ...Args> unspecified mem_fn(R (T::*)(Args...) volatile &); template<class R, class T, class ...Args> unspecified mem_fn(R (T::*)(Args...) const volatile &); template<class R, class T, class ...Args> unspecified mem_fn(R (T::*)(Args...) &&); template<class R, class T, class ...Args> unspecified mem_fn(R (T::*)(Args...) const &&); template<class R, class T, class ...Args> unspecified mem_fn(R (T::*)(Args...) volatile &&); template<class R, class T, class ...Args> unspecified mem_fn(R (T::*)(Args...) const volatile &&);
Change the prototype list of 22.10.16 [func.memfn] as follows [NB: The following text, most notably p.2 and p.3 which discuss influence of the cv-qualification on the definition of the base class's first template parameter remains unchanged. ]:
template<class R, class T> unspecified mem_fn(R T::* pm); template<class R, class T, class ...Args> unspecified mem_fn(R (T::* pm)(Args...)); template<class R, class T, class ...Args> unspecified mem_fn(R (T::* pm)(Args...) const); template<class R, class T, class ...Args> unspecified mem_fn(R (T::* pm)(Args...) volatile); template<class R, class T, class ...Args> unspecified mem_fn(R (T::* pm)(Args...) const volatile); template<class R, class T, class ...Args> unspecified mem_fn(R (T::* pm)(Args...) &); template<class R, class T, class ...Args> unspecified mem_fn(R (T::* pm)(Args...) const &); template<class R, class T, class ...Args> unspecified mem_fn(R (T::* pm)(Args...) volatile &); template<class R, class T, class ...Args> unspecified mem_fn(R (T::* pm)(Args...) const volatile &); template<class R, class T, class ...Args> unspecified mem_fn(R (T::* pm)(Args...) &&); template<class R, class T, class ...Args> unspecified mem_fn(R (T::* pm)(Args...) const &&); template<class R, class T, class ...Args> unspecified mem_fn(R (T::* pm)(Args...) volatile &&); template<class R, class T, class ...Args> unspecified mem_fn(R (T::* pm)(Args...) const volatile &&);
Remove 22.10.16 [func.memfn]/5:
Remarks: Implementations may implementmem_fn
as a set of overloaded function templates.
Section: 21.4.3 [ratio.ratio] Status: C++11 Submitter: Pablo Halpern Opened: 2008-10-07 Last modified: 2016-01-28
Priority: Not Prioritized
View all other issues in [ratio.ratio].
View all issues with C++11 status.
Discussion:
The compile-time functions that operate on ratio<N,D>
require the
cumbersome and error-prone "evaluation" of a type
member using a
meta-programming style that predates the invention of template aliases.
Thus, multiplying three ratios a
, b
, and c
requires the expression:
ratio_multiply<a, ratio_multiply<b, c>::type>::type
The simpler expression:
ratio_multiply<a, ratio_multiply<b, c>>
Could be used by if template aliases were employed in the definitions.
[ Post Summit: ]
Jens: not a complete proposed resolution: "would need to make similar change"
Consensus: We agree with the direction of the issue.
Recommend Open.
[ 2009-05-11 Daniel adds: ]
Personally I'm not in favor for the addition of:
typedef ratio type;For a reader of the standard it's usage or purpose is unclear. I haven't seen similar examples of attempts to satisfy non-feature complete compilers.
[ 2009-05-11 Pablo adds: ]
The addition of type to the
ratio
template allows the previous style (i.e., in the prototype implementations) to remain valid and permits the use of transitional library implementations for C++03 compilers. I do not feel strongly about its inclusion, however, and leave it up to the reviewers to decide.
[ Batavia (2009-05): ]
Bill asks for additional discussion in the issue that spells out more details of the implementation. Howard points us to issue 948(i) which has at least most of the requested details. Tom is strongly in favor of overflow-checking at compile time. Pete points out that there is no change of functionality implied. We agree with the proposed resolution, but recommend moving the issue to Review to allow time to improve the discussion if needed.
[ 2009-07-21 Alisdair adds: ]
[ 2009-10 Santa Cruz: ]
Move to Ready.
Proposed resolution:
In 21.4 [ratio] p.3 change as indicated:
// ratio arithmetic template <class R1, class R2>structusing ratio_add = see below; template <class R1, class R2>structusing ratio_subtract = see below; template <class R1, class R2>structusing ratio_multiply = see below; template <class R1, class R2>structusing ratio_divide = see below;
In 21.4.3 [ratio.ratio], change as indicated:
namespace std { template <intmax_t N, intmax_t D = 1> class ratio { public: typedef ratio type; static const intmax_t num; static const intmax_t den; }; }
In 21.4.4 [ratio.arithmetic] change as indicated:
template <class R1, class R2>structusing ratio_add = see below{ typedef see below type; };1 The
nested typedeftyperatio_add<R1, R2>
shall be a synonym forratio<T1, T2>
whereT1
has the valueR1::num * R2::den + R2::num * R1::den
andT2
has the valueR1::den * R2::den
.
template <class R1, class R2>structusing ratio_subtract = see below{ typedef see below type; };2 The
nested typedeftyperatio_subtract<R1, R2>
shall be a synonym forratio<T1, T2>
whereT1
has the valueR1::num * R2::den - R2::num * R1::den
andT2
has the valueR1::den * R2::den
.
template <class R1, class R2>structusing ratio_multiply = see below{ typedef see below type; };3 The
nested typedeftyperatio_multiply<R1, R2>
shall be a synonym forratio<T1, T2>
whereT1
has the valueR1::num * R2::num
andT2
has the valueR1::den * R2::den
.
template <class R1, class R2>structusing ratio_divide = see below{ typedef see below type; };4 The
nested typedeftyperatio_divide<R1, R2>
shall be a synonym forratio<T1, T2>
whereT1
has the valueR1::num * R2::den
andT2
has the valueR1::den * R2::num
.
In 30.5.2 [time.duration.cons] p.4 change as indicated:
Requires:
treat_as_floating_point<rep>::value
shall be true orratio_divide<Period2, period>::
shall be 1.[..]type::den
In 30.5.8 [time.duration.cast] p.2 change as indicated:
Returns: Let CF be
ratio_divide<Period, typename ToDuration::period>
, and [..]::type
Section: B [implimits] Status: C++11 Submitter: Sohail Somani Opened: 2008-10-11 Last modified: 2016-02-01
Priority: Not Prioritized
View all issues with C++11 status.
Discussion:
Addresses DE 24
With respect to the section 22.10.15.5 [func.bind.place]:
TR1 dropped some suggested implementation quantities for the number of placeholders. The purpose of this defect is to put these back for C++0x.
[ Post Summit: ]
see DE 24
Recommend applying the proposed resolution from DE 24, with that Tentatively Ready.
Original proposed resolution:
Add 22.10.15.5 [func.bind.place] p.2:
While the exact number of placeholders (
_M
) is implementation defined, this number shall be at least 10.
Proposed resolution:
Add to B [implimits]:
Section: 32.5 [atomics] Status: Resolved Submitter: Herb Sutter Opened: 2008-10-17 Last modified: 2016-01-28
Priority: Not Prioritized
View all other issues in [atomics].
View all issues with Resolved status.
Discussion:
Right now, C++0x doesn't have atomic<float>
. We're thinking of adding
the words to support it for TR2 (note: that would be slightly post-C++0x). If we
need it, we could probably add the words.
Proposed resolutions: Using atomic<FP>::compare_exchange
(weak or
strong) should be either:
I propose Option 1 for C++0x for expediency. If someone wants to argue
for Option 2, they need to say what exactly they want compare_exchange
to mean in this case (IIRC, C++0x doesn't even assume IEEE 754).
[ Summit: ]
Move to open. Blocked until concepts for atomics are addressed.
[ Post Summit Anthony adds: ]
Recommend NAD. C++0x does have
std::atomic<float>
, and bothcompare_exchange_weak
andcompare_exchange_strong
are well-defined in this case. Maybe change the note in 32.5.8.2 [atomics.types.operations] paragraph 20 to:[Note: The effect of the compare-and-exchange operations is
if (!memcmp(object,expected,sizeof(*object))) *object = desired; else *expected = *object;This may result in failed comparisons for values that compare equal if the underlying type has padding bits or alternate representations of the same value. -- end note]
[ 2009-10 Santa Cruz: ]
NAD EditorialResolved. Addressed by N2992.
Proposed resolution:
Change the note in 32.5.8.2 [atomics.types.operations] paragraph 20 to:
[Note: The effect of the compare-and-exchange operations is
if (*object == *expected!memcmp(object,expected,sizeof(*object))) *object = desired; else *expected = *object;This may result in failed comparisons for values that compare equal if the underlying type has padding bits or alternate representations of the same value. -- end note]
Section: 32.5 [atomics] Status: Resolved Submitter: Herb Sutter Opened: 2008-10-17 Last modified: 2016-01-28
Priority: Not Prioritized
View all other issues in [atomics].
View all issues with Resolved status.
Discussion:
Right now, the compare_exchange_weak
loop should rapidly converge on the
padding contents. But compare_exchange_strong
will require a bit more
compiler work to ignore padding for comparison purposes.
Note that this isn't a problem for structs with no padding, and we do
already have one portable way to ensure that there is no padding that
covers the key use cases: Have elements be the same type. I suspect that
the greatest need is for a structure of two pointers, which has no
padding problem. I suspect the second need is a structure of a pointer
and some form of an integer. If that integer is intptr_t
, there will be
no padding.
Related but separable issue: For unused bitfields, or other unused
fields for that matter, we should probably say it's the programmer's
responsibility to set them to zero or otherwise ensure they'll be
ignored by memcmp
.
Proposed resolution: Using
atomic<struct-with-padding>::compare_exchange_strong
should be either:
I propose Option 1 for C++0x for expediency, though I'm not sure how to
say it. I would be happy with Option 2, which I believe would mean that
compare_exchange_strong
would be implemented to avoid comparing padding
bytes, or something equivalent such as always zeroing out padding when
loading/storing/comparing. (Either implementation might require compiler
support.)
[ Summit: ]
Move to open. Blocked until concepts for atomics are addressed.
[ Post Summit Anthony adds: ]
The resolution of LWG 923(i) should resolve this issue as well.
[ 2009-10 Santa Cruz: ]
NAD EditorialResolved. Addressed by N2992.
Proposed resolution:
shared_ptr
's explicit conversion from unique_ptr
Section: 20.3.2.2.2 [util.smartptr.shared.const] Status: C++11 Submitter: Rodolfo Lima Opened: 2008-10-12 Last modified: 2016-01-28
Priority: Not Prioritized
View other active issues in [util.smartptr.shared.const].
View all other issues in [util.smartptr.shared.const].
View all issues with C++11 status.
Discussion:
The current working draft
(N2798),
section 20.3.2.2.2 [util.smartptr.shared.const] declares
shared_ptr
's constructor that takes a rvalue reference to unique_ptr
and
auto_ptr
as being explicit, affecting several valid smart pointer use
cases that would take advantage of this conversion being implicit, for
example:
class A; std::unique_ptr<A> create(); void process(std::shared_ptr<A> obj); int main() { process(create()); // use case #1 std::unique_ptr<A> uobj = create(); process(std::move(uobj)); // use case #2 return 0; }
If unique_ptr
to shared_ptr
conversions are explicit, the above lines
should be written:
process(std::shared_ptr<A>(create())); // use case #1 process(std::shared_ptr<A>(std::move(uobj))); // use case #2
The extra cast required doesn't seems to give any benefits to the user, nor protects him of any unintended conversions, this being the raison d'etre of explicit constructors.
It seems that this constructor was made explicit to mimic the conversion
from auto_ptr
in pre-rvalue reference days, which accepts both lvalue and
rvalue references. Although this decision was valid back then, C++0x
allows the user to express in a clear and non verbose manner when he wants
move semantics to be employed, be it implicitly (use case 1) or explicitly
(use case 2).
[ Batavia (2009-05): ]
Howard and Alisdair like the motivating use cases and the proposed resolution.
Move to Tentatively Ready.
Proposed resolution:
In both 20.3.2.2 [util.smartptr.shared] paragraph 1 and 20.3.2.2.2 [util.smartptr.shared.const] change:
template <class Y>explicitshared_ptr(auto_ptr<Y> &&r); template <class Y, class D>explicitshared_ptr(unique_ptr<Y, D> &&r);
Section: 32.4.3.3 [thread.thread.constr] Status: C++11 Submitter: Anthony Williams Opened: 2008-10-23 Last modified: 2016-01-28
Priority: Not Prioritized
View all other issues in [thread.thread.constr].
View all issues with C++11 status.
Discussion:
Addresses UK 323
The thread
constructor for starting a new thread with a function and
arguments is overly constrained by the signature requiring rvalue
references for func
and args
and the CopyConstructible
requirements
for the elements of args
. The use of an rvalue reference for the
function restricts the potential use of a plain function name, since
the type of the bound parameter will be deduced to be a function
reference and decay to pointer-to-function will not happen. This
therefore complicates the implementation in order to handle a simple
case. Furthermore, the use of rvalue references for args prevents the
array to pointer decay. Since arrays are not CopyConstructible
or even
MoveConstructible
, this essentially prevents the passing of arrays as
parameters. In particular it prevents the passing of string literals.
Consequently a simple case such as
void f(const char*); std::thread t(f,"hello");
is ill-formed since the type of the string literal is const char[6]
.
By changing the signature to take all parameters by value we can
eliminate the CopyConstructible
requirement and permit the use of
arrays, as the parameter passing semantics will cause the necessary
array-to-pointer decay. They will also cause the function name to
decay to a pointer to function and allow the implementation to handle
functions and function objects identically.
The new signature of the thread
constructor for a function and
arguments is thus:
template<typename F,typename... Args> thread(F,Args... args);
Since the parameter pack Args
can be empty, the single-parameter
constructor that takes just a function by value is now redundant.
[ Howard adds: ]
I agree with everything Anthony says in this issue. However I believe we can optimize in such a way as to get the pass-by-value behavior with the pass-by-rvalue-ref performance. The performance difference is that the latter removes a
move
when passing in an lvalue.This circumstance is very analogous to
make_pair
(22.3 [pairs]) where we started with passing by const reference, changed to pass by value to get pointer decay, and then changed to pass by rvalue reference, but modified withdecay<T>
to retain the pass-by-value behavior. If we were to apply the same solution here it would look like:template <class F> explicit thread(F f);template <class F, class ...Args> thread(F&& f, Args&&... args);-4- Requires:
F
and eachTi
inArgs
shall beCopyConstructible
if an lvalue and otherwiseMoveConstructible
.INVOKE(f, w1, w2, ..., wN)
(22.10.4 [func.require]) shall be a valid expression for some valuesw1, w2, ... , wN,
whereN == sizeof...(Args)
.-5- Effects: Constructs an object of type
thread
and executes. Constructs the following objects in memory which is accessible to a new thread of execution as if:INVOKE(f, t1, t2, ..., tN)
in a new thread of execution, wheret1, t2, ..., tN
are the values inargs...
typename decay<F>::type g(std::forward<F>(f)); tuple<typename decay<Args>::type...> w(std::forward<Args>(args)...);The new thread of execution executes
INVOKE(g, wi...)
where thewi...
refers to the elements stored in thetuple w
. Any return value fromg
is ignored.IfIf the evaluation off
terminates with an uncaught exception,std::terminate()
shall be called.INVOKE(g, wi...)
terminates with an uncaught exception,std::terminate()
shall be called [Note:std::terminate()
could be called before enteringg
. -- end note]. Any exception thrown before the evaluation ofINVOKE
has started shall be catchable in the calling thread.Text referring to when
terminate()
is called was contributed by Ganesh.
[ Batavia (2009-05): ]
We agree with the proposed resolution, but would like the final sentence to be reworded since "catchable" is not a term of art (and is used nowhere else).
[ 2009-07 Frankfurt: ]
This is linked to N2901.
Howard to open a separate issue to remove (1176(i)).
In Frankfurt there is no consensus for removing the variadic constructor.
[ 2009-10 Santa Cruz: ]
We want to move forward with this issue. If we later take it out via 1176(i) then that's ok too. Needs small group to improve wording.
[ 2009-10 Santa Cruz: ]
Stefanus provided revised wording. Moved to Review Here is the original wording:
Modify the class definition of
std::thread
in 32.4.3 [thread.thread.class] to remove the following signature:template<class F> explicit thread(F f);template<class F, class ... Args> explicit thread(F&& f, Args&& ... args);Modify 32.4.3.3 [thread.thread.constr] to replace the constructors prior to paragraph 4 with the single constructor as above. Replace paragraph 4 - 6 with the following:
-4- Requires:
F
and eachTi
inArgs
shall beCopyConstructible
if an lvalue and otherwiseMoveConstructible
.INVOKE(f, w1, w2, ..., wN)
(22.10.4 [func.require]) shall be a valid expression for some valuesw1, w2, ... , wN,
whereN == sizeof...(Args)
.-5- Effects: Constructs an object of type
thread
and executes. Constructs the following objects:INVOKE(f, t1, t2, ..., tN)
in a new thread of execution, wheret1, t2, ..., tN
are the values inargs...
typename decay<F>::type g(std::forward<F>(f)); tuple<typename decay<Args>::type...> w(std::forward<Args>(args)...);and executes
INVOKE(g, wi...)
in a new thread of execution. These objects shall be destroyed when the new thread of execution completes. Any return value fromg
is ignored.IfIf the evaluation off
terminates with an uncaught exception,std::terminate()
shall be called.INVOKE(g, wi...)
terminates with an uncaught exception,std::terminate()
shall be called [Note:std::terminate()
could be called before enteringg
. -- end note]. Any exception thrown before the evaluation ofINVOKE
has started shall be catchable in the calling thread.-6- Synchronization: The invocation of the constructor happens before the invocation of
f
g
.
[ 2010-01-19 Moved to Tentatively Ready after 5 positive votes on c++std-lib. ]
Proposed resolution:
Modify the class definition of std::thread
in 32.4.3 [thread.thread.class] to remove the
following signature:
template<class F> explicit thread(F f);template<class F, class ... Args> explicit thread(F&& f, Args&& ... args);
Modify 32.4.3.3 [thread.thread.constr] to replace the constructors prior to paragraph 4 with the single constructor as above. Replace paragraph 4 - 6 with the following:
Given a function as follows:
template<typename T> typename decay<T>::type decay_copy(T&& v) { return std::forward<T>(v); }-4- Requires:
F
and eachTi
inArgs
shallbesatisfy theCopyConstructible
if an lvalue and otherwiseMoveConstructible
requirements.INVOKE(f, w1, w2, ..., wN)
(22.10.4 [func.require]) shall be a valid expression for some valuesw1, w2, ... , wN,
whereN == sizeof...(Args)
.INVOKE(decay_copy(std::forward<F>(f)), decay_copy(std::forward<Args>(args))...)
(22.10.4 [func.require]) shall be a valid expression.-5- Effects: Constructs an object of type
thread
and executesThe new thread of execution executesINVOKE(f, t1, t2, ..., tN)
in a new thread of execution, wheret1, t2, ..., tN
are the values inargs...
. Any return value fromf
is ignored. Iff
terminates with an uncaught exception,std::terminate()
shall be called.INVOKE(decay_copy(std::forward<F>(f)), decay_copy(std::forward<Args>(args))...)
with the calls todecay_copy()
being evaluated in the constructing thread. Any return value from this invocation is ignored. [Note: this implies any exceptions not thrown from the invocation of the copy off
will be thrown in the constructing thread, not the new thread. — end note]. If the invocation ofINVOKE(decay_copy(std::forward<F>(f)), decay_copy(std::forward<Args>(args))...)
terminates with an uncaught exception,std::terminate
shall be called.-6- Synchronization: The invocation of the constructor happens before the invocation of the copy of
f
.
extent<T, I>
Section: 21.3.5.4 [meta.unary.prop] Status: C++11 Submitter: Yechezkel Mett Opened: 2008-11-04 Last modified: 2016-01-28
Priority: Not Prioritized
View other active issues in [meta.unary.prop].
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Discussion:
The draft (N2798) says in 21.3.5.4 [meta.unary.prop] Table 44:
Table 44 -- Type property queries Template Value template <class T, unsigned I = 0> struct extent;
If T
is not an array type (8.3.4), or if it has rank less thanI
, or ifI
is 0 andT
has type "array of unknown bound ofU
", then 0; otherwise, the size of theI
'th dimension ofT
Firstly it isn't clear from the wording if I
is 0-based or 1-based
("the I
'th dimension" sort of implies 1-based). From the following
example it is clear that the intent is 0-based, in which case it
should say "or if it has rank less than or equal to I
".
Sanity check:
The example says assert((extent<int[2], 1>::value) == 0);
Here the rank is 1 and I
is 1, but the desired result is 0.
[ Post Summit: ]
Do not use "size" or "value", use "bound". Also, move the cross-reference to 8.3.4 to just after "bound".
Recommend Tentatively Ready.
Proposed resolution:
In Table 44 of 21.3.5.4 [meta.unary.prop], third row, column "Value", change the cell content:
Table 44 -- Type property queries Template Value template <class T, unsigned I = 0> struct extent;
If T
is not an array type(8.3.4), or if it has rank less than or equal toI
, or ifI
is 0 andT
has type "array of unknown bound ofU
", then 0; otherwise, thesizebound (8.3.4) of theI
'th dimension ofT
, where indexing ofI
is zero-based.
[ Wording supplied by Daniel. ]
unique_ptr(pointer p)
for pointer deleter typesSection: 20.3.1.3.2 [unique.ptr.single.ctor] Status: Resolved Submitter: Howard Hinnant Opened: 2008-11-26 Last modified: 2016-01-28
Priority: Not Prioritized
View all other issues in [unique.ptr.single.ctor].
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Discussion:
Addresses US 79
20.3.1.3.2 [unique.ptr.single.ctor]/5 no longer requires for D
not to be a pointer type. I believe this restriction was accidently removed
when we relaxed the completeness reuqirements on T
. The restriction
needs to be put back in. Otherwise we have a run time failure that could
have been caught at compile time:
{ unique_ptr<int, void(*)(void*)> p1(malloc(sizeof(int))); // should not compile } // p1.~unique_ptr() dereferences a null function pointer unique_ptr<int, void(*)(void*)> p2(malloc(sizeof(int)), free); // ok
[ Post Summit: ]
Recommend Tentatively Ready.
[ 2009-07 Frankfurt ]
Moved from Tentatively Ready to Open only because the wording needs to be improved for
enable_if
type constraining, possibly following Robert's formula.
[ 2009-07 Frankfurt: ]
We need to consider whether some requirements in the Requires paragraphs of [unique.ptr] should instead be Remarks.
Leave Open. Howard to provide wording, and possibly demonstrate how this can be implemented using enable_if.
[ 2009-07-27 Howard adds: ]
The two constructors to which this issue applies are not easily constrained with
enable_if
as they are not templated:unique_ptr(); explicit unique_ptr(pointer p);To "SFINAE" these constructors away would take heroic effort such as specializing the entire
unique_ptr
class template on pointer deleter types. There is insufficient motivation for such heroics. Here is the expected and reasonable implementation for these constructors:unique_ptr() : ptr_(pointer()) { static_assert(!is_pointer<deleter_type>::value, "unique_ptr constructed with null function pointer deleter"); } explicit unique_ptr(pointer p) : ptr_(p) { static_assert(!is_pointer<deleter_type>::value, "unique_ptr constructed with null function pointer deleter"); }I.e. just use
static_assert
to verify that the constructor is not instantiated with a function pointer for a deleter. The compiler will automatically take care of issuing a diagnostic if the deleter is a reference type (uninitialized reference error).In keeping with our discussions in Frankfurt, I'm moving this requirement on the implementation from the Requires paragraph to a Remarks paragraph.
[ 2009-08-17 Daniel adds: ]
It is insufficient to require a diagnostic. This doesn't imply an ill-formed program as of 3.18 [defns.diagnostic] (a typical alternative would be a compiler warning), but exactly that seems to be the intend. I suggest to use the following remark instead:
Remarks: The program shall be ill-formed if this constructor is instantiated when
D
is a pointer type or reference type.Via the general standard rules of 4.1 [intro.compliance] the "diagnostic required" is implied.
[ 2009-10 Santa Cruz: ]
Moved to Ready.
[ 2010-03-14 Howard adds: ]
We moved N3073 to the formal motions page in Pittsburgh which should obsolete this issue. I've moved this issue to NAD Editorial, solved by N3073.
Rationale:
Solved by N3073.
Proposed resolution:
Change the description of the default constructor in 20.3.1.3.2 [unique.ptr.single.ctor]:
unique_ptr();-1- Requires:
D
shall be default constructible, and that construction shall not throw an exception.D
shall not be a reference type or pointer type (diagnostic required)....
Remarks: The program shall be ill-formed if this constructor is instantiated when
D
is a pointer type or reference type.
Add after 20.3.1.3.2 [unique.ptr.single.ctor]/8:
unique_ptr(pointer p);...
Remarks: The program shall be ill-formed if this constructor is instantiated when
D
is a pointer type or reference type.
duration
is missing operator%
Section: 30.5 [time.duration] Status: C++11 Submitter: Terry Golubiewski Opened: 2008-11-30 Last modified: 2016-01-28
Priority: Not Prioritized
View all other issues in [time.duration].
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Discussion:
Addresses US 81
duration
is missing operator%
. This operator is convenient
for computing where in a time frame a given duration
lies. A
motivating example is converting a duration
into a "broken-down"
time duration such as hours::minutes::seconds:
class ClockTime { typedef std::chrono::hours hours; typedef std::chrono::minutes minutes; typedef std::chrono::seconds seconds; public: hours hours_; minutes minutes_; seconds seconds_; template <class Rep, class Period> explicit ClockTime(const std::chrono::duration<Rep, Period>& d) : hours_ (std::chrono::duration_cast<hours> (d)), minutes_(std::chrono::duration_cast<minutes>(d % hours(1))), seconds_(std::chrono::duration_cast<seconds>(d % minutes(1))) {} };
[ Summit: ]
Agree except that there is a typo in the proposed resolution. The member operators should be
operator%=
.
[ Batavia (2009-05): ]
We agree with the proposed resolution. Move to Tentatively Ready.
[ 2009-07 Frankfurt ]
Moved from Tentatively Ready to Open only because the wording needs to be improved for enable_if type constraining, possibly following Robert's formula.
[ 2009-07 Frankfurt: ]
Howard to open a separate issue (1177(i)) to handle the removal of member functions from overload sets, provide wording, and possibly demonstrate how this can be implemented using enable_if (see 947(i)).
Move to Ready.
Proposed resolution:
Add to the synopsis in 30 [time]:
template <class Rep1, class Period, class Rep2> duration<typename common_type<Rep1, Rep2>::type, Period> operator%(const duration<Rep1, Period>& d, const Rep2& s); template <class Rep1, class Period1, class Rep2, class Period2> typename common_type<duration<Rep1, Period1>, duration<Rep2, Period2>>::type operator%(const duration<Rep1, Period1>& lhs, const duration<Rep2, Period2>& rhs);
Add to the synopsis of duration
in 30.5 [time.duration]:
template <class Rep, class Period = ratio<1>> class duration { public: ... duration& operator%=(const rep& rhs); duration& operator%=(const duration& d); ... };
Add to 30.5.4 [time.duration.arithmetic]:
duration& operator%=(const rep& rhs);Effects:
rep_ %= rhs
.Returns:
*this
.duration& operator%=(const duration& d);Effects:
rep_ %= d.count()
.Returns:
*this
.
Add to 30.5.6 [time.duration.nonmember]:
template <class Rep1, class Period, class Rep2> duration<typename common_type<Rep1, Rep2>::type, Period> operator%(const duration<Rep1, Period>& d, const Rep2& s);Requires:
Rep2
shall be implicitly convertible toCR(Rep1, Rep2)
andRep2
shall not be an instantiation ofduration
. Diagnostic required.Returns:
duration<CR, Period>(d) %= s
.template <class Rep1, class Period1, class Rep2, class Period2> typename common_type<duration<Rep1, Period1>, duration<Rep2, Period2>>::type operator%(const duration<Rep1, Period1>& lhs, const duration<Rep2, Period2>& rhs);Returns:
common_type<duration<Rep1, Period1>, duration<Rep2, Period2>>::type(lhs) %= rhs
.
default_delete<T[]>::operator()
should only accept T*
Section: 20.3.1.2.3 [unique.ptr.dltr.dflt1] Status: C++11 Submitter: Howard Hinnant Opened: 2008-12-07 Last modified: 2016-01-28
Priority: Not Prioritized
View all issues with C++11 status.
Discussion:
Consider:
derived* p = new derived[3];
std::default_delete<base[]> d;
d(p); // should fail
Currently the marked line is a run time failure. We can make it a compile
time failure by "poisoning" op(U*)
.
[ Post Summit: ]
Recommend Review.
[ Batavia (2009-05): ]
We agree with the proposed resolution. Move to Tentatively Ready.
Proposed resolution:
Add to 20.3.1.2.3 [unique.ptr.dltr.dflt1]:
namespace std { template <class T> struct default_delete<T[]> { void operator()(T*) const; template <class U> void operator()(U*) const = delete; }; }
std::identity
and reference-to-temporariesSection: 22.2.4 [forward] Status: C++11 Submitter: Alisdair Meredith Opened: 2008-12-11 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
std::identity
takes an argument of type T const &
and returns a result of T const &
.
Unfortunately, this signature will accept a value of type other than T
that
is convertible-to-T
, and then return a reference to the dead temporary. The
constraint in the concepts version simply protects against returning
reference-to-void
.
Solutions:
i/ Return-by-value, potentially slicing bases and rejecting non-copyable types
ii/ Provide an additional overload:
template< typename T > template operator( U & ) = delete;This seems closer on intent, but moves beyond the original motivation for the operator, which is compatibility with existing (non-standard) implementations.
iii/ Remove the
operator()
overload. This restores the original definition of theidentity
, although now effectively a type_trait rather than part of the perfect forwarding protocol.iv/ Remove
std::identity
completely; its original reason to exist is replaced with theIdentityOf
concept.
My own preference is somewhere between (ii) and (iii) - although I stumbled over the issue with a specific application hoping for resolution (i)!
[ Batavia (2009-05): ]
We dislike options i and iii, and option ii seems like overkill. If we remove it (option iv), implementers can still provide it under a different name.
Move to Open pending wording (from Alisdair) for option iv.
[ 2009-05-23 Alisdair provided wording for option iv. ]
[ 2009-07-20 Alisdair adds: ]
I'm not sure why this issue was not discussed at Frankfurt (or I missed the discussion) but the rationale is now fundamentally flawed. With the removal of concepts,
std::identity
again becomes an important library type so we cannot simply remove it.At that point, we need to pick one of the other suggested resolutions, but have no guidance at the moment.
[ 2009-07-20 Howard adds: ]
I believe the rationale for not addressing this issue in Frankfurt was that it did not address a national body comment.
I also believe that removal of
identity
is still a practical option as my latest reformulation offorward
, which is due to comments suggested at Summit, no longer usesidentity
. :-)template <class T, class U, class = typename enable_if < !is_lvalue_reference<T>::value || is_lvalue_reference<T>::value && is_lvalue_reference<U>::value >::type, class = typename enable_if < is_same<typename remove_all<T>::type, typename remove_all<U>::type>::value >::type> inline T&& forward(U&& t) { return static_cast<T&&>(t); }[ The above code assumes acceptance of 1120(i) for the definition of
remove_all
. This is just to make the syntax a little more palatable. Without this trait the above is still very implementable. ]Paper with rationale is on the way ... really, I promise this time! ;-)
[ 2009-07-30 Daniel adds: See 823(i) for an alternative resolution. ]
[ 2009-10 Santa Cruz: ]
Move to Ready. Howard will update proposed wording to reflect current draft.
Proposed resolution:
Strike from 22.2 [utility]:
template <class T> struct identity;
Remove from 22.2.4 [forward]:
template <class T> struct identity { typedef T type; const T& operator()(const T& x) const; };const T& operator()(const T& x) const;
-2- Returns:x
std::distance
Section: 24.4.3 [iterator.operations] Status: Resolved Submitter: Thomas Opened: 2008-12-14 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
Addresses UK 270
Regarding the std::distance
- function, 24.4.3 [iterator.operations]
p.4 says:
Returns the number of increments or decrements needed to get from first to last.
This sentence is completely silent about the sign of the return value. 24.4.3 [iterator.operations] p.1 gives more information about the underlying operations, but again no inferences about the sign can be made. Strictly speaking, that is taking that sentence literally, I think this sentence even implies a positive return value in all cases, as the number of increments or decrements is clearly a ratio scale variable, with a natural zero bound.
Practically speaking, my implementations did what common sense and
knowledge based on pointer arithmetic forecasts, namely a positive sign
for increments (that is, going from first
to last
by operator++
), and a
negative sign for decrements (going from first
to last
by operator--
).
Here are my two questions:
First, is that paragraph supposed to be interpreted in the way what I called 'common sense', that is negative sign for decrements ? I am fairly sure that's the supposed behavior, but a double-check here in this group can't hurt.
Second, is the present wording (2003 standard version - no idea about the draft for the upcoming standard) worth an edit to make it a bit more sensible, to mention the sign of the return value explicitly ?
[ Daniel adds: ]
My first thought was that resolution 204(i) would already cover the issue report, but it seems that current normative wording is in contradiction to that resolution:
Referring to N2798, 24.4.3 [iterator.operations]/ p.4 says:
Effects: Returns the number of increments or decrements needed to get from
first
tolast
.IMO the part " or decrements" is in contradiction to p. 5 which says
Requires:
last
shall be reachable fromfirst
.because "reachable" is defined in 24.3.4 [iterator.concepts]/7 as
An iterator
j
is called reachable from an iteratori
if and only if there is a finite sequence of applications of the expression++i
that makesi == j
.[..]Here is wording that would be consistent with this definition of "reachable":
Change 24.4.3 [iterator.operations] p4 as follows:
Effects: Returns the number of increments
or decrementsneeded to get fromfirst
tolast
.
Thomas adds more discussion and an alternative view point here.
[ Summit: ]
The proposed wording below was verbally agreed to. Howard provided.
[ Batavia (2009-05): ]
Pete reports that a recent similar change has been made for the
advance()
function.We agree with the proposed resolution. Move to Tentatively Ready.
[ 2009-07 Frankfurt ]
Moved from Tentatively Ready to Open only because the wording needs to be tweaked for concepts removal.
[ 2009-07 Frankfurt: ]
Leave Open pending arrival of a post-Concepts WD.
[ 2009-10-14 Daniel provided de-conceptified wording. ]
[ 2009-10 Santa Cruz: ]
Move to Ready, replacing the Effects clause in the proposed wording with "If InputIterator meets the requirements of random access iterator then returns (last - first), otherwise returns the number of increments needed to get from first to list.".
[ 2010 Pittsburgh: ]
Moved to
NAD EditorialResolved. Rationale added below.
Rationale:
Solved by N3066.
Proposed resolution:
Change 24.3.5.7 [random.access.iterators], Table 105 as indicated [This change is not
essential but it simplifies the specification] for the row with expression "b - a
"
and the column Operational semantics:
(a < b) ?distance(a,b): -distance(b,a)
Change 24.4.3 [iterator.operations]/4+5 as indicated:
template<class InputIterator> typename iterator_traits<InputIterator>::difference_type distance(InputIterator first, InputIterator last);4 Effects: If
InputIterator
meets the requirements of random access iterator then returns(last - first)
, otherwiseRreturns the number of incrementsor decrementsneeded to get fromfirst
tolast
.5 Requires: If
InputIterator
meets the requirements of random access iterator thenlast
shall be reachable fromfirst
orfirst
shall be reachable fromlast
, otherwiselast
shall be reachable fromfirst
.
ssize_t
undefinedSection: 99 [atomics.types.address] Status: C++11 Submitter: Holger Grund Opened: 2008-12-19 Last modified: 2016-01-28
Priority: Not Prioritized
View all other issues in [atomics.types.address].
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Discussion:
There is a row in "Table 122 - Atomics for standard typedef types"
in 99 [atomics.types.integral] with atomic_ssize_t
and ssize_t
. Unless, I'm missing something ssize_t
is not defined by the standard.
[ Summit: ]
Move to review. Proposed resolution: Remove the typedef. Note:
ssize_t
is a POSIX type.
[ Batavia (2009-05): ]
We agree with the proposed resolution. Move to Tentatively Ready.
Proposed resolution:
Remove the row containing ssize_t
from Table 119
"Atomics for standard typedef types" in 99 [atomics.types.address].
atomic<bool>
derive from atomic_bool
?Section: 32.5.8 [atomics.types.generic] Status: Resolved Submitter: Holger Grund Opened: 2008-12-19 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
I think it's fairly obvious that atomic<bool>
is supposed to be derived
from atomic_bool
(and otherwise follow the atomic<integral>
interface),
though I think the current wording doesn't support this. I raised this
point along with atomic<floating-point>
privately with Herb and I seem
to recall it came up in the resulting discussion on this list. However,
I don't see anything on the current libs issue list mentioning this
problem.
32.5.8 [atomics.types.generic]/3 reads
There are full specializations over the integral types on the atomic class template. For each integral type integral in the second column of table 121 or table 122, the specialization
atomic<integral>
shall be publicly derived from the corresponding atomic integral type in the first column of the table. These specializations shall have trivial default constructors and trivial destructors.
Table 121 does not include (atomic_bool
, bool
),
so that this should probably be mentioned explicitly in the quoted paragraph.
[ Summit: ]
Move to open. Lawrence will draft a proposed resolution. Also, ask Howard to fix the title.
[ Post Summit Anthony provided proposed wording. ]
[ 2009-10 Santa Cruz: ]
NAD EditorialResolved. Addressed by N2992.
Proposed resolution:
Replace paragraph 3 in 32.5.8 [atomics.types.generic] with
-3- There are full specializations over the integral types on the
atomic
class template. For each integral typeintegral
in the second column of table 121 or table 122, the specializationatomic<integral>
shall be publicly derived from the corresponding atomic integral type in the first column of the table. In addition, the specializationatomic<bool>
shall be publicly derived fromatomic_bool
. These specializations shall have trivial default constructors and trivial destructors.
duration
arithmetic: contradictory requirementsSection: 30.5.6 [time.duration.nonmember] Status: Resolved Submitter: Pete Becker Opened: 2008-12-20 Last modified: 2016-01-28
Priority: Not Prioritized
View all other issues in [time.duration.nonmember].
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Discussion:
In 30.5.6 [time.duration.nonmember], paragraph 8 says that calling
dur / rep
when rep
is an instantiation of duration
requires a diagnostic. That's followed by an operator/
that takes
two durations. So dur1 / dur2
is legal under the second version,
but requires a diagnostic under the first.
[ Howard adds: ]
Please see the thread starting with c++std-lib-22980 for more information.
[ Batavia (2009-05): ]
Move to Open, pending proposed wording (and preferably an implementation).
[ 2009-07-27 Howard adds: ]
I've addressed this issue under the proposed wording for 1177(i) which cleans up several places under 30.5 [time.duration] which used the phrase "diagnostic required".
For clarity's sake, here is an example implementation of the constrained
operator/
:template <class _Duration, class _Rep, bool = __is_duration<_Rep>::value> struct __duration_divide_result { }; template <class _Duration, class _Rep2, bool = is_convertible<_Rep2, typename common_type<typename _Duration::rep, _Rep2>::type>::value> struct __duration_divide_imp { }; template <class _Rep1, class _Period, class _Rep2> struct __duration_divide_imp<duration<_Rep1, _Period>, _Rep2, true> { typedef duration<typename common_type<_Rep1, _Rep2>::type, _Period> type; }; template <class _Rep1, class _Period, class _Rep2> struct __duration_divide_result<duration<_Rep1, _Period>, _Rep2, false> : __duration_divide_imp<duration<_Rep1, _Period>, _Rep2> { }; template <class _Rep1, class _Period, class _Rep2> inline typename __duration_divide_result<duration<_Rep1, _Period>, _Rep2>::type operator/(const duration<_Rep1, _Period>& __d, const _Rep2& __s) { typedef typename common_type<_Rep1, _Rep2>::type _Cr; duration<_Cr, _Period> __r = __d; __r /= static_cast<_Cr>(__s); return __r; }
__duration_divide_result
is basically a custom-builtenable_if
that will containtype
only ifRep2
is not aduration
and ifRep2
is implicitly convertible tocommon_type<typename Duration::rep, Rep2>::type
.__is_duration
is simply a private trait that answersfalse
, but is specialized forduration
to answertrue
.The constrained
operator%
works identically.
[ 2009-10 Santa Cruz: ]
Proposed resolution:
ratio
arithmetic tweakSection: 21.4.4 [ratio.arithmetic] Status: C++11 Submitter: Howard Hinnant Opened: 2008-12-26 Last modified: 2016-01-28
Priority: Not Prioritized
View all other issues in [ratio.arithmetic].
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Discussion:
N2800, 21.4.4 [ratio.arithmetic] lacks a paragraph from the proposal N2661:
ratio arithmetic [ratio.arithmetic]
... If the implementation is unable to form the indicated
ratio
due to overflow, a diagnostic shall be issued.
The lack of a diagnostic on compile-time overflow is a significant lack of functionality. This paragraph could be put back into the WP simply editorially. However in forming this issue I realized that we can do better than that. This paragraph should also allow alternative formulations which go to extra lengths to avoid overflow when possible. I.e. we should not mandate overflow when the implementation can avoid it.
For example:
template <class R1, class R2> struct ratio_multiply { typedef see below} type;The nested typedef type shall be a synonym for
ratio<T1, T2>
whereT1
has the valueR1::num * R2::num
andT2
has the valueR1::den * R2::den
.
Consider the case where intmax_t
is a 64 bit 2's complement signed integer,
and we have:
typedef std::ratio<0x7FFFFFFFFFFFFFFF, 0x7FFFFFFFFFFFFFF0> R1; typedef std::ratio<8, 7> R2; typedef std::ratio_multiply<R1, R2>::type RT;
According to the present formulation the implementaiton will multiply
0x7FFFFFFFFFFFFFFF * 8
which will result in an overflow and subsequently
require a diagnostic.
However if the implementation is first allowed to divde 0x7FFFFFFFFFFFFFFF
by 7
obtaining 0x1249249249249249 / 1
and divide
8
by 0x7FFFFFFFFFFFFFF0
obtaining 1 / 0x0FFFFFFFFFFFFFFE
,
then the exact result can then be computed without overflow:
[0x7FFFFFFFFFFFFFFF/0x7FFFFFFFFFFFFFF0] * [8/7] = [0x1249249249249249/0x0FFFFFFFFFFFFFFE]
Example implmentation which accomplishes this:
template <class R1, class R2> struct ratio_multiply { private: typedef ratio<R1::num, R2::den> _R3; typedef ratio<R2::num, R1::den> _R4; public: typedef ratio<__ll_mul<_R3::num, _R4::num>::value, __ll_mul<_R3::den, _R4::den>::value> type; };
[ Post Summit: ]
Recommend Tentatively Ready.
Proposed resolution:
Add a paragraph prior to p1 in 21.4.4 [ratio.arithmetic]:
Implementations may use other algorithms to compute the indicated ratios to avoid overflow. If overflow occurs, a diagnostic shall be issued.
owner_less
Section: 20.3.2.4 [util.smartptr.ownerless] Status: C++11 Submitter: Thomas Plum Opened: 2008-12-30 Last modified: 2016-01-28
Priority: Not Prioritized
View all issues with C++11 status.
Discussion:
20.3.2.4 [util.smartptr.ownerless] (class template owner_less
) says that
operator()(x,y)
shall return x.before(y)
.
However, shared_ptr
and weak_ptr
have an owner_before()
but not a
before()
, and there's no base class to provide a missing before()
.
Being that the class is named owner_less
, I'm guessing that
"before()
" should be "owner_before()
", right?
[ Herve adds: ]
Agreed with the typo, it should be "shall return
x.owner_before(y)
".
[ Post Summit: ]
Recommend Tentatively Ready.
Proposed resolution:
Change 20.3.2.4 [util.smartptr.ownerless] p2:
-2-
operator()(x,y)
shall returnx.owner_before(y)
. [Note: ...
unique_ptr
converting ctor shouldn't accept array formSection: 20.3.1.3.2 [unique.ptr.single.ctor] Status: Resolved Submitter: Howard Hinnant Opened: 2009-01-07 Last modified: 2016-01-28
Priority: Not Prioritized
View all other issues in [unique.ptr.single.ctor].
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Discussion:
unique_ptr
's of array type should not convert to
unique_ptr
's which do not have an array type.
struct Deleter
{
void operator()(void*) {}
};
int main()
{
unique_ptr<int[], Deleter> s;
unique_ptr<int, Deleter> s2(std::move(s)); // should not compile
}
[ Post Summit: ]
Walter: Does the "diagnostic required" apply to both arms of the "and"?
Tom Plum: suggest to break into several sentences
Walter: suggest "comma" before the "and" in both places
Recommend Review.
[ Batavia (2009-05): ]
The post-Summit comments have been applied to the proposed resolution. We now agree with the proposed resolution. Move to Tentatively Ready.
[ 2009-07 Frankfurt ]
Moved from Tentatively Ready to Open only because the wording needs to be improved for
enable_if
type constraining, possibly following Robert's formula.
[ 2009-08-01 Howard updates wording and sets to Review. ]
[ 2009-10 Santa Cruz: ]
Move to Ready.
[ 2010-02-27 Pete Opens: ]
The proposed replacement text doesn't make sense.
If
D
is a reference type, thenE
shall be the same type asD
, else this constructor shall not participate in overload resolution.This imposes two requirements. 1. If
D
is a reference type,E
has to beD
. 2. IfD
is not a reference type, the constructor shall not participate in overload resolution. If the latter apples, the language in the preceding paragraph that this constructor shall not throw an exception ifD
is not a reference type is superfluous. I suspect that's not the intention, but I can't parse this text any other way.
U
shall not be an array type, else this constructor shall not participate in overload resolution.I don't know what this means.
[ 2010-02-27 Peter adds: ]
I think that the intent is (proposed text):
Remarks: this constructor shall only participate in overload resolution if:
unique_ptr<U, E>::pointer
is implicitly convertible topointer
,U
is not an array type, and- if
D
is a reference type,E
is the same type asD
.
[ 2010-02-28 Howard adds: ]
I like Peter's proposal. Here is a tweak of it made after looking at my implementation. I believe this fixes a further defect not addressed by the current proposed wording:
Remarks: this constructor shall only participate in overload resolution if:
unique_ptr<U, E>::pointer
is implicitly convertible topointer
, andU
is not an array type, and- if
D
is a reference type,E
is the same type asD
, elseE
shall be implicitly convertible toD
.
[ 2010 Pittsburgh: Moved to NAD Editorial. Rationale added below. ]
Rationale:
Solved by N3073.
Proposed resolution:
Change 20.3.1.3.2 [unique.ptr.single.ctor]:
template <class U, class E> unique_ptr(unique_ptr<U, E>&& u);-20- Requires: If
D
is not a reference type, construction of the deleterD
from an rvalue of typeE
shall be well formed and shall not throw an exception.IfD
is a reference type, thenE
shall be the same type asD
(diagnostic required).unique_ptr<U, E>::pointer
shall be implicitly convertible topointer
. [Note: These requirements imply thatT
andU
are complete types. — end note]Remarks: If
D
is a reference type, thenE
shall be the same type asD
, else this constructor shall not participate in overload resolution.unique_ptr<U, E>::pointer
shall be implicitly convertible topointer
, else this constructor shall not participate in overload resolution.U
shall not be an array type, else this constructor shall not participate in overload resolution. [Note: These requirements imply thatT
andU
are complete types. — end note]
Change 20.3.1.3.4 [unique.ptr.single.asgn]:
template <class U, class E> unique_ptr& operator=(unique_ptr<U, E>&& u);-6- Requires: Assignment of the deleter
D
from an rvalueD
shall not throw an exception.unique_ptr<U, E>::pointer
shall be implicitly convertible topointer
. [Note: These requirements imply thatT
andU
are complete types. — end note]Remarks:
unique_ptr<U, E>::pointer
shall be implicitly convertible topointer
, else this operator shall not participate in overload resolution.U
shall not be an array type, else this operator shall not participate in overload resolution. [Note: These requirements imply thatT
andU
are complete types. — end note]
Section: 30.4.1 [time.traits.is.fp] Status: C++11 Submitter: Pete Becker Opened: 2009-01-07 Last modified: 2021-06-06
Priority: Not Prioritized
View all issues with C++11 status.
Discussion:
[time.traits.is_fp] says that the type Rep
"is
assumed to be ... a class emulating an integral type." What are the
requirements for such a type?
[ 2009-05-10 Howard adds: ]
IntegralLike
.
[ Batavia (2009-05): ]
As with issue 953(i), we recommend this issue be addressed in the context of providing concepts for the entire
thread
header.We look forward to proposed wording.
Move to Open.
[ 2009-08-01 Howard adds: ]
I have surveyed all clauses of [time.traits.duration_values], 30.4.3 [time.traits.specializations] and 30.5 [time.duration]. I can not find any clause which involves the use of a
duration::rep
type where the requirements on therep
type are not clearly spelled out. These requirements were carefully crafted to allow any arithmetic type, or any user-defined type emulating an arithmetic type.Indeed,
treat_as_floating_point
becomes completely superfluous ifduration::rep
can never be a class type.There will be some
Rep
types which will not meet the requirements of everyduration
operation. This is no different than the fact thatvector<T>
can easily be used for typesT
which are notDefaultConstructible
, even though some members ofvector<T>
requireT
to beDefaultConstructible
. This is why the requirements onRep
are specified for each operation individually.In [time.traits.is_fp] p1:
template <class Rep> struct treat_as_floating_point : is_floating_point<Rep> { };The
duration
template uses thetreat_as_floating_point
trait to help determine if aduration
object can be converted to anotherduration
with a different tick period. Iftreat_as_floating_point<Rep>::value
istrue
, thenRep
is a floating-point type and implicit conversions are allowed amongduration
s. Otherwise, the implicit convertibility depends on the tick periods of theduration
s. IfRep
is a class type which emulates a floating-point type, the author ofRep
can specializetreat_as_floating_point
so thatduration
will treat thisRep
as if it were a floating-point type. OtherwiseRep
is assumed to be an integral type or a class emulating an integral type.The phrases "a class type which emulates a floating-point type" and "a class emulating an integral type" are clarifying phrases which refer to the summation of all the requirements on the
Rep
type specified in detail elsewhere (and should not be repeated here).This specification has been implemented, now multiple times, and the experience has been favorable. The current specification clearly specifies the requirements at each point of use (though I'd be happy to fix any place I may have missed, but none has been pointed out).
I am amenable to improved wording of this paragraph (and any others), but do not have any suggestions for improved wording at this time. I am strongly opposed to changes which would significantly alter the semantics of the specification under 30 [time] without firmly grounded and documented rationale, example implementation, testing, and user experience which relates a positive experience.
I recommend NAD unless someone wants to produce some clarifying wording.
[ 2009-10 Santa Cruz: ]
Stefanus to provide wording to turn this into a note.
[ 2010-02-11 Stefanus provided wording. ]
[ 2010 Rapperswil: ]
Move to Ready.
[ Adopted at 2010-11 Batavia ]
Proposed resolution:
Change [time.traits.is_fp]/1:
1 The
duration
template uses thetreat_as_floating_point
trait to help determine if aduration
object can be converted to anotherduration
with a different tick period. Iftreat_as_floating_point<Rep>::value
istrue
, thenimplicit conversions are allowed amongRep
is a floating-point type andduration
s. Otherwise, the implicit convertibility depends on the tick periods of theduration
s.If[Note: The intention of this trait is to indicate whether a given class behaves like a floating point type, and thus allows division of one value by another with acceptable loss of precision. IfRep
is a class type which emulates a floating-point type, the author ofRep
can specializetreat_as_floating_point
so that duration will treat thisRep
as if it were a floating-point type. OtherwiseRep
is assumed to be an integral type or a class emulating an integral type.treat_as_floating_point<Rep>::value
isfalse
,Rep
will be treated as if it behaved like an integral type for the purpose of these conversions. — end note]
Section: 30.3 [time.clock.req] Status: Resolved Submitter: Pete Becker Opened: 2009-01-07 Last modified: 2016-01-28
Priority: Not Prioritized
View all other issues in [time.clock.req].
View all issues with Resolved status.
Discussion:
30.3 [time.clock.req] says that a clock's rep
member is "an
arithmetic type or a class emulating an arithmetic type." What are the
requirements for such a type?
[ 2009-05-10 Howard adds: ]
This wording was aimed directly at the
ArithmeticLike
concept.
[ Batavia (2009-05): ]
We recommend this issue be addressed in the context of providing concepts for the entire
thread
header.May resolve for now by specifying arithmetic types, and in future change to
ArithmeticLike
. However, Alisdair believes this is not feasible.Bill disagrees.
We look forward to proposed wording. Move to Open.
[ 2009-08-01 Howard adds: ]
[ 2009-10 Santa Cruz: ]
Stefanus to provide wording to turn this into a note.
[ 2010-02-11 Stephanus provided wording for 951(i) which addresses this issue as well. ]
[ 2010 Rapperswil: ]
Proposed resolution:
Section: 30.3 [time.clock.req] Status: C++11 Submitter: Pete Becker Opened: 2009-01-07 Last modified: 2016-01-28
Priority: Not Prioritized
View all other issues in [time.clock.req].
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Discussion:
Table 55 — Clock Requirements (in 30.3 [time.clock.req])
C1::time_point
require C1
and C2
to "refer to the same epoch", but "epoch" is not defined.
time_point
definition if it is
valid to compare their time_point
s by comparing their
respective duration
s." What does "valid" mean here? And, since
C1::rep
is "THE representation type of the native
duration
and time_point
" (emphasis added), there
doesn't seem to be much room for some other representation.
C1::is_monotonic
has type "const bool
". The
"const
" should be removed.
C1::period
has type ratio
. ratio
isn't a type,
it's a template. What is the required type?
[ 2009-05-10 Howard adds: ]
"epoch" is purposefully not defined beyond the common English
definition. The C standard
also chose not to define epoch, though POSIX did. I believe it is a strength
of the C standard that epoch is not defined. When it is known that two time_point
s
refer to the same epoch, then a definition of the epoch is not needed to compare
the two time_point
s, or subtract them.
A time_point
and a Clock
implicitly refer to an (unspecified) epoch.
The time_point
represents an offset (duration
) from an epoch.
The sentence:
Different clocks may share a
time_point
definition if it is valid to compare theirtime_point
s by comparing their respectiveduration
s.
is redundant and could be removed. I believe the sentence which follows the above:
C1
andC2
shall refer to the same epoch.
is sufficient. If two clocks share the same epoch, then by definition, comparing
their time_point
s is valid.
is_monotonic
is meant to never change (be const
). It is also
desired that this value be usable in compile-time computation and branching.
This should probably instead be worded:
An instantiation of
ratio
.
[ Batavia (2009-05): ]
Re (a): It is not clear to us whether "epoch" is a term of art.
Re (b), (c), and (d): We agree with Howard's comments, and would consider adding to (c) a
static constexpr
requirement.Move to Open pending proposed wording.
[ 2009-05-25 Daniel adds: ]
In regards to (d) I suggest to say "a specialization of ratio" instead of "An instantiation of ratio". This seems to be the better matching standard core language term for this kind of entity.
[ 2009-05-25 Ganesh adds: ]
Regarding (a), I found this paper on the ISO website using the term "epoch" consistently with the current wording:
which is part of ISO/IEC 18026 "Information technology -- Spatial Reference Model (SRM)".
[ 2009-08-01 Howard: Moved to Reivew as the wording requested in Batavia has been provided. ]
[ 2009-10 Santa Cruz: ]
Move to Ready.
Proposed resolution:
Change 30.3 [time.clock.req] p1:
-1- A clock is a bundle consisting of a native
duration
, a nativetime_point
, and a functionnow()
to get the currenttime_point
. The origin of the clock'stime_point
is referred to as the clock's epoch as defined in section 6.3 of ISO/IEC 18026. A clock shall meet the requirements in Table 45.
Remove the sentence from the time_point
row of the table "Clock Requirements":
C1::time_point
|
chrono::time_point<C1> or chrono::time_point<C2, C1::duration>
|
The native time_point type of the clock.
time_point definition if it is valid to compare their time_point s by comparing their respective duration s.C1 and C2 shall refer to the same epoch.
|
Change the row starting with C1::period
of the table "Clock Requirements":
C1::period
|
a specialization of ratio
|
The tick period of the clock in seconds. |
Section: 30.3 [time.clock.req] Status: C++11 Submitter: Pete Becker Opened: 2009-01-07 Last modified: 2016-01-28
Priority: Not Prioritized
View all other issues in [time.clock.req].
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Discussion:
30.3 [time.clock.req] uses the word "native" in several places,
but doesn't define it. What is a "native duration
"?
[ 2009-05-10 Howard adds: ]
The standard uses "native" in several places without defining it (e.g. 5.13.3 [lex.ccon]). It is meant to mean "that which is defined by the facility", or something along those lines. In this case it refers to the nested
time_point
andduration
types of the clock. Better wording is welcome.
[ Batavia (2009-05): ]
Move to Open pending proposed wording from Pete.
[ 2009-10-23 Pete provides wording: ]
[ 2009-11-18 Daniel adds: ]
I see that 32.6.4.3 [thread.timedmutex.requirements]/3 says:
Precondition: If the tick
period
ofrel_time
is not exactly convertible to the native tickperiod
, theduration
shall be rounded up to the nearest native tickperiod
.I would prefer to see that adapted as well. Following the same style as the proposed resolution I come up with
Precondition: If the tick
period
ofrel_time
is not exactly convertible to thenativetickperiod
of the execution environment, theduration
shall be rounded up to the nearestnativetickperiod
of the execution environment.
[ 2010-03-28 Daniel synced wording with N3092 ]
[ Post-Rapperswil, Howard provides wording: ]
Moved to Tentatively Ready with revised wording from Howard Hinnant after 5 positive votes on c++std-lib.
[ Adopted at 2010-11 Batavia ]
Proposed resolution:
Change 30.3 [time.clock.req]:
1 A clock is a bundle consisting of a
nativeduration
, anativetime_point
, and a functionnow()
to get the currenttime_point
. The origin of the clock'stime_point
is referred to as the clock's epoch. A clock shall meet the requirements in Table 56.2 ...
Table 56 — Clock requirements Expression Return type Operational semantics C1::rep
An arithmetic type or a class emulating an arithmetic type The representation type of the nativeC1::duration
.andtime_point
.C1::period
... ... C1::duration
chrono::duration<C1::rep, C1::period>
The nativeduration
type of the clock.C1::time_point
chrono::time_point<C1>
orchrono::time_point<C2, C1::duration>
The nativetime_point
type of the clock.C1
andC2
shall refer to the same epoch....
Section: 30.7.2 [time.clock.system] Status: C++11 Submitter: Pete Becker Opened: 2009-01-07 Last modified: 2016-01-28
Priority: Not Prioritized
View all other issues in [time.clock.system].
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Discussion:
30.7.2 [time.clock.system]: to_time_t
is overspecified. It
requires truncation, but should allow rounding. For example, suppose a
system has a clock that gives times in milliseconds, but time()
rounds
those times to the nearest second. Then system_clock
can't use any
resolution finer than one second, because if it did, truncating times
between half a second and a full second would produce the wrong time_t
value.
[ Post Summit Anthony Williams provided proposed wording. ]
[ Batavia (2009-05): ]
Move to Review pending input from Howard. and other stakeholders.
[ 2009-05-23 Howard adds: ]
I am in favor of the wording provided by Anthony.
[ 2009-10 Santa Cruz: ]
Move to Ready.
Proposed resolution:
In 30.7.2 [time.clock.system] replace paragraphs 3 and 4 with:
time_t to_time_t(const time_point& t);-3- Returns: A
time_t
object that represents the same point in time ast
when both values aretruncatedrestricted to the coarser of the precisions oftime_t
andtime_point
. It is implementation defined whether values are rounded or truncated to the required precision.time_point from_time_t(time_t t);-4- Returns: A
time_point
object that represents the same point in time ast
when both values aretruncatedrestricted to the coarser of the precisions oftime_t
andtime_point
. It is implementation defined whether values are rounded or truncated to the required precision.
Section: 32.7.4 [thread.condition.condvar] Status: Resolved Submitter: Pete Becker Opened: 2009-01-07 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
32.7.4 [thread.condition.condvar]: the specification for wait_for
with no predicate has an effects clause that says it calls wait_until
,
and a returns clause that sets out in words how to determine the return
value. Is this description of the return value subtly different from the
description of the value returned by wait_until
? Or should the effects
clause and the returns clause be merged?
[ Summit: ]
Move to open. Associate with LWG 859(i) and any other monotonic-clock related issues.
[ 2009-08-01 Howard adds: ]
I believe that 859(i) (currently Ready) addresses this issue, and that this issue should be marked NAD, solved by 859(i) (assuming it moves to WP).
[ 2009-10 Santa Cruz: ]
Mark as
NAD EditorialResolved, addressed by resolution of Issue 859(i).
Proposed resolution:
Section: 32.6.4 [thread.mutex.requirements] Status: C++11 Submitter: Pete Becker Opened: 2009-01-07 Last modified: 2016-01-28
Priority: Not Prioritized
View other active issues in [thread.mutex.requirements].
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Discussion:
32.6.4 [thread.mutex.requirements]: paragraph 4 is entitled "Error conditions", but according to 16.3.2.4 [structure.specifications], "Error conditions:" specifies "the error conditions for error codes reported by the function." It's not clear what this should mean when there is no function in sight.
[ Summit: ]
Move to open.
[ Beman provided proposed wording. ]
[ 2009-10 Santa Cruz: ]
Move to Ready. Fix the proposed wording with "functions of type Mutex" -> "functions of Mutex type"
Proposed resolution:
Change 32.6.4 [thread.mutex.requirements] Mutex requirements, paragraph 4 as indicated:
-4-
Error conditions:The error conditions for error codes, if any, reported by member functions of Mutex type shall be:
not_enough_memory
— if there is not enough memory to construct the mutex object.resource_unavailable_try_again
— if any native handle type manipulated is not available.operation_not_permitted
— if the thread does not have the necessary permission to change the state of the mutex object.device_or_resource_busy
— if any native handle type manipulated is already locked.invalid_argument
— if any native handle type manipulated as part of mutex construction is incorrect.
Section: 32.6.5.4.3 [thread.lock.unique.locking] Status: C++11 Submitter: Pete Becker Opened: 2009-01-07 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
32.6.5.4.3 [thread.lock.unique.locking]: unique_lock::lock
is
required to throw an object of type std::system_error
"when the
postcondition cannot be achieved." The postcondition is owns == true
,
and this is trivial to achieve. Presumably, the requirement is intended
to mean something more than that.
[ Summit: ]
Move to open.
[ Beman has volunteered to provide proposed wording. ]
[ 2009-07-21 Beman added wording to address 32.2.2 [thread.req.exception] in response to the Frankfurt notes in 859(i). ]
[ 2009-09-25 Beman: minor update to wording. ]
[ 2009-10 Santa Cruz: ]
Move to Ready.
Proposed resolution:
Change Exceptions 32.2.2 [thread.req.exception] as indicated:
Some functions described in this Clause are specified to throw exceptions of type
system_error
(19.5.5). Such exceptions shall be thrown if any of the Error conditions are detected or a call to an operating system or other underlying API results in an error that prevents the library function fromsatisfying its postconditions or from returning a meaningful valuemeeting its specifications. Failure to allocate storage shall be reported as described in 16.4.6.14 [res.on.exception.handling].
Change thread assignment 32.4.3.6 [thread.thread.member], join(), paragraph 8 as indicated:
Throws:
std::system_error
whenthe postconditions cannot be achievedan exception is required (32.2.2 [thread.req.exception]).
Change thread assignment 32.4.3.6 [thread.thread.member], detach(), paragraph 13 as indicated:
Throws:
std::system_error
whenthe effects or postconditions cannot be achievedan exception is required (32.2.2 [thread.req.exception]).
Change Mutex requirements 32.6.4 [thread.mutex.requirements], paragraph 11, as indicated:
Throws:
std::system_error
whenthe effects or postcondition cannot be achievedan exception is required (32.2.2 [thread.req.exception]).
Change unique_lock locking 32.6.5.4.3 [thread.lock.unique.locking], paragraph 3, as indicated:
Throws:
std::system_error
whenthe postcondition cannot be achievedan exception is required (32.2.2 [thread.req.exception]).
Change unique_lock locking 32.6.5.4.3 [thread.lock.unique.locking], paragraph 8, as indicated:
Throws:
std::system_error
whenthe postcondition cannot be achievedan exception is required (32.2.2 [thread.req.exception]).
Change unique_lock locking 32.6.5.4.3 [thread.lock.unique.locking], paragraph 13, as indicated:
Throws:
std::system_error
whenthe postcondition cannot be achievedan exception is required (32.2.2 [thread.req.exception]).
Change unique_lock locking 32.6.5.4.3 [thread.lock.unique.locking], paragraph 18, as indicated:
Throws:
std::system_error
whenthe postcondition cannot be achievedan exception is required (32.2.2 [thread.req.exception]).
Change unique_lock locking 32.6.5.4.3 [thread.lock.unique.locking], paragraph 22, as indicated:
Throws:
std::system_error
whenthe postcondition cannot be achievedan exception is required (32.2.2 [thread.req.exception]).
Change Function call_once 32.6.7.2 [thread.once.callonce], paragraph 4, as indicated
Throws:
std::system_error
whenthe effects cannot be achievedan exception is required (32.2.2 [thread.req.exception]), or any exception thrown byfunc
.
Change Class condition_variable 32.7.4 [thread.condition.condvar], paragraph 12, as indicated:
Throws:
std::system_error
whenthe effects or postcondition cannot be achievedan exception is required (32.2.2 [thread.req.exception]).
Change Class condition_variable 32.7.4 [thread.condition.condvar], paragraph 19, as indicated:
Throws:
std::system_error
whenthe effects or postcondition cannot be achievedan exception is required (32.2.2 [thread.req.exception]).
Change Class condition_variable_any 32.7.5 [thread.condition.condvarany], paragraph 10, as indicated:
Throws:
std::system_error
whenthe effects or postcondition cannot be achievedan exception is required (32.2.2 [thread.req.exception]).
Change Class condition_variable_any 32.7.5 [thread.condition.condvarany], paragraph 16, as indicated:
Throws:
std::system_error
whenthe returned value, effects, or postcondition cannot be achievedan exception is required (32.2.2 [thread.req.exception]).
Assuming issue 859(i), Monotonic Clock is Conditionally Supported?, has been applied to the working paper, change Change 32.7.4 [thread.condition.condvar] as indicated:
template <class Rep, class Period> bool wait_for(unique_lock<mutex>& lock, const chrono::duration<Rep, Period>& rel_time);...Throws:
std::system_error
whenthe effects or postcondition cannot be achievedan exception is required ([thread.req.exception]).
Assuming issue 859(i), Monotonic Clock is Conditionally Supported?, has been applied to the working paper, change Change 32.7.4 [thread.condition.condvar] as indicated:
template <class Rep, class Period, class Predicate> bool wait_for(unique_lock<mutex>& lock, const chrono::duration<Rep, Period>& rel_time, Predicate pred);...Throws:
std::system_error
whenthe effects or postcondition cannot be achievedan exception is required (32.2.2 [thread.req.exception]).
Assuming issue 859(i), Monotonic Clock is Conditionally Supported?, has been applied to the working paper, change 32.7.5 [thread.condition.condvarany] as indicated:
template <class Lock, class Rep, class Period> bool wait_for(Lock& lock, const chrono::duration<Rep, Period>& rel_time);...Throws:
std::system_error
whenthe returned value, effects or postcondition cannot be achievedan exception is required (32.2.2 [thread.req.exception]).
Assuming issue 859(i), Monotonic Clock is Conditionally Supported?, has been applied to the working paper, change 32.7.5 [thread.condition.condvarany] as indicated:
template <class Lock, class Rep, class Period, class Predicate> bool wait_for(Lock& lock, const chrono::duration<Rep, Period>& rel_time, Predicate pred);...Throws:
std::system_error
whenthe returned value, effects or postcondition cannot be achievedan exception is required (32.2.2 [thread.req.exception]).
Section: 32.4.3.6 [thread.thread.member] Status: C++11 Submitter: Pete Becker Opened: 2009-01-07 Last modified: 2016-01-28
Priority: Not Prioritized
View all other issues in [thread.thread.member].
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Discussion:
32.4.3.6 [thread.thread.member]: thread::detach
is required to
throw an exception if the thread is "not a detachable thread".
"Detachable" is never defined.
[ Howard adds: ]
Due to a mistake on my part, 3 proposed resolutions appeared at approximately the same time. They are all three noted below in the discussion.
[ Summit, proposed resolution: ]
In 32.4.3.6 [thread.thread.member] change:
void detach();...
-14- Error conditions:
no_such_process
-- if the thread is notavalidthread.invalid_argument
-- if the thread is nota detachablejoinablethread.
[ Post Summit, Jonathan Wakely adds: ]
A
thread
is detachable if it is joinable. As we've defined joinable, we can just use that.This corresponds to the pthreads specification, where pthread_detach fails if the thread is not joinable:
EINVAL: The implementation has detected that the value specified by thread does not refer to a joinable thread.
Jonathan recommends this proposed wording:
In 32.4.3.6 [thread.thread.member] change:
void detach();...
-14- Error conditions:
- ...
invalid_argument
-- not adetachablejoinable thread.
[ Post Summit, Anthony Williams adds: ]
This is covered by the precondition that
joinable()
betrue
.Anthony recommends this proposed wording:
In 32.4.3.6 [thread.thread.member] change:
void detach();...
-14- Error conditions:
- ...
invalid_argument
-- not a detachable thread.
[ 2009-10 Santa Cruz: ]
Mark as Ready with proposed resolution from Summit.
Proposed resolution:
In 32.4.3.6 [thread.thread.member] change:
void detach();...
-14- Error conditions:
no_such_process
-- if the thread is notavalidthread.invalid_argument
-- if the thread is nota detachablejoinablethread.
Section: 32.7.5 [thread.condition.condvarany] Status: Resolved Submitter: Pete Becker Opened: 2009-01-07 Last modified: 2016-01-28
Priority: Not Prioritized
View all other issues in [thread.condition.condvarany].
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Discussion:
The requirements for the constructor for condition_variable
has several
error conditions, but the requirements for the constructor for
condition_variable_any
has none. Is this difference intentional?
[ Summit: ]
Move to open, pass to Howard. If this is intentional, a note may be helpful. If the error conditions are to be copied from
condition_variable
, this depends on LWG 965(i).
[ Post Summit Howard adds: ]
The original intention (N2447) was to let the OS return whatever errors it was going to return, and for those to be translated into exceptions, for both
condition_variable
andcondition_variable_any
. I have not received any complaints about specific error conditions from vendors on non-POSIX platforms, but such complaints would not surprise me if they surfaced.
[ 2009-10 Santa Cruz: ]
Leave open. Benjamin to provide wording.
[ 2010 Pittsburgh: ]
We don't have throw clauses for condition variables.
This issue may be dependent on LWG 1268(i).
Leave open. Detlef will coordinate with Benjamin.
Consider merging LWG 964, 966(i), and 1268(i) into a single paper.
Proposed resolution:
Resolved 2011-03 Madrid meeting by paper N3278
Section: 32.7.4 [thread.condition.condvar] Status: C++11 Submitter: Pete Becker Opened: 2009-01-07 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
32.7.4 [thread.condition.condvar]: the constructor for
condition_variable
throws an exception with error code
device_or_resource_busy
"if attempting to initialize a
previously-initialized but as of yet undestroyed condition_variable
."
How can this occur?
[ Summit: ]
Move to review. Proposed resolution: strike the
device_or_resource_busy
error condition from the constructor ofcondition_variable
.
- This is a POSIX error that cannot occur in this interface because the C++ interface does not separate declaration from initialization.
[ Batavia (2009-05): ]
We agree with the proposed resolution. Move to Tentatively Ready.
Proposed resolution:
Change 32.7.4 [thread.condition.condvar] p3:
- ...
device_or_resource_busy
-- if attempting to initialize a previously-initialized but as of yet undestroyedcondition_variable
.
Section: 32.7.4 [thread.condition.condvar] Status: Resolved Submitter: Pete Becker Opened: 2009-01-07 Last modified: 2016-01-28
Priority: Not Prioritized
View all other issues in [thread.condition.condvar].
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Discussion:
32.7.4 [thread.condition.condvar]:
condition_variable::wait
and
condition_variable::wait_until
both have a postcondition that
lock
is locked by the calling thread, and a throws clause that
requires throwing an exception if this postcondition cannot be achieved.
How can the implementation detect that this lock
can never be
obtained?
[ Summit: ]
Move to open. Requires wording. Agreed this is an issue, and the specification should not require detecting deadlocks.
[ 2009-08-01 Howard provides wording. ]
The proposed wording is inspired by the POSIX spec which says:
- [EINVAL]
- The value specified by cond or mutex is invalid.
- [EPERM]
- The mutex was not owned by the current thread at the time of the call.
I do not believe [EINVAL] is possible without memory corruption (which we don't specify). [EPERM] is possible if this thread doesn't own the mutex, which is listed as a precondition. "May" is used instead of "Shall" because not all OS's are POSIX.
[ 2009-10 Santa Cruz: ]
Leave open, Detlef to provide improved wording.
[ 2009-10-23 Detlef Provided wording. ]
Detlef's wording put in Proposed resolution. Original wording here:
Change 32.7.4 [thread.condition.condvar] p12, p19 and 32.7.5 [thread.condition.condvarany] p10, p16:
Throws: May throw
std::system_error
if a precondition is not met.when the effects or postcondition cannot be achieved.
[ 2009-10 Santa Cruz: ]
Leave open, Detlef to provide improved wording.
[ 2009-11-18 Anthony adds: ]
condition_variable::wait
takes aunique_lock<mutex>
. We know whether or not aunique_lock
owns a lock, through use of itsowns_lock()
member.I would like to propose the following resolution:
Modify the first sentence of 32.7.4 [thread.condition.condvar] p9:
void wait(unique_lock<mutex>& lock);9 Precondition:
lock
is locked by the calling threadlock.owns_lock()
istrue
, and either...
Replace 32.7.4 [thread.condition.condvar] p11-13 with:
void wait(unique_lock<mutex>& lock);...
11 Postcondition:
lock
is locked by the calling threadlock.owns_lock()
istrue
.12 Throws:
std::system_error
when the effects or postcondition cannot be achievedif the implementation detects that the preconditions are not met or the effects cannot be achieved. Any exception thrown bylock.lock()
orlock.unlock()
.13 Error Conditions: The error conditions are implementation defined.
equivalent error condition fromlock.lock()
orlock.unlock()
.
[ 2010 Pittsburgh: ]
There are heavy conflicts with adopted papers.
This issue is dependent on LWG 1268(i).
Leave open pending outstanding edits to the working draft. Detlef will provide wording.
[2011-03-24 Madrid]
Rationale:
This has been resolved since filing, with the introduction of system_error
to the thread specification.
Proposed resolution:
Replace 32.7.4 [thread.condition.condvar] p12, p19 and 32.7.5 [thread.condition.condvarany] p10, p16:
Throws:std::system_error
when the effects or postcondition cannot be achieved.
Error conditions:
equivalent error condition fromlock.lock()
orlock.unlock()
.Throws: It is implementation-defined whether a
std::system_error
with implementation-defined error condition is thrown if the precondition is not met.
Section: 32.4.3.3 [thread.thread.constr] Status: C++11 Submitter: Pete Becker Opened: 2009-01-07 Last modified: 2016-01-28
Priority: Not Prioritized
View all other issues in [thread.thread.constr].
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Discussion:
the error handling for the constructor for condition_variable
distinguishes lack of memory from lack of other resources, but the error
handling for the thread constructor does not. Is this difference
intentional?
[ Beman has volunteered to provide proposed wording. ]
[ 2009-09-25 Beman provided proposed wording. ]
The proposed resolution assumes 962(i) has been accepted and its proposed resolution applied to the working paper.
[ 2009-10 Santa Cruz: ]
Move to Ready.
Proposed resolution:
Change Mutex requirements 32.6.4 [thread.mutex.requirements], paragraph 4, as indicated:
Error conditions:
not_enough_memory
— if there is not enough memory to construct the mutex object.resource_unavailable_try_again
— if any native handle type manipulated is not available.operation_not_permitted
— if the thread does not have the necessary permission to change the state of the mutex object.device_or_resource_busy
— if any native handle type manipulated is already locked.invalid_argument
— if any native handle type manipulated as part of mutex construction is incorrect.
Change Class condition_variable
32.7.4 [thread.condition.condvar],
default constructor, as indicated:
condition_variable();
Effects: Constructs an object of type
condition_variable
.Throws:
std::system_error
when an exception is required (32.2.2 [thread.req.exception]).Error conditions:
not_enough_memory
— if a memory limitation prevents initialization.resource_unavailable_try_again
— if some non-memory resource limitation prevents initialization.device_or_resource_busy
— if attempting to initialize a previously-initialized but as of yet undestroyedcondition_variable
.
Section: 32.6.4 [thread.mutex.requirements] Status: C++11 Submitter: Pete Becker Opened: 2009-01-07 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
32.6.4 [thread.mutex.requirements]: several functions are required to throw exceptions "if the thread does not have the necessary permission ...". "The necessary permission" is not defined.
[ Summit: ]
Move to open.
[ Beman has volunteered to provide proposed wording. ]
[ 2009-10 Santa Cruz: ]
Moved to Ready with minor word-smithing in the example.
Proposed resolution:
Change Exceptions 32.2.2 [thread.req.exception] as indicated:
Some functions described in this Clause are specified to throw exceptions of type
system_error
(19.5.5). Such exceptions shall be thrown if any of the Error conditions are detected or a call to an operating system or other underlying API results in an error that prevents the library function from meeting its specifications. [Note: See 16.4.6.14 [res.on.exception.handling] for exceptions thrown to report storage allocation failures. —end note][Example:
Consider a function in this clause that is specified to throw exceptions of type
system_error
and specifies Error conditions that includeoperation_not_permitted
for a thread that does not have the privilege to perform the operation. Assume that, during the execution of this function, anerrno
ofEPERM
is reported by a POSIX API call used by the implementation. Since POSIX specifies anerrno
ofEPERM
when "the caller does not have the privilege to perform the operation", the implementation mapsEPERM
to anerror_condition
ofoperation_not_permitted
(19.5 [syserr]) and an exception of typesystem_error
is thrown.—end example]
Editorial note: For the sake of exposition, the existing text above is shown with the changes proposed in issues 962 and 967. The proposed additional example is independent of whether or not the 962 and 967 proposed resolutions are accepted.
Change Mutex requirements 32.6.4 [thread.mutex.requirements], paragraph 4, as indicated:
—
operation_not_permitted
— if the thread does not have thenecessary permission to change the state of the mutex objectprivilege to perform the operation.
Change Mutex requirements 32.6.4 [thread.mutex.requirements], paragraph 12, as indicated:
—
operation_not_permitted
— if the thread does not have thenecessary permission to change the state of the mutexprivilege to perform the operation.
addressof
overload unneededSection: 20.2.11 [specialized.addressof] Status: C++11 Submitter: Howard Hinnant Opened: 2009-01-16 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
20.2.11 [specialized.addressof] specifies:
template <ObjectType T> T* addressof(T& r); template <ObjectType T> T* addressof(T&& r);
The two signatures are ambiguous when the argument is an lvalue. The second signature seems not useful: what does it mean to take the address of an rvalue?
[ Post Summit: ]
Recommend Review.
[ Batavia (2009-05): ]
We agree with the proposed resolution. Move to Tentatively Ready.
[ 2009-11-18 Moved from Pending WP to WP. Confirmed in N3000. ]
Proposed resolution:
Change 20.2.11 [specialized.addressof]:
template <ObjectType T> T* addressof(T& r);template <ObjectType T> T* addressof(T&& r);
duration<double>
should not implicitly convert to duration<int>
Section: 30.5.2 [time.duration.cons] Status: C++11 Submitter: Howard Hinnant Opened: 2009-01-21 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
The following code should not compile because it involves implicit truncation
errors (against the design philosophy of the duration
library).
duration<double> d(3.5);
duration<int> i = d; // implicit truncation, should not compile
This intent was codified in the example implementation which drove this proposal but I failed to accurately translate the code into the specification in this regard.
[ Batavia (2009-05): ]
We agree with the proposed resolution.
Move to Tentatively Ready.
[ 2009-07 Frankfurt ]
Moved from Tentatively Ready to Open only because the wording needs to be improved for
enable_if
type constraining, possibly following Robert's formula.
[ 2009-08-01 Howard adds: ]
[ 2009-10 Santa Cruz: ]
Proposed resolution:
Change 30.5.2 [time.duration.cons], p4:
template <class Rep2, class Period2> duration(const duration<Rep2, Period2>& d);-4- Requires:
treat_as_floating_point<rep>::value
shall betrue
or bothratio_divide<Period2, period>::type::den
shall be 1 andtreat_as_floating_point<Rep2>::value
shall befalse
. Diagnostic required. [Note: This requirement prevents implicit truncation error when converting between integral-basedduration
types. Such a construction could easily lead to confusion about the value of theduration
. — end note]
is_convertible
cannot be instantiated for non-convertible typesSection: 21.3.7 [meta.rel] Status: C++11 Submitter: Daniel Krügler Opened: 2009-01-25 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
Addresses UK 206
The current specification of std::is_convertible
(reference is draft
N2798)
is basically defined by 21.3.7 [meta.rel] p.4:
In order to instantiate the template
is_convertible<From, To>
, the following code shall be well formed:template <class T> typename add_rvalue_reference<T>::type create(); To test() { return create<From>(); }[Note: This requirement gives well defined results for reference types, void types, array types, and function types. — end note]
The first sentence can be interpreted, that e.g. the expression
std::is_convertible<double, int*>::value
is ill-formed because std::is_convertible<double, int*>
could not be
instantiated, or in more general terms: The wording requires that
std::is_convertible<X, Y>
cannot be instantiated for otherwise valid
argument types X
and Y
if X
is not convertible to Y
.
This semantic is both unpractical and in contradiction to what the last type traits paper N2255 proposed:
If the following
test
function is well formed codeb
istrue
, else it isfalse
.template <class T> typename add_rvalue_reference<T>::type create(); To test() { return create<From>(); }[Note: This definition gives well defined results for
reference
types,void
types, array types, and function types. — end note]
[ Post Summit: ]
Jens: Checking that code is well-formed and then returning
true
/false
sounds like speculative compilation. John Spicer would really dislike this. Please find another wording suggesting speculative compilation.Recommend Open.
[ Post Summit, Howard adds: ]
John finds the following wording clearer:
Template Condition Comments template <class From, class To>
struct is_convertible;see below From
andTo
shall be complete types, arrays of unknown bound, or (possibly cv-qualified)void
types.Given the following function prototype:
template <class T> typename add_rvalue_reference<T>::type create();
is_convertible<From, To>::value
shall betrue
if the return expression in the following code would be well-formed, including any implicit conversions to the return type of the function, elseis_convertible<From, To>::value
shall befalse
.To test() { return create<From>(); }
Original proposed wording:
In 21.3.7 [meta.rel]/4 change:
In order to instantiate the templateIf the following code is well formedis_convertible<From, To>
, the following code shall be well formedis_convertible<From, To>::value
istrue
, otherwisefalse
:[..]
Revision 2
In 21.3.7 [meta.rel] change:
Template Condition Comments ... ... ... template <class From, class To>
struct is_convertible;The code set out below shall be well formed.see belowFrom
andTo
shall be complete types, arrays of unknown bound, or (possibly cv-qualified)void
types.-4-
In order to instantiate the templateGiven the following function prototype:is_convertible<From, To>
, the following code shall be well formed:template <class T> typename add_rvalue_reference<T>::type create();
is_convertible<From, To>::value
inherits either directly or indirectly fromtrue_type
if the return expression in the following code would be well-formed, including any implicit conversions to the return type of the function, elseis_convertible<From, To>::value
inherits either directly or indirectly fromfalse_type
.To test() { return create<From>(); }[Note: This requirement gives well defined results for reference types, void types, array types, and function types. -- end note]
[ Batavia (2009-05): ]
We agree with the proposed resolution. Move to Tentatively Ready.
Proposed resolution:
In 21.3.7 [meta.rel] change:
Template Condition Comments ... ... ... template <class From, class To>
struct is_convertible;The code set out below shall be well formed.see belowFrom
andTo
shall be complete types, arrays of unknown bound, or (possibly cv-qualified)void
types.-4-
In order to instantiate the templateGiven the following function prototype:is_convertible<From, To>
, the following code shall be well formed:template <class T> typename add_rvalue_reference<T>::type create();the predicate condition for a template specialization
is_convertible<From, To>
shall be satisfied, if and only if the return expression in the following code would be well-formed, including any implicit conversions to the return type of the function.To test() { return create<From>(); }[Note: This requirement gives well defined results for reference types, void types, array types, and function types. — end note]
std::stack
should be movableSection: 23.6.6.2 [stack.defn] Status: Resolved Submitter: Daniel Krügler Opened: 2009-02-01 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
The synopsis given in 23.6.6.2 [stack.defn] does not show up
requires MoveConstructible<Cont> stack(stack&&); requires MoveAssignable<Cont> stack& operator=(stack&&);
although the other container adaptors do provide corresponding members.
[ Batavia (2009-05): ]
We agree with the proposed resolution.
Move to Tentatively Ready.
[ 2009-07 Frankfurt ]
Moved from Tentatively Ready to Open only because the wording needs to be tweaked for concepts removal.
[ 2009-08-18 Daniel updates the wording and Howard sets to Review. ]
[ 2009-08-23 Howard adds: ]
1194(i) also adds these move members using an editorially different style.
[ 2009-10 Santa Cruz: ]
Proposed resolution:
In the class stack synopsis of 23.6.6.2 [stack.defn] insert:
template <class T, class Container = deque<T> > class stack { [..] explicit stack(const Container&); explicit stack(Container&& = Container()); stack(stack&& s) : c(std::move(s.c)) {} stack& operator=(stack&& s) { c = std::move(s.c); return *this; } [..] };
Section: 22.10.19 [unord.hash] Status: C++11 Submitter: Alisdair Meredith Opened: 2009-02-02 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
Addresses UK 208
I don't see an open issue on supporting std::hash
for smart pointers
(unique_ptr
and shared_ptr
at least).
It seems reasonable to at least expect support for the smart pointers, especially as they support comparison for use in ordered associative containers.
[ Batavia (2009-05): ]
Howard points out that the client can always supply a custom hash function.
Alisdair replies that the smart pointer classes are highly likely to be frequently used as hash keys.
Bill would prefer to be conservative.
Alisdair mentions that this issue may also be viewed as a subissue or duplicate of issue 1025(i).
Move to Open, and recommend the issue be deferred until after the next Committee Draft is issued.
[ 2009-05-31 Peter adds: ]
Howard points out that the client can always supply a custom hash function.
Not entirely true. The client cannot supply the function that hashes the address of the control block (the equivalent of the old
operator<
, now proudly carrying the awkward name of 'owner_before
'). Only the implementation can do that, not necessarily via specializinghash<>
, of course.This hash function makes sense in certain situations for
shared_ptr
(when one needs to switch fromset/map
using ownership ordering tounordered_set/map
) and is the only hash function that makes sense forweak_ptr
.
[ 2009-07-28 Alisdair provides wording. ]
[ 2009-10 Santa Cruz: ]
Move to Ready.
[ 2009-11-16 Moved from Ready to Open: ]
Pete writes:
As far as I can see, "...suitable for using this type as key in unordered associative containers..." doesn't define any semantics. It's advice to the reader, and if it's present at all it should be in a note. But we have far too much of this sort of editorial commentary as it is.
And in the resolution of 978 it's clearly wrong: it says that if there is no hash specialization available for
D::pointer
, the implementation may providehash<unique_ptr<T,D>>
if the result is not suitable for use in unordered containers.Howard writes:
Is this a request to pull 978 from Ready?
Barry writes:
I read this as more than a request. The PE says it's wrong, so it can't be Ready.
[ 2010-01-31 Alisdair: related to 1245(i) and 1182(i). ]
[ 2010-02-08 Beman updates wording. ]
[ 2010-02-09 Moved to Tentatively Ready after 5 positive votes on c++std-lib. ]
Proposed resolution:
Add the following declarations to the synopsis of <memory>
in
20.2 [memory]
// [util.smartptr.hash] hash support template <class T> struct hash; template <class T, class D> struct hash<unique_ptr<T,D>>; template <class T> struct hash<shared_ptr<T>>;
Add a new subclause under [util.smartptr] called hash support
hash support [util.smartptr.hash]
template <class T, class D> struct hash<unique_ptr<T,D>>;Specialization meeting the requirements of class template
hash
(22.10.19 [unord.hash]). For an objectp
of typeUP
, whereUP
is a typeunique_ptr<T,D>
,hash<UP>()(p)
shall evaluate to the same value ashash<typename UP::pointer>()(p.get())
. The specializationhash<typename UP::pointer>
is required to be well-formed.template <class T> struct hash<shared_ptr<T>>;Specialization meeting the requirements of class template
hash
(22.10.19 [unord.hash]). For an objectp
of typeshared_ptr<T>
,hash<shared_ptr<T>>()(p)
shall evaluate to the same value ashash<T*>()(p.get())
.
initializer_list
supportSection: 23.2.8 [unord.req] Status: C++11 Submitter: Daniel Krügler Opened: 2009-02-08 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
Refering to
N2800
all container requirements tables (including those for
associative containers) provide useful member function overloads
accepting std::initializer_list
as argument, the only exception is
Table 87. There seems to be no reason for not providing them, because 23.5 [unord]
is already initializer_list
-aware. For the sake of
library interface consistency and user-expectations corresponding
overloads should be added to the table requirements of unordered
containers as well.
[ Batavia (2009-05): ]
We agree with the proposed resolution.
Move to Tentatively Ready.
Proposed resolution:
In 23.2.8 [unord.req]/9 insert:
...
[q1, q2)
is a valid range ina
,il
designates an object of typeinitializer_list<value_type>
,t
is a value of typeX::value_type
, ...
In 23.2.8 [unord.req], Table 87 insert:
Table 87 - Unordered associative container requirements (in addition to container) Expression Return type Assertion/note
pre-/post-conditionComplexity X(i, j)
X a(i, j)X
... ... X(il)
X
Same as X(il.begin(), il.end())
.Same as X(il.begin(), il.end())
.... ... ... ... a = b
X
... ... a = il
X&
a = X(il); return *this;
Same as a = X(il)
.... ... ... ... a.insert(i, j)
void
... ... a.insert(il)
void
Same as a.insert(il.begin(), il.end())
.Same as a.insert(il.begin(), il.end())
.
Section: 23.2.7 [associative.reqmts] Status: C++11 Submitter: Daniel Krügler Opened: 2009-02-08 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
According to
N2800,
the associative container requirements table 85 says
that assigning an initializer_list
to such a container is of
constant complexity, which is obviously wrong.
[ Batavia (2009-05): ]
We agree with the proposed resolution.
Move to Tentatively Ready.
Proposed resolution:
In 23.2.7 [associative.reqmts], Table 85 change:
Table 85 - Associative container requirements (in addition to container) Expression Return type Assertion/note
pre-/post-conditionComplexity a = il
X&
a = X(il);
return *this;constantSame asa = X(il)
.
unique_ptr
reference deleters should not be moved fromSection: 20.3.1.3 [unique.ptr.single] Status: Resolved Submitter: Howard Hinnant Opened: 2009-02-10 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
Dave brought to my attention that when a unique_ptr
has a non-const reference
type deleter, move constructing from it, even when the unique_ptr
containing
the reference is an rvalue, could have surprising results:
D d(some-state);
unique_ptr<A, D&> p(new A, d);
unique_ptr<A, D> p2 = std::move(p);
// has d's state changed here?
I agree with him. It is the unique_ptr
that is the rvalue, not the
deleter. When the deleter is a reference type, the unique_ptr
should
respect the "lvalueness" of the deleter.
Thanks Dave.
[ Batavia (2009-05): ]
Seems correct, but complicated enough that we recommend moving to Review.
[ 2009-10 Santa Cruz: ]
Move to Ready.
[ 2010-03-14 Howard adds: ]
We moved N3073 to the formal motions page in Pittsburgh which should obsolete this issue. I've moved this issue to NAD Editorial, solved by N3073.
Rationale:
Solved by N3073.
Proposed resolution:
Change 20.3.1.3.2 [unique.ptr.single.ctor], p20-21
template <class U, class E> unique_ptr(unique_ptr<U, E>&& u);-20- Requires: If
is not a reference type, construction of the deleter
DED
from an rvalue of typeE
shall be well formed and shall not throw an exception. OtherwiseE
is a reference type and construction of the deleterD
from an lvalue of typeE
shall be well formed and shall not throw an exception. IfD
is a reference type, thenE
shall be the same type asD
(diagnostic required).unique_ptr<U, E>::pointer
shall be implicitly convertible topointer
. [Note:
These requirements imply thatT
andU
are complete types. — end note]-21- Effects: Constructs a
unique_ptr
which owns the pointer whichu
owns (if any). If the deleterE
is not a reference type,itthis deleter is move constructed fromu
's deleter, otherwisethe referencethis deleter is copy constructed fromu
.'s deleter. After the construction,u
no longer owns a pointer. [Note: The deleter constructor can be implemented withstd::forward<
. — end note]DE>
Change 20.3.1.3.4 [unique.ptr.single.asgn], p1-3
unique_ptr& operator=(unique_ptr&& u);-1- Requires: If the deleter
D
is not a reference type,Aassignment of the deleterD
from an rvalueD
shall not throw an exception. Otherwise the deleterD
is a reference type, and assignment of the deleterD
from an lvalueD
shall not throw an exception.-2- Effects: reset(u.release()) followed by an
moveassignment fromu
's deleter to this deleterstd::forward<D>(u.get_deleter())
.-3- Postconditions: This
unique_ptr
now owns the pointer whichu
owned, andu
no longer owns it.[Note: IfD
is a reference type, then the referenced lvalue deleters are move assigned. — end note]
Change 20.3.1.3.4 [unique.ptr.single.asgn], p6-7
template <class U, class E> unique_ptr& operator=(unique_ptr<U, E>&& u);Requires: If the deleter
E
is not a reference type,Aassignment of the deleterD
from an rvalueshall not throw an exception. Otherwise the deleter
DEE
is a reference type, and assignment of the deleterD
from an lvalueE
shall not throw an exception.unique_ptr<U, E>::pointer
shall be implicitly convertible topointer
. [Note: These requirements imply thatT
andU>
are complete types. — end note]Effects:
reset(u.release())
followed by anmoveassignment fromu
's deleter to this deleterstd::forward<E>(u.get_deleter())
.If eitherD
orE
is a reference type, then the referenced lvalue deleter participates in the move assignment.
<cinttypes>
have macro guards?Section: 31.13 [c.files] Status: C++11 Submitter: Howard Hinnant Opened: 2009-02-12 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
The C standard says about <inttypes.h>
:
C++ implementations should define these macros only when
__STDC_FORMAT_MACROS
is defined before<inttypes.h>
is included.
The C standard has a similar note about <stdint.h>
. For <cstdint>
we adopted a "thanks but no thanks" policy and documented that fact in 17.4.1 [cstdint.syn]:
... [Note: The macros defined by
<stdint>
are provided unconditionally. In particular, the symbols__STDC_LIMIT_MACROS
and__STDC_CONSTANT_MACROS
(mentioned in C99 footnotes 219, 220, and 222) play no role in C++. — end note]
I recommend we put a similar note in 31.13 [c.files] regarding <cinttypes>
.
[ Batavia (2009-05): ]
We agree with the proposed resolution. Move to Tentatively Ready.
Proposed resolution:
Add to 31.13 [c.files]:
Table 112 describes header
<cinttypes>
. [Note: The macros defined by<cintypes>
are provided unconditionally. In particular, the symbol__STDC_FORMAT_MACROS
(mentioned in C99 footnote 182) plays no role in C++. — end note]
Section: 23.2.2 [container.requirements.general] Status: Resolved Submitter: Rani Sharoni Opened: 2009-02-12 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
Introduction
This proposal is meant to resolve potential regression of the N2800 draft, see next section, and to relax the requirements for containers of types with throwing move constructors.
The basic problem is that some containers operations, like push_back
,
have a strong exception safety
guarantee (i.e. no side effects upon exception) that are not achievable when
throwing move constructors are used since there is no way to guarantee revert
after partial move. For such operations the implementation can at most provide
the basic guarantee (i.e. valid but unpredictable) as it does with multi
copying operations (e.g. range insert).
For example, vector<T>::push_back()
(where T
has a move
constructor) might resize the vector
and move the objects to the new underlying
buffer. If move constructor throws it might
not be possible to recover the throwing object or to move the old objects back to
the original buffer.
The current draft is explicit by disallowing throwing move
for some operations (e.g. vector<>::reserve
) and not clear about other
operations mentioned in 23.2.2 [container.requirements.general]/10
(e.g. single element insert
): it guarantees strong exception
safety without explicitly disallowing a throwing move constructor.
Regression
This section only refers to cases in which the contained object is by itself a standard container.
Move constructors of standard containers are allowed to throw and therefore existing operations are broken, compared with C++03, due to move optimization. (In fact existing implementations like Dinkumware are actually throwing).
For example, vector< list<int> >::reserve
yields
undefined behavior since list<int>
's move constructor is allowed to throw.
On the other hand, the same operation has strong exception safety guarantee in
C++03.
There are few options to solve this regression:
Option 1 is suggested by proposal N2815 but it might not be applicable for existing implementations for which containers default constructors are throwing.
Option 2 limits the usage significantly and it's error prone
by allowing zombie objects that are nothing but destructible (e.g. no clear()
is allowed after move). It also potentially complicates the implementation by
introducing special state.
Option 3 is possible, for example, using default
construction and swap
instead of move for standard containers case. The
implementation is also free to provide special hidden operation for non
throwing move without forcing the user the cope with the limitation of option-2
when using the public move.
Option 4 impact the efficiency in all use cases due to rare throwing move.
The proposed wording will imply option 1 or 3 though option 2 is also achievable using more wording. I personally oppose to option 2 that has impact on usability.
Relaxation for user types
Disallowing throwing move constructors in general seems very restrictive
since, for example, common implementation of move will be default construction
+ swap
so move will throw if the
default constructor will throw. This is currently the case with the Dinkumware
implementation of node based containers (e.g. std::list
)
though this section doesn't refer to standard types.
For throwing move constructors it seem that the implementation should have no problems to provide the basic guarantee instead of the strong one. It's better to allow throwing move constructors with basic guarantee than to disallow it silently (compile and run), via undefined behavior.
There might still be cases in which the relaxation will break existing generic code that assumes the strong guarantee but it's broken either way given a throwing move constructor since this is not a preserving optimization.
[ Batavia (2009-05): ]
Bjarne comments (referring to his draft paper): "I believe that my suggestion simply solves that. Thus, we don't need a throwing move."
Move to Open and recommend it be deferred until after the next Committee Draft is issued.
[ 2009-10 Santa Cruz: ]
Should wait to get direction from Dave/Rani (N2983).
[ 2010-03-28 Daniel updated wording to sync with N3092. ]
The suggested change of 23.3.5.4 [deque.modifiers]/2 should be removed, because the current wording does say more general things:
2 Remarks: If an exception is thrown other than by the copy constructor, move constructor, assignment operator, or move assignment operator of
T
there are no effects. If an exception is thrown by the move constructor of a non-CopyConstructibleT
, the effects are unspecified.The suggested change of 23.3.13.3 [vector.capacity]/2 should be removed, because the current wording does say more general things:
2 Effects: A directive that informs a
vector
of a planned change in size, so that it can manage the storage allocation accordingly. Afterreserve()
,capacity()
is greater or equal to the argument ofreserve
if reallocation happens; and equal to the previous value ofcapacity()
otherwise. Reallocation happens at this point if and only if the current capacity is less than the argument ofreserve()
. If an exception is thrown other than by the move constructor of a non-CopyConstructible
type, there are no effects.
[2011-03-15: Daniel updates wording to sync with N3242 and comments]
The issue has nearly been resolved by previous changes to the working paper, in particular all suggested changes for
deque
andvector
are no longer necessary. The still remaining parts involve the unordered associative containers.
[2011-03-24 Madrid meeting]
It looks like this issue has been resolved already by noexcept
paper N3050
Rationale:
Resolved by N3050
Proposed resolution:
23.2.2 [container.requirements.general] paragraph 10 add footnote:
-10- Unless otherwise specified (see 23.2.7.2 [associative.reqmts.except], 23.2.8.2 [unord.req.except], 23.3.5.4 [deque.modifiers], and 23.3.13.5 [vector.modifiers]) all container types defined in this Clause meet the following additional requirements:
- …
[Note: for compatibility with C++ 2003, when "no effect" is required, standard containers should not use the
value_type
's throwing move constructor when the contained object is by itself a standard container. — end note]
23.2.8.2 [unord.req.except] change paragraph 2+4 to say:
-2- For unordered associative containers, if an exception is thrown by any operation other than the container's hash function from within an
[…] -4- For unordered associative containers, if an exception is thrown from within ainsert()
function inserting a single element, theinsert()
function has no effect unless the exception is thrown by the contained object move constructor.rehash()
function other than by the container's hash function or comparison function, therehash()
function has no effect unless the exception is thrown by the contained object move constructor.
Keep 23.3.5.4 [deque.modifiers] paragraph 2 unchanged [Drafting note: The originally proposed wording did suggest to add a last sentence as follows:
If an exception is thrown by
push_back()
oremplace_back()
function, that function has no effects unless the exception is thrown by the move constructor ofT
.
— end drafting note ]
-2- Remarks: If an exception is thrown other than by the copy constructor, move constructor, assignment operator, or move assignment operator of
T
there are no effects. If an exception is thrown by the move constructor of a non-CopyInsertableT
, the effects are unspecified.
Keep 23.3.13.3 [vector.capacity] paragraph 2 unchanged [Drafting note: The originally proposed wording did suggest to change the last sentence as follows:
If an exception is thrown, there are no effects unless the exception is thrown by the contained object move constructor.
— end drafting note ]
-2- Effects: A directive that informs a
vector
of a planned change in size, so that it can manage the storage allocation accordingly. Afterreserve()
,capacity()
is greater or equal to the argument ofreserve
if reallocation happens; and equal to the previous value ofcapacity()
otherwise. Reallocation happens at this point if and only if the current capacity is less than the argument ofreserve()
. If an exception is thrown other than by the move constructor of a non-CopyInsertable
type, there are no effects.
Keep 23.3.13.3 [vector.capacity] paragraph 12 unchanged [Drafting note: The originally proposed wording did suggest to change the old paragraph as follows:
-12- Requires:
IfIf an exception is thrown, there are no effects unless the exception is thrown by the contained object move constructor.value_type
has a move constructor, that constructor shall not throw any exceptions.
— end drafting note ]
-12- Requires: If an exception is thrown other than by the move constructor of a non-
CopyInsertable
T
there are no effects.
Keep 23.3.13.5 [vector.modifiers] paragraph 1 unchanged [Drafting note: The originally proposed wording did suggest to change the old paragraph as follows:
-1-
Requires: IfRemarks: If an exception is thrown byvalue_type
has a move constructor, that constructor shall not throw any exceptions.push_back()
oremplace_back()
function, that function has no effect unless the exception is thrown by the move constructor ofT
.
— end drafting note ]
-1- Remarks: Causes reallocation if the new size is greater than the old capacity. If no reallocation happens, all the iterators and references before the insertion point remain valid. If an exception is thrown other than by the copy constructor, move constructor, assignment operator, or move assignment operator of
T
or by anyInputIterator
operation there are no effects. If an exception is thrown by the move constructor of a non-CopyInsertable
T
, the effects are unspecified.
try_lock
contradictionSection: 32.6.6 [thread.lock.algorithm] Status: C++11 Submitter: Chris Fairles Opened: 2009-02-14 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
In 32.6.6 [thread.lock.algorithm], the generic try_lock
effects (p2) say that a failed
try_lock
is when it either returns false
or throws an exception. In
the event a call to try_lock
does fail, by either returning false
or
throwing an exception, it states that unlock
shall be called for all
prior arguments. Then the returns clause (p3) goes on to state
in a note that after returning, either all locks are locked or none
will be. So what happens if multiple locks fail on try_lock
?
Example:
#include <mutex> int main() { std::mutex m0, m1, m2; std::unique_lock<std::mutex> l0(m0, std::defer_lock); std::unique_lock<std::mutex> l1(m1); //throws on try_lock std::unique_lock<std::mutex> l2(m2); //throws on try_lock int result = std::try_lock(l0, l1, l2); assert( !l0.owns_lock() ); assert( l1.owns_lock() ); //?? assert( l2.owns_lock() ); //?? }
The first lock's try_lock
succeeded but, being a prior argument to a
lock whose try_lock
failed, it gets unlocked as per the effects clause
of 32.6.6 [thread.lock.algorithm]. However, 2 locks remain locked in this case but the return
clause states that either all arguments shall be locked or none will
be. This seems to be a contradiction unless the intent is for
implementations to make an effort to unlock not only prior arguments,
but the one that failed and those that come after as well. Shouldn't
the note only apply to the arguments that were successfully locked?
Further discussion and possible resolutions in c++std-lib-23049.
[ Summit: ]
Move to review. Agree with proposed resolution.
[ Batavia (2009-05): ]
We agree with the proposed resolution. Move to Tentatively Ready.
Proposed resolution:
Change 32.6.6 [thread.lock.algorithm], p2:
-2- Effects: Calls
try_lock()
for each argument in order beginning with the first until all arguments have been processed or a call totry_lock()
fails, either by returningfalse
or by throwing an exception. If a call totry_lock()
fails,unlock()
shall be called for all prior arguments and there shall be no further calls totry_lock()
.
Delete the note from 32.6.6 [thread.lock.algorithm], p3
-3- Returns: -1 if all calls to
try_lock()
returnedtrue
, otherwise a 0-based index value that indicates the argument for whichtry_lock()
returnedfalse
.[Note: On return, either all arguments will be locked or none will be locked. -- end note]
reference_wrapper
and function typesSection: 22.10.6 [refwrap] Status: C++11 Submitter: Howard Hinnant Opened: 2009-02-18 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
The synopsis in 22.10.6 [refwrap] says:
template <ObjectType T> class reference_wrapper ...
And then paragraph 3 says:
The template instantiation
reference_wrapper<T>
shall be derived fromstd::unary_function<T1, R>
only if the typeT
is any of the following:
- a function type or a pointer to function type taking one argument of type
T1
and returningR
But function types are not ObjectType
s.
Paragraph 4 contains the same contradiction.
[ Post Summit: ]
Jens: restricted reference to ObjectType
Recommend Review.
[ Post Summit, Peter adds: ]
In https://svn.boost.org/trac/boost/ticket/1846 however Eric Niebler makes the very reasonable point that
reference_wrapper<F>
, whereF
is a function type, represents a reference to a function, a legitimate entity. Soboost::ref
was changed to allow it.http://svn.boost.org/svn/boost/trunk/libs/bind/test/ref_fn_test.cpp
Therefore, I believe an alternative proposed resolution for issue 987 could simply allow
reference_wrapper
to be used with function types.
[ Post Summit, Howard adds: ]
I agree with Peter (and Eric). I got this one wrong on my first try. Here is code that demonstrates how easy (and useful) it is to instantiate
reference_wrapper
with a function type:#include <functional> template <class F> void test(F f); void f() {} int main() { test(std::ref(f)); }Output (link time error shows type of
reference_wrapper
instantiated with function type):Undefined symbols: "void test<std::reference_wrapper<void ()()> >(std::reference_wrapper<void ()()>)",...I've taken the liberty of changing the proposed wording to allow function types and set to Open. I'll also freely admit that I'm not positive
ReferentType
is the correct concept.
[ Batavia (2009-05): ]
Howard observed that
FunctionType
, a concept not (yet?) in the Working Paper, is likely the correct constraint to be applied. However, the proposed resolution provides an adequate approximation.Move to Review.
[ 2009-05-23 Alisdair adds: ]
By constraining to
PointeeType
we rule out the ability forT
to be a reference, and call in reference-collapsing. I'm not sure if this is correct and intended, but would like to be sure the case was considered.Is dis-allowing reference types and the implied reference collapsing the intended result?
[ 2009-07 Frankfurt ]
Moved from Review to Open only because the wording needs to be tweaked for concepts removal.
[ 2009-10-14 Daniel provided de-conceptified wording. ]
[ 2009-10 post-Santa Cruz: ]
Move to Tentatively Ready.
Proposed resolution:
Change 22.10.6 [refwrap]/1 as indicated:
reference_wrapper<T>
is aCopyConstructible
andCopyAssignable
wrapper around a reference to an object or function of typeT
.
monotonic_clock::is_monotonic
must be true
Section: 99 [time.clock.monotonic] Status: C++11 Submitter: Howard Hinnant Opened: 2009-03-09 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
There is some confusion over what the value of monotonic_clock::is_monotonic
when monotonic_clock
is a synonym for system_clock
. The
intent is that if monotonic_clock
exists, then monotonic_clock::is_monotonic
is true
.
[ Batavia (2009-05): ]
We agree with the proposed resolution.
Move to Tentatively Ready.
Proposed resolution:
Change 99 [time.clock.monotonic], p1:
-1- Objects of class
monotonic_clock
represent clocks for which values oftime_point
never decrease as physical time advances.monotonic_clock
may be a synonym forsystem_clock
if and only ifsystem_clock::is_monotonic
istrue
.
wstring_convert
Section: 99 [depr.conversions.string] Status: C++11 Submitter: P.J. Plauger Opened: 2009-03-03 Last modified: 2017-04-22
Priority: Not Prioritized
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Discussion:
Addresses JP-50 [CD1]
Add custom allocator parameter to wstring_convert
, since we cannot
allocate memory for strings from a custom allocator.
[ Batavia (2009-05): ]
We agree with the proposed resolution. Move to Tentatively Ready.
Proposed resolution:
Change [conversions.string]:
template<class Codecvt, class Elem = wchar_t, class Wide_alloc = std::allocator<Elem>, class Byte_alloc = std::allocator<char> > class wstring_convert { public: typedef std::basic_string<char, char_traits<char>, Byte_alloc> byte_string; typedef std::basic_string<Elem, char_traits<Elem>, Wide_alloc> wide_string; ...
Change [conversions.string], p3:
-3- The class template describes an ob ject that controls conversions between wide string ob jects of class
std::basic_string<Elem, char_traits<Elem>, Wide_alloc>
and byte string objects of classstd::basic_string<char, char_traits<char>, Byte_alloc>
(also known as.std::string
)
_Exit
needs better specificationSection: 17.5 [support.start.term] Status: C++11 Submitter: P.J. Plauger Opened: 2009-03-03 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
Addresses UK-188 [CD1]
The function _Exit
does not appear to be defined in this standard.
Should it be added to the table of functions included-by-reference to
the C standard?
[ 2009-05-09 Alisdair fixed some minor issues in the wording. ]
[ Batavia (2009-05): ]
We agree with the proposed resolution. Move to Tentatively Ready.
Proposed resolution:
Add to 17.5 [support.start.term] Table 20 (Header
<cstdlib>
synopsis) Functions:
_Exit
Add before the description of abort(void)
:
void _Exit [[noreturn]] (int status)The function
_Exit(int status)
has additional behavior in this International Standard:
- The program is terminated without executing destructors for objects of automatic, thread, or static storage duration and without calling the functions passed to
atexit()
(6.9.3.4 [basic.start.term]).
quick_exit
should terminate well-definedSection: 17.6.4.3 [new.handler] Status: C++11 Submitter: P.J. Plauger Opened: 2009-03-03 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
Addresses UK-193 [CD1]
quick_exit
has been added as a new valid way to terminate a program in a
well defined way.
[ Batavia (2009-05): ]
We agree with the proposed resolution. Move to Tentatively Ready.
Proposed resolution:
Change 17.6.4.3 [new.handler], p2:
-2- Required behavior: ...
- ...
call eitherterminate execution of the program without returning to the callerabort()
orexit();
Section: 16.3.2.4 [structure.specifications] Status: C++11 Submitter: Thomas Plum Opened: 2009-03-03 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
Addresses UK-163 [CD1]
Many functions are defined as "Effects: Equivalent to a...", which seems to also define the preconditions, effects, etc. But this is not made clear.
After studying the occurrences of "Effects: Equivalent to", I agree with the diagnosis but disagree with the solution. In 27.4.3.3 [string.cons] we find
14 Effects: If
InputIterator
is an integral type, equivalent tobasic_string(static_cast<size_type>(begin), static_cast<value_type>(end), a)
15 Otherwise constructs a string from the values in the range
[begin, end)
, as indicated in the Sequence Requirements table (see 23.1.3).
This would be devishly difficult to re-write with an explicit "Equivalent to:" clause. Instead, I propose the following, which will result in much less editorial re-work.
[ 2009-05-09 Alisdair adds: ]
[ Batavia (2009-05): ]
We agree with the proposed resolution. Move to Tentatively Ready.
Proposed resolution:
Add a new paragraph after 16.3.2.4 [structure.specifications], p3:
-3- Descriptions of function semantics contain the following elements (as appropriate):154
- Requires: the preconditions for calling the function
- Effects: the actions performed by the function
- Postconditions: the observable results established by the function
- Returns: a description of the value(s) returned by the function
- Throws: any exceptions thrown by the function, and the conditions that would cause the exception
- Complexity: the time and/or space complexity of the function
- Remarks: additional semantic constraints on the function
- Error conditions: the error conditions for error codes reported by the function.
- Notes: non-normative comments about the function
Whenever the Effects element specifies that the semantics of some function
F
are Equivalent to some code-sequence, then the various elements are interpreted as follows. IfF
's semantics specifies a Requires element, then that requirement is logically imposed prior to the equivalent-to semantics. Then, the semantics of the code-sequence are determined by the Requires, Effects, Postconditions, Returns, Throws, Complexity, Remarks, Error Conditions and Notes specified for the (one or more) function invocations contained in the code-sequence. The value returned fromF
is specified byF
's Returns element, or ifF
has no Returns element, a non-void
return fromF
is specified by the Returns elements in code-sequence. IfF
's semantics contains a Throws (or Postconditions, or Complexity) element, then that supersedes any occurrences of that element in the code-sequence.
Section: 20.3.1.3.6 [unique.ptr.single.modifiers] Status: C++11 Submitter: Pavel Minaev Opened: 2009-02-26 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
Consider the following (simplified) implementation of
std::auto_ptr<T>::reset()
:
void reset(T* newptr = 0) { if (this->ptr && this->ptr != newptr) { delete this->ptr; } this->ptr = newptr; }
Now consider the following code which uses the above implementation:
struct foo { std::auto_ptr<foo> ap; foo() : ap(this) {} void reset() { ap.reset(); } }; int main() { (new foo)->reset(); }
With the above implementation of auto_ptr, this results in U.B. at the point of auto_ptr::reset(). If this isn't obvious yet, let me explain how this goes step by step:
foo::reset()
entered
auto_ptr::reset()
entered
auto_ptr::reset()
tries to delete foo
foo::~foo()
entered, tries to destruct its members
auto_ptr::~auto_ptr()
executed - auto_ptr
is no longer a valid object!
foo::~foo()
left
auto_ptr::reset()
sets its "ptr" field to 0 <- U.B.! auto_ptr
is not a valid object here already!
[
Thanks to Peter Dimov who recognized the connection to unique_ptr
and
brought this to the attention of the LWG, and helped with the solution.
]
[ Howard adds: ]
To fix this behavior
reset
must be specified such that deleting the pointer is the last action to be taken withinreset
.
[ Alisdair adds: ]
The example providing the rationale for LWG 998(i) is poor, as it relies on broken semantics of having two object believing they are unique owners of a single resource. It should not be surprising that UB results from such code, and I feel no need to go out of our way to support such behaviour.
If an example is presented that does not imply multiple ownership of a unique resource, I would be much more ready to accept the proposed resolution.
[ Batavia (2009-05): ]
Howard summarizes:
This issue has to do with circular ownership, and affects
auto_ptr
, too (but we don't really care about that). It is intended to spell out the order in which operations must be performed so as to avoid the possibility of undefined behavior in the self-referential case.Howard points to message c++std-lib-23175 for another example, requested by Alisdair.
We agree with the issue and with the proposed resolution. Move to Tentatively Ready.
Proposed resolution:
Change 20.3.1.3.6 [unique.ptr.single.modifiers], p5 (Effects clause for reset
), and p6:
-5- Effects:
IfAssignsget() == nullptr
there are no effects. Otherwiseget_deleter()(get())
.p
to the storedpointer
, and then if the old value of thepointer
is not equal tonullptr
, callsget_deleter()(
the old value of thepointer)
. [Note: The order of these operations is significant because the call toget_deleter()
may destroy*this
. -- end note]-6- Postconditions:
get() == p
. [Note: The postcondition does not hold if the call toget_deleter()
destroys*this
sincethis->get()
is no longer a valid expression. -- end note]
Section: 26.11 [specialized.algorithms] Status: C++11 Submitter: Peter Dimov Opened: 2009-03-09 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
The same fix (reference 987(i)) may be applied to addressof
, which is also constrained to
ObjectType
. (That was why boost::ref
didn't work with functions - it
tried to apply boost::addressof
and the reinterpret_cast<char&>
implementation of addressof
failed.)
[ Batavia (2009-05): ]
We agree.
Move to Tentatively Ready.
[ 2009-07 Frankfurt ]
Moved from Tentatively Ready to Open only because the wording needs to be tweaked for concepts removal.
[ 2009-10-10 Daniel updates wording to concept-free. ]
[ 2009-10 post-Santa Cruz: ]
Move to Tentatively Ready.
Proposed resolution:
[
The resolution assumes that addressof
is reintroduced as described in
n2946
]
In 26.11 [specialized.algorithms] change as described:
template <class T> T* addressof(T& r);Returns: The actual address of the object or function referenced by
r
, even in the presence of an overloadedoperator&
.
Section: 16.4.5.8 [res.on.functions] Status: C++11 Submitter: Alisdair Meredith Opened: 2009-03-11 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
Addresses UK 179
According to the 4th bullet there is a problem if "if any replacement function or handler function or destructor operation throws an exception". There should be no problem throwing exceptions so long as they are caught within the function.
[ Batavia (2009-05): ]
The phrasing "throws an exception" is commonly used elsewhere to mean "throws or propagates an exception." Move to Open pending a possible more general resolution.
[ 2009-07 Frankfurt: ]
Replace "propagates" in the proposed resolution with the phrase "exits via" and move to Ready.
Proposed resolution:
Change the 4th bullet of 16.4.5.8 [res.on.functions], p2:
- if any replacement function or handler function or destructor operation
throwsexits via an exception, unless specifically allowed in the applicable Required behavior: paragraph.
operator delete
in garbage collected implementationSection: 17.6.3 [new.delete] Status: C++11 Submitter: Alisdair Meredith Opened: 2009-03-11 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
Addresses UK 190
It is not entirely clear how the current specification acts in the presence of a garbage collected implementation.
[ Summit: ]
Agreed.
[ 2009-05-09 Alisdair adds: ]
Proposed wording is too strict for implementations that do not support garbage collection. Updated wording supplied.
[ Batavia (2009-05): ]
We recommend advancing this to Tentatively Ready with the understanding that it will not be moved for adoption unless and until the proposed resolution to Core issue #853 is adopted.
Proposed resolution:
(Editorial note: This wording ties into the proposed resolution for Core #853)
Add paragraphs to 17.6.3.2 [new.delete.single]:
void operator delete(void* ptr) throw();void operator delete(void* ptr, const std::nothrow_t&) throw();[ The second signature deletion above is editorial. ]
Requires: If an implementation has strict pointer safety ( [basic.stc.dynamic.safety]) then
ptr
shall be a safely-derived pointer.-10- ...
void operator delete(void* ptr, const std::nothrow_t&) throw();Requires: If an implementation has strict pointer safety ( [basic.stc.dynamic.safety]) then
ptr
shall be a safely-derived pointer.-15- ...
Add paragraphs to 17.6.3.3 [new.delete.array]:
void operator delete[](void* ptr) throw();void operator delete[](void* ptr, const std::nothrow_t&) throw();[ The second signature deletion above is editorial. ]
Requires: If an implementation has strict pointer safety ( [basic.stc.dynamic.safety]) then
ptr
shall be a safely-derived pointer.-9- ...
void operator delete[](void* ptr, const std::nothrow_t&) throw();Requires: If an implementation has strict pointer safety ( [basic.stc.dynamic.safety]) then
ptr
shall be a safely-derived pointer.-13- ...
Add paragraphs to 17.6.3.4 [new.delete.placement]:
void operator delete(void* ptr, void*) throw();Requires: If an implementation has strict pointer safety ( [basic.stc.dynamic.safety]) then
ptr
shall be a safely-derived pointer.-7- ...
void operator delete[](void* ptr, void*) throw();Requires: If an implementation has strict pointer safety ( [basic.stc.dynamic.safety]) then
ptr
shall be a safely-derived pointer.-9- ...
next/prev
wrong iterator typeSection: 24.4.3 [iterator.operations] Status: C++11 Submitter: Alisdair Meredith Opened: 2009-03-11 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
Addresses UK 271
next/prev
return an incremented iterator without changing the value of
the original iterator. However, even this may invalidate an
InputIterator
. A ForwardIterator
is required to guarantee the
'multipass' property.
[ Batavia (2009-05): ]
We agree with the proposed resolution. Move to Tentatively Ready.
[ 2009-07 Frankfurt ]
Moved from Tentatively Ready to Open only because the wording needs to be tweaked for concepts removal.
[ 2009-10-14 Daniel provided de-conceptified wording. ]
[ 2009-10 Santa Cruz: ]
Moved to Ready.
Proposed resolution:
Change header <iterator>
synopsis 24.2 [iterator.synopsis] as indicated:
// 24.4.4, iterator operations: ... template <classInputForwardIterator>InputForwardIterator next(InputForwardIterator x, typename std::iterator_traits<InputForwardIterator>::difference_type n = 1);
Change 24.4.3 [iterator.operations] before p.6 as indicated:
template <classInputForwardIterator>InputForwardIterator next(InputForwardIterator x, typename std::iterator_traits<InputForwardIterator>::difference_type n = 1);
reverse_iterator
default ctor should value initializeSection: 24.5.1.4 [reverse.iter.cons] Status: C++11 Submitter: Alisdair Meredith Opened: 2009-03-11 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
Addresses UK 277
The default constructor default-initializes current, rather than value-initializes. This means that when Iterator corresponds to a trivial type, the current member is left un-initialized, even when the user explictly requests value intialization! At this point, it is not safe to perform any operations on the reverse_iterator other than assign it a new value or destroy it. Note that this does correspond to the basic definition of a singular iterator.
[ Summit: ]
Agree with option i.
[ Batavia (2009-05): ]
We believe this should be revisited in conjunction with issue 408(i), which nearly duplicates this issue. Move to Open.
[ 2009-07 post-Frankfurt: ]
Change "constructed" to "initialized" in two places in the proposed resolution.
Move to Tentatively Ready.
[ 2009 Santa Cruz: ]
Moved to Ready for this meeting.
Proposed resolution:
Change [reverse.iter.con]:
reverse_iterator();-1- Effects:
DefaultValue initializescurrent
. Iterator operations applied to the resulting iterator have defined behavior if and only if the corresponding operations are defined on adefault constructedvalue initialized iterator of typeIterator
.
Change [move.iter.op.const]:
move_iterator();-1- Effects: Constructs a
move_iterator
,defaultvalue initializingcurrent
. Iterator operations applied to the resulting iterator have defined behavior if and only if the corresponding operations are defined on a value initialized iterator of typeIterator
.
basic_regex
should be created/assigned from initializer listsSection: 28.6.7.2 [re.regex.construct] Status: C++11 Submitter: Alisdair Meredith Opened: 2009-03-11 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
Addresses UK 317 and JP 74
UK 317:
basic_string
has both a constructor and an assignment operator that accepts an initializer list,basic_regex
should have the same.
JP 74:
basic_regex & operator= (initializer_list<T>);
is not defined.
[ Batavia (2009-05): ]
UK 317 asks for both assignment and constructor, but the requested constructor is already present in the current Working Paper. We agree with the proposed resolution. Move to Tentatively Ready.
Proposed resolution:
Change 28.6.7 [re.regex]:
template <class charT, class traits = regex_traits<charT> > class basic_regex { ... basic_regex& operator=(const charT* ptr); basic_regex& operator=(initializer_list<charT> il); template <class ST, class SA> basic_regex& operator=(const basic_string<charT, ST, SA>& p); ... };
Add in 28.6.7.2 [re.regex.construct]:
-20- ...
basic_regex& operator=(initializer_list<charT> il);-21- Effects: returns
assign(il.begin(), il.end());
integral_constant
objects useable in integral-constant-expressionsSection: 21.3.4 [meta.help] Status: C++11 Submitter: Alisdair Meredith Opened: 2009-03-11 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
Addresses UK 205 [CD1]
integral_constant
objects should be usable in integral-constant-expressions.
The addition to the language of literal types and the enhanced rules for
constant expressions make this possible.
[ Batavia (2009-05): ]
We agree that the
static
data member ought be declaredconstexpr
, but do not see a need for the proposedoperator value_type()
. (A use case would be helpful.) Move to Open.
[ 2009-05-23 Alisdair adds: ]
The motivating case in my mind is that we can then use
true_type
andfalse_type
as integral Boolean expressions, for example inside astatic_assert
declaration. In that sense it is purely a matter of style.Note that Boost has applied the non-explicit conversion operator for many years as it has valuable properties for extension into other metaprogramming libraries, such as MPL. If additional rationale is desired I will poll the Boost lists for why this extension was originally applied. I would argue that explicit conversion is more appropriate for 0x though.
[ 2009-07-04 Howard adds: ]
Here's a use case which demonstrates the syntactic niceness which Alisdair describes:
#define requires(...) class = typename std::enable_if<(__VA_ARGS__)>::type template <class T, class U, requires(!is_lvalue_reference<T>() || is_lvalue_reference<T>() && is_lvalue_reference<U>()), requires(is_same<typename base_type<T>::type, typename base_type<U>::type>)> inline T&& forward(U&& t) { return static_cast<T&&>(t); }
[ 2009-07 post-Frankfurt: ]
Move to Tentatively Ready.
[ 2009 Santa Cruz: ]
Moved to Ready for this meeting.
Proposed resolution:
Add to the integral_constant
struct definition in 21.3.4 [meta.help]:
template <class T, T v> struct integral_constant { static constexpr T value = v; typedef T value_type; typedef integral_constant<T,v> type; constexpr operator value_type() { return value; } };
nullptr_t
assignments to unique_ptr
Section: 20.3.1.3.4 [unique.ptr.single.asgn] Status: C++11 Submitter: Alisdair Meredith Opened: 2009-03-11 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
Addresses UK 211 [CD1]
The nullptr_t
type was introduced to resolve the null pointer literal
problem. It should be used for the assignment operator, as with the
constructor and elsewhere through the library.
[ Batavia (2009-05): ]
We agree with the proposed resolution. Move to Tentatively Ready.
Proposed resolution:
Change the synopsis in 20.3.1.3 [unique.ptr.single]:
unique_ptr& operator=(unspecified-pointer-typenullptr_t);
Change 20.3.1.3.4 [unique.ptr.single.asgn]:
unique_ptr& operator=(unspecified-pointer-typenullptr_t);
Assigns from the literal 0 orNULL
. [Note: The unspecified-pointer-type is often implemented as a pointer to a private data member, avoiding many of the implicit conversion pitfalls. — end note]
Section: 99 [depr.util.smartptr.shared.atomic] Status: C++11 Submitter: Alisdair Meredith Opened: 2009-03-11 Last modified: 2017-11-29
Priority: Not Prioritized
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Discussion:
Addresses JP 44 [CD1]
The 1st parameter p
and 2nd parameter v
is now
shared_ptr<T>*
.
It should be shared_ptr<T>&
, or if these are
shared_ptr<T>*
then add the "p
shall not be a
null pointer" at the requires.
[ Summit: ]
Agree. All of the functions need a requirement that
p
(orv
) is a pointer to a valid object.
[ 2009-07 post-Frankfurt: ]
Lawrence explained that these signatures match the regular atomics. The regular atomics must not use references because these signatures are shared with C. The decision to pass shared_ptrs by pointer rather than by reference was deliberate and was motivated by the principle of least surprise.
Lawrence to write wording that requires that the pointers not be null.
[ 2009-09-20 Lawrence provided wording: ]
The parameter types for atomic shared pointer access were deliberately chosen to be pointers to match the corresponding parameters of the atomics chapter. Those in turn were deliberately chosen to match C functions, which do not have reference parameters.
We adopt the second suggestion, to require that such pointers not be null.
[ 2009-10 Santa Cruz: ]
Moved to Ready.
Proposed resolution:
In section "shared_ptr
atomic access"
[util.smartptr.shared.atomic], add to each function the
following clause.
Requires:
p
shall not be null.
thread::join()
effects?Section: 32.4.3.6 [thread.thread.member] Status: C++11 Submitter: Alberto Ganesh Barbati Opened: 2009-03-12 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
While looking at thread::join()
I think I spotted a couple of
possible defects in the specifications. I could not find a previous
issue or NB comment about that, but I might have missed it.
The postconditions clause for thread::join()
is:
Postconditions: If
join()
throws an exception, the value returned byget_id()
is unchanged. Otherwise,get_id() == id()
.
and the throws clause is:
Throws:
std::system_error
when the postconditions cannot be achieved.
Now... how could the postconditions not be achieved?
It's just a matter of resetting the value of get_id()
or leave it
unchanged! I bet we can always do that. Moreover, it's a chicken-and-egg
problem: in order to decide whether to throw or not I depend on the
postconditions, but the postconditions are different in the two cases.
I believe the throws clause should be:
Throws:
std::system_error
when the effects or postconditions cannot be achieved.
as it is in detach()
, or, even better, as the postcondition is
trivially satisfiable and to remove the circular dependency:
Throws:
std::system_error
if the effects cannot be achieved.
Problem is that... ehm... join()
has no "Effects" clause. Is that intentional?
[ See the thread starting at c++std-lib-23204 for more discussion. ]
[ Batavia (2009-05): ]
Pete believes there may be some more general language (in frontmatter) that can address this and related issues such as 962(i).
Move to Open.
[ 2009-11-18 Anthony provides wording. ]
[ 2010-02-12 Moved to Tentatively Ready after 5 positive votes on c++std-lib. ]
Proposed resolution:
Edit 32.4.3.6 [thread.thread.member] as indicated:
void join();5 Precondition:
joinable()
istrue
.Effects: Blocks until the thread represented by
*this
has completed.6 Synchronization: The completion of the thread represented by
*this
happens before (6.9.2 [intro.multithread])join()
returns. [Note: Operations on*this
are not synchronized. — end note]7 Postconditions:
IfThe thread represented byjoin()
throws an exception, the value returned byget_id()
is unchanged. Otherwise,*this
has completed.get_id() == id()
.8 ...
Section: 23.2.2 [container.requirements.general] Status: C++11 Submitter: Alisdair Meredith Opened: 2009-03-12 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
Addresses UK 222 [CD1]
It is not clear what purpose the Requirement tables serve in the
Containers clause. Are they the definition of a library Container? Or
simply a conventient shorthand to factor common semantics into a single
place, simplifying the description of each subsequent container? This
becomes an issue for 'containers' like array
, which does not meet the
default-construct-to-empty requirement, or forward_list
which does not
support the size operation. Are these components no longer containers?
Does that mean the remaining requirements don't apply? Or are these
contradictions that need fixing, despite being a clear design decision?
Recommend:
Clarify all the tables in 23.2 [container.requirements] are
there as a convenience for documentation, rather than a strict set of
requirements. Containers should be allowed to relax specific
requirements if they call attention to them in their documentation. The
introductory text for array
should be expanded to mention a
default constructed array
is not empty, and
forward_list
introduction should mention it does not provide
the required size
operation as it cannot be implemented
efficiently.
[ Summit: ]
Agree in principle.
[ 2009-07 post-Frankfurt: ]
We agree in principle, but we have a timetable. This group feels that the issue should be closed as NAD unless a proposed resolution is submitted prior to the March 2010 meeting.
[ 2009-10 Santa Cruz: ]
Looked at this and still intend to close as NAD in March 2010 unless there is proposed wording that we like.
[ 2010-02-02 Nicolai M. Josuttis updates proposed wording and adds: ]
I just came across issue #1034 (response to UK 222), which covers the role of container requirements. The reason I found this issue was that I am wondering why
array<>
is specified to be a sequence container. For me, currently, this follows from Sequence containers 23.2.4 [sequence.reqmts] saying:The library provides five basic kinds of sequence containers:
array
,vector
,forward_list
,list
, anddeque
. while later on in Table 94 "Sequence container requirements" are defined.IMO, you can hardly argue that this is NAD. We MUST say somewhere that either array is not a sequence container or does not provide all operations of a sequence container (even not all requirements of a container in general).
Here is the number of requirements
array<>
does not meet (AFAIK):general container requirements:
- a default constructed
array
is not emptyswap
has no constant complexityNote also that
swap
not only has linear complexity it also invalidates iterators (or to be more precise, assigns other values to the elements), which is different from the effect swap has for other containers. For this reason, I must say that i tend to propose to removeswap()
forarrays
.sequence container requirements:
- There is no constructor and assignment for a range
- There is no constructor and assignment for
n
copies oft
- There are no
emplace
,insert
,erase
,clear
,assign
operationsIn fact, out of all sequence container requirements
array<>
only provides the following operations: from sequence requirements (Table 94):X(il); a = il;and from optional requirements (Table 95):
[], at(), front(), back()This is almost nothing!
Note in addition, that due to the fact that
array
is an aggregate and not a container withinitializer_lists
a construction or assignment with an initializer list is valid for all sequence containers but not valid for array:vector<int> v({1,2,3}); // OK v = {4,5,6}; // OK array<int,3> a({1,2,3}); // Error array<int,3> a = {1,2,3}; // OK a = {4,5,6}; // ErrorBTW, for this reason, I am wondering, why
<array>
includes<initializer_list>
.IMO, we can't really say that
array
is a sequence container.array
is special. As the solution to this issue seemed to miss some proposed wording where all could live with, let me try to suggest some.
[ 2010-02-12 Moved to Tentatively Ready after 5 positive votes on c++std-lib. ]
[ 2010 Pittsburgh: Ok with move to Ready except for "OPEN:" part. ]
Proposed resolution:
In Sequence containers 23.2.4 [sequence.reqmts] modify paragraph 1 as indicated:
1 A sequence container organizes a finite set of objects, all of the same type, into a strictly linear arrangement. The library provides
fivefour basic kinds of sequence containers:array
,vector
,forward_list
,list
, anddeque
. In addition,array
is provided as a sequence container that only provides limited sequence operations because it has a fixed number of elements.ItThe library also provides container adaptors that make it easy to construct abstract data types, such asstack
s orqueue
s, out of the basic sequence container kinds (or out of other kinds of sequence containers that the user might define).
Modify paragraph 2 as follows (just editorial):
2 The
five basicsequence containers offer the programmer different complexity trade-offs and should be used accordingly.vector
orarray
is the type of sequence container that should be used by default.list
orforward_list
should be used when there are frequent insertions and deletions from the middle of the sequence.deque
is the data structure of choice when most insertions and deletions take place at the beginning or at the end of the sequence.
In Class template array 23.3.3 [array] modify paragraph 3 as indicated:
3
Unless otherwise specified, allAn array satisfies all of the requirements of a container and of a reversible container (given in two tables in 23.2 [container.requirements]) except that a default constructedarray
operations are as described in 23.2.array
is not empty,swap
does not have constant complexity, andswap
may throw exceptions. Anarray
satisfies some of the requirements of a sequence container (given in 23.2.4 [sequence.reqmts]). Descriptions are provided here only for operations onarray
that are not describedin that Clausein one of these tables or for operations where there is additional semantic information.
In array specialized algorithms 23.3.3.4 [array.special] add to the
specification of swap()
:
template <class T, size_t N> void swap(array<T,N>& x, array<T,N>& y);1 Effects: ...
Complexity: Linear in
N
.
match_results
as library containerSection: 23.2.4 [sequence.reqmts] Status: C++11 Submitter: Alisdair Meredith Opened: 2009-03-12 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
Addresses UK 232 [CD1]
match_results
may follow the requirements but is not listed a general
purpose library container.
Remove reference to match_results
against a[n]
operation.
[ Summit: ]
Agree.
operator[]
is defined elsewhere.
[ Batavia (2009-05): ]
We agree with the proposed resolution. Move to Tentatively Ready.
Proposed resolution:
In 23.2.4 [sequence.reqmts] Table 84, remove reference to
match_results
in the row describing the a[n]
operation.
Section: 23.2.4 [sequence.reqmts] Status: C++11 Submitter: Alisdair Meredith Opened: 2009-03-12 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
Addresses UK 233 [CD1]
Table 84 is missing references to several new container types.
[ Summit: ]
Agree.
[ Batavia (2009-05): ]
We agree with the proposed resolution. Move to Tentatively Ready.
Proposed resolution:
In 23.2.4 [sequence.reqmts] Table 84, Add reference to listed containers to the following rows:
Table 84 -- Optional sequence container operations Expression Return type Operational semantics Container a.front()
... ... vector, list, deque, basic_string, array, forward_list
a.back()
... ... vector, list, deque, basic_string, array
a.emplace_front(args)
... ... list, deque, forward_list
a.push_front(t)
... ... list, deque, forward_list
a.push_front(rv)
... ... list, deque, forward_list
a.pop_front()
... ... list, deque, forward_list
a[n]
... ... vector, deque, basic_string, array
a.at(n)
... ... vector, deque, basic_string, array
back
function should also support const_iterator
Section: 23.2.4 [sequence.reqmts] Status: C++11 Submitter: Alisdair Meredith Opened: 2009-03-12 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
Addresses UK 234 [CD1]
The reference to iterator
in semantics for back
should
also allow for const_iterator
when called on a const-qualified
container. This would be ugly to specify in the 03 standard, but is
quite easy with the addition of auto
in this new standard.
[ Summit: ]
Agree.
[ Batavia (2009-05): ]
We agree with the proposed resolution. Move to Tentatively Ready.
Proposed resolution:
In 23.2.4 [sequence.reqmts] Table 84, replace iterator with auto in semantics for back:
Table 84 — Optional sequence container operations Expression Return type Operational semantics Container a.back()
reference; const_reference
for constanta
{
iteratorauto tmp = a.end();
--tmp;
return *tmp; }vector, list, deque, basic_string
iterator
and const_iterator
Section: 23.2.7 [associative.reqmts] Status: C++11 Submitter: Alisdair Meredith Opened: 2009-03-12 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
Addresses UK 238 [CD1]
Leaving it unspecified whether or not iterator
and const_iterator
are the
same type is dangerous, as user code may or may not violate the One
Definition Rule by providing overloads for
both types. It is probably too late to specify a single behaviour, but
implementors should document what to expect. Observing that problems can be
avoided by users restricting themselves to using const_iterator
, add a note to that effect.
Suggest Change 'unspecified' to 'implementation defined'.
[ Summit: ]
Agree with issue. Agree with adding the note but not with changing the normative text. We believe the note provides sufficient guidance.
[ Batavia (2009-05): ]
We agree with the proposed resolution. Move to Tentatively Ready.
Proposed resolution:
In 23.2.7 [associative.reqmts] p6, add:
-6-
iterator
of an associative container meets the requirements of theBidirectionalIterator
concept. For associative containers where the value type is the same as the key type, bothiterator
andconst_iterator
are constant iterators. It is unspecified whether or notiterator
andconst_iterator
are the same type. [Note:iterator
andconst_iterator
have identical semantics in this case, anditerator
is convertible toconst_iterator
. Users can avoid violating the One Definition Rule by always usingconst_iterator
in their function parameter lists -- end note]
Section: 23.2.7 [associative.reqmts] Status: Resolved Submitter: Alisdair Meredith Opened: 2009-03-12 Last modified: 2020-09-06
Priority: Not Prioritized
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Discussion:
Addresses UK 239 [CD1]
It is not possible to take a move-only key out of an unordered
container, such as (multi
)set
or
(multi
)map
, or the new unordered containers.
Add below a.erase(q)
, a.extract(q)
, with the following notation:
a.extract(q)>
, Return type pair<key, iterator>
Extracts the element pointed to by q
and erases it from the
set
. Returns a pair
containing the value pointed to by
q
and an iterator
pointing to the element immediately
following q
prior to the element being erased. If no such
element exists,returns a.end()
.
[ Summit: ]
We look forward to a paper on this topic. We recommend no action until a paper is available. The paper would need to address exception safety.
[ Post Summit Alisdair adds: ]
Would
value_type
be a better return type thankey_type
?
[ 2009-07 post-Frankfurt: ]
Leave Open. Alisdair to contact Chris Jefferson about this.
[ 2009-09-20 Howard adds: ]
See the 2009-09-19 comment of 839(i) for an API which accomplishes this functionality and also addresses several other use cases which this proposal does not.
[ 2009-10 Santa Cruz: ]
Mark as NAD Future. No consensus to make the change at this time.
Original resolution [SUPERSEDED]:
In 23.2.7 [associative.reqmts] Table 85, add:
Table 85 -- Associative container requirements (in addition to container) Expression Return type Assertion/note
pre-/post-conditionComplexity a.erase(q)
... ... ... a.extract(q)
pair<key_type, iterator>
Extracts the element pointed to by q
and erases it from theset
. Returns apair
containing the value pointed to byq
and aniterator
pointing to the element immediately followingq
prior to the element being erased. If no such element exists, returnsa.end()
.amortized constant In 23.2.8 [unord.req] Table 87, add:
Table 87 -- Unordered associative container requirements (in addition to container) Expression Return type Assertion/note
pre-/post-conditionComplexity a.erase(q)
... ... ... a.extract(q)
pair<key_type, iterator>
Extracts the element pointed to by q
and erases it from theset
. Returns apair
containing the value pointed to byq
and aniterator
pointing to the element immediately followingq
prior to the element being erased. If no such element exists, returnsa.end()
.amortized constant
[08-2016, Post-Chicago]
Move to Tentatively Resolved
Proposed resolution:
This functionality is provided by P0083R3
compare_exchange
is not a read-modify-write operationSection: 32.5.8.2 [atomics.types.operations] Status: Resolved Submitter: Alisdair Meredith Opened: 2009-03-12 Last modified: 2016-01-28
Priority: Not Prioritized
View all other issues in [atomics.types.operations].
View all issues with Resolved status.
Discussion:
Addresses US 91 [CD1]
It is unclear whether or not a failed compare_exchange
is a RMW operation
(as used in 6.9.2 [intro.multithread]).
Suggested solution:
Make failing compare_exchange
operations not be RMW.
[ Anthony Williams adds: ]
In 32.5.8.2 [atomics.types.operations] p18 it says that "These operations are atomic read-modify-write operations" (final sentence). This is overly restrictive on the implementations of
compare_exchange_weak
andcompare_exchange_strong
on platforms without a native CAS instruction.
[ Summit: ]
Group agrees with the resolution as proposed by Anthony Williams in the attached note.
[ Batavia (2009-05): ]
We recommend the proposed resolution be reviewed by members of the Concurrency Subgroup.
[ 2009-07 post-Frankfurt: ]
This is likely to be addressed by Lawrence's upcoming paper. He will adopt the proposed resolution.
[ 2009-08-17 Handled by N2925. ]
[ 2009-10 Santa Cruz: ]
NAD EditorialResolved. Addressed by N2992.
Proposed resolution:
Change 32.5.8.2 [atomics.types.operations] p18:
-18- Effects: Atomically, compares the value pointed to by
object
or bythis
for equality with that inexpected
, and if true, replaces the value pointed to byobject
or bythis
with desired, and if false, updates the value inexpected
with the value pointed to byobject
or bythis
. Further, if the comparison is true, memory is affected according to the value ofsuccess
, and if the comparison is false, memory is affected according to the value offailure
. When only onememory_order
argument is supplied, the value ofsuccess
isorder
, and the value offailure
isorder
except that a value ofmemory_order_acq_rel
shall be replaced by the valuememory_order_acquire
and a value ofmemory_order_release
shall be replaced by the valuememory_order_relaxed
. If the comparison istrue
,Tthese operations are atomic read-modify-write operations (1.10). If the comparison isfalse
, these operations are atomic load operations.
constexpr
literalsSection: 32.6 [thread.mutex] Status: C++11 Submitter: Alisdair Meredith Opened: 2009-03-12 Last modified: 2016-01-28
Priority: Not Prioritized
View all other issues in [thread.mutex].
View all issues with C++11 status.
Discussion:
Addresses UK 325 [CD1]
We believe constexpr
literal values should be a more natural expression
of empty tag types than extern objects as it should improve the
compiler's ability to optimize the empty object away completely.
[ Summit: ]
Move to review. The current specification is a "hack", and the proposed specification is a better "hack".
[ Batavia (2009-05): ]
We agree with the proposed resolution. Move to Tentatively Ready.
Proposed resolution:
Change the synopsis in 32.6 [thread.mutex]:
struct defer_lock_t {}; struct try_to_lock_t {}; struct adopt_lock_t {};externconstexpr defer_lock_t defer_lock {};externconstexpr try_to_lock_t try_to_lock {};externconstexpr adopt_lock_t adopt_lock {};
unique_lock
constructorSection: 32.6.5.4.2 [thread.lock.unique.cons] Status: C++11 Submitter: Alisdair Meredith Opened: 2009-03-12 Last modified: 2016-01-28
Priority: Not Prioritized
View all other issues in [thread.lock.unique.cons].
View all issues with C++11 status.
Discussion:
Addresses UK 326 [CD1]
The precondition that the mutex is not owned by this thread offers introduces the risk of unnecessary undefined behaviour into the program. The only time it matters whether the current thread owns the mutex is in the lock operation, and that will happen subsequent to construction in this case. The lock operation has the identical pre-condition, so there is nothing gained by asserting that precondition earlier and denying the program the right to get into a valid state before calling lock.
[ Summit: ]
Agree, move to review.
[ Batavia (2009-05): ]
We agree with the proposed resolution. Move to Tentatively Ready.
Proposed resolution:
Strike 32.6.5.4.2 [thread.lock.unique.cons] p7:
unique_lock(mutex_type& m, defer_lock_t);
-7- Precondition: Ifmutex_type
is not a recursive mutex the calling thread does not own the mutex.
Section: 32.10 [futures] Status: Resolved Submitter: Alisdair Meredith Opened: 2009-03-12 Last modified: 2016-01-28
Priority: Not Prioritized
View all other issues in [futures].
View all issues with Resolved status.
Discussion:
Addresses UK 329 [CD1]
future
, promise
and packaged_task
provide a
framework for creating future values, but a simple function to tie all
three components together is missing. Note that we only need a simple
facility for C++0x. Advanced thread pools are to be left for TR2.
Simple Proposal:
Provide a simple function along the lines of:
template< typename F, typename ... Args > requires Callable< F, Args... > future< Callable::result_type > async( F&& f, Args && ... );
Semantics are similar to creating a thread
object with a packaged_task
invoking f
with forward<Args>(args...)
but details are left unspecified to allow different scheduling and thread
spawning implementations.
It is unspecified whether a task submitted to async is run on its own thread
or a thread previously used for another async task. If a call to async
succeeds, it shall be safe to wait for it from any thread.
The state of thread_local
variables shall be preserved during async
calls.
No two incomplete async tasks shall see the same value of
this_thread::get_id()
.
[Note: this effectively forces new tasks to be run on a new thread, or a
fixed-size pool with no queue. If the
library is unable to spawn a new thread or there are no free worker threads
then the async
call should fail. --end note]
[ Summit: ]
The concurrency subgroup has revisited this issue and decided that it could be considered a defect according to the Kona compromise. A task group was formed lead by Lawrence Crowl and Bjarne Stroustrup to write a paper for Frankfort proposing a simple asynchronous launch facility returning a
future
. It was agreed that the callable must be run on a separate thread from the caller, but not necessarily a brand-new thread. The proposal might or might not allow for an implementation that uses fixed-size or unlimited thread pools.Bjarne in c++std-lib-23121: I think that what we agreed was that to avoid deadlock
async()
would almost certainly be specified to launch in a different thread from the thread that executedasync()
, but I don't think it was a specific design constraint.
[ 2009-10 Santa Cruz: ]
Proposed resolution: see N2996 (Herb's and Lawrence's paper on Async). Move state to
NAD editorialResolved.
Proposed resolution:
get()
blocks when not readySection: 32.10.7 [futures.unique.future] Status: Resolved Submitter: Alisdair Meredith Opened: 2009-03-12 Last modified: 2021-06-06
Priority: Not Prioritized
View all other issues in [futures.unique.future].
View all issues with Resolved status.
Discussion:
Addresses UK 334 [CD1]
Behaviour of get()
is undefined if calling get()
while
not is_ready()
. The intent is that get()
is a blocking
call, and will wait for the future to become ready.
[ Summit: ]
Agree, move to Review.
[ 2009-04-03 Thomas J. Gritzan adds: ]
This issue also applies to
shared_future::get()
.Suggested wording:
Add a paragraph to [futures.shared_future]:
void shared_future<void>::get() const;Effects: If
is_ready()
would returnfalse
, block on the asynchronous result associated with*this
.
[ Batavia (2009-05): ]
It is not clear to us that this is an issue, because the proposed resolution's Effects clause seems to duplicate information already present in the Synchronization clause. Keep in Review status.
[ 2009-10 Santa Cruz: ]
NAD EditorialResolved. Addressed by N2997.
Proposed resolution:
Add a paragraph to [futures.unique_future]:
R&& unique_future::get(); R& unique_future<R&>::get(); void unique_future<void>::get();Note:...
Effects: If
is_ready()
would returnfalse
, block on the asynchronous result associated with*this
.Synchronization: if
*this
is associated with apromise
object, the completion ofset_value()
orset_exception()
to thatpromise
happens before (1.10)get()
returns.
std::unique_future
Section: 32.10.7 [futures.unique.future] Status: Resolved Submitter: Alisdair Meredith Opened: 2009-03-12 Last modified: 2021-06-06
Priority: Not Prioritized
View all other issues in [futures.unique.future].
View all issues with Resolved status.
Discussion:
Addresses UK 335 [CD1]
std::unique_future
is MoveConstructible
, so you can transfer the
association with an asynchronous result from one instance to another.
However, there is no way to determine whether or not an instance has
been moved from, and therefore whether or not it is safe to wait for it.
std::promise<int> p;
std::unique_future<int> uf(p.get_future());
std::unique_future<int> uf2(std::move(uf));
uf.wait(); // oops, uf has no result to wait for.
Suggest we add a waitable()
function to unique_future
(and shared_future
) akin to std::thread::joinable()
,
which returns true
if there is an associated result to wait for
(whether or not it is ready).
Then we can say:
if(uf.waitable()) uf.wait();
[ Summit: ]
Create an issue. Requires input from Howard. Probably NAD.
[ Post Summit, Howard throws in his two cents: ]
Here is a copy/paste of my last prototype of
unique_future
which was several years ago. At that time I was callingunique_future
future
:template <class R> class future { public: typedef R result_type; private: future(const future&);// = delete; future& operator=(const future&);// = delete; template <class R1, class F1> friend class prommise; public: future(); ~future(); future(future&& f); future& operator=(future&& f); void swap(future&& f); bool joinable() const; bool is_normal() const; bool is_exceptional() const; bool is_ready() const; R get(); void join(); template <class ElapsedTime> bool timed_join(const ElapsedTime&); };
shared_future
had a similar interface. I intentionally reused thethread
interface where possible to lessen the learning curve std::lib clients will be faced with.
[ 2009-10 Santa Cruz: ]
NAD EditorialResolved. Addressed by N2997.
Proposed resolution:
promise
invertedSection: 32.10.6 [futures.promise] Status: Resolved Submitter: Alisdair Meredith Opened: 2009-03-12 Last modified: 2016-01-28
Priority: Not Prioritized
View other active issues in [futures.promise].
View all other issues in [futures.promise].
View all issues with Resolved status.
Discussion:
Addresses UK 339 [CD1]
Move assignment is going in the wrong direction, assigning from
*this
to the passed rvalue, and then returning a reference to
an unusable *this
.
[ Summit: ]
Agree, move to Review.
[ Batavia (2009-05): ]
We recommend deferring this issue until after Detlef's paper (on futures) has been issued.
[ 2009-10 Santa Cruz: ]
NAD EditorialResolved. Addressed by N2997.
Proposed resolution:
Strike 32.10.6 [futures.promise] p6 and change p7:
promise& operator=(promise&& rhs);
-6- Effects: move assigns its associated state torhs
.-7- Postcondition:
associated state of*this
has no associated state.*this
is the same as the associated state ofrhs
before the call.rhs
has no associated state.
get_future()
Section: 32.10.6 [futures.promise] Status: Resolved Submitter: Alisdair Meredith Opened: 2009-03-12 Last modified: 2016-01-28
Priority: Not Prioritized
View other active issues in [futures.promise].
View all other issues in [futures.promise].
View all issues with Resolved status.
Discussion:
Addresses UK 340 [CD1]
There is an implied postcondition for get_future()
that the state of the
promise
is transferred into the future
leaving the promise
with no
associated state. It should be spelled out.
[ Summit: ]
Agree, move to Review.
[ 2009-04-03 Thomas J. Gritzan adds: ]
promise::get_future()
must not invalidate the state of the promise object.A promise is used like this:
promise<int> p; unique_future<int> f = p.get_future(); // post 'p' to a thread that calculates a value // use 'f' to retrieve the value.So
get_future()
must return an object that shares the same associated state with*this
.But still, this function should throw an
future_already_retrieved
error when it is called twice.
packaged_task::get_future()
throwsstd::bad_function_call
if itsfuture
was already retrieved. It should throwfuture_error(future_already_retrieved)
, too.Suggested resolution:
Replace p12/p13 32.10.6 [futures.promise]:
-12- Throws:
future_error
ifthe*this
has no associated statefuture
has already been retrieved.-13- Error conditions:
future_already_retrieved
ifthe*this
has no associated statefuture
associated with the associated state has already been retrieved.Postcondition: The returned object and
*this
share the associated state.Replace p14 32.10.10 [futures.task]:
-14- Throws:
if the future
std::bad_function_callfuture_errorassociated with the taskhas already been retrieved.Error conditions:
future_already_retrieved
if thefuture
associated with the task has already been retrieved.Postcondition: The returned object and
*this
share the associated task.
[ Batavia (2009-05): ]
Keep in Review status pending Detlef's forthcoming paper on futures.
[ 2009-10 Santa Cruz: ]
NAD EditorialResolved. Addressed by N2997.
Proposed resolution:
Add after p13 32.10.6 [futures.promise]:
unique_future<R> get_future();-13- ...
Postcondition:
*this
has no associated state.
reverse_iterator::operator->
should also support smart pointersSection: 24.5.1.6 [reverse.iter.elem] Status: Resolved Submitter: Alisdair Meredith Opened: 2009-03-12 Last modified: 2021-06-06
Priority: Not Prioritized
View all other issues in [reverse.iter.elem].
View all issues with Resolved status.
Duplicate of: 2775
Discussion:
Addresses UK 281 [CD1]
The current specification for return value for reverse_iterator::operator->
will always be a true pointer type, but reverse_iterator
supports proxy
iterators where the pointer type may be some kind of 'smart pointer'.
[ Summit: ]
move_iterator
avoids this problem by returning a value of the wrapped Iterator type. study group formed to come up with a suggested resolution.
move_iterator
solution shown in proposed wording.
[ 2009-07 post-Frankfurt: ]
Howard to deconceptize. Move to Review after that happens.
[ 2009-08-01 Howard deconceptized: ]
[ 2009-10 Santa Cruz: ]
We can't think of any reason we can't just define reverse iterator's pointer types to be the same as the underlying iterator's pointer type, and get it by calling the right arrow directly.
Here is the proposed wording that was replaced:
template <class Iterator> class reverse_iterator { […] typedeftypename iterator_traits<Iterator>::pointerpointer;Change [reverse.iter.opref]:
pointer operator->() const;Returns:
&(operator*());this->tmp = current; --this->tmp; return this->tmp;
[ 2010-03-03 Daniel opens: ]
- There is a minor problem with the exposition-only declaration of the private member
deref_tmp
which is modified in a const member function (and the same problem occurs in the specification ofoperator*
). The fix is to make it a mutable member.The more severe problem is that the resolution for some reasons does not explain in the rationale why it was decided to differ from the suggested fix (using
deref_tmp
instead oftmp
) in the [ 2009-10 Santa Cruz] comment:this->deref_tmp = current; --this->deref_tmp; return this->deref_tmp;combined with the change of
typedef typename iterator_traits<Iterator>::pointer pointer;to
typedef Iterator pointer;The problem of the agreed on wording is that the following rather typical example, that compiled with the wording before 1052 had been applied, won't compile anymore:
#include <iterator> #include <utility> int main() { typedef std::pair<int, double> P; P op; std::reverse_iterator<P*> ri(&op + 1); ri->first; // Error }Comeau online returns (if a correspondingly changed
reverse_iterator
is used):"error: expression must have class type return deref_tmp.operator->(); ^ detected during instantiation of "Iterator reverse_iterator<Iterator>::operator->() const [with Iterator=std::pair<int, double> *]""Thus the change will break valid, existing code based on
std::reverse_iterator
.IMO the suggestion proposed in the comment is a necessary fix, which harmonizes with the similar specification of
std::move_iterator
and properly reflects the recursive nature of the evaluation ofoperator->
overloads.Suggested resolution:
In the class template
reverse_iterator
synopsis of 24.5.1.2 [reverse.iterator] change as indicated:namespace std { template <class Iterator> class reverse_iterator : public iterator<typename iterator_traits<Iterator>::iterator_category, typename iterator_traits<Iterator>::value_type, typename iterator_traits<Iterator>::difference_type,typename iterator_traits<Iterator>::pointer, typename iterator_traits<Iterator>::reference> { public: [..] typedeftypename iterator_traits<Iterator>::pointerpointer; [..] protected: Iterator current; private: mutable Iterator deref_tmp; // exposition only };- Change [reverse.iter.opref]/1 as indicated:
pointer operator->() const;1
ReturnsEffects:&(operator*())
.deref_tmp = current; --deref_tmp; return deref_tmp;
[ 2010 Pittsburgh: ]
We prefer to make to use a local variable instead of
deref_tmp
withinoperator->()
. And although this means that themutable
change is no longer needed, we prefer to keep it because it is needed foroperator*()
anyway.Here is the proposed wording that was replaced:
Change [reverse.iter.opref]:
pointer operator->() const;Returns:
&(operator*());deref_tmp = current; --deref_tmp; return deref_tmp::operator->();
[ 2010-03-10 Howard adds: ]
Here are three tests that the current proposed wording passes, and no other solution I've seen passes all three:
Proxy pointer support:
#include <iterator> #include <cassert> struct X { int m; }; X x; struct IterX { typedef std::bidirectional_iterator_tag iterator_category; typedef X& reference; struct pointer { pointer(X& v) : value(v) {} X& value; X* operator->() const {return &value;} }; typedef std::ptrdiff_t difference_type; typedef X value_type; // additional iterator requirements not important for this issue reference operator*() const { return x; } pointer operator->() const { return pointer(x); } IterX& operator--() {return *this;} }; int main() { std::reverse_iterator<IterX> ix; assert(&ix->m == &(*ix).m); }Raw pointer support:
#include <iterator> #include <utility> int main() { typedef std::pair<int, double> P; P op; std::reverse_iterator<P*> ri(&op + 1); ri->first; // Error }Caching iterator support:
#include <iterator> #include <cassert> struct X { int m; }; struct IterX { typedef std::bidirectional_iterator_tag iterator_category; typedef X& reference; typedef X* pointer; typedef std::ptrdiff_t difference_type; typedef X value_type; // additional iterator requirements not important for this issue reference operator*() const { return value; } pointer operator->() const { return &value; } IterX& operator--() {return *this;} private: mutable X value; }; int main() { std::reverse_iterator<IterX> ix; assert(&ix->m == &(*ix).m); }
[ 2010 Pittsburgh: ]
Moved to NAD Future, rationale added.
[LEWG Kona 2017]
Recommend that we accept the "Alternate Proposed Resolution" from 2775(i).
[ Original rationale: ]
The LWG did not reach a consensus for a change to the WP.
Previous resolution [SUPERSEDED]:
In the class template
reverse_iterator
synopsis of 24.5.1.2 [reverse.iterator] change as indicated:namespace std { template <class Iterator> class reverse_iterator : public iterator<typename iterator_traits<Iterator>::iterator_category, typename iterator_traits<Iterator>::value_type, typename iterator_traits<Iterator>::difference_type,typename iterator_traits<Iterator&>::pointer, typename iterator_traits<Iterator>::reference> { public: [..] typedeftypename iterator_traits<Iterator&>::pointerpointer; [..] protected: Iterator current; private: mutable Iterator deref_tmp; // exposition only };- Change [reverse.iter.opref]/1 as indicated:
pointer operator->() const;1
ReturnsEffects:&(operator*())
.deref_tmp = current; --deref_tmp; return deref_tmp;[Alternate Proposed Resolution from 2775(i), which was closed as a dup of this issue]
This wording is relative to N4606.
Modify [reverse.iter.opref] as indicated:
constexpr pointer operator->() const;-1-
Returns:Effects: Ifaddressof(operator*())
.Iterator
is a pointer type, as if by:Iterator tmp = current; return --tmp;Otherwise, as if by:
Iterator tmp = current; --tmp; return tmp.operator->();
[2018-08-06; Daniel rebases to current working draft and reduces the proposed wording to that preferred by LEWG]
[2018-11-13; Casey Carter comments]
The acceptance of P0896R4 during the San Diego meeting resolves this
issue: The wording in [reverse.iter.elem] for operator->
is equivalent to the PR for LWG 1052.
Proposed resolution:
This wording is relative to N4762.
Modify 24.5.1.6 [reverse.iter.elem] as indicated:
constexpr pointer operator->() const;-2-
Returns:Effects: Ifaddressof(operator*())
.Iterator
is a pointer type, as if by:Iterator tmp = current; return --tmp;Otherwise, as if by:
Iterator tmp = current; --tmp; return tmp.operator->();
forward
brokenSection: 22.2.4 [forward] Status: Resolved Submitter: Howard Hinnant Opened: 2009-03-13 Last modified: 2016-01-28
Priority: Not Prioritized
View all other issues in [forward].
View all issues with Resolved status.
Discussion:
This is a placeholder issue to track the fact that we (well I) put the standard into an inconsistent state by requesting that we accept N2844 except for the proposed changes to [forward].
There will exist in the post meeting mailing N2835 which in its current state reflects the state of affairs prior to the Summit meeting. I hope to update it in time for the post Summit mailing, but as I write this issue I have not done so yet.
[ Batavia (2009-05): ]
Move to Open, awaiting the promised paper.
[ 2009-08-02 Howard adds: ]
My current preferred solution is:
template <class T> struct __base_type { typedef typename remove_cv<typename remove_reference<T>::type>::type type; }; template <class T, class U, class = typename enable_if< !is_lvalue_reference<T>::value || is_lvalue_reference<T>::value && is_lvalue_reference<U>::value>::type, class = typename enable_if< is_same<typename __base_type<T>::type, typename __base_type<U>::type>::value>::type> inline T&& forward(U&& t) { return static_cast<T&&>(t); }This has been tested by Bill, Jason and myself.
It allows the following lvalue/rvalue casts:
- Cast an lvalue
t
to an lvalueT
(identity).- Cast an lvalue
t
to an rvalueT
.- Cast an rvalue
t
to an rvalueT
(identity).It disallows:
- Cast an rvalue
t
to an lvalueT
.- Cast one type
t
to another typeT
(such asint
todouble
)."a." is disallowed as it can easily lead to dangling references. "b." is disallowed as this function is meant to only change the lvalue/rvalue characteristic of an expression.
Jason has expressed concern that "b." is not dangerous and is useful in contexts where you want to "forward" a derived type as a base type. I find this use case neither dangerous, nor compelling. I.e. I could live with or without the "b." constraint. Without it, forward would look like:
template <class T, class U, class = typename enable_if< !is_lvalue_reference<T>::value || is_lvalue_reference<T>::value && is_lvalue_reference<U>::value>::type> inline T&& forward(U&& t) { return static_cast<T&&>(t); }Or possibly:
template <class T, class U, class = typename enable_if< !is_lvalue_reference<T>::value || is_lvalue_reference<T>::value && is_lvalue_reference<U>::value>::type, class = typename enable_if< is_base_of<typename __base_type<U>::type, typename __base_type<T>::type>::value>::type> inline T&& forward(U&& t) { return static_cast<T&&>(t); }The "promised paper" is not in the post-Frankfurt mailing only because I'm waiting for the non-concepts draft. But I'm hoping that by adding this information here I can keep people up to date.
[ 2009-08-02 David adds: ]
forward
was originally designed to do one thing: perfect forwarding. That is, inside a function template whose actual argument can be a const or non-const lvalue or rvalue, restore the original "rvalue-ness" of the actual argument:template <class T> void f(T&& x) { // x is an lvalue here. If the actual argument to f was an // rvalue, pass static_cast<T&&>(x) to g; otherwise, pass x. g( forward<T>(x) ); }Attempting to engineer
forward
to accomodate uses other than perfect forwarding dilutes its idiomatic meaning. The solution proposed here declares thatforward<T>(x)
means nothing more thanstatic_cast<T&&>(x)
, with a patchwork of restrictions on whatT
andx
can be that can't be expressed in simple English.I would be happy with either of two approaches, whose code I hope (but can't guarantee) I got right.
Use a simple definition of
forward
that accomplishes its original purpose without complications to accomodate other uses:template <class T, class U> T&& forward(U& x) { return static_cast<T&&>(x); }Use a definition of
forward
that protects the user from as many potential mistakes as possible, by actively preventing all other uses:template <class T, class U> boost::enable_if_c< // in forward<T>(x), x is a parameter of the caller, thus an lvalue is_lvalue_reference<U>::value // in caller's deduced T&& argument, T can only be non-ref or lvalue ref && !is_rvalue_reference<T>::value // Must not cast cv-qualifications or do any type conversions && is_same<T&,U&>::value , T&&>::type forward(U&& a) { return static_cast<T&&>(a); }
[ 2009-09-27 Howard adds: ]
A paper, N2951, is available which compares several implementations (including David's) with respect to several use cases (including Jason's) and provides wording for one implementation.
[ 2009-10 Santa Cruz: ]
NAD EditorialResolved. Solved by N2951.
Proposed resolution:
Section: 21.3.8.7 [meta.trans.other] Status: Resolved Submitter: Alisdair Meredith Opened: 2009-03-12 Last modified: 2016-01-28
Priority: Not Prioritized
View all other issues in [meta.trans.other].
View all issues with Resolved status.
Discussion:
Addresses UK 98 [CD1]
It would be useful to be able to determine the underlying
type of an arbitrary enumeration type. This would allow
safe casting to an integral type (especially needed for
scoped enums, which do not promote), and would allow
use of numeric_limits
. In general it makes generic
programming with enumerations easier.
[ Batavia (2009-05): ]
Pete observes (and Tom concurs) that the proposed resolution seems to require compiler support for its implementation, as it seems necessary to look at the range of values of the enumerated type. To a first approximation, a library solution could give an answer based on the size of the type. If the user has specialized
numeric_limits
for the enumerated type, then the library might be able to do better, but there is no such requirement. Keep status as Open and solicit input from CWG.
[ 2009-05-23 Alisdair adds: ]
Just to confirm that the BSI originator of this comment assumed it did indeed imply a compiler intrinsic. Rather than request a Core extension, it seemed in keeping with that the type traits interface provides a library API to unspecified compiler features - where we require several other traits (e.g.
has_trivial_*
) to get the 'right' answer now, unlike in TR1.
[ Addressed in N2947. ]
[ 2009-10 Santa Cruz: ]
NAD EditorialResolved. Solved by N2984.
Proposed resolution:
Add a new row to the table in 21.3.8.7 [meta.trans.other]:
Table 41 -- Other transformations Template Condition Comments template< class T > struct enum_base;
T
shall be an enumeration type (9.8.1 [dcl.enum])The member typedef type
shall name the underlying type of the enumT
.
std
for implementationsSection: 16.4.2.2 [contents] Status: C++11 Submitter: Howard Hinnant Opened: 2009-03-15 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
Addresses UK 168 [CD1]
We should make it clear (either by note or normatively) that namespace
std
may contain inline namespaces, and that entities specified to be
defined in std may in fact be defined in one of these inline namespaces.
(If we're going to use them for versioning, eg when TR2 comes along,
we're going to need that.)
Replace "namespace std or namespaces nested within namespace std" with "namespace std or namespaces nested within namespace std or inline namespaces nested directly or indirectly within namespace std"
[ Summit: ]
adopt UK words (some have reservations whether it is correct)
[ 2009-05-09 Alisdair improves the wording. ]
[ Batavia (2009-05): ]
Bill believes there is strictly speaking no need to say that because no portable test can detect the difference. However he agrees that it doesn't hurt to say this.
Move to Tentatively Ready.
Proposed resolution:
Change 16.4.2.2 [contents] p2:
All library entities except macros,
operator new
andoperator delete
are defined within the namespacestd
or namespaces nested within namespacestd
. It is unspecified whether names declared in a specific namespace are declared directly in that namespace, or in an inline namespace inside that namespace. [Footnote: This gives implementers freedom to support multiple configurations of the library.]
[[noreturn]]
attribute in the librarySection: 17 [support] Status: C++11 Submitter: Howard Hinnant Opened: 2009-03-15 Last modified: 2021-06-06
Priority: Not Prioritized
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Discussion:
Addresses UK 189 and JP 27 [CD1]
The addition of the [[noreturn]]
attribute to the language will be an
important aid for static analysis tools.
The following functions should be declared in C++ with the
[[noreturn]]
attribute: abort
exit
quick_exit
terminate
unexpected
rethrow_exception
throw_with_nested
.
[ Summit: ]
Agreed.
[ Batavia (2009-05): ]
We agree with the proposed resolution. Move to Tentatively Ready.
Proposed resolution:
Change 17.5 [support.start.term] p3:
-2- ...
void abort [[noreturn]] (void)-3- ...
-6- ...
void exit [[noreturn]] (int status)-7- ...
-11- ...
void quick_exit [[noreturn]] (int status)-12- ...
Change the <exception>
synopsis in 17.9 [support.exception]:
void unexpected [[noreturn]] (); ... void terminate [[noreturn]] (); ... void rethrow_exception [[noreturn]] (exception_ptr p); ... template <class T> void throw_with_nested [[noreturn]] (T&& t);// [[noreturn]]
Change 99 [unexpected]:
void unexpected [[noreturn]] ();
Change 17.9.5.4 [terminate]:
void terminate [[noreturn]] ();
Change 17.9.7 [propagation]:
void rethrow_exception [[noreturn]] (exception_ptr p);
In the synopsis of 17.9.8 [except.nested] and the definition area change:
template <class T> void throw_with_nested [[noreturn]] (T&& t);// [[noreturn]]
Section: 22.10.17.3 [func.wrap.func] Status: C++11 Submitter: Howard Hinnant Opened: 2009-03-19 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
The synopsis in 22.10.17.3 [func.wrap.func] says:
template<Returnable R, CopyConstructible... ArgTypes> class function<R(ArgTypes...)> { ... template<class F> requires CopyConstructible<F> && Callable<F, ArgTypes...> && Convertible<Callable<F, ArgTypes...>::result_type, R> function(F); template<class F> requires CopyConstructible<F> && Callable<F, ArgTypes...> && Convertible<Callable<F, ArgTypes...>::result_type, R> function(F&&); ... template<class F, Allocator Alloc> function(allocator_arg_t, const Alloc&, F); template<class F, Allocator Alloc> function(allocator_arg_t, const Alloc&, F&&); ... template<class F> requires CopyConstructible<F> && Callable<F, ArgTypes..> && Convertible<Callable<F, ArgTypes...>::result_type function& operator=(F); template<class F> requires CopyConstructible<F> && Callable<F, ArgTypes...> && Convertible<Callable<F, ArgTypes...>::result_type, R> function& operator=(F&&); ... };
Each of the 3 pairs above are ambiguous. We need only one of each pair, and we
could do it with either one. If we choose the F&&
version we
need to bring decay
into the definition to get the pass-by-value behavior.
In the proposed wording I've gotten lazy and just used the pass-by-value signature.
[ 2009-05-01 Daniel adds: ]
[ Batavia (2009-05): ]
We briefly discussed whether we ought support moveable function objects, but decided that should be a separate issue if someone cares to propose it.
Move to Tentatively Ready.
Proposed resolution:
Change the synopsis of 22.10.17.3 [func.wrap.func], and remove the associated definitions in 22.10.17.3.2 [func.wrap.func.con]:
template<Returnable R, CopyConstructible... ArgTypes> class function<R(ArgTypes...)> { ... template<class F> requires CopyConstructible<F> && Callable<F, ArgTypes...> && Convertible<Callable<F, ArgTypes...>::result_type, R> function(F);template<class F> requires CopyConstructible<F> && Callable<F, ArgTypes...> && Convertible<Callable<F, ArgTypes...>::result_type, R> function(F&&);... template<class F, Allocator Alloc> function(allocator_arg_t, const Alloc&, F);template<class F, Allocator Alloc> function(allocator_arg_t, const Alloc&, F&&);... template<class F> requires CopyConstructible<F> && Callable<F, ArgTypes..> && Convertible<Callable<F, ArgTypes...>::result_type function& operator=(F);template<class F> requires CopyConstructible<F> && Callable<F, ArgTypes...> && Convertible<Callable<F, ArgTypes...>::result_type, R> function& operator=(F&&);... };
is_bind_expression
should derive from integral_constant<bool>
Section: 22.10.15.2 [func.bind.isbind] Status: C++11 Submitter: Alisdair Meredith Opened: 2009-03-19 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
Class template is_bind_expression 22.10.15.2 [func.bind.isbind]:
namespace std { template<class T> struct is_bind_expression { static const bool value = see below; }; }
is_bind_expression
should derive from std::integral_constant<bool>
like
other similar trait types.
[ Daniel adds: ]
We need the same thing for the trait
is_placeholder
as well.
[ 2009-03-22 Daniel provided wording. ]
[ Batavia (2009-05): ]
We recommend this be deferred until after the next Committee Draft is issued.
Move to Open.
[ 2009-05-31 Peter adds: ]
I am opposed to the proposed resolution and to the premise of the issue in general. The traits's default definitions should NOT derive from
integral_constant
, because this is harmful, as it misleads people into thinking thatis_bind_expression<E>
always derives fromintegral_constant
, whereas it may not.
is_bind_expression
andis_placeholder
allow user specializations, and in fact, this is their primary purpose. Such user specializations may not derive fromintegral_constant
, and the places whereis_bind_expression
andis_placeholder
are used intentionally do not require such derivation.The long-term approach here is to switch to
BindExpression<E>
andPlaceholder<P>
explicit concepts, of course, but until that happens, I say leave them alone.
[ 2009-10 post-Santa Cruz: ]
Move to Tentatively Ready. We are comfortable with requiring user specializations to derive from
integral_constant
.
Proposed resolution:
In 22.10.15.2 [func.bind.isbind] change as indicated:
namespace std { template<class T> struct is_bind_expression : integral_constant<bool, see below> { };{ static const bool value = see below; };}
In 22.10.15.2 [func.bind.isbind]/2 change as indicated:
static const bool value;-2-
Iftrue
ifT
is a type returned frombind
,false
otherwise.T
is a type returned frombind
,is_bind_expression<T>
shall be publicly derived fromintegral_constant<bool, true>
, otherwise it shall be publicly derived fromintegral_constant<bool, false>
.
In 22.10.15.3 [func.bind.isplace] change as indicated:
namespace std { template<class T> struct is_placeholder : integral_constant<int, see below> { };{ static const int value = see below; };}
In 22.10.15.3 [func.bind.isplace]/2 change as indicated:
static const int value;-2-
value isIfJ
ifT
is the type ofstd::placeholders::_J
, 0 otherwise.T
is the type ofstd::placeholders::_J
,is_placeholder<T>
shall be publicly derived fromintegral_constant<int, J>
otherwise it shall be publicly derived fromintegral_constant<int, 0>
.
allocator_arg
should be constexpr
Section: 20.2 [memory] Status: C++11 Submitter: Alisdair Meredith Opened: 2009-03-19 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
Declaration of allocator_arg
should be constexpr
to ensure constant
initialization.
[ Batavia (2009-05): ]
We agree with the proposed resolution. Move to Tentatively Ready.
Proposed resolution:
Change 20.2 [memory] p2:
// 20.8.1, allocator argument tag struct allocator_arg_t { }; constexpr allocator_arg_t allocator_arg = allocator_arg_t();
Section: 22 [utilities], 23 [containers] Status: Resolved Submitter: Alan Talbot Opened: 2009-03-20 Last modified: 2016-01-28
Priority: Not Prioritized
View all other issues in [utilities].
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Discussion:
Addresses US 65 and US 74.1 [CD1]
US 65:
Scoped allocators and allocator propagation traits add a small amount of utility at the cost of a great deal of machinery. The machinery is user visible, and it extends to library components that don't have any obvious connection to allocators, including basic concepts and simple components like
pair
andtuple
.Suggested resolution:
Sketch of proposed resolution: Eliminate scoped allocators, replace allocator propagation traits with a simple uniform rule (e.g. always propagate on copy and move), remove all mention of allocators from components that don't explicitly allocate memory (e.g. pair), and adjust container interfaces to reflect this simplification.
Components that I propose eliminating include
HasAllocatorType
,is_scoped_allocator
,allocator_propagation_map
,scoped_allocator_adaptor
, andConstructibleAsElement
.
US 74.1:
Scoped allocators represent a poor trade-off for standardization, since (1) scoped-allocator--aware containers can be implemented outside the C++ standard library but used with its algorithms, (2) scoped allocators only benefit a tiny proportion of the C++ community (since few C++ programmers even use today's allocators), and (3) all C++ users, especially the vast majority of the C++ community that won't ever use scoped allocators are forced to cope with the interface complexity introduced by scoped allocators.
In essence, the larger community will suffer to support a very small subset of the community who can already implement their own data structures outside of the standard library. Therefore, scoped allocators should be removed from the working paper.
Some evidence of the complexity introduced by scoped allocators:
22.3 [pairs], 22.4 [tuple]: Large increase in the number of pair and tuple constructors.
23 [containers]: Confusing "AllocatableElement" requirements throughout.
Suggested resolution:
Remove support for scoped allocators from the working paper. This includes at least the following changes:
Remove 99 [allocator.element.concepts]
Remove 20.6 [allocator.adaptor]
Remove [construct.element]
In Clause 23 [containers]: replace requirements naming the
AllocatableElement
concept with requirements namingCopyConstructible
,MoveConstructible
,DefaultConstructible
, orConstructible
, as appropriate.
[ Post Summit Alan moved from NAD to Open. ]
[ 2009-05-15 Ganesh adds: ]
The requirement
AllocatableElement
should not be replaced withConstructible
on theemplace_xxx()
functions as suggested. In the one-parameter case theConstructible
requirement is not satisfied when the constructor is explicit (as per [concept.map.fct], twelfth bullet) but we do want to allow explicit constructors in emplace, as the following example shows:vector<shared_ptr<int>> v; v.emplace_back(new int); // should be allowed
If the issue is accepted and scoped allocators are removed, I suggest to add a new pair of concepts to [concept.construct], namely:
auto concept HasExplicitConstructor<typename T, typename... Args> { explicit T::T(Args...); } auto concept ExplicitConstructible<typename T, typename... Args> : HasExplicitConstructor<T, Args...>, NothrowDestructible<T> { }We should then use
ExplicitConstructible
as the requirement for allemplace_xxx()
member functions.For coherence and consistency with the similar concepts
Convertible/ExplicitlyConvertible
, we might also consider changingConstructible
to:auto concept Constructible<typename T, typename... Args> : HasConstructor<T, Args...>, ExplicitConstructible<T, Args...> { }Moreover, all emplace-related concepts in [container.concepts] should also use
ExplicitConstructible
instead ofConstructible
in the definitions of their axioms. In fact the concepts in [container.concepts] should be corrected even if the issue is not accepted.On the other hand, if the issue is not accepted, the scoped allocator adaptors should be fixed because the following code:
template <typename T> using scoped_allocator = scoped_allocator_adaptor<allocator<T>>; vector<shared_ptr<int>, scoped_allocator<shared_ptr<int>>> v; v.emplace_back(new int); // ops! doesn't compile
doesn't compile, as the member function
construct()
of the scoped allocator requires non-explicit constructors through conceptConstructibleWithAllocator
. Fixing that is not difficult but probably more work than it's worth and is therefore, IMHO, one more reason in support of the complete removal of scoped allocators.
[ 2009-06-09 Alan adds: ]
I reopened this issue because I did not think that these National Body comments were adequately addressed by marking them NAD. My understanding is that something can be marked NAD if it is clearly a misunderstanding or trivial, but a substantive issue that has any technical merit requires a disposition that addresses the concerns.
The notes in the NB comment list (US 65 & US 74.1) say that:
- this issue has not introduced any new arguments not previously discussed,
- the vote (4-9-3) was not a consensus for removing scoped allocators,
- the issue is resolved by N2840.
My opinion is:
- there are new arguments in both comments regarding concepts (which were not present in the library when the scoped allocator proposal was voted in),
- the vote was clearly not a consensus for removal, but just saying there was a vote does not provide a rationale,
- I do not believe that N2840 addresses these comments (although it does many other things and was voted in with strong approval).
My motivation to open the issue was to ensure that the NB comments were adequately addressed in a way that would not risk a "no" vote on our FCD. If there are responses to the technical concerns raised, then perhaps they should be recorded. If the members of the NB who authored the comments are satisfied with N2840 and the other disposition remarks in the comment list, then I am sure they will say so. In either case, this issue can be closed very quickly in Frankfurt, and hopefully will have helped make us more confident of approval with little effort. If in fact there is controversy, my thought is that it is better to know now rather than later so there is more time to deal with it.
[ 2009-10 Santa Cruz: ]
NAD EditorialResolved. Addressed by N2982.
Proposed resolution:
Rationale:
Scoped allocators have been revised significantly.
RandomAccessIterator
's operator-
has nonsensical effects clauseSection: 24.3.5.7 [random.access.iterators] Status: C++11 Submitter: Doug Gregor Opened: 2009-03-20 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
Addresses UK 265
UK-265:
This effects clause is nonesense. It looks more like an axiom stating equivalence, and certainly an effects clause cannot change the state of two arguments passed by const reference
[ 2009-09-18 Alisdair adds: ]
For random access iterators, the definitions of
(b-a)
and(a<b)
are circular:From table Table 104 -- Random access iterator requirements:
b - a :==> (a < b) ? distance(a,b) : -distance(b,a) a < b :==> b - a > 0
[ 2009-10 Santa Cruz: ]
Moved to Ready.
[ 2010-02-13 Alisdair opens. ]
Looking again at LWG 1079(i), the wording in the issue no longer exists, and appears to be entirely an artefact of the concepts wording.
This issue is currently on our Ready list (not even Tentative!) but I think it has to be pulled as there is no way to apply the resolution.
Looking at the current paper, I think this issue is now "NAD, solved by the removal of concepts". Unfortunately it is too late to poll again, so we will have to perform that review in Pittsburgh.
[ 2010-02-13 Daniel updates the wording to address the circularity problem. ]
[ The previous wording is preserved here: ]
Modify 24.3.5.7 [random.access.iterators]p7-9 as follows:
difference_type operator-(const X& a, const X& b);-7- Precondition: there exists a value
-8-n
ofdifference_type
such thata == b + n
.Effects:-9- Returns:b == a + (b - a)
(a < b) ? distance(a,b) : -distance(b,a)
n
[ 2010 Pittsburgh: ]
Moved to Ready for Pittsburgh.
Proposed resolution:
Modify Table 105 in 24.3.5.7 [random.access.iterators]:
Table 105 — Random access iterator requirements (in addition to bidirectional iterator) Expression Return type Operational semantics Assertion/note
pre-/post-conditionb - a
Distance
return
distance(a,b)n
pre: there exists a value n
ofDistance
such thata + n == b
.b == a + (b - a)
.
std::promise
should provide non-member swap
overloadSection: 32.10.6 [futures.promise] Status: Resolved Submitter: Howard Hinnant Opened: 2009-03-22 Last modified: 2016-01-28
Priority: Not Prioritized
View other active issues in [futures.promise].
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Discussion:
Addresses UK 342 [CD1]
std::promise
is missing a non-member overload of swap
. This is
inconsistent with other types that provide a swap
member function.
Add a non-member overload void swap(promise&& x,promise&& y){ x.swap(y); }
[ Summit: ]
Create an issue. Move to review, attention: Howard. Detlef will also look into it.
[ Post Summit Daniel provided wording. ]
[ 2009-10 Santa Cruz: ]
NAD EditorialResolved. Addressed by N2997.
Proposed resolution:
In 32.10.6 [futures.promise], before p.1, immediately after class template promise add:
template <class R> void swap(promise<R>& x, promise<R>& y);
Change 32.10.6 [futures.promise]/10 as indicated (to fix a circular definition):
-10- Effects:
swap(*this, other)Swaps the associated state of*this
andother
Throws: Nothing.
After the last paragraph in 32.10.6 [futures.promise] add the following prototype description:
template <class R> void swap(promise<R>& x, promise<R>& y);Effects:
x.swap(y)
Throws: Nothing.
Section: 32 [thread] Status: C++11 Submitter: Howard Hinnant Opened: 2009-03-22 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
Addresses JP 76 [CD1]
A description for "Throws: Nothing." are not unified.
At the part without throw, "Throws: Nothing." should be described.
Add "Throws: Nothing." to the following.
[ Summit: ]
Pass on to editor.
[ Post Summit: Editor declares this non-editorial. ]
[ 2009-08-01 Howard provided wording: ]
The definition of "Throws: Nothing." that I added is probably going to be controversial, but I beg you to consider it seriously.
In C++ there are three "flow control" options for a function:
- It can return, either with a value, or with
void
.- It can call a function which never returns, such as
std::exit
orstd::terminate
.- It can throw an exception.
The above list can be abbreviated with:
- Returns.
- Ends program.
- Throws exception.
In general a function can have the behavior of any of these 3, or any combination of any of these three, depending upon run time data.
- R
- E
- T
- RE
- RT
- ET
- RET
A function with no throw spec, and no documentation, is in general a RET function. It may return, it may end the program, or it may throw. When we specify a function with an empty throw spec:
void f() throw();We are saying that
f()
is an RE function: It may return or end the program, but it will not throw.I posit that there are very few places in the library half of the standard where we intend for functions to be able to end the program (call
terminate
). And none of those places where we do sayterminate
could be called, do we currently say "Throws: Nothing.".I believe that if we define "Throws: Nothing." to mean R, we will both clarify many, many places in the standard, and give us a good rationale for choosing between "Throws: Nothing." (R) and
throw()
(RE) in the future. Indeed, this may give us motivation to change severalthrow()
s to "Throws: Nothing.".I did not add the following changes as JP 76 requested as I believe we want to allow these functions to throw:
Add a paragraph under 32.6.5.2 [thread.lock.guard] p4:
explicit lock_guard(mutex_type& m);Throws: Nothing.
Add a paragraph under 32.6.5.4.2 [thread.lock.unique.cons] p6:
explicit unique_lock(mutex_type& m);Throws: Nothing.
Add a paragraph under 32.7.5 [thread.condition.condvarany] p19, p21 and p25:
template <class Lock, class Rep, class Period> bool wait_for(Lock& lock, const chrono::duration<Rep, Period>& rel_time);Throws: Nothing.
template <class Lock, class Duration, class Predicate> bool wait_until(Lock& lock, const chrono::time_point<Clock, Duration>& rel_time, Predicate pred);Throws: Nothing.
template <class Lock, class Rep, class Period, class Predicate> bool wait_for(Lock& lock, const chrono::duration<Rep, Period>& rel_time, Predicate pred);Throws: Nothing.
[ 2009-10 Santa Cruz: ]
Defer pending further developments with exception restriction annotations.
[ 2010-02-11 Moved to Tentatively Ready after 5 positive votes on c++std-lib. ]
[ 2010-02-24 Pete moved to Open: ]
A "Throws: Nothing" specification is not the place to say that a function is not allowed to call
exit()
. While I agree with the thrust of the proposed resolution, "doesn't throw exceptions" is a subset of "always returns normally". If it's important to say that most library functions don't callexit()
, say so.
[ 2010 Pittsburgh: ]
Move to Ready except for the added paragraph to 16.3.2.4 [structure.specifications].
Proposed resolution:
Add a paragraph under 32.4.3.7 [thread.thread.static] p1:
unsigned hardware_concurrency();-1- Returns: ...
Throws: Nothing.
Add a paragraph under 32.7.4 [thread.condition.condvar] p7 and p8:
[Informational, not to be incluced in the WP: The POSIX spec allows only:
- [EINVAL]
- The value
cond
does not refer to an initialized condition variable. — end informational]void notify_one();-7- Effects: ...
Throws: Nothing.
void notify_all();-8- Effects: ...
Throws: Nothing.
Add a paragraph under 32.7.5 [thread.condition.condvarany] p6 and p7:
void notify_one();-6- Effects: ...
Throws: Nothing.
void notify_all();-7- Effects: ...
Throws: Nothing.
packaged_task
member swap
, missing non-member swap
Section: 32.10.10 [futures.task] Status: Resolved Submitter: Daniel Krügler Opened: 2009-03-22 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
Class template packaged_task
in 32.10.10 [futures.task] shows a member swap
declaration, but misses to document it's effects (No prototype provided). Further on this class
misses to provide a non-member swap.
[ Batavia (2009-05): ]
Alisdair notes that paragraph 2 of the proposed resolution has already been applied in the current Working Draft.
We note a pending
future
-related paper by Detlef; we would like to wait for this paper before proceeding.Move to Open.
[ 2009-05-24 Daniel removed part 2 of the proposed resolution. ]
[ 2009-10 post-Santa Cruz: ]
Move to Tentatively Ready, removing bullet 3 from the proposed resolution but keeping the other two bullets.
[ 2010 Pittsburgh: ]
Moved to
NAD EditorialResolved. Rationale added below.
Rationale:
Solved by N3058.
Proposed resolution:
In 32.10.10 [futures.task], immediately after the definition of class template packaged_task add:
template<class R, class... Argtypes> void swap(packaged_task<R(ArgTypes...)>&, packaged_task<R(ArgTypes...)>&);
At the end of 32.10.10 [futures.task] (after p. 20), add the following prototype description:
template<class R, class... Argtypes> void swap(packaged_task<R(ArgTypes...)>& x, packaged_task<R(ArgTypes...)>& y);Effects:
x.swap(y)
Throws: Nothing.
random_shuffle
algorithmSection: 26.7.13 [alg.random.shuffle] Status: Resolved Submitter: Alisdair Meredith Opened: 2009-03-22 Last modified: 2016-01-28
Priority: Not Prioritized
View all other issues in [alg.random.shuffle].
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Discussion:
There are a couple of issues with the declaration of the random_shuffle
algorithm accepting a random number engine.
RandomNumberEngine
concept is now provided by the random number
library
(n2836)
and the placeholder should be removed.
[ 2009-05-02 Daniel adds: ]
this issue completes adding necessary requirement to the third new
random_shuffle
overload. The current suggestion is:template<RandomAccessIterator Iter, UniformRandomNumberGenerator Rand> requires ShuffleIterator<Iter> void random_shuffle(Iter first, Iter last, Rand&& g);IMO this is still insufficient and I suggest to add the requirement
Convertible<Rand::result_type, Iter::difference_type>to the list (as the two other overloads already have).
Rationale:
Its true that this third overload is somewhat different from the remaining two. Nevertheless we know from
UniformRandomNumberGenerator
, that it'sresult_type
is an integral type and that it satisfiesUnsignedIntegralLike<result_type>
.To realize it's designated task, the algorithm has to invoke the
Callable
aspect ofg
and needs to perform some algebra involving it'smin()/max()
limits to compute another index value that at this point is converted intoIter::difference_type
. This is so, because 24.3.5.7 [random.access.iterators] uses this type as argument of it's algebraic operators. Alternatively consider the equivalent iterator algorithms in 24.4.3 [iterator.operations] with the same result.This argument leads us to the conclusion that we also need
Convertible<Rand::result_type, Iter::difference_type>
here.
[ Batavia (2009-05): ]
Alisdair notes that point (ii) has already been addressed.
We agree with the proposed resolution to point (i) with Daniel's added requirement.
Move to Review.
[ 2009-06-05 Daniel updated proposed wording as recommended in Batavia. ]
[ 2009-07-28 Alisdair adds: ]
Revert to Open, with a note there is consensus on direction but the wording needs updating to reflect removal of concepts.
[ 2009-10 post-Santa Cruz: ]
Leave Open, Walter to work on it.
[
2010 Pittsburgh: Moved to NAD EditorialResolved, addressed by
N3056.
]
Rationale:
Solved by N3056.
Proposed resolution:
Change in [algorithms.syn] and 26.7.13 [alg.random.shuffle]:
concept UniformRandomNumberGenerator<typename Rand> { }template<RandomAccessIterator Iter, UniformRandomNumberGenerator Rand> requires ShuffleIterator<Iter> && Convertible<Rand::result_type, Iter::difference_type> void random_shuffle(Iter first, Iter last, Rand&& g);
explicit operator bool() const
" in I/O librarySection: 31.5.4.4 [iostate.flags] Status: C++11 Submitter: P.J. Plauger Opened: 2009-03-24 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
Addresses JP 65 and JP 66 [CD1]
Switch from "unspecified-bool-type" to "explicit operator bool() const".
Replace operator unspecified-bool-type() const;
" with explicit operator bool() const;
[ Batavia (2009-05): ]
We agree with the proposed resolution. Move to Review.
[ 2009 Santa Cruz: ]
Moved to Ready.
Proposed resolution:
Change the synopis in 31.5.4 [ios]:
explicit operatorunspecified-bool-typebool() const;
Change 31.5.4.4 [iostate.flags]:
explicit operatorunspecified-bool-typebool() const;-1- Returns:
!fail()
Iffail()
then a value that will evaluate false in a boolean context; otherwise a value that will evaluate true in a boolean context. The value type returned shall not be convertible to int.
[Note: This conversion can be used in contexts where a bool is expected (e.g., anif
condition); however, implicit conversions (e.g., toint
) that can occur withbool
are not allowed, eliminating some sources of user error. One possible implementation choice for this type is pointer-to-member. -- end note]
Section: 16.4.5.10 [res.on.objects] Status: C++11 Submitter: Beman Dawes Opened: 2009-03-27 Last modified: 2016-01-28
Priority: Not Prioritized
View all other issues in [res.on.objects].
View all issues with C++11 status.
Discussion:
N2775, Small library thread-safety revisions, among other changes, removed a note from 16.4.5.10 [res.on.objects] that read:
[Note: This prohibition against concurrent non-const access means that modifying an object of a standard library type shared between threads without using a locking mechanism may result in a data race. --end note.]
That resulted in wording which is technically correct but can only be understood by reading the lengthy and complex 16.4.6.10 [res.on.data.races] Data race avoidance. This has the effect of making 16.4.5.10 [res.on.objects] unclear, and has already resulted in a query to the LWG reflector. See c++std-lib-23194.
[ Batavia (2009-05): ]
The proposed wording seems to need a bit of tweaking ("really bad idea" isn't quite up to standardese). We would like feedback as to whether the original Note's removal was intentional.
Change the phrase "is a really bad idea" to "risks undefined behavior" and move to Review status.
[ 2009-10 Santa Cruz: ]
Note: Change to read: "Modifying...", Delete 'thus', move to Ready
Proposed resolution:
Change 16.4.5.10 [res.on.objects] as indicated:
The behavior of a program is undefined if calls to standard library functions from different threads may introduce a data race. The conditions under which this may occur are specified in 17.6.4.7.
[Note: Modifying an object of a standard library type shared between threads risks undefined behavior unless objects of the type are explicitly specified as being sharable without data races or the user supplies a locking mechanism. --end note]
Section: 17.2 [support.types] Status: C++11 Submitter: Jens Maurer Opened: 2009-04-03 Last modified: 2016-01-28
Priority: Not Prioritized
View all other issues in [support.types].
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Discussion:
Addresses DE 18
Freestanding implementations do not (necessarily) have support for multiple threads (see 6.9.2 [intro.multithread]). Applications and libraries may want to optimize for the absence of threads. I therefore propose a preprocessor macro to indicate whether multiple threads can occur.
There is ample prior implementation experience for this
feature with various spellings of the macro name. For
example, gcc implicitly defines _REENTRANT
if multi-threading support is selected on the compiler
command-line.
While this is submitted as a library issue, it may be more appropriate to add the macro in 16.8 cpp.predefined in the core language.
See also N2693.
[ Batavia (2009-05): ]
We agree with the issue, and believe it is properly a library issue.
We prefer that the macro be conditionally defined as part of the
<thread>
header.Move to Review.
[ 2009-10 Santa Cruz: ]
Move to Ready.
[ 2010-02-25 Pete moved to Open: ]
The proposed resolution adds a feature-test macro named
__STDCPP_THREADS
, described after the following new text:The standard library defines the following macros; no explicit prior inclusion of any header file is necessary.
The correct term here is "header", not "header file". But that's minor. The real problem is that library entities are always defined in headers. If
__STDCPP_THREADS
is defined without including any header it's part of the language and belongs with the other predefined macros in the Preprocessor clause.Oddly enough, the comments from Batavia say "We prefer that the macro be conditionally defined as part of the
<thread>
header." There's no mention of a decision to change this.
[ 2010-02-26 Ganesh updates wording. ]
[ 2010 Pittsburgh: Adopt Ganesh's wording and move to Review. ]
[ 2010-03-08 Pete adds: ]
Most macros we have begin and end with with double underbars, this one only begins with double underbars.
[ 2010 Pittsburgh: Ganesh's wording adopted and moved to Ready for Pittsburgh. ]
Proposed resolution:
Change 16.4.2.5 [compliance]/3:
3 The supplied version of the header
<cstdlib>
shall declare at least the functionsabort()
,atexit()
, andexit()
(18.5). The supplied version of the header<thread>
either shall meet the same requirements as for a hosted implementation or including it shall have no effect. The other headers listed in this table shall meet the same requirements as for a hosted implementation.
Add the following line to table 15:
Table 15 — C++ headers for freestanding implementations Subclause Header(s) ... 32.4 [thread.threads] Threads <thread>
Add to the <thread>
synopsis in 32.4 [thread.threads]/1 the line:
namespace std { #define __STDCPP_THREADS __cplusplus class thread; ...
get_pointer_safety()
Section: 99 [util.dynamic.safety] Status: C++11 Submitter: Jens Maurer Opened: 2009-04-03 Last modified: 2016-01-28
Priority: Not Prioritized
View all other issues in [util.dynamic.safety].
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Discussion:
Addresses DE 18
In 99 [util.dynamic.safety], get_pointer_safety()
purports
to define behavior for
non-safely derived pointers ( [basic.stc.dynamic.safety]). However,
the cited core-language section in paragraph 4 specifies undefined behavior
for the use of such pointer values. This seems an unfortunate near-contradiction.
I suggest to specify the term relaxed pointer safety in
the core language section and refer to it from the library description.
This issue deals with the library part, the corresponding core issue (c++std-core-13940)
deals with the core modifications.
See also N2693.
[ Batavia (2009-05): ]
We recommend if this issue is to be moved, the issue be moved concurrently with the cited Core issue.
We agree with the intent of the proposed resolution. We would like input from garbage collection specialists.
Move to Open.
[ 2009-10 Santa Cruz: ]
The core issue is 853 and is in Ready status.
Proposed resolution:
In 99 [util.dynamic.safety] p16, replace the description of
get_pointer_safety()
with:
pointer_safety get_pointer_safety();
Returns: an enumeration value indicating the implementation's treatment of pointers that are not safely derived (3.7.4.3). Returnspointer_safety::relaxed
if pointers that are not safely derived will be treated the same as pointers that are safely derived for the duration of the program. Returnspointer_safety::preferred
if pointers that are not safely derived will be treated the same as pointers that are safely derived for the duration of the program but allows the implementation to hint that it could be desirable to avoid dereferencing pointers that are not safely derived as described. [Example:pointer_safety::preferred
might be returned to detect if a leak detector is running to avoid spurious leak reports. -- end note] Returnspointer_safety::strict
if pointers that are not safely derived might be treated differently than pointers that are safely derived.Returns: Returns
pointer_safety::strict
if the implementation has strict pointer safety ( [basic.stc.dynamic.safety]). It is implementation-defined whetherget_pointer_safety
returnspointer_safety::relaxed
orpointer_safety::preferred
if the implementation has relaxed pointer safety ( [basic.stc.dynamic.safety]).FootnoteThrows: nothing
Footnote)
pointer_safety::preferred
might be returned to indicate to the program that a leak detector is running so that the program can avoid spurious leak reports.
auto_ptr
to unique_ptr
conversionSection: 20.3.1.3.2 [unique.ptr.single.ctor] Status: Resolved Submitter: Howard Hinnant Opened: 2009-04-25 Last modified: 2016-01-28
Priority: Not Prioritized
View all other issues in [unique.ptr.single.ctor].
View all issues with Resolved status.
Discussion:
Message c++std-lib-23182 led to a discussion in which several people
expressed interest in being able to convert an auto_ptr
to a
unique_ptr
without the need to call release
. Below is
wording to accomplish this.
[ Batavia (2009-05): ]
Pete believes it not a good idea to separate parts of a class's definition. Therefore, if we do this, it should be part of
unique-ptr
's specification.Alisdair believes the lvalue overload may be not necessary.
Marc believes it is more than just sugar, as it does ease the transition to
unique-ptr
.We agree with the resolution as presented. Move to Tentatively Ready.
[ 2009-07 Frankfurt ]
Moved from Tentatively Ready to Open only because the wording needs to be tweaked for concepts removal.
[ 2009-08-01 Howard deconceptifies wording: ]
I also moved the change from 99 [depr.auto.ptr] to 20.3.1.3 [unique.ptr.single] per the Editor's request in Batavia (as long as I was making changes anyway). Set back to Review.
[ 2009-10 Santa Cruz: ]
Move to Ready.
[ 2010-03-14 Howard adds: ]
We moved N3073 to the formal motions page in Pittsburgh which should obsolete this issue. I've moved this issue to NAD Editorial, solved by N3073.
Rationale:
Solved by N3073.
Proposed resolution:
Add to 20.3.1.3 [unique.ptr.single]:
template <class T, class D> class unique_ptr { public: template <class U> unique_ptr(auto_ptr<U>& u); template <class U> unique_ptr(auto_ptr<U>&& u); };
Add to 20.3.1.3.2 [unique.ptr.single.ctor]:
template <class U> unique_ptr(auto_ptr<U>& u); template <class U> unique_ptr(auto_ptr<U>&& u);Effects: Constructs a
unique_ptr
withu.release()
.Postconditions:
get() ==
the valueu.get()
had before the construciton, modulo any required offset adjustments resulting from the cast fromU*
toT*
.u.get() == nullptr
.Throws: nothing.
Remarks:
U*
shall be implicitly convertible toT*
andD
shall be the same type asdefault_delete<T>
, else these constructors shall not participate in overload resolution.
system_error
constructor postcondition overly strictSection: 19.5.8.2 [syserr.syserr.members] Status: C++11 Submitter: Howard Hinnant Opened: 2009-04-25 Last modified: 2016-01-28
Priority: Not Prioritized
View all other issues in [syserr.syserr.members].
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Discussion:
19.5.8.2 [syserr.syserr.members] says:
system_error(error_code ec, const string& what_arg);Effects: Constructs an object of class
system_error
.Postconditions:
code() == ec
andstrcmp(runtime_error::what(), what_arg.c_str()) == 0
.
However the intent is for:
std::system_error se(std::errc::not_a_directory, "In FooBar"); ... se.what(); // returns something along the lines of: // "In FooBar: Not a directory"
The way the constructor postconditions are set up now, to achieve both
conformance, and the desired intent in the what()
string, the
system_error
constructor must store "In FooBar" in the base class,
and then form the desired output each time what()
is called. Or
alternatively, store "In FooBar" in the base class, and store the desired
what()
string in the derived system_error
, and override
what()
to return the string in the derived part.
Both of the above implementations seem suboptimal to me. In one I'm computing
a new string every time what()
is called. And since what()
can't propagate exceptions, the client may get a different string on different
calls.
The second solution requires storing two strings instead of one.
What I would like to be able to do is form the desired what()
string
once in the system_error
constructor, and store that in the
base class. Now I'm:
what()
only once.what()
definition is sufficient and nothrow.This is smaller code, smaller data, and faster.
ios_base::failure
has the same issue.
[
Comments about this change received favorable comments from the system_error
designers.
]
[ Batavia (2009-05): ]
We agree with the proposed resolution.
Move to Tentatively Ready.
Proposed resolution:
In 19.5.8.2 [syserr.syserr.members], change the following constructor postconditions:
system_error(error_code ec, const string& what_arg);-2- Postconditions:
code() == ec
and.
strcmp(runtime_error::what(), what_arg.c_str()) == 0string(what()).find(what_arg) != string::npossystem_error(error_code ec, const char* what_arg);-4- Postconditions:
code() == ec
and.
strcmp(runtime_error::what(), what_arg) == 0string(what()).find(what_arg) != string::npossystem_error(error_code ec);-6- Postconditions:
code() == ec
and.strcmp(runtime_error::what(), ""
system_error(int ev, const error_category& ecat, const string& what_arg);-8- Postconditions:
code() == error_code(ev, ecat)
and.
strcmp(runtime_error::what(), what_arg.c_str()) == 0string(what()).find(what_arg) != string::npossystem_error(int ev, const error_category& ecat, const char* what_arg);-10- Postconditions:
code() == error_code(ev, ecat)
and.
strcmp(runtime_error::what(), what_arg) == 0string(what()).find(what_arg) != string::npossystem_error(int ev, const error_category& ecat);-12- Postconditions:
code() == error_code(ev, ecat)
and.strcmp(runtime_error::what(), "") == 0
In 19.5.8.2 [syserr.syserr.members], change the description of what()
:
const char *what() const throw();-14- Returns: An NTBS incorporating
the arguments supplied in the constructor.runtime_error::what()
andcode().message()
[Note:
One possible implementation would be:The return NTBS might take the form:what_arg + ": " + code().message()
if (msg.empty()) { try { string tmp = runtime_error::what(); if (code()) { if (!tmp.empty()) tmp += ": "; tmp += code().message(); } swap(msg, tmp); } catch(...) { return runtime_error::what(); } return msg.c_str();— end note]
In [ios::failure], change the synopsis:
namespace std { class ios_base::failure : public system_error { public: explicit failure(const string& msg, const error_code& ec = io_errc::stream); explicit failure(const char* msg, const error_code& ec = io_errc::stream);virtual const char* what() const throw();}; }
In [ios::failure], change the description of the constructors:
explicit failure(const string& msg, , const error_code& ec = io_errc::stream);-3- Effects: Constructs an object of class
failure
by constructing the base class withmsg
andec
.
-4- Postcondition:code() == ec
andstrcmp(what(), msg.c_str()) == 0
explicit failure(const char* msg, const error_code& ec = io_errc::stream);-5- Effects: Constructs an object of class
failure
by constructing the base class withmsg
andec
.
-6- Postcondition:code() == ec and strcmp(what(), msg) == 0
In [ios::failure], remove what
(the base class definition
need not be repeated here).
const char* what() const;
-7- Returns: The messagemsg
with which the exception was created.
basic_ios::move
should accept lvaluesSection: 31.5.4.3 [basic.ios.members] Status: C++11 Submitter: Howard Hinnant Opened: 2009-04-25 Last modified: 2016-01-28
Priority: Not Prioritized
View all other issues in [basic.ios.members].
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Discussion:
With the rvalue reference changes in
N2844
basic_ios::move
no longer has the most convenient signature:
void move(basic_ios&& rhs);
This signature should be changed to accept lvalues. It does not need to be
overloaded to accept rvalues. This is a special case that only derived clients
will see. The generic move
still needs to accept rvalues.
[ Batavia (2009-05): ]
Tom prefers, on general principles, to provide both overloads. Alisdair agrees.
Howard points out that there is no backward compatibility issue as this is new to C++0X.
We agree that both overloads should be provided, and Howard will provide the additional wording. Move to Open.
[ 2009-05-23 Howard adds: ]
Added overload, moved to Review.
[ 2009 Santa Cruz: ]
Move to Ready.
Proposed resolution:
Add a signature to the existing prototype in the synopsis of 31.5.4 [ios] and in 31.5.4.3 [basic.ios.members]:
void move(basic_ios& rhs); void move(basic_ios&& rhs);
shared_future::get()
?Section: 32.10.8 [futures.shared.future] Status: Resolved Submitter: Thomas J. Gritzan Opened: 2009-04-03 Last modified: 2021-06-06
Priority: Not Prioritized
View all other issues in [futures.shared.future].
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Discussion:
It is not clear, if multiple threads are waiting in a
shared_future::get()
call, if each will rethrow the stored exception.
Paragraph 9 reads:
Throws: the stored exception, if an exception was stored and not retrieved before.
The "not retrieved before" suggests that only one exception is thrown,
but one exception for each call to get()
is needed, and multiple calls
to get()
even on the same shared_future
object seem to be allowed.
I suggest removing "and not retrieved before" from the Throws paragraph.
I recommend adding a note that explains that multiple calls on get()
are
allowed, and each call would result in an exception if an exception was
stored.
[ Batavia (2009-05): ]
We note there is a pending paper by Detlef on such
future
-related issues; we would like to wait for his paper before proceeding.Alisdair suggests we may want language to clarify that this
get()
function can be called from several threads with no need for explicit locking.Move to Open.
[ 2010-01-23 Moved to Tentatively NAD Editorial after 5 positive votes on c++std-lib. ]
Rationale:
Resolved by paper N2997.
Proposed resolution:
Change [futures.shared_future]:
const R& shared_future::get() const; R& shared_future<R&>::get() const; void shared_future<void>::get() const;...
-9- Throws: the stored exception, if an exception was stored
and not retrieved before. [Note: Multiple calls onget()
are allowed, and each call would result in an exception if an exception was stored. — end note]
Section: 32.2.2 [thread.req.exception] Status: C++11 Submitter: Christopher Kohlhoff Opened: 2009-04-25 Last modified: 2016-01-28
Priority: Not Prioritized
View all issues with C++11 status.
Discussion:
The current formulation of 32.2.2 [thread.req.exception]/2 reads:
The
error_category
of theerror_code
reported by such an exception'scode()
member function is as specified in the error condition Clause.
This constraint on the code's associated error_categor
means an
implementation must perform a mapping from the system-generated
error to a generic_category()
error code. The problems with this
include:
The original error produced by the operating system is lost.
The latter was one of Peter Dimov's main objections (in a private
email discussion) to the original error_code
-only design, and led to
the creation of error_condition
in the first place. Specifically,
error_code
and error_condition
are intended to perform
the following roles:
error_code
holds the original error produced by the operating
system.
error_condition
and the generic category provide a set of well
known error constants that error codes may be tested against.
Any mapping determining correspondence of the returned error code to the conditions listed in the error condition clause falls under the "latitude" granted to implementors in 19.5.3.5 [syserr.errcat.objects]. (Although obviously their latitude is restricted a little by the need to match the right error condition when returning an error code from a library function.)
It is important that this error_code/error_condition
usage is done
correctly for the thread library since it is likely to set the pattern for future
TR libraries that interact with the operating system.
[ Batavia (2009-05): ]
Move to Open, and recommend the issue be deferred until after the next Committee Draft is issued.
[ 2009-10 post-Santa Cruz: ]
Move to Tentatively Ready.
Proposed resolution:
Change 32.2.2 [thread.req.exception] p.2:
-2-
TheTheerror_category
(19.5.1.1) of theerror_code
reported by such an exception'scode()
member function is as specified in the error condition Clause.error_code
reported by such an exception'scode()
member function shall compare equal to one of the conditions specified in the function's error condition Clause. [Example: When the thread constructor fails:ec.category() == implementation-defined // probably system_category ec == errc::resource_unavailable_try_again // holds true— end example]
for_each
overconstrained?Section: 26.6.5 [alg.foreach] Status: C++11 Submitter: Alisdair Meredith Opened: 2009-04-29 Last modified: 2016-01-28
Priority: Not Prioritized
View other active issues in [alg.foreach].
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Discussion:
Quoting working paper for reference (26.6.5 [alg.foreach]):
template<InputIterator Iter, Callable<auto, Iter::reference> Function> requires CopyConstructible<Function> Function for_each(Iter first, Iter last, Function f);1 Effects: Applies f to the result of dereferencing every iterator in the range [first,last), starting from first and proceeding to last - 1.
2 Returns: f.
3 Complexity: Applies f exactly last - first times.
P2 implies the passed object f
should be invoked at each stage, rather than
some copy of f
. This is important if the return value is to usefully
accumulate changes. So the requirements are an object of type Function
can
be passed-by-value, invoked multiple times, and then return by value. In
this case, MoveConstructible
is sufficient. This would open support for
move-only functors, which might become important in concurrent code as you
can assume there are no other references (copies) of a move-only type and so
freely use them concurrently without additional locks.
[ See further discussion starting with c++std-lib-23686. ]
[ Batavia (2009-05): ]
Pete suggests we may want to look at this in a broader context involving other algorithms. We should also consider the implications of parallelism.
Move to Open, and recommend the issue be deferred until after the next Committee Draft is issued.
[ 2009-10-14 Daniel de-conceptified the proposed resolution. ]
The note in 26.1 [algorithms.general]/9 already says the right thing:
Unless otherwise specified, algorithms that take function objects as arguments are permitted to copy those function objects freely.
So we only need to ensure that the wording for
for_each
is sufficiently clear, which is the intend of the following rewording.
[ 2009-10-15 Daniel proposes: ]
Add a new Requires clause just after the prototype declaration (26.6.5 [alg.foreach]):
Requires:
Function
shall beMoveConstructible
( [moveconstructible]),CopyConstructible
is not required.Change 26.6.5 [alg.foreach]/2 as indicated:
Returns: std::move(f).
[ 2009-10 post-Santa Cruz: ]
Move to Tentatively Ready, using Daniel's wording without the portion saying "CopyConstructible is not required".
[ 2009-10-27 Daniel adds: ]
I see that during the Santa Cruz meeting the originally proposed addition
,
CopyConstructible
is not required.was removed. I don't think that this removal was a good idea. The combination of 26.1 [algorithms.general] p.9
[Note: Unless otherwise specified, algorithms that take function objects as arguments are permitted to copy those function objects freely.[..]
with the fact that
CopyConstructible
is a refinementMoveConstructible
makes it necessary that such an explicit statement is given. Even the existence of the usage ofstd::move
in the Returns clause doesn't help much, because this would still be well-formed for aCopyConstructible
without move constructor. Let me add that the originally proposed addition reflects current practice in the standard, e.g. 26.7.9 [alg.unique] p.5 usages a similar terminology.For similar wording need in case for auto_ptr see 973(i).
[ Howard: Moved from Tentatively Ready to Open. ]
[
2009-11-20 Howard restores "not CopyConstructible
" to the spec.
]
[ 2009-11-22 Moved to Tentatively Ready after 5 positive votes on c++std-lib. ]
Proposed resolution:
Add a new Requires clause just after the prototype declaration (26.6.5 [alg.foreach]):
Requires:
Function
shall meet the requirements ofMoveConstructible
( [moveconstructible]).Function
need not meet the requirements ofCopyConstructible
( [copyconstructible]).
Change 26.6.5 [alg.foreach]/2 as indicated:
Returns: std::move(f).
bitset::to_string
could be simplifiedSection: 22.9.2 [template.bitset] Status: C++11 Submitter: Alisdair Meredith Opened: 2009-05-09 Last modified: 2016-01-28
Priority: Not Prioritized
View other active issues in [template.bitset].
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Discussion:
In 853(i) our resolution is changing the signature by adding two defaulting arguments to 3 calls. In principle, this means that ABI breakage is not an issue, while API is preserved.
With that observation, it would be very nice to use the new ability to supply default template parameters to function templates to collapse all 3 signatures into 1. In that spirit, this issue offers an alternative resolution than that of 853(i).
[ Batavia (2009-05): ]
Move to Open, and look at the issue again after 853(i) has been accepted. We further recommend this be deferred until after the next Committee Draft.
[ 2009-10 post-Santa Cruz: ]
Move to Tentatively Ready.
Proposed resolution:
In 22.9.2 [template.bitset]/1 (class bitset) ammend:
template <class charT = char, class traits = char_traits<charT>, class Allocator = allocator<charT>> basic_string<charT, traits, Allocator> to_string(charT zero = charT('0'), charT one = charT('1')) const;template <class charT, class traits> basic_string<charT, traits, allocator<charT> > to_string() const; template <class charT> basic_string<charT, char_traits<charT>, allocator<charT> > to_string() const; basic_string<char, char_traits<char>, allocator<char> > to_string() const;
In 22.9.2.3 [bitset.members] prior to p35 ammend:
template <class charT = char, class traits = char_traits<charT>, class Allocator = allocator<charT>> basic_string<charT, traits, Allocator> to_string(charT zero = charT('0'), charT one = charT('1')) const;
Section: 21 [meta] Status: C++11 Submitter: Daniel Krügler Opened: 2009-05-12 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
Related to 975(i) and 1023(i).
The current wording in 21.3.2 [meta.rqmts] is still unclear concerning it's requirements on the type traits classes regarding ambiguities. Specifically it's unclear
true_type
/false_type
.
integral_constant
types making the contained names ambiguous
[ Batavia (2009-05): ]
Alisdair would prefer to factor some of the repeated text, but modulo a corner case or two, he believes the proposed wording is otherwise substantially correct.
Move to Open.
[ 2009-10 post-Santa Cruz: ]
Move to Tentatively Ready.
Proposed resolution:
[ The usage of the notion of a BaseCharacteristic below might be useful in other places - e.g. to define the base class relation in 22.10.6 [refwrap], 22.10.16 [func.memfn], or 22.10.17.3 [func.wrap.func]. In this case it's definition should probably be moved to Clause 17 ]
Change 21.3.2 [meta.rqmts] p.1 as indicated:
[..] It shall be
DefaultConstructible
,CopyConstructible
, and publicly and unambiguously derived, directly or indirectly, from its BaseCharacteristic, which is a specialization of the templateintegral_constant
(20.6.3), with the arguments to the templateintegral_constant
determined by the requirements for the particular property being described. The member names of the BaseCharacteristic shall be unhidden and unambiguously available in the UnaryTypeTrait.
Change 21.3.2 [meta.rqmts] p.2 as indicated:
[..] It shall be
DefaultConstructible
,CopyConstructible
, and publicly and unambiguously derived, directly or indirectly, froman instanceits BaseCharacteristic, which is a specialization of the templateintegral_constant
(20.6.3), with the arguments to the templateintegral_constant
determined by the requirements for the particular relationship being described. The member names of the BaseCharacteristic shall be unhidden and unambiguously available in the BinaryTypeTrait.
Change 21.3.5 [meta.unary] p.2 as indicated:
Each of these templates shall be a UnaryTypeTrait (20.6.1),
publicly derived directly or indirectly fromwhere its BaseCharacteristic shall betrue_type
if the corresponding condition is true, otherwise fromfalse_type
true_type
if the corresponding condition is true, otherwisefalse_type
.
Change 21.3.7 [meta.rel] p.2 as indicated:
Each of these templates shall be a BinaryTypeTrait (20.6.1),
publicly derived directly or indirectly fromwhere its BaseCharacteristic shall betrue_type
if the corresponding condition is true, otherwise fromfalse_type
true_type
if the corresponding condition is true, otherwisefalse_type
.
Section: 22.4.4 [tuple.tuple] Status: Resolved Submitter: Alisdair Meredith Opened: 2009-05-23 Last modified: 2016-01-28
Priority: Not Prioritized
View all other issues in [tuple.tuple].
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Discussion:
It is not currently possible to construct tuple
literal values,
even if the elements are all literal types. This is because parameters
are passed to constructor by reference.
An alternative would be to pass all constructor arguments by value, where it is known that *all* elements are literal types. This can be determined with concepts, although note that the negative constraint really requires factoring out a separate concept, as there is no way to provide an 'any of these fails' constraint inline.
Note that we will have similar issues with pair
(and
tuple
constructors from pair
) although I am steering
clear of that class while other constructor-related issues settle.
[ 2009-10 Santa Cruz: ]
NAD EditorialResolved. Solved by N2994.
Proposed resolution:
Ammend the tuple class template declaration in 22.4.4 [tuple.tuple] as follows
Add the following concept:
auto concept AllLiteral< typename ... Types > { requires LiteralType<Types>...; }ammend the constructor
template <class... UTypes> requires AllLiteral<Types...> && Constructible<Types, UTypes>... explicit tuple(UTypes...); template <class... UTypes> requires !AllLiteral<Types...> && Constructible<Types, UTypes&&>... explicit tuple(UTypes&&...);ammend the constructor
template <class... UTypes> requires AllLiteral<Types...> && Constructible<Types, UTypes>... tuple(tuple<UTypes...>); template <class... UTypes> requires !AllLiteral<Types...> && Constructible<Types, const UTypes&>... tuple(const tuple<UTypes...>&);
Update the same signatures in 22.4.4.2 [tuple.cnstr], paras 3 and 5.
tuple
copy constructorSection: 22.4.4.2 [tuple.cnstr] Status: Resolved Submitter: Alisdair Meredith Opened: 2009-05-23 Last modified: 2016-01-28
Priority: Not Prioritized
View other active issues in [tuple.cnstr].
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Discussion:
The copy constructor for the tuple
template is constrained. This seems an
unusual strategy, as the copy constructor will be implicitly deleted if the
constraints are not met. This is exactly the same effect as requesting an
=default;
constructor. The advantage of the latter is that it retains
triviality, and provides support for tuple
s as literal types if issue
1116(i) is also accepted.
Actually, it might be worth checking with core if a constrained copy constructor is treated as a constructor template, and as such does not suppress the implicit generation of the copy constructor which would hide the template in this case.
[ 2009-05-27 Daniel adds: ]
This would solve one half of the suggested changes in 801(i).
[ 2009-10 Santa Cruz: ]
NAD EditorialResolved. Solved by N2994.
Proposed resolution:
Change 22.4.4 [tuple.tuple] and 22.4.4.2 [tuple.cnstr] p4:
requires CopyConstructible<Types>...tuple(const tuple&) = default;
tuple
query APIs do not support cv-qualificationSection: 22.4.7 [tuple.helper] Status: C++11 Submitter: Alisdair Meredith Opened: 2009-05-23 Last modified: 2016-01-28
Priority: Not Prioritized
View all other issues in [tuple.helper].
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Discussion:
The APIs tuple_size
and tuple_element
do not support
cv-qualified tuple
s, pair
s or array
s.
The most generic solution would be to supply partial specializations once
for each cv-type in the tuple
header. However, requiring this header for
cv-qualified pair
s/array
s seems unhelpful. The BSI editorial
suggestion (UK-198/US-69,
N2533)
to merge tuple
into <utility>
would help with pair
,
but not array
. That might be resolved by making a dependency between the
<array>
header and <utility>
, or simply recognising
the dependency be fulfilled in a Remark.
[ 2009-05-24 Daniel adds: ]
All
tuple_size
templates with a base class need to derive publicly, e.g.template <IdentityOf T> class tuple_size< const T > : public tuple_size<T> {};The same applies to the tuple_element class hierarchies.
What is actually meant with the comment
this solution relies on 'metafunction forwarding' to inherit the nested typename type
?
I ask, because all base classes are currently unconstrained and their instantiation is invalid in the constrained context of the
tuple_element
partial template specializations.
[ 2009-05-24 Alisdair adds: ]
I think a better solution might be to ask Pete editorially to change all declarations of tupling APIs to use the struct specifier instead of class.
"metafunction forwarding" refers to the MPL metafunction protocol, where a metafunction result is declared as a nested typedef with the name "type", allowing metafunctions to be chained by means of inheritance. It is a neater syntax than repeatedly declaring a typedef, and inheritance syntax is slightly nicer when it comes to additional typename keywords.
The constrained template with an unconstrained base is a good observation though.
[ 2009-10 post-Santa Cruz: ]
Move to Open, Alisdair to provide wording. Once wording is provided, Howard will move to Review.
[ 2010-03-28 Daniel deconceptified wording. ]
[ Post-Rapperswil - Daniel provides wording: ]
The below given P/R reflects the discussion from the Rapperswil meeting that the wording should not constrain implementation freedom to realize the actual issue target. Thus the original code form was replaced by normative words.
While preparing this wording it turned out that several tuple_size
specializations as
that of pair
and array
are underspecified, because the underlying type of the member
value is not specified except that it is an integral type. For the specializations we could introduce a
canonical one - like size_t
- or we could use the same type as the specialization of the
unqualified type uses. The following wording follows the second approach.
The wording refers to N3126.
Moved to Tentatively Ready after 6 positive votes on c++std-lib.
[ Adopted at 2010-11 Batavia ]
Proposed resolution:
<tuple>
synopsis, as indicated:
// 20.4.2.5, tuple helper classes: template <class T> class tuple_size; // undefined template <class T> class tuple_size<const T>; template <class T> class tuple_size<volatile T>; template <class T> class tuple_size<const volatile T>; template <class... Types> class tuple_size<tuple<Types...> >; template <size_t I, class T> class tuple_element; // undefined template <size_t I, class T> class tuple_element<I, const T>; template <size_t I, class T> class tuple_element<I, volatile T>; template <size_t I, class T> class tuple_element<I, const volatile T>; template <size_t I, class... Types> class tuple_element<I, tuple<Types...> >;
template <class T> class tuple_size<const T>; template <class T> class tuple_size<volatile T>; template <class T> class tuple_size<const volatile T>;Let TS denote
tuple_size<T>
of the cv-unqualified typeT
. Then each of the three templates shall meet the UnaryTypeTrait requirements (20.7.1) with a BaseCharacteristic ofintegral_constant<remove_cv<decltype(TS::value)>::type, TS::value>
.
template <size_t I, class T> class tuple_element<I, const T>; template <size_t I, class T> class tuple_element<I, volatile T>; template <size_t I, class T> class tuple_element<I, const volatile T>;Let TE denote
tuple_element<I, T>
of the cv-unqualified typeT
. Then each of the three templates shall meet the TransformationTrait requirements (20.7.1) with a member typedeftype
that shall name the same type as the following type:
- for the first specialization, the type
add_const<TE::type>::type
,- for the second specialization, the type
add_volatile<TE::type>::type
, and- for the third specialization, the type
add_cv<TE::type>::type
constexpr
Section: 21.4.3 [ratio.ratio] Status: Resolved Submitter: Alisdair Meredith Opened: 2009-05-25 Last modified: 2016-01-28
Priority: Not Prioritized
View all other issues in [ratio.ratio].
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Discussion:
The values num
and den
in the ratio
template
should be declared constexpr
.
[ 2009-10 Santa Cruz: ]
NAD EditorialResolved. Solved by N2994.
Proposed resolution:
21.4.3 [ratio.ratio]
namespace std { template <intmax_t N, intmax_t D = 1> class ratio { public: static constexpr intmax_t num; static constexpr intmax_t den; }; }
Section: 31.5.2.2.6 [ios.init] Status: C++11 Submitter: James Kanze Opened: 2009-05-14 Last modified: 2021-06-06
Priority: Not Prioritized
View all other issues in [ios.init].
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Discussion:
As currently formulated, the standard doesn't require that there
is ever a flush of cout
, etc. (This implies, for example, that
the classical hello, world program may have no output.) In the
current draft
(N2798),
there is a requirement that the objects
be constructed before main
, and before the dynamic
initialization of any non-local objects defined after the
inclusion of <iostream>
in the same translation unit. The only
requirement that I can find concerning flushing, however, is in
[ios::Init], where the destructor of the last
std::ios_base::Init
object flushes. But there is, as far as I
can see, no guarantee that such an object ever exists.
Also, the wording in [iostreams.objects] says that:
The objects are constructed and the associations are established at some time prior to or during the first time an object of class
ios_base::Init
is constructed, and in any case before the body of main begins execution.
In [ios::Init], however, as an effect of the constructor, it says that
If
init_cnt
is zero, the function stores the value one ininit_cnt
, then constructs and initializes the objectscin
,cout
,cerr
,clog
wcin
,wcout
,wcerr
, andwclog
"
which seems to forbid earlier construction.
(Note that with these changes, the exposition only "static
int init_cnt
" in ios_base::Init
can be dropped.)
Of course, a determined programmer can still inhibit the flush with things like:
new std::ios_base::Init ; // never deleted
or (in a function):
std::ios_base::Init ensureConstruction ; // ... exit( EXIT_SUCCESS ) ;
Perhaps some words somewhere to the effect that all
std::ios_base::Init
objects should have static lifetime
would be in order.
[ 2009 Santa Cruz: ]
Moved to Ready. Some editorial changes are expected (in addition to the proposed wording) to remove
init_cnt
fromInit
.
Proposed resolution:
Change 31.4 [iostream.objects]/2:
-2- The objects are constructed and the associations are established at some time prior to or during the first time an object of class
ios_base::Init
is constructed, and in any case before the body of main begins execution.292 The objects are not destroyed during program execution.293If a translation unit includesThe results of including<iostream>
or explicitly constructs anios_base::Init
object, these stream objects shall be constructed before dynamic initialization of non-local objects defined later in that translation unit.<iostream>
in a translation unit shall be as if<iostream>
defined an instance ofios_base::Init
with static lifetime. Similarly, the entire program shall behave as if there were at least one instance ofios_base::Init
with static lifetime.
Change [ios::Init]/3:
Init();-3- Effects: Constructs an object of class
Init
.IfConstructs and initializes the objectsinit_cnt
is zero, the function stores the value one ininit_cnt
, then constructs and initializes the objectscin
,cout
,cerr
,clog
(27.4.1),wcin
,wcout
,wcerr
, andwclog
(27.4.2). In any case, the function then adds one to the value stored ininit_cnt
.cin
,cout
,cerr
,clog
,wcin
,wcout
,wcerr
andwclog
if they have not already been constructed and initialized.
Change [ios::Init]/4:
~Init();-4- Effects: Destroys an object of class
Init
.The function subtracts one from the value stored inIf there are no other instances of the class still in existance, callsinit_cnt
and, if the resulting stored value is one,cout.flush()
,cerr.flush()
,clog.flush()
,wcout.flush()
,wcerr.flush()
,wclog.flush()
.
istreambuff_iterator::equal
needs a const & parameterSection: 24.6.4.4 [istreambuf.iterator.ops] Status: C++11 Submitter: Alisdair Meredith Opened: 2009-05-28 Last modified: 2017-11-29
Priority: Not Prioritized
View all other issues in [istreambuf.iterator.ops].
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Discussion:
The equal
member function of istreambuf_iterator
is
declared const
, but takes its argument by non-const reference.
This is not compatible with the operator==
free function overload, which is
defined in terms of calling equal
yet takes both arguments by reference to
const.
[ The proposed wording is consistent with 110(i) with status TC1. ]
[ 2009-11-02 Howard adds: ]
Set to Tentatively Ready after 5 positive votes on c++std-lib.
Proposed resolution:
Ammend in both:
24.6.4 [istreambuf.iterator]
[istreambuf.iterator::equal]
bool equal(const istreambuf_iterator& b) const;
istream(buf)_iterator
should support literal sentinel valueSection: 24.6.2.2 [istream.iterator.cons], 24.6.4 [istreambuf.iterator] Status: Resolved Submitter: Alisdair Meredith Opened: 2009-05-30 Last modified: 2016-01-28
Priority: Not Prioritized
View all other issues in [istream.iterator.cons].
View all issues with Resolved status.
Discussion:
istream_iterator
and istreambuf_iterator
should support literal sentinel
values. The default constructor is frequently used to terminate ranges, and
could easily be a literal value for istreambuf_iterator
, and
istream_iterator
when iterating value types. A little more work using a
suitably sized/aligned char-array for storage (or an updated component like
boost::optional
proposed for TR2) would allow istream_iterator
to support
constexpr
default constructor in all cases, although we might leave this
tweak as a QoI issue. Note that requiring constexpr
be supported also
allows us to place no-throw guarantees on this constructor too.
[ 2009-06-02 Daniel adds: ]
I agree with the usefulness of the issue suggestion, but we need to ensure that
istream_iterator
can satisfy be literal if needed. Currently this is not clear, because 24.6.2 [istream.iterator]/3 declares a copy constructor and a destructor and explains their semantic in 24.6.2.2 [istream.iterator.cons]/3+4.The prototype semantic specification is ok (although it seems somewhat redundant to me, because the semantic doesn't say anything interesting in both cases), but for support of trivial class types we also need a trivial copy constructor and destructor as of 11 [class]/6. The current non-informative specification of these two special members suggests to remove their explicit declaration in the class and add explicit wording that says that if
T
is trivial a default constructed iterator is also literal, alternatively it would be possible to mark both as defaulted and add explicit (memberwise) wording that guarantees that they are trivial.Btw.: I'm quite sure that the
istreambuf_iterator
additions to ensure triviality are not sufficient as suggested, because the library does not yet give general guarantees that a defaulted special member declaration makes this member also trivial. Note that e.g. the atomic types do give a general statement!Finally there is a wording issue: There does not exist something like a "literal constructor". The core language uses the term "constexpr constructor" for this.
Suggestion:
Change 24.6.2 [istream.iterator]/3 as indicated:
constexpr istream_iterator(); istream_iterator(istream_type& s); istream_iterator(const istream_iterator<T,charT,traits,Distance>& x) = default; ~istream_iterator() = default;Change 24.6.2.2 [istream.iterator.cons]/1 as indicated:
constexpr istream_iterator();-1- Effects: Constructs the end-of-stream iterator. If
T
is a literal type, then this constructor shall be a constexpr constructor.Change 24.6.2.2 [istream.iterator.cons]/3 as indicated:
istream_iterator(const istream_iterator<T,charT,traits,Distance>& x) = default;-3- Effects: Constructs a copy of
x
. IfT
is a literal type, then this constructor shall be a trivial copy constructor.Change 24.6.2.2 [istream.iterator.cons]/4 as indicated:
~istream_iterator() = default;-4- Effects: The iterator is destroyed. If
T
is a literal type, then this destructor shall be a trivial destructor.Change 24.6.4 [istreambuf.iterator] before p. 1 as indicated:
constexpr istreambuf_iterator() throw(); istreambuf_iterator(const istreambuf_iterator&) throw() = default; ~istreambuf_iterator() throw() = default;Change 24.6.4 [istreambuf.iterator]/1 as indicated:
[..] The default constructor
istreambuf_iterator()
and the constructoristreambuf_iterator(0)
both construct an end of stream iterator object suitable for use as an end-of-range. All specializations ofistreambuf_iterator
shall have a trivial copy constructor, a constexpr default constructor and a trivial destructor.
[ 2009-10 Santa Cruz: ]
NAD EditorialResolved. Addressed by N2994.
Proposed resolution:
24.6.2 [istream.iterator] para 3
constexpr istream_iterator();
24.6.2.2 [istream.iterator.cons]
constexpr istream_iterator();-1- Effects: Constructs the end-of-stream iterator. If
T
is a literal type, then this constructor shall be a literal constructor.
24.6.4 [istreambuf.iterator]
constexpr istreambuf_iterator() throw();
copy_exception
name misleadingSection: 17.9.7 [propagation] Status: C++11 Submitter: Peter Dimov Opened: 2009-05-13 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
The naming of std::copy_exception
misleads almost everyone
(experts included!) to think that it is the function that copies an
exception_ptr
:
exception_ptr p1 = current_exception(); exception_ptr p2 = copy_exception( p1 );
But this is not at all what it does. The above actually creates an
exception_ptr p2
that contains a copy of p1
, not of
the exception to which p1
refers!
This is, of course, all my fault; in my defence, I used copy_exception
because I was unable to think of a better name.
But I believe that, based on what we've seen so far, any other name would be better.
Therefore, I propose copy_exception
to be renamed to
create_exception
:
template<class E> exception_ptr create_exception(E e);
with the following explanatory paragraph after it:
Creates an
exception_ptr
that refers to a copy ofe
.
[ 2009-05-13 Daniel adds: ]
What about
make_exception_ptrin similarity to
make_pair
andmake_tuple
,make_error_code
andmake_error_condition
, ormake_shared
? Or, if a stronger symmetry tocurrent_exception
is preferred:make_exceptionWe have not a single
create_*
function in the library, it was alwaysmake_*
used.
[ 2009-05-13 Peter adds: ]
make_exception_ptr
works for me.
[ 2009-06-02 Thomas J. Gritzan adds: ]
To avoid surprises and unwanted recursion, how about making a call to
std::make_exception_ptr
with anexception_ptr
illegal?It might work like this:
template<class E> exception_ptr make_exception_ptr(E e); template<> exception_ptr make_exception_ptr<exception_ptr>(exception_ptr e) = delete;
[ 2009 Santa Cruz: ]
Move to Review for the time being. The subgroup thinks this is a good idea, but doesn't want to break compatibility unnecessarily if someone is already shipping this. Let's talk to Benjamin and PJP tomorrow to make sure neither objects.
[ 2009-11-16 Jonathan Wakely adds: ]
GCC 4.4 shipped with
copy_exception
but we could certainly keep that symbol in the library (but not the headers) so it can still be found by any apps foolishly relying on the experimental C++0x mode being ABI stable.
[ 2009-11-16 Peter adopts wording supplied by Daniel. ]
[ 2009-11-16 Moved to Tentatively Ready after 5 positive votes on c++std-lib. ]
Proposed resolution:
Change 17.9 [support.exception]/1, header <exception>
synopsis as indicated:
exception_ptr current_exception(); void rethrow_exception [[noreturn]] (exception_ptr p); template<class E> exception_ptrcopy_exceptionmake_exception_ptr(E e);
Change 17.9.7 [propagation]:
template<class E> exception_ptrcopy_exceptionmake_exception_ptr(E e);-11- Effects: Creates an
exception_ptr
that refers to a copy ofe
, as iftry { throw e; } catch(...) { return current_exception(); }...
Change 32.10.6 [futures.promise]/7 as indicated:
Effects: if the associated state of
*this
is not ready, stores an exception object of typefuture_error
with an error code ofbroken_promise
as if bythis->set_exception(
. Destroys ...copy_exceptionmake_exception_ptr( future_error(future_errc::broken_promise))
alignment_of
Section: 21.3.5.4 [meta.unary.prop] Status: C++11 Submitter: Niels Dekker Opened: 2009-06-01 Last modified: 2016-01-28
Priority: Not Prioritized
View other active issues in [meta.unary.prop].
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Discussion:
The alignment_of
template is no longer necessary, now that the
core language will provide alignof
. Scott Meyers raised this
issue at comp.std.c++,
C++0x: alignof vs. alignment_of,
May 21, 2009. In a reply, Daniel Krügler pointed out that
alignof
was added to the working paper after
alignment_of
. So it appears that alignment_of
is only
part of the current Working Draft
(N2857)
because it is in TR1.
Having both alignof
and alignment_of
would cause
unwanted confusion. In general, I think TR1 functionality should not be
brought into C++0x if it is entirely redundant with other C++0x language
or library features.
[ 2009-11-16 Chris adds: ]
I would like to suggest the following new wording for this issue, based on recent discussions. Basically this doesn't delete
alignment_of
, it just makes it clear that it is just a wrapper foralignof
. This deletes the first part of the proposed resolution, changes the second part by leaving inalignof(T)
but changing the precondition and leaves the 3rd part unchanged.Suggested Resolution:
Change the first row of Table 44 ("Type property queries"), from Type properties 21.3.5.4 [meta.unary.prop]:
Table 44 — Type property queries template <class T> struct alignment_of;
alignof(T)
.
Precondition:T
shall be a complete type, a reference type, or an array of unknown bound, but shall not be a function type or (possibly cv-qualified)void
.alignof(T)
shall be definedChange text in Table 51 ("Other transformations"), from Other transformations 21.3.8.7 [meta.trans.other], as follows:
Table 51 — Other transformations … aligned_storage;
Len
shall not be zero.Align
shall be equal tofor some type
alignment_of<T>::valuealignof(T)T
or to default-alignment.…
[ 2010-01-30 Alisdair proposes that Chris' wording be moved into the proposed wording section and tweaks it on the way. ]
Original proposed wording saved here:
Remove from Header <type_traits> synopsis 21.3.3 [meta.type.synop]:
template <class T> struct alignment_of;Remove the first row of Table 44 ("Type property queries"), from Type properties 21.3.5.4 [meta.unary.prop]:
Table 44 — Type property queries template <class T> struct alignment_of;
alignof(T)
.
Precondition:T
shall be a complete type, a reference type, or an array of unknown bound, but shall not be a function type or (possibly cv-qualified)void
.Change text in Table 51 ("Other transformations"), from Other transformations 21.3.8.7 [meta.trans.other], as follows:
Table 51 — Other transformations … aligned_storage;
Len
shall not be zero. Align shall be equal tofor some type
alignment_of<T>::valuealignof(T)T
or to default-alignment.…
[ 2010-01-30 Moved to Tentatively Ready after 5 positive votes on c++std-lib. ]
Proposed resolution:
Change the first row of Table 43 ("Type property queries"), from Type properties 21.3.5.4 [meta.unary.prop]:
Table 43 — Type property queries template <class T> struct alignment_of;
alignof(T)
.
Precondition:T
shall be a complete type, a reference type, or an array of unknown bound, but shall not be a function type or (possibly cv-qualified)void
.alignof(T)
is a valid expression (7.6.2.6 [expr.alignof])
Change text in Table 51 ("Other transformations"), from Other transformations 21.3.8.7 [meta.trans.other], as follows:
Table 51 — Other transformations … aligned_storage;
Len
shall not be zero.Align
shall be equal tofor some type
alignment_of<T>::valuealignof(T)T
or to default-alignment.…
Section: 23.3.7.6 [forward.list.ops], 23.3.11.5 [list.ops] Status: C++11 Submitter: Alisdair Meredith Opened: 2009-05-09 Last modified: 2023-02-07
Priority: Not Prioritized
View all other issues in [forward.list.ops].
View all issues with C++11 status.
Discussion:
IIUC,
N2844
means that lvalues will no longer bind to rvalue references.
Therefore, the current specification of list::splice
(list
operations 23.3.11.5 [list.ops]) will be a breaking change of behaviour for existing
programs. That is because we changed the signature to swallow via an rvalue
reference rather than the lvalue reference used in 03.
Retaining this form would be safer, requiring an explicit move when splicing
from lvalues. However, this will break existing programs.
We have the same problem with forward_list
, although without the risk of
breaking programs so here it might be viewed as a positive feature.
The problem signatures:
void splice_after(const_iterator position, forward_list<T,Alloc>&& x); void splice_after(const_iterator position, forward_list<T,Alloc>&& x, const_iterator i); void splice_after(const_iterator position, forward_list<T,Alloc>&& x, const_iterator first, const_iterator last); void splice(const_iterator position, list<T,Alloc>&& x); void splice(const_iterator position, list<T,Alloc>&& x, const_iterator i); void splice(const_iterator position, list<T,Alloc>&& x, const_iterator first, const_iterator last);
Possible resolutions:
Option A. Add an additional (non-const) lvalue-reference overload in each case
Option B. Change rvalue reference back to (non-const) lvalue-reference overload in each case
Option C. Add an additional (non-const) lvalue-reference
overload in just the std::list
cases
I think (B) would be very unfortunate, I really like the forward_list
behaviour in (C) but feel (A) is needed for consistency.
My actual preference would be NAD, ship with this as a breaking change as it is a more explicit interface. I don't think that will fly though!
See the thread starting with c++std-lib-23725 for more discussion.
[ 2009-10-27 Christopher Jefferson provides proposed wording for Option C. ]
[ 2009-12-08 Jonathan Wakely adds: ]
As Bill Plauger pointed out,
list::merge
needs similar treatment.[ Wording updated. ]
[ 2009-12-13 Moved to Tentatively Ready after 5 positive votes on c++std-lib. ]
Proposed resolution:
In 23.3.11 [list]
Add lvalue overloads before rvalue ones:
void splice(const_iterator position, list<T,Allocator>& x); void splice(const_iterator position, list<T,Allocator>&& x); void splice(const_iterator position, list<T,Allocator>& x, const_iterator i); void splice(const_iterator position, list<T,Allocator>&& x, const_iterator i); void splice(const_iterator position, list<T,Allocator>& x, const_iterator first, const_iterator last); void splice(const_iterator position, list<T,Allocator>&& x, const_iterator first, const_iterator last); void merge(list<T,Allocator>& x); template <class Compare> void merge(list<T,Allocator>& x, Compare comp); void merge(list<T,Allocator>&& x); template <class Compare> void merge(list<T,Allocator>&& x, Compare comp);
In 23.3.11.5 [list.ops], similarly add lvalue overload before each rvalue one:
(After paragraph 2)
void splice(const_iterator position, list<T,Allocator>& x); void splice(const_iterator position, list<T,Allocator>&& x);
(After paragraph 6)
void splice(const_iterator position, list<T,Allocator>& x, const_iterator i); void splice(const_iterator position, list<T,Allocator>&& x, const_iterator i);
(After paragraph 10)
void splice(const_iterator position, list<T,Allocator>& x, const_iterator first, const_iterator last); void splice(const_iterator position, list<T,Allocator>&& x, const_iterator first, const_iterator last);
In 23.3.11.5 [list.ops], after paragraph 21
void merge(list<T,Allocator>& x); template <class Compare> void merge(list<T,Allocator>& x, Compare comp); void merge(list<T,Allocator>&& x); template <class Compare> void merge(list<T,Allocator>&& x, Compare comp);
<stdint.h>
, <fenv.h>
, <tgmath.h>
,
and maybe <complex.h>
Section: 29.7 [c.math], 99 [stdinth], 99 [fenv], 99 [cmplxh] Status: C++11 Submitter: Robert Klarer Opened: 2009-05-26 Last modified: 2017-06-15
Priority: Not Prioritized
View all other issues in [c.math].
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Discussion:
This is probably editorial.
The following items should be removed from the draft, because they're redundant with Annex D, and they arguably make some *.h headers non-deprecated:
99 [stdinth] (regarding <stdint.h>
)
99 [fenv] (regarding <fenv.h>
Line 3 of 29.7 [c.math] (regarding <tgmath.h>
)
99 [cmplxh] (regarding <complex.h>
, though the note in this subclause is not redundant)
[ 2009-06-10 Ganesh adds: ]
While searching for
stdint
in the CD, I found that<stdint.h>
is also mentioned in 6.8.2 [basic.fundamental] p.5. I guess it should refer to<cstdint>
instead.
[ 2009 Santa Cruz: ]
Real issue. Maybe just editorial, maybe not. Move to Ready.
Proposed resolution:
Remove the section 99 [stdinth].
Remove the section 99 [fenv].
Remove 29.7 [c.math], p3:
-3- The header<tgmath.h>
effectively includes the headers<complex.h>
and<math.h>
.
Remove the section 99 [cmplxh].
exception_ptr
should support contextual conversion to bool
Section: 17.9.7 [propagation] Status: Resolved Submitter: Daniel Krügler Opened: 2007-06-06 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
As of
N2857
17.9.7 [propagation] p.5, the implementation-defined type
exception_ptr
does provide the following ways to check whether
it is a null value:
void f(std::exception_ptr p) { p == nullptr; p == 0; p == exception_ptr(); }
This is rather cumbersome way of checking for the null value and I suggest to require support for evaluation in a boolean context like so:
void g(std::exception_ptr p) { if (p) {} !p; }
[ 2009 Santa Cruz: ]
Move to Ready. Note to editor: considering putting in a cross-reference to 7.3 [conv], paragraph 3, which defines the phrase "contextually converted to
bool
".
[ 2010-03-14 Howard adds: ]
We moved N3073 to the formal motions page in Pittsburgh which should obsolete this issue. I've moved this issue to NAD Editorial, solved by N3073.
Rationale:
Solved by N3073.
Proposed resolution:
In section 17.9.7 [propagation] insert a new paragraph between p.5 and p.6:
An object
e
of typeexception_ptr
can be contextually converted tobool
. The effect shall be as ife != exception_ptr()
had been evaluated in place ofe
. There shall be no implicit conversion to arithmetic type, to enumeration type or to pointer type.
nested_exception::rethrow_nested()
Section: 17.9.8 [except.nested] Status: C++11 Submitter: Daniel Krügler Opened: 2007-06-06 Last modified: 2016-01-28
Priority: Not Prioritized
View all other issues in [except.nested].
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Discussion:
It was recently mentioned in a newsgroup article
http://groups.google.de/group/comp.std.c++/msg/f82022aff68edf3d
that the specification of the member function rethrow_nested()
of the
class nested_exception
is incomplete, specifically it remains unclear
what happens, if member nested_ptr()
returns a null value. In
17.9.8 [except.nested] we find only the following paragraph related to that:
void rethrow_nested() const; // [[noreturn]]-4- Throws: the stored exception captured by this
nested_exception
object.
This is a problem, because it is possible to create an object of
nested_exception
with exactly such a state, e.g.
#include <exception> #include <iostream> int main() try { std::nested_exception e; // OK, calls current_exception() and stores it's null value e.rethrow_nested(); // ? std::cout << "A" << std::endl; } catch(...) { std::cout << "B" << std::endl; }
I suggest to follow the proposal of the reporter, namely to invoke
terminate()
if nested_ptr()
return a null value of exception_ptr
instead
of relying on the fallback position of undefined behavior. This would
be consistent to the behavior of a throw;
statement when no
exception is being handled.
[ 2009 Santa Cruz: ]
Move to Ready.
Proposed resolution:
Change around 17.9.8 [except.nested] p.4 as indicated:
-4- Throws: the stored exception captured by this
nested_exception
object, ifnested_ptr() != nullptr
- Remarks: If
nested_ptr() == nullptr
,terminate()
shall be called.
conj
and proj
Section: 29.4.10 [cmplx.over] Status: C++11 Submitter: Marc Steinbach Opened: 2009-06-11 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
In clause 1, the Working Draft (N2857) specifies overloads of the functions
arg, conj, imag, norm, proj, real
for non-complex arithmetic types (float
, double
,
long double
, and integers).
The only requirement (clause 2) specifies effective type promotion of arguments.
I strongly suggest to add the following requirement on the return types:
All the specified overloads must return real (i.e., non-complex) values, specifically, the nested
value_type
of effectively promoted arguments.
(This has no effect on arg
, imag
, norm
, real
:
they are real-valued anyway.)
Rationale:
Mathematically, conj()
and proj()
, like the transcendental functions, are
complex-valued in general but map the (extended) real line to itself.
In fact, both functions act as identity on the reals.
A typical user will expect conj()
and proj()
to preserve this essential
mathematical property in the same way as exp()
, sin()
, etc.
A typical use of conj()
, e.g., is the generic scalar product of n-vectors:
template<typename T> inline T scalar_product(size_t n, T const* x, T const* y) { T result = 0; for (size_t i = 0; i < n; ++i) result += x[i] * std::conj(y[i]); return result; }
This will work equally well for real and complex floating-point types T
if
conj()
returns T
. It will not work with real types if conj()
returns complex values.
Instead, the implementation of scalar_product
becomes either less efficient
and less useful (if a complex result is always returned), or unnecessarily
complicated (if overloaded versions with proper return types are defined).
In the second case, the real-argument overload of conj()
cannot be used.
In fact, it must be avoided.
Overloaded conj()
and proj()
are principally needed in generic programming.
All such use cases will benefit from the proposed return type requirement,
in a similar way as the scalar_product
example.
The requirement will not harm use cases where a complex return value
is expected, because of implicit conversion to complex.
Without the proposed return type guarantee, I find overloaded versions
of conj()
and proj()
not only useless but actually troublesome.
[ 2009-11-11 Moved to Tentatively Ready after 5 positive votes on c++std-lib. ]
Proposed resolution:
Insert a new paragraph after 29.4.10 [cmplx.over]/2:
All of the specified overloads shall have a return type which is the nested
value_type
of the effectively cast arguments.
operator+
Section: 27.4.4.1 [string.op.plus] Status: C++11 Submitter: Alisdair Meredith Opened: 2009-06-12 Last modified: 2021-06-06
Priority: Not Prioritized
View all issues with C++11 status.
Discussion:
Many of the basic_string operator+
overloads return an rvalue-reference. Is
that really intended?
I'm considering it might be a mild performance tweak to avoid making un-necessary copies of a cheaply movable type, but it opens risk to dangling references in code like:
auto && s = string{"x"} + string{y};
and I'm not sure about:
auto s = string{"x"} + string{y};
[ 2009-10-11 Howard updated Returns: clause for each of these. ]
[ 2009-11-05 Howard adds: ]
Moved to Tentatively Ready after 5 positive votes on c++std-lib.
Proposed resolution:
Strike the &&
from the return type in the following function
signatures:
27.4 [string.classes] p2 Header Synopsis
template<class charT, class traits, class Allocator> basic_string<charT,traits,Allocator>&&operator+(basic_string<charT,traits,Allocator>&& lhs, const basic_string<charT,traits,Allocator>& rhs); template<class charT, class traits, class Allocator> basic_string<charT,traits,Allocator>&&operator+(const basic_string<charT,traits,Allocator>& lhs, basic_string<charT,traits,Allocator>&& rhs); template<class charT, class traits, class Allocator> basic_string<charT,traits,Allocator>&&operator+(basic_string<charT,traits,Allocator>&& lhs, basic_string<charT,traits,Allocator>&& rhs); template<class charT, class traits, class Allocator> basic_string<charT,traits,Allocator>&&operator+(const charT* lhs, basic_string<charT,traits,Allocator>&& rhs); template<class charT, class traits, class Allocator> basic_string<charT,traits,Allocator>&&operator+(charT lhs, basic_string<charT,traits,Allocator>&& rhs); template<class charT, class traits, class Allocator> basic_string<charT,traits,Allocator>&&operator+(basic_string<charT,traits,Allocator>&& lhs, const charT* rhs); template<class charT, class traits, class Allocator> basic_string<charT,traits,Allocator>&&operator+(basic_string<charT,traits,Allocator>&& lhs, charT rhs);[string.op+]
template<class charT, class traits, class Allocator> basic_string<charT,traits,Allocator>&&operator+(basic_string<charT,traits,Allocator>&& lhs, const basic_string<charT,traits,Allocator>& rhs);Returns:
std::move(lhs.append(rhs))
template<class charT, class traits, class Allocator> basic_string<charT,traits,Allocator>&&operator+(const basic_string<charT,traits,Allocator>& lhs, basic_string<charT,traits,Allocator>&& rhs);Returns:
std::move(rhs.insert(0, lhs))
template<class charT, class traits, class Allocator> basic_string<charT,traits,Allocator>&&operator+(basic_string<charT,traits,Allocator>&& lhs, basic_string<charT,traits,Allocator>&& rhs);Returns:
std::move(lhs.append(rhs))
[Note: Or equivalentlystd::move(rhs.insert(0, lhs))
— end note]template<class charT, class traits, class Allocator> basic_string<charT,traits,Allocator>&&operator+(const charT* lhs, basic_string<charT,traits,Allocator>&& rhs);Returns:
std::move(rhs.insert(0, lhs))
.template<class charT, class traits, class Allocator> basic_string<charT,traits,Allocator>&&operator+(charT lhs, basic_string<charT,traits,Allocator>&& rhs);Returns:
std::move(rhs.insert(0, 1, lhs))
.template<class charT, class traits, class Allocator> basic_string<charT,traits,Allocator>&&operator+(basic_string<charT,traits,Allocator>&& lhs, const charT* rhs);Returns:
std::move(lhs.append(rhs))
.template<class charT, class traits, class Allocator> basic_string<charT,traits,Allocator>&&operator+(basic_string<charT,traits,Allocator>&& lhs, charT rhs);Returns:
std::move(lhs.append(1, rhs))
.
Section: 32.5 [atomics] Status: Resolved Submitter: LWG Opened: 2009-06-15 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
Addresses US 87, UK 311
The atomics chapter is not concept enabled.
Needs to also consider issues 923(i) and 924(i).
[ 2009-10 Santa Cruz: ]
NAD EditorialResolved. Addressed by N2992.
Proposed resolution:
Section: 17.5 [support.start.term] Status: C++11 Submitter: LWG Opened: 2009-06-16 Last modified: 2016-01-28
Priority: Not Prioritized
View other active issues in [support.start.term].
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Discussion:
Addresses UK 187
The term "thread safe" is not defined nor used in this context anywhere else in the standard.
Suggested action:
Clarify the meaning of "thread safe".
[ 2009 Santa Cruz: ]
The "thread safe" language has already been change in the WP. It was changed to "happen before", but the current WP text is still a little incomplete: "happen before" is binary, but the current WP text only mentions one thing.
Move to Ready.
Proposed resolution:
For the following functions in 17.5 [support.start.term].
extern "C" int at_quick_exit(void (*f)(void)); extern "C++" int at_quick_exit(void (*f)(void));
Edit paragraph 10 as follows. The intent is to provide the other half of the happens before relation; to note indeterminate ordering; and to clean up some formatting.
Effects: The
at_quick_exit()
functions register the function pointed to byf
to be called without arguments whenquick_exit
is called. It is unspecified whether a call toat_quick_exit()
that does nothappen-beforehappen before (1.10) all calls toquick_exit
will succeed. [Note: theat_quick_exit()
functions shall not introduce a data race (17.6.4.7).exitnote—end note] [Note: The order of registration may be indeterminate ifat_quick_exit
was called from more than one thread. —end note] [Note: Theat_quick_exit
registrations are distinct from theatexit
registrations, and applications may need to call both registration functions with the same argument. —end note]
For the following function.
void quick_exit [[noreturn]] (int status)
Edit paragraph 13 as follows. The intent is to note that thread-local variables may be different.
Effects: Functions registered by calls to
at_quick_exit
are called in the reverse order of their registration, except that a function shall be called after any previously registered functions that had already been called at the time it was registered. Objects shall not be destroyed as a result of callingquick_exit
. If control leaves a registered function called byquick_exit
because the function does not provide a handler for a thrown exception,terminate()
shall be called. [Note: Functions registered by one thread may be called by any thread, and hence should not rely on the identity of thread-storage-duration objects. —end note] After calling registered functions,quick_exit
shall call_Exit(status)
. [Note: The standard file buffers are not flushed. See: ISO C 7.20.4.4. —end note]
Section: 32.5 [atomics] Status: Resolved Submitter: LWG Opened: 2009-06-16 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
Addresses UK 312
The contents of the <stdatomic.h>
header are not listed anywhere,
and <cstdatomic>
is listed as a C99 header in chapter 17.
If we intend to use these for compatibility with a future C standard,
we should not use them now.
[ 2009-10 Santa Cruz: ]
NAD EditorialResolved. Solved by N2992.
Proposed resolution:
Remove <cstdatomic>
from the C99 headers in table 14.
Add a new header <atomic>
to the headers in table 13.
Update chapter 29 to remove reference to <stdatomic.h>
and replace the use of <cstdatomic>
with <atomic>
.
[ If and when WG14 adds atomic operations to C we can add corresponding headers to table 14 with a TR. ]
Section: 32.5.5 [atomics.lockfree] Status: Resolved Submitter: Jeffrey Yasskin Opened: 2009-06-16 Last modified: 2016-02-25
Priority: Not Prioritized
View all other issues in [atomics.lockfree].
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Discussion:
Addresses US 88
The "lockfree" facilities do not tell the programmer enough.
There are 2 problems here.
First, at least on x86,
it's less important to me whether some integral types are lock free
than what is the largest type I can pass to atomic and have it be lock-free.
For example, if long long
s are not lock-free,
ATOMIC_INTEGRAL_LOCK_FREE
is probably 1,
but I'd still be interested in knowing whether longs are always lock-free.
Or if long longs at any address are lock-free,
I'd expect ATOMIC_INTEGRAL_LOCK_FREE
to be 2,
but I may actually care whether I have access to
the cmpxchg16b
instruction.
None of the support here helps with that question.
(There are really 2 related questions here:
what alignment requirements are there for lock-free access;
and what processor is the program actually running on,
as opposed to what it was compiled for?)
Second, having atomic_is_lock_free
only apply to individual objects
is pretty useless
(except, as Lawrence Crowl points out,
for throwing an exception when an object is unexpectedly not lock-free).
I'm likely to want to use its result to decide what algorithm to use,
and that algorithm is probably going to allocate new memory
containing atomic objects and then try to act on them.
If I can't predict the lock-freedom of the new object
by checking the lock-freedom of an existing object,
I may discover after starting the algorithm that I can't continue.
[ 2009-06-16 Jeffrey Yasskin adds: ]
To solve the first problem, I think 2 macros would help:
MAX_POSSIBLE_LOCK_FREE_SIZE
andMAX_GUARANTEED_LOCK_FREE_SIZE
, which expand to the maximum value ofsizeof(T)
for which atomic may (or will, respectively) use lock-free operations. Lawrence points out that this "relies heavily on implementations using word-size compare-swap on sub-word-size types, which in turn requires address modulation." He expects that to be the end state anyway, so it doesn't bother him much.To solve the second, I think one could specify that equally aligned objects of the same type will return the same value from
atomic_is_lock_free()
. I don't know how to specify "equal alignment". Lawrence suggests an additional function,atomic_is_always_lock_free()
.
[ 2009-10-22 Benjamin Kosnik: ]
In the evolution discussion of N2925, "More Collected Issues with Atomics," there is an action item with respect to LWG 1146(i), US 88
This is stated in the paper as:
Relatedly, Mike Sperts will create an issue to propose adding a traits mechanism to check the compile-time properties through a template mechanism rather than macros
Here is my attempt to do this. I don't believe that a separate trait is necessary for this, and that instead
atomic_integral::is_lock_free
can be re-purposed with minimal work as follows.[ Howard: Put Benjamin's wording in the proposed wording section. ]
[ 2009-10-22 Alberto Ganesh Barbati: ]
Just a thought... wouldn't it be better to use a scoped enum instead of plain integers? For example:
enum class is_lock_free { never = 0, sometimes = 1, always = 2; };if compatibility with C is deemed important, we could use an unscoped enum with suitably chosen names. It would still be more descriptive than 0, 1 and 2.
Previous resolution [SUPERSEDED]:
Header
<cstdatomic>
synopsis [atomics.synopsis]Edit as follows:
namespace std { ... // 29.4, lock-free property#define ATOMIC_INTEGRAL_LOCK_FREE unspecified#define ATOMIC_CHAR_LOCK_FREE unspecified #define ATOMIC_CHAR16_T_LOCK_FREE unspecified #define ATOMIC_CHAR32_T_LOCK_FREE unspecified #define ATOMIC_WCHAR_T_LOCK_FREE unspecified #define ATOMIC_SHORT_LOCK_FREE unspecified #define ATOMIC_INT_LOCK_FREE unspecified #define ATOMIC_LONG_LOCK_FREE unspecified #define ATOMIC_LLONG_LOCK_FREE unspecified #define ATOMIC_ADDRESS_LOCK_FREE unspecifiedLock-free Property 32.5.5 [atomics.lockfree]
Edit the synopsis as follows.
namespace std {#define ATOMIC_INTEGRAL_LOCK_FREE unspecified#define ATOMIC_CHAR_LOCK_FREE unspecified #define ATOMIC_CHAR16_T_LOCK_FREE unspecified #define ATOMIC_CHAR32_T_LOCK_FREE unspecified #define ATOMIC_WCHAR_T_LOCK_FREE unspecified #define ATOMIC_SHORT_LOCK_FREE unspecified #define ATOMIC_INT_LOCK_FREE unspecified #define ATOMIC_LONG_LOCK_FREE unspecified #define ATOMIC_LLONG_LOCK_FREE unspecified #define ATOMIC_ADDRESS_LOCK_FREE unspecified }Edit paragraph 1 as follows.
The ATOMIC_...._LOCK_FREE macros
ATOMIC_INTEGRAL_LOCK_FREE and ATOMIC_ADDRESS_LOCK_FREEindicate the general lock-free property ofintegral and address atomicthe corresponding atomic integral types, with the signed and unsigned variants grouped together.The properties also apply to the corresponding specializations of the atomic template.A value of 0 indicates that the types are never lock-free. A value of 1 indicates that the types are sometimes lock-free. A value of 2 indicates that the types are always lock-free.Operations on Atomic Types 32.5.8.2 [atomics.types.operations]
Edit as follows.
voidstatic constexpr bool A::is_lock_free() const volatile;Returns: True if the
object'stypes's operations are lock-free, false otherwise. [Note: In the same way that<limits>
std::numeric_limits<short>::max()
is related to<limits.h>
__LONG_LONG_MAX__
,<atomic> std::atomic_short::is_lock_free
is related to<stdatomic.h>
andATOMIC_SHORT_LOCK_FREE
— end note]
[ 2009-10 Santa Cruz: ]
NAD EditorialResolved. Solved by N2992.
Proposed resolution:
Resolved by N2992.Section: 32.5.8.2 [atomics.types.operations] Status: Resolved Submitter: Jeffrey Yasskin Opened: 2009-06-16 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
Addresses US 90
The C++0X draft declares all of the functions dealing with atomics (section 32.5.8.2 [atomics.types.operations]) to take volatile arguments. Yet it also says (29.4-3),
[ Note: Many operations are volatile-qualified. The "volatile as device register" semantics have not changed in the standard. This qualification means that volatility is preserved when applying these operations to volatile objects. It does not mean that operations on non-volatile objects become volatile. Thus, volatile qualified operations on non-volatile objects may be merged under some conditions. — end note ]
I was thinking about how to implement this in gcc, and I believe that we'll want to overload most of the functions on volatile and non-volatile. Here's why:
To let the compiler take advantage of the permission
to merge non-volatile atomic operations and reorder atomics in certain,
we'll need to tell the compiler backend
about exactly which atomic operation was used.
So I expect most of the functions of the form atomic_<op>_explicit()
(e.g. atomic_load_explicit
, atomic_exchange_explicit
,
atomic_fetch_add_explicit
, etc.)
to become compiler builtins.
A builtin can tell whether its argument was volatile or not,
so those functions don't really need extra explicit overloads.
However, I don't expect that we'll want to add builtins
for every function in chapter 29,
since most can be implemented in terms of the _explicit
free functions:
class atomic_int {
__atomic_int_storage value;
public:
int fetch_add(int increment, memory_order order = memory_order_seq_cst) volatile {
// &value has type "volatile __atomic_int_storage*".
atomic_fetch_add_explicit(&value, increment, order);
}
...
};
But now this always calls
the volatile builtin version of atomic_fetch_add_explicit()
,
even if the atomic_int
wasn't declared volatile.
To preserve volatility and the compiler's permission to optimize,
I'd need to write:
class atomic_int {
__atomic_int_storage value;
public:
int fetch_add(int increment, memory_order order = memory_order_seq_cst) volatile {
atomic_fetch_add_explicit(&value, increment, order);
}
int fetch_add(int increment, memory_order order = memory_order_seq_cst) {
atomic_fetch_add_explicit(&value, increment, order);
}
...
};
But this is visibly different from the declarations in the standard
because it's now overloaded.
(Consider passing &atomic_int::fetch_add
as a template parameter.)
The implementation may already have permission to add overloads to the member functions:
16.4.6.5 [member.functions] An implementation may declare additional non-virtual member function signatures within a class:
...
- by adding a member function signature for a member function name.
but I don't see an equivalent permission to add overloads to the free functions.
[ 2009-06-16 Lawrence adds: ]
I recommend allowing non-volatile overloads.
[ 2009-10 Santa Cruz: ]
NAD EditorialResolved. Addressed by N2992.
Proposed resolution:
Section: 31.10.6 [fstream] Status: Resolved Submitter: LWG Opened: 2009-06-28 Last modified: 2018-07-02
Priority: Not Prioritized
View all issues with Resolved status.
Discussion:
Addresses JP 73
Description
It is a problem
from C++98, fstream
cannot appoint a filename of wide
character string(const wchar_t
and const wstring&
).
Suggestion
Add
interface corresponding to wchar_t
, char16_t
and char32_t
.
[ 2009-07-01 Alisdair notes that this is a duplicate of 454(i) which has more in-depth rationale. ]
[ 2009-09-21 Daniel adds: ]
I suggest to mark this issue as NAD Future with the intend to solve the issue with a single file path c'tor template assuming a provision of a TR2 filesystem library.
[ 2009 Santa Cruz: ]
[LEWG Kona 2017]
Recommend NAD: Needs a paper. Recommend providing overload that takes filesystem::path. Note that there are other similar issues elsewhere in the library, for example basic_filebuf. Handled by another issue.
Proposed resolution:
Resolved by the adoption of 2676(i).
Section: 16 [library] Status: Resolved Submitter: LWG Opened: 2009-06-28 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
Addresses US 63
Description
The behavior of the library in the presence of threads is incompletely specified.
For example, if thread 1 assigns to X
, then writes data
to file f
, which is read by thread 2, and then accesses
variable X
, is thread 2 guaranteed to be able to see the
value assigned to X
by thread 1? In other words, does the
write of the data "happen before" the read?
Another example: does simultaneous access using operator
at()
to different characters in the same non-const string
really introduce a data race?
Suggestion
Notes
17 SG: should go to threads group; misclassified in document
Concurrency SG: Create an issue. Hans will look into it.
[ 2009 Santa Cruz: ]
Move to "Open". Hans and the rest of the concurrency working group will study this. We can't make progress without a thorough review and a paper.
[ 2010 Pittsburgh: Moved to NAD Editorial. Rationale added below. ]
Rationale:
Solved by N3069.
Proposed resolution:
Section: 28.3.4.3.3.3 [facet.num.put.virtuals] Status: C++11 Submitter: Seungbeom Kim Opened: 2009-06-27 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
In Table 73 — Floating-point conversions, 28.3.4.3.3.3 [facet.num.put.virtuals], in N2914, we have the following entries:
State | stdio equivalent |
---|---|
floatfield == ios_base::fixed | ios_base::scientific && !uppercase |
%a |
floatfield == ios_base::fixed | ios_base::scientific |
%A |
These expressions are supposed to mean:
floatfield == (ios_base::fixed | ios_base::scientific) && !uppercase floatfield == (ios_base::fixed | ios_base::scientific)
but technically parsed as:
((floatfield == ios_base::fixed) | ios_base::scientific) && (!uppercase) ((floatfield == ios_base::fixed) | ios_base::scientific)
and should be corrected with additional parentheses, as shown above.
[ 2009-10-28 Howard: ]
Moved to Tentatively Ready after 5 positive votes on c++std-lib.
Proposed resolution:
Change Table 83 — Floating-point conversions in 28.3.4.3.3.3 [facet.num.put.virtuals]:
State | stdio equivalent |
---|---|
floatfield == (ios_base::fixed | ios_base::scientific) && !uppercase |
%a |
floatfield == (ios_base::fixed | ios_base::scientific) |
%A |
Section: 16.4.5.2.1 [namespace.std] Status: C++11 Submitter: LWG Opened: 2009-06-28 Last modified: 2016-01-28
Priority: Not Prioritized
View other active issues in [namespace.std].
View all other issues in [namespace.std].
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Discussion:
Addresses UK 175
Description
Local types can now be used to instantiate templates, but don't have external linkage.
Suggestion
Remove the reference to external linkage.
Notes
We accept the proposed solution. Martin will draft an issue.
[ 2009-07-28 Alisdair provided wording. ]
[ 2009-10 Santa Cruz: ]
Moved to Ready.
Proposed resolution:
16.4.5.2.1 [namespace.std]
Strike "of external linkage" in p1 and p2:
-1- The behavior of a C++ program is undefined if it adds declarations or definitions to namespace
std
or to a namespace within namespacestd
unless otherwise specified. A program may add a concept map for any standard library concept or a template specialization for any standard library template to namespacestd
only if the declaration depends on a user-defined typeof external linkageand the specialization meets the standard library requirements for the original template and is not explicitly prohibited.179-2- The behavior of a C++ program is undefined if it declares
- an explicit specialization of any member function of a standard library class template, or
- an explicit specialization of any member function template of a standard library class or class template, or
- an explicit or partial specialization of any member class template of a standard library class or class template.
A program may explicitly instantiate a template defined in the standard library only if the declaration depends on the name of a user-defined type
of external linkageand the instantiation meets the standard library requirements for the original template.
Section: 32.2.4 [thread.req.timing] Status: C++11 Submitter: LWG Opened: 2009-06-28 Last modified: 2016-01-28
Priority: Not Prioritized
View all other issues in [thread.req.timing].
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Discussion:
Addresses UK 322, US 96
Description
Not all systems can provide a monotonic clock. How are they expected to treat a _for function?
Suggestion
Add at least a note explaining the intent for systems that do not support a monotonic clock.
Notes
Create an issue, together with UK 96. Note that the specification as is already allows a non-monotonic clock due to the word “should” rather than “shall”. If this wording is kept, a footnote should be added to make the meaning clear.
[ 2009-06-29 Beman provided a proposed resolution. ]
[ 2009-10-31 Howard adds: ]
Set to Tentatively Ready after 5 positive votes on c++std-lib.
[ 2010-02-24 Pete moved to Open: ]
LWG 1158's proposed resolution replaces the ISO-specified normative term "should" with "are encouraged but not required to", which presumably means the same thing, but has no ISO normative status. The WD used the latter formulation in quite a few non-normative places, but only three normative ones. I've changed all the normative uses to "should".
[ 2010-03-06 Beman updates wording. ]
[ 2010 Pittsburgh: Moved to Ready. ]
Proposed resolution:
Change Timing specifications 32.2.4 [thread.req.timing] as indicated:
The member functions whose names end in _for
take an argument that
specifies a relative time. Implementations should use a monotonic clock to
measure time for these functions. [Note: Implementations are not
required to use a monotonic clock because such a clock may be unavailable.
— end note]
resource_deadlock_would_occur
Section: 32.6.5.4.3 [thread.lock.unique.locking] Status: C++11 Submitter: LWG Opened: 2009-06-28 Last modified: 2016-01-28
Priority: Not Prioritized
View all other issues in [thread.lock.unique.locking].
View all issues with C++11 status.
Duplicate of: 1219
Discussion:
Addresses UK 327, UK 328
UK 327 Description
Not clear what
the specification for error condition
resource_deadlock_would_occur
means. It is perfectly
possible for this thread to own the mutex without setting
owns to true on this specific lock object. It is also
possible for lock operations to succeed even if the thread
does own the mutex, if the mutex is recursive. Likewise, if
the mutex is not recursive and the mutex has been locked
externally, it is not always possible to know that this
error condition should be raised, depending on the host
operating system facilities. It is possible that 'i.e.' was
supposed to be 'e.g.' and that suggests that recursive
locks are not allowed. That makes sense, as the
exposition-only member owns is boolean and not a integer to
count recursive locks.
UK 327 Suggestion
Add a precondition !owns
. Change the 'i.e.'
in the error condition to be 'e.g.' to allow for this
condition to propogate deadlock detection by the host OS.
UK 327 Notes
Create an issue. Assigned to Lawrence Crowl. Note: not sure what try_lock means for recursive locks when you are the owner. POSIX has language on this, which should ideally be followed. Proposed fix is not quite right, for example, try_lock should have different wording from lock.
UK 328 Description
There is a missing precondition that owns
is true, or an if(owns)
test is missing from the effect
clause
UK 328 Suggestion
Add a
precondition that owns == true
. Add an error condition to
detect a violation, rather than yield undefined behaviour.
UK 328 Notes
Handle in same issue as UK 327. Also uncertain that the proposed resolution is the correct one.
[ 2009-11-11 Alisdair notes that this issue is very closely related to 1219(i), if not a dup. ]
[ 2010-02-12 Anthony provided wording. ]
[ 2010 Pittsburgh: ]
Wording updated and moved to Ready for Pittsburgh.
Proposed resolution:
Modify 32.6.5.4.3 [thread.lock.unique.locking] p3 to say:
void lock();...
3 Throws: Any exception thrown by
pm->lock()
.std::system_error
if an exception is required (32.2.2 [thread.req.exception]).std::system_error
with an error condition ofoperation_not_permitted
ifpm
is0
.std::system_error
with an error condition ofresource_deadlock_would_occur
if on entryowns
istrue
.std::system_error
when the postcondition cannot be achieved.
Remove 32.6.5.4.3 [thread.lock.unique.locking] p4 (Error condition clause).
Modify 32.6.5.4.3 [thread.lock.unique.locking] p8 to say:
bool try_lock();...
8 Throws: Any exception thrown by
pm->try_lock()
.std::system_error
if an exception is required (32.2.2 [thread.req.exception]).std::system_error
with an error condition ofoperation_not_permitted
ifpm
is0
.std::system_error
with an error condition ofresource_deadlock_would_occur
if on entryowns
istrue
.std::system_error
when the postcondition cannot be achieved.
Remove 32.6.5.4.3 [thread.lock.unique.locking] p9 (Error condition clause).
Modify 32.6.5.4.3 [thread.lock.unique.locking] p13 to say:
template <class Clock, class Duration> bool try_lock_until(const chrono::time_point<Clock, Duration>& abs_time);...
13 Throws: Any exception thrown by
pm->try_lock_until()
.std::system_error
if an exception is required (32.2.2 [thread.req.exception]).std::system_error
with an error condition ofoperation_not_permitted
ifpm
is0
.std::system_error
with an error condition ofresource_deadlock_would_occur
if on entryowns
istrue
.std::system_error
when the postcondition cannot be achieved.
Remove 32.6.5.4.3 [thread.lock.unique.locking] p14 (Error condition clause).
Modify 32.6.5.4.3 [thread.lock.unique.locking] p18 to say:
template <class Rep, class Period> bool try_lock_for(const chrono::duration<Rep, Period>& rel_time);...
18 Throws: Any exception thrown by
pm->try_lock_for()
.std::system_error
if an exception is required (32.2.2 [thread.req.exception]).std::system_error
with an error condition ofoperation_not_permitted
ifpm
is0
.std::system_error
with an error condition ofresource_deadlock_would_occur
if on entryowns
istrue
.std::system_error
when the postcondition cannot be achieved.
Remove 32.6.5.4.3 [thread.lock.unique.locking] p19 (Error condition clause).
future_error
public constructor is 'exposition only'Section: 32.10.4 [futures.future.error] Status: Resolved Submitter: LWG Opened: 2009-06-28 Last modified: 2021-06-06
Priority: Not Prioritized
View all issues with Resolved status.
Discussion:
Addresses UK 331
Description
Not clear what it means for a public constructor to be 'exposition only'. If the intent is purely to support the library calling this constructor then it can be made private and accessed through friendship. Otherwise it should be documented for public consumption.
Suggestion
Declare the constructor as private with a note about intended friendship, or remove the exposition-only comment and document the semantics.
Notes
Create an issue. Assigned to Detlef. Suggested resolution probably makes sense.
[ 2009-07 Frankfurt ]
Pending a paper from Anthony Williams / Detlef Vollmann.
[ 2009-10-14 Pending paper: N2967. ]
[ 2009-10 Santa Cruz: ]
NAD EditorialResolved. Solved by N2997.
Proposed resolution:
unique_future
limitationsSection: 32.10.7 [futures.unique.future] Status: Resolved Submitter: LWG Opened: 2009-06-28 Last modified: 2021-06-06
Priority: Not Prioritized
View all other issues in [futures.unique.future].
View all issues with Resolved status.
Discussion:
Addresses UK 336
Description
It is possible
to transfer ownership of the asynchronous result from one
unique_future instance to another via the move-constructor.
However, it is not possible to transfer it back, and nor is
it possible to create a default-constructed unique_future
instance to use as a later move target. This unduly limits
the use of unique_future
in code. Also, the lack of a
move-assignment operator restricts the use of unique_future
in containers such as std::vector
- vector::insert
requires
move-assignable for example.
Suggestion
Add a default constructor with the
semantics that it creates a unique_future
with no
associated asynchronous result. Add a move-assignment
operator which transfers ownership.
Notes
Create an issue. Detlef will look into it.
[ 2009-07 Frankfurt ]
Pending a paper from Anthony Williams / Detlef Vollmann.
[ 2009-10-14 Pending paper: N2967. ]
[ 2009-10 Santa Cruz: ]
NAD EditorialResolved. Addressed by N2997.
Proposed resolution:
shared_future
should support an efficient move constructorSection: 32.10.8 [futures.shared.future] Status: Resolved Submitter: LWG Opened: 2009-06-28 Last modified: 2021-06-06
Priority: Not Prioritized
View all other issues in [futures.shared.future].
View all issues with Resolved status.
Discussion:
Addresses UK 337
Description
shared_future
should support an efficient move constructor that can avoid
unnecessary manipulation of a reference count, much like
shared_ptr
Suggestion
Add a move constructor
Notes
Create an issue. Detlef will look into it.
[ 2009-07 Frankfurt ]
Pending a paper from Anthony Williams / Detlef Vollmann.
[ 2009-10-14 Pending paper: N2967. ]
[ 2009-10 Santa Cruz: ]
NAD EditorialResolved. Addressed by N2997.
Proposed resolution:
shared_future
is inconsistent with shared_ptr
Section: 32.10.8 [futures.shared.future] Status: Resolved Submitter: LWG Opened: 2009-06-28 Last modified: 2021-06-06
Priority: Not Prioritized
View all other issues in [futures.shared.future].
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Discussion:
Addresses UK 338
Description
shared_future
is currently
CopyConstructible
, but not CopyAssignable
. This is
inconsistent with shared_ptr
, and will surprise users.
Users will then write work-arounds to provide this
behaviour. We should provide it simply and efficiently as
part of shared_future. Note that since the shared_future
member functions for accessing the state are all declared
const, the original usage of an immutable shared_future
value that can be freely copied by multiple threads can be
retained by declaring such an instance as "const
shared_future
".
Suggestion
Remove "=delete"
from the copy-assignment operator of shared_future. Add a
move-constructor shared_future(shared_future&&
rhs)
, and a move-assignment operator shared_future&
operator=(shared_future&& rhs)
. The postcondition
for the copy-assignment operator is that *this
has the same
associated state as rhs
. The postcondition for the
move-constructor and move assignment is that *this
has the
same associated as rhs
had before the
constructor/assignment call and that rhs
has no associated
state.
Notes
Create an issue. Detlef will look into it.
[ 2009-07 Frankfurt ]
Pending a paper from Anthony Williams / Detlef Vollmann.
[ 2009-10-14 Pending paper: N2967. ]
[ 2009-10 Santa Cruz: ]
NAD EditorialResolved. Adressed by N2997.
Proposed resolution:
promise
move constructorSection: 32.10.6 [futures.promise] Status: Resolved Submitter: LWG Opened: 2009-06-28 Last modified: 2016-01-28
Priority: Not Prioritized
View other active issues in [futures.promise].
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Discussion:
Addresses UK 343
Description
The move constructor of a std::promise
object does not need to allocate any memory, so the
move-construct-with-allocator overload of the constructor
is superfluous.
Suggestion
Remove the constructor with the signature template <class
Allocator> promise(allocator_arg_t, const Allocator&
a, promise& rhs);
Notes
Create an issue. Detlef will look into it. Will solicit feedback from Pablo. Note that "rhs" argument should also be an rvalue reference in any case.
[ 2009-07 Frankfurt ]
Pending a paper from Anthony Williams / Detlef Vollmann.
[ 2009-10 Santa Cruz: ]
NAD EditorialResolved. Adressed by N2997.
Proposed resolution:
Section: 99 [allocator.propagation], 99 [allocator.propagation.map], 23 [containers] Status: Resolved Submitter: LWG Opened: 2009-06-28 Last modified: 2016-01-28
Priority: Not Prioritized
View all issues with Resolved status.
Discussion:
Addresses US 77
Description
Allocator-specific move and copy behavior for containers (N2525) complicates a little-used and already-complicated portion of the standard library (allocators), and breaks the conceptual model of move-constructor and move-assignment operations on standard containers being efficient operations. The extensions for allocator-specific move and copy behavior should be removed from the working paper.
With the introduction of rvalue references, we are teaching
programmers that moving from a standard container (e.g., a
vector<string>
) is an efficient, constant-time
operation. The introduction of N2525 removed that
guarantee; depending on the behavior of four different
traits (20.8.4), the complexity of copy and move operations
can be constant or linear time. This level of customization
greatly increases the complexity of standard containers,
and benefits only a tiny fraction of the C++ community.
Suggestion
Remove 20.8.4.
Remove 20.8.5.
Remove all references to the facilities in 20.8.4 and 20.8.5 from clause 23.
[ 2009-10 Santa Cruz: ]
NAD EditorialResolved. Addressed by N2982.
Proposed resolution:
num_get
not fully compatible with strto*
Section: 28.3.4.3.2.3 [facet.num.get.virtuals] Status: C++17 Submitter: Cosmin Truta Opened: 2009-07-04 Last modified: 2017-07-30
Priority: 3
View other active issues in [facet.num.get.virtuals].
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View all issues with C++17 status.
Discussion:
As specified in the latest draft,
N2914,
num_get
is still not fully compatible with the following C
functions: strtoul
, strtoull
,
strtof
and
strtod
.
In C, when conversion of a string to an unsigned integer type falls
outside the
representable range, strtoul
and strtoull
return
ULONG_MAX
and ULLONG_MAX
, respectively,
regardless
whether the input field represents a positive or a negative value.
On the other hand, the result of num_get
conversion of
negative
values to unsigned integer types is zero. This raises a compatibility
issue.
Moreover, in C, when conversion of a string to a floating-point type falls
outside the representable range, strtof
, strtod
and
strtold
return ±HUGE_VALF
,
±HUGE_VAL
and ±HUGE_VALL
, respectively.
On the other hand, the result of num_get
conversion of such
out-of-range floating-point values results in the most positive/negative
representable value.
Although many C library implementations do implement HUGE_VAL
(etc.) as the highest representable (which is, usually, the infinity),
this isn't required by the C standard. The C library specification makes no
statement regarding the value of HUGE_VAL
and friends, which
potentially raises the same compatibility issue as in the above case of
unsigned integers.
In addition, neither C nor C++ define symbolic constants for the maximum
representable floating-point values (they only do so only for the maximum
representable finite floating-point values), which raises a
usability
issue (it would be hard for the programmer to check the result of
num_get
against overflow).
As such, we propose to adjust the specification of num_get
to
closely follow the behavior of all of its underlying C functions.
[ 2010 Rapperswil: ]
Some concern that this is changing the specification for an existing C++03 function, but it was pointed out that this was underspecified as resolved by issue 23. This is clean-up for that issue in turn. Some concern that we are trying to solve the same problem in both clause 22 and 27.
Bill: There's a change here as to whether val is stored to in an error case.
Pablo: Don't think this changes whether val is stored to or not, but changes the value that is stored.
Bill: Remembers having skirmishes with customers and testers as to whether val is stored to, and the resolution was not to store in error cases.
Howard: Believes since C++03 we made a change to always store in overflow.
Everyone took some time to review the issue.
Pablo: C++98 definitely did not store any value during an error condition.
Dietmar: Depends on the question of what is considered an error, and whether overflow is an error or not, which was the crux of LWG 23.
Pablo: Yes, but given the "zero, if the conversion function fails to convert the entire field", we are requiring every error condition to store.
Bill: When did this happen?
Alisdair: One of the last two or three meetings.
Dietmar: To store a value in case of failure is a very bad idea.
Move to Open, needs more study.
[2011-03-24 Madrid meeting]
Move to deferred
[ 2011 Bloomington ]
The proposed wording looks good, no-one sure why this was held back before. Move to Review.
[2012,Kona]
Move to Open.
THe issues is what to do with -1
. Should it match 'C' or do the "sane" thing.
A fix here changes behavior, but is probably what we want.
Pablo to provide wording, with help from Howard.
[2015-05-06 Lenexa: Move to Ready]
STL: I like that this uses strtof, which I think is new in C99. that avoids truncation from using atof. I have another issue ...
MC: yes LWG 2403 (stof should call strtof)
PJP: the last line is horrible, you don't assign to err, you call setstate(ios_base::failbit). Ah, no, this is inside num_get so the caller does the setstate.
MC: we need all these words. are they the right words?
JW: I'd like to take a minute to check my impl. Technically this implies a change in behaviour (from always using strtold and checking the extracted floating point value, to using the right function). Oh, we already do exactly this.
MC: Move to Ready
6 in favor, none opposed, 1 abstention
Proposed resolution:
Change 28.3.4.3.2.3 [facet.num.get.virtuals] as follows:
Stage 3: The sequence of
char
s accumulated in stage 2 (the field) is converted to a numeric value by the rules of one of the functions declared in the header<cstdlib>
:
- For a signed integer value, the function
strtoll
.- For an unsigned integer value, the function
strtoull
.- For a
float
value, the functionstrtof
.- For a
double
value, the functionstrtod
.- For a
floating-pointlong double
value, the functionstrtold
.The numeric value to be stored can be one of:
- zero, if the conversion function fails to convert the entire field.
ios_base::failbit
is assigned toerr
.- the most positive (or negative) representable value, if the field to be converted to a signed integer type represents a value too large positive (or negative) to be represented in
val
.ios_base::failbit
is assigned toerr
.the most negative representable value or zero for an unsigned integer type, if the field represents a value too large negative to be represented inval
.ios_base::failbit
is assigned toerr
.- the most positive representable value, if the field to be converted to an unsigned integer type represents a value that cannot be represented in
val
.- the converted value, otherwise.
The resultant numeric value is stored in
val
. If the conversion function fails to convert the entire field, or if the field represents a value outside the range of representable values,ios_base::failbit
is assigned toerr
.
Section: 27.1 [strings.general] Status: C++11 Submitter: Beman Dawes Opened: 2009-06-22 Last modified: 2016-01-28
Priority: Not Prioritized
View all other issues in [strings.general].
View all issues with C++11 status.
Discussion:
Addresses UK 218
Prior to the introduction of constant expressions into the library,
basic_string
elements had to be POD types, and thus had to be both trivially
copyable and standard-layout. This ensured that they could be memcpy'ed and
would be compatible with other libraries and languages, particularly the C
language and its library.
N2349,
Constant Expressions in the Standard Library Revision 2, changed the
requirement in 21/1 from "POD type" to "literal type". That change had the
effect of removing the trivially copyable and standard-layout requirements from
basic_string
elements.
This means that basic_string
elements no longer are guaranteed to be
memcpy'able, and are no longer guaranteed to be standard-layout types:
3.9/p2 and 3.9/p3 both make it clear that a "trivially copyable type" is required for memcpy to be guaranteed to work.
Literal types (3.9p12) may have a non-trivial copy assignment operator, and that violates the trivially copyable requirements given in 9/p 6, bullet item 2.
Literal types (3.9p12) have no standard-layout requirement, either.
This situation probably arose because the wording for "Constant Expressions in the Standard Library" was in process at the same time the C++ POD deconstruction wording was in process.
Since trivially copyable types meet the C++0x requirements for literal types,
and thus work with constant expressions, it seems an easy fix to revert the
basic_string
element wording to its original state.
[ 2009-07-28 Alisdair adds: ]
When looking for any resolution for this issue, consider the definition of "character container type" in 3.10 [defns.character.container]. This does require the character type to be a POD, and this term is used in a number of places through clause 21 and 28. This suggests the PODness constraint remains, but is much more subtle than before. Meanwhile, I suspect the change from POD type to literal type was intentional with the assumption that trivially copyable types with non-trivial-but-constexpr constructors should serve as well. I don't believe the current wording offers the right guarantees for either of the above designs.
[ 2009-11-04 Howard modifies proposed wording to disallow array types as char-like types. ]
[ 2010-01-23 Moved to Tentatively Ready after 5 positive votes on c++std-lib. ]
Proposed resolution:
Change General 27.1 [strings.general] as indicated:
This Clause describes components for manipulating sequences of any
literalnon-array POD (3.9) type. In this Clause such types are called char-like types, and objects of char-like types are called char-like objects or simply characters.
Section: 30.5 [time.duration] Status: C++11 Submitter: Alisdair Meredith Opened: 2009-07-06 Last modified: 2016-01-28
Priority: Not Prioritized
View all other issues in [time.duration].
View all issues with C++11 status.
Discussion:
The duration
types in 30.5 [time.duration] are exactly the sort of type
that should be "literal types" in the new standard. Likewise,
arithmetic operations on duration
s should be declared constexpr
.
[ 2009-09-21 Daniel adds: ]
An alternative (and possibly preferable solution for potentially heap-allocating
big_int
representation types) would be to ask the core language to allow references toconst
literal types as feasible arguments forconstexpr
functions.
[ 2009-10-30 Alisdair adds: ]
I suggest this issue moves from New to Open.
Half of this issue was dealt with in paper n2994 on constexpr constructors.
The other half (duration arithmetic) is on hold pending Core support for
const &
inconstexpr
functions.
[ 2010-03-15 Alisdair updated wording to be consistent with N3078. ]
[ 2010 Rapperswil: ]
This issue was the motivation for Core adding the facility for
constexpr
functions to take parameters byconst &
. Move to Tentatively Ready.
[ Adopted at 2010-11 Batavia. ]
Proposed resolution:
Add constexpr
to declaration of following functions and constructors:
Modify p1 30 [time], and the prototype definitions in 30.5.6 [time.duration.nonmember], 30.5.7 [time.duration.comparisons], and 30.5.8 [time.duration.cast]:
Header
<chrono>
synopsis// duration arithmetic template <class Rep1, class Period1, class Rep2, class Period2> typename common_type<duration<Rep1, Period1>, duration<Rep2, Period2>>::type constexpr operator+(const duration<Rep1, Period1>& lhs, const duration<Rep2, Period2>& rhs); template <class Rep1, class Period1, class Rep2, class Period2> typename common_type<duration<Rep1, Period1>, duration<Rep2, Period2>>::type constexpr operator-(const duration<Rep1, Period1>& lhs, const duration<Rep2, Period2>& rhs); template <class Rep1, class Period, class Rep2> duration<typename common_type<Rep1, Rep2>::type, Period> constexpr operator*(const duration<Rep1, Period>& d, const Rep2& s); template <class Rep1, class Period, class Rep2> duration<typename common_type<Rep1, Rep2>::type, Period> constexpr operator*(const Rep1& s, const duration<Rep2, Period>& d); template <class Rep1, class Period, class Rep2> duration<typename common_type<Rep1, Rep2>::type, Period> constexpr operator/(const duration<Rep1, Period>& d, const Rep2& s); template <class Rep1, class Period1, class Rep2, class Period2> typename common_type<Rep1, Rep2>::type constexpr operator/(const duration<Rep1, Period1>& lhs, const duration<Rep2, Period2>& rhs); // duration comparisons template <class Rep1, class Period1, class Rep2, class Period2> constexpr bool operator==(const duration<Rep1, Period1>& lhs, const duration<Rep2, Period2>& rhs); template <class Rep1, class Period1, class Rep2, class Period2> constexpr bool operator!=(const duration<Rep1, Period1>& lhs, const duration<Rep2, Period2>& rhs); template <class Rep1, class Period1, class Rep2, class Period2> constexpr bool operator< (const duration<Rep1, Period1>& lhs, const duration<Rep2, Period2>& rhs); template <class Rep1, class Period1, class Rep2, class Period2> constexpr bool operator<=(const duration<Rep1, Period1>& lhs, const duration<Rep2, Period2>& rhs); template <class Rep1, class Period1, class Rep2, class Period2> constexpr bool operator> (const duration<Rep1, Period1>& lhs, const duration<Rep2, Period2>& rhs); template <class Rep1, class Period1, class Rep2, class Period2> constexpr bool operator>=(const duration<Rep1, Period1>& lhs, const duration<Rep2, Period2>& rhs); // duration_cast template <class ToDuration, class Rep, class Period> constexpr ToDuration duration_cast(const duration<Rep, Period>& d);
Change 30.5 [time.duration]:
template <class Rep, class Period = ratio<1>> class duration { ... public: ... constexpr duration(const duration&) = default; ... };
[
Note - this edit already seems assumed by definition of the duration static members
zero/min/max
. They cannot meaningfully be constexpr
without
this change.
]
select_on_container_(copy|move)_construction
over-constrainedSection: 99 [allocator.concepts.members] Status: Resolved Submitter: Alberto Ganesh Barbati Opened: 2009-07-08 Last modified: 2016-01-28
Priority: Not Prioritized
View all issues with Resolved status.
Discussion:
I believe the two functions
select_on_container_(copy|move)_construction()
are over-constrained. For
example, the return value of the "copy" version is (see
99 [allocator.concepts.members]/21):
Returns:
x
if the allocator should propagate from the existing container to the new container on copy construction, otherwiseX()
.
Consider the case where a user decides to provide an explicit concept
map for Allocator to adapt some legacy allocator class, as he wishes to
provide customizations that the LegacyAllocator
concept map template
does not provide. Now, although it's true that the legacy class is
required to have a default constructor, the user might have reasons to
prefer a different constructor to implement
select_on_container_copy_construction()
. However, the current wording
requires the use of the default constructor.
Moreover, it's not said explicitly that x
is supposed to be the
allocator of the existing container. A clarification would do no harm.
[ 2009-10 Santa Cruz: ]
NAD EditorialResolved. Addressed by N2982.
Proposed resolution:
Replace 99 [allocator.concepts.members]/21 with:
X select_on_container_copy_construction(const X& x);-21- Returns:
an allocator object to be used by the new container on copy construction. [Note:x
if the allocator should propagate from the existing container to the new container on copy construction, otherwiseX()
.x
is the allocator of the existing container that is being copied. The most obvious choices for the return value arex
, if the allocator should propagate from the existing container, andX()
. — end note]
Replace 99 [allocator.concepts.members]/25 with:
X select_on_container_move_construction(X&& x);-25- Returns:
an allocator object to be used by the new container on move construction. [Note:move(x)
if the allocator should propagate from the existing container to the new container on move construction, otherwiseX()
.x
is the allocator of the existing container that is being moved. The most obvious choices for the return value aremove(x)
, if the allocator should propagate from the existing container, andX()
. — end note]
Section: 21.3.5.4 [meta.unary.prop] Status: Resolved Submitter: Jason Merrill Opened: 2009-07-16 Last modified: 2016-01-28
Priority: Not Prioritized
View other active issues in [meta.unary.prop].
View all other issues in [meta.unary.prop].
View all issues with Resolved status.
Discussion:
I've been implementing compiler support for is_standard_layout
, and
noticed a few nits about 21.3.5.4 [meta.unary.prop]:
has_trivial_assign
&&
has_trivial_copy_constructor
&& has_trivial_destructor
is similar, but
not identical, specifically with respect to const types.
has_trivial_copy_constructor
and has_trivial_assign
lack the "or an
array of such a class type" language that most other traits in that
section, including has_nothrow_copy_constructor
and has_nothrow_assign
,
have; this seems like an oversight.
[ See the thread starting with c++std-lib-24420 for further discussion. ]
[ Addressed in N2947. ]
[ 2009-10 Santa Cruz: ]
NAD EditorialResolved. Solved by N2984.
Proposed resolution:
Section: 30.5 [time.duration] Status: C++11 Submitter: Howard Hinnant Opened: 2009-07-18 Last modified: 2016-01-28
Priority: Not Prioritized
View all other issues in [time.duration].
View all issues with C++11 status.
Discussion:
"diagnostic required" has been used (by me) for code words meaning "use
enable_if
to constrain templated functions. This needs to be
improved by referring to the function signature as not participating in
the overload set, and moving this wording to a Remarks paragraph.
[ 2009-10 Santa Cruz: ]
Moved to Ready.
[ 2009-11-19 Pete opens: ]
Oh, and speaking of 1177, most of the changes result in rather convoluted prose. Instead of saying
A shall be B, else C
it should be
C if A is not B
That is:
Rep2
shall be implicitly convertible toCR(Rep1, Rep2)
, else this signature shall not participate in overload resolution.should be
This signature shall not participate in overload resolution if
Rep2
is not implicitly convertible toCR(Rep1, Rep2)
.That is clearer, and eliminates the false requirement that
Rep2
"shall be" convertible.
[ 2009-11-19 Howard adds: ]
I've updated the wording to match Pete's suggestion and included bullet 16 from 1195(i).
[ 2009-11-19 Jens adds: ]
Further wording suggestion using "unless":
This signature shall not participate in overload resolution unless
Rep2
is implicitly convertible toCR(Rep1, Rep2)
.
[ 2009-11-20 Howard adds: ]
I've updated the wording to match Jens' suggestion.
[ 2009-11-22 Moved to Tentatively Ready after 5 positive votes on c++std-lib. ]
Proposed resolution:
[ This proposed resolution addresses 947(i) and 974(i). ]
Change 30.5.2 [time.duration.cons] (and reorder the Remarks paragraphs per 16.3.2.4 [structure.specifications]):
template <class Rep2> explicit duration(const Rep2& r);
Requires:Remarks: This constructor shall not participate in overload resolution unlessRep2
shall beis implicitly convertible torep
and
treat_as_floating_point<rep>::value
shall beistrue
, ortreat_as_floating_point<Rep2>::value
shall beisfalse
.
Diagnostic required[Example:duration<int, milli> d(3); // OK duration<int, milli> d(3.5); // error— end example]
Effects: Constructs an object of type
duration
.Postcondition:
count() == static_cast<rep>(r)
.template <class Rep2, class Period2> duration(const duration<Rep2, Period2>& d);
Requires:Remarks: This constructor shall not participate in overload resolution unlesstreat_as_floating_point<rep>::value
shall beistrue
orratio_divide<Period2, period>::type::den
shall beis 1.Diagnostic required.[Note: This requirement prevents implicit truncation error when converting between integral-based duration types. Such a construction could easily lead to confusion about the value of the duration. — end note] [Example:duration<int, milli> ms(3); duration<int, micro> us = ms; // OK duration<int, milli> ms2 = us; // error— end example]
Effects: Constructs an object of type
duration
, constructingrep_
fromduration_cast<duration>(d).count()
.
Change the following paragraphs in 30.5.6 [time.duration.nonmember]:
template <class Rep1, class Period, class Rep2> duration<typename common_type<Rep1, Rep2>::type, Period> operator*(const duration<Rep1, Period>& d, const Rep2& s);
RequiresRemarks: This operator shall not participate in overload resolution unlessRep2
shall beis implicitly convertible toCR(Rep1, Rep2)
.Diagnostic required.template <class Rep1, class Period, class Rep2> duration<typename common_type<Rep1, Rep2>::type, Period> operator*(const Rep1& s, const duration<Rep2, Period>& d);
RequiresRemarks: This operator shall not participate in overload resolution unlessRep1
shall beis implicitly convertible toCR(Rep1, Rep2)
.Diagnostic required.template <class Rep1, class Period, class Rep2> duration<typename common_type<Rep1, Rep2>::type, Period> operator/(const duration<Rep1, Period>& d, const Rep2& s);
RequiresRemarks: This operator shall not participate in overload resolution unlessRep2
shall beis implicitly convertible toCR(Rep1, Rep2)
andRep2
shall not beis not an instantiation ofduration
.Diagnostic required.template <class Rep1, class Period, class Rep2> duration<typename common_type<Rep1, Rep2>::type, Period> operator%(const duration<Rep1, Period>& d, const Rep2& s);
RequiresRemarks: This operator shall not participate in overload resolution unlessRep2
shall beis implicitly convertible toCR(Rep1, Rep2)
andRep2
shall not beis not an instantiation ofduration
.Diagnostic required.
Change the following paragraphs in 30.5.8 [time.duration.cast]:
template <class ToDuration, class Rep, class Period> ToDuration duration_cast(const duration<Rep, Period>& d);
RequiresRemarks: This function shall not participate in overload resolution unlessToDuration
shall beis an instantiation ofduration
.Diagnostic required.
Change 30.6.2 [time.point.cons]/3 as indicated:
Requires:Duration2
shall be implicitly convertible toduration
. Diagnostic required.Remarks: This constructor shall not participate in overload resolution unless
Duration2
is implicitly convertible toduration
.
Change the following paragraphs in 30.6.8 [time.point.cast]:
template <class ToDuration, class Clock, class Duration> time_point<Clock, ToDuration> time_point_cast(const time_point<Clock, Duration>& t);
RequiresRemarks: This function shall not participate in overload resolution unlessToDuration
shall beis an instantiation ofduration
.Diagnostic required.
Section: 16.4.6.2 [res.on.headers] Status: C++11 Submitter: Beman Dawes Opened: 2009-07-18 Last modified: 2016-01-28
Priority: Not Prioritized
View all issues with C++11 status.
Discussion:
See Frankfurt notes of 1001(i).
Proposed resolution:
Change 16.4.6.2 [res.on.headers], Headers, paragraph 1, as indicated:
A C++ header may include other C++ headers.
[footnote]A C++ header shall provide the declarations and definitions that appear in its synopsis (6.3 [basic.def.odr]). A C++ header shown in its synopsis as including other C++ headers shall provide the declarations and definitions that appear in the synopses of those other headers.
[footnote] C++ headers must include a C++ header that contains any needed definition (3.2).
string_type
member typedef in class sub_match
Section: 28.6.8.2 [re.submatch.members] Status: C++11 Submitter: Daniel Krügler Opened: 2009-07-25 Last modified: 2016-01-28
Priority: Not Prioritized
View all issues with C++11 status.
Discussion:
The definition of class template sub_match
is strongly dependent
on the type basic_string<value_type>
, both in interface and effects,
but does not provide a corresponding typedef string_type
, as e.g.
class match_results
does, which looks like an oversight to me that
should be fixed.
[ 2009-11-15 Moved to Tentatively Ready after 5 positive votes on c++std-lib. ]
Proposed resolution:
In the class template sub_match
synopsis 28.6.8 [re.submatch]/1
change as indicated:
template <class BidirectionalIterator> class sub_match : public std::pair<BidirectionalIterator, BidirectionalIterator> { public: typedef typename iterator_traits<BidirectionalIterator>::value_type value_type; typedef typename iterator_traits<BidirectionalIterator>::difference_type difference_type; typedef BidirectionalIterator iterator; typedef basic_string<value_type> string_type; bool matched; difference_type length() const; operatorbasic_string<value_type>string_type() const;basic_string<value_type>string_type str() const; int compare(const sub_match& s) const; int compare(constbasic_string<value_type>string_type& s) const; int compare(const value_type* s) const; };
In 28.6.8.2 [re.submatch.members]/2 change as indicated:
operatorbasic_string<value_type>string_type() const;Returns:
matched ?
.basic_string<value_type>string_type(first, second) :basic_string<value_type>string_type()
In 28.6.8.2 [re.submatch.members]/3 change as indicated:
basic_string<value_type>string_type str() const;Returns:
matched ?
.basic_string<value_type>string_type(first, second) :basic_string<value_type>string_type()
In 28.6.8.2 [re.submatch.members]/5 change as indicated:
int compare(constbasic_string<value_type>string_type& s) const;
sub_match
comparison operatorsSection: 28.6.8.3 [re.submatch.op] Status: C++11 Submitter: Daniel Krügler Opened: 2009-07-25 Last modified: 2016-01-28
Priority: Not Prioritized
View all other issues in [re.submatch.op].
View all issues with C++11 status.
Discussion:
Several heterogeneous comparison operators of class template
sub_match
are specified by return clauses that are not valid
in general. E.g. 28.6.8.3 [re.submatch.op]/7:
template <class BiIter, class ST, class SA> bool operator==( const basic_string< typename iterator_traits<BiIter>::value_type, ST, SA>& lhs, const sub_match<BiIter>& rhs);Returns:
lhs == rhs.str()
.
The returns clause would be ill-formed for all cases where
ST != std::char_traits<iterator_traits<BiIter>::value_type>
or SA != std::allocator<iterator_traits<BiIter>::value_type>
.
The generic character of the comparison was intended, so
there are basically two approaches to fix the problem: The
first one would define the semantics of the comparison
using the traits class ST
(The semantic of basic_string::compare
is defined in terms of the compare function of the corresponding
traits class), the second one would define the semantics of the
comparison using the traits class
std::char_traits<iterator_traits<BiIter>::value_type>
which is essentially identical to
std::char_traits<sub_match<BiIter>::value_type>
I suggest to follow the second approach, because
this emphasizes the central role of the sub_match
object as part of the comparison and would also
make sure that a sub_match
comparison using some
basic_string<char_t, ..>
always is equivalent to
a corresponding comparison with a string literal
because of the existence of further overloads (beginning
from 28.6.8.3 [re.submatch.op]/19). If users really want to
take advantage of their own traits::compare
, they can
simply write a corresponding compare function that
does so.
[ Post-Rapperswil ]
The following update is a result of the discussion during the Rapperswil meeting, the P/R expresses all comparisons by
delegating to sub_match's compare functions. The processing is rather mechanical: Only ==
and <
where defined by referring to sub_match
's compare function, all remaining ones where replaced by the canonical
definitions in terms of these two.
Moved to Tentatively Ready after 5 positive votes on c++std-lib.
[ Adopted at 2010-11 Batavia ]
Proposed resolution:
The wording refers to N3126.
template <class BiIter, class ST, class SA> bool operator==( const basic_string< typename iterator_traits<BiIter>::value_type, ST, SA>& lhs, const sub_match<BiIter>& rhs);7 Returns:
.
lhs == rhs.str()rhs.compare(lhs.c_str()) == 0
template <class BiIter, class ST, class SA> bool operator!=( const basic_string< typename iterator_traits<BiIter>::value_type, ST, SA>& lhs, const sub_match<BiIter>& rhs);8 Returns:
.
lhs != rhs.str()!(lhs == rhs)
template <class BiIter, class ST, class SA> bool operator<( const basic_string< typename iterator_traits<BiIter>::value_type, ST, SA>& lhs, const sub_match<BiIter>& rhs);9 Returns:
.
lhs < rhs.str()rhs.compare(lhs.c_str()) > 0
template <class BiIter, class ST, class SA> bool operator>( const basic_string< typename iterator_traits<BiIter>::value_type, ST, SA>& lhs, const sub_match<BiIter>& rhs);10 Returns:
.
lhs > rhs.str()rhs < lhs
template <class BiIter, class ST, class SA> bool operator>=( const basic_string< typename iterator_traits<BiIter>::value_type, ST, SA>& lhs, const sub_match<BiIter>& rhs);11 Returns:
.
lhs >= rhs.str()!(lhs < rhs)
template <class BiIter, class ST, class SA> bool operator<=( const basic_string< typename iterator_traits<BiIter>::value_type, ST, SA>& lhs, const sub_match<BiIter>& rhs);12 Returns:
.
lhs <= rhs.str()!(rhs < lhs)
template <class BiIter, class ST, class SA> bool operator==(const sub_match<BiIter>& lhs, const basic_string< typename iterator_traits<BiIter>::value_type, ST, SA>& rhs);13 Returns:
.
lhs.str() == rhslhs.compare(rhs.c_str()) == 0
template <class BiIter, class ST, class SA> bool operator!=(const sub_match<BiIter>& lhs, const basic_string< typename iterator_traits<BiIter>::value_type, ST, SA>& rhs);14 Returns:
.
lhs.str() != rhs!(lhs == rhs)
template <class BiIter, class ST, class SA> bool operator<(const sub_match<BiIter>& lhs, const basic_string< typename iterator_traits<BiIter>::value_type, ST, SA>& rhs);15 Returns:
.
lhs.str() < rhslhs.compare(rhs.c_str()) < 0
template <class BiIter, class ST, class SA> bool operator>(const sub_match<BiIter>& lhs, const basic_string< typename iterator_traits<BiIter>::value_type, ST, SA>& rhs);16 Returns:
.
lhs.str() > rhsrhs < lhs
template <class BiIter, class ST, class SA> bool operator>=(const sub_match<BiIter>& lhs, const basic_string< typename iterator_traits<BiIter>::value_type, ST, SA>& rhs);17 Returns:
.
lhs.str() >= rhs!(lhs < rhs)
template <class BiIter, class ST, class SA> bool operator<=(const sub_match<BiIter>& lhs, const basic_string< typename iterator_traits<BiIter>::value_type, ST, SA>& rhs);18 Returns:
.
lhs.str() <= rhs!(rhs < lhs)
template <class BiIter> bool operator==(typename iterator_traits<BiIter>::value_type const* lhs, const sub_match<BiIter>& rhs);19 Returns:
.
lhs == rhs.str()rhs.compare(lhs) == 0
template <class BiIter> bool operator!=(typename iterator_traits<BiIter>::value_type const* lhs, const sub_match<BiIter>& rhs);20 Returns:
.
lhs != rhs.str()!(lhs == rhs)
template <class BiIter> bool operator<(typename iterator_traits<BiIter>::value_type const* lhs, const sub_match<BiIter>& rhs);21 Returns:
.
lhs < rhs.str()rhs.compare(lhs) > 0
template <class BiIter> bool operator>(typename iterator_traits<BiIter>::value_type const* lhs, const sub_match<BiIter>& rhs);22 Returns:
.
lhs > rhs.str()rhs < lhs
template <class BiIter> bool operator>=(typename iterator_traits<BiIter>::value_type const* lhs, const sub_match<BiIter>& rhs);23 Returns:
.
lhs >= rhs.str()!(lhs < rhs)
template <class BiIter> bool operator<=(typename iterator_traits<BiIter>::value_type const* lhs, const sub_match<BiIter>& rhs);24 Returns:
.
lhs <= rhs.str()!(rhs < lhs)
template <class BiIter> bool operator==(const sub_match<BiIter>& lhs, typename iterator_traits<BiIter>::value_type const* rhs);25 Returns:
.
lhs.str() == rhslhs.compare(rhs) == 0
template <class BiIter> bool operator!=(const sub_match<BiIter>& lhs, typename iterator_traits<BiIter>::value_type const* rhs);26 Returns:
.
lhs.str() != rhs!(lhs == rhs)
template <class BiIter> bool operator<(const sub_match<BiIter>& lhs, typename iterator_traits<BiIter>::value_type const* rhs);27 Returns:
.
lhs.str() < rhslhs.compare(rhs) < 0
template <class BiIter> bool operator>(const sub_match<BiIter>& lhs, typename iterator_traits<BiIter>::value_type const* rhs);28 Returns:
.
lhs.str() > rhsrhs < lhs
template <class BiIter> bool operator>=(const sub_match<BiIter>& lhs, typename iterator_traits<BiIter>::value_type const* rhs);29 Returns:
.
lhs.str() >= rhs!(lhs < rhs)
template <class BiIter> bool operator<=(const sub_match<BiIter>& lhs, typename iterator_traits<BiIter>::value_type const* rhs);30 Returns:
.
lhs.str() <= rhs!(rhs < lhs)
template <class BiIter> bool operator==(typename iterator_traits<BiIter>::value_type const& lhs, const sub_match<BiIter>& rhs);
31 Returns:basic_string<typename iterator_traits<BiIter>::value_type>(1, lhs) == rhs.str()
.
31 Returns:rhs.compare(typename sub_match<BiIter>::string_type(1, lhs)) == 0
.
template <class BiIter> bool operator!=(typename iterator_traits<BiIter>::value_type const& lhs, const sub_match<BiIter>& rhs);32 Returns:
.
basic_string<typename iterator_traits<BiIter>::value_type>(1, lhs) != rhs.str()!(lhs == rhs)
template <class BiIter> bool operator<(typename iterator_traits<BiIter>::value_type const& lhs, const sub_match<BiIter>& rhs);
33 Returns:basic_string<typename iterator_traits<BiIter>::value_type>(1, lhs) < rhs.str()
.
33 Returns:rhs.compare(typename sub_match<BiIter>::string_type(1, lhs)) > 0
.
template <class BiIter> bool operator>(typename iterator_traits<BiIter>::value_type const& lhs, const sub_match<BiIter>& rhs);34 Returns:
.
basic_string<typename iterator_traits<BiIter>::value_type>(1, lhs) > rhs.str()rhs < lhs
template <class BiIter> bool operator>=(typename iterator_traits<BiIter>::value_type const& lhs, const sub_match<BiIter>& rhs);35 Returns:
.
basic_string<typename iterator_traits<BiIter>::value_type>(1, lhs) >= rhs.str()!(lhs < rhs)
template <class BiIter> bool operator<=(typename iterator_traits<BiIter>::value_type const& lhs, const sub_match<BiIter>& rhs);36 Returns:
.
basic_string<typename iterator_traits<BiIter>::value_type>(1, lhs) <= rhs.str()!(rhs < lhs)
template <class BiIter> bool operator==(const sub_match<BiIter>& lhs, typename iterator_traits<BiIter>::value_type const& rhs);
37 Returns:lhs.str() == basic_string<typename iterator_traits<BiIter>::value_type>(1, rhs)
.
37 Returns:lhs.compare(typename sub_match<BiIter>::string_type(1, rhs)) == 0
.
template <class BiIter> bool operator!=(const sub_match<BiIter>& lhs, typename iterator_traits<BiIter>::value_type const& rhs);38 Returns:
.
lhs.str() != basic_string<typename iterator_traits<BiIter>::value_type>(1, rhs)!(lhs == rhs)
template <class BiIter> bool operator<(const sub_match<BiIter>& lhs, typename iterator_traits<BiIter>::value_type const& rhs);
39 Returns:lhs.str() < basic_string<typename iterator_traits<BiIter>::value_type>(1, rhs)
.
39 Returns:lhs.compare(typename sub_match<BiIter>::string_type(1, rhs)) < 0
.
template <class BiIter> bool operator>(const sub_match<BiIter>& lhs, typename iterator_traits<BiIter>::value_type const& rhs);40 Returns:
.
lhs.str() > basic_string<typename iterator_traits<BiIter>::value_type>(1, rhs)rhs < lhs
template <class BiIter> bool operator>=(const sub_match<BiIter>& lhs, typename iterator_traits<BiIter>::value_type const& rhs);41 Returns:
.
lhs.str() >= basic_string<typename iterator_traits<BiIter>::value_type>(1, rhs)!(lhs < rhs)
template <class BiIter> bool operator<=(const sub_match<BiIter>& lhs, typename iterator_traits<BiIter>::value_type const& rhs);42 Returns:
.
lhs.str() <= basic_string<typename iterator_traits<BiIter>::value_type>(1, rhs)!(rhs < lhs)
Section: 22.10.19 [unord.hash] Status: C++11 Submitter: Alisdair Meredith Opened: 2009-07-28 Last modified: 2016-01-28
Priority: Not Prioritized
View all other issues in [unord.hash].
View all issues with C++11 status.
Discussion:
Addresses UK 324
The implied library dependencies created by spelling out all the hash
template specializations in the <functional>
synopsis are unfortunate.
The potential coupling is greatly reduced if the hash
specialization is
declared in the appropriate header for each library type, as it is much
simpler to forward declare the primary template and provide a single
specialization than it is to implement a hash
function for a string
or
vector
without providing a definition for the whole string/vector
template in order to access the necessary bits.
Note that the proposed resolution purely involves moving the declarations of a few specializations, it specifically does not make any changes to 22.10.19 [unord.hash].
[ 2009-09-15 Daniel adds: ]
I suggest to add to the current existing proposed resolution the following items.
Add to the very first strike-list of the currently suggested resolution the following lines:
template <> struct hash<std::error_code>;template <> struct hash<std::thread::id>;
Add the following declarations to 19.5 [syserr], header
<system_error>
synopsis after // 19.5.4:
// 19.5.x hash support template <class T> struct hash; template <> struct hash<error_code>;
Add a new clause 19.5.X (probably after 19.5.4):
19.5.X Hash support [syserr.hash]
template <> struct hash<error_code>;An explicit specialization of the class template hash (22.10.19 [unord.hash]) shall be provided for the type
error_code
suitable for using this type as key in unordered associative containers (23.5 [unord]).
Add the following declarations to 32.4.3.2 [thread.thread.id] just after the declaration of the comparison operators:
template <class T> struct hash; template <> struct hash<thread::id>;
Add a new paragraph at the end of 32.4.3.2 [thread.thread.id]:
template <> struct hash<thread::id>;An explicit specialization of the class template hash (22.10.19 [unord.hash]) shall be provided for the type
thread::id
suitable for using this type as key in unordered associative containers (23.5 [unord]).
std::hash<std::thread::id>
to header <thread>
.
[ 2009-11-13 Alisdair adopts Daniel's suggestion and the extended note from 889(i). ]
[ 2010-01-31 Alisdair: related to 1245(i) and 978(i). ]
[ 2010-02-07 Proposed wording updated by Beman, Daniel, Alisdair and Ganesh. ]
[ 2010-02-09 Moved to Tentatively Ready after 5 positive votes on c++std-lib. ]
Proposed resolution:
Strike the following specializations declared in the <functional>
synopsis p2 22.10 [function.objects]
template <> struct hash<std::string>;template <> struct hash<std::u16string>;template <> struct hash<std::u32string>;template <> struct hash<std::wstring>;template <> struct hash<std::error_code>;template <> struct hash<std::thread::id>;template <class Allocator> struct hash<std::vector<bool, Allocator> >;template <std::size_t N> struct hash<std::bitset<N> >;
Add the following at the end of 22.10.19 [unord.hash]:
template <> struct hash<bool>; template <> struct hash<char>; template <> struct hash<signed char>; template <> struct hash<unsigned char>; template <> struct hash<char16_t>; template <> struct hash<char32_t>; template <> struct hash<wchar_t>; template <> struct hash<short>; template <> struct hash<unsigned short>; template <> struct hash<int>; template <> struct hash<unsigned int>; template <> struct hash<long>; template <> struct hash<long long>; template <> struct hash<unsigned long>; template <> struct hash<unsigned long long>; template <> struct hash<float>; template <> struct hash<double>; template <> struct hash<long double>; template<class T> struct hash<T*>;Specializations meeting the requirements of class template
hash
22.10.19 [unord.hash].
Add the following declarations to 19.5 [syserr], header <system_error>
synopsis after // 19.5.4:
// [syserr.hash] hash support template <class T> struct hash; template <> struct hash<error_code>;
Add a new clause 19.5.X (probably after 19.5.4):
19.5.X Hash support [syserr.hash]
template <> struct hash<error_code>;Specialization meeting the requirements of class template
hash
22.10.19 [unord.hash].
Add the following declarations to the synopsis of <string>
in 27.4 [string.classes]
// [basic.string.hash] hash support template <class T> struct hash; template <> struct hash<string>; template <> struct hash<u16string>; template <> struct hash<u32string>; template <> struct hash<wstring>;
Add a new clause 21.4.X
21.4.X Hash support [basic.string.hash]>
template <> struct hash<string>; template <> struct hash<u16string>; template <> struct hash<u32string>; template <> struct hash<wstring>;Specializations meeting the requirements of class template
hash
22.10.19 [unord.hash].
Add the following declarations to the synopsis of <vector>
in
23.3 [sequences]
// 21.4.x hash support template <class T> struct hash; template <class Allocator> struct hash<vector<bool, Allocator>>;
Add a new paragraph to the end of 23.3.14 [vector.bool]
template <class Allocator> struct hash<vector<bool, Allocator>>;Specialization meeting the requirements of class template
hash
22.10.19 [unord.hash].
Add the following declarations to the synopsis of <bitset>
in 22.9.2 [template.bitset]
// [bitset.hash] hash support template <class T> struct hash; template <size_t N> struct hash<bitset<N> >;
Add a new subclause 20.3.7.X [bitset.hash]
20.3.7.X bitset hash support [bitset.hash]
template <size_t N> struct hash<bitset<N> >;Specialization meeting the requirements of class template
hash
22.10.19 [unord.hash].
Add the following declarations to 32.4.3.2 [thread.thread.id] synopsis just after the declaration of the comparison operators:
template <class T> struct hash; template <> struct hash<thread::id>;
Add a new paragraph at the end of 32.4.3.2 [thread.thread.id]:
template <> struct hash<thread::id>;Specialization meeting the requirements of class template
hash
22.10.19 [unord.hash].
Change Header <typeindex> synopsis 17.7.6 [type.index.synopsis] as indicated:
namespace std { class type_index; // [type.index.hash] hash support template <class T> struct hash; template<> struct hash<type_index>;: public unary_function<type_index, size_t> { size_t operator()(type_index index) const; }}
Change Template specialization hash<type_index> [type.index.templ] as indicated:
20.11.4
Template specialization hash<type_index> [type.index.templ]Hash support [type.index.hash]size_t operator()(type_index index) const;
Returns:index.hash_code()
template<> struct hash<type_index>;Specialization meeting the requirements of class template
hash
[unord.hash]. For an objectindex
of typetype_index
,hash<type_index>()(index)
shall evaluate to the same value asindex.hash_code()
.
basic_ios::set_rdbuf
may break class invariantsSection: 31.5.4.3 [basic.ios.members] Status: C++11 Submitter: Daniel Krügler Opened: 2009-07-28 Last modified: 2016-01-28
Priority: Not Prioritized
View all other issues in [basic.ios.members].
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Discussion:
The protected member function set_rdbuf
had been added during the
process of adding move and swap semantics to IO classes. A relevant
property of this function is described by it's effects in
31.5.4.3 [basic.ios.members]/19:
Effects: Associates the
basic_streambuf
object pointed to by sb with this stream without callingclear()
.
This means that implementors of or those who derive from existing IO classes
could cause an internal state where the stream buffer could be 0, but the
IO class has the state good()
. This would break several currently existing
implementations which rely on the fact that setting a stream buffer via the
currently only ways, i.e. either by calling
void init(basic_streambuf<charT,traits>* sb);
or by calling
basic_streambuf<charT,traits>* rdbuf(basic_streambuf<charT,traits>* sb);
to set rdstate()
to badbit
, if the buffer is 0. This has the effect that many
internal functions can simply check rdstate()
instead of rdbuf()
for being 0.
I therefore suggest that a requirement is added for callers of set_rdbuf
to
set a non-0 value.
[ 2009-10 Santa Cruz: ]
Moved to Open. Martin volunteers to provide new wording, where
set_rdbuf()
sets thebadbit
but does not cause an exception to be thrown like a call toclear()
would.
[ 2009-10-20 Martin provides wording: ]
Change 31.5.4.3 [basic.ios.members] around p. 19 as indicated:
void set_rdbuf(basic_streambuf<charT, traits>* sb);
Effects: Associates thebasic_streambuf
object pointed to bysb
with this stream without callingclear()
. Postconditions:rdbuf() == sb
.Effects: As if:
iostate state = rdstate(); try { rdbuf(sb); } catch(ios_base::failure) { if (0 == (state & ios_base::badbit)) unsetf(badbit); }Throws: Nothing.
Rationale:
We need to be able to call set_rdbuf()
on stream objects
for which (rdbuf() == 0
) holds without causing ios_base::failure
to
be thrown. We also don't want badbit
to be set as a result of
setting rdbuf()
to 0 if it wasn't set before the call. This changed
Effects clause maintains the current behavior (as of N2914) without
requiring that sb
be non-null.
[ Post-Rapperswil ]
Several reviewers and the submitter believe that the best solution would be to add a pre-condition that the buffer shall not be a null pointer value.
Moved to Tentatively Ready with revised wording provided by Daniel after 5 positive votes on c++std-lib.
[ Adopted at 2010-11 Batavia ]
Proposed resolution:
void set_rdbuf(basic_streambuf<charT, traits>* sb);?? Requires:
23 Effects: Associates thesb != nullptr
.basic_streambuf
object pointed to bysb
with this stream without callingclear()
.24 Postconditions:
rdbuf() == sb
.25 Throws: Nothing.
Rationale:
We believe that setting a nullptr
stream buffer can be prevented.
Section: 24.3 [iterator.requirements] Status: Resolved Submitter: Alisdair Meredith Opened: 2009-07-31 Last modified: 2016-01-28
Priority: Not Prioritized
View all other issues in [iterator.requirements].
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Discussion:
(wording relative to N2723 pending new working paper)
According to p3 24.3 [iterator.requirements], Forward iterators, Bidirectional iterators and Random Access iterators all satisfy the requirements for an Output iterator:
XXX iterators satisfy all the requirements of the input and output iterators and can be used whenever either kind is specified ...
Meanwhile, p4 goes on to contradict this:
Besides its category, a forward, bidirectional, or random access iterator can also be mutable or constant...
... Constant iterators do not satisfy the requirements for output iterators
The latter seems to be the overriding concern, as the iterator tag
hierarchy does not define forward_iterator_tag
as multiply derived from
both input_iterator_tag
and output_iterator_tag
.
The work on concepts for iterators showed us that output iterator really is fundamentally a second dimension to the iterator categories, rather than part of the linear input -> forward -> bidirectional -> random-access sequence. It would be good to clear up these words to reflect that, and separately list output iterator requirements in the requires clauses for the appropriate algorithms and operations.
[
2010 Pittsburgh: Moved to NAD EditorialResolved. Rationale added below.
]
Rationale:
Solved by N3066.
Proposed resolution:
std::decay
Section: 21.3.8.7 [meta.trans.other] Status: C++11 Submitter: Jason Merrill Opened: 2009-08-07 Last modified: 2016-01-28
Priority: Not Prioritized
View all other issues in [meta.trans.other].
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Discussion:
I notice that std::decay
is specified to strip the cv-quals from
anything but an array or pointer. This seems incorrect for values of
class type, since class rvalues can have cv-qualified type (7.2.1 [basic.lval]/9).
[ 2009-08-09 Howard adds: ]
See the thread starting with c++std-lib-24568 for further discussion. And here is a convenience link to the original proposal. Also see the closely related issue 705(i).
[ 2010 Pittsburgh: Moved to Ready. ]
Proposed resolution:
Add a note to decay
in 21.3.8.7 [meta.trans.other]:
[Note: This behavior is similar to the lvalue-to-rvalue (4.1), array-to-pointer (4.2), and function-to-pointer (4.3) conversions applied when an lvalue expression is used as an rvalue, but also strips cv-qualifiers from class types in order to more closely model by-value argument passing. — end note]
Section: 23.2.8 [unord.req], 23.5 [unord] Status: C++11 Submitter: Matt Austern Opened: 2009-08-10 Last modified: 2016-01-28
Priority: Not Prioritized
View other active issues in [unord.req].
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Discussion:
Consider a typical use case: I create an unordered_map
and then start
adding elements to it one at a time. I know that it will eventually need
to store a few million elements, so, for performance reasons, I would
like to reserve enough capacity that none of the calls to insert
will
trigger a rehash.
Unfortunately, the existing interface makes this awkward. The user
naturally sees the problem in terms of the number of elements, but the
interface presents it as buckets. If m
is the map and n
is the expected
number of elements, this operation is written m.rehash(n /
m.max_load_factor())
— not very novice friendly.
[ 2009-09-30 Daniel adds: ]
I recommend to replace "
resize
" by a different name like "reserve
", because that would better match the intended use-case. Rational: Any existing resize function has the on-success post-condition that the provided size is equal tosize()
, which is not satisfied for the proposal. Reserve seems to fit the purpose of the actual renaming suggestion.
[ 2009-10-28 Ganesh summarizes alternative resolutions and expresses a strong preference for the second (and opposition to the first): ]
In the unordered associative container requirements (23.2.8 [unord.req]), remove the row for rehash and replace it with:
Table 87 — Unordered associative container requirements (in addition to container) Expression Return type Assertion/note pre-/post-condition Complexity a.
rehashreserve(n)void
Post: a.bucket_count > max(a.size(), n) / a.max_load_factor()
and.a.bucket_count() >= n
Average case linear in a.size()
, worst case quadratic.Make the corresponding change in the class synopses in 23.5.3 [unord.map], 23.5.4 [unord.multimap], 23.5.6 [unord.set], and 23.5.7 [unord.multiset].
In 23.2.8 [unord.req]/9, table 98, append a new row after the last one:
Table 87 — Unordered associative container requirements (in addition to container) Expression Return type Assertion/note pre-/post-condition Complexity a.rehash(n)
void
Post: a.bucket_count > a.size() / a.max_load_factor()
anda.bucket_count() >= n
.Average case linear in a.size()
, worst case quadratic.a.reserve(n)
void
Same as a.rehash(ceil(n / a.max_load_factor()))
Average case linear in a.size()
, worst case quadratic.In 23.5.3 [unord.map]/3 in the definition of class template
unordered_map
, in 23.5.4 [unord.multimap]/3 in the definition of class templateunordered_multimap
, in 23.5.6 [unord.set]/3 in the definition of class templateunordered_set
and in 23.5.7 [unord.multiset]/3 in the definition of class templateunordered_multiset
, add the following line after member functionrehash()
:void reserve(size_type n);
[ 2009-10-28 Howard: ]
Moved to Tentatively Ready after 5 votes in favor of Ganesh's option 2 above. The original proposed wording now appears here:
Informally: instead of providing
rehash(n)
provideresize(n)
, with the semantics "make the container a good size forn
elements".In the unordered associative container requirements (23.2.8 [unord.req]), remove the row for rehash and replace it with:
Table 87 — Unordered associative container requirements (in addition to container) Expression Return type Assertion/note pre-/post-condition Complexity a.
rehashresize(n)void
Post: a.bucket_count > max(a.size(), n) / a.max_load_factor()
and.a.bucket_count() >= n
Average case linear in a.size()
, worst case quadratic.Make the corresponding change in the class synopses in 23.5.3 [unord.map], 23.5.4 [unord.multimap], 23.5.6 [unord.set], and 23.5.7 [unord.multiset].
Proposed resolution:
In 23.2.8 [unord.req]/9, table 98, append a new row after the last one:
Table 87 — Unordered associative container requirements (in addition to container) Expression Return type Assertion/note pre-/post-condition Complexity a.rehash(n)
void
Post: a.bucket_count > a.size() / a.max_load_factor()
anda.bucket_count() >= n
.Average case linear in a.size()
, worst case quadratic.a.reserve(n)
void
Same as a.rehash(ceil(n / a.max_load_factor()))
Average case linear in a.size()
, worst case quadratic.
In 23.5.3 [unord.map]/3 in the definition of class template unordered_map
, in
23.5.4 [unord.multimap]/3 in the definition of class template unordered_multimap
, in
23.5.6 [unord.set]/3 in the definition of class template unordered_set
and in
23.5.7 [unord.multiset]/3 in the definition of class template unordered_multiset
, add the
following line after member function rehash()
:
void reserve(size_type n);
tuple get
API should respect rvaluesSection: 22.4.8 [tuple.elem] Status: C++11 Submitter: Alisdair Meredith Opened: 2009-08-18 Last modified: 2016-01-28
Priority: Not Prioritized
View all other issues in [tuple.elem].
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Discussion:
The tuple get
API should respect rvalues. This would allow for moving a
single element out of a tuple
-like type.
[ 2009-10-30 Alisdair adds: ]
The issue of rvalue overloads of get for tuple-like types was briefly discussed in Santa Cruz.
The feedback was this would be welcome, but we need full wording for the other types (
pair
andarray
) before advancing.I suggest the issue moves to Open from New as it has been considered, feedback given, and it has not (yet) been rejected as NAD.
[ 2010 Rapperswil: ]
Note that wording has been provided, and this issue becomes more important now that we have added a function to support forwarding argument lists as
tuple
s. Move to Tentatively Ready.
[ Adopted at 2010-11 Batavia ]
Proposed resolution:
Add the following signature to p2 22.4.1 [tuple.general]
template <size_t I, class ... Types> typename tuple_element<I, tuple<Types...> >::type&& get(tuple<Types...> &&);
And again to 22.4.8 [tuple.elem].
template <size_t I, class ... Types> typename tuple_element<I, tuple<Types...> >::type&& get(tuple<Types...>&& t);Effects: Equivalent to
return std::forward<typename tuple_element<I, tuple<Types...> >::type&&>(get<I>(t));
[Note: If a
T
inTypes
is some reference typeX&
, the return type isX&
, notX&&
. However, if the element type is non-reference typeT
, the return type isT&&
. — end note]
Add the following signature to p1 22.2 [utility]
template <size_t I, class T1, class T2> typename tuple_element<I, pair<T1,T2> >::type&& get(pair<T1, T2>&&);
And to p5 22.3.4 [pair.astuple]
template <size_t I, class T1, class T2> typename tuple_element<I, pair<T1,T2> >::type&& get(pair<T1, T2>&& p);Returns: If
I == 0
returnsstd::forward<T1&&>(p.first)
; ifI == 1
returnsstd::forward<T2&&>(p.second)
; otherwise the program is ill-formed.Throws: Nothing.
Add the following signature to 23.3 [sequences] <array>
synopsis
template <size_t I, class T, size_t N> T&& get(array<T,N> &&);
And after p8 23.3.3.7 [array.tuple]
template <size_t I, class T, size_t N> T&& get(array<T,N> && a);Effects: Equivalent to
return std::move(get<I>(a));
basic_string
missing definitions for cbegin
/ cend
/ crbegin
/ crend
Section: 27.4.3.4 [string.iterators] Status: C++11 Submitter: Jonathan Wakely Opened: 2009-08-14 Last modified: 2016-01-28
Priority: Not Prioritized
View all issues with C++11 status.
Discussion:
Unlike the containers in clause 23, basic_string
has definitions for
begin()
and end()
, but these have not been updated to include cbegin
,
cend
, crbegin
and crend
.
[ 2009-10-28 Howard: ]
Moved to Tentatively NAD after 5 positive votes on c++std-lib. Added rationale.
[ 2009-10-28 Alisdair disagrees: ]
I'm going to have to speak up as the dissenting voice.
I agree the issue could be handled editorially, and that would be my preference if Pete feels this is appropriate. Failing that, I really think this issue should be accepted and moved to ready. The other begin/end functions all have a semantic definition for this template, and it is confusing if a small few are missing.
I agree that an alternative would be to strike all the definitions for
begin/end/rbegin/rend
and defer completely to the requirements tables in clause 23. I think that might be confusing without a forward reference though, as those tables are defined in a later clause than thebasic_string
template itself. If someone wants to pursue this I would support it, but recommend it as a separate issue.So my preference is strongly to move Ready over NAD, and a stronger preference for NAD Editorial if Pete is happy to make these changes.
[ 2009-10-29 Howard: ]
Moved to Tentatively Ready after 5 positive votes on c++std-lib. Removed rationale to mark it NAD. :-)
Proposed resolution:
Add to 27.4.3.4 [string.iterators]
iterator begin(); const_iterator begin() const; const_iterator cbegin() const;...
iterator end(); const_iterator end() const; const_iterator cend() const;...
reverse_iterator rbegin(); const_reverse_iterator rbegin() const; const_reverse_iterator crbegin() const;...
reverse_iterator rend(); const_reverse_iterator rend() const; const_reverse_iterator crend() const;
default_delete
cannot be instantiated with incomplete typesSection: 20.3.1.2 [unique.ptr.dltr] Status: C++11 Submitter: Daniel Krügler Opened: 2009-08-18 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
According to the general rules of 16.4.5.8 [res.on.functions] p 2 b 5 the effects are undefined, if an incomplete type is used to instantiate a library template. But neither in 20.3.1.2 [unique.ptr.dltr] nor in any other place of the standard such explicit allowance is given. Since this template is intended to be instantiated with incomplete types, this must be fixed.
[ 2009-11-15 Moved to Tentatively Ready after 5 positive votes on c++std-lib. ]
[ 2009-11-17 Alisdair Opens: ]
LWG 1193 tries to support
unique_ptr
for incomplete types. I believe the proposed wording goes too far:The template parameter
T
ofdefault_delete
may be an incomplete type.Do we really want to support
cv-void
? Suggested ammendment:The template parameter
T
ofdefault_delete
may be an incomplete type other thancv-void
.We might also consider saying something about arrays of incomplete types.
Did we lose support for
unique_ptr<function-type>
when the concept-enabled work was shelved? If so, we might want adefault_delete
partial specialization for function types that does nothing. Alternatively, function types should not be supported by default, but there is no reason a user cannot support them via their own deletion policy.Function-type support might also lead to conditionally supporting a function-call operator in the general case, and that seems way too inventive at this stage to me, even if we could largely steal wording directly from
reference_wrapper
.shared_ptr
would have similar problems too.
[ 2010-01-24 Moved to Tentatively Ready after 5 positive votes on c++std-lib. ]
Proposed resolution:
Add two new paragraphs directly to 20.3.1.2 [unique.ptr.dltr] (before 20.3.1.2.2 [unique.ptr.dltr.dflt]) with the following content:
The class template
default_delete
serves as the default deleter (destruction policy) for the class templateunique_ptr
.The template parameter
T
ofdefault_delete
may be an incomplete type.
queue
constructorSection: 23.6 [container.adaptors] Status: C++11 Submitter: Howard Hinnant Opened: 2009-08-20 Last modified: 2016-01-28
Priority: Not Prioritized
View all other issues in [container.adaptors].
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Discussion:
23.6.3.1 [queue.defn] has the following queue
constructor:
template <class Alloc> explicit queue(const Alloc&);
This will be implemented like so:
template <class Alloc> explicit queue(const Alloc& a) : c(a) {}
The issue is that Alloc
can be anything that a container will construct
from, for example an int
. Is this intended to compile?
queue<int> q(5);
Before the addition of this constructor, queue<int>(5)
would not compile.
I ask, not because this crashes, but because it is new and appears to be
unintended. We do not want to be in a position of accidently introducing this
"feature" in C++0X and later attempting to remove it.
I've picked on queue
. priority_queue
and stack
have
the same issue. Is it useful to create a priority_queue
of 5
identical elements?
[ Daniel, Howard and Pablo collaborated on the proposed wording. ]
[ 2009-10 Santa Cruz: ]
Move to Ready.
Proposed resolution:
[ This resolution includes a semi-editorial clean up, giving definitions to members which in some cases weren't defined since C++98. This resolution also offers editorially different wording for 976(i), and it also provides wording for 1196(i). ]
Change [ontainer.adaptor], p1:
The container adaptors each take a
Container
template parameter, and each constructor takes aContainer
reference argument. This container is copied into theContainer
member of each adaptor. If the container takes an allocator, then a compatible allocator may be passed in to the adaptor's constructor. Otherwise, normal copy or move construction is used for the container argument.[Note: it is not necessary for an implementation to distinguish between the one-argument constructor that takes aContainer
and the one- argument constructor that takes an allocator_type. Both forms use their argument to construct an instance of the container. — end note]
Change [ueue.def], p1:
template <class T, class Container = deque<T> > class queue { public: typedef typename Container::value_type value_type; typedef typename Container::reference reference; typedef typename Container::const_reference const_reference; typedef typename Container::size_type size_type; typedef Container container_type; protected: Container c; public: explicit queue(const Container&); explicit queue(Container&& = Container()); queue(queue&& q);: c(std::move(q.c)) {}template <class Alloc> explicit queue(const Alloc&); template <class Alloc> queue(const Container&, const Alloc&); template <class Alloc> queue(Container&&, const Alloc&); template <class Alloc> queue(queue&&, const Alloc&); queue& operator=(queue&& q);{ c = std::move(q.c); return *this; }bool empty() const { return c.empty(); } ... };
Add a new section after 23.6.3.1 [queue.defn], [queue.cons]:
queue
constructors [queue.cons]explicit queue(const Container& cont);Effects: Initializes
c
withcont
.explicit queue(Container&& cont = Container());Effects: Initializes
c
withstd::move(cont)
.queue(queue&& q)Effects: Initializes
c
withstd::move(q.c)
.For each of the following constructors, if
uses_allocator<container_type, Alloc>::value
isfalse
, then the constructor shall not participate in overload resolution.template <class Alloc> explicit queue(const Alloc& a);Effects: Initializes
c
witha
.template <class Alloc> queue(const container_type& cont, const Alloc& a);Effects: Initializes
c
withcont
as the first argument anda
as the second argument.template <class Alloc> queue(container_type&& cont, const Alloc& a);Effects: Initializes
c
withstd::move(cont)
as the first argument anda
as the second argument.template <class Alloc> queue(queue&& q, const Alloc& a);Effects: Initializes
c
withstd::move(q.c)
as the first argument anda
as the second argument.queue& operator=(queue&& q);Effects: Assigns
c
withstd::move(q.c)
.Returns:
*this
.
Add to 23.6.4.2 [priqueue.cons]:
priority_queue(priority_queue&& q);Effects: Initializes
c
withstd::move(q.c)
and initializescomp
withstd::move(q.comp)
.For each of the following constructors, if
uses_allocator<container_type, Alloc>::value
isfalse
, then the constructor shall not participate in overload resolution.template <class Alloc> explicit priority_queue(const Alloc& a);Effects: Initializes
c
witha
and value-initializescomp
.template <class Alloc> priority_queue(const Compare& compare, const Alloc& a);Effects: Initializes
c
witha
and initializescomp
withcompare
.template <class Alloc> priority_queue(const Compare& compare, const Container& cont, const Alloc& a);Effects: Initializes
c
withcont
as the first argument anda
as the second argument, and initializescomp
withcompare
.template <class Alloc> priority_queue(const Compare& compare, Container&& cont, const Alloc& a);Effects: Initializes
c
withstd::move(cont)
as the first argument anda
as the second argument, and initializescomp
withcompare
.template <class Alloc> priority_queue(priority_queue&& q, const Alloc& a);Effects: Initializes
c
withstd::move(q.c)
as the first argument anda
as the second argument, and initializescomp
withstd::move(q.comp)
.priority_queue& operator=(priority_queue&& q);Effects: Assigns
c
withstd::move(q.c)
and assignscomp
withstd::move(q.comp)
.Returns:
*this
.
Change 23.6.6.2 [stack.defn]:
template <class T, class Container = deque<T> > class stack { public: typedef typename Container::value_type value_type; typedef typename Container::reference reference; typedef typename Container::const_reference const_reference; typedef typename Container::size_type size_type; typedef Container container_type; protected: Container c; public: explicit stack(const Container&); explicit stack(Container&& = Container()); stack(stack&& s); template <class Alloc> explicit stack(const Alloc&); template <class Alloc> stack(const Container&, const Alloc&); template <class Alloc> stack(Container&&, const Alloc&); template <class Alloc> stack(stack&&, const Alloc&); stack& operator=(stack&& s); bool empty() const { return c.empty(); } ... };
Add a new section after 23.6.6.2 [stack.defn], [stack.cons]:
stack
constructors [stack.cons]stack(stack&& s);Effects: Initializes
c
withstd::move(s.c)
.For each of the following constructors, if
uses_allocator<container_type, Alloc>::value
isfalse
, then the constructor shall not participate in overload resolution.template <class Alloc> explicit stack(const Alloc& a);Effects: Initializes
c
witha
.template <class Alloc> stack(const container_type& cont, const Alloc& a);Effects: Initializes
c
withcont
as the first argument anda
as the second argument.template <class Alloc> stack(container_type&& cont, const Alloc& a);Effects: Initializes
c
withstd::move(cont)
as the first argument anda
as the second argument.template <class Alloc> stack(stack&& s, const Alloc& a);Effects: Initializes
c
withstd::move(s.c)
as the first argument anda
as the second argument.stack& operator=(stack&& s);Effects: Assigns
c
withstd::move(s.c)
.Returns:
*this
.
Section: 16 [library] Status: C++11 Submitter: Daniel Krügler Opened: 2009-08-18 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
Several parts of the library use the notion of "Diagnostic required" to indicate that in the corresponding situation an error diagnostic should occur, e.g. 20.3.1.2.2 [unique.ptr.dltr.dflt]/2
void operator()(T *ptr) const;Effects: calls
delete
onptr
. A diagnostic is required ifT
is an incomplete type.
The problem with this approach is that such a requirement is insufficient to prevent undefined behavior, if this situation occurs. According to 3.18 [defns.diagnostic] a diagnostic message is defined as
a message belonging to an implementation-defined subset of the implementation's output messages.
which doesn't indicate any relation to an ill-formed program. In fact, "compiler warnings" are a typical expression of such diagnostics. This means that above wording can be interpreted by compiler writers that they satisfy the requirements of the standard if they just produce such a "warning", if the compiler happens to compile code like this:
#include <memory> struct Ukn; // defined somewhere else Ukn* create_ukn(); // defined somewhere else int main() { std::default_delete<Ukn>()(create_ukn()); }
In this and other examples discussed here it was the authors intent to guarantee that the program is ill-formed with a required diagnostic, therefore such wording should be used instead. According to the general rules outlined in 4.1 [intro.compliance] it should be sufficient to require that these situations produce an ill-formed program and the "diagnostic required" part should be implied. The proposed resolution also suggests to remove several redundant wording of "Diagnostics required" to ensure that the absence of such saying does not cause a misleading interpretation.
[ 2009 Santa Cruz: ]
Move to NAD.
It's not clear that there's any important difference between "ill-formed" and "diagnostic required". From 4.1 [intro.compliance], 3.26 [defns.ill.formed], and 3.68 [defns.well.formed] it appears that an ill-formed program is one that is not correctly constructed according to the syntax rules and diagnosable semantic rules, which means that... "a conforming implementation shall issue at least one diagnostic message." The author's intent seems to be that we should be requiring a fatal error instead of a mere warning, but the standard just doesn't have language to express that distinction. The strongest thing we can ever require is a "diagnostic".
The proposed rewording may be a clearer way of expressing the same thing that the WP already says, but such a rewording is editorial.
[ 2009 Santa Cruz: ]
Considered again. Group disagrees that the change is technical, but likes it editorially. Moved to NAD Editorial.
[ 2009-11-19: Moved from NAD Editorial to Open. Please see the thread starting with Message c++std-lib-25916. ]
[ 2009-11-20 Daniel updated wording. ]
The following resolution differs from the previous one by avoiding the unusual and misleading term "shall be ill-formed", which does also not follow the core language style. This resolution has the advantage of a minimum impact on the current wording, but I would like to mention that a more intrusive solution might be preferrable - at least as a long-term solution: Jens Maurer suggested the following approach to get rid of the usage of the term "ill-formed" from the library by introducing a new category to existing elements to the list of 16.3.2.4 [structure.specifications]/3, e.g. "type requirements" or "static constraints" that define conditions that can be checked during compile-time and any violation would make the program ill-formed. As an example, the currently existing phrase 22.4.7 [tuple.helper]/1
Requires:
I < sizeof...(Types)
. The program is ill-formed ifI
is out of bounds.could then be written as
Static constraints:
I < sizeof...(Types)
.
[ 2009-11-21 Daniel updated wording. ]
[ 2009-11-22 Moved to Tentatively Ready after 5 positive votes on c++std-lib. ]
Proposed resolution:
Change 21.4 [ratio]/2 as indicated:
Throughout this subclause, if the template argument types
R1
andR2
shall beare not specializations of theratio
template, the program is ill-formed.Diagnostic required.
Change 21.4.3 [ratio.ratio]/1 as indicated:
If tThe template argument D
shall not
be is zero, and or the absolute values of
the template arguments N
and D
shall be are
not representable by type intmax_t
, the program is
ill-formed. Diagnostic required. [..]
Change 21.4.4 [ratio.arithmetic]/1 as indicated:
Implementations may use other algorithms to compute these values. If overflow occurs, the program is ill-formed
a diagnostic shall be issued.
Change 21.4.5 [ratio.comparison]/2 as indicated:
[...] Implementations may use other algorithms to compute this relationship to avoid overflow. If overflow occurs, the program is ill-formed
a diagnostic is required.
Change 20.3.1.2.2 [unique.ptr.dltr.dflt]/2 as indicated:
Effects: calls
delete
onptr
.A diagnostic is required ifT
is an incomplete type.Remarks: If
T
is an incomplete type, the program is ill-formed.
Change 20.3.1.2.3 [unique.ptr.dltr.dflt1]/1 as indicated:
void operator()(T* ptr) const;Effects:
callsoperator()
delete[]
onptr
.A diagnostic is required ifT
is an incomplete type.Remarks: If
T
is an incomplete type, the program is ill-formed.
Change 20.3.1.3.2 [unique.ptr.single.ctor] as indicated: [Note: This editorially improves the currently suggested wording of 932(i) by replacing
"shall be ill-formed" by "is ill-formed"]
[If N3025 is accepted this bullet is applied identically in that paper as well.]
-1- Requires:
D
shall be default constructible, and that construction shall not throw an exception.D
shall not be a reference type or pointer type (diagnostic required)....
Remarks: If this constructor is instantiated with a pointer type or reference type for the template argument
D
, the program is ill-formed.
Change 20.3.1.3.2 [unique.ptr.single.ctor]/8 as indicated: [Note: This editorially improves the currently suggested wording of 932(i) by replacing
"shall be ill-formed" by "is ill-formed"]
[If N3025 is accepted this bullet is applied identically in that paper as well.]
unique_ptr(pointer p);...
Remarks: If this constructor is instantiated with a pointer type or reference type for the template argument
D
, the program is ill-formed.
Change 20.3.1.3.2 [unique.ptr.single.ctor]/13 as indicated:
[..] If
d
is an rvalue, it will bind to the second constructor of this pair and the program is ill-formed.That constructor shall emit a diagnostic.[Note: The diagnostic could be implemented using astatic_assert
which assures thatD
is not a reference type. — end note] Elsed
is an lvalue and will bind to the first constructor of this pair. [..]
Change 20.3.1.4 [unique.ptr.runtime]/1 as indicated:
A specialization for array types is provided with a slightly altered interface.
- Conversions among different types of
unique_ptr<T[], D>
or to or from the non-array forms ofunique_ptr
are disallowed (diagnostic required)produce an ill-formed program.- ...
Change 30.5 [time.duration]/2-4 as indicated:
2 Requires:
Rep
shall be an arithmetic type or a class emulating an arithmetic type.If a program instantiatesduration
with aduration
type for the template argumentRep
a diagnostic is required.3 Remarks: If
duration
is instantiated with aduration
type for the template argumentRep
, the program is ill-formed.
34RequiresRemarks: IfPeriod
shall beis not a specialization ofratio
,diagnostic requiredthe program is ill-formed.
45RequiresRemarks: IfPeriod::num
shall beis not positive,diagnostic requiredthe program is ill-formed.
Change 30.6 [time.point]/2 as indicated:
If
Duration
shall beis not an instance ofduration
, the program is ill-formed.Diagnostic required.
Section: 23.6.4.2 [priqueue.cons] Status: Resolved Submitter: Alisdair Meredith Opened: 2009-08-19 Last modified: 2016-01-28
Priority: Not Prioritized
View all issues with Resolved status.
Discussion:
The class template priority_queue
declares signatures for a move
constructor and move assignment operator in its class definition.
However, it does not provide a definition (unlike std::queue
, and
proposed resolution for std::stack
.) Nor does it provide a text clause
specifying their behaviour.
[ 2009-08-23 Daniel adds: ]
[ 2009-10 Santa Cruz: ]
Proposed resolution:
bucket_count() == 0
?Section: 23.2.8 [unord.req] Status: C++11 Submitter: Howard Hinnant Opened: 2009-08-24 Last modified: 2016-01-28
Priority: Not Prioritized
View other active issues in [unord.req].
View all other issues in [unord.req].
View all issues with C++11 status.
Discussion:
Table 97 "Unordered associative container requirements" in 23.2.8 [unord.req] says:
Table 97 — Unordered associative container requirements (in addition to container) Expression Return type Assertion/note pre-/post-condition Complexity b.bucket(k)
size_type
Returns the index of the bucket in which elements with keys equivalent to k
would be found, if any such element existed. Post: the return value shall be in the range[0, b.bucket_count())
.Constant
What should b.bucket(k)
return if b.bucket_count() == 0
?
I believe allowing b.bucket_count() == 0
is important. It is a
very reasonable post-condition of the default constructor, or of a moved-from
container.
I can think of several reasonable results from b.bucket(k)
when
b.bucket_count() == 0
:
numeric_limits<size_type>::max()
.
domain_error
.
b.bucket_count() != 0
.
[ 2009-08-26 Daniel adds: ]
A forth choice would be to add the pre-condition "
b.bucket_count() != 0
" and thus imply undefined behavior if this is violated.[ Howard: I like this option too, added to the list. ]
Further on here my own favorite solution (rationale see below):
Suggested resolution:
[Rationale: I suggest to follow choice (1). The main reason is that all associative container functions which take a key argument, are basically free of pre-conditions and non-disrupting, therefore excluding choices (3) and (4). Option (2) seems a bit unexpected to me. It would be more natural, if several similar functions would exist which would also justify the existence of a symbolic constant like npos for this situation. The value 0 is both simple and consistent, it has exactly the same role as a past-the-end iterator value. A typical use-case is:
size_type pos = m.bucket(key); if (pos != m.bucket_count()) { ... } else { ... }— end Rationale]
- Change Table 97 in 23.2.8 [unord.req] as follows (Row b.bucket(k), Column "Assertion/..."):
Table 97 — Unordered associative container requirements (in addition to container) Expression Return type Assertion/note pre-/post-condition Complexity b.bucket(k)
size_type
Returns the index of the bucket in which elements with keys equivalent to k
would be found, if any such element existed. Post: if b.bucket_count() != 0, the return value shall be in the range[0, b.bucket_count())
, otherwise 0.Constant
[ 2010-01-25 Choice 4 put into proposed resolution section. ]
[ 2010-01-31 Moved to Tentatively Ready after 5 positive votes on c++std-lib. ]
Proposed resolution:
Change Table 97 in 23.2.8 [unord.req] as follows (Row b.bucket(k), Column "Assertion/..."):
Table 97 — Unordered associative container requirements (in addition to container) Expression Return type Assertion/note pre-/post-condition Complexity b.bucket(k)
size_type
Pre: b.bucket_count() > 0
Returns the index of the bucket in which elements with keys equivalent tok
would be found, if any such element existed. Post: the return value shall be in the range[0, b.bucket_count())
.Constant
Section: 23.6 [container.adaptors] Status: C++11 Submitter: Pablo Halpern Opened: 2009-08-26 Last modified: 2016-01-28
Priority: Not Prioritized
View all other issues in [container.adaptors].
View all issues with C++11 status.
Discussion:
Under 23.6 [container.adaptors] of
N2914
the member function of swap
of queue
and stack
call:
swap(c, q.c);
But under 23.6 [container.adaptors] of N2723 these members are specified to call:
c.swap(q.c);
Neither draft specifies the semantics of member swap
for
priority_queue
though it is declared.
Although the distinction between member swap
and non-member
swap
is not important when these adaptors are adapting standard
containers, it may be important for user-defined containers.
We (Pablo and Howard) feel that
it is more likely for a user-defined container to support a namespace scope
swap
than a member swap
, and therefore these adaptors
should use the container's namespace scope swap
.
[ 2009-09-30 Daniel adds: ]
The outcome of this issue should be considered with the outcome of 774(i) both in style and in content (e.g. 774(i) bullet 9 suggests to define the semantic of
void priority_queue::swap(priority_queue&)
in terms of the memberswap
of the container).
[ 2010-03-28 Daniel update to diff against N3092. ]
[ 2010 Rapperswil: ]
Preference to move the wording into normative text, rather than inline function definitions in the class synopsis. Move to Tenatively Ready.
[ Adopted at 2010-11 Batavia ]
Proposed resolution:
Change 23.6.3.1 [queue.defn]:
template <class T, class Container = deque<T> > class queue { ... void swap(queue& q) { using std::swap;c.swap(c, q.c); } ... };
Change 23.6.4 [priority.queue]:
template <class T, class Container = vector<T>, class Compare = less<typename Container::value_type> > class priority_queue { ... void swap(priority_queue& q);{ using std::swap; swap(c, q.c); swap(comp, q.comp); } ... };
Change 23.6.6.2 [stack.defn]:
template <class T, class Container = deque<T> > class stack { ... void swap(stack& s) { using std::swap;c.swap(c, s.c); } ... };
Section: 23.6 [container.adaptors] Status: C++11 Submitter: Pablo Halpern Opened: 2009-08-26 Last modified: 2020-11-29
Priority: Not Prioritized
View all other issues in [container.adaptors].
View all issues with C++11 status.
Discussion:
queue
has a constructor:
template <class Alloc> queue(queue&&, const Alloc&);
but it is missing a corresponding constructor:
template <class Alloc> queue(const queue&, const Alloc&);
The same is true of priority_queue
, and stack
. This
"extended copy constructor" is needed for consistency and to ensure that the
user of a container adaptor can always specify the allocator for his adaptor.
[ 2010-02-01 Moved to Tentatively Ready after 5 positive votes on c++std-lib. ]
Proposed resolution:
[ This resolution has been harmonized with the proposed resolution to issue 1194(i) ]
Change 23.6.3.1 [queue.defn], p1:
template <class T, class Container = deque<T> > class queue { public: typedef typename Container::value_type value_type; typedef typename Container::reference reference; typedef typename Container::const_reference const_reference; typedef typename Container::size_type size_type; typedef Container container_type; protected: Container c; public: explicit queue(const Container&); explicit queue(Container&& = Container()); queue(queue&& q); template <class Alloc> explicit queue(const Alloc&); template <class Alloc> queue(const Container&, const Alloc&); template <class Alloc> queue(Container&&, const Alloc&); template <class Alloc> queue(const queue&, const Alloc&); template <class Alloc> queue(queue&&, const Alloc&); queue& operator=(queue&& q); bool empty() const { return c.empty(); } ... };
To the new section 23.6.3.2 [queue.cons], introduced in 1194(i), add:
template <class Alloc> queue(const queue& q, const Alloc& a);Effects: Initializes
c
withq.c
as the first argument anda
as the second argument.
Change 23.6.4 [priority.queue] as follows (I've an included an editorial change to move the poorly-placed move-assignment operator):
template <class T, class Container = vector<T>, class Compare = less<typename Container::value_type> > class priority_queue { public: typedef typename Container::value_type value_type; typedef typename Container::reference reference; typedef typename Container::const_reference const_reference; typedef typename Container::size_type size_type; typedef Container container_type; protected: Container c; Compare comp; public: priority_queue(const Compare& x, const Container&); explicit priority_queue(const Compare& x = Compare(), Container&& = Container()); template <class InputIterator> priority_queue(InputIterator first, InputIterator last, const Compare& x, const Container&); template <class InputIterator> priority_queue(InputIterator first, InputIterator last, const Compare& x = Compare(), Container&& = Container()); priority_queue(priority_queue&&);priority_queue& operator=(priority_queue&&);template <class Alloc> explicit priority_queue(const Alloc&); template <class Alloc> priority_queue(const Compare&, const Alloc&); template <class Alloc> priority_queue(const Compare&, const Container&, const Alloc&); template <class Alloc> priority_queue(const Compare&, Container&&, const Alloc&); template <class Alloc> priority_queue(const priority_queue&, const Alloc&); template <class Alloc> priority_queue(priority_queue&&, const Alloc&); priority_queue& operator=(priority_queue&&); ... };
Add to 23.6.4.2 [priqueue.cons]:
template <class Alloc> priority_queue(const priority_queue& q, const Alloc& a);Effects: Initializes
c
withq.c
as the first argument anda
as the second argument, and initializescomp
withq.comp
.
Change 23.6.6.2 [stack.defn]:
template <class T, class Container = deque<T> > class stack { public: typedef typename Container::value_type value_type; typedef typename Container::reference reference; typedef typename Container::const_reference const_reference; typedef typename Container::size_type size_type; typedef Container container_type; protected: Container c; public: explicit stack(const Container&); explicit stack(Container&& = Container()); stack(stack&& s); template <class Alloc> explicit stack(const Alloc&); template <class Alloc> stack(const Container&, const Alloc&); template <class Alloc> stack(Container&&, const Alloc&); template <class Alloc> stack(const stack&, const Alloc&); template <class Alloc> stack(stack&&, const Alloc&); stack& operator=(stack&& s); bool empty() const { return c.empty(); } ... };
To the new section 23.6.6.3 [stack.cons], introduced in 1194(i), add:
template <class Alloc> stack(const stack& s, const Alloc& a);Effects: Initializes
c
withs.c
as the first argument anda
as the second argument.
ref
-wrappers in make_tuple
Section: 22.4.5 [tuple.creation], 22.3 [pairs] Status: Resolved Submitter: Alisdair Meredith Opened: 2009-09-05 Last modified: 2018-10-05
Priority: Not Prioritized
View all other issues in [tuple.creation].
View all issues with Resolved status.
Discussion:
Spotting a recent thread on the boost lists regarding collapsing
optional representations in optional<optional<T>>
instances, I wonder if
we have some of the same issues with make_tuple
, and now make_pair
?
Essentially, if my generic code in my own library is handed a
reference_wrapper
by a user, and my library in turn delegates some logic
to make_pair
or make_tuple
, then I am going to end up with a pair
/tuple
holding a real reference rather than the intended reference wrapper.
There are two things as a library author I can do at this point:
std::make_tuple
make_tuple
that does not unwrap rereferences, a lost
opportunity to re-use the standard library.
(There may be some metaprogramming approaches my library can use to wrap
the make_tuple
call, but all will be significantly more complex than
simply implementing a simplified make_tuple
.)
Now I don't propose we lose this library facility, I think unwrapping
references will be the common behaviour. However, we might want to
consider adding another overload that does nothing special with
ref
-wrappers. Note that we already have a second overload of
make_tuple
in the library, called tie
.
[ 2009-09-30 Daniel adds: ]
I suggest to change the currently proposed paragraph for
make_simple_pair
template<typename... Types> pair<typename decay<Types>::type...> make_simple_pair(Types&&... t);
Type requirements:Remarks: The program shall be ill-formed, ifsizeof...(Types) == 2
.sizeof...(Types) != 2
....
or alternatively (but with a slightly different semantic):
Remarks: If
sizeof...(Types) != 2
, this function shall not participate in overload resolution.to follow a currently introduced style and because the library does not have yet a specific "Type requirements" element. If such thing would be considered as useful this should be done as a separate issue. Given the increasing complexity of either of these wordings it might be preferable to use the normal two-argument-declaration style again in either of the following ways:
template<class T1, class T2> pair<typename decay<T1>::type, typename decay<T2>::type> make_simple_pair(T1&& t1, T2&& t2); template<class T1, class T2> pair<V1, V2> make_simple_pair(T1&& t1, T2&& t2);Let
V1
betypename decay<T1>::type
andV2
betypename decay<T2>::type
.
[ 2009-10 post-Santa Cruz: ]
Mark as Tentatively NAD Future.
[2018-10-05 Status to Resolved]
Alisdair notes that this is solved by CTAD that was added in C++17
Rationale:
Does not have sufficient support at this time. May wish to reconsider for a future standard.
Proposed resolution:
Add the following function to 22.3 [pairs] and signature in appropriate synopses:
template<typename... Types> pair<typename decay<Types>::type...> make_simple_pair(Types&&... t);Type requirements:
sizeof...(Types) == 2
.Returns:
pair<typename decay<Types>::type...>(std::forward<Types>(t)...)
.
[
Draughting note: I chose a variadic representation similar to make_tuple
rather than naming both types as it is easier to read through the
clutter of metaprogramming this way. Given there are exactly two
elements, the committee may prefer to draught with two explicit template
type parameters instead
]
Add the following function to 22.4.5 [tuple.creation] and signature in appropriate synopses:
template<typename... Types> tuple<typename decay<Types>::type...> make_simple_tuple(Types&&... t);Returns:
tuple<typename decay<Types>::type...>(std::forward<Types>(t)...)
.
Section: 31.7.6.6 [ostream.rvalue], 31.7.5.6 [istream.rvalue] Status: C++20 Submitter: Howard Hinnant Opened: 2009-09-06 Last modified: 2021-02-25
Priority: 2
View all other issues in [ostream.rvalue].
View all issues with C++20 status.
Discussion:
31.7.6.6 [ostream.rvalue] was created to preserve the ability to insert into (and extract from 31.7.5.6 [istream.rvalue]) rvalue streams:
template <class charT, class traits, class T> basic_ostream<charT, traits>& operator<<(basic_ostream<charT, traits>&& os, const T& x);1 Effects:
os << x
2 Returns:
os
This is good as it allows code that wants to (for example) open, write to, and
close an ofstream
all in one statement:
std::ofstream("log file") << "Some message\n";
However, I think we can easily make this "rvalue stream helper" even easier to use. Consider trying to quickly create a formatted string. With the current spec you have to write:
std::string s = static_cast<std::ostringstream&>(std::ostringstream() << "i = " << i).str();
This will store "i = 10
" (for example) in the string s
. Note
the need to cast the stream back to ostringstream&
prior to using
the member .str()
. This is necessary because the inserter has cast
the ostringstream
down to a more generic ostream
during the
insertion process.
I believe we can re-specify the rvalue-inserter so that this cast is unnecessary. Thus our customer now has to only type:
std::string s = (std::ostringstream() << "i = " << i).str();
This is accomplished by having the rvalue stream inserter return an rvalue of
the same type, instead of casting it down to the base class. This is done by
making the stream generic, and constraining it to be an rvalue of a type derived
from ios_base
.
The same argument and solution also applies to the inserter. This code has been implemented and tested.
[ 2009 Santa Cruz: ]
NAD Future. No concensus for change.
[LEWG Kona 2017]
Recommend Open: Design looks right.
[ 2018-05-25, Billy O'Neal requests this issue be reopened and provides P/R rebased against N4750 ]
Billy O'Neal requests this issue be reopened, as changing operator>> and operator<< to use perfect forwarding as described here is necessary to implement LWG 2534(i) which was accepted. Moreover, this P/R also resolves LWG 2498(i).
Previous resolution [SUPERSEDED]:
Change 31.7.5.6 [istream.rvalue]:
template <classcharT, class traitsIstream, class T>basic_istream<charT, traits>&Istream&& operator>>(basic_istream<charT, traits>Istream&& is, T& x);1 Effects:
is >> x
2 Returns:
std::move(is)
3 Remarks: This signature shall participate in overload resolution if and only if
Istream
is not an lvalue reference type and is derived fromios_base
.Change 31.7.6.6 [ostream.rvalue]:
template <classcharT, class traitsOstream, class T>basic_ostream<charT, traits>&Ostream&& operator<<(basic_ostream<charT, traits>Ostream&& os, const T& x);1 Effects:
os << x
2 Returns:
std::move(os)
3 Remarks: This signature shall participate in overload resolution if and only if
Ostream
is not an lvalue reference type and is derived fromios_base
.
[2018-12-03, Ville comments]
The implementation in libstdc++ doesn't require derivation from ios_base
, it
requires convertibility to basic_istream/basic_ostream
. This has been found to be
important to avoid regressions with existing stream wrappers.
basic_istream/basic_ostream
specialization
the template parameter converts to. This was done in order to try and be closer to the earlier
specification's return type, which specified basic_ostream<charT, traits>&
and basic_istream<charT, traits>&
. So we detected convertibility to
(a type convertible to) those, and returned the result of that conversion. That doesn't seem to
be necessary, and probably bothers certain chaining cases. Based on recent experiments, it seems
that this return-type dance (as opposed to just returning what the p/r suggests) is unnecessary,
and doesn't trigger any regressions.
[2019-01-20 Reflector prioritization]
Set Priority to 2
Previous resolution [SUPERSEDED]:
This resolution is relative to N4750.
Change 31.7.5.6 [istream.rvalue] as follows:
template <classcharT, class traitsIstream, class T>basic_istream<charT, traits>&Istream&& operator>>(basic_istream<charT, traits>Istream&& is, T&& x);-1- Effects: Equivalent to:
is >> std::forward<T>(x)
return std::move(is);-2- Remarks: This function shall not participate in overload resolution unless the expression
is >> std::forward<T>(x)
is well-formed,Istream
is not an lvalue reference type, andIstream
is derived fromios_base
.Change 31.7.6.6 [ostream.rvalue]:
template <classcharT, class traitsOstream, class T>basic_ostream<charT, traits>&Ostream&& operator<<(basic_ostream<charT, traits>Ostream&& os, const T& x);-1- Effects: As if by:
os << x;
-2- Returns:
std::move(os)
-3- Remarks: This signature shall not participate in overload resolution unless the expression
os << x
is well-formed,Ostream
is not an lvalue reference type, andOstream
is derived fromios_base
.
[2019-03-17; Daniel comments and provides updated wording]
After discussion with Ville it turns out that the "derived from ios_base
" approach works fine and
no breakages were found in regression tests. As result of that discussion the wording was rebased to the
most recent working draft and the "overload resolution participation" wording was replaced by a
corresponding Constraints: element.
[2020-02 Status to Immediate on Friday morning in Prague.]
Proposed resolution:
This wording is relative to N4810.
Change 31.7.5.6 [istream.rvalue] as follows:
template <classcharT, class traitsIstream, class T>basic_istream<charT, traits>&Istream&& operator>>(basic_istream<charT, traits>Istream&& is, T&& x);-?- Constraints: The expression
-1- Effects: Equivalent to:is >> std::forward<T>(x)
is well-formed andIstream
is publicly and unambiguously derived fromios_base
.is >> std::forward<T>(x)
; return std::move(is);
-2- Remarks: This function shall not participate in overload resolution unless the expressionis >> std::forward<T>(x)
is well-formed.
Change 31.7.6.6 [ostream.rvalue]:
template <classcharT, class traitsOstream, class T>basic_ostream<charT, traits>&Ostream&& operator<<(basic_ostream<charT, traits>Ostream&& os, const T& x);-?- Constraints: The expression
-1- Effects: As if by:os << x
is well-formed andOstream
is publicly and unambiguously derived fromios_base
.os << x;
-2- Returns:std::move(os)
.-3- Remarks: This signature shall not participate in overload resolution unless the expressionos << x
is well-formed.
Section: 16.4.5.9 [res.on.arguments] Status: C++11 Submitter: Howard Hinnant Opened: 2009-09-12 Last modified: 2016-01-28
Priority: Not Prioritized
View all other issues in [res.on.arguments].
View all issues with C++11 status.
Discussion:
When a library function binds an rvalue reference parameter to an argument, the library must be able to assume that the bound argument is a temporary, and not a moved-from lvalue. The reason for this is that the library function must be able to modify that argument without concern that such modifications will corrupt the logic of the calling code. For example:
template <class T, class A> void vector<T, A>::push_back(value_type&& v) { // This function should move from v, potentially modifying // the object v is bound to. }
If v
is truly bound to a temporary, then push_back
has the
only reference to this temporary in the entire program. Thus any
modifications will be invisible to the rest of the program.
If the client supplies std::move(x)
to push_back
, the onus is
on the client to ensure that the value of x
is no longer important to
the logic of his program after this statement. I.e. the client is making a statement
that push_back
may treat x
as a temporary.
The above statement is the very foundation upon which move semantics is based.
The standard is currently lacking a global statement to this effect. I propose the following addition to 16.4.5.9 [res.on.arguments]:
Each of the following statements applies to all arguments to functions defined in the C++ standard library, unless explicitly stated otherwise.
- If an argument to a function has an invalid value (such as a value outside the domain of the function, or a pointer invalid for its intended use), the behavior is undefined.
- If a function argument is described as being an array, the pointer actually passed to the function shall have a value such that all address computations and accesses to objects (that would be valid if the pointer did point to the first element of such an array) are in fact valid.
- If a function argument binds to an rvalue reference parameter, the C++ standard library may assume that this parameter is a unique reference to this argument. If the parameter is a generic parameter of the form
T&&
, and an lvalue of typeA
is bound, then the binding is considered to be to an lvalue reference (13.10.3.2 [temp.deduct.call]) and thus not covered by this clause. [Note: If a program casts an lvalue to an rvalue while passing that lvalue to a library function (e.g.move(x)
), then the program is effectively asking the library to treat that lvalue as a temporary. The library is at liberty to optimize away aliasing checks which might be needed if the argument were an lvalue. — end note]
Such a global statement will eliminate the need for piecemeal statements such as 23.2.2 [container.requirements.general]/13:
An object bound to an rvalue reference parameter of a member function of a container shall not be an element of that container; no diagnostic required.
Additionally this clarifies that move assignment operators need not perform the
traditional if (this != &rhs)
test commonly found (and needed) in
copy assignment operators.
[ 2009-09-13 Niels adds: ]
Note: This resolution supports the change of 31.10.3.3 [filebuf.assign]/1, proposed by LWG 900(i).
[ 2009 Santa Cruz: ]
Move to Ready.
Proposed resolution:
Add a bullet to 16.4.5.9 [res.on.arguments]:
Each of the following statements applies to all arguments to functions defined in the C++ standard library, unless explicitly stated otherwise.
- If an argument to a function has an invalid value (such as a value outside the domain of the function, or a pointer invalid for its intended use), the behavior is undefined.
- If a function argument is described as being an array, the pointer actually passed to the function shall have a value such that all address computations and accesses to objects (that would be valid if the pointer did point to the first element of such an array) are in fact valid.
- If a function argument binds to an rvalue reference parameter, the C++ standard library may assume that this parameter is a unique reference to this argument. If the parameter is a generic parameter of the form
T&&
, and an lvalue of typeA
is bound, then the binding is considered to be to an lvalue reference (13.10.3.2 [temp.deduct.call]) and thus not covered by this clause. [Note: If a program casts an lvalue to an rvalue while passing that lvalue to a library function (e.g.move(x)
), then the program is effectively asking the library to treat that lvalue as a temporary. The library is at liberty to optimize away aliasing checks which might be needed if the argument were an lvalue. — end note]
Delete 23.2.2 [container.requirements.general]/13:
An object bound to an rvalue reference parameter of a member function of a container shall not be an element of that container; no diagnostic required.
Section: 26 [algorithms] Status: C++11 Submitter: Alisdair Meredith Opened: 2009-09-13 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
There are a number of algorithms whose result might depend on the handling of an empty range. In some cases the result is not clear, while in others it would help readers to clearly mention the result rather than require some subtle intuition of the supplied wording.
[alg.all_of]
Returns:
true
ifpred(*i)
istrue
for every iteratori
in the range[first,last)
, ...
What does this mean if the range is empty?
I believe that we intend this to be true
and suggest a
non-normative note to clarify:
Add to p1 [alg.all_of]:
[Note: Returns
true
if[first,last)
is empty. — end note]
[alg.none_of]
Returns:
true
ifpred(*i)
isfalse
for every iteratori
in the range[first,last)
, ...
What does this mean if the range empty?
I believe that we intend this to be true
and suggest a
non-normative note to clarify:
Add to p1 [alg.none_of]:
[Note: Returns
true
if[first,last)
is empty. — end note]
[alg.any_of]
The specification for an empty range is actually fairly clear in this
case, but a note wouldn't hurt and would be consistent with proposals
for all_of
/none_of
algorithms.
Add to p1 [alg.any_of]:
[Note: Returns
false
if[first,last)
is empty. — end note]
26.6.8 [alg.find.end]
what does this mean if [first2,last2)
is empty?
I believe the wording suggests the algorithm should return
last1
in this case, but am not 100% sure. Is this in fact the
correct result anyway? Surely an empty range should always match and the
naive expected result would be first1
?
My proposed wording is a note to clarify the current semantic:
Add to p2 26.6.8 [alg.find.end]:
[Note: Returns
last1
if[first2,last2)
is empty. — end note]
I would prefer a normative wording treating empty ranges specially, but do not believe we can change semantics at this point in the process, unless existing implementations actually yield this result:
Alternative wording: (NOT a note)
Add to p2 26.6.8 [alg.find.end]:
Returns
first1
if[first2,last2)
is empty.
26.6.9 [alg.find.first.of]
The phrasing seems precise when [first2, last2)
is empty, but a small
note to confirm the reader's understanding might still help.
Add to p2 26.6.9 [alg.find.first.of]
[Note: Returns
last1
if[first2,last2)
is empty. — end note]
26.6.15 [alg.search]
What is the expected result if [first2, last2)
is empty?
I believe the wording suggests the algorithm should return last1
in this
case, but am not 100% sure. Is this in fact the correct result anyway?
Surely an empty range should always match and the naive expected result
would be first1
?
My proposed wording is a note to clarify the current semantic:
Add to p2 26.6.15 [alg.search]:
[Note: Returns
last1
if[first2,last2)
is empty. — end note]
Again, I would prefer a normative wording treating empty ranges specially, but do not believe we can change semantics at this point in the process, unless existing implementations actually yield this result:
Alternative wording: (NOT a note)
Add to p2 26.6.15 [alg.search]:
Returns
first1
if[first2,last2)
is empty.
26.8.5 [alg.partitions]
Is an empty range partitioned or not?
Proposed wording:
Add to p1 26.8.5 [alg.partitions]:
[Note: Returns
true
if[first,last)
is empty. — end note]
26.8.7.2 [includes]
Returns:
true
if every element in the range[first2,last2)
is contained in the range[first1,last1)
. ...
I really don't know what this means if [first2,last2)
is empty.
I could loosely guess that this implies empty ranges always match, and
my proposed wording is to clarify exactly that:
Add to p1 26.8.7.2 [includes]:
[Note: Returns
true
if[first2,last2)
is empty. — end note]
26.8.8.3 [pop.heap]
The effects clause is invalid if the range [first,last)
is empty, unlike
all the other heap alogorithms. The should be called out in the
requirements.
Proposed wording:
Revise p2 26.8.8.3 [pop.heap]
Requires: The range
[first,last)
shall be a valid non-empty heap.
[Editorial] Reverse order of 26.8.8.3 [pop.heap] p1 and p2.
26.8.9 [alg.min.max]
minmax_element
does not clearly specify behaviour for an empty
range in the same way that min_element
and max_element
do.
Add to p31 26.8.9 [alg.min.max]:
Returns
make_pair(first, first)
iffirst == last
.
26.8.11 [alg.lex.comparison]
The wording here seems quite clear, especially with the sample algorithm implementation. A note is recommended purely for consistency with the rest of these issue resolutions:
Add to p1 26.8.11 [alg.lex.comparison]:
[Note: An empty sequence is lexicographically less than any other non-empty sequence, but not to another empty sequence. — end note]
[
2009-11-11 Howard changes Notes to Remarks and changed search
to
return first1
instead of last1
.
]
[ 2009-11-11 Moved to Tentatively Ready after 5 positive votes on c++std-lib. ]
Proposed resolution:
Add to [alg.all_of]:
Remarks: Returns
true
if[first,last)
is empty.
Add to [alg.any_of]:
Remarks: Returns
false
if[first,last)
is empty.
Add to [alg.none_of]:
Remarks: Returns
true
if[first,last)
is empty.
Add to 26.6.8 [alg.find.end]:
Remarks: Returns
last1
if[first2,last2)
is empty.
Add to 26.6.9 [alg.find.first.of]
Remarks: Returns
last1
if[first2,last2)
is empty.
Add to 26.6.15 [alg.search]:
Remarks: Returns
first1
if[first2,last2)
is empty.
Add to 26.8.5 [alg.partitions]:
Remarks: Returns
true
if[first,last)
is empty.
Add to 26.8.7.2 [includes]:
Remarks: Returns
true
if[first2,last2)
is empty.
Revise p2 26.8.8.3 [pop.heap]
Requires: The range
[first,last)
shall be a valid non-empty heap.
[Editorial]
Reverse order of 26.8.8.3 [pop.heap] p1 and p2.
Add to p35 26.8.9 [alg.min.max]:
template<class ForwardIterator, class Compare> pair<ForwardIterator, ForwardIterator> minmax_element(ForwardIterator first, ForwardIterator last, Compare comp);Returns:
make_pair(m, M)
, wherem
is the first iterator in[first,last)
such that no iterator in the range refers to a smaller element, and whereM
is the last iterator in[first,last)
such that no iterator in the range refers to a larger element. Returnsmake_pair(first, first)
iffirst == last
.
Add to 26.8.11 [alg.lex.comparison]:
Remarks: An empty sequence is lexicographically less than any other non-empty sequence, but not less than another empty sequence.
move_backward
and copy_backward
Section: 26.7.2 [alg.move] Status: C++11 Submitter: Howard Hinnant Opened: 2009-09-13 Last modified: 2016-01-28
Priority: Not Prioritized
View all issues with C++11 status.
Discussion:
26.7.2 [alg.move], p6 says:
template<class BidirectionalIterator1, class BidirectionalIterator2> BidirectionalIterator2 move_backward(BidirectionalIterator1 first, BidirectionalIterator1 last, BidirectionalIterator2 result);...
Requires:
result
shall not be in the range[first,last)
.
This is essentially an "off-by-one" error.
When result == last
, which
is allowed by this specification, then the range [first, last)
is being move assigned into the range [first, last)
. The move
(forward) algorithm doesn't allow self move assignment, and neither should
move_backward
. So last
should be included in the range which
result
can not be in.
Conversely, when result == first
, which is not allowed by this
specification, then the range [first, last)
is being move assigned into the range [first - (last-first), first)
.
I.e. into a non-overlapping range. Therefore first
should
not be included in the range which result
can not be in.
The same argument applies to copy_backward
though copy assigning elements
to themselves (result == last
) should be harmless (though is disallowed
by copy
).
[ 2010 Pittsburgh: Moved to Ready. ]
Proposed resolution:
Change 26.7.2 [alg.move], p6:
template<class BidirectionalIterator1, class BidirectionalIterator2> BidirectionalIterator2 move_backward(BidirectionalIterator1 first, BidirectionalIterator1 last, BidirectionalIterator2 result);...
Requires:
result
shall not be in the range.
[(first,last])
Change 26.7.1 [alg.copy], p13:
template<class BidirectionalIterator1, class BidirectionalIterator2> BidirectionalIterator2 copy_backward(BidirectionalIterator1 first, BidirectionalIterator1 last, BidirectionalIterator2 result);...
Requires:
result
shall not be in the range.
[(first,last])
std::list
operations?Section: 23.3.11.5 [list.ops] Status: C++11 Submitter: Loïc Joly Opened: 2009-09-13 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
It looks to me like some operations of std::list
(sort
, reverse
, remove
, unique
&
merge
) do not specify the validity of iterators, pointers &
references to elements of the list after those operations. Is it implied
by some other text in the standard?
I believe sort
& reverse
do not invalidating
anything, remove
& unique
only invalidates what
refers to erased elements, merge
does not invalidate anything
(with the same precision as splice
for elements who changed of
container). Are those assumptions correct ?
[ 2009-12-08 Jonathan Wakely adds: ]
23.2.2 [container.requirements.general] paragraph 11 says iterators aren't invalidated unless specified, so I don't think it needs to be repeated on every function that doesn't invalidate iterators.
list::unique
says it "eliminates" elements, that should probably be "erases" because IMHO that term is used elsewhere and so makes it clearer that iterators to the erased elements are invalidated.
list::merge
coud use the same wording aslist::splice
w.r.t iterators and references to moved elements.Suggested resolution:
In 23.3.11.5 [list.ops] change paragraph 19
void unique(); template <class BinaryPredicate> void unique(BinaryPredicate binary_pred);Effects:
EliminatesErases all but the first element from every consecutive group ...Add to the end of paragraph 23
void merge(list<T,Allocator>&& x); template <class Compare> void merge(list<T,Allocator>&& x, Compare comp);...
Effects: ... that is, for every iterator
i
, in the range other than the first, the conditioncomp(*i, *(i - 1)
will be false. Pointers and references to the moved elements ofx
now refer to those same elements but as members of*this
. Iterators referring to the moved elements will continue to refer to their elements, but they now behave as iterators into*this
, not intox
.
[ 2009-12-12 Loïc adds wording. ]
[ 2010-02-10 Moved to Tentatively Ready after 5 positive votes on c++std-lib. ]
[ 2010-02-10 Alisdair opens: ]
I object to the current resolution of #1207. I believe it is overly strict with regard to
list
end iterators, being the only mutating operations to require such stability.More importantly, the same edits need to be applied to
forward_list
, which uses slightly different words to describe some of these operations so may require subtly different edits (not checked.)I am prepared to pick up the
end()
iterator as a separate (new) issue, as part of the FCD ballot review (BSI might tell me 'no' first ;~) but I do want to seeforward_list
adjusted at the same time.
[ 2010-03-28 Daniel adds the first 5 bullets in an attempt to address Alisdair's concerns. ]
[ 2010 Rapperswil: ]
The wording looks good. Move to Tentatively Ready.
[ Adopted at 2010-11 Batavia ]
Proposed resolution:
Change [forwardlist.ops]/12 as indicated:
void remove(const T& value); template <class Predicate> void remove_if(Predicate pred);12 Effects: Erases all the elements in the list referred by a list iterator
i
for which the following conditions hold:*i == value (for remove()), pred(*i)
is true (for remove_if()
). This operation shall be stable: the relative order of the elements that are not removed is the same as their relative order in the original list. Invalidates only the iterators and references to the erased elements.
Change [forwardlist.ops]/15 as indicated:
template <class BinaryPredicate> void unique(BinaryPredicate pred);15 Effects::
EliminatesErases all but the first element from every consecutive group of equal elements referred to by the iteratori
in the range[first + 1,last)
for which*i == *(i-1)
(for the version with no arguments) orpred(*i, *(i - 1))
(for the version with a predicate argument) holds. Invalidates only the iterators and references to the erased elements.
Change [forwardlist.ops]/19 as indicated:
void merge(forward_list<T,Allocator>&& x); template <class Compare> void merge(forward_list<T,Allocator>&& x, Compare comp)[..]
19 Effects:: Merges
x
into*this
. This operation shall be stable: for equivalent elements in the two lists, the elements from*this
shall always precede the elements fromx
.x
is empty after the merge. If an exception is thrown other than by a comparison there are no effects. Pointers and references to the moved elements ofx
now refer to those same elements but as members of*this
. Iterators referring to the moved elements will continue to refer to their elements, but they now behave as iterators into*this
, not intox
.
Change [forwardlist.ops]/22 as indicated:
void sort(); template <class Compare> void sort(Compare comp);[..]
22 Effects:: Sorts the list according to the
operator<
or thecomp
function object. This operation shall be stable: the relative order of the equivalent elements is preserved. If an exception is thrown the order of the elements in*this
is unspecified. Does not affect the validity of iterators and references.
Change [forwardlist.ops]/24 as indicated:
void reverse();24 Effects:: Reverses the order of the elements in the list. Does not affect the validity of iterators and references.
Change 23.3.11.5 [list.ops], p15:
void remove(const T& value); template <class Predicate> void remove_if(Predicate pred);Effects: Erases all the elements in the list referred by a list iterator
i
for which the following conditions hold:*i == value, pred(*i) != false
. Invalidates only the iterators and references to the erased elements.
Change 23.3.11.5 [list.ops], p19:
void unique(); template <class BinaryPredicate> void unique(BinaryPredicate binary_pred);Effects:
EliminatesErases all but the first element from every consecutive group of equal elements referred to by the iteratori
in the range[first + 1,last)
for which*i == *(i-1)
(for the version ofunique
with no arguments) orpred(*i, *(i - 1))
(for the version ofunique
with a predicate argument) holds. Invalidates only the iterators and references to the erased elements.
Change 23.3.11.5 [list.ops], p23:
void merge(list<T,Allocator>&& x); template <class Compare> void merge(list<T,Allocator>&& x, Compare comp);Effects: If
(&x == this)
does nothing; otherwise, merges the two sorted ranges[begin(), end())
and[x.begin(), x.end())
. The result is a range in which the elements will be sorted in non-decreasing order according to the ordering defined bycomp
; that is, for every iteratori
, in the range other than the first, the conditioncomp(*i, *(i - 1)
will be false. Pointers and references to the moved elements ofx
now refer to those same elements but as members of*this
. Iterators referring to the moved elements will continue to refer to their elements, but they now behave as iterators into*this
, not intox
.
Change 23.3.11.5 [list.ops], p26:
void reverse();Effects: Reverses the order of the elements in the list. Does not affect the validity of iterators and references.
Change 23.3.11.5 [list.ops], p30:
void sort(); template <class Compare> void sort(Compare comp);Effects: Sorts the list according to the
operator<
or aCompare
function object. Does not affect the validity of iterators and references.
valarray initializer_list
constructor has incorrect effectsSection: 29.6.2.2 [valarray.cons] Status: C++11 Submitter: Howard Hinnant Opened: 2009-09-23 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
29.6.2.2 [valarray.cons] says:
valarray(initializer_list<T> il);Effects: Same as
valarray(il.begin(), il.end())
.
But there is no valarray
constructor taking two const T*
.
[ 2009-10-29 Howard: ]
Moved to Tentatively Ready after 6 positive votes on c++std-lib.
Proposed resolution:
Change 29.6.2.2 [valarray.cons]:
valarray(initializer_list<T> il);Effects: Same as
valarray(il.begin(), il.
.endsize())
match_results
should be moveableSection: 28.6.9.2 [re.results.const] Status: C++11 Submitter: Stephan T. Lavavej Opened: 2009-09-15 Last modified: 2016-01-28
Priority: Not Prioritized
View all other issues in [re.results.const].
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Discussion:
In Working Draft
N2914,
match_results
lacks a move constructor and move
assignment operator. Because it owns dynamically allocated memory, it
should be moveable.
As far as I can tell, this isn't tracked by an active issue yet; Library
Issue 723(i) doesn't talk about match_results
.
[ 2009-09-21 Daniel provided wording. ]
[ 2009-11-18: Moved to Tentatively Ready after 5 positive votes on c++std-lib. ]
Proposed resolution:
Add the following member declarations to 28.6.9 [re.results]/3:
// 28.10.1, construct/copy/destroy: explicit match_results(const Allocator& a = Allocator()); match_results(const match_results& m); match_results(match_results&& m); match_results& operator=(const match_results& m); match_results& operator=(match_results&& m); ~match_results();
Add the following new prototype descriptions to 28.6.9.2 [re.results.const]
using the table numbering of
N3000
(referring to the table titled "match_results
assignment operator effects"):
match_results(const match_results& m);4 Effects: Constructs an object of class
match_results
, as a copy ofm
.match_results(match_results&& m);5 Effects: Move-constructs an object of class
match_results
fromm
satisfying the same postconditions as Table 131. Additionally the storedAllocator
value is move constructed fromm.get_allocator()
. After the initialization of*this
setsm
to an unspecified but valid state.6 Throws: Nothing if the allocator's move constructor throws nothing.
match_results& operator=(const match_results& m);7 Effects: Assigns
m
to*this
. The postconditions of this function are indicated in Table 131.match_results& operator=(match_results&& m);8 Effects: Move-assigns
m
to*this
. The postconditions of this function are indicated in Table 131. After the assignment,m
is in a valid but unspecified state.9 Throws: Nothing.
Section: 24.3 [iterator.requirements] Status: Resolved Submitter: Alisdair Meredith Opened: 2009-09-18 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
p6 Iterator requirements 24.3 [iterator.requirements]
An iterator
j
is called reachable from an iteratori
if and only if there is a finite sequence of applications of the expression++i
that makesi == j
. Ifj
is reachable fromi
, they refer to the same container.
A good example would be stream iterators, which do not refer to a container. Typically, the end iterator from a range of stream iterators will compare equal for many such ranges. I suggest striking the second sentence.
An alternative wording might be:
If
j
is reachable fromi
, and bothi
andj
are dereferencable iterators, then they refer to the same range.
[
2010 Pittsburgh: Moved to NAD EditorialResolved. Rationale added below.
]
Rationale:
Solved by N3066.
Proposed resolution:
Change 24.3 [iterator.requirements], p6:
An iterator
j
is called reachable from an iteratori
if and only if there is a finite sequence of applications of the expression++i
that makesi == j
.Ifj
is reachable fromi
, they refer to the same container.
Section: 24.5.4.2 [move.iterator] Status: Resolved Submitter: Alisdair Meredith Opened: 2009-09-18 Last modified: 2016-05-14
Priority: Not Prioritized
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Discussion:
I contend that while we can support both bidirectional and random access
traversal, the category of a move iterator should never be better than
input_iterator_tag
.
The contentious point is that you cannot truly have a multipass property when values are moved from a range. This is contentious if you view a moved-from object as still holding a valid value within the range.
The second reason comes from the Forward Iterator requirements table:
Forward iterators 24.3.5.5 [forward.iterators]
Table 102 — Forward iterator requirements
For expression
*a
the return type is: "T&
ifX
is mutable, otherwiseconst T&
"
There is a similar constraint on a->m
.
There is no support for rvalue references, nor do I believe their should
be. Again, opinions may vary but either this table or the definition of
move_iterator
need updating.
Note: this requirement probably need updating anyway if we wish to support proxy iterators but I am waiting to see a new working paper before filing that issue.
[ 2009-10 post-Santa Cruz: ]
Move to Open. Howard to put his rationale mentioned above into the issue as a note.
[ 2009-10-26 Howard adds: ]
vector::insert(pos, iter, iter)
is significantly more effcient wheniter
is a random access iterator, as compared to when it is an input iterator.When
iter
is an input iterator, the best algorithm is to append the inserted range to the end of thevector
usingpush_back
. This may involve several reallocations before the input range is exhausted. After the append, then one can usestd::rotate
to place the inserted range into the correct position in the vector.But when
iter
is a random access iterator, the best algorithm is to first compute the size of the range to be inserted (last - first
), do a buffer reallocation if necessary, scoot existing elements in thevector
down to make the "hole", and then insert the new elements directly to their correct place.The insert-with-random-access-iterators algorithm is considerably more efficient than the insert-with-input-iterators algorithm
Now consider:
vector<A> v; // ... build up a large vector of A ... vector<A> temp; // ... build up a large temporary vector of A to later be inserted ... typedef move_iterator<vector<A>::iterator> MI; // Now insert the temporary elements: v.insert(v.begin() + N, MI(temp.begin()), MI(temp.end()));A major motivation for using
move_iterator
in the above example is the expectation thatA
is cheap to move but expensive to copy. I.e. the customer is looking for high performance. If we allowvector::insert
to subtract twoMI
's to get the distance between them, the customer enjoys substantially better performance, compared to if we say thatvector::insert
can not subtract twoMI
's.I can find no rationale for not giving this performance boost to our customers. Therefore I am strongly against restricting
move_iterator
to theinput_iterator_tag
category.I believe that the requirement that forward iterators have a dereference that returns an lvalue reference to cause unacceptable pessimization. For example
vector<bool>::iterator
also does not return abool&
on dereference. Yet I am not aware of a single vendor that is willing to shipvector<bool>::iterator
as an input iterator. Everyone classifies it as a random access iterator. Not only does this not cause any problems, it prevents significant performance problems.
Previous resolution [SUPERSEDED]:
Class template move_iterator 24.5.4.2 [move.iterator]
namespace std { template <class Iterator> class move_iterator { public: ... typedeftypename iterator_traits<Iterator>::iterator_categoryinput_iterator_tag iterator_category;
[
2010 Pittsburgh: Moved to NAD EditorialResolved. Rationale added below.
]
Rationale:
Solved by N3066.
Proposed resolution:
See N3066.
Section: 24.3 [iterator.requirements] Status: Resolved Submitter: Alisdair Meredith Opened: 2009-09-18 Last modified: 2016-01-28
Priority: Not Prioritized
View all other issues in [iterator.requirements].
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Discussion:
Forward iterator and bidirectional iterator place different requirements on the result of post-increment/decrement operator. The same form should be used in each case.
Merging row from:
Table 102 -- Forward iterator requirements Table 103 -- Bidirectional iterator requirements r++ : convertible to const X& r-- : convertible to const X& *r++ : T& if X is mutable, otherwise const T& *r-- : convertible to T
[
2010 Pittsburgh: Moved to NAD EditorialResolved. Rationale added below.
]
Rationale:
Solved by N3066.
Proposed resolution:
Section: 23.2.7 [associative.reqmts] Status: C++14 Submitter: Daniel Krügler Opened: 2009-09-20 Last modified: 2016-01-28
Priority: Not Prioritized
View other active issues in [associative.reqmts].
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Discussion:
Scott Meyers' mentions on a recent posting on c.s.c++ some arguments that point to an incomplete resolution of 103(i) and to an inconsistency of requirements on keys in ordered and unordered associative containers:
1) 103(i) introduced the term immutable without defining it in a unique manner in 23.2.7 [associative.reqmts]/5:
[..] Keys in an associative container are immutable.
According to conventional dictionaries immutable is an unconditional way of saying that something cannot be changed. So without any further explicit allowance a user always runs into undefined behavior if (s)he attempts to modify such a key. IMO this was not the intend of the committee to resolve 103(i) in that way because the comments suggest an interpretation that should give any user the freedom to modify the key in an explicit way provided it would not affect the sort order in that container.
2) Another observation was that surprisingly no similar 'safety guards' exists against unintentional key changes for the unordered associative containers, specifically there is no such requirement as in 23.2.7 [associative.reqmts]/6 that "both
iterator
andconst_iterator
are constant iterators". But the need for such protection against unintentional changes as well as the constraints in which manner any explicit changes may be performed are both missing and necessary, because such changes could potentially change the equivalence of keys that is measured by thehasher
andkey_equal
.I suggest to fix the unconditional wording involved with "immutable keys" by at least adding a hint for the reader that users may perform such changes in an explicit manner and to perform similar wording changes as 103(i) did for the ordered associative containers also for the unordered containers.
[ 2010-03-27 Daniel provides wording. ]
This update attempts to provide normative wording that harmonizes the key and function object constraints of associative and unordered containers.
[ 2010 Batavia: ]
We're uncomfortable with the first agenda item, and we can live with the second agenda item being applied before or after Madrid.
[ 2011 Bloomington ]
Further discussion persuades us this issue is Ready (and so moved). We may need a further issue clarifying the notion of key value vs. key object, as object identity appears to be important to this wording.
Proposed resolution:
Change 23.2.7 [associative.reqmts]/2 as indicated: [This ensures that associative containers make better clear what this "arbitrary" type is, as the unordered containers do in 23.2.8 [unord.req]/3]
2 Each associative container is parameterized on
Key
and an ordering relationCompare
that induces a strict weak ordering (25.4) on elements ofKey
. In addition,map
andmultimap
associate an arbitrary mapped typetypeT
with theKey
. The object of typeCompare
is called the comparison object of a container.
Change 23.2.7 [associative.reqmts]/5 as indicated: [This removes the too strong requirement that keys must not be changed at all and brings this line in sync with 23.2.8 [unord.req]/7. We take care about the real constraints by the remaining suggested changes. The rationale provided by LWG 103(i) didn't really argue why that addition is necessary, and I believe the remaining additions make it clear that any user changes have strong restrictions]:
5 For
set
andmultiset
the value type is the same as the key type. Formap
andmultimap
it is equal topair<const Key, T>
.Keys in an associative container are immutable.
Change 23.2.8 [unord.req]/3+4 as indicated: [The current sentence of p.4 has doesn't say something really new and this whole subclause misses to define the concepts of the container-specific hasher object and predicate object. We introduce the term key equality predicate which is already used in the requirements table. This change does not really correct part of this issue, but is recommended to better clarify the nomenclature and the difference between the function objects and the function object types, which is important, because both can potentially be stateful.]
3 Each unordered associative container is parameterized by
Key
, by a function object typeHash
that meets theHash
requirements (20.2.4) and acts as a hash function for argument values of typeKey
, and by a binary predicatePred
that induces an equivalence relation on values of typeKey
. Additionally,unordered_map
andunordered_multimap
associate an arbitrary mapped typeT
with theKey
.4 The container's object of type
Hash
- denoted byhash
- is called the hash function of the container. The container's object of typePred
- denoted bypred
- is called the key equality predicate of the container.A hash function is a function object that takes a single argument of type.Key
and returns a value of typestd::size_t
Change 23.2.8 [unord.req]/5 as indicated: [This adds a similar safe-guard as the last sentence of 23.2.7 [associative.reqmts]/3]
5 Two values
k1
andk2
of typeKey
are considered equivalent if the container's key equality predicatereturnskey_equal
function objecttrue
when passed those values. Ifk1
andk2
are equivalent, the container's hash function shall return the same value for both. [Note: thus, when an unordered associative container is instantiated with a non-defaultPred
parameter it usually needs a non-defaultHash
parameter as well. — end note] For any two keysk1
andk2
in the same container, callingpred(k1, k2)
shall always return the same value. For any keyk
in a container, callinghash(k)
shall always return the same value.
After 23.2.8 [unord.req]/7 add the following new paragraph: [This ensures the same level of compile-time protection that we already require for associative containers. It is necessary for similar reasons, because any change in the stored key which would change it's equality relation to others or would change it's hash value such that it would no longer fall in the same bucket, would break the container invariants]
7 For
unordered_set
andunordered_multiset
the value type is the same as the key type. Forunordered_map
andunordered_multimap
it isstd::pair<const Key, T>
.For unordered containers where the value type is the same as the key type, both
iterator
andconst_iterator
are constant iterators. It is unspecified whether or notiterator
andconst_iterator
are the same type. [Note:iterator
andconst_iterator
have identical semantics in this case, anditerator
is convertible toconst_iterator
. Users can avoid violating the One Definition Rule by always usingconst_iterator
in their function parameter lists. — end note]
list::merge
with unequal allocatorsSection: 23.3.11.5 [list.ops] Status: C++11 Submitter: Pablo Halpern Opened: 2009-09-24 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
In Bellevue (I think), we passed
N2525,
which, among other things, specifies that the behavior of
list::splice
is undefined if the allocators of the two lists
being spliced do not compare equal. The same rationale should apply to
list::merge
. The intent of list::merge
(AFAIK) is to
move nodes from one sorted list
into another sorted
list
without copying the elements. This is possible only if the
allocators compare equal.
Proposed resolution:
Relative to the August 2009 WP, N2857, change 23.3.11.5 [list.ops], paragraphs 22-25 as follows:
void merge(list&& x); template <class Compare> void merge(list&& x, Compare comp);Requires: both the list and the argument list shall be sorted according to operator< or comp.
Effects: If
(&x == this)
does nothing; otherwise, merges the two sorted ranges[begin(), end())
and[x.begin(), x.end())
. The result is a range in which the elements will be sorted in non-decreasing order according to the ordering defined bycomp
; that is, for every iteratori
, in the range other than thefirst
, the conditioncomp(*i, *(i - 1))
will befalse
.Remarks: Stable. If
(&x != this)
the range[x.begin(), x.end())
is empty after the merge. No elements are copied by this operation. The behavior is undefined ifthis->get_allocator() != x.get_allocator()
.Complexity: At most
size() + x.size() - 1
applications ofcomp
if(&x != this)
; otherwise, no applications ofcomp
are performed. If an exception is thrown other than by a comparison there are no effects.
Section: 17.9.8 [except.nested] Status: C++11 Submitter: Pete Becker Opened: 2009-09-25 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
LWG 1066(i) adds [[noreturn]]
to a bunch of things.
It doesn't add it to rethrow_nested()
, which seems like an obvious
candidate. I've made the changes indicated in the issue, and haven't
changed rethrow_nested()
.
[ 2009 Santa Cruz: ]
Move to Ready.
Proposed resolution:
Add [[noreturn]]
to rethrow_nested()
in 17.9.8 [except.nested].
Section: 32.6.4 [thread.mutex.requirements] Status: C++11 Submitter: Jeffrey Yasskin Opened: 2009-09-30 Last modified: 2016-01-28
Priority: Not Prioritized
View other active issues in [thread.mutex.requirements].
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Discussion:
If an object *o
contains a mutex mu
and a
correctly-maintained reference count c
, is the following code
safe?
o->mu.lock(); bool del = (--(o->c) == 0); o->mu.unlock(); if (del) { delete o; }
If the implementation of mutex::unlock()
can touch the mutex's
memory after the moment it becomes free, this wouldn't be safe, and
"Construction and destruction of an object of a Mutex type need not be
thread-safe" 32.6.4 [thread.mutex.requirements] may imply that
it's not safe. Still, it's useful to allow mutexes to guard reference
counts, and if it's not allowed, users are likely to write bugs.
[ 2009-11-18: Moved to Tentatively Ready after 5 positive votes on c++std-lib. ]
Proposed resolution:
Add a new paragraph after 32.6.4.2.2 [thread.mutex.class] p1:
1 The class
mutex
provides a non-recursive mutex ...[Note: After a thread
A
has calledunlock()
, releasing the mutex, it is possible for another threadB
to lock the same mutex, observe that it is no longer in use, unlock and destroy it, before threadA
appears to have returned from its unlock call. Implementations are required to handle such scenarios correctly, as long as threadA
doesn't access the mutex after the unlock call returns. These cases typically occur when a reference-counted object contains a mutex that is used to protect the reference count. — end note]
condition_variable
wait on?Section: 32.7 [thread.condition] Status: C++11 Submitter: Jeffrey Yasskin Opened: 2009-09-30 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
"Class condition_variable
provides a condition variable that can only
wait on an object of type unique_lock
" should say "...object of type
unique_lock<mutex>
"
[ 2009-11-06 Howard adds: ]
Moved to Tentatively Ready after 5 positive votes on c++std-lib.
Proposed resolution:
Change 32.7 [thread.condition], p1:
Condition variables provide synchronization primitives used to block a thread until notified by some other thread that some condition is met or until a system time is reached. Class
condition_variable
provides a condition variable that can only wait on an object of typeunique_lock<mutex>
, allowing maximum efficiency on some platforms. Classcondition_variable_any
provides a general condition variable that can wait on objects of user-supplied lock types.
condition_variable
wordingSection: 32.7.4 [thread.condition.condvar] Status: C++11 Submitter: Jeffrey Yasskin Opened: 2009-09-30 Last modified: 2016-01-28
Priority: Not Prioritized
View all other issues in [thread.condition.condvar].
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Discussion:
32.7.4 [thread.condition.condvar] says:
~condition_variable();Precondition: There shall be no thread blocked on
*this
. [Note: That is, all threads shall have been notified; they may subsequently block on the lock specified in the wait. Beware that destroying acondition_variable
object while the corresponding predicate isfalse
is likely to lead to undefined behavior. — end note]
The text hasn't introduced the notion of a "corresponding predicate" yet.
[ 2010-02-11 Anthony provided wording. ]
[ 2010-02-12 Moved to Tentatively Ready after 5 positive votes on c++std-lib. ]
Proposed resolution:
Modify 32.7.4 [thread.condition.condvar]p4 as follows:
~condition_variable();4 Precondition: There shall be no thread blocked on
*this
. [Note: That is, all threads shall have been notified; they may subsequently block on the lock specified in the wait.Beware that destroying aThe user must take care to ensure that no threads wait oncondition_variable
object while the corresponding predicate is false is likely to lead to undefined behavior.*this
once the destructor has been started, especially when the waiting threads are calling the wait functions in a loop or using the overloads ofwait
,wait_for
orwait_until
that take a predicate. — end note]
Section: 32.7 [thread.condition] Status: C++11 Submitter: Jeffrey Yasskin Opened: 2009-09-30 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
32.7.4 [thread.condition.condvar] says:
void wait(unique_lock<mutex>& lock);...
Effects:
- ...
- If the function exits via an exception,
lock.unlock()
shall be called prior to exiting the function scope.
Should that be lock.lock()
?
[ 2009-11-17 Moved to Tentatively Ready after 5 positive votes on c++std-lib. ]
Proposed resolution:
Change 32.7.4 [thread.condition.condvar] p10:
void wait(unique_lock<mutex>& lock);...
Effects:
- ...
- If the function exits via an exception,
lock.
shall be called prior to exiting the function scope.unlock()
And make a similar change in p16, and in 32.7.5 [thread.condition.condvarany], p8 and p13.
result_of
issue Section: 99 [func.ret] Status: Resolved Submitter: Sebastian Gesemann Opened: 2009-10-05 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
I think the text about std::result_of
could be a little more precise.
Quoting from
N2960...
99 [func.ret] Function object return types
template<class> class result_of; template<class Fn, class... ArgTypes> class result_of<Fn(ArgTypes...)> { public: typedef see below type; };Given an rvalue
fn
of typeFn
and valuest1, t2, ..., tN
of typesT1, T2, ... TN
inArgTypes
respectivly, thetype
member is the result type of the expressionfn(t1,t2,...,tN)
. the valuesti
are lvalues when the corresponding typeTi
is an lvalue-reference type, and rvalues otherwise.
This text doesn't seem to consider lvalue reference types for Fn
.
Also, it's not clear whether this class template can be used for
"SFINAE" like std::enable_if
. Example:
template<typename Fn, typename... Args> typename std::result_of<Fn(Args...)>::type apply(Fn && fn, Args && ...args) { // Fn may be an lvalue reference, too return std::forward<Fn>(fn)(std::forward<Args>(args)...); }
Either std::result_of<...>
can be instantiated and simply may not have
a typedef "type
" (-->SFINAE) or instantiating the class template for
some type combinations will be a "hard" compile-time error.
[ 2010-02-14 Daniel adds: ]
This issue should be considered resolved by 1255(i) and 1270(i). The wish to change
result_of
into a compiler-support trait was beyond the actual intention of the submitter Sebastian.
[
2010 Pittsburgh: Moved to NAD EditorialResolved, rationale added below.
]
Rationale:
Proposed resolution:
[ These changes will require compiler support ]
Change 99 [func.ret]:
template<class> class result_of; // undefined template<class Fn, class... ArgTypes> class result_of<Fn(ArgTypes...)> { public:typedefsee belowtype;};
Given an rvaluefn
of typeFn
and valuest1, t2, ..., tN
of typesT1, T2, ... TN
inArgTypes
respectivly, thetype
member is the result type of the expressionfn(t1,t2,...,tN)
. the valuesti
are lvalues when the corresponding typeTi
is an lvalue-reference type, and rvalues otherwise.The class template
result_of
shall meet the requirements of a TransformationTrait: Given the typesFn
,T1
,T2
, ...,TN
every template specializationresult_of<Fn(T1,T2,...,TN)>
shall define the member typedef type equivalent todecltype(RE)
if and only if the expressionRE
value<Fn>() ( value<T1>(), value<T2>(), ... value<TN>() )would be well-formed. Otherwise, there shall be no member typedef
type
defined.
[
The value<>
helper function is a utility Daniel Krügler
proposed in
N2958.
]
Section: 32.10.3 [futures.errors] Status: Resolved Submitter: Daniel Krügler Opened: 2009-10-05 Last modified: 2016-01-28
Priority: Not Prioritized
View all issues with Resolved status.
Discussion:
Defect issue 890(i) overlooked to adapt the future_category
from
32.10.1 [futures.overview] and 32.10.3 [futures.errors]:
extern const error_category* const future_category;
which should be similarly transformed into function form.
[ 2009-10-27 Howard: ]
Moved to Tentatively Ready after 5 positive votes on c++std-lib.
[ 2009-11-11 Daniel adds: ]
I just observe that the proposed resolution of this issue is incomplete and needs to reworded. The problem is that the corresponding declarations
constexpr error_code make_error_code(future_errc e); constexpr error_condition make_error_condition(future_errc e);as constexpr functions are incompatible to the requirements of constexpr functions given their specified implementation. Note that the incompatibility is not a result of the modifications proposed by the issue resolution, but already existed within the N2960 state where we have
extern const error_category* const future_category;combined with
constexpr error_code make_error_code(future_errc e);3 Returns:
error_code(static_cast<int>(e), *future_category)
.constexpr error_code make_error_condition(future_errc e);4 Returns:
error_condition(static_cast<int>(e), *future_category)
.Neither is any of the constructors of
error_code
anderror_condition
constexpr, nor does the expression*future_category
satisfy the requirements for a constant expression (7.7 [expr.const]/2 bullet 6 in N3000).The simple solution is just to remove the constexpr qualifiers for both functions, which makes sense, because none of the remaining
make_error_*
overloads in the library is constexpr. One might consider to realize that thosemake_*
functions could satisfy the constexpr requirements, but this looks not like an easy task to me, because it would need to rely on a not yet existing language feature. If such a change is wanted, a new issue should be opened after the language extension approval (if at all) [1].If no-one complaints I would like to ask Howard to add the following modifications to this issue, alternatively a new issue could be opened but I don't know what the best solution is that would cause as little overhead as possible.
What-ever the route is, the following is my proposed resolution for this issue interaction part of the story:
In 32.10.1 [futures.overview]/1, Header
<future>
synopsis and in 32.10.3 [futures.errors]/3+4 change as indicated:constexprerror_code make_error_code(future_errc e);constexprerror_condition make_error_condition(future_errc e);[1] Let me add that we have a related NAD issue here: 832(i) so the chances for realization are little IMO.
[ Howard: I've updated the proposed wording as Daniel suggests and set to Review. ]
[ 2009-11-13 Moved to Tentatively Ready after 5 positive votes on c++std-lib. ]
[ 2010 Pittsburgh: ]
Moved to
NAD EditorialResolved. Rationale added below.
Rationale:
Solved by N3058.
Proposed resolution:
Change in 32.10.1 [futures.overview], header <future>
synopsis:
externconst error_category&* constfuture_category();
In 32.10.1 [futures.overview]/1, Header <future>
synopsis
change as indicated:
constexprerror_code make_error_code(future_errc e);constexprerror_condition make_error_condition(future_errc e);
Change in 32.10.3 [futures.errors]:
externconst error_category&* constfuture_category();
1-future_category
shall point to a statically initialized object of a type derived from classerror_category
.1- Returns: A reference to an object of a type derived from class
error_category
.constexprerror_code make_error_code(future_errc e);3 Returns:
error_code(static_cast<int>(e),
.*future_category())constexprerror_codecondition make_error_condition(future_errc e);4 Returns:
error_condition(static_cast<int>(e),
.*future_category())
<bitset>
synopsis overspecifiedSection: 22.9.2 [template.bitset] Status: C++11 Submitter: Bo Persson Opened: 2009-10-05 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
The resolutions to some library defect reports, like 1178(i)
requires that #includes
in each synopsis should be taken
literally. This means that the <bitset>
header now
must include <stdexcept>
, even though none of the
exceptions are mentioned in the <bitset>
header.
Many other classes are required to throw exceptions like
invalid_argument
and out_of_range
, without explicitly
including <stdexcept>
in their synopsis. It is totally
possible for implementations to throw the needed exceptions from utility
functions, whose implementations are not visible in the headers.
I propose that <stdexcept>
is removed from the
<bitset>
header.
[ 2009-10 Santa Cruz: ]
Moved to Ready.
Proposed resolution:
Change 22.9.2 [template.bitset]:
#include <cstddef> // for size_t #include <string>#include <stdexcept> // for invalid_argument,// out_of_range, overflow_error#include <iosfwd> // for istream, ostream namespace std { ...
error_code operator=
typoSection: 19.5.4.3 [syserr.errcode.modifiers] Status: Resolved Submitter: Stephan T. Lavavej Opened: 2009-10-08 Last modified: 2016-01-28
Priority: Not Prioritized
View all issues with Resolved status.
Discussion:
N2960 19.5.4.1 [syserr.errcode.overview] and 19.5.4.3 [syserr.errcode.modifiers] say:
template <class ErrorCodeEnum> typename enable_if<is_error_code_enum<ErrorCodeEnum>::value>::type& operator=(ErrorCodeEnum e);
They should say:
template <class ErrorCodeEnum> typename enable_if<is_error_code_enum<ErrorCodeEnum>::value, error_code>::type& operator=(ErrorCodeEnum e);
Or (I prefer this form):
template <class ErrorCodeEnum> typename enable_if<is_error_code_enum<ErrorCodeEnum>::value, error_code&>::type operator=(ErrorCodeEnum e);
This is because enable_if
is declared as (21.3.8.7 [meta.trans.other]):
template <bool B, class T = void> struct enable_if;
So, the current wording makes operator=
return
void&
, which is not good.
19.5.4.3 [syserr.errcode.modifiers]/4 says
Returns:
*this
.
which is correct.
Additionally,
19.5.5.1 [syserr.errcondition.overview]/1 says:
template<typename ErrorConditionEnum> typename enable_if<is_error_condition_enum<ErrorConditionEnum>, error_code>::type & operator=( ErrorConditionEnum e );
Which contains several problems (typename
versus class
inconsistency, lack of ::value
, error_code
instead of
error_condition
), while 19.5.5.3 [syserr.errcondition.modifiers] says:
template <class ErrorConditionEnum> typename enable_if<is_error_condition_enum<ErrorConditionEnum>::value>::type& operator=(ErrorConditionEnum e);
Which returns void&
. They should both say:
template <class ErrorConditionEnum> typename enable_if<is_error_condition_enum<ErrorConditionEnum>::value, error_condition>::type& operator=(ErrorConditionEnum e);
Or (again, I prefer this form):
template <class ErrorConditionEnum> typename enable_if<is_error_condition_enum<ErrorConditionEnum>::value, error_condition&>::type operator=(ErrorConditionEnum e);
Additionally, 19.5.5.3 [syserr.errcondition.modifiers] lacks a
"Returns: *this
." paragraph, which is presumably
necessary.
[ 2009-10-18 Beman adds: ]
The proposed resolution for issue 1237(i) makes this issue moot, so it should become NAD.
[ 2009-10 Santa Cruz: ]
Proposed resolution:
Change 19.5.4.1 [syserr.errcode.overview] and 19.5.4.3 [syserr.errcode.modifiers]:
template <class ErrorCodeEnum> typename enable_if<is_error_code_enum<ErrorCodeEnum>::value, error_code&>::type&operator=(ErrorCodeEnum e);
Change 19.5.5.1 [syserr.errcondition.overview]:
template<typenameclass ErrorConditionEnum> typename enable_if<is_error_condition_enum<ErrorConditionEnum>::value, error_conditionde&>::type&operator=( ErrorConditionEnum e );
Change 19.5.5.3 [syserr.errcondition.modifiers]:
template <class ErrorConditionEnum> typename enable_if<is_error_condition_enum<ErrorConditionEnum>::value, error_condition&>::type&operator=(ErrorConditionEnum e);Postcondition:
*this == make_error_condition(e)
.Returns:
*this
.Throws: Nothing.
weak_ptr
comparisons incompletely resolvedSection: 20.3.2.3.6 [util.smartptr.weak.obs] Status: C++11 Submitter: Daniel Krügler Opened: 2009-10-10 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
The
n2637
paper suggested several updates of the ordering semantics of shared_ptr
and weak_ptr
, among those the explicit comparison operators of weak_ptr
were
removed/deleted, instead a corresponding functor owner_less
was added.
The problem is that
n2637
did not clearly enough specify, how the previous wording parts describing
the comparison semantics of weak_ptr
should be removed.
[ 2009-11-06 Howard adds: ]
Moved to Tentatively Ready after 5 positive votes on c++std-lib.
Proposed resolution:
Change 20.3.2.3 [util.smartptr.weak]/2 as described, the intention is to fix
the now no longer valid requirement that weak_ptr
is LessComparable
[Note the deleted comma]:
Specializations of
weak_ptr
shall beCopyConstructible
,andCopyAssignable
,andallowing their use in standard containers.LessThanComparable
,
In 20.3.2.3.6 [util.smartptr.weak.obs] remove the paragraphs 9-11 including prototype:
template<class T, class U> bool operator<(const weak_ptr<T>& a, const weak_ptr<U>& b);Returns: an unspecified value such that
operator<
is a strict weak ordering as described in 25.4;under the equivalence relation defined byoperator<
,!(a < b) && !(b < a)
, twoweak_ptr
instances are equivalent if and only if they share ownership or are both empty.
Throws: nothing.
[Note: Allowsweak_ptr
objects to be used as keys in associative containers. — end note]
NULL
Section: 23.2.4 [sequence.reqmts] Status: C++11 Submitter: Matt Austern Opened: 2009-10-09 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
On g++ 4.2.4 (x86_64-linux-gnu), the following file gives a compile error:
#include <vector> void foo() { std::vector<int*> v(500L, NULL); }
Is this supposed to work?
The issue: if NULL
happens to be defined as 0L
, this is an invocation of
the constructor with two arguments of the same integral type.
23.2.4 [sequence.reqmts]/14
(N3035)
says that this will behave as if the overloaded constructor
X(size_type, const value_type& = value_type(), const allocator_type& = allocator_type())
were called instead, with the arguments
static_cast<size_type>(first)
, last
and
alloc
, respectively. However, it does not say whether this
actually means invoking that constructor with the exact textual form of
the arguments as supplied by the user, or whether the standard permits
an implementation to invoke that constructor with variables of the same
type and value as what the user passed in. In most cases this is a
distinction without a difference. In this particular case it does make a
difference, since one of those things is a null pointer constant and the
other is not.
Note that an implementation based on forwarding functions will use the latter interpretation.
[ 2010 Pittsburgh: Moved to Open. ]
[ 2010-03-19 Daniel provides wording. ]
- Adapts the numbering used in the discussion to the recent working paper N3035.
- Proposes a resolution that requires implementations to use sfinae-like means to possibly filter away the too generic template c'tor. In fact this resolution is equivalent to that used for the
pair-NULL
problem (811(i)), the only difference is, that issue 1234 was already a C++03 problem.
[ Post-Rapperswil ]
Wording was verified to match with the most recent WP. Jonathan Wakely and Alberto Barbati observed that the current
WP has a defect that should be fixed here as well: The functions signatures fx1
and fx3
are
incorrectly referring to iterator
instead of const_iterator
.
Moved to Tentatively Ready with revised wording after 7 positive votes on c++std-lib.
[ Adopted at 2010-11 Batavia ]
Proposed resolution:
Change 23.2.3 [sequence.reqmts]/14+15 as indicated:
14 For every sequence container defined in this Clause and in Clause 21:
If the constructor
template <class InputIterator> X(InputIterator first, InputIterator last, const allocator_type& alloc = allocator_type())is called with a type
InputIterator
that does not qualify as an input iterator, then the constructor shall not participate in overload resolution.will behave as if the overloaded constructor:X(size_type, const value_type& = value_type(), const allocator_type& = allocator_type())
were called instead, with the arguments.static_cast<size_type>(first)
,last
andalloc
, respectivelyIf the member functions of the forms:
template <class InputIterator> // such as insert() rt fx1(const_iterator p, InputIterator first, InputIterator last); template <class InputIterator> // such as append(), assign() rt fx2(InputIterator first, InputIterator last); template <class InputIterator> // such as replace() rt fx3(const_iterator i1, const_iterator i2, InputIterator first, InputIterator last);are called with a type
InputIterator
that does not qualify as an input iterator, then these functions shall not participate in overload resolution.will behave as if the overloaded member functions:rt fx1(iterator, size_type, const value_type&);rt fx2(size_type, const value_type&);rt fx3(iterator, iterator, size_type, const value_type&);
were called instead, with the same arguments.
15 In the previous paragraph the alternative binding will fail iffirst
is not implicitly convertible toX::size_type
or iflast
is not implicitly convertible toX::value_type
.
error_code
/error_condition members
Section: 19.5 [syserr] Status: C++11 Submitter: Daniel Krügler Opened: 2009-10-14 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
I'm just reflecting on the now SFINAE-constrained constructors
and assignment operators of error_code
and error_condition
:
These are the only library components that are pro-actively
announcing that they are using std::enable_if
as constraining tool,
which has IMO several disadvantages:
With the availability of template default arguments and
decltype, using enable_if
in the C++0x standard library, seems
unnecessary restricting implementation freedom. E.g. there
should be no need for a useless specification of a dummy
default function argument, which only confuses the reader.
A more reasonable implementation could e.g. be
template <class ErrorCodeEnum class = typename enable_if<is_error_code_enum<ErrorCodeEnum>::value>::type> error_code(ErrorCodeEnum e);
As currently specified, the function signatures are so unreadable, that errors quite easily happen, see e.g. 1229(i).
We have a lot of constrained functions in other places, that now have a standard phrase that is easily understandable:
Remarks: This constructor/function shall participate in overload resolution if and only if X.
where X describes the condition. Why should these components deviate?
If enable_if
would not be explicitly specified, the standard library
is much better prepared for the future. It would also be possible, that
libraries with partial support for not-yet-standard-concepts could provide
a much better diagnostic as is possible with enable_if
. This again
would allow for experimental concept implementations in the wild,
which as a result would make concept standardization a much more
natural thing, similar to the way as templates were standardized
in C++.
In summary: I consider it as a library defect that error_code
and
error_condition
explicitly require a dependency to enable_if
and
do limit implementation freedom and I volunteer to prepare a
corresponding resolution.
[ 2009-10-18 Beman adds: ]
I support this proposed resolution, and thank Daniel for writing it up.
[ 2009-10 Santa Cruz: ]
Moved to Ready.
Proposed resolution:
[ Should this resolution be accepted, I recommend to resolve 1229(i) as NAD ]
In 19.5.4.1 [syserr.errcode.overview]/1, class error_code
,
change as indicated:
// 19.5.2.2 constructors: error_code(); error_code(int val, const error_category& cat); template <class ErrorCodeEnum> error_code(ErrorCodeEnum e, typename enable_if<is_error_code_enum<ErrorCodeEnum>::value>::type * = 0); // 19.5.2.3 modifiers: void assign(int val, const error_category& cat); template <class ErrorCodeEnum>typename enable_if<is_error_code_enum<ErrorCodeEnum>::value>::typeerror_code& operator=(ErrorCodeEnum e); void clear();
Change 19.5.4.2 [syserr.errcode.constructors] around the prototype before p. 7:
template <class ErrorCodeEnum> error_code(ErrorCodeEnum e, typename enable_if<is_error_code_enum<ErrorCodeEnum>::value>::type * = 0);Remarks: This constructor shall not participate in overload resolution, unless
is_error_code_enum<ErrorCodeEnum>::value == true
.
Change 19.5.4.3 [syserr.errcode.modifiers] around the prototype before p. 3:
template <class ErrorCodeEnum>typename enable_if<is_error_code_enum<ErrorCodeEnum>::value>::typeerror_code& operator=(ErrorCodeEnum e);Remarks: This operator shall not participate in overload resolution, unless
is_error_code_enum<ErrorCodeEnum>::value == true
.
In 19.5.5.1 [syserr.errcondition.overview]/1, class error_condition
, change
as indicated:
// 19.5.3.2 constructors: error_condition(); error_condition(int val, const error_category& cat); template <class ErrorConditionEnum> error_condition(ErrorConditionEnum e, typename enable_if<is_error_condition_enum<ErrorConditionEnum>::type* = 0); // 19.5.3.3 modifiers: void assign(int val, const error_category& cat); template<typenameclass ErrorConditionEnum>typename enable_if<is_error_condition_enum<ErrorConditionEnum>, error_code>::typeerror_condition & operator=( ErrorConditionEnum e ); void clear();
Change 19.5.5.2 [syserr.errcondition.constructors] around the prototype before p. 7:
template <class ErrorConditionEnum> error_condition(ErrorConditionEnum e, typename enable_if<is_error_condition_enum<ErrorConditionEnum>::value>::type* = 0);Remarks: This constructor shall not participate in overload resolution, unless
is_error_condition_enum<ErrorConditionEnum>::value == true
.
Change 19.5.5.3 [syserr.errcondition.modifiers] around the prototype before p. 3:
template <class ErrorConditionEnum>typename enable_if<is_error_condition_enum<ErrorConditionEnum>::value>::typeerror_condition& operator=(ErrorConditionEnum e);Remarks: This operator shall not participate in overload resolution, unless
is_error_condition_enum<ErrorConditionEnum>::value == true
.Postcondition:
*this == make_error_condition(e)
.Returns:
*this
std::function
not neededSection: 22.10.17.3 [func.wrap.func] Status: C++11 Submitter: Daniel Krügler Opened: 2009-10-18 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
The class template std::function
contains the following member
declarations:
// deleted overloads close possible hole in the type system template<class R2, class... ArgTypes2> bool operator==(const function<R2(ArgTypes2...)>&) = delete; template<class R2, class... ArgTypes2> bool operator!=(const function<R2(ArgTypes2...)>&) = delete;
The leading comment here is part of the history of std::function
, which
was introduced with
N1402.
During that time no explicit conversion functions existed, and the
"safe-bool" idiom (based on pointers-to-member) was a popular
technique. The only disadvantage of this idiom was that given two
objects f1
and f2
of type std::function
the expression
f1 == f2;
was well-formed, just because the built-in operator==
for pointer to member
was considered after a single user-defined conversion. To fix this, an
overload set of undefined comparison functions was added,
such that overload resolution would prefer those ending up in a linkage error.
The new language facility of deleted functions provided a much better
diagnostic mechanism to fix this issue.
The central point of this issue is, that with the replacement of the
safe-bool idiom by explicit conversion to bool the original "hole in the
type system" does no longer exist and therefore the comment is wrong and
the superfluous function definitions should be removed as well. An
explicit conversion function is considered in direct-initialization
situations only, which indirectly contain the so-called "contextual
conversion to bool" (7.3 [conv]/3). These conversions are not considered for
==
or !=
as defined by the core language.
[ Post-Rapperswil ]
Moved to Tentatively Ready after 5 positive votes on c++std-lib.
[ Adopted at 2010-11 Batavia ]
Proposed resolution:
In 22.10.17.3 [func.wrap.func]/1, class function change as indicated:
// 20.7.15.2.3, function capacity: explicit operator bool() const;// deleted overloads close possible hole in the type systemtemplate<class R2, class... ArgTypes2>bool operator==(const function<R2(ArgTypes2...)>&) = delete;template<class R2, class... ArgTypes2>bool operator!=(const function<R2(ArgTypes2...)>&) = delete;
unique_copy
needs to require EquivalenceRelation
Section: 26.7.9 [alg.unique] Status: C++11 Submitter: Daniel Krügler Opened: 2009-10-17 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
A lot of fixes were silently applied during concept-time and we should
not lose them again. The Requires clause of 26.7.9 [alg.unique]/5
doesn't mention that ==
and the predicate need to satisfy an
EquivalenceRelation
, as it is correctly said for unique
.
This was intentionally fixed during conceptification, were we had:
template<InputIterator InIter, class OutIter> requires OutputIterator<OutIter, RvalueOf<InIter::value_type>::type> && EqualityComparable<InIter::value_type> && HasAssign<InIter::value_type, InIter::reference> && Constructible<InIter::value_type, InIter::reference> OutIter unique_copy(InIter first, InIter last, OutIter result); template<InputIterator InIter, class OutIter, EquivalenceRelation<auto, InIter::value_type> Pred> requires OutputIterator<OutIter, RvalueOf<InIter::value_type>::type> && HasAssign<InIter::value_type, InIter::reference> && Constructible<InIter::value_type, InIter::reference> && CopyConstructible<Pred> OutIter unique_copy(InIter first, InIter last, OutIter result, Pred pred);
Note that EqualityComparable implied an equivalence relation.
[
N.B. adjacent_find
was also specified to require
EquivalenceRelation
, but that was considered as a defect in
concepts, see 1000(i)
]
[ 2009-10-31 Howard adds: ]
Moved to Tentatively Ready after 5 positive votes on c++std-lib.
Proposed resolution:
Change 26.7.9 [alg.unique]/5 as indicated:
template<class InputIterator, class OutputIterator> OutputIterator unique_copy(InputIterator first, InputIterator last, OutputIterator result); template<class InputIterator, class OutputIterator, class BinaryPredicate> OutputIterator unique_copy(InputIterator first, InputIterator last, OutputIterator result, BinaryPredicate pred);Requires: The comparison function shall be an equivalence relation. The ranges
[first,last)
and[result,result+(last-first))
shall not overlap. The expression*result = *first
shall be valid. If neitherInputIterator
norOutputIterator
meets the requirements of forward iterator then the value type ofInputIterator
shall beCopyConstructible
(34) andCopyAssignable
(table 36). OtherwiseCopyConstructible
is not required.
wait_*()
in *future
for synchronous functionsSection: 32.10 [futures] Status: Resolved Submitter: Detlef Vollmann Opened: 2009-10-22 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
With the addition of async()
, a future
might be
associated with a function that is not running in a different thread but
is stored to by run synchronously on the get()
call. It's not
clear what the wait()
functions should do in this case.
Suggested resolution:
Throw an exception.
[
2010 Pittsburgh: Moved to NAD EditorialResolved. Rationale added below.
]
Rationale:
Solved by N3058.
Proposed resolution:
std::hash<string>
& coSection: 22.10.19 [unord.hash] Status: C++11 Submitter: Paolo Carlini Opened: 2009-10-22 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
In 22.10.19 [unord.hash], operator()
is specified as
taking the argument by value. Moreover, it is said that operator()
shall
not throw exceptions.
However, for the specializations for class types, like string
,
wstring
, etc, the former requirement seems suboptimal from the
performance point of view (a specific PR has been filed about this in the GCC Bugzilla)
and, together with the latter requirement, hard if not impossible to
fulfill. It looks like pass by const reference should be allowed in such
cases.
[ 2009-11-18: Ganesh updates wording. ]
I've removed the list of types for which
hash
shall be instantiated because it's already explicit in the synopsis of header<functional>
in 22.10 [function.objects]/2.
[ 2009-11-18: Original wording here: ]
Add to 22.10.19 [unord.hash]/2:
namespace std { template <class T> struct hash : public std::unary_function<T, std::size_t> { std::size_t operator()(T val) const; }; }The return value of
operator()
is unspecified, except that equal arguments shall yield the same result.operator()
shall not throw exceptions. It is also unspecified whetheroperator()
ofstd::hash
specializations for class types takes its argument by value or const reference.
[ 2009-11-19 Moved to Tentatively Ready after 5 positive votes on c++std-lib. ]
[ 2009-11-24 Ville Opens: ]
I have received community requests to ask for this issue to be reopened. Some users feel that mandating the inheritance is overly constraining.
[ 2010-01-31 Alisdair: related to 978(i) and 1182(i). ]
[ 2010-02-07 Proposed resolution updated by Beman, Daniel and Ganesh. ]
[ 2010-02-09 Moved to Tentatively Ready after 5 positive votes on c++std-lib. ]
Proposed resolution:
Insert a new subclause either before or after the current 16.4.4.6 [allocator.requirements]:
Hash
Requirements [hash.requirements]This subclause defines the named requirement
Hash
, used in several clauses of the C++ standard library. A typeH
meets theHash
requirement if
it is a function object type (22.10 [function.objects]).
it satisfies the requirements of
CopyConstructible
, andDestructible
(16.4.4.2 [utility.arg.requirements]),the expressions shown in the following table are valid and have the indicated semantics, and
it satisfies all other requirements of this subclause.
Given
Key
is an argument type for function objects of typeH
, in the table belowh
is a value of type (possiblyconst
)H
,u
is an lvalue of typeKey
, andk
is a value of a type convertible to (possiblyconst
)Key
:
Table ? — Hash
requirementsExpression Return type Requirement h(k)
size_t
Shall not throw exceptions. The value returned shall depend only on the argument k
. [Note: Thus all evaluations of the expressionh(k)
with the same value fork
yield the same result. — end note] [Note: Fort1
andt2
of different values, the probability thath(t1)
andh(t2)
compare equal should be very small, approaching(1.0/numeric_limits<size_t>::max())
. — end note] Comment (not to go in WP): The wording for the second note is based on a similar note in 28.3.4.5.1.3 [locale.collate.virtuals]/3h(u)
size_t
Shall not modify u
.
Change 22.10.19 [unord.hash] as indicated:
1 The unordered associative containers defined in Clause 23.5 [unord] use specializations of the class template
hash
as the default hash function. For all object typesT
for which there exists a specializationhash<T>
, the instantiationhash<T>
shall:
- satisfy the
Hash
requirements([hash.requirements]), withT
as the function call argument type, theDefaultConstructible
requirements ([defaultconstructible]), theCopyAssignable
requirements ([copyassignable]), and theSwappable
requirements ([swappable]),- provide two nested types
result_type
andargument_type
which shall be synonyms forsize_t
andT
, respectively,- satisfy the requirement that if
k1 == k2
istrue
,h(k1) == h(k2)
istrue
, whereh
is an object of typehash<T>
, andk1
,k2
are objects of typeT
.
This class template is only required to be instantiable for integer types (6.8.2 [basic.fundamental]), floating-point types (6.8.2 [basic.fundamental]), pointer types (9.3.4.2 [dcl.ptr]), andstd::string
,std::u16string
,std::u32string
,std::wstring
,std::error_code
,std::thread::id
,std::bitset
, andstd::vector<bool>
.namespace std { template <class T> struct hash : public std::unary_function<T, std::size_t> { std::size_t operator()(T val) const; }; }
2 The return value ofoperator()
is unspecified, except that equal arguments shall yield the same result.operator()
shall not throw exceptions.
Change Unordered associative containers 23.2.8 [unord.req] as indicated:
Each unordered associative container is parameterized by
Key
, by a function object typeHash
([hash.requirements]) that acts as a hash function for argument values of typeKey
, and by a binary predicatePred
that induces an equivalence relation on values of typeKey
. Additionally,unordered_map
andunordered_multimap
associate an arbitrary mapped typeT
with theKey
.A hash function is a function object that takes a single argument of type
Key
and returns a value of typestd::size_t
.Two values
k1
andk2
of typeKey
are considered equal if the container's equality function object returnstrue
when passed those values. Ifk1
andk2
are equal, the hash function shall return the same value for both. [Note: Thus supplying a non-defaultPred
parameter usually implies the need to supply a non-defaultHash
parameter. — end note]
auto_ptr
is overspecifiedSection: 99 [auto.ptr] Status: C++11 Submitter: Alisdair Meredith Opened: 2009-10-24 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
This issue is extracted as the ongoing point-of-interest from earlier issue 463(i).
auto_ptr
is overspecified as the auto_ptr_ref
implementation detail is formally specified, and the technique is
observable so workarounds for compiler defects can cause a working
implementation of the primary auto_ptr
template become
non-conforming.
auto_ptr_ref
is a documentation aid to describe a possible
mechanism to implement the class. It should be marked exposition only,
as per similar classes, e.g., istreambuf_iterator::proxy
[ 2009-10-25 Daniel adds: ]
I wonder, whether the revised wording shouldn't be as straight as for
istream_buf
by adding one further sentence:An implementation is permitted to provide equivalent functionality without providing a class with this name.
[ 2009-11-06 Alisdair adds Daniel's suggestion to the proposed wording. ]
[ 2009-11-06 Howard moves issue to Review. ]
[ 2009-11-14 Moved to Tentatively Ready after 5 positive votes on c++std-lib. ]
Proposed resolution:
Add the term "exposition only" in the following two places:
Ammend 99 [auto.ptr]p2:
The exposition only class
Ttemplateauto_ptr_ref
holds a reference to anauto_ptr
. It is used by theauto_ptr
conversions to allowauto_ptr
objects to be passed to and returned from functions. An implementation is permitted to provide equivalent functionality without providing a class with this name.namespace std { template <class Y> struct auto_ptr_ref { }; // exposition only
Section: 23.5 [unord] Status: Resolved Submitter: Herb Sutter Opened: 2009-10-25 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
See N2986.
[ 2010-01-22 Alisdair Opens. ]
[ 2010-01-24 Alisdair provides wording. ]
[
2010 Pittsburgh: Moved to NAD EditorialResolved. Rationale added below.
]
Rationale:
Solved by N3068.
Proposed resolution:
Apply paper N2986.
basic_ios
default ctorSection: 31.5.4.2 [basic.ios.cons] Status: C++11 Submitter: Martin Sebor Opened: 2009-10-25 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
The basic_ios
default ctor is required to leave the objects members
uninitialized (see below). The paragraph says the object must be
initialized by calling basic_ios::init()
before it's destroyed but
I can't find a requirement that it be initialized before calling
any of the class other member functions. Am I not looking in the
right place or that an issue?
[ 2009-10-25 Daniel adds: ]
I agree, that your wording makes that clearer, but suggest to write
... calling
basic_ios::init
before ...()Doing so, I recommend to adapt that of
ios_base();
as well, where we have:Effects: Each
ios_base
member has an indeterminate value after construction. These members shall be initialized by callingbasic_ios::init
. If anios_base
object is destroyed before these initializations have taken place, the behavior is undefined.
[ Post-Rapperswil: ]
Moved to Tentatively Ready after 5 positive votes on c++std-lib.
[ Adopted at 2010-11 Batavia ]
Proposed resolution:
Change 31.5.2.8 [ios.base.cons] p1:
ios_base();Effects: Each
ios_base
member has an indeterminate value after construction.TheseThe object's members shall be initialized by callingbasic_ios::init
before the object's first use or before it is destroyed, whichever comes first; otherwise the behavior is undefined..If anios_base
object is destroyed before these initializations have taken place, the behavior is undefined.
Change 31.5.4.2 [basic.ios.cons] p2:
basic_ios();Effects: Constructs an object of class
basic_ios
(27.5.2.7) leaving its member objects uninitialized. The object shall be initialized by callingitsbasic_ios::init
before its first use or before it is destroyed, whichever comes first; otherwise the behavior is undefined.member function. If it is destroyed before it has been initialized the behavior is undefined.
<bitset>
still overspecifiedSection: 22.9.2 [template.bitset] Status: C++11 Submitter: Martin Sebor Opened: 2009-10-29 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
Issue 1227(i) — <bitset>
synopsis overspecified makes the observation
that std::bitset
, and in fact the whole library, may be implemented
without needing to #include <stdexcept>
in any library header. The
proposed resolution removes the #include <stdexcept>
directive from
the header.
I'd like to add that the <bitset>
header (as well as the rest of
the library) has also been implemented without #including the
<cstddef>
header in any library header. In the case of std::bitset
,
the template is fully usable (i.e., it may be instantiated and all
its member functions may be used) without ever mentioning size_t
.
In addition, just like no library header except for <bitset>
#includes <stdexcept>
in its synopsis, no header but <bitset>
#includes <cstddef>
either.
Thus I suggest that the #include <cstddef>
directive be similarly
removed from the synopsis of <bitset>
.
[ 2010-02-08 Moved to Tentatively Ready after 6 positive votes on c++std-lib. ]
Proposed resolution:
Change 22.9.2 [template.bitset]:
#include <cstddef> // for size_t#include <string> #include <iosfwd> // for istream, ostream namespace std { ...
wbuffer_convert::state_type
inconsistencySection: 99 [depr.conversions.buffer] Status: C++11 Submitter: Bo Persson Opened: 2009-10-21 Last modified: 2017-04-22
Priority: Not Prioritized
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Discussion:
The synopsis for wbuffer_convert
[conversions.buffer]/2 contains
typedef typename Tr::state_type state_type;
making state_type
a synonym for (possibly) some
char_traits<x>::state_type
.
However, in paragraph 9 of the same section, we have
typedef typename Codecvt::state_type state_type;The type shall be a synonym for
Codecvt::state_type
.
From what I can see, it might be hard to implement wbuffer_convert
if
the types were not both std::mbstate_t
, but I cannot find a requirement
that they must be the same type.
[ Batavia 2010: ]
Howard to draft wording, move to Review. Run it by Bill. Need to move this in Madrid.
[2011-03-06: Howard drafts wording]
[2011-03-24 Madrid meeting]
Moved to Immediate
Proposed resolution:
Modify the state_type
typedef in the synopsis of [conversions.buffer] p.2 as shown
[This makes the synopsis consistent with [conversions.buffer] p.9]:
namespace std { template<class Codecvt, class Elem = wchar_t, class Tr = std::char_traits<Elem> > class wbuffer_convert : public std::basic_streambuf<Elem, Tr> { public: typedef typenameTrCodecvt::state_type state_type; […] }; }
emplace
vs. insert
inconsistence in assoc. containersSection: 23.2.7 [associative.reqmts] Status: C++11 Submitter: Boris Dušek Opened: 2009-10-24 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
In the latest published draft
N2960,
section 23.2.7 [associative.reqmts], paragraph 8, it is specified
that that insert
does not invalidate any iterators. As per
23.2.2 [container.requirements.general], paragraph 12, this holds
true not only for insert
, but emplace
as well. This
gives the insert
member a special treatment w.r.t.
emplace
member in 23.2.7 [associative.reqmts], par. 8,
since both modify the container. For the sake of consistency, in 23.2.7 [associative.reqmts], par. 8: either reference to insert
should be removed (i.e. count on 23.2.2 [container.requirements.general],
par. 12), or reference to emplace
be added (i.e. mention all
members of assoc. containers that modify it).
[ 2009-11-18 Chris provided wording. ]
This suggested wording covers both the issue discussed, and a number of other identical issues (namely
insert
being discussed withoutemplace
). I'm happy to go back and split and introduce a new issue if appropriate, but I think the changes are fairly mechanical and obvious.
[
2010-01-23 Daniel Krügler and J. Daniel García updated wording to
make the use of hint
consistent with insert
.
]
[
2011-02-23 Daniel Krügler adapts wording to numbering changes to match the N3225 draft. During this
action it was found that 23.2.8 [unord.req] had been changed considerably
due to acceptance of N3068
during the Pittsburgh meeting, such that the suggested wording change to
p. 6 can no longer be applied. The new wording is more general and should
now include insert()
and emplace()
calls as well ("mutating operations").
]
[2011-03-06 Daniel Krügler adapts wording to numbering changes to match the N3242 draft]
Proposed resolution:
Modify bullet 1 of 23.2.2 [container.requirements.general], p. 10:
10 Unless otherwise specified (see [associative.reqmts.except], [unord.req.except], [deque.modifiers], and [vector.modifiers]) all container types defined in this Clause meet the following additional requirements:
insert()
or
emplace()
function while inserting a single element, that
function has no effects.
Modify 23.2.7 [associative.reqmts], p. 4:
4 An associative container supports unique keys if it may contain at most one element for each key. Otherwise, it supports equivalent keys. The
set
andmap
classes support unique keys; themultiset
andmultimap
classes support equivalent keys. Formultiset
andmultimap
,insert
,emplace
, anderase
preserve the relative ordering of equivalent elements.
Modify Table 102 — Associative container requirements in 23.2.7 [associative.reqmts]:
Table 102 — Associative container requirements (in addition to container) Expression Return type Assertion/note
pre-/post-conditionComplexity […] a_eq.emplace(args)
iterator
[…] Effects: Inserts a T
objectt
constructed withstd::forward<Args>(args)...
and returns the iterator pointing to the newly inserted element. If a range containing elements equivalent tot
exists ina_eq
,t
is inserted at the end of that range.logarithmic a.emplace_hint(p, args)
iterator
equivalent to a.emplace(std::forward<Args>(args)...)
. Return value is an iterator pointing to the element with the key equivalent to the newly inserted element.TheThe element is inserted as close as possible to the position just prior toconst_iterator p
is a hint pointing to where the search should start.p
.Implementations are permitted to ignore the hint.logarithmic in general, but amortized constant if the element is inserted right afterbeforep
[…]
Modify 23.2.7 [associative.reqmts], p. 9:
9 The
insert
andemplace
members shall not affect the validity of iterators and references to the container, and theerase
members shall invalidate only iterators and references to the erased elements.
Modify 23.2.7.2 [associative.reqmts.except], p. 2:
2 For associative containers, if an exception is thrown by any operation from within an
insert()
oremplace()
function inserting a single element, theinsertion has no effect.insert()
function
Modify 23.2.8 [unord.req], p. 13 and p. 14:
6 An unordered associative container supports unique keys if it may contain at most one element for each key. Otherwise, it supports equivalent keys.
unordered_set
andunordered_map
support unique keys.unordered_multiset
andunordered_multimap
support equivalent keys. In containers that support equivalent keys, elements with equivalent keys are adjacent to each other in the iteration order of the container. Thus, although the absolute order of elements in an unordered container is not specified, its elements are grouped into equivalent-key groups such that all elements of each group have equivalent keys. Mutating operations on unordered containers shall preserve the relative order of elements within each equivalent-key group unless otherwise specified.[…]
13 The
insert
andemplace
members shall not affect the validity of references to container elements, but may invalidate all iterators to the container. The erase members shall invalidate only iterators and references to the erased elements.14 The
insert
andemplace
members shall not affect the validity of iterators if(N+n) < z * B
, whereN
is the number of elements in the container prior to the insert operation,n
is the number of elements inserted,B
is the container's bucket count, andz
is the container's maximum load factor.
Modify 23.2.8.2 [unord.req.except], p. 2:
2 For unordered associative containers, if an exception is thrown by any operation other than the container's hash function from within an
insert()
oremplace()
function inserting a single element, theinsertioninsert()
functionhas no effect.
vector<bool>::flip
Section: 23.3.14 [vector.bool] Status: C++11 Submitter: Christopher Jefferson Opened: 2009-11-01 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
The effects of vector<bool>::flip
has the line:
It is unspecified whether the function has any effect on allocated but unused bits.
While this is technically true, it is misleading, as any member function in any standard container may change unused but allocated memory. Users can never observe such changes as it would also be undefined behaviour to read such memory.
[ 2009-11-14 Moved to Tentatively Ready after 5 positive votes on c++std-lib. ]
Proposed resolution:
Strike second sentence from the definition of vector<bool>::flip()
,
23.3.14 [vector.bool], paragraph 5.
Effects: Replaces each element in the container with its complement.
It is unspecified whether the function has any effect on allocated but unused bits.
declval
should be added to the librarySection: 22.2 [utility] Status: C++11 Submitter: Daniel Krügler Opened: 2009-11-03 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
During the Santa Cruz meeting it was decided to split off the provision
of the library utility value()
proposed in
N2979
from the concrete request of the
UK 300
comment.
The provision of a new library component that allows the production of
values in unevaluated expressions is considered as important
to realize constrained templates in C++0x where concepts are not
available.
The following proposed resolution is an improvement over that suggested in N2958, because the proposed component can now be defined without loss of general usefulness and any use by user-code will make the program ill-formed. A possible prototype implementation that satisfies the core language requirements can be written as:
template<class T>
struct declval_protector {
static const bool stop = false;
static typename std::add_rvalue_reference<T>::type delegate(); // undefined
};
template<class T>
typename std::add_rvalue_reference<T>::type declval() {
static_assert(declval_protector<T>::stop, "declval() must not be used!");
return declval_protector<T>::delegate();
}
Further-on the earlier suggested name value()
has been changed to declval()
after discussions with committee members.
Finally the suggestion shown below demonstrates that it can simplify
existing standard wording by directly using it in the library
specification, and that it also improves an overlooked corner case for
common_type
by adding support for cv void
.
[ 2009-11-19 Moved to Tentatively Ready after 6 positive votes on c++std-lib. ]
Proposed resolution:
[ The proposed resolution has been updated to N3000 numbering and wording ]
Change 22.2 [utility], header <utility>
synopsis
as indicated:
// 20.3.3, forward/move: template <class T> struct identity; template <class T, class U> T&& forward(U&&); template <class T> typename remove_reference<T>::type&& move(T&&); // 20.3.4, declval: template <class T> typename add_rvalue_reference<T>::type declval(); // as unevaluated operand
Immediately after the current section 22.2.4 [forward] insert a new section:
20.3.4 Function template declval [declval]
The library provides the function template declval
to simplify
the definition of expressions which occur as
unevaluated operands (7 [expr]). The
template parameter T
of declval
may
be an incomplete type.
template <class T> typename add_rvalue_reference<T>::type declval(); // as unevaluated operand
Remarks: If this function is used according to 6.3 [basic.def.odr], the program is ill-formed.
[Example:
template<class To, class From> decltype(static_cast<To>(declval<From>())) convert(From&&);declares a function template
convert
, which only participates in overloading if the typeFrom
can be explicitly cast to typeTo
. For another example see class templatecommon_type
(21.3.8.7 [meta.trans.other]). — end example]
This bullet just makes clear that after applying
N2984,
the changes in 21.3.5.4 [meta.unary.prop], before
table Type property queries should not use declval
,
because the well-formedness requirement of the specification of
is_constructible
would become more complicated, because we
would need to make sure that the expression CE is checked in an
unevaluated context.
Also 21.3.7 [meta.rel]/4 is not modified similar to the previous bullet,
because with the stricter requirements of not using declval()
the well-formedness condition
would be harder to specify. The following changes are only editorial ones (e.g.
the removal of the duplicate declaration of create()
):
Given the following function prototype:
template <class T> typename add_rvalue_reference<T>::type create();the predicate condition for a template specialization
is_convertible<From, To>
shall be satisfied if and only if the return expression in the following code would be well-formed, including any implicit conversions to the return type of the function:template <class T> typename add_rvalue_reference<T>::type create();To test() { return create<From>(); }
Change the entry in column "Comments" for common_type
in Table 51 —
Other transformations (21.3.8.7 [meta.trans.other]):
[
NB: This wording change extends the type domain of common_type
for cv
void => cv void
transformations and thus makes common_type
usable for
all binary type combinations that are supported by is_convertible
]
The member typedef
type
shall be defined as set out below. All types in the parameter packT
shall be complete or (possibly cv-qualified)void
. A program may specialize this trait if at least one template parameter in the specialization is a user-defined type. [Note: Such specializations are needed when only explicit conversions are desired among the template arguments. — end note]
Change 21.3.8.7 [meta.trans.other]/3 as indicated:
[
NB: This wording change is more than an editorial simplification of
the definition of common_type
: It also extends its usefulness for cv
void
types as outlined above
]
The nested typedef
common_type::type
shall be defined as follows:[..]
template <class T, class U> struct common_type<T, U> {private: static T&& __t(); static U&& __u(); public:typedef decltype(true ?__tdeclval<T>() :__udeclval<U>()) type; };
Change 99 [func.ret]/1 as indicated [This part solves some main aspects of issue 1225(i)]:
namespace std { template <class> class result_of; // undefined template <class Fn, class... ArgTypes> class result_of<Fn(ArgTypes...)> { public :// typestypedefsee belowdecltype(declval<Fn>() ( declval<ArgTypes>()... )) type; }; }
1 Given an rvaluefn
of typeFn
and valuest1, t2, ..., tN
of typesT1, T2, ..., TN
inArgTypes
, respectively, thetype
member is the result type of the expressionfn(t1, t2, ...,tN)
. The valuesti
are lvalues when the corresponding typeTi
is an lvalue-reference type, and rvalues otherwise.
weak_ptr
comparison functions should be removedSection: 20.3.2.3 [util.smartptr.weak] Status: C++11 Submitter: Daniel Krügler Opened: 2009-11-04 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
Additional to the necessary cleanup of the description of the the
weak_ptr
component from 20.3.2.3 [util.smartptr.weak]
described in 1231(i) it turns out that the currently deleted
comparison functions of weak_ptr
are not needed at all: There
is no safe-bool conversion from weak_ptr
, and it won't silently
chose a conversion to shared_ptr
.
[ 2009-11-14 Moved to Tentatively Ready after 5 positive votes on c++std-lib. ]
Proposed resolution:
Change 20.3.2.3 [util.smartptr.weak]/1 as indicated:
namespace std { template<class T> class weak_ptr { public: ...// comparisonstemplate<class Y> bool operator<(weak_ptr<Y> const&) const = delete;template<class Y> bool operator<=(weak_ptr<Y> const&) const = delete;template<class Y> bool operator>(weak_ptr<Y> const&) const = delete;template<class Y> bool operator>=(weak_ptr<Y> const&) const = delete;}; ...
concept_map
Section: 31.5 [iostreams.base] Status: C++11 Submitter: Beman Dawes Opened: 2009-11-04 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
The current WP still contains a concept_map
.
[ 2009-11-11 Moved to Tentatively Ready after 5 positive votes on c++std-lib. ]
Proposed resolution:
Change Iostreams base classes 31.5 [iostreams.base], Header <ios> synopsis, as indicated:
concept_map ErrorCodeEnum<io_errc> { };template <> struct is_error_code_enum<io_errc> : true_type { } error_code make_error_code(io_errc e); error_condition make_error_condition(io_errc e); const error_category& iostream_category();
Section: 22.10.17.3.3 [func.wrap.func.mod] Status: Resolved Submitter: Daniel Krügler Opened: 2009-11-05 Last modified: 2016-01-28
Priority: Not Prioritized
View all issues with Resolved status.
Discussion:
As of 22.10.17.3.3 [func.wrap.func.mod]/2+ we have the following prototype description:
template<class F, Allocator Alloc> void assign(F, const Alloc&);Effects:
function(f, a).swap(*this)
Two things: First the concept debris needs to be removed, second and
much more importantly, the effects clause is now impossible to satisfy,
because there is no constructor that would match the parameter sequence
(FunctionObject
, Allocator
) [plus the fact that no
f
and no a
is part of the signature]. The most
probable candidate is
template<class F, class A> function(allocator_arg_t, const A&, F);
and the effects clause needs to be adapted to use this signature.
[ 2009-11-13 Daniel brought wording up to date. ]
[ 2009-11-15 Moved to Tentatively Ready after 5 positive votes on c++std-lib. ]
[ 2010-02-11 Moved to Tentatively NAD Editorial after 5 positive votes on c++std-lib. It was noted that this issue was in partial conflict with 1288(i), and the two issues were merged in 1288(i). ]
Rationale:
Proposed resolution:
Change in 22.10.17.3.3 [func.wrap.func.mod] the complete prototype description as indicated
[ Question to the editor: Shouldn't there a paragraph number in front of the Effects clause? ]
template<class F,Allocator Allocclass A> void assign(F f, const Alloc& a);3 Effects:
function(
f, aallocator_arg, a, f).swap(*this)
is_constructible<int*,void*>
reports trueSection: 21.3.5.4 [meta.unary.prop] Status: Resolved Submitter: Peter Dimov Opened: 2009-11-07 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
The specification of is_constructible<T,Args...>
in
N3000
uses
static_cast<T>(create<Args>()...)
for the one-argument case, but static_cast
also permits
unwanted conversions such as void*
to T*
and
Base*
to Derived*
.
[ Post-Rapperswil: ]
Moved to
NAD EditorialResolved, this issue is addressed by paper n3047
Proposed resolution:
Change 21.3.5.4 [meta.unary.prop], p6:
the predicate condition for a template specialization
is_constructible<T, Args>
shall be satisfied, if and only if the followingexpression CEvariable definition would be well-formed:
if
sizeof...(Args) == 0
1, the expression:static_cast<T>(create<Args>()...)T t;otherwise
the expression:T t(create<Args>()...);
to_string
/ to_wstring
Section: 27.4.5 [string.conversions] Status: C++11 Submitter: Christopher Jefferson Opened: 2009-11-10 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
Reported on the gcc mailing list.
The code "
int i; to_string(i);
" fails to compile, as 'int
' is ambiguous between 'long long
' and 'long long unsigned
'. It seems unreasonable to expect users to cast numbers up to a larger type just to useto_string
.
[ 2009-11-14 Moved to Tentatively Ready after 5 positive votes on c++std-lib. ]
Proposed resolution:
27.4 [string.classes], change to_string
and
to_wstring
to:
string to_string(int val); string to_string(unsigned val); string to_string(long val); string to_string(unsigned long val); string to_string(long long val); string to_string(unsigned long long val); string to_string(float val); string to_string(double val); string to_string(long double val); wstring to_wstring(int val); wstring to_wstring(unsigned val); wstring to_wstring(long val); wstring to_wstring(unsigned long val); wstring to_wstring(long long val); wstring to_wstring(unsigned long long val); wstring to_wstring(float val); wstring to_wstring(double val); wstring to_wstring(long double val);
In 27.4.5 [string.conversions], paragraph 7, change to:
string to_string(int val); string to_string(unsigned val); string to_string(long val); string to_string(unsigned long val); string to_string(long long val); string to_string(unsigned long long val); string to_string(float val); string to_string(double val); string to_string(long double val);7 Returns: each function returns a
string
object holding the character representation of the value of its argument that would be generated by callingsprintf(buf, fmt, val)
with a format specifier of"%d"
,"%u"
,"%ld"
,"%lu"
,"%lld"
,"%llu"
,"%f"
,"%f"
, or"%Lf"
, respectively, wherebuf
designates an internal character buffer of sufficient size.
In 27.4.5 [string.conversions], paragraph 14, change to:
wstring to_wstring(int val); wstring to_wstring(unsigned val); wstring to_wstring(long val); wstring to_wstring(unsigned long val); wstring to_wstring(long long val); wstring to_wstring(unsigned long long val); wstring to_wstring(float val); wstring to_wstring(double val); wstring to_wstring(long double val);14 Returns: Each function returns a
wstring
object holding the character representation of the value of its argument that would be generated by callingswprintf(buf, buffsz, fmt, val)
with a format specifier ofL"%d"
,L"%u"
,L"%ld"
,L"%lu"
,L"%lld"
,L"%llu"
,L"%f"
,L"%f"
, orL"%Lf"
, respectively, wherebuf
designates an internal character buffer of sufficient sizebuffsz
.
std::less<std::shared_ptr<T>>
is underspecifiedSection: 20.3.2.2.8 [util.smartptr.shared.cmp] Status: C++11 Submitter: Jonathan Wakely Opened: 2009-11-10 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
20.3.2.2.8 [util.smartptr.shared.cmp]/5 says:
For templates
greater
,less
,greater_equal
, andless_equal
, the partial specializations forshared_ptr
shall yield a total order, even if the built-in operators<
,>
,<=
, and>=
do not. Moreover,less<shared_ptr<T> >::operator()(a, b)
shall returnstd::less<T*>::operator()(a.get(), b.get())
.
This is necessary in order to use shared_ptr
as the key in associate
containers because
n2637
changed operator<
on shared_ptr
s to be
defined in terms of operator<
on the stored pointers (a mistake IMHO
but too late now.) By 7.6.9 [expr.rel]/2 the result of comparing builtin
pointers is unspecified except in special cases which generally do not
apply to shared_ptr
.
Earlier versions of the WP (n2798, n2857) had the following note on that paragraph:
[Editor's note: It's not clear to me whether the first sentence is a requirement or a note. The second sentence seems to be a requirement, but it doesn't really belong here, under
operator<
.]
I agree completely - if partial specializations are needed they should be properly specified.
20.3.2.2.8 [util.smartptr.shared.cmp]/6 has a note saying the comparison operator
allows shared_ptr
objects to be used as keys in associative
containers, which is misleading because something else like a
std::less
partial specialization is needed. If it is not correct that
note should be removed.
20.3.2.2.8 [util.smartptr.shared.cmp]/3 refers to 'x
' and
'y
' but the prototype has parameters 'a
' and
'b
' - that needs to be fixed even if the rest of the issue is
NAD.
I see two ways to fix this, I prefer the first because it removes the
need for any partial specializations and also fixes operator>
and
other comparisons when defined in terms of operator<
.
Replace 20.3.2.2.8 [util.smartptr.shared.cmp]/3 with the following and remove p5:
template<class T, class U> bool operator<(const shared_ptr<T>& a, const shared_ptr<U>& b);3 Returns:
x.get() < y.get()
.std::less<V>()(a.get(), b.get())
, whereV
is the composite pointer type (7.6.9 [expr.rel]).4 Throws: nothing.
5 For templatesgreater
,less
,greater_equal
, andless_equal
, the partial specializations forshared_ptr
shall yield a total order, even if the built-in operators<
,>
,<=
, and>=
do not. Moreover,less<shared_ptr<T> >::operator()(a, b)
shall returnstd::less<T*>::operator()(a.get(), b.get())
.6 [Note: Defining a comparison operator allows
shared_ptr
objects to be used as keys in associative containers. — end note]
Add to 20.3.2.2 [util.smartptr.shared]/1 (after the shared_ptr
comparisons)
template<class T> struct greater<shared_ptr<T>>; template<class T> struct less<shared_ptr<T>>; template<class T> struct greater_equal<shared_ptr<T>>; template<class T> struct less_equal<shared_ptr<T>>;
Remove 20.3.2.2.8 [util.smartptr.shared.cmp]/5 and /6 and replace with:
template<class T, class U> bool operator<(const shared_ptr<T>& a, const shared_ptr<U>& b);3 Returns:
.
xa.get() <yb.get()4 Throws: nothing.
5 For templatesgreater
,less
,greater_equal
, andless_equal
, the partial specializations forshared_ptr
shall yield a total order, even if the built-in operators<
,>
,<=
, and>=
do not. Moreover,less<shared_ptr<T> >::operator()(a, b)
shall returnstd::less<T*>::operator()(a.get(), b.get())
.
6 [Note: Defining a comparison operator allowsshared_ptr
objects to be used as keys in associative containers. — end note]template<class T> struct greater<shared_ptr<T>> : binary_function<shared_ptr<T>, shared_ptr<T>, bool> { bool operator()(const shared_ptr<T>& a, const shared_ptr<T>& b) const; };
operator()
returnsgreater<T*>()(a.get(), b.get())
.template<class T> struct less<shared_ptr<T>> : binary_function<shared_ptr<T>, shared_ptr<T>, bool> { bool operator()(const shared_ptr<T>& a, const shared_ptr<T>& b) const; };
operator()
returnsless<T*>()(a.get(), b.get())
.template<class T> struct greater_equal<shared_ptr<T>> : binary_function<shared_ptr<T>, shared_ptr<T>, bool> { bool operator()(const shared_ptr<T>& a, const shared_ptr<T>& b) const; };
operator()
returnsgreater_equal<T*>()(a.get(), b.get())
.template<class T> struct less_equal<shared_ptr<T>> : binary_function<shared_ptr<T>, shared_ptr<T>, bool> { bool operator()(const shared_ptr<T>& a, const shared_ptr<T>& b) const; };
operator()
returnsless_equal<T*>()(a.get(), b.get())
.
[ 2009-11-18: Moved to Tentatively Ready after 5 positive votes on c++std-lib. ]
Proposed resolution:
Replace 20.3.2.2.8 [util.smartptr.shared.cmp]/3 with the following and remove p5:
template<class T, class U> bool operator<(const shared_ptr<T>& a, const shared_ptr<U>& b);3 Returns:
x.get() < y.get()
.less<V>()(a.get(), b.get())
, whereV
is the composite pointer type (7.6.9 [expr.rel]) ofT*
andU*
.4 Throws: nothing.
5 For templatesgreater
,less
,greater_equal
, andless_equal
, the partial specializations forshared_ptr
shall yield a total order, even if the built-in operators<
,>
,<=
, and>=
do not. Moreover,less<shared_ptr<T> >::operator()(a, b)
shall returnstd::less<T*>::operator()(a.get(), b.get())
.6 [Note: Defining a comparison operator allows
shared_ptr
objects to be used as keys in associative containers. — end note]
quick_exit
support for freestanding implementationsSection: 16.4.2.5 [compliance] Status: C++11 Submitter: Alisdair Meredith Opened: 2009-11-12 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
Addresses UK 172
This issue is a response to NB comment UK-172
The functions quick_exit
and at_quick_exit
should be
added to the required features of <cstdlib>
in a
freestanding implementation.
This comment was rejected in Summit saying neither at_exit
nor
at_quick_exit
should be required. This suggests the comment was
misread, as atexit
is already required to be supported. If the LWG
really did wish to not require the registration functions be supported,
then a separate issue should be opened to change the current standard.
Given both exit
and atexit
are required, the UK panel feels it is
appropriate to require the new quick_exit
facility is similarly
supported.
[ 2009-12-11 Moved to Tentatively Ready after 5 positive votes on c++std-lib. ]
Proposed resolution:
Ammend p3 Freestanding implementations 16.4.2.5 [compliance]
3 The supplied version of the header
<cstdlib>
shall declare at least the functionsabort
,()atexit
,()at_quick_exit
,andexit
, and()quick_exit
(17.5 [support.start.term]). The other headers listed in this table shall meet the same requirements as for a hosted implementation.
shared_future::get
and deferred async
functionsSection: 32.10.8 [futures.shared.future] Status: Resolved Submitter: Anthony Williams Opened: 2009-11-17 Last modified: 2021-06-06
Priority: Not Prioritized
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Discussion:
If a shared_future
is constructed with the result of an async
call with a
deferred function, and two or more copies of that shared_future
are created,
with multiple threads calling get()
, it is not clear which thread runs the
deferred function. [futures.shared_future]p22 from
N3000
says (minus editor's note):
Effects: if the associated state contains a deferred function, executes the deferred function. Otherwise, blocks until the associated state is ready.
In the absence of wording to the contrary, this implies that every thread that
calls wait()
will execute the deferred function.
[
2010 Pittsburgh: Moved to NAD EditorialResolved. Rationale added below.
]
Rationale:
Solved by N3058.
Proposed resolution:
Replace [futures.shared_future]p22 with the following:
Effects: If the associated state
contains a deferred function, executes the deferred function. Otherwise, blocks until the associated state is ready.was created by apromise
orpackaged_task
object, block until the associated state is ready. If the associated state is associated with a thread created for anasync
call (32.10.9 [futures.async]), as ifassociated-thread.join()
.If the associated state contains a deferred function, calls to
wait()
on allshared_future
objects that share the same associated state are serialized. The first call towait()
that shares a given associated state executes the deferred function and stores the return value or exception in the associated state.Synchronization: if the associated state was created by a
promise
object, the completion ofset_value()
orset_exception()
to thatpromise
happens before (6.9.2 [intro.multithread])wait()
returns. If the associated state was created by apackaged_task
object, the completion of the associated task happens beforewait()
returns. If the associated state is associated with a thread created for anasync
call (32.10.9 [futures.async]), the completion of the associated thread happens-beforewait()
returns.If the associated state contained a deferred function, the invocation of the deferred function happens-before any call to
wait()
on afuture
that shares that state returns.
condition_variable_any::wait_for
Section: 32.7.5 [thread.condition.condvarany] Status: C++11 Submitter: Anthony Williams Opened: 2009-11-17 Last modified: 2016-01-28
Priority: Not Prioritized
View all other issues in [thread.condition.condvarany].
View all issues with C++11 status.
Discussion:
32.7.5 [thread.condition.condvarany]p18 and 32.7.5 [thread.condition.condvarany]p27 specify incorrect preconditions for
condition_variable_any::wait_for
. The stated preconditions require that
lock
has a mutex()
member function, and that this produces the
same result for all concurrent calls to wait_for()
. This is
inconsistent with wait()
and wait_until()
which do not impose
such a requirement.
[ 2009-12-24 Moved to Tentatively Ready after 5 positive votes on c++std-lib. ]
Proposed resolution:
Remove 32.7.5 [thread.condition.condvarany]p18 and 32.7.5 [thread.condition.condvarany]p27.
template <class Lock, class Rep, class Period> cv_status wait_for(Lock& lock, const chrono::duration<Rep, Period>& rel_time);
18 Precondition: lock is locked by the calling thread, and either
no other thread is waiting on thiscondition_variable
object orlock.mutex()
returns the same value for each of the lock arguments supplied by all concurrently waiting (viawait
,wait_for
, orwait_until
) threads....
template <class Lock, class Rep, class Period, class Predicate> bool wait_for(Lock& lock, const chrono::duration<Rep, Period>& rel_time, Predicate pred);
27 Precondition: lock is locked by the calling thread, and either
no other thread is waiting on thiscondition_variable
object orlock.mutex()
returns the same value for each of the lock arguments supplied by all concurrently waiting (viawait
,wait_for
, orwait_until
) threads.
Section: 32.6 [thread.mutex] Status: Resolved Submitter: Anthony Williams Opened: 2009-11-17 Last modified: 2016-01-28
Priority: Not Prioritized
View all other issues in [thread.mutex].
View all issues with Resolved status.
Discussion:
The Mutex
requirements in 32.6.4 [thread.mutex.requirements] and
32.6.4.3 [thread.timedmutex.requirements] confuse the requirements on the
behaviour of std::mutex
et al with the requirements on
Lockable
types for use with std::unique_lock
,
std::lock_guard
and std::condition_variable_any
.
[ 2010 Pittsburgh: ]
Concepts of threads chapter and issue presentation are: Lockable < Mutex < TimedMutex and Lockable < TimedLockable < TimedMutex.
Typo in failed deletion of Mutex in 30.4.4 p4 edits.
Lockable requirements are too weak for condition_variable_any, but the Mutex requirements are too strong.
Need subset of Lockable requirements for condition_variable_any that does not include try_lock. E.g. CvLockable < Lockable.
Text needs updating to recent draft changes.
Needs to specify exception behavior in Lockable.
The current standard is fine for what it says, but it places requirements that are too strong on authors of mutexes and locks.
Move to open status. Suggest Anthony look at condition_variable_any requirements. Suggest Anthony refine requirements/concepts categories.
[ 2010-03-28 Daniel synced with N3092. ]
[ 2010-10-25 Daniel adds: ]
Accepting n3130 would solve this issue.
[ 2010-11 Batavia: ]
Resolved by adopting n3197.
Proposed resolution:
Add a new section to 32.2 [thread.req] after 32.2.4 [thread.req.timing] as follows:
30.2.5 Requirements for Lockable types
The standard library templates
unique_lock
(32.6.5.4 [thread.lock.unique]),lock_guard
(32.6.5.2 [thread.lock.guard]),lock
,try_lock
(32.6.6 [thread.lock.algorithm]) andcondition_variable_any
(32.7.5 [thread.condition.condvarany]) all operate on user-suppliedLockable
objects. Such an object must support the member functions specified for either theLockable
Requirements or theTimedLockable
requirements as appropriate to acquire or release ownership of alock
by a giventhread
. [Note: the nature of any lock ownership and any synchronization it may entail are not part of these requirements. — end note]30.2.5.1 Lockable Requirements
In order to qualify as a
Lockable
type, the following expressions must be supported, with the specified semantics, wherem
denotes a value of typeL
that supports theLockable
:The expression
m.lock()
shall be well-formed and have the following semantics:
- Effects:
- Block until a lock can be acquired for the current thread.
- Return type:
void
The expression
m.try_lock()
shall be well-formed and have the following semantics:
- Effects:
- Attempt to acquire a lock for the current thread without blocking.
- Return type:
bool
- Returns:
true
if the lock was acquired,false
otherwise.The expression
m.unlock()
shall be well-formed and have the following semantics:
- Effects:
- Release a lock on
m
held by the current thread.- Return type:
void
- Throws:
- Nothing if the current thread holds a lock on
m
.30.2.5.2
TimedLockable
RequirementsFor a type to qualify as
TimedLockable
it must meet theLockable
requirements, and additionally the following expressions must be well-formed, with the specified semantics, wherem
is an instance of a typeTL
that supports theTimedLockable
requirements,rel_time
denotes instantiation ofduration
(30.5 [time.duration]) andabs_time
denotes an instantiation oftime_point
(30.6 [time.point])The expression
m.try_lock_for(rel_time)
shall be well-formed and have the following semantics:
- Effects:
- Attempt to acquire a lock for the current thread within the specified time period.
- Return type:
bool
- Returns:
true
if the lock was acquired,false
otherwise.The expression
m.try_lock_until(abs_time)
shall be well-formed and have the following semantics:
- Effects:
- Attempt to acquire a lock for the current thread before the specified point in time.
- Return type:
bool
- Returns:
true
if the lock was acquired,false
otherwise.
Replace 32.6.4 [thread.mutex.requirements] paragraph 2 with the following:
2 This section describes requirements on
template argument types used to instantiate templates defined inthe mutex types supplied by the C++ standard library.The template definitions in the C++ standard library referThese types shall conform to the namedMutex
requirements whose details are set out below. In this description,m
is an object ofaone of the standard library mutex typesMutex
typestd::mutex
,std::recursive_mutex
,std::timed_mutex
orstd::recursive_timed_mutex
..
Add the following paragraph after 32.6.4 [thread.mutex.requirements] paragraph 2:
A
Mutex
type shall conform to theLockable
requirements (30.2.5.1).
Replace 32.6.4.3 [thread.timedmutex.requirements] paragraph 1 with the following:
The C++ standard library
TimedMutex
typesstd::timed_mutex
andstd::recursive_timed_mutex
Ashall meet the requirements for aTimedMutex
typeMutex
type. In addition,itthey shall meet the requirements set outin this Clause 30.4.2below, whererel_time
denotes an instantiation ofduration
(30.5 [time.duration]) andabs_time
denotes an instantiation oftime_point
(30.6 [time.point]).
Add the following paragraph after 32.6.4.3 [thread.timedmutex.requirements] paragraph 1:
A
TimedMutex
type shall conform to theTimedLockable
requirements (30.2.5.1).
Add the following paragraph following 32.6.5.2 [thread.lock.guard] paragraph 1:
The supplied
Mutex
type shall meet theLockable
requirements (30.2.5.1).
Add the following paragraph following 32.6.5.4 [thread.lock.unique] paragraph 1:
The supplied
Mutex
type shall meet theLockable
requirements (30.2.5.1).unique_lock<Mutex>
meets theLockable
requirements. IfMutex
meets theTimedLockable
requirements (30.2.5.2) thenunique_lock<Mutex>
also meets theTimedLockable
requirements.
Replace the use of "mutex" or "mutex object" with "lockable object" throughout clause 32.6.5 [thread.lock] paragraph 1:
1 A lock is an object that holds a reference to a
mutexlockable object and may unlock themutexlockable object during the lock's destruction (such as when leaving block scope). A thread of execution may use a lock to aid in managingmutexownership of a lockable object in an exception safe manner. A lock is said to own amutexlockable object if it is currently managing the ownership of thatmutexlockable object for a thread of execution. A lock does not manage the lifetime of themutexlockable object it references. [ Note: Locks are intended to ease the burden of unlocking themutexlockable object under both normal and exceptional circumstances. — end note ]
32.6.5 [thread.lock] paragaph 2:
2 Some lock constructors take tag types which describe what should be done with the
mutexlockable object during the lock's constuction.
32.6.5.2 [thread.lock.guard] paragaph 1:
1 An object of type
lock_guard
controls the ownership of amutexlockable object within a scope. Alock_guard
object maintains ownership of amutexlockable object throughout thelock_guard
object's lifetime. The behavior of a program is undefined if themutexlockable object referenced bypm
does not exist for the entire lifetime (3.8) of thelock_guard
object.Mutex
shall meet theLockable
requirements (30.2.5.1).
32.6.5.4 [thread.lock.unique] paragaph 1:
1 An object of type
unique_lock
controls the ownership of amutexlockable object within a scope.MutexoOwnership of the lockable object may be acquired at construction or after construction, and may be transferred, after acquisition, to anotherunique_lock
object. Objects of typeunique_lock
are not copyable but are movable. The behavior of a program is undefined if the contained pointerpm
is not null and the mutex pointed to bypm
does not exist for the entire remaining lifetime (3.8) of theunique_lock
object.Mutex
shall meet theLockable
requirements (30.2.5.1).
Add the following to the precondition of unique_lock(mutex_type&
m, const chrono::time_point<Clock, Duration>& abs_time)
in
32.6.5.4.2 [thread.lock.unique.cons] paragraph 18:
template <class Clock, class Duration> unique_lock(mutex_type& m, const chrono::time_point<Clock, Duration>& abs_time);18 Requires: If
mutex_type
is not a recursive mutex the calling thread does not own the mutex. The suppliedmutex_type
type shall meet theTimedLockable
requirements (30.2.5.2).
Add the following to the precondition of unique_lock(mutex_type&
m,
const chrono::duration<Rep, Period>& rel_time)
in
32.6.5.4.2 [thread.lock.unique.cons]
paragraph 22
22 Requires: If
mutex_type
is not a recursive mutex the calling thread does not own the mutex. The suppliedmutex_type
type shall meet theTimedLockable
requirements (30.2.5.2).
Add the following as a precondition of bool try_lock_until(const
chrono::time_point<Clock, Duration>& abs_time)
before
32.6.5.4.3 [thread.lock.unique.locking] paragraph 8
template <class Clock, class Duration> bool try_lock_until(const chrono::time_point<Clock, Duration>& abs_time);Requires: The supplied
mutex_type
type shall meet theTimedLockable
requirements (30.2.5.2).
Add the following as a precondition of bool try_lock_for(const
chrono::duration<Rep, Period>& rel_time)
before
32.6.5.4.3 [thread.lock.unique.locking] paragraph 12
template <class Rep, class Period> bool try_lock_for(const chrono::duration<Rep, Period>& rel_time);Requires: The supplied
mutex_type
type shall meet theTimedLockable
requirements (30.2.5.2).
Replace 32.6.6 [thread.lock.algorithm] p1 with the following:
template <class L1, class L2, class... L3> int try_lock(L1&, L2&, L3&...);1 Requires: Each template parameter type shall meet the
requirements (30.2.5.1).
MutexLockable, except that a call to[Note: Thetry_lock()
may throw an exception.unique_lock
class template meets these requirements when suitably instantiated. — end note]
Replace 32.6.6 [thread.lock.algorithm] p4 with the following:
template <class L1, class L2, class... L3> void lock(L1&, L2&, L3&...);4 Requires: Each template parameter type shall meet the
Mutex
requirements (30.2.5.1).MutexLockable, except that a call to[Note: Thetry_lock()
may throw an exception.unique_lock
class template meets these requirements when suitably instantiated. — end note]
Replace 32.7.5 [thread.condition.condvarany] paragraph 1 with:
1 A
Lock
type shall meet therequirements for aMutex
typeLockable
requirements (30.2.5.1), except thattry_lock
is not required. [Note: All of the standard mutex types meet this requirement. — end note]
async
Section: 32.10.5 [futures.state] Status: Resolved Submitter: Anthony Williams Opened: 2009-11-18 Last modified: 2016-01-28
Priority: Not Prioritized
View all other issues in [futures.state].
View all issues with Resolved status.
Discussion:
The current description of the associated state in 32.10.5 [futures.state]
does not allow for futures created by an async
call. The description
therefore needs to be extended to cover that.
[
2010 Pittsburgh: Moved to NAD EditorialResolved. Rationale added below.
]
Rationale:
Solved by N3058.
Proposed resolution:
Add a new sentence to 32.10.5 [futures.state] p2:
2 This associated state consists of some state information and some (possibly not yet evaluated) result, which can be a (possibly
void
) value or an exception. If the associated state was created by a call toasync
(32.10.9 [futures.async]) then it may also contain a deferred function or an associatedthread
.
Add an extra bullet to 32.10.5 [futures.state] p3:
The result of an associated state can be set by calling:
promise::set_value
,promise::set_exception
,or- packaged_task::operator()
., or- a call to
async
(32.10.9 [futures.async]).
result_of
should be moved to <type_traits>
Section: 99 [func.ret] Status: C++11 Submitter: Alisdair Meredith Opened: 2009-11-19 Last modified: 2016-01-28
Priority: Not Prioritized
View all other issues in [func.ret].
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Discussion:
Addresses UK 198
NB Comment: UK-198 makes this request among others. It refers to a more detailed issue that BSI did not manage to submit by the CD1 ballot deadline though.
result_of
is essentially a metafunction to return the type of an
expression, and belongs with the other library metafunctions in
<type_traits>
rather than lurking in <functional>
.
The current definition in <functional>
made sense when
result_of
was nothing more than a protocol to enable several components
in <functional>
to provide their own result types, but it has
become a more general tool. For instance, result_of
is now used in the
threading and futures components.
Now that <type_traits>
is a required header for free-standing
implementations it will be much more noticeable (in such environments) that a
particularly useful trait is missing, unless that implementation also chooses to
offer <functional>
as an extension.
The simplest proposal is to simply move the wording (editorial direction below) although a more consistent form for type_traits would reformat this as a table.
Following the acceptance of 1255(i), result_of
now
depends on the declval
function template, tentatively provided
in <utility>
which is not (yet) required of a
free-standing implementation.
This dependency is less of an issue when result_of
continues to
live in <functional>
.
Personally, I would prefer to clean up the dependencies so both
result_of
and declval
are available in a free-standing
implementation, but that would require slightly more work than suggested
here. A minimal tweak would be to require <utility>
in a
free-standing implementation, although there are a couple of subtle
issues with make_pair
, which uses reference_wrapper
in
its protocol and that is much harder to separate cleanly from
<functional>
.
An alternative would be to enact the other half of
N2979
and create a new minimal header for the new C++0x library facilities to
be added to the freestanding requirements (plus swap
.)
I have a mild preference for the latter, although there are clearly
reasons to consider better library support for free-standing in general,
and adding the whole of <utility>
could be considered a step in that
direction. See NB comment
JP-23
for other suggestions (array
, ratio
)
[ 2010-01-27 Beman updated wording. ]
The original wording is preserved here:
Move 99 [func.ret] to a heading below 21 [meta]. Note that in principle we should not change the tag, although this is a new tag for 0x. If it has been stable since TR1 it is clearly immutable though.
This wording should obviously adopt any other changes currently in (Tentatively) Ready status that touch this wording, such as 1255(i).
[ 2010-02-09 Moved to Tentatively Ready after 5 positive votes on c++std-lib. ]
Proposed resolution:
From Function objects 22.10 [function.objects], Header <functional>
synopsis, remove:
// 20.7.4 result_of: template <class> class result_of; // undefined template <class F, class... Args> class result_of<F(ArgTypes...)>;
Remove Function object return types 99 [func.ret] in its entirety. This sub-section reads:
namespace std { template <class> class result_of; // undefined template <class Fn, class... ArgTypes> class result_of<Fn(ArgTypes...)> { public : // types typedef see below type; }; }Given an rvalue
fn
of typeFn
and valuest1, t2, ..., tN
of types T1, T2, ..., TN inArgTypes
, respectively, the type member is the result type of the expressionfn(t1, t2, ...,tN)
. The valuesti
are lvalues when the corresponding typeTi
is an lvalue-reference type, and rvalues otherwise.
To Header <type_traits> synopsis 21.3.3 [meta.type.synop], add at the indicated location:
template <class T> struct underlying_type; template <class T> struct result_of; // not defined template <class Fn, class... ArgTypes> struct result_of<Fn(ArgTypes...)>;
To Other transformations 21.3.8.7 [meta.trans.other], Table 51 — Other transformations, add:
Template Condition Comments template <class T>
struct underlying_type;T
shall be an enumeration type (7.2)The member typedef type
shall name the underlying type ofT
.template <class Fn, class... ArgTypes> struct result_of<Fn(ArgTypes...)>;
Fn
shall be a function object type 22.10 [function.objects], reference to function, or reference to function object type.decltype(declval<Fn>()(declval<ArgTypes>()...))
shall be well formed.The member typedef type
shall name the typedecltype(declval<Fn>()(declval<ArgTypes>()...))
.
At the end of Other transformations 21.3.8.7 [meta.trans.other] add:
[Example: Given these definitions:
typedef bool(&PF1)(); typedef short(*PF2)(long); struct S { operator PF2() const; double operator()(char, int&); };the following assertions will hold:
static_assert(std::is_same<std::result_of<S(int)>::type, short>::value, "Error!"); static_assert(std::is_same<std::result_of<S&(unsigned char, int&)>::type, double>::value, "Error!"); static_assert(std::is_same<std::result_of<PF1()>::type, bool>::value, "Error!");— end example]
CR
undefined in duration operatorsSection: 30.5.6 [time.duration.nonmember] Status: C++11 Submitter: Daniel Krügler Opened: 2009-11-21 Last modified: 2016-01-28
Priority: Not Prioritized
View all other issues in [time.duration.nonmember].
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Discussion:
IMO CR
alone is not really defined (it should be CR(Rep1,
Rep2)
).
[ 2009-12-24 Moved to Tentatively Ready after 5 positive votes on c++std-lib. ]
Proposed resolution:
Change 30.5.6 [time.duration.nonmember] paragraphs 9 and 12:
template <class Rep1, class Period, class Rep2> duration<typename common_type<Rep1, Rep2>::type, Period> operator/(const duration<Rep1, Period>& d, const Rep2& s);9 Returns:
duration<CR(Rep1, Rep2), Period>(d) /= s
.template <class Rep1, class Period, class Rep2> duration<typename common_type<Rep1, Rep2>::type, Period> operator%(const duration<Rep1, Period>& d, const Rep2& s);12 Returns:
duration<CR(Rep1, Rep2), Period>(d) %= s
.
promise::set_value
Section: 32.10.6 [futures.promise] Status: Resolved Submitter: Jonathan Wakely Opened: 2009-11-22 Last modified: 2016-01-28
Priority: Not Prioritized
View other active issues in [futures.promise].
View all other issues in [futures.promise].
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Discussion:
The definitions of promise::set_value
need tidying up, the
synopsis says:
// setting the result void set_value(const R& r); void set_value(see below);
Why is the first one there? It implies it is always present for all specialisations of promise, which is not true.
The definition says:
void set_value(const R& r); void promise::set_value(R&& r); void promise<R&>::set_value(R& r); void promise<void>::set_value();
The lack of qualification on the first one again implies it's present for all specialisations, again not true.
[
2010 Pittsburgh: Moved to NAD EditorialResolved. Rationale added below.
]
Rationale:
Solved by N3058.
Proposed resolution:
Change the synopsis in 32.10.6 [futures.promise]:
// setting the resultvoid set_value(const R& r);void set_value(see below);
And the definition be changed by qualifying the first signature:
void promise::set_value(const R& r); void promise::set_value(R&& r); void promise<R&>::set_value(R& r); void promise<void>::set_value();
future::valid
should be callable on an invalid futureSection: 32.10.7 [futures.unique.future] Status: Resolved Submitter: Jonathan Wakely Opened: 2009-11-22 Last modified: 2021-06-06
Priority: Not Prioritized
View all other issues in [futures.unique.future].
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Discussion:
[futures.unique_future]/3 says:
The effect of calling any member function other than the destructor or the move-assignment operator on a
future
object for whichvalid() == false
is undefined.
This means calling future::valid()
is undefined unless it will
return true
, so you can only use it if you know the answer!
[ 2009-12-08 Moved to Tentatively Ready after 5 positive votes on c++std-lib. ]
[ 2010 Pittsburgh: ]
Moved to
NAD EditorialResolved. Rationale added below.
Rationale:
Solved by N3058.
Proposed resolution:
Change [futures.unique_future]/3:
The effect of calling any member function other than the destructor, or the move-assignment operator, or
valid
, on afuture
object for whichvalid() == false
is undefined.
atomic_future
constructorSection: 99 [futures.atomic_future] Status: Resolved Submitter: Jonathan Wakely Opened: 2009-11-22 Last modified: 2016-01-28
Priority: Not Prioritized
View all other issues in [futures.atomic_future].
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Discussion:
In 99 [futures.atomic_future] this constructor:
atomic_future(future<R>&&);
is declared in the synopsis, but not defined. Instead n2997 defines:
atomic_future(const future<R>&& rhs);
and n3000 defines
atomic_future(atomic_future<R>&& rhs);
both of which are wrong. The constructor definition should be changed to match the synopsis.
[ 2009-12-12 Moved to Tentatively Ready after 5 positive votes on c++std-lib. ]
[ 2010 Pittsburgh: ]
Moved to
NAD EditorialResolved. Rationale added below.
Rationale:
Solved by N3058.
Proposed resolution:
Adjust the signature above 99 [futures.atomic_future]/6 like so:
atomic_future(atomic_future<R>&& rhs);
Section: 32.10 [futures] Status: Resolved Submitter: Jonathan Wakely Opened: 2009-11-22 Last modified: 2016-01-28
Priority: Not Prioritized
View all other issues in [futures].
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Discussion:
[futures.unique_future]/1 should be updated to mention
async
.
[futures.shared_future]/1 should also be updated for
async
. That paragraph also says
... Its value or exception can be set by use of a
shared_future
,promise
(32.10.6 [futures.promise]), orpackaged_task
(32.10.10 [futures.task]) object that shares the same associated state.
How can the value be set by a shared_future
?
99 [futures.atomic_future]/1 says
An
atomic_future
object can only be created by use of apromise
(32.10.6 [futures.promise]) orpackaged_task
(32.10.10 [futures.task]) object.
which is wrong, it's created from a std::future
, which could
have been default-constructed. That paragraph should be closer to the
text of [futures.shared_future]/1, and should also mention
async
.
[
2010 Pittsburgh: Moved to NAD EditorialResolved. Rationale added below.
]
Rationale:
Solved by N3058.
Proposed resolution:
forwardlist
missing allocator constructorsSection: 23.3.7 [forward.list] Status: C++11 Submitter: Daniel Krügler Opened: 2009-12-12 Last modified: 2023-02-07
Priority: Not Prioritized
View all other issues in [forward.list].
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Discussion:
I found that forward_list has only
forward_list(const forward_list<T,Allocator>& x); forward_list(forward_list<T,Allocator>&& x);
but misses
forward_list(const forward_list& x, const Allocator&); forward_list(forward_list&& x, const Allocator&);
Note to other reviewers: I also checked the container adaptors for similar inconsistencies, but as far as I can see these are already handled by the current active issues 1194(i) and 1199(i).
[ 2010-01-14 Moved to Tentatively Ready after 5 positive votes on c++std-lib. ]
Proposed resolution:
In [forwardlist]/3, class template forward_list synopsis change as indicated:
forward_list(const forward_list<T,Allocator>& x); forward_list(forward_list<T,Allocator>&& x); forward_list(const forward_list&, const Allocator&); forward_list(forward_list&&, const Allocator&);
std::thread::id
should be trivially copyableSection: 32.4.3.2 [thread.thread.id] Status: C++11 Submitter: Anthony Williams Opened: 2009-11-24 Last modified: 2016-01-28
Priority: Not Prioritized
View all other issues in [thread.thread.id].
View all issues with C++11 status.
Discussion:
The class definition of std::thread::id
in
N3000
is:
class thread::id { public: id(); };
Typically, I expect that the internal data members will either be
pointers or integers, so that in practice the class will be trivially
copyable. However, I don't think the current wording guarantees it, and
I think it would be useful. In particular, I can see a use for
std::atomic<std::thread::id>
to allow a thread
to claim ownership of a data structure atomicly, and
std::atomic<T>
requires that T
is trivially
copyable.
[ 2010-02-12 Moved to Tentatively Ready after 7 positive votes on c++std-lib. ]
Proposed resolution:
Add a new sentence to 32.4.3.2 [thread.thread.id] p1:
1 An object of type
thread::id
provides a unique identifier for each thread of execution and a single distinct value for allthread
objects that do not represent a thread of execution (32.4.3 [thread.thread.class]). Each thread of execution has an associatedthread::id
object that is not equal to thethread::id
object of any other thread of execution and that is not equal to thethread::id
object of anystd::thread
object that does not represent threads of execution. The library may reuse the value of athread::id
of a terminated thread that can no longer be joined.thread::id
shall be a trivially copyable class (11 [class]).
forward_list::insert_after
Section: 23.3.7.5 [forward.list.modifiers] Status: C++11 Submitter: Bo Persson Opened: 2009-11-25 Last modified: 2023-02-07
Priority: Not Prioritized
View all other issues in [forward.list.modifiers].
View all issues with C++11 status.
Discussion:
After applying LDR149(i), forward_list
now has 5
overloads of insert_after
, all returning an iterator.
However, two of those - inserting a single object - return "An iterator
pointing to a copy of x
[the inserted object]" while the other
three - inserting zero or more objects - return an iterator equivalent
to the position parameter, pointing before any possibly inserted
objects.
Is this the intended change?
I don't really know what insert_after(position, empty_range)
should really return, but always returning position
seems less
than useful.
[ 2010-02-04 Howard adds: ]
I agree this inconsistency will be error prone and needs to be fixed. Additionally
emplace_after
's return value is unspecified.
[ 2010-02-04 Nico provides wording. ]
[ 2010 Pittsburgh: ]
We prefer to return an iterator to the last inserted element. Modify the proposed wording and then set to Ready.
[ 2010-03-15 Howard adds: ]
Wording updated and set to Ready.
Proposed resolution:
In forward_list
modifiers [forwardlist.modifiers]
make the following modifications:
iterator insert_after(const_iterator position, size_type n, const T& x);...
10 Returns:
position.An iterator pointing to the last inserted copy ofx
orposition
ifn == 0
.template <class InputIterator> iterator insert_after(const_iterator position, InputIterator first, InputIterator last);...
13 Returns:
position.An iterator pointing to the last inserted element orposition
iffirst == last
.iterator insert_after(const_iterator position, initializer_list<T> il);...
15 Returns:
position.An iterator pointing to the last inserted element orposition
ifil
is empty.template <class... Args> iterator emplace_after(const_iterator position, Args&&... args);...
17 ...
Returns: An iterator pointing to the new constructed element from args.
[u|bi]nary_function
specializationSection: 99 [depr.base] Status: C++11 Submitter: Alberto Ganesh Barbati Opened: 2009-11-30 Last modified: 2016-01-28
Priority: Not Prioritized
View all other issues in [depr.base].
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Discussion:
A program should not be allowed to add specialization of class templates
unary_function
and binary_function
, in force of 16.4.5.2.1 [namespace.std]/1.
If a program were allowed to specialize these templates, the library could no
longer rely on them to provide the intended typedefs or there might be other
undesired interactions.
[ 2010-03-27 Daniel adds: ]
Accepting issue 1290(i) would resolve this issue as NAD editorial.
[ 2010-10-24 Daniel adds: ]
Accepting n3145 would resolve this issue as NAD editorial.
[ 2010 Batavia: ]
Pete: Is this issue actually addressed by N3198, or did deprecating unary/binary_function?
We determined that this issue is NOT resolved and that it must be resolved or else N3198 could break code that does specialize unary/binary function. Matt: don't move to NAD Howard: I suggest we go further and move 1279 to ready for Madrid. Group: Agrees move 1279 to ready for Madrid
Previous proposed resolution:
1 The following
classesclass templates are provided to simplify the typedefs of the argument and result types:. A program shall not declare specializations of these templates.
[2011-03-06 Daniel comments]
This meeting outcome was not properly reflected in the proposed resolution. I also adapted the suggested wording to the N3242 numbering and content state. During this course of action it turned out that the first suggested wording change has already been applied.
Proposed resolution:
Change paragraph 99 [depr.base]/1 as follows:
1 The class templates
unary_function
andbinary_function
are deprecated. A program shall not declare specializations of these templates.
Section: 24.6.2.2 [istream.iterator.cons], 24.6.3.2 [ostream.iterator.cons.des] Status: C++11 Submitter: Jonathan Wakely Opened: 2009-12-04 Last modified: 2016-01-28
Priority: Not Prioritized
View all other issues in [istream.iterator.cons].
View all issues with C++11 status.
Discussion:
24.6.2.2 [istream.iterator.cons] describes the effects in terms of:
basic_istream<charT,traits>* in_stream; // exposition only3 Effects: Initializes in_stream with
s
.
That should be &s
and similarly for 24.6.3.2 [ostream.iterator.cons.des].
[ 2009-12-23 Moved to Tentatively Ready after 6 positive votes on c++std-lib. ]
Proposed resolution:
Change 24.6.2.2 [istream.iterator.cons] like so:
istream_iterator(istream_type& s);3 Effects: Initializes in_stream with
&s
. value ...
And 24.6.3.2 [ostream.iterator.cons.des] like so:
ostream_iterator(ostream_type& s);1 Effects: Initializes out_stream with
&s
and delim with null.ostream_iterator(ostream_type& s, const charT* delimiter);2 Effects: Initializes out_stream with
&s
and delim withdelimiter
.
Section: 21.4.3 [ratio.ratio] Status: Resolved Submitter: Vicente Juan Botet Escribá Opened: 2009-12-07 Last modified: 2016-01-28
Priority: Not Prioritized
View all other issues in [ratio.ratio].
View all issues with Resolved status.
Discussion:
CopyConstruction and Assignment between ratio
s having the same
normalized form. Current
N3000
do not allows to copy-construct or assign ratio
instances of
ratio
classes having the same normalized form.
Two ratio
classes ratio<N1,D1>
and
ratio<N2,D2>
have the same normalized form if
ratio<N1, D1>::num == ratio<N2, D2>::num && ratio<N1, D1>::den == ratio<N2, D2>::den
This simple example
ratio<1,3> r1; ratio<3,9> r2; r1 = r2; // (1)
fails to compile in (1). Other example
ratio<1,3> r1; ratio_subtract<ratio<2,3>, ratio<1,3>>::type r2; r1 = r2;
The nested type of ratio_subtract<ratio<2,3>,
ratio<1,3>>
could be ratio<3,9>
so the compilation
could fail. It could also be ratio<1,3>
and the compilation
succeeds.
In 21.4.4 [ratio.arithmetic] 3 and similar clauses
3 The nested typedef
type
shall be a synonym forratio<T1, T2>
whereT1
has the valueR1::num * R2::den - R2::num * R1::den
andT2
has the valueR1::den * R2::den
.
the meaning of synonym let think that the result shall be a normalized
ratio
equivalent to ratio<T1, T2>
, but there is not an
explicit definition of what synonym means in this context.
Additionally we should add a typedef for accessing the normalized
ratio
, and change 21.4.4 [ratio.arithmetic] to return only this
normalized result.
[ 2010 Pittsburgh: ]
There is no consensus to add the converting copy constructor or converting copy assignment operator. However there was consensus to add the typedef.
Proposed wording modified. Original proposed wording preserved here. Moved to Review.
Make
ratio
default constructible, copy-constructible and assignable from anyratio
which has the same reduced form.Add to 21.4.3 [ratio.ratio] synopsis
template <intmax_t N, intmax_t D = 1> class ratio { public: static constexpr intmax_t num; static constexpr intmax_t den; typedef ratio<num, den> type; ratio() = default; template <intmax_t N2, intmax_t D2> ratio(const ratio<N2, D2>&); template <intmax_t N2, intmax_t D2> ratio& operator=(const ratio<N2, D2>&); };Add to 21.4.3 [ratio.ratio]:
Two ratio classes
ratio<N1,D1>
andratio<N2,D2>
have the same reduced form ifratio<N1,D1>::type
is the same type asratio<N2,D2>::type
Add a new section: [ratio.cons]
Construction and assignment [ratio.cons]
template <intmax_t N2, intmax_t D2> ratio(const ratio<N2, D2>& r);Effects: Constructs a
ratio
object.Remarks: This constructor shall not participate in overload resolution unless
r
has the same reduced form as*this
.template <intmax_t N2, intmax_t D2> ratio& operator=(const ratio<N2, D2>& r);Effects: None.
Returns:
*this
.Remarks: This operator shall not participate in overload resolution unless
r
has the same reduced form as*this
.Change 21.4.4 [ratio.arithmetic]
Implementations may use other algorithms to compute these values. If overflow occurs, a diagnostic shall be issued.
template <class R1, class R2> struct ratio_add { typedef see below type; };The nested typedef
type
shall be a synonym forratio<T1, T2>::type
whereT1
has the valueR1::num * R2::den + R2::num * R1::den
andT2
has the valueR1::den * R2::den
.template <class R1, class R2> struct ratio_subtract { typedef see below type; };The nested typedef
type
shall be a synonym forratio<T1, T2>::type
whereT1
has the valueR1::num * R2::den - R2::num * R1::den
andT2
has the valueR1::den * R2::den
.template <class R1, class R2> struct ratio_multiply { typedef see below type; };The nested typedef
type
shall be a synonym forratio<T1, T2>::type
whereT1
has the valueR1::num * R2::num
andT2
has the valueR1::den * R2::den
.template <class R1, class R2> struct ratio_divide { typedef see below type; };The nested typedef
type
shall be a synonym forratio<T1, T2>::type
whereT1
has the valueR1::num * R2::den
andT2
has the valueR1::den * R2::num
.
[ 2010-03-27 Howard adds: ]
Daniel brought to my attention the recent addition of the typedef
type
to the FCD N3092:typedef ratio type;This issue was discussed in Pittsburgh, and the decision there was to accept the typedef as proposed and move to Review. Unfortunately the issue was accidently applied to the FCD, and incorrectly. The FCD version of the typedef refers to
ratio<N, D>
, but the typedef is intended to refer toratio<num, den>
which in general is not the same type.I've updated the wording to diff against N3092.
[Batavia: NAD EditorialResolved - see rationale below]
Rationale:
Already fixed in working draft
Proposed resolution:
Add to 21.4.3 [ratio.ratio] synopsis
template <intmax_t N, intmax_t D = 1> class ratio { public: static constexpr intmax_t num; static constexpr intmax_t den; typedef ratio<num, den> type; };
MoveConstructible
and MoveAssignable
need clarification
of moved-from stateSection: 16.4.4.2 [utility.arg.requirements] Status: Resolved Submitter: Howard Hinnant Opened: 2009-12-12 Last modified: 2016-01-28
Priority: Not Prioritized
View all other issues in [utility.arg.requirements].
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Discussion:
Addresses UK 150
There is on going confusion over what one can and can not do with a moved-from object (e.g. UK 150, 910(i)). This issue attempts to clarify that moved-from objects are valid objects with an unknown state.
[ 2010-01-22 Wording tweaked by Beman. ]
[ 2010-01-22 Moved to Tentatively Ready after 5 positive votes on c++std-lib. ]
[ 2010-01-23 Alisdair opens: ]
I'm afraid I must register an objection.
My primary objection is that I have always been opposed to this kind of a resolution as over-constraining. My preferred example is a call implementing the pImpl idiom via
unique_ptr
. Once the pImpl has been moved from, it is no longer safe to call the vast majority of the object's methods, yet I see no reason to make such a type unusable in the standard library. I would prefer a resolution along the lines suggested in the UK comment, which only requires that the object can be safely destroyed, and serve as the target of an assignment operator (if supported.)However, I will not hold the issue up if I am a lone dissenting voice on this (yes, that is a call to hear more support, or I will drop that objection in Pittsburgh)
With the proposed wording, I'm not clear what the term 'valid object' means. In my example above, is a pImpl holding a null pointer 'valid'? What about a float holding a signalling NaN? What determines if an object is valid? Without a definition of a valid/invalid object, I don't think this wording adds anything, and this is an objection that I do want resolved.
[ 2010-01-24 Alisdair removes his objection. ]
[ 2010-01-24 Moved to Tentatively Ready after 5 positive votes on c++std-lib. ]
[ 2010-02-10 Reopened. The wording here has been merged into 1309(i). ]
[
2010-02-10 Moved to Tentatively NAD EditorialResolved after 5 postive votes on
c++std-lib. Rationale added below.
]
Rationale:
This issue is now addressed by 1309(i).
Proposed resolution:
Change the follwing tables in 16.4.4.2 [utility.arg.requirements] as shown:
Table 33 — MoveConstructible
requirements [moveconstructible]Expression Post-condition T t(rv)
t
is equivalent to the value ofrv
before the construction.[Note: There is no requirement on the value ofrv
after the construction.rv
remains a valid object. Its state is unspecified. — end note]
Table 35 — MoveAssignable
requirements [moveassignable]Expression Return type Return value Post-condition t = rv
T&
t
t
is equivalent to the value ofrv
before the assigment.[Note: There is no requirement on the value ofrv
after the assignment.rv
remains a valid object. Its state is unspecified. — end note]
vector<bool> initializer_list
constructor missing an allocator argumentSection: 23.3.14 [vector.bool] Status: C++11 Submitter: Bo Persson Opened: 2009-12-09 Last modified: 2016-01-28
Priority: Not Prioritized
View other active issues in [vector.bool].
View all other issues in [vector.bool].
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Discussion:
The specialization for vector<bool>
(23.3.14 [vector.bool])
has a constructor
vector(initializer_list<bool>);
which differs from the base template's constructor (and other containers) in
that it has no allocator
parameter.
[ 2009-12-16 Moved to Tentatively Ready after 5 positive votes on c++std-lib. ]
Proposed resolution:
Change the signature in the synopsis of 23.3.14 [vector.bool] to
vector(initializer_list<bool>, const Allocator& = Allocator());
allocator_traits
call to new
Section: 20.2.9.3 [allocator.traits.members] Status: C++11 Submitter: Howard Hinnant Opened: 2009-12-10 Last modified: 2016-01-28
Priority: Not Prioritized
View all other issues in [allocator.traits.members].
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Discussion:
LWG issue 402(i) added "::
" to the call to new
within allocator::construct
. I suspect we want to retain that fix.
[ 2009-12-13 Moved to Tentatively Ready after 7 positive votes on c++std-lib. ]
Proposed resolution:
Change 16.4.4.6 [allocator.requirements], table 40 "Allocator requirements":
Table 40 — Allocator requirements Expression Return type Assertion/note
pre-/post-conditionDefault a.construct(c,args)
(not used) Effect: Constructs an object of type C
atc
::new ((void*)c) C(forward<Args>(args)...)
Change 20.2.9.3 [allocator.traits.members], p4:
template <class T, class... Args> static void construct(Alloc& a, T* p, Args&&... args);4 Effects: calls
a.construct(p, std::forward<Args>(args)...)
if that call is well-formed; otherwise, invokes::new (static_cast<void*>(p)) T(std::forward<Args>(args)...)
.
allocator_traits::select_on_container_copy_construction
type-oSection: 20.2.9.3 [allocator.traits.members] Status: C++11 Submitter: Howard Hinnant Opened: 2009-12-10 Last modified: 2016-01-28
Priority: Not Prioritized
View all other issues in [allocator.traits.members].
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Discussion:
allocator_traits::select_on_container_copy_construction
refers to an
unknown "a
":
static Alloc select_on_container_copy_construction(const Alloc& rhs);7 Returns:
rhs.select_on_container_copy_construction(a)
if that expression is well-formed; otherwise,rhs
.
[ 2009-12-13 Moved to Tentatively Ready after 5 positive votes on c++std-lib. ]
Proposed resolution:
Change 20.2.9.3 [allocator.traits.members], p7:
static Alloc select_on_container_copy_construction(const Alloc& rhs);7 Returns:
rhs.select_on_container_copy_construction(
if that expression is well-formed; otherwise,a)rhs
.
std::function
requires CopyConstructible
target objectSection: 22.10.17.3.2 [func.wrap.func.con] Status: C++11 Submitter: Jonathan Wakely Opened: 2009-12-13 Last modified: 2016-01-28
Priority: Not Prioritized
View all other issues in [func.wrap.func.con].
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Discussion:
I think std::function
should require CopyConstructible
for the
target object.
I initially thought that MoveConstructible
was enough, but it's not. If
F
is move-only then function's copy constructor cannot be called, but
because function uses type erasure, F
is not known and so the copy
constructor cannot be disabled via enable_if
. One option would be to
throw an exception if you try to copy a function with a non-copyable target
type, but I think that would be a terrible idea.
So although the constructors require that the target be initialised by
std::move(f)
, that's only an optimisation, and a copy constructor is
required.
[ 2009-12-24 Moved to Tentatively Ready after 5 positive votes on c++std-lib. ]
Proposed resolution:
Add to 22.10.17.3.2 [func.wrap.func.con] paragraph 9:
template<class F> function(F f); template <class F, class A> function(allocator_arg_t, const A& a, F f);9 Requires:
F
shall beCopyConstructible
.f
shall be callable for argument typesArgTypes
and return typeR
. The copy constructor and destructor ofA
shall not throw exceptions.
std::function
assignment from rvaluesSection: 22.10.17.3.2 [func.wrap.func.con] Status: C++11 Submitter: Jonathan Wakely Opened: 2009-12-13 Last modified: 2016-01-28
Priority: Not Prioritized
View all other issues in [func.wrap.func.con].
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Discussion:
In 22.10.17.3.2 [func.wrap.func.con]
template<class F> function& operator=(F f);20 Effects:
function(f).swap(*this);
21 Returns:
*this
This assignment operator can be called such that F
is an rvalue-reference e.g.
func.operator=<F&&>(f);
There are two issues with this.
f
is passed as an lvalue and so there will be an
unnecessary copy. The argument should be forwarded, so that the copy can be
avoided.
F
is a deduced context it can be made to work with either lvalues or rvalues.
The same issues apply to function::assign
.
N.B. this issue is not related to 1287(i) and applies whether that issue is resolved or not. The wording below assumes the resolution of LWG 1258(i) has been applied.
[ 2009-12-16 Moved to Tentatively Ready after 5 positive votes on c++std-lib. ]
[ 201002-11 Opened by Alisdair for the purpose of merging 1258(i) into this issue as there is a minor conflict. ]
[ 2010-02-11 Moved to Tentatively Ready after 5 positive votes on c++std-lib. ]
Proposed resolution:
In 22.10.17.3.2 [func.wrap.func.con]
template<class F> function& operator=(F&& f);20 Effects:
function(std::forward<F>(f)).swap(*this);
21 Returns:
*this
In 22.10.17.3.3 [func.wrap.func.mod]
template<class F,Allocator Allocclass A> void assign(F&& f, const Alloc& a);3 Effects:
function(
f, aallocator_arg, a, std::forward<F>(f)).swap(*this);
Update member function signature for class template in 22.10.17.3 [func.wrap.func]
template<class F> function& operator=(F&&); template<class F, class A> void assign(F&&, const A&);
[u|bi]nary_function
inheritanceSection: 22.10 [function.objects] Status: Resolved Submitter: Daniel Krügler Opened: 2009-12-14 Last modified: 2025-03-13
Priority: Not Prioritized
View all other issues in [function.objects].
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Discussion:
This issue is a follow-up of the discussion on issue 870(i) during the 2009 Santa Cruz meeting.
The class templates unary_function
and binary_function
are
actually very simple typedef providers,
namespace std { template <class Arg, class Result> struct unary_function { typedef Arg argument_type; typedef Result result_type; }; template <class Arg1, class Arg2, class Result> struct binary_function { typedef Arg1 first_argument_type; typedef Arg2 second_argument_type; typedef Result result_type; }; }
which may be used as base classes (similarly to the iterator template),
but were originally not intended as a customization point. The SGI
documentation introduced the concept
Adaptable Unary
Function as function objects "with nested typedefs that define its argument
type and result type" and a similar definition for
Adaptable Binary
Function related to binary_function
. But as of TR1 a protocol was
introduced that relies on inheritance relations based on these types. 22.10.6 [refwrap]/3 b. 3 requires that a specialization of
reference_wrapper<T>
shall derive from unary_function
,
if type T
is "a class type that is derived from
std::unary_function<T1, R>
" and a similar inheritance-based rule
for binary_function
exists as well.
As another disadvantage it has been pointed out in the TR1 issue list, N1837
(see section 10.39), that the requirements of mem_fn
22.10.16 [func.memfn]/2+3 to derive from
std::unary_function/std::binary_function
under circumstances, where the
provision of corresponding typedefs would be sufficient, unnecessarily prevent
implementations that take advantage of empty-base-class optimizations.
Both requirements should be relaxed in the sense that the
reference_wrapper
should provide typedef's argument_type
,
first_argument_type
, and second_argument_type
based on similar
rules as the weak result type rule (22.10.4 [func.require]/3) does
specify the presence of result_type
member types.
For a related issue see also 1279(i).
[ 2010-10-24 Daniel adds: ]
Accepting n3145 would resolve this issue as NAD editorial.
[ 2010-11 Batavia: Solved by N3198 ]
Resolved by adopting n3198.
Previous proposed resolution:
[ The here proposed resolution is an attempt to realize the common denominator of the reflector threads c++std-lib-26011, c++std-lib-26095, and c++std-lib-26124. ]
Change [base]/1 as indicated: [The intend is to provide an alternative fix for issue 1279(i) and some editorial harmonization with existing wording in the library, like 99 [iterator.basic]/1]
1 The following class templates are provided to simplify the definition of typedefs of the argument and result types for function objects. The behavior of a program that adds specializations for any of these templates is undefined.
:namespace std { template <class Arg, class Result> struct unary_function { typedef Arg argument_type; typedef Result result_type; }; } namespace std { template <class Arg1, class Arg2, class Result> struct binary_function { typedef Arg1 first_argument_type; typedef Arg2 second_argument_type; typedef Result result_type; }; }Change 22.10.6 [refwrap], class template
reference_wrapper
synopsis as indicated: [The intent is to remove the requirement thatreference_wrapper
derives fromunary_function
orbinary_function
if the situation requires the definition of the typedefsargument_type
,first_argument_type
, orsecond_argument_type
. This change is suggested, because the new way of definition uses the same strategy as the weak result type specification applied to argument types, which provides the following advantages: It creates less potential conflicts between[u|bi]nary_function
bases and typedefs in a function object and it ensures that user-defined function objects which provide typedefs but no such bases are handled as first class citizens.]namespace std { template <class T> class reference_wrapper: public unary_function<T1, R> // see below: public binary_function<T1, T2, R> // see below{ public : // types typedef T type; typedef see below result_type; // not always defined typedef see below argument_type; // not always defined typedef see below first_argument_type; // not always defined typedef see below second_argument_type; // not always defined // construct/copy/destroy ... };Change 22.10.6 [refwrap]/3 as indicated: [The intent is to remove the requirement that
reference_wrapper
derives fromunary_function
if the situation requires the definition of the typedefargument_type
andresult_type
. Note that this clause does concentrate onargument_type
alone, because theresult_type
is already ruled by p. 2 via the weak result type specification. The new way of specifyingargument_type
is equivalent to the weak result type specification]3 The template instantiation
reference_wrapper<T>
shallbe derived fromdefine a nested type namedstd::unary_function<T1, R>
argument_type
as a synonym forT1
only if the typeT
is any of the following:
- a function type or a pointer to function type taking one argument of type
T1
and returningR
- a pointer to member function
R T0::f
cv (where cv represents the member function's cv-qualifiers); the typeT1
is cvT0*
- a class type
that is derived fromwith a member typestd::unary_function<T1, R>
argument_type
; the typeT1
isT::argument_type
Change 22.10.6 [refwrap]/4 as indicated: [The intent is to remove the requirement that
reference_wrapper
derives frombinary_function
if the situation requires the definition of the typedeffirst_argument_type
,second_argument_type
, andresult_type
. Note that this clause does concentrate onfirst_argument_type
andsecond_argument_type
alone, because theresult_type
is already ruled by p. 2 via the weak result type specification. The new way of specifyingfirst_argument_type
andsecond_argument_type
is equivalent to the weak result type specification]The template instantiation
reference_wrapper<T>
shallbe derived fromdefine two nested types namedstd::binary_function<T1, T2, R>
first_argument_type
andsecond_argument_type
as a synonym forT1
andT2
, respectively, only if the typeT
is any of the following:
- a function type or a pointer to function type taking two arguments of types
T1
andT2
and returningR
- a pointer to member function
R T0::f(T2)
cv (where cv represents the member function's cv-qualifiers); the typeT1
is cvT0*
- a class type
that is derived fromwith member typesstd::binary_function<T1, T2, R>
first_argument_type
andsecond_argument_type
; the typeT1
isT::first_argument_type
and the typeT2
isT::second_argument_type
Change 22.10.16 [func.memfn]/2+3 as indicated: [The intent is to remove the requirement that mem_fn's return type has to derive from
[u|bi]nary_function
. The reason for suggesting the change here is to better support empty-base-class optimization choices as has been pointed out in N1837]2 The simple call wrapper shall
be derived fromdefine two nested types namedstd::unary_function<cv T*, Ret>
argument_type
andresult_type
as a synonym forcv T*
andRet
, respectively, whenpm
is a pointer to member function with cv-qualifier cv and taking no arguments, whereRet
ispm
's return type.3 The simple call wrapper shall
be derived fromdefine three nested types namedstd::binary_function<cv T*, T1, Ret>
first_argument_type
,second_argument_type
, andresult_type
as a synonym forcv T*
,T1
, andRet
, respectively, whenpm
is a pointer to member function with cv-qualifier cv and taking one argument of typeT1
, whereRet
ispm
's return type.
Proposed resolution:
Addressed by paper n3198.
promise::set_value
Section: 32.10.6 [futures.promise] Status: Resolved Submitter: Jonathan Wakely Opened: 2009-12-18 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
In 32.10.6 [futures.promise]
Does promise<R>::set_value
return normally if the copy/move
constructor of R
throws?
The exception could be caught and set using
promise<R>::set_exception
, or it could be allowed to leave the
set_value
call, but it's not clear which is intended. I suggest the
exception should not be caught.
N.B. This doesn't apply to promise<R&>::set_value
or
promise<void>::set_value
because they don't construct a new
object.
[
2010 Pittsburgh: Moved to NAD EditorialResolved. Rationale added below.
]
Rationale:
Solved by N3058.
Proposed resolution:
Change 32.10.6 [futures.promise]/18:
18 Throws:
future_error
if its associated state is already ready or, for the first version an exception thrown by the copy constructor ofR
, or for the second version an exception thrown by the move constructor ofR
.
std::function
should support all callable typesSection: 22.10.17.3.2 [func.wrap.func.con] Status: C++11 Submitter: Daniel Krügler Opened: 2009-12-19 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
Some parts of the specification of std::function
is unnecessarily
restricted to a subset of all callable types (as defined in 22.10.3 [func.def]/3), even though the intent clearly is to be usable for
all of them as described in 22.10.17.3 [func.wrap.func]/1. This
argument becomes strengthened by the fact that current C++0x-compatible
compilers work fine with them:
#include <functional> #include <iostream> struct A { int foo(int i) const {return i+1;} }; struct B { int mem; }; int main() { std::function<int(const A&, int)> f(&A::foo); A a; std::cout << f(a, 1) << '\n'; std::cout << f.target_type().name() << '\n'; typedef int (A::* target_t)(int) const; target_t* p = f.target<target_t>(); std::cout << (p != 0) << '\n'; std::function<int(B&)> f2(&B::mem); B b = { 42 }; std::cout << f2(b) << '\n'; std::cout << f2.target_type().name() << '\n'; typedef int (B::* target2_t); target2_t* p2 = f2.target<target2_t>(); std::cout << (p2 != 0) << '\n'; }
The problematic passages are 22.10.17.3.2 [func.wrap.func.con]/10:
template<class F> function(F f); template <class F, class A> function(allocator_arg_t, const A& a, F f);...
10 Postconditions:
!*this
if any of the following hold:
f
is aNULL
function pointer.f
is aNULL
member function pointer.F
is an instance of the function class template, and!f
because it does not consider pointer to data member and all constraints based on function objects which like 22.10.17.3 [func.wrap.func]/2 or 22.10.17.3.6 [func.wrap.func.targ]/3. The latter two will be resolved by the proposed resolution of 870(i) and are therefore not handled here.
[ Post-Rapperswil: ]
Moved to Tentatively Ready after 5 positive votes on c++std-lib.
[ Adopted at 2010-11 Batavia ]
Proposed resolution:
Change 22.10.17.3.2 [func.wrap.func.con]/10+11 as indicated:
template<class F> function(F f); template <class F, class A> function(allocator_arg_t, const A& a, F f);...
10 Postconditions:
!*this
if any of the following hold:
f
is aNULL
function pointer.f
is aNULL
pointer to memberfunction pointer.F
is an instance of the function class template, and!f
11 Otherwise,
*this
targets a copy off
or, initialized withstd::move(f)
if. [Note: implementations are encouraged to avoid the use of dynamically allocated memory for small function objects, for example, wheref
is not a pointer to member function, and targets a copy ofmem_fn(f)
iff
is a pointer to member functionf
's target is an object holding only a pointer or reference to an object and a member function pointer. — end note]
unique_ptr<T[], D>
needs to get rid of unspecified-pointer-typeSection: 20.3.1.4 [unique.ptr.runtime] Status: Resolved Submitter: Daniel Krügler Opened: 2009-12-20 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
Addresses UK 211
As a response to UK 211 LWG issue 1021(i) has replaced the
unspecified-pointer-type by nullptr_t
to allow assignment of
type-safe null-pointer literals in the non-array form of
unique_ptr::operator=
, but did not the same for the specialization for
arrays of runtime length. But without this parallel change of the signature we
have a status quo, where unique_ptr<T[], D>
declares a member
function which is completely unspecified.
[ 2009-12-21 Moved to Tentatively Ready after 5 positive votes on c++std-lib. ]
[ 2010-03-14 Howard adds: ]
We moved N3073 to the formal motions page in Pittsburgh which should obsolete this issue. I've moved this issue to NAD Editorial, solved by N3073.
Rationale:
Solved by N3073.
Proposed resolution:
In 20.3.1.4 [unique.ptr.runtime], class template unique_ptr<T[],
D>
synopsis, change as indicated:
// assignment unique_ptr& operator=(unique_ptr&& u); unique_ptr& operator=(unspecified-pointer-typenullptr_t);
Section: 22.10.4 [func.require] Status: C++11 Submitter: Jens Maurer Opened: 2009-12-21 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
The current wording in the standard makes it hard to discriminate the difference between a "call wrapper" as defined in 22.10.3 [func.def]/5+6:
5 A call wrapper type is a type that holds a callable object and supports a call operation that forwards to that object.
6 A call wrapper is an object of a call wrapper type.
and a "forwarding call wrapper" as defined in 22.10.4 [func.require]/4:
4 [..] A forwarding call wrapper is a call wrapper that can be called with an argument list. [Note: in a typical implementation forwarding call wrappers have an overloaded function call operator of the form
template<class... ArgTypes> R operator()(ArgTypes&&... args) cv-qual;— end note]
Reason for this lack of clear difference seems to be that the wording adaption to variadics and rvalues that were applied after it's original proposal in N1673:
[..] A forwarding call wrapper is a call wrapper that can be called with an argument list
t1, t2, ..., tN
where eachti
is an lvalue. The effect of calling a forwarding call wrapper with one or more arguments that are rvalues is implementation defined. [Note: in a typical implementation forwarding call wrappers have overloaded function call operators of the formtemplate<class T1, class T2, ..., class TN> R operator()(T1& t1, T2& t2, ..., TN& tN) cv-qual;— end note]
combined with the fact that the word "forward" has two different meanings in this context. This issue attempts to clarify the difference better.
[ 2010-09-14 Daniel provides improved wording and verified that it is correct against N3126. Previous resolution is shown here: ]
4 [..] A forwarding call wrapper is a call wrapper that can be called with an arbitrary argument list and uses perfect forwarding to deliver the arguments to the wrapped callable object. [Note: in a typical implementation forwarding call wrappers have an overloaded function call operator of the form
template<class... ArgTypes> R operator()(ArgTypes&&... args) cv-qual;— end note]
[ Adopted at 2010-11 Batavia ]
Proposed resolution:
Change 22.10.4 [func.require]/4 as indicated:
[..] A forwarding call wrapper is a call wrapper that can be called with an arbitrary argument list and delivers the arguments as references to the wrapped callable object. This forwarding step shall ensure that rvalue arguments are delivered as rvalue-references and lvalue arguments are delivered as lvalue-references. [Note: in a typical implementation forwarding call wrappers have an overloaded function call operator of the form
template<class... UnBoundArgs> R operator()(UnBoundArgs&&... unbound_args) cv-qual;— end note ]
Section: 22.10.4 [func.require] Status: C++11 Submitter: Daniel Krügler Opened: 2009-12-22 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
22.10.4 [func.require]/3 b 1 says
3 If a call wrapper (22.10.3 [func.def]) has a weak result type the type of its member type
result_type
is based on the typeT
of the wrapper's target object (22.10.3 [func.def]):
- if
T
is a function, reference to function, or pointer to function type,result_type
shall be a synonym for the return type ofT
;- [..]
The first two enumerated types (function and reference to function)
can never be valid types for T
, because
22.10.3 [func.def]/7
7 A target object is the callable object held by a call wrapper.
and 22.10.3 [func.def]/3
3 A callable type is a pointer to function, a pointer to member function, a pointer to member data, or a class type whose objects can appear immediately to the left of a function call operator.
exclude functions and references to function as "target objects".
[ Post-Rapperswil: ]
Moved to Tentatively Ready after 5 positive votes on c++std-lib.
[ Adopted at 2010-11 Batavia ]
Proposed resolution:
Change 22.10.4 [func.require]/3 b 1 as indicated:
3 If a call wrapper (22.10.3 [func.def]) has a weak result type the type of its member type
result_type
is based on the typeT
of the wrapper's target object (22.10.3 [func.def]):
- if
T
is afunction, reference to function, orpointer to function type,result_type
shall be a synonym for the return type ofT
;- [..]
unique_ptr
's relational operator functions should induce a total orderSection: 20.3.1.6 [unique.ptr.special] Status: Resolved Submitter: Daniel Krügler Opened: 2009-12-23 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
The comparison functions of unique_ptr
currently directly delegate to
the underlying comparison functions of unique_ptr<T, D>::pointer
.
This is disadvantageous, because this would not guarantee to induce a total
ordering for native pointers and it is hard to define a total order for mixed
types anyway.
The currently suggested resolution for shared_ptr
comparison as of
1262(i) uses a normalization strategy: They perform the comparison on
the composite pointer type (7.6.9 [expr.rel]). This is not
exactly possible for unique_ptr
in the presence of user-defined
pointer-like types but the existing definition of std::duration
comparison as of 30.5.7 [time.duration.comparisons] via
common_type
of both argument types demonstrates a solution of this
problem. The approach can be seen as the general way to define a composite
pointer type and this is the approach which is used for here suggested
wording change.
For consistency reasons I would have preferred the same normalization strategy
for ==
and !=
, but Howard convinced me not to do so (now).
[ 2010-11-03 Daniel comments and adjustes the currently proposed wording changes: ]
Issue 1401(i) is remotely related. Bullet A of its proposed resolution
provides an alternative solution for issue discussed here and addresses NB comment GB-99.
Additionally I updated the below suggested wording in regard to the following:
It is an unncessary requirement that the below defined effective composite pointer-like
type CT
satisfies the LessThanComparable
requirements. All what is
needed is, that the function object type less<CT>
induces a strict
weak ordering on the pointer values.
[2011-03-24 Madrid meeting]
Proposed resolution:
Change 20.3.1.6 [unique.ptr.special]/4-7 as indicated: [The implicit
requirements and remarks imposed on the last three operators are the same as for
the first one due to the normative "equivalent to" usage within a Requires
element, see 16.3.2.4 [structure.specifications]/4. The effects of this
change are that all real pointers wrapped in a unique_ptr
will order
like shared_ptr
does.]
template <class T1, class D1, class T2, class D2> bool operator<(const unique_ptr<T1, D1>& x, const unique_ptr<T2, D2>& y);? Requires: Let
CT
becommon_type<unique_ptr<T1, D1>::pointer, unique_ptr<T2, D2>::pointer>::type
. Then the specializationless<CT>
shall be a function object type ([function.objects]) that induces a strict weak ordering ([alg.sorting]) on the pointer values.4 Returns:
less<CT>()(x.get(), y.get())
.x.get() < y.get()? Remarks: If
unique_ptr<T1, D1>::pointer
is not implicitly convertible toCT
orunique_ptr<T2, D2>::pointer
is not implicitly convertible toCT
, the program is ill-formed.template <class T1, class D1, class T2, class D2> bool operator<=(const unique_ptr<T1, D1>& x, const unique_ptr<T2, D2>& y);5 Effects: Equivalent to
return !(y < x)
Returns:.x.get() <= y.get()
template <class T1, class D1, class T2, class D2> bool operator>(const unique_ptr<T1, D1>& x, const unique_ptr<T2, D2>& y);6 Effects: Equivalent to
return (y < x)
Returns:.x.get() > y.get()
template <class T1, class D1, class T2, class D2> bool operator>=(const unique_ptr<T1, D1>& x, const unique_ptr<T2, D2>& y);7 Effects: Equivalent to
return !(x < y)
Returns:.x.get() >= y.get()
ctype_byname<char>
Section: 28.3.2 [locale.syn] Status: C++11 Submitter: Alisdair Meredith Opened: 2009-12-25 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
The <locale>
synopsis in 28.3.2 [locale.syn] calls out an
explicit specialization for ctype_byname<char>
, however no such
specialization is defined in the standard. The only reference I can find to
ctype_byname<char>
is 28.3.3.1.2.2 [locale.facet]:Table 77
— Required specializations (for facets) which also refers to
ctype_byname<wchar_t>
which has no special consideration.
Is the intent an explicit instantiation which would use a slightly different syntax? Should the explicit specialization simply be struck?
[ 2010-01-31 Moved to Tentatively Ready after 5 positive votes on c++std-lib. ]
Proposed resolution:
28.3.2 [locale.syn]
Strike the explicit specialization for
ctype_byname<char>
from the<locale>
synopsis... template <class charT> class ctype_byname;template <> class ctype_byname<char>; // specialization...
get_time
Section: 31.7.8 [ext.manip] Status: C++11 Submitter: Alisdair Meredith Opened: 2009-12-25 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
Extended Manipulators 31.7.8 [ext.manip] p8 defines the semantics of
get_time
in terms of a function f
.
template <class charT, class traits> void f(basic_ios<charT, traits>& str, struct tm* tmb, const charT* fmt) { typedef istreambuf_iterator<charT, traits> Iter; typedef time_get<charT, Iter> TimeGet; ios_base::iostate err = ios_base::goodbit; const TimeGet& tg = use_facet<TimeGet>(str.getloc()); tm.get(Iter(str.rdbuf()), Iter(), str, err, tmb, fmt, fmt + traits::length(fmt)); if (err != ios_base::goodbit) str.setstate(err): }
Note the call to tm.get
. This is clearly an error, as tm
is a
type and not an object. I believe this should be tg.get
, rather than
tm
, but this is not my area of expertise.
[ 2010-01-14 Moved to Tentatively Ready after 5 positive votes on c++std-lib. ]
Proposed resolution:
Change 31.7.8 [ext.manip] p8:
template <class charT, class traits> void f(basic_ios<charT, traits>& str, struct tm* tmb, const charT* fmt) { typedef istreambuf_iterator<charT, traits> Iter; typedef time_get<charT, Iter> TimeGet; ios_base::iostate err = ios_base::goodbit; const TimeGet& tg = use_facet<TimeGet>(str.getloc()); tgm.get(Iter(str.rdbuf()), Iter(), str, err, tmb, fmt, fmt + traits::length(fmt)); if (err != ios_base::goodbit) str.setstate(err): }
promise::swap
Section: 32.10.6 [futures.promise] Status: Resolved Submitter: Jonathan Wakely Opened: 2009-12-26 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
32.10.6 [futures.promise]/12 defines the effects of
promise::swap(promise&)
as
void swap(promise& other);12 Effects:
swap(*this, other)
and 32.10.6 [futures.promise]/25 defines swap(promise<R&>,
promise<R>&)
as
template <class R> void swap(promise<R>& x, promise<R>& y);25 Effects:
x.swap(y)
.
[ 2010-01-13 Daniel added "Throws: Nothing." ]
[ 2010-01-14 Moved to Tentatively Ready after 5 positive votes on c++std-lib. ]
[ 2010 Pittsburgh: ]
Moved to
NAD EditorialResolved. Rationale added below.
Rationale:
Solved by N3058.
Proposed resolution:
Change 32.10.6 [futures.promise] paragraph 12
void swap(promise& other);12 Effects:
Exchanges the associated states ofswap(*this, other)
*this
andother
.13 ...
Throws: Nothing.
shared_ptr
, unique_ptr
, and rvalue references v2Section: 20.3.1.3 [unique.ptr.single], 20.3.2.2 [util.smartptr.shared] Status: C++11 Submitter: Stephan T. Lavavej Opened: 2010-01-23 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
N3000 20.3.2.2 [util.smartptr.shared]/1 still says:
template <class Y, class D> explicit shared_ptr(const unique_ptr<Y, D>& r) = delete; template <class Y, class D> shared_ptr& operator=(const unique_ptr<Y, D>& r) = delete;
I believe that this is unnecessary now that "rvalue references v2" prevents rvalue references from binding to lvalues, and I didn't see a Library Issue tracking this.
[ 2010-02-12 Moved to Tentatively Ready after 5 positive votes on c++std-lib. ]
Proposed resolution:
Strike from 20.3.1.3 [unique.ptr.single]:
template <class T, class D = default_delete<T>> class unique_ptr { ... unique_ptr(const unique_ptr&) = delete;template <class U, class E> unique_ptr(const unique_ptr<U, E>&) = delete;unique_ptr& operator=(const unique_ptr&) = delete;template <class U, class E> unique_ptr& operator=(const unique_ptr<U, E>&) = delete;};
Strike from 20.3.2.2 [util.smartptr.shared]:
template<class T> class shared_ptr { ...template <class Y, class D> explicit shared_ptr(const unique_ptr<Y, D>& r) = delete;...template <class Y, class D> shared_ptr& operator=(const unique_ptr<Y, D>& r) = delete;... };
shared_future
Section: 32.10.8 [futures.shared.future] Status: Resolved Submitter: Alisdair Meredith Opened: 2010-01-23 Last modified: 2021-06-06
Priority: Not Prioritized
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Discussion:
The revised futures package in the current working paper simplified the
is_ready/has_exception/has_value
set of APIs, replacing them with a
single 'valid' method. This method is used in many places to signal pre- and
post- conditions, but that edit is not complete. Each method on a
shared_future
that requires an associated state should have a
pre-condition that valid() == true
.
[ 2010-01-28 Moved to Tentatively Ready after 5 positive votes on c++std-lib. ]
[ 2010 Pittsburgh: ]
Moved to
NAD EditorialResolved. Rationale added below.
Rationale:
Solved by N3058.
Proposed resolution:
Insert the following extra paragraphs:
In [futures.shared_future]
shared_future();4 Effects: constructs ...
Postcondition:
valid() == false
.Throws: nothing.
void wait() const;Requires:
valid() == true
.22 Effects: if the associated ...
template <class Rep, class Period> bool wait_for(const chrono::duration<Rep, Period>& rel_time) const;Requires:
valid() == true
.23 Effects: if the associated ...
template <class Clock, class Duration> bool wait_until(const chrono::time_point<Clock, Duration>& abs_time) const;Requires:
valid() == true
.25 Effects: blocks until ...
atomic_future
Section: 99 [futures.atomic_future] Status: Resolved Submitter: Alisdair Meredith Opened: 2010-01-23 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
The revised futures package in the current working paper simplified the
is_ready/has_exception/has_value
set of APIs, replacing them with a
single 'valid' method. This method is used in many places to signal pre- and
post- conditions, but that edit is not complete.
Atomic future retains the extended earlier API, and provides defined,
synchronized behaviour for all calls. However, some preconditions and throws
clauses are missing, which can easily be built around the new valid()
api. Note that for consistency, I suggest is_ready/has_exception/has_value
throw
an exception if valid()
is not true
, rather than
return false
. I think this is implied by the existing pre-condition on
is_ready
.
[ 2010-01-23 See discussion starting with Message c++std-lib-26666. ]
[
2010 Pittsburgh: Moved to NAD EditorialResolved. Rationale added below.
]
Rationale:
Solved by N3058.
Proposed resolution:
Insert the following extra paragraphs:
In 99 [futures.atomic_future]
bool is_ready() const;17
PreconditionRequires:valid() == true
.18 Returns:
true
only if the associated state is ready.Throws:
future_error
with an error condition ofno_state
if the precondition is not met.
bool has_exception() const;Requires:
valid() == true
.19 Returns:
true
only if the associated state is ready and contains an exception.Throws:
future_error
with an error condition ofno_state
if the precondition is not met.
bool has_value() const;Requires:
valid() == true
.20 Returns:
true
only if the associated state is ready and contains a value.Throws:
future_error
with an error condition ofno_state
if the precondition is not met.
void wait() const;Requires:
valid() == true
.22 Effects: blocks until ...
Throws:
future_error
with an error condition ofno_state
if the precondition is not met.
template <class Rep, class Period> bool wait_for(const chrono::duration<Rep, Period>& rel_time) const;Requires:
valid() == true
.23 Effects: blocks until ...
24 Returns:
true
only if ...Throws:
future_error
with an error condition ofno_state
if the precondition is not met.
template <class Clock, class Duration> bool wait_until(const chrono::time_point<Clock, Duration>& abs_time) const;Requires:
valid() == true
.25 Effects: blocks until ...
26 Returns:
true
only if ...Throws:
future_error
with an error condition ofno_state
if the precondition is not met.
pointer
and const_pointer
for <array>
Section: 23.3.3 [array] Status: C++11 Submitter: Nicolai Josuttis Opened: 2010-01-24 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
Class <array>
is the only sequence container class that has no
types pointer
and const_pointer
defined. You might argue that
this makes no sense because there is no allocator support, but on the other
hand, types reference
and const_reference
are defined for
array
.
[ 2010-02-11 Moved to Tentatively Ready after 6 positive votes on c++std-lib. ]
Proposed resolution:
Add to Class template array 23.3.3 [array]:
namespace std { template <class T, size_t N > struct array { ... typedef T value_type; typedef T * pointer; typedef const T * const_pointer; ... }; }
exception_ptr
and allocator
pointers don't understand !=Section: 17.9.7 [propagation] Status: Resolved Submitter: Daniel Krügler Opened: 2010-01-26 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
The current requirements for a conforming implementation of
std::exception_ptr
(17.9.7 [propagation]/1-6) does not clarify
whether the expression
e1 != e2 e1 != nullptr
with e1
and e2
being two values of type
std::exception_ptr
are supported or not. Reason for this oddity is that
the concept EqualityComparable
does not provide operator !=
.
For the same reason programmers working against the types X::pointer
,
X::const_pointer
, X::void_pointer
, and
X::const_void_pointer
of any allocator concept X
(16.4.4.6 [allocator.requirements]/4 + Table 40) in a generic context can not rely
on the availability of the != operation, which is rather unnatural and
error-prone.
[ 2010 Pittsburgh: Moved to NAD Editorial. Rationale added below. ]
Rationale:
Solved by N3073.
Proposed resolution:
Move/CopyConstructible
Section: 16.4.4.2 [utility.arg.requirements] Status: C++11 Submitter: Daniel Krügler Opened: 2010-02-03 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
Table 33 — MoveConstructible requirements [moveconstructible] and Table 34 — CopyConstructible requirements [copyconstructible] support solely the following expression:
T t(rv)
where rv
is defined to be as "non-const rvalue of type T
" and
t
as a "modifiable lvalue of type T
" in 16.4.4.2 [utility.arg.requirements]/1.
This causes two different defects:
We cannot move/copy-initialize a const lvalue of type T
as in:
int get_i(); const int i1(get_i());
both in Table 33 and in Table 34.
The single support for
T t(rv)
in case of CopyConstructible
means that we cannot provide an
lvalue as a source of a copy as in
const int& get_lri(); int i2(get_lri());
I believe this second defect is due to the fact that this single expression supported both initialization situations according to the old (implicit) lvalue reference -> rvalue reference conversion rules.
Finally [copyconstructible] refers to some name u
which is not part of
the expression, and both [copyconstructible] and [moveconstructible] should
support construction expressions from temporaries - this would be a stylistic
consequence in the light of the new DefaultConstructible
requirements
and compared with existing requirements (see e.g. Container requirements or the
output/forward iterator requirements)..
[ 2010-02-09 Moved to Tentatively Ready after 5 positive votes on c++std-lib. ]
[ 2010-02-10 Reopened. The proposed wording of 1283(i) has been merged here. ]
[ 2010-02-10 Moved to Tentatively Ready after 5 positive votes on c++std-lib. ]
Proposed resolution:
Change 16.4.4.2 [utility.arg.requirements]/1 as indicated: [This change
suggestion is motivated to make type descriptions clearer: First, a
,
b
, and c
may also be non-const T
. Second, u
is described in a manner consistent with the container requirements tables.]
1 The template definitions in the C++ standard library refer to various named requirements whose details are set out in tables 31-38. In these tables,
T
is an object or reference type to be supplied by a C++ program instantiating a template;a
,b
, andc
are values of type (possiblyconst) T
;s
andt
are modifiable lvalues of typeT
;u
denotes an identifier;is a value of type (possiblyconst
)T
; andrv
is annon-constrvalue of typeT
; andv
is an lvalue of type (possiblyconst
)T
or an rvalue of typeconst T
.
In 16.4.4.2 [utility.arg.requirements] Table 33 ([moveconstructible])
change as indicated [Note: The symbol u
is defined to be either a
const or a non-const value and is the right one we need here]:
Table 33 — MoveConstructible
requirements [moveconstructible]Expression Post-condition T
tu(rv);is equivalent to the value of
turv
before the constructionT(rv)
T(rv)
is equivalent to the value ofrv
before the construction[Note: There is no requirement on the value ofrv
after the construction.rv
remains a valid object. Its state is unspecified. — end note]
In 16.4.4.2 [utility.arg.requirements] Table 34 ([copyconstructible])
change as indicated [Note: The symbol u
is defined to be either a
const or a non-const value and is the right one we need here. The expressions
using a
are recommended to ensure that lvalues are supported as sources
of the copy expression]:
Table 34 — CopyConstructible
requirements [copyconstructible]
(in addition toMoveConstructible
)Expression Post-condition T
tu(rv);the value of is unchanged and is equivalent to
uv
tuT(v)
the value of v
is unchanged and is equivalent toT(v)
[Note: A type that satisfies theCopyConstructible
requirements also satisfies theMoveConstructible
requirements. — end note]
In Table 35 — MoveAssignable requirements [moveassignable] change as indicated:
Table 35 — MoveAssignable
requirements [moveassignable]Expression Return type Return value Post-condition t = rv
T&
t
t
is equivalent to the value ofrv
before the assigment.[Note: There is no requirement on the value ofrv
after the assignment.rv
remains a valid object. Its state is unspecified. — end note]
In 16.4.4.2 [utility.arg.requirements] change Table 36 as indicated:
Table 36 — CopyAssignable
requirements [copyassignable]
(in addition toMoveAssignable
)Expression Return type Return value Post-condition t =
uvT&
t
t
is equivalent to, the value of
uvis unchanged
uv[Note: A type that satisfies theCopyAssignable
requirements also satisfies theMoveAssignable
requirements. — end note]
forward_list splice_after
from lvaluesSection: 23.3.7.6 [forward.list.ops] Status: C++11 Submitter: Howard Hinnant Opened: 2010-02-05 Last modified: 2023-02-07
Priority: Not Prioritized
View all other issues in [forward.list.ops].
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Discussion:
We've moved 1133(i) to Tentatively Ready and I'm fine with that.
1133(i) adds lvalue-references to the splice
signatures for list
. So now
list
can splice
from lvalue and rvalue lists (which was the intent of the
original move papers btw). During the discussion of this issue it was mentioned
that if we want to give the same treatment to forward_list
, that should be a
separate issue.
This is that separate issue.
Consider the following case where you want to splice elements from one place in
a forward_list
to another. Currently this must be coded like so:
fl.splice_after(to_here, std::move(fl), from1, from2);
This looks pretty shocking to me. I would expect to be able to code instead:
fl.splice_after(to_here, fl, from1, from2);
but we currently don't allow it.
When I say move(fl)
, I consider that as saying that I don't care about
the value of fl
any more (until I assign it a new value). But in the
above example, this simply isn't true. I do care about the value of fl
after the move, and I'm not assigning it a new value. I'm merely permuting its
current value.
I propose adding forward_list&
overloads to the 3
splice_after
members. For consistency's sake (principal of least
surprise) I'm also proposing to overload merge
this way as well.
Proposed resolution:
Add to the synopsis of [forwardlist.overview]:
template <class T, class Allocator = allocator<T> > class forward_list { public: ... // [forwardlist.ops], forward_list operations: void splice_after(const_iterator p, forward_list& x); void splice_after(const_iterator p, forward_list&& x); void splice_after(const_iterator p, forward_list& x, const_iterator i); void splice_after(const_iterator p, forward_list&& x, const_iterator i); void splice_after(const_iterator p, forward_list& x, const_iterator first, const_iterator last); void splice_after(const_iterator p, forward_list&& x, const_iterator first, const_iterator last); ... void merge(forward_list& x); void merge(forward_list&& x); template <class Compare> void merge(forward_list& x, Compare comp); template <class Compare> void merge(forward_list&& x, Compare comp); ... };
Add to the signatures of [forwardlist.ops]:
void splice_after(const_iterator p, forward_list& x); void splice_after(const_iterator p, forward_list&& x);1 ...
void splice_after(const_iterator p, forward_list& x, const_iterator i); void splice_after(const_iterator p, forward_list&& x, const_iterator i);4 ...
void splice_after(const_iterator p, forward_list& x, const_iterator first, const_iterator last); void splice_after(const_iterator p, forward_list&& x, const_iterator first, const_iterator last);7 ...
void merge(forward_list& x); void merge(forward_list&& x); template <class Compare> void merge(forward_list& x, Compare comp); template <class Compare> void merge(forward_list&& x, Compare comp);16 ...
Section: 24.3.5.5 [forward.iterators] Status: Resolved Submitter: Alisdair Meredith Opened: 2010-02-07 Last modified: 2016-01-28
Priority: Not Prioritized
View all other issues in [forward.iterators].
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Discussion:
The following example demonstrates code that would meet the guarantees of a Forward Iterator, but only permits a single traversal of the underlying sequence:
template< typename ForwardIterator> struct bad_iterator { shared_ptr<ForwardIterator> impl; bad_iterator( ForwardIterator iter ) { : impl{new ForwardIterator{iter} } { } auto operator*() const -> decltype(*ForwardIterator{}) { return **impl; } auto operator->() const -> ForwardIterator { return *impl; } auto operator==(bad_iterator const & rhs) const -> bool { return impl == rhs.impl; } auto operator++() -> bad_iterator& { ++(*impl); return *this; } // other operations as necessary... };
Here, we use shared_ptr
to wrap a forward iterator, so all iterators
constructed from the same original iterator share the same 'value', and
incrementing any one copy increments all others.
There is a missing guarantee, expressed by the following code sequence
FwdIter x = seq.begin(); // obtain forward iterator from a sequence FwdIter y = x; // copy the iterator assert(x == y); // iterators must be the same ++x; // increment *just one* iterator assert(x != y); // iterators *must now be different* ++y; // increment the other iterator assert(x == y); // now the iterators must be the same again
That inequality in the middle is an essential guarantee. Note that this list is
simplified, as each assertion should also note that they refer to exactly the
same element (&*x == &*y)
but I am not complicating the issue
with tests to support proxy iterators, or value types overloading unary
operator+
.
I have not yet found a perverse example that can meet this additional constraint, and not meet the multi-pass expectations of a Forward Iterator without also violating other Forward Iterator requirements.
Note that I do not yet have standard-ready wording to resolve the problem, as saying this neatly and succinctly in 'standardese' is more difficult.
[
2010 Pittsburgh: Moved to NAD EditorialResolved. Rationale added below.
]
Rationale:
Solved by N3066.
Proposed resolution:
vector::data
no longer returns a raw pointerSection: 23.3.13.4 [vector.data] Status: C++11 Submitter: Alisdair Meredith Opened: 2010-02-07 Last modified: 2016-01-28
Priority: Not Prioritized
View all other issues in [vector.data].
View all issues with C++11 status.
Discussion:
The original intent of vector::data
was to match array::data
in providing a simple API with direct access to the contiguous buffer of
elements that could be passed to a "classic" C API. At some point, the return
type became the 'pointer
' typedef, which is not derived from the
allocator
via allocator traits - it is no longer specified to precisely
T *
. The return type of this function should be corrected to no longer
use the typedef.
[ 2010-02-10 Moved to Tentatively Ready after 5 positive votes on c++std-lib. ]
Proposed resolution:
23.3.13 [vector]
Update the class definition in p2:
// 23.3.6.3 data accesspointerT * data();const_pointerconst T * data() const;
23.3.13.4 [vector.data]
Adjust signatures:
pointerT * data();const_pointerconst T * data() const;
scoped_allocator_adaptor operator==
has no definitionSection: 20.6 [allocator.adaptor] Status: C++11 Submitter: Pablo Halpern Opened: 2009-02-11 Last modified: 2016-01-28
Priority: Not Prioritized
View all other issues in [allocator.adaptor].
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Discussion:
The WP (N3000) contains these declarations:
template <class OuterA1, class OuterA2, class... InnerAllocs> bool operator==(const scoped_allocator_adaptor<OuterA1, InnerAllocs...>& a, const scoped_allocator_adaptor<OuterA2, InnerAllocs...>& b); template <class OuterA1, class OuterA2, class... InnerAllocs> bool operator!=(const scoped_allocator_adaptor<OuterA1, InnerAllocs...>& a, const scoped_allocator_adaptor<OuterA2, InnerAllocs...>& b);
But does not define what the behavior of these operators are.
[ Post-Rapperswil: ]
Moved to Tentatively Ready after 5 positive votes on c++std-lib.
[ Adopted at 2010-11 Batavia ]
Proposed resolution:
Add a new section after 20.6.4 [allocator.adaptor.members]:
Scoped allocator operators [scoped.adaptor.operators]
template <class OuterA1, class OuterA2, class... InnerAllocs> bool operator==(const scoped_allocator_adaptor<OuterA1, InnerAllocs...>& a, const scoped_allocator_adaptor<OuterA2, InnerAllocs...>& b);Returns:
a.outer_allocator() == b.outer_allocator()
ifsizeof...(InnerAllocs)
is zero; otherwise,a.outer_allocator() == b.outer_allocator() && a.inner_allocator() == b.inner_allocator()
.template <class OuterA1, class OuterA2, class... InnerAllocs> bool operator!=(const scoped_allocator_adaptor<OuterA1, InnerAllocs...>& a, const scoped_allocator_adaptor<OuterA2, InnerAllocs...>& b);Returns:
!(a == b)
.
Section: 23.2.2 [container.requirements.general] Status: C++11 Submitter: Alisdair Meredith Opened: 2010-02-16 Last modified: 2016-01-28
Priority: Not Prioritized
View other active issues in [container.requirements.general].
View all other issues in [container.requirements.general].
View all issues with C++11 status.
Discussion:
The requirements on container iterators are spelled out in 23.2.2 [container.requirements.general], table 91.
Table 91 — Container requirements Expression Return type Operational semantics Assertion/note
pre-/post-conditionComplexity ...
X::iterator
iterator type whose value type is T
any iterator category except output iterator. Convertible to X::const_iterator
.compile time X::const_iterator
constant iterator type whose value type is T
any iterator category except output iterator compile time ...
As input iterators do not have the multi-pass guarantee, they are not suitable
for iterating over a container. For example, taking two calls to
begin()
, incrementing either iterator might invalidate the other.
While data structures might be imagined where this behaviour produces
interesting and useful results, it is very unlikely to meet the full set of
requirements for a standard container.
[ Post-Rapperswil: ]
Daniel notes: I changed the currently suggested P/R slightly, because it is not robust in regard to new fundamental iterator catagories. I recommend to say instead that each container::iterator shall satisfy (and thus may refine) the forward iterator requirements.
Moved to Tentatively Ready with revised wording after 5 positive votes on c++std-lib.
[ Adopted at 2010-11 Batavia ]
Proposed resolution:
Table 93 — Container requirements Expression Return type Operational
semanticsAssertion/note
pre-/post-conditionComplexity ...
X::iterator
iterator type
whose value
type isT
any iterator category except output iterator
that meets the forward iterator requirements. convertible
toX::const_iterator
compile time X::const_iterator
constant iterator type
whose value
type isT
any iterator category except output iterator
that meets the forward iterator requirements.compile time ...
scoped_allocator_adaptor construct
and destroy
don't
use allocator_traits
Section: 20.6.4 [allocator.adaptor.members] Status: Resolved Submitter: Howard Hinnant Opened: 2010-02-16 Last modified: 2016-01-28
Priority: Not Prioritized
View all other issues in [allocator.adaptor.members].
View all issues with Resolved status.
Discussion:
20.6.4 [allocator.adaptor.members] p8-9 says:
template <class T, class... Args> void construct(T* p, Args&&... args);8 Effects: let
OUTERMOST(x)
bex
ifx
does not have anouter_allocator()
function andOUTERMOST(x.outer_allocator())
otherwise.
- If
uses_allocator<T, inner_allocator_type>::value
isfalse
andis_constructible<T, Args...>::value
istrue
, callsOUTERMOST(*this).construct(p, std::forward<Args>(args)...)
.- Otherwise, if
uses_allocator<T, inner_allocator_type>::value
istrue
andis_constructible<T, allocator_arg_t, inner_allocator_type, Args...>::value
istrue
, callsOUTERMOST(*this).construct(p, allocator_arg, inner_allocator(),std::forward<Args>(args)...)
.- Otherwise, if
uses_allocator<T, inner_allocator_type>::value
istrue
andis_constructible<T, Args..., inner_allocator_type>::value
istrue
, callsOUTERMOST(*this).construct(p, std::forward<Args>(args)..., inner_allocator())
.- Otherwise, the program is ill-formed. [Note: an error will result if
uses_allocator
evaluates totrue
but the specific constructor does not take an allocator. This definition prevents a silent failure to pass an inner allocator to a contained element. — end note]template <class T> void destroy(T* p);9 Effects: calls
outer_allocator().destroy(p)
.
In all other calls where applicable scoped_allocator_adaptor
does not
call members of an allocator directly, but rather does so indirectly via
allocator_traits
. For example:
size_type max_size() const;7 Returns:
allocator_traits<OuterAlloc>::max_size(outer_allocator())
.
Indeed, without the indirection through allocator_traits
the
definitions for construct
and destroy
are likely to fail at
compile time since the outer_allocator()
may not have the members
construct
and destroy
.
[ The proposed wording is a product of Pablo, Daniel and Howard. ]
[ 2010 Pittsburgh: Moved to NAD Editorial. Rationale added below. ]
Rationale:
Solved by N3059.
Proposed resolution:
In 20.6.4 [allocator.adaptor.members] move and change p8 as indicated, and change p9 as indicated:
Let
OUTERMOST(x)
bex
ifx
does not have anouter_allocator()
member function andOUTERMOST(x.outer_allocator())
otherwise. LetOUTERMOST_ALLOC_TRAITS(x)
beallocator_traits<decltype(OUTERMOST(x))>
. [Note:OUTERMOST(x)
andOUTERMOST_ALLOC_TRAITS(x)
are recursive operations. It is incumbent upon the definition ofouter_allocator()
to ensure that the recursion terminates. It will terminate for all instantiations ofscoped_allocator_adaptor
. — end note]template <class T, class... Args> void construct(T* p, Args&&... args);8 Effects:
letOUTERMOST(x)
bex
ifx
does not have anouter_allocator()
function andOUTERMOST(x.outer_allocator())
otherwise.
- If
uses_allocator<T, inner_allocator_type>::value
isfalse
andis_constructible<T, Args...>::value
istrue
, calls.
OUTERMOST(*this).OUTERMOST_ALLOC_TRAITS(outer_allocator())::construct( OUTERMOST(outer_allocator()), p, std::forward<Args>(args)... )- Otherwise, if
uses_allocator<T, inner_allocator_type>::value
istrue
andis_constructible<T, allocator_arg_t, inner_allocator_type, Args...>::value
istrue
, calls.
OUTERMOST(*this).OUTERMOST_ALLOC_TRAITS(outer_allocator())::construct( OUTERMOST(outer_allocator()), p, allocator_arg, inner_allocator(), std::forward<Args>(args)... )- Otherwise, if
uses_allocator<T, inner_allocator_type>::value
istrue
andis_constructible<T, Args..., inner_allocator_type>::value
istrue
, calls.
OUTERMOST(*this).OUTERMOST_ALLOC_TRAITS(outer_allocator())::construct( OUTERMOST(outer_allocator()), p, std::forward<Args>(args)..., inner_allocator() )- Otherwise, the program is ill-formed. [Note: an error will result if
uses_allocator
evaluates totrue
but the specific constructor does not take an allocator. This definition prevents a silent failure to pass an inner allocator to a contained element. — end note]template <class T> void destroy(T* p);9 Effects: calls
.
outer_allocator().OUTERMOST_ALLOC_TRAITS(outer_allocator())::destroy( OUTERMOST(outer_allocator()), p)
CopyConstructible
requirements are insufficientSection: 16.4.4.2 [utility.arg.requirements] Status: Resolved Submitter: Daniel Krügler Opened: 2010-02-16 Last modified: 2016-01-28
Priority: Not Prioritized
View all other issues in [utility.arg.requirements].
View all issues with Resolved status.
Discussion:
With the acceptance of library defect 822(i) only
direct-initialization is supported, and not copy-initialization in the
requirement sets MoveConstructible
and CopyConstructible
. This
is usually a good thing, if only the library implementation needs to obey these
restrictions, but the Empire strikes back quickly:
Affects user-code: std::exception_ptr
is defined purely via
requirements, among them CopyConstructible
. A strict reading of the
standard would make implementations conforming where std::exception_ptr
has an explicit copy-c'tor and user-code must code defensively. This is a very
unwanted effect for such an important component like
std::exception_ptr
.
Wrong re-use: Recently proposed requirement sets
(NullablePointer
as of
N3025,
Hash) or cleanup of existing requirement sets (e.g. iterator requirements as of
N3046)
tend to reuse existing requirement sets, so reusing CopyConstructible
is attempting, even in cases, where the intend is to support copy-initialization
as well.
Inconsistency: The current iterator requirements set Table 102 (output iterator requirements) and Table 103 (forward iterator requirements) demonstrate quite clearly a strong divergence of copy-semantics: The specified semantics of
X u(a); X u = a;
are underspecified compared to the most recent clarifications of the
CopyConstructible
requirements, c.f. issue 1309(i) which is
very unsatisfactory. This will become worse for each further issue that involves
the CopyConstructible
specification (for possible directions see 1173(i)).
The suggested resolution is to define two further requirements
implicit-MoveConstructible
and implicit-CopyConstructible
(or
any other reasonable name like MoveConvertible
and
CopyConvertible
) each with a very succinct but precise meaning solving
all three problems mentioned above.
[Batavia: Resolved by accepting n3215.]
Proposed resolution:
Add the following new table ?? after Table 34 — MoveConstructible
requirements [moveconstructible]:
Table ?? — Implicit MoveConstructible
requirements [implicit.moveconstructible] (in addition toMoveConstructible
)Expression Operational Semantics T u = rv;
Equivalent to: T u(rv);
Add the following new table ?? after Table 35 — CopyConstructible
requirements [copyconstructible]:
Table ?? — Implicit CopyConstructible
requirements [implicit.copyconstructible] (in addition toCopyConstructible
)Expression Operational Semantics T u = v;
Equivalent to: T u(v);
Change 16.4.4.4 [nullablepointer.requirements]/1 as follows:
A
NullablePointer
type is a pointer-like type that supports null values. A typeP
meets the requirements ofNullablePointer
if:
P
satisfies the requirements ofEqualityComparable
,DefaultConstructible
,implicit CopyConstructible
,CopyAssignable
, andDestructible
,- [..]
Change 16.4.4.5 [hash.requirements]/1 as indicated: [explicit copy-constructible functors could not be provided as arguments to any algorithm that takes these by value. Also a typo is fixed.]
1 A type
H
meets the Hash requirements if:
- it is a function object type (20.8),
- it satisfies
ifesthe requirements ofimplicit CopyConstructible
andDestructible
(20.2.1),- [..]
Change 21.3.2 [meta.rqmts]/1+2 as indicated:
1 A UnaryTypeTrait describes a property of a type. It shall be a class template that takes one template type argument and, optionally, additional arguments that help define the property being described. It shall be
DefaultConstructible
,implicit CopyConstructible
, [..]2 A
BinaryTypeTrait
describes a relationship between two types. It shall be a class template that takes two template type arguments and, optionally, additional arguments that help define the relationship being described. It shall beDefaultConstructible
,implicit CopyConstructible
, and [..]
Change 22.10.4 [func.require]/4 as indicated: [explicit copy-constructible functors could not be provided as arguments to any algorithm that takes these by value]
4 Every call wrapper (20.8.1) shall be
implicit MoveConstructible
. A simple call wrapper is a call wrapper that isimplicit CopyConstructible
andCopyAssignable
and whose copy constructor, move constructor, and assignment operator do not throw exceptions. [..]
Change 22.10.6 [refwrap]/1 as indicated:
1
reference_wrapper<T>
is animplicit
CopyConstructible
andCopyAssignable
wrapper around a reference to an object or function of typeT
.
Change 22.10.15.4 [func.bind.bind]/5+9 as indicated:
5 Remarks: The return type shall satisfy the requirements of
implicit MoveConstructible
. If all ofFD
andTiD
satisfy the requirements ofCopyConstructible
, then the return type shall satisfy the requirements ofimplicit CopyConstructible
. [Note: this implies that all ofFD
andTiD
areMoveConstructible
. — end note][..]
9 Remarks: The return type shall satisfy the requirements of
implicit MoveConstructible
. If all ofFD
andTiD
satisfy the requirements ofCopyConstructible
, then the return type shall satisfy the requirements ofimplicit CopyConstructible
. [Note: this implies that all ofFD
andTiD
areMoveConstructible
. — end note]
Change 22.10.15.5 [func.bind.place] as indicated:
1 All placeholder types shall be
DefaultConstructible
andimplicit CopyConstructible
, and [..]
Change 20.3.1 [unique.ptr]/5 as indicated:
5 Each object of a type
U
instantiated form theunique_ptr
template specified in this subclause has the strict ownership semantics, specified above, of a unique pointer. In partial satisfaction of these semantics, each suchU
isimplicit MoveConstructible
andMoveAssignable
, but is notCopyConstructible
norCopyAssignable
. The template parameterT
ofunique_ptr
may be an incomplete type.
Change 20.3.2.2 [util.smartptr.shared]/2 as indicated:
2 Specializations of
shared_ptr
shall beimplicit CopyConstructible
,CopyAssignable
, andLessThanComparable
, [..]
Change 20.3.2.3 [util.smartptr.weak]/2 as indicated:
2 Specializations of
weak_ptr
shall beimplicit CopyConstructible
andCopyAssignable
, allowing their use in standard containers. The template parameterT
ofweak_ptr
may be an incomplete type.
Change 24.3.5.2 [iterator.iterators]/2 as indicated: [This fixes a defect in the Iterator requirements. None of the usual algorithms accepting iterators would be usable with iterators with explicit copy-constructors]
2 A type
X
satisfies the Iterator requirements if:
X
satisfies theimplicit CopyConstructible
,CopyAssignable
, andDestructible
requirements (20.2.1) and lvalues of typeX
are swappable (20.2.2), and [..]- ...
Change 99 [auto.ptr]/3 as indicated:
3 [..] Instances of
auto_ptr
meet the requirements ofimplicit MoveConstructible
andMoveAssignable
, but do not meet the requirements ofCopyConstructible
andCopyAssignable
. — end note]
basic_string::replace
should use const_iterator
Section: 27.4.3.7.6 [string.replace] Status: C++11 Submitter: Daniel Krügler Opened: 2010-02-19 Last modified: 2016-11-12
Priority: Not Prioritized
View all other issues in [string.replace].
View all issues with C++11 status.
Discussion:
In contrast to all library usages of purely positional iterator values several
overloads of std::basic_string::replace
still use iterator instead of
const_iterator
arguments. The paper
N3021
quite nicely visualizes the purely positional responsibilities of the function
arguments.
This should be fixed to make the library consistent, the proposed changes are quite mechanic.
[ Post-Rapperswil: ]
Moved to Tentatively Ready after 5 positive votes on c++std-lib.
[ Adopted at 2010-11 Batavia ]
Proposed resolution:
In 27.4.3 [basic.string], class template basic_string
synopsis
change as indicated:
// 21.4.6 modifiers: ... basic_string& replace(const_iterator i1, const_iterator i2, const basic_string& str); basic_string& replace(const_iterator i1, const_iterator i2, const charT* s, size_type n); basic_string& replace(const_iterator i1, const_iterator i2, const charT* s); basic_string& replace(const_iterator i1, const_iterator i2, size_type n, charT c); template<class InputIterator> basic_string& replace(const_iterator i1, const_iterator i2, InputIterator j1, InputIterator j2); basic_string& replace(const_iterator, const_iterator, initializer_list<charT>);
In 27.4.3.7.6 [string.replace] before p.18, change the following signatures as indicated:
basic_string& replace(const_iterator i1, const_iterator i2, const basic_string& str);
In 27.4.3.7.6 [string.replace] before p.21, change the following signatures as indicated:
basic_string& replace(const_iterator i1, const_iterator i2, const charT* s, size_type n);
In 27.4.3.7.6 [string.replace] before p.24, change the following signatures as indicated:
basic_string& replace(const_iterator i1, const_iterator i2, const charT* s);
In 27.4.3.7.6 [string.replace] before p.27, change the following signatures as indicated:
basic_string& replace(const_iterator i1, const_iterator i2, size_type n, charT c);
In 27.4.3.7.6 [string.replace] before p.30, change the following signatures as indicated:
template<class InputIterator> basic_string& replace(const_iterator i1, const_iterator i2, InputIterator j1, InputIterator j2);
In 27.4.3.7.6 [string.replace] before p.33, change the following signatures as indicated:
basic_string& replace(const_iterator i1, const_iterator i2, initializer_list<charT> il);
pair
and tuple
Section: 22.3.2 [pairs.pair], 22.4.4.2 [tuple.cnstr] Status: Resolved Submitter: Daniel Krügler Opened: 2010-03-20 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
In analogy to library defect 811(i), tuple
's variadic constructor
template <class... UTypes> explicit tuple(UTypes&&... u);
creates the same problem as pair:
#include <tuple> int main() { std::tuple<char*> p(0); }
produces a similar compile error for a recent gcc implementation.
I suggest to follow the same resolution path as has been applied to
pair
's corresponding c'tor, that is require that these c'tors should
not participate in overload resolution, if the arguments are not implicitly
convertible to the element types.
Further-on both pair
and tuple
provide converting constructors
from different pairs
/tuples
that should be not available, if
the corresponding element types are not implicitly convertible. It seems
astonishing that in the following example
struct A { explicit A(int); }; A a = 1; // Error std::tuple<A> ta = std::make_tuple(1); // # OK?
the initialization marked with # could be well-formed.
[ Only constraints on constructors are suggested. Adding similar constraints on assignment operators is considered as QoI, because the assigments wouldn't be well-formed anyway. ]
Following 22.3.2 [pairs.pair]/5 add a new Remarks element:
template<class U, class V> pair(const pair<U, V>& p);5 Effects: Initializes members from the corresponding members of the argument
, performing implicit conversions as needed.Remarks: This constructor shall not participate in overload resolution unless
U
is implicitly convertible tofirst_type
andV
is implicitly convertible tosecond_type
.
Following 22.3.2 [pairs.pair]/6 add a new Remarks element:
template<class U, class V> pair(pair<U, V>&& p);6 Effects: The constructor initializes
first
withstd::move(p.first)
and second withstd::move(p.second)
.Remarks: This constructor shall not participate in overload resolution unless
U
is implicitly convertible tofirst_type
andV
is implicitly convertible tosecond_type
.
Following 22.4.4.2 [tuple.cnstr]/7 add a new Remarks element:
template <class... UTypes> explicit tuple(UTypes&&... u);6 Requires: Each type in
Types
shall satisfy the requirements ofMoveConstructible
(Table 33) from the corresponding type inUTypes
.sizeof...(Types) == sizeof...(UTypes)
.7 Effects: Initializes the elements in the
tuple
with the corresponding value instd::forward<UTypes>(u)
.Remarks: This constructor shall not participate in overload resolution unless each type in
UTypes
is implicitly convertible to its corresponding type inTypes
.
Following 22.4.4.2 [tuple.cnstr]/13 add a new Remarks element:
template <class... UTypes> tuple(const tuple<UTypes...>& u);12 Requires: Each type in
Types
shall be constructible from the corresponding type inUTypes
.sizeof...(Types) == sizeof...(UTypes)
.13 Effects: Constructs each element of
*this
with the corresponding element ofu
.Remarks: This constructor shall not participate in overload resolution unless each type in
UTypes
is implicitly convertible to its corresponding type inTypes
.14 [Note:
enable_if
can be used to make the converting constructor and assignment operator exist only in the cases where the source and target have the same number of elements. — end note]
Following 22.4.4.2 [tuple.cnstr]/16 add a new Remarks element:
template <class... UTypes> tuple(tuple<UTypes...>&& u);15 Requires: Each type in
Types
shall shall satisfy the requirements ofMoveConstructible
(Table 33) from the corresponding type inUTypes
.sizeof...(Types) == sizeof...(UTypes)
.16 Effects: Move-constructs each element of
*this
with the corresponding element ofu
.Remarks: This constructor shall not participate in overload resolution unless each type in
UTypes
is implicitly convertible to its corresponding type inTypes
.[Note:
enable_if
can be used to make the converting constructor and assignment operator exist only in the cases where the source and target have the same number of elements. — end note]
Following 22.4.4.2 [tuple.cnstr]/18 add a new Remarks element:
template <class U1, class U2> tuple(const pair<U1, U2>& u);17 Requires: The first type in
Types
shall be constructible fromU1
and the second type inTypes
shall be constructible fromU2
.sizeof...(Types) == 2
.18 Effects: Constructs the first element with
u.first
and the second element withu.second
.Remarks: This constructor shall not participate in overload resolution unless
U1
is implicitly convertible to the first type inTypes
andU2
is implicitly convertible to the second type inTypes
.
Following 22.4.4.2 [tuple.cnstr]/20 add a new Remarks element:
template <class U1, class U2> tuple(pair<U1, U2>&& u);19 Requires: The first type in
Types
shall shall satisfy the requirements ofMoveConstructible
(Table 33) fromU1
and the second type inTypes
shall be move-constructible fromU2
.sizeof...(Types) == 2
.20 Effects: Constructs the first element with
std::move(u.first)
and the second element withstd::move(u.second)
Remarks: This constructor shall not participate in overload resolution unless
U1
is implicitly convertible to the first type inTypes
andU2
is implicitly convertible to the second type inTypes
.
[ 2010-10-24 Daniel adds: ]
Accepting n3140 would solve this issue.
Proposed resolution:
See n3140.
bitset
Section: 22.9.2.2 [bitset.cons] Status: C++11 Submitter: Christopher Jefferson Opened: 2010-03-07 Last modified: 2016-01-28
Priority: Not Prioritized
View all other issues in [bitset.cons].
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Discussion:
As mentioned on the boost mailing list:
The following code, valid in C++03, is broken in C++0x due to ambiguity
between the "unsigned long long
" and "char*
"
constructors.
#include <bitset> std::bitset<10> b(0);
[ The proposed resolution has been reviewed by Stephan T. Lavavej. ]
[ Post-Rapperswil ]
The proposed resolution has two problems:
it fails to provide support for non-terminated strings, which could be easily added and constitutes an important use-case. For example, the following code would invoke UB with the current P/R:
because it requires the evaluation (under the as-if rule, to be fair, but it doesn't matter) ofchar s[4] = { '0', '1', '0', '1' }; // notice: not null-terminated! bitset<4> b(s, 0, 4);
basic_string<char>(s)
it promotes a consistency between the two bitset
constructors that take a const std::string&
and a
const char*
, respectively, while practice established by
std::basic_string
would recommend a different set of
parameters. In particular, the constructor of
std::basic_string
that takes a const char*
does
not have a pos
parameter
Moved to Tentatively Ready with revised wording provided by Alberto Ganesh Babati after 5 positive votes on c++std-lib.
[ Adopted at 2010-11 Batavia ]
Proposed resolution:
<bitset>
in
22.9.2 [template.bitset]/1, replace the fourth bitset constructor:
explicit bitset(const char *str);template <class charT> explicit bitset( const charT *str, typename basic_string<charT>::size_type n = basic_string<charT>::npos, charT zero = charT('0'), charT one = charT('1'));
explicit bitset(const char *str);template <class charT> explicit bitset(const charT *str, typename basic_string<charT>::size_type n = basic_string<charT>::npos, charT zero = charT('0'), charT one = charT('1'));
Effects: Constructs an object of class
bitset<N>
as if by
bitset(string(str)).
bitset( n == basic_string<charT>::npos ? basic_string<charT>(str) : basic_string<charT>(str, n), 0, n, zero, one)
pair
and tuple
functionsSection: 22.3.2 [pairs.pair], 22.4.4.2 [tuple.cnstr] Status: Resolved Submitter: Daniel Krügler Opened: 2010-03-07 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
There are several constructors and creation functions of std::tuple that impose requirements on it's arguments, that are unnecessary restrictive and don't match the intention for the supported argument types. This is related to the fact that tuple is supposed to accept both object types and lvalue-references and the usual MoveConstructible and CopyConstructible requirements are bad descriptions for non-const references. Some examples:
22.4.4.2 [tuple.cnstr] before p.4 and p.8, resp.:
explicit tuple(const Types&...);4 Requires: Each type in
Types
shall be copy constructible.tuple(const tuple& u) = default;8 Requires: Each type in
Types
shall satisfy the requirements ofCopyConstructible
(Table 34).
A tuple that contains lvalue-references to non-const can never
satisfy the CopyConstructible
requirements. CopyConstructible
requirements refine the MoveConstructible
requirements and
this would require that these lvalue-references could bind
rvalues. But the core language does not allow that. Even, if we
would interpret that requirement as referring to the underlying
non-reference type, this requirement would be wrong as well,
because there is no reason to disallow a type such as
struct NoMoveNoCopy { NoMoveNoCopy(NoMoveNoCopy&&) = delete; NoMoveNoCopy(const NoMoveNoCopy&) = delete; ... }:
for the instantiation of std::tuple<NoMoveNoCopy&>
and
that of it's copy constructor.
A more reasonable requirement for this example would be to require that
"is_constructible<Ti, const Ti&>::value
shall
evaluate to true for all Ti
in Types
". In this case
the special reference-folding and const-merging rules of references
would make this well-formed in all cases. We could also add the further
constraint "if Ti
is an object type, it shall satisfy the
CopyConstructible
requirements", but this additional
requirement seems not really to help here. Ignoring it would only mean
that if a user would provide a curious object type C
that
satisfies the std::is_constructible<C, const C&>
test, but not the "C
is CopyConstructible
" test would
produce a tuple<C>
that does not satisfy the
CopyConstructible
requirements as well.
22.4.4.2 [tuple.cnstr] before p.6 and p.10, resp.:
template <class... UTypes> explicit tuple(UTypes&&... u);6 Requires: Each type in
Types
shall satisfy the requirements ofMoveConstructible
(Table 33) from the corresponding type inUTypes
.sizeof...(Types) == sizeof...(UTypes)
.tuple(tuple&& u);10 Requires: Each
type
inTypes
shall shall satisfy the requirements ofMoveConstructible
(Table 33).
We have a similar problem as in (a): Non-const lvalue-references
are intended template arguments for std::tuple
, but cannot satisfy
the MoveConstructible
requirements. In this case the correct
requirements would be
is_constructible<Ti, Ui>::value
shall evaluate to true for allTi
inTypes
and for allUi
inUTypes
and
is_constructible<Ti, Ti>::value
shall evaluate to true for allTi
inTypes
respectively.
Many std::pair
member functions do not add proper requirements, e.g.
the default c'tor does not require anything. This is corrected within the
suggested resolution. Further-on the P/R has been adapted to the FCD numbering.
[ 2010-03-25 Daniel updated wording: ]
The issue became updated to fix some minor inconsistencies and to ensure a similarly required fix for
std::pair
, which has the same specification problem asstd::tuple
, sincepair
became extended to support reference members as well.
[Original proposed resolution:]
Change 22.3.2 [pairs.pair]/1 as indicated [The changes for the effects elements are not normative changes, they just ensure harmonization with existing wording style]:
constexpr pair();Requires:
first_type
andsecond_type
shall satisfy theDefaultConstructible
requirements.1 Effects: Value-initializes
first
andsecond
.Initializes its members as if implemented:pair() : first(), second() { }
.
Change 22.3.2 [pairs.pair]/2 as indicated:
pair(const T1& x, const T2& y);Requires:
is_constructible<T1, const T1&>::value
istrue
andis_constructible<T2, const T2&>::value
istrue
.2 Effects: The constructor initializes
first
withx
andsecond
withy
.
Change 22.3.2 [pairs.pair]/3 as indicated:
template<class U, class V> pair(U&& x, V&& y);Requires:
is_constructible<first_type, U>::value
istrue
andis_constructible<second_type, V>::value
istrue
.3 Effects: The constructor initializes
first
withstd::forward<U>(x)
andsecond
withstd::forward<V>(y)
.4 Remarks: If
U
is not implicitly convertible tofirst_type
orV
is not implicitly convertible tosecond_type
this constructor shall not participate in overload resolution.
Change 22.3.2 [pairs.pair]/5 as indicated [The change in the effects element should be non-normatively and is in compatible to the change suggestion of 1324(i)]:
template<class U, class V> pair(const pair<U, V>& p);Requires:
is_constructible<first_type, const U&>::value
istrue
andis_constructible<second_type, const V&>::value
istrue
.5 Effects: Initializes members from the corresponding members of the argument
, performing implicit conversions as needed.
Change 22.3.2 [pairs.pair]/6 as indicated:
template<class U, class V> pair(pair<U, V>&& p);Requires:
is_constructible<first_type, U>::value
istrue
andis_constructible<second_type, V>::value
istrue
.6 Effects: The constructor initializes
first
withstd::
andmoveforward<U>(p.first)second
withstd::
.moveforward<V>(p.second)
Change 22.3.2 [pairs.pair]/7+8 as indicated [The deletion in the effects element should be non-normatively]:
template<class... Args1, class... Args2> pair(piecewise_construct_t, tuple<Args1...> first_args, tuple<Args2...> second_args);7 Requires:
is_constructible<first_type, Args1...>::value
istrue
andis_constructible<second_type, Args2...>::value
istrue
.All the types inArgs1
andArgs2
shall beCopyConstructible
(Table 35).T1
shall be constructible fromArgs1
.T2
shall be constructible fromArgs2
.8 Effects: The constructor initializes
first
with arguments of typesArgs1...
obtained by forwarding the elements offirst_args
and initializessecond
with arguments of typesArgs2...
obtained by forwarding the elements ofsecond_args
.(Here, forwarding an elementThis form of construction, whereby constructor arguments forx
of typeU
within atuple
object means callingstd::forward<U>(x)
.)first
andsecond
are each provided in a separatetuple
object, is called piecewise construction.
Change 22.3.2 [pairs.pair] before 12 as indicated:
pair& operator=(pair&& p);Requires:
first_type
andsecond_type
shall satisfy theMoveAssignable
requirements.12 Effects: Assigns to
first
withstd::move(p.first)
and tosecond
withstd::move(p.second)
.13 Returns:
*this
.
Change [pairs.pair] before 14 as indicated: [The heterogeneous usage of MoveAssignable is actually not defined, but the library uses it at several places, so we follow this tradition until a better term has been agreed on. One alternative could be to write "first_type shall be assignable from an rvalue of U [..]"]
template<class U, class V> pair& operator=(pair<U, V>&& p);Requires:
first_type
shall beMoveAssignable
fromU
andsecond_type
shall beMoveAssignable
fromV
.14 Effects: Assigns to
first
withstd::move(p.first)
and tosecond
withstd::move(p.second)
.15 Returns:
*this
.
Change 22.4.4.2 [tuple.cnstr]/4+5 as indicated:
explicit tuple(const Types&...);4 Requires:
is_constructible<Ti, const Ti&>::value == true
for eEach typeTi
inTypes
shall be copy constructible.5 Effects:
Copy iInitializes each element with the value of the corresponding parameter.
Change 22.4.4.2 [tuple.cnstr]/6 as indicated:
template <class... UTypes> explicit tuple(UTypes&&... u);6 Requires:
is_constructible<Ti, Ui>::value == true
for eEach typeTi
inTypes
shall satisfy the requirements ofand for the corresponding typeMoveConstructible
(Table 33) fromUi
inUTypes
.sizeof...(Types) == sizeof...(UTypes)
.7 Effects: Initializes the elements in the
tuple
with the corresponding value instd::forward<UTypes>(u)
.
Change 22.4.4.2 [tuple.cnstr]/8+9 as indicated:
tuple(const tuple& u) = default;8 Requires:
is_constructible<Ti, const Ti&>::value == true
for eEach typeTi
inTypes
shall satisfy the requirements of.CopyConstructible
(Table 34)9 Effects: Initializes
Copy constructseach element of*this
with the corresponding element ofu
.
Change 22.4.4.2 [tuple.cnstr]/10+11 as indicated:
tuple(tuple&& u);10 Requires: Let
i
be in[0, sizeof...(Types))
and letTi
be thei
th type inTypes
. Thenis_constructible<Ti, Ti>::value
shall betrue
for alli
.Each type in.Types
shall shall satisfy the requirements ofMoveConstructible
(Table 34)11 Effects: For each
Ti
inTypes
, initializes thei
thMove-constructs eachelement of*this
withthe corresponding element ofstd::forward<Ti>(get<i>(
u
))
.
Change 22.4.4.2 [tuple.cnstr]/15+16 as indicated:
template <class... UTypes> tuple(tuple<UTypes...>&& u);15 Requires: Let
i
be in[0, sizeof...(Types))
,Ti
be thei
th type inTypes
, andUi
be thei
th type inUTypes
. Thenis_constructible<Ti, Ui>::value
shall betrue
for alli
.Each type in.Types
shall shall satisfy the requirements ofMoveConstructible
(Table 34) from the corresponding type inUTypes
sizeof...(Types) == sizeof...(UTypes)
.16 Effects: For each type
Ti
, initializes thei
thMove-constructs eachelement of*this
withthe corresponding element ofstd::forward<Ui>(get<i>(
u
))
.
Change 22.4.4.2 [tuple.cnstr]/19+20 as indicated:
template <class U1, class U2> tuple(pair<U1, U2>&& u);19 Requires:
is_constructible<T1, U1>::value == true
fort
he first typeT
T1
inTypes
shall shall satisfy the requirements ofandMoveConstructible
(Table 33) fromU1
is_constructible<T2, U2>::value == true
for the second typeT2
inTypes
shall be move-constructible from.U2
sizeof...(Types) == 2
.20 Effects: Initializes
Constructsthe first element withstd::forward<U1>
and the second element withmove(u.first)std::forward<U2>
.move(u.second)
Change 22.4.5 [tuple.creation]/9-16 as indicated:
template <class... TTypes, class... UTypes> tuple<TTypes..., UTypes...> tuple_cat(const tuple<TTypes...>& t, const tuple<UTypes...>& u);9 Requires:
is_constructible<Ti, const Ti&>::value == true
for each typeTi
All the typesinTTypes
shall be.CopyConstructible
(Table 34)is_constructible<Ui, const Ui&>::value == true
for each typeUi
All the typesinUTypes
shall be.CopyConstructible
(Table 34)10 Returns: A
tuple
object constructed by initializingcopy constructingits firstsizeof...(TTypes)
elements from the corresponding elements oft
and initializingcopy constructingits lastsizeof...(UTypes)
elements from the corresponding elements ofu
.template <class... TTypes, class... UTypes> tuple<TTypes..., UTypes...> tuple_cat(tuple<TTypes...>&& t, const tuple<UTypes...>& u);11 Requires: Let
i
be in[0, sizeof...(TTypes))
,Ti
be thei
th type inTypes
,j
be in[0, sizeof...(UTypes))
, andUj
be thej
th type inUTypes
.is_constructible<Ti, Ti>::value
shall betrue
for each typeTi
andis_constructible<Uj, const Uj&>::value
shall betrue
for each typeUj
All the types in.TTypes
shall beMoveConstructible
(Table 34). All the types inUTypes
shall beCopyConstructible
(Table 35)12 Returns: A
tuple
object constructed by initializing thei
th element withstd::forward<Ti>(get<i>(t))
for allTi
inTTypes
and initializing the(j+sizeof...(TTypes))
th element withget<j>(u)
for allUj
inUTypes
.move constructing its first.sizeof...(TTypes)
elements from the corresponding elements oft
and copy constructing its lastsizeof...(UTypes)
elements from the corresponding elements ofu
template <class... TTypes, class... UTypes> tuple<TTypes..., UTypes...> tuple_cat(const tuple<TTypes...>& t, tuple<UTypes...>&& u);13 Requires: Let
i
be in[0, sizeof...(TTypes))
,Ti
be thei
th type inTypes
,j
be in[0, sizeof...(UTypes))
, andUj
be thej
th type inUTypes
.is_constructible<Ti, const Ti&>::value
shall betrue
for each typeTi
andis_constructible<Uj, Uj>::value
shall betrue
for each typeUj
All the types in.TTypes
shall beCopyConstructible
(Table 35). All the types inUTypes
shall beMoveConstructible
(Table 34)14 Returns: A
tuple
object constructed by initializing thei
th element withget<i>(t)
for each typeTi
and initializing the(j+sizeof...(TTypes))
th element withstd::forward<Uj>(get<j>(u))
for each typeUj
copy constructing its first.sizeof...(TTypes)
elements from the corresponding elements oft
and move constructing its lastsizeof...(UTypes)
elements from the corresponding elements ofu
template <class... TTypes, class... UTypes> tuple<TTypes..., UTypes...> tuple_cat(tuple<TTypes...>&& t, tuple<UTypes...>&& u);15 Requires: Let
i
be in[0, sizeof...(TTypes))
,Ti
be thei
th type inTypes
,j
be in[0, sizeof...(UTypes))
, andUj
be thej
th type inUTypes
.is_constructible<Ti, Ti>::value
shall betrue
for each typeTi
andis_constructible<Uj, Uj>::value
shall betrue
for each typeUj
All the types in.TTypes
shall beMoveConstructible
(Table 34). All the types inUTypes
shall beMoveConstructible
(Table 34)16 Returns: A
tuple
object constructed by initializing thei
th element withstd::forward<Ti>(get<i>(t))
for each typeTi
and initializing the(j+sizeof...(TTypes))
th element withstd::forward<Uj>(get<j>(u))
for each typeUj
move constructing its first.sizeof...(TTypes)
elements from the corresponding elements oft
and move constructing its lastsizeof...(UTypes)
elements from the corresponding elements ofu
[ 2010-10-24 Daniel adds: ]
Accepting n3140 would solve this issue.
Proposed resolution:
See n3140.
<cmath>
replacing C macros with the same nameSection: 29.7 [c.math] Status: Resolved Submitter: Michael Wong Opened: 2010-03-07 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
In 29.7 [c.math]p12
The templates defined in <cmath>
replace the C99 macros
with the same names. The templates have the following declarations:
namespace std { template <class T> bool signbit(T x); template <class T> int fpclassify(T x); template <class T> bool isfinite(T x); template <class T> bool isinf(T x); template <class T> bool isnan(T x); template <class T> bool isnormal(T x); template <class T> bool isgreater(T x, T y); template <class T> bool isgreaterequal(T x, T y); template <class T> bool isless(T x, T y); template <class T> bool islessequal(T x, T y); template <class T> bool islessgreater(T x, T y); template <class T> bool isunordered(T x, T y); }
and p13:
13 The templates behave the same as the C99 macros with corresponding names defined in C99 7.12.3, Classification macros, and C99 7.12.14, Comparison macros in the C standard.
The C Std versions look like this:
7.12.14.1/p1:
Synopsis
1
#include <math.h>
int isgreaterequal(real-floating x, real-floating y);
which is not necessarily the same types as is required by C++ since the two parameters may be different. Would it not be better if it were truly aligned with C?
[ 2010 Pittsburgh: Bill to ask WG-14 if heterogeneous support for the two-parameter macros is intended. ]
[ 2010-09-13 Daniel comments: ]
I recommend to resolve this issue as NAD Editorial because the accepted resolution for NB comment US-136 by motion 27 does address this.
[ 2010-09-14 Bill comments: ]
Motion 27 directly addresses LWG 1327 and solves the problem presented there. Moreover, the solution has been aired before WG14 with no dissent. These functions now behave the same for mixed-mode calls in both C and C++
Proposed resolution:
Apply proposed resolution for US-136
failbit
if eofbit
is already setSection: 31.7.5.2.4 [istream.sentry] Status: Resolved Submitter: Paolo Carlini Opened: 2010-02-17 Last modified: 2021-06-06
Priority: Not Prioritized
View all other issues in [istream.sentry].
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Discussion:
Basing on the recent discussion on the library reflector, see c++std-lib-27728 and follow ups, I hereby formally ask for LWG 419 to be re-opened, the rationale being that according to the current specifications, per n3000, it seems actually impossible to seek away from end of file, contrary to the rationale which led 342(i) to its closure as NAD. My request is also supported by Martin Sebor, and I'd like also to add, as tentative proposed resolution for the re-opened issue, the wording suggested by Sebor, thus, change the beginning of [istream::sentry]/2, to:
2 Effects: If
(!noskipws && !is.good())
is, calls
falsetrueis.setstate(failbit)
. Otherwise prepares for formatted or unformatted input. ...
[ 2010-10 Batavia ]
Resolved by adopting n3168.
Previous proposed resolution:
Change [istream::sentry] p.2:2 Effects: If
(!noskipws && !is.good())
is, calls
falsetrueis.setstate(failbit)
. Otherwise prepares for formatted or unformatted input. ...
Proposed resolution:
Addressed by paper n3168.
vector<bool>
Section: 23.2.3 [container.requirements.dataraces] Status: Resolved Submitter: Jeffrey Yaskin Opened: 2010-03-09 Last modified: 2016-01-28
Priority: Not Prioritized
View all other issues in [container.requirements.dataraces].
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Discussion:
The common implementation of vector<bool>
is as an
unsynchronized bitfield. The addition of 23.2.3 [container.requirements.dataraces]/2 would require either a
change in representation or a change in access synchronization, both of
which are undesireable with respect to compatibility and performance.
[
2010 Pittsburgh: Moved to NAD EditorialResolved. Rationale added below.
]
Rationale:
Solved by N3069.
Proposed resolution:
Container data races 23.2.3 [container.requirements.dataraces]
Paragraph 1 is unchanged as follows:
1 For purposes of avoiding data races (17.6.4.8), implementations shall consider the following functions to be
const
:begin
,end
,rbegin
,rend
,front
,back
,data
,find
,lower_bound
,upper_bound
,equal_range
, and, except in associative containers,operator[]
.
Edit paragraph 2 as follows:
2 Notwithstanding (17.6.4.8), implementations are required to avoid data races when the contents of the contained object in different elements in the same sequence, excepting
vector<bool>
, are modified concurrently.
Edit paragraph 3 as follows:
3 [Note: For a
vector<int> x
with a size greater than one,x[1] = 5
and*x.begin() = 10
can be executed concurrently without a data race, butx[0] = 5
and*x.begin() = 10
executed concurrently may result in a data race. As an exception to the general rule, for avector<bool> y
,y[i] = true
may race withy[j] = true
. —end note]
Section: 16.4.4.5 [hash.requirements] Status: C++11 Submitter: Daniel Krügler Opened: 2010-03-26 Last modified: 2025-03-13
Priority: Not Prioritized
View all other issues in [hash.requirements].
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Discussion:
The currently added Hash
requirements demand in Table 40 — Hash
requirements [hash]:
Table 40 — Hash requirements [hash] Expression Return type Requirement h(k)
size_t
Shall not throw exceptions. [..]
While it surely is a generally accepted idea that hash function objects should not throw exceptions, this basic constraint for such a fundamental requirement set does neither match the current library policy nor real world cases:
The new definition goes beyond the original hash requirements as specified by SGI library in regard to the exception requirement:
noexcept
language facility libraries can still take
advantage of no-throw guarantees of hasher functions with stricter guarantees.
Even though the majority of all known move, swap, and hash functions won't throw and in some cases must not throw, it seems like unnecessary over-constraining the definition of a Hash functor not to propagate exceptions in any case and it contradicts the general principle of C++ to impose such a requirement for this kind of fundamental requirement.
[ 2010-11-11 Daniel asks the working group whether they would prefer a replacement for the second bullet of the proposed resolution (a result of discussing this with Alberto) of the form: ]
Add to 22.10.19 [unord.hash]/1 a new bullet:
1 The unordered associative containers defined in Clause 23.5 use specializations of the class template
hash
as the default hash function. For all object typesKey
for which there exists a specializationhash<Key>
, the instantiationhash<Key>
shall:
- satisfy the
Hash
requirements (20.2.4), withKey
as the function call argument type, theDefaultConstructible
requirements (33), theCopyAssignable
requirements (37),- be swappable (20.2.2) for lvalues,
- provide two nested types
result_type
andargument_type
which shall be synonyms forsize_t
andKey
, respectively,- satisfy the requirement that if
k1 == k2
is true,h(k1) == h(k2)
is also true, whereh
is an object of typehash<Key>
andk1
andk2
are objects of typeKey
,.- satisfy the requirement
noexcept(h(k)) == true
, whereh
is an object of typehash<Key>
andk
is an object of typeKey
, unlesshash<Key>
is a user-defined specialization that depends on at least one user-defined type.
[Batavia: Closed as NAD Future, then reopened. See the wiki for Tuesday.]
Proposed resolution:
Change Table 26 — Hash
requirements [tab:hash] as indicated:
Table 26 — Hash
requirements [tab:hash]Expression Return type Requirement h(k)
size_t
Shall not throw exceptions.[…]
Add to 22.10.19 [unord.hash] p. 1 a new bullet:
1 The unordered associative containers defined in Clause 23.5 [unord] use specializations of the class template
hash
as the default hash function. For all object typesKey
for which there exists a specializationhash<Key>
, the instantiationhash<Key>
shall:
- satisfy the
Hash
requirements ([hash.requirements]), withKey
as the function call argument type, theDefaultConstructible
requirements (Table [defaultconstructible]), theCopyAssignable
requirements (Table [copyassignable]),- be swappable ([swappable.requirements]) for lvalues,
- provide two nested types
result_type
andargument_type
which shall be synonyms forsize_t
andKey
, respectively,- satisfy the requirement that if
k1 == k2
is true,h(k1) == h(k2)
is also true, whereh
is an object of typehash<Key>
andk1
andk2
are objects of typeKey
,.- satisfy the requirement that the expression
h(k)
, whereh
is an object of typehash<Key>
andk
is an object of typeKey
, shall not throw an exception, unlesshash<Key>
is a user-defined specialization that depends on at least one user-defined type.
std::function
invocationSection: 22.10.17.3.5 [func.wrap.func.inv] Status: C++11 Submitter: Daniel Krügler Opened: 2010-03-26 Last modified: 2016-01-28
Priority: Not Prioritized
View all other issues in [func.wrap.func.inv].
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Discussion:
The current wording of 22.10.17.3.5 [func.wrap.func.inv]/1:
R operator()(ArgTypes... args) constEffects:
INVOKE(f, t1, t2, ..., tN, R)
(20.8.2), wheref
is the target object (20.8.1) of*this
andt1, t2, ..., tN
are the values inargs...
.
uses an unclear relation between the actual args and the used variables
ti
. It should be made clear, that std::forward
has to be used
to conserve the expression lvalueness.
[ Post-Rapperswil: ]
Moved to Tentatively Ready after 5 positive votes on c++std-lib.
[ Adopted at 2010-11 Batavia ]
Proposed resolution:
Change 22.10.17.3.5 [func.wrap.func.inv]/1+2 as indicated:
R operator()(ArgTypes... args) const1 Effects::
INVOKE(f, std::forward<ArgTypes>(args)...
(20.8.2), wheret1, t2, ..., tN, R)f
is the target object (20.8.1) of*this
and.t1, t2, ..., tN
are the values inargs...
2 Returns: Nothing if
R
isvoid
, otherwise the return value ofINVOKE(f, std::forward<ArgTypes>(args)...
.t1, t2, ..., tN, R)3 Throws:
bad_function_call
if!*this
; otherwise, any exception thrown by the wrapped callable object.
Section: 24.5.2.2.2 [back.insert.iter.ops], 24.5.2.3.2 [front.insert.iter.ops], 24.5.2.4.2 [insert.iter.ops] Status: C++11 Submitter: Daniel Krügler Opened: 2010-03-28 Last modified: 2023-02-07
Priority: Not Prioritized
View all other issues in [back.insert.iter.ops].
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Discussion:
In C++03 this was valid code:
#include <vector> #include <iterator> int main() { typedef std::vector<bool> Cont; Cont c; std::back_insert_iterator<Cont> it = std::back_inserter(c); *it = true; }
In C++0x this code does no longer compile because of an ambiguity error for this
operator=
overload pair:
back_insert_iterator<Container>& operator=(typename Container::const_reference value); back_insert_iterator<Container>& operator=(typename Container::value_type&& value);
This is so, because for proxy-containers like std::vector<bool>
the const_reference
usually is a non-reference type and in this case
it's identical to Container::value_type
, thus forming the ambiguous
overload pair
back_insert_iterator<Container>& operator=(bool value); back_insert_iterator<Container>& operator=(bool&& value);
The same problem exists for std::back_insert_iterator
,
std::front_insert_iterator
, and std::insert_iterator
.
One possible fix would be to require that const_reference
of a proxy
container must not be the same as the value_type
, but this would break
earlier valid code. The alternative would be to change the first signature to
back_insert_iterator<Container>& operator=(const typename Container::const_reference& value);
This would have the effect that this signature always expects an lvalue or rvalue, but it would not create an ambiguity relative to the second form with rvalue-references. [For all non-proxy containers the signature will be the same as before due to reference-collapsing and const folding rules]
[ Post-Rapperswil ]
This problem is not restricted to the unspeakable vector<bool>
, but is already existing for other proxy
containers like gcc's rope
class. The following code does no longer work ([Bug libstdc++/44963]):
#include <iostream> #include <ext/rope> using namespace std; int main() { __gnu_cxx::crope line("test"); auto ii(back_inserter(line)); *ii++ = 'm'; // #1 *ii++ = 'e'; // #2 cout << line << endl; }
Both lines marked with #1 and #2 issue now an error because the library has properly implemented the current wording state (Thanks to Paolo Calini for making me aware of this real-life example).
The following P/R is a revision of the orignal P/R and was initially suggested by Howard Hinnant. Paolo verified that the approach works in gcc.
Moved to Tentatively Ready with revised wording after 6 positive votes on c++std-lib.
[ Adopted at 2010-11 Batavia ]
Proposed resolution:
The wording refers to N3126.
back_insert_iterator
synopsis as indicated:
template <class Container> class back_insert_iterator : public iterator<output_iterator_tag,void,void,void,void> { protected: Container* container; public: [..] back_insert_iterator<Container>& operator=(const typename Container::const_referencevalue_type& value); back_insert_iterator<Container>& operator=(typename Container::value_type&& value); [..] };
back_insert_iterator<Container>& operator=(const typename Container::const_referencevalue_type& value);1 Effects:
container->push_back(value)
;
2 Returns:*this
.
front_insert_iterator
synposis as indicated:
template <class Container> class front_insert_iterator : public iterator<output_iterator_tag,void,void,void,void> { protected: Container* container; public: [..] front_insert_iterator<Container>& operator=(const typename Container::const_referencevalue_type& value); front_insert_iterator<Container>& operator=(typename Container::value_type&& value); [..] };
front_insert_iterator<Container>& operator=(const typename Container::const_referencevalue_type& value);1 Effects:
container->push_front(value)
;
2 Returns:*this
.
template <class Container> class insert_iterator : public iterator<output_iterator_tag,void,void,void,void> { protected: Container* container; typename Container::iterator iter; public: [..] insert_iterator<Container>& operator=(const typename Container::const_referencevalue_type& value); insert_iterator<Container>& operator=(typename Container::value_type&& value); [..] };
insert_iterator<Container>& operator=(const typename Container::const_referencevalue_type& value);1 Effects:
iter = container->insert(iter, value); ++iter;2 Returns:
*this
.
tuple::operator<()
Section: 22.4.9 [tuple.rel] Status: C++11 Submitter: Joe Gottman Opened: 2010-05-15 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
The requirements section for std::tuple
says the following:
Requires: For all
i
, where0 <= i and i < sizeof...(Types)
,get<i>(t) < get<i>(u)
is a valid expression returning a type that is convertible tobool
.sizeof...(TTypes) == sizeof...(UTypes)
.
This is necessary but not sufficient, as the algorithm for comparing
tuple
s also computes get<i>(u) < get<i>(t)
(note the order)
[ Post-Rapperswil ]
Moved to Tentatively Ready with updated wording correcting change-bars after 6 positive votes on c++std-lib.
[ Adopted at 2010-11 Batavia ]
Proposed resolution:
template<class... TTypes, class... UTypes> bool operator<(const tuple<TTypes...>& t, const tuple<UTypes...>& u);Requires: For all
i
, where0 <= i
andi < sizeof...(Types)
,get<i>(t) < get<i>(u)
andget<i>(u) < get<i>(t)
is a valid expression returning a type that isare valid expressions returning types that are convertible tobool
.sizeof...(TTypes) == sizeof...(UTypes)
.
regex_traits::isctype
Section: 28.6.6 [re.traits] Status: C++11 Submitter: Howard Hinnant Opened: 2010-06-21 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
28.6.6 [re.traits]/12 describes regex_traits::isctype
in terms
of ctype::is(c, m)
, where c
is a charT
and m
is a ctype_base::mask
. Unfortunately 28.3.4.2.2.2 [locale.ctype.members]
specifies this function as ctype::is(m, c)
[ Post-Rapperswil: ]
Moved to Tentatively Ready after 5 positive votes on c++std-lib.
[ Adopted at 2010-11 Batavia ]
Proposed resolution:
Change 28.6.6 [re.traits] p.12:
bool isctype(charT c, char_class_type f) const;11 ...
12 Returns: Converts
f
into a valuem
of typestd::ctype_base::mask
in an unspecified manner, and returnstrue
ifuse_facet<ctype<charT> >(getloc()).is(
iscm,mc)true
. Otherwise returnstrue
iff
bitwise or'ed with the result of callinglookup_classname
with an iterator pair that designates the character sequence"w"
is not equal to 0 andc == '_'
, or iff
bitwise or'ed with the result of callinglookup_classname
with an iterator pair that designates the character sequence"blank"
is not equal to 0 andc
is one of an implementation-defined subset of the characters for whichisspace(c, getloc())
returnstrue
, otherwise returnsfalse
.
Section: 26.6.15 [alg.search] Status: C++11 Submitter: Howard Hinnant Opened: 2010-06-25 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
LWG 1205(i) (currently in WP) clarified the return value of several algorithms when dealing with empty ranges. In particular it recommended for 26.6.15 [alg.search]:
template<class ForwardIterator1, class ForwardIterator2> ForwardIterator1 search(ForwardIterator1 first1, ForwardIterator1 last1, ForwardIterator2 first2, ForwardIterator2 last2); template<class ForwardIterator1, class ForwardIterator2, class BinaryPredicate> ForwardIterator1 search(ForwardIterator1 first1, ForwardIterator1 last1, ForwardIterator2 first2, ForwardIterator2 last2, BinaryPredicate pred);1 Effects: ...
2 Returns: ... Returns
last1
if no such iterator is found.3 Remarks: Returns
first1
if[first2,last2)
is empty.
Unfortunately this got translated to an incorrect specification for what gets returned when no such iterator is found (N3092):
template<class ForwardIterator1, class ForwardIterator2> ForwardIterator1 search(ForwardIterator1 first1, ForwardIterator1 last1, ForwardIterator2 first2, ForwardIterator2 last2); template<class ForwardIterator1, class ForwardIterator2, class BinaryPredicate> ForwardIterator1 search(ForwardIterator1 first1, ForwardIterator1 last1, ForwardIterator2 first2, ForwardIterator2 last2, BinaryPredicate pred);1 Effects: ...
2 Returns: ... Returns
first1
if[first2,last2)
is empty or if no such iterator is found.
LWG 1205(i) is correct and N3092 is not equivalent nor correct.
I have not reviewed the other 10 recommendations of 1205(i).
[ Post-Rapperswil ]
It was verified that none of the remaining possibly affected algorithms does have any similar problems and a concrete P/R was added that used a similar style as has been applied to the other cases.
Moved to Tentatively Ready after 5 positive votes on c++std-lib.
[ Adopted at 2010-11 Batavia ]
Proposed resolution:
template<class ForwardIterator1, class ForwardIterator2> ForwardIterator1 search(ForwardIterator1 first1, ForwardIterator1 last1, ForwardIterator2 first2, ForwardIterator2 last2); template<class ForwardIterator1, class ForwardIterator2, class BinaryPredicate> ForwardIterator1 search(ForwardIterator1 first1, ForwardIterator1 last1, ForwardIterator2 first2, ForwardIterator2 last2, BinaryPredicate pred);1 - [...]
2 - Returns: The first iteratori
in the range[first1,last1 - (last2-first2))
such that for any nonnegative integern
less thanlast2 - first2
the following corresponding conditions hold:*(i + n) == *(first2 + n)
,pred(*(i + n), *(first2 + n)) != false
. Returnsfirst1
if[first2,last2)
is emptyor, otherwise returnslast1
if no such iterator is found.
uninitialized_fill_n
should return the end of its rangeSection: 26.11.7 [uninitialized.fill] Status: C++11 Submitter: Jared Hoberock Opened: 2010-07-14 Last modified: 2017-06-15
Priority: Not Prioritized
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Discussion:
N3092's
specification of uninitialized_fill_n
discards useful information and
is inconsistent with other algorithms such as fill_n
which accept an
iterator and a size. As currently specified, unintialized_fill_n
requires an additional linear traversal to find the end of the range.
Instead of returning void
, unintialized_fill_n
should return
one past the last iterator it dereferenced.
[ Post-Rapperswil: ]
Moved to Tentatively Ready after 5 positive votes on c++std-lib.
[ Adopted at 2010-11 Batavia ]
Proposed resolution:
In section 20.2 [memory] change:,
template <class ForwardIterator, class Size, class T>voidForwardIterator uninitialized_fill_n(ForwardIterator first, Size n, const T& x);
In section [uninitialized.fill.n] change,
template <class ForwardIterator, class Size, class T>voidForwardIterator uninitialized_fill_n(ForwardIterator first, Size n, const T& x);1 Effects:
for (; n--; ++first) ::new (static_cast<void*>(&*first)) typename iterator_traits<ForwardIterator>::value_type(x); return first;
forward_list::resize
take the object to be copied by value?Section: 23.3.7.5 [forward.list.modifiers] Status: C++11 Submitter: James McNellis Opened: 2010-07-16 Last modified: 2023-02-07
Priority: Not Prioritized
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Discussion:
In
N3092
[forwardlist.modifiers], the resize()
member function is
declared as:
void resize(size_type sz, value_type c);
The other sequence containers (list
, deque
, and
vector
) take 'c'
by const reference.
Is there a reason for this difference? If not, then resize()
should
be declared as:
void resize(size_type sz, const value_type& c);
The declaration would need to be changed both at its declaration in the class definition at [forwardlist]/3 and where its behavior is specified at [forwardlist.modifiers]/22.
This would make forward_list
consistent with the CD1 issue 679(i).
[ Post-Rapperswil ]
Daniel changed the P/R slightly, because one paragraph number has been changed since the issue
had been submitted. He also added a similar Requires element that exists in all other containers with
a resize
member function. He deliberately did not touch the wrong usage of "default-constructed" because that
will be taken care of by LWG issue 868(i).
Moved to Tentatively Ready with revised wording after 5 positive votes on c++std-lib.
[ Adopted at 2010-11 Batavia ]
Proposed resolution:
forward_list
synopsis, as indicated:
... void resize(size_type sz); void resize(size_type sz, const value_type& c); void clear(); ...
void resize(size_type sz); void resize(size_type sz, const value_type& c);27 Effects: If
28 - Requires:sz < distance(begin(), end())
, erases the lastdistance(begin(), end()) - sz
elements from the list. Otherwise, insertssz - distance(begin(), end())
elements at the end of the list. For the first signature the inserted elements are default constructed, and for the second signature they are copies ofc
.T
shall beDefaultConstructible
for the first form and it shall beCopyConstructible
for the second form.
throw()
with noexcept
Section: 16 [library] Status: Resolved Submitter: BSI Opened: 2010-08-25 Last modified: 2016-01-28
Priority: Not Prioritized
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Duplicate of: 1351
Discussion:
Addresses GB-60, CH-16
Dynamic exception specifications are deprecated; the
library should recognise this by replacing all non-throwing
exception specifications of the form throw()
with the
noexcept
form.
[ Resolution proposed by ballot comment: ]
Replace all non-throwing exception specifications of the form 'throw()' with the 'noexcept' form.
[ 2010-10-31 Daniel comments: ]
The proposed resolution of n3148 would satisfy this request.
[ 2010-Batavia: ]
Resolved by adopting n3148.
Proposed resolution:
See n3148 See n3150 See n3195 See n3155 See n3156
noexcept
move operationsSection: 16 [library] Status: Resolved Submitter: BSI Opened: 2010-08-25 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
Addresses GB-61
All library types should have non-throwing move
constructors and move-assignment operators unless
wrapping a type with a potentially throwing move operation.
When such a type is a class-template, these
operations should have a conditional noexcept
specification.
There are many other places where a noexcept
specification may be considered, but the move operations
are a special case that must be called out, to effectively
support the move_if_noexcept
function template.
[ Resolution proposed by ballot comment: ]
Review every class and class template in the library. If noexcept
move constructor/assignment operators can be implicitly declared, then they
should be implicitly declared, or explicitly defaulted. Otherwise, a move
constructor/move assignment operator with a noexcept
exception
specification should be provided.
[ 2010-10-31 Daniel comments: ]
The proposed resolution of n3157 would satisfy this request.
[2011-03-24 Madrid meeting]
Resolved by papers to be listed later
Proposed resolution:
See n3157
noexcept
where library specification does not permit exceptionsSection: 16 [library] Status: Resolved Submitter: BSI Opened: 2010-08-25 Last modified: 2016-01-28
Priority: Not Prioritized
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Duplicate of: 1352
Discussion:
Addresses GB-62, CH-17
Issues with efficiency and unsatisfactory semantics mean many library functions document they do not throw exceptions with a Throws: Nothing clause, but do not advertise it with an exception specification. The semantic issues are largely resolved with the new 'noexcept' specifications, and the noexcept operator means we will want to detect these guarantees programatically in order to construct programs taking advantage of the guarantee.
[ Resolution proposed by ballot comment: ]
Add a noexcept
exception specification on each
libary API that offers an unconditional Throws:
Nothing guarantee. Where the guarantee is
conditional, add the appropriate
noexcept(constant-expression)
if an appropriate
constant expression exists.
[ 2010-10-31 Daniel comments: ]
The proposed resolution of n3149 would satisfy this request.
[ 2010-Batavia: ]
Resolved by adopting n3149.
Proposed resolution:
This issue is resolved by the adoption of n3195
noexcept
judiciously throughout the librarySection: 16 [library] Status: Resolved Submitter: BSI Opened: 2010-08-25 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
Addresses GB-63, US-80
Since the newly introduced operator noexcept
makes it
easy (easier than previously) to detect whether or not a
function has been declared with the empty exception
specification (including noexcept
) library functions that
cannot throw should be decorated with the empty
exception specification. Failing to do so and leaving it as a
matter of QoI would be detrimental to portability and
efficiency.
[ Resolution proposed by ballot comment ]
Review the whole library, and apply the noexcept
specification where it is appropriate.
[ 2010-10-31 Daniel comments: ]
The proposed resolution of the combination of n3155, n3156, n3157, n3167 would satisfy this request. The paper n3150 is related to this as well.
[ 2010 Batavia: ]
While the LWG expects to see further papers in this area, sufficient action was taken in Batavia to close the issue as Resolved by the listed papers.
Proposed resolution:
See n3155, n3156, n3157, n3167 and remotely n3150
swap
should not throwSection: 16 [library] Status: C++11 Submitter: BSI Opened: 2010-08-25 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
Addresses GB-65
Nothrowing swap
operations are key to many C++ idioms,
notably the common copy/swap idiom to provide the
strong exception safety guarantee.
[ Resolution proposed by ballot comment ]
Where possible, all library types should provide a
swap
operation with an exception specification
guaranteeing no exception shall propagate.
Where noexcept(true)
cannot be guaranteed to
not terminate the program, and the swap
in
questions is a template, an exception specification
with the appropriate conditional expression could
be specified.
[2011-03-13: Daniel comments and drafts wording]
During a survey of the library some main categories for
potential noexcept
swap
function could be isolated:
swap
functions that are specified in terms of already
noexcept
swap
member functions, like that of valarray
.swap
of std::function
, and member and free swap
functions of stream buffers and streams where considered but rejected as good candidates,
because of the danger to potentially impose requirements of existing implementations.
These functions could be reconsidered as candidates in the future.Negative list:
iter_swap
, have not been touched,
because there are no fundamental exceptions constraints on iterator operations in general
(only for specific types, like library container iterators)While evaluating the current state of swap
functions
of library components it was observed that several conditional noexcept
functions have turned into unconditional ones, e.g. in the
header <utility>
synopsis:
template<class T> void swap(T& a, T& b) noexcept;
The suggested resolution shown below also attempts to fix these cases.
[2011-03-22 Daniel redrafts to satisfy new criteria for applying noexcept
.
Parts resolved by N3263-v2 and D3267 are not added here.]
Proposed resolution:
Edit 22.2 [utility] p. 2, header <utility>
synopsis and
22.2.2 [utility.swap] before p. 1, as indicated (The intent is to fix an editorial
omission):
template<class T> void swap(T& a, T& b) noexcept(see below);
Edit the prototype declaration in 22.3.2 [pairs.pair] before p. 34 as indicated (The intent is to fix an editorial omission):
void swap(pair& p) noexcept(see below);
Edit 22.4.1 [tuple.general] p. 2 header <tuple>
synopsis and
22.4.12 [tuple.special] before p. 1 as indicated (The intent is to fix an editorial omission):
template <class... Types> void swap(tuple<Types...>& x, tuple<Types...>& y) noexcept(see below);
Edit 22.4.4 [tuple.tuple], class template tuple
synopsis and
22.4.4.4 [tuple.swap] before p. 1 as indicated (The intent is to fix an editorial omission):
void swap(tuple&) noexcept(see below);
Edit 20.2.2 [memory.syn] p. 1, header <memory>
synopsis as indicated (The
intent is to fix an editorial omission of the proposing paper N3195).
template<class T> void swap(shared_ptr<T>& a, shared_ptr<T>& b) noexcept;
Edit header <valarray>
synopsis, 29.6.1 [valarray.syn] and
29.6.3.4 [valarray.special] before p. 1 as indicated
[Drafting comment: The corresponding member swap is already noexcept]:
template<class T> void swap(valarray<T>&, valarray<T>&) noexcept;
Section: 16 [library] Status: Resolved Submitter: Switzerland Opened: 2010-08-25 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
Addresses CH-18
The general approach on moving is that a library object after moving out is in a "valid but unspecified state". But this is stated at the single object specifications, which is error prone (especially if the move operations are implicit) and unnecessary duplication.
[ Resolution proposed by ballot comment ]
Consider putting a general statement to the same effect into clause 17.
[2010-11-05 Beman provides exact wording. The wording was inspired by Dave Abrahams' message c++std-lib-28958, and refined with help from Alisdair, Daniel, and Howard. ]
[2011-02-25 P/R wording superseded by N3241. ]
Proposed resolution:
Resolved by N3241
Section: 3.16 [defns.deadlock] Status: C++11 Submitter: BSI Opened: 2010-08-25 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
Addresses GB-52
The definition of deadlock in 17.3.7 excludes cases involving a single thread making it incorrect.
[ Resolution proposed by ballot comment ]
The definition should be corrected.
[ 2010 Batavia Concurrency group provides a Proposed Resolution ]
[ Adopted at 2010-11 Batavia ]
Proposed resolution:
Change 3.16 [defns.deadlock] as indicated:
deadlock
twoone or more threads are unable to continue execution because each is blocked waiting for one or more of the others to satisfy some condition.
Section: 99 [defns.move.assign.op] Status: Resolved Submitter: BSI Opened: 2010-08-25 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
Addresses GB-50
This definition of move-assignment operator is redundant and confusing now that the term move-assignment operator is defined by the core language in subclause 12.8p21.
[ 2010-10-24 Daniel adds: ]
Accepting n3142 provides a superior resolution.
[ Resolution proposed by ballot comment ]
Strike subclause 99 [defns.move.assign.op]. Add a cross-reference to (12.8) to 17.3.12.
Proposed resolution:
Resolved by paper n3142.
Section: 99 [defns.move.ctor] Status: Resolved Submitter: BSI Opened: 2010-08-25 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
Addresses GB-51
This definition of move-constructor is redundant and confusing now that the term constructor is defined by the core language in subclause 12.8p3.
[ 2010-10-24 Daniel adds: ]
Accepting n3142 provides a superior resolution.
[ 2010 Batavia: resolved as NAD Editorial by adopting paper n3142. ]
Original proposed resolution preserved for reference:
Strike subclause 17.3.14, [defns.move.ctor]
17.3.14 [defns.move.ctor]
move constructor a constructor which accepts only an rvalue argument of the type being constructed and might modify the argument as a side effect during construction.
Proposed resolution:
Resolved by paper n3142.
Section: 16.3.3.3.3 [bitmask.types] Status: Resolved Submitter: BSI Opened: 2010-08-25 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
Addresses GB-53
The bitmask types defined in 27.5.2 and 28.5 contradict the bitmask type requirements in 17.5.2.1.3, and have missing or incorrectly defined operators.
[ Resolution proposed by ballot comment ]
See Appendix 1 - Additional Details
[ 2010 - Rapperswil ]
The paper n3110 was made available during the meeting to resolve this comment, but withdrawn from formal motions to give others time to review the document. There was no technical objection, and it is expected that this paper will go forward at the next meeting.
Proposed resolution:
See n3110.
<atomic>
to free-standing implementationsSection: 16.4.2.5 [compliance] Status: C++11 Submitter: BSI Opened: 2010-08-25 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
Addresses GB-57
The atomic operations facility is closely tied to clause 1 and the memory model. It is not easily supplied as an after-market extension, and should be trivial to implement of a single-threaded serial machine. The consequence of not having this facility will be poor interoperability with future C++ libraries that memory model concerns seriously, and attempt to treat them in a portable way.
[ Resolved in Rapperswil by a motion to directly apply the words from the ballot comment in N3102. ]
Proposed resolution:
Add <atomic>
to table 15, headers required for a
free-standing implementation.
Section: 16.4.5.9 [res.on.arguments] Status: C++11 Submitter: INCITS Opened: 2010-08-25 Last modified: 2017-03-21
Priority: Not Prioritized
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Discussion:
Addresses US-82
16.4.5.9 [res.on.arguments] p.1. b.3: The second Note can benefit by adopting recent nomenclature.
[ Resolution proposed by the ballot comment: ]
Rephrase the Note in terms of xvalue.
[ Pre-Batavia: ]
Walter Brown provides wording.
[Batavia: Immediate]
[ Adopted at 2010-11 Batavia ]
Proposed resolution:
Amend the note in 17.6.3.9 [res.on.arguments] p1 bullet 3.
[ Note: If a program casts an lvalue to an
rvaluexvalue while passing that lvalue to a library function (e.g. by calling the function with the argumentmove(x)
), the program is effectively asking that function to treat that lvalue as a temporary. The implementation is free to optimize away aliasing checks which might be needed if the argument was anlvalue. — end note]
offsetof
should be marked noexcept
Section: 17.2 [support.types] Status: C++11 Submitter: BSI Opened: 2010-08-25 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
Addresses GB-68
There is no reason for the offsetof macro to invoke potentially throwing operations, so the result of noexcept(offsetof(type,member-designator)) should be true.
[ Resolved in Rapperswil by a motion to directly apply the words from the ballot comment in N3102. ]
Proposed resolution:
Add to the end of 18.2p4:
No operation invoked by the offsetof macro shall throw an exception, and
noexcept(offsetof(type,member-designator))
shall be true.
exception_ptr
is synchronizedSection: 17.9.7 [propagation] Status: Resolved Submitter: Switzerland Opened: 2010-08-25 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
Addresses CH-19
It is not clear how exception_ptr
is synchronized.
[ Resolution proposed by ballot comment ]
Make clear that accessing in different threads
multiple exception_ptr
objects that all refer to the
same exception introduce a race.
[2011-03-08: Lawrence comments and drafts wording]
I think fundamentally, this issue is NAD, but clarification would not hurt.
Proposed resolution
Add a new paragraph to 17.9.7 [propagation] after paragraph 6 as follows:
[Note: Exception objects have no synchronization requirements, and expressions using them may conflict (6.9.2 [intro.multithread]). — end note]
Proposed resolution:
Resolved 2011-03 Madrid meeting by paper N3278
Section: 17.6.4 [alloc.errors] Status: Resolved Submitter: BSI Opened: 2010-08-25 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
Addresses GB-71
The thread safety of std::set_new_handler()
,
std::set_unexpected()
, std::set_terminate()
, is
unspecified making the the functions impossible to use in a thread
safe manner.
[ Resolution proposed by ballot comment: ]
The thread safety guarantees for the functions must be specified and new interfaces should be provided to make it possible to query and install handlers in a thread safe way.
[ 2010-10-31 Daniel comments: ]
The proposed resolution of n3122 partially addresses this request. This issue is related to 1366(i).
[ 2010-Batavia: ]
Resolved by adopting n3189.
Proposed resolution:
Resolved in Batavia by accepting n3189.
Section: 17.6.3.5 [new.delete.dataraces] Status: Resolved Submitter: DIN Opened: 2010-08-25 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
Addresses DE-14
It is unclear how a user replacement function can
simultaneously satisfy the race-free conditions imposed in
this clause and query the new-handler in case of a failed
allocation with the only available, mutating interface
std::set_new_handler
.
[ Resolution proposed by ballot comment: ]
Offer a non-mutating interface to query the current new-handler.
[ 2010-10-24 Daniel adds: ]
Accepting n3122 would solve this issue. This issue is related to 1365(i).
[ 2010-Batavia: ]
Resolved by adopting n3189.
Proposed resolution:
Resolved in Batavia by accepting n3189.
Section: 99 [exception.unexpected] Status: C++11 Submitter: BSI Opened: 2010-08-25 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
Addresses GB-72
Dynamic exception specifications are deprecated, so
clause 18.8.2 that describes library support for this facility
should move to Annex D, with the exception of the
bad_exception
class which is retained to indicate other
failures in the exception dispatch mechanism (e.g. calling
current_exception()
).
[ Resolved in Rapperswil by a motion to directly apply the words from the ballot comment in N3102. ]
Proposed resolution:
With the exception of 18.8.2.1 [bad.exception], move clause 18.8.2 directly to Annex D. [bad.exception] should simply become the new 18.8.2.
std::uncaught_exception()
Section: 99 [depr.uncaught] Status: C++11 Submitter: BSI Opened: 2010-08-25 Last modified: 2017-06-15
Priority: Not Prioritized
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Discussion:
Addresses GB-73
The thread safety std::uncaught_exception()
and the
result of the function when multiple threads throw
exceptions at the same time are unspecified. To make the
function safe to use in the presence of exceptions in
multiple threads the specification needs to be updated.
[ Resolution proposed by ballot comment ]
Update this clause to support safe calls from multiple threads without placing synchronization requirements on the user.
[ 2010 Batavia Concurrency group provides a Proposed Resolution ]
[ Adopted at 2010-11 Batavia ]
Proposed resolution:
Change 99 [uncaught] p. 1 as follows:
Returns: true
after the current thread has initialized initializing
an exception object (15.1) until a handler for the exception (including unexpected()
or terminate()
)
is activated (15.3). [ Note: This includes stack unwinding (15.2). — end note ]
throw_with_nested
should not use perfect forwardingSection: 17.9.8 [except.nested] Status: C++11 Submitter: INCITS Opened: 2010-08-25 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
Addresses US-84
The throw_with_nested
specification passes in its argument as
T&&
(perfect forwarding pattern), but then discusses
requirements on T
without taking into account that T
may be an lvalue-reference type. It is also not clear in the spec that
t
is intended to be perfectly forwarded.
[ Resolution proposed by ballot comment ]
Patch [except.nested] p6-7 to match the intent with regards to
requirements on T
and the use of
std::forward<T>(t)
.
[ 2010-10-24 Daniel adds: ]
Accepting n3144 would solve this issue.
[2010-11-10 Batavia: LWG accepts Howard's updated wording with corrected boo boos reported by Sebastian Gesemann and Pete Becker, which is approved for Immediate adoption this meeting.]
[ Adopted at 2010-11 Batavia ]
Proposed resolution:
Change 18.8.7 nested_exception [except.nested] as indicated:
[[noreturn]] template <class T> void throw_with_nested(T&& t);Let
U
beremove_reference<T>::type
6 Requires:
shall be
TUCopyConstructible
.7 Throws: If
is a non-union class type not derived from
TUnested_exception
, an exception of unspecified type that is publicly derived from bothand
TUnested_exception
and constructed fromstd::forward<T>(t)
, otherwise throwsstd::forward<T>(t)
.
Section: 19.5.3.5 [syserr.errcat.objects] Status: C++11 Submitter: BSI Opened: 2010-08-25 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
Addresses GB-76
The C++0x FCD recommends, in a note (see 19.5.1.1/1), that users
create a single error_category
object for each user defined error
category and specifies error_category
equality comparsions based on
equality of addresses (19.5.1.3). The Draft apparently ignores this
when specifying standard error category objects in section 19.5.1.5,
by allowing the generic_category()
and system_category()
functions to return distinct objects for each invocation.
[ Resolved in Rapperswil by a motion to directly apply the words from the ballot comment in N3102. ]
Proposed resolution:
Append a new sentence to 19.5.3.5 [syserr.errcat.objects]/1, and append the same sentence to 19.5.1.5/3.
All calls of this function return references to the same object.
forward
is not compatible with access-controlSection: 22.2 [utility] Status: Resolved Submitter: INCITS Opened: 2010-08-25 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
Addresses US-90
In n3090, at variance with previous iterations of the idea
discussed in papers and incorporated in WDs,
std::forward
is constrained via std::is_convertible
,
thus is not robust wrt access control. This causes problems in
normal uses as implementation detail of member
functions. For example, the following snippet leads to a
compile time failure, whereas that was not the case for an
implementation along the lines of n2835 (using enable_if
s
instead of concepts for the constraining, of course)
#include <utility> struct Base { Base(Base&&); }; struct Derived : private Base { Derived(Derived&& d) : Base(std::forward<Base>(d)) { } };
In other terms, LWG 1054 can be resolved in a better way, the present status is not acceptable.
[ 2010-10-24 Daniel adds: ]
Accepting n3143 would solve this issue.
Proposed resolution:
Resolved as NAD Editorial by paper n3143.
pair
and tuple
have too many conversionsSection: 22.3 [pairs] Status: Resolved Submitter: DIN Opened: 2010-08-25 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
Addresses DE-15
Several function templates of pair
and tuple
allow for too
many implicit conversions, for example:
#include <tuple> std::tuple<char*> p(0); // Error? struct A { explicit A(int){} }; A a = 1; // Error std::tuple<A> ta = std::make_tuple(1); // OK?
[ Resolution proposed by ballot comment ]
Consider to add wording to constrain these function templates.
[ 2010-10-24 Daniel adds: ]
Accepting n3140 would solve this issue.
Proposed resolution:
See n3140.
pair
copy-assignment not consistent for referencesSection: 22.3.2 [pairs.pair] Status: Resolved Submitter: INCITS Opened: 2010-08-25 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
Addresses US-95
Copy-assignment for pair
is defaulted and does not work
for pairs with reference members. This is inconsistent with
conversion-assignment, which deliberately succeeds even
if one or both elements are reference types, just as for
tuple
. The copy-assignment operator should be
consistent with the conversion-assignment operator and
with tuple
's assignment operators.
[ 2010-10-24 Daniel adds: ]
Accepting n3140 would provide a superior resolution, because
pair
does not depend on the semantic requirements ofCopyAssignable
.
[ 2010-11 Batavia ]
Resolved by adopting n3140.
Proposed resolution:
Add to pair
synopsis:
pair& operator=(const pair& p);
Add before paragraph 9:
pair& operator=(const pair& p);
Requires:
T1
andT2
shall satisfy the requirements ofCopyAssignable
.Effects: Assigns
p.first
tofirst
andp.second
tosecond
. Returns:*this
.
pair
and tuple
of references need to better specify move-semanticsSection: 22.3 [pairs] Status: Resolved Submitter: DIN Opened: 2010-08-25 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
Addresses DE-16
Several pair
and tuple
functions in regard to move
operations are incorrectly specified if the member types
are references, because the result of a std::move
cannot
be assigned to lvalue-references. In this context the usage
of the requirement sets MoveConstructible
and
CopyConstructible
also doesn't make sense, because
non-const lvalue-references cannot satisfy these requirements.
[ Resolution proposed by ballot comment ]
Replace the usage of std::move
by that of
std::forward
and replace MoveConstructible
and
CopyConstructible
requirements by other requirements.
[ 2010-10-24 Daniel adds: ]
Accepting n3140 would solve this issue.
[ 2010-11 Batavia: ]
Resolved by adopting n3140.
Proposed resolution:
See n3140.
pair
's range support by proper range facilitySection: 99 [pair.range] Status: C++11 Submitter: BSI Opened: 2010-08-25 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
Addresses GB-85
While std::pair
may happen to hold a pair of iterators
forming a valid range, this is more likely a coincidence
than a feature guaranteed by the semantics of the pair
template. A distinct range-type should be supplied to
enable the new for-loop syntax rather than overloading an
existing type with a different semantic.
If a replacement facility is required for C++0x, consider n2995.
[ Resolved in Rapperswil by a motion to directly apply the words from the ballot comment in N3102. ]
Proposed resolution:
Strike 20.3.5.4 and the matching declarations in 20.3 header synopsis.
pair
and tuple
constructors should forward
argumentsSection: 22.3 [pairs] Status: Resolved Submitter: INCITS Opened: 2010-08-25 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
Addresses US-96
pair
and tuple
constructors and assignment operators use
std::move
when they should use std::forward
. This
causes lvalue references to be erroneously converted to
rvalue references. Related requirements clauses are also
wrong.
[ Resolution proposed by ballot comment ]
See Appendix 1 - Additional Details
[ 2010-10-24 Daniel adds: ]
Accepting n3140 would solve this issue.
[ 2010-11 Batavia ]
Resolved by adopting n3140.
Proposed resolution:
See n3140.
pair
and tuple
Section: 22.3 [pairs], 22.4 [tuple] Status: Resolved Submitter: INCITS Opened: 2010-08-25 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
Addresses US-97
pair
's class definition in N3092 22.3.2 [pairs.pair]
contains "pair(const pair&) = default;
" and
"pair& operator=(pair&& p);
". The latter is described by
22.3.2 [pairs.pair] p.12-13.
pair(const pair&) = default;
" is a user-declared explicitly defaulted
copy constructor. According to [class.copy]/10, this inhibits
the implicitly-declared move constructor. pair
should be move constructible.
( [class.copy]/7 explains that "pair(pair<U, V>&& p)
"
will never be instantiated to move pair<T1, T2>
to pair<T1, T2>
.)pair& operator=(pair&& p);
" is a user-provided move
assignment operator (according to 9.6.2 [dcl.fct.def.default]/4: "A
special member function is user-provided if it is user-declared and not explicitly defaulted
on its first declaration."). According to [class.copy]/20, this inhibits
the implicitly-declared copy assignment operator. pair
should be copy assignable, and was in C++98/03. (Again,
[class.copy]/7 explains that "operator=(const pair<U, V>& p)
"
will never be instantiated to copy pair<T1, T2>
to pair<T1, T2>
.)pair& operator=(pair&& p);
" is
unconditionally defined, whereas according to [class.copy]/25,
defaulted copy/move assignment operators are defined as
deleted in several situations, such as when non-static data
members of reference type are present.
If "pair(const pair&) = default;
" and "pair& operator=(pair&& p);
"
were removed from pair
's class definition in 22.3.2 [pairs.pair] and from
22.3.2 [pairs.pair]/12-13, pair
would
receive implicitly-declared copy/move constructors and
copy/move assignment operators, and [class.copy]/25 would
apply. The implicitly-declared copy/move constructors
would be trivial when T1
and T2
have trivial copy/move
constructors, according to [class.copy]/13, and similarly for the
assignment operators, according to [class.copy]/27. Notes could
be added as a reminder that these functions would be
implicitly-declared, but such notes would not be necessary
(the Standard Library specification already assumes a
high level of familiarity with the Core Language, and
casual readers will simply assume that pair
is copyable
and movable).
Alternatively, pair
could be given explicitly-defaulted
copy/move constructors and copy/move assignment
operators. This is a matter of style.
tuple
is also affected. tuple
's class definition in 22.4 [tuple] contains:
tuple(const tuple&) = default; tuple(tuple&&); tuple& operator=(const tuple&); tuple& operator=(tuple&&);
They should all be removed or all be explicitly-defaulted,
to be consistent with pair
. Additionally, 22.4.4.2 [tuple.cnstr]/8-9 specifies the
behavior of an explicitly defaulted function, which is currently inconsistent with
pair
.
[ Resolution proposed by ballot comment: ]
Either remove "
pair(const pair&) = default;
" and "pair& operator=(pair&& p);
" frompair
's class definition in 22.3.2 [pairs.pair] and from 22.3.2 [pairs.pair] p.12-13, or give pair explicitly-defaulted copy/move constructors and copy/move assignment operators.
Changetuple
to match.
[ 2010-10-24 Daniel adds: ]
Accepting n3140 would solve this issue: The move/copy constructor will be defaulted, but the corresponding assignment operators need a non-default implementation because they are supposed to work for references as well.
Proposed resolution:
See n3140.
pack_arguments
is poorly namedSection: 22.4.5 [tuple.creation] Status: C++11 Submitter: INCITS Opened: 2010-08-25 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
Addresses US-98
pack_arguments
is poorly named. It does not reflect the
fact that it is a tuple creation function and that it forwards
arguments.
[ Resolved in Rapperswil by a motion to directly apply the words from the ballot comment in N3102. ]
Proposed resolution:
Rename pack_arguments
to forward_as_tuple
throughout the standard.
tuple_cat
should be a single variadic signatureSection: 22.4.5 [tuple.creation] Status: C++11 Submitter: BSI Opened: 2010-08-25 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
Addresses GB-88
The tuple_cat
template consists of four overloads and that
can concatenate only two tuple
s. A single variadic
signature that can concatenate an arbitrary number of
tuple
s would be preferred.
[ Resolution proposed by ballot comment: ]
Adopt a simplified form of the proposal in n2975, restricted to
tuple
s and neither requiring nor outlawing support for othertuple
-like types.
[ 2010 Rapperswil: Alisdair to provide wording. ]
[ 2010-11-06: Daniel comments and proposes some alternative wording: ]
There are some problems in the wording: First, even though the result type tuple<see below>
implies it, the specification of the contained tuple element types is missing. Second, the term "tuple
protocol" is not defined anywhere and I see no reason why this normative wording should not be a non-normative
note. We could at least give a better approximation, maybe "tuple-like protocol" as indicated from header
<utility>
synopsis. Further, it seems to me that the effects need to contain a combination of std::forward
with the call of get
. Finally I suggest to replace the requirements Move/CopyConstructible
by proper usage of is_constructible
, as indicated by n3140.
[ 2010 Batavia ]
Moved to Ready with Daniel's improved wording.
Proposed resolution:
Note: This alternate proposed resolution works only if 1191(i) has been accepted.
<tuple>
synopsis, as indicated:
namespace std { ... // 20.4.2.4, tuple creation functions: const unspecified ignore; template <class... Types> tuple<VTypes...> make_tuple(Types&&...); template <class... Types> tuple<ATypes...> forward_as_tuple(Types&&...); template<class... Types> tuple<Types&...> tie(Types&...);template <class... TTypes, class... UTypes> tuple<TTypes..., UTypes...> tuple_cat(const tuple<TTypes...>&, const tuple<UTypes...>&); template <class... TTypes, class... UTypes> tuple<TTypes..., UTypes...> tuple_cat(tuple<TTypes...>&&, const tuple<UTypes...>&); template <class... TTypes, class... UTypes> tuple<TTypes..., UTypes...> tuple_cat(const tuple<TTypes...>&, tuple<UTypes...>&&); template <class... TTypes, class... UTypes> tuple<TTypes..., UTypes...> tuple_cat(tuple<TTypes...>&&, tuple<UTypes...>&&);template <class... Tuples> tuple<CTypes...> tuple_cat(Tuples&&...); ...
template <class... TTypes, class... UTypes> tuple<TTypes..., UTypes...> tuple_cat(const tuple<TTypes...>& t, const tuple<UTypes...>& u);
8 Requires: All the types inTTypes
shall beCopyConstructible
(Table 35). All the types inUTypes
shall beCopyConstructible
(Table 35).
9 Returns: Atuple
object constructed by copy constructing its firstsizeof...(TTypes)
elements from the corresponding elements oft
and copy constructing its lastsizeof...(UTypes)
elements from the corresponding elements ofu
.template <class... TTypes, class... UTypes> tuple<TTypes..., UTypes...> tuple_cat(tuple<TTypes...>&& t, const tuple<UTypes...>& u);
10 Requires: All the types inTTypes
shall beMoveConstructible
(Table 34). All the types inUTypes
shall beCopyConstructible
(Table 35).
11 Returns: Atuple
object constructed by move constructing its firstsizeof...(TTypes)
elements from the corresponding elements oft
and copy constructing its lastsizeof...(UTypes)
elements from the corresponding elements ofu
.template <class... TTypes, class... UTypes> tuple<TTypes..., UTypes...> tuple_cat(const tuple<TTypes...>& t, tuple<UTypes...>&& u);
12 Requires: All the types inTTypes
shall beCopyConstructible
(Table 35). All the types inUTypes
shall beMoveConstructible
(Table 34).
13 Returns: Atuple
object constructed by copy constructing its firstsizeof...(TTypes)
elements from the corresponding elements oft
and move constructing its lastsizeof...(UTypes)
elements from the corresponding elements ofu
.template <class... TTypes, class... UTypes> tuple<TTypes..., UTypes...> tuple_cat(tuple<TTypes...>&& t, tuple<UTypes...>&& u);
14 Requires: All the types inTTypes
shall beMoveConstructible
(Table 34). All the types inUTypes
shall beMoveConstructible
(Table 34).
15 Returns: Atuple
object constructed by move constructing its firstsizeof...(TTypes)
elements from the corresponding elements oft
and move constructing its lastsizeof...(UTypes)
elements from the corresponding elements ofu
.template <class... Tuples> tuple<CTypes...> tuple_cat(Tuples&&... tpls);8 Let
Ti
be thei
th type inTuples
,Ui
beremove_reference<Ti>::type
, andtpi
be thei
th parameter in the function parameter packtpls
, where all indexing is zero-based in the following paragraphs of this sub-clause [tuple.creation].9 Requires: For all
i
,Ui
shall be the type cvi
tuple<Argsi...>
, where cvi
is the (possibly empty)i
th cv-qualifier-seq, andArgsi
is the parameter pack representing the element types inUi
. LetAik
be theki
th type inArgsi
, then for allAik
the following requirements shall be satisfied: IfTi
is deduced as an lvalue reference type, thenis_constructible<Aik, cvi Aik&>::value == true
, otherwiseis_constructible<Aik, cvi Aik&&>::value == true
.10 Remarks: The types in
CTypes
shall be equal to the ordered sequence of the expanded typesArgs0..., Args1..., Argsn-1...
, wheren
equalssizeof...(Tuples)
. Letei...
be thei
th ordered sequence of tuple elements of the resulttuple
object corresponding to the type sequenceArgsi
.11 Returns: A
tuple
object constructed by initializing theki
th type elementeik
inei...
withget<ki>(std::forward<Ti>(tpi))
for each validki
and each element groupei
in order.12 [Note: An implementation may support additional types in the parameter pack
Tuples
, such aspair
andarray
that support thetuple
-like protocol. — end note]
pack_arguments
overly complexSection: 22.4.5 [tuple.creation] Status: C++11 Submitter: INCITS Opened: 2010-08-25 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
Addresses US-99
pack_arguments
is overly complex.
[ Resolved in Rapperswil by a motion to directly apply the words from the ballot comment in N3102. ]
Proposed resolution:
This issue resulted from a lack of understanding of
how references are forwarded. The definition of
pack_arguments
should be simply:
template <class... Types>
tuple<
ATypes&&>
pack_arguments(Types&&...t);
Types: Let Ti
be each type in Types
....
Effects: ...
Returns:
tuple<
ATypes&&...>(std::forward<Types>(t)...)
The synopsis should also change to reflect this simpler signature.
tuple
should be removedSection: 99 [tuple.range] Status: C++11 Submitter: BSI Opened: 2010-08-25 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
Addresses GB-87
There is no compelling reason to assume a
heterogeneous tuple of two elements holds a pair of
iterators forming a valid range. Unlike std::pair
, there are
no functions in the standard library using this as a return
type with a valid range, so there is even less reason to try
to adapt this type for the new for-loop syntax.
[ Resolved in Rapperswil by a motion to directly apply the words from the ballot comment in N3102. ]
Proposed resolution:
Strike 20.4.2.10 and the matching declarations in the header synopsis in 20.4.
Section: 21.4.3 [ratio.ratio] Status: C++11 Submitter: INCITS Opened: 2010-08-25 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
Addresses US-100
LWG 1281 was discussed in Pittsburgh, and the decision
there was to accept the typedef as proposed and move to
Review. Unfortunately the issue was accidentally applied
to the FCD, and incorrectly. The FCD version of the
typedef refers to ratio<N, D>
, but the typedef is intended
to refer to ratio<num, den>
which in general is not the
same type.
[ Resolved in Rapperswil by a motion to directly apply the words from the ballot comment in N3102. ]
Proposed resolution:
Accept the current proposed wording of LWG 1281(i) which adds:
typedef ratio<num, den> type;
Section: 21.4.4 [ratio.arithmetic] Status: Resolved Submitter: BSI Opened: 2010-08-25 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
Addresses GB-89
The alias representations of the ratio
arithmetic templates
do not allow implementations to avoid overflow, since they
explicitly specify the form of the aliased template
instantiation. For example
ratio_multiply
, ratio<2, LLONG_MAX>
is required to
alias ratio<2*LLONG_MAX, LLONG_MAX*2>
, which
overflows, so is ill-formed. However, this is trivially equal
to ratio<1, 1>
. It also contradicts the opening statement of
21.4.4 [ratio.arithmetic] p. 1 "implementations may use other algorithms to
compute these values".
[ 2010-10-25 Daniel adds: ]
Accepting n3131 would solve this issue.
[Batavia: Resolved by accepting n3210.]
Proposed resolution:
Change the wording in 21.4.4 [ratio.arithmetic] p. 2-5 as follows:
template <class R1, class R2> using ratio_add = see below;2 The type
ratio_add<R1, R2>
shall be a synonym forratio<T1, T2>
ratio<U, V>
such thatratio<U, V>::num
andratio<U, V>::den
are the same as the corresponding members ofratio<T1, T2>
would be in the absence of arithmetic overflow whereT1
has the valueR1::num * R2::den + R2::num * R1::den
andT2
has the valueR1::den * R2::den
. If the required values ofratio<U, V>::num
andratio<U, V>::den
cannot be represented inintmax_t
then the program is ill-formed.
template <class R1, class R2> using ratio_subtract = see below;3 The type
ratio_subtract<R1, R2>
shall be a synonym forratio<T1, T2>
ratio<U, V>
such thatratio<U, V>::num
andratio<U, V>::den
are the same as the corresponding members ofratio<T1, T2>
would be in the absence of arithmetic overflow whereT1
has the valueR1::num * R2::den - R2::num * R1::den
andT2
has the valueR1::den * R2::den
. If the required values ofratio<U, V>::num
andratio<U, V>::den
cannot be represented inintmax_t
then the program is ill-formed.
template <class R1, class R2> using ratio_multiply = see below;4 The type
ratio_multiply<R1, R2>
shall be a synonym forratio<T1, T2>
ratio<U, V>
such thatratio<U, V>::num
andratio<U, V>::den
are the same as the corresponding members ofratio<T1, T2>
would be in the absence of arithmetic overflow whereT1
has the valueR1::num * R2::num
andT2
has the valueR1::den * R2::den
. If the required values ofratio<U, V>::num
andratio<U, V>::den
cannot be represented inintmax_t
then the program is ill-formed.
template <class R1, class R2> using ratio_divide = see below;5 The type
ratio_divide<R1, R2>
shall be a synonym forratio<T1, T2>
ratio<U, V>
such thatratio<U, V>::num
andratio<U, V>::den
are the same as the corresponding members ofratio<T1, T2>
would be in the absence of arithmetic overflow whereT1
has the valueR1::num * R2::den
andT2
has the valueR1::den * R2::num
. If the required values ofratio<U, V>::num
andratio<U, V>::den
cannot be represented inintmax_t
then the program is ill-formed.
Section: 21 [meta] Status: Resolved Submitter: DIN Opened: 2010-08-25 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
Addresses DE-17
Speculative compilation for std::is_constructible
and
std::is_convertible
should be limited, similar to the core
language (see 14.8.2 paragraph 8).
[ 2010-10-24 Daniel adds: ]
Accepting n3142 would solve this issue.
Proposed resolution:
Resolved by paper n3142.
Section: 21 [meta] Status: Resolved Submitter: DIN Opened: 2010-08-25 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
Addresses DE-18
Several type traits require compiler support, e.g.
std::is_constructible
or std::is_convertible
.
Their current specification seems to imply, that the corresponding
test expressions should be well-formed, even in absense of access:
class X { X(int){} }; constexpr bool test = std::is_constructible<X, int>::value;
The specification does not clarify the context of this test and because it already goes beyond normal language rules, it's hard to argue by means of normal language rules what the context and outcome of the test should be.
[ Resolution proposed by ballot comment ]
Specify that std::is_constructible
and
std::is_convertible
will return true
only for
public constructors/conversion functions.
[ 2010-10-24 Daniel adds: ]
Accepting n3142 would solve this issue.
Proposed resolution:
Resolved by paper n3142.
result_of
should support pointer-to-data-memberSection: 21.3.5 [meta.unary] Status: Resolved Submitter: INCITS Opened: 2010-08-25 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
Addresses US-102
Despite Library Issue 520's ("result_of
and pointers to
data members") resolution of CD1, the FCD's result_of
supports neither pointers to member functions nor
pointers to data members. It should.
[ Resolution proposed by ballot comment ]
Ensure result_of
supports pointers to member
functions and pointers to data members.
[ 2010-10-24 Daniel adds: ]
Accepting n3123 would solve this issue.
Proposed resolution:
Resolved by n3123.
noexcept
Section: 21.3.5.4 [meta.unary.prop] Status: Resolved Submitter: BSI Opened: 2010-08-25 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
Addresses GB-92
Trivial functions implicitly declare a noexcept
exception
specification, so the references to has_trivial_*
traits in the
has_nothrow_*
traits are redundant, and should be struck for clarity.
[ Resolution proposed by ballot comment ]
For each of the has_nothrow_something
traits,
remove all references to the matching has_trivial_something
traits.
[ 2010-10-24 Daniel adds: ]
Accepting n3142 would solve this issue.
Proposed resolution:
Resolved by n3142.
is_constructible
reports false positivesSection: 21.3.5.4 [meta.unary.prop] Status: Resolved Submitter: DIN Opened: 2010-08-25 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
Addresses DE-19
The fundamental trait is_constructible
reports false
positives, e.g.
is_constructible<char*, void*>::value
evaluates to true, even though a corresponding variable initialization would be ill-formed.
[ Resolved in Rapperswil by paper N3047. ]
Proposed resolution:
Remove all false positives from the domain of is_constructible
.
Section: 22.10 [function.objects] Status: Resolved Submitter: BSI Opened: 2010-08-25 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
Addresses GB-95
The adaptable function protocol supported by
unary_function/binary_function
has been superceded by
lambda expressions and std::bind
. Despite the name, the
protocol is not very adaptable as it requires intrusive
support in the adaptable types, rather than offering an
external traits-like adaption mechanism. This protocol and
related support functions should be deprecated, and we
should not make onerous requirements for the
specification to support this protocol for callable types
introduced in this standard revision, including those
adopted from TR1. It is expected that high-quality
implementations will provide such support, but we should
not have to write robust standard specifications mixing this
restricted support with more general components such as
function
, bind
and reference_wrapper
.
[ Resolution proposed by ballot comment ]
Move clauses 20.8.3, 20.8.9, 20.8.11 and 20.8.12 to Annex D.
Remove the requirements to conditionally derive fromunary/binary_function
from function
,
reference_wrapper
, and the results of calling mem_fn
and bind
.
[ 2010-10-24 Daniel adds: ]
Accepting n3145 would solve this issue.
Proposed resolution:
Resolved by paper N3198.
function
does not need an explicit
default constructorSection: 22.10.17.3 [func.wrap.func] Status: C++11 Submitter: Japan Opened: 2010-08-25 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
Addresses JP-3
explicit
default contructor is defined in std::function
.
Although it is allowed according to 12.3.1, it seems
unnecessary to qualify the constructor as explicit
.
If it is explicit
, there will be a limitation in initializer_list
.
[ Resolved in Rapperswil by a motion to directly apply the words from the ballot comment in N3102. ]
Proposed resolution:
Remove explicit
.
namespace std { template<class> class function; // undefined template<class R, class... ArgTypes> class function<R(ArgTypes...)> : public unary_function<T1, R> // iff sizeof...(ArgTypes) == 1 and ArgTypes contains T1 : public binary_function<T1, T2, R> // iff sizeof...(ArgTypes) == 2 and ArgTypes contains T1 andT2 { public:typedef R result_type; // 20.8.14.2.1, construct/copy/destroy:explicitfunction();
function
does not need an explicit
default constructorSection: 22.10.17.3.2 [func.wrap.func.con] Status: C++11 Submitter: Japan Opened: 2010-08-25 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
Addresses JP-4
Really does the function
require that default constructor is explicit
?
[ Resolved in Rapperswil by a motion to directly apply the words from the ballot comment in N3102. ]
Proposed resolution:
Remove explicit
.
function(); template <class A> function(allocator_arg_t, const A& a);
unique_ptr<T> == nullptr
Section: 20.2 [memory] Status: C++11 Submitter: BSI Opened: 2010-08-25 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
Addresses GB-99
One reason that the unique_ptr
constructor taking a
nullptr_t
argument is not explicit
is to allow conversion
of nullptr
to unique_ptr
in contexts like equality
comparison. Unfortunately operator==
for unique_ptr
is a
little more clever than that, deducing template parameters for both
arguments. This means that nullptr
does not get deduced
as unique_ptr
type, and there are no other comparison
functions to match.
[ Resolution proposed by ballot comment: ]
Add the following signatures to 20.2 [memory] p.1,
<memory>
header synopsis:template<typename T, typename D> bool operator==(const unique_ptr<T, D> & lhs, nullptr_t); template<typename T, typename D> bool operator==(nullptr_t, const unique_ptr<T, D> & rhs); template<typename T, typename D> bool operator!=(const unique_ptr<T, D> & lhs, nullptr_t); template<typename T, typename D> bool operator!=(nullptr_t, const unique_ptr<T, D> & rhs);
[ 2010-11-02 Daniel comments and provides a proposed resolution: ]
The same problem applies to
The number of overloads triple the current number, but I think it is much clearer to provide them explicitly instead of adding wording that attempts to say that "sufficient overloads" are provided. The following proposal makes the declarations explicit. Additionally, the proposal adds the missing declarations for someshared_ptr
as well: In both cases there are no conversions considered because the comparison functions are templates. I agree with the direction of the proposed resolution, but I believe it would be very surprising and inconsistent, if given a smart pointer objectp
, the expressionp == nullptr
would be provided, but notp < nullptr
and the other relational operators. According to 7.6.9 [expr.rel] they are defined if null pointer values meet other pointer values, even though the result is unspecified for all except some trivial ones. But null pointer values are nothing special here: The Library already defines the relational operators for bothunique_ptr
andshared_ptr
and the outcome of comparing non-null pointer values will be equally unspecified. If the idea of supportingnullptr_t
arguments for relational operators is not what the committee prefers, I suggest at least to consider to remove the existing relational operators for bothunique_ptr
andshared_ptr
for consistency. But that would not be my preferred resolution of this issue.shared_ptr
comparison functions for consistency.
[ 2010-11-03 Daniel adds: ]
Issue 1297(i) is remotely related. The following proposed resolution splits this bullet into sub-bullets A and B. Sub-bullet A would also solve 1297(i), but sub-bullet B would not.
A further remark in regard to the proposed semantics of the ordering ofnullptr
against other pointer(-like) values: One might think that the following definition might
be superior because of simplicity:
template<class T> bool operator<(const shared_ptr<T>& a, nullptr_t); template<class T> bool operator>(nullptr_t, const shared_ptr<T>& a);Returns:
false
.
The underlying idea behind this approach is the assumption that nullptr corresponds to the least ordinal pointer value. But this assertion does not hold for all supported architectures, therefore this approach was not followed because it would lead to the inconsistency, that the following assertion could fire:
shared_ptr<int> p(new int); shared_ptr<int> null; bool v1 = p < nullptr; bool v2 = p < null; assert(v1 == v2);
[2011-03-06: Daniel comments]
The current issue state is not acceptable, because the Batavia meeting did not give advice whether choice A or B of bullet 3 should be applied. Option B will now be removed and if this resolution is accepted, issue 1297(i) should be declared as resolved by 1401(i). This update also resyncs the wording with N3242.
Proposed resolution:
Wording changes are against N3242.
<memory>
synopsis as indicated.
noexcept
specifications are only added, where the guarantee exists, that the function
shall no throw an exception (as replacement of "Throws: Nothing". Note that
the noexcept
additions to the shared_ptr
comparisons are editorial, because
they are already part of the accepted paper n3195:
namespace std { […] // [unique.ptr] Class unique_ptr: template <class T> class default_delete; template <class T> class default_delete<T[]>; template <class T, class D = default_delete<T>> class unique_ptr; template <class T, class D> class unique_ptr<T[], D>; template <class T1, class D1, class T2, class D2> bool operator==(const unique_ptr<T1, D1>& x, const unique_ptr<T2, D2>& y); template <class T1, class D1, class T2, class D2> bool operator!=(const unique_ptr<T1, D1>& x, const unique_ptr<T2, D2>& y); template <class T1, class D1, class T2, class D2> bool operator<(const unique_ptr<T1, D1>& x, const unique_ptr<T2, D2>& y); template <class T1, class D1, class T2, class D2> bool operator<=(const unique_ptr<T1, D1>& x, const unique_ptr<T2, D2>& y); template <class T1, class D1, class T2, class D2> bool operator>(const unique_ptr<T1, D1>& x, const unique_ptr<T2, D2>& y); template <class T1, class D1, class T2, class D2> bool operator>=(const unique_ptr<T1, D1>& x, const unique_ptr<T2, D2>& y); template <class T, class D> bool operator==(const unique_ptr<T, D>& x, nullptr_t) noexcept; template <class T, class D> bool operator==(nullptr_t, const unique_ptr<T, D>& x) noexcept; template <class T, class D> bool operator!=(const unique_ptr<T, D>& x, nullptr_t) noexcept; template <class T, class D> bool operator!=(nullptr_t, const unique_ptr<T, D>& x) noexcept; template <class T, class D> bool operator<(const unique_ptr<T, D>& x, nullptr_t); template <class T, class D> bool operator<(nullptr_t, const unique_ptr<T, D>& x); template <class T, class D> bool operator<=(const unique_ptr<T, D>& x, nullptr_t); template <class T, class D> bool operator<=(nullptr_t, const unique_ptr<T, D>& x); template <class T, class D> bool operator>(const unique_ptr<T, D>& x, nullptr_t); template <class T, class D> bool operator>(nullptr_t, const unique_ptr<T, D>& x); template <class T, class D> bool operator>=(const unique_ptr<T, D>& x, nullptr_t); template <class T, class D> bool operator>=(nullptr_t, const unique_ptr<T, D>& x); // [util.smartptr.weakptr], Class bad_weak_ptr: class bad_weak_ptr; // [util.smartptr.shared], Class template shared_ptr: template<class T> class shared_ptr; // [util.smartptr.shared.cmp], shared_ptr comparisons: template<class T, class U> bool operator==(shared_ptr<T> const& a, shared_ptr<U> const& b) noexcept; template<class T, class U> bool operator!=(shared_ptr<T> const& a, shared_ptr<U> const& b) noexcept; template<class T, class U> bool operator<(shared_ptr<T> const& a, shared_ptr<U> const& b) noexcept; template<class T, class U> bool operator>(shared_ptr<T> const& a, shared_ptr<U> const& b) noexcept; template<class T, class U> bool operator<=(shared_ptr<T> const& a, shared_ptr<U> const& b) noexcept; template<class T, class U> bool operator>=(shared_ptr<T> const& a, shared_ptr<U> const& b) noexcept; template<class T> bool operator==(shared_ptr<T> const& a, nullptr_t) noexcept; template<class T> bool operator==(nullptr_t, shared_ptr<T> const& a) noexcept; template<class T> bool operator!=(shared_ptr<T> const& a, nullptr_t) noexcept; template<class T> bool operator!=(nullptr_t, shared_ptr<T> const& a) noexcept; template<class T> bool operator<(shared_ptr<T> const& a, nullptr_t) noexcept; template<class T> bool operator<(nullptr_t, shared_ptr<T> const& a) noexcept; template>class T> bool operator>(shared_ptr<T> const& a, nullptr_t) noexcept; template>class T> bool operator>(nullptr_t, shared_ptr<T> const& a) noexcept; template<class T> bool operator<=(shared_ptr<T> const& a, nullptr_t) noexcept; template<class T> bool operator<=(nullptr_t, shared_ptr<T> const& a) noexcept; template>class T> bool operator>=(shared_ptr<T> const& a, nullptr_t) noexcept; template>class T> bool operator>=(nullptr_t, shared_ptr<T> const& a) noexcept; […] }
namespace std { […] template <class T1, class D1, class T2, class D2> bool operator==(const unique_ptr<T1, D1>& x, const unique_ptr<T2, D2>& y); template <class T1, class D1, class T2, class D2> bool operator!=(const unique_ptr<T1, D1>& x, const unique_ptr<T2, D2>& y); template <class T1, class D1, class T2, class D2> bool operator<(const unique_ptr<T1, D1>& x, const unique_ptr<T2, D2>& y); template <class T1, class D1, class T2, class D2> bool operator<=(const unique_ptr<T1, D1>& x, const unique_ptr<T2, D2>& y); template <class T1, class D1, class T2, class D2> bool operator>(const unique_ptr<T1, D1>& x, const unique_ptr<T2, D2>& y); template <class T1, class D1, class T2, class D2> bool operator>=(const unique_ptr<T1, D1>& x, const unique_ptr<T2, D2>& y); template <class T, class D> bool operator==(const unique_ptr<T, D>& x, nullptr_t) noexcept; template <class T, class D> bool operator==(nullptr_t, const unique_ptr<T, D>& x) noexcept; template <class T, class D> bool operator!=(const unique_ptr<T, D>& x, nullptr_t) noexcept; template <class T, class D> bool operator!=(nullptr_t, const unique_ptr<T, D>& x) noexcept; template <class T, class D> bool operator<(const unique_ptr<T, D>& x, nullptr_t); template <class T, class D> bool operator<(nullptr_t, const unique_ptr<T, D>& x); template <class T, class D> bool operator<=(const unique_ptr<T, D>& x, nullptr_t); template <class T, class D> bool operator<=(nullptr_t, const unique_ptr<T, D>& x); template <class T, class D> bool operator>(const unique_ptr<T, D>& x, nullptr_t); template <class T, class D> bool operator>(nullptr_t, const unique_ptr<T, D>& x); template <class T, class D> bool operator>=(const unique_ptr<T, D>& x, nullptr_t); template <class T, class D> bool operator>=(nullptr_t, const unique_ptr<T, D>& x); }
Change 20.3.1.6 [unique.ptr.special] p. 4-7 as indicated and add a series of prototype descriptions:
template <class T1, class D1, class T2, class D2> bool operator<(const unique_ptr<T1, D1>& x, const unique_ptr<T2, D2>& y);? Requires: Let
CT
becommon_type<unique_ptr<T1, D1>::pointer, unique_ptr<T2, D2>::pointer>::type
. Then the specializationless<CT>
shall be a function object type ([function.objects]) that induces a strict weak ordering ([alg.sorting]) on the pointer values.4 Returns:
less<CT>()(x.get(), y.get())
.x.get() < y.get()? Remarks: If
unique_ptr<T1, D1>::pointer
is not implicitly convertible toCT
orunique_ptr<T2, D2>::pointer
is not implicitly convertible toCT
, the program is ill-formed.template <class T1, class D1, class T2, class D2> bool operator<=(const unique_ptr<T1, D1>& x, const unique_ptr<T2, D2>& y);5 Returns:
!(y < x)
.x.get() <= y.get()template <class T1, class D1, class T2, class D2> bool operator>(const unique_ptr<T1, D1>& x, const unique_ptr<T2, D2>& y);6 Returns:
(y < x)
.x.get() > y.get()template <class T1, class D1, class T2, class D2> bool operator>=(const unique_ptr<T1, D1>& x, const unique_ptr<T2, D2>& y);7 Returns:
!(x < y)
.x.get() >= y.get()
template <class T, class D> bool operator==(const unique_ptr<T, D>& x, nullptr_t) noexcept; template <class T, class D> bool operator==(nullptr_t, const unique_ptr<T, D>& x) noexcept;? Returns:
!x
.
template <class T, class D> bool operator!=(const unique_ptr<T, D>& x, nullptr_t) noexcept; template <class T, class D> bool operator!=(nullptr_t, const unique_ptr<T, D>& x) noexcept;? Returns:
(bool) x
.
template <class T, class D> bool operator<(const unique_ptr<T, D>& x, nullptr_t); template <class T, class D> bool operator>(nullptr_t, const unique_ptr<T, D>& x);? Requires: The specialization
less<unique_ptr<T, D>::pointer>
shall be a function object type ([function.objects]) that induces a strict weak ordering ([alg.sorting]) on the pointer values.? Returns:
less<unique_ptr<T, D>::pointer>()(x.get(), nullptr)
.
template <class T, class D> bool operator<(nullptr_t, const unique_ptr<T, D>& x); template <class T, class D> bool operator>(const unique_ptr<T, D>& x, nullptr_t);? Requires: The specialization
less<unique_ptr<T, D>::pointer>
shall be a function object type ([function.objects]) that induces a strict weak ordering ([alg.sorting]) on the pointer values.? Returns:
less<unique_ptr<T, D>::pointer>()(nullptr, x.get())
.
template <class T, class D> bool operator<=(const unique_ptr<T, D>& x, nullptr_t); template <class T, class D> bool operator>=(nullptr_t, const unique_ptr<T, D>& x);? Returns:
!(nullptr < x)
.
template <class T, class D> bool operator<=(nullptr_t, const unique_ptr<T, D>& x); template <class T, class D> bool operator>=(const unique_ptr<T, D>& x, nullptr_t);? Returns:
!(x < nullptr)
.
Change 20.3.2.2 [util.smartptr.shared] p. 1, class template shared_ptr
synopsis as indicated. For consistency reasons the remaining normal relation
operators are added as well:
namespace std { […] // [util.smartptr.shared.cmp], shared_ptr comparisons: template<class T, class U> bool operator==(const shared_ptr<T>& a, const shared_ptr<U>& b) noexcept; template<class T, class U> bool operator!=(const shared_ptr<T>& a, const shared_ptr<U>& b) noexcept; template<class T, class U> bool operator<(const shared_ptr<T>& a, const shared_ptr<U>& b) noexcept; template<class T, class U> bool operator>(const shared_ptr<T>& a, const shared_ptr<U>& b) noexcept; template<class T, class U> bool operator<=(const shared_ptr<T>& a, const shared_ptr<U>& b) noexcept; template<class T, class U> bool operator>=(const shared_ptr<T>& a, const shared_ptr<U>& b) noexcept; template<class T> bool operator==(const shared_ptr<T>& a, nullptr_t) noexcept; template<class T> bool operator==(nullptr_t, const shared_ptr<T>& a) noexcept; template<class T> bool operator!=(const shared_ptr<T>& a, nullptr_t) noexcept; template<class T> bool operator!=(nullptr_t, const shared_ptr<T>& a) noexcept; template<class T> bool operator<(const shared_ptr<T>& a, nullptr_t) noexcept; template<class T> bool operator<(nullptr_t, const shared_ptr<T>& a) noexcept; template>class T> bool operator>(const shared_ptr<T>& a, nullptr_t) noexcept; template>class T> bool operator>(nullptr_t, const shared_ptr<T>& a) noexcept; template<class T> bool operator<=(const shared_ptr<T>& a, nullptr_t) noexcept; template<class T> bool operator<=(nullptr_t, const shared_ptr<T>& a) noexcept; template>class T> bool operator>=(const shared_ptr<T>& a, nullptr_t) noexcept; template>class T> bool operator>=(nullptr_t, const shared_ptr<T>& a) noexcept; […] }
template<class T> bool operator==(const shared_ptr<T>& a, nullptr_t) noexcept; template<class T> bool operator==(nullptr_t, const shared_ptr<T>& a) noexcept;? Returns:
!a
.
template<class T> bool operator!=(const shared_ptr<T>& a, nullptr_t) noexcept; template<class T> bool operator!=(nullptr_t, const shared_ptr<T>& a) noexcept;? Returns:
(bool) a
.
template<class T> bool operator<(const shared_ptr<T>& a, nullptr_t) noexcept; template<class T> bool operator>(nullptr_t, const shared_ptr<T>& a) noexcept;? Returns:
less<T*>()(a.get(), nullptr)
.
template<class T> bool operator<(nullptr_t, const shared_ptr<T>& a) noexcept; template<class T> bool operator>(const shared_ptr<T>& a, nullptr_t) noexcept;? Returns:
less<T*>()(nullptr, a.get())
.
template<class T> bool operator<=(const shared_ptr<T>& a, nullptr_t) noexcept; template<class T> bool operator>=(nullptr_t, const shared_ptr<T>& a) noexcept;? Returns:
!(nullptr < a)
.
template<class T> bool operator<=(nullptr_t, const shared_ptr<T>& a) noexcept; template<class T> bool operator>=(const shared_ptr<T>& a, nullptr_t) noexcept;? Returns:
!(a < nullptr)
.
nullptr
constructors for smart pointers should be constexpr
Section: 20.2 [memory] Status: C++11 Submitter: BSI Opened: 2010-08-25 Last modified: 2016-01-28
Priority: Not Prioritized
View all other issues in [memory].
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Discussion:
Addresses GB-100
The unique_ptr
and shared_ptr
constructors taking
nullptr_t
delegate to a constexpr
constructor, and could be
constexpr
themselves.
[ Resolved in Rapperswil by a motion to directly apply the words from the ballot comment in N3102. ]
Proposed resolution:
In the 20.3.1.3 [unique.ptr.single] synopsis add
"constexpr" to unique_ptr(nullptr_t)
.
In the 20.3.1.4 [unique.ptr.runtime] synopsis add
"constexpr" to unique_ptr(nullptr_t)
.
In the 20.3.2.2 [util.smartptr.shared] synopsis
add "constexpr" to shared_ptr(nullptr_t)
.
allocator_arg
Section: 20.2.7 [allocator.tag] Status: C++11 Submitter: Japan Opened: 2010-08-25 Last modified: 2016-01-28
Priority: Not Prioritized
View all issues with C++11 status.
Discussion:
Addresses JP-85
There are inconsistent definitions for allocator_arg
.
In 20.2 [memory] paragraph 1,
constexpr allocator_arg_t allocator_arg = allocator_arg_t();
and in 20.9.1,
const allocator_arg_t allocator_arg = allocator_arg_t();
[ Resolved in Rapperswil by a motion to directly apply the words from the ballot comment in N3102. ]
Proposed resolution:
Change "const" to "constexpr" in 20.9.1 as follows.
constexpr allocator_arg_t allocator_arg = allocator_arg_t();
pointer_traits
should have a size_type
memberSection: 20.2.3 [pointer.traits] Status: C++11 Submitter: INCITS Opened: 2010-08-25 Last modified: 2016-01-28
Priority: Not Prioritized
View all other issues in [pointer.traits].
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Discussion:
Addresses US-106
pointer_traits
should have a size_type
member for completeness.
Add typedef see below size_type;
to the generic
pointer_traits
template and typedef size_t
size_type;
to pointer_traits<T*>
. Use
pointer_traits::size_type
and
pointer_traits::difference_type
as the defaults for
allocator_traits::size_type
and
allocator_traits::difference_type
.
See Appendix 1 - Additional Details
[ Post-Rapperswil, Pablo provided wording: ]
The original ballot comment reads simply: "pointer_traits
should have a
size_type
for completeness." The additional details reveal, however,
that the desire for a size_type
is actually driven by the needs
of allocator_traits
. The allocator_traits
template should get its
default difference_type
from pointer_traits
but if it did,
it should get its size_type
from the same source. Unfortunately,
there is no obvious meaning for size_type
in pointer_traits
.
Alisdair suggested, however, that the natural relationship between
difference_type
and size_type
can be expressed simply by the
std::make_unsigned<T>
metafunction. Using this metafunction,
we can easily define size_type
for allocator_traits
without
artificially adding size_type
to pointer_traits
.
Moved to Tentatively Ready after 6 positive votes on c++std-lib.
[ Adopted at 2010-11 Batavia ]
Proposed resolution:
In [allocator.requirements], Table 42, change two rows as follows:
X::size_type
unsigned integral type a type that can represent the size of the largest object in the allocation model
size_tmake_unsigned<X::difference_type>::typeX::difference_type
signed integral type a type that can represent the difference between any two pointers in the allocation model
ptrdiff_tpointer_traits<X::pointer>::difference_type
In [allocator.traits.types], Change the definition of difference_type
and
size_type
as follows:
typedef
see belowdifference_type;
Type:
Alloc::difference_type
if such a type exists, else.
ptrdiff_tpointer_traits<pointer>::difference_type
typedef
see belowsize_type;
Type:
Alloc::size_type
if such a type exists, else.
size_tmake_unsigned<difference_type>::type
scoped_allocator_adaptor
into separate headerSection: 20.6 [allocator.adaptor] Status: Resolved Submitter: INCITS Opened: 2010-08-25 Last modified: 2016-01-28
Priority: Not Prioritized
View all other issues in [allocator.adaptor].
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Discussion:
Addresses US-107
scoped_allocator_adaptor
should have its own header.
[ Resolved in Rapperswil by a motion to directly apply the words from the ballot comment in N3102. ]
Proposed resolution:
See Appendix 1 - Additional Details
shared_ptr
constructors taking movable typesSection: 20.3.2.2.2 [util.smartptr.shared.const] Status: Resolved Submitter: INCITS Opened: 2010-08-25 Last modified: 2016-01-28
Priority: Not Prioritized
View other active issues in [util.smartptr.shared.const].
View all other issues in [util.smartptr.shared.const].
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Discussion:
Addresses US-108
shared_ptr
should have the same policy for constructing
from auto_ptr
as unique_ptr
. Currently it does not.
[ Resolved in Rapperswil by paper N3109. ]
Proposed resolution:
Add
template <class Y> explicit shared_ptr(auto_ptr<Y>&);
to [util.smartptr.shared.const] (and to the synopsis).
undeclare_no_pointers
Section: 99 [util.dynamic.safety] Status: C++11 Submitter: BSI Opened: 2010-08-25 Last modified: 2016-01-28
Priority: Not Prioritized
View all other issues in [util.dynamic.safety].
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Discussion:
Addresses GB-103
The precondition to calling declare_no_pointers
is that no
bytes in the range "have been previously registered" with
this call. As written, this precondition includes bytes in
ranges, even after they have been explicitly unregistered
with a later call to undeclare_no_pointers
.
Proposed resolution:
Update 99 [util.dynamic.safety] p.9:
void declare_no_pointers(char *p, size_t n);
9
Requires: No bytes in the specified rangehave been previously registeredare currently registered withdeclare_no_pointers()
. If the specified range is in an allocated object, then it must be entirely within a single allocated object. The object must be live until the correspondingundeclare_no_pointers()
call. [..]
monotonic_clock
is a distinct type or a typedefSection: 99 [time.clock.monotonic] Status: Resolved Submitter: INCITS Opened: 2010-08-25 Last modified: 2016-01-28
Priority: Not Prioritized
View all other issues in [time.clock.monotonic].
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Discussion:
Addresses US-111
What it means for monotonic_clock
to be a synonym is
undefined. If it may or may not be a typedef, then certain
classes of programs become unportable.
[ Resolution proposed in ballot comment: ]
Require that it be a distinct class type.
[ 2010-11-01 Daniel comments: ]
Paper n3128 addresses this issue by replacing
monotonic_clock
withsteady_clock
, which is not a typedef.
Proposed resolution:
This is resolved by n3191.
monotonic_clock
Section: 99 [time.clock.monotonic] Status: Resolved Submitter: BSI Opened: 2010-08-25 Last modified: 2016-01-28
Priority: Not Prioritized
View all other issues in [time.clock.monotonic].
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Duplicate of: 1411
Discussion:
Addresses GB-107, DE-20
4.1 [intro.compliance] p.9 states that which conditionally
supported constructs are available should be provided in the
documentation for the implementation. This doesn't help programmers trying
to write portable code, as they must then rely on
implementation-specific means to determine the
availability of such constructs. In particular, the presence
or absence of std::chrono::monotonic_clock
may require
different code paths to be selected. This is the only
conditionally-supported library facility, and differs from the
conditionally-supported language facilities in that it has
standard-defined semantics rather than implementation-defined
semantics.
[ Resolution proposed in ballot comment: ]
Provide feature test macro for determining the
presence of std::chrono::monotonic_clock
. Add
_STDCPP_HAS_MONOTONIC_CLOCK
to the
<chrono>
header, which is defined if
monotonic_clock
is present, and not defined if it is
not present.
[ 2010-11-01 Daniel comments: ]
Paper n3128 addresses this issue by replacing
monotonic_clock
withsteady_clock
, which is not conditionally supported, so there is no need to detect it.
Proposed resolution:
This is resolved by n3191.
Section: 99 [time.clock.monotonic] Status: Resolved Submitter: Switzerland Opened: 2010-08-25 Last modified: 2016-01-28
Priority: Not Prioritized
View all other issues in [time.clock.monotonic].
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Discussion:
Addresses CH-21
Monotonic clocks are generally easy to provide on all systems and are implicitely required by some of the library facilities anyway.
[ 2010-11-01 Daniel comments: ]
Paper n3128 addresses this issue by replacing
monotonic_clock
withsteady_clock
, which is mandatory.
[ 2010-11-13 Batavia meeting: ]
This is resolved by adopting n3191. The original resolution is preserved for reference:
Make monotonic clocks mandatory.
Strike 99 [time.clock.monotonic] p.2
2
The classmonotonic_clock
is conditionally supported.Change 32.2.4 [thread.req.timing] p.2 accordingly
The member functions whose names end in
_for
take an argument that specifies a relative time. Implementations should use a monotonic clock to measure time for these functions.[ Note: Implementations are not required to use a monotonic clock because such a clock may not be available. — end note ]
Proposed resolution:
This is resolved by n3191.
POS_T
and OFF_T
Section: 27.2.4.4 [char.traits.specializations.char16.t] Status: C++11 Submitter: BSI Opened: 2010-08-25 Last modified: 2021-06-06
Priority: Not Prioritized
View all other issues in [char.traits.specializations.char16.t].
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Duplicate of: 1444
Discussion:
Addresses GB-109, GB-123
It is not clear what the specification means for
u16streampos
, u32streampos
or wstreampos
when they
refer to the requirements for POS_T
in 21.2.2, as there
are no longer any such requirements. Similarly the annex
D.7 refers to the requirements of type POS_T
in 27.3 that
no longer exist either.
Clarify the meaning of all cross-reference to the
removed type POS_T
.
[ Post-Rapperswil, Daniel provides the wording. ]
When preparing the wording for this issue I first thought about adding both u16streampos
and u32streampos
to the [iostream.forward] header <iosfwd>
synopsis similar to streampos
and wstreampos
,
but decided not to do so, because the IO library does not yet actively support the char16_t
and char32_t
character types. Adding those would misleadingly imply that they would be part of the iostreams. Also, the addition
would make them also similarly equal to a typedef to fpos<mbstate_t>
, as for streampos
and
wstreampos
, so there is no loss for users that would like to use the proper fpos
instantiation for
these character types.
Additionally the way of referencing was chosen to follow the style suggested by NB comment GB 108.
Moved to Tentatively Ready with proposed wording after 5 positive votes on c++std-lib.
[ Adopted at 2010-11 Batavia ]
Proposed resolution:
The following wording changes are against N3126.
Change [char.traits.specializations.char16_t]p.1 as indicated:
1 - The type
u16streampos
shall be an implementation-defined type that satisfies the requirements forPOS_T
in 21.2.2pos_type
in [iostreams.limits.pos].
Change [char.traits.specializations.char32_t]p.1 as indicated:
1 - The type
u32streampos
shall be an implementation-defined type that satisfies the requirements forPOS_T
in 21.2.2pos_type
in [iostreams.limits.pos].
Change [char.traits.specializations.wchar.t]p.2 as indicated:
2 - The type
wstreampos
shall be an implementation-defined type that satisfies the requirements forPOS_T
in 21.2.2pos_type
in [iostreams.limits.pos].
Change [fpos.operations], Table 124 — Position type requirements as indicated:
Table 124 — Position type requirements Expression Return type ...
...
...
...
O(p)
OFF_T
streamoff
... ...
...
...
o = p - q
OFF_T
streamoff
...
streamsize(o)
O(sz)
streamsize
OFF_T
streamoff
...
Change [depr.ios.members]p.1 as indicated:
namespace std { class ios_base { public: typedef T1 io_state; typedef T2 open_mode; typedef T3 seek_dir; typedefOFF_Timplementation-defined streamoff; typedefPOS_Timplementation-defined streampos; // remainder unchanged }; }
Change [depr.ios.members]p.5+6 as indicated:
5 - The type
streamoff
is an implementation-defined type that satisfies the requirements oftypeOFF_T
(27.5.1)off_type
in [iostreams.limits.pos].6 - The type
streampos
is an implementation-defined type that satisfies the requirements oftypePOS_T
(27.3)pos_type
in [iostreams.limits.pos].
forward_list::erase_after
should not be allowed to throwSection: 23.2 [container.requirements] Status: C++11 Submitter: DIN Opened: 2010-08-25 Last modified: 2016-01-28
Priority: Not Prioritized
View all other issues in [container.requirements].
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Discussion:
Addresses DE-21
23.2.1/11 provides a general no-throw guarantee for erase() container functions, exceptions from this are explicitly mentioned for individual containers. Because of its different name, forward_list's erase_after() function is not ruled by this but should so.
[ Resolved in Rapperswil by a motion to directly apply the words from the ballot comment in N3102. ]
Proposed resolution:
Add a "Throws: Nothing" clause to both
erase_after
overloads in 23.3.3.4, [forwardlist.modifiers].
front/back
on a zero-sized array
should be undefinedSection: 23.3.3.5 [array.zero] Status: C++11 Submitter: BSI Opened: 2010-08-25 Last modified: 2016-01-28
Priority: Not Prioritized
View all other issues in [array.zero].
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Discussion:
Addresses GB-112
Should the effect of calling front/back
on a zero-sized
array
really be implementation defined i.e. require the
implementor to define behaviour?
[ Resolved in Rapperswil by a motion to directly apply the words from the ballot comment in N3102. ]
Proposed resolution:
Change "implementation defined" to "undefined"
resize(size())
on a deque
Section: 23.3.5.3 [deque.capacity] Status: C++11 Submitter: BSI Opened: 2010-08-25 Last modified: 2017-03-21
Priority: Not Prioritized
View all other issues in [deque.capacity].
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Discussion:
Addresses GB-113
There is no mention of what happens if sz==size()
. While
it obviously does nothing I feel a standard needs to say
this explicitely.
[ 2010 Batavia ]
Accepted with a simplified resolution turning one of the <
comparisons into <=.
Proposed resolution:
Ammend [deque.capacity]
void resize(size_type sz);
Effects: If sz <= size()
, equivalent to erase(begin() +
sz, end());
. If size() < sz
, appends sz - size()
default
constructedvalue initialized elements to the sequence.
resize(size())
on a list
Section: 23.3.11.3 [list.capacity] Status: C++11 Submitter: BSI Opened: 2010-08-25 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
Addresses GB-115
There is no mention of what happens if sz==size()
. While
it obviously does nothing I feel a standard needs to say
this explicitely.
[ Resolution proposed in ballot comment ]
Express the semantics as pseudo-code similarly
to the way it is done for the copying overload that
follows (in p3). Include an else clause that does
nothing and covers the sz==size()
case.
[ 2010 Batavia ]
Accepted with a simplified resolution turning one of the <
comparisons into <=.
Proposed resolution:
Ammend [list.capacity] p1:
void resize(size_type sz);
Effects: If
sz <= size()
, equivalent tolist<T>::iterator it = begin(); advance(it, sz); erase(it, end());
. Ifsize() < sz
, appendssz - size()
default constructedvalue initialized elements to the sequence.
Section: 23.6 [container.adaptors] Status: Resolved Submitter: DIN Opened: 2010-08-25 Last modified: 2016-01-28
Priority: Not Prioritized
View all other issues in [container.adaptors].
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Duplicate of: 1350
Discussion:
Addresses DE-22, CH-15
With the final acceptance of move operations as special
members and introduction of corresponding suppression
rules of implicitly generated copy operations the some
library types that were copyable in C++03 are no longer
copyable (only movable) in C++03, among them queue
,
priority_queue
, and stack
.
[ 2010-10-26: Daniel comments: ]
Accepting n3112 should fix this.
[2011-02-17: Lawrence comments:]
The only open issue in CH 15 with respect to the concurrency group
was the treatment of atomic_future
. Since we removed atomic_future
in Batavia, I think all that remains is to remove the open issue from
N3112 and adopt it.
[2011-03-23 Madrid meeting]
Resolved by N3264
Proposed resolution:
See n3112
map
constructor accepting an allocator as single parameter should be explicitSection: 23.4.3 [map] Status: C++11 Submitter: Japan Opened: 2010-08-25 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
Addresses JP-6
Constructor accepting an allocator as a single parameter should be qualified as explicit.
namespace std { template <class Key, class T, class Compare = less<Key>, class Allocator = allocator<pair<const Key, T> > > class map { public: ... map(const Allocator&);
[ Resolved in Rapperswil by a motion to directly apply the words from the ballot comment in N3102. ]
Proposed resolution:
Add explicit.
namespace std { template <class Key, class T, class Compare = less<Key>, class Allocator = allocator<pair<const Key, T> > > class map { public: ... explicit map(const Allocator&);
multimap
constructor accepting an allocator as a single parameter should be explicitSection: 23.4.4 [multimap] Status: C++11 Submitter: Japan Opened: 2010-08-25 Last modified: 2016-01-28
Priority: Not Prioritized
View all issues with C++11 status.
Discussion:
Addresses JP-7
Constructor accepting an allocator as a single parameter should be qualified as explicit.
[ Resolved in Rapperswil by a motion to directly apply the words from the ballot comment in N3102. ]
Proposed resolution:
Add explicit.
namespace std { template <class Key, class T, class Compare = less<Key>, class Allocator = allocator<pair<const Key, T> > > class multimap { public: ... explicit multimap(const Allocator&);
set
constructor accepting an allocator as a single parameter should be explicitSection: 23.4.6 [set] Status: C++11 Submitter: Japan Opened: 2010-08-25 Last modified: 2016-01-28
Priority: Not Prioritized
View all other issues in [set].
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Discussion:
Addresses JP-8
Constructor accepting an allocator as a single parameter should be qualified as explicit.
[ Resolved in Rapperswil by a motion to directly apply the words from the ballot comment in N3102. ]
Proposed resolution:
Add explicit.
namespace std { template <class Key, class Compare = less<Key>, class Allocator = allocator<Key> > class set { public: ... explicit set(const Allocator&);
multiset
constructor accepting an allocator as a single parameter should be explicitSection: 23.4.7 [multiset] Status: C++11 Submitter: Japan Opened: 2010-08-25 Last modified: 2016-01-28
Priority: Not Prioritized
View all issues with C++11 status.
Discussion:
Addresses JP-9
Constructor accepting an allocator as a single parameter should be qualified as explicit.
[ Resolved in Rapperswil by a motion to directly apply the words from the ballot comment in N3102. ]
Proposed resolution:
Add explicit.
namespace std { template <class Key, class Compare = less<Key>, class Allocator = allocator<Key> > class multiset { public: ... explicit multiset(const Allocator&);
unordered_map
constructor accepting an allocator as a single parameter should be explicitSection: 23.5.3 [unord.map] Status: C++11 Submitter: Japan Opened: 2010-08-25 Last modified: 2016-01-28
Priority: Not Prioritized
View all other issues in [unord.map].
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Discussion:
Addresses JP-10
Constructor accepting an allocator as a single parameter should be qualified as explicit.
[ Resolved in Rapperswil by a motion to directly apply the words from the ballot comment in N3102. ]
Proposed resolution:
Add explicit.
namespace std { template <class Key, template <class Key, class T, class Hash = hash<Key>, class Pred = std::equal_to<Key>, class Alloc = std::allocator<std::pair<const Key, T> > > class unordered_map { public: ... explicit unordered_map(const Allocator&);
unordered_multimap
constructor accepting an allocator as a single parameter should be explicitSection: 23.5.4 [unord.multimap] Status: C++11 Submitter: Japan Opened: 2010-08-25 Last modified: 2016-01-28
Priority: Not Prioritized
View all issues with C++11 status.
Discussion:
Addresses JP-11
Constructor accepting an allocator as a single parameter should be qualified as explicit.
[ Resolved in Rapperswil by a motion to directly apply the words from the ballot comment in N3102. ]
Proposed resolution:
Add explicit.
namespace std { template <class Key, class T, class Hash = hash<Key>, class Pred = std::equal_to<Key>, class Alloc = std::allocator<std::pair<const Key, T> > > class unordered_multimap { public: ... explicit unordered_multimap(const Allocator&);
unordered_set
constructor accepting an allocator as a single parameter should be explicitSection: 23.5.6 [unord.set] Status: C++11 Submitter: Japan Opened: 2010-08-25 Last modified: 2016-01-28
Priority: Not Prioritized
View all issues with C++11 status.
Discussion:
Addresses JP-12
Constructor accepting an allocator as a single parameter should be qualified as explicit.
[ Resolved in Rapperswil by a motion to directly apply the words from the ballot comment in N3102. ]
Proposed resolution:
Add explicit.
namespace std { template <class Key, class Hash = hash<Key>, class Pred = std::equal_to<Key>, class Alloc = std::allocator<Key> > class unordered_set { public: ... explicit unordered_set(const Allocator&);
unordered_multiset
constructor accepting an allocator as a single parameter should be explicitSection: 23.5.7 [unord.multiset] Status: C++11 Submitter: Japan Opened: 2010-08-25 Last modified: 2016-01-28
Priority: Not Prioritized
View all issues with C++11 status.
Discussion:
Addresses JP-13
Constructor accepting an allocator as a single parameter should be qualified as explicit.
[ Resolved in Rapperswil by a motion to directly apply the words from the ballot comment in N3102. ]
Proposed resolution:
Add explicit.
namespace std { template <class Key, class Hash = hash<Key>, class Pred = std::equal_to<Key>, class Alloc = std::allocator<Key> > class unordered_multiset { public: ... explicit unordered_multiset(const Allocator&);
is_permutation
must be more restrictiveSection: 26.6.14 [alg.is.permutation] Status: C++11 Submitter: INCITS Opened: 2010-08-25 Last modified: 2021-06-06
Priority: Not Prioritized
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Discussion:
Addresses US-120
is_permutation
is underspecified for anything but the
simple case where both ranges have the same value type
and the comparison function is an equivalence relation.
[ Resolved in Rapperswil by a motion to directly apply the words from the ballot comment in N3102. ]
Proposed resolution:
Restrict is_permutation
to the case where it is well
specified. See Appendix 1 - Additional Details
random_shuffle
signatures are inconsistentSection: 26.7.13 [alg.random.shuffle] Status: C++11 Submitter: INCITS Opened: 2010-08-25 Last modified: 2016-01-28
Priority: Not Prioritized
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Duplicate of: 1433
Discussion:
Addresses US-121, GB-119
random_shuffle
and shuffle
should be consistent in how
they accept their source of randomness: either both by rvalue reference or
both by lvalue reference.
[ Post-Rapperswil, Daniel provided wording ]
The signatures of the shuffle
and random_shuffle
algorithms are different
in regard to the support of rvalues and lvalues of the provided generator:
template<class RandomAccessIterator, class RandomNumberGenerator> void random_shuffle(RandomAccessIterator first, RandomAccessIterator last, RandomNumberGenerator&& rand);
template<class RandomAccessIterator, class UniformRandomNumberGenerator> void shuffle(RandomAccessIterator first, RandomAccessIterator last, UniformRandomNumberGenerator& g);
The first form uses the perfect forwarding signature and that change compared to
C++03
was done intentionally as shown in the first rvalue proposal
papers.
While it is true, that random generators are excellent examples of stateful functors, there still exist good reasons to support rvalues as arguments:
std::generate
and std::generate_n
)
accept both rvalues and lvalues as well.
C++03
. In the specific
cases, where rvalues are provided, the argument will be accepted instead of being rejected.
Arguments have been raised that accepting rvalues is error-prone or even fundamentally wrong. The author of this proposal disagrees with that position for two additional reasons:
instead of writingmy_generator get_generator(int size);
they will just writestd::vector<int> v = ...; std::shuffle(v.begin(), v.end(), get_generator(v.size()));
and feel annoyed about the need for it.std::vector<int> v = ...; auto gen = get_generator(v.size()); std::shuffle(v.begin(), v.end(), gen);
CopyConstructible
and this is obviously a generally useful property for such objects. It is also useful and sometimes necessary to start a
generator with exactly a specific seed again and again and thus to provide a new generator (or a copy) for each call. The
CopyConstructible
requirements allow providing rvalues of generators and thus this idiom must be useful as well.
Therefore preventing [random_]shuffle
to accept rvalues is an unnecessary restriction which doesn't prevent any
user-error, if there would exist one.
Thus this proposal recommends to make both shuffle
functions consistent and perfectly forward-able.
Moved to Tentatively Ready after 6 positive votes on c++std-lib.
[ Adopted at 2010-11 Batavia ]
Proposed resolution:
<algorithm>
synopsis as indicated:
template<class RandomAccessIterator, class UniformRandomNumberGenerator> void shuffle(RandomAccessIterator first, RandomAccessIterator last, UniformRandomNumberGenerator&& rand);
template<class RandomAccessIterator, class UniformRandomNumberGenerator> void shuffle(RandomAccessIterator first, RandomAccessIterator last, UniformRandomNumberGenerator&& rand);
Section: 29.4.7 [complex.value.ops] Status: C++11 Submitter: BSI Opened: 2010-08-25 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
Addresses GB-120
The complex number functions added for compatibility with the C99 standard library are defined purely as a cross-reference, with no hint of what they should return. This is distinct from the style of documentation for the functions in the earlier standard. In the case of the inverse-trigonometric and hyperbolic functions, a reasonable guess of the functionality may be made from the name, this is not true of the cproj function, which apparently returns the projection on the Reimann Sphere. A single line description of each function, associated with the cross-reference, will greatly improve clarity.
[2010-11-06 Beman provides proposed resolution wording.]
[ 2010 Batavia: The working group concurred with the issue's Proposed Resolution ]
[ Adopted at 2010-11 Batavia ]
Proposed resolution:
Change 26.4.7 complex value operations [complex.value.ops] as indicated:
template<class T> complex<T> proj(const complex<T>& x);
Returns: the projection of
x
onto the Riemann sphere.
Effects:Remarks: Behaves the same as the C functioncproj
, defined in 7.3.9.4.
Change 26.4.8 complex transcendentals [complex.transcendentals] as indicated:
template<class T> complex<T> acos(const complex<T>& x);
Returns: the complex arc cosine of
x
.
Effects:Remarks: Behaves the same as the C functioncacos
, defined in 7.3.5.1.
Change 26.4.8 complex transcendentals [complex.transcendentals] as indicated:
template<class T> complex<T> asin(const complex<T>& x);
Returns: the complex arc sine of
x
.
Effects:Remarks: Behaves the same as the C functioncasin
, defined in 7.3.5.2.
Change 26.4.8 complex transcendentals [complex.transcendentals] as indicated:
template<class T> complex<T> atan(const complex<T>& x);
Returns: the complex arc tangent of
x
.
Effects:Remarks: Behaves the same as the C functioncatan
, defined in 7.3.5.3.
Change 26.4.8 complex transcendentals [complex.transcendentals] as indicated:
template<class T> complex<T> acosh(const complex<T>& x);
Returns: the complex arc hyperbolic cosine of
x
.
Effects:Remarks: Behaves the same as the C functioncacosh
, defined in 7.3.6.1.
Change 26.4.8 complex transcendentals [complex.transcendentals] as indicated:
template<class T> complex<T> asinh(const complex<T>& x);
Returns: the complex arc hyperbolic sine of
x
.
Effects:Remarks: Behaves the same as the C functioncasinh
, defined in 7.3.6.2.
Change 26.4.8 complex transcendentals [complex.transcendentals] as indicated:
template<class T> complex<T> atanh(const complex<T>& x);
Returns: the complex arc hyperbolic tangent of
x
.
Effects:Remarks: Behaves the same as the C functioncatanh
, defined in 7.3.6.2.
Section: 29.5.4 [rand.eng] Status: C++11 Submitter: BSI Opened: 2010-08-25 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
Addresses GB-121
All the random number engine types in this clause have a constructor taking an unsigned integer type, and a constructor template for seed sequences. This means that an attempt to create a random number engine seeded by an integer literal must remember to add the appropriate unsigned suffix to the literal, as a signed integer will attempt to use the seed sequence template, yielding undefined behaviour, as per 26.5.1.1p1a. It would be helpful if at least these anticipated cases produced a defined behaviour, either an erroneous program with diagnostic, or a conversion to unsigned int forwarding to the appropriate constructor.
[ 2010-11-03 Daniel comments and provides a proposed resolution: ]
I suggest to apply a similar solution as recently suggested for 1234(i). It is basically a requirement for an implementation to constrain the template.
[
2010-11-04 Howard suggests to use !is_convertible<Sseq, result_type>::value
as minimum requirement instead of the originally proposed !is_scalar<Sseq>::value
.
This would allow for a user-defined type BigNum
, that is convertible to result_type
,
to be used as argument for a seed instead of a seed sequence. The wording has been updated to
reflect this suggestion.
]
[ 2010 Batavia: There were some initial concerns regarding the portability and reproducibility of results when seeded with a negative signed value, but these concerns were allayed after discussion. Thus, after reviewing the issue, the working group concurred with the issue's Proposed Resolution. ]
[ Adopted at 2010-11 Batavia ]
Proposed resolution:
Add the following paragraph at the end of 29.5.4 [rand.eng]:
5 Each template specified in this section [rand.eng] requires one or more relationships, involving the value(s) of its non-type template parameter(s), to hold. A program instantiating any of these templates is ill-formed if any such required relationship fails to hold.
? For every random number engine and for every random number engine adaptor
X
defined in this sub-clause [rand.eng] and in sub-clause [rand.adapt]:
- If the constructor
template<class Sseq> explicit X(Sseq& q);is called with a type
Sseq
that does not qualify as a seed sequence, then this constructor shall not participate in overload resolution.- If the member function
template<class Sseq> void seed(Sseq& q);is called with a type
Sseq
that does not qualify as a seed sequence, then this function shall not participate in overload resolution.The extent to which an implementation determines that a type cannot be a seed sequence is unspecified, except that as a minimum a type shall not qualify as seed sequence, if it is implicitly convertible to
X::result_type
.
Section: 29.5.4.3 [rand.eng.mers] Status: C++11 Submitter: INCITS Opened: 2010-08-25 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
Addresses US-124
The Mersenne twister algorithm is meaningless for word sizes less than two, as there are then insufficient bits available to be “twisted”.
[ Resolution proposed by ballot comment: ]
Insert the following among the relations that are required to hold:
2u < w
.
[ 2010 Batavia: The working group concurred with the issue's Proposed Resolution ]
[ Adopted at 2010-11 Batavia ]
Proposed resolution:
Change 29.5.4.3 [rand.eng.mers] p. 4 as indicated:
4 The following relations shall hold:
0 < m
,m <= n
,2u < w
,r <= w
,u <= w
,s <= w
,t <= w
,l <= w
,w <= numeric_limits<UIntType>::digits
,a <= (1u<<w) - 1u
,b <= (1u<<w) - 1u
,c <= (1u<<w) - 1u
,d <= (1u<<w) - 1u
, andf <= (1u<<w) - 1u
.
base()
Section: 29.5.5.2 [rand.adapt.disc] Status: C++11 Submitter: INCITS Opened: 2010-08-25 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
Addresses US-126
Each adaptor has a member function called base()
which has no definition.
[ Resolution proposed by ballot comment: ]
Give it the obvious definition.
[ 2010-11-03 Daniel comments and provides a proposed resolution: ]
The following proposal adds noexcept
specifiers to the declarations of
the base()
functions as replacement for a "Throws: Nothing" element.
[ 2010 Batavia: The working group reviewed this issue, and recommended to add the following to the Proposed Resolution. ]
a.base()
shall be valid and shall return a const reference to a
's base engine.
After further review, the working group concurred with the Proposed Resolution.
[Batavia: waiting for WEB to review wording]
Proposed resolution:
A random number engine adaptor (commonly shortened to adaptor)
a
of typeA
is a random number engine that takes values produced by some other random number engine, and applies an algorithm to those values in order to deliver a sequence of values with different randomness properties. An engineb
of typeB
adapted in this way is termed a base engine in this context. The expressiona.base()
shall be valid and shall return a const reference toa
's base engine.
discard_block_engine
synopsis, the following declaration:
// property functions const Engine& base() const noexcept;
const Engine& base() const noexcept;? Returns:
e
.
independent_bits_engine
synopsis, the following declaration:
// property functions const Engine& base() const noexcept;
const Engine& base() const noexcept;? Returns:
e
.
shuffle_order_engine
synopsis, the following declaration:
// property functions const Engine& base() const noexcept;
const Engine& base() const noexcept;? Returns:
e
.
densities()
functions?Section: 29.5.9.6.2 [rand.dist.samp.pconst] Status: C++11 Submitter: INCITS Opened: 2010-08-25 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
Addresses US-134
These two distributions have a member function called
densities()
which returns a vector<double>
. The
distribution is templated on RealType
. The distribution
also has another member called intervals()
which returns
a vector<RealType>
. Why doesn't densities return
vector<RealType>
as well? If RealType
is long double
,
the computed densities property isn't being computed to
the precision the client desires. If RealType
is float
, the
densities vector is taking up twice as much space as the client desires.
[ Resolution proposed by ballot comment: ]
Change the piecewise constant and linear distributions to hold / return the densities in a
If this is not done, at least correct 29.5.9.6.2 [rand.dist.samp.pconst] p. 13 which describes the return of densities as avector<result_type>
.vector<result_type>
.
[ Batavia 2010: After reviewing this issue, the working group concurred with the first of the suggestions proposed by the NB comment: "Change the piecewise constant and linear distributions to hold/return the densities in a vector. " ]
[ Adopted at 2010-11 Batavia ]
Proposed resolution:
piecewise_constant_distribution
synopsis
and the prototype description 29.5.9.6.2 [rand.dist.samp.pconst] before p. 13 as indicated:
vector<doubleresult_type> densities() const;
piecewise_linear_distribution
synopsis
and the prototype description 29.5.9.6.3 [rand.dist.samp.plinear] before p. 13 as indicated:
vector<doubleresult_type> densities() const;
piecewise_linear_distribution
Section: 29.5.9.6.3 [rand.dist.samp.plinear] Status: C++11 Submitter: INCITS Opened: 2010-08-25 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
Addresses US-135
This paragraph says: Let bk = xmin+k·δ for k = 0,...,n, and wk = fw(bk +δ) for k = 0,...,n. However I believe that fw(bk) would be far more desirable. I strongly suspect that this is nothing but a type-o.
[ Resolution proposed by ballot comment: ]
Change p. 10 to read:
Let bk = xmin+k·δ for k = 0,...,n, and wk = fw(bk) for k = 0,...,n.
[ 2010-11-02 Daniel translates into a proposed resolution ]
[ 2010 Batavia: The working group concurred with the issue's Proposed Resolution ]
[ Adopted at 2010-11 Batavia ]
Proposed resolution:
Change 29.5.9.6.3 [rand.dist.samp.plinear] p. 10 as indicated:
10 Effects: Constructs a
piecewise_linear_distribution
object with parameters taken or calculated from the following values: Letbk = xmin+k·δ
fork = 0, . . . , n
, andwk = fw(bk
for+δ)k = 0, . . . , n
.
Section: 29.7 [c.math] Status: C++11 Submitter: INCITS Opened: 2010-08-25 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
Addresses US-136
Floating-point test functions are incorrectly specified.
[ Resolved in Rapperswil by a motion to directly apply the words from the ballot comment in N3102. ]
Proposed resolution:
See Appendix 1 - Additional Details
Section: 31.7 [iostream.format] Status: Resolved Submitter: INCITS/PJ Plauger Opened: 2010-08-25 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
Addresses US-137
Several iostreams member functions are incorrectly specified.
[ Resolution proposed by ballot comment: ]
See Appendix 1 - Additional Details
[ 2010-10-24 Daniel adds: ]
Accepting n3168 would solve this issue.
Proposed resolution:
Addressed by paper n3168.
Section: 31.7 [iostream.format] Status: Resolved Submitter: INCITS Opened: 2010-08-25 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
Addresses US-139
Resolve issue LWG 1328 one way or the other, but
preferably in the direction outlined in the proposed
resolution, which, however, is not complete as-is: in any
case, the sentry must not set ok_ = false
if is.good() == false
,
otherwise istream::seekg
, being an unformatted
input function, does not take any action because the
sentry object returns false when converted to type bool
.
Thus, it remains impossible to seek away from end of file.
Proposed resolution:
Addressed by paper n3168.
basic_stringbuf::str(basic_string)
postconditionsSection: 31.8.2.4 [stringbuf.members] Status: C++11 Submitter: BSI Opened: 2010-08-25 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
Addresses GB-124
N3092 31.8.2.4 [stringbuf.members] contains this text specifying the postconditions of
basic_stringbuf::str(basic_string)
:
Postconditions: If
mode & ios_base::out
istrue
,pbase()
points to the first underlying character andepptr() >= pbase() + s.size()
holds; in addition, ifmode & ios_base::in
istrue
,pptr() == pbase() + s.data()
holds, otherwisepptr() == pbase()
istrue
. [...]
Firstly, there's a simple mistake: It should be pbase() + s.length()
,
not pbase() + s.data()
.
Secondly, it doesn't match existing implementations. As far as I can tell,
GCC 4.5 does not test for mode & ios_base::in
in the second part
of that sentence, but for mode & (ios_base::app | ios_base_ate)
,
and Visual C++ 9 for mode & ios_base::app
. Besides, the wording of
the C++0x draft doesn't make any sense to me. I suggest changing the second part
of the sentence to one of the following:
Replace ios_base::in
with (ios_base::ate | ios_base::app)
,
but this would require Visual C++ to change (replacing only with
ios_base::ate
would require GCC to change, and would make
ios_base::app
completely useless with stringstreams
):
in addition, if mode & (ios_base::ate | ios_base::app)
is true
,
pptr() == pbase() + s.length()
holds, otherwise pptr() == pbase()
is true
.
Leave pptr()
unspecified if mode & ios_base::app
, but not
mode & ios_base::ate
(implementations already differ in this case, and it
is always possible to use ios_base::ate
to get the effect of appending, so it
is not necessary to require any implementation to change):
in addition, if mode & ios_base::ate
is true
,
pptr() == pbase() + s.length()
holds, if neither mode & ios_base::ate
nor mode & ios_base::app
is true
, pptr() == pbase()
holds,
otherwise pptr() >= pbase() && pptr() <= pbase() + s.length()
(which of the values in this range is unspecified).
Slightly stricter:
in addition, if mode & ios_base::ate
is true
,
pptr() == pbase() + s.length()
holds, if neither
mode & ios_base::ate
nor mode & ios_base::app
is true
,
pptr() == pbase()
holds, otherwise pptr() == pbase() || pptr() == pbase() + s.length()
(which of these two values is unspecified). A small table might help to better explain the three cases.
BTW, at the end of the postconditions is this text: "egptr() == eback() + s.size()
hold".
Is there a perference for basic_string::length
or basic_string::size
? It doesn't really
matter, but it looks a bit inconsistent.
[2011-03-09: Nicolai Josuttis comments and drafts wording]
First, it seems the current wording is just an editorial mistake. When we added issue 432(i) to the draft standard (in n1733), the wording in the issue:
If
mode & ios_base::out
is true, initializes the output sequence such thatpbase()
points to the first underlying character,epptr()
points one past the last underlying character, and if(mode & ios_base::ate)
is true,pptr()
is set equal toepptr()
elsepptr()
is set equal topbase()
.
became:
If
mode & ios_base::out
is true, initializes the output sequence such thatpbase()
points to the first underlying character,epptr()
points one past the last underlying character, andpptr()
is equal toepptr()
ifmode & ios_base::in
is true, otherwisepptr()
is equal topbase()
.
which beside some changes of the order of words changed
ios_base::ate
into
ios_base::in
So, from this point of view, clearly mode & ios_base::ate
was meant.
Nevertheless, with this proposed resolution we'd have no wording regarding ios_base::app
.
Currently the only statements about app
in the Standard are just in two tables:
openmode
effects" says that the effect of
app
is "seek to end before each write"
app
is "a"
Indeed we seem to have different behavior currently in respect to app
: For
stringstream s2(ios_base::out|ios_base::in|ios_base::app); s2.str("s2 hello"); s1 << "more";
"moreello"
)"s2 hellomore"
)BTW, for fstreams, both implementations append when app
is set:
If f2.txt
has contents "xy"
,
fstream f2("f2.txt",ios_base::out|ios_base::in|ios_base::app); f1 << "more";
appends "more"
so that the contents is "xymore"
.
So IMO app
should set the write pointer to the end so that each writing
appends.
str()
of stringbuffer.
Nevertheless, it doesn't hurt IMO if we clarify the behavior of str()
here in respect to app
.
[2011-03-10: P.J.Plauger comments:]
I think we should say nothing special about app
at construction
time (thus leaving the write pointer at the beginning of the buffer).
Leave implementers wiggle room to ensure subsequent append writes as they see
fit, but don't change existing rules for initial seek position.
[Madrid meeting: It was observed that a different issue should be opened that
clarifies the meaning of app
for stringstream
]
Proposed resolution:
Change 31.8.2.4 [stringbuf.members] p. 3 as indicated:
void str(const basic_string<charT,traits,Allocator>& s);2 Effects: Copies the content of
3 Postconditions: Ifs
into thebasic_stringbuf
underlying character sequence and initializes the input and output sequences according tomode
.mode & ios_base::out
is true,pbase()
points to the first underlying character andepptr() >= pbase() + s.size()
holds; in addition, ifmode &
is true,ios_base::inios_base::atepptr() == pbase() +
holds, otherwises.data()s.size()pptr() == pbase()
is true. Ifmode & ios_base::in
is true,eback()
points to the first underlying character, and bothgptr() == eback() and egptr() == eback() + s.size()
hold.
<cinttypes>
Section: 31.8.3 [istringstream] Status: C++11 Submitter: Canada Opened: 2010-08-25 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
Addresses CA-4
Subclause 27.9.2 [c.files] specifies that <cinttypes> has declarations for abs() and div(); however, the signatures are not present in this subclause. The signatures proposed under TR1 ([tr.c99.inttypes]) are not present in FCD (unless if intmax_t happened to be long long). It is unclear as to which, if any of the abs() and div() functions in [c.math] are meant to be declared by <cinttypes>. This subclause mentions imaxabs() and imaxdiv(). These functions, among other things, are not specified in FCD to be the functions from Subclause 7.8 of the C Standard. Finally, <cinttypes> is not specified in FCD to include <cstdint> (whereas <inttypes.h> includes <stdint.h> in C).
[ Post-Rapperswil, Daniel provides wording ]
Subclause [c.files] specifies that <cinttypes>
has declarations for abs()
and div()
;
however, the signatures are not present in this subclause. The signatures proposed under TR1 ([tr.c99.inttypes]) are not
present in FCD (unless if intmax_t
happened to be long long
). It is unclear as to which, if any of the
abs()
and div()
functions in [c.math] are meant to be declared by <cinttypes>
. This
subclause mentions imaxabs()
and imaxdiv()
. These functions, among other things, are not specified in
FCD to be the functions from subclause 7.8 of the C
Standard. Finally, <cinttypes>
is not specified
in FCD to include <cstdint>
(whereas <inttypes.h>
includes <stdint.h>
in C
).
Moved to Tentatively Ready with proposed wording after 5 positive votes on c++std-lib.
[ Adopted at 2010-11 Batavia ]
Proposed resolution:
The wording refers to N3126.
Table 132 describes header
2 - The contents of header<cinttypes>
. [Note: The macros defined by<cinttypes>
are provided unconditionally. In particular, the symbol__STDC_FORMAT_MACROS
, mentioned in footnote 182 of theC
standard, plays no role inC++
. — end note ]<cinttypes>
are the same as the StandardC
library header<inttypes.h>
, with the following changes: 3 - The header<cinttypes>
includes the header<cstdint>
instead of<stdint.h>
. 4 - If and only if the typeintmax_t
designates an extended integer type ([basic.fundamental]), the following function signatures are added:intmax_t abs(intmax_t); imaxdiv_t div(intmax_t, intmax_t);which shall have the same semantics as the function signatures
intmax_t imaxabs(intmax_t)
andimaxdiv_t imaxdiv(intmax_t, intmax_t)
, respectively.
regex_constants
Section: 28.6.4.3 [re.matchflag] Status: C++14 Submitter: BSI Opened: 2010-08-25 Last modified: 2016-01-28
Priority: 3
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Discussion:
Addresses GB-127
The Bitmask Type requirements in 16.3.3.3.3 [bitmask.types] p.3 say that
all elements on a bitmask type have distinct values, but
28.6.4.3 [re.matchflag] defines regex_constants::match_default
and
regex_constants::format_default
as elements of the
bitmask type regex_constants::match_flag_type
, both with
value 0. This is a contradiction.
[ Resolution proposed by ballot comment: ]
One of the bitmask elements should be removed from the declaration and should be defined separately, in the same manner as
ios_base::adjustfield
,ios_base::basefield
andios_base::floatfield
are defined by [ios::fmtflags] p.2 and Table 120. These are constants of a bitmask type, but are not distinct elements, they have more than one value set in the bitmask.regex_constants::format_default
should be specified as a constant with the same value asregex_constants::match_default
.
[ 2010-10-31 Daniel comments: ]
Strictly speaking, a bitmask type cannot have any element of value 0 at all, because any such value would contradict the requirement expressed in 16.3.3.3.3 [bitmask.types] p. 3:
for any pair Ci and Cj, Ci & Ci is nonzero
So, actually both regex_constants::match_default
and
regex_constants::format_default
are only constants of the type
regex_constants::match_flag_type
, and no bitmask elements.
[ 2010-11-03 Daniel comments and provides a proposed resolution: ]
The proposed resolution is written against N3126 and considered as a further improvement of the fixes suggested by n3110.
Add the following sentence to 28.6.4.3 [re.matchflag] paragraph 1:
1 The type
regex_constants::match_flag_type
is an implementation-defined bitmask type (17.5.2.1.3). Matching a regular expression against a sequence of characters [first,last) proceeds according to the rules of the grammar specified for the regular expression object, modified according to the effects listed in Table 136 for any bitmask elements set. Typeregex_constants::match_flag_type
also defines the constantsregex_constants::match_default
andregex_constants::format_default
.
[ 2011 Bloomington ]
It appears the key problem is the phrasing of the bitmask requirements. Jeremiah supplies updated wording.
Pete Becker has also provided an alternative resolution.
Ammend 16.3.3.3.3 [bitmask.types]:
Change the list of values for "enum bit mask" in p2 from
V0 = 1 << 0, V1 = 1 << 1, V2 = 1 << 2, V3 = 1 << 3, ...
.
to
V0 = 0, V1 = 1 << 0, V2 = 1 << 1, V3 = 1 << 2, ...
.
Here, the names C0, C1, etc. represent bitmask elements for this particular
bitmask type. All such non-zero elements have distinct values such that, for any pair
Ci and Cj where i != j, Ci & Ci is nonzero
and Ci & Cj is zero.
Change bullet 3 of paragraph 4:
TheA non-zero value Y is set in the object X if the expression X & Y is nonzero.
[2014-02-13 Issaquah:]
Proposed resolution:
Ammend 16.3.3.3.3 [bitmask.types] p3:
Here, the names C0, C1, etc. represent bitmask elements for this particular bitmask type. All such elements have distinct, nonzero values such that, for any pair Ci and Cj where i != j, Ci & Ci is nonzero and Ci & Cj is zero. Additionally, the value 0 is used to represent an empty bitmask, in which no bitmask elements are set.
Add the following sentence to 28.6.4.3 [re.matchflag] paragraph 1:
1 The type
regex_constants::match_flag_type
is an implementation-defined bitmask type (17.5.2.1.3). The constants of that type, except formatch_default
andformat_default
, are bitmask elements. Thematch_default
andformat_default
constants are empty bitmasks. Matching a regular expression against a sequence of characters [first,last) proceeds according to the rules of the grammar specified for the regular expression object, modified according to the effects listed in Table 136 for any bitmask elements set.
match_results
behavior for certain operations Section: 28.6.9.5 [re.results.acc] Status: Resolved Submitter: BSI Opened: 2010-08-25 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
Addresses GB-126
It's unclear how match_results should behave if it has been default-constructed. The sub_match objects returned by operator[], prefix and suffix cannot point to the end of the sequence that was searched if no search was done. The iterators held by unmatched sub_match objects might be singular.
[ Resolution proposed by ballot comment: ]
Add to match_results::operator[], match_results::prefix and match_results::suffix:
Requires: !empty()
[ 2010-10-24 Daniel adds: ]
Accepting n3158 would solve this issue.
Proposed resolution:
Addressed by paper n3158.
Section: 32.5 [atomics] Status: Resolved Submitter: Switzerland Opened: 2010-08-25 Last modified: 2016-01-28
Priority: Not Prioritized
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Duplicate of: 1454
Discussion:
Addresses CH-22, GB-128
WG14 currently plans to introduce atomic facilities that are intended to be compatible with the facilities of clause 29. They should be compatible.
[ Resolution proposed by ballot comment ]
Make sure the headers in clause 29 are defined in a way that is compatible with the planned C standard.
[ 2010 Batavia ]
Resolved by adoption of n3193.
Proposed resolution:
Solved by n3193.
Section: 32.5.2 [atomics.syn] Status: Resolved Submitter: BSI Opened: 2010-08-25 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
Addresses GB-130
The synopsis for the <atomic>
header lists the macros
ATOMIC_INTEGRAL_LOCK_FREE
and ATOMIC_ADDRESS_LOCK_FREE
.
The ATOMIC_INTEGRAL_LOCK_FREE
macro has been replaced with a set of macros
for each integral type, as listed in 32.5.5 [atomics.lockfree].
[Proposed resolution as of comment]
Against FCD, N3092:
In [atomics.syn], header
<atomic>
synopsis replace as indicated:// 29.4, lock-free property#define ATOMIC_INTEGRAL_LOCK_FREE unspecified#define ATOMIC_CHAR_LOCK_FREE implementation-defined #define ATOMIC_CHAR16_T_LOCK_FREE implementation-defined #define ATOMIC_CHAR32_T_LOCK_FREE implementation-defined #define ATOMIC_WCHAR_T_LOCK_FREE implementation-defined #define ATOMIC_SHORT_LOCK_FREE implementation-defined #define ATOMIC_INT_LOCK_FREE implementation-defined #define ATOMIC_LONG_LOCK_FREE implementation-defined #define ATOMIC_LLONG_LOCK_FREE implementation-defined #define ATOMIC_ADDRESS_LOCK_FREE unspecified
[ 2010-10-26: Daniel adds: ]
The proposed resolution below is against the FCD working draft. After application of the editorial issues US-144 and US-146 the remaining difference against the working draft is the usage of implementation-defined instead of unspecified, effectively resulting in this delta:
// 29.4, lock-free property #define ATOMIC_CHAR_LOCK_FREEunspecifiedimplementation-defined #define ATOMIC_CHAR16_T_LOCK_FREEunspecifiedimplementation-defined #define ATOMIC_CHAR32_T_LOCK_FREEunspecifiedimplementation-defined #define ATOMIC_WCHAR_T_LOCK_FREEunspecifiedimplementation-defined #define ATOMIC_SHORT_LOCK_FREEunspecifiedimplementation-defined #define ATOMIC_INT_LOCK_FREEunspecifiedimplementation-defined #define ATOMIC_LONG_LOCK_FREEunspecifiedimplementation-defined #define ATOMIC_LLONG_LOCK_FREEunspecifiedimplementation-defined #define ATOMIC_ADDRESS_LOCK_FREE unspecified
It is my understanding that the intended wording should be unspecified as for ATOMIC_ADDRESS_LOCK_FREE
but if this is right, we need to use the same wording in 32.5.5 [atomics.lockfree], which consequently uses
the term implementation-defined. I recommend to keep 32.5.2 [atomics.syn] as it currently is and to
fix 32.5.5 [atomics.lockfree] instead as indicated:
[2011-02-24 Reflector discussion]
Moved to Tentatively Ready after 5 votes.
[2011-02-20: Daniel adapts the proposed wording to N3225 and fixes an editorial omission of applying N3193]
[2011-03-06: Daniel adapts the wording to N3242]
[Proposed Resolution]
Change 32.5.5 [atomics.lockfree] as indicated:
#define ATOMIC_CHAR_LOCK_FREEimplementation-definedunspecified #define ATOMIC_CHAR16_T_LOCK_FREEimplementation-definedunspecified #define ATOMIC_CHAR32_T_LOCK_FREEimplementation-definedunspecified #define ATOMIC_WCHAR_T_LOCK_FREEimplementation-definedunspecified #define ATOMIC_SHORT_LOCK_FREEimplementation-definedunspecified #define ATOMIC_INT_LOCK_FREEimplementation-definedunspecified #define ATOMIC_LONG_LOCK_FREEimplementation-definedunspecified #define ATOMIC_LLONG_LOCK_FREEimplementation-definedunspecified
Proposed resolution:
Resolved 2011-03 Madrid meeting by paper N3278
bool
should be addedSection: 32.5.5 [atomics.lockfree] Status: Resolved Submitter: INCITS Opened: 2010-08-25 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
Addresses US-154
There is no ATOMIC_BOOL_LOCK_FREE
macro.
Proposed resolution suggested by the NB comment:
Add ATOMIC_BOOL_LOCK_FREE
to 32.5.5 [atomics.lockfree] and to 32.5.2 [atomics.syn]:
[..] #define ATOMIC_BOOL_LOCK_FREE unspecified #define ATOMIC_CHAR_LOCK_FREE unspecified #define ATOMIC_CHAR16_T_LOCK_FREE unspecified #define ATOMIC_CHAR32_T_LOCK_FREE unspecified [..]
[2011-03-12: Lawrence comments and drafts wording]
Point: We were missing a macro test for bool
.
Comment: The atomic<bool>
type is the easiest to make lock-free.
There is no harm in providing a macro.
Action: Add an ATOMIC_BOOL_LOCK_FREE
.
Point: We were missing a macro test for pointers.
Comment: The national body comment noting the missing macro
for bool
did not note the lack of a macro for pointers because
ATOMIC_ADDRESS_LOCK_FREE
was present at the time of the comment.
Its removal appears to be an overzealous consequence of removing
atomic_address
.
Action: Add an ATOMIC_POINTER_LOCK_FREE
.
Point: Presumably atomic_is_lock_free()
will be an inline function
producing a constant in those cases in which the macros are useful.
Comment: The point is technically correct, but could use some exposition. Systems providing forward binary compatibility, e.g. mainstream desktop and server systems, would likely have these functions as inline constants only when the answer is true. Otherwise, the function should defer to a platform-specific dynamic library to take advantage of future systems that do provide lock-free support.
Comment: Such functions are not useful in the preprocessor, and not
portably useful in static_assert
.
Action: Preserve the macros.
Point: The required explicit instantiations are atomic<X>
for each
of the types X
in Table 145. Table 145 does not list bool
, so
atomic<bool>
is not a required instantiation.
Comment: I think specialization was intended in the point instead of
instantiation. In any event, 32.5.8 [atomics.types.generic] paragraph 5 does
indirectly require a specialization for atomic<bool>
. Confusion
arises because the specialization for other integral types have a
wider interface than the generic atomic<T>
, but
atomic<bool>
does not.
Action: Add clarifying text.
Point: The name of Table 145, "atomic integral typedefs", is perhaps misleading, since the types listed do not contain all of the "integral" types.
Comment: Granted, though the table describe those with extra operations.
Action: Leave the text as is.
Point: The macros correspond to the types in Table 145, "with the
signed and unsigned variants grouped together". That's a rather
inartful way of saying that ATOMIC_SHORT_LOCK_FREE
applies to
signed short
and unsigned short
. Presumably this also means that
ATOMIC_CHAR_LOCK_FREE
applies to all three char types.
Comment: Yes, it is inartful.
Comment: Adding additional macros to distinguish signed and unsigned would provide no real additional information given the other constraints in the language.
Comment: Yes, it applies to all three char
types.
Action: Leave the text as is.
Point: The standard says that "There are full specializations over the
integral types (char
, signed char
, ...)" bool
is not in the list. But this text is not normative. It simply tells you
that "there are" specializations, not "there shall be" specializations, which would
impose a requirement. The requirement, to the extent that there is
one, is in the header synopsis, which, in N3242, sort of pulls in the
list of types in Table 145.
Comment: The intent was for the specializations to be normative. Otherwise the extra member functions could not be present.
Action: Clarify the text.
[Proposed Resolution]
Edit header
<atomic>
synopsis 32.5.2 [atomics.syn]:namespace std { // 29.3, order and consistency enum memory_order; template <class T> T kill_dependency(T y); // 29.4, lock-free property #define ATOMIC_BOOL_LOCK_FREE unspecified #define ATOMIC_CHAR_LOCK_FREE unspecified #define ATOMIC_CHAR16_T_LOCK_FREE unspecified #define ATOMIC_CHAR32_T_LOCK_FREE unspecified #define ATOMIC_WCHAR_T_LOCK_FREE unspecified #define ATOMIC_SHORT_LOCK_FREE unspecified #define ATOMIC_INT_LOCK_FREE unspecified #define ATOMIC_LONG_LOCK_FREE unspecified #define ATOMIC_LLONG_LOCK_FREE unspecified #define ATOMIC_POINTER_LOCK_FREE unspecified // 29.5, generic types template<class T> struct atomic; template<> struct atomic<integral>; template<class T> struct atomic<T*>; // 29.6.1, general operations on atomic types // In the following declarations, atomic_type is either // atomic<T> or a named base class for T from // Table 145 or inferred from // Table 146 or from bool. […] }Edit the synopsis of 32.5.5 [atomics.lockfree] and paragraph 1 as follows:
#define ATOMIC_BOOL_LOCK_FREE unspecified #define ATOMIC_CHAR_LOCK_FREEimplementation-definedunspecified #define ATOMIC_CHAR16_T_LOCK_FREEimplementation-definedunspecified #define ATOMIC_CHAR32_T_LOCK_FREEimplementation-definedunspecified #define ATOMIC_WCHAR_T_LOCK_FREEimplementation-definedunspecified #define ATOMIC_SHORT_LOCK_FREEimplementation-definedunspecified #define ATOMIC_INT_LOCK_FREEimplementation-definedunspecified #define ATOMIC_LONG_LOCK_FREEimplementation-definedunspecified #define ATOMIC_LLONG_LOCK_FREEimplementation-definedunspecified #define ATOMIC_POINTER_LOCK_FREE unspecified1 The
ATOMIC_…_LOCK_FREE
macros indicate the lock-free property of the corresponding atomic types, with the signed and unsigned variants grouped together. The properties also apply to the corresponding (partial) specializations of theatomic
template. A value of 0 indicates that the types are never lock-free. A value of 1 indicates that the types are sometimes lock-free. A value of 2 indicates that the types are always lock-free.Edit 32.5.8 [atomics.types.generic] paragraph 3, 4, and 6-8 as follows:
2 The semantics of the operations on specializations of
3 Specializations and instantiations of theatomic
are defined in 32.5.8.2 [atomics.types.operations].atomic
template shall have a deleted copy constructor, a deleted copy assignment operator, and aconstexpr
value constructor. 4 Thereareshall be full specializationsoverfor the integral types(char
,signed char
,unsigned char
,short
,unsigned short
,int
,unsigned int
,long
,unsigned long
,long long
,unsigned long long
,char16_t
,char32_t
, andwchar_t
, and any other types needed by the typedefs in the header<cstdint>
)on theatomic
class template. For each integral type integral, the specializationatomic<integral>
provides additional atomic operations appropriate to integral types.[Editor's note: I'm guessing that this is the correct rendering of the text in the paper; if this sentence was intended to impose a requirement, rather than a description, it will have to be changed.]There shall be a specializationatomic<bool>
which provides the general atomic operations as specified in [atomics.types.operations.general]. 5 The atomic integral specializations and the specializationatomic<bool>
shall have standard layout. They shall each have a trivial default constructor and a trivial destructor. They shall each support aggregate initialization syntax. 6 Thereareshall be pointer partial specializationsonof theatomic
class template. These specializations shall have trivial default constructors and trivial destructors. 7 Thereareshall be named types corresponding to the integral specializations ofatomic
, as specified in Table 145. In addition, there shall be named typeatomic_bool
corresponding to the specializationatomic<bool>
. Each named type is either a typedef to the corresponding specialization or a base class of the corresponding specialization. If it is a base class, it shall support the same member functions as the corresponding specialization. 8 Thereareshall be atomic typedefs corresponding to the typedefs in the header<inttypes.h>
as specified in Table 146.
Proposed resolution:
Resolved 2011-03 Madrid meeting by paper N3278
atomic_bool
Section: 99 [atomics.types.integral] Status: Resolved Submitter: BSI Opened: 2010-08-25 Last modified: 2016-01-28
Priority: Not Prioritized
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Duplicate of: 1463
Discussion:
Addresses GB-132, US-157
The atomic_itype
types and atomic_address
have two
overloads of operator=
; one is volatile
qualified, and the
other is not. atomic_bool
only has the volatile
qualified
version:
bool operator=(bool) volatile;
On a non-volatile
-qualified object this is ambiguous with
the deleted copy-assignment operator
atomic_bool& operator=(atomic_bool const&) = delete;
due to the need for a single standard conversion in each
case when assigning a bool to an atomic_bool
as in:
atomic_bool b; b = true;
The conversions are:
atomic_bool& → atomic_bool volatile&
vs
bool → atomic_bool
[ Proposed resolution as of NB comment: ]
Change 99 [atomics.types.integral] as indicated:
namespace std { typedef struct atomic_bool { [..] bool operator=(bool) volatile; bool operator=(bool); } atomic_bool; [..] }
[ 2010-10-27 Daniel adds: ]
Accepting n3164 would solve this issue by replacing
atomic_bool
byatomic<bool>
.
[ 2010 Batavia ]
Resolved by adoption of n3193.
Proposed resolution:
Solved by n3193.
Section: 99 [atomics.types.integral] Status: Resolved Submitter: INCITS Opened: 2010-08-25 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
Addresses US-160
The last sentence of 99 [atomics.types.integral] p.1 says:
Table 143 shows typedefs to atomic integral classes and the corresponding
<cstdint>
typedefs.
That's nice, but nothing says these are supposed to be part of the implementation, and they are not listed in the synopsis.
[ Proposed resolution as of NB comment ]
1 The name
atomic_itype
and the functions operating on it in the preceding synopsis are placeholders for a set of classes and functions. Throughout the preceding synopsis,atomic_itype
should be replaced by each of the class names in Table 142 and integral should be replaced by the integral type corresponding to the class name.Table 143 shows typedefs to atomic integral classes and the corresponding<cstdint>
typedefs.
[ 2010-10-27 Daniel adds: ]
Accepting n3164 would solve this issue.
[ 2010-11 Batavia ]
Resolved by adopting n3193.
Proposed resolution:
Solved by n3193.
atomic_address
Section: 99 [atomics.types.address] Status: Resolved Submitter: INCITS Opened: 2010-08-25 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
Addresses US-161
atomic_address
has operator+=
and operator-=
, but no
operator++
or operator--
. The template specialization
atomic<Ty*>
has all of them.
[ 2010-10-27 Daniel adds: ]
Accepting n3164 would solve this issue by replacing
atomic_address
byatomic<void*>
.
[ Resolved in Batavia by accepting n3193. ]
Proposed resolution:
Change 99 [atomics.types.address], class atomic_address
synopsis, as indicated:
namespace std { typedef struct atomic_address { […] void* operator=(const void*) volatile; void* operator=(const void*); void* operator++(int) volatile; void* operator++(int); void* operator--(int) volatile; void* operator--(int); void* operator++() volatile; void* operator++(); void* operator--() volatile; void* operator--(); void* operator+=(ptrdiff_t) volatile; void* operator+=(ptrdiff_t); void* operator-=(ptrdiff_t) volatile; void* operator-=(ptrdiff_t); } atomic_address; […] }
const
breakage by compare_exchange_*
member functionsSection: 99 [atomics.types.address] Status: Resolved Submitter: INCITS Opened: 2010-08-25 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
Addresses US-162
The compare_exchange_weak
and compare_exchange_strong
member functions that take
const void*
arguments lead to a silent removal of const
, because the load
member function and other acessors return the stored value as a void*
.
[ Proposed resolution as of NB comment: ]
Change 99 [atomics.types.address], class atomic_address
synopsis, as indicated:
namespace std { typedef struct atomic_address { [..]bool compare_exchange_weak(const void*&, const void*, memory_order, memory_order) volatile;bool compare_exchange_weak(const void*&, const void*, memory_order, memory_order);bool compare_exchange_strong(const void*&, const void*, memory_order, memory_order) volatile;bool compare_exchange_strong(const void*&, const void*, memory_order, memory_order);bool compare_exchange_weak(const void*&, const void*, memory_order = memory_order_seq_cst) volatile;bool compare_exchange_weak(const void*&, const void*, memory_order = memory_order_seq_cst);bool compare_exchange_strong(const void*&, const void*, memory_order = memory_order_seq_cst) volatile;bool compare_exchange_strong(const void*&, const void*, memory_order = memory_order_seq_cst);[..] } atomic_address; [..] }
[ 2010-10-27 Daniel adds: ]
Accepting n3164 would solve this issue by replacing
atomic_address
byatomic<void*>
.
[ Resolved in Batavia by accepting n3193. ]
Proposed resolution:
Solved by n3193.
atomic<T*>
from atomic_address
breaks type safetySection: 99 [atomics.types.address] Status: Resolved Submitter: INCITS Opened: 2010-08-25 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
Addresses US-163
Requiring atomic<T*>
to be derived from atomic_address
breaks type safety:
atomic<double*> ip; char ch; atomic_store(&ip, &ch); *ip.load() = 3.14159;
The last line overwrites ch
with a value of type double
.
[ 2010-10-27 Daniel adds: ]
Resolving this issue will also solve 1469(i)
Accepting n3164 would solve this issue by removingatomic_address
.
[ Resolved in Batavia by accepting n3193. ]
Proposed resolution:
atomic<T*>
synopsis, as indicated:
namespace std { template <class T> struct atomic<T*>: atomic_address{ [..] }; [..] }
4 There are pointer partial specializations on the
atomic
class template.These specializations shall be publicly derived fromThe unit of addition/subtraction for these specializations shall be the size of the referenced type. These specializations shall have trivial default constructors and trivial destructors.atomic_address
.
atomic_address::compare_exchange_*
member functions should match atomic_compare_exchange_*
free functionsSection: 99 [atomics.types.address] Status: Resolved Submitter: INCITS Opened: 2010-08-25 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
Addresses US-164
atomic_address
has member functions compare_exchange_weak
and
compare_exchange_strong
that take arguments of type const void*
,
in addition to the void*
versions. If these member functions survive,
there should be corresponding free functions.
[ 2010-10-27 Daniel adds: ]
Accepting n3164 would solve this issue differently by removing the overloads with
const void*
arguments, because they break type-safety.
[ Resolved in Batavia by accepting n3193. ]
Proposed resolution:
Extend the synopsis around atomic_address
in 99 [atomics.types.address]
as indicated:
namespace std { [..] bool atomic_compare_exchange_weak(volatile atomic_address*, void**, void*); bool atomic_compare_exchange_weak(atomic_address*, void**, void*); bool atomic_compare_exchange_strong(volatile atomic_address*, void**, void*); bool atomic_compare_exchange_strong(atomic_address*, void**, void*); bool atomic_compare_exchange_weak_explicit(volatile atomic_address*, void**, void*, memory_order, memory_order); bool atomic_compare_exchange_weak_explicit(atomic_address*, void**, void*, memory_order, memory_order); bool atomic_compare_exchange_strong_explicit(volatile atomic_address*, void**, void*, memory_order, memory_order); bool atomic_compare_exchange_strong_explicit(atomic_address*, void**, void*, memory_order, memory_order); bool atomic_compare_exchange_weak(volatile atomic_address*, const void**, const void*); bool atomic_compare_exchange_weak(atomic_address*, const void**, const void*); bool atomic_compare_exchange_strong(volatile atomic_address*, const void**, const void*); bool atomic_compare_exchange_strong(atomic_address*, const void**, const void*); bool atomic_compare_exchange_weak_explicit(volatile atomic_address*, const void**, const void*, memory_order, memory_order); bool atomic_compare_exchange_weak_explicit(atomic_address*, const void**, const void*, memory_order, memory_order); bool atomic_compare_exchange_strong_explicit(volatile atomic_address*, const void**, const void*, memory_order, memory_order); bool atomic_compare_exchange_strong_explicit(volatile atomic_address*, const void**, const void*, memory_order, memory_order); bool atomic_compare_exchange_strong_explicit(atomic_address*, const void**, const void*, memory_order, memory_order); [..] }
atomic<T*>
inheritance from atomic_address
breaks type safetySection: 32.5.8 [atomics.types.generic] Status: Resolved Submitter: BSI Opened: 2010-08-25 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
Addresses GB-133
The free functions that operate on atomic_address
can be
used to store a pointer to an unrelated type in an atomic<T*>
without a cast. e.g.
int i; atomic<int*> ai(&i); string s; atomic_store(&ai,&s);
Overload the atomic_store
, atomic_exchange
and
atomic_compare_exchange_[weak/strong]
operations for
atomic<T*>
to allow storing only pointers to T
.
[ 2010-10-27 Daniel adds: ]
Resolving this issue will also solve 1467(i)
Accepting n3164 would solve this issue by removingatomic_address
.
[Resolved in Batavia by accepting n3193. ]
Proposed resolution:
Add the following overloads to 32.5.8 [atomics.types.generic], the synopsis around the specialization
atomic<T*>
, as indicated:
namespace std { [..] template <class T> struct atomic<T*> : atomic_address { [..] }; template<typename T> void atomic_store(atomic<T*>&,T*); template<typename T> void atomic_store(atomic<T*>&,void*) = delete; template<typename T> void atomic_store_explicit(atomic<T*>&,T*,memory_order); template<typename T> void atomic_store_explicit(atomic<T*>&,void*,memory_order) = delete; template<typename T> T* atomic_exchange(atomic<T*>&,T*); template<typename T> T* atomic_exchange(atomic<T*>&,void*) = delete; template<typename T> T* atomic_exchange_explicit(atomic<T*>&,T*,memory_order); template<typename T> T* atomic_exchange_explicit(atomic<T*>&,void*,memory_order) = delete; template<typename T> T* atomic_compare_exchange_weak(atomic<T*>&,T**,T*); template<typename T> T* atomic_compare_exchange_weak(atomic<T*>&,void**,void*) = delete; template<typename T> T* atomic_compare_exchange_weak_explicit(atomic<T*>&,T**,T*,memory_order); template<typename T> T* atomic_compare_exchange_weak_explicit(atomic<T*>&,void**,void*,memory_order) = delete; template<typename T> T* atomic_compare_exchange_strong(atomic<T*>&,T**,T*); template<typename T> T* atomic_compare_exchange_strong(atomic<T*>&,void**,void*) = delete; template<typename T> T* atomic_compare_exchange_strong_explicit(atomic<T*>&,T**,T*,memory_order); template<typename T> T* atomic_compare_exchange_strong_explicit(atomic<T*>&,void**,void*,memory_order) = delete; }
Section: 32.5.8.2 [atomics.types.operations] Status: C++11 Submitter: INCITS Opened: 2010-08-25 Last modified: 2017-06-15
Priority: Not Prioritized
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Duplicate of: 1470, 1475, 1476, 1477
Discussion:
Addresses US-175, US-165, CH-23, GB-135
99 [atomics.types.operations.req] p. 25: The first sentence is grammatically incorrect.
[ 2010-10-28 Daniel adds: ]
Duplicate issue 1475(i) also has a proposed resolution, but both issues are resolved with below proposed resolution.
[ 2011-02-15 Howard fixes numbering, Hans improves the wording ]
[2011-02-24 Reflector discussion]
Moved to Tentatively Ready after 6 votes.
Proposed resolution:
Change 99 [atomics.types.operations.req] p. 23 as indicated:
[ Note: For example, t
The effect ofthe compare-and-exchange operationsatomic_compare_exchange_strong
isif (memcmp(object, expected, sizeof(*object)) == 0) memcpy(object, &desired, sizeof(*object)); else memcpy(expected, object, sizeof(*object));— end note ] [..]
Change 99 [atomics.types.operations.req] p. 25 as indicated:
25 Remark:
When a compare-and-exchange is in a loop, the weak version will yield better performance on some platforms. When a weak compare-and-exchange would require a loop and a strong one would not, the strong one is preferable. — end note ]The weak compare-and-exchange operations may fail spuriously, that is, return false while leaving the contents of memory pointed to byA weak compare-and-exchange operation may fail spuriously. That is, even when the contents of memory referred to byexpected
before the operation is the same that same as that of theobject
and the same as that ofexpected
after the operationexpected
andobject
are equal, it may return false and store back toexpected
the same memory contents that were originally there.. [ Note: This spurious failure enables implementation of compare-and-exchange on a broader class of machines, e.g., loadlocked store-conditional machines. A consequence of spurious failure is that nearly all uses of weak compare-and-exchange will be in a loop.
Section: 32.5.8.2 [atomics.types.operations] Status: C++11 Submitter: BSI Opened: 2010-08-25 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
Addresses GB-136
GB requests normative clarification in 32.5.8.2 [atomics.types.operations] p.4 that concurrent access constitutes a race, as already done on p.6 and p.7.
[ Resolution proposed in ballot comment: ]
Initialisation of atomics:
We believe the intent is that for any atomics there is a distinguished initialisation write, but that this need not happens-before all the other operations on that atomic - specifically so that the initialisation write might be non-atomic and hence give rise to a data race, and hence undefined behaviour, in examples such as this (from Hans):atomic<atomic<int> *> p f() | { atomic<int>x; | W_na x p.store(&x,mo_rlx); | W_rlx p=&x } |(where na is nonatomic and rlx is relaxed). We suspect also that no other mixed atomic/nonatomic access to the same location is intended to be permitted. Either way, a note would probably help.
[2011-02-26: Hans comments and drafts wording]
I think the important point here is to clarify that races on atomics
are possible, and can be introduced as a result of non-atomic
initialization operations. There are other parts of this that remain unclear
to me, such as whether there are other ways to introduce data races on atomics,
or whether the races with initialization also introduce undefined behavior
by the 3.8 lifetime rules. But I don't think that it is necessary to resolve
those issues before releasing the standard. That's particularly true
since we've introduced atomic_init
, which allows easier ways to
construct initialization races.
[2011-03 Madrid]
Accepted to be applied immediately to the WP
Proposed resolution:
Update 99 [atomics.types.operations.req] p. 5 as follows:
constexpr A::A(C desired);5 Effects: Initializes the object with the value
desired
.[ Note: Construction is not atomic. — end note ]Initialization is not an atomic operation (1.10) [intro.multithread]. [Note: It is possible to have an access to an atomic objectA
race with its construction, for example by communicating the address of the just-constructed objectA
to another thread viamemory_order_relaxed
atomic operations on a suitable atomic pointer variable, and then immediately accessingA
in the receiving thread. This results in undefined behavior. — end note]
In response to the editor comment to 99 [atomics.types.operations.req] p. 8: The first Effects element is the correct and intended one:
void atomic_init(volatile A *object, C desired); void atomic_init(A *object, C desired);8 Effects: Non-atomically initializes
*object
with valuedesired
. This function shall only be applied to objects that have been default constructed, and then only once. [ Note: these semantics ensure compatibility with C. — end note ] [ Note: Concurrent access from another thread, even via an atomic operation, constitutes a data race. — end note ][Editor's note: The preceding text is from the WD as amended by N3196. N3193 makes different changes, marked up in the paper as follows:] Effects: Dynamically initializes an atomic variable. Non-atomically That is, non-atomically assigns the value desired to*object
. [ Note: this operation may need to initialize locks. — end note ] Concurrent access from another thread, even via an atomic operation, constitutes a data race.
extern "C"
Section: 32.5.11 [atomics.fences] Status: C++11 Submitter: INCITS Opened: 2010-08-25 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
Addresses US-179
The fence functions (32.5.11 [atomics.fences] p.5 + p.6) should be extern "C"
, for C
compatibility.
[2011-02-16 Reflector discussion]
Moved to Tentatively Ready after 6 votes.
Proposed resolution:
<atomic>
synopsis as indicated:
namespace std { [..] // 29.8, fences extern "C" void atomic_thread_fence(memory_order); extern "C" void atomic_signal_fence(memory_order); }
Change 32.5.11 [atomics.fences], p. 5 and p. 6 as indicated:
extern "C" void atomic_thread_fence(memory_order);5 Effects: depending on the value of
order
, this operation: [..]
extern "C" void atomic_signal_fence(memory_order);6 Effects: equivalent to
atomic_thread_fence(order)
, except that synchronizes with relationships are established only between a thread and a signal handler executed in the same thread.
Section: 32.5.11 [atomics.fences] Status: C++11 Submitter: BSI Opened: 2010-08-25 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
Addresses GB-137
Thread fence not only establish synchronizes with relationships,
there are semantics of fences that are expressed not in
terms of synchronizes with relationships (for example see 32.5.4 [atomics.order] p.5).
These semantics also need to apply to the use of
atomic_signal_fence
in a restricted way.
[Batavia: Concurrency group discussed issue, and is OK with the proposed resolution.]
[2011-02-26 Reflector discussion]
Moved to Tentatively Ready after 5 votes.
Proposed resolution:
Change 32.5.11 [atomics.fences] p. 6 as indicated:
void atomic_signal_fence(memory_order);6 Effects: equivalent to
atomic_thread_fence(order)
, except thatsynchronizes with relationshipsthe resulting ordering constraints are established only between a thread and a signal handler executed in the same thread.
Section: 32.2 [thread.req] Status: Resolved Submitter: BSI Opened: 2010-08-25 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
Addresses GB-138
The FCD combines the requirements for lockable objects with those for the standard mutex objects. These should be separate. This is LWG issue 1268(i).
[ Resolution proposed by ballot comment: ]
See attached Appendix 1 - Additional Details
[ 2010-11-01 Daniel comments: ]
Paper n3130 addresses this issue.
Proposed resolution:
Resolved by n3197.
Section: 32.2.4 [thread.req.timing] Status: Resolved Submitter: INCITS Opened: 2010-08-25 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
Addresses US-181
The timeout operations are under-specified.
[ Resolution proposed by ballot comment: ]
Define precise semantics for
timeout_until
andtimeout_for
. See n3141 page 193 - Appendix 1 - Additional Details
[ 2010-11-01 Daniel comments: ]
Accepting n3128 would solve this issue.
Proposed resolution:
Resolved by n3191.
Section: 32.4.5 [thread.thread.this] Status: C++11 Submitter: Switzerland Opened: 2010-08-25 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
Addresses CH-25
Clock related operations are currently not required not to throw. So "Throws: Nothing." is not always true.
[ Resolution proposed by ballot comment: ]
Either require clock related operations not to throw (in 20.10) or change the Throws clauses in 30.3.2. Also possibly add a note that
abs_time
in the past or negativerel_time
is allowed.
[2011-02-10: Howard Hinnant provides a resolution proposal]
[Previous proposed resolution:]
Change the Operational semantics of C1::now()
in 30.3 [time.clock.req],
Table 59 — Clock
requirements as follows:
Table 59 — Clock
requirementsExpression Return type Operational semantics C1::now()
C1::time_point
Returns a time_point
object
representing the current point in time.
Shall not throw an exception.
[2011-02-19: Daniel comments and suggests an alternative wording]
Imposing the no-throw requirement on C1::now()
of any clock time
is an overly radical step: It has the indirect consequences that representation
types for C1::rep
can never by types with dynamic memory managment,
e.g. my big_int
, which are currently fully supported by the time
utilities. Further-on this strong constraint does not even solve the problem
described in the issue, because we are still left with the fact that any
of the arithmetic operations of C1::rep
, C1::duration
,
and C1::time_point
may throw exceptions.
The alternative proposal uses the following strategy: The general Clock
requirements remain untouched, but we require that any functions of the library-provided
clocks from sub-clause 30.7 [time.clock] and their associated types shall not
throw exceptions. Second, we replace existing noexcept
specifications of
functions from Clause 30 that depend on durations, clocks, or time points by wording that
clarifies that these functions can only throw, if the operations of user-provided durations,
clocks, or time points used as arguments to these functions throw exceptions.
[2011-03-23 Daniel and Peter check and simplify the proposed resolution resulting in this paper]
There is an inherent problem with std::time_point
that it doesn't seem to have an equivalent value
for ((time_t)-1)
that gets returned by C's time()
function to signal a problem, e.g., because
the underlying hardware is unavailable. After a lot of thinking and checks we came to the resolution that
timepoint::max()
should be the value to serve as a value signaling errors in cases where we
prefer to stick with no-throw conditions. Of-course, user-provided representation types don't need to
follow this approach if they prefer exceptions to signal such failures.
now()
and from_time_t()
can remain noexcept
with the solution to
return timepoint::max()
in case the current time cannot be determined or (time_t)-1
is passed
in, respectively.
Based on the previous proposed solution to LWG 1487 we decided that the new TrivialClock
requirements
should define that now()
mustn't throw and return timepoint::max()
to signal a problem. That
is in line with the C standard where (time_t)-1
signals a problem. Together with a fix to a - we assumed -
buggy specifcation in 20.11.3 p2 which uses "happens-before" relationship with something that isn't any action:
2 In Table 59
C1
andC2
denote clock types.t1
andt2
are values returned byC1::now()
where the call returningt1
happens before (1.10) the call returningt2
and both of these calls happen beforeC1::time_point::max()
.
[2011-03-23 Review with Concurrency group suggested further simplifications and Howard pointed out, that we do not need time_point::max() as a special value.]
also the second "happens before" will be changed to "occurs before" in the english meaning. this is to allow a steady clock to wrap.
Peter updates issue accordingly to discussion.[Note to the editor: we recommend removal of the following redundant paragraphs in 32.7.5 [thread.condition.condvarany] p. 18 to p. 21, p. 27, p. 28, p. 30, and p. 31 that are defining details for the wait functions that are given by the Effects element. ]
[Note to the editor: we recommend removal of the following redundant paragraphs in
32.7.4 [thread.condition.condvar]: p24-p26, p33-p34, and p36-p37 that are defining details for the
wait_for
functions. We believe these paragraphs are redundant with respect to the Effects clauses that
define semantics based on wait_until
. An example of such a specification is the wait()
with a predicate.
]
Proposed resolution:
Change p2 in 20.11.3 [time.clock.req] as follows
2 In Table 59
C1
andC2
denote clock types.t1
andt2
are values returned byC1::now()
where the call returningt1
happens before (1.10) the call returningt2
and both of these callshappenoccur beforeC1::time_point::max()
. [ Note: This meansC1
didn't wrap around betweent1
andt2
— end note ]
Add the following new requirement set at the end of sub-clause 30.3 [time.clock.req]: [Comment:
This requirement set is intentionally incomplete. The reason for
this incompleteness is the based on the fact, that if we would make it right for C++0x, we would end up defining
something like a complete ArithmeticLike
concept for TC::rep
, TC::duration
, and TC::time_point
.
But this looks out-of scope for C++0x to me. The effect is that we essentially do not exactly say, which arithmetic
or comparison operations can be used in the time-dependent functions from Clause 30, even though I expect that
all declared functions of duration
and time_point
are well-formed and well-defined. — end comment]
3 [ Note: the relative difference in durations between those reported by a given clock and the SI definition is a measure of the quality of implementation. — end note ]
? A type
TC
meets theTrivialClock
requirements if:
TC
satisfies theClock
requirements (30.3 [time.clock.req]),the types
TC::rep
,TC::duration
, andTC::time_point
satisfy the requirements ofEqualityComparable
( [equalitycomparable]),LessThanComparable
( [lessthancomparable]),DefaultConstructible
( [defaultconstructible]),CopyConstructible
( [copyconstructible]),CopyAssignable
( [copyassignable]),Destructible
( [destructible]), and of numeric types ([numeric.requirements]) [Note: This means in particular, that operations of these types will not throw exceptions — end note ],lvalues of the types
TC::rep
,TC::duration
, andTC::time_point
are swappable (16.4.4.3 [swappable.requirements]),the function
TC::now()
does not throw exceptions, andthe type
TC::time_point::clock
meets theTrivialClock
requirements, recursively.
Modify 30.7 [time.clock] p. 1 as follows:
1 - The types defined in this subclause shall satisfy the
TrivialClock
requirements (20.11.1).
Modify 30.7.2 [time.clock.system] p. 1, class system_clock
synopsis, as follows:
class system_clock { public: typedef see below rep; typedef ratio<unspecified , unspecified > period; typedef chrono::duration<rep, period> duration; typedef chrono::time_point<system_clock> time_point; static const bool is_monotonic is_steady = unspecified; static time_point now() noexcept; // Map to C API static time_t to_time_t (const time_point& t) noexcept; static time_point from_time_t(time_t t) noexcept; };
Modify the prototype declarations in 30.7.2 [time.clock.system] p. 3 + p. 4 as indicated (This
edit also fixes the miss of the static
specifier in these prototype declarations):
static time_t to_time_t(const time_point& t) noexcept;static time_point from_time_t(time_t t) noexcept;
Modify 30.7.7 [time.clock.steady] p. 1, class steady_clock
synopsis, as follows:
class steady_clock { public: typedef unspecified rep; typedef ratio<unspecified , unspecified > period; typedef chrono::duration<rep, period> duration; typedef chrono::time_point<unspecified, duration> time_point; static const bool is_monotonic is_steady = true; static time_point now() noexcept; };
Modify 30.7.8 [time.clock.hires] p. 1, class high_resolution_clock
synopsis, as follows:
class high_resolution_clock { public: typedef unspecified rep; typedef ratio<unspecified , unspecified > period; typedef chrono::duration<rep, period> duration; typedef chrono::time_point<unspecified, duration> time_point; static const bool is_monotonic is_steady = unspecified; static time_point now() noexcept; };
Add a new paragraph at the end of 32.2.4 [thread.req.timing]:
6 The resolution of timing provided by an implementation depends on both operating system and hardware. The finest resolution provided by an implementation is called the native resolution.
? Implementation-provided clocks that are used for these functions shall meet the
TrivialClock
requirements (30.3 [time.clock.req]).
Edit the synopsis of 32.4.5 [thread.thread.this] before p. 1. [Note: this duplicates edits also in D/N3267]:
template <class Clock, class Duration> void sleep_until(const chrono::time_point<Clock, Duration>& abs_time)noexcept; template <class Rep, class Period> void sleep_for(const chrono::duration<Rep, Period>& rel_time)noexcept;
Modify the prototype specifications in 32.4.5 [thread.thread.this] before p. 4 and p. 6 and re-add a Throws element following the Synchronization elements at p. 5 and p. 7:
template <class Clock, class Duration> void sleep_until(const chrono::time_point<Clock, Duration>& abs_time)noexcept;4 - [...]
5 - Synchronization: None. ? - Throws: Nothing ifClock
satisfies theTrivialClock
requirements (30.3 [time.clock.req]) and operations ofDuration
do not throw exceptions. [Note: Instantiations of time point types and clocks supplied by the implementation as specified in 30.7 [time.clock] do not throw exceptions. — end note]
template <class Rep, class Period> void sleep_for(const chrono::duration<Rep, Period>& rel_time)noexcept;6 [...]
7 Synchronization: None. ? Throws: Nothing if operations ofchrono::duration<Rep, Period>
do not throw exceptions. [Note: Instantiations of duration types supplied by the implementation as specified in 30.7 [time.clock] do not throw exceptions. — end note]
Fix a minor incorrectness in p. 5: Duration types need to compare against duration<>::zero()
,
not 0
:
3 The expression
[...] 5 Effects: The function attempts to obtain ownership of the mutex within the relative timeout (30.2.4) specified bym.try_lock_for(rel_time)
shall be well-formed and have the following semantics:rel_time
. If the time specified byrel_time
is less than or equal to0
rel_time.zero()
, the function attempts to obtain ownership without blocking (as if by callingtry_lock()
). The function shall return within the timeout specified byrel_time
only if it has obtained ownership of the mutex object. [ Note: As withtry_lock()
, there is no guarantee that ownership will be obtained if the lock is available, but implementations are expected to make a strong effort to do so. — end note ]
Modify the class timed_mutex
synopsis in 32.6.4.3.2 [thread.timedmutex.class] as indicated:
[Note: this duplicates edits also in D/N3267]:
class timed_mutex { public: [...] template <class Rep, class Period> bool try_lock_for(const chrono::duration<Rep, Period>& rel_time)noexcept; template <class Clock, class Duration> bool try_lock_until(const chrono::time_point<Clock, Duration>& abs_time)noexcept; [...] };
Modify the class recursive_timed_mutex
synopsis in 32.6.4.3.3 [thread.timedmutex.recursive] as indicated:
[Note: this duplicates edits also in D/N3267]:
class recursive_timed_mutex { public: [...] template <class Rep, class Period> bool try_lock_for(const chrono::duration<Rep, Period>& rel_time)noexcept; template <class Clock, class Duration> bool try_lock_until(const chrono::time_point<Clock, Duration>& abs_time)noexcept; [...] };
Modify the class template unique_lock
synopsis in 32.6.5.4 [thread.lock.unique] as indicated.
[Note: this duplicates edits also in D/N3267]:
template <class Mutex> class unique_lock { public: [...] template <class Clock, class Duration> unique_lock(mutex_type& m, const chrono::time_point<Clock, Duration>& abs_time)noexcept; template <class Rep, class Period> unique_lock(mutex_type& m, const chrono::duration<Rep, Period>& rel_time)noexcept; [...] };
Modify the constructor prototypes in 32.6.5.4.2 [thread.lock.unique.cons] before p. 14 and p. 17 [Note: this duplicates edits also in D/N3267]:
template <class Clock, class Duration> unique_lock(mutex_type& m, const chrono::time_point<Clock, Duration>& abs_time)noexcept;
template <class Rep, class Period> unique_lock(mutex_type& m, const chrono::duration<Rep, Period>& rel_time)noexcept;
Section: 32.6.4 [thread.mutex.requirements] Status: Resolved Submitter: Switzerland Opened: 2010-08-25 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
Addresses CH-27
The mutex requirements force try_lock
to be
noexcept(true)
. However, where they are used by the
generic algorithms, those relax this requirement and say
that try_lock
may throw. This means the requirement is
too stringent, also a non-throwing try_lock
does not allow
for a diagnostic such as system_error
that lock()
will give us.
[ Resolution proposed by ballot comment: ]
delete p18, adjust 30.4.4 p1 and p4 accordingly
[ 2010-11-01 Daniel comments: ]
Accepting n3130 would solve this issue.
Proposed resolution:
Resolved by n3197.
try_lock
does not guarantee forward progressSection: 32.6.4 [thread.mutex.requirements] Status: Resolved Submitter: INCITS Opened: 2010-08-25 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
Addresses US-186
try_lock
does not provide a guarantee of forward progress
because it is allowed to spuriously fail.
[ Resolution proposed by ballot comment: ]
The standard mutex types must not fail spuriously in
try_lock
. See n3141 page 205 - Appendix 1 - Additional Details
[ 2010-11-01 Daniel comments: ]
Paper n3152 addresses this issue.
Proposed resolution:
Resolved by n3209.
Section: 32.6.4 [thread.mutex.requirements] Status: Resolved Submitter: INCITS Opened: 2010-08-25 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
Addresses US-188
Mutex requirements should not be bound to threads.
[ Resolution proposed by ballot comment: ]
See Appendix 1 of n3141 - Additional Details, p. 208.
[ 2010-10-24 Daniel adds: ]
Accepting n3130 would solve this issue.
Proposed resolution:
Resolved by n3197.
Section: 32.6.7.2 [thread.once.callonce] Status: C++11 Submitter: INCITS Opened: 2010-08-25 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
Addresses US-190
The term "are serialized" is never defined (32.6.7.2 [thread.once.callonce] p. 2).
[ Resolution proposed by ballot comment: ]
Remove the sentence with "are serialized" from
paragraph 2. Add "Calls to call_once
on the same
once_flag
object shall not introduce data races
(17.6.4.8)." to paragraph 3.
[ 2010-11-01 Daniel translates NB comment into wording ]
[ 2011-02-17: Hans proposes an alternative resolution ]
[ 2011-02-25: Hans, Clark, and Lawrence update the suggested wording ]
[2011-02-26 Reflector discussion]
Moved to Tentatively Ready after 5 votes.
Proposed resolution:
Change 32.6.7.2 [thread.once.callonce] p.2+3 as indicated:
template<class Callable, class ...Args> void call_once(once_flag& flag, Callable&& func, Args&&... args);[..]
2 Effects:Calls toAn execution ofcall_once
on the sameonce_flag
object are serialized. If there has been a prior effective call tocall_once
on the sameonce_flag object
, the call tocall_once
returns without invokingfunc
. If there has been no prior effective call tocall_once
on the sameonce_flag
object,INVOKE(decay_copy( std::forward<Callable>(func)), decay_copy(std::forward<Args>(args))...)
is executed. The call tocall_once
is effective if and only ifINVOKE(decay_copy( std::forward<Callable>(func)), decay_copy(std::forward<Args>(args))...)
returns without throwing an exception. If an exception is thrown it is propagated to the caller.call_once
that does not call itsfunc
is a passive execution. An execution ofcall_once
that calls itsfunc
is an active execution. An active execution shall callINVOKE(decay_copy(std::forward<Callable>(func)), decay_copy(std::forward<Args>(args))...)
. If such a call tofunc
throws an exception, the execution is exceptional, otherwise it is returning. An exceptional execution shall propagate the exception to the caller ofcall_once
. Among all executions ofcall_once
for any givenonce_flag
: at most one shall be a returning execution; if there is a returning execution, it shall be the last active execution; and there are passive executions only if there is a returning execution. [Note: Passive executions allow other threads to reliably observe the results produced by the earlier returning execution. — end note] 3 Synchronization:The completion of an effective call toFor any givencall_once
on aonce_flag
object synchronizes with (6.9.2 [intro.multithread]) all subsequent calls tocall_once
on the sameonce_flag
object.once_flag
: all active executions occur in a total order; completion of an active execution synchronizes with (6.9.2 [intro.multithread]) the start of the next one in this total order; and the returning execution synchronizes with the return from all passive executions.
lock()
postcondition can not be generally achievedSection: 32.7 [thread.condition] Status: C++11 Submitter: Switzerland Opened: 2010-08-25 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
Addresses CH-30
If lock.lock()
throws an exception, the postcondition can not be generally achieved.
[ Resolution proposed by ballot comment: ]
Either state that the postcondition might not be achieved, depending on the error condition, or state that
terminate()
is called in this case.
[ 2010-08-13 Peter Sommerlad comments and provides wording ]
32.7.4 [thread.condition.condvar], 32.7.5 [thread.condition.condvarany]
p. 13, last bullet, and corresponding paragraphs in all wait functions Problem:
Condition variable wait might fail, because the lock cannot be acquired when notified. CH-30 says: "If lock.lock() throws an exception, the postcondition can not be generally achieved." CH-30 proposes: "Either state that the postcondition might not be achieved, depending on the error condition, or state that terminate() is called in this case." The discussion in Rapperswil concluded that callingterminate()
might be too drastic in this case and a corresponding exception should be thrown/passed on and one should use a lock type that allows querying its status, whichunique_lock
allows forstd::condition_variable
We also had some additional observations while discussing in Rapperswil:
- in 32.7.4 [thread.condition.condvar]
wait
with predicate andwait_until
with predicate lack the precondition, postcondition and Error conditions sections. the lack of the precondition would allow to callpred()
without holding the lock.- in 32.7.4 [thread.condition.condvar]
wait_until
andwait_for
and 32.7.5 [thread.condition.condvarany]wait_for
still specify an error condition for a violated precondition. This should be removed.and add the following proposed solution:
[2011-02-27: Daniel adapts numbering to n3225]
Proposed resolution:
void wait(unique_lock<mutex>& lock);
[..]9 Requires:
lock
.owns_lock()
istrue
andlock.mutex()
is locked by the calling thread, and either
- no other thread is waiting on this
condition_variable
object orlock.mutex()
returns the same value for each of thelock
arguments supplied by all concurrently waiting (viawait
ortimed_wait
) threads.
[..]11 Postcondition:
lock
.owns_lock()
istrue
andlock.mutex()
is locked by the calling thread.
template <class Predicate> void wait(unique_lock<mutex>& lock, Predicate pred);
?? Requires:
lock.owns_lock()
istrue
andlock.mutex()
is locked by the calling thread, and either
- no other thread is waiting on this
condition_variable
object orlock.mutex()
returns the same value for each of thelock
arguments supplied by all concurrently waiting (viawait
ortimed_wait
) threads.
14 Effects:
while (!pred()) wait(lock);
?? Postcondition:
lock.owns_lock()
istrue
andlock.mutex()
is locked by the calling thread.
?? Throws:
std::system_error
when an exception is required (30.2.2).
?? Error conditions:
- equivalent error condition from
lock.lock()
orlock.unlock()
.
template <class Clock, class Duration> cv_status wait_until(unique_lock<mutex>& lock, const chrono::time_point<Clock, Duration>& abs_time);
15 Requires:
lock
.owns_lock()
istrue
andlock.mutex()
is locked by the calling thread, and either
- no other thread is waiting on this
condition_variable
object orlock.mutex()
returns the same value for each of thelock
arguments supplied by all concurrently waiting (viawait
,wait_for
, orwait_until
) threads.
[..]
[..]17 Postcondition:
lock
.owns_lock()
istrue
andlock.mutex()
is locked by the calling thread.
20 Error conditions:
operation_not_permitted
— if the thread does not own the lock.- equivalent error condition from
lock.lock()
orlock.unlock()
.
template <class Rep, class Period> cv_status wait_for(unique_lock<mutex>& lock, const chrono::duration<Rep, Period>& rel_time);
21 Requires:
lock
.owns_lock()
istrue
andlock.mutex()
is locked by the calling thread, and either
- no other thread is waiting on this
condition_variable
object orlock.mutex()
returns the same value for each of thelock
arguments supplied by all concurrently waiting (viawait
,wait_for
, orwait_until
) threads.
[..]
[..]24 Postcondition:
lock
.owns_lock()
istrue
andlock.mutex()
is locked by the calling thread.
26 Error conditions:
operation_not_permitted
— if the thread does not own the lock.- equivalent error condition from
lock.lock()
orlock.unlock()
.
template <class Clock, class Duration, class Predicate> bool wait_until(unique_lock<mutex>& lock, const chrono::time_point<Clock, Duration>& abs_time, Predicate pred);
?? Requires:
lock.owns_lock()
istrue
andlock.mutex()
is locked by the calling thread, and either
- no other thread is waiting on this
condition_variable
object orlock.mutex()
returns the same value for each of thelock
arguments supplied by all concurrently waiting (viawait
ortimed_wait
) threads.
27 Effects:
while (!pred()) if (wait_until(lock, abs_time) == cv_status::timeout) return pred(); return true;
28 Returns:
pred()
?? Postcondition:
lock.owns_lock()
istrue
andlock.mutex()
is locked by the calling thread.
29 [ Note: The returned value indicates whether the predicate evaluates to true regardless of whether the timeout was triggered. — end note ]
?? Throws:
std::system_error
when an exception is required (30.2.2).
?? Error conditions:
- equivalent error condition from
lock.lock()
orlock.unlock()
.
template <class Rep, class Period, class Predicate> bool wait_for(unique_lock<mutex>& lock, const chrono::duration<Rep, Period>& rel_time, Predicate pred);
30 Requires:
lock
.owns_lock()
istrue
andlock.mutex()
is locked by the calling thread, and either
- no other thread is waiting on this
condition_variable
object orlock.mutex()
returns the same value for each of thelock
arguments supplied by all concurrently waiting (viawait
,wait_for
, orwait_until
) threads.
[..]
33 Postcondition:
lock
.owns_lock()
istrue
andlock.mutex()
is locked by the calling thread.
[..]
37 Error conditions:
operation_not_permitted
— if the thread does not own the lock.- equivalent error condition from
lock.lock()
orlock.unlock()
.
template <class Lock, class Predicate> void wait(Lock& lock, Predicate pred);
[Note: if any of the wait functions exits with an exception it is indeterminate if the
Lock
is held. One can use aLock
type that allows to query that, such as theunique_lock
wrapper. — end note]
11 Effects:
while (!pred()) wait(lock);
[..]
31 Error conditions:
operation_not_permitted
— if the thread does not own the lock.- equivalent error condition from
lock.lock()
orlock.unlock()
.
Section: 32.7 [thread.condition] Status: Resolved Submitter: Switzerland Opened: 2010-08-25 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
Addresses CH-29
It is unclear if a spurious wake-up during the loop and reentering
of the blocked state due to a repeated execution
of the loop will adjust the timer of the blocking with the
respect to the previously specified rel_time
value.
[ Resolution proposed by ballot comment: ]
Make it clear (e.g. by a note) that when reexecuting the loop the waiting time when blocked will be adjusted with respect to the elapsed time of the previous loop executions.
[ 2010-08-13 Peter Sommerlad comments and provides wording: ]
Problem: It is unclear if a spurious wake-up during the loop and re-entering of the blocked state due to a repeated execution of the loop will adjust the timer of the blocking with the respect to the previously specified
Proposed Resolution from CH29: Make it clear (e.g. by a note) that when re-executing the loop the waiting time when blocked will be adjusted with respect to the elapsed time of the previous loop executions. Discussion in Rapperswil: Assuming the introduction of a mandatoryrel_time
value.steady_clock
proposed by US-181 to the FCD the specification ofcondition_variable::wait_for
can be defined in terms ofwait_until
using thesteady_clock
. This is also interleaving with US-181, because that touches the same paragraph (30.5.1 p 25, p34 and 30.5.2 p 20, p 28 in n3092.pdf) (The "as if" in the proposed solutions should be confirmed by the standardization terminology experts)
[ 2010-11 Batavia: Resolved by applying n3191 ]
- Change 32.7.4 [thread.condition.condvar] paragraph 25,
wait_for
Effects as indicated:template <class Rep, class Period> cv_status wait_for(unique_lock<mutex>& lock, const chrono::duration<Rep, Period>& rel_time);[..]
25 Effects: as ifreturn wait_until(lock, chrono::steady_clock::now() + rel_time);
Atomically callslock.unlock()
and blocks on*this
.When unblocked, callslock.lock()
(possibly blocking on the lock), then returns.The function will unblock when signaled by a call tonotify_one()
or a call tonotify_all()
, by the elapsed timerel_time
passing (30.2.4), or spuriously.If the function exits via an exception,lock.lock()
shall be called prior to exiting the function scope.- Change 32.7.4 [thread.condition.condvar] paragraph 34,
wait_for
with predicate Effects as indicated:template <class Rep, class Period, class Predicate> bool wait_for(unique_lock<mutex>& lock, const chrono::duration<Rep, Period>& rel_time, Predicate pred);[..]
34 Effects: as ifreturn wait_until(lock, chrono::steady_clock::now() + rel_time, std::move(pred));
Executes a loop: Within the loop the function first evaluatespred()
and exits the loop if the result istrue
.Atomically callslock.unlock()
and blocks on*this
.When unblocked, callslock.lock()
(possibly blocking on the lock).The function will unblock when signaled by a call tonotify_one()
or a call tonotify_all()
, by the elapsed timerel_time
passing (30.2.4), or spuriously.If the function exits via an exception,lock.lock()
shall be called prior to exiting the function scope.The loop terminates whenpred()
returnstrue
or when the time duration specified byrel_time
has elapsed.- Change 32.7.5 [thread.condition.condvarany] paragraph 20,
wait_for
Effects as indicated:template <class Lock, class Rep, class Period> cv_status wait_for(Lock& lock, const chrono::duration<Rep, Period>& rel_time);20 Effects: as if
return wait_until(lock, chrono::steady_clock::now() + rel_time);
Atomically callslock.unlock()
and blocks on*this
.When unblocked, callslock.lock()
(possibly blocking on the lock), then returns.The function will unblock when signaled by a call tonotify_one()
or a call tonotify_all()
, by the elapsed timerel_time
passing (30.2.4), or spuriously.If the function exits via an exception,lock.unlock()
shall be called prior to exiting the function scope.- Change 32.7.5 [thread.condition.condvarany] paragraph 28,
wait_for
with predicate Effects as indicated:template <class Lock, class Rep, class Period, class Predicate> bool wait_for(Lock& lock, const chrono::duration<Rep, Period>& rel_time, Predicate pred);28 Effects: as if
return wait_until(lock, chrono::steady_clock::now() + rel_time, std::move(pred));
Executes a loop: Within the loop the function first evaluatespred()
and exits the loop if the result istrue
.Atomically callslock.unlock()
and blocks on*this
.When unblocked, callslock.lock()
(possibly blocking on the lock).The function will unblock when signaled by a call tonotify_one()
or a call tonotify_all()
, by the elapsed timerel_time
passing (30.2.4), or spuriously.If the function exits via an exception,lock.unlock()
shall be called prior to exiting the function scope.The loop terminates whenpred()
returnstrue
or when the time duration specified byrel_time
has elapsed.
Proposed resolution:
Resolved by n3191.
Section: 32.10 [futures] Status: Resolved Submitter: INCITS Opened: 2010-08-25 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
Addresses US-194
The specification for managing associated asynchronous state is confusing, sometimes omitted, and redundantly specified.
[ Resolution proposed by ballot comment: ]
Define terms-of-art for releasing, making ready, and abandoning an associated asynchronous state. Use those terms where appropriate. See Appendix 1 - Additional Details
Proposed resolution:
Resolved in Batavia by accepting n3192.
Section: 32.10.5 [futures.state] Status: Resolved Submitter: INCITS Opened: 2010-08-25 Last modified: 2016-01-28
Priority: Not Prioritized
View all other issues in [futures.state].
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Discussion:
Addresses US-195
The intent and meaning of 32.10.5 [futures.state] p10 is not apparent.
10 Accesses to the same shared state conflict (1.10).
[ 2011-03-07 Jonathan Wakely adds: ]
It's not clear which paragraph this refers to, I had to go to the ballot comments where US-195 reveals it's para 8, which in the FCD (N3092) says:
Accesses to the same associated asynchronous state conflict (1.10).
This is now para 10 in N3242:
Accesses to the same shared state conflict (1.10).
[2011-03-07: Lawrence comments and drafts wording]
The intent of this paragraph is to deal with operations,
such as shared_future::get()
, that return a reference
to a value held in the shared state. User code could potentially
conflict when accessing that value.
Lawrence proposed resolution:
Modify 32.10.5 [futures.state] p10 as follows:
10
Accesses to the same shared state conflict (6.9.2 [intro.multithread]).Some operations, e.g.shared_future::get()
( [futures.shared_future]), may return a reference to a value held in their shared state. Accesses and modifications through those references by concurrent threads to the same shared state may potentially conflict (6.9.2 [intro.multithread]). [Note: As a consequence, accesses must either use read-only operations or provide additional synchronization. — end note]
[2011-03-19: Detlef suggests an alternative resolution, shown below.]
Proposed Resolution
Modify 32.10.5 [futures.state] p10 as follows:
10 Accesses to the same shared state conflict (6.9.2 [intro.multithread]). [Note: This explicitely specifies that the shared state is visible in the objects that reference this state in the sense of data race avoidance 16.4.6.10 [res.on.data.races]. — end note]
Proposed resolution:
Resolved 2011-03 Madrid meeting by paper N3278
Section: 32.10.6 [futures.promise] Status: Resolved Submitter: INCITS Opened: 2010-08-25 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
Addresses US-196
The term "are serialized" is not defined (32.10.6 [futures.promise] p. 21, 25).
[ Resolution proposed by ballot comment: ]
Replace "are serialized" with "shall not introduce a data race (17.6.4.8)".
[ 2010-11-02 Daniel translates proposal into proper wording changes ]
[2011-03-19: Detlef comments]
The proposed resolution for 1507(i) would cover this issue as well.
[Proposed Resolution]
- Change 32.10.6 [futures.promise] p. 21 as indicated:
21 Synchronization: calls to
set_value
andset_exception
on a singlepromise
objectare serializedshall not introduce a data race ([res.on.data.races]). [ Note: and they synchronize and serialize with other functions through the referred associated asynchronous state. — end note ]- Change 32.10.6 [futures.promise] p. 25 as indicated:
25 Synchronization: calls to
set_value
andset_exception
on a singlepromise
objectare serializedshall not introduce a data race ([res.on.data.races]). [ Note: and they synchronize and serialize with other functions through the referred associated asynchronous state. — end note ]
Proposed resolution:
Resolved 2001-03 Madrid by issue 1507.
promise::set_value
and future::get
Section: 32.10.6 [futures.promise] Status: Resolved Submitter: INCITS Opened: 2010-08-25 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
Addresses US-197
There is no defined synchronization between promise::set_value
and future::get
(32.10.6 [futures.promise] p. 21, 25).
[ Resolution proposed by ballot comment: ]
Replace "[Note: and they synchronize and serialize with other functions through the referred associated asynchronous state. — end note]" with the normative "They synchronize with (1.10) any operation on a future object with the same associated asynchronous state marked ready."
[ 2010-11-02 Daniel translates proposal into proper wording changes ]
[2011-03-19: Detlef comments]
The proposed resolution for 1507(i) would cover this issue as well. Effectively it will reject the request but a clarification is added that the normative wording is already in 32.10.5 [futures.state].
- Change 32.10.6 [futures.promise] p. 21 as indicated:
21 Synchronization: calls to
set_value
andset_exception
on a singlepromise
object are serialized.[ Note: and they synchronize and serialize with other functions through the referred associated asynchronous state. — end note ]They synchronize with ([intro.multithread]) any operation on a future object with the same associated asynchronous state marked ready.- Change 32.10.6 [futures.promise] p. 25 as indicated:
25 Synchronization: calls to
set_value
andset_exception
on a singlepromise
object are serialized.[ Note: and they synchronize and serialize with other functions through the referred associated asynchronous state. — end note ]They synchronize with ([intro.multithread]) any operation on a future object with the same associated asynchronous state marked ready.
Proposed resolution:
Resolved 2001-03 Madrid by issue 1507.
promise::XXX_at_thread_exit
functions have no
synchronization requirementsSection: 32.10.6 [futures.promise] Status: Resolved Submitter: INCITS Opened: 2010-08-25 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
Addresses US-199
promise::XXX_at_thread_exit
functions have no
synchronization requirements. Specifying synchronization
for these member functions requires coordinating with the
words in 32.10.6 [futures.promise]/21 and 25, which give synchronization
requirements for promise::set_value
and
promise::set_exception
(32.10.6 [futures.promise] p. 26 ff., p. 29 ff.).
[ Resolution proposed by ballot comment: ]
Change 32.10.6 [futures.promise]/21 to mention
set_value_at_thread_exit
andset_exception_at_thread_exit;
with this text, replace 32.10.6 [futures.promise]/25 and add two new paragraphs, after 32.10.6 [futures.promise]/28 and 32.10.6 [futures.promise]/31.
[2011-03-8: Lawrence comments and drafts wording]
This comment applies as well to other *_at_thread_exit
functions. The following resolution adds synchronization paragraphs
to all of them and edits a couple of related synchronization
paragraphs.
[2011-03-09: Hans and Anthony add some improvements]
[2011-03-19: Detlef comments]
In regard to the suggested part:
These operations do not provide any ordering guarantees with respect to other operations, except through operations on futures that reference the same shared state.
I would like this to change to:
These operations do not provide any ordering guarantees with respect to other operations on the same promise object. [Note: They synchronize with calls to operations on objects that refer to the same shared state according to 32.10.5 [futures.state]. — end note]
The current proposed resolution has exactly the same paragraph at for places. I propose to have it only once as new paragraph 2.
This also covers 1504(i) (US-196) and 1505(i) (US-197). US-197 is essentially rejected with this resolution, but a clarification is added that the normative wording is already in 32.10.5 [futures.state].
Proposed Resolution
Edit 32.6.4.2 [thread.mutex.requirements.mutex] paragraph 5 as follows:
5 The implementation shall provide lock and unlock operations, as described below.
The implementation shall serialize those operations.For purposes of determining the existence of a data race, these behave as atomic operations (6.9.2 [intro.multithread]). The lock and unlock operations on a single mutex shall appear to occur in a single total order. [Note: this can be viewed as the modification order (6.9.2 [intro.multithread]) of the mutex. — end note] [ Note: Construction and destruction of an object of a mutex type need not be thread-safe; other synchronization should be used to ensure that mutex objects are initialized and visible to other threads. — end note ]Edit 32.7 [thread.condition] paragraphs 6-9 as follows:
void notify_all_at_thread_exit(condition_variable& cond, unique_lock<mutex> lk);-6- Requires:
lk
is locked by the calling thread and either
- no other thread is waiting on
cond
, orlk.mutex()
returns the same value for each of the lock arguments supplied by all concurrently waiting (viawait
,wait_for
, orwait_until
) threads.-7- Effects: transfers ownership of the lock associated with
lk
into internal storage and schedulescond
to be notified when the current thread exits, after all objects of thread storage duration associated with the current thread have been destroyed. This notification shall be as iflk.unlock(); cond.notify_all();-?- Synchronization: The call to
-8- Note: The supplied lock will be held until the thread exits, and care must be taken to ensure that this does not cause deadlock due to lock ordering issues. After callingnotify_all_at_thread_exit
and the completion of the destructors for all the current thread's variables of thread storage duration synchronize with (6.9.2 [intro.multithread]) calls to functions waiting oncond
.notify_all_at_thread_exit
it is recommended that the thread should be exited as soon as possible, and that no blocking or time-consuming tasks are run on that thread. -9- Note: It is the user's responsibility to ensure that waiting threads do not erroneously assume that the thread has finished if they experience spurious wakeups. This typically requires that the condition being waited for is satisfied while holding the lock onlk
, and that this lock is not released and reacquired prior to callingnotify_all_at_thread_exit
.Edit 32.10.6 [futures.promise], paragraphs 14-27 as follows:
void promise::set_value(const R& r); void promise::set_value(R&& r); void promise<R&>::set_value(R& r); void promise<void>::set_value();-14- Effects: atomically stores the value
-15- Throws:r
in the shared state and makes that state ready (32.10.5 [futures.state]).
future_error
if its shared state already has a stored value or exception, or- for the first version, any exception thrown by the copy constructor of
R
, or- for the second version, any exception thrown by the move constructor of
R
.-16- Error conditions:
promise_already_satisfied
if its shared state already has a stored value or exception.no_state
if*this
has no shared state.-17- Synchronization:
calls toFor purposes of determining the existence of a data race,set_value
andset_exception
on a singlepromise
object are serialized. [ Note: And they synchronize and serialize with other functions through the referred shared state. — end note ]set_value
,set_exception
,set_value_at_thread_exit
, andset_exception_at_thread_exit
behave as atomic operations (6.9.2 [intro.multithread]) on the memory location associated with thepromise
. Calls to these operations on a single promise shall appear to occur in a single total order. [Note: this can be viewed as the modification order (6.9.2 [intro.multithread]) of the promise. — end note] These operations do not provide any ordering guarantees with respect to other operations, except through operations on futures that reference the same shared state.void set_exception(exception_ptr p);-18- Effects: atomically stores the exception pointer
p
in the shared state and makes that state ready (32.10.5 [futures.state]).-19- Throws:
future_error
if its shared state already has a stored value or exception.-20- Error conditions:
promise_already_satisfied
if its shared state already has a stored value or exception.no_state
if*this
has no shared state.-21- Synchronization:
calls toFor purposes of determining the existence of a data race,set_value
andset_exception
on a singlepromise
object are serialized. [ Note: And they synchronize and serialize with other functions through the referred shared state. — end note ]set_value
,set_exception
,set_value_at_thread_exit
, andset_exception_at_thread_exit
behave as atomic operations (6.9.2 [intro.multithread]) on the memory location associated with thepromise
. Calls to these operations on a single promise shall appear to occur in a single total order. [Note: this can be viewed as the modification order (6.9.2 [intro.multithread]) of the promise. — end note] These operations do not provide any ordering guarantees with respect to other operations, except through operations on futures that reference the same shared state.void promise::set_value_at_thread_exit(const R& r); void promise::set_value_at_thread_exit(R&& r); void promise<R&>::set_value_at_thread_exit(R& r); void promise<void>::set_value_at_thread_exit();-22- Effects: Stores the value
-23- Throws:r
in the shared state without making that state ready immediately. Schedules that state to be made ready when the current thread exits, after all objects of thread storage duration associated with the current thread have been destroyed.future_error
if an error condition occurs. -24- Error conditions:
promise_already_satisfied
if its shared state already has a stored value or exception.no_state
if*this
has no shared state.-??- Synchronization: For purposes of determining the existence of a data race,
set_value
,set_exception
,set_value_at_thread_exit
, andset_exception_at_thread_exit
behave as atomic operations (6.9.2 [intro.multithread]) on the memory location associated with thepromise
. Calls to these operations on a single promise shall appear to occur in a single total order. [Note: this can be viewed as the modification order (6.9.2 [intro.multithread]) of the promise. — end note] These operations do not provide any ordering guarantees with respect to other operations, except through operations on futures that reference the same shared state.void promise::set_exception_at_thread_exit(exception_ptr p);-25- Effects: Stores the exception pointer
-26- Throws:p
in the shared state without making that state ready immediately. Schedules that state to be made ready when the current thread exits, after all objects of thread storage duration associated with the current thread have been destroyed.future_error
if an error condition occurs. -27- Error conditions:
promise_already_satisfied
if its shared state already has a stored value or exception.no_state
if*this
has no shared state.-??- Synchronization: For purposes of determining the existence of a data race,
set_value
,set_exception
,set_value_at_thread_exit
, andset_exception_at_thread_exit
behave as atomic operations (6.9.2 [intro.multithread]) on the memory location associated with thepromise
. Calls to these operations on a single promise shall appear to occur in a single total order. [Note: this can be viewed as the modification order (6.9.2 [intro.multithread]) of the promise. — end note] These operations do not provide any ordering guarantees with respect to other operations, except through operations on futures that reference the same shared state.Edit 32.10.10.2 [futures.task.members], paragraph 15-21 as follows:
void operator()(ArgTypes... args);-15- Effects:
-16- Throws: aINVOKE(f, t1, t2, ..., tN, R)
, wheref
is the stored task of*this
andt1
,t2
,...
,tN
are the values inargs...
. If the task returns normally, the return value is stored as the asynchronous result in the shared state of*this
, otherwise the exception thrown by the task is stored. The shared state of*this
is made ready, and any threads blocked in a function waiting for the shared state of*this
to become ready are unblocked.future_error
exception object if there is no shared state or the stored task has already been invoked. -17- Error conditions:
promise_already_satisfied
if the shared state is already ready.no_state
if*this
has no shared state.-18- Synchronization: a successful call to
operator()
synchronizes with (6.9.2 [intro.multithread]) a call to any member function of afuture
orshared_future
object that shares the shared state of*this
. The completion of the invocation of the stored task and the storage of the result (whether normal or exceptional) into the shared state synchronizes with (6.9.2 [intro.multithread]) the successful return from any member function that detects that the state is set to ready. [ Note:operator()
synchronizes and serializes with other functions through the shared state. — end note ]void make_ready_at_thread_exit(ArgTypes... args);-19- Effects:
-20- Throws:INVOKE(f, t1, t2, ..., tN, R)
, wheref
is the stored task andt1
,t2
,...
,tN
are the values inargs...
. If the task returns normally, the return value is stored as the asynchronous result in the shared state of*this
, otherwise the exception thrown by the task is stored. In either case, this shall be done without making that state ready (32.10.5 [futures.state]) immediately. Schedules the shared state to be made ready when the current thread exits, after all objects of thread storage duration associated with the current thread have been destroyed.future_error
if an error condition occurs. -21- Error conditions:
promise_already_satisfied
if the shared state already has a stored value or exception.no_state
if*this
has no shared state.-??- Synchronization: a successful call to
make_ready_at_thread_exit
synchronizes with (6.9.2 [intro.multithread]) a call to any member function of afuture
orshared_future
object that shares the shared state of*this
. The completion of
the invocation of the stored task and the storage of the result (whether normal or exceptional) into the shared state
the destructors for all the current thread's variables of thread storage duration
synchronize with (6.9.2 [intro.multithread]) the successful return from any member function that detects that the state is set to ready. [Note:
make_ready_at_thread_exit
synchronizes and serializes with other functions through the shared state. — end note]
Proposed resolution:
Resolved 2011-03 Madrid meeting by paper N3278
packaged_task::operator bool()
Section: 32.10.10 [futures.task] Status: Resolved Submitter: INCITS Opened: 2010-08-25 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
Addresses US-201
packaged_task
provides operator bool()
to check whether
an object has an associated asynchronous state. The various future
types provide a member function valid()
that does the same thing.
The names of these members should be the same.
[ Resolution proposed by ballot comment: ]
Replaced the name
packaged_task::operator bool()
withpackaged_task::valid()
in the synopsis (32.10.10 [futures.task]/2) and the member function specification (before 32.10.10.2 [futures.task.members]/15).
[
2010-11-02 Daniel translates proposed wording changes into a proper proposed resolution
and verified that no other places implicitly take advantage of packaged_task
conversion to bool.
]
[Resolved in Batavia by accepting n3194. ]
Proposed resolution:
packaged_task
synopsis as indicated:
template<class R, class... ArgTypes> class packaged_task<R(ArgTypes...)> { public: typedef R result_type; [..]explicit operatorbool valid() const; [..] };
explicit operatorbool valid() const;15 Returns: true only if
16 Throws: nothing.*this
has an associated asynchronous state.
Section: 32.10 [futures] Status: Resolved Submitter: Switzerland Opened: 2010-08-25 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
Addresses CH-36
Providing only three different possible values for the enum
launch
and saying that launch::any
means either
launch::sync
or launch::async
is very restricting. This
hinders future implementors to provide clever
infrastructures that can simply by used by a call to
async(launch::any,...)
. Also there is no hook for an
implementation to provide additional alternatives to launch
enumeration and no useful means to combine those (i.e.
interpret them like flags). We believe something like
async(launch::sync | launch::async, ...)
should be allowed
and can become especially useful if one could say also
something like async(launch::any & ~launch::sync, ....)
respectively. This flexibility might limit the features usable
in the function called through async()
, but it will allow a
path to effortless profit from improved hardware/software
without complicating the programming model when just
using async(launch::any,...)
[ Resolution proposed by ballot comment: ]
Change in 32.10.1 [futures.overview] 'enum class launch' to allow
further implementation defined values and provide
the following bit-operators on the launch values
(operator|
, operator&
, operator~
delivering a
launch
value).
launch
enums,
but we shouldn't limit the standard to just 32 or 64
available bits in that case and also should keep
the launch enums in their own enum namespace.
Change [future.async] p3 according to the
changes to enum launch
. change --launch::any
to
"the implementation may choose any of the
policies it provides." Note: this can mean that an
implementation may restrict the called function to
take all required information by copy in case it will
be called in a different address space, or even, on
a different processor type. To ensure that a call is
either performed like launch::async
or
launch::sync
describe one should call
async(launch::sync|launch::async,...)
[ 2010-11-02 Daniel comments: ]
The new paper n3113 provides concrete wording.
Proposed resolution:
Resolved by n3188.
packaged_task
constructors need reviewSection: 32.10.10.2 [futures.task.members] Status: C++11 Submitter: INCITS Opened: 2010-08-25 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
Addresses US-207
The constructor that takes R(*)(ArgTypes...)
is not
needed; the constructor that takes a callable type works
for this argument type. More generally, the constructors
for packaged_task should parallel those for function.
[ US-207 Suggested Resolution: ]
Review the constructors for packaged_task and provide the same ones as function, except where inappropriate.
[ 2010-10-22 Howard provides wording, as requested by the LWG in Rapperswil. ]
[2011-02-10 Reflector discussion]
Moved to Tentatively Ready after 5 votes.
Proposed resolution:
Alter the list of constructors in both 32.10.10 [futures.task] and in 32.10.10.2 [futures.task.members] as indicated:
template <class F> explicit packaged_task(F f); template <class F, class Allocator> explicit packaged_task(allocator_arg_t, const Allocator& a, F f); explicit packaged_task(R(*f)(ArgTypes...));template <class F> explicit packaged_task(F&& f); template <class F, class Allocator> explicit packaged_task(allocator_arg_t, const Allocator& a, F&& f);
packaged_task::make_ready_at_thread_exit
has no synchronization requirementsSection: 32.10.10.2 [futures.task.members] Status: Resolved Submitter: INCITS Opened: 2010-08-25 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
Addresses US-208
packaged_task::make_ready_at_thread_exit
has no synchronization requirements.
[ Resolution proposed by ballot comment: ]
Figure out what the synchronization requirements should be and write them.
[2011-02-09 Anthony provides a proposed resolution]
[2011-02-19 Additional edits by Hans, shown in the proposed resolution section]
[2011-02-22 Reflector discussion]
Moved to Tentatively Ready after 5 votes.
Proposed Resolution
Add a new paragraph following 32.10.10.2 [futures.task.members] p. 19:
void make_ready_at_thread_exit(ArgTypes... args);19 - ...
?? - Synchronization: Following a successful call to
make_ready_at_thread_exit
, the destruction of all objects with thread storage duration associated with the current thread happens before the associated asynchronous state is made ready. The marking of the associated asynchronous state as ready synchronizes with (6.9.2 [intro.multithread]) the successful return from any function that detects that the state is set to ready.
Proposed resolution:
Resolved 2011-03 Madrid meeting by paper N3278
auto_ptr
Section: 99 [depr.auto.ptr] Status: C++11 Submitter: BSI Opened: 2010-08-25 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
Addresses GB-142
auto_ptr
does not appear in the <memory>
synopsis and
[depr.auto.ptr] doesn't say which header declares it.
Conversely, the deprecated binders bind1st
etc. are in the
<functional>
synopsis, this is inconsistent
Either auto_ptr
should be declared in the
<memory>
synopsis, or the deprecated binders
should be removed from the <functional>
synopsis
and appendix D should say which header declares
the binders and auto_ptr
.
[ Post-Rapperswil ]
Moved to Tentatively Ready after 5 positive votes on c++std-lib.
[ Adopted at 2010-11 Batavia ]
Proposed resolution:
Add the following lines to the synopsis of header <memory>
in [memory]/1:
// [depr.auto.ptr], Class auto_ptr (deprecated): template <class X> class auto_ptr;
Section: 20.3.1.2.2 [unique.ptr.dltr.dflt] Status: C++11 Submitter: Daniel Krügler Opened: 2010-09-12 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
The current working draft does specify the default c'tor of default_delete
in a manner
to guarantee static initialization for default-constructed objects of static storage duration
as a consequence of the acceptance of the proposal n2976
but this paper overlooked the fact that the suggested declaration does not ensure that the type
will be a trivial type. The type default_delete
was always considered as a simple wrapper for
calling delete
or delete[]
, respectivly and should be a trivial type.
In agreement with the new settled core language rules this easy to realize by just changing the declaration to
constexpr default_delete() = default;
This proposal also automatically solves the problem, that the semantics of the default constructor of the
partial specialization default_delete<T[]>
is not specified at all. By defaulting its default constructor
as well, the semantics are well-defined.
[ Post-Rapperswil ]
Moved to Tentatively Ready after 5 positive votes on c++std-lib.
[ Adopted at 2010-11 Batavia ]
Proposed resolution:
The following wording changes are against N3126.
default_delete
in [unique.ptr.dltr.dflt] as indicated:
namespace std { template <class T> struct default_delete { constexpr default_delete() = default; template <class U> default_delete(const default_delete<U>&); void operator()(T*) const; }; }
default_delete
default constructor in [unique.ptr.dltr.dflt]/1. This
brings it in harmony with the style used in the partial specialization default_delete<T[]>
. Since there are
neither implied nor explicit members, there is no possibility to misinterpret what the constructor does:
constexpr default_delete();
1 Effects: Default constructs adefault_delete
object.
default_delete
in [unique.ptr.dltr.dflt1] as indicated:
namespace std { template <class T> struct default_delete<T[]> { constexpr default_delete() = default; void operator()(T*) const; template <class U> void operator()(U*) const = delete; }; }
Section: 32.10 [futures] Status: C++11 Submitter: Alberto Ganesh Barbati Opened: 2010-09-14 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
The current WP N3126 contains ambiguous statements about the
behaviour of functions wait_for
/wait_until
in
case the future refers to a deferred function. Moreover, I believe
it describes a disputable intent, different from the one contained
in the original async proposals, that may have been introduced
inadvertently during the "async cleanup" that occurred recently.
Consider the following case:
int f(); future<int> x = async(launch::deferred, f); future_status s = x.wait_for(chrono::milliseconds(100));
This example raises two questions:
f
invoked?s
?According to the current WP, the answer to question 1 is yes,
because 30.6.9/3 says "The first call to a function waiting for the
associated asynchronous state created by this async call to become
ready shall invoke the deferred function in the thread that called
the waiting function". The answer to question 2, however, is not as
clear. According to 30.6.6/23, s should be
future_status::deferred
because x
refers to a
deferred function that is not running, but it should also be
future_status::ready
because after executing f
(and we saw that f
is always executed) the state becomes
ready. By the way, the expression "deferred function that is not
running" is very unfortunate in itself, because it may apply to
both the case where the function hasn't yet started, as well as the
case where it was executed and completed.
While we clearly have a defect in the WP answering to question
2, it is my opinion that the answer to question 1 is wrong, which
is even worse. Consider that the execution of the function
f
can take an arbitrarily long time. Having
wait_for()
invoke f
is a potential violation of
the reasonable expectation that the execution of
x.wait_for(chrono::milliseconds(100))
shall take at most
100 milliseconds plus a delay dependent on the quality of implementation
and the quality of management (as described in paper N3128).
In fact, previous versions of the WP
clearly specified that only function wait()
is required to
execute the deferred function, while wait_for()
and
wait_until()
shouldn't.
The proposed resolution captures the intent that
wait_for()
and wait_until()
should never attempt
to invoke the deferred function. In other words, the P/R provides
the following answers to the two questions above:
future_status::deferred
In order to simplify the wording, the definition of deferred function has been tweaked so that the function is no longer considered deferred once its evaluation has started, as suggested by Howard.
Discussions in the reflector questioned whether
wait_for()
and wait_until()
should return
immediately or actually wait hoping for a second thread to execute
the deferred function. I believe that waiting could be useful only
in a very specific scenario but detrimental in the general case and
would introduce another source of ambiguity: should
wait_for()
return future_status::deferred
or
future_status::timeout
after the wait? Therefore the P/R
specifies that wait_for
/wait_until
shall return
immediately, which is simpler, easier to explain and more useful in
the general case.
[ Post-Rapperswil ]
Moved to Tentatively Ready after 5 positive votes on c++std-lib.
[ Adopted at 2010-11 Batavia ]
Proposed resolution:
The proposed wording changes are relative to the Final Committee Draft, N3126.
Note to the editor: the proposed wording is meant not be in conflict with any change proposed by paper N3128 "C++ Timeout Specification". Ellipsis are deliberately used to avoid any unintended overlapping.
In [futures.unique_future] 30.6.6/22:
Effects: none if the associated asynchronous state contains a deferred function (30.6.9), otherwise blocks until the associated asynchronous state is ready or [...].
In [futures.unique_future] 30.6.6/23 first bullet:
— future_status::deferred if the associated asynchronous
state contains a deferred function that is not
running.
In [futures.unique_future] 30.6.6/25:
Effects: none if the associated asynchronous state contains a deferred function (30.6.9), otherwise blocks until the associated asynchronous state is ready or [...].
In [futures.unique_future] 30.6.6/26 first bullet:
— future_status::deferred if the associated asynchronous
state contains a deferred function that is not
running.
In [futures.shared_future] 30.6.7/27
Effects: none if the associated asynchronous state contains a deferred function (30.6.9), otherwise blocks until the associated asynchronous state is ready or [...].
In [futures.unique_future] 30.6.7/28 first bullet:
— future_status::deferred if the associated asynchronous
state contains a deferred function that is not
running.
In [futures.shared_future] 30.6.6/30:
Effects: none if the associated asynchronous state contains a deferred function (30.6.9), otherwise blocks until the associated asynchronous state is ready or [...].
In [futures.unique_future] 30.6.7/31 first bullet:
— future_status::deferred if the associated asynchronous
state contains a deferred function that is not
running.
In [futures.atomic_future] 30.6.8/23
Effects: none if the associated asynchronous state contains a deferred function (30.6.9), otherwise blocks until the associated asynchronous state is ready or [...].
In [futures.unique_future] 30.6.8/24 first bullet:
— future_status::deferred if the associated asynchronous
state contains a deferred function that is not
running.
In [futures.atomic_future] 30.6.8/27:
Effects: none if the associated asynchronous state contains a deferred function (30.6.9), otherwise blocks until the associated asynchronous state is ready or [...].
In [futures.unique_future] 30.6.8/28 first bullet:
— future_status::deferred if the associated asynchronous
state contains a deferred function that is not
running.
In [futures.async] 30.6.9/3 second bullet:
[...] The first call to a function
waitingrequiring a non-timed wait for the
associated asynchronous state created by this async call to become
ready shall invoke the deferred function in the thread that called
the waiting function; once evaluation of INVOKE(g,
xyz)
begins, the function is no longer considered
deferred all other calls waiting for the same associated
asynchronous state to become ready shall block until the deferred
function has completed.
Section: 23.5.3 [unord.map], 23.5.4 [unord.multimap], 23.5.7 [unord.multiset] Status: C++11 Submitter: Nicolai Josuttis Opened: 2010-10-09 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
While bucket_size()
is const for unordered_set
, for all other unordered containers it is not defined as
constant member function.
[ Post-Rapperswil ]
Moved to Tentatively Ready after 5 positive votes on c++std-lib.
[ Adopted at 2010-11 Batavia ]
Proposed resolution:
The wording refers to N3126.
namespace std { template <class Key, class T, class Hash = hash<Key>, class Pred = std::equal_to<Key>, class Alloc = std::allocator<std::pair<const Key, T> > > class unordered_map { public: [..] // bucket interface size_type bucket_count() const; size_type max_bucket_count() const; size_type bucket_size(size_type n) const; [..]
namespace std { template <class Key, class T, class Hash = hash<Key>, class Pred = std::equal_to<Key>, class Alloc = std::allocator<std::pair<const Key, T> > > class unordered_multimap { public: [..] // bucket interface size_type bucket_count() const; size_type max_bucket_count() const; size_type bucket_size(size_type n) const; [..]
namespace std { template <class Key, class Hash = hash<Key>, class Pred = std::equal_to<Key>, class Alloc = std::allocator<Key> > class unordered_multiset { public: [..] // bucket interface size_type bucket_count() const; size_type max_bucket_count() const; size_type bucket_size(size_type n) const; [..]
INVOKE
on member data pointer with too many argumentsSection: 22.10.4 [func.require] Status: C++11 Submitter: Howard Hinnant Opened: 2010-10-10 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
20.8.2 [func.require] p1 says:
1 Define
INVOKE(f, t1, t2, ..., tN)
as follows:
(t1.*f)(t2, ..., tN)
whenf
is a pointer to a member function of a classT
andt1
is an object of typeT
or a reference to an object of typeT
or a reference to an object of a type derived fromT
;((*t1).*f)(t2, ..., tN)
whenf
is a pointer to a member function of a classT
andt1
is not one of the types described in the previous item;t1.*f
whenf
is a pointer to member data of a classT
andt1
is an object of typeT
or a reference to an object of typeT
or a reference to an object of a type derived fromT
;(*t1).*f
whenf
is a pointer to member data of a classT
andt1
is not one of the types described in the previous item;f(t1, t2, ..., tN)
in all other cases.
The question is: What happens in the 3rd and
4th bullets when N > 1
?
Does the presence of t2, ..., tN
get ignored, or does it make the
INVOKE
ill formed?
Here is sample code which presents the problem in a concrete example:
#include <functional> #include <cassert> struct S { char data; }; typedef char S::*PMD; int main() { S s; PMD pmd = &S::data; std::reference_wrapper<PMD> r(pmd); r(s, 3.0) = 'a'; // well formed? assert(s.data == 'a'); }
Without the "3.0
" the example is well formed.
[Note: Daniel provided wording to make it explicit that the above example is ill-formed. — end note ]
[ Post-Rapperswil ]
Moved to Tentatively Ready after 5 positive votes on c++std-lib.
[ Adopted at 2010-11 Batavia ]
Proposed resolution:
The wording refers to N3126.
Change 20.8.2 [func.require]/1 as indicated:
1 Define
INVOKE(f, t1, t2, ..., tN)
as follows:
- ...
- ...
t1.*f
whenN == 1
andf
is a pointer to member data of a classT
andt1
is an object of typeT
or a reference to an object of typeT
or a reference to an object of a type derived fromT
;(*t1).*f
whenN == 1
andf
is a pointer to member data of a classT
andt1
is not one of the types described in the previous item;- ...
conj
specification is now nonsenseSection: 29.4.10 [cmplx.over] Status: C++11 Submitter: P.J. Plauger Opened: 2010-10-14 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
In Pittsburgh, we accepted the resolution of library issue 1137(i), to add a sentence 3 to [cmplx.over]:
All the specified overloads shall have a return type which is the nested
value_type
of the effectively cast arguments.
This was already true for four of the six functions except conj
and
proj
. It is not completely unreasonable to make proj
return
the real value only, but the IEC specification does call for an imaginary part
of -0 in some circumstances. The people who care about these distinctions really
care, and it is required by an international standard.
Making conj
return just the real part breaks it horribly, however. It is
well understood in mathematics that conj(re + i*im)
is (re - i*im)
,
and it is widely used. The accepted new definition makes conj
useful only
for pure real operations. This botch absolutely must be fixed.
[ 2010 Batavia: The working group concurred with the issue's Proposed Resolution ]
[ Adopted at 2010-11 Batavia ]
Proposed resolution:
Remove the recently added paragraph 3 from [cmplx.over]:
3 All the specified overloads shall have a return type which is the nestedvalue_type
of the effectively cast arguments.
noexcept
for Clause 29Section: 32.5 [atomics] Status: Resolved Submitter: Hans Boehm Opened: 2010-11-13 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
Addresses GB-63 for Clause 29
Clause 29 does not specify noexcept for any of the atomic operations. It probably should, though that's not completely clear. In particular, atomics may want to throw in implementations that support transactional memory.
Proposed resolution:
Apply paper N3251, noexcept for the Atomics Library.
Section: 17.6.3.5 [new.delete.dataraces] Status: C++11 Submitter: Hans Boehm Opened: 2011-02-26 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
Addresses US-34
Technical details:
When the same unit of storage is allocated and deallocated repeatedly, operations on it can't be allowed to race between the allocator and the user program. But I don't see any mention of happens-before in the descriptions of allocation and deallocation functions. Proposed resolution (not wording yet):The call to an allocation function returning a pointer P
must happen-before the matching
deallocation call with P
as a parameter. Otherwise the behavior is undefined. I don't know whether
receiving P
with memory_order_consume
fits this requirement. memory_order_relaxed
does not.
If some memory is passed to a deallocation function, the implementation must ensure that the deallocation call happens-before any allocation call that returns the same memory address.
[2011-02-26: Hans comments and drafts wording]
The second requirement already exists, almost verbatim, as 17.6.3.5 [new.delete.dataraces] p. 1. I think this is where the statement belongs. However, this paragraph requires work to correctly address the first part of the issue.
[Adopted at Madrid, 2011-03]
Proposed resolution:
Change 17.6.3.5 [new.delete.dataraces] p. 1 as follows:
1
The library versions ofFor purposes of determining the existence of data races, the library versions ofoperator new
andoperator delete
, user replacement versions of globaloperator new
andoperator delete
, and the C standard library functionscalloc
,malloc
,realloc
, andfree
shall not introduce data races (6.9.2 [intro.multithread]) as a result of concurrent calls from different threads.operator new
, user replacement versions of globaloperator new
, and the C standard library functionscalloc
andmalloc
shall behave as though they accessed and modified only the storage referenced by the return value. The library versions ofoperator delete
, user replacement versions ofoperator delete
, and the C standard library functionfree
shall behave as though they accessed and modified only the storage referenced by their first argument. The C standard libraryrealloc
function shall behave as though it accessed and modified only the storage referenced by its first argument and by its return value. Calls to these functions that allocate or deallocate a particular unit of storage shall occur in a single total order, and each such deallocation call shall happen before the next allocation (if any) in this order.
resize(size())
on a vector
Section: 23.3.13.3 [vector.capacity] Status: C++11 Submitter: BSI Opened: 2011-03-24 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
Addresses GB-117
23.3.13.3 [vector.capacity] p. 9 (Same as for 23.3.5.3 [deque.capacity] p. 1 i.e.
deque::resize
). There is no mention of what happens if sz==size()
. While
it obviously does nothing I feel a standard needs to say this explicitely.
Suggested resolution:
Append "If sz == size()
, does nothing" to the effects.
[2011-03-24 Daniel comments]
During the edit of this issue some non-conflicting overlap with 2033(i) became obvious.
CopyInsertable
should be MoveInsertable
and there is missing the DefaultConstructible
requirements, but this should be fixed by 2033(i).
Proposed resolution:
Change 23.3.13.3 [vector.capacity] p. 9 as follows:
void resize(size_type sz);9 Effects: If
10 Requires:sz <= size()
, equivalent toerase(begin() + sz, end());
. Ifsize() < sz
, appendssz - size()
value-initialized elements to the sequence.T
shall beCopyInsertable
into*this
.
Section: 16.4.6.10 [res.on.data.races] Status: Resolved Submitter: BSI Opened: 2011-03-24 Last modified: 2016-01-28
Priority: 3
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Discussion:
Addresses GB-111
Section 16.4.6.10 [res.on.data.races], Data Race Avoidance, requires the C++ Standard Library to avoid data races that might otherwise result from two threads making calls to C++ Standard Library functions on distinct objects. The C standard library is part of the C++ Standard Library and some C++ Standary library functions (parts of the Localization library, as well as Numeric Conversions in 21.5), are specified to make use of the C standard library. Therefore, the C++ standard indirectly imposes a requirement on the thread safety of the C standard library. However, since the C standard does not address the concept of thread safety conforming C implementations exist that do no provide such guarantees. This conflict needs to be reconciled.
Suggested resolution by national body comment:
remove the requirement to make use of
strtol()
andsprintf()
since these functions depend on the global C locale and thus cannot be made thread safe.
[2011-03-24 Madrid meeting]
Deferred
[ 2011 Bloomington ]
Alisdair: PJ, does this cause a problem in C?
PJ: Every implementation know of is thread safe.
Pete: There a couple of effects that are specified on strtol() and sprintf() which is a problem.
PJ: When C++ talks about C calls it should be "as if" calling the function.
Pete: Culprit is to string stuff. My fault.
PJ: Not your fault. You did what you were told. Distinct resolution to change wording.
Dietmar: What would we break if we change it back?
Pete: Nothing. If implemented on top of thread safe C library you are just fine.
Alisdair: Anyone want to clean up wording and put it back to what Pete gave us?
Alisdair: No volunteers. Do we want to mark as NAD? We could leave it as deferred.
Stefanus: Did original submitter care about this?
Lawrence: There is some work to make local calls thread safe. The resolution would be to call those thread safe version.
Pete: "As if called under single threaded C program"
Action Item (Alisdair): Write wording for this issue.
[2012, Kona]
Re-opened at the request of the concurrency subgroup, who feel there is an issue that needs clarifying for the (planned) 2017 standard.
Rationale:
No consensus to make a change at this time
[2012, Portland]
The concurrency subgroup decided to encourage the LWG to consider a change to 16.2 [library.c] or thereabouts
to clarify that we are requiring C++-like thread-safety for setlocale
, so that races are not introduced
by C locale accesses, even when the C library allows it. This would require e.g. adding "and data race avoidance"
at the end of 16.2 [library.c] p1:
"The C++ standard library also makes available the facilities of the C standard library, suitably adjusted to ensure static type safety and data race avoidance.",
with some further clarifications in the sections mentioned in 1526(i).
This seems to be consistent with existing implementations. This would technically not be constraining C implementation, but it would be further constraining C libraries used for both C and C++.
[Lenexa 2015-05-05: Move to Resolved]
JW: it's a bit odd that the issue title says sould not impose requirements on C libs, then the P/R does exactly that. Does make sense though, previously we imposed an implicit requirement which would not have been met. Now we say it explicitly and require it is met.
STL: I think this is Resolved, it has been fixed in the working paper [support.runtime]/6 is an example where we call out where things can race. That implies that for everything else they don't create races.
JW: I'm not sure, I think we still need the "and data race avoidance" to clarify that the features from C avoid races, even though C99 says no such thing.
STL: [library.c] says that something like sqrt is part of the C++ Standard LIbrary. [res.on.data.races] then applies to them. Would be OK with a note there, but am uncomfortable with "and data race avoidance" which sounds like it's making a very strong guarantee.
ACTION ITEM JW to editorially add note to [library.c] p1: "Unless otherwise specified, the C Standard Library functions shall meet the requirements for data race avoidance (xref [res.on.data.races])"
Move to Resolved?
10 in favor, 0 opposed, 3 abstentions
Proposed resolution:
This wording is relative to N3376.
Change 16.2 [library.c] p1 as indicated:
-1- The C++ standard library also makes available the facilities of the C standard library, suitably adjusted to ensure static type safety and data race avoidance.
packaged_task
specialization of uses_allocator
Section: 32.10.10.3 [futures.task.nonmembers] Status: C++11 Submitter: Howard Hinnant Opened: 2010-08-29 Last modified: 2016-01-28
Priority: Not Prioritized
View all issues with C++11 status.
Discussion:
[futures.task.nonmembers]/3 says:
template <class R, class Alloc> struct uses_allocator<packaged_task<R>, Alloc>;
This is a declaration, but should be a definition.
Proposed resolution:
Change [futures.task.nonmembers]/3:
template <class R, class Alloc> struct uses_allocator<packaged_task<R>, Alloc>;: true_type {};
basic_regex
uses non existent string_type
Section: 28.6.7.3 [re.regex.assign] Status: C++11 Submitter: Volker Lukas Opened: 2010-10-21 Last modified: 2016-01-28
Priority: Not Prioritized
View all other issues in [re.regex.assign].
View all issues with C++11 status.
Discussion:
In working draft N3126, subclause 28.6.7.3 [re.regex.assign], paragraphs 12, 13 and 19,
the name string_type
is used. This is presumably a typedef for basic_string<value_type>
, where
value_type
is the character type used by basic_regex
. The basic_regex
template however defines no such typedef, and neither does the <regex>
header or the <initializer_list>
header included by <regex>
.
[ 2010-11-03 Daniel comments and suggests alternative wording: ]
The proposed resolution needs to use
basic_string<charT>
instead ofbasic_string<char>
Previous Proposed Resolution:
Make the following changes to [re.regex.assign]:basic_regex& assign(const charT* ptr, flag_type f = regex_constants::ECMAScript);12 Returns:
assign(
.string_typebasic_string<charT>(ptr), f)basic_regex& assign(const charT* ptr, size_t len, flag_type f = regex_constants::ECMAScript);13 Returns:
assign(
.string_typebasic_string<charT>(ptr, len), f)[..] template <class InputIterator> basic_regex& assign(InputIterator first, InputIterator last, flag_type f = regex_constants::ECMAScript);18 Requires: The type
InputIterator
shall satisfy the requirements for an Input Iterator (24.2.3).19 Returns:
assign(
.string_typebasic_string<charT>(first, last), f)
[ 2010 Batavia ]
Unsure if we should just give basic_regex
a string_type
typedef. Looking for when string_type
was
introduced into regex
. Howard to draft wording for typedef typename traits::string_type string_type
, then move to Review.
[ 2011-02-16: Daniel comments and provides an alternative resolution. ]
I'm strongly in favour with the Batavia idea to provide a separate string_type
within
basic_regex
, but it seems to me that the issue resultion should add one more
important typedef, namely that of the traits type! Currently, basic_regex
is the
only template that does not publish the type of the associated traits type. Instead
of opening a new issue, I added this suggestion as part of the proposed wording.
[2011-02-24 Reflector discussion]
Moved to Tentatively Ready after 6 votes.
Proposed resolution:
Change the class template basic_regex
synopsis, 28.6.7 [re.regex] p. 3, as indicated:
namespace std { template <class charT, class traits = regex_traits<charT> > class basic_regex { public: // types: typedef charT value_type; typedef traits traits_type; typedef typename traits::string_type string_type; typedef regex_constants::syntax_option_type flag_type; typedef typename traits::locale_type locale_type; [..] }; }
match_results
does not specify the semantics of operator==
Section: 28.6.9.9 [re.results.nonmember] Status: Resolved Submitter: Daniel Krügler Opened: 2010-10-24 Last modified: 2016-01-28
Priority: Not Prioritized
View all issues with Resolved status.
Discussion:
The Returns element of operator==
says:
true
only if the two objects refer to the same match
It is not really clear what this means: The current specification would allow for an
implementation to return true
, only if the address values of m1
and
m2
are the same. While this approach is unproblematic in terms of used operations
this is also a bit unsatisfactory. With identity equality alone there seems to be no convincing
reason to provide this operator at all. It could for example also refer to an comparison based
on iterator values. In this case a user should better know that this will be done, because
there is no guarantee at all that inter-container comparison of iterators
is a feasible operation. This was a clear outcome of the resolution provided in
N3066
for LWG issue 446(i).
It could also mean that a character-based comparison of the individual sub_match
elements should be done - this would be equivalent to applying operator==
to
the subexpressions, prefix and suffix.
Proposed resolution:
Addressed by paper n3158.
Section: 27.4.3.2 [string.require] Status: C++14 Submitter: José Daniel García Sánchez Opened: 2010-10-21 Last modified: 2016-11-12
Priority: 0
View all other issues in [string.require].
View all issues with C++14 status.
Discussion:
Clause 21.4.1 [string.require]p3 states:
No
erase()
orpop_back()
member function shall throw any exceptions.
However in 21.4.6.5 [string.erase] p2 the first version of erase
has
Throws:
out_of_range
ifpos > size()
.
[2011-03-24 Madrid meeting]
Beman: Don't want to just change this, can we just say "unless otherwise specified"?
Alisdair: Leave open, but update proposed resolution to say something like "unless otherwise specified". General agreement that it should be corrected but not a stop-ship. Action: Update proposed wording for issue 2003 as above, but leave Open.[2014-02-12 Issaquah meeting]
Jeffrey: Madrid meeting's proposed wording wasn't applied, and it's better than the original proposed wording. However, this sentence is only doing 3 functions' worth of work, unlike the similar paragraphs in 23.2.2 [container.requirements.general]. Suggest just putting "Throws: Nothing" on the 3 functions.
[2014-02-13 Issaquah meeting]
Move as Immmediate
Proposed resolution:
Remove [string.require]p/3:
3 Noerase()
orpop_back()
member function shall throw any exceptions.
Add to the specifications of iterator erase(const_iterator p);
, iterator erase(const_iterator first, const_iterator last);
,
and void pop_back();
in 27.4.3.7.5 [string.erase]:
Throws: Nothing
duration::operator*
has template parameters in funny orderSection: 30.5.6 [time.duration.nonmember] Status: C++11 Submitter: P.J. Plauger Opened: 2010-10-14 Last modified: 2016-01-28
Priority: Not Prioritized
View all other issues in [time.duration.nonmember].
View all issues with C++11 status.
Discussion:
In [time] and [time.duration.nonmember] we have:
template <class Rep1, class Period, class Rep2> duration<typename common_type<Rep1, Rep2>::type, Period> operator*(const Rep1& s, const duration<Rep2, Period>& d);
Everywhere else, we always have <rep, period>
in that order for a given
type. But here, we have Period
and Rep2
in reverse order for
<Rep2, Period>
. This is probably of little importance, since the
template parameters are seldom spelled out for a function like this. But changing it
now will eliminate a potential source of future errors and confusion.
Proposed resolution:
Change the signature in [time] and [time.duration.nonmember] to:
template <class Rep1, classPeriodRep2, classRep2Period> duration<typename common_type<Rep1, Rep2>::type, Period> operator*(const Rep1& s, const duration<Rep2, Period>& d);
unordered_map::insert(T&&)
protection should apply to map
tooSection: 23.4.3.4 [map.modifiers], 23.4.4.3 [multimap.modifiers], 23.5.3.4 [unord.map.modifiers], 23.5.4.3 [unord.multimap.modifiers] Status: C++14 Submitter: P.J. Plauger Opened: 2010-10-14 Last modified: 2016-01-28
Priority: Not Prioritized
View all other issues in [map.modifiers].
View all issues with C++14 status.
Discussion:
In [unord.map.modifiers], the signature:
template <class P> pair<iterator, bool> insert(P&& obj);
now has an added Remarks paragraph:
Remarks: This signature shall not participate in overload resolution unless
P
is implicitly convertible tovalue_type
.
The same is true for unordered_multimap
.
map
nor multimap
have this constraint, even though it is a
Good Thing(TM) in those cases as well.
[ The submitter suggests: Add the same Remarks clause to [map.modifiers] and [multimap.modifiers]. ]
[ 2010-10-29 Daniel comments: ]
I believe both paragraphs need more cleanup: First, the current Requires element conflict with the Remark;
second, it seems to me that the whole single Requires element is intended to be split into a Requires
and an Effects element; third, the reference to tuple
is incorrect (noticed by Paolo Carlini);
fourth, it refers to some non-existing InputIterator
parameter relevant for a completely different
overload; sixth, the return type of the overload with hint is wrong.
The following proposed resolution tries to solve these issues as well and uses similar wording as for
the corresponding unordered containers. Unfortunately it has some redundancy over Table 99, but I did
not remove the specification because of the more general template parameter P
- the Table 99
requirements apply only for an argument identical to value_type
.
- Change 23.4.3.4 [map.modifiers] around p. 1 as indicated:
template <class P> pair<iterator, bool> insert(P&& x); template <class P>pair<iterator, bool>insert(const_iterator position, P&& x);1 Requires:
P
shall be convertible tovalue_type
is constructible fromstd::forward<P>(x)
..IfP
is instantiated as a reference type, then the argumentx
is copied from. Otherwisex
is considered to be an rvalue as it is converted tovalue_type
and inserted into the map. Specifically, in such casesCopyConstructible
is not required ofkey_type
ormapped_type
unless the conversion fromP
specifically requires it (e.g., ifP
is atuple<const key_type, mapped_type>
, thenkey_type
must beCopyConstructible
). The signature takingInputIterator
parameters does not requireCopyConstructible
of eitherkey_type
ormapped_type
if the dereferencedInputIterator
returns a non-const rvaluepair<key_type,mapped_type>
. OtherwiseCopyConstructible
is required for bothkey_type
andmapped_type
.
? Effects: Insertsx
converted tovalue_type
if and only if there is no element in the container with key equivalent to the key ofvalue_type(x)
. For the second form, the iteratorposition
is a hint pointing to where the search should start. ? Returns: For the first form, thebool
component of the returnedpair
object indicates whether the insertion took place and the iterator component - or for the second form the returned iterator - points to the element with key equivalent to the key ofvalue_type(x)
. ? Complexity: Logarithmic in general, but amortized constant ifx
is inserted right beforeposition
. ? Remarks: These signatures shall not participate in overload resolution unlessP
is implicitly convertible tovalue_type
.- Change 23.4.4.3 [multimap.modifiers] around p. 1 as indicated:
template <class P> iterator insert(P&& x); template <class P> iterator insert(const_iterator position, P&& x);1 Requires:
P
shall be convertible tovalue_type
is constructible fromstd::forward<P>(x)
.IfP
is instantiated as a reference type, then the argumentx
is copied from. Otherwisex
is considered to be an rvalue as it is converted tovalue_type
and inserted into the map. Specifically, in such casesCopyConstructible
is not required ofkey_type
ormapped_type
unless the conversion fromP
specifically requires it (e.g., ifP
is atuple<const key_type, mapped_type>
, thenkey_type
must beCopyConstructible
). The signature takingInputIterator
parameters does not requireCopyConstructible
of eitherkey_type
ormapped_type
if the dereferencedInputIterator
returns a non-const rvaluepair<key_type, mapped_type>
. OtherwiseCopyConstructible
is required for bothkey_type
andmapped_type
.
? Effects: Insertsx
converted tovalue_type
. For the second form, the iteratorposition
is a hint pointing to where the search should start. ? Returns: An iterator that points to the element with key equivalent to the key ofvalue_type(x)
. ? Complexity: Logarithmic in general, but amortized constant ifx
is inserted right beforeposition
. ? Remarks: These signatures shall not participate in overload resolution unlessP
is implicitly convertible tovalue_type
.
[ 2010 Batavia: ]
We need is_convertible
, not is_constructible
, both in ordered and unordered containers.
[ 2011 Bloomington ]
The effects of these inserts can be concisely stated in terms of emplace(). Also, the correct term is "EmplaceConstructible", not "constructible".
New wording by Pablo, eliminating duplicate requirements already implied by the effects clause. Move to Review.
[ 2011-10-02 Daniel comments and refines the proposed wording ]
Unfortunately the template constraints expressed as "
P
is implicitly convertible tovalue_type
" reject the intended effect to support move-only key types, which was the original intention when the library became move-enabled through the rvalue-reference proposals by Howard (This can clearly be deduced from existing carefully selected wording that emphasizes thatCopyConstructible
is only required for special situations involving lvalues or const rvalues as arguments). The root of the problem is based on current core rules, where an "implicitly converted" value has copy-initialization semantics. Consider a move-only key typeKM
, some mapped typeT
, and a source valuep
of typeP
equal tostd::pair<KM, T>
, this is equivalent to:std::pair<const KM, T> dest = std::move(p);Now 9.5 [dcl.init] p16 b6 sb2 says that the effects of this heterogeneous copy-initialization (
But the actual code that is required (with the default allocator) is simply a direct-initialization fromp
has a different type thandest
) are as-if a temporary of the target typestd::pair<const KM, T>
is produced from the rvaluep
of typeP
(which is fine), and this temporary is used to initializedest
. This second step cannot succeed, because we cannot move fromconst KM
toconst KM
. This means thatstd::is_convertible<P, std::pair<const KM, T>>::value
is false.P
tovalue_type
, so imposing an implicit conversion is more than necessary. Therefore I strongly recommend to reduce the "overload participation" constraint tostd::is_constructible<std::pair<const KM, T>, P>::value
instead. This change is the only change that has been performed to the previous proposed wording from Pablo shown below.
[2012, Kona]
Moved to Tentatively Ready by the post-Kona issues processing subgroup, after much discussion on Daniel's analysis of Copy Initialization and move semantics, which we ultimately agreed with.
[2012, Portland: applied to WP]
Proposed resolution:
template <class P> pair<iterator, bool> insert(P&& x); template <class P>pair<iterator, bool>insert(const_iterator position, P&& x);1 Requires:P
shall be convertible tovalue_type
.IfP
is instantiated as a reference type, then the argumentx
is copied from. Otherwisex
is considered to be an rvalue as it is converted tovalue_type
and inserted into the map. Specifically, in such casesCopyConstructible
is not required ofkey_type
ormapped_type
unless the conversion fromP
specifically requires it (e.g., ifP
is atuple<const key_type, mapped_type>
, thenkey_type
must beCopyConstructible
). The signature takingInputIterator
parameters does not requireCopyConstructible
of eitherkey_type
ormapped_type
if the dereferencedInputIterator
returns a non-const rvaluepair<key_type,mapped_type>
. OtherwiseCopyConstructible
is required for bothkey_type
andmapped_type
.
? Effects: The first form is equivalent toreturn emplace(std::forward<P>(x))
. The second form is equivalent toreturn emplace_hint(position, std::forward<P>(x))
. ? Remarks: These signatures shall not participate in overload resolution unlessstd::is_constructible<value_type, P&&>::value
is true.
template <class P> iterator insert(P&& x); template <class P> iterator insert(const_iterator position, P&& x);1 Requires:P
shall be convertible tovalue_type
.IfP
is instantiated as a reference type, then the argumentx
is copied from. Otherwisex
is considered to be an rvalue as it is converted tovalue_type
and inserted into the map. Specifically, in such casesCopyConstructible
is not required ofkey_type
ormapped_type
unless the conversion fromP
specifically requires it (e.g., ifP
is atuple<const key_type, mapped_type>
, thenkey_type
must beCopyConstructible
). The signature takingInputIterator
parameters does not requireCopyConstructible
of eitherkey_type
ormapped_type
if the dereferencedInputIterator
returns a non-const rvaluepair<key_type, mapped_type>
. OtherwiseCopyConstructible
is required for bothkey_type
andmapped_type
.
? Effects: The first form is equivalent toreturn emplace(std::forward<P>(x))
. The second form is equivalent toreturn emplace_hint(position, std::forward<P>(x))
. ? Remarks: These signatures shall not participate in overload resolution unlessstd::is_constructible<value_type, P&&>::value
is true.
template <class P> pair<iterator, bool> insert(P&& obj);1 Requires:2 Effects: equivalent tovalue_type
is constructible fromstd::forward<P>(obj)
.return emplace(std::forward<P>(obj))
.Inserts obj converted tovalue_type
if and only if there is no element in the container with key equivalent to the key ofvalue_type(obj)
.3 Returns: The bool component of the returned pair object indicates whether the insertion took place and the iterator component points to the element with key equivalent to the key ofvalue_type(obj)
.4 Complexity: Average case O(1), worst case O(size()).53 Remarks: This signature shall not participate in overload resolution unlessP
is implicitly convertible tovalue_type
std::is_constructible<value_type, P&&>::value
is true.template <class P> iterator insert(const_iterator hint, P&& obj);6 Requires:value_type
is constructible fromstd::forward<P>(obj)
.7? Effects: equivalent toreturn emplace_hint(hint, std::forward<P>(obj))
.Inserts obj converted tovalue_type
if and only if there is no element in the container with key equivalent to the key ofvalue_type(obj)
. The iterator hint is a hint pointing to where the search should start.8 Returns: An iterator that points to the element with key equivalent to the key ofvalue_type(obj)
.9 Complexity: Average case O(1), worst case O(size()).10? Remarks: This signature shall not participate in overload resolution unlessP
is implicitly convertible tovalue_type
std::is_constructible<value_type, P&&>::value
is true.
template <class P> iterator insert(P&& obj);1 Requires:2 Effects: equivalent tovalue_type
is constructible fromstd::forward<P>(obj)
.return emplace(std::forward<P>(obj))
.Inserts obj converted tovalue_type
.3 Returns: An iterator that points to the element with key equivalent to the key ofvalue_type(obj)
.4 Complexity: Average case O(1), worst case O(size()).53 Remarks: This signature shall not participate in overload resolution unlessP
is implicitly convertible tovalue_type
std::is_constructible<value_type, P&&>::value
is true.template <class P> iterator insert(const_iterator hint, P&& obj);6 Requires:value_type
is constructible fromstd::forward<P>(obj)
.7? Effects: equivalent toreturn emplace_hint(hint, std::forward<P>(obj))
.Inserts obj converted tovalue_type
. The iterator hint is a hint pointing to where the search should start.8 Returns: An iterator that points to the element with key equivalent to the key ofvalue_type
(obj).9 Complexity: Average case O(1), worst case O(size()).10? Remarks: This signature shall not participate in overload resolution unlessP
is implicitly convertible tovalue_type
std::is_constructible<value_type, P&&>::value
is true.
map<>::at()
Section: 23.4.3.3 [map.access] Status: C++11 Submitter: Matt Austern Opened: 2010-11-01 Last modified: 2016-01-28
Priority: Not Prioritized
View all other issues in [map.access].
View all issues with C++11 status.
Discussion:
In [map.access]/9, the Returns clause for map<Key, T>::at(x)
says
that it returns "a reference to the element whose key is equivalent to x
." That can't be right.
The signature for at()
says that its return type is T
, but the elements
of map<Key, T>
have type pair<const K, T>
. (I checked [unord.map.elem]
and found that its specification of at()
is correct. This is a problem for map
only.)
Proposed resolution:
Change the wording in [map.access]/9 so it's identical to what we already say for operator[]
,
which is unambiguous and correct.
Returns: A reference to the
element whose key is equivalentmapped_type
corresponding tox
in*this
.
packaged_task::operator()
Section: 32.10.10.2 [futures.task.members] Status: C++11 Submitter: Pete Becker Opened: 2010-06-21 Last modified: 2016-01-28
Priority: Not Prioritized
View all other issues in [futures.task.members].
View all issues with C++11 status.
Discussion:
The Throws clause for packaged_task::operator()
says that it throws "a
future_error
exception object if there is no associated asynchronous
state or the stored task has already been invoked." However, the Error
Conditions clause does not define an error condition when the stored task has
already been invoked, only when the associated state is already ready (i.e. the
invocation has completed).
[2011-02-17 Anthony provides an alternative resolution]
Previous proposed resolution:
Change the first bullet item in 32.10.10.2 [futures.task.members] /22:
void operator()(ArgTypes... args);20 ...
21 ...
22 Error conditions:
promise_already_satisfied
ifthe associated asynchronous state is already readyoperator()
has already been called.no_state
if*this
has no associated asynchronous state.
[Adopted at Madrid, 2011-03]
Proposed resolution:
Change the first bullet item in 32.10.10.2 [futures.task.members] p. 17:
void operator()(ArgTypes... args);15 ...
16 ...
17 Error conditions:
promise_already_satisfied
if theassociated asynchronous state is already readystored task has already been invoked.no_state
if*this
has no associated asynchronous state.
Change the first bullet item in 32.10.10.2 [futures.task.members] p. 21:
void make_ready_at_thread_exit(ArgTypes... args);19 ...
20 ...
21 Error conditions:
promise_already_satisfied
if theassociated asynchronous state already has a stored value or exceptionstored task has already been invoked.no_state
if*this
has no associated asynchronous state.
Section: 27.4.5 [string.conversions] Status: C++14 Submitter: Alisdair Meredith Opened: 2010-07-19 Last modified: 2016-01-28
Priority: Not Prioritized
View all other issues in [string.conversions].
View all issues with C++14 status.
Discussion:
The functions (w
)stoi
and (w
)stof
are specified in terms of calling C library APIs for potentially wider
types. The integer and floating-point versions have subtly different
behaviour when reading values that are too large to convert. The
floating point case will throw out_of_bound
if the read value
is too large to convert to the wider type used in the implementation,
but behaviour is undefined if the converted value cannot narrow to a
float. The integer case will throw out_of_bounds
if the
converted value cannot be represented in the narrower type, but throws
invalid_argument
, rather than out_of_range
, if the
conversion to the wider type fails due to overflow.
Suggest that the Throws clause for both specifications should be consistent, supporting the same set of fail-modes with the matching set of exceptions.
Proposed resolution:
21.5p3 [string.conversions]
int stoi(const string& str, size_t *idx = 0, int base = 10); long stol(const string& str, size_t *idx = 0, int base = 10); unsigned long stoul(const string& str, size_t *idx = 0, int base = 10); long long stoll(const string& str, size_t *idx = 0, int base = 10); unsigned long long stoull(const string& str, size_t *idx = 0, int base = 10);...
3 Throws:
invalid_argument
ifstrtol
,strtoul
,strtoll
, orstrtoull
reports that no conversion could be performed. Throwsout_of_range
ifstrtol
,strtoul
,strtoll
orstrtoull
setserrno
toERANGE
, or if the converted value is outside the range of representable values for the return type.
21.5p6 [string.conversions]
float stof(const string& str, size_t *idx = 0); double stod(const string& str, size_t *idx = 0); long double stold(const string& str, size_t *idx = 0);...
6 Throws:
invalid_argument
ifstrtod
orstrtold
reports that no conversion could be performed. Throwsout_of_range
ifstrtod
orstrtold
setserrno
toERANGE
or if the converted value is outside the range of representable values for the return type.
is_* traits
for binding operations can't be meaningfully specializedSection: 22.10.15.2 [func.bind.isbind] Status: C++14 Submitter: Sean Hunt Opened: 2010-07-19 Last modified: 2016-01-28
Priority: Not Prioritized
View all other issues in [func.bind.isbind].
View all issues with C++14 status.
Discussion:
22.10.15.2 [func.bind.isbind] says for is_bind_expression
:
Users may specialize this template to indicate that a type should be treated as a subexpression in a
bind
call.
But it also says:
If
T
is a type returned frombind
,is_bind_expression<T>
shall be publicly derived fromintegral_constant<bool, true>
, otherwise fromintegral_constant<bool, false>
.
This means that while the user is free to specialize, any specialization
would have to be false
to avoid violating the second
requirement. A similar problem exists for is_placeholder
.
[ 2010 Batavia (post meeting session) ]
Alisdair recognises this is clearly a bug introduced by some wording he
wrote, the sole purpose of this metafunction is as a customization point
for users to write their own bind
-expression types that participate
in the standard library bind
protocol. The consensus was that this
should be fixed in Madrid, moved to Open.
[2011-05-13 Jonathan Wakely comments and provides proposed wording]
The requirements are that is_bind_expression<T>::value
is true when T
is a type returned from bind
, false for any other type, except when
there's a specialization involving a user-defined type (N.B. 16.4.5.2.1 [namespace.std]
means we don't need to say e.g. is_bind_expression<string>
is false.)
integral_constant<bool, false>
and for implementations
to provide specializations for the unspecified types returned from
bind
. User-defined specializations can do whatever they like, as long
as is_bind_expression::value
is sane. There's no reason to forbid
users from defining is_bind_expression<user_defined_type>::value=false
if that's what they want to do.
Similar reasoning applies to is_placeholder
, but a further issue is
that 22.10.15.2 [func.bind.isbind] contains wording for is_placeholder
but
contains no definition of it and the sub-clause name only refers to
is_bind_expression
. The wording below proposes splitting paragraphs 3
and 4 of 22.10.15.2 [func.bind.isbind] into a new sub-clause covering
is_placeholder
.
If the template specializations added by the proposed wording are too
vague then they could be preceded by "for exposition only" comments
[2011-05-18 Daniel comments and provides some refinements to the P/R]
Both bind
-related type traits should take advantage of the
UnaryTypeTrait requirements. Additionally, the updated wording does not
imply that the implementation provides several specializations. Wording was
used similar to the specification of the uses_allocator
type trait
(which unfortunately is not expressed in terms of BinaryTypeTrait requirements).
[Bloomington, 2011]
Move to Ready
Proposed resolution:
This wording is relative to the FDIS.
Change 22.10.15.2 [func.bind.isbind] to:
namespace std { template<class T> struct is_bind_expression; // see below: integral_constant<bool, see below> { };}-1-
-2-is_bind_expression
can be used to detect function objects generated bybind
.bind
usesis_bind_expression
to detect subexpressions.Users may specialize this template to indicate that a type should be treated as a subexpression in abind
call.IfInstantiations of theT
is a type returned frombind
,is_bind_expression<T>
shall be publicly derived fromintegral_constant<bool, true>
, otherwise fromintegral_constant<bool, false>
is_bind_expression
template shall meet the UnaryTypeTrait requirements ([meta.rqmts]). The implementation shall provide a definition that has a BaseCharacteristic oftrue_type
ifT
is a type returned frombind
, otherwise it shall have a BaseCharacteristic offalse_type
. A program may specialize this template for a user-defined typeT
to have a BaseCharacteristic oftrue_type
to indicate thatT
should be treated as a subexpression in abind
call..-3-is_placeholder
can be used to detect the standard placeholders_1
,_2
, and so on.bind
usesis_placeholder
to detect placeholders. Users may specialize this template to indicate a placeholder type.-4- IfT
is the type ofstd::placeholders::_J
,is_placeholder<T>
shall be publicly derived fromintegral_constant<int, J>
, otherwise fromintegral_constant<int, 0>
.
Insert a new sub-clause immediately following sub-clause 22.10.15.2 [func.bind.isbind], the suggested sub-clause tag is [func.bind.isplace]:
is_placeholder
[func.bind.isplace]namespace std { template<class T> struct is_placeholder; // see below }-?-
-?- Instantiations of theis_placeholder
can be used to detect the standard placeholders_1
,_2
, and so on.bind
usesis_placeholder
to detect placeholders.is_placeholder
template shall meet the UnaryTypeTrait requirements ([meta.rqmts]). The implementation shall provide a definition that has a BaseCharacteristic ofintegral_constant<int, J>
ifT
is the type ofstd::placeholders::_J
, otherwise it shall have a BaseCharacteristic ofintegral_constant<int, 0>
. A program may specialize this template for a user-defined typeT
to have a BaseCharacteristic ofintegral_constant<int, N>
withN > 0
to indicate thatT
should be treated as a placeholder type.
Section: 27.4.4.4 [string.io] Status: C++14 Submitter: James Kanze Opened: 2010-07-23 Last modified: 2016-01-28
Priority: Not Prioritized
View all other issues in [string.io].
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Discussion:
What should the following code output?
#include <string> #include <iostream> #include <iomanip> int main() { std::string test("0X1Y2Z"); std::cout.fill('*'); std::cout.setf(std::ios::internal, std::ios::adjustfield); std::cout << std::setw(8) << test << std::endl; }
I would expect "**0X1Y2Z
", and this is what the compilers I have access
to (VC++, g++ and Sun CC) do. But according to the standard, it should be
"0X**1Y2Z
":
27.4.4.4 [string.io]/5:
template<class charT, class traits, class Allocator> basic_ostream<charT, traits>& operator<<(basic_ostream<charT, traits>& os, const basic_string<charT,traits,Allocator>& str);Effects: Behaves as a formatted output function (31.7.6.3.1 [ostream.formatted.reqmts]). After constructing a
sentry
object, if this object returnstrue
when converted to a value of typebool
, determines padding as described in 28.3.4.3.3.3 [facet.num.put.virtuals], then inserts the resulting sequence of characters seq as if by callingos.rdbuf()->sputn(seq, n)
, wheren
is the larger ofos.width()
andstr.size()
; then callsos.width(0)
.
28.3.4.3.3.3 [facet.num.put.virtuals]/5:
[…]
Stage 3: A local variable is initialized as
fmtflags adjustfield= (flags & (ios_base::adjustfield));The location of any padding is determined according to Table 88.
If
str.width()
is nonzero and the number ofcharT
's in the sequence after stage 2 is less thanstr.width()
, then enough fill characters are added to the sequence at the position indicated for padding to bring the length of the sequence tostr.width()
.str.width(0)
is called.
Table 88 — Fill padding State Location adjustfield == ios_base::left
pad after adjustfield == ios_base::right
pad before adjustfield == internal
and a sign occurs in the representationpad after the sign adjustfield == internal
and representation after stage 1 began with 0x or 0Xpad after x or X otherwise pad before
Although it's not 100% clear what "the sequence after stage 2" should mean here,
when there is no stage 2, the only reasonable assumption is that it is the
contents of the string being output. In the above code, the string being output
is "0X1Y2Z
", which starts with "0X
", so the padding should be
inserted "after x or X", and not before the string. I believe that this is a
defect in the standard, and not in the three compilers I tried.
[ 2010 Batavia (post meeting session) ]
Consensus that all known implementations are consistent, and disagree with the standard. Preference is to fix the standard before implementations start trying to conform to the current spec, as the current implementations have the preferred form. Howard volunteered to drught for Madrid, move to Open.
[2011-03-24 Madrid meeting]
Daniel Krügler volunteered to provide wording, interacting with Dietmar and Bill.
[2011-06-24 Daniel comments and provides wording]
The same problem applies to the output provided by const char*
and similar
character sequences as of 31.7.6.3.4 [ostream.inserters.character] p. 5. and even for
single character output (!) as described in 31.7.6.3.4 [ostream.inserters.character] p. 1,
just consider the character value '-' where '-' is the sign character. In this case
Table 91 — "Fill padding" requires to pad after the sign, i.e. the output
for the program
#include <iostream> #include <iomanip> int main() { char c = '-'; std::cout.fill('*'); std::cout.setf(std::ios::internal, std::ios::adjustfield); std::cout << std::setw(2) << c << std::endl; }
According to the current wording this program should output "-*
", but
all tested implementations output "*-
" instead.
money_put
functions.
[ 2011 Bloomington ]
Move to Review, the resolution seems correct but it would be nice if some factoring of the common words were proposed.
[2012, Kona]
Moved to Tentatively Ready by the post-Kona issues processing subgroup.
While better factoring of the common words is desirable, it is also editorial and should not hold up the progress of this issue. As the edits impact two distinct clauses, it is not entirely clear what a better factoring should look like.
[2012, Portland: applied to WP]
Proposed resolution:
The new wording refers to the FDIS numbering.
Change 27.4.4.4 [string.io]/5 as indicated:
template<class charT, class traits, class Allocator> basic_ostream<charT, traits>& operator<<(basic_ostream<charT, traits>& os, const basic_string<charT,traits,Allocator>& str);-5- Effects: Behaves as a formatted output function ([ostream.formatted.reqmts]). After constructing a sentry object, if this object returns
true
when converted to a value of typebool
, determines padding asdescribed in [facet.num.put.virtuals],follows: AcharT
character sequence is produced, initially consisting of the elements defined by the range[str.begin(), str.end())
. Ifstr.size()
is less thanos.width()
, then enough copies ofos.fill()
are added to this sequence as necessary to pad to a width ofos.width()
characters. If(os.flags() & ios_base::adjustfield) == ios_base::left
istrue
, the fill characters are placed after the character sequence; otherwise, they are placed before the character sequence. Tthen inserts the resulting sequence of charactersseq
as if by callingos.rdbuf()->sputn(seq, n)
, wheren
is the larger ofos.width()
andstr.size()
; then callsos.width(0)
.
Change 31.7.6.3.4 [ostream.inserters.character]/1 as indicated (An additional editorial fix is suggested for the first prototype declaration):
template<class charT, class traits> basic_ostream<charT,traits>& operator<<(basic_ostream<charT,traits>& out, charT c}); template<class charT, class traits> basic_ostream<charT,traits>& operator<<(basic_ostream<charT,traits>& out, char c); // specialization template<class traits> basic_ostream<char,traits>& operator<<(basic_ostream<char,traits>& out, char c); // signed and unsigned template<class traits> basic_ostream<char,traits>& operator<<(basic_ostream<char,traits>& out, signed char c); template<class traits> basic_ostream<char,traits>& operator<<(basic_ostream<char,traits>& out, unsigned char c);-1- Effects: Behaves like a formatted inserter (as described in [ostream.formatted.reqmts]) of
out
. After a sentry object is constructed it inserts characters. In casec
has typechar
and the character type of the stream is notchar
, then the character to be inserted isout.widen(c)
; otherwise the character isc
. Padding is determined asdescribed in [facet.num.put.virtuals]follows: A character sequence is produced, initially consisting of the insertion character. Ifout.width()
is greater than one, then enough copies ofout.fill()
are added to this sequence as necessary to pad to a width ofout.width()
characters. If(out.flags() & ios_base::adjustfield) == ios_base::left
istrue
, the fill characters are placed after the insertion character; otherwise, they are placed before the insertion character.The insertion character and any required padding are inserted intowidth(0)
is called.out
; then callsos.width(0)
.
Change 31.7.6.3.4 [ostream.inserters.character]/5 as indicated:
template<class charT, class traits> basic_ostream<charT,traits>& operator<<(basic_ostream<charT,traits>& out, const charT* s); template<class charT, class traits> basic_ostream<charT,traits>& operator<<(basic_ostream<charT,traits>& out, const char* s); template<class traits> basic_ostream<char,traits>& operator<<(basic_ostream<char,traits>& out, const char* s); template<class traits> basic_ostream<char,traits>& operator<<(basic_ostream<char,traits>& out, const signed char* s); template<class traits> basic_ostream<char,traits>& operator<<(basic_ostream<char,traits>& out, const unsigned char* s);[…]
-5- Padding is determined asdescribed in [facet.num.put.virtuals]. Thefollows: A character sequence is produced, initially consisting of the elements defined by then
characters starting ats
are widened usingout.widen
([basic.ios.members])n
characters starting ats
widened usingout.widen
([basic.ios.members]). Ifn
is less thanout.width()
, then enough copies ofout.fill()
are added to this sequence as necessary to pad to a width ofout.width()
characters. If(out.flags() & ios_base::adjustfield) == ios_base::left
istrue
, the fill characters are placed after the character sequence; otherwise, they are placed before the character sequence. The widened characters and any required padding are inserted intoout
. Callswidth(0)
.
pair
, not tuple
Section: 23.4 [associative] Status: Resolved Submitter: Paolo Carlini Opened: 2010-10-29 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
I'm seeing something strange in the paragraphs 23.4.3.4 [map.modifiers] and 23.4.4.3 [multimap.modifiers]:
they both talk about tuple<const key_type, mapped_type>
but I think they
should be talking about pair<const key_type, mapped_type>
because, among
other reasons, a tuple
is not convertible to a pair
. If I replace tuple
with pair
everything makes sense to me.
[ 2010-11-07 Daniel comments ]
This is by far not the only necessary fix within both sub-clauses. For details see the 2010-10-29 comment in 2005(i).
[2011-03-24 Madrid meeting]
Paolo: Don't think we can do it now.
Daniel K: Agrees.[ 2011 Bloomington ]
Consensus that this issue will be resolved by 2005(i), but held open until that issue is resolved.
Proposed resolution:
Apply the resolution proposed by the 2010-10-29 comment in 2005(i).
constexpr
?Section: 16.4.6.7 [constexpr.functions] Status: C++14 Submitter: Matt Austern Opened: 2010-11-12 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
Suppose that a particular function is not tagged as constexpr in the standard, but that, in some particular implementation, it is possible to write it within the constexpr constraints. If an implementer tags such a function as constexpr, is that a violation of the standard or is it a conforming extension?
There are two questions to consider. First, is this allowed under the as-if rule? Second, if it does not fall under as-if, is there (and should there be) any special license granted to implementers to do this anyway, sort of the way we allow elision of copy constructors even though it is detectable by users?
I believe that this does not fall under "as-if", so implementers probably don't have that freedom today. I suggest changing the WP to grant it. Even if we decide otherwise, however, I suggest that we make it explicit.
[ 2011 Bloomington ]
General surprise this was not already in 'Ready' status, and so moved.
[ 2012 Kona ]
Some concern expressed when presented to full committee for the vote to WP status that this issue had been resolved without sufficient thought of the consequences for diverging library implementations, as users may use SFINAE to observe different behavior from otherwise identical code. Issue moved back to Review status, and will be discussed again in Portland with a larger group. Note for Portland: John Spicer has agreed to represent Core's concerns during any such discussion within LWG.
[2013-09 Chicago]
Straw poll: LWG strongly favoured to remove from implementations the freedom to add constexpr
.
Matt provides new wording.
[2013-09 Chicago]
Move to Immediate after reviewing Matt's new wording, apply the new wording to the Working Paper.
Proposed resolution:
In 16.4.6.7 [constexpr.functions], change paragraph 1 to:
This standard explicitly requires that certain standard library functions are
constexpr
[dcl.constexpr]. An implementation shall not declare any standard library function signature asconstexpr
except for those where it is explicitly required. Within any header that provides any non-defining declarations ofconstexpr
functions or constructors an implementation shall provide corresponding definitions.
Section: 16.4.5.3.3 [macro.names] Status: C++11 Submitter: Alberto Ganesh Barbati Opened: 2010-11-16 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
A program is currently forbidden to use keywords as macro names. This restriction should be strengthened to include all identifiers
that could be used by the library as attribute-tokens (for example noreturn
, which is used by header <cstdlib>
)
and the special identifiers introduced recently for override control (these are not currently used in the library public interface,
but could potentially be used by the implementation or in future revisions of the library).
[2011-02-10 Reflector discussion]
Moved to Tentatively Ready after 5 votes.
Proposed resolution:
Modify 16.4.5.3.3 [macro.names] paragraph 2 as follows:
A translation unit shall not
#define
or#undef
names lexically identical to keywords, to the identifiers listed in Table X [Identifiers with special meaning], or to the attribute-tokens described in clause 7.6 [dcl.attr].
Section: 21.3.5 [meta.unary] Status: C++14 Submitter: Nikolay Ivchenkov Opened: 2010-11-08 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
According to N3126 ‑ 3.9/9,
"Scalar types, trivial class types (Clause 9), arrays of such types and cv‑qualified versions of these types (3.9.3) are collectively called trivial types."
Thus, an array (possibly of unknown bound) can be trivial type, non‑trivial type, or an array type whose triviality cannot be determined because its element type is incomplete.
According to N3126 ‑ Table 45, preconditions for std::is_trivial
are
defined as follows:
"T
shall be a complete type, (possibly cv-qualified) void
,
or an array of unknown bound"
It seems that "an array of unknown bound" should be changed to "an
array of unknown bound of a complete element type". Preconditions for
some other templates (e.g., std::is_trivially_copyable
,
std::is_standard_layout
, std::is_pod
, and std::is_literal_type
) should
be changed similarly.
On the other hand, some preconditions look too restrictive. For
example, std::is_empty
and std::is_polymorphic
might accept any
incomplete non‑class type.
[2011-02-18: Daniel provides wording proposal]
While reviewing the individual preconditions I could find three different groups of either too weakening or too strengthening constraints:
is_empty/is_polymorphic/is_abstract/has_virtual_destructor:
These traits can only apply for non‑union class types, otherwise the result must always be false
is_base_of:
Similar to the previous bullet, but the current wording comes already near to that ideal, it only misses to add the non‑union aspect.
is_trivial/is_trivially_copyable/is_standard_layout/is_pod/is_literal_type:
These traits always require that std::remove_all_extents<T>::type
to be cv void
or
a complete type.
[Bloomington, 2011]
Move to Ready
Proposed resolution:
Modify the pre-conditions of the following type traits in 21.3.5.4 [meta.unary.prop], Table 48 — Type property predicates:
Table 48 — Type property predicates Template Condition Preconditions ... template <class T>
struct is_trivial;T
is a trivial type (3.9)remove_all_extents<T>::type
shall be a complete type,or (possibly
cv-qualified)void
, or an array of.
unknown boundtemplate <class T>
struct is_trivially_copyable;T
is a trivially copyable
type (3.9)remove_all_extents<T>::type
shall be a complete type,or (possibly
cv-qualified)void
, or an array of.
unknown boundtemplate <class T>
struct is_standard_layout;T
is a standard-layout
type (3.9)remove_all_extents<T>::type
shall be a complete type,or (possibly
cv-qualified)void
, or an array of.
unknown boundtemplate <class T>
struct is_pod;T
is a POD type (3.9)remove_all_extents<T>::type
shall be a complete type,or (possibly
cv-qualified)void
, or an array of.
unknown boundtemplate <class T>
struct is_literal_type;T
is a literal type (3.9)remove_all_extents<T>::type
shall be a complete type,or (possibly
cv-qualified)void
, or an array of.
unknown boundtemplate <class T>
struct is_empty;T
is a class type, but not a
union type, with no
non-static data members
other than bit-fields of
length 0, no virtual
member functions, no
virtual base classes, and
no base class B for which
is_empty<B>::value
is
false.IfT
shall be a complete type,
(possibly cv-qualified)void
, or
an array of unknown boundT
is a non‑union class type,T
shall be a complete type.template <class T>
struct is_polymorphic;T
is a polymorphic
class (10.3)IfT
shall be a complete type,
type, (possibly cv-qualified)void
, or
an array of unknown boundT
is a non‑union class type,T
shall be a complete type.template <class T>
struct is_abstract;T
is an abstract
class (10.4)IfT
shall be a complete type,
type, (possibly cv-qualified)void
, or
an array of unknown boundT
is a non‑union class type,T
shall be a complete type.... template <class T>
struct has_virtual_destructor;T
has a virtual
destructor (12.4)IfT
shall be a complete type,
(possibly cv-qualified)void
, or
an array of unknown boundT
is a non‑union class type,T
shall be a complete type.
Modify the pre-conditions of the following type traits in 21.3.7 [meta.rel], Table 50 — Type relationship predicates:
Table 50 — Type relationship predicates Template Condition Comments ... template <class Base, class
Derived>
struct is_base_of;Base
is a base class of
Derived
(10) without
regard to cv-qualifiers
orBase
andDerived
are not unions and
name the same class
type without regard to
cv-qualifiersIf Base
andDerived
are
non‑union class types
and are different types
(ignoring possible cv-qualifiers)
thenDerived
shall be a complete
type. [ Note: Base classes that
are private, protected, or
ambigious are, nonetheless, base
classes. — end note ]...
Allocators
must be no-throw swappableSection: 16.4.4.6 [allocator.requirements] Status: C++17 Submitter: Daniel Krügler Opened: 2010-11-17 Last modified: 2017-07-30
Priority: 2
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Discussion:
During the Batavia meeting it turned out that there is a definition
hole for types satisfying the Allocators
requirements: The problem
became obvious when it was discussed whether all swap
functions
of Containers
with internal data handles can be safely tagged
with noexcept
or not. While it is correct that the implicit
swap
function of an allocator is required to be a no-throw
operation (because move/copy-constructors and assignment operators are
required to be no-throw functions), there are no such requirements
for specialized swap
overloads for a particular allocator.
Containers
are
required to support swappable Allocators
, when the value
allocator_traits<>::propagate_on_container_swap
evaluates
to true
.
[2011-02-10 Alberto, Daniel, and Pablo collaborated on the proposed wording]
The proposed resolution (based on N3225) attempts to solve the following problems:
X::propagate_on_container_copy_assignment
, X::propagate_on_container_move_assignment
, and
X::propagate_on_container_swap
only describe operations, but no requirements. In fact, if and only
if these compile-time predicates evaluate to true
, the additional requirements
CopyAssignable
, no-throw MoveAssignable
, and no-throw lvalue Swappable
,
respectively, are imposed on the allocator types.a.get_allocator() == b.get_allocator()
or allocator_traits<allocator_type>::propagate_on_container_swap::value == true
), which should be cleaned up.[2011-04-08 Pablo comments]
I'm implementing a version of list now and I actually do find it impossible to write an exception-safe assignment operator unless I can assume that allocator assignment does not throw. (The problem is that I use a sentinel node and I need to allocate a new sentinel using the new allocator without destroying the old one -- then swap the allocator and sentinel pointer in atomically, without risk of an exception leaving one inconsistent with the other.
Please update the proposed resolution to add the nothrow requirement to copy-assignment.[2014-02-14 Issaquah: Move to Ready]
Fix a couple of grammar issues related to calling swap
and move to Ready.
Proposed resolution:
Adapt the following three rows from Table 44 — Allocator requirements:
Table 44 — Allocator requirements Expression Return type Assertion/note
pre-/post-conditionDefault X::propagate_on_container_copy_assignment
Identical to or derived from true_type
orfalse_type
true_type
only if an allocator of typeX
should be copied
when the client container is copy-assigned. See Note B, below.false_type
X::propagate_on_container_move_assignment
Identical to or derived from true_type
orfalse_type
true_type
only if an allocator of typeX
should be moved
when the client container is move-assigned. See Note B, below.false_type
X::propagate_on_container_swap
Identical to or derived from true_type
orfalse_type
true_type
only if an allocator of typeX
should be swapped
when the client container is swapped. See Note B, below.false_type
Following 16.4.4.6 [allocator.requirements] p. 3 insert a new normative paragraph:
Note B: If
X::propagate_on_container_copy_assignment::value
is true,X
shall satisfy theCopyAssignable
requirements (Table 39 [copyassignable]) and the copy operation shall not throw exceptions. IfX::propagate_on_container_move_assignment::value
is true,X
shall satisfy theMoveAssignable
requirements (Table 38 [moveassignable]) and the move operation shall not throw exceptions. IfX::propagate_on_container_swap::value
is true, lvalues ofX
shall be swappable (16.4.4.3 [swappable.requirements]) and theswap
operation shall not throw exceptions.
Modify 23.2.2 [container.requirements.general] p. 8 and p. 9 as indicated:
8 - [..] The allocator may be replaced only via assignment or
9 - The expressionswap()
. Allocator replacement is performed by copy assignment, move assignment, or swapping of the allocator only ifallocator_traits<allocator_type>::propagate_on_container_copy_assignment::value
,allocator_traits<allocator_type>::propagate_on_container_move_assignment::value
, orallocator_traits<allocator_type>::propagate_on_container_swap::value
is true within the implementation of the corresponding container operation.The behavior of a call to a container's. In all container types defined in this Clause, the memberswap
function is undefined unless the objects being swapped have allocators that compare equal orallocator_traits<allocator_type>::propagate_on_container_swap::value
is trueget_allocator()
returns a copy of the allocator used to construct the container or, if that allocator has been replaced, a copy of the most recent replacement.a.swap(b)
, for containersa
andb
of a standard container type other thanarray
, shall exchange the values ofa
andb
without invoking any move, copy, or swap operations on the individual container elements. Lvalues of aAnyCompare
,Pred
, orHash
objectstypes belonging toa
andb
shall be swappable and shall be exchanged byunqualified calls to non-membercallingswap
as described in 16.4.4.3 [swappable.requirements]. Ifallocator_traits<allocator_type>::propagate_on_container_swap::value
istrue
, then lvalues ofallocator_type
shall be swappable and the allocators ofa
andb
shall also be exchangedusing an unqualified call to non-memberby callingswap
as described in 16.4.4.3 [swappable.requirements]. Otherwise,theythe allocators shall not be swapped, and the behavior is undefined unlessa.get_allocator() == b.get_allocator()
. Every iterator referring to an element in one container before the swap shall refer to the same element in the other container after the swap. It is unspecified whether an iterator with valuea.end()
before the swap will have valueb.end()
after the swap.
std::reference_wrapper
makes incorrect usage of std::result_of
Section: 22.10.6 [refwrap] Status: C++11 Submitter: Nikolay Ivchenkov Opened: 2010-11-15 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
std::reference_wrapper
's function call operator uses wrong
type encoding for rvalue-arguments. An rvalue-argument of type T
must
be encoded as T&&
, not as just T
.
#include <functional> #include <iostream> #include <string> #include <type_traits> #include <utility> template <class F, class... Types> typename std::result_of<F (Types...)>::type f1(F f, Types&&... params) { return f(std::forward<Types...>(params...)); } template <class F, class... Types> typename std::result_of<F (Types&&...)>::type f2(F f, Types&&... params) { return f(std::forward<Types...>(params...)); } struct Functor { template <class T> T&& operator()(T&& t) const { return static_cast<T&&>(t); } }; int main() { typedef std::string const Str; std::cout << f1(Functor(), Str("1")) << std::endl; // (1) std::cout << f2(Functor(), Str("2")) << std::endl; // (2) }
Lets consider the function template f1
(which is similar to
std::reference_wrapper
's function call operator). In the invocation
(1) F
is deduced as 'Functor
' and Types
is deduced as type sequence
which consists of one type 'std::string const
'. After the substitution
we have the following equivalent:
template <> std::result_of<F (std::string const)>::type f1<Functor, std::string const>(Functor f, std::string const && params) { return f(std::forward<const std::string>(params)); }
The top-level cv-qualifier in the parameter type of 'F (std::string const)
' is removed, so we have
template <> std::result_of<F (std::string)>::type f1<Functor, std::string const>(Functor f, std::string const && params) { return f(std::forward<const std::string>(params)); }
Let r
be an rvalue of type 'std::string
' and cr
be an rvalue of type
'std::string const
'. The expression Str("1")
is cr
. The corresponding
return type for the invocation
Functor().operator()(r)
is 'std::string &&
'. The corresponding return type for the invocation
Functor().operator()(cr)
is 'std::string const &&
'.
std::result_of<Functor (std::string)>::type
is the same type as the
corresponding return type for the invocation Functor().operator()(r)
,
i.e. it is 'std::string &&
'. As a consequence, we have wrong reference
binding in the return statement in f1
.
Now lets consider the invocation (2) of the function template f2
. When
the template arguments are substituted we have the following equivalent:
template <> std::result_of<F (std::string const &&)>::type f2<Functor, std::string const>(Functor f, std::string const && params) { return f(std::forward<const std::string>(params)); }
std::result_of<F (std::string const &&)>::type
is the same type as
'std::string const &&
'. This is correct result.
[ 2010-12-07 Jonathan Wakely comments and suggests a proposed resolution ]
I agree with the analysis and I think this is a defect in the standard, it would be a shame if it can't be fixed.
In the following example one would expectf(Str("1"))
and
std::ref(f)(Str("2"))
to be equivalent but the current wording makes
the invocation through reference_wrapper
ill-formed:
#include <functional> #include <string> struct Functor { template <class T> T&& operator()(T&& t) const { return static_cast<T&&>(t); } }; int main() { typedef std::string const Str; Functor f; f( Str("1") ); std::ref(f)( Str("2") ); // error }
[ 2010-12-07 Daniel comments and refines the proposed resolution ]
There is one further defect in the usage of result_of
within
reference_wrapper
's function call operator: According to 22.10.6.5 [refwrap.invoke] p. 1
the invokable entity of type T
is provided as lvalue, but
result_of
is fed as if it were an rvalue. This does not only lead to
potentially incorrect result types, but it will also have the effect that
we could never use the function call operator with a function type,
because the type encoding used in result_of
would form an invalid
function type return a function type. The following program demonstrates
this problem:
#include <functional> void foo(int) {} int main() { std::ref(foo)(0); // error }
The correct solution is to ensure that T
becomes T&
within result_of
, which solves both problems at once.
[2011-02-24 Reflector discussion]
Moved to Tentatively Ready after 5 votes.
Proposed resolution:
Change the synopsis in 22.10.6 [refwrap] paragraph 1:
namespace std { template <class T> class reference_wrapper { public : [...] // invocation template <class... ArgTypes> typename result_of<T&(ArgTypes&&...)>::type operator() (ArgTypes&&...) const; }; }
Change the signature in 22.10.6.5 [refwrap.invoke] before paragraph 1
template <class... ArgTypes> typename result_of<T&(ArgTypes&&... )>::type operator()(ArgTypes&&... args) const;1 Returns:
INVOKE(get(), std::forward<ArgTypes>(args)...)
. (20.8.2)
regex_traits::isctype
Returns clause is wrongSection: 28.6.6 [re.traits] Status: C++14 Submitter: Jonathan Wakely Opened: 2010-11-16 Last modified: 2016-01-28
Priority: Not Prioritized
View all other issues in [re.traits].
View all issues with C++14 status.
Discussion:
Addresses GB 10
28.6.6 [re.traits] p. 12 says:
returns true if
f
bitwise or'ed with the result of callinglookup_classname
with an iterator pair that designates the character sequence "w" is not equal to0
andc == '_'
If the bitmask value corresponding to "w" has a non-zero value (which
it must do) then the bitwise or with any value is also non-zero, and
so isctype('_', f)
returns true for any f
. Obviously this is wrong,
since '_'
is not in every ctype
category.
There's a similar problem with the following phrases discussing the "blank" char class.
[2011-05-06: Jonathan Wakely comments and provides suggested wording]
DR 2019(i) added isblank
support to <locale>
which simplifies the
definition of regex_traits::isctype
by removing the special case for the "blank" class.
regex_traits::lookup_classname
.
I then refer to that table in the Returns clause of regex_traits::isctype
to expand on the "in an unspecified manner" wording which is too vague. The conversion
can now be described using the "is set" term defined by 16.3.3.3.3 [bitmask.types] and
the new table to convey the intented relationship between e.g.
[[:digit:]] and ctype_base::digit
, which is not actually stated in the
FDIS.
The effects of isctype
can then most easily be described in code,
given an "exposition only" function prototype to do the not-quite-so-unspecified conversion
from char_class_type
to ctype_base::mask
.
The core of LWG 2018 is the "bitwise or'ed" wording which gives the
wrong result, always evaluating to true for all values of f
. That is
replaced by the condition (f&x) == x
where x
is the result of calling
lookup_classname
with "w". I believe that's necessary, because the
"w" class could be implemented by an internal "underscore" class i.e.
x = _Alnum|_Underscore
in which case (f&x) != 0
would give the wrong
result when f==_Alnum
.
The proposed resolution also makes use of ctype::widen
which addresses
the problem that the current wording only talks about "w" and '_' which assumes
charT
is char. There's still room for improvement here:
the regex grammar in 28.6.12 [re.grammar] says that the class names in the
table should always be recognized, implying that e.g. U"digit" should
be recognized by regex_traits<char32_t>
, but the specification of
regex_traits::lookup_classname
doesn't cover that, only mentioning
char
and wchar_t
. Maybe the table should not distinguish narrow and
wide strings, but should just have one column and add wording to say
that regex_traits
widens the name as if by using use_facet<ctype<charT>>::widen()
.
Another possible improvement would be to allow additional
implementation-defined extensions in isctype
. An implementation is
allowed to support additional class names in lookup_classname
, e.g.
[[:octdigit:]] for [0-7] or [[:bindigit:]] for [01], but the current
definition of isctype provides no way to use them unless ctype_base::mask
also supports them.
[2011-05-10: Alberto and Daniel perform minor fixes in the P/R]
[ 2011 Bloomington ]
Consensus that this looks to be a correct solution, and the presentation as a table is a big improvement.
Concern that the middle section wording is a little muddled and confusing, Stefanus volunteered to reword.
[ 2013-09 Chicago ]
Stefanus provides improved wording (replaced below)
[ 2013-09 Chicago ]
Move as Immediate after reviewing Stefanus's revised wording, apply the new wording to the Working Paper.
Proposed resolution:
This wording is relative to the FDIS.
Modify 28.6.6 [re.traits] p. 10 as indicated:
template <class ForwardIterator> char_class_type lookup_classname( ForwardIterator first, ForwardIterator last, bool icase = false) const;-9- Returns: an unspecified value that represents the character classification named by the character sequence designated by the iterator range [
-10- Remarks: Forfirst
,last
). If the parametericase
is true then the returned mask identifies the character classification without regard to the case of the characters being matched, otherwise it does honor the case of the characters being matched.(footnote 335) The value returned shall be independent of the case of the characters in the character sequence. If the name is not recognized then returns a value that compares equal to0
.regex_traits<char>
, at least thenames "d", "w", "s", "alnum", "alpha", "blank", "cntrl", "digit", "graph", "lower", "print", "punct", "space", "upper" and "xdigit"narrow character names in Table X shall be recognized. Forregex_traits<wchar_t>
, at least thenames L"d", L"w", L"s", L"alnum", L"alpha", L"blank", L"cntrl", L"digit", L"graph", L"lower", L"print", L"punct", L"space", L"upper" and L"xdigit"wide character names in Table X shall be recognized.
Modify 28.6.6 [re.traits] p. 12 as indicated:
bool isctype(charT c, char_class_type f) const;-11- Effects: Determines if the character
-12- Returns:c
is a member of the character classification represented byf
.ConvertsGiven an exposition-only function prototypef
into a valuem
of typestd::ctype_base::mask
in an unspecified manner, and returns true ifuse_facet<ctype<charT> >(getloc()).is(m, c)
is true. Otherwise returns true iff
bitwise or'ed with the result of callinglookup_classname
with an iterator pair that designates the character sequence "w" is not equal to0
andc == '_'
, or iff
bitwise or'ed with the result of callinglookup_classname
with an iterator pair that designates the character sequence "blank" is not equal to0
andc
is one of an implementation-defined subset of the characters for whichisspace(c, getloc())
returns true, otherwise returns false.template<class C> ctype_base::mask convert(typename regex_traits<C>::char_class_type f);that returns a value in which each
ctype_base::mask
value corresponding to a value inf
named in Table X is set, then the result is determined as if by:ctype_base::mask m = convert<charT>(f); const ctype<charT>& ct = use_facet<ctype<charT>>(getloc()); if (ct.is(m, c)) { return true; } else if (c == ct.widen('_')) { charT w[1] = { ct.widen('w') }; char_class_type x = lookup_classname(w, w+1); return (f&x) == x; } else { return false; }[Example:
regex_traits<char> t; string d("d"); string u("upper"); regex_traits<char>::char_class_type f; f = t.lookup_classname(d.begin(), d.end()); f |= t.lookup_classname(u.begin(), u.end()); ctype_base::mask m = convert<char>(f); // m == ctype_base::digit|ctype_base::upper— end example]
[Example:
regex_traits<char> t; string w("w"); regex_traits<char>::char_class_type f; f = t.lookup_classname(w.begin(), w.end()); t.isctype('A', f); // returns true t.isctype('_', f); // returns true t.isctype(' ', f); // returns false— end example]
At the end of 28.6.6 [re.traits] add a new "Table X — Character class names and corresponding ctype masks":
Table X — Character class names and corresponding ctype masks Narrow character name Wide character name Corresponding ctype_base::mask
value"alnum"
L"alnum"
ctype_base::alnum
"alpha"
L"alpha"
ctype_base::alpha
"blank"
L"blank"
ctype_base::blank
"cntrl"
L"cntrl"
ctype_base::cntrl
"digit"
L"digit"
ctype_base::digit
"d"
L"d"
ctype_base::digit
"graph"
L"graph"
ctype_base::graph
"lower"
L"lower"
ctype_base::lower
"print"
L"print"
ctype_base::print
"punct"
L"punct"
ctype_base::punct
"space"
L"space"
ctype_base::space
"s"
L"s"
ctype_base::space
"upper"
L"upper"
ctype_base::upper
"w"
L"w"
ctype_base::alnum
"xdigit"
L"xdigit"
ctype_base::xdigit
isblank
not supported by std::locale
Section: 28.3.3.3.1 [classification] Status: C++11 Submitter: Jonathan Wakely Opened: 2010-11-16 Last modified: 2016-01-28
Priority: Not Prioritized
View all issues with C++11 status.
Discussion:
C99 added isblank
and iswblank
to <locale.h>
but <locale>
does not
provide any equivalent.
[2011-02-24 Reflector discussion]
Moved to Tentatively Ready after 6 votes.
Proposed resolution:
Add to 28.3.3.3.1 [classification] synopsis:
template <class charT> bool isgraph (charT c, const locale& loc); template <class charT> bool isblank (charT c, const locale& loc);
Add to 28.3.4.2 [category.ctype] synopsis:
static const mask xdigit = 1 << 8; static const mask blank = 1 << 9; static const mask alnum = alpha | digit; static const mask graph = alnum | punct;
constexpr
functions have invalid effectsSection: 30.5.6 [time.duration.nonmember] Status: C++11 Submitter: Daniel Krügler Opened: 2010-12-06 Last modified: 2016-01-28
Priority: Not Prioritized
View all other issues in [time.duration.nonmember].
View all issues with C++11 status.
Discussion:
As of issue 1171(i) several time-utility functions have been marked constexpr
.
Alas this was done without adapting the corresponding return elements, which has the effect that
none of current arithmetic functions of class template duration
marked as constexpr
can ever be constexpr
functions (which makes them ill-formed, no diagnostics required as
of recent core rules), because they invoke a non-constant expression, e.g. 30.5.6 [time.duration.nonmember]/2:
template <class Rep1, class Period1, class Rep2, class Period2> constexpr typename common_type<duration<Rep1, Period1>, duration<Rep2, Period2>{>}::type operator+(const duration<Rep1, Period1>& lhs, const duration<Rep2, Period2>& rhs); 2 Returns: CD(lhs) += rhs.
The real problem is, that we cannot defer to as-if rules here: The returns element
specifies an indirect calling contract of a potentially user-defined function. This cannot be
the +=
assignment operator of such a user-defined type, but must be the corresponding
immutable binary operator+
(unless we require that +=
shall be an immutable function
which does not really makes sense).
[2011-02-17 Reflector discussion]
Moved to Tentatively Ready after 5 votes.
Proposed resolution:
The suggested wording changes are against the working draft N3242. Additional to the normative wording changes some editorial fixes are suggested.
In 30.5.6 [time.duration.nonmember], change the following arithmetic function specifications as follows:
template <class Rep1, class Period1, class Rep2, class Period2> constexpr typename common_type<duration<Rep1, Period1>, duration<Rep2, Period2>{>}::type operator+(const duration<Rep1, Period1>& lhs, const duration<Rep2, Period2>& rhs);2 Returns:
CD(lhs) += rhs
CD(CD(lhs).count() + CD(rhs).count())
.
template <class Rep1, class Period1, class Rep2, class Period2> constexpr typename common_type<duration<Rep1, Period1>, duration<Rep2, Period2>{>}::type operator-(const duration<Rep1, Period1>& lhs, const duration<Rep2, Period2>& rhs);3 Returns:
CD(lhs) -= rhs
CD(CD(lhs).count() - CD(rhs).count())
.
template <class Rep1, class Period, class Rep2> constexpr duration<typename common_type<Rep1, Rep2>::type, Period> operator*(const duration<Rep1, Period>& d, const Rep2& s);4 Remarks: This operator shall not participate in overload resolution unless
5 Returns:Rep2
is implicitly convertible toCR(Rep1, Rep2)
.duration<CR(Rep1, Rep2), Period>(d) *= s
CD(CD(d).count() * s)
.
[...]
template <class Rep1, class Period, class Rep2> constexpr duration<typename common_type<Rep1, Rep2>::type, Period> operator/(const duration<Rep1, Period>& d, const Rep2& s);8 Remarks: This operator shall not participate in overload resolution unless
9 Returns:Rep2
is implicitly convertible toCR(Rep1, Rep2)
andRep2
is not an instantiation ofduration
.duration<CR(Rep1, Rep2), Period>(d) /= s
CD(CD(d).count() / s)
.
[...]
template <class Rep1, class Period, class Rep2> constexpr duration<typename common_type<Rep1, Rep2>::type, Period> operator%(const duration<Rep1, Period>& d, const Rep2& s);11 Remarks: This operator shall not participate in overload resolution unless
12 Returns:Rep2
is implicitly convertible toCR(Rep1, Rep2)
andRep2
is not an instantiation ofduration
.duration<CR(Rep1, Rep2), Period>(d) %= s
CD(CD(d).count() % s)
template <class Rep1, class Period1, class Rep2, class Period2> constexpr typename common_type<duration<Rep1, Period1>, duration<Rep2, Period2>>::type operator%(const duration<Rep1, Period1>& lhs, const duration<Rep2, Period2>& rhs);13 Returns:
common_type<duration<Rep1, Period1>, duration<Rep2, Period2> >::type(lhs) %= rhs
CD(CD(lhs).count() % CD(rhs).count())
.
result_of
Section: 22.10.15.4 [func.bind.bind], 32.10.1 [futures.overview], 32.10.9 [futures.async] Status: C++14 Submitter: Daniel Krügler Opened: 2010-12-07 Last modified: 2016-01-28
Priority: Not Prioritized
View all other issues in [func.bind.bind].
View all issues with C++14 status.
Discussion:
Issue 2017(i) points out some incorrect usages of result_of
in the
declaration of the function call operator overload of reference_wrapper
,
but there are more such specification defects:
[..] The effect of
g(u1, u2, ..., uM)
shall beINVOKE(fd, v1, v2, ..., vN, result_of<FD cv (V1, V2, ..., VN)>::type)
[..]
but fd
is defined as "an lvalue of type FD
constructed from std::forward<F>(f)
". This means that
the above usage must refer to result_of<FD cv & (V1, V2, ..., VN)>
instead.
Similar in 22.10.15.4 [func.bind.bind] p. 10 bullet 2 we have:
if the value of
is_bind_expression<TiD>::value
is true, the argument istid(std::forward<Uj>(uj)...)
and its typeVi
isresult_of<TiD cv (Uj...)>::type
Again, tid
is defined as "lvalue of type TiD
constructed from std::forward<Ti>(ti)
". This means that
the above usage must refer to result_of<TiD cv & (Uj...)>
instead. We also have similar defect as in
2017(i) in regard to the argument types, this leads us to the further corrected form
result_of<TiD cv & (Uj&&...)>
. This is not the end: Since the Vi
are similar sensitive to the argument problem, the last part must say:
Vi
is result_of<TiD cv & (Uj&&...)>::type &&"
(The bound arguments Vi
can never be void
types, therefore we don't need
to use the more defensive std::add_rvalue_reference
type trait)
The function template async
is declared as follows (the other overload has the same problem):
template <class F, class... Args> future<typename result_of<F(Args...)>::type> async(F&& f, Args&&... args);
This usage has the some same problems as we have found in reference_wrapper
(2017(i)) and more: According to
the specification in 32.10.9 [futures.async] the effective result type is that of the call of
INVOKE(decay_copy(std::forward<F>(f)), decay_copy(std::forward<Args>(args))...)
First, decay_copy
potentially modifies the effective types to decay<F>::type
and decay<Args>::type...
.
Second, the current specification is not really clear, what the value category of callable type or the arguments shall be: According
to the second bullet of 32.10.9 [futures.async] p. 3:
Invocation of the deferred function evaluates
INVOKE(g, xyz)
whereg
is the stored value ofdecay_copy(std::forward<F>(f))
andxyz
is the stored copy ofdecay_copy(std::forward<Args>(args))...
.
This seems to imply that lvalues are provided in contrast to the direct call expression of 32.10.9 [futures.async] p. 2 which implies rvalues instead. The specification needs to be clarified.
[2011-06-13: Daniel comments and refines the proposed wording changes]
The feedback obtained following message c++std-lib-30745 and follow-ups point to the intention, that
the implied provision of lvalues due to named variables in async
should be provided as rvalues to support
move-only types, but the functor type should be forwarded as lvalue in bind
.
bind
were newly invented, the value strategy could be improved, because now we have a preference of
ref &
qualified function call operator overloads. But such a change seems to be too late now.
User-code that needs to bind a callable object with an ref &&
qualified function call
operator (or conversion function to function pointer) needs to use a corresponding wrapper similar to reference_wrapper
that forwards the reference as rvalue-reference instead.
The wording has been adapted to honor these observations and to fit to FDIS numbering as well.
[Bloomington, 2011]
Move to Ready
Proposed resolution:
The suggested wording changes are against the FDIS.
Change 22.10.15.4 [func.bind.bind] p. 3 as indicated:
template<class F, class... BoundArgs> unspecified bind(F&& f, BoundArgs&&... bound_args);-2- Requires:
-3- Returns: A forwarding call wrapperis_constructible<FD, F>::value
shall be true. For eachTi
inBoundArgs
,is_constructible<TiD, Ti>::value
shall be true.INVOKE(fd, w1, w2, ..., wN)
(20.8.2) shall be a valid expression for some valuesw1
,w2
, ...,wN
, whereN == sizeof...(bound_args)
.g
with a weak result type (20.8.2). The effect ofg(u1, u2, ..., uM)
shall beINVOKE(fd, std::forward<V1>(v1), std::forward<V2>(v2), ..., std::forward<VN>(vN), result_of<FD cv & (V1, V2, ..., VN)>::type)
, where cv represents the cv-qualifiers ofg
and the values and types of the bound argumentsv1
,v2
, ...,vN
are determined as specified below. […]
Change 22.10.15.4 [func.bind.bind] p. 7 as indicated:
template<class R, class F, class... BoundArgs> unspecified bind(F&& f, BoundArgs&&... bound_args);-6- Requires:
-7- Returns: A forwarding call wrapperis_constructible<FD, F>::value
shall be true. For eachTi
inBoundArgs
,is_constructible<TiD, Ti>::value
shall be true.INVOKE(fd, w1, w2, ..., wN)
shall be a valid expression for some valuesw1
,w2
, ...,wN
, whereN == sizeof...(bound_args)
.g
with a nested typeresult_type
defined as a synonym forR
. The effect ofg(u1, u2, ..., uM)
shall beINVOKE(fd, std::forward<V1>(v1), std::forward<V2>(v2), ..., std::forward<VN>(vN), R)
, where the values and types of the bound argumentsv1
,v2
, ...,vN
are determined as specified below. […]
Change 22.10.15.4 [func.bind.bind] p. 10 as indicated:
-10- The values of the bound arguments
v1
,v2
, ...,vN
and their corresponding typesV1
,V2
, ...,VN
depend on the typesTiD
derived from the call to bind and the cv-qualifiers cv of the call wrapperg
as follows:
- if
TiD
isreference_wrapper<T>
, the argument istid.get()
and its typeVi
isT&
;- if the value of
is_bind_expression<TiD>::value
istrue
, the argument istid(std::forward<Uj>(uj)...)
and its typeVi
isresult_of<TiD cv & (Uj&&...)>::type&&
;- if the value
j
ofis_placeholder<TiD>::value
is not zero, the argument isstd::forward<Uj>(uj)
and its typeVi
isUj&&
;- otherwise, the value is
tid
and its typeVi
isTiD cv &
.
This resolution assumes that the wording of 32.10.9 [futures.async] is intended to provide rvalues
as arguments of INVOKE
.
Change the function signatures in header <future>
synopsis 32.10.1 [futures.overview] p. 1
and in 32.10.9 [futures.async] p. 1 as indicated:
template <class F, class... Args> future<typename result_of<typename decay<F>::type(typename decay<Args>::type...)>::type> async(F&& f, Args&&... args); template <class F, class... Args> future<typename result_of<typename decay<F>::type(typename decay<Args>::type...)>::type> async(launch policy, F&& f, Args&&... args);
Change 32.10.9 [futures.async] as indicated: (Remark: There is also a tiny editorial correction
in p. 4 that completes one ::
scope specifier)
-3- Effects: […]
- […]
- if
policy & launch::deferred
is non-zero — StoresDECAY_COPY(std::forward<F>(f))
andDECAY_COPY(std::forward<Args>(args))...
in the shared state. These copies off
andargs
constitute a deferred function. Invocation of the deferred function evaluatesINVOKE(std::move(g), std::move(xyz))
whereg
is the stored value ofDECAY_COPY(std::forward<F>(f))
andxyz
is the stored copy ofDECAY_COPY(std::forward<Args>(args))...
. The shared state is not made ready until the function has completed. The first call to a non-timed waiting function (30.6.4) on an asynchronous return object referring to this shared state shall invoke the deferred function in the thread that called the waiting function. Once evaluation ofINVOKE(std::move(g), std::move(xyz))
begins, the function is no longer considered deferred. [ Note: If this policy is specified together with other policies, such as when using apolicy
value oflaunch::async | launch::deferred
, implementations should defer invocation or the selection of the policy when no more concurrency can be effectively exploited. — end note ]
-4- Returns: an object of type
future<typename result_of<typename decay<F>::type(typename decay<Args>::type...)>::type>
that refers to the associated asynchronous state created by this call toasync
.
reference_wrapper<T>::result_type
is underspecifiedSection: 22.10.6 [refwrap] Status: C++11 Submitter: Daniel Krügler Opened: 2010-12-08 Last modified: 2016-01-28
Priority: Not Prioritized
View all other issues in [refwrap].
View all issues with C++11 status.
Discussion:
Issue 1295(i) correctly removed function types and references to function types from the bullet 1 of 22.10.4 [func.require] p. 3 because neither function types nor function references satisfy the requirements for a target object which is defined to be an object of a callable type. This has the effect that the reference in 22.10.6 [refwrap] p. 2
reference_wrapper
has a weak result type (20.8.2).
is insufficient as a reference to define the member type result_type
when the template argument
T
is a function type.
Extend the definition of a weak result type in 22.10.4 [func.require] p. 3 to both function types and references thereof. This extension must be specified independend from the concept of a call wrapper, though.
Add one extra sentence to 22.10.6 [refwrap] p. 2 that simply defines the member type
result_type
for reference_wrapper<T>
, when T
is a function type.
I checked the current usages of weak result type to have a base to argue for one or the other
approach. It turns out, that there is no further reference to this definition in regard to
function types or references thereof. The only other reference can be found in
22.10.15.4 [func.bind.bind] p. 3, where g
is required to be a class type.
[2011-02-23 Reflector discussion]
Moved to Tentatively Ready after 5 votes.
Proposed resolution:
The suggested wording changes are against the working draft N3242.
Change 22.10.6 [refwrap] p. 2 as indicated:
2
reference_wrapper<T>
has a weak result type (20.8.2). IfT
is a function type,result_type
shall be a synonym for the return type ofT
.
lock_guard
and unique_lock
Section: 32.6.5.2 [thread.lock.guard], 32.6.5.4 [thread.lock.unique] Status: Resolved Submitter: Daniel Krügler Opened: 2010-12-08 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
There are two different *Lockable
requirements imposed on template arguments
of the class template lock_guard
as of 32.6.5.2 [thread.lock.guard] p. 1+2:
1 [..] The supplied
Mutex
type shall meet theBasicLockable
requirements (30.2.5.2).
2 The supplied
Mutex
type shall meet theLockable
requirements (30.2.5.3).
The Lockable
requirements include the availability of a member function try_lock()
,
but there is no operational semantics in the specification of lock_guard
that would rely
on such a function. It seems to me that paragraph 2 should be removed.
Mutex
should
always provide the try_lock()
member function, because several member functions of
unique_lock
(unique_lock(mutex_type& m, try_to_lock_t)
or bool try_lock()
)
take advantage of such a function without adding extra requirements for this.
It seems that the requirement subset BasicLockable
should be removed.
I searched for further possible misusages of the *Lockable
requirements, but could not
find any more.
[2011-02-23]
Howard suggests an alternative approach in regard to unique_lock
: The current
minimum requirements on its template argument should better be reduced to BasicLockable
instead of the current Lockable
, including ammending member-wise constraints where required.
This suggestions was supported by Anthony, Daniel, and Pablo.
[2011-02-24 Reflector discussion]
Moved to Tentatively Ready after 5 votes.
The suggested wording changes are against the working draft N3242.
Remove 32.6.5.2 [thread.lock.guard] p. 2 completely:
2 The suppliedMutex
type shall meet theLockable
requirements (30.2.5.3).Change 32.6.5.4 [thread.lock.unique] p. 1-3 as indicated. The intend is to make
BasicLockable
the fundamental requirement forunique_lock
. We also update the note to reflect these changes and synchronize one remaining reference of 'mutex' by the proper term 'lockable object' in sync to the wording changes oflock_guard
:1 [..] The behavior of a program is undefined if the contained pointer
pm
is not null and themutexlockable object pointed to bypm
does not exist for the entire remaining lifetime (3.8) of theunique_lock
object. The suppliedMutex
type shall meet theBasicLockable
requirements (30.2.5.2).[Editor's note: BasicLockable is redundant, since the following additional paragraph requires Lockable.]
2 The suppliedMutex
type shall meet theLockable
requirements (30.2.5.3).3 [ Note:
unique_lock<Mutex>
meets theBasicLockable
requirements. IfMutex
meets theLockable
requirements ([thread.req.lockable.req]),unique_lock<Mutex>
also meets theLockable
requirements and ifMutex
meets theTimedLockable
requirements (30.2.5.4),unique_lock<Mutex>
also meets theTimedLockable
requirements. — end note ]Modify 32.6.5.4.2 [thread.lock.unique.cons] to add the now necessary member-wise additional constraints for
Lockable
:unique_lock(mutex_type& m, try_to_lock_t) noexcept;8 Requires: The supplied
9 Effects: Constructs an object of typeMutex
type shall meet theLockable
requirements ([thread.req.lockable.req]). Ifmutex_type
is not a recursive mutex the calling thread does not own the mutex.unique_lock
and callsm.try_lock()
.Modify 32.6.5.4.3 [thread.lock.unique.locking] to add the now necessary member-wise additional constraints for
Lockable
:bool try_lock();? Requires: The supplied
4 Effects:Mutex
type shall meet theLockable
requirements ([thread.req.lockable.req]).pm->try_lock()
Proposed resolution:
Resolved 2011-03 Madrid meeting by paper N3278
atomic<integral>
and atomic<T*>
Section: 32.5.8 [atomics.types.generic] Status: Resolved Submitter: Daniel Krügler Opened: 2010-12-08 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
Paragraph 5 and 6 of 32.5.8 [atomics.types.generic] impose different requirements on implementations for
specializations of the atomic
class template for integral types and for pointer types:
5 The atomic integral specializations and the specialization
atomic<bool>
shall have standard layout. They shall each have a trivial default constructor and a trivial destructor. They shall each support aggregate initialization syntax.
6 There are pointer partial specializations on the
atomic
class template. These specializations shall have trivial default constructors and trivial destructors.
It looks like an oversight to me, that for pointer specializations the requirements for standard layout and support for aggregate initialization syntax are omitted. In fact, this been confirmed by the N3193 proposal author. I suggest to impose the same implementation requirements for pointer types as for integral types, this should not impose unrealistic requirements on implementations.
[2011-02-10 Reflector discussion]
Moved to Tentatively Ready after 5 votes.
Proposed Resolution
The suggested wording changes are against the working draft N3242.
Change 32.5.8 [atomics.types.generic] p. 6 as indicated:
6 There are pointer partial specializations on the
atomic
class template. These specializations shall have standard layout, trivial default constructors, and trivial destructors. They shall each support aggregate initialization syntax.
Proposed resolution:
Resolved 2011-03 Madrid meeting by paper N3278
packaged_task
Section: 32.10.10.2 [futures.task.members] Status: Resolved Submitter: Daniel Krügler Opened: 2010-12-08 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
According to 32.10.10.2 [futures.task.members] p. 7 bullet 2:
packaged_task& operator=(packaged_task&& other);7 Effects:
[...]
packaged_task<R, ArgTypes...>(other).swap(*this)
.
The argument other
given to the move constructor is an lvalue and must be converted into
an rvalue via appropriate usage of std::move
.
Proposed Resolution
The suggested wording changes are against the working draft N3242.
Change 32.10.10.2 [futures.task.members] p. 7 bullet 2 as indicated:
packaged_task& operator=(packaged_task&& other);7 Effects:
[...]
packaged_task(std::move(other)).swap(*this)
.
Proposed resolution:
Resolved 2011-03 Madrid meeting by paper N3278
packaged_task
Section: 32.10.10.2 [futures.task.members] Status: C++11 Submitter: Alberto Ganesh Barbati Opened: 2011-02-09 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
Related with LWG issue 1514(i).
The move constructor of packaged_task
does not specify how the stored task is constructed.
The obvious way is to move-construct it using the task stored in the argument. Moreover, the
constructor should be provided with a throws clause similar to one used for the other constructors,
as the move constructor of the stored task is not required to be nothrow.
As for the other constructors, the terms "stores a copy of f
" do not reflect the intent, which is
to allow f
to be moved when possible.
[2011-02-25: Alberto updates wording]
[2011-02-26 Reflector discussion]
Moved to Tentatively Ready after 5 votes.
Proposed resolution:
(wording written assuming LWG 1514(i) is also accepted)
Change 32.10.10.2 [futures.task.members] paragraph 3:
3 Effects: constructs a new
packaged_task
object with an associated asynchronous state andstores a copy ofinitializes the object's stored task withf
as the object's stored taskstd::forward<F>(f)
. The constructors that take anAllocator
argument use it to allocate memory needed to store the internal data structures.
Change 32.10.10.2 [futures.task.members] paragraph 5:
5 Effects: constructs a new
packaged_task
object and transfers ownership ofother
's associated asynchronous state to*this
, leavingother
with no associated asynchronous state. Moves the stored task fromother
to*this
.
messages_base::catalog
overspecifiedSection: 28.3.4.8.2 [locale.messages] Status: C++14 Submitter: Howard Hinnant Opened: 2011-02-14 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
In 28.3.4.8.2 [locale.messages], messages_base::catalog
is specified to be a typedef to int
.
This type is subsequently used to open, access and close catalogs.
However, an OS may have catalog/messaging services that are indexed and managed by types other than int
.
For example POSIX
, publishes the following messaging API:
typedef unspecified nl_catd; nl_catd catopen(const char* name , int oflag); char* catgets(nl_catd catd, int set_id, int msg_id, const char* s); int catclose(nl_catd catd);
I.e., the catalog is managed with an unspecified type, not necessarily an int
.
Mac OS uses a void*
for nl_catd
(which is conforming to the POSIX
standard).
The current messages_base
spec effectively outlaws using the built-in OS messaging service
supplied for this very purpose!
[2011-02-24: Chris Jefferson updates the proposed wording, changing unspecified to unspecified signed integral type]
[2011-03-02: Daniel updates the proposed wording, changing unspecified signed integral type to
unspecified signed integer type (We don't want to allow for bool
or char
)]
[2011-03-24 Madrid meeting]
Consensus that this resolution is the direction we would like to see.
Proposed resolution:
Modify 28.3.4.8.2 [locale.messages]:
namespace std { class messages_base { public: typedefintunspecified signed integer type catalog; }; ... }
noexcept
' on basic_regex
move-assignment operatorSection: 28.6.7 [re.regex] Status: C++11 Submitter: Jonathan Wakely Opened: 2011-02-16 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
N3149 replaced
the "Throws: nothing
" clause on basic_regex::assign(basic_regex&&)
with
the noexcept
keyword. The effects of the move-assignment operator are defined in terms of
the assign()
function, so the "Throws: nothing
" applied there too, and a
noexcept
-specification should be added there too.
[2011-02-24 Reflector discussion]
Moved to Tentatively Ready after 7 votes.
Proposed resolution:
Modify the basic_regex
synopsis in 28.6.7 [re.regex] p. 3:
namespace std { template <class charT, class traits = regex_traits<charT> > class basic_regex { public: ... basic_regex& operator=(const basic_regex&); basic_regex& operator=(basic_regex&&) noexcept; basic_regex& operator=(const charT* ptr); ... }; }
Modify 28.6.7.3 [re.regex.assign] p. 2:
basic_regex& operator=(basic_regex&& e) noexcept;2 Effects: returns
assign(std::move(e))
.
packaged_task::result_type
should be removedSection: 32.10.10 [futures.task] Status: C++11 Submitter: Anthony Williams Opened: 2010-11-12 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
packaged_task::operator()
always returns void
, regardless of the return
type of the wrapped task. However, packaged_task::result_type
is a
typedef to the return type of the wrapped task. This is inconsistent
with other uses of result_type
in the standard, where it matches the
return type of operator()
(e.g. function
, owner_less
). This is confusing.
It also violates the TR1 result_of
protocol, and thus makes
packaged_task
harder to use with anything that respects that protocol.
Finally, it is of little use anyway.
packaged_task::result_type
should therefore be removed.
[2011-02-24 Reflector discussion]
Moved to Tentatively Ready after 5 votes.
Proposed resolution:
Alter the class definition of packaged_task
in 32.10.10 [futures.task] p. 2 as follows:
template<class R, class... ArgTypes> class packaged_task<R(ArgTypes...)> { public:typedef R result_type;[...] };
std::future<>::share()
only applies to rvaluesSection: 32.10.7 [futures.unique.future] Status: C++11 Submitter: Anthony Williams Opened: 2011-02-17 Last modified: 2021-06-06
Priority: Not Prioritized
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Discussion:
As specified, future<>::share()
has the signature
shared_future<R> share() &&;
This means that it can only be applied to rvalues. One of the key benefits of share()
is
that it can be used with the new auto
facility:
std::promise<some_long_winded_type_name> some_promise; auto f = some_promise.get_future(); // std::future auto sf = std::move(f).share();
share()
is sufficiently explicit that the move should not be required. We should be able to write:
auto sf = f.share();
[2011-02-22 Reflector discussion]
Moved to Tentatively Ready after 5 votes.
Proposed resolution:
Alter the declaration of share()
to remove the "&&" rvalue qualifier in
[futures.unique_future] p. 3, and [futures.unique_future] p. 11:
shared_future<R> share()&&;
async
functionSection: 32.10.9 [futures.async] Status: C++11 Submitter: Alberto Ganesh Barbati Opened: 2011-02-17 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
Clause 32.10.9 [futures.async] has undergone significant rewording in Batavia. Due to co-presence of at least three different sources of modification there is a part where changes have overlapped (marked by an Editor's note), which should be reconciled. Moreover, I believe that a few non-overlapping sentences are now incorrect and should be fixed, so the problem cannot be handled editorially. (See c++std-lib-29667.)
[Adopted in Madrid, 2011-03]
Proposed resolution:
Edit 32.10.5 [futures.state], paragraph 3 as follows.
An asynchronous return object is an object that reads results from an associated asynchronous state. A waiting function of an asynchronous return object is one that potentially blocks to wait for the associated asynchronous state to be made ready. If a waiting function can return before the state is made ready because of a timeout (30.2.5), then it is a timed waiting function, otherwise it is a non-timed waiting function.
Edit within 32.10.9 [futures.async] paragraph 3 bullet 2 as follows.
Effects: [...]
- if
policy & launch::deferred
is non-zero — [...] The associated asynchronous state is not made ready until the function has completed. The first call to a non-timed waiting function (30.6.4 [futures.state])requiring a non-timed waiton an asynchronous return object referring tothethis associated asynchronous statecreated by thisshall invoke the deferred function in the thread that called the waiting functionasync
call to become ready;.onceOnce evaluation ofINVOKE(g, xyz)
begins, the function is no longer considered deferred. [...]
Edit 32.10.9 [futures.async] paragraph 5 as follows.
Synchronization: Regardless of the provided
policy
argument,
- the invocation of
async
synchronizes with (1.10) the invocation off
. [Note: this statement applies even when the corresponding future object is moved to another thread. —end note]; and- the completion of the function
f
is sequenced before (1.10) the associated asynchronous state is made ready. [Note:f
might not be called at all, so its completion might never happen. —end note]
IfIf the implementation chooses thepolicy & launch::async
is non-zero,launch::async
policy,
- a call to a waiting function on an asynchronous return object that shares the associated asynchronous state created by this
async
call shall block until the associated thread has completed, as if joined (30.3.1.5);thethe associated thread completion synchronizes with (1.10) the return from the first function that successfully detects the ready status of the associated asynchronous state or with the return from the last function that releases the associated asynchronous statejoin()
on the created thread objectreturns, whichever happens first.[Editor's note: N3196 changes the following sentence as indicated. N3188 removes the sentence. Please pick one.] If the invocation is deferred, the completion of the invocation of the deferred function synchronizes with the successful return from a call to a waiting function on the associated asynchronous state.
reserve
, shrink_to_fit
, and resize
functionsSection: 23.3.13.3 [vector.capacity], 23.3.5.3 [deque.capacity] Status: C++14 Submitter: Nikolay Ivchenkov Opened: 2011-02-20 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
I have several questions with regard to the working paper N3225 (C++0x working draft):
Where the working draft specifies preconditions for shrink_to_fit
member function of std::vector
and std::deque
?
Where the working draft specifies preconditions for 'void reserve(size_type n)
'
member function of std::vector
?
Does a call to 'void resize(size_type sz)
' of std::vector
require
the element type to be DefaultConstructible
? If yes, why such
requirement is not listed in the Requires paragraph?
Does a call to 'void resize(size_type sz)
' of std::vector
require
the element type to be MoveAssignable
because the call erase(begin() + sz, end())
mentioned in the Effects paragraph would require the element type to be MoveAssignable
?
Why CopyInsertable
requirement is used for 'void resize(size_type sz)
' of std::vector
instead of MoveInsertable
requirement?
[2011-06-12: Daniel comments and provides wording]
According to my understanding of the mental model of vector (and to some parts for deque) the some requirements are missing in the standard as response to above questions:
shrink_to_fit
for both std::vector
and std::deque
should impose the MoveInsertable
requirements. The reason for this is, that these containers
can host move-only types. For a container type X
the C++03 idiom X(*this).swap(*this)
imposes the CopyInsertable
requirements which would make the function call ill-formed,
which looks like an unacceptable restriction to me. Assuming the committee decides to support the
move-only case, further wording has to be added for the situation where such a move-only type could
throw an exception, because this can leave the object in an unspecified state. This seems consistent
with the requirements of reserve
, which seems like a very similar function to me (for
vector
). And this brings us smoothly to the following bullet:
I agree that we are currently missing to specify the preconditions of the reserve
function.
My interpretation of the mental model of this function is that it should work for move-only types, which
seems to be supported by the wording used in 23.3.13.3 [vector.capacity] p2:
[…] If an exception is thrown other than by the move constructor of a non-CopyInsertable type, there are no effects.
Given this statement, the appropriate requirement is MoveInsertable
into the vector
.
vector::resize(size_type)
misses to list the DefaultConstructible
requirements.
erase
implies the MoveAssignable
requirements. I don't think that this implication is intended. This function requires "append" and
"pop-back" effects, respectively, where the former can be realized in terms of MoveInsertable
requirements. The same fix in regard to using pop_back
instead of erase
is necessary
for the two argument overload of resize
as well (no MoveAssignable
is required).
CopyInsertable
requirement is incorrect and should be MoveInsertable
instead.In addition to above mentioned items, the proposed resolution adds a linear complexity bound for
shrink_to_fit
and attempts to resolve the related issue 2066(i).
[ 2011 Bloomington ]
Move to Ready.
Note for editor: we do not normally refer to 'linear time' for complexity requirements, but there is agreement that any clean-up of such wording is editorial.
Proposed resolution:
This wording is relative to the FDIS.
Edit 23.3.5.3 [deque.capacity] as indicated [Remark: The suggested change of p4 is
not redundant, because CopyInsertable
is not necessarily a refinement of MoveInsertable
in contrast to the fact that CopyConstructible
is a refinement of MoveConstructible
]:
void resize(size_type sz);-1- Effects: If
-2- Requires:sz <= size()
, equivalent tocallingerase(begin() + sz, end());
pop_back()
size() - sz
times. Ifsize() < sz
, appendssz - size()
value-initialized elements to the sequence.T
shall beMoveInsertable
into*this
andDefaultConstructible
.
void resize(size_type sz, const T& c);-3- Effects: If
sz <= size()
, equivalent to callingpop_back()
size() - sz
times. Ifsize() < sz
, appendssz - size()
copies ofc
to the sequence.if (sz > size()) insert(end(), sz-size(), c); else if (sz < size()) erase(begin()+sz, end()); else ; // do nothing-4- Requires:
T
shall beMoveInsertable
into*this
andCopyInsertable
into*this
.
void shrink_to_fit();-?- Requires:
-?- Complexity: Takes at most linear time in the size of the sequence. -5- Remarks:T
shall beMoveInsertable
into*this
.shrink_to_fit
is a non-binding request to reduce memory use but does not change the size of the sequence. [ Note: The request is non-binding to allow latitude for implementation-specific optimizations. — end note ]
Edit 23.3.13.3 [vector.capacity] as indicated including edits that also resolve 2066(i)
[Remark: The combined listing of MoveInsertable
and CopyInsertable
before p12 is not redundant,
because CopyInsertable
is not necessarily a refinement of MoveInsertable
in contrast to the
fact that CopyConstructible
is a refinement of MoveConstructible
]:
[…]
void reserve(size_type n);-?- Requires:
-2- Effects: A directive that informs a vector of a planned change in size, so that it can manage the storage allocation accordingly. AfterT
shall beMoveInsertable
into*this
.reserve()
,capacity()
is greater or equal to the argument of reserve if reallocation happens; and equal to the previous value ofcapacity()
otherwise. Reallocation happens at this point if and only if the current capacity is less than the argument ofreserve()
. If an exception is thrown other than by the move constructor of a non-CopyInsertable
type, there are no effects. -3- Complexity: It does not change the size of the sequence and takes at most linear time in the size of the sequence. -4- Throws:length_error
ifn > max_size()
.[footnote 266] -5- Remarks: Reallocation invalidates all the references, pointers, and iterators referring to the elements in the sequence. It is guaranteed that no reallocation takes place during insertions that happen after a call toreserve()
until the time when an insertion would make the size of the vector greater than the value ofcapacity()
.
void shrink_to_fit();-?- Requires:
-?- Complexity: Takes at most linear time in the size of the sequence. -6- Remarks:T
shall beMoveInsertable
into*this
.shrink_to_fit
is a non-binding request to reducecapacity()
tosize()
. [ Note: The request is non-binding to allow latitude for implementation-specific optimizations. — end note ] If an exception is thrown other than by the move constructor of a non-CopyInsertable
T
there are no effects.
[…]
void resize(size_type sz);-9- Effects: If
-10- Requires:sz <= size()
, equivalent tocallingerase(begin() + sz, end());
pop_back()
size() - sz
times. Ifsize() < sz
, appendssz - size()
value-initialized elements to the sequence.T
shall beinto
CopyMoveInsertable*this
andDefaultConstructible
. -??- Remarks: If an exception is thrown other than by the move constructor of a non-CopyInsertable
T
there are no effects.
void resize(size_type sz, const T& c);-11- Effects: If
sz <= size()
, equivalent to callingpop_back()
size() - sz
times. Ifsize() < sz
, appendssz - size()
copies ofc
to the sequence.if (sz > size()) insert(end(), sz-size(), c); else if (sz < size()) erase(begin()+sz, end()); else ; // do nothing-??- Requires:
-12-T
shall beMoveInsertable
into*this
andCopyInsertable
into*this
.RequiresRemarks: If an exception is thrown other than by the move constructor of a non-CopyInsertable
T
there are no effects.
Section: 32.5.4 [atomics.order] Status: Resolved Submitter: Hans Boehm Opened: 2011-02-26 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
This violates the core intent of the memory model, as stated in the note in 6.9.2 [intro.multithread] p. 21.
This was discovered by Mark Batty, and pointed out in their POPL 2011 paper, "Mathematizing C++ Concurrency", section 4, "Sequential consistency of SC atomics". The problem is quite technical, but well-explained in that paper.
This particular issue was not understood at the time the FCD comments were generated. But it is closely related to a number of FCD comments. It should have arisen from US-171, though that's not the actual history.
This issue has been under discussion for several months in a group that included a half dozen or so of the most interested committee members. The P/R represents a well-considered consensus among us:
[2011-03-16: Jens updates wording]
Modify 32.5.4 [atomics.order] p.3, so that the normative part reads:
3 There shall be a single total order S on all
memory_order_seq_cst
operations, consistent with the "happens before" order and modification orders for all affected locations, such that eachmemory_order_seq_cst
operation that loads a value observes either the last preceding modification according to this order S, A (if any), or the result of an operation X thatis notdoes not happen before A and that is notmemory_order_seq_cst
. [ Note: Although it is not explicitly required that S include locks, it can always be extended to an order that does include lock and unlock operations, since the ordering between those is already included in the "happens before" ordering. — end note ]
Proposed resolution:
Resolved 2011-03 Madrid meeting by paper N3278
atomic
free functions incorrectly specifiedSection: 32.5.2 [atomics.syn] Status: Resolved Submitter: Pete Becker Opened: 2011-03-01 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
In earlier specifications of atomics the template specialization atomic<integer>
was derived from atomic_integer
(e.g. atomic<int>
was derived from atomic_int
),
and the working draft required free functions such as
int atomic_load(const atomic_int*)
for each of the atomic_integer
types. This worked fine with normal function overloading.
For the post-Batavia working draft, N3193 removed the requirement that atomic<integer>
be derived from atomic_integer
and replaced the free functions taking pointers to
atomic_integer
with template functions taking atomic_type*
, such as
template <class T> T atomic_load(const atomic_type*);
and a code comment explaining that atomic_type
can be either atomic<T>
or a
named base class of atomic<T>
. The latter possibility is supposed to allow existing
implementations based on the previous specification to continue to conform.
From history, this allowance seems to imply that functions like atomic_load
can be non-template
free functions, as they were before. The explicit requirements do not allow this, and, by requiring that
they be templates, make them far more complicated. As the specification is currently written, code that
uses an implementation that uses a base class would have to provide an explicit template type:
atomic<int> my_atomic_int; atomic_load<int>(&my_atomic_int);
That type argument isn't needed when atomic_type
is atomic<T>
, but cautious
users would always provide it to make their code portable across different implementations of the
standard library.
One possibility for the implementor would be to do some template meta-programming to infer the type
T
when there are no function parameters of type T
, but without running afoul of the
prohibition on adding parameters with default values (16.4.6.4 [global.functions]/3).
So the promise that implementations of the previous specification continue to conform has not been met. The specification of these free functions should be rewritten to support library code written to the previous specification or the vacuous promise should be removed.
[2011-03-08: Lawrence comments and drafts wording:]
One of the goals is to permit atomics code to compile under both C and C++. Adding explicit template arguments would defeat that goal.
The intent was to permit the normal function overloads foratomic_int
when atomic_int
is distinct from atomic<int>
. That intent was not reflected in the wording.
Proposed Resolution
Explicitly permit free functions.
Edit within the header
<atomic>
synopsis 32.5.2 [atomics.syn] as follows:// 29.6.1, general operations on atomic types// In the following declarations, atomic_type is either //// In the following declarations, atomic-type is either //atomic<T>
or a named base class forT
from // Table 145 or inferred from // Table 146.atomic<T>
or a named base class forT
from // Table 145 or inferred from // Table 146. // If it isatomic<T>
, then the declaration is a template // declaration prefixed withtemplate <class T>
template <class T>bool atomic_is_lock_free(const volatileatomic_typeatomic-type*);template <class T>bool atomic_is_lock_free(constatomic_typeatomic-type*);template <class T>void atomic_init(volatileatomic_typeatomic-type*, T);template <class T>void atomic_init(atomic_typeatomic-type*, T);template <class T>void atomic_store(volatileatomic_typeatomic-type*, T);template <class T>void atomic_store(atomic_typeatomic-type*, T);template <class T>void atomic_store_explicit(volatileatomic_typeatomic-type*, T, memory_order);template <class T>void atomic_store_explicit(atomic_typeatomic-type*, T, memory_order);template <class T>T atomic_load(const volatileatomic_typeatomic-type*);template <class T>T atomic_load(constatomic_typeatomic-type*);template <class T>T atomic_load_explicit(const volatileatomic_typeatomic-type*, memory_order);template <class T>T atomic_load_explicit(constatomic_typeatomic-type*, memory_order);template <class T>T atomic_exchange(volatileatomic_typeatomic-type*, T);template <class T>T atomic_exchange(atomic_typeatomic-type*, T);template <class T>T atomic_exchange_explicit(volatileatomic_typeatomic-type*, T, memory_order);template <class T>T atomic_exchange_explicit(atomic_typeatomic-type*, T, memory_order);template <class T>bool atomic_compare_exchange_weak(volatileatomic_typeatomic-type*, T*, T);template <class T>bool atomic_compare_exchange_weak(atomic_typeatomic-type*, T*, T);template <class T>bool atomic_compare_exchange_strong(volatileatomic_typeatomic-type*, T*, T);template <class T>bool atomic_compare_exchange_strong(atomic_typeatomic-type*, T*, T);template <class T>bool atomic_compare_exchange_weak_explicit(volatileatomic_typeatomic-type*, T*, T, memory_order, memory_order);template <class T>bool atomic_compare_exchange_weak_explicit(atomic_typeatomic-type*, T*, T. memory_order, memory_order);template <class T>bool atomic_compare)exchange_strong_explicit(volatileatomic_typeatomic-type*, T*, T, memory_order, memory_order);template <class T>bool atomic_compare_exchange_strong_explicit(atomic_typeatomic-type*, T*, T, memory_order, memory_order); // 29.6.2, templated operations on atomic types// In the following declarations, atomic_type is either //template <class T> T atomic_fetch_add(volatileatomic<T>
or a named base class forT
from // Table 145 or inferred from // Table 146.atomic-typeatomic<T>*, T); template <class T> T atomic_fetch_add(atomic-typeatomic<T>*, T); template <class T> T atomic_fetch_add_explicit(volatileatomic-typeatomic<T>*, T, memory_order); template <class T> T atomic_fetch_add_explicit(atomic-typeatomic<T>*, T, memory_order); template <class T> T atomic_fetch_sub(volatileatomic-typeatomic<T>*, T); template <class T> T atomic_fetch_sub(atomic-typeatomic<T>*, T); template <class T> T atomic_fetch_sub_explicit(volatileatomic-typeatomic<T>*, T, memory_order); template <class T> T atomic_fetch_sub_explicit(atomic-typeatomic<T>*, T, memory_order); template <class T> T atomic_fetch_and(volatileatomic-typeatomic<T>*, T); template <class T> T atomic_fetch_and(atomic-typeatomic<T>*, T); template <class T> T atomic_fetch_and_explicit(volatileatomic-typeatomic<T>*, T, memory_order); template <class T> T atomic_fetch_and_explicit(atomic-typeatomic<T>*, T, memory_order); template <class T> T atomic_fetch_or(volatileatomic-typeatomic<T>*, T); template <class T> T atomic_fetch_or(atomic-typeatomic<T>*, T); template <class T> T atomic_fetch_or_explicit(volatileatomic-typeatomic<T>*, T, memory_order); template <class T> T atomic_fetch_or_explicit(atomic-typeatomic<T>*, T, memory_order); template <class T> T atomic_fetch_xor(volatileatomic-typeatomic<T>*, T); template <class T> T atomic_fetch_xor(atomic-typeatomic<T>*, T); template <class T> T atomic_fetch_xor_explicit(volatileatomic-typeatomic<T>*, T, memory_order); template <class T> T atomic_fetch_xor_explicit(atomic-typeatomic<T>*, T, memory_order); // 29.6.3, arithmetic operations on atomic types // In the following declarations, atomic-integral is either //atomic<T>
or a named base class forT
from // Table 145 or inferred from // Table 146. // If it isatomic<T>
, // then the declaration is a template specialization declaration prefixed with //template <>
template <>integral atomic_fetch_add(volatile atomic-integral*, integral);template <>integral atomic_fetch_add(atomic-integral*, integral);template <>integral atomic_fetch_add_explicit(volatile atomic-integral*, integral, memory_order);template <>integral atomic_fetch_add_explicit(atomic-integral*, integral, memory_order);template <>integral atomic_fetch_sub(volatile atomic-integral*, integral);template <>integral atomic_fetch_sub(atomic-integral*, integral);template <>integral atomic_fetch_sub_explicit(volatile atomic-integral*, integral, memory_order);template <>integral atomic_fetch_sub_explicit(atomic-integral*, integral, memory_order);template <>integral atomic_fetch_and(volatile atomic-integral*, integral);template <>integral atomic_fetch_and(atomic-integral*, integral);template <>integral atomic_fetch_and_explicit(volatile atomic-integral*, integral, memory_order);template <>integral atomic_fetch_and_explicit(atomic-integral*, integral, memory_order);template <>integral atomic_fetch_or(volatile atomic-integral*, integral);template <>integral atomic_fetch_or(atomic-integral*, integral);template <>integral atomic_fetch_or_explicit(atomic-integral*, integral, memory_order);template <>integral atomic_fetch_or_explicit(atomic-integral*, integral, memory_order);template <>integral atomic_fetch_xor(volatile atomic-integral*, integral);template <>integral atomic_fetch_xor(atomic-integral*, integral);template <>integral atomic_fetch_xor_explicit(volatile atomic-integral*, integral, memory_order);template <>integral atomic_fetch_xor_explicit(atomic-integral*, integral, memory_order);Edit [atomics.types.operations.general] paragraph 1+2 as follows:
-1- The implementation shall provide the functions and function templates identified as "general operations on atomic types" in 32.5.2 [atomics.syn].
-2- In the declarations of these functions and function templates, the name atomic-type refers to eitheratomic<T>
or to a named base class forT
from Table 145 or inferred from Table 146.In [atomics.types.operations.templ] delete paragraph 2:
-1- The implementation shall declare but not define the function templates identified as "templated operations on atomic types" in 32.5.2 [atomics.syn].
-2- In the declarations of these templates, the name atomic-type refers to eitheratomic<T>
or to a named base class forT
from Table 145 or inferred from Table 146.Edit [atomics.types.operations.arith] paragraph 1+2 as follows:
-1- The implementation shall provide the functions and function template specializations identified as "arithmetic operations on atomic types" in 32.5.2 [atomics.syn].
-2- In the declarations of these functions and function template specializations, the name integral refers to an integral type and the name atomic-integral refers to eitheratomic<
integral>
or to a named base class for integral from Table 145 or inferred from Table 146.
Proposed resolution:
Resolved 2011-03 Madrid meeting by paper N3278
std::reverse
and std::copy_if
Section: 26.7.1 [alg.copy], 26.7.10 [alg.reverse] Status: C++14 Submitter: Nikolay Ivchenkov Opened: 2011-03-02 Last modified: 2016-01-28
Priority: Not Prioritized
View other active issues in [alg.copy].
View all other issues in [alg.copy].
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Discussion:
In the description of std::reverse
Effects: For each non-negative integer
i <= (last - first)/2
, appliesiter_swap
to all pairs of iteratorsfirst + i
,(last - i) - 1
.
should be changed to
Effects: For each non-negative integer
i < (last - first)/2
, appliesiter_swap
to all pairs of iteratorsfirst + i
,(last - i) - 1
.
Here i
shall be strictly less than (last - first)/2
.
In the description of std::copy_if
Returns paragraph is missing.
[2011-03-02: Daniel drafts wording]
Proposed resolution:
Modify 26.7.10 [alg.reverse] p. 1 as indicated:
1 Effects: For each non-negative integer
i <
, applies=(last - first)/2iter_swap
to all pairs of iteratorsfirst + i
,(last - i) - 1
.
Add the following Returns element after 26.7.1 [alg.copy] p. 9:
template<class InputIterator, class OutputIterator, class Predicate> OutputIterator copy_if(InputIterator first, InputIterator last, OutputIterator result, Predicate pred);8 Requires: The ranges
9 Effects: Copies all of the elements referred to by the iterator[first,last)
and[result,result + (last - first))
shall not overlap.i
in the range[first,last)
for whichpred(*i)
is true. ?? Returns: The end of the resulting range. 10 Complexity: Exactlylast - first
applications of the corresponding predicate. 11 Remarks: Stable.
is_convertible
Section: 21 [meta] Status: Resolved Submitter: Daniel Krügler Opened: 2011-03-03 Last modified: 2020-09-06
Priority: Not Prioritized
View other active issues in [meta].
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Discussion:
When n3142 was suggested, it concentrated on constructions, assignments, and destructions, but overlooked to complement the single remaining compiler-support trait
template <class From, class To> struct is_convertible;
with the no-throw and triviality related aspects as it had been done with the other expression-based traits. Specifically, the current specification misses to add the following traits:
template <class From, class To> struct is_nothrow_convertible; template <class From, class To> struct is_trivially_convertible;
In particular the lack of is_nothrow_convertible
is severely restricting. This
was recently recognized when the proposal for decay_copy
was prepared by
n3255.
There does not exist a portable means to define the correct conditional noexcept
specification for the decay_copy
function template, which is declared as:
template <class T> typename decay<T>::type decay_copy(T&& v) noexcept(???);
The semantics of decay_copy
bases on an implicit conversion which again
influences the overload set of functions that are viable here. In most circumstances
this will have the same effect as comparing against the trait
std::is_nothrow_move_constructible
, but there is no guarantee for that being
the right answer. It is possible to construct examples, where this would lead
to the false result, e.g.
struct S { S(const S&) noexcept(false); template<class T> explicit S(T&&) noexcept(true); };
std::is_nothrow_move_constructible
will properly honor the explicit template
constructor because of the direct-initialization context which is part of the
std::is_constructible
definition and will in this case select it, such that
std::is_nothrow_move_constructible<S>::value == true
, but if we had
the traits is_nothrow_convertible
, is_nothrow_convertible<S, S>::value
would evaluate to false
, because it would use the copy-initialization context
that is part of the is_convertible
definition, excluding any explicit
constructors and giving the opposite result.
The decay_copy
example is surely not one of the most convincing examples, but
is_nothrow_convertible
has several use-cases, and can e.g. be used to express
whether calling the following implicit conversion function could throw an exception or not:
template<class T, class U> T implicit_cast(U&& u) noexcept(is_nothrow_convertible<U, T>::value) { return std::forward<U>(u); }
Therefore I suggest to add the missing trait is_nothrow_convertible
and for
completeness also the missing trait is_trivially_convertible
to 21 [meta].
[2011-03-24 Madrid meeting]
Daniel K: This is a new feature so out of scope.
Pablo: Any objections to moving 2040 to Open? No objections.[Bloomington, 2011]
Move to NAD Future, this would be an extension to existing functionality.
[LEWG, Kona 2017]
Fallen through the cracks since 2011, but we should discuss it. Alisdair points out that triviality is
about replacing operations with memcpy
, so be sure this is false for int
->float
.
[Rapperswil, 2018]
Resolved by the adoption of p0758r1.
Proposed resolution:
Ammend the following declarations to the header <type_traits>
synopsis
in 21.3.3 [meta.type.synop]:
namespace std { … // 20.9.6, type relations: template <class T, class U> struct is_same; template <class Base, class Derived> struct is_base_of; template <class From, class To> struct is_convertible; template <class From, class To> struct is_trivially_convertible; template <class From, class To> struct is_nothrow_convertible; … }
Modify Table 51 — "Type relationship predicates" as indicated. The removal of the
remaining traces of the trait is_explicitly_convertible
is an editorial
step, it was removed by n3047:
Table 51 — Type relationship predicates Template Condition Comments … template <class From, class To>
struct is_convertible;see below From
andTo
shall be complete
types, arrays of unknown bound, or
(possibly cv-qualified)void
types.template <class From, class To>
struct is_explicitly_convertible;is_constructible<To, From>::value
a synonym for a two-argument
version ofis_constructible
.
An implementation may define it
as an alias template.template <class From, class To>
struct is_trivially_convertible;is_convertible<From,
is
To>::valuetrue
and the
conversion, as defined by
is_convertible
, is known
to call no operation that is
not trivial ([basic.types], [special]).From
andTo
shall be complete
types, arrays of unknown bound,
or (possibly cv-qualified)void
types.template <class From, class To>
struct is_nothrow_convertible;is_convertible<From,
is
To>::valuetrue
and the
conversion, as defined by
is_convertible
, is known
not to throw any
exceptions ([expr.unary.noexcept]).From
andTo
shall be complete
types, arrays of unknown bound,
or (possibly cv-qualified)void
types.…
Section: 28.3.4.3.2.3 [facet.num.get.virtuals] Status: C++11 Submitter: Howard Hinnant Opened: 2011-03-09 Last modified: 2016-01-28
Priority: Not Prioritized
View other active issues in [facet.num.get.virtuals].
View all other issues in [facet.num.get.virtuals].
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Discussion:
num_get
Stage 2 accumulation changed between C++03 and the current C++0x working draft. The sentences:
If it is not discarded, then a check is made to determine if
c
is allowed as the next character of an input field of the conversion specifier returned by stage 1. If so it is accumulated.
have been dropped from 28.3.4.3.2.3 [facet.num.get.virtuals], Stage 2, paragraph 3 that begins:
If
discard
is true, […]
Consider this code:
#include <sstream> #include <iostream> int main(void) { std::istringstream s("8cz"); long i = 0; char c; s >> i; if (!s.fail()) std::cout << "i = " << i << '\n'; else { std::cout << "s >> i failed\n"; s.clear(); } s >> c; if (!s.fail()) std::cout << "c = " << c << '\n'; else std::cout << "s >> c failed\n"; }
C++0x currently prints out:
s >> i failed c = z
However C++03 conforming implementations will output:
i = 8 c = c
I believe we need to restore C++03 compatibility.
Proposed resolution:
Add to 28.3.4.3.2.3 [facet.num.get.virtuals], Stage 2:
If
If the character is either discarded or accumulated then in is advanced bydiscard
is true, then if'.'
has not yet been accumulated, then the position of the character is remembered, but the character is otherwise ignored. Otherwise, if'.'
has already been accumulated, the character is discarded and Stage 2 terminates. If it is not discarded, then a check is made to determine ifc
is allowed as the next character of an input field of the conversion specifier returned by stage 1. If so it is accumulated.++in
and processing returns to the beginning of stage 2.
forward_list::before_begin()
to forward_list::end()
Section: 23.3.7.3 [forward.list.iter] Status: C++11 Submitter: Joe Gottman Opened: 2011-03-13 Last modified: 2023-02-07
Priority: Not Prioritized
View all issues with C++11 status.
Discussion:
For an object c
of type forward_list<X, Alloc>
, the iterators
c.before_begin()
and c.end()
are part of the same underlying sequence,
so the expression c.before_begin() == c.end()
must be well-defined.
But the standard says nothing about what the result of this expression
should be. The forward iterator requirements says no dereferenceable
iterator is equal to a non-dereferenceable iterator and that two
dereferenceable iterators are equal if and only if they point to the
same element. But since before_begin()
and end()
are both
non-dereferenceable, neither of these rules applies.
Many forward_list
methods, such as insert_after()
, have a
precondition that the iterator passed to them must not be equal to
end()
. Thus, user code might look like the following:
void foo(forward_list<int>& c, forward_list<int>::iterator it) { assert(it != c.end()); c.insert_after(it, 42); }
Conversely, before_begin()
was specifically designed to be used with
methods like insert_after()
, so if c.before_begin()
is passed to
this function the assertion must not fail.
[2011-03-14: Daniel comments and updates the suggested wording]
The suggested wording changes are necessary but not sufficient. Since there
does not exist an equivalent semantic definition of cbefore_begin()
as
we have for cbegin()
, this still leaves the question open whether
the normative remark applies to cbefore_begin()
as well. A simple fix
is to define the operational semantics of cbefore_begin()
in terms of
before_begin()
.
[2011-03-24 Madrid meeting]
General agreement that this is a serious bug.
Pablo: Any objections to moving 2042 to Immediate? No objections.Proposed resolution:
Add to the definition of forward_list::before_begin()
[forwardlist.iter]
the following:
iterator before_begin(); const_iterator before_begin() const; const_iterator cbefore_begin() const;-1- Returns: A non-dereferenceable iterator that, when incremented, is equal to the iterator returned by
-?- Effects:begin()
.cbefore_begin()
is equivalent toconst_cast<forward_list const&>(*this).before_begin()
. -?- Remarks:before_begin() == end()
shall equal false.
Section: 16.4.6.8 [algorithm.stable] Status: C++14 Submitter: Pablo Halpern Opened: 2011-03-24 Last modified: 2016-01-28
Priority: Not Prioritized
View all issues with C++14 status.
Discussion:
16.4.6.8 [algorithm.stable] specified the meaning of "stable" when applied to the different types of algorithms. The second bullet says:
— For the remove algorithms the relative order of the elements that are not removed is preserved.
There is no description of what "stable" means for copy algorithms, even though the term is
applied to copy_if
(and perhaps others now or in the future). Thus, copy_if
is using the term without a precise definition.
[Bloomington, 2011]
Move to Ready
Proposed resolution:
This wording is relative to the FDIS.
In 16.4.6.8 [algorithm.stable] p. 1 change as indicated:
When the requirements for an algorithm state that it is “stable” without further elaboration, it means:
- For the sort algorithms the relative order of equivalent elements is preserved.
- For the remove and copy algorithms the relative order of the elements that are not removed is preserved.
- For the merge algorithms, for equivalent elements in the original two ranges, the elements from the first range precede the elements from the second range.
forward_list::merge
and forward_list::splice_after
with unequal allocatorsSection: 23.3.7.6 [forward.list.ops] Status: C++14 Submitter: Pablo Halpern Opened: 2011-03-24 Last modified: 2023-02-07
Priority: Not Prioritized
View all other issues in [forward.list.ops].
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Discussion:
list::merge
and list::splice
have the requirement that the two lists being merged or
spliced must use the same allocator. Otherwise, moving list nodes from one container to the other would
corrupt the data structure. The same requirement is needed for forward_list::merge
and
forward_list::splice_after
.
[ 2011 Bloomington ]
Move to Ready.
Proposed resolution:
This wording is relative to the FDIS.
In [forwardlist.ops] p. 1 change as indicated:
void splice_after(const_iterator position, forward_list<T,Allocator>& x); void splice_after(const_iterator position, forward_list<T,Allocator>&& x);1 - Requires:
position
isbefore_begin()
or is a dereferenceable iterator in the range [begin()
,end()
).get_allocator() == x.get_allocator()
.&x != this
.
In [forwardlist.ops] p. 5 change as indicated:
void splice_after(const_iterator position, forward_list<T,Allocator>& x, const_iterator i); void splice_after(const_iterator position, forward_list<T,Allocator>&& x, const_iterator i);5 - Requires:
position
isbefore_begin()
or is a dereferenceable iterator in the range [begin()
,end()
). The iterator followingi
is a dereferenceable iterator inx
.get_allocator() == x.get_allocator()
.
In [forwardlist.ops] p. 9 change as indicated:
void splice_after(const_iterator position, forward_list<T,Allocator>& x, const_iterator first, const_iterator last); void splice_after(const_iterator position, forward_list<T,Allocator>&& x, const_iterator first, const_iterator last);9 - Requires:
position
isbefore_begin()
or is a dereferenceable iterator in the range [begin()
,end()
). (first
,last
) is a valid range inx
, and all iterators in the range (first
,last
) are dereferenceable.position
is not an iterator in the range (first
,last
).get_allocator() == x.get_allocator()
.
In [forwardlist.ops] p. 18 change as indicated:
void merge(forward_list<T,Allocator>& x); void merge(forward_list<T,Allocator>&& x); template <class Compare> void merge(forward_list<T,Allocator>& x, Compare comp); template <class Compare> void merge(forward_list<T,Allocator>&& x, Compare comp);18 - Requires:
comp
defines a strict weak ordering ([alg.sorting]), and*this
andx
are both sorted according to this ordering.get_allocator() == x.get_allocator()
.
unique_ptr
Section: 20.3.1.3.4 [unique.ptr.single.asgn] Status: C++14 Submitter: Daniel Krügler Opened: 2011-04-16 Last modified: 2016-01-28
Priority: Not Prioritized
View all other issues in [unique.ptr.single.asgn].
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Discussion:
The semantics described in 20.3.1.3.4 [unique.ptr.single.asgn] p. 6
Effects: Transfers ownership from
u
to*this
as if […] followed by an assignment fromstd::forward<D>(u.get_deleter())
.
contradicts to the pre-conditions described in p. 4:
Requires: If E is not a reference type, assignment of the deleter from an rvalue of type
E
shall be well-formed and shall not throw an exception. Otherwise,E
is a reference type and assignment of the deleter from an lvalue of typeE
shall be well-formed and shall not throw an exception.
Either the pre-conditions are incorrect or the semantics should be an assignment from
std::forward<E>(u.get_deleter())
, instead.
It turns out that this contradiction is due to an incorrect transcription from the proposed
resolution of 983(i) to the finally accepted proposal
n3073 (see
bullet 12) as confirmed by Howard Hinnant, thus the type argument provided to std::forward
must be fixed as indicated.
[Bloomington, 2011]
Move to Ready
Proposed resolution:
This wording is relative to the FDIS.
Edit 20.3.1.3.4 [unique.ptr.single.asgn] p. 6 as indicated:
template <class U, class E> unique_ptr& operator=(unique_ptr<U, E>&& u) noexcept;4 - Requires: If
5 - Remarks: This operator shall not participate in overload resolution unless:E
is not a reference type, assignment of the deleter from an rvalue of typeE
shall be well-formed and shall not throw an exception. Otherwise,E
is a reference type and assignment of the deleter from an lvalue of typeE
shall be well-formed and shall not throw an exception.
unique_ptr<U, E>::pointer
is implicitly convertible topointer
andU
is not an array type.6 - Effects: Transfers ownership from
7 - Returns:u
to*this
as if by callingreset(u.release())
followed by an assignment fromstd::forward<
.DE>(u.get_deleter())*this
.
mem_fn
overloadsSection: 22.10 [function.objects], 22.10.16 [func.memfn] Status: C++14 Submitter: Jonathan Wakely Opened: 2011-04-18 Last modified: 2016-01-28
Priority: Not Prioritized
View all other issues in [function.objects].
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Discussion:
The mem_fn
overloads for member functions are redundant and misleading
and should be removed from the post-C++11 WP.
mem_fn
was specified by
a single signature:
template<class R, class T> unspecified mem_fn(R T::* pm);
and was accompanied by the remark "Implementations may implement mem_fn
as a set of overloaded function templates." This remark predates variadic templates
and was presumably to allow implementations to provide overloads for a limited
number of function parameters, to meet the implementation-defined limit on numbers of
template parameters.
CopyConstructible
concept
(those overloads first appeared in N2322.) The overloads failed to
account for varargs member functions (i.e. those declared with an
ellipsis in the parameter-declaration-clause) e.g.
struct S { int f(int, ...); };
Syntactically such a function would be handled by the original
mem_fn(R T::* pm)
signature, the only minor drawback being that there
would be no CopyConstructible
requirement on the parameter list. (Core
DR 547 clarifies that partial specializations can be written to match
cv-qualified and ref-qualified functions to support the case where R T::*
matches a pointer to member function type.)
mem_fn(R T::* pm)
signature.
Concepts were removed from the draft and N3000 restored the original
single signature and accompanying remark.
LWG 1230(i) was opened to strike the remark again and to add an overload
for member functions (this overload was unnecessary for syntactic reasons and
insufficient as it didn't handle member functions with cv-qualifiers and/or
ref-qualifiers.)
920(i) (and 1230(i)) were resolved by restoring a full set of
(non-concept-enabled) overloads for member functions with cv-qualifiers and ref-qualifiers,
but as in the concept-enabled draft there were no overloads for member functions with
an ellipsis in the parameter-declaration-clause. This is what is present in the FDIS.
Following the thread beginning with message c++std-lib-30675, it is my
understanding that all the mem_fn
overloads for member functions are
unnecessary and were only ever added to allow concept requirements.
I'm not aware of any reason implementations cannot implement mem_fn
as
a single function template. Without concepts the overloads are
redundant, and the absence of overloads for varargs functions can be
interpreted to imply that varargs functions are not intended to work
with mem_fn
. Clarifying the intent by adding overloads for varargs
functions would expand the list of 12 redundant overloads to 24, it
would be much simpler to remove the 12 redundant overloads entirely.
[Bloomington, 2011]
Move to Review.
The issue and resolution appear to be correct, but there is some concern that the wording of INVOKE may be different depending on whether you pass a pointer-to-member-data or pointer-to-member-function. That might make the current wording necessary after all, and then we might need to add the missing elipsis overloads.
There was some concern that the Remark confirming implementors had freedom to implement this as a set of overloaded functions may need to be restored if we delete the specification for these overloads.
[2012, Kona]
Moved to Tentatively Ready by the post-Kona issues processing subgroup.
[2012, Portland: applied to WP]
Proposed resolution:
This wording is relative to the FDIS.
Change the <functional>
synopsis 22.10 [function.objects] p. 2 as follows:
namespace std { […] // [func.memfn], member function adaptors: template<class R, class T> unspecified mem_fn(R T::*);template<class R, class T, class... Args> unspecified mem_fn(R (T::*)(Args...)); template<class R, class T, class... Args> unspecified mem_fn(R (T::*)(Args...) const); template<class R, class T, class... Args> unspecified mem_fn(R (T::*)(Args...) volatile); template<class R, class T, class... Args> unspecified mem_fn(R (T::*)(Args...) const volatile); template<class R, class T, class... Args> unspecified mem_fn(R (T::*)(Args...) &); template<class R, class T, class... Args> unspecified mem_fn(R (T::*)(Args...) const &); template<class R, class T, class... Args> unspecified mem_fn(R (T::*)(Args...) volatile &); template<class R, class T, class... Args> unspecified mem_fn(R (T::*)(Args...) const volatile &); template<class R, class T, class... Args> unspecified mem_fn(R (T::*)(Args...) &&); template<class R, class T, class... Args> unspecified mem_fn(R (T::*)(Args...) const &&); template<class R, class T, class... Args> unspecified mem_fn(R (T::*)(Args...) volatile &&); template<class R, class T, class... Args> unspecified mem_fn(R (T::*)(Args...) const volatile &&);[…] }
Change 22.10.16 [func.memfn] as follows:
template<class R, class T> unspecified mem_fn(R T::*);template<class R, class T, class... Args> unspecified mem_fn(R (T::*)(Args...)); template<class R, class T, class... Args> unspecified mem_fn(R (T::*)(Args...) const); template<class R, class T, class... Args> unspecified mem_fn(R (T::*)(Args...) volatile); template<class R, class T, class... Args> unspecified mem_fn(R (T::*)(Args...) const volatile); template<class R, class T, class... Args> unspecified mem_fn(R (T::*)(Args...) &); template<class R, class T, class... Args> unspecified mem_fn(R (T::*)(Args...) const &); template<class R, class T, class... Args> unspecified mem_fn(R (T::*)(Args...) volatile &); template<class R, class T, class... Args> unspecified mem_fn(R (T::*)(Args...) const volatile &); template<class R, class T, class... Args> unspecified mem_fn(R (T::*)(Args...) &&); template<class R, class T, class... Args> unspecified mem_fn(R (T::*)(Args...) const &&); template<class R, class T, class... Args> unspecified mem_fn(R (T::*)(Args...) volatile &&); template<class R, class T, class... Args> unspecified mem_fn(R (T::*)(Args...) const volatile &&);
is_destructible
is underspecifiedSection: 21.3.5.4 [meta.unary.prop] Status: C++14 Submitter: Daniel Krügler Opened: 2011-04-18 Last modified: 2016-01-28
Priority: Not Prioritized
View other active issues in [meta.unary.prop].
View all other issues in [meta.unary.prop].
View all issues with C++14 status.
Discussion:
The conditions for the type trait is_destructible
to be true
are described in Table 49 — Type property predicates:
For a complete type
T
and given
template <class U> struct test { U u; };
,
test<T>::~test()
is not deleted.
This specification does not say what the result would be for function types or for abstract types:
X
the instantiation test<X>
is already ill-formed, so we cannot say anything about whether the destructor
would be deleted or not.which has the same consequence as for abstract types, namely that the corresponding instantiation ofIf a declaration acquires a function type through a type dependent on a template-parameter and this causes a declaration that does not use the syntactic form of a function declarator to have function type, the program is ill-formed.
[ Example:template<class T> struct A { static T t; }; typedef int function(); A<function> a; // ill-formed: would declare A<function>::t // as a static member function— end example ]
test
is already ill-formed and we cannot say anything
about the destructor.
To solve this problem, I suggest to specify function types as trivially and nothrowing destructible, because above mentioned rule is very special for templates. For non-templates, a typedef can be used to introduce a member as member function as clarified in 9.3.4.6 [dcl.fct] p. 10.
For abstract types, two different suggestions have been brought to my attention: Either declare them as unconditionally non-destructible or check whether the expression
std::declval<T&>().~T()
is well-formed in an unevaluated context. The first solution is very easy to specify, but the second version has the advantage for providing more information to user-code. This information could be quite useful, if generic code is supposed to invoke the destructor of a reference to a base class indirectly via a delete expression, as suggested by Howard Hinnant:
template <class T> my_pointer<T>::~my_pointer() noexcept(is_nothrow_destructible<T>::value) { delete ptr_; }
Additional to the is_destructible
traits, its derived forms is_trivially_destructible
and is_nothrow_destructible
are similarly affected, because their wording refers to "the indicated
destructor" and probably need to be adapted as well.
[ 2011 Bloomington ]
After discussion about to to handle the exceptional cases of reference types, function types (available by defererencing a function pointer)
and void
types, Howard supplied proposed wording.
[ 2011-08-20 Daniel comments and provides alternatives wording ]
The currently proposed wording would have the consequence that every array type is not destructible, because the pseudo-destructor requires a scalar type with the effect that the expression
std::declval<T&>().~T()
is not well-formed for e.g. T
equal to int[3]
. The intuitive
solution to fix this problem would be to adapt the object type case to refer to
the expression
std::declval<U&>().~U()
with U
equal to remove_all_extents<T>::type
, but that
would have the effect that arrays of unknown bounds would be destructible, if
the element type is destructible, which was not the case before (This was
intentionally covered by the special "For a complete type T" rule in
the FDIS).
Let
U
beremove_all_extents<T>::type
.
For incomplete types and function types,is_destructible<T>::value
isfalse
.
For object types, if the expressionstd::declval<U&>().~U()
is well-formed
when treated as an unevaluated operand (Clause 5), thenis_destructible<T>::value
istrue
, otherwise it isfalse
.
For reference types,is_destructible<T>::value
istrue
.
This wording also harmonizes with the "unevaluated operand" phrase used in other places, there does not exist the definition of an "unevaluated context"
Note: In the actually proposed wording this wording has been slightly reordered with the same effects.Howard's (old) proposed resolution:
Update 21.3.5.4 [meta.unary.prop], table 49:
template <class T> struct is_destructible;
For a complete typeT
and giventemplate <class U> struct test { U u; };
,test<T>::~test()
is not deleted.
For object types, if the expression:std::declval<T&>().~T()
is well-formed in an unevaluated context thenis_destructible<T>::value
istrue
, otherwise it isfalse
.
Forvoid
types,is_destructible<T>::value
isfalse
.
For reference types,is_destructible<T>::value
istrue
.
For function types,is_destructible<T>::value
isfalse
.T
shall be a complete type, (possibly cv-qualified)void
, or an array of unknown bound.
[2012, Kona]
Moved to Tentatively Ready by the post-Kona issues processing subgroup.
[2012, Portland: applied to WP]
Proposed resolution:
Update 21.3.5.4 [meta.unary.prop], table 49:
template <class T>
struct is_destructible; |
T and given template <class U> struct test { U u; }; , test<T>::~test() is not deleted.
For reference types, is_destructible<T>::value is true .For incomplete types and function types, is_destructible<T>::value is false .For object types and given U equal to remove_all_extents<T>::type ,if the expression std::declval<U&>().~U() is well-formed when treated as anunevaluated operand (Clause 7 [expr]), then is_destructible<T>::value is true ,otherwise it is false . |
T shall be a complete type, (possibly cv-qualified) void , or an array of unknown bound. |
allocator_traits
to define member typesSection: 23.5 [unord] Status: C++14 Submitter: Tom Zieberman Opened: 2011-04-29 Last modified: 2016-01-28
Priority: Not Prioritized
View all other issues in [unord].
View all issues with C++14 status.
Discussion:
The unordered associative containers define their member types reference
,
const_reference
, pointer
, const_pointer
in terms of
their template parameter Allocator
(via allocator_type
typedef). As
a consequence, only the allocator types, that provide sufficient typedefs, are usable
as allocators for unordered associative containers, while other containers do not have
this deficiency. In addition to that, the definitions of said typedefs are different
from ones used in the other containers. This is counterintuitive and introduces a certain
level of confusion. These issues can be fixed by defining pointer
and
const_pointer
typedefs in terms of allocator_traits<Allocator>
and by defining reference
and const_reference
in terms of
value_type
as is done in the other containers.
[ 2011 Bloomington. ]
Move to Ready.
Proposed resolution:
This wording is relative to the FDIS.
Change 23.5.3.1 [unord.map.overview] paragraph 3 as indicated:
namespace std { template <class Key, class T, class Hash = hash<Key>, class Pred = std::equal_to<Key>, class Allocator = std::allocator<std::pair<const Key, T> > > class unordered_map { public: // types typedef Key key_type; typedef std::pair<const Key, T> value_type; typedef T mapped_type; typedef Hash hasher; typedef Pred key_equal; typedef Allocator allocator_type; typedef typenameallocator_typeallocator_traits<Allocator>::pointer pointer; typedef typenameallocator_typeallocator_traits<Allocator>::const_pointer const_pointer; typedeftypename allocator_type::referencevalue_type& reference; typedeftypename allocator_type::const_referenceconst value_type& const_reference; typedef implementation-defined size_type; typedef implementation-defined difference_type; […] }; }
Change 23.5.4.1 [unord.multimap.overview] paragraph 3 as indicated:
namespace std { template <class Key, class T, class Hash = hash<Key>, class Pred = std::equal_to<Key>, class Allocator = std::allocator<std::pair<const Key, T> > > class unordered_multimap { public: // types typedef Key key_type; typedef std::pair<const Key, T> value_type; typedef T mapped_type; typedef Hash hasher; typedef Pred key_equal; typedef Allocator allocator_type; typedef typenameallocator_typeallocator_traits<Allocator>::pointer pointer; typedef typenameallocator_typeallocator_traits<Allocator>::const_pointer const_pointer; typedeftypename allocator_type::referencevalue_type& reference; typedeftypename allocator_type::const_referenceconst value_type& const_reference; typedef implementation-defined size_type; typedef implementation-defined difference_type; […] }; }
Change 23.5.6.1 [unord.set.overview] paragraph 3 as indicated:
namespace std { template <class Key, class Hash = hash<Key>, class Pred = std::equal_to<Key>, class Allocator = std::allocator<Key> > class unordered_set { public: // types typedef Key key_type; typedef Key value_type; typedef Hash hasher; typedef Pred key_equal; typedef Allocator allocator_type; typedef typenameallocator_typeallocator_traits<Allocator>::pointer pointer; typedef typenameallocator_typeallocator_traits<Allocator>::const_pointer const_pointer; typedeftypename allocator_type::referencevalue_type& reference; typedeftypename allocator_type::const_referenceconst value_type& const_reference; typedef implementation-defined size_type; typedef implementation-defined difference_type; […] }; }
Change 23.5.7.1 [unord.multiset.overview] paragraph 3 as indicated:
namespace std { template <class Key, class Hash = hash<Key>, class Pred = std::equal_to<Key>, class Allocator = std::allocator<Key> > class unordered_multiset { public: // types typedef Key key_type; typedef Key value_type; typedef Hash hasher; typedef Pred key_equal; typedef Allocator allocator_type; typedef typenameallocator_typeallocator_traits<Allocator>::pointer pointer; typedef typenameallocator_typeallocator_traits<Allocator>::const_pointer const_pointer; typedeftypename allocator_type::referencevalue_type& reference; typedeftypename allocator_type::const_referenceconst value_type& const_reference; typedef implementation-defined size_type; typedef implementation-defined difference_type; […] }; }
tuple
constructors for more than one parameterSection: 22.4.4 [tuple.tuple], 22.4.4.2 [tuple.cnstr] Status: Resolved Submitter: Ville Voutilainen Opened: 2011-05-01 Last modified: 2016-01-28
Priority: 2
View all other issues in [tuple.tuple].
View all issues with Resolved status.
Discussion:
One of my constituents wrote the following:
-------snip------------ So far the only use I've found forstd::tuple
is as an ad-hoc type to emulate
multiple return values. If the tuple ctor was made non-explicit one could
almost think C++ supported multiple return values especially when combined
with std::tie()
.
// assume types line_segment and point // assume function double distance(point const&, point const&) std::tuple<point, point> closest_points(line_segment const& a, line_segment const& b) { point ax; point bx; /* some math */ return {ax, bx}; } double distance(line_segment const& a, line_segment const& b) { point ax; point bx; std::tie(ax, bx) = closest_points(a, b); return distance(ax, bx); }
-------snap----------
See also the messages starting from lib-29330. Some notes:pair
allows such a returndecltype
refuses {1, 2}
I would recommend making non-unary tuple
constructors non-explicit.
[Bloomington, 2011]
Move to NAD Future, this would be an extension to existing functionality.
[Portland, 2012]
Move to Open at the request of the Evolution Working Group.
[Lenexa 2015-05-05]
VV: While in the area of tuples, LWG 2051 should have status of WP, it is resolved by Daniel's "improving pair and tuple" paper.
MC: status Resolved, by N4387
Proposed resolution:
Resolved by the adoption of N4387.mapped_type
and value_type
for associative containersSection: 23.2.7 [associative.reqmts] Status: Resolved Submitter: Marc Glisse Opened: 2011-05-04 Last modified: 2016-01-28
Priority: 2
View other active issues in [associative.reqmts].
View all other issues in [associative.reqmts].
View all issues with Resolved status.
Discussion:
(this is basically reopening the first part of issue 2006(i), as discussed in the thread starting at c++std-lib-30698 )
Section 23.2.7 [associative.reqmts] In Table 102, several uses ofT
(which means mapped_type
here) should
be value_type
instead. This is almost editorial. For instance:
a_uniq.emplace(args)Requires:
Effects: Inserts aT
shall beEmplaceConstructible
intoX
from args.T
objectt
constructed withstd::forward<Args>(args)...
if and only if there is no element in the container with key equivalent to the key oft
. Thebool
component of the returned pair is true if and only if the insertion takes place, and the iterator component of the pair points to the element with key equivalent to the key oft
.
[ 2011 Bloomington ]
Not even an exhaustive list of problem locations. No reason to doubt issue.
Pablo agrees to provide wording.
[ 2011-09-04 Pablo Halpern provides improved wording ]
[2014-02-15 post-Issaquah session : move to Resolved]
AJM to replace this note with a 'Resolved By...' after tracking down the exact sequence of events, but clearly resolved in C++14 DIS.
Proposed resolution:
In both section 23.2.7 [associative.reqmts] Table 102 and 23.2.8 [unord.req], Table 103, make the following text replacements:
Original text, in FDIS | Replacement text |
T is CopyInsertable into X and CopyAssignable . |
value_type is CopyInsertable into X , key_type is CopyAssignable , and
mapped_type is CopyAssignable (for containers having a mapped_type ) |
T is CopyInsertable |
value_type is CopyInsertable |
T shall be CopyInsertable |
value_type shall be CopyInsertable |
T shall be MoveInsertable |
value_type shall be MoveInsertable |
T shall be EmplaceConstructible |
value_type shall be EmplaceConstructible |
T object |
value_type object |
[
Notes to the editor: The above are carefully selected
phrases that can be used for global search-and-replace within
the specified sections without accidentally making changes to
correct uses T
.
]
regex
bitmask typesSection: 28.6.4 [re.const] Status: C++14 Submitter: Jonathan Wakely Opened: 2011-05-09 Last modified: 2016-01-28
Priority: Not Prioritized
View all other issues in [re.const].
View all issues with C++14 status.
Discussion:
When N3110 was applied to the WP some redundant "static" keywords were added and one form of initializer which isn't valid for enumeration types was replaced with another form of invalid initializer.
[ 2011 Bloomington. ]
Move to Ready.
Proposed resolution:
This wording is relative to the FDIS.
Change 28.6.4.2 [re.synopt] as indicated:
namespace std { namespace regex_constants { typedef T1 syntax_option_type;staticconstexpr syntax_option_type icase = unspecified ;staticconstexpr syntax_option_type nosubs = unspecified ;staticconstexpr syntax_option_type optimize = unspecified ;staticconstexpr syntax_option_type collate = unspecified ;staticconstexpr syntax_option_type ECMAScript = unspecified ;staticconstexpr syntax_option_type basic = unspecified ;staticconstexpr syntax_option_type extended = unspecified ;staticconstexpr syntax_option_type awk = unspecified ;staticconstexpr syntax_option_type grep = unspecified ;staticconstexpr syntax_option_type egrep = unspecified ; } }
Change 28.6.4.3 [re.matchflag] as indicated:
namespace std { namespace regex_constants { typedef T2 match_flag_type;staticconstexpr match_flag_type match_default= 0{};staticconstexpr match_flag_type match_not_bol = unspecified ;staticconstexpr match_flag_type match_not_eol = unspecified ;staticconstexpr match_flag_type match_not_bow = unspecified ;staticconstexpr match_flag_type match_not_eow = unspecified ;staticconstexpr match_flag_type match_any = unspecified ;staticconstexpr match_flag_type match_not_null = unspecified ;staticconstexpr match_flag_type match_continuous = unspecified ;staticconstexpr match_flag_type match_prev_avail = unspecified ;staticconstexpr match_flag_type format_default= 0{};staticconstexpr match_flag_type format_sed = unspecified ;staticconstexpr match_flag_type format_no_copy = unspecified ;staticconstexpr match_flag_type format_first_only = unspecified ; } }
Change 28.6.4.4 [re.err] as indicated:
namespace std { namespace regex_constants { typedef T3 error_type;staticconstexpr error_type error_collate = unspecified ;staticconstexpr error_type error_ctype = unspecified ;staticconstexpr error_type error_escape = unspecified ;staticconstexpr error_type error_backref = unspecified ;staticconstexpr error_type error_brack = unspecified ;staticconstexpr error_type error_paren = unspecified ;staticconstexpr error_type error_brace = unspecified ;staticconstexpr error_type error_badbrace = unspecified ;staticconstexpr error_type error_range = unspecified ;staticconstexpr error_type error_space = unspecified ;staticconstexpr error_type error_badrepeat = unspecified ;staticconstexpr error_type error_complexity = unspecified ;staticconstexpr error_type error_stack = unspecified ; } }
time_point
constructors need to be constexpr
Section: 30.6 [time.point] Status: Resolved Submitter: Anthony Williams Opened: 2011-05-13 Last modified: 2016-01-28
Priority: Not Prioritized
View all issues with Resolved status.
Discussion:
In 30.6 [time.point], time_point::min()
and time_point::max()
are listed as constexpr
. However, time_point
has no constexpr
constructors,
so is not a literal type, and so these functions cannot be constexpr
without adding a
constexpr
constructor for implementation purposes.
constexpr
to the constructors of time_point
. The effects of
the constructor template basically imply that the member function time_since_epoch()
is
intended to be constexpr
as well.
[2012, Portland]
Resolved by adopting paper n3469.
Proposed resolution:
This wording is relative to the FDIS.
Alter the class template definition in 30.6 [time.point] as follows:
template <class Clock, class Duration = typename Clock::duration> class time_point { […] public: // 20.11.6.1, construct: constexpr time_point(); // has value epoch constexpr explicit time_point(const duration& d); // same as time_point() + d template <class Duration2> constexpr time_point(const time_point<clock, Duration2>& t); // 20.11.6.2, observer: constexpr duration time_since_epoch() const; […] };
Alter the declarations in 30.6.2 [time.point.cons]:
constexpr time_point();-1- Effects: Constructs an object of type
time_point
, initializingd_
withduration::zero()
. Such atime_point
object represents the epoch.
constexpr explicit time_point(const duration& d);-2- Effects: Constructs an object of type
time_point
, initializingd_
withd
. Such atime_point
object represents the epoch+ d
.
template <class Duration2> constexpr time_point(const time_point<clock, Duration2>& t);-3- Remarks: This constructor shall not participate in overload resolution unless
-4- Effects: Constructs an object of typeDuration2
is implicitly convertible toduration
.time_point
, initializingd_
witht.time_since_epoch()
.
Alter the declaration in 30.6.3 [time.point.observer]:
constexpr duration time_since_epoch() const;-1- Returns: d_.
std::move
in std::accumulate
and other algorithmsSection: 26.10 [numeric.ops] Status: Resolved Submitter: Chris Jefferson Opened: 2011-01-01 Last modified: 2020-09-06
Priority: 3
View all other issues in [numeric.ops].
View all issues with Resolved status.
Discussion:
The C++0x draft says std::accumulate
uses: acc = binary_op(acc, *i)
.
acc = binary_op(std::move(acc), *i)
can lead to massive improvements (particularly,
it means accumulating strings is linear rather than quadratic).
Consider the simple case, accumulating a bunch of strings of length 1 (the same argument holds for other length buffers).
For strings s
and t
, s+t
takes time length(s)+length(t)
, as you have to copy
both s
and t
into a new buffer.
So in accumulating n
strings, step i
adds a string of length i-1
to a string of length
1, so takes time i
.
Therefore the total time taken is: 1+2+3+...+n
= O(n2
)
std::move(s)+t
, for a "good" implementation, is amortized time length(t)
, like vector
,
just copy t
onto the end of the buffer. So the total time taken is:
1+1+1+...+1
(n
times) = O(n
). This is the same as push_back
on a vector
.
I'm trying to decide if this implementation might already be allowed. I suspect it might not
be (although I can't imagine any sensible code it would break). There are other algorithms
which could benefit similarly (inner_product
, partial_sum
and
adjacent_difference
are the most obvious).
Is there any general wording for "you can use rvalues of temporaries"?
The reflector discussion starting with message c++std-lib-29763 came to the conclusion
that above example is not covered by the "as-if" rules and that enabling this behaviour
would seem quite useful.
[ 2011 Bloomington ]
Moved to NAD Future. This would be a larger change than we would consider for a simple TC.
[2017-02 in Kona, LEWG responds]
Like the goal.
What is broken by adding std::move() on the non-binary-op version?
A different overload might be selected, and that would be a breakage. Is it breakage that we should care about?
We need to encourage value semantics.
Need a paper. What guidance do we give?
Use std::reduce() (uses generalized sum) instead of accumulate which doesn’t suffer it.
Inner_product and adjacent_difference also. adjacent_difference solves it half-way for “val” object, but misses the opportunity for passing acc as std::move(acc)
.
[2017-06-02 Issues Telecon]
Ville to encourage Eelis to write a paper on the algorithms in <numeric>, not just for accumulate.
Howard pointed out that this has already been done for the algorithms in <algorithm>
Status to Open; Priority 3
[2017-11-11, Albuquerque]
This was resolved by the adoption of P0616r0.
Proposed resolution:
future_errc
enums start with value 0 (invalid value for broken_promise
)Section: 32.10.1 [futures.overview] Status: C++14 Submitter: Nicolai Josuttis Opened: 2011-05-18 Last modified: 2016-01-28
Priority: Not Prioritized
View all other issues in [futures.overview].
View all issues with C++14 status.
Discussion:
In 32.10.1 [futures.overview] enum class future_errc
is defined as follows:
enum class future_errc { broken_promise, future_already_retrieved, promise_already_satisfied, no_state };
With this declaration broken_promise
has value 0, which means that
for a future_error f
with this code
f.code().operator bool()
yields false, which makes no sense. 0 has to be reserved for "no error". So, the enums defined here have to start with 1.
Howard, Anthony, and Jonathan have no objections.[Discussion in Bloomington 2011-08-16]
Previous resolution:
This wording is relative to the FDIS.
In 32.10.1 [futures.overview], header
<future>
synopsis, fix the declaration offuture_errc
as follows:namespace std { enum class future_errc {broken_promise,future_already_retrieved = 1, promise_already_satisfied, no_state, broken_promise }; […] }
Is this resolution overspecified? These seem to be all implementation-defined. How do users add new values and not conflict with established error codes?
PJP proxy says: over-specified. boo.
Other error codes: look for is_error_code_enum
specializations. Only one exists io_errc
Peter: I don't see any other parts of the standard that specify error codes where we have to do something similar.
Suggest that for every place where we add an error code, the following:
[2012, Kona]
Moved to Tentatively Ready by the post-Kona issues processing subgroup.
[2012, Portland: applied to WP]
Proposed resolution:
This wording is relative to the FDIS.
In 32.10.1 [futures.overview], header <future>
synopsis, fix
the declaration of future_errc
as follows:
namespace std { enum class future_errc { broken_promise = implementation defined, future_already_retrieved = implementation defined, promise_already_satisfied = implementation defined, no_state = implementation defined }; […] }
In 32.10.1 [futures.overview], header <future>
synopsis, add a paragraph after paragraph 2 as follows:
future_errc
are distinct and not zero.
time_point + duration
semantics should be made constexpr
conformingSection: 30.6.6 [time.point.nonmember] Status: Resolved Submitter: Daniel Krügler Opened: 2011-05-21 Last modified: 2016-01-28
Priority: Not Prioritized
View all other issues in [time.point.nonmember].
View all issues with Resolved status.
Discussion:
It has been observed by LWG 2054(i) that the specification of some time_point
member functions
already imply that time_point
needs to be a literal type and suggests to specify the constructors
and the member function time_since_epoch()
as constexpr
functions at the
minimum necessary. Adding further constexpr
specifier to other operations should
clearly be allowed and should probably be done as well. But to allow for further constexpr
functions in the future requires that their semantics is compatible to operations allowed in constexpr
functions. This is already fine for all operations, except this binary plus operator:
template <class Clock, class Duration1, class Rep2, class Period2> time_point<Clock, typename common_type<Duration1, duration<Rep2, Period2>>::type> operator+(const time_point<Clock, Duration1>& lhs, const duration<Rep2, Period2>& rhs);-1- Returns:
CT(lhs) += rhs
, whereCT
is the type of the return value.
for similar reasons as those mentioned in 2020(i). The semantics should be fixed to allow
for making them constexpr
. This issue should also be considered as a placeholder for a request
to make the remaining time_point
operations similarly constexpr
as had been done for
duration
.
[2012, Portland]
Resolved by adopting paper n3469.
Proposed resolution:
This wording is relative to the FDIS.
In 30.6.6 [time.point.nonmember], p.1 change the Returns element semantics as indicated:
template <class Clock, class Duration1, class Rep2, class Period2> time_point<Clock, typename common_type<Duration1, duration<Rep2, Period2>>::type> operator+(const time_point<Clock, Duration1>& lhs, const duration<Rep2, Period2>& rhs);-1- Returns:
, where
CT(lhs) += rhsCT(lhs.time_since_epoch() + rhs)CT
is the type of the return value.
valarray
and begin/end
Section: 29.6 [numarray] Status: C++14 Submitter: Gabriel Dos Reis Opened: 2011-05-17 Last modified: 2016-01-28
Priority: Not Prioritized
View all other issues in [numarray].
View all issues with C++14 status.
Discussion:
It was just brought to my attention that the pair of functions
begin/end
were added to valarray
component.
Those additions strike me as counter to the long standing agreement
that valarray<T>
is not yet another container. Valarray values
are in general supposed to be treated as a whole, and as such
has a loose specification allowing expression template techniques.
begin/end
- or at least for the
const valarray<T>&
version. I strongly believe those
are defects.
[This issue was discussed on the library reflector starting from c++std-lib-30761. Some of the key conclusions of this discussion were:]
begin/end
members were added to allow valarray
to participate
in the new range-based for-loop by n2930
and not to make them container-like.begin/end
become invalidated. To fix this, these invalidation rules need at
least to reflect the invalidation rules of the references returned by the
operator[]
overloads of valarray
(29.6.2.4 [valarray.access]).
begin/end
, if the
replacement type technique is used (which was clearly part of the design of valarray
).
Providing such additional overloads would also lead to life-time problems in examples like
begin(x + y)
where x
and y
are expressions involving valarray
objects. To fix this, the begin/end
overloads could be explicitly excluded from the
general statements of 29.6.1 [valarray.syn] p.3-5. This would make it unspecified
whether the expression begin(x + y)
would be well-formed, portable code would
need to write this as begin(std::valarray<T>(x + y))
.[ 2011 Bloomington ]
The intent of these overloads is entirely to support the new for syntax, and not to create new containers.
Stefanus provides suggested wording.
[2012, Kona]
Moved to Tenatively Ready by post-meeting issues processing group, after confirmation from Gaby.
[2012, Portland: applied to WP]
Proposed resolution:
In 29.6.1 [valarray.syn]/4, make the following insertion:
4 Implementations introducing such replacement types shall provide additional functions and operators as follows:
const valarray<T>&
other than begin
and end
(29.6.10 [valarray.range]), identical functions taking the replacement types shall be added;
const valarray<T>&
arguments, identical functions taking every combination
of const valarray<T>&
and replacement types shall be added.
In 29.6.10 [valarray.range], make the following insertion:
1 In the begin
and end
function templates that follow, unspecified1 is a type that meets
the requirements of a mutable random access iterator (24.2.7) whose value_type
is the template parameter
T
and whose reference
type is T&
. unspecified2 is a type that meets the
requirements of a constant random access iterator (24.2.7) whose value_type
is the template parameter
T
and whose reference
type is const T&
.
2 The iterators returned by begin
and end
for an array are guaranteed to be valid until the
member function resize(size_t, T)
(29.6.2.8 [valarray.members]) is called for that array or until
the lifetime of that array ends, whichever happens first.
map::erase
Section: 23.4.3 [map] Status: C++17 Submitter: Christopher Jefferson Opened: 2011-05-18 Last modified: 2017-07-30
Priority: 3
View all other issues in [map].
View all issues with C++17 status.
Discussion:
map::erase
(and several related methods) took an iterator in C++03, but take a const_iterator
in C++0x. This breaks code where the map's key_type
has a constructor which accepts an iterator
(for example a template constructor), as the compiler cannot choose between erase(const key_type&)
and erase(const_iterator)
.
#include <map> struct X { template<typename T> X(T&) {} }; bool operator<(const X&, const X&) { return false; } void erasor(std::map<X,int>& s, X x) { std::map<X,int>::iterator it = s.find(x); if (it != s.end()) s.erase(it); }
[ 2011 Bloomington ]
This issue affects only associative container erase
calls, and is not more general, as these are the
only functions that are also overloaded on another single arguement that might cause confusion - the erase
by key method. The complete resolution should simply restore the iterator
overload in addition to the
const_iterator
overload for all eight associative containers.
Proposed wording supplied by Alan Talbot, and moved to Review.
[2012, Kona]
Moved back to Open by post-meeting issues processing group.
Pablo very unhappy about case of breaking code with ambiguous conversion between both iterator types.
Alisdair strongly in favor of proposed resolution, this change from C++11 bit Chris in real code, and it took a while to track down the cause.
Move to open, bring in front of a larger group
Proposed wording from Jeremiah:
erase(key)
shall not participate in overload resolution if iterator
is
convertible to key
.
Note that this means making erase(key)
a template-method
Poll Chris to find out if he already fixed his code, or fixed his library
Jeremiah - allow both overloads, but enable_if
the const_iterator
form as
a template, requiring is_same
to match only const_iterator
.
Poll PJ to see if he has already applied this fix?
[2015-02 Cologne]
AM: To summarize, we changed a signature and code broke. At what point do we stop and accept breakage in increasingly obscure code? VV: libc++ is still broken, but libstdc++ works, so they've fixed this — perhaps using this PR? [Checks] Yes, libstdc++ uses this solution, and has a comment pointing to LWG 2059. AM: This issue hasn't been looked at since Kona. In any case, we already have implementation experience now.
AM: I'd say let's ship it. We already have implementation experience (libstdc++ and MSVS). MC: And "tentatively ready" lets me try to implement this and see how it works.Proposed resolution:
Editorial note: The following things are different between 23.2.7 [associative.reqmts] p.8 and 23.2.8 [unord.req] p.10. These should probably be reconciled.
- First uses the convention "denotes"; second uses the convention "is".
- First redundantly says: "If no such element exists, returns a.end()." in erase table entry, second does not.
23.2.7 [associative.reqmts] Associative containers
8 In Table 102, X
denotes an associative container class, a
denotes a value of X
, a_uniq
denotes a value of X
when X
supports unique keys, a_eq
denotes a value of X
when
X
supports multiple keys, u
denotes an identifier, i
and j
satisfy input iterator
requirements and refer to elements implicitly convertible to value_type
, [i,j)
denotes a valid range,
p
denotes a valid const iterator to a
, q
denotes a valid dereferenceable const iterator to a
,
r
denotes a valid dereferenceable iterator to a, [q1, q2)
denotes a valid range of const iterators
in a
, il
designates an object of type initializer_list<value_type>
, t
denotes a value of
X::value_type
, k
denotes a value of X::key_type
and c
denotes a value of type
X::key_compare
. A
denotes the storage allocator used by X
, if any, or
std::allocator<X::value_type>
otherwise, and m
denotes an allocator of a type convertible to A
.
23.2.7 [associative.reqmts] Associative containers Table 102
Add row:
a.erase(r) |
iterator |
erases the element pointed to by r . Returns an iterator pointing to the element immediately following r
prior to the element being erased. If no such element exists, returns a.end() .
|
amortized constant |
23.2.8 [unord.req] Unordered associative containers
10 In table 103: X
is an unordered associative container class, a
is an object of type X
,
b
is a possibly const object of type X
, a_uniq
is an object of type X
when
X
supports unique keys, a_eq
is an object of type X
when X
supports equivalent keys,
i
and j
are input iterators that refer to value_type
, [i, j)
is a valid range,
p
and q2
are valid const iterators to a
, q
and q1
are valid dereferenceable
const iterators to a
, r
is a valid dereferenceable iterator to a, [q1,q2)
is a
valid range in a
, il
designates an object of type initializer_list<value_type>
,
t
is a value of type X::value_type
, k
is a value of type key_type
, hf
is a
possibly const value of type hasher
, eq
is a possibly const value of type key_equal
,
n
is a value of type size_type
, and z
is a value of type float
.
23.2.8 [unord.req] Unordered associative containers Table 103
Add row:
a.erase(r) |
iterator |
Erases the element pointed to by r . Returns the iterator immediately following r prior to the erasure.
|
Average case O(1), worst case O(a.size() ). |
23.4.3.1 [map.overview] Class template map overview p. 2
iterator erase(iterator position); iterator erase(const_iterator position); size_type erase(const key_type& x); iterator erase(const_iterator first, const_iterator last);
23.4.4.1 [multimap.overview] Class template multimap overview p. 2
iterator erase(iterator position); iterator erase(const_iterator position); size_type erase(const key_type& x); iterator erase(const_iterator first, const_iterator last);
23.4.6.1 [set.overview] Class template set overview p. 2
iterator erase(iterator position); iterator erase(const_iterator position); size_type erase(const key_type& x); iterator erase(const_iterator first, const_iterator last);
23.4.7.1 [multiset.overview] Class template multiset overview
iterator erase(iterator position); iterator erase(const_iterator position); size_type erase(const key_type& x); iterator erase(const_iterator first, const_iterator last);
23.5.3.1 [unord.map.overview] Class template unordered_map overview p. 3
iterator erase(iterator position); iterator erase(const_iterator position); size_type erase(const key_type& x); iterator erase(const_iterator first, const_iterator last);
23.5.4.1 [unord.multimap.overview] Class template unordered_multimap overview p. 3
iterator erase(iterator position); iterator erase(const_iterator position); size_type erase(const key_type& x); iterator erase(const_iterator first, const_iterator last);
23.5.6.1 [unord.set.overview] Class template unordered_set overview p. 3
iterator erase(iterator position); iterator erase(const_iterator position); size_type erase(const key_type& x); iterator erase(const_iterator first, const_iterator last);
23.5.7.1 [unord.multiset.overview] Class template unordered_multiset overview p. 3
iterator erase(iterator position); iterator erase(const_iterator position); size_type erase(const key_type& x); iterator erase(const_iterator first, const_iterator last);
C.6.12 [diff.cpp03.containers] C.2.12 Clause 23: containers library
23.2.3, 23.2.4
Change: Signature changes: from iterator to const_iterator parameters
Rationale: Overspecification. Effects: The signatures of the following member functions changed from taking an iterator to taking a const_iterator:
Valid C++ 2003 code that uses these functions may fail to compile with this International Standard.
make_move_iterator
and arraysSection: 24.2 [iterator.synopsis], 24.5.4 [move.iterators] Status: C++14 Submitter: Marc Glisse Opened: 2011-05-28 Last modified: 2016-01-28
Priority: Not Prioritized
View all other issues in [iterator.synopsis].
View all issues with C++14 status.
Discussion:
The standard library always passes template iterators by value and never by reference,
which has the nice effect that an array decays to a pointer. There is one exception:
make_move_iterator
.
#include <iterator> int main(){ int a[]={1,2,3,4}; std::make_move_iterator(a+4); std::make_move_iterator(a); // fails here }
[ 2011 Bloomington. ]
Move to Ready.
Proposed resolution:
This wording is relative to the FDIS.
Modify the header <iterator>
synopsis in 24.2 [iterator.synopsis]:
namespace std { […] template <class Iterator> move_iterator<Iterator> make_move_iterator(const Iterator&Iterator i); […] }
Modify the class template move_iterator
synopsis in 24.5.4.2 [move.iterator]:
namespace std { […] template <class Iterator> move_iterator<Iterator> make_move_iterator(const Iterator&Iterator i); }
Modify 24.5.4.9 [move.iter.nonmember]:
template <class Iterator> move_iterator<Iterator> make_move_iterator(const Iterator&Iterator i);-3- Returns:
move_iterator<Iterator>(i)
.
std::function
swapsSection: 22.10.17.3 [func.wrap.func], 22.10.17.3.3 [func.wrap.func.mod] Status: C++17 Submitter: Daniel Krügler Opened: 2011-05-28 Last modified: 2020-09-06
Priority: 2
View all other issues in [func.wrap.func].
View all issues with C++17 status.
Discussion:
Howard Hinnant observed in reflector message c++std-lib-30841 that 22.10.17.3 [func.wrap.func]
makes the member swap noexcept
, even though the non-member swap is not noexcept
.
noexcept
specifier at the member swap is incorrect and should be removed.
But if we allow for a potentially throwing member swap of std::function
, this causes
another conflict with the exception specification for the following member function:
template<class F> function& operator=(reference_wrapper<F> f) noexcept;
Effects:
function(f).swap(*this);
Note that in this example the sub-expression function(f)
does not cause any problems,
because of the nothrow-guarantee given in 22.10.17.3.2 [func.wrap.func.con] p. 10. The problem
is located in the usage of the swap which could potentially throw given the general latitude.
std::function
should be noexcept), or this function needs to be adapted as well,
e.g. by taking the exception-specification away or by changing the semantics.
One argument for "swap-may-throw" would be to allow for small-object optimization techniques
where the copy of the target may throw. But given the fact that the swap function has been guaranteed
to be "Throws: Nothing" from TR1 on, it seems to me that that there would still be opportunities to
perform small-object optimizations just restricted to the set of target copies that cannot throw.
In my opinion member swap of std::function
has always been intended to be no-throw, because
otherwise there would be no good technical reason to specify the effects of several member
functions in terms of the "construct-swap" idiom (There are three functions that are defined
this way), which provides the strong exception safety in this case. I suggest to enforce that both
member swap and non-member swap of std::function
are nothrow functions as it had been guaranteed
since TR1 on.
[ 2011 Bloomington ]
Dietmar: May not be swappable in the first place.
Alisdair: This is wide contact. Then we should be taking noexcept off instead of putting it on. This is preferred resolution.
Pablo: This is bigger issue. Specification of assignment in terms of swap is suspect to begin with. It is over specification. How this was applied to string is a better example to work from.
Pablo: Two problems: inconsistency that should be fixed (neither should have noexcept), the other issues is that assignment should not be specified in terms of swap. There are cases where assignment should succeed where swap would fail. This is easier with string as it should follow container rules.
Action Item (Alisdair): There are a few more issues found to file.
Dave: This is because of allocators? The allocator makes this not work.
Howard: There is a type erased allocator in shared_ptr. There is a noexcept allocator in shared_ptr.
Pablo: shared_ptr is a different case. There are shared semantics and the allocator does move around. A function does not have shared semantics.
Alisdair: Function objects think they have unique ownership.
Howard: In function we specify semantics with copy construction and swap.
Action Item (Pablo): Write this up better (why assignment should not be defined in terms of swap)
Howard: Not having trouble making function constructor no throw.
Dietmar: Function must allocate memory.
Howard: Does not put stuff that will throw on copy or swap in small object optimization. Put those on heap. Storing allocator, but has to be no throw copy constructable.
Pablo: Are you allowed to or required to swap or move allocators in case or swap or move.
Dave: An allocator that is type erased should be different...
Pablo: it is
Dave: Do you need to know something about allocator types? But only at construction time.
Pablo: You could have allocators that are different types.
Dave: Swap is two ended operation.
Pablo: Opinion is that both have to say propagate on swap for them to swap.
John: It is not arbitrary. If one person says no. No is no.
Howard: Find noexcept swap to be very useful. Would like to move in that direction and bring container design along.
Dave: If you have something were allocator must not propagate you can detect that at construction time.
...
Pablo: Need to leave this open and discuss in smaller group.
Alisdair: Tried to add boost::any as TR2 proposal and ran into this issue. Only the first place where we run into issues with type erased allocators. Suggest we move it to open.
Action Item: Move to open.
Action Item (Pablo works with Howard and Daniel): Address the more fundamental issue (which may be multiple issues) and write up findings.
Previous resolution [SUPERSEDED]:
This wording is relative to the FDIS.
Modify the header
<functional>
synopsis in 22.10 [function.objects] as indicated:namespace std { […] template<class R, class... ArgTypes> void swap(function<R(ArgTypes...)>&, function<R(ArgTypes...)>&) noexcept; […] }Modify the class template
function
synopsis in 22.10.17.3 [func.wrap.func] as indicated:namespace std { […] // [func.wrap.func.alg], specialized algorithms: template<class R, class... ArgTypes> void swap(function<R(ArgTypes...)>&, function<R(ArgTypes...)>&) noexcept; […] }Modify 22.10.17.3.8 [func.wrap.func.alg] as indicated:
template<class R, class... ArgTypes> void swap(function<R(ArgTypes...)>& f1, function<R(ArgTypes...)>& f2) noexcept;-1- Effects:
f1.swap(f2);
[2014-02-28 (Post Issaquah), Pablo provides more information]
For cross-referencing purposes: The resolution of this issue should be
harmonized with any resolution to LWG 2370(i), which addresses
inappropriate noexcept
s in some function constructors.
We have the following choices:
swap()
does not throw
Discussion: This definition is desirable, and allows assignment to be implemented with the strong exception guarantee, but it does have consequences: The implementation cannot use the small-object optimization for a function-object
F
unlessF
isNothrowMovable
(nothrow-swappable is unimportant becauseF
is not swapped with anotherF
). Note that many functors written before C++11 will not have move constructors decorated withnoexcept
, so this limitation could affect a lot of code.It is not clear what other implementation restrictions might be needed. Allocators are required not to throw on move or copy. Is that sufficient?
swap()
can throw
Discussion: This definition gives maximum latitude to implementation to use small-object optimization. However, the strong guarantee on assignment is difficult to achieve. Should we consider giving up on the strong guarantee? How much are we willing to pessimize code for exceptions?
swap()
will not throw if both functions have NoThrowMoveable
functors
Discussion: This definition is similar to option 2, but gives slightly stronger guarantees. Here,
swap()
can throw, but the programmer can theoretically prevent that from happening. This should be straight-forward to implement and gives the implementation a lot of latitude for optimization. However, because this is a dynamic decision, the program is not as easy to reason about. Also, the strong guarantee for assignment is compromized as in option 2.
[2016-08-02, Ville, Billy, and Billy comment and reinstantiate the original P/R]
We (Ville, Billy, and Billy) propose to require that function
's swap
is noexcept
in all cases.
libstdc++ does not throw in their swap
. It is not noexcept
today, but the small functor
optimization only engages for trivially copyable types.
msvc++ checks is_nothrow_move_constructible
before engaging the small functor optimization and marks
its swap
noexcept
libc++ marks swap
noexcept
(though I have not looked at its implementation)
Moreover, many of the concerns that were raised by providing this guarantee are no longer applicable now that
P0302 has been accepted, which removes allocator support from std::function
.
[2016-08 Chicago]
Tues PM: Move to Tentatively Ready
Proposed resolution:
This wording is relative to N4606.
Modify the header <functional>
synopsis in 22.10 [function.objects] as indicated:
namespace std { […] template<class R, class... ArgTypes> void swap(function<R(ArgTypes...)>&, function<R(ArgTypes...)>&) noexcept; […] }
Modify the class template function
synopsis in 22.10.17.3 [func.wrap.func] as indicated:
namespace std { […] // [func.wrap.func.alg], specialized algorithms: template<class R, class... ArgTypes> void swap(function<R(ArgTypes...)>&, function<R(ArgTypes...)>&) noexcept; […] }
Modify 22.10.17.3.8 [func.wrap.func.alg] as indicated:
template<class R, class... ArgTypes> void swap(function<R(ArgTypes...)>& f1, function<R(ArgTypes...)>& f2) noexcept;-1- Effects: As if by:
f1.swap(f2);
Section: 27.4.3 [basic.string] Status: C++17 Submitter: Howard Hinnant Opened: 2011-05-29 Last modified: 2017-07-30
Priority: 3
View other active issues in [basic.string].
View all other issues in [basic.string].
View all issues with C++17 status.
Discussion:
27.4.3.2 [string.require]/p4 says that basic_string
is an "allocator-aware"
container and behaves as described in 23.2.2 [container.requirements.general].
allocator_traits<allocator_type>::propagate_on_container_move_assignment::value
is false, and if the allocators stored in the lhs and rhs sides are not equal, then move
assigning a string has the same semantics as copy assigning a string as far as resources are
concerned (resources can not be transferred). And in this event, the lhs may have to acquire
resources to gain sufficient capacity to store a copy of the rhs.
However 27.4.3.3 [string.cons]/p22 says:
basic_string<charT,traits,Allocator>& operator=(basic_string<charT,traits,Allocator>&& str) noexcept;Effects: If
*this
andstr
are not the same object, modifies*this
as shown in Table 71. [Note: A valid implementation isswap(str)
. — end note ]
These two specifications for basic_string::operator=(basic_string&&)
are in conflict with
each other. It is not possible to implement a basic_string
which satisfies both requirements.
basic_string
is defined as:
basic_string& assign(basic_string&& str) noexcept;Effects: The function replaces the string controlled by
*this
with a string of lengthstr.size()
whose elements are a copy of the string controlled bystr
. [ Note: A valid implementation isswap(str)
. — end note ]
It seems contradictory that this member can be sensitive to propagate_on_container_swap
instead
of propagate_on_container_move_assignment
. Indeed, there is a very subtle chance for undefined
behavior here: If the implementation implements this in terms of swap
, and if
propagate_on_container_swap
is false, and if the two allocators are unequal, the behavior
is undefined, and will likely lead to memory corruption. That's a lot to go wrong under a member
named "assign".
[ 2011 Bloomington ]
Alisdair: Can this be conditional noexcept
?
Pablo: We said we were not going to put in many conditional noexcept
s. Problem is not allocator, but non-normative definition. It says swap is a valid operation which it is not.
Dave: Move assignment is not a critical method.
Alisdair: Was confusing assignment and construction.
Dave: Move construction is critical for efficiency.
Kyle: Is it possible to test for noexcept
.
Alisdair: Yes, query the noexcept
operator.
Alisdair: Agreed there is a problem that we cannot unconditionally mark these operations as noexcept
.
Pablo: How come swap is not defined in alloc
Alisdair: It is in utility.
Pablo: Swap has a conditional noexcept
. Is no throw move constructable, is no throw move assignable.
Pablo: Not critical for strings or containers.
Kyle: Why?
Pablo: They do not use the default swap.
Dave: Important for deduction in other types.
Alisdair: Would change the policy we adopted during FDIS mode.
Pablo: Keep it simple and get some vendor experience.
Alisdair: Is this wording correct? Concerned with bullet 2.
Pablo: Where does it reference containers section.
Alisdair: String is a container.
Alisdair: We should not remove redundancy piecemeal.
Pablo: I agree. This is a deviation from rest of string. Missing forward reference to containers section.
Pablo: To fix section 2. Only the note needs to be removed. The rest needs to be a forward reference to containers.
Alisdair: That is a new issue.
Pablo: Not really. Talking about adding one sentence, saying that basic string is a container.
Dave: That is not just a forward reference, it is a semantic change.
PJ: We intended to make it look like a container, but it did not satisfy all the requirements.
Pablo: Clause 1 is correct. Clause 2 is removing note and noexcept
(do not remove the rest). Clause 3 is correct.
Alisdair: Not sure data() is correct (in clause 2).
Conclusion: Move to open, Alisdair and Pablo volunteered to provide wording
[ originally proposed wording: ]
This wording is relative to the FDIS.
Modify the class template basic_string
synopsis in 27.4.3 [basic.string]:
namespace std { template<class charT, class traits = char_traits<charT>, class Allocator = allocator<charT> > class basic_string { public: […] basic_string& operator=(basic_string&& str)noexcept; […] basic_string& assign(basic_string&& str)noexcept; […] }; }
Remove the definition of the basic_string
move assignment operator from 27.4.3.3 [string.cons]
entirely, including Table 71 — operator=(const basic_string<charT, traits, Allocator>&&)
.
This is consistent with how we define move assignment for the containers in Clause 23:
basic_string<charT,traits,Allocator>& operator=(basic_string<charT,traits,Allocator>&& str) noexcept;
-22- Effects: If*this
andstr
are not the same object, modifies*this
as shown in Table 71. [ Note: A valid implementation isswap(str)
. — end note ]-23- If*this
andstr
are the same object, the member has no effect.-24- Returns:*this
Table 71 —operator=(const basic_string<charT, traits, Allocator>&&)
ElementValuedata()
points at the array whose first element was pointed at bystr.data()
size()
previous value ofstr.size()
capacity()
a value at least as large assize()
Modify the paragraphs prior to 27.4.3.7.3 [string.assign] p.3 as indicated (The first insertion recommends a separate paragraph number for the indicated paragraph):
basic_string& assign(basic_string&& str)noexcept;-?- Effects: Equivalent to
-3- Returns:*this = std::move(str)
.The function replaces the string controlled by*this
with a string of lengthstr.size()
whose elements are a copy of the string controlled bystr
. [ Note: A valid implementation isswap(str)
. — end note ]*this
[ 2012-08-11 Joe Gottman observes: ]
One of the effects of
basic_string
's move-assignment operator (27.4.3.3 [string.cons], Table 71) is
Element Value data()
points at the array whose first element was pointed at by str.data()
If a string implementation uses the small-string optimization and the input string
str
is small enough to make use of it, this effect is impossible to achieve. To use the small string optimization, a string has to be implemented using something likeunion { char buffer[SMALL_STRING_SIZE]; char *pdata; };When the string is small enough to fit inside
Resolution proposal: Change Table 71 to read:buffer
, thedata()
member function returnsstatic_cast<const char *>(buffer)
, and sincebuffer
is an array variable, there is no way to implement move so that the moved-to string'sbuffer
member variable is equal tothis->buffer
.
Element Value data()
points at the array whose first element was pointed at bythat contains the same characters in the same order asstr.data()
str.data()
contained beforeoperator=()
was called
[2015-05-07, Lenexa]
Howard suggests improved wording
Move to ImmediateProposed resolution:
This wording is relative to N4431.
Modify the class template basic_string
synopsis in 27.4.3 [basic.string]:
namespace std { template<class charT, class traits = char_traits<charT>, class Allocator = allocator<charT> > class basic_string { public: […] basic_string& assign(basic_string&& str) noexcept( allocator_traits<Allocator>::propagate_on_container_move_assignment::value || allocator_traits<Allocator>::is_always_equal::value); […] }; }
Change 27.4.3.3 [string.cons]/p21-23:
basic_string& operator=(basic_string&& str) noexcept( allocator_traits<Allocator>::propagate_on_container_move_assignment::value || allocator_traits<Allocator>::is_always_equal::value);-21- Effects:
IfMove assigns as a sequence container ([container.requirements]), except that iterators, pointers and references may be invalidated.*this
andstr
are not the same object, modifies*this
as shown in Table 71. [ Note: A valid implementation isswap(str)
. — end note ]-22- If-23- Returns:*this
andstr
are the same object, the member has no effect.*this
Table 71 —operator=(basic_string&&)
effectsElementValuedata()
points at the array whose first element was pointed at bystr.data()
size()
previous value ofstr.size()
capacity()
a value at least as large assize()
Modify the paragraphs prior to 27.4.3.7.3 [string.assign] p.3 as indicated
basic_string& assign(basic_string&& str) noexcept( allocator_traits<Allocator>::propagate_on_container_move_assignment::value || allocator_traits<Allocator>::is_always_equal::value);-3- Effects: Equivalent to
-4- Returns:*this = std::move(str)
.The function replaces the string controlled by*this
with a string of lengthstr.size()
whose elements are a copy of the string controlled bystr
. [ Note: A valid implementation isswap(str)
. — end note ]*this
noexcept
issues in basic_string
Section: 27.4.3 [basic.string] Status: C++14 Submitter: Howard Hinnant Opened: 2011-05-29 Last modified: 2016-11-12
Priority: Not Prioritized
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Discussion:
The following inconsistencies regarding noexcept
for basic_string
are noted.
noexcept
:
void swap(basic_string& str);
But the global swap is marked noexcept
:
template<class charT, class traits, class Allocator>
void swap(basic_string<charT,traits,Allocator>& lhs,
basic_string<charT,traits,Allocator>& rhs) noexcept;
But only in the definition, not in the synopsis.
All comparison operators are markednoexcept
in their definitions, but not in the synopsis.
The compare function that takes a pointer:
int compare(const charT *s) const;
is not marked noexcept
. But some of the comparison functions which are marked noexcept
(only in their definition) are specified to call the throwing compare operator:
template<class charT, class traits, class Allocator> bool operator==(const basic_string<charT,traits,Allocator>& lhs, const charT* rhs) noexcept;
Returns:
lhs.compare(rhs) == 0
.
All functions with a narrow contract should not be declared as noexcept
according to
the guidelines presented in n3279.
Among these narrow contract functions are the swap
functions (23.2.2 [container.requirements.general] p. 8)
and functions with non-NULL
const charT*
parameters.
[2011-06-08 Daniel provides wording]
[Bloomington, 2011]
Move to Ready
Proposed resolution:
This wording is relative to the FDIS. Both move-assignment operator and the moving assign
function are not touched by this issue, because they are handled separately by issue 2063(i).
Modify the header <string>
synopsis in 27.4 [string.classes] as
indicated (Rationale: Adding noexcept
to these specific overloads is in sync with
applying the same rule to specific overloads of the member functions find
, compare
, etc.
This approach deviates from that taken in n3279,
but seems more consistent given similar application for comparable member functions):
#include <initializer_list> namespace std { […] template<class charT, class traits, class Allocator> bool operator==(const basic_string<charT,traits,Allocator>& lhs, const basic_string<charT,traits,Allocator>& rhs) noexcept; […] template<class charT, class traits, class Allocator> bool operator!=(const basic_string<charT,traits,Allocator>& lhs, const basic_string<charT,traits,Allocator>& rhs) noexcept; […] template<class charT, class traits, class Allocator> bool operator<(const basic_string<charT,traits,Allocator>& lhs, const basic_string<charT,traits,Allocator>& rhs) noexcept; […] template<class charT, class traits, class Allocator> bool operator>(const basic_string<charT,traits,Allocator>& lhs, const basic_string<charT,traits,Allocator>& rhs) noexcept; […] template<class charT, class traits, class Allocator> bool operator<=(const basic_string<charT,traits,Allocator>& lhs, const basic_string<charT,traits,Allocator>& rhs) noexcept; […] template<class charT, class traits, class Allocator> bool operator>=(const basic_string<charT,traits,Allocator>& lhs, const basic_string<charT,traits,Allocator>& rhs) noexcept; […] }
Modify the class template basic_string
synopsis in 27.4.3 [basic.string] as
indicated (Remark 1: The noexcept
at the move-constructor is fine, because even for a
small-object optimization there is no problem here, because basic_string::value_type
is required to be a non-array POD as of 27.1 [strings.general] p1, Remark 2: This
proposal removes the noexcept
at single character overloads of find
, rfind
,
etc. because they are defined in terms of potentially allocating functions. It seems like
an additional issue to me to change the semantics in terms of non-allocating functions and
adding noexcept
instead):
namespace std { template<class charT, class traits = char_traits<charT>, class Allocator = allocator<charT> > class basic_string { public: […] // [string.ops], string operations: […] size_type find (charT c, size_type pos = 0) constnoexcept; […] size_type rfind(charT c, size_type pos = npos) constnoexcept; […] size_type find_first_of(charT c, size_type pos = 0) constnoexcept; […] size_type find_last_of (charT c, size_type pos = npos) constnoexcept; […] size_type find_first_not_of(charT c, size_type pos = 0) constnoexcept; […] size_type find_last_not_of (charT c, size_type pos = npos) constnoexcept; […] }; }
Modify 27.4.3.8.2 [string.find] before p5 and before p7 as indicated:
size_type find(const charT* s, size_type pos = 0) constnoexcept; […] size_type find(charT c, size_type pos = 0) constnoexcept;-7- Returns:
find(basic_string<charT,traits,Allocator>(1,c), pos)
.
Modify [string.rfind] before p7 as indicated:
size_type rfind(charT c, size_type pos = npos) constnoexcept;-7- Returns:
rfind(basic_string<charT,traits,Allocator>(1,c),pos)
.
Modify [string.find.first.of] before p7 as indicated:
size_type find_first_of(charT c, size_type pos = 0) constnoexcept;-7- Returns:
find_first_of(basic_string<charT,traits,Allocator>(1,c), pos)
.
Modify [string.find.last.of] before p7 as indicated:
size_type find_last_of(charT c, size_type pos = npos) constnoexcept;-7- Returns:
find_last_of(basic_string<charT,traits,Allocator>(1,c),pos)
.
Modify [string.find.first.not.of] before p7 as indicated:
size_type find_first_not_of(charT c, size_type pos = 0) constnoexcept;-7- Returns:
find_first_not_of(basic_string(1, c), pos)
.
Modify [string.find.last.not.of] before p7 as indicated:
size_type find_last_not_of(charT c, size_type pos = npos) constnoexcept;-7- Returns:
find_last_not_of(basic_string(1, c), pos)
.
Modify [string.operator==] before p2+p3 as indicated:
template<class charT, class traits, class Allocator> bool operator==(const charT* lhs, const basic_string<charT,traits,Allocator>& rhs)noexcept; […] template<class charT, class traits, class Allocator> bool operator==(const basic_string<charT,traits,Allocator>& lhs, const charT* rhs)noexcept;
Modify [string.op!=] before p2+p3 as indicated:
template<class charT, class traits, class Allocator> bool operator!=(const charT* lhs, const basic_string<charT,traits,Allocator>& rhs)noexcept; […] template<class charT, class traits, class Allocator> bool operator!=(const basic_string<charT,traits,Allocator>& lhs, const charT* rhs)noexcept;
Modify [string.op<] before p2+p3 as indicated:
template<class charT, class traits, class Allocator> bool operator<(const charT* lhs, const basic_string<charT,traits,Allocator>& rhs)noexcept; […] template<class charT, class traits, class Allocator> bool operator<(const basic_string<charT,traits,Allocator>& lhs, const charT* rhs)noexcept;
Modify [string.op>] before p2+p3 as indicated:
template<class charT, class traits, class Allocator> bool operator>(const charT* lhs, const basic_string<charT,traits,Allocator>& rhs)noexcept; […] template<class charT, class traits, class Allocator> bool operator>(const basic_string<charT,traits,Allocator>& lhs, const charT* rhs)noexcept;
Modify [string.op<=] before p2+p3 as indicated:
template<class charT, class traits, class Allocator> bool operator<=(const charT* lhs, const basic_string<charT,traits,Allocator>& rhs)noexcept; […] template<class charT, class traits, class Allocator> bool operator<=(const basic_string<charT,traits,Allocator>& lhs, const charT* rhs)noexcept;
Modify [string.op>=] before p2+p3 as indicated:
template<class charT, class traits, class Allocator> bool operator>=(const charT* lhs, const basic_string<charT,traits,Allocator>& rhs)noexcept; […] template<class charT, class traits, class Allocator> bool operator>=(const basic_string<charT,traits,Allocator>& lhs, const charT* rhs)noexcept;
Modify 27.4.4.3 [string.special] as indicated (Remark: The change of the semantics guarantees as of 16.3.2.4 [structure.specifications] p4 that the "Throws: Nothing" element of member swap is implied):
template<class charT, class traits, class Allocator> void swap(basic_string<charT,traits,Allocator>& lhs, basic_string<charT,traits,Allocator>& rhs)noexcept;-1- Effects: Equivalent to
lhs.swap(rhs);
Section: 16.4.4.6 [allocator.requirements] Status: C++14 Submitter: Jonathan Wakely Opened: 2011-06-06 Last modified: 2016-01-28
Priority: Not Prioritized
View other active issues in [allocator.requirements].
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Discussion:
The example in 16.4.4.6 [allocator.requirements] says SimpleAllocator
satisfies
the requirements of Table 28 — Allocator requirements, but it doesn't support comparison
for equality/inequality.
[Bloomington, 2011]
Move to Ready
Proposed resolution:
This wording is relative to the FDIS.
Modify the example in 16.4.4.6 [allocator.requirements] p5 as indicated:
-5- […]
[ Example: the following is an allocator class template supporting the minimal interface that satisfies the requirements of Table 28:template <class Tp> struct SimpleAllocator { typedef Tp value_type; SimpleAllocator(ctor args); template <class T> SimpleAllocator(const SimpleAllocator<T>& other); Tp *allocate(std::size_t n); void deallocate(Tp *p, std::size_t n); }; template <class T, class U> bool operator==(const SimpleAllocator<T>&, const SimpleAllocator<U>&); template <class T, class U> bool operator!=(const SimpleAllocator<T>&, const SimpleAllocator<U>&);— end example ]
vector::resize(size_type)
Section: 23.3.13.3 [vector.capacity] Status: Resolved Submitter: Rani Sharoni Opened: 2011-03-29 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
In C++1x (N3090) there are two version of vector::resize
— 23.3.13.3 [vector.capacity]:
void resize(size_type sz); void resize(size_type sz, const T& c);
The text in 23.3.13.3 [vector.capacity]/12 only mentions "no effects on throw" for the two args version of resize:
Requires: If an exception is thrown other than by the move constructor of a non-
CopyConstructible
T
there are no effects.
This seems like unintentional oversight since resize(size)
is
semantically the same as resize(size, T())
.
Additionally, the C++03 standard only specify single version of resize
with default for the second argument - 23.2.4:
void resize(size_type sz, T c = T());
Therefore not requiring same guarantees for both version of resize is in fact a regression.
[2011-06-12: Daniel comments]
The proposed resolution for issue 2033(i) should solve this issue as well.
[ 2011 Bloomington ]
This issue will be resolved by issue 2033(i), and closed when this issue is applied.
[2012, Kona]
Resolved by adopting the resolution in issue 2033(i) at this meeting.
Proposed resolution:
Apply the proposed resolution of issue 2033(i)
packaged_task
should have deleted copy c'tor with const parameterSection: 32.10.10 [futures.task] Status: C++14 Submitter: Daniel Krügler Opened: 2011-06-16 Last modified: 2016-01-28
Priority: Not Prioritized
View all other issues in [futures.task].
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Discussion:
Class template packaged_task
is a move-only type with the following form of the
deleted copy operations:
packaged_task(packaged_task&) = delete; packaged_task& operator=(packaged_task&) = delete;
Note that the argument types are non-const. This does not look like a typo to me, this form seems to exist from the very first proposing paper on N2276. Using either of form of the copy-constructor did not make much difference before the introduction of defaulted special member functions, but it makes now an observable difference. This was brought to my attention by a question on a German C++ newsgroup where the question was raised why the following code does not compile on a recent gcc:
#include <utility>
#include <future>
#include <iostream>
#include <thread>
int main(){
std::packaged_task<void()> someTask([]{ std::cout << std::this_thread::get_id() << std::endl; });
std::thread someThread(std::move(someTask)); // Error here
// Remainder omitted
}
It turned out that the error was produced by the instantiation of some return type
of std::bind
which used a defaulted copy-constructor, which leads to a
const declaration conflict with [class.copy] p8.
packaged_task
to prevent such problems.
A similar problem exists for class template basic_ostream
in 31.7.6.2 [ostream]:
namespace std { template <class charT, class traits = char_traits<charT> > class basic_ostream : virtual public basic_ios<charT,traits> { […] // 27.7.3.3 Assign/swap basic_ostream& operator=(basic_ostream& rhs) = delete; basic_ostream& operator=(const basic_ostream&& rhs); void swap(basic_ostream& rhs); };
albeit this could be considered as an editorial swap of copy and move assignment operator, I suggest to fix this as part of this issue as well.
[ 2011 Bloomington. ]
Move to Ready.
Proposed resolution:
This wording is relative to the FDIS.
Modify the class template basic_ostream
synopsis in 31.7.6.2 [ostream]
as indicated (Note: The prototype signature of the move assignment operator in 31.7.6.2.3 [ostream.assign]
is fine):
namespace std { template <class charT, class traits = char_traits<charT> > class basic_ostream : virtual public basic_ios<charT,traits> { […] // 27.7.3.3 Assign/swap basic_ostream& operator=(const basic_ostream& rhs) = delete; basic_ostream& operator=(constbasic_ostream&& rhs); void swap(basic_ostream& rhs); };
Modify the class template packaged_task
synopsis in 32.10.10 [futures.task] p2
as indicated:
namespace std { template<class> class packaged_task; // undefined template<class R, class... ArgTypes> class packaged_task<R(ArgTypes...)> { public: […] // no copy packaged_task(const packaged_task&) = delete; packaged_task& operator=(const packaged_task&) = delete; […] }; […] }
basic_string
move constructorSection: 27.4.3.3 [string.cons] Status: C++14 Submitter: Bo Persson Opened: 2011-07-01 Last modified: 2016-01-28
Priority: Not Prioritized
View all other issues in [string.cons].
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Discussion:
Sub-clause 27.4.3.3 [string.cons] contains these constructors in paragraphs 2 and 3:
basic_string(const basic_string<charT,traits,Allocator>& str); basic_string(basic_string<charT,traits,Allocator>&& str) noexcept;[…]
-3- Throws: The second form throws nothing if the allocator's move constructor throws nothing.
How can it ever throw anything if it is marked noexcept
?
[2011-07-11: Daniel comments and suggests wording changes]
Further, according to paragraph 18 of the same sub-clause:
basic_string(const basic_string& str, const Allocator& alloc); basic_string(basic_string&& str, const Allocator& alloc);[…]
-18- Throws: The second form throws nothing ifalloc == str.get_allocator()
unless the copy constructor forAllocator
throws.
The constraint "unless the copy constructor for Allocator
throws"
is redundant, because according to Table 28 — Allocator requirements, the expressions
X a1(a); X a(b);
impose the requirement: "Shall not exit via an exception".
[ 2011 Bloomington. ]
Move to Ready.
Proposed resolution:
This wording is relative to the FDIS.
Change 27.4.3.3 [string.cons] p3 as indicated (This move constructor has a wide
contract and is therefore safely marked as noexcept
):
basic_string(const basic_string<charT,traits,Allocator>& str); basic_string(basic_string<charT,traits,Allocator>&& str) noexcept;-2- Effects: Constructs an object of class
basic_string
as indicated in Table 64. In the second form,str
is left in a valid state with an unspecified value.-3- Throws: The second form throws nothing if the allocator's move constructor throws nothing.
Change 27.4.3.3 [string.cons] p18 as indicated (This move-like constructor may throw, if the allocators don't compare equal, but not because of a potentially throwing allocator copy constructor, only because the allocation attempt may fail and throw an exception):
basic_string(const basic_string& str, const Allocator& alloc); basic_string(basic_string&& str, const Allocator& alloc);[…]
-18- Throws: The second form throws nothing ifalloc == str.get_allocator()
unless the copy constructor for.Allocator
throws
allocate_shared
should use allocator_traits<A>::construct
Section: 20.3.2.2.7 [util.smartptr.shared.create] Status: Resolved Submitter: Jonathan Wakely Opened: 2011-07-11 Last modified: 2017-07-16
Priority: 2
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Discussion:
20.3.2.2.7 [util.smartptr.shared.create] says:
-2- Effects: Allocates memory suitable for an object of type
T
and constructs an object in that memory via the placement new expression::new (pv) T(std::forward<Args>(args)...)
. The templateallocate_shared
uses a copy of a to allocate memory. If an exception is thrown, the functions have no effect.
This explicitly requires placement new rather than using
allocator_traits<A>::construct(a, (T*)pv, std::forward<Args>(args)...)
In most cases that would result in the same placement new expression,
but would allow more control over how the object is constructed e.g.
using scoped_allocator_adaptor
to do uses-allocator construction, or
using an allocator declared as a friend to construct objects with no
public constructors.
[2011-08-16 Bloomington:]
Agreed to fix in principle, but believe that make_shared
and
allocate_shared
have now diverged enough that their descriptions
should be separated. Pablo and Stefanus to provide revised wording.
Daniel's (old) proposed resolution:
This wording is relative to the FDIS.
Change the following paragraphs of 20.3.2.2.7 [util.smartptr.shared.create] as indicated (The suggested removal of the last sentence of p1 is not strictly required to resolve this issue, but is still recommended, because it does not say anything new but may give the impression that it says something new):
template<class T, class... Args> shared_ptr<T> make_shared(Args&&... args); template<class T, class A, class... Args> shared_ptr<T> allocate_shared(const A& a, Args&&... args);-1- Requires: For the template
-2- Effects: Allocates memory suitable for an object of typemake_shared
, tThe expression::new (pv) T(std::forward<Args>(args)...)
, wherepv
has typevoid*
and points to storage suitable to hold an object of typeT
, shall be well formed. For the templateallocate_shared
, the expressionallocator_traits<A>::construct(a, pt, std::forward<Args>(args)...)
, wherept
has typeT*
and points to storage suitable to hold an object of typeT
, shall be well formed.A
shall be an allocator ([allocator.requirements]).The copy constructor and destructor ofA
shall not throw exceptions.T
and constructs an object in that memory. The templatemake_shared
constructs the object via the placement new expression::new (pv) T(std::forward<Args>(args)...)
. The templateallocate_shared
uses a copy ofa
to allocate memory and constructs the object by callingallocator_traits<A>::construct(a, pt, std::forward<Args>(args)...)
. If an exception is thrown, the functions have no effect. -3- Returns: Ashared_ptr
instance that stores and owns the address of the newly constructed object of typeT
. -4- Postconditions:get() != 0 && use_count() == 1
-5- Throws:bad_alloc
, or, for the templatemake_shared
, an exception thrown from the constructor ofT
, or, for the templateallocate_shared
, an exception thrown fromA::allocate
or fromallocator_traits<A>::construct
from the constructor of. -6- Remarks: Implementations are encouraged, but not required, to perform no more than one memory allocation. [ Note: This provides efficiency equivalent to an intrusive smart pointer. — end note ] -7- [ Note: These functions will typically allocate more memory thanT
sizeof(T)
to allow for internal bookkeeping structures such as the reference counts. — end note ]
[2011-12-04: Jonathan and Daniel improve wording]
See also c++std-lib-31796
[2013-10-13, Ville]
This issue is related to 2089(i).
[2014-02-15 post-Issaquah session : move to Tentatively NAD]
STL: This takes an allocator, but then ignores its construct. That's squirrely.
Alisdair: The convention is when you take an allocator, you use its construct.
STL: 23.2.2 [container.requirements.general]/3, argh! This fills me with despair, but I understand it now.
STL: Ok, this is some cleanup.
STL: You're requiring b
to be of type A
and not being rebound, is that an overspecification?
Pablo: Good point. Hmm, that's only a requirement on what must be well-formed.
STL: If it's just a well-formed requirement, then why not just use a directly?
Pablo: Yeah, the well-formed requirement is overly complex. It's not a real call, we could just use a directly. It makes it harder to read.
Alisdair: b
should be an allocator in the same family as a
.
Pablo: This is a well-formed requirement, I wonder if it's the capital A that's the problem here. It doesn't matter here, this is way too much wording.
Alisdair: It's trying to tie the constructor arguments into the allocator requirements.
Pablo: b
could be struck, that's a runtime quality. The construct will work with anything that's in the family of A
.
Alisdair: The important part is the forward
of Args
.
Pablo: A
must be an allocator, and forward
Args
must work with that.
Alisdair: First let's nail down A
.
Pablo: Then replace b
with a
, and strike the rest.
STL: You need pt
's type, at least.
Pablo: There's nothing to be said about runtime constraints here, this function doesn't even take a pt
.
STL: Looking at the Effects, I believe b
is similarly messed up, we can use a2
to construct an object.
Alisdair: Or any allocator in the family of a
.
STL: We say this stuff for the deallocate too, it should be lifted up.
STL: "owns the address" is weird.
Alisdair: shared_ptr owns pointers, although it does sound funky.
Walter: "to destruct" is ungrammatical.
STL: "When ownership is given up" is not what we usually say.
Alisdair: I think the Returns clause is the right place to say this.
STL: The right place to say this is shared_ptr
's dtor, we don't want to use Core's "come from" convention.
Alisdair: I'm on the hook to draft cleaner wording.
[2015-10, Kona Saturday afternoon]
AM: I was going to clean up the wording, but haven't done it yet.
Defer until we have new wording.
[2016-03, Jacksonville]
Alisdair: we need to figure out whether we should call construct or not; major implementation divergence
STL: this does not grant friendship, does it?
Jonathan: some people want it.
Thomas: scoped allocator adapter should be supported, so placement new doesn't work
Alisdair: this makes the make_ functions impossible
Thomas: you don't want to use those though.
Alisdair: but people use that today, at Bloomberg
Alisdair: and what do we do about fancy pointers?
Jonathan: we constrain it to only non-fancy pointers.
STL: shared_ptr has never attempted to support fancy pointers; seems like a paper is needed.
Poll: call construct:6 operator new: 0 don't care: 4
Poll: should we support fancy pointers? Yes: 1 No: 4 don't care: 4
STL: 20.8.2.2.6p2: 'and pv->~T()' is bogus for void
STL: 20.8.2.2.6p4: is this true even if we're going to allocate a bit more?
Alisdair: yes
Alisdair: coming up with new wording
[2016-08, Chicago Monday PM]
Alisdair to provide new wording this week
[2017-07 Toronto]
Resolved by the adoption of P0674R1 in Toronto
Proposed resolution:
This wording is relative to the FDIS.
Change the following paragraphs of 20.3.2.2.7 [util.smartptr.shared.create] as indicated:
template<class T, class... Args> shared_ptr<T> make_shared(Args&&... args);template<class T, class A, class... Args> shared_ptr<T> allocate_shared(const A& a, Args&&... args);
-1- Requires: The expression
::new (pv) T(std::forward<Args>(args)...)
, where pv
has type void*
and points to storage suitable to hold an object of type T
, shall be well
formed. A
shall be an allocator (16.4.4.6 [allocator.requirements]). The copy constructor
and destructor of A
shall not throw exceptions.
return allocate_shared<T>(allocator<T>(), std::forward<Args>(args)...);
Allocates memory suitable for an object of type
T
and constructs an object in that memory via the placement new expression
::new (pv) T(std::forward<Args>(args)...)
. The template allocate_shared
uses a copy
of a
to allocate memory. If an exception is thrown, the functions have no effect.
std::allocator
may not be instantiated, the expressions ::new (pv) T(std::forward<Args>(args)...)
and
pv->~T()
may be evaluated directly — end note].
shared_ptr
instance that stores and owns the address of the newly constructed
object of type T
.get() != 0 && use_count() == 1
bad_alloc
, or an exception thrown from A::allocate
or from the
constructor of T
.sizeof(T)
to allow
for internal bookkeeping structures such as the reference counts. — end note]Add the following set of new paragraphs immediately following the previous paragraph 7 of 20.3.2.2.7 [util.smartptr.shared.create]:
template<class T, class A, class... Args> shared_ptr<T> allocate_shared(const A& a, Args&&... args);
-?- Requires: The expressions
allocator_traits<A>::construct(b, pt, std::forward<Args>(args)...)
and
allocator_traits<A>::destroy(b, pt)
shall be well-formed and well-defined,
where b
has type A
and is a copy of a
and where pt
has type T*
and points to storage suitable to hold an object of type T
.
A
shall meet the allocator requirements (16.4.4.6 [allocator.requirements]).
a2
of type allocator_traits<A>::rebind_alloc<unspecified>
that compares equal to
a
to allocate memory suitable for an object of type T
.
Uses a copy b
of type A
from a
to construct an object of type T
in
that memory by calling allocator_traits<A>::construct(b, pt, std::forward<Args>(args)...)
.
If an exception is thrown, the function has no effect.
-?- Returns: A shared_ptr
instance that stores and owns the address of the newly constructed
object of type T
. When ownership is given up, the effects are as follows: Uses a copy b2
of type A
from a
to destruct an object of type T
by calling
allocator_traits<A>::destroy(b2, pt2)
where pt2
has type T*
and refers to the newly constructed object. Then uses an object of type
allocator_traits<A>::rebind_alloc<unspecified>
that compares equal to
a
to deallocate the allocated memory.
-?- Postconditions: get() != 0 && use_count() == 1
-?- Throws: Nothing unless memory allocation or allocator_traits<A>::construct
throws an exception.
-?- Remarks: Implementations are encouraged, but not required, to perform no more than one memory
allocation. [Note: Such an implementation provides efficiency equivalent to an intrusive smart
pointer. — end note]
-?- [Note: This function will typically allocate more memory than sizeof(T)
to allow for internal
bookkeeping structures such as the reference counts. — end note]
std::valarray
move-assignmentSection: 29.6.2.3 [valarray.assign] Status: C++14 Submitter: Paolo Carlini Opened: 2011-05-05 Last modified: 2016-01-28
Priority: Not Prioritized
View all other issues in [valarray.assign].
View all issues with C++14 status.
Discussion:
Yesterday I noticed that the language we have in the FDIS about std::valarray
move assignment
is inconsistent with the resolution of LWG 675. Indeed, we guarantee constant complexity (vs linear
complexity). We also want it to be noexcept, that is more subtle, but again it's at variance with all
the containers.
std::valarray
? Shall we maybe just strike or fix the as-if, consider it
some sort of pasto from the copy-assignment text, thus keep the noexcept and constant complexity requirements
(essentially the whole operation would boild down to a swap of POD data members). Or LWG 675(i) should be
explicitly extended to std::valarray
too? In that case both noexcept and constant complexity
would go, I think, and the operation would boil down to the moral equivalent of clear()
(which
doesn't really exist in this case) + swap
?
Howard: I agree the current wording is incorrect. The complexity should be linear in size()
(not
v.size()
) because the first thing this operator needs to do is resize(0)
(or clear()
as you put it).
noexcept
.
As for proper wording, here's a first suggestion:
Effects:
Complexity: linear.*this
obtains the value ofv
. The value ofv
after the assignment is not specified.
See also reflector discussion starting with c++std-lib-30690.
[2012, Kona]
Move to Ready.
Some discussion on the types supported by valarray
concludes that the wording is
trying to say something similar to the core wording for trivial types, but significantly
predates it, and does allow for types with non-trivial destructors. Howard notes that
the only reason for linear complexity, rather than constant, is to support types with
non-trivial destructors.
AJM suggests replacing the word 'value' with 'state', but straw poll prefers moving forward with the current wording, 5 to 2.
[2012, Portland: applied to WP]
Proposed resolution:
This wording is relative to the FDIS.
In 29.6.2.3 [valarray.assign] update as follows:
valarray<T>& operator=(valarray<T>&& v) noexcept;3 Effects:
4 Complexity:*this
obtains the value ofv
.If the length ofThe value ofv
is not equal to the length of*this
, resizes*this
to make the two arrays the same length, as if by callingresize(v.size())
, before performing the assignment.v
after the assignment is not specified.ConstantLinear.
Section: 99 [depr.temporary.buffer] Status: C++17 Submitter: Kazutoshi Satoda Opened: 2011-08-10 Last modified: 2025-03-13
Priority: 3
View all other issues in [depr.temporary.buffer].
View all issues with C++17 status.
Discussion:
According to [temporary.buffer] p1+2:
template <class T> pair<T*, ptrdiff_t> get_temporary_buffer(ptrdiff_t n) noexcept;-1- Effects: Obtains a pointer to storage sufficient to store up to
-2- Returns: A pair containing the buffer's address and capacity (in the units ofn
adjacentT
objects. It is implementation-defined whether over-aligned types are supported (3.11).sizeof(T)
), or a pair of 0 values if no storage can be obtained or ifn <= 0
.
I read this as prohibiting to return a buffer of which capacity is less than n
, because
such a buffer is not sufficient to store n
objects.
(for the return value, a pair
P
) [...] the buffer pointed to byP.first
is large enough to holdP.second
objects of typeT
.P.second
is greater than or equal to 0, and less than or equal tolen
.
There seems to be two different targets of the "up to n" modification: The capacity of obtained buffer, and the actual number that the caller will store into the buffer.
First I read as the latter, and got surprised seeing that libstdc++ implementation can return a smaller buffer. I started searching aboutget_temporary_buffer()
. After reading a quote from TC++PL at
stackoverflow,
I realized that the former is intended.
Such misinterpretation seems common:
JIS standard (Japanese translation of ISO/IEC standard) says nothing
like "up to". I think the editor misinterpreted the original wording,
and omitted words for "up to" as it is redundant. (If a buffer is
sufficient to store n
objects, it is also sufficient to store
up to n
objects.)
Rogue Wave implementation doesn't return smaller buffer, instead, it can return larger buffer on some circumstances. Apache STDCXX is a derived version of that implementation, and publicly accessible:
Specializations of the
get_temporary_buffer()
function template attempt to allocate a region of storage sufficiently large to store at leastn
adjacent objects of typeT
.
I know one commercial compiler package based on Rogue Wave implementation, and its implementation is essentially same as the above.
[2014-05-18, Daniel comments and suggests concrete wording]
The provided wording attempts to clarify the discussed capacity freedom, but it also makes it clearer that the returned
memory is just "raw memory", which is currently not really clear. In addition the wording clarifies that the deallocating
return_temporary_buffer
function does not throw exceptions, which I believe is the intention when the preconditions
of the functions are satisfied. Then, my understanding is that we can provide to return_temporary_buffer
a
null pointer value if that was the value, get_temporary_buffer()
had returned. Furthermore, as STL noticed, the current
wording seemingly allows multiple invocations of return_temporary_buffer
with the same value returned by
get_temporary_buffer
; this should be constrained similar to the wording we have for operator delete
(unfortunately
we miss such wording for allocators).
[2015-05, Lenexa]
MC: move to ready? in favor: 14, opposed: 0, abstain: 0
Proposed resolution:
This wording is relative to N3936.
Change [temporary.buffer] as indicated:
template <class T> pair<T*, ptrdiff_t> get_temporary_buffer(ptrdiff_t n) noexcept;-1- Effects: Obtains a pointer to uninitialized, contiguous storage for
-?- Remarks: CallingN
adjacent objects of typeT
, for some non-negative numberN
.Obtains a pointer to storage sufficient to store up toIt is implementation-defined whether over-aligned types are supported (3.11).n
adjacentT
objects.get_temporary_buffer
with a positive numbern
is a non-binding request to return storage forn
objects of typeT
. In this case, an implementation is permitted to return instead storage for a non-negative numberN
of such objects, whereN != n
(includingN == 0
). [Note: The request is non-binding to allow latitude for implementation-specific optimizations of its memory management. — end note]. -2- Returns: Ifn <= 0
or if no storage could be obtained, returns a pairP
such thatP.first
is a null pointer value andP.second == 0
; otherwise returns a pairP
such thatP.first
refers to the address of the uninitialized storage andP.second
refers to its capacityN
(in the units ofsizeof(T)
).Apair
containing the buffer's address and capacity (in the units ofsizeof(T)
), or a pair of 0 values if no storage can be obtained or ifn <= 0
.template <class T> void return_temporary_buffer(T* p);-3- Effects: Deallocates the
-4- Requires:buffer to whichstorage referenced byp
pointsp
.The buffer shall have been previously allocated byp
shall be a pointer value returned by an earlier call toget_temporary_buffer
which has not been invalidated by an intervening call toreturn_temporary_buffer(T*)
. -?- Throws: Nothing.
std::reverse_copy
Section: 26.7.10 [alg.reverse] Status: C++14 Submitter: Peter Miller Opened: 2011-08-17 Last modified: 2016-01-28
Priority: Not Prioritized
View all other issues in [alg.reverse].
View all issues with C++14 status.
Discussion:
The output of the program below should be:
"three two one null \n"
But when std::reverse_copy
is implemented as described in N3291 26.7.10 [alg.reverse]
it's:
"null three two one \n"
because there's an off by one error in 26.7.10 [alg.reverse]/4; the definition should read:
*(result + (last - first) - 1 - i) = *(first + i)
Test program:
#include <algorithm> #include <iostream> template <typename BiIterator, typename OutIterator> auto reverse_copy_as_described_in_N3291( BiIterator first, BiIterator last, OutIterator result ) -> OutIterator { // 25.3.10/4 [alg.reverse]: // "...such that for any non-negative integer i < (last - first)..." for ( unsigned i = 0; i < ( last - first ); ++i ) // "...the following assignment takes place:" *(result + (last - first) - i) = *(first + i); // 25.3.10/6 return result + (last - first); } int main() { using std::begin; using std::end; using std::cout; static const char*const in[3] { "one", "two", "three" }; const char* out[4] { "null", "null", "null", "null" }; reverse_copy_as_described_in_N3291( begin( in ), end( in ), out ); for ( auto s : out ) cout << s << ' '; cout << std::endl; return 0; }
[2012, Kona]
Move to Ready.
[2012, Portland: applied to WP]
Proposed resolution:
This wording is relative to the FDIS.
Change 26.7.10 [alg.reverse] p4 as follows:
template<class BidirectionalIterator, class OutputIterator> OutputIterator reverse_copy(BidirectionalIterator first, BidirectionalIterator last, OutputIterator result);-4- Effects: Copies the range [
-5- Requires: The ranges [first,last
) to the range [result,result+(last-first)
) such that for any non-negative integeri < (last - first)
the following assignment takes place:*(result + (last - first) - 1 - i) = *(first + i)
.first,last
) and [result,result+(last-first)
) shall not overlap. -6- Returns:result + (last - first)
. -7- Complexity: Exactlylast - first
assignments.
Section: 6.9.2 [intro.multithread], 32.5.5 [atomics.lockfree], 99 [atomics.types.operations.req] Status: Resolved Submitter: Torvald Riegel Opened: 2011-08-18 Last modified: 2016-01-28
Priority: Not Prioritized
View all other issues in [intro.multithread].
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Discussion:
According to 6.9.2 [intro.multithread] p2:
"Implementations should ensure that all unblocked threads eventually make progress."
Which assumptions can an implementation make about the thread scheduling? This is relevant for how implementations implement compare-exchange with load-linked / store conditional (LL-SC), and atomic read-modifiy-write operations with load...compare-exchange-weak loops.
32.5.5 [atomics.lockfree] p2 declares the lock-free property for a particular object. However, "lock-free" is never defined, and in discussions that I had with committee members it seemed as if the standard's lock-free would be different from what lock-free means in other communities (eg, research, text books on concurrent programming, etc.).
Following 99 [atomics.types.operations.req] p7 is_lock_free()
returns "true if the object is lock-free". What is returned if the object is only
sometimes lock-free?
Basically, I would like to see clarifications for the progress guarantees so that users know what they can expect from implementations (and what they cannot expect!), and to give implementors a clearer understanding of which user expectations they have to implement.
Elaborate on the intentions of the progress guarantee in 6.9.2 [intro.multithread] p2. As I don't know about your intentions, it's hard to suggest a resolution.
Define the lock-free property. The definition should probably include the following points:
[2011-12-01: Hans comments]
6.9.2 [intro.multithread] p2 was an intentional compromise, and it was understood at the time that it was not a precise statement. The wording was introduced by N3209, which discusses some of the issues. There were additional reflector discussions.
This is somewhat separable from the question of what lock-free means, which is probably a more promising question to focus on.[2012, Kona]
General direction: lock-free means obstruction-free. Leave the current "should" recommendation for progress. It would take a lot of effort to try to do better.
[2012, Portland: move to Open]
The current wording of 6.9.2 [intro.multithread] p2 doesn't really say very much. As far as we can tell the term lock-free is nowhere defined in the standard.
James: we would prefer a different way to phrase it.
Hans: the research literature includes the term abstraction-free which might be a better fit.
Detlef: does Posix define a meaning for blocking (or locking) that we could use?
Hans: things like compare-exchange-strong can wait indefinitely.
Niklas: what about spin-locks -- still making no progress.
Hans: suspect we can only give guidance, at best. The lock-free meaning from the theoretical commmunity (forard progress will be made) is probably too strong here.
Atrur: what about livelocks?
Hans: each atomic modification completes, even if the whole thing is blocked.
Moved to open.
[2013-11-06: Jason Hearne-McGuiness comments]
Related to this issue, the wording in 32.5.5 [atomics.lockfree] p2,
In any given program execution, the result of the lock-free query shall be consistent for all pointers of the same type.
should be made clearer, because the object-specific (non-static) nature of the is_lock_free()
functions
from 32.5.8 [atomics.types.generic] and 32.5.8.2 [atomics.types.operations] imply that for different
instances of pointers of the same type the returned value could be different.
Proposed resolution:
[2014-02-16 Issaquah: Resolved by paper n3927]
CopyConstructible
requirement in set constructorsSection: 23.4.6.2 [set.cons] Status: C++17 Submitter: Jens Maurer Opened: 2011-08-20 Last modified: 2017-07-30
Priority: 3
View all issues with C++17 status.
Discussion:
23.4.6.2 [set.cons] paragraph 4 says:
Requires: If the iterator's dereference operator returns an lvalue or a non-const rvalue, then
Key
shall beCopyConstructible
.
I'm confused why a "non-const rvalue" for the return value of the iterator
would require CopyConstructible
; isn't that exactly the situation
when you'd want to apply the move constructor?
multimap
seems better in that regard
([multimap.cons] paragraph 3):
Requires: If the iterator's dereference operator returns an lvalue or a const rvalue
pair<key_type, mapped_type>
, then bothkey_type
and mapped_type shall beCopyConstructible
.
Obviously, if I have a const rvalue, I can't apply the move constructor (which will likely attempt modify its argument).
Dave Abrahams: I think you are right. Proposed resolution: drop "non-" from 23.4.6.2 [set.cons] paragraph 3.[2012, Kona]
The wording is in this area will be affected by Pablo's paper being adopted at this meeting. Wait for that paper to be applied before visiting this issue - deliberately leave in New status until the next meeting.
Proposed resolution from Kona 2012:
This wording is relative to the FDIS.
Change 23.4.6.2 [set.cons] p3 as follows:
template <class InputIterator> set(InputIterator first, InputIterator last, const Compare& comp = Compare(), const Allocator& = Allocator());-3- Effects: Constructs an empty set using the specified comparison object and allocator, and inserts elements from the range [
-4- Requires: If the iterator's dereference operator returns an lvalue or afirst,last
).non-const rvalue, thenKey
shall beCopyConstructible
. -5- Complexity: Linear inN
if the range [first,last
) is already sorted usingcomp
and otherwiseN logN
, whereN
islast - first
.
[2014-05-18, Daniel comments]
According to Pablo, the current P/R correctly incorporates the changes from his paper (which was adopted in Kona)
[2014-06-10, STL comments and suggests better wording]
N1858 was voted into WP N2284 but was "(reworded)", introducing the "non-const" damage.
N1858 wanted to add this for map:Requires: Does not require
CopyConstructible
of eitherkey_type
ormapped_type
if the dereferencedInputIterator
returns a non-const rvaluepair<key_type, mapped_type>
. OtherwiseCopyConstructible
is required for bothkey_type
andmapped_type
.
And this for set:
Requires:
Key
must beCopyConstructible
only if the dereferencedInputIterator
returns an lvalue or const rvalue.
(And similarly for multi.)
This was reworded to N2284 23.3.1.1 [map.cons]/3 and N2284 23.3.3.1 [set.cons]/4, and it slightly changed over the years into N3936 23.4.4.2 [map.cons]/3 and N3936 23.4.6.2 [set.cons]/4. In 2005/2007, this was the best known way to say "hey, we should try to move this stuff", as the fine-grained element requirements were taking shape. Then in 2010, N3173 was voted into WP N3225, adding the definition ofEmplaceConstructible
and modifying the container requirements tables to make the range constructors require
EmplaceConstructible
.
After looking at this history and double-checking our implementation (where map
/set
range construction goes through
emplacement, with absolutely no special-casing for map
's pairs), I am convinced that N3173 superseded N1858 here.
(Range-insert()
and the unordered containers are unaffected.)
Previous resolution [SUPERSEDED]:
This wording is relative to the N3936.
Change 23.4.6.2 [set.cons] p4 as follows:
template <class InputIterator> set(InputIterator first, InputIterator last, const Compare& comp = Compare(), const Allocator& = Allocator());-3- Effects: Constructs an empty
-4- Requires: If the iterator's indirection operator returns an lvalue or aset
using the specified comparison object and allocator, and inserts elements from the range [first,last
).non-const rvalue, thenKey
shall beCopyInsertible
into*this
. -5- Complexity: Linear inN
if the range [first,last
) is already sorted usingcomp
and otherwiseN logN
, whereN
islast - first
.
[2015-02 Cologne]
GR: Do requirements supersede rather than compose [container requirements and per-function requirements]? AM: Yes, they supersede.
AM: This looks good. Ready? Agreement.Proposed resolution:
This wording is relative to the N4296.
Remove 23.4.3.2 [map.cons] p3:
template <class InputIterator> map(InputIterator first, InputIterator last, const Compare& comp = Compare(), const Allocator& = Allocator());[…]
-3- Requires: If the iterator's indirection operator returns an lvalue or aconst
rvaluepair<key_type, mapped_type>
, then bothkey_type
andmapped_type
shall beCopyInsertible
into*this
.
Remove 23.4.4.2 [multimap.cons] p3:
template <class InputIterator> multimap(InputIterator first, InputIterator last, const Compare& comp = Compare(), const Allocator& = Allocator());[…]
-3- Requires: If the iterator's indirection operator returns an lvalue or aconst
rvaluepair<key_type, mapped_type>
, then bothkey_type
andmapped_type
shall beCopyInsertible
into*this
.
Remove 23.4.6.2 [set.cons] p4:
template <class InputIterator> set(InputIterator first, InputIterator last, const Compare& comp = Compare(), const Allocator& = Allocator());[…]
-4- Requires: If the iterator's indirection operator returns an lvalue or a non-const
rvalue, thenKey
shall beCopyInsertible
into*this
.
Remove 23.4.7.2 [multiset.cons] p3:
template <class InputIterator> multiset(InputIterator first, InputIterator last, const Compare& comp = Compare(), const Allocator& = Allocator());[…]
-3- Requires: If the iterator's indirection operator returns an lvalue or aconst
rvalue, thenKey
shall beCopyInsertible
into*this
.
async()
incompleteSection: 32.10.9 [futures.async] Status: C++14 Submitter: Nicolai Josuttis Opened: 2011-08-29 Last modified: 2016-01-28
Priority: Not Prioritized
View other active issues in [futures.async].
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Discussion:
The current throw specification of async()
does state:
-6- Throws:
system_error
if policy islaunch::async
and the implementation is unable to start a new thread.
First it seems not clear whether this only applies if policy equals
launch::async
of if the async
launch mode flag is set
(if policy|launch::async!=0
)
More generally, I think what we want to say is that if the implementation cannot successfully execute on one of the policies allowed, then it must choose another. The principle would apply to implementation-defined policies as well.
Peter Sommerlad:
Should not throw. That was the intent. "is async" meat exactly.
[2012, Portland: move to Tentatively NAD Editorial]
If no launch policy, it is undefined behavior.
Agree with Lawrence, should try all the allowed policies. We will rephrase so that
the policy argument should be lauch::async
. Current wording seems good enough.
We believe this choice of policy statement is really an editorial issue.
[2013-09 Chicago]
If all the implementors read it and can't get it right - it is not editorial. Nico to provide wording
No objections to revised wording, so moved to Immediate.
Accept for Working Paper
Proposed resolution:
This wording is relative to N3691.
Change 32.10.9 [futures.async] p6, p7 as indicated:
-6- Throws:
system_error
ifpolicy
is==
launch::async
and the implementation is unable to start a new thread.-7- Error conditions:
resource_unavailable_try_again
— ifpolicy
is==
launch::async
and the system is unable to start a new thread.
once_flag
becomes invalidSection: 32.6.7 [thread.once] Status: C++14 Submitter: Nicolai Josuttis Opened: 2011-08-30 Last modified: 2016-01-28
Priority: Not Prioritized
View all issues with C++14 status.
Discussion:
In function call_once
32.6.7.2 [thread.once.callonce]
paragraph 4 and 5 specify for call_once()
:
Throws:
Error conditions:system_error
when an exception is required (32.2.2 [thread.req.exception]), or any exception thrown byfunc
.
invalid_argument
— if theonce_flag
object is no longer valid.
However, nowhere in 32.6.7 [thread.once] is specified, when a once-flag becomes invalid.
As far as I know this happens if the flag is used for different functions. So we either have to have to insert a sentence/paragraph in30.4.4.2 Function call_once [thread.once.callonce]
or
30.4.4 Call once [thread.once]
explaining when a once_flag
becomes invalidated or we should state as error condition something like:
invalid_argument
— if the func
used in combination with the once_flag
is different
from a previously passed func
for the same once_flag
Anthony Williams:
A
If the library can detect that this is the case then it will throw this exception. If it cannot detect such a case then it will never be thrown.once_flag
is invalidated if you destroy it (e.g. it is an automatic object, or heap allocated and deleted, etc.)
Jonathan Wakely:
I have also wondered how that error can happen in C++, where the type system will reject a non-callable type being passed to
If acall_once()
and should prevent aonce_flag
being used after its destructor runs.once_flag
is used after its destructor runs then it is indeed undefined behaviour, so implementations are already free to throw any exception (or set fire to a printer) without the standard saying so. My assumption was that it's an artefact of basing the API on pthreads, which says:The
pthread_once()
function may fail if:[EINVAL]
If eitheronce_control
orinit_routine
is invalid.
Pete Becker:
Yes, probably. We had to clean up several UNIXisms that were in the original design.
[2012, Kona]
Remove error conditions, move to Review.
[2012, Portland: move to Tentatively Ready]
Concurrency move to Ready, pending LWG review.
LWG did not have time to perform the final review in Portland, so moving to tentatively ready to reflect the Concurrency belief that the issue is ready, but could use a final inspection from library wordsmiths.
[2013-04-20 Bristol]
Proposed resolution:
This wording is relative to N3337.
Change 32.6.7.2 [thread.once.callonce] as indicated:
template<class Callable, class ...Args> void call_once(once_flag& flag, Callable&& func, Args&&... args);[…]
-4- Throws:system_error
when an exception is required (30.2.2), or any exception thrown byfunc
.-5- Error conditions:
invalid_argument
— if theonce_flag
object is no longer valid.
Allocator
requirements should include CopyConstructible
Section: 16.4.4.6 [allocator.requirements] Status: C++14 Submitter: Jonathan Wakely Opened: 2011-08-30 Last modified: 2016-01-28
Priority: Not Prioritized
View other active issues in [allocator.requirements].
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Discussion:
As discussed in c++std-lib-31054 and c++std-lib-31059, the Allocator
requirements implicitly require CopyConstructible
because
a.select_on_container_copy_construction()
and
container.get_allocator()
both return a copy by value, but the
requirement is not stated explicitly anywhere.
CopyConstructible
.
[2012, Kona]
Move to Ready.
[2012, Portland: applied to WP]
Proposed resolution:
This wording is relative to the FDIS.
Change Table 28 — Allocator requirements in 16.4.4.6 [allocator.requirements]:
Expression | Return type | Assertion/note pre-/post-condition | Default |
---|---|---|---|
X a1(a);
|
Shall not exit via an exception. post: a1 == a
|
||
…
|
|||
X a1(move(a));
|
Shall not exit via an exception. post: a1 equals the prior valueof a .
|
Change 16.4.4.6 [allocator.requirements] paragraph 4:
An allocator type
X
shall satisfy the requirements ofCopyConstructible
(16.4.4.2 [utility.arg.requirements]). TheX::pointer
,X::const_pointer
,X::void_pointer
, andX::const_void_pointer
types shall satisfy the requirements ofNullablePointer
(16.4.4.4 [nullablepointer.requirements]). No constructor, comparison operator, copy operation, move operation, or swap operation on these types shall exit via an exception.X::pointer
andX::const_pointer
shall also satisfy the requirements for a random access iterator (24.3 [iterator.requirements]).
weak_ptr::owner_before
Section: 20.3.2.3 [util.smartptr.weak], 20.3.2.3.6 [util.smartptr.weak.obs] Status: C++14 Submitter: Ai Azuma Opened: 2011-09-06 Last modified: 2016-01-28
Priority: Not Prioritized
View all other issues in [util.smartptr.weak].
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Discussion:
Is there any reason why weak_ptr::owner_before
member function templates are not const-qualified?
Daniel Krügler:
I don't think so. To the contrary, without these to be const member function templates, the semantics of the specializations
It is amusing to note that this miss has remain undetected from the accepted paper n2637 on. For the suggested wording changes see below.owner_less<weak_ptr<T>>
andowner_less<shared_ptr<T>>
described in 20.3.2.4 [util.smartptr.ownerless] is unclear.
[2012, Kona]
Move to Ready.
[2012, Portland: applied to WP]
Proposed resolution:
This wording is relative to the FDIS.
Change the class template weak_ptr
synopsis in 20.3.2.3 [util.smartptr.weak]
as indicated:
namespace std { template<class T> class weak_ptr { public: typedef T element_type; […] template<class U> bool owner_before(shared_ptr<U> const& b) const; template<class U> bool owner_before(weak_ptr<U> const& b) const; }; […] }
Change the prototypes in 20.3.2.3.6 [util.smartptr.weak.obs] before p6 as indicated:
template<class U> bool owner_before(shared_ptr<U> const& b) const; template<class U> bool owner_before(weak_ptr<U> const& b) const;
basic_istream::ignore
Section: 31.7.5.4 [istream.unformatted] Status: C++14 Submitter: Krzysztof Żelechowski Opened: 2011-09-11 Last modified: 2016-08-09
Priority: Not Prioritized
View all other issues in [istream.unformatted].
View all issues with C++14 status.
Discussion:
31.7.5.4 [istream.unformatted] in N3242 currently has the following to say about the
semantics of basic_istream::ignore
:
[..]. Characters are extracted until any of the following occurs:
- if
n != numeric_limits<streamsize>::max()
(18.3.2),n
characters are extracted
This statement, apart from being slightly ungrammatical, indicates that if
(n == numeric_limits<streamsize>::max()
), the method returns without
extracting any characters.
[..]. Characters are extracted until any of the following occurs:
(n != numeric_limits<streamsize>::max())
(18.3.2) and (n
) characters have been extracted so far.
[2013-04-20, Bristol]
Resolution: Ready.
[2013-09-29, Chicago]
Apply to Working Paper
Proposed resolution:
This wording is relative to the FDIS.
Change 31.7.5.4 [istream.unformatted] p25 as indicated:
basic_istream<charT,traits>& ignore(streamsize n = 1, int_type delim = traits::eof());-25- Effects: Behaves as an unformatted input function (as described in 31.7.5.4 [istream.unformatted], paragraph 1). After constructing a
sentry
object, extracts characters and discards them. Characters are extracted until any of the following occurs:
ifn != numeric_limits<streamsize>::max()
( [limits.numeric]),andn
charactersarehave been extracted so far- end-of-file occurs on the input sequence (in which case the function calls
setstate(eofbit)
, which may throwios_base::failure
(31.5.4.4 [iostate.flags]));traits::eq_int_type(traits::to_int_type(c), delim)
for the next available input characterc
(in which casec
is extracted).
Section: 29.7 [c.math] Status: C++14 Submitter: Daniel Krügler Opened: 2011-09-22 Last modified: 2016-01-28
Priority: Not Prioritized
View all other issues in [c.math].
View all issues with C++14 status.
Discussion:
29.7 [c.math] ends with a description of a rule set for "sufficient overloads" in p11:
Moreover, there shall be additional overloads sufficient to ensure:
- If any argument corresponding to a
double
parameter has typelong double
, then all arguments corresponding todouble
parameters are effectively cast tolong double
.- Otherwise, if any argument corresponding to a
double
parameter has typedouble
or an integer type, then all arguments corresponding todouble
parameters are effectively cast todouble
.- Otherwise, all arguments corresponding to
double
parameters are effectively cast tofloat
.
My impression is that this rule set is probably more generic as intended, my assumption is that it is written to mimic the C99/C1x rule set in 7.25 p2+3 in the "C++" way:
-2- Of the
-3- Use of the macro invokes a function whose generic parameters have the corresponding real type determined as follows:<math.h>
and<complex.h>
functions without anf
(float
) orl
(long double
) suffix, several have one or more parameters whose corresponding real type isdouble
. For each such function, exceptmodf
, there is a corresponding type-generic macro. (footnote 313) The parameters whose corresponding real type isdouble
in the function synopsis are generic parameters. Use of the macro invokes a function whose corresponding real type and type domain are determined by the arguments for the generic parameters. (footnote 314)
- First, if any argument for generic parameters has type
long double
, the type determined islong double
.- Otherwise, if any argument for generic parameters has type
double
or is of integer type, the type determined isdouble
.- Otherwise, the type determined is
float
.
where footnote 314 clarifies the intent:
If the type of the argument is not compatible with the type of the parameter for the selected function, the behavior is undefined.
The combination of the usage of the unspecific term "cast" with otherwise no further constraints (note that C constraints the valid set to types that C++ describes as arithmetic types, but see below for one important difference) has the effect that it requires the following examples to be well-formed and well-defined:
#include <cmath> enum class Ec { }; struct S { explicit operator long double(); }; void test(Ec e, S s) { std::sqrt(e); // OK, behaves like std::sqrt((float) e); std::sqrt(s); // OK, behaves like std::sqrt((float) s); }
GCC 4.7 does not accept any of these examples.
I found another example where the C++ rule differs from the C set, but in this case I'm not so sure, which direction C++ should follow. The difference is located in the fact, that in C enumerated types are integer types as described in 6.2.5 p17 (see e.g. n1569 or n1256): "The type char, the signed and unsigned integer types, and the enumerated types are collectively called integer types. The integer and real floating types are collectively called real types." This indicates that in C the following code#include <math.h> enum E { e }; void test(void) { sqrt(e); // OK, behaves like sqrt((double) e); }
seems to be well-defined and e
is cast to double
, but in C++
referring to
#include <cmath> enum E { e }; void test() { std::sqrt(e); // OK, behaves like sqrt((float) e); }
is also well-defined (because of our lack of constraints) but we
must skip bullet 2 (because E is not an integer type) and effectively
cast e
to float
. Accepting this, we would introduce
a silent, but observable runtime difference for C and C++.
Howard provided wording to solve the issue.
[2012, Kona]
Moved to Ready. The proposed wording reflects both original intent from TR1, and current implementations.
[2012, Portland: applied to WP]
Proposed resolution:
This wording is relative to the FDIS.
Change 29.7 [c.math] p11 as indicated:
Moreover, there shall be additional overloads sufficient to ensure:
- If any arithmetic argument corresponding to a
double
parameter has typelong double
, then all arithmetic arguments corresponding todouble
parameters are effectively cast tolong double
.- Otherwise, if any arithmetic argument corresponding to a
double
parameter has typedouble
or an integer type, then all arithmetic arguments corresponding todouble
parameters are effectively cast todouble
.- Otherwise, all arithmetic arguments corresponding to
double
parametersare effectively cast tohave typefloat
.
iostream_category()
and noexcept
Section: 31.5 [iostreams.base] Status: C++14 Submitter: Nicolai Josuttis Opened: 2011-09-22 Last modified: 2016-01-28
Priority: Not Prioritized
View all other issues in [iostreams.base].
View all issues with C++14 status.
Discussion:
In <system_error>
we have:
const error_category& generic_category() noexcept; const error_category& system_category() noexcept;
In <future>
we have:
const error_category& future_category() noexcept;
But in <ios>
we have:
const error_category& iostream_category();
Is there any reason that iostream_category()
is not declared with
noexcept
or is this an oversight?
Daniel:
This looks like an oversight to me. We made the above mentioned changes as part of noexcept-ifying the thread library butiostream_category()
was skipped, so it seems
to be forgotten. There should be no reason, why it cannot
be noexcept
. When doing so, we should also make these functions
noexcept
(similar to corresponding overloads):
error_code make_error_code(io_errc e); error_condition make_error_condition(io_errc e);
Suggested wording provided by Daniel.
[2013-04-20, Bristol]
Unanimous.
Resolution: move to tentatively ready.[2013-09-29, Chicago]
Apply to Working Paper
Proposed resolution:
This wording is relative to the FDIS.
Change [iostreams.base.overview], header <ios>
synopsis
as indicated:
#include <iosfwd> namespace std { […] error_code make_error_code(io_errc e) noexcept; error_condition make_error_condition(io_errc e) noexcept; const error_category& iostream_category() noexcept; }
Change the prototype declarations in 31.5.6 [error.reporting] as indicated:
error_code make_error_code(io_errc e) noexcept;
-1- Returns:
error_code(static_cast<int>(e), iostream_category())
.
error_condition make_error_condition(io_errc e) noexcept;
-2- Returns:
error_condition(static_cast<int>(e), iostream_category())
.
const error_category& iostream_category() noexcept;
-3- Returns: A reference to an object of a type derived from class
-4- The object’serror_category
.default_error_condition
andequivalent
virtual functions shall behave as specified for the classerror_category
. The object’sname
virtual function shall return a pointer to the string"iostream"
.
std::terminate
problemSection: 17.9.5 [exception.terminate] Status: Resolved Submitter: Daniel Krügler Opened: 2011-09-25 Last modified: 2024-11-07
Priority: 3
View all issues with Resolved status.
Discussion:
Andrzej Krzemienski reported the following on comp.std.c++:
In N3290, which is to become the official standard, in 17.9.5.4 [terminate], paragraph 1 reads
Remarks: Called by the implementation when exception handling must be abandoned for any of several reasons (15.5.1), in effect immediately after evaluating the throw-expression (18.8.3.1). May also be called directly by the program.
It is not clear what is "in effect". It was clear in previous drafts where paragraphs 1 and 2 read:
Called by the implementation when exception handling must be abandoned for any of several reasons (15.5.1). May also be called directly by the program.
Effects: Calls theterminate_handler
function in effect immediately after evaluating the throw-expression (18.8.3.1), if called by the implementation, or calls the currentterminate_handler
function, if called by the program.It was changed by N3189. The same applies to function unexpected (D. 11.4, paragraph 1).
Assuming the previous wording is still intended, the wording can be read "unlessstd::terminate
is called by the program, we will use the handler that was in effect immediately after evaluating the throw-expression". This assumes that there is some throw-expression connected to every situation that triggers the call tostd::terminate
. But this is not the case:
- In case
std::thread
is assigned to or destroyed while being joinable there is no throw-expression involved.- In case
std::unexpected
is called by the program,std::terminate
is triggered by the implementation - no throw-expression involved.- In case a destructor throws during stack unwinding we have two throw-expressions involved.
Which one is referred to?
In casestd::nested_exception::rethrow_nested
is called for an object that has captured no exception, there is no throw-expression involved directly (and may no throw be involved even indirectly). Next, 17.9.5.1 [terminate.handler], paragraph 2 saysRequired behavior: A
terminate_handler
shall terminate execution of the program without returning to the caller.This seems to allow that the function may exit by throwing an exception (because word "return" implies a normal return).
One could argue that words "terminate execution of the program" are sufficient, but then why "without returning to the caller" would be mentioned. In case such handler throws, noexcept specification in functionstd::terminate
is violated, andstd::terminate
would be called recursively - shouldstd::abort
not be called in case of recursivestd::terminate
call? On the other hand some controlled recursion could be useful, like in the following technique.
The here mentioned wording changes by N3189 in regard to 17.9.5.4 [terminate] p1
were done for a better separation of effects (Effects element) and additional normative
wording explanations (Remarks element), there was no meaning change intended. Further,
there was already a defect existing in the previous wording, which was not updated when
further situations where defined, when std::terminate
where supposed to be
called by the implementation.
terminate_handler
function, so should be moved just after
"Effects: Calls the current terminate_handler
function."
It seems ok to allow a termination handler to exit via an exception, but the
suggested idiom should better be replaced by a more simpler one based on
evaluating the current exception pointer in the terminate handler, e.g.
void our_terminate (void) { std::exception_ptr p = std::current_exception(); if (p) { ... // OK to rethrow and to determine it's nature } else { ... // Do something else } }
[2011-12-09: Daniel comments]
[2012, Kona]
Move to Open.
There is an interaction with Core issues in this area that Jens is already supplying wording for. Review this issue again once Jens wording is available.
Alisdair to review clause 15.5 (per Jens suggestion) and recommend any changes, then integrate Jens wording into this issue.
[2024-11-07 Status changed: Open → Resolved.]
Resolved by the resolution of LWG 2111(i).
Proposed resolution:
std::allocator::construct
should use uniform initializationSection: 20.2.10.2 [allocator.members] Status: Resolved Submitter: David Krauss Opened: 2011-10-07 Last modified: 2020-09-06
Priority: 2
View all other issues in [allocator.members].
View all issues with Resolved status.
Discussion:
When the EmplaceConstructible
(23.2.2 [container.requirements.general]/13) requirement is used
to initialize an object, direct-initialization occurs. Initializing an aggregate or using a std::initializer_list
constructor with emplace requires naming the initialized type and moving a temporary. This is a result of
std::allocator::construct
using direct-initialization, not list-initialization (sometimes called "uniform
initialization") syntax.
std::allocator<T>::construct
to use list-initialization would, among other things, give
preference to std::initializer_list
constructor overloads, breaking valid code in an unintuitive and
unfixable way — there would be no way for emplace_back
to access a constructor preempted by
std::initializer_list
without essentially reimplementing push_back
.
std::vector<std::vector<int>> v; v.emplace_back(3, 4); // v[0] == {4, 4, 4}, not {3, 4} as in list-initialization
The proposed compromise is to use SFINAE with std::is_constructible
, which tests whether direct-initialization
is well formed. If is_constructible
is false, then an alternative std::allocator::construct
overload
is chosen which uses list-initialization. Since list-initialization always falls back on direct-initialization, the
user will see diagnostic messages as if list-initialization (uniform-initialization) were always being used, because
the direct-initialization overload cannot fail.
std::initializer_list
satisfy a constructor, such as trying to emplace-insert a value of {3, 4}
in
the above example. The workaround is to explicitly specify the std::initializer_list
type, as in
v.emplace_back(std::initializer_list<int>(3, 4))
. Since this matches the semantics as if
std::initializer_list
were deduced, there seems to be no real problem here.
The other case is when arguments intended for aggregate initialization satisfy a constructor. Since aggregates cannot
have user-defined constructors, this requires that the first nonstatic data member of the aggregate be implicitly
convertible from the aggregate type, and that the initializer list have one element. The workaround is to supply an
initializer for the second member. It remains impossible to in-place construct an aggregate with only one nonstatic
data member by conversion from a type convertible to the aggregate's own type. This seems like an acceptably small
hole.
The change is quite small because EmplaceConstructible
is defined in terms of whatever allocator is specified,
and there is no need to explicitly mention SFINAE in the normative text.
[2012, Kona]
Move to Open.
There appears to be a real concern with initializing aggregates, that can be performed only using brace-initialization. There is little interest in the rest of the issue, given the existence of 'emplace' methods in C++11.
Move to Open, to find an acceptable solution for intializing aggregates. There is the potential that EWG may have an interest in this area of language consistency as well.
[2013-10-13, Ville]
This issue is related to 2070(i).
[2015-02 Cologne]
Move to EWG, Ville to write a paper.
[2015-09, Telecon]
Ville: N4462 reviewed in Lenexa. EWG discussion to continue in Kona.
[2016-08 Chicago]
See N4462
The notes in Lenexa say that Marshall & Jonathan volunteered to write a paper on this
[2018-08-23 Batavia Issues processing]
P0960 (currently in flight) should resolve this.
[2020-01 Resolved by the adoption of P0960 in Kona.]
Proposed resolution:
This wording is relative to the FDIS.
Change 20.2.10.2 [allocator.members] p12 as indicated:
template <class U, class... Args> void construct(U* p, Args&&... args);12 Effects:
::new((void *)p) U(std::forward<Args>(args)...)
ifis_constructible<U, Args...>::value
istrue
, else::new((void *)p) U{std::forward<Args>(args)...}
m.try_lock_for()
Section: 32.6.4.3 [thread.timedmutex.requirements] Status: C++14 Submitter: Pete Becker Opened: 2011-10-18 Last modified: 2016-01-28
Priority: Not Prioritized
View all other issues in [thread.timedmutex.requirements].
View all issues with C++14 status.
Discussion:
32.6.4.3 [thread.timedmutex.requirements]/4 says, in part,
"Requires: If the tick period of [the argument] is not exactly convertible … [it] shall be rounded up …"
This doesn't belong in the requires clause. It's an effect. It belongs in paragraph 5. Nitpickingly, this would be a technical change: as written it imposes an obligation on the caller, while moving it imposes an obligation on the callee. Although that's certainly not what was intended.
Peter Dimov comments: Not to mention that it should round down, not up. :-) Incidentally, I see that the wrongtry_lock
requirement that the caller shall not own
the mutex has entered the standard, after all. Oh well. Let's hope that the real world
continues to ignore it.
[2012, Kona]
Remove the offending sentence from the requirements clause. Do not add it back anywhere else. The implementation already must have license to wake up late, so the rounding is invisible.
Move to Review.[2012, Portland]
Concurrency move to Ready.
[2013-04-20 Bristol]
Proposed resolution:
This wording is relative to N3337.
Change 32.6.4.3 [thread.timedmutex.requirements]/4 as indicated:
-3- The expression
-4- Requires:m.try_lock_for(rel_time)
shall be well-formed and have the following semantics:If the tick period ofIfrel_time
is not exactly convertible to the native tick period, the duration shall be rounded up to the nearest native tick period.m
is of typestd::timed_mutex
, the calling thread does not own the mutex.
condition_variable_any
Section: 32.7.5 [thread.condition.condvarany] Status: C++14 Submitter: Pete Becker Opened: 2011-10-20 Last modified: 2016-01-28
Priority: Not Prioritized
View all other issues in [thread.condition.condvarany].
View all issues with C++14 status.
Discussion:
32.7.5 [thread.condition.condvarany]/4 says, in part, that
condition_variable_any()
throws an exception
"if any native handle type manipulated is not available".
condition_variable()
[32.7.4 [thread.condition.condvar]/4],
"if some non-memory resource limitation prevents initialization"? If not,
it should be worded the same way.
[2012, Kona]
Copy the corresponding wording from the condition_variable
constructor in 32.7.4 [thread.condition.condvar] p4.
[2012, Portland]
Concurrency move to Ready.
[2013-04-20 Bristol]
Proposed resolution:
This wording is relative to N3337.
Change 32.6.4.3 [thread.timedmutex.requirements]/4 as indicated:
condition_variable_any();[…]
-4- Error conditions:
resource_unavailable_try_again
—if any native handle type manipulated is not availableif some non-memory resource limitation prevents initialization.operation_not_permitted
— if the thread does not have the privilege to perform the operation.
condition_variable::wait
with predicateSection: 32.7.4 [thread.condition.condvar] Status: C++14 Submitter: Alberto Ganesh Barbati Opened: 2011-10-27 Last modified: 2015-10-20
Priority: Not Prioritized
View all other issues in [thread.condition.condvar].
View all issues with C++14 status.
Discussion:
the Throws: clause of condition_variable::wait/wait_xxx
functions that
take a predicate argument is:
Throws:
system_error
when an exception is required (32.2.2 [thread.req.exception]).
If executing the predicate throws an exception, I would expect such exception to propagate unchanged
to the caller, but the throws clause seems to indicate that it gets mutated into a system_error
.
That's because of 16.3.2.4 [structure.specifications]/4:
F
's semantics contains a Throws:, Postconditions:, or Complexity: element, then that supersedes
any occurrences of that element in the code sequence."
Is my interpretation correct? Does it match the intent?
Daniel comments:
I don't think that this interpretation is entirely correct, the wording does not say that
std::system_error
or a derived class must be thrown, it simply is underspecified
in this regard (The extreme interpretation is that the behaviour would be undefined, but
that would be too far reaching I think). We have better wording for this in
32.6.7.2 [thread.once.callonce] p4, where it says:
"Throws: system_error
when an exception is required (32.2.2 [thread.req.exception]),
or any exception thrown by func
."
or in 32.4.5 [thread.thread.this] p6/p9:
"Throws: Nothing if Clock
satisfies the TrivialClock
requirements
(30.3 [time.clock.req]) and operations of Duration
do not throw exceptions.
[ Note: instantiations of time point types and clocks supplied by the implementation
as specified in 30.7 [time.clock] do not throw exceptions. — end note ]"
So, the here discussed Throws elements should add lines along the lines of
"Any exception thrown by operations of pred
."
and similar wording for time-related operations:
"Any exception thrown by operations of Duration
",
"Any exception thrown by operations of chrono::duration<Rep, Period>
"
[2011-11-28: Ganesh comments and suggests wording]
As for the discussion about the exception thrown by the manipulation of time-related objects, I believe the argument applies to all functions declared in 32 [thread]. Therefore, instead of adding wording to each member, I would simply move those requirements from 32.4.5 [thread.thread.this] p6/p9 to a new paragraph in 32.2.4 [thread.req.timing].
As for 32.7.5 [thread.condition.condvarany], the member functionswait()
and
wait_until()
are described only in terms of the Effects: clause (so strictly speaking,
they need no changes), however, wait_for()
is described with a full set of clauses
including Throws: and Error conditions:. Either we should add those clauses to wait/wait_until
with changes similar to the one above, or remove paragraphs 29 to 34 entirely. By the way,
even paragraph 26 could be removed IMHO.
[2012, Kona]
We like the idea behind the proposed resolution.
Modify the first addition to read instead: "Functions that specify a timeout, will throw if an operation on a clock, time point, or time duration throws an exception." In the note near the bottom change "even if the timeout has already expired" to "or if the timeout has already expired". (This is independent, but the original doesn't seem to make sense.) Move to Review.[2012, Portland]
Concurrency move to Ready with slightly ammended wording.
[2013-04-20 Bristol]
Proposed resolution:
This wording is relative to N3337.
Add a new paragraph at the end of 32.2.4 [thread.req.timing]:
[…]
-6- The resolution of timing provided by an implementation depends on both operating system and hardware. The finest resolution provided by an implementation is called the native resolution. -7- Implementation-provided clocks that are used for these functions shall meet theTrivialClock
requirements (30.3 [time.clock.req]). -?- Functions that specify a timeout, will throw if, during the execution of this function, a clock, time point, or time duration throws an exception. [ Note: instantiations of clock, time point and duration types supplied by the implementation as specified in 30.7 [time.clock] do not throw exceptions. — end note]
Change 32.4.5 [thread.thread.this] as indicated:
template <class Clock, class Duration> void sleep_until(const chrono::time_point<Clock, Duration>& abs_time);;-4- Effects: Blocks the calling thread for the absolute timeout (32.2.4 [thread.req.timing]) specified by
-5- Synchronization: None. -6- Throws: timeout-related exceptions (32.2.4 [thread.req.timing]).abs_time
.Nothing ifClock
satisfies theTrivialClock
requirements (30.3 [time.clock.req]) and operations ofDuration
do not throw exceptions. [ Note: instantiations of time point types and clocks supplied by the implementation as specified in 30.7 [time.clock] do not throw exceptions. — end note]
template <class Rep, class Period> void sleep_for(const chrono::duration<Rep, Period>& rel_time);;-7- Effects: Blocks the calling thread for the relative timeout (32.2.4 [thread.req.timing]) specified by
-8- Synchronization: None. -9- Throws: timeout-related exceptions (32.2.4 [thread.req.timing]).rel_time
.Nothing if operations ofchrono::duration<Rep, Period>
do not throw exceptions. [ Note: instantiations of time point types and clocks supplied by the implementation as specified in 30.7 [time.clock] do not throw exceptions. — end note]
Change 32.6.4.3 [thread.timedmutex.requirements] as indicated:
-3- The expression m.try_lock_for(rel_time)
shall be well-formed and have the following semantics:
rel_time
. If the time specified by rel_time
is less than or equal to rel_time.zero()
, the
function attempts to obtain ownership without blocking (as if by calling try_lock()
). The function
shall return within the timeout specified by rel_time
only if it has obtained ownership of the mutex
object. [Note: As with try_lock()
, there is no guarantee that ownership will be obtained if the lock
is available, but implementations are expected to make a strong effort to do so. — end note]
[…]
-8- Synchronization: If try_lock_for()
returns true
, prior unlock()
operations on the same object
synchronize with (6.9.2 [intro.multithread]) this operation.
-9- Throws: timeout-related exceptions (32.2.4 [thread.req.timing]).m.try_lock_until(abs_time)
shall be well-formed and have the following semantics:
[…]
-12- Effects: The function attempts to obtain ownership of the mutex. If abs_time
has already passed, the
function attempts to obtain ownership without blocking (as if by calling try_lock()
). The function
shall return before the absolute timeout (32.2.4 [thread.req.timing]) specified by abs_time
only
if it has obtained ownership of the mutex object. [Note: As with try_lock()
, there is no guarantee
that ownership will be obtained if the lock is available, but implementations are expected to make a strong effort
to do so. — end note]
[…]
-15- Synchronization: If try_lock_until()
returns true, prior unlock()
operations on the same object
synchronize with (6.9.2 [intro.multithread]) this operation.
-16- Throws: timeout-related exceptions (32.2.4 [thread.req.timing]).Change 32.7.4 [thread.condition.condvar] as indicated:
template <class Predicate> void wait(unique_lock<mutex>& lock, Predicate pred);[…]
-15- Effects: Equivalent to:while (!pred()) wait(lock);[…]
-17- Throws:std::system_error
when an exception is required (32.2.2 [thread.req.exception]), timeout-related exceptions (32.2.4 [thread.req.timing]), or any exception thrown bypred
. […]
template <class Clock, class Duration> cv_status wait_until(unique_lock<mutex>& lock, const chrono::time_point<Clock, Duration>& abs_time);[…]
-23- Throws:system_error
when an exception is required (32.2.2 [thread.req.exception]) or timeout-related exceptions (32.2.4 [thread.req.timing]). […]
template <class Rep, class Period> cv_status wait_for(unique_lock<mutex>& lock, const chrono::duration<Rep, Period>& rel_time);[…]
-26- Effects:as ifEquivalent to:return wait_until(lock, chrono::steady_clock::now() + rel_time);[…]
-29- Throws:system_error
when an exception is required (32.2.2 [thread.req.exception]) or timeout-related exceptions (32.2.4 [thread.req.timing]). […]
template <class Clock, class Duration, class Predicate> bool wait_until(unique_lock<mutex>& lock, const chrono::time_point<Clock, Duration>& abs_time, Predicate pred);[…]
-32- Effects: Equivalent to:while (!pred()) if (wait_until(lock, abs_time) == cv_status::timeout) return pred(); return true;[…] -36- Throws:
-33- Returns:pred()
std::system_error
when an exception is required (32.2.2 [thread.req.exception]), timeout-related exceptions (32.2.4 [thread.req.timing]), or any exception thrown bypred
. […]
template <class Rep, class Period, class Predicate> bool wait_for(unique_lock<mutex>& lock, const chrono::duration<Rep, Period>& rel_time, Predicate pred);[…]
-39- Effects:as ifEquivalent to:return wait_until(lock, chrono::steady_clock::now() + rel_time, std::move(pred));[…]
-42- Returns:[…] -44- Throws:pred()
system_error
when an exception is required (32.2.2 [thread.req.exception]), timeout-related exceptions (32.2.4 [thread.req.timing]), or any exception thrown bypred
. […]
Change 32.7.5 [thread.condition.condvarany] as indicated:
template <class Lock, class Predicate> void wait(Lock& lock, Predicate pred);-14- Effects: Equivalent to:
while (!pred()) wait(lock);
template <class Lock, class Clock, class Duration> cv_status wait_until(Lock& lock, const chrono::time_point<Clock, Duration>& abs_time);[…]
-18- Throws:system_error
when an exception is required (32.2.2 [thread.req.exception]) or any timeout-related exceptions (32.2.4 [thread.req.timing]). […]
template <class Lock, class Rep, class Period> cv_status wait_for(Lock& lock, const chrono::duration<Rep, Period>& rel_time);[…]
-20- Effects:as ifEquivalent to:return wait_until(lock, chrono::steady_clock::now() + rel_time);[…]
-23- Throws:system_error
when an exception is required (32.2.2 [thread.req.exception]) or any timeout-related exceptions (32.2.4 [thread.req.timing]). […]
template <class Lock, class Clock, class Duration, class Predicate> bool wait_until(Lock& lock, const chrono::time_point<Clock, Duration>& abs_time, Predicate pred);-25- Effects: Equivalent to:
while (!pred()) if (wait_until(lock, abs_time) == cv_status::timeout) return pred(); return true;-26-
-27- [Note: The returned value indicates whether the predicate evaluates toReturns:[Note: There is no blocking ifpred()
pred()
is initiallytrue
, or if the timeout has already expired. — end note]true
regardless of whether the timeout was triggered. end note]
template <class Lock, class Rep, class Period, class Predicate> bool wait_for(Lock& lock, const chrono::duration<Rep, Period>& rel_time, Predicate pred);-28- Effects:
as ifEquivalent to:return wait_until(lock, chrono::steady_clock::now() + rel_time, std::move(pred));
-29- [Note: There is no blocking ifpred()
is initiallytrue
, even if the timeout has already expired. — end note]-30- Postcondition:lock
is locked by the calling thread.-31- Returns:pred()
-32- [Note: The returned value indicates whether the predicate evaluates totrue
regardless of whether the timeout was triggered. — end note]-33- Throws:system_error
when an exception is required (32.2.2 [thread.req.exception]).-34- Error conditions:
equivalent error condition fromlock.lock()
orlock.unlock()
.
duration
conversion overflow shouldn't participate in overload resolutionSection: 30.5.2 [time.duration.cons] Status: C++14 Submitter: Vicente J. Botet Escriba Opened: 2011-10-31 Last modified: 2016-01-28
Priority: Not Prioritized
View all other issues in [time.duration.cons].
View all issues with C++14 status.
Discussion:
30.5.2 [time.duration.cons] says:
template <class Rep2, class Period2> constexpr duration(const duration<Rep2, Period2>& d);Remarks: This constructor shall not participate in overload resolution unless
treat_as_floating_point<rep>::value
istrue
or bothratio_divide<Period2, period>::den
is1
andtreat_as_floating_point<Rep2>::value
isfalse
.
The evaluation of ratio_divide<Period2, period>::den
could make
ratio_divide<Period2, period>::num
overflow.
period
=ratio<1,1000>
)
from an exa-second (Period2
=ratio<1018>
).
ratio_divide<ratio<1018>, ratio<1,1000>>::num
is
1021
which overflows which makes the compiler error.
If the function f
is overloaded with milliseconds and seconds
void f(milliseconds); void f(seconds);
The following fails to compile.
duration<int,exa> r(1); f(r);
While the conversion to seconds work, the conversion to milliseconds make the program fail at compile time.
In my opinion, this program should be well formed and the constructor from duration<int,exa>
to milliseconds shouldn't participate in overload resolution as the result can not be represented.
[2012, Kona]
Move to Review.
Pete: The wording is not right.
Howard: Will implement this to be sure it works.
Jeffrey: If ratio needs a new hook, should it be exposed to users for their own uses?
Pete: No.
Move to Review, Howard to implement in a way that mere mortals can understand.
[2013-04-18, Bristol]
Proposed resolution:
This wording is relative to the FDIS.
Change the following paragraphs of 30.5.2 [time.duration.cons] p4 indicated:
template <class Rep2, class Period2> constexpr duration(const duration<Rep2, Period2>& d);Remarks: This constructor shall not participate in overload resolution unless no overflow is induced in the conversion and
treat_as_floating_point<rep>::value
istrue
or bothratio_divide<Period2, period>::den
is1
andtreat_as_floating_point<Rep2>::value
isfalse
. [ Note: This requirement prevents implicit truncation error when converting between integral-based duration types. Such a construction could easily lead to confusion about the value of the duration. — end note ]
future::get
in regard to MoveAssignable
Section: 32.10.7 [futures.unique.future] Status: C++14 Submitter: Daniel Krügler Opened: 2011-11-02 Last modified: 2021-06-06
Priority: Not Prioritized
View all other issues in [futures.unique.future].
View all issues with C++14 status.
Discussion:
[futures.unique_future] paragraph 15 says the following:
R future::get();
…
-15- Returns:future::get()
returns the value stored in the object’s shared state. If the type of the value is
MoveAssignable
the returned value is moved, otherwise it is copied.
There are some problems with the description:
"If the type of the value isMoveAssignable
the returned value is moved, otherwise it is copied."
MoveAssignable
? This should be
based solely on a pure expression-based requirement, if this is an requirement for implementations.
MoveAssignable
to the plain expression part std::is_move_assignable
would solvs (1), but raises another question, namely why a move-assignment should be relevant
for a function return based on the value stored in the future state? We would better fall back to
std::is_move_constructible
instead.
The last criticism I have is about the part
"the returned value is moved, otherwise it is copied" because an implementation won't be able to recognize what the user-defined type will do during an expression that is prepared by the implementation. I think the wording is intended to allow a move by seeding with an rvalue expression viastd::move
(or equivalent), else the result will be an effective
copy construction.
[2011-11-28 Moved to Tentatively Ready after 5 positive votes on c++std-lib.]
Proposed resolution:
This wording is relative to the FDIS.
Change [futures.unique_future] paragraph 15 as indicated:
R future::get();
…
-15- Returns:future::get()
returns the value v
stored in the object’s shared
state as std::move(v)
. If the type of the value is
MoveAssignable
the returned value is moved, otherwise it is copied.
packaged_task
constructors should be constrainedSection: 32.10.10.2 [futures.task.members] Status: C++14 Submitter: Jonathan Wakely Opened: 2011-11-02 Last modified: 2016-01-28
Priority: Not Prioritized
View all other issues in [futures.task.members].
View all issues with C++14 status.
Discussion:
With the proposed resolution of 2067(i), this no longer selects the copy constructor:
std::packaged_task<void()> p1; std::packaged_task<void()> p2(p1);
Instead this constructor is a better match:
template <class F> explicit packaged_task(F&& f);
This attempts to package a packaged_task
, which internally tries to
copy p2
, which fails because the copy constructor is deleted. For at
least one implementation the resulting error message is much less
helpful than the expected "cannot call deleted function" because it
happens after instantiating several more templates rather than in the
context where the constructor is called.
F
cannot be deduced as (possibly cv)
packaged_task&
or packaged_task
. It could be argued
this constraint is already implied because packaged_task
is not
copyable and the template constructors require that "invoking a copy of f
shall behave the same as invoking f
".
Daniel points out that the variadic constructor of std::thread
described in 32.4.3.3 [thread.thread.constr] has a similar problem and
suggests a similar wording change, which has been integrated below.
An alternative is to declare thread(thread&)
and
packaged_task(packaged_task&)
as deleted.
[2012, Portland]
This issue appears to be more about library specification than technical concurrency issues, so should be handled in LWG.
[2013, Chicago]
Move to Immediate resolution.
Howard volunteered existing implementation experience with the first change, and saw no issue that the second would introduce any new issue.
Proposed resolution:
This wording is relative to the FDIS.
Insert a new Remarks element to 32.4.3.3 [thread.thread.constr] around p3 as indicated:
template <class F, class ...Args> explicit thread(F&& f, Args&&... args);
-3- Requires: F
and each Ti
in Args
shall satisfy the MoveConstructible
requirements. INVOKE(DECAY_COPY ( std::forward<F>(f)), DECAY_COPY (std::forward<Args>(args))...)
(20.8.2) shall be a valid expression.
decay<F>::type
is the same type as std::thread
.
Insert a new Remarks element to 32.10.10.2 [futures.task.members] around p2 as indicated:
template <class F> packaged_task(F&& f); template <class F, class Allocator> explicit packaged_task(allocator_arg_t, const Allocator& a, F&& f);
-2- Requires: INVOKE(f, t1, t2, ..., tN, R)
, where t1, t2, ..., tN
are values of the corresponding
types in ArgTypes...
, shall be a valid expression. Invoking a copy of f
shall behave the same as invoking f
.
decay<F>::type
is the same type as std::packaged_task<R(ArgTypes...)>
.
promise::set_value
and promise::set_value_at_thread_exit
Section: 32.10.6 [futures.promise] Status: C++14 Submitter: Pete Becker Opened: 2011-11-14 Last modified: 2016-01-28
Priority: Not Prioritized
View other active issues in [futures.promise].
View all other issues in [futures.promise].
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Discussion:
32.10.6 [futures.promise]/16 says that promise::set_value(const R&)
throws any exceptions
thrown by R
's copy constructor, and that promise_set_value(R&&)
throws any exceptions
thrown by R
's move constructor.
promise::set_value_at_thread_exit
. It
has no corresponding requirements, only that these functions throw "future_error
if an error condition
occurs."
Daniel suggests wording to fix this: The approach is a bit more ambitious and also attempts to fix wording glitches
of 32.10.6 [futures.promise]/16, because it would be beyond acceptable efforts of implementations to
determine whether a constructor call of a user-defined type will indeed call a copy constructor or move constructor
(in the first case it might be a template constructor, in the second case it might also be a copy-constructor,
if the type has no move constructor).
[2012, Portland: move to Review]
Moved to Review by the concurrency working group, with no further comments.
[2013-04-20, Bristol]
Accepted for the working paper
Proposed resolution:
This wording is relative to the FDIS.
Change 32.10.6 [futures.promise]/16 as indicated:
void promise::set_value(const R& r); void promise::set_value(R&& r); void promise<R&>::set_value(R& r); void promise<void>::set_value();[…]
-16- Throws:
future_error
if its shared state already has a stored value or exception, or- for the first version, any exception thrown by the
copy constructor ofconstructor selected to copy an object ofR
, or- for the second version, any exception thrown by the
move constructor ofconstructor selected to move an object ofR
.
Change 32.10.6 [futures.promise]/22 as indicated:
void promise::set_value_at_thread_exit(const R& r); void promise::set_value_at_thread_exit(R&& r); void promise<R&>::set_value_at_thread_exit(R& r); void promise<void>::set_value_at_thread_exit();[…]
-16- Throws:future_error
if an error condition occurs.
future_error
if its shared state already has a stored value or exception, or- for the first version, any exception thrown by the constructor selected to copy an object of
R
, or- for the second version, any exception thrown by the constructor selected to move an object of
R
.
va_start()
usageSection: 17.14 [support.runtime] Status: C++14 Submitter: Daniel Krügler Opened: 2011-11-12 Last modified: 2016-01-28
Priority: Not Prioritized
View all other issues in [support.runtime].
View all issues with C++14 status.
Discussion:
In 17.14 [support.runtime] p3 we find (emphasis mine):
The restrictions that ISO C places on the second parameter to the
va_start()
macro in header<stdarg.h>
are different in this International Standard. The parameterparmN
is the identifier of the rightmost parameter in the variable parameter list of the function definition (the one just before the ...).227 If the parameterparmN
is declared with a function, array, or reference type, or with a type that is not compatible with the type that results when passing an argument for which there is no parameter, the behavior is undefined.
It seems astonishing that the constraints on function types and array types imposes these
on the declared parameter parmN
, not to the adjusted one (which would
not require this extra wording, because that is implicit). This seems to say that a function
definition of the form (Thanks to Johannes Schaub for this example)
#include <stdarg.h> void f(char const paramN[], ...) { va_list ap; va_start(ap, paramN); va_end(ap); }
would produce undefined behaviour when used.
Similar wording exists in C99 and in the most recent C11 draft in 7.16.1.4 p4 In my opinion the constraints in regard to array types and function types are unnecessary and should be relaxed. Are there really implementations out in the wild that would (according to my understanding incorrectly) provide the declared and not the adjusted type ofparamN
as deduced type to va_start()
?
[2012, Kona]
Move to Ready.
[2012, Portland: applied to WP]
Proposed resolution:
This wording is relative to the FDIS.
Change 17.14 [support.runtime] p3 as indicated:
The restrictions that ISO C places on the second parameter to the
va_start()
macro in header<stdarg.h>
are different in this International Standard. The parameterparmN
is the identifier of the rightmost parameter in the variable parameter list of the function definition (the one just before the ...).227 If the parameterparmN
isdeclared withof afunction, array, orreference type, orwithof a type that is not compatible with the type that results when passing an argument for which there is no parameter, the behavior is undefined.
launch::async
policy usedSection: 32.10.9 [futures.async] Status: C++14 Submitter: Jonathan Wakely Opened: 2011-11-14 Last modified: 2016-01-28
Priority: Not Prioritized
View other active issues in [futures.async].
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Discussion:
32.10.9 [futures.async] p5 says
If the implementation chooses the
launch::async
policy,
- a call to a waiting function on an asynchronous return object that shares the shared state created by this
async
call shall block until the associated thread has completed, as if joined (32.4.3.6 [thread.thread.member]);
That should say a non-timed waiting function, otherwise, calling a timed waiting function can block indefinitely waiting for the associated thread to complete, rather than timing out after the specified time.
Sincestd::thread
does not provide a timed_join()
function (nor does
Pthreads, making it impossible on many platforms) there is no way for a timed waiting
function to try to join but return early due to timeout, therefore timed waiting
functions either cannot guarantee to timeout or cannot be used to meet the requirement
to block until the thread is joined. In order to allow timed waiting functions to
timeout the requirement should only apply to non-timed waiting functions.
[2012, Portland: move to Review]
Detlef: Do we actually need this fix — is it detectable?
Yes — you will never get a timeout. Should we strike the whole paragraph?
Hans: issue with thread local destruction.
Niklas: I have a strong expectation that a timed wait will respect the timeout
agreed
Detlef: we want a timed wait that does not time out to return like a non-timed wait; but is this implementable?
Pablo: Could we simply append ", or else time out"
Detlef: the time out on the shared state needs implementing anyway, even if the underlying O/S does not support a timed join.
Hans: the net effect is the timeout does not cover the thread local destruction... ah, I see what you're doing
Detlef: happy with Pablo's proposal
Wording proposed is to append after the word "joined" add ", or else time out"
Moved to review with this wording.
[2013, Bristol]
"Non-timed" made the new wording redundant and the result overly weak. Remove it.
Attempted to move to add this to the working paper (Concurrency motion 2) without the addition of "non-timed". Motion was withdrawn after Jonathan Wakely expressed implementability concerns.[2013-09, Chicago]
Discussion of interaction with the absence of a Posix timed join.
Jonathan Wakely withdrew his objection, so moved to Immediate. Accept for Working PaperProposed resolution:
[This wording is relative to the FDIS.]
Change 32.10.9 [futures.async] p5 as indicated:
If the implementation chooses the
launch::async
policy,
- a call to a waiting function on an asynchronous return object that shares the shared state created by this
async
call shall block until the associated thread has completed, as if joined, or else time out (32.4.3.6 [thread.thread.member]);
Section: 21.3.8 [meta.trans] Status: C++17 Submitter: Daniel Krügler Opened: 2011-11-18 Last modified: 2017-07-30
Priority: 3
View all issues with C++17 status.
Discussion:
Table 53 — "Reference modifications" says in regard to the type trait
add_lvalue_reference
(emphasize mine)
If
T
names an object or function type then the member typedef type shall nameT&
;
The problem with this specification is that function types with cv-qualifier or ref-qualifier, like
void() const void() &
are also affected by the first part of the rule, but this would essentially mean, that
instantiating add_lvalue_reference
with such a type would attempt to form
a type that is not defined in the C++ type system, namely
void(&)() const void(&)() &
The general policy for TransformationTraits is to define always some meaningful
mapping type, but this does not hold for add_lvalue_reference
, add_rvalue_reference
,
and in addition to these two for add_pointer
as well. The latter one would
attempt to form the invalid types
void(*)() const void(*)() &
A possible reason why those traits were specified in this way is that in C++03 (and that means for TR1), cv-qualifier were underspecified in the core language and several compilers just ignored them during template instantiations. This situation became fixed by adopting CWG issues 295 and 547.
While there is possibly some core language clarification needed (see reflector messages starting from c++std-core-20740), it seems also clear that the library should fix the specification. The suggested resolution follows the style of the specification of the support conceptsPointeeType
and ReferentType
defined in
N2914.
[2012-02-10, Kona]
Move to NAD.
These cv- and ref-qualified function types are aberrations in the type system, and do
not represent any actual entity defined by the language. The notion of cv- and ref-
qualification applies only to the implicit *this
reference in a member function.
However, these types can be produced by quirks of template metaprogramming, the question
remains what the library should do about it. For example, add_reference
returns
the original type if passed a reference type, or a void
type. Conversely,
add_pointer
will return a pointer to the referenced type when passed a reference.
It is most likely that the 'right' answer in any case will depend on the context that the question is being asked, in terms of forming these obscure types. The best the LWG can do is allow an error to propagate back to the user, so they can provide their own meaningful answer in their context - with additional metaprogramming on their part. The consensus is that if anyone is dangerous enough with templates to get themselves into this problem, they will also have the skills to resolve the problem themselves. This is not going to trip up the non-expert developer.
Lastly, it was noted that this problem arises only because the language is inconsistent in providing us these nonsense types that do no really represent anything in the language. There may be some way Core or Evolution could give us a more consistent type system so that the LWG does not need to invent an answer at all, should this question need resolving. This is another reason to not specify anything at the LWG trait level at this time, leaving the other working groups free to produce the 'right' answer that we can then follow without changing the meaning of existing, well-defined programs.
[2012-02-10, post-Kona]
Move back to Open. Daniel is concerned that this is not an issue we can simply ignore, further details to follow.
[2012-10-06, Daniel comments]
This issue really should be resolved as a defect: First, the argument that "forming these obscure types"
should "allow an error to propagate" is inconsistent with the exact same "obscure type" that would be formed
when std::add_lvalue_reference<void>
wouldn't have an extra rules for void
types, which
also cannot form references. The originally proposed resolution attempts to apply the same solution for the same
common property of void
types and function types with cv-qualifiers or ref-qualifier.
These functions had the property of ReferentType
during concept time (see
CWG 749 bullet three for the final
wording).
void
types shows, that this extra rule is not so unexpected. Further, any usage
of the result types of these traits as argument types or return types of functions would make these ill-formed
(and in a template context would be sfinaed away), so the expected effects are rarely unnoticed. Checking
all existing explicit usages of the traits add_rvalue_reference
, add_lvalue_reference
, and
add_pointer
didn't show any example where the error would be silent: add_rvalue_reference
is used to specify the return value of declval()
and the instantiation of declval<void() const>()
would be invalid, because of the attempt to return a function type. Similarly, add_lvalue_reference
is used to specify the return type of unique_ptr<T>::operator*()
. Again, any instantiation with
void() const
wouldn't remain unnoticed. The trait add_pointer
is used to specify the trait
std::decay
and this is an interesting example, because it is well-formed when instantiated with void
types, too, and is heavily used throughout the library specification. All use-cases would not be negatively affected
by the suggested acceptance of function types with cv-qualifiers or ref-qualifier, because they involve
types that are either function arguments, function parameters or types were references are formed from.
The alternative would be to add an additional extra rule that doesn't define a type member 'type' when
we have a function type with cv-qualifiers or ref-qualifier. This is better than the
current state but it is not superior than the proposal to specify the result as the original type, because
both variants are sfinae-friendly. A further disadvantage of the "non-type" approach here would be that any
usage of std::decay
would require special protection against these function types, because
instantiating std::decay<void() const>
again would lead to a deep, sfinae-unfriendly error.
The following example demonstrates the problem: Even though the second f
template is the best final
match here, the first one will be instantiated. During that process std::decay<T>::type
becomes instantiated as well and will raise a deep error, because as part of the implementation the trait
std::add_pointer<void() const>
becomes instantiated:
#include <type_traits> template<class T> typename std::decay<T>::type f(T&& t); template<class T, class U> U f(U u); int main() { f<void() const>(0); }
When the here proposed resolution would be applied this program would be well-formed and selects the expected function.
Previous resolution from Daniel [SUPERSEDED]:
Change Table 53 — "Reference modifications" in 21.3.8.3 [meta.trans.ref] as indicated:
Table 53 — Reference modifications Template Comments …
template <class T>
struct
add_lvalue_reference;If T
names an objecttype
or ifT
names a function type that does not have
cv-qualifiers or a ref-qualifier then the member typedeftype
shall nameT&
; otherwise, ifT
names a type "rvalue reference toT1
" then
the member typedeftype
shall nameT1&
; otherwise,type
shall nameT
.template <class T>
struct
add_rvalue_reference;If T
names an objecttype
or ifT
names a function type that does not have
cv-qualifiers or a ref-qualifier then the member typedeftype
shall nameT&&
; otherwise,type
shall nameT
. [Note: This rule reflects
the semantics of reference collapsing (9.3.4.3 [dcl.ref]). For example, when a typeT
names a typeT1&
, the typeadd_rvalue_reference<T>::type
is not an
rvalue reference. — end note]Change Table 56 — "Pointer modifications" in 21.3.8.6 [meta.trans.ptr] as indicated:
Table 56 — Pointer modifications Template Comments …
template <class T>
struct add_pointer;The member typedeftype
shall name the same type as
IfT
names a function type that has cv-qualifiers or a ref-qualifier
then the member typedeftype
shall nameT
; otherwise, it
shall name the same type asremove_reference<T>::type*
.
The following revised proposed resolution defines - in the absence of a proper core language definition - a new
term referenceable type as also suggested by the resolution for LWG 2196(i) as an
umbrella of the negation of void
types and function types with cv-qualifiers or ref-qualifier.
This simplifies and minimizes the requires wording changes.
[ 2013-09-26, Daniel synchronizes wording with recent draft ]
[ 2014-05-18, Daniel synchronizes wording with recent draft and comments ]
My impression is that this urgency of action this issue attempts to point out is partly caused by the fact that even for the most recent C++14 compilers the implementations have just recently changed to adopt the core wording. Examples for these are bug reports to gcc or clang.
Occasionally the argument has been presented to me that the suggested changes to the traits affected by this issue would lead to irregularities compared to other traits, especially the lack of guarantee thatadd_pointer
might not return
a pointer or that add_(l/r)value_reference
might not return a reference type. I would like to point out that this
kind of divergence is actually already present in most add/remove
traits: For example, we have no guarantee that
add_const
returns a const type (Reference types or function types get special treatments), or that add_rvalue_reference
returns an rvalue-reference (e.g. when applied to an lvalue-reference type).
Zhihao Yuan brought to my attention, that the originally proposing paper
N1345 carefully discussed these design choices.
[2015-05, Lenexa]
MC: move to Ready: in favor: 16, opposed: 0, abstain: 1
STL: have libstdc++, libc++ implemented this? we would need to change your implementation
Proposed resolution:
This wording is relative to N3936.
Change Table 53 — "Reference modifications" in 21.3.8.3 [meta.trans.ref] as indicated:
Template | Comments |
---|---|
…
|
|
template <class T>
|
If T names then the member typedef type shall name T& ; otherwise, T names a type "rvalue reference to T1 " thenthe member typedef type shall name T1& ; otherwise,type shall name T .[Note: This rule reflects the semantics of reference collapsing (9.3.4.3 [dcl.ref]). — end note] |
template <class T>
|
If T names then the member typedef type shall name T&& ; otherwise, type shall name T . [Note: This rule reflectsthe semantics of reference collapsing (9.3.4.3 [dcl.ref]). For example, when a type T names a type T1& , the type add_rvalue_reference_t<T> is not anrvalue reference. — end note] |
Change Table 56 — "Pointer modifications" in 21.3.8.6 [meta.trans.ptr] as indicated:
Template | Comments |
---|---|
…
|
|
template <class T>
|
If T names a referenceable type or a (possibly cv-qualified) void type thentype shall name the same type asremove_reference_t<T>* ; otherwise, type shall name T .
|
std::launch
an implementation-defined type?Section: 32.10.1 [futures.overview] Status: C++14 Submitter: Jonathan Wakely Opened: 2011-11-20 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
32.10.1 [futures.overview] says std::launch
is an
implementation-defined bitmask type, which would usually mean the
implementation can choose whether to define an enumeration type, or a
bitset
, or an integer type. But in the case of std::launch
it's
required to be a scoped enumeration type,
enum class launch : unspecified { async = unspecified, deferred = unspecified, implementation-defined };
so what is implementation-defined about it, and what is an implementation supposed to document about its choice?
[2011-12-02 Moved to Tentatively Ready after 6 positive votes on c++std-lib.]
Proposed resolution:
This wording is relative to the FDIS.
Change 32.10.1 [futures.overview] paragraph 2 as indicated:
The enum type
launch
isan implementation-defineda bitmask type (16.3.3.3.3 [bitmask.types]) withlaunch::async
andlaunch::deferred
denoting individual bits. [ Note: Implementations can provide bitmasks to specify restrictions on task interaction by functions launched byasync()
applicable to a corresponding subset of available launch policies. Implementations can extend the behavior of the first overload ofasync()
by adding their extensions to the launch policy under the “as if” rule. — end note ]
std::allocator_traits<std::allocator<T>>::propagate_on_container_move_assignment
Section: 20.2.10 [default.allocator] Status: C++14 Submitter: Ai Azuma Opened: 2011-11-08 Last modified: 2016-01-28
Priority: Not Prioritized
View other active issues in [default.allocator].
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Discussion:
"std::allocator_traits<std::allocator<T>>::propagate_on_container_move_assignment::value
"
is specified as "false", according to (20.2.10 [default.allocator]) and (20.2.9.2 [allocator.traits.types]).
However, according to (23.2.2 [container.requirements.general]), this specification leads to the unneeded requirements
(MoveInsertable
and MoveAssignable
of the value type) on the move assignment operator of containers
with the default allocator.
typedef std::true_type propagate_on_container_move_assignment;
"
in the definition of std::allocator
class template,
std::allocator_traits
" class template for "std::allocator
"
class template, in which "propagate_on_container_move_assignment
"
nested typedef is specified as "std::true_type
".
Pablo prefers the first resolution.
[2011-12-02: Pablo comments]
This issue has potentially some overlap with 2108(i). Should the trait always_compare_equal
been added, this issue's resolution should be improved based on that.
[2012, Kona]
Move to Ready.
[2012, Portland: applied to WP]
Proposed resolution:
This wording is relative to the FDIS.
Change 20.2.10 [default.allocator], the class template allocator
synopsis as indicated:
namespace std { template <class T> class allocator; // specialize forvoid
: template <> class allocator<void> { public: typedef void* pointer; typedef const void* const_pointer; // reference-to-void
members are impossible. typedef void value_type; template <class U> struct rebind { typedef allocator<U> other; }; }; template <class T> class allocator { public: typedef size_t size_type; typedef ptrdiff_t difference_type; typedef T* pointer; typedef const T* const_pointer; typedef T& reference; typedef const T& const_reference; typedef T value_type; template <class U> struct rebind { typedef allocator<U> other; }; typedef true_type propagate_on_container_move_assignment; […] }; }
unique_lock
move-assignment should not be noexcept
Section: 32.6.5.4 [thread.lock.unique] Status: C++14 Submitter: Anthony Williams Opened: 2011-11-27 Last modified: 2017-07-05
Priority: Not Prioritized
View all issues with C++14 status.
Discussion:
I just noticed that the unique_lock
move-assignment operator is declared noexcept
. This
function may call unlock()
on the wrapped mutex, which may throw.
noexcept
specification from unique_lock::operator=(unique_lock&&)
in 32.6.5.4 [thread.lock.unique] and 32.6.5.4.2 [thread.lock.unique.cons].
Daniel:
I think the situation is actually a bit more complex as it initially looks.
First, the effects of the move-assignment operator are (emphasize mine):
Effects: If
owns
callspm->unlock()
.
Now according to the BasicLockable
requirements:
3 Requires: The current execution agent shall hold a lock on
m.unlock()
m
. 4 Effects: Releases a lock onm
held by the current execution agent. Throws: Nothing.
This shows that unlock itself is a function with narrow contract and for this reasons no unlock function of a mutex or lock itself does have a noexcept specifier according to our mental model.
Now the move-assignment operator attempts to satisfy these requirement of the function and calls it only when it assumes that the conditions are ok, so from the view-point of the caller of the move-assignment operator it looks as if the move-assignment operator would in total a function with a wide contract. The problem with this analysis so far is, that it depends on the assumed correctness of the state "owns". Looking at the construction or state-changing functions, there do exist several ones that depend on caller-code satisfying the requirements and there is one guy, who looks most suspicious:11 Requires: The calling thread own the mutex.
unique_lock(mutex_type& m, adopt_lock_t);
[…]
13 Postconditions:pm == &m
andowns == true
.
because this function does not even call lock()
(which may, but is not
required to throw an exception if the calling thread does already own the mutex).
So we have in fact still a move-assignment operator that might throw an exception,
if the mutex was either constructed or used (call of lock) incorrectly.
[Issaquah 2014-02-11: Move to Immediate after SG1 review]
Proposed resolution:
This wording is relative to the FDIS.
Change 32.6.5.4 [thread.lock.unique], class template unique_lock
synopsis as indicated:
namespace std { template <class Mutex> class unique_lock { public: typedef Mutex mutex_type; […] unique_lock(unique_lock&& u) noexcept; unique_lock& operator=(unique_lock&& u)noexcept; […] }; }
Change 32.6.5.4.2 [thread.lock.unique.cons] around p22 as indicated:
unique_lock& operator=(unique_lock&& u)noexcept;-22- Effects: If
-23- Postconditions:owns
callspm->unlock()
.pm == u_p.pm
andowns == u_p.owns
(whereu_p
is the state ofu
just prior to this construction),u.pm == 0
andu.owns == false
. -24- [Note: With a recursive mutex it is possible for both*this
andu
to own the same mutex before the assignment. In this case,*this
will own the mutex after the assignment andu
will not. — end note] -??- Throws: Nothing.
const_iterator
's value_type
Section: 23.2.2 [container.requirements.general] Status: C++14 Submitter: Jeffrey Yasskin Opened: 2011-11-28 Last modified: 2016-01-28
Priority: Not Prioritized
View other active issues in [container.requirements.general].
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Discussion:
In the FDIS, Table 96 specifies X::const_iterator
as a "constant iterator type
whose value type is T
". However, Table 97 specifies X::const_reverse_iterator
as an "iterator type whose value type is const T
" and which is defined as
reverse_iterator<const_iterator>
. But reverse_iterator::value_type
is
just "typename iterator_traits<Iterator>::value_type
" 24.5.1.2 [reverse.iterator],
so const_iterator
and const_reverse_iterator
must have the same value_type
.
The resolution to issue 322(i) implies that
const_reverse_iterator
should change.
[2012, Kona]
Move to Ready.
[2012, Portland: applied to WP]
Proposed resolution:
This wording is relative to the FDIS.
Change Table 97 — "Reversible container requirements" as indicated
Expression | Return type | Assertion/note pre-/post-condition | Complexity |
---|---|---|---|
X::reverse_-
|
iterator type whose value type is T
|
reverse_iterator<iterator>
|
compile time |
X::const_-
|
constant iterator type whose value type is
|
reverse_iterator<const_iterator>
|
compile time |
move_iterator
wrapping iterators returning prvaluesSection: 24.5.4 [move.iterators] Status: C++17 Submitter: Dave Abrahams Opened: 2011-11-30 Last modified: 2017-07-30
Priority: 3
View all other issues in [move.iterators].
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Discussion:
Shouldn't move_iterator
be specialized so that if the iterator it wraps
returns a prvalue when dereferenced, the move_iterator
also returns by
value? Otherwise, it creates a dangling reference.
move_iterator<I>::reference
would do.
A direction might be testing on is_reference<iterator_traits<I>::reference>
,
or is_reference<decltype(*declval<I>())>
.
Daniel: I would prefer to use a consistent style among the iterator adaptors, so I
suggest to keep with the iterator_traits
typedefs if possible.
using reference = typename conditional< is_reference<typename iterator_traits<Iterator>::reference>::value, value_type&&, value_type >::type;
We might also want to ensure that if Iterator
's reference
type is
a reference, the referent is equal to value_type
(after removal of cv-qualifiers).
In general we have no such guarantee.
value_type&&
, should we use
value_type
or should we keep the reference
type of the wrapped iterator?
Daniel: This suggestion looks appealing at first, but the problem here is that using this typedef
can make it impossible for move_iterator
to satisfy its contract, which means returning
an rvalue of the value type (Currently it says rvalue-reference, but this must be fixed as of
this issue anyway). I think that user-code can reasonably expect that when it has constructed
an object m
of move_iterator<It>
, where It
is a valid
mutable iterator type, the expression
It::value_type&& rv = *m;
is well-formed.
Let's setR
equal to iterator_traits<Iterator>::reference
in the following. We can discuss the following situations:
R
is a reference type: We can only return the corresponding xvalue of R
,
if value_type
is reference-related to the referent type, else this is presumably no
forward iterator and we cannot say much about it, except that it must be convertible to
value_type
, so it better should return a prvalue.R
is not a reference type: In this case we can rely on a conversion to
value_type
again, but not much more. Assume we would return R
directly,
this might turn out to have a conversion to an lvalue-reference type of the value type (for
example). If that is the case, this would indirectly violate the contract of
move_iterator
.
In regard to the first scenario I suggest that implementations are simply required to
check that V2 = remove_cv<remove_reference<R>::type>::type
is equal
to the value type V1
as a criterion to return this reference as an xvalue, in all other
cases it should return the value type directly as prvalue.
reference
has
the correct cv-qualification, if R
is a real reference.
It is possible to improve this a bit by indeed supporting reference-related types,
this would require to test is_same<V1, V2>::value || is_base_of<V1, V2>::value
instead. I'm unsure whether (a) this additional effort is worth it and (b) a strict reading of
the forward iterator requirements seems not to allow to return a reference-related type (Whether
this is a defect or not is another question).
[2011-12-05: Marc Glisse comments and splits into two resolution alternatives]
I guess I am looking at the speed of:
value_type x; x = *m;
(copy construction would likely trigger copy elision and thus be neutral) instead of the validity of:
value_type&& x = *m;
In this sense, Daniels earlier proposition that ignored value_type
and just did
switch_lvalue_ref_to_rvalue_ref<reference>
was easier to understand (and it didn't
require thinking about reference related types).
[2012, Kona]
Move to Review.
Alisdair: This only applies to input iterators, so keep that in mind when thinking about this.
STL: I see what B is doing, but not A.
Howard: I agree.
Alisdair: Should we use add_rvalue_reference
?
STL: No, we do not want reference collapsing.
STL: Re A, messing with the CV qualification scares me.
Alisdair: Agree. That would break my intent.
STL: Actually I don't think it's actually wrong, but I still don't see what it's doing.
Alisdair: A is picking the value type, B is picking the proxy type.
Howard: I like returning the proxy type.
STL: Returning a reference (B) seems right, because the requirements say "reference". I suspect that B works correctly if you have a move iterator wrapping a move iterator wrapping a thing. I think that A would mess up the type in the middle.
Considerable discussion about which version is correct, checking various examples.
STL: Still think B is right. Still don't understand A. In A we are losing the proxyness.
Howard: Agree 100%. We don't want to lose the proxy. If it's const, so be it.
STL: B is also understandable by mortals.
Howard: Remove to review, keep A but move it out of the proposed resolution area (but keep it for rational).
Alisdair: Adding an explanatory note might be a good idea, if someone wants to write one.
Walter: Concerned about losing the word "reference" in p.1.
Howard: move_iterator
will return an xvalue or a prvalue, both of which are rvalues.
[Proposed resolution A, rejected in preference to the currently proposed resolution (B)
Change 24.5.4 [move.iterators] p1 as indicated:
Class template Change 24.5.4.2 [move.iterator], class template Immediately following the class template
-?- Let
]move_iterator
is an iterator adaptor with the same behavior as the underlying iterator
except that its dereference operator implicitly converts the value returned by the underlying iterator's
dereference operator to an rvalue referenceof the value type. Some generic algorithms
can be called with move iterators to replace copying with moving.
move_iterator
synopsis, as indicated:
namespace std {
template <class Iterator>
class move_iterator {
public:
typedef Iterator iterator_type;
typedef typename iterator_traits<Iterator>::difference_type difference_type;
typedef Iterator pointer;
typedef typename iterator_traits<Iterator>::value_type value_type;
typedef typename iterator_traits<Iterator>::iterator_category iterator_category;
typedef
value_type&&see below reference;
[…]
};
}
move_iterator
synopsis in
24.5.4.2 [move.iterator] insert a new paragraph as indicated:R
be iterator_traits<Iterator>::reference
and
let V
be iterator_traits<Iterator>::value_type
. If
is_reference<R>::value
is true
and if
remove_cv<remove_reference<R>::type>::type
is the same type as V
,
the template instantiation move_iterator<Iterator>
shall define the nested type
named reference
as a synonym for remove_reference<R>::type&&
,
otherwise as a synonym for V
.
[2012, Portland: Move to Tentatively Ready]
AJM wonders if the implied trait might be useful elsewhere, and worth adding to type traits as a transformation type trait.
Suspicion that the Range SG might find such a trait useful, but wait until there is clear additional use of such a trait before standardizing.
Minor wording tweak to use add_rvalue_reference
rather than manually adding the &&
,
then move to Tentatively Ready.
[2013-01-09 Howard Hinnant comments]
I believe the P/R for LWG 2106 is incorrect (item 3). The way it currently reads, move_iterator<I>::reference
is always an lvalue reference. I.e. if R
is an lvalue reference type, then reference becomes
add_rvalue_reference<R>::type
which is just R
. And if R
is not a reference type,
then reference becomes R
(which is also just R
;-)).
I believe the correct wording is what was there previously:
-?- Let R
be iterator_traits<Iterator>::reference
. If is_reference<R>::value
is true, the template instantiation move_iterator<Iterator>
shall define the nested type named
reference
as a synonym for remove_reference<R>::type&&
, otherwise as a synonym for
R
.
Additionally Marc Glisse points out that move_iterator<I>::operator*()
should return
static_cast<reference>(*current)
, not std::move(*current)
.
Previous resolution:
This wording is relative to the FDIS.
Change 24.5.4 [move.iterators] p1 as indicated:
Class template
move_iterator
is an iterator adaptor with the same behavior as the underlying iterator except that its dereference operator implicitly converts the value returned by the underlying iterator's dereference operator to an rvaluereference. Some generic algorithms can be called with move iterators to replace copying with moving.Change 24.5.4.2 [move.iterator], class template
move_iterator
synopsis, as indicated:namespace std { template <class Iterator> class move_iterator { public: typedef Iterator iterator_type; typedef typename iterator_traits<Iterator>::difference_type difference_type; typedef Iterator pointer; typedef typename iterator_traits<Iterator>::value_type value_type; typedef typename iterator_traits<Iterator>::iterator_category iterator_category; typedefvalue_type&&see below reference; […] }; }Immediately following the class template
move_iterator
synopsis in 24.5.4.2 [move.iterator] insert a new paragraph as indicated:-?- Let
R
beiterator_traits<Iterator>::reference
. Ifis_reference<R>::value
istrue
, the template instantiationmove_iterator<Iterator>
shall define the nested type namedreference
as a synonym foradd_rvalue_reference<R>::type
, otherwise as a synonym forR
.
[2014-05-19, Daniel comments]
The term instantiation has been changed to specialization in the newly added paragraph as suggested by STL and much preferred by myself.
[2014-05-19 Library reflector vote]
The issue has been identified as Tentatively Ready based on five votes in favour.
Proposed resolution:
This wording is relative to N3936.
Change 24.5.4 [move.iterators] p1 as indicated:
Class template
move_iterator
is an iterator adaptor with the same behavior as the underlying iterator except that its indirection operator implicitly converts the value returned by the underlying iterator's indirection operator to an rvaluereference. Some generic algorithms can be called with move iterators to replace copying with moving.
Change 24.5.4.2 [move.iterator], class template move_iterator
synopsis, as indicated:
namespace std { template <class Iterator> class move_iterator { public: typedef Iterator iterator_type; typedef typename iterator_traits<Iterator>::difference_type difference_type; typedef Iterator pointer; typedef typename iterator_traits<Iterator>::value_type value_type; typedef typename iterator_traits<Iterator>::iterator_category iterator_category; typedefvalue_type&&see below reference; […] }; }
Immediately following the class template move_iterator
synopsis in
24.5.4.2 [move.iterator] insert a new paragraph as indicated:
-?- Let
R
beiterator_traits<Iterator>::reference
. Ifis_reference<R>::value
istrue
, the template specializationmove_iterator<Iterator>
shall define the nested type namedreference
as a synonym forremove_reference<R>::type&&
, otherwise as a synonym forR
.
Edit [move.iter.op.star] p1 as indicated:
reference operator*() const;-1- Returns:
.
std::movestatic_cast<reference>(*current)
Section: 16.4.4.6 [allocator.requirements] Status: Resolved Submitter: Jonathan Wakely Opened: 2011-12-01 Last modified: 2018-12-03
Priority: 3
View other active issues in [allocator.requirements].
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Discussion:
Whether two allocator objects compare equal affects the complexity of
container copy and move assignments and also the possibility of an
exception being thrown by container move assignments. The latter point
means container move assignment cannot be noexcept
when
propagate_on_container_move_assignment
(POCMA) is false for the
allocator because there is no way to detect at compile-time if two
allocators will compare equal. LWG 2013(i) means this affects all
containers using std::allocator
, but even if that is resolved, this
affects all stateless allocators which do not explicitly define POCMA
to true_type
.
allocator_traits
, but that would be duplicating information that is
already defined by the type's equality operator if that operator
always returns true. Requiring users to write operator==
that simply
returns true and also explicitly override a trait to repeat the same
information would be unfortunate and risk user errors that allow the
trait and actual operator==
to disagree.
Dave Abrahams suggested a better solution in message c++std-lib-31532,
namely to allow operator==
to return true_type
, which is convertible
to bool
but also detectable at compile-time. Adopting this as the
recommended way to identify allocator types that always compare equal
only requires a slight relaxation of the allocator requirements so
that operator==
is not required to return bool
exactly.
The allocator requirements do not make it clear that it is well-defined
to compare non-const values, that should be corrected too.
In message c++std-lib-31615 Pablo Halpern suggested an always_compare_equal
trait that could still be defined, but with a sensible default value rather
than requiring users to override it, and using that to set sensible values for
other allocator traits:
Do we still need
[…] One point that I want to ensure doesn't get lost is that if we adopt some sort ofalways_compare_equal
if we can have anoperator==
that returnstrue_type
? What would its default value be?is_empty<A> || is_convertible<decltype(a == a), true_type>::value
, perhaps? One benefit I see to such a definition is that stateless C++03 allocators that don't use thetrue_type
idiom will still benefit from the new trait.always_compare_equal
-like trait, thenpropagate_on_container_swap
andpropagate_on_container_move_assignment
should default toalways_compare_equal
. Doing this will eliminate unnecessary requirements on the container element type, as per [LWG 2103(i)].
Optionally, operator==
for std::allocator
could be made to return
true_type
, however if LWG 2103(i) is adopted that is less important.
always_compare_equal
,
all_objects_(are_)equivalent
, or all_objects_compare_equal
.
[2014-11-07 Urbana]
Resolved by N4258
Proposed resolution:
This wording is relative to the FDIS.
Change Table 27 — "Descriptive variable definitions" in 16.4.4.6 [allocator.requirements]:
Variable | Definition |
---|---|
a3, a4
|
const ) type X
|
b
|
a value of (possibly const ) type Y
|
Change Table 28 — "Allocator requirements" in 16.4.4.6 [allocator.requirements]:
Expression | Return type | Assertion/note pre-/post-condition | Default |
---|---|---|---|
|
convertible to bool
|
returns true only if storage allocated from each can be deallocated via the other. operator== shall be reflexive,symmetric, and transitive, and shall not exit via an exception. |
|
|
convertible to bool
|
same as
|
|
a3 == b
|
convertible to bool
|
same as a3 ==
|
|
a3 != b
|
convertible to bool
|
same as !(a3 == b)
|
|
[…]
|
|||
a.select_on_-
|
X
|
Typically returns either a orX()
|
return a;
|
X::always_compares_equal
|
Identical to or derived from true_type orfalse_type
|
true_type if the expression x1 == x2 isguaranteed to be true for any two (possiblyconst ) values x1, x2 of type X , whenimplicitly converted to bool . See Note B, below.
|
true_type , if is_empty<X>::value is true or ifdecltype(declval<const X&>() == declval<const X&>()) is convertible to true_type , otherwise false_type .
|
[…]
|
Note A: […]
Note B: IfX::always_compares_equal::value
or XX::always_compares_equal::value
evaluate
to true
and an expression equivalent to x1 == x2
or x1 != x2
for any two values
x1, x2
of type X
evaluates to false
or true
, respectively, the behaviour
is undefined.
Change class template allocator_traits
synopsis, 20.2.9 [allocator.traits] as indicated:
namespace std { template <class Alloc> struct allocator_traits { typedef Alloc allocator_type; […] typedef see below always_compares_equal; typedef see below propagate_on_container_copy_assignment; […] }; }
Insert the following between 20.2.9.2 [allocator.traits.types] p6 and p7 as indicated:
typedef see below always_compares_equal;-?- Type:
Alloc::always_compares_equal
if such a type exists; otherwise,true_type
ifis_empty<Alloc>::value
istrue
or ifdecltype(declval<const Alloc&>() == declval<const Alloc&>())
is convertible totrue_type
; otherwise,false_type
.
typedef see below propagate_on_container_copy_assignment;-7- Type:
Alloc::propagate_on_container_copy_assignment
if such a type exits, otherwisefalse_type
.
Change class template allocator
synopsis, 20.2.10 [default.allocator] as indicated:
namespace std { template <class T> class allocator; // specialize forvoid
: template <> class allocator<void> { public: typedef void* pointer; typedef const void* const_pointer; // reference-to-void
members are impossible. typedef void value_type; template <class U> struct rebind { typedef allocator<U> other; }; }; template <class T> class allocator { public: typedef size_t size_type; typedef ptrdiff_t difference_type; typedef T* pointer; typedef const T* const_pointer; typedef T& reference; typedef const T& const_reference; typedef T value_type; template <class U> struct rebind { typedef allocator<U> other; }; typedef true_type always_compares_equal; […] }; }
hash
specializationsSection: 19.5.7 [syserr.hash], 20.3.3 [util.smartptr.hash], 22.10.19 [unord.hash], 17.7.6 [type.index.synopsis], 27.4.6 [basic.string.hash], 23.3.14 [vector.bool], 32.4.3.2 [thread.thread.id] Status: C++14 Submitter: Daniel Krügler Opened: 2011-12-04 Last modified: 2016-01-28
Priority: Not Prioritized
View all issues with C++14 status.
Discussion:
20.3.3 [util.smartptr.hash] p2 is specified as follows:
Requires: the template specializations shall meet the requirements of class template
hash
(20.8.12).
The problem here is the usage of a Requires element, which is actually a pre-condition that a user of a component has to satisfy. But the intent of this wording is actually to be a requirement on implementations. The Requires element should be removed here and the wording should be improved to say what it was intended for.
We have similar situations in basically all other places where the specification of library-providedhash
specializations is defined. Usually, the Requires element is incorrect. In the
special case of hash<unique_ptr<T, D>>
the implementation depends on
the behaviour of hash
specializations, that could be user-provided. In this case
the specification needs to separate the requirements on these specializations and those
that are imposed on the implementation.
[2012, Kona]
Update wording and move to Review.
Believe a simpler formulation is to simply string the term Requires: and leave the current wording intact, rather than strike the whole clause and replace it.
[Originally proposed wording for reference
Change 19.5.7 [syserr.hash] as indicated:
-1- Change 22.9.3 [bitset.hash] as indicated:
-1- Change 20.3.3 [util.smartptr.hash] as indicated:
-1-
template <> struct hash<error_code>;
Requires: the template specialization shall meet the requirements
of class template The header
hash
(22.10.19 [unord.hash])<system_error>
provides a definition for a specialization of the
template hash<error_code>
. The requirements for the members of
this specialization are given in sub-clause 22.10.19 [unord.hash].
template <size_t N> struct hash<bitset<N> >;
Requires: the template specialization shall meet the requirements
of class template The header
hash
(22.10.19 [unord.hash])<bitset>
provides a definition for a partial specialization of the
hash
class template for specializations of class template bitset<N>
.
The requirements for the members of instantiations of this specialization are given
in sub-clause 22.10.19 [unord.hash].
template <class T, class D> struct hash<unique_ptr<T, D> >;
Requires: the template specialization shall meet the requirements
of class template The header
hash
(22.10.19 [unord.hash])<memory>
provides a definition for a partial specialization of the
hash
class template for specializations of class template unique_ptr<T, D>
.
The requirements for the members of instantiations of this specialization are given
in sub-clause 22.10.19 [unord.hash]. For an object p
of type
UP
, where UP
is unique_ptr<T, D>
,
hash<UP>()(p)
shall evaluate to the same value as
hash<typename UP::pointer>()(p.get())
. The specialization
hash<typename UP::pointer>
shall be well-formed.hash<typename UP::pointer>
shall be well-formed and well-defined [Note: the general requirements
of class template hash
(22.10.19 [unord.hash]) are implied —
end note].
template <class T> struct hash<shared_ptr<T> >;-2-
Requires: the template specialization shall meet the requirements of class templateThe headerhash
(22.10.19 [unord.hash])<memory>
provides a definition for a partial specialization of thehash
class template for specializations of class templateshared_ptr<T>
. The requirements for the members of instantiations of this specialization are given in sub-clause 22.10.19 [unord.hash]. For an objectp
of typeshared_ptr<T>
,hash<shared_ptr<T> >()(p)
shall evaluate to the same value ashash<T*>()(p.get())
.
Change 22.10.19 [unord.hash] p2 as indicated: [Comment: For unknown reasons the extended integer types are not mentioned here, which looks like an oversight to me and makes also the wording more complicated. See 2119(i) for this part of the problem. — end comment]
template <> struct hash<bool>; template <> struct hash<char>; […] template <> struct hash<long double>; template <class T> struct hash<T*>;-2-
Requires: the template specializations shall meet the requirements of class templateThe headerhash
(22.10.19 [unord.hash])<functional>
provides definitions for specializations of thehash
class template for each cv-unqualified arithmetic type except for the extended integer types. This header also provides a definition for a partial specialization of thehash
class template for any pointer type. The requirements for the members of these specializations are given in sub-clause 22.10.19 [unord.hash].
Change [type.index.hash] p1 as indicated:
template <> struct hash<type_index>;-1-
Requires: the template specialization shall meet the requirements of class templateThe headerhash
(22.10.19 [unord.hash])<typeindex>
provides a definition for a specialization of the templatehash<type_index>
. The requirements for the members of this specialization are given in sub-clause 22.10.19 [unord.hash]. For an objectindex
of typetype_index
,hash<type_index>()(index)
shall evaluate to the same result asindex.hash_code()
.
Change 27.4.6 [basic.string.hash] p1 as indicated:
template <> struct hash<string>; template <> struct hash<u16string>; template <> struct hash<u32string>; template <> struct hash<wstring>;-1-
Requires: the template specializations shall meet the requirements of class templateThe headerhash
(22.10.19 [unord.hash])<string>
provides definitions for specializations of thehash
class template for the typesstring
,u16string
,u32string
, andwstring
. The requirements for the members of these specializations are given in sub-clause 22.10.19 [unord.hash].
Change 23.3.14 [vector.bool] p7 as indicated:
template <class Allocator> struct hash<vector<bool, Allocator> >;-7-
Requires: the template specialization shall meet the requirements of class templateThe headerhash
(22.10.19 [unord.hash])<vector>
provides a definition for a partial specialization of thehash
class template for specializations of class templatevector<bool, Allocator>
. The requirements for the members of instantiations of this specialization are given in sub-clause 22.10.19 [unord.hash].
Change 32.4.3.2 [thread.thread.id] p14 as indicated:
template <> struct hash<thread::id>;-14-
Requires: the template specialization shall meet the requirements of class templateThe headerhash
(22.10.19 [unord.hash])<thread>
provides a definition for a specialization of the templatehash<thread::id>
. The requirements for the members of this specialization are given in sub-clause 22.10.19 [unord.hash].
[2012, Portland: Move to Tentatively Ready]
No further wording issues, so move to Tentatively Ready (post meeting issues processing).
[2013-04-20 Bristol]
Proposed resolution:
Change 19.5.7 [syserr.hash] as indicated:
template <> struct hash<error_code>;-1-
Requires: tThe template specialization shall meet the requirements of class templatehash
(22.10.19 [unord.hash].
Change 22.9.3 [bitset.hash] as indicated:
template <size_t N> struct hash<bitset<N> >;-1-
Requires: tThe template specialization shall meet the requirements of class templatehash
(22.10.19 [unord.hash]).
Change 20.3.3 [util.smartptr.hash] as indicated:
template <class T, class D> struct hash<unique_ptr<T, D> >;-1-
-?- Requires: The specializationRequires: tThe template specialization shall meet the requirements of class templatehash
(22.10.19 [unord.hash]). For an objectp
of typeUP
, whereUP
isunique_ptr<T, D>
,hash<UP>()(p)
shall evaluate to the same value ashash<typename UP::pointer>()(p.get())
.The specializationhash<typename UP::pointer>
shall be well-formed.hash<typename UP::pointer>
shall be well-formed and well-defined, and shall meet the requirements of class templatehash
(22.10.19 [unord.hash]).
template <class T> struct hash<shared_ptr<T> >;-2-
Requires: tThe template specialization shall meet the requirements of class templatehash
(22.10.19 [unord.hash]). For an objectp
of typeshared_ptr<T>
,hash<shared_ptr<T> >()(p)
shall evaluate to the same value ashash<T*>()(p.get())
.
Change 22.10.19 [unord.hash] p2 as indicated: [Comment: For unknown reasons the extended integer types are not mentioned here, which looks like an oversight to me and makes also the wording more complicated. See 2119(i) for this part of the problem. — end comment]
template <> struct hash<bool>; template <> struct hash<char>; […] template <> struct hash<long double>; template <class T> struct hash<T*>;-2-
Requires: tThe template specializations shall meet the requirements of class templatehash
(22.10.19 [unord.hash]).
Change [type.index.hash] p1 as indicated:
template <> struct hash<type_index>;-1-
Requires: tThe template specialization shall meet the requirements of class templatehash
(22.10.19 [unord.hash]). For an objectindex
of typetype_index
,hash<type_index>()(index)
shall evaluate to the same result asindex.hash_code()
.
Change 27.4.6 [basic.string.hash] p1 as indicated:
template <> struct hash<string>; template <> struct hash<u16string>; template <> struct hash<u32string>; template <> struct hash<wstring>;-1-
Requires: tThe template specializations shall meet the requirements of class templatehash
(22.10.19 [unord.hash]).
Change 23.3.14 [vector.bool] p7 as indicated:
template <class Allocator> struct hash<vector<bool, Allocator> >;-7-
Requires: tThe template specialization shall meet the requirements of class templatehash
(22.10.19 [unord.hash]).
Change 32.4.3.2 [thread.thread.id] p14 as indicated:
template <> struct hash<thread::id>;-14-
Requires: tThe template specialization shall meet the requirements of class templatehash
(22.10.19 [unord.hash]).
remove
can't swap but note says it mightSection: 26.7.8 [alg.remove] Status: C++14 Submitter: Howard Hinnant Opened: 2011-12-07 Last modified: 2016-01-28
Priority: Not Prioritized
View all other issues in [alg.remove].
View all issues with C++14 status.
Discussion:
26.7.8 [alg.remove]/p1 says:
1 Requires: The type of
*first
shall satisfy theMoveAssignable
requirements (Table 22).
This means that remove
/remove_if
can only use move assignment to permute the sequence. But then
26.7.8 [alg.remove]/p6 (non-normatively) contradicts p1:
6 Note: each element in the range
[ret,last)
, whereret
is the returned value, has a valid but unspecified state, because the algorithms can eliminate elements by swapping with or moving from elements that were originally in that range.
[2012, Kona]
Move to Ready.
Alisdair notes we could extend permission to use swap
if it is available, but there
is no interest. Accept the proposed resolution as written.
[2012, Portland: applied to WP]
Proposed resolution:
This wording is relative to the FDIS.
Change 26.7.8 [alg.remove] as indicated:
template<class ForwardIterator, class T> ForwardIterator remove(ForwardIterator first, ForwardIterator last, const T& value); template<class ForwardIterator, class Predicate> ForwardIterator remove_if(ForwardIterator first, ForwardIterator last, Predicate pred);[…]
-6-Note: each element in the range[ret,last)
, whereret
is the returned value, has a valid but unspecified state, because the algorithms can eliminate elements byswapping with ormoving from elements that were originally in that range.
unexpected
/terminate
handler is called from the exception handling runtime?Section: 17.9.5.4 [terminate], 99 [unexpected] Status: C++17 Submitter: Howard Hinnant Opened: 2011-12-06 Last modified: 2017-07-30
Priority: 3
View all other issues in [terminate].
View all issues with C++17 status.
Discussion:
Prior to N3242, modified by N3189, we said this about unexpected()
:
Effects: Calls the
unexpected_handler
function in effect immediately after evaluating the throw-expression (D.13.1), if called by the implementation, or calls the currentunexpected_handler
, if called by the program.
and this about terminate()
:
Effects: Calls the
terminate_handler
function in effect immediately after evaluating the throw-expression (18.8.3.1), if called by the implementation, or calls the currentterminate_handler
function, if called by the program.
But now in both places we say:
Calls the current
unexpected_handler
function.
and:
Calls the current
terminate
function.
The difference is that in C++98/03 if a destructor reset a handler during stack unwinding, that new handler was
not called if the unwinding later led to unexpected()
or terminate()
being called. But these new
words say that this new handler is called. This is an ABI-breaking change in the way exceptions are handled.
Was this change intentional?
[2011-12-09: Daniel comments]
There was no such semantic change intended. It was an unfortunate side effect when trying to better separate different responsibilities in the previous wording.
A related issue is 2088(i).[2012-01-30: Howard comments]
The C++98/03 wording is somewhat ambiguous:
Calls the terminate_handler function in effect immediately after evaluating the throw-expression...
There are potentially two throw-expressions being referred to here, and it is not clear if this sentence is referring to just the first or both:
throw assignment-expression;
throw;
There is ample evidence in current implementations that it is understood that only 1. was meant. But clearly both 1 and 2 could have been meant. We need a clarification. Does an execution of a rethrow (throw;) update which handlers can potentially be called?
throw;
// update handlers to get_xxx()?My opinion: Go with existing practice, and clarify what that practice is, if surveys find that everyone does the same thing. Gcc 4.2 and Apple do 1. only, and do not reset the handlers to the current handlers on throw;.
If current practice is not unanimously one way or the other, I have no strong opinion. I have not found a motivating use case for the use of any particular handler. Most applications set the handlers once at the beginning of the program and then do not change them, and so will not be impacted by whatever decision is made here.[2014-02-15 Issaquah: Move to Review]
STL: Original change in N3242 came from trying to make set/get exception handler thread safe. The issue requests we revert to 98/03, which Howard notes was already ambiguous.
Alisdair: Issue author thinks we made this change in C++11 without taking into account Itanium ABI, which cannot implement the new semantic (without breaking compatibility).
Alisdair: original change in N3242 was trying to solve the problem of which handler is called when the handler is changing in another thread, but this turns out to be an issue in even the single-threaded case.
Pablo: despite wanting to make it thread safe, you are still changing a global
STL and Marshall confirm that there is real implementation divergance on the question, so we cannot pick just one behavior if we want to avoid breaking exisitng practice.
Alisdair: not sure who to talk to across all library vendors to fix, need more information for progress (IBM and Sun)
STL: Howard did identify a problem with the wording as well: throw;
is a throw expression,
but we typically want to re-activate the in-flight exception, not throw a new copy.
Pablo: wondering why all of this wording is here (N3189)? It looks like we were trying to handle another thread
changing handler between a throw
and terminate
in current thread.
Alisdair: Anything working with exception handling should have used only thread-local resources, but that ship has sailed. We must account for the same exception object being re-thrown in multiple threads simultaneously, with no happens-before relationships.
Room: Why on earth would we care about exactly which way the program dies when the terminate calls are racing?!
Pablo: Reasonable to set the handler once (in main
) and never change it.
Pablo: If willing to put lots of work into this, you could say at point of a throw
these handlers become
thread local but that is overkill. We want destructors to be able to change these handlers (if only for backwards
compatibility).
Alisdair: the "do it right" is to do something per thread, but that is more work than vendors will want to do. Want to say setting handler while running multiple threads is unspecified.
Pablo: possible all we need to do is say it is always the current handler
STL: That prevents an implementation or single threaded program from calling a new handler after a throw
,
probably should say if terminate
is called by the implementation (during EH), any handler that was
current can be called. Leaves it up in the air as to when the handler is captured, supporting the diverging
existing practices.
Jeffrey: use happens before terminology to avoid introducing races
STL: Give this to concurrency?
Jeffrey: It is in clause 18, generally LWG and not SG1 territory.
Alisdair: Concerned about introducing happens before into fundamental exception handling since it would affect single threaded performance as well. Want to give to concurrency or LEWG/EWG, we are into language design here.
Jeffrey: suspect LEWG won't have a strong opinion. I don't want it to be ours!!!
Pablo: Might be a case for core>
Alisdair: Would be happier if at least one core person were in the discussion.
STL: No sympathy for code that tries to guard the terminate handler.
Alisdair: We are back to set it once, globally. Want to be clear that if set_terminate
is called just once,
when EH is not active, and never changed again, then the user should get the handler from that specific call.
AlisdairM: "unspecified which handler is called if an exception is active when set_terminate
is called."
This supports existing behaviors, and guarantees which handler is called in non-conentious situations. Implicit
assumption that a funtion becomes a handler only after a successful call to set_handler
, so we are not
leaving a door open to the implementation inventing entirely new handlers of its own.
Consensus.
Poll to confirm status as P1: new consensus is P3
Action: Alisdair provides new wording. Drop from P1 to P3, and move to Review.
[2015-05, Lenexa]
HH: we accidentally changed semantics of which handler gets called during exception unwinding. This was attempt to put it back.
Discovered implementations don't really do anything. […] Fine with unspecified behavior to move this week.
STL/MC: observed different behavior
STL: legitimizes all implementations and tells users to not do this
Move to ready? 9/0/1
Proposed resolution:
Amend 17.9.5.4 [terminate] as indicated:
[[noreturn]] void terminate() noexcept;
Remarks: Called by the implementation when exception handling must be abandoned for any of several reasons (15.5.1)
, in effect immediately after throwing the exception. May also be called directly by the program.
Effects: Calls a
terminate_handler
function. It is unspecified whichterminate_handler
function will be called if an exception is active during a call toset_terminate
. Otherwise cCalls the currentterminate_handler
function. [Note: A defaultterminate_handler
is always considered a callable handler in this context. — end note]
Amend 99 [unexpected] as indicated:
[[noreturn]] void unexpected();
Remarks: Called by the implementation when a function exits via an exception not allowed by its exception-specification (15.5.2)
, in effect after evaluating the throw-expression (D.11.1). May also be called directly by the program.
Effects: Calls an
unexpected_handler
function. It is unspecified whichunexpected_handler
function will be called if an exception is active during a call toset_unexpected
. Otherwise cCalls the currentunexpected_handler
function. [Note: A defaultunexpected_handler
is always considered a callable handler in this context. — end note]
Section: 16.4.6 [conforming], 20.2.9 [allocator.traits], 20.6.1 [allocator.adaptor.syn] Status: C++14 Submitter: Daniel Krügler Opened: 2011-11-30 Last modified: 2016-01-28
Priority: 1
View all other issues in [conforming].
View all issues with C++14 status.
Discussion:
It is a very established technique for implementations to derive internally from user-defined class types that are used to customize some library component, e.g. deleters and allocators are typical candidates. The advantage of this approach is to possibly take advantage of the empty-base-class optimization (EBCO).
Whether or whether not libraries did take advantage of such a detail didn't much matter in C++03. Even though there did exist a portable idiom to prevent that a class type could be derived from, this idiom has never reached great popularity: The technique required to introduce a virtual base class and it did not really prevent the derivation, but only any construction of such a type. Further, such types are not empty as defined by thestd::is_empty
trait, so
could easily be detected by implementations from TR1 on.
With the new C++11 feature of final classes and final member functions it is now very easy to define an empty,
but not derivable from class type. From the point of the user it is quite natural to use this feature for
types that he or she did not foresee to be derivable from.
On the other hand, most library implementations (including third-party libraries) often take advantage of EBCO
applied to user-defined types used to instantiate library templates internally. As the time of submitting this
issue the following program failed to compile on all tested library implementations:
#include <memory>
struct Noop final {
template<class Ptr>
void operator()(Ptr) const {}
};
std::unique_ptr<int, Noop> up;
In addition, many std::tuple
implementations with empty, final classes as element types failed as well,
due to a popular inheritance-based implementation technique. EBCO has also a long tradition to be
used in library containers to efficiently store potentially stateless, empty allocators.
__is_final
or __is_derivable
to make EBCO possible in the current form but excluding non-derivable class types. As of this writing this
seems to happen already. Problem is that without a std::is_derivable
trait, third-party libraries
have no portable means to do the same thing as standard library implementations. This should be a good
reason to make such a trait public available soon, but seems not essential to have now. Further, this issue
should also be considered as a chance to recognice that EBCO has always been a very special corner case
(There exist parallels to the previously existing odd core language rule that did make the interplay
between std::auto_ptr
and std::auto_ptr_ref
possible) and that it would be better to
provide explicit means for space-efficient storage, not necessarily restricted to inheritance relations,
e.g. by marking data members with a special attribute.
At least two descriptions in the current standard should be fixed now for better clarification:
As mentioned by Ganesh, 20.2.9 [allocator.traits] p1 currently contains a (non-normative) note "Thus, it is always possible to create a derived class from an allocator." which should be removed.
As pointed out by Howard, the specification of scoped_allocator_adaptor
as of
20.6.1 [allocator.adaptor.syn] already requires derivation from OuterAlloc
, but
only implies indirectly the same for the inner allocators due to the exposition-only
description of member inner
. This indirect implication should be normatively required for
all participating allocators.
[2012, Kona]
What we really need is a type trait to indicate if a type can be derived from. Howard reports Clang and libc++ have had success with this approach.
Howard to provide wording, and AJM to alert Core that we may be wanting to add a new trait that requires compiler support.
[2014-02, Issaquah: Howard and Daniel comment and provide wording]
Several existing C++11 compilers do already provide an internal __is_final
intrinsic (e.g. clang and gcc) and therefore
we believe that this is evidence enough that this feature is implementable today.
is_final
query should result in a true outcome
if and only if the current existing language definition holds that a complete class type (either union or non-union)
has been marked with the class-virt-specifier final
— nothing more.
The following guidelines lead to the design decision and the wording choice given below:
It has been expressed several times that a high-level trait such as "is_derivable
" would be preferred and
would be more useful for non-experts. One problem with that request is that it is astonishingly hard to find a common denominator
for what the precise definition of this trait should be, especially regarding corner-cases. Another example of getting very
differing points of view is to ask a bunch of C++ experts what the best definition of the is_empty
trait
should be (which can be considered as a kind of higher-level trait).
Once we have a fundamental trait like is_final
available, we can easily define higher-level traits in the future
on top of this by a proper logical combination of the low-level traits.
A critical question is whether providing such a low-level compile-time introspection might be considered as disadvantageous,
because it could constrain the freedom of existing implementations even further and whether a high-level trait would solve
this dilemma. We assert that since C++11 the static introspection capabilities are already very large and we believe that
making the presence or absence of the final
keyword testable does not make the current situation worse.
Below code example demonstrates the intention and the implementability of this feature:
#include <type_traits> namespace std { template <class T> struct is_final : public integral_constant<bool, __is_final(T)> {}; } // std // test it union FinalUnion final { }; union NonFinalUnion { }; class FinalClass final { }; struct NonFinalClass { }; class Incomplete; int main() { using std::is_final; static_assert( is_final<const volatile FinalUnion>{}, ""); static_assert(!is_final<FinalUnion[]>{}, ""); static_assert(!is_final<FinalUnion[1]>{}, ""); static_assert(!is_final<NonFinalUnion>{}, ""); static_assert( is_final<FinalClass>{}, ""); static_assert(!is_final<FinalClass&>{}, ""); static_assert(!is_final<FinalClass*>{}, ""); static_assert(!is_final<NonFinalClass>{}, ""); static_assert(!is_final<void>{}, ""); static_assert(!is_final<int>{}, ""); static_assert(!is_final<Incomplete>{}, ""); // error incomplete type 'Incomplete' used in type trait expression }
[2014-02-14, Issaquah: Move to Immediate]
This is an important issue, that we really want to solve for C++14.
Move to Immediate after polling LEWG, and then the NB heads of delegation.
Proposed resolution:
This wording is relative to N3797.
Change 21.3.3 [meta.type.synop], header <type_traits>
synopsis, as indicated
namespace std { […] // 20.10.4.3, type properties: […] template <class T> struct is_empty; template <class T> struct is_polymorphic; template <class T> struct is_abstract; template <class T> struct is_final; […] }
Change 21.3.5.4 [meta.unary.prop], Table 49 — Type property predicates, as indicated
Template | Condition | Preconditions |
---|---|---|
template <class T> struct is_abstract;
|
[…] | […] |
template <class T> struct is_final;
|
T is a class type marked with the class-virt-specifier final (11 [class]).[Note: A union is a class type that can be marked with final . — end note]
|
If T is a class type, T shall be a complete type
|
After 21.3.5.4 [meta.unary.prop] p5 add one further example as indicated:
[Example:
// Given: struct P final { }; union U1 { }; union U2 final { }; // the following assertions hold: static_assert(!is_final<int>::value, "Error!"); static_assert( is_final<P>::value, "Error!"); static_assert(!is_final<U1>::value, "Error!"); static_assert( is_final<U2>::value, "Error!");— end example]
bool
" requirementsSection: 16.4.4.4 [nullablepointer.requirements], 24.3.5.3 [input.iterators], 24.3.5.7 [random.access.iterators], 26.1 [algorithms.general], 26.8 [alg.sorting], 32.2.1 [thread.req.paramname] Status: Resolved Submitter: Daniel Krügler Opened: 2011-12-09 Last modified: 2025-03-13
Priority: 3
View all other issues in [nullablepointer.requirements].
View all issues with Resolved status.
Discussion:
As of 16.4.4.2 [utility.arg.requirements] Table 17/18, the return types of the expressions
a == b
or
a < b
for types satisfying the EqualityComparable
or LessThanComparable
types, respectively, are required to be "convertible to bool
" which corresponds to
a copy-initialization context. But several newer parts of the library that refer to
such contexts have lowered the requirements taking advantage of the new terminology of
"contextually convertible to bool
" instead, which corresponds to a
direct-initialization context (In addition to "normal" direct-initialization constructions,
operands of logical operations as well as if
or switch
conditions also
belong to this special context).
EqualityComparable
but also specify that the expression
a != b
shall be just "contextually convertible to bool
". The same discrepancy
exists for requirement set NullablePointer
in regard to several equality-related expressions.
a < b
contextually convertible tobool
as well as for all derived comparison functions, so strictly speaking we could have a random access
iterator that does not satisfy the LessThanComparable
requirements, which looks like an
artifact to me.
LessThanComparable
or
EqualityComparable
we still would have the problem that some current specifications
are actually based on the assumption of implicit convertibility instead of "explicit convertibility", e.g.
20.3.1.6 [unique.ptr.special] p3:
template <class T1, class D1, class T2, class D2> bool operator!=(const unique_ptr<T1, D1>& x, const unique_ptr<T2, D2>& y);-3- Returns:
x.get() != y.get()
.
Similar examples exist in 20.3.1.3.3 [unique.ptr.single.dtor] p2, 20.3.1.3.4 [unique.ptr.single.asgn] p9, 20.3.1.3.5 [unique.ptr.single.observers] p1+3+8, etc.
In all these places the expressions involving comparison functions (but not those of the conversion of aNullablePointer
to bool
!) assume to be "convertible to bool
". I think this
is a very natural assumption and all delegations of the comparison functions of some type X
to some
other API type Y
in third-party code does so assuming that copy-initialization semantics will
just work.
The actual reason for using the newer terminology can be rooted back to LWG 556(i). My hypotheses
is that the resolution of that issue also needs a slight correction. Why so?
The reason for opening that issue were worries based on the previous "convertible to bool
"
wording. An expressions like "!pred(a, b)
" might not be well-formed in those situations, because
operator!
might not be accessible or might have an unusual semantics (and similarly for other logical
operations). This can indeed happen with unusual proxy return types, so the idea was that the evaluation of
Predicate
, BinaryPredicate
(26.1 [algorithms.general] p8+9), and Compare
(26.8 [alg.sorting] p2) should be defined based on contextual conversion to bool
.
Unfortunately this alone is not sufficient: In addition, I think, we also want the predicates
to be (implicitly) convertible to bool
! Without this wording, several conditions are plain wrong,
e.g. 26.6.6 [alg.find] p2, which talks about "pred(*i) != false
" (find_if
) and
"pred(*i) == false
" (find_if_not
). These expressions are not within a boolean context!
While we could simply fix all these places by proper wording to be considered in a "contextual conversion to
bool
", I think that this is not the correct solution: Many third-party libraries already refer to
the previous C++03 Predicate
definition — it actually predates C++98 and is as old as the
SGI specification. It seems to be a high price to
pay to switch to direct initialization here instead of fixing a completely different specification problem.
A final observation is that we have another definition for a Predicate
in 32.2.1 [thread.req.paramname] p2:
If a parameter is
Predicate
,operator()
applied to the actual template argument shall return a value that is convertible tobool
.
The problem here is not that we have two different definitions of Predicate
in the standard — this
is confusing, but this fact alone is not a defect. The first (minor) problem is that this definition does not properly
apply to function objects that are function pointers, because operator()
is not defined in a strict sense.
But the actually worse second problem is that this wording has the very same
problem that has originally lead to
LWG 556(i)! We only need to look at 32.7.4 [thread.condition.condvar] p15 to recognice this:
while (!pred()) wait(lock);
The negation expression here looks very familiar to the example provided in LWG 556(i) and is sensitive
to the same "unusual proxy" problem. Changing the 32.2.1 [thread.req.paramname] wording to a corresponding
"contextual conversion to bool
" wouldn't work either, because existing specifications rely on "convertible
to bool
", e.g. 32.7.4 [thread.condition.condvar] p32+33+42 or 32.7.5 [thread.condition.condvarany]
p25+26+32+33.
bool
" the actual problem of that
issue has not been fixed. What actually needs to be required here is some normative wording that basically
expresses something along the lines of:
The semantics of any contextual conversion to
bool
shall be equivalent to the semantics of any implicit conversion tobool
.
This is still not complete without having concepts, but it seems to be a better approximation. Another way of solving this issue would be to define a minimum requirements table with equivalent semantics. The proposed wording is a bit simpler but attempts to express the same thing.
[2012, Kona]
Agree with Daniel that we potentially broke some C++03 user code, accept the changes striking "contextually" from tables. Stefan to provide revised wording for section 25, and figure out changes to section 30.
Move to open, and then to Review when updated wording from Stefan is available.
[2012-10-12, STL comments]
The current proposed resolution still isn't completely satisfying. It would certainly be possible for the Standard to
require these various expressions to be implicitly and contextually convertible to bool
, but that would have
a subtle consequence (which, I will argue, is undesirable - regardless of the fact that it dates all the way back to
C++98/03). It would allow users to provide really wacky types to the Standard Library, with one of two effects:
Standard Library implementations would have to go to great lengths to respect such wacky types, essentially using
static_cast<bool>
when invoking any predicates or comparators.
Otherwise, such wacky types would be de facto nonportable, because they would make Standard Library implementations explode.
Effect B is the status quo we're living with today. What Standard Library implementations want to do with pred(args)
goes beyond "if (pred(args))
" (C++03), contextually converting pred(args)
to bool
(C++11), or
implicitly and contextually converting pred(args)
to bool
(the current proposed resolution).
Implementations want to say things like:
if (pred(args)) if (!pred(args)) if (cond && pred(args)) if (cond && !pred(args))
These are real examples taken from Dinkumware's implementation. There are others that would be realistic
("pred(args) && cond
", "cond || pred(args)
", etc.)
pred(args)
to be implicitly and contextually convertible to bool
doesn't prevent operator!()
from being overloaded and returning std::string
(as a wacky example). More
ominously, it doesn't prevent operator&&()
and operator||()
from being overloaded and destroying
short-circuiting.
I would like LWG input before working on Standardese for a new proposed resolution. Here's an outline of what I'd like to do:
Introduce a new "concept" in 16.4.4 [utility.requirements], which I would call BooleanTestable
in the
absence of better ideas.
Centralize things and reduce verbosity by having everything simply refer to BooleanTestable
when necessary.
I believe that the tables could say "Return type: BooleanTestable
", while Predicate/BinaryPredicate/Compare
would need the incantation "shall satisfy the requirements of BooleanTestable".
Resolve the tug-of-war between users (who occasionally want to do weird things) and implementers (who don't want to have to contort their code) by requiring that:
Given a BooleanTestable x
, x
is both implicitly and contextually convertible to bool
.
Given a BooleanTestable x
, !x
is BooleanTestable
. (This is intentionally "recursive".)
Given a BooleanTestable x
, bool t = x, t2(x), f = !x;
has the postcondition t == t2 && t != f
.
Given a BooleanTestable x
and a BooleanTestable y
of possibly different types, "x && y
"
and "x || y
" invoke the built-in operator&&()
and operator||()
, triggering short-circuiting.
bool
is BooleanTestable
.
I believe that this simultaneously gives users great latitude to use types other than bool
, while allowing
implementers to write reasonable code in order to get their jobs done. (If I'm forgetting anything that implementers
would want to say, please let me know.)
About requirement (I): As Daniel patiently explained to me, we need to talk about both implicit conversions and
contextual conversions, because it's possible for a devious type to have both "explicit operator bool()
"
and "operator int()
", which might behave differently (or be deleted, etc.).
About requirement (IV): This is kind of tricky. What we'd like to say is, "BooleanTestable
can't ever trigger
an overloaded logical operator". However, given a perfectly reasonable type Nice
- perhaps even bool
itself! -
other code (perhaps a third-party library) could overload operator&&(Nice, Evil)
. Therefore, I believe
that the requirement should be "no first use" - the Standard Library will ask for various BooleanTestable
types
from users (for example, the result of "first != last
" and the result of "pred(args)
"), and as long
as they don't trigger overloaded logical operators with each other, everything is awesome.
About requirement (V): This is possibly redundant, but it's trivial to specify, makes it easier for users to understand
what they need to do ("oh, I can always achieve this with bool
"), and provides a "base case" for requirement
(IV) that may or may not be necessary. Since bool
is BooleanTestable
, overloading
operator&&(bool, Other)
(etc.) clearly makes the Other
type non-BooleanTestable
.
This wording is relative to the FDIS.
Change Table 25 — "
NullablePointer
requirements" in 16.4.4.4 [nullablepointer.requirements] as indicated:
Table 25 — NullablePointer
requirementsExpression Return type Operational semantics […]
a != b
contextuallyconvertible tobool
!(a == b)
a == np
np == acontextuallyconvertible tobool
a == P()
a != np
np != acontextuallyconvertible tobool
!(a == np)
Change Table 107 — "Input iterator requirements" in 24.3.5.3 [input.iterators] as indicated:
Table 107 — Input iterator requirements (in addition to Iterator) Expression Return type Operational semantics Assertion/note
pre-/post-conditiona != b
contextuallyconvertible tobool
!(a == b)
pre: (a, b)
is in the domain of==
.[…]
Change Table 111 — "Random access iterator requirements" in 24.3.5.7 [random.access.iterators] as indicated:
Table 111 — Random access iterator requirements (in addition to bidirectional iterator) Expression Return type Operational semantics Assertion/note
pre-/post-condition[…]
a < b
contextuallyconvertible tobool
b - a > 0
<
is a total ordering relationa > b
contextuallyconvertible tobool
b < a
>
is a total ordering relation opposite to<
.a >= b
contextuallyconvertible tobool
!(a < b)
a <= b
contextuallyconvertible tobool
!(a > b)
Change 26.1 [algorithms.general] p8+9 as indicated:
-8- The
-9- ThePredicate
parameter is used whenever an algorithm expects a function object (22.10 [function.objects]) that, when applied to the result of dereferencing the corresponding iterator, returns a value testable astrue
. In other words, if an algorithm takesPredicate pred
as its argument and first as its iterator argument, it should work correctly in the constructpred(*first)
implicitly or contextually converted tobool
(Clause 7.3 [conv]). The function objectpred
shall not apply any non-constant function through the dereferenced iterator.BinaryPredicate
parameter is used whenever an algorithm expects a function object that when applied to the result of dereferencing two corresponding iterators or to dereferencing an iterator and typeT
whenT
is part of the signature returns a value testable astrue
. In other words, if an algorithm takesBinaryPredicate binary_pred
as its argument andfirst1
andfirst2
as its iterator arguments, it should work correctly in the constructbinary_pred(*first1, *first2)
implicitly or contextually converted tobool
(Clause 7.3 [conv]).BinaryPredicate
always takes the first iterator'svalue_type
as its first argument, that is, in those cases whenT
value is part of the signature, it should work correctly in the constructbinary_pred(*first1, value)
implicitly or contextually converted tobool
(Clause 7.3 [conv]).binary_pred
shall not apply any non-constant function through the dereferenced iterators.Change 26.8 [alg.sorting] p2 as indicated:
-2-
Compare
is a function object type (22.10 [function.objects]). The return value of the function call operation applied to an object of typeCompare
, when implicitly or contextually converted tobool
(7.3 [conv]), yieldstrue
if the first argument of the call is less than the second, andfalse
otherwise.Compare comp
is used throughout for algorithms assuming an ordering relation. It is assumed thatcomp
will not apply any non-constant function through the dereferenced iterator.Change 32.2.1 [thread.req.paramname] p2 as indicated:
-2-
If a parameter isPredicate
, operator() applied to the actual template argument shall return a value that is convertible tobool
Predicate
is a function object type (22.10 [function.objects]). The return value of the function call operation applied to an object of typePredicate
, when implicitly or contextually converted tobool
(7.3 [conv]), yieldstrue
if the corresponding test condition is satisfied, andfalse
otherwise.
[2014-05-20, Daniel suggests concrete wording based on STL's proposal]
The presented wording follows relatively closely STL's outline with the following notable exceptions:
A reference to BooleanTestable
in table "Return Type" specifications seemed very unusual to me and
I found no "prior art" for this in the Standard. Instead I decided to follow the usual style to add a symbol
with a specific meaning to a specific paragraph that specifies symbols and their meanings.
STL's requirement IV suggested to directly refer to built-in operators &&
and ||
. In my
opinion this concrete requirement isn't needed if we simply require that two BooleanTestable
operands behave
equivalently to two those operands after conversion to bool
(each of them).
I couldn't find a good reason to require normatively that type bool
meets the requirements of BooleanTestable
: My
assertion is that after having defined them, the result simply falls out of this. But to make this a bit clearer, I added
also a non-normative note to these effects.
[2014-06-10, STL comments]
In the current wording I would like to see changed the suggested changes described by bullet #6:
In 23.2.2 [container.requirements.general] p4 undo the suggested change
Then change the 7 occurrences of "convertible to bool
" in the denoted tables to "bool
".
[2015-05-05 Lenexa]
STL: Alisdair wanted to do something here, but Daniel gave us updated wording.
[2015-07 Telecon]
Alisdair: Should specify we don't break short circuiting.
Ville: Looks already specified because that's the way it works for bool.
Geoffrey: Maybe add a note about the short circuiting.
B2/P2 is somewhat ambiguous. It implies that B has to be both implicitly convertible to bool and contextually convertible to bool.
We like this, just have nits.
Status stays Open.
Marshall to ping Daniel with feedback.
[2016-02-27, Daniel updates wording]
The revised wording has been updated from N3936 to N4567.
To satisfy the Kona 2015 committee comments, the wording in
[booleantestable.requirements] has been improved to better separate the two different requirements of "can be
contextually converted to bool
" and "can be implicitly converted to bool
. Both are necessary because
it is possible to define a type that has the latter property but not the former, such as the following one:
2016-08-07, Daniel: The below example has been corrected to reduce confusion about the performed conversions as indicated by the delta markers:
using Bool = int; struct OddBoolean { explicit operator bool() const = delete; operator Bool() const;OddBoolean(bool) = delete; OddBoolean(Bool){}} ob; bool b2 = ob; // OK bool b1(ob); // ErrorOddBoolean b2 = true; // OK OddBoolean b1(true); // Error
In [booleantestable.requirements] a note has been added to ensure that an implementation is not allowed to break any short-circuiting semantics.
I decided to separate LWG 2587(i)/2588(i) from this issue. Both these issues aren't exactly the same but depending on the committee's position, their resolution might benefit from the new vocabulary introduced here.
[2021-06-25; Daniel comments]
The paper P2167R1 is provided to resolve this issue.
[2022-05-01; Daniel comments]
The paper P2167R2 is provided to resolve this issue.
Previous resolution [SUPERSEDED]:
This wording is relative to N4567.
Change 16.4.4.2 [utility.arg.requirements] p1, Table 17 — "EqualityComparable requirements", and Table 18 — "LessThanComparable requirements" as indicated:
-1- […] In these tables,
[…]T
is an object or reference type to be supplied by a C++ program instantiating a template;a
,b
, andc
are values of type (possiblyconst
)T
;s
andt
are modifiable lvalues of typeT
;u
denotes an identifier;rv
is an rvalue of typeT
;andv
is an lvalue of type (possiblyconst
)T
or an rvalue of typeconst T
; andBT
denotes a type that meets theBooleanTestable
requirements ([booleantestable.requirements]).
Table 17 — EqualityComparable
requirements [equalitycomparable]Expression Return type Requirement a == b
convertible to
bool
BT
==
is an equivalence relation, that is, it has the following properties: […][…]
Table 18 — LessThanComparable
requirements [lessthancomparable]Expression Return type Requirement a < b
convertible to
bool
BT
<
is a strict weak ordering relation (26.8 [alg.sorting])Between 16.4.4.3 [swappable.requirements] and 16.4.4.4 [nullablepointer.requirements] insert a new sub-clause as indicated:
?.?.?.?BooleanTestable
requirements [booleantestable.requirements]-?- A
An objectBooleanTestable
type is a boolean-like type that also supports conversions tobool
. A typeB
meets theBooleanTestable
requirements if the expressions described in Table ?? are valid and have the indicated semantics, and ifB
also satisfies all the other requirements of this sub-clause [booleantestable.requirements].b
of typeB
can be implicitly converted tobool
and in addition can be contextually converted tobool
(Clause 4). The result values of both kinds of conversions shall be equivalent. [Example: The typesbool
,std::true_type
, andstd::bitset<>::reference
areBooleanTestable
types. — end example] For the purpose of Table ??, letB2
andBn
denote types (possibly both equal toB
or to each other) that meet theBooleanTestable
requirements, letb1
denote a (possiblyconst
) value ofB
, letb2
denote a (possiblyconst
) value ofB2
, and lett1
denote a value of typebool
. [Note: These rules ensure what an implementation can rely on but doesn't grant it license to break short-circuiting behavior of aBooleanTestable
type. — end note]Somewhere within the new sub-clause [booleantestable.requirements] insert the following new Table (?? denotes the assigned table number):
Table ?? — BooleanTestable
requirements [booleantestable]Expression Return type Operational semantics bool(b1)
bool
Remarks: bool(b1) == t1
for every value
b1
implicitly converted tot1
.!b1
Bn
Remarks: bool(b1) == !bool(!b1)
for
every valueb1
.b1 && b2
bool
bool(b1) && bool(b2)
b1 || b2
bool
bool(b1) || bool(b2)
Change 16.4.4.4 [nullablepointer.requirements] p5 and Table 25 — "NullablePointer requirements" as indicated:
[…]
-5- In Table 25,u
denotes an identifier,t
denotes a non-const
lvalue of typeP
,a
andb
denote values of type (possiblyconst
)P
,andnp
denotes a value of type (possiblyconst
)std::nullptr_t
, andBT
denotes a type that meets theBooleanTestable
requirements ([booleantestable.requirements]). […]
Table 25 — NullablePointer
requirements [nullablepointer]Expression Return type Operational semantics …
a != b
contextually convertible tobool
BT
[…] a == np
np == a
contextually convertible tobool
BT
[…] a != np
np != a
contextually convertible tobool
BT
[…] Change 22.4.9 [tuple.rel] as indicated;
template<class... TTypes, class... UTypes> constexpr bool operator==(const tuple<TTypes...>& t, const tuple<UTypes...>& u);-1- Requires: For all
[…]i
, where0 <= i
andi < sizeof...(TTypes)
,get<i>(t) == get<i>(u)
is a valid expression returning a type thatis convertible tomeets thebool
BooleanTestable
requirements ([booleantestable.requirements]).sizeof...(TTypes) == sizeof...(UTypes)
.template<class... TTypes, class... UTypes> constexpr bool operator<(const tuple<TTypes...>& t, const tuple<UTypes...>& u);-4- Requires: For all
[…]i
, where0 <= i
andi < sizeof...(TTypes)
,get<i>(t) < get<i>(u)
andget<i>(u) < get<i>(t)
are valid expressions returning types thatare convertible tomeet thebool
BooleanTestable
requirements ([booleantestable.requirements]).sizeof...(TTypes) == sizeof...(UTypes)
.Change 23.2.2 [container.requirements.general], Table 95 — "Container requirements", and Table 97 — "Optional container operations" as indicated:
-4- In Tables 95, 96, and 97
X
denotes a container class containing objects of typeT
,a
andb
denote values of typeX
,u
denotes an identifier,r
denotes a non-const
value of typeX
,andrv
denotes a non-const
rvalue of typeX
, andBT
denotes a type that meets theBooleanTestable
requirements ([booleantestable.requirements]).
Table 95 — Container requirements Expression Return type […] …
a == b
convertible to
bool
BT
[…] a != b
convertible to
bool
BT
[…] …
a.empty()
convertible to
bool
BT
[…] […]
Table 97 — Optional container requirements Expression Return type […] …
a < b
convertible to
bool
BT
[…] a > b
convertible to
bool
BT
[…] a <= b
convertible to
bool
BT
[…] a >= b
convertible to
bool
BT
[…] Change 24.3.1 [iterator.requirements.general], Table 106 — "Input iterator requirements", and Table 110 — "Random access iterator requirements" as indicated:
-12- In the following sections,
a
andb
denote values of typeX
orconst X
,difference_type
andreference
refer to the typesiterator_traits<X>::difference_type
anditerator_traits<X>::reference
, respectively,n
denotes a value ofdifference_type
,u
,tmp
, andm
denote identifiers,r
denotes a value ofX&
,t
denotes a value of value typeT
,o
denotes a value of some type that is writable to the output iterator, andBT
denotes a type that meets theBooleanTestable
requirements ([booleantestable.requirements]).
Table 106 — Input iterator requirements Expression Return type […] a != b
contextually convertible to
bool
BT
[…] […]
Table 110 — Random access iterator requirements Expression Return type […] …
a < b
contextually convertible to
bool
BT
[…] a > b
contextually convertible to
bool
BT
[…] a >= b
contextually convertible to
bool
BT
[…] a <= b
contextually convertible to
bool
BT
[…] Change 26.1 [algorithms.general] p8+p9 as indicated:
[Drafting note: The wording changes below also fix (a) unusual wording forms used ("should work") which are unclear in which sense they are imposing normative requirements and (b) the problem, that the current wording seems to allow that the predicate may mutate a call argument, if that is not a dereferenced iterator. Upon applying the new wording it became obvious that the both the previous and the new wording has the effect that currently algorithms such as
adjacent_find
,search_n
,unique
, andunique_copy
are not correctly described (because they have no iterator argument namedfirst1
), which could give raise to a new library issue. — end drafting note]-8- The
-9- ThePredicate
parameter is used whenever an algorithm expects a function object (20.9) that, when applied to the result of dereferencing the corresponding iterator, returns a value testable astrue
.In other words, iIf an algorithm takesPredicate pred
as its argument andfirst
as its iterator argument,it should work correctly in the constructthe expressionpred(*first)
contextually converted tobool
(Clause 4)pred(*first)
shall have a type that meets theBooleanTestable
requirements ( [booleantestable.requirements]). The function objectpred
shall not apply any non-constant function throughthe dereferenced iteratorits argument.BinaryPredicate
parameter is used whenever an algorithm expects a function object that when applied to the result of dereferencing two corresponding iterators or to dereferencing an iterator and typeT
whenT
is part of the signature returns a value testable astrue
.In other words, iIf an algorithm takesBinaryPredicate binary_pred
as its argument andfirst1
andfirst2
as its iterator arguments,it should work correctly in the constructthe expressionbinary_pred(*first1, *first2)
contextually converted tobool
(Clause 4)binary_pred(*first1, *first2)
shall have a type that meets theBooleanTestable
requirements ( [booleantestable.requirements]).BinaryPredicate
always takes the first iterator'svalue_type
as its first argument, that is, in those cases whenT
value is part of the signature,it should work correctly in the constructthe expressionbinary_pred(*first1, value)
contextually converted tobool
(Clause 4)binary_pred(*first1, value)
shall have a type that meets theBooleanTestable
requirements ( [booleantestable.requirements]).binary_pred
shall not apply any non-constant function throughthe dereferenced iteratorsany of its arguments.Change 26.8 [alg.sorting] p2 as indicated:
[…]
-2-Compare
is a function object type (20.9).The return value of the function call operation applied to an object of typeCompare
, when contextually converted tobool
(Clause 4), yieldstrue
if the first argument of the call is less than the second, andfalse
otherwise.Compare comp
is used throughout for algorithms assuming an ordering relation. Leta
andb
denote two argument values whose types depend on the corresponding algorithm. Then the expressioncomp(a, b)
shall have a type that meets theBooleanTestable
requirements ( [booleantestable.requirements]). The return value ofcomp(a, b)
, converted tobool
, yieldstrue
if the first argumenta
is less than the second argumentb
, andfalse
otherwise. It is assumed thatcomp
will not apply any non-constant function throughthe dereferenced iteratorany of its arguments. […]Change 31.5.3.3 [fpos.operations] and Table 126 — "Position type requirements" as indicated:
-1- Operations specified in Table 126 are permitted. In that table,
P
refers to an instance offpos
,[…]
o
refers to a value of typestreamoff
,
BT
refers to a type that meets theBooleanTestable
requirements ([booleantestable.requirements]),[…]
Table 126 — Position type requirements Expression Return type […] …
p == q
convertible tobool
BT
[…] p != q
convertible tobool
BT
[…] Change 32.2.1 [thread.req.paramname] p1 as indicated:
-1- Throughout this Clause, the names of template parameters are used to express type requirements.
If a template parameter is namedPredicate
,operator()
applied to the template argument shall return a value that is convertible tobool
Predicate
is a function object type (22.10 [function.objects]). Letpred
denote an lvalue of typePredicate
. Then the expressionpred()
shall have a type that meets theBooleanTestable
requirements ( [booleantestable.requirements]). The return value ofpred()
, converted tobool
, yieldstrue
if the corresponding test condition is satisfied, andfalse
otherwise.
[2022-11-05; Daniel comments]
The paper P2167R3 has been reviewed and accepted by LWG and would solve this issue.
[2022-11-22 Resolved by P2167R3 accepted in Kona. Status changed: Open → Resolved.]
Proposed resolution:
unique_ptr
for array does not support cv qualification conversion of actual argumentSection: 20.3.1.4 [unique.ptr.runtime] Status: Resolved Submitter: Alf P. Steinbach Opened: 2011-12-16 Last modified: 2017-09-07
Priority: 1
View all other issues in [unique.ptr.runtime].
View all issues with Resolved status.
Discussion:
Addresses US 16
N3290 20.3.1.4.2 [unique.ptr.runtime.ctor] "unique_ptr
constructors":
These constructors behave the same as in the primary template except that they do not accept pointer types which are convertible to
pointer
. [Note: One implementation technique is to create private templated overloads of these members. — end note]
This language excludes even pointer
itself as type for the actual argument.
#include <memory>
using namespace std;
struct T {};
T* foo() { return new T; }
T const* bar() { return foo(); }
int main()
{
unique_ptr< T const > p1( bar() ); // OK
unique_ptr< T const [] > a1( bar() ); // OK
unique_ptr< T const > p2( foo() ); // OK
unique_ptr< T const [] > a2( foo() ); // ? this is line #15
}
The intent seems to be clearly specified in 20.3.1.4 [unique.ptr.runtime]/1 second bullet:
— Pointers to types derived from
T
are rejected by the constructors, and byreset
.
But the following language in 20.3.1.4.2 [unique.ptr.runtime.ctor] then rejects far too much...
Proposed new wording of N3290 20.3.1.4.2 [unique.ptr.runtime.ctor] "unique_ptr
constructors":
These constructors behave the same as in the primary template except that actual argument pointers
p
to types derived fromT
are rejected by the constructors. [Note: One implementation technique is to create private templated overloads of these members. — end note]
This will possibly capture the intent better, and avoid the inconsistency between the non-array and array
versions of unique_ptr
, by using nearly the exact same phrasing as for the paragraph explaining
the intent.
[2012-08-25 Geoffrey Romer comments in c++std-lib-32978]
The current P/R seems to intend to support at least two different implementation techniques — additional unusable templates that catch forbidden arguments or replacing existing constructors by templates that ensure ill-formed code inside the template body, when the requirements are not met. It seems unclear whether the current wording allows the second approach, though. It should be considered to allow both strategies or if that is not possible the note should be clearer.
The very same problem exists for the reset
member function, but even worse, because the current
specification is more than clear that the deleted reset
function will catch all cases not equal to
pointer
. It seems confusing at best to have different policies for the constructor and for the reset
function. In this case, the question in regard to implementation freedom mentioned above is even more important.
It's awkward to refer to "the constructors" twice in the same sentence; I suggest revising the sentence as
"...except that they do not accept argument pointers p
to types derived from T
"
[2012-12-20: Geoffrey Romer comments and provides a revised resolution]
The array specialization of unique_ptr
differs from the primary template in several ways,
including the following:
unique_ptr<T[], D>
cannot be constructed from a plain pointer whose type is not
exactly unique_ptr<T[], D>::pointer
or nullptr_t
.
unique_ptr<T[], D>
cannot be constructed from a unique_ptr<U[], E>&&
unless U
is exactly T
and E
is exactly D
.
unique_ptr<T[], D>
cannot be moveassigned from a unique_ptr<U[], E>&&
unless U
is exactly T
and E
is exactly D
.
unique_ptr<T[], D>::reset
cannot take an argument whose type is not exactly
unique_ptr<T[], D>::pointer
or nullptr_t
.
default_delete<T[]>
cannot be constructed from a default_delete<U[]>
unless
U
is exactly T
.
default_delete<T[]>::operator()
cannot be called on a pointer whose type is not
exactly T*
.
The common intent of all these restrictions appears to be to disallow implicit conversions from pointer-to-derived-class to pointer-to-base-class in contexts where the pointer is known to point to an array, because such conversions are inherently unsafe; deleting or subscripting the result of such a conversion leads to undefined behavior (see also CWG 1504). However, these restrictions have the effect of disallowing all implicit conversions in those contexts, including most notably cv-qualification, but also user-defined conversions, and possibly others. This PR narrows all those restrictions, to disallow only unsafe pointer-to-derived to pointer-to-base conversions, while allowing all others.
I removed the nebulous language stating that certain functions "will not accept" certain arguments. Instead I use explicitly deleted template functions, which participate in overload resolution only for pointer-to-derived to pointer-to-base conversions. This is more consistent with the existing text and easier to express correctly than an approach based on declaring certain types of calls to be ill-formed, but may produce inferior compiler diagnostics. Wherever possible, this PR defines the semantics of template specializations in terms of their differences from the primary template. This improves clarity and minimizes the risk of unintended differences (e.g. LWG 2169(i), which this PR also fixes). This PR also makes it explicit that the specialization inherits the description of all members, not just member functions, from the primary template and, in passing, clarifies the default definition of pointer in the specialization. This resolution only disallows pointer-to-derived to pointer-to-base conversions between ordinary pointer types; if user-defined pointer types provide comparable conversions, it is their responsibility to ensure they are safe. This is consistent with C++'s general preference for expressive power over safety, and for assuming the user knows what they're doing; furthermore, enforcing such a restriction on user-defined types appears to be impractical without cooperation from the user. The "base class without regard to cv-qualifiers" language is intended to parallel the specification ofstd::is_base_of
.
Jonathan Wakely has a working implementation of this PR patched into libstdc++.
Previous resolution:
This wording is relative to the FDIS.
Change 20.3.1.4.2 [unique.ptr.runtime.ctor] as indicated:
explicit unique_ptr(pointer p) noexcept; unique_ptr(pointer p, see below d) noexcept; unique_ptr(pointer p, see below d) noexcept;These constructors behave the same as in the primary template except that
they do not accept pointer types which are convertible toargument pointerspointer
p
to types derived fromT
are rejected by the constructors. [Note: One implementation technique is to create private templated overloads of these members. — end note]
[2014-02, Issaquah]
GR: want to prevent unsafe conversions. Standard is inconsistent how it does this. for reset()
has deleted function
template capturing everything except the known-safe cases. Other functions use SFINAE. Main reason this is hard is that
unique_ptr
supports fancy pointers. Have to figure out how to handle them and what requirements to put on them.
Requirements are minimal, not even required to work with pointer_traits
.
pointer_traits
doesn't work
GR: ways to get fancy pointers to work is to delegate responsibility for preventing unsafe conversions to the fancy pointers themselves.
Howard doesn't like that, he wants even fancy pointers to be prevented from doing unsafe conversions in unique_ptr
contexts.
AM: Howard says unique_ptr
was meant to be very very safe under all conditions, this open a hole in that. Howard wants to
eke forward and support more, but not if we open any holes in type safety.
GR: do we need to be typesafe even for fancy types with incorrect pointer_traits
?
AM: that would mean it's only unsafe for people who lie by providing a broken specialization of pointer_traits
GR: probably can't continue with ambiguity between using SFINAE and ill-formedness. Would appreciate guidance on direction used for that.
STL: difference is observable in convertibility using type traits.
STL: for reset()
which doesn't affect convertibility ill-formed allows static_assert
, better diagnostic.
For assignment it's detectable and has traits, constraining them is better.
EN: I strongly prefer constraints than static_asserts
STL: if we could rely on pointer_traits
that might be good. Alternatively could we add more machinery to deleter?
make deleter say conversions are allowed, otherwise we lock down all conversions. basically want to know if converting U
to
T
is safe.
Previous resolution [SUPERSEDED]:
This wording is relative to N3485.
Revise 20.3.1.2.3 [unique.ptr.dltr.dflt1] as follows
namespace std { template <class T> struct default_delete<T[]> { constexpr default_delete() noexcept = default; template <class U> default_delete(const default_delete<U>&) noexcept; void operator()(T*) const; template <class U> void operator()(U*) const = delete; }; }-?- Descriptions are provided below only for member functions that have behavior different from the primary template.
template <class U> default_delete(const default_delete<U>&) noexcept;-?- This constructor behaves the same as in the primary template except that it shall not participate in overload resolution unless:
U
is an array type, and
V*
is implicitly convertible toT*
, and
T
is not a base class ofV
(without regard to cv-qualifiers),where
V
is the array element type ofU
.void operator()(T* ptr) const;-1- Effects: calls
-2- Remarks: Ifdelete[]
onptr
.T
is an incomplete type, the program is ill-formed.template <class U> void operator()(U*) const = delete;-?- Remarks: This function shall not participate in overload resolution unless
T
is a base class ofU
(without regard to cv-qualifiers).Revise 20.3.1.3 [unique.ptr.single]/3 as follows:
If the type
remove_reference<D>::type::pointer
exists, thenunique_ptr<T, D>::pointer
shall be a synonym forremove_reference<D>::type::pointer
. Otherwiseunique_ptr<T, D>::pointer
shall be a synonym for. The type
Telement_type*unique_ptr<T, D>::pointer
shall satisfy the requirements ofNullablePointer
(16.4.4.4 [nullablepointer.requirements]).Revise 20.3.1.4 [unique.ptr.runtime] as follows:
namespace std { template <class T, class D> class unique_ptr<T[], D> { public: typedef see below pointer; typedef T element_type; typedef D deleter_type; // 20.7.1.3.1, constructors constexpr unique_ptr() noexcept; explicit unique_ptr(pointer p) noexcept; template <class U> explicit unique_ptr(U* p) = delete; unique_ptr(pointer p, see below d) noexcept; template <class U> unique_ptr(U* p, see below d) = delete; unique_ptr(pointer p, see below d) noexcept; template <class U> unique_ptr(U* p, see below d) = delete; unique_ptr(unique_ptr&& u) noexcept; constexpr unique_ptr(nullptr_t) noexcept : unique_ptr() { } template <class U, class E> unique_ptr(unique_ptr<U, E>&& u) noexcept; // destructor ~unique_ptr(); // assignment unique_ptr& operator=(unique_ptr&& u) noexcept; template <class U, class E> unique_ptr& operator=(unique_ptr<U, E>&& u) noexcept; unique_ptr& operator=(nullptr_t) noexcept; // 20.7.1.3.2, observers T& operator[](size_t i) const; pointer get() const noexcept; deleter_type& get_deleter() noexcept; const deleter_type& get_deleter() const noexcept; explicit operator bool() const noexcept; // 20.7.1.3.3 modifiers pointer release() noexcept; void reset(pointer p = pointer()) noexcept;void reset(nullptr_t) noexcept;template <class U> void reset(U*) = delete; void swap(unique_ptr& u) noexcept; // disable copy from lvalue unique_ptr(const unique_ptr&) = delete; unique_ptr& operator=(const unique_ptr&) = delete; }; }-1- A specialization for array types is provided with a slightly altered interface.
Conversions
between different types offromunique_ptr<T[], D>
unique_ptr<Derived[]>
tounique_ptr<Base[]>
, whereBase
is a base class ofDerived
, fromauto_ptr
, or to or from the non-array forms ofunique_ptr
produce an ill-formed program.Pointers to types derived from
T
are rejected by the constructors, and byreset
.The observers
operator*
andoperator->
are not provided.The indexing observer
operator[]
is provided.The default deleter will call
delete[]
.-2- Descriptions are provided below only for
-3- The template argumentmember functions that have behavior differentmembers that differ from the primary template.T
shall be a complete type.Revise 20.3.1.4.2 [unique.ptr.runtime.ctor] as follows:
explicit unique_ptr(pointer p) noexcept; unique_ptr(pointer p, see below d) noexcept; unique_ptr(pointer p, see below d) noexcept;template <class U> explicit unique_ptr(U* p) = delete; template <class U> unique_ptr(U* p, see below d) = delete; template <class U> unique_ptr(U* p, see below d) = delete;
These constructors behave the same as in the primary template except that they do not accept pointer types which are convertible to pointer. [Note: One implementation technique is to create private templated overloads of these members. — end note]These constructors shall not participate in overload resolution unless:
pointer
is a pointer type, and
U*
is implicitly convertible topointer
, and
T
is a base class ofU
(without regard to cv-qualifiers).The type of
d
is determined as in the corresponding non-deleted constructors.template <class U, class E> unique_ptr(unique_ptr<U, E>&& u) noexcept;-?- This constructor behaves the same as in the primary template, except that it shall not participate in overload resolution unless:
unique_ptr<U, E>::pointer
is implicitly convertible topointer
, and
U
is an array type, andeither
D
is a reference type andE
is the same type asD
, orD
is not a reference type andE
is implicitly convertible toD
, andeither at least one of
pointer
andunique_ptr<U, E>::pointer
is not a pointer type, orT
is not a base class of the array element type ofU
(without regard to cv-qualifiers).Insert a new sub-clause following 20.3.1.4.2 [unique.ptr.runtime.ctor] as follows:
??
unique_ptr
assignment [unique.ptr.runtime.asgn]template <class U, class E> unique_ptr& operator=(unique_ptr<U, E>&& u) noexcept;-?- This operator behaves the same as in the primary template, except that it shall not participate in overload resolution unless:
unique_ptr<U, E>::pointer
is implicitly convertible topointer
, and
U
is an array type, andeither
D
is a reference type andE
is the same type asD
, orD
is not a reference type andE
is implicitly convertible toD
, andeither at least one of
pointer
andunique_ptr<U, E>::pointer
is not a pointer type, orT
is not a base class of the array element type ofU
(without regard to cv-qualifiers).Revise 20.3.1.4.5 [unique.ptr.runtime.modifiers] as follows:
void reset(pointer p = pointer()) noexcept; void reset(nullptr_t p) noexcept;template <class U> void reset(U*) = delete;
-1- Effects: Ifget() == nullptr
there are no effects. Otherwiseget_deleter()(get())
.-2- Postcondition:-?- This function shall not participate in overload resolution unless:get() == p
.
pointer
is a pointer type, and
U*
is implicitly convertible topointer
, and
T
is a base class ofU
(without regard to cv-qualifiers).
[2014-06 Rapperswil]
Discussion of N4042 and general agreement that this paper resolves the substance of this issue and should be adopted with minor edits. Geoffrey Romer will provide an updated paper.
[2014-06 post-Rapperswil]
As described in N4089.
[2014-11-07 Urbana]
Resolved by N4089
Proposed resolution:
See proposed wording in N4089.
hash
specializations for extended integer typesSection: 22.10.19 [unord.hash] Status: C++17 Submitter: Daniel Krügler Opened: 2011-12-16 Last modified: 2017-07-30
Priority: 3
View all other issues in [unord.hash].
View all issues with C++17 status.
Discussion:
According to the header <functional>
synopsis 22.10 [function.objects]
and to the explicit description in 22.10.19 [unord.hash] class template
hash
specializations shall be provided for all arithmetic types that are
not extended integer types. This is not explicitly mentioned, but neither the list
nor any normative wording does include them, so it follows by implication.
numeric_limits
corresponding specializations are required. I would
expect that an unordered_map
with key type std::uintmax_t
would
just work, but that depends now on whether this type is an extended integer type
or not.
This issue is not asking for also providing specializations for the
cv-qualified arithmetic types. While this is surely a nice-to-have feature,
I consider that restriction as a more secondary problem in practice.
The proposed resolution also fixes a problem mentioned in 2109(i) in regard
to confusing requirements on user-defined types and those on implementations.
[2012, Kona]
Move to Open.
Agreed that it's a real issue and that the proposed wording fixes it. However, the wording change is not minimal and isn't consistent with the way we fixed hash wording elsewhere.
Alisdair will provide updated wording.
[2014-05-06 Geoffrey Romer suggests alternative wording]
Previous resolution from Daniel [SUPERSEDED]:
This wording is relative to the FDIS.
Change 22.10.19 [unord.hash] p2 as indicated:
template <> struct hash<bool>; template <> struct hash<char>; […] template <> struct hash<long double>; template <class T> struct hash<T*>;-2-
Requires: the template specializations shall meet the requirements of class templateThe headerhash
(22.10.19 [unord.hash])<functional>
provides definitions for specializations of thehash
class template for each cv-unqualified arithmetic type. This header also provides a definition for a partial specialization of thehash
class template for any pointer type. The requirements for the members of these specializations are given in sub-clause 22.10.19 [unord.hash].
[2015-05, Lenexa]
STL: the new PR is very simple and could resolve that nicely
MC: the older PR is rather longish
anybody have any objections to this approach?
what people want to have as a status?
STL: I want to have Ready
MC: move to ready: in favor: 13, opposed: 0, abstain: 4
Proposed resolution:
This wording is relative to N3936.
Change 22.10.19 [unord.hash] p1 as indicated:
The unordered associative containers defined in 23.5 use specializations of the class template
hash
as the default hash function. For all object typesKey
for which there exists a specializationhash<Key>
, and for all integral and enumeration types (7.2)Key
, the instantiationhash<Key>
shall: […]
async
do if neither 'async
' nor 'deferred
' is set in policy?Section: 32.10.9 [futures.async] Status: C++14 Submitter: Jonathan Wakely Opened: 2012-01-01 Last modified: 2016-01-28
Priority: Not Prioritized
View other active issues in [futures.async].
View all other issues in [futures.async].
View all issues with C++14 status.
Discussion:
Implementations already disagree, one returns an invalid future with
no shared state, one chooses policy == async
and one chooses policy ==
deferred
, see c++std-lib-30839, c++std-lib-30840 and c++std-lib-30844.
It's not clear if returning an invalid future is allowed by the current wording.
If the intention is to allow an empty future to be returned, then 32.10.9 [futures.async] p3 and p4 should be adjusted to clarify that a shared state might not be created and an invalid future might be returned.
If the intention is that a valid future is always returned, p3 should say something about the case where none of the conditions applies.
[2012, Portland: move to Review]
We could make it undefined if no launch policy is defined.
Hans: If no launch policy is specified the behaviour is undefined
Artur: or implementation defined?
Hans: no: we don't want people to do this
[Proposed wording]
This wording is relative to N3376
Add a third bullet to the end of the list in 30.6.8p3
"if no valid launch policy is provided the behaviour is undefined"
Moved to review
[2013-04-19, Bristol]
Detlef provides new wording
Previous wording:
[This wording is relative to N3376]
Add a third bullet to the end of the list in 32.10.9 [futures.async]p3
– if no valid launch policy is provided the behaviour is undefined
[2013-09 Chicago]
If no policy is given, it should be undefined, so moved to Immediate.
Accept for Working Paper
Proposed resolution:
[This wording is relative to N3485]
Add a third bullet to the end of the list in 32.10.9 [futures.async]p3
– If no value is set in the launch policy, or a value is set that is neither specified in this International Standard or by the implementation, the behaviour is undefined.
merge()
stability for lists versus forward listsSection: 23.3.11.5 [list.ops], 23.3.7.6 [forward.list.ops] Status: C++14 Submitter: Nicolai Josuttis Opened: 2012-01-15 Last modified: 2023-02-07
Priority: Not Prioritized
View all other issues in [list.ops].
View all issues with C++14 status.
Discussion:
forward_list::merge()
is specified in [forwardlist.ops], p19 as follows:
This operation shall be stable: for equivalent elements in the two lists, the elements from
*this
shall always precede the elements fromx
.
But list::merge()
is only specified in 23.3.11.5 [list.ops], p24 as follows:
Remarks: Stable.
Note that in general we define "stable" only for algorithms (see 3.60 [defns.stable] and 16.4.6.8 [algorithm.stable]) so for member function we should explain it everywhere we use it.
Thus for lists we have to add:Stable: for equivalent elements in the two lists, the elements from the list always precede the elements from the argument list.
This, BTW, was the specification we had with C++03.
In addition, I wonder whether we also have some guarantees regarding stability saying that the order of equivalent elements of each list merged remains stable (which would be my interpretation of just saying "stable", BTW). Thus, I'd expect that for equivalent elements we guarantee that*this
(in the same order as on entry)[2012, Kona]
Move to Open.
STL says we need to fix up 17.6.5.7 to be stronger, and then the remarks for merge should just say "Remarks: Stable (see 17.6.5.7)"
Assigned to STL for word-smithing.
[ 2013-04-14 STL provides rationale and improved wording ]
Step 1: Centralize all specifications of stability to 16.4.6.8 [algorithm.stable].
Step 2: 3.60 [defns.stable] and 16.4.6.8 [algorithm.stable] talk about "algorithms", without mentioning "container member functions". There's almost no potential for confusion here, but there's a simple way to increase clarity without increasing verbosity: make the container member functions explicitly cite 16.4.6.8 [algorithm.stable]. For consistency, we can also update the non-member functions.
Step 3: Fix the "so obvious, we forgot to say it" bug in 16.4.6.8 [algorithm.stable]: a "stable" merge of equivalent elements A B C D and W X Y Z produces A B C D W X Y Z, never D C B A X W Z Y.
Step 3.1: Say "(preserving their original order)" to be consistent with "the relative order [...] is preserved" in 16.4.6.8 [algorithm.stable]'s other bullet points.
Step 4: Copy part of list::merge()
's wording to forward_list::merge()
, in order to properly connect
with 16.4.6.8 [algorithm.stable]'s "first range" and "second range".
[2013-04-18, Bristol]
Original wording saved here:
This wording is relative to the FDIS.
Change 23.3.11.5 [list.ops] as indicated:
void merge(list<T,Allocator>& x); void merge(list<T,Allocator>&& x); template <class Compare> void merge(list<T,Allocator>& x, Compare comp); template <class Compare> void merge(list<T,Allocator>&& x, Compare comp);[…]
-24- Remarks:StableThis operation shall be stable: for equivalent elements in the two lists, the elements from*this
shall always precede the elements fromx
and the order of equivalent elements of*this
andx
remains stable. If(&x != this)
the range[x.begin(), x.end())
is empty after the merge. No elements are copied by this operation. The behavior is undefined ifthis->get_allocator() != x.get_allocator()
.Change [forwardlist.ops] as indicated:
void merge(forward_list<T,Allocator>& x); void merge(forward_list<T,Allocator>&& x); template <class Compare> void merge(forward_list<T,Allocator>& x, Compare comp); template <class Compare> void merge(forward_list<T,Allocator>&& x, Compare comp);[…]
-19- Effects: Mergesx
into*this
. This operation shall be stable: for equivalent elements in the two lists, the elements from*this
shall always precede the elements fromx
and the order of equivalent elements of*this
andx
remains stable.x
is empty after the merge. If an exception is thrown other than by a comparison there are no effects. Pointers and references to the moved elements ofx
now refer to those same elements but as members of*this
. Iterators referring to the moved elements will continue to refer to their elements, but they now behave as iterators into*this
, not intox
.
Proposed resolution:
This wording is relative to the N3485.
Change 16.4.6.8 [algorithm.stable]/1 as indicated:
When the requirements for an algorithm state that it is “stable” without further elaboration, it means:
[…]
- For the merge algorithms, for equivalent elements in the original two ranges, the elements from the first range (preserving their original order) precede the elements from the second range (preserving their original order).
Change [forwardlist.ops] as indicated:
void remove(const T& value); template <class Predicate> void remove_if(Predicate pred);-12- Effects: Erases all the elements in the list referred by a list iterator
-13- Throws: Nothing unless an exception is thrown by the equality comparison or the predicate. -??- Remarks: Stable (16.4.6.8 [algorithm.stable]). […]i
for which the following conditions hold:*i == value
(forremove()
),pred(*i)
is true (forremove_if()
).This operation shall be stable: the relative order of the elements that are not removed is the same as their relative order in the original list.Invalidates only the iterators and references to the erased elements.void merge(forward_list& x); void merge(forward_list&& x); template <class Compare> void merge(forward_list& x, Compare comp) template <class Compare> void merge(forward_list&& x, Compare comp)[…]
-19- Effects: Mergesthe two sorted rangesx
into*this
[begin(), end())
and[x.begin(), x.end())
.This operation shall be stable: for equivalent elements in the two lists, the elements from*this
shall always precede the elements fromx
.x
is empty after the merge. If an exception is thrown other than by a comparison there are no effects. Pointers and references to the moved elements ofx
now refer to those same elements but as members of*this
. Iterators referring to the moved elements will continue to refer to their elements, but they now behave as iterators into*this
, not intox
. -20- Remarks: Stable (16.4.6.8 [algorithm.stable]). The behavior is undefined ifthis->get_allocator() != x.get_allocator()
. […]void sort(); template <class Compare> void sort(Compare comp);[…]
-23- Effects: Sorts the list according to theoperator<
or thecomp
function object.This operation shall be stable: the relative order of the equivalent elements is preserved.If an exception is thrown the order of the elements in*this
is unspecified. Does not affect the validity of iterators and references. -??- Remarks: Stable (16.4.6.8 [algorithm.stable]). […]
Change 23.3.11.5 [list.ops] as indicated:
void remove(const T& value); template <class Predicate> void remove_if(Predicate pred);[…]
-17- Remarks: Stable (16.4.6.8 [algorithm.stable]). […]void merge(list& x); void merge(list&& x); template <class Compare> void merge(list& x, Compare comp) template <class Compare> void merge(list&& x, Compare comp)[…]
-24- Remarks: Stable (16.4.6.8 [algorithm.stable]). […] […]void sort(); template <class Compare> void sort(Compare comp);[…]
-30- Remarks: Stable (16.4.6.8 [algorithm.stable]). […]
Change 26.7.1 [alg.copy]/12 as indicated:
template<class InputIterator, class OutputIterator, class Predicate> OutputIterator copy_if(InputIterator first, InputIterator last, OutputIterator result, Predicate pred);[…]
-12- Remarks: Stable (16.4.6.8 [algorithm.stable]).
Change 26.7.8 [alg.remove] as indicated:
template<class ForwardIterator, class T> ForwardIterator remove(ForwardIterator first, ForwardIterator last, const T& value); template<class ForwardIterator, class Predicate> ForwardIterator remove_if(ForwardIterator first, ForwardIterator last, Predicate pred);[…]
-4- Remarks: Stable (16.4.6.8 [algorithm.stable]). […]template<class InputIterator, class OutputIterator, class T> OutputIterator remove_copy(InputIterator first, InputIterator last, OutputIterator result, const T& value); template<class InputIterator, class OutputIterator, class Predicate> OutputIterator remove_copy_if(InputIterator first, InputIterator last, OutputIterator result, Predicate pred);[…]
-11- Remarks: Stable (16.4.6.8 [algorithm.stable]).
Change 26.8.2.2 [stable.sort]/4 as indicated:
template<class RandomAccessIterator> void stable_sort(RandomAccessIterator first, RandomAccessIterator last); template<class RandomAccessIterator, class Compare> void stable_sort(RandomAccessIterator first, RandomAccessIterator last, Compare comp);[…]
-4- Remarks: Stable (16.4.6.8 [algorithm.stable]).
Change 26.8.6 [alg.merge] as indicated:
template<class InputIterator1, class InputIterator2, class OutputIterator> OutputIterator merge(InputIterator1 first1, InputIterator1 last1, InputIterator2 first2, InputIterator2 last2, OutputIterator result); template<class InputIterator1, class InputIterator2, class OutputIterator, class Compare> OutputIterator merge(InputIterator1 first1, InputIterator1 last1, InputIterator2 first2, InputIterator2 last2, OutputIterator result, Compare comp);[…]
-5- Remarks: Stable (16.4.6.8 [algorithm.stable]). […]template<class BidirectionalIterator> void inplace_merge(BidirectionalIterator first, BidirectionalIterator middle, BidirectionalIterator last); template<class BidirectionalIterator, class Compare> void inplace_merge(BidirectionalIterator first, BidirectionalIterator middle, BidirectionalIterator last, Compare comp);[…]
-9- Remarks: Stable (16.4.6.8 [algorithm.stable]).
merge()
allocator requirements for lists versus forward listsSection: 23.3.7.6 [forward.list.ops] Status: C++14 Submitter: Nicolai Josuttis Opened: 2012-01-15 Last modified: 2023-02-07
Priority: Not Prioritized
View all other issues in [forward.list.ops].
View all issues with C++14 status.
Discussion:
Sub-clause 23.3.11.5 [list.ops], p24 states for lists:
The behavior is undefined if
this->get_allocator() != x.get_allocator()
.
But there is nothing like that for forward lists in [forwardlist.ops], although I would expect the same undefined behavior there.
[2012, Kona]
Move to Ready.
[2012, Portland: applied to WP]
Proposed resolution:
This wording is relative to the FDIS.
Add a new paragraph after [forwardlist.ops] p19 as indicated:
void merge(forward_list<T,Allocator>& x); void merge(forward_list<T,Allocator>&& x); template <class Compare> void merge(forward_list<T,Allocator>& x, Compare comp); template <class Compare> void merge(forward_list<T,Allocator>&& x, Compare comp);[…]
-19- Effects: […] -?- Remarks: The behavior is undefined ifthis->get_allocator() != x.get_allocator()
.
raw_storage_iterator
Section: 99 [depr.storage.iterator] Status: C++17 Submitter: Jonathan Wakely Opened: 2012-01-23 Last modified: 2017-07-30
Priority: 3
View all other issues in [depr.storage.iterator].
View all issues with C++17 status.
Discussion:
Aliaksandr Valialkin pointed out that raw_storage_iterator
only supports constructing
a new object from lvalues so cannot be used to construct move-only types:
template <typename InputIterator, typename T> void move_to_raw_buffer(InputIterator first, InputIterator last, T *raw_buffer) { std::move(first, last, std::raw_storage_iterator<T *, T>(raw_buffer)); }
This could easily be solved by overloading operator=
for rvalues.
raw_storage_iterator
causes exception-safety problems when used with any
generic algorithm. I suggest leaving it alone and not encouraging its use.
[2014-11-11, Jonathan provides improved wording]
In Urbana LWG decided to explicitly say the value is constructed from an rvalue.
Previous resolution from Jonathan [SUPERSEDED]:This wording is relative to N3337.
Add a new signature to the synopsis in [storage.iterator] p1:
namespace std { template <class OutputIterator, class T> class raw_storage_iterator : public iterator<output_iterator_tag,void,void,void,void> { public: explicit raw_storage_iterator(OutputIterator x); raw_storage_iterator<OutputIterator,T>& operator*(); raw_storage_iterator<OutputIterator,T>& operator=(const T& element); raw_storage_iterator<OutputIterator,T>& operator=(T&& element); raw_storage_iterator<OutputIterator,T>& operator++(); raw_storage_iterator<OutputIterator,T> operator++(int); }; }Insert the new signature and a new paragraph before p4:
raw_storage_iterator<OutputIterator,T>& operator=(const T& element); raw_storage_iterator<OutputIterator,T>& operator=(T&& element);-?- Requires: For the first signature
-4- Effects: Constructs a value fromT
shall beCopyConstructible
. For the second signatureT
shall beMoveConstructible
.element
at the location to which the iterator points. -5- Returns: A reference to the iterator.
[2015-05, Lenexa]
MC: Suggestion to move it to Ready for incorporation on Friday
MC: move to ready: in favor: 12, opposed: 0, abstain: 3
Proposed resolution:
This wording is relative to N4140.
Add a new signature to the synopsis in [storage.iterator] p1:
namespace std { template <class OutputIterator, class T> class raw_storage_iterator : public iterator<output_iterator_tag,void,void,void,void> { public: explicit raw_storage_iterator(OutputIterator x); raw_storage_iterator<OutputIterator,T>& operator*(); raw_storage_iterator<OutputIterator,T>& operator=(const T& element); raw_storage_iterator<OutputIterator,T>& operator=(T&& element); raw_storage_iterator<OutputIterator,T>& operator++(); raw_storage_iterator<OutputIterator,T> operator++(int); }; }
Insert a new paragraph before p4:
raw_storage_iterator<OutputIterator,T>& operator=(const T& element);-?- Requires:
-4- Effects: Constructs a value fromT
shall beCopyConstructible
.element
at the location to which the iterator points. -5- Returns: A reference to the iterator.
Insert the new signature and a new paragraph after p5:
raw_storage_iterator<OutputIterator,T>& operator=(T&& element);-?- Requires:
-?- Effects: Constructs a value fromT
shall beMoveConstructible
.std::move(element)
at the location to which the iterator points. -?- Returns: A reference to the iterator.
cbegin/cend
Section: 24.2 [iterator.synopsis], 24.7 [iterator.range] Status: C++14 Submitter: Dmitry Polukhin Opened: 2012-01-23 Last modified: 2016-01-28
Priority: Not Prioritized
View all other issues in [iterator.synopsis].
View all issues with C++14 status.
Discussion:
All standard containers support cbegin/cend
member functions but corresponding global functions are
missing. Proposed resolution it to add global cbegin/cend
functions by analogy with global begin/end
functions. This addition will unify things for users.
[2012, Kona]
STL: Range-based for loops do not use global begin
/end
(anymore).
Alisdair: We will have to make sure these will be available through many headers.
STL: Do this, including r
and cr
. This won't add any additional work.
Matt: Users will find it strange if these are not all available.
Alisdair: Should we have these available everywhere begin/end are available?
Marshall: Yes. Not any extra work.
Howard: Adding all of these means we need all of <iterator>
.
STL: We already need it all.
Matt: We have to be careful what we are requiring if we include the r
versions.
Jeffrey: If we include r
, should they adapt if the container does not define reverse iteration?
STL: No. No special behavior. Should fail to compile. Up to user to add the reverse code--it's easy.
Howard: Anyway it will SFINAE out.
Alisdair: Error messages due to SFINAE are harder to understand than simple failure to compile.
STL: Agrees that SFINAE makes error messages much worse.
Action: STL to provide additional wording for the r
variants.
Move to Review once that wording is availalbe.
[ 2013-04-14 STL provides rationale and improved wording ]
Step 1: Implement std::cbegin/cend()
by calling std::begin/end()
. This has numerous advantages:
cbegin/cend()
members were invented.initializer_list
, which is extremely minimal and lacks cbegin/cend()
members.Step 2: Like std::begin/end()
, implement std::rbegin/rend()
by calling c.rbegin/rend()
.
Note that C++98/03 had the Reversible Container Requirements
.
Step 3: Also like std::begin/end()
, provide overloads of std::rbegin/rend()
for arrays.
Step 4: Provide overloads of std::rbegin/rend()
for initializer_list
, because it lacks
rbegin/rend()
members. These overloads follow 17.11.5 [support.initlist.range]'s signatures. Note that
because these overloads return reverse_iterator
, they aren't being specified in <initializer_list>
.
Step 5: Like Step 1, implement std::crbegin/crend()
by calling std::rbegin/rend()
.
Original wording saved here:
This wording is relative to N3337.
In 24.2 [iterator.synopsis], header iterator synopsis, add the following declarations:
namespace std { […] // 24.6.5, range access: template <class C> auto begin(C& c) -> decltype(c.begin()); template <class C> auto begin(const C& c) -> decltype(c.begin()); template <class C> auto end(C& c) -> decltype(c.end()); template <class C> auto end(const C& c) -> decltype(c.end()); template <class C> auto cbegin(const C& c) -> decltype(c.cbegin()); template <class C> auto cend(const C& c) -> decltype(c.cend()); template <class T, size_t N> T* begin(T (&array)[N]); template <class T, size_t N> T* end(T (&array)[N]); template <class T, size_t N> const T* cbegin(T (&array)[N]); template <class T, size_t N> const T* cend(T (&array)[N]); }In 24.7 [iterator.range] after p5 add the following series of paragraphs:
template <class C> auto cbegin(const C& c) -> decltype(c.cbegin());-?- Returns:
c.cbegin()
.template <class C> auto cend(const C& c) -> decltype(c.cend());-?- Returns:
c.cend()
.template <class T, size_t N> const T* cbegin(T (&array)[N]);-?- Returns:
array
.template <class T, size_t N> const T* cend(T (&array)[N]);-?- Returns:
array + N
.
[2013-04-18, Bristol]
Proposed resolution:
This wording is relative to N3485.
In 24.2 [iterator.synopsis], header iterator synopsis, add the following declarations:
namespace std { […] // 24.6.5, range access: template <class C> auto begin(C& c) -> decltype(c.begin()); template <class C> auto begin(const C& c) -> decltype(c.begin()); template <class C> auto end(C& c) -> decltype(c.end()); template <class C> auto end(const C& c) -> decltype(c.end()); template <class T, size_t N> T* begin(T (&array)[N]); template <class T, size_t N> T* end(T (&array)[N]); template <class C> auto cbegin(const C& c) -> decltype(std::begin(c)); template <class C> auto cend(const C& c) -> decltype(std::end(c)); template <class C> auto rbegin(C& c) -> decltype(c.rbegin()); template <class C> auto rbegin(const C& c) -> decltype(c.rbegin()); template <class C> auto rend(C& c) -> decltype(c.rend()); template <class C> auto rend(const C& c) -> decltype(c.rend()); template <class T, size_t N> reverse_iterator<T*> rbegin(T (&array)[N]); template <class T, size_t N> reverse_iterator<T*> rend(T (&array)[N]); template <class E> reverse_iterator<const E*> rbegin(initializer_list<E> il); template <class E> reverse_iterator<const E*> rend(initializer_list<E> il); template <class C> auto crbegin(const C& c) -> decltype(std::rbegin(c)); template <class C> auto crend(const C& c) -> decltype(std::rend(c)); }
At the end of 24.7 [iterator.range], add:
template <class C> auto cbegin(const C& c) -> decltype(std::begin(c));-?- Returns:
std::begin(c)
.template <class C> auto cend(const C& c) -> decltype(std::end(c));-?- Returns:
std::end(c)
.template <class C> auto rbegin(C& c) -> decltype(c.rbegin()); template <class C> auto rbegin(const C& c) -> decltype(c.rbegin());-?- Returns:
c.rbegin()
.template <class C> auto rend(C& c) -> decltype(c.rend()); template <class C> auto rend(const C& c) -> decltype(c.rend());-?- Returns:
c.rend()
.template <class T, size_t N> reverse_iterator<T*> rbegin(T (&array)[N]);-?- Returns:
reverse_iterator<T*>(array + N)
.template <class T, size_t N> reverse_iterator<T*> rend(T (&array)[N]);-?- Returns:
reverse_iterator<T*>(array)
.template <class E> reverse_iterator<const E*> rbegin(initializer_list<E> il);-?- Returns:
reverse_iterator<const E*>(il.end())
.template <class E> reverse_iterator<const E*> rend(initializer_list<E> il);-?- Returns:
reverse_iterator<const E*>(il.begin())
.template <class C> auto crbegin(const C& c) -> decltype(std::rbegin(c));-?- Returns:
std::rbegin(c)
.template <class C> auto crend(const C& c) -> decltype(std::rend(c));-?- Returns:
std::rend(c)
.
std::initializer_list
Section: 16.4.5.2.1 [namespace.std], 17.11 [support.initlist] Status: C++17 Submitter: Richard Smith Opened: 2012-01-18 Last modified: 2017-07-30
Priority: 3
View other active issues in [namespace.std].
View all other issues in [namespace.std].
View all issues with C++17 status.
Discussion:
Since the implementation is intended to magically synthesize instances of std::initializer_list
(rather than by a constructor call, for instance), user specializations of this type can't generally be
made to work. I can't find any wording which makes such specializations ill-formed, though, which leads
me to suspect that they're technically legal under the provisions of 16.4.5.2.1 [namespace.std] p1.
[2012, Kona]
This sounds correct, but we need wording for a resolution.
Marshall Clow volunteers to produce wording.
[2014-02-19, Jonathan Wakely provides proposed wording]
[2014-03-27, Library reflector vote]
The issue has been identified as Tentatively Ready based on six votes in favour.
Proposed resolution:
This wording is relative to N3936.
Add new new paragraph below 17.11 [support.initlist] p2:
-2- An object of type
-?- If an explicit specialization or partial specialization ofinitializer_list<E>
provides access to an array of objects of typeconst E
. […]initializer_list
is declared, the program is ill-formed.
Section: 32.5.4 [atomics.order] Status: C++14 Submitter: Mark Batty Opened: 2012-02-22 Last modified: 2016-01-28
Priority: Not Prioritized
View other active issues in [atomics.order].
View all other issues in [atomics.order].
View all issues with C++14 status.
Discussion:
C11 issue 407
It seems that both C11 and C++11 are missing the following two derivatives of this rule:
For atomic modifications
A
andB
of an atomic objectM
, if there is amemory_order_seq_cst
fenceX
such thatA
is sequenced beforeX
, andX
precedesB
inS
, thenB
occurs later thanA
in the modification order ofM
.
For atomic modifications
A
andB
of an atomic objectM
, if there is amemory_order_seq_cst
fenceY
such thatY
is sequenced beforeB
, andA
precedesY
inS
, thenB
occurs later thanA
in the modification order ofM
.
Above wording has been suggested for the Technical Corrigendum of C11 via issue 407, details can be found here.
[2012-03-19: Daniel proposes a slightly condensed form to reduce wording duplications]
[2012-03-20: Hans comments]
The usage of the term atomic operations in 32.5.4 [atomics.order] p7 is actually incorrect and should better be replaced by atomic modifications as used in the C11 407 wording.
There seems to be a similar wording incorrectness used in 6.9.2 [intro.multithread] p17 which should be corrected as well.[2012, Portland: move to Review]
Olivier: does the fence really participate in the modifications?
Hans: S is the total set of all sequentially consistent operations, and sequentially consistent fences are in S.
Olivier: this sort of combination of a pair of half-open rules seems to imply the write must make it to main memory
But not all implementations treat a fence as a memory operation; cannot observe the half-open rule.
Hans: not sure this is actually prevented here. You could defer until the next load. What the wording doesn't quite show is that the third bullet in the new wording is already in the standard.
Hans: it is the interaction between fences on one side and other memory modifications on the other that is being defined here.
Pablo: S is not directly observable; it is a hypothetic ordering.
Moved to review
Hans: to alert C liaison
[2013-04-20, Bristol]
Accepted for the working paper
Proposed resolution:
This wording is relative to N3376.
[Drafting note: The project editor is kindly asked to consider to replace in 6.9.2 [intro.multithread] p17 the phrase "before an operation B on M" by "before a modification B of M".]
Change 32.5.4 [atomics.order] paragraph 7 as indicated: [Drafting note: Note that the wording change intentionally does also replace the term atomic operation by atomic modification]
-7- For atomic operations A and B on an atomic object M, if there are
For atomic modifications A and B of an atomic object M, B occurs
later than A in the modification order of M if:
memory_order_seq_cst
fences X and Y such that A is sequenced before X,
Y is sequenced before B, and X precedes Y in S, then B
occurs later than A in the modification order of M.
memory_order_seq_cst
fence X such that A is sequenced before X,
and X precedes B in S, or
memory_order_seq_cst
fence Y such that Y is sequenced before B,
and A precedes Y in S, or
memory_order_seq_cst
fences X and Y such that A is sequenced
before X, Y is sequenced before B, and X precedes Y in S.
memory_order_seq_cst
ensures sequential consistency only for a program that is free of data races
and uses exclusively memory_order_seq_cst
operations. Any use of weaker ordering will invalidate this
guarantee unless extreme care is used. In particular, memory_order_seq_cst
fences ensure a total order
only for the fences themselves. Fences cannot, in general, be used to restore sequential consistency for atomic
operations with weaker ordering specifications. — end note ]
std::function
ambiguitySection: 22.10.17.3.2 [func.wrap.func.con] Status: C++14 Submitter: Ville Voutilainen Opened: 2012-02-28 Last modified: 2016-01-28
Priority: 2
View all other issues in [func.wrap.func.con].
View all issues with C++14 status.
Discussion:
Consider the following:
#include <functional> void f(std::function<void()>) {} void f(std::function<void(int)>) {} int main() { f([]{}); f([](int){}); }
The calls to f
in main
are ambiguous. Apparently because the
conversion sequences to std::function
from the lambdas are identical.
The standard specifies that the function object given to std::function
"shall be Callable (20.8.11.2) for argument types ArgTypes
and
return type R
." It doesn't say that if this is not the case, the
constructor isn't part of the overload set.
invoke
is possible but was deferred for a later point in time. Defining a type trait for
the Callable requirement would also be possible, so there seem to be no technical
reasons why the template constructor of std::function
should not be
constrained. The below suggested wording does this without introducing a special
trait for this. This corresponds to the way that has been used to specify the
result_of
trait. Note that the definition of the Callable
requirement is perfectly suitable for this, because it is a pure syntactically
based requirement and can be directly transformed into a constrained template.
The suggested resolution also applies such wording to the "perfectly forwarding"
assignment operator
template<class F> function& operator=(F&&);
The positive side-effect of this is that it automatically implements a solution to a problem similar to that mentioned in issue 1234(i).
It would be possible to apply similar constraints to the member signaturestemplate<class F> function& operator=(reference_wrapper<F>); template<class F, class A> void assign(F&&, const A&);
as well. At this point there does not seem to be a pestering reason to do so.
[2012-10 Portland: Move to Review]
STL: This is a real issue, but does not like a resolution relying on a SFINAEable metafunction that is not specified and available to the users.
packaged_task
has the same issue.
STL strongly wants to see an is_callable
type trait to clarify the proposed wording.
Jeremiah concerned about holding up what appears to be a correct resolution for a hypothetical better one later - the issue is real.
Why must f
by CopyConstructible? Surely MoveConstructible would be sufficient?
Answer: because function
is CopyConstructible, and the bound functor is type-erased
so must support all the properties of function
itself.
Replace various applications of declval
in the proposed resolution with simply using
the passed functor object, f
.
Alisdair to apply similar changes to packaged_task
.
[2012-11-09, Vicente J. Botet Escriba provides another example]
Consider the following:
class AThreadWrapper { public: explicit operator std::thread(); ... }; std::thread th = std::thread(AThreadWrapper); // call to conversion operator intended
The call to the conversion operator is overloaded with the thread constructor. But thread constructor requirement
makes it fail as AThreadWrapper
is not a Callable and the compiler tries to instantiate the thread
constructor and fails.
[2014-02-14 Issaquah meeting: Move to Immediate]
Proposed resolution:
This wording is relative to N3376.
Change the following paragraphs in 22.10.17.3.2 [func.wrap.func.con]:
[Editorial comment: The removal of the seemingly additional no-throw
requirements of copy constructor and destructor of A
is recommended,
because they are already part of the Allocator requirements. Similar clean-up
has been suggested by 2070(i) — end comment]
template<class F> function(F f); template<class F, class A> function(allocator_arg_t, const A& a, F f);-7- Requires:
-?- Remarks: These constructors shall not participate in overload resolution unlessF
shall beCopyConstructible
.f
shall be Callable (22.10.17.3 [func.wrap.func]) for argument typesArgTypes
and return typeR
. The copy constructor and destructor ofA
shall not throw exceptions.f
is Callable (22.10.17.3 [func.wrap.func]) for argument typesArgTypes...
and return typeR
.[…]
template<class F> function& operator=(F&& f);-18- Effects:
-19- Returns:function(std::forward<F>(f)).swap(*this);
*this
-?- Remarks: This assignment operator shall not participate in overload resolution unlessdeclval<typename decay<F>::type&>()
is Callable (22.10.17.3 [func.wrap.func]) for argument typesArgTypes...
and return typeR
.
Section: 16.4.6.4 [global.functions] Status: C++17 Submitter: Yakov Galka Opened: 2012-01-25 Last modified: 2017-07-30
Priority: 3
View all other issues in [global.functions].
View all issues with C++17 status.
Discussion:
16.4.6.4 [global.functions] says
Unless otherwise specified, global and non-member functions in the standard library shall not use functions from another namespace which are found through argument-dependent name lookup (3.4.2).
This sounds clear enough. There are just two problems:
Both implementations I tested (VS2005 and GCC 3.4.3) do unqualified calls to the comma operator in some parts of the library with operands of user-defined types.
The standard itself does this in the description of some algorithms. E.g. uninitialized_copy
is defined as:
Effects:
for (; first != last; ++result, ++first) ::new (static_cast<void*>(&*result)) typename iterator_traits<ForwardIterator>::value_type(*first);
If understood literally, it is required to call operator,(ForwardIterator, InputIterator)
.
[2013-03-15 Issues Teleconference]
Moved to Open.
There are real questions here, that may require a paper to explore and answer properly.
[2014-05-18, Daniel comments and suggests concrete wording]
Other issues, such as 2114(i) already follow a similar spirit as the one suggested by bullet 2 of the issue submitter. I assert that consideration of possible user-provided overloads of the comma-operator were not intended by the original wording and doing so afterwards would unnecessarily complicate a future conceptualization of the library and would needlessly restrict implementations.
I don't think that a paper is needed to solve this issue, there is a simply way to ensure that the code-semantics excludes consideration of user-provided comma operators. The provided wording below clarifies this by explicitly casting the first argument of the operator tovoid
.
[2015-05, Lenexa]
DK: is putting it in the middle the right place for it?
STL: either works, but visually putting it in the middle is clearer, and for "++it1, ++i2, ++it3" it needs to be
done after the second comma, so "++it1, (void) ++i2, (void) ++it3" is better than "(void) ++it1, ++i2, (void) ++it3"
ZY: for INVOKE
yesterday we used static_cast<void>
but here we're using C-style cast, why?
STL: for INVOKE
I want to draw attention that there's an intentional coercion to void
because that's
the desired type. Here we only do it because that's the best way to prevent the problem, not because we specifically want a
void
type.
Move to Ready: 9 in favor, none opposed, 1 abstention
Proposed resolution:
This wording is relative to N3936.
Change 26.11.5 [uninitialized.copy] as indicated:
template <class InputIterator, class ForwardIterator> ForwardIterator uninitialized_copy(InputIterator first, InputIterator last, ForwardIterator result);-1- Effects:
for (; first != last; ++result, (void) ++first) ::new (static_cast<void*>(&*result)) typename iterator_traits<ForwardIterator>::value_type(*first);[…]
template <class InputIterator, class Size,class ForwardIterator> ForwardIterator uninitialized_copy_n(InputIterator first, Size n, ForwardIterator result);-3- Effects:
for (; n > 0; ++result, (void) ++first, --n) ::new (static_cast<void*>(&*result)) typename iterator_traits<ForwardIterator>::value_type(*first);
Change 26.8.11 [alg.lex.comparison] p3 as indicated:
template<class InputIterator1, class InputIterator2> bool lexicographical_compare(InputIterator1 first1, InputIterator1 last1, InputIterator2 first2, InputIterator2 last2); template<class InputIterator1, class InputIterator2, class Compare> bool lexicographical_compare(InputIterator1 first1, InputIterator1 last1, InputIterator2 first2, InputIterator2 last2, Compare comp);-3- Remarks: […]
for ( ; first1 != last1 && first2 != last2 ; ++first1, (void) ++first2) { if (*first1 < *first2) return true; if (*first2 < *first1) return false; } return first1 == last1 && first2 != last2;
condition_variable::wait()
Section: 32.7.4 [thread.condition.condvar], 32.7.5 [thread.condition.condvarany] Status: C++14 Submitter: Pete Becker Opened: 2012-03-06 Last modified: 2015-10-03
Priority: Not Prioritized
View all other issues in [thread.condition.condvar].
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Discussion:
condition_varible::wait()
(and, presumably, condition_variable_any::wait()
, although
I haven't looked at it) says that it calls lock.unlock()
, and if condition_variable::wait()
exits by an exception it calls lock.lock()
on the way out. But if the initial call to
lock.unlock()
threw an exception, does it make sense to call lock.lock()
? We simply
don't know the state of that lock object, and it's probably better not to touch it.
wait()
call has been unblocked, it calls lock.lock()
. If lock.lock()
throws an exception, what happens? The requirement is:
If the function exits via an exception,
lock.lock()
shall be called prior to exiting the function scope.
That can be read in two different ways. One way is as if it said "lock.lock()
shall have been called …",
i.e. the original, failed, call to lock.lock()
is all that's required. But a more natural reading is
that wait has to call lock.lock()
again, even though it already failed.
lock.unlock()
and the final call to lock.lock()
. Each one should have its own requirement.
Lumping them together muddles things.
[2012, Portland: move to Open]
Pablo: unlock
failing is easy -- the call leaves it locked.
The second case, trying to lock
fails -- what can you do?
This is an odd state as we had it locked before was called wait.
Maybe we should call terminate
as we cannot meet the post-conditions.
We could throw a different exception.
Hans: calling terminate
makes sense as we're likely to call it soon anyway
and at least we have some context.
Detlef: what kind of locks might be being used?
Pablo: condition variables are 'our' locks so this is less of a problem.
condition_variable_any
might be more problematic.
The general direction is to call terminate
if the lock cannot be reacquired.
Pablo: Can we change the wording to 'leaves the mutex locked' ?
Hans: so if the unlock
throws we simply propagate the exception.
Move the issue to open and add some formal wording at a later time.
[2013-09 Chicago: Resolved]
Detlef improves wording. Daniel suggests to introduce a Remarks element for the special "If the function fails to meet the postcondition..." wording and applies this to the proposed wording.
Proposed resolution:
This wording is relative to N3691.
Edit 32.7.4 [thread.condition.condvar] as indicated:
void wait(unique_lock<mutex>& lock);[…]
-10- Effects:
Atomically calls
lock.unlock()
and blocks on*this
.When unblocked, calls
lock.lock()
(possibly blocking on the lock), then returns.The function will unblock when signaled by a call to
notify_one()
or a call tonotify_all()
, or spuriously.
If the function exits via an exception,lock.lock()
shall be called prior to exiting the function scope.-?- Remarks: If the function fails to meet the postcondition,
-11- Postcondition:std::terminate()
shall be called (14.6.2 [except.terminate]). [Note: This can happen if the re-locking of the mutex throws an exception. — end note]lock.owns_lock()
is true andlock.mutex()
is locked by the calling thread. -12- Throws: Nothing.system_error
when an exception is required (30.2.2)-13- Error conditions:
equivalent error condition fromlock.lock()
orlock.unlock()
.template <class Predicate> void wait(unique_lock<mutex>& lock, Predicate pred);[…]
-?- Remarks: If the function fails to meet the postcondition,std::terminate()
shall be called (14.6.2 [except.terminate]). [Note: This can happen if the re-locking of the mutex throws an exception. — end note] -16- Postcondition:lock.owns_lock()
is true andlock.mutex()
is locked by the calling thread. -17- Throws:timeout-related exceptions (30.2.4)system_error
when an exception is required (30.2.2),,or any exception thrown bypred
.-18- Error conditions:
equivalent error condition fromlock.lock()
orlock.unlock()
.template <class Clock, class Duration> cv_status wait_until(unique_lock<mutex>& lock, const chrono::time_point<Clock, Duration>& abs_time);[…]
-20- Effects:
[…]
If the function exits via an exception,
lock.lock()
shall be called prior to exiting the functionscope.-?- Remarks: If the function fails to meet the postcondition,
-21- Postcondition:std::terminate()
shall be called (14.6.2 [except.terminate]). [Note: This can happen if the re-locking of the mutex throws an exception. — end note]lock.owns_lock()
is true andlock.mutex()
is locked by the calling thread. […] -23- Throws:timeout-related exceptions (30.2.4).system_error
when an exception is required (30.2.2) or-24- Error conditions:
equivalent error condition fromlock.lock()
orlock.unlock()
.template <class Rep, class Period> cv_status wait_for(unique_lock<mutex>& lock, const chrono::duration<Rep, Period>& rel_time);[…]
-?- Remarks: If the function fails to meet the postcondition,std::terminate()
shall be called (14.6.2 [except.terminate]). [Note: This can happen if the re-locking of the mutex throws an exception. — end note] -28- Postcondition:lock.owns_lock()
is true andlock.mutex()
is locked by the calling thread. […] -29- Throws:timeout-related exceptions (30.2.4).system_error
when an exception is required (30.2.2) or-30- Error conditions:
equivalent error condition fromlock.lock()
orlock.unlock()
.template <class Clock, class Duration, class Predicate> bool wait_until(unique_lock<mutex>& lock, const chrono::time_point<Clock, Duration>& abs_time, Predicate pred);[…]
-?- Remarks: If the function fails to meet the postcondition,std::terminate()
shall be called (14.6.2 [except.terminate]). [Note: This can happen if the re-locking of the mutex throws an exception. — end note] -33- Postcondition:lock.owns_lock()
is true andlock.mutex()
is locked by the calling thread. […] -35- Throws:timeout-related exceptions (30.2.4)system_error
when an exception is required (30.2.2),,or any exception thrown bypred
.-36- Error conditions:
equivalent error condition fromlock.lock()
orlock.unlock()
.template <class Rep, class Period, class Predicate> bool wait_for(unique_lock<mutex>& lock, const chrono::duration<Rep, Period>& rel_time, Predicate pred);[…]
-?- Remarks: If the function fails to meet the postcondition,std::terminate()
shall be called (14.6.2 [except.terminate]). [Note: This can happen if the re-locking of the mutex throws an exception. — end note] -40- Postcondition:lock.owns_lock()
is true andlock.mutex()
is locked by the calling thread. […] -42- Throws:timeout-related exceptions (30.2.4)system_error
when an exception is required (30.2.2),,or any exception thrown bypred
.-43- Error conditions:
equivalent error condition fromlock.lock()
orlock.unlock()
.
Edit 32.7.5 [thread.condition.condvarany] as indicated:
template<class Lock> void wait(Lock& lock);[…]
-10- Effects:
Atomically calls
lock.unlock()
and blocks on*this
.When unblocked, calls
lock.lock()
(possibly blocking on the lock) and returns.The function will unblock when signaled by a call to
notify_one()
, a call tonotify_all()
, or spuriously.
If the function exits via an exception,lock.lock()
shall be called prior to exiting the function scope.-?- Remarks: If the function fails to meet the postcondition,
-11- Postcondition:std::terminate()
shall be called (14.6.2 [except.terminate]). [Note: This can happen if the re-locking of the mutex throws an exception. — end note]lock
is locked by the calling thread. -12- Throws: Nothing.system_error
when an exception is required (30.2.2)-13- Error conditions:
equivalent error condition fromlock.lock()
orlock.unlock()
.template <class Lock, class Clock, class Duration> cv_status wait_until(Lock& lock, const chrono::time_point<Clock, Duration>& abs_time);[…]
-15- Effects:
[…]
If the function exits via an exception,
lock.lock()
shall be called prior to exiting the functionscope.-?- Remarks: If the function fails to meet the postcondition,
-16- Postcondition:std::terminate()
shall be called (14.6.2 [except.terminate]). [Note: This can happen if the re-locking of the mutex throws an exception. — end note]lock
is locked by the calling thread. […] -18- Throws:timeout-related exceptions (30.2.4).system_error
when an exception is required (30.2.2) or-19- Error conditions:
equivalent error condition fromlock.lock()
orlock.unlock()
.template <class Lock, class Rep, class Period> cv_status wait_for(Lock& lock, const chrono::duration<Rep, Period>& rel_time);[…]
-?- Remarks: If the function fails to meet the postcondition,std::terminate()
shall be called (14.6.2 [except.terminate]). [Note: This can happen if the re-locking of the mutex throws an exception. — end note] -22- Postcondition:lock
is locked by the calling thread. […] -23- Throws:timeout-related exceptions (30.2.4).system_error
when an exception is required (30.2.2) or-24- Error conditions:
equivalent error condition fromlock.lock()
orlock.unlock()
.
atomic_flag::clear
should not accept memory_order_consume
Section: 32.5.10 [atomics.flag] Status: C++14 Submitter: Ben Viglietta Opened: 2012-03-08 Last modified: 2016-01-28
Priority: Not Prioritized
View all other issues in [atomics.flag].
View all issues with C++14 status.
Discussion:
N3376 32.5.10 [atomics.flag]/7 says
this about atomic_flag::clear
:
Requires: The
order
argument shall not bememory_order_acquire
ormemory_order_acq_rel
.
In addition, memory_order_consume
should be disallowed, since it doesn't meaningfully apply to store operations.
It's already disallowed on the analogous atomic<T>::store
. The proposed updated text would be:
Requires: The
order
argument shall not bememory_order_consume
,memory_order_acquire
, ormemory_order_acq_rel
.
[2012, Portland: move to Review]
Hans: this is a clear oversight.
Moved to review
[2013-04-20, Bristol]
Accepted for the working paper
Proposed resolution:
[This wording is relative to N3376.]
void atomic_flag_clear(volatile atomic_flag *object) noexcept; void atomic_flag_clear(atomic_flag *object) noexcept; void atomic_flag_clear_explicit(volatile atomic_flag *object, memory_order order) noexcept; void atomic_flag_clear_explicit(atomic_flag *object, memory_order order) noexcept; void atomic_flag::clear(memory_order order = memory_order_seq_cst) volatile noexcept; void atomic_flag::clear(memory_order order = memory_order_seq_cst) noexcept;-7- Requires: The
-8- Effects: Atomically sets the value pointed to byorder
argument shall not bememory_order_consume
,memory_order_acquire
, ormemory_order_acq_rel
.object
or bythis
to false. Memory is affected according to the value oforder
.
Section: 16.4.5.2.1 [namespace.std], 19.5 [syserr], 20.2.8.1 [allocator.uses.trait], 22.10.15.2 [func.bind.isbind], 22.10.15.3 [func.bind.isplace], 22.10.19 [unord.hash], 21.3.8.7 [meta.trans.other], 28.3.3.1 [locale], 28.3.4.2.5 [locale.codecvt], 28.6.11.1.5 [re.regiter.incr] Status: C++20 Submitter: Loïc Joly Opened: 2012-03-08 Last modified: 2021-02-25
Priority: 4
View other active issues in [namespace.std].
View all other issues in [namespace.std].
View all issues with C++20 status.
Discussion:
The expression "user-defined type" is used in several places in the standard, but I'm not sure what it means. More specifically, is a type defined in the standard library a user-defined type?
From my understanding of English, it is not. From most of the uses of this term in the standard, it seem to be considered as user defined. In some places, I'm hesitant, e.g. 16.4.5.2.1 [namespace.std] p1:A program may add a template specialization for any standard library template to namespace
std
only if the declaration depends on a user-defined type and the specialization meets the standard library requirements for the original template and is not explicitly prohibited.
Does it mean we are allowed to add in the namespace std
a specialization for
std::vector<std::pair<T, U>>
, for instance?
[ 2012-10 Portland: Move to Deferred ]
The issue is real, in that we never define this term and rely on a "know it when I see it" intuition. However, there is a fear that any attempt to pin down a definition is more likely to introduce bugs than solve them - getting the wording for this precisely correct is likely far more work than we are able to give it.
There is unease at simple closing as NAD, but not real enthusiasm to provide wording either. Move to Deferred as we are not opposed to some motivated individual coming back with full wording to review, but do not want to go out of our way to encourage someone to work on this in preference to other issues.
[2014-02-20 Re-open Deferred issues as Priority 4]
[2015-03-05 Jonathan suggests wording]
I dislike the suggestion to change to "user-provided" type because I already find the difference between user-declared / user-provided confusing for special member functions, so I think it would be better to use a completely different term. The core language uses "user-defined conversion sequence" and "user-defined literal" and similar terms for things which the library provides, so I think we should not refer to "user" at all to distinguish entities defined outside the implementation from things provided by the implementation.
I propose "program-defined type" (and "program-defined specialization"), defined below. The P/R below demonstrates the scope of the changes required, even if this name isn't adopted. I haven't proposed a change for "User-defined facets" in [locale].[Lenexa 2015-05-06]
RS, HT: The core language uses "user-defined" in a specific way, including library things but excluding core language things, let's use a different term.
MC: Agree.
RS: "which" should be "that", x2
RS: Is std::vector<MyType> a "program-defined type"?
MC: I think it should be.
TK: std::vector<int> seems to take the same path.
JW: std::vector<MyType> isn't program-defined, we don't need it to be, anything that depends on that also depends on =MyType.
TK: The type defined by an "explicit template specialization" should be a program-defined type.
RS: An implicit instantiation of a "program-defined partial specialization" should also be a program-defined type.
JY: This definition formatting is horrible and ugly, can we do better?
RS: Checking ISO directives.
RS: Define "program-defined type" and "program-defined specialization" instead, to get rid of the angle brackets.
JW redrafting.
[2017-09-12]
Jonathan revises wording as per Lenexa discussion
Previous resolution [SUPERSEDED]:This wording is relative to N4296.
Add a new sub-clause to [definitions]:
17.3.? [defns.program.defined]
program-defined
<type> a class type or enumeration type which is not part of the C++ standard library and not defined by the implementation. [Note: Types defined by the implementation include extensions (4.1 [intro.compliance]) and internal types used by the library. — end note]program-defined
<specialization> an explicit template specialization or partial specialization which is not part of the C++ standard library and not defined by the implementation.Change 16.4.5.2.1 [namespace.std] paragraph 1+2:
-1- The behavior of a C++ program is undefined if it adds declarations or definitions to namespace
-2- The behavior of a C++ program is undefined if it declares […] A program may explicitly instantiate a template defined in the standard library only if the declaration depends on the name of astd
or to a namespace within namespacestd
unless otherwise specified. A program may add a template specialization for any standard library template to namespacestd
only if the declaration depends on auserprogram-defined type and the specialization meets the standard library requirements for the original template and is not explicitly prohibited.userprogram-defined type and the instantiation meets the standard library requirements for the original template.Change 19.5 [syserr] paragraph 4:
-4- The
is_error_code_enum
andis_error_condition_enum
may be specialized foruserprogram-defined types to indicate that such types are eligible for classerror_code
and classerror_condition
automatic conversions, respectively.Change 20.2.8.1 [allocator.uses.trait] paragraph 1:
-1- Remarks: automatically detects […]. A program may specialize this template to derive from
true_type
for auserprogram-defined typeT
that does not have a nestedallocator_type
but nonetheless can be constructed with an allocator where either: […]Change 22.10.15.2 [func.bind.isbind] paragraph 2:
-2- Instantiations of the
is_bind_expression
template […]. A program may specialize this template for auserprogram-defined typeT
to have aBaseCharacteristic
oftrue_type
to indicate thatT
should be treated as a subexpression in abind
call.Change 22.10.15.3 [func.bind.isplace] paragraph 2:
-2- Instantiations of the
is_placeholder
template […]. A program may specialize this template for auserprogram-defined typeT
to have aBaseCharacteristic
ofintegral_constant<int, N>
withN > 0
to indicate thatT
should be treated as a placeholder type.Change 22.10.19 [unord.hash] paragraph 1:
The unordered associative containers defined in 23.5 use specializations of the class template
hash
[…], the instantiationhash<Key>
shall:
[…]
[…]
[…]
[…]
satisfy the requirement that the expression
h(k)
, whereh
is an object of typehash<Key>
andk
is an object of typeKey
, shall not throw an exception unlesshash<Key>
is auserprogram-defined specialization that depends on at least oneuserprogram-defined type.Change 21.3.8.6 [meta.trans.ptr] Table 57 (
common_type
row):
Table 57 — Other transformations Template Condition Comments …
template <class... T>
struct common_type;The member typedef type
shall be
defined or omitted as specified below.
[…]. A program may
specialize this trait if at least one
template parameter in the
specialization is auserprogram-defined type.
[…]…
Change 28.3.4.2.5 [locale.codecvt] paragraph 3:
-3- The specializations required in Table 81 (22.3.1.1.1) […]. Other encodings can be converted by specializing on a
userprogram-definedstateT
type.[…]Change 28.6.11.1.5 [re.regiter.incr] paragraph 8:
-8- [Note: This means that a compiler may call an implementation-specific search function, in which case a
userprogram-defined specialization ofregex_search
will not be called. — end note]
[2018-3-14 Wednesday evening issues processing; move to Ready]
After this lands, we need to audit Annex C to find "user-defined type" Example: [diff.cpp03.containers]/3
[2018-06 Rapperswil: Adopted]
Proposed resolution:
This wording is relative to N4687.
Add a new sub-clause to [definitions]:
20.3.? [defns.program.defined.spec]
program-defined specialization
explicit template specialization or partial specialization that is not part of the C++ standard library and not defined by the implementation20.3.? [defns.program.defined.type]
program-defined type
class type or enumeration type that is not part of the C++ standard library and not defined by the implementation, or an instantiation of a program-defined specialization[Drafting note: ISO directives say the following Note should be labelled as a "Note to entry" but the C++ WP doesn't follow that rule (yet). — end drafting note]
[Note: Types defined by the implementation include extensions (4.1 [intro.compliance]) and internal types used by the library. — end note]
Change 16.4.5.2.1 [namespace.std] paragraph 1+2:
-1- The behavior of a C++ program is undefined if it adds declarations or definitions to namespace std
or to a
namespace within namespace std
unless otherwise specified. A program may add a template specialization
for any standard library template to namespace std
only if the declaration depends on a
userprogram-defined type and the specialization meets the standard library requirements for the
original template and is not explicitly prohibited.
Change 19.5 [syserr] paragraph 4:
-4- The is_error_code_enum
and is_error_condition_enum
may be specialized for
userprogram-defined types to indicate that such types are eligible for class error_code
and class error_condition
automatic conversions, respectively.
Change 20.2.8.1 [allocator.uses.trait] paragraph 1:
-1- Remarks: automatically detects […]. A program may specialize this template to derive from
true_type
for a userprogram-defined type T
that does not have a nested
allocator_type
but nonetheless can be constructed with an allocator where either: […]
Change 22.10.15.2 [func.bind.isbind] paragraph 2:
-2- Instantiations of the is_bind_expression
template […]. A program may specialize
this template for a userprogram-defined type T
to have a BaseCharacteristic
of true_type
to indicate that T
should be treated as a subexpression in a bind
call.
Change 22.10.15.3 [func.bind.isplace] paragraph 2:
-2- Instantiations of the is_placeholder
template […]. A program may specialize this template for a
userprogram-defined type T
to have a BaseCharacteristic
of
integral_constant<int, N>
with N > 0
to indicate that T
should be
treated as a placeholder type.
Change 22.10.19 [unord.hash] paragraph 1:
The unordered associative containers defined in 23.5 use specializations of the class template hash
[…],
the instantiation hash<Key>
shall:
[…]
[…]
[…]
[…]
satisfy the requirement that the expression h(k)
, where h
is an object of type
hash<Key>
and k
is an object of type Key
, shall not throw an exception unless
hash<Key>
is a userprogram-defined specialization that depends on at least one
userprogram-defined type.
Change 21.3.8.6 [meta.trans.ptr] Table 57 (common_type
row):
Table 57 — Other transformations Template Condition Comments …
template <class... T>
struct common_type;The member typedef type
shall be
defined or omitted as specified below.
[…]. A program may
specialize this trait if at least one
template parameter in the
specialization is auserprogram-defined type.
[…]…
Change 28.3.4.2.5 [locale.codecvt] paragraph 3:
-3- The specializations required in Table 81 (22.3.1.1.1) […]. Other encodings can be converted
by specializing on a userprogram-defined stateT
type.[…]
Change 28.6.11.1.5 [re.regiter.incr] paragraph 8:
-8- [Note: This means that a compiler may call an implementation-specific search function, in which case
a userprogram-defined specialization of regex_search
will not be called. —
end note]
notify_all_at_thread_exit
synchronization requirement?Section: 32.7 [thread.condition] Status: C++14 Submitter: Pete Becker Opened: 2012-03-06 Last modified: 2016-01-28
Priority: Not Prioritized
View all other issues in [thread.condition].
View all issues with C++14 status.
Discussion:
notify_all_at_thread_exit
has the following synchronization requirement:
Synchronization: The call to
notify_all_at_thread_exit
and the completion of the destructors for all the current thread's variables of thread storage duration synchronize with (6.9.2 [intro.multithread]) calls to functions waiting oncond
.
The functions waiting on cond
have already been called, otherwise they wouldn't be waiting. So how can a subsequent
call to notify_all_at_thread_exit
synchronize with them?
[2012-03-09 Jeffrey Yasskin comments:]
I think the text should say that "notify_all_at_thread_exit
and destructor calls are sequenced before
the lk.unlock()
", and leave it at that, unless there's a funny implementation I haven't thought of.
[2012-03-19 Hans Boehm comments:]
I think the synchronization clause should just be replaced with (modulo wording tweaks):
"The impliedlk.unlock()
call is sequenced after the destruction of all objects with thread storage duration
associated with the current thread."
as Jeffrey suggested.
To use this correctly, the notifying thread has to essentially acquire the lock, set a variable indicating it's done,
call notify_all_at_thread_exit()
, while the waiting thread acquires the lock, and repeatedly waits on the
cv until the variable is set, and then releases the lock. That ensures that we have the proper synchronizes with
relationship as a result of the lock.
[2012, Portland: move to Review]
The lk.unlock()
refers back to the wording the previous paragraph.
Moved to review
[2013-04-20, Bristol]
Accepted for the working paper
Proposed resolution:
This wording is relative to N3376.
Modify 32.7 [thread.condition] p8 as indicated:
void notify_all_at_thread_exit(condition_variable& cond, unique_lock<mutex> lk);[…]
-8- Synchronization:The call toThe impliednotify_all_at_thread_exit
and the completion of the destructors for all the current thread's variables of thread storage duration synchronize with (6.9.2 [intro.multithread]) calls to functions waiting oncond
lk.unlock()
call is sequenced after the destruction of all objects with thread storage duration associated with the current thread.
common_type
trait produces reference typesSection: 21.3.8.7 [meta.trans.other] Status: C++14 Submitter: Doug Gregor Opened: 2012-03-11 Last modified: 2016-01-28
Priority: Not Prioritized
View all other issues in [meta.trans.other].
View all issues with C++14 status.
Discussion:
The type computation of the common_type
type trait is defined as
template <class T, class U> struct common_type<T, U> { typedef decltype(true ? declval<T>() : declval<U>()) type; };
This means that common_type<int, int>::type
is int&&
, because
declval<int>()
returns int&&
decltype
returns T&&
when its expression is an xvalue (9.2.9.3 [dcl.type.simple] p4)
Users of common_type
do not expect to get a reference type as the result; the expectation is that
common_type
will return a non-reference type to which all of the types can be converted.
std::unique_ptr
's
operator<
in 20.3.1.6 [unique.ptr.special] (around p4) is also broken: In the most typical case
(with default deleter), the determination of the common pointer type CT will instantiate
std::less<CT>
which can now be std::less<T*&&>
, which will
not be the specialization of pointer types that guarantess a total order.
Given the historic constext of common_type
original specification, the proper resolution to me
seems to be using std::decay
instead of std::remove_reference
:
template <class T, class U> struct common_type<T, U> { typedef typename decay<decltype(true ? declval<T>() : declval<U>())>::type type; };
At that time rvalues had no identity in this construct and rvalues of non-class types have no cv-qualification. With this change we would ensure that
common_type<int, int>::type == common_type<const int, const int>::type == int
Note that this harmonizes with the corresponding heterogenous case, which has already the exact same effect:
common_type<int, long>::type == common_type<const int, const long>::type == long
[2012-10-11 Daniel comments]
While testing the effects of applying the proposed resolution I noticed that this will have the effect that the unary
form of common_type
, like
common_type<int>
is not symmetric to the n-ary form (n > 1). This is unfortunate, because this difference comes especially to effect when
common_type
is used with variadic templates. As an example consider the following make_array
template:
#include <array>
#include <type_traits>
#include <utility>
template<class... Args>
std::array<typename std::common_type<Args...>::type, sizeof...(Args)>
make_array(Args&&... args)
{
typedef typename std::common_type<Args...>::type CT;
return std::array<CT, sizeof...(Args)>{static_cast<CT>(std::forward<Args>(args))...};
}
int main()
{
auto a1 = make_array(0); // OK: std::array<int, 1>
auto a2 = make_array(0, 1.2); // OK: std::array<double, 2>
auto a3 = make_array(5, true, 3.1415f, 'c'); // OK: std::array<float, 4>
int i = 0;
auto a1b = make_array(i); // Error, attempt to form std::array<int&, 1>
auto a2b = make_array(i, 1.2); // OK: std::array<double, 2>
auto a2c = make_array(i, 0); // OK: std::array<int, 2>
}
The error for a1b
only happens in the unary case and it is easy that it remains unnoticed
during tests. You cannot explain that reasonably to the user here.
std::decay
to the result of the
std::common_type
deduction. But if this is necessary here, I wonder why it should also be applied to
the binary case, where it gives the wrong illusion of a complete type decay? The other way around: Why is
std::decay
not also applied to the unary case as well?
This problem is not completely new and was already observed for the original std::common_type
specification.
At this time the decltype
rules had a similar asymmetric effect when comparing
std::common_type<const int, const int>::type
(equal to 'int
' at this time)
with:
std::common_type<const int>::type
(equal to 'const int
')
and I wondered whether the unary form shouldn't also perform the same "decay" as the n-ary form.
This problem makes me think that the current resolution proposal might not be ideal and I expect differences in implementations (for those who consider to apply this proposed resolution already). I see at least three reasonable options:Accept the current wording suggestion for LWG 2141 as it is and explain that to users.
Keep std::common_type
as currently specified in the Standard and tell users to use
std::decay
where needed. Also fix other places in the library, e.g. the comparison
functions of std::unique_ptr
or a most of the time library functions.
Apply std::decay
also in the unary specialization of std::common_type
with
the effect that std::common_type<const int&>::type
returns int
.
[2012-10-11 Marc Glisse comments]
If we are going with decay everywhere, I wonder whether we should also decay in the 2-argument version before
and not only after. So if I specialize common_type<mytype, double>
,
common_type<const mytype, volatile double&>
would automatically work.
[2012-10-11 Daniel provides wording for bullet 3 of his list:]
Change 21.3.8.7 [meta.trans.other] p3 as indicated:
template <class T> struct common_type<T> { typedef typename decay<T>::type type; }; template <class T, class U> struct common_type<T, U> { typedef typename decay<decltype(true ? declval<T>() : declval<U>())>::type type; };
[2013-03-15 Issues Teleconference]
Moved to Review.
Want to carefully consider the effect of decay
vs. remove_reference
with respect
to constness before adopting, although this proposed resolution stands for review in Bristol.
[2013-04-18, Bristol meeting]
Previous wording:
This wording is relative to N3376.
In 21.3.8.7 [meta.trans.other] p3, change the
common_type
definition totemplate <class T, class U> struct common_type<T, U> { typedef typename decay<decltype(true ? declval<T>() : declval<U>())>::type type; };
[2013-04-18, Bristol]
Move to Ready
[2013-09-29, Chicago]
Accepted for the working paper
Proposed resolution:
This wording is relative to N3485.
Change 21.3.8.7 [meta.trans.other] p3 as indicated:
template <class T> struct common_type<T> { typedef typename decay<T>::type type; }; template <class T, class U> struct common_type<T, U> { typedef typename decay<decltype(true ? declval<T>() : declval<U>())>::type type; };
packaged_task::operator()
synchronization too broad?Section: 32.10.10.2 [futures.task.members] Status: C++14 Submitter: Pete Becker Opened: 2012-03-12 Last modified: 2016-01-28
Priority: Not Prioritized
View all other issues in [futures.task.members].
View all issues with C++14 status.
Discussion:
According to 32.10.10.2 [futures.task.members] p.18:
[A] successful call to [
packaged_task::
]operator()
synchronizes with a call to any member function of afuture
orshared_future
object that shares the shared state of*this
.
This requires that the call to operator()
synchronizes with calls to future::wait_for
,
future::wait_until
, shared_future::wait_for
, and shared_future::wait_until
,
even when these functions return because of a timeout.
[2012, Portland: move to Open]
If it said "a successful return from" (or "a return from" to cover exceptions) the problem would be more obvious.
Detlef: will ask Anthony Williams to draft some wording.
Moved to open (Anthony drafted to draft)
[2013-09, Chicago: move to Ready]
Anthony's conclusion is that the offending paragraph is not needed. Already included in the statement that the state is made ready.
Recommendation: Remove 32.10.10.2 [futures.task.members] p18 (the synchronization clause). Redundant because of 32.10.5 [futures.state] p9.Moved to Ready
Proposed resolution:
This wording is relative to N3691.
Remove 32.10.10.2 [futures.task.members] p18 as indicated:
void operator()(ArgTypes... args);[…]
-18- Synchronization: a successful call tooperator()
synchronizes with (1.10) a call to any member function of afuture
orshared_future
object that shares the shared state of*this
. The completion of the invocation of the stored task and the storage of the result (whether normal or exceptional) into the shared state synchronizes with (1.10) the successful return from any member function that detects that the state is set to ready. [Note:operator()
synchronizes and serializes with other functions through the shared state. — end note]
ios_base::xalloc
should be thread-safeSection: 31.5.2 [ios.base] Status: C++14 Submitter: Alberto Ganesh Barbati Opened: 2012-03-14 Last modified: 2016-01-28
Priority: Not Prioritized
View all other issues in [ios.base].
View all issues with C++14 status.
Discussion:
The static function ios_base::xalloc()
could be called from multiple threads and is not covered by
16.4.5.10 [res.on.objects] and 16.4.6.10 [res.on.data.races]. Adding a thread-safety requirement
should not impose a significant burden on implementations, as the function can be easily implemented with
hopefully lock-free atomics.
[2013-04-20, Bristol]
Unanimous.
Resolution: move tentatively ready. (Inform Bill about this issue.)[2013-09-29, Chicago]
Apply to Working Paper
Proposed resolution:
This wording is relative to N3376.
In 31.5.2.6 [ios.base.storage] add a new paragraph after paragraph 1:
static int xalloc();-1- Returns:
-?- Remarks: Concurrent access to this function by multiple threads shall not result in a data race (6.9.2 [intro.multithread]).index ++
.
noexcept
specification in type_index
Section: 17.7.7 [type.index] Status: C++14 Submitter: Daniel Krügler Opened: 2012-03-18 Last modified: 2016-01-28
Priority: Not Prioritized
View all other issues in [type.index].
View all issues with C++14 status.
Discussion:
The class type type_index
is a thin wrapper of type_info
to
adapt it as a valid associative container element. Similar to type_info
,
all member functions have an effective noexcept(true)
specification, with the
exception of hash_code()
and name()
. The actual effects of these
functions is a direct call to type_info
's hash_code()
and name
function, but according to 17.7 [support.rtti] these are both noexcept
functions, so there is no reason for not declaring them as noexcept
, too. In fact,
one of the suggested changes of the original proposing paper
N2530
specifically was to ensure that type_info
would get a hash_code()
function that guarantees not to throw exceptions (during that time the hash
requirements did not allow to exit with an exception). From this we can conclude that
type_index::hash_code()
was intended to be nothrow.
noexcept
.
[2013-03-15 Issues Teleconference]
Moved to Tentatively Ready.
[2013-04-20 Bristol]
Proposed resolution:
This wording is relative to N3376.
Modify the class type_index
synopsis, [type.index.overview] as indicated:
namespace std { class type_index { public: type_index(const type_info& rhs) noexcept; bool operator==(const type_index& rhs) const noexcept; bool operator!=(const type_index& rhs) const noexcept; bool operator< (const type_index& rhs) const noexcept; bool operator<= (const type_index& rhs) const noexcept; bool operator> (const type_index& rhs) const noexcept; bool operator>= (const type_index& rhs) const noexcept; size_t hash_code() const noexcept; const char* name() const noexcept; private: const type_info* target; // exposition only // Note that the use of a pointer here, rather than a reference, // means that the default copy/move constructor and assignment // operators will be provided and work as expected. }; }
Modify the prototype definitions in [type.index.members] as indicated:
size_t hash_code() const noexcept;-8- Returns:
target->hash_code()
const char* name() const noexcept;-9- Returns:
target->name()
error_category
default constructorSection: 19.5.3 [syserr.errcat] Status: C++14 Submitter: Howard Hinnant Opened: 2012-03-21 Last modified: 2016-01-28
Priority: Not Prioritized
View all other issues in [syserr.errcat].
View all issues with C++14 status.
Discussion:
Should error_category
have a default constructor?
Classes may be derived from
error_category
to support categories of errors in addition to those defined in this International Standard.
How shall classes derived from error_category
construct their base?
error_category
was default-constructible. That is still the case in
N2241, because no other
constructor is declared. Then later N2422
(issue 6) declares
the copy constructor as deleted, but doesn't add a default constructor, causing it to be no longer
default-constructible. That looks like an oversight to me, and I think there should be a public default
constructor.
Daniel: A default-constructor indeed should be provided to allow user-derived classes as described by the
standard. I suggest this one to be both noexcept
and constexpr
. The latter allows
user-derived non-abstract classes to take advantage of the special constant initialization rule
of [basic.start.init] p2 b2 for objects with static (or thread) storage duration in namespace
scope. Note that a constexpr
constructor is feasible here, even though there exists a non-trivial
destructor and even though error_category
is not a literal type (see std::mutex
for a similar
design choice).
In addition to that the proposed resolution fixes another minor glitch: According to 16.3.3.5 [functions.within.classes]
virtual destructors require a semantics description.
Alberto Ganesh Barbati: I would suggest to remove =default
from the constructor instead.
Please consider that defaulting a constructor or destructor may actually define them as deleted under certain
conditions (see 11.4.5 [class.ctor]/5 and 11.4.7 [class.dtor]/5). Removing =default
is easier than providing wording to ensures that such conditions do not occur.
[2012-10 Portland: move to Ready]
The issue is real and the resolution looks good.
Are there similar issues elsewhere in this clause?
Potential to add constexpr
to more constructors, but clearly a separable issue.
[2013-04-20 Bristol]
Proposed resolution:
This wording is relative to N3376.
Modify the class error_category
synopsis, 19.5.3.1 [syserr.errcat.overview] as indicated:
[Drafting note: According to the general
noexcept
library guidelines
destructors should not have any explicit exception specification. This destructor was overlooked during the paper
analysis — end note]
namespace std { class error_category { public: constexpr error_category() noexcept; virtual ~error_category()noexcept; error_category(const error_category&) = delete; error_category& operator=(const error_category&) = delete; virtual const char* name() const noexcept = 0; virtual error_condition default_error_condition(int ev) const noexcept; virtual bool equivalent(int code, const error_condition& condition) const noexcept; virtual bool equivalent(const error_code& code, int condition) const noexcept; virtual string message(int ev) const = 0; bool operator==(const error_category& rhs) const noexcept; bool operator!=(const error_category& rhs) const noexcept; bool operator<(const error_category& rhs) const noexcept; }; }
Before 19.5.3.2 [syserr.errcat.virtuals] p1 insert a new prototype description as indicated:
virtual ~error_category();-?- Effects: Destroys an object of class
error_category
.
Before 19.5.3.3 [syserr.errcat.nonvirtuals] p1 insert a new prototype description as indicated:
constexpr error_category() noexcept;-?- Effects: Constructs an object of class
error_category
.
Allocator
's allocate
functionSection: 16.4.4.6 [allocator.requirements] Status: C++14 Submitter: Daniel Krügler Opened: 2012-03-05 Last modified: 2016-01-28
Priority: Not Prioritized
View other active issues in [allocator.requirements].
View all other issues in [allocator.requirements].
View all issues with C++14 status.
Discussion:
According to Table 28 — "Allocator requirements", the expression
a.allocate(n, u)
expects as second argument a value u
that is described in Table 27 as:
a value of type
YY::const_pointer
obtained by callingYY::allocate
, or elsenullptr
.
This description leaves it open, whether or whether not a value of type YY::const_void_pointer
is
valid or not. The corresponding wording in C++03 is nearly the same, but in C++03 there did not exist the concept of
a general void_pointer
for allocators. There is some evidence for support of void pointers because
the general allocator_traits
template declares
static pointer allocate(Alloc& a, size_type n, const_void_pointer hint);
and the corresponding function for std::allocator<T>
is declared as:
pointer allocate(size_type, allocator<void>::const_pointer hint = 0);
As an additional minor wording glitch (especially when comparing with the NullablePointer
requirements imposed on
const_pointer
and const_void_pointer
), the wording seems to exclude lvalues of type
std::nullptr_t
, which looks like an unwanted artifact to me.
[ 2012-10 Portland: Move to Ready ]
No strong feeling that this is a big issue, but consensus that the proposed resolution is strictly better than the current wording, so move to Ready.
[2013-04-20 Bristol]
Proposed resolution:
This wording is relative to N3376.
Change Table 27 — "Descriptive variable definitions" in 16.4.4.6 [allocator.requirements]:
Variable | Definition |
---|---|
u
|
a value of type YY::const_pointer obtained by calling YY::allocate , or else
nullptr XX::const_void_pointer obtained by conversion from a result
value of YY::allocate , or else a value of type (possibly const) std::nullptr_t .
|
std::hash
Section: 22.10.19 [unord.hash] Status: C++14 Submitter: Ville Voutilainen Opened: 2012-04-10 Last modified: 2016-08-03
Priority: Not Prioritized
View all other issues in [unord.hash].
View all issues with C++14 status.
Discussion:
The paper proposes various hashing improvements. What it doesn't mention is hashing of enums; enums are integral types, and users expect them to have built-in hashing support, rather than having to convert enums to ints for uses with unordered containers and other uses of hashes. Daniel Krügler explains in c++std-lib-32412 that this is not achievable with a SFINAEd hash specialization because it would require a partial specialization with a type parameter and a non-type parameter with a default argument, which is currently not allowed, and hence the fixes in N3333 should be adopted instead.
[2012-10 Portland: Move to Open]
We agree this is a real issue that should be resolved, by specifying such a hash.
It is not clear that we should specify this as calling hash on the underlying_type
,
or whether that is overspecification and we merely require that the hash be supplied.
STL already has shipped an implementation, and is keen to provide wording.
[ 2013-04-14 STL provides rationale and improved wording ]
Rationale:
This can be achieved by inserting a very small tweak to the Standardese. We merely have to require that hash<Key>
be valid when Key
is an "enumeration type" (which includes both scoped and unscoped enums). This permits, but does
not require, hash<Enum>
to behave identically to hash<underlying_type<Enum>::type>
, following
existing precedent — note that when unsigned int
and unsigned long
are the same size,
hash<unsigned int>
is permitted-but-not-required to behave identically to hash<unsigned long>
.
static_assert
nicely, explode horribly at compiletime or runtime, etc.
While we're in the neighborhood, this proposed resolution contains an editorial fix. The 22.10 [function.objects]
synopsis says "base template", which doesn't appear anywhere else in the Standard, and could confuse users into
thinking that they need to derive from it. The proper phrase is "primary template".
[2013-04-18, Bristol]
Proposed resolution:
This wording is relative to N3485.
In 22.10 [function.objects], header functional synopsis, edit as indicated:
namespace std { […] // 20.8.12, hash functionbaseprimary template: template <class T> struct hash; […] }
In 22.10.19 [unord.hash]/1 edit as indicated:
-1- The unordered associative containers defined in 23.5 [unord] use specializations of the class template
hash
as the defaulthash
function. For all object typesKey
for which there exists a specializationhash<Key>
, and for all enumeration types (9.8.1 [dcl.enum]) Key, the instantiationhash<Key>
shall: […]
Section: 22.10 [function.objects] Status: C++14 Submitter: Scott Meyers Opened: 2012-02-15 Last modified: 2016-01-28
Priority: Not Prioritized
View all other issues in [function.objects].
View all issues with C++14 status.
Discussion:
22.10 [function.objects] p5 says:
To enable adaptors and other components to manipulate function objects that take one or two arguments it is required that the function objects correspondingly provide typedefs
argument_type
andresult_type
for function objects that take one argument andfirst_argument_type
,second_argument_type
, andresult_type
for function objects that take two arguments.
I have two concerns about this paragraph. First, the wording appears to prescribe a requirement for all
function objects in valid C++ programs, but it seems unlikely that that is the intent. As such, the scope
of the requirement is unclear. For example, there is no mention of these typedefs in the specification for
closures (5.1.2), and Daniel Krügler has explained in the thread at
http://tinyurl.com/856plkn that conforming implementations can
detect the difference between closures with and without these typedefs. (Neither gcc 4.6 nor VC10 appear
to define typedefs such as result_type
for closure types. I have not tested other compilers.)
std::bind
, as Howard Hinnant explains in the thread at http://tinyurl.com/6q5bos4.
From what I can tell, the standard already defines which adaptability typedefs must be provided by various
kinds of function objects in the specifications for those objects. Examples include the function objects
specified in 22.10.6 [refwrap]- [negators]. I therefore suggest that
22.10 [function.objects]/5 simply be removed from the standard. I don't think it adds anything
except opportunities for confusion.
[2012-10 Portland: Move to Open]
This wording caused confusion earlier in the week when reviewing Stefan's paper on greater<>
.
This phrasing sounds normative, but is actually descriptive but uses unfortunate wording.
The main reason this wording exists is to document the protocol required to support the legacy binders in Annex D.
Stefan points out that unary_negate
and binary_negate
have not been deprecated and rely
on this. He plans a paper to remove this dependency.
Consensus that this wording is inadequate, confusing, and probably should be removed. However, that leaves a big hole in the specification for the legacy binders, that needs filling.
While not opposed to striking this paragraph, we will need the additional wording to fix the openning hole before this issue can move forward.
[ 2013-04-14 STL provides rationale ]
Rationale:
I've concluded that Scott's original proposed resolution was correct and complete. There are two sides to this story: the producers and the consumers of these typedefs.
Producers: As Scott noted, the Standard clearly documents which function objects must provide these
typedefs. Some function objects must provide them unconditionally (e.g. plus<T>
(for T != void
),
22.10.7 [arithmetic.operations]/1), some conditionally (e.g. reference_wrapper<T>
,
22.10.6 [refwrap]/2-4), and some don't have to provide them at all (e.g. lambdas, 7.5.6 [expr.prim.lambda]).
These requirements are clear, so we shouldn't change them or even add informative notes. Furthermore, because these
typedefs aren't needed in the C++11 world with decltype
/perfect forwarding/etc., we shouldn't add more
requirements to provide them.
Consumers: This is what we were concerned about at Portland. However, the consumers also clearly document
their requirements in the existing text. For example, reference_wrapper<T>
is also a conditional consumer,
and 22.10.6 [refwrap] explains what typedefs it's looking for. We were especially concerned about the old negators
and the deprecated binders, but they're okay too. [negators] clearly says that
unary_negate<Predicate>
requires Predicate::argument_type
to be a type, and
binary_negate<Predicate>
requires Predicate::first_argument_type
and Predicate::second_argument_type
to be types. (unary_negate
/binary_negate
provide result_type
but they don't consume it.)
99 [depr.lib.binders] behaves the same way with Fn::first_argument_type
, Fn::second_argument_type
,
and Fn::result_type
. No additional wording is necessary.
A careful reading of 22.10 [function.objects]/5 reveals that it wasn't talking about anything beyond the mere
existence of the mentioned typedefs — for example, it didn't mention that the function object's return type should be
result_type
, or even convertible to result_type
. As the producers and consumers are certainly talking about
the existence of the typedefs (in addition to clearly implying semantic requirements), we lose nothing by deleting the
unnecessary paragraph.
[2013-04-18, Bristol]
Previous wording:
Remove 22.10 [function.objects] p5:
To enable adaptors and other components to manipulate function objects that take one or two arguments it is required that the function objects correspondingly provide typedefsargument_type
andresult_type
for function objects that take one argument andfirst_argument_type
,second_argument_type
, andresult_type
for function objects that take two arguments.
Proposed resolution:
This wording is relative to N3485.
Edit 22.10 [function.objects] p5:
[Note:To enable adaptors and other components to manipulate function objects that take one or two arguments
it is required that the function objectsmany of the function objects in this clause correspondingly provide typedefsargument_type
andresult_type
for function objects that take one argument andfirst_argument_type
,second_argument_type
, andresult_type
for function objects that take two arguments.— end note]
find_end
Section: 26.6.8 [alg.find.end] Status: C++14 Submitter: Andrew Koenig Opened: 2012-03-28 Last modified: 2016-01-28
Priority: Not Prioritized
View all issues with C++14 status.
Discussion:
26.6.8 [alg.find.end] describes the behavior of find_end as returning:
The last iterator
i
in the range[first1,last1 - (last2 - first2))
such that for any nonnegative integern < (last2 - first2)
, the following corresponding conditions hold:*(i + n) == *(first2 + n), pred(*(i + n), *(first2 + n)) != false
.
Does "for any" here mean "for every" or "there exists a"? I think it means the former, but it could be interpreted either way.
Daniel: The same problem exists for the following specifications from Clause 26 [algorithms]:[2013-04-20, Bristol]
Unanimous agreement on the wording.
Resolution: move to tentatively ready[2013-09-29, Chicago]
Apply to Working Paper
Proposed resolution:
This wording is relative to N3376.
Change 26.6.8 [alg.find.end] p2 as indicated:
template<class ForwardIterator1, class ForwardIterator2> ForwardIterator1 find_end(ForwardIterator1 first1, ForwardIterator1 last1, ForwardIterator2 first2, ForwardIterator2 last2); template<class ForwardIterator1, class ForwardIterator2, class BinaryPredicate> ForwardIterator1 find_end(ForwardIterator1 first1, ForwardIterator1 last1, ForwardIterator2 first2, ForwardIterator2 last2, BinaryPredicate pred);[…]
-2- Returns: The last iteratori
in the range[first1,last1 - (last2 - first2))
such that foranyevery nonnegative integern < (last2 - first2)
, the following corresponding conditions hold:*(i + n) == *(first2 + n), pred(*(i + n), *(first2 + n)) != false
. Returnslast1
if[first2,last2)
is empty or if no such iterator is found.
Change 26.6.15 [alg.search] p2 and p6 as indicated:
template<class ForwardIterator1, class ForwardIterator2> ForwardIterator1 search(ForwardIterator1 first1, ForwardIterator1 last1, ForwardIterator2 first2, ForwardIterator2 last2); template<class ForwardIterator1, class ForwardIterator2, class BinaryPredicate> ForwardIterator1 search(ForwardIterator1 first1, ForwardIterator1 last1, ForwardIterator2 first2, ForwardIterator2 last2, BinaryPredicate pred);[…]
-2- Returns: The first iteratori
in the range[first1,last1 - (last2-first2))
such that foranyevery nonnegative integern
less thanlast2 - first2
the following corresponding conditions hold:*(i + n) == *(first2 + n), pred(*(i + n), *(first2 + n)) != false
. Returnsfirst1
if[first2,last2)
is empty, otherwise returnslast1
if no such iterator is found.
[…]
template<class ForwardIterator, class Size, class T> ForwardIterator search_n(ForwardIterator first, ForwardIterator last, Size count, const T& value); template<class ForwardIterator, class Size, class T, class BinaryPredicate> ForwardIterator search_n(ForwardIterator first, ForwardIterator last, Size count, const T& value, BinaryPredicate pred);[…]
-6- Returns: The first iteratori
in the range[first,last-count)
such that foranyevery non-negative integern
less thancount
the following corresponding conditions hold:*(i + n) == value, pred(*(i + n),value) != false
. Returnslast
if no such iterator is found.
Change 26.7.10 [alg.reverse] p4 as indicated:
template<class BidirectionalIterator, class OutputIterator> OutputIterator reverse_copy(BidirectionalIterator first, BidirectionalIterator last, OutputIterator result);[…]
-4- Effects: Copies the range[first,last)
to the range[result,result+(last-first))
such that foranyevery non-negative integeri < (last - first)
the following assignment takes place:*(result + (last - first) - i) = *(first + i)
.
Change 26.8.5 [alg.partitions] p5 and p9 as indicated:
template<class ForwardIterator, class Predicate> ForwardIterator partition(ForwardIterator first, ForwardIterator last, Predicate pred);[…]
-5- Returns: An iteratori
such that foranyevery iteratorj
in the range[first,i) pred(*j) != false
, and foranyevery iteratork
in the range[i,last), pred(*k) == false
.
[…]
template<class BidirectionalIterator, class Predicate> BidirectionalIterator stable_partition(BidirectionalIterator first, BidirectionalIterator last, Predicate pred);[…]
-9- Returns: An iteratori
such that foranyevery iteratorj
in the range[first,i), pred(*j) != false
, and foranyevery iteratork
in the range[i,last), pred(*k) == false
. The relative order of the elements in both groups is preserved.
Change 26.8 [alg.sorting] p5 as indicated:
-5- A sequence is sorted with respect to a comparator
comp
if foranyevery iteratori
pointing to the sequence andanyevery non-negative integern
such thati + n
is a valid iterator pointing to an element of the sequence,comp(*(i + n), *i) == false
.
Change 26.8.3 [alg.nth.element] p1 as indicated:
template<class RandomAccessIterator> void nth_element(RandomAccessIterator first, RandomAccessIterator nth, RandomAccessIterator last); template<class RandomAccessIterator, class Compare> void nth_element(RandomAccessIterator first, RandomAccessIterator nth, RandomAccessIterator last, Compare comp);-1- After
nth_element
the element in the position pointed to bynth
is the element that would be in that position if the whole range were sorted. Also foranyevery iteratori
in the range[first,nth)
andanyevery iteratorj
in the range[nth,last)
it holds that:!(*i > *j)
orcomp(*j, *i) == false
.
Change 26.8.4.2 [lower.bound] p2 as indicated:
template<lass ForwardIterator, class T> ForwardIterator lower_bound(ForwardIterator first, ForwardIterator last, const T& value); template<class ForwardIterator, class T, class Compare> ForwardIterator lower_bound(ForwardIterator first, ForwardIterator last, const T& value, Compare comp);[…]
-2- Returns: The furthermost iteratori
in the range[first,last]
such that foranyevery iteratorj
in the range[first,i)
the following corresponding conditions hold:*j < value
orcomp(*j, value) != false
.
Change 26.8.4.3 [upper.bound] p2 as indicated:
template<lass ForwardIterator, class T> ForwardIterator upper_bound(ForwardIterator first, ForwardIterator last, const T& value); template<class ForwardIterator, class T, class Compare> ForwardIterator upper_bound(ForwardIterator first, ForwardIterator last, const T& value, Compare comp);[…]
-2- Returns: The furthermost iteratori
in the range[first,last]
such that foranyevery iteratorj
in the range[first,i)
the following corresponding conditions hold:!(value < *j)
orcomp(value, *j) == false
.
Change 26.8.9 [alg.min.max] p21 and p23 as indicated:
template<class ForwardIterator> ForwardIterator min_element(ForwardIterator first, ForwardIterator last); template<class ForwardIterator, class Compare> ForwardIterator min_element(ForwardIterator first, ForwardIterator last, Compare comp);-21- Returns: The first iterator
i
in the range[first,last)
such that foranyevery iteratorj
in the range[first,last)
the following corresponding conditions hold:!(*j < *i)
orcomp(*j, *i) == false
. Returnslast
iffirst == last
.
[…]
template<class ForwardIterator> ForwardIterator max_element(ForwardIterator first, ForwardIterator last); template<class ForwardIterator, class Compare> ForwardIterator max_element(ForwardIterator first, ForwardIterator last, Compare comp);-23- Returns: The first iterator
i
in the range[first,last)
such that foranyevery iteratorj
in the range[first,last)
the following corresponding conditions hold:!(*i < *j)
orcomp(*i, *j) == false
. Returnslast
iffirst == last
.
basic_string<>::swap
semantics ignore allocatorsSection: 27.4.3.2 [string.require] Status: Resolved Submitter: Robert Shearer Opened: 2012-04-13 Last modified: 2018-11-25
Priority: 3
View all other issues in [string.require].
View all issues with Resolved status.
Discussion:
In C++11, basic_string
is not described as a "container", and is not governed by the allocator-aware
container semantics described in sub-clause 23.2 [container.requirements]; as a result, and
requirements or contracts for the basic_string
interface must be documented in Clause
27 [strings].
swap
member function with no requirements, and
with guarantees to execute in constant time without throwing. Fulfilling such a contract is not reasonable
in the presence of unequal non-propagating allocators.
In contrast, 23.2.2 [container.requirements.general] p7 declares the behavior of member swap
for containers with unequal non-propagating allocators to be undefined.
Resolution proposal:
Additional language from Clause 23 [containers] should probably be copied to Clause
27 [strings]. I will refrain from an exactly recommendation, however, as I am raising further
issues related to the language in Clause 23 [containers].
[2013-03-15 Issues Teleconference]
Moved to Open.
Alisdair has offered to provide wording.
Telecon notes that 23.2.2 [container.requirements.general]p13 says that string
is an
allocator-aware container.
Resolved by the adoption of P1148 in San Diego.
Proposed resolution:
Section: 29.5.3.3 [rand.req.urng] Status: Resolved Submitter: John Salmon Opened: 2012-04-26 Last modified: 2020-10-06
Priority: 4
View all other issues in [rand.req.urng].
View all issues with Resolved status.
Discussion:
The expressions G::min()
and G::max()
in Table 116 in 29.5.3.3 [rand.req.urng] are specified
as having "compile-time" complexity.
static int min();
then is the method required to have a constexpr
qualifier? I believe the standard would benefit from
clarification of this point.
[2018-12-08; Tim Song comments]
This issue was resolved by P0898R3 and the subsequent editorial rewrite of this subclause.
[2020-10-06; Reflector discussions]
Resolved by P0898R3 voted in Rapperswil.
Rationale:
Resolved by P0898R3.
Proposed resolution:
__bool_true_false_are_defined
should be removedSection: 17.14 [support.runtime] Status: Resolved Submitter: Thomas Plum Opened: 2012-04-30 Last modified: 2020-11-09
Priority: 4
View all other issues in [support.runtime].
View all issues with Resolved status.
Discussion:
Since C99, the C standard describes a macro named __bool_true_false_are_defined
.
#if !defined(__bool_true_false_are_defined) #define bool int /* or whatever */ #define true 1 #define false 0 #endif
Then when the project was compiled by a "new" compiler that implemented bool
as defined by the
evolving C++98 or C99 standards, those lines would be skipped; but when compiled by an "old" compiler that
didn't yet provide bool
, true
, and false
, then the #define
's would provide a
simulation that worked for most purposes.
Headers
<csetjmp>
(nonlocal jumps),<csignal>
(signal handling),<cstdalign>
(alignment),<cstdarg>
(variable arguments),<cstdbool>
(__bool_true_false_are_defined
).<cstdlib>
(runtime environmentgetenv()
,system()
), and<ctime>
(system clockclock()
,time()
) provide further compatibility with C code.
However, para 2 says
"The contents of these headers are the same as the Standard C library headers
<setjmp.h>
,<signal.h>
,<stdalign.h>
,<stdarg.h>
,<stdbool.h>
,<stdlib.h>
, and<time.h>
, respectively, with the following changes:",
and para 8 says
"The header
<cstdbool>
and the header<stdbool.h>
shall not define macros namedbool
,true
, orfalse
."
Thus para 8 doesn't exempt the C++ implementation from the arguably clear requirement of the C standard, to
provide a macro named __bool_true_false_are_defined
defined to be 1.
__bool_true_false_are_defined
. In that case, the name belongs to implementers to provide, or not, as
they choose.
[2013-03-15 Issues Teleconference]
Moved to Open.
While not strictly necessary, the clean-up look good.
We would like to hear from our C liaison before moving on this issue though.
[2015-05 Lenexa]
LWG agrees. Jonathan provides wording.
[2017-03-04, Kona]
The reference to <cstdbool> in p2 needs to be struck as well. Continue the discussion on the reflector once the DIS is available.
Previous resolution [SUPERSEDED]:
This wording is relative to N4296.
Edit the footnote on 16.4.2.3 [headers] p7:
176) In particular, including any of the standard headers
<stdbool.h>
,<cstdbool>
,<iso646.h>
or<ciso646>
has no effect.Edit 17.14 [support.runtime] p1 as indicated (and remove the index entry for
__bool_true_false_are_defined
):-1- Headers
<csetjmp>
(nonlocal jumps),<csignal>
(signal handling),<cstdalign>
(alignment),<cstdarg>
(variable arguments),<cstdbool>
,(__bool_true_false_are_defined
).<cstdlib>
(runtime environmentgetenv()
,system()
), and<ctime>
(system clockclock()
,time()
) provide further compatibility with C code.Remove Table 38 — Header
<cstdbool>
synopsis [tab:support.hdr.cstdbool] from 17.14 [support.runtime]
Table 38 — Header <cstdbool>
synopsisType Name(s) Macro: __bool_true_false_are_defined
[2019-03-18; Daniel comments and eliminates previously proposed wording]
With the approval of P0619R4 in Rapperswil, the offending wording has now been eliminated from the working draft. I suggest to close this issue as: Resolved by P0619R4.
[2020-11-09 Resolved for C++20. Status changed: Tentatively Resolved → Resolved.]
Rationale:
Resolved by P0619R4.Proposed resolution:
reserve(n)
reserves for n-1
elementsSection: 23.2.8 [unord.req] Status: C++17 Submitter: Daniel James Opened: 2012-05-07 Last modified: 2017-07-30
Priority: 3
View other active issues in [unord.req].
View all other issues in [unord.req].
View all issues with C++17 status.
Discussion:
I think that unordered containers' reserve
doesn't quite do what it should. I'd expect after calling
x.reserve(n)
to be able to insert n
elements without invalidating iterators. But as
the standard is written (I'm looking at n3376), I think the guarantee only holds for n-1
elements.
max_load_factor
of 1
, reserve(n)
is equivalent to
rehash(ceil(n/1))
, ie. rehash(n)
. rehash(n)
requires that the bucket
count is >= n
, so it can be n
(Table 103). The rule is that insert
shall not affect the validity of iterators if (N + n) < z * B
(23.2.8 [unord.req] p15).
But for this case the two sides of the equation are equal, so insert
can affect the validity of iterators.
[2013-03-16 Howard comments and provides wording]
Given the following:
LF := load_factor() MLF := max_load_factor() S := size() B := bucket_count() LF == S/B
The container has an invariant:
LF <= MLF
Therefore:
MLF >= S/B S <= MLF * B B >= S/MLF
[2013-03-15 Issues Teleconference]
Moved to Open.
Howard to provide rationale and potentally revised wording.
[2012-02-12 Issaquah : recategorize as P3]
Jonathan Wakely: submitter is Boost.Hash maintainer. Think it's right.
Marshall Clow: even if wrong it's more right than what we have now
Geoffrey Romer: issue is saying rehash should not leave container in such a state that a notional insertion of zero elements should not trigger a rehash
AJM: e.g. if you do a range insert from an empty range
AJM: we don't have enough brainpower to do this now, so not priority zero
Recategorised as P3
[Lenexa 2015-05-06: Move to Ready]
Proposed resolution:
This wording is relative to N3485.
In 23.2.8 [unord.req] Table 103 — Unordered associative container requirements, change the post-condition
in the row for a.rehash(n)
to:
Post:a.bucket_count() >= a.size() / a.max_load_factor()
anda.bucket_count() >= n
.
In 23.2.8 [unord.req]/p15 change
Theinsert
andemplace
members shall not affect the validity of iterators if(N+n) <= z * B
, whereN
is the number of elements in the container prior to the insert operation,n
is the number of elements inserted,B
is the container's bucket count, andz
is the container's maximum load factor.
atomic_flag
initializationSection: 32.5.10 [atomics.flag] Status: C++14 Submitter: Alberto Ganesh Barbati Opened: 2012-05-24 Last modified: 2016-01-28
Priority: Not Prioritized
View all other issues in [atomics.flag].
View all issues with C++14 status.
Discussion:
32.5.10 [atomics.flag]/4 describes the ATOMIC_FLAG_INIT
, but it's not quite clear about a
couple of points:
it's said that ATOMIC_FLAG_INIT
"can be used to initialize an object of type atomic_flag
"
and the following example:
std::atomic_flag guard = ATOMIC_FLAG_INIT;
is presented. It's not clear whether the macro can also be used in the other initialization contexts:
std::atomic_flag guard ATOMIC_FLAG_INIT; std::atomic_flag guard {ATOMIC_FLAG_INIT}; struct A { std::atomic_flag flag; A(); }; A::A() : flag (ATOMIC_FLAG_INIT); A::A() : flag {ATOMIC_FLAG_INIT};
Please also note that examples are non-normative, according to the ISO directives, meaning that the wording presents no normative way to use the macro.
it's said that "It is unspecified whether an uninitialized atomic_flag
object has an initial state
of set or clear.". I believe the use of "uninitialized" is inappropriate. First of all, if an object is
uninitialized it is obvious that we cannot assert anything about its state. Secondly, it doesn't address the
following cases:
std::atomic_flag a; // object is "initialized" by trivial default constructor std::atomic_flag a {}; // object is value-initialized static std::atomic_flag a; // object is zero-initialized
strictly speaking a trivial constructor "initializes" the object, although it doesn't actually initialize the sub-objects.
it's said that "For a static-duration object, that initialization shall be static.". Considering the following example:
struct A { A(); // user-provided, not constexpr std::atomic_flag flag = ATOMIC_FLAG_INIT; // possibly other non-static data members }; static A a;
The object a.flag
(as a sub-object of the object a
) has static-duration, yet the initialization
has to be dynamic because A::A
is not constexpr
.
[2012, Portland]
We would like to be able to allow more initialisation contexts for example:
However we need further input from experts with implementation specific knowledge to identify which additional contexts (if any) would be universally valid.
Moved to open
[2013, Chicago]
Move to Immediate, following review.
Some discussion over the explicit use of only copy initialization, and not direct initialization. This is necessary to
allow the implementation of atomic_flag
as an aggregate, and may be further reviewed in the future.
Accept for Working Paper
Proposed resolution:
[This wording is relative to N3376.]
Change 32.5.10 [atomics.flag]/4 as follows:
The macro
ATOMIC_FLAG_INIT
shall be defined in such a way that it can be used to initialize an object of typeatomic_flag
to the clear state. The macro can be used in the form:atomic_flag guard = ATOMIC_FLAG_INIT;It is unspecified whether the macro can be used in other initialization contexts. For a complete static-duration object, that initialization shall be static.
It is unspecified whether an uninitializedUnless initialized withATOMIC_FLAG_INIT
, it is unspecified whether anatomic_flag
object has an initial state of set or clear.[ Example:atomic_flag guard = ATOMIC_FLAG_INIT;
— end example ]
resize
Section: 23.3.13.3 [vector.capacity] Status: C++17 Submitter: Daniel Krügler Opened: 2012-06-07 Last modified: 2017-07-30
Priority: 1
View other active issues in [vector.capacity].
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View all issues with C++17 status.
Discussion:
As part of resolving LWG issue 2033(i) a wording change was done for resize()
to
respect the problem mentioned in the question:
Does a call to 'void resize(size_type sz)' of
std::vector
require the element type to beMoveAssignable
because the callerase(begin() + sz, end())
mentioned in the Effects paragraph would require the element type to beMoveAssignable
?
The wording change was to replace in 23.3.5.3 [deque.capacity] and 23.3.13.3 [vector.capacity]:
-1- Effects: If
sz <= size()
, equivalent toerase(begin() + sz, end())
; […]
by:
-1- Effects: If
sz <= size()
, equivalent to callingpop_back() size() - sz
times. […]
The overlooked side-effect of this wording change is that this implies a destruction order
of the removed elements to be in reverse order of construction, but the previous version
did not impose any specific destruction order due to the way how the semantics of erase
is specified in Table 100.
#include <vector>
#include <iostream>
struct Probe {
int value;
Probe() : value(0) {}
Probe(int value) : value(value) {}
~Probe() { std::cout << "~Probe() of " << value << std::endl; }
};
int main() {
std::vector<Probe> v;
v.push_back(Probe(1));
v.push_back(Probe(2));
v.push_back(Probe(3));
std::cout << "---" << std::endl;
v.resize(0);
}
the last three lines of the output for every compiler I tested was:
~Probe() of 1 ~Probe() of 2 ~Probe() of 3
but a conforming implementation would now need to change this order to
~Probe() of 3 ~Probe() of 2 ~Probe() of 1
This possible stringent interpretation makes sense, because one can argue that sequence containers
(or at least std::vector
) should have the same required destruction order of it's elements,
as elements of a C array or controlled by memory deallocated with an array delete
have.
I also learned that libc++ does indeed implement std::vector::resize
in a way that the
second output form is observed.
std::vector
this was not required in C++03 and this request may be too strong. My current
suggestion would be to restore the effects of the previous wording in regard to the destruction order,
because otherwise several currently existing implementations would be broken just because of this
additional requirement.
[2013-03-15 Issues Teleconference]
Moved to Open.
Jonathan says that he believes this is a valid issue.
Walter wonders if this was intended when we made the previous change - if so, this would be NAD.
Jonathan said that Issue 2033(i) doesn't mention ordering.
Walter then asked if anyone is really unhappy that we're destroying items in reverse order of construction.
Jonathan points out that this conflicts with existing practice (libstc++, but not libc++).
Jonathan asked for clarification as to whether this change was intended by 2033(i).
[2014-06 Rapperswil]
Daniel points out that the ordering change was not intended.
General agreement that implementations should not be required to change.[2014-06-28 Daniel provides alternative wording]
[Urbana 2014-11-07: Move to Ready]
Proposed resolution:
This wording is relative to N3936.
Change 23.3.5.3 [deque.capacity] as indicated: [Drafting note: The chosen wording form is similar to that for
forward_list
. Note that the existing Requires element already specifies the necessary operational requirements
on the value type. — end drafting note]
void resize(size_type sz);-1- Effects: If
[…]sz <
, erases the last=size()size() - sz
elements from the sequenceequivalent to calling. Otherwisepop_back() size() - sz
timesIf, appendssize() <= sz
sz - size()
default-inserted elements to the sequence.void resize(size_type sz, const T& c);-3- Effects: If
[…]sz <
, erases the last=size()size() - sz
elements from the sequenceequivalent to calling. Otherwisepop_back() size() - sz
timesIf, appendssize() < sz
sz - size()
copies ofc
to the sequence.
Change 23.3.13.3 [vector.capacity] as indicated: [Drafting note: See deque
for the rationale of the
used wording. — end drafting note]
void resize(size_type sz);-12- Effects: If
[…]sz <
, erases the last=size()size() - sz
elements from the sequenceequivalent to calling. Otherwisepop_back() size() - sz
timesIf, appendssize() < sz
sz - size()
default-inserted elements to the sequence.void resize(size_type sz, const T& c);-15- Effects: If
[…]sz <
, erases the last=size()size() - sz
elements from the sequenceequivalent to calling. Otherwisepop_back() size() - sz
timesIf, appendssize() < sz
sz - size()
copies ofc
to the sequence.
allocator_traits::max_size
missing noexcept
Section: 16.4.4.6 [allocator.requirements], 20.2.9.3 [allocator.traits.members], 20.2.9 [allocator.traits] Status: C++14 Submitter: Bo Persson Opened: 2012-07-03 Last modified: 2016-01-28
Priority: Not Prioritized
View other active issues in [allocator.requirements].
View all other issues in [allocator.requirements].
View all issues with C++14 status.
Discussion:
N3376 describes in 20.2.9.3 [allocator.traits.members]/7
static size_type max_size(Alloc& a);Returns:
a.max_size()
if that expression is well-formed; otherwise,numeric_limits<size_type>::max()
.
The max_size
function is supposed to call one of two functions that are both noexcept
.
To make this intermediate function useful for containers, it should preserve the noexcept
attribute.
static size_type max_size(Alloc& a) noexcept;
[2012-08-05 Daniel comments]
On the first sight this does not seem like a defect of the specification, because the Allocator requirements in
16.4.4.6 [allocator.requirements] (Table 28) do not impose a no-throw requirement onto max_size()
;
the table just describes the fall-back implementation for max_size()
if a given allocator does
not provide such a function.
std::allocator
as a special model of this concept and is allowed to increase the exception-guarantees
for max_size()
, but this does not imply a corresponding rules for other allocators.
Furthermore, max_size()
of Containers is not specified in terms of
Allocator::max_size()
, so again this is not a real contradiction.
Nonetheless I think that the following stronger decision should be considered:
Require that for all Allocators (as specified in 16.4.4.6 [allocator.requirements]) max_size()
never throws an exception. This would it make much more useful to call this function in situations where no
exception should leave the context.
Require that for all Allocators (as specified in 16.4.4.6 [allocator.requirements]) max_size()
can be called on const allocator object. Together with the previous item this would allow an implementation
of a container's max_size()
function to delegate to the allocator's max_size()
function.
In regard to the second statement it should be mentioned that there are two current specification deviations from that in the draft:
The synopsis of 20.2.9 [allocator.traits] uses a const allocator argument as part of the signature
of the max_size
function.
Both the synopsis of 20.6.1 [allocator.adaptor.syn] and the member specification in
20.6.4 [allocator.adaptor.members] p8 declare scoped_allocator_adaptor::max_size
as const member function, but this function delegates to
allocator_traits<OuterAlloc>::max_size(outer_allocator())
where outer_allocator()
resolves to the member function overload returning a
const outer_allocator_type&
.
The question arises whether these current defects actually point to a defect in the Allocator requirements and should be fixed there.
[ 2012-10 Portland: Move to Review ]
Consensus that the change seems reasonable, and that for any given type the template is intantiated with the contract should be 'wide' so this meets the guidelines we agreed in Madrid for C++11.
Some mild concern that while we don't imagine many allocator implementations throwing on this method,
it is technically permited by current code that we would not be breaking, by turning throw
expressions into disguised terminate
calls. In this case, an example might be an
instrumented 'logging' allocator that writes every function call to a log file or database, and might
throw if that connection/file were no longer available.
Another option would be to make exception spefication a conditional no-except, much like we do for
some swap
functions and assignment operators. However, this goes against the intent of the
Madrid adoption of noexcept
which is that vendors are free to add such extensions, but we
look for a clear line in the library specification, and do not want to introduce conditional-noexcept
piecemeal. A change in our conventions here would require a paper addressing the library specification
as a whole.
Consensus was to move forward, but move the issue only to Review rather than Ready to allow time for further comments. This issue should be considered 'Ready' next time it is reviewed unless we get such comments in the meantime.
[2013-04-18, Bristol]
Proposed resolution:
In 20.2.9 [allocator.traits] and 20.2.9.3 [allocator.traits.members]/7, change the function signature to
static size_type max_size(Alloc& a) noexcept;
nth_element
requires inconsistent post-conditionsSection: 26.8.3 [alg.nth.element] Status: C++14 Submitter: Peter Sommerlad Opened: 2012-07-06 Last modified: 2016-01-28
Priority: Not Prioritized
View all other issues in [alg.nth.element].
View all issues with C++14 status.
Discussion:
The specification of nth_element refers to operator<
whereas all sorting without a compare function is based on
operator<
. While it is true that for all regular types both operators should be defined accordingly, all other
sorting algorithms only rely on existence of operator<
. So I guess the paragraph p1
After
nth_element
the element in the position pointed to bynth
is the element that would be in that position if the whole range were sorted. Also for any iteratori
in the range[first,nth)
and any iteratorj
in the range[nth,last)
it holds that:!(*i > *j)
orcomp(*j, *i) == false
.
should read
After
nth_element
the element in the position pointed to bynth
is the element that would be in that position if the whole range were sorted. Also for any iteratori
in the range[first,nth)
and any iteratorj
in the range[nth,last)
it holds that:!(*j < *i)
orcomp(*j, *i) == false
.
Note only "!(*i > *j)
" was changed to "!(*j < *i)
" and it would be more symmetric with
comp(*j, *i)
as well.
[ 2012-10 Portland: Move to Ready ]
This is clearly correct by inspection, moved to Ready by unanimous consent.
[2013-04-20 Bristol]
Proposed resolution:
This wording is relative to N3376.
Change 26.8.3 [alg.nth.element] p1 as indicated:
template<class RandomAccessIterator> void nth_element(RandomAccessIterator first, RandomAccessIterator nth, RandomAccessIterator last); template<class RandomAccessIterator, class Compare> void nth_element(RandomAccessIterator first, RandomAccessIterator nth, RandomAccessIterator last, Compare comp);-1- After
nth_element
the element in the position pointed to bynth
is the element that would be in that position if the whole range were sorted. Also for any iteratori
in the range[first,nth)
and any iteratorj
in the range[nth,last)
it holds that:or
!(*i > *j)!(*j < *i)comp(*j, *i) == false
.
vector.emplace(vector.begin(), vector.back())
?Section: 23.3.13.5 [vector.modifiers], 23.2 [container.requirements] Status: C++20 Submitter: Howard Hinnant Opened: 2012-07-07 Last modified: 2021-02-25
Priority: 2
View all other issues in [vector.modifiers].
View all issues with C++20 status.
Discussion:
Nikolay Ivchenkov recently brought the following example on the std-discussion newsgroup, asking whether the following program well-defined:
#include <iostream> #include <vector> int main() { std::vector<int> v; v.reserve(4); v = { 1, 2, 3 }; v.emplace(v.begin(), v.back()); for (int x : v) std::cout << x << std::endl; }
Nikolay Ivchenkov:
I think that an implementation ofvector
's 'emplace' should initialize an intermediate object with
v.back()
before any shifts take place, then perform all necessary shifts and finally replace the
value pointed to by v.begin()
with the value of the intermediate object. So, I would expect the
following output:
3 1 2 3
GNU C++ 4.7.1 and GNU C++ 4.8.0 produce other results:
2 1 2 3
Howard Hinnant:
I believe Nikolay is correct that vector should initialize an intermediate object withv.back()
before any shifts take place. As Nikolay pointed out in another email, this appears to be the only way to
satisfy the strong exception guarantee when an exception is not thrown by T
's copy constructor,
move constructor, copy assignment operator, or move assignment operator as specified by
23.3.13.5 [vector.modifiers]/p1. I.e. if the emplace construction throws, the vector must remain unaltered.
That leads to an implementation that tolerates objects bound to the function parameter pack of the emplace
member function may be elements or sub-objects of elements of the container.
My position is that the standard is correct as written, but needs a clarification in this area. Self-referencing
emplace
should be legal and give the result Nikolay expects. The proposed resolution of LWG 760(i)
is not correct.
[2015-02 Cologne]
LWG agrees with the analysis including the assessment of LWG 760(i) and would appreciate a concrete wording proposal.
[2015-04-07 dyp comments]
The Standard currently does not require that creation of such intermediate objects is legal. 23.2.4 [sequence.reqmts] Table 100 — "Sequence container requirements" currently specifies:
Table 100 — Sequence container requirements Expression Return type Assertion/note
pre-/post-condition…
a.emplace(p, args);
iterator
Requires: T
isEmplaceConstructible
intoX
fromargs
. Forvector
anddeque
,T
is alsoMoveInsertable
intoX
andMoveAssignable
. […]…
The EmplaceConstructible
concept is defined via
allocator_traits<A>::construct
in 23.2.2 [container.requirements.general] p15.5 That's surprising to me
since the related concepts use the suffix Insertable
if they
refer to the allocator. An additional requirement such as
std::is_constructible<T, Args...>
is necessary to allow
creation of intermediate objects.
The creation of intermediate objects also affects other functions, such
as vector.insert
. Since aliasing the vector is only allowed for
the single-element forms of insert
and emplace
(see
526(i)), the range-forms are not affected. Similarly,
aliasing is not allowed for the rvalue-reference overload. See also LWG
2266(i).
There might be a problem with a requirement of
std::is_constructible<T, Args...>
related to the issues
described in LWG 2461(i). For example, a scoped allocator
adapter passes additional arguments to the constructor of the value
type. This is currently not done in recent implementations of libstdc++
and libc++ when creating the intermediate objects, they simply create
the intermediate object by perfectly forwarding the arguments. If such
an intermediate object is then moved to its final destination in the
vector, a change of the allocator instance might be required —
potentially leading to an expensive copy. One can also imagine worse
problems, such as run-time errors (allocators not comparing equal at
run-time) or compile-time errors (if the value type cannot be created
without the additional arguments). I have not looked in detail into this
issue, but I'd be reluctant adding a requirement such as
std::is_constructible<T, Args...>
without further
investigation.
It should be noted that the creation of intermediate objects currently
is inconsistent in libstdc++ vs libc++. For example, libstdc++ creates
an intermediate object for vector.insert
, but not
vector.emplace
, whereas libc++ does the exact opposite in this
respect.
A live demo of the inconsistent creation of intermediate objects can be found here.
[2015-10, Kona Saturday afternoon]
HH: If it were easy, it'd have wording. Over the decades I have flipped 180 degrees on this. My current position is that it should work even if the element is in the same container.
TK: What's the implentation status? JW: Broken in GCC. STL: Broken in MSVS. Users complain about this every year.
MC: 526 says push_back has to work.
HH: I think you have to make a copy of the element anyway for reasons of exception safety. [Discussion of exception guarantees]
STL: vector has strong exception guarantees. Could we not just provide the Basic guarantee here.
HH: It would terrify me to relax that guarantee. It'd be an ugly, imperceptible runtime error.
HH: I agree if we had a clean slate that strong exception safety is costing us here, and we shouldn't provide it if it costs us.
STL: I have a mail here, "how can vector provide the strong guarantee when inserting in the middle".
HH: The crucial point is that you only get the strong guarantee if the exception is thrown by something other than the copy and move operations that are used to make the hole.
STL: I think we need to clean up the wording. But it does mandate currently that the self-emplacement must work, because nothings says that you can't do it. TK clarifies that a) self-emplacement must work, and b) you get the strong guarantee only if the operations for making the hole don't throw, otherwise basic. HH agrees. STL wants this to be clear in the Standard.
STL: Should it work for deque, too? HH: Yes.
HH: I will attempt wording for this.
TK: Maybe mail this to the reflector, and maybe someone has a good idea?
JW: I will definitely not come up with anything better, but I can critique wording.
Moved to Open; Howard to provide wording, with feedback from Jonathan.
[2017-01-25, Howard suggests wording]
[2018-1-26 issues processing telecon]
Status to 'Tentatively Ready' after adding a period to Howard's wording.
[2018-3-17 Adopted in Jacksonville]
Proposed resolution:
This wording is relative to N4713.
Modify in 23.2.4 [sequence.reqmts] Table 87 — "Sequence container requirements" as indicated:
Table 87 — Sequence container requirements (in addition to container) Expression Return type Assertion/note
pre-/post-condition[…]
a.emplace(p, args)
iterator
Requires: T
isEmplaceConstructible
intoX
fromargs
. Forvector
anddeque
,T
is also
MoveInsertable
intoX
andMoveAssignable
.
Effects: Inserts an object of typeT
constructed with
std::forward<Args>(args)...
beforep
.
[Note:args
may directly or indirectly refer to a value
ina
. — end note]
std::atomic<X>
requires X
to be nothrow default constructibleSection: 32.5.8 [atomics.types.generic], 32.5.8.2 [atomics.types.operations] Status: Resolved Submitter: Jonathan Wakely Opened: 2012-07-19 Last modified: 2016-01-28
Priority: 4
View all other issues in [atomics.types.generic].
View all issues with Resolved status.
Discussion:
As raised in c++std-lib-32781, this fails to compile even though the default constructor is not used:
#include <atomic> struct X { X() noexcept(false) {} X(int) { } }; std::atomic<X> x(3);
This is because atomic<T>
's default constructor is declared to be non-throwing and
is explicitly-defaulted on its first declaration:
atomic() noexcept = default;
This is ill-formed if the implicitly-declared default constructor would not be non-throwing.
Possible solutions:atomic<T>
atomic<T>::atomic()
from the overload set if T
is not nothrow default constructible.
noexcept
from atomic<T>::atomic()
, allowing it to be
deduced (but the default constructor is intended to be always noexcept)
atomic<T>::atomic()
on its first declaration (but makes the default constructor
user-provided and so prevents atomic<T>
being trivial)
[2012, Portland: move to Core]
Recommend referring to core to see if the constructor noexcept
mismatch
can be resolved there. The issue is not specific to concurrency.
[2015-04-09 Daniel comments]
CWG issue 1778, which had been created in behalf of this LWG issue, has been resolved as a defect.
[2015-10, Kona]
Mark as resolved
Proposed resolution:
Section: 26.8.8 [alg.heap.operations] Status: C++17 Submitter: Peter Sommerlad Opened: 2012-07-09 Last modified: 2017-07-30
Priority: 3
View all other issues in [alg.heap.operations].
View all issues with C++17 status.
Discussion:
Another similar issue to the operator<
vs greater in nth_element
but not as direct occurs
in 26.8.8 [alg.heap.operations]:
-1- A heap is a particular organization of elements in a range between two random access iterators
[a,b)
. Its two key properties are:
- There is no element greater than
*a
in the range and*a
may be removed bypop_heap()
, or a new element added bypush_heap()
, in O(log(N
)) time.
As noted by Richard Smith, it seems that the first bullet should read:
*a
is not less than any element in the range
Even better the heap condition could be stated here directly, instead of leaving it unspecified, i.e.,
Each element at
(a+2*i+1)
and(a+2*i+2)
is less than the element at(a+i)
, if those elements exist, fori>=0
.
But may be that was may be intentional to allow other heap organizations?
See also follow-up discussion of c++std-lib-32780.[2016-08 Chicago]
Walter provided wording
Tues PM: Alisdair & Billy(MS) to improve the wording.
[2016-08-02 Chicago LWG]
Walter provides initial Proposed Resolution. Alisdair objects to perceived complexity of the mathematical phrasing.
Previous resolution [SUPERSEDED]:
[Note to editor: As a drive-by editorial adjustment, please replace the current enumerated list format by the numbered bullet items shown below.]
Change [alg.heap.operations]:
1 A heap is a particular organization of elements in a range between two random access iterators [a, b)
. Its two key properties aresuch that:(1.1) --
There is no element greater than*a
in the range and
For alli >= 0
,
comp(a[i], a[L])
is false whenever L = 2*i+1 < b-a,
and
comp(a[i], a[R])
is false whenever R = 2*i+2 < b-a.
(1.2) --
*a
may be removed bypop_heap()
, or a new element added bypush_heap()
, in O(log(N)) time.
[2016-08-03 Chicago LWG]
Walter and Billy O'Neal provide revised Proposed Resolution, superseding yesterday's.
Thurs PM: Moved to Tentatively Ready
Proposed resolution:
This wording is relative to N4606.
Change 26.8.8 [alg.heap.operations] as indicated:
Note to project editor: As a drive-by editorial adjustment, please replace the current enumerated list format by numbered bullet items.
-1- A heap is a particular organization of elements in a range between two random access iterators
[a, b)
. Its two key properties aresuch that:
(1.1) —
There is no element greater thanWith , for all , ,*a
in the range andcomp(a[
], a[])
isfalse
.[Note to the project editor: In LaTeX the above insertion should be expressed as follows:
With $N =b
-a
$, for all $i$, $0 < i < N$,comp(a[$\left \lfloor{\frac{i-1}{2}}\right \rfloor$], a[$i$])
isfalse
.](1.2) —
*a
may be removed bypop_heap()
, or a new element added bypush_heap()
, in 𝒪(log(N)) time.
uniform_real_distribution
constructorSection: 29.5.9.2.2 [rand.dist.uni.real] Status: C++17 Submitter: Marshall Clow Opened: 2012-07-14 Last modified: 2017-07-30
Priority: 3
View all issues with C++17 status.
Discussion:
uniform_real says in 29.5.9.2.2 [rand.dist.uni.real] p1:
A
uniform_real_distribution
random number distribution produces random numbersx
,a ≤ x < b
,
but also that (29.5.9.2.2 [rand.dist.uni.real] p2):
explicit uniform_real_distribution(RealType a = 0.0, RealType b = 1.0);-2- Requires:
a ≤ b
andb - a ≤ numeric_limits<RealType>::max()
.
If you construct a uniform_real_distribution<RealType>(a, b)
where there are no representable
numbers between 'a' and 'b' (using RealType
's representation) then you cannot satisfy
29.5.9.2.2 [rand.dist.uni.real].
a == b
.
[2014-11-04 Urbana]
Jonathan provides wording.
[2014-11-08 Urbana]
Moved to Ready with the note.
There remains concern that the constructors are permitting values that may (or may not) be strictly outside the domain of the function, but that is a concern that affects the design of the random number facility as a whole, and should be addressed by a paper reviewing and addressing the whole clause, not picked up in the issues list one distribution at a time. It is still not clear that such a paper would be uncontroversial.
Proposed resolution:
This wording is relative to N4140.
Add a note after paragraph 1 before the synopsis in 29.5.9.2.2 [rand.dist.uni.real]:
-1- A
uniform_real_distribution
random number distribution produces random numbers , , distributed according to the constant probability density function.
[Note: This implies that is undefined when
a == b
. — end note]
Drafting note: should be in math font, and
a == b
should be in code font.
reset()
requirements in unique_ptr
specializationSection: 20.3.1.4.5 [unique.ptr.runtime.modifiers] Status: C++14 Submitter: Geoffrey Romer Opened: 2012-07-16 Last modified: 2016-01-28
Priority: Not Prioritized
View all other issues in [unique.ptr.runtime.modifiers].
View all issues with C++14 status.
Discussion:
In 20.3.1.4.5 [unique.ptr.runtime.modifiers]/p1-2 of N3376, the description of reset()
in the
array specialization of unique_ptr
partially duplicates the description of the base template method
(as specified in 20.3.1.3.6 [unique.ptr.single.modifiers]/p3-5), but lacks some significant requirements.
Specifically, the text introduced in LWG 998(i), and item 13 of LWG 762(i), is present
only in the base template, not the specialization.
reset()
operation order addressed by LWG 998(i)
applies just as much to the derived template as to the base template, and the derived template has just as
much need to rely on get_deleter()(get())
being well-defined, well-formed, and not throwing exceptions
(arguably some of those properties follow from the fact that T
is required to be a complete type, but
not all).
Assuming the derived template's reset()
semantics are intended to be identical to the base template's,
there is no need to explicitly specify the semantics of reset(pointer p)
at all (since
20.3.1.4 [unique.ptr.runtime]/3 specifies "Descriptions are provided below only for member functions that
have behavior different from the primary template."), and reset(nullptr_t p)
can be specified by
reference to the 'pointer' overload. This is more concise, and eliminates any ambiguity about intentional vs.
accidental discrepancies.
[2012-10 Portland: Move to Ready]
This resolution looks blatantly wrong, as it seems to do nothing but defer to primary template
where we should describe the contract here.
Ongoing discussion points out that the primary template has a far more carefully worded semantic
for reset(p)
that we would want to copy here.
STL points out that we need the nullptr
overload for this dynamic-array form, as there is
a deleted member function template that exists to steal overloads of pointer-to-derived, avoiding
undifined behavior, so we need the extra overload.
Finally notice that there is blanket wording further up the clause saying we describe only changes from the primary template, so the proposed wording is in fact exactly correct. Move to Ready.
[2013-04-20 Bristol]
Proposed resolution:
This wording is relative to N3376.
Change 20.3.1.4.5 [unique.ptr.runtime.modifiers] as indicated:
void reset(pointer p = pointer()) noexcept;void reset(nullptr_t p) noexcept;-1- Effects:
IfEquivalent toget() == nullptr
there are no effects. Otherwiseget_deleter()(get())
reset(pointer())
.-2- Postcondition:get() == p
.
DefaultConstructible
Section: 16.4.4.2 [utility.arg.requirements] Status: C++17 Submitter: Daniel Krügler Opened: 2012-07-19 Last modified: 2017-07-30
Priority: 2
View all other issues in [utility.arg.requirements].
View all issues with C++17 status.
Discussion:
The lack of the definition of the DefaultConstructible
requirements in C++03 was fixed
by LWG 724(i) at a time where the core rules of list-initialization were slightly
different than today, at that time value-initialization (shortly) was the primary rule for
class types, i.e. just before applying CWG 1301,
CWG 1324, and
CWG 1368.
DefaultConstructible
requirements anymore, because we require that
T u{};
value-initializes the object u
.
[ 2012-10 Portland: Move to Core ]
We are not qualified to pick apart the Core rules quickly at this point, but the consensus is that if the core language has changed in this manner, then the fix should similarly be applied in Core - this is not something that we want users of the language to have to say every time they want to Value initialize (or aggregate initialize) an object.
More to Open until we get a clear response from Core, Alisdair to file an issue with Mike.
[2013-04 Bristol: Back to Library]
The Core Working group opened, discussed, and resolved CWG 1578 as NAD for this library-related problem: Empty aggregate initialization and value-initialization are different core language concepts, and this difference can be observed (e.g. for a type with a deleted default-constructor).
[2014-02-15 Issaquah: Move to Ready]
AM: core says still LWG issue, wording has been non-controversial, move to ready?
NJ: what about durations? think they are ok
Ville: pair
and a few other have value initialize
AM: look at core 1578
AM: value initialize would require ()
, remove braces from third row?
STL: no
PH: core has new issue on aggregates and non-aggregates.
AM: right, they said does not affect this issue
NJ: why ok with pair
and tuple
?
STL: will use ()
, tuple
of aggregates with deleted constructor is ill-formed
Ville: aggregate with reference can't have ()
STL: {}
would be an issue too
Ville: aggregate with reference will have ()
deleted implicitly
Move to Ready.
Proposed resolution:
This wording is relative to N3691.
Change Table 19 in 16.4.4.2 [utility.arg.requirements] as indicated:
Expression | Post-condition |
---|---|
T t;
|
object t is default-initialized
|
T u{};
|
object u is value-initialized or aggregate-initialized
|
T() T{}
|
a temporary object of type T is value-initialized or aggregate-initialized
|
atomic_compare_exchange_*
accept v == nullptr
arguments?Section: 99 [depr.util.smartptr.shared.atomic] Status: C++14 Submitter: Howard Hinnant Opened: 2012-07-28 Last modified: 2017-11-29
Priority: Not Prioritized
View all other issues in [depr.util.smartptr.shared.atomic].
View all issues with C++14 status.
Discussion:
Looking at [util.smartptr.shared.atomic]/p31
template<class T> bool atomic_compare_exchange_strong_explicit(shared_ptr<T>* p, shared_ptr<T>* v, shared_ptr<T> w, memory_order success, memory_order failure);-31- Requires:
p
shall not be null.
What about v
? Can it be null? And if so, what happens?
Requires:
p
shall not be null.
It looks like a simple oversight to me that we did not add for the atomic_compare_exchange_*
:
Requires:
p
shall not be null andv
shall not be null.
[2012-10 Portland: Move to Ready]
This is clearly the right thing to do, and Lawrence concurs.
[2013-04-20 Bristol]
Proposed resolution:
This wording is relative to N3376.
Change [util.smartptr.shared.atomic] as indicated:
template<class T> bool atomic_compare_exchange_weak( shared_ptr<T>* p, shared_ptr<T>* v, shared_ptr<T> w);-27- Requires:
[…]p
shall not be null andv
shall not be null.template<class T> bool atomic_compare_exchange_weak_explicit( shared_ptr<T>* p, shared_ptr<T>* v, shared_ptr<T> w, memory_order success, memory_order failure); template<class T> bool atomic_compare_exchange_strong_explicit( shared_ptr<T>* p, shared_ptr<T>* v, shared_ptr<T> w, memory_order success, memory_order failure);-31- Requires:
[…]p
shall not be null andv
shall not be null.
wstring_convert::converted()
should be noexcept
Section: 99 [depr.conversions.string] Status: C++14 Submitter: Jonathan Wakely Opened: 2012-08-02 Last modified: 2017-04-22
Priority: Not Prioritized
View other active issues in [depr.conversions.string].
View all other issues in [depr.conversions.string].
View all issues with C++14 status.
Discussion:
There is no reason wstring_convert::converted()
shouldn't be noexcept
.
wstring_convert::state()
and wbuffer_convert::state()
to be noexcept
too, depending on the requirements on mbstate_t
.
[2013-03-15 Issues Teleconference]
Moved to Tentatively Ready.
Defer the separate discsussion of state()
to another issue, if anyone is ever motivated
to file one.
[2013-04-20 Bristol]
Proposed resolution:
This wording is relative to N3376.
Edit in the class template wstring_convert
synopsis [conversions.string] p2:
size_t converted() const noexcept;
Edit the signature before [conversions.string] p6:
size_t converted() const noexcept;
wstring_convert
and wbuffer_convert
validitySection: 99 [depr.conversions.string], 99 [depr.conversions.buffer] Status: C++14 Submitter: Jonathan Wakely Opened: 2012-08-02 Last modified: 2017-09-07
Priority: Not Prioritized
View other active issues in [depr.conversions.string].
View all other issues in [depr.conversions.string].
View all issues with C++14 status.
Discussion:
See discussion following c++std-lib-32710.
It's not specified what happens ifwstring_convert
and wbuffer_convert
objects are constructed
with null Codecvt
pointers.
Should the constructors have preconditions that the pointers are not null? If not, are conversions expected to
fail, or is it undefined to attempt conversions if the pointers are null?
There are no observer functions to check whether objects were constructed with valid Codecvt
pointers.
If the types are made movable such observers would be necessary even if the constructors require non-null
pointers (see also LWG 2176(i)).
[2013-03-15 Issues Teleconference]
Moved to Tentatively Ready.
[2013-04-20 Bristol]
Proposed resolution:
This wording is relative to N3376.
Insert a new paragraph before [conversions.string] paragraph 16:
wstring_convert(Codecvt *pcvt = new Codecvt); wstring_convert(Codecvt *pcvt, state_type state); wstring_convert(const byte_string& byte_err, const wide_string& wide_err = wide_string());-?- Requires: For the first and second constructors
-16- Effects: The first constructor shall storepcvt != nullptr
.pcvt
incvtptr
and default values incvtstate
,byte_err_string
, andwide_err_string
. The second constructor shall storepcvt
incvtptr
,state
incvtstate
, and default values inbyte_err_string
andwide_err_string
; moreover the stored state shall be retained between calls tofrom_bytes
andto_bytes
. The third constructor shall storenew Codecvt
incvtptr
,state_type()
in cvtstate,byte_err
inbyte_err_string
, andwide_err
inwide_err_string
.
Insert a new paragraph before [conversions.buffer] paragraph 10:
wbuffer_convert(std::streambuf *bytebuf = 0, Codecvt *pcvt = new Codecvt, state_type state = state_type());-?- Requires:
-10- Effects: The constructor constructs a stream buffer object, initializespcvt != nullptr
.bufptr
tobytebuf
, initializescvtptr
topcvt
, and initializescvtstate
tostate
.
wstring_convert
and wbuffer_convert
Section: 99 [depr.conversions.string], 99 [depr.conversions.buffer] Status: C++14 Submitter: Jonathan Wakely Opened: 2012-08-02 Last modified: 2017-09-07
Priority: Not Prioritized
View other active issues in [depr.conversions.string].
View all other issues in [depr.conversions.string].
View all issues with C++14 status.
Discussion:
See discussion following c++std-lib-32699.
The constructors forwstring_convert
and wbuffer_convert
should be explicit, to avoid
implicit conversions which take ownership of a Codecvt
pointer and delete it unexpectedly.
Secondly, [conversions.buffer] p11 describes a destructor which is not declared in the class
synopsis in p2.
Finally, and most importantly, the definitions in [conversions.string] and
[conversions.buffer] imply implicitly-defined copy constructors and assignment operators, which
would do shallow copies of the owned Codecvt
objects and result in undefined behaviour in the
destructors.
Codecvt
is not required to be CopyConstructible
, so deep copies are not possible.
The wstring_convert
and wstring_buffer
types could be made move-only, but the proposed
resolution below doesn't do so because of the lack of preconditions regarding null Codecvt
pointers
and the absence of observer functions that would allow users to check preconditions (see also LWG 2175(i)).
[2013-03-15 Issues Teleconference]
Moved to Review.
Jonathan pointed out that you can have an implicit constructor that takes ownership of a heap reference, which would result an unexpected deletion.
No-one really likes the 'naked new' in the interface here, either.
[2013-04-18, Bristol]
Proposed resolution:
This wording is relative to N3376.
Edit the class template wstring_convert
synopsis in [conversions.string] p2:
explicit wstring_convert(Codecvt *pcvt = new Codecvt); wstring_convert(Codecvt *pcvt, state_type state); explicit wstring_convert(const byte_string& byte_err, const wide_string& wide_err = wide_string()); ~wstring_convert(); wstring_convert(const wstring_convert&) = delete; wstring_convert& operator=(const wstring_convert&) = delete;
Edit the signatures before [conversions.string] p16:
explicit wstring_convert(Codecvt *pcvt = new Codecvt); wstring_convert(Codecvt *pcvt, state_type state); explicit wstring_convert(const byte_string& byte_err, const wide_string& wide_err = wide_string());
Edit the class template wbuffer_convert
synopsis in [conversions.buffer] p2:
explicit wbuffer_convert(std::streambuf *bytebuf = 0, Codecvt *pcvt = new Codecvt, state_type state = state_type()); ~wbuffer_convert(); wbuffer_convert(const wbuffer_convert&) = delete; wbuffer_convert& operator=(const wbuffer_convert&) = delete;
Edit the signature before [conversions.buffer] p10:
explicit wbuffer_convert(std::streambuf *bytebuf = 0, Codecvt *pcvt = new Codecvt, state_type state = state_type());
Copy/MoveInsertable
Section: 23.2.2 [container.requirements.general] Status: C++14 Submitter: Loïc Joly Opened: 2012-08-10 Last modified: 2017-09-07
Priority: Not Prioritized
View other active issues in [container.requirements.general].
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Discussion:
See also discussion following c++std-lib-32883 and c++std-lib-32897.
The requirements onCopyInsertable
and MoveInsertable
are either incomplete, or complete but hard to
figure out.
From e-mail c++std-lib-32897:
Pablo Halpern:
I agree that we need semantic requirements for all of the *Insertable
concepts analogous to the requirements
we have on similar concepts.
Howard Hinnant:
I've come to believe that the standard is actually correct as written in this area. But it is really hard
to read. I would have no objection whatsoever to clarifications to CopyInsertable
as you suggest (such as the
post-conditions on v
). And I do agree with you that the correct approach to the clarifications is to
confirm that CopyInsertable
implies MoveInsertable
.
[2012, Portland: Move to Tentatively Ready]
Move to Tentatively Ready by unanimous consent.
[2013-04-20 Bristol]
Proposed resolution:
This wording is relative to N3376.
Edit 23.2.2 [container.requirements.general] p13 as indicated:
-13- […] Given a container type
X
having anallocator_type
identical toA
and avalue_type
identical toT
and given an lvaluem
of typeA
, a pointerp
of typeT*
, an expressionv
of type (possiblyconst
)T
, and an rvaluerv
of typeT
, the following terms are defined. IfX
is not allocator-aware, the terms below are defined as ifA
werestd::allocator<T>
— no allocator object needs to be created and user specializations ofstd::allocator<T>
are not instantiated:
T
isDefaultInsertable
intoX
means that the following expression is well-formed:allocator_traits<A>::construct(m, p);An element of
X
is default-inserted if it is initialized by evaluation of the expressionallocator_traits<A>::construct(m, p);where
p
is the address of the uninitialized storage for the element allocated withinX
.
T
isinto
CopyMoveInsertableX
means that the following expression is well-formed:allocator_traits<A>::construct(m, p, rv);and when evaluated the following postconditions hold: The value of
*p
is equivalent to the value ofrv
before the evaluation. [Note:rv
remains a valid object. Its state is unspecified — end note]
T
isinto
MoveCopyInsertableX
means that, in addition to satisfying theMoveInsertable
requirements, the following expression is well-formed:allocator_traits<A>::construct(m, p,rv);and when evaluated the following postconditions hold: The value of
v
is unchanged and is equivalent to*p
.
T
isEmplaceConstructible
intoX
fromargs
, for zero or more argumentsargs
, means that the following expression is well-formed:allocator_traits<A>::construct(m, p, args);
T
isErasable
fromX
means that the following expression is well-formed:allocator_traits<A>::destroy(m, p);[Note: A container calls
allocator_traits<A>::construct(m, p, args)
to construct an element atp
usingargs
. The default construct instd::allocator
will call::new((void*)p) T(args)
, but specialized allocators may choose a different definition. — end note]
enable_shared_from_this
and construction from raw pointersSection: 20.3.2.7 [util.smartptr.enab], 20.3.2.2.2 [util.smartptr.shared.const] Status: Resolved Submitter: Daniel Krügler Opened: 2012-08-16 Last modified: 2017-09-07
Priority: 3
View all other issues in [util.smartptr.enab].
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Discussion:
On reflector message c++std-lib-32927, Matt Austern asked whether the following example should be well-defined or not
struct X : public enable_shared_from_this<X> { }; auto xraw = new X; shared_ptr<X> xp1(xraw); shared_ptr<X> xp2(xraw);
pointing out that 20.3.2.2.2 [util.smartptr.shared.const] does not seem to allow it, since
xp1
and xp2
aren't allowed to share ownership, because each of them is required to have
use_count() == 1
. Despite this wording it might be reasonable (and technical possible)
to implement that request.
The
shared_ptr
constructors that create unique pointers can detect the presence of anenable_shared_from_this
base and assign the newly createdshared_ptr
to its__weak_this member
.
Now according to the specification in 20.3.2.2.2 [util.smartptr.shared.const] p3-7:
template<class Y> explicit shared_ptr(Y* p);
the notion of creating unique pointers can be read to be included by this note, because the post-condition
of this constructor is unique() == true
. Evidence for this interpretation seems to be weak, though.
auto xraw = new X; shared_ptr<X> xp1(xraw); shared_ptr<X> xp2(xraw);
He also pointed out that the current post-conditions of the affected shared_ptr
constructor
would need to be reworded.
shared_ptr
objects to prevent them from owning the same underlying object without sharing the
ownership. It might be useful to add such a requirement.
[2013-03-15 Issues Teleconference]
Moved to Open.
More discussion is needed to pick a direction to guide a proposed resolution.
[2013-05-09 Jonathan comments]
The note says the newly created shared_ptr
is assigned to the weak_ptr
member. It doesn't
say before doing that the shared_ptr
should check if the weak_ptr
is non-empty and possibly
share ownership with some other pre-existing shared_ptr
.
[2015-08-26 Daniel comments]
LWG issue 2529(i) is independent but related to this issue.
[2016-03-16, Alisdair comments]
This issues should be closed as Resolved
by paper p0033r1 at Jacksonville.
Proposed resolution:
std::seed_seq
operationsSection: 29.5.8.1 [rand.util.seedseq] Status: C++14 Submitter: Daniel Krügler Opened: 2012-08-18 Last modified: 2016-01-28
Priority: Not Prioritized
View all other issues in [rand.util.seedseq].
View all issues with C++14 status.
Discussion:
29.5.8.1 [rand.util.seedseq] p1 says upfront:
No function described in this section 29.5.8.1 [rand.util.seedseq] throws an exception.
This constraint seems non-implementable to me when looking especially at the members
template<class T> seed_seq(initializer_list<T> il); template<class InputIterator> seed_seq(InputIterator begin, InputIterator end);
which have the effect of invoking v.push_back()
for the exposition-only member of
type std::vector
(or its equivalent) over all elements of the provided range, so
out-of-memory exceptions are always possible and the seed_seq
object doesn't seem
to be constructible this way.
InputIterator
might also throw exceptions.
Aside to that it should me mentioned, that a default constructor of vector<uint_least32_t>
in theory can also throw exceptions, even though this seems less of a problem to me in this context, because
such an implementation could easily use a different internal container in seed_seq
that can hold
this no-throw exception guarantee.
Secondly, a slightly different problem category related to exceptions occurs for the member templates
template<class RandomAccessIterator> void generate(RandomAccessIterator begin, RandomAccessIterator end); template<class OutputIterator> void param(OutputIterator dest) const;
where the actual operations performed by the implementation would never need to throw, but since they invoke operations of a user-provided customization point, the overall operation, like for example
copy(v.begin(), v.end(), dest);
could also throw exceptions. In this particular example we can just think of a std::back_insert_iterator
applied to a container that needs to allocate its elements used as the type for OutputIterator
.
std::seed_seq
except the template generate
is actually needed within the library implementation, as mentioned in the
discussion of LWG 2124(i).
I suggest to remove the general no-exception constraints for operations of std::seed_seq
except for
member size()
and the default constructor and to provide specific wording for generate()
and
param()
to ensure that the algorithm itself is a nothrow operation, which is especially for
generate()
important, because the templates specified in 29.5.4 [rand.eng] and
29.5.5 [rand.adapt] also depend on this property indirectly, which is further discussed in LWG
2181(i).
Howard:
I suggest to use a different form for the exception specification, something similar to
22.10.15.4 [func.bind.bind] p4:
Throws: Nothing unless an operation on
RandomAccessIterator
throws an exception.
Daniel:
The currently suggested "what and when" form seems a bit more specific and harmonizes with the form used for function templategenerate_canonical
from 29.5.8.2 [rand.util.canonical].
[2013-04-20, Bristol]
Open an editorial issue on the exception wording ("Throws: What and when").
Solution: move to tentatively ready.[2013-09-29, Chicago]
Apply to Working Paper
Proposed resolution:
This wording is relative to N3376.
Edit 29.5.8.1 [rand.util.seedseq] p1 as indicated:
-1- No function described in this section 29.5.8.1 [rand.util.seedseq] throws an exception.
Edit 29.5.8.1 [rand.util.seedseq] around p2 as indicated:
seed_seq();-2- Effects: Constructs a
-?- Throws: Nothing.seed_seq
object as if by default-constructing its memberv
.
Edit 29.5.8.1 [rand.util.seedseq] around p7 as indicated:
template<class RandomAccessIterator> void generate(RandomAccessIterator begin, RandomAccessIterator end);-7- Requires:
-8- Effects: Does nothing ifRandomAccessIterator
shall meet the requirements of a mutable random access iterator (Table 111) type. Moreover,iterator_traits<class RandomAccessIterator>::value_type
shall denote an unsigned integer type capable of accommodating 32-bit quantities.begin == end
. Otherwise, withs = v.size()
andn = end - begin
, fills the supplied range[begin, end)
according to the following algorithm […] -?- Throws: What and whenRandomAccessIterator
operations ofbegin
andend
throw.
Edit 29.5.8.1 [rand.util.seedseq] around p9 as indicated:
size_t size() const;-9- Returns: The number of 32-bit units that would be returned by a call to
-??- Throws: Nothing. -10- Complexity: constant time.param()
.
Edit 29.5.8.1 [rand.util.seedseq] around p11 as indicated:
template<class OutputIterator> void param(OutputIterator dest) const;-11- Requires:
-12- Effects: Copies the sequence of prepared 32-bit units to the given destination, as if by executing the following statement:OutputIterator
shall satisfy the requirements of an output iterator (Table 108) type. Moreover, the expression*dest = rt
shall be valid for a valuert
of typeresult_type
.copy(v.begin(), v.end(), dest);-??- Throws: What and when
OutputIterator
operations ofdest
throw.
Section: 29.5.3.2 [rand.req.seedseq], 29.5.4 [rand.eng], 29.5.5 [rand.adapt] Status: C++17 Submitter: Daniel Krügler Opened: 2012-08-18 Last modified: 2017-07-30
Priority: 3
View all other issues in [rand.req.seedseq].
View all issues with C++17 status.
Discussion:
LWG issue 2180(i) points out some deficiences in regard to the specification of the library-provided
type std::seed_seq
regarding exceptions, but there is another specification problem
in regard to general types satisfying the seed sequence constraints (named SSeq
) as described in
29.5.3.2 [rand.req.seedseq].
Except where specified otherwise, no function described in this section 29.5.4 [rand.eng]/29.5.5 [rand.adapt] throws an exception.
This constraint causes problems, because the described templates in these sub-clauses depend on operations of
SSeq::generate()
which is a function template, that depends both on operations provided by the
implementor of SSeq
(e.g. of std::seed_seq
), and those of the random access iterator type
provided by the caller. With class template linear_congruential_engine
we have just one example for a user
of SSeq::generate()
via:
template<class Sseq> linear_congruential_engine<>::linear_congruential_engine(Sseq& q); template<class Sseq> void linear_congruential_engine<>::seed(Sseq& q);
None of these operations has an exclusion rule for exceptions.
As described in 2180(i) the wording forstd::seed_seq
should and can be fixed to ensure that
operations of seed_seq::generate()
won't throw except from operations of the provided iterator range,
but there is no corresponding "safety belt" for user-provided SSeq
types, since 29.5.3.2 [rand.req.seedseq]
does not impose no-throw requirements onto operations of seed sequences.
A quite radical step to fix this problem would be to impose general no-throw requirements on the expression
q.generate(rb,re)
from Table 115, but this is not as simple as it looks initially, because this
function again depends on general types that are mutable random access iterators. Typically, we do not
impose no-throw requirements on iterator operations and this would restrict general seed sequences where
exceptions are not a problem. Furthermore, we do not impose comparable constraints for other expressions,
like that of the expression g()
in Table 116 for good reasons, e.g. random_device::operator()
explicitly states when it throws exceptions.
A less radical variant of the previous suggestion would be to add a normative requirement on the expression
q.generate(rb,re)
from Table 115 that says: "Throws nothing if operations of rb
and re
do not throw exceptions". Nevertheless we typically do not describe conditional Throws elements in proper
requirement sets elsewhere (Container requirements excluded, they just describe the containers from Clause 23)
and this may exclude resonable implementations of seed sequences that could throw exceptions under rare
situations.
The iterator arguments provided to SSeq::generate()
for operations in templates of 29.5.4 [rand.eng] and
29.5.5 [rand.adapt] are under control of implementations, so we could impose stricter exceptions requirements
on SSeq::generate()
for SSeq
types that are used to instantiate member templates in 29.5.4 [rand.eng]
and 29.5.5 [rand.adapt] solely.
We simply add extra wording to the introductive parts of 29.5.4 [rand.eng] and 29.5.5 [rand.adapt]
that specify that operations of the engine (adaptor) templates that depend on a template parameter SSeq
throw no exception unless SSeq::generate()
throws an exception.
Given these options I would suggest to apply the variant described in the fourth bullet.
The proposed resolution attempts to reduce a lot of the redundancies of requirements in the introductory paragraphs of 29.5.4 [rand.eng] and 29.5.5 [rand.adapt] by introducing a new intermediate sub-clause "Engine and engine adaptor class templates" following sub-clause 29.5.2 [rand.synopsis]. This approach also solves the problem that currently 29.5.4 [rand.eng] also describes requirements that apply for 29.5.5 [rand.adapt] (Constrained templates involving theSseq
parameters).
[2013-04-20, Bristol]
Remove the first bullet point:
?- Throughout this sub-clause general requirements and conventions are described that apply to every class template specified in sub-clause 29.5.4 [rand.eng] and 29.5.5 [rand.adapt]. Phrases of the form "in those sub-clauses" shall be interpreted as equivalent to "in sub-clauses 29.5.4 [rand.eng] and 29.5.5 [rand.adapt]".
Replace "in those sub-clauses" with "in sub-clauses 29.5.4 [rand.eng] and 29.5.5 [rand.adapt]".
Find another place for the wording. Daniel: These are requirements on the implementation not on the types. I'm not comfortable in moving it to another place without double checking. Improve the text (there are 4 "for"s): for copy constructors, for copy assignment operators, for streaming operators, and for equality and inequality operators are not shown in the synopses. Move the information of this paragraph to the paragraphs it refers to:"-?- Descriptions are provided in those sub-clauses only for engine operations that are not described in 29.5.3.4 [rand.req.eng], for adaptor operations that are not described in 29.5.3.5 [rand.req.adapt], or for operations where there is additional semantic information. In particular, declarations for copy constructors, for copy assignment operators, for streaming operators, and for equality and inequality operators are not shown in the synopses."
Alisdair: I prefer duplication here than consolidation/reference to these paragraphs.
The room showed weakly favjust or for duplication.Previous resolution from Daniel [SUPERSEDED]:
Add a new sub-clause titled "Engine and engine adaptor class templates" following sub-clause 29.5.2 [rand.synopsis] (but at the same level) and add one further sub-clause "General" as child of the new sub-clause as follows:
Engine and engine adaptor class templates [rand.engadapt] General [rand.engadapt.general]-?- Throughout this sub-clause general requirements and conventions are described that apply to every class template specified in sub-clause 29.5.4 [rand.eng] and 29.5.5 [rand.adapt]. Phrases of the form "in those sub-clauses" shall be interpreted as equivalent to "in sub-clauses 29.5.4 [rand.eng] and 29.5.5 [rand.adapt]".
-?- Except where specified otherwise, the complexity of each function specified in those sub-clauses is constant. -?- Except where specified otherwise, no function described in those sub-clauses throws an exception. -?- Every function described in those sub-clauses that has a function parameterq
of typeSSeq&
for a template type parameter namedSSeq
that is different from typestd::seed_seq
throws what and when the invocation ofq.generate
throws. -?- Descriptions are provided in those sub-clauses only for engine operations that are not described in 29.5.3.4 [rand.req.eng], for adaptor operations that are not described in 29.5.3.5 [rand.req.adapt], or for operations where there is additional semantic information. In particular, declarations for copy constructors, for copy assignment operators, for streaming operators, and for equality and inequality operators are not shown in the synopses. -?- Each template specified in those sub-clauses requires one or more relationships, involving the value(s) of its non-type template parameter(s), to hold. A program instantiating any of these templates is ill-formed if any such required relationship fails to hold. -?- For every random number engine and for every random number engine adaptorX
defined in those sub-clauses:
if the constructor
template <class Sseq> explicit X(Sseq& q);is called with a type
Sseq
that does not qualify as a seed sequence, then this constructor shall not participate in overload resolution;if the member function
template <class Sseq> void seed(Sseq& q);is called with a type
Sseq
that does not qualify as a seed sequence, then this function shall not participate in overload resolution;The extent to which an implementation determines that a type cannot be a seed sequence is unspecified, except that as a minimum a type shall not qualify as a seed sequence if it is implicitly convertible to
X::result_type
.Edit the contents of sub-clause 29.5.4 [rand.eng] as indicated:
-1- Each type instantiated from a class template specified in this section 29.5.4 [rand.eng] satisfies the requirements of a random number engine (29.5.3.4 [rand.req.eng]) type and the general implementation requirements specified in sub-clause [rand.engadapt.general].
-2- Except where specified otherwise, the complexity of each function specified in this section 29.5.4 [rand.eng] is constant.-3- Except where specified otherwise, no function described in this section 29.5.4 [rand.eng] throws an exception.-4- Descriptions are provided in this section 29.5.4 [rand.eng] only for engine operations that are not described in 29.5.3.4 [rand.req.eng] […]-5- Each template specified in this section 29.5.4 [rand.eng] requires one or more relationships, involving the value(s) of its non-type template parameter(s), to hold. […]-6- For every random number engine and for every random number engine adaptorX
defined in this subclause (29.5.4 [rand.eng]) and in sub-clause 29.5.4 [rand.eng]: […]Edit the contents of sub-clause 29.5.5.1 [rand.adapt.general] as indicated:
-1- Each type instantiated from a class template specified in this section
29.5.4 [rand.eng]29.5.5 [rand.adapt] satisfies the requirements of a random number engine adaptor (29.5.3.5 [rand.req.adapt]) type and the general implementation requirements specified in sub-clause [rand.engadapt.general].-2- Except where specified otherwise, the complexity of each function specified in this section 29.5.5 [rand.adapt] is constant.-3- Except where specified otherwise, no function described in this section 29.5.5 [rand.adapt] throws an exception.-4- Descriptions are provided in this section 29.5.5 [rand.adapt] only for engine operations that are not described in 29.5.3.5 [rand.req.adapt] […]-5- Each template specified in this section 29.5.5 [rand.adapt] requires one or more relationships, involving the value(s) of its non-type template parameter(s), to hold. […]
[2014-02-09, Daniel provides alternative resolution]
[Lenexa 2015-05-07: Move to Ready]
LWG 2181 exceptions from seed sequence operations
STL: Daniel explained that I was confused. I said, oh, seed_seq says it can throw if the RanIt throws. Daniel says the RanIts are provided by the engine. Therefore if you give a seed_seq to an engine, it cannot throw, as implied by the current normative text. So what Daniel has in the PR is correct, if slightly unnecessary. It's okay to have explicitly non-overlapping Standardese even if overlapping would be okay.
Marshall: And this is a case where the std:: on seed_seq is a good thing.
STL: Meh.
STL: And that was my only concern with this PR. I like the latest PR much better than the previous.
Marshall: Yes. There's a drive-by fix for referencing the wrong section. Other than that, the two are the same.
STL: Alisdair wanted the repetition instead of centralization, and I agree.
Marshall: Any other opinions?
Jonathan: I'll buy it.
STL: For a dollar?
Hwrd: I'll buy that for a nickel.
Marshall: Any objections to Ready? I don't see a point in Immediate.
Jonathan: Absolutely agree.
Marshall: 7 for ready, 0 opposed, 0 abstain.
[2014-05-22, Daniel syncs with recent WP]
[2015-10-31, Daniel comments and simplifies suggested wording changes]
Upon Walter Brown's suggestion the revised wording does not contain any wording changes that could be considered as editorial.
Previous resolution from Daniel [SUPERSEDED]:
This wording is relative to N3936.
Edit the contents of sub-clause 29.5.4 [rand.eng] as indicated:
-1- Each type instantiated from a class template specified in this section 29.5.4 [rand.eng] satisfies the requirements of a random number engine (29.5.3.4 [rand.req.eng]) type.
-2- Except where specified otherwise, the complexity of each function specified in this section 29.5.4 [rand.eng] is constant. -3- Except where specified otherwise, no function described in this section 29.5.4 [rand.eng] throws an exception. -?- Every function described in this section 29.5.4 [rand.eng] that has a function parameterq
of typeSseq&
for a template type parameter namedSseq
that is different from typestd::seed_seq
throws what and when the invocation ofq.generate
throws. -4- Descriptions are provided in this section 29.5.4 [rand.eng] only for engine operations that are not described in 29.5.3.4 [rand.req.eng] or for operations where there is additional semantic information. In particular, declarations for copy constructors,forcopy assignment operators,forstreaming operators,and forequality operators, and inequality operators are not shown in the synopses. -5- Each template specified in this section 29.5.4 [rand.eng] requires one or more relationships, involving the value(s) of its non-type template parameter(s), to hold. A program instantiating any of these templates is ill-formed if any such required relationship fails to hold. -6- For every random number engine and for every random number engine adaptorX
defined in this subclause (29.5.4 [rand.eng]) and in sub-clause 29.5.5 [rand.adapt]:
if the constructor
template <class Sseq> explicit X(Sseq& q);is called with a type
Sseq
that does not qualify as a seed sequence, then this constructor shall not participate in overload resolution;if the member function
template <class Sseq> void seed(Sseq& q);is called with a type
Sseq
that does not qualify as a seed sequence, then this function shall not participate in overload resolution;The extent to which an implementation determines that a type cannot be a seed sequence is unspecified, except that as a minimum a type shall not qualify as a seed sequence if it is implicitly convertible to
X::result_type
.Edit the contents of sub-clause 29.5.5.1 [rand.adapt.general] as indicated:
-1- Each type instantiated from a class template specified in this section
-2- Except where specified otherwise, the complexity of each function specified in this section 29.5.5 [rand.adapt] is constant. -3- Except where specified otherwise, no function described in this section 29.5.5 [rand.adapt] throws an exception. -?- Every function described in this section 29.5.5 [rand.adapt] that has a function parameter29.5.4 [rand.eng]29.5.5 [rand.adapt] satisfies the requirements of a random number engine adaptor (29.5.3.5 [rand.req.adapt]) type.q
of typeSseq&
for a template type parameter namedSseq
that is different from typestd::seed_seq
throws what and when the invocation ofq.generate
throws. -4- Descriptions are provided in this section 29.5.5 [rand.adapt] only for adaptor operations that are not described in section 29.5.3.5 [rand.req.adapt] or for operations where there is additional semantic information. In particular, declarations for copy constructors,forcopy assignment operators,forstreaming operators,and forequality operators, and inequality operators are not shown in the synopses. -5- Each template specified in this section 29.5.5 [rand.adapt] requires one or more relationships, involving the value(s) of its non-type template parameter(s), to hold. A program instantiating any of these templates is ill-formed if any such required relationship fails to hold.Edit the contents of sub-clause 29.5.9.1 [rand.dist.general] p2 as indicated: [Drafting note: These editorial changes are just for consistency with those applied to 29.5.4 [rand.eng] and 29.5.5.1 [rand.adapt.general] — end drafting note]
-2- Descriptions are provided in this section 29.5.9 [rand.dist] only for distribution operations that are not described in 29.5.3.6 [rand.req.dist] or for operations where there is additional semantic information. In particular, declarations for copy constructors,
forcopy assignment operators,forstreaming operators,and forequality operators, and inequality operators are not shown in the synopses.
Proposed resolution:
This wording is relative to N4527.
Edit the contents of sub-clause 29.5.4 [rand.eng] as indicated:
-1- […]
-2- Except where specified otherwise, the complexity of each function specified in this section 29.5.4 [rand.eng] is constant. -3- Except where specified otherwise, no function described in this section 29.5.4 [rand.eng] throws an exception. -?- Every function described in this section 29.5.4 [rand.eng] that has a function parameterq
of typeSseq&
for a template type parameter namedSseq
that is different from typestd::seed_seq
throws what and when the invocation ofq.generate
throws. […]
Edit the contents of sub-clause 29.5.5.1 [rand.adapt.general] as indicated:
-1- […]
-2- Except where specified otherwise, the complexity of each function specified in this section 29.5.5 [rand.adapt] is constant. -3- Except where specified otherwise, no function described in this section 29.5.5 [rand.adapt] throws an exception. -?- Every function described in this section 29.5.5 [rand.adapt] that has a function parameterq
of typeSseq&
for a template type parameter namedSseq
that is different from typestd::seed_seq
throws what and when the invocation ofq.generate
throws.
Container::[const_]reference
types are misleadingly specifiedSection: 23.2.2 [container.requirements.general] Status: C++14 Submitter: Daniel Krügler Opened: 2012-08-20 Last modified: 2017-07-05
Priority: 0
View other active issues in [container.requirements.general].
View all other issues in [container.requirements.general].
View all issues with C++14 status.
Discussion:
According to Table 96 (Container requirements) the return type of X::reference
and
X::const_reference
is "lvalue of T
" and "const
lvalue of T
",
respectively. This does not make much sense, because an lvalue is an expression category, not a type.
It could also refer to an expression that has a type, but this doesn't make sense either in this
context, because obviously X::[const_]reference
are intended to refer to types.
vector<bool>
has no real reference type for X::[const_]reference
and this definition presumably is intended to cover such situations as well, one might think that the wording is
just a sloppy form of "type that represents a [const] lvalue of T
". But this is also problematic,
because basically all proxy reference expressions are rvalues.
It is unclear what the intention is. A straightward way of fixing this wording could make
X::[const_]reference
identical to [const] T&
. This holds for all Library containers
except for vector<bool>
.
Another way of solving this definition problem would be to impose a requirement that holds for both
references and reference-like proxies. Both X::reference
and X::const_reference
would need to be convertible to const T&
. Additionally X::reference
would need to
support for a mutable container an assignment expression of the form
declval<X::reference>() = declval<T>()
(this presentation intentionally does not require
declval<X::reference&>() = declval<T>()
).
Further, the Table 96 does not impose any relations between X::reference
and X::const_reference
.
It seems that at least X::reference
needs to be convertible to X::const_reference
.
A related question is whether X::reference
is supposed to be a mutable reference-like type,
irrespective of whether the container is an immutable container or not. The way, type match_results
defines reference
identical to const_reference
indicates one specific interpretation (similarly,
the initializer_list
template also defines member type reference
equal to const value_type&
).
Note that this can be a different decision as that for iterator
and const_iterator
,
e.g. for sets the type X::reference
still is a mutable reference, even though iterator
is described as constant iterator.
The proposed resolution is incomplete in regard to the last question.
[2013-03-15 Issues Teleconference]
Moved to Review.
Alisdair notes that this looks like wording in the right direction. Wonders about congruence of these typedefs and the similar ones for iterators.
[2013-09 Chicago]
Moved to Ready.
Consensus that the requirements should require real references, just like iterators, as containers are required to support at least ForwardIterators, which have the same restriction on references.
Matt will file a new issue for some additional concerns with regex match_results
.
[2014-02-10, Daniel comments]
The new issue opened by Matt is LWG 2306(i).
[Issaquah 2014-02-11: Move to Immediate]
Issue should have been Ready in pre-meeting mailing.
Proposed resolution:
This wording is relative to N3376.
Change Table 96 — "Container requirements" as indicated:
Expression | Return type | Operational Semantics |
Assertion/note pre-/post-condition |
Complexity |
---|---|---|---|---|
X::reference
|
T&
|
compile time | ||
X::const_reference
|
const T&
|
compile time |
match_results
constructorsSection: 28.6.9.2 [re.results.const], 28.6.9.7 [re.results.all] Status: C++20 Submitter: Pete Becker Opened: 2012-08-29 Last modified: 2021-02-25
Priority: 3
View all other issues in [re.results.const].
View all issues with C++20 status.
Discussion:
28.6.9.2 [re.results.const] p1 says:
In all
match_results
constructors, a copy of theAllocator
argument shall be used for any memory allocation performed by the constructor or member functions during the lifetime of the object.
There are three constructors:
match_results(const Allocator& = Allocator()); match_results(const match_results& m); match_results(match_results&& m) noexcept;
The second and third constructors do no have an Allocator
argument, so despite the "all match_results
constructors", it is not possible to use "the Allocator
argument" for the second and third constructors.
Allocator
value is move constructed from
m.get_allocator()
, but doesn't require using that allocator to allocate memory.
The same basic problem recurs in 28.6.9.7 [re.results.all], which gives the required return value for
get_allocator()
:
Returns: A copy of the
Allocator
that was passed to the object's constructor or, if that allocator has been replaced, a copy of the most recent replacement.
Again, the second and third constructors do not take an Allocator
, so there is nothing that meets this
requirement when those constructors are used.
[2018-06-02, Daniel comments and provides wording]
The introductory wording of match_results
says in 28.6.9 [re.results] p2:
The class template
match_results
satisfies the requirements of an allocator-aware container and of a sequence container (26.2.1, 26.2.3) except that only operations defined for const-qualified sequence containers are supported and that the semantics of comparison functions are different from those required for a container.
This wording essentially brings us to 23.2.2 [container.requirements.general] which describes in p8 in general the usage of allocators:
[…] Copy constructors for these container types obtain an allocator by calling
allocator_traits<allocator_ type>::select_on_container_copy_construction
on the allocator belonging to the container being copied. Move constructors obtain an allocator by move construction from the allocator belonging to the container being moved. […]
The constructors referred to in the issue discussion are the copy constructor and move constructor of match_results
,
so we know already what the required effects are supposed to be.
[…] All other constructors for these container types take a
const allocator_type&
argument. [Note: If an invocation of a constructor uses the default value of an optional allocator argument, then the Allocator type must support value-initialization. — end note] A copy of this allocator is used for any memory allocation and element construction performed, by these constructors and by all member functions, during the lifetime of each container object or until the allocator is replaced. […]
Further requirements imposed on two of the three match_results
constructors can be derived from Table 80 —
"Allocator-aware container requirements" via the specified expressions
X() X(m) X(rv)
In other words: The existing wording does already say everything that it said by 28.6.9.2 [re.results.const] p1 (end even more), except for possibly the tiny problem that
match_results(const Allocator& a = Allocator());
uses "const Allocator&
" instead of "const allocator_type&
" in the signature, albeit even
that deviation shouldn't change the intended outcome, which is IMO crystal-clear when looking at sub-clauses
23.2.2 [container.requirements.general] and 23.2.4 [sequence.reqmts] as a whole.
Either strike 28.6.9.2 [re.results.const] p1 completely; or
Replace 28.6.9.2 [re.results.const] p1 by referring to the specification of allocators in 23.2.2 [container.requirements.general] and 23.2.4 [sequence.reqmts].
My suggestion is to favour for the first option, because attempting to provide extra wording that refers to allocators
and the three constructors may lead to the false impression that no further allocator-related
requirements hold for type match_results
which are not explicitly repeated here again.
[2018-06, Rapperswil]
The group agrees with the provided resolution. Move to Ready.
[2018-11, Adopted in San Diego]
Proposed resolution:
This wording is relative to N4750.
Edit 28.6.9.2 [re.results.const] as indicated:
-1- In allmatch_results
constructors, a copy of theAllocator
argument shall be used for any memory allocation performed by the constructor or member functions during the lifetime of the object.
match_results
assignmentsSection: 28.6.9.2 [re.results.const], 28.6.9.7 [re.results.all] Status: C++20 Submitter: Pete Becker Opened: 2012-08-29 Last modified: 2021-02-25
Priority: 3
View all other issues in [re.results.const].
View all issues with C++20 status.
Discussion:
The effects of the two assignment operators are specified in Table 141. Table 141 makes no mention of allocators,
so, presumably, they don't touch the target object's allocator. That's okay, but it leaves the question:
match_results::get_allocator()
is supposed to return "A copy of the Allocator that was passed to the
object's constructor or, if that allocator has been replaced, a copy of the most recent replacement"; if assignment
doesn't replace the allocator, how can the allocator be replaced?
[2018-06-04, Daniel comments and provides wording]
Similar to the reasoning provided in the 2018-06-02 comment in LWG 2183(i), it is possible to refer to
the introductory wording of match_results
which says in 28.6.9 [re.results] p2:
The class template
match_results
satisfies the requirements of an allocator-aware container and of a sequence container (26.2.1, 26.2.3) except that only operations defined for const-qualified sequence containers are supported and that the semantics of comparison functions are different from those required for a container.
Again, similar to LWG 2183(i), this allows us to deduce the required effects of the copy/move assignment operators discussed here, because 23.2.2 [container.requirements.general] p8 also says:
[…] The allocator may be replaced only via assignment or
swap()
. Allocator replacement is performed by copy assignment, move assignment, or swapping of the allocator only ifallocator_traits<allocator_type>::propagate_on_container_copy_assignment::value
,allocator_traits<allocator_type>::propagate_on_container_move_assignment::value
, orallocator_traits<allocator_type>::propagate_on_container_swap::value
istrue
within the implementation of the corresponding container operation. In all container types defined in this Clause, the memberget_allocator()
returns a copy of the allocator used to construct the container or, if that allocator has been replaced, a copy of the most recent replacement. […]
So this wording already specifies everything we need, except for the problem that
28.6.9 [re.results] p2 quoted above restricts to operations supported by a const-qualified sequence
container, which of-course would exclude the copy assignment and the move assignment operators.
But given that these mutable definitions are defined for match_results
, it seems that the only fix
needed is to adjust 28.6.9 [re.results] p2 a bit to ensure that both assignment operators are
covered (again) by the general allocator-aware container wording.
[2018-06, Rapperswil]
The group generally likes the suggested direction, but would prefer the changed wording to say effectively "except that only copy assignment, move assignment, and operations defined...". Once applied, move to ready.
Previous resolution [SUPERSEDED]:
This wording is relative to N4750.
Edit 28.6.9 [re.results] as indicated:
-2- The class template
match_results
satisfies the requirements of an allocator-aware container and of a sequence container (23.2.2 [container.requirements.general], 23.2.4 [sequence.reqmts]) except that besides copy assignment and move assignment only operations defined for const-qualified sequence containers are supported and that the semantics of comparison functions are different from those required for a container.
[2018-06-06, Daniel updates wording]
[2018-11, Adopted in San Diego]
Proposed resolution:
This wording is relative to N4750.
Edit 28.6.9 [re.results] as indicated:
-2- The class template
match_results
satisfies the requirements of an allocator-aware container and of a sequence container (23.2.2 [container.requirements.general], 23.2.4 [sequence.reqmts]) except that only copy assignment, move assignment, and operations defined for const-qualified sequence containers are supported and that the semantics of comparison functions are different from those required for a container.
future/shared_future::wait_for/wait_until
Section: 32.10.7 [futures.unique.future], 32.10.8 [futures.shared.future] Status: C++14 Submitter: Vicente J. Botet Escriba Opened: 2012-09-20 Last modified: 2021-06-06
Priority: Not Prioritized
View all other issues in [futures.unique.future].
View all issues with C++14 status.
Discussion:
The functions future::wait_for
, future::wait_until
, shared_future::wait_for
, and
shared_future::wait_for
can throw any timeout-related exceptions. It would be better if the wording
could be more explicit. This is in line with the changes proposed in LWG 2093(i)'s Throws element
of condition_variable::wait
with predicate.
[2012, Portland: move to Review]
The phrase timeout-related exception does not exist.
2093(i) was put in review, and there is some dependency here with this issue.
If you provide a user-defined clock that throws, we need to put back an exception to allow that to be done.
We will put this in review and say that this cannot go in before 2093(i).
[2013-04-20, Bristol]
Accepted for the working paper
Proposed resolution:
[This resolution should not be adopted before resolving 2093(i)]
[This wording is relative to N3376.]
Change [futures.unique_future] as indicated:
template <class Rep, class Period> future_status wait_for(const chrono::duration<Rep, Period>& rel_time) const;-21- Effects: none if the shared state contains a deferred function (32.10.9 [futures.async]), otherwise blocks until the shared state is ready or until the relative timeout (32.2.4 [thread.req.timing]) specified by
-22- Returns: […] -??- Throws: timeout-related exceptions (32.2.4 [thread.req.timing]).rel_time
has expired.template <class Clock, class Duration> future_status wait_until(const chrono::time_point<Clock, Duration>& abs_time) const;-23- Effects: none if the shared state contains a deferred function (32.10.9 [futures.async]), otherwise blocks until the shared state is ready or until the absolute timeout (32.2.4 [thread.req.timing]) specified by
-24- Returns: […] -??- Throws: timeout-related exceptions (32.2.4 [thread.req.timing]).abs_time
has expired.
Change [futures.shared_future] as indicated:
template <class Rep, class Period> future_status wait_for(const chrono::duration<Rep, Period>& rel_time) const;-23- Effects: none if the shared state contains a deferred function (32.10.9 [futures.async]), otherwise blocks until the shared state is ready or until the relative timeout (32.2.4 [thread.req.timing]) specified by
-24- Returns: […] -??- Throws: timeout-related exceptions (32.2.4 [thread.req.timing]).rel_time
has expired.template <class Clock, class Duration> future_status wait_until(const chrono::time_point<Clock, Duration>& abs_time) const;-25- Effects: none if the shared state contains a deferred function (32.10.9 [futures.async]), otherwise blocks until the shared state is ready or until the absolute timeout (32.2.4 [thread.req.timing]) specified by
-26- Returns: […] -??- Throws: timeout-related exceptions (32.2.4 [thread.req.timing]).abs_time
has expired.
async/launch::deferred
Section: 32.10.9 [futures.async] Status: C++14 Submitter: Vicente J. Botet Escriba Opened: 2012-09-20 Last modified: 2017-07-05
Priority: Not Prioritized
View other active issues in [futures.async].
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Discussion:
The description of the effects of async
when the launch policy is launch::deferred
doesn't
state what is done with the result of the deferred function invocation and the possible exceptions as it is done
for the asynchronous function when the policy is launch::async
.
[2012, Portland: move to Open]
Detlef: agree with the problem but not with the resolution. The wording should be applied to all launch policies rather than having to be separately specified for each one.
Hans: we should redraft to factor out the proposed text outside the two bullets. Needs to be carefully worded to be compatible with the resolution of 2120(i) (see above).
Moved to open
[Issaquah 2014-02-11: Move to Immediate after SG1 review]
Proposed resolution:
[This wording is relative to N3376.]
Change 32.10.9 [futures.async] p3 bullet 2 as indicated:
template <class F, class... Args> future<typename result_of<typename decay<F>::type(typename decay<Args>::type...)>::type> async(F&& f, Args&&... args); template <class F, class... Args> future<typename result_of<typename decay<F>::type(typename decay<Args>::type...)>::type> async(launch policy, F&& f, Args&&... args);-2- Requires: […]
-3- Effects:: The first function behaves the same as a call to the second function with apolicy
argument oflaunch::async | launch::deferred
and the same arguments forF
andArgs
. […] The further behavior of the second function depends on thepolicy
argument as follows (if more than one of these conditions applies, the implementation may choose any of the corresponding policies):
if
policy & launch::async
is non-zero […]if
policy & launch::deferred
is non-zero — StoresDECAY_COPY(std::forward<F>(f))
andDECAY_COPY(std::forward<Args>(args))...
in the shared state. These copies off
andargs
constitute a deferred function. Invocation of the deferred function evaluatesINVOKE(std::move(g), std::move(xyz))
whereg
is the stored value ofDECAY_COPY(std::forward<F>(f))
andxyz
is the stored copy ofDECAY_COPY(std::forward<Args>(args))...
. Any return value is stored as the result in the shared state. Any exception propagated from the execution of the deferred function is stored as the exceptional result in the shared state. The shared state is not made ready until the function has completed. The first call to a non-timed waiting function (32.10.5 [futures.state]) on an asynchronous return object referring to this shared state shall invoke the deferred function in the thread that called the waiting function. Once evaluation ofINVOKE(std::move(g), std::move(xyz))
begins, the function is no longer considered deferred. [Note: If this policy is specified together with other policies, such as when using a policy value oflaunch::async | launch::deferred
, implementations should defer invocation or the selection of thepolicy
when no more concurrency can be effectively exploited. — end note]
vector<bool>
is missing emplace
and emplace_back
member functionsSection: 23.3.14 [vector.bool] Status: C++14 Submitter: Nevin Liber Opened: 2012-09-21 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
It should have them so that it more closely matches the vector<T>
interface, as this helps when
writing generic code.
[2012, Portland: Move to Tentatively Ready]
Question on whether the variadic template is really needed, but it turns out to be needed to support
emplace
of no arguments.
[2013-04-20 Bristol]
Proposed resolution:
This wording is relative to N3376.
Change the class template vector<bool>
synopsis, 23.3.14 [vector.bool] p1, as indicated:
namespace std { template <class Allocator> class vector<bool, Allocator> { public: […] // modifiers: template <class... Args> void emplace_back(Args&&... args); void push_back(const bool& x); void pop_back(); template <class... Args> iterator emplace(const_iterator position, Args&&... args); iterator insert(const_iterator position, const bool& x); […] }; }
operator&
Section: 24.5.1.6 [reverse.iter.elem] Status: C++14 Submitter: Alisdair Meredith Opened: 2012-09-23 Last modified: 2021-06-06
Priority: 1
View all other issues in [reverse.iter.elem].
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Discussion:
The specification for reverse_iterator::operator->
returns the address of the object yielded by dereferencing
with operator*
, but does not have the usual
wording about returning the true address of the object. As
reverse_iterator
requires the adapted iterator have
at least the bidirectional iterator category, we know that
the returned reference is a true reference, and not a proxy,
hence we can use std::addressof
on the reference
to get the right answer.
This will most likely show itself as an issue with a list
or vector
of a type with such an overloaded operator,
where algorithms are likely to work with a forward iteration, but
not with reverse iteration.
[2013-04-20, Bristol]
Resolution: Goes to open now and move to review as soon as Daniel proposes a new wording.
[2014-02-12 Issaquah meeting]
Use std::addressof
as the library uses elsewhere, then move as Immediate.
Proposed resolution:
Revise [reverse.iter.opref] p1, as indicated:
Returns: addressof
.
&(operator*())
Section: 32.7 [thread.condition] Status: C++14 Submitter: Hans Boehm Opened: 2012-09-25 Last modified: 2016-01-28
Priority: Not Prioritized
View all other issues in [thread.condition].
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Discussion:
The condition variable specification possibly leaves it unclear whether the effect of a notify_one()
call can effectively be delayed, so that a call unblocks a wait()
call that happens after the
notify_one
call. (For notify_all()
this is not detectable, since it only results in spurious
wake-ups.) Although this may at first glance seem like a contrived interpretation, it gains relevance since
glibc in fact allows the analogous behavior (see here)
and it is currently controversial whether this is correct and the Posix specification allows it (see
here).
The following proposed resolution disallows the glibc implementation, remaining consistent with the believed intent of C++11. To make that clear, we require that the "unspecified total order" O from 32.7 [thread.condition] p4 be consistent with happens-before. We also intend that the 3 components of a wait occur in order in O, but stating that explicitly seems too pedantic. Since they are numbered, it appears clear enough that they are sequenced one after the other.
Another uncertainty with the current phrasing is whether there is a single total order that includes all c.v. accesses, or one total order per c.v. We believe it actually doesn't matter, because there is no way to tell the difference, but this requires a bit more thought. We resolved it one way, just to remove the potential ambiguity.
[2012, Portland: Move to Review]
This is linked to a glibc issue, and a POSIX specification issue.
We believe the proposed wording fixes the ambiguity in C++ and is compatible with the proposed resolution for Posix (which confirms the glibc behaviour as illegal).
Moved to review (Detlef hopes to send some improved wording to the reflector).
[2013-04-20, Bristol]
Accepted for the working paper
Proposed resolution:
This wording is relative to N3376.
Change 32.7 [thread.condition] p4 as indicated:
-4- The implementation shall behave as if all executions of
notify_one
,notify_all
, and each part of thewait
,wait_for
, andwait_until
executions are executed insomea single unspecified total order consistent with the "happens before" order.
match_results(match_results&&)
Section: 28.6.9.2 [re.results.const] Status: C++23 Submitter: Pete Becker Opened: 2012-10-02 Last modified: 2023-11-22
Priority: 4
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Discussion:
28.6.9.2 [re.results.const]/3: "Move-constructs an object of class match_results
satisfying the same
postconditions as Table 141."
Table 141 lists various member functions and says that their results should be the results of the corresponding member
function calls on m
. But m
has been moved from, so the actual requirement ought to be based on the
value that m
had before the move construction, not on m
itself.
In addition to that, the requirements for the copy constructor should refer to Table 141.
Ganesh: Also, the requirements for move-assignment should refer to Table 141. Further it seems as if in Table 141 all phrases of "for all integersn < m.size()
" should be replaced by "for all unsigned integers
n < m.size()
".
[2019-03-26; Daniel comments and provides wording]
The previous Table 141 (Now Table 128 in N4810) has been modified to cover now the effects of move/copy constructors and move/copy assignment operators. Newly added wording now clarifies that for move operations the corresponding values refer to the values of the move source before the operation has started.
Re Ganesh's proposal: Note that no further wording is needed for the move-assignment operator, because in the current working draft the move-assignment operator's Effects: element refers already to Table 128. The suggested clarification of unsigned integers has been implemented by referring to non-negative integers instead. Upon suggestion from Casey, the wording also introduces Ensures: elements that refer to Table 128 and as drive-by fix eliminates a "Throws: Nothing." element from anoexcept
function.
Previous resolution [SUPERSEDED]:
This wording is relative to N4810.
Add a new paragraph at the beginning of 28.6.9.2 [re.results.const] as indicated:
-?- Table 128 lists the postconditions of
match_results
copy/move constructors and copy/move assignment operators. For move operations, the results of the expressions depending on the parameterm
denote the values they had before the respective function calls.Modify 28.6.9.2 [re.results.const] as indicated:
match_results(const match_results& m);-3- Effects: Constructs
-?- Ensures: As indicated in Table 128.an object of classa copy ofmatch_results
, asm
.match_results(match_results&& m) noexcept;-4- Effects: Move constructs
-?- Ensures: As indicated in Table 128.an object of classfrommatch_results
m
satisfying the same postconditions as Table 128. Additionally, the storedAllocator
value is move constructed fromm.get_allocator()
.-5- Throws: Nothing.match_results& operator=(const match_results& m);-6- Effects: Assigns
-?- Ensures: As indicated in Table 128.m
to*this
.The postconditions of this function are indicated in Table 128.match_results& operator=(match_results&& m);-7- Effects: Move
-?- Ensures: As indicated in Table 128.-assignsm
to*this
.The postconditions of this function are indicated in Table 128.Modify 28.6.9.2 [re.results.const], Table 128 — "
match_results
assignment operator effects", as indicated:
Table 128 — match_results
assignment operator effectscopy/move operation postconditionsElement Value ready()
m.ready()
size()
m.size()
str(n)
m.str(n)
for all non-negative integersn < m.size()
prefix()
m.prefix()
suffix()
m.suffix()
(*this)[n]
m[n]
for all non-negative integersn < m.size()
length(n)
m.length(n)
for all non-negative integersn < m.size()
position(n)
m.position(n)
for all non-negative integersn < m.size()
[2021-06-25; Daniel comments and provides new wording]
The revised wording has been rebased to N4892. It replaces the now obsolete Ensures: element by the Postconditions: element and also restores the copy constructor prototype that has been eliminated by P1722R2 as anchor point for the link to Table 140.
[2021-06-30; Reflector poll]
Set status to Tentatively Ready after six votes in favour during reflector poll.
[2021-10-14 Approved at October 2021 virtual plenary. Status changed: Voting → WP.]
Proposed resolution:
This wording is relative to N4892.
Add a new paragraph at the beginning of 28.6.9.2 [re.results.const] as indicated:
-?- Table 140 [tab:re.results.const] lists the postconditions of
match_results
copy/move constructors and copy/move assignment operators. For move operations, the results of the expressions depending on the parameterm
denote the values they had before the respective function calls.
Modify 28.6.9.2 [re.results.const] as indicated:
explicit match_results(const Allocator& a);-1- Postconditions:
ready()
returnsfalse
.size()
returns0
.match_results(const match_results& m);-?- Postconditions: As specified in Table 140.
match_results(match_results&& m) noexcept;-2- Effects: The stored
-3- Postconditions: As specified in Table 140.Allocator
value is move constructed fromm.get_allocator()
.match_results& operator=(const match_results& m);-4- Postconditions: As specified in Table 140.
match_results& operator=(match_results&& m);-5- Postconditions: As specified in Table 140.
Modify 28.6.9.2 [re.results.const], Table 140 — "match_results
assignment operator effects",
[tab:re.results.const], as indicated:
Table 140 — match_results
assignment operator effectscopy/move operation postconditionsElement Value ready()
m.ready()
size()
m.size()
str(n)
m.str(n)
for all non-negative integersn < m.size()
prefix()
m.prefix()
suffix()
m.suffix()
(*this)[n]
m[n]
for all non-negative integersn < m.size()
length(n)
m.length(n)
for all non-negative integersn < m.size()
position(n)
m.position(n)
for all non-negative integersn < m.size()
std::abs(0u)
is unclearSection: 29.7 [c.math] Status: C++17 Submitter: Daniel Krügler Opened: 2012-10-02 Last modified: 2017-07-30
Priority: 2
View all other issues in [c.math].
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Discussion:
In C++03 the following two programs are invalid:
#include <cmath> int main() { std::abs(0u); }
#include <cstdlib> int main() { std::abs(0u); }
because none of the std::abs()
overloads is a best match.
In C++11 the additional "sufficient overload" rule from 29.7 [c.math] p11 (see also LWG
2086(i)) can be read to be applicable to the std::abs()
overloads as well, which
can lead to the following possible conclusions:
The program
#include <type_traits> #include <cmath> static_assert(std::is_same<decltype(std::abs(0u)), double>(), "Oops"); int main() { std::abs(0u); // Calls std::abs(double) }
is required to be well-formed, because of sub-bullet 2 ("[..] or an integer type [..]") of 29.7 [c.math] p11 (Note that the current resolution of LWG 2086(i) doesn't fix this problem).
Any translation unit including both <cmath>
and <cstdlib>
might be ill-formed because of two conflicting requirements for the return type of the overload
std::abs(int)
.
It seems to me that at least the second outcome is not intended, personally I think that both
are unfortunate: In contrast to all other floating-point functions explicitly listed in sub-clause
29.7 [c.math], the abs
overloads have a special and well-defined meaning for
signed integers and thus have explicit overloads returning a signed integral type. I also believe that
there is no problem accepting that std::fabs(0u)
is well-defined with return type double
,
because the leading 'f' clearly signals that we have a floating point function here. But the expected
return type of std::abs(0u)
seems less than clear to me. A very reasonable answer could be that
this has the same type as its argument type, alternatively it could be a reasonably chosen signed
integer type, or a floating point type. It should also be noted, that the corresponding
"generic type function" rule set from C99/C1x in 7.25 p2+3 is restricted to the floating-point functions
from <math.h>
and <complex.h>
, so cannot be applied to the abs
functions (but to the fabs
functions!).
unsigned int
, but there would be no clear answer for the input type std::uintmax_t
.
Based on this it seems to me that the C++03 state in regard to unsigned integer values was the better situation, alerting the user that this code is ambigious at the moment (This might be change with different core-language rules as described in N3387).
[2013-04-20, Bristol]
Resolution: leave as new and bring it back in Chicago.
[2013-09 Chicago]
This issue also relates to LWG 2294(i)
STL: these two issues should be bundled Stefanus: do what Pete says, and add overloads for unsigned to return directly STL: agree Consensus that this is an issue Walter: motion to move to Open STL: no wording for 2294(i) Stefanus: move to open and note the 2 issues are related and should be moved together Stefanus: add and define unsigned versions ofabs()
[2014-02-03 Howard comments]
Defining abs()
for unsigned integers is a bad idea. Doing so would turn compile time errors into run time errors,
especially in C++ where we have templates, and the types involved are not always apparent to the programmer at design time.
For example, consider:
template <class Int>
Int
analyze(Int x, Int y)
{
// ...
if (std::abs(x - y) < threshold)
{
// ...
}
// ...
}
std::abs(expr)
is often used to ask: Are these two numbers sufficiently close? When the assumption is that
the two numbers are signed (either signed integral, or floating point), the logic is sound. But when the same logic is
accidentally used with an arithmetic type not capable of representing negative numbers, and especially if unsigned overflow
will silently happen, then the logic is no longer correct:
auto i = analyze(20u, 21u); // Today a compile time error // But withabs(unsigned)
becomes a run time error
This is not idle speculation. Search the net for "abs unsigned
"
here or
here.
chrono
duration
s and time_point
s are allowed to be based on unsigned integers. Taking the
absolute value of the difference of two such time_point
s would be easy to accidentally do (say in code templated on
time_point
s), and would certainly be a logic bug, caught at compile time unless we provide the error prone abs(unsigned)
.
[2015-02, Cologne]
GR: Do we want to make the changes to both <cmath>
and <cstdlib>
?
AM: I think so; we should provide consistent overloads.
GR: Then we're imposing restrictions on what users put in the global namespace.
AM: I'm not so worried about that. Users already know not to use C library names.
VV: So what are we going to do about unsigned integers? AM: We will say that they are ill-formed.
AM: Does anyone volunteer to send updated wording to Daniel? GR, can you do it? GR: Sure.
GR: To clarify: we want to make unsigned types ill-formed?
AM: With promotion rank at least unsigned int
.
GR: And NL suggests to just list those types.
This wording is relative to N3376.
Change 29.7 [c.math] p11 as indicated:
-11- Moreover, except for the
[…]abs
functions, there shall be additional overloads sufficient to ensure:
[2015-03-03, Geoffrey Romer provides improved wording]
In the following I've drafted combined wording to resolve LWG 2192(i) and 2294(i). Note that the first two paragraphs are taken verbatim from the P/R of LWG 2294(i), but the third is newly drafted:
[2015-05-05 Lenexa: Howard to draft updated wording]
[2015-09-11: Howard updated wording]
[2015-10, Kona Saturday afternoon]
HH: abs() for unsigned types is really dangerous. People often use abs(x - y), which would be a disaster.
TK: That's why you need a two-argument abs_diff(x, y), especially for unsigned types.
JW: Lawrence has a proposal for abs_diff in the mailing.
STL: As an alternative to considering promotions, I would just ban all unsigned types, even unsigned char.
STL: Does the PR change any implementation? Is the final paragraph just a consequence?
HH: It's a consequence. It could just be a note.
VV: Ship it as is.
STL: Editorial: capitalize the first letter in the Note.
Move to Tentatively ready.
Proposed resolution:
This wording is relative to N4527.
Insert the following new paragraphs after 29.7 [c.math] p7:
-6- In addition to the
-7- The added signatures are:int
versions of certain math functions in<cstdlib>
, C++ addslong
andlong long
overloaded versions of these functions, with the same semantics.long abs(long); // labs() long long abs(long long); // llabs() ldiv_t div(long, long); // ldiv() lldiv_t div(long long, long long); // lldiv()-?- To avoid ambiguities, C++ also adds the following overloads of
abs()
to<cstdlib>
, with the semantics defined in<cmath>
:float abs(float); double abs(double); long double abs(long double);-?- To avoid ambiguities, C++ also adds the following overloads of
abs()
to<cmath>
, with the semantics defined in<cstdlib>
:int abs(int); long abs(long); long long abs(long long);-?- If
abs()
is called with an argument of typeX
for whichis_unsigned<X>::value
istrue
and ifX
cannot be converted toint
by integral promotion (7.3.7 [conv.prom]), the program is ill-formed. [Note: arguments that can be promoted toint
are permitted for compatibility with C. — end note]
Section: 23 [containers] Status: C++14 Submitter: Richard Smith Opened: 2012-10-04 Last modified: 2017-07-05
Priority: 1
View other active issues in [containers].
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Discussion:
Most (all?) of the standard library containers have explicit default constructors. Consequently:
std::set<int> s1 = { 1, 2 }; // ok std::set<int> s2 = { 1 }; // ok std::set<int> s3 = {}; // ill-formed, copy-list-initialization selected an explicit constructor
Note that Clang + libc++ rejects the declaration of s3
for this reason. This cannot possibly match the intent.
Suggested fix: apply this transformation throughout the standard library:
set() : set(Compare()) {} explicit set(const Compare& comp= Compare(), const Allocator& = Allocator());
[ 2012-10-06: Daniel adds concrete wording. ]
[2012, Portland: Move to Open]
This may be an issue better solved by a core language tweak. Throw the issue over to EWG and see whether they believe the issue is better resolved in Core or Library.
AJM suggest we spawn a new status of 'EWG' to handle such issues - and will move this issue appropriately when the software can record such resolutions.
[2013-08-27, Joaquín M López Muñoz comments:]
For the record, I'd like to point out that the resolution proposed by the submitter, namely replacing
explicit basic_string(const Allocator& a = Allocator());
by
basic_string() : basic_string(Allocator()) {} explicit basic_string(const Allocator& a);
(and similarly for other container and container-like classes) might introduce a potential backwards-compatibility problem related with explicit instantiation. Consider for instance
struct my_allocator { my_allocator(...); // no default ctor ... }; template class std::basic_string<char, std::char_traits<char>, my_allocator<char>>;
This (which I understand is currently a valid explicit instantiation of std::basic_string
) will break if
std::basic_string
ctors are modified as proposed by this issue, since my_allocator
doesn't have
a default ctor.
[2013-10-06, Daniel comments:]
Issue 2303(i) describes the more general problem related to explicit instantiation requests in the current library and may help to solve this problem here as well.
[2014-02-13, Issaquah, Jonathan revises wording]
Previous resolution from Daniel [SUPERSEDED]:
This wording is relative to N3376.
The more general criterion for performing the suggested transformation was: Any type with an initializer-list constructor that also has an explicit default constructor.
Change class template
basic_string
synopsis, 27.4.3 [basic.string] p5 as indicated:basic_string() : basic_string(Allocator()) {} explicit basic_string(const Allocator& a= Allocator());Change 27.4.3.3 [string.cons] before p1 as indicated:
explicit basic_string(const Allocator& a= Allocator());Change class template
deque
synopsis, 23.3.5.1 [deque.overview] p2 as indicated:deque() : deque(Allocator()) {} explicit deque(const Allocator&= Allocator());Change 23.3.5.2 [deque.cons] before p1 as indicated:
explicit deque(const Allocator&= Allocator());Change class template
forward_list
synopsis, [forwardlist.overview] p3 as indicated:forward_list() : forward_list(Allocator()) {} explicit forward_list(const Allocator&= Allocator());Change [forwardlist.cons] before p1 as indicated:
explicit forward_list(const Allocator&= Allocator());Change class template
list
synopsis, 23.3.11.1 [list.overview] p2 as indicated:list() : list(Allocator()) {} explicit list(const Allocator&= Allocator());Change 23.3.11.2 [list.cons] before p1 as indicated:
explicit list(const Allocator&= Allocator());Change class template
vector
synopsis, 23.3.13.1 [vector.overview] p2 as indicated:vector() : vector(Allocator()) {} explicit vector(const Allocator&= Allocator());Change 23.3.13.2 [vector.cons] before p1 as indicated:
explicit vector(const Allocator&= Allocator());Change class template specialization
vector<bool>
synopsis, 23.3.14 [vector.bool] p1 as indicated:vector() : vector(Allocator()) {} explicit vector(const Allocator&= Allocator());Change class template
map
synopsis, 23.4.3.1 [map.overview] p2 as indicated:map() : map(Compare()) {} explicit map(const Compare& comp= Compare(), const Allocator& = Allocator());Change 23.4.3.2 [map.cons] before p1 as indicated:
explicit map(const Compare& comp= Compare(), const Allocator& = Allocator());Change class template
multimap
synopsis, 23.4.4.1 [multimap.overview] p2 as indicated:multimap() : multimap(Compare()) {} explicit multimap(const Compare& comp= Compare(), const Allocator& = Allocator());Change 23.4.4.2 [multimap.cons] before p1 as indicated:
explicit multimap(const Compare& comp= Compare(), const Allocator& = Allocator());Change class template
set
synopsis, 23.4.6.1 [set.overview] p2 as indicated:set() : set(Compare()) {} explicit set(const Compare& comp= Compare(), const Allocator& = Allocator());Change 23.4.6.2 [set.cons] before p1 as indicated:
explicit set(const Compare& comp= Compare(), const Allocator& = Allocator());Change class template
multiset
synopsis, 23.4.7.1 [multiset.overview] p2 as indicated:multiset() : multiset(Compare()) {} explicit multiset(const Compare& comp= Compare(), const Allocator& = Allocator());Change 23.4.7.2 [multiset.cons] before p1 as indicated:
explicit multiset(const Compare& comp= Compare(), const Allocator& = Allocator());Change class template
unordered_map
synopsis, 23.5.3.1 [unord.map.overview] p3 as indicated:unordered_map() : unordered_map(see below) {} explicit unordered_map(size_type n= see below, const hasher& hf = hasher(), const key_equal& eql = key_equal(), const allocator_type& a = allocator_type());Change 23.5.3.2 [unord.map.cnstr] before p1 as indicated:
unordered_map() : unordered_map(see below) {} explicit unordered_map(size_type n= see below, const hasher& hf = hasher(), const key_equal& eql = key_equal(), const allocator_type& a = allocator_type());Change class template
unordered_multimap
synopsis, 23.5.4.1 [unord.multimap.overview] p3 as indicated:unordered_multimap() : unordered_multimap(see below) {} explicit unordered_multimap(size_type n= see below, const hasher& hf = hasher(), const key_equal& eql = key_equal(), const allocator_type& a = allocator_type());Change 23.5.4.2 [unord.multimap.cnstr] before p1 as indicated:
unordered_multimap() : unordered_multimap(see below) {} explicit unordered_multimap(size_type n= see below, const hasher& hf = hasher(), const key_equal& eql = key_equal(), const allocator_type& a = allocator_type());Change class template
unordered_set
synopsis, 23.5.6.1 [unord.set.overview] p3 as indicated:unordered_set() : unordered_set(see below) {} explicit unordered_set(size_type n= see below, const hasher& hf = hasher(), const key_equal& eql = key_equal(), const allocator_type& a = allocator_type());Change 23.5.6.2 [unord.set.cnstr] before p1 as indicated:
unordered_set() : unordered_set(see below) {} explicit unordered_set(size_type n= see below, const hasher& hf = hasher(), const key_equal& eql = key_equal(), const allocator_type& a = allocator_type());Change class template
unordered_multiset
synopsis, 23.5.7.1 [unord.multiset.overview] p3 as indicated:unordered_multiset() : unordered_multiset(see below) {} explicit unordered_multiset(size_type n= see below, const hasher& hf = hasher(), const key_equal& eql = key_equal(), const allocator_type& a = allocator_type());Change 23.5.7.2 [unord.multiset.cnstr] before p1 as indicated:
unordered_multiset() : unordered_multiset(see below) {} explicit unordered_multiset(size_type n= see below, const hasher& hf = hasher(), const key_equal& eql = key_equal(), const allocator_type& a = allocator_type());
[Issaquah 2014-02-11: Move to Immediate after final review]
Proposed resolution:
This wording is relative to N3376.
The more general criterion for performing the suggested transformation was: Any type with an initializer-list constructor that also has an explicit default constructor.
Change class template basic_string
synopsis, 27.4.3 [basic.string] p5 as indicated:
basic_string() : basic_string(Allocator()) { } explicit basic_string(const Allocator& a= Allocator());
Change 27.4.3.3 [string.cons] before p1 as indicated:
explicit basic_string(const Allocator& a= Allocator());
Change class template deque
synopsis, 23.3.5.1 [deque.overview] p2 as indicated:
deque() : deque(Allocator()) { } explicit deque(const Allocator&= Allocator());
Change 23.3.5.2 [deque.cons] before p1 as indicated:
explicit deque(const Allocator&= Allocator());
Change class template forward_list
synopsis, [forwardlist.overview] p3 as indicated:
forward_list() : forward_list(Allocator()) { } explicit forward_list(const Allocator&= Allocator());
Change [forwardlist.cons] before p1 as indicated:
explicit forward_list(const Allocator&= Allocator());
Change class template list
synopsis, 23.3.11.1 [list.overview] p2 as indicated:
list() : list(Allocator()) { } explicit list(const Allocator&= Allocator());
Change 23.3.11.2 [list.cons] before p1 as indicated:
explicit list(const Allocator&= Allocator());
Change class template vector
synopsis, 23.3.13.1 [vector.overview] p2 as indicated:
vector() : vector(Allocator()) { } explicit vector(const Allocator&= Allocator());
Change 23.3.13.2 [vector.cons] before p1 as indicated:
explicit vector(const Allocator&= Allocator());
Change class template specialization vector<bool>
synopsis, 23.3.14 [vector.bool] p1 as indicated:
vector() : vector(Allocator()) { } explicit vector(const Allocator&= Allocator());
Change class template map
synopsis, 23.4.3.1 [map.overview] p2 as indicated:
map() : map(Compare()) { } explicit map(const Compare& comp= Compare(), const Allocator& = Allocator());
Change 23.4.3.2 [map.cons] before p1 as indicated:
explicit map(const Compare& comp= Compare(), const Allocator& = Allocator());
Change class template multimap
synopsis, 23.4.4.1 [multimap.overview] p2 as indicated:
multimap() : multimap(Compare()) { } explicit multimap(const Compare& comp= Compare(), const Allocator& = Allocator());
Change 23.4.4.2 [multimap.cons] before p1 as indicated:
explicit multimap(const Compare& comp= Compare(), const Allocator& = Allocator());
Change class template set
synopsis, 23.4.6.1 [set.overview] p2 as indicated:
set() : set(Compare()) { } explicit set(const Compare& comp= Compare(), const Allocator& = Allocator());
Change 23.4.6.2 [set.cons] before p1 as indicated:
explicit set(const Compare& comp= Compare(), const Allocator& = Allocator());
Change class template multiset
synopsis, 23.4.7.1 [multiset.overview] p2 as indicated:
multiset() : multiset(Compare()) { } explicit multiset(const Compare& comp= Compare(), const Allocator& = Allocator());
Change 23.4.7.2 [multiset.cons] before p1 as indicated:
explicit multiset(const Compare& comp= Compare(), const Allocator& = Allocator());
Change class template unordered_map
synopsis, 23.5.3.1 [unord.map.overview] p3 as indicated:
unordered_map(); explicit unordered_map(size_type n= see below, const hasher& hf = hasher(), const key_equal& eql = key_equal(), const allocator_type& a = allocator_type());
Change 23.5.3.2 [unord.map.cnstr] before p1 as indicated:
unordered_map() : unordered_map(size_type(see below)) { } explicit unordered_map(size_type n-1- Effects: Constructs an empty= see below, const hasher& hf = hasher(), const key_equal& eql = key_equal(), const allocator_type& a = allocator_type());unordered_map
using the specified hash function, key equality func-
tion, and allocator, and using at least n buckets.If n is not provided,For the default constructor
the number of buckets is implementation-defined.max_load_factor()
returns 1.0.
Change class template unordered_multimap
synopsis, 23.5.4.1 [unord.multimap.overview] p3 as indicated:
unordered_multimap(); explicit unordered_multimap(size_type n= see below, const hasher& hf = hasher(), const key_equal& eql = key_equal(), const allocator_type& a = allocator_type());
Change 23.5.4.2 [unord.multimap.cnstr] before p1 as indicated:
unordered_multimap() : unordered_multimap(size_type(see below)) { } explicit unordered_multimap(size_type n-1- Effects: Constructs an empty= see below, const hasher& hf = hasher(), const key_equal& eql = key_equal(), const allocator_type& a = allocator_type());unordered_multimap
using the specified hash function, key equality
function, and allocator, and using at least n buckets.If n is not provided,For the default constructor
the number of buckets is implementation-defined.max_load_factor()
returns 1.0.
Change class template unordered_set
synopsis, 23.5.6.1 [unord.set.overview] p3 as indicated:
unordered_set(); explicit unordered_set(size_type n= see below, const hasher& hf = hasher(), const key_equal& eql = key_equal(), const allocator_type& a = allocator_type());
Change 23.5.6.2 [unord.set.cnstr] before p1 as indicated:
unordered_set() : unordered_set(size_type(see below)) { } explicit unordered_set(size_type n-1- Effects: Constructs an empty= see below, const hasher& hf = hasher(), const key_equal& eql = key_equal(), const allocator_type& a = allocator_type());unordered_set
using the specified hash function, key equality func-
tion, and allocator, and using at least n buckets.If n is not provided,For the default constructor
the number of buckets is implementation-defined.max_load_factor()
returns 1.0.
Change class template unordered_multiset
synopsis, 23.5.7.1 [unord.multiset.overview] p3 as indicated:
unordered_multiset(); explicit unordered_multiset(size_type n= see below, const hasher& hf = hasher(), const key_equal& eql = key_equal(), const allocator_type& a = allocator_type());
Change 23.5.7.2 [unord.multiset.cnstr] before p1 as indicated:
unordered_multiset() : unordered_multiset(size_type(see below)) { } explicit unordered_multiset(size_type n-1- Effects: Constructs an empty= see below, const hasher& hf = hasher(), const key_equal& eql = key_equal(), const allocator_type& a = allocator_type());unordered_multiset
using the specified hash function, key equality
function, and allocator, and using at least n buckets.If n is not provided,For the default constructor
the number of buckets is implementation-defined.max_load_factor()
returns 1.0.
Section: 23.6 [container.adaptors] Status: C++14 Submitter: Sebastian Mach Opened: 2012-10-05 Last modified: 2016-01-28
Priority: Not Prioritized
View all other issues in [container.adaptors].
View all issues with C++14 status.
Discussion:
The stack
class template does not have an member type iterator
, and therefore instantiations do not
meet the general container requirements as described in 23.2.2 [container.requirements.general]. But
23.6.1 [container.adaptors.general] p1 says:
The headers
<queue>
and<stack>
define the container adaptorsqueue
,priority_queue
, andstack
. These container adaptors meet the requirements for sequence containers.
Since sequence containers is a subset of general containers, this imposes requirements on the container adaptors that are not satisfied.
Daniel Krügler: The wording change was performed as an editorial reorganization as requested by GB 116 occuring first in N3242, as a side-effect it does now make the previous implicit C++03 classification to [lib.sequences]/1 more obvious. As the NB comment noticed, the adaptors really are not sequences nor containers, so this wording needs to be fixed. The most simple way to realize that is to strike the offending sentence.
[ Daniel adds concrete wording. ]
[2013-04-20, Bristol]
Unanimous consensus that queue
and stack
are not meant to be sequences.
[2013-09-29, Chicago]
Apply to Working Paper
Proposed resolution:
This wording is relative to N3376.
Change 23.6.1 [container.adaptors.general] p1 as indicated:
-1- The headers
<queue>
and<stack>
define the container adaptorsqueue
,priority_queue
, andstack
.These container adaptors meet the requirements for sequence containers.
match_results
Section: 28.6.9 [re.results] Status: C++23 Submitter: Daniel Krügler Opened: 2012-10-06 Last modified: 2023-11-22
Priority: 3
View all other issues in [re.results].
View all issues with C++23 status.
Discussion:
The requirement expressed in 28.6.9 [re.results] p2
The class template
match_results
shall satisfy the requirements of an allocator-aware container and of a sequence container, as specified in 23.2.4 [sequence.reqmts], except that only operations defined for const-qualified sequence containers are supported.
can be read to require the existence of the described constructors from as well, but they do not exist in the synopsis.
The missing sequence constructors are:match_results(initializer_list<value_type>); match_results(size_type, const value_type&); template<class InputIterator> match_results(InputIterator, InputIterator);
The missing allocator-aware container constructors are:
match_results(const match_results&, const Allocator&); match_results(match_results&&, const Allocator&);
It should be clarified, whether (a) constructors are an exception of above mentioned operations or (b) whether
at least some of them (like those accepting a match_results
value and an allocator) should be added.
[2013-04-20, Bristol]
Check current implementations to see what they do and, possibly, write a paper.
[2013-09 Chicago]
Ask Daniel to update the proposed wording to include the allocator copy and move constructors.
[2014-01-18 Daniel changes proposed resolution]
Previous resolution from Daniel [SUPERSEDED]:
Change 28.6.9 [re.results] p2 as indicated:
The class template
match_results
shall satisfy the requirements of an allocator-aware container and of a sequence container, as specified in 23.2.4 [sequence.reqmts], except that only operations defined for const-qualified sequence containers that are not constructors are supported.
[2015-05-06 Lenexa]
MC passes important knowledge to EF.
VV, RP: Looks good.
TK: Second form should be conditionally noexcept
JY: Sequence constructors are not here, but mentioned in the issue writeup. Why?
TK: That would have been fixed by the superseded wording.
JW: How does this interact with Mike Spertus' allocator-aware regexes? [...] Perhaps it doesn't.
JW: Can't create match_results, want both old and new resolution.
JY: It's problematic that users can't create these, but not this issue.
VV: Why conditional noexcept?
MC: Allocator move might throw.
JW: Update superseded wording to "only non-constructor operations that are"?
MC: Only keep superseded, but append "and the means of constructing match_results are limited to [...]"?
JY: Bullet 4 paragraph 2 needs to address the allocator constructor.
Assigned to JW for drafting.
[2015-10, Kona Saturday afternoon]
STL: I want Mike Spertus to be aware of this issue.
Previous resolution from Daniel [SUPERSEDED]:This wording is relative to N3936.
Change 28.6.9 [re.results] p4, class template
match_results
synopsis, as indicated:[…] // 28.10.1, construct/copy/destroy: explicit match_results(const Allocator& a = Allocator()); match_results(const match_results& m); match_results(const match_results& m, const Allocator& a); match_results(match_results&& m) noexcept; match_results(match_results&& m, const Allocator& a) noexcept; […]Change 28.6.9.2 [re.results.const] as indicated: [Drafting note: Paragraph 6 as currently written, makes not much sense, because the
noexcept
does not allow any exception to propagate. Further-on, the allocator requirements do not allow for throwing move constructors. Deleting it seems to be near to editorial — end drafting note]match_results(const match_results& m); match_results(const match_results& m, const Allocator& a);-4- Effects: Constructs an object of class
match_results
, as a copy ofm
.match_results(match_results&& m) noexcept; match_results(match_results&& m, const Allocator& a) noexcept;-5- Effects: Move-constructs an object of class
match_results
fromm
satisfying the same postconditions as Table 142.AdditionallyFor the first form, the storedAllocator
value is move constructed fromm.get_allocator()
.-6- Throws: Nothing if the allocator's move constructor throws nothing.
[2019-03-27 Jonathan updates proposed resolution]
Previous resolution [SUPERSEDED]:
This wording is relative to N4810.
These edits overlap with the proposed resolution of 2191(i) but it should be obvious how to resolve the conflicts. Both resolutions remove the word "Additionally" from p4. Issue 2191 removes the entire Throws: element in p5 but this issue replaces it with different text that applies to the new constructor only.
Change 28.6.9 [re.results] p4, class template
match_results
synopsis, as indicated:[…] // 30.10.1, construct/copy/destroy: explicit match_results(const Allocator& a = Allocator()); match_results(const match_results& m); match_results(const match_results& m, const Allocator& a); match_results(match_results&& m) noexcept; match_results(match_results&& m, const Allocator& a); […]Change 28.6.9.2 [re.results.const] as indicated:
match_results(const match_results& m); match_results(const match_results& m, const Allocator& a);-3- Effects: Constructs an object of class
match_results
, as a copy ofm
. For the second form, the storedAllocator
value is constructed froma
.match_results(match_results&& m) noexcept; match_results(match_results&& m, const Allocator& a);-4- Effects: Move-constructs an object of class
match_results
fromm
satisfying the same postconditions as Table 128.AdditionallyFor the first form, the storedAllocator
value is move constructed fromm.get_allocator()
. For the second form, the storedAllocator
value is constructed froma
.-6- Throws:
Nothing.The second form throws nothing ifa == m.get_allocator()
.
[2022-11-06; Daniel syncs wording with recent working draft]
To ensure that all constructors are consistent in regard to the information about how the stored
allocator is constructed, more wording is added. This harmonizes with the way how we specify the
individual container constructors (Such as vector
) even though 23.2.2.5 [container.alloc.reqmts]
already provides some guarantees. For the copy-constructor we intentionally refer to
23.2.2.2 [container.reqmts] so that we don't need to repeat what is said there.
[Kona 2022-11-08; Move to Ready]
[2023-02-13 Approved at February 2023 meeting in Issaquah. Status changed: Voting → WP.]
Proposed resolution:
This wording is relative to N4917.
Change 28.6.9 [re.results], class template match_results
synopsis, as indicated:
[…] // 28.6.9.2 [re.results.const], construct/copy/destroy: match_results() : match_results(Allocator()) {} explicit match_results(const Allocator& a); match_results(const match_results& m); match_results(const match_results& m, const Allocator& a); match_results(match_results&& m) noexcept; match_results(match_results&& m, const Allocator& a); […]
Change 28.6.9.2 [re.results.const] as indicated:
explicit match_results(const Allocator& a);-?- Effects: The stored
-2- Postconditions:Allocator
value is constructed froma
.ready()
returnsfalse
.size()
returns0
.match_results(const match_results& m); match_results(const match_results& m, const Allocator& a);-?- Effects: For the first form, the stored
-3- Postconditions: As specified in Table 142 [tab:re.results.const].Allocator
value is obtained as specified in 23.2.2.2 [container.reqmts]. For the second form, the storedAllocator
value is constructed froma
.match_results(match_results&& m) noexcept; match_results(match_results&& m, const Allocator& a);-4- Effects: For the first form, t
-5- Postconditions: As specified in Table 142 [tab:re.results.const]. -?- Throws: The second form throws nothing ifThe storedAllocator
value is move constructed fromm.get_allocator()
. For the second form, the storedAllocator
value is constructed froma
.a == m.get_allocator()
istrue
.
is_*[copy/move]_[constructible/assignable]
unclear for non-referencable typesSection: 21.3.5.4 [meta.unary.prop] Status: C++14 Submitter: Daniel Krügler Opened: 2012-10-06 Last modified: 2016-01-28
Priority: Not Prioritized
View other active issues in [meta.unary.prop].
View all other issues in [meta.unary.prop].
View all issues with C++14 status.
Discussion:
The pre-conditions for the type is_copy_constructible
allow for template argument types were the language forbids
forming a reference, namely void
types and function types that have cv-qualifiers or a ref-qualifier.
is_constructible<T, const T&>::value
istrue
.
leaves it open whether such argument types would (a) create a well-formed instantiation of the trait template or if so (b) what
the outcome of the trait evaluation would be, as an example consider std::is_copy_constructible<void>
.
void
type or of function type at all, so it would be surprising
to return a positive result for copy or move construction if no other construction could succeed. It is also not
possible to assign to a any of these values (because there is no way to form lvalues of them), so the same argumentation
can be applied to the is_copy/move_assignable
traits as well.
To reduce the amount of wording changes and repetitions, I suggest to define the term referenceable type in
sub-clause [definitions] or alternatively in the core language to describe types to which references
can be created via a typedef name. This definition corresponds to what the support concept ReferentType
intended
to describe during concept time.
In addition, LWG issue 2101(i) can also take advantage of the definition of a referenceable type.
If the proposed resolution for LWG issue 2101(i) would be accepted, there is an alternative solution possible
with the same effects. Now we would be able to use the now always well-formed instantiation of
std::add_lvalue_reference
to modify the current definition of is_copy_constructible
to
is_constructible<T,
is true.
typename add_lvalue_reference<
typename add_const<T>::type>::type>::value
and similar changes for the other affected traits.
[2012-10 Portland: Move to Open]
Referencable-type should be defined as "something that can be bound into a reference" or similar, rather than a list of types where that is true today. We can then provide the list of known types that cannot be bound as examples that do not qualify in a note.
Otherwise we are happy with the wording. AJM to redraft the definition and move to Review.
[2013-04-18, Bristol]
Proposed resolution:
This wording is relative to N3376.
Add the following new definition to [definitions] as indicated:
referenceable type [defns.referenceable]
An object type, a function type that does not have cv-qualifiers or a ref-qualifier, or a reference type. [Note: The term describes a type to which a reference can be created, including reference types. — end note]
Change Table 49 as indicated:
Template | Condition | Preconditions |
---|---|---|
template <class T> struct is_copy_constructible;
|
For a referenceable type T , the same result asis_constructible<T, const T&>::value |
T shall be a complete type,(possibly cv-qualified) void , or anarray of unknown bound. |
template <class T> struct is_move_constructible;
|
For a referenceable type T , the same result asis_constructible<T, T&&>::value |
T shall be a complete type,(possibly cv-qualified) void , or anarray of unknown bound. |
…
|
||
template <class T> struct is_copy_assignable;
|
For a referenceable type T , the same result asis_assignable<T&, const T&>::value |
T shall be a complete type,(possibly cv-qualified) void , or anarray of unknown bound. |
template <class T> struct is_move_assignable;
|
For a referenceable type T , the same result asis_assignable<T&, T&&>::value |
T shall be a complete type,(possibly cv-qualified) void , or anarray of unknown bound. |
…
|
||
template <class T> struct is_trivially_copy_constructible;
|
For a referenceable type T , the same result asis_trivially_constructible<T, const T&>::value |
T shall be a complete type,(possibly cv-qualified) void , or anarray of unknown bound. |
template <class T> struct is_trivially_move_constructible;
|
For a referenceable type T , the same result asis_trivially_constructible<T, T&&>::value |
T shall be a complete type,(possibly cv-qualified) void , or anarray of unknown bound. |
…
|
||
template <class T> struct is_trivially_copy_assignable;
|
For a referenceable type T , the same result asis_trivially_assignable<T&, const T&>::value |
T shall be a complete type,(possibly cv-qualified) void , or anarray of unknown bound. |
template <class T> struct is_trivially_move_assignable;
|
For a referenceable type T , the same result asis_trivially_assignable<T&, T&&>::value |
T shall be a complete type,(possibly cv-qualified) void , or anarray of unknown bound. |
…
|
||
template <class T> struct is_nothrow_copy_constructible;
|
For a referenceable type T , the same result asis_nothrow_constructible<T, const T&>::value |
T shall be a complete type,(possibly cv-qualified) void , or anarray of unknown bound. |
template <class T> struct is_nothrow_move_constructible;
|
For a referenceable type T , the same result asis_nothrow_constructible<T, T&&>::value |
T shall be a complete type,(possibly cv-qualified) void , or anarray of unknown bound. |
…
|
||
template <class T> struct is_nothrow_copy_assignable;
|
For a referenceable type T , the same result asis_nothrow_assignable<T&, const T&>::value |
T shall be a complete type,(possibly cv-qualified) void , or anarray of unknown bound. |
template <class T> struct is_nothrow_move_assignable;
|
For a referenceable type T , the same result asis_nothrow_assignable<T&, T&&>::value |
T shall be a complete type,(possibly cv-qualified) void , or anarray of unknown bound. |
is_[un]signed
unclear for non-arithmetic typesSection: 21.3.5.4 [meta.unary.prop] Status: C++14 Submitter: Daniel Krügler Opened: 2012-10-07 Last modified: 2016-01-28
Priority: Not Prioritized
View other active issues in [meta.unary.prop].
View all other issues in [meta.unary.prop].
View all issues with C++14 status.
Discussion:
The pre-conditions for the trait is_signed
allow for any types as template arguments,
including non-arithmetic ones.
is_arithmetic<T>::value && T(-1) < T(0)
looks like real code and so leaves it open whether such argument types would create a well-formed instantiation of the trait template or not. As written this definition would lead to a hard instantiation error for a non-arithmetic type like e.g.
struct S {};
I would suggest that the wording clarifies that the instantiation would be valid for such types as well, by means of a specification that is not an exact code pattern. This also reflects how existing implementations behave.
[2013-03-15 Issues Teleconference]
Moved to Tentatively Ready.
[2013-04-20 Bristol]
Proposed resolution:
This wording is relative to N3376.
Change Table 49 as indicated:
Template | Condition | Preconditions |
---|---|---|
template <class T> struct is_signed;
|
If is_arithmetic<T>::value is true, the same result asintegral_constant<bool, T(-1) < T(0)>::value ;otherwise, false. |
|
template <class T> struct is_unsigned;
|
If is_arithmetic<T>::value is true, the same result asintegral_constant<bool, T(0) < T(-1)>::value ;otherwise, false. |
Section: 23.2.3 [container.requirements.dataraces] Status: C++14 Submitter: Jonathan Wakely Opened: 2012-10-17 Last modified: 2016-01-28
Priority: Not Prioritized
View all other issues in [container.requirements.dataraces].
View all issues with C++14 status.
Discussion:
23.2.3 [container.requirements.dataraces]/2 says "[…] implementations are
required to avoid data races when the contents of the contained object in different
elements in the same sequence, excepting vector<bool>
, are modified
concurrently."
This should say "same container" instead of "same sequence", to avoid the interpretation that it only applies to sequence containers.
[2013-03-15 Issues Teleconference]
Moved to Tentatively Ready.
[2013-04-20 Bristol]
Proposed resolution:
This wording is relative to N3376.
Change 23.2.3 [container.requirements.dataraces]/2 as indicated:
-2- Notwithstanding (16.4.6.10 [res.on.data.races]), implementations are required to avoid data races when the contents of the contained object in different elements in the same
-3- [Note: For asequencecontainer, exceptingvector<bool>
, are modified concurrently.vector<int> x
with a size greater than one,x[1] = 5
and*x.begin() = 10
can be executed concurrently without a data race, butx[0] = 5
and*x.begin() = 10
executed concurrently may result in a data race. As an exception to the general rule, for avector<bool> y, y[0] = true
may race withy[1] = true
. — end note ]
scoped_allocator_adaptor
uses wrong argument types for piecewise constructionSection: 20.6.4 [allocator.adaptor.members] Status: C++14 Submitter: Jonathan Wakely Opened: 2012-10-19 Last modified: 2016-01-28
Priority: Not Prioritized
View all other issues in [allocator.adaptor.members].
View all issues with C++14 status.
Discussion:
In 20.6.4 [allocator.adaptor.members] paragraph 11 the effects
clause says a tuple should be constructed with inner_allocator_type()
,
but that creates an rvalue which cannot bind to inner_allocator_type&
,
and would also be wrong if this->inner_allocator() != inner_allocator_type()
.
This could be considered editorial, since the current wording doesn't even compile.
Secondly, in the same paragraph, the tuple objects xprime
and yprime
seem to be lvalues and might be constructed by copying x
and y
. This
prevents using scoped_allocator
to construct pairs from arguments of
move-only types. I believe the tuple_cast()
expressions should use
std::move(x)
and std::move(y)
to move from the incoming arguments
(which are passed by value to candidates for moving) and the final sentence of the paragraph
should be:
then calls OUTERMOST_ALLOC_TRAITS(*this)::construct(OUTERMOST (*this), p, piecewise_construct,
std::move(xprime), std::move(yprime))
.
so that the objects are passed to std::pair
's piecewise constructor as rvalues and
are eligible for moving into the constructor arguments. This could also be considered editorial,
as the current wording prevents certain uses which were intended to be supported.
[2013-03-15 Issues Teleconference]
Moved to Review.
The resolution looks good, with wording provided by a recent implementer. However, it will take more time than the telecon allows to review with confidence, and we would like Pablo to at least take a look over the resolution and confirm that it matches the design intent.
[2013-04-18, Bristol]
Proposed resolution:
This wording is relative to N3376.
Change 20.6.4 [allocator.adaptor.members] paragraph 11 as indicated:
-11- Effects: Constructs a
tuple
objectxprime
fromx
by the following rules:
If
uses_allocator<T1, inner_allocator_type>::value
isfalse
andis_constructible<T1, Args1...>::value
istrue
, thenxprime
isx
.Otherwise, if
uses_allocator<T1, inner_allocator_type>::value
istrue
andis_constructible<T1, allocator_arg_t, inner_allocator_type, Args1...>::value
istrue
, thenxprime
istuple_cat(tuple<allocator_arg_t, inner_allocator_type&>( allocator_arg, inner_allocator
._type()), std::move(x))Otherwise, if
uses_allocator<T1, inner_allocator_type>::value
istrue
andis_constructible<T1, Args1..., inner_allocator_type>::value
istrue
, thenxprime
istuple_cat(std::move(x), tuple<inner_allocator_type&>(inner_allocator
._type()))Otherwise, the program is ill-formed.
and constructs a
tuple
objectyprime
fromy
by the following rules:
If
uses_allocator<T2, inner_allocator_type>::value
isfalse
andis_constructible<T2, Args2...>::value
istrue
, thenyprime
isy
.Otherwise, if
uses_allocator<T2, inner_allocator_type>::value
istrue
andis_constructible<T2, allocator_arg_t, inner_allocator_type, Args2...>::value
istrue
, thenyprime
istuple_cat(tuple<allocator_arg_t, inner_allocator_type&>( allocator_arg, inner_allocator
._type()), std::move(y))Otherwise, if
uses_allocator<T2, inner_allocator_type>::value
istrue
andis_constructible<T2, Args2..., inner_allocator_type>::value
istrue
, thenyprime
istuple_cat(std::move(y), tuple<inner_allocator_type&>(inner_allocator
._type()))Otherwise, the program is ill-formed.
then calls
OUTERMOST_ALLOC_TRAITS(*this)::construct(OUTERMOST(*this), p, piecewise_construct, std::move(xprime), std::move(yprime))
.
regex_match
and regex_search
Section: 28.6.10.2 [re.alg.match], 28.6.10.3 [re.alg.search] Status: C++14 Submitter: Pete Becker Opened: 2012-10-24 Last modified: 2017-07-05
Priority: 0
View all other issues in [re.alg.match].
View all issues with C++14 status.
Discussion:
Table 142 lists post-conditions on the match_results
object when a call to regex_match
succeeds.
regex_match
is required to match the entire target sequence. The post-condition for m[0].matched
is "true if a full match was found." Since these are conditions for a successful search which is, by definition,
a full match, the post-condition should be simply "true".
There's an analogous probem in Table 143: the condition for m[0].matched
is "true if a match was found,
false otherwise." But Table 143 gives post-conditions for a successful match, so the condition should be simply
"true".
Furthermore, they have explicit requirements for m[0].first
, m[0].second
, and m[0].matched
.
They also have requirements for the other elements of m
, described as m[n].first
, m[n].second
,
and m[n].matched
, in each case qualifying the value of n
as "for n < m.size()
". Since
there is an explicit description for n == 0
, this qualification should be "for 0 < n < m.size()
"
in all 6 places.
[Issaquah 2014-02-11: Move to Immediate]
Proposed resolution:
This wording is relative to N3376.
Change Table 142 as indicated:
Element | Value |
---|---|
…
|
|
m[0].first
|
first
|
m[0].second
|
last
|
m[0].matched
|
true |
m[n].first
|
For all integers 0 < n < m.size() , the start of the sequence
that matched sub-expression n .Alternatively, if subexpression n did not participate in the match, then last .
|
m[n].second
|
For all integers 0 < n < m.size() , the end of the sequence that
matched sub-expression n .Alternatively, if sub-expression n did not participate in the match, then last .
|
m[n].matched
|
For all integers 0 < n < m.size() , true if sub-expression
n participated in the match, false otherwise.
|
Change Table 143 as indicated:
Element | Value |
---|---|
…
|
|
m[0].first
|
The start of the sequence of characters that matched the regular expression |
m[0].second
|
The end of the sequence of characters that matched the regular expression |
m[0].matched
|
true |
m[n].first
|
For all integers 0 < n < m.size() , the start of the sequence
that matched sub-expression n .Alternatively, if subexpression n did not participate in the match, then last .
|
m[n].second
|
For all integers 0 < n < m.size() , the end of the sequence that
matched sub-expression n .Alternatively, if sub-expression n did not participate in the match, then last .
|
m[n].matched
|
For all integers 0 < n < m.size() , true if sub-expression
n participated in the match, false otherwise.
|
basic_string::at
should not have a Requires clauseSection: 27.4.3.6 [string.access] Status: C++14 Submitter: Nevin Liber Opened: 2012-10-26 Last modified: 2016-01-28
Priority: Not Prioritized
View all other issues in [string.access].
View all issues with C++14 status.
Discussion:
basic_string::at()
has a wide contract and should not have a "Requires" clause on it.
[2013-01-17, Juan Soulie comments]
This issue would also apply to every member function of basic_string
that throws
out_of_range
, and to some cases where a length_error
can be thrown.
[2013-03-15 Issues Teleconference]
Moved to Review.
While this could simply move to Ready on inspection, there is concern that this will not be the only such case. Alisdair volunteers to review clause 21/23 for more of such issues for Bristol, and update the proposed resolution as necessary.
[2013-04-18, Bristol]
Proposed resolution:
This wording is relative to N3376.
Remove 27.4.3.6 [string.access] p5:
const_reference at(size_type pos) const; reference at(size_type pos);-6- Throws:
-5- Requires:pos < size()
out_of_range
ifpos >= size()
. -7- Returns:operator[](pos)
.
std::reverse_iterator
should be a literal typeSection: 24.5.1 [reverse.iterators] Status: Resolved Submitter: Jeffrey Yasskin Opened: 2012-10-30 Last modified: 2017-03-12
Priority: 3
View all other issues in [reverse.iterators].
View all issues with Resolved status.
Discussion:
std::reverse_iterator::reverse_iterator(Iterator)
should be constexpr
so that other constexpr functions can return reverse_iterator
s. Of the
other methods, the other constructors, base()
, operator+
, operator-
,
operator[]
, and the non-member operators can probably also be
constexpr
.
operator*
cannot be constexpr because it involves an assignment to a
member variable. Discussion starting with c++std-lib-33282 indicated
that it would be useful to make reverse_iterator a literal type
despite this restriction on its use at compile time.
Proposed resolution:
This issue was Resolved by paper P0031R0 adopted at Jacksonville, 2016.assign()
overspecified for sequence containersSection: 23.3 [sequences] Status: C++14 Submitter: Jonathan Wakely Opened: 2012-10-31 Last modified: 2016-01-28
Priority: Not Prioritized
View all other issues in [sequences].
View all issues with C++14 status.
Discussion:
DR 704(i) ensures allocator-aware containers can reuse existing
elements during copy/move assignment, and sequence containers can do
the same for assign()
.
std::list
(which was changed by DR 320(i)) the sequence
containers define the Effects of assign()
in terms of clear()
followed
by insert
. A user-defined allocator can easily tell whether all old
elements are cleared and then new elements inserted or whether existing elements are assigned
to, so those Effects clauses cannot be ignored via the as-if rule.
The descriptions of the assign()
members for deque
, forward_list
and
vector
should be removed. Their intended effects are entirely described by the
sequence container requirements table, and the specific definitions of them are worse than
redundant, they're contradictory (if the operations are defined in terms of erase
and
insert
then there's no need for elements to be assignable.) The descriptions of
assign()
for list
are correct but redundant, so should be removed too.
[2013-03-15 Issues Teleconference]
Moved to Tentatively Ready.
[2013-04-20 Bristol]
Proposed resolution:
This wording is relative to N3376.
Edit 23.3.5.2 [deque.cons] to remove everything after paragraph 10:
template <class InputIterator> void assign(InputIterator first, InputIterator last);
-11- Effects:erase(begin(), end()); insert(begin(), first, last);void assign(size_type n, const T& t);
-12- Effects:erase(begin(), end()); insert(begin(), n, t);
Edit [forwardlist.cons] to remove everything after paragraph 10:
template <class InputIterator> void assign(InputIterator first, InputIterator last);
-11- Effects:clear(); insert_after(before_begin(), first, last);
void assign(size_type n, const T& t);
-12- Effects:clear(); insert_after(before_begin(), n, t);
Edit 23.3.11.2 [list.cons] to remove everything after paragraph 10:
template <class InputIterator> void assign(InputIterator first, InputIterator last);
-11- Effects: Replaces the contents of the list with the range[first, last)
.void assign(size_type n, const T& t);
-12- Effects: Replaces the contents of the list withn
copies oft
.
Edit 23.3.13.2 [vector.cons] to remove everything after paragraph 10:
template <class InputIterator> void assign(InputIterator first, InputIterator last);
-11- Effects:erase(begin(), end()); insert(begin(), first, last);void assign(size_type n, const T& t);
-12- Effects:erase(begin(), end()); insert(begin(), n, t);
Section: 23.3 [sequences], 23.4 [associative], 23.5 [unord] Status: C++14 Submitter: Jonathan Wakely Opened: 2012-11-01 Last modified: 2016-01-28
Priority: Not Prioritized
View all other issues in [sequences].
View all issues with C++14 status.
Discussion:
The forward_list(size_type)
constructor has no allocator-extended
equivalent, preventing the following code from compiling:
#include <forward_list> #include <vector> #include <scoped_allocator> using namespace std; int main() { using list = forward_list<int>; vector<list, scoped_allocator_adaptor<list::allocator_type>> v; v.emplace_back(1u); }
The very same problem exists for all allocator-aware sequence containers.
In addition it exists for associative containers. For example, it's possible to constructstd::set<int>{0, 1, 2}
but not std::set<int>{{0, 1, 2}, alloc}
,
and possible to construct std::set<int>{begin, end}
but not
std::set<int>{begin, end, alloc}
.
This makes the following program fail when SCOPED
is defined:
#include <set> #include <vector> #include <scoped_allocator> #if SCOPED using A = std::scoped_allocator_adaptor<std::allocator<int>>; #else using A = std::allocator<int>; #endif int main() { int values[] = {0, 1, 2}; std::vector<std::set<int>, A> v; v.emplace_back(std::begin(values), std::end(values)); }
[2013-03-15 Issues Teleconference]
Moved to Review.
Jonathan: There are lots of places where this is missing.
Howard: We should ping Pablo, this might be a deliberate design decision.
[2013-04-18, Bristol]
Proposed resolution:
This wording is relative to N3485.
Edit the synopsis in 23.3.5.1 [deque.overview]/2:
namespace std { template <class T, class Allocator = allocator<T> > class deque { public: […] explicit deque(const Allocator& = Allocator()); explicit deque(size_type n, const Allocator& = Allocator()); […] }; }
Edit 23.3.5.2 [deque.cons]/2:
explicit deque(size_type n, const Allocator& = Allocator());-3- Effects: Constructs a
deque
withn
default-inserted elements using the specified allocator.
Edit the synopsis in [forwardlist.overview]/3:
namespace std { template <class T, class Allocator = allocator<T> > class forward_list { public: […] explicit forward_list(const Allocator& = Allocator()); explicit forward_list(size_type n, const Allocator& = Allocator()); […] }; }
Edit [forwardlist.cons]/3:
explicit forward_list(size_type n, const Allocator& = Allocator());-3- Effects: Constructs a
forward_list
object withn
default-inserted elements using the specified allocator.
Edit the synopsis in 23.3.11.1 [list.overview]/2:
namespace std { template <class T, class Allocator = allocator<T> > class list { public: […] explicit list(const Allocator& = Allocator()); explicit list(size_type n, const Allocator& = Allocator()); […] }; }
Edit 23.3.11.2 [list.cons]/3:
explicit list(size_type n, const Allocator& = Allocator());-3- Effects: Constructs a
list
withn
default-inserted elements using the specified allocator.
Edit the synopsis in 23.3.13.1 [vector.overview]/2:
namespace std { template <class T, class Allocator = allocator<T> > class vector { public: […] explicit vector(const Allocator& = Allocator()); explicit vector(size_type n, const Allocator& = Allocator()); […] }; }
Edit 23.3.13.2 [vector.cons]/3:
explicit vector(size_type n, const Allocator& = Allocator());-3- Effects: Constructs a
vector
withn
default-inserted elements using the specified allocator.
Edit the synopsis in 23.3.14 [vector.bool]/1:
namespace std { template <class Allocator> class vector<bool, Allocator> { class vector { public: […] explicit vector(const Allocator& = Allocator()); explicit vector(size_type n, const Allocator& = Allocator());explicitvector(size_type n, const bool& value= bool(), const Allocator& = Allocator()); […] }; }
Add to the synopsis in 23.4.3.1 [map.overview] p2:
namespace std { template <class Key, class T, class Compare = less<Key>, class Allocator = allocator<pair<const Key, T> > > { class map { public: […] map(initializer_list<value_type>, const Compare& = Compare(), const Allocator& = Allocator()); template <class InputIterator> map(InputIterator first, InputIterator last, const Allocator& a) : map(first, last, Compare(), a) { } map(initializer_list<value_type> il, const Allocator& a) : map(il, Compare(), a) { } ~map(); […] }; }
Add to the synopsis in 23.4.4.1 [multimap.overview] p2:
namespace std { template <class Key, class T, class Compare = less<Key>, class Allocator = allocator<pair<const Key, T> > > { class multimap { public: […] multimap(initializer_list<value_type>, const Compare& = Compare(), const Allocator& = Allocator()); template <class InputIterator> multimap(InputIterator first, InputIterator last, const Allocator& a) : multimap(first, last, Compare(), a) { } multimap(initializer_list<value_type> il, const Allocator& a) : multimap(il, Compare(), a) { } ~multimap(); […] }; }
Add to the synopsis in 23.4.6.1 [set.overview] p2:
namespace std { template <class Key, class Compare = less<Key>, class Allocator = allocator<Key> > { class set { public: […] set(initializer_list<value_type>, const Compare& = Compare(), const Allocator& = Allocator()); template <class InputIterator> set(InputIterator first, InputIterator last, const Allocator& a) : set(first, last, Compare(), a) { } set(initializer_list<value_type> il, const Allocator& a) : set(il, Compare(), a) { } ~set(); […] }; }
Add to the synopsis in 23.4.7.1 [multiset.overview] p2:
namespace std { template <class Key, class Compare = less<Key>, class Allocator = allocator<Key> > { class multiset { public: […] multiset(initializer_list<value_type>, const Compare& = Compare(), const Allocator& = Allocator()); template <class InputIterator> multiset(InputIterator first, InputIterator last, const Allocator& a) : multiset(first, last, Compare(), a) { } multiset(initializer_list<value_type> il, const Allocator& a) : multiset(il, Compare(), a) { } ~multiset(); […] }; }
Add to the synopsis in 23.5.3.1 [unord.map.overview] p3:
namespace std { template <class Key, class T, class Hash = hash<Key>, class Pred = std::equal_to<Key>, class Allocator = std::allocator<std::pair<const Key, T> > > { class unordered_map { public: […] unordered_map(initializer_list<value_type>, size_type = see below, const hasher& hf = hasher(), const key_equal& eql = key_equal(), const allocator_type& a = allocator_type()); unordered_map(size_type n, const allocator_type& a) : unordered_map(n, hasher(), key_equal(), a) { } unordered_map(size_type n, const hasher& hf, const allocator_type& a) : unordered_map(n, hf, key_equal(), a) { } template <class InputIterator> unordered_map(InputIterator f, InputIterator l, size_type n, const allocator_type& a) : unordered_map(f, l, n, hasher(), key_equal(), a) { } template <class InputIterator> unordered_map(InputIterator f, InputIterator l, size_type n, const hasher& hf, const allocator_type& a) : unordered_map(f, l, n, hf, key_equal(), a) { } unordered_map(initializer_list<value_type> il, size_type n, const allocator_type& a) : unordered_map(il, n, hasher(), key_equal(), a) { } unordered_map(initializer_list<value_type> il, size_type n, const hasher& hf, const allocator_type& a) : unordered_map(il, n, hf, key_equal(), a) { } ~unordered_map(); […] }; }
Add to the synopsis in 23.5.4.1 [unord.multimap.overview] p3:
namespace std { template <class Key, class T, class Hash = hash<Key>, class Pred = std::equal_to<Key>, class Allocator = std::allocator<std::pair<const Key, T> > > { class unordered_multimap { public: […] unordered_multimap(initializer_list<value_type>, size_type = see below, const hasher& hf = hasher(), const key_equal& eql = key_equal(), const allocator_type& a = allocator_type()); unordered_multimap(size_type n, const allocator_type& a) : unordered_multimap(n, hasher(), key_equal(), a) { } unordered_multimap(size_type n, const hasher& hf, const allocator_type& a) : unordered_multimap(n, hf, key_equal(), a) { } template <class InputIterator> unordered_multimap(InputIterator f, InputIterator l, size_type n, const allocator_type& a) : unordered_multimap(f, l, n, hasher(), key_equal(), a) { } template <class InputIterator> unordered_multimap(InputIterator f, InputIterator l, size_type n, const hasher& hf, const allocator_type& a) : unordered_multimap(f, l, n, hf, key_equal(), a) { } unordered_multimap(initializer_list<value_type> il, size_type n, const allocator_type& a) : unordered_multimap(il, n, hasher(), key_equal(), a) { } unordered_multimap(initializer_list<value_type> il, size_type n, const hasher& hf, const allocator_type& a) : unordered_multimap(il, n, hf, key_equal(), a) { } ~unordered_multimap(); […] }; }
Add to the synopsis in 23.5.6.1 [unord.set.overview] p3:
namespace std { template <class Key, class Hash = hash<Key>, class Pred = std::equal_to<Key>, class Allocator = std::allocator<Key> > { class unordered_set { public: […] unordered_set(initializer_list<value_type>, size_type = see below, const hasher& hf = hasher(), const key_equal& eql = key_equal(), const allocator_type& a = allocator_type()); unordered_set(size_type n, const allocator_type& a) : unordered_set(n, hasher(), key_equal(), a) { } unordered_set(size_type n, const hasher& hf, const allocator_type& a) : unordered_set(n, hf, key_equal(), a) { } template <class InputIterator> unordered_set(InputIterator f, InputIterator l, size_type n, const allocator_type& a) : unordered_set(f, l, n, hasher(), key_equal(), a) { } template <class InputIterator> unordered_set(InputIterator f, InputIterator l, size_type n, const hasher& hf, const allocator_type& a) : unordered_set(f, l, n, hf, key_equal(), a) { } unordered_set(initializer_list<value_type> il, size_type n, const allocator_type& a) : unordered_set(il, n, hasher(), key_equal(), a) { } unordered_set(initializer_list<value_type> il, size_type n, const hasher& hf, const allocator_type& a) : unordered_set(il, n, hf, key_equal(), a) { } ~unordered_set(); […] }; }
Add to the synopsis in 23.5.7.1 [unord.multiset.overview] p3:
namespace std { template <class Key, class Hash = hash<Key>, class Pred = std::equal_to<Key>, class Allocator = std::allocator<Key> > { class unordered_multiset { public: […] unordered_multiset(initializer_list<value_type>, size_type = see below, const hasher& hf = hasher(), const key_equal& eql = key_equal(), const allocator_type& a = allocator_type()); unordered_multiset(size_type n, const allocator_type& a) : unordered_multiset(n, hasher(), key_equal(), a) { } unordered_multiset(size_type n, const hasher& hf, const allocator_type& a) : unordered_multiset(n, hf, key_equal(), a) { } template <class InputIterator> unordered_multiset(InputIterator f, InputIterator l, size_type n, const allocator_type& a) : unordered_multiset(f, l, n, hasher(), key_equal(), a) { } template <class InputIterator> unordered_multiset(InputIterator f, InputIterator l, size_type n, const hasher& hf, const allocator_type& a) : unordered_multiset(f, l, n, hf, key_equal(), a) { } unordered_multiset(initializer_list<value_type> il, size_type n, const allocator_type& a) : unordered_multiset(il, n, hasher(), key_equal(), a) { } unordered_multiset(initializer_list<value_type> il, size_type n, const hasher& hf, const allocator_type& a) : unordered_multiset(il, n, hf, key_equal(), a) { } ~unordered_multiset(); […] }; }
Section: 23.2.2 [container.requirements.general] Status: C++14 Submitter: Jonathan Wakely Opened: 2012-11-07 Last modified: 2016-01-28
Priority: Not Prioritized
View other active issues in [container.requirements.general].
View all other issues in [container.requirements.general].
View all issues with C++14 status.
Discussion:
23.2.2 [container.requirements.general]/7 says:
All other constructors for these container types take an
Allocator&
argument (16.4.4.6 [allocator.requirements]), an allocator whose value type is the same as the container's value type.
This is a strange place to state the requirement on the allocator's
value_type
, because the allocator is a property (and template
parameter) of the container type not of some of its constructors.
It's also unclear whether "Allocator&
" refers to the concept (as
implied by the cross-reference to the allocator requirements in Clause 17)
or to the container's template parameter (as implied by the fact
it's shown as an lvalue-reference type.) I believe the latter is
intended, because those constructors can't take any model of the
allocator concept, they can only take the container's allocator_type
.
allocator_type
. There is
already a cross-reference to the allocator requirements earlier in the paragraph,
so it doesn't need to be repeated in another place where it causes confusion.
[2013-03-15 Issues Teleconference]
Moved to Tentatively Ready.
[2013-04-20 Bristol]
Proposed resolution:
This wording is relative to N3485.
Edit 23.2.2 [container.requirements.general] paragraph 7:
Unless otherwise specified, all containers defined in this clause obtain memory using an allocator (see 16.4.4.6 [allocator.requirements]). Copy constructors for these container types obtain an allocator by calling
allocator_traits<allocator_type>::select_on_container_copy_construction
on their first parameters. Move constructors obtain an allocator by move construction from the allocator belonging to the container being moved. Such move construction of the allocator shall not exit via an exception. All other constructors for these container types takeanaAllocator&
argument (16.4.4.6 [allocator.requirements]), an allocator whose value type is the same as the container's value typeconst allocator_type&
argument. [Note: If an invocation of a constructor uses the default value of an optional allocator argument, then theAllocator
type must support value initialization. — end note] A copy of this allocator is used for any memory allocation performed, by these constructors and by all member functions, during the lifetime of each container object or until the allocator is replaced. […]
tuple_size
for const pair
request <tuple>
headerSection: 22.2 [utility] Status: C++17 Submitter: Alisdair Meredith Opened: 2012-11-09 Last modified: 2017-07-30
Priority: 3
View all other issues in [utility].
View all issues with C++17 status.
Discussion:
The <utility>
header declares sufficient of the tuple API to specialize
the necessary templates for pair
, notably tuple_size
and
tuple_element
. However, it does not make available the partial specializations
that support cv-qualified template arguments, so while I can write the following after
including only <utility>
:
#include <utility> using TestType = std::pair<int, int>; static_assert(2 == std::tuple_size<TestType>(), "Pairs have two elements"); std::tuple_element<0, TestType>::type var{1};
the following may fail to compile unless I also include <tuple>
:
#include <utility> using TestType = const std::pair<int, int>; static_assert(2 == std::tuple_size<TestType>(), "Pairs have two elements"); std::tuple_element<0, TestType>::type var{1};
Note, however, that the latter may compile with some standard library implementations but not others, leading to subtle portability issues.
[2013-03-15 Issues Teleconference]
Moved to Open.
Howard notes that we have the same issue with array, so any resolution should apply to that header too.
[2013-10-18 Daniel provides wording]
The suggested wording uses a similar approach as we already have in 24.7 [iterator.range] to ensure that the range access templates are available when at least one of an enumerated list of header files is included.
I also think that the restricted focus ontuple_size
of this issue is too narrow and should be extended to
the similar partial template specializations of tuple_element
as well. Therefore the suggested wording
ensures this as well.
[2014-03-27 Library reflector vote]
The issue has been identified as Tentatively Ready based on eight votes in favour.
Proposed resolution:
This wording is relative to N3936.
Change 22.4.7 [tuple.helper] as indicated:
template <class T> class tuple_size<const T>; template <class T> class tuple_size<volatile T>; template <class T> class tuple_size<const volatile T>;-3- Let
TS
denotetuple_size<T>
of the cv-unqualified typeT
. Then each of the three templates shall meet theUnaryTypeTrait
requirements (20.10.1) with aBaseCharacteristic
ofintegral_constant<size_t, TS::value>-?- In addition to being available via inclusion of the
<tuple>
header, each of the three templates are available when any of the headers<array>
or<utility>
are included.
template <size_t I, class T> class tuple_element<I, const T>; template <size_t I, class T> class tuple_element<I, volatile T>; template <size_t I, class T> class tuple_element<I, const volatile T>;-?- Let
TE
denotetuple_element<I, T>
of the cv-unqualified typeT
. Then each of the three templates shall meet theTransformationTrait
requirements (20.10.1) with a member typedeftype
that names the following type:
for the first specialization,
add_const<TE::type>::type
,for the second specialization,
add_volatile<TE::type>::type
, andfor the third specialization,
add_cv<TE::type>::type
.-?- In addition to being available via inclusion of the
<tuple>
header, each of the three templates are available when any of the headers<array>
or<utility>
are included.
std::regex_replace
Section: 28.6.10.4 [re.alg.replace] Status: C++14 Submitter: Pete Becker Opened: 2012-11-08 Last modified: 2017-07-05
Priority: 0
View all other issues in [re.alg.replace].
View all issues with C++14 status.
Discussion:
In 28.6.10.4 [re.alg.replace], the first two variants of std::regex_replace
take an output iterator named
"out" as their first argument. Paragraph 2 of that section says that the functions return "out". When I first implemented
this, many years ago, I wrote it to return the value of the output iterator after all the insertions (cf. std::copy
),
which seems like the most useful behavior. But looking at the requirement now, it like the functions should return the
original value of "out" (i.e. they have to keep a copy of the iterator for no reason except to return it). Is that
really what was intended?
[Issaquah 2014-02-11: Move to Immediate]
Proposed resolution:
This wording is relative to N3485.
Edit 28.6.10.4 [re.alg.replace] as indicated:
template <class OutputIterator, class BidirectionalIterator, class traits, class charT, class ST, class SA> OutputIterator regex_replace(OutputIterator out, BidirectionalIterator first, BidirectionalIterator last, const basic_regex<charT, traits>& e, const basic_string<charT, ST, SA>& fmt, regex_constants::match_flag_type flags = regex_constants::match_default); template <class OutputIterator, class BidirectionalIterator, class traits, class charT> OutputIterator regex_replace(OutputIterator out, BidirectionalIterator first, BidirectionalIterator last, const basic_regex<charT, traits>& e, const charT* fmt, regex_constants::match_flag_type flags = regex_constants::match_default);-1- Effects: Constructs a
-2- Returns:regex_iterator
objecti
as if byregex_iterator<BidirectionalIterator, charT, traits> i(first, last, e, flags)
, and usesi
to enumerate through all of the matchesm
of typematch_results<BidirectionalIterator>
that occur within the sequence[first, last)
. If no such matches are found and!(flags & regex_constants ::format_no_copy)
then callsout = std::copy(first, last, out)
. If any matches are found then, for each such match, if!(flags & regex_constants::format_no_copy)
, callsout = std::copy(m.prefix().first, m.prefix().second, out)
, and then callsout = m.format(out, fmt, flags)
for the first form of the function andout = m.format(out, fmt, fmt + char_traits<charT>::length(fmt), flags)
for the second. Finally, if such a match is found and!(flags & regex_constants ::format_no_copy)
, callsout = std::copy(last_m.suffix().first, last_m.suffix().second, out)
wherelast_m
is a copy of the last match found. Ifflags & regex_constants::format_first_only
is non-zero then only the first match found is replaced.out
.
operator==(sub_match, string)
slices on embedded '\0'
sSection: 28.6.8.3 [re.submatch.op] Status: C++17 Submitter: Jeffrey Yasskin Opened: 2012-11-26 Last modified: 2017-07-30
Priority: 2
View all other issues in [re.submatch.op].
View all issues with C++17 status.
Discussion:
template <class BiIter, class ST, class SA> bool operator==( const basic_string< typename iterator_traits<BiIter>::value_type, ST, SA>& lhs, const sub_match<BiIter>& rhs);
is specified as:
Returns:
rhs.compare(lhs.c_str()) == 0
.
This is odd because sub_match::compare(basic_string)
is defined to
honor embedded '\0'
characters. This could allow a sub_match
to ==
or
!=
a std::string
unexpectedly.
[Daniel:]
This wording change was done intentionally as of LWG 1181(i), but the here mentioned slicing effect was not considered at that time. It seems best to use another overload of compare to fix this problem:
Returns:
rhs.str().compare(0, rhs.length(), lhs.data(), lhs.size()) == 0
.
or
Returns:
rhs.compare(sub_match<BiIter>::string_type(lhs.data(), lhs.size())) == 0
.
[2013-10-17: Daniel provides concrete wording]
The original wording was suggested to reduce the need to allocate memory during comparisons. The specification would be
very much easier, if sub_match
would provide an additional compare
overload of the form:
int compare(const value_type* s, size_t n) const;
But given the fact that currently all of basic_string
's compare
overloads are defined in terms
of temporary string constructions, the following proposed wording does follow the same string-construction route as
basic_string
does (where needed to fix the embedded zeros issue) and to hope that existing implementations
ignore to interpret this semantics in the literal sense.
Returns:rhs.compare(sub_match<BiIter>::string_type(lhs.data(), lhs.size())) == 0
.
because it already reflects the existing style used in 28.6.8.3 [re.submatch.op] p31.
[2014-02-15 post-Issaquah session : move to Tentatively Ready]
Proposed resolution:
This wording is relative to N3691.
Change 28.6.8.3 [re.submatch.op] as indicated:
template <class BiIter, class ST, class SA> bool operator==( const basic_string< typename iterator_traits<BiIter>::value_type, ST, SA>& lhs, const sub_match<BiIter>& rhs);-7- Returns:
rhs.compare(
.lhs.c_str()typename sub_match<BiIter>::string_type(lhs.data(), lhs.size())) == 0
[…]
template <class BiIter, class ST, class SA> bool operator<( const basic_string< typename iterator_traits<BiIter>::value_type, ST, SA>& lhs, const sub_match<BiIter>& rhs);-9- Returns:
rhs.compare(
.lhs.c_str()typename sub_match<BiIter>::string_type(lhs.data(), lhs.size())) > 0
[…]
template <class BiIter, class ST, class SA> bool operator==(const sub_match<BiIter>& lhs, const basic_string< typename iterator_traits<BiIter>::value_type, ST, SA>& rhs);-13- Returns:
lhs.compare(
.rhs.c_str()typename sub_match<BiIter>::string_type(rhs.data(), rhs.size())) == 0
[…]
template <class BiIter, class ST, class SA> bool operator<(const sub_match<BiIter>& lhs, const basic_string< typename iterator_traits<BiIter>::value_type, ST, SA>& rhs);-15- Returns:
lhs.compare(
.rhs.c_str()typename sub_match<BiIter>::string_type(rhs.data(), rhs.size())) < 0
allocator_traits::construct()
Section: 23.2.2 [container.requirements.general] Status: C++17 Submitter: Jonathan Wakely Opened: 2012-11-27 Last modified: 2017-07-30
Priority: 3
View other active issues in [container.requirements.general].
View all other issues in [container.requirements.general].
View all issues with C++17 status.
Discussion:
Firstly, 23.2.2 [container.requirements.general]/7 says a container's allocator is used to obtain memory, but it isn't stated explicitly that the same allocator is used to construct and destroy elements, as opposed to a value-initialized allocator of the same type.
Secondly, 23.2.2 [container.requirements.general]/3 says elements "shall be
constructed using the allocator_traits<allocator_type>::construct
function and destroyed using the allocator_traits<allocator_type>::destroy
function" and
23.2.2 [container.requirements.general]/13 defines CopyInsertable
etc. in
terms of an allocator A
which is identical to the container's allocator_type
.
The intent of making construct()
and destroy()
function templates was
that containers would be permitted to use allocator_traits<A>::construct()
instead of
allocator_traits<allocator_type>::construct()
, where A
is
allocator_traits<allocator_type>::rebind_alloc<U>
for some other type
U
. This allows node-based containers to store an allocator of the right type for
allocating nodes and to use the same object to construct elements in aligned storage within
those nodes, avoiding rebinding and copying the stored allocator every time an element needs
to be constructed.
[2013-03-15 Issues Teleconference]
Moved to Open.
Jonathan: point 2 in the proposed resolution is definitely needed.
[2014-11-28, Jonathan improves wording]
In the first set of edits to paragraph 3 both pieces inserting "rebind_alloc<U>::
"
should be replaced by "rebind_traits<U>::
"
[2015-05, Lenexa]
STL: You want to permit but not require rebinding?
Wakely: The current wording forces me to use the original allocator, not the rebound one.
STL: Oh, I see. Yeah, we immediately rebind.
Wakely: The edits clarify that we don't use some other allocator. The third diff is because the definitions of
EmplaceConstructible/etc. happen with the same types. The diff to the note is because it doesn't require the value
of the allocator was the one passed in.
STL: After looking at this, I think I'm comfortable with the edits. The previous Standardese was nonsense so it's
pretty easy to improve upon.
Marshall: Any other opinions?
Marshall: Any objections to moving it to Ready? Review? Ready in Kona?
Wakely: My preference would be Ready. We all know this is what we're doing anyways.
Nevin: The intent won't change.
STL: I think this is the right fix.
Hwrd: I third Ready. Even if Jonathan retracts his.
Marshall: Ready!
Proposed resolution:
This wording is relative to N3485.
Edit 23.2.2 [container.requirements.general] paragraph 3:
For the components affected by this subclause that declare an
allocator_type
, objects stored in these components shall be constructed using theallocator_traits<allocator_type>::rebind_traits<U>::construct
function and destroyed using theallocator_traits<allocator_type>::rebind_traits<U>::destroy
function (20.2.9.3 [allocator.traits.members]), whereU
is eitherallocator_type::value_type
or an internal type used by the container. These functions are called only for the container's element type, not for internal types used by the container. [ Note: This means, for example, that a node-based container might need to construct nodes containing aligned buffers and call construct to place the element into the buffer. — end note ]
Edit 23.2.2 [container.requirements.general] paragraph 7:
[…] A copy of this allocator is used for any memory allocation and element construction performed, by these constructors and by all member functions, during the lifetime of each container object or until the allocator is replaced. […]
Edit 23.2.2 [container.requirements.general] paragraph 13:
[…] Given an allocator type
[…] [ Note: A container callsA
and given a container typeX
havinganaallocator_type
identical toA
andvalue_type
identical toT
and anallocator_type
identical toallocator_traits<A>::rebind_alloc<T>
and given an lvaluem
of typeA
, a pointerp
of typeT*
, an expressionv
of type (possiblyconst
)T
, and an rvaluerv
of typeT
, the following terms are defined.allocator_traits<A>::construct(m, p, args)
to construct an element atp
usingargs
, withm == get_allocator()
. The defaultconstruct
instd::allocator
will call::new((void*)p) T(args)
, but specialized allocators may choose a different definition. — end note ]
INVOKE
-ing a pointer to member with a reference_wrapper
as the object expressionSection: 22.10.4 [func.require] Status: C++17 Submitter: Jonathan Wakely Opened: 2012-11-28 Last modified: 2017-07-30
Priority: 2
View other active issues in [func.require].
View all other issues in [func.require].
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Discussion:
The standard currently requires this to be invalid:
#include <functional> struct X { int i; } x; auto f = &X::i; auto t1 = std::ref(x); int i = std::mem_fn(f)(t1);
The call expression on the last line is equivalent to INVOKE(f, std::ref(x))
which according to 22.10.4 [func.require]p1 results in the invalid expression (*t1).*f
because reference_wrapper<X>
is neither an object of type X
nor a reference
to an object of type X
nor a reference to an object of a type derived from X
.
The same argument applies to pointers to member functions, and if they don't work with INVOKE
it becomes harder to do all sorts of things such as:
call_once(o, &std::thread::join, std::ref(thr))
or
async(&std::list<int>::sort, std::ref(list));
The definition of INVOKE
should be extended to handle reference wrappers.
[2013-03-15 Issues Teleconference]
Moved to Review.
The wording seems accurate, but verbose. If possible, we would like to define the kind of thing being specified so carefully as one of a number of potential language constructs in a single place. It is also possible that this clause is that single place.
[2013-04-18, Bristol]
Jonathan comments:
In the proposed resolution in the first bullet (t1.*f)
is not valid if t1
is a
reference_wrapper
, so we probably need a separate bullet to handle the
reference_wrapper
case.
[2014-02-14, Issaquah, Mike Spertus supplies wording]
Previous resolution from Jonathan [SUPERSEDED]:
This wording is relative to N3485.
Edit 22.10.4 [func.require]:
Define
INVOKE(f, t1, t2, ..., tN)
as follows:
(t1.*f)(t2, ..., tN)
whenf
is a pointer to a member function of a classT
andt1
is an object of typeT
or a reference to an object of typeT
or a reference to an object of a type derived fromT
U
or an object of typereference_wrapper<U>
or a reference to an object of typereference_wrapper<U>
whereU
is either the typeT
or a type derived fromT
;
((*t1).*f)(t2, ..., tN)
whenf
is a pointer to a member function of a classT
andt1
is not one of the types described in the previous item;
t1.*f
whenN == 1
andf
is a pointer to member data of a classT
andt1
is an object of typeT
or a reference to an object of typeT
or a reference to an object of a type derived fromT
U
or an object of typereference_wrapper<U>
or a reference to an object of typereference_wrapper<U>
whereU
is either the typeT
or a type derived fromT
;
(*t1).*f
whenN == 1
andf
is a pointer to member data of a classT
andt1
is not one of the types described in the previous item;
f(t1, t2, ..., tN)
in all other cases.
[2014-10-01, STL adds discussion and provides an improved resolution]
Because neither t1.*f
nor (*t1).*f
will compile when t1
is reference_wrapper<U>
for any U
, we don't need to inspect U
carefully. We can bluntly detect all reference_wrapper
s
and use get()
for them.
reference_wrapper
itself.
Fortunately, we don't. First, it doesn't have user-visible data members. Second, users technically can't take the
addresses of its member functions (this is a consequence of 16.4.6.5 [member.functions], the Implementer's Best Friend).
While we're in the neighborhood, I recommend simplifying and clarifying the wording used to detect base/derived objects.
Previous resolution from Mike Spertus [SUPERSEDED]:
This wording is relative to N3936.
Edit 22.10.4 [func.require]:
Define
INVOKE(f, t1, t2, ..., tN)
as follows:
(t1.*f)(t2, ..., tN)
whenf
is a pointer to a member function of a classT
andt1
is an object of typeT
or a reference to an object of typeT
or a reference to an object of a type derived fromT
;
(t1.get().*f)(t2, ..., tN)
whenf
is a pointer to a member function of classT
andt1
is an object of typereference_wrapper<U>
whereU
is either the typeT
or a type derived fromT
.
((*t1).*f)(t2, ..., tN)
whenf
is a pointer to a member function of a classT
andt1
is not one of the types described in the previous item;
t1.*f
whenN == 1
andf
is a pointer to member data of a classT
andt1
is an object of typeT
or a reference to an object of typeT
or a reference to an object of a type derived fromT
;
t1.get().*f
whenN == 1
andf
is a pointer to member data of a classT
andt1
is an object of typereference_wrapper<U>
whereU
is either the typeT
or a type derived fromT
.
(*t1).*f
whenN == 1
andf
is a pointer to member data of a classT
andt1
is not one of the types described in the previous item;
f(t1, t2, ..., tN)
in all other cases.
[2015-02, Cologne]
Waiting for implementation experience.
[2015-05, Lenexa]
STL: latest note from Cologne, waiting for implementation experience
STL: don't think this is harder than anything else we do
MC: it does involve mem_fn
and invoke
STL: my simplication was not to attempt fine-grained
STL: can ignore pmf
STL: can't invoke pmf to reference wrapper
STL: wording dated back to TR1 when there was no decltype
MC: should decay_t<decltype(t1)>
be pulled out since it is in multiple places
STL: it could be handled editorially
STL: we fix function, bind, invoke
STL: have not implemented this but believe it is fine
MC: Eric F, you have worked in invoke
EF: yes, looks ok
MC: consensus move to ready
Proposed resolution:
This wording is relative to N3936.
Change 22.10.4 [func.require] p1 as depicted:
Define
INVOKE(f, t1, t2, ..., tN)
as follows:
(t1.*f)(t2, ..., tN)
whenf
is a pointer to a member function of a classT
andt1
is an object of typeT
or a reference to an object of typeT
or a reference to an object of a type derived fromT
is_base_of<T, decay_t<decltype(t1)>>::value
is true;
(t1.get().*f)(t2, ..., tN)
whenf
is a pointer to a member function of a classT
anddecay_t<decltype(t1)>
is a specialization ofreference_wrapper
;
((*t1).*f)(t2, ..., tN)
whenf
is a pointer to a member function of a classT
andt1
is not one of the types described in the previous itemdoes not satisfy the previous two items;
t1.*f
whenN == 1
andf
is a pointer to member data of a classT
andt1
is an object of typeT
or a reference to an object of typeT
or a reference to an object of a type derived fromT
is_base_of<T, decay_t<decltype(t1)>>::value
is true;
t1.get().*f
whenN == 1
andf
is a pointer to member data of a classT
anddecay_t<decltype(t1)>
is a specialization ofreference_wrapper
;
(*t1).*f
whenN == 1
andf
is a pointer to member data of a classT
andt1
is not one of the types described in the previous itemdoes not satisfy the previous two items;
f(t1, t2, ..., tN)
in all other cases.
nullptr
Section: 31.7.6 [output.streams] Status: C++17 Submitter: Matt Austern Opened: 2012-12-07 Last modified: 2017-07-30
Priority: 3
View all issues with C++17 status.
Discussion:
When I write
std::cout << nullptr << std::endl;
I get a compilation error, "ambiguous overload for 'operator<<
' in 'std::cout << nullptr
'".
As far as I can tell, the compiler is right to issue that error. There are inserters for const void*
,
const char*
, const signed char*
, and const unsigned char*
, and none for
nullptr_t
, so the expression really is ambiguous.
nullptr_t
overload, which would be defined something like
template<class C, class T> basic_ostream<C, T>& operator<<(basic_ostream<C, T>& os, nullptr_t) { return os << (void*) nullptr; }
We might also consider addressing this at a core level: add a special-case language rule that addresses all
cases where you write f(nullptr)
and f
is overloaded on multiple pointer types. (Perhaps
a tiebreaker saying that void*
is preferred in such cases.)
[2016-01-18, comments from Mike and Ville collected by Walter Brown]
Mike Miller: "Changing overload resolution sounds like something that should be considered by EWG before CWG […]"
Ville: "Agreed, such a change would be Evolutionary. Personally, I think it would also be wrong, because I don't see howvoid*
is the right choice to prefer in the case of code that is currently ambiguous.
Sure, it would solve this particular library issue, but it seemingly has wider repercussions. If LWG really wants
to, EWG can certainly discuss this issue, but I would recommend solving it on the LWG side (which doesn't mean
that the standard necessarily needs to change, I wouldn't call it far-fetched to NAD it)."
[2016-08 Chicago]
Zhihao recommends NAD:
nullptr
is printable if being treated as void*
, but causes
UB if being treated as char cv*
. Capturing this ambigurity
at compile time and avoid a runtime UB is a good thing.
[2016-08 Chicago]
Tues PM: General agreement on providing the overload; discussion on what it should say.
Polls:
Matt's suggestion (in the issue): 2/0/6/2/2/
Unspecified output: 3/2/5/0/1
Specified output: 1/1/6/3/0
Move to Open
[2016-08 Chicago]
The group consensus is that we only output nullptr
because
it is of a fundamental type, causing problems in functions doing
forwarding, and we don't want to read it back.
Fri PM: Move to Tentatively Ready
Proposed resolution:
This wording is relative to N4606
Insert the signature into 31.7.6.2 [ostream], class template basic_ostream
synopsis, as follows:
[Drafting notes: Why member? Don't want to define a new category of inserters just for this.]
namespace std { template <class charT, class traits = char_traits<charT> > class basic_ostream : virtual public basic_ios<charT, traits> { public: […] basic_ostream<charT, traits>& operator<<(const void* p); basic_ostream<charT, traits>& operator<<(nullptr_t); basic_ostream<charT, traits>& operator<<( basic_streambuf<char_type, traits>* sb); […] };
Append the following new paragraphs to 31.7.6.3.3 [ostream.inserters]:
basic_ostream<charT, traits>& operator<< (basic_streambuf<charT, traits>* sb);[…]
-10- Returns:*this
.basic_ostream<charT, traits>& operator<<(nullptr_t);-??- Effects: Equivalent to
return *this << s;
wheres
is an implementation-defined NTCTS.
forward_list::splice_after
single-element overloadSection: 23.3.7.6 [forward.list.ops] Status: C++14 Submitter: Edward Catmur Opened: 2012-12-11 Last modified: 2023-02-07
Priority: Not Prioritized
View all other issues in [forward.list.ops].
View all issues with C++14 status.
Discussion:
[forwardlist.ops] p6 has
void splice_after(const_iterator position, forward_list& x, const_iterator i); void splice_after(const_iterator position, forward_list&& x, const_iterator i);Effects: Inserts the element following
i
into*this
, followingposition
, and removes it fromx
. The result is unchanged ifposition == i
orposition == ++i
. Pointers and references to*i
continue to refer to the same element but as a member of*this
. Iterators to*i
(includingi
itself) continue to refer to the same element, but now behave as iterators into*this
, not intox
.
This overload splices the element following i
from x
to *this
, so the
language in the two latter sentences should refer to ++i
:
Pointers and references to
*++i
continue to refer to the same element but as a member of*this
. Iterators to*++i
continue to refer to the same element, but now behave as iterators into*this
, not intox
.
[2013-03-15 Issues Teleconference]
Moved to Tentatively Ready.
[2013-04-20 Bristol]
Proposed resolution:
This wording is relative to N3485.
Edit [forwardlist.ops] p6 as indicated:
void splice_after(const_iterator position, forward_list& x, const_iterator i); void splice_after(const_iterator position, forward_list&& x, const_iterator i);-5- Requires:
-6- Effects: Inserts the element followingposition
isbefore_begin()
or is a dereferenceable iterator in the range[begin(),end())
. The iterator followingi
is a dereferenceable iterator inx
.get_allocator() == x.get_allocator()
.i
into*this
, followingposition
, and removes it fromx
. The result is unchanged ifposition == i
orposition == ++i
. Pointers and references to*++i
continue to refer to the same element but as a member of*this
. Iterators to*++i
(includingcontinue to refer to the same element, but now behave as iterators intoi
itself)*this
, not intox
.
shrink_to_fit
effect on iterator validitySection: 23.3.13.3 [vector.capacity] Status: C++17 Submitter: Juan Soulie Opened: 2012-12-17 Last modified: 2017-07-30
Priority: 2
View other active issues in [vector.capacity].
View all other issues in [vector.capacity].
View all issues with C++17 status.
Discussion:
After the additions by 2033(i), it appears clear that the intended effect includes a reallocation and thus the potential effect on iterators should be explicitly added to the text in order to not contradict 23.2.2 [container.requirements.general]/11, or at the very least, explicitly state that a reallocation may happen.
Taking consistency with "reserve" into consideration, I propose:that the current "Remarks" are made its "Effect" instead, inserting "Reallocation happens at this point if and only if the function effectively reduces the capacity." after the note on non-bindingness.
adding a "Remarks" paragraph, similar to that of reserve: "Reallocation invalidates all the references, pointers, and iterators referring to the elements in the sequence."
BTW, while we are at it, I believe the effect on iterators should also be explicitly stated in the other instance a reallocation may happen: 23.3.13.5 [vector.modifiers]/1 — even if obvious, it only contradicts 23.2.2 [container.requirements.general]/11 implicitly.
I propose to also insert "Reallocation invalidates all the references, pointers, and iterators referring to the elements in the sequence." at the appropriate location in its "Remarks".[2012-12-19: Jonathan Wakely comments]
The described problem also affects std::basic_string
and std::deque
.
[2013-03-15 Issues Teleconference]
Moved to Review.
[2013-04-18, Bristol]
Daniel extends the P/R.
Rationale:
The wording in 27.4.3.5 [string.capacity] combined with 27.4.3.2 [string.require]
seems to say the necessary things. We cannot impose all requirements as we do for vector
, because
we want to allow the short-string-optimization.
[2014-02-15 post-Issaquah session]
STL: I think that shrink_to_fit
should be a no-op when called twice.
STL: Do we ever define reallocation for deque
? Nope, all mentions of "reallocation" are in vector
.
We define what it means in vector::reserve()
, but not for deque
.
STL: Oh duh, they define reallocate in the PR. But I think we can do better here.
STL: Optimally, deque shrinking just allocates a new map of pointers, and drops empty blocks, but preserves pointers/references to elements.
Alisdair: That's like unordered containers, invalidating only iterators.
Pablo: It doesn't make sense to reduce capacity()
to size()
, because deque
doesn't have capacity!
STL: For vector
, "effectively reduces the capacity" is unnecessary, the capacity there is observable.
STL: There is a strong reason to provide an optimal shrink to fit for deque
, since only the library implementer can do this.
STL: The other thing I don't like the repeated definition of reallocation for vector
, we define it once and use it in a bunch of places.
At most we can lift it up to the vector
synopsis.
STL: I'll write new wording.
[2014-10-01, STL adds discussion and provides new wording]
Compared to the previous proposed resolution:
I'm changing basic_string
's wording because (1) we should guarantee that capacity won't increase, (2) we should mention
that it's linear complexity, and (3) we can provide a better invalidation guarantee than 27.4.3.2 [string.require]/5.
(As previously noted, we already have the strong exception guarantee.) This introduces the term "reallocation" into
basic_string
, but immediately explains what it means for iterator validity. As far as I can tell, the Small String
Optimization doesn't complicate the wording here; it's a reason why an implementation might not honor the request, but if
the capacity is reduced, we are definitely reallocating buffers and will invalidate everything (including when the destination
is the small buffer).
Between N3485 and N3936, deque
's wording was updated to avoid talking about capacity()
which it doesn't have.
Since the container's capacity is unobservable, I'm saying that invalidation is unconditional.
In vector
's wording, I'm also guaranteeing that capacity won't increase, and that iterators/etc. remain valid if the
capacity is unchanged.
My wording doesn't directly say that shrink_to_fit()
should be a no-op when called twice in a row. (Indirectly,
if the first call reduces capacity()
to size()
, the second call must preserve iterators/etc.) I considered
rewording the complexity to say "linear if reallocation happens", but that's potentially problematic (what if we copy almost
all N
elements, then one throws and we have to unwind? There are no effects, so reallocation didn't happen, yet we
took longer than constant time). Implementers can always do better than the stated complexity bounds.
deque
's requirements, so implementations remain free to reallocate the elements themselves.
I didn't attempt to centralize vector's reallocation wording. That can be done editorially, if someone is sufficiently motivated.
Previous resolution from Juan Soulie/Daniel [SUPERSEDED]:
This wording is relative to N3485.
Keep 27.4.3.5 [string.capacity] around p14 unchanged, because we don't speak about reallocations and we give the strong exception guarantee in 27.4.3.2 [string.require] (Invalidation specification also at that place):
void shrink_to_fit();-14- Remarks:
shrink_to_fit
is a non-binding request to reducecapacity()
tosize()
. [Note: The request is non-binding to allow latitude for implementation-specific optimizations. — end note ].Edit 23.3.5.3 [deque.capacity] around p7 as indicated:
void shrink_to_fit();-5- Requires:
-?- Effects:T
shall beMoveInsertable
into*this
.shrink_to_fit
is a non-binding request to reducecapacity()
tosize()
. [Note: The request is non-binding to allow latitude for implementation-specific optimizations. — end note ] Reallocation happens at this point if and only if the function effectively reduces the capacity. If an exception is thrown other than by the move constructor of a non-CopyInsertable
T
there are no effects. -6- Complexity: Linear in the size of the sequence. -7- Remarks:Reallocation invalidates all the references, pointers, and iterators referring to the elements in the sequence.shrink_to_fit
is a non-binding request to reducecapacity()
tosize()
. [Note: The request is non-binding to allow latitude for implementation-specific optimizations. — end note ] If an exception is thrown other than by the move constructor of a non-CopyInsertable
T
there are no effects.Edit 23.3.13.3 [vector.capacity] around p7 as indicated:
void shrink_to_fit();-7- Requires:
-?- Effects:T
shall beMoveInsertable
into*this
.shrink_to_fit
is a non-binding request to reducecapacity()
tosize()
. [Note: The request is non-binding to allow latitude for implementation-specific optimizations. — end note ] Reallocation happens at this point if and only if the function effectively reduces the capacity. If an exception is thrown other than by the move constructor of a non-CopyInsertable
T
there are no effects. -8- Complexity: Linear in the size of the sequence. -9- Remarks:Reallocation invalidates all the references, pointers, and iterators referring to the elements in the sequence.shrink_to_fit
is a non-binding request to reducecapacity()
tosize()
. [Note: The request is non-binding to allow latitude for implementation-specific optimizations. — end note ] If an exception is thrown other than by the move constructor of a non-CopyInsertable
T
there are no effects.Edit 23.3.13.5 [vector.modifiers] p1 as indicated:
iterator insert(const_iterator position, const T& x); iterator insert(const_iterator position, T&& x); iterator insert(const_iterator position, size_type n, const T& x); template <class InputIterator> iterator insert(const_iterator position, InputIterator first, InputIterator last); iterator insert(const_iterator position, initializer_list<T>); template <class... Args> void emplace_back(Args&&... args); template <class... Args> iterator emplace(const_iterator position, Args&&... args); void push_back(const T& x); void push_back(T&& x);-1- Remarks: Causes reallocation if the new size is greater than the old capacity. Reallocation invalidates all the references, pointers, and iterators referring to the elements in the sequence. If no reallocation happens, all the iterators and references before the insertion point remain valid. If an exception is thrown other than by the copy constructor, move constructor, assignment operator, or move assignment operator of
T
or by anyInputIterator
operation there are no effects. If an exception is thrown by the move constructor of a non-CopyInsertable
T
, the effects are unspecified.
[2015-02 Cologne]
GR: I'm concerned that shrink_to_fit
may cause reallocation without changing the capacity. […]
It's about correctness. The statement about invalidation is useless if I cannot detect whether reallocation has happened?
reserve()
invalidates? AM: It should say that in the container requirements.
VV: vector specifies in reserve
that there's reallocation if and only if the capacity changes. GR: I can't find
anything in the container requirements about reserve
. DK: No, it's specified for every container separately.
GR: It isn't specified for string.
GR: I'm noticing that the issue touches on shrink_to_fit
for a bunch of containers. Anyway, I think the
reserve issue [re string] is in scope for this issue. This change is touching on a lot of members.
AM: Landing this change will provide clarity for what we should do with basic_string
. GR: We're already asking
for changes; we should fix string as well. AM: If one of the changes is ready before the other, I'd like to land the
finished part first, but if both are ready for Lenexa, I'm equally happy to fix them in one go.
DK will reword this.
Conclusion: Update wording, revisit in Lenexa.
[2016-08 Chicago]
Monday PM: Move to Tentatively Ready
Proposed resolution:
This wording is relative to N3936.
Change 27.4.3.5 [string.capacity] p14 as depicted:
void shrink_to_fit();-14-
-?- Complexity: Linear in the size of the sequence. -?- Remarks: Reallocation invalidates all the references, pointers, and iterators referring to the elements in the sequence. If no reallocation happens, they remain valid.RemarksEffects:shrink_to_fit
is a non-binding request to reducecapacity()
tosize()
. [Note: The request is non-binding to allow latitude for implementation-specific optimizations. — end note] It does not increasecapacity()
, but may reducecapacity()
by causing reallocation.
Change 23.3.5.3 [deque.capacity] p5-p7 as depicted:
void shrink_to_fit();-5- Requires:
-?- Effects:T
shall beMoveInsertable
into*this
.shrink_to_fit
is a non-binding request to reduce memory use but does not change the size of the sequence. [Note: The request is non-binding to allow latitude for implementation-specific optimizations. — end note] If an exception is thrown other than by the move constructor of a non-CopyInsertable
T
there are no effects. -6- Complexity: Linear in the size of the sequence. -7- Remarks:shrink_to_fit
is a non-binding request to reduce memory use but does not change the size of the sequence. [Note: The request is non-binding to allow latitude for implementation-specific optimizations. — end note]shrink_to_fit
invalidates all the references, pointers, and iterators referring to the elements in the sequence.
Change 23.3.13.3 [vector.capacity] p7-p9 as depicted:
void shrink_to_fit();-7- Requires:
-?- Effects:T
shall beMoveInsertable
into*this
.shrink_to_fit
is a non-binding request to reducecapacity()
tosize()
. [Note: The request is non-binding to allow latitude for implementation-specific optimizations. — end note] It does not increasecapacity()
, but may reducecapacity()
by causing reallocation. If an exception is thrown other than by the move constructor of a non-CopyInsertable
T
there are no effects. -8- Complexity: Linear in the size of the sequence. -9- Remarks:Reallocation invalidates all the references, pointers, and iterators referring to the elements in the sequence. If no reallocation happens, they remain valid.shrink_to_fit
is a non-binding request to reducecapacity()
tosize()
. [Note: The request is non-binding to allow latitude for implementation-specific optimizations. — end note] If an exception is thrown other than by the move constructor of a non-CopyInsertable
T
there are no effects.
Change 23.3.13.5 [vector.modifiers] p1 as depicted:
-1- Remarks: Causes reallocation if the new size is greater than the old capacity. Reallocation invalidates all the references, pointers, and iterators referring to the elements in the sequence. If no reallocation happens, all the iterators and references before the insertion point remain valid. […]
Section: 16.4.5.10 [res.on.objects] Status: C++17 Submitter: Geoffrey Romer Opened: 2012-12-17 Last modified: 2017-09-07
Priority: 2
View all other issues in [res.on.objects].
View all issues with C++17 status.
Discussion:
The standard currently does not discuss when library objects may be accessed, except in a non-normative
note pertaining to synchronization in [res.on.objects], leaving it ambiguous whether single-threaded
code can access a library object during its construction or destruction. For example, there is a
reasonable question as to what happens if the deleter supplied to a unique_ptr
transitively
accesses the unique_ptr
itself during unique_ptr
's destruction; a straightforward
reading suggests that this is permitted, and that the deleter will see the unique_ptr
still
holding the originally stored pointer, but consensus on the LWG reflector indicates this was not the
intent (see discussion beginning with
c++std-lib-33362).
[2013-03-15 Issues Teleconference]
Moved to Open.
Geoffrey will provide an example that clearly highlights the issue.
[2013-03-19 Geoffrey provides revised resolution and an example]
I contend that the most straightforward reading of the current standard requires the following example code to print
"good" (because ~unique_ptr
is not specified to modify the state of the internal pointer), but the consensus
on the reflector was that its behavior should be undefined.
#include <memory> #include <iostream> class A; struct B { std::unique_ptr<A> a; }; struct A { B* b; ~A() { if (b->a.get() == this) { std::cout << "good" << std::endl; } } }; int main() { B b; b.a.reset(new A); b.a->b = &b; }
Previous resolution:
Change the title of sub-clause 16.4.5.10 [res.on.objects] as indicated:
Shared objects and the libraryLibrary object access [res.on.objects]Edit 16.4.5.10 [res.on.objects] p2 as indicated:
-2-
[Note: In particular, the program is required to ensure that completion of the constructor of any object of a class type defined in the standard library happens before any other member function invocation on that object and, unless otherwise specified, to ensure that completion of any member function invocation other than destruction on such an object happens before destruction of that object. This applies even to objects such as mutexes intended for thread synchronization. — end note]If an object of a standard library type is accessed outside of the object's lifetime (6.7.4 [basic.life]), the behavior is undefined unless otherwise specified.
[2014 Urbana]
STL: is this resolved by our change to the reeentrancy rules? [LWG 2414(i)]
GR: don't think that solves the multi-threaded case
MC: I like changing the note to normative text
GR: uses the magic "happens before" words, and "access" is magic too
JW: I like this. strict improvement, uses the right wording we have to say this properly
STL: I like the last sentence of the note, could we add that as a new note at the end?
So add "[Note: This applies even to objects such as mutexes intended for thread synchronization.]" to the end and move to Ready
Proposed resolution:
This wording is relative to N3485.
Change the title of sub-clause 16.4.5.10 [res.on.objects] as indicated:
Shared objects and the libraryLibrary object access [res.on.objects]
Edit 16.4.5.10 [res.on.objects] p2 as indicated: [Editorial remark: The motivation, is to be more precise about the meaning of "outside the object's lifetime" in the presence of threads — end editorial remark]
-2- [Note: In particular, the program is required to ensure that completion of the constructor
of any object of a class type defined in the standard library happens before any other member function
invocation on that object and, unless otherwise specified, to ensure that completion of any member function
invocation other than destruction on such an object happens before destruction of that object. This applies
even to objects such as mutexes intended for thread synchronization. — end note]
If an object of a standard library type is accessed, and the beginning of the object's lifetime
(6.7.4 [basic.life]) does not happen before the access, or the access does not happen before the end
of the object's lifetime, the behavior is undefined unless otherwise specified. [Note: This applies even to
objects such as mutexes intended for thread synchronization. — end note]
Section: 16.4.3.2 [using.headers] Status: C++14 Submitter: Richard Smith Opened: 2012-12-18 Last modified: 2016-01-28
Priority: Not Prioritized
View all other issues in [using.headers].
View all issues with C++14 status.
Discussion:
16.4.3.2 [using.headers]/3 says:
A translation unit shall include a header only outside of any external declaration or definition, and shall include the header lexically before the first reference in that translation unit to any of the entities declared in that header. Per 4.1 [intro.compliance]/1, programs which violate this rule are ill-formed, and a conforming implementation is required to produce a diagnostic. This does not seem to match reality. Presumably, this paragraph is missing a "no diagnostic is required".[2013-03-15 Issues Teleconference]
Moved to Tentatively Ready.
[2013-04-20 Bristol]
Proposed resolution:
This wording is relative to N3485.
Edit 16.4.3.2 [using.headers] p3 as indicated:
-3- A translation unit shall include a header only outside of any external declaration or definition, and shall include the header lexically before the first reference in that translation unit to any of the entities declared in that header. No diagnostic is required.
unique_ptr
templated assignmentSection: 20.3.1.3.4 [unique.ptr.single.asgn] Status: Resolved Submitter: Geoffrey Romer Opened: 2012-12-20 Last modified: 2016-01-28
Priority: 3
View all other issues in [unique.ptr.single.asgn].
View all issues with Resolved status.
Discussion:
20.3.1.3.4 [unique.ptr.single.asgn]/5 permits unique_ptr
's templated assignment operator to participate
in overload resolution even when incompatibilities between D
and E
will render the result ill-formed,
but the corresponding templated copy constructor is removed from the overload set in those situations (see the third
bullet point of 20.3.1.3.2 [unique.ptr.single.ctor]/19). This asymmetry is confusing, and presumably unintended;
it may lead to situations where constructing one unique_ptr
from another is well-formed, but assigning from
the same unique_ptr
would be ill-formed.
Previous resolution [SUPERSEDED]:
This wording is relative to N3485.
Revise 20.3.1.3.4 [unique.ptr.single.asgn] p5 as follows:
template <class U, class E> unique_ptr& operator=(unique_ptr<U, E>&& u) noexcept;-4- Requires: If
-5- Remarks: This operator shall not participate in overload resolution unless:E
is not a reference type, assignment of the deleter from an rvalue of typeE
shall be well-formed and shall not throw an exception. Otherwise,E
is a reference type and assignment of the deleter from an lvalue of typeE
shall be well-formed and shall not throw an exception.
unique_ptr<U, E>::pointer
is implicitly convertible topointer
and
U
is not an array type., andeither
D
is a reference type andE
is the same type asD
, orD
is not a reference type andE
is implicitly convertible toD
.-6- Effects: Transfers ownership from
-7- Returns:u
to*this
as if by callingreset(u.release())
followed by an assignment fromstd::forward<E>(u.get_deleter())
.*this
.
[2013-03-15 Issues Teleconference]
Moved to Review.
The wording looks good, but we want a little more time than the telecon permits to be truly comfortable. We expect this issue to resolve fairly easily in Bristol.
[2015-05-18, Howard comments]
Updated proposed wording has been provided in N4366.
[2015-05, Lenexa]
Straw poll: send N4366 to full committee,
with both fixes from the sections "What is the correct fix?" and "unique_ptr<T[]>
needs the correct fix too"
Proposed resolution:
Resolved by accepting N4366.
Section: 99 [depr.locale.stdcvt] Status: C++14 Submitter: Beman Dawes Opened: 2012-12-30 Last modified: 2017-04-22
Priority: Not Prioritized
View all other issues in [depr.locale.stdcvt].
View all issues with C++14 status.
Discussion:
The only specification for the non-inherited members of classes codecvt_utf8
,
codecvt_utf16
, and codecvt_utf8_utf16
is a comment line in the synopsis
that says // unspecified
. There is no further indication of functionality,
so a user does not know if one of these classes can be constructed or destroyed.
The proposed resolution adds a constructor that mimics the class codecvt
constructor, and also adds a destructor. Following the practice of class codecvt
,
the semantics are not specified.
The only existing implementation I could find was libc++, and it does supply the proposed constructor and destructor for each of the three classes.
[2013-03-15 Issues Teleconference]
Moved to Review.
There was concern about the unspecified semantics - but that matches what is done in codecvt
.
Jonathan: Should these constructor/destructors be public? Proposed wording is private. Base class constructor is public.
Howard noted that other facets do not have specified constructors.
Alisdair noted that this whole section was new in C++11.
Howard suggested looking at section 28.3.3.1.2.2 [locale.facet]p2/p3 for more info.
[2013-04-18, Bristol]
Proposed resolution:
In [locale.stdcvt] paragraph 2, Header codecvt synopsis:
template<class Elem, unsigned long Maxcode = 0x10ffff, codecvt_mode Mode = (codecvt_mode)0> class codecvt_utf8 : public codecvt<Elem, char, mbstate_t> {// unspecifiedpublic: explicit codecvt_utf8(size_t refs = 0); ~codecvt_utf8(); }; template<class Elem, unsigned long Maxcode = 0x10ffff, codecvt_mode Mode = (codecvt_mode)0> class codecvt_utf16 : public codecvt<Elem, char, mbstate_t> {// unspecifiedpublic: explicit codecvt_utf16(size_t refs = 0); ~codecvt_utf16(); }; template<class Elem, unsigned long Maxcode = 0x10ffff, codecvt_mode Mode = (codecvt_mode)0> class codecvt_utf8_utf16 : public codecvt<Elem, char, mbstate_t> {// unspecifiedpublic: explicit codecvt_utf8_utf16(size_t refs = 0); ~codecvt_utf8_utf16(); };
Section: 23.5 [unord] Status: C++17 Submitter: Jonathan Wakely Opened: 2013-01-06 Last modified: 2017-07-30
Priority: 4
View all other issues in [unord].
View all issues with C++17 status.
Discussion:
The unordered_map
class definition in 23.5.3.1 [unord.map.overview] declares an
initializer-list constructor that says "see below":
unordered_map(initializer_list<value_type>, size_type = see below, const hasher& hf = hasher(), const key_equal& eql = key_equal(), const allocator_type& a = allocator_type());
But that constructor isn't defined below. The same problem exists for the other unordered associative containers.
[2013-09 Chicago]
STL: ordered are also missing declarations, but issue is forthcoming
Walter: how does adding a signature address issue? — nevermind Jayson: in his wording, isn't he just dropping thesize_type
?
Walter: partial fix is to introduce the name
Stefanus: explanation of requiring name because of n buckets
STL: solution for his issue satisfies both ordered and unordered and is simplier than provided wording
STL: patches general table instead
STL: proposes adding extra rows instead of extra declarations
Stefanus: clarify n
in the synopsis
Walter: general rule, name is optional in declaration
Stefanus: how to proceed
Walter: significant overlap with forthcoming issue, suggestion to defer
[2014-02-20 Re-open Deferred issues as Priority 4]
[2014-03-27 Jonathan improves proposed wording]
[2014-05-20 STL and Jonathan communicate]
STL: With 2322(i) resolved, is there anything left for this issue to fix?
Jonathan: The synopsis still says "see below" and it's not immediately clear that "see below" means "see the definition of a different constructor, which defines the behaviour of this one due to a table defined much earlier".[2014-05-23 Library reflector vote]
The issue has been identified as Tentatively Ready based on five votes in favour.
Proposed resolution:
This wording is relative to N3936.
Edit 23.5.3.1 [unord.map.overview], class template unordered_map
synopsis, as follows:
[…] unordered_map(initializer_list<value_type> il, size_type n = see below, const hasher& hf = hasher(), const key_equal& eql = key_equal(), const allocator_type& a = allocator_type()); […]
Edit 23.5.3.2 [unord.map.cnstr] as follows:
template <class InputIterator> unordered_map(InputIterator f, InputIterator l, size_type n = see below, const hasher& hf = hasher(), const key_equal& eql = key_equal(), const allocator_type& a = allocator_type()); unordered_map(initializer_list<value_type> il, size_type n = see below, const hasher& hf = hasher(), const key_equal& eql = key_equal(), const allocator_type& a = allocator_type());-3- Effects: Constructs an empty
unordered_map
using the specified hash function, key equality function, and allocator, and using at leastn
buckets. Ifn
is not provided, the number of buckets is implementation-defined. Then inserts elements from the range[f, l)
for the first form, or from the range[il.begin(), il.end())
for the second form.max_load_factor()
returns1.0
.
Edit 23.5.4.1 [unord.multimap.overview], class template unordered_multimap
synopsis, as follows:
[…] unordered_multimap(initializer_list<value_type> il, size_type n = see below, const hasher& hf = hasher(), const key_equal& eql = key_equal(), const allocator_type& a = allocator_type()); […]
Edit 23.5.4.2 [unord.multimap.cnstr] as follows:
template <class InputIterator> unordered_multimap(InputIterator f, InputIterator l, size_type n = see below, const hasher& hf = hasher(), const key_equal& eql = key_equal(), const allocator_type& a = allocator_type()); unordered_multimap(initializer_list<value_type> il, size_type n = see below, const hasher& hf = hasher(), const key_equal& eql = key_equal(), const allocator_type& a = allocator_type());-3- Effects: Constructs an empty
unordered_multimap
using the specified hash function, key equality function, and allocator, and using at leastn
buckets. Ifn
is not provided, the number of buckets is implementation-defined. Then inserts elements from the range[f, l)
for the first form, or from the range[il.begin(), il.end())
for the second form.max_load_factor()
returns1.0
.
Edit 23.5.6.1 [unord.set.overview], class template unordered_set
synopsis, as follows:
[…] unordered_set(initializer_list<value_type> il, size_type n = see below, const hasher& hf = hasher(), const key_equal& eql = key_equal(), const allocator_type& a = allocator_type()); […]
Edit 23.5.6.2 [unord.set.cnstr] as follows:
template <class InputIterator> unordered_set(InputIterator f, InputIterator l, size_type n = see below, const hasher& hf = hasher(), const key_equal& eql = key_equal(), const allocator_type& a = allocator_type()); unordered_set(initializer_list<value_type> il, size_type n = see below, const hasher& hf = hasher(), const key_equal& eql = key_equal(), const allocator_type& a = allocator_type());-3- Effects: Constructs an empty
unordered_set
using the specified hash function, key equality function, and allocator, and using at leastn
buckets. Ifn
is not provided, the number of buckets is implementation-defined. Then inserts elements from the range[f, l)
for the first form, or from the range[il.begin(), il.end())
for the second form.max_load_factor()
returns1.0
.
Edit 23.5.7.1 [unord.multiset.overview], class template unordered_multiset
synopsis, as follows:
[…] unordered_multiset(initializer_list<value_type> il, size_type n = see below, const hasher& hf = hasher(), const key_equal& eql = key_equal(), const allocator_type& a = allocator_type()); […]
Edit 23.5.7.2 [unord.multiset.cnstr] as follows:
template <class InputIterator> unordered_multiset(InputIterator f, InputIterator l, size_type n = see below, const hasher& hf = hasher(), const key_equal& eql = key_equal(), const allocator_type& a = allocator_type()); unordered_multiset(initializer_list<value_type> il, size_type n = see below, const hasher& hf = hasher(), const key_equal& eql = key_equal(), const allocator_type& a = allocator_type());-3- Effects: Constructs an empty
unordered_multiset
using the specified hash function, key equality function, and allocator, and using at leastn
buckets. Ifn
is not provided, the number of buckets is implementation-defined. Then inserts elements from the range[f, l)
for the first form, or from the range[il.begin(), il.end())
for the second form.max_load_factor()
returns1.0
.
clear()
Section: 23.2.4 [sequence.reqmts] Status: C++14 Submitter: Jonathan Wakely Opened: 2012-12-30 Last modified: 2016-01-28
Priority: Not Prioritized
View other active issues in [sequence.reqmts].
View all other issues in [sequence.reqmts].
View all issues with C++14 status.
Discussion:
From the question at stackoverflow.
Were we aware that the resolution to LWG 704(i) means there is no complexity guarantee for
clear()
on most sequence containers? Previously it was implied by defining it in terms of
erase(begin(), end())
but we no longer do that.
There are explicit complexity requirements for std::list::clear()
, but not the other sequence containers.
Daniel:
The idea was that the notion of "destroys all elements in a
" would imply a linear complexity, but the wording
needs to be clearer, because this doesn't say that this step is the actual complexity bound.
[2013-03-15 Issues Teleconference]
Moved to Tentatively Ready.
[2013-04-20 Bristol]
Proposed resolution:
This wording is relative to N3485.
Change Table 100 as indicated:
Table 100 — Sequence container requirements (in addition to container) (continued) Expression Return type Assertion/note pre-/post-condition …
a.clear()
void
Destroys all elements in a
. Invalidates all
references, pointers, and iterators referring to
the elements ofa
and may invalidate the
past-the-end iterator.
post:a.empty()
returnstrue
complexity: linear…
char_traits
specializations should declare their length()
, compare()
, and
find()
members constexprSection: 27.2.4 [char.traits.specializations] Status: Resolved Submitter: Jeffrey Yasskin Opened: 2012-12-24 Last modified: 2020-09-06
Priority: Not Prioritized
View all other issues in [char.traits.specializations].
View all issues with Resolved status.
Discussion:
Addresses ES 14, US 19
These functions have easy recursive constexpr implementations that, unfortunately, aren't efficient at runtime. EWG is still figuring out how to solve this problem in general (e.g., N3444 isn't sufficient to avoid stack overflows in debug builds or to get the optimal assembly-based implementations at runtime), so users can't portably solve this problem for themselves, but implementations can use compiler-specific techniques to choose the right implementation inside their standard libraries.
The LWG is still undecided about whether individual implementations can add constexpr to these functions, so we
need to add constexpr
to the standard here for implementations to be able to improve this.
[2013-03-15 Issues Teleconference]
Moved to Open.
There are a number of people who have a strong interest in this issue not available for the telecon.
It also plays at the heart of a discussion about library freedoms for constexpr
and specifying
a library that may depend on unspecified compiler intrinsics to be implementable.
[2013-09 Chicago]
Moved to NAD Future.
While it is clear that this feature can be implemented using only C++14 constexpr
features,
there is real concern that we cannot call the efficient, highly optimized, C implementations of these
functions under a C++14 constexpr
implementation, nor implement similar ourselves as this
typically involves use of inline asm
instructions.
Clang and libc++ have some experience of using intrinsics to try to address the performance issue, but
the current intrinsics are not general enough to support char_traits
. The intrinsics support
only operations on character string literals, and the string literal is no longer visible as a
literal after passing as a const char *
to the char_traits
functions.
Additional concern was raised that these operations are unlikely to be useful anyway, as the only client
is basic_string
which relies on dynamic memory allocation, and so cannot effectively be made a
literal type. Jeffrey then pointed out the pending string_view
library that will also use
char_traits
and would most certainly benefit from being a literal type.
Given the choice of giving up performance on a critical library component, or requiring a compiler intrinsic with only unsuccessful implementation experience, the consensus is to not reject this, unless compelling implementation experience is demonstrated. NAD Future seems the appropriate resolution.
[2017-06-02 Issues Telecon]
Resolved by P0426R1, adopted in Issaquah.
Proposed resolution:
This wording is relative to N3691.
In 27.2.4.2 [char.traits.specializations.char], [char.traits.specializations.char16_t], [char.traits.specializations.char32_t], and 27.2.4.6 [char.traits.specializations.wchar.t]:
static constexpr int compare(const char_type* s1, const char_type* s2, size_t n); static constexpr size_t length(const char_type* s); static constexpr const char_type* find(const char_type* s, size_t n, const char_type& a);
bad_function_call::what()
unhelpfulSection: 22.10.17.2 [func.wrap.badcall] Status: C++17 Submitter: Jonathan Wakely Opened: 2013-01-05 Last modified: 2017-09-07
Priority: 3
View all issues with C++17 status.
Discussion:
A strict reading of the standard implies std::bad_function_call{}.what()
returns the same string as
std::exception{}.what()
which doesn't help to know what happened if you catch an exception by reference
to std::exception
.
For consistency with bad_weak_ptr::what()
it should return "bad_function_call"
.
See c++std-lib-33515 for other details.
There was a considerable support on the reflector to instead change the specification of both bad_weak_ptr::what()
and bad_function_call::what()
to return an implementation-defined string instead.
[2013-03-15 Issues Teleconference]
Moved to Open.
Consensus that we want consistency in how this is treated. Less consensus on what the common direction should be.
Alisdair to provide wording proposing that all string literals held by standard exception objects are either unspecified, or implmentation defined.
[2014-02-15 Issauqah]
STL: I think it should be an implementation-defined NTBS, same on bad_weak_ptr
. I will write a PR.
[2014-03-27, STL provides improved wording]
The new wording reflects better the general agreement of the committee, see also issue 2376(i) for similar wording.
[2014-03-28 Library reflector vote]
The issue has been identified as Tentatively Ready based on five votes in favour.
Proposed resolution:
This wording is relative to N3936.
Edit [func.wrap.badcall.const]:
bad_function_call() noexcept;-1- Effects: constructs a
-?- Postconditions:bad_function_call object
.what()
returns an implementation-defined NTBS.
assert()
should allow usage in constant expressionsSection: 19.3 [assertions] Status: C++17 Submitter: Daniel Krügler Opened: 2013-01-12 Last modified: 2017-07-30
Priority: 2
View all other issues in [assertions].
View all issues with C++17 status.
Discussion:
It is unclear from the current specification whether assert()
expressions can be used in
(potential) constant expressions. As an example consider the implementation of a constexpr
function:
#include <cassert> template<class T, unsigned N> struct array { T data[N]; constexpr const T& operator[](unsigned i) const { return assert(i < N), data[i]; } }; int main() { constexpr array<int, 3> ai = {1, 2, 3}; constexpr int i = ai[0]; int j = ai[0]; // constexpr int k = ai[5]; }
The first question is whether this program is guaranteed well-formed? A second question is whether is would guaranteed to be
ill-formed, if we uncomment the last code line in main()
?
The wording in 19.3 [assertions] doesn't add anything significant to the C99 wording. From the C99 specification (7.2 p1 and 7.2.1.1 p2) we get already some valuable guarantees:
The expression assert(e)
is a void
expression for all expressions e
independent of
the definition of NDEBUG
.
If NDEBUG
is defined, assert(e)
is equivalent to the expression void()
(or anything that cannot be distinguished from that).
The current wording does not yet guarantee that assert
expressions can be used in constant expressions,
but all tested implementations (gcc, MSVC) would already support this use-case. It seems to me that this should be possible
without giving assert
a special meaning for the core language.
constexpr
functions and literal
types. The most
interesting one (making void
a literal types and allowing for expression-statements) would simplify the motivating
example implementation of operator[]
to:
constexpr const T& operator[](unsigned i) const { assert(i < N); return data[i]; };
[2013-03-15 Issues Teleconference]
Moved to Open.
We are still gaining experience with constexpr
as a language feature, and there may
be work in Evolution that would help address some of these concerns. Defer discussion until
we have a group familiar with any evolutionary direction.
[2014-06-08, Daniel comments and suggests wording]
After approval of N3652,
void
is now a literal type and constexpr
functions can contain multiple statements, so
this makes the guarantee that assert
expressions are per-se constexpr
-friendly even more
relevant. A possible wording form could be along the lines of:
For every core constant expression e of scalar type that evaluates to
true
after being contextually converted tobool
, the expressionassert(e)
shall be a prvalue core constant expression of typevoid
.
Richard Smith pointed out some weaknesses of this wording form, for example it would not guarantee to require the following example to work:
constexpr void check(bool b) { assert(b); }
because b
is not a core constant expression in this context.
[Lenexa 2015-05-05]
MC : ran into this
Z : Is it guaranteed to be an expression?
MC : clarifies that assert runs at runtime, not sure what it does at compile time
STL : c standard guarantees its an expression and not a whole statement, so comma chaining it is ok
HH : Some implementations work as author wants it to
STL : also doing this as constexpr
DK/STL : discussing how this can actually work
HH : GCC 5 also implements it. We have implementor convergence
MC : Wants to do this without giving assert a special meaning
STL : NDEBUG being defined where assert appears is not how assert works. This is bug in wording. Should be "when assert is defined" or something like that. ... is a constant subexpression if NDEBUG is defined at the point where assert is last defined or redefined."
Would like to strike the "either" because ok if both debug or assertion is true. We want inclusive-or here
MC : is redefined needed?
STL : my mental model is its defined once and then redefined
HH : wants to up to P2
Z/STL : discussing how wording takes care of how/when assert is defined/redefefined
STL/WB : discussing whether to move to ready or review. -> Want to move it to ready.
ask for updated wording
p3 -> p2
plan to go to ready after checking wording
[Telecon 2015-06-30]
HH: standardizing existing practice
MC: what about the comment from Lenexa about striking "either"?
HH: all three implementations accept it
MC: update issue to strike "either" and move to Tentatively Ready
Proposed resolution:
This wording is relative to N3936.
Previous resolution [SUPERSEDED]:
Introduce the following new definition to the existing list in [definitions]: [Drafting note: If LWG 2296(i) is accepted before this issue, the accepted wording for the new definition should be used instead — end drafting note]
constant subexpression [defns.const.subexpr]
an expression whose evaluation as subexpression of a conditional-expression CE (7.6.16 [expr.cond]) would not prevent CE from being a core constant expression (7.7 [expr.const]).Insert a new paragraph following 19.3 [assertions] p1 as indicated:
-?- An expression
assert(E)
is a constant subexpression (3.15 [defns.const.subexpr]), if either
NDEBUG
is defined at the point whereassert(E)
appears, or
E
contextually converted tobool
(7.3 [conv]), is a constant subexpression that evaluates to the valuetrue
.
Introduce the following new definition to the existing list in [definitions]: [Drafting note: If LWG 2296(i) is accepted before this issue, the accepted wording for the new definition should be used instead — end drafting note]
constant subexpression [defns.const.subexpr]
an expression whose evaluation as subexpression of a conditional-expression CE (7.6.16 [expr.cond]) would not prevent CE from being a core constant expression (7.7 [expr.const]).
Insert a new paragraph following 19.3 [assertions] p1 as indicated:
-?- An expression
assert(E)
is a constant subexpression (3.15 [defns.const.subexpr]), if
NDEBUG
is defined at the point whereassert(E)
appears, or
E
contextually converted tobool
(7.3 [conv]), is a constant subexpression that evaluates to the valuetrue
.
basic_string
constructorsSection: 27.4.3.3 [string.cons] Status: C++14 Submitter: Juan Soulie Opened: 2013-01-17 Last modified: 2016-01-28
Priority: Not Prioritized
View all other issues in [string.cons].
View all issues with C++14 status.
Discussion:
In 27.4.3.3 [string.cons], I believe tighter requirements should be imposed on basic_string
's constructors
taking an s
argument (or, a behavior should be provided for the undefined cases).
These requirements are properly stated in the other members functions taking s
arguments (append
,
assign
, insert
,...).
basic_string(const charT* s, size_type n, const Allocator& a = Allocator());
Relative to N3485, 27.4.3.3 [string.cons]/6 says "Requires: s
shall not be a null pointer and n < npos
",
where it should say: "Requires: s
points to an array of at least n
elements of charT
"
basic_string(const charT* s, const Allocator& a = Allocator());
27.4.3.3 [string.cons]/8 says "Requires: s
shall not be a null pointer.", where it should say:
"Requires: s
points to an array of at least traits::length(s) + 1
elements of charT
"
Daniel:
I think that 16.4.5.9 [res.on.arguments] p1 b2 basically requires this already, but the wording is indeed worth improving it.
[2013-03-15 Issues Teleconference]
Moved to Review.
The resolution could be worded more cleanly, and there is some concern about redundancy between Requirements and Effects clauses. Consensus that we do want to say something like this for the Requirements though.
[2013-04-18, Bristol]
Move to Ready
[2013-09-29, Bristol]
Apply to the Working Paper
Proposed resolution:
This wording is relative to N3485.
Change 27.4.3.3 [string.cons]/6 as indicated:
basic_string(const charT* s, size_type n, const Allocator& a = Allocator());-6- Requires:
s
shall not be a null pointer andpoints to an array of at leastn < npos
n
elements ofcharT
.
Change 27.4.3.3 [string.cons]/8 as indicated:
basic_string(const charT* s, const Allocator& a = Allocator());-8- Requires:
s
shall not be a null pointerpoints to an array of at leasttraits::length(s) + 1
elements ofcharT
.
min/max/minmax
requirementsSection: 26.8.9 [alg.min.max] Status: C++17 Submitter: Juan Soulie Opened: 2013-01-26 Last modified: 2017-07-30
Priority: 3
View other active issues in [alg.min.max].
View all other issues in [alg.min.max].
View all issues with C++17 status.
Discussion:
26.8.9 [alg.min.max] requires type T
in min
, max
, and minmax
to be
LessThanComparable
, but I don't believe this should be required for the versions that take a Compare
argument.
Compare
being required to induce a strict weak ordering here.
Further, min
and max
also lack formal complexity guarantees.
[2014-06-07 Daniel comments and provides wording]
Certainly, the functions with Compare
should not impose LessThanComparable
requirements.
Compare
requirements, I would like to point out that this is requirement is in fact needed, because the specification of
the normative Remarks elements (e.g. "Returns the first argument when the arguments are equivalent.") do depend
on the existence of a equivalence relation that can be relied on and this is also consistent with the same
strict weak ordering requirement that is indirectly imposed by the LessThanComparable
requirement set for
functions referring to operator<
(Let me note that the very same StrictWeakOrder
language
concept had intentionally been required for similar reasons during "concept-time" in
N2914).
[2015-02 Cologne]
JY: We have library-wide requirements that Comp
induce a strict weak ordering.
[2015-03-30 Daniel comments]
The Complexity element of p16 is correct, but some others involving initializer_list
arguments are wrong.
[2015-04-02 Library reflector vote]
The issue has been identified as Tentatively Ready based on six votes in favour.
Proposed resolution:
This wording is relative to N4296.
Change 26.8.9 [alg.min.max] as indicated:
template<class T> constexpr const T& min(const T& a, const T& b); template<class T, class Compare> constexpr const T& min(const T& a, const T& b, Compare comp);-1- Requires: For the first form, type
-2- Returns: The smaller value. -3- Remarks: Returns the first argument when the arguments are equivalent. -?- Complexity: Exactly one comparison.T
shall beTypeT
isLessThanComparable
(Table 18).template<class T> constexpr T min(initializer_list<T> t); template<class T, class Compare> constexpr T min(initializer_list<T> t, Compare comp);-4- Requires:
-5- Returns: […] -6- Remarks: […] -?- Complexity: ExactlyT
isshall beLessThanComparable
andCopyConstructible
andt.size() > 0
. For the first form, typeT
shall beLessThanComparable
.t.size() - 1
comparisons.template<class T> constexpr const T& max(const T& a, const T& b); template<class T, class Compare> constexpr const T& max(const T& a, const T& b, Compare comp);-7- Requires: For the first form, type
-8- Returns: […] -9- Remarks: […] -?- Complexity: Exactly one comparison.T
shall beTypeT
isLessThanComparable
(Table 18).template<class T> constexpr T max(initializer_list<T> t); template<class T, class Compare> constexpr T max(initializer_list<T> t, Compare comp);-10- Requires:
-11- Returns: […] -12- Remarks: […] -?- Complexity: ExactlyT
isshall beLessThanComparable
andCopyConstructible
andt.size() > 0
. For the first form, typeT
shall beLessThanComparable
.t.size() - 1
comparisons.template<class T> constexpr pair<const T&, const T&> minmax(const T& a, const T& b); template<class T, class Compare> constexpr pair<const T&, const T&> minmax(const T& a, const T& b, Compare comp);-13- Requires: For the first form, t
-14- Returns: […] -15- Remarks: […] -16- Complexity: Exactly one comparison.TypeT
shall beLessThanComparable
(Table 18).template<class T> constexpr pair<T, T> minmax(initializer_list<T> t); template<class T, class Compare> constexpr pair<T, T> minmax(initializer_list<T> t, Compare comp);-17- Requires:
-18- Returns: […] -19- Remarks: […] -20- Complexity: At mostT
isshall beLessThanComparable
andCopyConstructible
andt.size() > 0
. For the first form, typeT
shall beLessThanComparable
.(3/2) * t.size()
applications of the corresponding predicate.
Section: 32.7.4 [thread.condition.condvar], 32.7.5 [thread.condition.condvarany] Status: Resolved Submitter: FrankHB1989 Opened: 2013-02-03 Last modified: 2016-01-28
Priority: Not Prioritized
View all other issues in [thread.condition.condvar].
View all issues with Resolved status.
Discussion:
All usages of "function scope" in 32.7.4 [thread.condition.condvar] and 32.7.5 [thread.condition.condvarany], such as 32.7.4 [thread.condition.condvar] p10 b4:
If the function exits via an exception, lock.lock() shall be called prior to exiting the function scope.
seem to be inappropriate compared to the actual core language definition of [basic.funscope]:
Labels (6.1) have function scope and may be used anywhere in the function in which they are declared. Only labels have function scope.
Probably the intended meaning is "outermost block scope of the function".
[2013-09 Chicago: Resolved by proposed resolution of LWG 2135(i)]
Proposed resolution:
Resolved by proposed resolution of LWG 2135(i).
<cstdalign>
and #define
of alignof
Section: 17.14 [support.runtime] Status: Resolved Submitter: Richard Smith Opened: 2013-02-14 Last modified: 2020-09-06
Priority: 2
View all other issues in [support.runtime].
View all issues with Resolved status.
Discussion:
According to 17.14 [support.runtime] p2:
The contents of these headers are the same as the Standard C library headers [..],
<stdalign.h>
, [..]
Since our base C standard is C99, which doesn't have a <stdalign.h>
, the reference to a non-existing
C header is irritating (In this context <stdalign.h>
doesn't refer to the deprecated C++ header
<stdalign.h>
described in [depr.c.headers]).
alignof
, which C11 also defines
in this header.
Currently we only have the following guarantee as part of 17.14 [support.runtime] p7:
The header
<cstdalign>
and the header<stdalign.h>
shall not define a macro namedalignas
.
It is unclear what the better strategy is: Striking the reference to <stdalign.h>
in
17.14 [support.runtime] p2 or upgrading to C11 as new base C standard.
[2014-02-15 Issaquah]
STL: related to earlier issue on C4, 2201(i), and now we get a C11 header
JY: find _Alignof as keyword C11 FDIS has four defines in stdalign.h
AM: need paper for C11 as base library we should really do that
STL: really need vendor input
STL: don't think we need to do anything right now not P1
AM: any objections to downscale to P2 (no objections)
[2016-03 Jacksonville]
Walter: this is on track to go away if we adopt Clark's paper to rebase to C11
Room: tentatively resolved; revisit after C11 paper: P0063
[2016-03 Oulu]
P0063 was adopted.
Change status to Tentatively Resolved
Proposed resolution:
istream::putback
problemSection: 31.7.5.4 [istream.unformatted] Status: C++20 Submitter: Juan Soulie Opened: 2013-03-01 Last modified: 2021-02-25
Priority: 3
View all other issues in [istream.unformatted].
View all issues with C++20 status.
Discussion:
In 31.7.5.4 [istream.unformatted] / 34, when describing putback
, it says that "rdbuf->sputbackc()
"
is called. The problem are not the obvious typos in the expression, but the fact that it may lead to different
interpretations, since nowhere is specified what the required argument to sputbackc
is.
rdbuf()->sputbackc(c)
", but "rdbuf()->sputbackc(char_type())
" or
just anything would be as conforming (or non-conforming) as the first guess.
[2017-12-12, Jonathan comments and provides wording]
Fix the bogus expression, and change sputbackc()
to just sputbackc
since we're talking about the function, not an expression sputbackc()
(which
isn't a valid expression any more than rdbuf->sputbackc()
is). Make the
corresponding change to the equivalent wording in p36 too.
[ 2017-12-14 Moved to Tentatively Ready after 6 positive votes on c++std-lib. ]
[2018-3-17 Adopted in Jacksonville]
Proposed resolution:
This wording is relative to N4713.
Change 31.7.5.4 [istream.unformatted] as shown:
basic_istream<charT, traits>& putback(char_type c);-34- Effects: Behaves as an unformatted input function (as described above), except that the function first clears
-35- Returns:eofbit
. After constructing a sentry object, if!good()
callssetstate(failbit)
which may throw an exception, and return. Ifrdbuf()
is not null, callsrdbuf()->sputbackc(c)
. Ifrdbuf()
is null, or ifsputbackc
returns()traits::eof()
, callssetstate(badbit)
(which may throwios_base::failure
(31.5.4.4 [iostate.flags])). [Note: This function extracts no characters, so the value returned by the next call togcount()
is0
. — end note]*this
.basic_istream<charT, traits>& unget();-36- Effects: Behaves as an unformatted input function (as described above), except that the function first clears
-37- Returns:eofbit
. After constructing a sentry object, if!good()
callssetstate(failbit)
which may throw an exception, and return. Ifrdbuf()
is not null, callsrdbuf()->sungetc()
. Ifrdbuf()
is null, or ifsungetc
returns()traits::eof()
, callssetstate(badbit)
(which may throwios_base::failure
(31.5.4.4 [iostate.flags])). [Note: This function extracts no characters, so the value returned by the next call togcount()
is0
. — end note]*this
.
basic_istream::seekg
Section: 31.7.5.4 [istream.unformatted] Status: C++17 Submitter: Juan Soulie Opened: 2013-03-04 Last modified: 2017-07-30
Priority: 3
View all other issues in [istream.unformatted].
View all issues with C++17 status.
Discussion:
When issue 1445(i) was resolved by adopting
N3168, it exposed the need to
modify both overloads of basic_istream::seekg
(by inserting "the function clears eofbit," after "except that"),
but the fix applied to the text apparently forgets the second overload at 31.7.5.4 [istream.unformatted] p43.
[2013-10-17: Daniel provides concrete wording]
It seems that the tiny sentence "SIMILARLY for 27.7.1.3/43 (seekg
)." had been overlooked. I agree that the wording needs to be
applied here as well.
[2015-05-06 Lenexa: Move to Ready]
MC: This was just missed when we added "the function first clears eofbit" to the other overload, Daniel agrees. Editing mistake.
Move to Ready, consensus.
Proposed resolution:
This wording is relative to N3691.
Change 31.7.5.4 [istream.unformatted] p43 as indicated:
basic_istream<charT,traits>& seekg(off_type off, ios_base::seekdir dir);-43- Effects: Behaves as an unformatted input function (as described in 27.7.2.3, paragraph 1), except that the function first clears
eofbit
, it does not count the number of characters extracted, and does not affect the value returned by subsequent calls togcount()
. […]
packaged_task::reset()
memory allocationSection: 32.10.10.2 [futures.task.members] Status: Resolved Submitter: Jonathan Wakely Opened: 2013-03-05 Last modified: 2017-03-20
Priority: 3
View all other issues in [futures.task.members].
View all issues with Resolved status.
Discussion:
The effects of packaged_task::reset()
result in memory allocation, but
don't allow a user to provide an allocator.
packaged_task::reset()
needs to be overloaded like so:
template<class Alloc> void reset(const Alloc&);
Alternatively, the effects of reset()
need to require the same allocator is used
as at construction, which would require the constructor to store the allocator for later use.
[2015-02 Cologne]
Handed over to SG1.
[2015-05 Lenexa, SG1 response]
No strong opinions in SG1, and this is really an LWG issue. Back to you.
[2016-08-02 Chicago, Billy O'Neal comments and suggests concrete wording]
Talked this over with Alasdair, who says there's little desire to allow the packaged_task
to
be change allocators after initial construction, making what libstdc++ does already the "right thing."
A clarification note is still necessary to indicate that the allocator supplied to the
allocator_arg_t
constructor is to be used.
Wed PM: Move to Tentatively Ready
[2016-09-08]
Alisdair requests change to Review.
[2017-03-03, Kona]
This was resolved by adopting 2921(i), which removed the constructors that take allocators.
Proposed resolution:
This wording is relative to N4606
Change 32.10.10.2 [futures.task.members] as indicated:
void reset();-22- Effects:
if the shared state associated with
*this
was created via thepackaged_task(F&& f)
constructor, aAs if*this = packaged_task(std::move(f))
, wheref
is the task stored in*this
.if the shared state associated with
*this
was created via thepackaged_task(allocator_arg_t, Allocator& a, F&&)
constructor, as if*this = packaged_task(allocator_arg, a, std::move(f))
, wherea
is the allocator used to allocate the shared state associated with*this
, andf
is the task stored in*this
.[Note: This constructs a new shared state for
-23- Throws:*this
. The old state is abandoned (30.6.4). — end note]
if no allocator was used,
bad_alloc
if memory for the new shared state could not be allocated.if an allocator was used, any exception thrown by
std::allocator_traits<Allocator>::template rebind_traits<unspecified>::allocate
.any exception thrown by the move constructor of the task stored in the shared state.
future_error
with an error condition ofno_state
if*this
has no shared state.
unique_ptr
assignment effects w.r.t. deleterSection: 20.3.1.3.4 [unique.ptr.single.asgn] Status: C++14 Submitter: Jonathan Wakely Opened: 2013-03-13 Last modified: 2016-01-28
Priority: Not Prioritized
View all other issues in [unique.ptr.single.asgn].
View all issues with C++14 status.
Discussion:
The Effects clauses for unique_ptr
assignment don't make sense, what
is the target of "an assignment from std::forward<D>(u.get_deleter())
"?
[2013-04-20, Bristol]
Move to Ready
[2013-09-29, Chicago]
Apply to Working Paper
Proposed resolution:
This wording is relative to N3485.
Edit 20.3.1.3.4 [unique.ptr.single.asgn] paragraph 2:
unique_ptr& operator=(unique_ptr&& u) noexcept;[…]
-2- Effects: Transfers ownership fromu
to*this
as if by callingreset(u.release())
followed byan assignment fromget_deleter() = std::forward<D>(u.get_deleter())
.
Edit 20.3.1.3.4 [unique.ptr.single.asgn] paragraph 6:
template <class U, class E> unique_ptr& operator=(unique_ptr<U, E>&& u) noexcept;[…]
-6- Effects: Transfers ownership fromu
to*this
as if by callingreset(u.release())
followed byan assignment fromget_deleter() = std::forward<E>(u.get_deleter())
.
std::nullptr_t
Section: 21.3.5.2 [meta.unary.cat] Status: C++14 Submitter: Joe Gottman Opened: 2013-03-15 Last modified: 2016-01-28
Priority: Not Prioritized
View all issues with C++14 status.
Discussion:
According to 21.3.5.2 [meta.unary.cat], for every type T
, exactly one of the primary type traits is true.
So which is true for the type std::nullptr_t
? By 5.13.8 [lex.nullptr] std::nullptr_t
is not a
pointer type or a pointer-to-member type, so is_pointer
, is_member_object_pointer
and
is_member_function_pointer
can't be true for std::nullptr_t
, and none of the other primary type traits
seem to apply.
[2013-04-20, Bristol]
Rename to is_null_pointer
, move to Ready
Previous wording:
This wording is relative to N3485.
Edit 21.3.3 [meta.type.synop], header
<type_traits>
synopsis:namespace std { […] // 20.9.4.1, primary type categories: template <class T> struct is_void; template <class T> struct is_nullptr; template <class T> struct is_integral; template <class T> struct is_floating_point; […] }Edit Table 47 — "Primary type category predicates" as indicated:
Table 47 — Primary type category predicates Template Condition Comments …
template <class T>
struct is_nullptr;T
isstd::nullptr_t
([basic.fundamental])…
[2013-09-29, Chicago]
Apply to the Working Paper
Proposed resolution:
This wording is relative to N3485.
Edit 21.3.3 [meta.type.synop], header <type_traits>
synopsis:
namespace std { […] // 20.9.4.1, primary type categories: template <class T> struct is_void; template <class T> struct is_null_pointer; template <class T> struct is_integral; template <class T> struct is_floating_point; […] }
Edit Table 47 — "Primary type category predicates" as indicated:
Table 47 — Primary type category predicates Template Condition Comments …
template <class T>
struct is_null_pointer;T
isstd::nullptr_t
([basic.fundamental])…
gets
from <cstdio>
Section: 31.13 [c.files] Status: Resolved Submitter: Jonathan Wakely Opened: 2013-04-17 Last modified: 2016-10-31
Priority: Not Prioritized
View all other issues in [c.files].
View all issues with Resolved status.
Discussion:
Addresses GB 9
In 31.13 [c.files] the current C++ standard claims that <cstdio>
defines a
function called "gets" but it has no declaration or semantics, because it was removed from C11,
having been deprecated since C99. We should remove it for C++14.
[2013-09 Chicago]
Will resolve with the wording in the NB comment.
Proposed resolution:
Resolved by resolution as suggested by NB comment GB 9
Section: 22.9.2.2 [bitset.cons], 22.9.2.3 [bitset.members], 27.4.3.3 [string.cons], 27.4.3.7 [string.modifiers], 27.4.3.8 [string.ops] Status: C++17 Submitter: Frank Birbacher Opened: 2013-04-18 Last modified: 2017-07-30
Priority: 3
View all other issues in [bitset.cons].
View all issues with C++17 status.
Discussion:
Similar to LWG 2207(i) there are several other places where the "Requires" clause precludes the "Throws" condition.
Searching for the out_of_range
exception to be thrown, the following have been found (based on the working draft
N3485):
22.9.2.2 [bitset.cons] p3+4
22.9.2.3 [bitset.members] p13+14 (set
)
22.9.2.3 [bitset.members] p19+20 (reset
)
22.9.2.3 [bitset.members] p27+28 (flip
)
22.9.2.3 [bitset.members] p41+42 (test
)
27.4.3.3 [string.cons] p3+4
27.4.3.7.2 [string.append] p3+4
27.4.3.7.3 [string.assign] p4+5
27.4.3.7.4 [string.insert] p1+2, p5+6, p9+10 (partially)
27.4.3.7.5 [string.erase] p1+2
27.4.3.7.6 [string.replace] p1+2, p5+6, p9+10 (partially)
27.4.3.7.7 [string.copy] p1+2
27.4.3.8.3 [string.substr] p1+2
[2013-10-15: Daniel provides wording]
In addition to the examples mentioned in the discussion, a similar defect exists for thread
's join()
and detach
functions (see 32.4.3.6 [thread.thread.member]). The suggested wording applies a similar fix for these
as well.
[2015-05, Lenexa]
STL : likes it
DK : does it change behavior?
Multiple : no
Move to ready? Unanimous
Proposed resolution:
This wording is relative to N3936.
Modify 22.9.2.2 [bitset.cons] as indicated: [Editorial comment: The wording form used to ammend the Throws element is borrowed from a similar style used in 27.4.3.7.6 [string.replace] p10]
template <class charT, class traits, class Allocator> explicit bitset(const basic_string<charT, traits, Allocator>& str, typename basic_string<charT, traits, Allocator>::size_type pos = 0, typename basic_string<charT, traits, Allocator>::size_type n = basic_string<charT, traits, Allocator>::npos, charT zero = charT('0'), charT one = charT('1'));-4- Throws:
-3- Requires:pos <= str.size()
.out_of_range
ifpos > str.size()
orinvalid_argument
if an invalid character is found (see below). -5- Effects: Determines the effective lengthrlen
of the initializing string as the smaller ofn
andstr.size() - pos
. The function then throwsinvalid_argument
if any of therlen
characters instr
beginning at positionpos
is other thanzero
orone
. The function usestraits::eq()
to compare the character values. […]
Modify 22.9.2.3 [bitset.members] as indicated:
bitset<N>& set(size_t pos, bool val = true);-14- Throws:
-13- Requires:pos
is validout_of_range
ifpos
does not correspond to a valid bit position. […]
bitset<N>& reset(size_t pos);-20- Throws:
-19- Requires:pos
is validout_of_range
ifpos
does not correspond to a valid bit position. […]
bitset<N>& flip(size_t pos);-28- Throws:
-27- Requires:pos
is validout_of_range
ifpos
does not correspond to a valid bit position. […]
bool test(size_t pos) const;-42- Throws:
-41- Requires:pos
is validout_of_range
ifpos
does not correspond to a valid bit position. […]
Modify 27.4.3.3 [string.cons] as indicated:
basic_string(const basic_string& str, size_type pos, size_type n = npos, const Allocator& a = Allocator());-4- Throws:
-3- Requires:pos <= str.size()
out_of_range
ifpos > str.size()
.
Modify 27.4.3.5 [string.capacity] as indicated:
void resize(size_type n, charT c);-7- Throws:
-6- Requires:n <= max_size()
length_error
ifn > max_size()
.
Modify 27.4.3.7.2 [string.append] as indicated:
basic_string& append(const basic_string& str, size_type pos, size_type n = npos);-4- Throws:
-3- Requires:pos <= str.size()
out_of_range
ifpos > str.size()
.
Modify 27.4.3.7.3 [string.assign] as indicated:
basic_string& assign(const basic_string& str, size_type pos, size_type n = npos);-6- Throws:
-5- Requires:pos <= str.size()
out_of_range
ifpos > str.size()
.
Modify 27.4.3.7.4 [string.insert] as indicated: [Editorial note: The first change suggestion is also a bug fix
of the current wording, because (a) the function has parameter pos1
but the semantics refers to pos
and (b)
it is possible that this function can throw length_error
, see p10]
basic_string& insert(size_type pos1, const basic_string& str);
-1- Requires:pos <= size()
.-2- Throws:-3- Effects:out_of_range
ifpos > size()
.CallsEquivalent to:return insert(pos, str.data(), str.size());
.-4- Returns:*this
.
basic_string& insert(size_type pos1, const basic_string& str, size_type pos2, size_type n = npos);-6- Throws:
-5- Requires:pos1 <= size()
andpos2 <= str.size()
.out_of_range
ifpos1 > size()
orpos2 > str.size()
. […]
basic_string& insert(size_type pos, const charT* s, size_type n);-9- Requires:
-10- Throws:s
points to an array of at leastn
elements ofcharT
and.pos <= size()
out_of_range
ifpos > size()
orlength_error
ifsize() + n > max_size()
. […]
basic_string& insert(size_type pos, const charT* s);-13- Requires:
-14- Effects: Equivalent topos <= size()
ands
points to an array of at leasttraits::length(s) + 1
elements ofcharT
.return insert(pos, s, traits::length(s));
.-15- Returns:*this
.
Modify 27.4.3.7.5 [string.erase] as indicated:
basic_string& erase(size_type pos = 0, size_type n = npos);-2- Throws:
-1- Requires:pos <= size()
out_of_range
ifpos > size()
. […]
Modify 27.4.3.7.6 [string.replace] as indicated: [Editorial note: The first change suggestion is also a bug fix
of the current wording, because it is possible that this function can throw length_error
, see p10]
basic_string& replace(size_type pos1, size_type n1, const basic_string& str);
-1- Requires:pos1 <= size()
.-2- Throws:-3- Effects:out_of_range
ifpos1 > size()
.CallsEquivalent toreturn replace(pos1, n1, str.data(), str.size());
.-4- Returns:*this
.
basic_string& replace(size_type pos1, size_type n1, const basic_string& str, size_type pos2, size_type n = npos);-6- Throws:
-5- Requires:pos1 <= size()
andpos2 <= str.size()
.out_of_range
ifpos1 > size()
orpos2 > str.size()
. […]
basic_string& replace(size_type pos1, size_type n1, const charT* s, size_type n2);-9- Requires:
-10- Throws:pos1 <= size()
ands
points to an array of at leastn2
elements ofcharT
.out_of_range
ifpos1 > size()
orlength_error
if the length of the resulting string would exceedmax_size()
(see below). […]
basic_string& replace(size_type pos, size_type n, const charT* s);-13- Requires:
-14- Effects: Equivalent topos <= size()
ands
points to an array of at leasttraits::length(s) + 1
elements ofcharT
.return replace(pos, n, s, traits::length(s));
.-15- Returns:*this
.
Modify 27.4.3.7.7 [string.copy] as indicated:
size_type copy(charT* s, size_type n, size_type pos = 0) const;-2- Throws:
-1- Requires:pos <= size()
out_of_range
ifpos > size()
. […]
Modify 27.4.3.8.3 [string.substr] as indicated:
basic_string substr(size_type pos = 0, size_type n = npos) const;-2- Throws:
-1- Requires:pos <= size()
out_of_range
ifpos > size()
. […]
Modify 32.4.3.6 [thread.thread.member] as indicated:
void join();[…] -7- Throws:
-3- Requires:joinable()
is true.system_error
when an exception is required (30.2.2). -8- Error conditions:
[…]
invalid_argument
— if the thread is not joinable.
void detach();[…] -12- Throws:
-9- Requires:joinable()
is true.system_error
when an exception is required (30.2.2). -13- Error conditions:
[…]
invalid_argument
— if the thread is not joinable.
vector::push_back()
still broken with C++11?Section: 23.3.13.5 [vector.modifiers] Status: C++14 Submitter: Nicolai Josuttis Opened: 2013-04-21 Last modified: 2016-01-28
Priority: Not Prioritized
View all other issues in [vector.modifiers].
View all issues with C++14 status.
Discussion:
According to my understanding, the strong guarantee of push_back()
led to the introduction of noexcept
and to the typical implementation that vectors usually copy their elements on reallocation unless the move operations of
their element type guarantees not to throw.
Unless otherwise specified (see 23.2.4.1, 23.2.5.1, 23.3.3.4, and 23.3.6.5) all container types defined in this Clause meet the following additional requirements:
- […]
- if an exception is thrown by a
push_back()
orpush_front()
function, that function has no effects.
However, 23.3.13.5 [vector.modifiers] specifies for vector modifiers, including push_back()
:
If an exception is thrown other than by the copy constructor, move constructor, assignment operator, or move assignment operator of
T
or by anyInputIterator
operation there are no effects. If an exception is thrown by the move constructor of a non-CopyInsertable
T
, the effects are unspecified.
I would interpret this as an "otherwise specified" behavior for push_back()
, saying that the strong guarantee
is only given if constructors and assignments do not throw.
emplace_back()
and emplace_front()
. These are similar single-element additions and should provide the same
strong guarantee.
Daniel adds:
It seems the error came in when N2350
and N2345 became accepted and where
integrated into the working draft N2369.
The merge resulted in a form that changed the previous meaning and as far as I understand it, this effect was not intended.
[2013-09-16, Nico provides concrete wording]
[2013-09-26, Nico improves wording]
The new proposed resolution is driven as follows:
In the container requirements section, there shall be general statements that single element insertions
and push_back()
, pop_back
, emplace_front()
, and emplace_back()
have no effect
on any exception.
emplace_front()
and emplace_back()
, which
are missing.
Formulate only the exceptions from that (or where other general statements might lead to the impression, that the blanket statement no longer applies):
remove the statement in list::push_back()
saying again that exceptions have to effect.
Clarify that all single-element insertions at either end of a deque
have the strong guarantee.
Clarify that all single-element insertions at the end of a vector
have the strong guarantee.
Proposed resolution:
This wording is relative to N3691.
Edit 23.2.2 [container.requirements.general] p10 b2 as indicated:
if an exception is thrown by an insert()
or emplace()
function while inserting a single element, that
function has no effects.
if an exception is thrown by a push_back()
or, push_front()
, emplace_back()
, or
emplace_front()
function, that function has no effects.
Edit 23.3.5.4 [deque.modifiers] as indicated:
iterator insert(const_iterator position, const T& x); iterator insert(const_iterator position, T&& x); iterator insert(const_iterator position, size_type n, const T& x); template <class InputIterator> iterator insert(const_iterator position, InputIterator first, InputIterator last); iterator insert(const_iterator position, initializer_list<T>); template <class... Args> void emplace_front(Args&&... args); template <class... Args> void emplace_back(Args&&... args); template <class... Args> iterator emplace(const_iterator position, Args&&... args); void push_front(const T& x); void push_front(T&& x); void push_back(const T& x); void push_back(T&& x);-1- Effects: An insertion in the middle of the deque invalidates all the iterators and references to elements of the deque. An insertion at either end of the deque invalidates all the iterators to the deque, but has no effect on the validity of references to elements of the deque.
-2- Remarks: If an exception is thrown other than by the copy constructor, move constructor, assignment operator, or move assignment operator of
T
there are no effects. If an exception is thrown while inserting a single element at either end, there are no effects.IfOtherwise, if an exception is thrown by the move constructor of a non-CopyInsertable
T
, the effects are unspecified.-3- Complexity: The complexity is linear in the number of elements inserted plus the lesser of the distances to the beginning and end of the deque. Inserting a single element either at the beginning or end of a deque always takes constant time and causes a single call to a constructor of
T
.
Edit 23.3.13.5 [vector.modifiers] as indicated:
iterator insert(const_iterator position, const T& x); iterator insert(const_iterator position, T&& x); iterator insert(const_iterator position, size_type n, const T& x); template <class InputIterator> iterator insert(const_iterator position, InputIterator first, InputIterator last); iterator insert(const_iterator position, initializer_list<T>); template <class... Args> void emplace_back(Args&&... args); template <class... Args> iterator emplace(const_iterator position, Args&&... args); void push_back(const T& x); void push_back(T&& x);-1- Remarks: Causes reallocation if the new size is greater than the old capacity. If no reallocation happens, all the iterators and references before the insertion point remain valid. If an exception is thrown other than by the copy constructor, move constructor, assignment operator, or move assignment operator of
T
or by anyInputIterator
operation there are no effects. If an exception is thrown while inserting a single element at the end andT
isCopyInsertable
oris_nothrow_move_constructible<T>::value
istrue
, there are no effects.IfOtherwise, if an exception is thrown by the move constructor of a non-CopyInsertable
T
, the effects are unspecified.-2- Complexity: The complexity is linear in the number of elements inserted plus the distance to the end of the vector.
Section: 23.2.2 [container.requirements.general] Status: C++14 Submitter: Marshall Clow Opened: 2013-05-29 Last modified: 2016-01-28
Priority: Not Prioritized
View other active issues in [container.requirements.general].
View all other issues in [container.requirements.general].
View all issues with C++14 status.
Discussion:
Currently (n3690) Table 96 says, in the row for "a == b
", that the
Operational semantics are:
==
is an equivalence relation.
distance(a.begin(), a.end()) == distance(b.begin(), b.end()) && equal(a.begin(), a.end(),b.begin())
Given the extension of equal for C++14, this can be simplified to:
==
is an equivalence relation.
equal(a.begin(), a.end(), b.begin(), b.end())
[ Alisdair notes that a similar edit would apply to the unordered containers requirements. ]
Previous resolution from Marshall Clow:
Ammend the Operational Semantics for 23.2.2 [container.requirements.general], Table 96, row "
a == b
"==
is an equivalence relation.
distance(a.begin(), a.end()) == distance(b.begin(), b.end()) &&equal(a.begin(), a.end(), b.begin(), b.end())Ammend 23.2.8 [unord.req] p12:
Two unordered containersa
andb
compare equal ifa.size() == b.size()
and, for every equivalent-key group[Ea1,Ea2)
obtained froma.equal_range(Ea1)
, there exists an equivalent-key group[Eb1,Eb2)
obtained fromb.equal_range(Ea1)
, such thatdistance(Ea1, Ea2) == distance(Eb1, Eb2)
andis_permutation(Ea1, Ea2, Eb1, Eb2)
returnstrue
. For ...
[2013-09 Chicago]
Marshall improves wording
[2013-09 Chicago (evening issues)]
Moved to ready, after confirming latest wording reflects the discussion earlier in the day.
Proposed resolution:
Ammend 23.2.2 [container.requirements.general], Table 96 as indicated:
Table 96 — Container requirements (continued) Expression Return type Operational
semanticsAssertion/note
pre-/post-conditionComplexity …
a == b
convertible to bool
==
is an equivalence relation.
distance(a.begin(),
a.end()) ==
distance(b.begin(),
b.end()) &&
equal(a.begin(),
a.end(),
b.begin(), b.end())Requires: T
is
EqualityComparable
Constant if a.size() != b.size()
, linear otherwise…
Ammend 23.2.8 [unord.req] p12:
Two unordered containersa
andb
compare equal ifa.size() == b.size()
and, for every equivalent-key group[Ea1,Ea2)
obtained froma.equal_range(Ea1)
, there exists an equivalent-key group[Eb1,Eb2)
obtained fromb.equal_range(Ea1)
, such thatdistance(Ea1, Ea2) == distance(Eb1, Eb2)
andis_permutation(Ea1, Ea2, Eb1, Eb2)
returnstrue
. For […]
Amend [forwardlist.overview] p2:
-2- A
forward_list
satisfies all of the requirements of a container (Table 96), except that thesize()
member function is not provided andoperator==
has linear complexity. […]
a.erase(q1, q2)
unable to directly return q2
Section: 23.2.7 [associative.reqmts] Status: C++14 Submitter: Geoff Alexander Opened: 2013-05-11 Last modified: 2016-01-28
Priority: 0
View other active issues in [associative.reqmts].
View all other issues in [associative.reqmts].
View all issues with C++14 status.
Discussion:
Section 23.2.7 [associative.reqmts], Table 102, page 743 of the C++ 2011 Standard states that a.erase(q1, q2)
returns q2
. The problem is that a.erase(q1, q2)
cannot directly return q2
as the return type,
iterator
, differs from that of q2
, const_iterator
.
[2013-09 Chicago (evening issues group)]
The wording looks good, but is worded slightly differently to how we say the same for sequence containers, and for unordered associative containers. We should apply consistent wording in all three cases.
Alisdair to provide the wording.
[2014-02-12 Issaquah meeting]
Move a Immediate.
Proposed resolution:
In the specification of a.erase(q1, q2)
in sub-clause 23.2.7 [associative.reqmts], Table 102 change as indicated:
Table 102 — Associative container requirements (in addition to container) (continued) Expression Return type Assertion/note pre-/post-condition Complexity …
a.erase(q1, q2)
iterator
erases all the elements in the range [q1,q2)
. Returnsan iterator pointing to the element pointed to byq2
q2
prior to any elements being erased. If no such element exists,a.end()
is returned.log(a.size()) + N
whereN
has the valuedistance(q1, q2)
.…
Section: 16.4.6.5 [member.functions] Status: C++17 Submitter: Richard Smith Opened: 2013-05-12 Last modified: 2017-07-30
Priority: 3
View all other issues in [member.functions].
View all issues with C++17 status.
Discussion:
16.4.6.5 [member.functions] p2 says:
"An implementation may declare additional non-virtual member function signatures within a class:
- by adding arguments with default values to a member function signature; [Footnote: Hence, the address of a member function of a class in the C++ standard library has an unspecified type.] [Note: An implementation may not add arguments with default values to virtual, global, or non-member functions. — end note]
- by replacing a member function signature with default values by two or more member function signatures with equivalent behavior; and
- by adding a member function signature for a member function name."
This wording is not using the correct terminology. "by adding arguments with default values" presumably means "by adding parameters with default arguments", and likewise throughout.
This paragraph only allows an implementation to declare "additional" signatures, but the first bullet is talking about replacing a standard signature with one with additional parameters.
None of these bullets allows a member function with no ref-qualifier to be replaced by signatures with ref-qualifiers (a situation which was just discussed on std-proposals), and likewise for cv-qualifiers. Presumably that is not intentional, and such changes should be permissible.
I think the first two items are probably editorial, since the intent is clear.
[2013-12-11 Richard provides concrete wording]
[2015-05, Lenexa]
JW: I don't like that this loses the footnote about the address of member functions having an unspecified type,
the footnote is good to be able to point to as an explicit clarification of one consequence of the normative wording.
MC: so we want to keep the footnote
STL: doesn't need to be a footnote, can be an inline Note
JW: does this have any impact on our ability to add totally different functions with unrelated names, not described in
the standard?
MC: no, the old wording didn't refer to such functions anyway
Move to Ready and include in motion on Friday?
9 in favor, 0 opposed, 2 abstention
Proposed resolution:
This wording is relative to N3797.
Merge 16.4.6.5 [member.functions]p2+3 as indicated:
-2-
An implementation may declare additional non-virtual member function signatures within a class:
by adding arguments with default values to a member function signature;188 [Note: An implementation may not add arguments with default values to virtual, global, or non-member functions. — end note]
by replacing a member function signature with default values by two or more member function signatures with equivalent behavior; and
by adding a member function signature for a member function name.
-3- A call to a member function signature described in the C++ standard library behaves as if the implementation declares no additional member function signatures.[Footnote: A valid C++ program always calls the expected library member function, or one with equivalent behavior. An implementation may also define additional member functions that would otherwise not be called by a valid C++ program.]For a non-virtual member function described in the C++ standard library, an implementation may declare a different set of member function signatures, provided that any call to the member function that would select an overload from the set of declarations described in this standard behaves as if that overload were selected. [Note: For instance, an implementation may add parameters with default values, or replace a member function with default arguments with two or more member functions with equivalent behavior, or add additional signatures for a member function name. — end note]
Allocator::pointer
Section: 16.4.4.6 [allocator.requirements] Status: C++17 Submitter: Jonathan Wakely Opened: 2013-05-14 Last modified: 2017-07-30
Priority: 3
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Discussion:
For an allocator A<T>
which defines A<T>::pointer
to a class type,
i.e. not T*
, I see no requirement that A<T>::pointer
is convertible to
A<U>::pointer
, even if T*
is convertible to U*
. Such conversions are
needed in containers to convert from e.g. ListNodeBase*
to ListNode<T>*
.
The obvious way to do such conversions appears to be
pointer_traits::pointer_to()
, but that's ill-formed if the static
member function A<T>::pointer::pointer_to()
doesn't exist and the
allocator requirements don't mention that function, so you need to
cast A<T>::pointer
to A<T>::void_pointer
then cast that to
A<U>::pointer
.
Is converting via void_pointer
really intended, or are we missing a requirement that
pointer_traits<A<T>::pointer>::pointer_to()
be well-formed?
Proposed resolution:
Add to the Allocator requirements table the following requirement:
The expression
pointer_traits<XX::pointer>::pointer_to(r)
is well-defined.
[2013-09 Chicago]
Pablo to come back with proposed wording
[2015-07 Telecon]
Marshall to ping Pablo for proposed wording and disable current wording.
Previous resolution [SUPERSEDED]:
Edit Table 28 as indicated:
Table 28 — Allocator requirements (continued) Expression Return type Assertion/note pre-/post-condition Default …
static_cast<X::const_pointer>(z)
X::const_pointer
static_cast<X::const_pointer>(z) == q
pointer_traits<X::pointer>::pointer_to(r)
X::pointer
…
[2016-11-12, Issaquah]
Sat PM: Restore original P/R and move to tentatively ready.
Proposed resolution:
Edit Table 28 as indicated:
Table 28 — Allocator requirements (continued) Expression Return type Assertion/note pre-/post-condition Default …
static_cast<X::const_pointer>(z)
X::const_pointer
static_cast<X::const_pointer>(z) == q
pointer_traits<X::pointer>::pointer_to(r)
X::pointer
…
pointer
' type internally?Section: 23.2 [container.requirements] Status: C++17 Submitter: Jonathan Wakely Opened: 2013-05-14 Last modified: 2017-07-30
Priority: 2
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Discussion:
Is a container C
only supposed to refer to allocated memory (blocks of
contiguous storage, nodes, etc.) through objects of type C::pointer
rather than C::value_type*
?
I don't see anything explicitly requiring this, so a container could
immediately convert the result of get_allocator().allocate(1)
to a
built-in pointer of type value_type*
and only deal with the built-in
pointer until it needs to deallocate it again, but that removes most
of the benefit of allowing allocators to use custom pointer types.
[2014-06-12, Jonathan comments]
This issue is basically the same issue as LWG 1521(i), which agrees it's an issue, to be dealt with in the future, so I request that 2261(i) not be closed as a dup unless we reopen 1521(i).
[2016-08, Zhihao comments]
The pointer
types are not exposed in the container interface,
and we consider that the memory allocation constraints
"all containers defined in this clause obtain memory using an
allocator" already implies the reasonable expectation. We
propose the fix as non-normative.
[2016-08 Chicago]
Tues PM: General agreement on direction, Alisdair and Billy to update wording
Fri AM: Moved to Tentatively Ready
Proposed resolution:
This wording is relative to N4606.
[Drafting notes: if people prefer this to be normative, strip the "Note" markups.]
Modify 23.2.2 [container.requirements.general]/8 as indicated:
Unless otherwise specified, all containers defined in this clause obtain memory using an allocator (see 16.4.4.6 [allocator.requirements]). [Note: In particular, containers and iterators do not store references to allocated elements other than through the allocator's pointer type, i.e., as objects of type
P
orpointer_traits<P>::template rebind<unspecified>
, whereP
isallocator_traits<allocator_type>::pointer
. — end note]
Section: 16.4.4.6 [allocator.requirements], 23.2 [container.requirements] Status: C++14 Submitter: Howard Hinnant Opened: 2013-06-25 Last modified: 2016-01-28
Priority: 1
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Discussion:
This ancient issue 179(i) says one ought to be able to compare iterators with const_iterators
from any given container. I'm having trouble finding words that guarantee this in C++11. This impacts not only a
container's iterators, but also the allocator requirements in [llocator.requirements] surrounding
pointer
, const_pointer
, void_pointer
and const_void_pointer
. E.g. can one
compare a pointer
with a const_pointer
?
allocator::pointer
and const_pointer
are required to be random access iterators, one could
expect that the 179(i) guarantees apply for them as well.
[ Daniel comments: ]
The wording for 179(i) was part of several working drafts (e.g. also in N3092) over some time and suddenly got lost in N3242, presumably by accident. Whatever we decide for allocator pointers, I expect that we need to restore the 179(i) wording as part of the overall resolution:
Reinsert after 23.2 [container.requirements] p6:
-6-
-?- In the expressionsbegin()
returns an iterator referring to the first element in the container.end()
returns an iterator which is the past-the-end value for the container. If the container is empty, thenbegin() == end()
;i == j i != j i < j i <= j i >= j i > j i - jwhere
i
andj
denote objects of a container'siterator
type, either or both may be replaced by an object of the container'sconst_iterator
type referring to the same element with no change in semantics.
[2014-02-13 Issaquah, Daniel comments and suggests wording]
First, I didn't originally move the seemingly lost wording to the resolution section because I wanted to ensure that the committee double-checks the reason of this loss.
Second, albeit restoring this wording will restore the comparability ofconst_iterator
and iterator
of
containers specified in Clause 23, but this alone would not imply that this guarantee automatically extends to
all other iterators, simply because there is no fundamental relation between a mutable iterator and a constant
iterator by itself. This relation only exists under specific conditions, for example for containers which provide two such
typedefs
of that kind. Thus the wording restoration would not ensure that allocator pointer
and
const_pointer
would be comparable with each other. To realize that, we would need additional guarantees added
to the allocator requirements. In fact, it is crucial to separate these things, because allocators are not
restricted to be used within containers, they have their own legitimate use for other places as well (albeit containers
presumably belong to the most important use-cases), and this is also stated in the introduction of 16.4.4.6 [allocator.requirements],
where it says:
All of the string types (Clause 21), containers (Clause 23) (except array), string buffers and string streams (Clause 27), and
match_results
(Clause 28) are parameterized in terms of allocators.
[2014-02-12 Issaquah meeting]
Move a Immediate.
Proposed resolution:
Insert after 16.4.4.6 [allocator.requirements] p4 as indicated:
-4- An allocator type
-?- LetX
shall satisfy the requirements ofCopyConstructible
(17.6.3.1). TheX::pointer
,X::const_pointer
,X::void_pointer
, andX::const_void_pointer
types shall satisfy the requirements ofNullablePointer
(17.6.3.3). No constructor, comparison operator, copy operation, move operation, or swap operation on these types shall exit via an exception.X::pointer
andX::const_pointer
shall also satisfy the requirements for a random access iterator (24.2).x1
andx2
denote objects of (possibly different) typesX::void_pointer
,X::const_void_pointer
,X::pointer
, orX::const_pointer
. Then,x1
andx2
are equivalently-valued pointer values, if and only if bothx1
andx2
can be explicitly converted to the two corresponding objectspx1
andpx2
of typeX::const_pointer
, using a sequence ofstatic_cast
s using only these four types, and the expressionpx1 == px2
evaluates totrue
.Drafting note: This wording uses the seemingly complicated route via
X::const_pointer
, because these are (contrary toX::const_void_pointer
) random access iterators and we can rely here for dereferenceable values on the fundamental pointee equivalence of 24.3.5.5 [forward.iterators] p6:If
a
andb
are both dereferenceable, thena == b
if and only if*a
and*b
are bound to the same object.while for null pointer values we can rely on the special equality relation induced by 16.4.4.4 [nullablepointer.requirements].
-?- Let
w1
andw2
denote objects of typeX::void_pointer
. Then for the expressionsw1 == w2 w1 != w2either or both objects may be replaced by an equivalently-valued object of type
-?- LetX::const_void_pointer
with no change in semantics.p1
andp2
denote objects of typeX::pointer
. Then for the expressionsp1 == p2 p1 != p2 p1 < p2 p1 <= p2 p1 >= p2 p1 > p2 p1 - p2either or both objects may be replaced by an equivalently-valued object of type
X::const_pointer
with no change in semantics.
Reinsert after 23.2 [container.requirements] p6:
-6-
-?- In the expressionsbegin()
returns an iterator referring to the first element in the container.end()
returns an iterator which is the past-the-end value for the container. If the container is empty, thenbegin() == end()
;i == j i != j i < j i <= j i >= j i > j i - jwhere
i
andj
denote objects of a container'siterator
type, either or both may be replaced by an object of the container'sconst_iterator
type referring to the same element with no change in semantics.
vector
and deque
have incorrect insert
requirementsSection: 23.2.4 [sequence.reqmts] Status: C++17 Submitter: Ahmed Charles Opened: 2013-05-17 Last modified: 2017-07-30
Priority: 2
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Discussion:
According to Table 100 in n3485 23.2.4 [sequence.reqmts]/4 the notes for the expression a.insert(p,i,j)
say:
Requires:
Each iterator in the rangeT
shall beEmplaceConstructible
intoX
from*i
. Forvector
, if the iterator does not meet the forward iterator requirements (24.2.5),T
shall also beMoveInsertable
intoX
andMoveAssignable
.[i,j)
shall be dereferenced exactly once. pre:i
andj
are not iterators intoa
. Inserts copies of elements in[i, j)
beforep
There are two problems with that wording: First, the special constraints for vector
, that are expressed to be valid for
forward iterators only, are necessary for all iterator categories. Second, the same special constraints are needed for deque
, too.
[2013-10-05, Stephan T. Lavavej comments and provides alternative wording]
In Chicago, we determined that the original proposed resolution was correct, except that it needed additional requirements.
When vector
insert(p, i, j)
is called with input-only iterators, it can't know how many elements will be inserted,
which is obviously problematic for insertion anywhere other than at the end. Therefore, implementations typically append elements
(geometrically reallocating), followed by rotate()
. Given forward+ iterators, some implementations append and
rotate()
when they determine that there is sufficient capacity. Additionally, deque
insert(p, i, j)
is
typically implemented with prepending/appending, with a possible call to reverse()
, followed by a call to rotate()
.
Note that rotate()
's requirements are strictly stronger than reverse()
's.
rotate()
's requirements. Note that this does not physically affect code
(implementations were already calling rotate()
here), and even in Standardese terms it is barely noticeable — if an
element is MoveInsertable
and MoveAssignable
then it is almost certainly MoveConstructible
and swappable.
However, this patch is necessary to be strictly correct.
Previous resolution from Ahmed Charles:
Change Table 100 as indicated:
Table 100 — Sequence container requirements (in addition to container) (continued) Expression Return type Assertion/note pre-/post-condition …
a.insert(p,i,j)
iterator
Requires: T
shall beEmplaceConstructible
intoX
from*i
. Forvector
anddeque
,if the iterator does not meet the forward iterator requirements (24.2.5),T
shall also beMoveInsertable
intoX
andMoveAssignable
.
Each iterator in the range[i,j)
shall be dereferenced exactly once.
pre:i
andj
are not iterators intoa
.
Inserts copies of elements in[i, j)
beforep
…
[2014-02-15 post-Issaquah session : move to Tentatively Ready]
Pablo: We might have gone too far with the fine-grained requirements. Typically these things come in groups.
Alisdair: I think the concepts folks assumed we would take their guidance.
Move to Tentatively Ready.
Proposed resolution:
Change Table 100 as indicated:
Table 100 — Sequence container requirements (in addition to container) (continued) Expression Return type Assertion/note pre-/post-condition …
a.insert(p,i,j)
iterator
Requires: T
shall beEmplaceConstructible
intoX
from*i
. Forvector
anddeque
,if the iterator
does not meet the forward iterator requirements (24.2.5),T
shall also be
MoveInsertable
intoX
,MoveConstructible
,
andMoveAssignable
, and swappable (16.4.4.3 [swappable.requirements]).
Each iterator in the range[i,j)
shall be dereferenced exactly once.
pre:i
andj
are not iterators intoa
.
Inserts copies of elements in[i, j)
beforep
…
assign
of std::basic_string
Section: 27.4.3 [basic.string] Status: C++14 Submitter: Vladimir Grigoriev Opened: 2013-06-26 Last modified: 2016-11-12
Priority: Not Prioritized
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Discussion:
Constructors and member functions assign
of class std::basic_string
have one to one relation (except the
explicit constructor that creates an empty string). The following list shows this relation:
explicit basic_string(const Allocator& a = Allocator()); basic_string(const basic_string& str); basic_string& assign(const basic_string& str); basic_string(basic_string&& str) noexcept; basic_string& assign(basic_string&& str) noexcept; basic_string(const basic_string& str, size_type pos, size_type n = npos, const Allocator& a = Allocator()); basic_string& assign(const basic_string& str, size_type pos, size_type n); basic_string(const charT* s, size_type n, const Allocator& a = Allocator()); basic_string& assign(const charT* s, size_type n); basic_string(const charT* s, const Allocator& a = Allocator()); basic_string& assign(const charT* s); basic_string(size_type n, charT c, const Allocator& a = Allocator()); basic_string& assign(size_type n, charT c); template<class InputIterator> basic_string(InputIterator begin, InputIterator end, const Allocator& a = Allocator()); template<class InputIterator> basic_string& assign(InputIterator first, InputIterator last); basic_string(initializer_list<charT>, const Allocator& = Allocator()); basic_string& assign(initializer_list<charT>);
So in fact any creating of an object of type std::basic_string
using any of the above constructors
except the explicit constructor can be substituted for creating a (possibly non-empty) string and
then applying to it the corresponding method assign.
std::string s("Hello World");
and
std::string s; s.assign("Hello World");
However there is one exception that has no a logical support. It is the pair of the following constructor and member function
assign
basic_string(const basic_string& str, size_type pos, size_type n = npos, const Allocator& a = Allocator()); basic_string& assign(const basic_string& str, size_type pos, size_type n);
The third parameter of the constructor has a default argument while in the assign
function it is absent. So it is impossible
one to one to substitute the following code snippet
std::string s("Hello World"); std::string t(s, 6);
by
std::string s("Hello World"); std::string t; t.assign(s, 6); // error: no such function
To get an equivalent result using the assign
function the programmer has to complicate the code that is error-prone
std::string s("Hello World"); std::string t; t.assign(s, 6, s.size() - 6);
To fix that, the declaration of the member function assign
should be changed in such a way that its declaration
would be fully compatible with the declaration of the corresponding constructor, that is to specify the same default argument
for the third parameter of the assign
.
The assign
function is not the only function that requires to be revised.
assign
one more member function
append
. We will get:
explicit basic_string(const Allocator& a = Allocator()); basic_string(const basic_string& str); basic_string& assign(const basic_string& str); basic_string& append(const basic_string& str); basic_string(basic_string&& str) noexcept; basic_string& assign(basic_string&& str) noexcept; basic_string(const basic_string& str, size_type pos, size_type n = npos, const Allocator& a = Allocator()); basic_string& assign(const basic_string& str, size_type pos, size_type n); basic_string& append(const basic_string& str, size_type pos, size_type n); basic_string(const charT* s, size_type n, const Allocator& a = Allocator()); basic_string& assign(const charT* s, size_type n); basic_string& append(const charT* s, size_type n); basic_string(const charT* s, const Allocator& a = Allocator()); basic_string& assign(const charT* s); basic_string& append(const charT* s); basic_string(size_type n, charT c, const Allocator& a = Allocator()); basic_string& assign(size_type n, charT c); basic_string& append(size_type n, charT c); template<class InputIterator> basic_string(InputIterator begin, InputIterator end, const Allocator& a = Allocator()); template<class InputIterator> basic_string& assign(InputIterator first, InputIterator last); template<class InputIterator> basic_string& append(InputIterator first, InputIterator last); basic_string(initializer_list<charT>, const Allocator& = Allocator()); basic_string& assign(initializer_list<charT>); basic_string& append(initializer_list<charT>);
As it seen from this record:
basic_string(const basic_string& str, size_type pos, size_type n = npos,
const Allocator& a = Allocator());
basic_string& assign(const basic_string& str, size_type pos,
size_type n);
basic_string& append(const basic_string& str, size_type pos,
size_type n);
it is obvious that the function append
also should have the default argument that is that it should be declared as:
basic_string& append(const basic_string& str, size_type pos, size_type n = npos);
In fact there is no a great difference in using assign
or append
especially when the string is empty:
std::string s("Hello World"); std::string t; t.assign(s, 6); std::string s("Hello World"); std::string t; t.append(s, 6);
In both cases the result will be the same. So the assign
and append
will be interchangeable from the point
of view of used arguments.
std::basic_string
that could be brought in conformity with considered above functions.
They are member functions insert
, replace
, and compare
.
So it is suggested to substitute the following declarations of insert
, replace
, and compare
:
basic_string& insert(size_type pos1, const basic_string& str, size_type pos2, size_type n); basic_string& replace(size_type pos1, size_type n1, const basic_string& str, size_type pos2, size_type n2); int compare(size_type pos1, size_type n1, const basic_string& str, size_type pos2, size_type n2) const;
by the declarations:
basic_string& insert(size_type pos1, const basic_string& str, size_type pos2, size_type n = npos); basic_string& replace(size_type pos1, size_type n1, const basic_string& str, size_type pos2, size_type n2 = npos); int compare(size_type pos1, size_type n1, const basic_string& str, size_type pos2, size_type n2 = npos) const;
[2013-09 Chicago]
Howard: Are we positive this won't conflict with any other overloads?
They all appear to be unambiguous. Alisdair: Ok, move to Ready.Proposed resolution:
Change class template basic_string
synopsis, 27.4.3 [basic.string] p5, as indicated:
namespace std { template<class charT, class traits = char_traits<charT>, class Allocator = allocator<charT> > class basic_string { public: […] basic_string& append(const basic_string& str, size_type pos, size_type n = npos); […] basic_string& assign(const basic_string& str, size_type pos, size_type n = npos); […] basic_string& insert(size_type pos1, const basic_string& str, size_type pos2, size_type n = npos); […] basic_string& replace(size_type pos1, size_type n1, const basic_string& str, size_type pos2, size_type n2 = npos); […] int compare(size_type pos1, size_type n1, const basic_string& str, size_type pos2, size_type n2 = npos) const; […] }; }
Change 27.4.3.7.2 [string.append] before p3 as indicated:
basic_string& append(const basic_string& str, size_type pos, size_type n = npos);
Change 27.4.3.7.3 [string.assign] before p4 as indicated:
basic_string& assign(const basic_string& str, size_type pos, size_type n = npos);
Change 27.4.3.7.4 [string.insert] before p5 as indicated:
basic_string& insert(size_type pos1, const basic_string& str, size_type pos2, size_type n = npos);
Change 27.4.3.7.6 [string.replace] before p5 as indicated:
basic_string& replace(size_type pos1, size_type n1, const basic_string& str, size_type pos2, size_type n2 = npos);
Change 27.4.3.8.4 [string.compare] before p4 as indicated:
int compare(size_type pos1, size_type n1, const basic_string& str, size_type pos2, size_type n2 = npos) const;
regex_traits::lookup_classname
specification unclearSection: 28.6.6 [re.traits] Status: C++14 Submitter: Jonathan Wakely Opened: 2013-07-02 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
28.6.6 [re.traits] p9 says that regex_traits::lookup_classname
should
return a value that compares equal to 0, but there is no requirement
that a bitmask type is equality comparable with 0, e.g. 16.3.3.3.3 [bitmask.types] says bitmask types
can be implemented using std::bitset
.
[2013-09 Chicago]
Stefanus: Resolution looks good, doesn't seem to need fixing anywhere else from a quick look through the draft.
Any objection to Ready? No objection. Action: Move to Ready.Proposed resolution:
This wording is relative to N3691.
Edit 28.6.6 [re.traits] p9:
template <class ForwardIterator> char_class_type lookup_classname( ForwardIterator first, ForwardIterator last, bool icase = false) const;-9- Returns: an unspecified value that represents the character classification named by the character sequence designated by the iterator range
[first,last)
. If the parametericase
is true then the returned mask identifies the character classification without regard to the case of the characters being matched, otherwise it does honor the case of the characters being matched. The value returned shall be independent of the case of the characters in the character sequence. If the name is not recognized then returnsa value that compares equal to 0char_class_type()
.
quoted
should use char_traits::eq
for character comparisonSection: 31.7.9 [quoted.manip] Status: C++14 Submitter: Marshall Clow Opened: 2013-07-12 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
In 31.7.9 [quoted.manip] p2 b2:
— Each character in
s
. If the character to be output is equal toescape
ordelim
, as determined byoperator==
, first outputescape
.
In 31.7.9 [quoted.manip] p3 b1 sb1:
— If the first character extracted is equal to
delim
, as determined byoperator==
, […]
these should both use traits::eq
.
Also, I believe that 31.7.9 [quoted.manip] p3 implies that:
std::ostream _stream; std::string _string; _stream << _string; _stream << quoted(_string);
should both compile, or both fail to compile, based on whether or not their char_traits
match.
But I believe that the standard should say that explicitly.
[ 2013-09 Chicago ]
Marshall Clow improved the wording with support from Stefanus.
[ 2013-09 Chicago (late night issues) ]
Moved to Ready, after confirming wording correctly reflects discussion earlier in the day.
Proposed resolution:
This wording is relative to N3691.
Change 31.7.9 [quoted.manip] p2+3 as indicated:
template <class charT> unspecified quoted(const charT* s, charT delim=charT('"'), charT escape=charT('\\')); template <class charT, class traits, class Allocator> unspecified quoted(const basic_string<charT, traits, Allocator>& s, charT delim=charT('"'), charT escape=charT('\\'));-2- Returns: An object of unspecified type such that if
out
is an instance ofbasic_ostream
with member typechar_type
the same ascharT
and with member typetraits_type
, which in the second form is the same astraits
, then the expressionout << quoted(s, delim, escape)
behaves as if it inserts the following characters intoout
using character inserter function templates (27.7.3.6.4), which may throwios_base::failure
(27.5.3.1.1):
delim
Each character in
s
. If the character to be output is equal toescape
ordelim
, as determined byoperator==
traits_type::eq
, first outputescape
.
delim
template <class charT, class traits, class Allocator> unspecified quoted(basic_string<charT, traits, Allocator>& s, charT delim=charT('"'), charT escape=charT('\\'));-3- Returns: An object of unspecified type such that:
If
in
is an instance ofbasic_istream
with member typeschar_type
andtraits_type
the same ascharT
andtraits
, respectively, then the expressionin >> quoted(s, delim, escape)
behaves as if it extracts the following characters from in usingbasic_istream::operator>>
(27.7.2.2.3) which may throwios_base::failure
(27.5.3.1.1):
If the first character extracted is equal to
delim
, as determined byoperator==
traits_type::eq
, then: […]If
out
is an instance ofbasic_ostream
with member typeschar_type
andtraits_type
the same ascharT
andtraits
, respectively, then the expressionout << quoted(s, delim, escape)
behaves as specified for theconst basic_string<charT, traits, Allocator>&
overload of the quoted function.
regex_match
ambiguitySection: 28.6.10.2 [re.alg.match] Status: C++17 Submitter: Howard Hinnant Opened: 2013-07-14 Last modified: 2017-07-30
Priority: 2
View all other issues in [re.alg.match].
View all issues with C++17 status.
Discussion:
28.6.10.2 [re.alg.match] p2 in describing regex_match says:
-2- Effects: Determines whether there is a match between the regular expression
e
, and all of the character sequence[first,last)
. The parameterflags
is used to control how the expression is matched against the character sequence. Returns true if such a match exists, false otherwise.
It has come to my attention that different people are interpreting the first sentence of p2 in different ways:
If a search of the input string using the regular expression e
matches the entire input string,
regex_match
should return true.
Search the input string using the regular expression e
. Reject all matches that do not match the
entire input string. If a such a match is found, return true.
The difference between these two subtly different interpretations is found using the following ECMAScript example:
std::regex re("Get|GetValue");
Using regex_search
, this re
can never match the input string "GetValue"
, because ECMA
specifies that alternations are ordered, not greedy. As soon as "Get"
is matched in the left alternation,
the matching algorithm stops.
regex_match
would return false for an input string of "GetValue"
.
However definition 2 alters the grammar and appears equivalent to augmenting the regex with a trailing '$'
,
which is an anchor that specifies, reject any matches which do not come at the end of the input sequence.
So, using definition 2, regex_match
would return true for an input string of "GetValue"
.
My opinion is that it would be strange to have regex_match
return true for a string/regex
pair that regex_search
could never find. I.e. I favor definition 1.
John Maddock writes:
The intention was always that regex_match
would reject any match candidate which didn't match the entire
input string. So it would find GetValue
in this case because the "Get"
alternative had already
been rejected as not matching. Note that the comparison with ECMA script is somewhat moot, as ECMAScript defines
the regex grammar (the bit we've imported), it does not define anything like regex_match
, nor do we import
from ECMAScript the behaviour of that function. So IMO the function should behave consistently regardless of the
regex dialect chosen. Saying "use awk regexes" doesn't cut it, because that changes the grammar in other ways.
(John favors definition 2).
We need to clarify 28.6.10.2 [re.alg.match]/p2 in one of these two directions.
[2014-06-21, Rapperswil]
AM: I think there's a clear direction and consensus we agree with John Maddock's position, and if noone else thinks we need the other function I won't ask for it.
Marshall Clow and STL to draft.[2015-06-10, Marshall suggests concrete wording]
[2015-01-11, Telecon]
Move to Tenatatively Ready
Proposed resolution:
This wording is relative to N4527.
Change 28.6.10.2 [re.alg.match]/2, as follows:
template <class BidirectionalIterator, class Allocator, class charT, class traits> bool regex_match(BidirectionalIterator first, BidirectionalIterator last, match_results<BidirectionalIterator, Allocator>& m, const basic_regex<charT, traits>& e, regex_constants::match_flag_type flags = regex_constants::match_default);-1- Requires: The type
-2- Effects: Determines whether there is a match between the regular expressionBidirectionalIterator
shall satisfy the requirements of a Bidirectional Iterator (24.2.6).e
, and all of the character sequence[first,last)
. The parameterflags
is used to control how the expression is matched against the character sequence. When determining if there is a match, only potential matches that match the entire character sequence are considered. Returnstrue
if such a match exists,false
otherwise. [Example:std::regex re("Get|GetValue"); std::cmatch m; regex_search("GetValue", m, re); // returns true, and m[0] contains "Get" regex_match ("GetValue", m, re); // returns true, and m[0] contains "GetValue" regex_search("GetValues", m, re); // returns true, and m[0] contains "Get" regex_match ("GetValues", m, re); // returns false— end example]
[…]
map::operator[]
value-initialize or default-insert a missing element?Section: 23.4.3.3 [map.access], 23.5.3.3 [unord.map.elem] Status: Resolved Submitter: Andrzej Krzemieński Opened: 2013-07-16 Last modified: 2015-10-22
Priority: 3
View all other issues in [map.access].
View all issues with Resolved status.
Discussion:
Suppose that I provide a custom allocator for type int
, that renders value 1 rather than 0 in default-insertion:
struct Allocator1 : std::allocator<int> { using super = std::allocator<int>; template<typename Up, typename... Args> void construct(Up* p, Args&&... args) { super::construct(p, std::forward<Args>(args)...); } template<typename Up> void construct(Up* p) { ::new((void*)p) Up(1); } };
Now, if I use this allocator with std::map
, and I use operator[]
to access a not-yet-existent value,
what value of the mapped_type
should be created? 0 (value-initialization) or 1 (default-insertion):
map<string, int, less<string>, Allocator1> map; cout << map["cat"];
N3960 is not very clear. 23.4.3.3 [map.access] in para 1 says:
"If there is no key equivalent to
x
in the map, insertsvalue_type(x, T())
into the map."
So, it requires value-initialization.
But para 2 says:"
mapped_type
shall beDefaultInsertable
into*this
."
This implies default-insertion, because if not, why the requirement. Also similar functions like
vector::resize
already require default-insertion wherever they put DefaultInsertable
requirements.
mapped_type
.
[2013-09 Chicago]
Alisdair: Matters only for POD or trivial types
Marshall: issue might show up elsewhere other thanmap<>
Alisdair: initialize elements in any containers — by calling construct on allocator traits
Marshall: existing wording is clear
Alisdair: main concern is difference in wording, discusses default initialization
Nico: different requirement needed
Alisdair: gut is issue is NAD, brings up DefaultInsertable
definition — discusses definition
Nico: why do we have the requirement?
Alisdair: other containers have this requirement
Marshall: this applies to many other containers
Nico: deque<>
in particular
Alisdair: discusses allocator construct
Alisdair: wording raises concerns that aren't said in existing standard
Nico: sees no benefit to change
Marshall: leery of change
Alisdair: can be made clearer; might need to add note to DefaultInsertable
; borderline editorial,
comfortable without note, willing to wait until other issues arise. close issue as NAD
[2015-01-20: Tomasz Kamiński comments]
With the addition of the try_emplace
method the behavior of the operator[]
for the maps,
may be defined as follows:
T& operator[](const key_type& x);Effects: Equivalent to:
try_emplace(x).first->second
;T& operator[](key_type&& x);Effects: Equivalent to
try_emplace(std::move(x)).first->second
;
This would simplify the wording and also after resolution of the issue 2464(i), this wording would also address this issue.
[2015-02 Cologne]
Wait until 2464(i) and 2469(i) are in, which solve this.
[2015-05-06 Lenexa: This is resolved by 2469(i).]
Proposed resolution:
This wording is relative to N3691.
Change 23.4.3.3 [map.access] p1+p5 as indicated:
T& operator[](const key_type& x);-1- Effects: If there is no key equivalent to
-2- Requires:x
in the map, insertsinto the map a value withvalue_type(x, T())
into the mapkey_type
initialized using expressionx
andmapped_type
initialized by default-insertion.key_type
shall beCopyInsertable
andmapped_type
shall beDefaultInsertable
into*this
. […]T& operator[](key_type&& x);-5- Effects: If there is no key equivalent to
-6- Requires:x
in the map, insertsinto the map a value withvalue_type(std::move(x), T())
into the mapkey_type
initialized using expressionstd::move(x)
andmapped_type
initialized by default-insertion.mapped_type
shall beDefaultInsertable
into*this
.
Change 23.5.3.3 [unord.map.elem] p2 as indicated:
mapped_type& operator[](const key_type& k); mapped_type& operator[](key_type&& k);-1- Requires:
-2- Effects: If themapped_type
shall beDefaultInsertable
into*this
. For the first operator,key_type
shall beCopyInsertable
into*this
. For the second operator,key_type
shall beMoveConstructible
.unordered_map
does not already contain an element whose key is equivalent tok
, the first operator insertsthe valuea value withvalue_type(k, mapped_type())
key_type
initialized using expressionx
andmapped_type
initialized by default-insertion and the second operator insertsthe valuea value withvalue_type(std::move(k), mapped_type())
key_type
initialized using expressionstd::move(x)
andmapped_type
initialized by default-insertion.
forward_as_tuple
not constexpr
?Section: 22.4.5 [tuple.creation] Status: C++14 Submitter: Marshall Clow Opened: 2013-07-30 Last modified: 2017-09-07
Priority: Not Prioritized
View all other issues in [tuple.creation].
View all issues with C++14 status.
Discussion:
Addresses ES 11
In n3471, a bunch of routines
from header <tuple>
were made constexpr
.
make_tuple/tuple_cat/get<>(tuple)/
relational operators — all these were "constexpr-ified".
But not forward_as_tuple
.
Why not?
This was discussed in Portland, and STL opined that this was "an omission" (along with tuple_cat
, which was added)
In discussion on lib@lists.isocpp.org list, Pablo agreed that forward_as_tuple
should be constexpr
.
[2013-09 Chicago]
Moved to Immediate, this directly addresses an NB comment and the wording is non-controversial.
Accept for Working Paper
Proposed resolution:
This wording is relative to N3691.
Change header <tuple>
synopsis, 22.4.1 [tuple.general] as indicated:
template <class... Types> constexpr tuple<Types&&...> forward_as_tuple(Types&&...) noexcept;
Change 22.4.5 [tuple.creation] before p5 as indicated:
template <class... Types> constexpr tuple<Types&&...> forward_as_tuple(Types&&... t) noexcept;
std::promise::set_exception
Section: 32.10 [futures] Status: C++17 Submitter: Jonathan Wakely Opened: 2013-07-30 Last modified: 2017-07-30
Priority: Not Prioritized
View all other issues in [futures].
View all issues with C++17 status.
Discussion:
The standard does not specify the behaviour of this program:
#include <future> #include <cassert> struct NonTrivial { NonTrivial() : init(true) { } ~NonTrivial() { assert(init); } bool init; }; int main() { std::promise<NonTrivial> p; auto f = p.get_future(); p.set_exception(std::exception_ptr()); f.get(); }
The standard doesn't forbid making the state ready with a null
exception_ptr
, so what should get()
return? There's no stored
exception to throw, but it can't return a value because none was initialized.
set_exception()
makes the state ready that it must store a value or exception, so
cannot store "nothing", but that isn't explicit.
The promise::set_exception()
and promise::set_exception_at_thread_exit()
members should require p != nullptr
or should state the type of exception thrown
if p
is null.
[2015-02 Cologne]
Handed over to SG1.
[2015-05 Lenexa, SG1 response]
SG1 provides P/R and requests move to SG1-OK status: Add Requires clauses for promise (30.6.5 [futures.promise]) set_exception (before p18) and set_exception_at_thread_exit (before p24): Requires: p is not null.
[2015-10, Kona issue prioritization]
Priority 0, move to Ready
Proposed resolution:
This wording is relative to N4431.
Add Requires clauses for promise (32.10.6 [futures.promise])set_exception
(before p18) and
set_exception_at_thread_exit
(before p24): Requires: p
is not null.
Change 32.10.6 [futures.promise] as depicted:
void set_exception(exception_ptr p);-??- Requires:
p
is not null.-18- Effects: atomically stores the exception pointer
p
in the shared state and makes that state ready (30.6.4).
void set_exception_at_thread_exit(exception_ptr p);-??- Requires:
p
is not null.-24- Effects: Stores the exception pointer
p
in the shared state without making that state ready immediately. […]
Section: 30.2 [time.syn], 27.4 [string.classes] Status: C++14 Submitter: Howard Hinnant Opened: 2013-07-22 Last modified: 2017-09-07
Priority: Not Prioritized
View all other issues in [time.syn].
View all issues with C++14 status.
Discussion:
This paper adds user-defined literals
for string
, complex
and chrono
types. It puts each new literal signature in an inline namespace
inside of std. Section 3.1 of the paper gives the rationale for doing this:
As a common schema this paper proposes to put all suffixes for user defined literals in separate inline namespaces that are below the inline namespace
std::literals
. [Note: This allows a user either to do ausing namespace std::literals;
to import all literal operators from the standard available through header file inclusion, or to useusing namespace std::string_literals;
to just obtain the literals operators for a specific type. — end note]
This isn't how inline namespaces work.
9.9.2 [namespace.def]/p8 says in part:
Members of an inline namespace can be used in most respects as though they were members of the enclosing namespace. Specifically, the inline namespace and its enclosing namespace are both added to the set of associated namespaces used in argument-dependent lookup (3.4.2) whenever one of them is, and a using- directive (7.3.4) that names the inline namespace is implicitly inserted into the enclosing namespace as for an unnamed namespace (7.3.1.1). […]
I.e. these literals will appear to the client to already be imported into namespace std
. The rationale in
the paper appears to indicate that this is not the intended behavior, and that instead the intended behavior is
to require the user to say:
using namespace std::literals;
or:
using namespace std::literals::string_literals;
prior to use. To get this behavior non-inlined (normal) namespaces must be used.
Originally proposed resolution:
Strike the use of "inline" from each use associated withliterals
, string_literals
,
chrono_literals
.
My opinion is that this must be done prior to publishing C++14, otherwise we are stuck with this
(apparently unwanted) decision forever.
Marshall Clow:
The rationale that I recall was that:
Users could write "
using namespace std::literals;
" to get all the literal suffixes, orUsers could write "
using namespace std::literals::string_literals;
" or "using namespace std::literals::chrono_literals;
" to get a subset of the suffixes.To accomplish that, I believe that:
Namespace "
std::literals
" should not beinline
Namespaces "
std::literals::string_literals
" and "std::literals::chrono_literals
" should beinline
Further details see also reflector message c++std-lib-34256.
Previous resolution from Marshall Clow:
Modify header
<chrono>
synopsis, 30.2 [time.syn], as indicated:namespace std { namespace chrono { […] } // namespace chronoinlinenamespace literals { inline namespace chrono_literals { […] } // namespace chrono_literals } // namespace literals } // namespace stdModify header
<string>
synopsis, 27.4 [string.classes] p1, as indicated:#include <initializer_list> namespace std { […]inlinenamespace literals { inline namespace string_literals { […] } } }
[2013-09 Chicago]
After a discussion about intent, the conclusion was that if you hoist a type with a "using" directive, then you should also get the associated literal suffixes with the type.
This is accomplished by marking namespacestd::literals
as inline
, but for types in their own namespace inside
std
, then they will need to do this as well. The only case in the current library is chrono
.
Marshall Clow provides alternative wording.
[2013-09 Chicago (late night issues)]
Moved to Ready, after confirming wording reflects the intent of the earlier discussion.
Proposed resolution:
This wording is relative to N3691.
Modify header <chrono>
synopsis, 30.2 [time.syn], as indicated:
namespace std { […] inline namespace literals { inline namespace chrono_literals { […] constexpr chrono::duration<unspecified , nano> operator "" ns(long double); } // namespace chrono_literals } // namespace literals namespace chrono { using namespace literals::chrono_literals; } // namespace chrono } // namespace std
begin/end
for arrays should be constexpr
and noexcept
Section: 24.7 [iterator.range] Status: C++14 Submitter: Andy Sawyer Opened: 2013-08-22 Last modified: 2016-01-28
Priority: Not Prioritized
View other active issues in [iterator.range].
View all other issues in [iterator.range].
View all issues with C++14 status.
Discussion:
The array forms of std::begin
and std::end
should be constexpr
and noexcept
.
Previous resolution from Andy Sawyer:
Edit header
<iterator>
synopsis, 24.2 [iterator.synopsis] as indicated:[…] template <class T, size_t N> constexpr T* begin(T (&array)[N]) noexcept; template <class T, size_t N> constexpr T* end(T (&array)[N]) noexcept; template <class C> constexpr auto cbegin(const C& c) -> decltype(std::begin(c)); template <class C> constexpr auto cend(const C& c) -> decltype(std::end(c)); […]Edit 24.7 [iterator.range] before p4+5 as indicated:
template <class T, size_t N> constexpr T* begin(T (&array)[N]) noexcept;-4- Returns:
array
.template <class T, size_t N> constexpr T* end(T (&array)[N]) noexcept;-5- Returns:
array + N
.template <class C> constexpr auto cbegin(const C& c) -> decltype(std::begin(c));-6- Returns:
std::begin(c)
.template <class C> constexpr auto cend(const C& c) -> decltype(std::end(c));-7- Returns:
std::end(c)
.
[2013-09 Chicago]
Add noexcept(noexcept(std::begin/end(c)))
to cbegin
and cend
, move to ready
Proposed resolution:
This wording is relative to N3797.
Edit header <iterator>
synopsis, 24.2 [iterator.synopsis] as indicated:
[…] template <class T, size_t N> constexpr T* begin(T (&array)[N]) noexcept; template <class T, size_t N> constexpr T* end(T (&array)[N]) noexcept; template <class C> constexpr auto cbegin(const C& c) noexcept(noexcept(std::begin(c))) -> decltype(std::begin(c)); template <class C> constexpr auto cend(const C& c) noexcept(noexcept(std::end(c))) -> decltype(std::end(c)); […]
Edit 24.7 [iterator.range] before p4+5 as indicated:
template <class T, size_t N> constexpr T* begin(T (&array)[N]) noexcept;-4- Returns:
array
.template <class T, size_t N> constexpr T* end(T (&array)[N]) noexcept;-5- Returns:
array + N
.template <class C> constexpr auto cbegin(const C& c) noexcept(noexcept(std::begin(c))) -> decltype(std::begin(c));-6- Returns:
std::begin(c)
.template <class C> constexpr auto cend(const C& c) noexcept(noexcept(std::end(c))) -> decltype(std::end(c));-7- Returns:
std::end(c)
.
is_assignable
constraint in optional::op=(U&&)
Section: 5.3.3 [fund.ts::optional.object.assign] Status: Resolved Submitter: Howard Hinnant Opened: 2013-08-25 Last modified: 2015-10-26
Priority: Not Prioritized
View all other issues in [fund.ts::optional.object.assign].
View all issues with Resolved status.
Discussion:
Addresses: fund.ts
Minor wording nit in 5.3.3 [fund.ts::optional.object.assign]/p15:
template <class U> optional<T>& operator=(U&& v);-15- Requires:
is_constructible<T, U>::value
is true andis_assignable<U, T>::value
is true.
Should be:
template <class U> optional<T>& operator=(U&& v);-15- Requires:
is_constructible<T, U>::value
is true andis_assignable<T&, U>::value
is true.
[2013-09 Chicago:]
Move to Deferred. This feature will ship after C++14 and should be revisited then.
[2014-06-06 pre-Rapperswill]
This issue has been reopened as fundamentals-ts.
[2014-06-07 Daniel comments]
This issue should be set to Resolved, because the wording fix is already applied in the last fundamentals working draft.
[2014-06-16 Rapperswill]
Confirmed that this issue is resolved in the current Library Fundamentals working paper.
Proposed resolution:
This wording is relative to N3691.
Edit 5.3.3 [fund.ts::optional.object.assign] p15 as indicated:
template <class U> optional<T>& operator=(U&& v);-15- Requires:
is_constructible<T, U>::value
is true andis_assignable<
is true.U, TT&, U>::value
optional
declares and then does not define an operator<()
Section: 5.9 [fund.ts::optional.comp_with_t] Status: Resolved Submitter: Howard Hinnant Opened: 2013-08-26 Last modified: 2021-06-06
Priority: Not Prioritized
View all issues with Resolved status.
Discussion:
Addresses: fund.ts
In 22.5.2 [optional.syn] there is:
template <class T> constexpr bool operator<(const T&, const optional<T>&);
But I can find no definition for this signature.
[2013-09 Chicago:]
Move to Deferred. This feature will ship after C++14 and should be revisited then.
[2014-06-06 pre-Rapperswill]
This issue has been reopened as fundamentals-ts.
[2014-06-07 Daniel comments]
This issue should be set to Resolved, because the wording fix is already applied in the last fundamentals working draft.
[2014-06-16 Rapperswill]
Confirmed that this issue is resolved in the current Library Fundamentals working paper.
Proposed resolution:
This wording is relative to N3691.
Add to 5.9 [fund.ts::optional.comp_with_t]:
template <class T> constexpr bool operator<(const T& v, const optional<T>& x);-?- Returns:
bool(x) ? less<T>{}(v, *x) : false
.
allocator_traits::max_size
Section: 20.2.9 [allocator.traits] Status: C++14 Submitter: Marshall Clow Opened: 2013-08-27 Last modified: 2018-08-21
Priority: Not Prioritized
View all issues with C++14 status.
Discussion:
Section 20.2.9 [allocator.traits] says:
static size_type max_size(const Alloc& a) noexcept;
Section 20.2.9.3 [allocator.traits.members] says:
static size_type max_size(Alloc& a) noexcept;
These should be the same.
Discussion:
Pablo (who I believe wrote the allocator_traits
proposal) says "The function should take a const
reference."
[2013-09 Chicago]
No objections, so moved to Immediate.
Accept for Working Paper
Proposed resolution:
This wording is relative to N3691.
Change 20.2.9.3 [allocator.traits.members] as follows:
static size_type max_size(const Alloc& a) noexcept;
make_reverse_iterator
Section: 24.5.1 [reverse.iterators] Status: C++14 Submitter: Zhihao Yuan Opened: 2013-08-27 Last modified: 2016-01-28
Priority: Not Prioritized
View all other issues in [reverse.iterators].
View all issues with C++14 status.
Discussion:
We have make_move_iterator
, but not make_reverse_iterator
, which
is also useful when dealing with some types without an rbegin
/rend
support (like, C strings).
[2013-09 Chicago]
Billy: reviewed it last night STL: has suggested prior, but denied for complexity
Billy: Alisdair wanted to review forreverse(reverse())
;
STL: likes the issue, was like him
Stefanus: likes definitions, places where things should be
STL: for consistency with make_move_iterator
Stefanus: minor editorial issue - subdivision in these 2 sections is different from [move.iter].
See 24.5.4.9 [move.iter.nonmember]
STL: motion to move to Ready
Move to Ready
Proposed resolution:
This wording is relative to N3691.
Change header <iterator>
synopsis, 24.2 [iterator.synopsis] as indicated:
namespace std { […] template <class Iterator> reverse_iterator<Iterator> operator+( typename reverse_iterator<Iterator>::difference_type n, const reverse_iterator<Iterator>& x); template <class Iterator> reverse_iterator<Iterator> make_reverse_iterator(Iterator i); }
Change class template reverse_iterator
synopsis, 24.5.1.2 [reverse.iterator] as indicated:
namespace std { […] template <class Iterator> reverse_iterator<Iterator> operator+( typename reverse_iterator<Iterator>::difference_type n, const reverse_iterator<Iterator>& x); template <class Iterator> reverse_iterator<Iterator> make_reverse_iterator(Iterator i); }
After [reverse.iter.opsum] add the following new sub-clause to [reverse.iter.ops]:
template <class Iterator> reverse_iterator<Iterator> make_reverse_iterator(Iterator i);-?- Returns:
reverse_iterator<Iterator>(i)
.
optional
copy assignment operatorSection: 5.3.3 [fund.ts::optional.object.assign] Status: Resolved Submitter: Howard Hinnant Opened: 2013-08-16 Last modified: 2015-10-26
Priority: Not Prioritized
View all other issues in [fund.ts::optional.object.assign].
View all issues with Resolved status.
Discussion:
Addresses: fund.ts
The Exception safety paragraph of 5.3.3 [fund.ts::optional.object.assign] calls out T
's copy constructor when
it should refer to T
's copy assignment operator.
[2013-09 Chicago:]
Move to Deferred. This feature will ship after C++14 and should be revisited then.
[2014-06-06 pre-Rapperswill]
This issue has been reopened as fundamentals-ts.
[2014-06-07 Daniel comments]
This issue should be set to Resolved, because the wording fix is already applied in the last fundamentals working draft.
[2014-06-16 Rapperswill]
Confirmed that this issue is resolved in the current Library Fundamentals working paper.
Proposed resolution:
This wording is relative to N3691.
Change 5.3.3 [fund.ts::optional.object.assign] as indicated:
optional<T>& operator=(const optional<T>& rhs);[…]
-8- Exception safety: If any exception is thrown, the values ofinit
andrhs.init
remain unchanged. If an exception is thrown during the call toT
's copy constructor, no effect. If an exception is thrown during the call toT
's copy assignment, the state of its contained value is as defined by the exception safety guarantee ofT
's copyconstructorassignment.
Section: 32.6.4.4 [thread.sharedmutex.requirements] Status: C++14 Submitter: Daniel Krügler Opened: 2013-08-30 Last modified: 2016-01-28
Priority: Not Prioritized
View all issues with C++14 status.
Discussion:
Albeit shared mutex types refine timed mutex types, the requirements imposed on the corresponding required member function expressions are inconsistent in several aspects, most probably because failing synchronisation with wording changes for timed mutexes applied by some issues:
Due to acceptance of N3568 a wording phrase came in 32.6.4.4 [thread.sharedmutex.requirements] p26,
Effects: If the tick period of
rel_time
is not exactly convertible to the native tick period, the duration shall be rounded up to the nearest native tick period. […]
while a very similar one had been removed for 32.6.4.3 [thread.timedmutex.requirements] by LWG 2091(i).
Having this guaranteed effect fortry_lock_shared_for
but not for try_lock_for
seems inconsistent
and astonishing.
If the actual intended restriction imposed onto the implementation is to forbid early wakeups here, we should ensure that
to hold for timed mutex's try_lock_for
as well. Note that the rationale provided for LWG 2091(i) was
a potential late wakeup situation, but it seems that there is no implementation restriction that prevents early wakeups.
The shared-lock requirements for any *lock*()
functions don't provide the guarantee that "If an exception is thrown then
a lock shall not have been acquired for the current execution agent.". For other mutex types this guarantee can be derived from
the corresponding TimedLockable
requirements, but there are no SharedLockable
requirements.
The shared-lock requirements for *lock_for/_until()
functions require "Throws: Nothing."
instead of "Throws: Timeout-related exceptions (30.2.4)." which had been added by LWG 2093(i), because user-provided
clocks, durations, or time points may throw exceptions.
With the addition of std::shared_mutex
, the explicit lists of 32.6.4.2 [thread.mutex.requirements.mutex] p7+15,
Requires: If
m
is of typestd::mutex
orstd::timed_mutex
, the calling thread does not own the mutex.
and of 32.6.4.3 [thread.timedmutex.requirements] p4+11,
Requires: If
m
is of typestd::timed_mutex
, the calling thread does not own the mutex.
are incomplete and should add the non-recursive std::shared_mutex
as well.
[2014-02-16: Moved as Immediate]
Proposed resolution:
This wording is relative to N3691.
Change 32.6.4.2 [thread.mutex.requirements.mutex] p7+15 as indicated:
-6- The expression
m.lock()
shall be well-formed and have the following semantics:-7- Requires: If
m
is of typestd::mutex
or,std::timed_mutex
, orstd::shared_mutex
, the calling thread does not own the mutex.[…]
-14- The expression
m.try_lock()
shall be well-formed and have the following semantics:-15- Requires: If
m
is of typestd::mutex
or,std::timed_mutex
, orstd::shared_mutex
, the calling thread does not own the mutex.
Change 32.6.4.3 [thread.timedmutex.requirements] p4+11 as indicated:
-3- The expression
m.try_lock_for(rel_time)
shall be well-formed and have the following semantics:-4- Requires: If
m
is of typestd::timed_mutex
orstd::shared_mutex
, the calling thread does not own the mutex.[…]
-10- The expression
m.try_lock_until(abs_time)
shall be well-formed and have the following semantics:-11- Requires: If
m
is of typestd::timed_mutex
orstd::shared_mutex
, the calling thread does not own the mutex.
Change 32.6.4.4 [thread.sharedmutex.requirements] as indicated:
-3- The expression
m.lock_shared()
shall be well-formed and have the following semantics:-4- Requires: The calling thread has no ownership of the mutex.
-5- Effects: Blocks the calling thread until shared ownership of the mutex can be obtained for the calling thread. If an exception is thrown then a shared lock shall not have been acquired for the current thread. […]-24- The expression
m.try_lock_shared_for(rel_time)
shall be well-formed and have the following semantics:-25- Requires: The calling thread has no ownership of the mutex.
-26- Effects:If the tick period ofAttempts to obtain shared lock ownership for the calling thread within the relative timeout (30.2.4) specified byrel_time
is not exactly convertible to the native tick period, the duration shall be rounded up to the nearest native tick period.rel_time
. If the time specified byrel_time
is less than or equal torel_time.zero()
, the function attempts to obtain ownership without blocking (as if by callingtry_lock_shared()
). The function shall return within the timeout specified byrel_time
only if it has obtained shared ownership of the mutex object. [Note: As withtry_lock()
, there is no guarantee that ownership will be obtained if the lock is available, but implementations are expected to make a strong effort to do so. — end note] If an exception is thrown then a shared lock shall not have been acquired for the current thread. […] -30- Throws:NothingTimeout-related exceptions (32.2.4 [thread.req.timing]).-31- The expression
m.try_lock_shared_until(abs_time)
shall be well-formed and have the following semantics:-32- Requires: The calling thread has no ownership of the mutex.
-33- Effects: The function attempts to obtain shared ownership of the mutex. Ifabs_time
has already passed, the function attempts to obtain shared ownership without blocking (as if by callingtry_lock_shared()
). The function shall return before the absolute timeout (30.2.4) specified byabs_time
only if it has obtained shared ownership of the mutex object. [Note: As withtry_lock()
, there is no guarantee that ownership will be obtained if the lock is available, but implementations are expected to make a strong effort to do so. — end note] If an exception is thrown then a shared lock shall not have been acquired for the current thread. […] -37- Throws:NothingTimeout-related exceptions (32.2.4 [thread.req.timing]).
std::hash
is vulnerable to collision DoS attackSection: 16.4.4.5 [hash.requirements] Status: C++14 Submitter: Zhihao Yuan Opened: 2013-09-02 Last modified: 2016-01-28
Priority: Not Prioritized
View all other issues in [hash.requirements].
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Discussion:
For a non-cryptographic hash function, it's possible to pre-calculate massive inputs with the same hashed value to algorithmically slow down the unordered containers, and results in a denial-of-service attack. Many languages with built-in hash table support have fixed this issue. For example, Perl has universal hashing, Python 3 uses salted hashes.
However, for C++, in 16.4.4.5 [hash.requirements] p2, Table 26:The value returned shall depend only on the argument
k
. [Note: Thus all evaluations of the expressionh(k)
with the same value fork
yield the same result. — end note]
The wording is not clear here: does that mean all the standard library implementations must use the same hash function for a same type? Or it is not allowed for an implementation to change its hash function?
I suggest to explicitly allow the salted hash functions.[2013-09 Chicago]
Moved to Ready.
There is some concern that the issue of better hashing, especially standardizing any kind of secure hashing, is a feature that deserves attention in LEWG
The proposed resolution is much simpler than the larger issue though, merely clarifying a permission that many implementers believe they already have, without mandating a change to more straight forward implementations.
Move to Ready, rather than Immediate, as even the permission has been contentious in reflector discussion, although the consensus in Chicago is to accept as written unless we hear a further strong objection.
Proposed resolution:
This wording is relative to N3691.
Edit 16.4.4.5 [hash.requirements] p2, Table 26, as indicated: [Editorial note: We can consider adding some additional guideline here. Unlike N3333, this proposed change makes the hashing per-execution instead of per-process. The standard does not discuss OS processes. And, practically, a per-process hashing makes a program unable to share an unordered container to a child process. — end note ]
Table 26 — Hash requirements [hash] Expression Return type Requirement h(k)
size_t
The value returned shall depend only on the argument k
for the duration of the program.
[Note: Thus all evaluations of the expressionh(k)
with the
same value fork
yield the same result for a given
execution of the program. — end note]
…
Section: 16.3.2.4 [structure.specifications] Status: Resolved Submitter: Jeffrey Yasskin Opened: 2013-09-03 Last modified: 2020-05-12
Priority: 3
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Discussion:
The C++14 CD has 25 sections including the phrase "X shall not participate in overload resolution ...". Most of these uses are double negatives, which are hard to interpret. "shall not ... unless" tends to be the easiest to read, since the condition is true when the function is available, but we also have a lot of "if X is not Y, then Z shall not participate", which actually means "You can call Z if X is Y." The current wording is also clumsy and long-winded. We should find a better and more concise phrasing.
As an initial proposal, I'd suggest using "X is enabled if and only if Y" in prose and adding an "Enabled If: ..." element to 16.3.2.4 [structure.specifications]. Daniel: I suggest to name this new specification element for 16.3.2.4 [structure.specifications] as "Template Constraints:" instead, because the mentioned wording form was intentionally provided starting with LWG 1237(i) to give implementations more freedom to realize the concrete constraints. Instead of the originalstd::enable_if
-based specifications
we can use better forms of "SFINAE" constraints today and it eases the path to possible language-based
constraints in the future.
[2019-03-21; Daniel comments ]
Apparently the adoption of P0788R3 at the Rapperswil 2018 meeting has resolved this issue by introduction of the new Constraints: element.
[2020-05-10; Reflector discussions]
Resolved by P0788R3 and the Mandating paper series culminating with P1460R1.
Rationale:
Resolved by P0788R3 and finally by P1460R1.Proposed resolution:
num_put::do_put
Section: 28.3.4.3.3.3 [facet.num.put.virtuals] Status: C++14 Submitter: Juan Soulie Opened: 2013-09-04 Last modified: 2017-07-05
Priority: 0
View other active issues in [facet.num.put.virtuals].
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Discussion:
At the end of 28.3.4.3.3.3 [facet.num.put.virtuals] (in p6), the return value is said to be obtained by calling
truename
or falsename
on the wrong facet: ctype
should be replaced by numpunct
.
[Issaquah 2014-02-11: Move to Immediate]
Proposed resolution:
This wording is relative to N3691.
Edit 28.3.4.3.3.3 [facet.num.put.virtuals] p6 as indicated:
-6- Returns: If
(str.flags() & ios_base::boolalpha) == 0
returnsdo_put(out, str, fill, (int)val)
, otherwise obtains a strings
as if bystring_type s = val ? use_facet<ctypenumpunct<charT> >(loc).truename() : use_facet<ctypenumpunct<charT> >(loc).falsename();and then inserts each character c of s into out via *out++ = c and returns out.
<cstdlib>
should declare abs(double)
Section: 29.7 [c.math] Status: Resolved Submitter: Pete Becker Opened: 2013-09-04 Last modified: 2017-03-12
Priority: 2
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Discussion:
… and abs(float)
and abs(long double)
. And <cmath>
should declare
abs(int)
, abs(long)
, and abs(long long)
.
#include <cstdlib> int main() { double d = -1.23; double dd = std::abs(d); return 0; }
The call is ambiguous because of the various integer overloads, that's because <cstdlib>
provides
abs(int)
but not abs(double)
.
abs
is dangerous, and to recommend using fabs
instead.
In general, it makes sense to declare overloaded functions that take user-defined types in the same header as the
definition of the user-defined types; it isn't necessary to declare all of the overloads in the same place. But
here we're not dealing with any user-defined types; we're dealing with builtin types, which are always defined;
all of the overloads should be defined in the same place, to avoid mysterious problems like the one in the code above.
The standard library has six overloads for abs
:
int abs(int); // <cstdlib> long abs(long); // <cstdlib> long long abs(long long); // <cstdlib> float abs(float); // <cmath> double abs(double); // <cmath> long double abs(long double); // <cmath>
These should all be declared in both headers.
I have no opinion on<stdlib.h>
and <math.h>
.
[2013-09 Chicago]
This issue is related to LWG 2192(i)
Move to open[2014-02-13 Issaquah — Nicolai Josuttis suggest wording]
[2015-03-03, Geoffrey Romer provides improved wording]
See proposed resolution of LWG 2192(i).
[2015-09-11, Telecon]
Geoff provided combined wording for 2192(i) after Cologne, Howard to provide updated wording for Kona.
Howard: my notes say I wanted to use is_unsigned
instead of 'unsigned integral type'.
Edit 29.7 [c.math] after p7 as indicated:
-6- In addition to the
-7- The added signatures are:int
versions of certain math functions in<cstdlib>
, C++ addslong
andlong long
overloaded versions of these functions, with the same semantics.long abs(long); // labs() long long abs(long long); // llabs() ldiv_t div(long, long); // ldiv() lldiv_t div(long long, long long); // lldiv()-?- To avoid ambiguities, C++ also adds the following overloads of
abs()
to<cstdlib>
, with the semantics defined in<cmath>
:float abs(float); double abs(double); long double abs(long double);-?- To avoid ambiguities, C++ also adds the following overloads of
abs()
to<cmath>
, with the semantics defined in<cstdlib>
:int abs(int); long abs(long); long long abs(long long);
[2015-08 Chicago]
Proposed resolution:
See proposed resolution of LWG 2192(i).
Facet
is a nullptr
Section: 28.3.3.1.3 [locale.cons] Status: C++23 Submitter: Juan Soulie Opened: 2013-09-04 Last modified: 2023-11-22
Priority: 3
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Discussion:
28.3.3.1.3 [locale.cons] p14 ends with:
"[…] If
f
is null, the resulting object is a copy ofother
."
but the next line p15 says:
"Remarks: The resulting locale has no name."
But both can't be true when other
has a name and f
is null.
Daniel Krügler:
As currently written, the Remarks element applies unconditionally for all cases and thus should "win". The question arises whether the introduction of this element by LWG 424(i) had actually intended to change the previous Note to a Remarks element. In either case the wording should be improved to clarify this special case.[2022-02-14; Daniel comments]
This issue seems to have some overlap with LWG 3676(i) so both should presumably be resolved in a harmonized way.
[2022-11-01; Jonathan provides wording]
This also resolves 3673(i) and 3676(i).
[2022-11-04; Jonathan revises wording after feedback]
Revert an incorrect edit to p8, which was incorrectly changed to:
"If cats
is equal to locale::none
, the resulting locale
has the same name as locale(std_name)
. Otherwise, the locale
has a name if and only if other
has a name."
[Kona 2022-11-08; Move to Ready status]
[2023-02-13 Approved at February 2023 meeting in Issaquah. Status changed: Voting → WP.]
Proposed resolution:
This wording is relative to N4917.
Modify 28.3.3.1.3 [locale.cons] as indicated:
explicit locale(const char* std_name);-2- Effects: Constructs a locale using standard C locale names, e.g.,
"POSIX"
. The resulting locale implements semantics defined to be associated with that name.-3- Throws:
runtime_error
if the argument is not valid, or is null.-4- Remarks: The set of valid string argument values is
"C"
,""
, and any implementation-defined values.explicit locale(const string& std_name);-5- Effects:
The same asEquivalent tolocale(std_name.c_str())
.locale(const locale& other, const char* std_name, category cats);-?- Preconditions:
cats
is a validcategory
value (28.3.3.1.2.1 [locale.category]).-6- Effects: Constructs a locale as a copy of
other
except for the facets identified by thecategory
argument, which instead implement the same semantics aslocale(std_name)
.-7- Throws:
runtime_error
if the second argument is not valid, or is null.-8- Remarks: The locale has a name if and only if
other
has a name.locale(const locale& other, const string& std_name, category cats);-9- Effects:
The same asEquivalent tolocale(other, std_name.c_str(), cats)
.template<class Facet> locale(const locale& other, Facet* f);-10- Effects: Constructs a locale incorporating all facets from the first argument except that of type
Facet
, and installs the second argument as the remaining facet. Iff
is null, the resulting object is a copy ofother
.-11- Remarks: If
f
is null, the resulting locale has the same name asother
. Otherwise, theTheresulting locale has no name.locale(const locale& other, const locale& one, category cats);-?- Preconditions:
cats
is a validcategory
value.-12- Effects: Constructs a locale incorporating all facets from the first argument except for those that implement
cats
, which are instead incorporated from the second argument.-13- Remarks: If
cats
is equal tolocale::none
, the resulting locale has a name if and only if the first argument has a name. Otherwise, theThelocale has a name if and only if the first two arguments both have names.
std::addressof
should be constexpr
Section: 20.2.11 [specialized.addressof] Status: C++17 Submitter: Daryle Walker Opened: 2013-09-08 Last modified: 2017-07-30
Priority: 3
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Discussion:
I'm writing a function that needs to be constexpr
and I wanted to take the address of its input. I was
thinking of using std::addressof
to be safe, but it isn't currently constexpr
. A
sample implementation
couldn't be constexpr
under the C++11 rules, though.
std::addressof
implementations are not valid in constant expressions, therefore it seems more like a defect than a feature request to ask for
the guarantee that std::addressof
is a constexpr
function. It should be added that a similar requirement
already exists for offsetof
indirectly via the C99 standard as of 7.17 p3:
The macros are […]
offsetof(type, member-designator)which expands to an integer constant expression that has type
size_t
[…]
combined with the noted property in C++11 that:
"
offsetof
is required to work as specified even if unaryoperator&
is overloaded for any of the types involved"
Therefore implementations should already be able without heroic efforts to realize this functionality by
some intrinsic. The wording needs at least to ensure that for any lvalue core constant expression e
the expression std::addressof(e)
is a core constant expression.
[2013-09 Chicago]
[2014-06-08, Daniel improves wording]
It has been ensured that the wording is in sync with the recent working paper and the usage of "any" has been improved to say "every" instead (the fix is similar to that applied by LWG 2150(i)).
Previous resolution from Daniel [SUPERSEDED]:
Change header
<memory>
synopsis, 20.2.2 [memory.syn] as indicated:namespace std { […] // 26.11 [specialized.algorithms], specialized algorithms: template <class T> constexpr T* addressof(T& r) noexcept; […] }Change 20.2.11 [specialized.addressof] as indicated:
template <class T> constexpr T* addressof(T& r) noexcept;-1- Returns: The actual address of the object or function referenced by
-?- Remarks: For every lvalue core constant expressionr
, even in the presence of an overloadedoperator&
.e
(7.7 [expr.const]), the expressionstd::addressof(e)
is a core constant expression.
[2014-06-09, further improvements]
A new wording form is now used similar to the approach used by LWG 2234(i), which is a stricter way to impose the necessary implementation requirements.
[2015-05, Lenexa]
STL: the intent of this change is good; I think the wording is good
- I'm a bit worried about asking for a compiler hook
- if every implementer says: yes they can do it we should be good
EB: there is missing the word "a" before "subexpression" (in multiple places)
MC: the editor should do - we rely on our editors
MC: move to Review with a note stating that we wait for implementation experience first
- in favor: 13, opposed: 0, abstain: 2
HB: N4430 will bring something which is addressing this issue
MC: good we didn't go to ready then
Proposed resolution:
This wording is relative to N3936.
Introduce the following new definition to the existing list in [definitions]: [Drafting note: If LWG 2234(i) is accepted before this issue, the accepted wording for the new definition should be used instead — end drafting note]
constant subexpression [defns.const.subexpr]
an expression whose evaluation as a subexpression of a conditional-expression CE (7.6.16 [expr.cond]) would not prevent CE from being a core constant expression (7.7 [expr.const]).
Change header <memory>
synopsis, 20.2.2 [memory.syn] as indicated:
namespace std { […] // 26.11 [specialized.algorithms], specialized algorithms: template <class T> constexpr T* addressof(T& r) noexcept; […] }
Change 20.2.11 [specialized.addressof] as indicated:
template <class T> constexpr T* addressof(T& r) noexcept;-1- Returns: The actual address of the object or function referenced by
-?- Remarks: An expressionr
, even in the presence of an overloadedoperator&
.std::addressof(E)
is a constant subexpression (3.15 [defns.const.subexpr]), ifE
is an lvalue constant subexpression.
is_nothrow_constructible
is always false because of create<>
Section: 21.3.5.4 [meta.unary.prop] Status: C++14 Submitter: Daniel Krügler Opened: 2013-09-24 Last modified: 2016-01-28
Priority: Not Prioritized
View other active issues in [meta.unary.prop].
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Discussion:
Addresses US 18
The trait is_constructible<T, Args...>
is defined in terms of a helper template, create<>
,
that is identical to std::declval<>
except for the latter's noexcept
clause.
noexcept
is critical to this definition, insert a Note of explanation; otherwise, excise
create<>
and reformulate in terms of declval<>
the definition of is_constructible
.
[2013-09-24 Daniel comments and provides resolution suggestion]
Replacing create<>
by std::declval<>
would make the situation worse, because the definition of
is_constructible
is based on a well-formed variable definition and there is no way to specify a variable definition
without odr-using its initializer arguments. It should also be added, that there is another problem with the specification of
all existing is_trivially_*
traits, because neither create<>
nor std::declval<>
are considered as trivial functions, but this should be solved by a different issue.
[2013-09-26 Nico improves wording]
The additional change is just to keep both places were create()
is defined consistent.
[2013-09 Chicago]
No objections, so moved to Immediate.
Accept for Working Paper
Proposed resolution:
This wording is relative to N3691.
Change 21.3.5.4 [meta.unary.prop] around p6 as indicated:
-6- Given the following function prototype:
template <class T> typename add_rvalue_reference<T>::type create() noexcept;the predicate condition for a template specialization
is_constructible<T, Args...>
shall be satisfied if and only if the following variable definition would be well-formed for some invented variablet
:T t(create<Args>()...);[…]
Change 21.3.5.4 [meta.unary.prop] around p4 as indicated:
-4- Given the following function prototype:
template <class T> typename add_rvalue_reference<T>::type create() noexcept;the predicate condition for a template specialization
is_convertible<From, To>
shall be satisfied if and only if the return expression in the following code would be well-formed, including any implicit conversions to the return type of the function:To test() { return create<From>(); }[…]
key_compare::is_transparent
type are not clearSection: 23.2.7 [associative.reqmts] Status: C++14 Submitter: Daniel Krügler Opened: 2013-09-24 Last modified: 2016-01-28
Priority: 1
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Discussion:
Addresses ES 16
The condition "X::key_compare::is_transparent
exists" does not specify that the type be publicly accessible.
X::key_compare::is_transparent
and whether its potential inaccessibility
should be banned for a compliant key_compare
type.
[2013-09-24 Daniel provides resolution suggestion]
[2013-09-25 Chicago]
Daniel's wording is good, advance to Immediate to respond to NB comment.
[2013-09-26 Chicago]
Moved back to Review as Daniel would like another look at the words, and to confirm implementability.
Previous resolution from Daniel [SUPERSEDED]:
Change 23.2.7 [associative.reqmts] p8 as indicated:
-8- In Table 102,
X
denotes an associative container class,a
denotes a value ofX
,a_uniq
denotes a value ofX
whenX
supports unique keys,a_eq
denotes a value ofX
whenX
supports multiple keys,a_tran
denotes a value ofX
whenthea publicly accessible typeX::key_compare::is_transparent
exists whose name is unambiguous and not hidden, […]Change 23.2.7 [associative.reqmts] p13 as indicated:
The member function templates
find
,count
,lower_bound
,upper_bound
, andequal_range
shall not participate in overload resolution unlessthea publicly accessible typeCompare::is_transparent
exists whose name is unambiguous and not hidden.
[2014-02-10 Daniel comments provides alternative wording]
I could confirm that my previous concerns were unwarranted, because they turned out to be due to a compiler-bug. Nonetheless I would suggest to replace the previously suggested replication of core-wording situations (access, ambiguity, hidden) by a single more robust phrase based on "valid type".
[2014-02-12 Issaquah: Move to Immediate]
STL: This uses "valid type", which is a Phrase Of Power in Core, and Daniel has a citation for the term.
Jonathan: It's nice to rely on Core.
Proposed resolution:
This wording is relative to N3797.
Change 23.2.7 [associative.reqmts] p8 as indicated:
-8- In Table 102,
X
denotes an associative container class,a
denotes a value ofX
,a_uniq
denotes a value ofX
whenX
supports unique keys,a_eq
denotes a value ofX
whenX
supports multiple keys,a_tran
denotes a value ofX
when thetypequalified-idX::key_compare::is_transparent
existsis valid and denotes a type (13.10.3 [temp.deduct]), […]
Change 23.2.7 [associative.reqmts] p13 as indicated:
The member function templates
find
,count
,lower_bound
,upper_bound
, andequal_range
shall not participate in overload resolution unless thetypequalified-idCompare::is_transparent
existsis valid and denotes a type (13.10.3 [temp.deduct]).
map
and multimap
members should be removedSection: 23.4.3 [map], 23.4.4 [multimap] Status: C++14 Submitter: Daniel Krügler Opened: 2013-09-25 Last modified: 2017-06-15
Priority: Not Prioritized
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Discussion:
Addresses ES 17
Sections are redundant with general associative container requirements at 23.2.7 [associative.reqmts], Table 102.
Suggested action: Delete sections.[2013-09-25 Daniel provides resolution suggestion]
[2013-09-25 Chicago]
Daniel's wording is good, move to Immediate to resolve NB comment.
[2013-09-29 Chicago]
Accept for Working Paper
Proposed resolution:
This wording is relative to N3691.
Change the header <map>
synopsis, 23.4.3.1 [map.overview] p2 as indicated:
//23.4.4.5,map operations: iterator find(const key_type& x); const_iterator find(const key_type& x) const; template <class K> iterator find(const K& x); template <class K> const_iterator find(const K& x) const;
Delete the complete sub-clause [map.ops]:
23.4.4.5 map operations [map.ops]
iterator find(const key_type& x); const_iterator find(const key_type& x) const; iterator lower_bound(const key_type& x); const_iterator lower_bound(const key_type& x) const; iterator upper_bound(const key_type& x); const_iterator upper_bound(const key_type &x) const; pair<iterator, iterator> equal_range(const key_type &x); pair<const_iterator, const_iterator> equal_range(const key_type& x) const;
-1- Thefind
,lower_bound
,upper_bound
andequal_range
member functions each have two versions, one const and the other non-const. In each case the behavior of the two functions is identical except that the const version returns aconst_iterator
and the non-const version aniterator
(23.2.4).
Delete the complete sub-clause [multimap.ops]:
23.4.5.4 multimap operations [multimap.ops]
iterator find(const key_type &x); const_iterator find(const key_type& x) const; iterator lower_bound(const key_type& x); const_iterator lower_bound(const key_type& x) const; pair<iterator, iterator> equal_range(const key_type &x); pair<const_iterator, const_iterator> equal_range(const key_type& x) const;
-1- Thefind
,lower_bound
,upper_bound
andequal_range
member functions each have two versions, one const and one non-const. In each case the behavior of the two versions is identical except that the const version returns aconst_iterator
and the non-const version aniterator
(23.2.4).
std::tie
not constexpr
?Section: 22.4.5 [tuple.creation] Status: C++14 Submitter: Rein Halbersma Opened: 2013-09-11 Last modified: 2016-01-28
Priority: 2
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Discussion:
In N3471, a bunch of routines from header
<tuple>
were made constexpr
.
make_tuple/tuple_cat/get<>(tuple)/
relational operators — all these were "constexpr
-ified".
But not tie
. This is similar to Issue 2275(i), where the same observation was made about forward_as_tuple
.
[2014-02-13 Issaquah: Move as Immediate]
Proposed resolution:
This wording is relative to N3691.
Change the header <tuple>
synopsis, 22.4.1 [tuple.general] p2 as indicated:
template<class... Types> constexpr tuple<Types&...> tie(Types&...) noexcept;
Change 22.4.5 [tuple.creation] around p7 as indicated:
template<class... Types> constexpr tuple<Types&...> tie(Types&... t) noexcept;
count
in unordered associative containersSection: 23.2.8 [unord.req] Status: C++14 Submitter: Joaquín M López Muñoz Opened: 2013-09-20 Last modified: 2017-07-05
Priority: 0
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Discussion:
Table 103 in 23.2.8 [unord.req] states that the complexity of b.count(k)
is average case 𝒪(1
) rather
than linear with the number of equivalent elements, which seems to be a typo as this requires holding an internal
count of elements in each group of equivalent keys, something which hardly looks the intent of the standard and no
(known by the submitter) stdlib implementation is currently doing.
[Issaquah 2014-02-11: Move to Immediate]
Proposed resolution:
This wording is relative to N3691.
Change Table 103 as indicated:
Table 103 — Unordered associative container requirements (in addition to container) Expression Return type Assertion/note pre-/post-condition Complexity …
b.count(k)
size_type
Returns the number of elements with key equivalent to k
.Average case 𝒪( ), worst case 𝒪(
1b.count(k)b.size()
).…
match_results::reference
should be value_type&
, not const value_type&
Section: 28.6.9 [re.results] Status: C++14 Submitter: Matt Austern Opened: 2013-09-25 Last modified: 2017-07-05
Priority: 4
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Discussion:
The match_results class synopsis has
typedef const value_type& const_reference; typedef const_reference reference;
We're getting too enthusiastic about types here by insisting that reference
is a const reference, even
though match_results
is a read-only container. In the container requirements table (Table 96, in section
23.2.2 [container.requirements.general] we say that Container::reference
is "lvalue of T
" and
Container::const_reference
is "const lvalue of T
".
That phrasing in the container requirements table is admittedly a little fuzzy and ought to be clarified (as discussed in
lwg issue 2182(i)), but in context it's clear that Container::reference
ought to be a T&
even for constant containers. In the rest of Clause 23 we see that Container::reference
is T&
, not
const T&
, even for const-qualified containers and that it's T&
, not const T&
, even
for containers like set
and unordered_set
that provide const iterators only.
match_results
)
there are no operations that return Container::reference
. That's already the case, so this issue is complaining
about an unused typedef.
[2013-10-17: Daniel comments]
The std::initializer_list
synopsis, 17.11 [support.initlist] shows a similar problem:
template<class E> class initializer_list { public: typedef E value_type; typedef const E& reference; typedef const E& const_reference; […] }
Given the fact that std::initializer_list
doesn't meet the container requirements anyway (and is such a core-language related
type) I recommend to stick with the current state.
[Issaquah 2014-02-11: Move to Immediate]
Proposed resolution:
This wording is relative to N3691.
Change the class template match_results
header synopsis, 28.6.9 [re.results] p4 as indicated:
typedef const value_type& const_reference; typedefconst_referencevalue_type& reference;
std::array
Section: 23.2.2 [container.requirements.general] Status: C++14 Submitter: Jonathan Wakely Opened: 2013-09-26 Last modified: 2017-07-05
Priority: 0
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Discussion:
It has been suggested that Table 96 — "Container requirements" makes
confusing requirements for the destructor of std::array
:
a
; all the memory is deallocated."
Since std::array
obtains no memory, there is none to deallocate,
arguably making it unclear what the requirement means for std::array::~array()
.
[Issaquah 2014-02-11: Move to Immediate]
Proposed resolution:
This wording is relative to N3691.
Change in 23.2.2 [container.requirements.general], Table 96 — "Container requirements",
the "Assertion/note/pre-/post-condition" for the expression "(&a)->~X()
" as indicated:
note: the destructor is applied to every element of
a
;all theany memory obtained is deallocated.
mutex::lock()
should not throw device_or_resource_busy
Section: 32.6.4.2 [thread.mutex.requirements.mutex] Status: C++17 Submitter: Detlef Vollmann Opened: 2013-09-27 Last modified: 2017-07-30
Priority: 0
View all other issues in [thread.mutex.requirements.mutex].
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Discussion:
As discussed during the Chicago meeting in
SG1
the only reasonable reasons for throwing device_or_resource_busy
seem to be:
The thread currently already holds the mutex, the mutex is not recursive, and the implementation detects this.
In this case resource_deadlock_would_occur
should be thrown.
Priority reasons. At least std::mutex
(and possibly all standard mutex types)
should not be setup this way, otherwise we have real problems with condition_variable::wait()
.
[2014-06-17 Rapperswil]
Detlef provides wording
[2015-02 Cologne]
Handed over to SG1.
[2015-05 Lenexa, SG1 response]
We believe we were already done with it. Should be in SG1-OK status.
[2015-10 pre-Kona]
SG1 hands this over to LWG for wording review
[2015-10 Kona]
Geoffrey provides new wording.
Previous resolution [SUPERSEDED]:
This wording is relative to N3936.
Change 32.6.4.2 [thread.mutex.requirements.mutex] as indicated:
-13- Error conditions:
operation_not_permitted
— if the thread does not have the privilege to perform the operation.
resource_deadlock_would_occur
— if the implementation detects that a deadlock would occur.
device_or_resource_busy
— if the mutex is already locked and blocking is not possible.
Proposed resolution:
This wording is relative to N4527.
Change 32.6.4.2 [thread.mutex.requirements.mutex] as indicated:
[…]
-4- The error conditions for error codes, if any, reported by member functions of the mutex types shall be:
(4.1) —
resource_unavailable_try_again
— if any native handle type manipulated is not available.(4.2) —
operation_not_permitted
— if the thread does not have the privilege to perform the operation.
(4.3) —device_or_resource_busy
— if any native handle type manipulated is already locked.[…]
-13- Error conditions:
(13.1) —
operation_not_permitted
— if the thread does not have the privilege to perform the operation.(13.2) —
resource_deadlock_would_occur
— if the implementation detects that a deadlock would occur.
(13.3) —device_or_resource_busy
— if the mutex is already locked and blocking is not possible.
Change 32.6.4.4 [thread.sharedmutex.requirements] as indicated:
[…]
-10- Error conditions:
(10.1) —
operation_not_permitted
— if the thread does not have the privilege to perform the operation.(10.2) —
resource_deadlock_would_occur
— if the implementation detects that a deadlock would occur.
(10.3) —device_or_resource_busy
— if the mutex is already locked and blocking is not possible.[…]
std::array
Section: 23.3.3.1 [array.overview] Status: C++17 Submitter: Jonathan Wakely Opened: 2013-09-30 Last modified: 2017-07-30
Priority: 4
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Discussion:
23.3.3.1 [array.overview] shows std::array
with an "exposition only" data member, elems
.
std::array::elems
(or its equivalent) must be public in order for
std::array
to be an aggregate.
If the intention is that std::array::elems
places requirements on the
implementation to provide "equivalent external behavior" to a public
array member, then 16.3.3.6 [objects.within.classes] needs to cover public
members too, or some other form should be used in 23.3.3.1 [array.overview].
[Urbana 2014-11-07: Move to Open]
[Kona 2015-10: Link to 2516(i)]
[2015-11-14, Geoffrey Romer provides wording]
[2016-02-04, Tim Song improves the P/R]
Instead of the build-in address-operator, std::addressof
should be used.
[2016-03 Jacksonville]
Move to Ready.Proposed resolution:
This wording is relative to N4582.
Edit 23.3.3.1 [array.overview] as indicated
[…]
-3- An array […]namespace std { template <class T, size_t N> struct array { […]T elems[N]; // exposition only[…] }; }
-4- [Note: The member variableelems
is shown for exposition only, to emphasize that array is a class aggregate. The nameelems
is not part of array's interface. — end note]
Edit [array.data] as follows:
constexpr T* data() noexcept; constexpr const T* data() const noexcept;-1- Returns:
A pointer such thatelems
[data(), data() + size())
is a valid range, anddata() == addressof(front())
.
tuple
's constructor constraints need to be phrased more preciselySection: 22.4.4.2 [tuple.cnstr] Status: C++17 Submitter: Stephan T. Lavavej Opened: 2013-09-21 Last modified: 2017-07-30
Priority: 2
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Discussion:
Consider the following code:
void meow(tuple<long, long>) { puts("Two"); } void meow(tuple<long, long, long>) { puts("Three"); } tuple<int, int, int> t(0, 0, 0); meow(t);
This should compile and print "Three" because tuple<long, long>
's constructor from
const tuple<int, int, int>&
should remove itself from overload resolution.
Implementations sensibly do this, but the Standard doesn't actually say it!
Types
is "long, long
" and UTypes
is "int, int, int
". 22.4.4.2 [tuple.cnstr]/3
says "let i
be in the range [0,sizeof...(Types))
in order", which is [0, 2)
. Then /17 says
"Remark: This constructor shall not participate in overload resolution unless const Ui&
is implicitly
convertible to Ti
for all i
." Interpreted literally, this is true! /15 says
"Requires: sizeof...(Types) == sizeof...(UTypes)
." but requiring the sizes to be identical doesn't help.
Only the special phrase "shall not participate in overload resolution unless" mandates SFINAE/enable_if
machinery.
The wording that we need is almost available in the Requires paragraphs, except that the Requires paragraphs say
"is_constructible
" while the Remark paragraphs say "is implicitly convertible", which is the correct thing for the SFINAE
constraints to check. My proposed resolution is to unify the Requires and Remark paragraphs, after which there
will be no need for Requires (when a constructor participates in overload resolution if and only if X
is true,
then there's no need for it to Require that X
is true).
Note: 21.3.5.4 [meta.unary.prop]/6 specifies is_constructible<To, From>
and 21.3.7 [meta.rel]/4 specifies
is_convertible<From, To>
. Both are specified in terms of
"template <class T> typename add_rvalue_reference<T>::type create();
".
Therefore, passing From
and From&&
is equivalent, regardless of whether From
is an object type,
an lvalue reference, or an rvalue reference.
Also note that 22.4.4.2 [tuple.cnstr]/3 defines T0
and T1
so we don't need to repeat their definitions.
[2014-10-05, Daniel comments]
This issue is closely related to LWG 2419(i).
[2015-02, Cologne]
AM: Howard wants to do something in this space and I want to wait for him to get a paper in.
Postponed.[2015-05, Lenexa]
MC: handled by Daniel's tuple paper N4387
STL: look at status after N4387 applied.
[2015-05-05, Daniel comments]
N4387 doesn't touch these area intentionally. I agree with Howard that a different option exists that would introduce a TupleLike concept. Some implementations currently take advantage of this choice and this P/R would forbid them, which seems unfortunate to me.
[2015-09, Telecon]
Proposed resolution is obsolete.
Howard has considered writing a paper.
Status quo gives more implementation freedom.
Previous resolution [SUPERSEDED]:
This wording is relative to N3691.
Edit 22.4.4.2 [tuple.cnstr] as indicated:
template <class... UTypes> explicit constexpr tuple(UTypes&&... u);[…] -10- Remark: This constructor shall not participate in overload resolution unless
-8- Requires:sizeof...(Types) == sizeof...(UTypes)
.is_constructible<Ti, Ui&&>::value
is true for all i.each type inUTypes
is implicitly convertible to its corresponding type inTypes
sizeof...(Types) == sizeof...(UTypes)
and bothis_constructible<Ti, Ui>::value
andis_convertible<Ui, Ti>::value
are true for all i.[…]
template <class... UTypes> constexpr tuple(const tuple<UTypes...>& u);[…] -17- Remark: This constructor shall not participate in overload resolution unless
-15- Requires:sizeof...(Types) == sizeof...(UTypes)
.is_constructible<Ti, const Ui&>::value
is true for all i.const Ui&
is implicitly convertible toTi
sizeof...(Types) == sizeof...(UTypes)
and bothis_constructible<Ti, const Ui&>::value
andis_convertible<const Ui&, Ti>::value
are true for all i.template <class... UTypes> constexpr tuple(tuple<UTypes...>&& u);[…] -20- Remark: This constructor shall not participate in overload resolution unless
-18- Requires:sizeof...(Types) == sizeof...(UTypes)
.is_constructible<Ti, Ui&&>::value
is true for all i.each type inUTypes
is implicitly convertible to its corresponding type inTypes
sizeof...(Types) == sizeof...(UTypes)
and bothis_constructible<Ti, Ui>::value
andis_convertible<Ui, Ti>::value
are true for all i.template <class U1, class U2> constexpr tuple(const pair<U1, U2>& u);[…] -23- Remark: This constructor shall not participate in overload resolution unless
-21- Requires:sizeof...(Types) == 2
.is_constructible<T0, const U1&>::value
is true for the first typeT0
in Types andis_constructible<T1, const U2&>::value
is true for the second typeT1
inTypes
.const U1&
is implicitly convertible toT0
andconst U2&
is implicitly convertible toT1
sizeof...(Types) == 2 && is_constructible<T0, const U1&>::value && is_constructible<T1, const U2&>::value && is_convertible<const U1&, T0>::value && is_convertible<const U2&, T1>::value
is true.template <class U1, class U2> constexpr tuple(pair<U1, U2>&& u);[…] -26- Remark: This constructor shall not participate in overload resolution unless
-24- Requires:sizeof...(Types) == 2
.is_constructible<T0, U1&&>::value
is true for the first typeT0
in Types andis_constructible<T1, U2&&>::value
is true for the second typeT1
inTypes
.U1
is implicitly convertible toT0
andU2
is implicitly convertible toT1
sizeof...(Types) == 2 && is_constructible<T0, U1>::value && is_constructible<T1, U2>::value && is_convertible<U1, T0>::value && is_convertible<U2, T1>::value
is true.
[2016-03, Jacksonville]
STL provides improved wording.
[2016-06 Oulu]
Tuesday: Adopt option 1, drop option B of 2549(i), and move to Ready.
Friday: status to Immediate
Proposed resolution:
This wording is relative to N4567.
Edit 22.4.4.2 [tuple.cnstr] as indicated:
template <class... UTypes> EXPLICIT constexpr tuple(UTypes&&... u);[…] -10- Remarks: This constructor shall not participate in overload resolution unless
-8- Requires:sizeof...(Types) == sizeof...(UTypes)
.sizeof...(Types) >= 1
andsizeof...(Types) == sizeof...(UTypes)
andis_constructible<Ti, Ui&&>::value
istrue
for alli
. The constructor is explicit if and only ifis_convertible<Ui&&, Ti>::value
isfalse
for at least onei
.[…]
template <class... UTypes> EXPLICIT constexpr tuple(const tuple<UTypes...>& u);[…] -17- Remarks: This constructor shall not participate in overload resolution unless
-15- Requires:sizeof...(Types) == sizeof...(UTypes)
.sizeof...(Types) == sizeof...(UTypes)
andis_constructible<Ti, const Ui&>::value
istrue
for alli
. The constructor is explicit if and only ifis_convertible<const Ui&, Ti>::value
isfalse
for at least onei
.template <class... UTypes> EXPLICIT constexpr tuple(tuple<UTypes...>&& u);[…] -20- Remarks: This constructor shall not participate in overload resolution unless
-18- Requires:sizeof...(Types) == sizeof...(UTypes)
.sizeof...(Types) == sizeof...(UTypes)
andis_constructible<Ti, Ui&&>::value
istrue
for alli
. The constructor is explicit if and only ifis_convertible<Ui&&, Ti>::value
isfalse
for at least onei
.template <class U1, class U2> EXPLICIT constexpr tuple(const pair<U1, U2>& u);[…] -23- Remarks: This constructor shall not participate in overload resolution unless
-21- Requires:sizeof...(Types) == 2
.sizeof...(Types) == 2
andis_constructible<T0, const U1&>::value
istrue
andis_constructible<T1, const U2&>::value
istrue
. The constructor is explicit if and only ifis_convertible<const U1&, T0>::value
isfalse
oris_convertible<const U2&, T1>::value
isfalse
.template <class U1, class U2> EXPLICIT constexpr tuple(pair<U1, U2>&& u);[…] -26- Remarks: This constructor shall not participate in overload resolution unless
-24- Requires:sizeof...(Types) == 2
.sizeof...(Types) == 2
andis_constructible<T0, U1&&>::value
istrue
andis_constructible<T1, U2&&>::value
istrue
. The constructor is explicit if and only ifis_convertible<U1&&, T0>::value
isfalse
oris_convertible<U2&&, T1>::value
isfalse
.
tuple_size
should always derive from integral_constant<size_t, N>
Section: 22.4.7 [tuple.helper] Status: C++14 Submitter: Stephan T. Lavavej Opened: 2013-09-21 Last modified: 2017-07-05
Priority: 2
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Discussion:
In 22.4.7 [tuple.helper], the "primary template" is depicted as:
template <class... Types> class tuple_size<tuple<Types...> > : public integral_constant<size_t, sizeof...(Types)> { };
However, 22.3.4 [pair.astuple]/1-2 and 23.3.3.7 [array.tuple]/1-2 are underspecified, saying:
tuple_size<pair<T1, T2> >::valueReturns: Integral constant expression.
Value:2
.
tuple_size<array<T, N> >::valueReturn type: integral constant expression.
Value:N
They should be required to behave like the "primary template". This is more than a stylistic decision — it allows
tuple_size
to be passed to a function taking integral_constant
.
tuple_size<cv T>
to derive from
integral_constant<remove_cv<decltype(TS::value)>::type, TS::value>
. This is unnecessarily overgeneralized.
tuple_size
is primarily for tuples, where it is required to be size_t
, and it has been extended to handle
pairs and arrays, which (as explained above) should also be guaranteed to be size_t
. tuple_size<cv T>
works with cv-qualified tuples, pairs, arrays, and user-defined types that also want to participate in the tuple_size
system. It would be far simpler and perfectly reasonable to expect that user-defined types supporting the "tuple-like protocol"
should have tuple_sizes
of size_t
.
[Issaquah 2014-02-11: Move to Immediate]
Proposed resolution:
This wording is relative to N3691.
Edit 22.3.4 [pair.astuple]/1-2 as indicated:
tuple_size<pair<T1, T2> >::valuetemplate <class T1, class T2> struct tuple_size<pair<T1, T2>> : integral_constant<size_t, 2> { };
-1- Returns: Integral constant expression.-2- Value:2
.
Edit 23.3.3.7 [array.tuple]/1-2 as indicated:
tuple_size<array<T, N> >::valuetemplate <class T, size_t N> struct tuple_size<array<T, N>> : integral_constant<size_t, N> { };
-1- Returns: Integral constant expression.-2- Value:N
.
Edit 22.4.7 [tuple.helper]/p1-p3 as indicated:
template <class T> struct tuple_size;-?- Remarks: All specializations of
tuple_size<T>
shall meet theUnaryTypeTrait
requirements (21.3.2 [meta.rqmts]) with aBaseCharacteristic
ofintegral_constant<size_t, N>
for someN
.
template <class... Types> struct tuple_size<tuple<Types...> > : integral_constant<size_t, sizeof...(Types)> { }; template <size_t I, class... Types> class tuple_element<I, tuple<Types...> > { public: typedef TI type; };-1- Requires:
[…]I < sizeof...(Types)
. The program is ill-formed ifI
is out of bounds.
template <class T> class tuple_size<const T>; template <class T> class tuple_size<volatile T>; template <class T> class tuple_size<const volatile T>;-3- Let TS denote
tuple_size<T>
of the cv-unqualified typeT
. Then each of the three templates shall meet theUnaryTypeTrait
requirements (21.3.2 [meta.rqmts]) with aBaseCharacteristic
ofintegral_constant<remove_cv<decltype(TS::value)>::typesize_t, TS::value>
apply()
should return decltype(auto)
and use decay_t
before tuple_size
Section: 21.2.1 [intseq.general] Status: C++14 Submitter: Stephan T. Lavavej Opened: 2013-09-21 Last modified: 2017-07-05
Priority: 0
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Discussion:
The example in 21.2.1 [intseq.general]/2 depicts apply_impl()
and apply()
as returning auto
.
This is incorrect because it will trigger decay and will not preserve F
's return type. For example, if invoking the
functor returns const int&
, apply_impl()
and apply()
will return int
. decltype(auto)
should be used for "perfect returning".
Additionally, this depicts apply()
as taking Tuple&&
, then saying "std::tuple_size<Tuple>::value
".
This is incorrect because when apply()
is called with lvalue tuples, perfect forwarding will deduce Tuple
to be cv
tuple&
, but 22.4.7 [tuple.helper] says that tuple_size
handles only cv tuple
, not
references to tuples. Using remove_reference_t
would avoid this problem, but so would decay_t
, which has a
significantly shorter name. (The additional transformations that decay_t
does are neither beneficial nor harmful here.)
[Issaquah 2014-02-11: Move to Immediate]
Proposed resolution:
This wording is relative to N3691.
Edit the example code in 21.2.1 [intseq.general]/2 as indicated:
template<class F, class Tuple, std::size_t... I>autodecltype(auto) apply_impl(F&& f, Tuple&& t, index_sequence<I...>) { return std::forward<F>(f)(std::get<I>(std::forward<Tuple>(t))...); } template<class F, class Tuple>autodecltype(auto) apply(F&& f, Tuple&& t) { using Indices = make_index_sequence<std::tuple_size<std::decay_t<Tuple>>::value>; return apply_impl(std::forward<F>(f), std::forward<Tuple>(t), Indices()); }
weak_ptr
should be movableSection: 20.3.2.3 [util.smartptr.weak] Status: C++14 Submitter: Stephan T. Lavavej Opened: 2013-09-21 Last modified: 2016-01-28
Priority: 2
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Discussion:
Like shared_ptr
, weak_ptr
should be movable to avoid unnecessary atomic increments/decrements of the weak refcount.
[2014-02-13 Issaquah: Move to Immediate]
Proposed resolution:
This wording is relative to N3691.
Edit 20.3.2.3 [util.smartptr.weak]/1, class template weak_ptr
synopsis, as indicated:
namespace std { template<class T> class weak_ptr { public: typedef T element_type; // 20.9.2.3.1, constructors constexpr weak_ptr() noexcept; template<class Y> weak_ptr(shared_ptr<Y> const& r) noexcept; weak_ptr(weak_ptr const& r) noexcept; template<class Y> weak_ptr(weak_ptr<Y> const& r) noexcept; weak_ptr(weak_ptr&& r) noexcept; template<class Y> weak_ptr(weak_ptr<Y>&& r) noexcept; […] // 20.9.2.3.3, assignment weak_ptr& operator=(weak_ptr const& r) noexcept; template<class Y> weak_ptr& operator=(weak_ptr<Y> const& r) noexcept; template<class Y> weak_ptr& operator=(shared_ptr<Y> const& r) noexcept; weak_ptr& operator=(weak_ptr&& r) noexcept; template<class Y> weak_ptr& operator=(weak_ptr<Y>&& r) noexcept; }; }
Add the following new paragraphs at the end of sub-clause 20.3.2.3.2 [util.smartptr.weak.const]:
weak_ptr(weak_ptr&& r) noexcept; template<class Y> weak_ptr(weak_ptr<Y>&& r) noexcept;-?- Remark: The second constructor shall not participate in overload resolution unless
-?- Effects: Move-constructs aY*
is implicitly convertible toT*
.weak_ptr
instance fromr
. -?- Postconditions:*this
shall contain the old value ofr
.r
shall be empty.r.use_count() == 0
.
Edit 20.3.2.3.4 [util.smartptr.weak.assign] as indicated:
weak_ptr& operator=(const weak_ptr& r) noexcept; template<class Y> weak_ptr& operator=(const weak_ptr<Y>& r) noexcept; template<class Y> weak_ptr& operator=(const shared_ptr<Y>& r) noexcept;-1- Effects: […]
-2- Remarks: […] -?- Returns:*this
.
weak_ptr& operator=(weak_ptr&& r) noexcept; template<class Y> weak_ptr& operator=(weak_ptr<Y>&& r) noexcept;-?- Effects: Equivalent to
-?- Returns:weak_ptr(std::move(r)).swap(*this)
.*this
.
weak_ptr::lock()
should be atomicSection: 20.3.2.3.6 [util.smartptr.weak.obs] Status: C++14 Submitter: Stephan T. Lavavej Opened: 2013-09-21 Last modified: 2017-07-05
Priority: 0
View all other issues in [util.smartptr.weak.obs].
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Discussion:
20.3.2.2 [util.smartptr.shared]/4 says: "For purposes of determining the presence of a data race, member functions shall
access and modify only the shared_ptr
and weak_ptr
objects themselves and not objects they refer to. Changes
in use_count()
do not reflect modifications that can introduce data races." This requires shared_ptr/weak_ptr
implementations to protect their strong and weak refcounts with atomic operations, without the Standardese having to say this
elsewhere. However, 20.3.2.3.6 [util.smartptr.weak.obs]/5 describes weak_ptr::lock()
with
"Returns: expired() ? shared_ptr<T>() : shared_ptr<T>(*this)
."
Even after considering the blanket wording about
data races, this specification is insufficient. If this conditional expression were literally implemented, the use_count()
could change from nonzero to zero after testing expired()
, causing shared_ptr<T>(*this)
to throw
bad_weak_ptr
when the intention is for weak_ptr::lock()
to return empty or nonempty without throwing — indeed,
weak_ptr::lock()
is marked as noexcept
.
We all know what weak_ptr::lock()
should do, the Standardese just doesn't say it.
shared_ptr(const weak_ptr<Y>&)
's specification is not really affected because
20.3.2.2.2 [util.smartptr.shared.const]/23-27 describes the behavior with English instead of code.
[Issaquah 2014-02-11: Move to Immediate]
Proposed resolution:
This wording is relative to N3691.
Edit 20.3.2.3.6 [util.smartptr.weak.obs]/5 as indicated:
shared_ptr<T> lock() const noexcept;-5- Returns:
expired() ? shared_ptr<T>() : shared_ptr<T>(*this)
, executed atomically.
UnaryTypeTraits
returning size_t
Section: 21.3.6 [meta.unary.prop.query] Status: C++14 Submitter: Stephan T. Lavavej Opened: 2013-09-21 Last modified: 2017-07-05
Priority: 0
View all issues with C++14 status.
Discussion:
The sibling sections 21.3.5 [meta.unary], 21.3.7 [meta.rel], and 21.3.8 [meta.trans] respectively specify
UnaryTypeTraits
, BinaryTypeTraits
, and TransformationTraits
, as stated by each /2 paragraph. However,
21.3.6 [meta.unary.prop.query] is underspecified. alignment_of
, rank
, and extent
are said to produce
"Values", but the type of that Value is not specified, and the struct templates are not required to derive from integral_constant
.
Such derivation is more than stylistic — it allows the structs to be passed to functions taking integral_constant
.
alignment_of
returns alignof(T)
which is size_t
(7.6.2.6 [expr.alignof]/2). extent
returns
an array bound, which is clearly size_t
. rank
returns "the number of dimensions" of an array, so any type could
be chosen, with size_t
being a reasonable choice. (Another choice would be unsigned int
, to match extent
's
template parameter I
.)
[Issaquah 2014-02-11: Move to Immediate]
Proposed resolution:
This wording is relative to N3691.
Following 21.3.6 [meta.unary.prop.query]/1 add a new paragraph as indicated:
Each of these templates shall be a
UnaryTypeTrait
(21.3.2 [meta.rqmts]) with aBaseCharacteristic
ofintegral_constant<size_t, Value>
.
basic_string
's wording has confusing relics from the copy-on-write eraSection: 27.4.3 [basic.string] Status: Resolved Submitter: Stephan T. Lavavej Opened: 2013-09-21 Last modified: 2018-11-25
Priority: 4
View other active issues in [basic.string].
View all other issues in [basic.string].
View all issues with Resolved status.
Discussion:
27.4.3.5 [string.capacity]/8 specifies basic_string::resize(n, c)
with:
Effects: Alters the length of the string designated by
*this
as follows:
If
n <= size()
, the function replaces the string designated by*this
with a string of lengthn
whose elements are a copy of the initial elements of the original string designated by*this
.If
n > size()
, the function replaces the string designated by*this
with a string of lengthn
whose firstsize()
elements are a copy of the original string designated by*this
, and whose remaining elements are all initialized toc
.
This wording is a relic of the copy-on-write era. In addition to being extremely confusing, it has undesirable implications.
Saying "replaces the string designated by *this
with a string of length n
whose elements are a copy" suggests
that the trimming case can reallocate. Reallocation during trimming should be forbidden, like vector
.
*this
". (27.4.3.7.7 [string.copy]/3
is different — it "replaces the string designated by s
".)
Of the affected paragraphs, resize()
and erase()
are the most important to fix because they should forbid
reallocation during trimming.
Resolved by the adoption of P1148 in San Diego.
Proposed resolution:
select_on_container_copy_construction()
takes allocators, not containersSection: 23.2.2 [container.requirements.general] Status: C++14 Submitter: Stephan T. Lavavej Opened: 2013-09-21 Last modified: 2017-07-05
Priority: 0
View other active issues in [container.requirements.general].
View all other issues in [container.requirements.general].
View all issues with C++14 status.
Discussion:
23.2.2 [container.requirements.general]/7 says "Copy constructors for these container types obtain an allocator by calling
allocator_traits<allocator_type>::select_on_container_copy_construction
on their first parameters." However,
20.2.9.3 [allocator.traits.members]/8 says that this takes const Alloc&
, not a container.
23.2.2 [container.requirements.general]/7 goes on to say "Move constructors obtain an allocator by move construction from
the allocator belonging to the container being moved." so we can follow that wording.
[Issaquah 2014-02-11: Move to Immediate]
Proposed resolution:
This wording is relative to N3691.
In 23.2.2 [container.requirements.general]/7 change as indicated:
-7- Unless otherwise specified, all containers defined in this clause obtain memory using an allocator (see 17.6.3.5). Copy constructors for these container types obtain an allocator by calling
allocator_traits<allocator_type>::select_on_container_copy_construction
ontheir first parametersthe allocator belonging to the container being copied. Move constructors obtain an allocator by move construction from the allocator belonging to the container being moved. […]
initializer_list
, stuff) constructors are underspecifiedSection: 23.2.7 [associative.reqmts], 23.2.8 [unord.req] Status: C++14 Submitter: Stephan T. Lavavej Opened: 2013-09-21 Last modified: 2017-07-05
Priority: 0
View other active issues in [associative.reqmts].
View all other issues in [associative.reqmts].
View all issues with C++14 status.
Discussion:
23.2.7 [associative.reqmts] specifies both X(i,j)
and X(i,j,c)
, but only X(il)
.
23.4.3.1 [map.overview] declares "map(initializer_list<value_type>, const Compare& = Compare(),
const Allocator& = Allocator());
" but 23.4.3.2 [map.cons] intentionally doesn't explain it, relying
on the big table's requirements. As a result, map(il, c)
's behavior is not actually specified by the Standard.
(All of the other ordered associative containers also provide such constructors.)
[Issaquah 2014-02-11: Move to Immediate]
Proposed resolution:
This wording is relative to N3691.
Edit 23.2.7 [associative.reqmts], Table 102 — "Associative container requirements", as indicated:
Table 102 — Associative container requirements (in addition to container) (continued) Expression Return type Assertion/note pre-/post-condition Complexity …
X(il);
Same as X(il.begin(), il.end())
.sSame asX(il.begin(), il.end())
.X(il, c);
Same as X(il.begin(), il.end(), c)
.Same as X(il.begin(), il.end(), c)
.…
Edit 23.2.8 [unord.req], Table 103 "Unordered associative container requirements", as indicated:
Table 103 — Unordered associative container requirements (in addition to container) Expression Return type Assertion/note pre-/post-condition Complexity …
X(il)
X
Same as X(il.begin(), il.end())
.Same as X(il.begin(), il.end())
.X(il, n)
X
Same as X(il.begin(), il.end(), n)
.Same as X(il.begin(), il.end(), n)
.X(il, n, hf)
X
Same as X(il.begin(), il.end(), n, hf)
.Same as X(il.begin(), il.end(), n, hf)
.X(il, n, hf, eq)
X
Same as X(il.begin(), il.end(), n, hf, eq)
.Same as X(il.begin(), il.end(), n, hf, eq)
.…
vector::resize(n, t)
's specification should be simplifiedSection: 23.3.13.3 [vector.capacity], 23.3.5.3 [deque.capacity] Status: C++14 Submitter: Stephan T. Lavavej Opened: 2013-09-21 Last modified: 2017-07-05
Priority: 0
View other active issues in [vector.capacity].
View all other issues in [vector.capacity].
View all issues with C++14 status.
Discussion:
First, 23.3.5.3 [deque.capacity]/4 and 23.3.13.3 [vector.capacity]/16 say that resize(size_type sz, const T& c)
"Requires: T
shall be MoveInsertable
into *this
and CopyInsertable
into *this
."
The CopyInsertable
requirement is correct (because sz
might be size() + 2
or more), but the
MoveInsertable
requirement is redundant due to 23.2.2 [container.requirements.general]/13: "T
is
CopyInsertable
into X
means that, in addition to T
being MoveInsertable
into X
, the [...]".
(LWG 2033(i)'s resolution said that this was "not redundant, because CopyInsertable
is not necessarily a refinement
of MoveInsertable
" which was true at the time, but then LWG 2177(i)'s resolution made it a refinement.)
CopyInsertable
T
there are no effects." This is confusing because T
is required to be
CopyInsertable
. (/14 says the same thing for resize(size_type sz)
, where it is correct because that overload
requires only MoveInsertable
and DefaultInsertable
.)
[Issaquah 2014-02-11: Move to Immediate]
Proposed resolution:
This wording is relative to N3691.
Edit 23.3.5.3 [deque.capacity]/4 as indicated:
void resize(size_type sz, const T& c);[…]
-4- Requires:T
shall beMoveInsertable
into*this
andCopyInsertable
into*this
.
Edit 23.3.13.3 [vector.capacity]/16+17 as indicated:
void resize(size_type sz, const T& c);[…]
-16- Requires:T
shall beMoveInsertable
into*this
andCopyInsertable
into*this
. -17- Remarks: If an exception is thrownother than by the move constructor of a non-there are no effects.CopyInsertable
T
addressof()
Section: 24.5.2.2.2 [back.insert.iter.ops], 24.5.2.3.2 [front.insert.iter.ops], 24.5.2.4.2 [insert.iter.ops] Status: C++14 Submitter: Stephan T. Lavavej Opened: 2013-09-21 Last modified: 2021-06-06
Priority: 0
View all other issues in [back.insert.iter.ops].
View all issues with C++14 status.
Discussion:
[back.insert.iter.cons]/1, [front.insert.iter.cons]/1, and [insert.iter.cons]/1
say "Initializes container with &x
", which doesn't defend against containers overloading operator&()
.
Containers are now required to have such defenses for their elements, so we may as well be consistent here.
[Issaquah 2014-02-11: Move to Immediate]
Proposed resolution:
This wording is relative to N3691.
Edit [back.insert.iter.cons]/1 as indicated:
explicit back_insert_iterator(Container& x);-1- Effects: Initializes
container
with.
&xstd::addressof(x)
Edit [front.insert.iter.cons]/1 as indicated:
explicit front_insert_iterator(Container& x);-1- Effects: Initializes
container
with.
&xstd::addressof(x)
Edit [insert.iter.cons]/1 as indicated:
insert_iterator(Container& x, typename Container::iterator i);-1- Effects: Initializes
container
withand
&xstd::addressof(x)iter
withi
.
minmax_element()
's behavior differing from max_element()
's should be notedSection: 26.8.9 [alg.min.max] Status: C++17 Submitter: Stephan T. Lavavej Opened: 2013-09-21 Last modified: 2017-07-30
Priority: 3
View other active issues in [alg.min.max].
View all other issues in [alg.min.max].
View all issues with C++17 status.
Discussion:
26.8.9 [alg.min.max]/23 says that max_element()
finds the first biggest element, while /25 says that
minmax_element()
finds the last biggest element. This significant difference is unusual — it means that
minmax_element(args)
is not equivalent to make_pair(min_element(args), max_element(args))
, whereas the other
major "two for one" algorithm equal_range(args)
is equivalent to make_pair(lower_bound(args), upper_bound(args))
.
minmax_element()
's behavior is intentional — it is a fundamental consequence of the 3N/2
algorithm —
but the Standardese does not draw attention to this in any way. This wording came from LWG 715(i)'s resolution (which
changed the semantics but didn't mention it), citing CLRS for the algorithm — but CLRS doesn't mention the behavior for
equivalent elements! The wording here deeply confused me (as an STL maintainer fixing an incorrect implementation) until I walked
through the algorithm by hand and figured out the fundamental reason. It would be really nice for the Standard to provide a hint
that something magical is happening here.
[2014-06-06 Library reflector vote]
The issue has been identified as Tentatively Ready based on six votes in favour.
Proposed resolution:
This wording is relative to N3691.
Add a footnote to 26.8.9 [alg.min.max]/25 as indicated:
template<class ForwardIterator> pair<ForwardIterator, ForwardIterator> minmax_element(ForwardIterator first, ForwardIterator last); template<class ForwardIterator, class Compare> pair<ForwardIterator, ForwardIterator> minmax_element(ForwardIterator first, ForwardIterator last, Compare comp);-25- Returns:
make_pair(first, first)
if[first,last)
is empty, otherwisemake_pair(m, M)
, wherem
is the first iterator in[first,last)
such that no iterator in the range refers to a smaller element, and whereM
is the last iterator [Footnote: This behavior intentionally differs frommax_element()
.] in[first,last)
such that no iterator in the range refers to a larger element.
Section: 31.7.5.6 [istream.rvalue] Status: C++17 Submitter: Stephan T. Lavavej Opened: 2013-09-21 Last modified: 2017-07-30
Priority: 3
View all other issues in [istream.rvalue].
View all issues with C++17 status.
Discussion:
31.7.5.6 [istream.rvalue] declares operator>>(basic_istream<charT, traits>&& is, T& x)
.
However, [istream::extractors]/7 declares operator>>(basic_istream<charT,traits>& in, charT* s)
,
plus additional overloads for unsigned char*
and signed char*
. This means that
"return_rvalue_istream() >> &arr[0]
" won't compile, because T&
won't bind to the rvalue
&arr[0]
.
[2014-02-12 Issaquah : recategorize as P3]
Jonathan Wakely: Bill was certain the change is right, I think so with less certainty
Jeffrey Yaskin: I think he's right, hate that we need this
Jonathan Wakely: is this the security issue Jeffrey raised on lib reflector?
Move to P3
[2015-05-06 Lenexa: Move to Review]
WEB, MC: Proposed wording changes one signature (in two places) to take a forwarding reference.
TK: Should be consistent with an istream rvalue?
MC: This is the case where you pass the stream by rvalue reference.
RP: I would try it before standardizing.
TK: Does it break anything?
RP, TK: It will take all arguments, will be an exact match for everything.
RS, TK: This adapts streaming into an rvalue stream to make it act like streaming into an lvalue stream.
RS: Should this really return the stream by lvalue reference instead of by rvalue reference? ⇒ new LWG issue.
RP: Security issue?
MC: No. That's istream >> char*
, C++ version of gets()
. Remove it, as we did for
gets()
? ⇒ new LWG issue.
RS: Proposed resolution looks correct to me.
MC: Makes me (and Jonathan Wakely) feel uneasy.
Move to Review, consensus.
[2016-01-15, Daniel comments and suggests improved wording]
It has been pointed out by Tim Song, that the initial P/R (deleting the Returns paragraph and making the Effects
"equivalent to return is >> /* stuff */;
") breaks cases where the applicable operator>>
doesn't
return a type that is convertible to a reference to the stream:
struct A{}; void operator>>(std::istream&, A&){ } void f() { A a; std::istringstream() >> a; // was OK, now error }
This seems like an unintended wording artifact of the "Equivalent to" Phrase Of Power and could be easily fixed by changing the second part of the P/R slightly as follows:
template <class charT, class traits, class T> basic_istream<charT, traits>& operator>>(basic_istream<charT, traits>&& is, T&& x);-1- Effects: Equivalent to:
is >>x
is >> std::forward<T>(x); return is;
-2- Returns:is
Previous resolution [SUPERSEDED]:
This wording is relative to N4567.
Edit [iostream.format.overview], header
<istream>
synopsis, as indicated:namespace std { […] template <class charT, class traits, class T> basic_istream<charT, traits>& operator>>(basic_istream<charT, traits>&& is, T&& x); }Edit 31.7.5.6 [istream.rvalue] as indicated:
template <class charT, class traits, class T> basic_istream<charT, traits>& operator>>(basic_istream<charT, traits>&& is, T&& x);-1- Effects: Equivalent to
return
is >>
xstd::forward<T>(x)-2- Returns:is
[2016-03 Jacksonville: Move to Ready]
Proposed resolution:
This wording is relative to N4567.
Edit [iostream.format.overview], header <istream>
synopsis, as indicated:
namespace std { […] template <class charT, class traits, class T> basic_istream<charT, traits>& operator>>(basic_istream<charT, traits>&& is, T&& x); }
Edit 31.7.5.6 [istream.rvalue] as indicated:
template <class charT, class traits, class T> basic_istream<charT, traits>& operator>>(basic_istream<charT, traits>&& is, T&& x);-1- Effects: Equivalent to:
is >>x
is >> std::forward<T>(x); return is;
-2- Returns:is
regex_match()/regex_search()
with match_results
should forbid temporary stringsSection: 28.6.3 [re.syn] Status: C++14 Submitter: Stephan T. Lavavej Opened: 2013-09-21 Last modified: 2016-01-28
Priority: 2
View all other issues in [re.syn].
View all issues with C++14 status.
Discussion:
Consider the following code:
const regex r(R"(meow(\d+)\.txt)"); smatch m; if (regex_match(dir_iter->path().filename().string(), m, r)) { DoSomethingWith(m[1]); }
This occasionally crashes. The problem is that dir_iter->path().filename().string()
returns a temporary string,
so the match_results
contains invalidated iterators into a destroyed temporary string.
regex_match/regex_search(str, reg)
to accept temporary strings, because they just return bool
.
However, the overloads taking match_results
should forbid temporary strings.
[2014-02-13 Issaquah: Move as Immediate]
Proposed resolution:
This wording is relative to N3691.
Edit 28.6.3 [re.syn], header <regex>
synopsis, as indicated:
#include <initializer_list> namespace std { […] // 28.11.2, function template regex_match: […] template <class ST, class SA, class Allocator, class charT, class traits> bool regex_match(const basic_string<charT, ST, SA>&&, match_results< typename basic_string<charT, ST, SA>::const_iterator, Allocator>&, const basic_regex<charT, traits>&, regex_constants::match_flag_type = regex_constants::match_default) = delete; // 28.11.3, function template regex_search: […] template <class ST, class SA, class Allocator, class charT, class traits> bool regex_search(const basic_string<charT, ST, SA>&&, match_results< typename basic_string<charT, ST, SA>::const_iterator, Allocator>&, const basic_regex<charT, traits>&, regex_constants::match_flag_type = regex_constants::match_default) = delete; […] }
regex("meow", regex::icase)
is technically forbidden but should be permittedSection: 28.6.4.2 [re.synopt] Status: C++14 Submitter: Stephan T. Lavavej Opened: 2013-09-21 Last modified: 2017-07-05
Priority: 0
View all other issues in [re.synopt].
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Discussion:
28.6.4.2 [re.synopt]/1 says "A valid value of type syntax_option_type
shall have exactly one of the elements
ECMAScript
, basic
, extended
, awk
, grep
, egrep
, set."
icase
by itself! Users should not be required to pass
regex::ECMAScript | regex::icase
. (Note that the cost of an additional check for no grammar being explicitly requested
is completely irrelevant, as regex construction is so much more expensive.)
[Issaquah 2014-02-11: Move to Immediate]
Proposed resolution:
This wording is relative to N3691.
Edit 28.6.4.2 [re.synopt] as indicated:
-1- The type
syntax_option_type
is an implementation-defined bitmask type (17.5.2.1.3). Setting its elements has the effects listed in table 138. A valid value of typesyntax_option_type
shall haveexactlyat most one of the grammar elementsECMAScript
,basic
,extended
,awk
,grep
,egrep
, set. If no grammar element is set, the default grammar isECMAScript
.
regex_iterator/regex_token_iterator
should forbid temporary regexesSection: 28.6.11 [re.iter] Status: C++14 Submitter: Stephan T. Lavavej Opened: 2013-09-21 Last modified: 2016-01-28
Priority: 2
View all other issues in [re.iter].
View all issues with C++14 status.
Discussion:
Users can write "for(sregex_iterator i(s.begin(), s.end(), regex("meow")), end; i != end; ++i)
", binding a temporary
regex
to const regex&
and storing a pointer to it. This will compile silently, triggering undefined behavior
at runtime. We now have the technology to prevent this from compiling, like how reference_wrapper
refuses to bind to
temporaries.
[2014-02-14 Issaquah meeting: Move to Immediate]
Proposed resolution:
This wording is relative to N3691.
Change 28.6.11.1 [re.regiter]/1, class template regex_iterator
synopsis, as indicated:
regex_iterator(); regex_iterator(BidirectionalIterator a, BidirectionalIterator b, const regex_type& re, regex_constants::match_flag_type m = regex_constants::match_default); regex_iterator(BidirectionalIterator a, BidirectionalIterator b, const regex_type&& re, regex_constants::match_flag_type m = regex_constants::match_default) = delete;
Change 28.6.11.2 [re.tokiter]/6, class template regex_token_iterator
synopsis, as indicated:
regex_token_iterator(); regex_token_iterator(BidirectionalIterator a, BidirectionalIterator b, const regex_type& re, int submatch = 0, regex_constants::match_flag_type m = regex_constants::match_default); regex_token_iterator(BidirectionalIterator a, BidirectionalIterator b, const regex_type& re, const std::vector<int>& submatches, regex_constants::match_flag_type m = regex_constants::match_default); regex_token_iterator(BidirectionalIterator a, BidirectionalIterator b, const regex_type& re, initializer_list<int> submatches, regex_constants::match_flag_type m = regex_constants::match_default); template <std::size_t N> regex_token_iterator(BidirectionalIterator a, BidirectionalIterator b, const regex_type& re, const int (&submatches)[N], regex_constants::match_flag_type m = regex_constants::match_default); regex_token_iterator(BidirectionalIterator a, BidirectionalIterator b, const regex_type&& re, int submatch = 0, regex_constants::match_flag_type m = regex_constants::match_default) = delete; regex_token_iterator(BidirectionalIterator a, BidirectionalIterator b, const regex_type&& re, const std::vector<int>& submatches, regex_constants::match_flag_type m = regex_constants::match_default) = delete; regex_token_iterator(BidirectionalIterator a, BidirectionalIterator b, const regex_type&& re, initializer_list<int> submatches, regex_constants::match_flag_type m = regex_constants::match_default) = delete; template <std::size_t N> regex_token_iterator(BidirectionalIterator a, BidirectionalIterator b, const regex_type&& re, const int (&submatches)[N], regex_constants::match_flag_type m = regex_constants::match_default) = delete;
optional<T>
objectsSection: 5.11 [fund.ts::optional.hash] Status: Resolved Submitter: Jonathan Wakely Opened: 2013-10-03 Last modified: 2015-10-26
Priority: Not Prioritized
View all issues with Resolved status.
Discussion:
Addresses: fund.ts
The spec for hash<optional<T>>
doesn't say anything about disengaged
objects, so 3.65 [defns.undefined] would imply it's undefined behaviour,
but that's very unhelpful to users.
std::hash<void*>()(nullptr)
, but leaving
it unspecified would permit users to specialize hash<optional<UserDefinedType>>
so
that the hash value for a disengaged object is distinct from any value returned by
hash<UserDefinedType>
.
[2014-06-06 pre-Rapperswill]
This issue has been reopened as fundamentals-ts.
[2014-06-07 Daniel comments]
This issue should be set to Resolved, because the wording fix is already applied in the last fundamentals working draft.
[2014-06-16 Rapperswill]
Confirmed that this issue is resolved in the current Library Fundamentals working paper.
Proposed resolution:
This wording is relative to N3691.
Add to 5.11 [fund.ts::optional.hash]/3
template <class T> struct hash<optional<T>>;[…]
-3- For an objecto
of typeoptional<T>
, ifbool(o) == true
,hash<optional<T>>()(o)
shall evaluate to the same value ashash<T>()(*o)
otherwise it evaluates to an unspecified value.
atomic
's default constructor requires "uninitialized" state even for types with non-trivial default-constructorSection: 32.5.8.2 [atomics.types.operations] Status: Resolved Submitter: Daniel Krügler Opened: 2013-10-03 Last modified: 2019-11-19
Priority: Not Prioritized
View all other issues in [atomics.types.operations].
View all issues with Resolved status.
Discussion:
According to 99 [atomics.types.operations.req] p4,
A::A() noexcept = default;Effects: leaves the atomic object in an uninitialized state. [Note: These semantics ensure compatibility with
C
. — end note]
This implementation requirement is OK for POD types, like int
, but 32.5.8 [atomics.types.generic] p1
intentionally allows template arguments of atomic
with a non-trivial default constructor ("The type of the template argument
T
shall be trivially copyable (3.9)"), so this wording can be read in a way that makes the behaviour of the following code
undefined:
#include <atomic> #include <iostream> struct S { S() noexcept : v(42) {} int v; }; int main() { std::atomic<S> as; // Default-initialization std::cout << as.load().v << std::endl; // ? }
For a user-defined emulation of atomic
the expected outcome would be defined and the program would output "42",
but existing implementations differ and the result value is a "random number" for at least one implementation. This seems
very surprising to me.
S
.
According to my understanding, the non-normative note in 99 [atomics.types.operations.req] p4 is intended to
refer to types that are valid C
-types, but the example type S
is not such a type.
To make the mental model of atomic
's default constructor more intuitive for user-code, I suggest to clarify the wording
to have the effects of default-initialization instead. The current state seems more like an unintended effect of imprecise
language used here and has some similarities to wording that was incorrectly used to specify atomic_flag
initialization
as described by LWG 2159(i).
[2014-05-17, Daniel comments and provides alternative wording]
The current wording was considered controversial as expressed by reflector discussions. To me, the actual problem is not newly introduced by that wording, but instead is already present in basically all paragraphs specifying semantics of atomic types, since the wording never clearly distinguishes the value of the actual atomic type A and the value of the "underlying", corresponding non-atomic type C. The revised proposed wording attempts to improve the current ambiguity of these two kinds of values.
Previous resolution from Daniel [SUPERSEDED]:
This wording is relative to N3691.
Modify 99 [atomics.types.operations.req] p4 as indicated: [Editorial note: There is no exposition-only member in
atomic
, which makes it a bit hard to specify what actually is initialized, but the usage of the term "value" seems consistent with similar wording used to specify the effects of the atomicload
functions]A ::A () noexcept = default;-4- Effects:
leaves the atomic object in an uninitialized stateThe value of the atomic object is default-initialized (9.5 [dcl.init]). [Note: These semantics ensure compatibility withC
. — end note]
[2015-02 Cologne]
Handed over to SG1.
[2017-07 Toronto]
SG1 reviewed the PR below:
Previous resolution [SUPERSEDED]:This wording is relative to N3936.
Modify 99 [atomics.types.operations.req] p2 as indicated: [Editorial note: This is a near-to editorial change not directly affecting this issue, but
atomic_address
does no longer exist and the pointed to definition is relevant in the context of this issue resolution.]-2- In the following operation definitions:
an A refers to one of the atomic types.
a C refers to its corresponding non-atomic type.
Theatomic_address
atomic type corresponds to thevoid*
non-atomic type.[…]
Modify 99 [atomics.types.operations.req] p4 and the following as indicated: [Editorial note: There is no exposition-only member in
atomic
, which makes it a bit hard to specify what actually is initialized, but the introductory wording of 99 [atomics.types.operations.req] p2 b2 defines: "a C refers to its corresponding non-atomic type." which helps to specify the semantics in terms of "the C value referred to by the atomic object"]A::A() noexcept = default;-4- Effects:
leaves the atomic object in an uninitialized stateDefault-initializes (9.5 [dcl.init]) the C value referred to by the atomic object. [Note: These semantics ensure compatibility withC
. — end note]constexpr A::A(C desired) noexcept;-5- Effects: Direct-i
[…]Initializes the C value referred to by the atomic object with the valuedesired
. Initialization is not an atomic operation (1.10). […]void atomic_init(volatile A* object, C desired) noexcept; void atomic_init(A* object, C desired) noexcept;-8- Effects: Non-atomically initializes the C value referred to by
*object
with valuedesired
. […]void atomic_store(volatile A* object, C desired) noexcept; […] void A::store(C desired, memory_order order = memory_order_seq_cst) noexcept;-9- […]
-10- Effects: Atomically replaces the C value pointed to byobject
or bythis
with the value ofdesired
. […] […]C atomic_load(const volatile A* object) noexcept; […] C A::load(memory_order order = memory_order_seq_cst) const noexcept;-13- […]
-14- […] -15- Returns: Atomically returns the C value pointed to byobject
or bythis
. […]C atomic_exchange(volatile A* object, C desired) noexcept; […] C A::exchange(C desired, memory_order order = memory_order_seq_cst) noexcept;-18- Effects: Atomically replaces the C value pointed to by
-19- Returns: Atomically returns the C value pointed to byobject
or bythis
withdesired
. […]object
or bythis
immediately before the effects. […]C atomic_fetch_key(volatile A* object, M operand) noexcept; […] C A::fetch_key(M operand, memory_order order = memory_order_seq_cst) noexcept;-28- Effects: Atomically replaces the C value pointed to by
-29- Returns: Atomicallyobject
or bythis
with the result of the computation applied to the C value pointed to byobject
or bythis
and the givenoperand
. […],returns the C value pointed to byobject
or bythis
immediately before the effects. […]Modify 32.5.10 [atomics.flag] p5 and the following as indicated:
bool atomic_flag_test_and_set(volatile atomic_flag* object) noexcept; […] bool atomic_flag::test_and_set(memory_order order = memory_order_seq_cst) noexcept;-5- Effects: Atomically sets the bool value pointed to by
-6- Returns: Atomicallyobject
or bythis
totrue
. […],returns the bool valueof thepointed to byobject
or bythis
immediately before the effects.void atomic_flag_clear(volatile atomic_flag* object) noexcept; […] void atomic_flag::clear(memory_order order = memory_order_seq_cst) noexcept;-7- […]
-8- Effects: Atomically sets the bool value pointed to byobject
or bythis
tofalse
. […]
SG1 also reviewed another PR from Lawrence Crowl. Lawrence's feedback was that turning atomic<T> into a container of T was a mistake, even if we allow the implementation of atomic to contain a T. SG1 agreed with Lawrence, but his PR (http://wiki.edg.com/bin/view/Wg21toronto2017/DefaultInitNonContainer) had massive merge conflicts caused by the adoption of P0558. Billy O'Neal supplied a new PR, which SG1 agreed to and which LWG looked at informally. This change also makes it clearer that initialization of an atomic is not an atomic operation in all forms, changes the C compatibility example to actually be compatible with C, and removes "initialization-compatible" which is not defined anywhere.
SG1 considered moving ATOMIC_VAR_INIT
into Annex D, as their understanding at this time is that WG14 is considering removal of that macro. However, consensus was that moving things between clauses would require a paper, and that we should wait to remove that until WG14 actually does so.
[2019-02, Monday in Kona]
While discussing Richard's P1286 paper, we noted that this issue's resolution needs to be updated based on that discussion.
Also, the idea that atomic() noexcept = default
is ok will not fly for implementors who store additional information inside the atomic variable.
Previous resolution [SUPERSEDED]:
This wording is relative to N4659.
Modify 32.5.8.2 [atomics.types.operations] as indicated:
-?- Initialization of an atomic object is not an atomic operation (6.9.2 [intro.multithread]). [Note: It is possible to have an access to an atomic object A race with its construction, for example by communicating the address of the just-constructed object A via a
memory_order_relaxed
operations on a suitable atomic pointer variable, and then immediately accessing A in the recieving thread. This results in undefined behavior. — end note]-1- [Note: Many operations are volatile-qualified. The "volatile as device register" semantics have not changed in the standard. This qualification means that volatility is preserved when applying these operations to volatile objects. It does not mean that operations on non-volatile objects become volatile. — end note]
atomic() noexcept = default;-2- Effects:
Leaves the atomic object in an uninitialized state. [Note: These semantics ensure compatibility with C. — end note]Initializes the atomic object with a default-initialized (9.5 [dcl.init]) value of typeT
. [Note: The default-initialized value may not be pointer-interconvertible with the atomic object. — end note]constexpr atomic(T desired) noexcept;-3- Effects: Initializes the atomic object with the value
desired
.Initialization is not an atomic operation (6.9.2 [intro.multithread]). [Note: It is possible to have an access to an atomic object A race with its construction, for example by communicating the address of the just-constructed object A to another thread viamemory_order_relaxed
operations on a suitable atomic pointer variable, and then immediately accessing A in the receiving thread. This results in undefined behavior — end note]#define ATOMIC_VAR_INIT(value)see below{value}-4-
The macro expands to a token sequence suitable for constant initialization of an atomic variable of static storage duration of a type that is initialization-compatible with value. [Note: This operation may need to initialize locks. — end note] Concurrent access to the variable being initialized, even via an atomic operation, constitutes a data race.[Note: This macro ensures compatibility with C. — end note]
[Example:
atomic<int>atomic_int v = ATOMIC_VAR_INIT(5);
— end example]
[2019-11; Resolved by the adoption of P0883 in Belfast.]
Proposed resolution:
Resolved by P0883.
is_trivially_constructible/is_trivially_assignable
traits are always falseSection: 21.3.5.4 [meta.unary.prop] Status: C++17 Submitter: Daniel Krügler Opened: 2013-10-01 Last modified: 2017-07-30
Priority: 3
View other active issues in [meta.unary.prop].
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Discussion:
In 21.3.5.4 [meta.unary.prop] we have traits to allow testing for triviality of specific operations, such as
is_trivially_constructible
and is_trivially_assignable
(and their derived forms), which are specified
in terms of the following initialization and assignment, respectively:
T t(create<Args>()...); declval<T>() = declval<U>()
The wording that describes the way how triviality is deduced, is in both cases of the same form:
[… ] and the variable definition/assignment, as defined by
is_constructible/is_assignable
, is known to call no operation that is not trivial (3.9, 12).
The problematic part of this wording is that both definitions are specified in terms of an "object construction" function
create
or declval
, respectively, (The former being a conceptual function, the latter being a library function),
but for none of these functions we can assume that they could be considered as trivial — only special member functions can
have this property and none of these is one. This problem became obvious, when the similar issue LWG 2298(i)
in regard to is_nothrow_constructible
was opened.
create
, which
is currently needed for the is_convertible
and the is_constructible
traits, because both traits are specified in
terms of contexts where technically the corresponding "object construction" function would be considered as odr-used. This is problematic,
because these traits are defined in terms of well-formed code and odr-using declval
would make the program ill-formed (see
22.2.6 [declval]). So extending above blanket statement to consider std::declval<T>()
as not odr-used
in the context of the corresponding trait definition would allow for replacing create
by declval
.
[2015-05, Lenexa]
STL: would you consider moving the change to 20.10 as editorial or are you uncomfortable with it?
JW: this sounds a viable editorial change
VV: I guarantee you that moving it doesn't change anything
MC: how about this: we move it to Ready as is and if we conclude moving it is editorial we can do it and if not open an issue
STL: I would like to guarantee that the lifting happens
JW: I do that! If it goes in I move it up
MC: move to Ready: in favor: 15, opposed: 0, abstain: 1
Proposed resolution:
This wording is relative to N3936.
Add a new paragraph after 21.3.5.4 [meta.unary.prop] p3 as indicated: [Editorial note: The first change in 21.3.5.4 [meta.unary.prop] p3 is recommended, because technically a Clause is always a "main chapter" — such as Clause 20 — but every child of a Clause or sub-clause is a sub-clause]
[…]
-3- For all of the class templatesX
declared in thisClausesub-clause, instantiating that template with a template-argument that is a class template specialization may result in the implicit instantiation of the template argument if and only if the semantics ofX
require that the argument must be a complete type. -?- For the purpose of defining the templates in this sub-clause, a function call expressiondeclval<T>()
for any typeT
is considered to be a trivial (6.8 [basic.types], 11.4.4 [special]) function call that is not an odr-use (6.3 [basic.def.odr]) ofdeclval
in the context of the corresponding definition notwithstanding the restrictions of 22.2.6 [declval]. […]
Modify 21.3.5.4 [meta.unary.prop] p7 as indicated:
-7-
Given the following function prototype:template <class T> typename add_rvalue_reference<T>::type create();
tThe predicate condition for a template specializationis_constructible<T, Args...>
shall be satisfied if and only if the following variable definition would be well-formed for some invented variablet
:T t(createdeclval<Args>()...);[…]
Add a new paragraph after 21.3.7 [meta.rel] p2 as indicated: [Editorial note: Technically we don't need the guarantee of "a trivial function call" for the type relationship predicates at the very moment, but it seems more robust and consistent to have the exact same guarantee here as well]
[…]
-2- […] -?- For the purpose of defining the templates in this sub-clause, a function call expressiondeclval<T>()
for any typeT
is considered to be a trivial (6.8 [basic.types], 11.4.4 [special]) function call that is not an odr-use (6.3 [basic.def.odr]) ofdeclval
in the context of the corresponding definition notwithstanding the restrictions of 22.2.6 [declval]. […]
Modify 21.3.7 [meta.rel] p4 as indicated:
-4-
Given the following function prototype:template <class T> typename add_rvalue_reference<T>::type create();
tThe predicate condition for a template specializationis_convertible<From, To>
shall be satisfied if and only if the return expression in the following code would be well-formed, including any implicit conversions to the return type of the function:To test() { returncreatedeclval<From>(); }[…]
nth_element
Section: 26.8.3 [alg.nth.element] Status: C++14 Submitter: Christopher Jefferson Opened: 2013-10-19 Last modified: 2017-07-05
Priority: 0
View all other issues in [alg.nth.element].
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Discussion:
The wording of nth_element
says:
template<class RandomAccessIterator> void nth_element(RandomAccessIterator first, RandomAccessIterator nth, RandomAccessIterator last);After
nth_element
the element in the position pointed to bynth
is the element that would be in that position if the whole range were sorted. Also for every iteratori
in the range[first,nth)
and every iteratorj
in the range[nth,last)
it holds that:!(*j < *i)
orcomp(*j, *i) == false
.
That wording, to me, implies that there must be an element at 'nth
'.
However, gcc at least accepts nth == last
, and returns without effect
(which seems like the sensible option).
nth == last
? If so, then I would suggest adding
this to the wording explicitly, say:
After
nth_element
the element in the position pointed to bynth
, if any, is the element that would be in that position if the whole range were sorted. Also for every iteratori
in the range[first,nth)
and every iteratorj
in the range[nth,last)
it holds that:!(*j < *i)
orcomp(*j, *i) == false
.
[Issaquah 2014-02-11: Move to Immediate]
Proposed resolution:
This wording is relative to N3797.
Modify 26.8.3 [alg.nth.element]/1 as indicated:
template<class RandomAccessIterator> void nth_element(RandomAccessIterator first, RandomAccessIterator nth, RandomAccessIterator last); template<class RandomAccessIterator, class Compare> void nth_element(RandomAccessIterator first, RandomAccessIterator nth, RandomAccessIterator last, Compare comp);-1- After
nth_element
the element in the position pointed to bynth
is the element that would be in that position if the whole range were sorted, unlessnth == last
. Also for every iteratori
in the range[first,nth)
and every iteratorj
in the range[nth,last)
it holds that:!(*j < *i)
orcomp(*j, *i) == false
.
Section: 16.4.5.6 [replacement.functions] Status: C++17 Submitter: David Majnemer Opened: 2013-10-20 Last modified: 2017-07-30
Priority: 2
View all other issues in [replacement.functions].
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Discussion:
N3290 16.4.5.6 [replacement.functions]/p3 says:
The program's definitions shall not be specified as
inline
.
This seems to permit declarations of replacement allocation functions that are specified as inline
so long
as they aren't used. This behavior seems more like a bug than a feature, I propose that we do the following:
The program's
definitionsdeclarations shall not be specified asinline
.
[2014-02-15 Issaquah : Move to Ready]
Proposed resolution:
This wording is relative to N3797.
Modify 16.4.5.6 [replacement.functions]/3 as indicated:
-3- The program's definitions are used instead of the default versions supplied by the implementation (18.6). Such replacement occurs prior to program startup (3.2, 3.6). The program's
definitionsdeclarations shall not be specified asinline
. No diagnostic is required.
basic_ostream::seekp(pos)
and basic_ostream::seekp(off, dir)
Section: 31.7.6.2.5 [ostream.seeks] Status: C++14 Submitter: Marshall Clow Opened: 2013-10-21 Last modified: 2017-07-05
Priority: 0
View all issues with C++14 status.
Discussion:
In 31.7.6.2.5 [ostream.seeks], we have:
basic_ostream<charT,traits>& seekp(pos_type pos);-3- Effects: If
-4- Returns:fail() != true
, executesrdbuf()->pubseekpos(pos, ios_base::out)
. In case of failure, the function callssetstate(failbit)
(which may throwios_base::failure
).*this
.
basic_ostream<charT,traits>& seekp(off_type off, ios_base::seekdir dir);-5- Effects: If
-6- Returns:fail() != true
, executesrdbuf()->pubseekoff(off, dir, ios_base::out)
.*this
.
The first call is required to set the failbit
on failure, but the second is not
os1
and os2
) the following code (confusingly) works:
os1.seekp(-1); assert(os1.fail()); os2.seekp(-1, std::ios_base::beg); assert(os2.good());
Note that the description of basic_istream<charT,traits>& seekg(off_type off, ios_base::seekdir dir)
in
31.7.5.4 [istream.unformatted] p43 does require setting failbit
.
[Issaquah 2014-02-11: Move to Immediate]
Proposed resolution:
This wording is relative to N3797.
Modify 31.7.6.2.5 [ostream.seeks]p5 as indicated:
basic_ostream<charT,traits>& seekp(off_type off, ios_base::seekdir dir);-5- Effects: If
-6- Returns:fail() != true
, executesrdbuf()->pubseekoff(off, dir, ios_base::out)
. In case of failure, the function callssetstate(failbit)
(which may throwios_base::failure
).*this
.
Section: 28.6.12 [re.grammar] Status: Resolved Submitter: Nayuta Taga Opened: 2013-10-30 Last modified: 2017-03-12
Priority: 2
View other active issues in [re.grammar].
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Discussion:
In the following "Multiline" is the value of the ECMA-262 RegExp object's multiline property.
In ECMA-262, there are some definitions that relate to Multiline:ECMA-262 15.10.2.6:
If Multiline is true, ^ matches just after LineTerminator.
If Multiline is false, ^ does not match just after LineTerminator. If Multiline is true, $ matches just before LineTerminator. If Multiline is false, $ does not match just before LineTerminator.
ECMA-262 15.10.4.1, 15.10.7.4:
By default, Multiline is false.
So, the C++11 standard says that Multiline is false. As it is false, ^ matches only the beginning of the string, and $ matches only the end of the string.
However, two flags are defined in 28.6.4.3 [re.matchflag] Table 139:
match_not_bol
: the character ^ in the regular expression shall not match[first,first)
.match_not_eol
: the character "$" in the regular expression shall not match[last,last)
.
As Multiline is false, the match_not_bol
and the match_not_eol
are
meaningless because they only make ^ and $ match none.
libstdc++ r206594
libc++ r199174
Multiline=true:
Visual Studio Express 2013
boost 1.55
[2015-05-22, Daniel comments]
This issue interacts with LWG 2503(i).
[2016-08 Chicago]
Resolving 2503(i) will resolve this as well.
Proposed resolution:
quoted()
's interaction with padding is unclearSection: 31.7.9 [quoted.manip] Status: C++14 Submitter: Stephan T. Lavavej Opened: 2013-11-01 Last modified: 2016-01-28
Priority: 1
View all other issues in [quoted.manip].
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Discussion:
Given this code:
cout << "[" << left << setfill('x') << setw(20) << R"("AB \"CD\" EF")" << "]" << endl; cout << "[" << left << setfill('y') << setw(20) << quoted(R"(GH "IJ" KL)") << "]" << endl;
The first line prints ["AB \"CD\" EF"xxxxxx]
. The second line should probably print ["GH \"IJ\" KL"yyyyyy]
, but
31.7.9 [quoted.manip]/2 doesn't say whether or how quoted()
should interact with padding. All it says is that
"
out << quoted(s, delim, escape)
behaves as if it inserts the following characters into out using character inserter function templates (27.7.3.6.4)".
31.7.6.3.4 [ostream.inserters.character] specifies both single-character and null-terminated inserters, both referring to
31.7.6.3.1 [ostream.formatted.reqmts]/3 for padding. Literally implementing quoted()
with single-character inserters
would result in padding being emitted after the first character, with undesirable effects for ios_base::left
.
os << str
"Behaves as a formatted output function (27.7.3.6.1) of
os
. Forms a character sequenceseq
, initially consisting of the elements defined by the range[str.begin(), str.end())
. Determines padding forseq
as described in 27.7.3.6.1. Then insertsseq
as if by callingos.rdbuf()->sputn(seq, n)
, wheren
is the larger ofos.width()
andstr.size();
then callsos.width(0)
."
Additionally, saying that it's a "formatted output function" activates 31.7.6.3.1 [ostream.formatted.reqmts]/1's wording for sentry objects.
[2014-02-14 Issaquah meeting: Move to Immediate]
Proposed resolution:
This wording is relative to N3797.
Edit 31.7.9 [quoted.manip] as follows:
template <class charT> unspecified quoted(const charT* s, charT delim=charT('"'), charT escape=charT('\\')); template <class charT, class traits, class Allocator> unspecified quoted(const basic_string<charT, traits, Allocator>& s, charT delim=charT('"'), charT escape=charT('\\'));-2- Returns: An object of unspecified type such that if
out
is an instance ofbasic_ostream
with member typechar_type
the same ascharT
, then the expressionout << quoted(s, delim, escape)
behaves asif it inserts the following characters intoa formatted output function (31.7.6.3.1 [ostream.formatted.reqmts]) ofout
using character inserter function templates (27.7.3.6.4), which may throwios_base::failure
(27.5.3.1.1)out
. This forms a character sequenceseq
, initially consisting of the following elements:
delim
.Each character in
s
. If the character to be output is equal to escape ordelim
, as determined byoperator==
, first outputescape
.
delim
.Let
x
be the number of elements initially inseq
. Then padding is determined forseq
as described in 31.7.6.3.1 [ostream.formatted.reqmts],seq
is inserted as if by callingout.rdbuf()->sputn(seq, n)
, wheren
is the larger ofout.width()
andx
, andout.width(0)
is called. The expressionout << quoted(s, delim, escape)
shall have typebasic_ostream<charT, traits>&
and valueout
.
integral_constant
's member functions should be marked noexcept
Section: 21.3.4 [meta.help] Status: C++14 Submitter: Stephan T. Lavavej Opened: 2013-11-05 Last modified: 2017-07-05
Priority: 0
View all other issues in [meta.help].
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Discussion:
Obvious.
[Issaquah 2014-02-11: Move to Immediate]
Proposed resolution:
This wording is relative to N3797.
Edit 21.3.4 [meta.help] as indicated:
namespace std { template<class T, T v> struct integral_constant { static constexpr T value = v; typedef T value_type; typedef integral_constant<T,v> type; constexpr operator value_type() const noexcept { return value; } constexpr value_type operator()() const noexcept { return value; } }; […] }
Section: 31.7.5.3.1 [istream.formatted.reqmts] Status: Resolved Submitter: Zhihao Yuan Opened: 2013-12-06 Last modified: 2022-11-22
Priority: 3
View all other issues in [istream.formatted.reqmts].
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Discussion:
The formatted input function requirement says in 31.7.5.3.1 [istream.formatted.reqmts]:
"If an exception is thrown during input then
ios::badbit
is turned on in*this
's error state. If(exceptions()&badbit) != 0
then the exception is rethrown."
while some formatted function may throw an exception from basic_ios::clear
, for example
in 22.9.4 [bitset.operators] p6:
"If no characters are stored in
str
, callsis.setstate(ios_base::failbit)
(which may throwios_base::failure
)"
So should this exception be considered as "an exception [...] thrown during input"? And here is an implementation divergence (or you can read the following as "a bug libc++ only has" :)
cin.exceptions(ios_base::failbit); bitset<N> b; try { cin >> b; // type 'a' and return } catch (...) {}
Now cin.rdstate()
is just failbit
in libstdc++ (and Dinkumware, by
PJ), but failbit & badbit
libc++. Similar difference found in other
places, like eofbit & badbid
after std::getline
.
badbit
+ rethrow) is
"to signify an exception arising in user code, not the iostreams package".
In addition, I found the following words in unformatted input
function's requirements (31.7.5.4 [istream.unformatted]):
If an exception is thrown during input then
ios::badbit
is turned on in*this
's error state. (Exceptions thrown frombasic_ios<>::clear()
are not caught or rethrown.) If(exceptions()&badbit) != 0
then the exception is rethrown.
The content within the parenthesis is added by LWG defect 61(i), and does fix the ambiguity. However, it only fixed the 1 of 4 requirements, and it lost some context (the word "rethrown" is not seen before this sentence within this section).
[Lenexa 2015-05-07: Marshall to research and report]
[Kona 2022-11-08; this would be resolved by P1264]
[2022-11-22 Resolved by P1264R2 accepted in Kona. Status changed: Open → Resolved.]
Proposed resolution:
This wording is relative to N3797.
[Drafting note: The editor is kindly asked to introduce additional spaces at the following marked occurrences ofoperator&
— end drafting note]
Modify 31.7.5.3.1 [istream.formatted.reqmts] p1 as indicated:
-1- Each formatted input function begins execution by constructing an object of class
sentry
with thenoskipws
(second) argument false. If thesentry
object returns true, when converted to a value of typebool
, the function endeavors to obtain the requested input. If an exception, other than the ones thrown fromclear()
, if any, is thrown during input thenios::badbit
is turned on[Footnote 314] in*this
's error state. If(exceptions() & badbit) != 0
then the exception is rethrown. In any case, the formatted input function destroys thesentry
object. If no exception has been thrown, it returns*this
.
Modify 31.7.6.3.1 [ostream.formatted.reqmts] p1 as indicated:
-1- Each formatted output function begins execution by constructing an object of class
sentry
. If this object returns true when converted to a value of typebool
, the function endeavors to generate the requested output. If the generation fails, then the formatted output function doessetstate(ios_base::failbit)
, which might throw an exception. If an exception, other than the ones thrown fromclear()
, if any, is thrown during output, thenios::badbit
is turned on[Footnote 327] in*this
's error state. If(exceptions() & badbit) != 0
then the exception is rethrown. Whether or not an exception is thrown, thesentry
object is destroyed before leaving the formatted output function. If no exception is thrown, the result of the formatted output function is*this
.
Modify 31.7.6.4 [ostream.unformatted] p1 as indicated:
-1- Each unformatted output function begins execution by constructing an object of class
sentry
. If this object returns true, while converting to a value of typebool
, the function endeavors to generate the requested output. If an exception, other than the ones thrown fromclear()
, if any, is thrown during output, then ios::badbit is turned on[Footnote 330] in*this
's error state. If(exceptions() & badbit) != 0
then the exception is rethrown. In any case, the unformatted output function ends by destroying thesentry
object, then, if no exception was thrown, returning the value specified for the unformatted output function.
Modify 31.7.5.4 [istream.unformatted] p1 as indicated:
-1- Each unformatted input function begins execution by constructing an object of class
sentry
with the default argumentnoskipws
(second) argument true. If thesentry
object returns true, when converted to a value of typebool
, the function endeavors to obtain the requested input. Otherwise, if thesentry
constructor exits by throwing an exception or if the sentry object returns false, when converted to a value of typebool
, the function returns without attempting to obtain any input. In either case the number of extracted characters is set to0
; unformatted input functions taking a character array of non-zero size as an argument shall also store a null character (usingcharT()
) in the first location of the array. If an exception, other than the ones thrown fromclear()
, if any, is thrown during input thenios::badbit
is turned on[Footnote 317] in*this
's error state.(Exceptions thrown fromIfbasic_ios<>::clear()
are not caught or rethrown.)(exceptions() & badbit) != 0
then the exception is rethrown. It also counts the number of characters extracted. If no exception has been thrown it ends by storing the count in a member object and returning the value specified. In any event thesentry
object is destroyed before leaving the unformatted input function.
min
, max
, and minmax
should be constexpr
Section: 26.8.9 [alg.min.max] Status: C++14 Submitter: Ville Voutilainen Opened: 2013-12-15 Last modified: 2016-01-28
Priority: 1
View other active issues in [alg.min.max].
View all other issues in [alg.min.max].
View all issues with C++14 status.
Discussion:
Having min
, max
, and minmax
constexpr
was a large part of the motivation to allow reference-to-const arguments for
constexpr
functions as per
N3039.
Furthermore, initializer_lists
are immutable and not-movable-from
for large part in order to allow using them in constexpr
contexts
and other hoisting-optimizations. In N3797
version of the draft none of these functions are constexpr
, and they should be made
constexpr
.
Proposed resolution:
This wording is relative to N3797.
In 26.1 [algorithms.general], header <algorithm>
synopsis, and 26.8.9 [alg.min.max],
change as indicated (add constexpr
to every signature before min_element
):
template<class T> constexpr const T& min(const T& a, const T& b); template<class T, class Compare> constexpr const T& min(const T& a, const T& b, Compare comp); […] template<class T> constexpr T min(initializer_list<T> t); template<class T, class Compare> constexpr T min(initializer_list<T> t, Compare comp); […] template<class T> constexpr const T& max(const T& a, const T& b); template<class T, class Compare> constexpr const T& max(const T& a, const T& b, Compare comp); […] template<class T> constexpr T max(initializer_list<T> t); template<class T, class Compare> constexpr T max(initializer_list<T> t, Compare comp); […] template<class T> constexpr pair<const T&, const T&> minmax(const T& a, const T& b); template<class T, class Compare> constexpr pair<const T&, const T&> minmax(const T& a, const T& b, Compare comp); […] template<class T> constexpr pair<T, T> minmax(initializer_list<T> t); template<class T, class Compare> constexpr pair<T, T> minmax(initializer_list<T> t, Compare comp);
std::next
is over-constrainedSection: 24.4.3 [iterator.operations] Status: C++17 Submitter: Eric Niebler Opened: 2013-12-24 Last modified: 2017-09-07
Priority: 4
View other active issues in [iterator.operations].
View all other issues in [iterator.operations].
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Discussion:
In LWG 1011(i), std::next
and std::prev
were changed
from accepting InputIterator
to accepting
ForwardIterator
. This needlessly excludes perfectly legitimate use
cases. Consider the following hypothetical range-based implementation of
drop, which creates a view of a range without the first n
elements:
template<typename Distance, typename InputRange> iterator_range<range_iterator_t<InputRange>> drop(Distance n, InputRange& rng) { return make_iterator_range( std::next(std::begin(rng), n), std::end(rng) ); }
I believe this to be a legitimate use case that is currently outlawed by the standard without cause. See the discussion beginning at c++std-lib-35313 for an in-depth discussion of the issue, in which Howard Hinnant agreed that it was a defect.
(Some discussion then ensued about whether an overload should be added that only accepts rvalueInputIterators
to avoid the surprise that issue
1011(i) sought to address. I make no such attempt, nor do I believe it to
be necessary.)
Suggested resolution:
Back out the resolution of 1011(i).
[Lenexa 2015-05-07: Move to Ready]
Proposed resolution:
This wording is relative to N3797.
Change 24.2 [iterator.synopsis], header <iterator>
synopsis, and 24.4.3 [iterator.operations]
before p.6 as indicated:
template <classForwardInputIterator>ForwardInputIterator next(ForwardInputIterator x, typename std::iterator_traits<ForwardInputIterator>::difference_type n = 1);
Section: 23.4.3.1 [map.overview], 23.4.4.1 [multimap.overview], 23.5.3.1 [unord.map.overview], 23.5.4.1 [unord.multimap.overview] Status: C++17 Submitter: Geoffrey Romer Opened: 2014-01-08 Last modified: 2017-10-13
Priority: 2
View all other issues in [map.overview].
View all issues with C++17 status.
Discussion:
The rvalue-reference insert()
members of map
, multimap
, unordered_map
, and
unordered_multimap
are specified as function templates, where the rvalue-reference parameter type
depends on the template parameter.
As a consequence, these overloads cannot be invoked via braced-initializer syntax (e.g. my_map.insert({key, value})
),
because the template argument cannot be deduced from a braced-init-list. Such calls instead resolve to the
const lvalue reference overload, which forces a non-elidable copy of the argument, despite the fact that the
argument is an rvalue, and so should be eligible for moving and copy elision.
value_type&&
overload for each affected
member template. Simply declaring these members in the class synopses should be sufficient; their semantics are
already dictated by the container concepts (c.f. the corresponding lvalue-reference overloads, which have no
additional discussion beyond being listed in the synopsis).
[2014-02-13 Issaquah]
AJM: Is this not better solved by emplace
?
Nico: emplace
was a mistake, it breaks a uniform pattern designed into the STL.
Hence, this fix is important, it should be the preferred way to do this.
JonW: emplace
is still more efficient, as this form must make a non-elidable copy.
GeoffR: Also, cannot move from a const
key, must always make a copy.
Poll for adopting the proposed wording:
SF: 1 WF: 4 N: 4 WA: 1 SA: 0Move to Ready, pending implementation experience.
Proposed resolution:
This wording is relative to N3797.
Change 23.4.3.1 [map.overview], class template map
synopsis, as indicated:
[…] pair<iterator, bool> insert(const value_type& x); pair<iterator, bool> insert(value_type&& x); template <class P> pair<iterator, bool> insert(P&& x); iterator insert(const_iterator position, const value_type& x); iterator insert(const_iterator position, value_type&& x); template <class P> iterator insert(const_iterator position, P&&); […]
Change 23.4.4.1 [multimap.overview], class template multimap
synopsis, as indicated:
[…] iterator insert(const value_type& x); iterator insert(value_type&& x); template <class P> iterator insert(P&& x); iterator insert(const_iterator position, const value_type& x); iterator insert(const_iterator position, value_type&& x); template <class P> iterator insert(const_iterator position, P&& x); […]
Change 23.5.3.1 [unord.map.overview], class template unordered_map
synopsis, as indicated:
[…] pair<iterator, bool> insert(const value_type& obj); pair<iterator, bool> insert(value_type&& obj); template <class P> pair<iterator, bool> insert(P&& obj); iterator insert(const_iterator hint, const value_type& obj); iterator insert(const_iterator hint, value_type&& obj); template <class P> iterator insert(const_iterator hint, P&& obj); […]
Change 23.5.4.1 [unord.multimap.overview], class template unordered_multimap
synopsis, as indicated:
[…] iterator insert(const value_type& obj); iterator insert(value_type&& obj); template <class P> iterator insert(P&& obj); iterator insert(const_iterator hint, const value_type& obj); iterator insert(const_iterator hint, value_type&& obj); template <class P> iterator insert(const_iterator hint, P&& obj); […]
Section: 23.2.8 [unord.req] Status: C++14 Submitter: Joaquín M López Muñoz Opened: 2014-01-21 Last modified: 2016-01-28
Priority: 2
View other active issues in [unord.req].
View all other issues in [unord.req].
View all issues with C++14 status.
Discussion:
Issue 518(i) resolution for unordered associative containers, modelled after that of issue 371(i), which is related to associative containers, states that insertion, erasure and rehashing preserve the relative ordering of equivalent elements. Unfortunately, this is not sufficient to guarantee the validity of code such as this:
std::unordered_multimap<int, int> m; auto i = m.begin(); while (i != m.end()) { if (pred(i)) m.erase(i++); else ++i; }
(which is a direct translation from multimap
to unordered_multimap
of the motivating example in 371(i)),
or even this:
std::unordered_multimap<int, int> m; auto p = m.equal_range(k); while (p.first != p.second) { if (pred(p.first)) m.erase((p.first)++); else ++(p.first); }
because the relative ordering of non-equivalent elements elements could potentially change after erasure (not that any actual implementation does that, anyway). Such an underspecification does not happen for regular associative containers, where the relative ordering of non-equivalent elements is kept by design.
[2014-02-13 Issaquah: Move to Immediate]
Proposed resolution:
This wording is relative to N3797.
Modify 23.2.8 [unord.req], p14 as indicated:
-14- The
insert
andemplace
members shall not affect the validity of references to container elements, but may invalidate all iterators to the container. Theerase
members shall invalidate only iterators and references to the erased elements, and preserve the relative order of the elements that are not erased.
Assignable
" requirementSection: 26.8.5 [alg.partitions] Status: C++14 Submitter: Daniel Krügler Opened: 2014-02-01 Last modified: 2017-07-05
Priority: 0
View all other issues in [alg.partitions].
View all issues with C++14 status.
Discussion:
The Requires element of partition_copy
says (emphasis mine):
Requires:
InputIterator
's value type shall beAssignable
, and …
The C++03 term Assignable
was replaced by CopyAssignable
, remaining cleanups happened via LWG issue
972(i), but algorithm partition_copy
was not affected at that time (during that time the requirements
of partition_copy
didn't mention writable nor assignable, but I cannot track down at the moment where these requirements
had been added). Presumably this requirement should be corrected similarly to the approach used in 972(i).
CopyAssignable
is needed here, given the fact that we already require "writable to" an
OutputIterator
which is defined in 24.3.1 [iterator.requirements.general] and does already impose the necessary
statement
*out = *in;
Given the fact that partition_copy
never touches any input value twice, there is no reason why anything more than
writable to should be necessary.
[Issaquah 2014-02-11: Move to Immediate]
Proposed resolution:
This wording is relative to N3797.
Modify 26.8.5 [alg.partitions], p12 as indicated:
-12- Requires:
InputIterator
's value type shall beCopyAssignable
, and shall be writable to theout_true
andout_false
OutputIterator
s, and shall be convertible toPredicate
's argument type. The input range shall not overlap with either of the output ranges.
regex_constants::nosubs
affect basic_regex::mark_count()
?Section: 28.6.4.2 [re.synopt] Status: C++14 Submitter: Jonathan Wakely Opened: 2014-02-01 Last modified: 2017-09-07
Priority: 0
View all other issues in [re.synopt].
View all issues with C++14 status.
Discussion:
As discussed in c++std-lib-35399 and its replies,
I can see two possible interpretations of the effects of regex_constants::nosubs
:
The effect of nosubs
only applies during matching. Parentheses are
still recognized as marking a sub-expression by the basic_regex
compiler, and basic_regex::mark_count()
still returns the number of
marked sub-expressions, but anything they match is not stored in the
results. This means it is not always true that results.size() == r.mark_count() + 1
for a successful match.
nosubs
affects how a regular expression is compiled, altering the
state of the std::basic_regex
object so that mark_count()
returns
zero. This also affects any subsequent matching.
The definition of nosubs
should make this clear.
nosubs
only has
effects during matching, which is (1), but all known implementations
do (2). John Maddock confirmed that (2) was intended.
[Issaquah 2014-02-12: Move to Immediate]
Proposed resolution:
This wording is relative to N3797.
Apply the following edit to the table in 28.6.4.2 [re.synopt]/1
Specifies that no sub-expressions shall be considered to be marked, so that when a regular expression is matched against a character container sequence, no sub-expression matches shall be stored in the supplied
match_results
structure.
reverse_iterator::operator*()
is unimplementableSection: 24.5.1.2 [reverse.iterator] Status: C++14 Submitter: Stephan T. Lavavej Opened: 2014-02-07 Last modified: 2014-02-27
Priority: 1
View all other issues in [reverse.iterator].
View all issues with C++14 status.
Discussion:
Previously, C++03 24.4.1.3.3 [lib.reverse.iter.op.star] required:
reference operator*() const;Effects:
Iterator tmp = current; return *--tmp;
Now, N3797 24.5.1.1 [reverse.iterator] depicts:
private: Iterator deref_tmp; // exposition only };
And 24.5.1.3.4 [reverse.iter.op.star] requires:
reference operator*() const;Effects:
deref_tmp = current; --deref_tmp; return *deref_tmp;[Note: This operation must use an auxiliary member variable rather than a temporary variable to avoid returning a reference that persists beyond the lifetime of its associated iterator. (See 24.2.) — end note]
As written, this won't compile, because operator*()
is const
yet it's modifying (via assignment and decrement)
the deref_tmp
data member. So what happens if you say "mutable Iterator deref_tmp;
"?
const
member functions to be callable from multiple threads simultaneously. This is
16.4.6.10 [res.on.data.races]/3: "A C++ standard library function shall not directly or indirectly modify objects (1.10)
accessible by threads other than the current thread unless the objects are accessed directly or indirectly via the function's
non-const arguments, including this
."
Multiple threads simultaneously modifying deref_tmp
will trigger data races, so both mutable
and some form of
synchronization (e.g. mutex
or atomic
) are actually necessary!
Here's what implementations currently do: Dinkumware/VC follows C++03 and doesn't use deref_tmp
(attempting to
implement it is what led me to file this issue). According to Jonathan Wakely, libstdc++ also follows C++03 (see
PR51823 which is suspended until LWG 2204(i) is resolved). According to
Marshall Clow, libc++ uses deref_tmp
with mutable
but without synchronization, so it can trigger data races.
This deref_tmp
Standardese was added by LWG 198(i) "Validity of pointers and references unspecified after
iterator destruction" and is present in Working Papers going back to N1638
on April 11, 2004, long before C++ recognized the existence of multithreading and developed the "const
means simultaneously
readable" convention.
A related issue is LWG 1052(i) "reverse_iterator::operator->
should also support smart pointers" which
mentioned the need to depict mutable
in the Standardese, but it was resolved NAD Future and no change was made.
Finally, LWG 2204(i) "reverse_iterator
should not require a second copy of the base iterator" talked about
removing deref_tmp
, but without considering multithreading.
I argue that deref_tmp
must be removed. Its existence has highly undesirable consequences: either no synchronization
is used, violating the Standard's usual multithreading guarantees, or synchronization is used, adding further costs for all
users that benefit almost no iterators.
deref_tmp
is attempting to handle iterators that return references to things "inside themselves", which I usually call
"stashing iterators" (as they have a secret stash). Note that these are very unusual, and are different from proxy iterators like
vector<bool>::iterator
. While vector<bool>::iterator
's operator*()
does not return a true
reference, it refers to a bit that is unrelated to the iterator's lifetime.
[2014-02-14 Issaquah meeting: Move to Immediate]
Strike superfluous note to avoid potential confusion, and move to Immediate.
Proposed resolution:
This wording is relative to N3797.
Change class template reverse_iterator
synopsis, 24.5.1.2 [reverse.iterator], as indicated:
[…] protected: Iterator current;private: Iterator deref_tmp; // exposition only};
Change [reverse.iter.op.star] as indicated:
reference operator*() const;-1- Effects:
deref_tmp = current; --deref_tmp; return *deref_tmp;Iterator tmp = current; return *--tmp;
-2- [Note: This operation must use an auxiliary member variable rather than a temporary variable to avoid returning a reference that persists beyond the lifetime of its associated iterator. (See 24.2.) — end note]
Section: 20.3.1.3 [unique.ptr.single], 20.2.3.2 [pointer.traits.types], 20.2.8.1 [allocator.uses.trait], 20.2.9.2 [allocator.traits.types], 23.2.4 [sequence.reqmts] Status: C++17 Submitter: Jonathan Wakely Opened: 2014-02-14 Last modified: 2017-07-30
Priority: Not Prioritized
View other active issues in [unique.ptr.single].
View all other issues in [unique.ptr.single].
View all issues with C++17 status.
Discussion:
LWG 2299(i) addressed a N.B. comment pointing out that recently added wording about a type existing was not clear what happens if the type exists but is inaccessible. There are 16 pre-existing uses of the same language in the library that should use the same wording used to resolve 2299.
The relevant paragraphs are:
20.3.1.3 [unique.ptr.single]
20.2.3.2 [pointer.traits.types] 20.2.8.1 [allocator.uses.trait] 20.2.9.2 [allocator.traits.types] 23.2.4 [sequence.reqmts][2014-05-16, Daniel provides wording]
[2014-05-18 Library reflector vote]
The issue has been identified as Tentatively Ready based on six votes in favour.
Proposed resolution:
This wording is relative to N3936.
Change 20.2.3.2 [pointer.traits.types] as indicated:
typedef see below element_type;-1- Type:
Ptr::element_type
ifsuch a type existsthe qualified-idPtr::element_type
is valid and denotes a type (13.10.3 [temp.deduct]); otherwise,T
ifPtr
is a class template instantiation of the formSomePointer<T, Args>
, whereArgs
is zero or more type arguments; otherwise, the specialization is ill-formed.typedef see below difference_type;-2- Type:
Ptr::difference_type
ifsuch a type existsthe qualified-idPtr::difference_type
is valid and denotes a type (13.10.3 [temp.deduct]); otherwise,std::ptrdiff_t
.template <class U> using rebind = see below;-3- Alias template:
Ptr::rebind<U>
ifsuch a type existsthe qualified-idPtr::rebind<U>
is valid and denotes a type (13.10.3 [temp.deduct]); otherwise,SomePointer<U, Args>
ifPtr
is a class template instantiation of the formSomePointer<T, Args>
, whereArgs
is zero or more type arguments; otherwise, the instantiation ofrebind
is ill-formed.
Change 20.2.8.1 [allocator.uses.trait] p1 as indicated:
template <class T, class Alloc> struct uses_allocator;-1- Remarks: automatically detects whether
T
has a nestedallocator_type
that is convertible fromAlloc
. Meets theBinaryTypeTrait
requirements (20.10.1). The implementation shall provide a definition that is derived fromtrue_type
ifa typethe qualified-idT::allocator_type
existsis valid and denotes a type (13.10.3 [temp.deduct]) andis_convertible<Alloc, T::allocator_type>::value != false
, otherwise it shall be derived fromfalse_type
. […]
Change 20.2.9.2 [allocator.traits.types] as indicated:
typedef see below pointer;-1- Type:
Alloc::pointer
ifsuch a type existsthe qualified-idAlloc::pointer
is valid and denotes a type (13.10.3 [temp.deduct]); otherwise,value_type*
.typedef see below const_pointer;-2- Type:
Alloc::const_pointer
ifsuch a type existsthe qualified-idAlloc::const_pointer
is valid and denotes a type (13.10.3 [temp.deduct]); otherwise,pointer_traits<pointer>::rebind<const value_type>
.typedef see below void_pointer;-3- Type:
Alloc::void_pointer
ifsuch a type existsthe qualified-idAlloc::void_pointer
is valid and denotes a type (13.10.3 [temp.deduct]); otherwise,pointer_traits<pointer>::rebind<void>
.typedef see below const_void_pointer;-4- Type:
Alloc::const_void_pointer
ifsuch a type existsthe qualified-idAlloc::const_void_pointer
is valid and denotes a type (13.10.3 [temp.deduct]); otherwise,pointer_traits<pointer>::rebind<const void>
.typedef see below difference_type;-5- Type:
Alloc::difference_type
ifsuch a type existsthe qualified-idAlloc::difference_type
is valid and denotes a type (13.10.3 [temp.deduct]); otherwise,pointer_traits<pointer>::difference_type
.typedef see below size_type;-6- Type:
Alloc::size_type
ifsuch a type existsthe qualified-idAlloc::size_type
is valid and denotes a type (13.10.3 [temp.deduct]); otherwise,make_unsigned_t<difference_type>
.typedef see below propagate_on_container_copy_assignment;-7- Type:
Alloc::propagate_on_container_copy_assignment
ifsuch a type existsthe qualified-idAlloc::propagate_on_container_copy_assignment
is valid and denotes a type (13.10.3 [temp.deduct]); otherwise,false_type
.typedef see below propagate_on_container_move_assignment;-8- Type:
Alloc::propagate_on_container_move_assignment
ifsuch a type existsthe qualified-idAlloc::propagate_on_container_move_assignment
is valid and denotes a type (13.10.3 [temp.deduct]); otherwise,false_type
.typedef see below propagate_on_container_swap;-9- Type:
Alloc::propagate_on_container_swap
ifsuch a type existsthe qualified-idAlloc::propagate_on_container_swap
is valid and denotes a type (13.10.3 [temp.deduct]); otherwise,false_type
.template <class T> using rebind_alloc = see below;-10- Alias template:
Alloc::rebind<T>::other
ifsuch a type existsthe qualified-idAlloc::rebind<T>::other
is valid and denotes a type (13.10.3 [temp.deduct]); otherwise,Alloc<T, Args>
ifAlloc
is a class template instantiation of the formAlloc<U, Args>
, whereArgs
is zero or more type arguments; otherwise, the instantiation ofrebind_alloc
is ill-formed.
Change 20.3.1.3 [unique.ptr.single] p3 as indicated:
-3- If the
typequalified-idremove_reference_t<D>::pointer
existsis valid and denotes a type (13.10.3 [temp.deduct]), thenunique_ptr<T, D>::pointer
shall be a synonym forremove_reference_t<D>::pointer
. […]
Change 23.2.4 [sequence.reqmts] p3 as indicated:
-3- In Tables 100 and 101,
X
denotes a sequence container class,a
denotes a value ofX
containing elements of typeT
,A
denotesX::allocator_type
ifit existsthe qualified-idX::allocator_type
is valid and denotes a type (13.10.3 [temp.deduct]) andstd::allocator<T>
if it doesn't, […]
Section: 32.6.4.5.2 [thread.sharedtimedmutex.class] Status: Resolved Submitter: Richard Smith Opened: 2014-02-16 Last modified: 2021-05-18
Priority: 2
View all issues with Resolved status.
Discussion:
32.6.4.5.2 [thread.sharedtimedmutex.class] paragraph 2:
The class
shared_timed_mutex
shall satisfy all of theSharedTimedMutex
requirements (30.4.1.4). It shall be a standard layout class (Clause 9).
There's no SharedTimedMutex
requirements; this name doesn't appear anywhere else in the standard. (Prior to N3891,
this was SharedMutex
, which was equally undefined.)
SharedMutex
or
SharedTimedMutex
were defined it could be reused here).
[2014-05-22, Daniel comments]
As for SharedTimedMutex
, there exists a similar problem in regard to TimedMutex
referred to in
32.6.4.3.2 [thread.timedmutex.class] p2 and in 32.6.4.3.3 [thread.timedmutex.recursive] p2, but nowhere defined.
mutex
shall satisfy all the
Mutex
requirements (32.6.4 [thread.mutex.requirements]).", but there are no concrete Mutex
requirements,
32.6.4 [thread.mutex.requirements] — titled as "Mutex requirements" — describes mutex types,
timed mutex types, and shared timed mutex types.
[2014-06-08, Daniel comments and provides wording]
The presented wording adds to the existing mutex types, timed mutex types, and shared timed mutex types
terms a new set of corresponding MutexType
, TimedMutexType
, and SharedTimedMutexType
requirements.
Lockable
requirements, which are not restricted to a explicitly enumerated set of library types). Second, using
**MutexType
over **Mutex
provides the additional advantage that it reduces the chances of confusing named
requirements from template parameters named Mutex
(such as for unique_lock
or shared_lock
).
Nonetheless the here presented wording has one unfortunate side-effect: Once applied it would have the effect that types
used to instantiate std::shared_lock
cannot be user-defined shared mutex types due to 32.6.5.5 [thread.lock.shared].
The reason is based on the currently lack of an existing SharedLockable
requirement set, which would complete the
existing BasicLockable
and Lockable
requirements (which are "real" requirements). This restriction is not
actually a problem introduced by the provided resolution but instead one that existed before but becomes more obvious now.
[2015-02 Cologne]
Handed over to SG1.
[2015-05 Lenexa, SG1 response]
Thanks to Daniel, and please put it in SG1-OK status. Perhaps open another issue for the remaining problem Daniel points out?
[2015-10 pre-Kona]
SG1 hands this over to LWG for wording review
[2015-10-21 Kona, Daniel comments and adjusts wording to to untimed shared mutex types]
The new wording reflects the addition of the new shared mutex types. The approach used for shared_lock
is similar to the one used for unique_lock
: The template argument Mutex
has a reduced requirement set that is not
sufficient for all operations. Only those members that require stronger requirements of SharedTimedMutexType
specify that additionally in the Requires element of the corresponding prototype specifications.
SharedLockable
and SharedTimedLockable
types which could be satisfied by user-provided types as well, because the
SharedMutexType
and SharedTimedMutexType
requirements are essentially restricted to an enumerated set of
types provided by the Standard Library. But this extension seemed too large for this issue and can be easily fixed later
without any harm.
Previous resolution [SUPERSEDED]:
This wording is relative to N3936.
Change 32.6.4.2 [thread.mutex.requirements.mutex] as indicated:
-1- The mutex types are the standard library types
std::mutex
,std::recursive_mutex
,std::timed_mutex
,std::recursive_timed_mutex
, andstd::shared_timed_mutex
. They shall meet theMutexType
requirements set out in this section. In this description,m
denotes an object of a mutex type.Change 32.6.4.2.2 [thread.mutex.class] as indicated:
-3- The class
mutex
shall satisfy all theMutexType
requirements (32.6.4.2 [thread.mutex.requirements.mutex]32.6.4 [thread.mutex.requirements]). It shall be a standard-layout class (Clause 9).Change 32.6.4.2.3 [thread.mutex.recursive] as indicated:
-2- The class
recursive_mutex
shall satisfy all theMutexMutexType
requirements (32.6.4.2 [thread.mutex.requirements.mutex]32.6.4 [thread.mutex.requirements]). It shall be a standard-layout class (Clause 9).Change 32.6.4.3 [thread.timedmutex.requirements] as indicated:
-1- The timed mutex types are the standard library types
std::timed_mutex
,std::recursive_timed_mutex
, andstd::shared_timed_mutex
. They shall meet theTimedMutexType
requirements set out below. In this description,m
denotes an object of a mutex type,rel_time
denotes an object of an instantiation ofduration
(20.12.5), andabs_time
denotes an object of an instantiation oftime_point
(20.12.6).Change 32.6.4.3.2 [thread.timedmutex.class] as indicated:
-2- The class
timed_mutex
shall satisfy all of theTimedMutexType
requirements (32.6.4.3 [thread.timedmutex.requirements]). It shall be a standard-layout class (Clause 9).Change 32.6.4.3.3 [thread.timedmutex.recursive] as indicated:
-2- The class
recursive_timed_mutex
shall satisfy all of theTimedMutexType
requirements (32.6.4.3 [thread.timedmutex.requirements]). It shall be a standard-layout class (Clause 9).Change 32.6.4.5 [thread.sharedtimedmutex.requirements] as indicated: [Drafting note: The reference to the timed mutex types requirements has been moved after introducing the new requirement set to ensure that
SharedTimedMutexType
refineTimedMutexType
.]-1- The standard library type
-?- The shared timed mutex types shall meet thestd::shared_timed_mutex
is a shared timed mutex type. Shared timed mutex types shall meet theSharedTimedMutexType
requirementsof timed mutex types (32.6.4.3 [thread.timedmutex.requirements]), and additionally shall meet the requirementsset out below. In this description,m
denotes an object of a mutex type,rel_type
denotes an object of an instantiation ofduration
(20.12.5), andabs_time
denotes an object of an instantiation oftime_point
(20.12.6).TimedMutexType
requirements (32.6.4.3 [thread.timedmutex.requirements]).Change 32.6.4.5.2 [thread.sharedtimedmutex.class] as indicated:
-2- The class
shared_timed_mutex
shall satisfy all of theSharedTimedMutexType
requirements (32.6.4.5 [thread.sharedtimedmutex.requirements]). It shall be a standard-layout class (Clause 9).Change 32.6.5.5 [thread.lock.shared] as indicated: [Drafting note: Once N3995 has been applied, the following reference should be changed to the new
SharedMutexType
requirements ([thread.sharedmutex.requirements]) or even better to some newSharedLockable
requirements (to be defined) — end drafting note]-1- […] The supplied
-2- [Note:Mutex
type shall meet theshared mutexSharedTimedMutexType
requirements (32.6.4.5 [thread.sharedtimedmutex.requirements]).shared_lock<Mutex>
meets theTimedLockable
requirements (30.2.5.4). — end note]
[2016-02 Jacksonville]
Marshall to review wording.
[2018-08-23 Batavia Issues processing]
Tim to redraft.
[2021-05-18 Resolved by the adoption of P2160R1 at the February 2021 plenary. Status changed: Open → Resolved.]
Proposed resolution:
This wording is relative to N4527.
Change 32.6.4.2 [thread.mutex.requirements.mutex] as indicated:
-1- The mutex types are the standard library types
-2- The mutex types shall meet thestd::mutex
,std::recursive_mutex
,std::timed_mutex
,std::recursive_timed_mutex
,std::shared_mutex
, andstd::shared_timed_mutex
. They shall meet theMutexType
requirements set out in this section. In this description,m
denotes an object of a mutex type.Lockable
requirements (32.2.5.3 [thread.req.lockable.req]).
Change 32.6.4.2.2 [thread.mutex.class] as indicated:
-3- The class
mutex
shall satisfy all theMutexType
requirements (32.6.4.2 [thread.mutex.requirements.mutex]32.6.4 [thread.mutex.requirements]). It shall be a standard-layout class (Clause 9).
Change 32.6.4.2.3 [thread.mutex.recursive] as indicated:
-2- The class
recursive_mutex
shall satisfy all theMutexMutexType
requirements (32.6.4.2 [thread.mutex.requirements.mutex]32.6.4 [thread.mutex.requirements]). It shall be a standard-layout class (Clause 9).
Change 32.6.4.3 [thread.timedmutex.requirements] as indicated:
-1- The timed mutex types are the standard library types
-2- The timed mutex types shall meet thestd::timed_mutex
,std::recursive_timed_mutex
, andstd::shared_timed_mutex
. They shall meet theTimedMutexType
requirements set out below. In this description,m
denotes an object of a mutex type,rel_time
denotes an object of an instantiation ofduration
(20.12.5), andabs_time
denotes an object of an instantiation oftime_point
(20.12.6).TimedLockable
requirements (32.2.5.4 [thread.req.lockable.timed]).
Change 32.6.4.3.2 [thread.timedmutex.class] as indicated:
-2- The class
timed_mutex
shall satisfy all of theTimedMutexType
requirements (32.6.4.3 [thread.timedmutex.requirements]). It shall be a standard-layout class (Clause 9).
Change 32.6.4.3.3 [thread.timedmutex.recursive] as indicated:
-2- The class
recursive_timed_mutex
shall satisfy all of theTimedMutexType
requirements (32.6.4.3 [thread.timedmutex.requirements]). It shall be a standard-layout class (Clause 9).
Change 32.6.4.4 [thread.sharedmutex.requirements] as indicated: [Drafting note: The reference to the
mutex types requirements has been moved after introducing the new requirement set to ensure that
SharedMutexType
refines MutexType
.]
-1- The standard library types
-?- The shared mutex types shall meet thestd::shared_mutex
andstd::shared_timed_mutex
are shared mutex types. Shared mutex types shall meet theSharedMutexType
requirementsof mutex types (32.6.4.2 [thread.mutex.requirements.mutex]), and additionally shall meet the requirementsset out below. In this description,m
denotes an object of a shared mutex type.MutexType
requirements (32.6.4.2 [thread.mutex.requirements.mutex]).
Change 32.6.4.4.2 [thread.sharedmutex.class] as indicated:
-2- The class
shared_mutex
shall satisfy all of theSharedMutexType
requirementsfor shared mutexes(32.6.4.4 [thread.sharedmutex.requirements]). It shall be a standard-layout class (Clause 9).
Change 32.6.4.5 [thread.sharedtimedmutex.requirements] as indicated: [Drafting note: The reference to the
timed mutex types requirements has been moved after introducing the new requirement set to ensure that
SharedTimedMutexType
refines TimedMutexType
and SharedMutexType
.]
-1- The standard library type
-?- The shared timed mutex types shall meet thestd::shared_timed_mutex
is a shared timed mutex type. Shared timed mutex types shall meet theSharedTimedMutexType
requirementsof timed mutex types (32.6.4.3 [thread.timedmutex.requirements]), shared mutex types (32.6.4.4 [thread.sharedmutex.requirements]), and additionally shall meet the requirementsset out below. In this description,m
denotes an object of a shared timed mutex type,rel_type
denotes an object of an instantiation ofduration
(20.12.5), andabs_time
denotes an object of an instantiation oftime_point
(20.12.6).TimedMutexType
requirements (32.6.4.3 [thread.timedmutex.requirements]) and theSharedMutexType
requirements (32.6.4.4 [thread.sharedmutex.requirements]).
Change 32.6.4.5.2 [thread.sharedtimedmutex.class] as indicated:
-2- The class
shared_timed_mutex
shall satisfy all of theSharedTimedMutexType
requirementsfor shared timed mutexes(32.6.4.5 [thread.sharedtimedmutex.requirements]). It shall be a standard-layout class (Clause 9).
Change 32.6.5.5 [thread.lock.shared] as indicated:
-1- […] The supplied
-2- [Note:Mutex
type shall meet theshared mutexSharedMutexType
requirements (32.6.4.5 [thread.sharedtimedmutex.requirements]32.6.4.4 [thread.sharedmutex.requirements]).shared_lock<Mutex>
meets theTimedLockable
requirements (30.2.5.4). — end note]
Change 32.6.5.5.2 [thread.lock.shared.cons] as indicated:
template <class Clock, class Duration> shared_lock(mutex_type& m, const chrono::time_point<Clock, Duration>& abs_time);-14- Requires: The supplied
-15- Effects: Constructs an object of typeMutex
type shall meet theSharedTimedMutexType
requirements (32.6.4.5 [thread.sharedtimedmutex.requirements]). The calling thread does not own the mutex for any ownership mode.shared_lock
and callsm.try_lock_shared_until(abs_time)
. […]template <class Rep, class Period> shared_lock(mutex_type& m, const chrono::duration<Rep, Period>& rel_time);-17- Requires: The supplied
-18- Effects: Constructs an object of typeMutex
type shall meet theSharedTimedMutexType
requirements (32.6.4.5 [thread.sharedtimedmutex.requirements]). The calling thread does not own the mutex for any ownership mode.shared_lock
and callsm.try_lock_shared_for(rel_time)
. […]
Change 32.6.5.5.3 [thread.lock.shared.locking] as indicated:
template <class Clock, class Duration> bool try_lock_until(const chrono::time_point<Clock, Duration>& abs_time);-?- Requires: The supplied
-8- Effects:Mutex
type shall meet theSharedTimedMutexType
requirements (32.6.4.5 [thread.sharedtimedmutex.requirements]).pm->try_lock_shared_until(abs_time)
. […]template <class Rep, class Period> bool try_lock_for(const chrono::duration<Rep, Period>& rel_time);-?- Requires: The supplied
-12- Effects:Mutex
type shall meet theSharedTimedMutexType
requirements (32.6.4.5 [thread.sharedtimedmutex.requirements]).pm->try_lock_shared_for(rel_time)
. […]
deque
and vector
pop_back
don't specify iterator invalidation requirementsSection: 23.3.5.4 [deque.modifiers], 23.3.13.5 [vector.modifiers] Status: C++17 Submitter: Deskin Miller Opened: 2014-02-17 Last modified: 2017-07-30
Priority: 0
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Discussion:
I think it's obvious that vector::pop_back
invalidates the path-the-end iterator, but I cannot find language that says so to
my satisfaction in the Standard. N3797 23.2.4 [sequence.reqmts] Table 101 lists a.pop_back()
semantics as "Destroys the
last element", but nowhere do I see this required to invalidate the end iterator (or iterators previously referring to the last element).
[container.reqmts.general]/11 states "Unless otherwise specified (either explicitly or by defining a function in terms of
other functions), invoking a container member function or passing a container as an argument to a library function shall not
invalidate iterators to, or change the values of, objects within that container." 23.3.13.5 [vector.modifiers]/3 says that each
flavor of vector::erase
"Invalidates iterators and references at or after the point of the erase", but pop_back
isn't
discussed, and it wasn't specified in terms of erase
.
Similarly for std::deque
, 23.2.4 [sequence.reqmts] Table 101 and [container.reqmts.general]/11 both apply.
Yet 23.3.5.4 [deque.modifiers] likewise doesn't discuss pop_back
nor pop_front
. Furthermore paragraph 4 fails to
specify the iterator-invalidation guarantees when erasing the first element but not the last.
std::vector
and std::deque
are in contrast to std::list
, which says in 23.3.11.4 [list.modifiers]/3
regarding pop_back
(as well as all forms of erase
, pop_front
, and clear
) "Effects: Invalidates only
the iterators and references to the erased elements."
[2014-06-16 Jonathan comments and improves wording]
I believe this reflects our preferred form discussed earlier,
specifically putting the signatures with the erase
signatures, so that
the full specification of erase()
applies to the pop_xxx()
functions.
This covers the case for deque
where pop_front()
erases the only
element (which is both the first and last element).
pop_front
and pop_back
clearly covered by "erase operations"? I
believe so, as 23.3.5.1 [deque.overview]/1 and other places talk about "insert
and erase operations" which covers push
/pop
functions too. I've added
a note which could be used to clarify that if desired.
Previous resolution [SUPERSEDED]:
This wording is relative to N3936.
Change 23.3.5.4 [deque.modifiers] as indicated:
iterator erase(const_iterator position); iterator erase(const_iterator first, const_iterator last);-4- Effects: An erase operation that erases the last element of a deque invalidates only the past-the-end iterator and all iterators and references to the erased elements. An erase operation that erases the first element of a deque but not the last element invalidates only iterators and references to the erased elements. An erase operation that erases neither the first element nor the last element of a deque invalidates the past-the-end iterator and all iterators and references to all the elements of the deque.
-5- […] -6- […]void pop_front(); void pop_back();-?- Effects:
pop_front
invalidates iterators and references to the first element of thedeque
.pop_back
invalidates the past-the-end iterator, and all iterators and references to the last element of thedeque
.Change 23.3.13.5 [vector.modifiers] as indicated:
-5- […]
void pop_back();-?- Effects: Invalidates the past-the-end iterator, and iterators and references to the last element of the vector.
[2014-06-21 Rapperswil]
Tony van Eerd: Would be good to define "an erase operation is ..." somewhere.
AM: The containers clause is known to be suboptimal in many ways. Looks good[Urbana 2014-11-07: Move to Ready]
Proposed resolution:
This wording is relative to N3936.
Change 23.3.5.4 [deque.modifiers] as indicated:
iterator erase(const_iterator position); iterator erase(const_iterator first, const_iterator last); void pop_front(); void pop_back();-4- Effects: An erase operation that erases the last element of a deque invalidates only the past-the-end iterator and all iterators and references to the erased elements. An erase operation that erases the first element of a deque but not the last element invalidates only iterators and references to the erased elements. An erase operation that erases neither the first element nor the last element of a deque invalidates the past-the-end iterator and all iterators and references to all the elements of the deque. [Note:
pop_front
andpop_back
are erase operations — end note]
Change 23.3.13.5 [vector.modifiers] as indicated:
iterator erase(const_iterator position); iterator erase(const_iterator first, const_iterator last); void pop_back();-3- Effects: Invalidates iterators and references at or after the point of the erase.
noexcept
in shared_ptr::shared_ptr(nullptr_t)
Section: 20.3.2.2 [util.smartptr.shared] Status: C++17 Submitter: Cassio Neri Opened: 2014-02-13 Last modified: 2017-07-30
Priority: Not Prioritized
View all other issues in [util.smartptr.shared].
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Discussion:
The declaration and definition of shared_ptr::shared_ptr(nullptr_t)
, given in 20.3.2.2 [util.smartptr.shared], is
constexpr shared_ptr(nullptr_t) : shared_ptr() { }
The intention seems clear: this constructor should have the same semantics of the default constructor. However, contrarily to the
default constructor, this one is not noexcept
. In contrast, unique_ptr::unique_ptr(nullptr_t)
is noexcept
,
as per 20.3.1.3 [unique.ptr.single]:
constexpr unique_ptr(nullptr_t) noexcept : unique_ptr() { }
Both libstdc++ and libc++ have added noexcept
to shared_ptr::shared_ptr(nullptr_t)
. Microsoft's STL has not.
[2014-03-26 Library reflector vote]
The issue has been identified as Tentatively Ready based on six votes in favour.
Proposed resolution:
This wording is relative to N3936.
Change class template shared_ptr
synopsis, 20.3.2.2 [util.smartptr.shared], as indicated:
constexpr shared_ptr(nullptr_t) noexcept : shared_ptr() { }
pair
and tuple
are not correctly implemented for is_constructible
with no argsSection: 21.3.5.4 [meta.unary.prop] Status: C++17 Submitter: Howard Hinnant Opened: 2014-02-19 Last modified: 2017-07-30
Priority: 3
View other active issues in [meta.unary.prop].
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Discussion:
Consider:
struct X { X() = delete; }; int main() { typedef std::pair<int, X> P; static_assert(!std::is_constructible<P>::value, ""); static_assert(!std::is_default_constructible<P>::value, ""); typedef std::tuple<int, X> T; static_assert(!std::is_constructible<T>::value, ""); static_assert(!std::is_default_constructible<T>::value, ""); }
For me these static_asserts
fail. And worse than that, even asking the question fails (as opposed to gets the wrong answer):
assert(!std::is_constructible<P>::value);
In file included from test.cpp:2:
error: call to deleted constructor of 'X' pair() : first(), second() {} ^ note: function has been explicitly marked deleted here X() = delete; ^ 1 error generated.
This can be solved by specializing is_constructible
on pair
and tuple
for zero Args:
template <class T, class U> struct is_constructible<pair<T, U>> : integral_constant<bool, is_default_constructible<T>::value && is_default_constructible<U>::value> {}; template <class ...T> struct is_constructible<tuple<T...>> : integral_constant<bool, __all<is_default_constructible<T>::value...>::value> {};
Now everything just works.
[2014-05-14, Daniel comments]
The proposed resolution is incomplete, because it wouldn't work for cv-qualified objects of
pair
or for references of them during reference-initialization.
is_constructible
:
template<class U1 = T1, class U2 = T2, typename enable_if< is_default_constructible<U1>::value && is_default_constructible<U2>::value , bool>::type = false > constexpr pair();
The new wording proposal represents an alternative wording change that I would strongly prefer.
Previous resolution from Howard [SUPERSEDED]:This wording is relative to N3936.
Add to 22.3.3 [pairs.spec]:
template <class T, class U> struct is_constructible<pair<T, U>> : integral_constant<bool, is_default_constructible<T>::value && is_default_constructible<U>::value> {};Add to 22.4.12 [tuple.special]:
template <class ...T> struct is_constructible<tuple<T...>> : integral_constant<bool, see below> {};-?- The second argument to
integral_constant
shall be true if for eachT
,is_default_constructible<T>::value
is true.
[2015-05, Lenexa]
STL: I object to this resolution due to British spelling of behavior
JW: we already have other places of this spelling
VV: the easy resolution is to remove the notes
MC: if that's all we want to change: put it in and make the editorial change of removing the note
VV: the other paper doesn't make any of these changes so it would be consistent
JW: this make me want even more the features of having constructors doing the Right Thing
- I haven't written up the request to do something like that
VV: so it would be an aggregate reflecting the properties of the constituting types
JW: I should write that up
MC: any objection to move to ready? in favor: 16, opposed: 0, abstain: 1
Proposed resolution:
This wording is relative to N3936.
Change 22.3.2 [pairs.pair] around p3 as indicated:
constexpr pair();-4- Effects: Value-initializes first and second. -?- Remarks: This constructor shall not participate in overload resolution unless
-3- Requires:is_default_constructible<first_type>::value
is true andis_default_constructible<second_type>::value
is true.is_default_constructible<first_type>::value
is true andis_default_constructible<second_type>::value
is true. [Note: This behaviour can be implemented by a constructor template with default template arguments — end note].
Change 22.4.4.2 [tuple.cnstr] around p4 as indicated:
constexpr tuple();-5- Effects: Value initializes each element. -?- Remarks: This constructor shall not participate in overload resolution unless
-4- Requires:is_default_constructible<Ti>::value
is true for all i.is_default_constructible<Ti>::value
is true for all i. [Note: This behaviour can be implemented by a constructor template with default template arguments — end note].
operator new
Section: 17.6.3 [new.delete] Status: Resolved Submitter: Stephen Clamage Opened: 2014-02-20 Last modified: 2020-09-06
Priority: 2
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Discussion:
Section 17.6.3 [new.delete] and subsections shows:
void* operator new(std::size_t size); void* operator new[](std::size_t size);
That is, without exception-specifications. (Recall that C++03 specified these functions with throw(std::bad_alloc)
.)
Any other functions defined in the C++ standard library that do not have an exception-specification may throw implementation-defined exceptions unless otherwise specified. An implementation may strengthen this implicit exception-specification by adding an explicit one.
For example, an implementation could provide C++03-compatible declarations of operator new
.
operator new
functions. But how can you write the definition of these functions when
the exception specification can vary among implementations? For example, the declarations
void* operator new(std::size_t size) throw(std::bad_alloc); void* operator new(std::size_t size);
are not compatible.
From what I have been able to determine, gcc has a hack for the special case ofoperator new
to ignore the differences in
(at least) the two cases I show above. But can users expect all compilers to quietly ignore the incompatibility?
The blanket permission to add any explicit exception specification could cause a problem for any user-overridable function.
Different implementations could provide incompatible specifications, making portable code impossible to write.
[2016-03, Jacksonville]
STL: Core changes to remove dynamic exception specs would make this moot
Room: This is on track to be resolved by P0003, or may be moot.
[2016-07, Toronto Thursday night issues processing]
Resolved by P0003.
Proposed resolution:
constexpr max(initializer_list)
vs max_element
Section: 26.8.9 [alg.min.max] Status: C++17 Submitter: Marc Glisse Opened: 2014-02-21 Last modified: 2017-07-30
Priority: 3
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Discussion:
As part of the resolution for LWG issue 2350(i), max(initializer_list)
was marked as constexpr
. Looking
at two implementations of this function (libstdc++ and libc++), both implement it in terms of max_element
, which is not
marked as constexpr
. This is inconsistent and forces some small amount of code duplication in the implementation. Unless we
remove constexpr
from this overload of max
, I believe we should add constexpr
to max_element
.
[2015-02 Cologne]
AM: Can we implement this with the C++14 constexpr
rules? JM: Yes. AM: Ready? [Yes]
Proposed resolution:
This wording is relative to N3936.
In 26.1 [algorithms.general], header <algorithm>
synopsis, and 26.8.9 [alg.min.max], change as
indicated (add constexpr
to every signature from the first min_element
to the second minmax_element
)::
template<class ForwardIterator> constexpr ForwardIterator min_element(ForwardIterator first, ForwardIterator last); template<class ForwardIterator, class Compare> constexpr ForwardIterator min_element(ForwardIterator first, ForwardIterator last, Compare comp); […] template<class ForwardIterator> constexpr ForwardIterator max_element(ForwardIterator first, ForwardIterator last); template<class ForwardIterator, class Compare> constexpr ForwardIterator max_element(ForwardIterator first, ForwardIterator last, Compare comp); […] template<class ForwardIterator> constexpr pair<ForwardIterator, ForwardIterator> minmax_element(ForwardIterator first, ForwardIterator last); template<class ForwardIterator, class Compare> constexpr pair<ForwardIterator, ForwardIterator> minmax_element(ForwardIterator first, ForwardIterator last, Compare comp);
noexcept
in std::function
Section: 22.10.17.3 [func.wrap.func] Status: Resolved Submitter: Pablo Halpern Opened: 2014-02-27 Last modified: 2020-09-06
Priority: 3
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Discussion:
The following constructors in 22.10.17.3 [func.wrap.func] are declared noexcept
, even
though it is not possible for an implementation to guarantee that they will not throw:
template <class A> function(allocator_arg_t, const A&) noexcept; template <class A> function(allocator_arg_t, const A&, nullptr_t) noexcept;
In addition, the following functions are guaranteed not to throw if the target
is a function pointer or a reference_wrapper
:
template <class A> function(allocator_arg_t, const A& a, const function& f); template <class F, class A> function(allocator_arg_t, const A& a, F f);
In all of the above cases, the function object might need to allocate memory
(an operation that can throw) in order to hold a copy of the type-erased
allocator itself. The first two constructors produce an empty function
object, but the allocator is still needed in case the object is later assigned
to. In this case, we note that the propagation of allocators on assignment is
underspecified for std::function
. There are three possibilities:
The allocator is never copied on copy-assignment, moved on move-assignment, or swapped on swap.
The allocator is always copied on copy-assignment, moved on move-assignment, and swapped on swap.
Whether or not the allocator is copied, moved, or swapped is determined at
run-time based on the propagate_on_container_copy_assignment
and
propagate_on_container_move_assignment
traits of the allocators at
construction of the source function, the target function, or both.
Although the third option seems to be the most consistent with existing wording in the containers section of the standard, it is problematic in a number of respects. To begin with, the propagation behavior is determined at run time based on a pair of type-erased allocators, instead of at compile time. Such run-time logic is not consistent with the rest of the standard and is hard to reason about. Additionally, there are two allocator types involved, rather than one. Any set of rules that attempts to rationally interpret the propagation traits of both allocators is likely to be arcane at best, and subtly wrong for some set of codes at worst.
The second option is a non-starter. Historically, and in the vast majority of existing code, an allocator does not change after an object is constructed. The second option, if adopted, would undermine the programmer's ability to construct, e.g., an array of function objects, all using the same allocator.
The first option is (in Pablo's opinion) the simplest and best. It is consistent with historical use of allocators, is easy to understand, and requires minimal wording. It is also consistent with the wording in N3916, which formalizes type-erased allocators.
For cross-referencing purposes: The resolution of this issue should be
harmonized with any resolution to LWG 2062(i), which questions the noexcept
specification on the following member functions of std::function:
template <class F> function& operator=(reference_wrapper<F>) noexcept; void swap(function&) noexcept;
[2015-05 Lenexa]
MC: change to P3 and status to open.
STL: note thatnoexcept
is an issue and large chunks of allocator should be destroyed.
[2015-12-16, Daniel comments]
See 2564(i) for a corresponding issue addressing library fundamentals v2.
Previous resolution [SUPERSEDED]:
This wording is relative to N3936.
Change 22.10.17.3 [func.wrap.func], class template
function
synopsis, as indicated:template <class A> function(allocator_arg_t, const A&)noexcept; template <class A> function(allocator_arg_t, const A&, nullptr_t)noexcept;Change 22.10.17.3.2 [func.wrap.func.con] as indicated:
-1- When any function constructor that takes a first argument of type
allocator_arg_t
is invoked, the second argument shall have a type that conforms to the requirements forAllocator
(Table 17.6.3.5). A copy of the allocator argument is used to allocate memory, if necessary, for the internal data structures of the constructed function object. For the remaining constructors, an instance ofallocator<T>
, for some suitable typeT
, is used to allocate memory, if necessary, for the internal data structures of the constructed function object.function() noexcept; template <class A> function(allocator_arg_t, const A&)noexcept;-2- Postconditions:
!*this
.function(nullptr_t) noexcept; template <class A> function(allocator_arg_t, const A&, nullptr_t)noexcept;-3- Postconditions:
!*this
.function(const function& f);template <class A> function(allocator_arg_t, const A& a, const function& f);-4- Postconditions:
!*this
if!f
; otherwise,*this
targets a copy off.target()
.-5- Throws: shall not throw exceptions if
f
's target is a callable object passed viareference_wrapper
or a function pointer. Otherwise, may throwbad_alloc
or any exception thrown by the copy constructor of the stored callable object. [Note: Implementations are encouraged to avoid the use of dynamically allocated memory for small callable objects, for example, wheref
's target is an object holding only a pointer or reference to an object and a member function pointer. — end note]template <class A> function(allocator_arg_t, const A& a, const function& f);-?- Postconditions:
!*this
if!f
; otherwise,*this
targets a copy off.target()
.function(function&& f); template <class A> function(allocator_arg_t, const A& a, function&& f);-6- Effects: If
!f
,*this
has no target; otherwise, move-constructs the target off
into the target of*this
, leavingf
in a valid state with an unspecified value. If an allocator is not specified, the constructed function will use the same allocator asf
.template<class F> function(F f); template <class F, class A> function(allocator_arg_t, const A& a, F f);-7- Requires:
-8- Remarks: These constructors shall not participate in overload resolution unlessF
shall beCopyConstructible
.f
is Callable (20.9.11.2) for argument typesArgTypes...
and return typeR
.-9- Postconditions:
!*this
if any of the following hold:
f
is a null function pointer value.
f
is a null member pointer value.
F
is an instance of the function class template, and!f
-10- Otherwise,
*this
targets a copy off
initialized withstd::move(f)
. [Note: Implementations are encouraged to avoid the use of dynamically allocated memory for small callable objects, for example, wheref
's target is an object holding only a pointer or reference to an object and a member function pointer. — end note]-11- Throws: shall not throw exceptions when an allocator is not specified and
f
is a function pointer or areference_wrapper<T>
for someT
. Otherwise, may throwbad_alloc
or any exception thrown byF
's copy or move constructor or byA
's allocate function.
[2016-08 Chicago]
Tues PM: Resolved by P0302R1
Proposed resolution:
Resolved by acceptance of P0302R1.
Section: 3.3.1 [fund.ts::meta.type.synop] Status: TS Submitter: Joe Gottman Opened: 2014-03-07 Last modified: 2017-07-30
Priority: 0
View all issues with TS status.
Discussion:
Addresses: fund.ts
The library fundamentals specification defines two new type trait template classes: invocation_type
and raw_invocation_type
.
But it does not define the corresponding template aliases. Note that both of these classes define a member typedef type
and no
other public members, so according to the argument in N3887
the template aliases should be defined.
[2013-06-21 Rapperswil]
Accept for Library Fundamentals TS Working Paper
Proposed resolution:
This wording is relative to N3908.
Add the following to section 3.3.1[meta.type.synop] of the Library Fundamentals specification as indicated:
namespace std { namespace experimental { inline namespace fundamentals_v1 { […] // 3.3.2, Other type transformations template <class> class invocation_type; // not defined template <class F, class... ArgTypes> class invocation_type<F(ArgTypes...)>; template <class> class raw_invocation_type; // not defined template <class F, class... ArgTypes> class raw_invocation_type<F(ArgTypes...)>; template <class T> using invocation_type_t = typename invocation_type<T>::type; template <class T> using raw_invocation_type_t = typename raw_invocation_type<T>::type; } // namespace fundamentals_v1 } // namespace experimental } // namespace std
optional::to_value
are too restrictiveSection: 5.3.5 [fund.ts::optional.object.observe] Status: TS Submitter: Jonathan Wakely Opened: 2014-03-25 Last modified: 2017-07-30
Priority: 0
View all issues with TS status.
Discussion:
Addresses: fund.ts
In Bristol I think I claimed that the remarks for optional::to_value()
were unimplementable and the function could only be constexpr
if both
constructors that could be called were constexpr
, but I was wrong.
The remarks should be reverted to the original pre-n3793 form.
[2013-06-21 Rapperswil]
Accept for Library Fundamentals TS Working Paper
Proposed resolution:
This wording is relative to N3908.
Change [optional.object.observe] p23 of the Library Fundamentals specification as indicated:
template <class U> constexpr T value_or(U&& v) const &;[…]
-23- Remarks: Ifboth constructors ofthe selected constructor ofT
which could be selected areconstexpr
constructorsT
is aconstexpr
constructor, this function shall be aconstexpr
function.
Section: 24.3.1 [iterator.requirements.general] Status: Resolved Submitter: Marshall Clow Opened: 2014-03-25 Last modified: 2021-06-23
Priority: 3
View all other issues in [iterator.requirements.general].
View all issues with Resolved status.
Discussion:
24.3.1 [iterator.requirements.general] p9 says:
Destruction of an iterator may invalidate pointers and references previously obtained from that iterator.
But the resolution of LWG issue 2360(i) specifically advocates returning *--temp;
where temp
is a
local variable.
If
a
andb
are both dereferenceable, thena == b
if and only if*a
and*b
are bound to the same object.
which disallows "stashing" iterators (i.e, iterators that refer to data inside themselves).
So, I suspect that the restriction in p9 should only apply to input iterators, and can probably be moved into 24.3.5.3 [input.iterators] instead of 24.3.1 [iterator.requirements.general].[2014-05-22, Daniel comments]
Given that forward iterators (and beyond) are refinements of input iterator, moving this constraint to input iterators won't help much because it would still hold for all refined forms.
[2021-06-23 Resolved by adoption of P0896R4 in San Diego. Status changed: New → Resolved.]
Proposed resolution:
bad_weak_ptr::what()
overspecifiedSection: 20.3.2.1 [util.smartptr.weak.bad] Status: C++17 Submitter: Jonathan Wakely Opened: 2014-03-27 Last modified: 2017-07-30
Priority: Not Prioritized
View all issues with C++17 status.
Discussion:
[util.smartptr.weakptr] p2 requires bad_weak_ptr
to return precisely
the string "bad_weak_ptr"
.
bad_weak_ptr
consistent with other
exception types such as bad_alloc
and bad_cast
.
If accepted, the P/R for issue 2233(i), which currently uses similar
wording to bad_weak_ptr
, could be updated appropriately.
[2014-03-27 Library reflector vote]
The issue has been identified as Tentatively Ready based on six votes in favour.
Proposed resolution:
This wording is relative to N3936.
Edit [util.smartptr.weakptr]:
bad_weak_ptr() noexcept;-2- Postconditions:
what()
returnsan implementation-defined NTBS."bad_weak_ptr"
std::align
requirements overly strictSection: 20.2.5 [ptr.align] Status: C++17 Submitter: Peter Dimov Opened: 2014-03-30 Last modified: 2017-07-30
Priority: 0
View all other issues in [ptr.align].
View all issues with C++17 status.
Discussion:
std::align
requires that its alignment
argument shall be "a fundamental alignment value or an
extended alignment value supported by the implementation in this context".
std::align
does not depend on the
requirement that alignment
be a fundamental or an extended alignment value; any power of two would be handled
the same way.
In addition, it is not possible for the user to even determine whether a value is "a fundamental alignment value
or an extended alignment value supported by the implementation in this context". One would expect values coming
from alignof
to be fine, but I'm not sure whether even that is guaranteed in the presence of alignas
.
Therefore, I propose that
Requires:
alignment
shall be a fundamental alignment value or an extended alignment value supported by the implementation in this context
be changed to
Requires:
alignment
shall be a power of two
[2014-06-16 Rapperswil]
Move to Ready
Proposed resolution:
This wording is relative to N3936.
Edit 20.2.5 [ptr.align] p2 as indicated:
void* align(std::size_t alignment, std::size_t size, void*& ptr, std::size_t& space);-1- […]
-2- Requires:
alignment
shall be afundamental alignment value or an extended alignment value supported by the implementation in this contextpower of two
Section: 17.6.4.1 [bad.alloc], 17.6.4.2 [new.badlength], 17.7.4 [bad.cast], 17.7.5 [bad.typeid], 17.9.4 [bad.exception] Status: C++17 Submitter: Andy Sawyer Opened: 2014-03-31 Last modified: 2017-07-30
Priority: 0
View all issues with C++17 status.
Discussion:
I think we have an issue with the specification of some of the standard exception types.
In particular, several of them have default constructors with remarks to the effect that
"The result of calling what()
on the newly constructed object is implementation-defined".
(In some cases this is contradictory to a further specification of what()
, which
is specified to return an implementation-defined NTBS.)
Previous resolution from Andy [SUPERSEDED]:
This wording is relative to N3936.
Edit 17.6.4.1 [bad.alloc] p3 as indicated:
bad_alloc() noexcept;[…]
-3- Remarks:The result of callingwhat()
on the newly constructed object is implementation-definedwhat()
returns an implementation-defined NTBS.Edit 17.6.4.2 [new.badlength] p3 as indicated: [Drafting note: Added the Postcondition, since we don't say anything else about
bad_array_new_length::what()
— end of note]bad_array_new_length() noexcept;[…]
-3-RemarksPostcondition:The result of callingwhat()
on the newly constructed object is implementation-definedwhat()
returns an implementation-defined NTBS.Edit 17.7.4 [bad.cast] p3 as indicated:
bad_cast() noexcept;[…]
-3- Remarks: The result of calling.what()
on the newly constructed object is implementation-defined.Edit 17.7.5 [bad.typeid] p3 as indicated:
bad_typeid() noexcept;[…]
-3- Remarks: The result of calling.what()
on the newly constructed object is implementation-defined.Edit 17.9.4 [bad.exception] p3 as indicated:
bad_exception() noexcept;[…]
-3- Remarks: The result of calling.what()
on the newly constructed object is implementation-defined.
[2014-06-17, Rapperswil]
Jonathan provides alternative wording.
[2015-02, Cologne]
NJ: I don't know why we need the explict statement about what()
here, since bad_array_new_length
already derives.
AM: yes.
NJ: Then "what()" is missing from the synopsis.
AM: Yes, that's an error and it needs to be added.
Proposed resolution:
This wording is relative to N4296.
Edit 17.6.4.1 [bad.alloc] p3 as indicated:
bad_alloc() noexcept;[…]
-3- Remarks: The result of callingwhat()
on the newly constructed object is implementation-defined.
Edit 17.6.4.1 [bad.alloc] p5 as indicated:
virtual const char* what() const noexcept;-5- Returns: An implementation-defined NTBS.
-?- Remarks: The message may be a null-terminated multibyte string (17.5.2.1.4.2), suitable for conversion and display as awstring
(21.3, 22.4.1.4).
Edit class bad_array_new_length
synopsis 17.6.4.2 [new.badlength] as indicated:
namespace std { class bad_array_new_length : public bad_alloc { public: bad_array_new_length() noexcept; virtual const char* what() const noexcept; }; }
Edit 17.6.4.2 [new.badlength] as indicated:
bad_array_new_length() noexcept;[…]
-3- Remarks: The result of callingwhat()
on the newly constructed object is implementation-defined.virtual const char* what() const noexcept;-?- Returns: An implementation-defined NTBS.
-?- Remarks: The message may be a null-terminated multibyte string (17.5.2.1.4.2), suitable for conversion and display as awstring
(21.3, 22.4.1.4).
Edit 17.7.4 [bad.cast] p3 as indicated:
bad_cast() noexcept;[…]
-3- Remarks: The result of calling.what()
on the newly constructed object is implementation-defined.
Edit 17.7.5 [bad.typeid] p3 as indicated:
bad_typeid() noexcept;[…]
-3- Remarks: The result of calling.what()
on the newly constructed object is implementation-defined.
Edit 17.9.4 [bad.exception] p3 as indicated:
bad_exception() noexcept;[…]
-3- Remarks: The result of calling.what()
on the newly constructed object is implementation-defined.
<cstdlib>
provide long ::abs(long)
and long long ::abs(long long)
?Section: 16.4.2.3 [headers] Status: C++17 Submitter: Richard Smith Opened: 2014-03-31 Last modified: 2017-07-30
Priority: 2
View all other issues in [headers].
View all issues with C++17 status.
Discussion:
[depr.c.headers] p3 says:
[Example: The header
<cstdlib>
assuredly provides its declarations and definitions within the namespacestd
. It may also provide these names within the global namespace. The header<stdlib.h>
assuredly provides the same declarations and definitions within the global namespace, much as in the C Standard. It may also provide these names within the namespacestd
. — end example]
This suggests that <cstdlib>
may provide ::abs(long)
and ::abs(long long)
. But this seems like
it might contradict the normative wording of 16.4.2.3 [headers] p4:
Except as noted in Clauses 18 through 30 and Annex D, the contents of each header
cname
shall be the same as that of the corresponding headername.h
, as specified in the C standard library (1.2) or the C Unicode TR, as appropriate, as if by inclusion. In the C++ standard library, however, the declarations (except for names which are defined as macros in C) are within namespace scope (3.3.6) of the namespacestd
. It is unspecified whether these names are first declared within the global namespace scope and are then injected into namespacestd
by explicit using-declarations (7.3.3).
Note that this allows <cstdlib>
to provide ::abs(int)
, but does not obviously allow ::abs(long)
nor ::abs(long long)
, since they are not part of the header stdlib.h
as specified in the C standard library.
std::abs(long)
and std::abs(long long)
, but not in a way that seems
to allow ::abs(long)
and ::abs(long long)
to be provided.
I think the right approach here would be to allow <cstdlib>
to either provide no ::abs
declaration, or
to provide all three declarations from namespace std
, but it should not be permitted to provide only int abs(int)
.
Suggestion:
Change in 16.4.2.3 [headers] p4:
[…]. It is unspecified whether these names (including any overloads added in Clauses 18 through 30 and Annex D) are first declared within the global namespace scope and are then injected into namespace
std
by explicit using-declarations (7.3.3).
[2015-05, Lenexa]
MC: do we need to defer this?
PJP: just need to get my mind around it, already playing dirty games here, my reaction is just do it as it will help C++
STL: this is safe
TP: would be surprising if using abs didn't bring in all of the overloads
MC: that's Richard's argument
MC: move to ready
Proposed resolution:
This wording is relative to N3936.
Modify 16.4.2.3 [headers] p4 as indicated:
Except as noted in Clauses 18 through 30 and Annex D, the contents of each header
cname
shall be the same as that of the corresponding headername.h
, as specified in the C standard library (1.2) or the C Unicode TR, as appropriate, as if by inclusion. In the C++ standard library, however, the declarations (except for names which are defined as macros in C) are within namespace scope (3.3.6) of the namespacestd
. It is unspecified whether these names (including any overloads added in Clauses 18 through 30 and Annex D) are first declared within the global namespace scope and are then injected into namespacestd
by explicit using-declarations (7.3.3).
Section: 28.3.4.3.2.3 [facet.num.get.virtuals] Status: C++23 Submitter: Marshall Clow Opened: 2014-04-30 Last modified: 2023-11-22
Priority: 2
View other active issues in [facet.num.get.virtuals].
View all other issues in [facet.num.get.virtuals].
View all issues with C++23 status.
Discussion:
In 28.3.4.3.2.3 [facet.num.get.virtuals] we have:
Stage 3: The sequence of chars accumulated in stage 2 (the field) is converted to a numeric value by the rules of one of the functions declared in the header
<cstdlib>
:
For a signed integer value, the function
strtoll
.For an unsigned integer value, the function
strtoull
.For a floating-point value, the function
strtold
.
This implies that for many cases, this routine should return true:
bool is_same(const char* p) { std::string str{p}; double val1 = std::strtod(str.c_str(), nullptr); std::stringstream ss(str); double val2; ss >> val2; return std::isinf(val1) == std::isinf(val2) && // either they're both infinity std::isnan(val1) == std::isnan(val2) && // or they're both NaN (std::isinf(val1) || std::isnan(val1) || val1 == val2); // or they're equal }
and this is indeed true, for many strings:
assert(is_same("0")); assert(is_same("1.0")); assert(is_same("-1.0")); assert(is_same("100.123")); assert(is_same("1234.456e89"));
but not for others
assert(is_same("0xABp-4")); // hex float assert(is_same("inf")); assert(is_same("+inf")); assert(is_same("-inf")); assert(is_same("nan")); assert(is_same("+nan")); assert(is_same("-nan")); assert(is_same("infinity")); assert(is_same("+infinity")); assert(is_same("-infinity"));
These are all strings that are correctly parsed by std::strtod
, but not by the stream extraction operators.
They contain characters that are deemed invalid in stage 2 of parsing.
strtold
, then we should accept all the things that
strtold
accepts.
[2016-04, Issues Telecon]
People are much more interested in round-tripping hex floats than handling inf
and nan
. Priority changed to P2.
Marshall says he'll try to write some wording, noting that this is a very closely specified part of the standard, and has remained unchanged for a long time. Also, there will need to be a sample implementation.
[2016-08, Chicago]
Zhihao provides wording
The src
array in Stage 2 does narrowing only. The actual
input validation is delegated to strtold
(independent from
the parsing in Stage 3 which is again being delegated
to strtold
) by saying:
[...] If it is not discarded, then a check is made to determine if c is allowed as the next character of an input field of the conversion specifier returned by Stage 1.
So a conforming C++11 num_get
is supposed to magically
accept an hexfloat without an exponent
0x3.AB
because we refers to C99, and the fix to this issue should be
just expanding the src
array.
Support for Infs and NaNs are not proposed because of the complexity of nan(n-chars).
[2016-08, Chicago]
Tues PM: Move to Open
[2016-09-08, Zhihao Yuan comments and updates proposed wording]
Examples added.
[2018-08-23 Batavia Issues processing]
Needs an Annex C entry. Tim to write Annex C.
Previous resolution [SUPERSEDED]:
This wording is relative to N4606.
Change 28.3.4.3.2.3 [facet.num.get.virtuals]/3 Stage 2 as indicated:
static const char src[] = "0123456789abcdefpxABCDEFPX+-";
Append the following examples to 28.3.4.3.2.3 [facet.num.get.virtuals]/3 Stage 2 as indicated:
[Example:
Given an input sequence of
"0x1a.bp+07p"
,
if Stage 1 returns
%d
,"0"
is accumulated;if Stage 1 returns
%i
,"0x1a"
are accumulated;if Stage 1 returns
%g
,"0x1a.bp+07"
are accumulated.In all cases, leaving the rest in the input.
— end example]
[2021-05-18 Tim updates wording]
Based on the git history, libc++ appears to have always included
p
and P
in src
.
[2021-09-20; Reflector poll]
Set status to Tentatively Ready after eight votes in favour during reflector poll.
[2021-10-14 Approved at October 2021 virtual plenary. Status changed: Voting → WP.]
Proposed resolution:
This wording is relative to N4885.
Change 28.3.4.3.2.3 [facet.num.get.virtuals]/3 Stage 2 as indicated:
— Stage 2:
Ifin == end
then stage 2 terminates. Otherwise acharT
is taken fromin
and local variables are initialized as if bychar_type ct = *in; char c = src[find(atoms, atoms + sizeof(src) - 1, ct) - atoms]; if (ct == use_facet<numpunct<charT>>(loc).decimal_point()) c = '.'; bool discard = ct == use_facet<numpunct<charT>>(loc).thousands_sep() && use_facet<numpunct<charT>>(loc).grouping().length() != 0;where the values
src
andatoms
are defined as if by:static const char src[] = "0123456789abcdefpxABCDEFPX+-"; char_type atoms[sizeof(src)]; use_facet<ctype<charT>>(loc).widen(src, src + sizeof(src), atoms);for this value of
Ifloc
.discard
is true, then if'.'
has not yet been accumulated, then the position of the character is remembered, but the character is otherwise ignored. Otherwise, if'.'
has already been accumulated, the character is discarded and Stage 2 terminates. If it is not discarded, then a check is made to determine ifc
is allowed as the next character of an input field of the conversion specifier returned by Stage 1. If so, it is accumulated. If the character is either discarded or accumulated thenin
is advanced by++in
and processing returns to the beginning of stage 2. [Example:Given an input sequence of
"0x1a.bp+07p"
,
if the conversion specifier returned by Stage 1 is
%d
,"0"
is accumulated;if the conversion specifier returned by Stage 1 is
%i
,"0x1a"
are accumulated;if the conversion specifier returned by Stage 1 is
%g
,"0x1a.bp+07"
are accumulated.In all cases, the remainder is left in the input.
— end example]
Add the following new subclause to C.6 [diff.cpp03]:
C.4.? [locale]: localization library [diff.cpp03.locale]
Affected subclause: 28.3.4.3.2.3 [facet.num.get.virtuals]
Change: Thenum_get
facet recognizes hexadecimal floating point values.
Rationale: Required by new feature.
Effect on original feature: Valid C++2003 code may have different behavior in this revision of C++.
deallocate
function needs better specificationSection: 16.4.4.6 [allocator.requirements] Status: C++17 Submitter: Daniel Krügler Opened: 2014-05-19 Last modified: 2017-07-30
Priority: 3
View other active issues in [allocator.requirements].
View all other issues in [allocator.requirements].
View all issues with C++17 status.
Discussion:
According to Table 28, 16.4.4.6 [allocator.requirements], an Allocator
's deallocate
function is specified as follows:
All
n
T
objects in the area pointed to byp
shall be destroyed prior to this call.n
shall match the value passed to allocate to obtain this memory. Does not throw exceptions. [Note:p
shall not be singular. — end note]
This wording is confusing in regard to the following points:
This specification does not make clear that the result of an allocate
call can only be returned once to the deallocate
function. This is much
clearer expressed for operator delete
(17.6.3.2 [new.delete.single] p12, emphasis mine):
Requires:
ptr
shall be a null pointer or its value shall be a value returned by an earlier call to the (possibly replaced)operator new(std::size_t)
oroperator new(std::size_t,const std::nothrow_t&)
which has not been invalidated by an intervening call tooperator delete(void*)
.
The intended meaning of that wording was to say that deallocate
shall accept every result value
that had been returned by a corresponding call to allocate
, this includes also a possible result of a
null pointer value, which is possible ("[Note: If n == 0
, the return value is unspecified.
— end note]"). Unfortunately the deallocate
function uses a non-normative note ("p
shall not be
singular.") which refers to the fuzzy term singular
, that is one of the most unclear and misunderstood terms
of the library, as pointed out in 1213(i). The occurrence of this term has lead to the possible understanding,
that this function would never allow null pointer values. Albeit for allocators the intention had not been to require the support
in general that a null pointer value can be provided to deallocate
(as it is allowed for std::free
and operator delete
), the mental model was that every returned value of allocate
shall be an
acceptable argument type of the corresponding deallocate
function.
This issue is not intending to enforce a specific meaning of singular iterator values, but the assertion is
that this note does more harm than good. In addition to wording from operator delete
there is no longer any need
to obfuscate the normative wording.
[2014-05-24 Alisdair comments]
Now that I am reading it very precisely, there is another mis-stated assumption
as a precondition for deallocate
:
All
n T
objects in the area pointed to byp
shall be destroyed prior to this call.
This makes a poor assumption that every possible object in the allocated buffer
was indeed constructed, but this is often not the case, e.g., a vector
that is not
filled to capacity. We should require calling the destructor for only those objects
actually constructed in the buffer, which may be fewer than n
, or even 0.
deallocate
though. Are we really so concerned about leaking objects that might not manage
resources? Should this not be the proper concern of the library managing the
objects and memory?
[2014-06-05 Daniel responds and improves wording]
I fully agree with the last comment and I think that this requirement should be removed. We have no such
requirements for comparable functions such as operator delete
or return_temporary_buffer()
,
and this wording seems to be a wording rudiment that exists since C++98.
[2015-05, Lenexa]
Marshall: What do people think about this?
PJP: Sure.
Wakely: Love it.
Marshall: Ready?
Everyone agrees.
Proposed resolution:
This wording is relative to N3936.
Change Table 28 ("Allocator
requirements") as indicated:
Table 28 — Allocator
requirementsExpression Return type Assertion/note
pre-/post-conditionDefault …
a.deallocate(p,n)
(not used) Pre: p
shall be a value returned by an earlier
call toallocate
which has not been invalidated by
an intervening call todeallocate
.n
shall
match the value passed toallocate
to obtain this
memory.Alln T
objects in the area pointed to by
p
shall be destroyed prior to this call.
Throws: Nothing.n
shall match the value passed to
allocate to obtain this
memory. Does not throw
exceptions. [Note:p
shall not
be singular. — end note]…
function::assign
allocator argument doesn't make senseSection: 22.10.17.3 [func.wrap.func] Status: C++17 Submitter: Pablo Halpern Opened: 2014-05-23 Last modified: 2017-07-30
Priority: 2
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Discussion:
The definition of function::assign
in N3936 is:
template<class F, class A> void assign(F&& f, const A& a);Effects:
function(allocator_arg, a, std::forward<F>(f)).swap(*this)
This definition is flawed in several respects:
The interface implies that the intent is to replace the allocator in *this
with the specified allocator, a
. Such functionality is unique in the
standard and is problematic when creating, e.g. a container of function
objects, all using the same allocator.
The current definition of swap()
makes it unclear whether the objects being
swapped can have different allocators. The general practice is that
allocators must be equal in order to support swap, and this practice is
reinforced by the proposed library TS. Thus, the definition of assign
would
have undefined behavior unless the allocator matched the allocator already
within function.
The general rule for members of function is to supply the allocator before
the functor, using the allocator_arg
prefix. Supplying the allocator as a
second argument, without the allocator_arg
prefix is error prone and
confusing.
I believe that this ill-conceived interface was introduced in the effort to
add allocators to parts of the standard where it had been missing, when we
were unpracticed in the right way to accomplish that. Allocators were added
to function at a time when the allocator model was in flux and it was the
first class in the standard after shared_ptr
to use type-erased allocators, so
it is not surprising to see some errors in specification here. (shared_ptr
is
a special case because of its shared semantics, and so is not a good model.)
*this
:
function temp(allocator_arg, a, std::forward<F>(f)); this->~function(); ::new(this) function(std::move(temp));
(The temp variable is needed for exception safety). The ugliness of this specification underscores the ugliness of the concept. What is the purpose of this member other than to reconstruct the object from scratch, a facility that library classes do not generally provide? Programmers are always free to destroy and re-construct objects — there is no reason why function should make that especially easy.
I propose, therefore, that we make no attempt at giving the current interface a meaningful definition of questionable utility, but simply get rid of it all together. This leaves us with only two questions:Should we deprecate the interface or just remove it?
Should we replace it with an assign(f)
member that doesn't take an
allocator?
Of these four combinations of binary answers to the above questions, I think
the ones that make the most sense are (remove, no) and (deprecate, yes). The
proposed new interface provides nothing that operator=
does not already
provide. However, if the old (deprecated) interface remains, then having the
new interface will guide the programmer away from it.
Previous resolution [SUPERSEDED]:
This wording is relative to N3936.
Change class template
function
synopsis, 22.10.17.3 [func.wrap.func], as indicated:template<class R, class... ArgTypes> class function<R(ArgTypes...)> { […] // 20.9.11.2.2, function modifiers: void swap(function&) noexcept; template<class F, class A> void assign(F&&, const A&); […] };Change 22.10.17.3.3 [func.wrap.func.mod] as indicated:
template<class F, class A> void assign(F&& f, const A& a);Effects:
*this = forward<F>(f);
function(allocator_arg, a, std::forward<F>(f)).swap(*this)To deprecation section [depr.function.objects], add the following new sub-clause:
Old
assign
member of polymorphic function wrappers [depr.function.objects.assign]namespace std{ template<class R, class... ArgTypes> class function<R(ArgTypes...)> { // remainder unchanged template<class F, class A> void assign(F&& f, const A& a); […] }; }The two-argument form of
assign
is defined as follows:template<class F, class A> void assign(F&& f, const A& a);Requires:
Effects:a
shall be equivalent to the allocator used to construct*this
.this->assign(forward<F>(f));
[2015-05, Lenexa]
STL: I would ask, does anybody oppose removing this outright?
Wakely: I don't even have this signature.
Hwrd: And I think this doesn't go far enough, even more should be removed. This is a step in the right direction.
PJP: I'm in favor of removal.
Wakely: We've already got TS1 that has a new function that does it right. We could wait for feedback on that.
I think this issue should be taken now.
Marshall: Then the goal will be to move to ready.
Proposed resolution:
This wording is relative to N4431.
Change class template function
synopsis, 22.10.17.3 [func.wrap.func], as indicated:
template<class R, class... ArgTypes> class function<R(ArgTypes...)> { […] // 20.9.12.2.2, function modifiers: void swap(function&) noexcept;template<class F, class A> void assign(F&&, const A&);[…] };
Change 22.10.17.3.3 [func.wrap.func.mod] as indicated:
template<class F, class A> void assign(F&& f, const A& a);
-2- Effects:function(allocator_arg, a, std::forward<F>(f)).swap(*this)
Section: 22.10.4 [func.require], 22.10.6 [refwrap] Status: C++17 Submitter: Jonathan Wakely Opened: 2014-05-23 Last modified: 2017-07-30
Priority: Not Prioritized
View other active issues in [func.require].
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Discussion:
Further to 2299(i) and 2361(i), 22.10.4 [func.require] p3 and 22.10.6 [refwrap] p3 and p4 talk about member types without any mention of being accessible and unambiguous.
[2014-06-05 Daniel provides wording]
[2014-06-06 Library reflector vote]
The issue has been identified as Tentatively Ready based on six votes in favour.
Proposed resolution:
This wording is relative to N3936.
Change 22.10.4 [func.require] p3 as indicated:
-3- If a call wrapper (20.9.1) has a weak result type the type of its member type
result_type
is based on the typeT
of the wrapper's target object (20.9.1):
if
T
is a pointer to function type,result_type
shall be a synonym for the return type ofT
;if
T
is a pointer to member function,result_type
shall be a synonym for the return type ofT
;if
T
is a class type and the qualified-idT::result_type
is valid and denotes a type (13.10.3 [temp.deduct])with a member type, thenresult_type
result_type
shall be a synonym forT::result_type
;otherwise
result_type
shall not be defined.
Change 22.10.6 [refwrap] p3+p4 as indicated:
-3- The template instantiation
reference_wrapper<T>
shall define a nested type namedargument_type
as a synonym forT1
only if the typeT
is any of the following:
a function type or a pointer to function type taking one argument of type
T1
a pointer to member function
R T0::f
cv (where cv represents the member function_s cv-qualifiers); the typeT1
is cvT0*
a class type where the qualified-id
T::argument_type
is valid and denotes a type (13.10.3 [temp.deduct])with a member type; the typeargument_type
T1
isT::argument_type
.-4- The template instantiation
reference_wrapper<T>
shall define two nested types namedfirst_argument_type
andsecond_argument_type
as synonyms forT1
andT2
, respectively, only if the typeT
is any of the following:
a function type or a pointer to function type taking two arguments of types
T1
andT2
a pointer to member function
R T0::f(T2)
cv (where cv represents the member function's cv-qualifiers); the typeT1
is cvT0*
a class type where the qualified-ids
T::first_argument_type
andT::second_argument_type
are both valid and both denote types (13.10.3 [temp.deduct])with member types; the typefirst_argument_type
andsecond_argument_type
T1
isT::first_argument_type
.and the typeT2
isT::second_argument_type
.
function::operator=
is over-specified and handles allocators incorrectlySection: 4.2.1 [fund.ts::func.wrap.func.con] Status: TS Submitter: Pablo Halpern Opened: 2014-05-23 Last modified: 2017-07-30
Priority: 2
View all issues with TS status.
Discussion:
Addresses: fund.ts
This issue against the TS is similar to LWG 2386(i), which is against the standard. The Effects clauses for the assignment
operator for class template function
are written as code that constructs a temporary function
and then swaps it
with *this
.
The intention appears to be that assignment should have the strong exception guarantee, i.e., *this
is not modified if
an exception is thrown. The description in the standard is incorrect when *this
was originally constructed using an
allocator. The TS attempts to correct the problem, but the correction is incomplete.
get_memory_resource()
to construct a temporary function
object with the same allocator as the left-hand size
(lhs) of the assignment. The intended result of using this pattern was that the allocator for *this
would be unchanged,
but it doesn't quite work. The problem is that the allocator returned by get_memory_resource()
is not the same type as
the type-erased allocator used to construct the function
object, but rather a type-erased distillation of that type that
is insufficient for making a true copy of the allocator. The rules for type-erased allocators in the TS
([memory.type.erased.allocator])
specify that the lifetime of the object returned by get_memory_resource()
is sometimes tied to the lifetime of *this
,
which might cause the (single copy of) the allocator to be destroyed if the swap
operation destroys and reconstructs *this
,
as some implementations do (and are allowed to do).
The desired behavior is that assignment would leave the allocator of the lhs unchanged. The way to achieve this behavior is to
construct the temporary function
using the original allocator. Unfortunately, we cannot describe the desired behavior in
pure code, because get_memory_resource()
does not really name the type-erased allocator, as mentioned above. The PR below,
therefore, uses pseudo-code, inventing a fictitious ALLOCATOR_OF(f)
expression that evaluates to the actual allocator
type, even if that allocator was type erased. I have implemented this PR successfully.
[2014-06-21, Rapperswil]
Apply to Library Fundamentals TS (after removing the previous "Throws: Nothing" element to prevent an editorial conflict with 2401(i)).
Proposed resolution:
This wording is relative to N3908.
Change in [mods.func.wrap] in the Library TS as indicated:
In the following descriptions, let
ALLOCATOR_OF(f)
be the allocator specified in the construction offunction
f
, orallocator<char>()
if no allocator was specified.function& operator=(const function& f);-5- Effects:
[…]function(allocator_arg,
get_memory_resource()ALLOCATOR_OF(*this), f).swap(*this);function& operator=(function&& f);-8- Effects:
[…]function(allocator_arg,
get_memory_resource()ALLOCATOR_OF(*this), std::move(f)).swap(*this);function& operator=(nullptr_t);-11- Effects: If
-12- Postconditions:*this != NULL
, destroys the target ofthis
.!(*this)
. The memory resource returned byget_memory_resource()
after the assignment is equivalent to the memory resource before the assignment. [Note: the address returned byget_memory_resource()
might change — end note] -13- Returns:*this
template<class F> function& operator=(F&& f);-15- Effects:
[…]function(allocator_arg,
get_memory_resource()ALLOCATOR_OF(*this), std::forward<F>(f)).swap(*this);template<class F> function& operator=(reference_wrapper<F> f);-18- Effects:
[…]function(allocator_arg,
get_memory_resource()ALLOCATOR_OF(*this), f).swap(*this);
Section: 3.3.2 [fund.ts::meta.trans.other] Status: TS Submitter: Michael Spertus Opened: 2014-05-26 Last modified: 2017-07-30
Priority: Not Prioritized
View all other issues in [fund.ts::meta.trans.other].
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Discussion:
Addresses: fund.ts
invocation_type
falls short of its stated goals in the following case. Using the notation of
Invocation type traits,
consider
void f(int const& i); more_perfect_forwarding_async(f, int(7)); // Oops. Dangling reference because rvalue gone when async runs
This was always the advertised intent of the proposal, but while the language reflects this for the first parameter in the case of a member pointer, it failed to include corresponding language for other parameters.
[2014-06-18, Rapperswil]
Mike Spertus, Richard Smith, Jonathan Wakely, and Jeffrey Yasskin suggest improved wording.
Previous resolution [SUPERSEDED]:This wording is relative to N3908.
Change Table 3, [meta.trans.other] in the Library TS as indicated:
Table 3 — Other type transformations Template Condition Comments …
template <class Fn, class... ArgTypes>
struct invocation_type<Fn(ArgTypes...)>;Fn
and all types in the parameter packArgTypes
shall be complete types, (possibly cv-qualified)void
,
or arrays of unknown bound.If A1, A2,...
denotesArgTypes...
and
raw_invocation_type<Fn(ArgTypes...)>::type
is the function typeR(T1, T2, ...)
then letUi
be
decay<Ai>::type
ifAi
is an rvalue otherwiseTi
.
IfandFn
is a pointer to member type andT1
is
an rvalue reference, then letU1
be.
R(decay<T1>::type,
T2, ...)
Otherwise,raw_invocation_type<Fn(ArgTypes...)>::type
The member typedeftype
shall equalR(U1, U2, ...)
.
[2013-06-21 Rapperswil]
Accept for Fundamentals TS Working Paper
Proposed resolution:
This wording is relative to N4023.
Change Table 3, [meta.trans.other] in the Library TS as indicated:
Table 3 — Other type transformations Template Condition Comments …
template <class Fn, class... ArgTypes>
struct invocation_type<Fn(ArgTypes...)>;Fn
and all types in the parameter packArgTypes
shall be complete types, (possibly cv-qualified)void
,
or arrays of unknown bound.The nested typedef invocation_type<Fn(ArgTypes...)>::type
shall be defined as follows. If
raw_invocation_type<Fn(ArgTypes...)>::type
does not exist, there shall be no member typedeftype
. Otherwise:
Let
A1, A2,
… denoteArgTypes...
Let
R(T1, T2, …)
denote
raw_invocation_type_t<Fn(ArgTypes...)>
Then the member typedef
type
shall name the function
typeR(U1, U2, …)
whereUi
isdecay_t<Ai>
ifdeclval<Ai>()
is an rvalue otherwiseTi
.Ifraw_invocation_type<Fn(ArgTypes...)>::type
is the function typeR(T1, T2, …)
andFn
is a pointer to member type andT1
is
an rvalue reference, thenR(decay<T1>::type,
.
T2, …)
Otherwise,raw_invocation_type<Fn(ArgTypes...)>::type
.
basic_string
is missing non-const
data()
Section: 27.4.3 [basic.string] Status: Resolved Submitter: Michael Bradshaw Opened: 2014-05-27 Last modified: 2017-03-12
Priority: 3
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Discussion:
Regarding 27.4.3 [basic.string], std::basic_string<charT>::data()
returns a const charT*
27.4.3.8.1 [string.accessors]. While this method is convenient, it doesn't quite match std::array<T>::data()
[array.data] or std::vector<T>::data()
23.3.13.4 [vector.data], both of which provide two
versions (that return T*
or const T*
). An additional data()
method can be added to
std::basic_string
that returns a charT*
so it can be used in similar situations that std::array
and
std::vector
can be used. Without a non-const
data()
method, std::basic_string
has to be treated
specially in code that is otherwise oblivious to the container type being used.
charT*
return type to data()
would be equivalent to doing &str[0]
or &str.front()
.
Small discussion on the issue can be found here
and in the std-discussion thread
(which didn't get too much attention).
This requires a small change to std::basic_string
's definition in 27.4.3 [basic.string] to add the method to
std::basic_string
, and another small change in 27.4.3.8.1 [string.accessors] to define the new method.
[2015-02 Cologne]
Back to LEWG.
[2016-05-22]
Marshall says: this issue has been resolved by P0272R1.
Proposed resolution:
This wording is relative to N3936.
Change class template basic_string
synopsis, 27.4.3 [basic.string], as indicated:
namespace std { template<class charT, class traits = char_traits<charT>, class Allocator = allocator<charT> > class basic_string { public: […] // 21.4.7, string operations: const charT* c_str() const noexcept; const charT* data() const noexcept; charT* data() noexcept; allocator_type get_allocator() const noexcept; […] }; }
Add the following sequence of paragraphs following 27.4.3.8.1 [string.accessors] p3, as indicated:
charT* data() noexcept;-?- Returns: A pointer
-?- Complexity: Constant time. -?- Requires: The program shall not alter the value stored atp
such thatp + i == &operator[](i)
for eachi
in[0,size()]
.p + size()
.
Section: 3.36 [defns.ntcts], 28.3.3.1.2.1 [locale.category], 31.2.3 [iostreams.limits.pos], 31.7.6.3.1 [ostream.formatted.reqmts], 31.7.6.3.4 [ostream.inserters.character] Status: WP Submitter: Jeffrey Yasskin Opened: 2014-06-01 Last modified: 2023-11-22
Priority: 3
View all issues with WP status.
Discussion:
The term "character type" is used in 3.36 [defns.ntcts], 28.3.3.1.2.1 [locale.category], 31.2.3 [iostreams.limits.pos], 31.7.6.3.1 [ostream.formatted.reqmts], and 31.7.6.3.4 [ostream.inserters.character], but the core language only defines "narrow character types" (6.8.2 [basic.fundamental]).
"wide-character type" is used in 99 [depr.locale.stdcvt], but the core language only defines a "wide-character set" and "wide-character literal".[2023-06-14; Varna; Daniel comments and provides wording]
Given the resolution of P2314 which had introduced to 6.8.2 [basic.fundamental] p11 a definition of "character type":
The types
char
,wchar_t
,char8_t
,char16_t
,char32_t
are collectively called character types.
one might feel tempted to have most parts of this issue resolved here, but I think that this actually is a red herring.
First, as Jonathan already pointed out, for two places, 31.7.6.3.1 [ostream.formatted.reqmts] and 31.7.6.3.4 [ostream.inserters.character], this clearly doesn't work, instead it seems as if we should replace "character type of the stream" here by "char_type
of the stream".
To me "char_type
of the stream" sounds a bit odd (we usually refer to char_type
in terms of a qualified name such as X::char_type
instead unless we are specifying
a member of some X
, where we can omit the qualification) and in the suggested
wording below I'm taking advantage of the already defined term "character container type"
(3.10 [defns.character.container]) instead, which seems to fit its intended purpose here.
Second, on further inspection it turns out that actually only one usage of the
term "character type" seems to be intended to refer to the actual core language meaning (See
the unchanged wording for 28.3.4.3.3.3 [facet.num.put.virtuals] in the proposed wording
below), all other places quite clearly must refer to the above mentioned
"character container type".
For the problem related to the missing definition of "wide-character type" (used two times in
99 [depr.locale.stdcvt]) I would like to suggest a less general and less inventive
approach to solve the definition problem here, because it only occurs in an already deprecated
component specification: My suggestion is to simply get rid of that term
by just identifying Elem
with being one of wchar_t
, char16_t
, or,
char32_t
. (This result is identical to identifying "wide-character type" with
a "character type that is not a narrow character type (6.8.2 [basic.fundamental])", but this
seemingly more general definition doesn't provide a real advantage.)
[Varna 2023-06-14; Move to Ready]
[2023-06-25; Daniel comments]
During the Varna LWG discussions of this issue it had been pointed out that the wording change applied to
[depr.locale.stdcvt.req] bullet (1.1) could exclude now the previously allowed support
of narrow character types as a "wide-character" with e.g. a Maxcode
value of 255. First,
I don't think that the revised wording really forbids this. Second, the originating proposal
N2401 doesn't indicate what the actual intend here was and it seems questionable to
assign LEWG to this issue given that the relevant wording is part of deprecated components, especially
given their current position expressed here
to eliminate the specification of the affected components as suggested by P2871.
[2023-11-11 Approved at November 2023 meeting in Kona. Status changed: Voting → WP.]
Proposed resolution:
This wording is relative to N4950.
[Drafting note: All usages of "character type" in 28.5 [format] seem to be without problems.]
Modify 28.3.3.1.2.1 [locale.category] as indicated:
[Drafting note: The more general interpretation of "character container type" instead of character type by the meaning of the core language seems safe here. It seems reasonable that an implementation allows more than the core language character types, but still could impose additional constraints imposed on them. Even if an implementation does never intend to support anything beyond
char
andwchar_t
, the wording below is harmless. One alternative could be here to use the even more general term "char-like types" from 27.1 [strings.general], but I'm unconvinced that this buys us much]
-6- […] A template parameter with name
C
represents the set of types containingchar
,wchar_t
, and any other implementation-defined character container types (3.10 [defns.character.container]) that meet the requirements for a character on which any of the iostream components can be instantiated. […]
Keep 28.3.4.3.3.3 [facet.num.put.virtuals] of Stage 1 following p4 unchanged:
[Drafting note: The wording here seems to refer to the pure core language wording meaning of a character type.]
[…] For conversion from an integral type other than a character type, the function determines the integral conversion specifier as indicated in Table 110.
Modify 31.2.3 [iostreams.limits.pos] as indicated:
[Drafting note: Similar to 28.3.3.1.2.1 [locale.category] above the more general interpretation of "character container type" instead of character type by the meaning of the core language seems safe here. ]
-3- In the classes of Clause 31, a template parameter with name
charT
represents a member of the set of types containingchar
,wchar_t
, and any other implementation-defined character container types (3.10 [defns.character.container]) that meet the requirements for a character on which any of the iostream components can be instantiated.
Modify 31.7.6.3.1 [ostream.formatted.reqmts] as indicated:
-3- If a formatted output function of a stream
os
determines padding, it does so as follows. Given acharT
character sequenceseq
wherecharT
is the character container type of the stream, […]
Modify 31.7.6.3.4 [ostream.inserters.character] as indicated:
template<class charT, class traits> basic_ostream<charT, traits>& operator<<(basic_ostream<charT, traits>& out, charT c); template<class charT, class traits> basic_ostream<charT, traits>& operator<<(basic_ostream<charT, traits>& out, char c); // specialization template<class traits> basic_ostream<char, traits>& operator<<(basic_ostream<char, traits>& out, char c); // signed and unsigned template<class traits> basic_ostream<char, traits>& operator<<(basic_ostream<char, traits>& out, signed char c); template<class traits> basic_ostream<char, traits>& operator<<(basic_ostream<char, traits>& out, unsigned char c);-1- Effects: Behaves as a formatted output function (31.7.6.3.1 [ostream.formatted.reqmts]) of
out
. Constructs a character sequenceseq
. Ifc
has typechar
and the character container type of the stream is notchar
, thenseq
consists ofout.widen(c)
; otherwiseseq
consists ofc
. Determines padding forseq
as described in 31.7.6.3.1 [ostream.formatted.reqmts]. Insertsseq
intoout
. Callsos.width(0)
.
Modify [depr.locale.stdcvt.req] as indicated:
(1.1) —
Elem
is one ofthe wide-character type, such aswchar_t
,char16_t
, orchar32_t
.(1.2) —
Maxcode
is the largestwide-character codevalue ofElem
converted tounsigned long
that the facet will read or write without reporting a conversion error.[…]
std::function
's Callable definition is brokenSection: 22.10.17.3 [func.wrap.func] Status: C++17 Submitter: Daniel Krügler Opened: 2014-06-03 Last modified: 2017-07-30
Priority: 2
View all other issues in [func.wrap.func].
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Discussion:
The existing definition of std::function
's Callable requirements provided in 22.10.17.3 [func.wrap.func]
p2,
A callable object
f
of typeF
is Callable for argument typesArgTypes
and return typeR
if the expressionINVOKE(f, declval<ArgTypes>()..., R)
, considered as an unevaluated operand (Clause 5), is well formed (20.9.2).
is defective in several aspects:
The wording can be read to be defined in terms of callable objects, not of callable types.
Contrary to that, 22.10.17.3.6 [func.wrap.func.targ] p2 speaks of "T
shall be a type that is Callable
(20.9.11.2) for parameter types ArgTypes
and return type R
."
The required value category of the callable object during the call expression (lvalue or rvalue) strongly depends on
an interpretation of the expression f
and therefore needs to be specified unambiguously.
The intention of original proposal
(see IIIa. Relaxation of target requirements) was to refer to both types and values ("we say that the function object f
(and its type F
) is Callable […]"), but that mental model is not really deducible from the
existing wording. An improved type-dependence wording would also make the sfinae-conditions specified in 22.10.17.3.2 [func.wrap.func.con]
p8 and p21 ("[…] shall not participate in overload resolution unless f
is Callable (20.9.11.2)
for argument types ArgTypes...
and return type R
.") easier to interpret.
std::function
invokes the call operator of its target via an lvalue. The required value-category is relevant,
because it allows to reflect upon whether an callable object such as
struct RVF { void operator()() const && {} };
would be a feasible target object for std::function<void()>
or not.
A callable
objecttype (22.10.3 [func.def])f
ofF
is Callable for argument typesArgTypes
and return typeR
if the expressionINVOKE(
, considered as an unevaluated operand (Clause 5), is well formed (20.9.2).fdeclval<F&>(), declval<ArgTypes>()..., R)
It seems appealing to move such a general Callable definition to a more "fundamental" place (e.g. as another
paragraph of 22.10.3 [func.def]), but the question arises, whether such a more general concept should impose
the requirement that the call expression is invoked on an lvalue of the callable object — such a
special condition would also conflict with the more general definition of the result_of
trait, which
is defined for either lvalues or rvalues of the callable type Fn
. In this context I would like to point out that
"Lvalue-Callable" is not the one and only Callable requirement in the library. Counter examples are
std::thread
, call_once
, or async
, which depend on "Rvalue-Callable", because they
all act on functor rvalues, see e.g. 32.4.3.3 [thread.thread.constr]:
[…] The new thread of execution executes
INVOKE(DECAY_COPY(std::forward<F>(f)), DECAY_COPY(std::forward<Args>(args))...)
[…]
For every callable object F
, the result of DECAY_COPY
is an rvalue. These implied rvalue function calls are
no artifacts, but had been deliberately voted for by a Committee decision (see LWG 2021(i), 2011-06-13 comment)
and existing implementations respect these constraints correctly. Just to give an example,
#include <thread> struct LVF { void operator()() & {} }; int main() { LVF lf; std::thread t(lf); t.join(); }
is supposed to be rejected.
The below presented wording changes are suggested to be minimal (still local tostd::function
), but the used approach
would simplify a future (second) conceptualization or any further generalization of Callable requirements of the Library.
[2015-02 Cologne]
Related to N4348. Don't touch with a barge pole.
[2015-09 Telecon]
N4348 not going anywhere, can now touch with or without barge poles
Ville: where is Lvalue-Callable defined?
Jonathan: this is the definition. It's replacing Callable with a new term and defining that. Understand why it's needed, hate the change.
Geoff: punt to an LWG discussion in Kona
[2015-10 Kona]
STL: I like this in general. But we also have an opportunity here to add a precondition. By adding static assertions, we can make implementations better. Accept the PR but reinstate the requirement.
MC: Status Review, to be moved to TR at the next telecon.[2015-10-28 Daniel comments and provides alternative wording]
The wording has been changed as requested by the Kona result. But I would like to provide the following counter-argument for this changed resolution: Currently the following program is accepted by three popular Standard libraries, Visual Studio 2015, gcc 6 libstdc++, and clang 3.8.0 libc++:
#include <functional> #include <iostream> #include <typeinfo> #include "boost/function.hpp" void foo(int) {} int main() { std::function<void(int)> f(foo); std::cout << f.target<double>() << std::endl; boost::function<void(int)> f2(foo); std::cout << f2.target<double>() << std::endl; }
and outputs the implementation-specific result for two null pointer values.
Albeit this code is not conforming, it is probable that similar code exists in the wild. The current boost documentation does not indicate any precondition for calling thetarget
function, so it is natural
that programmers would expect similar specification and behaviour.
Standardizing the suggested change requires a change of all implementations and I don't see any advantage
for the user. With that change previously working code could now cause instantiation errors, I don't see how
this could be considered as an improvement of the status quo. The result value of target
is
always a pointer, so a null-check by the user code is already required, therefore I really see no reason
what kind of problem could result out of the current implementation behaviour, since the implementation
never is required to perform a C cast to some funny type.
Previous resolution [SUPERSEDED]:
This wording is relative to N3936.
Change 22.10.17.3 [func.wrap.func] p2 as indicated:
-2- A callable
objecttype (22.10.3 [func.def])f
ofF
is Lvalue-Callable for argument typesArgTypes
and return typeR
if the expressionINVOKE(
, considered as an unevaluated operand (Clause 5), is well formed (20.9.2).fdeclval<F&>(), declval<ArgTypes>()..., R)Change 22.10.17.3.2 [func.wrap.func.con] p8+p21 as indicated:
template<class F> function(F f); template <class F, class A> function(allocator_arg_t, const A& a, F f);[…]
-8- Remarks: These constructors shall not participate in overload resolution unlessis
fFLvalue-Callable
(20.9.11.2) for argument typesArgTypes...
and return typeR
.[…]
template<class F> function& operator=(F&& f);[…]
-21- Remarks: This assignment operator shall not participate in overload resolution unlessis
declval<typename decay<F>::type&>()decay_t<F>Lvalue-Callable
(20.9.11.2) for argument typesArgTypes...
and return typeR
.Change 22.10.17.3.6 [func.wrap.func.targ] p2 as indicated: [Editorial comment: Instead of adapting the preconditions for the naming change I recommend to strike it completely, because the
target()
functions do not depend on it; the corresponding wording exists since its initial proposal and it seems without any advantage to me. Assume that some template argumentT
is provided, which does not satisfy the requirements: The effect will be that the result is a null pointer value, but that case can happen in other (valid) situations as well. — end comment]template<class T> T* target() noexcept; template<class T> const T* target() const noexcept;-3- Returns: If
-2- Requires:T
shall be a type that isCallable
(20.9.11.2) for parameter typesArgTypes
and return typeR
.target_type() == typeid(T)
a pointer to the stored function target; otherwise a null pointer.
[2015-10, Kona Saturday afternoon]
GR explains the current short-comings. There's no concept in the standard that expresses rvalue member function qualification, and so, e.g. std::function cannot be forbidden from wrapping such functions. TK: Although it wouldn't currently compile.
GR: Implementations won't change as part of this. We're just clearing up the wording.
STL: I like this in general. But we also have an opportunity here to add a precondition. By adding static assertions, we can make implementations better. Accept the PR but reinstate the requirement.
JW: I hate the word "Lvalue-Callable". I don't have a better suggestion, but it'd be terrible to teach. AM: I like the term. I don't like that we need it, but I like it. AM wants the naming not to get in the way with future naming. MC: We'll review it.
TK: Why don't we also add Rvalue-Callable? STL: Because nobody consumes it.
Discussion whether "tentatively ready" or "review". The latter would require one more meeting. EF: We already have implementation convergence. MC: I worry about a two-meeting delay. WEB: All that being said, I'd be slightly more confident with a review since we'll have new wording, but I wouldn't object. MC: We can look at it in a telecon and move it.
STL reads out email to Daniel.
Status Review, to be moved to TR at the next telecon.
[2016-01-31, Daniel comments and suggests less controversive resolution]
It seems that specifically the wording changes for 22.10.17.3.6 [func.wrap.func.targ] p2 prevent this issue from making make progress. Therefore the separate issue LWG 2591(i) has been created, that focuses solely on this aspect. Furtheron the current P/R of this issue has been adjusted to the minimal possible one, where the term "Callable" has been replaced by the new term "Lvalue-Callable".
Previous resolution II [SUPERSEDED]:
This wording is relative to N4527.
Change 22.10.17.3 [func.wrap.func] p2 as indicated:
-2- A callable
objecttype (22.10.3 [func.def])f
ofF
is Lvalue-Callable for argument typesArgTypes
and return typeR
if the expressionINVOKE(
, considered as an unevaluated operand (Clause 5), is well formed (20.9.2).fdeclval<F&>(), declval<ArgTypes>()..., R)Change 22.10.17.3.2 [func.wrap.func.con] p8+p21 as indicated:
template<class F> function(F f); template <class F, class A> function(allocator_arg_t, const A& a, F f);[…]
-8- Remarks: These constructors shall not participate in overload resolution unlessis
fFLvalue-Callable
(20.9.11.2) for argument typesArgTypes...
and return typeR
.[…]
template<class F> function& operator=(F&& f);[…]
-21- Remarks: This assignment operator shall not participate in overload resolution unlessis
declval<typename decay<F>::type&>()decay_t<F>Lvalue-Callable
(20.9.11.2) for argument typesArgTypes...
and return typeR
.Change 22.10.17.3.6 [func.wrap.func.targ] p2 as indicated:
template<class T> T* target() noexcept; template<class T> const T* target() const noexcept;-2- Remarks: If
-3- Returns: IfT
is a type that is notLvalue-Callable
(20.9.11.2) for parameter typesArgTypes
and return typeR
, the program is ill-formedRequires:.T
shall be a type that isCallable
(20.9.11.2) for parameter typesArgTypes
and return typeR
target_type() == typeid(T)
a pointer to the stored function target; otherwise a null pointer.
[2016-03 Jacksonville]
Move to Ready.Proposed resolution:
This wording is relative to N4567.
Change 22.10.17.3 [func.wrap.func] p2 as indicated:
-2- A callable
objecttype (22.10.3 [func.def])f
ofF
is Lvalue-Callable for argument typesArgTypes
and return typeR
if the expressionINVOKE(
, considered as an unevaluated operand (Clause 5), is well formed (20.9.2).fdeclval<F&>(), declval<ArgTypes>()..., R)
Change 22.10.17.3.2 [func.wrap.func.con] p8+p21 as indicated:
template<class F> function(F f); template <class F, class A> function(allocator_arg_t, const A& a, F f);[…]
-8- Remarks: These constructors shall not participate in overload resolution unlessis
fFLvalue-Callable
(22.10.17.3 [func.wrap.func]) for argument typesArgTypes...
and return typeR
.[…]
template<class F> function& operator=(F&& f);[…]
-21- Remarks: This assignment operator shall not participate in overload resolution unlessis
declval<typename decay<F>::type&>()decay_t<F>Lvalue-Callable
(22.10.17.3 [func.wrap.func]) for argument typesArgTypes...
and return typeR
.
Change 22.10.17.3.6 [func.wrap.func.targ] p2 as indicated:
template<class T> T* target() noexcept; template<class T> const T* target() const noexcept;-2- Requires:
-3- Returns: IfT
shall be a type that isLvalue-Callable
(22.10.17.3 [func.wrap.func]) for parameter typesArgTypes
and return typeR
.target_type() == typeid(T)
a pointer to the stored function target; otherwise a null pointer.
locale::name
specification unclear — what is implementation-defined?Section: 28.3.3.1.4 [locale.members] Status: C++17 Submitter: Richard Smith Opened: 2014-06-09 Last modified: 2017-07-30
Priority: 3
View all other issues in [locale.members].
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Discussion:
28.3.3.1.4 [locale.members] p5 says:
Returns: The name of
*this
, if it has one; otherwise, the string"*"
. If*this
has a name, thenlocale(name().c_str())
is equivalent to*this
. Details of the contents of the resulting string are otherwise implementation-defined.
So… what is implementation-defined here, exactly? The first sentence completely defines the behavior of this function in all cases.
Also, the second sentence says (effectively) that all locales with the same name are equivalent: givenL1
and L2
that have the same name N
, they are both equivalent to locale(N)
, and since there is no definition of
"equivalent" specific to locale
, I assume it's the normal transitive equivalence property, which would imply that
L1
is equivalent to L2
. I'm not sure why this central fact is in the description of locale::name
, nor
why it's written in this roundabout way.
[2016-08-03 Chicago LWG]
Walter, Nevin, and Jason provide initial Proposed Resolution.
[2016-08 - Chicago]
Thurs PM: Moved to Tentatively Ready
Proposed resolution:
This wording is relative to N4606.
Change 28.3.3.1.4 [locale.members] as indicated:
basic_string<char> name() const;-5- Returns: The name of
*this
, if it has one; otherwise, the string"*"
.If*this
has a name, thenlocale(name().c_str())
is equivalent to*this
. Details of the contents of the resulting string are otherwise implementation-defined.
Section: 4.2 [fund.ts::func.wrap.func], 8.5.3 [fund.ts::memory.resource.priv], 8.6.2 [fund.ts::memory.polymorphic.allocator.ctor], 8.6.3 [fund.ts::memory.polymorphic.allocator.mem], 8.9.3 [fund.ts::memory.resource.pool.ctor], 8.10.2 [fund.ts::memory.resource.monotonic.buffer.ctor] Status: TS Submitter: Zhihao Yuan Opened: 2014-06-09 Last modified: 2017-07-30
Priority: 2
View all issues with TS status.
Discussion:
Addresses: fund.ts
This element has been introduced by N3916, but the standard does not define it. The standard defines Requires: to indicate a precondition (16.3.2.4 [structure.specifications] p3).
Proposed wording: Substitute all Preconditions: with Requires:.[2013-06-21 Rapperswil]
Accept for Fundamentals TS Working Paper
Proposed resolution:
This wording is relative to N4023.
Substitute all Preconditions: with Requires:.
underlying_type
doesn't say what to do for an incomplete enumeration typeSection: 21.3.8.7 [meta.trans.other] Status: C++17 Submitter: Richard Smith Opened: 2014-06-12 Last modified: 2017-07-30
Priority: 0
View all other issues in [meta.trans.other].
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Discussion:
Consider:
enum E { e = std::underlying_type<E>::type(1) };
Clearly this should be ill-formed, but the library section doesn't appear to ban it. Suggestion:
Change in 21.3.8.7 [meta.trans.other] Table 57:Template:
Condition:template<class T> struct underlying_type;
T
shall be a completeanenumeration type (7.2) Comments: […]
[2014-06-16 Rapperswil]
Move to Ready
Proposed resolution:
This wording is relative to N3936.
Change Table 57 — "Other transformations" as indicated:
Table 3 — Other type transformations Template Condition Comments …
template <class T>
struct underlying_type;T shall be a complete anenumeration type (7.2)[…]
map<K, V>::emplace
and explicit V
constructorsSection: 21.3.8.7 [meta.trans.other] Status: Resolved Submitter: Peter Dimov Opened: 2014-06-12 Last modified: 2015-05-05
Priority: 1
View all other issues in [meta.trans.other].
View all issues with Resolved status.
Discussion:
Please consider the following example:
#include <map> #include <atomic> int main() { std::map<int, std::atomic<int>> map_; map_.emplace(1, 0); // fail map_.emplace(1); // fail map_.emplace(1, {}); // fail map_.emplace(std::piecewise_construct, std::tuple<int>(1), std::tuple<>()); // OK }
The first three calls represent attempts by an ordinary programmer (in which role I appear today) to construct
a map element. Since std::atomic<int>
is non-copyable and immovable, I was naturally drawn to
emplace()
because it constructs in-place and hence doesn't need to copy or move. The logic behind the
attempts was that K=int
would be constructed from '1
', and V=std::atomic<int>
would be
(directly) constructed by '0
', default constructed, or constructed by '{}
'.
map::emplace
or pair
.
Ville:
There exists a related EWG issue for this.
Daniel:
If the proposal N4387 would be accepted, it would solve the first problem mentioned above.
[2015-02, Cologne]
AM: I think Peter's expectation is misguided that the second and third "//fail" cases should work.
DK: Howard's paper [note: which hasn't been written yet] will make the second case work... AM: ...but
the third one will never work without core changes.
[2015-05, Lenexa]
STL: think this is covered with N4387
MC: this was accepted in Cologne
STL: only want to fix the first emplace
MC: leave alone and mark as closed by N4387
Proposed resolution:
Resolved by acceptance of N4387.
shared_ptr
's constructor from unique_ptr
should be constrainedSection: 20.3.2.2.2 [util.smartptr.shared.const] Status: C++17 Submitter: Stephan T. Lavavej Opened: 2014-06-14 Last modified: 2017-07-30
Priority: 0
View other active issues in [util.smartptr.shared.const].
View all other issues in [util.smartptr.shared.const].
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Discussion:
Consider the following code:
#include <iostream> #include <memory> #include <string> using namespace std; void meow(const shared_ptr<int>& sp) { cout << "int: " << *sp << endl; } void meow(const shared_ptr<string>& sp) { cout << "string: " << *sp << endl; } int main() { meow(make_unique<int>(1729)); meow(make_unique<string>("kitty")); }
This fails to compile due to ambiguous overload resolution, but we can easily make this work. (Note: shared_ptr
's
constructor from auto_ptr
is also affected, but I believe that it's time to remove auto_ptr
completely.)
[2014-06-16 Rapperswil]
Move to Ready
Proposed resolution:
This wording is relative to N3936.
Change 20.3.2.2.2 [util.smartptr.shared.const] around p33 as indicated:
template <class Y, class D> shared_ptr(unique_ptr<Y, D>&& r);-?- Remark: This constructor shall not participate in overload resolution unless
-33- Effects: Equivalent tounique_ptr<Y, D>::pointer
is convertible toT*
.shared_ptr(r.release(), r.get_deleter())
whenD
is not a reference type, otherwiseshared_ptr(r.release(), ref(r.get_deleter()))
.
shared_ptr
's get_deleter()
should use addressof()
Section: 20.3.2.2.11 [util.smartptr.getdeleter] Status: C++17 Submitter: Stephan T. Lavavej Opened: 2014-06-14 Last modified: 2017-07-30
Priority: 0
View all other issues in [util.smartptr.getdeleter].
View all issues with C++17 status.
Discussion:
The Standard Library should consistently use addressof()
to defend itself against overloaded operator&()
.
0
to nullptr
.
[2014-06-16 Rapperswil]
Move to Ready
Proposed resolution:
This wording is relative to N3936.
Change 20.3.2.2.11 [util.smartptr.getdeleter] as indicated:
template <class D, class T> get_deleter(const shared_ptr<T>& p) noexcept;-1- Returns: If
p
owns a deleterd
of type cv-unqualifiedD
, returns; otherwise returns
&std::addressof(d). […]
0nullptr
std::function
needs more noexcept
Section: 22.10.17.3 [func.wrap.func] Status: C++17 Submitter: Stephan T. Lavavej Opened: 2014-06-14 Last modified: 2017-07-30
Priority: 0
View all other issues in [func.wrap.func].
View all issues with C++17 status.
Discussion:
There are two issues here:
std::function
's constructor from nullptr_t
is marked as noexcept
, but its assignment operator
from nullptr_t
isn't. This assignment can and should be marked as noexcept
.
std::function
's comparisons with nullptr_t
are marked as noexcept
in two out of three places.
[2014-06-16 Rapperswil]
Move to Ready
Proposed resolution:
This wording is relative to N3936.
Change 22.10 [function.objects] p2, header <functional>
synopsis, as indicated:
namespace std { […] // 20.9.11 polymorphic function wrappers: […] template<class R, class... ArgTypes> bool operator==(const function<R(ArgTypes...)>&, nullptr_t) noexcept; template<class R, class... ArgTypes> bool operator==(nullptr_t, const function<R(ArgTypes...)>&) noexcept; template<class R, class... ArgTypes> bool operator!=(const function<R(ArgTypes...)>&, nullptr_t) noexcept; template<class R, class... ArgTypes> bool operator!=(nullptr_t, const function<R(ArgTypes...)>&) noexcept; […] }
Change 22.10.17.3 [func.wrap.func], class template function
synopsis, as indicated:
[…] // 20.9.11.2.1, construct/copy/destroy: […] function& operator=(nullptr_t) noexcept; […]
Change 22.10.17.3.2 [func.wrap.func.con] before p16 as indicated:
function& operator=(nullptr_t) noexcept;
stof()
should call strtof()
and wcstof()
Section: 27.4.5 [string.conversions] Status: C++17 Submitter: Stephan T. Lavavej Opened: 2014-06-14 Last modified: 2017-07-30
Priority: 2
View all other issues in [string.conversions].
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Discussion:
stof()
is currently specified to call strtod()
/wcstod()
(which converts the given string to
double
) and then it's specified to convert that double
to float
. This performs rounding twice,
which introduces error. Here's an example written up by James McNellis:
X
:
1.999999821186065729339276231257827021181583404541015625 (X)
This number is exactly representable in binary as:
1.111111111111111111111101000000000000000000000000000001 * ^1st ^23rd ^52nd
I've marked the 23rd and 52nd fractional bits. These are the least significant bits for float
and double
,
respectively.
float
, we take the 23 most significant bits:
1.11111111111111111111110
The next bit is a one and the tail is nonzero (the 54th fractional bit is a one), so we round up. This gives us the correctly rounded result:
1.11111111111111111111111
So far so good. But... If we convert X
to double
, we take the 52 most significant bits:
1.1111111111111111111111010000000000000000000000000000 (Y)
The next bit is a zero, so we round down (truncating the value). If we then convert Y
to float
, we take
its 23 most significant bits:
1.11111111111111111111110
The next bit is a one and the tail is zero, so we round to even (leaving the value unchanged). This is off by 1ulp from the correctly rounded result.
[2014-06 Rapperswil]
Marshall Clow will look at this.
[Urbana 2014-11-07: Move to Ready]
Proposed resolution:
This wording is relative to N3936.
Change 27.4.5 [string.conversions] p4+p6 as indicated:
float stof(const string& str, size_t* idx = 0); double stod(const string& str, size_t* idx = 0); long double stold(const string& str, size_t* idx = 0);-4- Effects:
[…] -6- Throws:the first twoThese functions callstrtof(str.c_str(), ptr)
,strtod(str.c_str(), ptr)
, andthe third function callsstrtold(str.c_str(), ptr)
, respectively. Each function returns the converted result, if any. […]invalid_argument
ifstrtof
,strtod
, orstrtold
reports that no conversion could be performed. Throwsout_of_range
ifstrtof
,strtod
, orstrtold
setserrno
toERANGE
or if the converted value is outside the range of representable values for the return type.
Change 27.4.5 [string.conversions] p11+p13 as indicated:
float stof(const wstring& str, size_t* idx = 0); double stod(const wstring& str, size_t* idx = 0); long double stold(const wstring& str, size_t* idx = 0);-11- Effects:
[…] -13- Throws:the first twoThese functions callwcstof(str.c_str(), ptr)
,wcstod(str.c_str(), ptr)
, andthe third function callswcstold(str.c_str(), ptr)
, respectively. Each function returns the converted result, if any. […]invalid_argument
ifwcstof
,wcstod
, orwcstold
reports that no conversion could be performed. Throwsout_of_range
ifwcstof
,wcstod
, orwcstold
setserrno
toERANGE
.
mismatch()
's complexity needs to be updatedSection: 99 [mismatch] Status: C++17 Submitter: Stephan T. Lavavej Opened: 2014-06-14 Last modified: 2017-07-30
Priority: 0
View all other issues in [mismatch].
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Discussion:
N3671 updated the complexities of
equal()
and is_permutation()
, but not mismatch()
.
[2014-06-16 Rapperswil]
Move to Ready
Proposed resolution:
This wording is relative to N3936.
Change 99 [mismatch] p3 as indicated:
-3- Complexity: At most
min(last1 - first1, last2 - first2)
applications of the corresponding predicate.
negative_binomial_distribution
should reject p == 1
Section: 29.5.9.3.4 [rand.dist.bern.negbin] Status: C++17 Submitter: Stephan T. Lavavej Opened: 2014-06-14 Last modified: 2017-07-30
Priority: 3
View all other issues in [rand.dist.bern.negbin].
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Discussion:
29.5.9.3.4 [rand.dist.bern.negbin] p2 requires "0 < p <= 1
". Consider what happens when p == 1
.
The discrete probability function specified by p1 involves "* p^k * (1 - p)^i
". For p == 1
, this is
"* 1^k * 0^i
", so every integer i >= 0
is produced with zero probability. (Let's avoid thinking about
0^0
.)
p
must be within
(0, 1)
, exclusive on both sides.
Previous resolution [SUPERSEDED]:
Change 29.5.9.3.4 [rand.dist.bern.negbin] p2 as indicated: [Drafting note: This should be read as: Replace the symbol "
≤
" by "<
" — end drafting note]explicit negative_binomial_distribution(IntType k = 1, double p = 0.5);-2- Requires:
0 < p
and≤< 10 < k
.
[2014-11 Urbana]
SG6 suggests better wording.
[2014-11-08 Urbana]
Moved to Ready with the node.
There remains concern that the constructors are permitting values that may (or may not) be strictly outside the domain of the function, but that is a concern that affects the design of the random number facility as a whole, and should be addressed by a paper reviewing and addressing the whole clause, not picked up in the issues list one distribution at a time. It is still not clear that such a paper would be uncontroversial.
Proposed resolution:
This wording is relative to N4140.
Add a note after paragraph 1 before the synopsis in 29.5.9.3.4 [rand.dist.bern.negbin]:
A
negative_binomial_distribution
random number distribution produces random integers distributed according to the discrete probability function.
[Note: This implies that is undefined when
p == 1
. — end note]
Drafting note: should be in math font, and
p == 1
should be in code font.
packaged_task(allocator_arg_t, const Allocator&, F&&)
should neither be constrained nor
explicit
Section: 32.10.10.2 [futures.task.members] Status: C++17 Submitter: Stephan T. Lavavej Opened: 2014-06-14 Last modified: 2017-07-30
Priority: Not Prioritized
View all other issues in [futures.task.members].
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Discussion:
LWG 2097(i)'s resolution was slightly too aggressive. It constrained
packaged_task(allocator_arg_t, const Allocator&, F&&)
, but that's unnecessary because
packaged_task
doesn't have any other three-argument constructors. Additionally, it's marked as
explicit
(going back to WP N2798 when packaged_task
first appeared) which is unnecessary.
[2015-02 Cologne]
Handed over to SG1.
[2015-05 Lenexa, SG1 response]
Back to LWG; not an SG1 issue.
[2015-05 Lenexa]
STL improves proposed wording by restoring the constraint again.
Proposed resolution:
This wording is relative to N3936.
Change 32.10.10 [futures.task] p2, class template packaged_task
as indicated:
template <class F> explicit packaged_task(F&& f); template <class F, class Allocator>explicitpackaged_task(allocator_arg_t, const Allocator& a, F&& f);
Change 32.10.10.2 [futures.task.members] as indicated:
template <class F> packaged_task(F&& f); template <class F, class Allocator>explicitpackaged_task(allocator_arg_t, const Allocator& a, F&& f);[…]
-3- Remarks: These constructors shall not participate in overload resolution ifdecay_t<F>
is the same type asstd::packaged_task<R(ArgTypes...)>
.
common_type/iterator_traits
is missing in C++14Section: 21.3.8.7 [meta.trans.other], 24.3.2.3 [iterator.traits] Status: C++17 Submitter: Daniel Krügler Opened: 2014-06-19 Last modified: 2017-07-30
Priority: Not Prioritized
View all other issues in [meta.trans.other].
View all issues with C++17 status.
Discussion:
During the Rapperswil meeting the proposal N4041 had been discussed and there seemed to be strong consensus to apply the SFINAE-friendly definitions that currently exist within the fundamentals-ts to the C++ Standard working draft. This issue requests this change to happen.
Proposed resolution:
This wording is relative to N3936.
Change 21.3.8.7 [meta.trans.other] p3 as indicated:
-3- For the
common_type
trait applied to a parameter packT
of types, the membertype
shall be either defined or not present as follows:
If
sizeof...(T)
is zero, there shall be no membertype
.If
sizeof...(T)
is one, letT0
denote the sole type comprisingT
. The member typedeftype
shall denote the same type asdecay_t<T0>
.If
sizeof...(T)
is greater than one, letT1
,T2
, andR
, respectively, denote the first, second, and (pack of) remaining types comprisingT
. [Note:sizeof...(R)
may be zero. — end note] Finally, letC
denote the type, if any, of an unevaluated conditional expression (7.6.16 [expr.cond]) whose first operand is an arbitrary value of typebool
, whose second operand is an xvalue of typeT1
, and whose third operand is an xvalue of typeT2
. If there is such a typeC
, the member typedeftype
shall denote the same type, if any, ascommon_type_t<C,R...>
. Otherwise, there shall be no membertype
.The nested typedefcommon_type::type
shall be defined as follows:template <class ...T> struct common_type; template <class T> struct common_type<T> { typedef decay_t<T> type; }; template <class T, class U> struct common_type<T, U> { typedef decay_t<decltype(true ? declval<T>() : declval<U>())> type; }; template <class T, class U, class... V> struct common_type<T, U, V...> { typedef common_type_t<common_type_t<T, U>, V...> type; };
Change 24.3.2.3 [iterator.traits] p2 as indicated:
-2- The template
iterator_traits<Iterator>
is defined asshall have the following as publicly accessible members, and have no other members, if and only ifIterator
has valid (13.10.3 [temp.deduct]) member typesdifference_type
,value_type
,pointer
,reference
, anditerator_category
; otherwise, the template shall have no members:namespace std { template<class Iterator> struct iterator_traits {typedef typename Iterator::difference_type difference_type; typedef typename Iterator::value_type value_type; typedef typename Iterator::pointer pointer; typedef typename Iterator::reference reference; typedef typename Iterator::iterator_category iterator_category;}; }
common_type/iterator_traits
should be removed from the fundamental-tsSection: 3.3.2 [fund.ts::meta.trans.other], 99 [fund.ts::iterator.traits] Status: TS Submitter: Daniel Krügler Opened: 2014-06-19 Last modified: 2017-07-30
Priority: Not Prioritized
View all other issues in [fund.ts::meta.trans.other].
View all issues with TS status.
Discussion:
Addresses: fund.ts
During the Rapperswil meeting the proposal N4041 had been discussed and there seemed to be strong consensus to apply the SFINAE-friendly definitions that currently exist within the fundamentals-ts to the C++17 working draft. If this happens, the fundamentals-ts needs to remove its own specification regarding these templates. This issue requests this change to happen.
[2013-06-21 Rapperswil]
Accept for Fundamentals TS Working Paper
Proposed resolution:
This wording is relative to N4023 in regard to fundamental-ts changes.
In fundamental-ts, change Table 2 — "Significant features in this technical specification" as indicated:
Table 2 — Significant features in this technical specification Doc.
No.Title Primary
SectionMacro Name Suffix Value Header …
N3843
A SFINAE-
Friendlycommon_type
2.4 [mods.meta.trans.other]
common_type_sfinae
201402
<type_traits>
N3843
A SFINAE-
Friendlyiterator_traits
2.5 [mods.iterator.traits]
iterator_traits_sfinae
201402
<iterator>
…
In fundamental-ts, remove the existing sub-clause 2.4 [mods.meta.trans.other] in its entirety:
2.4 Changes to
std::common_type
[mods.meta.trans.other]
-1- […]
In fundamental-ts, remove the existing sub-clause 2.5 [mods.iterator.traits] in its entirety:
2.5 Changes to
std::iterator_traits
[mods.iterator.traits]
-1- […]
shared_ptr<array>
's constructor from unique_ptr
should be constrainedSection: 8.2.1.1 [fund.ts::memory.smartptr.shared.const] Status: TS Submitter: Jeffrey Yasskin Opened: 2014-06-16 Last modified: 2017-07-30
Priority: 0
View all issues with TS status.
Discussion:
Addresses: fund.ts
The proposed resolution for LWG 2399(i) doesn't apply cleanly to the Fundamentals TS, but the issue is still present.
[2015-02, Cologne]
Unanimous consent.
Proposed resolution:
This wording is relative to N4023 in regard to fundamental-ts changes.
In fundamental-ts, change [mods.util.smartptr.shared.const] p34 as indicated:
template <class Y, class D> shared_ptr(unique_ptr<Y, D>&& r);-34-
-35- Effects: Equivalent toRequiresRemarks: This constructor shall not participate in overload resolution unlessY*
shall beis compatible withT*
.shared_ptr(r.release(), r.get_deleter())
whenD
is not a reference type, otherwiseshared_ptr(r.release(), ref(r.get_deleter()))
. -36- Exception safety: If an exception is thrown, the constructor has no effect.
shared_ptr
is only contextually convertible to bool
Section: 20.3.2.2 [util.smartptr.shared] Status: C++17 Submitter: Jonathan Wakely Opened: 2014-06-21 Last modified: 2017-07-30
Priority: 0
View all other issues in [util.smartptr.shared].
View all issues with C++17 status.
Discussion:
N3920 made this edit, which is correct but unrelated to the support for arrays:
Change 20.7.2.2 [util.smartptr.shared] p2 as follows:
Specializations of
shared_ptr
shall beCopyConstructible
,CopyAssignable
, andLessThanComparable
, allowing their use in standard containers. Specializations ofshared_ptr
shall be contextually convertible tobool
, allowing their use in boolean expressions and declarations in conditions. […]
That change is actually fixing a defect in the current wording and should be applied directly to the working paper, not just to the
Library Fundamentals TS. The declarations of the conversion operator in 20.3.2.2 [util.smartptr.shared] and
20.3.2.2.6 [util.smartptr.shared.obs] are explicit
which contradicts the "convertible to bool
" statement. The
intention is definitely for shared_ptr
to only be contextually convertible to bool
.
[Urbana 2014-11-07: Move to Ready]
Proposed resolution:
This wording is relative to N3936.
Change 20.3.2.2 [util.smartptr.shared] p2 as indicated:
-2- Specializations of
shared_ptr
shall beCopyConstructible
,CopyAssignable
, andLessThanComparable
, allowing their use in standard containers. Specializations ofshared_ptr
shall be contextually convertible tobool
, allowing their use in boolean expressions and declarations in conditions. The template parameterT
ofshared_ptr
may be an incomplete type.
promise::set_value()
and promise::get_future()
should not raceSection: 32.10.6 [futures.promise], 32.10.10.2 [futures.task.members] Status: C++20 Submitter: Jonathan Wakely Opened: 2014-06-23 Last modified: 2021-02-25
Priority: 3
View other active issues in [futures.promise].
View all other issues in [futures.promise].
View all issues with C++20 status.
Discussion:
The following code has a data race according to the standard:
std::promise<void> p; std::thread t{ []{ p.get_future().wait(); }}; p.set_value(); t.join();
The problem is that both promise::set_value()
and
promise::get_future()
are non-const member functions which modify the
same object, and we only have wording saying that the set_value()
and
wait()
calls (i.e. calls setting and reading the shared state) are
synchronized.
get_future()
does not conflict with calling the various functions that
make the shared state ready, but clarify with a note that this does
not imply any synchronization or "happens before", only being free
from data races.
[2015-02 Cologne]
Handed over to SG1.
[2016-10-21, Nico comments]
After creating a promise or packaged task one thread can call get_future()
while another thread can set values/exceptions (either directly or via function call).
This happens very easily.
promise<string> p; thread t(doSomething, ref(p)); cout << "result: " << p.get_future().get() << endl;
AFAIK, this is currently UB due to a data race (calling get_future()
for the
promise might happen while setting the value in the promise).
promise<string> p; future<string> f(p.get_future()); thread t(doSomething, ref(p)); cout << "result: " << f.get() << endl;
but I would like to have get_future()
and setters be synchronized to avoid this UB.
vector<packaged_task<int(char)>> tasks; packaged_task<int(char)> t1(func); // start separate thread to run all tasks: auto futCallTasks = async(launch::async, callTasks, ref(tasks)); for (auto& fut : tasksResults) { cout << "result: " << fut.get_future().get() << endl; // OOPS: UB }
Again, AFAIK, this program currently is UB due to a data race. Instead, currently I'd have to program, which is a lot less convenient:
vector<packaged_task<int(char)>> tasks; vector<future<int>> tasksResults; packaged_task<int(char)> t1(func); tasksResults.push_back(t1.getFuture())); tasks.push_back(move(t1)); // start separate thread to run all tasks: auto futCallTasks = async(launch::async, callTasks, ref(tasks)); for (auto& fut : tasksResults) { cout << "result: " << fut.get() << endl; }
With my naive thinking I see not reason not to guarantee
that these calls synchronize (as get_future
returns an "address/reference"
while all setters set the values there).
Previous resolution [SUPERSEDED]:
This wording is relative to N3936.
Change 32.10.6 [futures.promise] around p12 as indicated:
future<R> get_future();-12- Returns: A
-?- Synchronization: Calls to this function do not conflict (6.9.2 [intro.multithread]) with calls tofuture<R>
object with the same shared state as*this
.set_value
,set_exception
,set_value_at_thread_exit
, orset_exception_at_thread_exit
. [Note: Such calls need not be synchronized, but implementations must ensure they do not introduce data races. — end note] -13- Throws:future_error
if*this
has no shared state or ifget_future
has already been called on apromise
with the same shared state as*this
. -14- Error conditions: […]Change 32.10.10.2 [futures.task.members] around p13 as indicated:
future<R> get_future();-13- Returns: A
-?- Synchronization: Calls to this function do not conflict (6.9.2 [intro.multithread]) with calls tofuture<R>
object that shares the same shared state as*this
.operator()
ormake_ready_at_thread_exit
. [Note: Such calls need not be synchronized, but implementations must ensure they do not introduce data races. — end note] -14- Throws: afuture_error
object if an error occurs. -15- Error conditions: […]
[2017-02-28, Kona]
SG1 has updated wording for LWG 2412. SG1 voted to move this to Ready status by unanimous consent.
[2017-03-01, Kona, SG1]
GeoffR to forward revised wording.
[2018-06, Rapperswil, Wednesday evening session]
JW: lets move on and I'll file another issue to make the wording better
BO: the current wording is better than what there before
JM: ACTION I should file an editorial issue to clean up on how to refer to [res.on.data.races]: raised editorial issue 2097
ACTION: move to Ready
[2018-11, Adopted in San Diego]
Proposed resolution:
This wording is relative to N4750.
Change 32.10.6 [futures.promise] around p12 as indicated:
future<R> get_future();-12- Returns: A
-?- Synchronization: Calls to this function do not introduce data races (6.9.2 [intro.multithread]) with calls tofuture<R>
object with the same shared state as*this
.set_value
,set_exception
,set_value_at_thread_exit
, orset_exception_at_thread_exit
. [Note: Such calls need not synchronize with each other. — end note] -13- Throws:future_error
if*this
has no shared state or ifget_future
has already been called on apromise
with the same shared state as*this
. -14- Error conditions: […]
Change 32.10.10.2 [futures.task.members] around p13 as indicated:
future<R> get_future();-13- Returns: A
-?- Synchronization: Calls to this function do not introduce data races (6.9.2 [intro.multithread]) with calls tofuture
object that shares the same shared state as*this
.operator()
ormake_ready_at_thread_exit
. [Note: Such calls need not synchronize with each other. — end note] -14- Throws: afuture_error
object if an error occurs. -15- Error conditions: […]
unique_ptr
and shared_ptr
Section: 20.3.2.2.2 [util.smartptr.shared.const] Status: C++17 Submitter: Jonathan Wakely Opened: 2014-07-03 Last modified: 2017-07-30
Priority: 2
View other active issues in [util.smartptr.shared.const].
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View all issues with C++17 status.
Discussion:
unique_ptr
guarantees that it will not invoke its deleter if it stores
a null pointer, which is useful for deleters that must not be called
with a null pointer e.g.
unique_ptr<FILE, int(*)(FILE*)> fptr(file, &::fclose);
However, shared_ptr
does invoke the deleter if it owns a null pointer,
which is a silent change in behaviour when transferring
ownership from unique_ptr
to shared_ptr
. That means the following
leads to undefined behaviour:
std:shared_ptr<FILE> fp = std::move(fptr);
Peter Dimov's suggested fix is to construct an empty shared_ptr
from a
unique_ptr
that contains a null pointer.
[2015-01-18 Library reflector vote]
The issue has been identified as Tentatively Ready based on eight votes in favour.
Proposed resolution:
This wording is relative to N4296.
Change 20.3.2.2.2 [util.smartptr.shared.const] p29 as indicated:
template <class Y, class D> shared_ptr(unique_ptr<Y, D>&& r);[…]
-29- Effects: Ifr.get() == nullptr
, equivalent toshared_ptr()
. Otherwise, ifD
is not a reference type, equivalent toshared_ptr(r.release(), r.get_deleter())
. Otherwise, equivalent toshared_ptr(r.release(), ref(r.get_deleter()))
Equivalent to.shared_ptr(r.release(), r.get_deleter())
whenD
is not a reference type, otherwiseshared_ptr(r.release(), ref(r.get_deleter()))
std::experimental::any allocator
support is unimplementableSection: 6.3 [fund.ts::any.class] Status: Resolved Submitter: Jonathan Wakely Opened: 2014-06-16 Last modified: 2015-10-26
Priority: Not Prioritized
View all issues with Resolved status.
Discussion:
Addresses: fund.ts
The allocator-extended copy constructor for any
requires an
arbitrary template parameter to be available in a type-erased context
where the dynamic type of the contained object is known. This is not
believed to be possible in C++.
[Urbana, 2014-11]
Resolved by paper N4270.
Proposed resolution:
apply
does not work with member pointersSection: 3.2.2 [fund.ts::tuple.apply] Status: TS Submitter: Zhihao Yuan Opened: 2014-07-08 Last modified: 2017-07-30
Priority: 0
View all issues with TS status.
Discussion:
Addresses: fund.ts
The definition of apply
present in §3.2.2 [tuple.apply] prevents this
function template to be used with pointer to members type passed as the first argument.
Effects:
[…]
return std::forward<F>(f)(std::get<I>(std::forward<Tuple>(t))...);
This makes this utility inconsistent with other standard library components and limits its usability.
We propose to define its functionally in terms ofINVOKE
.
[2015-02, Cologne]
DK: We should use the new std::invoke
.
TK: Is this a defect?
AM: std::invoke
goes into C++17, and this is a defect against a TS based on C++14. We can change this later,
but now leave it as INVOKE.
GR: The TS lets you have Editor's Notes, so leave a note to make that change for C++17.
index_sequence
.
Then someone looked at it and said, "why isn't this in the Standard". NJ to VV: Why are you against useful steps?
We are trying to converge on a consistent standard across multiple documents. The alternative is to reopen this
in a later discussion.Proposed resolution:
This wording is relative to N4081 in regard to fundamental-ts changes.
Edit §3.2.2 [tuple.apply] paragraph 2:
template <class F, class Tuple> constexpr decltype(auto) apply(F&& f, Tuple&& t);-2- Effects: Given the exposition only function
template <class F, class Tuple, size_t... I> constexpr decltype(auto) apply_impl( // exposition only F&& f, Tuple&& t, index_sequence<I...>) { return INVOKE(std::forward<F>(f)(, std::get<I>(std::forward<Tuple>(t))...); }[…]
std::tuple
Section: 22.4.4.2 [tuple.cnstr] Status: Resolved Submitter: Akim Demaille Opened: 2014-07-11 Last modified: 2018-06-23
Priority: Not Prioritized
View other active issues in [tuple.cnstr].
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Discussion:
The issue has been submitted after exchanges with the clang++ team as a consequence of two PR I sent:
Issue 20174 Issue 20175 The short version is shown in the program below:#include <iostream> #include <tuple> struct base { void out(const std::tuple<char, char>& w) const { std::cerr << "Tuple: " << std::get<0>(w) << std::get<1>(w) << '\n'; } }; struct decorator { base b_; template <typename... Args> auto out(Args&&... args) -> decltype(b_.out(args...)) { return b_.out(args...); } void out(const char& w) { std::cerr << "char: " << w << '\n'; } }; int main() { decorator d{base{}}; char l = 'a'; d.out(l); }
This is a stripped down version of a real world case where I wrap objects in decorators. These decorators contributes some functions, and forward all the rest of the API to the wrapped object using perfect forwarding. There can be overloaded names.
Here the inner object provides anout(const std::tuple<char, char>&) -> void
function, and the wrappers, in addition to perfect forwarding, provides
out(const char&) -> void
The main function then call out(l)
where l
is a char
lvalue.
char
overload is run. With (clang++'s) libc++ it is the tuple
version which is run.
$ g++-mp-4.9 -std=c++11 bar.cc && ./a.out char: a $ clang++-mp-3.5 -std=c++11 bar.cc -Wall && ./a.out Tuple: a
It turns out that this is the result of an extension of std::tuple
in libc++ where they accept constructors with fewer values that
tuple elements.
[2014-10-05, Daniel comments]
This issue is closely related to LWG 2312(i).
[2014-11 Urbana]
Moved to LEWG.
Extensions to tuple
's design are initially a question for LEWG.
Proposed resolution:
This was resolved by the adoption of 2312(i) and 2549(i).
function<void(ArgTypes...)>
does not discard the return value of the target objectSection: 22.10.17.3 [func.wrap.func] Status: C++17 Submitter: Agustín Bergé Opened: 2014-07-12 Last modified: 2017-07-30
Priority: 1
View all other issues in [func.wrap.func].
View all issues with C++17 status.
Discussion:
function<void(ArgTypes...)>
should discard the return value of the target object. This behavior was
in the original proposal, and it was removed (accidentally?) by the resolution of LWG 870(i).
Previous resolution [SUPERSEDED]:
Edit 22.10.17.3 [func.wrap.func] paragraph 2:
A callable object
f
of typeF
is Callable for argument typesArgTypes
and return typeR
if the expressionINVOKE(f, declval<ArgTypes>()...
, considered as an unevaluated operand (Clause 5), is well formed (22.10.4 [func.require]) and, if, R)R
is notvoid
, implicitly convertible toR
.
[2014-10-05 Daniel comments]
This side-effect was indeed not intended by 870(i).
[2015-05, Lenexa]
STL provides improved wording. It replaces the current PR, and intentionally leaves 22.10.17.3 [func.wrap.func] unchanged.
Due to 7 [expr]/6,static_cast<void>
is correct even when R
is const void
.
Proposed resolution:
This wording is relative to N4431.
Edit 22.10.4 [func.require] as depicted:
-2- Define
INVOKE(f, t1, t2, ..., tN, R)
asstatic_cast<void>(INVOKE(f, t1, t2, ..., tN))
ifR
is cvvoid
, otherwiseINVOKE(f, t1, t2, ..., tN)
implicitly converted toR
.
Change 22.10.17.3.5 [func.wrap.func.inv] as depicted:
R operator()(ArgTypes... args) const;-1-
EffectsReturns:INVOKE(f, std::forward<ArgTypes>(args)..., R)
(20.9.2), wheref
is the target object (20.9.1) of*this
.-2- Returns: Nothing ifR
isvoid
, otherwise the return value ofINVOKE(f, std::forward<ArgTypes>(args)..., R)
.
std::numeric_limits<T>::is_modulo
description: "most machines" errataSection: 17.3.5.2 [numeric.limits.members] Status: C++17 Submitter: Melissa Mears Opened: 2014-08-06 Last modified: 2017-07-30
Priority: 2
View all other issues in [numeric.limits.members].
View all issues with C++17 status.
Discussion:
The seemingly non-normative (?) paragraph 61 (referring to N3936) describing how "most machines" define
std::numeric_limits<T>::is_modulo
in [numeric.limits.members] appears to have some issues, in my opinion.
-61- On most machines, this is
false
for floating types,true
for unsigned integers, andtrue
for signed integers.
Issues I see:
Very minor: change clause 2 to "this is false
for floating point types". Other uses of the term
say "floating point types" rather than just "floating types" — see nearby is_iec559
, tinyness_before
,
etc.
is_modulo
must be true
for unsigned integers in order to be compliant with the Standard;
this is not just for "most machines". For reference, this requirement is from [basic.fundamental] paragraph 4 with its
footnote 48.
Depending on the definition of "most machines", is_modulo
could be false
for most machines' signed
integer types. GCC, Clang and Visual Studio, the 3 most popular C++ compilers by far, by default treat signed integer overflow
as undefined.
As an additional note regarding the definition of is_modulo
, it seems like it should be explicitly mentioned that
on an implementation for which signed integer overflow is undefined, is_modulo
shall be false
for signed
integer types. It took bugs filed for all three of these compilers before they finally changed (or planned to change)
is_modulo
to false
for signed types.
[2014-12 telecon]
HH: agree with the proposal, don't like the phrasing
AM: second note feels a bit wooly
WB: not even happy with the first note, notes shouldn't say "shall"
JW: the original isn't very prescriptive because "on most machines" is not something the standard controls.
AM: "On most machines" should become a note too?
AM: first note is repeating something defined in core, shouldn't say it normatively here. Change "shall" to "is"?
MC: don't like "signed integer overflow is left undefined" ... it's just plain undefined.
AM: implementations can define what they do in that case and provide guarantees.
WB: in paragraph 61, would like to see "this" replaced by "is_modulo"
AM: Move to Open
[2015-05-05 Lenexa]
Marshall: I will contact the submitter to see if she can re-draft the Proposed Resolution
Previous resolution [SUPERSEDED]:
Edit 17.3.5.2 [numeric.limits.members] around p60 as indicated:
static constexpr bool is_modulo;-60- True if the type is modulo.(footnote) A type is modulo if, for any operation involving
-??- [Note:+
,-
, or*
on values of that type whose result would fall outside the range[min(), max()]
, the value returned differs from the true value by an integer multiple ofmax() - min() + 1
.is_modulo
shall betrue
for unsigned integer types (6.8.2 [basic.fundamental]). — end note] -??- [Note:is_modulo
shall befalse
for types for which overflow is undefined on the implementation, because such types cannot meet the modulo requirement. Often, signed integer overflow is left undefined on implementations. — end note] -61- On most machines, this isfalse
for floating point types,. -62- Meaningful for all specializations.true
for unsigned integers, andtrue
for signed integers
[2016-05-21 Melissa Mears comments and provides improved wording]
GCC and Clang have -fwrapv
and MSVC (as of a May 2016 preview build)
has /d2UndefIntOverflow-
as compiler command line flags to define
signed integer overflow as wrapping. Such implementations can't set
is_modulo
to true
for signed integer types when these options are
enabled, because if translation units using opposite settings were
linked together, the One Definition Rule would be violated.
Previous resolution [SUPERSEDED]:
Edit 17.3.5.2 [numeric.limits.members] around p60 as indicated:
static constexpr bool is_modulo;-60- True if the type is modulo.(footnote) A type is modulo if, for any operation involving
-??- [Note:+
,-
, or*
on values of that type whose result would fall outside the range[min(), max()]
, the value returned differs from the true value by an integer multiple ofmax() - min() + 1
.is_modulo
istrue
for unsigned integer types (6.8.2 [basic.fundamental]). — end note] -??- [Note:is_modulo
isfalse
for signed integer types (6.8.2 [basic.fundamental]) unless an implementation, as an extension to this International Standard, defines signed integer overflow to wrap as specified by the reference mentioned in footnote 217. — end note] -??- [Note:is_modulo
isfalse
for floating point types on most machines. — end note]-61- On most machines, this is. -62- Meaningful for all specializations.false
for floating types,true
for unsigned integers, andtrue
for signed integers
[2016-06, Oulu]
We believe that the notes about unsigned integer types and floating point types are already evident from what the standard describes and should be removed. Furthermore the suggested note for signed integer types really shouldn't refer to a footnote in the own document, because footnotes have no stable identifiers. The below given revised resolution has been changed accordingly.
[2016-06 Oulu]
Added rationalization for changes. Moved to Ready.
Friday: status to Immediate
Proposed resolution:
Edit 17.3.5.2 [numeric.limits.members] around p60 as indicated:
static constexpr bool is_modulo;-60- True if the type is modulo.(footnote) A type is modulo if, for any operation involving
-??- [Example:+
,-
, or*
on values of that type whose result would fall outside the range[min(), max()]
, the value returned differs from the true value by an integer multiple ofmax() - min() + 1
.is_modulo
isfalse
for signed integer types (6.8.2 [basic.fundamental]) unless an implementation, as an extension to this International Standard, defines signed integer overflow to wrap. — end example]-61- On most machines, this is. -62- Meaningful for all specializations.false
for floating types,true
for unsigned integers, andtrue
for signed integers
Section: 32.5.8 [atomics.types.generic] Status: Resolved Submitter: Jens Maurer Opened: 2014-08-14 Last modified: 2017-03-12
Priority: 2
View all other issues in [atomics.types.generic].
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Discussion:
Otherwise, one could use memcpy
to save and restore the value according to 3.9p2.
[2014-11 Urbana]
Lawrence:Definition of "trivially copyable" has been changing.
Doesn't hurt to add proposed change, even if the sentence is redundant
Move to Review.
[2015-02 Cologne]
GR has a minor problem with the style of the wording. VV has major issues with implementability.
[2015-03-22, Jens Maurer responses to Cologne discussion]
A library implementation could provide a partial specialization for is_trivially_copyable<atomic<T>>
,
to ensure that any such type query would return false
.
atomic
specialization would actually be trivially copyable if there
is no way to call the (deleted) copy constructor or copy assignment operator?
The sole effect of the suggested addition of the constraining sentence is that it would make a user program
non-conforming that attempts to invoke memcpy
(and the like) on atomic
types, since that
would invoke undefined behaviour.
[2015-05 Lenexa, SG1 response]
SG1 is fine with P/R (and agrees it's needed), but LWG may want to check the details; it's not entirely an SG1 issue.
[2015-05-05 Lenexa]
Marshall: This was discussed on the telecon. Alisdair was going to write something to Mike and send it to Core.
Hwrd: Core says that deleted copies are trivially copyable, which makes no sense to Library people.
STL: There doesn't appear to be a Core issue about it.
[2015-09-11 Telecon]
Howard: currently std::is_trivially_copyable<std::atomic>
is true, so this resolution would contradict reality
Jonathan: changing that is good, we don't want it to be trivially copyable, otherwise users can memcpy them, which we really don't want
Howard: is it reasonable to memcpy something that isn't trivially copy constructible or trivially assignable?
Jonathan: no, it's not, but Core says you can, so this resolution is needed to stop people memcpying atomic
Howard: we should fix the core rule
Marshall: there is a separate issue of whether trivially-copyable makes sense, but this resolution is a net good purely because it stops memcpy of atomics
Howard: so should implementations specialize is_trivially_copyable
the trait to meet this?
Jonathan: or add an empty, user-defined destructor.
Howard: should the spec specify that then?
Over-specification.
Howard: without that I fear implementation divergence.
Ville and Jonathan to investigate potential implementation options.
Ville: request a note on the issue saying we need review other types such as condition_variable
to see if they are also unintentionally trivially-copyable. N4460 mentions some such types.
[2016-03 Jacksonville]
We think there is something coming from Core to resolve that, and that this will be NAD.
Until then, defer.
[2016-03 Jacksonville]
This was resolved by Core Issue 1496
[2017-01-19 Jens Maurer comments, issue state to Review]
The previous entry "[2016-03 Jacksonville] This was resolved by Core Issue 1496" is wrong; Core issue 1496 only modified the definition of "trivial class", not of "trivially copyable". However, as Ville Voutilainen observed, Core Issue 1734 made all atomics not trivially copyable, because they do not have at least one non-deleted copy/move constructor or copy/move assignment operator.
Previous resolution [SUPERSEDED]:
Change 32.5.8 [atomics.types.generic]p3 as indicated:
Specializations and instantiations of the
atomic
template shall have a deleted copy constructor, a deleted copy assignment operator, and a constexpr value constructor. They are not trivially copyable types (6.8 [basic.types]).
[2017-01-27 Telecon]
Resolved as NAD.
[2017-02-02 Daniel comments and adjusts status]
The NAD resolution is inappropriate, because the group didn't argue against the actual issue, instead the situation was that core wording changes in an unrelated area had resolved the previous problem indirectly. In this cases the correct resolution is Resolved by core wording changes as described by Jens Maurer in the 2017-01-19 comment.
Proposed resolution:
operator delete(void*, size_t)
doesn't invalidate pointers sufficientlySection: 17.6.3 [new.delete] Status: C++17 Submitter: Richard Smith Opened: 2014-08-29 Last modified: 2017-07-30
Priority: 0
View all other issues in [new.delete].
View all issues with C++17 status.
Discussion:
17.6.3.2 [new.delete.single]/12 says:
Requires:
ptr
shall be a null pointer or its value shall be a value returned by an earlier call to the (possibly replaced)operator new(std::size_t)
oroperator new(std::size_t,const std::nothrow_t&)
which has not been invalidated by an intervening call tooperator delete(void*)
.
This should say:
[…] by an intervening call to
operator delete(void*)
oroperator delete(void*, std::size_t)
.
Likewise at the end of 17.6.3.3 [new.delete.array]/11, operator delete[](void*, std::size_t)
.
[Urbana 2014-11-07: Move to Ready]
Proposed resolution:
Change 17.6.3.2 [new.delete.single]p12 as indicated:
-12- Requires:
ptr
shall be a null pointer or its value shall be a value returned by an earlier call to the (possibly replaced)operator new(std::size_t)
oroperator new(std::size_t,const std::nothrow_t&)
which has not been invalidated by an intervening call tooperator delete(void*)
oroperator delete(void*, std::size_t)
.
Change 17.6.3.3 [new.delete.array]p11 as indicated:
-11- Requires:
ptr
shall be a null pointer or its value shall be the value returned by an earlier call tooperator new[](std::size_t)
oroperator new[](std::size_t,const std::nothrow_t&)
which has not been invalidated by an intervening call tooperator delete[](void*)
oroperator delete[](void*, std::size_t)
.
compare_exchange
Section: 32.5.8.2 [atomics.types.operations] Status: C++17 Submitter: Hans Boehm Opened: 2014-08-25 Last modified: 2017-07-30
Priority: 1
View all other issues in [atomics.types.operations].
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Discussion:
The standard is either ambiguous or misleading about the nature of accesses through the expected
argument
to the compare_exchange_*
functions in 99 [atomics.types.operations.req]p21.
expected
is itself atomic (intent clearly no) and exactly when the implementation
is allowed to read or write it. These affect the correctness of reasonable code.
Herb Sutter, summarizing a complaint from Duncan Forster wrote:
Thanks Duncan,
I think we have a bug in the standardese wording and the implementations are legal, but let's check with the designers of the feature. Let me try to summarize the issue as I understand it:
What I think was intended: Lawrence, I believe you championed having
compare_exchange_*
take theexpected
value by reference, and updateexpected
on failure to expose the old value, but this was only for convenience to simplify the calling loops which would otherwise always have to write an extra "reload" line of code. Lawrence, did I summarize your intent correctly?What I think Duncan is trying to do: However, it turns out that, now that
expected
is an lvalue, it has misled(?) Duncan into trying to use the success ofcompare_exchange_*
to hand off ownership ofexpected
itself to another thread. For that to be safe, if thecompare_exchange_*
succeeds then the thread that performed it must no longer read or write fromexpected
else his technique contains a race. Duncan, did I summarize your usage correctly? Is that the only use that is broken?What the standard says: I can see why Duncan thinks the standard supports his use, but I don't think this was intended (I don't remember this being discussed but I may have been away for that part) and unless you tell me this was intended I think it's a defect in the standard. From 99 [atomics.types.operations.req]/21:
-21- Effects: Atomically, compares the contents of the memory pointed to by
object
or bythis
for equality with that inexpected
, and if true, replaces the contents of the memory pointed to byobject
or bythis
with that indesired
, and if false, updates the contents of the memory inexpected
with the contents of the memory pointed to byobject
or bythis
. […]I think we have a wording defect here in any case, because the "atomically" should not apply to the entire sentence — I'm pretty sure we never intended the atomicity to cover the write to
As a case in point, borrowing from Duncan's mail below, I think the following implementation is intended to be legal:expected
.inline int _Compare_exchange_seq_cst_4(volatile _Uint4_t *_Tgt, _Uint4_t *_Exp, _Uint4_t _Value) { /* compare and exchange values atomically with sequentially consistent memory order */ int _Res; _Uint4_t _Prev = _InterlockedCompareExchange((volatile long *)_Tgt, _Value, *_Exp); if (_Prev == *_Exp) //!!!!! Note the unconditional read from *_Exp here _Res = 1; else { /* copy old value */ _Res = 0; *_Exp = _Prev; } return (_Res); }
I think this implementation is intended to be valid — I think the only code that could be broken with the "!!!!!" read of
*_Exp
is Duncan's use of treatinga.compare_exchange_*(expected, desired) == true
as implyingexpected
got handed off, because then another thread could validly be using*_Exp
— but we never intended this use, right?
In a different thread Richard Smith wrote about the same problem:
The
atomic_compare_exchange
functions are described as follows:"Atomically, compares the contents of the memory pointed to by
object
or bythis
for equality with that inexpected
, and if true, replaces the contents of the memory pointed to byobject
or bythis
with that indesired
, and if false, updates the contents of the memory inexpected
with the contents of the memory pointed to byobject
or bythis
. Further, if the comparison is true, memory is affected according to the value ofsuccess
, and if the comparison is false, memory is affected according to the value offailure
."I think this is less clear than it could be about the effects of these operations on
*expected
in the failure case:
We have "Atomically, compares […] and updates the contents of the memory in
expected
[…]". The update to the memory inexpected
is clearly not atomic, and yet this wording parallels the success case, in which the memory update is atomic.The wording suggests that memory (including
*expected
) is affected according to the value offailure
. In particular, the failure order could bememory_order_seq_cst
, which might lead someone to incorrectly think they'd published the value of*expected
.I think this can be clarified with no change in meaning by reordering the wording a little:
"Atomically, compares the contents of the memory pointed to by
object
or bythis
for equality with that inexpected
, and if true, replaces the contents of the memory pointed to byobject
or bythis
with that indesired
, and if. If the comparison is true, memory is affected according to the value ofsuccess
, and if the comparison is false, memory is affected according to the value offailure
. Further, if the comparison is false,updatesreplaces the contents of the memory inexpected
with thecontents ofvalue that was atomically read from the memory pointed to byobject
or bythis
.Further, if the comparison is true, memory is affected according to the value of"success
, and if the comparison is false, memory is affected according to the value offailure
.
Jens Maurer add:
I believe this is an improvement.
I like to see the following additional improvements:
"contents of the memory" is strange phrasing, which doesn't say how large the memory block is. Do we compare the values or the value representation of the lvalue
*object
(or*this
)?32.5.4 [atomics.order] defines memory order based on the "affected memory location". It would be better to say something like "If the comparison is true, the memory synchronization order for the affected memory location
*object
is […]"
There was also a discussion thread involving Herb Sutter, Hans Boehm, and Lawrence Crowl, resulting in proposed wording along the lines of:
-21- Effects: Atomically with respect to
expected
and the memory pointed to byobject
or bythis
, compares the contents of the memory pointed to byobject
or bythis
for equality with that inexpected
, and if and only if true, replaces the contents of the memory pointed to byobject
or bythis
with that in desired, and if and only if false, updates the contents of the memory in expected with the contents of the memory pointed to byobject
or bythis
.At the end of paragraph 23, perhaps add
[Example: Because the
expected
value is updated only on failure, code releasing the memory containing theexpected
value on success will work. E.g. list head insertion will act atomically and not have a data race in the following code.do { p->next = head; // make new list node point to the current head } while(!head.compare_exchange_weak(p->next, p)); // try to insert— end example]
Hans objected that this still gives the misimpression that the update to expected
is atomic.
[2014-11 Urbana]
Proposed resolution was added after Redmond.
Recommendations from SG1:
(wording edits not yet applied)
[2015-02 Cologne]
Handed over to SG1.
[2015-05 Lenexa, SG1 response]
We believed we were done with it, but it was kicked back to us, with the wording we suggested not yet applied. It may have been that our suggestions were unclear. Was that the concern?
[2016-02 Jacksonville]
Applied the other half of the "if and only if" response from SG1, and moved to Ready.
Proposed resolution:
Edit 99 [atomics.types.operations.req] p21 as indicated:
-21- Effects: Retrieves the value in
expected
. It then aAtomically,compares the contents of the memory pointed to byobject
or bythis
for equality with thatinpreviously retrieved fromexpected
, and if true, replaces the contents of the memory pointed to byobject
or bythis
with that indesired
, and if false, updates the contents of the memory in expected with the contents of the memory pointed to by. If and only if the comparison is true, memory is affected according to the value ofobject
or bythis
. Further, ifsuccess
, and if the comparison is false, memory is affected according to the value offailure
. When only onememory_order
argument is supplied, the value ofsuccess
isorder
, and the value offailure
isorder
except that a value ofmemory_order_acq_rel
shall be replaced by the valuememory_order_acquire
and a value ofmemory_order_release
shall be replaced by the valuememory_order_relaxed
. If and only if the comparison is false then, after the atomic operation, the contents of the memory inexpected
are replaced by the value read fromobject
or bythis
during the atomic comparison. If the operation returns true, these operations are atomic read-modify-write operations (1.10) on the memory pointed to bythis
orobject
. Otherwise, these operations are atomic load operations on that memory.
Add the following example to the end of 99 [atomics.types.operations.req] p23:
-23- [Note: […] — end note] [Example: […] — end example]
[Example: Because the expected value is updated only on failure, code releasing the memory containing theexpected
value on success will work. E.g. list head insertion will act atomically and would not introduce a data race in the following code:do { p->next = head; // make new list node point to the current head } while(!head.compare_exchange_weak(p->next, p)); // try to insert— end example]
Section: 23.3.1 [sequences.general] Status: C++17 Submitter: Tim Song Opened: 2014-08-29 Last modified: 2017-07-30
Priority: 0
View all issues with C++17 status.
Discussion:
LWG 2194(i) removed "These container adaptors meet the requirements for sequence containers." from 23.6.1 [container.adaptors.general].
However, N3936 23.3.1 [sequences.general]/p2 still says "The headers<queue>
and <stack>
define container adaptors (23.6) that also meet the requirements for sequence containers." I assume this is just an oversight.
[Urbana 2014-11-07: Move to Ready]
Proposed resolution:
Delete paragraph 2 of 23.3.1 [sequences.general] as indicated:
-2- The headers<queue>
and<stack>
define container adaptors (23.6) that also meet the requirements for sequence containers.
Section: 16.4.3.2 [using.headers] Status: C++17 Submitter: Tim Song Opened: 2014-09-03 Last modified: 2017-07-30
Priority: 0
View all other issues in [using.headers].
View all issues with C++17 status.
Discussion:
16.4.3.2 [using.headers]/3 says
A translation unit shall include a header only outside of any external declaration or definition […]
This wording appears to be borrowed from the C standard. However, the term "external declaration" is not defined in the C++ standard, and in fact is only used here as far as I can tell, so it is unclear what it means. The C standard does define external declarations as (WG14 N1570 6.9 External definitions/4-5):
As discussed in 5.1.1.1, the unit of program text after preprocessing is a translation unit, which consists of a sequence of external declarations. These are described as "external" because they appear outside any function (and hence have file scope). [...] An external definition is an external declaration that is also a definition of a function (other than an inline definition) or an object.
The corresponding description of a translation unit in C++ is "A translation unit consists of a sequence of declarations." (6.6 [basic.link]/3).
So it appears that the C++ counterpart of "external declaration" in C is simply a "declaration" at file scope. There is no need to specifically limit the statement in 16.4.3.2 [using.headers]/3 to file-scope declarations, however, since every non-file-scope declaration is necessarily inside a file-scope declaration, so banning including a header inside file-scope declarations necessarily bans including one inside non-file-scope declarations as well.[Urbana 2014-11-07: Move to Ready]
Proposed resolution:
This wording is relative to N3936.
Edit 16.4.3.2 [using.headers] as indicated:
A translation unit shall include a header only outside of any
externaldeclaration or definition, and shall include the header lexically before the first reference in that translation unit to any of the entities declared in that header. No diagnostic is required.
uninitialized_copy()
/etc. should tolerate overloaded operator&
Section: 26.11 [specialized.algorithms] Status: C++17 Submitter: Stephan T. Lavavej Opened: 2014-10-01 Last modified: 2017-07-30
Priority: 0
View other active issues in [specialized.algorithms].
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Discussion:
This restriction isn't necessary anymore. In fact, this is the section that defines addressof()
.
(Editorial note: We can depict these algorithms as calling addressof()
instead of std::addressof()
thanks to 16.4.2.2 [contents]/3 "Whenever a name x
defined in the standard library is mentioned, the name
x
is assumed to be fully qualified as ::std::x
, unless explicitly described otherwise.")
[Urbana 2014-11-07: Move to Ready]
Proposed resolution:
This wording is relative to N3936.
Change 26.11 [specialized.algorithms] p1 as depicted:
-1-
All the iterators that are used as formal template parameters in the following algorithms are required to have theirIn the algorithmoperator*
return an object for whichoperator&
is defined and returns a pointer toT
.uninitialized_copy
, the formal template parameterInputIterator
is required to satisfy the requirements of an input iterator (24.2.3). In all of the following algorithms, the formal template parameterForwardIterator
is required to satisfy the requirements of a forward iterator (24.2.5), and is required to have the property that no exceptions are thrown from increment, assignment, comparison, or indirection through valid iterators. In the following algorithms, if an exception is thrown there are no effects.
Change 26.11.5 [uninitialized.copy] p1 as depicted:
-1- Effects:
for (; first != last; ++result, ++first) ::new (static_cast<void*>(addressof(&*result))) typename iterator_traits<ForwardIterator>::value_type(*first);
Change 26.11.5 [uninitialized.copy] p3 as depicted:
-3- Effects:
for (; n > 0; ++result, ++first, --n) { ::new (static_cast<void*>(addressof(&*result))) typename iterator_traits<ForwardIterator>::value_type(*first); }
Change 26.11.7 [uninitialized.fill] p1 as depicted:
-1- Effects:
for (; first != last; ++first) ::new (static_cast<void*>(addressof(&*first))) typename iterator_traits<ForwardIterator>::value_type(x);
Change [uninitialized.fill.n] p1 as depicted:
-1- Effects:
for (; n--; ++first) ::new (static_cast<void*>(addressof(&*first))) typename iterator_traits<ForwardIterator>::value_type(x); return first;
shared_ptr::use_count()
is efficientSection: 20.3.2.2.6 [util.smartptr.shared.obs] Status: C++17 Submitter: Stephan T. Lavavej Opened: 2014-10-01 Last modified: 2017-07-30
Priority: 0
View all other issues in [util.smartptr.shared.obs].
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Discussion:
shared_ptr
and weak_ptr
have Notes that their use_count()
might be inefficient.
This is an attempt to acknowledge reflinked implementations (which can be used by Loki smart pointers, for
example). However, there aren't any shared_ptr
implementations that use reflinking, especially
after C++11 recognized the existence of multithreading. Everyone uses atomic refcounts, so use_count()
is just an atomic load.
[Urbana 2014-11-07: Move to Ready]
Proposed resolution:
This wording is relative to N3936.
Change 20.3.2.2.6 [util.smartptr.shared.obs] p7-p10 as depicted:
long use_count() const noexcept;-7- Returns: the number of
shared_ptr
objects,*this
included, that share ownership with*this
, or 0 when*this
is empty.-8- [Note:use_count()
is not necessarily efficient. — end note]bool unique() const noexcept;-9- Returns:
-10- [Note:use_count() == 1
.If you are usingunique()
may be faster thanuse_count()
.unique()
to implement copy on write, do not rely on a specific value whenget() == 0
. — end note]
Change 20.3.2.3.6 [util.smartptr.weak.obs] p1-p4 as depicted:
long use_count() const noexcept;-1- Returns: 0 if
*this
is empty; otherwise, the number ofshared_ptr
instances that share ownership with*this
.-2- [Note:use_count()
is not necessarily efficient. — end note]bool expired() const noexcept;-3- Returns:
use_count() == 0
.-4- [Note:expired()
may be faster thanuse_count()
. — end note]
reference_wrapper::operator()
's Remark should be deletedSection: 22.10.6.5 [refwrap.invoke] Status: C++17 Submitter: Stephan T. Lavavej Opened: 2014-10-01 Last modified: 2017-07-30
Priority: 4
View all other issues in [refwrap.invoke].
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Discussion:
22.10.6.5 [refwrap.invoke]/2 is no longer useful. (It was originally TR1 2.1.2.4 [tr.util.refwrp.invoke]/2.) First, we already have the As If Rule (6.9.1 [intro.execution]/1) and the STL Implementers Can Be Sneaky Rule (16.4.6.5 [member.functions]). Second, with variadic templates and other C++11/14 tech, this can be implemented exactly as depicted.
[2015-05, Lenexa]
DK: I don't see a defect here
STL: the issue is that the standard is overly verbose, we don't need this sentence. It's redundant.
MC: does anyone think this paragraph has value?
JW: it has negative value. reading it makes me wonder if there's some reason I would want to provide a set of overloaded
functions, maybe there's some problem with doing it the obvious way that I'm not clever enough to see.
Move to Ready status: 8 in favor, none against.
Proposed resolution:
This wording is relative to N3936.
Change 22.10.6.5 [refwrap.invoke] p2 as depicted:
template <class... ArgTypes> result_of_t<T&(ArgTypes&&...)> operator()(ArgTypes&&... args) const;-1- Returns:
INVOKE(get(), std::forward<ArgTypes>(args)...)
. (20.9.2)-2- Remark:operator()
is described for exposition only. Implementations are not required to provide an actualreference_wrapper::operator()
. Implementations are permitted to supportreference_wrapper
function invocation through multiple overloaded operators or through other means.
CopyConstructible
Section: 23.2.7 [associative.reqmts], 23.2.8 [unord.req] Status: C++17 Submitter: Stephan T. Lavavej Opened: 2014-10-01 Last modified: 2018-06-23
Priority: 2
View other active issues in [associative.reqmts].
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Discussion:
The associative container requirements attempt to permit comparators that are DefaultConstructible
but non-CopyConstructible
. However, the Standard contradicts itself. 23.4.3.1 [map.overview] depicts
map() : map(Compare()) { }
which requires both DefaultConstructible
and CopyConstructible
.
CopyConstructible
comparators.
(Note that DefaultConstructible
should remain optional; this is not problematic for implementers, and
allows users to use lambdas.)
Key equality predicates for unordered associative containers are also affected. However, 16.4.4.5 [hash.requirements]/1
already requires hashers to be CopyConstructible
, so 23.2.8 [unord.req]'s redundant wording should
be removed.
[2015-02, Cologne]
GR: I prefer to say "Compare
" rather than "X::key_compare
", since the former is what the user supplies.
JY: It makes sense to use "Compare
" when we talk about requirements but "key_compare
" when we use it.
CopyConstructible
comparator. But the simplification is probably worth it.
GR: I don't care about unmovable containers. But I do worry that people might want to move they comparators. MC: How do
you reconcile that with the function that says "give me the comparator"? GR: That one returns by value? JY: Yes. [To MC]
You make it a requirement of that function. [To GR] And it [the key_comp()
function] is missing its requirements.
We need to add them everywhere. GR: map already has the right requirements.
JM: I dispute this. If in C++98 a type wasn't copyable, it had some interesting internal state, but in C++98 you wouldn't
have been able to pass it into the container since you would have had to make a copy. JY: No, you could have
default-constructed it and never moved it, e.g. a mutex. AM: So, it's a design change, but one that we should make.
That's probably an LEWG issue. AM: There's a contradiction in the Standard here, and we need to fix it one way or another.
Conclusion: Move to LEWG
[LEWG: 2016-03, Jacksonville]
Adding CopyConstructible
requirement OK.
MoveConstructible
. A moved-from set<>
might still contain elements,
and using them would become undefined if the comparator changed behavior.
Proposed resolution:
This wording is relative to N3936.
Change 23.2.7 [associative.reqmts] Table 102 as indicated (Editorial note: For "expression" X::key_compare
"defaults to" is redundant with the class definitions for map/etc.):
Table 102 — Associative container requirements Expression Return type Assertion/note pre-/post-condition Complexity …
X::key_compare
Compare
defaults toless<key_type>
Requires:key_compare
isCopyConstructible
.compile time …
X(c)
X a(c);Requires:key_compare
isCopyConstructible
.
Effects: Constructs an empty container. Uses a copy ofc
as a comparison object.constant …
X(i,j,c)
X a(i,j,c);Requires: key_compare
isCopyConstructible
.
value_type
isEmplaceConstructible
intoX
from*i
.
Effects: Constructs […][…] …
Change 23.2.8 [unord.req] Table 103 as indicated:
Table 103 — Unordered associative container requirements (in addition to container) Expression Return type Assertion/note pre-/post-condition Complexity …
X::key_equal
Pred
Requires: Pred
isCopyConstructible
.
Pred
shall be a binary predicate that takes two arguments of typeKey
.
Pred
is an equivalence relation.compile time …
X(n, hf, eq)
X a(n, hf, eq);X
Requires:hasher
andkey_equal
areCopyConstructible
.
Effects: Constructs […][…] …
X(n, hf)
X a(n, hf);X
Requires: hasher
isCopyConstructible
and
key_equal
isDefaultConstructible
Effects: Constructs […][…] …
X(i, j, n, hf, eq)
X a(i, j, n, hf, eq);X
Requires: hasher
andkey_equal
areCopyConstructible
.
value_type
isEmplaceConstructible
intoX
from*i
.
Effects: Constructs […][…] …
X(i, j, n, hf)
X a(i, j, n, hf);X
Requires: hasher
isCopyConstructible
and
key_equal
isDefaultConstructible
value_type
isEmplaceConstructible
intoX
from*i
.
Effects: Constructs […][…] …
iterator_traits<OutIt>::reference
can and can't be void
Section: 24.3.5.2 [iterator.iterators] Status: C++17 Submitter: Stephan T. Lavavej Opened: 2014-10-01 Last modified: 2017-07-30
Priority: 3
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Discussion:
24.3.5.2 [iterator.iterators]/2 requires an Iterator
's *r
to return reference
,
i.e. iterator_traits<X>::reference
according to 24.3.1 [iterator.requirements.general]/11.
OutputIterator
's *r = o
to do its job,
so *r
clearly can't return void
.
24.3.2.3 [iterator.traits]/1 says: "In the case of an output iterator, the types
iterator_traits<Iterator>::difference_type iterator_traits<Iterator>::value_type iterator_traits<Iterator>::reference iterator_traits<Iterator>::pointer
may be defined as void
."
Iterator
to
InputIterator
, and making Iterator
say that *r
returns an unspecified type. This will
have the following effects:
Output-only iterators will inherit Iterator's "*r
returns unspecified" requirement, while
24.3.2.3 [iterator.traits]/1 clearly permits reference/etc. to be void
.
Input-or-stronger iterators (whether constant or mutable) are unaffected — they still have to satisfy
"*r
returns reference", they're just getting that requirement from InputIterator
instead of
Iterator
.
[2015-02 Cologne]
EF: This is related to 2438(i). MC: I'd like to take up 2438 right after this.
AM: Does anyone think this is wrong? GR: Why do we give output iterators to have reference type void? AM: we've mandated that certain output iterators define it as void since 1998. GR: Oh OK, I'm satisfied. Accepted. And 2438(i) is already Ready.Proposed resolution:
This wording is relative to N3936.
In 24.3.5.2 [iterator.iterators] Table 106 "Iterator requirements" change as indicated:
Table 106 — Iterator requirements Expression Return type Operational
semanticsAssertion/note pre-/post-condition *r
unspecifiedreference
pre: r
is dereferenceable.…
In 24.3.5.3 [input.iterators] Table 107 "Input iterator requirements" change as indicated:
Table 107 — Input iterator requirements (in addition to Iterator) Expression Return type Operational
semanticsAssertion/note pre-/post-condition …
*a
reference
, convertible toT
[…] …
std::iterator
inheritance shouldn't be mandatedSection: D.17 [depr.iterator] Status: C++17 Submitter: Stephan T. Lavavej Opened: 2014-10-01 Last modified: 2023-02-07
Priority: 3
View all issues with C++17 status.
Discussion:
For LWG convenience, nine STL iterators are depicted as deriving from std::iterator
to get their
iterator_category
/etc. typedefs. Unfortunately (and unintentionally), this also mandates the
inheritance, which is observable (not just through is_base_of
, but also overload resolution).
This is unfortunate because it confuses users, who can be misled into thinking that their own iterators
must derive from std::iterator
, or that overloading functions to take std::iterator
is
somehow meaningful. This is also unintentional because the STL's most important iterators, the container
iterators, aren't required to derive from std::iterator
. (Some are even allowed to be raw pointers.)
Finally, this unnecessarily constrains implementers, who may not want to derive from std::iterator
.
(For example, to simplify debugger views.)
std::iterator
if they want.
(Editorial note: The order of the typedefs follows the order of std::iterator
's template parameters.)
[Urbana 2014-11-07: Move to Ready]
Proposed resolution:
This wording is relative to N3936.
Change [storage.iterator], class template raw_storage_iterator
synopsis, as depicted:
template <class OutputIterator, class T> class raw_storage_iterator: public iterator<output_iterator_tag,void,void,void,void>{ public: typedef output_iterator_tag iterator_category; typedef void value_type; typedef void difference_type; typedef void pointer; typedef void reference; explicit raw_storage_iterator(OutputIterator x); […] };
Change 24.5.1.2 [reverse.iterator], class template reverse_iterator
synopsis, as depicted
(editorial note: this reorders "reference
, pointer
" to "pointer
, reference
"
and aligns whitespace):
template <class Iterator> class reverse_iterator: public iterator<typename iterator_traits<Iterator>::iterator_category, typename iterator_traits<Iterator>::value_type, typename iterator_traits<Iterator>::difference_type, typename iterator_traits<Iterator>::pointer, typename iterator_traits<Iterator>::reference>{ public:typedef Iterator iterator_type; typedef typename iterator_traits<Iterator>::difference_type difference_type; typedef typename iterator_traits<Iterator>::reference reference; typedef typename iterator_traits<Iterator>::pointer pointer;typedef Iterator iterator_type; typedef typename iterator_traits<Iterator>::iterator_category iterator_category; typedef typename iterator_traits<Iterator>::value_type value_type; typedef typename iterator_traits<Iterator>::difference_type difference_type; typedef typename iterator_traits<Iterator>::pointer pointer; typedef typename iterator_traits<Iterator>::reference reference; reverse_iterator(); […] };
Change 24.5.2.2 [back.insert.iterator], class template back_insert_iterator
synopsis, as depicted:
template <class Container> class back_insert_iterator: public iterator<output_iterator_tag,void,void,void,void>{ protected: Container* container; public: typedef output_iterator_tag iterator_category; typedef void value_type; typedef void difference_type; typedef void pointer; typedef void reference; typedef Container container_type; explicit back_insert_iterator(Container& x); […] };
Change 24.5.2.3 [front.insert.iterator], class template front_insert_iterator
synopsis, as depicted:
template <class Container> class front_insert_iterator: public iterator<output_iterator_tag,void,void,void,void>{ protected: Container* container; public: typedef output_iterator_tag iterator_category; typedef void value_type; typedef void difference_type; typedef void pointer; typedef void reference; typedef Container container_type; explicit front_insert_iterator(Container& x); […] };
Change 24.5.2.4 [insert.iterator], class template insert_iterator
synopsis, as depicted:
template <class Container> class insert_iterator: public iterator<output_iterator_tag,void,void,void,void>{ protected: Container* container; typename Container::iterator iter; public: typedef output_iterator_tag iterator_category; typedef void value_type; typedef void difference_type; typedef void pointer; typedef void reference; typedef Container container_type; insert_iterator(Container& x, typename Container::iterator i); […] };
Change 24.6.2 [istream.iterator], class template istream_iterator
synopsis, as depicted:
template <class T, class charT = char, class traits = char_traits<charT>, class Distance = ptrdiff_t> class istream_iterator: public iterator<input_iterator_tag, T, Distance, const T*, const T&>{ public: typedef input_iterator_tag iterator_category; typedef T value_type; typedef Distance difference_type; typedef const T* pointer; typedef const T& reference; […] };
Change 24.6.3 [ostream.iterator], class template ostream_iterator
synopsis, as depicted:
template <class T, class charT = char, class traits = char_traits<charT>> class ostream_iterator: public iterator<output_iterator_tag, void, void, void, void>{ public: typedef output_iterator_tag iterator_category; typedef void value_type; typedef void difference_type; typedef void pointer; typedef void reference; […] };
Change 24.6.4 [istreambuf.iterator], class template istreambuf_iterator
synopsis, as depicted:
template <class charT = char, class traits = char_traits<charT> > class istreambuf_iterator: public iterator<input_iterator_tag, charT, typename traits::off_type, unspecified, charT>{ public: typedef input_iterator_tag iterator_category; typedef charT value_type; typedef typename traits::off_type difference_type; typedef unspecified pointer; typedef charT reference; […] };
Change 24.6.5 [ostreambuf.iterator], class template ostreambuf_iterator
synopsis, as depicted
(editorial note: this removes a redundant "public:"):
template <class charT = char, class traits = char_traits<charT>> class ostreambuf_iterator: public iterator<output_iterator_tag, void, void, void, void>{ public: typedef output_iterator_tag iterator_category; typedef void value_type; typedef void difference_type; typedef void pointer; typedef void reference; […]public:[…] };
unique_copy()
sometimes can't fall back to reading its outputSection: 26.7.9 [alg.unique] Status: C++17 Submitter: Stephan T. Lavavej Opened: 2014-10-01 Last modified: 2017-07-30
Priority: 3
View other active issues in [alg.unique].
View all other issues in [alg.unique].
View all issues with C++17 status.
Discussion:
unique_copy()
's wording says that if it's given input-only and output-only iterators, it needs
the input's value type to be copyable. This is correct, because in this case the algorithm must have a
local element copy in order to detect duplicates.
InputIterator
that's forward or stronger, the input's
value type doesn't have to be copyable. This is also correct, because in this case the algorithm can reread
the input in order to detect duplicates.
Finally, the wording says that if it's given an input-only iterator with an OutputIterator
that's
forward or stronger, the input's value type doesn't have to be copyable. This is telling the algorithm to
compare its input to its output in order to detect duplicates, but that isn't always possible! If the input
and output have the same value type, then they can be compared (as long as *result = *first
behaves
sanely; see below). If they have different value types, then we can't compare them.
This could be resolved by requiring heterogeneous value types to be comparable in this situation, but that
would be extremely tricky to wordsmith (as it would challenge the concept of "group of equal elements" used
by the Effects). It will be vastly simpler and more effective to extend the "local element copy" requirement
to this scenario.
Note that the input-only, output forward-or-stronger, identical value types scenario needs a bit of work too.
We always require *result = *first to be "valid", but in this case we need to additionally require that the
assignment actually transfers the value. (Otherwise, we'd be allowing an op=()
that ignores *first
and always sets *result
to zero, or other unacceptable behavior.) This is just CopyAssignable
.
(What happens when unique_copy()
is given a move_iterator
is a separate issue.)
To summarize:
input forward+: no additional requirements
input-only, output forward+, same value types: needsCopyAssignable
input-only, output forward+, different value types: needsCopyConstructible
andCopyAssignable
input-only, output-only: needsCopyConstructible
andCopyAssignable
[Urbana 2014-11-07: Move to Ready]
Proposed resolution:
This wording is relative to N3936.
Change 26.7.9 [alg.unique] p5, as depicted:
template<class InputIterator, class OutputIterator> OutputIterator unique_copy(InputIterator first, InputIterator last, OutputIterator result); template<class InputIterator, class OutputIterator, class BinaryPredicate> OutputIterator unique_copy(InputIterator first, InputIterator last, OutputIterator result, BinaryPredicate pred);-5- Requires: The comparison function shall be an equivalence relation. The ranges [
first,last
) and [result,result+(last-first)
) shall not overlap. The expression*result = *first
shall be valid.If neitherLetInputIterator
norOutputIterator
meets the requirements of forward iterator then the value type ofInputIterator
shall beCopyConstructible
(Table 21) andCopyAssignable
(Table 23). OtherwiseCopyConstructible
is not required.T
be the value type ofInputIterator
. IfInputIterator
meets the forward iterator requirements, then there are no additional requirements forT
. Otherwise, ifOutputIterator
meets the forward iterator requirements and its value type is the same asT
, thenT
shall beCopyAssignable
(Table 23). Otherwise,T
shall be bothCopyConstructible
(Table 21) andCopyAssignable
.
seed_seq::size()
should be noexcept
Section: 29.5.8.1 [rand.util.seedseq] Status: C++17 Submitter: Stephan T. Lavavej Opened: 2014-10-01 Last modified: 2017-07-30
Priority: 0
View all other issues in [rand.util.seedseq].
View all issues with C++17 status.
Discussion:
Obvious.
[Urbana 2014-11-07: Move to Ready]
Proposed resolution:
This wording is relative to N3936.
Change 29.5.8.1 [rand.util.seedseq], class seed_seq
synopsis, as depicted:
class seed_seq { public: […] size_t size() const noexcept; […] };
Change 29.5.8.1 [rand.util.seedseq] around p10, as depicted:
size_t size() const noexcept;-10- Returns: The number of 32-bit units that would be returned by a call to
param()
.-11- Throws: Nothing.-12- Complexity: Constant time.
Section: 32.5.8 [atomics.types.generic] Status: C++17 Submitter: Stephan T. Lavavej Opened: 2014-10-01 Last modified: 2017-07-30
Priority: 0
View all other issues in [atomics.types.generic].
View all issues with C++17 status.
Discussion:
<atomic>
doesn't provide counterparts for <inttypes.h>
's most useful typedefs, possibly
because they're quasi-optional. We can easily fix this.
[2014-11, Urbana]
Typedefs were transitional compatibility hack. Should use _Atomic
macro or template.
E.g. _Atomic(int8_t)
. BUT _Atomic
disappeared!
Detlef will look for _Atomic
macro. If missing, will open issue.
[2014-11-25, Hans comments]
There is no _Atomic
in C++. This is related to the much more general unanswered question of whether C++17
should reference C11, C99, or neither.
[2015-02 Cologne]
AM: I think this is still an SG1 issue; they need to deal with it before we do.
[2015-05 Lenexa, SG1 response]
Move to SG1-OK status. This seems like an easy short-term fix. We probably need a paper on C/C++ atomics compatibility
to deal with _Atomic
, but that's a separable issue.
[2015-10 pre-Kona]
SG1 hands this over to LWG for wording review
Proposed resolution:
This wording is relative to N3936.
Change 32.5.8 [atomics.types.generic] p8 as depicted:
-8- There shall be atomic typedefs corresponding to the typedefs in the header
<inttypes.h>
as specified in Table 147.atomic_intN_t
,atomic_uintN_t
,atomic_intptr_t
, andatomic_uintptr_t
shall be defined if and only ifintN_t
,uintN_t
,intptr_t
, anduintptr_t
are defined, respectively.
Change 99 [atomics.types.operations.req], Table 147 ("atomic
<inttypes.h>
typedefs"), as depicted:
Table 147 — atomic
<inttypes.h>
typedefsAtomic typedef <inttypes.h>
typeatomic_int8_t
int8_t
atomic_uint8_t
uint8_t
atomic_int16_t
int16_t
atomic_uint16_t
uint16_t
atomic_int32_t
int32_t
atomic_uint32_t
uint32_t
atomic_int64_t
int64_t
atomic_uint64_t
uint64_t
…
call_once()
shouldn't DECAY_COPY()
Section: 32.6.7.2 [thread.once.callonce] Status: C++17 Submitter: Stephan T. Lavavej Opened: 2014-10-01 Last modified: 2017-07-30
Priority: Not Prioritized
View all other issues in [thread.once.callonce].
View all issues with C++17 status.
Discussion:
When LWG 891(i) overhauled call_once()
's specification, it used decay_copy()
, following
LWG 929(i)'s overhaul of thread
's constructor.
thread
's constructor, this is necessary and critically important. 32.4.3.3 [thread.thread.constr]/5
"The new thread of execution executes INVOKE(DECAY_COPY(std::forward<F>(f)),
DECAY_COPY(std::forward<Args>(args))...)
with the calls to DECAY_COPY
being evaluated in the constructing thread." requires the parent thread
to copy arguments for the child thread to access.
In call_once()
, this is unnecessary and harmful. It's unnecessary because call_once()
doesn't transfer
arguments between threads. It's harmful because:
decay_copy()
returns a prvalue. Given meow(int&)
, meow(i)
can be called directly,
but call_once(flag, meow, i)
won't compile.
decay_copy()
moves from modifiable rvalues. Given purr(const unique_ptr<int>&)
,
purr(move(up))
won't modify up
. (This is observable, because moved-from unique_ptr
s are
guaranteed empty.) However, call_once(flag, purr, move(up))
will leave up
empty after the first active
execution. Observe the behavioral difference — if purr()
is directly called like this repeatedly until it
doesn't throw an exception, each call will observe up
unchanged. With call_once()
, the second active
execution will observe up
to be empty.
call_once()
should use perfect forwarding without decay_copy()
, in order to avoid interfering with the call like this.
[2015-02 Cologne]
Handed over to SG1.
[2015-05 Lenexa, SG1 response]
Looks good to us, but this is really an LWG issue.
[2015-05-07 Lenexa: Move Immediate]
LWG 2442 call_once shouldn't decay_copy
STL summarizes the SG1 minutes.
Marshall: Jonathan updated all the issues with SG1 status last night. Except this one.
STL summarizes the issue.
Dietmar: Of course, call_once has become useless.
STL: With magic statics.
Jonathan: Magic statics can't be per object, which I use in future.
Marshall: I see why you are removing the MoveConstructible on the arguments, but what about Callable?
STL: That's a type named Callable, which we will no longer decay_copy. We're still requiring the INVOKE expression to be valid.
Marshall: Okay. Basically, ripping the decay_copy out of here.
STL: I recall searching the Standard for other occurrences and I believe this is the only inappropriate use of decay_copy.
Marshall: We do the decay_copy.
Jonathan: Us too.
Marshall: What do people think?
Jonathan: I think STL's right. In the use I was mentioning inside futures, I actually pass them by reference_wrapper and pointers, to avoid the decay causing problems. Inside the call_once, I then extract the args. So I've had to work around this and didn't realize it was a defect.
Marshall: What do people think is the right resolution?
STL: I would like to see Immediate.
Hwrd: No objections to Immediate.
Marshall: Bill is nodding.
PJP: He said it. Everything STL says applies to our other customers.
Marshall: Any objections to Immediate?
Jonathan: I can't see any funky implementations where a decay_copy would be necessary?
Marshall: 6 votes for Immediate, 0 opposed, 0 abstaining.
Proposed resolution:
This wording is relative to N3936.
Change 32.6.7.2 [thread.once.callonce] p1+p2 as depicted:
template<class Callable, class ...Args> void call_once(once_flag& flag, Callable&& func, Args&&... args);-1- Requires:
-2- Effects; […] An active execution shall callCallable
and eachTi
inArgs
shall satisfy theMoveConstructible
requirements.INVOKE(
(20.9.2) shall be a valid expression.DECAY_COPY(std::forward<Callable>(func)),DECAY_COPY(std::forward<Args>(args))...)INVOKE(
. […]DECAY_COPY(std::forward<Callable>(func)),DECAY_COPY(std::forward<Args>(args))...)
std::array
member functions should be constexpr
Section: 23.3.3 [array] Status: Resolved Submitter: Peter Sommerlad Opened: 2014-10-06 Last modified: 2020-09-06
Priority: Not Prioritized
View all other issues in [array].
View all issues with Resolved status.
Discussion:
When experimenting with C++14 relaxed constexpr
functions I made the observation that I couldn't
use std::array
to create a table of data at compile time directly using loops in a function.
However, a simple substitute I could use instead:
template <typename T, size_t n> struct ar { T a[n]; constexpr ar() : a{{}}{} constexpr auto data() const { return &a[0];} constexpr T const & operator[](size_t i) const { return a[i]; } constexpr T & operator[](size_t i) { return a[i]; } }; template <size_t n> using arr = ar<size_t, n>; // std::array<size_t, n>; template <size_t n> constexpr auto make_tab(){ arr<n> result; for(size_t i=0; i < n; ++i) result[i] = (i+1)*(i+1); // cannot define operator[] for mutable array... return result; } template <size_t n> constexpr auto squares=make_tab< n>(); int main() { int dummy[squares<5>[3]]; }
Therefore, I suggest that all member functions of std::array
should be made constexpr
to make the type usable in constexpr
functions.
fill
, which would require
fill_n
to be constexpr
as well.
[2014-11 Urbana]
Move to LEWG
The extent to which constexpr
becomes a part of the Library design is a policy
matter best handled initially by LEWG.
[08-2016, Post-Chicago]
Move to Tentatively Resolved
Proposed resolution:
This functionality is provided by P0031R0
std::sort_heap
Section: 26.8.8.5 [sort.heap] Status: C++20 Submitter: François Dumont Opened: 2014-10-07 Last modified: 2021-02-25
Priority: 3
View all issues with C++20 status.
Discussion:
While creating complexity tests for the GNU libstdc++ implementation I
stumbled across a surprising requirement for the std::sort_heap
algorithm.
Complexity: At most
N log(N)
comparisons (whereN == last - first
).
As stated on the libstdc++ mailing list by Marc Glisse sort_heap
can be implemented by N
calls to
pop_heap
. As max number of comparisons of pop_heap
is 2 * log(N)
then sort_heap
max limit should be 2 * log(1) + 2 * log(2) + .... + 2 * log(N)
that is to say 2 * log(N!)
.
In terms of log(N)
we can also consider that this limit is also cap by 2 * N * log(N)
which is surely what the Standard wanted to set as a limit.
Complexity: At most
2N log(N)
comparisons (whereN == last - first
).
[2015-02 Cologne]
Marshall will research the maths and report back in Lenexa.
[2015-05-06 Lenexa]
STL: I dislike exact complexity requirements, they prevent one or two extra checks in debug mode. Would it be better to say O(N log(N)) not at most?
[2017-03-04, Kona]
Move to Tentatively Ready. STL may write a paper (with Thomas & Robert) offering guidance about Big-O notation vs. exact requirements.
Proposed resolution:
This wording is relative to N3936.
In 26.8.8.5 [sort.heap] p3 the Standard states:
template<class RandomAccessIterator> void sort_heap(RandomAccessIterator first, RandomAccessIterator last); template<class RandomAccessIterator, class Compare> void sort_heap(RandomAccessIterator first, RandomAccessIterator last, Compare comp);[…]
-3- Complexity: At most2N log(N)
comparisons (whereN == last - first
).
Section: 99 [depr.util.smartptr.shared.atomic], 99 [atomics.types.operations.req] Status: Resolved Submitter: JF Bastien Opened: 2014-10-08 Last modified: 2017-11-29
Priority: Not Prioritized
View all other issues in [depr.util.smartptr.shared.atomic].
View all issues with Resolved status.
Discussion:
The definitions of compare and exchange in [util.smartptr.shared.atomic] p32 and 99 [atomics.types.operations.req] p20 state:
Requires: The failure argument shall not be
memory_order_release
normemory_order_acq_rel
. Thefailure
argument shall be no stronger than thesuccess
argument.
The term "stronger" isn't defined by the standard.
It is hinted at by 99 [atomics.types.operations.req] p21:When only one
memory_order
argument is supplied, the value ofsuccess
isorder
, and the value offailure
isorder
except that a value ofmemory_order_acq_rel
shall be replaced by the valuememory_order_acquire
and a value ofmemory_order_release
shall be replaced by the valuememory_order_relaxed
.
Should the standard define a partial ordering for memory orders, where consume and acquire are incomparable with release?
[2014-11 Urbana]
Move to SG1.
[2016-11-12, Issaquah]
Resolved by P0418R2
Proposed resolution:
volatile
-qualified value typesSection: 16.4.4.6 [allocator.requirements] Status: C++17 Submitter: Daniel Krügler Opened: 2014-10-16 Last modified: 2017-07-30
Priority: 4
View other active issues in [allocator.requirements].
View all other issues in [allocator.requirements].
View all issues with C++17 status.
Discussion:
According to Table 27 — "Descriptive variable definitions" which is used to define the symbols used in the
allocator requirements table within 16.4.4.6 [allocator.requirements] we have the following constraints for
the types T, U, C
:
any non-const object type (3.9)
This wording can be read to allow instead a volatile
-qualified value type such as volatile int
.
volatile
" as additional constraint to this table
row.
Another choice would be to think of requiring that allocators must be capable to handle any cv
-qualified
value types. This would make all currently existing allocators non-conforming that can't handle cv
-qualified
value types, so I'm not suggesting to follow that route.
A less radical step would be to allow cv
-qualified types just for C
(which is used to specify the
functions construct
and destroy
and where does not even exist any requirement that C
actually
is related to the value type of the allocator at all). This seemingly extension would be harmless because as of p8 of the
same sub-clause "An allocator may constrain the types on which it can be instantiated and the arguments for which its
construct
member may be called."
This differs from the requirements imposed on the types T
and U
which both refer to value types of allocators.
The proposed wording attempts to separate the two classes of requirements.
Previous resolution [SUPERSEDED]:
This wording is relative to N4140.
Change 16.4.4.6 [allocator.requirements], Table 27 — "Descriptive variable definitions", as indicated:
Table 27 — Descriptive variable definitions Variable Definition T, U
, Cany non- constconst
and non-volatile
object type (3.9)C
any object type …
Change 16.4.4.6 [allocator.requirements] p8 as indicated: (This wording change is intended to fix an obvious asymmetry between
construct
anddestroy
which I believe is not intended)-8- An allocator may constrain the types on which it can be instantiated and the arguments for which its
construct
ordestroy
members may be called. If a type cannot be used with a particular allocator, the allocator class or the call toconstruct
ordestroy
may fail to instantiate.
[2014-11, Urbana]
JW: say "cv-unqualified" instead?
JW: very nervous about allowing construct on const-types, because of the cast to (non-const) void*
MA: should we just make the minimal fix?
STL: don't break C
out for special treatment
New proposed resolution: just change "non-const" to "cv-unqualified". Keep addition of destroy
later.
[2015-02 Cologne]
GR: It makes me nervous that someone at some point decided to not add "non-volatile
".
AM: That was over ten years ago. It was a deliberate, minuted choice to support volatile
. We are now reversing that decision.
It would be good to poll our vendors, none of which are in the room. This is a bit more work than we expect of a P0 issue.
VV: libstdc++ and libc++ seem to support volatile
template parameters for the standard allocator.
AM: To clarify, the proposed resolution here would remove the requirement to support volatile
. Implementations could still
choose to support volatile
.
DK: I'm happy to drop this and open a new issue in regard to the destroy
member specification.
AM: I just think this is harder than a P0. Let's reprioritize.
[2015-04-01 Daniel comments]
The less controversial part of the issue related to constraints imposed on destroy
has be handed over to the new
issue 2470(i).
[2015-05-06 Lenexa: Move to Ready]
Proposed resolution:
This wording is relative to N4431.
Change 16.4.4.6 [allocator.requirements], Table 27 — "Descriptive variable definitions", as indicated:
Table 27 — Descriptive variable definitions Variable Definition T, U, C
any non-constcv-unqualified object type (3.9)…
Section: 23.2.2 [container.requirements.general] Status: C++17 Submitter: Daniel Krügler Opened: 2014-10-18 Last modified: 2025-03-13
Priority: 0
View other active issues in [container.requirements.general].
View all other issues in [container.requirements.general].
View all issues with C++17 status.
Discussion:
According to Table 96 — "Container requirements" the specification:
note: the destructor is applied to every element of
a
; any memory obtained is deallocated.
The initial "note:" can be read as if that part of the specification would not be normative (This note form differs from footnotes in tables, which have normative meaning).
It seems that this initial part of the specification exists since C++98. But comparing with the similar SGI Container specification there is no evidence for that being intended to be non-normative.[2015-02, Cologne]
NJ: If we fix this, we should also fix it elsewhere. Oh, this is the only place?
GR: If this is intended to be different from elsewhere, we should make sure.
AM: valarray
specifies this without the "note:".
DK: valarray
requires trivially destructible types!
GR: That's good enough for me.
NJ: First time valarray
has been useful for something!
Proposed resolution:
This wording is relative to N4140.
Change 23.2.2 [container.requirements.general], Table 96 — "Container requirements", as indicated:
Table 96 — Container requirements Expression Return type Operational
semanticsAssertion/note
pre-/post-conditionComplexity …
(&a)->~X()
void
note:the destructor
is applied to every
element ofa
; any
memory obtained is deallocated.linear …
(greater|less|greater_equal|less_equal)<void>
do not yield a total order for pointersSection: 22.10.8 [comparisons] Status: C++17 Submitter: Joaquín M López Muñoz Opened: 2014-10-30 Last modified: 2017-07-30
Priority: 2
View other active issues in [comparisons].
View all other issues in [comparisons].
View all issues with C++17 status.
Discussion:
less<void>::operator(t, u)
(and the same applies to the rest of void
specializations for standard
comparison function objects) returns t < u
even if t
and u
are pointers, which by
7.6.9 [expr.rel]/3 is undefined except if both pointers point to the same array or object. This might be
regarded as a specification defect since the intention of N3421 is that less<>
can substitute for
less<T>
in any case where the latter is applicable. less<void>
can be rewritten in
the following manner to cope with pointers:
template<> struct less<void> { typedef unspecified is_transparent; template <class T, class U> struct pointer_overload : std::is_pointer<std::common_type_t<T, U>> {}; template < class T, class U, typename std::enable_if<!pointer_overload<T, U>::value>::type* = nullptr > auto operator()(T&& t, U&& u) const -> decltype(std::forward<T>(t) < std::forward<U>(u)) { return std::forward<T>(t) < std::forward<U>(u); } template < class T, class U, typename std::enable_if<pointer_overload<T, U>::value>::type* = nullptr > auto operator()(T&& t, U&& u) const -> decltype(std::declval<std::less<std::common_type_t<T, U>>>()(std::forward<T>(t), std::forward<U>(u))) { std::less<std::common_type_t<T, U>> l; return l(std::forward<T>(t), std::forward<U>(u)); } };
This wording is relative to N4140.
Change 22.10.8 [comparisons] p14 as indicated:
-14- For templates
greater
,less
,greater_equal
, andless_equal
, the specializations for any pointer type yield a total order, even if the built-in operators<
,>
,<=
,>=
do not. For template specializationsgreater<void>
,less<void>
,greater_equal<void>
, andless_equal<void>
, the call operator with arguments whose common typeCT
is a pointer yields the same value as the corresponding comparison function object class specialization forCT
.
[2015-02, Cologne]
AM: Is there any way this will be resolved elsewhere? VV: No. AM: Then we should bite the bullet and deal with it here.
MC: These diamond operators are already ugly. Making them more ugly isn't a big problem. JY found some issue with types that are convertible, and will reword. Jeffrey suggests improved wording.[2015-05, Lenexa]
STL: when diamond functions designed, this was on purpose
STL: this does go against the original design
STL: library is smarter and can give a total order
MC: given that the original design rejected this, give back to LEWG
STL: original proposal did not talk about total order
STL: don't feel strongly about changing the design
STL: no objections to taking this issue with some wording changes if people want it
MC: not happy with wording, comparing pointers — what does that mean?
STL: needs careful attention to wording
STL: want to guarantee that nullptr
participates in total ordering
STL: all hooks into composite pointer type
MC: move from new to open with better wording
STL: to check updates to issue after Lenexa
[2015-06, Telecon]
MC: STL on the hook to update. He's shipping something today so not here.
MC: also add link to N4229
[2015-10, Kona Saturday afternoon]
STL was on the hook for wording, but STL: I don't care. The architecture on which this is an issue does not exist.
STL: We will also need to incorporate nullptr. TK: I think that's implied, since the wording is in terms of the resulting operation, not the deduced types.
STL: Seems legit. MC: I guess I'm OK with this. TK: I'm weakly in favour, so that we can get people to use transparent comparators without worrying.
STL: There's no change to implementations.
Move to Tentatively ready.
Proposed resolution:
This wording is relative to N4296.
Change 22.10.8 [comparisons] p14 as indicated:
-14- For templates
greater
,less
,greater_equal
, andless_equal
, the specializations for any pointer type yield a total order, even if the built-in operators<
,>
,<=
,>=
do not. For template specializationsgreater<void>
,less<void>
,greater_equal<void>
, andless_equal<void>
, if the call operator calls a built-in operator comparing pointers, the call operator yields a total order.
optional<T>
should 'forward' T
's implicit conversionsSection: 5.3 [fund.ts.v2::optional.object] Status: TS Submitter: Geoffrey Romer Opened: 2014-10-31 Last modified: 2018-07-08
Priority: Not Prioritized
View all other issues in [fund.ts.v2::optional.object].
View all issues with TS status.
Discussion:
Addresses: fund.ts.v2
Code such as the following is currently ill-formed (thanks to STL for the compelling example):
optional<string> opt_str = "meow";
This is because it would require two user-defined conversions (from const char*
to string
,
and from string
to optional<string>
) where the language permits only one. This is
likely to be a surprise and an inconvenience for users.
optional<T>
should be implicitly convertible from any U
that is implicitly convertible
to T
. This can be implemented as a non-explicit constructor template optional(U&&)
,
which is enabled via SFINAE only if is_convertible_v<U, T>
and is_constructible_v<T, U>
,
plus any additional conditions needed to avoid ambiguity with other constructors (see
N4064, particularly the
"Odd" example, for why is_convertible
and is_constructible
are both needed; thanks to Howard
Hinnant for spotting this).
In addition, we may want to support explicit construction from U
, which would mean providing a corresponding
explicit constructor with a complementary SFINAE condition (this is the single-argument case of the "perfect
initialization" pattern described in N4064).
[2015-10, Kona Saturday afternoon]
STL: This has status LEWG, but it should be priority 1, since we cannot ship an IS without this.
TK: We assigned our own priorities to LWG-LEWG issues, but haven't actually processed any issues yet.
MC: This is important.
[2016-02-17, Ville comments and provides concrete wording]
I have prototype-implemented this wording in libstdc++. I didn't edit
the copy/move-assignment operator tables into the new
operator=
templates that take optional
s of a different
type; there's a drafting note that suggests copying them
from the existing tables.
[LEWG: 2016-03, Jacksonville]
Discussion of whether variant
supports this. We think it does.
Proposed resolution:
This wording is relative to N4562.
Edit 22.5.3 [optional.optional] as indicated:
template <class T> class optional { public: typedef T value_type; // 5.3.1, Constructors constexpr optional() noexcept; constexpr optional(nullopt_t) noexcept; optional(const optional&); optional(optional&&) noexcept(see below); constexpr optional(const T&); constexpr optional(T&&); template <class... Args> constexpr explicit optional(in_place_t, Args&&...); template <class U, class... Args> constexpr explicit optional(in_place_t, initializer_list<U>, Args&&...); template <class U> constexpr optional(U&&); template <class U> constexpr optional(const optional<U>&); template <class U> constexpr optional(optional<U>&&); […] // 5.3.3, Assignment optional& operator=(nullopt_t) noexcept; optional& operator=(const optional&); optional& operator=(optional&&) noexcept(see below); template <class U> optional& operator=(U&&); template <class U> optional& operator=(const optional<U>&); template <class U> optional& operator=(optional<U>&&); template <class... Args> void emplace(Args&&...); template <class U, class... Args> void emplace(initializer_list<U>, Args&&...); […] };
In 5.3.1 [fund.ts.v2::optional.object.ctor], insert new signature specifications after p33:
[Note: The following constructors are conditionally specified as
explicit
. This is typically implemented by declaring two such constructors, of which at most one participates in overload resolution. — end note]template <class U> constexpr optional(U&& v);-?- Effects: Initializes the contained value as if direct-non-list-initializing an object of type
-?- Postconditions:T
with the expressionstd::forward<U>(v)
.*this
contains a value. -?- Throws: Any exception thrown by the selected constructor ofT
. -?- Remarks: IfT
's selected constructor is aconstexpr
constructor, this constructor shall be aconstexpr
constructor. This constructor shall not participate in overload resolution unlessis_constructible_v<T, U&&>
istrue
andU
is not the same type asT
. The constructor isexplicit
if and only ifis_convertible_v<U&&, T>
isfalse
.template <class U> constexpr optional(const optional<U>& rhs);-?- Effects: If
-?- Postconditions:rhs
contains a value, initializes the contained value as if direct-non-list-initializing an object of typeT
with the expression*rhs
.bool(rhs) == bool(*this)
. -?- Throws: Any exception thrown by the selected constructor ofT
. -?- Remarks: IfT
's selected constructor is aconstexpr
constructor, this constructor shall be aconstexpr
constructor. This constructor shall not participate in overload resolution unlessis_constructible_v<T, const U&>
istrue
,is_same<decay_t<U>, T>
isfalse
,is_constructible_v<T, const optional<U>&>
isfalse
andis_convertible_v<const optional<U>&, T>
isfalse
. The constructor isexplicit
if and only ifis_convertible_v<const U&, T>
isfalse
.template <class U> constexpr optional(optional<U>&& rhs);-?- Effects: If
-?- Postconditions:rhs
contains a value, initializes the contained value as if direct-non-list-initializing an object of typeT
with the expressionstd::move(*rhs)
.bool(rhs)
is unchanged.bool(rhs) == bool(*this)
. -?- Throws: Any exception thrown by the selected constructor ofT
. -?- Remarks: IfT
's selected constructor is aconstexpr
constructor, this constructor shall be aconstexpr
constructor. This constructor shall not participate in overload resolution unlessis_constructible_v<T, U&&>
istrue
,is_same<decay_t<U>, T>
isfalse
,is_constructible_v<T, optional<U>&&>
isfalse
andis_convertible_v<optional<U>&&, T>
isfalse
andU
is not the same type asT
. The constructor isexplicit
if and only ifis_convertible_v<U&&, T>
isfalse
.
In 5.3.3 [fund.ts.v2::optional.object.assign], change as indicated:
template <class U> optional<T>& operator=(U&& v);-22- Remarks: If any exception is thrown, the result of the expression
bool(*this)
remains unchanged. If an exception is thrown during the call toT
's constructor, the state ofv
is determined by the exception safety guarantee ofT
's constructor. If an exception is thrown during the call toT
's assignment, the state of*val
andv
is determined by the exception safety guarantee ofT
's assignment. The function shall not participate in overload resolution unlessdecay_t<U>
is notnullopt_t
anddecay_t<U>
is not a specialization ofoptional
.is_same_v<decay_t<U>, T>
istrue
-23- Notes: The reason for providing such generic assignment and then constraining it so that effectivelyT == U
is to guarantee that assignment of the formo = {}
is unambiguous.template <class U> optional<T>& operator=(const optional<U>& rhs);-?- Requires:
-?- Effects:is_constructible_v<T, const U&>
istrue
andis_assignable_v<T&, const U&>
istrue
.
Table ? — optional::operator=(const optional<U>&)
effects*this
contains a value*this
does not contain a valuerhs
contains a valueassigns *rhs
to the contained valueinitializes the contained value as if direct-non-list-initializing an object of type T
with*rhs
rhs
does not contain a valuedestroys the contained value by calling val->T::~T()
no effect -?- Returns:
-?- Postconditions:*this
.bool(rhs) == bool(*this)
. -?- Remarks: If any exception is thrown, the result of the expressionbool(*this)
remains unchanged. If an exception is thrown during the call toT
's constructor, the state of*rhs.val
is determined by the exception safety guarantee ofT
's constructor. If an exception is thrown during the call toT
's assignment, the state of*val
and*rhs.val
is determined by the exception safety guarantee ofT
's assignment. The function shall not participate in overload resolution unlessis_same_v<decay_t<U>, T>
isfalse
.template <class U> optional<T>& operator=(optional<U>&& rhs);-?- Requires:
-?- Effects: The result of the expressionis_constructible_v<T, U>
istrue
andis_assignable_v<T&, U>
istrue
.bool(rhs)
remains unchanged.
Table ? — optional::operator=(optional<U>&&)
effects*this
contains a value*this
does not contain a valuerhs
contains a valueassigns std::move(*rhs)
to the contained valueinitializes the contained value as if direct-non-list-initializing an object of type T
withstd::move(*rhs)
rhs
does not contain a valuedestroys the contained value by calling val->T::~T()
no effect -?- Returns:
-?- Postconditions:*this
.bool(rhs) == bool(*this)
. -?- Remarks: If any exception is thrown, the result of the expressionbool(*this)
remains unchanged. If an exception is thrown during the call toT
's constructor, the state of*rhs.val
is determined by the exception safety guarantee ofT
's constructor. If an exception is thrown during the call toT
's assignment, the state of*val
and*rhs.val
is determined by the exception safety guarantee ofT
's assignment. The function shall not participate in overload resolution unlessis_same_v<decay_t<U>, T>
isfalse
.
raw_storage_iterator::base()
memberSection: 99 [depr.storage.iterator] Status: C++17 Submitter: Jonathan Wakely Opened: 2014-11-11 Last modified: 2017-07-30
Priority: 0
View all other issues in [depr.storage.iterator].
View all issues with C++17 status.
Discussion:
Eric Niebler pointed out that raw_storage_iterator
should give access
to the OutputIterator
it wraps.
[begin, raw.base())
[2015-02 Cologne]
NJ: Is this "const" correct [in "base()"]? DK: Yes, we always do that. NJ: And the output iterator is not qualifying in any way? AM/DK: That wouldn't make sense. NJ: OK.
VV: What did LEWG say about this feature request? In other words, why is this a library issue? AM: LEWG/JY thought this wouldn't be a contentious issue. NJ: I really hope the split of LEWG and LWG will be fixed soon, since it's only wasting time. VV: So you want to spend even more of your time on discussions that LEWG has? AM: I think this specified correctly. I'm not wild about it. But no longer bothered to stand in its way. GR: Why do we need to repeat the type in "Returns" even though it's part of the synopsis? AM: Good point, but not worth fixing. NJ: Why is "base()
" for reverse_iterator
commented with "// explicit"? AM: I guess in 1998 that was the
only way to say this.
AM: So, it's tentatively ready.
Proposed resolution:
This wording is relative to N4140.
Add a new function to the synopsis in [storage.iterator] p1:
namespace std { template <class OutputIterator, class T> class raw_storage_iterator : public iterator<output_iterator_tag,void,void,void,void> { public: explicit raw_storage_iterator(OutputIterator x); raw_storage_iterator<OutputIterator,T>& operator*(); raw_storage_iterator<OutputIterator,T>& operator=(const T& element); raw_storage_iterator<OutputIterator,T>& operator++(); raw_storage_iterator<OutputIterator,T> operator++(int); OutputIterator base() const; }; }
Insert the new function and a new paragraph series after p7:
OutputIterator base() const;-?- Returns: An iterator of type
OutputIterator
that points to the same value as*this
points to.
Section: 16.4.4.6 [allocator.requirements] Status: C++17 Submitter: Pablo Halpern Opened: 2014-11-11 Last modified: 2017-07-30
Priority: Not Prioritized
View other active issues in [allocator.requirements].
View all other issues in [allocator.requirements].
View all issues with C++17 status.
Discussion:
16.4.4.6 [allocator.requirements]/4 in the 2014-10 WP (N4140), says:
An allocator type
X
shall satisfy the requirements ofCopyConstructible
(17.6.3.1). TheX::pointer
,X::const_pointer
,X::void_pointer
, andX::const_void_pointer
types shall satisfy the requirements ofNullablePointer
(17.6.3.3). No constructor, comparison operator, copy operation, move operation, or swap operation on these types shall exit via an exception.X::pointer
andX::const_pointer
shall also satisfy the requirements for a random access iterator (24.2).
The words "these types" would normally apply only to the previous sentence only, i.e., only to the pointer types.
However, an alternative reading would be that the allocator constructors themselves cannot throw. The change to
the vector
and string
default constructors, making them unconditionally noexcept
depends
on this alternative reading.
noexcept
specifications for the string
and vector
default constructors
should be changed to make them conditional.
[2015-01-18 Library reflector vote]
The issue has been identified as Tentatively Ready based on six votes in favour.
Proposed resolution:
Change 16.4.4.6 [allocator.requirements] p4 as indicated:
An allocator type
X
shall satisfy the requirements ofCopyConstructible
(17.6.3.1). TheX::pointer
,X::const_pointer
,X::void_pointer
, andX::const_void_pointer
types shall satisfy the requirements ofNullablePointer
(17.6.3.3). No constructor, comparison operator, copy operation, move operation, or swap operation on these pointer types shall exit via an exception.X::pointer
andX::const_pointer
shall also satisfy the requirements for a random access iterator (24.2).
Change 27.4.3 [basic.string] following p5, class template basic_string
synopsis, as indicated:
(This change assumes that N4258 has been applied, as voted on in Urbana on 2014-11-08)
// 21.4.2, construct/copy/destroy: basic_string() noexcept(noexcept(Allocator())) : basic_string(Allocator()) { }
An alternative formulation of the above would be:
// 21.4.2, construct/copy/destroy: basic_string() noexcept(is_nothrow_default_constructible<Allocator>{}) : basic_string(Allocator()) { }
Change 23.3.13.1 [vector.overview] following p2, class template vector
synopsis, as indicated:
(This change assumes that N4258 has been applied, as voted on in Urbana on 2014-11-08)
// 23.3.6.2, construct/copy/destroy: vector() noexcept(noexcept(Allocator())) : vector(Allocator()) { }
An alternative formulation of the above would be:
// 23.3.6.2, construct/copy/destroy: vector() noexcept(is_nothrow_default_constructible<Allocator>{}) : vector(Allocator()) { }
swap
' throughout librarySection: 22.2 [utility], 22.3.2 [pairs.pair], 22.4 [tuple], 23.3.3 [array], 23.6.3 [queue], 23.6.4 [priority.queue], 23.6.6 [stack] Status: Resolved Submitter: Richard Smith Opened: 2014-11-14 Last modified: 2016-03-07
Priority: 1
View all other issues in [utility].
View all issues with Resolved status.
Discussion:
We have this antipattern in various library classes:
void swap(priority_queue& q) noexcept( noexcept(swap(c, q.c)) && noexcept(swap(comp, q.comp)))
This doesn't work. The unqualified lookup for 'swap' finds the member named 'swap', and that suppresses ADL, so the exception specification is always ill-formed because you can't call the member 'swap' with two arguments.
Relevant history on the core language side: This used to be ill-formed due to 6.4.7 [basic.scope.class] p1 rule 2: "A nameN
used in a class
S
shall refer to the same declaration in its context and when re-evaluated in the completed scope of S
.
No diagnostic is required for a violation of this rule."
Core issue 1330 introduced delay-parsing for exception specifications. Due to the 6.4.7 [basic.scope.class] rules,
this shouldn't have changed the behavior of any conforming programs. But it changes the behavior in the non-conforming
case from "no diagnostic required" to "diagnostic required", so implementations that implement core issue 1330 are now
required to diagnose the ill-formed declarations in the standard library.
Suggested resolution:
Add an is_nothrow_swappable
trait, and use it throughout the library in place of these noexcept
expressions.
[2015-02, Cologne]
No action for now; we intend to have papers for Lenexa.
[2015-05, Lenexa]
Move to Open.
Daniel: A first paper (N4426) exists to suggest some ways of solving this issue.[2015-10, Kona, Daniel comments]
A revised paper (N4511) has been provided
[2015-12-16, Daniel comments]
Revision 2 (P0185R0) will available for the mid February 2016 mailing.
[2016-03, Jacksonville]
P0185R1 was adopted in Jacksonville.Proposed resolution:
Section: 17.6 [support.dynamic], 17.6.3.2 [new.delete.single], 17.6.3.3 [new.delete.array] Status: C++17 Submitter: Richard Smith Opened: 2014-11-23 Last modified: 2017-07-30
Priority: 2
View all other issues in [support.dynamic].
View all issues with C++17 status.
Discussion:
N3778 added the following sized deallocation signatures to the library:
void operator delete(void* ptr, std::size_t size) noexcept; void operator delete[](void* ptr, std::size_t size) noexcept; void operator delete(void* ptr, std::size_t size, const std::nothrow_t&) noexcept; void operator delete[](void* ptr, std::size_t size, const std::nothrow_t&) noexcept;
The former two are an essential part of the proposal. The latter two seem
spurious — they are not called when new (std::nothrow) X
fails due to
X::X()
throwing, because the core language rules for selecting a placement
deallocation function do not consider passing a size
argument. Instead, the
above would be the matching deallocation functions for:
void *operator new(std::size_t size, std::size_t size_again, const std::nothrow_t&) noexcept; void *operator new[](std::size_t size, std::size_t size_again, const std::nothrow_t&) noexcept;
... which don't exist.
Since they're not implicitly called, the only other possible use for those functions would be to perform an explicitly non-throwing deallocation. But... the first two overloads are already explicitly non-throwing and are required to be semantically identical to the second two. So there's no point in making an explicit call to the second pair of functions either. It seems to me that we should remove the(void*, size_t, nothrow_t)
overloads, because
the core working group decided during the Urbana 2014 meeting, that no change to the core
language was warranted.
[2014-11-23, Daniel suggests concrete wording changes]
[2015-02 Cologne]
Nobody can call those overloads, since the nothrow allocation functions cannot throw. JY: Ship it. GR: Should we do due diligence and make sure we're deleting what we mean to be deleting? [Some checking, everything looks good.]
Accepted.Proposed resolution:
This wording is relative to N4140.
Change 17.6 [support.dynamic], header <new>
synopsis, as indicated:
[…] void operator delete(void* ptr, std::size_t size) noexcept;void operator delete(void* ptr, std::size_t size, const std::nothrow_t&) noexcept;[…] void operator delete[](void* ptr, std::size_t size) noexcept;void operator delete[](void* ptr, std::size_t size, const std::nothrow_t&) noexcept;[…]
Change 17.6.3.2 [new.delete.single], starting before p19, as indicated:
void operator delete(void* ptr, const std::nothrow_t&) noexcept;void operator delete(void* ptr, std::size_t size, const std::nothrow_t&) noexcept;[…]
-20- Replaceable: a C++ program may define a function with signaturevoid operator delete(void* ptr, const std::nothrow_t&) noexcept
that displaces the default version defined by the C++ standard library.If this function (without[…]size
parameter) is defined, the program should also definevoid operator delete(void* ptr, std::size_t size, const std::nothrow_t&) noexcept
. If this function withsize
parameter is defined, the program shall also define the version without thesize
parameter. [Note: The default behavior below may change in the future, which will require replacing both deallocation functions when replacing the allocation function. — end note]-22- Requires: If present, thestd::size_t size
argument must equal thesize
argument passed to the allocation function that returnedptr
.-23- Required behavior: Calls to-24- Default behavior:operator delete(void* ptr, std::size_t size, const std::nothrow_t&)
may be changed to calls tooperator delete(void* ptr, const std::nothrow_t&)
without affecting memory allocation. [Note: A conforming implementation is foroperator delete(void* ptr, std::size_t size, const std::nothrow_t&)
to simply calloperator delete(void* ptr, const std::nothrow_t&)
. — end note]operator delete(void* ptr, std::size_t size, const std::nothrow_t&)
callsoperator delete(ptr, std::nothrow)
, andoperator delete(void* ptr, const std::nothrow_t&)
callsoperator delete(ptr)
.
Change 17.6.3.3 [new.delete.array], starting before p16, as indicated:
void operator delete[](void* ptr, const std::nothrow_t&) noexcept;void operator delete[](void* ptr, std::size_t size, const std::nothrow_t&) noexcept;[…]
-17- Replaceable: a C++ program may define a function with signaturevoid operator delete[](void* ptr, const std::nothrow_t&) noexcept
that displaces the default version defined by the C++ standard library.If this function (without[…]size
parameter) is defined, the program should also definevoid operator delete[](void* ptr, std::size_t size, const std::nothrow_t&) noexcept
. If this function withsize
parameter is defined, the program shall also define the version without thesize
parameter. [Note: The default behavior below may change in the future, which will require replacing both deallocation functions when replacing the allocation function. — end note]-19- Requires: If present, thestd::size_t size
argument must equal thesize
argument passed to the allocation function that returnedptr
.-20- Required behavior: Calls to-21- Default behavior:operator delete[](void* ptr, std::size_t size, const std::nothrow_t&)
may be changed to calls tooperator delete[](void* ptr, const std::nothrow_t&)
without affecting memory allocation. [Note: A conforming implementation is foroperator delete[](void* ptr, std::size_t size, const std::nothrow_t&)
to simply calloperator delete[](void* ptr, const std::nothrow_t&)
. — end note]operator delete[](void* ptr, std::size_t size, const std::nothrow_t&)
callsoperator delete[](ptr, std::nothrow)
, andoperator delete[](void* ptr, const std::nothrow_t&)
callsoperator delete[](ptr)
.
std::polar
should require a non-negative rhoSection: 29.4.7 [complex.value.ops] Status: C++17 Submitter: Marshall Clow Opened: 2014-10-22 Last modified: 2017-07-30
Priority: 0
View all other issues in [complex.value.ops].
View all issues with C++17 status.
Discussion:
Different implementations give different answers for the following code:
#include <iostream> #include <complex> int main () { std::cout << std::polar(-1.0, -1.0) << '\n'; return 0; }
One implementation prints:
(nan, nan)
Another:
(-0.243068, 0.243068)
Which is correct? Or neither?
In this list, Howard Hinnant wrote:I've read this over several times. I've consulted C++11, C11, and IEC 10967-3. [snip]
I'm finding:
The magnitude of a complex number
== abs(c) == hypot(c.real(), c.imag())
and is always non-negative (by all three of the above standards).Therefore no complex number exists for which
abs(c) < 0
.Therefore when the first argument to
std::polar
(which is calledrho
) is negative, no complex number can be formed which meets the post-conidtion thatabs(c) == rho
.One could argue that this is already covered in 29.4 [complex.numbers]/3, but I think it's worth making explicit.
[2015-02, Cologne]
Discussion on whether theta should also be constrained.
TK: infinite theta doesn't make sense, whereas infinite rho does (theta is on a compact domain, rho is on a non-compact domain).
AM: We already have a narrow contract, so I don't mind adding further requirements. Any objections to requiring that theta be finite?
Some more discussion, but general consensus. Agreement that if someone finds the restrictions problematic, they should write
a proper paper to address how std::polar
should behave. For now, we allow infinite rho (but not NaN and not negative),
and require finite theta.
Proposed resolution:
This wording is relative to N4296.
Change 29.4.7 [complex.value.ops] around p9 as indicated
template<class T> complex<T> polar(const T& rho, const T& theta = 0);-?- Requires:
-9- Returns: The complex value corresponding to a complex number whose magnitude isrho
shall be non-negative and non-NaN.theta
shall be finite.rho
and whose phase angle istheta
.
Section: 21.3.8.7 [meta.trans.other], 24.3.2.3 [iterator.traits] Status: C++17 Submitter: Richard Smith Opened: 2014-11-19 Last modified: 2017-07-30
Priority: 2
View all other issues in [meta.trans.other].
View all issues with C++17 status.
Discussion:
LWG issue 2408(i) changes the meat of the specification of common_type
to compute:
[…] the type, if any, of an unevaluated conditional expression (5.16) whose first operand is an arbitrary value of type
bool
, whose second operand is anxvalue
of typeT1
, and whose third operand is an xvalue of typeT2
.
This has an effect on the specification that I think was unintended. It used to be the case that
common_type<T&, U&&>
would consider the type of a conditional between an
lvalue of type T
and an xvalue of type U
. It's now either invalid (because there is
no such thing as an xvalue of reference type) or considers the type of a conditional between an xvalue
of type T
and an xvalue of type U
, depending on how you choose to read it.
typedef decay_t<decltype(true ? declval<T>() : declval<U>())> type;
to:
typedef decay_t<decltype(true ? declval<remove_reference_t<T>>() : declval<remove_reference_t<U>>())> type;
It also makes common_type
underspecified in the case where one of the operands is of type void
;
in that case, the resulting type depends on whether the expression is a throw-expression, which is not
specified (but used to be).
iterator_traits<T>
"shall have no members" in some cases. That's wrong. It's a class type;
it always has at least a copy constructor, a copy assignment operator, and a destructor. Plus this
removes the usual library liberty to add additional members with names that don't collide with normal
usage (for instance, if a later version of the standard adds members, they can't be present here as a
conforming extension). Perhaps this should instead require that the class doesn't have members with any
of those five names? That's what 2408(i) does for common_type
's type member.
[2016-08 Chicago]
This issue has two parts, one dealing with common_type
, the other with iterator_traits
.
The first of these is resolved by 2465(i). See below for the proposed resolution for the other one.
Wed PM: Move to Tentatively Ready
Proposed resolution:
Change 24.3.2.3 [iterator.traits] p.2:
[…] as publicly accessible members and no other members:
[…]
Otherwise, iterator_traits<Iterator>
shall have no members by any of the above names.
std::ios_base::failure
is overspecifiedSection: 31.5.2 [ios.base], 31.5.2.2.1 [ios.failure] Status: C++17 Submitter: Jonathan Wakely Opened: 2014-12-15 Last modified: 2023-02-07
Priority: 3
View all other issues in [ios.base].
View all issues with C++17 status.
Discussion:
31.5.2 [ios.base] defines ios_base::failure
as a nested class:
namespace std { class ios_base { public: class failure; […] }; […] }
This means it is valid to use an elaborated-type-specifier to
refer to ios_base::failure
:
using F = class std::ios_base::failure; throw F();
Therefore implementations are not permitted to define
ios_base::failure
as a typedef e.g.
class ios_base { public: #if __cplusplus < 201103L class failure_cxx03 : public exception {...}; typedef failure_cxx03 failure; #else class failure_cxx11 : public system_error {...}; typedef failure_cxx11 failure; #endif […] };
This constrains implementations, making it harder to manage the ABI
change to ios_base::failure
between C++03 and C++11.
[2015-05-06 Lenexa: Move to Ready]
JW: the issue is that users are currently allowed to write "class failure" with an elaborated-type-specifier and it must be well-formed, I want the freedom to make that type a typedef, so they can't necessarily use an elaborated-type-specifier (which there is no good reason to use anyway)
JW: ideally I'd like this everywhere for all nested classes, but that's a paper not an issue, I only need this type fixed right now.
RD: is a synonym the same as an alias?
JW: dcl.typedef says a typedef introduces a synonym for another type, so I think this is the right way to say this
JW: I already shipped this last month
PJP: we're going to have to break ABIs again some time, we need all the wiggle room we can get to make that easier. This helps.
MC: do we want this at all? Ready?
9 in favor, none opose or abstaining
Proposed resolution:
This wording is relative to N4296.
Change the synopsis in 31.5.2 [ios.base] as indicated:
namespace std { class ios_base { public: class failure; // see below […] }; […] };
Change 31.5.2 [ios.base] paragraph 1:
ios_base
defines several member types:
a
classtypefailure
failure
, defined as either a class derived fromsystem_error
or a synonym for a class derived fromsystem_error
;
Change [ios::failure] paragraph 1:
-1- An implementation is permitted to define
ios_base::failure
as a synonym for a class with equivalent functionality to classios_base::failure
shown in this subclause. [Note: Whenios_base::failure
is a synonym for another type it shall provide a nested typefailure
, to emulate the injected class name. — end note] The classfailure
defines the base class for the types of all objects thrown as exceptions, by functions in the iostreams library, to report errors detected during stream buffer operations.
sample()
algorithmSection: 10.3 [fund.ts::alg.random.sample] Status: TS Submitter: Joe Gottman Opened: 2014-12-17 Last modified: 2017-07-30
Priority: 0
View all issues with TS status.
Discussion:
Addresses: fund.ts
According to paragraph 10.1 of the Library Fundamentals 1 draft, the complexity of the new
std::experimental::sample
template function is 𝒪(n
). Note that n
is actually
a parameter of this function, corresponding to the sample size. But both common algorithms for
sampling, the selection algorithm and the reservoir algorithm, are linear with respect to the
population size, which is often many orders of magnitude bigger than the sample size.
[2015-02, Cologne]
AM: I suggest we make this a DR against the Fundamentals TS.
GR: Agreed, this is a no-brainer.
Proposed resolution:
This wording is relative to N4335 in regard to fundamental-ts changes.
Change 10.3 [fund.ts::alg.random.sample] p5 to read:
-5- Complexity: 𝒪(
).
nlast - first
try_emplace
and insert_or_assign
misspecifiedSection: 23.4.3.4 [map.modifiers], 23.5.3.4 [unord.map.modifiers] Status: C++17 Submitter: Thomas Koeppe Opened: 2014-12-17 Last modified: 2017-07-30
Priority: 2
View all other issues in [map.modifiers].
View all issues with C++17 status.
Discussion:
The specification of the try_emplace
and insert_or_assign
member functions in N4279
contains the following errors and omissions:
In insert_or_assign
, each occurrence of std::forward<Args>(args)...
should be std::forward<M>(obj)
; this is was a mistake introduced in editing.
In try_emplace
, the construction of the value_type
is misspecified, which
is a mistake that was introduced during the evolution from a one-parameter to a variadic form.
As written, value_type(k, std::forward<Args>(args)...)
does not do the right thing;
it can only be used with a single argument, which moreover must be convertible to a mapped_type
.
The intention is to allow direct-initialization from an argument pack, and the correct constructor
should be value_type(piecewise_construct, forward_as_tuple(k),
forward_as_tuple(std::forward<Args>(args)...)
.
Both try_emplace
and insert_or_assign
are missing requirements on the
argument types. Since the semantics of these functions are specified independent of other functions,
they need to include their requirements.
[2015-02, Cologne]
This issue is related to 2469(i).
AM: The repeated references to "first and third forms" and "second and fourth forms" is a bit cumbersome. Maybe split the four functions?[2015-03-26, Thomas provides improved wording]
The approach is to split the descriptions of the various blocks of four functions into two blocks each so as to make the wording easier to follow.
Previous resolution [SUPERSEDED]:This wording is relative to N4296.
Apply the following changes to section 23.4.3.4 [map.modifiers] p3:
template <class... Args> pair<iterator, bool> try_emplace(const key_type& k, Args&&... args); template <class... Args> pair<iterator, bool> try_emplace(key_type&& k, Args&&... args); template <class... Args> iterator try_emplace(const_iterator hint, const key_type& k, Args&&... args); template <class... Args> iterator try_emplace(const_iterator hint, key_type&& k, Args&&... args);-?- Requires: For the first and third forms,
-3- Effects:value_type
shall beEmplaceConstructible
into map frompiecewise_construct
,forward_as_tuple(k)
,forward_as_tuple(forward<Args>(args)...)
. For the second and fourth forms,value_type
shall beEmplaceConstructible
into map frompiecewise_construct
,forward_as_tuple(move(k))
,forward_as_tuple(forward<Args>(args)...)
.If the keyIf the map does already contain an element whose key is equivalent tok
already exists in the map, there is no effect. Otherwise, inserts an element into the map. In the first and third forms, the element is constructed from the arguments asvalue_type(k, std::forward<Args>(args)...)
. In the second and fourth forms, the element is constructed from the arguments asvalue_type(std::move(k), std::forward<Args>(args)...)
. In the first two overloads, thebool
component of the returned pair istrue
if and only if the insertion took place. The returned iterator points to the element of the map whose key is equivalent tok
k
, there is no effect. Otherwise for the first and third forms inserts avalue_type
objectt
constructed withpiecewise_construct
,forward_as_tuple(k)
,forward_as_tuple(forward<Args>(args)...)
, for the second and fourth forms inserts avalue_type
objectt
constructed withpiecewise_construct
,forward_as_tuple(move(k))
,forward_as_tuple(forward<Args>(args)...)
. -?- Returns: In the first two overloads, thebool
component of the returned pair istrue
if and only if the insertion took place. The returned iterator points to the map element whose key is equivalent tok
.Apply the following changes to section 23.4.3.4 [map.modifiers] p5:
template <class M> pair<iterator, bool> insert_or_assign(const key_type& k, M&& obj); template <class M> pair<iterator, bool> insert_or_assign(key_type&& k, M&& obj); template <class M> iterator insert_or_assign(const_iterator hint, const key_type& k, M&& obj); template <class M> iterator insert_or_assign(const_iterator hint, key_type&& k, M&& obj);-?- Requires:
-5- Effects:is_assignable<mapped_type&, M&&>::value
shall be true. For the first and third forms,value_type
shall beEmplaceConstructible
into map fromk
,forward<M>(obj)
. For the second and fourth forms,value_type
shall beEmplaceConstructible
into map frommove(k), forward<M>(obj)
.If the keyIf the map does already contain an element whose key is equivalent tok
does not exist in the map, inserts an element into the map. In the first and third forms, the element is constructed from the arguments asvalue_type(k, std::forward<Args>(args)...)
. In the second and fourth forms, the element is constructed from the arguments asvalue_type(std::move(k), std::forward<Args>(args)...)
. If the key already exists,std::forward<M>(obj)
is assigned to themapped_type
corresponding to the key. In the first two overloads, thebool
component of the returned value is true if and only if the insertion took place. The returned iterator points to the element that was inserted or updatedk
,forward<M>(obj)
is assigned to themapped_type
corresponding to the key. Otherwise the first and third forms inserts avalue_type
objectt
constructed withk
,forward<M>(obj)
, the second and fourth forms inserts avalue_type
objectt
constructed withmove(k), forward<M>(obj)
. -?- Returns: In the first two overloads, thebool
component of the returned pair istrue
if and only if the insertion took place. The returned iterator points to the element of the map whose key is equivalent tok
.Apply the following changes to section 23.5.3.4 [unord.map.modifiers] p5:
template <class... Args> pair<iterator, bool> try_emplace(const key_type& k, Args&&... args); template <class... Args> pair<iterator, bool> try_emplace(key_type&& k, Args&&... args); template <class... Args> iterator try_emplace(const_iterator hint, const key_type& k, Args&&... args); template <class... Args> iterator try_emplace(const_iterator hint, key_type&& k, Args&&... args);-?- Requires: For the first and third forms,
-5- Effects:value_type
shall beEmplaceConstructible
into unordered_map frompiecewise_construct
,forward_as_tuple(k)
,forward_as_tuple(forward<Args>(args)...)
. For the second and fourth forms,value_type
shall beEmplaceConstructible
into unordered_map frompiecewise_construct
,forward_as_tuple(move(k))
,forward_as_tuple(forward<Args>(args)...)
.If the keyIf the unordered_map does already contain an element whose key is equivalent tok
already exists in the map, there is no effect. Otherwise, inserts an element into the map. In the first and third forms, the element is constructed from the arguments asvalue_type(k, std::forward<Args>(args)...)
. In the second and fourth forms, the element is constructed from the arguments asvalue_type(std::move(k), std::forward<Args>(args)...)
. In the first two overloads, thebool
component of the returned pair istrue
if and only if the insertion took place. The returned iterator points to the element of the map whose key is equivalent tok
k
, there is no effect. Otherwise for the first and third forms inserts avalue_type
objectt
constructed withpiecewise_construct
,forward_as_tuple(k)
,forward_as_tuple(forward<Args>(args)...)
, for the second and fourth forms inserts avalue_type
objectt
constructed withpiecewise_construct
,forward_as_tuple(move(k))
,forward_as_tuple(forward<Args>(args)...)
. -?- Returns: In the first two overloads, thebool
component of the returned pair istrue
if and only if the insertion took place. The returned iterator points to the element of the unordered_map whose key is equivalent tok
.Apply the following changes to section 23.5.3.4 [unord.map.modifiers] p7:
template <class M> pair<iterator, bool> insert_or_assign(const key_type& k, M&& obj); template <class M> pair<iterator, bool> insert_or_assign(key_type&& k, M&& obj); template <class M> iterator insert_or_assign(const_iterator hint, const key_type& k, M&& obj); template <class M> iterator insert_or_assign(const_iterator hint, key_type&& k, M&& obj);-?- Requires:
-7- Effects:is_assignable<mapped_type&, M&&>::value
shall be true. For the first and third forms,value_type
shall beEmplaceConstructible
into unordered_map fromk
,forward<M>(obj)
. For the second and fourth forms,value_type
shall beEmplaceConstructible
into unordered_map frommove(k), forward<M>(obj)
.If the keyIf the unordered_map does already contain an element whose key is equivalent tok
does not exist in the map, inserts an element into the map. In the first and third forms, the element is constructed from the arguments asvalue_type(k, std::forward<Args>(args)...)
. In the second and fourth forms, the element is constructed from the arguments asvalue_type(std::move(k), std::forward<Args>(args)...)
. If the key already exists,std::forward<M>(obj)
is assigned to themapped_type
corresponding to the key. In the first two overloads, thebool
component of the returned value is true if and only if the insertion took place. The returned iterator points to the element that was inserted or updatedk
,forward<M>(obj)
is assigned to themapped_type
corresponding to the key. Otherwise the first and third forms inserts avalue_type
objectt
constructed withk
,forward<M>(obj)
, the second and fourth forms inserts avalue_type
objectt
constructed withmove(k), forward<M>(obj)
. -?- Returns: In the first two overloads, thebool
component of the returned pair istrue
if and only if the insertion took place. The returned iterator points to the element of the unordered_map whose key is equivalent tok
.
[2015-05, Lenexa]
STL: existing wording is horrible, this is Thomas' wording and his issue
STL: already implemented the piecewise part
MC: ok with changes
STL: changes are mechanical
STL: believe this is P1, it must be fixed, we have wording
PJP: functions are sensible
STL: has been implemented
MC: consensus is to move to ready
Proposed resolution:
This wording is relative to N4296.
Apply the following changes to 23.4.3.4 [map.modifiers] p3+p4:
template <class... Args> pair<iterator, bool> try_emplace(const key_type& k, Args&&... args);template <class... Args> pair<iterator, bool> try_emplace(key_type&& k, Args&&... args);template <class... Args> iterator try_emplace(const_iterator hint, const key_type& k, Args&&... args);template <class... Args> iterator try_emplace(const_iterator hint, key_type&& k, Args&&... args);-?- Requires:
-3- Effects:value_type
shall beEmplaceConstructible
intomap
frompiecewise_construct, forward_as_tuple(k), forward_as_tuple(forward<Args>(args)...)
.If the keyIf the map already contains an element whose key is equivalent tok
already exists in the map, there is no effect. Otherwise, inserts an element into the map. In the first and third forms, the element is constructed from the arguments asvalue_type(k, std::forward<Args>(args)...)
. In the second and fourth forms, the element is constructed from the arguments asvalue_type(std::move(k), std::forward<Args>(args)...)
. In the first two overloads, thebool
component of the returned pair istrue
if and only if the insertion took place. The returned iterator points to the element of the map whose key is equivalent tok
k
, there is no effect. Otherwise inserts an object of typevalue_type
constructed withpiecewise_construct, forward_as_tuple(k), forward_as_tuple(forward<Args>(args)...)
. -?- Returns: In the first overload, thebool
component of the returned pair istrue
if and only if the insertion took place. The returned iterator points to the map element whose key is equivalent tok
. -4- Complexity: The same asemplace
andemplace_hint
, respectively.template <class... Args> pair<iterator, bool> try_emplace(key_type&& k, Args&&... args); template <class... Args> iterator try_emplace(const_iterator hint, key_type&& k, Args&&... args);-?- Requires:
-?- Effects: If the map already contains an element whose key is equivalent tovalue_type
shall beEmplaceConstructible
intomap
frompiecewise_construct, forward_as_tuple(move(k)), forward_as_tuple(forward<Args>(args)...)
.k
, there is no effect. Otherwise inserts an object of typevalue_type
constructed withpiecewise_construct, forward_as_tuple(move(k)), forward_as_tuple(forward<Args>(args)...)
. -?- Returns: In the first overload, thebool
component of the returned pair istrue
if and only if the insertion took place. The returned iterator points to the map element whose key is equivalent tok
. -?- Complexity: The same asemplace
andemplace_hint
, respectively.
Apply the following changes to 23.4.3.4 [map.modifiers] p5+p6:
template <class M> pair<iterator, bool> insert_or_assign(const key_type& k, M&& obj);template <class M> pair<iterator, bool> insert_or_assign(key_type&& k, M&& obj);template <class M> iterator insert_or_assign(const_iterator hint, const key_type& k, M&& obj);template <class M> iterator insert_or_assign(const_iterator hint, key_type&& k, M&& obj);-?- Requires:
-5- Effects:is_assignable<mapped_type&, M&&>::value
shall betrue
.value_type
shall beEmplaceConstructible
intomap
fromk, forward<M>(obj)
.If the keyIf the map already contains an elementk
does not exist in the map, inserts an element into the map. In the first and third forms, the element is constructed from the arguments asvalue_type(k, std::forward<Args>(args)...)
. In the second and fourth forms, the element is constructed from the arguments asvalue_type(std::move(k), std::forward<Args>(args)...)
. If the key already exists,std::forward<M>(obj)
is assigned to themapped_type
corresponding to the key. In the first two overloads, thebool
component of the returned value is true if and only if the insertion took place. The returned iterator points to the element that was inserted or updatede
whose key is equivalent tok
, assignsforward<M>(obj)
toe.second
. Otherwise inserts an object of typevalue_type
constructed withk, forward<M>(obj)
. -?- Returns: In the first overload, thebool
component of the returned pair istrue
if and only if the insertion took place. The returned iterator points to the map element whose key is equivalent tok
. -6- Complexity: The same asemplace
andemplace_hint
, respectively.template <class M> pair<iterator, bool> insert_or_assign(key_type&& k, M&& obj); template <class M> iterator insert_or_assign(const_iterator hint, key_type&& k, M&& obj);-?- Requires:
-?- Effects: If the map already contains an elementis_assignable<mapped_type&, M&&>::value
shall betrue
.value_type
shall beEmplaceConstructible
intomap
frommove(k), forward<M>(obj)
.e
whose key is equivalent tok
, assignsforward<M>(obj)
toe.second
. Otherwise inserts an object of typevalue_type
constructed withmove(k), forward<M>(obj)
. -?- Returns: In the first overload, thebool
component of the returned pair istrue
if and only if the insertion took place. The returned iterator points to the map element whose key is equivalent tok
. -?- Complexity: The same asemplace
andemplace_hint
, respectively.
Apply the following changes to 23.5.3.4 [unord.map.modifiers] p5+p6:
template <class... Args> pair<iterator, bool> try_emplace(const key_type& k, Args&&... args);template <class... Args> pair<iterator, bool> try_emplace(key_type&& k, Args&&... args);template <class... Args> iterator try_emplace(const_iterator hint, const key_type& k, Args&&... args);template <class... Args> iterator try_emplace(const_iterator hint, key_type&& k, Args&&... args);-?- Requires:
-5- Effects:value_type
shall beEmplaceConstructible
intounordered_map
frompiecewise_construct, forward_as_tuple(k), forward_as_tuple(forward<Args>(args)...)
.If the keyIf the map already contains an element whose key is equivalent tok
already exists in the map, there is no effect. Otherwise, inserts an element into the map. In the first and third forms, the element is constructed from the arguments asvalue_type(k, std::forward<Args>(args)...)
. In the second and fourth forms, the element is constructed from the arguments asvalue_type(std::move(k), std::forward<Args>(args)...)
. In the first two overloads, thebool
component of the returned pair istrue
if and only if the insertion took place. The returned iterator points to the element of the map whose key is equivalent tok
k
, there is no effect. Otherwise inserts an object of typevalue_type
constructed withpiecewise_construct, forward_as_tuple(k), forward_as_tuple(forward<Args>(args)...)
. -?- Returns: In the first overload, thebool
component of the returned pair istrue
if and only if the insertion took place. The returned iterator points to the map element whose key is equivalent tok
. -6- Complexity: The same asemplace
andemplace_hint
, respectively.template <class... Args> pair<iterator, bool> try_emplace(key_type&& k, Args&&... args); template <class... Args> iterator try_emplace(const_iterator hint, key_type&& k, Args&&... args);-?- Requires:
-?- Effects: If the map already contains an element whose key is equivalent tovalue_type
shall beEmplaceConstructible
intounordered_map
frompiecewise_construct, forward_as_tuple(move(k)), forward_as_tuple(forward<Args>(args)...)
.k
, there is no effect. Otherwise inserts an object of typevalue_type
constructed withpiecewise_construct, forward_as_tuple(move(k)), forward_as_tuple(forward<Args>(args)...)
. -?- Returns: In the first overload, thebool
component of the returned pair istrue
if and only if the insertion took place. The returned iterator points to the map element whose key is equivalent tok
. -?- Complexity: The same asemplace
andemplace_hint
, respectively.
Apply the following changes to 23.5.3.4 [unord.map.modifiers] p7+p8:
template <class M> pair<iterator, bool> insert_or_assign(const key_type& k, M&& obj);template <class M> pair<iterator, bool> insert_or_assign(key_type&& k, M&& obj);template <class M> iterator insert_or_assign(const_iterator hint, const key_type& k, M&& obj);template <class M> iterator insert_or_assign(const_iterator hint, key_type&& k, M&& obj);-?- Requires:
-7- Effects:is_assignable<mapped_type&, M&&>::value
shall betrue
.value_type
shall beEmplaceConstructible
intounordered_map
fromk, forward<M>(obj)
.If the keyIf the map already contains an elementk
does not exist in the map, inserts an element into the map. In the first and third forms, the element is constructed from the arguments asvalue_type(k, std::forward<Args>(args)...)
. In the second and fourth forms, the element is constructed from the arguments asvalue_type(std::move(k), std::forward<Args>(args)...)
. If the key already exists,std::forward<M>(obj)
is assigned to themapped_type
corresponding to the key. In the first two overloads, thebool
component of the returned value is true if and only if the insertion took place. The returned iterator points to the element that was inserted or updatede
whose key is equivalent tok
, assignsforward<M>(obj)
toe.second
. Otherwise inserts an object of typevalue_type
constructed withk, forward<M>(obj)
. -?- Returns: In the first overload, thebool
component of the returned pair istrue
if and only if the insertion took place. The returned iterator points to the map element whose key is equivalent tok
. -8- Complexity: The same asemplace
andemplace_hint
, respectively.template <class M> pair<iterator, bool> insert_or_assign(key_type&& k, M&& obj); template <class M> iterator insert_or_assign(const_iterator hint, key_type&& k, M&& obj);-?- Requires:
-?- Effects: If the map already contains an elementis_assignable<mapped_type&, M&&>::value
shall betrue
.value_type
shall beEmplaceConstructible
intounordered_map
frommove(k), forward<M>(obj)
.e
whose key is equivalent tok
, assignsforward<M>(obj)
toe.second
. Otherwise inserts an object of typevalue_type
constructed withmove(k), forward<M>(obj)
. -?- Returns: In the first overload, thebool
component of the returned pair istrue
if and only if the insertion took place. The returned iterator points to the map element whose key is equivalent tok
. -?- Complexity: The same asemplace
andemplace_hint
, respectively.
common_type
is nearly impossible to specialize
correctly and regresses key functionalitySection: 21.3.8.7 [meta.trans.other] Status: Resolved Submitter: Eric Niebler Opened: 2015-01-12 Last modified: 2017-09-07
Priority: 2
View all other issues in [meta.trans.other].
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Discussion:
I think there's a defect regarding common_type
and its specializations.
Unless I've missed it, there is nothing preventing folks from
instantiating common_type
with cv-qualified types or reference types. In
fact, the wording in N3797 explicitly mentions cv void
, so presumably at
least cv qualifications are allowed.
common_type
when at least of of
the types is user-defined. (A separate issue is the meaning of
user-defined. In core, I believe this is any class/struct/union/enum,
but in lib, I think it means any type not defined in std, right?) There
is at least one place in the standard that specializes common_type
(time.traits.specializations) on time_point and duration. But the
specializations are only for non-cv-qualified and non-reference
specializations of time_point
and duration
.
If the user uses, say, common_type<duration<X,Y> const, duration<A,B>
const>
, they're not going to get the behavior they expect.
Suggest we clarify the requirements of common_type
's template
parameters. Also, perhaps we can add blanket wording that common_type<A
[cv][&], B [cv][&]>
is required to be equivalent to
common_type<A,B>
(if that is in fact the way we intent this to work).
Also, the change to make common_type
SFINAE-friendly regressed key
functionality, as noted by Agustín K-ballo Bergé in
c++std-lib-37178.
Since decay_t
is not applied until the very end of the type computation,
user specializations are very likely to to be found.
Agustín says:
Consider the following snippet:
struct X {}; struct Y { explicit Y(X){} }; namespace std { template<> struct common_type<X, Y> { typedef Y type; }; template<> struct common_type<Y, X> { typedef Y type; }; } static_assert(is_same<common_type_t<X, Y>, Y>()); // (A) static_assert(is_same<common_type_t<X, Y, Y>, Y>()); // (B) static_assert(is_same<common_type_t<X, X, Y>, Y>()); // (C)Under the original wording, all three assertion holds. Under the current wording,
(A) picks the user-defined specialization, so the assertion holds.
(B) goes to the third bullet and, ignoring the user-defined specialization, looks for
decltype(true ? declval<X>() : declval<Y>())
; since it is ill-formed there is no common type.(C) goes to the third bullet and yields
common_type_t<X&&, Y>
, which again misses the user-defined specialization.
The discussion following c++std-lib-35636
seemed to cohere around the idea that the primary common_type
specialization should have the effect
of stripping top-level ref and cv qualifiers by applying std::decay_t
to its arguments and,
if any of them change as a result of that transformation, re-dispatching to common_type
on those transformed
arguments, thereby picking up any user-defined specializations. This change to common_type
would make
the specializations in time.traits.specializations sufficient.
namespace detail { template<typename T, typename U> using default_common_t = decltype(true? std::declval<T>() : std::declval<U>()); template<typename T, typename U, typename Enable = void> struct common_type_if {}; template<typename T, typename U> struct common_type_if<T, U, void_t<default_common_t<T, U>>> { using type = decay_t<default_common_t<T, U>>; }; template<typename T, typename U, typename TT = decay_t<T>, typename UU = decay_t<U>> struct common_type2 : common_type<TT, UU> // Recurse to catch user specializations {}; template<typename T, typename U> struct common_type2<T, U, T, U> : common_type_if<T, U> {}; template<typename Meta, typename Enable = void> struct has_type : std::false_type {}; template<typename Meta> struct has_type<Meta, void_t<typename Meta::type>> : std::true_type {}; template<typename Meta, typename...Ts> struct common_type_recurse : common_type<typename Meta::type, Ts...> {}; template<typename Meta, typename...Ts> struct common_type_recurse_if : std::conditional< has_type<Meta>::value, common_type_recurse<Meta, Ts...>, empty >::type {}; } template<typename ...Ts> struct common_type {}; template<typename T> struct common_type<T> { using type = std::decay_t<T>; }; template<typename T, typename U> struct common_type<T, U> : detail::common_type2<T, U> {}; template<typename T, typename U, typename... Vs> struct common_type<T, U, Vs...> : detail::common_type_recurse_if<common_type<T, U>, Vs...> {};
[2016-08 Chicago]
Walter and Nevin provide wording.
Previous resolution [SUPERSEDED]:
[This also resolves the first part of 2460(i)]
In Table 46 of N4604, entry for
common_type
:... may specialize this trait if at least one template parameter in the specialization is a user-defined type and no template parameter is cv-qualified.
In [meta.trans.other] bullet 3.3:
... whose second operand is an xvalue of type
T1
decay_t<T1>
, and whose third operand is an xvalue of typeT2
decay_t<T2>
. If ...
[2016-08-02, Chicago: Walt, Nevin, Rob, and Hal provide revised wording]
Previous resolution [SUPERSEDED]:
This wording is relative to N4606.
[This also resolves the first part of LWG 2460(i)]
In Table 46 — "Other transformations" edit the entry for
common_type
:
Table 46 — Other transformations Template Comments …
template <class... T>
struct common_type;The member typedef type
shall be defined or omitted as specified below.
If it is omitted, there shall be no membertype
. All types in the
parameter packT
shall be complete or (possibly cv)void
.
A program may specialize this trait for two cv-unqualified non-reference types
if at least onetemplate parameter in the specializationof them
is a user-defined type. [Note: Such specializations are
needed when only explicit conversions are desired among the template
arguments. — end note]…
Edit 21.3.8.7 [meta.trans.other] p3 (and its subbullets) as shown below
For the
common_type
trait applied to a parameter packT
of types, the membertype
shall be either defined or not present as follows:
If
sizeof...(T)
is zero, there shall be no membertype
.If
sizeof...(T)
is one, letT0
denote the sole type in the packT
. The member typedeftype
shall denote the same type asdecay_t<T0>
.If
sizeof...(T)
is two, letT1
andT2
, respectively, denote the first and second types comprisingT
, and letD1
andD2
, respectively, denotedecay_t<T1>
anddecay_t<T2>
.
If
is_same_v<T1, D1>
andis_same_v<T2, D2>
, and if there is no specializationcommon_type<T1, T2>
, letC
denote the type, if any, of an unevaluated conditional expression (7.6.16 [expr.cond]) whose first operand is an arbitrary value of typebool
, whose second operand is an xvalue of typeD1
, and whose third operand is an xvalue of typeD2
. If there is such a typeC
, the member typedeftype
shall denoteC
. Otherwise, there shall be no membertype
.If
not is_same_v<T1, D1>
ornot is_same_v<T2, D2>
, the member typedeftype
shall denote the same type, if any, ascommon_type_t<D1, D2>
. Otherwise, there shall be no membertype
.If
sizeof...(T)
is greater thanonetwo, letT1
,T2
, andR
, respectively, denote the first, second, and (pack of) remaining types comprisingT
.[Note:Letsizeof...(R)
may be zero. — end note] LetC
denote the type, if any, of an unevaluated conditional expression (7.6.16 [expr.cond]) whose first operand is an arbitrary value of typebool
, whose second operand is an xvalue of typeT1
, and whose third operand is an xvalue of typeT2
.C
denotecommon_type_t<T1, T2>
. If there is such a typeC
, the member typedeftype
shall denote the same type, if any, ascommon_type_t<C, R...>
. Otherwise, there shall be no membertype
.
[2016-08-03 Chicago LWG]
LWG asks for minor wording tweaks and for an added Note. Walter revises the Proposed Resolution accordingly.
Previous resolution [SUPERSEDED]:
This wording is relative to N4606.
[This also resolves the first part of LWG 2460(i)]
In Table 46 — "Other transformations" edit the entry for
common_type
:
Table 46 — Other transformations Template Comments …
template <class... T>
struct common_type;The member typedef type
shall be defined or omitted as specified below.
If it is omitted, there shall be no membertype
. All types in the
parameter packT
shall be complete or (possibly cv)void
.
A program may specialize this trait for two cv-unqualified non-reference types
if at least onetemplate parameter in the specializationof them
is a user-defined type. [Note: Such specializations are
needed when only explicit conversions are desired among the template
arguments. — end note]…
Edit 21.3.8.7 [meta.trans.other] p3 (and its subbullets) as shown below
For the
common_type
trait applied to a parameter packT
of types, the membertype
shall be either defined or not present as follows:
(3.1) — If
sizeof...(T)
is zero, there shall be no membertype
.(3.2) — If
sizeof...(T)
is one, letT0
denote the sole type in the packT
. The member typedeftype
shall denote the same type asdecay_t<T0>
.(3.3) — If
sizeof...(T)
is two, letT1
andT2
, respectively, denote the first and second types comprisingT
, and letD1
andD2
, respectively, denotedecay_t<T1>
anddecay_t<T2>
.
(3.3.1) — If
is_same_v<T1, D1>
andis_same_v<T2, D2>
, letC
denote the type of an unevaluated conditional expression (7.6.16 [expr.cond]) whose first operand is an arbitrary value of typebool
, whose second operand is an xvalue of typeD1
, and whose third operand is an xvalue of typeD2
. [Note: This will not apply if there is a specializationcommon_type<D1, D2>
. — end note](3.3.2) — Otherwise, let
C
denote the typecommon_type_t<D1, D2>
.In either case, if there is such a type
C
, the member typedeftype
shall denoteC
. Otherwise, there shall be no membertype
.(3.4) — If
sizeof...(T)
is greater thanonetwo, letT1
,T2
, andR
, respectively, denote the first, second, and (pack of) remaining types comprisingT
.[Note:Letsizeof...(R)
may be zero. — end note] LetC
denote the type, if any, of an unevaluated conditional expression (7.6.16 [expr.cond]) whose first operand is an arbitrary value of typebool
, whose second operand is an xvalue of typeT1
, and whose third operand is an xvalue of typeT2
.C
denotecommon_type_t<T1, T2>
. If there is such a typeC
, the member typedeftype
shall denote the same type, if any, ascommon_type_t<C, R...>
. Otherwise, there shall be no membertype
.
[2016-08-04 Chicago LWG]
Alisdair notes that 16.4.5.2.1 [namespace.std] p.1 seems to prohibit some kinds of specializations that we want to permit here and asks that the Table entry be augmented so as to specify the precise rules that a specialization is required to obey. Walter revises Proposed Resolution accordingly.
[2016-08-03 Chicago]
Fri PM: Move to Tentatively Ready
[2016-08-11 Daniel comments]
LWG 2763(i) presumably provides a superiour resolution that also fixes another bug in the Standard.
[2016-08-12]
Howard request to reopen this issue because of the problem pointed out by LWG 2763(i).
[2016-08-13 Tim Song comments]
In addition to the issue pointed out in LWG 2763(i), the current P/R no longer decays the type
of the conditional expression. However, that seems harmless since 7 [expr]/5 means that the
"type of an expression" is never a reference type, and 7.6.16 [expr.cond]'s rules appear to ensure that
the type of the conditional expression will never be "decay-able" when fed with two xvalues of cv-unqualified
non-array object type. Nonetheless, a note along the lines of "[Note: C
is never a reference,
function, array, or cv-qualified type. — end note]" may be appropriate, similar to the note
at the end of [dcl.decomp]/1.
[2016-11-12, Issaquah]
Resolved by P0435R1
Proposed resolution:
This wording is relative to N4606.
[This also resolves the first part of LWG 2460(i)]
In Table 46 — "Other transformations" edit the entry for common_type
:
Table 46 — Other transformations Template Comments …
template <class... T>
struct common_type;Unless this trait is specialized (as specified in Note B, below), t The
member typedeftype
shall be defined or omitted as specified in Note A, below.
If it is omitted, there shall be no membertype
. All types in the
parameter packT
shall be complete or (possibly cv)void
.
A program may specialize this trait
if at least one template parameter in the specialization
is a user-defined type. [Note: Such specializations are
needed when only explicit conversions are desired among the template
arguments. — end note]…
Edit 21.3.8.7 [meta.trans.other] p3 (and its subbullets) as shown below
-3- Note A: For the
common_type
trait applied to a parameter packT
of types, the membertype
shall be either defined or not present as follows:
(3.1) — If
sizeof...(T)
is zero, there shall be no membertype
.(3.2) — If
sizeof...(T)
is one, letT0
denote the sole type in the packT
. The member typedeftype
shall denote the same type asdecay_t<T0>
.(3.3) — If
sizeof...(T)
is two, letT1
andT2
, respectively, denote the first and second types comprisingT
, and letD1
andD2
, respectively, denotedecay_t<T1>
anddecay_t<T2>
.
(3.3.1) — If
is_same_v<T1, D1>
andis_same_v<T2, D2>
, letC
denote the type of an unevaluated conditional expression (7.6.16 [expr.cond]) whose first operand is an arbitrary value of typebool
, whose second operand is an xvalue of typeD1
, and whose third operand is an xvalue of typeD2
. [Note: This will not apply if there is a specializationcommon_type<D1, D2>
. — end note](3.3.2) — Otherwise, let
C
denote the typecommon_type_t<D1, D2>
.In either case, if there is such a type
C
, the member typedeftype
shall denoteC
. Otherwise, there shall be no membertype
.(3.4) — If
sizeof...(T)
is greater thanonetwo, letT1
,T2
, andR
, respectively, denote the first, second, and (pack of) remaining types comprisingT
.[Note:Letsizeof...(R)
may be zero. — end note] LetC
denote the type, if any, of an unevaluated conditional expression (7.6.16 [expr.cond]) whose first operand is an arbitrary value of typebool
, whose second operand is an xvalue of typeT1
, and whose third operand is an xvalue of typeT2
.C
denotecommon_type_t<T1, T2>
. If there is such a typeC
, the member typedeftype
shall denote the same type, if any, ascommon_type_t<C, R...>
. Otherwise, there shall be no membertype
.-?- Note B: A program may specialize the
-4- [Example: Given these definitions: […]common_type
trait for two cv-unqualified non-reference types if at least one of them is a user-defined type. [Note: Such specializations are needed when only explicit conversions are desired among the template arguments. — end note] Such a specialization need not have a member namedtype
, but if it does, that member shall be a typedef-name for a cv-unqualified non-reference type that need not otherwise meet the specification set forth in Note A, above.
allocator_traits::max_size()
default behavior is incorrectSection: 16.4.4.6 [allocator.requirements], 20.2.9.3 [allocator.traits.members] Status: C++17 Submitter: Howard Hinnant Opened: 2015-01-17 Last modified: 2017-07-30
Priority: 3
View other active issues in [allocator.requirements].
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Discussion:
Table 28 — "Allocator requirements" says that default behavior for a.max_size()
is
numeric_limits<size_type>::max()
. And this is consistent with the matching statement
for allocator_traits
in 20.2.9.3 [allocator.traits.members]/p7:
static size_type max_size(const Alloc& a) noexcept;Returns:
a.max_size()
if that expression is well-formed; otherwise,numeric_limits<size_type>::max()
.
However, when allocating memory, an allocator must allocate n*sizeof(value_type)
bytes, for example:
value_type* allocate(std::size_t n) { return static_cast<value_type*>(::operator new (n * sizeof(value_type))); }
When n == numeric_limits<size_type>::max(), n * sizeof(value_type)
is guaranteed
to overflow except when sizeof(value_type) == 1
.
numeric_limits<size_type>::max() / sizeof(value_type)
.
[2015-05, Lenexa]
Marshall: Is this the right solution?
PJP: I think it's gilding the lily.
STL: I think this is right, and it doesn't interact with the incomplete container stuff because it's in a member function.
Marshall: Objections to this?
STL: Spaces around binary operators.
Hwrd: It's completely wrong without spaces.
Marshall: All in favor of Ready?
Lots.
Proposed resolution:
This wording is relative to N4296.
Change 16.4.4.6 [allocator.requirements], Table 28 — "Allocator requirements", as indicated:
Table 28 — Allocator requirements Expression Return type Assertion/note
pre-/post-conditionDefault …
a.max_size()
X::size_type
the largest value that can
meaningfully be passed to
X::allocate()
numeric_limits<size_type>::max()/sizeof(value_type)
…
Change 20.2.9.3 [allocator.traits.members]/p7 as indicated:
static size_type max_size(const Alloc& a) noexcept;Returns:
a.max_size()
if that expression is well-formed; otherwise,numeric_limits<size_type>::max()/sizeof(value_type)
.
is_always_equal
has slightly inconsistent defaultSection: 16.4.4.6 [allocator.requirements], 20.2.9.2 [allocator.traits.types] Status: C++17 Submitter: Howard Hinnant Opened: 2015-01-18 Last modified: 2017-07-30
Priority: 0
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Discussion:
Table 28 — "Allocator requirements" says that X::is_always_equal
has a default value
of is_empty<X>
, and this is consistent with the return type description:
Identical to or derived from
true_type
orfalse_type
is_empty<X>
is guaranteed to be derived from either true_type
or false_type
.
So far so good.
typedef see below is_always_equal;Type:
Alloc::is_always_equal
if the qualified-idAlloc::is_always_equal
is valid and denotes a type (14.8.2); otherwiseis_empty<Alloc>::type
.
This is subtly different than what Table 28 says is the default: is_empty<Alloc>::type
is
not is_empty<Alloc>
, but is rather one of true_type
or false_type
.
Change Table 28 to say: is_empty<X>::type
.
Change 20.2.9.2 [allocator.traits.types]/p10:
Type:
Alloc::is_always_equal
if the qualified-idAlloc::is_always_equal
is valid and denotes a type (14.8.2); otherwiseis_empty<Alloc>
.::type
Both options are correct, and I see no reason to prefer one fix over the other. But Table 28 and 20.2.9.2 [allocator.traits.types]/p10 should be consistent with one another.
[2015-02 Cologne]
DK: We should accept the first bullet. GR: Why does is_empty
even have a type member? AM: All type traits
have a type member. I agree with DK's preference for the first type.
Proposed resolution:
This wording is relative to N4296.
Change 16.4.4.6 [allocator.requirements], Table 28 — "Allocator requirements" as presented:
Table 28 — Allocator requirements Expression Return type Assertion/note
pre-/post-conditionDefault …
X::is_always_equal
Identical to or derived
fromtrue_type
or
false_type
[…] is_empty<X>::type
…
Section: 16.4.5.9 [res.on.arguments], 16.4.4.2 [utility.arg.requirements], 16.4.6.17 [lib.types.movedfrom], 23.2.2 [container.requirements.general] Status: C++17 Submitter: Matt Austern Opened: 2015-01-22 Last modified: 2017-07-30
Priority: 2
View all other issues in [res.on.arguments].
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Discussion:
Suppose we write
vector<string> v{"a", "b", "c", "d"}; v = move(v);
What should be the state of v
be? The standard doesn't say anything specific about self-move-assignment.
There's relevant text in several parts of the standard, and it's not clear how to reconcile them.
MoveAssignable
requirements table in
16.4.4.2 [utility.arg.requirements] writes that, given t = rv
, t
's state is equivalent to
rv
's from before the assignment and rv
's state is unspecified (but valid). For containers
specifically, the requirements table in 23.2.2 [container.requirements.general] says that, given a = rv
,
a
becomes equal to what rv
was before the assignment (and doesn't say anything about rv
's
state post-assignment).
Taking each of these pieces in isolation, without reference to the other two:
16.4.5.9 [res.on.arguments] would clearly imply that the effect of v = move(v)
is undefined.
16.4.4.2 [utility.arg.requirements] would clearly imply that v = move(v)
has defined behavior.
It might be read to imply that this is a no-op, or might be read to imply that it leaves v
in a valid but
unspecified state; I'm not sure which reading is more natural.
23.2.2 [container.requirements.general] would clearly imply that v = move(v)
is a no-op.
It's not clear from the text how to put these pieces together, because it's not clear which one takes precedence.
Maybe 16.4.5.9 [res.on.arguments] wins (it imposes an implicit precondition that isn't mentioned in the
MoveAssignable
requirements, so v = move(v)
is undefined), or maybe
23.2.2 [container.requirements.general] wins (it explicitly gives additional guarantees for
Container::operator=
beyond what's guaranteed for library functions in general, so v = move(v)
is a no-op), or maybe something else.
v = move(v)
appeared to clear the vector;
it didn't leave the vector unchanged and it didn't cause a crash.
Proposed wording:
Informally: change the MoveAssignable
and Container requirements tables (and any other requirements tables
that mention move assignment, if any) to make it explicit that x = move(x)
is defined behavior and it leaves
x
in a valid but unspecified state. That's probably not what the standard says today, but it's probably what
we intended and it's consistent with what we've told users and with what implementations actually do.
[2015-10, Kona Saturday afternoon]
JW: So far, the library forbids self-assignment since it assumes that anything bound to an rvalue reference has no aliases. But self-assignment can happen in real code, and it can be implemented. So I want to add an exception to the Standard that this should be allowed and leave the object in a valid-but-unspecified state.
STL: When this is resolved, I want to see a) VBU for library types after self-move, but also b) requirements on user types for self-moves. E.g. should algorithms be required to avoid self-assignments (since a user-defined type might blow up)? HH: In other words, should we require that you can assign from moved-from values.
WEB: What can one generally do with moved-from values?
VV: Call any member function that has no preconditions.
JW: That's certainly the library requirement, and it's also good guidance for user types.
JW: I'm writing wording. I care about this.
Move to Open; Jonathan to provide wording
[2016-08-01, Howard provided wording]
[2016-08 Chicago]
Tuesday AM: Move to Tentatively Ready
Previous resolution [SUPERSEDED]:
In 16.4.4.3 [swappable.requirements], modify Table 23 —
MoveAssignable
requirements [moveassignable]:
Table 23 — MoveAssignable
requirements [moveassignable]Expression Return type Return value Post-condition t = rv
T&
t
If addressof(t) != addressof(rv)
,t
is equivalent to the value ofrv
before the assignmentrv
's state is unspecified. [Note:rv
must still meet the requirements of the library component that is using it, whether or notaddressof(t) == addressof(rv)
. The operations listed in those requirements must work as specified whetherrv
has been moved from or not. — end note]
[2016-08-07, Daniel reopens]
With the acceptance of LWG 2598(i), the proposed wording is invalid code, because it attempts to
call std::addressof
with an rvalue argument. It should be pointed out that the new restriction
caused by 2598(i) doesn't affect real code, because any identity test within a move assignment
operator (or any comparable function) would act on the current function argument, which is an lvalue in the
context of the function body. The existing wording form of the issue could still be kept, if a helper variable
would be introduced such as:
Let
refrv
denote a reference initialized as if byconst T& refrv = rv;
. Then ifaddressof(t) != addressof(refrv)
,t
is equivalent to the value ofrv
before the assignment
But it seems to me that the same effect could be much easier realized by replacing the code form by a non-code English phrase that realizes the same effect.
[2016-09-09 Issues Resolution Telecon]
Move to Tentatively Ready
[2016-10-05, Tim Song comments]
The current P/R of LWG 2468 simply adds to MoveAssignable
the requirement to tolerate self-move-assignment,
but that doesn't actually do much about self-move-assignment of library types. Very few types in the library are
explicitly required to satisfy MoveAssignable
, so as written the restriction in 16.4.5.9 [res.on.arguments]
would seem to still apply for any type that's not explicitly required to be CopyAssignable
or MoveAssignable
.
Proposed resolution:
This wording is relative to N4606.
In 16.4.4.3 [swappable.requirements], modify Table 23 — MoveAssignable
requirements [moveassignable]:
Table 23 — MoveAssignable
requirements [moveassignable]Expression Return type Return value Post-condition t = rv
T&
t
If t
andrv
do not refer to the same object,t
is equivalent to the value ofrv
before the assignmentrv
's state is unspecified. [Note:rv
must still meet the requirements of the library component that is using it, whether or nott
andrv
refer to the same object. The operations listed in those requirements must work as specified whetherrv
has been moved from or not. — end note]
operator[]
for map
and unordered_map
Section: 23.4.3.3 [map.access], 23.5.3.3 [unord.map.elem] Status: C++17 Submitter: Tomasz Kamiński Opened: 2015-01-21 Last modified: 2017-07-30
Priority: 3
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Discussion:
The "Requires:" clause for the operator[]
for the unordered_map
and map
, are defining
separate requirements for insertability into container of mapped_type
and key_type
.
T& operator[](const key_type& x);
Requires: key_type
shall be CopyInsertable
and mapped_type
shall be DefaultInsertable
into *this
.
23.4.3.3 [map.access] p6: // T& operator[](key_type&& x)
Requires: mapped_type
shall be DefaultInsertable
into *this
.
23.5.3.3 [unord.map.elem] p1: // mapped_type& operator[](const key_type& k);
mapped_type& operator[](key_type&& k);
Requires: mapped_type
shall be DefaultInsertable
into *this
. For the first operator,
key_type
shall be CopyInsertable
into *this
. For the second operator, key_type
shall
be MoveConstructible
.
Definition of the appropriate requirements: 23.2.2 [container.requirements.general] p15.
T
is DefaultInsertable
into X
means that the following expression is well-formed: //p15.1
allocator_traits<A>::construct(m, p)
T
is MoveInsertable
into X
means that the following expression is well-formed: //p15.3
allocator_traits<A>::construct(m, p, rv)
T
is CopyInsertable
into X
means that, in addition to T
being MoveInsertable
into X
, the following expression is well-formed: //p15.4
allocator_traits<A>::construct(m, p, v)
In the context of above definition the requirement "key_type
shall be CopyInsertable
into *this
"
would mean that the key element of the pair<const key_type, mapped_type>
(value_type
of the map)
should be constructed using separate call to the construct
method, the same applies for the mapped_type
.
Such behavior is explicitly prohibited by 23.2.2 [container.requirements.general] p3.
For the components affected by this sub-clause that declare an allocator_type, objects stored in these components shall be constructed using the
allocator_traits<allocator_type>::construct
function and destroyed using theallocator_traits<allocator_type>::destroy
function (20.7.8.2). These functions are called only for the container's element type, not for internal types used by the container.
It clearly states that element_type
of the map, must be constructed using allocator for value type, which
disallows using of separate construction of first and second element, regardless of the fact if it can be actually
performed without causing undefined behavior.
MoveInsertable
and similar requirements may only be expressed in terms of value_type
,
not its members types.
[2015-02 Cologne]
This issue is related to 2464(i).
GR: Effects should say "returns ...". DK: Or just have a Returns clause? MC: A Returns clause is a directive to implementers. TK/DK: This PR fails to address the requirements about which it complained in the first place. DK: I can reword this. TK can help.[2015-03-29, Daniel provides improved wording]
The revised wording fixes the proper usage of the magic "Equivalent to" wording, which automatically induces Requires:, Returns:, and Complexity: elements (and possibly more). This allows us to strike all the remaining elements, because they fall out from the semantics of the wording defined by 2464(i). In particular it is important to realize that the wording form
value_type
shall beEmplaceConstructible
intomap
frompiecewise_construct
,forward_as_tuple(k)
,forward_as_tuple(forward<Args>(args)...)
degenerates for the empty pack expansion args
to:
value_type
shall beEmplaceConstructible
intomap
frompiecewise_construct
,forward_as_tuple(k)
,forward_as_tuple()
which again means that such a pair
construction (assuming std::allocator
) would copy k
into member first
and would value-initialize member second
.
Previous resolution [SUPERSEDED]:
This wording is relative to N4296.
Accept resolution of the issue issue 2464(i) and define
operator[]
as follows (This would also address issue 2274(i)):
Change 23.4.3.3 [map.access] as indicated:
T& operator[](const key_type& x);-1- Effects:
[…]If there is no key equivalent toEquivalent to:x
in the map, insertsvalue_type(x, T())
into the maptry_emplace(x).first->second
.T& operator[](key_type&& x);-5- Effects:
If there is no key equivalent toEquivalent to:x
in the map, insertsvalue_type(std::move(x), T())
into the maptry_emplace(move(x)).first->second
.Change 23.5.3.3 [unord.map.elem] as indicated:
mapped_type& operator[](const key_type& k); mapped_type& operator[](key_type&& k);[…]
-2- Effects:If theFor the first operator, equivalent to:unordered_map
does not already contain an element whose key is equivalent tok
, the first operator inserts the valuevalue_type(k, mapped_type())
and the second operator inserts the valuevalue_type(std::move(k), mapped_type())
try_emplace(k).first->second
; for the second operator, equivalent to:try_emplace(move(k)).first->second
.
Previous resolution [SUPERSEDED]:
This wording is relative to N4296.
Accept resolution of the issue issue 2464(i) and define
operator[]
as follows (This would also address issue 2274(i)):
Change 23.4.3.3 [map.access] as indicated:
T& operator[](const key_type& x);-1- Effects:
If there is no key equivalent toEquivalent to:x
in the map, insertsvalue_type(x, T())
into the map.return try_emplace(x).first->second;
-2- Requires:key_type
shall beCopyInsertable
andmapped_type
shall beDefaultInsertable
into*this
.-3- Returns: A reference to themapped_type
corresponding tox
in*this
.-4- Complexity: Logarithmic.T& operator[](key_type&& x);-5- Effects:
If there is no key equivalent toEquivalent to:x
in the map, insertsvalue_type(std::move(x), T())
into the map.return try_emplace(move(x)).first->second;
-6- Requires:mapped_type
shall beDefaultInsertable
into*this
.-7- Returns: A reference to themapped_type
corresponding tox
in*this
.-8- Complexity: Logarithmic.Change 23.5.3.3 [unord.map.elem] as indicated:
mapped_type& operator[](const key_type& k); mapped_type& operator[](key_type&& k);-2- Effects:
-1- Requires:mapped_type
shall beDefaultInsertable
into*this
. For the first operator,key_type
shall beCopyInsertable
into*this
. For the second operator,key_type
shall beMoveConstructible
.If theFor the first operator, equivalent to:unordered_map
does not already contain an element whose key is equivalent tok
, the first operator inserts the valuevalue_type(k, mapped_type())
and the second operator inserts the valuevalue_type(std::move(k), mapped_type())
return try_emplace(k).first->second;for the second operator, equivalent to:
return try_emplace(move(k)).first->second;
-3- Returns: A reference tox.second
, wherex
is the (unique) element whose key is equivalent tok
.-4- Complexity: Average case 𝒪(1
), worst case 𝒪(size()
).
Proposed resolution:
This wording is relative to N4431.
Accept resolution of the issue issue 2464(i) and define operator[]
as follows
(This would also address issue 2274(i)):
Change 23.4.3.3 [map.access] as indicated:
T& operator[](const key_type& x);-1- Effects:
If there is no key equivalent toEquivalent to:x
in the map, insertsvalue_type(x, T())
into the map.return try_emplace(x).first->second;
-2- Requires:key_type
shall beCopyInsertable
andmapped_type
shall beDefaultInsertable
into*this
.-3- Returns: A reference to themapped_type
corresponding tox
in*this
.-4- Complexity: Logarithmic.T& operator[](key_type&& x);-5- Effects:
If there is no key equivalent toEquivalent to:x
in the map, insertsvalue_type(std::move(x), T())
into the map.return try_emplace(move(x)).first->second;
-6- Requires:mapped_type
shall beDefaultInsertable
into*this
.-7- Returns: A reference to themapped_type
corresponding tox
in*this
.-8- Complexity: Logarithmic.
Change 23.5.3.3 [unord.map.elem] as indicated:
mapped_type& operator[](const key_type& k);mapped_type& operator[](key_type&& k);-2- Effects: Equivalent to
-1- Requires:mapped_type
shall beDefaultInsertable
into*this
. For the first operator,key_type
shall beCopyInsertable
into*this
. For the second operator,key_type
shall beMoveConstructible
.return try_emplace(k).first->second;
If theunordered_map
does not already contain an element whose key is equivalent tok
, the first operator inserts the valuevalue_type(k, mapped_type())
and the second operator inserts the valuevalue_type(std::move(k), mapped_type())
-3- Returns: A reference tox.second
, wherex
is the (unique) element whose key is equivalent tok
.-4- Complexity: Average case 𝒪(1
), worst case 𝒪(size()
).mapped_type& operator[](key_type&& k);-?- Effects: Equivalent to
return try_emplace(move(k)).first->second;
destroy
function should be allowed to fail to instantiateSection: 16.4.4.6 [allocator.requirements] Status: C++17 Submitter: Daniel Krügler Opened: 2015-03-22 Last modified: 2017-07-30
Priority: Not Prioritized
View other active issues in [allocator.requirements].
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Discussion:
This issue is a spin-off of issue LWG 2447(i): It focuses on the observation that
16.4.4.6 [allocator.requirements] p9 (based on the numbering of working draft N4296) gives
the template member construct
more relaxations than the template member destroy
:
An allocator may constrain the types on which it can be instantiated and the arguments for which its
construct
member may be called. If a type cannot be used with a particular allocator, the allocator class or the call toconstruct
may fail to instantiate.
Construction and destruction of a type T
are usually intimately related to each other, so it
seems similarly useful to allow the destroy
member to fail to instantiate for a possible sub-set
of instantiation types.
[2015-04-01 Library reflector vote]
The issue has been identified as Tentatively Ready based on six votes in favour.
Proposed resolution:
This wording is relative to N4296.
Change 16.4.4.6 [allocator.requirements] p9 as indicated:
-8- An allocator may constrain the types on which it can be instantiated and the arguments for which its
construct
ordestroy
members may be called. If a type cannot be used with a particular allocator, the allocator class or the call toconstruct
ordestroy
may fail to instantiate.
basic_filebuf
's relation to C FILE
semanticsSection: 31.10.3.5 [filebuf.virtuals] Status: C++17 Submitter: Aaron Ballman Opened: 2015-02-09 Last modified: 2017-07-30
Priority: 2
View all other issues in [filebuf.virtuals].
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Discussion:
The restrictions on reading and writing a sequence controlled by an object of class
basic_filebuf<charT, traits>
are the same as for reading and writing with
the Standard C library FILE
s. One of the restrictions placed by C is on the behavior of
a stream that is opened for input and output. See the C99 standard, 7.19.5.3p6 for more
details, but the gist is that when opened in update mode, reads and writes must have an
intervening file positioning or flushing call to not trigger UB.
31.10.3.5 [filebuf.virtuals] p13 specifies that basic_filebuf::seekoff()
calls
std::fseek()
. However, there is no mention of std::fseek()
in
basic_filebuf::seekpos()
, and no mention of std::fflush()
in
basic_filebuf::sync()
, which seem like an oversight.
Previous resolution [SUPERSEDED]:
This wording is relative to N4296.
Change 31.10.3.5 [filebuf.virtuals] p16 as follows [Editorial note: A footnote referring to
fseek
is not needed, because this is already covered by the existing footnote 334]:-16- Alters the file position, if possible, to correspond to the position stored in
sp
(as described below). Altering the file position performs as follows:
if
(om & ios_base::out) != 0
, then update the output sequence and write any unshift sequence;set the file position to
sp
as if by callingstd::fseek(file, sp, SEEK_SET)
;if
(om & ios_base::in) != 0
, then update the input sequence;where
om
is the open mode passed to the last call toopen()
. The operation fails ifis_open()
returns false.Change 31.10.3.5 [filebuf.virtuals] p19 as follows and add a new footnote that mimics comparable footnotes in 31.10.3.4 [filebuf.members] and 31.10.3.5 [filebuf.virtuals]:
-19- Effects: If a put area exists, calls
filebuf::overflow
to write the characters to the file, then flushes the file as if by callingstd::fflush(file)
[Footnote: The function signaturefflush(FILE*)
is declared in<cstdio>
(27.9.2).]. If a get area exists, the effect is implementation-defined.
[2015-05, Lenexa]
Aaron provides improved wording by removing the params from std::fseek()
due to the concerns
regarding the parameters on systems where fseek
uses 32-bit parameters.
drops the std::
drops the footnote for fflush
replaces fseek
with fsetpos
Previous resolution [SUPERSEDED]:
This wording is relative to N4431.
Change 31.10.3.5 [filebuf.virtuals] p16 as follows [Editorial note: A footnote referring to
fseek
is not needed, because this is already covered by the existing footnote 334]:-16- Alters the file position, if possible, to correspond to the position stored in
sp
(as described below). Altering the file position performs as follows:
if
(om & ios_base::out) != 0
, then update the output sequence and write any unshift sequence;set the file position to
sp
as if by a call tostd::fseek
;if
(om & ios_base::in) != 0
, then update the input sequence;where
om
is the open mode passed to the last call toopen()
. The operation fails ifis_open()
returns false.Change 31.10.3.5 [filebuf.virtuals] p19 as follows and add a new footnote that mimics comparable footnotes in 31.10.3.4 [filebuf.members] and 31.10.3.5 [filebuf.virtuals]:
-19- Effects: If a put area exists, calls
filebuf::overflow
to write the characters to the file, then flushes the file as if by callingstd::fflush(file)
[Footnote: The function signaturefflush(FILE*)
is declared in<cstdio>
(27.9.2).]. If a get area exists, the effect is implementation-defined.
Proposed resolution:
This wording is relative to N4431.
Change 31.10.3.5 [filebuf.virtuals] p16 as follows:
-16- Alters the file position, if possible, to correspond to the position stored in
sp
(as described below). Altering the file position performs as follows:
if
(om & ios_base::out) != 0
, then update the output sequence and write any unshift sequence;set the file position to
sp
as if by a call tofsetpos
;if
(om & ios_base::in) != 0
, then update the input sequence;where
om
is the open mode passed to the last call toopen()
. The operation fails ifis_open()
returns false.
Change 31.10.3.5 [filebuf.virtuals] p19 as follows:
-19- Effects: If a put area exists, calls
filebuf::overflow
to write the characters to the file, then flushes the file as if by callingfflush(file)
. If a get area exists, the effect is implementation-defined.
std::basic_string
terminator with charT()
to allow
cleaner interoperation with legacy APIsSection: 27.4.3.6 [string.access] Status: C++17 Submitter: Matt Weber Opened: 2015-02-21 Last modified: 2017-07-30
Priority: 3
View all other issues in [string.access].
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Discussion:
It is often desirable to use a std::basic_string
object as a buffer when interoperating with libraries
that mutate null-terminated arrays of characters. In many cases, these legacy APIs write a null terminator at
the specified end of the provided buffer. Providing such a function with an appropriately-sized
std::basic_string
results in undefined behavior when the charT
object at the size()
position is overwritten, even if the value remains unchanged.
std::vectors
of charT
for interoperating with the legacy API, and then
copying the std::vector
to a std::basic_string
; providing an oversized std::basic_string
object and then calling resize()
later.
A trivial example:
#include <string> #include <vector> void legacy_function(char *out, size_t count) { for (size_t i = 0; i < count; ++i) { *out++ = '0' + (i % 10); } *out = '\0'; // if size() == count, this results in undefined behavior } int main() { std::string s(10, '\0'); legacy_function(&s[0], s.size()); // undefined behavior std::vector<char> buffer(11); legacy_function(&buffer[0], buffer.size() - 1); std::string t(&buffer[0], buffer.size() - 1); // potentially expensive copy std::string u(11, '\0'); legacy_function(&u[0], u.size() - 1); u.resize(u.size() - 1); // needlessly complicates the program's logic }
A slight relaxation of the requirement on the returned object from the element access operator would allow for this interaction with no semantic change to existing programs.
[2016-08 Chicago]
Tues PM: This should also apply to non-const data()
. Billy to update wording.
Fri PM: Move to Tentatively Ready
Proposed resolution:
This wording is relative to N4296.
Edit 27.4.3.6 [string.access] as indicated:
const_reference operator[](size_type pos) const; reference operator[](size_type pos);-1- Requires: […]
-2- Returns:*(begin() + pos)
ifpos < size()
. Otherwise, returns a reference to an object of typecharT
with valuecharT()
, where modifying the object to any value other thancharT()
leads to undefined behavior. […]
scoped_allocator_adaptor
is not assignableSection: 20.6.1 [allocator.adaptor.syn] Status: C++17 Submitter: Jonathan Wakely Opened: 2015-03-02 Last modified: 2017-07-30
Priority: 0
View all issues with C++17 status.
Discussion:
The class definition in 20.6.1 [allocator.adaptor.syn] declares a move constructor, which means that the copy assignment operator is defined as deleted, and no move assignment operator is declared.
This means ascoped_allocator_adaptor
is not assignable, and a
container using scoped_allocator_adaptor<A...>
may not be
CopyAssignable
or MoveAssignable
(depending on the
propagate_on_container_xxxx_assignment
traits of the outer and inner
allocator types).
[2015-04-03 Howard comments]
If the contained allocators are not assignable, I think we need the ability of = default
to automagically become
= delete
. My concern is that is_copy_assignable<scoped_allocator_adaptor<CustomAllocator>>::value
get the right answer for both cases:
is_copy_assignable<CustomAllocator>::value
is true
.
is_copy_assignable<CustomAllocator>::value
is false
.
If we allow the vendor to declare and provide the copy assignment operator, the chance of getting #2 correct goes to zero.
Previous resolution [SUPERSEDED]:
This wording is relative to N4296.
Add to the synopsis in 20.6.1 [allocator.adaptor.syn]/1 [Editorial remark: The proposed wording does not explicitly specify the semantics of the added copy/move assignment operators, based on 16.3.3.5 [functions.within.classes] p1, which says:
"For the sake of exposition, Clauses 18 through 30 and Annex D do not describe copy/move constructors, assignment operators, or (non-virtual) destructors with the same apparent semantics as those that can be generated by default (12.1, 12.4, 12.8)." — end remark]:[…] template <class OuterA2> scoped_allocator_adaptor( scoped_allocator_adaptor<OuterA2, InnerAllocs...>&& other) noexcept; scoped_allocator_adaptor& operator=(const scoped_allocator_adaptor&); scoped_allocator_adaptor& operator=(scoped_allocator_adaptor&&); ~scoped_allocator_adaptor(); […]
[2015-05, Lenexa]
Move to Immediate.
Proposed resolution:
This wording is relative to N4296.
Add to the synopsis in 20.6.1 [allocator.adaptor.syn]/1:
[…] template <class OuterA2> scoped_allocator_adaptor( scoped_allocator_adaptor<OuterA2, InnerAllocs...>&& other) noexcept; scoped_allocator_adaptor& operator=(const scoped_allocator_adaptor&) = default; scoped_allocator_adaptor& operator=(scoped_allocator_adaptor&&) = default; ~scoped_allocator_adaptor(); […]
std::vector::erase()
and std::deque::erase()
Section: 23.3.5.4 [deque.modifiers], 23.3.13.5 [vector.modifiers] Status: C++17 Submitter: Anton Savin Opened: 2015-03-03 Last modified: 2017-07-30
Priority: 0
View other active issues in [deque.modifiers].
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Discussion:
In the latest draft N4296, and in all drafts up to at least N3337:
23.3.5.4 [deque.modifiers]/5 (regardingdeque::erase()
):
Complexity: The number of calls to the destructor is the same as the number of elements erased, but the number of calls to the assignment operator is no more than the lesser of the number of elements before the erased elements and the number of elements after the erased elements.
23.3.13.5 [vector.modifiers]/4 (regarding vector::erase()
):
Complexity: The destructor of
T
is called the number of times equal to the number of the elements erased, but the move assignment operator ofT
is called the number of times equal to the number of elements in the vector after the erased elements.
Is there any reason for explicit mentioning of move assignment for std::vector::erase()
?
Shouldn't these two wordings be the same with this regard?
std::deque
, it's not clear from the text which destructors and assignment operators are called.
[2015-05, Lenexa]
Move to Immediate.
Proposed resolution:
This wording is relative to N4296.
Change 23.3.5.4 [deque.modifiers]/5 to:
-5- Complexity: The number of calls to the destructor of T
is the same as the number of
elements erased, but the number of calls to the assignment operator of T
is no more than the
lesser of the number of elements before the erased elements and the number of elements after the erased elements.
Change 23.3.13.5 [vector.modifiers]/4 to:
-4- Complexity: The destructor of T
is called the number of times equal to the number of the elements
erased, but the move assignment operator of T
is called the number of times equal to the number of
elements in the vector after the erased elements.
Section: 27.5 [c.strings] Status: C++17 Submitter: S. B.Tam Opened: 2015-01-18 Last modified: 2017-07-30
Priority: Not Prioritized
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Discussion:
N4296 Table 73 mentions the functions mbsrtowc
and wcsrtomb
, which are not defined in ISO C
or ISO C++. Presumably they should be mbsrtowcs
and wcsrtombs
instead.
[2015-04-02 Library reflector vote]
The issue has been identified as Tentatively Ready based on six votes in favour.
Proposed resolution:
This wording is relative to N4296.
Table 33 — Potential mbstate_t
data racesmbrlen
mbrtowc
mbsrtowcs
mbtowc
wcrtomb
wcsrtombs
wctomb
throw_with_nested()
should use is_final
Section: 17.9.8 [except.nested] Status: C++17 Submitter: Stephan T. Lavavej Opened: 2015-03-27 Last modified: 2017-07-30
Priority: 2
View all other issues in [except.nested].
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Discussion:
When N2559 was voted into the Working Paper,
it said "This function template must take special case to handle non-class types, unions and [[final]]
classes that cannot
be derived from, and [...]". However, its Standardese didn't handle final
classes, and this was never revisited. Now that
we have is_final
, we can achieve this proposal's original intention.
U
is nested_exception
itself. is_base_of
's wording
handles this and ignores cv-qualifiers. (Note that is_class
detects "non-union class type".)
[2015-05, Lenexa]
STL, MC and JW already do this
MC: move to Ready, bring to motion on Friday
7 in favor, none opposed
Proposed resolution:
This wording is relative to N4296.
Change 17.9.8 [except.nested] as depicted:
template <class T> [[noreturn]] void throw_with_nested(T&& t);-6- Let
-7- Requires:U
beremove_reference_t<T>
.U
shall beCopyConstructible
. -8- Throws: ifU
is a non-union class type not derived fromnested_exception
is_class<U>::value && !is_final<U>::value && !is_base_of<nested_exception, U>::value
istrue
, an exception of unspecified type that is publicly derived from bothU
andnested_exception
and constructed fromstd::forward<T>(t)
, otherwisestd::forward<T>(t)
.
rethrow_if_nested()
is doubly unimplementableSection: 17.9.8 [except.nested] Status: C++17 Submitter: Stephan T. Lavavej Opened: 2015-03-27 Last modified: 2017-07-30
Priority: 2
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Discussion:
rethrow_if_nested()
wants to determine "If the dynamic type of e
is publicly and unambiguously derived
from nested_exception
", but 7.6.1.7 [expr.dynamic.cast] specifies that dynamic_cast
has a couple
of limitations.
E
is int
, the dynamic type can't possibly derive from
nested_exception
. Implementers need to detect this and avoid dynamic_cast
, which would be ill-formed
due to 7.6.1.7 [expr.dynamic.cast]/2.) The Standardese is defective when E
is a nonpolymorphic class.
Consider the following example:
struct Nonpolymorphic { }; struct MostDerived : Nonpolymorphic, nested_exception { }; MostDerived md; const Nonpolymorphic& np = md; rethrow_if_nested(np);
According to 3.19 [defns.dynamic.type], the dynamic type of np
is MostDerived
. However, it's
physically impossible to discover this, and attempting to do so will lead to an ill-formed dynamic_cast
(7.6.1.7 [expr.dynamic.cast]/6). The Standardese must be changed to say that if E
is nonpolymorphic, nothing happens.
struct Nested1 : nested_exception { }; struct Nested2 : nested_exception { }; struct Ambiguous : Nested1, Nested2 { }; Ambiguous amb; const Nested1& n1 = amb; rethrow_if_nested(n1);
Here, the static type Nested1
is good (i.e. publicly and unambiguously derived from nested_exception
), but
the dynamic type Ambiguous
is bad. The Standardese currently says that we have to detect the dynamic badness, but
dynamic_cast
won't let us. 7.6.1.7 [expr.dynamic.cast]/3 and /5 are special cases (identity-casting and upcasting,
respectively) that activate before the "run-time check" behavior that we want (/6 and below). Statically good inputs succeed
(regardless of the dynamic type) and statically bad inputs are ill-formed (implementers must use type traits to avoid this).
dynamic_cast
),
but implementers shouldn't be asked to do so much work for such an unimportant case. (This case is pathological because the
usual way of adding nested_exception
inheritance is throw_with_nested()
, which avoids creating bad inheritance.)
The Standardese should be changed to say that statically good inputs are considered good.
Finally, we want is_base_of
's "base class or same class" semantics. If the static type is nested_exception
,
we have to consider it good due to dynamic_cast
's identity-casting behavior. And if the dynamic type is
nested_exception
, it is definitely good.
[2015-05, Lenexa]
WB: so the is_polymorphic
trait must be used?
STL and JW: yes, that must be used to decide whether to try using dynamic_cast
or not.
JW: I'd already made this fix in our implementation
STL: the harder case also involves dynamic_cast
. should not try using dynamic_cast
if we can
statically detect it is OK, doing the dynamic_cast
might fail.
STL: finally, want "is the same or derived from" behaviour of is_base_of
WB: should there be an "else no effect" at the end? We have "Otherwise, if ..." and nothing saying what if the condition is false.
TP I agree.
MC: move to Ready and bring to motion on Friday
7 in favor, none opposed
Proposed resolution:
This wording is relative to N4296.
Change 17.9.8 [except.nested] as depicted:
template <class E> void rethrow_if_nested(const E& e);-9- Effects: If
E
is not a polymorphic class type, there is no effect. Otherwise, if the static type or the dynamic type ofe
isnested_exception
or is publicly and unambiguously derived fromnested_exception
, callsdynamic_cast<const nested_exception&>(e).rethrow_nested()
.
get()
should be overloaded for const tuple&&
Section: 22.4.8 [tuple.elem] Status: C++17 Submitter: Stephan T. Lavavej Opened: 2015-03-27 Last modified: 2017-07-30
Priority: 1
View all other issues in [tuple.elem].
View all issues with C++17 status.
Discussion:
const
rvalues are weird, but they're part of the type system. Consider the following code:
#include <functional> #include <string> #include <tuple> using namespace std; string str1() { return "one"; } const string str2() { return "two"; } tuple<string> tup3() { return make_tuple("three"); } const tuple<string> tup4() { return make_tuple("four"); } int main() { // cref(str1()); // BAD, properly rejected // cref(str2()); // BAD, properly rejected // cref(get<0>(tup3())); // BAD, properly rejected cref(get<0>(tup4())); // BAD, but improperly accepted! }
As tuple
is a fundamental building block (and the only convenient way to have variadic data members), it should
not open a hole in the type system. get()
should imitate 7.6.1.5 [expr.ref]'s rules for accessing data members.
(This is especially true for pair
, where both get<0>()
and .first
are available.)
[2015-05, Lenexa]
TP: for the existing overloads there's no change to the code, just descriptions?
STL: right.
JW: I love it
MC: in favor of moving to Ready and bringing up for vote on Friday
7 in favor, none opposed
Proposed resolution:
This wording is relative to N4296.
Change 22.2 [utility]/2 "Header <utility>
synopsis" as depicted:
[…] template<size_t I, class T1, class T2> constexpr tuple_element_t<I, pair<T1, T2>>& get(pair<T1, T2>&) noexcept; template<size_t I, class T1, class T2> constexpr tuple_element_t<I, pair<T1, T2>>&& get(pair<T1, T2>&&) noexcept; template<size_t I, class T1, class T2> constexpr const tuple_element_t<I, pair<T1, T2>>& get(const pair<T1, T2>&) noexcept; template<size_t I, class T1, class T2> constexpr const tuple_element_t<I, pair<T1, T2>>&& get(const pair<T1, T2>&&) noexcept; template <class T, class U> constexpr T& get(pair<T, U>& p) noexcept; template <class T, class U> constexpr const T& get(const pair<T, U>& p) noexcept; template <class T, class U> constexpr T&& get(pair<T, U>&& p) noexcept; template <class T, class U> constexpr const T&& get(const pair<T, U>&& p) noexcept; template <class T, class U> constexpr T& get(pair<U, T>& p) noexcept; template <class T, class U> constexpr const T& get(const pair<U, T>& p) noexcept; template <class T, class U> constexpr T&& get(pair<U, T>&& p) noexcept; template <class T, class U> constexpr const T&& get(const pair<U, T>&& p) noexcept; […]
Change 22.4.1 [tuple.general]/2 "Header <tuple>
synopsis" as depicted:
[…] // 20.4.2.6, element access: template <size_t I, class... Types> constexpr tuple_element_t<I, tuple<Types...>>& get(tuple<Types...>&) noexcept; template <size_t I, class... Types> constexpr tuple_element_t<I, tuple<Types...>>&& get(tuple<Types...>&&) noexcept; template <size_t I, class... Types> constexpr const tuple_element_t<I, tuple<Types...>>& get(const tuple<Types...>&) noexcept; template <size_t I, class... Types> constexpr const tuple_element_t<I, tuple<Types...>>&& get(const tuple<Types...>&&) noexcept; template <class T, class... Types> constexpr T& get(tuple<Types...>& t) noexcept; template <class T, class... Types> constexpr T&& get(tuple<Types...>&& t) noexcept; template <class T, class... Types> constexpr const T& get(const tuple<Types...>& t) noexcept; template <class T, class... Types> constexpr const T&& get(const tuple<Types...>&& t) noexcept; […]
Change 23.3.1 [sequences.general]/2 "Header <array>
synopsis" as depicted:
[…] template <size_t I, class T, size_t N> constexpr T& get(array<T, N>&) noexcept; template <size_t I, class T, size_t N> constexpr T&& get(array<T, N>&&) noexcept; template <size_t I, class T, size_t N> constexpr const T& get(const array<T, N>&) noexcept; template <size_t I, class T, size_t N> constexpr const T&& get(const array<T, N>&&) noexcept; […]
Change 22.3.4 [pair.astuple] as depicted:
template<size_t I, class T1, class T2> constexpr tuple_element_t<I, pair<T1, T2>>& get(pair<T1, T2>& p) noexcept; template<size_t I, class T1, class T2> constexpr const tuple_element_t<I, pair<T1, T2>>& get(const pair<T1, T2>& p) noexcept;
-3- Returns: IfI == 0
returnsp.first
; ifI == 1
returnsp.second
; otherwise the program is ill-formed.template<size_t I, class T1, class T2> constexpr tuple_element_t<I, pair<T1, T2>>&& get(pair<T1, T2>&& p) noexcept; template<size_t I, class T1, class T2> constexpr const tuple_element_t<I, pair<T1, T2>>&& get(const pair<T1, T2>&& p) noexcept;-4- Returns: If
I == 0
returns a reference to; if
std::forward<T1&&>(p.first)I == 1
returns a reference to; otherwise the program is ill-formed.
std::forward<T2&&>(p.second)template <class T, class U> constexpr T& get(pair<T, U>& p) noexcept; template <class T, class U> constexpr const T& get(const pair<T, U>& p) noexcept;
-5- Requires:T
andU
are distinct types. Otherwise, the program is ill-formed.-6- Returns:get<0>(p)
;template <class T, class U> constexpr T&& get(pair<T, U>&& p) noexcept; template <class T, class U> constexpr const T&& get(const pair<T, U>&& p) noexcept;-7- Requires:
-8- Returns: A reference toT
andU
are distinct types. Otherwise, the program is ill-formed.p.first
.get<0>(std::move(p))
;template <class T, class U> constexpr T& get(pair<U, T>& p) noexcept; template <class T, class U> constexpr const T& get(const pair<U, T>& p) noexcept;
-9- Requires:T
andU
are distinct types. Otherwise, the program is ill-formed.-10- Returns:get<1>(p)
;template <class T, class U> constexpr T&& get(pair<U, T>&& p) noexcept; template <class T, class U> constexpr const T&& get(const pair<U, T>&& p) noexcept;-11- Requires:
-12- Returns: A reference toT
andU
are distinct types. Otherwise, the program is ill-formed.p.second
.get<1>(std::move(p))
;
Change 22.4.8 [tuple.elem] as depicted:
template <size_t I, class... Types> constexpr tuple_element_t<I, tuple<Types...> >& get(tuple<Types...>& t) noexcept;
-1- Requires:I < sizeof...(Types)
. The program is ill-formed ifI
is out of bounds.-2- Returns: A reference to theI
th element oft
, where indexing is zero-based.template <size_t I, class... Types> constexpr tuple_element_t<I, tuple<Types...> >&& get(tuple<Types...>&& t) noexcept; // Note A
-3- Effects: Equivalent to return std::forward<typename tuple_element<I, tuple<Types...> >::type&&>(get<I>(t));-4- Note: if aT
inTypes
is some reference typeX&
, the return type isX&
, notX&&
. However, if the element type is a non-reference typeT
, the return type isT&&
.template <size_t I, class... Types> constexpr tuple_element_t<I, tuple<Types...> > const& get(const tuple<Types...>& t) noexcept; // Note B template <size_t I, class... Types> constexpr const tuple_element_t<I, tuple<Types...> >&& get(const tuple<Types...>&& t) noexcept;-5- Requires:
-6- Returns: AI < sizeof...(Types)
. The program is ill-formed ifI
is out of bounds.constreference to theI
th element oft
, where indexing is zero-based. -?- [Note A: if aT
inTypes
is some reference typeX&
, the return type isX&
, notX&&
. However, if the element type is a non-reference typeT
, the return type isT&&
. — end note] -7- [Note B: Constness is shallow. If aT
inTypes
is some reference typeX&
, the return type isX&
, notconst X&
. However, if the element type is non-reference typeT
, the return type isconst T&
. This is consistent with how constness is defined to work for member variables of reference type. — end note]template <class T, class... Types> constexpr T& get(tuple<Types...>& t) noexcept; template <class T, class... Types> constexpr T&& get(tuple<Types...>&& t) noexcept; template <class T, class... Types> constexpr const T& get(const tuple<Types...>& t) noexcept; template <class T, class... Types> constexpr const T&& get(const tuple<Types...>&& t) noexcept;-8- Requires: The type
-9- Returns: A reference to the element ofT
occurs exactly once inTypes...
. Otherwise, the program is ill-formed.t
corresponding to the typeT
inTypes...
. […]
Change 23.3.3.7 [array.tuple] as depicted:
template <size_t I, class T, size_t N> constexpr T& get(array<T, N>& a) noexcept;
-3- Requires:I < N
. The program is ill-formed ifI
is out of bounds.-4- Returns: A reference to theI
th element ofa
, where indexing is zero-based.template <size_t I, class T, size_t N> constexpr T&& get(array<T, N>&& a) noexcept;
-5- Effects: Equivalent toreturn std::move(get<I>(a));
template <size_t I, class T, size_t N> constexpr const T& get(const array<T, N>& a) noexcept; template <size_t I, class T, size_t N> constexpr const T&& get(const array<T, N>&& a) noexcept;-6- Requires:
-7- Returns: AI < N
. The program is ill-formed ifI
is out of bounds.constreference to theI
th element ofa
, where indexing is zero-based.
mem_fn()
should be required to use perfect forwardingSection: 22.10.4 [func.require] Status: C++17 Submitter: Stephan T. Lavavej Opened: 2015-03-27 Last modified: 2017-07-30
Priority: 0
View other active issues in [func.require].
View all other issues in [func.require].
View all issues with C++17 status.
Discussion:
22.10.4 [func.require]/4 defines "simple call wrapper" and "forwarding call wrapper". Only mem_fn()
is
specified to be a "simple call wrapper", by 22.10.16 [func.memfn]/1: "A simple call wrapper (20.9.1) fn
such that the expression fn(t, a2, ..., aN)
is equivalent to INVOKE(pm, t, a2, ..., aN)
(20.9.2)."
R (T::*)(Arg)
where Arg
is passed by value — if the mem_fn()
wrapper's function call
operator takes Arg
by value, an extra copy/move will be observable. We should require perfect forwarding here.
[2015-05, Lenexa]
Move to Immediate.
Proposed resolution:
This wording is relative to N4296.
Change 22.10.4 [func.require] as depicted [Editorial remark: This simply adds "A simple call wrapper is a forwarding call wrapper", then moves the sentence. — end of remark]:
-4- Every call wrapper (20.9.1) shall be
MoveConstructible
.A simple call wrapper is a call wrapper that isA forwarding call wrapper is a call wrapper that can be called with an arbitrary argument list and delivers the arguments to the wrapped callable object as references. This forwarding step shall ensure that rvalue arguments are delivered as rvalue-references and lvalue arguments are delivered as lvalue-references. A simple call wrapper is a forwarding call wrapper that isCopyConstructible
andCopyAssignable
and whose copy constructor, move constructor, and assignment operator do not throw exceptions.CopyConstructible
andCopyAssignable
and whose copy constructor, move constructor, and assignment operator do not throw exceptions. [Note: In a typical implementation […] — end note]
bind()
should be const
-overloaded, not cv-overloadedSection: 22.10.15.4 [func.bind.bind] Status: C++17 Submitter: Stephan T. Lavavej Opened: 2015-03-27 Last modified: 2017-07-30
Priority: 2
View all other issues in [func.bind.bind].
View all issues with C++17 status.
Discussion:
The Standard currently requires bind()
to return something with a cv-overloaded function call operator.
const
is great, but volatile
is not. First, the Library almost always ignores volatile
's
existence (with <type_traits>
and <atomic>
being rare exceptions). Second, implementations
typically store bound arguments in a tuple
, but get()
isn't overloaded for volatile tuple
. Third,
when a bound argument is a reference_wrapper
, we have to call tid.get()
, but that won't compile for a
volatile reference_wrapper
. Finally, const
and volatile
don't always have to be handled symmetrically
— for example, lambda function call operators are const
by default, but they can't ever be volatile
.
[2015-05, Lenexa]
JW: why would a reference_wrapper
be volatile?
STL: if a bound argument is a reference_wrapper
then in a volatile-qualified operator() that
member will be volatile so you can't call get() on it
STL: worded like this it's a conforming extension to kep overloading on volatile
HH: libc++ doesn't overload on volatile
JW: libstdc++ does overload for volatile
MC: move to Ready and bring motion on Friday
10 in favor, none opposed
Proposed resolution:
This wording is relative to N4296.
Change 22.10.15.4 [func.bind.bind] as depicted:
template<class F, class... BoundArgs> unspecified bind(F&& f, BoundArgs&&... bound_args);-2- Requires:
[…]is_constructible<FD, F>::value
shall betrue
. For eachTi
inBoundArgs
,is_constructible<TiD, Ti>::value
shall betrue
.INVOKE(fd, w1, w2, ..., wN)
(20.9.2) shall be a valid expression for some valuesw1
,w2
, ...,wN
, whereN == sizeof...(bound_args)
. The cv-qualifiers cv of the call wrapperg
, as specified below, shall be neithervolatile
norconst volatile
.template<class R, class F, class... BoundArgs> unspecified bind(F&& f, BoundArgs&&... bound_args);-6- Requires:
[…]is_constructible<FD, F>::value
shall betrue
. For eachTi
inBoundArgs
,is_constructible<TiD, Ti>::value
shall betrue
.INVOKE(fd, w1, w2, ..., wN)
shall be a valid expression for some valuesw1
,w2
, ...,wN
, whereN == sizeof...(bound_args)
. The cv-qualifiers cv of the call wrapperg
, as specified below, shall be neithervolatile
norconst volatile
.
constexpr
Section: 22.10.15.5 [func.bind.place] Status: C++17 Submitter: Stephan T. Lavavej Opened: 2015-03-27 Last modified: 2017-07-30
Priority: 2
View all other issues in [func.bind.place].
View all issues with C++17 status.
Discussion:
piecewise_construct
(22.3.5 [pair.piecewise]), allocator_arg
(20.2.7 [allocator.tag]),
and adopt_lock
/defer_lock
/try_to_lock
(32.6.5 [thread.lock]) are all required to be
constexpr
with internal linkage. bind()
's placeholders should be allowed to follow this modern practice,
for increased consistency and reduced implementer headaches (header-only is easier than separately-compiled).
[2015-05-07 Lenexa: Move to Immediate]
STL: I'd like this one immediate.
Jonathan: I want to think about forcing constexpr, but the current issue, I have no objections. I'd say ready, but I won't object to immediate if STL wants it.
Marshall: You should report in Kona how it worked out.
STL: We went around a bit on the reflector about how to phrase the encouragement.
Jonathan: I think the shall may be not quite right.
Marshall: I see, you can change your implementation, but you don't.
Jonathan: There's precedent for the shall, but it's wrong, see editorial issue 493.
STL: I would prefer not to ask Daniel to reword, and 493 can fix it later.
Marshall: I remove my objection to immediate because it doesn't force implementations to change.
Marshall: Any other discussion?
Marshall: Immediate vote: 6. Opposed, 0. Abstain, 1.
Proposed resolution:
This wording is relative to N4296.
Change 22.10 [function.objects] p2 "Header <functional>
synopsis" as depicted:
namespace placeholders { // M is the implementation-defined number of placeholders see belowextern unspecified_1; see belowextern unspecified_2; ... see belowextern unspecified_M; }
Change 22.10.15.5 [func.bind.place] p2 as depicted:
namespace std::placeholders { // M is the implementation-defined number of placeholders see belowextern unspecified_1; see belowextern unspecified_2; . . . see belowextern unspecified_M; }-1- All placeholder types shall be
-?- Placeholders should be defined as:DefaultConstructible
andCopyConstructible
, and their default constructors and copy/move constructors shall not throw exceptions. It is implementation-defined whether placeholder types areCopyAssignable
.CopyAssignable
placeholders' copy assignment operators shall not throw exceptions.constexpr unspecified _1{};If they are not, they shall be declared as:
extern unspecified _1;
mem_fn()
should be noexcept
Section: 22.10.16 [func.memfn] Status: C++17 Submitter: Stephan T. Lavavej Opened: 2015-03-27 Last modified: 2017-07-30
Priority: 0
View all other issues in [func.memfn].
View all issues with C++17 status.
Discussion:
mem_fn()
is wide contract and doesn't do anything that could throw exceptions, so it should be marked noexcept
.
mem_fn()
is perfectly happy to wrap a null PMF/PMD, it just can't be invoked later. This is exactly like
std::function
, which can be constructed from null PMFs/PMDs. Therefore, mem_fn()
will remain wide contract forever.
[2015-05, Lenexa]
Move to Immediate.
Proposed resolution:
This wording is relative to N4296.
Change 22.10 [function.objects] p2 "Header <functional>
synopsis" as depicted:
[…] // 20.9.11, member function adaptors: template<class R, class T> unspecified mem_fn(R T::*) noexcept; […]
Change 22.10.16 [func.memfn] as depicted:
template<class R, class T> unspecified mem_fn(R T::* pm) noexcept;[…]
-4- Throws: Nothing.
comp
Section: 26.8 [alg.sorting] Status: C++17 Submitter: Anton Savin Opened: 2015-04-14 Last modified: 2017-07-30
Priority: 0
View all other issues in [alg.sorting].
View all issues with C++17 status.
Discussion:
N4296 26.8 [alg.sorting]/3 reads:
For all algorithms that take
Compare
, there is a version that usesoperator<
instead. That is,comp(*i,*j) != false
defaults to*i < *j != false
. For algorithms other than those described in 25.4.3 to work correctly,comp
has to induce a strict weak ordering on the values.
So it's not specified clearly what happens if comp
or operator<
don't induce a strict weak ordering.
Is it undefined or implementation-defined behavior? It seems that it should be stated more clearly that the behavior is
undefined.
[2015-05, Lenexa]
Move to Immediate.
Proposed resolution:
This wording is relative to N4431.
Change 26.8 [alg.sorting]/3 to the following:
For all algorithms that take
Compare
, there is a version that usesoperator<
instead. That is,comp(*i, *j) != false
defaults to*i < *j != false
. For algorithms other than those described in 25.4.3to work correctly,comp
shallhas toinduce a strict weak ordering on the values.
ostream_joiner
needs noexcept
Section: 10.2 [fund.ts.v2::iterator.ostream.joiner] Status: TS Submitter: Nate Wilson Opened: 2015-05-03 Last modified: 2017-07-30
Priority: 0
View all issues with TS status.
Discussion:
Addresses: fund.ts.v2
In Library Fundamentals 2 N4481, [iterator.ostream.joiner], all operations are no-ops other than the assignment operator.
So, they should be marked asnoexcept
.
[2015-05, Lenexa]
Move to Immediate.
Proposed resolution:
This wording is relative to N4481 in regard to fundamental-ts-2 changes.
Change class template ostream_joiner
synopsis, [iterator.ostream.joiner] p2, as indicated:
namespace std { namespace experimental { inline namespace fundamentals_v2 { template <class DelimT, class charT = char, class traits = char_traits<charT> > class ostream_joiner { public: […] ostream_joiner<DelimT, charT,traits>& operator*() noexcept; ostream_joiner<DelimT, charT,traits>& operator++() noexcept; ostream_joiner<DelimT, charT,traits>& operator++(int) noexcept; […] }; } // inline namespace fundamentals_v2 } // namespace experimental } // namespace std
Change [iterator.ostream.joiner.ops] p3+5, as indicated:
ostream_joiner<DelimT, charT, traits>& operator*() noexcept;[…]
ostream_joiner<DelimT, charT, traits>& operator++() noexcept; ostream_joiner<DelimT, charT, traits>& operator++(int) noexcept;
Section: 20.3.2.2.2 [util.smartptr.shared.const] Status: C++17 Submitter: Jonathan Wakely Opened: 2015-05-05 Last modified: 2017-07-30
Priority: 0
View other active issues in [util.smartptr.shared.const].
View all other issues in [util.smartptr.shared.const].
View all issues with C++17 status.
Discussion:
20.3.2.2.2 [util.smartptr.shared.const] includes several "Exception safety" elements, but that is not one of the elements defined in 17.5.1.4 16.3.2.4 [structure.specifications]. We should either define what it means, or just move those sentences into the Effects: clause.
[2015-06, Telecon]
Move to Tentatively Ready.
Proposed resolution:
This wording is relative to N4431.
Change 20.3.2.2.2 [util.smartptr.shared.const] as follows:
template<class Y> explicit shared_ptr(Y* p);[…]
-4- Effects: Constructs ashared_ptr
object that owns the pointerp
. If an exception is thrown,delete p
is called. […]-7- Exception safety: If an exception is thrown,delete p
is called.template <class Y, class D> shared_ptr(Y* p, D d); template <class Y, class D, class A> shared_ptr(Y* p, D d, A a); template <class D> shared_ptr(nullptr_t p, D d); template <class D, class A> shared_ptr(nullptr_t p, D d, A a);[…][…]
-9- Effects: Constructs ashared_ptr
object that owns the objectp
and the deleterd
. The second and fourth constructors shall use a copy ofa
to allocate memory for internal use. If an exception is thrown,d(p)
is called. […]-12- Exception safety: If an exception is thrown,d(p)
is called.template <class Y> explicit shared_ptr(const weak_ptr<Y>& r);[…]
-24- Effects: Constructs ashared_ptr
object that shares ownership withr
and stores a copy of the pointer stored inr
. If an exception is thrown, the constructor has no effect. […]-27- Exception safety: If an exception is thrown, the constructor has no effect.template <class Y, class D> shared_ptr(unique_ptr<Y, D>&& r);[…]
-29- Effects: Equivalent toshared_ptr(r.release(), r.get_deleter())
whenD
is not a reference type, otherwiseshared_ptr(r.release(), ref(r.get_deleter()))
. If an exception is thrown, the constructor has no effect.-30- Exception safety: If an exception is thrown, the constructor has no effect.
operator>>(basic_istream&&, T&&)
returns basic_istream&
, but should probably return
basic_istream&&
Section: 31.7.5.6 [istream.rvalue] Status: Resolved Submitter: Richard Smith Opened: 2015-05-08 Last modified: 2020-11-09
Priority: 3
View all other issues in [istream.rvalue].
View all issues with Resolved status.
Discussion:
Consider:
auto& is = make_istream() >> x; // oops, istream object is already gone
With a basic_istream&&
return type, the above would be ill-formed, and generally we'd
preserve the value category properly.
[2015-06, Telecon]
JW: think this needs proper consideration, it would make
stream() >> x >> y >> zgo from 3 operator>> calls to 6 operator>> calls, and wouldn't prevent dangling references (change the example to auto&&)
[2020-02 Resolved by the adoption of 1203(i) in Prague.]
[2020-11-09 Resolved for C++20. Status changed: Tentatively Resolved → Resolved.]
Proposed resolution:
operator>>(basic_istream&, CharT*)
makes it hard to avoid buffer overflowsSection: 31.7.5.3.3 [istream.extractors] Status: Resolved Submitter: Richard Smith Opened: 2015-05-08 Last modified: 2018-11-12
Priority: 2
View all other issues in [istream.extractors].
View all issues with Resolved status.
Discussion:
We removed gets()
(due to an NB comment and C11 — bastion of backwards compatibility — doing the same).
Should we remove this too?
gets()
, there are legitimate uses:
char buffer[32]; char text[32] = // ... ostream_for_buffer(text) >> buffer; // ok, can't overrun buffer
… but the risk from constructs like "std::cin >> buffer
" seems to outweigh the benefit.
[2015-06, Telecon]
VV: Request a paper to deprecate / remove anything
[2015-10, Kona Saturday afternoon]
STL: This overload is evil and should probably die.
VV: I agree with that, even though I don't care.
STL: Say that we either remove it outright following the gets() rationale, or at least deprecate it.
Move to Open; needs a paper.
[2016-08, Chicago: Zhihao Yuan comments and provides wording]
Rationale:
I would like to keep some reasonable code working;
Reasonable code includes two cases:
width() > 0
, any pointer argument
width() >= 0
, array argument
For a), banning bad code will become a silent behavior change at runtime; for b), it breaks at compile time.
I propose to replace these signatures with references to arrays. An implementation may want to ship the old instantiatations in the binary without exposing the old signatures.
[2016-08, Chicago]
Tues PM: General agreement on deprecating the unsafe call, but no consensus for the P/R.
General feeling that implementation experience would be useful.
[2018-08-23 Batavia Issues processing]
Will be resolved by the adoption of P0487.
[2018-11-11 Resolved by P0487R1, adopted in San Diego.]
Proposed resolution:
This wording is relative to N4606.
Modify 31.7.5.3.3 [istream.extractors] as indicated:
template<class charT, class traits, size_t N> basic_istream<charT, traits>& operator>>(basic_istream<charT, traits>& in,charT* scharT (&s)[N]); template<class traits, size_t N> basic_istream<char, traits>& operator>>(basic_istream<char, traits>& in,unsigned char* sunsigned char (&s)[N]); template<class traits, size_t N> basic_istream<char, traits>& operator>>(basic_istream<char, traits>& in,signed char* ssigned char (&s)[N]);-7- Effects: Behaves like a formatted input member (as described in 31.7.5.3.1 [istream.formatted.reqmts]) of
in
. After asentry
object is constructed,operator>>
extracts characters and stores them intosuccessive locations of an array whose first element is designated bys
. Ifwidth()
is greater than zero,n
is. Otherwise
width()min(size_t(width()), N)n
isthe number of elements of the largest array ofchar_type
that can store a terminatingcharT()
N
.n
is the maximum number of characters stored.
void
Section: 8.2.1.2 [fund.ts.v2::memory.smartptr.shared.obs] Status: TS Submitter: Jeffrey Yasskin Opened: 2015-05-11 Last modified: 2017-07-30
Priority: 0
View all issues with TS status.
Discussion:
Addresses: fund.ts.v2
S. B. Tam reported this here.
N3920 changedoperator*()
in
[util.smartptr.shared.obs] as:
Remarks: When
T
is an array type or cv-qualifiedvoid
, it is unspecified whether this member function is declared. …
This excludes cv-unqualified void
, which is probably unintended.
[2015-09-11, Telecon]
Move to Tentatively Ready
Proposed resolution:
In the library fundamentals v2, [memory.smartptr.shared.obs] p2, change as indicated:
Remarks: When
T
is an array type or (possibly cv-qualified)void
, it is unspecified whether this member function is declared. If it is declared, it is unspecified what its return type is, except that the declaration (although not necessarily the definition) of the function shall be well formed.
std::function
requires POCMA/POCCASection: 22.10.17.3 [func.wrap.func] Status: Resolved Submitter: David Krauss Opened: 2015-05-20 Last modified: 2020-09-06
Priority: 3
View all other issues in [func.wrap.func].
View all issues with Resolved status.
Discussion:
The idea behind propagate_on_container_move_assignment
is that you can keep an allocator attached to a container.
But it's not really designed to work with polymorphism, which introduces the condition where the current allocator is non-POCMA
and the RHS of assignment, being POCMA, wants to replace it. If function were to respect the literal meaning, any would-be
attached allocator is at the mercy of every assignment operation. So, std::function
is inherently POCMA, and passing
a non-POCMA allocator should be ill-formed.
noexcept
.
The same applies to propagate_on_container_copy_assignment
. This presents more difficulty because std::allocator
does not set this to true. Perhaps it should. For function to respect this would require inspecting the POCCA of the source allocator,
slicing the target from the erasure of the source, slicing the allocation from the erasure of the destination, and performing a
copy with the destination's allocator with the source's target. This comes out of the blue for the destination allocator, which
might not support the new type anyway. Theoretically possible, but brittle and not very practical. Again, current implementations
quietly ignore the issue but this isn't very clean.
The following code example is intended to demonstrate the issue here:
#include <functional> #include <iostream> #include <vector> template <typename T> struct diag_alloc { std::string name; T* allocate(std::size_t n) const { std::cout << '+' << name << '\n'; return static_cast<T*>(::operator new(n * sizeof(T))); } void deallocate(T* p, std::size_t) const { std::cout << '-' << name << '\n'; return ::operator delete(p); } template <typename U> operator diag_alloc<U>() const { return {name}; } friend bool operator==(const diag_alloc& a, const diag_alloc& b) { return a.name == b.name; } friend bool operator!=(const diag_alloc& a, const diag_alloc& b) { return a.name != b.name; } typedef T value_type; template <typename U> struct rebind { typedef diag_alloc<U> other; }; }; int main() { std::cout << "VECTOR\n"; std::vector<int, diag_alloc<int>> foo({1, 2}, {"foo"}); // +foo std::vector<int, diag_alloc<int>> bar({3, 4}, {"bar"}); // +bar std::cout << "move\n"; foo = std::move(bar); // no message std::cout << "more foo\n"; foo.reserve(40); // +foo -foo std::cout << "more bar\n"; bar.reserve(40); // +bar -bar std::cout << "\nFUNCTION\n"; int bigdata[100]; auto bigfun = [bigdata]{}; typedef decltype(bigfun) ft; std::cout << "make fizz\n"; std::function<void()> fizz(std::allocator_arg, diag_alloc<ft>{"fizz"}, bigfun); // +fizz std::cout << "another fizz\n"; std::function<void()> fizz2; fizz2 = fizz; // +fizz as if POCCA std::cout << "make buzz\n"; std::function<void()> buzz(std::allocator_arg, diag_alloc<ft>{"buzz"}, bigfun); // +buzz std::cout << "move\n"; buzz = std::move(fizz); // -buzz as if POCMA std::cout << "\nCLEANUP\n"; }
[2016-08, Chicago]
Tues PM: Resolved by P0302R1.
Proposed resolution:
Resolved by P0302R1.
std::function
does not use allocator::construct
Section: 22.10.17.3 [func.wrap.func] Status: Resolved Submitter: David Krauss Opened: 2015-05-20 Last modified: 2020-09-06
Priority: 3
View all other issues in [func.wrap.func].
View all issues with Resolved status.
Discussion:
It is impossible for std::function
to construct its target object using the construct
method of a type-erased
allocator. More confusingly, it is possible when the allocator and the target are created at the same time. The means
of target construction should be specified.
[2016-08 Chicago]
Tues PM: Resolved by P0302R1.
Proposed resolution:
Resolved by P0302R1.
syntax_option_type
Section: 28.6.4.2 [re.synopt] Status: C++17 Submitter: Nozomu Katō Opened: 2015-05-22 Last modified: 2017-07-30
Priority: 2
View all other issues in [re.synopt].
View all issues with C++17 status.
Discussion:
The specification of ECMAScript defines the Multiline property for its
RegExp and the regular expressions ^ and $ behave differently according
to the value of this property. Thus, this property should be available
also in the ECMAScript compatible engine in std::regex
.
[2015-05-22, Daniel comments]
This issue interacts somewhat with LWG 2343(i).
[Telecon 2015-07]
Set the priority to match LWG 2343(i).
[2016-08, Chicago]
Monday PM: Moved to Tentatively Ready. This also resolves 2343(i)
Proposed resolution:
This wording is relative to N4431.
Change 28.6.4.2 [re.synopt] as indicated:
namespace std::regex_constants { typedef T1 syntax_option_type; constexpr syntax_option_type icase = unspecified ; constexpr syntax_option_type nosubs = unspecified ; constexpr syntax_option_type optimize = unspecified ; constexpr syntax_option_type collate = unspecified ; constexpr syntax_option_type ECMAScript = unspecified ; constexpr syntax_option_type basic = unspecified ; constexpr syntax_option_type extended = unspecified ; constexpr syntax_option_type awk = unspecified ; constexpr syntax_option_type grep = unspecified ; constexpr syntax_option_type egrep = unspecified ; constexpr syntax_option_type multiline = unspecified ; }
Change 28.6.4.3 [re.matchflag], Table 138 — "syntax_option_type
effects" as indicated:
Table 138 — syntax_option_type
effectsElement Effect(s) if set …
multiline
Specifies that ^
shall match the beginning of a line and$
shall match the end of a line, if the ECMAScript engine is selected.…
auto_ptr_ref
creation requirements underspecifiedSection: 99 [auto.ptr.conv] Status: Resolved Submitter: Hubert Tong Opened: 2015-05-28 Last modified: 2020-09-06
Priority: 4
View all other issues in [auto.ptr.conv].
View all issues with Resolved status.
Discussion:
In C++14 sub-clause 99 [auto.ptr.conv], there appears to be no requirement that the formation of an
auto_ptr_ref<Y>
from an auto_ptr<X>
is done only when X*
can be implicitly
converted to Y*
.
auto_ptr_ref<A>
from the prvalue of type auto_ptr<B>
to be invalid in the case below (but the wording does not seem to be there):
#include <memory> struct A { }; struct B { } b; std::auto_ptr<B> apB() { return std::auto_ptr<B>(&b); } int main() { std::auto_ptr<A> apA(apB()); apA.release(); }
The behaviour of the implementation in question on the case presented above is to compile and execute it successfully
(which is what the C++14 wording implies). The returned value from apA.release()
is essentially
reinterpret_cast<A*>(&b)
.
template <class X> template <class Y> operator auto_ptr<X>::auto_ptr_ref<Y>() throw();
which implies that X*
should be implicitly convertible to Y*
.
reinterpret_cast
interpretation even when Y
is an accessible,
unambiguous base class of X
; the result thereof is that no offset adjustment is performed.
[2015-07, Telecon]
Marshall to resolve.
[2016-03-16, Alisdair Meredith comments]
This issue is a defect in a component we have actively removed from the standard. I can't think of a clearer example of something that is no longer a defect!
[2016-08-03, Alisdair Meredith comments]
As C++17 removes auto_ptr
, I suggest closing this issue as closed by paper
N4190.
Previous resolution [SUPERSEDED]:
This wording is relative to ISO/IEC 14882:2014(E).
Change 99 [auto.ptr.conv] as indicated:
template<class Y> operator auto_ptr_ref<Y>() throw();-?- Requires:
-3- Returns: AnX*
can be implicitly converted toY*
.auto_ptr_ref<Y>
that holds*this
. -?- Notes: Becauseauto_ptr_ref
is present for exposition only, the only way to invoke this function is by calling one of theauto_ptr
conversions which take anauto_ptr_ref
as an argument. Since all such conversions will callrelease()
on*this
(in the form of theauto_ptr
that theauto_ptr_ref
holds a reference to), an implementation of this function may cause instantiation of saidrelease()
function without changing the semantics of the program.template<class Y> operator auto_ptr<Y>() throw();-?- Requires:
-4- Effects: CallsX*
can be implicitly converted toY*
.release()
. -5- Returns: Anauto_ptr<Y>
that holds the pointer returned fromrelease()
.
[2016-08 - Chicago]
Thurs AM: Moved to Tentatively Resolved
Proposed resolution:
Resolved by acceptance of N4190.
any_cast
doesn't work with rvalue reference targets and cannot move with a value targetSection: 6.4 [fund.ts.v2::any.nonmembers] Status: TS Submitter: Ville Voutilainen Opened: 2015-06-13 Last modified: 2017-07-30
Priority: 2
View all issues with TS status.
Discussion:
Addresses: fund.ts.v2
In Library Fundamentals v1, [any.nonmembers]/5 says:
For the first form,
*any_cast<add_const_t<remove_reference_t<ValueType>>>(&operand)
. For the second and third forms,*any_cast<remove_reference_t<ValueType>>(&operand)
.
This means that
any_cast<Foo&&>(whatever_kind_of_any_lvalue_or_rvalue);
is always ill-formed. That's unfortunate, because forwarding such a cast
result of an any
is actually useful, and such uses do not want to copy/move
the underlying value just yet.
Another problem is that that same specification prevents an implementation from moving to the target when
ValueType any_cast(any&& operand);
is used. The difference is observable, so an implementation can't perform
an optimization under the as-if rule. We are pessimizing every CopyConstructible
and MoveConstructible
type because we are not using the move when
we can. This unfortunately includes types such as the library containers,
and we do not want such a pessimization!
[2015-07, Telecon]
Jonathan to provide wording
[2015-10, Kona Saturday afternoon]
Eric offered to help JW with wording
Move to Open
[2016-01-30, Ville comments and provides wording]
Drafting note: the first two changes add support for types that have explicitly deleted move constructors. Should we choose not to support such types at all, the third change is all we need. For the second change, there are still potential cases where Requires is fulfilled but Effects is ill-formed, if a suitably concocted type is thrown into the mix.
Proposed resolution:
This wording is relative to N4562.
In 6.3.1 [fund.ts.v2::any.cons] p11+p12, edit as follows:
template<class ValueType> any(ValueType&& value);-10- Let
-11- Requires:T
be equal todecay_t<ValueType>
.T
shall satisfy theCopyConstructible
requirements, except for the requirements forMoveConstructible
. Ifis_copy_constructible_v<T>
isfalse
, the program is ill-formed. -12- Effects: Ifis_constructible_v<T, ValueType&&>
is true, cConstructs an object of typeany
that contains an object of typeT
direct-initialized withstd::forward<ValueType>(value)
. Otherwise, constructs an object of typeany
that contains an object of typeT
direct-initialized withvalue
. […]
In 6.4 [fund.ts.v2::any.nonmembers] p5, edit as follows:
template<class ValueType> ValueType any_cast(const any& operand); template<class ValueType> ValueType any_cast(any& operand); template<class ValueType> ValueType any_cast(any&& operand);-4- Requires:
-5- Returns: For the first form,is_reference_v<ValueType>
istrue
oris_copy_constructible_v<ValueType>
istrue
. Otherwise the program is ill-formed.*any_cast<add_const_t<remove_reference_t<ValueType>>>(&operand)
. For the secondand thirdforms,*any_cast<remove_reference_t<ValueType>>(&operand)
. For the third form, ifis_move_constructible_v<ValueType>
istrue
andis_lvalue_reference_v<ValueType>
isfalse
,std::move(*any_cast<remove_reference_t<ValueType>>(&operand))
, otherwise,*any_cast<remove_reference_t<ValueType>>(&operand)
. […]
DefaultConstructible
Section: 17.6 [support.dynamic], 22.2 [utility], 22.3.5 [pair.piecewise], 20.2.2 [memory.syn], 20.2.7 [allocator.tag], 32.6 [thread.mutex] Status: C++17 Submitter: Ville Voutilainen Opened: 2015-06-13 Last modified: 2017-07-30
Priority: 2
View all other issues in [support.dynamic].
View all issues with C++17 status.
Discussion:
std::experimental::optional
, for certain reasons, specifies its nullopt
type
to not be DefaultConstructible
. It doesn't do so for its tag type in_place_t
and neither does the standard proper for any of its tag types. That turns
out to be very unfortunate, consider the following:
#include <memory> #include <array> void f(std::array<int, 1>, int) {} // #1 void f(std::allocator_arg_t, int) {} // #2 int main() { f({}, 666); // #3 }
The call at #3 is ambiguous. What's even worse is that if the overload #1 is removed, the call works just fine. The whole point of a tag type is that it either needs to mentioned in a call or it needs to be a forwarded argument, so being able to construct a tag type like that makes no sense.
Making the types have an explicit default constructor might have helped, but CWG 1518 is going against that idea. [optional.nullopt]/3 solves this problem fornullopt
:
Type
nullopt_t
shall not have a default constructor. It shall be a literal type. Constantnullopt
shall be initialized with an argument of literal type.
[2015-06, Telecon]
Move to Tentatively Ready.
[2015-10, Kona Saturday afternoon]
Move back to Open
JW: The linked Core issue (CWG 1518) gives us a better tool to solve this (explicit default constructors). [The CWG Issue means that an explicit default constructor will no longer match "{}".] JW explains that it's important that tag types cannot be constructed from "{}" (e.g. the allocator tag in the tuple constructors).
WEB: Should we now go back and update our constructors? JW: For tag types, yes.
VV: The guideline is that anything that does not mention the type name explicitly should not invoke an explicit constructor.
Ville will provide wording.
Discussion about pair/tuple's default constructor - should they now be explicit?
[2016-01-31]
Ville provides revised wording.
Previous resolution [SUPERSEDED]:
This wording is relative to N4527.
In 17.6 [support.dynamic]/1, change the header
<new>
synopsis:[…] struct nothrow_t{}; see below extern const nothrow_t nothrow; […]Add a new paragraph after 17.6 [support.dynamic]/1 (following the header
<new>
synopsis):-?- Type
nothrow_t
shall not have a default constructor.In 22.2 [utility]/2, change the header
<utility>
synopsis:[…] // 20.3.5, pair piecewise construction struct piecewise_construct_t{ }; see below constexpr piecewise_construct_t piecewise_construct{ unspecified }; […]Add a new paragraph after 22.2 [utility]/2 (following the header
<utility>
synopsis):-?- Type
piecewise_construct_t
shall not have a default constructor. It shall be a literal type. Constantpiecewise_construct
shall be initialized with an argument of literal type.In 22.3.5 [pair.piecewise], apply the following edits:
struct piecewise_construct_t{ }; constexpr piecewise_construct_t piecewise_construct{ unspecified };In 20.2.2 [memory.syn]/1, change the header
<memory>
synopsis:[…] // 20.7.6, allocator argument tag struct allocator_arg_t{ }; see below constexpr allocator_arg_t allocator_arg{ unspecified }; […]Add a new paragraph after 20.2.2 [memory.syn]/1 (following the header
<memory>
synopsis):-?- Type
allocator_arg_t
shall not have a default constructor. It shall be a literal type. Constantallocator_arg
shall be initialized with an argument of literal type.In 20.2.7 [allocator.tag], apply the following edits:
namespace std { struct allocator_arg_t{ }; constexpr allocator_arg_t allocator_arg{ unspecified }; }Editorial drive-by:
piecewise_construct_t
is written, in 22.3.5 [pair.piecewise] likestruct piecewise_construct_t { }; constexpr piecewise_construct_t piecewise_construct{};whereas other tag types such as
allocator_construct_t
are, in e.g. 20.2.7 [allocator.tag], written likenamespace std { struct allocator_arg_t { }; constexpr allocator_arg_t allocator_arg{}; }We should decide whether or not to write out the
std
namespace in such paragraphs. I would suggest not to write it out.In 32.6 [thread.mutex]/1, change the header
<mutex>
synopsis:[…] struct defer_lock_t{ }; see below struct try_to_lock_t{ }; see below struct adopt_lock_t{ }; see below constexpr defer_lock_t defer_lock { unspecified }; constexpr try_to_lock_t try_to_lock { unspecified }; constexpr adopt_lock_t adopt_lock { unspecified }; […]Add three new paragraphs after [thread.mutex]/1 (following the header
<mutex>
synopsis):-?- Type
-?- Typedefer_lock_t
shall not have a default constructor. It shall be a literal type. Constantdefer_lock
shall be initialized with an argument of literal type.try_to_lock_t
shall not have a default constructor. It shall be a literal type. Constanttry_to_lock
shall be initialized with an argument of literal type. -?- Typeadopt_lock_t
shall not have a default constructor. It shall be a literal type. Constantadopt_lock
shall be initialized with an argument of literal type.
[2016-03 Jacksonville]
AM: should have note about compatibility in Annex C
HH: like this idiom well enough that I've started using it in my own code
AM: why are pair and tuple involved here?
GR: they are the only types which forward explicitness with EXPLICIT
AM: British spelling of behaviour
AM: happy to drop my issue about Annex C
[2016-06 Oulu]
This is waiting on Core issue 1518
Saturday: Core 1518 was resolved in Oulu
[2016-07 Chicago]
Monday PM: Moved to Tentatively Ready
Proposed resolution:
This wording is relative to N4567.
In 17.6 [support.dynamic]/1, change the header <new>
synopsis:
[…] struct nothrow_t { explicit nothrow_t() = default; }; extern const nothrow_t nothrow; […]
In 22.2 [utility]/2, change the header <utility>
synopsis:
[…] // 20.3.5, pair piecewise construction struct piecewise_construct_t { explicit piecewise_construct_t() = default; }; constexpr piecewise_construct_t piecewise_construct{}; […]
In 22.3.2 [pairs.pair], change the class template pair
synopsis:
[…] pair(pair&&) = default; EXPLICIT constexpr pair(); EXPLICIT constexpr pair(const T1& x, const T2& y); […]
Around 22.3.2 [pairs.pair] p3, apply the following edits:
EXPLICIT constexpr pair();-3- Effects: Value-initializes
-4- Remarks: This constructor shall not participate in overload resolution unlessfirst
andsecond
.is_default_constructible<first_type>::value
istrue
andis_default_constructible<second_type>::value
istrue
. [Note: This behaviour can be implemented by a constructor template with default template arguments. — end note] The constructor is explicit if and only if eitherfirst_type
orsecond_type
is not implicitly default-constructible. [Note: This behaviour can be implemented with a trait that checks whether aconst first_type&
or aconst second_type&
can be initialized with{}
. — end note]
In 22.3.5 [pair.piecewise], apply the following edits:
struct piecewise_construct_t { explicit piecewise_construct_t() = default; }; constexpr piecewise_construct_t piecewise_construct{};
In 22.4.4 [tuple.tuple], change the class template tuple
synopsis:
[…] // 20.4.2.1, tuple construction EXPLICIT constexpr tuple(); EXPLICIT constexpr tuple(const Types&...); // only if sizeof...(Types) >= 1 […]
Around 22.4.4.2 [tuple.cnstr] p4, apply the following edits:
EXPLICIT constexpr tuple();-4- Effects: Value initializes each element.
-5- Remarks: This constructor shall not participate in overload resolution unlessis_default_constructible<Ti>::value
istrue
for all i. [Note: This behaviour can be implemented by a constructor template with default template arguments. — end note] The constructor is explicit if and only ifTi
is not implicitly default-constructible for at least one i. [Note: This behaviour can be implemented with a trait that checks whether aconst Ti&
can be initialized with{}
. — end note]
In 20.2.2 [memory.syn]/1, change the header <memory>
synopsis:
[…] // 20.7.6, allocator argument tag struct allocator_arg_t { explicit allocator_arg_t() = default; }; constexpr allocator_arg_t allocator_arg{}; […]
In 20.2.7 [allocator.tag], apply the following edits:
namespace std { struct allocator_arg_t { explicit allocator_arg_t() = default; }; constexpr allocator_arg_t allocator_arg{}; }
Editorial drive-by:
piecewise_construct_t
is written, in 22.3.5 [pair.piecewise] likestruct piecewise_construct_t { }; constexpr piecewise_construct_t piecewise_construct{};whereas other tag types such as
allocator_construct_t
are, in e.g. 20.2.7 [allocator.tag], written likenamespace std { struct allocator_arg_t { }; constexpr allocator_arg_t allocator_arg{}; }We should decide whether or not to write out the
std
namespace in such paragraphs. I would suggest not to write it out.
In 32.6 [thread.mutex]/1, change the header <mutex>
synopsis:
[…] struct defer_lock_t { explicit defer_lock_t() = default; }; struct try_to_lock_t { explicit try_to_lock_t() = default; }; struct adopt_lock_t { explicit adopt_lock_t() = default; }; constexpr defer_lock_t defer_lock { }; constexpr try_to_lock_t try_to_lock { }; constexpr adopt_lock_t adopt_lock { }; […]
scoped_allocator_adaptor
piecewise construction does not require CopyConstructible
Section: 20.6.4 [allocator.adaptor.members] Status: Resolved Submitter: David Krauss Opened: 2015-06-16 Last modified: 2020-09-06
Priority: 3
View all other issues in [allocator.adaptor.members].
View all issues with Resolved status.
Discussion:
20.6.4 [allocator.adaptor.members]/10 requires that the argument types in the piecewise-construction tuples
all be CopyConstructible
. These tuples are typically created by std::forward_as_tuple
, such as in
¶13. So they will be a mix of lvalue and rvalue references, the latter of which are not CopyConstructible
.
CopyConstructible
was specified to feed the tuple_cat
, before that function could
handle rvalues. Since the argument tuple is already moved in ¶11, the requirement is obsolete. It should either
be changed to MoveConstructible
, or perhaps better, convert the whole tuple to references (i.e. form
tuple<Args1&&...>
) so nothing needs to be moved. After all, this is a facility for handling non-movable
types.
It appears that the resolution of DR 2203(i), which added std::move
to ¶11, simply omitted the
change to ¶10.
[2016-11-08, Jonathan comments]
My paper P0475R0 provides a proposed resolution.
[2018-06 set to 'Resolved']
P0475R1 was adopted in Rapperswil.
Proposed resolution:
final
Section: 21.3.2 [meta.rqmts] Status: C++17 Submitter: Jonathan Wakely Opened: 2015-07-03 Last modified: 2017-07-30
Priority: 3
View all other issues in [meta.rqmts].
View all issues with C++17 status.
Discussion:
We should make it clear that all standard UnaryTypeTraits
,
BinaryTypeTraits
and TransformationTraits
are not final
.
template<typename C1, typename C2> struct conjunction : conditional_t<C1::value, C2, C1> { };
[2016-08-03 Chicago LWG]
Walter, Nevin, and Jason provide initial Proposed Resolution.
Previous resolution [SUPERSEDED]:
This wording is relative to N4606.
Change 21.3.2 [meta.rqmts] as indicated:
-1- A UnaryTypeTrait describes a property of a type. It shall be a non-
-2- A BinaryTypeTrait describes a relationship between two types. It shall be a non-final
class template […]final
class template […] -3- A TransformationTrait modifies a property of a type. It shall be a non-final
class template […]
[2016-08-04 Chicago LWG]
LWG discusses and expresses preference for a more general, Library-wide, resolution. Walter and Nevin provide a new Proposed Resolution consistent with such guidance.
[2016-08 - Chicago]
Thurs PM: Moved to Tentatively Ready
Proposed resolution:
This wording is relative to N4606.
Add a new paragraph add the end of 16.4.6.13 [derivation] as indicated:
-?- All types specified in the C++ standard library shall be non-
final
types unless otherwise specified.
observer_ptr
do not match synopsisSection: 8.12.6 [fund.ts.v2::memory.observer.ptr.special] Status: TS Submitter: Tim Song Opened: 2015-07-07 Last modified: 2017-07-30
Priority: 0
View all issues with TS status.
Discussion:
Addresses: fund.ts.v2
In N4529
[memory.observer.ptr.special] paragraphs 15, 17 and 19, the >
, <=
and
>=
operators of observer_ptr
are shown as
template <class W> bool operator>(observer_ptr<W> p1, observer_ptr<W> p2);
whereas in [header.memory.synop] they are shown as
template <class W1, class W2> bool operator>(observer_ptr<W1> p1, observer_ptr<W2> p2);
Given that the specification of operator<
took special care to support hetergeneous types,
presumably the second version is intended.
[2015-07, Telecon]
Move to Tentatively Ready.
Proposed resolution:
This wording is relative to N4529.
Edit 8.12.6 [fund.ts.v2::memory.observer.ptr.special] as indicated:
-15- template <class W1, class W2> bool operator>(observer_ptr<W1> p1, observer_ptr<W2> p2);[…]
-17- template <class W1, class W2> bool operator<=(observer_ptr<W1> p1, observer_ptr<W2> p2);[…]
-19- template <class W1, class W2> bool operator>=(observer_ptr<W1> p1, observer_ptr<W2> p2);
observer_ptr
Section: 8.12.1 [fund.ts.v2::memory.observer.ptr.overview] Status: TS Submitter: Tim Song Opened: 2015-07-07 Last modified: 2017-07-30
Priority: 2
View all issues with TS status.
Discussion:
Addresses: fund.ts.v2
In N4529
[memory.observer.ptr.overview], the public member typedefs pointer
and reference
are marked
"//exposition-only".
Clause 17 of the standard, which the TS incorporates by reference, only defines "exposition only" for private members (see 16.3.3.6 [objects.within.classes], also compare LWG 2310(i)). These types should be either made private, or not exposition only.
[2015-07, Telecon]
Geoffrey: Will survey "exposition-only" use and come up with wording to define "exposition-only" for data members, types, and concepts.
[2015-10, Kona Saturday afternoon]
GR: I cannot figure out what "exposition only" means. JW explained it to me once as meaning that "it has the same normative effect as if the member existed, without the effect of there actually being a member". But I don't know what is the "normative effect of the existence of a member" is.
MC: The "exposition-only" member should be private. GR: But it still has normative effect.
GR, STL: Maybe the member is public so it can be used in the public section? WEB (original worder): I don't know why the member is public. I'm not averse to changing it. GR: A telecon instructed me to find out how [exposition-only] is used elsewhere, and I came back with this issue because I don't understand what it means.
STL: We use exposition-only in many places. We all know what it means. GR: I don't know precisely what means.
STL: I know exactly what it means. STL suggests a rewording: the paragraph already says what we want, it's just a little too precise. GR not sure.
WEB: Are there any non-members in the Standard that are "exposition only"? STL: DECAY_COPY and INVOKE.
GR: There are a few enums.
HH: We have nested class types that are exposition-only. HH: We're about to make "none_t" an exposition-only type. [Everyone lists some examples from the Standard that are EO.] GR: "bitmask" contains some hypothetical types. That potentially contains user requirements (since regexp requires certain trait members to be "bitmask types").
STL: What's the priority of this issue? Does this have any effect on implementations? Is there anything mysterious that could happen?
AM: 2.
STL: How did that happen? It's at best priority 4. We have so many better things we could be doing.
WEB: "exposition only" in [???] is just used as plain English, not as words of power. GR: That gives me enough to make wording to fix this.
STL: I wouldn't mind if we didn't fix this ever.
MC: @JW, please write up wording for 2310(i) to make the EO members private. JW: I can't do that, because that'd make the class a non-aggregate.
WEB: Please cross-link both issues and move 2516 to "open".
[2015-11-14, Geoffrey Romer provides wording]
[2016-03 Jacksonville]
Move to ready. Note that this modifies both the Draft Standard and LFTS 2
Proposed resolution:
This part of the wording is against the working draft of the standard for Programming Language C++, the wording is relative to N4567.
Edit 16.3.3.6 [objects.within.classes]/p2 as follows:
-2-
Objects of certain classes are sometimes required by the external specifications of their classes to store data, apparently in member objects.For the sake of exposition, some subclauses provide representative declarations, and semantic requirements, for private membersobjectsof classes that meet the external specifications of the classes. The declarations for such membersobjects and the definitions of related member typesare followed by a comment that ends with exposition only, as in:streambuf* sb; // exposition only
This part of the wording is against the working draft of the C++ Extensions for Library Fundamentals, Version 2, the wording is relative to N4529
Edit the synopsis in 8.12.1 [fund.ts.v2::memory.observer.ptr.overview] as follows:
namespace std { namespace experimental { inline namespace fundamentals_v2 { template <class W> class observer_ptr { using pointer = add_pointer_t<W>; // exposition-only using reference = add_lvalue_reference_t<W> // exposition-only public: // publish our template parameter and variations thereof using element_type = W;using pointer = add_pointer_t<W>; // exposition-only using reference = add_lvalue_reference_t<W> // exposition-only// 8.12.2, observer_ptr constructors // default c'tor constexpr observer_ptr() noexcept; […] }; // observer_ptr<> } // inline namespace fundamentals_v2 } // namespace experimental } // namespace std
propagate_const
assignment operators have incorrect return typeSection: 3.7.5 [fund.ts.v2::propagate_const.assignment] Status: TS Submitter: Tim Song Opened: 2015-07-08 Last modified: 2017-07-30
Priority: 0
View all issues with TS status.
Discussion:
Addresses: fund.ts.v2
N4529
[propagate_const.assignment] depicts the two operator=
s described as returning by value.
This is obviously incorrect. The class synopsis correctly shows them as returning by reference.
[2015-06, Telecon]
Move to Tentatively Ready.
Proposed resolution:
This wording is relative to N4529.
Edit [propagate_const.assignment] as indicated:
-1- template <class U> constexpr propagate_const& operator=(propagate_const<U>&& pu)[…]
-5- template <class U> constexpr propagate_const& operator=(U&& u)
swap
for propagate_const
should call member swap
Section: 3.7.10 [fund.ts.v2::propagate_const.algorithms] Status: TS Submitter: Tim Song Opened: 2015-07-08 Last modified: 2017-07-30
Priority: 3
View all issues with TS status.
Discussion:
Addresses: fund.ts.v2
For consistency with the rest of the standard library, the non-member swap
for propagate_const
should
call member swap
.
[2015-07, Telecon]
Both P3 and NAD were suggested.
[2016-02-20, Ville comments]
Feedback from an implementation:
The implementation ofpropagate_const
in libstdc++ calls propagate_const
's
member swap
from the non-member swap
.
[2016-11-08, Issaquah]
Adopted during NB comment resolution
Proposed resolution:
This wording is relative to N4529.
Edit [propagate_const.algorithms] as indicated:
-1- template <class T> constexpr void swap(propagate_const<T>& pt1, propagate_const<T>& pt2) noexcept(see below)-2- The constant-expression in the exception-specification is
-3- Effects:noexcept(
.swap(pt1.t_, pt2.t_)pt1.swap(pt2)).
swap(pt1.t_, pt2.t_)pt1.swap(pt2)
operator-=
has gratuitous undefined behaviourSection: 24.3.5.7 [random.access.iterators] Status: C++17 Submitter: Hubert Tong Opened: 2015-07-15 Last modified: 2017-07-30
Priority: 2
View all other issues in [random.access.iterators].
View all issues with C++17 status.
Discussion:
In subclause 24.3.5.7 [random.access.iterators], Table 110, the operational semantics for the expression "r -= n
"
are defined as
return r += -n;
Given a difference_type
of a type int
with range [-32768, 32767], if the value of n
is -32768,
then the evaluation of -n
causes undefined behaviour (Clause 5 [expr] paragraph 4).
r -= n
" with:
{ difference_type m = n; if (m >= 0) while (m--) --r; else while (m++) ++r; return r; }
Jonathan Wakely:
I'm now convinced we don't want to change the definition of-=
and
instead we should explicitly state the (currently implicit)
precondition that n != numeric_limits<difference_type>::min()
.
[2016-08, Chicago]
Monday PM: Move to Tentatively Ready
Proposed resolution:
This wording is relative to N4527.
Change Table 110 "Random access iterator requirements (in addition to bidirectional iterator)" as indicated:
Table 110 — Random access iterator requirements (in addition to bidirectional iterator) Expression Return type Operational
semanticsAssertion/note
pre-/post-condition…
r -= n
X&
return r += -n;
pre: the absolute value of n
is in the range of representable values ofdifference_type
.…
unique_ptr<T[]>
from a nullptr
Section: 20.3.1.4.2 [unique.ptr.runtime.ctor] Status: C++17 Submitter: Ville Voutilainen Opened: 2015-07-19 Last modified: 2017-07-30
Priority: 2
View all issues with C++17 status.
Discussion:
According to the wording in 20.3.1.4.2 [unique.ptr.runtime.ctor]/1, this won't work:
unique_ptr<int[], DeleterType> x{nullptr, DeleterType{}};
U
is not the same type as pointer
, so the first bullet will not do.
U
is not a pointer type, so the second bullet will not do.
U
is the same type as pointer
, or
U
is nullptr_t
, or
pointer
is the same type as element_type*
, […]
[2015-10, Kona Saturday afternoon]
MC: Is it the right fix?
GR: It'd be awefully surprising if we had an interface that accepts null pointer values but not std::nullptr_t. I think the PR is good.
STL: Are any of the assignments and reset affected? [No, they don't operate on explicit {pointer, deleter} pairs.]
VV: This is already shipping, has been implemented, has been tested and works fine.
Move to Tentatively ready.
Proposed resolution:
This wording is relative to N4527.
Change 20.3.1.4.2 [unique.ptr.runtime.ctor] as indicated:
template <class U> explicit unique_ptr(U p) noexcept; template <class U> unique_ptr(U p, see below d) noexcept; template <class U> unique_ptr(U p, see below d) noexcept;-1- These constructors behave the same as the constructors that take a
pointer
parameter in the primary template except that they shall not participate in overload resolution unless either
U
is the same type aspointer
, or
U
isnullptr_t
, or
pointer
is the same type aselement_type*
,U
is a pointer typeV*
, andV(*)[]
is convertible toelement_type(*)[]
.
weak_ptr
's converting move constructor should be modified as well for array supportSection: 8.2.2.1 [fund.ts.v2::memory.smartptr.weak.const] Status: TS Submitter: Tim Song Opened: 2015-07-25 Last modified: 2017-07-30
Priority: 2
View all issues with TS status.
Discussion:
Addresses: fund.ts.v2
[memory.smartptr.weak.const] altered the constraints on weak_ptr
's constructor from const weak_ptr<Y>&
and const shared_ptr<Y>&
. The constraints on the converting move constructor from weak_ptr<Y>&&
was not, but should be, similarly modified.
[2015-10-26]
Daniel adjusts wording to lib. fund. v2. As link to the originating proposal: The discussion in this issue refers to wording changes that were requested by N3920.
[2016-11-08, Issaquah]
Adopted during NB comment resolution
Proposed resolution:
This wording is relative to N4529.
At the end of [memory.smartptr.weak.const], add:
[Drafting note: The current paragraph [memory.smartptr.weak.const] p2 is incorrectly declared as Requires element,
but it does not describe a requirement, instead it describes a "template constraint" which are elsewhere always
specified within a Remarks element because it describes constraints that an implementation (and not the user) has to meet.
See LWG 2292(i) for a suggestion to introduce a separate new specification element for this situation.
This has also been fixed in the current working draft. —
end drafting note]
weak_ptr(weak_ptr&& r) noexcept; template<class Y> weak_ptr(weak_ptr<Y>&& r) noexcept;-?- Remark: The second constructor shall not participate in overload resolution unless
-?- Effects: Move-constructs aY*
is compatible withT*
.weak_ptr
instance fromr
. -?- Postconditions:*this
shall contain the old value ofr
.r
shall be empty.r.use_count() == 0
.
set_default_resource
specificationSection: 8.8 [fund.ts.v2::memory.resource.global] Status: TS Submitter: Tim Song Opened: 2015-07-28 Last modified: 2017-07-30
Priority: 2
View all issues with TS status.
Discussion:
Addresses: fund.ts.v2
[memory.resource.global]/p7-8 says that the effects of set_default_resource(r)
are
If
r
is non-null, sets the value of the default memory resource pointer tor
, otherwise sets the default memory resource pointer tonew_delete_resource()
.
and the operation has the postcondition
get_default_resource() == r
.
When r
is null, however, the postcondition cannot be met, since the call sets the default memory resource pointer to
new_delete_resource()
, and so get_default_resource()
would return the value of new_delete_resource()
, which
is obviously not null and so cannot compare equal to r
.
This wording is relative to N4480.
Edit [memory.resource.global]/p8 as follows:
-6- memory_resource* set_default_resource(memory_resource* r) noexcept;-7- Effects: If
-8- Postconditions:r
is non-null, sets the value of the default memory resource pointer tor
, otherwise sets the default memory resource pointer tonew_delete_resource()
.get_default_resource() == r
ifr
is non-null; otherwise,get_default_resource() == new_delete_resource()
. […]
[2015-09-15 Geoffrey Romer comments and suggests alternative wording]
Let's just strike 8.8 [fund.ts.v2::memory.resource.global]/p8. The problem is that p8 is restating p7 incorrectly, but the solution is not to restate p7 correctly, it's to stop trying to restate p7 at all.
[2015-10, Kona Saturday afternoon]
Move to Tentatively ready
[2015-10-26]
Daniel adjusts wording to lib. fund. v2.
Proposed resolution:
This wording is relative to N4529.
Edit [memory.resource.global]/p8 as follows:
-6- memory_resource* set_default_resource(memory_resource* r) noexcept;-7- Effects: If
r
is non-null, sets the value of the default memory resource pointer tor
, otherwise sets the default memory resource pointer tonew_delete_resource()
.-8- Postconditions:[…]get_default_resource() == r
.
std::promise
synopsis shows two set_value_at_thread_exit()
's for no apparent reasonSection: 32.10.6 [futures.promise] Status: C++17 Submitter: Tim Song Opened: 2015-07-31 Last modified: 2017-07-30
Priority: 0
View other active issues in [futures.promise].
View all other issues in [futures.promise].
View all issues with C++17 status.
Discussion:
In 32.10.6 [futures.promise], the class synopsis shows
void set_value_at_thread_exit(const R& r); void set_value_at_thread_exit(see below);
There's no apparent reason for having void set_value_at_thread_exit(const R& r);
, especially as that
signature isn't really present in the specializations (particularly promise<void>
). Note that the similar
set_value
only has a void set_value(see below);
While we are here, 32.10.6 [futures.promise]/p1 says that the specializations "differ only in the argument type of the
member function set_value
", which missed set_value_at_thread_exit
.
[2015-10, Kona issue prioritization]
Priority 0, move to Ready
Proposed resolution:
This wording is relative to N4527.
Edit 32.10.6 [futures.promise], class template promise
synopsis, as indicated:
namespace std { template <class R> class promise { public: […] // setting the result void set_value(see below); void set_exception(exception_ptr p); // setting the result with deferred notificationvoid set_value_at_thread_exit(const R& r);void set_value_at_thread_exit(see below); void set_exception_at_thread_exit(exception_ptr p); }; }
Edit 32.10.6 [futures.promise]/1 as indicated:
-1- The implementation shall provide the template
promise
and two specializations,promise<R&>
andpromise<void>
. These differ only in the argument type of the member functionsset_value
andset_value_at_thread_exit
, as set out inits descriptiontheir descriptions, below.
generate_canonical
can occasionally return 1.0Section: 29.5.9.4.2 [rand.dist.pois.exp] Status: Resolved Submitter: Michael Prähofer Opened: 2015-08-20 Last modified: 2024-04-02
Priority: 2
View all issues with Resolved status.
Discussion:
Original title was: exponential_distribution<float>
sometimes returns inf.
The random number distribution class template exponential_distribution<float>
may return "inf" as can be seen from the following example program:
// compiled with // g++ -std=c++11 Error_exp_distr.cpp #include <iostream> #include <random> #include <bitset> int main(){ unsigned long long h; std::mt19937_64 mt1(1); std::mt19937_64 mt2(1); mt1.discard(517517); mt2.discard(517517); std::exponential_distribution<float> dis(1.0); h = mt2(); std::cout << std::bitset<64>(h) << " " << (float) -log(1 - h/pow(2, 64)) << " " << -log(1 - (float) h/pow(2, 64)) << " " << dis(mt1) << std::endl; h = mt2(); std::cout << std::bitset<64>(h) << " " << (float) -log(1 - h/pow(2, 64)) << " " << -log(1 - (float) h/pow(2, 64)) << " " << dis(mt1) << std::endl; }
output:
0110010110001001010011000111000101001100111110100001110011100001 0.505218 0.505218 0.505218 1111111111111111111111111101010011000110011110011000110101100110 18.4143 inf inf
The reason seems to be that converting a double x
in the range [0, 1)
to float
may result in 1.0f
if x
is close enough to 1
. I see two possibilities to fix that:
use internally double
(or long double
?) and then convert the result at the very end to float
.
take only 24 random bits and convert them to a float x
in the range [0, 1)
and then return -log(1 - x)
.
I have not checked if std::exponential_distribution<double>
has the same problem:
For float
on the average 1 out of 224 (~107) draws returns "inf", which is easily confirmed.
For double
on the average 1 out of 253 (~1016) draws might return "inf", which I have not tested.
Marshall:
I don't think the problem is in std::exponential_distribution
; but rather in generate_canonical
.
Consider:
which outputs:std::mt19937_64 mt2(1); mt2.discard(517517); std::cout << std::hexfloat << std::generate_canonical<float, std::numeric_limits<float>::digits>(mt2) << std::endl; std::cout << std::hexfloat << std::generate_canonical<float, std::numeric_limits<float>::digits>(mt2) << std::endl; std::cout << std::hexfloat << std::generate_canonical<float, std::numeric_limits<float>::digits>(mt2) << std::endl;
but0x1.962532p-2 0x1p+0 0x1.20d0cap-3
generate_canonical
is defined to return a result in the range [0, 1).
[2015-10, Kona Saturday afternoon]
Options:
WEB: The one thing we cannot tolerate is any output range other than [0, 1).
WEB: I believe there may be a documented algorithm for the generator, and perhaps it's possible to discover en-route that the algorithm produces the wrong result and fix it.
MC: No. I analyzed this once, and here it is: the algorithm is in [rand.util.canonical], and it's all fine until p5. The expression S/R^k
is mathematically less than one, but it may round to one.
GR: Could we change the rounding mode for the computation?
HH: No, because the rounding mode is global, not thread-local.
AM: SG1 wants to get rid of the floating point environment.
STL: The problem is that the standard specifies the implementation, and the implementation doesn't work.
MC: I'm not sure if nudging it down will introduce a subtle bias.
EF: I worry about how the user's choice of floating point environment affects the behaviour.
MS offers to run the topic past colleagues.
MC: Will set the status to open. STL wants to rename the issue. WEB wants to be able to find the issue by its original name still.
Mike Spertus to run the options past his mathematical colleagues, and report back.
[2017-11 Albuquerque Wednesday issue processing]
Choice: Rerun the algorithm if it gets 1.0.
Thomas K to provide wording; Marshall and STL to review.
[2018-08 Batavia Monday issue discussion]
Davis has a paper P0952 which resolves this.
[2024-04-02 Kona 2023; Resolved by P0952R2. Status changed: Open → Resolved.]
Proposed resolution:
get_memory_resource
should be const
and noexcept
Section: 4.2 [fund.ts.v2::func.wrap.func], 11.2 [fund.ts.v2::futures.promise], 11.3 [fund.ts.v2::futures.task] Status: TS Submitter: Tim Song Opened: 2015-08-04 Last modified: 2017-07-30
Priority: 3
View all other issues in [fund.ts.v2::func.wrap.func].
View all issues with TS status.
Discussion:
Addresses: fund.ts.v2
There doesn't seem to be any reason why this member function cannot be called on a const
object,
or why it would ever throw. I discussed this with Pablo Halpern, the author of N3916, and he agrees that
this appears to have been an oversight.
[2015-10-26]
Daniel adjusts wording to lib. fund. v2.
[2016-11-08, Issaquah]
Adopted during NB comment resolution
Proposed resolution:
This wording is relative to N4529.
Edit each of the synposes in 4.2 [fund.ts.v2::func.wrap.func], 11.2 [fund.ts.v2::futures.promise], and 11.3 [fund.ts.v2::futures.task] as indicated:
pmr::memory_resource* get_memory_resource() const noexcept;
experimental::function::swap
Section: 4.2.2 [fund.ts.v2::func.wrap.func.mod] Status: TS Submitter: Tim Song Opened: 2015-08-04 Last modified: 2017-07-30
Priority: 0
View all issues with TS status.
Discussion:
Addresses: fund.ts.v2
4.2.2 [fund.ts.v2::func.wrap.func.mod] says that the precondition of swap
is
this->get_memory_resource() == other->get_memory_resource()
That compares two pointers and so requires the memory resource used by *this
and other
to be
the exact same object. That doesn't seem correct (for one, it would essentially outlaw swapping all function
s
constructed with "plain" allocators, since they would each have their own resource_adaptor
object
and so the pointers will not compare equal). Presumably the intent is to compare the memory_resource
s for equality.
Also, other
is a reference, not a pointer.
[2015-09-11, Telecon]
Move to Tentatively Ready
[2015-10-26]
Daniel adjusts wording to lib. fund. v2.
Proposed resolution:
This wording is relative to N4529.
Edit 4.2.2 [fund.ts.v2::func.wrap.func.mod] as indicated:
void swap(function& other);-2- Requires:
-3- Effects: Interchanges the targets of*this->get_memory_resource() == *other
.->.get_memory_resource()*this
andother
. -4- Remarks: The allocators of*this
andother
are not interchanged.
ALLOCATOR_OF
for function::operator=
has incorrect defaultSection: 4.2.1 [fund.ts.v2::func.wrap.func.con] Status: TS Submitter: Tim Song Opened: 2015-08-04 Last modified: 2017-07-30
Priority: 3
View all other issues in [fund.ts.v2::func.wrap.func.con].
View all issues with TS status.
Discussion:
Addresses: fund.ts.v2
According to the table in 8.3 [fund.ts.v2::memory.type.erased.allocator], if no allocator argument is specified at the time
of construction, the memory resource pointer used is the value of experimental::pmr::get_default_resource()
at
the time of construction.
Yet in 4.2.1 [fund.ts.v2::func.wrap.func.con], ALLOCATOR_OF
is specified to return allocator<char>()
if no allocator was specified at the time of construction, which seems incorrect, especially as the user can change the default
memory resource pointer to something other than new_delete_resource()
.
[2015-10-26]
Daniel adjusts wording to lib. fund. v2.
[2016-11-08, Issaquah]
Adopted during NB comment resolution
Proposed resolution:
This wording is relative to N4529.
Edit 4.2.1 [fund.ts.v2::func.wrap.func.con]/p2 as indicated:
-2- In the following descriptions, let
ALLOCATOR_OF(f)
be the allocator specified in the construction offunction
f
, orthe value ofallocator<char>()
experimental::pmr::get_default_resource()
at the time of the construction off
if no allocator was specified.
enable_shared_from_this::__weak_this
twiceSection: 20.3.2.7 [util.smartptr.enab] Status: Resolved Submitter: Jonathan Wakely Opened: 2015-08-26 Last modified: 2020-09-06
Priority: 3
View all other issues in [util.smartptr.enab].
View all issues with Resolved status.
Discussion:
It is unclear what should happen if a pointer to an object with an
enable_shared_from_this
base is passed to two different shared_ptr
constructors.
#include <memory> using namespace std; int main() { struct X : public enable_shared_from_this<X> { }; auto xraw = new X; shared_ptr<X> xp1(xraw); // #1 { shared_ptr<X> xp2(xraw, [](void*) { }); // #2 } xraw->shared_from_this(); // #3 }
This is similar to LWG 2179(i), but involves no undefined behaviour due
to the no-op deleter, and the question is not whether the second
shared_ptr
should share ownership with the first, but which shared_ptr
shares ownership with the enable_shared_from_this::__weak_this
member.
std::shared_ptr
implementations the xp2
constructor modifies the __weak_this
member so the last line of the
program throws bad_weak_ptr
, even though all the requirements on the
shared_from_this()
function are met (20.3.2.7 [util.smartptr.enab])/7:
Requires:
enable_shared_from_this<T>
shall be an accessible base class ofT
.*this
shall be a subobject of an objectt
of typeT
. There shall be at least oneshared_ptr
instancep
that owns&t
.
Boost doesn't update __weak_this
, leaving it sharing with xp1
, so the
program doesn't throw. That change was made to boost::enable_shared_from_this
because
someone reported exactly this issue as a bug, see Boost issue 2584.
std::shared_ptr
implementations. We should
specify the behaviour of enable_shared_from_this
more precisely, and
resolve this issue one way or another.
[2016-03-16, Alisdair comments]
This issues should be closed as Resolved
by paper p0033r1 at Jacksonville.
Proposed resolution:
future::get
should explicitly state that the shared state is releasedSection: 32.10.7 [futures.unique.future] Status: C++17 Submitter: Agustín K-ballo Bergé Opened: 2015-09-03 Last modified: 2021-06-06
Priority: 3
View all other issues in [futures.unique.future].
View all issues with C++17 status.
Discussion:
The standard is usually very explicit on when a shared state is released, except for future::get
for which it only states valid() == false
as a postcondition.
[2016-08 - Chicago]
Thurs AM: Moved to Tentatively Ready
Proposed resolution:
This wording is relative to N4527.
Modify [futures.unique_future] as indicated:
R future::get(); R& future<R&>::get(); void future<void>::get();-14- Note: as described above, the template and its two required specializations differ only in the return type and return value of the member function
-15- Effects:get
.
wait()
s until the shared state is ready, then retrieves the value stored in the shared state.;releases any shared state (32.10.5 [futures.state]).
[…]
Section: 31.7.6.6 [ostream.rvalue], 31.7.5.6 [istream.rvalue] Status: C++17 Submitter: Robert Haberlach Opened: 2015-09-08 Last modified: 2017-07-30
Priority: 3
View all other issues in [ostream.rvalue].
View all issues with C++17 status.
Discussion:
The rvalue stream insertion and extraction operators should be constrained to not participate in overload resolution unless the expression they evaluate is well-formed. Programming code that tests the validity of stream insertions (or extractions) using SFINAE can result in false positives, as the present declarations accept virtually any right-hand side argument. Moreover, there is no need for pollution of the candidate set with ill-formed specializations.
[2016-08 - Chicago]
Thurs AM: Moved to Tentatively Ready
Proposed resolution:
This wording is relative to N4527.
Modify 31.7.6.6 [ostream.rvalue] as indicated:
template <class charT, class traits, class T> basic_ostream<charT, traits>& operator<<(basic_ostream<charT, traits>&& os, const T& x);-1- Effects:
-2- Returns:os << x
os
-?- Remarks: This function shall not participate in overload resolution unless the expressionos << x
is well-formed.
Modify 31.7.5.6 [istream.rvalue] as indicated:
template <class charT, class traits, class T> basic_istream<charT, traits>& operator>>(basic_istream<charT, traits>&& is, T& x);-1- Effects:
-2- Returns:is >> x
is
-?- Remarks: This function shall not participate in overload resolution unless the expressionis >> x
is well-formed.
<complex.h>
do?Section: 17.15 [support.c.headers] Status: C++17 Submitter: Richard Smith Opened: 2015-09-10 Last modified: 2023-02-07
Priority: 2
View all other issues in [support.c.headers].
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Discussion:
LWG issue 1134(i) removed the resolution of LWG 551(i), leaving an incorrect specification for
the behavior of <complex.h>
. This header is currently required to make std::complex
(and
associated functions) visible in the global namespace, but should not be so required.
[2016-09-09 Issues Resolution Telecon]
Move to Tentatively Ready
Proposed resolution:
This wording is relative to N4527.
Add a new paragraph before [depr.c.headers]/2:
-?- The header
<complex.h>
behaves as if it simply includes the header<ccomplex>
.
Change in [depr.c.headers]/2:
-2- Every other C header, each of which has a name of the form
name.h
, behaves as if each name placed in the standard library namespace by the correspondingcname
header is placed within the global namespace scope. It is unspecified whether these names are first declared or defined within namespace scope (3.3.6) of the namespacestd
and are then injected into the global namespace scope by explicit using-declarations (7.3.3).
priority_queue
taking allocators should call make_heap
Section: 23.6.4.3 [priqueue.cons.alloc] Status: C++17 Submitter: Eric Schmidt Opened: 2015-09-19 Last modified: 2017-07-30
Priority: 0
View all issues with C++17 status.
Discussion:
priority_queue
constructors taking both Container
and Alloc
arguments should
finish by calling make_heap
, just as with the constructors that do not have allocator parameters.
[2015-10, Kona issue prioritization]
Priority 0, move to Ready
Proposed resolution:
This wording is relative to N4527.
Change 23.6.4.3 [priqueue.cons.alloc] as indicated:
template <class Alloc> priority_queue(const Compare& compare, const Container& cont, const Alloc& a);-4- Effects: Initializes
c
withcont
as the first argument anda
as the second argument, and initializescomp
withcompare
; callsmake_heap(c.begin(), c.end(), comp)
.template <class Alloc> priority_queue(const Compare& compare, Container&& cont, const Alloc& a);-5- Effects: Initializes
c
withstd::move(cont)
as the first argument anda
as the second argument, and initializescomp
withcompare
; callsmake_heap(c.begin(), c.end(), comp)
.
invocation_trait
definition definition doesn't work for surrogate call functionsSection: 3.3.2 [fund.ts.v2::meta.trans.other] Status: TS Submitter: Mike Spertus Opened: 2015-09-25 Last modified: 2017-07-30
Priority: Not Prioritized
View all issues with TS status.
Discussion:
Addresses: fund.ts.v2
In Library Fundamentals 2 (N4529)
3.3.2p3 [meta.trans.other], the definition of invocation traits for a class object f
considers when f
is called via a function call operator that is matched by the arguments but ignores the possibility that f
may be called via a surrogate call function (C++14 12.2.2.2.3 [over.call.object] p2), in which case, the definition
of the invocation parameters may be either incorrect or even unsatisfiable.
[2015-10, Kona Saturday afternoon]
AM: Do we have this trait yet? JW: No, it cannot be implemented without compiler support.
Move to tentatively ready
Proposed resolution:
This wording is relative to N4529.
In Library Fundamentals 2, change [meta.trans.other] as indicated:
-3- Within this section, define the invocation parameters of
INVOKE(f, t1, t2, ..., tN)
as follows, in whichT1
is the possibly cv-qualified type oft1
andU1
denotesT1&
ift1
is an lvalue orT1&&
ift1
is an rvalue:
[…]
If
f
is a class object, the invocation parameters are the parameters matchingt1, ..., tN
of the best viable function (C++14 §13.3.3) for the argumentst1, ..., tN
among the function call operators and surrogate call functions off
.[…]
unordered_multimap::insert
hint iteratorSection: 23.2.8 [unord.req] Status: C++17 Submitter: Isaac Hier Opened: 2015-09-16 Last modified: 2017-07-30
Priority: 3
View other active issues in [unord.req].
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Discussion:
I have been wondering about the C++ standard requirements regarding the hint iterator for insertion into an
unordered_multimap
(and I imagine a similar question could be asked of unordered_map
, but
I have not researched that topic). As far as I can tell, it seems perfectly valid for an implementation to
allow only valid dereferencable iterators to be used as the hint argument for this member function. If that
is correct, it means that one could not expect the end iterator to be used as a valid hint nor could one use
the begin iterator of an empty unordered_multimap
as the hint. However, this essentially precludes
all uses of inserter on an empty unordered_multimap
seeing as the inserter functor requires that a
hint iterator be passed to its constructor.
The intent of the standard is that the iterator produced from container
It appears to me that you and the Bloomberg implementation have fallen victim to a type-o in the Unordered associative container requirements, Table 102. The row containing:c
byc.end()
is a valid (but non-dereferenceable) iterator into containerc
. It is reachable by every other iterator intoc
.a.insert(q, t);should read instead:
a.insert(p, t);The distinction is that
p
is valid, andq
is both valid and dereferenceable. The correction of this type-o would make unordered containerinsert
consistent with unorderedemplace_hint
, associativeinsert
, and associativeemplace_hint
.
[2016-08 - Chicago]
Thurs AM: Moved to Tentatively Ready
Proposed resolution:
Change the insert-with-hint row in Table 102 Unordered associative container requirements like so:
a.insert(
qp, t);iterator
Requires: If t
is a non-const
...Average Case
...
ExecutionPolicy
algorithm overloadsSection: 99 [parallel.ts::parallel.alg.overloads] Status: Resolved Submitter: Tim Song Opened: 2015-09-26 Last modified: 2017-11-13
Priority: 1
View all issues with Resolved status.
Discussion:
Addresses: parallel.ts
99 [parallel.ts::parallel.alg.overloads] provides parallel algorithm overloads for many algorithms in the standard library,
but I can't find any normative wording specifying which headers these new overloads live in. Presumably, if the original
algorithm is in <meow>
, the new overloads should be in <experimental/meow>
.
[2017-11 Albuquerque Saturday issues processing]
Resolved by P0776R1.
Proposed resolution:
const
requirements for associative containersSection: 23.2.7 [associative.reqmts] Status: C++17 Submitter: Daniel Krügler Opened: 2015-09-26 Last modified: 2017-07-30
Priority: 1
View other active issues in [associative.reqmts].
View all other issues in [associative.reqmts].
View all issues with C++17 status.
Discussion:
Table 101 — "Associative container requirements" and its associated legend paragraph
23.2.7 [associative.reqmts] p8 omits to impose constraints related to const
values, contrary to unordered containers as specified by Table 102 and its associated legend
paragraph p11.
key_comp()
, value_comp()
,
or count()
— as non-const
functions. In Table 102, "possibly const
"
values are exposed by different symbols, so the situation for unordered containers is clear
that these functions may be invoked by const
container objects.
For the above mentioned member functions this problem is only minor, because the synopses of the
actual Standard associative containers do declare these members as const
functions, but
nonetheless a wording fix is recommended to clean up the specification asymmetry between associative
containers and unordered containers.
The consequences of the ignorance of const
becomes much worse when we consider a code
example such as the following one from a recent
libstdc++ bug report:
#include <set>
struct compare
{
bool operator()(int a, int b) // Notice the non-const
member declaration!
{
return a < b;
}
};
int main() {
const std::set<int, compare> s;
s.find(0);
}
The current wording in 23.2.7 [associative.reqmts] can be read to require this code to be well-formed,
because there is no requirement that an object comp
of the ordering relation of type Compare
might be a const
value when the function call expression comp(k1, k2)
is evaluated.
const
comparison
function objects of associative containers need to support the predicate call expression. This becomes
more obvious when considering the member value_compare
of std::map
which provides (only) a
const operator()
overload which invokes the call expression of data member comp
.
[2016-02-20, Daniel comments and extends suggested wording]
It has been pointed out to me, that the suggested wording is a potentially breaking change and should therefore be mentioned in Annex C.
First, let me emphasize that this potentially breaking change is solely caused by the wording change in 23.2.7 [associative.reqmts] p8:[…] and
c
denotes a possiblyconst
value of typeX::key_compare
; […]
So, even if that proposal would be rejected, the rest of the suggested changes could (and should) be
considered for further evaluation, because the remaining parts do just repair an obvious mismatch between
the concrete associative containers (std::set
, std::map
, …) and the requirement
tables.
const operator()
. If the committee really intends to require
a Library to support comparison functors with non-const operator()
, this should be clarified by at least
an additional note to e.g. 23.2.7 [associative.reqmts] p8.
[2016-03, Jacksonville]
Move to Ready with Daniel's updated wordingProposed resolution:
This wording is relative to N4567.
Change 23.2.7 [associative.reqmts] p8 as indicated:
-8- In Table 101,
X
denotes an associative container class,a
denotes a value of typeX
,b
denotes a possiblyconst
value of typeX
,u
denotes the name of a variable being declared,a_uniq
denotes a value of typeX
whenX
supports unique keys,a_eq
denotes a value of typeX
whenX
supports multiple keys,a_tran
denotes a possiblyconst
value of typeX
when the qualified-idX::key_compare::is_transparent
is valid and denotes a type (14.8.2),i
andj
satisfy input iterator requirements and refer to elements implicitly convertible tovalue_type
,[i, j)
denotes a valid range,p
denotes a valid const iterator toa
,q
denotes a valid dereferenceable const iterator toa
,r
denotes a valid dereferenceable iterator toa
,[q1, q2)
denotes a valid range of const iterators ina
,il
designates an object of typeinitializer_list<value_type>
,t
denotes a value of typeX::value_type
,k
denotes a value of typeX::key_type
andc
denotes a possiblyconst
value of typeX::key_compare
;kl
is a value such thata
is partitioned (25.4) with respect toc(r, kl)
, withr
the key value ofe
ande
ina
;ku
is a value such thata
is partitioned with respect to!c(ku, r)
;ke
is a value such thata
is partitioned with respect toc(r, ke)
and!c(ke, r)
, withc(r, ke)
implying!c(ke, r)
.A
denotes the storage allocator used byX
, if any, orstd::allocator<X::value_type>
otherwise, andm
denotes an allocator of a type convertible toA
.
Change 23.2.7 [associative.reqmts], Table 101 — "Associative container requirements" as indicated:
[Editorial note: This issue doesn't touch the note column entries for the expressions related tokey_comp()
and value_comp()
(except for the symbolic correction), since these are already handled by LWG issues
2227(i) and 2215(i) — end editorial note]
Table 101 — Associative container requirements (in addition to container) (continued) Expression Return type Assertion/note pre-/post-condition Complexity …
ab.key_comp()X::key_compare
returns the comparison object
out of whichwas
ab
constructed.constant
ab.value_comp()X::value_compare
returns an object of
value_compare
constructed
out of the comparison objectconstant …
ab.find(k)iterator
;
const_iterator
for constant.
abreturns an iterator pointing to
an element with the key
equivalent tok
, orif
ab.end()
such an element is not foundlogarithmic …
ab.count(k)size_type
returns the number of elements
with key equivalent tok
log(
ab.size()) +ab.count(k)…
ab.lower_bound(k)iterator
;
const_iterator
for constant.
abreturns an iterator pointing to
the first element with key not
less thank
, orif such
ab.end()
an element is not found.logarithmic …
ab.upper_bound(k)iterator
;
const_iterator
for constant.
abreturns an iterator pointing to
the first element with key
greater thank
, orif
ab.end()
such an element is not found.logarithmic …
ab.equal_range(k)pair<iterator, iterator>
;
pair<const_iterator, const_iterator>
for
constantab.equivalent to make_-
.
pair(ab.lower_bound(k),
ab.upper_bound(k))logarithmic …
Add a new entry to Annex C, C.4 [diff.cpp14], as indicated:
C.4.4 Clause 23: containers library [diff.cpp14.containers]
23.2.4 Change: Requirements change: Rationale: Increase portability, clarification of associative container requirements. Effect on original feature: Valid 2014 code that attempts to use associative containers having a comparison object with non-const
function call operator may fail to compile:#include <set> struct compare { bool operator()(int a, int b) { return a < b; } }; int main() { const std::set<int, compare> s; s.find(0); }
Section: 22.10.19 [unord.hash] Status: Resolved Submitter: Ville Voutilainen Opened: 2015-09-27 Last modified: 2017-02-02
Priority: 2
View all other issues in [unord.hash].
View all issues with Resolved status.
Discussion:
The rationale in issue 2148(i) says:
This proposed resolution doesn't specify anything else about the primary template, allowing implementations to do whatever they want for non-enums:
static_assert
nicely, explode horribly at compiletime or runtime, etc.
libc++ seems to implement it by defining the primary template and
static_assert
ing is_enum
inside it. However, that brings forth
a problem; there are reasonable SFINAE uses broken by it:
#include <type_traits> #include <functional> class S{}; // No hash specialization template<class T> auto f(int) -> decltype(std::hash<T>(), std::true_type()); template<class T> auto f(...) -> decltype(std::false_type()); static_assert(!decltype(f<S>(0))::value, "");
MSVC doesn't seem to accept that code either.
There is a way to implement LWG 2148(i) so thathash
for enums is supported
without breaking that sort of SFINAE uses:
Derive the main hash
template from a library-internal uglified-named
base template that takes a type and a bool
, pass as argument for the base
the result of is_enum
.
Partially specialize that base template so that the false-case has a suitable set of private special member function declarations so that it's not an aggregate nor usable in almost any expression.
[2015-10, Kona Saturday afternoon]
EricWF to come back with wording; move to Open
[2016-05-08, Eric Fiselier & Ville provide wording]
[2016-05-25, Tim Song comments]
I see two issues with this P/R:
"for which neither the library nor the user provides an explicit specialization" should probably be "for which neither the library nor the user provides an explicit or partial specialization".
Saying that the specialization "is not DefaultConstructible
nor MoveAssignable
" is not enough to
guarantee that common SFINAE uses will work. Both of those requirements have several parts, and it's not too hard
to fail only some of them. For instance, not meeting the assignment postcondition breaks MoveAssignable
,
but is usually not SFINAE-detectible. And for DefaultConstructible
, it's easy to write something in a way
that breaks T()
but not T{}
(due to aggregate initialization in the latter case).
[2016-06-14, Daniel comments]
The problematic part of the P/R is that it describes constraints that would be suitable if they were constraints
for user-code, but they are not suitable as requirements imposed on implementations to provide certain guarantees
for clients of the Library. The guarantees should be written in terms of testable compile-time expressions, e.g. based on
negative results of is_default_constructible<hash<X>>::value
,
std::is_copy_constructible<hash<X>>::value
, and possibly also
std::is_destructible<hash<X>>::value
. How an implementation realizes these negative
results shouldn't be specified, though, but the expressions need to be well-formed and well-defined.
[2016-08-03, Ville provides revised wording as response to Daniel's previous comment]
Previous resolution [SUPERSEDED]:This wording is relative to N4582.
Insert a new paragraph after 22.10.19 [unord.hash]/2
-2- The template specializations shall meet the requirements of class template
-?- For any type that is not of integral or enumeration type, or for which neither the library nor the user provides an explicit specialization of the class templatehash
(20.12.14).hash
, the specialization ofhash
does not meet any of theHash
requirements, and is notDefaultConstructible
norMoveAssignable
. [Note: this means that the specialization ofhash
exists, but any attempts to use it as a Hash will be ill-formed. — end note]
[2016-08 - Chicago]
Thurs AM: Moved to Tentatively Ready
Previous resolution [SUPERSEDED]:This wording is relative to N4606.
Insert a new paragraph after 22.10.19 [unord.hash]/2
[Drafting note: I see no reason to specify whether
H<T>
is destructible. There's no practical use case for which that would need to be covered. libstdc++ makes it so thatH<T>
is destructible.]-2- The template specializations shall meet the requirements of class template
-?- For any typehash
(20.12.14).T
that is not of integral or enumeration type, or for which neither the library nor the user provides an explicit or partial specialization of the class template hash, the specialization ofhash<T>
has the following properties:
is_default_constructible_v<hash<T>>
isfalse
is_copy_constructible_v<hash<T>>
isfalse
is_move_constructible_v<hash<T>>
isfalse
is_copy_assignable_v<hash<T>>
isfalse
is_move_assignable_v<hash<T>>
isfalse
is_callable_v<hash<T>, T&>
isfalse
is_callable_v<hash<T>, const T&>
isfalse
[Note: this means that the specialization of
hash
exists, but any attempts to use it as aHash
will be ill-formed. — end note]
[2016-08-09 Daniel reopens]
As pointed out by Eric, the usage of is_callable
is incorrect.
Eric provides new wording.
[2016-09-09 Issues Resolution Telecon]
Move to Tentatively Ready
[2016-11-12, Issaquah]
Resolved by P0513R0
Proposed resolution:
This wording is relative to N4606.
Insert a new paragraph after 22.10.19 [unord.hash]/2
[Drafting note: I see no reason to specify whether
H<T>
is destructible. There's no practical use case for which that would need to be covered. libstdc++ makes it so thatH<T>
is destructible.]
-2- The template specializations shall meet the requirements of class template
-?- For any typehash
(20.12.14).T
that is not of integral or enumeration type, or for which neither the library nor the user provides an explicit or partial specialization of the class template hash, the specialization ofhash<T>
has the following properties:
is_default_constructible_v<hash<T>>
isfalse
is_copy_constructible_v<hash<T>>
isfalse
is_move_constructible_v<hash<T>>
isfalse
is_copy_assignable_v<hash<T>>
isfalse
is_move_assignable_v<hash<T>>
isfalse
hash<T>
is not a function object type (22.10 [function.objects])[Note: this means that the specialization of
hash
exists, but any attempts to use it as aHash
will be ill-formed. — end note]
istreambuf_iterator(basic_streambuf<charT, traits>* s)
effects unclear when s
is 0
Section: 24.6.4.3 [istreambuf.iterator.cons] Status: C++17 Submitter: S. B. Tam Opened: 2015-10-05 Last modified: 2017-07-30
Priority: 3
View all issues with C++17 status.
Discussion:
N4527 24.6.4.3 [istreambuf.iterator.cons] does not mention what the effect of calling
istreambuf_iterator(basic_streambuf<charT, traits>* s)
is when s
is a null pointer.
It should be made clear that this case is well-formed and the result is a end-of-stream iterator.
[…] The default constructor
istreambuf_iterator()
and the constructoristreambuf_iterator(0)
both construct an end-of-stream iterator object suitable for use as an end-of-range. […]
This indicates that the described constructor creates an end-of-stream iterator, but this wording is part of the introductory
wording and I recommend to make 24.6.4.3 [istreambuf.iterator.cons] clearer, because the existing specification is already
flawed, e.g. it never specifies when and how the exposition-only-member sbuf_
is initialized. The proposed wording
below attempts to solve these problems as well.
Previous resolution [SUPERSEDED]:
This wording is relative to N4527.
Change 24.6.4.3 [istreambuf.iterator.cons] as indicated:
[Editorial note: The proposed wording changes also performs some editorial clean-up of the existing mismatches of the declarations in the class template synopsis and the individual member specifications. The below wording intentionally does not say anything about the concrete value ofsbuf_
for end-of-stream iterator values, because that was never specified before; in theory, this could be some magic non-null pointer that can be used in constant expressions. But the wording could be drastically simplified by requiringsbuf_
to be a null pointer for an end-of-stream iterator value, since I have not yet seen any implementation where this requirement does not hold. — end editorial note]constexpr istreambuf_iterator() noexcept;-1- Effects: Constructs the end-of-stream iterator.
istreambuf_iterator(basic_istream<charT,traits>istream_type& s) noexcept;istreambuf_iterator(basic_streambuf<charT,traits>* s) noexcept;-2- Effects: If
s.rdbuf()
is a null pointer, constructs an end-of-stream iterator; otherwise initializessbuf_
withs.rdbuf()
and constructs anistreambuf_iterator
that uses thestreambuf_type
object*sbuf_
Constructs an.istreambuf_iterator<>
that uses thebasic_streambuf<>
object*(s.rdbuf())
, or*s
, respectively. Constructs an end-of-stream iterator ifs.rdbuf()
is nullistreambuf_iterator(streambuf_type* s) noexcept;-?- Effects: If
s
is a null pointer, constructs an end-of-stream iterator; otherwise initializessbuf_
withs
and constructs anistreambuf_iterator
that uses thestreambuf_type
object*sbuf_
.istreambuf_iterator(const proxy& p) noexcept;-3- Effects: Initializes
sbuf_
withp.sbuf_
and constructs anistreambuf_iterator
that uses thestreambuf_type
object*sbuf_
Constructs a.istreambuf_iterator<>
that uses thebasic_streambuf<>
object pointed to by theproxy
object's constructor argumentp
[2015-10-20, Daniel provides alternative wording]
[2016-08-03 Chicago]
Fri AM: Moved to Tentatively Ready
Proposed resolution:
This wording is relative to N4606.
Change 24.6.4.3 [istreambuf.iterator.cons] as indicated:
[Drafting note: The proposed wording changes also performs some editorial clean-up of the existing mismatches of the declarations in the class template synopsis and the individual member specifications. The below wording is simplified by requiring
sbuf_
to be a null pointer for an end-of-stream iterator value, since I have not yet seen any implementation where this requirement does not hold. Even if there were such an implementation, this would still be conforming, because concrete exposition-only member values are not part of public API. — end drafting note]
For each
istreambuf_iterator
constructor in this section, an end-of-stream iterator is constructed if and only if the exposition-only membersbuf_
is initialized with a null pointer value.constexpr istreambuf_iterator() noexcept;-1- Effects: Initializes
sbuf_
withnullptr
Constructs the end-of-stream iterator.istreambuf_iterator(basic_istream<charT,traits>istream_type& s) noexcept;istreambuf_iterator(basic_streambuf<charT,traits>* s) noexcept;-2- Effects: Initializes
sbuf_
withs.rdbuf()
Constructs an.istreambuf_iterator<>
that uses thebasic_streambuf<>
object*(s.rdbuf())
, or*s
, respectively. Constructs an end-of-stream iterator ifs.rdbuf()
is nullistreambuf_iterator(streambuf_type* s) noexcept;-?- Effects: Initializes
sbuf_
withs
.istreambuf_iterator(const proxy& p) noexcept;-3- Effects: Initializes
sbuf_
withp.sbuf_
Constructs a.istreambuf_iterator<>
that uses thebasic_streambuf<>
object pointed to by theproxy
object's constructor argumentp
bind
without explicitly specified return typeSection: 22.10.15.4 [func.bind.bind] Status: C++17 Submitter: Tomasz Kamiński Opened: 2015-10-05 Last modified: 2017-07-30
Priority: 3
View all other issues in [func.bind.bind].
View all issues with C++17 status.
Discussion:
The specification of the bind
overload without return type as of 22.10.15.4 [func.bind.bind] p3,
uses the following expression INVOKE(fd, std::forward<V1>(v1), std::forward<V2>(v2), ...,
std::forward<VN>(vN), result_of_t<FD cv & (V1, V2, ..., VN)>)
to describe effects of
invocation of returned function.
result_of_t<FD cv & (V1, V2, ..., VN)>
is equivalent to
decltype(INVOKE(declval<FD cv &>(), declval<V1>(), declval<V2>(), ..., declval<VN>()))
.
When combined with the definition of INVOKE
from 22.10.4 [func.require] p2, the expression
INVOKE(fd, std::forward<V1>(v1), std::forward<V2>(v2), ..., std::forward<VN>(vN),
result_of_t<FD cv & (V1, V2, ...., VN)>)
is equivalent to INVOKE(fd,
std::forward<V1>(v1), std::forward<V2>(v2), ..., std::forward<VN>(vN))
implicitly converted
to decltype(INVOKE(declval<FD cv &>(), declval<V1>(), declval<V2>(), ...,
declval<VN>()))
(itself).
It is also worth to notice that specifying the result type (R
) in INVOKE(f, args..., R)
does
not in any way affect the selected call. As a consequence the use of wording of the form
INVOKE(fd, std::forward<V1>(v1), std::forward<V2>(v2), ..., std::forward<VN>(vN),
result_of_t<FD cv & (V1, V2, ..., VN)>)
does not and cannot lead to call of different overload
than one invoked by INVOKE(fd, std::forward<V1>(v1), std::forward<V2>(v2), ...,
std::forward<VN>(vN))
.
In summary the form INVOKE(fd, std::forward<V1>(v1), std::forward<V2>(v2), ...,
std::forward<VN>(vN), result_of_t<FD cv & (V1, V2, ..., VN)>)
is a convoluted way of expressing
INVOKE(fd, std::forward<V1>(v1), std::forward<V2>(v2), ..., std::forward<VN>(vN))
,
that only confuses reader.
[2015-10, Kona Saturday afternoon]
STL: I most recently reimplemented std::bind from scratch, and I think this issue is correct and the solution is good.
Move to Tentatively ready.
Proposed resolution:
This wording is relative to N4527.
Change 22.10.15.4 [func.bind.bind] p3 as indicated:
template<class F, class... BoundArgs> unspecified bind(F&& f, BoundArgs&&... bound_args);[…]
-3- Returns: A forwarding call wrapperg
with a weak result type (20.9.2). The effect ofg(u1, u2, ..., uM)
shall beINVOKE(fd, std::forward<V1>(v1), std::forward<V2>(v2), ..., std::forward<VN>(vN)
, where, result_of_t<FD cv & (V1, V2, ..., VN)>)the values and types of the bound argumentscv
represents the cv-qualifiers ofg
andv1, v2, ..., vN
are determined as specified below. The copy constructor and move constructor of the forwarding call wrapper shall throw an exception if and only if the corresponding constructor ofFD
or of any of the typesTiD
throws an exception. […]
vfscanf
from <cstdio>
Section: 31.13 [c.files] Status: Resolved Submitter: Richard Smith Opened: 2015-10-09 Last modified: 2017-03-12
Priority: 3
View all other issues in [c.files].
View all issues with Resolved status.
Discussion:
C's vfscanf
function is not present in C++'s <cstdio>
, and presumably should be.
It looks like this is the only missing member of C's [v]{f,s,sn}[w]{printf,scanf}
family.
[2016-08 - Chicago]
Resolved by P0175R1
Thurs AM: Moved to Tentatively Resolved
Proposed resolution:
This wording is relative to N4527.
Modify Table 133 as follows:
ungetc
vfprintf
vfscanf
vprintf
vscanf
tuple
parameters end up taking references
to temporaries and will create dangling referencesSection: 22.4.4.2 [tuple.cnstr] Status: C++17 Submitter: Ville Voutilainen Opened: 2015-10-11 Last modified: 2017-07-30
Priority: 2
View other active issues in [tuple.cnstr].
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Discussion:
Consider this example:
#include <utility> #include <tuple> struct X { int state; // this has to be here to not allow tuple // to inherit from an empty X. X() { } X(X const&) { } X(X&&) { } ~X() { } }; int main() { X v; std::tuple<X> t1{v}; std::tuple<std::tuple<X>&&> t2{std::move(t1)}; // #1 std::tuple<std::tuple<X>> t3{std::move(t2)}; // #2 }
The line marked with #1
will use the constructor
template <class... UTypes> EXPLICIT constexpr tuple(tuple<UTypes...>&& u);
and will construct a temporary and bind an rvalue reference to it.
The line marked with #2
will move from a dangling reference.
Types...
is constructible
or convertible from the incoming tuple type and sizeof...(Types)
equals
one. libstdc++ already has this fix applied.
There is an additional check that needs to be done, in order to avoid
infinite meta-recursion during overload resolution, for a case where
the element type is itself constructible from the target tuple. An example
illustrating that problem is as follows:
#include <tuple> struct A { template <typename T> A(T) { } A(const A&) = default; A(A&&) = default; A& operator=(const A&) = default; A& operator=(A&&) = default; ~A() = default; }; int main() { auto x = A{7}; std::make_tuple(x); }
I provide two proposed resolutions, one that merely has a note encouraging trait-based implementations to avoid infinite meta-recursion, and a second one that avoids it normatively (implementations can still do things differently under the as-if rule, so we are not necessarily overspecifying how to do it.)
[2016-02-17, Ville comments]
It was pointed out at gcc bug 69853
that the fix for LWG 2549 is a breaking change. That is, it breaks code
that expects constructors inherited from tuple
to provide an implicit
base-to-derived conversion. I think that's just additional motivation
to apply the fix; that conversion is very much undesirable. The example
I wrote into the bug report is just one example of a very subtle temporary
being created.
Previous resolution from Ville [SUPERSEDED]:
Alternative 1:
In 22.4.4.2 [tuple.cnstr]/17, edit as follows:
template <class... UTypes> EXPLICIT constexpr tuple(const tuple<UTypes...>& u);[…]
-17- Remarks: This constructor shall not participate in overload resolution unless
is_constructible<Ti, const Ui&>::value
is true for alli
., and
sizeof...(Types) != 1
, oris_convertible<const tuple<UTypes...>&, Types...>::value
is false andis_constructible<Types..., const tuple<UTypes...>&>::value
is false.[Note: to avoid infinite template recursion in a trait-based implementation for the case where
The constructor is explicit if and only ifUTypes...
is a single type that has a constructor template that acceptstuple<Types...>
, implementations need to additionally check thatremove_cv_t<remove_reference_t<const tuple<UTypes...>&>>
andtuple<Types...>
are not the same type. — end note]is_convertible<const Ui&, Ti>::value
is false for at least onei
.In 22.4.4.2 [tuple.cnstr]/20, edit as follows:
template <class... UTypes> EXPLICIT constexpr tuple(tuple<UTypes...>&& u);[…]
-20- Remarks: This constructor shall not participate in overload resolution unless
is_constructible<Ti, Ui&&>::value
is true for alli
., and
sizeof...(Types) != 1
, oris_convertible<tuple<UTypes...>&&, Types...>::value
is false andis_constructible<Types..., tuple<UTypes...>&&>::value
is false.[Note: to avoid infinite template recursion in a trait-based implementation for the case where
The constructor is explicit if and only ifUTypes...
is a single type that has a constructor template that acceptstuple<Types...>
, implementations need to additionally check thatremove_cv_t<remove_reference_t<tuple<UTypes...>&&>>
andtuple<Types...>
are not the same type. — end note]is_convertible<Ui&&, Ti>::value
is false for at least onei
.Alternative 2 (do the sameness-check normatively):
In 22.4.4.2 [tuple.cnstr]/17, edit as follows:
template <class... UTypes> EXPLICIT constexpr tuple(const tuple<UTypes...>& u);[…]
-17- Remarks: This constructor shall not participate in overload resolution unless
is_constructible<Ti, const Ui&>::value
is true for alli
., and
sizeof...(Types) != 1
, oris_convertible<const tuple<UTypes...>&, Types...>::value
is false andis_constructible<Types..., const tuple<UTypes...>&>::value
is false andis_same<tuple<Types...>, remove_cv_t<remove_reference_t<const tuple<UTypes...>&>>>::value
is false.The constructor is explicit if and only if
is_convertible<const Ui&, Ti>::value
is false for at least onei
.In 22.4.4.2 [tuple.cnstr]/20, edit as follows:
template <class... UTypes> EXPLICIT constexpr tuple(tuple<UTypes...>&& u);[…]
-20- Remarks: This constructor shall not participate in overload resolution unless
is_constructible<Ti, Ui&&>::value
is true for alli
., and
sizeof...(Types) != 1
, oris_convertible<tuple<UTypes...>&&, Types...>::value
is false andis_constructible<Types..., tuple<UTypes...>&&>::value
is false andis_same<tuple<Types...>, remove_cv_t<remove_reference_t<tuple<UTypes...>&&>>>::value
is false.The constructor is explicit if and only if
is_convertible<Ui&&, Ti>::value
is false for at least onei
.
[2016-03, Jacksonville]
STL provides a simplification of Ville's alternative #2 (with no semantic changes), and it's shipping in VS 2015 Update 2.
Proposed resolution:
This wording is relative to N4567.
This approach is orthogonal to the Proposed Resolution for LWG 2312(i):
Edit 22.4.4.2 [tuple.cnstr] as indicated:
template <class... UTypes> EXPLICIT constexpr tuple(const tuple<UTypes...>& u);[…]
-17- Remarks: This constructor shall not participate in overload resolution unless
is_constructible<Ti, const Ui&>::value
istrue
for alli
, and
sizeof...(Types) != 1
, or (whenTypes...
expands toT
andUTypes...
expands toU
)!is_convertible_v<const tuple<U>&, T> && !is_constructible_v<T, const tuple<U>&> && !is_same_v<T, U>
istrue
.The constructor is explicit if and only if
is_convertible<const Ui&, Ti>::value
isfalse
for at least onei
.template <class... UTypes> EXPLICIT constexpr tuple(tuple<UTypes...>&& u);[…]
-20- Remarks: This constructor shall not participate in overload resolution unless
is_constructible<Ti, Ui&&>::value
istrue
for alli
, and
sizeof...(Types) != 1
, or (whenTypes...
expands toT
andUTypes...
expands toU
)!is_convertible_v<tuple<U>, T> && !is_constructible_v<T, tuple<U>> && !is_same_v<T, U>
istrue
.The constructor is explicit if and only if
is_convertible<Ui&&, Ti>::value
isfalse
for at least onei
.
This approach is presented as a merge with the parts of the Proposed Resolution for LWG 2312(i) with overlapping modifications in the same paragraph, to provide editorial guidance if 2312(i) would be accepted.
Edit 22.4.4.2 [tuple.cnstr] as indicated:
template <class... UTypes> EXPLICIT constexpr tuple(const tuple<UTypes...>& u);[…]
-17- Remarks: This constructor shall not participate in overload resolution unless
sizeof...(Types) == sizeof...(UTypes)
, and
is_constructible<Ti, const Ui&>::value
istrue
for alli
, and
sizeof...(Types) != 1
, or (whenTypes...
expands toT
andUTypes...
expands toU
)!is_convertible_v<const tuple<U>&, T> && !is_constructible_v<T, const tuple<U>&> && !is_same_v<T, U>
istrue
.The constructor is explicit if and only if
is_convertible<const Ui&, Ti>::value
isfalse
for at least onei
.template <class... UTypes> EXPLICIT constexpr tuple(tuple<UTypes...>&& u);[…]
-20- Remarks: This constructor shall not participate in overload resolution unless
sizeof...(Types) == sizeof...(UTypes)
, and
is_constructible<Ti, Ui&&>::value
istrue
for alli
, and
sizeof...(Types) != 1
, or (whenTypes...
expands toT
andUTypes...
expands toU
)!is_convertible_v<tuple<U>, T> && !is_constructible_v<T, tuple<U>> && !is_same_v<T, U>
istrue
.The constructor is explicit if and only if
is_convertible<Ui&&, Ti>::value
isfalse
for at least onei
.
clear()
method complexitySection: 23.2.8 [unord.req] Status: C++17 Submitter: Yegor Derevenets Opened: 2015-10-11 Last modified: 2017-07-30
Priority: 2
View other active issues in [unord.req].
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Discussion:
I believe, the wording of the complexity specification for the standard
unordered containers' clear()
method should be changed from "Linear" to
"Linear in a.size()
". As of N4527,
the change should be done in the Complexity column of row "a.clear()
..." in "Table 102 — Unordered
associative container requirements" in section Unordered associative containers 23.2.8 [unord.req].
[2016-03 Jacksonville]
GR: erase(begin. end) has to touch every element. clear() has the option of working with buckets instead. will be faster in some cases, slower in some.
clear() has to be at least linear in size as it has to run destructors.
MC: wording needs to say what it's linear in, either elements or buckets.
HH: my vote is the proposed resolution is correct.
Move to Ready.
Proposed resolution:
This wording is relative to N4527.
Change 23.2.8 [unord.req], Table 102 as indicated:
Table 102 — Unordered associative container requirements (in addition to container) Expression Return type Assertion/note pre-/post-condition Complexity …
a.clear()
void
Erases all elements in the
container. Post:a.empty()
returnstrue
Linear in a.size()
.…
Section: 8.2.1.1 [fund.ts.v2::memory.smartptr.shared.const] Status: TS Submitter: Daniel Krügler Opened: 2015-10-24 Last modified: 2017-07-30
Priority: 0
View all issues with TS status.
Discussion:
Addresses: fund.ts.v2
Similar to LWG 2495(i), the current library fundamentals working paper refers to several "Exception safety" elements without providing a definition for this element type.
Proposed resolution:
This wording is relative to N4529.
Change 8.2.1.1 [fund.ts.v2::memory.smartptr.shared.const] as indicated:
template<class Y> explicit shared_ptr(Y* p);[…]
-3- Effects: WhenT
is not an array type, constructs ashared_ptr
object that owns the pointerp
. Otherwise, constructs ashared_ptr
that ownsp
and a deleter of an unspecified type that callsdelete[] p
. If an exception is thrown,delete p
is called whenT
is not an array type,delete[] p
otherwise. […]-6- Exception safety: If an exception is thrown,delete p
is called whenT
is not an array type,delete[] p
otherwise.template<class Y, class D> shared_ptr(Y* p, D d); template<class Y, class D, class A> shared_ptr(Y* p, D d, A a); template <class D> shared_ptr(nullptr_t p, D d); template <class D, class A> shared_ptr(nullptr_t p, D d, A a);[…]
-9- Effects: Constructs ashared_ptr
object that owns the objectp
and the deleterd
. The second and fourth constructors shall use a copy ofa
to allocate memory for internal use. If an exception is thrown,d(p)
is called. […]-12- Exception safety: If an exception is thrown,d(p)
is called.[…]
template<class Y> explicit shared_ptr(const weak_ptr<Y>& r);[…]
-28- Effects: Constructs ashared_ptr
object that shares ownership withr
and stores a copy of the pointer stored inr
. If an exception is thrown, the constructor has no effect. […]-31- Exception safety: If an exception is thrown, the constructor has no effect.template <class Y, class D> shared_ptr(unique_ptr<Y, D>&& r);[…]
-34- Effects: Equivalent toshared_ptr(r.release(), r.get_deleter())
whenD
is not a reference type, otherwiseshared_ptr(r.release(), ref(r.get_deleter()))
. If an exception is thrown, the constructor has no effect.-35- Exception safety: If an exception is thrown, the constructor has no effect.
noexcept
Section: 22.2.2 [utility.swap] Status: Resolved Submitter: Orson Peters Opened: 2015-11-01 Last modified: 2020-09-06
Priority: 2
View all other issues in [utility.swap].
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Discussion:
The noexcept
specification for the std::swap
overload for arrays has the effect that
all multidimensional arrays — even those of build-in types — would be considered as non-noexcept
swapping, as described in the following
Stackoverflow article.
#include <utility> #include <iostream> int main() { int x[2][3]; int y[2][3]; using std::swap; std::cout << noexcept(swap(x, y)) << "\n"; }
Both clang 3.8.0 and gcc 5.2.0 return 0
.
std::swap
overload for arrays uses an expression and not a type trait. At the point where the expression is evaluated, only the non-array
std::swap
overload is in scope whose noexcept
specification evaluates to false
since arrays are neither
move-constructible nor move-assignable.
Daniel:
The here described problem is another example for the currently broken swap
exception specifications in the Standard
library as pointed out by LWG 2456(i). The paper
N4511 describes a resolution that would address
this problem. If the array swap
overload would be declared instead as follows,
template <class T, size_t N> void swap(T (&a)[N], T (&b)[N]) noexcept(is_nothrow_swappable<T>::value);
the expected outcome is obtained.
Revision 2 (P0185R0) of above mentioned paper will available for the mid February 2016 mailing.[2016-03-06, Daniel comments]
With the acceptance of revision 3 P0185R1 during the Jacksonville meeting, this
issue should be closed as "resolved": The expected program output is now 1
. The current gcc 6.0 trunk has
already implemented the relevant parts of P0185R1.
Proposed resolution:
optional
Section: 5.3 [fund.ts.v2::optional.object] Status: TS Submitter: Marshall Clow Opened: 2015-11-03 Last modified: 2018-07-08
Priority: 0
View all other issues in [fund.ts.v2::optional.object].
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Discussion:
Addresses: fund.ts.v2
22.5.3 [optional.optional] does not specify whether over-aligned types are supported. In other places where we specify allocation of user-supplied types, we state that "It is implementation-defined whether over-aligned types are supported (3.11)." (Examples: 7.6.2.8 [expr.new]/p1, 20.2.10.2 [allocator.members]/p5, [temporary.buffer]/p1). We should presumably do the same thing here.
Proposed resolution:
This wording is relative to N4562.
Edit 22.5.3 [optional.optional]/p1 as follows::
[…] The contained value shall be allocated in a region of the
optional<T>
storage suitably aligned for the typeT
. It is implementation-defined whether over-aligned types are supported (C++14 §3.11). When an object of typeoptional<T>
is contextually converted tobool
, the conversion returnstrue
if the object contains a value; otherwise the conversion returnsfalse
.
future::share()
Section: 32.10.7 [futures.unique.future] Status: C++17 Submitter: Agustín K-ballo Bergé Opened: 2015-11-05 Last modified: 2021-06-06
Priority: 3
View all other issues in [futures.unique.future].
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Discussion:
future::share()
is not noexcept
, it has a narrow contact requiring valid()
as per the blanket
wording in [futures.unique_future] p3. Its effects, however, are return shared_future<R>(std::move(*this))
,
which is noexcept
as it has a wide contract. If the source future
isn't valid then the target
shared_future
simply won't be valid either. There appears to be no technical reason preventing future::share()
from having a wide contract, and thus being noexcept
.
[2016-08-03 Chicago]
Fri AM: Moved to Tentatively Ready
Proposed resolution:
This wording is relative to N4567.
Change [futures.unique_future] as indicated:
-3- The effect of calling any member function other than the destructor, the move-assignment operator,
share
, orvalid
on afuture
object for whichvalid() == false
is undefined. [Note: Implementations are encouraged to detect this case and throw an object of typefuture_error
with an error condition offuture_errc::no_state
. — end note]namespace std { template <class R> class future { public: […] shared_future<R> share() noexcept; […] }; }[…]
shared_future<R> share() noexcept;-12- Returns:
-13- Postcondition:shared_future<R>(std::move(*this))
.valid() == false
.[…]
Section: 21.3.9 [meta.logical] Status: C++17 Submitter: Geoffrey Romer Opened: 2015-11-05 Last modified: 2017-07-30
Priority: 0
View all other issues in [meta.logical].
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Discussion:
The conjunction trait in 21.3.9 [meta.logical] seems intended to support invocation with zero arguments, e.g.
conjunction<>::value
, which is likely to be a useful feature. However, the specification doesn't
actually make sense in the zero-argument case. See 21.3.9 [meta.logical]/p3:
The BaseCharacteristic of a specialization
conjunction<B1, …, BN>
is the first typeB
in the listtrue_type
,B1
, …,BN
for whichB::value == false
, or if everyB::value != false
the BaseCharacteristic isBN
.
If "B1, ..., BN
" is an empty list, then every B::value != false
, so the BaseCharacteristic is BN
,
but there is no BN
in this case.
conjunction
intentionally requires at least one argument, I would appreciate their confirmation that
I can editorially remove the mention of true_type
, which seems to have no normative impact outside the zero-argument case.)
Similar comments apply to the disjunction
trait, and to the corresponding traits in the Fundamentals working
paper, see LWG 2558(i).
[2016-02, Issues Telecon]
P0; move to Tentatively Ready.
Proposed resolution:
This wording is relative to N4567.
Revise 21.3.9 [meta.logical] as follows:
template<class... B> struct conjunction : see below { };[…]
-3- The BaseCharacteristic of a specializationconjunction<B1, ..., BN>
is the first typeBi
in the listtrue_type, B1, ..., BN
for whichBi::value == false
, or if everyBi::value != false
, the BaseCharacteristic isthe last type in the list. [Note: This means a specialization ofBN
conjunction
does not necessarily have a BaseCharacteristic of eithertrue_type
orfalse_type
. — end note] […]template<class... B> struct disjunction : see below { };[…]
-6- The BaseCharacteristic of a specializationdisjunction<B1, ..., BN>
is the first typeBi
in the listfalse_type, B1, ..., BN
for whichBi::value != false
, or if everyBi::value == false
, the BaseCharacteristic isthe last type in the list. [Note: This means a specialization ofBN
disjunction
does not necessarily have a BaseCharacteristic of eithertrue_type
orfalse_type
. — end note] […]
Section: 3.3.3 [fund.ts.v2::meta.logical] Status: TS Submitter: Geoffrey Romer Opened: 2015-11-05 Last modified: 2017-07-30
Priority: 0
View all other issues in [fund.ts.v2::meta.logical].
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Discussion:
Addresses: fund.ts.v2
The conjunction trait in 3.3.3 [fund.ts.v2::meta.logical] seems intended to support invocation with zero arguments, e.g.
conjunction<>::value
, which is likely to be a useful feature. However, the specification doesn't
actually make sense in the zero-argument case. See 3.3.3 [fund.ts.v2::meta.logical]/p3:
The BaseCharacteristic of a specialization
conjunction<B1, …, BN>
is the first typeB
in the listtrue_type
,B1
, …,BN
for whichB::value == false
, or if everyB::value != false
the BaseCharacteristic isBN
.
If "B1, ..., BN
" is an empty list, then every B::value != false
, so the BaseCharacteristic is BN
,
but there is no BN
in this case.
conjunction
intentionally requires at least one argument, I would appreciate their confirmation that
I can editorially remove the mention of true_type
, which seems to have no normative impact outside the zero-argument case.)
Similar comments apply to the disjunction
trait, and to the corresponding traits in the C++ working paper,
see LWG 2557(i).
[2016-02, Issues Telecon]
P0; move to Tentatively Ready.
Proposed resolution:
This wording is relative to N4562.
Revise 3.3.3 [fund.ts.v2::meta.logical] as follows:
template<class... B> struct conjunction : see below { };[…]
-3- The BaseCharacteristic of a specializationconjunction<B1, ..., BN>
is the first typeBi
in the listtrue_type, B1, ..., BN
for whichBi::value == false
, or if everyBi::value != false
, the BaseCharacteristic isthe last type in the list. [Note: This means a specialization ofBN
conjunction
does not necessarily have a BaseCharacteristic of eithertrue_type
orfalse_type
. — end note] […]template<class... B> struct disjunction : see below { };[…]
-6- The BaseCharacteristic of a specializationdisjunction<B1, ..., BN>
is the first typeBi
in the listfalse_type, B1, ..., BN
for whichBi::value != false
, or if everyBi::value == false
, the BaseCharacteristic isthe last type in the list. [Note: This means a specialization ofBN
disjunction
does not necessarily have a BaseCharacteristic of eithertrue_type
orfalse_type
. — end note] […]
Section: 19.3 [assertions] Status: C++17 Submitter: Tim Song Opened: 2015-11-07 Last modified: 2017-07-30
Priority: 0
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Discussion:
The resolution of LWG 2234(i) says that assert(E)
is a constant subexpression if "NDEBUG
is defined
at the point where assert(E)
appears".
[2016-02, Issues Telecon]
P0; move to Tentatively Ready.
Proposed resolution:
This wording is relative to N4567.
Change 19.3 [assertions] p2 as indicated:
An expression
assert(E)
is a constant subexpression (3.15 [defns.const.subexpr]), if
NDEBUG
is defined at the point whereassert(E)
appearsassert
is last defined or redefined, or[…]
is_constructible
underspecified when applied to a function typeSection: 21.3.5.4 [meta.unary.prop] Status: C++17 Submitter: Richard Smith Opened: 2015-11-14 Last modified: 2017-07-30
Priority: 0
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Discussion:
What is is_constructible<void()>::value
? Per 21.3.5.4 [meta.unary.prop] p8:
The predicate condition for a template specialization
is_constructible<T, Args...>
shall be satisfied if and only if the following variable definition would be well-formed for some invented variablet
:T t(declval<Args>()...);[Note: These tokens are never interpreted as a function declaration. — end note]
The problem here is that substituting in T
as a function type doesn't give a variable definition that's not well-formed
(by 3.68 [defns.well.formed], well-formed means that it doesn't violate any syntactic or diagnosable semantic rules, and
it does not). Instead, it gives a logical absurdity: this wording forces us to imagine a variable of function type, which contradicts
the definition of "variable" in 3/6, but does so without violating any diagnosable language rule. So presumably the result must
be undefined behavior.
T
to be an object or reference type.
Daniel:
As one of the authors of N3142 I would like
to express that at least according to my mental model the intention for this trait was to be well-defined for T
being
a function type with the result of false
regardless of what the other type arguments are. It would seem like a very
unfortunate and unnecessary complication to keep the result as being undefined. First, this result value is symmetric to
the result of is_destructible<T>::value
(where the word covers function types explicitly). Second, if such a
resolution would be applied to the working paper, it wouldn't break existing implementations. I have tested clang 3.8.0,
gcc 5.x until gcc 6.0, and Visual Studio 2015, all of these implementations evaluate is_constructible<void()>::value
to false
.
[2016-02, Issues Telecon]
P0; move to Tentatively Ready.
Proposed resolution:
This wording is relative to N4567.
Change 21.3.5.4 [meta.unary.prop], Table 49 — "Type property predicates", as indicated:
Table 49 — Type property predicates Template Condition Preconditions …
template <class T, class... Args>
struct is_constructible;For a function type T
,
is_constructible<T, Args...>::value
isfalse
, otherwise see belowT
and all types in the
parameter packArgs
shall
be complete types,
(possibly cv-qualified)
void
, or arrays of
unknown bound.…
Section: 5.3.4 [fund.ts.v2::optional.object.swap], 3.7.8 [fund.ts.v2::propagate_const.modifiers] Status: Resolved Submitter: Daniel Krügler Opened: 2015-11-14 Last modified: 2022-07-28
Priority: 3
View all issues with Resolved status.
Discussion:
Addresses: fund.ts.v2
As pointed out in N4511,
the Library fundamentals are affected by a similar problem as described in LWG 2456(i). First, it is caused
by optional
's member swap
(5.3.4 [fund.ts.v2::optional.object.swap]):
void swap(optional<T>& rhs) noexcept(see below);
with
The expression inside
noexcept
is equivalent to:is_nothrow_move_constructible_v<T> && noexcept(swap(declval<T&>(), declval<T&>()))
Again, the unqualified lookup for swap
finds the member swap
instead of the result of a normal argument-depending
lookup, making this ill-formed.
propagate_const
template
with another member swap
(3.7.8 [fund.ts.v2::propagate_const.modifiers]):
constexpr void swap(propagate_const& pt) noexcept(see below);-2- The constant-expression in the exception-specification is
noexcept(swap(t_, pt.t_))
.
A working approach is presented in
N4511. By adding a new
trait to the standard library and referencing this by the library fundamentals (A similar approach had been applied in the
file system specification
where the quoted
manipulator from C++14 had been referred to, albeit the file system specification is generally based on the
C++11 standard), optional
's member swap
exception specification could be rephrased as follows:
The expression inside
noexcept
is equivalent to:is_nothrow_move_constructible_v<T> && is_nothrow_swappable_v<T>noexcept(swap(declval<T&>(), declval<T&>()))
and propagate_const
's member swap
exception specification could be rephrased as follows:
constexpr void swap(propagate_const& pt) noexcept(see below);-2- The constant-expression in the exception-specification is
is_nothrow_swappable_v<T>
.noexcept(swap(t_, pt.t_))
[2016-02-20, Ville comments]
Feedback from an implementation:
libstdc++ already applies the proposed resolution forpropagate_const
,
but not for optional
.
[2016-02-20, Daniel comments]
A recent paper update has been provided: P0185R0.
[2016-03, Jacksonville]
Add a link to 2456(i)[2016-11-08, Issaquah]
Not adopted during NB comment resolution
[2020-03-30; Daniel comments]
This has strong overlap with LWG 3413(i), which describes a sub-set of the problem here.
Rebasing of the library fundamentals on C++20 has removed the mentioned problem for
optional
s free swap
, so there are now no longer any further free swap
function
templates with conditionally noexcept
specifications except for propagate_const
(but now handled by LWG 3413(i)).
[2022-07-28 Resolved by P0966R1 and LWG 3413(i). Status changed: New → Resolved.]
Proposed resolution:
Section: 22.10.8 [comparisons] Status: C++17 Submitter: Casey Carter Opened: 2015-11-18 Last modified: 2017-07-30
Priority: 3
View other active issues in [comparisons].
View all other issues in [comparisons].
View all issues with C++17 status.
Discussion:
N4567 22.10.8 [comparisons]/14 specifies that the comparison functors provide a total ordering for pointer types:
For templates
greater
,less
,greater_equal
, andless_equal
, the specializations for any pointer type yield a total order, even if the built-in operators<
,>
,<=
,>=
do not.
It notably does not specify:
whether the specializations of all of the named templates for a given pointer type yield the same total order
whether the total order imposed respects the partial order imposed by the built-in operators
whether the total order imposed is consistent with the partition induced by ==
All of which are important for sane semantics and provided by common implementations, since the built-in operators provide a total order and the comparison functors yield that same order.
It would be extremely confusing — if not outright insane — for e.g.:less<int*>
and greater<int*>
to yield different orders
less<int*>
to disagree with <
on the relative order of two pointers for which <
is defined
less<int*>
to order a
before b
when a == b
, i.e., not preserve equality.
Consistent semantics for the various comparison functors and the built-in operators is so intuitive as to be assumed by most programmers.
Related issues: 2450(i), 2547(i).Previous resolution [SUPERSEDED]:
This wording is relative to N4567.
Alter 22.10.8 [comparisons]/14 to read:
For templates
greater
,less
,greater_equal
, andless_equal
, the specializations for any pointer type yieldathe same total order, even if the built-in operators<
,>
,<=
,>=
do not. The total order shall respect the partial order imposed by the built-in operators.
[2016-05-20, Casey Carter comments and suggests revised wording]
The new proposed wording is attempting to address the issue raised in the 2016-02-04 telecon.
The real issue I'm trying to address here is ensure that "weird" implementations provide the same kind of consistency for pointer orderings as "normal" implementations that use a flat address spaces and have totally ordered<
.
If a < b
is true for int
pointers a
and b
, then less<int*>(a, b)
,
less_equal<int*>(a, b)
, less<char*>(a, b)
, less<void*>(a, b)
, and
greater<int*>(b, a)
should all hold. I think this wording is sufficient to provide that.
Previous resolution [SUPERSEDED]:
This wording is relative to N4582.
Alter 22.10.8 [comparisons] to read:
-14- For templates
greater
,less
,greater_equal
, andless_equal
, the specializations for any pointer type yieldathe same total order. That total order is consistent with the partial order imposed by, even ifthe built-in operators<
,>
,≤
, and>
do not. [Note: Whena < b
is well-defined for pointersa
andb
of typeP
, this implies(a < b) == less<P>(a, b)
,(a > b) == greater<P>(a, b)
, and so forth. — end note] For template specializationsgreater<void>
,less<void>
,greater_equal<void>
, andless_equal<void>
, if the call operator calls a built-in operator comparing pointers, the call operator yields a total order.
[2016-08-04 Chicago LWG]
LWG discusses and concludes that we are trying to accomplish the following:
T* a = /* ... */; T* b = /* ... */;
if a < b
is valid, a < b == less<T*>(a, b)
, and analogously for >
,
<=
, >=
.
less<void>(a, b) == less<T*>(a, b); less<T*>(a, b) == greater<T*>(b, a);
etc.
less<T*>
produces a strict total ordering with which the other three
function objects are consistent
less<void>
when applied to pointers produces a strict total ordering with
which the other three are consistent
less<void>
when applied to pointers of the same type produces the same
strict total ordering as less<T*>
, and analogously for the other three
we are not addressing less<void>
(and the other three) when applied to
pointers of differing types
Walter and Nevin revise Proposed Wording accordingly.
[2016-08 - Chicago]
Thurs PM: Moved to Tentatively Ready
Proposed resolution:
This wording is relative to N4606.
Change 22.10.8 [comparisons] p14 as indicated:
-14- For templates
greater
,less
,greater_equal
, andless_equal
less
,greater
,less_equal
, andgreater_equal
, the specializations for any pointer type yield a strict total order that is consistent among those specializations and is also consistent with the partial order imposed by, even ifthe built-in operators<
,>
,<=
,>=
do not. [Note: Whena < b
is well-defined for pointersa
andb
of typeP
, this implies(a < b) == less<P>(a, b)
,(a > b) == greater<P>(a, b)
, and so forth. — end note] For template specializationsgreater<void>
,less<void>
,greater_equal<void>
, andless_equal<void>
less<void>
,greater<void>
,less_equal<void>
, andgreater_equal<void>
, if the call operator calls a built-in operator comparing pointers, the call operator yields a strict total order that is consistent among those specializations and is also consistent with the partial order imposed by those built-in operators.
std::experimental::function
constructors taking allocator arguments may throw exceptionsSection: 4.2 [fund.ts.v2::func.wrap.func] Status: Resolved Submitter: Tim Song Opened: 2015-12-05 Last modified: 2022-11-22
Priority: 3
View all other issues in [fund.ts.v2::func.wrap.func].
View all issues with Resolved status.
Discussion:
Addresses: fund.ts.v2
[This is essentially LWG 2370(i), but deals with the fundamentals TS version rather than the one in the standard]
In 4.2 [fund.ts.v2::func.wrap.func] of library fundamentals TS, the constructorstemplate<class A> function(allocator_arg_t, const A&) noexcept; template<class A> function(allocator_arg_t, const A&, nullptr_t) noexcept;
must type-erase and store the provided allocator, since the operator=
specification requires using the "allocator
specified in the construction of" the std::experimental::function
object. This may require a dynamic allocation
and so cannot be noexcept. Similarly, the following constructors
template<class A> function(allocator_arg_t, const A&, const function&); template<class A> function(allocator_arg_t, const A&, function&&); template<class F, class A> function(allocator_arg_t, const A&, F);
cannot satisfy the C++14 requirement that they "shall not throw exceptions if [the function
object to be stored]
is a callable object passed via reference_wrapper
or a function pointer" if they need to type-erase and store the
allocator.
[2016-11-08, Issaquah]
Not adopted during NB comment resolution
[2022-10-12 LWG telecon]
Set status to "Open". This would be resolved by P0987R2.
[2022-11-22 Resolved by P0897R2 accepted in Kona. Status changed: Open → Resolved.]
Proposed resolution:
This wording is relative to N4562.
Edit 4.2 [fund.ts.v2::func.wrap.func], class template function
synopsis, as follows:
namespace std { namespace experimental { inline namespace fundamentals_v2 { […] template<class R, class... ArgTypes> class function<R(ArgTypes...)> { public: […] template<class A> function(allocator_arg_t, const A&)noexcept; template<class A> function(allocator_arg_t, const A&, nullptr_t)noexcept; […] }; […] } // namespace fundamentals_v2 } // namespace experimental […] } // namespace std
Insert the following paragraphs after 4.2.1 [fund.ts.v2::func.wrap.func.con]/1:
[Drafting note: This just reproduces the wording from C++14 with the "shall not throw exceptions for
reference_wrapper
/function pointer" provision deleted. — end drafting note]
-1- When a
function
constructor that takes a first argument of typeallocator_arg_t
is invoked, the second argument is treated as a type-erased allocator (8.3). If the constructor moves or makes a copy of a function object (C++14 §20.9), including an instance of theexperimental::function
class template, then that move or copy is performed by using-allocator construction with allocatorget_memory_resource()
.template <class A> function(allocator_arg_t, const A& a); template <class A> function(allocator_arg_t, const A& a, nullptr_t);-?- Postconditions:
!*this
.template <class A> function(allocator_arg_t, const A& a, const function& f);-?- Postconditions:
-?- Throws: May throw!*this if !f
; otherwise,*this
targets a copy off.target()
.bad_alloc
or any exception thrown by the copy constructor of the stored callable object. [Note: Implementations are encouraged to avoid the use of dynamically allocated memory for small callable objects, for example, wheref
's target is an object holding only a pointer or reference to an object and a member function pointer. — end note]template <class A> function(allocator_arg_t, const A& a, function&& f);-?- Effects: If
!f
,*this
has no target; otherwise, move-constructs the target off
into the target of*this
, leavingf
in a valid state with an unspecified value.template <class F, class A> function(allocator_arg_t, const A& a, F f);-?- Requires:
-?- Remarks: This constructor shall not participate in overload resolution unlessF
shall beCopyConstructible
.f
is Callable (C++14 §20.9.11.2) for argument typesArgTypes...
and return typeR
. -?- Postconditions:!*this
if any of the following hold:-?- Otherwise,
f
is a null function pointer value.
f
is a null member pointer value.
F
is an instance of thefunction
class template, and!f
.*this
targets a copy off
initialized withstd::move(f)
. [Note: Implementations are encouraged to avoid the use of dynamically allocated memory for small callable objects, for example, wheref
's target is an object holding only a pointer or reference to an object and a member function pointer. — end note] -?- Throws: May throwbad_alloc
or any exception thrown byF
's copy or move constructor.-2- In the following descriptions, let
[…]ALLOCATOR_OF(f)
be the allocator specified in the construction offunction f
, orallocator<char>()
if no allocator was specified.
std::function
's move constructor should guarantee nothrow for reference_wrapper
s and function pointersSection: 22.10.17.3.2 [func.wrap.func.con] Status: C++17 Submitter: Tim Song Opened: 2015-12-05 Last modified: 2017-07-30
Priority: 0
View all other issues in [func.wrap.func.con].
View all issues with C++17 status.
Discussion:
22.10.17.3.2 [func.wrap.func.con]/5 guarantees that copying a std::function
whose "target is a callable object passed
via reference_wrapper
or a function pointer" does not throw exceptions, but the standard doesn't provide this guarantee for the move constructor,
which makes scant sense.
[2016-02, Issues Telecon]
P0; move to Tentatively Ready.
Proposed resolution:
This wording is relative to N4567.
[Drafting note: The inserted paragraph is a copy of 22.10.17.3.2 [func.wrap.func.con]/5, only changing "copy constructor" to "copy or move constructor". It does not attempt to fix the issue identified in LWG 2370(i), whose P/R will likely need updating if this wording is adopted.]
Insert after 22.10.17.3.2 [func.wrap.func.con]/6:
function(function&& f); template <class A> function(allocator_arg_t, const A& a, function&& f);-6- Effects: If
-?- Throws: Shall not throw exceptions if!f
,*this
has no target; otherwise, move-constructs the target off
into the target of*this
, leavingf
in a valid state with an unspecified value.f
's target is a callable object passed viareference_wrapper
or a function pointer. Otherwise, may throwbad_alloc
or any exception thrown by the copy or move constructor of the stored callable object. [Note: Implementations are encouraged to avoid the use of dynamically allocated memory for small callable objects, for example, wheref
's target is an object holding only a pointer or reference to an object and a member function pointer. — end note]
Section: 23.6 [container.adaptors] Status: C++17 Submitter: Tim Song Opened: 2015-12-08 Last modified: 2017-07-30
Priority: 0
View all other issues in [container.adaptors].
View all issues with C++17 status.
Discussion:
As noted in this
StackOverflow question, 23.6 [container.adaptors] doesn't seem to place any requirement on the first template parameter
(T
) of stack
, queue
, and priority_queue
: the only use of T
is in the default template
argument (which need not be used) for the second template parameter (Container
), while all of the operations of the adaptors
are defined using Container
's member typedefs.
queue<double, deque<std::string>>
or
priority_queue<std::nothrow_t, vector<int>>
, which presumably wasn't intended.
[2016-02, Issues Telecon]
P0; move to Tentatively Ready.
Proposed resolution:
This wording is relative to N4567.
Edit 23.6.1 [container.adaptors.general]/2 as indicated:
-2- The container adaptors each take a
Container
template parameter, and each constructor takes aContainer
reference argument. This container is copied into theContainer
member of each adaptor. If the container takes an allocator, then a compatible allocator may be passed in to the adaptor's constructor. Otherwise, normal copy or move construction is used for the container argument. The first template parameterT
of the container adaptors shall denote the same type asContainer::value_type
.
BaseCharacteristic
, which is defined only for UnaryTypeTraits
and BinaryTypeTraits
Section: 21.3.9 [meta.logical] Status: C++17 Submitter: Tim Song Opened: 2015-12-10 Last modified: 2017-07-30
Priority: 2
View all other issues in [meta.logical].
View all issues with C++17 status.
Discussion:
The specification of conjunction
and disjunction
uses the term BaseCharacteristic
, which
is problematic in several ways:
That term is defined in 21.3.2 [meta.rqmts], but only for UnaryTypeTraits
and BinaryTypeTraits
.
conjunction
and disjunction
seem to be neither.
21.3.2 [meta.rqmts] also requires the BaseCharacteristic
for both UnaryTypeTraits
and
BinaryTypeTraits
to be a specialization of integral_constant
, which is inconsistent with the current design of
conjunction
and disjunction
.
The requirement in 21.3.2 [meta.rqmts] that "member names of the BaseCharacteristic
shall not be hidden
and shall be unambiguously available" seems impossible to meet in every case, since the arbitrary base class from which a
specialization of conjunction
or disjunction
derives may contain members called conjunction
or
disjunction
that will necessarily be hidden.
[2016-08 Chicago]
Ville provided wording for both 2567(i) and 2568(i)
Previous resolution [SUPERSEDED]:
In [meta.logical]/3, edit as follows:
The
BaseCharacteristic of aspecializationconjunction<B1, ..., BN>
has a public and unambiguous base that is the first typeBi
in the listtrue_type, B1, ..., BN
for whichBi::value == false
, or if everyBi::value != false
, the aforementioned baseBaseCharacteristicis the last type in the list. [ Note: This means a specialization of conjunction does not necessarilyhave a BaseCharacteristic ofderive from either true_type or false_type. — end note ]In [meta.logical]/6, edit as follows:
The
BaseCharacteristic of aspecializationdisjunction<B1, ..., BN>
has a public and unambiguous base that is the first typeBi
in the listfalse_type, B1, ..., BN
for whichBi::value != false
, or if everyBi::value == false
, the aforementioned baseBaseCharacteristicis the last type in the list. [ Note: This means a specialization of disjunction does not necessarilyhave a BaseCharacteristic ofderive from either true_type or false_type. — end note ]
Previous resolution [SUPERSEDED]:
Merged the resolution of 2587(i) with this issue. This proposed resolution resolves both, and includes fixes from Daniel for negation. Last review of this with LWG turned up a true_type typo in the definition of disjunction, and some editorial changes.In [meta.logical]/3, edit as follows:
The
BaseCharacteristic of aspecializationconjunction<B1, ..., BN>
has a public and unambiguous base that is either
* the first typeBi
in the listtrue_type, B1, ..., BN
for whichBi::value == false
, or
* if there is no suchBi
, the last type in the list.
is the first typeBi
in the listtrue_type, B1, ..., BN
for whichBi::value == false
, or if everyBi::value != false
, the BaseCharacteristic is the last type in the list.
[ Note: This means a specialization ofconjunction
does not necessarilyhave a BaseCharacteristic ofderive from either true_type or false_type. — end note ]In [meta.logical]/6, edit as follows:
The
BaseCharacteristic of aspecializationdisjunction<B1, ..., BN>
has a public and unambiguous base that is either
* the first typeBi
in the listtrue_type, B1, ..., BN
for whichBi::value != false
, or
* if there is no suchBi
, the last type in the list.
is the first typeBi
in the listtrue_type, B1, ..., BN
for whichBi::value != false
, or if everyBi::value == false
, the BaseCharacteristic is the last type in the list.
[ Note: This means a specialization ofdisjunction
does not necessarilyhave a BaseCharacteristic ofderive from either true_type or false_type. — end note ]
Previous resolution [SUPERSEDED]:
This wording is relative to N4606.
In 21.3.9 [meta.logical] p3, edit as follows:
template<class... B> struct conjunction : see below { };-3- The
BaseCharacteristic of aspecializationconjunction<B1, ..., BN>
has a public and unambiguous base that is either
- the first type
Bi
in the listtrue_type, B1, ..., BN
for whichbool(Bi::value)
isfalse
, or- if there is no such
Bi
, the last type in the list.is the first typeBi
in the listtrue_type, B1, ..., BN
for whichBi::value == false
, or if everyBi::value != false
, the BaseCharacteristic is the last type in the list.-?- The member names of the base class, other than
conjunction
andoperator=
, shall not be hidden and shall be unambiguously available inconjunction
. [Note: This means a specialization ofconjunction
does not necessarilyhave a BaseCharacteristic ofinherit from eithertrue_type
orfalse_type
. —end note]In 21.3.9 [meta.logical] p6, edit as follows:
template<class... B> struct disjunction : see below { };-6- The
BaseCharacteristic of aspecializationdisjunction<B1, ..., BN>
has a public and unambiguous base that is either
- the first type
Bi
in the listtrue_type, B1, ..., BN
for whichbool(Bi::value)
istrue
, or,- if there is no such
Bi
, the last type in the list.is the first typeBi
in the listtrue_type, B1, ..., BN
for whichBi::value != false
, or if everyBi::value == false
, the BaseCharacteristic is the last type in the list.-?- The member names of the base class, other than
disjunction
andoperator=
, shall not be hidden and shall be unambiguously available indisjunction
. [Note: This means a specialization ofdisjunction
does not necessarilyhave a BaseCharacteristic ofinherit from eithertrue_type
orfalse_type
. —end note]In 21.3.9 [meta.logical] p8, edit as follows
template<class B> struct negation : bool_constant<!bool(B::value)> { };-8- The class template negation forms the logical negation of its template type argument. The type
negation<B>
is a UnaryTypeTrait with a BaseCharacteristic ofbool_constant<!bool(B::value)>
.
[2016-08-03 Chicago]
Fri AM: Moved to Tentatively Ready
Proposed resolution:
This wording is relative to N4606.
In 21.3.9 [meta.logical] p3, edit as follows:
template<class... B> struct conjunction : see below { };
[…]
-3- TheBaseCharacteristic of aspecializationconjunction<B1, ..., BN>
has a public and unambiguous base that is either
- — the first type
Bi
in the listtrue_type, B1, ..., BN
for whichbool(Bi::value)
isfalse
, or- — if there is no such
Bi
, the last type in the list.is the first type[Note: This means a specialization ofBi
in the listtrue_type, B1, ..., BN
for whichBi::value == false
, or if everyBi::value != false
, the BaseCharacteristic is the last type in the list.conjunction
does not necessarilyhave a BaseCharacteristic ofinherit from eithertrue_type
orfalse_type
. —end note]-?- The member names of the base class, other than
conjunction
andoperator=
, shall not be hidden and shall be unambiguously available inconjunction
.
In 21.3.9 [meta.logical] p6, edit as follows:
template<class... B> struct disjunction : see below { };
[…]
-6- TheBaseCharacteristic of aspecializationdisjunction<B1, ..., BN>
has a public and unambiguous base that is either
- — the first type
Bi
in the listfalse_type, B1, ..., BN
for whichbool(Bi::value)
istrue
, or,- — if there is no such
Bi
, the last type in the list.is the first type[Note: This means a specialization ofBi
in the listfalse_type, B1, ..., BN
for whichBi::value != false
, or if everyBi::value == false
, the BaseCharacteristic is the last type in the list.disjunction
does not necessarilyhave a BaseCharacteristic ofinherit from eithertrue_type
orfalse_type
. —end note]-?- The member names of the base class, other than
disjunction
andoperator=
, shall not be hidden and shall be unambiguously available indisjunction
.
In 21.3.9 [meta.logical] p8, edit as follows
template<class B> struct negation : see below{ };bool_constant<!B::value>
-8- The class template negation forms the logical negation of its template type argument. The type
negation<B>
is a UnaryTypeTrait with a BaseCharacteristic ofbool_constant<!bool(B::value)>
.
BaseCharacteristic
, which is defined only for
UnaryTypeTraits
and BinaryTypeTraits
Section: 3.3.3 [fund.ts.v2::meta.logical] Status: TS Submitter: Tim Song Opened: 2015-12-10 Last modified: 2020-09-06
Priority: 2
View all other issues in [fund.ts.v2::meta.logical].
View all issues with TS status.
Discussion:
Addresses: fund.ts.v2
The specification of conjunction
and disjunction
uses the term BaseCharacteristic
, which
is problematic in several ways:
That term is defined in 21.3.2 [meta.rqmts], but only for UnaryTypeTraits
and BinaryTypeTraits
.
conjunction
and disjunction
seem to be neither.
21.3.2 [meta.rqmts] also requires the BaseCharacteristic
for both UnaryTypeTraits
and
BinaryTypeTraits
to be a specialization of integral_constant
, which is inconsistent with the current design of
conjunction
and disjunction
.
The requirement in 21.3.2 [meta.rqmts] that "member names of the BaseCharacteristic
shall not be hidden
and shall be unambiguously available" seems impossible to meet in every case, since the arbitrary base class from which a
specialization of conjunction
or disjunction
derives may contain members called conjunction
or
disjunction
that will necessarily be hidden.
[2016-08 Chicago]
Ville provided wording for both 2567(i) and 2568(i).
[2016-08-07 Daniel provides wording borrowed from 2567(i)]
[2016-11-08, Issaquah]
Adopted during NB comment resolution
Proposed resolution:
This wording is relative to N4600.
In 3.3.3 [fund.ts.v2::meta.logical] p3, edit as follows:
template<class... B> struct conjunction : see below { };-2- The class template
-3- Theconjunction
forms the logical conjunction of its template type arguments. Every template type argument shall be usable as a base class and shall have a static data membervalue
which is convertible tobool
, is not hidden, and is unambiguously available in the type.BaseCharacteristic of aspecializationconjunction<B1, …, BN>
has a public and unambiguous base that is either
- — the first type
Bi
in the listtrue_type, B1, ..., BN
for whichbool(Bi::value)
isfalse
, or- — if there is no such
Bi
, the last type in the list.-?- The member names of the base class, other than
is the first type[Note: This means a specialization of conjunction does not necessarilyB
in the listtrue_type, B1, …, BN
for whichB::value == false
, or ifevery B::value != false
the BaseCharacteristic is the last type in the list.have a BaseCharacteristic ofinherit from eithertrue_type
orfalse_type
. — end note]conjunction
andoperator=
, shall not be hidden and shall be unambiguously available inconjunction
.
In 3.3.3 [fund.ts.v2::meta.logical] p6, edit as follows:
template<class... B> struct disjunction : see below { };-5- The class template
-6- Thedisjunction
forms the logical disjunction of its template type arguments. Every template type argument shall be usable as a base class and shall have a static data membervalue
which is convertible tobool
, is not hidden, and is unambiguously available in the type.BaseCharacteristic of aspecializationdisjunction<B1, …, BN>
has a public and unambiguous base that is either
- — the first type
Bi
in the listfalse_type, B1, ..., BN
for whichbool(Bi::value)
istrue
, or,- — if there is no such
Bi
, the last type in the list.-?- The member names of the base class, other than
is the first type[Note: This means a specialization of disjunction does not necessarilyB
in the listfalse_type, B1, …, BN
for whichB::value != false
, or ifevery B::value == false
the BaseCharacteristic is the last type in the list.have a BaseCharacteristic ofinherit from eithertrue_type
orfalse_type
. — end note]disjunction
andoperator=
, shall not be hidden and shall be unambiguously available indisjunction
.
In 3.3.3 [fund.ts.v2::meta.logical] p8, edit as follows:
template<class B> struct negation :integral_constant<bool, !B::value>see below { };-8- The class template
negation
forms the logical negation of its template type argument. The typenegation<B>
is a UnaryTypeTrait with a BaseCharacteristic ofintegral_constant<bool, !bool(B::value)>
.
conjunction
and disjunction
requirements are too strictSection: 21.3.9 [meta.logical] Status: C++17 Submitter: Tim Song Opened: 2015-12-11 Last modified: 2017-12-05
Priority: 2
View all other issues in [meta.logical].
View all issues with C++17 status.
Discussion:
21.3.9 [meta.logical]/2 and /5 impose the following requirement on the arguments of conjunction
and disjunction
:
Every template type argument shall be usable as a base class and shall have a static data member value which is convertible to
bool
, is not hidden, and is unambiguously available in the type.
Since the requirement is unconditional, it applies even to type arguments whose instantiation is not required due to short circuiting.
This seems contrary to the design intent, expressed in P0013R1,
that it is valid to write conjunction_v<is_class<T>, is_foo<T>>
even if instantiating
is_foo<T>::value
is ill-formed for non-class types.
[2016-08 Chicago]
Ville provided wording for both 2569(i) and 2570(i).
Tuesday AM: Move to Tentatively Ready
[2016-11-15, Reopen upon request of Dawn Perchik ]
The proposed wording requires an update, because the referenced issue LWG 2568(i) is still open.
[Dec 2017 - The resolution for this issue shipped in the C++17 standard; setting status to 'C++17']
Proposed resolution:
[We recommend applying the proposed resolution for LWG issues 2567 and 2568 before this proposed resolution, lest the poor editor gets confused.]
In [meta.logical],
- insert a new paragraph before paragraph 2:
The class template conjunction
forms the logical conjunction of its
template type arguments.
- move paragraph 4 before paragraph 2, and edit paragraph 2 as follows:
The class template
Every template type argument for which conjunction
forms the logical conjunction of its
template type arguments.Bi::value
is instantiated
shall be usable as a base class and shall have a member value which is
convertible to bool
, is not hidden, and is unambiguously available in the type.
- insert a new paragraph before paragraph 5:
The class template disjunction
forms the logical disjunction
of its template type arguments.
- move paragraph 7 before paragraph 5, and edit paragraph 5 as follows:
The class template
Every template type argument for which disjunction
forms the logical disjunction
of its template type arguments.Bi::value
is instantiated
shall be usable as a base class and shall have a member value which is
convertible to bool
, is not hidden, and is unambiguously available in the type.
conjunction
and disjunction
requirements are too strictSection: 3.3.3 [fund.ts.v2::meta.logical] Status: TS Submitter: Tim Song Opened: 2015-12-11 Last modified: 2017-07-30
Priority: 2
View all other issues in [fund.ts.v2::meta.logical].
View all issues with TS status.
Discussion:
Addresses: fund.ts.v2
3.3.3 [fund.ts.v2::meta.logical]/2 and /5 impose the following requirement on the arguments of conjunction
and disjunction
:
Every template type argument shall be usable as a base class and shall have a static data member value which is convertible to
bool
, is not hidden, and is unambiguously available in the type.
Since the requirement is unconditional, it applies even to type arguments whose instantiation is not required due to short circuiting.
This seems contrary to the design intent, expressed in P0013R1,
that it is valid to write conjunction_v<is_class<T>, is_foo<T>>
even if instantiating
is_foo<T>::value
is ill-formed for non-class types.
[2016-06 Oulu]
Alisdair has a paper in progress addressing this
[2016-08 Chicago]
Ville provided wording for both 2569(i) and 2570(i).
Tuesday AM: Move to Tentatively Ready
Proposed resolution:
insert(InputIterator, InputIterator)
Section: 23.4.3.4 [map.modifiers] Status: C++17 Submitter: Tim Song Opened: 2015-12-12 Last modified: 2017-07-30
Priority: 0
View all other issues in [map.modifiers].
View all issues with C++17 status.
Discussion:
The initial paragraphs of 23.4.3.4 [map.modifiers] currently read:
template <class P> pair<iterator, bool> insert(P&& x); template <class P> iterator insert(const_iterator position, P&& x); template <class InputIterator> void insert(InputIterator first, InputIterator last);-1- Effects: The first form is equivalent to
-2- Remarks: These signatures shall not participate in overload resolution unlessreturn emplace(std::forward<P>(x))
. The second form is equivalent toreturn emplace_hint(position, std::forward<P>(x))
.std::is_constructible<value_type, P&&>::value
istrue
.
Clearly, p2's requirement makes no sense for insert(InputIterator, InputIterator)
- it doesn't even have
a template parameter called P
.
InputIterator
parameters does not require
CopyConstructible
of either key_type
or mapped_type
if the dereferenced InputIterator
returns a non-const
rvalue pair<key_type,mapped_type>
. Otherwise CopyConstructible
is
required for both key_type
and mapped_type
", but that was removed by LWG 2005(i), whose PR
was written as if that overload didn't exist in the text.
It looks like the text addressing this overload is redundant to the requirements on a.insert(i, j)
in Table 102
that value_type
be EmplaceConstructible
from *i
. If so, then the signature should just be
deleted from this section.
[2016-02, Issues Telecon]
P0; move to Tentatively Ready.
Proposed resolution:
This wording is relative to N4567.
Edit 23.4.3.4 [map.modifiers] as indicated:
template <class P> pair<iterator, bool> insert(P&& x); template <class P> iterator insert(const_iterator position, P&& x);template <class InputIterator> void insert(InputIterator first, InputIterator last);-1- Effects: The first form is equivalent to
-2- Remarks: These signatures shall not participate in overload resolution unlessreturn emplace(std::forward<P>(x))
. The second form is equivalent toreturn emplace_hint(position, std::forward<P>(x))
.std::is_constructible<value_type, P&&>::value
istrue
.
shared_ptr::operator*
should apply to cv-qualified void
as wellSection: 20.3.2.2.6 [util.smartptr.shared.obs] Status: C++17 Submitter: Tim Song Opened: 2015-12-13 Last modified: 2017-07-30
Priority: 0
View all other issues in [util.smartptr.shared.obs].
View all issues with C++17 status.
Discussion:
20.3.2.2.6 [util.smartptr.shared.obs]/4 says for shared_ptr::operator*
Remarks: When
T
isvoid
, it is unspecified whether this member function is declared. If it is declared, it is unspecified what its return type is, except that the declaration (although not necessarily the definition) of the function shall be well formed.
This remark should also apply when T
is cv-qualified void
(compare LWG 2500(i)).
[2016-02, Issues Telecon]
P0; move to Tentatively Ready.
Proposed resolution:
This wording is relative to N4567.
Edit 20.3.2.2.6 [util.smartptr.shared.obs]/4 as indicated:
T& operator*() const noexcept;[…]
-4- Remarks: WhenT
is (possibly cv-qualified)void
, it is unspecified whether this member function is declared. If it is declared, it is unspecified what its return type is, except that the declaration (although not necessarily the definition) of the function shall be well formed.
std::hash<std::experimental::shared_ptr<T>>
does not work for arraysSection: 8.2.1 [fund.ts.v2::memory.smartptr.shared] Status: TS Submitter: Tim Song Opened: 2015-12-13 Last modified: 2017-07-30
Priority: 0
View all issues with TS status.
Discussion:
Addresses: fund.ts.v2
The library fundamentals TS does not provide a separate specification for std::hash<std::experimental::shared_ptr<T>>
,
deferring to the C++14 specification in §20.8.2.7/3:
template <class T> struct hash<shared_ptr<T> >;-3- The template specialization shall meet the requirements of class template
hash
(20.9.13). For an objectp
of typeshared_ptr<T>
,hash<shared_ptr<T> >()(p)
shall evaluate to the same value ashash<T*>()(p.get())
.
That specification doesn't work if T
is an array type (U[N]
or U[]
), as in this case get()
returns U*
, which cannot be hashed by std::hash<T*>
.
Proposed resolution:
This wording is relative to N4562.
Insert a new subclause after 8.2.1.3 [fund.ts.v2::memory.smartptr.shared.cast]:
[Note for the editor: The synopses in [header.memory.synop] and [memory.smartptr.shared] should be updated to refer to the new subclause rather than C++14 §20.8.2.7]
?.?.?.?
shared_ptr
hash support [memory.smartptr.shared.hash]template <class T> struct hash<experimental::shared_ptr<T>>;-1- The template specialization shall meet the requirements of class template
hash
(C++14 §20.9.12). For an objectp
of typeexperimental::shared_ptr<T>
,hash<experimental::shared_ptr<T>>()(p)
shall evaluate to the same value ashash<typename experimental::shared_ptr<T>::element_type*>()(p.get())
.
std::experimental::function::operator=(F&&)
should be constrainedSection: 4.2.1 [fund.ts.v2::func.wrap.func.con] Status: TS Submitter: Tim Song Opened: 2015-12-05 Last modified: 2017-07-30
Priority: 0
View all other issues in [fund.ts.v2::func.wrap.func.con].
View all issues with TS status.
Discussion:
Addresses: fund.ts.v2
LWG 2132(i) constrained std::function
's constructor and assignment operator from callable objects for C++14.
The constructors of std::experimental::function
isn't separately specified in the fundamentals TS and so inherited the
constraints from C++14, but the assignment operator is separately specified and presumably needs to be constrained.
[2016-02, Issues Telecon]
P0; move to Tentatively Ready.
Proposed resolution:
This wording is relative to N4562.
Insert a paragraph after 4.2.1 [fund.ts.v2::func.wrap.func.con]/15 as indicated:
template<class F> function& operator=(F&& f);-14- Effects:
-15- Returns:function(allocator_arg, ALLOCATOR_OF(*this), std::forward<F>(f)).swap(*this);
*this
. -?- Remarks: This assignment operator shall not participate in overload resolution unlessdeclval<decay_t<F>&>()
is Callable (C++14 §20.9.11.2) for argument typesArgTypes...
and return typeR
.
experimental::function::assign
should be removedSection: 4.2 [fund.ts.v2::func.wrap.func] Status: TS Submitter: Tim Song Opened: 2015-12-20 Last modified: 2017-07-30
Priority: 0
View all other issues in [fund.ts.v2::func.wrap.func].
View all issues with TS status.
Discussion:
Addresses: fund.ts.v2
Following the lead of LWG 2385(i), the assign(F&&, const A&)
member function template in
std::experimental::function
makes no sense (it causes undefined behavior unless the allocator passed compares equal
to the one already used by *this
) and should be removed.
[2016-02, Issues Telecon]
P0; move to Tentatively Ready.
Proposed resolution:
This wording is relative to N4562.
Edit 4.2 [fund.ts.v2::func.wrap.func], class template function
synopsis, as indicated:
namespace std { namespace experimental { inline namespace fundamentals_v2 { […] template<class R, class... ArgTypes> class function<R(ArgTypes...)> { public: […] void swap(function&);template<class F, class A> void assign(F&&, const A&);[…] }; […] } // namespace fundamentals_v2 } // namespace experimental […] } // namespace std
istream_iterator
and ostream_iterator
should use std::addressof
Section: 24.6 [stream.iterators] Status: C++17 Submitter: Tim Song Opened: 2016-01-01 Last modified: 2017-07-30
Priority: 0
View all other issues in [stream.iterators].
View all issues with C++17 status.
Discussion:
To defend against overloaded unary &
. This includes the constructors of both iterators, and
istream_iterator::operator->
.
{i,o}stream_type
are specializations of basic_{i,o}stream
, but the constructors might still
pick up an overloaded &
via the traits
template parameter. This change also provides consistency
with std::experimental::ostream_joiner
(which uses std::addressof
).
[2016-02, Issues Telecon]
P0; move to Tentatively Ready.
Proposed resolution:
This wording is relative to N4567.
Edit 24.6.2.2 [istream.iterator.cons]/3+4 as indicated:
istream_iterator(istream_type& s);-3- Effects: Initializes
-4- Postcondition:in_stream
with.
&saddressof(s)value
may be initialized during construction or the first time it is referenced.in_stream ==
.&saddressof(s)
Edit 24.6.2.3 [istream.iterator.ops]/2 as indicated:
const T* operator->() const;-2- Returns:
.
&addressof(operator*())
Edit 24.6.3.2 [ostream.iterator.cons.des]/1+2 as indicated:
ostream_iterator(ostream_type& s);-1- Effects: Initializes
out_stream
withand
&saddressof(s)delim
with null.ostream_iterator(ostream_type& s, const charT* delimiter);-2- Effects: Initializes
out_stream
withand
&saddressof(s)delim
withdelimiter
.
{shared,unique}_lock
should use std::addressof
Section: 32.6.5.4.2 [thread.lock.unique.cons], 32.6.5.5.2 [thread.lock.shared.cons] Status: C++17 Submitter: Tim Song Opened: 2016-01-01 Last modified: 2017-07-30
Priority: 0
View all other issues in [thread.lock.unique.cons].
View all issues with C++17 status.
Discussion:
So that they work with user-defined types that have overloaded unary &
.
[2016-02, Issues Telecon]
P0; move to Tentatively Ready.
Proposed resolution:
This wording is relative to N4567.
Edit 32.6.5.4.2 [thread.lock.unique.cons] as indicated:
explicit unique_lock(mutex_type& m);[…]
-5- Postconditions:pm ==
and&maddressof(m)owns == true
.unique_lock(mutex_type& m, defer_lock_t) noexcept;[…]
-7- Postconditions:pm ==
and&maddressof(m)owns == false
.unique_lock(mutex_type& m, try_to_lock_t);[…]
-10- Postconditions:pm ==
and&maddressof(m)owns == res
, whereres
is the value returned by the call tom.try_lock()
.unique_lock(mutex_type& m, adopt_lock_t);[…]
-13- Postconditions:pm ==
and&maddressof(m)owns == true
. -14- Throws: Nothing.template <class Clock, class Duration> unique_lock(mutex_type& m, const chrono::time_point<Clock, Duration>& abs_time);[…]
-17- Postconditions:pm ==
and&maddressof(m)owns == res
, whereres
is the value returned by the call tom.try_lock_until(abs_time)
.template <class Rep, class Period> unique_lock(mutex_type& m, const chrono::duration<Rep, Period>& rel_time);[…]
-20- Postconditions:pm ==
and&maddressof(m)owns == res
, whereres
is the value returned by the call tom.try_lock_for(rel_time)
.
Edit 32.6.5.5.2 [thread.lock.shared.cons] as indicated:
explicit shared_lock(mutex_type& m);[…]
-5- Postconditions:pm ==
and&maddressof(m)owns == true
.shared_lock(mutex_type& m, defer_lock_t) noexcept;[…]
-7- Postconditions:pm ==
and&maddressof(m)owns == false
.shared_lock(mutex_type& m, try_to_lock_t);[…]
-10- Postconditions:pm ==
and&maddressof(m)owns == res
whereres
is the value returned by the call tom.try_lock_shared()
.shared_lock(mutex_type& m, adopt_lock_t);[…]
-13- Postconditions:pm ==
and&maddressof(m)owns == true
.template <class Clock, class Duration> shared_lock(mutex_type& m, const chrono::time_point<Clock, Duration>& abs_time);[…]
-16- Postconditions:pm ==
and&maddressof(m)owns == res
whereres
is the value returned by the call tom.try_lock_shared_until(abs_time)
.template <class Rep, class Period> shared_lock(mutex_type& m, const chrono::duration<Rep, Period>& rel_time);[…]
-19- Postconditions:pm ==
and&maddressof(m)owns == res
whereres
is the value returned by the call tom.try_lock_shared_for(rel_time)
.
Section: 24.3 [iterator.requirements], 24.3.2.3 [iterator.traits] Status: C++17 Submitter: Ville Voutilainen Opened: 2016-01-05 Last modified: 2017-09-07
Priority: 3
View all other issues in [iterator.requirements].
View all issues with C++17 status.
Discussion:
See this reflector discussion for background.
24.3 [iterator.requirements] attempts to establish requirements for iterators, but 24.3.2.3 [iterator.traits]/1 establishes further requirements that must be met in order to author a portable iterator that works with existing implementations. Failing to meet the requirements of the latter will fail to work in practice. The former requirements should reference the latter, normatively.[2016-08-03 Chicago]
Fri AM: Moved to Tentatively Ready
Proposed resolution:
After [iterator.requirements.general]/5, insert the following new paragraph:-?- In addition to the requirements in this sub-clause, the nested typedef-names specified in ([iterator.traits]) shall be provided for the iterator type. [Note: Either the iterator type must provide the typedef-names directly (in which case iterator_traits pick them up automatically), or an iterator_traits specialization must provide them. -end note]
basic_string
assignment vs. basic_string::assign
Section: 27.4.3.7.3 [string.assign] Status: C++17 Submitter: Marshall Clow Opened: 2016-01-05 Last modified: 2017-07-30
Priority: 0
View all other issues in [string.assign].
View all issues with C++17 status.
Discussion:
In issue 2063(i), we changed the Effects of basic_string::assign(basic_string&&)
to match the
behavior of basic_string::operator=(basic_string&&)
, making them consistent.
basic_string::assign(const basic_string&)
, and its Effects differ from
operator=(const basic_string&)
.
Given the following definition:
typedef std::basic_string<char, std::char_traits<char>, MyAllocator<char>> MyString; MyAllocator<char> alloc1, alloc2; MyString string1("Alloc1", alloc1); MyString string2(alloc2);
the following bits of code are not equivalent:
string2 = string1; // (a) calls operator=(const MyString&) string2.assign(string1); // (b) calls MyString::assign(const MyString&)
What is the allocator for string2
after each of these calls?
If MyAllocator<char>::propagate_on_container_copy_assignment
is true, then it should be alloc2
,
otherwise it should be alloc1
.
alloc2
27.4.3.7.3 [string.assign]/1 says that (b) is equivalent to assign(string1, 0, npos)
, which eventually calls
assign(str.data() + pos, rlen)
. No allocator transfer there.
[2016-02, Issues Telecon]
P0; move to Tentatively Ready.
Proposed resolution:
This wording is relative to N4567.
Modify 27.4.3.7.3 [string.assign] p.1 as indicated:
basic_string& assign(const basic_string& str);-1- Effects: Equivalent to
-2- Returns:*this = str
.assign(str, 0, npos)*this
.
<type_traits>
variable templates should be prohibitedSection: 21.3.3 [meta.type.synop] Status: C++17 Submitter: Tim Song Opened: 2016-01-07 Last modified: 2017-07-30
Priority: 0
View other active issues in [meta.type.synop].
View all other issues in [meta.type.synop].
View all issues with C++17 status.
Discussion:
21.3.3 [meta.type.synop]/1 only prohibits adding specializations of class templates in <type_traits>
.
Now that we have _v
variable templates, this prohibition should apply to them as well.
[2016-02, Issues Telecon]
P0; move to Tentatively Ready.
Proposed resolution:
This wording is relative to N4567.
Edit 21.3.3 [meta.type.synop]/1 as indicated:
-1- The behavior of a program that adds specializations for any of the
classtemplates defined in this subclause is undefined unless otherwise specified.
Section: 21 [meta] Status: C++17 Submitter: Tim Song Opened: 2016-01-07 Last modified: 2017-07-30
Priority: 0
View other active issues in [meta].
View all other issues in [meta].
View all issues with C++17 status.
Discussion:
16.4.5.8 [res.on.functions]/2.5 says that the behavior is undefined "if an incomplete type is used as a template argument when instantiating a template component, unless specifically allowed for that component."
This rule should not apply to type traits — a literal application would makeis_same<void, void>
undefined behavior, since nothing in 21 [meta] (or elsewhere) "specifically allows" instantiating is_same
with incomplete types.
Traits that require complete types are already explicitly specified as such, so the proposed wording below simply negates
16.4.5.8 [res.on.functions]/2.5 for 21 [meta].
[2016-02, Issues Telecon]
P0; move to Tentatively Ready.
Proposed resolution:
This wording is relative to N4567.
Insert a new paragraph after 21.3.3 [meta.type.synop]/1:
-?- Unless otherwise specified, an incomplete type may be used to instantiate a template in this subclause.
basic_string(str, pos)
Section: 27.4.3.3 [string.cons] Status: C++17 Submitter: Pablo Halpern Opened: 2016-01-05 Last modified: 2017-07-30
Priority: 0
View all other issues in [string.cons].
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Discussion:
Container and string
constructors in the standard follow two general rules:
Every constructor needs a version with and without an allocator argument (possibly through the use of default arguments).
Every constructor except the copy constructor for which an allocator is not provided uses a default-constructed allocator.
The first rule ensures emplacing a string
into a container that uses a scoped allocator will correctly propagate
the container's allocator to the new element.
string
as basic_string(str, pos)
but not
basic_string(str, pos, alloc)
. This omission breaks the first rule and causes something like the following to fail:
typedef basic_string<char, char_traits<char>, A<char>> stringA; vector<stringA, scoped_allocator_adaptor<A<stringA>>> vs; stringA s; vs.emplace_back(s, 2); // Ill-formed
[2016-02, Issues Telecon]
P0; move to Tentatively Ready.
Proposed resolution:
This wording is relative to N4567.
Change 27.4.3 [basic.string], class template basic_string
synopsis, as indicated
basic_string(const basic_string& str, size_type pos,size_type n = npos,const Allocator& a = Allocator()); basic_string(const basic_string& str, size_type pos, size_type n, const Allocator& a = Allocator());
Change 27.4.3.3 [string.cons] as indicated
basic_string(const basic_string& str, size_type pos,size_type n = npos,const Allocator& a = Allocator());-3- Throws:
-4- Effects: Constructs an object of classout_of_range
ifpos > str.size()
.basic_string
and determines the effective lengthrlen
of the initial string value asthe smaller ofn
andstr.size() - pos
, as indicated in Table 65.basic_string(const basic_string& str, size_type pos, size_type n, const Allocator& a = Allocator());-?- Throws:
-?- Effects: Constructs an object of classout_of_range
ifpos > str.size()
.basic_string
and determines the effective lengthrlen
of the initial string value as the smaller ofn
andstr.size() - pos
, as indicated in Table 65.
Table 65 — basic_string(const basic_string&, size_type,
andsize_type,const Allocator&)basic_string(const basic_string&, size_type, size_type, const Allocator&)
effectsElement Value data()
points at the first element of an allocated copy of rlen
consecutive elements of the string controlled bystr
beginning at positionpos
size()
rlen
capacity()
a value at least as large as size()
<regex>
ECMAScript IdentityEscape
is ambiguousSection: 28.6.12 [re.grammar] Status: C++17 Submitter: Billy O'Neal III Opened: 2016-01-13 Last modified: 2017-07-30
Priority: 2
View other active issues in [re.grammar].
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Discussion:
Stephan and I are seeing differences in implementation for how non-special characters should be handled in the
IdentityEscape
part of the ECMAScript grammar. For example:
#include <stdio.h> #include <iostream> #ifdef USE_BOOST #include <boost/regex.hpp> using namespace boost; #else #include <regex> #endif using namespace std; int main() { try { const regex r("\\z"); cout << "Constructed \\z." << endl; if (regex_match("z", r)) cout << "Matches z" << endl; } catch (const regex_error& e) { cout << e.what() << endl; } }
libstdc++, boost, and browsers I tested with (Microsoft Edge, Google Chrome) all happily interpret \z
, which
otherwise has no meaning, as an identity character escape for the letter z
.
libc++ and msvc++ say that this is invalid, and throw regex_error
with error_escape
.
IdentityEscape :: SourceCharacter but not IdentifierPart IdentifierPart :: IdentifierStart UnicodeCombiningMark UnicodeDigit UnicodeConnectorPunctuation \ UnicodeEscapeSequence IdentifierStart :: UnicodeLetter $ _ \ UnicodeEscapeSequence
But this doesn't make any sense — it prohibits things like \$
which users absolutely need to be able to escape.
So let's look at ECMAScript 6. I believe this says much the same thing, but updates the spec to better handle Unicode by
referencing what the Unicode standard says is an identifier character:
IdentityEscape :: SyntaxCharacter / SourceCharacter but not UnicodeIDContinue UnicodeIDContinue :: any Unicode code point with the Unicode property "ID_Continue", "Other_ID_Continue", or "Other_ID_Start"
However, ECMAScript 6 has an appendix B defining "additional features for web browsers" which says:
IdentityEscape :: SourceCharacter but not c
which appears to agree with what libstdc++, boost, and browsers are doing.
What should be the correct behavior here?[2016-08, Chicago]
Monday PM: Move to tentatively ready
Proposed resolution:
This wording is relative to N4567.
Change 28.6.12 [re.grammar]/3 as indicated:
-3- The following productions within the ECMAScript grammar are modified as follows:
ClassAtom :: - ClassAtomNoDash ClassAtomExClass ClassAtomCollatingElement ClassAtomEquivalence IdentityEscape :: SourceCharacter but not c
forward_list::resize(size_type, const value_type&)
effects incorrectSection: 23.3.7.5 [forward.list.modifiers] Status: C++17 Submitter: Tim Song Opened: 2016-01-14 Last modified: 2023-02-07
Priority: 0
View all other issues in [forward.list.modifiers].
View all issues with C++17 status.
Discussion:
[forwardlist.modifiers]/29 says that the effects of forward_list::resize(size_type sz, const value_type& c)
are:
Effects: If
sz < distance(begin(), end())
, erases the lastdistance(begin(), end()) - sz
elements from the list. Otherwise, insertssz - distance(begin(), end())
elements at the end of the list such that each new element,e
, is initialized by a method equivalent to callingallocator_traits<allocator_type>::construct(get_allocator(), std::addressof(e), c)
.
In light of LWG 2218(i), the use of allocator_traits<allocator_type>::construct
is incorrect,
as a rebound allocator may be used. There's no need to repeat this information, in any event — no other specification
of resize()
does it.
[2016-02, Issues Telecon]
P0; move to Tentatively Ready.
[2016-02-11, Alisdair requests reopening]
I believe the standard is correct as written, and that by removing the clear direction to make the copy with uses-allocator-construction, we open ourselves to disputing this very point again at some point in the future.
The issue seems to be complaining that a rebound allocator may be used instead of the allocator returned byget_allocator()
call, and nailing us down to exactly which
instantiation of allocator_traits
is used. Given the requirements on allocators being
constructible from within the same template "family" though, and specifically that
copies compare equal and can allocate/deallocate on each other's behalf, this should
clearly fall under existing as-if freedom. The construct call is even more clear, as
there is no requirement that the allocator to construct be of a kind that can allocate
the specific type being constructed — a freedom granted precisely so this kind of code
can be written, and be correct, regardless of internal node type of any container and
the actual rebound allocator used internally.
I think the new wording is less clear than the current wording, and would prefer to
resolve as NAD.
Proposed resolution:
This wording is relative to N4567.
Edit [forwardlist.modifiers]/29 as indicated:
[Drafting note: "copies of
c
" is the phrase used byvector::resize
anddeque::resize
.]
void resize(size_type sz, const value_type& c);-29- Effects: If
sz < distance(begin(), end())
, erases the lastdistance(begin(), end()) - sz
elements from the list. Otherwise, insertssz - distance(begin(), end())
elementscopies ofc
at the end of the listsuch that each new element,.e
, is initialized by a method equivalent to callingallocator_traits<allocator_type>::construct(get_allocator(), std::addressof(e), c)
scoped_allocator_adaptor::construct()
Section: 20.6.4 [allocator.adaptor.members], 20.2.8.2 [allocator.uses.construction] Status: C++17 Submitter: Jonathan Wakely Opened: 2016-01-15 Last modified: 2017-07-30
Priority: 0
View all other issues in [allocator.adaptor.members].
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Discussion:
20.6.4 [allocator.adaptor.members] p9 says that the is_constructible
tests are done using inner_allocator_type
, which checks for
construction from an rvalue, but then the constructor is passed
inner_allocator()
which returns a non-const
lvalue reference. The
value categories should be consistent, otherwise this fails to
compile:
#include <memory> #include <scoped_allocator> struct X { using allocator_type = std::allocator<X>; X(std::allocator_arg_t, allocator_type&&) { } X(allocator_type&) { } }; int main() { std::scoped_allocator_adaptor<std::allocator<X>> sa; sa.construct(sa.allocate(1)); }
uses_allocator<X, decltype(sa)::inner_allocator_type>>
is true, because
it can be constructed from an rvalue of the allocator type, so bullet (9.1) doesn't apply.
is_constructible<X, allocator_arg_t, decltype(sa)::inner_allocator_type>
is true, so bullet (9.2) applies.
That means we try to construct the object passing it
sa.inner_allocator()
which is an lvalue reference, so it fails.
The is_constructible
checks should use an lvalue reference, as that's
what's actually going to be used.
I don't think the same problem exists in the related wording in
20.2.8.2 [allocator.uses.construction] if we assume that the value categories
of v1, v2, ..., vN
and alloc
are meant to be preserved, so that the
is_constructible
traits and the initialization expressions match.
However, it does say "an allocator alloc
of type Alloc
" and if Alloc
is an reference type then it's not an allocator, so I suggest a small tweak there too.
[2016-02, Issues Telecon]
Strike first paragraph of PR, and move to Tentatively Ready.
Original Resolution [SUPERSEDED]:
Change 20.2.8.2 [allocator.uses.construction] p1:
-1- Uses-allocator construction with allocator
Alloc
refers to the construction of an objectobj
of typeT
, using constructor argumentsv1, v2, ..., vN
of typesV1, V2, ..., VN
, respectively, and an allocator (or reference to an allocator)alloc
of typeAlloc
, according to the following rules:Change the 2nd and 3rd bullets in 20.6.4 [allocator.adaptor.members] p9 to add two lvalue-references:
(9.2) — Otherwise, if
uses_allocator<T, inner_allocator_type>::value
istrue
andis_constructible<T, allocator_arg_t, inner_allocator_type&, Args...>::value
istrue
, callsOUTERMOST_ALLOC_TRAITS(*this)::construct(OUTERMOST(*this), p, allocator_arg, inner_allocator(), std::forward<Args>(args)...)
.(9.3) — Otherwise, if
uses_allocator<T, inner_allocator_type>::value
istrue
andis_constructible<T, Args..., inner_allocator_type&>::value
istrue
, callsOUTERMOST_ALLOC_TRAITS(*this)::construct(OUTERMOST(*this), p, std::forward<Args>(args)..., inner_allocator())
.Change the 2nd, 3rd, 6th, and 7th bullets in 20.6.4 [allocator.adaptor.members] p11 to add four lvalue-references:
(11.2) — Otherwise, if
uses_allocator<T1, inner_allocator_type>::value
istrue
andis_constructible<T1, allocator_arg_t, inner_allocator_type&, Args1...>::value
istrue
, thenxprime
istuple_cat(tuple<allocator_arg_t, inner_allocator_type&>(allocator_arg, inner_allocator()), std::move(x))
.(11.3) — Otherwise, if
uses_allocator<T1, inner_allocator_type>::value
istrue
andis_constructible<T1, Args1..., inner_allocator_type&>::value
istrue
, thenxprime
istuple_cat(std::move(x), tuple<inner_allocator_type&>(inner_allocator()))
.[…]
(11.6) — Otherwise, if
uses_allocator<T2, inner_allocator_type>::value
istrue
andis_constructible<T2, allocator_arg_t, inner_allocator_type&, Args2...>::value
istrue
, thenyprime
istuple_cat(tuple<allocator_arg_t, inner_allocator_type&>(allocator_arg, inner_allocator()), std::move(y))
.(11.7) — Otherwise, if
uses_allocator<T2, inner_allocator_type>::value
istrue
andis_constructible<T2, Args2..., inner_allocator_type&>::value
istrue
, thenyprime
istuple_cat(std::move(y), tuple<inner_allocator_type&>(inner_allocator()))
.
[2016-02, Issues Telecon]
P0; move to Tentatively Ready.
Proposed resolution:
This wording is relative to N4567.
Change the 2nd and 3rd bullets in 20.6.4 [allocator.adaptor.members] p9 to add two lvalue-references:
(9.2) — Otherwise, if
uses_allocator<T, inner_allocator_type>::value
istrue
andis_constructible<T, allocator_arg_t, inner_allocator_type&, Args...>::value
istrue
, callsOUTERMOST_ALLOC_TRAITS(*this)::construct(OUTERMOST(*this), p, allocator_arg, inner_allocator(), std::forward<Args>(args)...)
.(9.3) — Otherwise, if
uses_allocator<T, inner_allocator_type>::value
istrue
andis_constructible<T, Args..., inner_allocator_type&>::value
istrue
, callsOUTERMOST_ALLOC_TRAITS(*this)::construct(OUTERMOST(*this), p, std::forward<Args>(args)..., inner_allocator())
.
Change the 2nd, 3rd, 6th, and 7th bullets in 20.6.4 [allocator.adaptor.members] p11 to add four lvalue-references:
(11.2) — Otherwise, if
uses_allocator<T1, inner_allocator_type>::value
istrue
andis_constructible<T1, allocator_arg_t, inner_allocator_type&, Args1...>::value
istrue
, thenxprime
istuple_cat(tuple<allocator_arg_t, inner_allocator_type&>(allocator_arg, inner_allocator()), std::move(x))
.(11.3) — Otherwise, if
uses_allocator<T1, inner_allocator_type>::value
istrue
andis_constructible<T1, Args1..., inner_allocator_type&>::value
istrue
, thenxprime
istuple_cat(std::move(x), tuple<inner_allocator_type&>(inner_allocator()))
.[…]
(11.6) — Otherwise, if
uses_allocator<T2, inner_allocator_type>::value
istrue
andis_constructible<T2, allocator_arg_t, inner_allocator_type&, Args2...>::value
istrue
, thenyprime
istuple_cat(tuple<allocator_arg_t, inner_allocator_type&>(allocator_arg, inner_allocator()), std::move(y))
.(11.7) — Otherwise, if
uses_allocator<T2, inner_allocator_type>::value
istrue
andis_constructible<T2, Args2..., inner_allocator_type&>::value
istrue
, thenyprime
istuple_cat(std::move(y), tuple<inner_allocator_type&>(inner_allocator()))
.
bool
" requirement in conjunction
and disjunction
Section: 21.3.9 [meta.logical] Status: C++17 Submitter: Tim Song Opened: 2016-01-18 Last modified: 2017-12-05
Priority: 3
View all other issues in [meta.logical].
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Discussion:
The specification of conjunction
and disjunction
in 21.3.9 [meta.logical] p2 and p5 requires
Bi::value
to be convertible to bool
, but nothing in the specification of the actual behavior of the
templates, which instead uses the expressions Bi::value == false
and Bi::value != false
instead,
actually requires this conversion.
Bi::value
directly to std::conditional
,
like the sample implementation in P0013R1:
template<class B1, class B2> struct and_<B1, B2> : conditional_t<B1::value, B2, B1> { };
then it's insufficient in at least two ways:
Nothing in the specification requires the result of comparing Bi::value
with false
to be consistent
with the result of the implicit conversion. This is similar to LWG 2114(i), though I don't think the
BooleanTestable
requirements in that issue's P/R covers Bi::value == false
and Bi::value != false
.
More importantly, the above implementation is ill-formed for, e.g.,
std::conjunction<std::integral_constant<int, 2>, std::integral_constant<int, 4>>
, because converting 2
to bool
is a narrowing conversion that is not allowed for non-type template arguments (see 7.7 [expr.const]/4).
(Note that GCC currently doesn't diagnose this error at all, and Clang doesn't diagnose it inside system headers.) It's not clear
whether such constructs are intended to be supported, but if they are not, the current wording doesn't prohibit it.
[2016-08-03 Chicago LWG]
Walter, Nevin, and Jason provide initial Proposed Resolution.
Previous resolution [SUPERSEDED]:
This wording is relative to N4606.
Change 21.3.9 [meta.logical] as indicated:
template<class... B> struct conjunction : see below { };[…]
-3- The BaseCharacteristic of a specializationconjunction<B1, ..., BN>
is the first typeBi
in the listtrue_type, B1, ..., BN
for which, or if every
Bi::value == false! bool(Bi::value), the BaseCharacteristic is the last type in the list. […] -4- For a specialization
Bi::value != falsebool(Bi::value)conjunction<B1, ..., BN>
, if there is a template type argumentBi
with, then instantiating […]
Bi::value == false! bool(Bi::value)template<class... B> struct disjunction : see below { };[…]
-6- The BaseCharacteristic of a specializationdisjunction<B1, ..., BN>
is the first typeBi
in the listfalse_type, B1, ..., BN
for which, or if every
Bi::value != falsebool(Bi::value), the BaseCharacteristic is the last type in the list. […] -7- For a specialization
Bi::value == false! bool(Bi::value)disjunction<B1, ..., BN>
, if there is a template type argumentBi
with, then instantiating […]
Bi::value != falsebool(Bi::value)template<class B> struct negation : bool_constant<!bool(B::value)> { };-8- The class template negation forms the logical negation of its template type argument. The type
negation<B>
is a UnaryTypeTrait with a BaseCharacteristic ofbool_constant<!bool(B::value)>
.
[Dec 2017 - The resolution for this issue shipped in the C++17 standard; setting status to 'C++17']
Proposed resolution:
The resolution for this issue was combined with the resolution for LWG 2567(i), so 2567(i) resolves this issue here as well.
bool
" requirement in conjunction
and disjunction
Section: 3.3.3 [fund.ts.v2::meta.logical] Status: TS Submitter: Tim Song Opened: 2016-01-18 Last modified: 2017-07-30
Priority: 3
View all other issues in [fund.ts.v2::meta.logical].
View all issues with TS status.
Discussion:
Addresses: fund.ts.v2
The specification of conjunction
and disjunction
in 3.3.3 [fund.ts.v2::meta.logical] p2 and p5 requires
Bi::value
to be convertible to bool
, but nothing in the specification of the actual behavior of the
templates, which instead uses the expressions Bi::value == false
and Bi::value != false
instead,
actually requires this conversion.
Bi::value
directly to std::conditional
,
like the sample implementation in P0013R1:
template<class B1, class B2> struct and_<B1, B2> : conditional_t<B1::value, B2, B1> { };
then it's insufficient in at least two ways:
Nothing in the specification requires the result of comparing Bi::value
with false
to be consistent
with the result of the implicit conversion. This is similar to LWG 2114(i), though I don't think the
BooleanTestable
requirements in that issue's P/R covers Bi::value == false
and Bi::value != false
.
More importantly, the above implementation is ill-formed for, e.g.,
std::conjunction<std::integral_constant<int, 2>, std::integral_constant<int, 4>>
, because converting 2
to bool
is a narrowing conversion that is not allowed for non-type template arguments (see 7.7 [expr.const]/4).
(Note that GCC currently doesn't diagnose this error at all, and Clang doesn't diagnose it inside system headers.) It's not clear
whether such constructs are intended to be supported, but if they are not, the current wording doesn't prohibit it.
[2016-11-08, Issaquah]
Adopted during NB comment resolution
Proposed resolution:
The resolution for this issue was combined with the resolution for LWG 2568(i), so 2568(i) resolves this issue here as well.
match_results
can't satisfy the requirements of a containerSection: 28.6.9 [re.results] Status: C++17 Submitter: S. B. Tam Opened: 2016-01-24 Last modified: 2017-07-30
Priority: 3
View all other issues in [re.results].
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Discussion:
N4567 28.6.9 [re.results] p2 mentions
The class template
match_results
shall satisfy the requirements of an allocator-aware container and of a sequence container, as specifed in 23.2.3, except that only operations defined for const-qualified sequence containers are supported.
However, this is impossible because match_results
has a operator==
whose semantics differs from the one
required in Table 95 — "Container requirements".
a == b
is an equivalence relation and means equal(a.begin(), a.end(), b.begin(), b.end())
.
But for match_results
, a == b
and equal(a.begin(), a.end(), b.begin(), b.end())
can give different
results. For example:
#include <iostream> #include <regex> #include <string> #include <algorithm> int main() { std::regex re("a*"); std::string target("baaab"); std::smatch a; std::regex_search(target, a, re); std::string target2("raaau"); std::smatch b; std::regex_search(target2, b, re); std::cout << std::boolalpha; std::cout << (a == b) << '\n'; // false std::cout << std::equal(a.begin(), a.end(), b.begin(), b.end()) << '\n'; // true }
[2016-02, Issues Telecon]
Marshall: The submitter is absolutely right, but the proposed resolution is insufficient. We should avoid "shall", for once.2016-05: Marshall cleans up the wording around the change
[2016-08 - Chicago]
Thurs AM: Moved to Tentatively Ready
Previous resolution [SUPERSEDED]:
This wording is relative to N4567.
Change 28.6.9 [re.results] p2 as indicated:
-2- The class template
match_results
shall satisfy the requirements of an allocator-aware container and of a sequence container, as specified in 23.2.3, except that only operations defined for const-qualified sequence containers are supported and that the semantics of comparison functions are different from those required for a container.
Proposed resolution:
This wording is relative to N4567.
Change 28.6.9 [re.results] p2 as indicated:
-2- The class template
match_results
shall satisfysatisfies the requirements of an allocator-aware container and of a sequence container,as specified in(23.2.3), except that only operations defined for const-qualified sequence containers are supported and the semantics of comparison functions are different from those required for a container.
[Drafting note: (Post-Issaquah) Due to the outdated N4567 wording the project editor accepted the following merge suggestion into N4606 wording: — end drafting note]
This wording is relative to N4606.
Change 28.6.9 [re.results] p2 as indicated:
-2- The class template
match_results
satisfies the requirements of an allocator-aware container and of a sequence container,as specified in(23.2.2 [container.requirements.general]and, 23.2.4 [sequence.reqmts])respectively, except that only operations defined for const-qualified sequence containers are supported and the semantics of comparison functions are different from those required for a container.
std::array
Section: 23.3.3.1 [array.overview] Status: C++17 Submitter: Robert Haberlach Opened: 2016-01-30 Last modified: 2017-07-30
Priority: 0
View other active issues in [array.overview].
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Discussion:
Similar to core issue 1270's resolution, 23.3.3.1 [array.overview]/2 should cover aggregate-initialization in general. As it stands, that paragraph solely mentions copy-list-initialization — i.e. it is unclear whether the following notation is (guaranteed to be) well-formed:
std::array<int, 1> arr{0};
[2016-02, Issues Telecon]
P0; move to Tentatively Ready.
Proposed resolution:
This wording is relative to N4567.
Change 23.3.3.1 [array.overview] p2 as indicated:
-2- An
array
is an aggregate (8.5.1) that can be list-initialized withthe syntaxarray<T, N> a = { initializer-list };
where initializer-list is a comma-separated list ofup toN
elements whose types are convertible toT
.
std::function
's member template target()
should not lead to undefined behaviourSection: 22.10.17.3.6 [func.wrap.func.targ] Status: C++17 Submitter: Daniel Krügler Opened: 2016-01-31 Last modified: 2017-09-07
Priority: 3
View all other issues in [func.wrap.func.targ].
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Discussion:
This issue is a spin-off of LWG 2393(i), it solely focuses on the pre-condition of 22.10.17.3.6 [func.wrap.func.targ] p2:
Requires:
T
shall be a type that is Callable (20.9.12.2) for parameter typesArgTypes
and return typeR
.
Originally, the author of this issue here had assumed that simply removing the precondition as a side-step of fixing LWG 2393(i) would be uncontroversial. Discussions on the library reflector indicated that this is not the case, although it seemed that there was agreement on removing the undefined behaviour edge-case.
There exist basically the following positions:The constraint should be removed completely, the function is considered as having a wide contract.
The pre-condition should be replaced by a Remarks element, that has the effect of making the code ill-formed,
if T
is a type that is not Lvalue-Callable (20.9.11.2) for parameter types ArgTypes
and return type R
.
Technically this approach is still conforming with a wide contract function, because the definition of this contract form
depends on runtime constraints.
Not yet explicitly discussed, but a possible variant of bullet (2) could be:
The pre-condition should be replaced by a Remarks element, that has the effect of SFINAE-constraining
this member: "This function shall not participate in overload resolution unless T
is a type that is
Lvalue-Callable (20.9.11.2) for parameter types ArgTypes
and return type R
".
The following describes a list of some selected arguments that have been provided for one or the other position using corresponding list items. Unless explicitly denoted, no difference has been accounted for option (3) over option (2).
It reflects existing implementation practice, Visual Studio 2015 SR1, gcc 6 libstdc++, and clang 3.8.0 libc++ do accept the following code:
#include <functional> #include <iostream> #include <typeinfo> #include "boost/function.hpp" void foo(int) {} int main() { std::function<void(int)> f(foo); std::cout << f.target<void(*)()>() << std::endl; boost::function<void(int)> f2(foo); std::cout << f2.target<void(*)()>() << std::endl; }
and consistently output the implementation-specific result for two null pointer values.
The current
Boost documentation
does not indicate any precondition for calling the target
function, so it is natural
that programmers would expect similar specification and behaviour for the corresponding standard component.
There is a consistency argument in regard to the free function template get_deleter
template<class D, class T> D* get_deleter(const shared_ptr<T>& p) noexcept;
This function also does not impose any pre-conditions on its template argument D
.
Programmers have control over the type they're passing to target<T>()
. Passing a non-callable type
can't possibly retrieve a non-null target, so it seems highly likely to be programmer error. Diagnosing that at
compile time seems highly preferable to allowing this to return null, always, at runtime.
If T
is a reference type then the return type T*
is ill-formed anyway. This implies that one can't
blindly call target<T>
without knowing what T
is.
It has been pointed out that some real world code, boiling down to
void foo() {} int main() { std::function<void()> f = foo; if (f.target<decltype(foo)>()) { // fast path } else { // slow path } }
had manifested as a performance issue and preparing a patch that made the library static_assert
in that
case solved this problem (Note that decltype(foo)
evaluates to void()
, but a proper argument of
target()
would have been the function pointer type void(*)()
, because a function type
void()
is not any Callable type).
It might be worth adding that if use case (2 c) is indeed an often occurring idiom, it would make sense to consider
to provide an explicit conversion to a function pointer (w/o template parameters that could be provided incorrectly),
if the std::function
object at runtime conditions contains a pointer to a real function, e.g.
R(*)(ArgTypes...) target_func_ptr() const noexcept;
[2016-08 Chicago]
Tues PM: Moved to Tentatively Ready
Proposed resolution:
This wording is relative to N4567.
Change 22.10.17.3.6 [func.wrap.func.targ] p2 as indicated:
template<class T> T* target() noexcept; template<class T> const T* target() const noexcept;-3- Returns: If
-2- Requires:T
shall be a type that isCallable
(22.10.17.3 [func.wrap.func]) for parameter typesArgTypes
and return typeR
.target_type() == typeid(T)
a pointer to the stored function target; otherwise a null pointer.
Section: 16.4.4.6 [allocator.requirements] Status: C++20 Submitter: David Krauss Opened: 2016-02-19 Last modified: 2021-02-25
Priority: 4
View other active issues in [allocator.requirements].
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Discussion:
16.4.4.6 [allocator.requirements] suggests that the moved-from state of an allocator may be unequal to its previous state. Such a move constructor would break most container implementations, which move-construct the embedded allocator along with a compressed pair. Even if a moved-from container is empty, it should still subsequently allocate from the same resource pool as it did before.
std::vector<int, pool> a(500, my_pool); auto b = std::move(a); // b uses my_pool too. a.resize(500); // should still use my_pool.
[2016-02, Jacksonville]
Marshall will see if this can be resolved editorially.
After discussion, the editors and I decided that this could not be handled editorially. The bit about a moved-from state of an allocator being the same as the original state is a normative change. I submitted a pull request to handle the mismatched variables in the table.
Previous resolution [SUPERSEDED]:
This wording is relative to N4567.
Change 16.4.4.6 [allocator.requirements], Table 28 — "Allocator requirements" as indicated:
Note there's an editorial error in Table 28 in that line and the surrounding ones. The left column was apparently updated to use
u
and the right column is still usinga
/a1
/b
.
Table 28 — Allocator requirements Expression Return type Assertion/note
pre-/post-conditionDefault …
X u(move(a));
X u = move(a);Shall not exit via an exception. post: u
is equal toa
and equal to the prior value ofa
.a1
equals the prior value ofa
…
[2016-06-20, Oulu, Daniel comments]
According to the current working draft, the situation has changed due to changes performed by the project editor, the revised resolution has been adjusted to N4594.
[2016-08 - Chicago]
Thurs AM: Moved to LEWG, as this decision (should allocators only be copyable, not movable) is design.
[2017-02 in Kona, LEWG responds]
Alisdair Meredith says that if you have a do-not-propagate-on-move-assignment, then the move of the allocator must compare equal to the original.
Have a data structure where all allocators are equal. Construct an element somewhere else moving from inside the container (which doesn't have a POCMA trait); you don't want the allocator of the moved-from element to now be different. So in that case, the allocator's move constructor must behave the same as copy.
We don't need to go as far as this issue, but going that far is ok for Bloomberg.
[2017-06-02 Issues Telecon]
We discussed containers that have sentinel nodes, etc, and so might have to allocate/deallocate using a moved-from allocator - and decided that we didn't want any part of that
Adjusted the wording slightly, and moved to Tentatively Ready
Previous resolution [SUPERSEDED]:
This wording is relative to N4594.
Change 16.4.4.6 [allocator.requirements], Table 28 — "Allocator requirements" as indicated:
Table 28 — Allocator requirements Expression Return type Assertion/note
pre-/post-conditionDefault …
X u(std::move(a));
X u = std::move(a);Shall not exit via an exception. post: u
is equal toa
and equal to the prior value ofa
.u
is equal to the prior value ofa
.…
Proposed resolution:
This wording is relative to N4594.
Change 16.4.4.6 [allocator.requirements], Table 28 — "Allocator requirements" as indicated:
Table 28 — Allocator requirements Expression Return type Assertion/note
pre-/post-conditionDefault …
X u(std::move(a));
X u = std::move(a);Shall not exit via an exception. post: The value of a
is unchanged and is equal tou
.u
is equal to the prior value ofa
…
vector::data()
should use addressof
Section: 23.3.13.4 [vector.data] Status: C++17 Submitter: Marshall Clow Opened: 2016-02-29 Last modified: 2017-07-30
Priority: 0
View all other issues in [vector.data].
View all issues with C++17 status.
Discussion:
In 23.3.13.4 [vector.data], we have:
Returns: A pointer such that
[data(),data() + size())
is a valid range. For a non-empty vector,data() == &front()
.
This should be:
Returns: A pointer such that
[data(),data() + size())
is a valid range. For a non-empty vector,data() == addressof(front())
.
Proposed resolution:
This wording is relative to N4582.
Change 23.3.13.4 [vector.data] p1 as indicated:
T* data() noexcept; const T* data() const noexcept;-1- Returns: A pointer such that
[data(), data() + size())
is a valid range. For a non-empty vector,data() == addressof(
.&front())
std::log
misspecified for complex numbersSection: 29.4.8 [complex.transcendentals] Status: C++20 Submitter: Thomas Koeppe Opened: 2016-03-01 Last modified: 2021-02-25
Priority: 3
View all other issues in [complex.transcendentals].
View all issues with C++20 status.
Discussion:
The current specification of std::log
is inconsistent for complex numbers, specifically, the Returns clause
(29.4.8 [complex.transcendentals]). On the one hand, it states that the imaginary part of the return value lies
in the closed interval [-i π, +i π]
. On the other hand, it says that "the branch
cuts are along the negative real axis" and "the imaginary part of log(x)
is +π
when x
is a negative real number".
The inconsistency lies in the difference between the mathematical concept of a branch cut and the nature of floating
point numbers in C++. The corresponding specification in the C standard makes it clearer that if x
is a real
number, then log(x + 0i) = +π
, but log(x - 0i) = -π
, i.e. they consider positive
and negative zero to represent the two different limits of approaching the branch cut from opposite directions. In
other words, the term "negative real number" is misleading, and in fact there are two distinct real numbers,
x + 0i
and x - 0i
, that compare equal but whose logarithms differ by 2 π i
.
The resolution should consist of two parts:
Double-check that our usage and definition of "branch cut" is sufficiently unambiguous. The C standard contains a lot more wording around this that we don't have in C++.
Change the Returns clause of log
appropriately. For example: "When x
is a negative real number,
imag(log(x + 0i))
is π
, and imag(log(x - 0i))
is -π
."
Current implementations seem to behave as described in (2). (Try-it-at-home link)
[2016-11-12, Issaquah]
Move to Open - Thomas to provide wording
[2016-11-15, Thomas comments and provides wording]
Following LWG discussion in Issaquah, I now propose to resolve this issue by removing the normative requirement on the function limits, and instead adding a note that the intention is to match the behaviour of C. This allows implementations to use the behaviour of C without having to specify what floating point numbers really are.
The change applies to bothstd::log
and std::sqrt
.
Updated try-at-home link, see here.
[2017-03-04, Kona]
Minor wording update and status to Tentatively Ready.
Previous resolution [SUPERSEDED]:
This wording is relative to N4606.
Change the "returns" element for
std::log
(29.4.8 [complex.transcendentals] p17):template<class T> complex<T> log(const complex<T>& x);-16- Remarks: The branch cuts are along the negative real axis.
-17- Returns: The complex natural (base-ℯ) logarithm ofx
. For allx
,imag(log(x))
lies in the interval[
-π, π]
, and when. [Note: The semantics ofx
is a negative real number,imag(log(x))
is πstd::log
are intended to be the same in C++ as they are forclog
in C. — end note]Change the "returns" element for
std::sqrt
(29.4.8 [complex.transcendentals] p25):template<class T> complex<T> sqrt(const complex<T>& x);-24- Remarks: The branch cuts are along the negative real axis.
-25- Returns: The complex square root ofx
, in the range of the right half-plane.If the argument is a negative real number, the value returned lies on the positive imaginary axis.[Note: The semantics ofstd::sqrt
are intended to be the same in C++ as they are forcsqrt
in C. — end note]
Proposed resolution:
This wording is relative to N4606.
Change the "returns" element for std::log
(29.4.8 [complex.transcendentals] p17):
template<class T> complex<T> log(const complex<T>& x);-16- Remarks: The branch cuts are along the negative real axis.
-17- Returns: The complex natural (base-ℯ) logarithm ofx
. For allx
,imag(log(x))
lies in the interval[
-π, π]
, and when. [Note: the semantics of this function are intended to be the same in C++ as they are forx
is a negative real number,imag(log(x))
is πclog
in C. — end note]
Change the "returns" element for std::sqrt
(29.4.8 [complex.transcendentals] p25):
template<class T> complex<T> sqrt(const complex<T>& x);-24- Remarks: The branch cuts are along the negative real axis.
-25- Returns: The complex square root ofx
, in the range of the right half-plane.If the argument is a negative real number, the value returned lies on the positive imaginary axis.[Note: The semantics of this function are intended to be the same in C++ as they are forcsqrt
in C. — end note]
addressof
works on temporariesSection: 20.2.11 [specialized.addressof] Status: C++17 Submitter: Brent Friedman Opened: 2016-03-06 Last modified: 2017-07-30
Priority: 3
View all other issues in [specialized.addressof].
View all issues with C++17 status.
Discussion:
LWG issue 970(i) removed the rvalue reference overload for addressof
. This allows const prvalues to
bind to a call to addressof
, which is dissimilar from the behavior of operator&
.
const vector<int> a(); void b() { auto x = addressof(a()); // "ok" auto y = addressof<const int>(0); // "ok" auto z = &a(); //error: cannot take address of a temporary }
[2016-08 Chicago]
Tues PM: Move to Tentatively Ready
Proposed resolution:
This wording is relative to N4582.
Change 20.2.2 [memory.syn], header <memory>
synopsis, as indicated:
[…] // 20.9.12, specialized algorithms: template <class T> constexpr T* addressof(T& r) noexcept; template <class T> const T* addressof(const T&& elem) = delete; […]
Change 20.2.11 [specialized.addressof] p1 as indicated:
template <class T> constexpr T* addressof(T& r) noexcept; template <class T> const T* addressof(const T&& elem) = delete;-1- Returns: The actual address of the object or function referenced by
r
, even in the presence of an overloadedoperator&
.
Section: 1 [filesys.ts::fs.scope] Status: TS Submitter: FI-5, US-5, GB-3, CH-6 Opened: 2014-01-20 Last modified: 2017-07-30
Priority: Not Prioritized
View all other issues in [filesys.ts::fs.scope].
View all issues with TS status.
Discussion:
Addresses: filesys.ts
The PDTS used a placeholder namespace "tbs" since the Standard Library policy for TS namespaces had not yet been fully articulated.
[2014-02-11 Issaquah: Project editor to make indicated changes to WP, post notice on lib and SG3 reflectors.]
Proposed resolution:
[2014-02-07: Beman Dawes]
Throughout the WP, change namespace "tbs" to "experimental" as described in the Library Fundamentals TS working paper, 1.3 Namespaces and headers [general.namespaces].
Section: 2.1 [filesys.ts::fs.conform.9945] Status: TS Submitter: FI-1 Opened: 2014-01-20 Last modified: 2017-07-30
Priority: Not Prioritized
View all other issues in [filesys.ts::fs.conform.9945].
View all issues with TS status.
Discussion:
Addresses: filesys.ts
It is unfortunate that error reporting for inability to provide reasonable behaviour is completely implementation-defined. This hurts portability in the sense that programmers have no idea how errors will be reported and cannot anticipate anything.
Change "If an implementation cannot provide any reasonable behavior, the implementation shall report an error in an implementation-defined manner." to "If an implementation cannot provide any reasonable behavior, the code using the facilities for which reasonable behaviour cannot be provided shall be ill-formed." and strike the note.
[2014-02-07, Beman Dawes suggests wording]
[2014-02-12, Daniel Krügler comments:]
In our code bases we routinely have to target different filesystems,
notably POSIX-based and Windows. While on Windows there is no meaning
in setting the owner_exec
permission, for example, this is required on
POSIX systems when the corresponding file should be executable. Still,
attempting to invoke this operation should be possible. It would be
OK, if we got a defined runtime response to this, such as a
specifically defined error code value that could be tested either via
the error_code& overload, but it would be IMO unacceptable for
end-users to tell them "Well, this code may not compile". I don't
think that we can teach people that code written using the filesystem
operations might or might not compile. It would be very valuable to
have at least a clear indication that implementers are required to
give a defined runtime-response if they do believe that this operation
cannot be implemented resulting in "reasonable behaviour".
[2014-02-12, Proposed wording updated to reflect LWG/SG-3 discussion in Issaquah.
Since the standardese to carry out the LWG/SG-3 "throw or error code" intent is best achieved by reference to the Error reporting section, a note was added by the project editor. Please review carefully. ][2014-02-13 LWG/SG-3 Issaquah: Proposed wording accepted.]
Proposed resolution:
Change 2.1:
If an implementation cannot provide any reasonable behavior, the implementation shall report an error
in an implementation-defined manneras specified in § 7 [fs.err.report]. [Note: This allows users to rely on an exception being thrown or an error code being set when an implementation cannot provide any reasonable behavior. — end note] .
Section: 4.7 [filesys.ts::fs.def.filename] Status: TS Submitter: CH-2 Opened: 2014-01-20 Last modified: 2017-07-30
Priority: Not Prioritized
View all issues with TS status.
Discussion:
Addresses: filesys.ts
Filename lengths are also implementation dependent. This is not the same as
FILENAME_MAX
that specifies the maximum length of pathnames.
Add a bullet: "Length of filenames."
[2014-02-07, Beman Dawes provides wording]
Proposed resolution:
Change 4.7 [fs.def.filename]:
The name of a file. Filenames dot and dot-dot have special meaning. The following characteristics of filenames are operating system dependent:
The permitted characters. [Example: Some operating systems prohibit the ASCII control characters (0x00-0x1F) in filenames. — end example].
Filenames that are not permitted.
Filenames that have special meaning.
Case awareness and sensitivity during path resolution.
Special rules that may apply to file types other than regular files, such as directories.
Section: 8.1 [filesys.ts::path.generic] Status: TS Submitter: CH-4 Opened: 2014-01-20 Last modified: 2017-07-30
Priority: Not Prioritized
View all issues with TS status.
Discussion:
Addresses: filesys.ts
8.1 [path.generic] says: "The filename dot-dot is treated as a reference to the parent directory." So it must be specified what "/.." and "/../.." refer to.
Add a statement what the parent directory of the root directory is.
[2014-02-07, Beman Dawes suggests wording]
[ 2014-02-11 Issaquah: Implementation defined. See wiki notes for rationale. Beman to provide wording for review next meeting. ]
[ 2014-05-22 Beman provides wording, taken directly from POSIX. See pubs.opengroup.org/onlinepubs/9699919799/xrat/V4_xbd_chap04.html#tag_21_04_12 ]
Proposed resolution:
Change 8.1 [path.generic]:
The filename dot is treated as a reference to the current directory. The filename dot-dot is treated as a reference to the parent directory. What the filename dot-dot refers to relative to root-directory is implementation-defined. Specific filenames may have special meanings for a particular operating system.
Section: 4.15 [filesys.ts::fs.def.path] Status: TS Submitter: CH-5 Opened: 2014-01-20 Last modified: 2017-07-30
Priority: Not Prioritized
View all issues with TS status.
Discussion:
Addresses: filesys.ts
Path depth is implementation dependent.
Add a paragraph: "The maximum length of the sequence (i.e. the maximum depth) is implementation dependent.
[2014-02-07, Beman Dawes comments]
"implementaton defined" and "operating system dependent" are well defined terms in this TS, but "implementation dependent" is not well defined. The path depth is operating system dependent, so that's the form used in the proposed wording.
[2014-02-07, Beman Dawes provides wording]
Proposed resolution:
Change 4.15 [fs.def.path]:
4.15 path [fs.def.path]
A sequence of elements that identify the location of a file within a filesystem. The elements are the root-nameopt , root-directoryopt , and an optional sequence of filenames.
The maximum number of elements in the sequence is operating system dependent.
struct space_info
Section: 6 [filesys.ts::fs.filesystem.synopsis], 15.32 [filesys.ts::fs.op.space] Status: TS Submitter: GB-4 Opened: 2014-01-20 Last modified: 2017-07-30
Priority: Not Prioritized
View all other issues in [filesys.ts::fs.filesystem.synopsis].
View all issues with TS status.
Discussion:
Addresses: filesys.ts
Use of the term a 'non-privileged' process. The comment for available in the struct space_info refers to: free space available to a non-privileged process. This seems quite specific to a POSIX implementation (on Windows, for instance, the equivalent data would be user-specific but not directly related to privilege)
Remove the comment and add a note to 15.32 [fs.op.space]: [Note: the precise meaning of available space is implementation dependent. — end note]
[2014-02-07, Beman Dawes comments]
"implementaton defined" and "operating system dependent"
are well defined terms in this TS, but "implementation dependent" is not well defined.
The meaning of available
is operating system dependent, so that's the form used
in the proposed wording.
[2014-02-07, Beman Dawes provides wording]
Proposed resolution:
Change 6 [fs.filesystem.synopsis]:
uintmax_t available;
// free space available to a non-privileged process
Add Remarks to 15.32 [fs.op.space]:
Remarks: The value of member
space_info::available
is operating system dependent. [Note:available
may be less thanfree
. — end note]
file_time_type
underspecifiedSection: 6 [filesys.ts::fs.filesystem.synopsis] Status: TS Submitter: CH-7 Opened: 2014-01-20 Last modified: 2017-07-30
Priority: Not Prioritized
View all other issues in [filesys.ts::fs.filesystem.synopsis].
View all issues with TS status.
Discussion:
Addresses: filesys.ts
Must the file_time_type
hold times before 1960 and after 2050?
Specify the requirements to unspecified-trivial-clock for file_time_type
.
[2014-02-10, Daniel suggests wording]
[ 2014-02-11 Issaquah: (1)Implementation-defined. See wiki notes for rationale. (2) Leave other additions in place, but insert "the" before "resolution" (3) Strike "unspecified-" from "unspecified-trivial-type" in two places. Beman to provide wording for review next meeting. ]
[2014-02-13 LWG/SG-3 Issaquah: Proposed wording accepted.]
Proposed resolution:
Modify 6 [fs.filesystem.synopsis] as indicated:
typedef chrono::time_point<unspecified-trivial-clock> file_time_type;
unspecified-trivial-clock is anunspecified type provided by the implementationimplementation-defined type that satisfies the TrivialClock requirements (C++11ISO 14882:2011 §20.12.3) and that is capable of representing and measuring file time values. Implementations should ensure that the resolution and range offile_time_type
reflect the operating system dependent resolution and range of file time values.
Section: 6 [filesys.ts::fs.filesystem.synopsis] Status: TS Submitter: FI-2 Opened: 2014-01-20 Last modified: 2017-07-30
Priority: Not Prioritized
View all other issues in [filesys.ts::fs.filesystem.synopsis].
View all issues with TS status.
Discussion:
Addresses: filesys.ts
It is unclear why the range-for support functions (begin()/end()) for
directory_iterator
and recursive_directory_iterator
return different types for begin()
and end()
, namely that begin()
returns a reference to const
and end()
returns a value.
[2014-02-07: Beman Dawes provides comments from the Boost implementation:]
// begin() and end() are only used by a range-based for statement in the context of // auto - thus the top-level const is stripped - so returning const is harmless and // emphasizes begin() is just a pass through.
[2014-02-08: Daniel responds to Beman]
The difference in return types becomes relevant, when testing whether e.g. directory_iterator
would satisfy the Traversable
requirements as currently specified in
N3763. Expressed in
code form these requirements impose that the following assertion holds:
static_assert(std::is_same< decltype(std::range_begin(std::declval<directory_iterator>())), decltype(std::range_end(std::declval<directory_iterator>())) >::value, "No Traversable type");
Both directory_iterator
and recursive_directory_iterator
won't satisfy this requirement
currently.
[ 2014-02-11 Issaquah: Change begin() argument and return to pass-by-value. See wiki notes for rationale. Beman to provide wording for review next meeting. ]
[2014-02-13 LWG/SG-3 Issaquah: Proposed wording accepted.]
Proposed resolution:
Change 6 [fs.filesystem.synopsis]:
class directory_iterator; // enable directory_iterator range-based for statementsconstdirectory_iterator&begin(constdirectory_iterator&iter) noexcept; directory_iterator end(const directory_iterator&) noexcept; class recursive_directory_iterator; // enable recursive_directory_iterator range-based for statementsconstrecursive_directory_iterator&begin(constrecursive_directory_iterator&iter) noexcept; recursive_directory_iterator end(const recursive_directory_iterator&) noexcept;
Change 13.2 [directory_iterator.nonmembers]:
These functions enable use of directory_iterator
with C++11
range-based for statements.
constdirectory_iterator&begin(constdirectory_iterator&iter) noexcept;
Returns:
iter
.
directory_iterator end(const directory_iterator&) noexcept;
Returns:
directory_iterator()
.
Change 14.2 [rec.dir.itr.nonmembers]:
These functions enable use of recursive_directory_iterator
with C++11 range-based for statements.
constrecursive_directory_iterator&begin(constrecursive_directory_iterator&iter) noexcept;
Returns:
iter
.
recursive_directory_iterator end(const recursive_directory_iterator&) noexcept;
Returns:
recursive_directory_iterator()
.
path
copy/move constructorSection: 8.4.1 [filesys.ts::path.construct] Status: TS Submitter: GB-7, CH-10 Opened: 2014-01-20 Last modified: 2017-07-30
Priority: Not Prioritized
View all other issues in [filesys.ts::path.construct].
View all issues with TS status.
Discussion:
Addresses: filesys.ts
The postconditions for the copy/move constructor for path are shown as "empty()". This appears to have been incorrectly copied from the default ctor.
Remove the 'postconditions' clause from the copy/move ctor.
[2014-02-07, Beman Dawes suggests wording]
Proposed resolution:
Change 8.4.1 [path.construct]:
path(const path& p); path(path&& p) noexcept;Effects: Constructs an object of class
path
withpathname
having the original value ofp.pathname
. In the second form,p
is left in a valid but unspecified state.
Postconditions: empty().
Section: 8.2.2 [filesys.ts::path.type.cvt], 8.4.1 [filesys.ts::path.construct] Status: TS Submitter: GB-8 Opened: 2014-01-20 Last modified: 2017-07-30
Priority: Not Prioritized
View all issues with TS status.
Discussion:
Addresses: filesys.ts
No specification for characters with no representation. The example at the end of 8.4.1 refers to "for other native narrow encodings some characters may have no representation" - what happens in such cases?
Suggested action:
Add some definition of the behaviour for characters with no representation.
[2014-02-12 Applied LWG/SG-3 Issaquah wording tweaks to make characters referred to plural.]
[2014-02-08, Beman Dawes provides wording]
Proposed resolution:
Add a paragraph at the end of 8.2.2 [path.type.cvt]:
If the encoding being converted to has no representation for source characters, the resulting converted characters, if any, are unspecified.
Section: 8.4.3 [filesys.ts::path.append] Status: TS Submitter: CH-11 Opened: 2014-01-20 Last modified: 2017-07-30
Priority: Not Prioritized
View all issues with TS status.
Discussion:
Addresses: filesys.ts
Is the added separator redundant in p1 /= p2, where p1 is empty? I.e. does the result start with a separator?
Suggested action:
Specify what behaviour is required.
[2014-02-07: Beman Dawes comments]
The second bullet item is supposed to deal with the empty()
condition.
[2014-02-12 LWG/SG-3 Issaquah: The text is correct as written, however adding a note will clarify this and address the NB comment.]
Proposed resolution:
Change 8.4.3 [path.append]:
Effects:
Appends
path::preferred_separator
topathname
unless:
an added separator would be redundant, or
would change a
nrelative path to anabsolute path [Note: An empty path is relative. — end note], or
p.empty()
, or
*p.native().cbegin()
is a directory separator.Then appends
p.native()
topathname
.
is_absolute()
return clause confusingSection: 8.4.10 [filesys.ts::path.query] Status: TS Submitter: FI-7 Opened: 2014-01-20 Last modified: 2017-07-30
Priority: Not Prioritized
View all issues with TS status.
Discussion:
Addresses: filesys.ts
is_absolute
says: "Returns: true if the elements of root_path()
uniquely identify
a file system location, else false." The "uniquely identify a location" seems confusing
in presence of symlinks.
Suggested action:
Clarify the returns clause so that there's no confusion about symlinks and 'location'.
[2014-02-10 Beman Dawes provides wording]
Proposed resolution:
Change 8.4.10 path query [path.query]:
bool is_absolute() const;Returns:
true
ifthe elements ofroot_path()
uniquely identify a file system locationpathname
contains an absolute path (4.1 [fs.def.absolute-path]) , elsefalse
.[Example:path("/").is_absolute()
istrue
for POSIX based operating systems, andfalse
for Windows based operating systems. — end example]
Section: 8.6.1 [filesys.ts::path.io] Status: TS Submitter: FI-8 Opened: 2014-01-20 Last modified: 2017-07-30
Priority: Not Prioritized
View all issues with TS status.
Discussion:
Addresses: filesys.ts
"[Note: Pathnames containing spaces require special handling by the user to avoid truncation when read by the extractor. — end note]" sounds like a quoted manipulator as specified in the C++14 draft in [quoted.manip] would be useful.
Consider using quoted manipulators for stream insertion and extraction.
[2014-02-10, Daniel suggests wording]
[2014-02-12 Applied LWG/SG-3 Issaquah wording tweak to use ISO doc number for reference to C++14.]
Proposed resolution:
Change 8.6.1 [path.io] as indicated:
template <class charT, class traits> basic_ostream<charT, traits>& operator<<(basic_ostream<charT, traits>& os, const path& p);Effects:
os << quoted(p.string<charT, traits>())
.[Note: Pathnames containing spaces require special handling by the user to avoid truncation when read by the extractor. — end note][Note: Thequoted
function is described in ISO 14882:2014 §27.7.6. — end note] Returns:os
template <class charT, class traits> basic_istream<charT, traits>& operator>>(basic_istream<charT, traits>& is, path& p);Effects:
basic_string<charT, traits> tmp; is >> quoted(tmp); p = tmp;
directory_entry operator==
needs clarificationSection: 12.3 [filesys.ts::directory_entry.obs] Status: TS Submitter: GB-12 Opened: 2014-01-20 Last modified: 2017-07-30
Priority: Not Prioritized
View all other issues in [filesys.ts::directory_entry.obs].
View all issues with TS status.
Discussion:
Addresses: filesys.ts
Since operator==
for directory_entry
does not check status, it would be worth
highlighting that operator==
only checks that the paths match.
Suggested action:
Add: [Note: does not include status values — end note]
[2014-02-09, Beman Dawes suggest wording]
[2014-02-13 LWG/SG-3 Issaquah: Proposed wording accepted. Typo /no/not/ fixed per suggestion.]
Proposed resolution:
To 12.3 [directory_entry.obs]operator==
add:
[Note: Status members do not participate in determining equality. — end note]
directory_iterator
underspecifiedSection: 13.1 [filesys.ts::directory_iterator.members] Status: TS Submitter: CH-13 Opened: 2014-01-20 Last modified: 2017-07-30
Priority: Not Prioritized
View all issues with TS status.
Discussion:
Addresses: filesys.ts
The behaviour of increment is underspecified: What happens if the implementation detects an endless loop (e.g. caused by links)? What happens with automounting and possible race conditions?
More information on this can be found here.
Suggested action:
Specify the required behaviour in these cases.
[2014-02-13 LWG/SG-3 Issaquah: STL will provide wording for next meeting for the endless loop case. The other cases are covered by existing wording in the front matter.]
[17 Jun 2014 At the request of the LWG, Beman provides wording for a note.]
Proposed resolution:
In 14 Class recursive_directory_iterator [class.rec.dir.itr], add:
[Note: If the directory structure being iterated over contains cycles then the end iterator may be unreachable. --end note]
copy
Section: 15.3 [filesys.ts::fs.op.copy] Status: TS Submitter: GB-14 Opened: 2014-01-20 Last modified: 2017-07-30
Priority: Not Prioritized
View all issues with TS status.
Discussion:
Addresses: filesys.ts
Incorrect effects clause for path copy — the effect clause for copy
[fs.op.copy]
includes "equivalent(f, t)
" — there is no equivalent()
function defined for variables
of this type (file_status
)
Suggested action:
Replace with "equivalent(from, to)
"
[2014-02-09, Beman Dawes suggests wording]
[2014-02-13 LWG/SG-3 Issaquah: Proposed wording accepted.]
Proposed resolution:
Change 15.3 [fs.op.copy]:Report an error as specified in Error reporting if:
!exists(f)
.equivalent(
.f, tfrom, to)is_other(f) || is_other(t)
.is_directory(f) && is_regular_file(t)
.
Section: 15.4 [filesys.ts::fs.op.copy_file] Status: TS Submitter: CH-15 Opened: 2014-01-20 Last modified: 2017-07-30
Priority: Not Prioritized
View all issues with TS status.
Discussion:
Addresses: filesys.ts
Even if to
and from
are different paths, they may be equivalent.
Specify what happens if (options & copy_options::overwrite_existing)
but from
and to
resolve to the same file.
[2014-02-09, Beman Dawes: Need advice on this issue:]
What do existing implentations do? Possible resolutions:[2014-02-13 LWG/SG-3 Issaquah: LWG/SG-3 decided to treat equivalence in this case as an error. Beman to provide wording.]
[2014-04-09 Beman provided wording as requested. The Effects were rewritten to increase clarity. Behavior remains unchanged except for treating equivalence as an error.]
[17 Jun 2014 Rapperswil LWG moves to Immediate. Jonathan Wakely will provide editorial changes to improve the presentation of bitmask values.]
Proposed resolution:
Change 15.4 [fs.op.copy_file]:Precondition: At most one constant from each
copy_options
option group ([enum.copy_options]) is present inoptions
.Effects:
Ifexists(to) &&
!(options & (copy_options::skip_existing | copy_options::overwrite_existing | copy_options::update_existing))
report a file already exists error as specified in Error reporting (7).
If!exists(to) || (options & copy_options::overwrite_existing) || ((options & copy_options::update_existing) && last_write_time(from) > last_write_time(to)) || !(options & (copy_options::skip_existing | copy_options::overwrite_existing | copy_options::update_existing))
copy the contents and attributes of the filefrom
resolves to the fileto
resolves to.Report a file already exists error as specified in Error reporting (7) if:
exists(to)
andequivalent(from, to)
, orexists(to)
and(options & (copy_options::skip_existing | copy_options::overwrite_existing | copy_options::update_existing)) == copy_options::none.
Otherwise copy the contents and attributes of the file
from
resolves to to the fileto
resolves to if:
!exists(to)
, orexists(to)
and(options & copy_options::overwrite_existing) != copy_options::none
, orexists(to)
and(options & copy_options::update_existing) != copy_options::none
andfrom
is more recent thanto
, determined as if by use of thelast_write_time
function.Otherwise no effects.
Returns:
true
if thefrom
file was copied, otherwisefalse
. The signature with argumentec
returnfalse
if an error occurs.Throws: As specified in Error reporting (7).
Complexity: At most one direct or indirect invocation of
status(to)
.
uintmax_t
on error?Section: 15.14 [filesys.ts::fs.op.file_size] Status: TS Submitter: FI-9 Opened: 2014-01-20 Last modified: 2017-07-30
Priority: Not Prioritized
View all issues with TS status.
Discussion:
Addresses: filesys.ts
"The signature with argument ec
returns static_cast<uintmax_t>(-1)
if an error
occurs.", one would expect that both signatures return that if an error occurs?
Clarify the Returns clause, and apply the same for every function that returns an
uintmax_t
where applicable.
[2014-02-13 LWG/SG-3 Issaquah:]
Discussion when around in circles for a while, until someone suggested the reference to 15.15 was wrong, and that the issue applied to the previous function in the WP, 15.14 File size [fs.op.file_size]. The NB Comment makes much more sense if it applies to file_size(), so the chair was directed to change the reference from 15.15 [fs.op.hard_lk_ct] to 15.14 [fs.op.file_size]. The intent that file_size() is only meaningful for regular_files. Beman to strike the the static_cast, changing it to "Otherwise an error is reported.".Proposed resolution:
Change 15.14 [fs.op.file_size]:
uintmax_t file_size(const path& p); uintmax_t file_size(const path& p, error_code& ec) noexcept;
Returns: If
!exists(p)
an error is reported (7). Otherwise, the size in bytes of the file&&|| !is_regular_file(p)p
resolves to, determined as if by the value of the POSIXstat
structure memberst_size
obtained as if by POSIXstat()
.Otherwise,The signature with argumentstatic_cast<uintmax_t>(-1)
.ec
returnsstatic_cast<uintmax_t>(-1)
if an error occurs.Throws: As specified in Error reporting (7).
read_symlink
on errorSection: 15.27 [filesys.ts::fs.op.read_symlink] Status: TS Submitter: GB-16 Opened: 2014-01-20 Last modified: 2017-07-30
Priority: Not Prioritized
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Discussion:
Addresses: filesys.ts
Unclear semantics of read_symlink on error: 15.27 [fs.op.read_symlink] has: Returns: If p resolves to a symbolic link, a path object containing the contents of that symbolic link. Otherwise path(). and also [Note: It is an error if p does not resolve to a symbolic link. -- end note]
I do not believe path() can be a valid return for the overload not taking error_code.
Strike "Otherwise path()
."
[2014-02-09, Beman Dawes provides wording]
[2014-02-13 LWG/SG-3 Issaquah: Proposed wording accepted.]
Proposed resolution:
Change 15.27 [fs.op.read_symlink]:Returns: If
p
resolves to a symbolic link, apath
object containing the contents of that symbolic link.OtherwiseThe signature with argumentpath()
.ec
returnspath()
if an error occurs.Throws: As specified in Error reporting. [Note: It is an error if
p
does not resolve to a symbolic link. — end note]
system_complete()
example needs clarificationSection: 15.36 [filesys.ts::fs.op.system_complete] Status: TS Submitter: FI-10 Opened: 2014-01-20 Last modified: 2017-07-30
Priority: Not Prioritized
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Discussion:
Addresses: filesys.ts
"[Example: For POSIX based operating systems, system_complete(p)
has the same semantics as complete(p, current_path())
." What is this
complete
that is referred here?
Clarify the example.
[2014-02-10 Beman Dawes suggests wording]
[2014-02-13 LWG/SG-3 Issaquah: Proposed wording accepted.]
Proposed resolution:
Change 15.36 [fs.op.system_complete]:
[Example: For POSIX based operating systems,system_complete(p)
has the same semantics as.
completeabsolute(p, current_path())
unique_path()
is a security vulnerabilitySection: 15 [filesys.ts::fs.op.funcs] Status: TS Submitter: CH-19 Opened: 2014-01-20 Last modified: 2017-07-30
Priority: Not Prioritized
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Discussion:
Addresses: filesys.ts
unique_path()
is a security vulnerability. As the Linux manual page for the similar
function tmpnam()
writes in the "BUGS" section: "Never use this function. Use mkstemp
(3)
or tmpfile
(3) instead." mkstemp()
and tmpfile()
avoid the inherent race condition of
unique_path()
by returning an open file descriptor or FILE*
.
There are two issues here:
- Confusion over what unique_path does and how it is used. The function is misleadingly named. These issue have arisen in the past, but apparently not been fully corrected. The suggested fix is to (1) rename the function and (2) provide an example of how to use the function safely with fstreams or even C I/O. See below for proposed wording.
- Very real security concerns. See 2654(i). The security concerns are probably best dealt with in the next File System TS, since a full-blown proposal is needed and will likely take several years to develop.
[ 2014-02-11 Issaquah: Strike the function. ]
[2014-02-12 The following Proposed resolution from CH-19 was moved here to avoid confusion with the final Proposed resolution wording from the WG/SG3.]
Remove this function. Consider providing a function create_unique_directory()
.
If it fits the scope of the proposed TS, consider providing functions
create_unique_file()
that returns ifstream
, ofstream
and iofstream
.
[ 2014-02-12 The following Proposed wording was moved here to avoid confusion with the final Proposed resolution wording from the WG/SG3. ]
[2014-02-10 Beman Dawes]
Previous resolution from Beman [SUPERSEDED]:
Change 15.38 [fs.op.unique_path]:
pathunique_pathgenerate_random_filename(const path& model="%%%%-%%%%-%%%%-%%%%"); pathunique_pathgenerate_random_filename(const path& model, error_code& ec);The
function generates a name suitable for temporary files, including directories. The name is based on a model that uses the percent sign character to specify replacement by a random hexadecimal digit.
unique_pathgenerate_random_filename[Note: The more bits of randomness in the generated name, the less likelihood of prior existence or being guessed. Each replacement hexadecimal digit in the model adds four bits of randomness. The default model thus provides 64 bits of randomness. --end note]
Returns: A path identical to
model
, except that each occurrence of the percent sign character is replaced by a random hexadecimal digit character in the range 0-9, a-f. The signature with argumentec
returnspath()
if an error occurs.Throws: As specified in Error reporting.
Remarks: Implementations are encouraged to obtain the required randomness via a cryptographically secure pseudo-random number generator, such as one provided by the operating system. [Note: Such generators may block until sufficient entropy develops. --end note]
Replace this example with one that opens a std::ofstream:
[Example:cout <<unique_pathgenerate_random_filename("test-%%%%%%%%%%%.txt") << endl;Typical output would be
"test-0db7f2bf57a.txt"
. Because 11 hexadecimal output characters are specified, 44 bits of randomness are supplied. -- end example]
Proposed resolution:
Remove the twounique_path
function signatures from 6 [fs.filesystem.synopsis].
Remove 15.38 [fs.op.unique_path] in its entirety.
[This removes all references the function from the working draft.]
enum class directory_options
has no summary
Section: 6 [filesys.ts::fs.filesystem.synopsis] Status: TS Submitter: P.J. Plauger Opened: 2014-01-30 Last modified: 2017-07-30
Priority: Not Prioritized
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Discussion:
Addresses: filesys.ts
enum class directory_options
has no summary.
Proposed resolution:
Change 6 [fs.filesystem.synopsis]:
enum class directory_options;{none,follow_directory_symlink,skip_permission_denied};
Add the following sub-section:
10.4 Enum class
directory_options
[enum.directory_options]The
enum class
typedirectory_options
is a bitmask type (C++11 §17.5.2.1.3) that specifies bitmask constants used to identify directory traversal options.
Name Value Meaning none
0
(Default) Skip directory symlinks, permission denied is error. follow_directory_symlink
1
Follow rather than skip directory symlinks. skip_permission_denied
2
Skip directories that would otherwise result in permission denied errors.
directory_options::skip_permission_denied
is not usedSection: 6 [filesys.ts::fs.filesystem.synopsis] Status: TS Submitter: P.J. Plauger Opened: 2014-01-30 Last modified: 2017-07-30
Priority: Not Prioritized
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Discussion:
Addresses: filesys.ts
directory_options::skip_permission_denied
is not used.
[2014-04-13 Beman: skip_permissions_denied
not being used is a
symptom of a more serious problem; two directory_itorator
constructors
are missing directory_options
arguments and a description of how they
are used. Proposed wording provided.]
[17 Jun 2014 LWG requests two signatures rather than one with default argument. Beman updates wording.]
Proposed resolution:
Change 13 [class.directory_iterator]:
directory_iterator() noexcept; explicit directory_iterator(const path& p); directory_iterator(const path& p, directory_options options); directory_iterator(const path& p, error_code& ec) noexcept; directory_iterator(const path& p, directory_options options, error_code& ec) noexcept; directory_iterator(const directory_iterator&) = default; directory_iterator(directory_iterator&&) = default; ~directory_iterator();
Change 13.1 directory_iterator members [directory_iterator.members]:
explicit directory_iterator(const path& p); directory_iterator(const path& p, directory_options options); directory_iterator(const path& p, error_code& ec) noexcept; directory_iterator(const path& p, directory_options options, error_code& ec) noexcept;Effects: For the directory that
p
resolves to, constructs an iterator for the first element in a sequence ofdirectory_entry
elements representing the files in the directory, if any; otherwise the end iterator.However, if
options & directory_options::skip_permissions_denied != directory_options::none
and construction encounters an error indicating that permission to accessp
is denied, constructs the end iterator and does not report an error.
Change 14
Class recursive_directory_iterator
[class.rec.dir.itr]
:
explicit recursive_directory_iterator(const path& p,directory_options options = directory_options::none); recursive_directory_iterator(const path& p, directory_options options); recursive_directory_iterator(const path& p, directory_options options, error_code& ec) noexcept; recursive_directory_iterator(const path& p, error_code& ec) noexcept;
Change 14.1 recursive_directory_iterator members [rec.dir.itr.members]:
explicit recursive_directory_iterator(const path& p,directory_options options = directory_options::none); recursive_directory_iterator(const path& p, directory_options options); recursive_directory_iterator(const path& p, directory_options options, error_code& ec) noexcept; recursive_directory_iterator(const path& p, error_code& ec) noexcept;Effects: Constructs a iterator representing the first entry in the directory
p
resolves to, if any; otherwise, the end iterator.However, if
options & directory_options::skip_permissions_denied != directory_options::none
and construction encounters an error indicating that permission to accessp
is denied, constructs the end iterator and does not report an error.
Change 14.1 recursive_directory_iterator members [rec.dir.itr.members]:
recursive_directory_iterator& operator++(); recursive_directory_iterator& increment(error_code& ec);Requires:
*this != recursive_directory_iterator()
.Effects: As specified by C++11 § 24.1.1 Input iterators, except that:
- If there are no more entries at this depth, then if
depth()!= 0
iteration over the parent directory resumes; otherwise*this = recursive_directory_iterator()
.- Otherwise if
recursion_pending() && is_directory(this->status()) && (!is_symlink(this->symlink_status()) || (options() & directory_options::follow_directory_symlink) !=
then either directory0directory_options::none)(*this)->path()
is recursively iterated into or, ifoptions() & directory_options::skip_permissions_denied != directory_options::none
and an error occurs indicating that permission to access directory(*this)->path()
is denied, then directory(*this)->path()
is treated as an empty directory and no error is reported .
copy_options::copy_symlinks
is not usedSection: 10.2 [filesys.ts::enum.copy_options] Status: TS Submitter: P.J. Plauger Opened: 2014-01-30 Last modified: 2017-07-30
Priority: Not Prioritized
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Discussion:
Addresses: filesys.ts
copy_options::copy_symlinks
is not used (should test it before calling copy_symlinks
in copy
).
[20 May 2014 Beman Dawes provides proposed wording.]
Proposed resolution:
Change 15.3 Copy [fs.op.copy]:
If
is_symlink(f)
, then:
- If
options & copy_options::skip_symlinks
, then return. Missing != copy_options::none fixed by issue 59- Otherwise if
!exists(t) && (options & copy_options::copy_symlinks) != copy_options::none
, thencopy_symlink(from, to, options)
.- Otherwise report an error as specified in Error reporting (7).
error_code
arguments should be noexcept
Section: 15.2 [filesys.ts::fs.op.canonical], 15.11 [filesys.ts::fs.op.current_path], 15.27 [filesys.ts::fs.op.read_symlink], 15.36 [filesys.ts::fs.op.system_complete], 15.37 [filesys.ts::fs.op.temp_dir_path], 99 [filesys.ts::fs.op.unique_path], 8.4 [filesys.ts::path.member] Status: TS Submitter: P.J. Plauger Opened: 2014-01-30 Last modified: 2017-07-30
Priority: Not Prioritized
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Discussion:
Addresses: filesys.ts
all functions with error_code
arguments should be noexcept
(see canonical
, current_path
, read_symlink
, system_complete
,
temp_directory_path
, unique_path
, plus member functions).
[2014-02-03: Stephan T. Lavavej comments:]
The declaration and definition of
"recursive_directory_iterator& increment(error_code& ec);
" should almost certainly
be marked noexcept.
[2014-02-08: Daniel comments]
All functions that return a path
value such as canonical
, current_path
,
read_symlink
, system_complete
, temp_directory_path
, unique_path
,
or any member function returning a path
value or basic_string
value might legally
throw an exception, if the allocator of the underlying string complains about insufficient
memory. This is an anti-pattern for noexcept
, because the exception-specification includes
the return statement of a function and given the fact that the language currently does declare RVO
as an optional action, we cannot rely on it.
noexcept
,
if this case can happen, see e.g. std::locale::name()
. To the contrary, enforcing them to be
noexcept
, would cause a similar problem as the unconditional noexcept
specifier of
the value-returning kill_dependency
as denoted by LWG 2236(i).
Basically this requirement conflicts with the section "Adopted Guidelines", second bullet of
N3279.
[Beman Dawes 2014-02-27]
Issues 2637(i), 2638(i), 2641(i), and 2649(i) are concerned with signatures which should or should not benoexcept
. I will provide unified proposed wording for these issues, possibly in a separate paper.
[21 May 2014 Beman Dawes reviewed all functions with error_code&
arguments,
and provided proposed wording for the one case found where the above guidelines were not met.
This was the case previously identified by Stephan T. Lavavej.
]
Proposed resolution:
Change 14 Class recursive_directory_iterator [class.rec.dir.itr]:
recursive_directory_iterator& increment(error_code& ec) noexcept;
Change 14.1 recursive_directory_iterator members [rec.dir.itr.members]:
recursive_directory_iterator& increment(error_code& ec) noexcept;
class directory_entry
should retain operator const path&()
from V2Section: 12 [filesys.ts::class.directory_entry] Status: TS Submitter: P.J. Plauger Opened: 2014-01-30 Last modified: 2017-07-30
Priority: Not Prioritized
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Discussion:
Addresses: filesys.ts
class directory_entry
should retain operator const path&()
from V2.
[2014-05-19 Beman Dawes supplied proposed wording.]
[2014-06-02 Beman Dawes provides rationale:]
This conversion operator was removed whenstatus()
andsymlink_status()
were added during the transition from V2 to V3 as it seemed a bit unusual to have a conversion operator for one of the several values held. Users complained as they had found the automatic conversion convenient in the most commondirectory_entry
use case.
[17 Jun 2014 Rapperswil LWG discusses in depth, accepts proposed wording.]
Proposed resolution:
Change 12 Class directory_entry [class.directory_entry]:
// observers const path& path() const noexcept; operator const path&() const noexcept; file_status status() const; file_status status(error_code& ec) const noexcept; file_status symlink_status() const; file_status symlink_status(error_code& ec) const noexcept;
Change 12.3 directory_entry observers [directory_entry.obs]:
const path& path() const noexcept; operator const path&() const noexcept;Returns:
m_path
directory_iterator
, recursive_directory_iterator
, move construct/assign should be noexcept
Section: 13 [filesys.ts::class.directory_iterator], 14 [filesys.ts::class.rec.dir.itr] Status: TS Submitter: P.J. Plauger Opened: 2014-01-30 Last modified: 2017-07-30
Priority: Not Prioritized
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Discussion:
Addresses: filesys.ts
class directory_iterator
move construct/assign should be noexcept
.
class recursive_directory_iterator
move construct/assign should be noexcept
.
[Beman Dawes 2014-02-27]
Issues 2637(i), 2638(i), 2641(i), and 2649(i) are concerned with signatures which should or should not benoexcept
. I will provide unified proposed wording for these issues, possibly in a separate paper.
[Daniel Krügler 2014-02-28]
directory_iterator begin(directory_iterator iter) noexcept; directory_iterator end(const directory_iterator&) noexcept;are noexcept, but we have no guarantee that at least the move-constructor is noexcept:
directory_iterator(const directory_iterator&) = default; directory_iterator(directory_iterator&&) = default;This means that either the above noexcept specifications are not warranted or that at least the move-constructor of
directory_iterator
is required to be noexcept.
The same applies to recursive_directory_iterator
.
[21 May 2014 Beman Dawes provided proposed resolution wording.]
[18 Jun 2014 "= default" removed per LWG discussion]
Proposed resolution:
Change 13 Class directory_iterator [class.directory_iterator]:
To 13.1 directory_iterator members [directory_iterator.members] add:directory_iterator(const directory_iterator& rhs)= default; directory_iterator(directory_iterator&& rhs) noexcept= default; ... directory_iterator& operator=(const directory_iterator& rhs)= default; directory_iterator& operator=(directory_iterator&& rhs) noexcept= default;
directory_iterator(const directory_iterator& rhs); directory_iterator(directory_iterator&& rhs) noexcept;Effects: Constructs an object of class
directory_iterator
.Postconditions:
*this
has the original value ofrhs
.directory_iterator& operator=(const directory_iterator& rhs); directory_iterator& operator=(directory_iterator&& rhs) noexcept;Effects: If
*this
andrhs
are the same object, the member has no effect.Postconditions:
Returns:*this
has the original value ofrhs
.*this
.
Change 14 Class recursive_directory_iterator [class.rec.dir.itr]:
To 14.1 recursive_directory_iterator members [rec.dir.itr.members] add:recursive_directory_iterator(const recursive_directory_iterator& rhs)= default; recursive_directory_iterator(recursive_directory_iterator&& rhs) noexcept= default; ... recursive_directory_iterator& operator=(const recursive_directory_iterator& rhs)= default; recursive_directory_iterator& operator=(recursive_directory_iterator&& rhs) noexcept= default;
recursive_directory_iterator(const recursive_directory_iterator& rhs);Effects: Constructs an object of class
recursive_directory_iterator
.Postconditions:
this->options() == rhs.options() && this->depth() == rhs.depth() && this->recursion_pending() == rhs.recursion_pending()
.recursive_directory_iterator(recursive_directory_iterator&& rhs) noexcept;Effects: Constructs an object of class
recursive_directory_iterator
.Postconditions:
this->options()
,this->depth()
, andthis->recursion_pending()
return the values thatrhs.options()
,rhs.depth()
, andrhs.recursion_pending()
, respectively, had before the function call.recursive_directory_iterator& operator=(const recursive_directory_iterator& rhs);Effects: If
*this
andrhs
are the same object, the member has no effect.Postconditions:
Returns:this->options() == rhs.options() && this->depth() == rhs.depth() && this->recursion_pending() == rhs.recursion_pending()
.*this
.recursive_directory_iterator& operator=(recursive_directory_iterator&& rhs) noexcept;Effects: If
*this
andrhs
are the same object, the member has no effect.Postconditions:
Returns:this->options()
,this->depth()
, andthis->recursion_pending()
return the values thatrhs.options()
,rhs.depth()
, andrhs.recursion_pending()
, respectively, had before the function call.*this
.
copy_options
and perms
should be bitmask typesSection: 10.2 [filesys.ts::enum.copy_options], 10.3 [filesys.ts::enum.perms] Status: TS Submitter: P.J. Plauger Opened: 2014-01-30 Last modified: 2017-07-30
Priority: Not Prioritized
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Discussion:
Addresses: filesys.ts
enum classes copy_options
and perms
should be bitmask types.
[2014-02-08 Daniel comments and suggests wording]
Since both enum types contain enumeration values that denote the value zero, they are currently not allowed to be described as bitmask types. This issue depends on LWG issue 1450, which — if it would be accepted — would allow for applying this concept to these enumeration types.
[2014-02-13 LWG/SG-3 Issaquah: Proposed wording accepted.]
Proposed resolution:
This wording is relative to SG3 working draft and assumes that LWG 1450 has been resolved.
Change [enum.copy_options] as indicated:
This enumerationTheenum class
typecopy_options
is a bitmask type (C++11 §17.5.2.1.3) that specifies bitmask constants used to control the semantics of copy operations. The constants are specified in option groups. […]
Change [enum.perms] as indicated:
This enumerationTheenum class
typeperms
is a bitmask type (C++11 §17.5.2.1.3) that specifies bitmask constants used to identify file permissions.
create_directory
should refer to perms::all
instead of Posix
S_IRWXU|S_IRWXG|S_IRWXO
Section: 15.7 [filesys.ts::fs.op.create_directory] Status: TS Submitter: P.J. Plauger Opened: 2014-01-30 Last modified: 2017-07-30
Priority: Not Prioritized
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Discussion:
Addresses: filesys.ts
create_directory
should refer to perms::all
instead of the Posix
S_IRWXU|S_IRWXG|S_IRWXO
.
[2014-02-28 Beman provided proposed wording.]
Proposed resolution:
Effects: Establishes the postcondition by attempting to create the
directory p
resolves to, as if by POSIX
mkdir()
with a second argument of S_IRWXU|S_IRWXG|S_IRWXO static_cast<int>(perms::all)
. Creation
failure because p
resolves to an existing directory shall not be
treated as an error.
last_write_time()
uses ill-formed castSection: 15.25 [filesys.ts::fs.op.last_write_time] Status: TS Submitter: P.J. Plauger Opened: 2014-01-30 Last modified: 2017-07-30
Priority: Not Prioritized
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Discussion:
Addresses: filesys.ts
In last_write_time
, static_cast<file_time_type>(-1)
is ill formed,
(possibly should be chrono<unspecified-trivial-clock>::from_time_t(-1))
.
[2014-02-08 Daniel comments and provides wording]
I agree that the current wording needs to be fixed, but it is unclear to me whether invoking
from_time_t
(which is only guaranteed to exist for std::chrono::system_clock
)
with a value of time_t(-1)
is a well-defined operation. Reason is that the C Standard
makes a difference between a calendar time and the value time_t(-1)
as an error
indication. But the specification of system_clock::from_time_t
only says "A time_point
object that represents the same point in time as t
[…]" and it is not clear whether
time_t(-1)
can be considered as a "point in time". Instead of relying on such a potentially
subtle semantics of the conversion result of time_t(-1)
to std::chrono::system_clock::time_point
with a further conversion to file_time_type
(which in theory could led to overflow or underflow and
thus another potential source of undefined behaviour), I suggest to change the current error value of
last_write_time
to one of the well-define limiting values of file_time_type
, e.g.
file_time_type::min()
.
[2014-02-13 LWG/SG-3 Issaquah: Proposed wording accepted.]
Proposed resolution:
This wording is relative to SG3 working draft.
Change the last write time prototype specification, 15.25 [fs.op.last_write_time], as indicated:
file_time_type last_write_time(const path& p); file_time_type last_write_time(const path& p, error_code& ec) noexcept;Returns: The time of last data modification of
p
, determined as if by the value of the POSIXstat
structure memberst_mtime
obtained as if by POSIXstat()
. The signature with argumentec
returnsif an error occurs.
static_cast<file_time_type>(-1)file_time_type::min()
path::template<class charT>string()
conversion rulesSection: 8.4.6 [filesys.ts::path.native.obs] Status: TS Submitter: P.J. Plauger Opened: 2014-01-30 Last modified: 2017-07-30
Priority: Not Prioritized
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Discussion:
Addresses: filesys.ts
path::template<class charT>string()
should promise to convert by the same
rules as u16string
for string<char16_t>
, etc. and one-for-one otherwise.
charT
is signed char
(or even float
)? I don't see where that choice
is disallowed, so we should either disallow it or make it be something that is quasi-sensible.
[2014-02-08 Daniel comments]
There are two relevant places in the wording that seem to clarify what is required here:
Most importantly, 5 [fs.req] says:
Throughout this Technical Specification,
Template parameters namedchar
,wchar_t
,char16_t
, andchar32_t
are collectively called encoded character types.charT
shall be one of the encoded character types. […] [Note: Use of an encoded character type implies an associated encoding. Sincesigned char
andunsigned char
have no implied encoding, they are not included as permitted types. — end note]
For both the mentioned string
function template and the specific non-template functions (such as u16string()
)
the same Remarks
element exists, that refers to character conversion:
Conversion, if any, is performed as specified by 8.2 [path.cvt].
The first quote excludes arbitrary types such as signed char
or float
as feasible template arguments.
The second quote makes clear that both the function template and the non-template functions do normatively refer to the same
wording in 8.2 [path.cvt].
charT
in this technical
specification, because 5 [fs.req] assigns a very special meaning to the constraints on charT
types that does not
exist to this extend in the rest of the standard library. A similar approach is used in Clause 22, where a special symbol C
is used for constrained character types (C++11 §22.3.1.1.1 [locale.category]):
A template formal parameter with name
C
represents the set of types containingchar
,wchar_t
, and any other implementation-defined character types that satisfy the requirements for a character on which any of theiostream
components can be instantiated.
[2014-02-28 Beman provides proposed resolution wording]
[18 Jun 2014 Beman changes proposed name from C to EcharT in response to tentative vote concern that C was insuficiently informative as a name.]
Proposed resolution:
In the entire Working Paper, except in the synopsis for path inserter and extractor and 8.6.1 path inserter and extractor [path.io],
change each instance of "charT
" to "EcharT
".
For example, in 5 [fs.req]:
Template parameters namedcharT
EcharT
shall be one of the encoded character types.
path
and directory_entry
move ctors should not be noexcept
Section: 8.4.1 [filesys.ts::path.construct], 12 [filesys.ts::class.directory_entry] Status: TS Submitter: Stephan T. Lavavej Opened: 2014-02-03 Last modified: 2017-07-30
Priority: Not Prioritized
View all other issues in [filesys.ts::path.construct].
View all issues with TS status.
Discussion:
Addresses: filesys.ts
path
's move ctor is marked noexcept
, but it contains a basic_string
.
Similarly, directory_entry
's move ctor is marked noexcept
, but it contains a path
.
This is affected by LWG 2319 "basic_string
's move constructor should not be noexcept
".
[Beman Dawes 2014-02-27]
Issues 2637(i), 2638(i), 2641(i), and 2649(i) are concerned with signatures which should or should not benoexcept
. I will provide unified proposed wording for these issues, possibly in a separate paper.
[21 May 2014 Beman Dawes provides wording. See 2319(i) for rationale. ]
[19 Jun 2014 LWG revises P/R to stay consistent with LWG issues.]
Proposed resolution:
Change 8 Class path [class.path]:Change 8.4.1 path constructors [path.construct]:path() noexcept;
Change 12 Class directory_entry [class.directory_entry]:path() noexcept;
directory_entry() noexcept = default;
path::compare(const string& s)
wrong argument typeSection: 8 [filesys.ts::class.path] Status: TS Submitter: Stephan T. Lavavej Opened: 2014-02-03 Last modified: 2017-07-30
Priority: Not Prioritized
View all other issues in [filesys.ts::class.path].
View all issues with TS status.
Discussion:
Addresses: filesys.ts
path
has "int compare(const string& s) const;
" but it should almost certainly take
const string_type&
, since the third overload takes const value_type*
.
[2014-02-08 Daniel comments and provides wording]
This issue is the same as 2643(i) and resolves that issue as well.
[2014-02-13 LWG/SG-3 Issaquah: Proposed wording accepted.]
Proposed resolution:
This wording is relative to SG3 working draft.
Change class path
synopsis, 8 [class.path], as indicated:
// compare int compare(const path& p) const noexcept; int compare(const string_type& s) const; int compare(const value_type* s) const;
Change path compare
prototype specification, 8.4.8 [path.compare], as indicated:
int compare(const string_type& s) const;
std::iterator
Section: 13 [filesys.ts::class.directory_iterator], 14 [filesys.ts::class.rec.dir.itr] Status: TS Submitter: Stephan T. Lavavej Opened: 2014-02-03 Last modified: 2017-07-30
Priority: Not Prioritized
View all other issues in [filesys.ts::class.directory_iterator].
View all issues with TS status.
Discussion:
Addresses: filesys.ts
Although the Standard has made this mistake almost a dozen times, I recommend not
depicting directory_iterator
and recursive_directory_iterator
as deriving from
std::iterator
, since that's a binding requirement on implementations.
Instead they should be depicted as having the appropriate typedefs, and leave it up to
implementers to decide how to provide them. (The difference is observable to users with
is_base_of
, not that they should be asking that question.)
[2014-02-08 Daniel comments and provides wording]
This issue is basically similar to the kind of solution that had been used to remove the
requirement to derive from unary_function
and friends as described by
N3198 and
I'm strongly in favour to follow that spirit here as well. I'd like to add that basically all
"newer" iterator types (such as the regex
related iterator don't derive from
std::iterator
either.
pointer
and reference
would be determined to value_type*
and value_type&
(that is mutable pointers and references), which conflicts with the specification
of the return types of the following members:
const directory_entry& operator*() const; const directory_entry* operator->() const;
The proposed fording fixes this by correcting these typedef
to corresponding const
access.
[2014-02-13 LWG/SG-3 Issaquah: Proposed wording accepted.]
Proposed resolution:
This wording is relative to SG3 working draft.
Change class directory_iterator
synopsis, [class.directory_iterator], as indicated:
namespace std { namespace tbd { namespace filesystem { class directory_iterator: public iterator<input_iterator_tag, directory_entry>{ public: typedef directory_entry value_type; typedef ptrdiff_t difference_type; typedef const directory_entry* pointer; typedef const directory_entry& reference; typedef input_iterator_tag iterator_category; // member functions […] }; } } } // namespaces std::tbd::filesystem
Change class recursive_directory_iterator
synopsis, [class.rec.dir.itr], as indicated:
namespace std { namespace tbd { namespace filesystem { class recursive_directory_iterator: public iterator<input_iterator_tag, directory_entry>{ public: typedef directory_entry value_type; typedef ptrdiff_t difference_type; typedef const directory_entry* pointer; typedef const directory_entry& reference; typedef input_iterator_tag iterator_category; // constructors and destructor […] // modifiers recursive_directory_iterator& operator=(const recursive_directory_iterator&) = default; recursive_directory_iterator& operator=(recursive_directory_iterator&&) = default; const directory_entry& operator*() const; const directory_entry* operator->() const; recursive_directory_iterator& operator++(); recursive_directory_iterator& increment(error_code& ec); […] }; } } } // namespaces std::tbd::filesystem
directory_entry
multithreading concernsSection: 12 [filesys.ts::class.directory_entry] Status: TS Submitter: Stephan T. Lavavej Opened: 2014-02-03 Last modified: 2017-07-30
Priority: Not Prioritized
View all other issues in [filesys.ts::class.directory_entry].
View all issues with TS status.
Discussion:
Addresses: filesys.ts
12 [class.directory_entry] depicts directory_entry
as having mutable m_status
and
m_symlink_status
data members. This is problematic because of C++11's multithreading
guarantees, which say that const
member functions are simultaneously callable.
As a result, mutable
data members must be protected with synchronization, which was
probably not considered during this design. The complexity and expense may be acceptable
in directory_entry
(since it can save filesystem queries, apparently) but it would be
very very nice to have a note about this.
[2014-02-13 LWG/SG-3 discussed in Issaquah: Beman and STL will work together to better understand the concerns, and to formulate a solution. This is not a blocking issue for finishing the TS, but is a blocking issue for merging into the IS]
[2014-04-17 Beman provides proposed wording]
[2014-06-19 Rapperswil LWG decides to remove mutable private members. Beman provides wording.]
Proposed resolution:
Change 12 [class.directory_entry] as indicated:namespace std { namespace experimental { namespace filesystem { inline namespace v1 { class directory_entry { public: // constructors and destructor directory_entry() = default; directory_entry(const directory_entry&) = default; directory_entry(directory_entry&&) noexcept = default; explicit directory_entry(const path& p);explicit directory_entry(const path& p, file_status st=file_status(),file_status symlink_st=file_status());~directory_entry(); // modifiers directory_entry& operator=(const directory_entry&) = default; directory_entry& operator=(directory_entry&&) noexcept = default; void assign(const path& p);void assign(const path& p, file_status st=file_status(),file_status symlink_st=file_status()); void replace_filename(const path& p);void replace_filename(const path& p, file_status st=file_status(),file_status symlink_st=file_status()); // observers const path& path() const noexcept; file_status status() const; file_status status(error_code& ec) const noexcept; file_status symlink_status() const; file_status symlink_status(error_code& ec) const noexcept; bool operator< (const directory_entry& rhs) const noexcept; bool operator==(const directory_entry& rhs) const noexcept; bool operator!=(const directory_entry& rhs) const noexcept; bool operator<=(const directory_entry& rhs) const noexcept; bool operator> (const directory_entry& rhs) const noexcept; bool operator>=(const directory_entry& rhs) const noexcept; private: path m_path; // for exposition onlymutable file_status m_status; // for exposition only; stat()-likemutable file_status m_symlink_status; // for exposition only; lstat()-like}; } } } } // namespaces std::experimental::filesystem::v1
A directory_entry
object stores a path
object,
a .
file_status
object for non-symbolic link status, and a file_status
object for symbolic link status
The
file_status
objects act as value caches.
[Note: Becausestatus()
on a pathname may be a relatively expensive operation, some operating systems provide status information as a byproduct of directory iteration. Caching such status information can result in significant time savings. Cached and non-cached results may differ in the presence of file system races. —end note]
directory_entry
constructors
[directory_entry.cons]
Add the description of this constructor in its entirety:
explicit directory_entry(const path& p);
Effects: Constructs an object of typedirectory_entry
Postcondition:
path() == p
.
explicit directory_entry(const path& p, file_status st=file_status(),file_status symlink_st=file_status());
Strike the description
directory_entry
modifiers
[directory_entry.mods]
Add the description of this function in its entirety:
void assign(const path& p);
Postcondition:
path() == p
.
void assign(const path& p, file_status st=file_status(),file_status symlink_st=file_status());
Strike the description
Add the description of this function in its entirety:
void replace_filename(const path& p);
Postcondition:
path() == x.parent_path() / p
wherex
is the value ofpath()
before the function is called.
void replace_filename(const path& p, file_status st=file_status(),file_status symlink_st=file_status());
Strike the description
directory_entry
observers
[directory_entry.obs]
const path& path() const noexcept;
Returns:
m_path
file_status status() const; file_status status(error_code& ec) const noexcept;
Effects: As if,if (!status_known(m_status)) { if (status_known(m_symlink_status) && !is_symlink(m_symlink_status)) { m_status = m_symlink_status; } else { m_status = status(m_path[, ec]); } }Returns:
.
m_statusstatus(path()[, ec])Throws: As specified in Error reporting (7).
file_status symlink_status() const; file_status symlink_status(error_code& ec) const noexcept;
Effects: As if,if (!status_known(m_symlink_status)) { m_symlink_status = symlink_status(m_path[, ec]); }Returns:
.
m_symlink_statussymlink_status(path()[, ec])Throws: As specified in Error reporting (7).
bool operator==(const directory_entry& rhs) const noexcept;
Returns:
m_path == rhs.m_path
.
[Note: Status members do not participate in determining equality. — end note]
bool operator!=(const directory_entry& rhs) const noexcept;
Returns:
m_path != rhs.m_path
.
bool operator< (const directory_entry& rhs) const noexcept;
Returns:
m_path < rhs.m_path
.
bool operator<=(const directory_entry& rhs) const noexcept;
Returns:
m_path <= rhs.m_path
.
bool operator> (const directory_entry& rhs) const noexcept;
Returns:
m_path > rhs.m_path
.
bool operator>=(const directory_entry& rhs) const noexcept;
Returns:
m_path >= rhs.m_path
.
Section: 7 [filesys.ts::fs.err.report] Status: TS Submitter: Beman Dawes Opened: 2014-01-20 Last modified: 2017-07-30
Priority: Not Prioritized
View all other issues in [filesys.ts::fs.err.report].
View all issues with TS status.
Discussion:
Addresses: filesys.ts
The proposed change below was suggested in Issaquah as part of the resolution of issue 13 to clarify the Error reporting section. LWG/SG3 liked the change, but since issue 13 was NAD requested that a separate issue be opened.
[2014-02-13 LWG/SG-3 Issaquah: Proposed wording accepted.]
Proposed resolution:
Change 7 [fs.err.report]:
Functions not having an argument of type
error_code&
report errors as follows, unless otherwise specified:
- When a call by the implementation to an operating system or other underlying API results in an error that prevents the function from meeting its specifications, an exception of type
filesystem_error
shall be thrown. For functions with a single path argument, that argument shall be passed to thefilesystem_error
constructor with a single path argument. For functions with two path arguments, the first of these arguments shall be passed to thefilesystem_error
constructor as thepath1
argument, and the second shall be passed as thepath2
argument. Thefilesystem_error
constructor'serror_code
argument is set as appropriate for the specific operating system dependent error.
- Failure to allocate storage is reported by throwing an exception as described in C++11 § 17.6.4.10.
- Destructors throw nothing.
Functions having an argument of type
error_code&
report errors as follows, unless otherwise specified:
- If a call by the implementation to an operating system or other underlying API results in an error that prevents the function from meeting its specifications, the
error_code&
argument is set as appropriate for the specific operating system dependent error. Otherwise,clear()
is called on theerror_code&
argument.
Section: 5 [filesys.ts::fs.req] Status: TS Submitter: Clark Nelson Opened: 2014-02-10 Last modified: 2017-07-30
Priority: Not Prioritized
View all other issues in [filesys.ts::fs.req].
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Discussion:
Addresses: filesys.ts
SD-6: SG10 Feature Test Recommendations recommends that each library header provide feature test macros. For Technical Specifications, the TS itself should specify recommended names for the feature test macros.[Beman Dawes 2014-02-28 provided the proposed resolution. Thanks to Vicente J. Botet Escriba, Richard Smith, and Clark Nelson for suggestions and corrections.]
Proposed resolution:
Add a new sub-section:
5.2 Feature test macros [fs.req.macros]
This macro allows users to determine which version of this Technical Specification is supported by header
<experimental/filesystem>
.Header
<experimental/filesystem>
shall supply the following macro definition:#define __cpp_lib_experimental_filesystem 201406[Note: The value of macro
__cpp_lib_experimental_filesystem
is yyyymm where yyyy is the year and mm the month when the version of the Technical Specification was completed. — end note]
Section: 2.1 [filesys.ts::fs.conform.9945] Status: TS Submitter: LWG/SG-3 Opened: 2014-02-13 Last modified: 2017-07-30
Priority: Not Prioritized
View all other issues in [filesys.ts::fs.conform.9945].
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Discussion:
Addresses: filesys.ts
§ 2.1 [fs.conform.9945] specifes "No diagnostic is required" for races. That prescription is used by the standard for the core language; it is not appropriate in library specifications. LWG/SG-3 wishes this be changed to undefined behavior.[Beman comments: The replacment wording is similar to that used by C++14 17.6.4.10]
Proposed resolution:
Change the following paragraph from § 2.1 [fs.conform.9945]Insert a new sub-section head before the modified paragraph:The behavior of functions described in this Technical Specification may differ from their specification in the presence of file system races ([fs.def.race]). No diagnostic is required.Behavior is undefined if calls to functions provided by this Technical Specification introduce a file system race (4.6 [fs.def.race]).
2.1 File system race behavior [fs.race.behavior]
utime()
is obsolescentSection: 15.25 [filesys.ts::fs.op.last_write_time] Status: TS Submitter: LWG/SG-3 Opened: 2014-02-13 Last modified: 2017-07-30
Priority: Not Prioritized
View all other issues in [filesys.ts::fs.op.last_write_time].
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Discussion:
Addresses: filesys.ts
POSIX now says: "Since the utimbuf
structure only contains time_t
variables
and is not accurate to fractions of a second, applications
should use the utimensat()
function instead of the obsolescent utime()
function."
Suggested resolution: Rewrite the last_write_time
setter description to utilize utimensat() or similar.
[20 May 2014 Beman Dawes supplies proposed wording. He comments:]
POSIX supplies two functions, futimens()
and utimensat()
,
as replacements for utime()
.
futimens()
is appropriate for the current TS.
utimensat()
will be appropriate for a future File System TS that adds the
functionality of the whole family of POSIX *at()
functions.
Proposed resolution:
Change 15.25 Last write time [fs.op.last_write_time]:
Effects: Sets the time of last data modification of the file resolved to by
p
tonew_time
, as if by POSIXstat()
followed by POSIXutime()
futimens()
.
absolute()
Section: 15.1 [filesys.ts::fs.op.absolute] Status: TS Submitter: Daniel Krügler Opened: 2014-02-28 Last modified: 2017-07-30
Priority: Not Prioritized
View all issues with TS status.
Discussion:
Addresses: filesys.ts
The Throws element [fs.op.absolute] says:
"If
base.is_absolute()
is true, throws only if memory allocation fails."
We can strike:
"If
base.is_absolute()
is true," and the wording will still hold. Note that:
- None of the involved functions has requirements.
- In every case potentially memory allocation occurs, even for "
return p
", so this allocation can fail.
[2014-03-02 Beman Dawes comments and provides P/R]
The Throws element should follow the same form as similar Throws elements in the TS. There isn't enough special about this function to justify special wording and by referencing Error reporting (7) we ensureabsolute()
follows
the overall policy for handling memory allocation failures.
Proposed resolution:
[Change 15.1 [fs.op.absolute]:]
Throws:
IfAs specified in Error reporting (7).base.is_absolute()
is true, throws only if memory allocation fails.
Section: 5 [filesys.ts::fs.req] Status: TS Submitter: Daniel Krügler Opened: 2014-05-19 Last modified: 2017-07-30
Priority: Not Prioritized
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Discussion:
Addresses: filesys.ts
The SG3 WP refers in several places to a template parameter named "Allocator" but does not impose requirements on this type.
Proposed resolution:
Add the following to 5 Requirements [fs.req]:
Template parameters named
Allocator
shall meet the C++ Standard's library Allocator requirements (C++11 §17.6.3.5)
directory_entry
Section: 31.12.13.15 [fs.op.file.size] Status: Resolved Submitter: Gor Nishanov Opened: 2014-05-22 Last modified: 2021-06-06
Priority: 2
View all issues with Resolved status.
Discussion:
On Windows, the
structure, which is the underlying data type for FindFileData WIN32_FIND_DATAdirectory_entry
, contains the file size as one of the fields.
Thus efficient enumeration of files and getting their sizes is possible without doing a separate query for the file size.
[17 Jun 2014 Rapperswil LWG will investigate issue at a subsequent meeting.]
[23 Nov 2015 Editorally correct name of data structure mentioned in discussion.]
[Mar 2016 Jacksonville Beman to provide paper about this]
[Apr 2016 Issue updated to address the C++ Working Paper. Previously addressed File System TS]
Previous resolution [SUPERSEDED]
In [fs.class.directory_entry] Class
directory_entry
add the following observer declarations:uintmax_t file_size(); uintmax_t file_size(error_code& ec) noexcept;In
directory_entry
observers 31.12.10.4 [fs.dir.entry.obs] add the following:uintmax_t file_size(); uintmax_t file_size(error_code& ec) noexcept;Returns: if
*this
contains a cached file size, return it. Otherwise returnfile_size(path())
orfile_size(path(), ec)
respectively.Throws: As specified in Error reporting (7).
[2016-08, Beman comments]
This will be resolved by P0317R1, Directory Entry Caching for Filesystem.
Fri AM: Moved to Tentatively Resolved
Proposed resolution:
operator/
(and other append) semantics not useful if argument has rootSection: 31.12.6.5.3 [fs.path.append], 31.12.6.8 [fs.path.nonmember] Status: C++17 Submitter: Peter Dimov Opened: 2014-05-30 Last modified: 2017-07-30
Priority: 2
View all other issues in [fs.path.append].
View all issues with C++17 status.
Discussion:
In a recent discussion on the Boost developers mailing list, the semantics of operator /
and other append operations were questioned:
p1 / p2
is required to concatenate the
lexical representation of p1
and p2
, inserting a
preferred separator as needed.
This means that, for example, "c:\x" / "d:\y"
gives
"c:\x\d:\y"
, and that "c:\x" / "\\server\share"
gives "c:\x\\server\share"
. This is rarely, if ever, useful.
An alternative interpretation of p1 / p2
could be that it yields a
path that is the approximation of what p2
would mean if interpreted
in an environment in which p1
is the starting directory.
Under this interpretation, "c:\x" / "d:\y"
gives "d:\y"
,
which is more likely to match what was intended.
I am not saying that this second interpretation is the right one, but I do say
that we have reasons to suspect that the first one (lexical concatenation using
a separator) may not be entirely correct.
This leads me to think that the behavior of p1 / p2
, when p2
has a root, needs to be left implementation-defined, so that implementations are
not required to do the wrong thing, as above.
This change will not affect the ordinary use case in which p2
is a
relative, root-less, path.
[17 Jun 2014 Rapperswil LWG will investigate issue at a subsequent meeting.]
[2016-02, Jacksonville]
Beman to provide wording.
[2016-06-13, Beman provides wording and rationale]
Rationale: The purpose of the append operations is to provide a simple concatenation facility for users
wishing to extend a path by appending one or more additional elements, and to do so without worrying about the
details of when a separator is needed. In that context it makes no sense to provide an argument that has a
root-name. The simplest solution is simply to require !p.has_root_name()
.
The other suggested solutions IMO twist the functions into something harder to reason about
yet any advantages for users are purely speculative. The concatenation functions can
be used instead for corner cases.
[Apr 2016 Issue updated to address the C++ Working Paper. Previously addressed File System TS]
[2016-07-03, Daniel comments]
The same wording area is touched by LWG 2732(i).
Previous resolution [SUPERSEDED]:
This wording is relative to N4594.
Change 31.12.6.5.3 [fs.path.append] path appends as indicated:
path& operator/=(const path& p);
-?- Requires:
!p.has_root_name()
.-2- Effects: Appends
path::preferred_separator
topathname
unless:
an added directory-separator would be redundant, or
an added directory-separator would change a relative path to an absolute path [Note: An empty path is relative. — end note], or
p.empty()
istrue
, or
*p.native().cbegin()
is a directory-separator.Then appends
p.native()
topathname
.-3- Returns:
*this
.template <class Source>
path& operator/=(const Source& source);
template <class Source>
path& append(const Source& source);
template <class InputIterator>
path& append(InputIterator first, InputIterator last);-?- Requires:
-4- Effects: Appends!source.has_root_name()
or!*first.has_root_name()
, respectively.path::preferred_separator
topathname
, converting format and encoding if required (31.12.6.3 [fs.path.cvt]), unless:
an added directory-separator would be redundant, or
an added directory-separator would change a relative path to an absolute path, or
source.empty()
istrue
, or
*source.native().cbegin()
is a directory-separator.Then appends the effective range of
-5- Returns:source
(31.12.6.4 [fs.path.req]) or the range[first, last)
topathname
, converting format and encoding if required (31.12.6.3 [fs.path.cvt]).*this
.Change 31.12.6.8 [fs.path.nonmember] path non-member functions as indicated:
path operator/(const path& lhs, const path& rhs);
-?- Requires:
!rhs.has_root_name()
.-13- Returns:
path(lhs) /= rhs
.
[2016-08-03 Chicago]
After discussion on 2732(i), it was determined that the PR for that issue should be applied to this issue before it is accepted. That PR changes all the path appends to go through operator/=, so only one requires element remains necessary.
Fri AM: Moved to Tentatively Ready
Proposed resolution:
This wording is relative to N4606, and assumes that the PR for 2732(i) is applied.
path& operator/=(const path& p);
-?- Requires:
!p.has_root_name()
.-2- Effects: Appends
path::preferred_separator
topathname
unless:
- — an added directory-separator would be redundant, or
- — an added directory-separator would change a relative path to an absolute path [Note: An empty path is relative. — end note], or
- —
p.empty()
istrue
, or- —
*p.native().cbegin()
is a directory-separator.Then appends
p.native()
topathname
.-3- Returns:
*this
.
path operator/(const path& lhs, const path& rhs);
-13-
ReturnsEffects: Equivalent toreturn path(lhs) /= rhs;
.
remove_filename()
post condition is incorrectSection: 31.12.6.5.5 [fs.path.modifiers] Status: Resolved Submitter: Eric Fiselier Opened: 2014-06-07 Last modified: 2017-07-17
Priority: 1
View all issues with Resolved status.
Discussion:
remove_filename()
specifies !has_filename()
as the post condition.
This post condition is not correct. For example the path "/foo"
has a filename of "foo"
. If we remove the filename we get "/"
,
and "/"
has a filename of "/"
.
[2014-06-08 Beman supplies an Effects: element.]
[2014-06-17 Rapperswil LWG will investigate issue at a subsequent meeting.]
[2016-04 Issue updated to address the C++ Working Paper. Previously addressed File System TS]
[2016-04, Issues Telecon]
There was concern that the effects wording is not right. Jonathan provided updated wording.
Previous resolution [SUPERSEDED]:
path& remove_filename();
Postcondition:!has_filename()
.Effects:
*this = parent_path()
, except that ifparent_path() == root_path()
,clear()
.Returns:
*this
.[Example:
std::cout << path("/foo").remove_filename();
// outputs "/
" std::cout << path("/").remove_
filename(); // outputs ""—end example]
[2016-08 Chicago]
Wed AM: Move to Tentatively Ready
Previous resolution [SUPERSEDED]:
path& remove_filename();
Postcondition: !has_filename().Effects: If
*this == root_path()
, thenclear()
. Otherwise,*this = parent_path()
.Returns:
*this
.[Example:
std::cout << path("/foo").remove_filename();
// outputs "/
" std::cout << path("/").remove_
filename(); // outputs ""
—end example]
[2016-10-16, Eric reopens and provides improved wording]
The suggested PR is incorrect. root_path()
removes redundant directory separators.
*this == root_path()
will fail for "//foo///"
because root_path()
returns "//foo/"
. However using path::compare
instead would solve this problem.
[2016-11-21, Beman comments]
This issue is closely related to CD NB comments US 25, US 37, US 51, US 52, US 53, US 54, and US 60. The Filesystem SG in Issaquah recommended that these all be handled together, as they all revolve around the exact meaning of "filename" and path decomposition.
[2017-07-14, Davis Herring comments]
Changes in P0492R2 changed remove_filename()
and has_filename()
in such a fashion that the postcondition is now satisfied. (In the example in the issue description, "/foo"
does gets mapped to "/"
, but path("/").has_filename()
is false
.)
[2017-07, Toronto Thursday night issues processing]
This was resolved by the adoption of P0492R2.
Proposed resolution:
This wording is relative to N4606.
Modify 31.12.6.5.5 [fs.path.modifiers] as indicated:
path& remove_filename();-5-
-6- Returns:PostconditionEffects:If!has_filename()
this->compare(root_path()) == 0
, thenclear()
. Otherwise,*this = parent_path()
..*this
. -7- [Example:std::cout << path("/foo").remove_filename(); // outputs "/" std::cout << path("/").remove_filename(); // outputs ""— end example]
path::root_directory()
description is confusingSection: 31.12.6.5.9 [fs.path.decompose] Status: C++17 Submitter: Jonathan Wakely Opened: 2014-07-03 Last modified: 2017-07-30
Priority: 0
View all other issues in [fs.path.decompose].
View all issues with C++17 status.
Discussion:
31.12.6.5.9 [fs.path.decompose] p5 says:
If root-directory is composed of slash name, slash is excluded from the returned string.
but the grammar says that root-directory is just slash so that makes no sense.
[Apr 2016 Issue updated to address the C++ Working Paper. Previously addressed File System TS]
Proposed resolution:
Change 31.12.6.5.9 [fs.path.decompose] as indicated:
path root_directory() const;Returns: root-directory, if
pathname
includes root-directory, otherwisepath()
.
If root-directory is composed of slash name, slash is excluded from the returned string.
recursive_directory_iterator
effects refers to non-existent functions
Section: 31.12.12.2 [fs.rec.dir.itr.members] Status: C++17 Submitter: Jonathan Wakely Opened: 2014-07-10 Last modified: 2017-07-30
Priority: 0
View all other issues in [fs.rec.dir.itr.members].
View all issues with C++17 status.
Discussion:
operator++/increment
, second bullet item, ¶39 says
"Otherwise if
recursion_pending() && is_directory(this->status())
&& (!is_symlink(this->symlink_status())
..."
but recursive_directory_iterator
does not have status
or symlink_status
members.
[Apr 2016 Issue updated to address the C++ Working Paper. Previously addressed File System TS]
Proposed resolution:
Change 31.12.12.2 [fs.rec.dir.itr.members] ¶39 as indicated:
Otherwise if
recursion_pending() && is_directory((*this)->status()) && (!is_symlink((*this)->symlink_status())
..."
system_complete
refers to undefined variable 'base'
Section: 99 [fs.op.system_complete] Status: C++17 Submitter: Jonathan Wakely Opened: 2014-07-20 Last modified: 2017-07-30
Priority: 0
View all issues with C++17 status.
Discussion:
The example says "...or p
and base
have the same root_name()
.", but
base
is not defined. I believe it refers to the value returned by current_path()
.
[Apr 2016 Issue updated to address the C++ Working Paper. Previously addressed File System TS]
Proposed resolution:
Change 99 [fs.op.system_complete] as indicated:
For Windows based operating systems,
system_complete(p)
has the same semantics asabsolute(p, current_path())
ifp.is_absolute() || !p.has_root_name()
orp
andhave the same
basecurrent_path()root_name()
. Otherwise it acts likeabsolute(p, cwd)
is the current directory for thep.root_name()
drive. This will be the current directory for that drive the last time it was set, and thus may be residue left over from a prior program run by the command processor. Although these semantics are useful, they may be surprising.
Section: 31.12.13.4 [fs.op.copy] Status: C++17 Submitter: Jonathan Wakely Opened: 2014-07-28 Last modified: 2017-07-30
Priority: 0
View all other issues in [fs.op.copy].
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Discussion:
31.12.13.4 [fs.op.copy] paragraph 16 says:
Otherwise if
!exists(t) & (options & copy_options::copy_symlinks) != copy_options::none
, thencopy_symlink(from, to, options)
.
But there is no overload of copy_symlink
that takes a copy_options
;
it should be copy_symlink(from, to)
.
31.12.13.4 [fs.op.copy] Paragraph 26 says:
as if by
for (directory_entry& x : directory_iterator(from))
but the result of dereferencing a directory_iterator
is const; it should be:
as if by
for (const directory_entry& x : directory_iterator(from))
[Apr 2016 Issue updated to address the C++ Working Paper. Previously addressed File System TS]
Proposed resolution:
Change 31.12.13.4 [fs.op.copy] paragraph 16 as indicated:
Otherwise if
!exists(t) & (options & copy_options::copy_symlinks) != copy_options::none
, thencopy_symlink(from, to
., options)
Change 31.12.13.4 [fs.op.copy] paragraph 26 as indicated:
as if by
for (const directory_entry& x : directory_iterator(from))
Section: 31.12.13.20 [fs.op.is.empty] Status: C++17 Submitter: Jonathan Wakely Opened: 2014-08-01 Last modified: 2021-06-06
Priority: 3
View all issues with C++17 status.
Discussion:
The descriptions of most functions are explicit about the use of the ec
argument,
either saying something like "foo(p)
or foo(p, ec)
, respectively" or using the
ec
argument like foo(p[, ec])
, but is_empty
does not.
[fs.op.is_empty]/2 refers to ec
unconditionally, but more importantly
[fs.op.is_empty]/3 doesn't pass ec
to the directory_iterator
constructor or the file_size
function.
[ 9 Oct 2014 Beman supplies proposed wording. ]
[Apr 2016 Issue updated to address the C++ Working Paper. Previously addressed File System TS]
Previous resolution [SUPERSEDED]:
bool is_empty(const path& p); bool is_empty(const path& p, error_code& ec) noexcept;Effects:
- Determine
file_status s
, as if bystatus(p)
orstatus(p, ec)
, respectively.- For the signature with argument
ec
, returnfalse
if an error occurred.- Otherwise, if
is_directory(s)
:
- Create
directory_iterator itr
, as if bydirectory_iterator(p)
ordirectory_iterator(p, ec)
, respectively.- For the signature with argument
ec
, returnfalse
if an error occurred.- Otherwise, return
itr == directory_iterator()
.- Otherwise:
- Determine
uintmax_t sz
, as if byfile_size(p)
orfile_size(p, ec)
, respectively .- For the signature with argument
ec
, returnfalse
if an error occurred.- Otherwise, return
sz == 0
.Remarks: The temporary objects described in Effects are for exposition only. Implementations are not required to create them.
Returns:
See Effects.is_directory(s) ? directory_iterator(p) == directory_iterator() : file_size(p) == 0;
The signature with argumentec
returnsfalse
if an error occurs.Throws: As specified in Error reporting (7).
[2016-08 Chicago]
Wed PM: Move to Tentatively Ready
Proposed resolution:
bool is_empty(const path& p); bool is_empty(const path& p, error_code& ec) noexcept;Effects:
- Determine
file_status s
, as if bystatus(p)
orstatus(p, ec)
, respectively.- For the signature with argument
ec
, returnfalse
if an error occurred.- Otherwise, if
is_directory(s)
:
- Create
directory_iterator itr
, as if bydirectory_iterator(p)
ordirectory_iterator(p, ec)
, respectively.- For the signature with argument
ec
, returnfalse
if an error occurred.- Otherwise, return
itr == directory_iterator()
.- Otherwise:
- Determine
uintmax_t sz
, as if byfile_size(p)
orfile_size(p, ec)
, respectively .- For the signature with argument
ec
, returnfalse
if an error occurred.- Otherwise, return
sz == 0
.
Returns:is_directory(s) ? directory_iterator(p) == directory_iterator() : file_size(p) == 0;
The signature with argumentec
returnsfalse
if an error occurs.Throws: As specified in Error reporting (7).
status()
effects cannot be implemented as specifiedSection: 31.12.13.36 [fs.op.status] Status: C++17 Submitter: Jonathan Wakely Opened: 2015-09-15 Last modified: 2017-07-30
Priority: 0
View all other issues in [fs.op.status].
View all issues with C++17 status.
Discussion:
31.12.13.36 [fs.op.status] paragraph 2 says:
Effects: As if:
error_code ec; file_status result = status(p, ec); if (result == file_type::none) ...
This won't compile, there is no comparison operator for file_status
and file_type
,
and the conversion from file_type
to file_status
is explicit.
[Apr 2016 Issue updated to address the C++ Working Paper. Previously addressed File System TS]
Proposed resolution:
Change 31.12.13.36 [fs.op.status] paragraph 2:
Effects: As if:
error_code ec; file_status result = status(p, ec); if (result.type() == file_type::none) ...
path::iterator
is very expensiveSection: 31.12.6.6 [fs.path.itr] Status: C++17 Submitter: Jonathan Wakely Opened: 2015-09-15 Last modified: 2017-07-30
Priority: 2
View all other issues in [fs.path.itr].
View all issues with C++17 status.
Discussion:
31.12.6.6 [fs.path.itr] requires path::iterator
to be a BidirectionalIterator, which also implies
the ForwardIterator requirement in [forward.iterators] p6 for the following assertion
to pass:
path p("/"); auto it1 = p.begin(); auto it2 = p.begin(); assert( &*it1 == &*it2 );
This prevents iterators containing a path
, or constructing one on the fly when
dereferenced, the object they point to must exist outside the iterators and potentially
outlive them. The only practical way to meet the requirement is for p
to hold
a container of child path
objects so the iterators can refer to those
children. This makes a path
object much larger than would naïvely be
expected.
The Boost and MSVC implementations of Filesystem fail to meet this requirement. The
GCC implementation meets it, but it makes sizeof(path) == 64
(for 64-bit) or
sizeof(path) == 40
for 32-bit, and makes many path operations
more expensive.
[21 Nov 2015 Beman comments:]
The ForwardIterator requirement in
[forward.iterators] "If a
and b
are both dereferenceable, then a == b
if and only if
*a
and *b
are bound to the same object." will be removed by N4560, Working Draft, C++ Extensions for
Ranges. I see no point in requiring something for the File System TS that is expensive,
has never to my knowledge been requested by users, and is going to go away soon anyhow.
The wording I propose below removes the requirement.
[Apr 2016 Issue updated to address the C++ Working Paper. Previously addressed File System TS]
Proposed resolution:
This wording is relative to N4582.
Change 31.12.6.6 [fs.path.itr] paragraph 2:
A
path::iterator
is a constant iterator satisfying all the requirements of a bidirectional iterator (C++14 §24.1.4 Bidirectional iterators) except that there is no requirement that two equal iterators be bound to the same object. Itsvalue_type
ispath
.
filesystem::path
overloads for File-based streamsSection: 31.10 [file.streams] Status: C++17 Submitter: Beman Dawes Opened: 2016-03-16 Last modified: 2017-07-30
Priority: 2
View all other issues in [file.streams].
View all issues with C++17 status.
Discussion:
The constructor and open
functions for File-based streams in
27.9 [file.streams] currently provide overloads for const char*
and
const string&
arguments that specify the path for the file to be opened.
With the addition of the File System TS to the standard library, these
constructors and open
functions need to be overloaded for
const filesystem::path&
so that file-based streams can take advantage of class filesystem::path
features such as support for strings of character types wchar_t
,
char16_t
, and char32_t
.
The
const filesystem::path& p
overload for these functions is like the
existing const string&
overload; it simply calls the overload of
the same function that takes a C-style string.
For operating systems like POSIX
that traffic in char
strings for filenames, nothing more is
required. For operating systems like Windows that traffic in wchar_t
strings for filenames, an additional C-style string overload is required.
The overload's character type needs to be specified as
std::filesystem::path::value_type
to also support possible future
operating systems that traffic in char16_t
or char32_t
characters.
Not recommended:
Given the proposed constructor and open
signatures taking
const filesystem::path&
, it would in theory be possible to remove
some of the other signatures. This is not proposed because it would break ABI's, break
user code depending on user-defined automatic conversions to the existing argument types,
and invalidate existing documentation, books, and tutorials.
Implementation experience:
The Boost Filesystem library has provided header <boost/filesystem/fstream.hpp>
implementing the proposed resolution for over a decade.
The Microsoft/Dinkumware
implementation of standard library header <fstream>
has provided
the const wchar_t*
overloads for many years.
[2016-08-03 Chicago]
Fri PM: Move to Review
Proposed resolution:
At the end of 27.9.1 File streams [fstreams] add a paragraph:
In this subclause, member functions taking arguments of
const std::filesystem::path::value_type*
shall only be provided on systems
where std::filesystem::path::value_type
([fs.class.path]) is not
char
. [Note: These functions enable class path
support for systems with a wide native path character type, such as
wchar_t
. — end note]
Change 27.9.1.1 Class template basic_filebuf [filebuf] as indicated:
basic_filebuf<charT,traits>* open(const char* s, ios_base::openmode mode); basic_filebuf<charT,traits>* open(const std::filesystem::path::value_type* s, ios_base::openmode mode); // wide systems only; see [fstreams] basic_filebuf<charT,traits>* open(const string& s, ios_base::openmode mode); basic_filebuf<charT,traits>* open(const filesystem::path& p, ios_base::openmode mode);
Change 27.9.1.4 Member functions [filebuf.members] as indicated:
basic_filebuf<charT,traits>* open(const char* s, ios_base::openmode mode); basic_filebuf<charT,traits>* open(const std::filesystem::path::value_type* s, ios_base::openmode mode); // wide systems only; see [fstreams]
To 27.9.1.4 Member functions [filebuf.members] add:
basic_filebuf<charT,traits>* open(const filesystem::path& p, ios_base::openmode mode);
Returns:
open(p.c_str(), mode);
Change 27.9.1.6 Class template basic_ifstream [ifstream] as indicated:
explicit basic_ifstream(const char* s, ios_base::openmode mode = ios_base::in); explicit basic_ifstream(const std::filesystem::path::value_type* s, ios_base::openmode mode = ios_base::in); // wide systems only; see [fstreams] explicit basic_ifstream(const string& s, ios_base::openmode mode = ios_base::in); explicit basic_ifstream(const filesystem::path& p, ios_base::openmode mode = ios_base::in); ... void open(const char* s, ios_base::openmode mode = ios_base::in); void open(const std::filesystem::path::value_type* s, ios_base::openmode mode = ios_base::in); // wide systems only; see [fstreams] void open(const string& s, ios_base::openmode mode = ios_base::in); void open(const filesystem::path& p, ios_base::openmode mode = ios_base::in);
Change 27.9.1.7 basic_ifstream constructors [ifstream.cons] as indicated:
explicit basic_ifstream(const char* s, ios_base::openmode mode = ios_base::in); explicit basic_ifstream(const std::filesystem::path::value_type* s, ios_base::openmode mode = ios_base::in); // wide systems only; see [fstreams]
To 27.9.1.7 basic_ifstream constructors [ifstream.cons] add:
explicit basic_ifstream(const filesystem::path& p, ios_base::openmode mode = ios_base::in);
Effects: the same as
basic_ifstream(p.c_str(), mode)
.
Change 27.9.1.9 Member functions [ifstream.members] as indicated:
void open(const char* s, ios_base::openmode mode = ios_base::in); void open(const std::filesystem::path::value_type* s, ios_base::openmode mode = ios_base::in); // wide systems only; see [fstreams]
To 27.9.1.9 Member functions [ifstream.members] add:
void open(const filesystem::path& p, ios_base::openmode mode = ios_base::in);
Effects: calls
open(p.c_str(), mode)
.
Change 27.9.1.10 Class template basic_ofstream [ofstream] as indicated:
explicit basic_ofstream(const char* s, ios_base::openmode mode = ios_base::out); explicit basic_ofstream(const std::filesystem::path::value_type* s, ios_base::openmode mode = ios_base::out); // wide systems only; see [fstreams] explicit basic_ofstream(const string& s, ios_base::openmode mode = ios_base::out); explicit basic_ofstream(const filesystem::path& p, ios_base::openmode mode = ios_base::out); ... void open(const char* s, ios_base::openmode mode = ios_base::out); void open(const std::filesystem::path::value_type* s, ios_base::openmode mode = ios_base::out); // wide systems only; see [fstreams] void open(const string& s, ios_base::openmode mode = ios_base::out); void open(const filesystem::path& p, ios_base::openmode mode = ios_base::out);
Change 27.9.1.11 basic_ofstream constructors [ofstream.cons] as indicated:
explicit basic_ofstream(const char* s, ios_base::openmode mode = ios_base::out); explicit basic_ofstream(const std::filesystem::path::value_type* s, ios_base::openmode mode = ios_base::out); // wide systems only; see [fstreams]
To 27.9.1.11 basic_ofstream constructors [ofstream.cons] add:
explicit basic_ofstream(const filesystem::path& p, ios_base::openmode mode = ios_base::out);
Effects: the same as
basic_ofstream(p.c_str(), mode)
.
Change 27.9.1.13 Member functions [ofstream.members] as indicated:
void open(const char* s, ios_base::openmode mode = ios_base::out); void open(const std::filesystem::path::value_type* s, ios_base::openmode mode = ios_base::out); // wide systems only; see [fstreams]
To 27.9.1.13 Member functions [ofstream.members] add:
void open(const filesystem::path& p, ios_base::openmode mode = ios_base::out);
Effects: calls
open(p.c_str(), mode)
.
Change 27.9.1.14 Class template basic_fstream [fstream] as indicated:
explicit basic_fstream(const char* s, ios_base::openmode mode = ios_base::in|ios_base::out); explicit basic_fstream(const std::filesystem::path::value_type* s, ios_base::openmode mode = ios_base::in|ios_base::out); // wide systems only; see [fstreams] explicit basic_fstream(const string& s, ios_base::openmode mode = ios_base::in|ios_base::out); explicit basic_fstream(const filesystem::path& p, ios_base::openmode mode = ios_base::in|ios_base::out); ... void open(const char* s, ios_base::openmode mode = ios_base::in|ios_base::out); void open(const std::filesystem::path::value_type* s, ios_base::openmode mode = ios_base::in|ios_base::out); // wide systems only; see [fstreams] void open(const string& s, ios_base::openmode mode = ios_base::in|ios_base::out); void open(const filesystem::path& p, ios_base::openmode mode = ios_base::in|ios_base::out);
Change 27.9.1.15 basic_fstream constructors [fstream.cons] as indicated:
explicit basic_fstream(const char* s, ios_base::openmode mode = ios_base::in|ios_base::out); explicit basic_fstream(const std::filesystem::path::value_type* s, ios_base::openmode mode = ios_base::in|ios_base::out); // wide systems only; see [fstreams]
To 27.9.1.15 basic_fstream constructors [fstream.cons] add:
explicit basic_fstream(const filesystem::path& p, ios_base::openmode mode = ios_base::in|ios_base::out);
Effects: the same as
basic_fstream(p.c_str(), mode)
.
Change 27.9.1.17 Member functions [fstream.members] as indicated:
void open(const char* s, ios_base::openmode mode = ios_base::in|ios_base::out); void open(const std::filesystem::path::value_type* s, ios_base::openmode mode = ios_base::in|ios_base::out); // wide systems only; see [fstreams]
To 27.9.1.17 Member functions [fstream.members] add:
void open(const filesystem::path& p, ios_base::openmode mode = ios_base::in|ios_base::out);
Effects: calls
open(p.c_str(), mode)
.
directory_entry::status
is not allowed to be cached as a quality-of-implementation issueSection: 31.12.10.4 [fs.dir.entry.obs] Status: Resolved Submitter: Billy O'Neal Opened: 2016-03-03 Last modified: 2020-09-06
Priority: 2
View all other issues in [fs.dir.entry.obs].
View all issues with Resolved status.
Discussion:
To fix multi-threading problems in directory_entry
, caching behavior
was removed from the type. This is bad for performance reasons,
because the values can no longer be cached from the result of readdir
on POSIX platforms, or from FindFirstFile/FindNextFile
on Windows.
It appears that the intent was to allow implementers to fill in the
values for directory_entry::status
inside directory_iterator
, but as
currently specified:
Returns: status(path()[, ec])
.
This is not allowed to be cached, because the target of the path can change. For example, consider the following program:
#include <stdio.h> #include <stdlib.h> #include <filesystem> #include <fstream> using namespace std; namespace fs = ::std::filesystem; void verify(const bool b, const char * const msg) { if (!b) { printf("fail: %s\n", msg); exit(1); } } void touch_file(const char * const filename) { ofstream f(filename); f << '\n' << endl; verify(f.good(), "File write failed"); } int main() { fs::remove_all("subDir"); fs::create_directory("subDir"); fs::create_directory("subDir/child"); touch_file("subDir/child/child"); fs::current_path("./subDir"); fs::directory_iterator dir("."); ++dir; fs::directory_entry entry = *dir; verify(entry == "./child", "unexpected subdirectory"); //enumerating "subDir" returned the directory "child" fs::file_status status = entry.status(); verify(status.type() == fs::file_type::directory, "subDir/child was not a directory"); fs::current_path("./child"); status = entry.status(); // REQUIRED to re-stat() on POSIX, // GetFileAttributes() on Windows verify(status.type() == fs::file_type::regular, "subDir/child/child was not a regular file"); return 0; }
directory_entry
should be re-specified to allow implementers to cache
the value of status(path)
at the time irectory_iterator
was
incremented to avoid repeated stat()
/ GetFileAttributes
calls. (This
may mean additional constructors are necessary for directory_entry
as well)
[2016-04, Issues Telecon]
Beman is working on a paper to address this.
[2016-08, Beman comments]
This will be resolved by P0317R1, Directory Entry Caching for Filesystem.
Fri AM: Moved to Tentatively Resolved
Proposed resolution:
std::filesystem
enum classes overspecifiedSection: 31.12.8.2 [fs.enum.file.type], 31.12.8.3 [fs.enum.copy.opts], 31.12.8.6 [fs.enum.dir.opts] Status: C++17 Submitter: Richard Smith Opened: 2016-03-19 Last modified: 2021-06-06
Priority: 3
View all other issues in [fs.enum.file.type].
View all issues with C++17 status.
Discussion:
The enum class std::filesystem::file_type
specifies a collection of enumerators for describing the type of
a file, but also specifies (1) the numeric values of the enumerators, and (2) that an implementation cannot extend the
list with additional types known to it (and by extension, the standard cannot extend the list in future versions without
breakage).
These both seem like mistakes in the design. I would suggest we remove the specification of numerical values, and maybe also
we allow implementations to add new implementation-defined value
we replace std::filesystem::file_type::unknown
with a value that only
means "exists but type could not be determined" and use
implementation-specific values for other possibilities.
Similar overspecification exists for std::filesystem::copy_options
and
std::filesystem::directory_options
, where again precise numerical values are specified.
[2016-08 Chicago]
Wed AM: Move to Tentatively Ready
Proposed resolution:
Strike the "Value" column from:
[fs.enum.file_type], Table 145 Enum class file_type
31.12.8.3 [fs.enum.copy.opts], Table 146 Enum class copy_options
31.12.8.6 [fs.enum.dir.opts], Table 148 Enum class enum.directory_options
.
Change [fs.enum.file_type] Table 145 Enum class file_type
as indicated:
Constant Meaning none
The type of the file has not been determined or an error occurred while trying to determine the type. not_found
Pseudo-type indicating the file was not found. [ Note: The file not being found is not considered an error while determining the type of a file. —end note ] regular
Regular file directory
Directory file symlink
Symbolic link file block
Block special file character
Character special file fifo
FIFO or pipe file socket
Socket file implementation-
definedImplementations that support file systems having file types in addition to the above file_type types shall supply implementation-defined file_type
constants to separately identify each of those additional file typesunknown
The file does exist, but is of an operating system dependent type not covered by any of the other cases or the process does not have permission to query the file typeThe file exists but the type could not be determined
Section: 16.3.2.4 [structure.specifications] Status: C++17 Submitter: Dawn Perchik Opened: 2016-04-14 Last modified: 2017-07-30
Priority: 3
View other active issues in [structure.specifications].
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Discussion:
Some Effects: were intended to include the "Equivalent to" phrasing where no or different words were used.
See examples in std::filesystem
and throughout the library.
The wording in [structure.specifications]/4 is incorrect and Effects: throughout the library which use (or were intended to use) the "Equivalent to" phrasing need to be checked to make sure they fit the intended wording.
[2016-04, Issues Telecon]
The PR is fine; but bullet #1 in the report really should have a list of places to change.
Jonathan checked throughout the library for bullet #2 and found two problems in [string.access], which have been added to the PR.[2016-08-03 Chicago]
Fri PM: Move to Tentatively Ready
Proposed resolution:
Change [structure.specifications]/4 as indicated:
"[…] if
F
has no Returns: element, a non-void
return fromF
is specified by theReturns: elementsreturn statements in the code sequence."
Add two return
keywords to [string.access]:
const charT& front() const; charT& front();-7- Requires:
!empty()
.-8- Effects: Equivalent to
return operator[](0)
.const charT& back() const; charT& back();-9- Requires:
!empty()
.-10- Effects: Equivalent to
return operator[](size() - 1)
.
Section: 31.12 [filesystems] Status: C++17 Submitter: Beman Dawes Opened: 2016-04-15 Last modified: 2017-07-30
Priority: 2
View all other issues in [filesystems].
View all issues with C++17 status.
Discussion:
The IS project editors have requested that filesystem Effects elements specified purely as C++ code include the phrase "Equivalent to" if this is the intent. They do not consider this change to be editorial.
[2016-08 Chicago]
Wed AM: Move to Tentatively Ready
Proposed resolution:
path& replace_filename(const path& replacement);Effects: Equivalent to:
remove_filename(); operator/=(replacement);
void swap(path& lhs, path& rhs) noexcept;Effects: Equivalent to:
lhs.swap(rhs)
template <class charT, class traits> basic_ostream<charT, traits>& operator<<(basic_ostream<charT, traits>& os, const path& p);Effects: Equivalent to:
os << quoted(p.string<charT, traits>())
.
template <class charT, class traits> basic_istream<charT, traits>& operator>>(basic_istream<charT, traits>& is, path& p);Effects: Equivalent to:
basic_string<charT, traits> tmp; is >> quoted(tmp); p = tmp;
void copy(const path& from, const path& to); void copy(const path& from, const path& to, error_code& ec) noexcept;Effects: Equivalent to:
copy(from, to, copy_options::none)
orcopy(from, to, copy_options::none, ec)
, respectively.
void copy_symlink(const path& existing_symlink, const path& new_symlink); void copy_symlink(const path& existing_symlink, const path& new_symlink, error_code& ec) noexcept;Effects: Equivalent to:
function(read_symlink(existing_symlink), new_symlink)
orfunction(read_symlink(existing_symlink, ec), new_symlink, ec)
, respectively, wherefunction
iscreate_symlink
orcreate_directory_symlink
, as appropriate.
filesystem::copy()
cannot copy symlinksSection: 31.12.13.4 [fs.op.copy] Status: C++17 Submitter: Jonathan Wakely Opened: 2016-04-19 Last modified: 2017-07-30
Priority: 2
View all other issues in [fs.op.copy].
View all issues with C++17 status.
Discussion:
31.12.13.4 [fs.op.copy] paragraph 3 bullet (3.4) says that if is_symlink(f)
and
copy_options
is set in options then it should copy a symlink, but that
case cannot be reached.
is_symlink(f)
can only be true if f was set using
symlink_status(from)
, but that doesn't happen unless one of
create_symlinks
or skip_symlinks
is set in options. It should depend
on copy_symlinks
too.
I'm not sure what the correct behaviour is, but I don't think we
want to simply add a check for copy_symlinks
in bullet (3.1) because
that would mean that t = symlink_status(to)
, and I don't think we want
that. Consider 'touch file; mkdir dir; ln -s dir link;'
and then
filesystem::copy("file", "link",
filesystem::copy_options::copy_symlinks)
. If t = symlink_status(to)
then is_directory(t) == false
, and we don't use bullet (3.5.4) but go
to (3.5.5) instead and fail. So when copy_symlinks
is set we still
need to use t = status(to)
as specified today.
[2016-05 Issues Telecon]
This is related to 2682(i); and should be considered together.
[2016-08 Chicago]
Wed AM: Move to Tentatively Ready
Proposed resolution:
Modify paragraph 3 of 31.12.13.4 [fs.op.copy] as shown:
Effects: Before the first use of
f
andt
:
— If(options & copy_options::create_symlinks) != copy_options::none || (options & copy_options::skip_symlinks) != copy_options::none
thenauto f = symlink_status(from)
and if neededauto t = symlink_status(to)
.
— Otherwise, if(options & copy_options::copy_symlinks) != copy_options::none
thenauto f = symlink_status(from)
and if neededauto t = status(to)
.
— Otherwise,auto f = status(from)
and if neededauto t = status(to)
.
filesystem::copy()
won't create a symlink to a directorySection: 31.12.13.4 [fs.op.copy] Status: C++20 Submitter: Jonathan Wakely Opened: 2016-04-19 Last modified: 2021-02-25
Priority: 2
View all other issues in [fs.op.copy].
View all issues with C++20 status.
Discussion:
(First raised in c++std-lib-38544)
filesystem::copy
doesn't create a symlink to a directory in this case:
copy("/", "root", copy_options::create_symlinks);
If the first path is a file then a symlink is created, but I think my
implementation is correct to do nothing for a directory. We get to
bullet 31.12.13.4 [fs.op.copy] (3.6) where is_directory(f)
is true, but options
== create_symlinks
, so we go to the next bullet (3.7) which says
"Otherwise, no effects."
create_symlink
to create a symlink to a directory.
[2016-05 Issues Telecon]
This is related to 2681(i); and should be considered together.
[2016-08 Chicago]
Wed AM: Move to Tentatively Ready
Previous resolution [SUPERSEDED]:
Add a new bullet following (3.6) in 31.12.13.4 [fs.op.copy] as shown:
If
!exists(t)
, thencreate_directory(to, from)
.Then, iterate over the files in
from
, as if byfor (directory_entry& x : directory_iterator(from))
, and for each iterationcopy(x.path(), to/x.path().filename(), options | copy_options::unspecified )
Otherwise, if
is_directory(f) && (options & copy_options::create_symlinks) != copy_options::none
, then report an error with anerror_code
argument equal tomake_error_code(errc::is_a_directory)
.Otherwise, no effects.
[2016-10-16, Eric reopens and provides improved wording]
The current PR makes using copy(...)
to copy/create a directory symlink an error. For example, the following
is now an error:
copy("/", "root", copy_options::create_symlinks);
However the current PR doesn't handle the case where both copy_options::create_symlinks
and
copy_options::recursive
are specified. This case is still incorrectly handled by bullet (3.6) [fs.op.copy].
3.6 Otherwise if
is_directory(f) && (bool(options & copy_options::recursive) || ...)
3.X Otherwise ifis_directory(f) && bool(options & copy_options::create_symlinks)
So 3.6 catches create_symlinks | recursive
but I believe we want 3.X to handle it instead.
[2018-01-26 issues processing telecon]
Status to 'Review'; we think this is OK but want some implementation experience before adopting it.
[2018-01-29 Jonathan says]
The proposed resolution for LWG 2682 has been in GCC's Filesystem TS implementation for more than a year.
It's also in our std::filesystem
implementation on the subversion trunk.
[2018-06; Rapperswil Wednesday evening]
JW: can we use the words we are shipping already since two years?
BO: what we got is better than what we had before
no objection to moving to Ready
ACTION move to Ready
ACTION link 2682 and LWG 3057(i) and set a priority 2 and look at 3057 in San Diego
[2018-11, Adopted in San Diego]
Proposed resolution:
This wording is relative to N4750.
Add a new bullet before (4.8) in 31.12.13.4 [fs.op.copy] as shown:
(4.7) — Otherwise, if
is_regular_file(f)
, then:[…](4.?) — Otherwise, if
is_directory(f) && (options & copy_options::create_symlinks) != copy_options::nonethen report an error with an
error_code
argument equal tomake_error_code(errc::is_a_directory)
.(4.8) — Otherwise, if
is_directory(f) && ((options & copy_options::recursive) != copy_options::none || options == copy_options::none)then:
(4.8.1) — If
exists(t)
isfalse
, thencreate_directory(to, from)
.(4.8.2) — Then, iterate over the files in
from
, as if byfor (const directory_entry& x : directory_iterator(from)) copy(x.path(), to/x.path().filename(), options | copy_options::in-recursive-copy);where
in-recursive-copy
is a bitmask element ofcopy_options
that is not one of the elements in 31.12.8.3 [fs.enum.copy.opts].(4.9) — Otherwise, for the signature with argument
ec
,ec.clear()
.(4.10) — Otherwise, no effects.
filesystem::copy()
says "no effects"Section: 31.12.13.4 [fs.op.copy] Status: C++17 Submitter: Jonathan Wakely Opened: 2016-04-19 Last modified: 2017-07-30
Priority: 3
View all other issues in [fs.op.copy].
View all issues with C++17 status.
Discussion:
In 31.12.13.4 [fs.op.copy]/8 the final bullet says "Otherwise, no effects" which
implies there is no call to ec.clear()
if nothing happens, nor any
error condition, is that right?
[2016-06 Oulu]
Moved to P0/Ready during issues prioritization.
Friday: status to Immediate
Proposed resolution:
Change 31.12.13.4 [fs.op.copy]/8 as indicated:
Otherwise no effects.For the signature with argumentec
,ec.clear()
.
Section: 23.6.4 [priority.queue] Status: C++17 Submitter: Robert Haberlach Opened: 2016-05-02 Last modified: 2017-07-30
Priority: 0
View all other issues in [priority.queue].
View all issues with C++17 status.
Discussion:
The containers that take a comparison functor (set
, multiset
,
map
, and multimap
) have a typedef for the comparison functor.
priority_queue
does not.
Proposed resolution:
Augment [priority.queue] as indicated:
typedef Container container_type; typedef Compare value_compare;
shared_ptr
deleters must not not throw on move constructionSection: 20.3.2.2.2 [util.smartptr.shared.const] Status: C++17 Submitter: Jonathan Wakely Opened: 2016-05-03 Last modified: 2017-07-30
Priority: 0
View other active issues in [util.smartptr.shared.const].
View all other issues in [util.smartptr.shared.const].
View all issues with C++17 status.
Discussion:
In 20.3.2.2.2 [util.smartptr.shared.const] p8 the shared_ptr
constructors taking
a deleter say:
The copy constructor and destructor of
D
shall not throw exceptions.
It's been pointed out that this doesn't forbid throwing moves, which makes it difficult to avoid a leak here:
struct D { D() = default; D(const D&) noexcept = default; D(D&&) { throw 1; } void operator()(int* p) const { delete p; } }; shared_ptr<int> p{new int, D{}};
"The copy constructor" should be changed to reflect that the chosen constructor might not be a copy constructor, and that copies made using any constructor must not throw.
N.B. the same wording is used for the allocator argument, but that's
redundant because the Allocator
requirements already forbid exceptions
when copying or moving.
Proposed resolution:
[Drafting note: the relevant expressions we're concerned about are
enumerated in the CopyConstructible
and MoveConstructible
requirements, so I see no need to repeat them by saying something
clunky like "Initialization of an object of type D
from an expression
of type (possibly const) D
shall not throw exceptions", we can just
refer to them. An alternative would be to define
NothrowCopyConstructible
, which includes CopyConstructible
but
requires that construction and destruction do not throw.]
Change 20.3.2.2.2 [util.smartptr.shared.const] p8:
D
shall beCopyConstructible
and such construction shall not throw exceptions. Thecopy constructor anddestructor ofD
shall not throw exceptions.
std::hash
specialized for error_code
, but not error_condition
?Section: 19.5.2 [system.error.syn] Status: C++17 Submitter: Tim Song Opened: 2016-05-03 Last modified: 2021-06-06
Priority: 3
View all other issues in [system.error.syn].
View all issues with C++17 status.
Discussion:
Both error_code
and error_condition
have an operator<
overload,
enabling their use in associative containers without having to write a custom comparator.
However, only error_code
has a std::hash
specialization. So it's possible to have a
set<error_code>
, a set<error_condition>
,
an unordered_set<error_code>
, but not an
unordered_set<error_condition>
. This seems...odd.
[2016-08 - Chicago]
Thurs AM: Moved to Tentatively Ready
Proposed resolution:
This wording is relative to N4582.
Edit [system_error.syn], header <system_error>
synopsis, as indicated:
namespace std { // ... // 19.5.6 Hash support template<class T> struct hash; template<> struct hash<error_code>; template<> struct hash<error_condition>; // ... }
Edit 19.5.7 [syserr.hash] as indicated:
template <> struct hash<error_code>; template <> struct hash<error_condition>;-1- The template specializations shall meet the requirements of class template
hash
(20.12.14).
{inclusive,exclusive}_scan
misspecifiedSection: 26.10.8 [exclusive.scan], 26.10.9 [inclusive.scan], 26.10.10 [transform.exclusive.scan], 26.10.11 [transform.inclusive.scan] Status: C++17 Submitter: Tim Song Opened: 2016-03-21 Last modified: 2017-07-30
Priority: 1
View all other issues in [exclusive.scan].
View all issues with C++17 status.
Discussion:
When P0024R2 changed the description of {inclusive,exclusive}_scan
(and the transform_
version),
it seems to have introduced an off-by-one error. Consider what N4582 26.10.9 [inclusive.scan]/2 says of
inclusive_scan
:
Effects: Assigns through each iterator
i
in[result, result + (last - first))
the value of
GENERALIZED_NONCOMMUTATIVE_SUM(binary_op, init, *j, ...)
for everyj
in[first, first + (i - result))
ifinit
is provided, or
GENERALIZED_NONCOMMUTATIVE_SUM(binary_op, *j, ...)
for everyj
in[first, first + (i - result))
otherwise.
When i == result
, i - result = 0
, so the range [first, first + (i - result))
is
[first, first)
— which is empty. Thus according to the specification, we should assign
GENERALIZED_NONCOMMUTATIVE_SUM(binary_op, init)
if init
is provided, or
GENERALIZED_NONCOMMUTATIVE_SUM(binary_op)
otherwise, which doesn't seem "inclusive" — and isn't
even defined in the second case.
GENERALIZED_NONCOMMUTATIVE_SUM(binary_op, *first, ..., *(first + (i - result)))
—
which is a closed range, not a half-open one.
Similar issues affect exclusive_scan
, transform_inclusive_scan
, and transform_exclusive_scan
.
[2016-06 Oulu]
Voted to Ready 11-0 Tuesday evening in Oulu
Proposed resolution:
This wording is relative to N4582.
Edit 26.10.8 [exclusive.scan]/2 as indicated:
[Drafting note: when
i
isresult
,[first, first + (i - result))
is an empty range, so the value to be assigned reduces toGENERALIZED_NONCOMMUTATIVE_SUM(binary_op, init)
, which isinit
, so there's no need to special case this.]
template<class InputIterator, class OutputIterator, class T, class BinaryOperation> OutputIterator exclusive_scan(InputIterator first, InputIterator last, OutputIterator result, T init, BinaryOperation binary_op);-2- Effects: Assigns through each iterator
i
in[result, result + (last - first))
the value ofGENERALIZED_NONCOMMUTATIVE_SUM(binary_op, init, *j, ...)
for everyj
in[first, first + (i - result))
.
init
, ifi
isresult
, otherwise
GENERALIZED_NONCOMMUTATIVE_SUM(binary_op, init, *j, ...)
for everyj
in[first, first + (i - result) - 1)
.
Edit 26.10.9 [inclusive.scan]/2 as indicated:
template<class InputIterator, class OutputIterator, class BinaryOperation> OutputIterator inclusive_scan(InputIterator first, InputIterator last, OutputIterator result, BinaryOperation binary_op); template<class InputIterator, class OutputIterator, class BinaryOperation> OutputIterator inclusive_scan(InputIterator first, InputIterator last, OutputIterator result, BinaryOperation binary_op, T init);-2- Effects: Assigns through each iterator
i
in[result, result + (last - first))
the value of
GENERALIZED_NONCOMMUTATIVE_SUM(binary_op, init, *j, ...)
for everyj
in[first, first + (i - result + 1))
ifinit
is provided, or
GENERALIZED_NONCOMMUTATIVE_SUM(binary_op, *j, ...)
for everyj
in[first, first + (i - result + 1))
otherwise.
Edit 26.10.10 [transform.exclusive.scan]/1 as indicated:
template<class InputIterator, class OutputIterator, class UnaryOperation, class T, class BinaryOperation> OutputIterator transform_exclusive_scan(InputIterator first, InputIterator last, OutputIterator result, UnaryOperation unary_op, T init, BinaryOperation binary_op);-1- Effects: Assigns through each iterator
i
in[result, result + (last - first))
the value ofGENERALIZED_NONCOMMUTATIVE_SUM(binary_op, init, unary_op(*j), ...)
for everyj
in[first, first + (i - result))
.
init
, ifi
isresult
, otherwise
GENERALIZED_NONCOMMUTATIVE_SUM(binary_op, init, unary_op(*j), ...)
for everyj
in[first, first + (i - result) - 1)
.
Edit 26.10.11 [transform.inclusive.scan]/1 as indicated:
template<class InputIterator, class OutputIterator, class UnaryOperation, class BinaryOperation> OutputIterator transform_inclusive_scan(InputIterator first, InputIterator last, OutputIterator result, UnaryOperation unary_op, BinaryOperation binary_op); template<class InputIterator, class OutputIterator, class UnaryOperation, class BinaryOperation, class T> OutputIterator transform_inclusive_scan(InputIterator first, InputIterator last, OutputIterator result, UnaryOperation unary_op, BinaryOperation binary_op, T init);-1- Effects: Assigns through each iterator
i
in[result, result + (last - first))
the value of
GENERALIZED_NONCOMMUTATIVE_SUM(binary_op, init, unary_op(*j), ...)
for everyj
in[first, first + (i - result + 1))
ifinit
is provided, or
GENERALIZED_NONCOMMUTATIVE_SUM(binary_op, unary_op(*j), ...)
for everyj
in[first, first + (i - result + 1))
otherwise.
clamp
misses preconditions and has extraneous condition on resultSection: 26.8.10 [alg.clamp] Status: C++17 Submitter: Martin Moene Opened: 2016-03-23 Last modified: 2017-07-30
Priority: 0
View all issues with C++17 status.
Discussion:
In Jacksonville (2016), P0025R0 was voted in instead of the intended P0025R1. This report contains the necessary mending along with two other improvements.
This report:adds the precondition that misses from P0025R0 but is in P0025R1,
corrects the returns: specification that contains an extraneous condition,
replaces the now superfluous remark with a note on usage of clamp with float
or double
.
Thanks to Carlo Assink and David Gaarenstroom for making us aware of the extraneous condition in the returns: specification and for suggesting the fix and to Jeffrey Yasskin for suggesting to add a note like p3.
Previous resolution [SUPERSEDED]:
Edit 26.8.10 [alg.clamp] as indicated:
template<class T> constexpr const T& clamp(const T& v, const T& lo, const T& hi); template<class T, class Compare> constexpr const T& clamp(const T& v, const T& lo, const T& hi, Compare comp);-1- Requires: The value of
-2- Returns:lo
shall be no greater thanhi
. For the first form, typeT
shall beLessThanComparable
(Table 18).lo
ifv
is less thanlo
,hi
ifhi
is less thanv
, otherwisev
The larger value of. -3- Notev
andlo
ifv
is smaller thanhi
, otherwise the smaller value ofv
andhi
Remarks: If NaN is avoided,T
can befloat
ordouble
Returns the first argument when it is equivalent to one of the boundary arguments. -4- Complexity: At most two comparisons.
[2016-05 Issues Telecon]
Reworded p3 slightly.
Proposed resolution:
This wording is relative to N4582.
Edit 26.8.10 [alg.clamp] as indicated:
template<class T> constexpr const T& clamp(const T& v, const T& lo, const T& hi); template<class T, class Compare> constexpr const T& clamp(const T& v, const T& lo, const T& hi, Compare comp);-1- Requires: The value of
-2- Returns:lo
shall be no greater thanhi
. For the first form, typeT
shall beLessThanComparable
(Table 18).lo
ifv
is less thanlo
,hi
ifhi
is less thanv
, otherwisev
The larger value of. -3- Notev
andlo
ifv
is smaller thanhi
, otherwise the smaller value ofv
andhi
Remarks: If NaN is avoided,T
can be a floating point typeReturns the first argument when it is equivalent to one of the boundary arguments. -4- Complexity: At most two comparisons.
std::copy
and std::move
shouldn't be in orderSection: 26.7.1 [alg.copy], 26.7.2 [alg.move] Status: C++17 Submitter: Tim Song Opened: 2016-03-23 Last modified: 2017-07-30
Priority: 0
View other active issues in [alg.copy].
View all other issues in [alg.copy].
View all issues with C++17 status.
Discussion:
26.3.5 [algorithms.parallel.overloads]/2 says that "Unless otherwise specified, the semantics of
ExecutionPolicy
algorithm overloads are identical to their overloads without."
ExecutionPolicy
overloads for std::copy
and std::move
,
so the requirement that they "start[] from first and proceed[] to last" in the original algorithm's description would
seem to apply, which defeats the whole point of adding a parallel overload.
[2016-05 Issues Telecon]
Marshall noted that all three versions of copy have subtly different wording, and suggested that they should not.
Proposed resolution:
This wording is relative to N4582.
Insert the following paragraphs after 26.7.1 [alg.copy]/4:
template<class ExecutionPolicy, class InputIterator, class OutputIterator> OutputIterator copy(ExecutionPolicy&& policy, InputIterator first, InputIterator last, OutputIterator result);-?- Requires: The ranges
-?- Effects: Copies elements in the range[first, last)
and[result, result + (last - first))
shall not overlap.[first, last)
into the range[result, result + (last - first))
. For each non-negative integern < (last - first)
, performs*(result + n) = *(first + n)
. -?- Returns:result + (last - first)
. -?- Complexity: Exactlylast - first
assignments.
Insert the following paragraphs after 26.7.2 [alg.move]/4:
template<class ExecutionPolicy, class InputIterator, class OutputIterator> OutputIterator move(ExecutionPolicy&& policy, InputIterator first, InputIterator last, OutputIterator result);-?- Requires: The ranges
-?- Effects: Moves elements in the range[first, last)
and[result, result + (last - first))
shall not overlap.[first, last)
into the range[result, result + (last - first))
. For each non-negative integern < (last - first)
, performs*(result + n) = std::move(*(first + n))
. -?- Returns:result + (last - first)
. -?- Complexity: Exactlylast - first
assignments.
invoke<R>
Section: 22.10.5 [func.invoke] Status: Resolved Submitter: Zhihao Yuan Opened: 2016-03-25 Last modified: 2021-06-12
Priority: Not Prioritized
View all other issues in [func.invoke].
View all issues with Resolved status.
Discussion:
In N4169
the author dropped the invoke<R>
support by claiming
that it's an unnecessary cruft in TR1, obsoleted by C++11
type inference. But now we have some new business went
to *INVOKE
*(f, t1, t2, ..., tN, R)
, that is to discard the
return type when R
is void
. This form is very useful, or
possible even more useful than the basic form when
implementing a call wrapper. Also note that the optional
R
support is already in std::is_callable
and
std::is_nothrow_callable
.
[2016-07-31, Tomasz Kamiński comments]
The lack of invoke<R>
was basically a result of the concurrent publication of the never revision
of the paper and additional special semantics of INVOKE(f, args..., void)
.
std::invoke
function, the proposed invoke<R>
version is not
SFINAE friendly, as elimination of the standard version of invoke is guaranteed by std::result_of_t
in the result type that is missing for proposed invoke<R>
version. To provide this guarantee,
following remarks shall be added to the specification:
Remarks: This function shall not participate in overload resolution unless
is_callable_v<F(Args...), R>
istrue
.
[2016-08-01, Tomasz Kamiński and Zhihao Yuan update the proposed wording]
Previous resolution [SUPERSEDED]:
This wording is relative to N4606.
Modify 22.10 [function.objects]/2, header
<functional>
synopsis, as indicated:namespace std { // 20.12.3, invoke: template <class F, class... Args> result_of_t<F&&(Args&&...)> invoke(F&& f, Args&&... args); template <class R, class F, class... Args> R invoke(F&& f, Args&&... args);Add the following sequence of paragraphs after 22.10.5 [func.invoke]/1 as indicated:
template <class R, class F, class... Args> R invoke(F&& f, Args&&... args);-?- Returns:
-?- Remarks: This function shall not participate in overload resolution unlessINVOKE(std::forward<F>(f), std::forward<Args>(args)..., R)
(22.10.4 [func.require]).is_callable_v<F(Args...), R>
istrue
.
[2016-09-04, Tomasz Kamiński comments and improves wording]
The usage of is_callable_v<F(Args...), R>
causes problem in situation when either F
or Args
is an abstract type and the function type F(Args...)
cannot be formed or when one of the args is cv-qualified,
as top-level cv-qualification for function parameters is dropped by language rules. It should use
is_callable_v<F&&(Args&&...), R>
instead.
Previous resolution [SUPERSEDED]:
This wording is relative to N4606.
Modify 22.10 [function.objects]/2, header
<functional>
synopsis, as indicated:namespace std { // 20.12.3, invoke: template <class F, class... Args> result_of_t<F&&(Args&&...)> invoke(F&& f, Args&&... args); template <class R, class F, class... Args> R invoke(F&& f, Args&&... args);Add the following sequence of paragraphs after 22.10.5 [func.invoke]/1 as indicated:
template <class R, class F, class... Args> R invoke(F&& f, Args&&... args);-?- Returns:
-?- Remarks: This function shall not participate in overload resolution unlessINVOKE(std::forward<F>(f), std::forward<Args>(args)..., R)
(22.10.4 [func.require]).is_callable_v<F&&(Args&&...), R>
istrue
.
[2018-08-22, Zhihao Yuan provides improved wording]
Previous resolution [SUPERSEDED]:
This wording is relative to N4762.
Modify 22.10.2 [functional.syn], header
<functional>
synopsis, as indicated:namespace std { // 22.10.5 [func.invoke], invoke template<class F, class... Args> invoke_result_t<F, Args...> invoke(F&& f, Args&&... args) noexcept(is_nothrow_invocable_v<F, Args...>); template <class R, class F, class... Args> R invoke(F&& f, Args&&... args) noexcept(is_nothrow_invocable_r_v<R, F, Args...>);Add the following sequence of paragraphs after 22.10.5 [func.invoke]/1 as indicated:
template <class R, class F, class... Args> R invoke(F&& f, Args&&... args) noexcept(is_nothrow_invocable_r_v<R, F, Args...>);-?- Constraints:
-?- Returns:is_invocable_r_v<R, F, Args...>
.INVOKE<R>(std::forward<F>(f), std::forward<Args>(args)...)
(22.10.4 [func.require]).
[2021-06-12; Resolved by accepting P2136R3.]
Proposed resolution:
This wording is relative to N4849.
Modify 22.10.2 [functional.syn], header <functional>
synopsis, as indicated:
namespace std { // 22.10.5 [func.invoke], invoke template<class F, class... Args> constexpr invoke_result_t<F, Args...> invoke(F&& f, Args&&... args) noexcept(is_nothrow_invocable_v<F, Args...>); template <class R, class F, class... Args> constexpr R invoke(F&& f, Args&&... args) noexcept(is_nothrow_invocable_r_v<R, F, Args...>);
Add the following sequence of paragraphs after 22.10.5 [func.invoke]/1 as indicated:
template <class R, class F, class... Args> constexpr R invoke(F&& f, Args&&... args) noexcept(is_nothrow_invocable_r_v<R, F, Args...>);-?- Constraints:
-?- Returns:is_invocable_r_v<R, F, Args...>
.INVOKE<R>(std::forward<F>(f), std::forward<Args>(args)...)
(22.10.4 [func.require]).
constexpr
for various std::complex
arithmetic and value operatorsSection: 29.4 [complex.numbers] Status: Resolved Submitter: Oliver Rosten Opened: 2016-04-14 Last modified: 2018-06-12
Priority: 3
View all other issues in [complex.numbers].
View all issues with Resolved status.
Discussion:
This modification will allow complex-number arithmetic to be performed at compile time. From a mathematical
standpoint, it is natural (and desirable) to treat complex numbers on the same footing as the reals.
From a programming perspective, this change will broaden the scope in which std::complex
can be used,
allowing it to be smoothly incorporated into classes exploiting constexpr
.
std::complex
namespace should be made constexpr
:
Section 29.4.5 [complex.member.ops]: The member (arithmetic) operators
{+=
, -=
, /=
, *=
}
Section 29.4.6 [complex.ops]: The arithmetic operators unary operators {+
, -
}
and binary operators {+
, -
, /
, *
};
Section 29.4.7 [complex.value.ops]: The 'value' operators abs
, norm
, and conj
.
In terms of modification of the standard, all that is required is the insertion of the constexpr
specifier
in the relevant parts of:
Section 29.4.2 [complex.syn] (the Header synopsis), together with the corresponding entries in sections 29.4.6 [complex.ops] and 29.4.7 [complex.value.ops].
Sections 29.4.3 [complex], [complex.special] (class template and specializations), together with the corresponding entries in section 29.4.5 [complex.member.ops].
[2016-05 Issues Telecon]
This kind of work (new feature) has been being done via papers rather than via the issues list.
Walter believes that he knows someone who would be willing to write such a paper.
[2018-06 Rapperswil Wednesday issues processing]
This was resolved by P0415, which was adopted in Albequerque.
Proposed resolution:
Section: 28.3.3.1.2.2 [locale.facet] Status: C++17 Submitter: Tim Song Opened: 2016-04-15 Last modified: 2017-07-30
Priority: 3
View all other issues in [locale.facet].
View all issues with C++17 status.
Discussion:
[locale.facet]/1 (then-called [lib.locale.facet]) read in C++03:
Class
facet
is the base class for locale feature sets. A class is a facet if it is publicly derived from another facet, or if it is a class derived fromlocale::facet
and containing a publicly-accessible declaration as follows: [Footnote: This is a complete list of requirements; there are no other requirements. Thus, a facet class need not have a public copy constructor, assignment, default constructor, destructor, etc.]static ::std::locale::id id;Template parameters in this clause which are required to be facets are those named
Facet
in declarations. A program that passes a type that is not a facet, as an (explicit or deduced) template parameter to a locale function expecting a facet, is ill-formed.
LWG 436(i)'s intent appears to be to ban volatile
-qualified facets and permitting
const
-qualified ones. The PR was somewhat poorly worded, however, and the editor in applying
it deleted the whole first half of the paragraph, including the definition of facet and the requirement
for a static data member named id
.
id
member that's not defined anywhere in the working draft.
[2016-08-03 Chicago]
Fri AM: Moved to Tentatively Ready
Proposed resolution:
This wording is relative to N4582.
Insert the following paragraph before 28.3.3.1.2.2 [locale.facet]/1:
-?- Class
facet
is the base class for locale feature sets. A class is a facet if it is publicly derived from another facet, or if it is a class derived fromlocale::facet
and containing a publicly accessible declaration as follows: [Footnote: This is a complete list of requirements; there are no other requirements. Thus, a facet class need not have a public copy constructor, assignment, default constructor, destructor, etc.]static ::std::locale::id id;-1- Template parameters in this Clause which are required to be facets are those named
Facet
in declarations. A program that passes a type that is not a facet, or a type that refers to a volatile-qualified facet, as an (explicit or deduced) template parameter to a locale function expecting a facet, is ill-formed. A const-qualified facet is a valid template argument to any locale function that expects a Facet template parameter.
make_shared
and enable_shared_from_this
is underspecifiedSection: 20.3.2.2.7 [util.smartptr.shared.create] Status: C++17 Submitter: Joe Gottman Opened: 2016-04-02 Last modified: 2017-07-30
Priority: 2
View all other issues in [util.smartptr.shared.create].
View all issues with C++17 status.
Discussion:
For each public constructor of std::shared_ptr
, the standard says that constructor enables
shared_from_this
if that constructor is expected to initialize the internal weak_ptr
of a contained enable_shared_from_this<X>
object. But there are other ways to construct
a shared_ptr
than by using a public constructor. The template functions make_shared
and allocate_shared
both require calling a private constructor, since no public constructor
can fulfill the requirement that at most one allocation is made. The standard does not specify that
that private constructor enables shared_from_this
; therefore in the following code:
struct Foo : public std::enable_shared_from_this<Foo> {}; int main() { auto p = std::make_shared<Foo>(); assert(p == p->shared_from_this()); return 0; }
it is unspecified whether the assertion will fail.
[2016-05 Issues Telecon]
Jonathan Wakely to provide updated wording.
[2016-08 Chicago]
Monday PM: Move to Tentatively Ready
Proposed resolution:
This wording is relative to N4582.
Change 20.3.2.2.7 [util.smartptr.shared.create] indicated:
template<class T, class... Args> shared_ptr<T> make_shared(Args&&... args); template<class T, class A, class... Args> shared_ptr<T> allocate_shared(const A& a, Args&&... args);[…]
-6- Remarks: Theshared_ptr
constructor called by this function enablesshared_from_this
with the address of the newly constructed object of typeT
. Implementations should perform no more than one memory allocation. [Note: This provides efficiency equivalent to an intrusive smart pointer. — end note]
future/shared_future
unwrapping constructor when given an invalid future
Section: 99 [concurr.ts::futures.unique_future], 99 [concurr.ts::futures.shared_future] Status: C++23 Submitter: Tim Song Opened: 2016-04-22 Last modified: 2023-11-22
Priority: 2
View all issues with C++23 status.
Discussion:
Addresses: concurr.ts
In the concurrency TS, the future/shared_future unwrapping constructors
future(future<future<R>>&&) noexcept; shared_future(future<shared_future<R>>&& rhs) noexcept;
appear to implicitly require rhs
be valid (e.g., by referring to its shared state, and by requiring a
valid() == true
postcondition). However, they are also marked noexcept
, suggesting that they
are wide-contract, and also makes the usual suggested handling for invalid futures, throwing a
future_error
, impossible.
noexcept
should be removed, or the behavior with an invalid future should be specified.
Original resolution alternative #1 [NOT CHOSEN]:
This wording is relative to N4577.
Strike the
noexcept
on these constructors in 99 [concurr.ts::futures.unique_future]/1-2 and 99 [concurr.ts::futures.shared_future]/1-2, and optionally add a Requires:rhs.valid() == true
paragraph.
[2016-11-12, Issaquah]
Sat PM: We prefer alternative #2 - Move to review
[2018-06; Rapperswil, Wednesday evening session]
DR: there is a sentence ended followed by an entirely new sentence
JM: so the period should be a semicolon in both edits
MC: ACTION I can make the change editorially
ACTION move to Ready
[2018-11, Adopted in San Diego]
Proposed resolution:
This wording is relative to N4577.
Alternative #2: Specify that an empty (
shared_
)future
object is constructed ifrhs
is invalid, and adjust the postcondition accordingly.
Edit 99 [concurr.ts::futures.unique_future] as indicated:
future(future<future<R>>&& rhs) noexcept;-3- Effects: If
rhs.valid() == false
, constructs an emptyfuture
object that does not refer to a shared state. Otherwise, cConstructs afuture
object from the shared state referred to byrhs
. T; thefuture
becomes ready when one of the following occurs:
Both the
rhs
andrhs.get()
are ready. The value or the exception fromrhs.get()
is stored in thefuture
's shared state.
rhs
is ready butrhs.get()
is invalid. An exception of typestd::future_error
, with an error condition ofstd::future_errc::broken_promise
is stored in thefuture
's shared state.-4- Postconditions:
valid() == true
valid()
returns the same value asrhs.valid()
prior to the constructor invocation..
rhs.valid() == false
.
Edit 99 [concurr.ts::futures.shared_future] as indicated:
shared_future(future<shared_future<R>>&& rhs) noexcept;-3- Effects: If
rhs.valid() == false
, constructs an emptyshared_future
object that does not refer to a shared state. Otherwise, cConstructs ashared_future
object from the shared state referred to byrhs
. T; theshared_future
becomes ready when one of the following occurs:
Both the
rhs
andrhs.get()
are ready. The value or the exception fromrhs.get()
is stored in theshared_future
's shared state.
rhs
is ready butrhs.get()
is invalid. Theshared_future
stores an exception of typestd::future_error
, with an error condition ofstd::future_errc::broken_promise
.-4- Postconditions:
valid() == true
valid()
returns the same value asrhs.valid()
prior to the constructor invocation..
rhs.valid() == false
.
assign()
on iterators/pointers/referencesSection: 23.2.4 [sequence.reqmts] Status: C++17 Submitter: Tim Song Opened: 2016-04-25 Last modified: 2017-07-30
Priority: 0
View other active issues in [sequence.reqmts].
View all other issues in [sequence.reqmts].
View all issues with C++17 status.
Discussion:
The sequence container requirements table says nothing about the effect of assign()
on iterators,
pointers or references into the container. Before LWG 2209(i) (and LWG 320(i) for std::list
),
assign()
was specified as "erase everything then insert
", which implies wholesale invalidation from the
"erase everything" part. With that gone, the blanket "no invalidation" wording in
23.2.2 [container.requirements.general]/12 would seem to apply, which makes absolutely no sense.
[2016-05 Issues Telecon]
Proposed resolution:
This wording is relative to N4582.
In 23.2.4 [sequence.reqmts], edit Table 107 (Sequence container requirements) as indicated:
Table 107 — Sequence container requirements (in addition to container) Expression Return type Assertion/note
pre-/post-condition[…]
a.assign(i, j)
void
Requires: T
shall beEmplaceConstructible
intoX
from*i
and assignable from*i
.
Forvector
, if the iterator does not meet the forward iterator requirements (24.2.5),
T
shall also beMoveInsertable
intoX
.
Each iterator in the range[i, j)
shall be dereferenced exactly once.
pre:i
,j
are not iterators intoa
.
Replaces elements ina
with a copy of[i, j)
.
Invalidates all references, pointers and iterators referring to the elements ofa
.
Forvector
anddeque
, also invalidates the past-the-end iterator.[…]
a.assign(n, t)
void
Requires: T
shall beCopyInsertable
intoX
andCopyAssignable
.
pre:t
is not a reference intoa
.
Replaces elements ina
withn
copies oft
.
Invalidates all references, pointers and iterators referring to the elements ofa
.
Forvector
anddeque
, also invalidates the past-the-end iterator.
Section: 29.2 [numeric.requirements] Status: C++17 Submitter: Hubert Tong Opened: 2016-05-03 Last modified: 2017-07-30
Priority: 3
View all other issues in [numeric.requirements].
View all issues with C++17 status.
Discussion:
In N4582 subclause 29.2 [numeric.requirements], the "considerable flexibility in how arrays are initialized" do not appear to allow for replacement of calls to the default constructor with calls to the copy constructor with an appropriate source value.
[2016-08-03 Chicago]
Fri AM: Moved to Tentatively Ready
Proposed resolution:
This wording is relative to N4582.
Adjust 29.2 [numeric.requirements]/1 as indicated:
-1- The
complex
andvalarray
components are parameterized by the type of information they contain and manipulate. […]
(1.1) —
T
is not an abstract class (it has no pure virtual member functions);[…]
(1.8) — If
T
is a class, its assignment operator, copy and default constructors, and destructor shall correspond to each other in the following sense: Initialization of raw storage using the copy constructor on the value ofT()
, however obtained, is semantically equivalent to value initialization of the same raw storage. Initialization of raw storage using the default constructor, followed by assignment, is semantically equivalent to initialization of raw storage using the copy constructor. Destruction of an object, followed by initialization of its raw storage using the copy constructor, is semantically equivalent to assignment to the original object. [Note: This rule states, in part, that there shall not be any subtle differences in the semantics of initialization versus assignment. This gives an implementation considerable flexibility in how arrays are initialized. [Example: An implementation is allowed to initialize avalarray
by allocating storage using thenew
operator (which implies a call to the default constructor for each element) and then assigning each element its value. Or the implementation can allocate raw storage and use the copy constructor to initialize each element. — end example] If the distinction between initialization and assignment is important for a class, or if it fails to satisfy any of the other conditions listed above, the programmer should usevector
(23.3.11) instead ofvalarray
for that class; — end note]
recursive_directory_iterator
's members should require '*this
is dereferenceable'Section: 31.12.12.2 [fs.rec.dir.itr.members], 31.12.11.2 [fs.dir.itr.members] Status: C++17 Submitter: Eric Fiselier Opened: 2016-05-08 Last modified: 2017-07-30
Priority: 1
View all other issues in [fs.rec.dir.itr.members].
View all issues with C++17 status.
Discussion:
In 31.12.12.2 [fs.rec.dir.itr.members] the following members are specified as having the requirement
"*this != recursive_directory_iterator{}
":
options()
depth()
recursion_pending()
operator++
increment(...)
pop()
disable_recursion_pending()
This requirement is not strong enough since it still allows non-dereferenceable iterators to invoke these methods. For example:
recursive_directory_iterator it("."); recursive_directory_iterator it_copy(it); assert(it_copy.depth() == 0); // OK ++it; assert(it_copy.depth() == ???); // Not OK auto x = *it_copy; // Is this OK?
I believe these should instead require that *this
is dereferenceable, however the current specification
seems to say that all previous copies of it
are still dereferenceable although not what they dereference to.
The result of
operator*
on an end iterator is undefined behavior. For any other iterator value aconst recursive_directory_entry&
is returned. The result ofoperator->
on an end iterator is undefined behavior. For any other iterator value aconst directory_entry*
is returned.
Is the intention of this clause to make all non-end iterators dereferenceable?
One further complication with these methods comes from the specification ofrecursive_directory_iterator
's
copy/move constructors and assignment operators which specify the following post conditions:
this->options() == rhs.options()
this->depth() == rhs.depth()
this->recursion_pending() == rhs.recursion_pending()
If rhs
is the end iterator these post conditions are poorly stated.
[2016-06, Oulu — Daniel comments]
The loss of information caused by bullet three of the suggested wording below is corrected by 2726(i)'s wording.
Voted to Ready 7-0 Monday morning in Oulu
Proposed resolution:
This wording is relative to N4582.
[Drafting note: I have not attempted to fix the specification of the copy/move constructors and assignment operators for
recursive_directory_iterator
][Drafting note:
increment
directly specifies "Effects: As specified by Input iterators (24.2.3)", so no additional specification is needed.]
Change [fs.class.directory_iterator] p4 as indicated:
-4-
The result ofThe end iterator is not dereferenceable.operator*
on an end iterator is undefined behavior. For any other iterator value aconst directory_entry&
is returned. The result ofoperator->
on an end iterator is undefined behavior. For any other iterator value aconst directory_entry*
is returned
Add a new bullet after the class synopsis in 31.12.12 [fs.class.rec.dir.itr]:
-?- Callingoptions
,depth
,recursion_pending
,pop
ordisable_recursion_pending
on an iterator that is not dereferencable results in undefined behavior.
Change 31.12.12.2 [fs.rec.dir.itr.members] as indicated:
directory_options options() const;-17-
[…]Requires:*this != recursive_directory_iterator()
.int depth() const;-20-
[…]Requires:*this != recursive_directory_iterator()
.bool recursion_pending() const;-23-
[…]Requires:*this != recursive_directory_iterator()
.recursive_directory_iterator& operator++(); recursive_directory_iterator& increment(error_code& ec) noexcept;-26-
[…]Requires:*this != recursive_directory_iterator()
.void pop();-30-
[…]Requires:*this != recursive_directory_iterator()
.void disable_recursion_pending();-32-
[…]Requires:*this != recursive_directory_iterator()
.
recursive_directory_iterator::pop()
is under-specifiedSection: 31.12.12 [fs.class.rec.dir.itr] Status: C++17 Submitter: Eric Fiselier Opened: 2016-05-09 Last modified: 2017-07-30
Priority: 0
View all issues with C++17 status.
Discussion:
Unlike increment
, pop()
does not specify how it reports errors nor does it provide a
std::error_code
overload. However implementing pop()
all but requires performing an increment,
so it should handle errors in the same way.
Proposed resolution:
This wording is relative to N4582.
Change 31.12.12 [fs.class.rec.dir.itr], class recursive_directory_iterator
synopsis, as indicated:
namespace std::filesystem { class recursive_directory_iterator { public: […] void pop(); void pop(error_code& ec); void disable_recursion_pending(); […] }; }
Change 31.12.12.2 [fs.rec.dir.itr.members] as indicated:
void pop(); void pop(error_code& ec);-30- Requires:
-31- Effects: If*this != recursive_directory_iterator()
.depth() == 0
, set*this
torecursive_directory_iterator()
. Otherwise, cease iteration of the directory currently being iterated over, and continue iteration over the parent directory. -?- Throws: As specified in Error reporting (31.5.6 [error.reporting]).
path
construction and assignment should have "string_type&&
" overloadsSection: 31.12.6 [fs.class.path] Status: C++17 Submitter: Eric Fiselier Opened: 2016-05-09 Last modified: 2017-07-30
Priority: 0
View all other issues in [fs.class.path].
View all issues with C++17 status.
Discussion:
Currently construction of a path
from the native string_type
always performs a copy, even when
that string is passed as a rvalue. This is a large pessimization as paths are commonly constructed from temporary strings.
path foo(path const& p) { auto s = p.native(); mutateString(s); return s; }
Implementations should be allowed to move from s
and avoid an unnecessary allocation.
I believe string_type&&
constructor and assignment operator overloads should be added to support this.
Proposed resolution:
This wording is relative to N4582.
Change 31.12.6 [fs.class.path], class path
synopsis, as indicated:
[Drafting note: Making the
string_type&&
constructors and assignment operatorsnoexcept
would over-constrain implementations which may need to perform construct additional state]
namespace std::filesystem { class path { public: […] // 27.10.8.4.1, constructors and destructor path() noexcept; path(const path& p); path(path&& p) noexcept; path(string_type&& source); template <class Source> path(const Source& source); […] // 27.10.8.4.2, assignments path& operator=(const path& p); path& operator=(path&& p) noexcept; path& operator=(string_type&& source); path& assign(string_type&& source); template <class Source> path& operator=(const Source& source); template <class Source> path& assign(const Source& source) template <class InputIterator> path& assign(InputIterator first, InputIterator last); […] }; }
Add a new paragraph following 31.12.6.5.1 [fs.path.construct]/3:
path(string_type&& source);-?- Effects: Constructs an object of class
path
withpathname
having the original value ofsource
.source
is left in a valid but unspecified state.
Add a new paragraph following 31.12.6.5.2 [fs.path.assign]/4:
path& operator=(string_type&& source); path& assign(string_type&& source);-?- Effects: Modifies
-?- Returns:pathname
to have the original value ofsource
.source
is left in a valid but unspecified state.*this
offsetof
is unnecessarily impreciseSection: 17.2 [support.types] Status: C++17 Submitter: Richard Smith Opened: 2016-05-10 Last modified: 2017-07-30
Priority: 2
View all other issues in [support.types].
View all issues with C++17 status.
Discussion:
Per 17.2 [support.types]/4:
"The macro
offsetof(type, member-designator)
accepts a restricted set of type arguments in this International Standard. If type is not a standard-layout class (Clause 9), the results are undefined. [...]"
Implementations in practice interpret this as meaning that the program is ill-formed for classes that are not standard-layout, but several implementations allow additional types as an extension (rejected in their "strictly conforming" modes).
It would seem superior to specify that implementation-defined extensions are permitted, and that the implementation must give a correct answer for any type that it chooses to accept.[2016-05 Issues Telecon]
People were worried about the requirement to report errors implied by 'conditionally supported'.
[2016-06 Oulu]
The concern about errors appears to be less severe that we thought. Moving to Ready.
Friday: status to Immediate
Proposed resolution:
This wording is relative to N4582.
Change in 17.2 [support.types]/4:
-4- The macro
offsetof(type, member-designator)
accepts a restricted set of type arguments in this International Standard.IfUse of thetype
is notoffsetof
macro with atype
other than a standard-layout class (Clause 9), the results are undefinedis conditionally-supported.193 The expressionoffsetof(type, member-designator)
is never type-dependent (14.6.2.2) and it is value-dependent (14.6.2.3) if and only iftype
is dependent. The result of applying theoffsetof
macro to a static data member or a function member is undefined. No operation invoked by theoffsetof
macro shall throw an exception andnoexcept(offsetof(type, member-designator))
shall betrue
.
Section: 16.3.2.4 [structure.specifications] Status: C++17 Submitter: Kazutoshi Satoda Opened: 2016-05-08 Last modified: 2017-07-30
Priority: 0
View other active issues in [structure.specifications].
View all other issues in [structure.specifications].
View all issues with C++17 status.
Discussion:
From N4582 17.5.1.4 [structure.specifications] p3 and p4
-3- Descriptions of function semantics contain the following elements (as appropriate):
- Requires: the preconditions for calling the function
- Effects: the actions performed by the function
- Synchronization: the synchronization operations (1.10) applicable to the function
- Postconditions: the observable results established by the function
- Returns: a description of the value(s) returned by the function
- Throws: any exceptions thrown by the function, and the conditions that would cause the exception
- Complexity: the time and/or space complexity of the function
- Remarks: additional semantic constraints on the function
- Error conditions: the error conditions for error codes reported by the function.
- Notes: non-normative comments about the function
-4- Whenever the Effects: element specifies that the semantics of some function
F
are Equivalent to some code sequence, then the various elements are interpreted as follows. IfF
's semantics specifies a Requires: element, then that requirement is logically imposed prior to the equivalent-to semantics. Next, the semantics of the code sequence are determined by the Requires:, Effects:, Postconditions:, Returns:, Throws:, Complexity:, Remarks:, Error conditions:, and Notes: specified for the function invocations contained in the code sequence. The value returned fromF
is specified byF
's Returns: element, or ifF
has no Returns: element, a non-void
return fromF
is specified by the Returns: elements in the code sequence. IfF
's semantics contains a Throws:, Postconditions:, or Complexity: element, then that supersedes any occurrences of that element in the code sequence.
The third sentence of p4 says "the semantics of the code sequence are determined by ..." and lists all elements in p3 except "Synchronization:".
I think it was just an oversight because p4 was added by library issue 997(i), and its proposed resolution was drafted at the time (2009) before "Synchronization:" was added into p3 for C++11. However, I'm not definitely sure that it is really intended and safe to just supply "Synchronization:" in the list. (Could a library designer rely on this in writing new specifications, or could someone rely on this in writing user codes, after some years after C++11?)Proposed resolution:
This wording is relative to N4582.
Change 16.3.2.4 [structure.specifications] as indicated:
-4- Whenever the Effects: element specifies that the semantics of some function
F
are Equivalent to some code sequence, then the various elements are interpreted as follows. IfF
's semantics specifies a Requires: element, then that requirement is logically imposed prior to the equivalent-to semantics. Next, the semantics of the code sequence are determined by the Requires:, Effects:, Synchronization:, Postconditions:, Returns:, Throws:, Complexity:, Remarks:, Error conditions:, and Notes: specified for the function invocations contained in the code sequence. The value returned fromF
is specified byF
's Returns: element, or ifF
has no Returns: element, a non-void
return fromF
is specified by the Returns: elements in the code sequence. IfF
's semantics contains a Throws:, Postconditions:, or Complexity: element, then that supersedes any occurrences of that element in the code sequence.
path
is convertible from approximately everything under the sunSection: 31.12.6.5.1 [fs.path.construct] Status: C++17 Submitter: Tim Song Opened: 2016-05-10 Last modified: 2017-07-30
Priority: 1
View all issues with C++17 status.
Discussion:
The unconstrained template<class Source> path(const Source& source);
constructor defines an
implicit conversion from pretty much everything to path
. This can lead to surprising ambiguities in
overload resolution.
struct meow { operator const char *() const; }; std::ifstream purr(meow{});
because a meow
can be converted to either a path
or a const char*
by a user-defined
conversion, even though part of the stated goal of LWG 2676(i)'s P/R is to avoid "break[ing] user
code depending on user-defined automatic conversions to the existing argument types".
Previous resolution [SUPERSEDED]:
This wording is relative to N4582.
[Drafting note:
decay_t<Source>
handles both the input iterator case (31.12.6.4 [fs.path.req]/1.2) and the array case (31.12.6.4 [fs.path.req]/1.3). The level of constraints required is debatable; the following wording limits the constraint to "is abasic_string
or an iterator", but perhaps stronger constraints (e.g., aniterator_category
check in the second case, and/or avalue_type
check for both cases) would be desirable.]
Change 31.12.6.5.1 [fs.path.construct] as indicated:
template <class Source> path(const Source& source); template <class InputIterator> path(InputIterator first, InputIterator last);-4- Effects: Constructs an object of class
-?- Remarks: The first overload shall not participate in overload resolution unless eitherpath
, storing the effective range ofsource
(27.10.8.3) or the range[first, last)
inpathname
, converting format and encoding if required (27.10.8.2.1).Source
is a specialization ofbasic_string
or the qualified-iditerator_traits<decay_t<Source>>::value_type
is valid and denotes a type (13.10.3 [temp.deduct]).
[2016-05-28, Eric Fiselier comments suggests alternative wording]
Functions taking EcharT
or Source
parameter types often provide additional overloads with the
same arity and concrete types. In order to allow conversions to these concrete types in the interface we need to
explicitly disable the EcharT
and Source
overloads.
[2016-06-19, Eric and Tim improve the wording]
[2016-06, Oulu]
Voted to Ready 8-0 Monday morning in Oulu
Proposed resolution:
This wording is relative to N4594.
[Drafting note: Functions taking
EcharT
orSource
parameter types often provide additional overloads with the same arity and concrete types. In order to allow conversions to these concrete types in the interface we need to explicitly disable theEcharT
andSource
overloads.]
Change 31.12.3 [fs.req] as indicated:
-2-
Template parameters namedFunctions with template parameters namedEcharT
shall beEcharT
shall not participate in overload resolution unlessEcharT
is one of the encoded character types.
Insert a new paragraph between 31.12.6.4 [fs.path.req] p1 and p2 as indicated:
-?- Functions taking template parameters named
Source
shall not participate in overload resolution unless eitherSource
is a specialization ofbasic_string
or the qualified-iditerator_traits<decay_t<Source>>::value_type
is valid and denotes a possiblyconst
encoded character type (13.10.3 [temp.deduct]).
copy_file(from, to, ...)
has a number of unspecified error conditionsSection: 31.12.13.5 [fs.op.copy.file] Status: C++17 Submitter: Eric Fiselier Opened: 2016-05-10 Last modified: 2021-06-06
Priority: 2
View all other issues in [fs.op.copy.file].
View all issues with C++17 status.
Discussion:
There are a number of error cases that copy_file(from, to, ...)
does not take into account.
Specifically the cases where:
from
does not existfrom
is not a regular fileto
exists and is not a regular fileThese error cases should be specified as such.
[2016-05 Issues Telecon]
Eric to provide wording.
[2016-05-28, Eric Fiselier provides wording]
[2016-08 Chicago]
Wed AM: Move to Tentatively Ready
Proposed resolution:
This wording is relative to N4582.
Modify [fs.op.copy_file] as indicated:
bool copy_file(const path& from, const path& to, copy_options options); bool copy_file(const path& from, const path& to, copy_options options, error_code& ec) noexcept;-3- Requires: At most one constant from each
-4- Effects: Report a file already exists error as specified in Error reporting (27.5.6.5) if:copy_options
option group (27.10.10.2) is present inoptions
.
!is_regular_file(from)
, orexists(to)
and!is_regular_file(to)
, orexists(to)
andequivalent(from, to)
, orexists(to)
and(options & (copy_options::skip_existing | copy_options::overwrite_existing | copy_options::update_existing)) == copy_options::none
.
Section: 32.5.8 [atomics.types.generic] Status: Resolved Submitter: S. B. Tam Opened: 2016-05-24 Last modified: 2017-03-20
Priority: 3
View all other issues in [atomics.types.generic].
View all issues with Resolved status.
Discussion:
32.5.8 [atomics.types.generic] mentions 'aggregate initialization syntax'. It's unclear what it stands for, especially since atomic types are actually not aggregate types (they have user-provided constructors).
[2016-11-13, Jonathan comments and provides wording]
LWG agreed to simply strike those sentences. We believe their purpose is to require that the same syntax as C programs use is valid, e.g.
atomic_int i = { 1 };
But such syntax is already required to work by the constructor
definitions, so there is no need to require it again here with
redundant wording.
It's unnecessary to require that syntax for the atomic<T*>
specializations because they have no equivalent in C anyway (unlike
the named atomic types for integers, there are no typedefs for
atomic<T*>
).
atomic_int i = { 1 };
— end example]
[2016-11-12, Issaquah]
Sat PM: The key here is that we need to support = { ... }
JW to provide wording
[2017-03-04, Kona]
This is resolved by P0558.
Proposed resolution:
This wording is relative to N4606.
Edit 32.5.8 [atomics.types.generic] as indicated:
-5- The atomic integral specializations and the specialization
-6- There shall be pointer partial specializations of theatomic<bool>
shall have standard layout. They shall each have a trivial default constructor and a trivial destructor.They shall each support aggregate initialization syntax.atomic
class template. These specializations shall have standard layout, trivial default constructors, and trivial destructors.They shall each support aggregate initialization syntax.
shuffle
and sample
disallows lvalue URNG
sSection: 26.7.12 [alg.random.sample], 26.7.13 [alg.random.shuffle] Status: C++17 Submitter: Tim Song Opened: 2016-05-24 Last modified: 2017-07-30
Priority: 0
View all issues with C++17 status.
Discussion:
std::shuffle
and std::sample
each accepts a UniformRandomNumberGenerator&&
argument
and says that "UniformRandomNumberGenerator
shall meet the requirements of a uniform random number generator
(29.5.3.3 [rand.req.urng]) type".
G
satisfies the requirements of a uniform random
number generator if […]".
If an lvalue is passed, UniformRandomNumberGenerator
is a reference type, not a class, and in fact expressions
like G::min()
will not compile if G
is a reference type.
[2016-06 Oulu]
Moved to P0/Ready during issues prioritization.
Friday: status to Immediate
Proposed resolution:
This wording is relative to N4582.
Edit 26.7.12 [alg.random.sample]/1 as indicated:
template<class PopulationIterator, class SampleIterator, class Distance, class UniformRandomNumberGenerator> SampleIterator sample(PopulationIterator first, PopulationIterator last, SampleIterator out, Distance n, UniformRandomNumberGenerator&& g);-1- Requires::
- […]
- (1.6) —
remove_reference_t<UniformRandomNumberGenerator>
shall meet the requirements of a uniform random number generator type (26.6.1.3) whose return type is convertible toDistance
.- […]
Edit 26.7.13 [alg.random.shuffle]/2 as indicated:
template<class RandomAccessIterator, class UniformRandomNumberGenerator> void shuffle(RandomAccessIterator first, RandomAccessIterator last, UniformRandomNumberGenerator&& g);-1- […]
-2- Requires:RandomAccessIterator
shall satisfy the requirements ofValueSwappable
(17.6.3.2). The typeremove_reference_t<UniformRandomNumberGenerator>
shall meet the requirements of a uniform random number generator (26.6.1.3) type whose return type is convertible toiterator_traits<RandomAccessIterator>::difference_type
.
Section: 26.3.3 [algorithms.parallel.exec] Status: C++17 Submitter: Jared Hoberock Opened: 2016-05-25 Last modified: 2017-07-30
Priority: Not Prioritized
View all other issues in [algorithms.parallel.exec].
View all issues with C++17 status.
Discussion:
The paragraph describing the effect of composing sequential_execution_policy
with parallel algorithms says:
The invocations of element access functions in parallel algorithms invoked with an execution policy object of type
sequential_execution_policy
are indeterminately sequenced (1.9) in the calling thread.
The intended behavior of sequential_execution_policy
is to match the classic-style algorithm semantics, cf.
26.3.5 [algorithms.parallel.overloads] p2:
Unless otherwise specified, the semantics of
ExecutionPolicy
algorithm overloads are identical to their overloads without.
Because many classic-style algorithms execute element access functions in a specified sequence,
26.3.3 [algorithms.parallel.exec] p2 introduces unintentionally different semantics between classic-style
algorithms and parallel algorithms invoked with sequential_execution_policy
.
The invocations of element access functions in parallel algorithms invoked with an execution policy object of type
sequential_execution_policy
execute in sequential order in the calling thread.
to
The invocations of element access functions in parallel algorithms invoked with an execution policy object of type
sequential_execution_policy
are indeterminately sequenced (1.9) in the calling thread.
Suggested resolution:
To restore the originally intended behavior ofsequential_execution_policy
, Jens Maurer suggests
replacing 26.3.3 [algorithms.parallel.exec] p2 with:
The invocations of element access functions in parallel algorithms invoked with an execution policy object of type
sequential_execution_policy
all occur in the calling thread. [Note: The invocations are not interleaved; see 6.9.1 [intro.execution] — end note]
[2016-06 Oulu]
Could be P0 after SG1 gives OK
Tuesday, Oulu: Hans Ok'd this
Friday: status to Immediate
Proposed resolution:
This wording is relative to N4582.
Change 26.3.3 [algorithms.parallel.exec] as indicated:
The invocations of element access functions in parallel algorithms invoked with an execution policy object of type
sequential_execution_policy
all occurare indeterminately sequenced (1.9)in the calling thread. [Note: The invocations are not interleaved; see 6.9.1 [intro.execution] — end note]
permissions
function should not be noexcept
due to narrow contractSection: 31.12.13.27 [fs.op.permissions] Status: C++17 Submitter: Eric Fiselier Opened: 2016-05-28 Last modified: 2017-07-30
Priority: 0
View all other issues in [fs.op.permissions].
View all issues with C++17 status.
Discussion:
Currently the signatures for permissions
are:
void permissions(const path& p, perms prms); void permissions(const path& p, perms prms, error_code& ec) noexcept;
However both overloads have a narrow contract since due to the requires clause:
Requires:
!((prms & perms::add_perms) != perms::none && (prms & perms::remove_perms) != perms::none)
.
For this reason I believe the second overload of permissions
should not be marked noexcept
.
[2016-06 Oulu]
Moved to P0/Ready during issues prioritization.
Friday: status to Immediate
Proposed resolution:
This wording is relative to N4582.
Change 31.12.4 [fs.filesystem.syn] as indicated:
namespace std::filesystem { […] void permissions(const path& p, perms prms); void permissions(const path& p, perms prms, error_code& ec)noexcept; […] }
Change 31.12.13.27 [fs.op.permissions] as indicated:
void permissions(const path& p, perms prms); void permissions(const path& p, perms prms, error_code& ec)noexcept;-1- Requires:
!((prms & perms::add_perms) != perms::none && (prms & perms::remove_perms) != perms::none)
.
permissions
function incorrectly specified for symlinksSection: 31.12.13.27 [fs.op.permissions] Status: C++17 Submitter: Eric Fiselier Opened: 2016-05-28 Last modified: 2017-07-30
Priority: 2
View all other issues in [fs.op.permissions].
View all issues with C++17 status.
Discussion:
Currently when adding or removing permissions the permissions(p, prms, [...])
function always
determines the current permissions for a file p
using status(p).permissions()
. This
means that it resolves symlinks even when perms::resolve_symlinks
was not specified.
symlink_status(p).permissions()
should be used
unless perms::resolve_symlinks
is specified.
Previous resolution [SUPERSEDED]:
This wording is relative to N4582.
In 31.12.13.27 [fs.op.permissions] change Table 150 — "Effects of permission bits" as indicated:
Table 150 — Effects of permission bits Bits present in prms
Effective bits applied Neither add_perms
norremove_perms
prms & perms::mask
add_perms
andresolve_symlinks
status(p).permissions() | (prms & perms::mask)
remove_perms
andresolve_symlinks
status(p).permissions() & (prms & perms::mask)
add_perms
and notresolve_symlinks
symlink_status(p).permissions() | (prms & perms::mask)
remove_perms
and notresolve_symlinks
symlink_status(p).permissions() & ~(prms & perms::mask)
[2016-06, Oulu — Jonathan comments and provides alternative wording]
We agree there is an issue here, but I don't like the proposed
resolution. If Eric's P/R is accepted then it changes the default
behaviour (when users do not set the perms::resolve_symlinks
bit) to
modify the permissions of the symlink itself.
chmod
system call. To change permissions of a symlink with POSIX
you must use the newer fchmodat
function and the AT_SYMLINK_NOFOLLOW
flag, see here.
Changing permissions of a symlink is not possible using the GNU chmod
util, see
here:
"
chmod
never changes the permissions of symbolic links, since thechmod
system call cannot change their permissions. This is not a problem since the permissions of symbolic links are never used."
BSD chmod
does provide a switch to change a symlink's permissions, but
it's not the default.
filesystem::perms::resolve_symlinks
enumerator with
filesystem::perms::symlink_nofollow
(paint the bikeshed!), so that the
default is sensible, and the uncommon, useless alternative of changing
the symlink itself requires setting a bit in the flags explicitly.
resolve_symlinks
is unused in the spec today, the only mention is its
definition in Table 147.
[2016-06, Oulu]
There exists a slightly related issue, 2728(i).
[2016-06 Oulu]
Tuesday: Move to Ready. JW and Eric to implement and report back if problems found.
Friday: status to Immediate
Proposed resolution:
This wording is relative to N4594.
Change Table 147 — "Enum class perms
" as indicated:
Table 147 — Enum class perms
Name Value
(octal)POSIX
macroDefinition or notes symlink_nofollow
resolve_symlinks0x40000
permissions()
shall change the permissions of symbolic linksresolve symlinks
Edit 31.12.13.27 [fs.op.permissions]:
void permissions(const path& p, perms prms); void permissions(const path& p, perms prms, error_code& ec) noexcept;-1- Requires:
-2- Effects: Applies the effective permissions bits from!((prms & perms::add_perms) != perms::none && (prms & perms::remove_perms) != perms::none)
.prms
to the filep
resolves to, or if that file is a symbolic link andsymlink_nofollow
is not set inprms
, the file that it points to, as if by POSIXfchmodat()
. The effective permission bits are determined as specified in Table 150, wheres
is the result of(prms & perms::symlink_nofollow) != perms::none ? symlink_status(p) : status(p)
.
Change Table 150 — "Effects of permission bits" as indicated:
[Drafting note: Very recently the project editor had already fixed a typo in Table 150 editorially, the applied change effectively was:
status(p).permissions() & ~(prms & perms::mask)]
Table 150 — Effects of permission bits Bits present in prms
Effective bits applied Neither add_perms
norremove_perms
prms & perms::mask
add_perms
s
tatus(p).permissions() | (prms & perms::mask)remove_perms
s
tatus(p).permissions() & (prms & perms::mask)
remove_all
has incorrect post conditionsSection: 31.12.13.32 [fs.op.remove.all] Status: C++17 Submitter: Eric Fiselier Opened: 2016-05-28 Last modified: 2021-06-06
Priority: 3
View all issues with C++17 status.
Discussion:
The current post condition for remove_all(p, [...])
states:
Postcondition:
!exists(p)
This is not correct when p
is a symlink, since !exists(p)
reads through the symlink.
The postcondition should be changed to match that of remove
which states !exists(symlink_status(p))
.
[2016-06, Oulu — Eric clarifies the importance of the suggested change]
With the current post conditions remove_all(p)
could just not remove
dangling symlinks and still meet the post conditions.
Moved to Ready after Eric convinced the room.
Friday: status to Immediate
Proposed resolution:
This wording is relative to N4582.
Change [fs.op.remove_all] as indicated:
uintmax_t remove_all(const path& p); uintmax_t remove_all(const path& p, error_code& ec) noexcept;-1- Effects: Recursively deletes the contents of
-2- [Note: A symbolic link is itself removed, rather than the file it resolves to being removed. — end note] -3- Postcondition:p
if it exists, then deletes filep
itself, as if by POSIXremove()
.!exists(symlink_status(p))
.
equivalent
incorrectly specifies throws clauseSection: 31.12.13.13 [fs.op.equivalent] Status: C++17 Submitter: Eric Fiselier Opened: 2016-05-28 Last modified: 2017-07-30
Priority: 3
View all other issues in [fs.op.equivalent].
View all issues with C++17 status.
Discussion:
The spec for equivalent
has a throws clause which reads: [fs.op.equivalent]/5
Throws:
filesystem_error
if(!exists(s1) && !exists(s2)) || (is_other(s1) && is_other(s2))
, otherwise as specified in Error reporting (27.10.7).
This explicit requirement to throw is incorrect for the equivalent
overload which takes an error_code
.
Previous resolution [SUPERSEDED]:
This wording is relative to N4582.
Modify 31.12.13.13 [fs.op.equivalent] as follows:
bool equivalent(const path& p1, const path& p2); bool equivalent(const path& p1, const path& p2, error_code& ec) noexcept;-1- Effects: Determines
-2- Returns: Iffile_status s1
ands2
, as if bystatus(p1)
andstatus(p2)
, respectively.(!exists(s1) && !exists(s2)) || (is_other(s1) && is_other(s2))
an error is reported (31.12.5 [fs.err.report]). Otherwisetrue
, ifs1 == s2
andp1
andp2
resolve to the same file system entity, elsefalse
. The signature with argumentec
returnsfalse
if an error occurs. -3- Two paths are considered to resolve to the same file system entity if two candidate entities reside on the same device at the same location. This is determined as if by the values of the POSIXstat
structure, obtained as if bystat()
for the two paths, having equalst_dev
values and equalst_ino
values. -4- Throws:As specified infilesystem_error
if(!exists(s1) && !exists(s2)) || (is_other(s1) && is_other(s2))
, otherwise aEerror reporting (31.12.5 [fs.err.report]).
[2016-06 Oulu — Daniel provides wording improvements]
mc: do we have an error reporting clause?
jw: there is no such clause
gr: it should go into the effects clause
dk: we have the same issue for file_size
dk: the right place is the effects clause
[2016-08 Chicago]
Wed AM: Move to Tentatively Ready
Proposed resolution:
This wording is relative to N4594.
Modify 31.12.13.13 [fs.op.equivalent] as follows:
bool equivalent(const path& p1, const path& p2); bool equivalent(const path& p1, const path& p2, error_code& ec) noexcept;-1- Let
-2- Effects: Determiness1
ands2
befile_status
s, determined as if bystatus(p1)
andstatus(p2)
, respectively.s1
ands2
. If(!exists(s1) && !exists(s2)) || (is_other(s1) && is_other(s2))
an error is reported (31.12.5 [fs.err.report]). -3- Returns:true
, ifs1 == s2
andp1
andp2
resolve to the same file system entity, elsefalse
. The signature with argumentec
returnsfalse
if an error occurs. -4- Two paths are considered to resolve to the same file system entity if two candidate entities reside on the same device at the same location. This is determined as if by the values of the POSIXstat
structure, obtained as if bystat()
for the two paths, having equalst_dev
values and equalst_ino
values. -5- Throws:As specified infilesystem_error
if(!exists(s1) && !exists(s2)) || (is_other(s1) && is_other(s2))
, otherwise aEerror reporting (31.12.5 [fs.err.report]).
directory_iterator
and recursive_directory_iterator
become the end iterator upon error?Section: 31.12.11 [fs.class.directory.iterator], 31.12.12 [fs.class.rec.dir.itr] Status: C++17 Submitter: Eric Fiselier Opened: 2016-05-28 Last modified: 2021-06-06
Priority: 0
View all other issues in [fs.class.directory.iterator].
View all issues with C++17 status.
Discussion:
Constructing or performing an increment on directory iterator types can result in an error. Currently there is implementation divergence regarding the value of the iterator after an error occurs. Both boost and libc++ construct the end iterator. libstdc++ constructs a singular iterator which is not equal to the end iterator. For this reason we should clarify the state of the iterators after an error occurs.
[2016-06 Oulu]
Moved to P0/Ready during issues prioritization.
Friday: status to Immediate
Proposed resolution:
This wording is relative to N4582.
Modify [fs.class.directory_iterator] as follows:
-3- If an iterator of type
directory_iterator
reports an error or is advanced past the last directory element, that iterator shall become equal to the end iterator value. Thedirectory_iterator
default constructor shall create an iterator equal to the end iterator value, and this shall be the only valid iterator for the end condition.
memory_resource
should be privateSection: 20.5.2 [mem.res.class] Status: C++17 Submitter: Ville Voutilainen Opened: 2016-06-04 Last modified: 2017-07-30
Priority: 4
View all other issues in [mem.res.class].
View all issues with C++17 status.
Discussion:
memory_resource
doesn't define any behavior, it's just an interface.
Furthermore, we don't say whether the functions at [memory.resource.prot] should or should
not be defined by implementations. Presumably they should not. Those functions
are not designed to be called by derived classes, and thus should not be protected.
[2016-06 Oulu]
Looks fine, check with Pablo to make sure that was his intent.
Pablo replied that this was correct.
Friday: status to Immediate
Proposed resolution:
This wording is relative to N4582.
Modify [memory.resource.class] as indicated:
class memory_resource { […]protectedprivate: virtual void* do_allocate(size_t bytes, size_t alignment) = 0; virtual void do_deallocate(void* p, size_t bytes, size_t alignment) = 0; virtual bool do_is_equal(const memory_resource& other) const noexcept = 0; };
Modify [memory.resource.prot] as indicated:
[Drafting note: I don't know whether it's too late to change the section mnemonic [memory.resource.prot] to e.g. [memory.resource.priv] or perhaps [memory.resource.virt].]
memory_resource
protectedprivate virtual member functions [memory.resource.prot]
filesystem::exists(const path&, error_code&)
error reportingSection: 31.12.13.14 [fs.op.exists] Status: C++17 Submitter: Jonathan Wakely Opened: 2016-06-06 Last modified: 2017-07-30
Priority: 1
View all issues with C++17 status.
Discussion:
The filesystem::exists(const path&)
function does not throw an
exception if the file doesn't exist, but the corresponding function
taking an error_code&
argument does set it to indicate an error.
filesystem::exists(const path&, error_code&)
to
call ec.clear()
if status(p, ec).type() == file_type::not_found
.
[2016-06, Oulu — Jonathan comments and provides wording]
The sentence "The signature with argument ec
returns false
if an error
occurs." means that given a file such that status(p).type() == file_type::unknown
,
exists(p)
is true
but exists(p, ec)
is false
.
exists(p)
and exists(p, ec)
consistent, so that the latter clears ec
except when the former would
throw an exception, which is only for the file_type::none
case.
[2016-06, Oulu]
Prioritized as P1
Voted to Ready 7-0 Tuesday evening in Oulu
Proposed resolution:
This wording is relative to N4594.
Insert a new paragraph before 31.12.13.14 [fs.op.exists] p2 and edit it as shown:
bool exists(const path& p); bool exists(const path& p, error_code& ec) noexcept;-?- Let
-?- Effects: The signature with arguments
be afile_status
, determined as if bystatus(p)
orstatus(p, ec)
, respectively.ec
callsec.clear()
ifstatus_known(s)
. -2- Returns:exists(status(p))
orexists(status(p, ec))
, respectivelyexists(s)
.The signature with argument.ec
returnsfalse
if an error occurs
[recursive_]directory_iterator::increment(error_code&)
is underspecifiedSection: 31.12.11.2 [fs.dir.itr.members], 31.12.12.2 [fs.rec.dir.itr.members] Status: C++17 Submitter: Daniel Krügler Opened: 2016-06-20 Last modified: 2017-07-30
Priority: 0
View all other issues in [fs.dir.itr.members].
View all issues with C++17 status.
Discussion:
Setting X
as being either directory_iterator
or recursive_directory_iterator
there exists a member function
in X
,
X& increment(error_code& ec) noexcept;
whose effects are described as:
As specified by Input iterators (24.2.3).
which is somewhat surprising, because for input iterators there is no call expression naming increment
specified.
increment
as a another name for the prefix increment operator of iterators, but that
needs to be expressed somewhere.
[2016-06 Oulu]
Moved to P0/Ready during issues prioritization.
Friday: status to Immediate
Proposed resolution:
This wording is relative to N4594.
[Drafting note: The suggested wording for this issue also repairs the information loss that had been caused by the third bullet of the proposed resolution of 2704(i)]
Change 31.12.11.2 [fs.dir.itr.members] as indicated:
directory_iterator& operator++(); directory_iterator& increment(error_code& ec) noexcept;-10- Effects: As specified
byfor the prefix increment operation of Input iterators (24.3.5.3 [input.iterators]).
Change 31.12.12.2 [fs.rec.dir.itr.members] as indicated:
recursive_directory_iterator& operator++(); recursive_directory_iterator& increment(error_code& ec) noexcept;[…]
-27- Effects: As specifiedbyfor the prefix increment operation of Input iterators (24.3.5.3 [input.iterators]), except that: […]
constexpr
specifierSection: 26.1 [algorithms.general] Status: C++17 Submitter: Jonathan Wakely Opened: 2016-06-21 Last modified: 2017-07-30
Priority: 0
View all issues with C++17 status.
Discussion:
In LEWG we noticed some parallel algorithms are constexpr
. Jared said:
std::max_element
/std::minmax_element
. To my knowledge,
neither SG1 nor LWG ever explicitly considered whether a parallel algorithm should be constexpr
. I think the assumption
was that parallel algorithms would be regular old function templates without additional specifiers such as constexpr
.
[2016-06 Oulu]
Moved to P0/Ready during issues prioritization.
Friday: status to Immediate
Proposed resolution:
This wording is relative to N4594.
Change the <algorithm>
header synopsis, 26.1 [algorithms.general], as indicated,
to remove "constexpr
" from the six {min,max,minmax}_element
overloads with an ExecutionPolicy
argument:
namespace std { […] // 25.5.7, minimum and maximum: […] template<class ExecutionPolicy, class ForwardIterator>constexprForwardIterator min_element(ExecutionPolicy&& exec, // see 25.2.5 ForwardIterator first, ForwardIterator last); template<class ExecutionPolicy, class ForwardIterator, class Compare>constexprForwardIterator min_element(ExecutionPolicy&& exec, // see 25.2.5 ForwardIterator first, ForwardIterator last, Compare comp); […] template<class ExecutionPolicy, class ForwardIterator>constexprForwardIterator max_element(ExecutionPolicy&& exec, // see 25.2.5 ForwardIterator first, ForwardIterator last); template<class ExecutionPolicy, class ForwardIterator, class Compare>constexprForwardIterator max_element(ExecutionPolicy&& exec, // see 25.2.5 ForwardIterator first, ForwardIterator last, Compare comp); […] template<class ExecutionPolicy, class ForwardIterator>constexprpair<ForwardIterator, ForwardIterator> minmax_element(ExecutionPolicy&& exec, // see 25.2.5 ForwardIterator first, ForwardIterator last); template<class ExecutionPolicy, class ForwardIterator, class Compare>constexprpair<ForwardIterator, ForwardIterator> minmax_element(ExecutionPolicy&& exec, // see 25.2.5 ForwardIterator first, ForwardIterator last, Compare comp); […] }
status(p).permissions()
and symlink_status(p).permissions()
are not specifiedSection: 31.12.13.36 [fs.op.status], 31.12.13.38 [fs.op.symlink.status] Status: C++17 Submitter: Eric Fiselier Opened: 2016-06-19 Last modified: 2023-02-07
Priority: 0
View all other issues in [fs.op.status].
View all issues with C++17 status.
Discussion:
The current specification for status(p)
and symlink_status(p)
omits any mention on setting permissions()
on the returned file_status
.
Obviously they should be set, but as currently specified the
permissions()
will always be perms::unknown
.
[2016-06, Oulu]
[2016-06 Oulu]
Moved to P0/Ready during issues prioritization.
Friday: status to Immediate
Proposed resolution:
This wording is relative to N4594.
Change 31.12.13.36 [fs.op.status] as indicated:
file_status status(const path& p, error_code& ec) noexcept;-4- Effects: If possible, determines the attributes of the file
-?- Letp
resolves to, as if byPOSIXusing POSIXstat()
.stat()
to obtain a POSIXstruct stat
.. […]prms
denote the result of(m & perms::mask)
, wherem
is determined as if by converting thest_mode
member of the obtainedstruct stat
to the typeperms
. -5- Returns:
If
ec != error_code()
:
[…]
Otherwise
If the attributes indicate a regular file, as if by POSIX
S_ISREG
, returnsfile_status(file_type::regular, prms)
. […]Otherwise, if the attributes indicate a directory, as if by POSIX
S_ISDIR
, returnsfile_status(file_type::directory, prms)
. […]Otherwise, if the attributes indicate a block special file, as if by POSIX
S_ISBLK
, returnsfile_status(file_type::block, prms)
.Otherwise, if the attributes indicate a character special file, as if by POSIX
S_ISCHR
, returnsfile_status(file_type::character, prms)
.Otherwise, if the attributes indicate a fifo or pipe file, as if by POSIX
S_ISFIFO
, returnsfile_status(file_type::fifo, prms)
.Otherwise, if the attributes indicate a socket, as if by POSIX
S_ISSOCK
, returnsfile_status(file_type::socket, prms)
.Otherwise, returns
file_status(file_type::unknown, prms)
.
Change [fs.op.symlink_status] as indicated:
file_status symlink_status(const path& p); file_status symlink_status(const path& p, error_code& ec) noexcept;-1- Effects: Same as
-?- Letstatus()
, above, except that the attributes ofp
are determined as if byPOSIXusing POSIXlstat()
lstat()
to obtain a POSIXstruct stat
.prms
denote the result of(m & perms::mask)
, wherem
is determined as if by converting thest_mode
member of the obtainedstruct stat
to the typeperms
. -2- Returns: Same asstatus()
, above, except that if the attributes indicate a symbolic link, as if by POSIXS_ISLNK
, returnsfile_status(file_type::symlink, prms)
. The signature with argumentec
returnsfile_status(file_type::none)
if an error occurs.
std::pair::operator=
Section: 22.3.2 [pairs.pair], 22.4.4.3 [tuple.assign] Status: C++17 Submitter: Richard Smith Opened: 2016-06-07 Last modified: 2017-07-30
Priority: 2
View other active issues in [pairs.pair].
View all other issues in [pairs.pair].
View all issues with C++17 status.
Discussion:
std::is_copy_assignable<std::pair<int, std::unique_ptr<int>>>::value
is true
, and
should be false
. We're missing a "shall not participate in overload resolution unless" for pair
's
operator=
, and likewise for tuple
.
[2016-08-03 Chicago LWG]
Inspired by Eric Fiselier and Ville, Walter and Nevin provide initial Proposed Resolution.
[2016-08 - Chicago]
Thurs PM: Moved to Tentatively Ready
Lots of discussion, but no one had a better idea.
Proposed resolution:
This wording is relative to N4606.
Change 22.3.2 [pairs.pair] as indicated:
pair& operator=(const pair& p);-15-
[…]RequiresRemarks: This operator shall be defined as deleted unlessis_copy_assignable_v<first_type>
istrue
andis_copy_assignable_v<second_type>
istrue
.template<class U, class V> pair& operator=(const pair<U, V>& p);-18-
[…]RequiresRemarks: This operator shall not participate in overload resolution unlessis_assignable_v<first_type&, const U&>
istrue
andis_assignable_v<second_type&, const V&>
istrue
.pair& operator=(pair&& p) noexcept(see below);-21- Remarks: The expression inside
noexcept
is equivalent to:is_nothrow_move_assignable_v<T1> && is_nothrow_move_assignable_v<T2>-22-
[…]RequiresRemarks: This operator shall be defined as deleted unlessis_move_assignable_v<first_type>
istrue
andis_move_assignable_v<second_type>
istrue
.template<class U, class V> pair& operator=(pair<U, V>&& p);-25-
RequiresRemarks: This operator shall not participate in overload resolution unlessis_assignable_v<first_type&, U&&>
istrue
andis_assignable_v<second_type&, V&&>
istrue
.
Change 22.4.4.3 [tuple.assign] as indicated:
tuple& operator=(const tuple& u);-2-
[…]RequiresRemarks: This operator shall be defined as deleted unlessis_copy_assignable_v<Ti>
istrue
for alli
.tuple& operator=(tuple&& u) noexcept(see below);-5- Remark: The expression inside
noexcept
is equivalent to the logical AND of the following expressions:is_nothrow_move_assignable_v<Ti>where
-6-Ti
is theith
type inTypes
.RequiresRemarks: This operator shall be defined as deleted unlessis_move_assignable_v<Ti>
istrue
for alli
. […]template <class... UTypes> tuple& operator=(const tuple<UTypes...>& u);-9-
[…]RequiresRemarks: This operator shall not participate in overload resolution unlesssizeof...(Types) == sizeof...(UTypes)
andis_assignable_v<Ti&, const Ui&>
istrue
for alli
.template <class... UTypes> tuple& operator=(tuple<UTypes...>&& u);-12-
[…]RequiresRemarks: This operator shall not participate in overload resolution unlessis_assignable_v<Ti&, Ui&&> == true
for alli
.andsizeof...(Types) == sizeof...(UTypes)
.template <class U1, class U2> tuple& operator=(const pair<U1, U2>& u);-15-
[…]RequiresRemarks: This operator shall not participate in overload resolution unlesssizeof...(Types) == 2
.andis_assignable_v<T0&, const U1&>
istrue
for the first typeT0
inTypes
andis_assignable_v<T1&, const U2&>
istrue
for the second typeT1
inTypes
.template <class U1, class U2> tuple& operator=(pair<U1, U2>&& u);-18-
RequiresRemarks: This operator shall not participate in overload resolution unlesssizeof...(Types) == 2
.andis_assignable_v<T0&, U1&&>
istrue
for the first typeT0
inTypes
andis_assignable_v<T1&, U2&&>
istrue
for the second typeT1
inTypes
.
lock_guard<MutexTypes...>::mutex_type
typedef unclearSection: 32.6.5.2 [thread.lock.guard] Status: C++23 Submitter: Eric Fiselier Opened: 2016-06-13 Last modified: 2023-11-22
Priority: 3
View all other issues in [thread.lock.guard].
View all issues with C++23 status.
Discussion:
In the synopsis of 32.6.5.3 [thread.lock.scoped] the mutex_type
typedef is specified as follows:
template <class... MutexTypes> class scoped_lock { public: typedef Mutex mutex_type; // If MutexTypes... consists of the single type Mutex […] };
The comment seems ambiguous as it could mean either:
sizeof...(MutexTypes) == 1
.sizeof...(MutexTypes) >= 1
and every type in MutexTypes...
is the same type.I originally took the language to mean (2), but upon further review it seems that (1) is the intended interpretation, as suggested in the LEWG discussion in Lenexa.
I think the language should be clarified to prevent implementation divergence.[2016-07, Toronto Saturday afternoon issues processing]
General feeling that sizeof(MutexTypes...) == 1
is a better way to state the requirement.
Reworked the text to refer to scoped_lock
instead of lock_guard
Marshall and Eric to reword and discuss on reflector. Status to Open
[2018-3-14 Wednesday evening issues processing; general agreement to adopt once the wording is updated.]
2018-03-18 Marshall provides updated wording.
Previous resolution: [SUPERSEDED]Previous resolution: [SUPERSEDED]This wording is relative to N4594.
Edit 32.6.5.2 [thread.lock.guard]/1, class template
lock_guard
synopsis, as indicated:template <class... MutexTypes> class lock_guard { public: typedef Mutex mutex_type; // Only iIf MutexTypes...consists of theexpands to a single typeMutex
[…] };
This wording is relative to N4727.
Edit 32.6.5.2 [thread.lock.guard]/1, class template
lock_guard
synopsis, as indicated:template <class... MutexTypes> class scoped_lock { public: using mutex_type = Mutex; // Only iIfsizeof(MutexTypes...) == 1
[…] };MutexTypes...
consists of the single typeMutex
[2020-05-11; Daniel provides improved wording]
[2020-05-16 Reflector discussions]
Status to Tentatively Ready after five positive votes on the reflector.
[2020-11-09 Approved In November virtual meeting. Status changed: Tentatively Ready → WP.]
Proposed resolution:
This wording is relative to N4861.
Edit 32.6.5.3 [thread.lock.scoped], class template scoped_lock
synopsis, as indicated:
template <class... MutexTypes> class scoped_lock { public:using mutex_type = Mutex; // If MutexTypes... consists of the single type Mutexusing mutex_type = see below; // Only if sizeof...(MutexTypes) == 1 […] };-1- An object of type
scoped_lock
controls the ownership of lockable objects within a scope. Ascoped_lock
object maintains ownership of lockable objects throughout thescoped_lock
object's lifetime (6.7.4 [basic.life]). The behavior of a program is undefined if the lockable objects referenced bypm
do not exist for the entire lifetime of thescoped_lock
object.
WhenIfsizeof...(MutexTypes)
isone, let1
Mutex
denote the sole type constituting the packMutexTypes
., the suppliedMutex
typeshall meet the Cpp17BasicLockable requirements (32.2.5.2 [thread.req.lockable.basic]). The member typedef-namemutex_type
denotes the same type asMutex
.Otherwise,
each of the mutex typesall types in the template parameter packMutexTypes
shall meet the Cpp17Lockable requirements (32.2.5.3 [thread.req.lockable.req]) and there is no membermutex_type
.
path::operator/=
and path::append
Section: 31.12.6.5.3 [fs.path.append] Status: C++17 Submitter: Tim Song Opened: 2016-06-14 Last modified: 2017-07-30
Priority: 2
View all other issues in [fs.path.append].
View all issues with C++17 status.
Discussion:
The current specification of operator/=
taking a const Source&
parameter, and of
path::append
in 31.12.6.5.3 [fs.path.append] appears to require Source
to have a native()
and an empty()
member, and seemingly requires different behavior for append(empty_range)
and append(first, last)
when first == last
(the last two bullet points being specified with
source
alone), which doesn't make any sense.
operator/=(const path&)
overload.
[2016-07-03, Daniel comments]
The same wording area is affected by LWG 2664(i).
[2016-08 Chicago]
Wed AM: Move to Tentatively Ready
Friday AM, in discussing 2664(i) a comment about missing "equivalent to" language was made, so PR updated.
Previous Resolution [SUPERSEDED]
This wording is relative to N4594.
Edit 31.12.6.5.3 [fs.path.append]/4-5 as indicated:
template <class Source> path& operator/=(const Source& source); template <class Source> path& append(const Source& source);-?- Effects:
-?- Returns:operator/=(path(source))
*this
.template <class InputIterator> path& append(InputIterator first, InputIterator last);-4- Effects:
Appendspath::preferred_separator
topathname
, converting format and encoding if required (31.12.6.3 [fs.path.cvt]), unless:
an added directory-separator would be redundant, or
an added directory-separator would change an relative path to an absolute path, or
source.empty()
istrue
, or
*source.native().cbegin()
is a directory-separator.-5- Returns:
Then appends the effective range ofsource
(31.12.6.4 [fs.path.req]) or the range[first, last)
topathname
, converting format and encoding if required (31.12.6.3 [fs.path.cvt])operator/=(path(first, last))
.*this
.
Proposed resolution:
This wording is relative to N4606.
Edit 31.12.6.5.3 [fs.path.append]/4-5 as indicated:
template <class Source> path& operator/=(const Source& source); template <class Source> path& append(const Source& source);-?- Effects: Equivalent to
return operator/=(path(source));
.template <class InputIterator> path& append(InputIterator first, InputIterator last);-4- Effects: Equivalent to
return operator/=(path(first, last));
.Appendspath::preferred_separator
topathname
, converting format and encoding if required (31.12.6.3 [fs.path.cvt]), unless:
— an added directory-separator would be redundant, or— an added directory-separator would change an relative path to an absolute path, or—source.empty()
istrue
, or—*source.native().cbegin()
is a directory-separator.
Then appends the effective range ofsource
(31.12.6.4 [fs.path.req]) or the range[first, last)
topathname
, converting format and encoding if required (31.12.6.3 [fs.path.cvt]).
-5- Returns:*this
.
gcd
/ lcm
and bool
Section: 13.1.2 [fund.ts.v2::numeric.ops.gcd], 13.1.3 [fund.ts.v2::numeric.ops.lcm] Status: TS Submitter: Richard Smith Opened: 2016-06-15 Last modified: 2017-07-30
Priority: 4
View all other issues in [fund.ts.v2::numeric.ops.gcd].
View all issues with TS status.
Discussion:
Addresses: fund.ts.v2
According to N4562, gcd
and
lcm
support bool
as the operand type. The wording doesn't appear to cover the behavior for that case,
since bool
does not have a zero value and gcd
/ lcm
are not normally mathematically defined
over {false, true}
.
gcd
and lcm
shouldn't accept arguments of type bool
.
[2016-08-01, Walter Brown suggests wording]
A corresponding issue has been added addressing the WP, see LWG 2759(i).
[2016-08, Chicago]
Monday PM: Moved to Tentatively Ready
Proposed resolution:
This wording is relative to N4600.
Adjust 13.1.2 [fund.ts.v2::numeric.ops.gcd] p3 as indicated:
template<class M, class N> constexpr common_type_t<M, N> gcd(M m, N n);[…]
-3- Remarks: If eitherM
orN
is not an integer type, or if either is (possibly cv-qualified)bool
, the program is ill-formed.
Adjust 13.1.3 [fund.ts.v2::numeric.ops.lcm] p3 as indicated:
template<class M, class N> constexpr common_type_t<M, N> lcm(M m, N n);[…]
-3- Remarks: If eitherM
orN
is not an integer type, or if either is (possibly cv-qualified)bool
, the program is ill-formed.
Section: 31.12.6.5.4 [fs.path.concat] Status: Resolved Submitter: Tim Song Opened: 2016-06-16 Last modified: 2018-01-24
Priority: 2
View all other issues in [fs.path.concat].
View all issues with Resolved status.
Discussion:
31.12.6.5.4 [fs.path.concat] specifies that the postcondition for
path& operator+=(const path& x); path& operator+=(const string_type& x); path& operator+=(const value_type* x); path& operator+=(value_type x); template<class Source> path& operator+=(const Source& x); template<class EcharT> path& operator+=(EcharT x); template<class Source> path& concat(const Source& x); template<class InputIterator> path& concat(InputIterator first, InputIterator last);
is
native() == prior_native
+ effective-argument
where effective-argument is
x
is present and is const path&
, x.native()
; otherwisesource
is present, the effective range of source
(31.12.6.4 [fs.path.req]); otherwise,first
and last
are present, the range [first, last)
; otherwise,x
It also says that
If the value type of effective-argument would not be
path::value_type
, the actual argument or argument range is first converted (31.12.6.3.2 [fs.path.type.cvt]) so that effective-argument has value typepath::value_type
.
There are several problems with this specification:
First, there is no overload taking "source
" (note the lower case); all single-argument overloads take
"x
". Second, there's nothing that defines what it means to use operator+
on a string and an
iterator range; clearly concatentation is intended but there is no wording to that effect.
Third, the final portion uses "value type", but the "value type" of a single character is not a defined concept.
Also, the reference only to 31.12.6.3.2 [fs.path.type.cvt] seems to imply that any format conversion specified in
31.12.6.3.1 [fs.path.fmt.cvt] will not be performed, in seeming contradiction to the rule that native()
is to return the native pathname format (31.12.6.5.6 [fs.path.native.obs]/1). Is that intended?
[2016-11-10, Billy suggests wording]
The wording for LWG 2798(i) resolves this issue as well.
Proposed resolution:
This is resolved by p0492r2.
std::abs(short)
, std::abs(signed char)
and others should return int
instead of
double
in order to be compatible with C++98 and CSection: 29.7 [c.math] Status: C++17 Submitter: Jörn Heusipp Opened: 2016-06-16 Last modified: 2017-07-30
Priority: 3
View all other issues in [c.math].
View all issues with C++17 status.
Discussion:
Consider this C++98 program:
#include <cmath> #include <cstdlib> int main() { return std::abs(static_cast<short>(23)) % 42; }
This works fine with C++98 compilers. At the std::abs(short)
call, short gets promoted to int
and
std::abs(int)
is called.
Otherwise, if any argument of arithmetic type corresponding to a
double
parameter has typedouble
or an integer type, then all arguments of arithmetic type corresponding todouble
parameters are effectively cast todouble
.
C++17 draft additionally adds on page 1080 §26.9 p10 [c.math]:
If
abs()
is called with an argument of typeX
for whichis_unsigned<X>::value
istrue
and ifX
cannot be converted toint
by integral promotion (4.5), the program is ill-formed. [Note: Arguments that can be promoted toint
are permitted for compatibility with C. — end note]
It is somewhat confusing and probably even contradictory to on the one hand specify abs()
in terms of integral
promotion in §26.9 p10 and on the other hand demand all integral types to be converted to double
in
§26.9 p15 b2.
std::abs
and compile the code successfully. GCC 4.5-5.3 (for std::abs
but
not for ::abs
) as well as GCC >=6.0 (for both std::abs
and ::abs
) fail to compile in the following
way: Taking §26.9 p15 b2 literally and applying it to abs()
(which is listed in §26.9 p12) results in
abs(short)
returning double
, and with operator%
not being specified for double
, this
makes the programm ill-formed.
I do acknowledge the reason for the wording and semantics demanded by §26.9 p15 b2, i.e. being able to call math functions
with integral types or with partly floating point types and partly integral types. Converting integral types to double
certainly makes sense here for all the other floating point math functions.
However, abs()
is special. abs()
has overloads for the 3 wider integral types which return integral types.
abs()
originates in the C standard in stdlib.h
and had originally been specified for integral types only.
Calling it in C with a short argument returns an int
. Calling std::abs(short)
in C++98 also returns an
int
. Calling std::abs(short)
in C++11 and later with §26.9 p15 b2 applied to abs()
suddenly
returns a double
.
Additionally, this behaviour also breaks third-party C headers which contain macros or inline functions calling
abs(short)
.
As per discussion on std-discussion, my reading of the standard as well as GCC's interpretation seem valid.
However, as can be seen, this breaks existing code.
In addition to the compatibilty concerns, having std::abs(short)
return double
is also very confusing
and unintuitive.
The other (possibly, depending on their respective size relative to int
) affected types besides short
are signed char
, unsigned char
and unsigned short
, and also char
, char16_t
,
char32_t
and wchar_t
, (all of these are or may be promotable to int
). Wider integral types
are not affected because explicit overloads are specified for those types by §26.9 p6, §26.9 p7 and §26.9 p9.
div()
is also not affected because it is neither listed in §26.9 p12, nor does it actually provide
any overload for double
at all.
As far as I can see, the proposed or implemented solutions for LWG 2294(i), 2192(i) and/or
2086(i) do not resolve this issue.
I think both, §26.9 p10 [c.math] and §26.9 p15 [c.math] need some correction and clarification.
(Note: These changes would explicitly render the current implementation in GCC's libstdc++ non-conforming, which would
be a good thing, as outlined above.)
Previous resolution [SUPERSEDED]:
This wording is relative to N4594.
Modify 29.7 [c.math] as indicated:
-10- If
[…] -15- Moreover, there shall be additional overloads for these functions, with the exception ofabs()
is called with an argument of typeX
for whichis_unsigned<X>::value
istrue
and ifX
cannot be converted toint
by integral promotion (4.5), the program is ill-formed. Ifabs()
is called with an argument of typeX
which can be converted toint
by integral promotion (4.5), the argument is promoted toint
. [Note: Arguments that can be promoted toint
are promoted toint
in order to keeppermitted forcompatibility with C. — end note]abs()
, sufficient to ensure:
If any argument of arithmetic type corresponding to a
double
parameter has typelong double
, then all arguments of arithmetic type (3.9.1) corresponding todouble
parameters are effectively cast tolong double
.Otherwise, if any argument of arithmetic type corresponding to a
double
parameter has typedouble
or an integer type, then all arguments of arithmetic type corresponding todouble
parameters are effectively cast todouble
.Otherwise, all arguments of arithmetic type corresponding to
double
parameters have typefloat
.See also: ISO C 7.5, 7.10.2, 7.10.6.
[Note:abs()
is exempted from these rules in order to stay compatible with C. — end note]
[2016-07 Chicago]
Monday: Some of this has been changed in N4606; the rest of the changes may be editorial.
Fri PM: Move to Tentatively Ready
Proposed resolution:
This wording is relative to N4606.
Modify 29.7.1 [cmath.syn] as indicated:
-2- For each set of overloaded functions within
<cmath>
, with the exception ofabs
, there shall be additional overloads sufficient to ensure:
If any argument of arithmetic type corresponding to a
double
parameter has typelong double
, then all arguments of arithmetic type (3.9.1) corresponding todouble
parameters are effectively cast tolong double
.Otherwise, if any argument of arithmetic type corresponding to a
double
parameter has typedouble
or an integer type, then all arguments of arithmetic type corresponding todouble
parameters are effectively cast todouble
.Otherwise, all arguments of arithmetic type corresponding to
double
parameters have typefloat
.[Note:
abs
is exempted from these rules in order to stay compatible with C. — end note]See also: ISO C 7.5, 7.10.2, 7.10.6.
nullopt_t
insufficiently constrainedSection: 22.5.4 [optional.nullopt] Status: C++17 Submitter: Tim Song Opened: 2016-06-17 Last modified: 2017-07-30
Priority: 2
View all other issues in [optional.nullopt].
View all issues with C++17 status.
Discussion:
22.5.4 [optional.nullopt]/2 requires of nullopt_t
that
Type
nullopt_t
shall not have a default constructor. It shall be a literal type. Constantnullopt
shall be initialized with an argument of literal type.
This does not appear sufficient to foreclose the following implementation:
struct nullopt_t { constexpr nullopt_t(const nullopt_t&) = default; }; constexpr nullopt_t nullopt(nullopt_t{});
nullopt_t
has no default constructor because it has a user-declared (copy) constructor;nullopt_t
has a trivial destructor, is an aggregate, and is a literal type;nullopt
has been initialized with an argument of literal type, to wit, nullopt_t
.
But such a nullopt_t
is still constructible from {}
and so still makes opt = {}
ambiguous.
[2016-08 Chicago]
This is related to LWG 2510(i).
Monday PM: Ville to provide updated wording
Fri AM: Moved to Tentatively Ready
Proposed resolution:
This wording is relative to N4606.
Edit 22.5.4 [optional.nullopt]/2 as indicated:
[Drafting note:
{}
can do one of three things for a class type: it may be aggregate initialization, it may call a default constructor, or it may call an initializer-list constructor (see 9.5.5 [dcl.init.list], 12.2.2.8 [over.match.list]). The wording below forecloses all three possibilities. — end drafting note]
-2- Type
nullopt_t
shall not have a default constructor or an initializer-list constructor. It shall not be an aggregate and shall be a literal type. Constantnullopt
shall be initialized with an argument of literal type.
is_constructible
with void
typesSection: 21.3.5.4 [meta.unary.prop] Status: C++17 Submitter: S. B. Tam Opened: 2016-06-22 Last modified: 2017-07-30
Priority: Not Prioritized
View other active issues in [meta.unary.prop].
View all other issues in [meta.unary.prop].
View all issues with C++17 status.
Discussion:
LWG 2560(i) mention that there is no variable of function type. There's also no variable of void
type,
so should 21.3.5.4 [meta.unary.prop] also explicitly say that for a void
type T
,
is_constructible<T, Args...>::value
is false
?
[2016-07-03, Daniel provides wording]
[2016-08 Chicago]
Wed PM: Move to Tentatively Ready
Proposed resolution:
This wording is relative to N4594.
Change 21.3.5.4 [meta.unary.prop], Table 52 — "Type property predicates", as indicated:
Table 52 — Type property predicates Template Condition Preconditions …
template <class T, class... Args>
struct is_constructible;For a function type T
or for a (possibly cv-qualified)void
typeT
,
is_constructible<T, Args...>::value
isfalse
, otherwise see belowT
and all types in the
parameter packArgs
shall
be complete types,
(possibly cv-qualified)
void
, or arrays of
unknown bound.…
time_point
non-member subtraction with an unsigned durationSection: 30.6.6 [time.point.nonmember] Status: C++17 Submitter: Michael Winterberg Opened: 2016-06-23 Last modified: 2017-07-30
Priority: 0
View all other issues in [time.point.nonmember].
View all issues with C++17 status.
Discussion:
In N4594, 30.6.6 [time.point.nonmember], operator-(time_point, duration)
is specified as:
template <class Clock, class Duration1, class Rep2, class Period2> constexpr time_point<Clock, common_type_t<Duration1, duration<Rep2, Period2>>> operator-(const time_point<Clock, Duration1>& lhs, const duration<Rep2, Period2>& rhs);-3- Returns:
lhs + (-rhs)
.
When Rep2
is an unsigned integral type, the behavior is quite different with arithmetic of the underlying
integral types because of the requirement to negate the incoming duration and then add that. It also ends up
producing different results than the underlying durations as well as the non-member time_point::operator-=
.
#include <chrono> #include <iostream> #include <cstdint> using namespace std; using namespace std::chrono; int main() { const duration<uint32_t> unsignedSeconds{5}; auto someValue = system_clock::from_time_t(200); cout << system_clock::to_time_t(someValue) << '\n'; cout << system_clock::to_time_t(someValue - unsignedSeconds) << '\n'; someValue -= unsignedSeconds; cout << system_clock::to_time_t(someValue) << '\n'; std::chrono::seconds signedDur{200}; cout << signedDur.count() << '\n'; cout << (signedDur - unsignedSeconds).count() << '\n'; signedDur -= unsignedSeconds; cout << signedDur.count() << '\n'; }
The goal of the program is to compare the behavior of time_point
non-member operator-
,
time_point
member operator-=
, duration
non-member operator-
, and
duration
member operator-=
with basically the same inputs.
200 4294967491 195 200 195 195
On the other hand, libstdc++ produces this output, which is what I "intuitively" expect and behaves more consistently:
200 195 195 200 195 195
Given the seemingly brief coverage of durations with unsigned representations in the standard, this seems to be an oversight rather than a deliberate choice for this behavior. Additionally, there may be other "unexpected" behaviors with durations with an unsigned representation, this is just the one that I've come across.
[2016-07 Chicago]
Monday: P0 - tentatively ready
Proposed resolution:
This wording is relative to N4594.
Change 30.6.6 [time.point.nonmember] as indicated:
template <class Clock, class Duration1, class Rep2, class Period2> constexpr time_point<Clock, common_type_t<Duration1, duration<Rep2, Period2>>> operator-(const time_point<Clock, Duration1>& lhs, const duration<Rep2, Period2>& rhs);-3- Returns:
lhs + (-rhs)
CT(lhs.time_since_epoch() - rhs)
, whereCT
is the type of the return value.
constexpr optional<T>::operator->
Section: 22.5.3.7 [optional.observe] Status: C++17 Submitter: Agustín K-ballo Bergé Opened: 2016-07-02 Last modified: 2017-07-30
Priority: 0
View other active issues in [optional.observe].
View all other issues in [optional.observe].
View all issues with C++17 status.
Discussion:
optional<T>::operator->
s are constrained to be constexpr
functions
only when T
is not a type with an overloaded unary operator&
. This
constrain comes from the need to use addressof
(or a similar
mechanism), and the inability to do so in a constant expression in
C++14. Given that addressof
is now constexpr
, this constrain is no
longer needed.
[2016-07 Chicago]
Monday: P0 - tentatively ready
Proposed resolution:
This wording is relative to N4594.
Modify [optional.object.observe] as indicated:
constexpr T const* operator->() const; constexpr T* operator->();-1- Requires:
-2- Returns:*this
contains a value.val
. -3- Throws: Nothing. -4- Remarks:UnlessThese functions shall beT
is a user-defined type with overloaded unaryoperator&
, tconstexpr
functions.
is_partitioned
requirements need updatingSection: 26.8.5 [alg.partitions] Status: Resolved Submitter: Jonathan Wakely Opened: 2016-07-06 Last modified: 2020-05-12
Priority: 3
View all other issues in [alg.partitions].
View all issues with Resolved status.
Discussion:
Requires:
InputIterator
's value type shall be convertible toPredicate
's argument type.
This seems to date from the days of adaptable function objects with an argument_type
typedef, but in
modern C++ the predicate might not have an argument type. It could have a function template that accepts various
arguments, so it doesn't make sense to state requirements in terms of a type that isn't well defined.
[2016-07, Toronto Saturday afternoon issues processing]
The proposed resolution needs to be updated because the underlying wording has changed. Also, since the sequence is homogeneous, we shouldn't have to say that the expression is well-formed for all elements in the range; that implies that it need not be well-formed if the range is empty.
Marshall and JW to reword. Status to Open
Previous resolution [SUPERSEDED]:
This wording is relative to N4594.
Edit 26.8.5 [alg.partitions] as indicated:
template <class InputIterator, class Predicate> bool is_partitioned(InputIterator first, InputIterator last, Predicate pred);-1- Requires:
[…]The expressionInputIterator
's value type shall be convertible toPredicate
's argument typepred(*i)
shall be well-formed for alli
in[first, last)
.template <class InputIterator, class OutputIterator1, class OutputIterator2, class Predicate> pair<OutputIterator1, OutputIterator2> partition_copy(InputIterator first, InputIterator last, OutputIterator1 out_true, OutputIterator2 out_false, Predicate pred);-12- Requires:
[…]InputIterator
's value type shall beCopyAssignable
, and shall be writable (24.3.1 [iterator.requirements.general]) to theout_true
andout_false
OutputIterator
s, andshall be convertible tothe expressionPredicate
's argument typepred(*i)
shall be well-formed for alli
in[first, last)
. The input range shall not overlap with either of the output ranges.template<class ForwardIterator, class Predicate> ForwardIterator partition_point(ForwardIterator first, ForwardIterator last, Predicate pred);-16- Requires:
[…]The expressionForwardIterator
's value type shall be convertible toPredicate
's argument typepred(*i)
shall be well-formed for alli
in[first, last)
.[first, last)
shall be partitioned bypred
, i.e. all elements that satisfypred
shall appear before those that do not.
[2019-03-17; Daniel comments and removes previous wording]
In the recent working draft N4810 all the "shall be convertible to Predicate
's
argument type" are gone - they were cleaned up when "The One Range" proposal P0896R4
had been accepted in San Diego 2018.
pred(*i)
is well-formed), because this follows (in a more general way) by the expression
invoke(pred, invoke(proj, e))
that is applied to the elements e
of [first, last)
.
Therefore I'm suggesting that the resolution for this issue should be: Resolved by P0896R4.
[2020-05-12; Reflector discussions]
Resolved by P0896R4.
Rationale:
Resolved by P0896R4.Proposed resolution:
string
interface taking string_view
Section: 27.4.3.3 [string.cons] Status: C++17 Submitter: Richard Smith Opened: 2016-07-06 Last modified: 2020-09-06
Priority: 1
View all other issues in [string.cons].
View all issues with C++17 status.
Discussion:
Generally, basic_string
has a constructor matching each assign
function and vice versa (except the
constructor takes an allocator where assign
does not). P0254R2 violates
this by adding an assign(basic_string_view, size_type pos, size_type n = npos)
but no corresponding constructor.
[2016-08-04 Chicago LWG]
Robert Douglas provides initial wording.
We decided against another constructor overload to avoid the semantic confusion between:basic_string(char const*, size_type length, Allocator = Allocator())
and
template<class T, class Foo = is_convertible_v<T const&, basic_string_view<charT, traits>> basic_string(T const&, size_type pos, Allocator = Allocator())
where someone might call:
basic_string("HelloWorld", 5, 5);
and get "World", but then call
basic_string("HelloWorld", 5);
and instead get "Hello". The second parameter changes between length and position.
[08-2016, Chicago]
Fri PM: Move to Tentatively Ready
Proposed resolution:
This wording is relative to N4606.
In 27.4.3 [basic.string] add the following constructor overload:
[…] basic_string(const basic_string& str, size_type pos, const Allocator& a = Allocator()); basic_string(const basic_string& str, size_type pos, size_type n, const Allocator& a = Allocator()); template<class T> basic_string(const T& t, size_type pos, size_type n, const Allocator& a = Allocator()); explicit basic_string(basic_string_view<charT, traits> sv, const Allocator& a = Allocator()); […]
In 27.4.3.3 [string.cons] add the following ctor definition:
template<class T> basic_string(const T& t, size_type pos, size_type n, const Allocator& a = Allocator());-?- Effects: Creates a variable,
sv
, as if bybasic_string_view<charT, traits> sv = t;
and then behaves the same as:-?- Remarks: This constructor shall not participate in overload resolution unlessbasic_string(sv.substr(pos, n), a)is_convertible_v<const T&, basic_string_view<charT, traits>>
istrue
.
node_handle
private members missing "exposition only" commentSection: 23.2.5.1 [container.node.overview] Status: C++23 Submitter: Richard Smith Opened: 2016-07-08 Last modified: 2023-11-22
Priority: 3
View all other issues in [container.node.overview].
View all issues with C++23 status.
Discussion:
The private members of node_handle
are missing the usual "exposition only" comment. As a consequence,
ptr_
and alloc_
now appear to be names defined by the library (so programs defining these names
as macros before including a library header have undefined behavior).
node_handle
is reserved for library usage or not;
23.2.5.1 [container.node.overview]/3 says the implementation need not provide a type with this name, but
doesn't seem to rule out the possibility that an implementation will choose to do so regardless.
Daniel:
A similar problem seems to exist for the exposition-only typecall_wrapper
from
p0358r1, which exposes a private data member named fd
and
a typedef FD
.
[2016-07 Chicago]
Jonathan says that we need to make clear that the name node_handle
is not reserved
[2019-03-17; Daniel comments and provides wording]
Due to an editorial step, the previous name node_handle
/node_handle
has been replaced
by the artificial node-handle
name, so I see no longer any reason to talk about a
name node_handle
reservation. The provided wording therefore only takes care of the private
members.
[2020-05-16 Reflector discussions]
Status to Tentatively Ready after five positive votes on the reflector.
[2020-11-09 Approved In November virtual meeting. Status changed: Tentatively Ready → WP.]
Proposed resolution:
This wording is relative to N4810.
Change 23.2.5.1 [container.node.overview], exposition-only class template
node-handle
synopsis, as indicated:
template<unspecified> class node-handle { public: […] private: using container_node_type = unspecified; // exposition only using ator_traits = allocator_traits<allocator_type>; // exposition only typename ator_traits::template rebind_traits<container_node_type>::pointer ptr_; // exposition only optional<allocator_type> alloc_; // exposition only public: […] };
any
's in_place
constructorsSection: 22.7.4.2 [any.cons] Status: C++17 Submitter: Ville Voutilainen Opened: 2016-07-10 Last modified: 2017-07-30
Priority: 0
View all other issues in [any.cons].
View all issues with C++17 status.
Discussion:
The in_place
constructor that takes an initializer_list
has both a Requires:
for is_constructible
and a Remarks: for is_constructible
. The one
that takes just a pack has just a Requires: for is_constructible
.
in_place_t
will
not give a reasonable answer, and I utterly fail to see any implementation
burden in SFINAEing those constructors.
[2016-07 Chicago]
Monday: P0 - tentatively ready
Proposed resolution:
This wording is relative to N4606.
Modify 22.7.4.2 [any.cons] as indicated:
template<class ValueType> any(ValueType&& value);[…]
-7- Requires:T
shall satisfy theCopyConstructible
requirements. Ifis_copy_constructible_v<T>
isfalse
, the program is ill-formed. -8- Effects: Constructs an object of typeany
that contains an object of typeT
direct-initialized withstd::forward<ValueType>(value)
. -9- Remarks: This constructor shall not participate in overload resolution ifdecay_t<ValueType>
is the same type asany
or ifValueType
is a specialization ofin_place_type_t
. […]template <class T, class... Args> explicit any(in_place_type_t<T>, Args&&... args);-?- Remarks: This constructor shall not participate in overload resolution unless
-11- Requires:is_constructible_v<T, Args...>
istrue
.is_constructible_v<T, Args...>
istrue
[…]template <class T, class U, class... Args> explicit any(in_place_type_t<T>, initializer_list<U> il, Args&&... args);[…] -19- Remarks:
-15- Requires:is_constructible_v<T, initializer_list<U>&, Args...>
istrue
.The functionThis constructor shall not participate in overload resolution unlessis_constructible_v<T, initializer_list<U>&, Args...>
istrue
.
Section: 5.3 [fund.ts.v2::optional.object] Status: TS Submitter: Casey Carter Opened: 2016-07-10 Last modified: 2018-07-08
Priority: 0
View all other issues in [fund.ts.v2::optional.object].
View all issues with TS status.
Discussion:
Addresses: fund.ts.v2
LWG 2451(i) adds conditionally explicit converting constructors to optional<T>
that accept:
T
: template <class U> constexpr optional(T&&);
optional<U>
when U&&
is convertible to T
:
template <class U> constexpr optional(optional<U>&&);
const optional<U>
when const U&
is convertible to T
:
template <class U> constexpr optional(const optional<U>&);
All three of these constructors are required to be constexpr
"If T
's selected constructor is a
constexpr
constructor". While this is not problematic for #1, it is not possible in the current language
to implement signatures #2 and #3 as constexpr
functions for the same reasons that optional
's
non-converting constructors from optional<T>&&
and const optional<T>&
cannot be constexpr
.
constexpr
" specifier from the declarations of the conditionally explicit converting
constructors that accept optional<U>&&
and const optional<U>&
, and strike
the remarks requiring these constructors to be constexpr
.
[2016-07 Chicago]
Monday: P0 - tentatively ready
This needs to be considered for C++17 as well
Proposed resolution:
This wording is relative to N4600.
Wording relative to N4600 + LWG 2451(i), although it should be noted that this resolution should be applied wherever LWG 2451(i) is applied, be that to the fundamentals TS or the specification of
optional
in the C++ Working Paper.
Edit 22.5.3 [optional.optional] as indicated:
template <class T> class optional { public: typedef T value_type; // 5.3.1, Constructors […] template <class U> constexpr optional(U&&); template <class U>constexproptional(const optional<U>&); template <class U>constexproptional(optional<U<&&); […] };
In 5.3.1 [fund.ts.v2::optional.object.ctor], modify the new signature specifications added by LWG 2451(i)
template <class U>constexproptional(const optional<U>& rhs);[…]
-48- Remarks:IfThis constructor shall not participate in overload resolution unless […]T
's selected constructor is aconstexpr
constructor, this constructor shall be aconstexpr
constructor.template <class U>constexproptional(optional<U>&& rhs);[…]
-53- Remarks:IfThis constructor shall not participate in overload resolution unless […]T
's selected constructor is aconstexpr
constructor, this constructor shall be aconstexpr
constructor.
std::move
in [alg.foreach]Section: 26.6.5 [alg.foreach] Status: C++17 Submitter: Jonathan Wakely Opened: 2016-07-15 Last modified: 2017-07-30
Priority: 0
View other active issues in [alg.foreach].
View all other issues in [alg.foreach].
View all issues with C++17 status.
Discussion:
26.6.5 [alg.foreach] p3 says Returns: std::move(f)
.
f
is a function parameter overload resolution to select the constructor
for the return value is first performed as if for an rvalue, so the std::move
is redundant.
It could be argued that it isn't entirely redundant, because it says that implementations can't do something slightly different like return an lvalue reference that is bound to f
, which would prevent it being treated as an rvalue. We should discuss it.
[2016-07 Chicago]
Monday: P0 - tentatively ready
Proposed resolution:
This wording is relative to N4606.
Change 26.6.5 [alg.foreach] as indicated:
template<class InputIterator, class Function> Function for_each(InputIterator first, InputIterator last, Function f);[…]
-3- Returns:. […]
std::move(f)
swappable
traits for optional
sSection: 22.5.3.5 [optional.swap], 22.5.9 [optional.specalg] Status: C++17 Submitter: Agustín K-ballo Bergé Opened: 2016-07-19 Last modified: 2017-07-30
Priority: 0
View all issues with C++17 status.
Discussion:
optional
didn't benefit from the wording modifications by P0185 "Adding [nothrow_
]swappable
traits"; as such, it suffers from LWG 2456(i), and does not play nice with swappable traits.
[2016-07 Chicago]
Monday: P0 - tentatively ready
Proposed resolution:
This wording is relative to N4606.
Modify [optional.object.swap] as indicated:
void swap(optional<T>& rhs) noexcept(see below);[…]
-4- Remarks: The expression insidenoexcept
is equivalent to:is_nothrow_move_constructible_v<T> && is_nothrow_swappable_v<T>noexcept(swap(declval<T&>(), declval<T&>()))
Modify 22.5.9 [optional.specalg] as indicated:
template <class T> void swap(optional<T>& x, optional<T>& y) noexcept(noexcept(x.swap(y)));-1- Effects: Calls
-?- Remarks: This function shall not participate in overload resolution unlessx.swap(y)
.is_move_constructible_v<T>
istrue
andis_swappable_v<T>
istrue
.
swappable
traits for variant
sSection: 22.6.3.7 [variant.swap], 22.6.10 [variant.specalg] Status: C++17 Submitter: Agustín K-ballo Bergé Opened: 2016-07-19 Last modified: 2017-07-30
Priority: 1
View all issues with C++17 status.
Discussion:
variant
does not play nice with swappable traits, the non-member specialized swap
overload is not
SFINAE friendly. On the other hand, the member swap
is SFINAE friendly, albeit with an incomplete condition,
when arguably it shouldn't be. Given the Effects, Throws, and Remarks clauses, the SFINAE
condition should include is_move_constructible_v
and is_move_assignable_v
to account for the
involvement of variant
's move constructor and move assignment operator (the noexcept
specification is
correct as is, since the move assignment operator would only be called for variant
s with different alternatives).
This SFINAE condition should apply to the non-member swap
overload, while the member swap
should require
all alternatives are swappable (as defined by 16.4.4.3 [swappable.requirements]).
[2016-07 Chicago]
Monday: P1 - review later in the week
Fri PM: Move to Tentatively Ready
Previous resolution [SUPERSEDED]:
This wording is relative to N4606.
Modify 22.6.3.7 [variant.swap] as indicated:
void swap(variant& rhs) noexcept(see below);-?- Requires: Lvalues of type
[…] -3- Remarks:Ti
shall be swappable andis_move_constructible_v<Ti> && is_move_assignable_v<Ti>
istrue
for alli
.This function shall not participate in overload resolution unlessIf an exception is thrown during the call to functionis_swappable_v<Ti>
istrue
for alli
.swap(get<i>(*this), get<i>(rhs))
, the states of the contained values of*this
and ofrhs
are determined by the exception safety guarantee of swap for lvalues ofTi
withi
beingindex()
. If an exception is thrown during the exchange of the values of*this
andrhs
, the states of the values of*this
and ofrhs
are determined by the exception safety guarantee ofvariant
's move constructor and move assignment operator. The expression insidenoexcept
is equivalent to the logical AND ofis_nothrow_move_constructible_v<Ti> && is_nothrow_swappable_v<Ti>
for alli
.Modify 22.6.10 [variant.specalg] as indicated:
template <class... Types> void swap(variant<Types...>& v, variant<Types...>& w) noexcept(see below);-1- Effects: Equivalent to
-2- Remarks: This function shall not participate in overload resolution unlessv.swap(w)
.is_move_constructible_v<Ti> && is_move_assignable_v<Ti> && is_swappable_v<Ti>
istrue
for alli
. The expression insidenoexcept
is equivalent tonoexcept(v.swap(w))
.
[2016-08-13, Reopened by Casey Carter]
It is possible to exchange the value of two variants using only move construction on the alternative types, as if by
auto tmp = move(x); x.emplace<i>(move(y)); y.emplace<j>(move(tmp));where i is
y.index()
and j is tmp.index()
. Consequently, variant
's member swap
need not require move assignable alternatives.
[2016-09-09 Issues Resolution Telecon]
Move to Tentatively Ready
Proposed resolution:
This wording is relative to N4606.
Modify 22.6.3.7 [variant.swap] as indicated:
void swap(variant& rhs) noexcept(see below);-?- Requires: Lvalues of typeTi
shall be swappable andis_move_constructible_v<Ti>
shall betrue
for alli
. […] -2- Throws: Ifindex() == rhs.index()
, aAny exception thrown byswap(get<i>(*this), get<i>(rhs))
with i beingindex()
and. Otherwise, any exception thrown by the move constructor ofvariant
's move constructor and assignment operatorTi
orTj
withi
beingindex()
andj
beingrhs.index()
. -3- Remarks:This function shall not participate in overload resolution unlessIf an exception is thrown during the call to functionis_swappable_v<Ti>
istrue
for alli
.swap(get<i>(*this), get<i>(rhs))
, the states of the contained values of*this
and ofrhs
are determined by the exception safety guarantee of swap for lvalues ofTi
withi
beingindex()
. If an exception is thrown during the exchange of the values of*this
andrhs
, the states of the values of*this
and ofrhs
are determined by the exception safety guarantee ofvariant
's move constructorand move assignment operator. The expression insidenoexcept
is equivalent to the logical AND ofis_nothrow_move_constructible_v<Ti> && is_nothrow_swappable_v<Ti>
for alli
.
Modify 22.6.10 [variant.specalg] as indicated:
template <class... Types> void swap(variant<Types...>& v, variant<Types...>& w) noexcept(see below);-1- Effects: Equivalent to
-2- Remarks: This function shall not participate in overload resolution unlessv.swap(w)
.is_move_constructible_v<Ti> && is_swappable_v<Ti>
istrue
for alli
. The expression insidenoexcept
is equivalent tonoexcept(v.swap(w))
.
Section: 5.3.1 [fund.ts.v2::optional.object.ctor] Status: TS Submitter: Casey Carter Opened: 2016-07-20 Last modified: 2017-07-30
Priority: 0
View all issues with TS status.
Discussion:
Addresses: fund.ts.v2
LWG 2451(i) adds a converting constructor to optional
with signature:
template <class U> constexpr optional(U&& v);
and specifies that "This constructor shall not participate in overload resolution unless
is_constructible_v<T, U&&>
is true
and U
is not the same type as T
."
This suffices to avoid this constructor being selected by overload resolution for arguments that should match the
move constructor, but not for arguments that should match the copy constructor. The recent churn around tuple's
constructors suggests that we want this constructor to not participate in overload resolution if
remove_cv_t<remove_reference_t<U>>
is the same type as T
.
[2016-07 Chicago]
Monday: P0 - tentatively ready
Proposed resolution:
This wording is relative to N4600.
Wording relative to N4600 + LWG 2451(i), although it should be noted that this resolution should be applied wherever LWG 2451(i) is applied, be that to the fundamentals TS or the specification of
optional
in the C++ Working Paper.
In 5.3.1 [fund.ts.v2::optional.object.ctor], modify as indicated:
template <class U> constexpr optional(U&& v);[…]
-43- Remarks: IfT
's selected constructor is aconstexpr
constructor, this constructor shall be aconstexpr
constructor. This constructor shall not participate in overload resolution unlessis_constructible_v<T, U&&>
istrue
anddecay_t<U>
is not the same type asT
. The constructor is explicit if and only ifis_convertible_v<U&&, T>
isfalse
.
async
and packaged_task
are unimplementableSection: 32.10.9 [futures.async], 32.10.10.2 [futures.task.members] Status: C++17 Submitter: Billy Robert O'Neal III Opened: 2016-07-07 Last modified: 2017-07-30
Priority: 3
View other active issues in [futures.async].
View all other issues in [futures.async].
View all issues with C++17 status.
Discussion:
std::async
is a request from the user for type erasure; as any given function gets passed to async
which returns only future<ReturnType>
. Therefore, it needs to be able to allocate memory, as other issues
(e.g. LWG 2202(i)) indicate. However, async's Throws clause doesn't allow an implementation to do
this, as it permits only future_error
.
std::packaged_task
's constructor allocates memory using a user supplied allocator. An implementation needs to
call the user's allocate
to allocate such memory. The user's allocate function is not constrained to throwing
only bad_alloc
; it can raise whatever it wants, but packaged_task
's constructor prohibits this.
[2016-07 Chicago]
Alisdair thinks the third bullet is not quite right.
Previous resolution [SUPERSEDED]:
Change 32.10.9 [futures.async] p6 to:
Throws:
system_error
ifpolicy == launch::async
and the implementation is unable to start a new thread, orstd::bad_alloc
if memory for the internal data structures could not be allocated.Change 32.10.10.2 [futures.task.members] p5 to:
template <class F> packaged_task(F&& f); template <class F, class Allocator> packaged_task(allocator_arg_t, const Allocator& a, F&& f);Throws:
- (?) — A
any exceptions thrown by the copy or move constructor off
., or- (?) — For the first version,
std::bad_alloc
if memory for the internal data structures could not be allocated.- (?) — For the second version, any exceptions thrown by
std::allocator_traits<Allocator>::template rebind<unspecified>::allocate
.
[2016-08 Chicago]
Wed PM: Move to Tentatively Ready
Proposed resolution:
This wording is relative to N4606.
Change 32.10.9 [futures.async] p6 to:
Throws:
system_error
ifpolicy == launch::async
and the implementation is unable to start a new thread, orstd::bad_alloc
if memory for the internal data structures could not be allocated.
Change 32.10.10.2 [futures.task.members] p5 to:
template <class F> packaged_task(F&& f); template <class F, class Allocator> packaged_task(allocator_arg_t, const Allocator& a, F&& f);Throws:
- (?) — A
any exceptions thrown by the copy or move constructor off
., or- (?) — For the first version,
std::bad_alloc
if memory for the internal data structures could not be allocated.- (?) — For the second version, any exceptions thrown by
std::allocator_traits<Allocator>::template rebind_traits<unspecified>::allocate
.
Section: 22.5.3.2 [optional.ctor], 22.5.3.4 [optional.assign] Status: Resolved Submitter: Casey Carter Opened: 2016-07-22 Last modified: 2017-06-15
Priority: 0
View all other issues in [optional.ctor].
View all issues with Resolved status.
Discussion:
To use optional<T>
as if it were a T
in generic contexts, optional<T>
's "generic"
operations must behave as do those of T
under overload resolution. At minimum, optional
's constructors
and assignment operators should not participate in overload resolution with argument types that cannot be used to
construct/assign the contained T
so that is_constructible_v<optional<T>, Args...>
(respectively is_assignable_v<optional<T>&, RHS>
) is equivalent to
is_constructible_v<T, Args...>
(respectively is_assignable_v<T&, RHS>
).
In passing, note that the Requires element for optional
's in-place initializer_list
constructor
unnecessarily duplicates its Remarks element; it should be removed.
It should also be noted that the resolution of LWG 2451(i) adds constructors to optional
with
appropriate constraints, but does not constrain the additional assignment operators. If LWG chooses to apply the
resolution of 2451 to the WP, the Requires elements of the additional assignment operators should also be converted
to constraints as the wording herein does for the assignment operators in N4606.
[2016-07 Chicago]
Monday: P0 - tentatively ready
[2016-11-28 Post-Issaquah]
Resolved by the adoption of 2756(i)
Proposed resolution:
This wording is relative to N4606.
Remove [optional.object.ctor] p3, and add a new paragraph after p6:
optional(const optional<T>& rhs);[…] -?- Remarks: The function shall not participate in overload resolution unless
-3- Requires:is_copy_constructible_v<T>
istrue
.is_copy_constructible_v<T>
istrue
.
Remove [optional.object.ctor] p7, and change p11 to:
optional(optional<T>&& rhs) noexcept(see below);[…] -11- Remarks: The expression inside
-7- Requires:is_move_constructible_v<T>
istrue
.noexcept
is equivalent tois_nothrow_move_constructible_v<T>
. The function shall not participate in overload resolution unlessis_move_constructible_v<T>
istrue
.
Remove [optional.object.ctor] p12, and change p16 to:
constexpr optional(const T& v);[…] -16- Remarks: If
-12- Requires:is_copy_constructible_v<T>
istrue
.T
's selected constructor is aconstexpr
constructor, this constructor shall be aconstexpr
constructor. The function shall not participate in overload resolution unlessis_copy_constructible_v<T>
istrue
.
Remove [optional.object.ctor] p17, and change p21 to:
constexpr optional(T&& v);[…] -21- Remarks: If
-17- Requires:is_move_constructible_v<T>
istrue
.T
's selected constructor is aconstexpr
constructor, this constructor shall be aconstexpr
constructor. The function shall not participate in overload resolution unlessis_move_constructible_v<T>
istrue
.
Remove [optional.object.ctor] p22, and change p26 to:
template <class... Args> constexpr explicit optional(in_place_t, Args&&... args);[…] -26- Remarks: If
-22- Requires:is_constructible_v<T, Args&&...>
istrue
.T
's constructor selected for the initialization is aconstexpr
constructor, this constructor shall be aconstexpr
constructor. The function shall not participate in overload resolution unlessis_constructible_v<T, Args...>
istrue
.
Remove [optional.object.ctor] p27.
template <class U, class... Args> constexpr explicit optional(in_place_t, initializer_list<U> il, Args&&... args);[…]
-27- Requires:is_constructible_v<T, initializer_list<U>&, Args&&...>
istrue
.
Remove [optional.object.assign] p4, and change p8 to:
optional<T>& operator=(const optional<T>& rhs);[…] -8- Remarks: If any exception is thrown, the result of the expression
-4- Requires:is_copy_constructible_v<T>
istrue
andis_copy_assignable_v<T>
istrue
.bool(*this)
remains unchanged. If an exception is thrown during the call toT
's copy constructor, no effect. If an exception is thrown during the call toT
's copy assignment, the state of its contained value is as defined by the exception safety guarantee ofT
's copy assignment. The function shall not participate in overload resolution unlessis_copy_constructible_v<T> && is_copy_assignable_v<T>
istrue
.
Remove [optional.object.assign] p9, and add a new paragraph after p14:
optional<T>& operator=(optional<T>&& rhs) noexcept(see below);[…] -14- Remarks: […] If an exception is thrown during the call to
-9- Requires:is_move_constructible_v<T>
istrue
andis_move_assignable_v<T>
istrue
.T
's move assignment, the state of*val
and*rhs.val
is determined by the exception safety guarantee ofT
's move assignment. The function shall not participate in overload resolution unlessis_move_constructible_v<T> && is_move_assignable_v<T>
istrue
.
Remove [optional.object.assign] p15, and change p19 to (yes, this wording is odd - the intent is that it will "do the right thing" after incorporation of LWG 2451(i)):
template <class U> optional<T>& operator=(U&& v);[…] -19- Remarks: If any exception is thrown, the result of the expression
-15- Requires:is_constructible_v<T, U>
istrue
andis_assignable_v<T&, U>
istrue
.bool(*this)
remains unchanged. If an exception is thrown during the call toT
's constructor, the state ofv
is determined by the exception safety guarantee ofT
's constructor. If an exception is thrown during the call toT
's assignment, the state of*val
andv
is determined by the exception safety guarantee ofT
's assignment. The function shall not participate in overload resolution unlessis_same_v<decay_t<U>, T> && is_constructible_v<T, U> && is_assignable_v<T&, U>
istrue
.
in_place
constructors and emplace
functions added by P0032R3 don't require CopyConstructible
Section: 22.7.4.2 [any.cons], 22.7.4.3 [any.assign], 22.7.4.4 [any.modifiers] Status: Resolved Submitter: Ville Voutilainen Opened: 2016-07-05 Last modified: 2020-09-06
Priority: 1
View all other issues in [any.cons].
View all issues with Resolved status.
Discussion:
The in_place
constructors and emplace
functions
added by P0032R3 don't require CopyConstructible
.
any
that's made to hold a non-CopyConstructible
type must fail with a run-time error. Since that's crazy, we want to prevent
storing non-CopyConstructible
types in an any
.
Previously, the requirement for CopyConstructible
was just on the converting
constructor template and the converting assignment operator template on any
.
Now that we are adding two in_place
constructor overloads and two
emplace
overloads, it seems reasonable to require CopyConstructible
in some more
general location, in order to avoid repeating that requirement all over the place.
[2016-07 — Chicago]
Monday: P1
Tuesday: Ville/Billy/Billy provide wording
[2016-08-02: Daniel comments]
The P/R wording of this issue brought to my intention that the recently added emplace
functions
of std::any
introduced a breakage of a previous class invariant that only a decay
ed type could
be stored as object into an any
, this prevented storing arrays, references, functions, and cv-qualified
types. The new constraints added my Ville do prevent some of these types (e.g. neither arrays nor functions meet
the CopyConstructible
requirements), but we need to cope with cv-qualified types and reference types.
[2016-08-02: Agustín K-ballo Bergé comments]
Presumably the constructors any(in_place_type_t<T>, ...)
would need to be modified in the same way
the emplace
overloads were.
[2016-08-02: Ville adjusts the P/R to cope with the problems pointed out by Daniel's and Agustín's comments]
Ville notes that 2746(i), 2754(i) and 2756(i) all go together.
Previous resolution [SUPERSEDED]:
This wording is relative to N4606.
Drafting note: this P/R doesn't turn the Requires-clauses into Remarks-clauses. We might want to do that separately, because SFINAEing the constructors allows users to query for
is_constructible
and get the right answer. Failing to mandate the SFINAE will lead to non-portable answers foris_constructible
. Currently, libstdc++ SFINAEs. That should be done as a separate issue, as this issue is an urgent bug-fix but the mandated SFINAE is not.
Change 22.7.4 [any.class], class
any
synopsis, as indicated:class any { public: […] template <classTValueType, class... Args> explicit any(in_place_type_t<TValueType>, Args&&...); template <classTValueType, class U, class... Args> explicit any(in_place_type_t<TValueType>, initializer_list<U>, Args&&...); […] template <classTValueType, class... Args> void emplace(Args&& ...); template <classTValueType, class U, class... Args> void emplace(initializer_list<U>, Args&&...); […] };Change 22.7.4.2 [any.cons] as indicated:
template<class ValueType> any(ValueType&& value);-6- Let
-7- Requires:T
be equal todecay_t<ValueType>
.T
shall satisfy theCopyConstructible
requirements.If[…] -9- Remarks: This constructor shall not participate in overload resolutionis_copy_constructible_v<T>
isfalse
, the program is ill-formed.ifunlessdecay_t<ValueType>
is not the same type asany
andis_copy_constructible_v<T>
istrue
.template <classTValueType, class... Args> explicit any(in_place_type_t<TValueType>, Args&&... args);-?- Let
-11- Requires:T
be equal toremove_cv_t<ValueType>
.T
shall satisfy theCopyConstructible
requirements. […] -?- Remarks: This constructor shall not participate in overload resolution unlessis_constructible_v<T, Args...>
istrue
is_reference_v<T>
isfalse
,is_array_v<T>
isfalse
,is_function_v<T>
isfalse
,is_copy_constructible_v<T>
istrue
andis_constructible_v<T, Args...>
istrue
.template <classTValueType, class U, class... Args> explicit any(in_place_type_t<TValueType>, initializer_list<U> il, Args&&... args);-?- Let
-15- Requires:T
be equal toremove_cv_t<ValueType>
.T
shall satisfy theCopyConstructible
requirements. […] -19- Remarks: The function shall not participate in overload resolution unlessis_constructible_v<T, initializer_list<U>&, Args...>
istrue
is_reference_v<T>
isfalse
,is_array_v<T>
isfalse
,is_function_v<T>
isfalse
,is_copy_constructible_v<T>
istrue
andis_constructible_v<T, initializer_list<U>&, Args...>
istrue
.Change 22.7.4.3 [any.assign] as indicated:
template<class ValueType> any& operator=(ValueType&& rhs);-7- Let
-8- Requires:T
be equal todecay_t<ValueType>
.T
shall satisfy theCopyConstructible
requirements.If[…] -11- Remarks: This operator shall not participate in overload resolutionis_copy_constructible_v<T>
isfalse
, the program is ill-formed.ifunlessdecay_t<ValueType>
is not the same type asany
andis_copy_constructible_v<T>
istrue
.Change 22.7.4.4 [any.modifiers] as indicated:
template <classTValueType, class... Args> void emplace(Args&&... args);-?- Let
-1- Requires:T
be equal toremove_cv_t<ValueType>
.T
shall satisfy theCopyConstructible
requirements. […] -5- Remarks: If an exception is thrown during the call tois_constructible_v<T, Args...>
istrue
T
's constructor,*this
does not contain a value, and any previously contained object has been destroyed. This function shall not participate in overload resolution unlessis_reference_v<T>
isfalse
,is_array_v<T>
isfalse
,is_function_v<T>
isfalse
,is_copy_constructible_v<T>
istrue
andis_constructible_v<T, Args...>
istrue
.template <classTValueType, class U, class... Args> void emplace(initializer_list<U> il, Args&&... args);-?- Let
-?- Requires:T
be equal toremove_cv_t<ValueType>
.T
shall satisfy theCopyConstructible
requirements. -6- Effects: […] […] -9- Remarks: If an exception is thrown during the call toT
's constructor,*this
does not contain a value, and any previously contained object has been destroyed. The function shall not participate in overload resolution unlessis_reference_v<T>
isfalse
,is_array_v<T>
isfalse
,is_function_v<T>
isfalse
,is_copy_constructible_v<T>
istrue
andis_constructible_v<T, initializer_list<U>&, Args...>
istrue
.
[2016-08-03: Ville comments and revises his proposed wording]
After discussing the latest P/R, here's an update. What this update does is that:
It strikes the Requires-clauses and does not add
CopyConstructible
to the Requires-clauses.
any
doesn't care whether the type it holds satisfies the
semantic requirements of the CopyConstructible
concept. The syntactic
requirements are now SFINAE constraints in Requires-clauses.It reverts back towards decay_t
rather than remove_cv_t
, and does
not add the suggested SFINAE constraints for is_reference
/is_array
/is_function
.
any
decays by design. It's to some extent inconsistent
to not protect against decay in the ValueType
constructor/assignment operator, but to protect
against decay in the in_place_t
constructors and emplace
functions
I think it's saner to just decay than to potentially run into
situations where I need to remove_reference
inside in_place_t
.
Based on that, this P/R should supersede the previous one. We want to look at this new P/R in LWG and potentially send it to LEWG for verification. Personally, I think this P/R is the more conservative one, doesn't add significant new functionality, and is consistent, and is thus not really Library-Evolutionary.
Previous resolution [SUPERSEDED]:
This wording is relative to N4606.
Change 22.7.4 [any.class], class
any
synopsis, as indicated:class any { public: […] template <classTValueType, class... Args> explicit any(in_place_type_t<TValueType>, Args&&...); template <classTValueType, class U, class... Args> explicit any(in_place_type_t<TValueType>, initializer_list<U>, Args&&...); […] template <classTValueType, class... Args> void emplace(Args&& ...); template <classTValueType, class U, class... Args> void emplace(initializer_list<U>, Args&&...); […] };Change 22.7.4.2 [any.cons] as indicated:
template<class ValueType> any(ValueType&& value);-6- Let
T
be equal todecay_t<ValueType>
.-7- Requires:[…] -9- Remarks: This constructor shall not participate in overload resolutionT
shall satisfy theCopyConstructible
requirements. Ifis_copy_constructible_v<T>
isfalse
, the program is ill-formed.ifunlessdecay_t<ValueType>
is not the same type asany
andis_copy_constructible_v<T>
istrue
.template <classTValueType, class... Args> explicit any(in_place_type_t<TValueType>, Args&&... args);-?- Let
T
be equal todecay_t<ValueType>
.-11- Requires:. […] -?- Remarks: This constructor shall not participate in overload resolution unlessis_constructible_v<T, Args...>
istrue
is_copy_constructible_v<T>
istrue
andis_constructible_v<T, Args...>
istrue
.template <classTValueType, class U, class... Args> explicit any(in_place_type_t<TValueType>, initializer_list<U> il, Args&&... args);-?- Let
T
be equal todecay_t<ValueType>
.-15- Requires:. […] -19- Remarks: The function shall not participate in overload resolution unlessis_constructible_v<T, initializer_list<U>&, Args...>
istrue
is_copy_constructible_v<T>
istrue
andis_constructible_v<T, initializer_list<U>&, Args...>
istrue
.Change 22.7.4.3 [any.assign] as indicated:
template<class ValueType> any& operator=(ValueType&& rhs);-7- Let
T
be equal todecay_t<ValueType>
.-8- Requires:[…] -11- Remarks: This operator shall not participate in overload resolutionT
shall satisfy theCopyConstructible
requirements. Ifis_copy_constructible_v<T>
isfalse
, the program is ill-formed.ifunlessdecay_t<ValueType>
is not the same type asany
andis_copy_constructible_v<T>
istrue
.Change 22.7.4.4 [any.modifiers] as indicated:
template <classTValueType, class... Args> void emplace(Args&&... args);-?- Let
T
be equal todecay_t<ValueType>
.-1- Requires:. […] -5- Remarks: If an exception is thrown during the call tois_constructible_v<T, Args...>
istrue
T
's constructor,*this
does not contain a value, and any previously contained object has been destroyed. This function shall not participate in overload resolution unlessis_copy_constructible_v<T>
istrue
andis_constructible_v<T, Args...>
istrue
.template <classTValueType, class U, class... Args> void emplace(initializer_list<U> il, Args&&... args);-?- Let
-6- Effects: […] […] -9- Remarks: If an exception is thrown during the call toT
be equal todecay_t<ValueType>
.T
's constructor,*this
does not contain a value, and any previously contained object has been destroyed. The function shall not participate in overload resolution unlessis_copy_constructible_v<T>
istrue
andis_constructible_v<T, initializer_list<U>&, Args...>
istrue
.
[2016-08-03: Ville comments and revises his proposed wording]
This P/R brings back the CopyConstructible
parts of the relevant
Requires-clauses but removes the other parts of the Requires-clauses.
[2016-08 - Chicago]
Thurs PM: Moved to Tentatively Ready
[2016-11 - Issaquah]
Approved in plenary.
After plenary, there was concern about applying both this and 2744(i), so it was moved back to "Open". Then, when the concerns were resolved, moved to "Resolved".
Proposed resolution:
This wording is relative to N4606.
Change 22.7.4 [any.class], class any
synopsis, as indicated:
class any { public: […] template <classTValueType, class... Args> explicit any(in_place_type_t<TValueType>, Args&&...); template <classTValueType, class U, class... Args> explicit any(in_place_type_t<TValueType>, initializer_list<U>, Args&&...); […] template <classTValueType, class... Args> void emplace(Args&& ...); template <classTValueType, class U, class... Args> void emplace(initializer_list<U>, Args&&...); […] };
Change 22.7.4.2 [any.cons] as indicated:
template<class ValueType> any(ValueType&& value);-6- Let
-7- Requires:T
bedecay_t<ValueType>
.T
shall satisfy theCopyConstructible
requirements.If[…] -9- Remarks: This constructor shall not participate in overload resolutionis_copy_constructible_v<T>
isfalse
, the program is ill-formed.ifunlessT
is not the same type asdecay_t<ValueType>any
andis_copy_constructible_v<T>
istrue
.template <classTValueType, class... Args> explicit any(in_place_type_t<TValueType>, Args&&... args);-?- Let
-11- Requires:T
bedecay_t<ValueType>
.T
shall satisfy theCopyConstructible
requirements. […] -?- Remarks: This constructor shall not participate in overload resolution unlessis_constructible_v<T, Args...>
istrue
is_copy_constructible_v<T>
istrue
andis_constructible_v<T, Args...>
istrue
.template <classTValueType, class U, class... Args> explicit any(in_place_type_t<TValueType>, initializer_list<U> il, Args&&... args);-?- Let
-15- Requires:T
bedecay_t<ValueType>
.T
shall satisfy theCopyConstructible
requirements. […] -19- Remarks: The function shall not participate in overload resolution unlessis_constructible_v<T, initializer_list<U>&, Args...>
istrue
is_copy_constructible_v<T>
istrue
andis_constructible_v<T, initializer_list<U>&, Args...>
istrue
.
Change 22.7.4.3 [any.assign] as indicated:
template<class ValueType> any& operator=(ValueType&& rhs);-7- Let
-8- Requires:T
bedecay_t<ValueType>
.T
shall satisfy theCopyConstructible
requirements.If[…] -11- Remarks: This operator shall not participate in overload resolutionis_copy_constructible_v<T>
isfalse
, the program is ill-formed.ifunlessT
is not the same type asdecay_t<ValueType>any
andis_copy_constructible_v<T>
istrue
.
Change 22.7.4.4 [any.modifiers] as indicated:
template <classTValueType, class... Args> void emplace(Args&&... args);-?- Let
-1- Requires:T
bedecay_t<ValueType>
.T
shall satisfy theCopyConstructible
requirements. […] -5- Remarks: If an exception is thrown during the call tois_constructible_v<T, Args...>
istrue
T
's constructor,*this
does not contain a value, and any previously contained object has been destroyed. This function shall not participate in overload resolution unlessis_copy_constructible_v<T>
istrue
andis_constructible_v<T, Args...>
istrue
.template <classTValueType, class U, class... Args> void emplace(initializer_list<U> il, Args&&... args);-?- Let
-?- Requires:T
bedecay_t<ValueType>
.T
shall satisfy theCopyConstructible
requirements. -6- Effects: […] […] -9- Remarks: If an exception is thrown during the call toT
's constructor,*this
does not contain a value, and any previously contained object has been destroyed. The function shall not participate in overload resolution unlessis_copy_constructible_v<T>
istrue
andis_constructible_v<T, initializer_list<U>&, Args...>
istrue
.
basic_string_view::to_string
functionSection: 27.3.5 [string.view.io], 27.4.4.4 [string.io] Status: C++17 Submitter: Billy Baker Opened: 2016-07-26 Last modified: 2020-09-06
Priority: 0
View all issues with C++17 status.
Discussion:
In looking at N4606, [string.view.io] has an Effects clause that references basic_string_view::to_string
which no longer exists after the application of P0254R2.
[2016-07-26, Marshall suggests concrete wording]
[2016-07 Chicago LWG]
Monday: P0 - tentatively ready
Proposed resolution:
This wording is relative to N4606.
Modify 27.4.4.4 [string.io] as indicated:
template<class charT, class traits, class Allocator> basic_ostream<charT, traits>& operator<<(basic_ostream<charT, traits>& os, const basic_string<charT, traits, Allocator>& str);-5- Effects: Equivalent to:
return os << basic_string_view<charT, traits>(str);
Behaves as a formatted output function (31.7.6.3.1 [ostream.formatted.reqmts]) ofos
. Forms a character sequenceseq
, initially consisting of the elements defined by the range[str.begin(), str.end())
. Determines padding forseq
as described in 31.7.6.3.1 [ostream.formatted.reqmts]. Then insertsseq
as if by callingos.rdbuf()->sputn(seq, n)
, wheren
is the larger ofos.width()
andstr.size()
; then callsos.width(0)
.-6- Returns:os
Modify 27.3.5 [string.view.io] as indicated:
template<class charT, class traits> basic_ostream<charT, traits>& operator<<(basic_ostream<charT, traits>& os, basic_string_view<charT, traits> str);-1- Effects:
-?- Returns:Equivalent to:Behaves as a formatted output function (31.7.6.3.1 [ostream.formatted.reqmts]) ofreturn os << str.to_string();
os
. Forms a character sequenceseq
, initially consisting of the elements defined by the range[str.begin(), str.end())
. Determines padding forseq
as described in 31.7.6.3.1 [ostream.formatted.reqmts]. Then insertsseq
as if by callingos.rdbuf()->sputn(seq, n)
, wheren
is the larger ofos.width()
andstr.size()
; then callsos.width(0)
.os
optional<T>
should 'forward' T
's implicit conversionsSection: 22.5.3 [optional.optional] Status: C++17 Submitter: Casey Carter Opened: 2016-07-26 Last modified: 2017-07-30
Priority: 1
View all other issues in [optional.optional].
View all issues with C++17 status.
Discussion:
LWG 2451(i) adds converting constructors and assignment operators to optional
. The committee
voted to apply it to the Library Fundamentals 2 TS WP in Oulu as part of LWG Motion 3. In both LWG and LEWG
discussion of this issue, it was considered to be critical to apply to the specification of optional
before shipping C++17 — it was an oversight on the part of LWG that there was no motion brought to apply
this resolution to the C++ WP.
constexpr
specifier from the declarations of the
converting constructors from const optional<U>&
and optional<U>&&
since they are not implementable as constexpr
constructors in C++17.
This issue proposes application of the resolution of LWG 2451(i) as amended by LWG 2745(i)
to the specification of optional
in the C++ WP.
[2016-07 — Chicago]
Monday: Priority set to 1; will re-review later in the week
[2016-08-03, Tomasz comments]
Value forwarding constructor (template<typename U> optional(U&&)
) is underspecified.
For the following use code:
optional<T> o1; optional<T> o2(o1);
The second constructor will invoke value forwarding (U = optional<T>&)
constructor (better match)
instead of the optional<T>
copy constructor, in case if T
can be constructed from
optional<T>
. This happens for any type T
that has unconstrained perfect forwarding constructor,
especially optional<any>
.
The behavior of the construction of the optional<T> ot
from optional<U> ou
is
inconsistent for classes T
than can be constructed both from optional<U>
and U
.
There are two possible semantics for such operation:
unwrapping: if ou
is initialized (bool(ou)
), initialize contained value of ot
from *ou
, otherwise make ot
uninitialized (!bool(ot)
)
value forwarding: initialize contained value of ot
from ou
, ot
is always
initialized (bool(ot)
).
For example, if we consider following class:
struct Widget { Widget(int); Widget(optional<int>); };
Notice, that such set of constructor is pretty common in situation when the construction of the
Widget
from known value is common and usage of optional
version is rare. In such situation,
presence of Widget(int)
construction is an optimization used to avoid unnecessary empty checks and
construction optional<int>
.
optional<Widget> w1(make_optional(10)); optional<Widget> w2; w2 = make_optional(10);
The w1
will contain a value created using Widget(optional<int>)
constructor,
as corresponding unwrapping constructor (optional<U> const&
) is eliminated by
is_constructible_v<T, const optional<U>&>
(is_constructible_v<Widget, const optional<int>&>
) having a true
value.
In contrast w2
will contain a value created using Widget(int)
constructor, as corresponding
value forwarding assignment (U&&
) is eliminated by the fact that
std::decay_t<U>
(optional<int>
) is specialization of optional
.
struct Thingy { Thingy(optional<string>); }; optional<Thingy> t1(optional<string>("test"));
The t1
has a contained value constructed from using Thingy(optional<std::string>)
,
as unwrapping constructor (optional<U> const&
) are eliminated by the
is_constructible<T, U const&>
(is_constructible<Thingy, std::string const&>
)
being false
. However the following:
t1 = optional<std::string>("test2");
will not compile, because, the value forwarding assignment (U&&
) is eliminated by
std::decay_t<U>
(optional<std::string>
) being optional specialization and the
unwrapping assignment (optional<U> const&
) is ill-formed because
is_constructible<T, U const&>
(is_constructible<Thingy, std::string const&>
)
is false
.
The semantics of construction and assignment, of optional<optional<V>>
from
optional<U>
where U
is convertible to/ same as T
is also inconsistent. Firstly,
in this situation the optional<V>
is a type that can be constructed both from
optional<U>
and U
so it fails into set of problem described above. However in
addition we have following non-template constructor in optional<T>
:
optional(T const&);
Which for optional<optional<V>>
is equivalent to:
optional(optional<V> const&);
While there is no corresponding non-template assignment from T const&
, to make sure that
o = {};
always clear an optional o
.
optional<int> oi; optional<short> os; optional<optional<int>> ooi1(oi); optional<optional<int>> ooi2(os);
The ooi1
uses from-T
constructor, while ooi2
uses value forwarding constructor.
In this case both ooi1
and ooi2
are initialized and their contained values *ooi1
,
*ooi2
are uninitialized optional
s. However, if we will attempt to make construction consistent
with assignment, by preferring unwrapping (optional<U> const&
), then ooi2
will end up being uninitialized.
T
construction/assignment is to subtle to being handled as defect report and requires
a full paper analyzing possible design and their pros/cons.
Tuesday PM: Ville and Eric to implement and report back in Issaquah. Moved to Open
Ville notes that 2746(i), 2754(i) and 2756(i) all go together.
Previous resolution [SUPERSEDED]:
This wording is relative to N4606.
Modify 22.5.3 [optional.optional] as indicated:
template <class T> class optional { public: using value_type = T; // 20.6.3.1, Constructors constexpr optional() noexcept; constexpr optional(nullopt_t) noexcept; optional(const optional &); optional(optional &&) noexcept(see below); constexpr optional(const T &); constexpr optional(T &&); template <class... Args> constexpr explicit optional(in_place_t, Args &&...); template <class U, class... Args> constexpr explicit optional(in_place_t, initializer_list<U>, Args &&...); template <class U> constexpr optional(U &&); template <class U> optional(const optional<U> &); template <class U> optional(optional<U> &&); […] // 20.6.3.3, Assignment optional &operator=(nullopt_t) noexcept; optional &operator=(const optional &); optional &operator=(optional &&) noexcept(see below); template <class U> optional &operator=(U &&); template <class U> optional& operator=(const optional<U> &); template <class U> optional& operator=(optional<U> &&); template <class... Args> void emplace(Args &&...); template <class U, class... Args> void emplace(initializer_list<U>, Args &&...); […] };In [optional.object.ctor], insert new signature specifications after p31:
[Note: The following constructors are conditionally specified as
explicit
. This is typically implemented by declaring two such constructors, of which at most one participates in overload resolution. — end note]template <class U> constexpr optional(U&& v);-?- Effects: Initializes the contained value as if direct-non-list-initializing an object of type
-?- Postconditions:T
with the expressionstd::forward<U>(v)
.*this
contains a value. -?- Throws: Any exception thrown by the selected constructor ofT
. -?- Remarks: IfT
's selected constructor is aconstexpr
constructor, this constructor shall be aconstexpr
constructor. This constructor shall not participate in overload resolution unlessis_constructible_v<T, U&&>
istrue
andU
is not the same type asT
. The constructor isexplicit
if and only ifis_convertible_v<U&&, T>
isfalse
.template <class U> optional(const optional<U>& rhs);-?- Effects: If
-?- Postconditions:rhs
contains a value, initializes the contained value as if direct-non-list-initializing an object of typeT
with the expression*rhs
.bool(rhs) == bool(*this)
. -?- Throws: Any exception thrown by the selected constructor ofT
. -?- Remarks: This constructor shall not participate in overload resolution unlessis_constructible_v<T, const U&>
istrue
,is_same<decay_t<U>, T>
isfalse
,is_constructible_v<T, const optional<U>&>
isfalse
andis_convertible_v<const optional<U>&, T>
isfalse
. The constructor isexplicit
if and only ifis_convertible_v<const U&, T>
isfalse
.template <class U> optional(optional<U>&& rhs);-?- Effects: If
-?- Postconditions:rhs
contains a value, initializes the contained value as if direct-non-list-initializing an object of typeT
with the expressionstd::move(*rhs)
.bool(rhs)
is unchanged.bool(rhs) == bool(*this)
. -?- Throws: Any exception thrown by the selected constructor ofT
. -?- Remarks: This constructor shall not participate in overload resolution unlessis_constructible_v<T, U&&>
istrue
,is_same<decay_t<U>, T>
isfalse
,is_constructible_v<T, optional<U>&&>
isfalse
andis_convertible_v<optional<U>&&, T>
isfalse
andU
is not the same type asT
. The constructor isexplicit
if and only ifis_convertible_v<U&&, T>
isfalse
.In [optional.object.assign], change as indicated:
template <class U> optional<T>& operator=(U&& v);-22- Remarks: If any exception is thrown, the result of the expression
bool(*this)
remains unchanged. If an exception is thrown during the call toT
's constructor, the state ofv
is determined by the exception safety guarantee ofT
's constructor. If an exception is thrown during the call toT
's assignment, the state of*val
andv
is determined by the exception safety guarantee ofT
's assignment. The function shall not participate in overload resolution unlessdecay_t<U>
is notnullopt_t
anddecay_t<U>
is not a specialization ofoptional
.is_same_v<decay_t<U>, T>
istrue
-23- Notes: The reason for providing such generic assignment and then constraining it so that effectivelyT == U
is to guarantee that assignment of the formo = {}
is unambiguous.template <class U> optional<T>& operator=(const optional<U>& rhs);-?- Requires:
-?- Effects:is_constructible_v<T, const U&>
istrue
andis_assignable_v<T&, const U&>
istrue
.
Table ? — optional::operator=(const optional<U>&)
effects*this
contains a value*this
does not contain a valuerhs
contains a valueassigns *rhs
to the contained valueinitializes the contained value as if direct-non-list-initializing an object of type T
with*rhs
rhs
does not contain a valuedestroys the contained value by calling val->T::~T()
no effect -?- Returns:
-?- Postconditions:*this
.bool(rhs) == bool(*this)
. -?- Remarks: If any exception is thrown, the result of the expressionbool(*this)
remains unchanged. If an exception is thrown during the call toT
's constructor, the state of*rhs.val
is determined by the exception safety guarantee ofT
's constructor. If an exception is thrown during the call toT
's assignment, the state of*val
and*rhs.val
is determined by the exception safety guarantee ofT
's assignment. The function shall not participate in overload resolution unlessis_same_v<decay_t<U>, T>
isfalse
.template <class U> optional<T>& operator=(optional<U>&& rhs);-?- Requires:
-?- Effects: The result of the expressionis_constructible_v<T, U>
istrue
andis_assignable_v<T&, U>
istrue
.bool(rhs)
remains unchanged.
Table ? — optional::operator=(optional<U>&&)
effects*this
contains a value*this
does not contain a valuerhs
contains a valueassigns std::move(*rhs)
to the contained valueinitializes the contained value as if direct-non-list-initializing an object of type T
withstd::move(*rhs)
rhs
does not contain a valuedestroys the contained value by calling val->T::~T()
no effect -?- Returns:
-?- Postconditions:*this
.bool(rhs) == bool(*this)
. -?- Remarks: If any exception is thrown, the result of the expressionbool(*this)
remains unchanged. If an exception is thrown during the call toT
's constructor, the state of*rhs.val
is determined by the exception safety guarantee ofT
's constructor. If an exception is thrown during the call toT
's assignment, the state of*val
and*rhs.val
is determined by the exception safety guarantee ofT
's assignment. The function shall not participate in overload resolution unlessis_same_v<decay_t<U>, T>
isfalse
.
[2016-08-05 Chicago LWG]
Ville provides revised wording, that also fixes LWG 2753(i).
Rationale:The resolution of LWG 2753(i) makes special member functions defined as deleted in case the desired constraints aren't met.
There is no decay for the converting
constructor optional(U&&)
, there is a remove_reference
instead.
The target type may hold a cv-qualified type, and the incoming
type may hold a cv-qualified type, but neither can hold a reference.
Thus, remove_reference
is what we need, remove_cv
would
be wrong, and decay
would be wrong.
There is no decay
or remove_reference
for converting constructors like
optional(optional<U>)
, because none is needed.
For optional(U&&)
, an added constraint is that U
is
not a specialization of optional
[2016-08, Chicago]
Fri PM: Move to Tentatively Ready
Previous resolution [SUPERSEDED]:
This wording is relative to N4606.
Modify 22.5.3 [optional.optional] as indicated:
template <class T> class optional { public: using value_type = T; // 20.6.3.1, Constructors constexpr optional() noexcept; constexpr optional(nullopt_t) noexcept; optional(const optional &); optional(optional &&) noexcept(see below); constexpr optional(const T &); constexpr optional(T &&); template <class... Args> constexpr explicit optional(in_place_t, Args &&...); template <class U, class... Args> constexpr explicit optional(in_place_t, initializer_list<U>, Args &&...); template <class U> EXPLICIT constexpr optional(U &&); template <class U> EXPLICIT optional(const optional<U> &); template <class U> EXPLICIT optional(optional<U> &&); […] // 20.6.3.3, Assignment optional &operator=(nullopt_t) noexcept; optional &operator=(const optional &); optional &operator=(optional &&) noexcept(see below); template <class U> optional &operator=(U &&); template <class U> optional& operator=(const optional<U> &); template <class U> optional& operator=(optional<U> &&); template <class... Args> void emplace(Args &&...); template <class U, class... Args> void emplace(initializer_list<U>, Args &&...); […] };Change [optional.object.ctor] as indicated:
optional(const optional<T>& rhs);[…] -?- Remarks: This constructor shall be defined as deleted unless
-3- Requires:is_copy_constructible_v<T>
istrue
.is_copy_constructible_v<T>
istrue
.optional(optional<T>&& rhs) noexcept(see below);[…] -11- Remarks: The expression inside
-7- Requires:is_move_constructible_v<T>
istrue
.noexcept
is equivalent tois_nothrow_move_constructible_v<T>
. This constructor shall be defined as deleted unlessis_move_constructible_v<T>
istrue
.constexpr optional(const T& v);[…] -16- Remarks: If
-12- Requires:is_copy_constructible_v<T>
istrue
.T
's selected constructor is aconstexpr
constructor, this constructor shall be aconstexpr
constructor. This constructor shall not participate in overload resolution unlessis_copy_constructible_v<T>
istrue
.constexpr optional(T&& v);[…] -21- Remarks: If
-17- Requires:is_move_constructible_v<T>
istrue
.T
's selected constructor is aconstexpr
constructor, this constructor shall be aconstexpr
constructor. This constructor shall not participate in overload resolution unlessis_move_constructible_v<T>
istrue
.template <class... Args> constexpr explicit optional(in_place_t, Args&&... args);[…] -26- Remarks: If
-22- Requires:is_constructible_v<T, Args&&...>
istrue
.T
's constructor selected for the initialization is aconstexpr
constructor, this constructor shall be aconstexpr
constructor. This constructor shall not participate in overload resolution unlessis_constructible_v<T, Args...>
istrue
.template <class U, class... Args> constexpr explicit optional(in_place_t, initializer_list<U> il, Args&&... args);[…] -31- Remarks: The
-27- Requires:is_constructible_v<T, initializer_list<U>&, Args&&...>
istrue
.functionconstructor shall not participate in overload resolution unlessis_constructible_v<T, initializer_list<U>&, Args&&...>
istrue
. IfT
's constructor selected for the initialization is aconstexpr
constructor, this constructor shall be aconstexpr
constructor.[Note: The following constructors are conditionally specified as explicit. This is typically implemented by declaring two such constructors, of which at most one participates in overload resolution. — end note]
template <class U> EXPLICIT constexpr optional(U&& v);-?- Effects: Initializes the contained value as if direct-non-list-initializing an object of type
-?- Postconditions:T
with the expressionstd::forward<U>(v)
.*this
contains a value. -?- Throws: Any exception thrown by the selected constructor ofT
. -?- Remarks: IfT
's selected constructor is aconstexpr
constructor, this constructor shall be aconstexpr
constructor. This constructor shall not participate in overload resolution unlessis_constructible_v<T, U&&>
istrue
,remove_reference_t<U>
is not the same type asT
, andU
is not a specialization ofoptional
. The constructor is explicit if and only ifis_convertible_v<U&&, T>
isfalse
.template <class U> EXPLICIT optional(const optional<U>& rhs);-?- Effects: If
-?- Postconditions:rhs
contains a value, initializes the contained value as if direct-non-list-initializing an object of typeT
with the expression*rhs
.bool(rhs) == bool(*this)
. -?- Throws: Any exception thrown by the selected constructor ofT
. -?- Remarks: This constructor shall not participate in overload resolution unlessis_constructible_v<T, const U&>
istrue
andis_same_v<U, T>
isfalse
. The constructor is explicit if and only ifis_convertible_v<const U&, T>
isfalse
.template <class U> EXPLICIT optional(optional<U>&& rhs);-?- Effects: If
-?- Postconditions:rhs
contains a value, initializes the contained value as if direct-non-list-initializing an object of typeT
with the expressionstd::move(*rhs)
.bool(rhs)
is unchanged.bool(rhs) == bool(*this)
. -?- Throws: Any exception thrown by the selected constructor ofT
. -?- Remarks: This constructor shall not participate in overload resolution unlessis_constructible_v<T, U&&>
istrue
andis_same_v<U, T>
isfalse
. The constructor is explicit if and only ifis_convertible_v<U&&, T>
isfalse
.Change [optional.object.assign] as indicated:
optional<T>& operator=(const optional<T>& rhs);[…] -8- Remarks: If any exception is thrown, the result of the expression
-4- Requires:is_copy_constructible_v<T>
istrue
andis_copy_assignable_v<T>
istrue
.bool(*this)
remains unchanged. If an exception is thrown during the call toT
's copy constructor, no effect. If an exception is thrown during the call toT
's copy assignment, the state of its contained value is as defined by the exception safety guarantee ofT
's copy assignment. This operator shall be defined as deleted unlessis_copy_constructible_v<T>
istrue
andis_copy_assignable_v<T>
istrue
.optional<T>& operator=(optional<T>&& rhs) noexcept(see below);[…] -13- Remarks: The expression inside
-9- Requires:is_move_constructible_v<T>
istrue
andis_move_assignable_v<T>
istrue
.noexcept
is equivalent to:is_nothrow_move_assignable_v<T> && is_nothrow_move_constructible_v<T>-14- If any exception is thrown, the result of the expression
bool(*this)
remains unchanged. If an exception is thrown during the call toT
's move constructor, the state of*rhs.val
is determined by the exception safety guarantee ofT
's move constructor. If an exception is thrown during the call toT
's move assignment, the state of*val
and*rhs.val
is determined by the exception safety guarantee ofT
's move assignment. This operator shall be defined as deleted unlessis_move_constructible_v<T>
istrue
andis_move_assignable_v<T>
istrue
.template <class U> optional<T>& operator=(U&& v);[…] -19- Remarks: If any exception is thrown, the result of the expression
-15- Requires:is_constructible_v<T, U>
istrue
andis_assignable_v<T&, U>
istrue
.bool(*this)
remains unchanged. If an exception is thrown during the call toT
's constructor, the state ofv
is determined by the exception safety guarantee ofT
's constructor. If an exception is thrown during the call toT
's assignment, the state of*val
andv
is determined by the exception safety guarantee ofT
's assignment.TheThis function shall not participate in overload resolution unlessis_same_v<decay_t<U>, T>
decay_t<U>
is notnullopt_t
,decay_t<U>
is not a specialization ofoptional
,is_constructible_v<T, U>
istrue
andis_assignable_v<T&, U>
istrue
.-20- Notes: The reason for providing such generic assignment and then constraining it so that effectivelyT == U
is to guarantee that assignment of the formo = {}
is unambiguous.template <class U> optional<T>& operator=(const optional<U>& rhs);-?- Effects:
Table ? — optional::operator=(const optional<U>&)
effects*this
contains a value*this
does not contain a valuerhs
contains a valueassigns *rhs
to the contained valueinitializes the contained value as if direct-non-list-initializing an object of type T
with*rhs
rhs
does not contain a valuedestroys the contained value by calling val->T::~T()
no effect -?- Returns:
-?- Postconditions:*this
.bool(rhs) == bool(*this)
. -?- Remarks: If any exception is thrown, the result of the expressionbool(*this)
remains unchanged. If an exception is thrown during the call toT
's constructor, the state of*rhs.val
is determined by the exception safety guarantee ofT
's constructor. If an exception is thrown during the call toT
's assignment, the state of*val
and*rhs.val
is determined by the exception safety guarantee ofT
's assignment. The function shall not participate in overload resolution unlessis_constructible_v<T, const U&>
istrue
andis_assignable_v<T&, const U&>
istrue
andis_same_v<U, T>
isfalse
.template <class U> optional<T>& operator=(optional<U>&& rhs);-?- Effects: The result of the expression
bool(rhs)
remains unchanged.
Table ? — optional::operator=(optional<U>&&)
effects*this
contains a value*this
does not contain a valuerhs
contains a valueassigns std::move(*rhs)
to the contained valueinitializes the contained value as if direct-non-list-initializing an object of type T
withstd::move(*rhs)
rhs
does not contain a valuedestroys the contained value by calling val->T::~T()
no effect -?- Returns:
-?- Postconditions:*this
.bool(rhs) == bool(*this)
. -?- Remarks: If any exception is thrown, the result of the expressionbool(*this)
remains unchanged. If an exception is thrown during the call toT
's constructor, the state of*rhs.val
is determined by the exception safety guarantee ofT
's constructor. If an exception is thrown during the call toT
's assignment, the state of*val
and*rhs.val
is determined by the exception safety guarantee ofT
's assignment. The function shall not participate in overload resolution unlessis_constructible_v<T, U>
istrue
andis_assignable_v<T&, U>
istrue
andis_same_v<U, T>
isfalse
.
[2016-08-08 Ville reopens and provides improved wording]
This alternative proposed wording also resolves 2753(i).
The constructors that take a constT&
or T&&
are replaced by
a constructor template that takes a U&&
and defaults U = T
. This
allows copy-list-initialization with empty braces to still work:
optional<whatever> o = {}; // equivalent to initializingoptional
withnullopt
This resolution makes converting constructors and assignments have the same capabilities, including
using arguments that can't be deduced. That is achieved by using a perfect-forwarding constructor
and an assignment operator that default their argument to T
. We don't need separate
overloads for T
, the overload for U
does the job:
optional<vector<int>> ovi{{1, 2, 3}}; // still works ovi = {4, 5, 6, 7}; // now works, didn't work before
Furthermore, this proposed wording makes optional
"always unwrap". That is, the result
of the following initializations is the same:
optional<optional<int>> oi = optional<int>(); optional<optional<int>> oi = optional<short>();
Both of those initializations initialize the optional
wrapping
another optional
as if initializing with nullopt
. Assignments
do the same. These changes solve the issues pointed out by Tomasz Kamiński.
optional
.
[2016-08-08 Ville and Tomasz collaborate and improve wording]
The suggested wording retains optional
's converting constructors and assignment
operators, but provides sane results for the types Tomasz Kaminski depicts
in previous discussions.
[2016-09-08 Casey Carter finetunes existing resolution for move members]
[2016-09-09 Issues Resolution Telecon]
Move to Tentatively Ready
[2016-10-06 Ville Voutilainen finetunes the resolution for assignment from scalars]
Previous resolution [SUPERSEDED]:
This wording is relative to N4606.
Modify 22.5.3 [optional.optional] as indicated:
template <class T> class optional { public: using value_type = T; // 20.6.3.1, Constructors constexpr optional() noexcept; constexpr optional(nullopt_t) noexcept; optional(const optional &); optional(optional &&) noexcept(see below);constexpr optional(const T &);constexpr optional(T &&);template <class... Args> constexpr explicit optional(in_place_t, Args &&...); template <class U, class... Args> constexpr explicit optional(in_place_t, initializer_list<U>, Args &&...); template <class U = T> EXPLICIT constexpr optional(U &&); template <class U> EXPLICIT optional(const optional<U> &); template <class U> EXPLICIT optional(optional<U> &&); […] // 20.6.3.3, Assignment optional &operator=(nullopt_t) noexcept; optional &operator=(const optional &); optional &operator=(optional &&) noexcept(see below); template <class U = T> optional &operator=(U &&); template <class U> optional& operator=(const optional<U> &); template <class U> optional& operator=(optional<U> &&); template <class... Args> void emplace(Args &&...); template <class U, class... Args> void emplace(initializer_list<U>, Args &&...); […] };Change [optional.object.ctor] as indicated:
optional(const optional<T>& rhs);[…] -?- Remarks: This constructor shall be defined as deleted unless
-3- Requires:is_copy_constructible_v<T>
istrue
.is_copy_constructible_v<T>
istrue
.optional(optional<T>&& rhs) noexcept(see below);[…] -11- Remarks: The expression inside
-7- Requires:is_move_constructible_v<T>
istrue
.noexcept
is equivalent tois_nothrow_move_constructible_v<T>
. This constructor shall not participate in overload resolution unlessis_move_constructible_v<T>
istrue
.constexpr optional(const T& v);
-12- Requires:is_copy_constructible_v<T>
istrue
.-13- Effects: Initializes the contained value as if direct-non-list-initializing an object of typeT
with the expressionv
.-14- Postcondition:*this
contains a value.-15- Throws: Any exception thrown by the selected constructor ofT
.-16- Remarks: IfT
's selected constructor is aconstexpr
constructor, this constructor shall be aconstexpr
constructor.constexpr optional(T&& v);
-17- Requires:is_move_constructible_v<T>
istrue
.-18- Effects: Initializes the contained value as if direct-non-list-initializing an object of typeT
with the expressionstd::move(v)
.-19- Postcondition:*this
contains a value.-20- Throws: Any exception thrown by the selected constructor ofT
.-21- Remarks: IfT
's selected constructor is aconstexpr
constructor, this constructor shall be aconstexpr
constructor.template <class... Args> constexpr explicit optional(in_place_t, Args&&... args);[…] -26- Remarks: If
-22- Requires:is_constructible_v<T, Args&&...>
istrue
.T
's constructor selected for the initialization is aconstexpr
constructor, this constructor shall be aconstexpr
constructor. This constructor shall not participate in overload resolution unlessis_constructible_v<T, Args...>
istrue
.template <class U, class... Args> constexpr explicit optional(in_place_t, initializer_list<U> il, Args&&... args);[…] -31- Remarks:
-27- Requires:is_constructible_v<T, initializer_list<U>&, Args&&...>
istrue
.The functionThis constructor shall not participate in overload resolution unlessis_constructible_v<T, initializer_list<U>&, Args&&...>
istrue
. IfT
's constructor selected for the initialization is aconstexpr
constructor, this constructor shall be aconstexpr
constructor.[Note: The following constructors are conditionally specified as explicit. This is typically implemented by declaring two such constructors, of which at most one participates in overload resolution. — end note]
template <class U = T> EXPLICIT constexpr optional(U&& v);-?- Effects: Initializes the contained value as if direct-non-list-initializing an object of type
-?- Postconditions:T
with the expressionstd::forward<U>(v)
.*this
contains a value. -?- Throws: Any exception thrown by the selected constructor ofT
. -?- Remarks: IfT
's selected constructor is aconstexpr
constructor, this constructor shall be aconstexpr
constructor. This constructor shall not participate in overload resolution unlessis_constructible_v<T, U&&>
istrue
,is_same_v<U, in_place_t>
isfalse
, andis_same_v<optional<T>, decay_t<U>>
isfalse
. The constructor is explicit if and only ifis_convertible_v<U&&, T>
isfalse
.template <class U> EXPLICIT optional(const optional<U>& rhs);-?- Effects: If
-?- Postconditions:rhs
contains a value, initializes the contained value as if direct-non-list-initializing an object of typeT
with the expression*rhs
.bool(rhs) == bool(*this)
. -?- Throws: Any exception thrown by the selected constructor ofT
. -?- Remarks: This constructor shall not participate in overload resolution unlessis_constructible_v<T, const U&>
istrue
,is_constructible_v<T, optional<U>&>
isfalse
,is_constructible_v<T, const optional<U>&>
isfalse
,is_constructible_v<T, const optional<U>&&>
isfalse
,is_constructible_v<T, optional<U>&&>
isfalse
,is_convertible_v<optional<U>&, T>
isfalse
,is_convertible_v<const optional<U>&, T>
isfalse
,is_convertible_v<const optional<U>&&, T>
isfalse
, andis_convertible_v<optional<U>&&, T>
isfalse
. The constructor is explicit if and only ifis_convertible_v<const U&, T>
isfalse
.template <class U> EXPLICIT optional(optional<U>&& rhs);-?- Effects: If
-?- Postconditions:rhs
contains a value, initializes the contained value as if direct-non-list-initializing an object of typeT
with the expressionstd::move(*rhs)
.bool(rhs)
is unchanged.bool(rhs) == bool(*this)
. -?- Throws: Any exception thrown by the selected constructor ofT
. -?- Remarks: This constructor shall not participate in overload resolution unlessis_constructible_v<T, U&&>
istrue
,is_constructible_v<T, optional<U>&>
isfalse
,is_constructible_v<T, const optional<U>&>
isfalse
,is_constructible_v<T, const optional<U>&&>
isfalse
,is_constructible_v<T, optional<U>&&>
isfalse
,is_convertible_v<optional<U>&, T>
isfalse
,is_convertible_v<const optional<U>&, T>
isfalse
,is_convertible_v<const optional<U>&&, T>
isfalse
, andis_convertible_v<optional<U>&&, T>
isfalse
. The constructor is explicit if and only ifis_convertible_v<U&&, T>
isfalse
.Change [optional.object.assign] as indicated:
optional<T>& operator=(const optional<T>& rhs);[…] -8- Remarks: If any exception is thrown, the result of the expression
-4- Requires:is_copy_constructible_v<T>
istrue
andis_copy_assignable_v<T>
istrue
.bool(*this)
remains unchanged. If an exception is thrown during the call toT
's copy constructor, no effect. If an exception is thrown during the call toT
's copy assignment, the state of its contained value is as defined by the exception safety guarantee ofT
's copy assignment. This operator shall be defined as deleted unlessis_copy_constructible_v<T>
istrue
andis_copy_assignable_v<T>
istrue
.optional<T>& operator=(optional<T>&& rhs) noexcept(see below);[…] -13- Remarks: The expression inside
-9- Requires:is_move_constructible_v<T>
istrue
andis_move_assignable_v<T>
istrue
.noexcept
is equivalent to:is_nothrow_move_assignable_v<T> && is_nothrow_move_constructible_v<T>-14- If any exception is thrown, the result of the expression
bool(*this)
remains unchanged. If an exception is thrown during the call toT
's move constructor, the state of*rhs.val
is determined by the exception safety guarantee ofT
's move constructor. If an exception is thrown during the call toT
's move assignment, the state of*val
and*rhs.val
is determined by the exception safety guarantee ofT
's move assignment. This operator shall not participate in overload resolution unlessis_move_constructible_v<T>
istrue
andis_move_assignable_v<T>
istrue
.template <class U = T> optional<T>& operator=(U&& v);[…] -19- Remarks: If any exception is thrown, the result of the expression
-15- Requires:is_constructible_v<T, U>
istrue
andis_assignable_v<T&, U>
istrue
.bool(*this)
remains unchanged. If an exception is thrown during the call toT
's constructor, the state ofv
is determined by the exception safety guarantee ofT
's constructor. If an exception is thrown during the call toT
's assignment, the state of*val
andv
is determined by the exception safety guarantee ofT
's assignment.TheThis function shall not participate in overload resolution unlessis_same_v<decay_t<U>, T>
is_same_v<optional<T>, decay_t<U>>
isfalse
,is_constructible_v<T, U>
istrue
, andis_assignable_v<T&, U>
istrue
.-20- Notes: The reason for providing such generic assignment and then constraining it so that effectivelyT == U
is to guarantee that assignment of the formo = {}
is unambiguous.template <class U> optional<T>& operator=(const optional<U>& rhs);-?- Effects: See Table ?.
Table ? — optional::operator=(const optional<U>&)
effects*this
contains a value*this
does not contain a valuerhs
contains a valueassigns *rhs
to the contained valueinitializes the contained value as if direct-non-list-initializing an object of type T
with*rhs
rhs
does not contain a valuedestroys the contained value by calling val->T::~T()
no effect -?- Returns:
-?- Postconditions:*this
.bool(rhs) == bool(*this)
. -?- Remarks: If any exception is thrown, the result of the expressionbool(*this)
remains unchanged. If an exception is thrown during the call toT
's constructor, the state of*rhs.val
is determined by the exception safety guarantee ofT
's constructor. If an exception is thrown during the call toT
's assignment, the state of*val
and*rhs.val
is determined by the exception safety guarantee ofT
's assignment. This function shall not participate in overload resolution unlessis_constructible_v<T, const U&>
istrue
,is_assignable_v<T&, const U&>
istrue
,is_constructible_v<T, optional<U>&>
isfalse
,is_constructible_v<T, const optional<U>&>
isfalse
,is_constructible_v<T, const optional<U>&&>
isfalse
,is_constructible_v<T, optional<U>&&>
isfalse
,is_convertible_v<optional<U>&, T>
isfalse
,is_convertible_v<const optional<U>&, T>
isfalse
,is_convertible_v<const optional<U>&&, T>
isfalse
,is_convertible_v<optional<U>&&, T>
isfalse
,is_assignable_v<T&, optional<U>&>
isfalse
,is_assignable_v<T&, const optional<U>&>
isfalse
,is_assignable_v<T&, const optional<U>&&>
isfalse
, andis_assignable_v<T&, optional<U>&&>
isfalse
.template <class U> optional<T>& operator=(optional<U>&& rhs);-?- Effects: See Table ?. The result of the expression
bool(rhs)
remains unchanged.
Table ? — optional::operator=(optional<U>&&)
effects*this
contains a value*this
does not contain a valuerhs
contains a valueassigns std::move(*rhs)
to the contained valueinitializes the contained value as if direct-non-list-initializing an object of type T
withstd::move(*rhs)
rhs
does not contain a valuedestroys the contained value by calling val->T::~T()
no effect -?- Returns:
-?- Postconditions:*this
.bool(rhs) == bool(*this)
. -?- Remarks: If any exception is thrown, the result of the expressionbool(*this)
remains unchanged. If an exception is thrown during the call toT
's constructor, the state of*rhs.val
is determined by the exception safety guarantee ofT
's constructor. If an exception is thrown during the call toT
's assignment, the state of*val
and*rhs.val
is determined by the exception safety guarantee ofT
's assignment. This function shall not participate in overload resolution unlessis_constructible_v<T, U>
istrue
,is_assignable_v<T&, U>
istrue
,is_constructible_v<T, optional<U>&>
isfalse
,is_constructible_v<T, const optional<U>&>
isfalse
,is_constructible_v<T, const optional<U>&&>
isfalse
,is_constructible_v<T, optional<U>&&>
isfalse
,is_convertible_v<optional<U>&, T>
isfalse
,is_convertible_v<const optional<U>&, T>
isfalse
,is_convertible<const optional<U>&&, T>
isfalse
,is_convertible<optional<U>&&, T>
isfalse
,is_assignable_v<T&, optional<U>&>
isfalse
,is_assignable_v<T&, const optional<U>&>
isfalse
,is_assignable_v<T&, const optional<U>&&>
isfalse
, andis_assignable_v<T&, optional<U>&&>
isfalse
.
Proposed resolution:
This wording is relative to N4606.
Modify 22.5.3 [optional.optional] as indicated:
template <class T> class optional { public: using value_type = T; // 20.6.3.1, Constructors constexpr optional() noexcept; constexpr optional(nullopt_t) noexcept; optional(const optional &); optional(optional &&) noexcept(see below);constexpr optional(const T &);constexpr optional(T &&);template <class... Args> constexpr explicit optional(in_place_t, Args &&...); template <class U, class... Args> constexpr explicit optional(in_place_t, initializer_list<U>, Args &&...); template <class U = T> EXPLICIT constexpr optional(U &&); template <class U> EXPLICIT optional(const optional<U> &); template <class U> EXPLICIT optional(optional<U> &&); […] // 20.6.3.3, Assignment optional &operator=(nullopt_t) noexcept; optional &operator=(const optional &); optional &operator=(optional &&) noexcept(see below); template <class U = T> optional &operator=(U &&); template <class U> optional& operator=(const optional<U> &); template <class U> optional& operator=(optional<U> &&); template <class... Args> void emplace(Args &&...); template <class U, class... Args> void emplace(initializer_list<U>, Args &&...); […] };
Change [optional.object.ctor] as indicated:
optional(const optional<T>& rhs);[…] -?- Remarks: This constructor shall be defined as deleted unless
-3- Requires:is_copy_constructible_v<T>
istrue
.is_copy_constructible_v<T>
istrue
.optional(optional<T>&& rhs) noexcept(see below);[…] -11- Remarks: The expression inside
-7- Requires:is_move_constructible_v<T>
istrue
.noexcept
is equivalent tois_nothrow_move_constructible_v<T>
. This constructor shall not participate in overload resolution unlessis_move_constructible_v<T>
istrue
.constexpr optional(const T& v);
-12- Requires:is_copy_constructible_v<T>
istrue
.-13- Effects: Initializes the contained value as if direct-non-list-initializing an object of typeT
with the expressionv
.-14- Postcondition:*this
contains a value.-15- Throws: Any exception thrown by the selected constructor ofT
.-16- Remarks: IfT
's selected constructor is aconstexpr
constructor, this constructor shall be aconstexpr
constructor.constexpr optional(T&& v);
-17- Requires:is_move_constructible_v<T>
istrue
.-18- Effects: Initializes the contained value as if direct-non-list-initializing an object of typeT
with the expressionstd::move(v)
.-19- Postcondition:*this
contains a value.-20- Throws: Any exception thrown by the selected constructor ofT
.-21- Remarks: IfT
's selected constructor is aconstexpr
constructor, this constructor shall be aconstexpr
constructor.template <class... Args> constexpr explicit optional(in_place_t, Args&&... args);[…] -26- Remarks: If
-22- Requires:is_constructible_v<T, Args&&...>
istrue
.T
's constructor selected for the initialization is aconstexpr
constructor, this constructor shall be aconstexpr
constructor. This constructor shall not participate in overload resolution unlessis_constructible_v<T, Args...>
istrue
.template <class U, class... Args> constexpr explicit optional(in_place_t, initializer_list<U> il, Args&&... args);[…] -31- Remarks:
-27- Requires:is_constructible_v<T, initializer_list<U>&, Args&&...>
istrue
.The functionThis constructor shall not participate in overload resolution unlessis_constructible_v<T, initializer_list<U>&, Args&&...>
istrue
. IfT
's constructor selected for the initialization is aconstexpr
constructor, this constructor shall be aconstexpr
constructor.[Note: The following constructors are conditionally specified as explicit. This is typically implemented by declaring two such constructors, of which at most one participates in overload resolution. — end note]
template <class U = T> EXPLICIT constexpr optional(U&& v);-?- Effects: Initializes the contained value as if direct-non-list-initializing an object of type
-?- Postconditions:T
with the expressionstd::forward<U>(v)
.*this
contains a value. -?- Throws: Any exception thrown by the selected constructor ofT
. -?- Remarks: IfT
's selected constructor is aconstexpr
constructor, this constructor shall be aconstexpr
constructor. This constructor shall not participate in overload resolution unlessis_constructible_v<T, U&&>
istrue
,is_same_v<U, in_place_t>
isfalse
, andis_same_v<optional<T>, decay_t<U>>
isfalse
. The constructor is explicit if and only ifis_convertible_v<U&&, T>
isfalse
.template <class U> EXPLICIT optional(const optional<U>& rhs);-?- Effects: If
-?- Postconditions:rhs
contains a value, initializes the contained value as if direct-non-list-initializing an object of typeT
with the expression*rhs
.bool(rhs) == bool(*this)
. -?- Throws: Any exception thrown by the selected constructor ofT
. -?- Remarks: This constructor shall not participate in overload resolution unlessis_constructible_v<T, const U&>
istrue
,is_constructible_v<T, optional<U>&>
isfalse
,is_constructible_v<T, const optional<U>&>
isfalse
,is_constructible_v<T, const optional<U>&&>
isfalse
,is_constructible_v<T, optional<U>&&>
isfalse
,is_convertible_v<optional<U>&, T>
isfalse
,is_convertible_v<const optional<U>&, T>
isfalse
,is_convertible_v<const optional<U>&&, T>
isfalse
, andis_convertible_v<optional<U>&&, T>
isfalse
. The constructor is explicit if and only ifis_convertible_v<const U&, T>
isfalse
.template <class U> EXPLICIT optional(optional<U>&& rhs);-?- Effects: If
-?- Postconditions:rhs
contains a value, initializes the contained value as if direct-non-list-initializing an object of typeT
with the expressionstd::move(*rhs)
.bool(rhs)
is unchanged.bool(rhs) == bool(*this)
. -?- Throws: Any exception thrown by the selected constructor ofT
. -?- Remarks: This constructor shall not participate in overload resolution unlessis_constructible_v<T, U&&>
istrue
,is_constructible_v<T, optional<U>&>
isfalse
,is_constructible_v<T, const optional<U>&>
isfalse
,is_constructible_v<T, const optional<U>&&>
isfalse
,is_constructible_v<T, optional<U>&&>
isfalse
,is_convertible_v<optional<U>&, T>
isfalse
,is_convertible_v<const optional<U>&, T>
isfalse
,is_convertible_v<const optional<U>&&, T>
isfalse
, andis_convertible_v<optional<U>&&, T>
isfalse
. The constructor is explicit if and only ifis_convertible_v<U&&, T>
isfalse
.
Change [optional.object.assign] as indicated:
optional<T>& operator=(const optional<T>& rhs);[…] -8- Remarks: If any exception is thrown, the result of the expression
-4- Requires:is_copy_constructible_v<T>
istrue
andis_copy_assignable_v<T>
istrue
.bool(*this)
remains unchanged. If an exception is thrown during the call toT
's copy constructor, no effect. If an exception is thrown during the call toT
's copy assignment, the state of its contained value is as defined by the exception safety guarantee ofT
's copy assignment. This operator shall be defined as deleted unlessis_copy_constructible_v<T>
istrue
andis_copy_assignable_v<T>
istrue
.optional<T>& operator=(optional<T>&& rhs) noexcept(see below);[…] -13- Remarks: The expression inside
-9- Requires:is_move_constructible_v<T>
istrue
andis_move_assignable_v<T>
istrue
.noexcept
is equivalent to:is_nothrow_move_assignable_v<T> && is_nothrow_move_constructible_v<T>-14- If any exception is thrown, the result of the expression
bool(*this)
remains unchanged. If an exception is thrown during the call toT
's move constructor, the state of*rhs.val
is determined by the exception safety guarantee ofT
's move constructor. If an exception is thrown during the call toT
's move assignment, the state of*val
and*rhs.val
is determined by the exception safety guarantee ofT
's move assignment. This operator shall not participate in overload resolution unlessis_move_constructible_v<T>
istrue
andis_move_assignable_v<T>
istrue
.template <class U = T> optional<T>& operator=(U&& v);[…] -19- Remarks: If any exception is thrown, the result of the expression
-15- Requires:is_constructible_v<T, U>
istrue
andis_assignable_v<T&, U>
istrue
.bool(*this)
remains unchanged. If an exception is thrown during the call toT
's constructor, the state ofv
is determined by the exception safety guarantee ofT
's constructor. If an exception is thrown during the call toT
's assignment, the state of*val
andv
is determined by the exception safety guarantee ofT
's assignment.TheThis function shall not participate in overload resolution unlessis_same_v<decay_t<U>, T>
is_same_v<optional<T>, decay_t<U>>
isfalse
, conjunction_v<is_scalar<T>, is_same<T, decay_t<U>>> isfalse
,is_constructible_v<T, U>
istrue
, andis_assignable_v<T&, U>
istrue
.-20- Notes: The reason for providing such generic assignment and then constraining it so that effectivelyT == U
is to guarantee that assignment of the formo = {}
is unambiguous.template <class U> optional<T>& operator=(const optional<U>& rhs);-?- Effects: See Table ?.
Table ? — optional::operator=(const optional<U>&)
effects*this
contains a value*this
does not contain a valuerhs
contains a valueassigns *rhs
to the contained valueinitializes the contained value as if direct-non-list-initializing an object of type T
with*rhs
rhs
does not contain a valuedestroys the contained value by calling val->T::~T()
no effect -?- Returns:
-?- Postconditions:*this
.bool(rhs) == bool(*this)
. -?- Remarks: If any exception is thrown, the result of the expressionbool(*this)
remains unchanged. If an exception is thrown during the call toT
's constructor, the state of*rhs.val
is determined by the exception safety guarantee ofT
's constructor. If an exception is thrown during the call toT
's assignment, the state of*val
and*rhs.val
is determined by the exception safety guarantee ofT
's assignment. This function shall not participate in overload resolution unlessis_constructible_v<T, const U&>
istrue
,is_assignable_v<T&, const U&>
istrue
,is_constructible_v<T, optional<U>&>
isfalse
,is_constructible_v<T, const optional<U>&>
isfalse
,is_constructible_v<T, const optional<U>&&>
isfalse
,is_constructible_v<T, optional<U>&&>
isfalse
,is_convertible_v<optional<U>&, T>
isfalse
,is_convertible_v<const optional<U>&, T>
isfalse
,is_convertible_v<const optional<U>&&, T>
isfalse
,is_convertible_v<optional<U>&&, T>
isfalse
,is_assignable_v<T&, optional<U>&>
isfalse
,is_assignable_v<T&, const optional<U>&>
isfalse
,is_assignable_v<T&, const optional<U>&&>
isfalse
, andis_assignable_v<T&, optional<U>&&>
isfalse
.template <class U> optional<T>& operator=(optional<U>&& rhs);-?- Effects: See Table ?. The result of the expression
bool(rhs)
remains unchanged.
Table ? — optional::operator=(optional<U>&&)
effects*this
contains a value*this
does not contain a valuerhs
contains a valueassigns std::move(*rhs)
to the contained valueinitializes the contained value as if direct-non-list-initializing an object of type T
withstd::move(*rhs)
rhs
does not contain a valuedestroys the contained value by calling val->T::~T()
no effect -?- Returns:
-?- Postconditions:*this
.bool(rhs) == bool(*this)
. -?- Remarks: If any exception is thrown, the result of the expressionbool(*this)
remains unchanged. If an exception is thrown during the call toT
's constructor, the state of*rhs.val
is determined by the exception safety guarantee ofT
's constructor. If an exception is thrown during the call toT
's assignment, the state of*val
and*rhs.val
is determined by the exception safety guarantee ofT
's assignment. This function shall not participate in overload resolution unlessis_constructible_v<T, U>
istrue
,is_assignable_v<T&, U>
istrue
,is_constructible_v<T, optional<U>&>
isfalse
,is_constructible_v<T, const optional<U>&>
isfalse
,is_constructible_v<T, const optional<U>&&>
isfalse
,is_constructible_v<T, optional<U>&&>
isfalse
,is_convertible_v<optional<U>&, T>
isfalse
,is_convertible_v<const optional<U>&, T>
isfalse
,is_convertible<const optional<U>&&, T>
isfalse
,is_convertible<optional<U>&&, T>
isfalse
,is_assignable_v<T&, optional<U>&>
isfalse
,is_assignable_v<T&, const optional<U>&>
isfalse
,is_assignable_v<T&, const optional<U>&&>
isfalse
, andis_assignable_v<T&, optional<U>&&>
isfalse
.
std::string{}.insert(3, "ABCDE", 0, 1)
is ambiguousSection: 27.4.3.7.4 [string.insert] Status: Resolved Submitter: Marshall Clow Opened: 2016-07-30 Last modified: 2020-09-06
Priority: 1
View all other issues in [string.insert].
View all issues with Resolved status.
Discussion:
Before C++17, we had the following signature to std::basic_string
:
basic_string& insert(size_type pos1, const basic_string& str, size_type pos2, size_type n = npos);
Unlike most of the other member functions on std::basic_string
, there were not corresponding
versions that take a charT*
or (charT *, size)
.
basic_string& insert(size_type pos1, basic_string_view<charT, traits> sv, size_type pos2, size_type n = npos);
which made the code above ambiguous. There are two conversions from "const charT*
",
one to basic_string
, and the other to basic_string_view
, and they're both equally
good (in the view of the compiler).
assign(const basic_string& str, size_type pos, size_type n = npos); assign(basic_string_view<charT, traits> sv, size_type pos, size_type n = npos);
but I will file a separate issue (2758(i)) for that.
A solution is to add even more overloads toinsert
, to make it match all the other member
functions of basic_string
, which come in fours (string
, pointer
, pointer + size
,
string_view
).
[2016-08-03, Chicago, Robert Douglas provides wording]
Previous resolution [SUPERSEDED]:
This wording is relative to N4606.
In 27.4.3 [basic.string] modify the synopsis for
basic_string
as follows:namespace std { template<class charT, class traits = char_traits<charT>, class Allocator = allocator<charT>> class basic_string { public: […] template<class T> basic_string& insert(size_type pos1,basic_string_view<charT, traits>T sv, size_type pos2, size_type n = npos); […] }; }In 27.4.3.7.4 [string.insert], modify
basic_string_view
overload as follows:template<class T> basic_string& insert(size_type pos1,basic_string_view<charT, traits>T sv, size_type pos2, size_type n = npos);[…]
-?- Remarks: This function shall not participate in overload resolution unlessis_same_v<T, basic_string_view<charT, traits>>
istrue
.
[2016-08-04, Chicago, Robert Douglas comments]
For the sake of simplicity, the previous wording suggestion has been merged into the proposed wording of LWG 2758(i).
[08-2016, Chicago]
Fri PM: Move to Tentatively Ready (along with 2758(i)).
[2016-09-09 Issues Resolution Telecon]
Since 2758(i) has been moved back to Open, move this one, too
[2016-10 Telecon]
Ville's wording for 2758(i) has been implemented in libstdc++ and libc++. Move 2758(i) to Tentatively Ready and this one to Tentatively Resolved
Proposed resolution:
This issue is resolved by the proposed wording for LWG 2758(i).
std::string{}.assign("ABCDE", 0, 1)
is ambiguousSection: 27.4.3.7.3 [string.assign] Status: C++17 Submitter: Marshall Clow Opened: 2016-07-30 Last modified: 2020-09-06
Priority: 1
View all other issues in [string.assign].
View all issues with C++17 status.
Discussion:
Before C++17, we had the following signature to std::basic_string
:
basic_string& assign(const basic_string& str, size_type pos, size_type n = npos);
Unlike most of the other member functions on std::basic_string
, there were not corresponding
versions that take a charT*
or (charT *, size)
.
basic_string& assign(basic_string_view<charT, traits> sv, size_type pos, size_type n = npos);
which made the code above ambiguous. There are two conversions from "const charT*
",
one to basic_string
, and the other to basic_string_view
, and they're both equally
good (in the view of the compiler).
insert(size_type pos1, const basic_string& str, size_type pos2, size_type n = npos); insert(size_type pos1, basic_string_view<charT, traits> sv, size_type pos2, size_type n = npos);
but I will file a separate issue (2757(i)) for that.
A solution is to add even more overloads toassign
, to make it match all the other member
functions of basic_string
, which come in fours (string
, pointer
, pointer + size
,
string_view
).
[2016-08-03, Chicago, Robert Douglas provides wording]
[2016-08-05, Tim Song comments]
On the assumption that the basic_string
version is untouchable, I like the updated P/R, with a couple comments:
If it's constraining on is_convertible
to basic_string_view
, then I think it should take
by reference to avoid copying T
, which can be arbitrarily expensive. Both const T&
and
T&&
should work; the question is whether to accommodate non-const operator basic_string_view()
s
(which arguably shouldn't exist).
Minor issue: compare
tests is_convertible
and then uses direct-initialization syntax
(which is is_constructible
). They should match, because it's possible to have basic_string_view sv = t;
succeed yet basic_string_view sv(t);
fail.
[2016-08-05, Chicago LWG]
char const*
to basic_string_view
if possibleGiven feedback from discussion, we wish to go with the is_convertible_v
method, but change:
T const&
for each overloadis_convertible_v
to T const&
sv(t)
to sv = t
[2016-08, Chicago]
Fri PM: Move to Tentatively Ready
[2016-08-16, Jonathan Wakely reopens]
The P/R is not correct, the new overloads get chosen in preference to
the overloads taking const char*
when passed a char*
:
#include <string> int main () { std::string str("a"); char c = 'b'; str.replace(0, 1, &c, 1); if (str[0] != 'b') __builtin_abort(); }
With the resolution of 2758 this is now equivalent to:
str.replace(0, 1, string_view{&c, 1}, 1);
which replaces the character with string_view{"b", 1}.substr(1, npos)
i.e. an empty string.
value_type*
arguments.
I've implemented an alternative resolution, which would be specified as:
Remarks: This function shall not participate in overload resolution unless
is_convertible_v<const T&, basic_string_view<charT, traits>>
istrue
andis_convertible_v<const T&, const charT*>
isfalse
.
All the overloads have the same problem.
This program prints no output in C++14, and we should make sure it prints no output in C++17 too:#include <string> int main() { std::string str("a"); char c[1] = { 'b' }; str.append(c, 1); if (str != "ab") puts("bad append"); str.assign(c, 1); if (str != "b") puts("bad assign"); str.insert(0, c, 1); if (str != "bb") puts("bad insert"); str.replace(0, 2, c, 1); if (str != "b") puts("bad replace"); if (str.compare(0, 1, c, 1)) puts("bad compare"); }
Ville and I considered "is_same_v<decay_t<T>, char*>
is false
" but
that would still select the wrong overload for an array of const char
,
because the new function template would be preferred to doing an array-to-pointer conversion to
call the old overload.
[2016-09-09 Issues Resolution Telecon]
Ville to provide updated wording; Marshall to implement
[2016-10-05]
Ville provides revised wording.
[2016-10 Telecon]
Ville's wording has been implemented in libstdc++ and libc++. Move this to Tentatively Ready and 2757(i) to Tentatively Resolved
Previous resolution [SUPERSEDED]:
This wording is relative to N4606.
In 27.4.3 [basic.string] modify the synopsis for
basic_string
as follows:namespace std { template<class charT, class traits = char_traits<charT>, class Allocator = allocator<charT>> class basic_string { public: […] template<class T> basic_string& append(basic_string_view<charT, traits> svconst T& t, size_type pos, size_type n = npos); […] template<class T> basic_string& assign(basic_string_view<charT, traits> svconst T& t, size_type pos, size_type n = npos); […] template<class T> basic_string& insert(size_type pos1,basic_string_view<charT, traits> svconst T& t, size_type pos2, size_type n = npos); […] template<class T> basic_string& replace(size_type pos1, size_type n1,basic_string_view<charT, traits> svconst T& t, size_type pos2, size_type n2 = npos); […] template<class T> int compare(size_type pos1, size_type n1,basic_string_view<charT, traits> svconst T& t, size_type pos2, size_type n2 = npos) const; […] }; }In 27.4.3.7.2 [string.append], modify
basic_string_view
overload as follows:template<class T> basic_string& append(basic_string_view<charT, traits> svconst T& t, size_type pos, size_type n = npos);-7- Throws:
-8- Effects: Creates a variable,out_of_range
ifpos > sv.size()
.sv
, as if bybasic_string_view<charT, traits> sv = t
. Determines the effective lengthrlen
of the string to append as the smaller ofn
andsv.size() - pos
and callsappend(sv.data() + pos, rlen)
. -?- Remarks: This function shall not participate in overload resolution unlessis_convertible_v<const T&, basic_string_view<charT, traits>>
istrue
. -9- Returns:*this
.In 27.4.3.7.3 [string.assign], modify
basic_string_view
overload as follows:template<class T> basic_string& assign(basic_string_view<charT, traits> svconst T& t, size_type pos, size_type n = npos);-9- Throws:
-10- Effects: Creates a variable,out_of_range
ifpos > sv.size()
.sv
, as if bybasic_string_view<charT, traits> sv = t
. Determines the effective lengthrlen
of the string to assign as the smaller ofn
andsv.size() - pos
and callsassign(sv.data() + pos, rlen)
. -?- Remarks: This function shall not participate in overload resolution unlessis_convertible_v<const T&, basic_string_view<charT, traits>>
istrue
. -11- Returns:*this
.In 27.4.3.7.4 [string.insert], modify
basic_string_view
overload as follows:template<class T> basic_string& insert(size_type pos1,basic_string_view<charT, traits> svconst T& t, size_type pos2, size_type n = npos);-6- Throws:
-7- Effects: Creates a variable,out_of_range
ifpos1 > size()
orpos2 > sv.size()
.sv
, as if bybasic_string_view<charT, traits> sv = t
. Determines the effective lengthrlen
of the string to assign as the smaller ofn
andsv.size() - pos2
and callsinsert(pos1, sv.data() + pos2, rlen)
. -?- Remarks: This function shall not participate in overload resolution unlessis_convertible_v<const T&, basic_string_view<charT, traits>>
istrue
. -8- Returns:*this
.In 27.4.3.7.6 [string.replace], modify
basic_string_view
overload as follows:template<class T> basic_string& replace(size_type pos1, size_type n1,basic_string_view<charT, traits> svconst T& t, size_type pos2, size_type n2 = npos);-6- Throws:
-7- Effects: Creates a variable,out_of_range
ifpos1 > size()
orpos2 > sv.size()
.sv
, as if bybasic_string_view<charT, traits> sv = t
. Determines the effective lengthrlen
of the string to be inserted as the smaller ofn2
andsv.size() - pos2
and callsreplace(pos1, n1, sv.data() + pos2, rlen)
. -?- Remarks: This function shall not participate in overload resolution unlessis_convertible_v<const T&, basic_string_view<charT, traits>>
istrue
. -8- Returns:*this
.In 27.4.3.8.4 [string.compare], modify
basic_string_view
overload as follows:template<class T> int compare(size_type pos1, size_type n1,basic_string_view<charT, traits> svconst T& t, size_type pos2, size_type n2 = npos) const;-4- Effects: Equivalent to:
{ basic_string_view<charT, traits> sv = t; return basic_string_view<charT, traits>(this.data(), pos1, n1).compare(sv, pos2, n2); }-?- Remarks: This function shall not participate in overload resolution unless
is_convertible_v<const T&, basic_string_view<charT, traits>>
istrue
.
Proposed resolution:
This wording is relative to N4606.
In 27.4.3 [basic.string] modify the synopsis for basic_string
as follows:
namespace std { template<class charT, class traits = char_traits<charT>, class Allocator = allocator<charT>> class basic_string { public: […] template<class T> basic_string& append(basic_string_view<charT, traits> svconst T& t, size_type pos, size_type n = npos); […] template<class T> basic_string& assign(basic_string_view<charT, traits> svconst T& t, size_type pos, size_type n = npos); […] template<class T> basic_string& insert(size_type pos1,basic_string_view<charT, traits> svconst T& t, size_type pos2, size_type n = npos); […] template<class T> basic_string& replace(size_type pos1, size_type n1,basic_string_view<charT, traits> svconst T& t, size_type pos2, size_type n2 = npos); […] template<class T> int compare(size_type pos1, size_type n1,basic_string_view<charT, traits> svconst T& t, size_type pos2, size_type n2 = npos) const; […] }; }
In 27.4.3.7.2 [string.append], modify basic_string_view
overload as follows:
template<class T> basic_string& append(basic_string_view<charT, traits> svconst T& t, size_type pos, size_type n = npos);-7- Throws:
-8- Effects: Creates a variable,out_of_range
ifpos > sv.size()
.sv
, as if bybasic_string_view<charT, traits> sv = t
. Determines the effective lengthrlen
of the string to append as the smaller ofn
andsv.size() - pos
and callsappend(sv.data() + pos, rlen)
. -?- Remarks: This function shall not participate in overload resolution unlessis_convertible_v<const T&, basic_string_view<charT, traits>>
istrue
andis_convertible_v<const T&, const charT*>
isfalse
. -9- Returns:*this
.
In 27.4.3.7.3 [string.assign], modify basic_string_view
overload as follows:
template<class T> basic_string& assign(basic_string_view<charT, traits> svconst T& t, size_type pos, size_type n = npos);-9- Throws:
-10- Effects: Creates a variable,out_of_range
ifpos > sv.size()
.sv
, as if bybasic_string_view<charT, traits> sv = t
. Determines the effective lengthrlen
of the string to assign as the smaller ofn
andsv.size() - pos
and callsassign(sv.data() + pos, rlen)
. -?- Remarks: This function shall not participate in overload resolution unlessis_convertible_v<const T&, basic_string_view<charT, traits>>
istrue
andis_convertible_v<const T&, const charT*>
isfalse
. -11- Returns:*this
.
In 27.4.3.7.4 [string.insert], modify basic_string_view
overload as follows:
template<class T> basic_string& insert(size_type pos1,basic_string_view<charT, traits> svconst T& t, size_type pos2, size_type n = npos);-6- Throws:
-7- Effects: Creates a variable,out_of_range
ifpos1 > size()
orpos2 > sv.size()
.sv
, as if bybasic_string_view<charT, traits> sv = t
. Determines the effective lengthrlen
of the string to assign as the smaller ofn
andsv.size() - pos2
and callsinsert(pos1, sv.data() + pos2, rlen)
. -?- Remarks: This function shall not participate in overload resolution unlessis_convertible_v<const T&, basic_string_view<charT, traits>>
istrue
andis_convertible_v<const T&, const charT*>
isfalse
. -8- Returns:*this
.
In 27.4.3.7.6 [string.replace], modify basic_string_view
overload as follows:
template<class T> basic_string& replace(size_type pos1, size_type n1,basic_string_view<charT, traits> svconst T& t, size_type pos2, size_type n2 = npos);-6- Throws:
-7- Effects: Creates a variable,out_of_range
ifpos1 > size()
orpos2 > sv.size()
.sv
, as if bybasic_string_view<charT, traits> sv = t
. Determines the effective lengthrlen
of the string to be inserted as the smaller ofn2
andsv.size() - pos2
and callsreplace(pos1, n1, sv.data() + pos2, rlen)
. -?- Remarks: This function shall not participate in overload resolution unlessis_convertible_v<const T&, basic_string_view<charT, traits>>
istrue
andis_convertible_v<const T&, const charT*>
isfalse
. -8- Returns:*this
.
In 27.4.3.8.4 [string.compare], modify basic_string_view
overload as follows:
[Drafting note: The wording changes below are for the same part of the standard as 2771(i). However, they do not conflict. This one changes the definition of the routine, while the other changes the "Effects". — end drafting note]
template<class T> int compare(size_type pos1, size_type n1,basic_string_view<charT, traits> svconst T& t, size_type pos2, size_type n2 = npos) const;-4- Effects: Equivalent to:
{ basic_string_view<charT, traits> sv = t; return basic_string_view<charT, traits>(this.data(), pos1, n1).compare(sv, pos2, n2); }-?- Remarks: This function shall not participate in overload resolution unless
is_convertible_v<const T&, basic_string_view<charT, traits>>
istrue
andis_convertible_v<const T&, const charT*>
isfalse
.
gcd
/ lcm
and bool
for the WPSection: 26.10.14 [numeric.ops.gcd], 26.10.15 [numeric.ops.lcm] Status: C++17 Submitter: Walter Brown Opened: 2016-08-01 Last modified: 2017-07-30
Priority: Not Prioritized
View all other issues in [numeric.ops.gcd].
View all issues with C++17 status.
Discussion:
With the acceptance of gcd
and lcm
in the working draft, the same problem as pointed out by
LWG 2733(i) exists here as well and should be fixed accordingly.
[2016-08, Chicago]
Monday PM: Moved to Tentatively Ready
Proposed resolution:
This wording is relative to N4606.
Adjust 26.10.14 [numeric.ops.gcd] p2 as indicated:
template<class M, class N> constexpr common_type_t<M, N> gcd(M m, N n);[…]
-2- Remarks: If eitherM
orN
is not an integer type, or if either is (possibly cv-qualified)bool
, the program is ill-formed.
Adjust 26.10.15 [numeric.ops.lcm] p2 as indicated:
template<class M, class N> constexpr common_type_t<M, N> lcm(M m, N n);[…]
-2- Remarks: If eitherM
orN
is not an integer type, or if either is (possibly cv-qualified)bool
, the program is ill-formed.
basic_string::data
should not invalidate iteratorsSection: 27.4.3.2 [string.require] Status: C++17 Submitter: Billy Baker Opened: 2016-08-03 Last modified: 2017-07-30
Priority: Not Prioritized
View all other issues in [string.require].
View all issues with C++17 status.
Discussion:
27.4.3.2 [string.require]/4 does not list non-const basic_string::data()
as being a
function that may be called with the guarantee that it will not invalidate references, pointers, and
iterators to elements of a basic_string
object.
[2016-08 Chicago]
Wed PM: Move to Tentatively Ready
Proposed resolution:
This wording is relative to N4606.
Change 27.4.3.2 [string.require]/4 as indicated:
-4- References, pointers, and iterators referring to the elements of a
basic_string
sequence may be invalidated by the following uses of thatbasic_string
object:
as an argument to any standard library function taking a reference to non-const
basic_string
as an argument.(footnote 230)Calling non-const member functions, except
operator[]
,at
, data,front
,back
,begin
,rbegin
,end
, andrend
.
unique_ptr operator*()
should be noexcept
Section: 20.3.1.3.5 [unique.ptr.single.observers] Status: C++23 Submitter: Ville Voutilainen Opened: 2016-08-04 Last modified: 2023-11-22
Priority: 3
View all other issues in [unique.ptr.single.observers].
View all issues with C++23 status.
Discussion:
See LWG 2337(i). Since we aren't removing noexcept
from shared_ptr
's
operator*
, we should consider adding noexcept
to unique_ptr
's operator*
.
[2016-08 — Chicago]
Thurs PM: P3, and status to 'LEWG'
[2016-08-05 Chicago]
Ville provides an initial proposed wording.
[LEWG Kona 2017]
->Open: Believe these should be noexcept
for consistency. We like these. We agree with the proposed resolution.
operator->()
already has noexcept
.
Also adds optional::operator*
Alisdair points out that fancy pointers might intentionally throw from operator*
, and we don't want to prohibit that.
Go forward with conditional noexcept(noexcept(*decltype()))
.
Previous resolution [SUPERSEDED]:
This wording is relative to N4606.
[Drafting note: since this issue is all about consistency,
optional
's pointer-like operators are additionally included.]
In 20.3.1.3 [unique.ptr.single] synopsis, edit as follows:
add_lvalue_reference_t<T> operator*() const noexcept;Before 20.3.1.3.5 [unique.ptr.single.observers]/1, edit as follows:
add_lvalue_reference_t<T> operator*() const noexcept;In 22.5.3 [optional.optional] synopsis, edit as follows:
constexpr T const *operator->() const noexcept; constexpr T *operator->() noexcept; constexpr T const &operator*() const & noexcept; constexpr T &operator*() & noexcept; constexpr T &&operator*() && noexcept; constexpr const T &&operator*() const && noexcept;Before [optional.object.observe]/1, edit as follows:
constexpr T const* operator->() const noexcept; constexpr T* operator->() noexcept;Before [optional.object.observe]/5, edit as follows:
constexpr T const& operator*() const & noexcept; constexpr T& operator*() & noexcept;Before [optional.object.observe]/9, edit as follows:
constexpr T&& operator*() && noexcept; constexpr const T&& operator*() const && noexcept;
[2021-06-19 Tim updates wording]
[2021-06-23; Reflector poll]
Set status to Tentatively Ready after seven votes in favour during reflector poll.
[2021-10-14 Approved at October 2021 virtual plenary. Status changed: Voting → WP.]
Proposed resolution:
This wording is relative to N4892.
[Drafting note: since this issue is all about consistency,
optional
's pointer-like operators are additionally included.]
Edit 20.3.1.3.1 [unique.ptr.single.general], class template unique_ptr
synopsis, as indicated:
namespace std { template<class T, class D = default_delete<T>> class unique_ptr { public: […] // 20.3.1.3.5 [unique.ptr.single.observers], observers add_lvalue_reference_t<T> operator*() const noexcept(see below); […] }; }
Edit 20.3.1.3.5 [unique.ptr.single.observers] as indicated:
add_lvalue_reference_t<T> operator*() const noexcept(noexcept(*declval<pointer>()));-1- Preconditions:
-2- Returns:get() != nullptr
.*get()
.
Edit 22.5.3.1 [optional.optional.general], class template optional
synopsis, as indicated:
namespace std { template<class T> class optional { public: […] // 22.5.3.7 [optional.observe], observers constexpr const T* operator->() const noexcept; constexpr T* operator->() noexcept; constexpr const T& operator*() const & noexcept; constexpr T& operator*() & noexcept; constexpr T&& operator*() && noexcept; constexpr const T&& operator*() const && noexcept; […] }; }
Edit 22.5.3.7 [optional.observe] as indicated:
constexpr const T* operator->() const noexcept; constexpr T* operator->() noexcept;-1- Preconditions:
-2- Returns:*this
contains a value.val
.-3- Throws: Nothing.-4- Remarks: These functions are constexpr functions.constexpr const T& operator*() const & noexcept; constexpr T& operator*() & noexcept;-5- Preconditions:
-6- Returns:*this
contains a value.*val
.-7- Throws: Nothing.-8- Remarks: These functions are constexpr functions.constexpr T&& operator*() && noexcept; constexpr const T&& operator*() const && noexcept;-9- Preconditions:
-10- Effects: Equivalent to:*this
contains a value.return std::move(*val);
common_type_t<void, void>
is undefinedSection: 21.3.8.7 [meta.trans.other] Status: Resolved Submitter: Tim Song Opened: 2016-08-10 Last modified: 2016-11-21
Priority: 2
View all other issues in [meta.trans.other].
View all issues with Resolved status.
Discussion:
There are no xvalues of type cv void
(see [basic.lval]/6), so the current wording appears
to mean that there is no common_type_t<void, void>
. Is that intended?
[2016-08-11, Daniel comments]
This is strongly related to LWG 2465(i). It should be considered to resolve 2465(i) by this revised wording.
[2016-11-12, Issaquah]
Resolved by P0435R1
Proposed resolution:
This wording is relative to N4606.
Edit 21.3.8.7 [meta.trans.other]/3 as indicated:
[Drafting note: The proposed wording below simply goes back to using
declval
, which already does the right thing. To describe this in words would be something like "ifD1
isvoid
, a prvalue of typevoid
that is not a (possibly parenthesized) throw-expression, otherwise an xvalue of typeD1
", which seems unnecessarily convoluted at best. — end drafting note]
For the
common_type
trait applied to a parameter packT
of types, the membertype
shall be either defined or not present as follows:
(3.1) — If
sizeof...(T)
is zero, there shall be no membertype
.(3.2) — If
sizeof...(T)
is one, letT0
denote the sole type in the packT
. The member typedeftype
shall denote the same type asdecay_t<T0>
.(3.3) — If
sizeof...(T)
is greater than two, letT1
,T2
, andR
, respectively, denote the first, second, and (pack of) remaining types comprisingT
. [Note:sizeof...(R)
may be zero. — end note] LetC
denote the type, if any, of an unevaluated conditional expression (7.6.16 [expr.cond]) whose first operand is an arbitrary value of typebool
, whose second operand isan xvalue of typeT1
declval<T1>()
, and whose third operand isan xvalue of typeT2
declval<T2>()
. If there is such a typeC
, the member typedeftype
shall denote the same type, if any, ascommon_type_t<C, R...>
. Otherwise, there shall be no membertype
.
The following wording is a merge of the above with the current proposed resolution of 2465(i), to provide editorial guidance if both proposed resolutions are accepted:
-3- Note A: For the
common_type
trait applied to a parameter packT
of types, the membertype
shall be either defined or not present as follows:
(3.1) — If
sizeof...(T)
is zero, there shall be no membertype
.(3.2) — If
sizeof...(T)
is one, letT0
denote the sole type in the packT
. The member typedeftype
shall denote the same type asdecay_t<T0>
.(3.3) — If
sizeof...(T)
is two, letT1
andT2
, respectively, denote the first and second types comprisingT
, and letD1
andD2
, respectively, denotedecay_t<T1>
anddecay_t<T2>
.
(3.3.1) — If
is_same_v<T1, D1>
andis_same_v<T2, D2>
, letC
denote the type of an unevaluated conditional expression (7.6.16 [expr.cond]) whose first operand is an arbitrary value of typebool
, whose second operand isdeclval<D1>()
, and whose third operand isdeclval<D2>()
. [Note: This will not apply if there is a specializationcommon_type<D1, D2>
. — end note](3.3.2) — Otherwise, let
C
denote the typecommon_type_t<D1, D2>
.In either case, if there is such a type
C
, the member typedeftype
shall denoteC
. Otherwise, there shall be no membertype
.(3.4) — If
sizeof...(T)
is greater thanonetwo, letT1
,T2
, andR
, respectively, denote the first, second, and (pack of) remaining types comprisingT
.[Note:Letsizeof...(R)
may be zero. — end note] LetC
denote the type, if any, of an unevaluated conditional expression (7.6.16 [expr.cond]) whose first operand is an arbitrary value of typebool
, whose second operand is an xvalue of typeT1
, and whose third operand is an xvalue of typeT2
.C
denotecommon_type_t<T1, T2>
. If there is such a typeC
, the member typedeftype
shall denote the same type, if any, ascommon_type_t<C, R...>
. Otherwise, there shall be no membertype
.-?- Note B: A program may specialize the
-4- [Example: Given these definitions: […]common_type
trait for two cv-unqualified non-reference types if at least one of them is a user-defined type. [Note: Such specializations are needed when only explicit conversions are desired among the template arguments. — end note] Such a specialization need not have a member namedtype
, but if it does, that member shall be a typedef-name for a cv-unqualified non-reference type that need not otherwise meet the specification set forth in Note A, above.
Section: 31.5.2.2.6 [ios.init] Status: C++17 Submitter: Richard Smith Opened: 2016-08-13 Last modified: 2021-06-06
Priority: 0
View all other issues in [ios.init].
View all issues with C++17 status.
Discussion:
1123(i) fixed a bug where users of <iostream>
were not guaranteed to have their streams flushed
on program shutdown. However, it also added this rule:
"Similarly, the entire program shall behave as if there were at least one instance of
ios_base::Init
with static lifetime."
This seems pointless: it only affects the behavior of programs that never include <iostream>
(because programs
that do include it are already guaranteed at least one such instance), and those programs do not need an implicit flush
because they cannot have written to the relevant streams.
[2016-09-09 Issues Resolution Telecon]
P0; move to Tentatively Ready
Proposed resolution:
This wording is relative to N4606.
Modify [ios::Init] p3 as indicated:
-3- The objects are constructed and the associations are established at some time prior to or during the first time an object of class
ios_base::Init
is constructed, and in any case before the body ofmain
begins execution.(footnote 293) The objects are not destroyed during program execution.(footnote 294) The results of including<iostream>
in a translation unit shall be as if<iostream>
defined an instance ofios_base::Init
with static storage duration.Similarly, the entire program shall behave as if there were at least one instance ofios_base::Init
with static storage duration.
not_fn call_wrapper
can form invalid typesSection: 22.10.13 [func.not.fn] Status: C++17 Submitter: Jonathan Wakely Opened: 2016-08-19 Last modified: 2021-06-06
Priority: 0
View all other issues in [func.not.fn].
View all issues with C++17 status.
Discussion:
The definition of the call_wrapper
type in the C++17 CD means this
fails to compile:
#include <functional> struct abc { virtual void f() const = 0; }; struct derived : abc { void f() const { } }; struct F { bool operator()(abc&) { return false; } }; derived d; bool b = std::not_fn(F{})(static_cast<abc&&>(d));
The problem is that the return types use result_of_t<F(abc)>
and
F(abc)
is not a valid function type, because it takes an abstract
class by value.
result_of_t<F(Args&&...)>
instead.
[2016-09-09 Issues Resolution Telecon]
P0; move to Tentatively Ready
Proposed resolution:
This wording is relative to N4606.
Modify [func.not_fn], class call_wrapper
synopsis, as indicated:
class call_wrapper { […] template<class... Args> auto operator()(Args&&...) & -> decltype(!declval<result_of_t<FD&(Args&&...)>>()); template<class... Args> auto operator()(Args&&...) const& -> decltype(!declval<result_of_t<FD const&(Args&&...)>>()); template<class... Args> auto operator()(Args&&...) && -> decltype(!declval<result_of_t<FD(Args&&...)>>()); template<class... Args> auto operator()(Args&&...) const&& -> decltype(!declval<result_of_t<FD const(Args&&...)>>()); […] };
Modify the prototype declarations of [func.not_fn] as indicated:
template<class... Args> auto operator()(Args&&... args) & -> decltype(!declval<result_of_t<FD&(Args&&...)>>()); template<class... Args> auto operator()(Args&&... args) const& -> decltype(!declval<result_of_t<FD const&(Args&&...)>>());[…]
template<class... Args> auto operator()(Args&&... args) && -> decltype(!declval<result_of_t<FD(Args&&...)>>()); template<class... Args> auto operator()(Args&&... args) const&& -> decltype(!declval<result_of_t<FD const(Args&&...)>>());
any_cast
and move semanticsSection: 22.7.5 [any.nonmembers] Status: C++17 Submitter: Casey Carter Opened: 2016-08-27 Last modified: 2017-07-30
Priority: 0
View all other issues in [any.nonmembers].
View all issues with C++17 status.
Discussion:
LWG 2509(i) made two changes to the specification of any
in v2 of the library fundamentals TS:
any_cast(any&&)
overload to enable moving the value out of the any
object and/or obtaining an rvalue reference to the contained value.
Change 1 has very desirable effects; I propose that we apply the sane part of LWG 2509(i) to any
in the C++17 WP, for all of the reasons cited in the discussion of LWG 2509(i).
[2016-09-09 Issues Resolution Telecon]
P0; move to Tentatively Ready
Previous resolution [SUPERSEDED]:
This wording is relative to N4606.
In 22.7.5 [any.nonmembers] p5, edit as follows:
template<class ValueType> ValueType any_cast(const any& operand); template<class ValueType> ValueType any_cast(any& operand); template<class ValueType> ValueType any_cast(any&& operand);-4- Requires:
is_reference_v<ValueType>
istrue
oris_copy_constructible_v<ValueType>
istrue
. Otherwise the program is ill-formed.-5- Returns: For the first form,
*any_cast<add_const_t<remove_reference_t<ValueType>>>(&operand)
. For the secondand thirdforms,*any_cast<remove_reference_t<ValueType>>(&operand)
. For the third form,std::forward<ValueType>(*any_cast<remove_reference_t<ValueType>>(&operand))
.[…]
[Issues Telecon 16-Dec-2016]
Move to Tentatively Ready
Proposed resolution:
Resolved by the wording provided by LWG 2769(i).
const
in the return type of any_cast(const any&)
Section: 22.7.5 [any.nonmembers] Status: C++17 Submitter: Casey Carter Opened: 2016-09-02 Last modified: 2017-07-30
Priority: 0
View all other issues in [any.nonmembers].
View all issues with C++17 status.
Discussion:
The overload of any_cast
that accepts a reference to constant any
:
template<class ValueType> ValueType any_cast(const any& operand);
is specified to return *any_cast<add_const_t<remove_reference_t<ValueType>>>(&operand)
in [any.nonmembers]/5. This calls the pointer-to-constant overload of any_cast
:
template<class ValueType> const ValueType* any_cast(const any* operand) noexcept;
which is specified as:
Returns: Ifoperand != nullptr && operand->type() == typeid(ValueType)
, a pointer to the object contained byoperand
; otherwise,nullptr
.
Since typeid(T) == typeid(const T)
for all types T
, any_cast<add_const_t<T>>(&operand)
is equivalent to any_cast<T>(&operand)
for all types T
when operand is a constant lvalue any
.
The add_const_t
in the return specification of the first overload above is therefore redundant.
[2016-09-09 Issues Resolution Telecon]
P0; move to Tentatively Ready
Casey will provide combined wording for this and 2768(i), since they modify the same paragraph.
Previous resolution [SUPERSEDED]:
This wording is relative to N4606.
Modify 22.7.5 [any.nonmembers] as indicated:
template<class ValueType> ValueType any_cast(const any& operand); template<class ValueType> ValueType any_cast(any& operand); template<class ValueType> ValueType any_cast(any&& operand);-4- Requires:
-5- Returns:is_reference_v<ValueType>
istrue
oris_copy_constructible_v<ValueType>
istrue
. Otherwise the program is ill-formed.For the first form,*any_cast<add_const_t<remove_reference_t<ValueType>>>(&operand)
. For the second and third forms,*any_cast<remove_reference_t<ValueType>>(&operand)
. […]
[2016-09-09 Casey improves wording as determined by telecon]
The presented resolution is intended as the common wording for both LWG 2768(i) and LWG 2769(i).
Previous resolution [SUPERSEDED]:
This wording is relative to N4606.
Modify 22.7.5 [any.nonmembers] as indicated:
template<class ValueType> ValueType any_cast(const any& operand); template<class ValueType> ValueType any_cast(any& operand); template<class ValueType> ValueType any_cast(any&& operand);-4- Requires:
-5- Returns: For the firstis_reference_v<ValueType>
istrue
oris_copy_constructible_v<ValueType>
istrue
. Otherwise the program is ill-formed.form,and second*any_cast<add_const_t<remove_reference_t<ValueType>>>(&operand)
. For theand thirdforms,*any_cast<remove_reference_t<ValueType>>(&operand)
. For the third form,std::forward<ValueType>(*any_cast<remove_reference_t<ValueType>>(&operand))
. […]
[2016-10-05, Tomasz and Casey reopen and improve the wording]
The constraints placed on the non-pointer any_cast
overloads are neither necessary nor sufficient to
guarantee that the specified effects are well-formed. The current PR for LWG 2769(i) also makes it
possible to retrieve a dangling lvalue reference to a temporary any with e.g. any_cast<int&>(any{42})
,
which should be forbidden.
[2016-10-16, Eric made some corrections to the wording]
Previous resolution [SUPERSEDED]:
This wording is relative to N4606.
Modify 22.7.5 [any.nonmembers] as indicated:
template<class ValueType> ValueType any_cast(const any& operand); template<class ValueType> ValueType any_cast(any& operand); template<class ValueType> ValueType any_cast(any&& operand);-4- Requires:
-5- Returns:For the first overload,is_reference_v<ValueType>
istrue
oris_copy_constructible_v<ValueType>
istrue
.is_constructible_v<ValueType, const remove_cv_t<remove_reference_t<ValueType>>&>
istrue
. For the second overload,is_constructible_v<ValueType, remove_cv_t<remove_reference_t<ValueType>>&>
istrue
. For the third overload,is_constructible_v<ValueType, remove_cv_t<remove_reference_t<ValueType>>>
istrue
. Otherwise the program is ill-formed.For the first form,For the first and second overload,*any_cast<add_const_t<remove_reference_t<ValueType>>>(&operand)
. For the second and third forms,*any_cast<remove_reference_t<ValueType>>(&operand)
.static_cast<ValueType>(*any_cast<remove_cv_t<remove_reference_t<ValueType>>>(&operand))
. For the third overload,static_cast<ValueType>(std::move(*any_cast<remove_cv_t<remove_reference_t<ValueType>>>(&operand)))
. […]
[2016-11-10, LWG asks for simplification of the wording]
[Issues Telecon 16-Dec-2016]
Move to Tentatively Ready
Proposed resolution:
This wording is relative to N4606.
Modify 22.7.5 [any.nonmembers] as indicated:
template<class ValueType> ValueType any_cast(const any& operand); template<class ValueType> ValueType any_cast(any& operand); template<class ValueType> ValueType any_cast(any&& operand);-?- Let
U
be the typeremove_cv_t<remove_reference_t<ValueType>>
.-4- Requires:
-5- Returns:For the first overload,is_reference_v<ValueType>
istrue
oris_copy_constructible_v<ValueType>
istrue
.is_constructible_v<ValueType, const U&>
istrue
. For the second overload,is_constructible_v<ValueType, U&>
istrue
. For the third overload,is_constructible_v<ValueType, U>
istrue
. Otherwise the program is ill-formed.For the first form,For the first and second overload,*any_cast<add_const_t<remove_reference_t<ValueType>>>(&operand)
. For the second and third forms,*any_cast<remove_reference_t<ValueType>>(&operand)
.static_cast<ValueType>(*any_cast<U>(&operand))
. For the third overload,static_cast<ValueType>(std::move(*any_cast<U>(&operand)))
. […]
tuple_size<const T>
specialization is not SFINAE compatible and breaks decomposition declarationsSection: 22.4.7 [tuple.helper] Status: C++17 Submitter: Richard Smith Opened: 2016-08-15 Last modified: 2017-07-30
Priority: 1
View all other issues in [tuple.helper].
View all issues with C++17 status.
Discussion:
Consider:
#include <utility> struct X { int a, b; }; const auto [x, y] = X();
This is ill-formed: it triggers the instantiation of std::tuple_size<const X>
, which hits a hard error
because it tries to use tuple_size<X>::value
, which does not exist. The code compiles if
<utility>
(or another header providing tuple_size
) is not included.
tuple_size
partial specializations for cv-qualifiers SFINAE-compatible.
The latter seems like the better option to me, and like a good idea regardless of decomposition declarations.
[2016-09-05, Daniel comments]
This is partially related to LWG 2446(i).
[2016-09-09 Issues Resolution Telecon]
Geoffrey to provide wording
[2016-09-14 Geoffrey provides wording]
[2016-10 Telecon]
Alisdair to add his concerns here before Issaquah. Revisit then. Status to 'Open'
Proposed resolution:
This wording is relative to N4606.
Edit 22.4.7 [tuple.helper] as follows:
template <class T> class tuple_size<const T>; template <class T> class tuple_size<volatile T>; template <class T> class tuple_size<const volatile T>;-4- Let
TS
denotetuple_size<T>
of the cv-unqualified typeT
. If the expressionTS::value
is well-formed when treated as an unevaluated operand, tThen each of the three templates shall meet theUnaryTypeTrait
requirements (20.15.1) with aBaseCharacteristic
ofintegral_constant<size_t, TS::value>Otherwise, they shall have no member
value
.Access checking is performed as if in a context unrelated to
-5- In addition to being available via inclusion of theTS
andT
. Only the validity of the immediate context of the expression is considered. [Note: The compilation of the expression can result in side effects such as the instantiation of class template specializations and function template specializations, the generation of implicitly-defined functions, and so on. Such side effects are not in the "immediate context" and can result in the program being ill-formed. — end note]<tuple>
header, the three templates are available when either of the headers<array>
or<utility>
are included.
basic_string::compare
functions in terms of basic_string_view
Section: 27.4.3.8.4 [string.compare] Status: C++17 Submitter: Daniel Krügler Opened: 2016-09-05 Last modified: 2017-07-30
Priority: 1
View all issues with C++17 status.
Discussion:
Some basic_string::compare
functions are specified in terms of a non-existing basic_string_view
constructor, namely 27.4.3.8.4 [string.compare] p3,
return basic_string_view<charT, traits>(this.data(), pos1, n1).compare(sv);
and 27.4.3.8.4 [string.compare] p4:
return basic_string_view<charT, traits>(this.data(), pos1, n1).compare(sv, pos2, n2);
because there doesn't exist a basic_string_view
constructor with three arguments.
constexpr basic_string_view(const charT* str, size_type len);
with the additional member function
constexpr basic_string_view substr(size_type pos = 0, size_type n = npos) const;
it should be decided whether adding the seemingly natural constructor
constexpr basic_string_view(const charT* str, size_type pos, size_type n);
could simplify matters. A counter argument for this addition might be, that basic_string
doesn't provide this constructor either.
basic_string_view::compare
overload that would match the signature:
constexpr int compare(basic_string_view str, size_type pos1, size_type n1) const;
[2016-09-09 Issues Resolution Telecon]
Marshall to investigate using P/R vs. adding the missing constructor.
[2016-10 Issues Resolution Telecon]
Marshall reports that P/R is better. Status to Tentatively Ready
Proposed resolution:
This wording is relative to N4606.
[Drafting note: The wording changes below are for the same part of the standard as 2758(i). However, they do not conflict. This one changes the "Effects" of the routine, while the other changes the definition. — end drafting note]
Change 27.4.3.8.4 [string.compare] as indicated:
int compare(size_type pos1, size_type n1, basic_string_view<charT, traits> sv) const;-3- Effects: Equivalent to:
return basic_string_view<charT, traits>(this.data(), size()).substr(pos1, n1).compare(sv);int compare(size_type pos1, size_type n1, basic_string_view<charT, traits> sv, size_type pos2, size_type n2 = npos) const;-4- Effects: Equivalent to:
return basic_string_view<charT, traits>(this.data(), size()).substr(pos1, n1).compare(sv,.substr(pos2, n2));
std::ignore constexpr
Section: 22.4.1 [tuple.general] Status: C++17 Submitter: Vincent Reverdy Opened: 2016-09-10 Last modified: 2017-07-30
Priority: 0
View all other issues in [tuple.general].
View all issues with C++17 status.
Discussion:
Currently std::ignore
is not specified constexpr
according to the C++ draft N4606 in the paragraph
22.4.1 [tuple.general]. It prevents some use in constexpr
context: for example declaring a
constexpr
variable equals to the result of a function to which std::ignore
has been passed as a parameter:
constexpr int i = f(std::ignore); // Won't compile
If there is no fundamental reason preventing std::ignore
to be constexpr
, then we propose to declare
it as constexpr
instead of as const
.
[Issues processing Telecon 2016-10-7]
P0; set to Tentatively Ready
Proposed resolution:
This wording is relative to N4606.
Modify 22.4.1 [tuple.general] as indicated:
// 20.5.2.4, tuple creation functions:constconstexpr unspecified ignore;
std::function
construction vs assignmentSection: 22.10.17.3.2 [func.wrap.func.con] Status: C++23 Submitter: Barry Revzin Opened: 2016-09-14 Last modified: 2023-11-22
Priority: 3
View all other issues in [func.wrap.func.con].
View all issues with C++23 status.
Discussion:
I think there's a minor defect in the std::function
interface. The constructor template is:
template <class F> function(F f);
while the assignment operator template is
template <class F> function& operator=(F&& f);
The latter came about as a result of LWG 1288(i), but that one was dealing with a specific issue that
wouldn't have affected the constructor. I think the constructor should also take f
by forwarding reference,
this saves a move in the lvalue/xvalue cases and is also just generally more consistent. Should just make sure
that it's stored as std::decay_t<F>
instead of F
.
[2019-07-26 Tim provides PR.]
Previous resolution [SUPERSEDED]:
This wording is relative to N4820.
Edit 22.10.17.3 [func.wrap.func], class template
function
synopsis, as indicated:namespace std { template<class> class function; // not defined template<class R, class... ArgTypes> { public: using result_type = R; // 22.10.17.3.2 [func.wrap.func.con], construct/copy/destroy function() noexcept; function(nullptr_t) noexcept; function(const function&); function(function&&) noexcept; template<class F> function(F&&); […] }; […] }Edit 22.10.17.3.2 [func.wrap.func.con] p7-11 as indicated:
template<class F> function(F&& f);
-7- Requires:LetF
shall be Cpp17CopyConstructibleFD
bedecay_t<F>
.-8-
Remarks: This constructor template shall not participate in overload resolution unlessConstraints:F
(8.1) —
is_same_v<FD, function>
isfalse
; and(8.2) —
FD
is Lvalue-Callable (22.10.17.3 [func.wrap.func]) for argument typesArgTypes...
and return typeR
.-?- Expects:
FD
meets the Cpp17CopyConstructible requirements.-9- Ensures:
!*this
if any of the following hold:
(9.1) —
f
is a null function pointer value.(9.2) —
f
is a null member pointer value.(9.3) —
F
is an instanceremove_cvref_t<F>
is a specialization of thefunction
class template, and!f
istrue
.-10- Otherwise,
*this
targetsa copy ofan object of typef
FD
direct-non-list-initialized withstd::move(f)
std::forward<F>(f)
. [Note: Implementations should avoid the use of dynamically allocated memory for small callable objects, for example, wheref
isrefers to an object holding only a pointer or reference to an object and a member function pointer. — end note]-11- Throws:
ShallDoes not throw exceptions whenf
FD
is a function pointer type or a specialization ofreference_wrapper
. Otherwise, may throw<T>
for someT
bad_alloc
or any exception thrown bythe initialization of the target object.F
’s copy or move constructor
[2020-11-01; Daniel comments and improves the wording]
The proposed wording should — following the line of Marshall's "Mandating" papers — extract from the Cpp17CopyConstructible precondition a corresponding Constraints: element and in addition to that the wording should replace old-style elements such as Expects: by the recently agreed on elements.
See also the related issue LWG 3493(i).Previous resolution [SUPERSEDED]:
This wording is relative to N4868.
Edit 22.10.17.3 [func.wrap.func], class template
function
synopsis, as indicated:namespace std { template<class> class function; // not defined template<class R, class... ArgTypes> { public: using result_type = R; // 22.10.17.3.2 [func.wrap.func.con], construct/copy/destroy function() noexcept; function(nullptr_t) noexcept; function(const function&); function(function&&) noexcept; template<class F> function(F&&); […] }; […] }Edit 22.10.17.3.2 [func.wrap.func.con] as indicated:
template<class F> function(F&& f);Let
-8- Constraints:FD
bedecay_t<F>
.
(8.1) —
is_same_v<remove_cvref_t<F>, function>
isfalse
,(8.2) —
FD
is Lvalue-Callable (22.10.17.3.1 [func.wrap.func.general]) for argument typesF
ArgTypes...
and return typeR
,(8.3) —
is_copy_constructible_v<FD>
istrue
, and(8.4) —
is_constructible_v<FD, F>
istrue
.-9- Preconditions:
-10- Postconditions:meets the Cpp17CopyConstructible requirements.
FFD!*this
if any of the following hold:
(10.1) —
f
is a null function pointer value.(10.2) —
f
is a null member pointer value.(10.3) —
F
is an instanceremove_cvref_t<F>
is a specialization of the function class template, and!f
istrue
.-11- Otherwise,
-12- Throws: Nothing if*this
targetsa copy ofan object of typef
FD
direct-non-list-initialized with.
std::move(f)std::forward<F>(f)is a specialization of
fFDreference_wrapper
or a function pointer type. Otherwise, may throwbad_alloc
or any exception thrown bythe initialization of the target object. -13- Recommended practice: Implementations should avoid the use of dynamically allocated memory for small callable objects, for example, whereF
's copy or move constructorf
isrefers to an object holding only a pointer or reference to an object and a member function pointer.
[2021-05-17; Tim comments and revises the wording]
The additional constraints added in the previous wording can induce constraint recursion, as noted in the discussion of LWG 3493(i). The wording below changes them to Mandates: instead to allow this issue to make progress independently of that issue.
The proposed resolution below has been implemented and tested on top of libstdc++.[2021-05-20; Reflector poll]
Set status to Tentatively Ready after five votes in favour during reflector poll.
[2021-06-07 Approved at June 2021 virtual plenary. Status changed: Voting → WP.]
Proposed resolution:
This wording is relative to N4885.
Edit 22.10.17.3.1 [func.wrap.func.general], class template function
synopsis, as indicated:
namespace std { template<class> class function; // not defined template<class R, class... ArgTypes> { public: using result_type = R; // 22.10.17.3.2 [func.wrap.func.con], construct/copy/destroy function() noexcept; function(nullptr_t) noexcept; function(const function&); function(function&&) noexcept; template<class F> function(F&&); […] }; […] }
Edit 22.10.17.3.2 [func.wrap.func.con] as indicated:
template<class F> function(F&& f);Let
-8- Constraints:FD
bedecay_t<F>
.
(8.1) —
is_same_v<remove_cvref_t<F>, function>
isfalse
, and(8.2) —
FD
is Lvalue-Callable (22.10.17.3.1 [func.wrap.func.general]) for argument typesF
ArgTypes...
and return typeR
.-?- Mandates:
(?.1) —
is_copy_constructible_v<FD>
istrue
, and(?.2) —
is_constructible_v<FD, F>
istrue
.-9- Preconditions:
-10- Postconditions:meets the Cpp17CopyConstructible requirements.
FFD!*this
istrue
if any of the following hold:
(10.1) —
f
is a null function pointer value.(10.2) —
f
is a null member pointer value.(10.3) —
F
is an instanceremove_cvref_t<F>
is a specialization of thefunction
class template, and!f
istrue
.-11- Otherwise,
-12- Throws: Nothing if*this
targetsa copy ofan object of typef
FD
direct-non-list-initialized with.
std::move(f)std::forward<F>(f)is a specialization of
fFDreference_wrapper
or a function pointer type. Otherwise, may throwbad_alloc
or any exception thrown bythe initialization of the target object. -13- Recommended practice: Implementations should avoid the use of dynamically allocated memory for small callable objects, for example, whereF
's copy or move constructorf
isrefers to an object holding only a pointer or reference to an object and a member function pointer.
shared_ptr unique()
and use_count()
Section: 20.3.2.2.6 [util.smartptr.shared.obs] Status: Resolved Submitter: Hans Boehm Opened: 2016-09-22 Last modified: 2018-06-12
Priority: 2
View all other issues in [util.smartptr.shared.obs].
View all issues with Resolved status.
Discussion:
The removal of the "debug only" restriction for use_count()
and unique()
in shared_ptr
by LWG 2434(i) introduced a bug. In order for unique()
to produce a useful and reliable value,
it needs a synchronize clause to ensure that prior accesses through another reference are visible to the successful
caller of unique()
. Many current implementations use a relaxed load, and do not provide this guarantee,
since it's not stated in the standard. For debug/hint usage that was OK. Without it the specification is unclear
and probably misleading.
unique()
use memory_order_acquire
, and specifying that reference count
decrement operations synchronize with unique()
. That still doesn't give us sequential consistency by default,
like we're supposed to have. But the violations seem sufficiently obscure that I think it's OK. All uses that
anybody should care about will work correctly, and the bad uses are clearly bad. I agree with Peter that this
version of unique()
may be quite useful.
I would prefer to specify use_count()
as only providing an unreliable hint of the actual count (another way
of saying debug only). Or deprecate it, as JF suggested. We can't make use_count()
reliable without adding
substantially more fencing. We really don't want someone waiting for use_count() == 2
to determine that
another thread got that far. And unfortunately, I don't think we currently say anything to make it clear that's a
mistake.
This would imply that use_count()
normally uses memory_order_relaxed
, and unique
is
neither specified nor implemented in terms of use_count()
.
[2016-10-27 Telecon]
Priority set to 2
[2018-06 Rapperswil Thursday issues processing]
This was resolved by P0521, which was adopted in Jacksonville.
Proposed resolution:
basic_string_view::copy
should use char_traits::copy
Section: 27.3.3.8 [string.view.ops], 27.4.3.7.7 [string.copy] Status: C++17 Submitter: Billy Robert O'Neal III Opened: 2016-09-27 Last modified: 2017-07-30
Priority: 0
View all other issues in [string.view.ops].
View all issues with C++17 status.
Discussion:
basic_string_view::copy
is inconsistent with basic_string::copy
, in that the
former uses copy_n
and the latter uses traits::copy
. We should have this
handling be consistent.
basic_string::copy
description is excessively roundabout due to
copy-on-write era wording.
[Issues processing Telecon 2016-10-7]
P0; set to Tentatively Ready
Removed "Note to project editor", since the issue there has been fixed in the current draft.
Proposed resolution:
This wording is relative to N4606.
Change 27.4.3.7.7 [string.copy] as indicated:
size_type copy(charT* s, size_type n, size_type pos = 0) const;
-?- Let rlen
be the smaller of n
and size() - pos
.
-2- Throws: out_of_range
if pos > size()
.
-?- Requires: [s, s + rlen)
is a valid range.
-3- Effects: Determines the effective length Equivalent to: rlen
of the string
to copy as the smaller of n
and size() - pos
. s
shall
designate an array of at least rlen
elements.traits::copy(s, data() + pos, rlen)
.
[Note: This does not terminate s
with a null object. — end note]
The function then replaces the string designated by
s
with a string of
length rlen
whose elements are a copy of the string controlled by
*this
beginning at position pos
.
The function does not append a null object to the string designated by
s
.
-4- Returns: rlen
.
Change 27.3.3.8 [string.view.ops] as indicated:
size_type copy(charT* s, size_type n, size_type pos = 0) const;
-1- Let rlen
be the smaller of n
and size() - pos
.
-2- Throws: out_of_range
if pos > size()
.
-3- Requires: [s, s + rlen)
is a valid range.
-4- Effects: Equivalent to: copy_n(begin() + pos, rlen, s)
traits::copy(s,
data() + pos, rlen)
-5- Returns: rlen
.
-6- Complexity: 𝒪(rlen
).
basic_string_view
is missing constexpr
Section: 27.3 [string.view] Status: C++17 Submitter: Billy Robert O'Neal III Opened: 2016-09-30 Last modified: 2017-07-30
Priority: 0
View other active issues in [string.view].
View all other issues in [string.view].
View all issues with C++17 status.
Discussion:
basic_string_view
was not updated to account for other library
machinery made constexpr
in Oulu. Now that reverse_iterator
is
constexpr
there's no reason the reverse range functions can't be.
Also, now that we have C++14 relaxed constexpr
, we can also take care
of the assignment operator and copy
.
[Issues processing Telecon 2016-10-7]
split off the copy into its' own issue 2780(i)
P0; set what's left to Tentatively Ready
Previous resolution [SUPERSEDED]:
This wording is relative to N4606.
In 27.3.3 [string.view.template], add
constexpr
to the assignment operator:constexpr basic_string_view& operator=(const basic_string_view&) noexcept = default;In 27.3.3 [string.view.template], add
constexpr
to the reverse range functions:constexpr const_reverse_iterator rbegin() const noexcept; constexpr const_reverse_iterator rend() const noexcept; constexpr const_reverse_iterator crbegin() const noexcept; constexpr const_reverse_iterator crend() const noexcept;In 27.3.3.4 [string.view.iterators], add
constexpr
to the reverse range functions:constexpr const_reverse_iterator rbegin() const noexcept; constexpr const_reverse_iterator crbegin() const noexcept;-6- Returns:
const_reverse_iterator(end())
constexpr const_reverse_iterator rend() const noexcept; constexpr const_reverse_iterator crend() const noexcept;-7- Returns:
const_reverse_iterator(begin())
In 27.3.3 [string.view.template], add
constexpr
tocopy
:constexpr size_type copy(charT* s, size_type n, size_type pos = 0) const;In 27.3.3.8 [string.view.ops], add
constexpr
tocopy
:constexpr size_type copy(charT* s, size_type n, size_type pos = 0) const;
Proposed resolution:
This wording is relative to N4606.
In 27.3.3 [string.view.template], add constexpr
to the assignment operator:
constexpr basic_string_view& operator=(const basic_string_view&) noexcept = default;
In 27.3.3 [string.view.template], add constexpr
to the reverse range functions:
constexpr const_reverse_iterator rbegin() const noexcept; constexpr const_reverse_iterator rend() const noexcept; constexpr const_reverse_iterator crbegin() const noexcept; constexpr const_reverse_iterator crend() const noexcept;
In 27.3.3.4 [string.view.iterators], add constexpr
to the reverse range functions:
constexpr const_reverse_iterator rbegin() const noexcept; constexpr const_reverse_iterator crbegin() const noexcept;-6- Returns:
const_reverse_iterator(end())
constexpr const_reverse_iterator rend() const noexcept; constexpr const_reverse_iterator crend() const noexcept;-7- Returns:
const_reverse_iterator(begin())
Section: 16.2.1 [networking.ts::buffer.reqmts.mutablebuffersequence] Status: C++23 Submitter: Vinnie Falco Opened: 2016-10-05 Last modified: 2023-11-22
Priority: Not Prioritized
View all issues with C++23 status.
Discussion:
Addresses: networking.ts
We propose to relax the ForwardIterator requirements of buffer sequences
in [networking.ts] by allowing buffer sequence iterators to return rvalues when
dereferenced, and skip providing operator->
.
A paraphrased explanation of why the referential equality rules of ForwardIterator
are harmful to the buffer sequence requirements comes from N4128,
3.3.7 "Ranges For The Standard Library":
The [networking.ts] dependence on ForwardIterator in the buffer sequence requirements ties together the traversal and access properties of iterators. For instance, no forward iterator may return an rvalue proxy when it is dereferenced; the ForwardIterator concept requires that unary
operator*
return an lvalue. This problem has serious consequences for lazy evaluation that applies transformations to buffer sequence elements on the fly. If the transformation function does not return an lvalue, the range's iterator can model no concept stronger than InputIterator, even if the resulting iterator could in theory support BidirectionalIterator. The result in practice is that most range adaptors today will not be compatible with [networking.ts], thereby limiting the types that [networking.ts] can be passed, for no good reason.
Consider a user defined function trim
which lazily adapts a
ConstBufferSequence
, such that when iterating the buffers in the new
sequence, each buffer appears one byte shorter than in the underlying sequence:
#include <boost/range/adaptor/transformed.hpp> struct trim { using result_type = const_buffer; result_type operator()(const_buffer b) { return const_buffer{b.data(), b.size() - 1}; } }; template <ConstBufferSequence> auto trim(ConstBufferSequence const& buffers) { using namespace boost::adaptors; return buffers | transformed(trim{}); }
trim
returns a BidirectionalRange, whose
const_iterator
returns an rvalue when dereferenced. This breaks the
requirements of ForwardIterator. A solution that meets the referential equality
rules of ForwardIterator, would be to evaluate the transformed
sequence upon construction (for example, by storing each transformed
const_buffer
in a vector
). Unfortunately this work-around is
more expensive since it would add heap allocation which the original example avoids.
The requirement of InputIterator operator->
is also
unnecessary for buffer sequence iterators, and should be removed. Because
[networking.ts] only requires that a buffer sequence iterator's
value_type
be convertible to const_buffer
or
mutable_buffer
, implementations of [networking.ts] cannot assume the
existence of any particular member functions or data members other than an
implicit conversion to const_buffer
or mutable_buffer
.
Removing the requirement for operator->
to be present, provides
additional relief from the referential equality requirements of
ForwardIterator and allows transformations of buffer sequences to meet
the requirements of buffer sequences.
This proposal imposes no changes on existing implementations of [networking.ts]. It does not change anything in the standard. The proposal is precise, minimal, and allows range adapters to transform buffer sequences in optimized, compatible ways.
[Issues processing Telecon 2016-10-07]
Status set to LEWG
[2017-02-21, Jonathan comments]
The use of the term "strict aliasing" in the issue discussion is
misleading as that refers to type-based alias analysis in compilers,
but the rule for ForwardIterator
s is related to referential equality
and not strict aliasing.
[2017-02-22, Vinnie Falco comments]
We have eliminated the use of the term "strict aliasing" from the discussion.
[2017-07-10, Toronto, LEWG comments]
Status change: LEWG → Open.
Forward to LWG with the note that they may want to use "input +" instead of "bidirectional -". Unanimous yes.
[2017-07 Toronto Wednesday night issue processing]
Status to Ready
Proposed resolution:
This wording is relative to N4588.
Modify 16.2.1 [networking.ts::buffer.reqmts.mutablebuffersequence] as indicated:
An iterator type meeting the requirements for bidirectional iterators (C++Std [bidirectional.iterators]) whose value type is convertible tomutable_buffer
An iterator type whose
reference
type is convertible tomutable_buffer
, and which satisfies all the requirements for bidirectional iterators (C++Std [bidirectional.iterators]) except that:
- there is no requirement that
operator->
is provided, and- there is no requirement that
reference
be a reference type.
Modify 16.2.2 [networking.ts::buffer.reqmts.constbuffersequence] as indicated:
An iterator type meeting the requirements for bidirectional iterators (C++Std [bidirectional.iterators]) whose value type is convertible toconst_buffer
.An iterator type whose
reference
type is convertible toconst_buffer
, and which satisfies all the requirements for bidirectional iterators (C++Std [bidirectional.iterators]) except that:
- there is no requirement that
operator->
is provided, and- there is no requirement that
reference
be a reference type.
basic_string_view::copy
is missing constexpr
Section: 27.3 [string.view] Status: Resolved Submitter: Billy Robert O'Neal III Opened: 2016-10-07 Last modified: 2018-11-25
Priority: 2
View other active issues in [string.view].
View all other issues in [string.view].
View all issues with Resolved status.
Discussion:
Now that we have C++14 relaxed constexpr
, we can also take care
of the assignment operator and copy
.
[Issues processing Telecon 2016-10-07]
[2016-11-12, Issaquah]
Sat PM: Status LEWG
[LEWG Kona 2017]
Recommend NAD: Don't want to spend the committee time. Can call std::copy if you want this, after LEWG159 goes into C++20.
[2018-06 Rapperswil Wednesday issues processing]
This will be addressed by P1032.
Resolved by the adoption of P1032 in San Diego.
Proposed resolution:
This wording is relative to N4606.
In 27.3.3 [string.view.template], add constexpr
to copy
:
constexpr size_type copy(charT* s, size_type n, size_type pos = 0) const;
In 27.3.3.8 [string.view.ops], add constexpr
to copy
:
constexpr size_type copy(charT* s, size_type n, size_type pos = 0) const;
std::function
and std::reference_wrapper
Section: 22.10.17.3.2 [func.wrap.func.con] Status: C++17 Submitter: Jonathan Wakely Opened: 2016-10-13 Last modified: 2017-07-30
Priority: 0
View all other issues in [func.wrap.func.con].
View all issues with C++17 status.
Discussion:
template<class F> function(F f)
says that the effects are "*this
targets a copy of f
" which seems pretty clear that if F
is
reference_wrapper<CallableType>
then the target is a
reference_wrapper<CallableType>
.
f
's target is a callable object passed via
reference_wrapper
or a function pointer." From the requirement above
it's impossible for the target to be "a callable object passed via
reference_wrapper
" because if the function was constructed with such a
type then the target is the reference_wrapper
not the callable object
it wraps.
This matters because it affects the result of function::target_type()
,
and we have implementation divergence. VC++ and libc++ store the
reference_wrapper
as the target, but libstdc++ and Boost.Function
(both written by Doug Gregor) unwrap it, so the following fails:
#include <functional> #include <cassert> int main() { auto f = []{}; std::function<void()> fn(std::ref(f)); assert(fn.target<std::reference_wrapper<decltype(f)>>() != nullptr); }
If std::function
is intended to deviate from boost::function
this way
then the Throws element for the copy and move constructors is
misleading, and should be clarified.
[2016-11-12, Issaquah]
Sat AM: Priority 0; move to Ready
Proposed resolution:
This wording is relative to N4606.
Modify 22.10.17.3.2 [func.wrap.func.con] p4 and p6 the same way, as shown:
function(const function& f);-3- Postconditions:
-4- Throws: shall not throw exceptions if!*this
if!f
; otherwise,*this
targets a copy off.target()
.f
's target is acallable object passed viaspecialization ofreference_wrapper
or a function pointer. Otherwise, may throwbad_alloc
or any exception thrown by the copy constructor of the stored callable object. [Note: Implementations are encouraged to avoid the use of dynamically allocated memory for small callable objects, for example, wheref
's target is an object holding only a pointer or reference to an object and a member function pointer. — end note]function(function&& f);-5- Effects: If
-6- Throws: shall not throw exceptions if!f
,*this
has no target; otherwise, move constructs the target off
into the target of*this
, leavingf
in a valid state with an unspecified value.f
's target is acallable object passed viaspecialization ofreference_wrapper
or a function pointer. Otherwise, may throwbad_alloc
or any exception thrown by the copy or move constructor of the stored callable object. [Note: Implementations are encouraged to avoid the use of dynamically allocated memory for small callable objects, for example, wheref
's target is an object holding only a pointer or reference to an object and a member function pointer. — end note]
scoped_allocator_adaptor
constructors must be constrainedSection: 20.6.3 [allocator.adaptor.cnstr] Status: C++17 Submitter: Jonathan Wakely Opened: 2016-10-14 Last modified: 2017-07-30
Priority: 0
View all issues with C++17 status.
Discussion:
The templated constructors of scoped_allocator_adaptor
need to be constrained, otherwise uses-allocator
construction gives the wrong answer and causes errors for code that should compile.
template<class T> struct Alloc1 { ... }; template<class T> struct Alloc2 { ... }; static_assert(!is_convertible_v<Alloc1<int>, Alloc2<int>>);
The unconstrained constructors give this bogus answer:
template<class T> using scoped = scoped_allocator_adaptor<T>; static_assert(is_convertible_v<scoped<Alloc1<int>>, scoped<Alloc2<int>>>);
This causes uses_allocator
to give the wrong answer for any specialization involving incompatible
scoped_allocator_adaptors
, which makes scoped_allocator_adaptor::construct()
take an ill-formed
branch e.g.
struct X { using allocator_type = scoped<Alloc2<int>>; X(const allocator_type&); X(); }; scoped<Alloc1<int>>{}.construct((X*)0);
This fails to compile, because uses_allocator<X, scoped_allocator_adaptor<Alloc2<int>>>
is
true
, so the allocator is passed to the X
constructor, but the conversion fails. The error is outside
the immediate context, and so is a hard error.
[2016-11-12, Issaquah]
Sat AM: Priority 0; move to Ready
Billy to open another issue about the confusion with the ctor
Proposed resolution:
This wording is relative to N4606.
Modify 20.6.3 [allocator.adaptor.cnstr] by converting "Requires" elements to "Remarks: shall not participate ..." constraints as shown:
template <class OuterA2> scoped_allocator_adaptor(OuterA2&& outerAlloc, const InnerAllocs&... innerAllocs) noexcept;-3- Effects: Initializes the
-2- Requires:OuterAlloc
shall be constructible fromOuterA2
.OuterAlloc
base class withstd::forward<OuterA2>(outerAlloc)
and inner withinnerAllocs...
(hence recursively initializing each allocator within the adaptor with the corresponding allocator from the argument list). -?- Remarks: This constructor shall not participate in overload resolution unlessis_constructible_v<OuterAlloc, OuterA2>
istrue
. […]template <class OuterA2> scoped_allocator_adaptor(const scoped_allocator_adaptor<OuterA2, InnerAllocs...>& other) noexcept;-7- Effects: Initializes each allocator within the adaptor with the corresponding allocator from
-6- Requires:OuterAlloc
shall be constructible fromOuterA2
.other
. -?- Remarks: This constructor shall not participate in overload resolution unlessis_constructible_v<OuterAlloc, const OuterA2&>
istrue
.template <class OuterA2> scoped_allocator_adaptor(scoped_allocator_adaptor<OuterA2, InnerAllocs...>&& other) noexcept;-9- Effects: Initializes each allocator within the adaptor with the corresponding allocator rvalue from
-8- Requires:OuterAlloc
shall be constructible fromOuterA2
.other
. -?- Remarks: This constructor shall not participate in overload resolution unlessis_constructible_v<OuterAlloc, OuterA2>
istrue
.
stack::emplace()
and queue::emplace()
should return decltype(auto)
Section: 23.6.3.1 [queue.defn], 23.6.6.2 [stack.defn] Status: C++20 Submitter: Jonathan Wakely Opened: 2016-10-14 Last modified: 2021-02-25
Priority: 2
View all issues with C++20 status.
Discussion:
The stack
and queue
adaptors are now defined as:
template <class... Args> reference emplace(Args&&... args) { return c.emplace_back(std::forward<Args>(args)...); }
This breaks any code using queue<UserDefinedSequence>
or stack<UserDefinedSequence>
until the user-defined containers are updated to meet the new C++17 requirements.
decltype(auto)
then we don't break any code. When used with std::vector
or std::deque
they will return reference
, as required, but when used with C++14-conforming containers
they will return void
, as before.
[2016-11-12, Issaquah]
Sat AM: P2
[2017-03-04, Kona]
Status to Tentatively Ready.
Proposed resolution:
This wording is relative to N4606.
Change return type of emplace
in class definition in 23.6.3.1 [queue.defn]:
template <class... Args>referencedecltype(auto) emplace(Args&&... args) { return c.emplace_back(std::forward<Args>(args)...); }
Change return type of emplace
in class definition in 23.6.6.2 [stack.defn]:
template <class... Args>referencedecltype(auto) emplace(Args&&... args) { return c.emplace_back(std::forward<Args>(args)...); }
Section: 17.9.8 [except.nested] Status: C++17 Submitter: Jonathan Wakely Opened: 2016-10-15 Last modified: 2017-07-30
Priority: 0
View all other issues in [except.nested].
View all issues with C++17 status.
Discussion:
The discussion notes for LWG 2484(i) point out there should be an "else, no effects" at the end, which didn't make it into the resolution. Without this it's unclear whether it should do nothing, or be ill-formed, or undefined.
Additionally, the precise effects of the Effects are hard to determine, because the conditions on the static type and dynamic type are intermingled, but must actually be checked separately (the static checks must be done statically and the dynamic checks must be done dynamically!) Furthermore, the obvious way to know if "the dynamic type ofe
is nested_exception
or is publicly and unambiguously derived from
nested_exception
" is to use dynamic_cast
, so we have to use dynamic_cast
to find out
whether to perform the dynamic_cast
expression specified in the Effects. It would make more sense
to specify it in terms of a dynamic_cast
to a pointer type, and only call rethrow_nested()
if
the result is not null.
The entire spec can be expressed in C++17 as:
if constexpr(is_polymorphic_v<E> && (!is_base_of_v<nested_exception, E> || is_convertible_v<E*, nested_exception*>)) if (auto p = dynamic_cast<const nested_exception*>(addressof(e))) p->rethrow_nested();
This uses traits to perform checks on the static type, then uses dynamic_cast
to perform the checks on the
dynamic type. I think the spec would be clearer if it had the same structure.
[2016-11-12, Issaquah]
Sat AM: Priority 0; move to Ready
Proposed resolution:
This wording is relative to N4606.
Modify 17.9.8 [except.nested] p9:
template <class E> void rethrow_if_nested(const E& e);-9- Effects: If
E
is not a polymorphic class type, or ifnested_exception
is an inaccessible or ambiguous base class ofE
, there is no effect. Otherwise,if the static type or the dynamic type ofperforms:e
isnested_exception
or is publicly and unambiguously derived fromnested_exception
, callsdynamic_cast<const nested_exception&>(e).rethrow_nested();if (auto p = dynamic_cast<const nested_exception*>(addressof(e))) p->rethrow_nested();
quoted
should work with basic_string_view
Section: 31.7.9 [quoted.manip] Status: C++17 Submitter: Marshall Clow Opened: 2016-10-27 Last modified: 2017-07-30
Priority: 0
View all other issues in [quoted.manip].
View all issues with C++17 status.
Discussion:
Quoted output for strings was added for C++14. But when we merged string_view
from the Library Fundamentals TS, we did not add support for quoted output of
basic_string_view
[2016-11-12, Issaquah]
Sat AM: Priority 0; move to Ready
13 -> 13 in paragraph modification; and T14 -> T15
Proposed resolution:
This wording is relative to N4606.
Add to the end of the <iomanip> synopsis in [iostream.format.overview]
template <class charT, class traits> T15 quoted(basic_string_view<charT, traits> s, charT delim = charT(’"’), charT escape = charT(’\\’));
Add to [quoted.manip] at the end of p2:
template <class charT, class traits> unspecified quoted(basic_string_view<charT, traits> s, charT delim = charT(’"’), charT escape = charT(’\\’));
Modify [quoted.manip]/3 as follows:
Returns: An object of unspecified type such that if out is an instance of
basic_ostream
with member type char_type
the same as charT
and with member type traits_type
which in the second and third forms
is the same as traits
, then the expression out << quoted(s, delim, escape)
behaves as a formatted output function (27.7.3.6.1) of out
. This forms a character
sequence seq
, initially consisting of the following elements:
shared_ptr
changes for array supportSection: C.4.9 [diff.cpp14.utilities] Status: C++17 Submitter: Jonathan Wakely Opened: 2016-10-18 Last modified: 2017-07-30
Priority: 0
View all issues with C++17 status.
Discussion:
This is valid in C++11 and C++14:
shared_ptr<int> s{unique_ptr<int[]>{new int[1]}};
The shared_ptr
copies the default_delete<int[]>
deleter from the unique_ptr
,
which does the right thing on destruction.
!is_convertible_v<int(*)[], int*>
.
The solution is to use shared_ptr<int[]>
, which doesn't work well in C++14, so there's no transition path.
This should be called out in Annex C.
[2016-11-12, Issaquah]
Sat AM: Priority 0; move to Ready
Proposed resolution:
This wording is relative to N4606.
Add to C.4.9 [diff.cpp14.utilities]:
20.3.2.2 [util.smartptr.shared]
Change: Different constraint on conversions fromunique_ptr
. Rationale: Adding array support toshared_ptr
, via the syntaxshared_ptr<T[]>
andshared_ptr<T[N]>
. Effect on original feature: Valid code may fail to compile or change meaning in this International Standard. For example:#include <memory> std::unique_ptr<int[]> arr(new int[1]); std::shared_ptr<int> ptr(std::move(arr)); // error: int(*)[] is not compatible with int*
Section: 31.12.9 [fs.class.file.status], 31.12.9.2 [fs.file.status.cons] Status: C++17 Submitter: Tim Song Opened: 2016-10-21 Last modified: 2021-06-06
Priority: 0
View all issues with C++17 status.
Discussion:
[fs.class.file_status] depicts:
explicit file_status(file_type ft = file_type::none, perms prms = perms::unknown) noexcept;
while [fs.file_status.cons] describes two constructors:
explicit file_status() noexcept; explicit file_status(file_type ft, perms prms = perms::unknown) noexcept;
It's also not clear why the default constructor needs to be explicit. Unlike tag types, there doesn't seem to be
a compelling reason to disallow constructing a file_status
without naming the type.
[2016-11-12, Issaquah]
Sat AM: Priority 0; move to Ready
Proposed resolution:
This wording is relative to N4606.
Edit [fs.class.file_status] as indicated:
class file_status { public: // 27.10.11.1, constructors and destructor: file_status() noexcept : file_status(file_type::none) {} explicit file_status(file_type ft= file_type::none, perms prms = perms::unknown) noexcept; […] };
Edit [fs.file_status.cons] as indicated:
explicit file_status() noexcept;
-1- Postconditions:type() == file_type::none
andpermissions() == perms::unknown
.
basic_string
range mutators unintentionally require a default constructible allocatorSection: 27.4.3.7.2 [string.append], 27.4.3.7.3 [string.assign], 27.4.3.7.4 [string.insert], 27.4.3.7.6 [string.replace] Status: C++17 Submitter: Billy O'Neal III Opened: 2016-10-25 Last modified: 2017-07-30
Priority: 2
View all other issues in [string.append].
View all issues with C++17 status.
Discussion:
Email discussion occurred on the lib reflector.
basic_string
's mutation functions show construction of temporary basic_string
instances,
without passing an allocator parameter. This says that basic_string
needs to use a default-initialized
allocator, which is clearly unintentional. The temporary needs to use the same allocator the current
basic_string
instance uses, if an implmentation needs to create a temporary at all.
libc++ already does this; I believe libstdc++ does as well (due to the bug report we got from a user that brought
this to our attention), but have not verified there. I implemented this in MSVC++'s STL and this change is scheduled
to ship in VS "15" RTW.
[2016-11-12, Issaquah]
Sat AM: Priority 2
Alisdair to investigate and (possibly) provide an alternate P/R
[2017-02-13 Alisdair responds:]
Looks good to me - no suggested alternative
[Kona 2017-02-28]
Accepted as Immediate.
Proposed resolution:
This wording is relative to N4606.
In 27.4.3.7.2 [string.append], add the allocator parameter to the range overload temporary:
template<class InputIterator> basic_string& append(InputIterator first, InputIterator last);-19- Requires:
-20- Effects: Equivalent to[first, last)
is a valid range.append(basic_string(first, last, get_allocator()))
. -21- Returns:*this
.
In 27.4.3.7.3 [string.assign], add the allocator parameter to the range overload temporary:
template<class InputIterator> basic_string& assign(InputIterator first, InputIterator last);-23- Effects: Equivalent to
-24- Returns:assign(basic_string(first, last, get_allocator()))
.*this
.
In 27.4.3.7.4 [string.insert], add the allocator parameter to the range overload temporary:
template<class InputIterator> iterator insert(const_iterator p, InputIterator first, InputIterator last);-23- Requires:
-24- Effects: Equivalent top
is a valid iterator on*this
.[first, last)
is a valid range.insert(p - begin(), basic_string(first, last, get_allocator()))
. -25- Returns: An iterator which refers to the copy of the first inserted character, orp
iffirst == last
.
In 27.4.3.7.6 [string.replace], add the allocator parameter to the range overload temporary:
template<class InputIterator> basic_string& replace(const_iterator i1, const_iterator i2, InputIterator j1, InputIterator j2);-32- Requires:
-33- Effects: Calls[begin(), i1)
,[i1, i2)
and[j1, j2)
are valid ranges.replace(i1 - begin(), i2 - i1, basic_string(j1, j2, get_allocator()))
. -34- Returns:*this
.
Section: 22.7.4 [any.class] Status: C++17 Submitter: Jonathan Wakely Opened: 2016-11-09 Last modified: 2017-07-30
Priority: 0
View all other issues in [any.class].
View all issues with C++17 status.
Discussion:
Addresses US 29
What does it mean for (the contained) objects to be "equivalent"? Suggested resolution: Add definition (note that usingoperator==()
involves complicated questions of overload
resolution).
[2016-11-08, Jonathan comments and suggests wording]
We can rephrase the copy constructor in terms of equivalence to construction from the contained object. We need to use in-place construction to avoid recursion in the case where the contained object is itself an any.
For the move constructor we don't simply want to construct from the contrained object, because when the contained object is stored in dynamic memory we don't actually construct anything, we just transfer ownership of a pointer.[Issues Telecon 16-Dec-2016]
Move to Tentatively Ready
Proposed resolution:
This wording is relative to N4606.
Change [any.cons] p2:
any(const any& other);Effects:
Constructs an object of typeIfany
with an equivalent state asother
.other.has_value()
isfalse
, constructs an object that has no value. Otherwise, equivalent toany(in_place<T>, any_cast<const T&>(other))
whereT
is the type of the contained object.
Change [any.cons] p4:
any(any&& other);Effects:
Constructs an object of typeIfany
with a state equivalent to the original state ofother
.other.has_value()
isfalse
, constructs an object that has no value. Otherwise, constructs an object of typeany
that contains either the contained object ofother
, or contains an object of the same type constructed from the contained object of other considering that contained object as an rvalue.
istreambuf_iterator::operator->
Section: 24.6.4 [istreambuf.iterator] Status: C++17 Submitter: Jonathan Wakely Opened: 2016-11-09 Last modified: 2017-07-30
Priority: 3
View other active issues in [istreambuf.iterator].
View all other issues in [istreambuf.iterator].
View all issues with C++17 status.
Discussion:
Addresses GB 59
There is no specification foristreambuf_iterator::operator->
. This
operator appears to have been added for C++11 by LWG issue 659(i),
which gave the signature, but also lacked specification.
[2016-11-08, Jonathan comments and suggests wording]
There is no good option here, and implementations either return
nullptr
, or return the address of a temporary, or don't even provide
the member at all. We took polls to decide whether to remove
istreambuf_iterator::operator->
, or specify it to return nullptr
, and
the preferred option was to remove it. It was noted that in the Ranges
TS input iterators no longer require operator->
anyway, and the
library never tries to use it.
[Issues Telecon 16-Dec-2016]
Move to Review
Proposed resolution:
This wording is relative to N4606.
Remove the note in paragraph 1 of 24.6.4 [istreambuf.iterator]:
The class template
istreambuf_iterator
defines an input iterator (24.2.3) that reads successive characters from the streambuf for which it was constructed.operator*
provides access to the current input character, if any.[Note:Each timeoperator->
may return a proxy. — end note]operator++
is evaluated, the iterator advances to the next input character. […]
Remove the member from the class synopsis in 24.6.4 [istreambuf.iterator]:
charT operator*() const;pointer operator->() const;istreambuf_iterator& operator++(); proxy operator++(int);
string_view
objects and strings should yield the same hash valuesSection: 27.3.6 [string.view.hash] Status: Resolved Submitter: Nicolai Josuttis Opened: 2016-11-09 Last modified: 2016-11-21
Priority: Not Prioritized
View all issues with Resolved status.
Discussion:
Under certain conditions we want to be able to use string_view
instead
of string
. As a consequence both types should behave the same as long we
talk about value specific behavior (if possible). For this reason, we should require
that both strings and string_view
yield the same hash values.
[2016-11-12, Issaquah]
Resolved by P0513R0
Proposed resolution:
This wording is relative to N4606.
A more formal formulation would be:
For any
sv
of typeSV
(beingstring_view
,u16string_view
,u32string_view
, orwstring_view
) ands
of the corresponding typeS
(beingstring
,u16string
,u32string
, orwstring
),hash<SV>()(sv) == hash<S>()(s)
.
Edit 27.3.6 [string.view.hash] as indicated:
template<> struct hash<string_view>; template<> struct hash<u16string_view>; template<> struct hash<u32string_view>; template<> struct hash<wstring_view>>;-1- The template specializations shall meet the requirements of class template
hash
(22.10.19 [unord.hash]). The hash values shall be the same as the hash values for the corresponding string class (27.4.6 [basic.string.hash]).
gcd
and lcm
should support a wider range of input valuesSection: 13.1.2 [fund.ts.v2::numeric.ops.gcd], 13.1.3 [fund.ts.v2::numeric.ops.lcm] Status: TS Submitter: Marshall Clow Opened: 2016-11-09 Last modified: 2020-09-06
Priority: Not Prioritized
View all other issues in [fund.ts.v2::numeric.ops.gcd].
View all issues with TS status.
Discussion:
Addresses fund.ts.v2: JP 010, JP 011
By the current definition,gcd((int64_t)1234, (int32_t)-2147483648)
is
ill-formed (because 2147483648
is not representable as a value of int32_t
.)
We want to change this case to be well-formed. As long as both |m|
and |n|
are representable as values of the common type, absolute values can
be calculate d
without causing unspecified behavior, by converting m
and n
to the common type before taking the negation.
Suggested resolution:
|m|
shall be representable as a value of typeM
and|n|
shall be representable as a value of typeN
|m|
and|n|
shall be representable as a value ofcommon_type_t<M, N>
.
[Issues Telecon 16-Dec-2016]
Resolved by N4616
Proposed resolution:
This wording is relative to N4600.
Edit 13.1.2 [fund.ts.v2::numeric.ops.gcd] as indicated:
template<class M, class N> constexpr common_type_t<M, N> gcd(M m, N n);-2- Requires:
|m|
shall be representable as a value of typeM
and|n|
shall be representable as a value of typeN
|m|
and|n|
shall be representable as a value ofcommon_type_t<M, N>
. [Note: These requirements ensure, for example, thatgcd(m, m) = |m|
is representable as a value of typeM
. — end note]
Edit 13.1.3 [fund.ts.v2::numeric.ops.lcm] as indicated:
template<class M, class N> constexpr common_type_t<M, N> lcm(M m, N n);-2- Requires:
|m|
shall be representable as a value of typeM
and|n|
shall be representable as a value of typeN
|m|
and|n|
shall be representable as a value ofcommon_type_t<M, N>
. The least common multiple of|m|
and|n|
shall be representable as a value of typecommon_type_t<M, N>
.
istream_iterator
Section: 24.6.2.2 [istream.iterator.cons] Status: C++17 Submitter: Erich Keane Opened: 2016-11-09 Last modified: 2020-09-06
Priority: Not Prioritized
View all other issues in [istream.iterator.cons].
View all issues with C++17 status.
Discussion:
Addresses GB 68, US 154, US 155
The term 'literal type' is dangerous and misleading, as text using this term really wants to require that a constexpr constructor/initialization is called with a constant expression, but does not actually tie the selected constructor to the type being 'literal'. Suggested resolution: Verify the uses of the term in the Core and Library specifications and replace with something more precise where appropriate. The conflation of trivial copy constructor and literal type is awkward. Not all literal types have trivial copy constructors, and not all types with trivial copy constructors are literal. Suggested resolution: Revise p5 as:Effects: Constructs a copy of
x
. IfT
has a trivial copy constructor, then this constructor shall be a trivial copy constructor. IfT
has a constexpr copy constructor, then this constructor shall beconstexpr
.
The requirement that the destructor is trivial if T
is a
literal type should be generalized to any type T
with a
trivial destructor — this encompasses all literal types,
as they are required to have a trivial destructor.
Effects: The iterator is destroyed. If
T
has a trivial destructor, then this destructor shall be a trivial destructor.
Previous resolution [SUPERSEDED]:
This wording is relative to N4606.
Change 24.6.2.2 [istream.iterator.cons] p1 as indicated:
see below istream_iterator();-1- Effects: Constructs the end-of-stream iterator. If
T
is a literal typeis_trivially_constructible_v<T> == true
, then this constructorshall be ais a trivial,constexpr
constructor.Change 24.6.2.2 [istream.iterator.cons] p5 as indicated:
istream_iterator(const istream_iterator& x) = default;-5- Effects: Constructs a copy of
x
. IfT
is a literal typeis_trivially_copyable_v<T> == true
, then this constructorshall beis a trivial copy constructor.Change 24.6.2.2 [istream.iterator.cons] p7 as indicated:
~istream_iterator() = default;-7- Effects: The iterator is destroyed. If
T
is a literal typeis_trivially_destructible_v<T> == true
, then this destructorshall beis a trivial destructor.
[Issues Telecon 16-Dec-2016]
Resolved by the adoption of P0503R0
Proposed resolution:
Resolve by accepting the wording suggested by P0503R0.
Section: 23.2.2 [container.requirements.general] Status: C++17 Submitter: Billy O'Neal III Opened: 2016-11-09 Last modified: 2017-07-30
Priority: 0
View other active issues in [container.requirements.general].
View all other issues in [container.requirements.general].
View all issues with C++17 status.
Discussion:
Addresses US 146
An allocator-aware contiguous container must require an allocator whose pointer type is a contiguous iterator. Otherwise, functions like data forbasic_string
and vector
do not work correctly, along with many
other expectations of the contiguous guarantee.
Suggested resolution:
Add a second sentence to 23.2.2 [container.requirements.general] p13:
An allocator-aware contiguous container requires
allocator_traits<Allocator>::pointer
is a contiguous iterator.
[2016-11-12, Issaquah]
Sat PM: Move to 'Tentatively Ready'
Proposed resolution:
This wording is relative to N4606.
In 16.4.4.6 [allocator.requirements]/5, edit as follows:
-5- An allocator type
X
shall satisfy the requirements ofCopyConstructible
(17.6.3.1). TheX::pointer
,X::const_pointer
,X::void_pointer
, andX::const_void_pointer
types shall satisfy the requirements ofNullablePointer
(17.6.3.3). No constructor, comparison operator, copy operation, move operation, or swap operation on these pointer types shall exit via an exception.X::pointer
andX::const_pointer
shall also satisfy the requirements for a random access iterator (24.3 [iterator.requirements]24.3.5.7 [random.access.iterators]) and of a contiguous iterator (24.3.1 [iterator.requirements.general]).
Section: 16.4.6.4 [global.functions] Status: C++17 Submitter: Jonathan Wakely Opened: 2016-11-09 Last modified: 2017-07-30
Priority: Not Prioritized
View all other issues in [global.functions].
View all issues with C++17 status.
Discussion:
Addresses GB 39
The example is supposed to highlight the 'otherwise specified' aspect of invoking ADL, yet there is no such specification. It is unlikely that we intend to explicitly qualify calls to operator functions, so they probably should be exempted from this restriction. Suggested resolution: Fix example (and referenced clause) to specify use of ADL, or exempt operators from this clause, and find a better example, probably usingswap
.
[2016-11-09, Jonathan comments and provides wording]
The current wording was added by DR 225(i).
[2016-11-10, Tim Song comments]
The "non-operator" seems to have been added at the wrong spot. The problem at issue is permission
to call operator functions found via ADL, not permission for operator functions in the standard
library to ADL all over the place. The problem is not unique to operator functions in the standard
library — a significant portion of <algorithm>
and <numeric>
uses some operator (==
, <
, +
, *
, etc.) that may be picked
up via ADL.
ostream_iterator::operator=
is a member function.
[2016-11-10, Tim Song and Jonathan agree on new wording]
The new wording still doesn't quite get it right:
"calls to non-operator, non-member functions in the standard library do not use functions from another namespace which are found through argument-dependent name lookup" can be interpreted as saying that if a user writes "
std::equal(a, b, c, d)
", that call will not use ADL'd "operator==
" because "std::equal(a, b, c, d)
" is a "call" to a "non-operator, non-member function in the standard library".
The key point here is that "in the standard library" should be modifying "calls", not "function".
Previous resolution [SUPERSEDED]:
This wording is relative to N4606.
Change 16.4.6.4 [global.functions] p4:
Unless otherwise specified, non-operator, non-member functions in the standard library shall not use functions from another namespace which are found through argument-dependent name lookup (3.4.2). [Note: The phrase "unless otherwise specified" applies to cases such as the swappable with requirements (16.4.4.3 [swappable.requirements]). The exception for overloaded operators allows
is intended to allowargument-dependent lookup in cases like that ofostream_iterator::operator=
(24.6.2.2):Effects:
*out_stream << value; if (delim != 0) *out_stream << delim ; return *this;— end note]
[Issues Telecon 16-Dec-2016]
Move to Tentatively Ready
Proposed resolution:
This wording is relative to N4606.
Change 16.4.6.4 [global.functions] p4:
Unless otherwise specified, calls made by functions in the standard library to non-operator, non-member functions
in the standard libraryshalldo not use functions from another namespace which are found through argument-dependent name lookup (3.4.2). [Note: The phrase "unless otherwise specified" applies to cases such as the swappable with requirements (16.4.4.3 [swappable.requirements]). The exception for overloaded operators allowsis intended to allowargument-dependent lookup in cases like that ofostream_iterator::operator=
(24.6.2.2):Effects:
*out_stream << value; if (delim != 0) *out_stream << delim ; return *this;— end note]
tuple
should be a literal typeSection: 22.4.1 [tuple.general] Status: C++17 Submitter: Jonathan Wakely Opened: 2016-11-09 Last modified: 2017-07-30
Priority: 2
View all other issues in [tuple.general].
View all issues with C++17 status.
Discussion:
Addresses US 109
tuple
should be a literal type if its elements are literal
types; it fails because the destructor is not necessarily trivial.
It should follow the form of optional and variant, and mandate a
trivial destructor if all types in Types...
have a trivial destructor. It is not
clear if pair
has the same issue, as pair
specifies data
members first
and second
, and appears to have an
implicitly declared and defined destructor.
Suggested resolution:
Document the destructor for tuple
, and mandate that
it is trivial if each of the elements in the tuple
has a
trivial destructor. Consider whether the same
specification is needed for pair
.
[2016-11-09, Jonathan provides wording]
[Issues Telecon 16-Dec-2016]
Move to Review; we think this is right, but are awaiting implementation experience.
Proposed resolution:
This wording is relative to N4606.
Add a new paragraph after 22.3.2 [pairs.pair] p2:
-2- The defaulted move and copy constructor, respectively, of
pair
shall be aconstexpr
function if and only if all required element-wise initializations for copy and move, respectively, would satisfy the requirements for aconstexpr
function. The destructor ofpair
shall be a trivial destructor if(is_trivially_destructible_v<T1> && is_trivially_destructible_v<T2>)
istrue
.
Add a new paragraph after the class synopsis in 22.4.4 [tuple.tuple]:
-?- The destructor of
tuple
shall be a trivial destructor if(is_trivially_destructible_v<Types> && ...)
istrue
.
Section: 21.3.3 [meta.type.synop] Status: Resolved Submitter: Russia Opened: 2016-11-09 Last modified: 2018-11-12
Priority: 2
View other active issues in [meta.type.synop].
View all other issues in [meta.type.synop].
View all issues with Resolved status.
Discussion:
Addresses RU 2
Failed prerequirement for the type trait must result in ill-formed program. Otherwise hard detectable errors will happen:#include <type_traits> struct foo; void damage_type_trait() { // must be ill-formed std::is_constructible<foo, foo>::value; } struct foo{}; int main() { static_assert( // produces invalid result std::is_constructible<foo, foo>::value, "foo must be constructible from foo" ); }
Suggested resolution:
Add to the end of the [meta.type.synop] section:Program is ill-formed if precondition for the type trait is violated.
[2016-11-09, Jonathan provides wording]
[Issues Telecon 16-Dec-2016]
Priority 2
[2017-03-02, Daniel comments]
LWG 2939(i) has been created to signal that some of our current type trait constraints are not quite correct and I recommend not to enforce the required diagnostics for traits that are sensitive to mismatches of the current approximate rules.
[2017-03-03, Kona Friday morning]
Unanimous consent to adopt this for C++17, but due to a misunderstanding, it wasn't on the ballot
Setting status to 'Ready' so we'll get it in immediately post-C++17
[2017-06-15 request from Daniel]
I don't believe that this should be "Ready"; I added the extra note to LWG 2797 *and* added the new issue 2939(i) exactly to *prevent* 2797 being accepted for C++17
Setting status back to 'Open'
[2018-08 Batavia Monday issue discussion]
Issues 2797(i), 2939(i), 3022(i), and 3099(i) are all closely related. Walter to write a paper resolving them.
[2018-11-11 Resolved by P1285R0, adopted in San Diego.]
Proposed resolution:
This wording is relative to N4606.
Add a new paragraph after 21.3.5.4 [meta.unary.prop] paragraph 3:
-?- If an instantiation of any template declared in this subclause fails to meet the preconditions, the program is ill-formed.
Change the specification for alignment_of
in Table 39 in 21.3.6 [meta.unary.prop.query]:
Table 39 — Type property queries template <class T> struct alignment_of;
alignof(T)
.
Requires:alignof(T)
shall be a valid expression (5.3.6), otherwise the program is ill-formed
Change the specification for is_base_of
, is_convertible
, is_callable
, and
is_nothrow_callable
in Table 40 in 21.3.7 [meta.rel]:
Table 40 — Type relationship predicates […]
template <class Base, class
Derived>
struct is_base_of;[…] If Base
andDerived
are
non-union class types and are
different types (ignoring possible
cv-qualifiers) thenDerived
shall
be a complete type, otherwise the program is ill-formed.
[Note: Base classes that
are private, protected, or ambiguous are,
nonetheless, base classes. — end note]template <class From, class To>
struct is_convertible;see below From
andTo
shall be complete
types, arrays of unknown bound,
or (possibly cv-qualified)void
types, otherwise the program is
ill-formed.template <class Fn, class...
ArgTypes, class R>
struct is_callable<
Fn(ArgTypes...), R>;[…] Fn
,R
, and all types in the
parameter packArgTypes
shall
be complete types, (possibly
cv-qualified)void
, or arrays of
unknown bound,
otherwise the program is ill-formed.template <class Fn, class...
ArgTypes, class R>
struct is_nothrow_callable<
Fn(ArgTypes...), R>;[…] Fn
,R
, and all types in the
parameter packArgTypes
shall
be complete types, (possibly
cv-qualified)void
, or arrays of
unknown bound,
otherwise the program is ill-formed.
Add a new paragraph after 21.3.8 [meta.trans] paragraph 2:
-2- Each of the templates in this subclause shall be a
-?- If an instantiation of any template declared in this subclause fails to meet the Requires: preconditions, the program is ill-formed.TransformationTrait
(20.15.1).
Section: 31.12.6 [fs.class.path] Status: Resolved Submitter: Billy O'Neal III Opened: 2016-11-10 Last modified: 2017-07-18
Priority: 2
View all other issues in [fs.class.path].
View all issues with Resolved status.
Discussion:
The explicit definition ofpath
in terms of a string
requires that the abstraction be leaky. Consider that
the meaning of the expression p += '/'
has very
different behavior in the case that p
is empty; that a
path
can uselessly contain null characters; and that
iterators must be constant to avoid having to reshuffle
the packed string.
Suggested resolution:
Define member functions to express apath
as a
string, but define its state in terms of the abstract
sequence of components (including the leading
special components) already described by the
iterator interface. Remove members that rely on
arbitrary manipulation of a string value.
[2016-11-12, Issaquah]
Sat PM: "Looks good"
[Issues Telecon 16-Dec-2016]
Priority 2; should be addressed by omnibus Filesystem paper.
[Kona, 2017-03]
This is resolved by p0492r2.
Proposed resolution:
This wording is relative to N4606.
Edit [fs.path.concat] as follows:
path& operator+=(const path& x); path& operator+=(const string_type& x); path& operator+=(basic_string_view<value_type> x); path& operator+=(const value_type * x); path& operator+=(value_type x); template <class Source> path& operator+=(const Source& x); template <class EcharT> path& operator+=(EcharT x); template <class Source> path& concat(const Source& x);template <class InputIterator> path& concat(InputIterator first, InputIterator last);-1-
Postcondition:native() == prior_native + effective-argument
, whereprior_native
isnative()
prior to the call tooperator+=
, andeffective-argument
is:
ifx
is present and isconst path&
,x.native()
; otherwise,ifsource
is present, the effective range ofsource
[ath.re]; otherwise,>iffirst
andlast
are present, the range[first, last)
; otherwise,x
.-2- Returns:
If the value type ofEffects: Equivalent toeffective-argument
would not bepath::value_type
, the acctual argument or argument range is first converted [ath.type.cv] so thateffective-argument
has value typepath::value_type
.pathname.append(path(x).native())
[Note: This directly manipulates the value ofnative()
and may not be portable between operating systems. — end note]*this
.template <class InputIterator> path& concat(InputIterator first, InputIterator last);-?- Effects: Equivalent to
-?- Returns:pathname.append(path(first, last).native())
[Note: This directly manipulates the value ofnative()
and may not be portable between operating systems. — end note]*this
.
noexcept
-specifications in shared_future
Section: 32.10.8 [futures.shared.future] Status: Resolved Submitter: Zhihao Yuan Opened: 2016-11-11 Last modified: 2021-06-06
Priority: 2
View all other issues in [futures.shared.future].
View all issues with Resolved status.
Discussion:
Addresses GB 62
There is an implicit precondition on mostshared_future
operations that
valid() == true
, 30.6.7p3. The list of exempted functions seems copied
directly from class future
, and would also include copy operations for
shared_futures
, which are copyable. Similarly, this would be a wide
contract that cannot throw, so those members would be marked noexcept
.
Suggested resolution:
Revise p3: "The effect of calling any member function other than the move constructor, the copy constructor, the destructor, the move-assignment operator, the copy-assignment operator, orvalid()
on a shared_future
object for which
valid() == false
is undefined." […]
Add noexcept
specification to the copy constructor and copy-assignment operator,
in the class definition and where those members are specified.
[2016-11-10, Zhihao Yuan comments and provides wording]
This resolution should close LWG 2697(i) as well, but that one was filed against concurrent TS.
[Issues Telecon 16-Dec-2016]
This is also addressed (but in a slightly different way by P0516R0
[2017-11 Albuquerque Wednesday issue processing]
Resolved by P0516, adopted in Issaquah.
Proposed resolution:
This wording is relative to N4606.
[Drafting note: This change supersedes the 1st among 3 edits in LWG 2556(i), please omit that edit and apply the one below.]
Modify [futures.unique_future]/3 as follows:
-3- The effect of calling
any member function other than the destructor, the move-assignment operator, orthe member functionsvalid
get
,wait
,wait_for
, orwait_until
on afuture
object for whichvalid() == false
is undefined. [Note: Implementations are encouraged to detect this case and throw an object of typefuture_error
with an error condition offuture_errc::no_state
. — end note]
Change [futures.shared_future] as indicated:
-3- The effect of calling
any member function other than the destructor, the move-assignment operator, orthe member functionsvalid()
get
,wait
,wait_for
, orwait_until
on ashared_future
object for whichvalid() == false
is undefined. [Note: Implementations are encouraged to detect this case and throw an object of typefuture_error
with an error condition offuture_errc::no_state
. — end note][…]namespace std { template <class R> class shared_future { public: shared_future() noexcept; shared_future(const shared_future& rhs) noexcept; shared_future(future<R>&&) noexcept; shared_future(shared_future&& rhs) noexcept; ~shared_future(); shared_future& operator=(const shared_future& rhs) noexcept; shared_future& operator=(shared_future&& rhs) noexcept; […] }; […] }shared_future(const shared_future& rhs) noexcept;[…]-7- Effects: […]
shared_future& operator=(const shared_future& rhs) noexcept;-14- Effects: […]
constexpr swap
Section: 22.2.2 [utility.swap] Status: Resolved Submitter: United States Opened: 2016-11-09 Last modified: 2020-09-06
Priority: 3
View all other issues in [utility.swap].
View all issues with Resolved status.
Discussion:
Addresses US 108
swap
is a critical function in the standard library, and
should be declared constexpr
to support more
widespread support for constexpr
in libraries. This
was proposed in P0202R1 which was reviewed
favourably at Oulu, but the widespread changes to
the <algorithm>
header were too risky and unproven
for C++17. We should not lose constexpr
support for
the much simpler (and more important) <utility>
functions because they were attached to a larger
paper. Similarly, the fundamental value wrappers, pair
and tuple
,
should have constexpr swap
functions,
and the same should be considered for optional
and
variant
. It is not possible to mark swap
for std::array
as constexpr
without adopting the rest of the
P0202R1 though, or rewriting the specification
for array swap to not use swap_ranges
.
Suggested resolution:
Adopt the changes to the<utility>
header proposed in
P0202R1, i.e., only bullets C, D, and E.
In addition, mark the swap
functions of pair
and
tuple
as constexpr
, and consider doing the same for
optional
and variant
.
[Issues Telecon 16-Dec-2016]
Priority 3
[2017-11 Albuquerque Wednesday issue processing]
Status to Open; we don't want to do this yet; gated on Core issue 1581. See also 2897(i).
[2017-11 Albuquerque Thursday]
It looks like 1581 is going to be resolved this week, so we should revisit soon.
[2018-06]
Resolved by the adoption of P0879R0
Proposed resolution:
unique_ptr
Section: 20.3.1.3.2 [unique.ptr.single.ctor] Status: C++17 Submitter: United States Opened: 2016-11-09 Last modified: 2017-07-30
Priority: 2
View all other issues in [unique.ptr.single.ctor].
View all issues with C++17 status.
Discussion:
Addresses US 122
unique_ptr
should not satisfy is_constructible_v<unique_ptr<T, D>>
unless D
is DefaultConstructible
and not a pointer type. This is
important for interactions with pair
, tuple
, and variant
constructors that rely on the is_default_constructible
trait.
Suggested resolution:
Add a Remarks: clause to constrain the default constructor to not exist unless the Requires clause is satisfied.[Issues Telecon 16-Dec-2016]
Priority 2; Howard and Ville to provide wording.
[2016-12-24: Howard and Ville provided wording.]
[2017-03-02, Kona, STL comments and tweaks the wording]
LWG discussed this issue, and we liked it, but we wanted to tweak the PR. I ran this past Ville (who drafted the PR with Howard), and he was in favor of tweaking this.
[Kona 2017-03-02]
Accepted as Immediate to resolve NB comment.
Proposed resolution:
Modify the synopsis in 20.3.1.3 [unique.ptr.single] as follows:
[…] constexpr unique_ptr(nullptr_t) noexcept;: unique_ptr() { }[…]
Modify 20.3.1.3.2 [unique.ptr.single.ctor] as follows:
constexpr unique_ptr() noexcept; constexpr unique_ptr(nullptr_t) noexcept;-1- Requires:
-2- Effects: Constructs aD
shall satisfy the requirements ofDefaultConstructible
(Table 22), and that construction shall not throw an exception.unique_ptr
object that owns nothing, value-initializing the stored pointer and the stored deleter. -3- Postconditions:get() == nullptr
.get_deleter()
returns a reference to the stored deleter. -4- Remarks:If this constructor is instantiated with a pointer type or reference type for the template argumentIfD
, the program is ill-formed.is_pointer_v<deleter_type>
istrue
oris_default_constructible_v<deleter_type>
isfalse
, this constructor shall not participate in overload resolution.explicit unique_ptr(pointer p) noexcept;[…]
-8- Remarks:
If this constructor is instantiated with a pointer type or reference type for the template argumentIfD
, the program is ill-formed.is_pointer_v<deleter_type>
istrue
oris_default_constructible_v<deleter_type>
isfalse
, this constructor shall not participate in overload resolution.
Modify the synopsis in 20.3.1.4 [unique.ptr.runtime] as follows:
[…] constexpr unique_ptr(nullptr_t) noexcept;: unique_ptr() { }[…]
Modify 20.3.1.4.2 [unique.ptr.runtime.ctor] as follows:
template <class U> explicit unique_ptr(U p) noexcept;This constructor behaves the same as the constructor that takes a pointer parameter in the primary template except that the following constraints are added for it to participate in overload resolution
U
is the same type aspointer
, or
pointer
is the same type aselement_type*
,U
is a pointer typeV*
, andV(*)[]
is convertible toelement_type(*)[]
.template <class U> unique_ptr(U p, see below d) noexcept; template <class U> unique_ptr(U p, see below d) noexcept;1 These constructors behave the same as the constructors that take a pointer parameter in the primary template except that they shall not participate in overload resolution unless either
shared_ptr
constructor requirements for a deleterSection: 20.3.2.2.2 [util.smartptr.shared.const] Status: C++17 Submitter: United States Opened: 2016-11-09 Last modified: 2020-09-06
Priority: 2
View other active issues in [util.smartptr.shared.const].
View all other issues in [util.smartptr.shared.const].
View all issues with C++17 status.
Discussion:
Addresses US 127
It should suffice for the deleterD
to be nothrow move-constructible.
However, to avoid potentially leaking the pointer p
if D
is also
copy-constructible when copying the argument by-value, we should continue
to require the copy constructor does not throw if D
is CopyConstructible
.
Proposed change:
Relax the requirement theD
be CopyConstructible
to simply require that D
be MoveConstructible
.
Clarify the requirement that construction of any of the arguments
passed by-value shall not throw exceptions. Note that we have library-wide
wording in clause 17 that says any type supported by the
library, not just this delete, shall not throw exceptions from its destructor,
so that wording could be editorially removed. Similarly, the requirements
that A
shall be an allocator satisfy that neither
constructor nor destructor for A
can throw.
[2016-12-16, Issues Telecon]
Priority 3; Jonathan to provide wording.
[2017-02-23, Jonathan comments and suggests wording]
I don't think the Clause 17 wording in [res.on.functions] is sufficient to require that the delete expression is well-formed. A class-specific deallocation function ([class.free]) would not be covered by [res.on.functions] and so could throw:
struct Y { void operator delete(void*) noexcept(false) { throw 1; } };
[Kona 2017-02-27]
Accepted as Immediate to resolve NB comment.
Proposed resolution:
This wording is relative to N4640.
Modify 20.3.2.2.2 [util.smartptr.shared.const] as indicated:
template<class Y, class D> shared_ptr(Y* p, D d); template<class Y, class D, class A> shared_ptr(Y* p, D d, A a); template <class D> shared_ptr(nullptr_t p, D d); template <class D, class A> shared_ptr(nullptr_t p, D d, A a);-8- Requires:
D
shall beand
CopyMoveConstructiblesuchconstruction ofd
and a deleter of typeD
initialized withstd::move(d)
shall not throw exceptions.The destructor ofThe expressionD
shall not throw exceptions.d(p)
shall be well formed, shall have well-defined behavior, and shall not throw exceptions.A
shall be an allocator (17.5.3.5).The copy constructor and destructor ofWhenA
shall not throw exceptions.T
is […].
hash
for arithmetic, pointer and standard library types should not throwSection: 22.10.19 [unord.hash] Status: Resolved Submitter: United States Opened: 2016-11-09 Last modified: 2020-09-06
Priority: 3
View all other issues in [unord.hash].
View all issues with Resolved status.
Discussion:
Addresses US 140
Specializations ofstd::hash
for arithmetic, pointer,
and standard library types should not be allowed to throw. The
constructors, assignment operators, and function call operator
should all be marked as noexcept
.
It might be reasonable to consider making this a
binding requirement on user specializations of the
hash
template as well (in p1) but that may be big a
change to make at this stage.
[Issues Telecon 16-Dec-2016]
Priority 2, Nico to provide wording.
[2017-02-07, Nico comments]
Concrete wording is provided in P0599.
[2017-03-12, post-Kona]
Resolved by P0599R0.
Proposed resolution:
constexpr
default constructor for istream_iterator
Section: 24.6.2.2 [istream.iterator.cons] Status: C++17 Submitter: United States Opened: 2016-11-09 Last modified: 2017-07-30
Priority: 0
View all other issues in [istream.iterator.cons].
View all issues with C++17 status.
Discussion:
Addresses US 152
see below for the default constructor should simply be spelledconstexpr
. The current declaration looks like a
member function, not a constructor, and the constexpr
keyword implicitly does not apply unless the instantiation could
make it so, under the guarantees al ready present in the Effects clause.
Proposed change:
Replace see below withconstexpr
in the declaration
of the default constructor for istream_iterator
in the
class definition, and function specification.
[Issues Telecon 16-Dec-2016]
Jonathan provides wording, Move to Tentatively Ready
Proposed resolution:
In the class synopsis in 24.6.1 [istream.iterator] change the default constructor:
see belowconstexpr istream_iterator(); istream_iterator(istream_type& s); istream_iterator(const istream_iterator& x) = default; ~istream_iterator() = default;
Change [istream.iterator.cons] before paragraph 1:
see belowconstexpr istream_iterator(); -1- Effects: ...
void
and reference type alternatives in variant
, variant<>
and
index()
Section: 22.6 [variant], 22.6.3 [variant.variant], 22.6.3.2 [variant.ctor] Status: Resolved Submitter: Switzerland Opened: 2016-11-10 Last modified: 2020-09-06
Priority: Not Prioritized
View all issues with Resolved status.
Discussion:
Addresses CH 3, CH 4, CH 5, CH 6, CH 8
variant
allows reference types as
alternatives; optional
explicitly forbids
to be instantiated for reference types.
This is inconsistent.
variant<int, void>
should be as
usable as variant<int>
.
variant<>
should not have an
index()
function.
Clarify the intended behavior of
variant
for alternative types that are
references.
Clarify variant
construction.
Proposed change:
Consider allowing reference types for both or none.
—
Consider specifying a specialization for variant<>
like:
template<> class variant<> { public: variant() = delete; variant(const variant&) = delete; variant& operator=(variant const&) = delete; };
Add a respective note.
Add a note that variant<>
cannot be constructed.
[Issues Telecon 16-Dec-2016]
Resolved by the adoption of P0501R0.
Proposed resolution:
bad_optional_access
Section: 22.5.5 [optional.bad.access] Status: C++17 Submitter: Great Britain Opened: 2016-11-09 Last modified: 2017-07-30
Priority: 1
View all issues with C++17 status.
Discussion:
Addresses GB 49
LEWG 72 suggests
changing the base class of std::bad_optional_access
, but the issue
appears to have been forgotten.
Proposed change:
Address LEWG issue 72, either changing it for C++17 or closing the issue.[Issues Telecon 16-Dec-2016]
Priority 1; Marshall provides wording
Not the most important bug in the world, but we can't change it after we ship C++17, hence the P1.
[Kona 2017-03-01]
Accepted as Immediate to resolve NB comment.
Proposed resolution:
Changes are relative to N4604.
In the class synopsis in 20.6.5 [optional.bad_optional_access] change:
class bad_optional_access : public exceptionlogic_error{
std::invoke
should use std::is_nothrow_callable
Section: 22.10.5 [func.invoke] Status: C++17 Submitter: Great Britain Opened: 2016-11-09 Last modified: 2020-09-06
Priority: 3
View all other issues in [func.invoke].
View all issues with C++17 status.
Discussion:
Addresses GB 53
std::invoke
can be made trivially noexcept
using the new std::is_nothrow_callable
trait:
Proposed change:
Add the exception specifiernoexcept(is_nothrow_callable_v<F&&(Args&&...)>)
to std::invoke
.
[Issues Telecon 16-Dec-2016]
Priority 3
[2017-02-28, Daniel comments and provides wording]
The following wording assumes that D0604R0 would be accepted, therefore uses is_nothrow_invocable_v
instead of the suggested current trait is_nothrow_callable_v
[Kona 2017-03-01]
Accepted as Immediate to resolve NB comment.
Proposed resolution:
This wording is relative to N4640.
Change 22.10.2 [functional.syn], header <functional>
synopsis, as indicated:
// 20.14.4, invoke template <class F, class... Args> invoke_result_t<F, Args...>result_of_t<F&&(Args&&...)>invoke(F&& f, Args&&... args) noexcept(is_nothrow_invocable_v<F, Args...>);
Change 22.10.5 [func.invoke] as indicated:
template <class F, class... Args> invoke_result_t<F, Args...>result_of_t<F&&(Args&&...)>invoke(F&& f, Args&&... args) noexcept(is_nothrow_invocable_v<F, Args...>);
fpos
and stateT
Section: 31.5.3.3 [fpos.operations] Status: Resolved Submitter: Great Britain Opened: 2016-11-09 Last modified: 2020-05-12
Priority: 4
View all other issues in [fpos.operations].
View all issues with Resolved status.
Discussion:
Addresses GB 60
The requirements on the stateT
type used
to instantiate class template fpos
are not
clear, and the following Table 108 — "Position
type requirements" is a bit of a mess. This is
old wording, and should be cleaned up with better
terminology from the Clause 17 Requirements. For example,
stateT
might be require CopyConstructible
?,
CopyAssignable
?, and Destructible
. Several
entries in the final column of the table appear to be
post-conditions, but without the post
markup to
clarify they are not assertions or preconditions. They
frequently refer to identifiers that do not apply to all
entries in their corresponding Expression
column, leaving some expressions without a clearly defined semantic.
If stateT
is a trivial type, is fpos
also a
trivial type, or is a default constructor not required/supported?
Proposed change:
Clarify the requirements and the table.[Issues Telecon 16-Dec-2016]
Priority 4; no consensus for any concrete change
[2019-03-17; Daniel comments]
With the acceptance of P0759R1 at the Rapperswil 2018 meeting this issue should be closed as Resolved (Please note that this paper resolves a historic NB comment that was originally written against C++17 but was at that time responded: "invite a paper if anybody wants to change it - no concensus for change").
[2020-05-12; Reflector discussions]
Resolved by P0759R1.
Rationale:
Resolved by P0759R1.Proposed resolution:
variant
hash requirementsSection: 22.6.12 [variant.hash] Status: Resolved Submitter: Great Britain Opened: 2016-11-09 Last modified: 2020-09-06
Priority: Not Prioritized
View all issues with Resolved status.
Discussion:
Addresses GB 69
The paragraph is really trying to say two different things, and should be split into two paragraphs, using standard terminology.
Proposed change:
The first sentence should become a Requires: clause, as it dictates requirements to callers. The second sentence should be a Remarks: clause, at is a normative requirement on the implementation.[Issues Telecon 16-Dec-2016]
Resolved by the adoption of P0513R0.
Proposed resolution:
use_count
and unique
in shared_ptr
Section: 20.3.2.2 [util.smartptr.shared] Status: Resolved Submitter: Marshall Clow Opened: 2016-11-09 Last modified: 2020-09-06
Priority: Not Prioritized
View all other issues in [util.smartptr.shared].
View all issues with Resolved status.
Discussion:
Addresses CA 14
The removal of the "debug only" restriction for
use_count()
and unique()
in shared_ptr
introduced a bug: in order for unique()
to produce
a useful and reliable value, it needs a synchronize clause to
ensure that prior accesses through another reference are visible to the
successful caller of unique()
. Many current implementations use
a relaxed load, and do not provide this guarantee, since it's not stated in the
Standard. For debug/hint usage that was OK. Without it the specification is
unclear and misleading.
Proposed change:
A solution could makeunique()
use memory_order_acquire
, and
specifying that reference count decrement operations synchronize with
unique()
. This won't provide sequential consistency but may be useful.
We could also specify use_count()
as only providing an unreliable hint
of the actual count, or deprecate it.
[Issues Telecon 16-Dec-2016]
Resolved by adoption of P0521R0.
Proposed resolution:
<string_view>
Section: 24.7 [iterator.range], 27.3.2 [string.view.synop] Status: C++17 Submitter: Johel Ernesto Guerrero Peña Opened: 2016-10-29 Last modified: 2017-07-30
Priority: 0
View other active issues in [iterator.range].
View all other issues in [iterator.range].
View all issues with C++17 status.
Discussion:
27.3.2 [string.view.synop]/1 states
The function templates defined in 20.2.2 and 24.7 are available when
<string_view>
is included.
24.7 [iterator.range], in p1 is missing <string_view>
.
[Issues Telecon 16-Dec-2016]
Move to Tentatively Ready
Proposed resolution:
This wording is relative to N4606.
Edit 24.7 [iterator.range] p1 as indicated:
-1- In addition to being available via inclusion of the
<iterator>
header, the function templates in 24.7 are available when any of the following headers are included:<array>
,<deque>
,<forward_list>
,<list>
,<map>
,<regex>
,<set>
,<string>
,<string_view>
,<unordered_map>
,<unordered_set>
, and<vector>
.
std::function
should not return dangling referencesSection: 22.10.17.3.2 [func.wrap.func.con] Status: Resolved Submitter: Brian Bi Opened: 2016-11-03 Last modified: 2022-11-25
Priority: 2
View all other issues in [func.wrap.func.con].
View all issues with Resolved status.
Discussion:
If a std::function
has a reference as a return type, and that reference binds to a prvalue
returned by the callable that it wraps, then the reference is always dangling. Because any use of such
a reference results in undefined behaviour, the std::function
should not be allowed to be
initialized with such a callable. Instead, the program should be ill-formed.
#include <functional> int main() { std::function<const int&()> F([]{ return 42; }); int x = F(); // oops! }
[2016-11-22, David Krauss comments and suggests wording]
Indirect bindings may also introduce temporaries inside std::function
, e.g.:
void f(std::function<long const&()>); // Retains an observer to a long. void g() { int v; f([&]()->int& { return v; } ); // int lvalue binds to long const& through a temporary. }
A fix has been implemented. Conversions that may be conversion operators are allowed, though, because those can produce legitimate glvalues. Before adopting this, it need to be considered considered whether there should be SFINAE or a hard error.
[Issues Telecon 16-Dec-2016]
Priority 2
[2016-07, Toronto Saturday afternoon issues processing]
Billy to work with Brian to rework PR. Status to Open
[2018-08-23 Batavia Issues processing]
Really needs a language change to fix this. Status to EWG.
[2022-08-24 Resolved by P2255R2. Status changed: EWG → Resolved.]
[2022-11-25; see EWG 1370]
Proposed resolution:
This wording is relative to N4618.
Add a second paragraph to the remarks section of 22.10.17.3.2 [func.wrap.func.con]:
template<class F> function(F f);-7- Requires:
-8- Remarks: This constructor template shall not participate in overload resolution unlessF
shall beCopyConstructible
.
F
is Lvalue-Callable (22.10.17.3 [func.wrap.func]) for argument typesArgTypes...
and return typeR
, andIf
R
is type "reference toT
" andINVOKE(ArgTypes...)
has value categoryV
and typeU
:
V
is a prvalue,U
is a class type, andT
is not reference-related (9.5.4 [dcl.init.ref]) toU
, and
V
is an lvalue or xvalue, and eitherU
is a class type orT
is reference-related toU
.[…]
to_array
should take rvalue reference as wellSection: 4.2.1 [fund.ts.v2::func.wrap.func.con] Status: Resolved Submitter: Zhihao Yuan Opened: 2016-11-07 Last modified: 2020-09-06
Priority: 3
View all other issues in [fund.ts.v2::func.wrap.func.con].
View all issues with Resolved status.
Discussion:
Addresses: fund.ts.v2
C++ doesn't have a prvalue expression of array type, but rvalue arrays can still come from different kinds of sources:
C99 compound literals (int[]) {2, 4}
,
std::move(arr)
,
Deduction to_array<int const>({ 2, 4 })
.
See also CWG 1591: Deducing array bound and element type from initializer list.
For 3), users are "abusing" to_array
to get access to uniform
initialization to benefit from initializing elements through
braced-init-list and/or better narrowing conversion support.
to_array
.
[Issues Telecon 16-Dec-2016]
Status to LEWG
[2017-02 in Kona, LEWG responds]
Would like a small paper; see examples before and after. How does this affect overload resolution?
[2017-06-02 Issues Telecon]
Leave status as LEWG; priority 3
Previous resolution [SUPERSEDED]:This wording is relative to N4600.
Add the following signature to 9.2.1 [fund.ts.v2::header.array.synop]:
// 9.2.2, Array creation functions template <class D = void, class... Types> constexpr array<VT, sizeof...(Types)> make_array(Types&&... t); template <class T, size_t N> constexpr array<remove_cv_t<T>, N> to_array(T (&a)[N]); template <class T, size_t N> constexpr array<remove_cv_t<T>, N> to_array(T (&&a)[N]);Modify 9.2.2 [fund.ts.v2::container.array.creation] as follows:
template <class T, size_t N> constexpr array<remove_cv_t<T>, N> to_array(T (&a)[N]); template <class T, size_t N> constexpr array<remove_cv_t<T>, N> to_array(T (&&a)[N]);-6- Returns: For all
i
,0 ≤ i < N
, aAnarray<remove_cv_t<T>, N>
such that each element is copy-initialized with the corresponding element ofinitialized witha
{ a[i]... }
for the first form, or{ std::move(a[i])... }
for the second form..
[2020-05-10; Reflector discussions]
Resolved by adoption of P0325R4 and P2081R1.
Rationale:
Resolved by P0325R4 and P2081R1.Proposed resolution:
resize_file
has impossible postconditionSection: 31.12.13.34 [fs.op.resize.file] Status: C++20 Submitter: Richard Smith Opened: 2016-11-07 Last modified: 2021-06-06
Priority: 3
View all issues with C++20 status.
Discussion:
resize_file
has this postcondition (after resolving late comment 42, see P0489R0):
Postcondition:
file_size(p) == new_size
.
This is impossible for an implementation to satisfy, due to the possibility of file system races. This is not actually a postcondition; rather, it is an effect that need no longer hold when the function returns.
[Issues Telecon 16-Dec-2016]
Priority 3
[2018-01-16, Jonathan provides wording]
[2018-1-26 issues processing telecon]
Status to 'Tentatively Ready'
[2018-3-17 Adopted in Jacksonville]
Proposed resolution:
This wording is relative to N4713.
[Drafting note: I considered a slightly more verbose form: "Causes the size in bytes of the file
p
resolves to, as determined byfile_size
( [fs.op.file_size]), to be equal tonew_size
, as if by POSIXtruncate
." but I don't think it's an improvement. The intent of the proposed wording is that if eitherfile_size(p)
ortruncate(p.c_str())
would fail then an error occurs, but no call tofile_size
is required, and file system races might change the size before any such call does occur.]
Modify [fs.op.resize_file] as indicated:
void resize_file(const path& p, uintmax_t new_size); void resize_file(const path& p, uintmax_t new_size, error_code& ec) noexcept;-1-
-2- Throws: As specified in 31.12.5 [fs.err.report].Postconditions:Effects: Causes the size that would be returned byfile_size(p) == new_size
file_size(p)
to be equal tonew_size
, as if by POSIXtruncate
.-3- Remarks: Achieves its postconditions as if by POSIXtruncate()
.
std::hash
for nullptr_t
Section: 22.10.19 [unord.hash] Status: Resolved Submitter: Marshall Clow Opened: 2016-11-10 Last modified: 2020-09-06
Priority: Not Prioritized
View all other issues in [unord.hash].
View all issues with Resolved status.
Discussion:
In the list of types that the library provides std::hash
specializations, there is an entry for
hash<T*>
, but none entry for hash<nullptr_t>
.
[2016-11-13, Daniel provides wording]
[2016-11-28]
Resolved by P0513R0
Proposed resolution:
This wording is relative to N4606.
Change 22.10 [function.objects], header <functional>
synopsis, as indicated:
[…] // Hash function specializations template <> struct hash<bool>; […] template <> struct hash<long double>; template <> struct hash<nullptr_t>; template<class T> struct hash<T*>; […]
Change 22.10.19 [unord.hash] before p2, as indicated:
template <> struct hash<bool>; […] template <> struct hash<long double>; template <> struct hash<nullptr_t>; template<class T> struct hash<T*>;
"::std::"
everywhere rule needs tweakingSection: 16.4.2.2 [contents] Status: C++23 Submitter: Tim Song Opened: 2016-11-11 Last modified: 2023-11-22
Priority: 2
View all other issues in [contents].
View all issues with C++23 status.
Discussion:
[contents]/3 says
Whenever a name
x
defined in the standard library is mentioned, the namex
is assumed to be fully qualified as::std::x
, unless explicitly described otherwise. For example, if the Effects section for library functionF
is described as calling library functionG
, the function::std::G
is meant.
With the introduction of nested namespaces inside std
, this rule needs tweaking. For instance,
time_point_cast
's Returns clause says "time_point<Clock,
ToDuration>(duration_cast<ToDuration>(t.time_since_epoch()))
"; that reference to duration_cast
obviously means ::std::chrono::duration_cast
, not ::std::duration_cast
, which doesn't exist.
[Issues Telecon 16-Dec-2016]
Priority 2; Jonathan to provide wording
[2019 Cologne Wednesday night]
Geoffrey suggested editing 16.4.2.2 [contents]/2 to mention the case when we're defining things in a sub-namespace.
Jonathan to word this.
[2020-02-14, Prague; Walter provides wording]
[2020-10-02; Issue processing telecon: new wording from Jens]
Use "Simplified suggestion" in 13 June 2020 email from Jens.
Previous resolution [SUPERSEDED]:
This wording is relative to N4849.
Modify 16.4.2.2 [contents] as indicated:
-3-
Whenever a nameLetx
defined in the standard library is mentioned, the namex
is assumed to be fully qualified as::std::x
, unless explicitly described otherwise. For example, if the Effects: element for library functionF
is described as calling library functionG
, the function::std::G
is meant.x
be a name specified by the standard library via a declaration in namespacestd
or in a subnamespace of namespacestd
. Wheneverx
is used as an unqualified name in a further specification, it is assumed to correspond to the samex
that would be found via unqualified name lookup (6.5.3 [basic.lookup.unqual]) performed at that point of use. Similarly, wheneverx
is used as a qualified name in a further specification, it is assumed to correspond to the samex
that would be found via qualified name lookup (6.5.5 [basic.lookup.qual]) performed at that point of use. [Note: Such lookups can never fail in a well-formed program. — end note] [Example: If an Effects: element for a library functionF
specifies that library functionG
is to be used, the function::std::G
is intended. — end example]
Previous resolution [SUPERSEDED]:
This wording is relative to N4849.
Modify 16.4.2.2 [contents] as indicated:
[Drafting note: Consider adding a note clarifying that the unqualified lookup does not perform ADL. ]
-3-
Whenever a nameWhenever an unqualified namex
defined in the standard library is mentioned, the namex
is assumed to be fully qualified as::std::x
, unless explicitly described otherwise. For example, if the Effects: element for library functionF
is described as calling library functionG
, the function::std::G
is meant.x
is used in the specification of a declarationD
in clauses 16-32, its meaning is established as-if by performing unqualified name lookup (6.5.3 [basic.lookup.unqual]) in the context ofD
. Similarly, the meaning of a qualified-id is established as-if by performing qualified name lookup (6.5.5 [basic.lookup.qual]) in the context ofD
. [Example: The reference tois_array_v
in the specification ofstd::to_array
(23.3.3.6 [array.creation]) refers to::std::is_array_v
. -- end example] [Note: Operators in expressions 12.2.2.3 [over.match.oper] are not so constrained; see 16.4.6.4 [global.functions]. -- end note]
[2020-11-04; Jens provides improved wording]
[2020-11-06; Reflector discussion]
Casey suggests to insert "or Annex D" after "in clauses 16-32". This insertion has been performed during reflector discussions immediately because it seemed editorial.
[2020-11-15; Reflector poll]
Set priority status to Tentatively Ready after seven votes in favour during reflector discussions.
[2020-11-22, Tim Song reopens]
The references to get in 25.5.4.1 [range.subrange.general] and
25.7.23.2 [range.elements.view] need to be qualified as they would otherwise
refer to std::ranges::get
instead of std::get
. Additionally,
[expos.only.func] needs to clarify that the lookup there also takes
place from within namespace std
.
Previous resolution [SUPERSEDED]:
This wording is relative to N4868.
Modify 16.4.2.2 [contents] as indicated:
-3-
Whenever a nameWhenever an unqualified namex
defined in the standard library is mentioned, the namex
is assumed to be fully qualified as::std::x
, unless explicitly described otherwise. For example, if the Effects: element for library functionF
is described as calling library functionG
, the function::std::G
is meant.x
is used in the specification of a declarationD
in clauses 16-32 or Annex D, its meaning is established as-if by performing unqualified name lookup (6.5.3 [basic.lookup.unqual]) in the context ofD
. [Note ?: Argument-dependent lookup is not performed. — end note] Similarly, the meaning of a qualified-id is established as-if by performing qualified name lookup (6.5.5 [basic.lookup.qual]) in the context ofD
. [Example: The reference tois_array_v
in the specification ofstd::to_array
(23.3.3.6 [array.creation]) refers to::std::is_array_v
. — end example] [Note ?: Operators in expressions (12.2.2.3 [over.match.oper]) are not so constrained; see 16.4.6.4 [global.functions]. — end note]Remove [fs.req.namespace] in its entirety:
29.11.3.2 Namespaces and headers [fs.req.namespace]-1- Unless otherwise specified, references to entities described in subclause 31.12 [filesystems] are assumed to be qualified with::std::filesystem::
.
[2021-05-20; Jens Maurer provides an updated proposed resolution]
[2021-05-23; Daniel provides some additional tweaks to the updated proposed resolution]
[2021-05-24; Reflector poll]
Set status to Tentatively Ready after six votes in favour during reflector poll.
[2021-06-07 Approved at June 2021 virtual plenary. Status changed: Voting → WP.]
Proposed resolution:
This wording is relative to N4885.
Modify [expos.only.func] as indicated:
-2- The following are defined for exposition only to aid in the specification of the library:
namespace std { template<class T> constexpr decay_t<T> decay-copy(T&& v) noexcept(is_nothrow_convertible_v<T, decay_t<T>>) // exposition only { return std::forward<T>(v); } constexpr auto synth-three-way = []<class T, class U>(const T& t, const U& u) requires requires { { t < u } -> boolean-testable; { u < t } -> boolean-testable; } { if constexpr (three_way_comparable_with<T, U>) { return t <=> u; } else { if (t < u) return weak_ordering::less; if (u < t) return weak_ordering::greater; return weak_ordering::equivalent; } }; template<class T, class U=T> using synth-three-way-result = decltype(synth-three-way(declval<T&>(), declval<U&>())); }
Modify 16.4.2.2 [contents] as indicated:
-3-
Whenever a nameWhenever an unqualified namex
defined in the standard library is mentioned, the namex
is assumed to be fully qualified as::std::x
, unless explicitly described otherwise. For example, if the Effects: element for library functionF
is described as calling library functionG
, the function::std::G
is meant.x
is used in the specification of a declarationD
in clauses 16-32 or Annex D, its meaning is established as-if by performing unqualified name lookup (6.5.3 [basic.lookup.unqual]) in the context ofD
. [Note ?: Argument-dependent lookup is not performed. — end note] Similarly, the meaning of a qualified-id is established as-if by performing qualified name lookup (6.5.5 [basic.lookup.qual]) in the context ofD
. [Example: The reference tois_array_v
in the specification ofstd::to_array
(23.3.3.6 [array.creation]) refers to::std::is_array_v
. — end example] [Note ?: Operators in expressions (12.2.2.3 [over.match.oper]) are not so constrained; see 16.4.6.4 [global.functions]. — end note]
Modify 25.5.4.1 [range.subrange.general] as indicated:
template<class T> concept pair-like = // exposition only !is_reference_v<T> && requires(T t) { typename tuple_size<T>::type; // ensures tuple_size<T> is complete requires derived_from<tuple_size<T>, integral_constant<size_t, 2>>; typename tuple_element_t<0, remove_const_t<T>>; typename tuple_element_t<1, remove_const_t<T>>; { std::get<0>(t) } -> convertible_to<const tuple_element_t<0, T>&>; { std::get<1>(t) } -> convertible_to<const tuple_element_t<1, T>&>; };
Modify 25.7.23.2 [range.elements.view] as indicated:
template<class T, size_t N> concept has-tuple-element = // exposition only requires(T t) { typename tuple_size<T>::type; requires N <tuple_size_v<T>; typename tuple_element_t<N, T>; { std::get<N>(t) } -> convertible_to<const tuple_element_t<N, T>&>; };
Modify 25.7.23.3 [range.elements.iterator] as indicated:
-2- The member typedef-name
iterator_category
is defined if and only ifBase
modelsforward_range
. In that case,iterator_category
is defined as follows: […]
(2.1) — If
std::get<N>(*current_)
is an rvalue,iterator_category
denotesinput_iterator_tag
.[…]
static constexpr decltype(auto) get-element(const iterator_t<Base>& i); // exposition only-3- Effects: Equivalent to:
if constexpr (is_reference_v<range_reference_t<Base>>) { return std::get<N>(*i); } else { using E = remove_cv_t<tuple_element_t<N, range_reference_t<Base>>>; return static_cast<E>(std::get<N>(*i)); }
Remove [fs.req.namespace] in its entirety:
29.11.3.2 Namespaces and headers [fs.req.namespace]-1- Unless otherwise specified, references to entities described in subclause 31.12 [filesystems] are assumed to be qualified with::std::filesystem::
.
<cstdint>
macrosSection: 17.4.1 [cstdint.syn] Status: C++23 Submitter: Thomas Köppe Opened: 2016-11-12 Last modified: 2023-11-22
Priority: 3
View all other issues in [cstdint.syn].
View all issues with C++23 status.
Discussion:
I would like clarification from LWG regarding the various limit macros like INT_8_MIN
in <cstdint>
,
in pursuit of editorial cleanup of this header's synopsis. I have two questions:
At present, macros like INT_8_MIN
that correspond to the optional type int8_t
are required
(unconditionally), whereas the underlying type to which they appertain is only optional. Is this deliberate?
Should the macros also be optional?
Is it deliberate that C++ only specifies sized aliases for the sizes 8, 16, 32 and 64, whereas the corresponding C header allows type aliases and macros for arbitrary sizes for implementations that choose to provide extended integer types? Is the C++ wording more restrictive by accident?
[2017-01-27 Telecon]
Priority 3
[2017-03-04, Kona]
C11 ties the macro names to the existence of the types. Marshall to research the second question.
Close 2764(i) as a duplicate of this issue.
[2017-03-18, Thomas comments and provides wording]
This is as close as I can get to the C wording without resolving part (a) of the issue (whether we deliberately don't
allow sized type aliases for sizes other than 8, 16, 32, 64, a departure from C). Once we resolve part (a), we need
to revisit <cinttypes>
and fix up the synopsis (perhaps to get rid of N
) and add similar
wording as the one below to make the formatting macros for the fixed-width types optional. For historical interest,
this issue is related to LWG 553(i) and LWG 841(i).
[2016-07, Toronto Saturday afternoon issues processing]
Status to Open
Previous resolution: [SUPERSEDED]This wording is relative to N4640.
Append the following content to 17.4.1 [cstdint.syn] p2:
-2- The header defines all types and macros the same as the C standard library header
<stdint.h>
. In particular, for each of the fixed-width types (int8_t
,int16_t
,int32_t
,int64_t
,uint8_t
,uint16_t
,uint32_t
,uint64_t
) the type alias and the corresponding limit macros are defined if and only if the implementation provides the corresponding type.
[2017-10-21, Thomas Köppe provides improved wording]
Previous resolution [SUPERSEDED]:
This wording is relative to N4687.
Change 17.4.1 [cstdint.syn], header
<cstdint>
synopsis, as indicated:[…] using int64_t = signed integer type; // optional using intN_t = see below; // optional, see below […] using int_fast64_t = signed integer type; using int_fastN_t = see below; // optional, see below […] using int_least64_t = signed integer type; using int_leastN_t = see below; // optional, see below […] using uint64_t = unsigned integer type; // optional using uintN_t = see below; // optional, see below […] using uint_fast64_t = unsigned integer type; using uint_fastN_t = see below; // optional, see below […] using uint_least64_t = unsigned integer type; using uint_leastN_t = see below; // optional, see below using uintmax_t = unsigned integer type; using uintptr_t = unsigned integer type; // optional #define INT_N_MIN see below; #define INT_N_MAX see below; #define UINT_N_MAX see below; #define INT_FASTN_MIN see below; #define INT_FASTN_MAX see below; #define UINT_FASTN_MAX see below; #define INT_LEASTN_MIN see below; #define INT_LEASTN_MAX see below; #define UINT_LEASTN_MAX see below; #define INTMAX_MIN see below; #define INTMAX_MAX see below; #define UINTMAX_MAX see below; #define INTPTR_MIN see below; #define INTPTR_MAX see below; #define UINTPTR_MAX see below; #define PTRDIFF_MIN see below; #define PTRDIFF_MAX see below; #define SIZE_MAX see below; #define SIGATOMIC_MIN see below; #define SIGATOMIC_MAX see below; #define WCHAR_MIN see below; #define WCHAR_MAX see below; #define WINT_MIN see below; #define WINT_MAX see below; #define INTN_C(value) see below; #define UINTN_C(value) see below; #define INTMAX_C(value) see below; #define UINTMAX_C(value) see below;
-1- The header also defines numerous macros of the form:INT_[FAST LEAST]{8 16 32 64}_MIN [U]INT_[FAST LEAST]{8 16 32 64}_MAX INT{MAX PTR}_MIN [U]INT{MAX PTR}_MAX {PTRDIFF SIG_ATOMIC WCHAR WINT}{_MAX _MIN} SIZE_MAX
plus function macros of the form:[U]INT{8 16 32 64 MAX}_C-2- The header defines all types and macros the same as the C standard library header
-?- In particular, all types that use the placeholder<stdint.h>
. See also: ISO C 7.20N
are optional whenN
is not 8, 16, 32 or 64. The exact-width typesintN_t
anduintN_t
forN
= 8, 16, 32, 64 are also optional; however, if an implementation provides integer types with the corresponding width, no padding bits, and (for the signed types) that have a two's complement representation, it defines the corresponding typedef names. Only those macros are defined that correspond to typedef names that the implementation actually provides. [Note: The macrosINTN_C
andUINTN_C
correspond to the typedef namesint_leastN_t
anduint_leastN_t
, respectively. — end note]Change 31.13.2 [cinttypes.syn] as indicated:
#define PRIdNN see below #define PRIiNN see below #define PRIoNN see below #define PRIuNN see below #define PRIxNN see below #define PRIXNN see below #define SCNdNN see below #define SCNiNN see below #define SCNoNN see below #define SCNuNN see below #define SCNxNN see below #define PRIdLEASTNN see below #define PRIiLEASTNN see below #define PRIoLEASTNN see below #define PRIuLEASTNN see below #define PRIxLEASTNN see below #define PRIXLEASTNN see below #define SCNdLEASTNN see below #define SCNiLEASTNN see below #define SCNoLEASTNN see below #define SCNuLEASTNN see below #define SCNxLEASTNN see below #define PRIdFASTNN see below #define PRIiFASTNN see below #define PRIoFASTNN see below #define PRIuFASTNN see below #define PRIxFASTNN see below #define PRIXFASTNN see below #define SCNdFASTNN see below #define SCNiFASTNN see below #define SCNoFASTNN see below #define SCNuFASTNN see below #define SCNxFASTNN see below […]-1- The contents and meaning of the header
-?- In particular, macros that use the placeholder<cinttypes>
[…]N
are defined if and only if the implementation actually provides the corresponding typedef name in 17.4.1 [cstdint.syn], and moreover, thefscanf
macros are provided unless the implementation does not have a suitablefscanf
length modifier for the type.
[2018-04-03; Geoffrey Romer suggests improved wording]
Previous resolution [SUPERSEDED]:
This wording is relative to N4727.
Change 17.4.1 [cstdint.syn], header
<cstdint>
synopsis, as indicated:[…] using int64_t = signed integer type; // optional using intN_t = see below; // optional, see below […] using int_fast64_t = signed integer type; using int_fastN_t = see below; // optional, see below […] using int_least64_t = signed integer type; using int_leastN_t = see below; // optional, see below […] using uint64_t = unsigned integer type; // optional using uintN_t = see below; // optional, see below […] using uint_fast64_t = unsigned integer type; using uint_fastN_t = see below; // optional, see below […] using uint_least64_t = unsigned integer type; using uint_leastN_t = see below; // optional, see below using uintmax_t = unsigned integer type; using uintptr_t = unsigned integer type; // optional #define INT_N_MIN see below; #define INT_N_MAX see below; #define UINT_N_MAX see below; #define INT_FASTN_MIN see below; #define INT_FASTN_MAX see below; #define UINT_FASTN_MAX see below; #define INT_LEASTN_MIN see below; #define INT_LEASTN_MAX see below; #define UINT_LEASTN_MAX see below; #define INTMAX_MIN see below; #define INTMAX_MAX see below; #define UINTMAX_MAX see below; #define INTPTR_MIN see below; #define INTPTR_MAX see below; #define UINTPTR_MAX see below; #define PTRDIFF_MIN see below; #define PTRDIFF_MAX see below; #define SIZE_MAX see below; #define SIGATOMIC_MIN see below; #define SIGATOMIC_MAX see below; #define WCHAR_MIN see below; #define WCHAR_MAX see below; #define WINT_MIN see below; #define WINT_MAX see below; #define INTN_C(value) see below; #define UINTN_C(value) see below; #define INTMAX_C(value) see below; #define UINTMAX_C(value) see below;
-1- The header also defines numerous macros of the form:INT_[FAST LEAST]{8 16 32 64}_MIN [U]INT_[FAST LEAST]{8 16 32 64}_MAX INT{MAX PTR}_MIN [U]INT{MAX PTR}_MAX {PTRDIFF SIG_ATOMIC WCHAR WINT}{_MAX _MIN} SIZE_MAX
plus function macros of the form:[U]INT{8 16 32 64 MAX}_C-2- The header defines all types and macros the same as the C standard library header
-?- In particular, all types that use the placeholder<stdint.h>
. See also: ISO C 7.20N
are optional whenN
is not 8, 16, 32 or 64. The exact-width typesintN_t
anduintN_t
forN
= 8, 16, 32, 64 are also optional; however, if an implementation provides integer types with the corresponding width, no padding bits, and (for the signed types) that have a two's complement representation, it defines the corresponding typedef names. Only those macros are defined that correspond to typedef names that the implementation actually provides. [Note: The macrosINTN_C
andUINTN_C
correspond to the typedef namesint_leastN_t
anduint_leastN_t
, respectively. — end note]Change 31.13.2 [cinttypes.syn] as indicated:
#define PRIdNN see below #define PRIiNN see below #define PRIoNN see below #define PRIuNN see below #define PRIxNN see below #define PRIXNN see below #define SCNdNN see below #define SCNiNN see below #define SCNoNN see below #define SCNuNN see below #define SCNxNN see below #define PRIdLEASTNN see below #define PRIiLEASTNN see below #define PRIoLEASTNN see below #define PRIuLEASTNN see below #define PRIxLEASTNN see below #define PRIXLEASTNN see below #define SCNdLEASTNN see below #define SCNiLEASTNN see below #define SCNoLEASTNN see below #define SCNuLEASTNN see below #define SCNxLEASTNN see below #define PRIdFASTNN see below #define PRIiFASTNN see below #define PRIoFASTNN see below #define PRIuFASTNN see below #define PRIxFASTNN see below #define PRIXFASTNN see below #define SCNdFASTNN see below #define SCNiFASTNN see below #define SCNoFASTNN see below #define SCNuFASTNN see below #define SCNxFASTNN see below […]-1- The contents and meaning of the header
-?-<cinttypes>
[…]PRI
macros that use the placeholderN
are defined if and only if the implementation actually provides the corresponding typedef name in 17.4.1 [cstdint.syn].SCN
macros that use the placeholderN
are defined if and only if the implementation actually provides the corresponding typedef name and the implementation has a suitablefscanf
length modifier for the type.
[2019-03-11; Reflector review and improved wording]
Wording simplifications due to new general two's complement requirements of integer types; removal of wording redundancies and applying some typo fixes in macro names.
[2019-03-16; Daniel comments and updates wording]
Hubert Tong pointed out that we do not have a statement about [U]INTPTR_{MIN|MAX}
being optional.
Interestingly, the C11 Standard does not say directly that the [U]INTPTR_{MIN|MAX}
macros are optional,
but this follows indirectly from the fact that intptr_t
and uintptr_t
are indeed optional.
The updated wording therefore realizes Hubert's suggestion.
If and only if the implementation defines such a typedef name, it also defines the corresponding macros.
to:
Each of the macros listed in this subclause is defined if and only if the implementation defines the corresponding typedef name.
and of
PRI
macros that use the placeholder N are defined if and only if the implementation actually defines the corresponding typedef name in 17.4.1 [cstdint.syn].SCN
macros that use the placeholderN
are defined if and only if the implementation actually defines the corresponding typedef name and the implementation has a suitablefscanf
length modifier for the type.
to:
Each of the macros listed in this subclause is defined if and only if the implementation actually defines the corresponding typedef name in 17.4.1 [cstdint.syn].
Those changes have been applied as well.
[2019-03-26; Reflector discussion and minor wording update]
Geoffrey pointed out that the revised wording has the effect that it requires an implementation to define SCN
macros for all mentioned typedefs, but the C11 standard says "the corresponding fscanf
macros shall be
defined unless the implementation does not have a suitable fscanf
length modifier for the type.". An additional
wording update repairs this problem below.
[2020-02-22; Reflector discussion]
Status set to Tentatively Ready after seven positive votes on the reflector.
[2020-11-09 Approved In November virtual meeting. Status changed: Tentatively Ready → WP.]
Proposed resolution:
This wording is relative to N4849.
Change 17.4.1 [cstdint.syn], header <cstdint>
synopsis, as indicated:
[…] using int64_t = signed integer type; // optional using intN_t = see below; // optional, see below […] using int_fast64_t = signed integer type; using int_fastN_t = see below; // optional, see below […] using int_least64_t = signed integer type; using int_leastN_t = see below; // optional, see below […] using uint64_t = unsigned integer type; // optional using uintN_t = see below; // optional, see below […] using uint_fast64_t = unsigned integer type; using uint_fastN_t = see below; // optional, see below […] using uint_least64_t = unsigned integer type; using uint_leastN_t = see below; // optional, see below using uintmax_t = unsigned integer type; using uintptr_t = unsigned integer type; // optional #define INTN_MIN see below #define INTN_MAX see below #define UINTN_MAX see below #define INT_FASTN_MIN see below #define INT_FASTN_MAX see below #define UINT_FASTN_MAX see below #define INT_LEASTN_MIN see below #define INT_LEASTN_MAX see below #define UINT_LEASTN_MAX see below #define INTMAX_MIN see below #define INTMAX_MAX see below #define UINTMAX_MAX see below #define INTPTR_MIN optional, see below #define INTPTR_MAX optional, see below #define UINTPTR_MAX optional, see below #define PTRDIFF_MIN see below #define PTRDIFF_MAX see below #define SIZE_MAX see below #define SIG_ATOMIC_MIN see below #define SIG_ATOMIC_MAX see below #define WCHAR_MIN see below #define WCHAR_MAX see below #define WINT_MIN see below #define WINT_MAX see below #define INTN_C(value) see below #define UINTN_C(value) see below #define INTMAX_C(value) see below #define UINTMAX_C(value) see below
-1- The header also defines numerous macros of the form:INT_[FAST LEAST]{8 16 32 64}_MIN [U]INT_[FAST LEAST]{8 16 32 64}_MAX INT{MAX PTR}_MIN [U]INT{MAX PTR}_MAX {PTRDIFF SIG_ATOMIC WCHAR WINT}{_MAX _MIN} SIZE_MAX
plus function macros of the form:[U]INT{8 16 32 64 MAX}_C-2- The header defines all types and macros the same as the C standard library header
-?- All types that use the placeholder<stdint.h>
. See also: ISO C 7.20N
are optional whenN
is not 8, 16, 32 or 64. The exact-width typesintN_t
anduintN_t
forN
= 8, 16, 32, 64 are also optional; however, if an implementation defines integer types with the corresponding width and no padding bits, it defines the corresponding typedef names. Each of the macros listed in this subclause is defined if and only if the implementation defines the corresponding typedef name. [Note: The macrosINTN_C
andUINTN_C
correspond to the typedef namesint_leastN_t
anduint_leastN_t
, respectively. — end note]
Change 31.13.2 [cinttypes.syn] as indicated:
#define PRIdNN see below #define PRIiNN see below #define PRIoNN see below #define PRIuNN see below #define PRIxNN see below #define PRIXNN see below #define SCNdNN see below #define SCNiNN see below #define SCNoNN see below #define SCNuNN see below #define SCNxNN see below #define PRIdLEASTNN see below #define PRIiLEASTNN see below #define PRIoLEASTNN see below #define PRIuLEASTNN see below #define PRIxLEASTNN see below #define PRIXLEASTNN see below #define SCNdLEASTNN see below #define SCNiLEASTNN see below #define SCNoLEASTNN see below #define SCNuLEASTNN see below #define SCNxLEASTNN see below #define PRIdFASTNN see below #define PRIiFASTNN see below #define PRIoFASTNN see below #define PRIuFASTNN see below #define PRIxFASTNN see below #define PRIXFASTNN see below #define SCNdFASTNN see below #define SCNiFASTNN see below #define SCNoFASTNN see below #define SCNuFASTNN see below #define SCNxFASTNN see below […]-1- The contents and meaning of the header
-?- Each of the<cinttypes>
[…]PRI
macros listed in this subclause is defined if and only if the implementation defines the corresponding typedef name in 17.4.1 [cstdint.syn]. Each of theSCN
macros listed in this subclause is defined if and only if the implementation defines the corresponding typedef name in 17.4.1 [cstdint.syn] and has a suitablefscanf
length modifier for the type.
std::launder()
should be marked as [[nodiscard]]
Section: 17.6.5 [ptr.launder] Status: Resolved Submitter: Tony van Eerd Opened: 2016-11-13 Last modified: 2020-09-06
Priority: 3
View all other issues in [ptr.launder].
View all issues with Resolved status.
Discussion:
As pointed out by Nevin: A use of std::launder
that does not make use of its return value is always
pointless; the function has no side effects.
[2017-01-27 Telecon]
Priority 3; Nico's upcoming paper P0532 should address this and other issues around launder.
[2017-03-04, Kona]
This should be handled post-C++17 by Nico's paper P0600.
[2017-11-11, Albuquerque]
This was resolved by the adoption of P0600R1.
Proposed resolution:
list::sort
should say that the order of elements is unspecified if an exception is thrownSection: 23.3.11.5 [list.ops] Status: C++17 Submitter: Tim Song Opened: 2016-11-16 Last modified: 2017-07-30
Priority: 0
View all other issues in [list.ops].
View all issues with C++17 status.
Discussion:
Sometime between N1638 and N1804 the sentence "If an exception is thrown the order of the elements in the list is indeterminate."
in the specification of list::sort
went missing. This suspiciously coincided with the editorial change that "consolidated definitions of "Stable" in the library clauses" (N1805).
forward_list::sort
says that "If an exception is thrown the order of the elements in *this
is unspecified";
list::sort
should do the same.
[2017-01-27 Telecon]
Priority 0
Proposed resolution:
This wording is relative to N4606.
Edit 23.3.11.5 [list.ops] p29 as indicated:
void sort(); template <class Compare> void sort(Compare comp);-28- Requires:
-29- Effects: Sorts the list according to theoperator<
(for the first version) orcomp
(for the second version) shall define a strict weak ordering (26.8 [alg.sorting]).operator<
or aCompare
function object. If an exception is thrown, the order of the elements in*this
is unspecified. Does not affect the validity of iterators and references. […]
optional
Section: 22.5.3 [optional.optional] Status: Resolved Submitter: Richard Smith Opened: 2016-11-24 Last modified: 2020-11-09
Priority: 2
View all other issues in [optional.optional].
View all issues with Resolved status.
Discussion:
LWG 2756(i) applies these changes:
constexpr optional(const T&); constexpr optional(T&&);template <class... Args> constexpr explicit optional(in_place_t, Args&&...); template <class U, class... Args> constexpr explicit optional(in_place_t, initializer_list<U>, Args&&...); template <class U = T> EXPLICIT constexpr optional(U&&); template <class U> EXPLICIT optional(const optional<U>&); template <class U> EXPLICIT optional(optional<U>&&);
These break the ability for optional
to perform class template argument deduction, as there
is now no way to map from optional
's argument to the template parameter T
.
[2017-01-27 Telecon]
Priority 2
[2017-01-30 Ville comments:]
Seems like the problem is resolved by a simple deduction guide:
template <class T> optional(T) -> optional<T>;
The paper p0433r0 seems to suggest a different guide,
template<class T> optional(T&& t) -> optional<remove_reference_t<T>>;
but I don't think the paper is up to speed with LWG 2756(i). There's no reason
to use such an universal reference in the guide and remove_reference
in its
target, just guide with T
, with the target optional<T>
, optional
's constructors
do the right thing once the type has been deduced.
[2020-05-29; Billy Baker comments]
At Kona 2017, P0433R2 was accepted and added Ville's deduction guide rather than one proposed in P0433R0.
[2020-11-09 Resolved for C++20. Status changed: Tentatively Resolved → Resolved.]
Rationale:
Resolved by P0433R2.Proposed resolution:
string_view
iterators use old wordingSection: 27.3.3.4 [string.view.iterators] Status: C++17 Submitter: Alisdair Meredith Opened: 2016-11-17 Last modified: 2017-07-30
Priority: 0
View all issues with C++17 status.
Discussion:
The wording for basic_string_view
was written before the definition of
a contiguous iterator was added to C++17 to avoid repeating redundant
wording.
Suggested modification of 27.3.3.4 [string.view.iterators] p1 (stealing words from valarray begin
/end
):
-1-
A constant random-access iterator type such that, for aA type that meets the requirements of a constant random access iterator (24.2.7) and of a contiguous iterator (24.2.1) whoseconst_iterator it
, if&*(it + N)
is valid, then&*(it + N) == (&*it) + N
value_type
is the template parametercharT
.
[2017-01-27 Telecon]
Priority 0
Proposed resolution:
This wording is relative to N4606.
Modify 27.3.3.4 [string.view.iterators] as indicated:
using const_iterator = implementation-defined;-1- A
-2- For aconstant random-access iterator type such that, for atype that meets the requirements of a constant random access iterator (24.3.5.7 [random.access.iterators]) and of a contiguous iterator (24.3.1 [iterator.requirements.general]) whoseconst_iterator it
, if&*(it + N)
is valid, then&*(it + N) == (&*it) + N
value_type
is the template parametercharT
.basic_string_view str
, any operation that invalidates a pointer in the range[str.data(), str.data() + str.size())
invalidates pointers, iterators, and references returned fromstr
's methods. -3- All requirements on container iterators (23.2 [container.requirements]) apply tobasic_string_view::const_iterator
as well.
insert_return_type
is only defined for containers with unique keysSection: 23.2.7 [associative.reqmts], 23.2.8 [unord.req] Status: Resolved Submitter: Daniel James Opened: 2016-11-24 Last modified: 2020-09-06
Priority: Not Prioritized
View other active issues in [associative.reqmts].
View all other issues in [associative.reqmts].
View all issues with Resolved status.
Discussion:
The requirements for associative and unordered associative containers list insert_return_type
for all
containers, but looking at the class description in 23.4.3.1 [map.overview], 23.4.4.1 [multimap.overview]
etc. it's only defined for containers with unique keys. Jonathan Wakely pointed out that this depends on the
resolution of LWG 2772(i), which might remove insert_return_type
completely.
insert_return_type
(map
and set
only)' and in 23.2.8 [unord.req]: 'insert_return_type
(unordered_map
and unordered_set
only)'. Or maybe something like
'(containers with unique keys only)'.
[Issues Telecon 16-Dec-2016]
Resolved by adoption of P0508R0.
Proposed resolution:
Hash
function objects have different behaviourSection: 23.2.8 [unord.req] Status: Resolved Submitter: Daniel James Opened: 2016-11-24 Last modified: 2020-09-06
Priority: Not Prioritized
View other active issues in [unord.req].
View all other issues in [unord.req].
View all issues with Resolved status.
Discussion:
In 23.2.8 [unord.req] paragraph 12, it says that the behaviour of operator==
is undefined unless the
Hash
and Pred
function objects respectively have the same behaviour. This makes comparing containers
with randomized hashes with different seeds undefined behaviour, but I think that's a valid use case. It's not much
more difficult to support it when the Hash
function objects behave differently. I did a little testing and
both libstdc++ and libc++ appear to support this correctly.
operator==
or operator!=
on unordered containers is undefined unless the Hash
and
Pred
function refinement
"
[2017-01-27 Telecon]
This is a design issue; send to LEWG
[2018-3-17 Resolved by P0809R0, which was adopted in Jacksonville.]
Proposed resolution:
This wording is relative to N4618.
Change 23.2.8 [unord.req] as indicated:
-12- Two unordered containers
a
andb
compare equal ifa.size() == b.size()
and, for every equivalent-key group[Ea1, Ea2)
obtained froma.equal_range(Ea1)
, there exists an equivalent-key group[Eb1, Eb2)
obtained fromb.equal_range(Ea1)
, such thatis_permutation(Ea1, Ea2, Eb1, Eb2)
returnstrue
. Forunordered_set
andunordered_map
, the complexity ofoperator==
(i.e., the number of calls to the==
operator of thevalue_type
, to the predicate returned bykey_eq()
, and to the hasher returned byhash_function()
) is proportional toN
in the average case and toN2
in the worst case, whereN
isa.size()
. Forunordered_multiset
andunordered_multimap
, the complexity ofoperator==
is proportional to ∑Ei2
in the average case and toN2
in the worst case, whereN
is a.size(), andEi
is the size of thei
th equivalent-key group ina
. However, if the respective elements of each corresponding pair of equivalent-key groupsEai
andEbi
are arranged in the same order (as is commonly the case, e.g., ifa
andb
are unmodified copies of the same container), then the average-case complexity forunordered_multiset
andunordered_multimap
becomes proportional toN
(but worst-case complexity remains 𝒪(N2
), e.g., for a pathologically bad hash function). The behavior of a program that usesoperator==
oroperator!=
on unordered containers is undefined unless theHash
andPred
function objects respectively havehas the same behavior for both containers and the equality comparison operator forKey
is a refinement(footnote 258) of the partition into equivalent-key groups produced byPred
.
P(i)
Section: 31.5.3.3 [fpos.operations] Status: Resolved Submitter: Jens Maurer Opened: 2016-11-24 Last modified: 2020-05-12
Priority: 3
View all other issues in [fpos.operations].
View all issues with Resolved status.
Discussion:
This is from editorial issue #1031.
The first row in Table 112 "Position type requirements" talks about the expressionP(i)
and then has an assertion
p == P(i)
. However, there are no constraints on p
other than being of type P
, so (on the face of it) this
seems to require that operator==
on P
always returns
true, which is non-sensical.
[2017-01-27 Telecon]
Priority 3
[2019-03-17; Daniel comments]
With the acceptance of P0759R1 at the Rapperswil 2018 meeting this issue should be closed as Resolved.
[2020-05-12; Reflector discussions]
Resolved by P0759R1.
Rationale:
Resolved by P0759R1.Proposed resolution:
Section: 27.4.3.5 [string.capacity], 23.3.5.3 [deque.capacity], 23.3.13.3 [vector.capacity] Status: C++17 Submitter: Thomas Koeppe Opened: 2016-11-29 Last modified: 2020-09-06
Priority: 0
View all other issues in [string.capacity].
View all issues with C++17 status.
Discussion:
The resolution of LWG 2223(i) added the wording "Reallocation invalidates all the references, pointers,
and iterators referring to the elements in the sequence." to a number of shrink_to_fit
operations.
string
, deque
, and vector
each, append as follows:
Reallocation invalidates all the references, pointers, and iterators referring to the elements in the sequence as
well as the past-the-end iterator.
[2017-01-27 Telecon]
Priority 0
Proposed resolution:
This wording is relative to N4618.
Edit 27.4.3.5 [string.capacity] as indicated:
void shrink_to_fit();[…]
-15- Remarks: Reallocation invalidates all the references, pointers, and iterators referring to the elements in the sequence as well as the past-the-end iterator. If no reallocation happens, they remain valid.
Edit 23.3.5.3 [deque.capacity] as indicated:
void shrink_to_fit();[…]
-8- Remarks:shrink_to_fit
invalidates all the references, pointers, and iterators referring to the elements in the sequence as well as the past-the-end iterator.
Edit 23.3.13.3 [vector.capacity] as indicated:
void shrink_to_fit();[…]
-10- Remarks: Reallocation invalidates all the references, pointers, and iterators referring to the elements in the sequence as well as the past-the-end iterator. If no reallocation happens, they remain valid.
<tgmath.h>
Section: 17.15 [support.c.headers], 99 [depr.ctgmath.syn] Status: C++17 Submitter: Thomas Koeppe Opened: 2016-11-29 Last modified: 2023-02-07
Priority: 0
View all other issues in [support.c.headers].
View all issues with C++17 status.
Discussion:
The resolution of LWG 2536(i) touches on the specification of C headers (of the form foo.h
), but
while it fixes the specification of complex.h
, it fails to address the specification of tgmath.h
.
complex.h
, tgmath.h
is not defined by C. Rather, our tgmath.h
behaves like <ctgmath>
, which is specified in [ctgmath.syn].
Suggested resolution:
Amend [depr.c.headers] p3 as follows:
3. The header
4. Every other C header […]<complex.h>
behaves as if it simply includes the header<ccomplex>
. The header<tgmath.h>
behaves as if it simply includes the header<ctgmath>
.
[2017-01-27 Telecon]
Priority 0
Proposed resolution:
This wording is relative to N4618.
Edit [depr.c.headers] as indicated:
[…]
-2- The use of any of the C++ headers […] is deprecated. -3- The header<complex.h>
behaves as if it simply includes the header<ccomplex>
. The header<tgmath.h>
behaves as if it simply includes the header<ctgmath>
. -4- Every other C header, […]
noexcept
Section: 27.4.3 [basic.string], 27.4.3.8.2 [string.find], 27.4.3.8.4 [string.compare] Status: Resolved Submitter: Jonathan Wakely Opened: 2016-12-05 Last modified: 2023-02-07
Priority: 2
View other active issues in [basic.string].
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Discussion:
Currently some overloads of basic_string::find
are noexcept
and some are not. Historically this
was because some were specified in terms of constructing a temporary basic_string
, which could throw. In
practice creating a temporary (and potentially allocating memory) is a silly implementation, and so they could be
noexcept
. In the C++17 draft most of them have been changed to create a temporary basic_string_view
instead, which can't throw anyway (P0254R2 made those changes).
find
, rfind
, find_first_of
, find_last_of
, find_first_not_of
and
find_last_not_of
overloads that are defined in terms of basic_string_view
should be noexcept
,
or "Throws: Nothing." for the ones with narrow contracts (even though those narrow contracts are not enforcable
or testable).
The remaining overloads that are still specified in terms of a temporary string could also be noexcept
. They
construct basic_string
of length one (which won't throw for an SSO implementation anyway), but can easily be
defined in terms of basic_string_view
instead.
There's one basic_string::compare
overload that is still defined in terms of a temporary basic_string
,
which should be basic_string_view
and so can also be noexcept
(the other compare
overloads can
throw out_of_range
).
[2016-12-15, Tim Song comments]
The following overloads invoking basic_string_view<charT>(s, n)
are implicitly narrow-contract
(the basic_string_view
constructor requires [s, s+n)
to be a valid range) and should be
"Throws: Nothing" rather than noexcept
:
size_type find(const charT* s, size_type pos, size_type n) const; size_type rfind(const charT* s, size_type pos, size_type n) const; size_type find_first_of(const charT* s, size_type pos, size_type n) const; size_type find_last_of(const charT* s, size_type pos, size_type n) const; size_type find_first_not_of(const charT* s, size_type pos, size_type n) const; size_type find_last_not_of(const charT* s, size_type pos, size_type n) const;
Similarly, the basic_string_view
constructor invoked by this overload of compare
requires
[s, s + traits::length(s))
to be a valid range, and so it should be "Throws: Nothing" rather
than noexcept
:
int compare(const charT* s) const;
[2017-01-27 Telecon]
Priority 2; Jonathan to provide updated wording
[2017-10-22, Marshall comments]
libc++ already has all of these member fns marked as noexcept
[2017-11 Albuquerque Saturday issues processing]
All the fns that take a const char *
are narrow contract, and so can't be noexcept. Should be "Throws: Nothing" instead. Alisdair to re-word.
[2018-08 mailing list discussion]
This will be resolved by Tim's string rework paper.
Resolved by the adoption of P1148 in San Diego.
Proposed resolution:
This wording is relative to N4618.
Change class template synopsis std::basic_string
, 27.4.3 [basic.string], as indicated:
size_type find (basic_string_view<charT, traits> sv, size_type pos = 0) const noexcept; size_type find (const basic_string& str, size_type pos = 0) const noexcept; size_type find (const charT* s, size_type pos, size_type n) const noexcept; size_type find (const charT* s, size_type pos = 0) const; size_type find (charT c, size_type pos = 0) const noexcept; size_type rfind(basic_string_view<charT, traits> sv, size_type pos = npos) const noexcept; size_type rfind(const basic_string& str, size_type pos = npos) const noexcept; size_type rfind(const charT* s, size_type pos, size_type n) const noexcept; size_type rfind(const charT* s, size_type pos = npos) const; size_type rfind(charT c, size_type pos = npos) const noexcept; size_type find_first_of(basic_string_view<charT, traits> sv, size_type pos = 0) const noexcept; size_type find_first_of(const basic_string& str, size_type pos = 0) const noexcept; size_type find_first_of(const charT* s, size_type pos, size_type n) const noexcept; size_type find_first_of(const charT* s, size_type pos = 0) const; size_type find_first_of(charT c, size_type pos = 0) const noexcept; size_type find_last_of (basic_string_view<charT, traits> sv, size_type pos = npos) const noexcept; size_type find_last_of (const basic_string& str, size_type pos = npos) const noexcept; size_type find_last_of (const charT* s, size_type pos, size_type n) const noexcept; size_type find_last_of (const charT* s, size_type pos = npos) const; size_type find_last_of (charT c, size_type pos = npos) const noexcept; size_type find_first_not_of(basic_string_view<charT, traits> sv, size_type pos = 0) const noexcept; size_type find_first_not_of(const basic_string& str, size_type pos = 0) const noexcept; size_type find_first_not_of(const charT* s, size_type pos, size_type n) const noexcept; size_type find_first_not_of(const charT* s, size_type pos = 0) const; size_type find_first_not_of(charT c, size_type pos = 0) const noexcept; size_type find_last_not_of (basic_string_view<charT, traits> sv, size_type pos = npos) const noexcept; size_type find_last_not_of (const basic_string& str, size_type pos = npos) const noexcept; size_type find_last_not_of (const charT* s, size_type pos, size_type n) const noexcept; size_type find_last_not_of (const charT* s, size_type pos = npos) const; size_type find_last_not_of (charT c, size_type pos = npos) const noexcept; basic_string substr(size_type pos = 0, size_type n = npos) const; int compare(basic_string_view<charT, traits> sv) const noexcept; int compare(size_type pos1, size_type n1, basic_string_view<charT, traits> sv) const; template<class T> int compare(size_type pos1, size_type n1, const T& t, size_type pos2, size_type n2 = npos) const; int compare(const basic_string& str) const noexcept; int compare(size_type pos1, size_type n1, const basic_string& str) const; int compare(size_type pos1, size_type n1, const basic_string& str, size_type pos2, size_type n2 = npos) const; int compare(const charT* s) const noexcept; int compare(size_type pos1, size_type n1, const charT* s) const; int compare(size_type pos1, size_type n1, const charT* s, size_type n2) const;
Change 27.4.3.8.2 [string.find] as indicated:
size_type find(const charT* s, size_type pos, size_type n) const noexcept;-5- Returns:
find(basic_string_view<charT, traits>(s, n), pos)
.size_type find(const charT* s, size_type pos = 0) const;-6- Requires:
-7- Returns:s
points to an array of at leasttraits::length(s) + 1
elements ofcharT
.find(basic_string_view<charT, traits>(s), pos)
. -?- Throws: Nothing.size_type find(charT c, size_type pos = 0) const noexcept;-8- Returns:
find(basic_string_view<charT, traits>(addressof(c), 1)
.(1, c), pos)
Change [string.rfind] as indicated:
size_type rfind(const charT* s, size_type pos, size_type n) const noexcept;-5- Returns:
rfind(basic_string_view<charT, traits>(s, n), pos)
.size_type rfind(const charT* s, size_type pos = npos) const;-6- Requires:
-7- Returns:s
points to an array of at leasttraits::length(s) + 1
elements ofcharT
.rfind(basic_string_view<charT, traits>(s), pos)
. -?- Throws: Nothing.size_type rfind(charT c, size_type pos = npos) const noexcept;-8- Returns:
rfind(basic_string_view<charT, traits>(addressof(c), 1)
.(1, c), pos)
Change [string.find.first.of] as indicated:
size_type find_first_of(const charT* s, size_type pos, size_type n) const noexcept;-5- Returns:
find_first_of(basic_string_view<charT, traits>(s, n), pos)
.size_type find_first_of(const charT* s, size_type pos = 0) const;-6- Requires:
-7- Returns:s
points to an array of at leasttraits::length(s) + 1
elements ofcharT
.find_first_of(basic_string_view<charT, traits>(s), pos)
. -?- Throws: Nothing.size_type find_first_of(charT c, size_type pos = 0) const noexcept;-8- Returns:
find_first_of(basic_string_view<charT, traits>(addressof(c), 1)
.(1, c), pos)
Change [string.find.last.of] as indicated:
size_type find_last_of(const charT* s, size_type pos, size_type n) const noexcept;-5- Returns:
find_last_of(basic_string_view<charT, traits>(s, n), pos)
.size_type find_last_of(const charT* s, size_type pos = npos) const;-6- Requires:
-7- Returns:s
points to an array of at leasttraits::length(s) + 1
elements ofcharT
.find_last_of(basic_string_view<charT, traits>(s), pos)
. -?- Throws: Nothing.size_type find_last_of(charT c, size_type pos = npos) const noexcept;-8- Returns:
find_last_of(basic_string_view<charT, traits>(addressof(c), 1)
.(1, c), pos)
Change [string.find.first.not.of] as indicated:
size_type find_first_not_of(const charT* s, size_type pos, size_type n) const noexcept;-5- Returns:
find_first_not_of(basic_string_view<charT, traits>(s, n), pos)
.size_type find_first_not_of(const charT* s, size_type pos = 0) const;-6- Requires:
-7- Returns:s
points to an array of at leasttraits::length(s) + 1
elements ofcharT
.find_first_not_of(basic_string_view<charT, traits>(s), pos)
. -?- Throws: Nothing.size_type find_first_not_of(charT c, size_type pos = 0) const noexcept;-8- Returns:
find_first_not_of(basic_string_view<charT, traits>(addressof(c), 1)
.(1, c), pos)
Change [string.find.last.not.of] as indicated:
size_type find_last_not_of(const charT* s, size_type pos, size_type n) const noexcept;-5- Returns:
find_last_not_of(basic_string_view<charT, traits>(s, n), pos)
.size_type find_last_not_of(const charT* s, size_type pos = npos) const;-6- Requires:
-7- Returns:s
points to an array of at leasttraits::length(s) + 1
elements ofcharT
.find_last_not_of(basic_string_view<charT, traits>(s), pos)
. -?- Throws: Nothing.size_type find_last_not_of(charT c, size_type pos = npos) const noexcept;-8- Returns:
find_last_not_of(basic_string_view<charT, traits>(addressof(c), 1)
.(1, c), pos)
Change 27.4.3.8.4 [string.compare] as indicated:
int compare(const charT* s) const noexcept;-9- Returns:
compare(basic_string_view<charT, traits>(s))
.
gcd
and lcm
should support a wider range of input valuesSection: 26.10.14 [numeric.ops.gcd], 26.10.15 [numeric.ops.lcm] Status: C++17 Submitter: Marshall Clow Opened: 2016-12-16 Last modified: 2020-09-06
Priority: 0
View all other issues in [numeric.ops.gcd].
View all issues with C++17 status.
Discussion:
This is a duplicate of 2792(i), which addressed LFTS 2.
By the current definition, gcd((int64_t)1234, (int32_t)-2147483648)
is
ill-formed (because 2147483648
is not representable as a value of int32_t
.)
We want to change this case to be well-formed. As long as both |m|
and |n|
are representable as values of the common type, absolute values can
be calculate d
without causing unspecified behavior, by converting m
and n
to the common type before taking the negation.
|m|
shall be representable as a value of typeM
and|n|
shall be representable as a value of typeN
|m|
and|n|
shall be representable as a value ofcommon_type_t<M, N>
.
[2017-01-27 Telecon]
Priority 0
Proposed resolution:
This wording is relative to N4604.
Edit 26.10.14 [numeric.ops.gcd] as indicated:
template<class M, class N> constexpr common_type_t<M, N> gcd(M m, N n);-2- Requires:
|m|
shall be representable as a value of typeM
and|n|
shall be representable as a value of typeN
|m|
and|n|
shall be representable as a value ofcommon_type_t<M, N>
. [Note: These requirements ensure, for example, thatgcd(m, m) = |m|
is representable as a value of typeM
. — end note]
Edit 26.10.15 [numeric.ops.lcm] as indicated:
template<class M, class N> constexpr common_type_t<M, N> lcm(M m, N n);-2- Requires:
|m|
shall be representable as a value of typeM
and|n|
shall be representable as a value of typeN
|m|
and|n|
shall be representable as a value ofcommon_type_t<M, N>
. The least common multiple of|m|
and|n|
shall be representable as a value of typecommon_type_t<M, N>
.
is_literal_type
specification needs a little cleanupSection: D.13 [depr.meta.types] Status: C++17 Submitter: Tim Song Opened: 2016-12-09 Last modified: 2020-09-06
Priority: 0
View all other issues in [depr.meta.types].
View all issues with C++17 status.
Discussion:
D.13 [depr.meta.types]/3 currently reads:
Effects:
is_literal_type
has a base-characteristic oftrue_type
ifT
is a literal type ([basic.types]), and a base-characteristic offalse_type
otherwise.
First, this doesn't seem like an Effects clause. Second, this wording fails to say that is_literal_type
is an UnaryTypeTrait
, and misspells BaseCharacteristic
— which is only defined for
UnaryTypeTraits
and BinaryTypeTraits
. Third, moving this to Annex D means that the general prohibition
against specializing type traits in [meta.type.synop]/1 no longer applies, which is presumably unintended.
[2017-01-27 Telecon]
Priority 0
Proposed resolution:
This wording is relative to N4618.
Edit D.13 [depr.meta.types] as indicated:
The header
<type_traits>
has the following addition:namespace std { template <class T> struct is_literal_type; template <class T> constexpr bool is_literal_type_v = is_literal_type<T>::value; }-2- Requires:
-3-remove_all_extents_t<T>
shall be a complete type or (possibly cv-qualified)void
.Effects:is_literal_type
has a base-characteristic oftrue_type
ifT
is a literal type (3.9), and a basecharacteristic offalse_type
otherwiseis_literal_type<T>
is aUnaryTypeTrait
(21.3.2 [meta.rqmts]) with aBaseCharacteristic
oftrue_type
ifT
is a literal type (6.8 [basic.types]), andfalse_type
otherwise. -?- The behavior of a program that adds specializations foris_literal_type
oris_literal_type_v
is undefined.
Section: 16.4.6.17 [lib.types.movedfrom], 16.4.5.9 [res.on.arguments], 23.2.2 [container.requirements.general] Status: C++23 Submitter: Tim Song Opened: 2016-12-09 Last modified: 2023-11-22
Priority: 2
View all issues with C++23 status.
Discussion:
LWG 2468(i)'s resolution added to MoveAssignable
the requirement to tolerate self-move-assignment,
but that does nothing for library types that aren't explicitly specified to meet MoveAssignable
other than make
those types not meet MoveAssignable
any longer.
[2017-01-27 Telecon]
Priority 2
[2018-1-26 issues processing telecon]
Status to 'Open'; Howard to reword using 'MoveAssignable'.
Previous resolution [SUPERSEDED]:
This wording is relative to N4618.
Add a new paragraph at the end of 16.4.6.17 [lib.types.movedfrom]:
-1- Objects of types defined in the C++ standard library may be moved from (12.8). Move operations may be explicitly specified or implicitly generated. Unless otherwise specified, such moved-from objects shall be placed in a valid but unspecified state.
-?- An object of a type defined in the C++ standard library may be move-assigned (11.4.6 [class.copy.assign]) to itself. Such an assignment places the object in a valid but unspecified state unless otherwise specified.Add a note at the end of 16.4.5.9 [res.on.arguments]/1, bullet 3, as indicated:
-1- Each of the following applies to all arguments to functions defined in the C++ standard library, unless explicitly stated otherwise.
(1.1) — […]
(1.2) — […]
(1.3) — If a function argument binds to an rvalue reference parameter, the implementation may assume that this parameter is a unique reference to this argument. [Note: If the parameter is a generic parameter of the form
T&&
and an lvalue of typeA
is bound, the argument binds to an lvalue reference (14.8.2.1) and thus is not covered by the previous sentence. — end note] [Note: If a program casts an lvalue to an xvalue while passing that lvalue to a library function (e.g. by calling the function with the argumentstd::move(x)
), the program is effectively asking that function to treat that lvalue as a temporary. The implementation is free to optimize away aliasing checks which might be needed if the argument was an lvalue. — end note] [Note: This does not apply to the argument passed to a move assignment operator (16.4.6.17 [lib.types.movedfrom]). — end note]Edit Table 83 "Container requirements" in 23.2.2 [container.requirements.general] as indicated:
Table 83 — Container requirements Expression Return type Operational
semanticsAssertion/note
pre-/post-conditionComplexity …
a = rv
T&
All existing elements of a
are either move
assigned to or
destroyedpost: If a
andrv
do not refer to the same object,
a
shall be equal to the value that
rv
had before this assignmentlinear …
Edit Table 86 "Allocator-aware container requirements" in 23.2.2 [container.requirements.general] as indicated:
Table 86 — Allocator-aware container requirements Expression Return type Assertion/note
pre-/post-conditionComplexity …
a = rv
T&
Requires: If allocator_traits<allocator_type
>::propagate_on_container_move_assignment::value
isfalse
,T
isMoveInsertable
intoX
andMoveAssignable
.
All existing elements ofa
are either
move assigned to or destroyed.
post: Ifa
andrv
do not refer
to the same object,a
shall be equal
to the value thatrv
had before this assignmentlinear …
[2018-08-16, Howard comments and provides updated wording]
I agreed to provide proposed wording for LWG 2839 that was reworded to use MoveAssignable
. The advantage of
this is that MoveAssignable
specifies the self-assignment case, thus we do not need to repeat ourselves.
[2018-08-23 Batavia Issues processing]
Howard and Tim to discuss a revised P/R.
Previous resolution [SUPERSEDED]:
This wording is relative to N4762.
Add a new subsection to 16.4.6 [conforming] after 16.4.6.5 [member.functions]:
Special members [conforming.special]
Class types defined by the C++ standard library and specified to be default constructible, move constructible, copy constructible, move assignable, copy assignable, or destructible, shall meet the associated requirements Cpp17DefaultConstructible, Cpp17MoveConstructible, Cpp17CopyConstructible, Cpp17MoveAssignable, Cpp17CopyAssignable, and Cpp17Destructible, respectively (16.4.4.2 [utility.arg.requirements]).
[2020-06-06 Tim restores and updates P/R following 2020-05-29 telecon discussion]
The standard doesn't define phrases like "default constructible" used in the
previous P/R. Moreover, the library provides a variety of wrapper types, and
whether these types meet the semantic requirements of Cpp17Meowable
(and maybe even syntactic, depending on how "copy constructible" is interpreted)
depends on the property of their underlying wrapped types, which might not even
be an object type (e.g., tuple
or pair
of references). This is
a large can of worms (see LWG 2146(i)) that we don't want to get into.
assignable_from
(18.4.8 [concept.assignable])
contains a note alluding to these cases and suggesting that they should be
considered to be outside the domain of =
entirely.
[2020-07-17; issue processing telecon]
LWG reviewed the latest proposed resolution. Unanimous consent to move to Ready.
[2020-11-09 Approved In November virtual meeting. Status changed: Ready → WP.]
Proposed resolution:
This wording is relative to N4861.
Add a new paragraph at the end of 16.4.6.17 [lib.types.movedfrom]:
-1- Objects of types defined in the C++ standard library may be moved from ( [clss.copy.ctor]). Move operations may be explicitly specified or implicitly generated. Unless otherwise specified, such moved-from objects shall be placed in a valid but unspecified state.
-?- An object of a type defined in the C++ standard library may be move-assigned (11.4.6 [class.copy.assign]) to itself. Unless otherwise specified, such an assignment places the object in a valid but unspecified state.
Edit 16.4.5.9 [res.on.arguments]/1, bullet 3, as indicated:
-1- Each of the following applies to all arguments to functions defined in the C++ standard library, unless explicitly stated otherwise.
(1.1) — […]
(1.2) — […]
(1.3) — If a function argument binds to an rvalue reference parameter, the implementation may assume that this parameter is a unique reference to this argument, except that the argument passed to a move-assignment operator may be a reference to
*this
(16.4.6.17 [lib.types.movedfrom]). [Note: If the type of a parameter is ageneric parameter of the formforwarding reference (13.10.3.2 [temp.deduct.call]) that is deduced to an lvalue reference type, then the argument is not bound to an rvalue reference. — end note] [Note: If a program casts an lvalue to an xvalue while passing that lvalue to a library function (e.g. by calling the function with the argumentT&&
and an lvalue of typeA
is bound, the argument binds to an lvalue reference (13.10.3.2 [temp.deduct.call]) and thus is not covered by the previous sentence.std::move(x)
), the program is effectively asking that function to treat that lvalue as a temporary. The implementation is free to optimize away aliasing checks which might be needed if the argument was an lvalue. — end note]
Edit Table 73 "Container requirements" in 23.2.2 [container.requirements.general] as indicated:
Table 73 — Container requirements Expression Return type Operational
semanticsAssertion/note
pre-/post-conditionComplexity …
a = rv
T&
All existing elements of a
are either move assigned to or destroyedPostconditions: If a
andrv
do not refer to the same object,a
is equal to the value thatrv
had before this assignment.linear …
Edit Table 76 "Allocator-aware container requirements" in 23.2.2 [container.requirements.general] as indicated:
Table 86 — Allocator-aware container requirements Expression Return type Assertion/note
pre-/post-conditionComplexity …
a = rv
T&
Preconditions: If allocator_traits<allocator_type>
is
::propagate_on_container_move_assignment::valuefalse
,
T
is Cpp17MoveInsertable intoX
and Cpp17MoveAssignable.
Effects: All existing elements ofa
are either move assigned to or destroyed.
Postconditions: Ifa
andrv
do not refer to the same object,a
is equal to the value thatrv
had before this assignment.linear …
directory_iterator::increment
is seemingly narrow-contract but marked noexcept
Section: 31.12.11.2 [fs.dir.itr.members], 31.12.12.2 [fs.rec.dir.itr.members] Status: Resolved Submitter: Tim Song Opened: 2016-12-09 Last modified: 2020-09-06
Priority: 2
View all other issues in [fs.dir.itr.members].
View all issues with Resolved status.
Discussion:
directory_iterator::increment
and recursive_directory_iterator::increment
are specified by
reference to the input iterator requirements, which is narrow-contract: it has a precondition that the iterator
is dereferenceable. Yet they are marked as noexcept
.
noexcept
(one of which was added by LWG 2637(i)) should be removed, or the
behavior of increment
when given a nondereferenceable iterator should be specified.
Currently, libstdc++ and MSVC report an error via the error_code
argument for a nondereferenceable
directory_iterator
, while libc++ and boost::filesystem
assert.
[2017-01-27 Telecon]
Priority 2; there are some problems with the wording in alternative B.
[2018-01-24 Tim Song comments]
LWG 3013(i) will remove this noexcept
(for a different reason). The behavior of
calling increment
on a nondereferenceble iterator should still be clarified, as I was informed
that LWG wanted it to be well-defined.
[2018-1-26 issues processing telecon]
Part A is already done by 3013(i).
Marshall to talk to Tim about Part B; it would be odd if operator++
was undefined behaviour, but increment
on the same iterator returned an error
[2018-8-15 Offline discussions]
Tim and Marshall recommend NAD for about Part B
[2018-08-20 Batavia Issues processing]
Status to resolved.
Proposed resolution:
This wording is relative to N4618.
Alternative A (remove the noexcept
):
Edit [fs.class.directory_iterator], class directory_iterator
synopsis, as indicated:
directory_iterator& operator++(); directory_iterator& increment(error_code& ec)noexcept;
Edit 31.12.11.2 [fs.dir.itr.members] before p10 as indicated:
directory_iterator& operator++(); directory_iterator& increment(error_code& ec)noexcept;-10- Effects: As specified for the prefix increment operation of Input iterators (24.2.3).
-11- Returns:*this
. -12- Throws: As specified in 27.10.7.
Edit 31.12.12 [fs.class.rec.dir.itr], class recursive_directory_iterator
synopsis, as indicated:
recursive_directory_iterator& operator++(); recursive_directory_iterator& increment(error_code& ec)noexcept;
Edit 31.12.12.2 [fs.rec.dir.itr.members] before p23 as indicated:
recursive_directory_iterator& operator++(); recursive_directory_iterator& increment(error_code& ec)noexcept;-23- Effects: As specified for the prefix increment operation of Input iterators (24.2.3), except that: […]
-24- Returns:*this
. -25- Throws: As specified in 27.10.7.
Alternative B (specify that increment
reports an error if the iterator is not dereferenceable):
Edit 31.12.11.2 [fs.dir.itr.members] p10 as indicated:
directory_iterator& operator++(); directory_iterator& increment(error_code& ec) noexcept;-10- Effects: As specified for the prefix increment operation of Input iterators (24.2.3), except that
-11- Returns:increment
reports an error if*this
is not dereferenceable.*this
. -12- Throws: As specified in 27.10.7.
Edit 31.12.12.2 [fs.rec.dir.itr.members] p23 as indicated:
recursive_directory_iterator& operator++(); recursive_directory_iterator& increment(error_code& ec) noexcept;-23- Effects: As specified for the prefix increment operation of Input iterators (24.2.3), except that:
increment
reports an error if*this
is not dereferenceable.If there are no more entries at the current depth, […]
Otherwise if […]
-24- Returns:
-25- Throws: As specified in 27.10.7.*this
.
Section: 27 [strings] Status: Resolved Submitter: Jeffrey Yasskin Opened: 2016-12-09 Last modified: 2018-11-25
Priority: 3
View all other issues in [strings].
View all issues with Resolved status.
Discussion:
Reported via editorial issue #165:
The following places should probably use the "Equivalent to" wording introduced in [structure.specifications]/4.
[string.cons]/18
Effects: Same as
basic_string(il.begin(), il.end(), a)
.[string.capacity]/3
Returns:
size()
.[string.capacity]/8
Effects: As if by
resize(n, charT())
.[string.capacity]/16
Effects: Behaves as if the function calls:
erase(begin(), end());
[string.capacity]/17
Returns:
size() == 0
.[string.insert]/27
Effects: As if by
Returns: An iterator which refers to the copy of the first inserted character, orinsert(p, il.begin(), il.end())
.p
ifi1
is empty.
[2017-01-27 Telecon]
Priority 3; Marshall to provide wording.
Resolved by the adoption of P1148 in San Diego.
Proposed resolution:
in_place_t
check for optional::optional(U&&)
should decay U
Section: 22.5.3.2 [optional.ctor] Status: C++17 Submitter: Tim Song Opened: 2016-12-13 Last modified: 2020-09-06
Priority: 0
View all other issues in [optional.ctor].
View all issues with C++17 status.
Discussion:
As in_place_t
is a normal tag type again, we need to decay U
before doing the is_same_v
check.
[2017-01-27 Telecon]
Priority 0
Proposed resolution:
This wording is relative to N4618.
Edit 22.5.3.2 [optional.ctor] as indicated:
template <class U = T> EXPLICIT constexpr optional(U&& v);[…]
-22- Remarks: IfT
's selected constructor is aconstexpr
constructor, this constructor shall be aconstexpr
constructor. This constructor shall not participate in overload resolution unlessis_constructible_v<T, U&&>
istrue
,is_same_v<decay_t<U>, in_place_t>
isfalse
, andis_same_v<optional<T>, decay_t<U>>
isfalse
. The constructor is explicit if and only ifis_convertible_v<U&&, T>
isfalse
.
std::pmr::memory_resource::do_allocate()
Section: 20.5.2.3 [mem.res.private] Status: C++20 Submitter: Jens Maurer Opened: 2016-12-13 Last modified: 2021-02-25
Priority: 3
View all other issues in [mem.res.private].
View all issues with C++20 status.
Discussion:
The specification of do_allocate()
(20.5.2.3 [mem.res.private] p2+p3) says:
Returns: A derived class shall implement this function to return a pointer to allocated storage (3.7.4.2) with a size of at least
Throws: A derived class implementation shall throw an appropriate exception if it is unable to allocate memory with the requested size and alignment.bytes
. The returned storage is aligned to the specified alignment, if such alignment is supported (3.11); otherwise it is aligned tomax_align
.
It is unclear whether a request for an unsupported alignment
(e.g. larger than max_align
) yields an exception or the returned
storage is silently aligned to max_align
.
[2017-01-27 Telecon]
Priority 3; Marshall to ping Pablo for intent and provide wording.
[2017-02-12 Pablo responds and provides wording]
The original intent was:
However, the description of do_allocate might have gone stale as the aligned-allocation proposal made its way into the standard.
The understanding I take from the definition of extended alignment in
(the current text of) 3.11/3 [basic.align] and "assembling an argument
list" in 5.3.4/14 [expr.new] is that it is intended that, when
allocating space for an object with extended alignment in a well-formed
program, the alignment will be honored and will not be
truncated to max_align. I think this is a change from earlier drafts of
the extended-alignment proposal, where silent truncation to max_align
was permitted (I could be wrong). Anyway, it seems wrong to ever ignore
the alignment parameter in do_allocate()
.
[2017-11 Albuquerque Wednesday issue processing]
Move to Ready.
[2018-3-17 Adopted in Jacksonville]
Proposed resolution:
Change the specification of do_allocate()
(20.5.2.3 [mem.res.private] p2+p3) as follows:
Returns: A derived class shall implement this function to return a pointer to allocated storage (3.7.4.2) with a size of at least
bytes
, aligned to the specifiedalignment
.The returned storage is aligned to the specified alignment, if such alignment is supported; otherwise it is aligned tomax_align
.Throws: A derived class implementation shall throw an appropriate exception if it is unable to allocate memory with the requested size and alignment.
!is_regular_file(from)
cause copy_file
to report a "file already exists" error?Section: 31.12.13.5 [fs.op.copy.file] Status: C++20 Submitter: Tim Song Opened: 2016-12-17 Last modified: 2021-06-06
Priority: 2
View all other issues in [fs.op.copy.file].
View all issues with C++20 status.
Discussion:
[fs.op.copy_file]/4 says that copy_file
reports "a file already exists error as specified in
[fs.err.report] if" any of several error conditions exist.
!is_regular_file(from)
, can be sensibly described
as "file already exists". Pretty much everywhere else in the filesystem specification just says "an error" without
further elaboration.
[2017-01-27 Telecon]
Priority 2; Jonathan to provide updated wording.
[2018-01-16, Jonathan comments]
I said I'd provide updated wording because I wanted to preserve the requirement that the reported error is "file already exists" for some of the error cases. I no longer think that's necessary, so I think the current P/R is fine. Please note that I don't think new wording is needed.
[ 2018-01-23 Moved to Tentatively Ready after 6 positive votes on c++std-lib. ]
[2018-3-17 Adopted in Jacksonville]
Proposed resolution:
This wording is relative to N4713.
Edit [fs.op.copy_file]/4 as indicated:
bool copy_file(const path& from, const path& to, copy_options options); bool copy_file(const path& from, const path& to, copy_options options, error_code& ec) noexcept;-4- Effects: As follows:
(4.1) — Report
a file already existsan error as specified in 31.12.5 [fs.err.report] if:
(4.1.1) —[…]
(4.2) —[…]
std::function
move constructor does unnecessary workSection: 22.10.17.3.2 [func.wrap.func.con] Status: C++17 Submitter: Geoffrey Romer Opened: 2016-12-15 Last modified: 2020-09-06
Priority: 0
View all other issues in [func.wrap.func.con].
View all issues with C++17 status.
Discussion:
Consider [func.wrap.func.con]/p5:
function(function&& f);Effects: If
!f
,*this
has no target; otherwise, move constructs the target off
into the target of*this
, leavingf
in a valid state with an unspecified value.
By my reading, this wording requires the move constructor of std::function
to construct an entirely new target object.
This is silly: in cases where the target is held in separately allocated memory (i.e. where the target doesn't fit in
std::function
's internal buffer, if any), std::function
's move constructor can be implemented by simply
transferring ownership of the existing target object (which is a simple pointer assignment), so this requirement forces an
unnecessary constructor invocation and probably an unnecessary allocation (the latter can be avoided with something like
double-buffering, but ew). Fixing this would technically be a visible change, but I have a hard time imagining reasonable
code that would be broken by it, especially since both libstdc++ and libc++ already do the sensible thing, constructing a
new target only if the target is held in an internal buffer, and otherwise assigning pointers.
[2017-01-27 Telecon]
Priority 0
Proposed resolution:
This wording is relative to N4618.
Edit 22.10.17.3.2 [func.wrap.func.con]/5 as indicated:
Drafting note: The "equivalent to ... before the construction" wording is based on the wording for
MoveConstructible
.
function(function&& f);-5-
EffectsPostconditions: If!f
,*this
has no target; otherwise,move constructs the target ofthe target off
into the target of*this
, leavingf
*this
is equivalent to the target off
before the construction, andf
is in a valid state with an unspecified value.
std::filesystem
enum classes are now underspecifiedSection: 31.12.8.2 [fs.enum.file.type], 31.12.8.3 [fs.enum.copy.opts], 31.12.8.6 [fs.enum.dir.opts] Status: C++20 Submitter: Tim Song Opened: 2016-12-18 Last modified: 2021-06-06
Priority: 2
View all other issues in [fs.enum.file.type].
View all issues with C++20 status.
Discussion:
LWG 2678(i) stripped the numerical values of the enumerators from three enum classes in 31.12.8 [fs.enum];
in doing so it also removed the implicit specification 1) of the bitmask elements for the two bitmask types (copy_options
and directory_options
) and 2) that the file_type
constants are distinct.
[2017-01-27 Telecon]
Priority 2; Jonathan to work with Tim to tweak wording.
[2018-01-16, Jonathan comments]
I no longer remember what I didn't like about Tim's P/R so I think we should accept the original P/R.
[2018-1-26 issues processing telecon]
Status to 'Tentatively Ready'
[2018-3-17 Adopted in Jacksonville]
Proposed resolution:
This wording is relative to N4713.
Edit [fs.enum.file_type]/1 as indicated:
This enum class specifies constants used to identify file types, with the meanings listed in Table 123. The values of the constants are distinct.
Edit 31.12.8.3 [fs.enum.copy.opts]/1 as indicated:
The
enum class
typecopy_options
is a bitmask type (16.3.3.3.3 [bitmask.types]) that specifies bitmask constants used to control the semantics of copy operations. The constants are specified in option groups with the meanings listed in Table 124. The constantnone
represents the empty bitmask, andConstantis shown in each option group for purposes of exposition; implementations shall provide only a single definition. Every other constant in the table represents a distinct bitmask element. Calling a library function with more than a single constant for an option group results in undefined behavior.none
Edit 31.12.8.6 [fs.enum.dir.opts]/1 as indicated:
The
enum class
typedirectory_options
is a bitmask type (16.3.3.3.3 [bitmask.types]) that specifies bitmask constants used to identify directory traversal options, with the meanings listed in Table 127. The constantnone
represents the empty bitmask; every other constant in the table represents a distinct bitmask element.
erase
in [vector.modifiers]Section: 23.3.13.5 [vector.modifiers] Status: C++17 Submitter: Gerard Stone Opened: 2017-01-16 Last modified: 2020-09-06
Priority: 0
View all other issues in [vector.modifiers].
View all issues with C++17 status.
Discussion:
In Table 87 (Sequence container requirements) erase(q)
and erase(q1, q2)
functions have the following requirements:
For
vector
anddeque
,T
shall beMoveAssignable
.
On the other hand, section [vector.modifiers] has the following specification for erase functions (emphasis mine):
Throws: Nothing unless an exception is thrown by the copy constructor, move constructor, assignment operator, or move assignment operator of
T
.
Note that Table 87 requires T
to be only MoveAssignable
, it says nothing about T
being copy- or move-constructible. It also says nothing about T
being CopyInsertable
and MoveInsertable
,
so why is this even there? The only reason might be so that vector could shrink, but in this case T
should be required
to be MoveInsertable
or CopyInsertable
into vector
.
T
's copy/move constructors
from [vector.modifiers] paragraph 5.
[2017-01-27 Telecon]
Priority 0
Proposed resolution:
This wording is relative to N4618.
Edit 23.3.13.5 [vector.modifiers] p5 as indicated:
iterator erase(const_iterator position); iterator erase(const_iterator first, const_iterator last); void pop_back();-3- Effects: Invalidates iterators and references at or after the point of the erase.
-4- Complexity: The destructor ofT
is called the number of times equal to the number of the elements erased, but the assignment operator ofT
is called the number of times equal to the number of elements in the vector after the erased elements. -5- Throws: Nothing unless an exception is thrown by thecopy constructor, move constructor,assignment operator,or move assignment operator ofT
.
std::throw_with_nested("string_literal")
Section: 17.9.8 [except.nested] Status: C++17 Submitter: Jonathan Wakely Opened: 2017-01-17 Last modified: 2020-09-06
Priority: 0
View all other issues in [except.nested].
View all issues with C++17 status.
Discussion:
[except.nested] says:
template <class T> [[noreturn]] void throw_with_nested(T&& t);Let
Requires:U
beremove_reference_t<T>
.U
shall beCopyConstructible
.
This forbids std::throw_with_nested("string literal")
because T
gets deduced as const char(&)[15]
and so U
is const char[15]
which is not CopyConstructible
.
throw_with_nested
also worked fine until I added a static_assert
to enforce the
CopyConstructible
requirement.
The same problem exists when throwing a function type, which should also decay:
#include <exception> void f() { } int main() { std::throw_with_nested(f); }
(Note: LWG 1370(i) added the remove_reference
, which was a step in the right direction but not far enough.)
[2017-01-27 Telecon]
Priority 0
Proposed resolution:
This wording is relative to N4618.
Edit 17.9.8 [except.nested] as indicated:
template <class T> [[noreturn]] void throw_with_nested(T&& t);-6- Let
-7- Requires:U
be.
remove_referencedecay_t<T>U
shall beCopyConstructible
.
std::async
should be marked as [[nodiscard]]
Section: 32.10.9 [futures.async] Status: Resolved Submitter: Andrey Davydov Opened: 2017-01-20 Last modified: 2020-09-06
Priority: 2
View other active issues in [futures.async].
View all other issues in [futures.async].
View all issues with Resolved status.
Discussion:
Because the destructor of the std::future
returned from the std::async
blocks until
the asynchronous operation completes, discarding the std::async
return value leads to the
synchronous code execution, which is pointless. For example, in the following code
void task1(); void task2(); void foo() { std::async(std::launch::async, task1), std::async(std::launch::async, task2); } void bar() { std::async(std::launch::async, task1); std::async(std::launch::async, task2); }
task1
and task2
will be concurrently executed in the function 'foo', but sequentially in the function 'bar'.
[2017-01-27 Telecon]
Priority 2; Nico to provide wording.
[2017-03-04, Kona]
This should be handled by Nico's paper P0600.
Proposed resolution:
Resolved by adoption of P0600 in Albuquerque
{variant,optional,any}::emplace
should return the constructed valueSection: 22.5 [optional], 22.6 [variant], 22.7 [any] Status: C++17 Submitter: Zhihao Yuan Opened: 2017-01-25 Last modified: 2020-09-06
Priority: 1
View all other issues in [optional].
View all issues with C++17 status.
Discussion:
When you want to continue operate on the new contained object constructed by emplace
, you probably
don't want to go through the type-safe machinery again.
[2017-01-27 Telecon]
Priority 1; send to LEWG.
[Kona 2017-03-02]
Accepted as Immediate because P1.
Proposed resolution:
This wording is relative to N4618.
Update the following signatures in 22.5.3 [optional.optional], class template optional
synopsis,
as indicated:
[…] // 22.5.3.4 [optional.assign], assignment […] template <class... Args>voidT& emplace(Args&&...); template <class U, class... Args>voidT& emplace(initializer_list<U>, Args&&...); […]
Modify 22.5.3.4 [optional.assign] as indicated:
template <class... Args>voidT& emplace(Args&&... args);[…]
-27- Postconditions:*this
contains a value. -?- Returns: A reference to the new contained value. -28- Throws: Any exception thrown by the selected constructor ofT
.template <class U, class... Args>voidT& emplace(initializer_list<U> il, Args&&... args);[…]
-31- Postconditions:*this
contains a value. -?- Returns: A reference to the new contained value. -32- Throws: Any exception thrown by the selected constructor ofT
.
Modify the following signatures in 22.6.3 [variant.variant], class template variant
synopsis, as indicated:
[…] // 22.6.3.5 [variant.mod], modifiers template <class T, class... Args>voidT& emplace(Args&&...); template <class T, class U, class... Args>voidT& emplace(initializer_list<U>, Args&&...); template <size_t I, class... Args>voidvariant_alternative_t<I, variant<Types...>>& emplace(Args&&...); template <size_t I, class U, class... Args>voidvariant_alternative_t<I, variant<Types...>>& emplace(initializer_list<U>, Args&&...); […]
Modify 22.6.3.5 [variant.mod] as indicated:
template <class T, class... Args>voidT& emplace(Args&&... args);-1- Effects: Equivalent to
[…]return emplace<I>(std::forward<Args>(args)...);
whereI
is the zero-based index ofT
inTypes...
.template <class T, class U, class... Args>voidT& emplace(initializer_list<U> il, Args&&... args);-3- Effects: Equivalent to
[…]return emplace<I>(il, std::forward<Args>(args)...);
whereI
is the zero-based index ofT
inTypes...
.template <size_t I, class... Args>voidvariant_alternative_t<I, variant<Types...>>& emplace(Args&&... args);[…]
-7- Postconditions:index()
isI
. -?- Returns: A reference to the new contained value. -8- Throws: Any exception thrown during the initialization of the contained value. […]template <size_t I, class U, class... Args>voidvariant_alternative_t<I, variant<Types...>>& emplace(initializer_list<U> il, Args&&... args);[…]
-12- Postconditions:index()
isI
. -?- Returns: A reference to the new contained value. -13- Throws: Any exception thrown during the initialization of the contained value. […]
Modify the following signatures in 22.7.4 [any.class], class any
synopsis, as indicated:
[…] // 22.7.4.4 [any.modifiers], modifiers template <class ValueType, class... Args>voiddecay_t<ValueType>& emplace(Args&& ...); template <class ValueType, class U, class... Args>voiddecay_t<ValueType>& emplace(initializer_list<U>, Args&&...); […]
Modify 22.7.4.4 [any.modifiers] as indicated:
template <class ValueType, class... Args>voiddecay_t<ValueType>& emplace(Args&&... args);[…]
-4- Postconditions:*this
contains a value. -?- Returns: A reference to the new contained value. -5- Throws: Any exception thrown by the selected constructor ofT
. […]template <class ValueType, class U, class... Args>voiddecay_t<ValueType>& emplace(initializer_list<U> il, Args&&... args);[…]
-10- Postconditions:*this
contains a value. -?- Returns: A reference to the new contained value. -11- Throws: Any exception thrown by the selected constructor ofT
. […]
Section: 17.6.5 [ptr.launder] Status: C++20 Submitter: Hubert Tong Opened: 2017-01-31 Last modified: 2021-02-25
Priority: 2
View all other issues in [ptr.launder].
View all issues with C++20 status.
Discussion:
Given:
struct A { int x, y; }; A a[100];
The bytes which compose a[3]
can be reached from &a[2].x
:
reinterpret_cast<A *>(&a[2].x) + 1
points to a[3]
,
however, the definition of "reachable" in [ptr.launder] does not encompass this case.
[2017-03-04, Kona]
Set priority to 2. Assign this (and 2860(i)) to Core.
[2017-08-14, CWG telecon note]
CWG is fine with the proposed resolution.
[2020-02 Status to Immediate on Thursday night in Prague.]
Proposed resolution:
This wording is relative to N4618.
Modify 17.6.5 [ptr.launder] as indicated:
template <class T> constexpr T* launder(T* p) noexcept;[…]
-3- Remarks: An invocation of this function may be used in a core constant expression whenever the value of its argument may be used in a core constant expression. A byte of storageb
is reachable through a pointer value that points to an objectY
if there is an objectZ
, pointer-interconvertible withY
, such thatb
itis within the storage occupied byZ
,Yan object that is pointer-interconvertible withor the immediately-enclosing array object ifY
,Z
is an array element. The program is ill-formed ifYT
is a function type or (possibly cv-qualified)void
.
basic_string
should require that charT
match traits::char_type
Section: 27.4.3.2 [string.require] Status: C++17 Submitter: United States Opened: 2017-02-02 Last modified: 2020-09-06
Priority: Not Prioritized
View all other issues in [string.require].
View all issues with C++17 status.
Discussion:
Addresses US 145
There is no requirement that traits::char_type
is charT
, although there is a requirement that
allocator::value_type
is charT
. This means that it might be difficult to honour both methods returning reference
(such as operator[]
) and charT&
(like front
/back
) when traits has a surprising
char_type
. It seems that the allocator should not rebind in such cases, making the reference-returning
signatures the problematic ones.
Suggested resolution: Add a requirement that is_same_v<typename traits::char_type, charT>
is true, and simplify so that value_type is just an alias for charT
.
[2017-02-02 Marshall adds]
In [string.require]/3, there's already a note that the types shall be the same. In [string.view.template]/1, it says "In every specialization basic_string_view<charT, traits, Allocator>
, the type traits
shall satisfy the character traits requirements (21.2), and the type traits::char_type
shall name the same type as charT
".
[Kona 2017-02-28]
Accepted as Immediate to resolve NB comment.
Proposed resolution:
Changes are based off of N4618
Modify [basic.string] as indicated (in the synopsis):
class basic_string { public: // types: using traits_type = traits; using value_type = charTtypename traits::char_type; using allocator_type = Allocator
Change [string.require]/3 as indicated:
-3- In every specialization basic_string<charT, traits, Allocator>
, the type allocator_traits<Allocator>::value_type
shall name the same type as charT
. Every object of type basic_string<charT, traits, Allocator>
shall use an object of type Allocator
to allocate and free storage for the contained charT
objects as needed. The Allocator
object used shall be obtained as described in 23.2.1. [ Note: In every specialization basic_string<charT, traits, Allocator>
, the type traits
shall satisfy the character traits requirements (21.2), and the type traits::char_type
shall namebe the same type as charT
; see 21.2. — end note ]
Section: 22.5 [optional] Status: Resolved Submitter: Finland Opened: 2017-02-03 Last modified: 2017-02-21
Priority: Not Prioritized
View all other issues in [optional].
View all issues with Resolved status.
Discussion:
Addresses FI 10
Adopt the proposed resolution of LWG 2756(i) into C++17, to provide converting constructors and
assignment operators for optional
.
Proposed change:
Adopt the latest proposed resolution of LWG 2756(i), which should be available by Issaquah.
Proposed resolution:
Resolved by resolving LWG 2756(i).
default_order
changes of maps and setsSection: 99 [func.default.traits] Status: Resolved Submitter: Finland Opened: 2017-02-03 Last modified: 2017-03-20
Priority: Not Prioritized
View all issues with Resolved status.
Discussion:
Addresses FI 18
It was thought that using default_order
as the default comparison for maps and sets was not
abi-breaking but this is apparently not the case.
Proposed change:
Revert the change to the default comparison of maps and sets.
[2016-10 Issaquah]
STL and AM want to revert whole paper.
[2017-03-12, post-Kona]
Resolved reverting P0181R1.
Proposed resolution:
shared_ptr
changes from Library Fundamentals to C++17Section: 20.3.2.2 [util.smartptr.shared], 20.3.2.3 [util.smartptr.weak] Status: Resolved Submitter: Finland Opened: 2017-02-03 Last modified: 2020-09-06
Priority: Not Prioritized
View all other issues in [util.smartptr.shared].
View all issues with Resolved status.
Discussion:
Addresses FI 19
The changes in the paper P0414 should be adopted into C++17.
Proposed change:
Adopt the changes in P0414.
Resolved by the adoption of P0414 in Issaquah
Proposed resolution:
Section: 16.4.6.13 [derivation] Status: C++17 Submitter: Great Britain Opened: 2017-02-03 Last modified: 2020-09-06
Priority: Not Prioritized
View all other issues in [derivation].
View all issues with C++17 status.
Discussion:
Addresses GB 36
For bullet (3.2), no base classes are described as non-virtual. Rather, base classes are not specified as virtual, a subtly different negative.
Proposed change:
Rewrite bullet 3.2:
Every base class not specified as virtual shall not be virtual;
[Kona 2017-03-01]
Accepted as Immediate to resolve NB comment.
Proposed resolution:
This wording is relative to N4640.
Modify 16.4.6.13 [derivation] paragraph 3.2 as indicated:
Every base class described as non-Every base class not specified asvirtual
shall not be virtual;virtual
shall not be virtual;
Section: 16.4.6.14 [res.on.exception.handling] Status: Resolved Submitter: Great Britain Opened: 2017-02-03 Last modified: 2020-09-06
Priority: Not Prioritized
View other active issues in [res.on.exception.handling].
View all other issues in [res.on.exception.handling].
View all issues with Resolved status.
Discussion:
Addresses GB 40
The freedom referenced in footnote 189 was curtailed in C++11 to allow only non-throwing specifications. The footnote is both wrong, and unnecessary.
Proposed change:
Strike footnote 189.
[2017-02-03, Daniel comments]
The referenced footnote had the following content in the ballot document N4604:
That is, an implementation may provide an explicit exception-specification that defines the subset of "any" exceptions thrown by that function. This implies that the implementation may list implementation-defined types in such an exception-specification.
In N4618 this footnote has already been removed, I therefore suggest to resolve the issue as "Resolved by editorial change".
[2017-02-13, Alisdair comments]
Just to confirm that issue 2867 was filed when it looked like the paper to remove exception specifications from the language had missed C++17. The very specific edit requested by the issue can be found in the paper P0003R5 that was applied.
I suggest we mark as resolved by the above paper, rather than "resolved editorially".Proposed resolution:
Resolved by accepting P0003R5.
bad_any_cast::what()
Section: 22.7.3 [any.bad.any.cast] Status: C++17 Submitter: Great Britain Opened: 2017-02-03 Last modified: 2021-06-06
Priority: Not Prioritized
View all issues with C++17 status.
Discussion:
Addresses GB 54
There is no specification for bad_any_cast.what
.
Proposed change:
Add a paragraphs:
const char* what() const noexcept override;Returns: An implementation-defined NTBS.
Remarks: The message may be a null-terminated multibyte string (17.5.2.1.4.2), suitable for conversion and display as awstring
(21.3, 22.4.1.4).
[Kona 2017-03-01]
Accepted as Immediate to resolve NB comment.
Proposed resolution:
This wording is relative to N4618.
Insert the following series of paragraphs to [any.bad_any_cast] as indicated:
const char* what() const noexcept override;-?- Returns: An implementation-defined NTBS.
-?- Remarks: The message may be a null-terminated multibyte string (16.3.3.3.4.3 [multibyte.strings]), suitable for conversion and display as awstring
(27.4 [string.classes], 28.3.4.2.5 [locale.codecvt]).
Section: 99 [depr.locale.stdcvt] Status: Resolved Submitter: Great Britain Opened: 2017-02-03 Last modified: 2017-04-22
Priority: Not Prioritized
View all other issues in [depr.locale.stdcvt].
View all issues with Resolved status.
Discussion:
Addresses GB 57
The contents of <codecvt>
are underspecified, and will take a reasonable
amount of work to identify and correct all of the issues. There appears to be a general
feeling that this is not the best way to address unicode transcoding in the first
place, and this library component should be retired to Annex D, along side
<strstream>
, until a suitable replacement is standardized.
Proposed change:
Deprecate and move the whole of clause [locale.stdcvt] to Annex D.
[2017-02 pre-Kona]
LEWG says Accept.
[2017-03-12, post-Kona]
Resolved by P0618R0.
Proposed resolution:
theta
of polar
should be dependentSection: 29.4.7 [complex.value.ops] Status: C++20 Submitter: Japan Opened: 2017-02-03 Last modified: 2021-02-25
Priority: Not Prioritized
View all other issues in [complex.value.ops].
View all issues with C++20 status.
Discussion:
Addresses JP 25
Parameter theta
of polar
has the type of the
template parameter. Therefore, it needs to change the default initial value
to T()
. The change of the declaration of this function in
29.4.2 [complex.syn] is accompanied by this change.
Proposed change:
template<class T> complex<T> polar(const T& rho, const T& theta =0T());
[2017-02 pre-Kona]
(twice)
[ 2017-06-27 Moved to Tentatively Ready after 7 positive votes on c++std-lib. ]
Proposed resolution:
This wording is relative to N4659.
Modify 29.4.2 [complex.syn], header <complex>
synopsis, as indicated:
template<class T> complex<T> polar(const T&, const T& =0T());
Modify 29.4.7 [complex.value.ops] as indicated:
template<class T> complex<T> polar(const T& rho, const T& theta =0T());
Section: 3 [intro.defs] Status: C++17 Submitter: United States Opened: 2017-02-03 Last modified: 2021-06-06
Priority: Not Prioritized
View all issues with C++17 status.
Discussion:
Addresses US 107
The term 'direct non-list initialization' needs to be incorporated from the Library Fundamentals TS, as several components added to C++17 rely on this definition.
Proposed change:
Add:
17.3.X direct-non-list-initialization [defns.direct-non-list-init]
A direct-initialization that is not list-initialization.
[Kona 2017-02-27]
Accepted as Immediate to resolve NB comment.
Proposed resolution:
This wording is relative to N4618.
Add the following to [definitions] as indicated:
17.3.? direct-non-list-initialization [defns.direct-non-list-init]
A direct-initialization that is not list-initialization.
noexcept
to several shared_ptr
related functionsSection: 20.3.2.2 [util.smartptr.shared] Status: C++17 Submitter: United States Opened: 2017-02-03 Last modified: 2020-09-06
Priority: Not Prioritized
View all other issues in [util.smartptr.shared].
View all issues with C++17 status.
Discussion:
Addresses US 124
Several shared_ptr
related functions have wide
contracts and cannot throw, so should be marked
unconditionally noexcept
.
Proposed change:
Add noexcept
to:
template<class U> bool shared_ptr::owner_before(shared_ptr<U> const& b) const noexcept; template<class U> bool shared_ptr::owner_before(weak_ptr<U> const& b) const noexcept; template<class U> bool weak_ptr::owner_before(shared_ptr<U> const& b) const noexcept; template<class U> bool weak_ptr::owner_before(weak_ptr<U> const& b) const noexcept; bool owner_less::operator()(A,B) const noexcept; // all versions
[2017-02-20, Marshall adds wording]
[Kona 2017-02-27]
Accepted as Immediate to resolve NB comment.
Proposed resolution:
This wording is relative to N4640.
Modify 20.3.2.2 [util.smartptr.shared] as indicated:
template<class U> bool owner_before(const shared_ptr<U>& b) const noexcept; template<class U> bool owner_before(const weak_ptr<U>& b) const noexcept;
Modify 20.3.2.2.6 [util.smartptr.shared.obs] as indicated:
template<class U> bool owner_before(const shared_ptr<U>& b) const noexcept; template<class U> bool owner_before(const weak_ptr<U>& b) const noexcept;
Modify 20.3.2.4 [util.smartptr.ownerless] as indicated:
template<class T> struct owner_less<shared_ptr<T>> { bool operator()(const shared_ptr<T>&, const shared_ptr<T>&) const noexcept; bool operator()(const shared_ptr<T>&, const weak_ptr<T>&) const noexcept; bool operator()(const weak_ptr<T>&, const shared_ptr<T>&) const noexcept; }; template<class T> struct owner_less<weak_ptr<T>> { bool operator()(const weak_ptr<T>&, const weak_ptr<T>&) const noexcept; bool operator()(const shared_ptr<T>&, const weak_ptr<T>&) const noexcept; bool operator()(const weak_ptr<T>&, const shared_ptr<T>&) const noexcept; }; template<> struct owner_less<void> { template<class T, class U> bool operator()(const shared_ptr<T>&, const shared_ptr<U>&) const noexcept; template<class T, class U> bool operator()(const shared_ptr<T>&, const weak_ptr<U>&) const noexcept; template<class T, class U> bool operator()(const weak_ptr<T>&, const shared_ptr<U>&) const noexcept; template<class T, class U> bool operator()(const weak_ptr<T>&, const weak_ptr<U>&) const noexcept; using is_transparent = unspecified; };
shared_ptr::shared_ptr(Y*)
should be constrainedSection: 20.3.2.2.2 [util.smartptr.shared.const] Status: C++17 Submitter: United States Opened: 2017-02-03 Last modified: 2020-09-06
Priority: Not Prioritized
View other active issues in [util.smartptr.shared.const].
View all other issues in [util.smartptr.shared.const].
View all issues with C++17 status.
Discussion:
Addresses US 125
Paragraph 4: This constructor should not participate in overload
resolution unless the Requires clause is satisfied. Note that this would
therefore apply to some assignment operator and reset
overloads, via
Effects: equivalent to some code wording.
Proposed change:
Add a Remarks: clause to constrain this constructor not to participate in overload resolution unless the Requires clause is satisfied.
[2017-02-23, Jonathan provides wording]
Previous resolution [SUPERSEDED]:
This wording is relative to N4640.
Modify 20.3.2.2.2 [util.smartptr.shared.const] as indicated:
[Drafting note: This also adds a hyphen to "well defined"]
template<class Y> explicit shared_ptr(Y* p);-4- Requires:
-5- Effects: […] -6- Postconditions: […] -7- Throws: […] -?- Remarks: WhenY
shall be a complete type. The expressiondelete[] p
, whenT
is an array type, ordelete p
, whenT
is not an array type,shall be well formed,shall have well-defined behavior, and shall not throw exceptions.WhenT
isU[N]
,Y(*)[N]
shall be convertible toT*
; whenT
isU[]
,Y(*)[]
shall be convertible toT*
; otherwise,Y*
shall be convertible toT*
.T
is an array type, this constructor shall not participate in overload resolution unless the expressiondelete[] p
is well-formed and eitherT
isU[N]
andY(*)[N]
is convertible toT*
, orY(*)[]
is convertible toT*
. WhenT
is not an array type, this constructor shall not participate in overload resolution unless the expressiondelete p
is well-formed andY*
is convertible toT*
.
[Kona 2017-02-27: Jonathan updates wording after LWG review]
[Kona 2017-02-27]
Accepted as Immediate to resolve NB comment.
Proposed resolution:
This wording is relative to N4640.
Modify 20.3.2.2.2 [util.smartptr.shared.const] as indicated:
[Drafting note: This also adds a hyphen to "well defined"]
template<class Y> explicit shared_ptr(Y* p);-4- Requires:
-5- Effects: […] -6- Postconditions: […] -7- Throws: […] -?- Remarks: WhenY
shall be a complete type. The expressiondelete[] p
, whenT
is an array type, ordelete p
, whenT
is not an array type,shall be well formed,shall have well-defined behavior, and shall not throw exceptions.WhenT
isU[N]
,Y(*)[N]
shall be convertible toT*
; whenT
isU[]
,Y(*)[]
shall be convertible toT*
; otherwise,Y*
shall be convertible toT*
.T
is an array type, this constructor shall not participate in overload resolution unless the expressiondelete[] p
is well-formed and eitherT
isU[N]
andY(*)[N]
is convertible toT*
, orT
isU[]
andY(*)[]
is convertible toT*
. WhenT
is not an array type, this constructor shall not participate in overload resolution unless the expressiondelete p
is well-formed andY*
is convertible toT*
.
shared_ptr::shared_ptr(Y*, D, […])
constructors should be constrainedSection: 20.3.2.2.2 [util.smartptr.shared.const] Status: C++17 Submitter: United States Opened: 2017-02-03 Last modified: 2020-09-06
Priority: Not Prioritized
View other active issues in [util.smartptr.shared.const].
View all other issues in [util.smartptr.shared.const].
View all issues with C++17 status.
Discussion:
Addresses US 126
Paragraph 8: This constructor should not participate in overload
resolution unless the Requires clause is satisfied.
Note that this would therefore apply to some assignment operator and
reset
overloads, via Effects: equivalent to some code wording.
Proposed change:
Add a Remarks: clause to constrain this constructor not to participate in overload resolution unless the Requires clause is satisfied.
[2017-02 pre-Kona]
[2017-02-23, Jonathan provides wording]
This wording is relative to N4640.
Modify 20.3.2.2.2 [util.smartptr.shared.const] as indicated:
If the proposed resolution of LWG 2802(i) has been accepted modify p8 as shown:
template<class Y, class D> shared_ptr(Y* p, D d); template<class Y, class D, class A> shared_ptr(Y* p, D d, A a); template <class D> shared_ptr(nullptr_t p, D d); template <class D, class A> shared_ptr(nullptr_t p, D d, A a);-8- Requires:
Construction ofD
shall beMoveConstructible
and cd
and a deleter of typeD
initialized withstd::move(d)
shall not throw exceptions. The expressiond(p)
shall be well formed,shall have well-defined behavior, and shall not throw exceptions.A
shall be an allocator (17.5.3.5).WhenT
isU[N]
,Y(*)[N]
shall be convertible toT*
; whenT
isU[]
,Y(*)[]
shall be convertible toT*
; otherwise,Y*
shall be convertible toT*
.
If the proposed resolution of LWG 2802(i) has not been accepted modify p8 as shown:
template<class Y, class D> shared_ptr(Y* p, D d); template<class Y, class D, class A> shared_ptr(Y* p, D d, A a); template <class D> shared_ptr(nullptr_t p, D d); template <class D, class A> shared_ptr(nullptr_t p, D d, A a);-8- Requires:
Construction ofD
shall beCopyConstructible
and such cd
and a copy ofd
shall not throw exceptions. The destructor ofD
shall not throw exceptions. The expressiond(p)
shall be well formed,shall have well defined behavior, and shall not throw exceptions.A
shall be an allocator (17.5.3.5). The copy constructor and destructor ofA
shall not throw exceptions.WhenT
isU[N]
,Y(*)[N]
shall be convertible toT*
; whenT
isU[]
,Y(*)[]
shall be convertible toT*
; otherwise,Y*
shall be convertible toT*
.
In either case, add a Remarks paragraph after p11:
[Drafting note: If LWG 2802(i) is not accepted, replace
is_move_constructible_v
withis_copy_constructible_v
.]
template<class Y, class D> shared_ptr(Y* p, D d); template<class Y, class D, class A> shared_ptr(Y* p, D d, A a); template <class D> shared_ptr(nullptr_t p, D d); template <class D, class A> shared_ptr(nullptr_t p, D d, A a);-8- Requires: […]
[…] -11- Throws: […] -?- Remarks: WhenT
is an array type, this constructor shall not participate in overload resolution unlessis_move_constructible_v<D>
istrue
, the expressiond(p)
is well-formed, and eitherT
isU[N]
andY(*)[N]
is convertible toT*
, orY(*)[]
is convertible toT*
. WhenT
is not an array type, this constructor shall not participate in overload resolution unlessis_move_constructible_v<D>
istrue
, the expressiond(p)
is well-formed, andY*
is convertible toT*
.
[Kona 2017-02-27: Jonathan updates wording after LWG review]
[Kona 2017-02-27]
Accepted as Immediate to resolve NB comment.
Proposed resolution:
This wording is relative to N4640 as modified by the proposed resolution of LWG 2802(i).
Modify 20.3.2.2.2 [util.smartptr.shared.const] as indicated:
template<class Y, class D> shared_ptr(Y* p, D d); template<class Y, class D, class A> shared_ptr(Y* p, D d, A a); template <class D> shared_ptr(nullptr_t p, D d); template <class D, class A> shared_ptr(nullptr_t p, D d, A a);-8- Requires:
Construction ofD
shall beMoveConstructible
and cd
and a deleter of typeD
initialized withstd::move(d)
shall not throw exceptions. The expressiond(p)
shall be well formed,shall have well-defined behavior, and shall not throw exceptions.A
shall be an allocator (17.5.3.5).WhenT
isU[N]
,Y(*)[N]
shall be convertible toT*
; whenT
isU[]
,Y(*)[]
shall be convertible toT*
; otherwise,Y*
shall be convertible toT*
.
Add a Remarks paragraph after p11:
template<class Y, class D> shared_ptr(Y* p, D d); template<class Y, class D, class A> shared_ptr(Y* p, D d, A a); template <class D> shared_ptr(nullptr_t p, D d); template <class D, class A> shared_ptr(nullptr_t p, D d, A a);-8- Requires: […]
[…] -11- Throws: […] -?- Remarks: WhenT
is an array type, this constructor shall not participate in overload resolution unlessis_move_constructible_v<D>
istrue
, the expressiond(p)
is well-formed, and eitherT
isU[N]
andY(*)[N]
is convertible toT*
, orT
isU[]
andY(*)[]
is convertible toT*
. WhenT
is not an array type, this constructor shall not participate in overload resolution unlessis_move_constructible_v<D>
istrue
, the expressiond(p)
is well-formed, andY*
is convertible toT*
.
shared_ptr::shared_ptr(const weak_ptr<Y>&)
constructor should be constrainedSection: 20.3.2.2.2 [util.smartptr.shared.const] Status: C++17 Submitter: United States Opened: 2017-02-03 Last modified: 2020-09-06
Priority: Not Prioritized
View other active issues in [util.smartptr.shared.const].
View all other issues in [util.smartptr.shared.const].
View all issues with C++17 status.
Discussion:
Addresses US 129
Paragraph 22: This constructor should not participate in overload
resolution unless the requirements are satisfied, in
order to give correct results from the is_constructible
trait.
Proposed change:
Add a Remarks: clause to constrain this constructor not to participate in overload resolution unless the Requires clause is satisfied.
[2017-02-23, Jonathan provides wording]
[Kona 2017-02-27]
Accepted as Immediate to resolve NB comment.
Proposed resolution:
This wording is relative to N4640.
Modify 20.3.2.2.2 [util.smartptr.shared.const] as indicated:
template<class Y> explicit shared_ptr(const weak_ptr<Y>& r);[…] -25- Throws: […] -?- Remarks: This constructor shall not participate in overload resolution unless
-22- Requires:Y*
shall be compatible withT*
.Y*
is compatible withT*
.
shared_ptr<T>
" in dynamic_pointer_cast
Section: 20.3.2.2.10 [util.smartptr.shared.cast] Status: Resolved Submitter: United States Opened: 2017-02-03 Last modified: 2020-09-06
Priority: Not Prioritized
View all other issues in [util.smartptr.shared.cast].
View all issues with Resolved status.
Discussion:
Addresses US 137
Paragraph (6.2): It is intuitive, but not specified, that the empty pointer
returned by a dynamic_pointer_cast
should point to null.
Proposed change:
Rephrase as:
Otherwise,
shared_ptr<T>()
.
Proposed resolution:
Resolved by P0414R2, which was adopted in Issaquah.
DefaultConstructible
requirement for istream_iterator
default constructorSection: 24.6.2.2 [istream.iterator.cons] Status: C++17 Submitter: United States Opened: 2017-02-03 Last modified: 2020-09-06
Priority: Not Prioritized
View all other issues in [istream.iterator.cons].
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Discussion:
Addresses US 153
istream_iterator
default constructor requires a DefaultConstructible T
.
Proposed change:
Add a new p1:
Requires:
T
isDefaultConstructible
.
Previous resolution [SUPERSEDED]:
This wording is relative to N4618.
Modify 24.6.2.2 [istream.iterator.cons] as indicated:
see below istream_iterator();-?- Requires:
-1- Effects: Constructs the end-of-stream iterator. IfT
isDefaultConstructible
.is_trivially_default_constructible_v<T>
istrue
, then this constructor is aconstexpr
constructor. -2- Postconditions:in_stream == 0
.
[Kona 2017-02-28: Jonathan provides updated wording as requested by LWG.]
[Kona 2017-03-02]
Accepted as Immediate to resolve NB comment.
Proposed resolution:
This wording is relative to N4618.
Modify 24.6.2 [istream.iterator] as indicated:
-1- The class template
istream_iterator
is an input iterator (24.2.3) that reads (usingoperator>>
) successive elements from the input stream for which it was constructed. After it is constructed, and every time++
is used, the iterator reads and stores a value ofT
. If the iterator fails to read and store a value ofT
(fail()
on the stream returnstrue
), the iterator becomes equal to the end-of-stream iterator value. The constructor with no argumentsistream_iterator()
always constructs an end-of-stream input iterator object, which is the only legitimate iterator to be used for the end condition. The result ofoperator*
on an end-of-stream iterator is not defined. For any other iterator value aconst T&
is returned. The result ofoperator->
on an end-of-stream iterator is not defined. For any other iterator value aconst T*
is returned. The behavior of a program that appliesoperator++()
to an end-of-stream iterator is undefined. It is impossible to store things into istream iterators. The typeT
shall meet theDefaultConstructible
,CopyConstructible
, andCopyAssignable
requirements.
Section: 17.14.4 [csignal.syn], 17.5 [support.start.term] Status: Resolved Submitter: Canada Opened: 2017-02-03 Last modified: 2020-09-06
Priority: Not Prioritized
View all issues with Resolved status.
Discussion:
Addresses CA 1P0270R1 went through SG1 and LWG but was too late to make it to the straw polls.
The problems it addresses stem from referring to C11, which came into C++17 at the last minute.
P0270R1 should have made it in with the C11 change.
Proposed change: Apply all of P0270R1, "Removing C dependencies from signal handler wording", to C++17.
[Kona 2017-03-04]
P0270R1 was adopted in Kona
Proposed resolution:
Section: 26.3.3 [algorithms.parallel.exec] Status: Resolved Submitter: Switzerland Opened: 2017-02-03 Last modified: 2017-06-26
Priority: Not Prioritized
View all other issues in [algorithms.parallel.exec].
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Discussion:
Addresses CH 10Parallel implementations of algorithms may be faster if not restricted to the complexity specifications of serial implementations.
Proposed change: Add a relaxation of complexity specifications for non-sequenced policies.
Proposed resolution:
Resolved by the adoption of P0523r1 in Kona
variant
constructionSection: 22.6.3.2 [variant.ctor] Status: Resolved Submitter: Switzerland Opened: 2017-02-03 Last modified: 2020-09-06
Priority: Not Prioritized
View other active issues in [variant.ctor].
View all other issues in [variant.ctor].
View all issues with Resolved status.
Discussion:
Addresses CH 8Clarify variant
construction
Proposed change: Add a note that variant<>
cannot be constructed.
[2017-02-03, Marshall comments]
[variant]/3 now says "A program that instantiates the definition of variant
with no template arguments is ill-formed."
[2017-02-21, Marshall comments]
It appears that the changes requested by this NB comment have already been applied — as part of P0510R0, adopted in Issaquah.
The NB comment is already marked as "accepted", so I suggest we mark the issue as "Resolved".Proposed resolution:
Resolved by applying corresponding wording changes through P0510R0.
lock_guard
Section: 32.6.5.2 [thread.lock.guard] Status: Resolved Submitter: Finland, Great Britain Opened: 2017-02-03 Last modified: 2017-04-22
Priority: Not Prioritized
View all other issues in [thread.lock.guard].
View all issues with Resolved status.
Discussion:
Addresses FI 8, GB 61The class template lock_guard
was made variadic. This is abi-breaking, and confusing because one-argument
lock_guard
s have a typedef mutex_type
but lock_guard
s with more than one argument don't. There's
no need to try to shoehorn this functionality into one type.
Proposed change: Revert the changes to lock_guard
, and introduce a new variadic class template
vlock_guard
that doesn't have the mutex_type
typedef at all.
[2017-02-02, Marshall notes]
This was the subject of intense discussion in Issaquah, and a joint LEG/LEWG session agreed on this approach.
[2017-03-12, post-Kona]
Resolved by P0156R2.
Proposed resolution:
Section: 22 [utilities], 32 [thread] Status: Resolved Submitter: Finland Opened: 2017-02-03 Last modified: 2017-03-20
Priority: Not Prioritized
View all other issues in [utilities].
View all issues with Resolved status.
Discussion:
Addresses FI 9The variables of library tag types need to be inline variables. Otherwise, using them in inline functions in multiple translation units is an ODR violation.
Proposed change: Make piecewise_construct
, allocator_arg
, nullopt
, (the in_place_tags
after they are made regular tags), defer_lock
, try_to_lock
and adopt_lock
inline.
[2017-02-03, Marshall notes]
[2017-02-25, Daniel comments]
There will be the paper p0607r0 provided for the Kona meeting that solves this issue.
[2017-03-12, post-Kona]
Resolved by p0607r0.
Proposed resolution:
constexpr
global variables as inline
Section: 22 [utilities], 26.3.6 [execpol] Status: Resolved Submitter: Great Britain Opened: 2017-02-03 Last modified: 2017-03-20
Priority: Not Prioritized
View all other issues in [utilities].
View all issues with Resolved status.
Discussion:
Addresses GB 28The C++ standard library provides many constexpr
global variables. These all create the risk of ODR violations
for innocent user code. This is especially bad for the new ExecutionPolicy
algorithms, since their constants are
always passed by reference, so any use of those algorithms from an inline function results in an ODR violation.
This can be avoided by marking the globals as inline
.
Proposed change: Add inline
specifier to: bind
placeholders _1, _2, ...
, nullopt
,
piecewise_construct
, allocator_arg
, ignore
, seq
, par
, par_unseq
in
<execution>
[2017-02-03, Marshall notes]
[2017-02-25, Daniel comments]
There will be the paper p0607r0 provided for the Kona meeting that solves this issue.
[2017-03-12, post-Kona]
Resolved by p0607r0.
Proposed resolution:
Section: 99 [defns.obj.state] Status: C++17 Submitter: Great Britain Opened: 2017-02-03 Last modified: 2020-09-06
Priority: Not Prioritized
View all other issues in [defns.obj.state].
View all issues with C++17 status.
Discussion:
Addresses GB 30The definition of 'object state' applies only to class types, implying that fundamental types and arrays do not have this property.
Proposed change: Replacing "an object state" with "a value of an object" in 3.67 [defns.valid] and dropping the definition of "object state" in 99 [defns.obj.state].
[2017-02-26, Scott Schurr provides wording]
[Kona 2017-03-01]
Accepted as Immediate to resolve NB comment.
Proposed resolution:
This wording is relative to N4640.
Modify 99 [defns.obj.state] as indicated:
17.3.16 [defns.obj.state]object statethe current value of all non-static class members of an object (9.2) [Note: The state of an object can be obtained by using one or more observer functions. — end note]
Modify 3.67 [defns.valid] as indicated:
17.3.25 [defns.valid]
valid but unspecified statean object statea value of an object that is not specified except that the object's invariants are met and operations on the object behave as specified for its type
std::apply()
is required to be constexpr
, but std::invoke()
isn'tSection: 22.10.5 [func.invoke] Status: Resolved Submitter: Great Britain Opened: 2017-02-03 Last modified: 2020-09-06
Priority: 3
View all other issues in [func.invoke].
View all issues with Resolved status.
Discussion:
Addresses GB 51The function template std::apply()
in 22.4.6 [tuple.apply] is required to be constexpr
,
but std::invoke()
in 22.10.5 [func.invoke] isn't. The most sensible implementation of apply_impl()
is exactly equivalent to std::invoke()
, so this requires implementations to have a constexpr
version of
invoke()
for internal use, and the public API std::invoke
, which must not be constexpr
even
though it is probably implemented in terms of the internal version.
Proposed change: Add constexpr
to std::invoke
.
[2017-02-20, Marshall adds wording]
[Kona 2017-03-01]
We think this needs CWG 1581 to work; accepted as Immediate to resolve NB comment.
Friday: CWG 1581 was not moved in Kona. Status back to Open.
[2017-07 Toronto Tuesday PM issue prioritization]
Priority 3
[2020-01 Resolved by the adoption of P1065 in Cologne.]
Proposed resolution:
This wording is relative to N4640.
Modify 22.10.5 [func.invoke] as indicated:
template <class F, class... Args> constexpr result_of_t<F&&(Args&&...)> invoke(F&& f, Args&&... args);
result_of
and is_callable
Section: 21.3.7 [meta.rel] Status: Resolved Submitter: Great Britain Opened: 2017-02-03 Last modified: 2020-09-06
Priority: Not Prioritized
View other active issues in [meta.rel].
View all other issues in [meta.rel].
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Discussion:
Addresses GB 55It is becoming more and more apparent that using a function type as the template argument to result_of
causes
annoying problems. That was done because C++03 didn't have variadic templates, so it allowed an arbitrary number of types
to be smuggled into the template via a single parameter, but it's a hack and unnecessary in C++ today.
result_of<F(Args...)>
has absolutely nothing to do with a function type that returns F
, and the
syntactic trickery using a function type has unfortunate consequences such as top-level cv-qualifiers and arrays
decaying (because those are the rules for function types).
It might be too late to change result_of
, but we should not repeat the same mistake for std::is_callable
.
Proposed change: Possibly get rid of the is_callable<Fn(ArgTypes?...), R>
specialization. Change the
primary template is_callable<class, class R = void> to is_callable<class Fn, class.. ArgTypes?>
and
define a separate template such as is_callable_r<class R, class Fn, class... ArgTypes?>
for the version
that checks the return type. The resulting inconsistency might need to be resolved/improved upon.
[2017-02, pre-Kona]
[2017-02-22, Daniel comments and provides concrete wording]
The approach chosen to resolve this issue is a merger with LWG 2928(i), that is the callable
traits are also renamed to invocable
.
Previous resolution [SUPERSEDED]:
This wording is relative to N4640.
Modify 21.3.3 [meta.type.synop], header
<type_traits>
synopsis, as indicated:[…] // 20.15.6, type relations […]template <class, class R = void> struct is_callable; // not defined template <class Fn, class... ArgTypes, class R> struct is_callable<Fn(ArgTypes...), R>;template <class Fn, class... ArgTypes> struct is_invocable; template <class R, class Fn, class... ArgTypes> struct is_invocable_r;template <class, class R = void> struct is_nothrow_callable; // not defined template <class Fn, class... ArgTypes, class R> struct is_nothrow_callable<Fn(ArgTypes...), R>;template <class Fn, class... ArgTypes> struct is_nothrow_invocable; template <class R, class Fn, class... ArgTypes> struct is_nothrow_invocable_r; […] // 20.15.6, type relations […]template <class T, class R = void> constexpr bool is_callable_v = is_callable<T, R>::value; template <class T, class R = void> constexpr bool is_nothrow_callable_v = is_nothrow_callable<T, R>::value;template <class Fn, class... ArgTypes> constexpr bool is_invocable_v = is_invocable<Fn, ArgTypes...>::value; template <class R, class Fn, class... ArgTypes> constexpr bool is_invocable_r_v = is_invocable_r<R, Fn, ArgTypes...>::value; template <class Fn, class... ArgTypes> constexpr bool is_nothrow_invocable_v = is_nothrow_invocable<Fn, ArgTypes...>::value; template <class R, class Fn, class... ArgTypes> constexpr bool is_nothrow_invocable_r_v = is_nothrow_invocable_r<R, Fn, ArgTypes...>::value; […]Modify 21.3.7 [meta.rel], Table 44 — "Type relationship predicates", as indicated:
Table 44 — Type relationship predicates […]
template <class Fn, class...
ArgTypes, class R>
struct is_invocablecallable<;
Fn(ArgTypes...), R>The expression
INVOKE(declval<Fn>(),
is well formed when treated
declval<ArgTypes>()...,)
R
as an unevaluated operandFn
,and all types in theR
,
parameter packArgTypes
shall
be complete types, cvvoid
, or
arrays of unknown bound.template <class R, class Fn, class...
ArgTypes>
struct is_invocable_r;The expression
INVOKE(declval<Fn>(),
is well formed when treated
declval<ArgTypes>()...,
R)
as an unevaluated operandFn
,R
, and all types in the
parameter packArgTypes
shall
be complete types, cvvoid
, or
arrays of unknown bound.template <class Fn, class...
ArgTypes, class R>
struct is_nothrow_invocablecallable<;
Fn(ArgTypes...), R>is_invocable
iscallable_v<
Fn, ArgTypes...Fn(ArgTypes...), R>
true
and the expression
INVOKE(declval<Fn>(),
is known not to throw any
declval<ArgTypes>()...,)
R
exceptionsFn
,and all types in theR
,
parameter packArgTypes
shall
be complete types, cvvoid
, or
arrays of unknown bound.template <class R, class Fn, class...
ArgTypes, class R>
struct is_nothrow_invocable_r;is_invocable_r_v<
is
R, Fn, ArgTypes...>
true
and the expression
INVOKE(declval<Fn>(),
is known not to throw any
declval<ArgTypes>()...,
R)
exceptionsFn
,R
, and all types in the
parameter packArgTypes
shall
be complete types, cvvoid
, or
arrays of unknown bound.
[2017-02-24, Daniel comments]
I suggest to apply the paper d0604r0 instead, available on the Kona LWG wiki.
[2017-03-12, post-Kona]
Resolved by p0604r0.
Proposed resolution:
array::iterator
and array::const_iterator
should be literal typesSection: 23.3.3.1 [array.overview] Status: Resolved Submitter: Russia Opened: 2017-02-03 Last modified: 2020-09-06
Priority: 2
View other active issues in [array.overview].
View all other issues in [array.overview].
View all issues with Resolved status.
Discussion:
Addresses RU 3Force the literal type requirement for the iterator
and const_iterator
in the std::array
so that iterators of std::array
could be used in constexpr
functions.
Proposed change: Add to the end of the [array.overview] section: iterator
and const_iterator
shall be literal types.
[2017-02-20 Marshall adds wording]
I used the formulation "are literal types", rather than "shall be", since that denotes a requirement on the user.
Previous resolution [SUPERSEDED]:
This wording is relative to N4640.
Add a new paragraph at the end of 23.3.3.1 [array.overview]:
-?-iterator
andconst_iterator
are literal types.
[2017-03-01, Kona]
Antony Polukhin provides revised wording.
[2017-03-02, Kona]
Wording tweaks suggested by LWG applied.
[2017-03-02, Tim Song comments]
I don't believe the blanket "all operations" wording is quite correct.
First,T t;
(required by DefaultConstructible
) isn't usable in a constant expression if the iterator is a
pointer, since it would leave it uninitialized.
Second, an explicit destructor call u.~T()
(required by Destructible
) isn't usable if the iterator is a
class type because it explicitly invokes a non-constexpr
function (the destructor); see [expr.const]/2.2.
[2017-03-04, Kona]
Set priority to 2. Lisa and Alisdair to work with Antony to come up with better wording. The same wording can be used for 2938(i).
Previous resolution [SUPERSEDED]:
This wording is relative to N4640.
Add a new paragraph at the end of 23.3.3.1 [array.overview]:
-3- An
-?- All operations onarray
satisfies all of the requirements of a container […]iterator
andconst_iterator
that are required to satisfy the random access iterator requirements are usable in constant expressions.
[2017-04-22, Antony Polukhin provides improved wording]
[2017-11 Albuquerque Wednesday issue processing]
Status to Open; We don't want to do this yet; gated on Core issue 1581 (affects swap). See also 2800(i).
Thomas to talk to Anthony about writing a paper detailing what it takes to be a constexpr iterator.
[2017-11 Albuquerque Thursday]
It looks like 1581 is going to be resolved this week, so we should revisit soon.
[2017-11 Albuquerque Saturday issues processing]
P0858R0 (adopted on Sat; to be moved in Jacksonville) will resolve this.
[2018-06 Rapperswil Wednesday issues processing]
This was resolved by P0858, which was adopted in Jacksonville.
Proposed resolution:
This wording is relative to N4659.
Add a new paragraph at the end of 24.3.1 [iterator.requirements.general] as indicated:
-12- An invalid iterator is an iterator that may be singular.(footnote: […])
Iterators are called constexpr iterators if all defined iterator category operations, except a pseudo-destructor call ( [expr.pseudo]) or the construction of an iterator with a singular value, are usable as constant expressions. [Note: For example, the types "pointer to int" andreverse_iterator<int*>
are constexpr random access iterators. — end note] -13- In the following sections,a
andb
denote […]
Add a new paragraph at the end of 23.3.3.1 [array.overview]:
-3- An
-?-array
satisfies all of the requirements of a container […]iterator
andconst_iterator
satisfy the constexpr iterator requirements (24.3.1 [iterator.requirements.general]).
is_(nothrow_)move_constructible
and tuple
, optional
and unique_ptr
Section: 22.4 [tuple], 22.5 [optional], 20.3.1.3.2 [unique.ptr.single.ctor] Status: C++20 Submitter: United States Opened: 2017-02-03 Last modified: 2021-02-25
Priority: 2
View all other issues in [tuple].
View all issues with C++20 status.
Discussion:
Addresses US 110The move constructors for tuple
, optional
, and unique_ptr
should return false for
is_(nothrow_)move_constructible_v<TYPE>
when their corresponding Requires clauses are not
satisfied, as there are now several library clauses that are defined in terms of these traits. The same concern
applies to the move-assignment operator. Note that pair
and variant
already satisfy this constraint.
[2017-02-26, Scott Schurr provides wording]
[ 2017-06-27 P2 after 5 positive votes on c++std-lib. ]
[2016-07, Toronto Thursday night issues processing]
The description doesn't match the resolution; Alisdair to investigate. Status to Open
Previous resolution [SUPERSEDED]:
This wording is relative to N4640.
Modify 22.4.4 [tuple.tuple] as indicated:
// 20.5.3.1, tuple construction EXPLICIT constexpr tuple(); EXPLICIT constexpr tuple(const Types&...); // only if sizeof...(Types) >= 1 template <class... UTypes> EXPLICIT constexpr tuple(UTypes&&...) noexcept(see below); // only if sizeof...(Types) >= 1 tuple(const tuple&) = default; tuple(tuple&&) = default; template <class... UTypes> EXPLICIT constexpr tuple(const tuple<UTypes...>&); template <class... UTypes> EXPLICIT constexpr tuple(tuple<UTypes...>&&) noexcept(see below); template <class U1, class U2> EXPLICIT constexpr tuple(const pair<U1, U2>&); // only if sizeof...(Types) == 2 template <class U1, class U2> EXPLICIT constexpr tuple(pair<U1, U2>&&) noexcept(see below); // only if sizeof...(Types) == 2 […] // 20.5.3.2, tuple assignment tuple& operator=(const tuple&); tuple& operator=(tuple&&) noexcept(see below); template <class... UTypes> tuple& operator=(const tuple<UTypes...>&); template <class... UTypes> tuple& operator=(tuple<UTypes...>&&) noexcept(see below); template <class U1, class U2> tuple& operator=(const pair<U1, U2>&); // only if sizeof...(Types) == 2 template <class U1, class U2> tuple& operator=(pair<U1, U2>&&) noexcept(see below); // only if sizeof...(Types) == 2Modify 22.4.4.2 [tuple.cnstr] as indicated:
template <class... UTypes> EXPLICIT constexpr tuple(UTypes&&... u) noexcept(see below);-8- Effects: Initializes the elements in the tuple with the corresponding value in
-9- Remarks: This constructor shall not participate in overload resolution unlessstd::forward<UTypes>(u)
.sizeof...(Types) == sizeof...(UTypes)
andsizeof...(Types) >= 1
andis_constructible_v<Ti, Ui&&>
istrue
for alli
. The constructor is explicit if and only ifis_convertible_v<Ui&&, Ti>
isfalse
for at least onei
. The expression insidenoexcept
is equivalent to the logical AND of the following expressions:is_nothrow_constructible_v<Ti, Ui&&>where
Ti
is thei
th type inTypes
, andUi
is thei
th type inUTypes
.[…]
template <class... UTypes> EXPLICIT constexpr tuple(tuple<UTypes...>&& u) noexcept(see below);-16- Effects: For all
-17- Remarks: This constructor shall not participate in overload resolution unless […] The constructor is explicit if and only ifi
, initializes thei
th element of*this
withstd::forward<Ui>(get<i>(u))
.is_convertible_v<Ui&&, Ti>
isfalse
for at least onei
. The expression insidenoexcept
is equivalent to the logical AND of the following expressions:is_nothrow_constructible_v<Ti, Ui&&>where
Ti
is thei
th type inTypes
, andUi
is thei
th type inUTypes
.[…]
template <class U1, class U2> EXPLICIT constexpr tuple(pair<U1, U2>&& u) noexcept(see below);-21- Effects: Initializes the first element with
-22- Remarks: This constructor shall not participate in overload resolution unlessstd::forward<U1>(u.first)
and the second element withstd::forward<U2>(u.second)
.sizeof...(Types) == 2
,is_constructible_v<T0, U1&&>
istrue
andis_constructible_v<T1, U2&&>
istrue
. -23- The constructor is explicit if and only ifis_convertible_v<U1&&, T0>
isfalse
oris_convertible_v<U2&&, T1>
isfalse
. The expression insidenoexcept
is equivalent to:is_nothrow_constructible_v<T0, U1&&> && is_nothrow_constructible_v<T1, U2&&>Modify 22.4.4.3 [tuple.assign] as indicated:
template <class... UTypes> tuple& operator=(tuple<UTypes...>&& u) noexcept(see below);-12- Effects: For all
-13- Remarks: This operator shall not participate in overload resolution unlessi
, assignsstd::forward<Ui>(get<i>(u))
toget<i>(*this)
.is_assignable_v<Ti&, Ui&&> == true
for alli
andsizeof...(Types) == sizeof...(UTypes)
. The expression insidenoexcept
is equivalent to the logical AND of the following expressions:is_nothrow_assignable_v<Ti&, Ui&&>where
Ti
is thei
th type inTypes
, andUi
is thei
th type inUTypes
.-14- Returns:
*this
.[…]
template <class U1, class U2> tuple& operator=(pair<U1, U2>&& u) noexcept(see below);-18- Effects: Assigns
-19- Remarks: This operator shall not participate in overload resolution unlessstd::forward<U1>(u.first)
to the first element of*this
andstd::forward<U2>(u.second)
to the second element of*this
.sizeof...(Types) == 2
andis_assignable_v<T0&, U1&&>
istrue
for the first typeT0
inTypes
andis_assignable_v<T1&, U2&&>
istrue
for the second typeT1
inTypes
. The expression insidenoexcept
is equivalent to:is_nothrow_assignable_v<T0&, U1&&> && is_nothrow_assignable_v<T1&, U2&&>-20- Returns:
*this
.
[2019-02-14; Jonathan comments and provides revised wording]
The suggested change was already made to std::optional
by LWG 2756(i). The current P/R for 2899
doesn't resolve the issue for std::tuple
or std::unique_ptr
. I hope the following alternative does.
[2019-02; Kona Wednesday night issue processing]
Status to Ready
Proposed resolution:
This wording is relative to N4800.
Modify 22.4.4.2 [tuple.cnstr] as indicated:
tuple(tuple&& u) = default;-13-
-14- Effects: For allRequiresConstraints:is_move_constructible_v<Ti>
istrue
for alli
.i
, initializes thei
th element of*this
withstd::forward<Ti>(get<i>(u))
.
Modify 20.3.1.3.2 [unique.ptr.single.ctor] as indicated:
unique_ptr(unique_ptr&& u) noexcept;-?- Constraints:
-15- Requires: Ifis_move_constructible_v<D>
istrue
.D
is not a reference type,D
shall satisfy the Cpp17MoveConstructible requirements (Table 26). Construction of the deleter from an rvalue of typeD
shall not throw an exception. […]
Modify 20.3.1.3.4 [unique.ptr.single.asgn] as indicated:
unique_ptr& operator=(unique_ptr&& u) noexcept;-?- Constraints:
-1- Requires: Ifis_move_assignable_v<D>
istrue
.D
is not a reference type,D
shall satisfy the Cpp17MoveAssignable requirements (Table 28) and assignment of the deleter from an rvalue of typeD
shall not throw an exception. Otherwise,D
is a reference type;remove_reference_t<D>
shall satisfy the Cpp17CopyAssignable requirements and assignment of the deleter from an lvalue of typeD
shall not throw an exception. -2- Effects: Callsreset(u.release())
followed byget_deleter() = std::forward<D>(u.get_deleter())
. […]
optional
are not constexpr
Section: 22.5.3 [optional.optional] Status: C++17 Submitter: United States Opened: 2017-02-03 Last modified: 2020-09-06
Priority: Not Prioritized
View all other issues in [optional.optional].
View all issues with C++17 status.
Discussion:
Addresses US 111The copy and move constructors of optional
are not constexpr
. However, the constructors taking a
const T&
or T&&
are constexpr
, and there is a precedent for having a constexpr
copy constructor in 29.4.3 [complex]. The defaulted copy and move constructors of pair
and tuple
are also conditionally constexpr
(see 20.4.2 [pairs.pair] p2 and 20.5.2.1 [tuple.cnstr] p2).
A strong motivating use-case is constexpr
functions returning optional values. This issue was discovered while
working on a library making heavy use of such.
Proposed change: Add constexpr to:
optional(const optional &); optional(optional &&) noexcept(see below);
[2017-02-23, Casey comments and suggests wording]
This issue corresponds to NB comment US 111, which requests that the move and copy constructors of
std::optional
be declared constexpr
. The PR simply suggests adding the constexpr
specifier to the declarations of the constructors. The PR fails to specify the most important thing —
and this has been a failing of Library in general — under what conditions is the thing that we've
declared constexpr actually expected to be usable in constant expression context? (I think the proper
standardese here is "under what conditions is the full expression of an initialization that would invoke
these constructors a constant subexpression?")
optional<T>
must store a T
in a union to provide constexpr constructors that either do [optional(T const&)
] or do not
[optional()
] initialize the contained T
. A general implementation of optional
's
copy/move constructors must statically choose which union member, if any, to activate in each constructor.
Since there is no way to change the active member of a union in a constant expression, and a constructor must
statically choose a union member to activate (i.e., without being affected by the runtime state of the
copy/move constructor's argument) it's not possible to implement a general constexpr copy/move constructor
for optional
.
It is, however, possible to copy/move construct a trivially copy constructible/trivially move constructible union
in constexpr
context, which effectively copies the union's object representation, resulting in a union
whose active member is the same as the source union's. Indeed, at least two major implementations of optional
(MSVC and libc++) already provide that behavior as a conforming optimization when T
is a trivially
copyable type. If we are to declare optional<T>
's copy and move constructors constexpr
,
we should additionally specify that those constructors are only required to have the "constexpr mojo" when T
is trivially copyable. (Note that I suggest "trivially copyable" here rather than "trivially copy constructible or
trivially move constructible" since the simpler requirement is simpler to implement, and I don't believe the more
complicated requirement provides any additional benefit: I've never seen a trivially copy constructible or
trivially move constructible type outside of a test suite that was not also trivially copyable.)
Previous resolution [SUPERSEDED]:
This wording is relative to N4618.
Edit 22.5.3 [optional.optional] as indicated:
constexpr optional(const optional &);
constexpr optional(optional &&) noexcept(see below);
Edit 22.5.3.2 [optional.ctor] paragraph as indicated:
constexpr optional(const optional &);
and
constexpr optional(optional &&) noexcept(see below);
[2017-02-23, Marshall comments]
This is related to LWG 2745(i).
[2017-02-28, Kona, Casey comments and improves wording]
Amended PR per LWG discussion in Kona: replace the "is trivially copyable" requirement with the more specific
is_trivially_copy/move_constructible<T>
. LWG was concerned that tuple
is a counter-example
to the assumption that all three traits are equivalent for real-world types.
Previous resolution [SUPERSEDED]:
This wording is relative to N4640.
Change the synopsis of class template
optional
in 22.5.3 [optional.optional] as follows:[…] // 20.6.3.1, constructors constexpr optional() noexcept; constexpr optional(nullopt_t) noexcept; constexpr optional(const optional&); constexpr optional(optional&&) noexcept(see below); […]Modify 22.5.3.2 [optional.ctor] as indicated:
constexpr optional(const optional& rhs);[…]
-6- Remarks: This constructor shall be defined as deleted unlessis_copy_constructible_v<T>
istrue
. IfT
is a trivially copyable type, this constructor shall be aconstexpr
constructor.constexpr optional(optional&& rhs) noexcept(see below);[…]
-10- Remarks: The expression insidenoexcept
is equivalent tois_nothrow_move_constructible_v<T>
. This constructor shall not participate in overload resolution unlessis_move_constructible_v<T>
istrue
. IfT
is a trivially copyable type, this constructor shall be aconstexpr
constructor.
[Kona 2017-02-27]
Accepted as Immediate to resolve NB comment.
Proposed resolution:
This wording is relative to N4640.
Change the synopsis of class template optional
in 22.5.3 [optional.optional] as follows:
[…] // 20.6.3.1, constructors constexpr optional() noexcept; constexpr optional(nullopt_t) noexcept; constexpr optional(const optional&); constexpr optional(optional&&) noexcept(see below); […]
Modify 22.5.3.2 [optional.ctor] as indicated:
constexpr optional(const optional& rhs);[…]
-6- Remarks: This constructor shall be defined as deleted unlessis_copy_constructible_v<T>
istrue
. Ifis_trivially_copy_constructible_v<T>
istrue
, this constructor shall be aconstexpr
constructor.constexpr optional(optional&& rhs) noexcept(see below);[…]
-10- Remarks: The expression insidenoexcept
is equivalent tois_nothrow_move_constructible_v<T>
. This constructor shall not participate in overload resolution unlessis_move_constructible_v<T>
istrue
. Ifis_trivially_move_constructible_v<T>
istrue
, this constructor shall be aconstexpr
constructor.
Section: 22.6.3 [variant.variant] Status: C++17 Submitter: United States Opened: 2017-02-03 Last modified: 2020-09-06
Priority: 0
View all other issues in [variant.variant].
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Discussion:
Addresses US 113Variants cannot properly support allocators, as any assignment of a subsequent value throws away the allocator used at
construction. This is not an easy problem to solve, so variant
would be better served dropping the illusion of
allocator support for now, leaving open the possibility to provide proper support once the problems are fully understood.
Proposed change: Strike the 8 allocator aware constructor overloads from the class definition, and strike
20.7.2.1 [variant.ctor] p34/35. Strike clause 20.7.12 [variant.traits]. Strike the specialization of
uses_allocator
for variant
in the <variant>
header synopsis, 20.7.1 [variant.general].
[2017-02-28, Kona, Casey provides wording]
[2017-06-29 Moved to Tentatively Ready after 7 positive votes on c++std-lib.]
[2017-07 Toronto Moved to Immediate]
Proposed resolution:
This wording is relative to N4659.
Change 22.6.2 [variant.syn], header <variant>
synopsis, as follows:
[…]// [variant.traits], allocator-related traits template <class T, class Alloc> struct uses_allocator; template <class... Types, class Alloc> struct uses_allocator<variant<Types...>, Alloc>;
Change 22.6.3 [variant.variant], class template variant
synopsis, as follows:
[…]// allocator-extended constructors template <class Alloc> variant(allocator_arg_t, const Alloc&); template <class Alloc> variant(allocator_arg_t, const Alloc&, const variant&); template <class Alloc> variant(allocator_arg_t, const Alloc&, variant&&); template <class Alloc, class T> variant(allocator_arg_t, const Alloc&, T&&); template <class Alloc, class T, class... Args> variant(allocator_arg_t, const Alloc&, in_place_type_t<T>, Args&&...); template <class Alloc, class T, class U, class... Args> variant(allocator_arg_t, const Alloc&, in_place_type_t<T>, initializer_list<U>, Args&&...); template <class Alloc, size_t I, class... Args> variant(allocator_arg_t, const Alloc&, in_place_index_t<I>, Args&&...); template <class Alloc, size_t I, class U, class... Args> variant(allocator_arg_t, const Alloc&, in_place_index_t<I>, initializer_list<U>, Args&&...);
Modify 22.6.3.2 [variant.ctor] as indicated:
// allocator-extended constructors template <class Alloc> variant(allocator_arg_t, const Alloc& a); template <class Alloc> variant(allocator_arg_t, const Alloc& a, const variant& v); template <class Alloc> variant(allocator_arg_t, const Alloc& a, variant&& v); template <class Alloc, class T> variant(allocator_arg_t, const Alloc& a, T&& t); template <class Alloc, class T, class... Args> variant(allocator_arg_t, const Alloc& a, in_place_type_t<T>, Args&&... args); template <class Alloc, class T, class U, class... Args> variant(allocator_arg_t, const Alloc& a, in_place_type_t<T>, initializer_list<U> il, Args&&... args); template <class Alloc, size_t I, class... Args> variant(allocator_arg_t, const Alloc& a, in_place_index_t<I>, Args&&... args); template <class Alloc, size_t I, class U, class... Args> variant(allocator_arg_t, const Alloc& a, in_place_index_t<I>, initializer_list<U> il, Args&&... args);
-34- Requires:Alloc
shall meet the requirements for an Allocator (16.4.4.6 [allocator.requirements]).-35- Effects: Equivalent to the preceding constructors except that the contained value is constructed with uses-allocator construction (20.2.8.2 [allocator.uses.construction]).
Modify [variant.traits] as indicated:
template <class... Types, class Alloc> struct uses_allocator<variant<Types...>, Alloc> : true_type { };
-1- Requires:Alloc
shall be an Allocator (16.4.4.6 [allocator.requirements]).-2- [Note: Specialization of this trait informs other library components that variant can be constructed with an allocator, even though it does not have a nestedallocator_type
. — end note]
Section: 22.6.3.2 [variant.ctor] Status: C++17 Submitter: United States Opened: 2017-02-03 Last modified: 2020-09-06
Priority: Not Prioritized
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Discussion:
Addresses US 118The form of initialization for the emplace-constructors is not specified. We are very clear to mandate "as if by direct
non-list initialization" for each constructor in optional
, so there is no ambiguity regarding parens vs. braces.
That wording idiom should be followed by variant
.
Proposed change: Insert the phrase "as if direct-non-list-initializing" at appropriate locations in paragraphs 19, 23, 27, and 31
[2017-02-20, Marshall adds wording]
[2017-02-27, Marshall adds wording to cover two more cases]
Previous resolution [SUPERSEDED]:
This wording is relative to N4640.
Modify 22.6.3.2 [variant.ctor] paragraph 19 as indicated:
Effects: Initializes the contained value as if direct-non-list-initializing an object of typeT
with the argumentsstd::forward<Args>(args)...
.Modify 22.6.3.2 [variant.ctor] paragraph 23 as indicated:
Effects: Initializes the contained value as if direct-non-list-initializingconstructingan object of typeT
with the argumentsil, std::forward<Args>(args)...
.Modify 22.6.3.2 [variant.ctor] paragraph 27 as indicated:
Effects: Initializes the contained value as if direct-non-list-initializingconstructingan object of typeTI
with the argumentsstd::forward<Args>(args)...
.Modify 22.6.3.2 [variant.ctor] paragraph 31 as indicated:
Effects: Initializes the contained value as if direct-non-list-initializingconstructingan object of typeTI
with the argumentsil, std::forward(args)...
.
[Kona 2017-02-28]
Accepted as Immediate to resolve NB comment.
Proposed resolution:
This wording is relative to N4640.
Modify 22.6.3.2 [variant.ctor] paragraph 19 as indicated:
Effects: Initializes the contained value as if direct-non-list-initializing an object of typeT
with the argumentsstd::forward<Args>(args)...
.
Modify 22.6.3.2 [variant.ctor] paragraph 23 as indicated:
Effects: Initializes the contained value as if direct-non-list-initializingconstructingan object of typeT
with the argumentsil, std::forward<Args>(args)...
.
Modify 22.6.3.2 [variant.ctor] paragraph 27 as indicated:
Effects: Initializes the contained value as if direct-non-list-initializingconstructingan object of typeTI
with the argumentsstd::forward<Args>(args)...
.
Modify 22.6.3.2 [variant.ctor] paragraph 31 as indicated:
Effects: Initializes the contained value as if direct-non-list-initializingconstructingan object of typeTI
with the argumentsil, std::forward(args)...
.
Modify 22.6.3.5 [variant.mod] paragraph 6 as indicated:
Effects: Destroys the currently contained value ifvalueless_by_exception()
is false. Thendirect-initializes the contained value as if direct-non-list-initializingconstructinga value of typeTI
with the argumentsstd::forward<Args>(args)...
.
Modify 22.6.3.5 [variant.mod] paragraph 11 as indicated:
Effects: Destroys the currently contained value ifvalueless_by_exception()
is false. Thendirect-initializes the contained value as if direct-non-list-initializingconstructinga value of typeTI
with the argumentsil, std::forward<Args>(args)...
.
variant
move-assignment more exception safeSection: 22.6.3.4 [variant.assign] Status: C++17 Submitter: United States Opened: 2017-02-03 Last modified: 2020-09-06
Priority: Not Prioritized
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Discussion:
Addresses US 119 and CH 7The copy-assignment operator is very careful to not destroy the contained element until after a temporary has been constructed, which can be safely moved from.
This makes the valueless_by_exception
state extremely rare, by design.
However, the same care and attention is not paid to the move-assignment operator, nor the assignment-from-deduced-value assignment template. This concern should be similarly important in these cases, especially the latter.
Proposed change: —
[2017-03-02, Kona, Casey comments and suggests wording]
The wording below has been developed with much input from Tomasz.
[Kona 2017-03-02]
Accepted as Immediate to resolve NB comment.
Proposed resolution:
This wording is relative to N4640.
Modify 22.6.3.4 [variant.assign] as indicated:
[Drafting note: Presentation of para 9 immediately below has been split into individual bullets.]
variant& operator=(const variant& rhs);Let
-1- Effects:j
berhs.index()
.
(1.1) — If neither
*this
norrhs
holds a value, there is no effect. Otherwise,(1.2) — if
*this
holds a value butrhs
does not, destroys the value contained in*this
and sets*this
to not hold a value. Otherwise,(1.3) — if
index() == j
, assigns the value contained inrhs.index()rhs
to the value contained in*this
. Otherwise,(1.?) — if
is_nothrow_copy_constructible_v<Tj> || !is_nothrow_move_constructible_v<Tj>
istrue
, equivalent toemplace<j>(get<j>(rhs))
. Otherwise,(1.4) — equivalent to
operator=(variant(rhs))
copies the value contained in.rhs
to a temporary, then destroys any value contained in*this
. Sets*this
to hold the same alternative index asrhs
and initializes the value contained in*this
as if direct-non-list-initializing an object of typeTj
withstd::forward<Tj>(TMP)
, withTMP
being the temporary andj
beingrhs.index()
-2- Returns:
-3- Postconditions:*this.
index() == rhs.index()
. -4- Remarks: This function shall not participate in overload resolution unlessis_copy_constructible_v<Ti>
is&& is_move_constructible_v<Ti>&& is_copy_assignable_v<Ti>true
for alli
.
(4.1) — If an exception is thrown during the call […]
(4.2) — If an exception is thrown during the call […]
(4.3) — If an exception is thrown during the call […]variant& operator=(variant&& rhs) noexcept(see below);Let
-5- Effects:j
berhs.index()
.
(5.1) — If neither
*this
norrhs
holds a value, there is no effect. Otherwise,(5.2) — if
*this
holds a value butrhs
does not, destroys the value contained in*this
and sets*this
to not hold a value. Otherwise,(5.3) — if
index() == j
, assignsrhs.index()get<j>(std::move(rhs))
to the value contained in*this
, with. Otherwise,j
beingindex()
(5.4) — equivalent to
emplace<j>(get<j>(std::move(rhs)))
destroys any value contained in.*this
. Sets*this
to hold the same alternative index asrhs
and initializes the value contained in*this
as if direct-non-list-initializing an object of typeTj
withget<j>(std::move(rhs))
withj
beingrhs.index()
[…]
[…]
template <class T> variant& operator=(T&& t) noexcept(see below);-8- […]
-9- Effects:
(9.1) — If *this holds a
Tj
, assignsstd::forward<T>(t)
to the value contained in*this
. Otherwise,(9.?) — if
is_nothrow_constructible_v<Tj, T> || !is_nothrow_move_constructible_v<Tj>
istrue
, equivalent toemplace<j>(std::forward<T>(t))
. Otherwise,(9.3) — equivalent to
operator=(variant(std::forward<T>(t)))
destroys any value contained in.*this
, sets*this
to hold the alternative typeTj
as selected by the imaginary function overload resolution described above, and direct-initializes the contained value as if direct-non-list-initializing it withstd::forward<T>(t)
is_constructible_v<unique_ptr<P, D>, P, D const &>
should be false when
D
is not copy constructibleSection: 20.3.1.3.2 [unique.ptr.single.ctor] Status: C++17 Submitter: United States Opened: 2017-02-03 Last modified: 2017-07-30
Priority: Not Prioritized
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Discussion:
Addresses US 123is_constructible_v<unique_ptr<P, D>, P, D const &>
should be false when D
is not
copy constructible, and similarly for D&&
when D
is not move constructible. This could be
achieved by the traditional 'does not participate in overload resolution' wording, or similar.
Proposed change: Add a Remarks: clause to constrain the appropriate constructors.
[2017-02-28, Jonathan comments and provides concrete wording]
As well as addressing the NB comment, this attempts to make some further improvements to the current wording, which is a little strange.
It incorrectly uses "d
" to mean the constructor argument that initializes the parameter d
, and unnecessarily explains how overload resolution works for lvalues and rvalues.
It refers to the copy/move constructor of D
, but the constructor that is selected to perform the initialization may not be a copy/move constructor (e.g. initializing a deleter object from an rvalue might use a copy constructor if there is no move constructor).
The condition "d
shall be reference compatible with one of the constructors" is bogus: reference compatible is a property of two types, not a value and a constructor, and again is trying to talk about the argument not the parameter.
Note that we could replace the "see below" in the signatures and paragraphs 9, 10 and 11 by declaring the constructors as:
unique_ptr(pointer p, const D& d) noexcept; unique_ptr(pointer p, remove_reference_t<D>&& d) noexcept;
I think this produces the same signatures in all cases. I haven't proposed that here, it could be changed separately if desired.
[Kona 2017-02-27]
Accepted as Immediate to resolve NB comment.
Proposed resolution:
Modify [unique.ptr.single.ctor] paragraphs 9-11 as shown:
unique_ptr(pointer p, see below d1) noexcept; unique_ptr(pointer p, see below d2) noexcept;-9- The signature of these constructors depends upon whether
D
is a reference type. IfD
is a non-reference typeA
, then the signatures areunique_ptr(pointer p, const A& d) noexcept; unique_ptr(pointer p, A&& d) noexcept;
-10- If
D
is an lvalue reference typeA&
, then the signatures are:unique_ptr(pointer p, A& d) noexcept; unique_ptr(pointer p, A&& d) = delete;
-11- If
D
is an lvalue reference typeconst A&
, then the signatures are:unique_ptr(pointer p, const A& d) noexcept; unique_ptr(pointer p, const A&& d) = delete;
Remove paragraph 12 entirely:
-12- Requires:
IfD
is not an lvalue reference type then
Ifd
is an lvalue or const rvalue then the first constructor of this pair will be selected.D
shall satisfy the requirements ofCopyConstructible
(Table 24), and the copy constructor ofD
shall not throw an exception. Thisunique_ptr
will hold a copy ofd
.Otherwise,d
is a non-const rvalue and the second constructor of this pair will be selected.D
shall satisfy the requirements ofMoveConstructible
(Table 23), and the move constructor ofD
shall not throw an exception. Thisunique_ptr
will hold a value move constructed fromd
.OtherwiseD
is an lvalue reference type.d
shall be reference-compatible with one of the constructors. Ifd
is an rvalue, it will bind to the second constructor of this pair and the program is ill-formed. [Note: The diagnostic could be implemented using a static_assert which assures thatD
is not a reference type. — end note] Elsed
is an lvalue and will bind to the first constructor of this pair. The type whichD
references need not beCopyConstructible
norMoveConstructible
. Thisunique_ptr
will hold aD
which refers to the lvalued
. [Note:D
may not be an rvalue reference type. — end note]
Modify paragraph 13 as shown:
-13- Effects: Constructs a
unique_ptr
object which ownsp
, initializing the stored pointer withp
and initializing the deleteras described abovefromstd::forward<decltype(d)>(d)
.
Add a new paragraph after paragraph 14 (Postconditions):
-?- Remarks: These constructors shall not participate in overload resolution unless
is_constructible_v<D, decltype(d)>
istrue
.
Section: 20.3.2.2.8 [util.smartptr.shared.cmp] Status: C++17 Submitter: United States Opened: 2017-02-03 Last modified: 2020-09-06
Priority: Not Prioritized
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Discussion:
Addresses US 135The less-than operator for shared pointers compares only those combinations that can form a composite pointer type.
With the C++17 wording for the diamond functor, less<>
, we should be able to support comparison of a wider
range of shared pointers, such that less<>::operator(shared_ptr<A>, shared_ptr<B>)
is consistent
with less<>::operator(A *, B *)
.
Proposed change: Replace less<V>
with just less<>
, and drop the reference to composite
pointer types.
[2017-03-02, Kona, STL comments and provides wording]
We talked about this in LWG, and I talked to Alisdair. The status quo is already maximally general (operator less-than in Core always forms a composite pointer type), but we can use diamond-less to significantly simplify the specification. (We can't use less-than directly, due to the total ordering issue, which diamond-less was recently "upgraded" to handle.)
[Kona 2017-03-02]
Accepted as Immediate to resolve NB comment.
Proposed resolution:
This wording is relative to N4640.
Change 20.3.2.2.8 [util.smartptr.shared.cmp] as depicted:
template<class T, class U> bool operator<(const shared_ptr<T>& a, const shared_ptr<U>& b) noexcept;-2- Returns:
less<
V>()(a.get(), b.get()), where.V
is the composite pointer type (Clause 5) ofshared_ptr<T>::element_type*
andshared_ptr<U>::element_type*
is_aggregate
type trait is neededSection: 21.3.5.4 [meta.unary.prop] Status: C++17 Submitter: United States Opened: 2017-02-03 Last modified: 2020-09-06
Priority: Not Prioritized
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Discussion:
Addresses US 143An is_aggregate
type trait is needed. The emplace idiom is now common throughout the library, but typically relies
on direct non-list initalization, which does not work for aggregates. With a suitable type trait, we could extend direct non-list-initlaization to perform aggregate-initalization on aggregate types.
Proposed change:
Add a new row to Table 38:
template <class T> struct is_aggregate;
T
is an aggregate type ([dcl.init.aggr])remove_all_extents_t<T>
shall be a complete type, an array type, or (possibly cv-qualified)void
.
[2017-02-26, Daniel comments and completes wording]
The proposed wording is incomplete, since it doesn't update the <type_traits>
header synopsis, also
ensuring that the corresponding is_aggregate_v
variable template is defined.
[2017-03-01, Daniel comments and adjusts wording]
There is only one minor change performed, namely to mark the constant in the is_aggregate_v
variable
template as inline
following the LWG preferences for "full inline" bullet list (B2) from
P0607R0.
[2017-03-03, Kona, LEWG]
No interest in doing the is-list-initializable instead. No objection to getting that proposal also.
Want this for C++17? SF | F | N | A | SA 2 | 8 | 1 | 3 | 0 Want this for C++20? SF | F | N | A | SA 6 | 5 | 2 | 0 | 0 Remember to correct the wording to: "remove_all_extents_t<T>
shall be a complete type or cv-void
."
[2017-03-03, Daniel updates wording]
In sync with existing writing, this was changed to "remove_all_extents_t<T>
shall be a complete type or cv void
"
[Kona 2017-03-02]
Accepted as Immediate to resolve NB comment.
This will require a new complier intrinsic; Alisdair checked with Core and they're OK with that
Proposed resolution:
This wording is relative to N4640.
Modify 21.3.3 [meta.type.synop], <type_traits>
header synopsis, as indicated:
// 20.15.4.3, type properties […] template <class T> struct is_final; template <class T> struct is_aggregate; […] // 20.15.4.3, type properties […] template <class T> constexpr bool is_final_v = is_final<T>::value; template <class T> inline constexpr bool is_aggregate_v = is_aggregate<T>::value; […]
Add a new row to Table 42 — "Type property predicates":
template <class T> struct is_aggregate;
T
is an aggregate type (9.5.2 [dcl.init.aggr])remove_all_extents_t<T>
shall be a complete type or cvvoid
.
duration
Section: 30.5 [time.duration] Status: Resolved Submitter: United States Opened: 2017-02-03 Last modified: 2020-09-06
Priority: Not Prioritized
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Discussion:
Addresses US 144Proposed change:
Add to <chrono>
synopsis:
template <class Rep, class Period> duration(const Rep &) -> duration<Rep>;
[2017-02-21; via email]
This should be addressed by P0433R2.
[2017-03-12, post-Kona]
Resolved by P0433R2.
Proposed resolution:
This wording is relative to N4618.
Add to the synopsis of <chrono>
:
template <class Rep, class Period> duration(const Rep &) -> duration<Rep>;
Section: 23 [containers] Status: Resolved Submitter: United States Opened: 2017-02-03 Last modified: 2017-03-20
Priority: Not Prioritized
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Discussion:
Addresses US 147One of the motivating features behind deduction guides was constructing containers from a pair of iterators, yet the standard
library does not provide any such deduction guides. They should be provided in header synopsis for each container in clause 23.
It is expected that the default arguments from the called constructors will provide the context to deduce any remaining class template arguments, such as the Allocator type, and default comparators/hashers for (unordered) associative containers. At this stage, we
do not recommend adding additional guides to deduce a (rebound) allocator, comparator etc. due to the likely large number of such
guides. It is noted that the requirements on iterator_traits
to be an empty type will produce a SFINAE condition to
allow correct deduction for vector in the case of the Do-The-Right-Thing clause, resolving ambiguity between two integers, and
two iterators.
Proposed change: For each container in clause 23, add to the header synopsis a deduction guide of the form
template <class Iterator> container(Iterator, Iterator) -> container<typename iterator_traits<Iterator>::value_type>;
[2017-03-12, post-Kona]
Resolved by P0433R2.
Proposed resolution:
std::array
does not support class-template deduction from initializersSection: 23.3.2 [array.syn] Status: Resolved Submitter: United States Opened: 2017-02-03 Last modified: 2020-09-06
Priority: Not Prioritized
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Discussion:
Addresses US 148std::array
does not support class-template deduction from initializers without a deduction guide.
Proposed change:
Add to <array>
synopsis:
template <class TYPES> array(TYPES&&...) -> array<common_type_t<TYPES...>, sizeof...(TYPES)>;
[2017-03-12, post-Kona]
Resolved by P0433R2.
Proposed resolution:
This wording is relative to N4618.
Add to the synopsis of <array>
:
template <class TYPES> array(TYPES&&...) -> array<common_type_t<TYPES...>, sizeof...(TYPES)>;
Section: 23.6 [container.adaptors] Status: Resolved Submitter: United States Opened: 2017-02-03 Last modified: 2017-03-20
Priority: Not Prioritized
View all other issues in [container.adaptors].
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Discussion:
Addresses US 150The three container adapters should each have a deduction guide allowing the deduction of the value type T
from the supplied container, potentially constrained to avoid confusion with deduction from a copy/move constructor.
Proposed change: For each container adapter, add a deduction guide of the form
template <class Container> adapter(const Container&) -> adapter<typename Container::value_type, Container>;
[2017-03-12, post-Kona]
Resolved by P0433R2.
Proposed resolution:
InputIterator
sSection: 26 [algorithms], 26.10 [numeric.ops] Status: Resolved Submitter: United States Opened: 2017-02-03 Last modified: 2020-09-06
Priority: Not Prioritized
View other active issues in [algorithms].
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Discussion:
Addresses US 156Parallel algorithms cannot easily work with InputIterator
s, as any attempt to partition the work is going to
invalidate iterators used by other sub-tasks. While this may work for the sequential execution policy, the goal of that
policy is to transparently switch between serial and parallel execution of code without changing semantics, so there
should not be a special case extension for this policy. There is a corresponding concern for writing through
OutputIterator
s. Note that the input iterator problem could be mitigated, to some extent, by serially copying/moving
data out of the input range and into temporary storage with a more favourable iterator category, and then the work of the
algorithm can be parallelized. If this is the design intent, a note to confirm that in the standard would avoid future
issues filed in this area. However, the requirement of an algorithm that must copy/move values into intermediate storage
may not be the same as those acting immediately on a dereferenced input iterator, and further issues would be likely.
It is not clear that anything can be done to improve the serial nature of writing to a simple output iterator though.
Proposed change: All algorithms in the <algorithm>
and <numeric>
headers that take an
execution policy and an InputIterator
type should update that iterator to a ForwardIterator
, and similarly
all such overloads taking an OutputIterator
should update that iterator to a ForwardIterator
.
(Conversely, if the design intent is confirmed to support input and output iterators, add a note to state that clearly and avoid confusion and more issues by future generations of library implementers.)
[2017-02-13, Alisdair comments]
The pre-Kona mailing has two competing papers that provide wording to address #2917, sequential constraints on parallel algorithms. They should probably be cross-refrenced by the issue:
P0467R1: Iterator Concerns for Parallel Algorithms
P0574R0: Algorithm Complexity Constraints and Parallel Overloads
[2017-03-12, post-Kona]
Resolved by P0467R2.
Proposed resolution:
inner_product
Section: 26.10.5 [inner.product] Status: Resolved Submitter: United States Opened: 2017-02-03 Last modified: 2017-03-20
Priority: Not Prioritized
View all issues with Resolved status.
Discussion:
Addresses US 161There is a surprising sequential operation applying BinaryOp1
in inner_product
that may, for example,
require additional storage for the parallel algorithms to enable effective distribution of work, and is likely to be a
performance bottleneck. GENERALIZED_SUM
is probably intended here for the parallel version of the algorithm,
with the corresponding strengthening on constraints on BinaryOp1
to allow arbitrary order of evaluation.
Proposed change: For the overloads taking an execution policy, copy the current specification, but replace algorithm in Effects with
GENERALIZED_SUM(plus<>(), init, multiplies<>(*i1, *i2), ...) GENERALIZED_SUM(binary_op1, init, binary_op2(*i1, *i2), ...)
[2017-03-12, post-Kona]
Resolved by P0623R0.
Proposed resolution:
adjacent_difference
has baked-in sequential semanticsSection: 26.10.12 [adjacent.difference] Status: Resolved Submitter: United States Opened: 2017-02-03 Last modified: 2020-09-06
Priority: Not Prioritized
View all other issues in [adjacent.difference].
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Discussion:
Addresses US 162The specification for adjacent_difference
has baked-in sequential semantics, in order to support reading/writing
through input/output iterators. There should a second specification more amenable to parallelization for the overloads taking
an execution policy.
Proposed change: Provide a specification for the overloads taking an execution policy this is more clearly suitable for parallel execution. (i.e., one that does not refer to an accumulated state.)
[2017-02-25, Alisdair comments]
Anthony Williams's paper on parallel algorithm complexity, p0574r0, includes wording that would resolve LWG issue 2919, and I suggest we defer initial triage to handling that paper.
[2017-03-12, post-Kona]
Resolved by P0467R2.
Proposed resolution:
shared_future
from a future
rvalueSection: 32.10.8 [futures.shared.future] Status: Resolved Submitter: United States Opened: 2017-02-03 Last modified: 2021-06-06
Priority: Not Prioritized
View all other issues in [futures.shared.future].
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Discussion:
Addresses US 164Add a deduction guide for creating a shared future from a future rvalue.
Proposed change:
Add to the <future>
synopsis:
template <class R> shared_future(future<R>&&) -> shared_future<R>;
[2017-03-12, post-Kona]
Resolved by P0433R2.
Proposed resolution:
This wording is relative to N4618.
Add to the synopsis of <future>
:
template <class R> shared_future(future<R>&&) -> shared_future<R>;
packaged_task
and type-erased allocatorsSection: 32.10.10 [futures.task] Status: C++17 Submitter: United States Opened: 2017-02-03 Last modified: 2020-09-06
Priority: Not Prioritized
View all other issues in [futures.task].
View all issues with C++17 status.
Discussion:
Addresses US 165The constructor that type-erases an allocator has all of the problems of the similar function constructor that was removed
for this CD. This constructor from packaged_task
should similarly be removed as well. If we prefer to keep this
constructor, the current wording is underspecified, as the Allocator
argument is not required to be type satisfying
the Allocator requirements, nor is allocator_traits
used.
Proposed change:
Strike
template <class F, class Allocator> packaged_task(allocator_arg_t, const Allocator& a, F&& f);
from the class definition in p2, and from 30.6.9.1 [futures.task.members] p2. Strike the last sentence of 30.6.9.1p4. In p3, revise "These constructors" to "This constructor"
[Kona 2017-03-02]
Accepted as Immediate to resolve NB comment.
Proposed resolution:
This wording is relative to N4618.
Modify 32.10.10 [futures.task] as follows:
Strike
template <class F, class Allocator> packaged_task(allocator_arg_t, const Allocator& a, F&& f);
from the class definition in p2, and from [futures.task.members] p2.
Modify 32.10.10.2 [futures.task.members]/3:
Remarks:These constructorsThis constructor shall not participate in overload resolution ifdecay_t<F>
is the same type aspackaged_task<R(ArgTypes...)>
.
Strike the last sentence of 32.10.10.2 [futures.task.members]/4:
The constructors that take anAllocator
argument use it to allocate memory needed to store the internal data structures.
ExecutionPolicy
overload for inner_product()
seems impracticalSection: 26.10 [numeric.ops] Status: Resolved Submitter: United States Opened: 2017-02-03 Last modified: 2017-03-20
Priority: Not Prioritized
View all other issues in [numeric.ops].
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Discussion:
Addresses US 184An ExecutionPolicy
overload for inner_product()
is specified in the synopsis of <numeric>
.
Such an overload seems impractical. inner_product()
is ordered and cannot be parallelized; this was the motivation for the introduction of transform_reduce()
.
Proposed change: Delete the ExecutionPolicy
overload for inner_product()
.
[2017-03-12, post-Kona]
Resolved by P0623R0.
Proposed resolution:
Section: 16 [library] Status: Resolved Submitter: United States Opened: 2017-02-03 Last modified: 2020-09-06
Priority: Not Prioritized
View other active issues in [library].
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Discussion:
Addresses US 7P0091R3 "Template argument deduction for class templates (Rev. 6)" was adopted for the core language, but the Standard Library makes no explicit use of this new feature, even though the promise of such use provided strong motivation for the feature.
Proposed change: Analyze the Standard Library's constructors to determine which classes would profit from explicit deduction guides. Formulate the appropriate guides for those classes and insert them in their respective types.
[2017-02-03, Marshall notes]
P0433 is an attempt to do exactly this.
[2017-03-12, post-Kona]
Resolved by P0433R2.
Proposed resolution:
INVOKE(f, t1, t2,... tN)
and INVOKE(f, t1, t2,... tN, R)
are too similarSection: 22.10.4 [func.require] Status: Resolved Submitter: United States Opened: 2017-02-03 Last modified: 2017-03-20
Priority: Not Prioritized
View other active issues in [func.require].
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Discussion:
Addresses US 84The distinction between INVOKE(f, t1, t2,... tN)
and INVOKE(f, t1, t2,... tN, R)
is too subtle.
If the last argument is an expression, it represents tN
, if it's a type, then it represents R
. Very clumsy.
Proposed change: Rename INVOKE(f, t1, t2,... tN, R)
to INVOKE_R(R, f, t1, t2,... tN)
and
adjust all uses of this form. (Approximately 10 occurrences of invoke would need to change.)
[2017-02-24, Daniel comments]
I suggest to apply the paper d0604r0, available on the Kona LWG wiki, implements this suggestions.
[2017-03-12, post-Kona]
Resolved by p0604r0.
Proposed resolution:
is_callable
and result_of
is fragileSection: 21.3.3 [meta.type.synop], 21.3.7 [meta.rel] Status: Resolved Submitter: United States Opened: 2017-02-03 Last modified: 2017-03-21
Priority: Not Prioritized
View other active issues in [meta.type.synop].
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Discussion:
Addresses US 85The trick of encoding a functor and argument types as a function signature for is_callable
and
result_of
loses cv information on argument types, fails for non-decayed function types, and is confusing. E.g.,
typedef int MyClass::*mp; result_of_t<mp(const MyClass)>; // should be const, but isn't typedef int F(double); is_callable<F(float)>; // ill-formed
Minimal change:
Replace is_callable<Fn(ArgTypes...)>
with
is_callable<Fn, ArgTypes...>
and replace is_callable<Fn(ArgTypes...), R>
with is_callable_r<R, Fn, ArgTypes...>
.
Do the same for is_nothrow_callable
.
Preferred change:
All of the above, plus deprecate result_of<Fn(ArgTypes...)>
and replace it with
result_of_invoke<Fn, ArgTypes...>
[2017-02-22, Daniel comments]
LWG 2895(i) provides now wording for this issue and for LWG 2928(i) as well.
[2017-02-24, Daniel comments]
I suggest to apply the paper d0604r0 instead, available on the Kona LWG wiki.
[2017-03-12, post-Kona]
Resolved by p0604r0.
Proposed resolution:
is_callable
is not a good nameSection: 21.3.3 [meta.type.synop], 21.3.7 [meta.rel] Status: Resolved Submitter: United States Opened: 2017-02-03 Last modified: 2020-09-06
Priority: Not Prioritized
View other active issues in [meta.type.synop].
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Discussion:
Addresses US 86is_callable
is not a good name because it implies F(A...)
instead of INVOKE(F, A...)
Proposed change: Rename is_callable
to is_invocable
and rename is_nothrow_callable
to
is_nothrow_invocable
.
[2017-02-22, Daniel comments and provides concrete wording]
I'm strongly in favour for this change to possibly allow for a future "pure" is_callable
trait that solely
describes function call-like expressions.
Previous resolution [SUPERSEDED]:
This wording is relative to N4640.
Modify 21.3.3 [meta.type.synop], header
<type_traits>
synopsis, as indicated:[…] // 20.15.6, type relations […] template <class, class R = void> struct is_invocablecallable; // not defined template <class Fn, class... ArgTypes, class R> struct is_invocablecallable<Fn(ArgTypes...), R>; template <class, class R = void> struct is_nothrow_invocablecallable; // not defined template <class Fn, class... ArgTypes, class R> struct is_nothrow_invocablecallable<Fn(ArgTypes...), R>; […] // 20.15.6, type relations […] template <class T, class R = void> constexpr bool is_invocablecallable_v = is_invocablecallable<T, R>::value; template <class T, class R = void> constexpr bool is_nothrow_invocablecallable_v = is_nothrow_invocablecallable<T, R>::value; […]Modify 21.3.7 [meta.rel], Table 44 — "Type relationship predicates", as indicated:
Table 44 — Type relationship predicates […]
template <class Fn, class...
ArgTypes, class R>
struct is_invocablecallable<
Fn(ArgTypes...), R>;[…] […] template <class Fn, class...
ArgTypes, class R>
struct is_nothrow_invocablecallable<
Fn(ArgTypes...), R>;is_invocable
iscallable_v<
Fn(ArgTypes...), R>
true
[…][…]
[2017-02-24, Daniel comments]
I suggest to apply the paper d0604r0 instead, available on the Kona LWG wiki.
[2017-03-12, post-Kona]
Resolved by p0604r0.
Proposed resolution:
basic_string
misuses "Effects: Equivalent to"Section: 27.4.3.7.3 [string.assign] Status: Resolved Submitter: Jonathan Wakely Opened: 2017-02-03 Last modified: 2020-09-06
Priority: 3
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Discussion:
basic_string::assign(size_type n, charT c);
says:
Effects: Equivalent to
assign(basic_string(n, c))
.
This requires that a new basic_string
is constructed, using a
default-constructed allocator, potentially allocating memory, and then
that new string is copy-assigned to *this
, potentially propagating the
allocator. This must be done even if this->capacity() > n
,
because memory allocation and allocator propagation are observable side
effects. If the allocator doesn't propagate and isn't equal to
this->get_allocator()
then a second allocation may be required. This
can't be right; it won't even compile if the allocator isn't default
constructible.
basic_string::assign(InputIterator first, InputIterator last)
has a
similar problem, even if the iterators are random access and
this->capacity() > distance(first, last)
.
basic_string::assign(std::initializer_list<charT>
doesn't say
"Equivalent to" so maybe it's OK to not allocate anything if the list
fits in the existing capacity.
basic_string::append(size_type, charT)
and
basic_string::append(InputIterator, InputIterator)
have the same
problem, although they don't propagate the allocator, but still
require at least one, maybe two allocations.
A partial fix would be to ensure all the temporaries are constructed
with get_allocator()
so that they don't require default constructible
allocators, and so propagation won't alter allocators. The problem of
observable side effects is still present (the temporary might need to
allocate memory, even if this->capacity()
is large) but arguably it's
unspecified when construction allocates, to allow for small-string
optimisations.
[2017-03-04, Kona]
Set priority to 3. Thomas to argue on reflector that this is NAD.
[2017-03-07, LWG reflector discussion]
Thomas and Jonathan remark that LWG 2788(i) fixed most cases except the allocator respectance and provide wording for this:
Resolved by the adoption of P1148 in San Diego.
Proposed resolution:
This wording is relative to N4659.
Change 27.4.3.7.3 [string.assign] as indicated:
basic_string& assign(size_type n, charT c);-22- Effects: Equivalent to
assign(basic_string(n, c, get_allocator()))
.
Section: 26.3.3 [algorithms.parallel.exec] Status: C++20 Submitter: Dietmar Kühl Opened: 2017-02-05 Last modified: 2021-02-25
Priority: Not Prioritized
View all other issues in [algorithms.parallel.exec].
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Discussion:
Section 26.3.3 [algorithms.parallel.exec] specifies constraints a user of the parallel algorithms has to obey. Notably, it specifies in paragraph 3 that executions of element access functions are indeterminately sequenced with respect to each other. Correspondingly, it is the user's obligation to ensure that these calls do not introduce data races (this is also clarified in a note on this section).
Unfortunately, there is no constraint that, at least, mutating element access functions likeoperator++()
on an
iterator are called on different objects. An odd implementation of a parallel algorithm could increment a shared iterator
from two threads without synchronisation of its own and the user would be obliged to make sure there is no data race!
For example:
template <typename FwdIt> FwdIt adjacent_find(std::execution::parallel_policy, FwdIt it, FwdIt end) { if (2 <= std::distance(it, end)) { FwdIt tmp(it); auto f1 = std::async([&tmp](){ ++tmp; }); auto f2 = std::async([&tmp](){ ++tmp; }); f1.get(); f2.get(); } return std::adjancent_find(it, end); }
This code is, obviously, a contrived example but with the current specification a legal implementation of adjacent_find()
.
The problem is that, e.g., for pointers there is a data race when incrementing tmp
, i.e., the function can't be used
on pointers. I don't think any of the containers makes a guarantee that their iterators can be incremented without synchronisation,
i.e., the standard library doesn't have any iterators which could be used with this algorithm!
ExecutionPolicy
template parameter there are broadly
four groups of template parameters:
Parameters with a known set of possible arguments: ExecutionPolicy
(execution policies listed in
26.3.6 [execpol]).
Parameters specifying types of objects which are expected not to change: BinaryOperation
, BinaryPredicate
,
Compare
, Function
, Predicate
, UnaryFunction
, UnaryOperation
, and T
(all but
the last one are function objects although I couldn't locate concepts for some of them — that may be a separate issue).
Parameters of mutable types which are also meant to be mutated: InputIterator
, OutputIterator
,
ForwardIterator
, BidirectionalIterator
, RandomAccessIterator
, and Size
(the concept for
Size
also seems to be unspecified).
Some algorithms use Generator
which seems to be a mutable function object. However, I couldn't locate a concept
for this parameter.
The problematic group is 3 and possibly 4: mutations on the objects are expected. It seem the general approach of disallowing calling non-const functions without synchronisation applies. Note, however, that prohibiting calling of any non-const function from the algorithms would put undue burden on the implementation of algorithms: any of the accessor functions may be non-const although the concept assume that the function would be const. The constraint should, thus, only apply to functions which may mutate the object according to their respective concept.
Suggested Resolution:
Add a statement prohibiting unsequenced calls to element access functions on the same object which are not applicable to const objects according to the corresponding concept. I'm not sure how to best specify the constraint in general, though.
Since the current algorithms use relatively few concepts there are fairly few operations actually affected. It may be reasonable at least for the initial version (and until we could refer to constructs in concepts in the language) to explicitly list the affected operations. I haven't done a full audit but iterator++
, --
, @=
(for
@
being any of the operators which can be combined with an assignment), and assignments on all objects may be
the set of affected element access functions whose use needs to be constrained.
Here is a concrete proposal for the change: In 26.3.2 [algorithms.parallel.user] add a paragraph:
Parallel algorithms are constrained when calling mutating element access functions without synchronisation: if any mutating
element access function is called on an object there shall be no other unsynchronised accesses to this object. The mutating
element access functions are those which are specified as mutating object in the concept, notably assignment on any object,
operators ++
, --
, +=
, and -=
on any of the iterator or Size
parameters, and
any @=
operators on the Size
parameters.
Previous resolution [SUPERSEDED]:
This wording is relative to N4640.
Modify 26.3.2 [algorithms.parallel.user] as indicated:
-1- Function objects passed into parallel algorithms as objects of type
-?- Parallel algorithms are constrained when calling mutating element access functions without synchronisation: If any mutating element access function is called on an object there shall be no other unsynchronised accesses to this object. The mutating element access functions are those which are specified as mutating object in the concept, notably assignment on any object, operatorsPredicate
,BinaryPredicate
,Compare
, andBinaryOperation
shall not directly or indirectly modify objects via their arguments.++
,--
,+=
, and-=
on any of the iterator orSize
parameters, and any@=
operators on theSize
parameters.
[2017-03-03, Kona]
Dietmar provides improved wording. Issues with the PR before the change:
The part before the colon is redundant: we don't need to state that.
Replace "notably" with "specifically"
swap()
needs to be in the list.
Not sure what "called on an object means"
The assignment side is overconstrained: the right hand side is allowed.
Previous resolution [SUPERSEDED]:
This wording is relative to N4640.
Modify 26.3.2 [algorithms.parallel.user] as indicated:
-1- Function objects passed into parallel algorithms as objects of type
-?- If an object is mutated by an element access function the algorithm will perform no other unsynchronized accesses to that object. The mutating element access functions are those which are specified as mutating the object in the relevant concept, such asPredicate
,BinaryPredicate
,Compare
, andBinaryOperation
shall not directly or indirectly modify objects via their arguments.swap()
,++
,--
,@=
, and assignments. For the assignment and@=
operators only the left argument is mutated.
[2017-03-03, Kona]
Dietmar finetunes wording after review by SG1.
[2017-03-03, Kona]
Move to Ready
Proposed resolution:
This wording is relative to N4640.
Add a new paragraph following 26.3.3 [algorithms.parallel.exec] p1 as indicated:
-1- Parallel algorithms have template parameters named
-?- If an object is modified by an element access function the algorithm will perform no other unsynchronized accesses to that object. The modifying element access functions are those which are specified as modifying the object in the relevant concept [Note: For example,ExecutionPolicy
(20.19) which describe the manner in which the execution of these algorithms may be parallelized and the manner in which they apply the element access functions.swap()
,++
,--
,@=
, and assignments modify the object. For the assignment and@=
operators only the left argument is modified. — end note]. -2- […]
Section: 22.4.5 [tuple.creation] Status: Resolved Submitter: Eric Fiselier Opened: 2017-02-06 Last modified: 2024-07-25
Priority: 3
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Discussion:
The current PR for LWG 2773(i) changes std::ignore
to be a constexpr
variable. However it says
nothing about whether using std::ignore
in std::tie
is a constant expression. I think the intent was clearly
to allow this. Therefore I suggest we update the resolution to explicitly call this out in a note. (I don't think new normative
wording is needed).
Keep the current changes proposed by the PR.
Add a note after [tuple.creation]/p7 (std::tie
):
[Note: The constructors and assignment operators provided by
ignore
shall beconstexpr
]
Perhaps LWG feels the existing wording is clear enough, but if not I think the above changes sufficiently clarify it.
The ability toconstexpr
assign to std::ignore
can be important: Here is an extremely contrived example:
constexpr bool foo() { auto res = std::tie(std::ignore); std::get<0>(res) =42; return true; } static_assert(foo());
[2017-03-04, Kona]
Set priority to 3. P/R is incorrect; it should NOT be a note. Marshall to work with Eric to get better wording. STL says "use an exposition-only class".
[2024-07-25 Status changed: New → Resolved.]
Resolved by P2968R2, approved in St. Louis.
Proposed resolution:
This wording is relative to N4640.
Modify 22.4.5 [tuple.creation] as indicated:
template<class... TTypes> constexpr tuple<TTypes&...> tie(TTypes&... t) noexcept;-7- Returns: […]
-?- [Note: The constructors and assignment operators provided byignore
shall beconstexpr
. — end note] -8- [Example: […] — end example]
optional<const T>
doesn't compare with T
Section: 22.5.8 [optional.comp.with.t] Status: C++17 Submitter: Ville Voutilainen Opened: 2017-02-17 Last modified: 2021-06-06
Priority: Not Prioritized
View all other issues in [optional.comp.with.t].
View all issues with C++17 status.
Discussion:
Consider:
optional<const int> x = 42; int y = 666; x == y; // ill-formed
The comparison is ill-formed, because in [optional.comp_with_t]/1,
template <class T> constexpr bool operator==(const optional<T>& x, const T& v);
the T
is deduced to be both const int
and int
, which is ill-formed.
Since it became apparent that the root cause of this issue is also causing difficulties with e.g.
comparing optional<string>
with literals etc., here's a p/r that
adds requirements for optional
's comparisons with T
turns optional
-optional
comparisons into two-template-parameter
templates, allowing comparing mixed optionals
turns optional
-T
comparisons into two-template-parameter templates,
allowing comparing optionals with T
and types comparable with T
[Kona 2017-02-28]
Accepted as Immediate to avoid having break ABI later.
Proposed resolution:
This wording is relative to N4640.
Modify 22.5.2 [optional.syn], header <optional>
synopsis, as indicated:
[…] // 22.5.6 [optional.relops], relational operators template <class T, class U> constexpr bool operator==(const optional<T>&, const optional<TU>&); template <class T, class U> constexpr bool operator!=(const optional<T>&, const optional<TU>&); template <class T, class U> constexpr bool operator<(const optional<T>&, const optional<TU>&); template <class T, class U> constexpr bool operator>(const optional<T>&, const optional<TU>&); template <class T, class U> constexpr bool operator<=(const optional<T>&, const optional<TU>&); template <class T, class U> constexpr bool operator>=(const optional<T>&, const optional<TU>&); […] // [optional.comp_with_t], comparison with T template <class T, class U> constexpr bool operator==(const optional<T>&, constTU&); template <class T, class U> constexpr bool operator==(constTU&, const optional<T>&); template <class T, class U> constexpr bool operator!=(const optional<T>&, constTU&); template <class T, class U> constexpr bool operator!=(constTU&, const optional<T>&); template <class T, class U> constexpr bool operator<(const optional<T>&, constTU&); template <class T, class U> constexpr bool operator<(constTU&, const optional<T>&); template <class T, class U> constexpr bool operator<=(const optional<T>&, constTU&); template <class T, class U> constexpr bool operator<=(constTU&, const optional<T>&); template <class T, class U> constexpr bool operator>(const optional<T>&, constTU&); template <class T, class U> constexpr bool operator>(constTU&, const optional<T>&); template <class T, class U> constexpr bool operator>=(const optional<T>&, constTU&); template <class T, class U> constexpr bool operator>=(constTU&, const optional<T>&);
Modify 22.5.6 [optional.relops] as indicated:
template <class T, class U> constexpr bool operator==(const optional<T>& x, const optional<TU>& y);[…]
template <class T, class U> constexpr bool operator!=(const optional<T>& x, const optional<TU>& y);[…]
template <class T, class U> constexpr bool operator<(const optional<T>& x, const optional<TU>& y);[…]
template <class T, class U> constexpr bool operator>(const optional<T>& x, const optional<TU>& y);[…]
template <class T, class U> constexpr bool operator<=(const optional<T>& x, const optional<TU>& y);[…]
template <class T, class U> constexpr bool operator>=(const optional<T>& x, const optional<TU>& y);[…]
Modify [optional.comp_with_t] as indicated:
template <class T, class U> constexpr bool operator==(const optional<T>& x, constTU& v);-?- Requires: The expression
-1- Effects: Equivalent to:*x == v
shall be well-formed and its result shall be convertible tobool
. [Note:T
need not beEqualityComparable
. — end note]return bool(x) ? *x == v : false;
template <class T, class U> constexpr bool operator==(constTU& v, const optional<T>& x);-?- Requires: The expression
-2- Effects: Equivalent to:v == *x
shall be well-formed and its result shall be convertible tobool
.return bool(x) ? v == *x : false;
template <class T, class U> constexpr bool operator!=(const optional<T>& x, constTU& v);-?- Requires: The expression
-3- Effects: Equivalent to:*x != v
shall be well-formed and its result shall be convertible tobool
.return bool(x) ? *x != v : true;
template <class T, class U> constexpr bool operator!=(constTU& v, const optional<T>& x);-?- Requires: The expression
-4- Effects: Equivalent to:v != *x
shall be well-formed and its result shall be convertible tobool
.return bool(x) ? v != *x : true;
template <class T, class U> constexpr bool operator<(const optional<T>& x, constTU& v);-?- Requires: The expression
-5- Effects: Equivalent to:*x < v
shall be well-formed and its result shall be convertible tobool
.return bool(x) ? *x < v : true;
template <class T, class U> constexpr bool operator<(constTU& v, const optional<T>& x);-?- Requires: The expression
-6- Effects: Equivalent to:v < *x
shall be well-formed and its result shall be convertible tobool
.return bool(x) ? v < *x : false;
template <class T, class U> constexpr bool operator<=(const optional<T>& x, constTU& v);-?- Requires: The expression
-7- Effects: Equivalent to:*x <= v
shall be well-formed and its result shall be convertible tobool
.return bool(x) ? *x <= v : true;
template <class T, class U> constexpr bool operator<=(constTU& v, const optional<T>& x);-?- Requires: The expression
-8- Effects: Equivalent to:v <= *x
shall be well-formed and its result shall be convertible tobool
.return bool(x) ? v <= *x : false;
template <class T, class U> constexpr bool operator>(const optional<T>& x, constTU& v);-?- Requires: The expression
-9- Effects: Equivalent to:*x > v
shall be well-formed and its result shall be convertible tobool
.return bool(x) ? *x > v : false;
template <class T, class U> constexpr bool operator>(constTU& v, const optional<T>& x);-?- Requires: The expression
-10- Effects: Equivalent to:v > *x
shall be well-formed and its result shall be convertible tobool
.return bool(x) ? v > *x : true;
template <class T, class U> constexpr bool operator>=(const optional<T>& x, constTU& v);-?- Requires: The expression
-11- Effects: Equivalent to:*x >= v
shall be well-formed and its result shall be convertible tobool
.return bool(x) ? *x >= v : false;
template <class T, class U> constexpr bool operator>=(constTU& v, const optional<T>& x);-?- Requires: The expression
-12- Effects: Equivalent to:v >= *x
shall be well-formed and its result shall be convertible tobool
.return bool(x) ? v >= *x : true;
create_directories
do when p
already exists but is not a directory?Section: 31.12.13.7 [fs.op.create.directories], 31.12.13.8 [fs.op.create.directory] Status: C++20 Submitter: Billy Robert O'Neal III Opened: 2017-02-15 Last modified: 2021-06-06
Priority: 0
View all issues with C++20 status.
Discussion:
The create_directory
and create_directories
functions have a postcondition that says
is_directory(p)
, but it is unclear how they are supposed to provide this. Both of their effects say that
they create a directory and return whether it was actually created. It is possible to interpret this as "if
creation fails due to the path already existing, issue another system call to see if the path is a directory,
and change the result if so" — but it seems unfortunate to require both Windows and POSIX to issue more
system calls in this case.
create_directories
'
Returns clause.
[2017-07 Toronto Thurs Issue Prioritization]
Priority 0; move to Ready
Proposed resolution:
This wording is relative to N4640.
Make the following edits to [fs.op.create_directories]:
bool create_directories(const path& p); bool create_directories(const path& p, error_code& ec) noexcept;
-1- Effects: Establishes the postcondition by calling Calls create_directory()
for anyeach element of p
that does not exist.
is_directory(p)
.true
if a new directory was created for the directory p
resolves to,
otherwise false
. The signature with argument ec
returns false
if an error occurs.
-4- Throws: As specified in 31.12.5 [fs.err.report].
-5- Complexity: 𝒪(n) where n is the number of elements of p
that do not exist.
Make the following edits to [fs.op.create_directory]:
bool create_directory(const path& p); bool create_directory(const path& p, error_code& ec) noexcept;
-1- Effects: Establishes the postcondition by attempting to createCreates the directory
p
resolves to, as if by POSIX mkdir()
with a second argument of static_cast<int>(perms::all)
.
Creation failure because p
resolves to an existing directory shall not be treated asalready exists
is not an error.
is_directory(p)
.true
if a new directory was created, otherwise false
. The signature with argument
ec
returns false
if an error occurs.
-4- Throws: As specified in 31.12.5 [fs.err.report].
Section: 31.12.6.5.8 [fs.path.compare] Status: C++20 Submitter: Billy Robert O'Neal III Opened: 2017-02-21 Last modified: 2021-02-25
Priority: 2
View all issues with C++20 status.
Discussion:
Currently, path comparison is defined elementwise, which implies a conversion from the native format (implied by
native()
returning const string&
). However, the conversion from the native format to the generic
format might not be information preserving. This would allow two paths a
and b
to say
a.compare(b) == 0
, but a.native().compare(b.native()) != 0
as a result of this missing information,
which is undesirable. We only want that condition to happen if there are redundant directory separators. We also don't
want to change the path comparison to be in terms of the native format, due to Peter Dimov's example where we want
path("a/b")
to sort earlier than path("a.b")
, and we want path("a/b") == path("a//////b")
.
path("a/b:ads") == path("a/b")
. I think I should consider the alternate data streams as part of the path
element though, so this case might be fine, so long as I make path("b:ads").native()
be "b:ads"
.
This might not work for our z/OS friends though, or for folks where the native format looks nothing like the generic format.
Additionally, this treats root-directory specially. For example, the current spec wants path("c:/a/b") == path("c:/a////b")
,
but path("c:/a/b") != path("c:///a/b")
, because native()
for the root-directory path element will literally
be the slashes or preferred separators.
This addresses similar issues to those raised in US 57 — it won't make absolute paths sort at the beginning or end
but it will make paths of the same kind sort together.
[2017-03-04, Kona Saturday morning]
We decided that this had seen so much churn that we would postpone looking at this until Toronto
[2017-07 Toronto Thurs Issue Prioritization]
Priority 2
[2016-07, Toronto Saturday afternoon issues processing]
Billy to reword after Davis researches history about ordering. Status to Open.
Previous resolution [SUPERSEDED]:
This wording is relative to N4640.
Make the following edits to 31.12.6.5.8 [fs.path.compare]:
int compare(const path& p) const noexcept;-1- Returns:
— Let
rootNameComparison
be the result ofthis->root_name().native().compare(p.root_name().native())
. IfrootNameComparison
is not0
,rootNameComparison
; otherwise,— If
this->has_root_directory()
and!p.has_root_directory()
, a value less than0
; otherwise,— If
!this->has_root_directory()
andp.has_root_directory()
, a value greater than0
; otherwise,— A value greater than, less than, or equal to 0, ordering the paths in a depth-first traversal order.
-?- [Note: For POSIX and Windows platforms, this is accomplished by lexicographically ordering the half-open ranges
[begin(), end())
ofthis->relative_path()
andp.relative_path()
as follows:— A value less than
0
, ifnative()
for the elements ofare lexicographically less than
*this->relative_path()native()
for the elements ofp.relative_path()
; otherwise,— a value greater than
0
, ifnative()
for the elements ofare lexicographically greater than
*this->relative_path()native()
for the elements ofp.relative_path()
; otherwise,—
0
.— end note]
-2- Remarks: The elements are determined as if by iteration over the half-open range[begin(), end())
for*this
andp
.int compare(const string_type& s) const int compare(basic_string_view<value_type> s) const;
-3- Returns:compare(path(s))
[Editor's note: Delete paragraph 3 entirely and merge the
value_type
overload with those above.]int compare(const value_type* s) const-4-
ReturnsEffects: Equivalent toreturn
compare(path(s));
.
[2018-01-26 issues processing telecon]
Status set to 'Review'. We like the wording, but would like to see some implementation experience.
Previous resolution [SUPERSEDED]:
This wording is relative to N4659.
Make the following edits to 31.12.6.5.8 [fs.path.compare]:
int compare(const path& p) const noexcept;-1- Returns:
— Let
rootNameComparison
be the result ofthis->root_name().native().compare(p.root_name().native())
. IfrootNameComparison
is not0
,rootNameComparison
; otherwise,— If
this->has_root_directory()
and!p.has_root_directory()
, a value less than0
; otherwise,— If
!this->has_root_directory()
andp.has_root_directory()
, a value greater than0
; otherwise,—
a value less thanIf0
, inative()
for the elements ofare lexicographically less than
*this->relative_path()native()
for the elements ofp.relative_path()
, a value less than0
; otherwise,—
a value greater thanIf0
, inative()
for the elements ofare lexicographically greater than
*this->relative_path()native()
for the elements ofp.relative_path()
, a value greater than0
; otherwise,—
0
.
-2- Remarks: The elements are determined as if by iteration over the half-open range[begin(), end())
for*this
andp
.int compare(const string_type& s) const int compare(basic_string_view<value_type> s) const;
-3- Returns:compare(path(s))
[Editor's note: Delete paragraph 3 entirely and merge the
value_type
overload with those above.]int compare(const value_type* s) const-4-
ReturnsEffects: Equivalent toreturn
compare(path(s));
.
[2018-02-13 Billy improves wording]
The revised wording has the effect to invert the ordering of the added new bullets (2) and (3), the effect of this change is that
path("c:/").compare("c:")
compares greater, not less.
[2018-06, Rapperswil Wednesday evening]
Agreement to move that to Ready, Daniel rebased to N4750.
[2018-11, Adopted in San Diego]
Proposed resolution:
This wording is relative to N4750.
Make the following edits to 31.12.6.5.8 [fs.path.compare]:
int compare(const path& p) const noexcept;-1- Returns:
— Let
rootNameComparison
be the result ofthis->root_name().native().compare(p.root_name().native())
. IfrootNameComparison
is not0
,rootNameComparison
; otherwise,— If
!this->has_root_directory()
andp.has_root_directory()
, a value less than0
; otherwise,— If
this->has_root_directory()
and!p.has_root_directory()
, a value greater than0
; otherwise,—
a value less thanIf0
, inative()
for the elements ofare lexicographically less than
*this->relative_path()native()
for the elements ofp.relative_path()
, a value less than0
; otherwise,—
a value greater thanIf0
, inative()
for the elements ofare lexicographically greater than
*this->relative_path()native()
for the elements ofp.relative_path()
, a value greater than0
; otherwise,—
0
.
-2- Remarks: The elements are determined as if by iteration over the half-open range[begin(), end())
for*this
andp
.int compare(const string_type& s) const int compare(basic_string_view<value_type> s) const;
-3- Returns:compare(path(s))
[Editor's note: Delete paragraph 3 entirely and merge the
value_type
overload with those above.]int compare(const value_type* s) const-4-
ReturnsEffects: Equivalent to:return
compare(path(s));
.
equivalent("existing_thing", "not_existing_thing")
an error?Section: 31.12.13.13 [fs.op.equivalent] Status: C++20 Submitter: Billy Robert O'Neal III Opened: 2017-02-27 Last modified: 2021-02-25
Priority: 0
View all other issues in [fs.op.equivalent].
View all issues with C++20 status.
Discussion:
See discussion on the LWG mailing list with subject "Is equivalent("existing_thing", "not_existing_thing") an error?", abreviated below:
Billy O'Neal:The existing "an error is reported" effects say that an error is reported for
It's also unfortunate that the current spec requires reporting an error for!exists(p1) && !exists(p2)
, but I'm not sure that treatingequivalent("existing_thing", "not_existing_thing")
as "false
, no error" makes any more sense than forequivalent("not_existing_thing", "not_existing_thing")
.is_other(p1) && is_other(p2)
— there's no reason that you can't give a sane answer for paths to NT pipes. (Do POSIX FIFOs give garbage answers here?)
Davis Herring:
I'm fine with an error if either path does not exist. See also Late 29: I would much prefer
file_identity identity(const path&, bool resolve = true);which would of course produce an error if the path did not exist (or, with the default
See Late 30 and 32 (31 has been resolved). FIFOs pose no trouble: you can evenresolve
, was a broken symlink).fstat(2)
on the naked file descriptors produced bypipe(2)
. (That said, I observe the strange inconsistency that Linux but not macOS gives both ends of a pipe the samest_ino
.) POSIX has no reason that I know of to treat any file type specially forequivalent()
.
Billy O'Neal:
I think such a
file_identity
feature would be useful but we can always add it in addition toequivalent
post-C++17.
Beman Dawes:
Looks good to me. Maybe submit this as an issue right away in the hopes it can go in C++17?
[2017-03-04, Kona]
Set priority to 0; Tentatively Ready
Proposed resolution:
This wording is relative to N4640.
Make the following edits to 31.12.13.13 [fs.op.equivalent]:
bool equivalent(const path& p1, const path& p2); bool equivalent(const path& p1, const path& p2, error_code& ec) noexcept;
-1- Lets1
ands2
befile_status
s determined as if bystatus(p1)
andstatus(p2)
, respectively.-2- Effects: Determines-3- Returns:s1
ands2
. If(!exists(s1) && !exists(s2)) || (is_other(s1) && is_other(s2))
an error is reported (27.10.7).true
, ifs1 == s2
andp1
andp2
resolve to the same file system entity, elsefalse
. The signature with argumentec
returnsfalse
if an error occurs. -4- Two paths are considered to resolve to the same file system entity if two candidate entities reside on the same device at the same location. [Note: On POSIX platforms, tThis is determined as if by the values of the POSIXstat
structure, obtained as if bystat()
for the two paths, having equalst_dev
values and equalst_ino
values. — end note] -?- Remarks:!exists(p1) || !exists(p2)
is an error. -5- Throws: As specified in 27.10.7.
basic_string_view::const_iterator
should be literal typesSection: 27.3.3 [string.view.template] Status: Resolved Submitter: Antony Polukhin Opened: 2017-03-01 Last modified: 2020-09-06
Priority: 2
View all other issues in [string.view.template].
View all issues with Resolved status.
Discussion:
Force the literal type requirement for the const_iterator
in the
std::basic_string_view
so that iterators of std::basic_string_view
could be used in constexpr
functions.
[2017-03-02, Kona]
Wording tweaks suggested by LWG applied.
[2017-03-02, Tim Song comments]
I don't believe the blanket "all operations" wording is quite correct.
First,T t;
(required by DefaultConstructible
) isn't usable in a constant expression if the iterator is a
pointer, since it would leave it uninitialized.
Second, an explicit destructor call u.~T()
(required by Destructible
) isn't usable if the iterator is a
class type because it explicitly invokes a non-constexpr
function (the destructor); see [expr.const]/2.2.
[2017-03-04, Kona]
Set priority to 2. Lisa and Alisdair to work with Antony to come up with better wording. The same wording can be used for 2897(i).
[2017-11 Albuquerque Saturday issues processing]
P0858R0 (adopted on Sat; to be moved in Jacksonville) will resolve this.
[2018-06 Rapperswil Wednesday issues processing]
This was resolved by P0858, which was adopted in Jacksonville.
Proposed resolution:
This wording is relative to N4640.
Add to the end of the 27.3.3 [string.view.template] section:
-1- In every specialization
-?- All operations onbasic_string_view<charT, traits>
, the type traits shall satisfy the character traits requirements (21.2), and the typetraits::char_type
shall name the same type ascharT
.iterator
andconst_iterator
that are required to satisfy the random access iterator requirements are usable in constant expressions.
result_of
specification also needs a little cleanupSection: D.13 [depr.meta.types] Status: C++20 Submitter: Daniel Krügler Opened: 2017-03-04 Last modified: 2021-02-25
Priority: 3
View all other issues in [depr.meta.types].
View all issues with C++20 status.
Discussion:
P0604R0 missed a similar adjustment that was performed for the
deprecated type trait templates is_literal_type
and is_literal_type_v
via LWG 2838(i):
result_of
to Annex D means that the general prohibition against specializing type traits
in [meta.type.synop]/1 does no longer exist, so should be explicitly spelled out. Wording will be provided
after publication of the successor of N4640.
[2017-03-04, Kona]
Set priority to 3
[2017-04-24, Daniel provides wording]
Instead of enumerating all the templates where adding a specialization is valid, a general exclusion rule is provided. Albeit currently not needed for the templates handled by this sub-clause, a potential exception case is constructed to ensure correctness for the first deprecated trait template that would permit specializations by the user.
[2017-06-10, Moved to Tentatively Ready after 6 positive votes on c++std-lib]
Proposed resolution:
This wording is relative to N4659.
Change D.13 [depr.meta.types] as indicated:
-4- The behavior of a program that adds specializations for any of the templates defined in this subclause
is undefined, unless explicitly permitted by the specification of the corresponding template.is_literal_type
oris_literal_type_v
Section: 32.2.4 [thread.req.timing] Status: C++20 Submitter: Jonathan Mcdougall Opened: 2017-03-07 Last modified: 2021-02-25
Priority: 0
View all other issues in [thread.req.timing].
View all issues with C++20 status.
Discussion:
In 32.2.4 [thread.req.timing], both /3 and /4 talk about "member
functions whose names end in _for
" and "_until
", but these clauses
also apply to this_thread::sleep_for()
and this_thread::sleep_until()
,
which are namespace-level functions (30.3.2).
[2017-07 Toronto Wed Issue Prioritization]
Priority 0; Move to Ready
Proposed resolution:
This wording is relative to N4640.
Modify 32.2.4 [thread.req.timing] as indicated::
[…]
-3- Thememberfunctions whose names end in_for
take an argument that specifies a duration. […] -4- Thememberfunctions whose names end in_until
take an argument that specifies a time point. […]
weak_ptr::owner_before
Section: 20.3.2.3.6 [util.smartptr.weak.obs] Status: C++20 Submitter: Tim Song Opened: 2017-03-09 Last modified: 2021-02-25
Priority: Not Prioritized
View all other issues in [util.smartptr.weak.obs].
View all issues with C++20 status.
Discussion:
The NB comment asked for noexcept
on shared_ptr::owner_before
, weak_ptr::owner_before
, and
owner_less::operator()
, but the PR of LWG 2873(i) only added it to the first and the third one.
[2017-06-03, Moved to Tentatively Ready after 6 positive votes on c++std-lib]
Proposed resolution:
This wording is relative to N4659.
Edit 20.3.2.3 [util.smartptr.weak], class template weak_ptr
synopsis, as indicated:
[…] // 20.3.2.3.6 [util.smartptr.weak.obs], observers […] template<class U> bool owner_before(const shared_ptr<U>& b) const noexcept; template<class U> bool owner_before(const weak_ptr<U>& b) const noexcept;
Edit 20.3.2.3.6 [util.smartptr.weak.obs] immediately before p4 as indicated:
template<class U> bool owner_before(const shared_ptr<U>& b) const noexcept; template<class U> bool owner_before(const weak_ptr<U>& b) const noexcept;-4- Returns: An unspecified value such that […]
basic_filebuf::open
Section: 31.10.3.4 [filebuf.members] Status: C++20 Submitter: Tim Song Opened: 2017-03-09 Last modified: 2021-02-25
Priority: 2
View all other issues in [filebuf.members].
View all issues with C++20 status.
Discussion:
LWG 2676(i) specified basic_filebuf::open(const std::filesystem::path::value_type* s, ios_base::openmode mode)
by simply reusing the specification for the const char*
overload, but that specification is incorrect for the wide overload:
it says that s
is an NTBS — a null-terminated byte string — which it isn't. Moreover, it specifies that
the file is opened as if by calling fopen(s, modstr)
, but that call is ill-formed if s
isn't a const char*
.
[2017-07 Toronto Wed Issue Prioritization]
Priority 2
[2017-11 Albuquerque Wednesday issue processing]
Status to Open; Jonathan to provide wording.
[2018-01-16; Jonathan and Tim Song provide wording]
We'll have to ask the Microsoft guys if "as by a call to fopen
" is OK for them. There are
paths that can be represented as a wide character string that can't reliably be converted to narrow
characters (because they become dependent on the current codepage, or some other Windows nonsense)
so they definitely won't use fopen
. But as long as they call something that behaves like
it (which should allow _fwopen
), I think they'll still meet the spirit of the wording.
[2018-08-14; Marshall corrects a grammar nit in the P/R]
The Microsoft guys note that "as by a call to fopen
" is OK by them.
Previous resolution [SUPERSEDED]:
This wording is relative to N4713.
Edit 31.10.3.4 [filebuf.members] as indicated:
basic_filebuf* open(const char* s, ios_base::openmode mode); basic_filebuf* open(const filesystem::path::value_type* s, ios_base::openmode mode); // wide systems only; see 31.10.1 [fstream.syn]-?- Requires:
-2- Effects: Ifs
shall point to a NTCTS (3.36 [defns.ntcts]).is_open() != false
, returns a null pointer. Otherwise, initializes thefilebuf
as required. It then opens the file to whichs
resolves, if possible, as if by a call tofopen
with the second argumenta file, if possible, whose name is the ntbsdetermined froms
(as if by callingfopen(s, modstr))
. The ntbsmodstr
ismode & ~ios_base::ate
as indicated in Table 117. Ifmode
is not some combination of flags shown in the table then the open fails. -3- If the open operation succeeds and(mode & ios_base::ate) != 0
, positions the file to the end (as if by callingfseek(file, 0, SEEK_END)
, wherefile
is the pointer returned by callingfopen
).(footnote 330) -4- If the repositioning operation fails, callsclose()
and returns a null pointer to indicate failure. -5- Returns:this
if successful, a null pointer otherwise.
[2018-08-23 Batavia Issues processing]
Adopted after changing 'Requires' -> 'Expects' in the P/R.
[2018-11, Adopted in San Diego]
Proposed resolution:
This wording is relative to N4713.
Edit 31.10.3.4 [filebuf.members] as indicated:
basic_filebuf* open(const char* s, ios_base::openmode mode); basic_filebuf* open(const filesystem::path::value_type* s, ios_base::openmode mode); // wide systems only; see 31.10.1 [fstream.syn]-?- Expects:
-2- Effects: Ifs
shall point to a NTCTS (3.36 [defns.ntcts]).is_open() != false
, returns a null pointer. Otherwise, initializes thefilebuf
as required. It then opens the file to whichs
resolves, if possible, as if by a call tofopen
with the second argumenta file, if possible, whose name is the ntbsdetermined froms
(as if by callingfopen(s, modstr))
. The ntbsmodstr
ismode & ~ios_base::ate
as indicated in Table 117. Ifmode
is not some combination of flags shown in the table then the open fails. -3- If the open operation succeeds and(mode & ios_base::ate) != 0
, positions the file to the end (as if by callingfseek(file, 0, SEEK_END)
, wherefile
is the pointer returned by callingfopen
).(footnote 330) -4- If the repositioning operation fails, callsclose()
and returns a null pointer to indicate failure. -5- Returns:this
if successful, a null pointer otherwise.
Section: 20.3.1.3.2 [unique.ptr.single.ctor] Status: C++20 Submitter: Tim Song Opened: 2017-03-11 Last modified: 2021-02-25
Priority: 0
View all other issues in [unique.ptr.single.ctor].
View all issues with C++20 status.
Discussion:
The wording simplification in LWG 2905(i) accidentally deleted the
requirement that construction of the deleter doesn't throw an
exception. While this isn't the end of the world since any exception
will just run into the noexcept on the constructor and result in a
call to std::terminate()
, the other unique_ptr
constructors still have
a similar no-exception Requires: clause, leaving us in the odd
situation where throwing an exception results in undefined behavior
for some constructors and terminate()
for others. If guaranteeing
std::terminate()
on exception is desirable, that should be done across
the board.
Copy/MoveConstructible
requirement. Wording for the
other alternative (guaranteed std::terminate()
) can be produced if
desired.
[2017-03-16, Daniel comments]
The publication of the new working draft is awaited, before proposed wording against that new working draft is formally possible.
[2017-05-03, Tim comments]
The suggested wording has been moved to the PR section now that the new working draft is available.
[2017-07 Toronto Wed Issue Prioritization]
Priority 0; Move to Ready
Proposed resolution:
This wording is relative to N4659.
Insert a paragraph after 20.3.1.3.2 [unique.ptr.single.ctor] p11:
unique_ptr(pointer p, see below d1) noexcept; unique_ptr(pointer p, see below d2) noexcept;-9- The signature of these constructors depends upon whether
[…] -10- IfD
is a reference type. IfD
is a non-reference typeA
, then the signatures are:D
is an lvalue reference typeA&
, then the signatures are: […] -11- IfD
is an lvalue reference typeconst A&
, then the signatures are: […] -??- Requires: For the first constructor, ifD
is not a reference type,D
shall satisfy the requirements ofCopyConstructible
and such construction shall not exit via an exception. For the second constructor, ifD
is not a reference type,D
shall satisfy the requirements ofMoveConstructible
and such construction shall not exit via an exception.
optional
comparisonsSection: 22.5.8 [optional.comp.with.t] Status: C++20 Submitter: Jonathan Wakely Opened: 2017-03-13 Last modified: 2021-06-06
Priority: 2
View all other issues in [optional.comp.with.t].
View all issues with C++20 status.
Discussion:
LWG 2934(i) added an additional template parameter to the comparison
operators for std::optional
, but the ones that compare U
with
optional<T>
have the parameters backwards compared to the function parameters:
template <class T, class U> constexpr bool operator==(const U&, const optional<T>&);
Ville confirmed there's no particular reason for this, it's just how he wrote the proposed resolution, but as this has normative effect we should consider if we really want the template parameters and function parameters to be in different orders or not.
[2017-07-13, Casey Carter provides wording]
[2016-07, Toronto Thursday night issues processing]
Status to Ready
Proposed resolution:
This wording is relative to N4659.
Modify 22.5.2 [optional.syn], <optional>
synopsis, as indicated:
// [optional.comp_with_t], comparison with T template <class T, class U> constexpr bool operator==(const optional<T>&, const U&); template <class T, class U> constexpr bool operator==(constUT&, const optional<TU>&); template <class T, class U> constexpr bool operator!=(const optional<T>&, const U&); template <class T, class U> constexpr bool operator!=(constUT&, const optional<TU>&); template <class T, class U> constexpr bool operator<(const optional<T>&, const U&); template <class T, class U> constexpr bool operator<(constUT&, const optional<TU>&); template <class T, class U> constexpr bool operator<=(const optional<T>&, const U&); template <class T, class U> constexpr bool operator<=(constUT&, const optional<TU>&); template <class T, class U> constexpr bool operator>(const optional<T>&, const U&); template <class T, class U> constexpr bool operator>(constUT&, const optional<TU>&); template <class T, class U> constexpr bool operator>=(const optional<T>&, const U&); template <class T, class U> constexpr bool operator>=(constUT&, const optional<TU>&);
Modify [optional.comp_with_t] as indicated:
template <class T, class U> constexpr bool operator==(constUT& v, const optional<TU>& x);-3- […]
[…]
template <class T, class U> constexpr bool operator!=(constUT& v, const optional<TU>& x);-7- […]
[…]
template <class T, class U> constexpr bool operator<(constUT& v, const optional<TU>& x);-11- […]
[…]
template <class T, class U> constexpr bool operator<=(constUT& v, const optional<TU>& x);-15- […]
[…]
template <class T, class U> constexpr bool operator>(constUT& v, const optional<TU>& x);-19- […]
[…]
template <class T, class U> constexpr bool operator>=(constUT& v, const optional<TU>& x);-23- […]
[…]
Section: 27.4.3.3 [string.cons], 27.4.3.7.2 [string.append], 27.4.3.7.3 [string.assign], 27.4.3.8 [string.ops] Status: C++20 Submitter: Daniel Krügler Opened: 2017-03-17 Last modified: 2021-02-25
Priority: 2
View all other issues in [string.cons].
View all issues with C++20 status.
Discussion:
LWG 2758(i) corrected newly introduced ambiguities of std::string::assign
and other functions
that got new overloads taking a basic_string_view
as argument, but the assignment operator of
basic_string
and other functions taking a parameter of type basic_string_view<charT, traits>
were not corrected. Similar to the previous issue the following operations lead now to an ambiguity as well:
#include <string> int main() { std::string s({"abc", 1}); s = {"abc", 1}; s += {"abc", 1}; s.append({"abc", 1}); s.assign({"abc", 1}); s.insert(0, {"abc", 1}); s.replace(0, 1, {"abc", 1}); s.replace(s.cbegin(), s.cbegin(), {"abc", 1}); s.find({"abc", 1}); s.rfind({"abc", 1}); s.find_first_of({"abc", 1}); s.find_last_of({"abc", 1}); s.find_first_not_of({"abc", 1}); s.find_last_not_of({"abc", 1}); s.compare({"abc", 1}); s.compare(0, 1, {"abc", 1}); }
The right fix is to convert all member functions taken a basic_string_view<charT, traits>
parameter
into constrained function templates.
noexcept
, but the wider range of
"string operation" functions taking a basic_string_view<charT, traits>
parameter are mostly
noexcept
because they had a wide contract. Now with the approach of LWG 2758(i), there are all
types allowed that are convertible to basic_string_view<charT, traits>
, but the conversion
occurs now in the function body, not outside of it. So, if these conversion would be potentially
exception-throwing, this would lead to a call to std::terminate
, which is a semantic change compared to
the previous specification. There are several options to handle this situation:
Ignore that and let std::terminate
come into action. This is a different way of saying that
we impose the requirement of a nothrowing operation.
Remove noexcept
from all the affected functions.
Make these functions conditionally noexcept
.
The submitter has a personal preference for option (3), except that this would complicate the wording a bit,
because unfortunately there exists yet no trait std::is_nothrow_convertible
(See LWG 2040(i)).
A seemingly low-hanging fruit would be the attempt to use std::is_nothrow_constructible
instead, but this
trait describes a potentially different initialization context and is therefore inappropriate. Option (1) would
conserve the existing noexcept
guarantee for all non-throwing conversions, but now these functions become
narrow-contract functions and at least according to the current noexcept
guidelines such functions should not be marked as noexcept
. But there are exceptions possible
for that rule, and the initially suggested proposed wording below argues that this exception is reasonable here,
because the required wording fixes just an unintended side-effects of transforming the functions into functions
templates, but it doesn't intend to change the actual functionality.
is_convertible_v<const T&, const charT*> == false
requirement, but the submitter of this issue
suggests a more advanced approach that should be applied in a synchronous wording adjustment combined with the
existing LWG 2758(i) wording: It would presumably life easier for implementations (which are allowed
to provide additional member function overloads as conforming extensions), when we would define a mini requirement
set for template parameter type T
below:
is_convertible_v<const T&, basic_string_view<charT, traits>>
is true
.
is_convertible_v<const T&, const charT*>
is false
.
The implicit conversion to basic_string_view<charT, traits>
shall not throw an exception.
But the corresponding slightly revised wording taking advantage of this "concept-like" requirements set will not be suggested before the upcoming working draft has been published to allow a simpler coordinated adjustment together with the LWG 2758(i) wording.
It should also be noted that these changes have impact on deduction behaviour and therefore may require further adjustments of the deduction rules.[2017-07-13, Toronto]
LWG preferred to remove in all functions with added nothrow constraints the noexcept
specifier (and the
constraint) and to possibly improve the situation later.
Previous resolution [SUPERSEDED]:
This wording is relative to N4640.
Edit 27.4.3 [basic.string], class template
basic_string
synopsis, as indicated:[…] // 21.3.2.2, construct/copy/destroy […] template<class T> explicit basic_string(basic_string_view<charT, traits> svconst T& t, const Allocator& a = Allocator()); […] template<class T> basic_string& operator=(basic_string_view<charT, traits> svconst T& t); basic_string& operator=(const charT* s); […] // 21.3.2.6, modifiers […] template<class T> basic_string& operator+=(basic_string_view<charT, traits> svconst T& t); […] template<class T> basic_string& append(basic_string_view<charT, traits> svconst T& t); […] template<class T> basic_string& assign(basic_string_view<charT, traits> svconst T& t); […] template<class T> basic_string& insert(size_type pos,basic_string_view<charT, traits> svconst T& t); […] template<class T> basic_string& replace(size_type pos1, size_type n1,basic_string_view<charT, traits> svconst T& t); […] template<class T> basic_string& replace(const_iterator i1, const_iterator i2,basic_string_view<charT, traits> svconst T& t); […] // 21.3.2.7, string operations […] template<class T> size_type find (basic_string_view<charT, traits> svconst T& t, size_type pos = 0) const noexcept; […] template<class T> size_type rfind(basic_string_view<charT, traits> svconst T& t, size_type pos = npos) const noexcept; […] template<class T> size_type find_first_of(basic_string_view<charT, traits> svconst T& t, size_type pos = 0) const noexcept; […] template<class T> size_type find_last_of (basic_string_view<charT, traits> svconst T& t, size_type pos = npos) const noexcept; […] template<class T> size_type find_first_not_of(basic_string_view<charT, traits> svconst T& t, size_type pos = 0) const noexcept; […] template<class T> size_type find_last_not_of (basic_string_view<charT, traits> svconst T& t, size_type pos = npos) const noexcept; […] template<class T> int compare(basic_string_view<charT, traits> svconst T& t) const noexcept; […] template<class T> int compare(size_type pos1, size_type n1,basic_string_view<charT, traits> svconst T& t) const; […]Edit 27.4.3.3 [string.cons] as indicated:
template<class T> explicit basic_string(basic_string_view<charT, traits> svconst T& t, const Allocator& a = Allocator());-9- Effects:
-?- Remarks: This constructor shall not participate in overload resolution unlessSame asCreates a variable,basic_string(sv.data(), sv.size(), a)
.sv
, as if bybasic_string_view<charT, traits> sv = t;
and then behaves the same asbasic_string(sv.data(), sv.size(), a)
.is_convertible_v<const T&, basic_string_view<charT, traits>>
istrue
andis_convertible_v<const T&, const charT*>
isfalse
.[…]
template<class T> basic_string& operator=(basic_string_view<charT, traits> svconst T& t);-25- Effects: Equivalent to:
{ basic_string_view<charT, traits> sv = t; return assign(sv); }-?- Remarks: This function shall not participate in overload resolution unless
is_convertible_v<const T&, basic_string_view<charT, traits>>
istrue
andis_convertible_v<const T&, const charT*>
isfalse
.Edit [string.op+=] as indicated:
template<class T> basic_string& operator+=(basic_string_view<charT, traits> svconst T& t);-3- Effects: Creates a variable,
-4- Returns:sv
, as if bybasic_string_view<charT, traits> sv = t;
and then cCallsappend(sv)
.*this
. -?- Remarks: This function shall not participate in overload resolution unlessis_convertible_v<const T&, basic_string_view<charT, traits>>
istrue
andis_convertible_v<const T&, const charT*>
isfalse
.Edit 27.4.3.7.2 [string.append] as indicated:
template<class T> basic_string& append(basic_string_view<charT, traits> svconst T& t);-6- Effects: Equivalent to:
{ basic_string_view<charT, traits> sv = t; return append(sv.data(), sv.size()); }-?- Remarks: This function shall not participate in overload resolution unless
is_convertible_v<const T&, basic_string_view<charT, traits>>
istrue
andis_convertible_v<const T&, const charT*>
isfalse
.Edit 27.4.3.7.3 [string.assign] as indicated:
template<class T> basic_string& assign(basic_string_view<charT, traits> svconst T& t);-8- Effects: Equivalent to:
{ basic_string_view<charT, traits> sv = t; return assign(sv.data(), sv.size()); }-?- Remarks: This function shall not participate in overload resolution unless
is_convertible_v<const T&, basic_string_view<charT, traits>>
istrue
andis_convertible_v<const T&, const charT*>
isfalse
.Edit 27.4.3.7.4 [string.insert] as indicated:
template<class T> basic_string& insert(size_type pos,basic_string_view<charT, traits> svconst T& t);-5- Effects: Equivalent to:
{ basic_string_view<charT, traits> sv = t; return insert(pos, sv.data(), sv.size()); }-?- Remarks: This function shall not participate in overload resolution unless
is_convertible_v<const T&, basic_string_view<charT, traits>>
istrue
andis_convertible_v<const T&, const charT*>
isfalse
.Edit 27.4.3.7.6 [string.replace] as indicated:
template<class T> basic_string& replace(size_type pos1, size_type n1,basic_string_view<charT, traits> svconst T& t);-5- Effects: Equivalent to:
{ basic_string_view<charT, traits> sv = t; return replace(pos1, n1, sv.data(), sv.size()); }-?- Remarks: This function shall not participate in overload resolution unless
is_convertible_v<const T&, basic_string_view<charT, traits>>
istrue
andis_convertible_v<const T&, const charT*>
isfalse
.[…]
template<class T> basic_string& replace(const_iterator i1, const_iterator i2,basic_string_view<charT, traits> svconst T& t);-21- Requires:
-22- Effects: Creates a variable,[begin(), i1)
and[i1, i2)
are valid ranges.sv
, as if bybasic_string_view<charT, traits> sv = t;
and then cCallsreplace(i1 - begin(), i2 - i1, sv)
. -23- Returns:*this
.-?- Remarks: This function shall not participate in overload resolution unless
is_convertible_v<const T&, basic_string_view<charT, traits>>
istrue
andis_convertible_v<const T&, const charT*>
isfalse
.Edit 27.4.3.8.2 [string.find] as indicated:
template<class T> size_type find(basic_string_view<charT, traits> svconst T& t, size_type pos = 0) const noexcept;-?- Requires: The initialization of
-1- Effects: Creates a variable,sv
, as specified below, shall not throw an exception.sv
, as if bybasic_string_view<charT, traits> sv = t;
and then dDetermines the lowest positionxpos
, if possible, such that both of the following conditions hold: […] -2- Returns:xpos
if the function can determine such a value forxpos
. Otherwise, returnsnpos
.-?- Remarks: This function shall not participate in overload resolution unless
is_convertible_v<const T&, basic_string_view<charT, traits>>
istrue
andis_convertible_v<const T&, const charT*>
isfalse
.Edit [string.rfind] as indicated:
template<class T> size_type rfind(basic_string_view<charT, traits> svconst T& t, size_type pos = npos) const noexcept;-?- Requires: The initialization of
-1- Effects: Creates a variable,sv
, as specified below, shall not throw an exception.sv
, as if bybasic_string_view<charT, traits> sv = t;
and then dDetermines the highest positionxpos
, if possible, such that both of the following conditions hold: […] -2- Returns:xpos
if the function can determine such a value forxpos
. Otherwise, returnsnpos
.-?- Remarks: This function shall not participate in overload resolution unless
is_convertible_v<const T&, basic_string_view<charT, traits>>
istrue
andis_convertible_v<const T&, const charT*>
isfalse
.Edit [string.find.first.of] as indicated:
template<class T> size_type find_first_of(basic_string_view<charT, traits> svconst T& t, size_type pos = 0) const noexcept;-?- Requires: The initialization of
-1- Effects: Creates a variable,sv
, as specified below, shall not throw an exception.sv
, as if bybasic_string_view<charT, traits> sv = t;
and then dDetermines the lowest positionxpos
, if possible, such that both of the following conditions hold: […] -2- Returns:xpos
if the function can determine such a value forxpos
. Otherwise, returnsnpos
.-?- Remarks: This function shall not participate in overload resolution unless
is_convertible_v<const T&, basic_string_view<charT, traits>>
istrue
andis_convertible_v<const T&, const charT*>
isfalse
.Edit [string.find.last.of] as indicated:
template<class T> size_type find_last_of(basic_string_view<charT, traits> svconst T& t, size_type pos = npos) const noexcept;-?- Requires: The initialization of
-1- Effects: Creates a variable,sv
, as specified below, shall not throw an exception.sv
, as if bybasic_string_view<charT, traits> sv = t;
and then dDetermines the highest positionxpos
, if possible, such that both of the following conditions hold: […] -2- Returns:xpos
if the function can determine such a value forxpos
. Otherwise, returnsnpos
.-?- Remarks: This function shall not participate in overload resolution unless
is_convertible_v<const T&, basic_string_view<charT, traits>>
istrue
andis_convertible_v<const T&, const charT*>
isfalse
.Edit [string.find.first.not.of] as indicated:
template<class T> size_type find_first_not_of(basic_string_view<charT, traits> svconst T& t, size_type pos = 0) const noexcept;-?- Requires: The initialization of
-1- Effects: Creates a variable,sv
, as specified below, shall not throw an exception.sv
, as if bybasic_string_view<charT, traits> sv = t;
and then dDetermines the lowest positionxpos
, if possible, such that both of the following conditions hold: […] -2- Returns:xpos
if the function can determine such a value forxpos
. Otherwise, returnsnpos
.-?- Remarks: This function shall not participate in overload resolution unless
is_convertible_v<const T&, basic_string_view<charT, traits>>
istrue
andis_convertible_v<const T&, const charT*>
isfalse
.Edit [string.find.last.not.of] as indicated:
template<class T> size_type find_last_not_of(basic_string_view<charT, traits> svconst T& t, size_type pos = npos) const noexcept;-?- Requires: The initialization of
-1- Effects: Creates a variable,sv
, as specified below, shall not throw an exception.sv
, as if bybasic_string_view<charT, traits> sv = t;
and then dDetermines the highest positionxpos
, if possible, such that both of the following conditions hold: […] -2- Returns:xpos
if the function can determine such a value forxpos
. Otherwise, returnsnpos
.-?- Remarks: This function shall not participate in overload resolution unless
is_convertible_v<const T&, basic_string_view<charT, traits>>
istrue
andis_convertible_v<const T&, const charT*>
isfalse
.Edit 27.4.3.8.4 [string.compare] as indicated:
template<class T> int compare(basic_string_view<charT, traits> svconst T& t) const noexcept;-?- Requires: The initialization of
-1- Effects: Creates a variable,sv
, as specified below, shall not throw an exception.sv
, as if bybasic_string_view<charT, traits> sv = t;
and then dDetermines the effective lengthrlen
of the strings to compare as the smaller ofsize()
andsv.size()
. The function then compares the two strings by callingtraits::compare(data(), sv.data(), rlen)
. -2- Returns: The nonzero result if the result of the comparison is nonzero. Otherwise, returns a value as indicated in Table 63. […]-?- Remarks: This function shall not participate in overload resolution unless
is_convertible_v<const T&, basic_string_view<charT, traits>>
istrue
andis_convertible_v<const T&, const charT*>
isfalse
.template<class T> int compare(size_type pos1, size_type n1,basic_string_view<charT, traits> svconst T& t) const;-3- Effects: Equivalent to:
{ basic_string_view<charT, traits> sv = t; return basic_string_view<charT, traits>(data(), size()).substr(pos1, n1).compare(sv); }-?- Remarks: This function shall not participate in overload resolution unless
is_convertible_v<const T&, basic_string_view<charT, traits>>
istrue
andis_convertible_v<const T&, const charT*>
isfalse
.
[2017-07-14, Toronto, Daniel refines wording]
To balance the loss of information about the removed noexcept
specifications, all affected functions
should get a new Throws: element saying that they won't throw unless this is caused by the conversion to
the local basic_string_view
object. The existing P/R has been updated to reflect that suggestion.
[2017-07 Toronto Wed Issue Prioritization]
Priority ; Marshall to investigate and if OK, will propose Tentatively Ready on reflector.
[2018-03-03: STL reported a related issue, LWG 3075(i).]
[2018-14: Wednesday night issues processing: both this and 3075(i) to status "Immediate".]
[2018-3-17 Adopted in Jacksonville]
Proposed resolution:
This wording is relative to N4656.
Edit 27.4.3 [basic.string], class template basic_string
synopsis, as indicated:
[…] // 21.3.2.2, construct/copy/destroy […] template<class T> explicit basic_string(basic_string_view<charT, traits> svconst T& t, const Allocator& a = Allocator()); […] template<class T> basic_string& operator=(basic_string_view<charT, traits> svconst T& t); basic_string& operator=(const charT* s); […] // 21.3.2.6, modifiers […] template<class T> basic_string& operator+=(basic_string_view<charT, traits> svconst T& t); […] template<class T> basic_string& append(basic_string_view<charT, traits> svconst T& t); […] template<class T> basic_string& assign(basic_string_view<charT, traits> svconst T& t); […] template<class T> basic_string& insert(size_type pos,basic_string_view<charT, traits> svconst T& t); […] template<class T> basic_string& replace(size_type pos1, size_type n1,basic_string_view<charT, traits> svconst T& t); […] template<class T> basic_string& replace(const_iterator i1, const_iterator i2,basic_string_view<charT, traits> svconst T& t); […] // 21.3.2.7, string operations […] template<class T> size_type find (basic_string_view<charT, traits> svconst T& t, size_type pos = 0) constnoexcept; […] template<class T> size_type rfind(basic_string_view<charT, traits> svconst T& t, size_type pos = npos) constnoexcept; […] template<class T> size_type find_first_of(basic_string_view<charT, traits> svconst T& t, size_type pos = 0) constnoexcept; […] template<class T> size_type find_last_of (basic_string_view<charT, traits> svconst T& t, size_type pos = npos) constnoexcept; […] template<class T> size_type find_first_not_of(basic_string_view<charT, traits> svconst T& t, size_type pos = 0) constnoexcept; […] template<class T> size_type find_last_not_of (basic_string_view<charT, traits> svconst T& t, size_type pos = npos) constnoexcept; […] template<class T> int compare(basic_string_view<charT, traits> svconst T& t) constnoexcept; […] template<class T> int compare(size_type pos1, size_type n1,basic_string_view<charT, traits> svconst T& t) const; […]
Edit 27.4.3.3 [string.cons] as indicated:
template<class T> explicit basic_string(basic_string_view<charT, traits> svconst T& t, const Allocator& a = Allocator());-9- Effects:
-?- Remarks: This constructor shall not participate in overload resolution unlessSame asCreates a variable,basic_string(sv.data(), sv.size(), a)
.sv
, as if bybasic_string_view<charT, traits> sv = t;
and then behaves the same asbasic_string(sv.data(), sv.size(), a)
.is_convertible_v<const T&, basic_string_view<charT, traits>>
istrue
andis_convertible_v<const T&, const charT*>
isfalse
.[…]
template<class T> basic_string& operator=(basic_string_view<charT, traits> svconst T& t);-25- Effects: Equivalent to:
{ basic_string_view<charT, traits> sv = t; return assign(sv); }-?- Remarks: This function shall not participate in overload resolution unless
is_convertible_v<const T&, basic_string_view<charT, traits>>
istrue
andis_convertible_v<const T&, const charT*>
isfalse
.
Edit [string.op+=] as indicated:
template<class T> basic_string& operator+=(basic_string_view<charT, traits> svconst T& t);-3- Effects: Creates a variable,
-4- Returns:sv
, as if bybasic_string_view<charT, traits> sv = t;
and then cCallsappend(sv)
.*this
. -?- Remarks: This function shall not participate in overload resolution unlessis_convertible_v<const T&, basic_string_view<charT, traits>>
istrue
andis_convertible_v<const T&, const charT*>
isfalse
.
Edit 27.4.3.7.2 [string.append] as indicated:
template<class T> basic_string& append(basic_string_view<charT, traits> svconst T& t);-6- Effects: Equivalent to:
{ basic_string_view<charT, traits> sv = t; return append(sv.data(), sv.size()); }-?- Remarks: This function shall not participate in overload resolution unless
is_convertible_v<const T&, basic_string_view<charT, traits>>
istrue
andis_convertible_v<const T&, const charT*>
isfalse
.
Edit 27.4.3.7.3 [string.assign] as indicated:
template<class T> basic_string& assign(basic_string_view<charT, traits> svconst T& t);-8- Effects: Equivalent to:
{ basic_string_view<charT, traits> sv = t; return assign(sv.data(), sv.size()); }-?- Remarks: This function shall not participate in overload resolution unless
is_convertible_v<const T&, basic_string_view<charT, traits>>
istrue
andis_convertible_v<const T&, const charT*>
isfalse
.
Edit 27.4.3.7.4 [string.insert] as indicated:
template<class T> basic_string& insert(size_type pos,basic_string_view<charT, traits> svconst T& t);-5- Effects: Equivalent to:
{ basic_string_view<charT, traits> sv = t; return insert(pos, sv.data(), sv.size()); }-?- Remarks: This function shall not participate in overload resolution unless
is_convertible_v<const T&, basic_string_view<charT, traits>>
istrue
andis_convertible_v<const T&, const charT*>
isfalse
.
Edit 27.4.3.7.6 [string.replace] as indicated:
template<class T> basic_string& replace(size_type pos1, size_type n1,basic_string_view<charT, traits> svconst T& t);-5- Effects: Equivalent to:
{ basic_string_view<charT, traits> sv = t; return replace(pos1, n1, sv.data(), sv.size()); }-?- Remarks: This function shall not participate in overload resolution unless
is_convertible_v<const T&, basic_string_view<charT, traits>>
istrue
andis_convertible_v<const T&, const charT*>
isfalse
.[…]
template<class T> basic_string& replace(const_iterator i1, const_iterator i2,basic_string_view<charT, traits> svconst T& t);-21- Requires:
-22- Effects: Creates a variable,[begin(), i1)
and[i1, i2)
are valid ranges.sv
, as if bybasic_string_view<charT, traits> sv = t;
and then cCallsreplace(i1 - begin(), i2 - i1, sv)
. -23- Returns:*this
.-?- Remarks: This function shall not participate in overload resolution unless
is_convertible_v<const T&, basic_string_view<charT, traits>>
istrue
andis_convertible_v<const T&, const charT*>
isfalse
.
Edit 27.4.3.8.2 [string.find] as indicated:
template<class T> size_type find(basic_string_view<charT, traits> svconst T& t, size_type pos = 0) constnoexcept;-1- Effects: Creates a variable,
-2- Returns:sv
, as if bybasic_string_view<charT, traits> sv = t;
and then dDetermines the lowest positionxpos
, if possible, such that both of the following conditions hold: […]xpos
if the function can determine such a value forxpos
. Otherwise, returnsnpos
.-?- Remarks: This function shall not participate in overload resolution unless
-?- Throws: Nothing unless the initialization ofis_convertible_v<const T&, basic_string_view<charT, traits>>
istrue
andis_convertible_v<const T&, const charT*>
isfalse
.sv
throws an exception.
Edit [string.rfind] as indicated:
template<class T> size_type rfind(basic_string_view<charT, traits> svconst T& t, size_type pos = npos) constnoexcept;-1- Effects: Creates a variable,
-2- Returns:sv
, as if bybasic_string_view<charT, traits> sv = t;
and then dDetermines the highest positionxpos
, if possible, such that both of the following conditions hold: […]xpos
if the function can determine such a value forxpos
. Otherwise, returnsnpos
.-?- Remarks: This function shall not participate in overload resolution unless
-?- Throws: Nothing unless the initialization ofis_convertible_v<const T&, basic_string_view<charT, traits>>
istrue
andis_convertible_v<const T&, const charT*>
isfalse
.sv
throws an exception.
Edit [string.find.first.of] as indicated:
template<class T> size_type find_first_of(basic_string_view<charT, traits> svconst T& t, size_type pos = 0) constnoexcept;-1- Effects: Creates a variable,
-2- Returns:sv
, as if bybasic_string_view<charT, traits> sv = t;
and then dDetermines the lowest positionxpos
, if possible, such that both of the following conditions hold: […]xpos
if the function can determine such a value forxpos
. Otherwise, returnsnpos
.-?- Remarks: This function shall not participate in overload resolution unless
-?- Throws: Nothing unless the initialization ofis_convertible_v<const T&, basic_string_view<charT, traits>>
istrue
andis_convertible_v<const T&, const charT*>
isfalse
.sv
throws an exception.
Edit [string.find.last.of] as indicated:
template<class T> size_type find_last_of(basic_string_view<charT, traits> svconst T& t, size_type pos = npos) constnoexcept;-1- Effects: Creates a variable,
-2- Returns:sv
, as if bybasic_string_view<charT, traits> sv = t;
and then dDetermines the highest positionxpos
, if possible, such that both of the following conditions hold: […]xpos
if the function can determine such a value forxpos
. Otherwise, returnsnpos
.-?- Remarks: This function shall not participate in overload resolution unless
-?- Throws: Nothing unless the initialization ofis_convertible_v<const T&, basic_string_view<charT, traits>>
istrue
andis_convertible_v<const T&, const charT*>
isfalse
.sv
throws an exception.
Edit [string.find.first.not.of] as indicated:
template<class T> size_type find_first_not_of(basic_string_view<charT, traits> svconst T& t, size_type pos = 0) constnoexcept;-1- Effects: Creates a variable,
-2- Returns:sv
, as if bybasic_string_view<charT, traits> sv = t;
and then dDetermines the lowest positionxpos
, if possible, such that both of the following conditions hold: […]xpos
if the function can determine such a value forxpos
. Otherwise, returnsnpos
.-?- Remarks: This function shall not participate in overload resolution unless
-?- Throws: Nothing unless the initialization ofis_convertible_v<const T&, basic_string_view<charT, traits>>
istrue
andis_convertible_v<const T&, const charT*>
isfalse
.sv
throws an exception.
Edit [string.find.last.not.of] as indicated:
template<class T> size_type find_last_not_of(basic_string_view<charT, traits> svconst T& t, size_type pos = npos) constnoexcept;-1- Effects: Creates a variable,
-2- Returns:sv
, as if bybasic_string_view<charT, traits> sv = t;
and then dDetermines the highest positionxpos
, if possible, such that both of the following conditions hold: […]xpos
if the function can determine such a value forxpos
. Otherwise, returnsnpos
.-?- Remarks: This function shall not participate in overload resolution unless
-?- Throws: Nothing unless the initialization ofis_convertible_v<const T&, basic_string_view<charT, traits>>
istrue
andis_convertible_v<const T&, const charT*>
isfalse
.sv
throws an exception.
Edit 27.4.3.8.4 [string.compare] as indicated:
template<class T> int compare(basic_string_view<charT, traits> svconst T& t) constnoexcept;-1- Effects: Creates a variable,
-2- Returns: The nonzero result if the result of the comparison is nonzero. Otherwise, returns a value as indicated in Table 63. […]sv
, as if bybasic_string_view<charT, traits> sv = t;
and then dDetermines the effective lengthrlen
of the strings to compare as the smaller ofsize()
andsv.size()
. The function then compares the two strings by callingtraits::compare(data(), sv.data(), rlen)
.-?- Remarks: This function shall not participate in overload resolution unless
-?- Throws: Nothing unless the initialization ofis_convertible_v<const T&, basic_string_view<charT, traits>>
istrue
andis_convertible_v<const T&, const charT*>
isfalse
.sv
throws an exception.template<class T> int compare(size_type pos1, size_type n1,basic_string_view<charT, traits> svconst T& t) const;-3- Effects: Equivalent to:
{ basic_string_view<charT, traits> sv = t; return basic_string_view<charT, traits>(data(), size()).substr(pos1, n1).compare(sv); }-?- Remarks: This function shall not participate in overload resolution unless
is_convertible_v<const T&, basic_string_view<charT, traits>>
istrue
andis_convertible_v<const T&, const charT*>
isfalse
.
unique_ptr
does not define operator<<
for stream outputSection: 20.3.1 [unique.ptr] Status: C++20 Submitter: Peter Dimov Opened: 2017-03-19 Last modified: 2021-02-25
Priority: 0
View all other issues in [unique.ptr].
View all issues with C++20 status.
Discussion:
shared_ptr
does define operator<<
, and unique_ptr
should too, for consistency and usability reasons.
[2017-07 Toronto Wed Issue Prioritization]
Priority 0; move to Ready
Proposed resolution:
This wording is relative to N4659.
Change 20.2.2 [memory.syn], header <memory>
synopsis, as indicated:
namespace std { […] // 20.3.1 [unique.ptr], class template unique_ptr […] template <class T, class D> bool operator>=(nullptr_t, const unique_ptr<T, D>& y); template<class E, class T, class Y, class D> basic_ostream<E, T>& operator<< (basic_ostream<E, T>& os, const unique_ptr<Y, D>& p); […] }
Change 20.3.1 [unique.ptr], class template unique_ptr
synopsis, as indicated:
namespace std { […] template <class T, class D> bool operator>=(nullptr_t, const unique_ptr<T, D>& y); template<class E, class T, class Y, class D> basic_ostream<E, T>& operator<< (basic_ostream<E, T>& os, const unique_ptr<Y, D>& p); }
Add a new subclause following subclause 20.3.1.6 [unique.ptr.special] as indicated:
23.11.1.??
unique_ptr
I/O [unique.ptr.io]template<class E, class T, class Y, class D> basic_ostream<E, T>& operator<< (basic_ostream<E, T>& os, const unique_ptr<Y, D>& p);-?- Effects: Equivalent to
-?- Returns:os << p.get();
os
. -?- Remarks: This function shall not participate in overload resolution unlessos << p.get()
is a valid expression.
std::byte
operations are misspecifiedSection: 17.2.5 [support.types.byteops] Status: C++20 Submitter: Thomas Köppe Opened: 2017-03-24 Last modified: 2021-02-25
Priority: 1
View all issues with C++20 status.
Discussion:
The operations for std::byte
(17.2.5 [support.types.byteops]) are currently specified to have undefined
behaviour in general cases, since the type of the expression expr that appears in return byte(expr)
is
obtained by the arithmetic conversion rules and has higher conversion rank than unsigned char
. Therefore, the
value of the expression may be outside the range of the enum (for example, consider ~0
), and by
7.6.1.9 [expr.static.cast] p10 the conversion results in undefined behaviour.
byte operator<<(byte b, IntType shift)Equivalent to:
return byte(static_cast<unsigned char>(static_cast<unsigned char>(b) << shift));
[ 2017-06-27 P1 after 5 positive votes on c++std-lib. ]
[2017-06-28, STL comments and provides wording]
This proposed resolution performs its work in unsigned int
, which is immune to promotion. For op=
,
I'm avoiding unnecessary verbosity.
static_cast
s instead of functional-style casts. All of the static_cast
s are
intentional, although not all of them are strictly necessary. I felt that it was simpler to always follow the
same pattern for type conversions, instead of skipping static_cast
s by taking advantage of the possible
ranges of values. (I could prepare an alternative PR to avoid unnecessary casts.) I'm not static_cast
ing
the shift arguments, because of how 7.6.7 [expr.shift] works.
For to_integer()
, there's a tiny question. According to 7.6.1.9 [expr.static.cast]/9,
static_cast
ing from [128, 255]
bytes to signed
(behavior) char
s triggers unspecified behavior,
whereas using unsigned char
as an intermediate type would produce implementation-defined behavior,
7.3.9 [conv.integral]/3. This question is basically theoretical, and it's unaffected by this proposed resolution
(which is just changing a functional-style cast to a static_cast
).
[2016-07, Toronto Thursday night issues processing]
Status to Ready
Proposed resolution:
This wording is relative to N4659.
Edit 17.2.5 [support.types.byteops] as indicated:
template <class IntType> constexpr byte& operator<<=(byte& b, IntType shift) noexcept;-1- Remarks: This function shall not participate in overload resolution unless
-2- Effects: Equivalent to:is_integral_v<IntType>
istrue
.return b = b << shift
byte(static_cast<unsigned char>(b) << shift);template <class IntType> constexpr byte operator<<(byte b, IntType shift) noexcept;-3- Remarks: This function shall not participate in overload resolution unless
-4- Effects: Equivalent to:is_integral_v<IntType>
istrue
.return static_cast<byte>(static_cast<unsigned char>(static_cast<unsigned int>(b) << shift))
byte(static_cast<unsigned char>(b) << shift);template <class IntType> constexpr byte& operator>>=(byte& b, IntType shift) noexcept;-5- Remarks: This function shall not participate in overload resolution unless
-6- Effects: Equivalent to:is_integral_v<IntType>
istrue
.return b = b >> shift
byte(static_cast<unsigned char>(b) >> shift);template <class IntType> constexpr byte operator>>(byte b, IntType shift) noexcept;-7- Remarks: This function shall not participate in overload resolution unless
-8- Effects: Equivalent to:is_integral_v<IntType>
istrue
.return static_cast<byte>(static_cast<unsigned char>(static_cast<unsigned int>(b) >> shift))
byte(static_cast<unsigned char>(b) >> shift);constexpr byte& operator|=(byte& l, byte r) noexcept;-9- Effects: Equivalent to:
return l = l | rbyte(static_cast<unsigned char>(l) | static_cast<unsigned char>(r));constexpr byte operator|(byte l, byte r) noexcept;-10- Effects: Equivalent to:
return static_cast<byte>(static_cast<unsigned char>(static_cast<unsigned int>(l) | static_cast<unsigned int>(r)))byte(static_cast<unsigned char>(l) | static_cast<unsigned char>(r));constexpr byte& operator&=(byte& l, byte r) noexcept;-11- Effects: Equivalent to:
return l = l & rbyte(static_cast<unsigned char>(l) & static_cast<unsigned char>(r));constexpr byte operator&(byte l, byte r) noexcept;-12- Effects: Equivalent to:
return static_cast<byte>(static_cast<unsigned char>(static_cast<unsigned int>(l) & static_cast<unsigned int>(r)))byte(static_cast<unsigned char>(l) & static_cast<unsigned char>(r));constexpr byte& operator^=(byte& l, byte r) noexcept;-13- Effects: Equivalent to:
return l = l ^ rbyte(static_cast<unsigned char>(l) ^ static_cast<unsigned char>(r));constexpr byte operator^(byte l, byte r) noexcept;-14- Effects: Equivalent to:
return static_cast<byte>(static_cast<unsigned char>(static_cast<unsigned int>(l) ^ static_cast<unsigned int>(r)))byte(static_cast<unsigned char>(l) ^ static_cast<unsigned char>(r));constexpr byte operator~(byte b) noexcept;-15- Effects: Equivalent to:
return static_cast<byte>(static_cast<unsigned char>(~static_cast<unsigned int>(b)))
byte(~static_cast<unsigned char>(b));template <class IntType> constexpr IntType to_integer(byte b) noexcept;-16- Remarks: This function shall not participate in overload resolution unless
-17- Effects: Equivalent to:is_integral_v<IntType>
istrue
.return static_cast<IntType>
IntType(b);
iterator_traits
should SFINAE for void*
and function pointersSection: 24.3.2.3 [iterator.traits] Status: Resolved Submitter: Billy Robert O'Neal III Opened: 2017-03-24 Last modified: 2020-09-06
Priority: 3
View all other issues in [iterator.traits].
View all issues with Resolved status.
Discussion:
A customer recently triggered an unexpected SFINAE condition with class path
. We constrain the constructor that takes
const Source&
by asking if iterator_traits<Source>::value_type
is one that's OK. This forms
iterator_traits<void*>
, which is a hard error, as it tries to form void&
.
atomic<void*>
/ atomic<int(*)(int)>
).
[2017-07 Toronto Wed Issue Prioritization]
Priority 3; Billy to look into arrays of unknown bounds
[2019-02; Kona Wednesday night issue processing]
This was resolved by the adoption of P0896 in San Diego.
Proposed resolution:
This wording is relative to N4659.
Change 24.3.2.3 [iterator.traits] as indicated:
-3- It is specialized for pointers as
namespace std { template<class T> struct iterator_traits<T*> { using difference_type = ptrdiff_t; using value_type = T; using pointer = T*; using reference = T&; using iterator_category = random_access_iterator_tag; }; }and for pointers to
const
asnamespace std { template<class T> struct iterator_traits<const T*> { using difference_type = ptrdiff_t; using value_type = T; using pointer = const T*; using reference = const T&; using iterator_category = random_access_iterator_tag; }; }-?- These partial specializations for pointers apply only when
T
is an object type.
iterator_traits
should work for pointers to cv T
Section: 24.3.2.3 [iterator.traits] Status: C++20 Submitter: Billy Robert O'Neal III Opened: 2017-03-27 Last modified: 2021-02-25
Priority: 0
View all other issues in [iterator.traits].
View all issues with C++20 status.
Discussion:
iterator_traits
accepts pointer to volatile T*
, but then says that the value_type
is
volatile T
, instead of T
, which is inconsistent for what it does for pointer to const T
.
We should either reject volatile
outright or give the right answer.
[2017-03-30, David Krauss comments]
volatile
pointers may not be well-behaved random-access iterators. When simple access incurs side effects,
the multiple-pass guarantee depends on underlying (hardware) semantics.
[2017-07 Toronto Wed Issue Prioritization]
Priority 0; move to Ready
Proposed resolution:
This wording is relative to N4659.
Change 24.2 [iterator.synopsis] as indicated:
// 24.4 [iterator.primitives], primitives template<class Iterator> struct iterator_traits; template<class T> struct iterator_traits<T*>;template<class T> struct iterator_traits<const T*>;
Change 24.3.2.3 [iterator.traits] as indicated:
-3- It is specialized for pointers as
namespace std { template<class T> struct iterator_traits<T*> { using difference_type = ptrdiff_t; using value_type = remove_cv_t<T>; using pointer = T*; using reference = T&; using iterator_category = random_access_iterator_tag; }; }
and for pointers toconst
asnamespace std { template<class T> struct iterator_traits<const T*> { using difference_type = ptrdiff_t; using value_type = T; using pointer = const T*; using reference = const T&; using iterator_category = random_access_iterator_tag; }; }
deque::erase
tooSection: 23.3.5.4 [deque.modifiers] Status: C++20 Submitter: Tim Song Opened: 2017-03-30 Last modified: 2021-02-25
Priority: 0
View other active issues in [deque.modifiers].
View all other issues in [deque.modifiers].
View all issues with C++20 status.
Discussion:
Most of the discussion of LWG 2853(i) applies, mutatis mutandis, to
deque::erase
. The relevant requirements table requires neither
Copy/MoveInsertable
nor Copy/MoveConstructible
for the erase
operations, so there's no way a copy/move constructor can safely be
called.
[2017-07 Toronto Wed Issue Prioritization]
Priority 0; Move to Ready
Proposed resolution:
This wording is relative to N4659.
Change 23.3.5.4 [deque.modifiers] as indicated:
iterator erase(const_iterator position); iterator erase(const_iterator first, const_iterator last); void pop_front(); void pop_back();-4- Effects: […]
-5- Complexity: The number of calls to the destructor ofT
is the same as the number of elements erased, but the number of calls to the assignment operator ofT
is no more than the lesser of the number of elements before the erased elements and the number of elements after the erased elements. -6- Throws: Nothing unless an exception is thrown by thecopy constructor, move constructor,assignment operator, or move assignment operatorofT
.
Section: 16.4.5 [constraints] Status: C++20 Submitter: Tim Song Opened: 2017-03-31 Last modified: 2021-02-25
Priority: Not Prioritized
View all issues with C++20 status.
Discussion:
There's currently no rule against specializing the various _v
convenience variable templates outside of <type_traits>
.
foo_v<T>
should be always equal to
foo<T>::value
. The correct way to influence, say,
is_placeholder_v<T>
should be to specialize is_placeholder
,
not is_placeholder_v
. Otherwise, the editorial changes to use the
_v
form to the specification would no longer be editorial but have
normative impact.
Adding a global prohibition in 16.4.5.2.1 [namespace.std] seems preferable to
adding individual prohibitions to each affected template; the PR below
carves out an exception for variable templates that are intended to be
specialized by users. As far as I know there are no such templates in
the current WP, but the Ranges TS does use them.
[2017-06-14, Moved to Tentatively Ready after 6 positive votes on c++std-lib]
Proposed resolution:
This wording is relative to N4659.
Add a paragraph to 16.4.5.2.1 [namespace.std], before p2:
-1- The behavior of a C++ program is undefined if it adds declarations or definitions to namespace
-?- The behavior of a C++ program is undefined if it declares an explicit or partial specialization of any standard library variable template, except where explicitly permitted by the specification of that variable template. -2- The behavior of a C++ program is undefined if it declares […]std
or to a namespace within namespacestd
unless otherwise specified. A program may add a template specialization for any standard library template to namespacestd
only if the declaration depends on a user-defined type and the specialization meets the standard library requirements for the original template and is not explicitly prohibited.(footnote: […])
to_chars
/ from_chars
depend on std::string
Section: 22.2 [utility] Status: Resolved Submitter: Jens Maurer Opened: 2017-04-11 Last modified: 2017-07-16
Priority: Not Prioritized
View all other issues in [utility].
View all issues with Resolved status.
Discussion:
We added from_chars_result
and to_chars_result
into <utility>
, but those contain
error_code
instances, which come from <system_error>
. This creates a circular dependency
issue which is somewhat difficult to resolve, as error_code
depends on error_category
which has virtuals
that return string
— it would effectively force putting all of <string>
into
<utility>
.
from_chars_result
/ to_chars_result
to an errc
instead of an error_code
(as a
fast-track DR against 17)?
[2017-06-27, Jens comments]
The paper P0682R0 "Repairing elementary string conversions" is available in the pre-Toronto mailing and provides wording.
[2017-07 Toronto]
Resolved by the adoption of P0682R1 in Toronto.
Proposed resolution:
filesystem::canonical()
still defined in terms of absolute(p, base)
Section: 31.12.13.3 [fs.op.canonical] Status: C++17 Submitter: Sergey Zubkov Opened: 2017-04-21 Last modified: 2020-09-06
Priority: 1
View all issues with C++17 status.
Discussion:
This is from editorial issue #1620:
Since the resolution of US-78 was applied (as part of P0492R2),std::filesystem::absolute(path, base)
no longer exists. However, std::filesystem::canonical
is still defined in terms of absolute(p, base)
.
[2017-06-27 P1 after 5 positive votes on c++std-lib]
Davis Herring: This needs to be P1 — due to a wording gap in P0492R2, 2 out of the
3 overloads of filesystem::canonical()
have bad signatures and are unimplementable.
[2017-07-14, Toronto, Davis Herring provides wording]
[2017-07-14, Toronto, Moved to Immediate]
Proposed resolution:
This wording is relative to N4659.
Edit 31.12.4 [fs.filesystem.syn] as indicated:
path canonical(const path& p, const path& base = current_path()); path canonical(const path& p, error_code& ec);path canonical(const path& p, const path& base, error_code& ec);
Edit 31.12.13.3 [fs.op.canonical] as indicated:
path canonical(const path& p, const path& base = current_path()); path canonical(const path& p, error_code& ec);path canonical(const path& p, const path& base, error_code& ec);-1- Effects: Converts
-2- Returns: A path that refers to the same file system object asp
, which must exist, to an absolute path that has no symbolic link, dot, or dot-dot elements in its pathname in the generic format.absolute(p
., base)For the overload without aThe signature with argumentbase
argument,base
iscurrent_path()
. Signaturesec
returnspath()
if an error occurs. […]
bind
's specification doesn't apply the cv-qualification of the call wrapper to the callable objectSection: 22.10.15.4 [func.bind.bind] Status: Resolved Submitter: Tim Song Opened: 2017-05-04 Last modified: 2020-09-06
Priority: 3
View all other issues in [func.bind.bind].
View all issues with Resolved status.
Discussion:
According to 22.10.15.4 [func.bind.bind]/1.2,
fd
is an lvalue of typeFD
constructed fromstd::forward<F>(f)
,
and then uses fd
throughout the specification, seemingly without
regard to the cv-qualification of the call wrapper g
. But this
definition means that fd
is always cv-unqualified, rather than having
its cv-qualification change with that of g
as intended.
fd
's cv-qualifier followed that of the call wrapper.
A similar issue affects the type of tdi
for nested binds.
[2017-07 Toronto Wed Issue Prioritization]
Priority 3
[2020-01 Resolved by the adoption of P1065 in Cologne.]
Proposed resolution:
This wording is relative to N4659.
Edit 22.10.15.4 [func.bind.bind] as indicated:
template<class F, class... BoundArgs> unspecified bind(F&& f, BoundArgs&&... bound_args);[…]
-3- Returns: A forwarding call wrapperg
(22.10.4 [func.require]). The effect ofg(u1, u2, ..., uM)
shall beINVOKE(static_cast<FD cv &>(fd), std::forward<V1>(v1), std::forward<V2>(v2), ..., std::forward<VN>(vN))where cv represents the cv-qualifiers of
[…]g
and the values and types of the bound argumentsv1, v2, . . . , vN
are determined as specified below. The copy constructor and move constructor of the forwarding call wrapper shall throw an exception if and only if the corresponding constructor ofFD
or of any of the typesTDi
throws an exception.template<class R, class F, class... BoundArgs> unspecified bind(F&& f, BoundArgs&&... bound_args);[…]
-7- Returns: A forwarding call wrapperg
(22.10.4 [func.require]). The effect ofg(u1, u2, ..., uM)
shall beINVOKE<R>(static_cast<FD cv &>(fd), std::forward<V1>(v1), std::forward<V2>(v2), ..., std::forward<VN>(vN))where cv represents the cv-qualifiers of
[…] -10- The values of the bound argumentsg
and the values and types of the bound argumentsv1, v2, . . . , vN
are determined as specified below. The copy constructor and move constructor of the forwarding call wrapper shall throw an exception if and only if the corresponding constructor ofFD
or of any of the typesTDi
throws an exception.v1, v2, ... , vN
and their corresponding typesV1, V2, ... , VN
depend on the typesTDi
derived from the call tobind
and the cv-qualifiers cv of the call wrapperg
as follows:
(10.1) — […]
(10.2) — if the value of
is_bind_expression_v<TDi>
istrue
, the argument isstatic_cast<TDi cv &>(tdi)(std::forward<Uj>(uj)...)
and its typeVi
isinvoke_result_t<TDi cv &, Uj...>&&
;(10.3) — […]
Section: 22.3.2 [pairs.pair], 22.4.4.3 [tuple.assign] Status: C++20 Submitter: Casey Carter Opened: 2017-05-05 Last modified: 2021-02-25
Priority: 2
View other active issues in [pairs.pair].
View all other issues in [pairs.pair].
View all issues with C++20 status.
Discussion:
LWG 2729(i) constrained many pair
& tuple
assignment operators to "not participate in overload resolution" and "be defined as deleted if." As discussed when LWG reviewed 2756(i), it is undesirable to require
that a move constructor or assignment operator be "defined as deleted," since it is unclear whether that operation should be
implicitly deleted, and therefore not participate in overload resolution, or explicitly deleted and therefore
impede overload resolution for usages that would otherwise find copies.
[2017-07 Toronto Wed Issue Prioritization]
Priority 2
[2017-11 Albuquerque Wednesday issue processing]
Move to Immediate
Proposed resolution:
This wording is relative to N4659.
Edit 22.3.2 [pairs.pair] as indicated:
pair& operator=(pair&& p) noexcept(see below);-21- Effects: Assigns to
-22- Remarks: This operator shallfirst
withstd::forward<first_type>(p.first)
and tosecond
withstd::forward<second_type>(p.second)
.be defined as deletednot participate in overload resolution unlessis_move_assignable_v<first_type>
istrue
andis_move_assignable_v<second_type>
istrue
. […]
Edit 22.4.4.3 [tuple.assign] as indicated:
tuple& operator=(tuple&& u) noexcept(see below);-5- Effects: For all
-6- Remarks: This operator shalli
, assignsstd::forward<Ti>(get<i>(u))
toget<i>(*this)
.be defined as deletednot participate in overload resolution unlessis_move_assignable_v<Ti>
istrue
for alli
. […]
nonesuch
is insufficiently uselessSection: 6.3 [fund.ts.v3::meta] Status: C++23 Submitter: Tim Song Opened: 2017-05-08 Last modified: 2023-11-22
Priority: 2
View all issues with C++23 status.
Discussion:
Addresses: fund.ts.v3
The definition of std::experimental::nonesuch
(with a deleted default
constructor, destructor, copy constructor, and copy assignment
operator) means that it is an aggregate, which means that it can be
initialized from {}
in contexts where the availability of the
destructor is not considered (e.g., overload resolution or a
new-expression).
struct such {}; void f(const such&); void f(const nonesuch&); f({});
For a real-life example of such ambiguity, see
GCC bug 79141,
involving libstdc++'s internal __nonesuch
type defined just like the
one in the fundamentals TS.
nonesuch
would be substantially more useful if the ICS
from {}
is gone. nonesuch
should have no default constructor (rather
than a deleted one), and it shouldn't be an aggregate.
[2017-11-20 Priority set to 2 after discussion on the reflector.]
Previous resolution [SUPERSEDED]:
This wording is relative to N4617.
Edit 99 [fund.ts.v3::meta.type.synop] as indicated, moving the definition of
nonesuch
to 6.3.3 [fund.ts.v3::meta.detect]://6.3.3 [fund.ts.v3::meta.detect], Detection idiom […] struct nonesuch{ nonesuch() = delete; ~nonesuch() = delete; nonesuch(nonesuch const&) = delete; void operator=(nonesuch const&) = delete; }; […]
Insert at the beginning of 6.3.3 [fund.ts.v3::meta.detect] the following paragraphs:
[Drafting note: The seemingly redundant statement about default and initializer-list constructors is intended to negate the usual leeway for implementations to declare additional member function signatures granted in 16.4.6.5 [member.functions]. — end drafting note]
struct nonesuch { ~nonesuch() = delete; nonesuch(nonesuch const&) = delete; void operator=(nonesuch const&) = delete; };-?-
nonesuch
has no default constructor (C++14 §[class.ctor]) or initializer-list constructor (C++14 §[dcl.init.list]), and is not an aggregate (C++14 §[dcl.init.aggr]).
[2018-08-23 Batavia Issues processing]
Change C++14 references to C++17, and all the LFTS2 references to LFTS3
Status to Tentatively Ready
[2018-11, Adopted in San Diego]
Proposed resolution:
This wording is relative to N4617.
Edit 99 [fund.ts.v3::meta.type.synop] as indicated, moving the definition of
nonesuch
to 6.3.3 [fund.ts.v3::meta.detect]:
//6.3.3 [fund.ts.v3::meta.detect], Detection idiom […] struct nonesuch{ nonesuch() = delete; ~nonesuch() = delete; nonesuch(nonesuch const&) = delete; void operator=(nonesuch const&) = delete; }; […]
Insert at the beginning of 6.3.3 [fund.ts.v3::meta.detect] the following paragraphs:
[Drafting note: The seemingly redundant statement about default and initializer-list constructors is intended to negate the usual leeway for implementations to declare additional member function signatures granted in 16.4.6.5 [member.functions]. — end drafting note]
struct nonesuch { ~nonesuch() = delete; nonesuch(nonesuch const&) = delete; void operator=(nonesuch const&) = delete; };-?-
nonesuch
has no default constructor (C++17 §[class.ctor]) or initializer-list constructor (C++17 §[dcl.init.list]), and is not an aggregate (C++17 §[dcl.init.aggr]).
set_default_resource
Section: 20.5.4 [mem.res.global] Status: C++20 Submitter: Casey Carter Opened: 2017-05-09 Last modified: 2021-02-25
Priority: Not Prioritized
View all other issues in [mem.res.global].
View all issues with C++20 status.
Discussion:
The specification of set_default_resource
in N4659
20.5.4 [mem.res.global] reads:
memory_resource* set_default_resource(memory_resource* r) noexcept;-4- Effects: If
-5- Postconditions:r
is non-null, sets the value of the default memory resource pointer tor
, otherwise sets the default memory resource pointer tonew_delete_resource()
.get_default_resource() == r
. -6- Returns: The previous value of the default memory resource pointer. -7- Remarks: […]
It is apparent that the effects specified in para 4 cannot — and indeed should not — achieve the
postcondition specified in para 5 when r
is null.
[2017-05-13, Tim comments]
This is the same issue as LWG 2522(i), which just missed the Fundamentals TS working draft used for the merge. A similar resolution (simply striking the Postconditions: paragraph) might be better.
Previous resolution [SUPERSEDED]:
This wording is relative to N4659.
Modify 20.5.4 [mem.res.global] as follows:
memory_resource* set_default_resource(memory_resource* r) noexcept;-4- Effects: If
-5- Postconditions: Ifr
is non-null, sets the value of the default memory resource pointer tor
, otherwise sets the default memory resource pointer tonew_delete_resource()
.r
is non-null,get_default_resource() == r
. Otherwise,get_default_resource() == new_delete_resource()
. -6- Returns: The previous value of the default memory resource pointer. -7- Remarks: […]
[2017-06-13, Casey Carter revises proposed wording]
I suggest to strike the Postconditions paragraph ([mem.res.global]/5) completely, as in LWG 2522(i)
[2017-06-15, Moved to Tentatively Ready after 6 positive votes on c++std-lib]
Proposed resolution:
This wording is relative to N4659.
Modify 20.5.4 [mem.res.global] as follows:
memory_resource* set_default_resource(memory_resource* r) noexcept;-4- Effects: If
r
is non-null, sets the value of the default memory resource pointer tor
, otherwise sets the default memory resource pointer tonew_delete_resource()
.-5- Postconditions:-6- Returns: The previous value of the default memory resource pointer. -7- Remarks: […]get_default_resource() == r
.
dynamic_pointer_cast
Section: 20.3.2.2.10 [util.smartptr.shared.cast] Status: C++20 Submitter: Tim Song Opened: 2017-05-11 Last modified: 2021-02-25
Priority: 0
View all other issues in [util.smartptr.shared.cast].
View all issues with C++20 status.
Discussion:
Currently 20.3.2.2.10 [util.smartptr.shared.cast]/4 says:
Requires: The expression
dynamic_cast<T*>((U*)nullptr)
shall be well formed and shall have well defined behavior.
A dynamic_cast
of a null pointer, if well-formed, always has
well-defined behavior: it returns a null pointer. The second part is
therefore redundant as currently worded. The C++14 version, on the
other hand, requires dynamic_cast<T*>(r.get())
to have well-defined
behavior, which actually adds something: it requires the user to not
trigger the undefined case in [class.cdtor]/5, for instance.
[2017-07 Toronto Monday issue prioritization]
Priority 0; move to Ready
Proposed resolution:
This wording is relative to N4659.
Edit 20.3.2.2.10 [util.smartptr.shared.cast] as indicated:
shared_ptr<T> dynamic_pointer_cast(const shared_ptr<U>& r) noexcept;-4- Requires: The expression
[…]dynamic_cast<T*>((U*)0)
shall be well formed. The expressiondynamic_cast<typename shared_ptr<T>::element_type*>(r.get())
shall be well formed and shall have well defined behavior.
path::native_string()
in filesystem_error::what()
specificationSection: 31.12.7.2 [fs.filesystem.error.members] Status: C++20 Submitter: Daniel Krügler Opened: 2017-05-22 Last modified: 2021-06-06
Priority: 0
View all other issues in [fs.filesystem.error.members].
View all issues with C++20 status.
Discussion:
As pointed out by Jonathan Wakely and Bo Persson, [filesystem_error.members]/7 refers to a non-existing
function path::native_string
:
Returns: A string containing
runtime_error::what()
. The exact format is unspecified. Implementations are encouraged but not required to includepath1.native_string()
if not empty,path2.native_string()
if not empty, andsystem_error::what()
strings in the returned string.
Existing implementations differ, as Jonathan also determined:
Boost.Filesystem uses path::string()
.
Libstdc++ uses path::string()
.
MSVC++/Dinkumware uses path::u8string()
.
It seems that libc++ doesn't include the paths in what()
.
We've had native_string()
in the spec since N3239 (where
it already didn't match any existing path
function at that time).
file_string()
in N1975
(within that specification path
was a template that was parametrized in the character type).
Since it can't be path::native()
because that might be the wrong type, one of path::string()
or
path::u8string()
seems appropriate.
Albeit the wording is just a non-binding encouragement to implementations, the decision on this matter should not be
considered editorially due to the existing implementation variance. Any official resolution of the current state could
cause a reconsideration of existing implementations, and therefore it should be documented.
Previous resolution [SUPERSEDED]:
This wording is relative to N4659.
Edit [filesystem_error.members] as indicated:
const char* what() const noexcept override;-7- Returns: A string containing
runtime_error::what()
. The exact format is unspecified. Implementations are encouraged but not required to includepath1.
if not empty,native_string()path2.
if not empty, andnative_string()system_error::what()
strings in the returned string.
[2017-05-25, Jonathan comments and suggests an alternative resolution]
The revised wording changes leave it up to the implementation which of the native format observers to use. The "if not empty" seems redundant, because if the path is empty then there's nothing to include anyway, but the proposed resolution preserves it.
[2017-07 Toronto Monday issue prioritization]
Priority 0; move to Ready
Proposed resolution:
This wording is relative to N4659.
Edit [filesystem_error.members] as indicated:
const char* what() const noexcept override;-7- Returns: A string containing
runtime_error::what()
. The exact format is unspecified. Implementations are encouraged but not required to includethepath1.native_string()
if not empty,path2.native_string()
if not empty, andsystem_error::what()
stringssystem_error::what()
string and the pathnames ofpath1
andpath2
in the native format in the returned string.
Section: 31.12.6.5.6 [fs.path.native.obs] Status: C++20 Submitter: Jonathan Wakely Opened: 2017-05-25 Last modified: 2021-02-25
Priority: Not Prioritized
View all issues with C++20 status.
Discussion:
P0492R1 US-74 failed to replace one occurrence of pathstring
,
in [fs.path.native.obs] p8. It should be changed to native()
, as with the other native format observers.
[2017-06-12, Moved to Tentatively Ready after 6 positive votes on c++std-lib]
Proposed resolution:
This wording is relative to N4659.
Edit 31.12.6.5.6 [fs.path.native.obs] as indicated:
std::string string() const; std::wstring wstring() const; std::string u8string() const; std::u16string u16string() const; std::u32string u32string() const;-8- Returns:
-9- Remarks: Conversion, if any, is performed as specified by 31.12.6.3 [fs.path.cvt]. The encoding of the string returned by.
pathstringnative()u8string()
is always UTF-8.
basic_string reserve
and vector/unordered_map/unordered_set reserve
functionsSection: 27.4.3.5 [string.capacity] Status: Resolved Submitter: Andrew Luo Opened: 2017-05-30 Last modified: 2020-09-06
Priority: 3
View all other issues in [string.capacity].
View all issues with Resolved status.
Discussion:
According to 27.4.3.5 [string.capacity] paragraph 11:
-11- Effects: After
reserve()
,capacity()
is greater or equal to the argument ofreserve
. [Note: Callingreserve()
with ares_arg
argument less thancapacity()
is in effect a non-binding shrink request. A call withres_arg <= size()
is in effect a non-binding shrink-to-fit request. — end note]
A call to basic_string
's reserve
function with res_arg <= size()
is taken as a non-binding
request to shrink the capacity, whereas for vector
(and similarly for unordered_map
and unordered_set
)
according to 23.3.13.3 [vector.capacity] p3:
-3- Effects: A directive that informs a
vector
of a planned change in size, so that it can manage the storage allocation accordingly. Afterreserve()
,capacity()
is greater or equal to the argument of reserve if reallocation happens; and equal to the previous value ofcapacity()
otherwise. Reallocation happens at this point if and only if the current capacity is less than the argument ofreserve()
. If an exception is thrown other than by the move constructor of a non-CopyInsertable
type, there are no effects.
The problem here is that the different behavior makes it that writing template code where the template argument type is a
container type (for example std::string
or std::vector<char>
) calls to reserve
can have
different meaning depending on which container type the template is instantiated with. It might be a minor issue but
it would be nice to fix the inconsistency. I ran into an issue around this when I was porting code from MSVC++ to G++
(For basic_string
, MSVC++'s STL implementation, based on Dinkumware, ignores the call if res_arg < capacity()
whereas GCC's STL implementation, libstdc++ will actually shrink the string. For the code I wrote this caused a huge
performance issue since we were reallocating the entire string with every call to reserve
. Of course we could have
worked around it by doing the res_arg < capacity()
check ourselves, but I think this inconsistency in the
standard isn't desirable).
-11- Effects: After
reserve()
,capacity()
is greater or equal to the argument ofreserve
if reallocation happens; and equal to the previous value ofcapacity()
otherwise. Reallocation happens at this point if and only if the current capacity is less than the argument ofreserve()
.
I realize that this causes the basic_string::reserve
to no longer have the secondary property of shrinking,
but this is what shrink_to_fit
is for.
[2017-07 Toronto Monday issue prioritization]
Priority 3; status to LEWG
[2018-3-17 Resolved by P0966, which was adopted in Jacksonville.]
Proposed resolution:
This wording is relative to N4659.
Edit 27.4.3.5 [string.capacity] as indicated:
void reserve(size_type res_arg=0);-10- The member function
-11- Effects: Afterreserve()
is a directive that informs abasic_string
object of a planned change in size, so that it can manage the storage allocation accordingly.reserve()
,capacity()
is greater or equal to the argument ofreserve
, if reallocation happens; and equal to the previous value ofcapacity()
otherwise. Reallocation happens at this point if and only if the current capacity is less than the argument ofreserve()
.[Note: Calling-12- Throws:reserve()
with ares_arg
argument less thancapacity()
is in effect a non-binding shrink request. A call withres_arg <= size()
is in effect a non-binding shrink-to-fit request. — end note]length_error
ifres_arg > max_size()
.
polymorphic_allocator::construct()
shouldn't pass resource()
Section: 20.5.3.3 [mem.poly.allocator.mem] Status: C++20 Submitter: Pablo Halpern Opened: 2017-05-30 Last modified: 2021-02-25
Priority: 2
View all other issues in [mem.poly.allocator.mem].
View all issues with C++20 status.
Discussion:
Section 20.5.3.3 [mem.poly.allocator.mem] defines the effect of polymorphic_allocator<T>::construct
as:
Effects: Construct a
T
object in the storage whose address is represented byp
by uses-allocator construction with allocatorresource()
and constructor argumentsstd::forward<Args>(args)...
.
The use of resource()
is a hold-over from the LFTS, which contains a modified definition of uses-allocator construction. This revised definition was not carried over into the C++17 WP when allocator_resource and polymorphic_allocator were moved over.
Previous resolution [SUPERSEDED]:
This wording is relative to N4659.
Edit 20.5.3.3 [mem.poly.allocator.mem] as indicated:
template <class T, class... Args> void construct(T* p, Args&&... args);-5- Requires: Uses-allocator construction of
-6- Effects: Construct aT
with allocator(see 20.2.8.2 [allocator.uses.construction]) and constructor arguments
resource()*thisstd::forward<Args>(args)...
is well-formed. [Note: Uses-allocator construction is always well formed for types that do not use allocators. — end note]T
object in the storage whose address is represented byp
by uses-allocator construction with allocatorand constructor arguments
resource()*thisstd::forward<Args>(args)...
. -7- Throws: Nothing unless the constructor forT
throws.template <class T1, class T2, class... Args1, class... Args2> void construct(pair<T1,T2>* p, piecewise_construct_t, tuple<Args1...> x, tuple<Args2...> y);-8- [Note: This method and the
-9- Effects: Letconstruct
methods that follow are overloads for piecewise construction of pairs (22.3.2 [pairs.pair]). — end note]xprime
be atuple
constructed fromx
according to the appropriate rule from the following list. [Note: The following description can be summarized as constructing apair<T1, T2>
object in the storage whose address is represented byp
, as if by separate uses-allocator construction with allocator(20.2.8.2 [allocator.uses.construction]) of
resource()*thisp->first
using the elements ofx
andp->second
using the elements ofy
. — end note] […]
[2017-06-12, Pablo comments]
The current description is correct and does not depend on changes to uses-allocator construction. It relies on the fact
that memory_resource*
is convertible to polymorphic_allocator
.
[2017-06-13, Tim Song reopens]
While it is true that memory_resource*
is convertible to polymorphic_allocator
,
uses-allocator construction still requires allocators, and a
memory_resource*
isn't an allocator.
pmr::vector<std::promise<int>>
, as specified,
will be attempting to uses-allocator construct a promise<int>
with a memory_resource*
, but
std::promise
's allocator-taking constructor expects something that satisfies the allocator requirements,
rather than a memory_resource*
.
[2017-06-13, Daniel and Tim restore and improve the previously proposed wording]
[2017-07 Toronto Monday issue prioritization]
Priority 2; Dietmar to check the P/R before Albuquerque.
[2017-11 Albuquerque Wednesday issue processing]
Move to Ready.
[2018-3-17 Adopted in Jacksonville]
Proposed resolution:
This wording is relative to N4659.
Edit 20.5.3.3 [mem.poly.allocator.mem] as indicated:
template <class T, class... Args> void construct(T* p, Args&&... args);-5- Requires: Uses-allocator construction of
-6- Effects: Construct aT
with allocator(see 20.2.8.2 [allocator.uses.construction]) and constructor arguments
resource()*thisstd::forward<Args>(args)...
is well-formed. [Note: Uses-allocator construction is always well formed for types that do not use allocators. — end note]T
object in the storage whose address is represented byp
by uses-allocator construction with allocatorand constructor arguments
resource()*thisstd::forward<Args>(args)...
. -7- Throws: Nothing unless the constructor forT
throws.template <class T1, class T2, class... Args1, class... Args2> void construct(pair<T1,T2>* p, piecewise_construct_t, tuple<Args1...> x, tuple<Args2...> y);-8- [Note: This method and the
-9- Effects: Letconstruct
methods that follow are overloads for piecewise construction of pairs (22.3.2 [pairs.pair]). — end note]xprime
be atuple
constructed fromx
according to the appropriate rule from the following list. [Note: The following description can be summarized as constructing apair<T1, T2>
object in the storage whose address is represented byp
, as if by separate uses-allocator construction with allocator(20.2.8.2 [allocator.uses.construction]) of
resource()*thisp->first
using the elements ofx
andp->second
using the elements ofy
. — end note]
(9.1) — If
uses_allocator_v<T1,
ismemory_resource*polymorphic_allocator>false
andis_constructible_v<T1,Args1...>
istrue
, thenxprime
isx
.(9.2) — Otherwise, if
uses_allocator_v<T1,
ismemory_resource*polymorphic_allocator>true
andis_constructible_v<T1,allocator_arg_t,
ismemory_resource*polymorphic_allocator,Args1...>true
, thenxprime
istuple_cat(make_tuple(allocator_arg,
.resource()*this), std::move(x))(9.3) — Otherwise, if
uses_allocator_v<T1,
ismemory_resource*polymorphic_allocator>true
andis_constructible_v<T1,Args1...,
ismemory_resource*polymorphic_allocator>true
, thenxprime
istuple_cat(std::move(x), make_tuple(
.resource()*this))(9.4) — Otherwise the program is ill formed.
Let
yprime
be a tuple constructed fromy
according to the appropriate rule from the following list:
(9.5) — If
uses_allocator_v<T2,
ismemory_resource*polymorphic_allocator>false
andis_constructible_v<T2,Args2...>
istrue
, thenyprime
isy
.(9.6) — Otherwise, if
uses_allocator_v<T2,
ismemory_resource*polymorphic_allocator>true
andis_constructible_v<T2,allocator_arg_t,
ismemory_resource*polymorphic_allocator,Args2...>true
, thenyprime
istuple_cat(make_tuple(allocator_arg,
.resource()*this), std::move(y))(9.7) — Otherwise, if
uses_allocator_v<T2,
ismemory_resource*polymorphic_allocator>true
andis_constructible_v<T2,Args2...,
ismemory_resource*polymorphic_allocator>true
, thenyprime
istuple_cat(std::move(y), make_tuple(
.resource()*this))(9.8) — Otherwise the program is ill formed.
std::visit
misspecifiedSection: 22.6.7 [variant.visit] Status: C++20 Submitter: Tim Song Opened: 2017-05-31 Last modified: 2021-02-25
Priority: 2
View all other issues in [variant.visit].
View all issues with C++20 status.
Discussion:
[variant.visit]/1 correctly uses "type and value category", but then
p3 describes the return type of visit
to be "the common type of all
possible INVOKE
expressions of the Effects: element."
The type of an expression is never a reference type, due to [expr]/5 removing the
referenceness "prior to any further analysis", so this wording as
written says that visit
always returns a non-reference type, which is
presumably not the intent.
[2017-07 Toronto Monday issue prioritization]
Priority 2; Matt to provide wording
[2018-01-11, Thomas Köppe comments and suggests wording]
The return type of std::visit
(originating by P0088R3
accepted during the Oulo 2016 meeting) is currently misspecified and refers only to the common
type of all the possible visitation calls, without attention to the value category. This
seems unintended, and we should preserve the value category.
[2017-01-24, Daniel comments]
This issue should be reviewed in common with LWG 3052(i).
[ 2018-02-23 Moved to Tentatively Ready after 6 positive votes on c++std-lib. ]
[2018-06 Rapperswil: Adopted]
Proposed resolution:
This wording is relative to N4727.
Modify 22.6.7 [variant.visit] as indicated:
template<class Visitor, class... Variants> constexpr see below visit(Visitor&& vis, Variants&&... vars);[…]
-3- Returns:e(m)
, wherem
is the pack for whichmi
isvarsi.index()
for all0 <= i < n
. The return type isthe type ofe(m)
decltype(e(m))
. […]
is_trivially_destructible_v<int>
?Section: 21.3.5.4 [meta.unary.prop] Status: C++20 Submitter: Richard Smith Opened: 2017-06-01 Last modified: 2021-02-25
Priority: Not Prioritized
View other active issues in [meta.unary.prop].
View all other issues in [meta.unary.prop].
View all issues with C++20 status.
Discussion:
The spec for is_trivially_destructible
says the value is true if "is_destructible_v<T>
is true
and the indicated destructor is known to be trivial."
is_trivially_destructible_v<int>
, there is no indicated destructor, so it's unclear what value
the trait would have but the most plausible reading of these words is that it should be false
. However, I'm confident
the intent is that this trait should yield true
in that situation, and that's what all the implementations I can
find actually do.
[2017-06-14, Daniel and Jonathan provide wording]
[2017-07-05 Moved to Tentatively Ready after 5 positive votes on c++std-lib.]
Proposed resolution:
This wording is relative to N4659.Change 21.3.5.4 [meta.unary.prop], Table 42 — "Type property predicates", as indicated:
Table 22 — Type property predicates Template Condition Preconditions …
template <class T>
struct is_trivially_destructible;is_destructible_v<T>
istrue
andthe indicated destructor is known to be trivialremove_all_extents_t<T>
is either a non-class type or a class type with a trivial destructor.T
shall be a complete type, cvvoid
, or an array of unknown bound.…
tuple_element
/variant_alternative
Section: 22.3.4 [pair.astuple], 22.6.4 [variant.helper] Status: C++20 Submitter: Agustín K-ballo Bergé Opened: 2017-06-10 Last modified: 2021-02-25
Priority: Not Prioritized
View all other issues in [pair.astuple].
View all issues with C++20 status.
Discussion:
Instantiating tuple_element
with an out-of-bounds index requires a diagnostic for tuple
and array
,
but not for pair
. The specification requires an out-of-bounds index for pair
to go to the base template
instead, which is undefined.
variant_alternative
with an out-of-bounds index violates a requirement, but it is not
required to be diagnosed.
[2017-06-12, Moved to Tentatively Ready after 5 positive votes on c++std-lib]
Proposed resolution:
This wording is relative to N4659.
Modify 22.2.1 [utility.syn], header <utility>
synopsis, as indicated:
[…] // 22.3.4 [pair.astuple], tuple-like access to pair template <class T> class tuple_size; template <size_t I, class T> class tuple_element; template <class T1, class T2> struct tuple_size<pair<T1, T2>>; template <size_t I, class T1, class T2> struct tuple_element<I, pair<T1, T2>>;template <class T1, class T2> struct tuple_element<0, pair<T1, T2>>; template <class T1, class T2> struct tuple_element<1, pair<T1, T2>>;[…]
Modify 22.3.4 [pair.astuple] as indicated:
tuple_element<0, pair<T1, T2>>::type
-1- Value: The typeT1
.tuple_element<1, pair<T1, T2>>::type
-2- Value: The typeT2
.tuple_element<I, pair<T1, T2>>::type-?- Requires:
-?- Value: The typeI < 2
. The program is ill-formed ifI
is out of bounds.T1
ifI == 0
, otherwise the typeT2
.
Modify 22.6.4 [variant.helper] as indicated:
variant_alternative<I, variant<Types...>>::type-4- Requires:
-5- Value: The typeI < sizeof...(Types)
. The program is ill-formed ifI
is out of bounds.TI
.
pair
construction in scoped and polymorphic allocatorsSection: 20.5.3.3 [mem.poly.allocator.mem], 20.6.4 [allocator.adaptor.members] Status: C++20 Submitter: Casey Carter Opened: 2017-06-13 Last modified: 2021-02-25
Priority: 3
View all other issues in [mem.poly.allocator.mem].
View all issues with C++20 status.
Discussion:
scoped_allocator_adaptor
([allocator.adaptor.syn]) and polymorphic_allocator
([mem.poly.allocator.class])
have identical families of members named construct
:
template <class T, class... Args> void construct(T* p, Args&&... args); template <class T1, class T2, class... Args1, class... Args2> void construct(pair<T1,T2>* p, piecewise_construct_t, tuple<Args1...> x, tuple<Args2...> y); template <class T1, class T2> void construct(pair<T1,T2>* p); template <class T1, class T2, class U, class V> void construct(pair<T1,T2>* p, U&& x, V&& y); template <class T1, class T2, class U, class V> void construct(pair<T1,T2>* p, const pair<U, V>& pr); template <class T1, class T2, class U, class V> void construct(pair<T1,T2>* p, pair<U, V>&& pr);
Both allocators perform uses_allocator
construction, and therefore need special handling for pair
constructions since pair
doesn't specialize uses_allocator
(tuple
gets all of that magic
and pair
is left out in the cold). Presumably, the intent is that the construct
overloads whose first
argument is a pointer to pair
capture all pair
constructions. This is not the case: invoking
construct
with a pair
pointer and a non-constant lvalue pair
resolves to the first
overload when it is viable: it's a better match than the pair
-pointer-and-const
-lvalue-pair
overload. The first overload notably does not properly perform piecewise uses_allocator
construction for
pair
s as intended.
[2017-07 Toronto Monday issue prioritization]
Priority 2; Marshall to work with Casey to reduce the negations in the wording.
Previous resolution [SUPERSEDED]:
Modify 20.5.3.3 [mem.poly.allocator.mem] as indicated:
template <class T, class... Args> void construct(T* p, Args&&... args);-5- Requires: Uses-allocator construction of
-6- Effects: Construct aT
with allocatorresource()
(see 20.2.8.2 [allocator.uses.construction]) and constructor argumentsstd::forward<Args>(args)...
is well-formed. [Note: Uses-allocator construction is always well formed for types that do not use allocators. — end note]T
object in the storage whose address is represented byp
by uses-allocator construction with allocatorresource()
and constructor argumentsstd::forward<Args>(args)...
. -7- Throws: Nothing unless the constructor forT
throws. -?- Remarks: This function shall not participate in overload resolution unlessT
is not a specialization ofpair
.Modify 20.6.4 [allocator.adaptor.members] as indicated:
template <class T, class... Args> void construct(T* p, Args&&... args);-9- Effects: […]
-?- Remarks: This function shall not participate in overload resolution unlessT
is not a specialization ofpair
.
[2017-11-02 Marshall and Casey provide updated wording]
[2017-11 Albuquerque Wednesday issue processing]
Move to Ready.
[2018-3-17 Adopted in Jacksonville]
Proposed resolution:
This wording is relative to N4659.
Modify 20.5.3.3 [mem.poly.allocator.mem] as indicated:
template <class T, class... Args> void construct(T* p, Args&&... args);-5- Requires: Uses-allocator construction of
-6- Effects: Construct aT
with allocatorresource()
(see 20.2.8.2 [allocator.uses.construction]) and constructor argumentsstd::forward<Args>(args)...
is well-formed. [Note: Uses-allocator construction is always well formed for types that do not use allocators. — end note]T
object in the storage whose address is represented byp
by uses-allocator construction with allocatorresource()
and constructor argumentsstd::forward<Args>(args)...
. -7- Throws: Nothing unless the constructor forT
throws. -?- Remarks: This function shall not participate in overload resolution ifT
is a specialization ofpair
.
Modify 20.6.4 [allocator.adaptor.members] as indicated:
template <class T, class... Args> void construct(T* p, Args&&... args);-9- Effects: […]
-?- Remarks: This function shall not participate in overload resolution ifT
is a specialization ofpair
.
uses_allocator
specialization for packaged_task
Section: 32.10.10 [futures.task], 32.10.2 [future.syn], 32.10.10.3 [futures.task.nonmembers] Status: C++20 Submitter: Tim Song Opened: 2017-06-13 Last modified: 2021-02-25
Priority: 3
View all other issues in [futures.task].
View all issues with C++20 status.
Discussion:
When LWG 2921(i) removed allocator support from packaged_task
, it forgot
to remove the uses_allocator
partial specialization.
[ 2017-06-26 Moved to Tentatively Ready after 6 positive votes on c++std-lib. ]
[2017-06-26, Billy O'Neal reopens]
I think 2921(i) was resolved in error. If promise<T>
can have an allocator, there's no
reason for packaged_task<T>
to not have one. If we remove it from packaged_task
we should
remove it from promise
as well.
std::function
" case. packaged_task
has none of the std::function
problems because
the function inside a given packaged_task
is not reassignable.
If LWG decides to remove allocator support here then there are more bits that need to be struck, e.g.
[futures.task.members] (5.3).
[2017-06-26, Tim updates P/R to remove more dangling bits.]
The additional point in the P/R effectively reverts the second part of the resolution of 2752(i).
The alternative resolution for this issue is, of course, to just revert the resolution of 2921. In that case 2245(i) needs to be reopened.[2016-07, Toronto Saturday afternoon issues processing]
Status to Ready
Proposed resolution:
This wording is relative to N4659.
Modify 32.10.2 [future.syn], header <future>
synopsis, and
32.10.10 [futures.task], class template packaged_task
synopsis, as indicated:
template <class R, class Alloc> struct uses_allocator<packaged_task<R>, Alloc>;
Modify 32.10.10.3 [futures.task.nonmembers] as indicated:
template <class R, class Alloc> struct uses_allocator<packaged_task<R>, Alloc> : true_type { };
-2- Requires:Alloc
shall be an Allocator (20.5.3.5).
Modify 32.10.10.2 [futures.task.members]/5 as indicated:
template <class F> packaged_task(F&& F);-2- Requires: […]
-3- Remarks: […]
-4- Effects: […]
-5- Throws:
— Aany exceptions thrown by the copy or move constructor off
., or
— For the first version,bad_alloc
if memory for the internal data structures could not be allocated.
— For the second version, any exceptions thrown byallocator_traits<Allocator>::template rebind_traits<unspecified>::allocate
.
unordered_meow::merge()
has incorrect Throws: clauseSection: 23.2.8 [unord.req] Status: C++20 Submitter: Tim Song Opened: 2017-06-14 Last modified: 2021-02-25
Priority: 0
View other active issues in [unord.req].
View all other issues in [unord.req].
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Discussion:
As pointed out in this StackOverflow question,
unordered_{map,multimap,set,multiset}::merge()
may need to rehash to maintain its
max_load_factor
invariant, which may require allocation, which may throw.
[2017-07 Toronto Monday issue prioritization]
Priority 0; move to Ready
Proposed resolution:
This wording is relative to N4659.
In 23.2.8 [unord.req], edit Table 91 "Unordered associative container requirements" as indicated:
Table 91 — Unordered associative container requirements (in addition to container) Expression Return type Assertion/note
pre-/post-conditionComplexity …
a.merge(a2)
void
Requires: a.get_allocator() == a2.get_allocator()
.
Attempts to extract each element ina2
and insert it intoa
using the hash function and key equality predicate ofa
. In containers with unique keys, if there is an element ina
with key equivalent to the key of an element froma2
, then that element is not extracted froma2
.
Postconditions: Pointers and references to the transferred elements ofa2
refer to those same elements but as members ofa
. Iterators referring to the transferred elements and all iterators referring toa
will be invalidated, but iterators to elements remaining ina2
will remain valid.
Throws: Nothing unless the hash function or key equality predicate throws.Average case 𝒪(N), where N is a2.size()
.
Worst case 𝒪(N*a.size()+
N).…
pmr::string
and friendsSection: 27.4.6 [basic.string.hash], 27.4.2 [string.syn] Status: C++20 Submitter: Tim Song Opened: 2017-06-14 Last modified: 2021-02-25
Priority: 0
View all other issues in [basic.string.hash].
View all issues with C++20 status.
Discussion:
In most cases, std::pmr::meow
is a drop-in replacement for std::meow
. The exception is
std::pmr::{,w,u16,u32}string
, because unlike their std::
counterparts, they don't come with enabled
std::hash
specializations.
The P/R below simply adds std::hash
specializations for those four typedefs. An alternative approach, for which wording can be
produced if desired, is to make the hash
specializations for basic_string
allocator-agnostic, similar to the partial
specialization of hash
for vector<bool>
.
[2017-07 Toronto Monday issue prioritization]
Priority 0; move to Ready
Proposed resolution:
This wording is relative to N4659.
Edit 27.4.2 [string.syn], header <string>
synopsis, as indicated:
namespace pmr { template <class charT, class traits = char_traits<charT>> using basic_string = std::basic_string<charT, traits, polymorphic_allocator<charT>>; using string = basic_string<char>; using u16string = basic_string<char16_t>; using u32string = basic_string<char32_t>; using wstring = basic_string<wchar_t>; } // 27.4.6 [basic.string.hash], hash support template<class T> struct hash; template<> struct hash<string>; template<> struct hash<u16string>; template<> struct hash<u32string>; template<> struct hash<wstring>; template<> struct hash<pmr::string>; template<> struct hash<pmr::u16string>; template<> struct hash<pmr::u32string>; template<> struct hash<pmr::wstring>;namespace pmr { template <class charT, class traits = char_traits<charT>> using basic_string = std::basic_string<charT, traits, polymorphic_allocator<charT>>; using string = basic_string<char>; using u16string = basic_string<char16_t>; using u32string = basic_string<char32_t>; using wstring = basic_string<wchar_t>; }
Edit 27.4.6 [basic.string.hash] as indicated:
template<> struct hash<string>; template<> struct hash<u16string>; template<> struct hash<u32string>; template<> struct hash<wstring>; template<> struct hash<pmr::string>; template<> struct hash<pmr::u16string>; template<> struct hash<pmr::u32string>; template<> struct hash<pmr::wstring>;-1- If
S
is one of these string types,SV
is the corresponding string view type, ands
is an object of typeS
, thenhash<S>()(s) == hash<SV>()(SV(s))
.
aligned_union
should require complete object typesSection: 21.3.8.7 [meta.trans.other] Status: C++20 Submitter: Tim Song Opened: 2017-06-14 Last modified: 2021-02-25
Priority: 0
View all other issues in [meta.trans.other].
View all issues with C++20 status.
Discussion:
aligned_union
's description doesn't, but should, require the types provided to be complete object types.
[2017-07 Toronto Monday issue prioritization]
Priority 0; move to Ready
Proposed resolution:
This wording is relative to N4659.
In 21.3.8.7 [meta.trans.other], edit Table 50 "Other transformations" as indicated:
Table 50 — Other transformations Template Comments […]
template <size_t Len, class... Types>
struct aligned_union;The member typedef type
shall be a POD type suitable for use as uninitialized storage for any object whose type is listed inTypes
; its size shall be at leastLen
. The static memberalignment_value
shall be an integral constant of typesize_t
whose value is the strictest alignment of all types listed inTypes
.
Requires: At least one type is provided. Each type in the parameter packTypes
shall be a complete object type.[…]
compare_exchange
empty pointersSection: 99 [depr.util.smartptr.shared.atomic] Status: C++20 Submitter: Alisdair Meredith Opened: 2017-06-15 Last modified: 2021-02-25
Priority: Not Prioritized
View all other issues in [depr.util.smartptr.shared.atomic].
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Discussion:
[util.smartptr.shared.atomic] p35 states that two shared pointers are equivalent if
they store the same pointer value, and share ownership. As empty shared pointers
never share ownership, it is not possible to replace an empty shared pointer using
the atomic compare_exchange
API.
[ 2017-06-26 Moved to Tentatively Ready after 5 positive votes on c++std-lib. ]
Proposed resolution:
This wording is relative to N4659.
Edit [util.smartptr.shared.atomic] as indicated:
template<class T> bool atomic_compare_exchange_weak_explicit( shared_ptr<T>* p, shared_ptr<T>* v, shared_ptr<T> w, memory_order success, memory_order failure); template<class T> bool atomic_compare_exchange_strong_explicit( shared_ptr<T>* p, shared_ptr<T>* v, shared_ptr<T> w, memory_order success, memory_order failure);[…]
-35- Remarks: Twoshared_ptr
objects are equivalent if they store the same pointer value and share ownership, or if they store the same pointer value and both are empty. The weak form may fail spuriously. See 32.6.1.
Section: 22.10.6 [refwrap], 32.6.5.2 [thread.lock.guard], 32.6.5.3 [thread.lock.scoped], 32.6.5.4 [thread.lock.unique], 32.6.5.5 [thread.lock.shared] Status: C++20 Submitter: Mike Spertus Opened: 2017-06-15 Last modified: 2021-02-25
Priority: 0
View all other issues in [refwrap].
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Discussion:
There are several deduction guides added to the standard library by P0433R2 that have no effect probably because LWG had not considered late changes to core wording that automatically adds a "copy deduction candidate" (12.2.2.9 [over.match.class.deduct]) that renders these explicit guides moot.
[2017-07 Toronto Monday issue prioritization]
Priority 0; move to Ready
Proposed resolution:
This wording is relative to N4659.
Edit 22.10.6 [refwrap], end of class template reference_wrapper
synopsis,
as indicated:
template<class T> reference_wrapper(reference_wrapper<T>) -> reference_wrapper<T>;
Edit 32.6.5.2 [thread.lock.guard], end of class template lock_guard
synopsis,
as indicated:
template<class Mutex> lock_guard(lock_guard<Mutex>) -> lock_guard<Mutex>;
Edit 32.6.5.3 [thread.lock.scoped], end of class template scoped_lock
synopsis,
as indicated:
template<class... MutexTypes> scoped_lock(scoped_lock<MutexTypes...>) -> scoped_lock<MutexTypes...>;
Edit 32.6.5.4 [thread.lock.unique], end of class template unique_lock
synopsis,
as indicated:
template<class Mutex> unique_lock(unique_lock<Mutex>) -> unique_lock<Mutex>;
Edit 32.6.5.5 [thread.lock.shared], end of class template shared_lock
synopsis,
as indicated:
template<class Mutex> shared_lock(shared_lock<Mutex>) -> shared_lock<Mutex>;
size_type
consistent in associative container deduction guidesSection: 23.5.6.1 [unord.set.overview], 23.5.7.1 [unord.multiset.overview] Status: C++20 Submitter: Mike Spertus Opened: 2017-06-16 Last modified: 2021-02-25
Priority: 2
View all issues with C++20 status.
Discussion:
Due to an incompletely implemented change in Kona, some of the size_type deduction guides say something like:
A
size_type
parameter type in anunordered_set
deduction guide refers to thesize_type
member type of the primaryunordered_set
template
while others say
A
size_type
parameter type in anunordered_map
deduction guide refers to thesize_type
member type of the type deduced by the deduction guide.
Clearly they should both be the same. My recollection is that the intent of the committee was to change them all to be the latter. Note, however, that this issue may be mooted if the suggestions in the upcoming P0433R3 paper are adopted as a DR in Toronto.
[2017-07 Toronto Monday issue prioritization]
Priority 2; Mike is preparing an updated paper — currently named P0433R3.
[2016-07, Toronto Saturday afternoon issues processing]
Status to Ready; Marshall to check with Mike about his paper
Proposed resolution:
This wording is relative to N4659.
Edit 23.5.6.1 [unord.set.overview] as indicated:
-4- A
size_type
parameter type in anunordered_set
deduction guide refers to thesize_type
member type of theprimarytype deduced by the deduction guide.unordered_set
template
Edit 23.5.7.1 [unord.multiset.overview] as indicated:
-4- A
size_type
parameter type in anunordered_multiset
deduction guide refers to thesize_type
member type of theprimarytype deduced by the deduction guide.unordered_multiset
template
typename
Section: 32.5.2 [atomics.syn] Status: C++20 Submitter: Jens Maurer Opened: 2017-06-25 Last modified: 2021-02-25
Priority: 0
View other active issues in [atomics.syn].
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Discussion:
P0558R1 missed updating one of the std::atomic_exchange
signatures to avoid independent deduction for T
on the second parameter.
[ 2017-06-26 Moved to Tentatively Ready after 6 positive votes on c++std-lib. ]
Proposed resolution:
This wording is relative to N4659.
Edit 32.5.2 [atomics.syn], header <atomic>
synopsis, as indicated:
template<class T> T atomic_exchange(volatile atomic<T>*, typename atomic<T>::value_type) noexcept;
path
's stream insertion operator lets you insert everything under the sunSection: 31.12.6.7 [fs.path.io] Status: C++20 Submitter: Billy O'Neal III Opened: 2017-06-27 Last modified: 2021-02-25
Priority: 2
View all issues with C++20 status.
Discussion:
The rules for converting a path
to a narrow character sequence aren't necessarily the same as that iostreams should use. Note that this program creates a temporary path and stream inserts that, which likely destroys information.
#include <iostream> #include <filesystem> #include <string> void foo() { using namespace std; using namespace std::experimental::filesystem::v1; wstring val(L"abc"); std::cout << val; }
Using the godbolt online compiler we get:
foo PROC sub rsp, 104 ; 00000068H lea rdx, OFFSET FLAT:$SG44895 lea rcx, QWORD PTR val$[rsp] call std::basic_string<wchar_t,std::char_traits<wchar_t>,std::allocator<wchar_t> >::basic_string<wchar_t,std::char_traits<wchar_t>,std::allocator<wchar_t> > lea rdx, QWORD PTR val$[rsp] lea rcx, QWORD PTR $T1[rsp] call ??$?0_WU?$char_traits@_W@std@@V?$allocator@_W@1@@path@v1@filesystem@experimental@std@@QEAA@AEBV?$basic_string@_WU?$char_traits@_W@std@@V?$allocator@_W@2@@4@@Z lea rdx, QWORD PTR $T1[rsp] lea rcx, OFFSET FLAT:std::cout call std::experimental::filesystem::v1::operator<<<char,std::char_traits<char> > lea rcx, QWORD PTR $T1[rsp] call std::experimental::filesystem::v1::path::~path lea rcx, QWORD PTR val$[rsp] call std::basic_string<wchar_t,std::char_traits<wchar_t>,std::allocator<wchar_t> >::~basic_string<wchar_t,std::char_traits<wchar_t>,std::allocator<wchar_t> > add rsp, 104 ; 00000068H ret 0 foo ENDP
This should either be disabled with a SFINAE constraint, use the auto_ptr
user-defined conversion
trick, or the stream insertion operators should be made "hidden friends" to prevent conversions to path
from being considered here.
[2017-07 Toronto Monday issue prioritization]
Priority 2
[2017-07 Toronto Saturday afternoon]
LWG confirmed they want the hidden friend solution, Billy O'Neal to provide wording.
[2018-1-26 issues processing telecon]
Status to 'Tentatively Ready'
[2018-3-17 Adopted in Jacksonville]
Proposed resolution:
This resolution is relative to N4659.
Edit 31.12.4 [fs.filesystem.syn], header <filesystem>
synopsis, as indicated:
// 31.12.6.7 [fs.path.io], path inserter and extractor template <class charT, class traits> basic_ostream<charT, traits>& operator<<(basic_ostream<charT, traits>& os, const path& p); template <class charT, class traits> basic_istream<charT, traits>& operator>>(basic_istream<charT, traits>& is, path& p);
Edit 31.12.6.7 [fs.path.io] as indicated:
[Drafting note: The project editor is kindly asked to consider to move sub-clause 31.12.6.7 [fs.path.io] before sub-clause 31.12.6.8 [fs.path.nonmember] (as a peer of it) — end drafting note]
template <class charT, class traits> friend basic_ostream<charT, traits>& operator<<(basic_ostream<charT, traits>& os, const path& p); […] template <class charT, class traits> friend basic_istream<charT, traits>& operator>>(basic_istream<charT, traits>& is, path& p);
Edit 31.12.6 [fs.class.path] p2, class path
synopsis, as indicated:
namespace std::filesystem { class path { public: […] iterator begin() const; iterator end() const; // 31.12.6.7 [fs.path.io] path inserter and extractor template <class charT, class traits> friend basic_ostream<charT, traits>& operator<<(basic_ostream<charT, traits>& os, const path& p); template <class charT, class traits> friend basic_istream<charT, traits>& operator>>(basic_istream<charT, traits>& is, path& p); }; }
reference_wrapper<T>
conversion from T&&
Section: 22.10.6 [refwrap] Status: C++20 Submitter: Tim Song Opened: 2017-06-28 Last modified: 2021-02-25
Priority: 3
View all other issues in [refwrap].
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Discussion:
reference_wrapper<T>
has a deleted constructor taking T&&
in order to
prevent accidentally wrapping an rvalue (which can otherwise happen with the
reference_wrapper(T&)
constructor if T
is a non-volatile
const
-qualified type). Unfortunately, a deleted constructor can still
be used to form implicit conversion sequences, so the deleted T&&
constructor has the (presumably unintended) effect of creating an implicit conversion sequence from
a T
rvalue to a reference_wrapper<T>
, even though such a conversion would be
ill-formed if actually used. This is visible in overload resolution:
void meow(std::reference_wrapper<int>); //#1
void meow(convertible_from_int); //#2
meow(0); // error, ambiguous; would unambiguously call #2 if #1 instead took int&
and in conditional expressions (and hence std::common_type
) after core issue 1895:
std::reference_wrapper<int> purr(); auto x = true? purr() : 0; // error, ambiguous: ICS exists from int prvalue to // reference_wrapper<int> and from reference_wrapper<int> to int using t = std::common_type_t<std::reference_wrapper<int>, int>; // error: no member 'type' because the conditional // expression is ill-formed
The latter in turn interferes with the use of reference_wrapper
as a
proxy reference type with proxy iterators.
T
rvalues to reference_wrapper<T>
, not just that the conversion will be
ill-formed when used. This can be done by using a suitably constrained
constructor template taking a forwarding reference instead of the
current pair of constructors taking T&
and T&&
.
[2017-06-29, Tim adds P/R and comments]
The draft P/R below uses a conditional noexcept
specification to ensure that converting a T&
to
a reference_wrapper<T>
remains noexcept
and make it not usable when the source type is a
reference_wrapper
of the same type so as to avoid affecting is_trivially_constructible
. It adds a deduction
guide as the new constructor template will not support class template argument deduction.
reference_wrapper<T>
convertible from everything
that is convertible to T&
. This implies, for instance, that reference_wrapper<int>
is now
convertible to reference_wrapper<const int>
when it wasn't before (the conversion would have required
two user-defined conversions previously). This more closely emulates the behavior of an actual reference, but does represent
a change to the existing behavior.
If perfectly emulating the existing behavior is desired, a conditionally-explicit constructor that is only implicit if
T
is reference-compatible with remove_reference_t<U>
(see 9.5.4 [dcl.init.ref]) can be used.
[2017-07 Toronto Tuesday PM issue prioritization]
Priority 3; what else in the library does this affect? ref
or cref
?
[2016-07, Toronto Saturday afternoon issues processing]
Status to Ready.
Proposed resolution:
This wording is relative to N4659.
Edit 22.10.6 [refwrap], class template reference_wrapper
synopsis, as indicated:
namespace std { template <class T> class reference_wrapper { […] // construct/copy/destroyreference_wrapper(T&) noexcept; reference_wrapper(T&&) = delete; // do not bind to temporary objectstemplate <class U> reference_wrapper(U&&) noexcept(see below); […] }; template <class T> reference_wrapper(T&) -> reference_wrapper<T>; […] }
Edit 22.10.6.2 [refwrap.const]/1 as indicated:
reference_wrapper(T& t) noexcept;
-1- Effects: Constructs areference_wrapper
object that stores a reference tot
.template<class U> reference_wrapper(U&& u) noexcept(see below);-?- Remarks: Let
FUN
denote the exposition-only functionsThis constructor shall not participate in overload resolution unless the expressionvoid FUN(T&) noexcept; void FUN(T&&) = delete;FUN(declval<U>())
is well-formed andis_same_v<decay_t<U>, reference_wrapper>
isfalse
. The expression insidenoexcept
is equivalent tonoexcept(FUN(declval<U>()))
. -?- Effects: Creates a variabler
as if byT& r = std::forward<U>(u)
, then constructs areference_wrapper
object that stores a reference tor
.
basic_string
and basic_string_view
Section: 27.2 [char.traits], 27.4.3.2 [string.require], 27.3.3 [string.view.template] Status: WP Submitter: Gennaro Prota Opened: 2017-07-03 Last modified: 2023-11-22
Priority: 3
View all other issues in [char.traits].
View all issues with WP status.
Discussion:
basic_string
and basic_string_view
involve undefined behavior in a few cases where it's
simple for the implementation to add a static_assert
and make the program ill-formed.
Traits::char_type
shall be the same asCharT
.
Here, the implementation can add a static_assert
using the is_same
type trait. Similar issues exist in 27.4.3.2 [string.require] and, for
basic_string_view
, in 27.3.3 [string.view.template]/1.
[2017-07 Toronto Tuesday PM issue prioritization]
Priority 3; need to check general container requirements
Partially by the adoption of P1148 in San Diego.
Tim opines: "the remainder deals with allocator value type mismatch, which I think is NAD."
Previous resolution [SUPERSEDED]:
This wording is relative to N4659.
Edit 27.2 [char.traits] as indicated:
-3- To specialize those templates to generate a string or iostream class to handle a particular character container type
CharT
, that and its related character traits classTraits
are passed as a pair of parameters to the string or iostream template as parameterscharT
andtraits
. IfTraits::char_type
shall be the sameis not the same type asCharT
, the program is ill-formed.Edit 27.4.3.2 [string.require] as indicated:
-3- In every specialization
basic_string<charT, traits, Allocator>
, ifthe typeallocator_traits<Allocator>::value_type
shall name the same typeis not the same type ascharT
, the program is ill-formed. Every object of typebasic_string<charT, traits, Allocator>
shall use an object of typeAllocator
to allocate and free storage for the containedcharT
objects as needed. TheAllocator
object used shall be obtained as described in 23.2.2 [container.requirements.general]. In every specializationbasic_string<charT, traits, Allocator>
, the typetraits
shall satisfy the character traits requirements (27.2 [char.traits]). If, and the typetraits::char_type
shall name the same typeis not the same type ascharT
, the program is ill-formed.Edit 27.3.3 [string.view.template] as indicated:
-1- In every specialization
basic_string_view<charT, traits>
, the typetraits
shall satisfy the character traits requirements (27.2 [char.traits]). If, and the typetraits::char_type
shall name the same typeis not the same type ascharT
, the program is ill-formed.
[2023-05-15; Jonathan Wakely provides improved wording]
As noted above, most of this issue was resolved by P1148R0.
The remainder is the change to make it ill-formed if the allocator has the wrong
value_type
, but since P1463R1 that is already part of
the Allocator-aware container requirements, which apply to basic_string
.
[2023-06-01; Reflector poll]
Set status to Tentatively Ready after eight votes in favour during reflector poll.
There was a request to editorially move the added text in Note 1 into Note 2 after this is approved.
[2023-06-17 Approved at June 2023 meeting in Varna. Status changed: Voting → WP.]
Proposed resolution:
This wording is relative to N4950.
Edit 27.4.3.2 [string.require] as indicated:
-3-
In every specializationEvery object of typebasic_string<charT, traits, Allocator>
, the typeallocator_traits<Allocator>::value_type
shall name the same type ascharT
.basic_string<charT, traits, Allocator>
shall use an object of typeAllocator
to allocate and free storage for the containedcharT
objects as needed. The In every specializationbasic_string<charT, traits, Allocator>
, the typetraits
shall satisfy the character traits requirements (27.2 [char.traits]).[Note 1: Every specialization
basic_string<charT, traits, Allocator>
is an allocator-aware container, but does not use the allocator’sconstruct
anddestroy
member functions (23.2.2 [container.requirements.general]). The program is ill-formed ifAllocator::value_type
is not the same type ascharT
. — end note][Note 2: The program is ill-formed if
traits::char_type
is not the same type ascharT
. — end note]
basic_stringbuf
default constructor forbids it from using SSO capacitySection: 31.8.2.2 [stringbuf.cons] Status: C++20 Submitter: Jonathan Wakely Opened: 2017-07-07 Last modified: 2021-02-25
Priority: 3
View all other issues in [stringbuf.cons].
View all issues with C++20 status.
Discussion:
[stringbuf.cons] says that the default constructor initializes the
base class as basic_streambuf()
which means the all the pointers to
the input and output sequences (pbase
, eback
etc) are all required to
be null.
stringbuf
that is implemented in terms of a Small
String Optimised std::basic_string
cannot make us of the string's
initial capacity, and so cannot avoid a call to the overflow virtual
function even for small writes. In other words, the following
assertions must pass:
#include <sstream> #include <cassert> bool overflowed = false; struct SB : std::stringbuf { int overflow(int c) { assert( pbase() == nullptr ); overflowed = true; return std::stringbuf::overflow(c); } }; int main() { SB sb; sb.sputc('1'); assert(overflowed); }
This is an unnecessary pessimisation. Implementations should be allowed to use the SSO buffer immediately and write to it without calling overflow. Libc++ already does this, so is non-conforming.
[2017-07 Toronto Tuesday PM issue prioritization]
Priority 3; is this affected by Peter Sommerlad's paper P0407R1?
[2018-06 Rapperswil Wednesday issues processing]
Status to Ready
[2018-11, Adopted in San Diego]
Proposed resolution:
This wording is relative to N4659.
Edit 31.8.2.2 [stringbuf.cons] as indicated:
explicit basic_stringbuf( ios_base::openmode which = ios_base::in | ios_base::out);-1- Effects: Constructs an object of class
-2- Postconditions:basic_stringbuf
, initializing the base class withbasic_streambuf()
(31.6.3.2 [streambuf.cons]), and initializingmode
withwhich
. It is implementation-defined whether the sequence pointers (eback()
,gptr()
,egptr()
,pbase()
,pptr()
,epptr()
) are initialized to null pointers.str() == ""
.
shared_ptr
operationsSection: 20.3.2.2 [util.smartptr.shared], 20.3.2.2.10 [util.smartptr.shared.cast] Status: C++20 Submitter: Geoffrey Romer Opened: 2017-07-07 Last modified: 2021-02-25
Priority: Not Prioritized
View all other issues in [util.smartptr.shared].
View all issues with C++20 status.
Discussion:
The shared_ptr
aliasing constructor and the shared_ptr
casts are specified to take a shared_ptr
by const
reference and construct a new shared_ptr
that shares ownership with it, and yet they have no
corresponding rvalue reference overloads. That results in an unnecessary refcount increment/decrement when those operations
are given an rvalue. Rvalue overloads can't be added as a conforming extension because they observably change semantics
(but mostly only for code that does unreasonable things like pass an argument by move and then rely on the fact that it's
unchanged), and [res.on.arguments]/p1.3 doesn't help because it only applies to rvalue reference parameters.
[2017-07 Toronto Tuesday PM issue prioritization]
Status LEWG
[2018-06 Rapperswil Monday AM]
Move to Ready; choosing the PR in the issue as opposed to P0390R0 and rebase wording to most recent working draft
[2018-11, Adopted in San Diego]
Proposed resolution:
This wording is relative to N4750.
Edit 20.2.2 [memory.syn], header <memory>
synopsis, as indicated:
[…] // 20.3.2.2.10 [util.smartptr.shared.cast], shared_ptr casts template<class T, class U> shared_ptr<T> static_pointer_cast(const shared_ptr<U>& r) noexcept; template<class T, class U> shared_ptr<T> static_pointer_cast(shared_ptr<U>&& r) noexcept; template<class T, class U> shared_ptr<T> dynamic_pointer_cast(const shared_ptr<U>& r) noexcept; template<class T, class U> shared_ptr<T> dynamic_pointer_cast(shared_ptr<U>&& r) noexcept; template<class T, class U> shared_ptr<T> const_pointer_cast(const shared_ptr<U>& r) noexcept; template<class T, class U> shared_ptr<T> const_pointer_cast(shared_ptr<U>&& r) noexcept; template<class T, class U> shared_ptr<T> reinterpret_pointer_cast(const shared_ptr<U>& r) noexcept; template<class T, class U> shared_ptr<T> reinterpret_pointer_cast(shared_ptr<U>&& r) noexcept; […]
Edit 20.3.2.2 [util.smartptr.shared], class template shared_ptr
synopsis, as indicated:
template<class T> class shared_ptr { public: […] // 20.3.2.2.2 [util.smartptr.shared.const], constructors […] template <class D, class A> shared_ptr(nullptr_t p, D d, A a); template<class Y> shared_ptr(const shared_ptr<Y>& r, element_type* p) noexcept; template<class Y> shared_ptr(shared_ptr<Y>&& r, element_type* p) noexcept; shared_ptr(const shared_ptr& r) noexcept; […] }; […]
Edit 20.3.2.2.2 [util.smartptr.shared.const] as indicated:
[Drafting note: the
use_count()
postcondition can safely be deleted because it is redundant with the "shares ownership" wording in the Effects. — end drafting note]
template<class Y> shared_ptr(const shared_ptr<Y>& r, element_type* p) noexcept; template<class Y> shared_ptr(shared_ptr<Y>&& r, element_type* p) noexcept;-14- Effects: Constructs a
-15- Postconditions:shared_ptr
instance that storesp
and shares ownership with the initial value ofr
.get() == p
. For the second overload,&& use_count() == r.use_count()r
is empty andr.get() == nullptr
. -16- [Note: To avoid the possibility of a dangling pointer, the user of this constructor must ensure thatp
remains valid at least until the ownership group ofr
is destroyed. — end note] -17- [Note: This constructor allows creation of an emptyshared_ptr
instance with a non-null stored pointer. — end note]
Edit 20.3.2.2.10 [util.smartptr.shared.cast] as indicated:
template<class T, class U> shared_ptr<T> static_pointer_cast(const shared_ptr<U>& r) noexcept; template<class T, class U> shared_ptr<T> static_pointer_cast(shared_ptr<U>&& r) noexcept;-1- Requires: The expression
-2- Returns:static_cast<T*>((U*)nullptr)
shall be well-formed., whereshared_ptr<T>(
rR, static_cast<typename shared_ptr<T>::element_type*>(r.get()))R
isr
for the first overload, andstd::move(r)
for the second. -3- [Note: The seemingly equivalent expressionshared_ptr<T>(static_cast<T*>(r.get()))
will eventually result in undefined behavior, attempting to delete the same object twice. — end note]template<class T, class U> shared_ptr<T> dynamic_pointer_cast(const shared_ptr<U>& r) noexcept; template<class T, class U> shared_ptr<T> dynamic_pointer_cast(shared_ptr<U>&& r) noexcept;-4- Requires: The expression
-5- Returns:dynamic_cast<T*>((U*)nullptr)
shall be well-formed. The expressiondynamic_cast<typename shared_ptr<T>::element_type*>(r.get())
shall be well formed and shall have well-defined behavior.
(5.1) — When
dynamic_cast<typename shared_ptr<T>::element_type*>(r.get())
returns a non-null valuep
,shared_ptr<T>(
, whererR, p)R
isr
for the first overload, andstd::move(r)
for the second.(5.2) — Otherwise,
shared_ptr<T>()
.-6- [Note: The seemingly equivalent expression
shared_ptr<T>(dynamic_cast<T*>(r.get()))
will eventually result in undefined behavior, attempting to delete the same object twice. — end note]template<class T, class U> shared_ptr<T> const_pointer_cast(const shared_ptr<U>& r) noexcept; template<class T, class U> shared_ptr<T> const_pointer_cast(shared_ptr<U>&& r) noexcept;-7- Requires: The expression
-8- Returns:const_cast<T*>((U*)nullptr)
shall be well-formed., whereshared_ptr<T>(
rR, const_cast<typename shared_ptr<T>::element_type*>(r.get()))R
isr
for the first overload, andstd::move(r)
for the second. -9- [Note: The seemingly equivalent expressionshared_ptr<T>(const_cast<T*>(r.get()))
will eventually result in undefined behavior, attempting to delete the same object twice. — end note]template<class T, class U> shared_ptr<T> reinterpret_pointer_cast(const shared_ptr<U>& r) noexcept; template<class T, class U> shared_ptr<T> reinterpret_pointer_cast(shared_ptr<U>&& r) noexcept;-10- Requires: The expression
-11- Returns:reinterpret_cast<T*>((U*)nullptr)
shall be well-formed., whereshared_ptr<T>(
rR, reinterpret_cast<typename shared_ptr<T>::element_type*>(r.get()))R
isr
for the first overload, andstd::move(r)
for the second. -12- [Note: The seemingly equivalent expressionshared_ptr<T>(reinterpret_cast<T*>(r.get()))
will eventually result in undefined behavior, attempting to delete the same object twice. — end note]
{forward_,}list::unique
Section: 23.3.11.5 [list.ops], 23.3.7.6 [forward.list.ops] Status: C++23 Submitter: Tim Song Opened: 2017-07-07 Last modified: 2023-11-22
Priority: 3
View all other issues in [list.ops].
View all issues with C++23 status.
Discussion:
There are various problems with the specification of list::unique
and its forward_list
counterpart, some of which are obvious even on cursory inspection:
first
and last
, which aren't even defined.i - 1
on non-random-access iterators - in the case of forward_list
, on forward-only iterators.std::unique
to require an equivalence relation
and adjusting the order of comparison, weren't applied to these member functions.list::unique
, was closed as NAD with the rationale that
"All implementations known to the author of this Defect Report comply with these assumption", and "no impact on current code is expected", i.e. there is no evidence of real-world confusion or harm.That implementations somehow managed to do the right thing in spite of obviously defective standardese doesn't seem like a good reason to not fix the defects.
[2017-07 Toronto Tuesday PM issue prioritization]
Priority 3; by the way, there's general wording in 26.2 [algorithms.requirements] p10 that lets us specify iterator arithmetic as if we were using random access iterators.
[2017-07-11 Tim comments]
I drafted the P/R fully aware of the general wording in 26.2 [algorithms.requirements] p10. However, that general wording is limited to Clause 28, so to make use of the shorthand permitted by that wording, we would need additional wording importing it to these subclauses.
Moreover, that general wording only definesa+n
and b-a
; it notably doesn't define a-n
, which is needed here. And one cannot merely define a-n
as
a+(-n)
since that has undefined behavior for forward iterators.
Previous resolution [SUPERSEDED]:
This wording is relative to N4659.
Edit 23.3.11.5 [list.ops] as indicated:
void unique(); template <class BinaryPredicate> void unique(BinaryPredicate binary_pred);-?- Requires: The comparison function shall be an equivalence relation.
-19- Effects: Ifempty()
, has no effects. Otherwise, eErases all but the first element from every consecutive group ofequalequivalent elements referred to by the iteratori
in the rangefor which
[first + 1, last)[next(begin()), end())(for the version of
*i == *(i-1)*j == *iunique
with no arguments) or(for the version of
pred(*i, *(i - 1))pred(*j, *i)unique
with a predicate argument) holds, wherej
is an iterator in[begin(), end())
such thatnext(j) == i
. Invalidates only the iterators and references to the erased elements. -20- Throws: Nothing unless an exception is thrown bythe equality comparison or the predicate. -21- Complexity: If*i == *(i-1)
orpred(*i, *(i - 1))
the range[first, last)
is not empty!empty()
, exactlyapplications of the corresponding predicate, otherwise no applications of the predicate.
(last - first) - 1size() - 1Edit [forwardlist.ops] as indicated:
void unique(); template <class BinaryPredicate> void unique(BinaryPredicate binary_pred);-?- Requires: The comparison function shall be an equivalence relation.
-16- Effects: Ifempty()
, has no effects. Otherwise, eErases all but the first element from every consecutive group ofequalequivalent elements referred to by the iteratori
in the rangefor which
[first + 1, last)[next(begin()), end())(for the version with no arguments) or
*i == *(i-1)*j == *i(for the version with a predicate argument) holds, where
pred(*i, *(i - 1))pred(*j, *i)j
is an iterator in[begin(), end())
such thatnext(j) == i
. Invalidates only the iterators and references to the erased elements. -17- Throws: Nothing unless an exception is thrown by the equality comparison or the predicate. -18- Complexity: Ifthe range[first, last)
is not empty!empty()
, exactlyapplications of the corresponding predicate, otherwise no applications of the predicate.
(last - first) - 1distance(begin(), end()) - 1
[2021-02-21 Tim redrafts]
The wording below incorporates editorial pull request 4465.
[2021-05-21; Reflector poll]
Set status to Tentatively Ready after five votes in favour during reflector poll.
[2021-06-07 Approved at June 2021 virtual plenary. Status changed: Voting → WP.]
Proposed resolution:
This wording is relative to N4878.
Edit 23.3.11.5 [list.ops] as indicated:
-1- Since lists allow fast insertion and erasing from the middle of a list, certain operations are provided specifically for them.222 In this subclause, arguments for a template parameter named
[…]Predicate
orBinaryPredicate
shall meet the corresponding requirements in 26.2 [algorithms.requirements]. The semantics ofi + n
andi - n
, wherei
is an iterator into the list andn
is an integer, are the same as those ofnext(i, n)
andprev(i, n)
, respectively. Formerge
andsort
, the definitions and requirements in 26.8 [alg.sorting] apply.void unique(); template<class BinaryPredicate> void unique(BinaryPredicate binary_pred);-?- Let
-?- Preconditions:binary_pred
beequal_to<>{}
for the first overload.binary_pred
is an equivalence relation. -20- Effects: Erases all but the first element from every consecutive group ofequalequivalent elements. That is, for a nonempty list, erases all elements referred to by the iteratori
in the rangefor which
[first + 1, last)[begin() + 1, end())*i == *(i-1)
(for the version ofunique
with no arguments) orbinary_pred(*i, *(i - 1))
istrue
(for the version of. Invalidates only the iterators and references to the erased elements. -21- Returns: The number of elements erased. -22- Throws: Nothing unless an exception is thrown byunique
with a predicate argument) holdsthe predicate. -23- Complexity: If*i == *(i-1)
orpred(*i, *(i - 1))
the range[first, last)
is not emptyempty()
isfalse
, exactlyapplications of the corresponding predicate, otherwise no applications of the predicate.
(last - first) - 1size() - 1
Edit [forwardlist.ops] as indicated:
-1- In this subclause, arguments for a template parameter named
[…]Predicate
orBinaryPredicate
shall meet the corresponding requirements in 26.2 [algorithms.requirements]. The semantics ofi + n
, wherei
is an iterator into the list andn
is an integer, are the same as those ofnext(i, n)
. The expressioni - n
, wherei
is an iterator into the list andn
is an integer, means an iteratorj
such thatj + n == i
istrue
. Formerge
andsort
, the definitions and requirements in 26.8 [alg.sorting] apply.void unique(); template<class BinaryPredicate> void unique(BinaryPredicate binary_pred);-?- Let
-?- Preconditions:binary_pred
beequal_to<>{}
for the first overload.binary_pred
is an equivalence relation. -18- Effects: Erases all but the first element from every consecutive group ofequalequivalent elements. That is, for a nonempty list, erases all elements referred to by the iteratori
in the rangefor which
[first + 1, last)[begin() + 1, end())*i == *(i-1)
(for the version with no arguments) orbinary_pred(*i, *(i - 1))
istrue
(for the version with a predicate argument) holds. Invalidates only the iterators and references to the erased elements. -19- Returns: The number of elements erased. -20- Throws: Nothing unless an exception is thrown bythe equality comparison orthe predicate. -21- Complexity: Ifthe range[first, last)
is not emptyempty()
isfalse
, exactlyapplications of the corresponding predicate, otherwise no applications of the predicate.
(last - first) - 1distance(begin(), end()) - 1
{forward_,}list
-specific algorithmsSection: 23.3.11.5 [list.ops], 23.3.7.6 [forward.list.ops] Status: C++20 Submitter: Tim Song Opened: 2017-07-07 Last modified: 2023-02-07
Priority: 0
View all other issues in [list.ops].
View all issues with C++20 status.
Discussion:
Some specialized algorithms for forward_list
and list
take template parameters named Predicate
,
BinaryPredicate
, or Compare
.
However, there's no wording importing the full requirements for template type parameters with such names from
26.2 [algorithms.requirements] and 26.8 [alg.sorting],
which means, for instance, that there appears to be no rule prohibiting Compare
from modifying its arguments,
because we only refer to 26.8 [alg.sorting] for the definition of strict weak ordering. Is that intended?
[2017-07 Toronto Tuesday PM issue prioritization]
Priority 0; status to Ready
Proposed resolution:
This wording is relative to N4659.
Edit 23.3.11.5 [list.ops] as indicated:
-1- Since lists allow fast insertion and erasing from the middle of a list, certain operations are provided specifically for them.259) In this subclause, arguments for a template parameter named
Predicate
orBinaryPredicate
shall meet the corresponding requirements in 26.2 [algorithms.requirements]. Formerge
andsort
, the definitions and requirements in 26.8 [alg.sorting] apply.
Edit 23.3.11.5 [list.ops] as indicated:
void merge(list& x); void merge(list&& x); template <class Compare> void merge(list& x, Compare comp); template <class Compare> void merge(list&& x, Compare comp);-22- Requires:
Both the list and the argument list shall be sortedcomp
shall define a strict weak ordering (26.8 [alg.sorting]), and baccording to this orderingwith respect to the comparatoroperator<
(for the first two overloads) orcomp
(for the last two overloads).
Delete 23.3.11.5 [list.ops]/28 as redundant:
void sort(); template <class Compare> void sort(Compare comp);
-28- Requires:operator<
(for the first version) orcomp
(for the second version) shall define a strict weak ordering (26.8 [alg.sorting]).
Insert a new paragraph at the beginning of [forwardlist.ops]:
-?- In this subclause, arguments for a template parameter named
Predicate
orBinaryPredicate
shall meet the corresponding requirements in 26.2 [algorithms.requirements]. Formerge
andsort
, the definitions and requirements in 26.8 [alg.sorting] apply.void splice_after(const_iterator position, forward_list& x); void splice_after(const_iterator position, forward_list&& x);[…]
Edit [forwardlist.ops] as indicated:
void merge(forward_list& x); void merge(forward_list&& x); template <class Compare> void merge(forward_list& x, Compare comp); template <class Compare> void merge(forward_list&& x, Compare comp);-22- Requires:
comp
defines a strict weak ordering (26.8 [alg.sorting]), and*this
andx
are both sortedaccording to this orderingwith respect to the comparatoroperator<
(for the first two overloads) orcomp
(for the last two overloads).get_allocator() == x.get_allocator()
.
Delete [forwardlist.ops]/23 as redundant:
void sort(); template <class Compare> void sort(Compare comp);
-23- Requires:operator<
(for the version with no arguments) orcomp
(for the version with a comparison argument) defines a strict weak ordering (26.8 [alg.sorting]).
Section: 16.3.3.2 [expos.only.entity] Status: Resolved Submitter: Marshall Clow Opened: 2017-07-11 Last modified: 2023-02-07
Priority: 3
View all other issues in [expos.only.entity].
View all issues with Resolved status.
Discussion:
[thread.decaycopy] says:
In several places in this Clause the operation
DECAY_COPY(x)
is used. All such uses mean call the functiondecay_copy(x)
and use the result, wheredecay_copy
is defined as follows:
but decay_copy
is not defined in any synopsis.
DECAY_COPY
, except that theirs is also constexpr
and noexcept
.
We should mark the function decay_copy
as "exposition only" and constexpr
and noexcept
.
[2017-07-16, Daniel comments]
Currently there exists no proper way to mark decay_copy
as conditionally noexcept
as explained in
N3255 section "Adding to the Standard". This is also slighly related to
the request to add an is_nothrow_convertible
trait, as requested by LWG 2040(i).
[ 2017-11-01 P3 as result of c++std-lib online vote. ]
[2019-03-22; Daniel comments]
Starting with N4800 have now a constexpr
and conditionally noexcept
decay-copy
in the working draft ( [expos.only.func]). The pre-condition for that specification
helper became possible by adoption of P0758R1, which introduced the missing
is_nothrow_convertible
trait.
[2020-05-03; Reflector discussions]
Resolved by editorial action starting with N4800.
Rationale:
Resolved by editorial creation ofdecay-copy
after acceptance of P0758R1.
Proposed resolution:
monotonic_memory_resource::do_is_equal
uses dynamic_cast
unnecessarilySection: 20.5.6.3 [mem.res.monotonic.buffer.mem] Status: C++20 Submitter: Pablo Halpern Opened: 2017-07-14 Last modified: 2021-02-25
Priority: 0
View all other issues in [mem.res.monotonic.buffer.mem].
View all issues with C++20 status.
Discussion:
Section [mem.res.monotonic.buffer.mem], paragraph 11 says
bool do_is_equal(const memory_resource& other) const noexcept override;Returns:
this == dynamic_cast<const monotonic_buffer_resource*>(&other)
.
The dynamic_cast
adds nothing of value. It is an incorrect cut-and-paste from an example do_is_equal
for a more complex resource.
[2017-07-16, Tim Song comments]
The pool resource classes appear to also have this issue.
[2017-09-18, Casey Carter expands PR to cover the pool resources.]
Previous resolution: [SUPERSEDED]
Edit 20.5.6.3 [mem.res.monotonic.buffer.mem] as indicated:
bool do_is_equal(const memory_resource& other) const noexcept override;Returns:
this ==
.dynamic_cast<const monotonic_buffer_resource*>(&other)
[ 2017-11-01 Moved to Tentatively Ready after 7 positive votes for P0 on c++std-lib. ]
[2018-3-17 Adopted in Jacksonville]
Proposed resolution:
This resolution is relative to N4687.
Edit 20.5.5.4 [mem.res.pool.mem] as indicated:
boolsynchronized_pool_resource::do_is_equal(const memory_resource& other) const noexcept override;const memory_resource& other) const noexcept override;Returns:
this ==
.dynamic_cast<const synchronized_pool_resource*>(&other)
Strike 20.5.5.4 [mem.res.pool.mem] paragraph 10, and the immediately preceding declaration of unsynchronized_pool_resource::do_is_equal
.
Edit 20.5.6.3 [mem.res.monotonic.buffer.mem] as indicated:
bool do_is_equal(const memory_resource& other) const noexcept override;Returns:
this ==
.dynamic_cast<const monotonic_buffer_resource*>(&other)
weak_ptr::element_type
needs remove_extent_t
Section: 20.3.2.3 [util.smartptr.weak] Status: C++20 Submitter: Stephan T. Lavavej Opened: 2017-07-14 Last modified: 2021-02-25
Priority: 0
View all other issues in [util.smartptr.weak].
View all issues with C++20 status.
Discussion:
C++17's shared_ptr<T>::element_type
is remove_extent_t<T>
, but weak_ptr<T>::element_type
is T
. They should be the same, but this was lost over time.
namespace std { namespace experimental { inline namespace fundamentals_v2 { template<class T> class weak_ptr { public: typedef typename remove_extent_t<T> element_type;
(The typename
here was spurious.)
8.2.2 Class template
weak_ptr
8.2.2.1
weak_ptr
constructors
This obscured the fact that the Library Fundamentals TS had altered weak_ptr::element_type
.
shared_ptr
changes from Library Fundamentals
to C++17" missed the change to weak_ptr::element_type
, so it wasn't applied to C++17.
Peter Dimov has confirmed that this was unintentionally lost, and that "boost::weak_ptr
defines
element_type
in the same way as shared_ptr
".
[ 2017-07-17 Moved to Tentatively Ready after 6 positive votes on c++std-lib. ]
Proposed resolution:
This resolution is relative to N4659.
Edit 20.3.2.3 [util.smartptr.weak], class template weak_ptr
synopsis, as indicated:
template<class T> class weak_ptr { public: using element_type = remove_extent_t<T>; […] };
basic_socket_acceptor::is_open()
isn't noexcept
Section: 18.9.4 [networking.ts::socket.acceptor.ops] Status: C++23 Submitter: Jonathan Wakely Opened: 2017-07-14 Last modified: 2023-11-22
Priority: 0
View all issues with C++23 status.
Discussion:
Addresses: networking.ts
basic_socket::is_open()
is noexcept
, but the corresponding function on
basic_socket_acceptor
is not. This is a simple observer with a wide contract and cannot fail.
[ 2017-11-01 Moved to Tentatively Ready after 7 positive votes for P0 on c++std-lib. ]
[2018-3-17 Adopted in Jacksonville]
Proposed resolution:
This resolution is relative to N4656.
Edit 18.9 [networking.ts::socket.acceptor], class template basic_socket_acceptor
synopsis, as indicated:
template<class AcceptableProtocol> class basic_socket_acceptor { public: […] bool is_open() const noexcept; […] };
Edit 18.9.4 [networking.ts::socket.acceptor.ops] as indicated:
bool is_open() const noexcept;-10- Returns: A
bool
indicating whether this acceptor was opened by a previous call toopen
orassign
.
capacity()
Section: 27.4.3.5 [string.capacity], 23.3.13.3 [vector.capacity] Status: C++20 Submitter: Andy Giese Opened: 2017-07-24 Last modified: 2021-02-25
Priority: 0
View all other issues in [string.capacity].
View all issues with C++20 status.
Discussion:
basic_string
and vector
both have a capacity
function that returns the size of the internally
allocated buffer. This function does not specify a time complexity nor does it have an implied time complexity. However,
given the complexities for data()
to be 𝒪(1)
and size()
to be 𝒪(1)
,
we can imagine that it's reasonable to also require capacity()
to be 𝒪(1)
, since the
implementation will most likely be caching the size of the allocated buffer so as to check for the need to reallocate
on insertion.
[ 2017-11-01 Moved to Tentatively Ready after 10 positive votes for P0 on c++std-lib. ]
[2018-3-17 Adopted in Jacksonville]
Proposed resolution:
This resolution is relative to N4659.
Edit 27.4.3.5 [string.capacity] as indicated:
size_type capacity() const noexcept;-9- Returns: The size of the allocated storage in the string.
-?- Complexity: Constant time.
Edit 23.3.13.3 [vector.capacity] as indicated:
size_type capacity() const noexcept;-1- Returns: The total number of elements that the vector can hold without requiring reallocation.
-?- Complexity: Constant time.
make_shared/allocate_shared
only recommended?Section: 20.3.2.2.7 [util.smartptr.shared.create] Status: C++20 Submitter: Richard Smith Opened: 2017-08-01 Last modified: 2021-02-25
Priority: 0
View all other issues in [util.smartptr.shared.create].
View all issues with C++20 status.
Discussion:
In [util.smartptr.shared.create]/7.9 we find this:
"When the lifetime of the object managed by the return value ends, or when the initialization of an array element throws an exception, the initialized elements should be destroyed in the reverse order of their construction."
Why is this only a "should be" and not a "shall be" (or, following usual conventions for how we write requirements on the implementation, "are")? Is there some problem that means we can't require an implementation to destroy in reverse construction order in all cases?
Previous resolution: [SUPERSEDED]This resolution is relative to N4687.
Edit 20.3.2.2.7 [util.smartptr.shared.create] as indicated:
template<class T, ...> shared_ptr<T> make_shared(args); template<class T, class A, ...> shared_ptr<T> allocate_shared(const A& a, args);[…]
-7- Remarks:
[…]
(7.9) — When the lifetime of the object managed by the return value ends, or when the initialization of an array element throws an exception, the initialized elements are
should bedestroyed in decreasing index orderthe reverse order of their construction.
[2017-11-01, Alisdair comments and suggests wording improvement]
I dislike the change of how we specify the order of destruction, which is clear (but non-normative) from the preceding paragraph making the order of construction explicit. We are replacing that with a different specification, so now I need to match up ""decreasing index order" to "ascending order of address" to infer the behavior that is worded more directly today.
P0 to change "should be" to "are" though. Previous resolution: [SUPERSEDED]This resolution is relative to N4700.
Edit 20.3.2.2.7 [util.smartptr.shared.create] as indicated:
template<class T, ...> shared_ptr<T> make_shared(args); template<class T, class A, ...> shared_ptr<T> allocate_shared(const A& a, args);[…]
-7- Remarks:
[…]
(7.9) — When the lifetime of the object managed by the return value ends, or when the initialization of an array element throws an exception, the initialized elements are
should bedestroyed in the reverse order of their construction.
[2017-11-01]
The rationale for the "decreasing index order" change in the original P/R suggested by Richard Smith had been presented
as that user code may destroy one or more array elements and construct new ones in their place. In those cases
shared_ptr
has no way of knowing the construction order, so it cannot ensure that the destruction order reverses
it.
Richard: Perhaps something like:
the initialized elements are
should bedestroyed in the reverse order of their original construction.
would work?
[ 2017-11-02 Moved to Tentatively Ready after 10 positive votes for P0 on c++std-lib. ]
[2018-3-17 Adopted in Jacksonville]
Proposed resolution:
This resolution is relative to N4700.
Edit 20.3.2.2.7 [util.smartptr.shared.create] as indicated:
template<class T, ...> shared_ptr<T> make_shared(args); template<class T, class A, ...> shared_ptr<T> allocate_shared(const A& a, args);[…]
-7- Remarks:
[…]
(7.9) — When the lifetime of the object managed by the return value ends, or when the initialization of an array element throws an exception, the initialized elements are
should bedestroyed in the reverse order of their original construction.
basic_stringbuf
from a string
— where does the allocator come from?Section: 31.8.2.2 [stringbuf.cons] Status: Resolved Submitter: Marshall Clow Opened: 2017-08-02 Last modified: 2020-11-09
Priority: 3
View all other issues in [stringbuf.cons].
View all issues with Resolved status.
Discussion:
Consider the following constructor from 31.8.2.2 [stringbuf.cons] before p3:
explicit basic_stringbuf( const basic_string<charT, traits, Allocator>& s, ios_base::openmode which = ios_base::in | ios_base::out);
In [stringbuf.cons]/3, it says:
Effects: Constructs an object of class
basic_stringbuf
, initializing the base class withbasic_streambuf()
(30.6.3.1), and initializingmode
withwhich
. Then callsstr(s)
.
But that doesn't mention where the allocator for the basic_stringbuf
comes from.
Until recently, most implementations just default constructed the allocator.
But that requires that the allocator be default constructible.
basic_stringbuf
should be constructed with a copy of the allocator from the string.
[2017-11-01, Marshall comments]
There exist two related proposals, namely P0407R1 and P0408R2.
[2017-11 Albuquerque Wednesday night issues processing]
Priority set to 3
[2020-05-02 Daniel comments]
It seems that this issue is now resolved by the proposal P0408R7, accepted in Cologne 2019.
The paper clarified that the allocator is provided based on the effect of initializing the exposition-onlybasic_string buf
with the provided
basic_string argument s
(Which again is specified in terms of the
allocator-related specification(27.4.3.2 [string.require] p3).
[2020-05-03 Reflector discussion]
Tentatively Resolved by P0408R7 adopted in Cologne 2019.
The change we approved is not what the issue recommended (and not what some implementations do), so we decided with leaving it as Tentatively Resolved to give people a chance to review it.[2020-11-09 Resolved for C++20. Status changed: Tentatively Resolved → Resolved.]
Rationale:
Resolved by P0408R7.Proposed resolution:
allocate_shared
should rebind allocator to cv-unqualified value_type
for constructionSection: 20.3.2.2.7 [util.smartptr.shared.create] Status: C++20 Submitter: Glen Joseph Fernandes Opened: 2017-08-06 Last modified: 2021-02-25
Priority: 0
View all other issues in [util.smartptr.shared.create].
View all issues with C++20 status.
Discussion:
The remarks for the allocate_shared
family of functions
specify that when constructing a (sub)object of type U
, it uses a
rebound copy of the allocator a
passed to
allocate_shared
such that its value_type
is
U
. However U
can be a const
or
volatile
qualified type, and [allocator.requirements] specify
that the value_type
must be cv-unqualified.
[ 2017-11-01 Moved to Tentatively Ready after 6 positive votes for P0 on c++std-lib. ]
[2018-3-17 Adopted in Jacksonville]
Proposed resolution:
This resolution is relative to N4687.
Edit 20.3.2.2.7 [util.smartptr.shared.create] as indicated:
template<class T, ...> shared_ptr<T> make_shared(args); template<class T, class A, ...> shared_ptr<T> allocate_shared(const A& a, args);[…]
-7- Remarks:
[…]
(7.5) — When a (sub)object of a non-array type
U
is specified to have an initial value ofv
, orU(l...)
, wherel...
is a list of constructor arguments,allocate_shared
shall initialize this (sub)object via the expression
(7.5.1) —
allocator_traits<A2>::construct(a2, pv, v)
or(7.5.2) —
allocator_traits<A2>::construct(a2, pv, l...)
respectively, where
pv
points to storage suitable to hold an object of typeU
anda2
of typeA2
is a rebound copy of the allocator a passed toallocate_shared
such that itsvalue_type
isremove_cv_t<U>
.(7.6) — When a (sub)object of non-array type
U
is specified to have a default initial value,make_shared
shall initialize this (sub)object via the expression::new(pv) U()
, wherepv
has typevoid*
and points to storage suitable to hold an object of typeU
.(7.7) — When a (sub)object of non-array type
U
is specified to have a default initial value,allocate_shared
shall initialize this (sub)object via the expressionallocator_traits<A2>::construct(a2, pv)
, wherepv
points to storage suitable to hold an object of typeU
anda2
of typeA2
is a rebound copy of the allocator a passed toallocate_shared
such that itsvalue_type
isremove_cv_t<U>
.[…]
make_shared
(sub)object destruction semantics are not specifiedSection: 20.3.2.2.7 [util.smartptr.shared.create] Status: C++20 Submitter: Glen Joseph Fernandes Opened: 2017-08-06 Last modified: 2021-02-25
Priority: 2
View all other issues in [util.smartptr.shared.create].
View all issues with C++20 status.
Discussion:
The remarks for the make_shared
and allocate_shared
functions
do not specify how the objects managed by the returned shared_ptr
are
destroyed. It is implied that when objects are constructed via a placement new
expression, they are destroyed by calling the destructor, and that when objects
are constructed via an allocator, they are destroyed using that allocator. This
should be explicitly specified.
[2017-11 Albuquerque Wednesday night issues processing]
Priority set to 2
Previous resolution [SUPERSEDED]:
This resolution is relative to N4687.
Edit 20.3.2.2.7 [util.smartptr.shared.create] as indicated:
template<class T, ...> shared_ptr<T> make_shared(args); template<class T, class A, ...> shared_ptr<T> allocate_shared(const A& a, args);[…]
-7- Remarks:
[…]
(7.9) — When the lifetime of the object managed by the return value ends, or when the initialization of an array element throws an exception, the initialized elements should be destroyed in the reverse order of their construction.
(7.?) — When a (sub)object of a non-array type
U
that was initialized bymake_shared
is to be destroyed, it shall be destroyed via the expressionpv->~U()
wherepv
points to that object of typeU
.(7.?) — When a (sub)object of a non-array type
U
that was initialized byallocate_shared
is to be destroyed, it shall be destroyed via the expressionallocator_traits<A2>::destroy(a2, pv)
wherepv
points to that object of type cv-unqualifiedU
anda2
of typeA2
is a rebound copy of the allocatora
passed toallocate_shared
such that itsvalue_type
isremove_cv_t<U>
.
[2018-06 Rapperswil Wednesday night issues processing]
CC: what is "of type cv-unqualified U
" and "remove_cv_T<U>
" about?
DK: again, it isn't new wording; it is in p 7.5.2
JW: but none of the words use "of type cv-unqualified U
"
CT: so we should also used remove_cv_T<U>
instead?
JW: I would like to talk to Glen
FB: does anybody know how it works for an array of arrays? It seems to cover the case
JW: we could leave it vague as it is now or specify it to exactly what it does
DK: I think we should split the thing into two parts and start with definitions
DK: ACTION I can refactor the wording
MC: there was a fairly long message thread when we talked about this
C
and pointer variable c
as well as
Table 31 expressions "a.construct(c, args)
" and "a.destroy(c)
"), therefore a
conforming implementation needs to effectively construct an object pointer that holds an object of type
remove_cv_T<U>
and similarly destroy such an object. Albeit it seems to be an artificial
restriction to construct and destroy only non-cv-qualified object types, this is, if any,
a different issue. But given this current state, the wording for allocate_shared
needs
to make a special wording dance via remove_cv_T<U>
.
For construct
the existing wording prevents to speak about that detail by using the more indirect
phrase "where pv
points to storage suitable to hold an object of type U
", but since
object types U
and const U
have exactly the same storage and alignment requirements,
this sentence is correct for remove_cv_T<U>
as well.
[2018-08-23 Batavia Issues processing]
Status to Tentatively Ready.
[2018-11, Adopted in San Diego]
Proposed resolution:
This resolution is relative to N4750.
Edit 20.3.2.2.7 [util.smartptr.shared.create] as indicated:
template<class T, ...> shared_ptr<T> make_shared(args); template<class T, class A, ...> shared_ptr<T> allocate_shared(const A& a, args);[…]
-7- Remarks:
[…]
(7.9) — When the lifetime of the object managed by the return value ends, or when the initialization of an array element throws an exception, the initialized elements are destroyed in the reverse order of their original construction.
(7.?) — When a (sub)object of a non-array type
U
that was initialized bymake_shared
is to be destroyed, it is destroyed via the expressionpv->~U()
wherepv
points to that object of typeU
.(7.?) — When a (sub)object of a non-array type
U
that was initialized byallocate_shared
is to be destroyed, it is destroyed via the expressionallocator_traits<A2>::destroy(a2, pv)
wherepv
points to that object of typeremove_cv_t<U>
anda2
of typeA2
is a rebound copy of the allocatora
passed toallocate_shared
such that itsvalue_type
isremove_cv_t<U>
.
<string_view>
doesn't provide std::size/empty/data
Section: 24.7 [iterator.range] Status: C++20 Submitter: Tim Song Opened: 2017-08-11 Last modified: 2021-06-06
Priority: 0
View other active issues in [iterator.range].
View all other issues in [iterator.range].
View all issues with C++20 status.
Discussion:
basic_string_view
has size()
, empty()
, and data()
members, but
including <string_view>
isn't guaranteed to give you access to the
corresponding free function templates. This seems surprising.
[ 2017-11-01 Moved to Tentatively Ready after 7 positive votes for P0 on c++std-lib. ]
[2018-3-17 Adopted in Jacksonville]
Proposed resolution:
This resolution is relative to N4687.
Edit [iterator.container] as indicated:
-1- In addition to being available via inclusion of the
<iterator>
header, the function templates in 27.8 are available when any of the following headers are included:<array>
,<deque>
,<forward_list>
,<list>
,<map>
,<regex>
,<set>
,<string>
,<string_view>
,<unordered_map>
,<unordered_set>
, and<vector>
.
uses_executor
says "if a type T::executor_type
exists"Section: 13.11.1 [networking.ts::async.uses.executor.trait] Status: C++23 Submitter: Jonathan Wakely Opened: 2017-08-17 Last modified: 2023-11-22
Priority: 0
View all issues with C++23 status.
Discussion:
Addresses: networking.ts
[async.uses.executor.trait] p1 says "if a type T::executor_type
exists" but we don't want it to be required
to detect private or ambiguous types.
[ 2017-11-01 Moved to Tentatively Ready after 7 positive votes for P0 on c++std-lib. ]
[2018-3-17 Adopted in Jacksonville]
Proposed resolution:
This resolution is relative to N4656.
Edit 13.11.1 [networking.ts::async.uses.executor.trait] as indicated:
-1- Remark: Detects whether
T
has a nestedexecutor_type
that is convertible fromExecutor
. Meets theBinaryTypeTrait
requirements (C++Std [meta.rqmts]). The implementation provides a definition that is derived fromtrue_type
ifa typethe qualified-idT::executor_type
existsis valid and denotes a type andis_convertible<Executor, T::executor_type>::value != false
, otherwise it is derived fromfalse_type
. A program may specialize this template […].
atomic<T>
is unimplementable for non-is_trivially_copy_constructible T
Section: 32.5.8 [atomics.types.generic] Status: C++20 Submitter: Billy O'Neal III Opened: 2017-08-16 Last modified: 2021-02-25
Priority: 2
View all other issues in [atomics.types.generic].
View all issues with C++20 status.
Discussion:
32.5.8 [atomics.types.generic] requires that T
for std::atomic
is trivially copyable.
Unfortunately, that's not sufficient to implement atomic
. Consider atomic<T>::load
, which
wants to look something like this:
template<class T> struct atomic { __compiler_magic_storage_for_t storage; T load(memory_order = memory_order_seq_cst) const { return __magic_intrinsic(storage); } };
Forming this return statement, though, requires that T
is copy constructible — trivially copyable things
aren't necessarily copyable! For example, the following is trivially copyable but breaks libc++, libstdc++, and msvc++:
struct NonAssignable { int i; NonAssignable() = delete; NonAssignable(int) : i(0) {} NonAssignable(const NonAssignable&) = delete; NonAssignable(NonAssignable&&) = default; NonAssignable& operator=(const NonAssignable&) = delete; NonAssignable& operator=(NonAssignable&&) = delete; ~NonAssignable() = default; };
All three standard libraries are happy as long as T
is trivially copy constructible, assignability is not required.
Casey Carter says that we might want to still require trivially copy assignable though, since what happens when you do an
atomic<T>::store
is morally an "assignment" even if it doesn't use the user's assignment operator.
[2017-11 Albuquerque Wednesday issue processing]
Status to Open; Casey and STL to work with Billy for better wording.
Should this include trivially copyable as well as trivially copy assignable?
2017-11-09, Billy O'Neal provides updated wording.
Previous resolution [SUPERSEDED]:
This resolution is relative to N4687.
Edit 32.5.8 [atomics.types.generic] as indicated:
-1- If
is_trivially_copy_constructible_v<T>
isfalse
, the program is ill-formedThe template argument for. [Note: Type arguments that are not also statically initializable may be difficult to use. — end note]T
shall be trivially copyable (6.8 [basic.types])
Previous resolution [SUPERSEDED]:
This resolution is relative to N4687.
Edit 32.5.8 [atomics.types.generic] as indicated:
-1- If
is_copy_constructible_v<T>
isfalse
or ifis_trivially_copyable_v<T>
isfalse
, the program is ill-formedThe template argument for. [Note: Type arguments that are not also statically initializable may be difficult to use. — end note]T
shall be trivially copyable (6.8 [basic.types])
[2017-11-12, Tomasz comments and suggests alternative wording]
According to my understanding during Albuquerque Saturday issue processing we agreed that we want the type used with the atomics to have non-deleted and trivial copy/move construction and assignment.
Wording note:CopyConstructible
and CopyAssignable
include semantic requirements that are not
checkable at compile time, so these are requirements imposed on the user and cannot be validated by an
implementation without heroic efforts.
[2018-11 San Diego Thursday night issue processing]
Status to Ready.
Proposed resolution:
This resolution is relative to N4700.
Edit 32.5.8 [atomics.types.generic] as indicated:
-1- The template argument for
T
shall meet theCopyConstructible
andCopyAssignable
requirements. Ifis_trivially_copyable_v<T> && is_copy_constructible_v<T> && is_move_constructible_v<T> && is_copy_assignable_v<T> && is_move_assignable_v<T>
isfalse
, the program is ill-formedbe trivially copyable (6.8 [basic.types]). [Note: Type arguments that are not also statically initializable may be difficult to use. — end note]
(recursive_)directory_iterator
construction and traversal should not be noexcept
Section: 31.12.11.2 [fs.dir.itr.members], 31.12.12.2 [fs.rec.dir.itr.members], 31.12.13.4 [fs.op.copy], 31.12.13.20 [fs.op.is.empty] Status: C++20 Submitter: Tim Song Opened: 2017-08-23 Last modified: 2023-02-07
Priority: 0
View all other issues in [fs.dir.itr.members].
View all issues with C++20 status.
Discussion:
Constructing a (recursive_)directory_iterator
from a path
requires, at a minimum, initializing its
underlying directory_entry
object with the path
formed from the supplied path
and the name of the
first entry, which requires a potentially throwing memory allocation; every implementation I've looked at also allocates
memory to store additional data as well.
increment()
needs to update the path
stored in directory_entry
object to refer
to the name of the next entry, which may require a memory allocation. While it might conceivably be possible to
postpone the update in this case until the iterator is dereferenced (the dereference operation is not noexcept
due to its narrow contract), it seems highly unlikely that such an implementation is intended (not to mention that it
would require additional synchronization as the dereference operations are const).
This further calls into question whether the error_code
overloads of copy
and is_empty
,
whose specification uses directory_iterator
, should be noexcept
. There might be a case for keeping
the noexcept
for is_empty
, although that would require changes in all implementations I checked
(libstdc++, libc++, and Boost). copy
appears to be relentlessly hostile to noexcept
, since its
specification forms a path
via operator/
in two places (bullets 4.7.4 and 4.8.2) in addition to the
directory_iterator
usage. The proposed resolution below removes both.
[ 2017-11-03 Moved to Tentatively Ready after 7 positive votes for P0 on c++std-lib. ]
[2018-3-17 Adopted in Jacksonville]
Proposed resolution:
This wording is relative to N4700.
Edit [fs.class.directory_iterator], class directory_iterator
synopsis, as indicated:
[…] explicit directory_iterator(const path& p); directory_iterator(const path& p, directory_options options); directory_iterator(const path& p, error_code& ec)noexcept; directory_iterator(const path& p, directory_options options, error_code& ec)noexcept; […] directory_iterator& operator++(); directory_iterator& increment(error_code& ec)noexcept;
Edit 31.12.11.2 [fs.dir.itr.members] before p2 as indicated:
explicit directory_iterator(const path& p); directory_iterator(const path& p, directory_options options); directory_iterator(const path& p, error_code& ec)noexcept; directory_iterator(const path& p, directory_options options, error_code& ec)noexcept;-2- Effects: […]
-3- Throws: As specified in 31.12.5 [fs.err.report]. -4- [Note: […] — end note]
Edit 31.12.11.2 [fs.dir.itr.members] before p10 as indicated:
directory_iterator& operator++(); directory_iterator& increment(error_code& ec)noexcept;-10- Effects: As specified for the prefix increment operation of Input iterators (24.2.3).
-11- Returns:*this
. -12- Throws: As specified in 31.12.5 [fs.err.report].
Edit 31.12.12 [fs.class.rec.dir.itr], class recursive_directory_iterator
synopsis, as indicated:
[…] explicit recursive_directory_iterator(const path& p); recursive_directory_iterator(const path& p, directory_options options); recursive_directory_iterator(const path& p, directory_options options, error_code& ec)noexcept; recursive_directory_iterator(const path& p, error_code& ec)noexcept; […] recursive_directory_iterator& operator++(); recursive_directory_iterator& increment(error_code& ec)noexcept;
Edit 31.12.12.2 [fs.rec.dir.itr.members] before p2 as indicated:
explicit recursive_directory_iterator(const path& p); recursive_directory_iterator(const path& p, directory_options options); recursive_directory_iterator(const path& p, directory_options options, error_code& ec)noexcept; recursive_directory_iterator(const path& p, error_code& ec)noexcept;-2- Effects: […]
-3- Postconditions: […] -4- Throws: As specified in 31.12.5 [fs.err.report]. -5- [Note: […] — end note] -6- [Note: […] — end note]
Edit 31.12.12.2 [fs.rec.dir.itr.members] before p23 as indicated:
recursive_directory_iterator& operator++(); recursive_directory_iterator& increment(error_code& ec)noexcept;-23- Effects: As specified for the prefix increment operation of Input iterators (24.2.3), except that: […]
-24- Returns:*this
. -25- Throws: As specified 31.12.5 [fs.err.report].
Edit 31.12.4 [fs.filesystem.syn], header <filesystem>
synopsis, as indicated:
namespace std::filesystem { […] void copy(const path& from, const path& to); void copy(const path& from, const path& to, error_code& ec)noexcept; void copy(const path& from, const path& to, copy_options options); void copy(const path& from, const path& to, copy_options options, error_code& ec)noexcept; […] bool is_empty(const path& p); bool is_empty(const path& p, error_code& ec)noexcept; […] }
Edit 31.12.13.4 [fs.op.copy] as indicated:
void copy(const path& from, const path& to, error_code& ec)noexcept;-2- Effects: Equivalent to
copy(from, to, copy_options::none, ec)
.void copy(const path& from, const path& to, copy_options options); void copy(const path& from, const path& to, copy_options options, error_code& ec)noexcept;-3- Requires: […]
-4- Effects: […] -5- Throws: […] -6- Remarks: […] -7- [Example: […] — end example]
Edit [fs.op.is_empty] as indicated:
bool is_empty(const path& p); bool is_empty(const path& p, error_code& ec)noexcept;-1- Effects: […]
-2- Throws: […]
noexcept
issues with filesystem operationsSection: 31.12.13.5 [fs.op.copy.file], 31.12.13.7 [fs.op.create.directories], 31.12.13.32 [fs.op.remove.all] Status: C++20 Submitter: Tim Song Opened: 2017-08-23 Last modified: 2021-06-06
Priority: Not Prioritized
View all other issues in [fs.op.copy.file].
View all issues with C++20 status.
Discussion:
create_directories
may need to create temporary path
s, and remove_all
may need to create temporary
path
s and/or directory_iterator
s. These operations may require a potentially throwing memory allocation.
copy_file
may wish to dynamically allocate the buffer used for copying when the underlying OS
doesn't supply a copy API directly. This can happen indirectly, e.g., by using <fstream>
facilities to
perform the copying without supplying a custom buffer. Unless LWG wishes to prohibit using a dynamically allocated buffer
in this manner, the noexcept
should be removed.
[2017-11 Albuquerque Wednesday night issues processing]
Moved to Ready
[2018-3-17 Adopted in Jacksonville]
Proposed resolution:
This wording is relative to N4687.
Edit 31.12.4 [fs.filesystem.syn], header <filesystem>
synopsis, as indicated:
namespace std::filesystem { […] bool copy_file(const path& from, const path& to); bool copy_file(const path& from, const path& to, error_code& ec)noexcept; bool copy_file(const path& from, const path& to, copy_options option); bool copy_file(const path& from, const path& to, copy_options option, error_code& ec)noexcept; […] bool create_directories(const path& p); bool create_directories(const path& p, error_code& ec)noexcept; […] uintmax_t remove_all(const path& p); uintmax_t remove_all(const path& p, error_code& ec)noexcept; […] }
Edit [fs.op.copy_file] as indicated:
bool copy_file(const path& from, const path& to); bool copy_file(const path& from, const path& to, error_code& ec)noexcept;-1- Returns: […]
-2- Throws: […]bool copy_file(const path& from, const path& to, copy_options options); bool copy_file(const path& from, const path& to, copy_options options, error_code& ec)noexcept;-3- Requires: […]
-4- Effects: […] -5- Returns: […] -6- Throws: […] -7- Complexity: […]
Edit [fs.op.create_directories] as indicated:
bool create_directories(const path& p); bool create_directories(const path& p, error_code& ec)noexcept;-1- Effects: […]
-2- Postconditions: […] -3- Returns: […] -4- Throws: […] -5- Complexity: […]
Edit [fs.op.remove_all] as indicated:
uintmax_t remove_all(const path& p); uintmax_t remove_all(const path& p, error_code& ec)noexcept;-1- Effects: […]
-2- Postconditions: […] -3- Returns: […] -4- Throws: […]
copy_options::unspecified
underspecifiedSection: 31.12.13.4 [fs.op.copy] Status: C++20 Submitter: Tim Song Opened: 2017-08-24 Last modified: 2021-02-25
Priority: 3
View all other issues in [fs.op.copy].
View all issues with C++20 status.
Discussion:
31.12.13.4 [fs.op.copy]/4.8.2 says in the copy-a-directory case filesystem::copy
performs:
Presumably this does not actually mean that the implementation is free to set whateverfor (const directory_entry& x : directory_iterator(from)) copy(x.path(), to/x.path().filename(), options | copy_options::unspecified);
copy_option
element it wishes
(directories_only
? recursive
? create_hard_links
?), or none
at all, or – since
unspecified behavior corresponds to the nondeterministic aspects of the abstract machine (6.9.1 [intro.execution]/3)
– a nondeterministically picked element for every iteration of the loop. That would be outright insane.
I'm fairly sure that what's intended here is to set an otherwise-unused bit in options
so as to prevent recursion in the options == copy_options::none
case.
[2017-11-08]
Priority set to 3 after five votes on the mailing list
Previous resolution: [SUPERSEDED]This wording is relative to N4687.
Edit 31.12.13.4 [fs.op.copy] p4, bullet 4.8.2 as indicated:
(4.7) — Otherwise, if
is_regular_file(f)
, then:[…](4.8) — Otherwise, if
is_directory(f) && ((options & copy_options::recursive) != copy_options::none || options == copy_options::none)then:
(4.8.1) — If
exists(t)
isfalse
, thencreate_directory(to, from)
.(4.8.2) — Then, iterate over the files in
from
, as if byfor (const directory_entry& x : directory_iterator(from)) copy(x.path(), to/x.path().filename(), options | copy_options::unspecifiedin-recursive-copy);where
in-recursive-copy
is an exposition-only bitmask element ofcopy_options
that is not one of the elements in 31.12.8.3 [fs.enum.copy.opts].(4.9) — Otherwise, for the signature with argument
ec
,ec.clear()
.(4.10) — Otherwise, no effects.
[2018-1-26 issues processing telecon]
Status to 'Tentatively Ready' after striking the words 'Exposition-only' from the added text
[2018-3-17 Adopted in Jacksonville]
Proposed resolution:
This wording is relative to N4687.
Edit 31.12.13.4 [fs.op.copy] p4, bullet 4.8.2 as indicated:
(4.7) — Otherwise, if
is_regular_file(f)
, then:[…](4.8) — Otherwise, if
is_directory(f) && ((options & copy_options::recursive) != copy_options::none || options == copy_options::none)then:
(4.8.1) — If
exists(t)
isfalse
, thencreate_directory(to, from)
.(4.8.2) — Then, iterate over the files in
from
, as if byfor (const directory_entry& x : directory_iterator(from)) copy(x.path(), to/x.path().filename(), options | copy_options::unspecifiedin-recursive-copy);where
in-recursive-copy
is an bitmask element ofcopy_options
that is not one of the elements in 31.12.8.3 [fs.enum.copy.opts].(4.9) — Otherwise, for the signature with argument
ec
,ec.clear()
.(4.10) — Otherwise, no effects.
list splice
functions should use addressof
Section: 23.3.7.6 [forward.list.ops], 23.3.11.5 [list.ops] Status: C++20 Submitter: Jonathan Wakely Opened: 2017-09-05 Last modified: 2023-02-07
Priority: 0
View all other issues in [forward.list.ops].
View all issues with C++20 status.
Discussion:
[forwardlist.ops] p1 and 23.3.11.5 [list.ops] p3 say &x != this
, but should use
addressof
. We really need front matter saying that when the library says &x
it means
std::addressof(x)
.
[ 2017-11-03 Moved to Tentatively Ready after 9 positive votes for P0 on c++std-lib. ]
[2018-3-17 Adopted in Jacksonville]
Proposed resolution:
This wording is relative to N4687.
Edit [forwardlist.ops] p1 as indicated:
void splice_after(const_iterator position, forward_list& x); void splice_after(const_iterator position, forward_list&& x);-1- Requires:
position
isbefore_begin()
or is a dereferenceable iterator in the range[begin(), end())
.get_allocator() == x.get_allocator()
..
&addressof(x) != this
Edit 23.3.11.5 [list.ops] p3 as indicated:
void splice(const_iterator position, list& x); void splice(const_iterator position, list&& x);-3- Requires:
.
&addressof(x) != this
Edit 23.3.11.5 [list.ops] p3 as indicated:
template <class Compare> void merge(list& x, Compare comp); template <class Compare> void merge(list&& x, Compare comp);-22- Requires:
-23- Effects: Ifcomp
shall define a strict weak ordering (26.8 [alg.sorting]), and both the list and the argument list shall be sorted according to this ordering.(
does nothing; otherwise, merges the two sorted ranges&addressof(x) == this)[begin(), end())
and[x.begin(), x.end())
. The result is a range in which the elements will be sorted in non-decreasing order according to the ordering defined bycomp
; that is, for every iteratori
, in the range other than the first, the conditioncomp(*i, *(i - 1))
will befalse
. Pointers and references to the moved elements ofx
now refer to those same elements but as members of*this
. Iterators referring to the moved elements will continue to refer to their elements, but they now behave as iterators into*this
, not intox
. -24- Remarks: Stable (16.4.6.8 [algorithm.stable]). If(
the range&addressof(x) != this)[x.begin(), x.end())
is empty after the merge. No elements are copied by this operation. The behavior is undefined ifget_allocator() != x.get_allocator()
. -25- Complexity: At mostsize() + x.size() - 1
applications ofcomp
if(
; otherwise, no applications of&addressof(x) != this)comp
are performed. If an exception is thrown other than by a comparison there are no effects.
shared_ptr
of function typeSection: 20.3.2.2 [util.smartptr.shared] Status: C++20 Submitter: Agustín K-ballo Bergé Opened: 2017-09-13 Last modified: 2021-02-25
Priority: 3
View all other issues in [util.smartptr.shared].
View all issues with C++20 status.
Discussion:
shared_ptr
has been designed to support use cases where it owns a pointer to function, and whose deleter
does something with it. This can be used, for example, to keep a dynamic library loaded for as long as their exported
functions are referenced.
T
in shared_ptr<T>
can be a function type. It isn't
immediately obvious from the standard, and it's not possible to tell from the wording that this is intentional.
[2017-11 Albuquerque Wednesday night issues processing]
Priority set to 3
Previous resolution [SUPERSEDED]:
This wording is relative to N4687.
Edit 20.3.2.2 [util.smartptr.shared] as indicated:
[…]
-2- Specializations ofshared_ptr
shall beCopyConstructible
,CopyAssignable
, andLessThanComparable
, allowing their use in standard containers. Specializations ofshared_ptr
shall be contextually convertible tobool
, allowing their use in boolean expressions and declarations in conditions.The template parameter-?- The template parameterT
ofshared_ptr
may be an incomplete type.T
ofshared_ptr
may be an incomplete type.T*
shall be an object pointer type or a function pointer type. […]
[2020-02-13; Prague]
LWG would prefer to make the new constraint a Mandates-like thing.
Original resolution [SUPERSEDED]:
This wording is relative to N4849.
Edit 20.3.2.2 [util.smartptr.shared] as indicated:
[…]
-2- Specializations ofshared_ptr
shall be Cpp17CopyConstructible, Cpp17CopyAssignable, and Cpp17LessThanComparable, allowing their use in standard containers. Specializations ofshared_ptr
shall be contextually convertible tobool
, allowing their use in boolean expressions and declarations in conditions.The template parameter-?- The template parameterT
ofshared_ptr
may be an incomplete type.T
ofshared_ptr
may be an incomplete type. The program is ill-formed unlessT*
is an object pointer type or a function pointer type. […]
[2020-02 Friday AM discussion in Prague.]
Marshall provides updated wording; status to Immediate
Proposed resolution:
This wording is relative to N4849.
Edit 20.3.2.2 [util.smartptr.shared] as indicated:
[…]
-2- Specializations ofshared_ptr
shall be Cpp17CopyConstructible, Cpp17CopyAssignable, and Cpp17LessThanComparable, allowing their use in standard containers. Specializations ofshared_ptr
shall be contextually convertible tobool
, allowing their use in boolean expressions and declarations in conditions.The template parameter-?- The template parameterT
ofshared_ptr
may be an incomplete type.T
ofshared_ptr
may be an incomplete type. [Note: T may be a function type. -- end note] […]
value_type
buffer sequence requirementSection: 16.2 [networking.ts::buffer.reqmts] Status: C++23 Submitter: Vinnie Falco Opened: 2017-09-20 Last modified: 2023-11-22
Priority: 0
View all other issues in [networking.ts::buffer.reqmts].
View all issues with C++23 status.
Discussion:
Addresses: networking.ts
The post-condition requirements for ConstBufferSequence
and MutableBufferSequence
refer to X::value_type
,
but no such nested type is required. The lambda expression passed to equal
can use auto const&
parameter types
instead.
This wording is relative to N4588.
Modify 16.2.1 [networking.ts::buffer.reqmts.mutablebuffersequence] Table 12 "
MutableBufferSequence
requirements" as indicated:
Table 12 — MutableBufferSequence
requirementsexpression return type assertion/note pre/post-condition […]
X u(x);
post:
equal( net::buffer_sequence_begin(x), net::buffer_sequence_end(x), net::buffer_sequence_begin(u), net::buffer_sequence_end(u), [](consttypename X::value_typeauto& v1, consttypename X::value_typeauto& v2) { mutable_buffer b1(v1); mutable_buffer b2(v2); return b1.data() == b2.data() && b1.size() == b2.size(); })Modify 16.2.2 [networking.ts::buffer.reqmts.constbuffersequence] Table 13 "
ConstBufferSequence
requirements" as indicated:
Table 13 — ConstBufferSequence
requirementsexpression return type assertion/note pre/post-condition […]
X u(x);
post:
equal( net::buffer_sequence_begin(x), net::buffer_sequence_end(x), net::buffer_sequence_begin(u), net::buffer_sequence_end(u), [](consttypename X::value_typeauto& v1, consttypename X::value_typeauto& v2) { const_buffer b1(v1); const_buffer b2(v2); return b1.data() == b2.data() && b1.size() == b2.size(); })
[2017-10-19, Peter Dimov provides improved wording]
The alternative wording prevents the need for auto
parameters because it takes advantage of the "convertible to"
requirement.
[ 2017-10-20 Moved to Tentatively Ready after 5 positive votes on c++std-lib. ]
[2018-3-17 Adopted in Jacksonville]
Proposed resolution:
This wording is relative to N4588.
Modify 16.2.1 [networking.ts::buffer.reqmts.mutablebuffersequence] Table 12 "MutableBufferSequence
requirements" as indicated:
Table 12 — MutableBufferSequence
requirementsexpression return type assertion/note pre/post-condition […]
X u(x);
post:
equal( net::buffer_sequence_begin(x), net::buffer_sequence_end(x), net::buffer_sequence_begin(u), net::buffer_sequence_end(u), [](consttypename X::value_type& v1mutable_buffer& b1, consttypename X::value_type& v2mutable_buffer& b2) {mutable_buffer b1(v1); mutable_buffer b2(v2);return b1.data() == b2.data() && b1.size() == b2.size(); })
Modify 16.2.2 [networking.ts::buffer.reqmts.constbuffersequence] Table 13 "ConstBufferSequence
requirements" as indicated:
Table 13 — ConstBufferSequence
requirementsexpression return type assertion/note pre/post-condition […]
X u(x);
post:
equal( net::buffer_sequence_begin(x), net::buffer_sequence_end(x), net::buffer_sequence_begin(u), net::buffer_sequence_end(u), [](consttypename X::value_type& v1const_buffer& b1, consttypename X::value_type& v2const_buffer& v2) {const_buffer b1(v1); const_buffer b2(v2);return b1.data() == b2.data() && b1.size() == b2.size(); })
is_convertible<derived*, base*>
may lead to ODRSection: 21.3.7 [meta.rel] Status: Resolved Submitter: Alisdair Meredith Opened: 2017-09-24 Last modified: 2018-11-12
Priority: 2
View other active issues in [meta.rel].
View all other issues in [meta.rel].
View all issues with Resolved status.
Discussion:
Given two incomplete types, base
and derived
, that will have
the expected base/derived relationship when complete, the trait
is_convertible
claims to support instantiation with pointers to
these types (as pointers to incomplete types are, themselves,
complete), yet will give a different answer when the types are
complete vs. when they are incomplete.
void
. We may also want some weasel-wording to
permit pointers to arrays-of-unknown-bound, and pointers
to cv-qualified variants of the same incomplete type.
[2017-11 Albuquerque Wednesday night issues processing]
Priority set to 2
[2018-08 Batavia Monday issue discussion]
Issues 2797(i), 2939(i), 3022(i), and 3099(i) are all closely related. Walter to write a paper resolving them.
[2018-11-11 Resolved by P1285R0, adopted in San Diego.]
Proposed resolution:
Section: 22.10.16 [func.memfn], 22.10.13 [func.not.fn], 22.10.15 [func.bind] Status: Resolved Submitter: Detlef Vollmann Opened: 2017-10-07 Last modified: 2021-06-06
Priority: 3
View all other issues in [func.memfn].
View all issues with Resolved status.
Discussion:
Even after the discussion on the reflector, starting with
this reflector message
it's not completely clear that unspecified as return type
of mem_fn
really means 'unspecified, but always the same'.
The same problem exists for bind()
and not_fn()
.
call_wrapper
object is a
simple call wrapper.
[2017-11 Albuquerque Wednesday night issues processing]
Priority set to 3. Tomasz to write a paper that will address this issue. See also 3015(i)
[2017-11-10, Tomasz comments and provides wording together with STL]
From the core language rules it is already required that same function
template specialization have the same return type. Given that the
invocation of mem_fn
/bind
/not_fn
will always return
the same wrapper type, if they are instantiated (called with) same parameters type.
However, the existence of this issue, shows that some library-wide
clarification note would be welcomed.
[2019-05-12; Tomasz comments]
I have realized that this issue indicates an real problem with the
usability of bind
as the replacement of the binder1st/binder2nd
.
Currently it is not required that a binding functor of the same type with
same argument, produces the same result, as the type of the call wrapper
may depend on the cv ref qualifiers of arguments. For example we are
not requiring that the types of f1, f2, f3, f4
are the same (and actually
they are not for clang):
auto func = [](std::string) {}; std::string s("foo"); auto f1 = std::bind(func, s); auto f2 = std::bind(std::as_const(func), std::as_const(s)); auto f3 = std::bind(func, std::string("bar")); auto f4 = std::bind(std::move(func), std::move(s)); // online link: https://wandbox.org/permlink/dcXJaITMJCnBWt7R
As a consequence, if the user creates a std::vector<decltype(std::bind(func,
std::string(), _2))>
(instead of std::vector<std::binder1st<FuncType,
std::string>>
) he may not be able to store the result of the binding func
with std::string
instance, if an copy of std::string
is made. That leads me
to conclusion that this issue actually require wording change, to provide such
guarantee, and is materially different from LWG 3015(i).
std::bind1st/std::bind2nd
(removed in C++17) to
std::bind
, the user may need to replace std::binder1st/std::binder2nd
with an appropriate decltype
of std::bind
invocation. For example:
FuncType func; std::string s; std::vector<std::binder1st<FuncType>> v; v.push_back(std::bind1st(func, s)); v.push_back(std::bind1st(func, std::string("text"))); needs to be replaced with: std::vector<decltype(std::bind(func, s, _1))> v; v.push_back(std::bind(func, s, _1)); v.push_back(std::bind(func, std::string("text"), _1));
but the last statement is not guaranteed to be well-formed.
Therefore I would like to withdraw my previously suggested wording change. Previous resolution [SUPERSEDED]:This wording is relative to N4700.
After section [expos.only.types] "Exposition-only types" add the following new section:
?.?.?.?.? unspecified types [unspecified.types]
[Note: Whenever the return type of a function template is declared as unspecified, the return type depends only on the template arguments of the specialization. Given the example:template<class T> unspecified f(T);the expressions
f(0)
andf(1)
have the same type. — end note]
[2020-01 Resolved by the adoption of P1065 in Cologne.]
Proposed resolution:
variant
's copies must be deleted instead of disabled via SFINAESection: 22.6.3.2 [variant.ctor] Status: C++20 Submitter: Casey Carter Opened: 2017-10-10 Last modified: 2021-02-25
Priority: Not Prioritized
View other active issues in [variant.ctor].
View all other issues in [variant.ctor].
View all issues with C++20 status.
Discussion:
The specification of variant
's copy constructor and copy assignment
operator require that those functions do not participate in overload resolution
unless certain conditions are satisfied. There is no mechanism in C++ that makes
it possible to prevent a copy constructor or copy assignment operator from
participating in overload resolution. These functions should instead be specified
to be defined as deleted unless the requisite conditions hold, as we did for the
copy constructor and copy assignment operator of optional
in LWG
2756(i).
[ 2017-10-11 Moved to Tentatively Ready after 5 positive votes on c++std-lib. ]
Proposed resolution:
This wording is relative to N4687.
Change 22.6.3.2 [variant.ctor] as indicated:
variant(const variant& w);-6- Effects: If
w
holds a value, initializes thevariant
to hold the same alternative asw
and direct-initializes the contained value withget<j>(w)
, wherej
isw.index()
. Otherwise, initializes thevariant
to not hold a value.-7- Throws: Any exception thrown by direct-initializing any
Ti
for all i.-8- Remarks: This
function shall not participate in overload resolutionconstructor shall be defined as deleted unlessis_copy_constructible_v<Ti>
is true for all i.
Change 22.6.3.4 [variant.assign] as indicated:
variant& operator=(const variant& rhs);[…]
-4- Postconditions:
index() == rhs.index()
.-5- Remarks: This
function shall not participate in overload resolutionoperator shall be defined as deleted unlessis_copy_constructible_v<Ti> && is_copy_assignable_v<Ti>
is true for all i.
pair<Key, T>
, not pair<const Key, T>
Section: 23.4.3.1 [map.overview], 23.4.4.1 [multimap.overview], 23.5.3.1 [unord.map.overview], 23.5.4.1 [unord.multimap.overview] Status: C++20 Submitter: Ville Voutilainen Opened: 2017-10-08 Last modified: 2021-02-25
Priority: 2
View all other issues in [map.overview].
View all issues with C++20 status.
Discussion:
With the deduction guides as specified currently, code like this doesn't work:
map m{pair{1, 1}, {2, 2}, {3, 3}};
Same problem occurs with multimap
, unordered_map
and unordered_multimap
.
The problem is in deduction guides like
template<class Key, class T, class Compare = less<Key>, class Allocator = allocator<pair<const Key, T>>> map(initializer_list<pair<const Key, T>>, Compare = Compare(), Allocator = Allocator()) -> map<Key, T, Compare, Allocator>;
The pair<const Key, T>
is not matched by a pair<int, int>
, because
int
can't match a const Key
. Dropping the const
from the parameter of the
deduction guide makes it work with no loss of functionality.
[2017-11-03, Zhihao Yuan comments]
The fix described here prevents
std::map m2{m0.begin(), m0.end()};
from falling back to direct-non-list-initialization. Treating a uniform initialization with >1 clauses
of the same un-cvref type as std::initializer_list
is the only consistent interpretation I found
so far.
[2017-11 Albuquerque Wednesday night issues processing]
Priority set to 2
[2018-08-23 Batavia Issues processing]
Status to Tentatively Ready
[2018-11, Adopted in San Diego]
Proposed resolution:
This wording is relative to N4687.
Change 23.4.3.1 [map.overview] p3, class template map
synopsis, as indicated:
[…] template<class Key, class T, class Compare = less<Key>, class Allocator = allocator<pair<const Key, T>>> map(initializer_list<pair<constKey, T>>, Compare = Compare(), Allocator = Allocator()) -> map<Key, T, Compare, Allocator>; […] template<class Key, class T, class Allocator> map(initializer_list<pair<constKey, T>>, Allocator) -> map<Key, T, less<Key>, Allocator>; […]
Change 23.4.4.1 [multimap.overview] p3, class template multimap
synopsis, as indicated:
[…] template<class Key, class T, class Compare = less<Key>, class Allocator = allocator<pair<const Key, T>>> multimap(initializer_list<pair<constKey, T>>, Compare = Compare(), Allocator = Allocator()) -> multimap<Key, T, Compare, Allocator>; […] template<class Key, class T, class Allocator> multimap(initializer_list<pair<constKey, T>>, Allocator) -> multimap<Key, T, less<Key>, Allocator>; […]
Change 23.5.3.1 [unord.map.overview] p3, class template unordered_map
synopsis, as indicated:
[…] template<class Key, class T, class Hash = hash<Key>, class Pred = equal_to<Key>, class Allocator = allocator<pair<const Key, T>>> unordered_map(initializer_list<pair<constKey, T>>, typename see below::size_type = see below, Hash = Hash(), Pred = Pred(), Allocator = Allocator()) -> unordered_map<Key, T, Hash, Pred, Allocator>; […] template<class Key, class T, typename Allocator> unordered_map(initializer_list<pair<constKey, T>>, typename see below::size_type, Allocator) -> unordered_map<Key, T, hash<Key>, equal_to<Key>, Allocator>; template<class Key, class T, typename Allocator> unordered_map(initializer_list<pair<constKey, T>>, Allocator) -> unordered_map<Key, T, hash<Key>, equal_to<Key>, Allocator>; template<class Key, class T, class Hash, class Allocator> unordered_map(initializer_list<pair<constKey, T>>, typename see below::size_type, Hash, Allocator) -> unordered_map<Key, T, Hash, equal_to<Key>, Allocator>; […]
Change 23.5.4.1 [unord.multimap.overview] p3, class template unordered_multimap
synopsis, as indicated:
[…] template<class Key, class T, class Hash = hash<Key>, class Pred = equal_to<Key>, class Allocator = allocator<pair<const Key, T>>> unordered_multimap(initializer_list<pair<constKey, T>>, typename see below::size_type = see below, Hash = Hash(), Pred = Pred(), Allocator = Allocator()) -> unordered_multimap<Key, T, Hash, Pred, Allocator>; […] template<class Key, class T, typename Allocator> unordered_multimap(initializer_list<pair<constKey, T>>, typename see below::size_type, Allocator) -> unordered_multimap<Key, T, hash<Key>, equal_to<Key>, Allocator>; template<class Key, class T, typename Allocator> unordered_multimap(initializer_list<pair<constKey, T>>, Allocator) -> unordered_multimap<Key, T, hash<Key>, equal_to<Key>, Allocator>; template<class Key, class T, class Hash, class Allocator> unordered_multimap(initializer_list<pair<constKey, T>>, typename see below::size_type, Hash, Allocator) -> unordered_multimap<Key, T, Hash, equal_to<Key>, Allocator>; […]
filesystem::weakly_canonical
still defined in terms of canonical(p, base)
Section: 31.12.13.40 [fs.op.weakly.canonical] Status: C++20 Submitter: Jonathan Wakely Opened: 2017-10-14 Last modified: 2021-06-06
Priority: 0
View all other issues in [fs.op.weakly.canonical].
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Discussion:
LWG 2956(i) fixed canonical
to no longer use a base path, but weakly_canonical
should have been
changed too:
Effects: Using
status(p)
orstatus(p, ec)
, respectively, to determine existence, return a path composed byoperator/=
from the result of callingcanonical()
without abase
argument and with a […]
Since canonical
doesn't accept a base
argument, it doesn't make sense
to talk about calling it without one.
[ 2017-10-16 Moved to Tentatively Ready after 5 positive votes on c++std-lib. ]
[2018-3-17 Adopted in Jacksonville]
Proposed resolution:
This wording is relative to N4687.
Change [fs.op.weakly_canonical] as indicated:
path weakly_canonical(const path& p); path weakly_canonical(const path& p, error_code& ec);-1- Returns: […]
-2- Effects: Usingstatus(p)
orstatus(p, ec)
, respectively, to determine existence, return a path composed byoperator/=
from the result of callingcanonical()
without awith a path argument composed of the leading elements ofbase
argument andp
that exist, if any, followed by the elements ofp
that do not exist, if any. For the first form,canonical()
is called without anerror_code
argument. For the second form,canonical()
is called with ec as anerror_code
argument, andpath()
is returned at the first error occurrence, if any. […]
const
and non-const
variablesSection: 23.2.2 [container.requirements.general] Status: C++23 Submitter: Jonathan Wakely Opened: 2017-10-17 Last modified: 2023-11-22
Priority: 3
View other active issues in [container.requirements.general].
View all other issues in [container.requirements.general].
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Discussion:
[container.requirements.general] p4 says:
In Tables 83, 84, and 85
X
denotes a container class containing objects of typeT
,a
andb
denote values of typeX
,u
denotes an identifier,r
denotes a non-const
value of typeX
, andrv
denotes a non-const
rvalue of typeX
.
This doesn't say anything about whether a
and b
are allowed to be
const
, or must be non-const
. In fact Table 83 uses them
inconsistently, e.g. the rows for "a = rv
" and "a.swap(b)
" most
certainly require them to be non-const
, but all other uses are valid
for either const
or non-const X
.
[2017-11 Albuquerque Wednesday night issues processing]
Priority set to 3; Jonathan to provide updated wording.
Wording needs adjustment - could use "possibly const values of type X"
Will distinguish between lvalue/rvalue
Previous resolution [SUPERSEDED]:
This wording is relative to N4687.
Change 23.2.2 [container.requirements.general] p4 as indicated:
-4- In Tables 83, 84, and 85
X
denotes a container class containing objects of typeT
,a
andb
denote values of typeX
,u
denotes an identifier,r
and s denotes anon-const
values of typeX
, andrv
denotes a non-const
rvalue of typeX
.Change 23.2.2 [container.requirements.general], Table 83 "Container requirements", as indicated:
Table 83 — Container requirements Expression Return type Operational
semanticsAssertion/note
pre/post-conditionComplexity […]
ar = rvX&
All existing elements
ofare either move
ar
assigned to or
destroyedshall be equal to
ar
the value thatrv
had
before this
assignmentlinear […]
ar.swap(bs)void
exchanges the
contents ofand
ar
bs(Note A) […]
swap(
ar,bs)void
ar.swap(bs)(Note A)
[2020-05-03; Daniel provides alternative wording]
Previous resolution [SUPERSEDED]:
This wording is relative to N4861.
Change 23.2.2 [container.requirements.general] as indicated:
[Drafting note:
The following presentation also transforms the current list into a bullet list as we already have in 23.2.8 [unord.req] p11
It has been decided to replace the symbol
r
bys
, because it is easy to confuse withrv
but means an lvalue instead, and the other container tables use it rarely and for something completely different (iterator value)A separate symbol
v
is introduced to unambigiously distinguish the counterpart of a non-const
rvalue (See 16.4.4.2 [utility.arg.requirements])Two separate symbols
b
andc
represent now "(possiblyconst
) values, while the existing symbola
represents an unspecified value, whose meaning becomes defined when context is provided, e.g. for overloads likebegin()
andend
-4- In Tables 73, 74, and 75:
(4.1) —
X
denotes a container class containing objects of typeT
,(4.2) —
a
anddenotes a valueb
sof typeX
,(4.2) —
b
andc
denote (possiblyconst
) values of typeX
,(4.3) —
i
andj
denote values of type (possiblyconst
)X::iterator
,(4.4) —
u
denotes an identifier,(?.?) —
v
denotes an lvalue of type (possiblyconst
)X
or an rvalue of typeconst X
,(4.5) —
r
s
andt
denotes anon-const
valuelvalues of typeX
, and(4.6) —
rv
denotes a non-const
rvalue of typeX
.Change 23.2.2 [container.requirements.general], Table 73 "Container requirements" [tab:container.req], as indicated:
[Drafting note: The following presentation also moves the copy-assignment expression just before the move-assignment expression]
Table 73: — Container requirements [tab:container.req] Expression Return type Operational
semanticsAssertion/note
pre/post-conditionComplexity […]
X(
av)Preconditions: T
is Cpp17CopyInsertable
intoX
(see below).
Postconditions:.
av == X(av)linear X u(
av);
X u =av;Preconditions: T
is Cpp17CopyInsertable
intoX
(see below).
Postconditions:u ==
.avlinear X u(rv);
X u = rv;Postconditions: u
is equal to the value
thatrv
had before this construction(Note B) t = v
X&
Postconditions: t == v
.linear
at = rvX&
All existing elements
ofare either move
at
assigned to or
destroyedshall be equal to
at
the value thatrv
had
before this
assignmentlinear […]
ac == bconvertible to bool
==
is an equivalence relation.
equal(
ac.begin(),
ac.end(),
b.begin(),
b.end())Preconditions: T
meets the
Cpp17EqualityComparable requirementsConstant if ,
ac.size() != b.size()
linear otherwise
ac != bconvertible to bool
Equivalent to !(
ac == b)linear
at.swap(bs)void
exchanges the
contents ofand
at
bs(Note A) swap(
at,bs)void
at.swap(bs)(Note A) r = a
X&
Postconditions:r == a
.linear
ac.size()size_type
distance(
ac.begin(),ac.end())constant
ac.max_size()size_type
distance(begin(), end())
for the largest
possible containerconstant
ac.empty()convertible to bool
ac.begin() ==ac.end()constant
[2022-04-20; Jonathan rebases the wording on the latest draft]
[2022-09-05; Reflector poll]
Set status to Tentatively Ready after five votes in favour during reflector poll in April 2022.
[2022-11-12 Approved at November 2022 meeting in Kona. Status changed: Voting → WP.]
Proposed resolution:
This wording is relative to N4910.
Change 23.2.2 [container.requirements.general] as indicated:
[Drafting note:
It has been decided to replace the symbol
r
bys
, because it is easy to confuse withrv
but means an lvalue instead, and the other container tables use it rarely and for something completely different (iterator value)A separate symbol
v
is introduced to unambigiously distinguish the counterpart of a non-const
rvalue (See 16.4.4.2 [utility.arg.requirements])Two separate symbols
b
andc
represent now "(possiblyconst
) values, while the existing symbola
represents an unspecified value, whose meaning becomes defined when context is provided, e.g. for overloads likebegin()
andend
-1- In subclause [container.gen.reqmts],
(1.1) —
X
denotes a container class containing objects of typeT
,(1.2) —
a
anddenotes a value of typeb
denote valuesX
,(?.?) —
b
andc
denote values of type (possiblyconst
)X
,(1.3) —
i
andj
denote values of type (possiblyconst
)X::iterator
,(1.4) —
u
denotes an identifier,(?.?) —
v
denotes an lvalue of type (possiblyconst
)X
or an rvalue of typeconst X
,(1.5) —
r
denotes as
andt
denote non-const
valuelvalues of typeX
, and(1.6) —
rv
denotes a non-const
rvalue of typeX
.
Change 23.2.2.2 [container.reqmts] as indicated:
[Drafting note: The following presentation also moves the copy-assignment expression just before the move-assignment expression]
X u(av); X u =av;-12- Preconditions:
T
is Cpp17CopyInsertable intoX
(see below).-13- Postconditions:
u ==
.av-14- Complexity: Linear.
X u(rv); X u = rv;-15- Postconditions:
u
is equal to the value thatrv
had before this construction.-14- Complexity: Linear for
array
and constant for all other standard containers.t = v-?- Result:
X&
.-?- Postconditions:
t == v
.-?- Complexity: Linear.
at = rv-17- Result:
X&
.-18- Effects: All existing elements of
are either move assigned to or destroyed.
at-19- Postconditions:
shall be equal to the value that
atrv
had before this assignment.-20- Complexity: Linear.
[…]
ab.begin()-24- Result:
iterator
;const_iterator
for constantab.-25- Returns: An iterator referring to the first element in the container.
-26- Complexity: Constant.
ab.end()-27- Result:
iterator
;const_iterator
for constantab.-28- Returns: An iterator which is the past-the-end value for the container.
-29- Complexity: Constant.
ab.cbegin()-30- Result:
const_iterator
.-31- Returns:
const_cast<X const&>(
ab).begin()-32- Complexity: Constant.
ab.cend()-33- Result:
const_iterator
.-34- Returns:
const_cast<X const&>(
ab).end()-35- Complexity: Constant.
[…]
ac == b-39- Preconditions:
T
meets the Cpp17EqualityComparable requirements.-40- Result: Convertible to
bool
.-41- Returns:
equal(
.ac.begin(),ac.end(), b.begin(), b.end())[Note 1: The algorithm
equal
is defined in 26.6.13 [alg.equal]. — end note]-42- Complexity: Constant if
, linear otherwise.
ac.size() != b.size()-43- Remarks:
==
is an equivalence relation.ac != b-44- Effects: Equivalent to
!(
.ac == b)at.swap(bs)-45- Result:
void
.-46- Effects: Exchanges the contents of
and
at.
bs-47- Complexity: Linear for
array
and constant for all other standard containers.swap(at,bs)-48- Effects: Equivalent to
at.swap(bs)r = a
-49- Result:X&
.
-50- Postconditions:r == a
.
-51- Complexity: Linear.ac.size()-52- Result:
size_type
.-53- Returns:
distance(
, i.e. the number of elements in the container.ac.begin(),ac.end())-54- Complexity: Constant.
-55- Remarks: The number of elements is defined by the rules of constructors, inserts, and erases.
ac.max_size()-56- Result:
size_type
.-57- Returns:
distance(begin(), end())
for the largest possible container.-58- Complexity: Constant.
ac.empty()-59- Result: Convertible to
bool
.-60- Returns:
ac.begin() ==ac.end())-61- Complexity: Constant.
-62- Remarks: If the container is empty, then
is
ac.empty()true
.
try_lock
?Section: 32.6.6 [thread.lock.algorithm] Status: C++20 Submitter: Jonathan Wakely Opened: 2017-11-07 Last modified: 2021-02-25
Priority: 0
View all other issues in [thread.lock.algorithm].
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Discussion:
32.6.6 [thread.lock.algorithm] says:
"If a call to
try_lock()
fails,unlock()
shall be called for all prior arguments and there shall be no further calls totry_lock()
."
We try to use "shall" for requirements on the user (e.g. as in the previous paragraph) which is absolutely not what is meant here.
[2017-11 Albuquerque Wednesday night issues processing]
Moved to Ready
[2018-3-17 Adopted in Jacksonville]
Proposed resolution:
This wording is relative to N4700.
Change 32.6.6 [thread.lock.algorithm] as indicated:
template <class L1, class L2, class... L3> int try_lock(L1&, L2&, L3&...);-1- Requires: […]
-2- Effects: Callstry_lock()
for each argument in order beginning with the first until all arguments have been processed or a call totry_lock()
fails, either by returningfalse
or by throwing an exception. If a call totry_lock()
fails,unlock()
shall beis called for all prior argumentsand there shall bewith no further calls totry_lock()
. […]template <class L1, class L2, class... L3> void lock(L1&, L2&, L3&...);-4- Requires: […]
-5- Effects: All arguments are locked via a sequence of calls tolock()
,try_lock()
, orunlock()
on each argument. The sequence of callsshalldoes not result in deadlock, but is otherwise unspecified. [Note: A deadlock avoidance algorithm such as try-and-back-off must be used, but the specific algorithm is not specified to avoid over-constraining implementations. — end note] If a call tolock()
ortry_lock()
throws an exception,unlock()
shall beis called for any argument that had been locked by a call tolock()
ortry_lock()
.
Section: 26.8 [alg.sorting] Status: C++20 Submitter: Jonathan Wakely Opened: 2017-11-08 Last modified: 2021-02-25
Priority: 2
View all other issues in [alg.sorting].
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Discussion:
This doesn't compile with any major implementation:
int i[1] = { }; std::stable_sort(i, i, [](int& x, int& y) { return x < y; });
The problem is that the Compare
expects non-const references. We say "It is assumed that
comp
will not apply any non-constant function through the dereferenced iterator" But that
isn't sufficient to forbid the example.
Compare
requirements use
comp(as_const(x), as_const(x))
but that would get very verbose to add to every expression
using comp
.
[2017-11 Albuquerque Wednesday night issues processing]
Priority set to 2; Jonathan to improve the statement of the problem.
[2018-02 David Jones provided this truly awful example:]
#include <algorithm> #include <iostream> #include <vector> struct Base { Base(int value) : v(value) {} friend bool operator<(const Base& l, const Base& r) { return l.v < r.v; } int v; }; struct Derived : public Base { using Base::Base; bool operator<(const Derived& o) /* no const here */ { return v > o.v; } }; int main(void) { std::vector<Base> b = {{1}, {5}, {0}, {3}}; std::vector<Derived> d = {{0}, {1}, {3}, {5}}; std::cout << std::lower_bound(d.begin(), d.end(), 4)->v << std::endl; std::sort(b.begin(), b.end()); for (const auto &x : b) std::cout << x.v << " "; std::cout << std::endl; std::sort(d.begin(), d.end()); for (const auto &x : d) std::cout << x.v << " "; std::cout << std::endl; } libc++: ===== $ bin/clang++ -std=c++11 -stdlib=libc++ tmp/ex.cc && ./a.out 5 0 1 3 5 0 1 3 5 ===== libstdc++: ===== $ bin/clang++ -std=c++11 -stdlib=libstdc++ tmp/ex.cc && ./a.out 0 0 1 3 5 5 3 1 0 =====
[2018-08 Batavia Monday issue discussion]
Tim to provide wording; status to 'Open'
[ 2018-08-20, Tim adds P/R based on Batavia discussion.]
Similar to the Ranges TS design, the P/R below requires Predicate
,
BinaryPredicate
, and Compare
to accept all mixes of
const
and non-const
arguments.
[2018-08-23 Batavia Issues processing]
Status to Tentatively Ready after minor wording nit (corrected in place)
[2018-11, Adopted in San Diego]
Proposed resolution:
This wording is relative to N4762.
Edit 26.2 [algorithms.requirements] p6-7 as indicated:
-6- The
-7- ThePredicate
parameter is used whenever an algorithm expects a function object (22.10 [function.objects]) that, when applied to the result of dereferencing the corresponding iterator, returns a value testable astrue
. In other words, if an algorithm takesPredicate pred
as its argument andfirst
as its iterator argument with value typeT
, it should work correctly in the constructpred(*first)
contextually converted tobool
(7.3 [conv]). The function objectpred
shall not apply any non-constant function through the dereferenced iterator. Given a glvalueu
of type (possiblyconst
)T
that designates the same object as*first
,pred(u)
shall be a valid expression that is equal topred(*first)
.BinaryPredicate
parameter is used whenever an algorithm expects a function object that when applied to the result of dereferencing two corresponding iterators or to dereferencing an iterator and typeT
whenT
is part of the signature returns a value testable astrue
. In other words, if an algorithm takesBinaryPredicate binary_pred
as its argument andfirst1
andfirst2
as its iterator arguments with respective value typesT1
andT2
, it should work correctly in the constructbinary_pred(*first1, *first2)
contextually converted tobool
(7.3 [conv]). Unless otherwise specified,BinaryPredicate
always takes the first iterator'svalue_type
as its first argument, that is, in those cases whenT value
is part of the signature, it should work correctly in the constructbinary_pred(*first1, value)
contextually converted tobool
(7.3 [conv]).binary_pred
shall not apply any non-constant function through the dereferenced iterators. Given a glvalueu
of type (possiblyconst
)T1
that designates the same object as*first1
, and a glvaluev
of type (possiblyconst
)T2
that designates the same object as*first2
,binary_pred(u, *first2)
,binary_pred(*first1, v)
, andbinary_pred(u, v)
shall each be a valid expression that is equal tobinary_pred(*first1, *first2)
, andbinary_pred(u, value)
shall be a valid expression that is equal tobinary_pred(*first1, value)
.
Edit 26.8 [alg.sorting] p2 as indicated:
Compare
is a function object type (22.10 [function.objects]) that meets the requirements for a template parameter namedBinaryPredicate
(26.2 [algorithms.requirements]). The return value of the function call operation applied to an object of typeCompare
, when contextually converted tobool
(7.3 [conv]), yieldstrue
if the first argument of the call is less than the second, andfalse
otherwise.Compare comp
is used throughout for algorithms assuming an ordering relation.It is assumed thatcomp
will not apply any non-constant function through the dereferenced iterator.
ValueSwappable
requirement missing for push_heap
and make_heap
Section: 26.8.8 [alg.heap.operations] Status: C++23 Submitter: Robert Douglas Opened: 2017-11-08 Last modified: 2023-11-22
Priority: 3
View all other issues in [alg.heap.operations].
View all issues with C++23 status.
Discussion:
In discussion of D0202R3 in Albuquerque, it was observed that pop_heap
and sort_heap
had
constexpr
removed for their requirement of ValueSwappable
. It was then observed that
push_heap
and make_heap
were not similarly marked as having the ValueSwappable
requirement.
The room believed this was likely a specification error, and asked to open an issue to track it.
[2017-11 Albuquerque Wednesday night issues processing]
Priority set to 3; Marshall to investigate
Previous resolution [SUPERSEDED]:
This wording is relative to N4700.
Change 26.8.8.2 [push.heap] as indicated:
template<class RandomAccessIterator> void push_heap(RandomAccessIterator first, RandomAccessIterator last); template<class RandomAccessIterator, class Compare> void push_heap(RandomAccessIterator first, RandomAccessIterator last, Compare comp);-1- Requires: The range
[first, last - 1)
shall be a valid heap.RandomAccessIterator
shall satisfy the requirements ofValueSwappable
(16.4.4.3 [swappable.requirements]). The type of*first
shall satisfy theMoveConstructible
requirements (Table 23) and theMoveAssignable
requirements (Table 25).Change 26.8.8.4 [make.heap] as indicated:
template<class RandomAccessIterator> void make_heap(RandomAccessIterator first, RandomAccessIterator last); template<class RandomAccessIterator, class Compare> void make_heap(RandomAccessIterator first, RandomAccessIterator last, Compare comp);-1- Requires:
RandomAccessIterator
shall satisfy the requirements ofValueSwappable
(16.4.4.3 [swappable.requirements]). The type of*first
shall satisfy theMoveConstructible
requirements (Table 23) and theMoveAssignable
requirements (Table 25).
[2022-11-06; Daniel comments and syncs wording with recent working draft]
For reference, the finally accepted paper was P0202R3 and the constexpr-ification of swap-related algorithms had been realized later by P0879R0 after resolution of CWG 1581 and more importantly CWG 1330.
[Kona 2022-11-09; Move to Ready]
[2023-02-13 Approved at February 2023 meeting in Issaquah. Status changed: Voting → WP.]
Proposed resolution:
This wording is relative to N4917.
Change 26.8.8.2 [push.heap] as indicated:
template<class RandomAccessIterator> constexpr void push_heap(RandomAccessIterator first, RandomAccessIterator last); template<class RandomAccessIterator, class Compare> constexpr void push_heap(RandomAccessIterator first, RandomAccessIterator last, Compare comp); template<random_access_iterator I, sentinel_for<I> S, class Comp = ranges::less, class Proj = identity> requires sortable<I, Comp, Proj> constexpr I ranges::push_heap(I first, S last, Comp comp = {}, Proj proj = {}); template<random_access_range R, class Comp = ranges::less, class Proj = identity> requires sortable<iterator_t<R>, Comp, Proj> constexpr borrowed_iterator_t<R> ranges::push_heap(R&& r, Comp comp = {}, Proj proj = {});-1- Let
-2- Preconditions: The rangecomp
beless{}
andproj
beidentity{}
for the overloads with no parameters by those names.[first, last - 1)
is a valid heap with respect tocomp
andproj
. For the overloads in namespacestd
,RandomAccessIterator
meets the Cpp17ValueSwappable requirements (16.4.4.3 [swappable.requirements]) and the type of*first
meets the Cpp17MoveConstructible requirements (Table 32) and the Cpp17MoveAssignable requirements (Table 34). -3- Effects: Places the value in the locationlast - 1
into the resulting heap[first, last)
. -4- Returns:last
for the overloads in namespaceranges
. -5- Complexity: At most log(last - first
) comparisons and twice as many projections.
Change 26.8.8.4 [make.heap] as indicated:
template<class RandomAccessIterator> constexpr void make_heap(RandomAccessIterator first, RandomAccessIterator last); template<class RandomAccessIterator, class Compare> constexpr void make_heap(RandomAccessIterator first, RandomAccessIterator last, Compare comp); template<random_access_iterator I, sentinel_for<I> S, class Comp = ranges::less, class Proj = identity> requires sortable<I, Comp, Proj> constexpr I ranges::make_heap(I first, S last, Comp comp = {}, Proj proj = {}); template<random_access_range R, class Comp = ranges::less, class Proj = identity> requires sortable<iterator_t<R>, Comp, Proj> constexpr borrowed_iterator_t<R> ranges::make_heap(R&& r, Comp comp = {}, Proj proj = {});-1- Let
-2- Preconditions: For the overloads in namespacecomp
beless{}
andproj
beidentity{}
for the overloads with no parameters by those names.std
,RandomAccessIterator
meets the Cpp17ValueSwappable requirements (16.4.4.3 [swappable.requirements]) and the type of*first
meets the Cpp17MoveConstructible (Table 32) and Cpp17MoveAssignable (Table 34) requirements. -3- Effects: Constructs a heap with respect tocomp
andproj
out of the range[first, last)
. -4- Returns:last
for the overloads in namespaceranges
. -5- Complexity: At most 3(last - first
) comparisons and twice as many projections.
Section: 21.3.8.7 [meta.trans.other] Status: C++20 Submitter: Casey Carter Opened: 2017-11-12 Last modified: 2021-02-25
Priority: 0
View all other issues in [meta.trans.other].
View all issues with C++20 status.
Discussion:
P0767R1 "Expunge POD" changed the requirement for several library types from "POD" to "trivial." Since these types no longer provide/require the standard-layout portion of "POD," the change breaks:
basic_string
and basic_string_view
that
expect character types to be both trivial and standard layout.The fix is straight-forward: apply an additional standard-layout requirement to the affected types:
max_align_t
type
member of specializations of aligned_storage
type
member of specializations of aligned_union
basic_string
and basic_string_view
.max_align_t
is admittedly small.)
[ 2017-11-14 Moved to Tentatively Ready after 8 positive votes for P0 on c++std-lib. ]
[2018-3-17 Adopted in Jacksonville]
Proposed resolution:
This wording is relative to N4700 + P0767R1.
Change in 17.2.4 [support.types.layout] paragraph 5:
The type max_align_t
is a trivial standard-layout type whose
alignment requirement is at least as great as that of every scalar type, and whose
alignment requirement is supported in every context (6.7.3 [basic.align]).
Change the table in 21.3.8.7 [meta.trans.other] as indicated:
aligned_storage
The member typedef type
shall be a trivial standard-layout
type suitable for use as uninitialized storage for any object whose size is at
most Len
and whose alignment is a divisor of Align
.
aligned_union
The member typedef type
shall be a trivial standard-layout
type suitable for use as uninitialized storage for any object whose type is listed
in Types
; its size shall be at least Len
.
Change 27.1 [strings.general] paragraph 1 as indicated:
This Clause describes components for manipulating sequences of any non-array trivial standard-layout (6.8 [basic.types]) type. Such types are called char-like types, and objects of char-like types are called char-like objects or simply characters.
std::allocator
's constructors should be constexpr
Section: 20.2.10 [default.allocator] Status: C++20 Submitter: Geoffrey Romer Opened: 2017-11-11 Last modified: 2021-02-25
Priority: 0
View other active issues in [default.allocator].
View all other issues in [default.allocator].
View all issues with C++20 status.
Discussion:
std::allocator
's constructors should be constexpr
. It's expected to be an empty class as far as I know,
so this should impose no implementation burden, and it would be useful to permit guaranteed static initialization of
objects that need to hold a std::allocator
, but don't have to actually use it until after construction.
[ 2017-11-25 Moved to Tentatively Ready after 7 positive votes for P0 on c++std-lib. ]
[2018-3-17 Adopted in Jacksonville]
Proposed resolution:
This wording is relative to N4700.
Change in 20.2.10 [default.allocator] as indicated:
namespace std { template <class T> class allocator { public: using value_type = T; using propagate_on_container_move_assignment = true_type; using is_always_equal = true_type; constexpr allocator() noexcept; constexpr allocator(const allocator&) noexcept; constexpr template <class U> allocator(const allocator<U>&) noexcept; ~allocator(); T* allocate(size_t n); void deallocate(T* p, size_t n); }; }
polymorphic_allocator::destroy
is extraneousSection: 20.5.3 [mem.poly.allocator.class] Status: C++23 Submitter: Casey Carter Opened: 2017-11-15 Last modified: 2023-11-22
Priority: 3
View all other issues in [mem.poly.allocator.class].
View all issues with C++23 status.
Discussion:
polymorphic_allocator
's member function destroy
is exactly
equivalent to the default implementation of destroy
in
allocator_traits
(20.2.9.3 [allocator.traits.members] para 6). It
should be struck from polymorphic_allocator
as it provides no value.
[28-Nov-2017 Mailing list discussion - set priority to P3]
PJ says that Dinkumware is shipping an implementation of polymorphic_allocator
with destroy
, so removing it would be a breaking change for him.
[2019-02; Kona Wednesday night issue processing]
Status to Open; revisit once P0339 lands. Poll taken was 5-3-2 in favor of removal.
[2020-10-05; Jonathan provides new wording]
Previous resolution [SUPERSEDED]:
Wording relative to N4700.
Strike the declaration of
destroy
from the synopsis of classpolymorphic_allocator
in 20.5.3 [mem.poly.allocator.class]:template <class T1, class T2, class U, class V> void construct(pair<T1,T2>* p, pair<U, V>&& pr);template <class T>void destroy(T* p);polymorphic_allocator select_on_container_copy_construction() const;Strike the specification of
destroy
in 20.5.3.3 [mem.poly.allocator.mem]:[…]template <class T>void destroy(T* p);[…]
14 Effects: As if byp->~T()
.
[2020-10-11; Reflector poll]
Moved to Tentatively Ready after seven votes in favour.
[2020-11-09 Approved In November virtual meeting. Status changed: Tentatively Ready → WP.]
Proposed resolution:
Wording relative to N4861.
Strike the declaration of destroy
from the synopsis of class
polymorphic_allocator
in 20.5.3 [mem.poly.allocator.class]:
template <class T1, class T2, class U, class V> void construct(pair<T1,T2>* p, pair<U, V>&& pr);template <class T>void destroy(T* p);polymorphic_allocator select_on_container_copy_construction() const;
Adjust the specification of delete_object
in 20.5.3.3 [mem.poly.allocator.mem]:
template <class T> void delete_object(T* p);-13- Effects: Equivalent to:
allocator_traits<polymorphic_allocator>::destroy(*this, p); deallocate_object(p);
Strike the specification of destroy
in 20.5.3.3 [mem.poly.allocator.mem]:
[…]template <class T>void destroy(T* p);[…]
-17- Effects: As if byp->~T()
.
Add a new subclause to Annex D:
D.?? Deprecated
polymorphic_allocator
member function-1- The following member is declared in addition to those members specified in 20.5.3.3 [mem.poly.allocator.mem]:
namespace std::pmr { template<class Tp = byte> class polymorphic_allocator { public: template <class T> void destroy(T* p); }; }template <class T> void destroy(T* p);-1- Effects: As if by
p->~T()
.
polymorphic_allocator
and incomplete typesSection: 20.5.3 [mem.poly.allocator.class] Status: C++20 Submitter: Casey Carter Opened: 2017-11-15 Last modified: 2021-02-25
Priority: 2
View all other issues in [mem.poly.allocator.class].
View all issues with C++20 status.
Discussion:
polymorphic_allocator
can trivially support the allocator completeness
requirements (16.4.4.6.2 [allocator.requirements.completeness]) just as does
the default allocator. Doing so imposes no implementation burden, and enables
pmr::forward_list
, pmr::list
, and pmr::vector
to
support incomplete types as do the non-pmr
equivalents.
[2018-01; Priority set to 2 after mailing list discussion]
[2018-08-23 Batavia Issues processing]
Status to Tentatively Ready
[2018-11, Adopted in San Diego]
Proposed resolution:
Wording relative to N4700.
Add a new paragraph in 20.5.3 [mem.poly.allocator.class] after para 1:
1 A specialization of class template
pmr::polymorphic_allocator
conforms to theAllocator
requirements [...]-?- All specializations of class template
pmr::polymorphic_allocator
satisfy the allocator completeness requirements (16.4.4.6.2 [allocator.requirements.completeness]).
polymorphic_allocator::allocate
should not allow integer overflow to create vulnerabilitiesSection: 20.5.3.3 [mem.poly.allocator.mem] Status: C++20 Submitter: Billy O'Neal III Opened: 2017-11-16 Last modified: 2021-02-25
Priority: 2
View all other issues in [mem.poly.allocator.mem].
View all issues with C++20 status.
Discussion:
At the moment polymorphic_allocator
is specified to do sizeof(T) * n
directly; this may allow an attacker
to cause this calculation to overflow, resulting in allocate()
not meeting its postcondition of returning a buffer
suitable to store n
copies of T
; this is a common bug described in
CWE-190.
memory_resource
underneath polymorphic_allocator
is going to have to throw bad_alloc
(or another exception) for a request
of SIZE_MAX
.
(There's also a minor editorial thing here that Returns should be Effects)
[2018-06 Rapperswil Thursday issues processing]
Consensus was that the overflow should be detected and an exception thrown rather than leaving that to the underlying memory resource. Billy to reword, and then get feedback on the reflector. Status to Open.
Previous resolution [SUPERSEDED]:
Wording relative to N4700.
Edit 20.5.3.3 [mem.poly.allocator.mem] as indicated:
Tp* allocate(size_t n);-1-
ReturnsEffects: Equivalent toreturn static_cast<Tp*>(memory_rsrc->allocate(SIZE_MAX / sizeof(Tp) < n ? SIZE_MAX : n * sizeof(Tp), alignof(Tp)));
[2018-08-23 Batavia Issues processing]
Status to Tentatively Ready with updated wording
Previous resolution [SUPERSEDED]:
Wording relative to N4762.
Edit 20.5.3.3 [mem.poly.allocator.mem] as indicated:
Tp* allocate(size_t n);-1- Effects: If
SIZE_MAX / sizeof(Tp) < n
, throwslength_error
, thenEequivalent to:return static_cast<Tp*>(memory_rsrc->allocate(n * sizeof(Tp), alignof(Tp)));
[2018-11, Adopted in San Diego]
Proposed resolution:
Wording relative to N4762.
Edit 20.5.3.3 [mem.poly.allocator.mem] as indicated:
Tp* allocate(size_t n);-1- Effects: If
SIZE_MAX / sizeof(Tp) < n
, throwslength_error
. OtherwiseEequivalent to:return static_cast<Tp*>(memory_rsrc->allocate(n * sizeof(Tp), alignof(Tp)));
decay
in thread
and packaged_task
Section: 32.4.3.3 [thread.thread.constr], 32.10.10.2 [futures.task.members] Status: C++20 Submitter: Stephan T. Lavavej Opened: 2017-11-17 Last modified: 2021-02-25
Priority: 0
View all other issues in [thread.thread.constr].
View all issues with C++20 status.
Discussion:
Following P0777R1 "Treating Unnecessary decay", more occurrences can be fixed.
When constraints are checking for the same type as thread
or a specialization of packaged_task
,
decaying functions to function pointers and arrays to object pointers can't affect the result.
[28-Nov-2017 Moved to Tentatively Ready after five positive votes on the ML.]
[2018-3-17 Adopted in Jacksonville]
Proposed resolution:
Wording relative to N4700.
Edit 32.4.3.3 [thread.thread.constr] as indicated:
template <class F, class... Args> explicit thread(F&& f, Args&&... args);-3- Requires: […]
-4- Remarks: This constructor shall not participate in overload resolution ifis the same type as
decay_tremove_cvref_t<F>std::thread
.
Edit 32.10.10.2 [futures.task.members] as indicated:
template <class F> packaged_task(F&& f);-2- Requires: […]
-3- Remarks: This constructor shall not participate in overload resolution ifis the same type as
decay_tremove_cvref_t<F>packaged_task<R(ArgTypes...)>
.
basic_string_view::starts_with
Effects are incorrectSection: 27.3.3.8 [string.view.ops] Status: C++20 Submitter: Marshall Clow Opened: 2017-11-29 Last modified: 2021-02-25
Priority: 0
View all other issues in [string.view.ops].
View all issues with C++20 status.
Discussion:
The effects of starts_with
are described as equivalent to return compare(0, npos, x) == 0
.
false
when you check to see if any sequence begins with the empty sequence.
(There are other failure cases, but that one's easy)
As a drive-by fix, we can make the Effects: for starts_with
and ends_with
clearer.
Those are the second and proposed third changes, and they are not required.
[ 2017-12-13 Moved to Tentatively Ready after 8 positive votes for P0 on c++std-lib. ]
Previous resolution: [SUPERSEDED]This wording is relative to N4713.
Change 27.3.3.8 [string.view.ops] p20 as indicated:
constexpr bool starts_with(basic_string_view x) const noexcept;-20- Effects: Equivalent to:
return size() >= x.size() && compare(0,
nposx.size(), x) == 0;Change 27.3.3.8 [string.view.ops] p21 as indicated:
constexpr bool starts_with(charT x) const noexcept;-21- Effects: Equivalent to:
return !empty() && traits::eq(front(), x)
starts_with(basic_string_view(&x, 1));Change 27.3.3.8 [string.view.ops] p24 as indicated:
constexpr bool ends_with(charT x) const noexcept;-24- Effects: Equivalent to:
return !empty() && traits::eq(back(), x)
ends_with(basic_string_view(&x, 1));
[2018-01-23, Reopening due to a comment of Billy Robert O'Neal III requesting a change of the proposed wording]
The currently suggested wording has:
Effects: Equivalent to:
return size() >= x.size() && compare(0, x.size(), x) == 0;
but compare()
already does the size() >= x.size()
check.
Effects: Equivalent to:
return substr(0, x.size()) == x;
[ 2018-10-29 Moved to Tentatively Ready after 5 positive votes for P0 on c++std-lib. ]
Proposed resolution:
This wording is relative to N4713.
Change 27.3.3.8 [string.view.ops] p20 as indicated:
constexpr bool starts_with(basic_string_view x) const noexcept;-20- Effects: Equivalent to:
return substr(0, x.size()) == x
compare(0, npos, x) == 0;
Change 27.3.3.8 [string.view.ops] p21 as indicated:
constexpr bool starts_with(charT x) const noexcept;-21- Effects: Equivalent to:
return !empty() && traits::eq(front(), x)
starts_with(basic_string_view(&x, 1));
Change 27.3.3.8 [string.view.ops] p24 as indicated:
constexpr bool ends_with(charT x) const noexcept;-24- Effects: Equivalent to:
return !empty() && traits::eq(back(), x)
ends_with(basic_string_view(&x, 1));
decay
in reference_wrapper
Section: 22.10.6.2 [refwrap.const] Status: C++20 Submitter: Agustín K-ballo Bergé Opened: 2017-12-04 Last modified: 2021-02-25
Priority: 0
View all other issues in [refwrap.const].
View all issues with C++20 status.
Discussion:
Another occurrence of unnecessary decay (P0777) was introduced by the
resolution of LWG 2993(i). A decayed function/array type will never be the same type as
reference_wrapper
.
[ 2018-01-09 Moved to Tentatively Ready after 9 positive votes on c++std-lib. ]
[2018-3-17 Adopted in Jacksonville]
Proposed resolution:
This wording is relative to N4713.
Change 22.10.6.2 [refwrap.const] as indicated:
template<class U> reference_wrapper(U&& u) noexcept(see below);-1- Remarks: Let
FUN
denote the exposition-only functionsvoid FUN(T&) noexcept; void FUN(T&&) = delete;This constructor shall not participate in overload resolution unless the expression
FUN(declval<U>())
is well-formed andis_same_v<
isdecay_tremove_cvref_t<U>, reference_wrapper>false
. The expression insidenoexcept
is equivalent tonoexcept(FUN(declval<U>()))
.
is_literal_type_v
should be inlineSection: D.13 [depr.meta.types] Status: C++20 Submitter: Tim Song Opened: 2017-12-06 Last modified: 2021-02-25
Priority: 0
View all other issues in [depr.meta.types].
View all issues with C++20 status.
Discussion:
P0607R0 forgot to look at D.13 [depr.meta.types] and make is_literal_type_v
inline.
[ 2018-01-08 Moved to Tentatively Ready after 8 positive votes on c++std-lib. ]
[2018-3-17 Adopted in Jacksonville]
Proposed resolution:
This wording is relative to N4713.
Change D.13 [depr.meta.types]p1 as indicated:
-1- The header <type_traits>
has the following addition:
namespace std { template<class T> struct is_literal_type; template<class T> inline constexpr bool is_literal_type_v = is_literal_type<T>::value; template<class> struct result_of; // not defined template<class Fn, class... ArgTypes> struct result_of<Fn(ArgTypes...)>; template<class T> using result_of_t = typename result_of<T>::type; template<class T> struct is_pod; template<class T> inline constexpr bool is_pod_v = is_pod<T>::value; }
filesystem_error
constructorSection: 31.12.7.2 [fs.filesystem.error.members] Status: C++20 Submitter: Tim Song Opened: 2017-12-06 Last modified: 2021-06-06
Priority: 0
View all other issues in [fs.filesystem.error.members].
View all issues with C++20 status.
Discussion:
[fs.filesystem_error.members] says that constructors of
filesystem_error
have the postcondition that runtime_error::what()
has the
value what_arg.c_str()
. That's obviously incorrect: these are pointers
to distinct copies of the string in any sane implementation and cannot
possibly compare equal.
runtime_error::what()
, but filesystem_error
has no direct control over the construction of its indirect
non-virtual base class runtime_error
. Instead, what is passed to runtime_error
's
constructor is determined by system_error
's constructor, which in many implementations
is an eagerly crafted error string. This is permitted by the specification of
system_error
(see 19.5.8 [syserr.syserr]) but would make the requirement unimplementable.
The proposed wording below adjusts the postcondition using the formula of system_error
's
constructor. As an editorial change, it also replaces the postcondition tables with normal postcondition clauses,
in the spirit of editorial issue 1875.
[ 2018-01-12 Moved to Tentatively Ready after 5 positive votes on c++std-lib. ]
[2018-3-17 Adopted in Jacksonville]
Proposed resolution:
This wording is relative to N4713.
Replace [fs.filesystem_error.members] p2-4, including Tables 119 through 121, with the following:
filesystem_error(const string& what_arg, error_code ec);
-2- Postconditions:code() == ec
,path1().empty() == true
,path2().empty() == true
, andstring_view(what()).find(what_arg) != string_view::npos
.
filesystem_error(const string& what_arg, const path& p1, error_code ec);
-3- Postconditions:code() == ec
,path1()
returns a reference to the stored copy ofp1
,path2().empty() == true
, andstring_view(what()).find(what_arg) != string_view::npos
.
filesystem_error(const string& what_arg, const path& p1, const path& p2, error_code ec);
-4- Postconditions:code() == ec
,path1()
returns a reference to the stored copy ofp1
,path2()
returns a reference to the stored copy ofp2
, andstring_view(what()).find(what_arg) != string_view::npos
.
Edit [fs.filesystem_error.members] p7 as indicated:
const char* what() const noexcept override;
-7- Returns:A string containingAn ntbs that incorporates theruntime_error::what()
.what_arg
argument supplied to the constructor. The exact format is unspecified. Implementations should include thesystem_error::what()
string and the pathnames ofpath1
andpath2
in the native format in the returned string.
atomic<floating-point>
doesn't have value_type
or difference_type
Section: 32.5.8.4 [atomics.types.float] Status: C++20 Submitter: Tim Song Opened: 2017-12-11 Last modified: 2021-02-25
Priority: 0
View all issues with C++20 status.
Discussion:
The atomic<floating-point>
specialization doesn't have the value_type
and difference_type
member typedefs, making it
unusable with most of the nonmember function templates. This doesn't seem to be the intent.
[ 2018-01-09 Moved to Tentatively Ready after 6 positive votes on c++std-lib. ]
[2018-3-17 Adopted in Jacksonville]
Proposed resolution:
This wording is relative to N4713.
Edit 32.5.8.4 [atomics.types.float] after p1, class template specialization atomic<floating-point>
synopsis, as indicated:
namespace std { template<> struct atomic<floating-point> { using value_type = floating-point; using difference_type = value_type; static constexpr bool is_always_lock_free = implementation-defined; […] }; }
transform_reduce(exec, first1, last1, first2, init)
discards execution policySection: 26.10.6 [transform.reduce] Status: C++20 Submitter: Billy Robert O'Neal III Opened: 2017-12-15 Last modified: 2021-02-25
Priority: 0
View all issues with C++20 status.
Discussion:
Since there exists only one common Effects element for both the parallel and the non-parallel form of
transform_reduce
without explicit operation parameters, the current specification of a function call
std::transform_reduce(exec, first1, last1, first2, init)
has the same effect as if the
ExecutionPolicy
would have been ignored. Presumably this effect is unintended.
[ 2018-01-15 Moved to Tentatively Ready after 5 positive votes on c++std-lib. ]
[2018-3-17 Adopted in Jacksonville]
Proposed resolution:
This wording is relative to N4713.
Modify 26.10.6 [transform.reduce] as indicated:
template<class InputIterator1, class InputIterator2, class T> T transform_reduce(InputIterator1 first1, InputIterator1 last1, InputIterator2 first2, T init);-?- Effects: Equivalent to:
return transform_reduce(first1, last1, first2, init, plus<>(), multiplies<>());template<class ExecutionPolicy, class ForwardIterator1, class ForwardIterator2, class T> T transform_reduce(ExecutionPolicy&& exec, ForwardIterator1 first1, ForwardIterator1 last1, ForwardIterator2 first2, T init);-1- Effects: Equivalent to:
return transform_reduce(std::forward<ExecutionPolicy>(exec), first1, last1, first2, init, plus<>(), multiplies<>());
chrono::duration
constructorSection: 30.5.2 [time.duration.cons], 30.5.6 [time.duration.nonmember] Status: C++20 Submitter: Barry Revzin Opened: 2018-01-22 Last modified: 2021-02-25
Priority: 3
View all other issues in [time.duration.cons].
View all issues with C++20 status.
Discussion:
The converting constructor in std::chrono::duration
is currently specified as
(30.5.2 [time.duration.cons] p1):
template<class Rep2> constexpr explicit duration(const Rep2& r);Remarks: This constructor shall not participate in overload resolution unless
Rep2
is implicitly convertible torep
and […]
But the parameter is of type Rep2 const
, so we should check that Rep2 const
is
implicitly convertible to rep
, not just Rep2
. This means that for a type like:
struct X { operator int64_t() /* not const */; };
std::is_constructible_v<std::chrono::seconds, X>
is true
, but actual
construction will fail to compile.
[2018-06 Rapperswil Thursday issues processing]
P3; Status to Open
[2018-11 San Diego Thursday night issue processing]
Jonathan to provide updated wording; the underlying text has changed.
[2018-12-05 Jonathan provides new wording]
In San Diego Geoff noticed that the current WP does not use CR
.
Jonathan provides new wording consistent with the
editorial changes
that removed CR
.
Previous resolution [SUPERSEDED]:This wording is relative to N4713.
Modify 30.5.2 [time.duration.cons] as indicated:
template<class Rep2> constexpr explicit duration(const Rep2& r);-1- Remarks: This constructor shall not participate in overload resolution unless
is_convertible_v<const Rep2&, rep>
istrue
andRep2
is implicitly convertible torep
[…] -2- Effects: Constructs an object of type
(1.1) —
treat_as_floating_point_v<rep>
istrue
or(1.2) —
treat_as_floating_point_v<Rep2>
isfalse
.duration
. -3- Postconditions:count() == static_cast<rep>(r)
.Modify 30.5.6 [time.duration.nonmember] as indicated:
template<class Rep1, class Period, class Rep2> constexpr duration<common_type_t<Rep1, Rep2>, Period> operator*(const duration<Rep1, Period>& d, const Rep2& s);-4- Remarks: This operator shall not participate in overload resolution unless
[…]is_convertible_v<const Rep2&, CR(Rep1, Rep2)>
istrue
.Rep2
is implicitly convertible toCR(Rep1, Rep2)
template<class Rep1, class Rep2, class Period> constexpr duration<common_type_t<Rep1, Rep2>, Period> operator*(const Rep1& s, const duration<Rep2, Period>& d);-6- Remarks: This operator shall not participate in overload resolution unless
[…]is_convertible_v<const Rep1&, CR(Rep1, Rep2)>
istrue
.Rep1
is implicitly convertible toCR(Rep1, Rep2)
template<class Rep1, class Period, class Rep2> constexpr duration<common_type_t<Rep1, Rep2>, Period> operator/(const duration<Rep1, Period>& d, const Rep2& s);[…]-8- Remarks: This operator shall not participate in overload resolution unless
is_convertible_v<const Rep2&, CR(Rep1, Rep2)>
istrue
andRep2
is implicitly convertible toCR(Rep1, Rep2)
Rep2
is not a specialization ofduration
.template<class Rep1, class Period, class Rep2> constexpr duration<common_type_t<Rep1, Rep2>, Period> operator%(const duration<Rep1, Period>& d, const Rep2& s);-11- Remarks: This operator shall not participate in overload resolution unless
[…]is_convertible_v<const Rep2&, CR(Rep1, Rep2)>
istrue
andRep2
is implicitly convertible toCR(Rep1, Rep2)
Rep2
is not a specialization ofduration
.
This wording is relative to N4791.
Modify 30.5.2 [time.duration.cons] as indicated:
template<class Rep2> constexpr explicit duration(const Rep2& r);-1- Remarks: This constructor shall not participate in overload resolution unless
is_convertible_v<const Rep2&, rep>
istrue
andRep2
is implicitly convertible torep
[…] -2- Effects: Constructs an object of type
(1.1) —
treat_as_floating_point_v<rep>
istrue
or(1.2) —
treat_as_floating_point_v<Rep2>
isfalse
.duration
. -3- Postconditions:count() == static_cast<rep>(r)
.Modify 30.5.6 [time.duration.nonmember] as indicated:
template<class Rep1, class Period, class Rep2> constexpr duration<common_type_t<Rep1, Rep2>, Period> operator*(const duration<Rep1, Period>& d, const Rep2& s);-4- Remarks: This operator shall not participate in overload resolution unless
[…]is_convertible_v<const Rep2&, common_type_t<Rep1, Rep2>>
istrue
.Rep2
is implicitly convertible tocommon_type_t<Rep1, Rep2>
template<class Rep1, class Rep2, class Period> constexpr duration<common_type_t<Rep1, Rep2>, Period> operator*(const Rep1& s, const duration<Rep2, Period>& d);-6- Remarks: This operator shall not participate in overload resolution unless
[…]is_convertible_v<const Rep1&, common_type_t<Rep1, Rep2>>
istrue
.Rep1
is implicitly convertible tocommon_type_t<Rep1, Rep2>
template<class Rep1, class Period, class Rep2> constexpr duration<common_type_t<Rep1, Rep2>, Period> operator/(const duration<Rep1, Period>& d, const Rep2& s);[…]-8- Remarks: This operator shall not participate in overload resolution unless
is_convertible_v<const Rep2&, common_type_t<Rep1, Rep2>>
istrue
andRep2
is implicitly convertible tocommon_type_t<Rep1, Rep2>
Rep2
is not a specialization ofduration
.template<class Rep1, class Period, class Rep2> constexpr duration<common_type_t<Rep1, Rep2>, Period> operator%(const duration<Rep1, Period>& d, const Rep2& s);-11- Remarks: This operator shall not participate in overload resolution unless
[…]is_convertible_v<const Rep2&, common_type_t<Rep1, Rep2>>
istrue
andRep2
is implicitly convertible tocommon_type_t<Rep1, Rep2>
Rep2
is not a specialization ofduration
.
[2020-02-13, Prague]
Rebase to most recent working draft
[2020-02 Status to Immediate on Thursday night in Prague.]
Proposed resolution:
This wording is relative to N4849.
Modify 30.5.2 [time.duration.cons] as indicated:
template<class Rep2> constexpr explicit duration(const Rep2& r);-1- Constraints:
is_convertible_v<const Rep2&, rep>
istrue
and[…] -2- Postconditions:
(1.1) —
treat_as_floating_point_v<rep>
istrue
or(1.2) —
treat_as_floating_point_v<Rep2>
isfalse
.count() == static_cast<rep>(r)
.
Modify 30.5.6 [time.duration.nonmember] as indicated:
template<class Rep1, class Period, class Rep2> constexpr duration<common_type_t<Rep1, Rep2>, Period> operator*(const duration<Rep1, Period>& d, const Rep2& s);-4- Constraints:
[…]is_convertible_v<const Rep2&, common_type_t<Rep1, Rep2>>
istrue
.template<class Rep1, class Rep2, class Period> constexpr duration<common_type_t<Rep1, Rep2>, Period> operator*(const Rep1& s, const duration<Rep2, Period>& d);-6- Constraints:
[…]is_convertible_v<const Rep1&, common_type_t<Rep1, Rep2>>
istrue
.template<class Rep1, class Period, class Rep2> constexpr duration<common_type_t<Rep1, Rep2>, Period> operator/(const duration<Rep1, Period>& d, const Rep2& s);[…]-8- Constraints:
is_convertible_v<const Rep2&, common_type_t<Rep1, Rep2>>
istrue
andRep2
is not a specialization ofduration
.template<class Rep1, class Period, class Rep2> constexpr duration<common_type_t<Rep1, Rep2>, Period> operator%(const duration<Rep1, Period>& d, const Rep2& s);-12- Constraints:
[…]is_convertible_v<const Rep2&, common_type_t<Rep1, Rep2>>
istrue
andRep2
is not a specialization ofduration
.
Section: 29.7.1 [cmath.syn] Status: C++20 Submitter: Thomas Köppe Opened: 2018-01-23 Last modified: 2021-02-25
Priority: 0
View other active issues in [cmath.syn].
View all other issues in [cmath.syn].
View all issues with C++20 status.
Discussion:
The paper P0175 was meant to be a purely editorial change to
spell out the synopses of the "C library" headers. However it contained the following inadvertent
normative change: The floating point classification functions isinf
, isfinite
,
signbit
, etc. (but not fpclassify
) used to be specified to return "bool
"
in C++14. In C, those are macros, but in C++ they have always been functions. During the preparation
of P0175, I recreated the function signatures copying the return type "int
" from C, but
failed to notice that we had already specified those functions differently, so the return type was
changed to "int
".
is...
and signbit
classification functions back to bool
. Alternatively, we could decide
that the return type should actually be int
, but that would be a larger discussion.
Proposed resolution for restoring the original wording: Replace return type "int
" with
return type "bool
" and for all the classification/comparison functions after
"// classification/comparison functions" in [cmath.syn] except the fpclassify
functions.
Related previous issue was LWG 1327(i) and the corresponding
NB comment US-136
resolution.
[ 2018-01-29 Moved to Tentatively Ready after 8 positive votes on c++std-lib. ]
[2018-3-17 Adopted in Jacksonville]
Proposed resolution:
This wording is relative to N4713.
Modify 29.7.1 [cmath.syn] as indicated:
[…] namespace std { […] // 29.7.5 [c.math.fpclass], classification / comparison functions int fpclassify(float x); int fpclassify(double x); int fpclassify(long double x); boolintisfinite(float x); boolintisfinite(double x); boolintisfinite(long double x); boolintisinf(float x); boolintisinf(double x); boolintisinf(long double x); boolintisnan(float x); boolintisnan(double x); boolintisnan(long double x); boolintisnormal(float x); boolintisnormal(double x); boolintisnormal(long double x); boolintsignbit(float x); boolintsignbit(double x); boolintsignbit(long double x); boolintisgreater(float x, float y); boolintisgreater(double x, double y); boolintisgreater(long double x, long double y); boolintisgreaterequal(float x, float y); boolintisgreaterequal(double x, double y); boolintisgreaterequal(long double x, long double y); boolintisless(float x, float y); boolintisless(double x, double y); boolintisless(long double x, long double y); boolintislessequal(float x, float y); boolintislessequal(double x, double y); boolintislessequal(long double x, long double y); boolintislessgreater(float x, float y); boolintislessgreater(double x, double y); boolintislessgreater(long double x, long double y); boolintisunordered(float x, float y); boolintisunordered(double x, double y); boolintisunordered(long double x, long double y); […] }
visit
is underconstrainedSection: 22.6.7 [variant.visit] Status: Resolved Submitter: Casey Carter Opened: 2018-01-23 Last modified: 2021-05-18
Priority: 2
View all other issues in [variant.visit].
View all issues with Resolved status.
Discussion:
std::visit
accepts a parameter pack of forwarding references named
vars
whose types are the parameter pack Variants
. Despite that:
Variants
(modified) to variant_size_v
,varsi.index()
,variant
in vars
is valueless_by_exception
,
andVariants0
"Variants
.
Notably, the Variants
are not required to be variant
s. This lack of constraints
appears to be simply an oversight.
[2018-01-24, Daniel comments]
This issue should be reviewed in common with LWG 2970(i).
[2018-06-18 after reflector discussion]
Priority set to 2; status to LEWG
[2020-11-18; this will be resolved by P2162.]
[2021-04-19 P2162R2 was adopted at February 2021 plenary. Status changed: Tentatively Resolved → Resolved.]
Proposed resolution:
This wording is relative to N4727.
Modify 22.6.7 [variant.visit] as indicated:
template<class Visitor, class... Variants> constexpr see below visit(Visitor&& vis, Variants&&... vars);[…]
-4- Throws:
bad_variant_access
ifanyvariant
invars
isvalueless_by_exception()
(vars.valueless_by_exception() || ...)
istrue
.-5- Complexity: […]
-?- Remarks: This function shall not participate in overload resolution unless
remove_cvref_t<Variantsi>
is a specialization ofvariant
for all0 <= i < n
.
uninitialized_copy
appears to not be able to meet its exception-safety guaranteeSection: 26.11.5 [uninitialized.copy] Status: C++20 Submitter: Jon Cohen Opened: 2018-01-24 Last modified: 2021-02-25
Priority: 2
View all other issues in [uninitialized.copy].
View all issues with C++20 status.
Discussion:
I believe that uninitialized_copy
is unable to meet its exception-safety guarantee in the
presence of throwing move constructors:
uninitialized_copy
:
the provided iterators satisfy the InputIterator
requirements (24.3.5.3 [input.iterators])
if an exception is thrown during the algorithm then there are no effects
Iter
. Then std::move_iterator<Iter>
appears
to also be an input iterator. Notably, it still satisfies that (void)*a, *a
is equivalent to
*a
for move iterator a
since the dereference only forms an rvalue reference, it
doesn't actually perform the move operation (24.3.5.3 [input.iterators] Table 95 — "Input iterator requirements").
Suppose also that we have a type T
whose move constructor can throw, a range of T
's
[tbegin, tend)
, and a pointer to an uninitialized buffer of T
's
buf
. Then std::uninitialized_copy(std::make_move_iterator(tbegin),
std::make_move_iterator(tend), buf)
can't possibly satisfy the property that it has
no effects if one of the moves throws — we'll have a T
left in a moved-from state with
no way of recovering.
See here for an example in code.
It seems like the correct specification for uninitialized_copy
should be that if
InputIterator
's operator*
returns an rvalue reference and
InputIterator::value_type
's move constructor is not marked noexcept
, then
uninitialized_copy
will leave the objects in the underlying range in a valid but
unspecified state.
[2018-01-24, Casey comments and provides wording]
This issue points out a particular hole in the "..if an exception is thrown in the following algorithms
there are no effects." wording for the "uninitialized" memory algorithms
(26.11 [specialized.algorithms]/1) and suggests a PR to patch over said hole. The true problem
here is that "no effects" is not and never has been implementable. For example, "first != last
"
may have observable effects that an implementation is required to somehow reverse if some later operation
throws an exception.
[2018-02-05, Priority set to 2 after mailing list discussion]
[2018-06 Rapperswil Thursday issues processing]
Status to Ready
[2018-11, Adopted in San Diego]
Proposed resolution:
This wording is relative to N4713.
Modify 26.11 [specialized.algorithms] as indicated:
-1- […]
Unless otherwise specified, if an exception is thrown in the following algorithms objects constructed by a placement new-expression (7.6.2.8 [expr.new]) are destroyed in an unspecified order before allowing the exception to propagatethere are no effects.
Modify 26.11.6 [uninitialized.move] as indicated (The removed paragraphs are now unnecessary):
template<class InputIterator, class ForwardIterator> ForwardIterator uninitialized_move(InputIterator first, InputIterator last, ForwardIterator result);[…]
-2- Remarks: If an exception is thrown, some objects in the range[first, last)
are left in a valid but unspecified state.template<class InputIterator, class Size, class ForwardIterator> pair<InputIterator, ForwardIterator> uninitialized_move_n(InputIterator first, Size n, ForwardIterator result);[…]
-4- Remarks: If an exception is thrown, some objects in the range[first, std::next(first, n))
are left in a valid but unspecified state.
path::operator+=(single-character)
misspecifiedSection: 31.12.6.5.4 [fs.path.concat] Status: C++20 Submitter: Tim Song Opened: 2018-01-24 Last modified: 2021-02-25
Priority: 3
View all other issues in [fs.path.concat].
View all issues with C++20 status.
Discussion:
31.12.6.5.4 [fs.path.concat] uses the expression path(x).native()
to specify the effects of
concatenating a single character x
. However, there is no path
constructor taking a single character.
[2018-06-18 after reflector discussion]
Priority set to 3
[2018-10-12 Tim updates PR to avoid suggesting the creation of a temporary path
.]
Previous resolution [SUPERSEDED]:
This wording is relative to N4713.
Modify 31.12.6.5.4 [fs.path.concat] as indicated:
path& operator+=(const path& x); path& operator+=(const string_type& x); path& operator+=(basic_string_view<value_type> x); path& operator+=(const value_type* x);path& operator+=(value_type x);template<class Source> path& operator+=(const Source& x);template<class EcharT> path& operator+=(EcharT x);template<class Source> path& concat(const Source& x);-1- Effects: […]
-2- Returns:*this
.path& operator+=(value_type x); template<class EcharT> path& operator+=(EcharT x);-?- Effects: Equivalent to:
return *this += path(&x, &x + 1);
.
[2019-02; Kona Wednesday night issue processing]
Status to Ready
Proposed resolution:
This wording is relative to N4762.
Modify 31.12.6.5.4 [fs.path.concat] as indicated:
path& operator+=(const path& x); path& operator+=(const string_type& x); path& operator+=(basic_string_view<value_type> x); path& operator+=(const value_type* x);path& operator+=(value_type x);template<class Source> path& operator+=(const Source& x);template<class EcharT> path& operator+=(EcharT x);template<class Source> path& concat(const Source& x);-1- Effects: […]
-2- Returns:*this
.path& operator+=(value_type x); template<class EcharT> path& operator+=(EcharT x);-?- Effects: Equivalent to:
return *this += basic_string_view(&x, 1);
adjacent_difference
shouldn't require creating temporariesSection: 26.10.12 [adjacent.difference] Status: C++20 Submitter: Billy O'Neal III Opened: 2018-02-02 Last modified: 2021-02-25
Priority: 3
View all other issues in [adjacent.difference].
View all issues with C++20 status.
Discussion:
Parallel adjacent_difference
is presently specified to "create a temporary object whose
type is ForwardIterator1
's value type". Serial adjacent_difference
does that
because it needs to work with input iterators, and needs to work when the destination range
exactly overlaps the input range. The parallel version requires forward iterators and doesn't
allow overlap, so it can avoid making these temporaries.
[2018-02-13, Priority set to 3 after mailing list discussion]
[2018-3-14 Wednesday evening issues processing; remove 'const' before minus
and move to Ready.]
Previous resolution [SUPERSEDED]:
This wording is relative to N4713.
Modify 26.10.12 [adjacent.difference] as indicated:
template<class InputIterator, class OutputIterator> OutputIterator adjacent_difference(InputIterator first, InputIterator last, OutputIterator result); template<class ExecutionPolicy, class ForwardIterator1, class ForwardIterator2> ForwardIterator2 adjacent_difference(ExecutionPolicy&& exec, ForwardIterator1 first, ForwardIterator1 last, ForwardIterator2 result); template<class InputIterator, class OutputIterator, class BinaryOperation> OutputIterator adjacent_difference(InputIterator first, InputIterator last, OutputIterator result, BinaryOperation binary_op); template<class ExecutionPolicy, class ForwardIterator1, class ForwardIterator2, class BinaryOperation> ForwardIterator2 adjacent_difference(ExecutionPolicy&& exec, ForwardIterator1 first, ForwardIterator1 last, ForwardIterator2 result, BinaryOperation binary_op);-?- Let
T
be the value type ofdecltype(first)
. For the overloads that do not take an argumentbinary_op
, letbinary_op
be an lvalue that denotes an object of typeconst minus<>
.-1- Requires:
-2- Effects: For the overloads with no
(1.1) — For the overloads with no
ExecutionPolicy
,InputIterator
’s value typeT
shall beMoveAssignable
(Table 25) and shall be constructible from the type of*first
.acc
(defined below) shall be writable (24.3.1 [iterator.requirements.general]) to theresult
output iterator. The result of the expressionval - std::move(acc)
orbinary_op(val, std::move(acc))
shall be writable to theresult
output iterator.(1.2) — For the overloads with an
ExecutionPolicy
, thevalue type ofresult of the expressionsForwardIterator1
shall beCopyConstructible
(Table 24), constructible from the expression*first - *first
orbinary_op(*first, *first)
, and assignable to the value type ofForwardIterator2
binary_op(*first, *first)
and*first
shall be writable toresult
.(1.3) — […]
ExecutionPolicy
and a non-empty range, the function creates an accumulatoracc
whose type isof typeInputIterator
’s value typeT
, initializes it with*first
, and assigns the result to*result
. For every iteratori
in[first + 1, last)
in order, creates an objectval
whose type isInputIterator
’s value typeT
, initializes it with*i
, computesval - std::move(acc)
orbinary_op(val, std::move(acc))
, assigns the result to*(result + (i - first))
, and move assigns fromval
toacc
. -3- For the overloads with anExecutionPolicy
and a non-empty range,first the function creates an object whose type isperformsForwardIterator1
's value type, initializes it with*first
, and assigns the result to*result
. Then for everyd
in[1, last - first - 1]
, creates an objectval
whose type isForwardIterator1
's value type, initializes it with*(first + d) - *(first + d - 1)
orbinary_op(*(first + d), *(first + d - 1))
, and assigns the result to*(result + d)
*result = *first
. Then, for everyd
in[1, last - first - 1]
, performs*(result + d) = binary_op(*(first + d), *(first + (d - 1)))
.
[2018-06 Rapperswil: Adopted]
Proposed resolution:
This wording is relative to N4713.
Modify 26.10.12 [adjacent.difference] as indicated:
template<class InputIterator, class OutputIterator> OutputIterator adjacent_difference(InputIterator first, InputIterator last, OutputIterator result); template<class ExecutionPolicy, class ForwardIterator1, class ForwardIterator2> ForwardIterator2 adjacent_difference(ExecutionPolicy&& exec, ForwardIterator1 first, ForwardIterator1 last, ForwardIterator2 result); template<class InputIterator, class OutputIterator, class BinaryOperation> OutputIterator adjacent_difference(InputIterator first, InputIterator last, OutputIterator result, BinaryOperation binary_op); template<class ExecutionPolicy, class ForwardIterator1, class ForwardIterator2, class BinaryOperation> ForwardIterator2 adjacent_difference(ExecutionPolicy&& exec, ForwardIterator1 first, ForwardIterator1 last, ForwardIterator2 result, BinaryOperation binary_op);-?- Let
T
be the value type ofdecltype(first)
. For the overloads that do not take an argumentbinary_op
, letbinary_op
be an lvalue that denotes an object of typeminus<>
.-1- Requires:
-2- Effects: For the overloads with no
(1.1) — For the overloads with no
ExecutionPolicy
,InputIterator
’s value typeT
shall beMoveAssignable
(Table 25) and shall be constructible from the type of*first
.acc
(defined below) shall be writable (24.3.1 [iterator.requirements.general]) to theresult
output iterator. The result of the expressionval - std::move(acc)
orbinary_op(val, std::move(acc))
shall be writable to theresult
output iterator.(1.2) — For the overloads with an
ExecutionPolicy
, thevalue type ofresult of the expressionsForwardIterator1
shall beCopyConstructible
(Table 24), constructible from the expression*first - *first
orbinary_op(*first, *first)
, and assignable to the value type ofForwardIterator2
binary_op(*first, *first)
and*first
shall be writable toresult
.(1.3) — […]
ExecutionPolicy
and a non-empty range, the function creates an accumulatoracc
whose type isof typeInputIterator
’s value typeT
, initializes it with*first
, and assigns the result to*result
. For every iteratori
in[first + 1, last)
in order, creates an objectval
whose type isInputIterator
’s value typeT
, initializes it with*i
, computesval - std::move(acc)
orbinary_op(val, std::move(acc))
, assigns the result to*(result + (i - first))
, and move assigns fromval
toacc
. -3- For the overloads with anExecutionPolicy
and a non-empty range,first the function creates an object whose type isperformsForwardIterator1
's value type, initializes it with*first
, and assigns the result to*result
. Then for everyd
in[1, last - first - 1]
, creates an objectval
whose type isForwardIterator1
's value type, initializes it with*(first + d) - *(first + d - 1)
orbinary_op(*(first + d), *(first + d - 1))
, and assigns the result to*(result + d)
*result = *first
. Then, for everyd
in[1, last - first - 1]
, performs*(result + d) = binary_op(*(first + d), *(first + (d - 1)))
.
compare_3way
?Section: 26.8.12 [alg.three.way] Status: Resolved Submitter: Richard Smith Opened: 2018-02-07 Last modified: 2020-09-06
Priority: 2
View all other issues in [alg.three.way].
View all issues with Resolved status.
Discussion:
The P0768R1 specification of compare_3way says:
template<class T, class U> constexpr auto compare_3way(const T& a, const U& b);-1- Effects: Compares two values and produces a result of the strongest applicable comparison category type:
(1.1) — Returns
a <=> b
if that expression is well-formed.(1.2) — Otherwise, if the expressions
a == b
anda < b
are each well-formed and convertible tobool
, returnsstrong_ordering::equal
whena == b
istrue
, otherwise returnsstrong_ordering::less
whena < b
istrue
, and otherwise returnsstrong_ordering::greater
.(1.3) — Otherwise, if the expression
a == b
is well-formed and convertible tobool
, returnsstrong_equality::equal
whena == b
istrue
, and otherwise returnsstrong_equality::nonequal
.(1.4) — Otherwise, the function shall be defined as deleted.
So, it returns strong_ordering::...
or strong_equality::
or a <=> b
.
By the normal core deduction rules, that means it's always ill-formed, because one return type
deduction deduces strong_ordering
and another deduces strong_equality
.
constexpr if
/ else. But I think you need to actually say that.
[Tomasz suggests proposed wording]
[2018-06-18 after reflector discussion]
Priority set to 2
[2018-11 San Diego Thursday night issue processing]
Spaceship is still in flux; revisit in Kona. Status to Open
[2020-01 Resolved by the adoption of P1614 in Cologne.]
Proposed resolution:
This wording is relative to N4713.
Modify 26.4 [algorithm.syn], header <algorithm>
synopsis, as indicated:
// 26.8.12 [alg.three.way], three-way comparison algorithms template<class T, class U> constexprautosee below compare_3way(const T& a, const U& b);
Modify 26.8.12 [alg.three.way] as indicated:
template<class T, class U> constexprautosee below compare_3way(const T& a, const U& b);-1- Effects: Compares two values and produces a result of the strongest applicable comparison category type:
(1.1) —
ReturnsIf the expressiona <=> b
if that expression is well-formeda <=> b
is well-formed, returns a value of typedecay_t<decltype(a <=> b)>
initialized froma <=> b
.(1.2) — Otherwise, if the expressions
a == b
anda < b
are each well-formed and convertible tobool
, returns a value of typestrong_ordering
equal tostrong_ordering::equal
whena == b
istrue
, otherwise returnsstrong_ordering::less
whena < b
istrue
, and otherwise returnsstrong_ordering::greater
.(1.3) — Otherwise, if the expression
a == b
is well-formed and convertible tobool
, returns a value of typestrong_equality
equal tostrong_equality::equal
whena == b
istrue
, and otherwise returnsstrong_equality::nonequal
.(1.4) — Otherwise, the return type is
void
and the function is defined as deleted.
decay_t
in is_execution_policy_v
should be remove_cvref_t
Section: 26.3.5 [algorithms.parallel.overloads] Status: C++20 Submitter: Billy O'Neal III Opened: 2018-02-07 Last modified: 2021-02-25
Priority: 0
View all issues with C++20 status.
Discussion:
Our compiler throughput friends were hissing at us about throughput regressions in C++17 mode caused by the addition of the parallel algorithms' signatures. One change to reduce the throughput impact would be to remove unnecessary decay here, as LWG has done in other places recently.
[ 2018-02-13 Moved to Tentatively Ready after 7 positive votes on c++std-lib. ]
[2018-06 Rapperswil: Adopted]
Proposed resolution:
This wording is relative to N4713.
Modify 26.3.5 [algorithms.parallel.overloads] as indicated:
-4- Parallel algorithms shall not participate in overload resolution unless
is_execution_policy_v<
isdecayremove_cvref_t<ExecutionPolicy>>true
.
Section: 26.11 [specialized.algorithms] Status: Resolved Submitter: Alisdair Meredith Opened: 2018-02-12 Last modified: 2024-11-28
Priority: 3
View other active issues in [specialized.algorithms].
View all other issues in [specialized.algorithms].
View all issues with Resolved status.
Discussion:
A typical specification of the algorithms for initializing raw memory in <memory>
looks like:
Effects: Equivalent to:
for (; first != last; ++first) ::new (static_cast<void*>(addressof(*first))) typename iterator_traits<ForwardIterator>::value_type;
However, this hides a nasty question:
How do we bind a reference to an uninitialized object when dereferencing our iterator, so thatstatic_cast<void*>(addressof(*first))
does not trigger undefined behavior on
the call to *first
?
When pointers are the only iterators we cared about, we could simply cast the iterator
value to void*
without dereferencing. I don't see how to implement this spec safely
without introducing another customization point for iterators that performs the same
function as casting a pointer to void*
in order to get the address of the element.
[2018-02-20, Priority set to 3 after mailing list discussion]
[2024-11-28 Status changed: New → Resolved.]
Resolved by CWG 453, accepted as a DR in March 2024.
Proposed resolution:
path
's other operators should be hidden friends as wellSection: 31.12.6.8 [fs.path.nonmember] Status: C++20 Submitter: Billy O'Neal III Opened: 2018-02-13 Last modified: 2021-02-25
Priority: 2
View all issues with C++20 status.
Discussion:
Consider the following program:
// See godbolt link #include <assert.h> #include <string> #include <filesystem> using namespace std; using namespace std::filesystem; int main() { bool b = L"a//b" == std::string("a/b"); assert(b); // passes. What?! return b; }
L"a"
gets converted into a path
, and the string gets converted into a path
,
and then those paths are compared for equality. But path
equality comparison doesn't work
anything like string equality comparison, leading to surprises.
path
's other operators should be made hidden friends as well, so that one side or the other
of a given operator is of type path
before those conversions apply.
[2018-02-20, Priority set to 2 after mailing list discussion]
[2018-08-23 Batavia Issues processing]
Status to Tentatively Ready
[2018-11, Adopted in San Diego]
Proposed resolution:
This wording is relative to N4713.
All drafting notes from LWG 2989(i) apply here too.Modify 31.12.4 [fs.filesystem.syn], header <filesystem>
synopsis, as indicated:
[…] // 31.12.6.8 [fs.path.nonmember], path non-member functions void swap(path& lhs, path& rhs) noexcept; size_t hash_value(const path& p) noexcept;bool operator==(const path& lhs, const path& rhs) noexcept; bool operator!=(const path& lhs, const path& rhs) noexcept; bool operator< (const path& lhs, const path& rhs) noexcept; bool operator<=(const path& lhs, const path& rhs) noexcept; bool operator> (const path& lhs, const path& rhs) noexcept; bool operator>=(const path& lhs, const path& rhs) noexcept; path operator/ (const path& lhs, const path& rhs);[…]
Modify 31.12.6.8 [fs.path.nonmember] as indicated:
[…] friend bool operator< (const path& lhs, const path& rhs) noexcept; […] friend bool operator<=(const path& lhs, const path& rhs) noexcept; […] friend bool operator> (const path& lhs, const path& rhs) noexcept; […] friend bool operator>=(const path& lhs, const path& rhs) noexcept; […] friend bool operator==(const path& lhs, const path& rhs) noexcept; […] friend bool operator!=(const path& lhs, const path& rhs) noexcept; […] friend path operator/ (const path& lhs, const path& rhs); […]
Modify 31.12.6 [fs.class.path], class path
synopsis, as indicated:
class path { public: […] // 31.12.6.5.5 [fs.path.modifiers], modifiers […] // 31.12.6.8 [fs.path.nonmember], non-member operators friend bool operator< (const path& lhs, const path& rhs) noexcept; friend bool operator<=(const path& lhs, const path& rhs) noexcept; friend bool operator> (const path& lhs, const path& rhs) noexcept; friend bool operator>=(const path& lhs, const path& rhs) noexcept; friend bool operator==(const path& lhs, const path& rhs) noexcept; friend bool operator!=(const path& lhs, const path& rhs) noexcept; friend path operator/ (const path& lhs, const path& rhs); // 31.12.6.5.6 [fs.path.native.obs], native format observers […] };
recursive_directory_iterator::pop
must invalidateSection: 31.12.12.2 [fs.rec.dir.itr.members] Status: C++20 Submitter: Casey Carter Opened: 2018-02-25 Last modified: 2021-02-25
Priority: 0
View all other issues in [fs.rec.dir.itr.members].
View all issues with C++20 status.
Discussion:
recursive_directory_iterator::pop
is effectively a "supercharged" operator++
: it
advances the iterator forward as many steps as are necessary to reach the next entry in the parent
directory. Just as is the case for operator++
, pop
must be allowed to invalidate
iterator copies to allow efficient implementation. The most efficient fix seems to be borrowing the
invalidation wording from 24.3.5.3 [input.iterators] Table 87's specification for the required
++r
expression for input iterators.
[ 2018-03-06 Moved to Tentatively Ready after 5 positive votes on c++std-lib. ]
[2018-06 Rapperswil: Adopted]
Proposed resolution:
This wording is relative to N4727.
Change 31.12.12.2 [fs.rec.dir.itr.members] as indicated:
void pop(); void pop(error_code& ec);-26- Effects: If
depth() == 0
, set*this
torecursive_directory_iterator()
. Otherwise, cease iteration of the directory currently being iterated over, and continue iteration over the parent directory.-?- Postconditions: Any copies of the previous value of
*this
are no longer required either to be dereferenceable or to be in the domain of==
.-27- Throws: As specified in 31.12.5 [fs.err.report].
path::lexically_relative
causes surprising results if a filename can also be a
root-nameSection: 31.12.6.5.11 [fs.path.gen] Status: C++20 Submitter: Billy O'Neal III Opened: 2018-02-23 Last modified: 2021-02-25
Priority: 2
View all other issues in [fs.path.gen].
View all issues with C++20 status.
Discussion:
path::lexically_relative
constructs the resulting path with operator/=
. If any of
the filename elements from *this
are themselves acceptable root-names, operator/=
will destroy any previous value, and take that root_name()
. For example:
path("/a:/b:").lexically_relative("/a:/c:")
On a POSIX implementation, this would return path("../b:")
, but on a Windows implementation, the
"b:"
element is interpreted as a root-name, and clobbers the entire result path
,
giving path("b:")
. We should detect this problematic condition and fail (by returning path()
).
[2019-01-20 Reflector prioritization]
Set Priority to 2
[2019 Cologne Wednesday night]
Status to Ready
Proposed resolution:
This wording is relative to N4727.
Change 31.12.6.5.11 [fs.path.gen] as indicated:
path lexically_relative(const path& base) const;-3- […]
-4- Effects: Ifroot_name() != base.root_name()
istrue
oris_absolute() != base.is_absolute()
istrue
or!has_root_directory() && base.has_root_directory()
istrue
or if any filename inrelative_path()
orbase.relative_path()
can be interpreted as a root-name, returnspath()
. [Note: On a POSIX implementation, no filename in a relative-path is acceptable as a root-name — end note] Determines the first mismatched element of*this
andbase
as if by:auto [a, b] = mismatch(begin(), end(), base.begin(), base.end());Then,
(4.1) — if
a == end()
andb == base.end()
, returnspath(".")
; otherwise(4.2) — let
n
be the number of filename elements in[b, base.end())
that are not dot or dot-dot minus the number that are dot-dot. Ifn < 0
, returnspath()
; otherwise(4.3) — returns an object of class
path
that is default-constructed, followed by
(4.3.1) — application of
operator/=(path(".."))
n
times, and then(4.3.2) — application of
operator/=
for each element in[a, end())
.
read_until
still refers to "input sequence"Section: 17.9 [networking.ts::buffer.read.until] Status: C++23 Submitter: Christopher Kohlhoff Opened: 2018-02-26 Last modified: 2023-11-22
Priority: 0
View all issues with C++23 status.
Discussion:
Addresses: networking.ts
When specifying DynamicBuffers
and their related operations, early drafts of the Networking TS described the buffers in terms of their "input sequence" and "output sequence". This was changed to "readable bytes" and "writable bytes" respectively. Unfortunately, some instances of "input sequence" were missed in section
17.9 [networking.ts::buffer.read.until].
[ 2018-03-06 Moved to Tentatively Ready after 5 positive votes on c++std-lib. ]
[2018-06 Rapperswil: Adopted]
Proposed resolution:
This wording is relative to N4711.
Change 17.9 [networking.ts::buffer.read.until] as indicated:
template<class SyncReadStream, class DynamicBuffer> size_t read_until(SyncReadStream& s, DynamicBuffer&& b, char delim); template<class SyncReadStream, class DynamicBuffer> size_t read_until(SyncReadStream& s, DynamicBuffer&& b, char delim, error_code& ec); template<class SyncReadStream, class DynamicBuffer> size_t read_until(SyncReadStream& s, DynamicBuffer&& b, string_view delim); template<class SyncReadStream, class DynamicBuffer> size_t read_until(SyncReadStream& s, DynamicBuffer&& b, string_view delim, error_code& ec);-1- Effects: Reads data from the buffer-oriented synchronous read stream (17.1.1 [networking.ts::buffer.stream.reqmts.syncreadstream]) object stream by performing zero or more calls to the stream's
-2- Data is placed into the dynamic buffer object b. A mutable buffer sequence (16.2.1) is obtained prior to eachread_some
member function, until theinput sequencereadable bytes of the dynamic buffer (16.2.4 [networking.ts::buffer.reqmts.dynamicbuffer]) objectb
containsthe specified delimiterdelim
.read_some
call usingb.prepare(N)
, whereN
is an unspecified value such thatN <= max_size() - size()
. [Note: Implementations are encouraged to useb.capacity()
when determiningN
, to minimize the number ofread_some
calls performed on the stream. — end note] After eachread_some
call, the implementation performsb.commit(n)
, wheren
is the return value fromread_some
. -3- The synchronousread_until
operation continues until:-4- On exit, if the
(3.1) — the
input sequencereadable bytes ofb
containsthe delimiterdelim
; or(3.2) —
b.size() == b.max_size()
; or(3.3) — an asynchronous
read_some
operation fails.input sequencereadable bytes ofb
containsthe delimiter,ec
is set such that!ec
istrue
. Otherwise, ifb.size() == b.max_size()
,ec
is set such thatec == stream_errc::not_found
. Ifb.size() < b.max_size()
,ec
contains theerror_code
from the most recentread_some
call. -5- Returns: The number ofbytes in the input sequence ofreadable bytes inb
up to and including the delimiter, if present. [Note: On completion, the buffer may contain additional bytes following the delimiter. — end note] Otherwise returns0
.
valarray
should only deduce from the valarray
Section: 29.6.3 [valarray.nonmembers] Status: C++20 Submitter: Jonathan Wakely Opened: 2018-02-28 Last modified: 2021-02-25
Priority: 0
View all issues with C++20 status.
Discussion:
The expression (std::valarray<double>{} * 2)
is ill-formed, because argument deduction
fails for:
template<class T> valarray<T> operator*(const valarray<T>&, const T&);
Is there any reason to try and deduce the argument from the scalar, instead of only deducing from the valarray and allowing implicit conversions to the scalar? i.e.
template<class T> valarray<T> operator*(const valarray<T>&, const typename valarray<T>::value_type&);
[ 2018-03-07 Moved to Tentatively Ready after 5 positive votes on c++std-lib. ]
[2018-06 Rapperswil: Adopted]
Proposed resolution:
This wording is relative to N4727.
Edit 29.6.1 [valarray.syn], header <valarray>
synopsis, as indicated:
[…] template<class T> valarray<T> operator* (const valarray<T>&, const valarray<T>&); template<class T> valarray<T> operator* (const valarray<T>&, constTtypename valarray<T>::value_type&); template<class T> valarray<T> operator* (constTtypename valarray<T>::value_type&, const valarray<T>&); template<class T> valarray<T> operator/ (const valarray<T>&, const valarray<T>&); template<class T> valarray<T> operator/ (const valarray<T>&, constTtypename valarray<T>::value_type&); template<class T> valarray<T> operator/ (constTtypename valarray<T>::value_type&, const valarray<T>&); template<class T> valarray<T> operator% (const valarray<T>&, const valarray<T>&); template<class T> valarray<T> operator% (const valarray<T>&, constTtypename valarray<T>::value_type&); template<class T> valarray<T> operator% (constTtypename valarray<T>::value_type&, const valarray<T>&); template<class T> valarray<T> operator+ (const valarray<T>&, const valarray<T>&); template<class T> valarray<T> operator+ (const valarray<T>&, constTtypename valarray<T>::value_type&); template<class T> valarray<T> operator+ (constTtypename valarray<T>::value_type&, const valarray<T>&); template<class T> valarray<T> operator- (const valarray<T>&, const valarray<T>&); template<class T> valarray<T> operator- (const valarray<T>&, constTtypename valarray<T>::value_type&); template<class T> valarray<T> operator- (constTtypename valarray<T>::value_type&, const valarray<T>&); template<class T> valarray<T> operator^ (const valarray<T>&, const valarray<T>&); template<class T> valarray<T> operator^ (const valarray<T>&, constTtypename valarray<T>::value_type&); template<class T> valarray<T> operator^ (constTtypename valarray<T>::value_type&, const valarray<T>&); template<class T> valarray<T> operator& (const valarray<T>&, const valarray<T>&); template<class T> valarray<T> operator& (const valarray<T>&, constTtypename valarray<T>::value_type&); template<class T> valarray<T> operator& (constTtypename valarray<T>::value_type&, const valarray<T>&); template<class T> valarray<T> operator| (const valarray<T>&, const valarray<T>&); template<class T> valarray<T> operator| (const valarray<T>&, constTtypename valarray<T>::value_type&); template<class T> valarray<T> operator| (constTtypename valarray<T>::value_type&, const valarray<T>&); template<class T> valarray<T> operator<<(const valarray<T>&, const valarray<T>&); template<class T> valarray<T> operator<<(const valarray<T>&, constTtypename valarray<T>::value_type&); template<class T> valarray<T> operator<<(constTtypename valarray<T>::value_type&, const valarray<T>&); template<class T> valarray<T> operator>>(const valarray<T>&, const valarray<T>&); template<class T> valarray<T> operator>>(const valarray<T>&, constTtypename valarray<T>::value_type&); template<class T> valarray<T> operator>>(constTtypename valarray<T>::value_type&, const valarray<T>&); template<class T> valarray<bool> operator&&(const valarray<T>&, const valarray<T>&); template<class T> valarray<bool> operator&&(const valarray<T>&, constTtypename valarray<T>::value_type&); template<class T> valarray<bool> operator&&(constTtypename valarray<T>::value_type&, const valarray<T>&); template<class T> valarray<bool> operator||(const valarray<T>&, const valarray<T>&); template<class T> valarray<bool> operator||(const valarray<T>&, constTtypename valarray<T>::value_type&); template<class T> valarray<bool> operator||(constTtypename valarray<T>::value_type&, const valarray<T>&); template<class T> valarray<bool> operator==(const valarray<T>&, const valarray<T>&); template<class T> valarray<bool> operator==(const valarray<T>&, constTtypename valarray<T>::value_type&); template<class T> valarray<bool> operator==(constTtypename valarray<T>::value_type&, const valarray<T>&); template<class T> valarray<bool> operator!=(const valarray<T>&, const valarray<T>&); template<class T> valarray<bool> operator!=(const valarray<T>&, constTtypename valarray<T>::value_type&); template<class T> valarray<bool> operator!=(constTtypename valarray<T>::value_type&, const valarray<T>&); template<class T> valarray<bool> operator< (const valarray<T>&, const valarray<T>&); template<class T> valarray<bool> operator< (const valarray<T>&, constTtypename valarray<T>::value_type&); template<class T> valarray<bool> operator< (constTtypename valarray<T>::value_type&, const valarray<T>&); template<class T> valarray<bool> operator> (const valarray<T>&, const valarray<T>&); template<class T> valarray<bool> operator> (const valarray<T>&, constTtypename valarray<T>::value_type&); template<class T> valarray<bool> operator> (constTtypename valarray<T>::value_type&, const valarray<T>&); template<class T> valarray<bool> operator<=(const valarray<T>&, const valarray<T>&); template<class T> valarray<bool> operator<=(const valarray<T>&, constTtypename valarray<T>::value_type&); template<class T> valarray<bool> operator<=(constTtypename valarray<T>::value_type&, const valarray<T>&); template<class T> valarray<bool> operator>=(const valarray<T>&, const valarray<T>&); template<class T> valarray<bool> operator>=(const valarray<T>&, constTtypename valarray<T>::value_type&); template<class T> valarray<bool> operator>=(constTtypename valarray<T>::value_type&, const valarray<T>&); template<class T> valarray<T> abs (const valarray<T>&); template<class T> valarray<T> acos (const valarray<T>&); template<class T> valarray<T> asin (const valarray<T>&); template<class T> valarray<T> atan (const valarray<T>&); template<class T> valarray<T> atan2(const valarray<T>&, const valarray<T>&); template<class T> valarray<T> atan2(const valarray<T>&, constTtypename valarray<T>::value_type&); template<class T> valarray<T> atan2(constTtypename valarray<T>::value_type&, const valarray<T>&); template<class T> valarray<T> cos (const valarray<T>&); template<class T> valarray<T> cosh (const valarray<T>&); template<class T> valarray<T> exp (const valarray<T>&); template<class T> valarray<T> log (const valarray<T>&); template<class T> valarray<T> log10(const valarray<T>&); template<class T> valarray<T> pow(const valarray<T>&, const valarray<T>&); template<class T> valarray<T> pow(const valarray<T>&, constTtypename valarray<T>::value_type&); template<class T> valarray<T> pow(constTtypename valarray<T>::value_type&, const valarray<T>&); […]
Edit 29.6.3.1 [valarray.binary] as indicated:
[…] template<class T> valarray<T> operator* (const valarray<T>&, constTtypename valarray<T>::value_type&); template<class T> valarray<T> operator* (constTtypename valarray<T>::value_type&, const valarray<T>&); template<class T> valarray<T> operator/ (const valarray<T>&, constTtypename valarray<T>::value_type&); template<class T> valarray<T> operator/ (constTtypename valarray<T>::value_type&, const valarray<T>&); template<class T> valarray<T> operator% (const valarray<T>&, constTtypename valarray<T>::value_type&); template<class T> valarray<T> operator% (constTtypename valarray<T>::value_type&, const valarray<T>&); template<class T> valarray<T> operator+ (const valarray<T>&, constTtypename valarray<T>::value_type&); template<class T> valarray<T> operator+ (constTtypename valarray<T>::value_type&, const valarray<T>&); template<class T> valarray<T> operator- (const valarray<T>&, constTtypename valarray<T>::value_type&); template<class T> valarray<T> operator- (constTtypename valarray<T>::value_type&, const valarray<T>&); template<class T> valarray<T> operator^ (const valarray<T>&, constTtypename valarray<T>::value_type&); template<class T> valarray<T> operator^ (constTtypename valarray<T>::value_type&, const valarray<T>&); template<class T> valarray<T> operator& (const valarray<T>&, constTtypename valarray<T>::value_type&); template<class T> valarray<T> operator& (constTtypename valarray<T>::value_type&, const valarray<T>&); template<class T> valarray<T> operator| (const valarray<T>&, constTtypename valarray<T>::value_type&); template<class T> valarray<T> operator| (constTtypename valarray<T>::value_type&, const valarray<T>&); template<class T> valarray<T> operator<<(const valarray<T>&, constTtypename valarray<T>::value_type&); template<class T> valarray<T> operator<<(constTtypename valarray<T>::value_type&, const valarray<T>&); template<class T> valarray<T> operator>>(const valarray<T>&, constTtypename valarray<T>::value_type&); template<class T> valarray<T> operator>>(constTtypename valarray<T>::value_type&, const valarray<T>&); […]
Edit 29.6.3.2 [valarray.comparison] as indicated:
[…] template<class T> valarray<bool> operator==(const valarray<T>&, constTtypename valarray<T>::value_type&); template<class T> valarray<bool> operator==(constTtypename valarray<T>::value_type&, const valarray<T>&); template<class T> valarray<bool> operator!=(const valarray<T>&, constTtypename valarray<T>::value_type&); template<class T> valarray<bool> operator!=(constTtypename valarray<T>::value_type&, const valarray<T>&); template<class T> valarray<bool> operator< (const valarray<T>&, constTtypename valarray<T>::value_type&); template<class T> valarray<bool> operator< (constTtypename valarray<T>::value_type&, const valarray<T>&); template<class T> valarray<bool> operator> (const valarray<T>&, constTtypename valarray<T>::value_type&); template<class T> valarray<bool> operator> (constTtypename valarray<T>::value_type&, const valarray<T>&); template<class T> valarray<bool> operator<=(const valarray<T>&, constTtypename valarray<T>::value_type&); template<class T> valarray<bool> operator<=(constTtypename valarray<T>::value_type&, const valarray<T>&); template<class T> valarray<bool> operator>=(const valarray<T>&, constTtypename valarray<T>::value_type&); template<class T> valarray<bool> operator>=(constTtypename valarray<T>::value_type&, const valarray<T>&); template<class T> valarray<bool> operator&&(const valarray<T>&, constTtypename valarray<T>::value_type&); template<class T> valarray<bool> operator&&(constTtypename valarray<T>::value_type&, const valarray<T>&); template<class T> valarray<bool> operator||(const valarray<T>&, constTtypename valarray<T>::value_type&); template<class T> valarray<bool> operator||(constTtypename valarray<T>::value_type&, const valarray<T>&); […]
Edit 29.6.3.3 [valarray.transcend] as indicated:
template<class T> valarray<T> abs (const valarray<T>&); template<class T> valarray<T> acos (const valarray<T>&); template<class T> valarray<T> asin (const valarray<T>&); template<class T> valarray<T> atan (const valarray<T>&); template<class T> valarray<T> atan2(const valarray<T>&, const valarray<T>&); template<class T> valarray<T> atan2(const valarray<T>&, constTtypename valarray<T>::value_type&); template<class T> valarray<T> atan2(constTtypename valarray<T>::value_type&, const valarray<T>&); template<class T> valarray<T> cos (const valarray<T>&); template<class T> valarray<T> cosh (const valarray<T>&); template<class T> valarray<T> exp (const valarray<T>&); template<class T> valarray<T> log (const valarray<T>&); template<class T> valarray<T> log10(const valarray<T>&); template<class T> valarray<T> pow (const valarray<T>&, const valarray<T>&); template<class T> valarray<T> pow (const valarray<T>&, constTtypename valarray<T>::value_type&); template<class T> valarray<T> pow (constTtypename valarray<T>::value_type&, const valarray<T>&); […]
basic_string
needs deduction guides from basic_string_view
Section: 27.4.3 [basic.string], 27.4.3.3 [string.cons] Status: C++20 Submitter: Stephan T. Lavavej Opened: 2018-03-03 Last modified: 2021-02-25
Priority: Not Prioritized
View other active issues in [basic.string].
View all other issues in [basic.string].
View all issues with C++20 status.
Discussion:
The Proposed Resolution for LWG 2946(i) appears to be correct and we've implemented it in MSVC, but it worsens a pre-existing problem with basic_string class template argument deduction.
The followings1
and s2
compiled in C++17 before LWG 2946's PR, fail
to compile after LWG 2946's PR, and are fixed by my PR:
basic_string s1("cat"sv); basic_string s2("cat"sv, alloc);
The following s4 failed to compile in C++17, and is fixed by my PR:
// basic_string s3("cat"sv, 1, 1); basic_string s4("cat"sv, 1, 1, alloc);
(s3 failed to compile in C++17, and would be fixed by my PR, but it is affected by a pre-existing and unrelated ambiguity which I am not attempting to fix here.)
As C++17 and LWG 2946's PR introduced templated constructors forbasic_string
from
basic_string_view
, we need to add corresponding deduction guides.
The constructors take const T&
that's convertible to basic_string_view
(the
additional constraint about not converting to const charT*
is irrelevant here). However, CTAD
can't deduce charT
and traits
from arbitrary user-defined types, so the deduction guides
need T
to be exactly basic_string_view
.
Additionally, we need to handle the size_type
parameters in the same way that the unordered
containers do. This PR has been implemented in MSVC.
[2018-14: Wednesday night issues processing: both this and 2946(i) to status "Immediate".]
[2018-3-17 Adopted in Jacksonville]
Proposed resolution:
This wording is relative to N4727.
Edit 27.4.3 [basic.string], class template basic_string
synopsis, as indicated:
[…] template<class InputIterator, class Allocator = allocator<typename iterator_traits<InputIterator>::value_type>> basic_string(InputIterator, InputIterator, Allocator = Allocator()) -> basic_string<typename iterator_traits<InputIterator>::value_type, char_traits<typename iterator_traits<InputIterator>::value_type>, Allocator>; template<class charT, class traits, class Allocator = allocator<charT>> explicit basic_string(basic_string_view<charT, traits>, const Allocator& = Allocator()) -> basic_string<charT, traits, Allocator>; template<class charT, class traits, class Allocator = allocator<charT>> basic_string(basic_string_view<charT, traits>, typename see below::size_type, typename see below::size_type, const Allocator& = Allocator()) -> basic_string<charT, traits, Allocator>; }-?- A
size_type
parameter type in abasic_string
deduction guide refers to thesize_type
member type of the type deduced by the deduction guide.
Edit 27.4.3.3 [string.cons] as indicated:
template<class InputIterator, class Allocator = allocator<typename iterator_traits<InputIterator>::value_type>> basic_string(InputIterator, InputIterator, Allocator = Allocator()) -> basic_string<typename iterator_traits<InputIterator>::value_type, char_traits<typename iterator_traits<InputIterator>::value_type>, Allocator>;-25- Remarks: Shall not participate in overload resolution if
InputIterator
is a type that does not qualify as an input iterator, or ifAllocator
is a type that does not qualify as an allocator (23.2.2 [container.requirements.general]).template<class charT, class traits, class Allocator = allocator<charT>> explicit basic_string(basic_string_view<charT, traits>, const Allocator& = Allocator()) -> basic_string<charT, traits, Allocator>; template<class charT, class traits, class Allocator = allocator<charT>> basic_string(basic_string_view<charT, traits>, typename see below::size_type, typename see below::size_type, const Allocator& = Allocator()) -> basic_string<charT, traits, Allocator>;-?- Remarks: Shall not participate in overload resolution if
Allocator
is a type that does not qualify as an allocator (23.2.2 [container.requirements.general]).
basic_string
CTAD ambiguitySection: 27.4.3.3 [string.cons] Status: C++20 Submitter: Stephan T. Lavavej Opened: 2018-03-03 Last modified: 2021-02-25
Priority: Not Prioritized
View all other issues in [string.cons].
View all issues with C++20 status.
Discussion:
The following code fails to compile for surprising reasons.
#include <string> #include <string_view> using namespace std; int main() { string s0; basic_string s1(s0, 1, 1); // WANT: basic_string(const basic_string&, size_type, size_type, const Allocator& = Allocator()) // CONFLICT: basic_string(size_type, charT, const Allocator&) basic_string s2("cat"sv, 1, 1); // WANT: basic_string(const T&, size_type, size_type, const Allocator& = Allocator()) // CONFLICT: basic_string(size_type, charT, const Allocator&) basic_string s3("cat", 1); // WANT: basic_string(const charT *, size_type, const Allocator& = Allocator()) // CONFLICT: basic_string(const charT *, const Allocator&) }
For s1
and s2
, the signature basic_string(size_type, charT, const Allocator&)
participates in CTAD. size_type
is non-deduced (it will be substituted later, so the compiler
can't immediately realize that s0
or "cat"sv
are totally non-viable arguments).
charT
is deduced to be int
(weird, but not the problem). Finally, Allocator
is deduced to be int
. Then the compiler tries to substitute for size_type
, but
this ends up giving int to allocator_traits
in a non-SFINAE context, so compilation fails.
s3
fails for a slightly different reason. basic_string(const charT *, const Allocator&)
participates in CTAD, deducing charT
to be char
(good) and Allocator
to be
int
. This is an exact match, which is better than the constructor that the user actually wants
(where int
would need to be converted to size_type
, which is unsigned). So CTAD deduces basic_string<char, char_traits<char>, int>
, which is the wrong type.
This problem appears to be unique to basic_string
and its heavily overloaded set of constructors.
I haven't figured out how to fix it by adding (non-greedy) deduction guides. The conflicting constructors
are always considered during CTAD, regardless of whether deduction guides are provided that correspond
to the desired or conflicting constructors. (That's because deduction guides are preferred as a late
tiebreaker in overload resolution; if a constructor provides a better match it will be chosen before
tiebreaking.) It appears that we need to constrain the conflicting constructors themselves; this will
have no effect on actual usage (where Allocator
will be an allocator) but will prevent CTAD
from considering them for non-allocators. As this is unusual, I believe it deserves a Note.
This has been implemented in MSVC.
[2018-3-14 Wednesday evening issues processing; move to Ready]
[2018-06 Rapperswil: Adopted]
Proposed resolution:
This wording is relative to N4727.
Edit 27.4.3.3 [string.cons] as indicated:
basic_string(const charT* s, const Allocator& a = Allocator());-14- Requires:
-15- Effects: Constructs an object of classs
points to an array of at leasttraits::length(s) + 1
elements ofcharT
.basic_string
and determines its initial string value from the array ofcharT
of lengthtraits::length(s)
whose first element is designated bys
. -16- Postconditions:data()
points at the first element of an allocated copy of the array whose first element is pointed at bys
,size()
is equal totraits::length(s)
, andcapacity()
is a value at least as large assize()
. -?- Remarks: Shall not participate in overload resolution ifAllocator
is a type that does not qualify as an allocator (23.2.2 [container.requirements.general]). [Note: This affects class template argument deduction. — end note]basic_string(size_type n, charT c, const Allocator& a = Allocator());-17- Requires:
-18- Effects: Constructs an object of classn < npos
.basic_string
and determines its initial string value by repeating the char-like objectc
for alln
elements. -19- Postconditions:data()
points at the first element of an allocated array ofn
elements, each storing the initial valuec
,size()
is equal ton
, andcapacity()
is a value at least as large assize()
. -?- Remarks: Shall not participate in overload resolution ifAllocator
is a type that does not qualify as an allocator (23.2.2 [container.requirements.general]). [Note: This affects class template argument deduction. — end note]
(push|emplace)_back
should invalidate the end
iteratorSection: 23.3.13.5 [vector.modifiers] Status: C++20 Submitter: Casey Carter Opened: 2018-03-10 Last modified: 2021-02-25
Priority: 3
View all other issues in [vector.modifiers].
View all issues with C++20 status.
Discussion:
23.3.13.5 [vector.modifiers] paragraph 1 specifies that emplace_back
and push_back
do not invalidate iterators before the insertion point when
reallocation is unnecessary:
Remarks: Causes reallocation if the new size is greater than the old capacity. Reallocation invalidates all the references, pointers, and iterators referring to the elements in the sequence. If no reallocation happens, all the iterators and references before the insertion point remain valid. […]This statement is redundant, given the blanket wording in 23.2.2 [container.requirements.general] paragraph 12:
Unless otherwise specified (either explicitly or by defining a function in terms of other functions), invoking a container member function or passing a container as an argument to a library function shall not invalidate iterators to, or change the values of, objects within that container.It seems that this second sentence (1) should be a note that reminds us that the blanket wording applies here when no reallocation occurs, and/or (2) actually intends to specify that iterators at and after the insertion point are invalidated.
Also, it seems intended that reallocation should invalidate the end
iterator as well.
[2018-06-18 after reflector discussion]
Priority set to 3
Previous resolution [SUPERSEDED]:
Edit 23.3.13.5 [vector.modifiers] as indicated:
-1- Remarks: Invalidates the past-the-end iterator. Causes reallocation if the new size is greater than the old capacity. Reallocation invalidates all the references, pointers, and iterators referring to the elements in the sequence. [Note: If no reallocation happens, all the iterators and references before the insertion point remain valid.—end note] If an exception is thrown […]
[2018-11-28 Casey provides an updated P/R]
Per discussion in the prioritization thread on the reflector.[2018-12-01 Status to Tentatively Ready after seven positive votes on the reflector.]
Proposed resolution:
This wording is relative to the post-San Diego working draft.
Change 27.4.3.5 [string.capacity] as indicated:
void shrink_to_fit();-11- Effects:
shrink_to_fit
is a non-binding request to reducecapacity()
tosize()
. [ Note: The request is non-binding to allow latitude for implementation-specific optimizations. — end note ] It does not increasecapacity()
, but may reducecapacity()
by causing reallocation.-12- Complexity: If the size is not equal to the old capacity, linear in the size of the sequence; otherwise constant.
-13- Remarks: Reallocation invalidates all the references, pointers, and iterators referring to the elements in the sequence, as well as the past-the-end iterator. [ Note: If no reallocation happens, they remain valid. — end note ]
Change 23.3.5.3 [deque.capacity] as indicated:
void shrink_to_fit();-5- Requires:
T
shall beCpp17MoveInsertable
into*this
.-6- Effects:
shrink_to_fit
is a non-binding request to reduce memory use but does not change the size of the sequence. [ Note: The request is non-binding to allow latitude for implementation-specific optimizations. —end note ] If the size is equal to the old capacity, or if an exception is thrown other than by the move constructor of a non-Cpp17CopyInsertable
T
, then there are no effects.-7- Complexity: If the size is not equal to the old capacity, linear in the size of the sequence; otherwise constant.
-8- Remarks:
If the size is not equal to the old capacity, then invalidates all the references, pointers, and iterators referring to the elements in the sequence, as well as the past-the-end iterator.shrink_to_fit
Change 23.3.13.3 [vector.capacity] as indicated:
void reserve(size_type n);[…]
-7- Remarks: Reallocation invalidates all the references, pointers, and iterators referring to the elements in the sequence, as well as the past-the-end iterator. [ Note: If no reallocation happens, they remain valid. — end note ] No reallocation shall take place during insertions that happen after a call to
reserve()
untilthe time whenan insertion would make the size of the vector greater than the value ofcapacity()
.void shrink_to_fit();[…]
-10- Complexity: If reallocation happens, linear in the size of the sequence.
-11- Remarks: Reallocation invalidates all the references, pointers, and iterators referring to the elements in the sequence, as well as the past-the-end iterator. [ Note: If no reallocation happens, they remain valid. — end note ]
Change 23.3.13.5 [vector.modifiers] as indicated:
-1- Remarks: Causes reallocation if the new size is greater than the old capacity. Reallocation invalidates all the references, pointers, and iterators referring to the elements in the sequence as well as the past-the-end iterator. If no reallocation happens,
all the iterators and referencesthen references, pointers, and iterators before the insertion point remain valid but those at or after the insertion point, including the past-the-end iterator, are invalidated. If an exception is thrown […]-2- Complexity:
The complexity isIf reallocation happens, linear in the number of elements of the resulting vector; otherwise linear in the number of elements inserted plus the distance to the end of the vector.
existing_p
overloads of create_directory
Section: 31.12.13.8 [fs.op.create.directory] Status: C++20 Submitter: Billy O'Neal III Opened: 2018-03-07 Last modified: 2021-06-06
Priority: 0
View all issues with C++20 status.
Discussion:
LWG 2935(i) clarified that create_directory
is not supposed to report an error if
exists(p)
, even if p
is not a directory. However, the P/R there missed the
existing_p
overloads.
[ 2018-03-27 Moved to Tentatively Ready after 6 positive votes on c++std-lib. ]
[2018-06 Rapperswil: Adopted]
Proposed resolution:
This wording is relative to N4727.
Edit [fs.op.create_directory] as indicated:
bool create_directory(const path& p, const path& existing_p); bool create_directory(const path& p, const path& existing_p, error_code& ec) noexcept;-4- Effects:
Establishes the postcondition by attempting to createCreates the directoryp
resolves to, with attributes copied from directoryexisting_p
. The set of attributes copied is operating system dependent. Creation failure becausep
resolves to an existing directory shall not be treated asalready exists is not an error. [Note: For POSIX-based operating systems, the attributes are those copied by native APIstat(existing_p.c_str(), &attributes_stat)
followed bymkdir(p.c_str(), attributes_stat.st_mode)
. For Windows-based operating systems, the attributes are those copied by native APICreateDirectoryExW(existing_p.c_str(), p.c_str(), 0)
. — end note]-5- Postconditions:[…]is_directory(p)
.
from_chars
pattern specification breaks round-trippingSection: 28.2.3 [charconv.from.chars] Status: C++20 Submitter: Greg Falcon Opened: 2018-03-12 Last modified: 2021-02-25
Priority: 0
View other active issues in [charconv.from.chars].
View all other issues in [charconv.from.chars].
View all issues with C++20 status.
Discussion:
from_chars
specifies that the '+'
character is never matched, but to_chars
specifies its output format in terms of printf()
, which puts a '+'
sign before
positive exponents.
strtod()
matches '+'
signs, it is also desirable to accept '+'
in
exponents, so that code currently using strtod()
can be migrated to from_chars()
without a breaking semantic change.
[ 2018-03-27 Moved to Tentatively Ready after 9 positive votes on c++std-lib. ]
[2018-06 Rapperswil: Adopted]
Proposed resolution:
This wording is relative to N4727.
Edit 28.2.3 [charconv.from.chars] as indicated:
from_chars_result from_chars(const char* first, const char* last, float& value, chars_format fmt = chars_format::general); from_chars_result from_chars(const char* first, const char* last, double& value, chars_format fmt = chars_format::general); from_chars_result from_chars(const char* first, const char* last, long double& value, chars_format fmt = chars_format::general);-6- Requires:
-7- Effects: The pattern is the expected form of the subject sequence in thefmt
has the value of one of the enumerators ofchars_format
."C"
locale, as described forstrtod
, except that
(7.1) — the
onlysign'+'
thatmay only appearisin the exponent part;'-'
(7.2) […]
(7.3) […]
(7.4) […]
ios::iword(-1)
do?Section: 31.5.2.6 [ios.base.storage] Status: C++20 Submitter: Jonathan Wakely Opened: 2018-03-16 Last modified: 2021-02-25
Priority: 0
View all other issues in [ios.base.storage].
View all issues with C++20 status.
Discussion:
Is calling iword
and pword
with a negative argument undefined, or should it cause
a failure condition (and return a valid reference)? What about INT_MAX
? What about 0
?
ios_base::xalloc()
. Others pointed out that the iwords and pwords could be stored in sparse
arrays, so that any value from INT_MIN
to INT_MAX
could be a valid key (which might
require the implementation to use keys outside that range for its own entries in the arrays).
If it's undefined we should add a Requires element to the spec. If invalid indices are supposed
to cause a failure we need to define which indices are invalid (and ensure that's something the
implementation can check), and specify that it causes a failure.
[ 2018-03-27 Moved to Tentatively Ready after 5 positive votes on c++std-lib. ]
[2018-06 Rapperswil: Adopted]
Proposed resolution:
This wording is relative to N4727.
Edit 31.5.2.6 [ios.base.storage] as indicated:
long& iword(int idx);-?- Requires:
-3- Effects: Ifidx
is a value obtained by a call toxalloc
.iarray
is a null pointer, […] […]void*& pword(int idx);-?- Requires:
-5- Effects: Ifidx
is a value obtained by a call toxalloc
.iarray
is a null pointer, […] […]
char_traits::copy
precondition too weakSection: 27.2.2 [char.traits.require] Status: C++23 Submitter: Jonathan Wakely Opened: 2018-03-16 Last modified: 2023-11-22
Priority: 2
View other active issues in [char.traits.require].
View all other issues in [char.traits.require].
View all issues with C++23 status.
Discussion:
Table 54, Character traits requirements, says that char_traits::move
allows the ranges to overlap,
but char_traits::copy
requires that p
is not in the range [s, s + n)
. This
appears to be an attempt to map to the requirements of memmove
and memcpy
respectively,
allowing those to be used to implement the functions, however the requirements for copy
are
weaker than those for memcpy
. The C standard says for memcpy
"If copying takes place
between objects that overlap, the behavior is undefined" which is a stronger requirement than the start
of the source range not being in the destination range.
memcpy
for char_traits<char>::copy
,
resulting in undefined behaviour in this example:
char p[] = "abc"; char* s = p + 1; std::char_traits<char>::copy(s, p, 2); assert(std::char_traits<char>::compare(p, "aab", 3) == 0);
If the intention is to allow memcpy
as a valid implementation then the precondition is
wrong (unfortunately nobody realized this when fixing char_traits::move
in LWG DR 7(i)).
If the intention is to require memmove
then it is strange to have separate copy
and
move
functions that both use memmove
.
std::copy
and std::copy_backward
are not valid implementations of
char_traits::copy
either, due to different preconditions.
Changing the precondition implicitly applies to basic_string::copy
(27.4.3.7.7 [string.copy]),
and basic_string_view::copy
(27.3.3.8 [string.view.ops]), which are currently required
to support partially overlapping ranges:
std::string s = "abc"; s.copy(s.data() + 1, s.length() - 1); assert(s == "aab");
[2018-04-03 Priority set to 2 after discussion on the reflector.]
[2018-08-23 Batavia Issues processing]
No consensus for direction; revisit in San Diego. Status to Open.
[2022-04-25; Daniel rebases wording on N4910]
Previous resolution [SUPERSEDED]:
This wording is relative to N4910.
Option A:
Edit 27.2.2 [char.traits.require], Table 75 — "Character traits requirements" [tab:char.traits.req], as indicated:
Table 75 — Character traits requirements [tab:char.traits.req] Expression Return type Assertion/note
pre/post-conditionComplexity […]
X::copy(s,p,n)
X::char_type*
Preconditions: The rangesp
not in[s,s+n)
[p,p+n)
and[s,s+n)
do not overlap.
Returns:s
.
for eachi
in[0,n)
, performs
X::assign(s[i],p[i])
.linear […]
Option B:
NAD (i.e. implementations need to be fixed, in practice
char_traits::copy
andchar_traits::move
might be equivalent).
[Kona 2022-11-11; Move to Ready]
LWG voted for Option A (6 for, 0 against, 1 netural)
[2023-02-13 Approved at February 2023 meeting in Issaquah. Status changed: Voting → WP.]
Proposed resolution:
Edit 27.2.2 [char.traits.require], Table 75 — "Character traits requirements" [tab:char.traits.req], as indicated:
Table 75 — Character traits requirements [tab:char.traits.req] Expression Return type Assertion/note
pre/post-conditionComplexity […]
X::copy(s,p,n)
X::char_type*
Preconditions: The rangesp
not in[s,s+n)
[p,p+n)
and[s,s+n)
do not overlap.
Returns:s
.
for eachi
in[0,n)
, performs
X::assign(s[i],p[i])
.linear […]
&x
in §[list.ops]Section: 23.3.11.5 [list.ops] Status: C++20 Submitter: Tim Song Opened: 2018-03-19 Last modified: 2021-02-25
Priority: 3
View all other issues in [list.ops].
View all issues with C++20 status.
Discussion:
LWG 3017(i) missed an instance of &x
in 23.3.11.5 [list.ops] p14.
[2018-06-18 after reflector discussion]
Priority set to 3
[2018-10-15 Status to Tentatively Ready after seven positive votes on the reflector.]
Proposed resolution:
This wording is relative to N4727.
Edit 23.3.11.5 [list.ops] as indicated:
void splice(const_iterator position, list& x, const_iterator first, const_iterator last); void splice(const_iterator position, list&& x, const_iterator first, const_iterator last);-11- Requires: […]
-12- Effects: […] -13- Throws: Nothing. -14- Complexity: Constant time if; otherwise, linear time.
&addressof(x) == this
forward_list::merge
behavior unclear when passed *this
Section: 23.3.7.6 [forward.list.ops] Status: C++23 Submitter: Tim Song Opened: 2018-03-19 Last modified: 2023-11-22
Priority: 3
View all other issues in [forward.list.ops].
View all issues with C++23 status.
Discussion:
LWG 300(i) changed list::merge
to be a no-op when passed *this
, but there's
no equivalent rule for forward_list::merge
.
Presumably the forward_list
proposal predated the adoption of LWG 300's PR and was never updated
for the change. Everything in the discussion of that issue applies mutatis mutandis to the current
specification of forward_list::merge
.
[2018-06-18 after reflector discussion]
Priority set to 3
[2019-07-30 Tim provides updated PR]
Per the comments during issue prioritization, the new PR tries to synchronize the wording
between list::merge
and forward_list::merge
.
Previous resolution [SUPERSEDED]:
This wording is relative to N4727.
Edit [forwardlist.ops] as indicated:
void merge(forward_list& x); void merge(forward_list&& x); template<class Compare> void merge(forward_list& x, Compare comp); template<class Compare> void merge(forward_list&& x, Compare comp);-20- Requires:
-21- Effects: If*this
andx
are both sorted with respect to the comparatoroperator<
(for the first two overloads) orcomp
(for the last two overloads), andget_allocator() == x.get_allocator()
istrue
.addressof(x) == this
, does nothing. Otherwise, mMerges the two sorted ranges[begin(), end())
and[x.begin(), x.end())
. The result is a range that is sorted with respect to the comparatoroperator<
(for the first two overloads) orcomp
(for the last two overloads).x
is empty after the merge. If an exception is thrown other than by a comparison there are no effects. Pointers and references to the moved elements ofx
now refer to those same elements but as members of*this
. Iterators referring to the moved elements will continue to refer to their elements, but they now behave as iterators into*this
, not intox
. -22- Remarks: Stable (16.4.6.8 [algorithm.stable]). The behavior is undefined ifget_allocator() != x.get_allocator()
. -23- Complexity: At mostdistance(begin(), end()) + distance(x.begin(), x.end()) - 1
comparisons ifaddressof(x) != this
; otherwise, no comparisons are performed.
[2021-05-22 Tim syncs wording to the current working draft]
[2022-01-31; Reflector poll]
Set status to Tentatively Ready after six votes in favour during reflector poll.
[2022-02-10 Approved at February 2022 virtual plenary. Status changed: Tentatively Ready → WP.]
Proposed resolution:
This wording is relative to N4901.
Edit 23.3.7.6 [forward.list.ops] as indicated:
void merge(forward_list& x); void merge(forward_list&& x); template<class Compare> void merge(forward_list& x, Compare comp); template<class Compare> void merge(forward_list&& x, Compare comp);-?- Let
-24- Preconditions:comp
beless<>{}
for the first two overloads.*this
andx
are both sorted with respect to the comparatoroperator<
(for the first two overloads) orcomp
(for the last two overloads), andget_allocator() == x.get_allocator()
istrue
. -25- Effects: Ifaddressof(x) == this
, there are no effects. Otherwise, mMerges the two sorted ranges[begin(), end())
and[x.begin(), x.end())
. The result is a range that is sorted with respect to the comparatorcomp
.Pointers and references to the moved elements ofx
is empty after the merge. If an exception is thrown other than by a comparison there are no effects.x
now refer to those same elements but as members of*this
. Iterators referring to the moved elements will continue to refer to their elements, but they now behave as iterators into*this
, not intox
. -26- Complexity: At mostdistance(begin(), end()) + distance(x.begin(), x.end()) - 1
comparisons ifaddressof(x) != this
; otherwise, no comparisons are performed. -27- Remarks: Stable (16.4.6.8 [algorithm.stable]). Ifaddressof(x) != this
,x
is empty after the merge. No elements are copied by this operation. If an exception is thrown other than by a comparison there are no effects.
Edit 23.3.11.5 [list.ops] as indicated:
void merge(list& x); void merge(list&& x); template<class Compare> void merge(list& x, Compare comp); template<class Compare> void merge(list&& x, Compare comp);-?- Let
-26- Preconditions:comp
beless<>{}
for the first two overloads.Both the list and the argument list shall be*this
andx
are both sorted with respect to the comparatoroperator<
(for the first two overloads) orcomp
(for the last two overloads), andget_allocator() == x.get_allocator()
istrue
. -27- Effects: Ifaddressof(x) == this
,does nothing; othere are no effects. Otherwise, merges the two sorted ranges[begin(), end())
and[x.begin(), x.end())
. The result is a rangein which the elements will be sorted in non-decreasing order according to the ordering defined bythat is sorted with respect to the comparatorcomp
; that is, for every iteratori
, in the range other than the first, the conditioncomp(*i, *(i - 1))
will befalse
comp
. Pointers and references to the moved elements ofx
now refer to those same elements but as members of*this
. Iterators referring to the moved elements will continue to refer to their elements, but they now behave as iterators into*this
, not intox
. -28- Complexity: At mostsize() + x.size() - 1
applications ofcomparisons ifcomp
addressof(x) != this
; otherwise, noapplications ofcomparisons are performed.comp
If an exception is thrown other than by a comparison there are no effects.-29- Remarks: Stable (16.4.6.8 [algorithm.stable]). Ifaddressof(x) != this
,the range[x.begin(), x.end())
x
is empty after the merge. No elements are copied by this operation. If an exception is thrown other than by a comparison there are no effects.
time_of_day
and durations that seconds
cannot convert toSection: 30.9 [time.hms] Status: Resolved Submitter: Richard Smith Opened: 2018-03-24 Last modified: 2021-06-06
Priority: 2
View all issues with Resolved status.
Discussion:
What should happen here:
const int bpm = 100; using beats = duration<int, ratio<60, 100>>; auto v = time_of_day<beats>(beats{2}).subseconds();
? 2 beats at 100bpm is 1.2
seconds. The time_of_day
constructor specification says:
seconds()
returns the integral number of secondssince_midnight
is after(00:00:00 + hours() + minutes())
.subseconds()
returns the integral number of fractional precision secondssince_midnight
is after(00:00:00 + hours() + minutes() + seconds())
.
But that's impossible. If seconds()
returns 1, we need to return a subseconds()
value representing 0.2s
of type precision
, but type precision
can only represent multiples of 0.6s
.
time_of_day
specialization only be available for the case where seconds
is convertible to
precision
? Or should the precision
type used by this specialization be
common_type_t<seconds, duration<Rep, Period>>
rather than merely duration<Rep, Period>
?
Either way I think we need a wording update to specify one of those two behaviors.
[2018-04-09 Priority set to 2 after discussion on the reflector.]
[2019 Cologne Wednesday night]
Status to Resolved (group voted on NAD, but Marshall changed it to Resolved)
Resolved by the adoption of P1466 in Cologne.
hh_mm_ss
is now specified such that subseconds must be a
non-positive power of 10 (e.g. 1/10s, 1/100s, milliseconds, etc.). In
this example 60/100 simplifies to 3/5, which can be exactly represented
with 1 fractional decimal digit. So in this example subseconds()
has
the value of 2ds
(2 deciseconds).
Proposed resolution:
Section: 30.5.11 [time.duration.io] Status: C++20 Submitter: Richard Smith Opened: 2018-04-02 Last modified: 2021-02-25
Priority: 0
View all other issues in [time.duration.io].
View all issues with C++20 status.
Discussion:
[time.duration.io]p4 says:
For streams where
charT
has an 8-bit representation,"µs"
should be encoded as UTF-8. Otherwise UTF-16 or UTF-32 is encouraged. The implementation may substitute other encodings, including"us"
.
This choice of encoding is not up to the <chrono>
library to decide or encourage. The basic execution character
set determines how a mu should be encoded in type char
, for instance, and it would be truly bizarre to use a UTF-8
encoding if that character set is, say, Latin-1 or EBCDIC.
"us"
an "encoding" of "µs"
; it's really just an alternative unit suffix. So how about replacing that
paragraph with this:
If
Period::type
ismicro
, but the character U+00B5 cannot be represented in the encoding used forcharT
, the unit suffix"us"
is used instead of"µs"
.
(This also removes the permission for an implementation to choose an arbitrary alternative "encoding", which seems undesirable.)
[ 2018-04-23 Moved to Tentatively Ready after 6 positive votes on c++std-lib. ]
[2018-06 Rapperswil: Adopted]
Proposed resolution:
This wording is relative to N4741.
Edit 30.5.11 [time.duration.io] as indicated:
template<class charT, class traits, class Rep, class Period> basic_ostream<charT, traits>& operator<<(basic_ostream<charT, traits>& os, const duration<Rep, Period>& d);-1- Requires: […]
-2- Effects: […] -3- The units suffix depends on the typePeriod::type
as follows:
[…]
(3.5) — Otherwise, if
Period::type
ismicro
, the suffix is"µs"
("\u00b5\u0073"
).[…]
(3.21) — Otherwise, the suffix is
"[num/den]s"
.[…]
-4-For streams whereIfcharT
has an 8-bit representation,"µs"
should be encoded as UTF-8. Otherwise UTF-16 or UTF-32 is encouraged. The implementation may substitute other encodings, including"us"
Period::type
ismicro
, but the character U+00B5 cannot be represented in the encoding used forcharT
, the unit suffix"us"
is used instead of"µs"
. -5- Returns:os
.
path::lexically_relative
is confused by trailing slashesSection: 31.12.6.5.11 [fs.path.gen] Status: C++20 Submitter: Jonathan Wakely Opened: 2018-04-04 Last modified: 2021-02-25
Priority: 2
View all other issues in [fs.path.gen].
View all issues with C++20 status.
Discussion:
filesystem::proximate("/dir", "/dir/")
returns "."
when "/dir"
exists, and ".."
otherwise. It should always be "."
because whether it exists shouldn't matter.
filesystem::path::lexically_relative
, as shown by:
path("/dir").lexically_relative("/dir/."); // yields "" path("/dir").lexically_relative("/dir/"); // yields ".."
The two calls should yield the same result, and when iteration of a path
with a trailing slash gave "." as the final
element they did yield the same result. In the final C++17 spec the trailing slash produces an empty filename in the
iteration sequence, and lexically_relative
doesn't handle that correctly.
[2018-04-10, Jonathan comments]
There are more inconsistencies with paths that are "obviously" equivalent to the human reader:
path("a/b/c").lexically_relative("a/b/c") // yields "." path("a/b/c").lexically_relative("a/b/c/") // yields ".." path("a/b/c").lexically_relative("a/b/c/.") // yields "" path("a/b/c/").lexically_relative("a/b/c") // yields "" path("a/b/c/.").lexically_relative("a/b/c") // yields "." path("a/b/c/.").lexically_relative("a/b/c/") // yields "../."
I think the right solution is:
when counting [b, base.end())
in bullet (4.2) handle empty filename elements (which can only occur as the last
element, due to a trailing slash) equivalently to dot elements; and
add a new condition for the case where n == 0
and [a, end())
contains no non-empty elements, i.e. the
paths are equivalent except for final dots or a final slash, which don't introduce any relative difference between the paths.
[2018-06-18 after reflector discussion]
Priority set to 2
[2018-08-23 Batavia Issues processing]
Status to Tentatively Ready
[2018-11, Adopted in San Diego]
Proposed resolution:
This wording is relative to N4727.
Edit 31.12.6.5.11 [fs.path.gen] as indicated:
path lexically_relative(const path& base) const;-3- Returns:
-4- Effects: If*this
made relative tobase
. Does not resolve (31.12.6 [fs.class.path]) symlinks. Does not first normalize (31.12.6.2 [fs.path.generic])*this
orbase
.root_name() != base.root_name()
istrue
oris_absolute() != base.is_absolute()
istrue
or!has_root_directory() && base.has_root_directory()
istrue
, returnspath()
. Determines the first mismatched element of*this
andbase
as if by:auto [a, b] = mismatch(begin(), end(), base.begin(), base.end());Then,
(4.1) — if
a == end()
andb == base.end()
, returnspath(".")
; otherwise(4.2) — let
n
be the number of filename elements in[b, base.end())
that are not dot or dot-dot or empty, minus the number that are dot-dot. Ifn<0
, returnspath()
; otherwise(4.?) — if
n == 0
and(a == end() || a->empty())
, returnspath(".")
; otherwise(4.3) — returns an object of class
path
that is default-constructed, followed by […]
span
" wordingSection: 23.7.2.2.2 [span.cons] Status: C++20 Submitter: Stephan T. Lavavej Opened: 2018-04-12 Last modified: 2021-02-25
Priority: 0
View all other issues in [span.cons].
View all issues with C++20 status.
Discussion:
The span
constructors have wording relics that mention an "empty span
". It's unnecessary (the
behavior is fully specified by the postconditions), but I left it there because I thought it was harmless. It was
later pointed out to me that this is actually confusing. Talking about an "empty span
" implies that there's just
one such thing, but span
permits empty()
to be true
while data()
can vary
(being null or non-null). (This behavior is very useful; consider how equal_range()
behaves.)
span
" wording should simply be removed, leaving the constructor behavior unchanged.
Editorially, there's also a missing paragraph number.
[ 2018-04-24 Moved to Tentatively Ready after 6 positive votes on c++std-lib. ]
[2018-06 Rapperswil: Adopted]
Proposed resolution:
This wording is relative to N4741.
Edit 23.7.2.2.2 [span.cons] as indicated:
constexpr span() noexcept;-2- Postconditions:
-1- Effects: Constructs an emptyspan
.size() == 0 && data() == nullptr
. -3- Remarks: This constructor shall not participate in overload resolution unlessExtent <= 0
istrue
.constexpr span(pointer ptr, index_type count);-4- Requires:
-5- Effects: Constructs a[ptr, ptr + count)
shall be a valid range. Ifextent
is not equal todynamic_extent
, thencount
shall be equal toextent
.span
that is a view over the range[ptr, ptr + count)
.If-6- Postconditions:count
is0
then an empty span is constructed.size() == count && data() == ptr
. -?- Throws: Nothing.constexpr span(pointer first, pointer last);-7- Requires:
-8- Effects: Constructs a[first, last)
shall be a valid range. Ifextent
is not equal todynamic_extent
, thenlast - first
shall be equal toextent
.span
that is a view over the range[first, last)
.If-9- Postconditions:last - first == 0
then an emptyspan
is constructed.size() == last - first && data() == first
. -10- Throws: Nothing.
span
's Container
constructors need another constraintSection: 23.7.2.2.2 [span.cons] Status: C++20 Submitter: Stephan T. Lavavej Opened: 2018-04-12 Last modified: 2021-02-25
Priority: 1
View all other issues in [span.cons].
View all issues with C++20 status.
Discussion:
When I overhauled span
's constructor constraints, I was careful about the built-in array, std::array
,
and converting span
constructors. These types contain bounds information, so we can achieve safety at
compile-time by permitting implicit conversions if and only if the destination extent is dynamic (this accepts
anything by recording the size at runtime) or the source and destination extents are identical. However, I missed
the fact that the Container
constructors are the opposite case. A Container
(e.g. a vector
)
has a size that's known only at runtime. It's safe to convert this to a span
with dynamic_extent
,
but for consistency and safety, this shouldn't implicitly convert to a span
with fixed extent. (The more
verbose (ptr, count)
and (first, last)
constructors are available to construct fixed extent spans
from runtime-length ranges. Note that debug precondition checks are equally possible with the Container
and
(ptr, count)
/(first, last)
constructors. The issue is that implicit conversions are notoriously
problematic, so they should be permitted only when they are absolutely known to be safe.)
[2018-04-24 Priority set to 1 after discussion on the reflector.]
[2018-06 Rapperswil Thursday issues processing]
Status to LEWG. Should this be ill-formed, or fail at runtime if the container is too small? Discussion on the reflector here.
[2018-11 San Diego Saturday]
LEWG said that they're fine with the proposed resolution. Status to Tentatively Ready.
Proposed resolution:
This wording is relative to N4741.
Edit 23.7.2.2.2 [span.cons] as indicated:
template<class Container> constexpr span(Container& cont); template<class Container> constexpr span(const Container& cont);-14- Requires:
-15- Effects: Constructs a[data(cont), data(cont) + size(cont))
shall be a valid range.Ifextent
is not equal todynamic_extent
, thensize(cont)
shall be equal toextent
.span
that is a view over the range[data(cont), data(cont) + size(cont))
. -16- Postconditions:size() == size(cont) && data() == data(cont)
. -17- Throws: What and whendata(cont)
andsize(cont)
throw. -18- Remarks: These constructors shall not participate in overload resolution unless:
(18.?) —
extent == dynamic_extent
,(18.1) —
Container
is not a specialization ofspan
,(18.2) —
Container
is not a specialization ofarray
,[…]
span iterator
and const_iterator
behaviorSection: 23.7.2.2.1 [span.overview] Status: C++20 Submitter: Stephan T. Lavavej Opened: 2018-04-12 Last modified: 2021-02-25
Priority: 0
View all other issues in [span.overview].
View all issues with C++20 status.
Discussion:
There are multiple issues with how span specifies its iterators:
const_iterator
isn't mentioned.
The relationship between iterator
and const_iterator
isn't specified. (span
isn't
a container, so it doesn't receive this automatically.)
The iterators should be specified to be constexpr.
By imitating 27.3.3.4 [string.view.iterators]/3 "All requirements on container iterators ([container.requirements])
apply to basic_string_view::const_iterator
as well.", we can specify that iterator
is convertible to
const_iterator
.
[ 2018-04-23 Moved to Tentatively Ready after 6 positive votes on c++std-lib. ]
[2018-06 Rapperswil: Adopted]
Proposed resolution:
This wording is relative to N4741.
Edit 23.7.2.2.1 [span.overview] as indicated:
-4- The iterator types
for span is a random access iterator and a contiguous iteratorspan::iterator
andspan::const_iterator
are random access iterators (24.3.5.7 [random.access.iterators]), contiguous iterators (24.3.1 [iterator.requirements.general]), and constexpr iterators (24.3.1 [iterator.requirements.general]). All requirements on container iterators (23.2 [container.requirements]) apply tospan::iterator
andspan::const_iterator
as well.
span
should be ill-formed where possibleSection: 23.7.2.2.4 [span.sub] Status: C++20 Submitter: Tomasz Kamiński Opened: 2018-04-13 Last modified: 2021-02-25
Priority: 3
View all issues with C++20 status.
Discussion:
Currently all out-of-bound/inputs errors in the functions taking an subview of span
lead to undefined behavior,
even in the situation when they could be detected at compile time. This is inconsistent with the behavior of the span
constructors, which make similar constructs ill-formed.
subspan
function, the following invocation:
span<T, N> s; // N > 0 s.subspan<O>(); // with O > 0
is ill-formed when O > N + 1
, as the return of the function is span<T, K>
with K < -1
.
However in case when O == N + 1
, runtime sized span
is returned (span<T, -1>
) instead and
the behavior of the function is undefined.
N == dynamic_extent
) and fixed sized (N > 0
) object s
of
type span<T, N>
, the following constructs should be ill-formed, instead of having undefined behavior:
s.first<C>()
with C < 0
s.last<C>()
with C < 0
s.subspan<O, E>
with O < 0
or E < 0
and E != dynamic_extent
.
This would follow span
specification, that make instantiation of span<T, N>
ill-formed for
N < 0
and N != dynamic_extent
.
s
of type
span<T, N>
(with N > 0
):
s.first<C>()
with C > N
s.last<C>()
with C > N
s.subspan<O, dynamic_extent>()
with O > N
s.subspan<O, C>()
with O + C > N
This will match the span
constructor that made construction of fixed size span<T, N>
from fixed
size span
of different size ill-formed.
[2018-04-24 Priority set to 3 after discussion on the reflector.]
[2018-11 San Diego Thursday night issue processing]
Tomasz to provide updated wording.
Previous resolution: [SUPERSEDED]This wording is relative to N4741.
Edit 23.7.2.2.4 [span.sub] as indicated:
template<ptrdiff_t Count> constexpr span<element_type, Count> first() const;-?- Remarks: If
-1- Requires:Count < 0 || (Extent != dynamic_extent && Count > Extent)
, the program is ill-formed.. -2- Effects: Equivalent to:
0 <= Count &&Count <= size()return {data(), Count};
template<ptrdiff_t Count> constexpr span<element_type, Count> last() const;-?- Remarks: If
-3- Requires:Count < 0 || (Extent != dynamic_extent && Count > Extent)
, the program is ill-formed.. -4- Effects: Equivalent to:
0 <= Count &&Count <= size()return {data() + (size() - Count), Count};
template<ptrdiff_t Offset, ptrdiff_t Count = dynamic_extent> constexpr span<element_type, see below> subspan() const;-?- Remarks: The program is ill-formed if:
-5- Requires:
Offset < 0 || (Count < 0 && Count != dynamic_extent)
, or
Extend != dynamic_extent && (Offset > Extent || (Count != dynamic_extent && Offset + Count > Extent))
.. -6- Effects: Equivalent to:
(0 <= Offset &&Offset <= size())&& (Count == dynamic_extent ||Count >= 0 &&Offset + Count <= size())return span<ElementType, see below>( data() + Offset, Count != dynamic_extent ? Count : size() - Offset);
-7- Remarks: The second template argument of the returnedspan
type is:Count != dynamic_extent ? Count : (Extent != dynamic_extent ? Extent - Offset : dynamic_extent)
[2018-11-09; Tomasz provides updated wording]
I have decided to replace all Requires: elements in the section 23.7.2.2.4 [span.sub] to preserve consistency.
Previous resolution: [SUPERSEDED]This wording is relative to N4778.
Edit 23.7.2.2.4 [span.sub] as indicated:
template<ptrdiff_t Count> constexpr span<element_type, Count> first() const;-?- Mandates:
-1-Count >= 0 && (Extent == dynamic_extent || Count <= Extent)
.RequiresExpects:. -2- Effects: Equivalent to:
0 <= Count &&Count <= size()return {data(), Count};
template<ptrdiff_t Count> constexpr span<element_type, Count> last() const;-?- Mandates:
-3-Count >= 0 && (Extent == dynamic_extent || Count <= Extent)
.RequiresExpects:. -4- Effects: Equivalent to:
0 <= Count &&Count <= size()return {data() + (size() - Count), Count};
template<ptrdiff_t Offset, ptrdiff_t Count = dynamic_extent> constexpr span<element_type, see below> subspan() const;-?- Mandates:
-5-Offset >= 0 && (Count >= 0 || Count == dynamic_extent) && (Extent == dynamic_extent || (Offset <= Extent && (Count == dynamic_extent || Offset + Count <= Extent)))
.RequiresExpects:. -6- Effects: Equivalent to:
(0 <= Offset &&Offset <= size())&& (Count == dynamic_extent ||Count >= 0 &&Offset + Count <= size())return span<ElementType, see below>( data() + Offset, Count != dynamic_extent ? Count : size() - Offset);
-7- Remarks: The second template argument of the returnedspan
type is:Count != dynamic_extent ? Count : (Extent != dynamic_extent ? Extent - Offset : dynamic_extent)constexpr span<element_type, dynamic_extent> first(index_type count) const;-8-
-9- Effects: Equivalent to:RequiresExpects:0 <= count && count <= size()
.return {data(), count};
constexpr span<element_type, dynamic_extent> last(index_type count) const;-10-
-11- Effects: Equivalent to:RequiresExpects:0 <= count && count <= size()
.return {data() + (size() - count), count};
constexpr span<element_type, dynamic_extent> subspan( index_type offset, index_type count = dynamic_extent) const;-12-
-13- Effects: Equivalent to:RequiresExpects:(0 <= offset && offset <= size()) && (count == dynamic_extent || count >= 0 && offset + count <= size())
return {data() + offset, count == dynamic_extent ? size() - offset : count};
[2019-06-23; Tomasz comments and provides updated wording]
The current proposed resolution no longer applies to the newest revision of the standard
(N4820), due changes introduced in
P1227 (making size()
and template parameters
of span
unsigned).
[2019 Cologne Wednesday night]
Status to Ready
Proposed resolution:
This wording is relative to N4820.
[Drafting note: This wording relies on observation, that the condition in formExtent == dynamic_extent || Count <= Extent
, can be simplified intoCount <= Extent
, becausedynamic_extent
is equal tonumeric_limits<size_t>::max()
, thussize() <= Extent
is always true, andExtent == dynamic_extent
implies thatCount <= Extent
. Furthermore we check thatCount != dynamic_extent || Count <= Extent - Offset
, as theOffset + Count <= Extent
may overflow (defined for unsigned integers) and produce false positive result. This change is also applied to Expects clause. ]
Edit 23.7.2.2.4 [span.sub] as indicated:
template<size_t Count> constexpr span<element_type, Count> first() const;-?- Mandates:
-1- Expects:Count <= Extent
istrue
.Count <= size()
istrue
. -2- Effects: Equivalent to:return {data(), Count};
template<size_t Count> constexpr span<element_type, Count> last() const;-?- Mandates:
-3- Expects:Count <= Extent
istrue
.Count <= size()
istrue
. -4- Effects: Equivalent to:return {data() + (size() - Count), Count};
template<size_t Offset, size_t Count = dynamic_extent> constexpr span<element_type, see below> subspan() const;[…]-?- Mandates:
-5- Expects:Offset <= Extent && (Count == dynamic_extent || Count <= Extent - Offset)
istrue
.Offset <= size() && (Count == dynamic_extent ||
isOffset + Count <= size()Count <= size() - Offset)true
. -6- Effects: Equivalent to:return span<ElementType, see below>(data() + Offset, Count != dynamic_extent ? Count : size() - Offset);
-7- Remarks: The second template argument of the returnedspan
type is:Count != dynamic_extent ? Count : (Extent != dynamic_extent ? Extent - Offset : dynamic_extent)constexpr span<element_type, dynamic_extent> subspan( index_type offset, index_type count = dynamic_extent) const;-12- Expects:
-13- Effects: Equivalent to:offset <= size() && (count == dynamic_extent ||
isoffset + count <= size()count <= size() - offset)true
.return {data() + offset, count == dynamic_extent ? size() - offset : count};
duration
divisionSection: 30.5.6 [time.duration.nonmember] Status: C++20 Submitter: Johel Ernesto Guerrero Peña Opened: 2018-04-17 Last modified: 2021-02-25
Priority: 0
View all other issues in [time.duration.nonmember].
View all issues with C++20 status.
Discussion:
[time.duration.nonmember]/1 states
In the function descriptions that follow,
CD
represents the return type of the function.
From what I could find, many definitions of CD
in the paragraphs of [time.duration.nonmember] were lifted to
[time.duration.nonmember]/1 as cited above. That works for all other paragraphs, but not for [time.duration.nonmember]/10,
which the change rendered ill-formed:
template<class Rep1, class Period1, class Rep2, class Period2> constexpr common_type_t<Rep1, Rep2> operator/(const duration<Rep1, Period1>& lhs, const duration<Rep2, Period2>& rhs);Returns:CD(lhs).count() / CD(rhs).count()
.
In this case, we want CD
to mean common_type_t<duration<Rep1, Period1>, duration<Rep2, Period2>>
.
That way, the division has the expected semantics of dividing two quantities of the same dimension.
[ 2018-04-24 Moved to Tentatively Ready after 6 positive votes on c++std-lib. ]
[2018-06 Rapperswil: Adopted]
Proposed resolution:
This wording is relative to N4741.
Edit 30.5.6 [time.duration.nonmember] as indicated:
-1- In the function descriptions that follow, unless stated otherwise, let
[…]CD
representsthe return type of the function.template<class Rep1, class Period1, class Rep2, class Period2> constexpr common_type_t<Rep1, Rep2> operator/(const duration<Rep1, Period1>& lhs, const duration<Rep2, Period2>& rhs);Let
-10- Returns:CD
becommon_type_t<duration<Rep1, Period1>, duration<Rep2, Period2>>
.CD(lhs).count() / CD(rhs).count()
.
Section: 24.3.1 [iterator.requirements.general] Status: Resolved Submitter: Marc Aldorasi Opened: 2018-05-07 Last modified: 2020-09-06
Priority: 3
View all other issues in [iterator.requirements.general].
View all issues with Resolved status.
Discussion:
In [iterator.requirements.general] paragraph 6, contiguous iterators are defined in terms of general iterators, not random-access iterators. Since the defining expressions require random-access and the original paper's introduction describes contiguous iterators as a refinement of random-access iterators, contiguous iterators should be defined in terms of random-access iterators.
[2018-06-18 after reflector discussion]
Priority set to 3
Previous resolution [SUPERSEDED]:
This wording is relative to N4741.
Edit 24.3.1 [iterator.requirements.general] as indicated:
-6- Random-access i
Iterators that further satisfy the requirement that, for integral valuesn
and dereferenceable iterator valuesa
and(a + n)
,*(a + n)
is equivalent to*(addressof(*a) + n)
, are called contiguous iterators. [Note: For example, the type "pointer toint
" is a contiguous iterator, butreverse_iterator<int *>
is not. For a valid iterator range[a, b)
with dereferenceable a, the corresponding range denoted by pointers is[addressof(*a), addressof(*a) + (b - a))
;b
might not be dereferenceable. — end note]
[2020-05-03 Reflector discussion]
Resolved by P0894R4.
Rationale:
Resolved by P0894R4Proposed resolution:
basic_string
constructorSection: 27.4.3.3 [string.cons] Status: Resolved Submitter: Andrzej Krzemienski Opened: 2018-05-09 Last modified: 2020-09-06
Priority: 2
View all other issues in [string.cons].
View all issues with Resolved status.
Discussion:
The following is the spec for basic_string
constructor taking a pointer and a size in
N4741 ([string.cons]/12-14):
basic_string(const charT* s, size_type n, const Allocator& a = Allocator());
(12) Requires:
s
points to an array of at leastn
elements ofcharT
.(13) Effects: Constructs an object of class
basic_string
and determines its initial string value from the array ofcharT
of lengthn
whose first element is designated bys
.(14) Postconditions:
data()
points at the first element of an allocated copy of the array whose first element is pointed at bys
,size()
is equal ton
, andcapacity()
is a value at least as large assize()
.
This implies that passing a null pointer and a zero size to this constructor is violating the precondition, as null
pointer cannot be described as "pointing to an array of at least n
elements of charT
".
On the other hand, being able to pass {nullptr, 0}
is essential for basic_string
to be
able to inter-operate with other containers that are allowed to use the null pointer value to represent sequences
of size zero:
std::vector<char> v{}; assert(v.data() == nullptr); // on some implementations std::string s(v.data(), v.size()); // nullptr on some implementations
This has been already acknowledged as a defect in issue 2235(i) and applied, but the resolution still implies a too strong precondition.
Previous resolution [SUPERSEDED]:
This wording is relative to N4741.
Edit 27.4.3.3 [string.cons] as indicated:
basic_string(const charT* s, size_type n, const Allocator& a = Allocator());-12- Requires: Unless
-13- Effects: Constructs an object of classn == 0
,s
points to an array of at leastn
elements ofcharT
.basic_string
and, unlessn == 0
, determines its initial string value from the array ofcharT
of lengthn
whose first element is designated bys
. -14- Postconditions: Ifn != 0
, thendata()
points at the first element of an allocated copy of the array whose first element is pointed at bys
,;size()
is equal ton
, andcapacity()
is a value at least as large assize()
.
[2016-06-04 Marshall provides alternate resolution]
[2018-06-18 after reflector discussion]
Priority set to 2
[2018-08 mailing list discussion]
This will be resolved by Tim's string rework paper.
Resolved by the adoption of P1148 in San Diego.
Proposed resolution:
This wording is relative to N4750.
Edit 27.4.3.3 [string.cons] as indicated:
basic_string(const charT* s, size_type n, const Allocator& a = Allocator());-12- Requires:
[s, s + n)
is a valid range.s
points to an array of at leastn
elements ofcharT
-13- Effects: Constructs an object of class
-14- Postconditions:basic_string
and determines its initial string value from the range[s, s + n)
array of.charT
of lengthn
whose first element is designated bys
data()
points at the first element of an allocated copy of the array whose first element is pointed at bys
,size()
is equal ton
,andcapacity()
is a value at least as large assize()
, andtraits::compare(data(), s, n) == 0
.
system_error
and filesystem_error
constructors taking a string
may not be able
to meet their postconditionsSection: 19.5.8.2 [syserr.syserr.members], 31.12.7.2 [fs.filesystem.error.members] Status: C++20 Submitter: Tim Song Opened: 2018-05-10 Last modified: 2021-06-06
Priority: 0
View all other issues in [syserr.syserr.members].
View all issues with C++20 status.
Discussion:
The constructors of system_error
and filesystem_error
taking a std::string
what_arg
are specified to have a postcondition of string(what()).find(what_arg) != string::npos
(or the equivalent with
string_view
). This is not possible if what_arg
contains an embedded null character.
[2019-01-20 Reflector prioritization]
Set Priority to 0 and status to Tentatively Ready
Proposed resolution:
This wording is relative to N4727.
Drafting note: This contains a drive-by editorial change to use
string_view
for these postconditions rather thanstring
.
Edit 19.5.8.2 [syserr.syserr.members] p1-4 as indicated:
system_error(error_code ec, const string& what_arg);-1- Effects: Constructs an object of class
-2- Postconditions:system_error
.code() == ec
andstring_view(what()).find(what_arg.c_str()) != string_view::npos
.system_error(error_code ec, const char* what_arg);-3- Effects: Constructs an object of class
-4- Postconditions:system_error
.code() == ec
andstring_view(what()).find(what_arg) != string_view::npos
.
Edit 19.5.8.2 [syserr.syserr.members] p7-10 as indicated:
system_error(int ev, const error_category& ecat, const std::string& what_arg);-7- Effects: Constructs an object of class
-8- Postconditions:system_error
.code() == error_code(ev, ecat)
andstring_view(what()).find(what_arg.c_str()) != string_view::npos
.system_error(int ev, const error_category& ecat, const char* what_arg);-9- Effects: Constructs an object of class
-10- Postconditions:system_error
.code() == error_code(ev, ecat)
andstring_view(what()).find(what_arg) != string_view::npos
.
Edit [fs.filesystem_error.members] p2-4 as indicated:
filesystem_error(const string& what_arg, error_code ec);-2- Postconditions:
code() == ec
,path1().empty() == true
,path2().empty() == true
, andstring_view(what()).find(what_arg.c_str()) != string_view::npos
.filesystem_error(const string& what_arg, const path& p1, error_code ec);-3- Postconditions:
code() == ec
,path1()
returns a reference to the stored copy ofp1
,path2().empty() == true
, andstring_view(what()).find(what_arg.c_str()) != string_view::npos
.filesystem_error(const string& what_arg, const path& p1, const path& p2, error_code ec);-4- Postconditions:
code() == ec
,path1()
returns a reference to the stored copy ofp1
,path2()
returns a reference to the stored copy ofp2
,string_view(what()).find(what_arg.c_str()) != string_view::npos
.
polymorphic_allocator::construct()
should more closely match scoped_allocator_adaptor::construct()
Section: 20.5.3.3 [mem.poly.allocator.mem] Status: Resolved Submitter: Arthur O'Dwyer Opened: 2018-05-14 Last modified: 2020-09-06
Priority: 3
View all other issues in [mem.poly.allocator.mem].
View all issues with Resolved status.
Discussion:
Based on my new understanding of how uses_allocator
and is_constructible
are supposed to play together,
I think there was a minor defect in the resolution of LWG 2969(i) "polymorphic_allocator::construct()
shouldn't pass resource()
".
is_constructible
in 20.5.3.3 [mem.poly.allocator.mem] (10.2), which were added to resolve LWG
2969(i), are missing an lvalue-qualification to match the lvalue-qualification of the parallel calls in
[llocator.adaptor.member] (9.2).
The latter talks about constructibility from inner_allocator_type&
; the former (after LWG 2969) talks about
constructibility from polymorphic_allocator
with no ref-qualification. But since we are perfect-forwarding
*this
through a tuple of references, we definitely are going to try to construct the target object from
a polymorphic_allocator&
and not from a polymorphic_allocator
. I believe that the wording in
20.5.3.3 [mem.poly.allocator.mem] (10.2) needs to be updated to make the program ill-formed in cases where
that construction is going to fail.
Orthogonally, I believe we need additional std::move
's in the sentence following 20.5.3.3 [mem.poly.allocator.mem] (10.8)
for two reasons:
We don't want to copy-construct the user's arguments that came in
by-value as part of x
and/or y
.
The value category of the argument to pair
's constructor is currently
unspecified; it could reasonably be either an xvalue or a prvalue. Adding the std::move
makes it clearer
that we really want it to be an xvalue.
[2018-06-18 after reflector discussion]
Priority set to 3
[2019-02; Kona Wednesday night issue processing]
This was resolved by the adoption of P0591 in San Diego.
Proposed resolution:
This wording is relative to N4750.
Edit 20.5.3.3 [mem.poly.allocator.mem] as indicated:
template<class T1, class T2, class... Args1, class... Args2> void construct(pair<T1, T2>* p, piecewise_construct_t, tuple<Args1...> x, tuple<Args2...> y);-9- […]
-10- Effects:: […]
(10.1) — […]
(10.2) — Otherwise, if
uses_allocator_v<T1,polymorphic_allocator>
istrue
andis_constructible_v<T1, allocator_arg_t, polymorphic_allocator&, Args1...>
istrue
, thenxprime
istuple_cat(
.make_tupletuple<allocator_arg_t, polymorphic_allocator&>(allocator_arg, *this), std::move(x))(10.3) — Otherwise, if
uses_allocator_v<T1, polymorphic_allocator>
istrue
andis_constructible_v<T1, Args1..., polymorphic_allocator&>
istrue
, thenxprime
istuple_cat(std::move(x),
.make_tupletuple<polymorphic_allocator&>(*this))(10.4) — Otherwise the program is ill formed.
Let
yprime
be a tuple constructed fromy
according to the appropriate rule from the following list:
(10.5) — […]
(10.6) — Otherwise, if
uses_allocator_v<T2, polymorphic_allocator>
istrue
andis_constructible_v<T2, allocator_arg_t, polymorphic_allocator&, Args2...>
istrue
, thenyprime
istuple_cat(
.make_tupletuple<allocator_arg_t, polymorphic_allocator&>(allocator_arg, *this), std::move(y))(10.7) — Otherwise, if
uses_allocator_v<T2, polymorphic_allocator>
istrue
andis_constructible_v<T2, Args2..., polymorphic_allocator&>
istrue
, thenyprime
istuple_cat(std::move(y),
.make_tupletuple<polymorphic_allocator&>(*this))(10.8) — Otherwise the program is ill formed.
Then, using
piecewise_construct
,std::move(xprime)
, andstd::move(yprime)
as the constructor arguments, this function constructs apair<T1, T2>
object in the storage whose address is represented byp
.
includes
Section: 26.8.7.2 [includes] Status: Resolved Submitter: Casey Carter Opened: 2018-05-24 Last modified: 2020-09-06
Priority: 3
View all other issues in [includes].
View all issues with Resolved status.
Discussion:
26.8.7.2 [includes]/1 states:
Returns:
true
if[first2, last2)
is empty or if every element in the range[first2, last2)
is contained in the range[first1, last1)
. Returnsfalse
otherwise.
but this program:
#include <algorithm> #include <array> int main() { std::array<int, 1> a{1}; std::array<int, 3> b{1,1,1}; return std::includes(a.begin(), a.end(), b.begin(), b.end()); }
returns 0
on every implementation I can find, despite
that every element in the range b
is contained in the range a
. The design intent of the algorithm is
actually to determine if the sorted intersection of the elements from the two ranges — as would be computed by the
set_intersection
algorithm — is the same sequence as the range [first2, last2)
.
The specification should say so.
The complexity bound in 26.8.7.2 [includes]/2 is also unnecessarily high: straightforward implementations perform
at most 2 * (last1 - first1)
comparisons.
Previous resolution [SUPERSEDED]:
This wording is relative to N4750.
Modify 26.8.7.2 [includes] as indicated:
template<class InputIterator1, class InputIterator2> constexpr bool includes(InputIterator1 first1, InputIterator1 last1, InputIterator2 first2, InputIterator2 last2); […]-1- Returns:
-2- Complexity: At mosttrue
if and only if[first2, last2)
isempty or if every element in the rangea subsequence of[first2, last2)
is contained in the range[first1, last1)
. Returnsfalse
otherwise[first1, last1)
.2 *
comparisons.((last1 - first1)+ (last2 - first2)) - 1
[2018-06-27 after reflector discussion]
Priority set to 3. Improved wording as result of that discussion.
[2018-11-13; Casey Carter comments]
The acceptance of P0896R4 during the San Diego meeting resolves this issue: The wording in [includes] includes the PR for LWG 3115.
Proposed resolution:
This wording is relative to N4750.
Modify 26.8.7.2 [includes] as indicated:
template<class InputIterator1, class InputIterator2> constexpr bool includes(InputIterator1 first1, InputIterator1 last1, InputIterator2 first2, InputIterator2 last2); […]-1- Returns:
-2- Complexity: At mosttrue
if and only if[first2, last2)
isempty or if every element in the rangea subsequence of[first2, last2)
is contained in the range[first1, last1)
. Returnsfalse
otherwise[first1, last1)
. [Note: A sequenceS
is a subsequence of another sequenceT
ifS
can be obtained fromT
by removing some, all, or none ofT
's elements and keeping the remaining elements in the same order. — end note].2 *
comparisons.((last1 - first1)+ (last2 - first2)) - 1
OUTERMOST_ALLOC_TRAITS
needs remove_reference_t
Section: 20.6.4 [allocator.adaptor.members] Status: C++20 Submitter: Tim Song Opened: 2018-06-04 Last modified: 2021-02-25
Priority: 0
View all other issues in [allocator.adaptor.members].
View all issues with C++20 status.
Discussion:
OUTERMOST_ALLOC_TRAITS(x)
is currently defined in 20.6.4 [allocator.adaptor.members]p1 as
allocator_traits<decltype(OUTERMOST(x))>
. However, OUTERMOST(x)
, as defined and used
in this subclause, is an lvalue for which decltype
produces an lvalue reference. That referenceness needs to be
removed before the type can be used with allocator_traits
.
OUTERMOST
uses the imprecise "if x
does not have an
outer_allocator()
member function". What we meant to check is the validity of the expression x.outer_allocator()
,
not whether x
has some (possibly ambiguous and/or inaccessible) member function named outer_allocator
.
[2018-06 Rapperswil Thursday issues processing]
Status to Ready
[2018-11, Adopted in San Diego]
Proposed resolution:
This wording is relative to N4750.
[Drafting note: The subclause only uses
OUTERMOST_ALLOC_TRAITS(*this)
and only in non-const
member functions, so the result is also non-const
. Thus,remove_reference_t
is sufficient; there's no need to further remove cv-qualification. — end drafting note]
Modify 20.6.4 [allocator.adaptor.members]p1 as indicated:
-1- In the
construct
member functions,OUTERMOST(x)
isx
ifx
does not have anouter_allocator()
member function andOUTERMOST(x.outer_allocator())
if the expressionx.outer_allocator()
is valid (13.10.3 [temp.deduct]) andx
otherwise;OUTERMOST_ALLOC_TRAITS(x)
isallocator_traits<remove_reference_t<decltype(OUTERMOST(x))>>
. [Note: […] — end note]
packaged_task
deduction guidesSection: 32.10.10 [futures.task] Status: C++23 Submitter: Marc Mutz Opened: 2018-06-08 Last modified: 2023-11-22
Priority: 3
View all other issues in [futures.task].
View all issues with C++23 status.
Discussion:
std::function
has deduction guides, but std::packaged_task
, which is otherwise very
similar, does not. This is surprising to users and I can think of no reason for the former
to be treated differently from the latter. I therefore propose to add deduction guides for
packaged task with the same semantics as the existing ones for function.
[2018-06-23 after reflector discussion]
Priority set to 3
Previous resolution [SUPERSEDED]:
This wording is relative to N4750.
Modify 32.10.10 [futures.task], class template
packaged_task
synopsis, as indicated:namespace std { […] template<class R, class... ArgTypes> class packaged_task<R(ArgTypes...)> { […] }; template<class R, class... ArgTypes> packaged_task(R (*)( ArgTypes ...)) -> packaged_task<R( ArgTypes...)>; template<class F> packaged_task(F) -> packaged_task<see below>; template<class R, class... ArgTypes> void swap(packaged_task<R(ArgTypes...)>& x, packaged_task<R(ArgTypes...)>& y) noexcept; }Modify 32.10.10.2 [futures.task.members] as indicated:
template<class F> packaged_task(F&& f);[…]template<class F> packaged_task(F) -> packaged_task<see below>;[…]-?- Remarks: This deduction guide participates in overload resolution only if
&F::operator()
is well-formed when treated as an unevaluated operand. In that case, ifdecltype(&F::operator())
is of the formR(G::*)(A...) cv &opt noexceptopt
for a class typeG
, then the deduced type ispackaged_task<R(A...)>
.packaged_task(packaged_task&& rhs) noexcept;
[2020-02-13; Prague]
LWG improves wording matching Marshall's Mandating paper.
[2020-02-14; Prague]
Do we want a feature test macro for this new feature?
F N A 1 7 6
[Status to Ready on Friday in Prague.]
[2020-11-09 Approved In November virtual meeting. Status changed: Ready → WP.]
Proposed resolution:
This wording is relative to N4849.
Modify 32.10.10 [futures.task], class template packaged_task
synopsis, as indicated:
namespace std { […] template<class R, class... ArgTypes> class packaged_task<R(ArgTypes...)> { […] }; template<class R, class... ArgTypes> packaged_task(R (*)(ArgTypes...)) -> packaged_task<R(ArgTypes...)>; template<class F> packaged_task(F) -> packaged_task<see below>; template<class R, class... ArgTypes> void swap(packaged_task<R(ArgTypes...)>& x, packaged_task<R(ArgTypes...)>& y) noexcept; }
Modify 32.10.10.2 [futures.task.members] as indicated:
template<class F> packaged_task(F&& f);[…]template<class F> packaged_task(F) -> packaged_task<see below>;[…]-?- Constraints:
-?- Remarks: The deduced type is&F::operator()
is well-formed when treated as an unevaluated operand anddecltype(&F::operator())
is of the formR(G::*)(A...) cv &opt noexceptopt
for a class typeG
.packaged_task<R(A...)>
.packaged_task(packaged_task&& rhs) noexcept;
fpos
equality comparison unspecifiedSection: 31.5.3.3 [fpos.operations] Status: C++23 Submitter: Jonathan Wakely Opened: 2018-06-04 Last modified: 2023-11-22
Priority: 4
View all other issues in [fpos.operations].
View all issues with C++23 status.
Discussion:
The fpos
requirements do not give any idea what is compared by operator==
(even after Daniel's
P0759R1 paper). I'd like something to make it clear that return true;
is not a valid implementation of operator==(const fpos<T>&, const fpos<T>&)
. Maybe in the
P(o)
row state that "p == P(o)
" and "p != P(o + 1)
", i.e. two fpos
objects
constructed from the same streamoff
values are equal, and two fpos
objects constructed from two
different streamoff
values are not equal.
[2018-06-23 after reflector discussion]
Priority set to 4
[2022-05-01; Daniel comments and provides wording]
The proposed wording does intentionally not use a form involving addition or subtraction to prevent the need
for extra wording that ensures that this computed value is well-defined. According to 31.2.2 [stream.types],
streamoff
is a signed basic integral type, so we know what equality means for such values.
Previous resolution [SUPERSEDED]:
This wording is relative to N4910.
Modify in 31.5.3.3 [fpos.operations] as indicated:
[Drafting note: The return type specification of
operator==
should be resolved in sync with D2167R2; see also LWG 2114(i).]
(1.1) — […]
[…]
(1.5) —
o
ando2
referstoavalues of typestreamoff
orconst streamoff
.
Table 119: Position type requirements [tab:fpos.operations] Expression Return type Operational
semanticsAssertion/note
pre-/post-condition… P p(o);
P p = o;Effects: Value-initializes the
state object.
Postconditions:p == P(o)
istrue
.… O(p)
streamoff
converts to offset
P(O(p)) == p
p == q
convertible to bool
Remarks: For any two values o
ando2
, if
p
is obtained fromo
converted toP
or from a copy
of suchP
value and ifq
is obtained fromo2
converted toP
or from a copy of suchP
value, then
bool(p == q)
istrue
only ifo == o2
istrue
.p != q
convertible to bool
!(p == q)
…
[2022-11-02; Daniel comments and improves wording]
LWG discussion of P2167R2
has shown preference to require that the equality operations of fpos
should be specified to have type
bool
instead of being specified as "convertible to bool
". This has been reflected in the most recent
paper revision P2167R3. The below wording changes
follow that direction to reduce the wording mismatch to a minimum.
[2022-11-02; LWG telecon]
Moved to Ready. For: 6, Against: 0, Neutral: 0
[2022-11-12 Approved at November 2022 meeting in Kona. Status changed: Voting → WP.]
Proposed resolution:
This wording is relative to N4917.
Modify in 31.5.3.3 [fpos.operations] as indicated:
(1.1) — […]
[…]
(1.5) —
o
ando2
referstoavalues of typestreamoff
orconst streamoff
.
Table 124: Position type requirements [tab:fpos.operations] Expression Return type Operational
semanticsAssertion/note
pre-/post-condition… P p(o);
P p = o;Effects: Value-initializes the
state object.
Postconditions:p == P(o)
istrue
.… O(p)
streamoff
converts to offset
P(O(p)) == p
p == q
bool
Remarks: For any two values o
ando2
, if
p
is obtained fromo
converted toP
or from a copy
of suchP
value and ifq
is obtained fromo2
converted toP
or from a copy of suchP
value, then
p == q
istrue
only ifo == o2
istrue
.p != q
convertible tobool
!(p == q)
…
Section: 3.42 [defns.prog.def.spec] Status: C++20 Submitter: Hubert Tong Opened: 2018-06-09 Last modified: 2021-06-06
Priority: 2
View all issues with C++20 status.
Discussion:
The description of closure types in 7.5.6.2 [expr.prim.lambda.closure] says:
An implementation may define the closure type differently […]
The proposed resolution to LWG 2139(i) defines a "program-defined type" to be a
class type or enumeration type that is not part of the C++ standard library and not defined by the implementation, or an instantiation of a program-defined specialization
I am not sure that the intent of whether closure types are or are not program-defined types is clearly conveyed by the wording.
[2018-06-23 after reflector discussion]
Priority set to 2
[2018-08-14 Casey provides additional discussion and a Proposed Resolution]
We use the term "program-defined" in the library specification to ensure that
two users cannot create conflicts in a component in namespace std
by
specifying different behaviors for the same type. For example, we allow users to
specialize common_type
when at least one of the parameters is a
program-defined type. Since two users cannot define the same program-defined
type, this rule prevents two users (or libraries) defining the same
specialization of std::common_type
.
Since it's guaranteed that even distinct utterances of identical lambda expressions produce closures with distinct types (7.5.6.2 [expr.prim.lambda.closure]), adding closure types to our term "program-defined type" is consistent with the intended use despite that such types are technically defined by the implementation.
Previous resolution [SUPERSEDED]:
This wording is relative to N4762.
Modify 3.43 [defns.prog.def.type] as follows:
program-defined type
class type or enumeration type that is not part of the C++ standard library and not defined by the implementation (except for closure types (7.5.6.2 [expr.prim.lambda.closure]) for program-defined lambda expressions), or an instantiation of a program-defined specialization
[2018-08-23 Batavia Issues processing]
Updated wording
[2018-11 San Diego Thursday night issue processing]
Status to Ready.
Proposed resolution:
This wording is relative to N4762.
Modify 3.43 [defns.prog.def.type] as follows:
program-defined type
non-closure class type or enumeration type that is not part of the C++ standard library and not defined by the implementation, or a closure type of a non-implementation-provided lambda expression, or an instantiation of a program-defined specialization
monotonic_buffer_resource::release()
Section: 20.5.6.3 [mem.res.monotonic.buffer.mem] Status: C++23 Submitter: Arthur O'Dwyer Opened: 2018-06-10 Last modified: 2023-11-22
Priority: 2
View all other issues in [mem.res.monotonic.buffer.mem].
View all issues with C++23 status.
Discussion:
The effects of monotonic_buffer_resource::release()
are defined as:
Calls
upstream_rsrc->deallocate()
as necessary to release all allocated memory.
This doesn't give any instruction on what to do with the memory controlled by the monotonic_buffer_resource
which
was not allocated, i.e., what to do with the initial buffer provided to its constructor.
release()
.
Arthur O'Dwyer's proposed pmr implementation for libc++ reuses the initial buffer after a release()
, on the
assumption that this is what the average library user will be expecting.
#include <memory_resource> int main() { char buffer[100]; { std::pmr::monotonic_buffer_resource mr(buffer, 100, std::pmr::null_memory_resource()); mr.release(); mr.allocate(60); // A } { std::pmr::monotonic_buffer_resource mr(buffer, 100, std::pmr::null_memory_resource()); mr.allocate(60); // B mr.release(); mr.allocate(60); // C } }
Assume that allocation "B" always succeeds.
With the proposed libc++ implementation, allocations "A" and "C" both succeed.
With Boost.Container's implementation, allocations "A" and "C" both fail.
Using another plausible implementation strategy, allocation "A" could succeed but allocation "C"
could fail. I have been informed that MSVC's implementation does this.
release()
which goes underspecified by the Standard is the effect of
release()
on next_buffer_size
. As currently written, my interpretation is that
release()
is not permitted to decrease current_buffer_size
; I'm not sure if this
is a feature or a bug.
Consider this test case (taken from here):
std::pmr::monotonic_buffer_resource mr(std::pmr::new_delete_resource()); for (int i=0; i < 100; ++i) { mr.allocate(1); // D mr.release(); }
Arthur believes it is important that the 100th invocation of line "D" does not attempt to allocate 2100 bytes from the upstream resource.
[2018-06-23 after reflector discussion]
Priority set to 2
Previous resolution [SUPERSEDED]:
This wording is relative to N4750.
[Drafting note: The resolution depicted below would make MSVC's and my-proposed-libc++'s implementations both conforming.]
Modify 20.5.6.3 [mem.res.monotonic.buffer.mem] as indicated:
void release();-1- Effects: Calls
-2- [Note: The memory is released back toupstream_rsrc->deallocate()
as necessary to release all allocated memory. Resets the state of the initial buffer.upstream_rsrc
even if some blocks that were allocated from this have not been deallocated from this. This function has an unspecified effect onnext_buffer_size
. — end note]
[2018-08-23 Batavia Issues processing]
We liked Pablo's wording from the reflector discussion. Status to Open.
Previous resolution [SUPERSEDED]:
This wording is relative to N4750.
Modify 20.5.6.3 [mem.res.monotonic.buffer.mem] as indicated:
void release();-1- Effects: Calls
upstream_rsrc->deallocate()
as necessary to release all allocated memory. Resets*this
to its initial state at construction.
[2020-10-03; Daniel comments and provides improved wording]
The recent wording introduces the very generic term "state" without giving a concrete definition of that term. During reflector discussions different interpretations of that term were expressed. The revised wording below gets rid of that word and replaces it by the actually involved exposition-only members.
[2020-10-06; moved to Tentatively Ready after seven votes in favour in reflector poll]
[2020-11-09 Approved In November virtual meeting. Status changed: Tentatively Ready → WP.]
Proposed resolution:
This wording is relative to N4861.
Modify 20.5.6.3 [mem.res.monotonic.buffer.mem] as indicated:
void release();-1- Effects: Calls
upstream_rsrc->deallocate()
as necessary to release all allocated memory. Resetscurrent_buffer
andnext_buffer_size
to their initial values at construction.
tuple
constructor constraints for UTypes&&...
overloadsSection: 22.4.4.2 [tuple.cnstr] Status: C++23 Submitter: Matt Calabrese Opened: 2018-06-12 Last modified: 2023-11-22
Priority: 2
View other active issues in [tuple.cnstr].
View all other issues in [tuple.cnstr].
View all issues with C++23 status.
Discussion:
Currently the tuple
constructors of the form:
template<class... UTypes> EXPLICIT constexpr tuple(UTypes&&...);
are not properly constrained in that in the 1-element tuple
case, the constraints do no short-circuit when
the constructor would be (incorrectly) considered as a possible copy/move constructor candidate. libc++ has a
workaround for this, but the additional short-circuiting does not actually appear in the working draft.
bool a = std::is_copy_constructible_v<std::tuple<any>>;
The above code will cause a compile error because of a recursive trait definition. The copy constructibility
check implies doing substitution into the UTypes&&...
constructor overloads, which in turn
will check if tuple<any>
is convertible to any, which in turn will check if tuple<any>
is copy constructible (and so the trait is dependent on itself).
sizeof...(UTypes) == 1
and the type, after applying remove_cvref_t
, is the tuple
type itself, then we should force
substitution failure rather than checking any further constraints.
[2018-06-23 after reflector discussion]
Priority set to 3
[2018-08-20, Jonathan provides wording]
[2018-08-20, Daniel comments]
The wording changes by this issue are very near to those suggested for LWG 3155(i).
[2018-11 San Diego Thursday night issue processing]
Jonathan to update wording - using conjunction. Priority set to 2
Previous resolution [SUPERSEDED]:
This wording is relative to N4762.
Modify 22.4.4.2 [tuple.cnstr] as indicated:
template<class... UTypes> explicit(see below) constexpr tuple(UTypes&&... u);-9- Effects: Initializes the elements in the tuple with the corresponding value in
-10- Remarks: This constructor shall not participate in overload resolution unlessstd::forward<UTypes>(u)
.sizeof...(Types) == sizeof...(UTypes)
andsizeof...(Types) >= 1
and(sizeof...(Types) > 1 || !is_same_v<remove_cvref_t<U0>, tuple>)
andis_constructible_v<Ti, Ui&&>
istrue
for alli
. The expression insideexplicit
is equivalent to:!conjunction_v<is_convertible<UTypes, Types>...>
[2021-05-20 Tim updates wording]
The new wording below also resolves LWG 3155(i), relating to an
allocator_arg_t
tag argument being treated by this constructor template
as converting to the first tuple element instead of as a tag. To minimize
collateral damage, this wording takes this constructor out of overload resolution
only if the tuple is of size 2 or 3, the first argument is an allocator_arg_t
,
but the first tuple element isn't of type allocator_arg_t
(in both cases
after removing cv/ref qualifiers). This avoids damaging tuples that actually
contain an allocator_arg_t
as the first element (which can be formed
during uses-allocator construction, thanks to uses_allocator_construction_args
).
[2021-08-20; LWG telecon]
Set status to Tentatively Ready after telecon review.
[2021-10-14 Approved at October 2021 virtual plenary. Status changed: Voting → WP.]
Proposed resolution:
This wording is relative to N4885, and also resolves LWG 3155(i).
Modify 22.4.4.2 [tuple.cnstr] as indicated:
template<class... UTypes> explicit(see below) constexpr tuple(UTypes&&... u);-?- Let
disambiguating-constraint
be:
(?.1) —
negation<is_same<remove_cvref_t<U0>, tuple>>
ifsizeof...(Types)
is 1;(?.2) — otherwise,
bool_constant<!is_same_v<remove_cvref_t<U0>, allocator_arg_t> || is_same_v<remove_cvref_t<T0>, allocator_arg_t>>
ifsizeof...(Types)
is 2 or 3;(?.3) — otherwise,
true_type
.-12- Constraints:
(12.1) —
sizeof...(Types)
equalssizeof...(UTypes)
,and(12.2) —
sizeof...(Types)
≥ 1, and(12.3) —
conjunction_v<disambiguating-constraint, is_constructible<Types, UTypes>...>
istrue
.is_constructible_v<Ti, Ui>
istrue
for all i-13- Effects: Initializes the elements in the tuple with the corresponding value in
-14- Remarks: The expression insidestd::forward<UTypes>(u)
.explicit
is equivalent to:!conjunction_v<is_convertible<UTypes, Types>...>
__cpp_lib_chrono_udls
was accidentally droppedSection: 17.3.1 [support.limits.general] Status: C++20 Submitter: Stephan T. Lavavej Opened: 2018-06-14 Last modified: 2021-02-25
Priority: 0
View all other issues in [support.limits.general].
View all issues with C++20 status.
Discussion:
Between P0941R0 and
P0941R1/P0941R2, the feature-test macro
__cpp_lib_chrono_udls
was dropped. It wasn't mentioned in the changelog, and Jonathan Wakely and I
believe that this was unintentional.
[2018-06-23 Moved to Tentatively Ready after 5 positive votes on c++std-lib.]
[2018-11, Adopted in San Diego]
Proposed resolution:
This wording is relative to the post-Rapperswil 2018 working draft.
In 17.3.1 [support.limits.general], "Table ??? - Standard library feature-test macros", add the following row:
Table ??? — Standard library feature-test macros Macro name Value Headers […]
__cpp_lib_chrono_udls
201304L
<chrono>
[…]
duration
constructor from representation shouldn't be effectively non-throwingSection: 30.5 [time.duration] Status: C++23 Submitter: Johel Ernesto Guerrero Peña Opened: 2018-06-22 Last modified: 2023-11-22
Priority: 3
View all other issues in [time.duration].
View all issues with C++23 status.
Discussion:
[time.duration]/4 states:
Members of duration shall not throw exceptions other than those thrown by the indicated operations on their representations.
Where representation is defined in the non-normative, brief description at [time.duration]/1:
[…] A duration has a representation which holds a count of ticks and a tick period. […]
[time.duration.cons]/2 doesn't indicate the operation undergone by its representation, merely stating a postcondition in [time.duration.cons]/3:
Effects: Constructs an object of type
Postconditions:duration
.count() == static_cast<rep>(r)
.
I suggest this reformulation that follows the format of [time.duration.cons]/5.
Effects: Constructs an object of type duration, constructing
rep_
fromr
.
Now it is clear why the constructor would throw.
Previous resolution [SUPERSEDED]:
This wording is relative to N4750.
Change 30.5.2 [time.duration.cons] as indicated:
template<class Rep2> constexpr explicit duration(const Rep2& r);-1- Remarks: This constructor shall not participate in overload resolution unless […]
-2- Effects: Constructs an object of typeduration
, constructingrep_
fromr
. -3- Postconditions:count() == static_cast<rep>(r)
.
[2018-06-27 after reflector discussion]
Priority set to 3. Improved wording as result of that discussion.
Previous resolution [SUPERSEDED]:
This wording is relative to N4750.
Change 30.5.2 [time.duration.cons] as indicated:
template<class Rep2> constexpr explicit duration(const Rep2& r);-1- Remarks: This constructor shall not participate in overload resolution unless […]
-2- Effects:Constructs an object of typeInitializesduration
rep_
withr
.-3- Postconditions:count() == static_cast<rep>(r)
.
[2020-05-02; Daniel resyncs wording with recent working draft]
[2021-09-20; Reflector poll]
Set status to Tentatively Ready after seven votes in favour during reflector poll.
[2021-10-14 Approved at October 2021 virtual plenary. Status changed: Voting → WP.]
Proposed resolution:
This wording is relative to N4861.
Change 30.5.2 [time.duration.cons] as indicated:
template<class Rep2> constexpr explicit duration(const Rep2& r);-1- Constraints:
is_convertible_v<const Rep2&, rep>
istrue
and
(1.1) —
treat_as_floating_point_v<rep>
istrue
or(1.2) —
treat_as_floating_point_v<Rep2>
isfalse
.[Example: […] end example]
-?- Effects: Initializesrep_
withr
.-2- Postconditions:count() == static_cast<rep>(r)
.
duration
streaming precondition should be a SFINAE conditionSection: 30.5.11 [time.duration.io] Status: Resolved Submitter: Johel Ernesto Guerrero Peña Opened: 2018-06-23 Last modified: 2020-11-09
Priority: 2
View all other issues in [time.duration.io].
View all issues with Resolved status.
Discussion:
30.5.11 [time.duration.io]/1 states:
template<class charT, class traits, class Rep, class Period> basic_ostream<charT, traits>& operator<<(basic_ostream<charT, traits>& os, const duration<Rep, Period>& d);Requires:
Rep
is an integral type whose integer conversion rank (6.8.6 [conv.rank]) is greater than or equal to that ofshort
, or a floating point type.charT
ischar
orwchar_t
.
I think the intention was to make this a compile-time error, since all the information to make it so is available at compile-time. But the wording doesn't suggest that.
[2018-06-29; Daniel comments]
The wording will be significantly simplified by the application of the new Library element Constraints: introduced by P0788R3 and available with the post-Rapperswil working draft.
[2018-07-20 Priority set to 2 after reflector discussion. NAD and P0 were also mentioned.]
[2018-08-23 Batavia Issues processing]
Marshall to talk to Howard about his intent. Status to Open
Previous resolution [SUPERSEDED]:This wording is relative to N4750.
Change 30.5.11 [time.duration.io] as indicated:
[Drafting note: The suggested wording changes include the insertion of two bullets into the existing running text to improve clarification of the logic arm of the "unless"].
template<class charT, class traits, class Rep, class Period> basic_ostream<charT, traits>& operator<<(basic_ostream<charT, traits>& os, const duration<Rep, Period>& d);-1-
RequiresRemarks: This function shall not participate in overload resolution unless:
Rep
is an integral type whose integer conversion rank (6.8.6 [conv.rank]) is greater than or equal to that ofshort
, or a floating point type., and
charT
ischar
orwchar_t
[…]
[2020-02-13, Prague]
Will be resolved by LWG 3317(i).
[2020-11-09 Resolved for C++20. Status changed: Tentatively Resolved → Resolved.]
Proposed resolution:
basic_osyncstream::rdbuf
needs a const_cast
Section: 31.11.3.1 [syncstream.osyncstream.overview] Status: C++20 Submitter: Tim Song Opened: 2018-06-29 Last modified: 2021-02-25
Priority: 0
View all other issues in [syncstream.osyncstream.overview].
View all issues with C++20 status.
Discussion:
The current specification of basic_osyncstream::rdbuf()
is
syncbuf_type* rdbuf() const noexcept { return &sb; }
This is ill-formed because the exposition-only member sb
is const
inside this const
member function,
but the return type is a pointer to non-const
syncbuf_type
. It needs to cast away the constness, consistent
with the other streams with embedded stream buffers (such as string and file streams).
[2018-07-20 Status set to Tentatively Ready after five positive votes on the reflector.]
[2018-11, Adopted in San Diego]
Proposed resolution:
This wording is relative to N4750.
Change 31.11.3.1 [syncstream.osyncstream.overview], class template basic_osyncstream
synopsis, as indicated:
namespace std { template<class charT, class traits, class Allocator> class basic_osyncstream : public basic_ostream<charT, traits> { public: […] // 31.11.3.3 [syncstream.osyncstream.members], member functions void emit(); streambuf_type* get_wrapped() const noexcept; syncbuf_type* rdbuf() const noexcept { return const_cast<syncbuf_type*>(&sb); } […] }; }
strstream::rdbuf
needs a const_cast
Section: 99 [depr.strstream.oper] Status: C++20 Submitter: Tim Song Opened: 2018-06-30 Last modified: 2021-02-25
Priority: 0
View all issues with C++20 status.
Discussion:
strstream::rdbuf
has the same issue with a missing const_cast
on &sb
.
istrstream::rdbuf
and ostrstream::rdbuf
got this right,
but each with a different style (see issue 252(i)).
[2018-07-20 Status to Tentatively Ready after five positive votes on the reflector.]
[2018-11, Adopted in San Diego]
Proposed resolution:
This wording is relative to N4750.
Change 99 [depr.strstream.oper] p1 as indicated:
strstreambuf* rdbuf() const;-1- Returns:
const_cast<strstreambuf*>(&sb)
.
regex_token_iterator
constructor uses wrong pointer arithmeticSection: 28.6.11.2.2 [re.tokiter.cnstr] Status: C++20 Submitter: Tim Song Opened: 2018-06-30 Last modified: 2021-02-25
Priority: 0
View all other issues in [re.tokiter.cnstr].
View all issues with C++20 status.
Discussion:
The specification of regex_token_iterator
for the overload taking a
const int (&submatchs)[N]
uses the range [&submatches, &submatches + N)
.
This is obviously incorrect; we want to perform pointer arithmetic on a pointer to the first element
of that array, not a pointer to the whole array.
[2018-07-20 Status to Tentatively Ready after five positive votes on the reflector.]
[2018-11, Adopted in San Diego]
Proposed resolution:
This wording is relative to N4750.
Change 28.6.11.2.2 [re.tokiter.cnstr] p3 as indicated:
regex_token_iterator(BidirectionalIterator a, BidirectionalIterator b, const regex_type& re, int submatch = 0, regex_constants::match_flag_type m = regex_constants::match_default); regex_token_iterator(BidirectionalIterator a, BidirectionalIterator b, const regex_type& re, const vector<int>& submatches, regex_constants::match_flag_type m = regex_constants::match_default); regex_token_iterator(BidirectionalIterator a, BidirectionalIterator b, const regex_type& re, initializer_list<int> submatches, regex_constants::match_flag_type m = regex_constants::match_default); template<size_t N> regex_token_iterator(BidirectionalIterator a, BidirectionalIterator b, const regex_type& re, const int (&submatches)[N], regex_constants::match_flag_type m = regex_constants::match_default);-2- Requires: […]
-3- Effects: The first constructor initializes the membersubs
to hold the single valuesubmatch
.The second constructor initializes the memberThe second, third and fourth constructors initialize the membersubs
to hold a copy of the argumentsubmatches
.subs
to hold a copy of the sequence of integer values pointed to by the iterator range[submatches.begin(), submatches.end())
and[&submatches, &submatches + N)
, respectively[begin(submatches), end(submatches))
. -4- […]
addressof
Section: 31 [input.output] Status: C++20 Submitter: Tim Song Opened: 2018-06-30 Last modified: 2021-02-25
Priority: 0
View all other issues in [input.output].
View all issues with C++20 status.
Discussion:
There are 27 instances of &sb
and one instance of &rhs
in
Clause 31 [input.output], each of which needs to use addressof
because
the operand has a user-provided template type parameter as an associated class and so
the use of unary &
is subject to ADL hijacking.
[2018-07-20 Status to Tentatively Ready after five positive votes on the reflector.]
[2018-11, Adopted in San Diego]
Proposed resolution:
This wording is relative to N4750.
Change 31.5.4.3 [basic.ios.members] p16 as indicated:
basic_ios& copyfmt(const basic_ios& rhs);-16- Effects: If
(this ==
does nothing. […]&addressof(rhs))
Change 31.8.3.2 [istringstream.cons] as indicated:
explicit basic_istringstream(ios_base::openmode which);-1- Effects: Constructs an object of class
basic_istringstream<charT, traits>
, initializing the base class withbasic_istream<charT, traits>(
(31.7.5.2 [istream]) and initializing&addressof(sb))sb
withbasic_stringbuf<charT, traits, Allocator>(which | ios_base::in)
(31.8.2.2 [stringbuf.cons]).explicit basic_istringstream( const basic_string<charT, traits, Allocator>& str, ios_base::openmode which = ios_base::in);-2- Effects: Constructs an object of class
basic_istringstream<charT, traits>
, initializing the base class withbasic_istream<charT, traits>(
(31.7.5.2 [istream]) and initializing&addressof(sb))sb
withbasic_stringbuf<charT, traits, Allocator>(str, which | ios_base::in)
(31.8.2.2 [stringbuf.cons]).basic_istringstream(basic_istringstream&& rhs);-3- Effects: Move constructs from the rvalue
rhs
. This is accomplished by move constructing the base class, and the containedbasic_stringbuf
. Nextbasic_istream<charT, traits>::set_rdbuf(
is called to install the contained&addressof(sb))basic_stringbuf
.
Change 31.8.3.4 [istringstream.members] p1 as indicated:
basic_stringbuf<charT, traits, Allocator>* rdbuf() const;-1- Returns:
const_cast<basic_stringbuf<charT, traits, Allocator>*>(
.&addressof(sb))
Change 31.8.4.2 [ostringstream.cons] as indicated:
explicit basic_ostringstream(ios_base::openmode which);-1- Effects: Constructs an object of class
basic_ostringstream<charT, traits>
, initializing the base class withbasic_ostream<charT, traits>(
(31.7.6.2 [ostream]) and initializing&addressof(sb))sb
withbasic_stringbuf<charT, traits, Allocator>(which | ios_base::out)
(31.8.2.2 [stringbuf.cons]).explicit basic_ostringstream( const basic_string<charT, traits, Allocator>& str, ios_base::openmode which = ios_base::out);-2- Effects: Constructs an object of class
basic_ostringstream<charT, traits>
, initializing the base class withbasic_ostream<charT, traits>(
(31.7.6.2 [ostream]) and initializing&addressof(sb))sb
withbasic_stringbuf<charT, traits, Allocator>(str, which | ios_base::out)
(31.8.2.2 [stringbuf.cons]).basic_ostringstream(basic_ostringstream&& rhs);-3- Effects: Move constructs from the rvalue
rhs
. This is accomplished by move constructing the base class, and the containedbasic_stringbuf
. Nextbasic_ostream<charT, traits>::set_rdbuf(
is called to install the contained&addressof(sb))basic_stringbuf
.
Change 31.8.4.4 [ostringstream.members] p1 as indicated:
basic_stringbuf<charT, traits, Allocator>* rdbuf() const;-1- Returns:
const_cast<basic_stringbuf<charT, traits, Allocator>*>(
.&addressof(sb))
Change 31.8.5.2 [stringstream.cons] as indicated:
explicit basic_stringstream(ios_base::openmode which);-1- Effects: Constructs an object of class
basic_stringstream<charT, traits>
, initializing the base class withbasic_iostream<charT, traits>(
(31.7.5.7.2 [iostream.cons]) and initializing&addressof(sb))sb
withbasic_stringbuf<charT, traits, Allocator>(which)
.explicit basic_stringstream( const basic_string<charT, traits, Allocator>& str, ios_base::openmode which = ios_base::out | ios_base::in);-2- Effects: Constructs an object of class
basic_stringstream<charT, traits>
, initializing the base class withbasic_iostream<charT, traits>(
(31.7.5.7.2 [iostream.cons]) and initializing&addressof(sb))sb
withbasic_stringbuf<charT, traits, Allocator>(str, which)
.basic_stringstream(basic_stringstream&& rhs);-3- Effects: Move constructs from the rvalue
rhs
. This is accomplished by move constructing the base class, and the containedbasic_stringbuf
. Nextbasic_istream<charT, traits>::set_rdbuf(
is called to install the contained&addressof(sb))basic_stringbuf
.
Change 31.8.5.4 [stringstream.members] p1 as indicated:
basic_stringbuf<charT, traits, Allocator>* rdbuf() const;-1- Returns:
const_cast<basic_stringbuf<charT, traits, Allocator>*>(
.&addressof(sb))
Change 31.10.4.2 [ifstream.cons] as indicated:
basic_ifstream();-1- Effects: Constructs an object of class
basic_ifstream<charT, traits>
, initializing the base class withbasic_istream<charT, traits>(
(31.7.5.2.2 [istream.cons]) and initializing&addressof(sb))sb
withbasic_filebuf<charT, traits>()
(31.10.3.2 [filebuf.cons]).explicit basic_ifstream(const char* s, ios_base::openmode mode = ios_base::in); explicit basic_ifstream(const filesystem::path::value_type* s, ios_base::openmode mode = ios_base::in); // wide systems only; see 31.10.1 [fstream.syn]-2- Effects: Constructs an object of class
basic_ifstream<charT, traits>
, initializing the base class withbasic_istream<charT, traits>(
(31.7.5.2.2 [istream.cons]) and initializing&addressof(sb))sb
withbasic_filebuf<charT, traits>()
(31.10.3.2 [filebuf.cons]), then callsrdbuf()->open(s, mode | ios_base::in)
. If that function returns a null pointer, callssetstate(failbit)
.[…]
basic_ifstream(basic_ifstream&& rhs);-4- Effects: Move constructs from the rvalue
rhs
. This is accomplished by move constructing the base class, and the containedbasic_filebuf
. Nextbasic_istream<charT, traits>::set_rdbuf(
is called to install the contained&addressof(sb))basic_filebuf
.
Change 31.10.4.4 [ifstream.members] p1 as indicated:
basic_filebuf<charT, traits>* rdbuf() const;-1- Returns:
const_cast<basic_filebuf<charT, traits>*>(
.&addressof(sb))
Change 31.10.5.2 [ofstream.cons] as indicated:
basic_ofstream();-1- Effects: Constructs an object of class
basic_ofstream<charT, traits>
, initializing the base class withbasic_ostream<charT, traits>(
(31.7.6.2.2 [ostream.cons]) and initializing&addressof(sb))sb
withbasic_filebuf<charT, traits>()
(31.10.3.2 [filebuf.cons]).explicit basic_ofstream(const char* s, ios_base::openmode mode = ios_base::out); explicit basic_ofstream(const filesystem::path::value_type* s, ios_base::openmode mode = ios_base::out); // wide systems only; see 31.10.1 [fstream.syn]-2- Effects: Constructs an object of class
basic_ofstream<charT, traits>
, initializing the base class withbasic_ostream<charT, traits>(
(31.7.6.2.2 [ostream.cons]) and initializing&addressof(sb))sb
withbasic_filebuf<charT, traits>()
(31.10.3.2 [filebuf.cons]), then callsrdbuf()->open(s, mode | ios_base::out)
. If that function returns a null pointer, callssetstate(failbit)
.[…]
basic_ofstream(basic_ofstream&& rhs);-4- Effects: Move constructs from the rvalue
rhs
. This is accomplished by move constructing the base class, and the containedbasic_filebuf
. Nextbasic_ostream<charT, traits>::set_rdbuf(
is called to install the contained&addressof(sb))basic_filebuf
.
Change 31.10.5.4 [ofstream.members] p1 as indicated:
basic_filebuf<charT, traits>* rdbuf() const;-1- Returns:
const_cast<basic_filebuf<charT, traits>*>(
.&addressof(sb))
Change 31.10.6.2 [fstream.cons] as indicated:
basic_fstream();-1- Effects: Constructs an object of class
basic_fstream<charT, traits>
, initializing the base class withbasic_iostream<charT, traits>(
(31.7.5.7.2 [iostream.cons]) and initializing&addressof(sb))sb
withbasic_filebuf<charT, traits>()
.explicit basic_fstream( const char* s, ios_base::openmode mode = ios_base::in | ios_base::out); explicit basic_fstream( const filesystem::path::value_type* s, ios_base::openmode mode = ios_base::in | ios_base::out); // wide systems only; see 31.10.1 [fstream.syn]-2- Effects: Constructs an object of class
basic_fstream<charT, traits>
, initializing the base class withbasic_iostream<charT, traits>(
(31.7.5.7.2 [iostream.cons]) and initializing&addressof(sb))sb
withbasic_filebuf<charT, traits>()
, then callsrdbuf()->open(s, mode)
. If that function returns a null pointer, callssetstate(failbit)
.[…]
basic_fstream(basic_fstream&& rhs);-4- Effects: Move constructs from the rvalue
rhs
. This is accomplished by move constructing the base class, and the containedbasic_filebuf
. Nextbasic_istream<charT, traits>::set_rdbuf(
is called to install the contained&addressof(sb))basic_filebuf
.
Change 31.10.6.4 [fstream.members] p1 as indicated:
basic_filebuf<charT, traits>* rdbuf() const;-1- Returns:
const_cast<basic_filebuf<charT, traits>*>(
.&addressof(sb))
Change 31.11.3.1 [syncstream.osyncstream.overview], class template basic_osyncstream
synopsis, as indicated:
[Drafting note: The text shown below assumes the application of the proposed resolution of issue 3127(i).]
namespace std { template<class charT, class traits, class Allocator> class basic_osyncstream : public basic_ostream<charT, traits> { public: […] // 31.11.3.3 [syncstream.osyncstream.members], member functions void emit(); streambuf_type* get_wrapped() const noexcept; syncbuf_type* rdbuf() const noexcept { return const_cast<syncbuf_type*>(&addressof(sb)); } […] }; }
Change 31.11.3.2 [syncstream.osyncstream.cons] p1 and p4 as indicated:
basic_osyncstream(streambuf_type* buf, const Allocator& allocator);-1- Effects: Initializes
-2- […] -3- […]sb
frombuf
andallocator
. Initializes the base class withbasic_ostream<charT, traits>(
.&addressof(sb))basic_osyncstream(basic_osyncstream&& other) noexcept;-4- Effects: Move constructs the base class and
-5- […]sb
from the corresponding subobjects ofother
, and callsbasic_ostream<charT, traits>::set_rdbuf(
.&addressof(sb))
addressof
all the thingsSection: 30.13 [time.parse], 27.4.3.8.1 [string.accessors], 27.3.3 [string.view.template], 23.2.2 [container.requirements.general], 24.3.5.4 [output.iterators], 24.3.5.6 [bidirectional.iterators], 28.6.6 [re.traits], 28.6.11.1 [re.regiter], 32.6.5.2 [thread.lock.guard] Status: C++20 Submitter: Tim Song Opened: 2018-06-30 Last modified: 2021-02-25
Priority: 0
View other active issues in [time.parse].
View all other issues in [time.parse].
View all issues with C++20 status.
Discussion:
Some additional instances where the library specification applies unary
operator &
when it should use addressof
.
[2018-07-20 Status to Tentatively Ready after five positive votes on the reflector.]
[2018-11, Adopted in San Diego]
Proposed resolution:
This wording is relative to N4750.
[Drafting note: Two uses of
&
in 24.5.1 [reverse.iterators] are not included in the wording below because the entire sentence is slated to be removed by a revision of P0896, the One Ranges Proposal.]
Change 30.13 [time.parse] p4-5 as indicated:
template<class charT, class traits, class Alloc, class Parsable> unspecified parse(const basic_string<charT, traits, Alloc>& fmt, Parsable& tp, basic_string<charT, traits, Alloc>& abbrev);-4- Remarks: This function shall not participate in overload resolution unless
from_stream(declval<basic_istream<charT, traits>&>(), fmt.c_str(), tp,&addressof(abbrev))is a valid expression.
-5- Returns: A manipulator that, when extracted from abasic_istream<charT, traits> is
, callsfrom_stream(is, fmt.c_str(), tp,
.&addressof(abbrev))
Change 30.13 [time.parse] p8-9 as indicated:
template<class charT, class traits, class Alloc, class Parsable> unspecified parse(const basic_string<charT, traits, Alloc>& fmt, Parsable& tp, basic_string<charT, traits, Alloc>& abbrev, minutes& offset);-8- Remarks: This function shall not participate in overload resolution unless
from_stream(declval<basic_istream<charT, traits>&>(), fmt.c_str(), tp,&addressof(abbrev), &offset)is a valid expression.
-9- Returns: A manipulator that, when extracted from abasic_istream<charT, traits> is
, callsfrom_stream(is, fmt.c_str(), tp,
.&addressof(abbrev), &offset)
Change 27.4.3.8.1 [string.accessors] p1 and p4 as indicated:
const charT* c_str() const noexcept; const charT* data() const noexcept;-1- Returns: A pointer
-2- Complexity: Constant time. -3- Requires: The program shall not alter any of the values stored in the character array.p
such thatp + i ==
for each&addressof(operator[](i))i
in[0, size()]
.charT* data() noexcept;-4- Returns: A pointer
-5- Complexity: Constant time. -6- Requires: The program shall not alter the value stored atp
such thatp + i ==
for each&addressof(operator[](i))i
in[0, size()]
.p + size()
.
Change 27.3.3.4 [string.view.iterators] p4 as indicated:
constexpr const_iterator begin() const noexcept; constexpr const_iterator cbegin() const noexcept;-4- Returns: An iterator such that
(4.1) — if!empty()
,, (4.2) — otherwise, an unspecified value such that
&addressof(*begin()) == data_[begin(), end())
is a valid range.
Change 27.3.3.8 [string.view.ops] p21 and p24 as indicated:
constexpr bool starts_with(charT x) const noexcept;-21- Effects: Equivalent to:
return starts_with(basic_string_view(
&addressof(x), 1));[…]
constexpr bool ends_with(charT x) const noexcept;-24- Effects: Equivalent to:
return ends_with(basic_string_view(
&addressof(x), 1));
Change 27.3.3.9 [string.view.find] p5 as indicated:
-5- Each member function of the form
constexpr return-type F(charT c, size_type pos);is equivalent to
return F(basic_string_view(
&addressof(c), 1), pos);
Edit 23.2.2 [container.requirements.general], Table 77 — "Container requirements", as indicated:
Table 77 — Container requirements Expression Return type Operational
semanticsAssertion/note
pre/post-conditionComplexity […]
(&a)->a.~X()void
the destructor is applied to every element of a
; any memory obtained is deallocated.linear […]
Edit 24.3.5.4 [output.iterators], Table 90 — "Output iterator requirements (in addition to Iterator)", as indicated:
Table 90 — Output iterator requirements (in addition to Iterator) Expression Return type Operational
semanticsAssertion/note
pre/post-condition[…]
++r
X&
.
&addressof(r) ==&addressof(++r)
[…][…]
Edit 24.3.5.6 [bidirectional.iterators], Table 92 — "Bidirectional iterator requirements (in addition to forward iterator)", as indicated:
Table 92 — Bidirectional iterator requirements (in addition to forward iterator) Expression Return type Operational
semanticsAssertion/note
pre/post-condition--r
X&
[…]
.
&addressof(r) ==&addressof(--r)[…]
Change 28.6.6 [re.traits] p6 as indicated:
template<class ForwardIterator> string_type transform(ForwardIterator first, ForwardIterator last) const;-6- Effects: As if by:
string_type str(first, last); return use_facet<collate<charT>>( getloc()).transform(&*str.begindata(),&*str.begindata() + str.length());
Change 28.6.11.1.2 [re.regiter.cnstr] p2 as indicated:
regex_iterator(BidirectionalIterator a, BidirectionalIterator b, const regex_type& re, regex_constants::match_flag_type m = regex_constants::match_default);-2- Effects: Initializes
begin
andend
toa
andb
, respectively, setspregex
to, sets
&addressof(re)flags
tom
, then callsregex_search(begin, end, match, *pregex, flags)
. If this call returnsfalse
the constructor sets*this
to the end-of-sequence iterator.
Change 28.6.11.1.4 [re.regiter.deref] p2 as indicated:
const value_type* operator->() const;-2- Returns:
.
&addressof(match)
Change 32.6.5.2 [thread.lock.guard] p2-7 as indicated:
explicit lock_guard(mutex_type& m);-2- Requires: If
-3- Effects:mutex_type
is not a recursive mutex, the calling thread does not own the mutexm
.As if byInitializespm
withm
. Callsm.lock()
.-4- Postconditions:&pm == &m
lock_guard(mutex_type& m, adopt_lock_t);-5- Requires: The calling thread owns the mutex
-6-m
.Postconditions:Effects: Initializes&pm == &m
pm
withm
. -7- Throws: Nothing.
expects
or ensures
Section: 16.4.5.3.3 [macro.names] Status: C++20 Submitter: Tim Song Opened: 2018-06-30 Last modified: 2021-02-25
Priority: 0
View other active issues in [macro.names].
View all other issues in [macro.names].
View all issues with C++20 status.
Discussion:
expects
and ensures
are not technically described as attribute-tokens when used
in a contract-attribute-specifier, so the existing prohibition in 16.4.5.3.3 [macro.names]
doesn't apply to them.
assert
is also a library name so falls under p1, default
is a keyword, and both axiom
and audit
were added to Table 4.
[2018-07-20 Status to Tentatively Ready after five positive votes on the reflector.]
[2018-11, Adopted in San Diego]
Proposed resolution:
This wording is relative to N4762.
Change 16.4.5.3.3 [macro.names] p2 as indicated:
-2- A translation unit shall not
#define
or#undef
names lexically identical to keywords, to the identifiers listed in Table 4,orto the attribute-tokens described in 9.13 [dcl.attr], or to the identifiersexpects
orensures
.
Section: 29.2 [numeric.requirements] Status: C++20 Submitter: Tim Song Opened: 2018-07-05 Last modified: 2021-02-25
Priority: 0
View all other issues in [numeric.requirements].
View all issues with C++20 status.
Discussion:
29.2 [numeric.requirements] contains some very old wording that hasn't been changed since C++98 except for issue 2699(i). As a result, it is at once over- and under-restrictive. For example:
operator&
for class types, but not enumeration types; andT
's special member functions, but
doesn't require that copying produces an equivalent value to the original.We can significantly clean up this wording by using the existing named requirements. For ease of review, the following table provides a side-by-side comparison of the current and proposed wording.
Before | After |
---|---|
A C++ program shall instantiate these components only with a type T
that satisfies the following requirements: [Footnote … ] |
A C++ program shall instantiate these components only with a cv-unqualified object type T
that satisfies the Cpp17DefaultConstructible, Cpp17CopyConstructible, Cpp17CopyAssignable,
and Cpp17Destructible |
(1.1) — T is not an abstract class (it has no pure virtual member functions); |
Cpp17DefaultConstructible |
(1.2) — T is not a reference type; (1.3) — T is not cv-qualified; |
Implied by "cv-unqualified object type" |
(1.4) — If T is a class, it has a public default constructor; |
Cpp17DefaultConstructible |
(1.5) — If T is a class, it has a public copy constructor
with the signature T::T(const T&) ; |
Cpp17CopyConstructible |
(1.6) — If T is a class, it has a public destructor; |
Cpp17Destructible |
(1.7) — If T is a class, it has a public copy assignment operator whose signature is either
T& T::operator=(const T&) or T& T::operator=(T); |
Cpp17CopyAssignable |
(1.8) — If T is a class, its assignment operator, copy and default constructors, and destructor
shall correspond to each other in the following sense:(1.8.1) — Initialization of raw storage using the copy constructor on the value of T() , however
obtained, is semantically equivalent to value-initialization of the same raw storage.(1.8.2) — Initialization of raw storage using the default constructor, followed by assignment, is semantically equivalent to initialization of raw storage using the copy constructor. (1.8.3) — Destruction of an object, followed by initialization of its raw storage using the copy constructor, is semantically equivalent to assignment to the original object. [Note: […] — end note] |
These requirements are implied by Cpp17CopyConstructible and Cpp17CopyAssignable's requirement that the value of the copy is equivalent to the source. |
(1.9) — If T is a class, it does not overload unary operator& . |
omitted now that we have std::addressof |
[2019-01-20 Reflector prioritization]
Set Priority to 0 and status to Tentatively Ready
Proposed resolution:
This wording is relative to the post-Rapperswil 2018 working draft.
Edit 29.2 [numeric.requirements] p1 as indicated, striking the entire bulleted list:
-1- The
complex
andvalarray
components are parameterized by the type of information they contain and manipulate. A C++ program shall instantiate these components only with a cv-unqualified object typeT
that satisfies the Cpp17DefaultConstructible, Cpp17CopyConstructible, Cpp17CopyAssignable, and Cpp17Destructiblefollowingrequirements (16.4.4.2 [utility.arg.requirements]).:[Footnote: … ]
(1.1) —T
is not an abstract class (it has no pure virtual member functions);[…](1.9) — IfT
is a class, it does not overload unaryoperator&
.
Edit 29.6.2.4 [valarray.access] p3-4 as indicated:
const T& operator[](size_t n) const; T& operator[](size_t n);-1- Requires:
-2- Returns: […] -3- Remarks: The expressionn < size()
.evaluates to
&addressof(a[i+j]) ==&addressof(a[i]) + jtrue
for allsize_t i
andsize_t j
such thati+j < a.size()
. -4- The expressionevaluates to
&addressof(a[i]) !=&addressof(b[j])true
for any two arraysa
andb
and for anysize_t i
andsize_t j
such thati < a.size()
andj < b.size()
. [Note: […] — end note ]
Section: 99 [fund.ts.v3::meta.type.synop] Status: Resolved Submitter: Thomas Köppe Opened: 2018-07-02 Last modified: 2020-09-06
Priority: 0
View all other issues in [fund.ts.v3::meta.type.synop].
View all issues with Resolved status.
Discussion:
Addresses: fund.ts.v3
The LFTSv3 prospective-working-paper N4758 lists a large number of type trait variable templates in [meta.type.synop] that are duplicates of corresponding ones in C++17. The paper P0996R1 that was meant to rebase the LFTS on C++17 appears to have missed them.
[2018-07-20 Status to Tentatively Ready after five positive votes on the reflector.]
[2018-11-11 Resolved by P1210R0, adopted in San Diego.]
Proposed resolution:
This wording is relative to N4758.
Delete from 99 [fund.ts.v3::meta.type.synop] all variable templates starting at is_void_v
up to and including
is_convertible_v
as indicated:
#include <type_traits> namespace std::experimental { inline namespace fundamentals_v3 {// See C++17 §23.15.4.1, primary type categories template <class T> constexpr bool is_void_v = is_void<T>::value; […] template <class From, class To> constexpr bool is_convertible_v = is_convertible<From, To>::value;// 3.3.2, Other type transformations template <class> class invocation_type; // not defined […] } // inline namespace fundamentals_v3 } // namespace std::experimental
Section: 99 [fund.ts.v3::meta.type.synop], 99 [fund.ts.v3::header.memory.synop] Status: Resolved Submitter: Thomas Köppe Opened: 2018-07-02 Last modified: 2020-05-03
Priority: 3
View all other issues in [fund.ts.v3::meta.type.synop].
View all issues with Resolved status.
Discussion:
Addresses: fund.ts.v3
The LFTSv3 prospective-working-paper N4758 contains two aliases that are already in C++17:
void_t
in 99 [fund.ts.v3::meta.type.synop]
uses_allocator_v
in 99 [fund.ts.v3::header.memory.synop]
I'd like to propose deleting both, but separate discussion is warranted:
void_t
belongs with the larger "detection idiom", which has not been merged into C++17. We may prefer
to keep our own local version of the alias.
uses_allocator_v
aliases a version of uses_allocator
that is modified by this TS. However,
as specified the alias may actually end up referring to std::uses_allocator
(because
<memory>
is included), not to std::experimental::uses_allocator
, as may have been intended.
[2018-07-20 Priority set to 3 after reflector discussion]
[2020-05-03 Reflector discussion]
Resolved by P1210R0 accepted during the San Diego 2018 meeting.
Rationale:
Resolved by P1210R0Proposed resolution:
propagate_const
requirementsSection: 6.1.2.3 [fund.ts.v3::propagate_const.class_type_requirements] Status: C++23 Submitter: Thomas Köppe Opened: 2018-07-02 Last modified: 2023-11-22
Priority: 3
View all issues with C++23 status.
Discussion:
Addresses: fund.ts.v3
In the LFTSv3 prospective-working-paper N4758, [propagate_const.class_type_requirements] uses a strange turn of phrase:
"In this sub-clause,
t
denotes a non-const
lvalue of typeT
,ct
is aconst T&
bound tot
, […]"
The last bit is strange: "ct
is a const T&
bound to t
" is not how we usually say things.
The specification-variables usually denote values, and values can't be references. Perhaps we could just say,
"ct
is as_const(t)
"?
[2018-07-20 Priority set to 3 after reflector discussion]
Previous resolution [SUPERSEDED]:
This wording is relative to N4758.
Edit 6.1.2.3 [fund.ts.v3::propagate_const.class_type_requirements] as indicated:
-1- If
T
is class type then it shall satisfy the following requirements. In this sub-clauset
denotes a non-const
lvalue of typeT
,ct
isaconst T&
bound tot
as_const(t)
,element_type
denotes an object type.
[2022-10-12; Jonathan provides improved wording]
[2022-10-19; Reflector poll]
Set status to "Tentatively Ready" after eight votes in favour in reflector poll.
[2022-11-12 Approved at November 2022 meeting in Kona. Status changed: Voting → WP.]
Proposed resolution:
This wording is relative to N4920.
Edit 6.1.2.2 [fund.ts.v3::propagate_const.requirements] as indicated:
-1-
T
shall be a cv-unqualified pointer-to-object typean object pointer typeor a cv-unqualified class type for whichdecltype(*declval<T&>())
is an lvalue reference to object type; otherwise the program is ill-formed.
-2- IfT
is an array type, reference type, pointer to function type or pointer to (possibly cv-qualified)void
, then the program is ill-formed.-3- [Note:
propagate_const<const int*>
is well-formed but propagate_const<int* const> is not. — end note]
Edit 6.1.2.3 [fund.ts.v3::propagate_const.class_type_requirements] as indicated:
-1- If
T
is class type then it shall satisfy the following requirements. In this sub-clauset
denotesa non-an lvalue of typeconst
T
,ct
is adenotesconst T&
bound tot
,element_type
denotes an object type.as_const(t)
.
__cpp_lib_to_chars
Section: 17.3.1 [support.limits.general] Status: C++20 Submitter: S. B. Tam Opened: 2018-07-03 Last modified: 2022-08-23
Priority: 0
View all other issues in [support.limits.general].
View all issues with C++20 status.
Discussion:
After acceptance of P0941R2 into the working draft, in [support.limits.general],
__cpp_lib_to_chars
is required to be in <utility>
. Since the relevant feature
(std::to_chars
and std::from_chars
) is now in the header <charconv>
,
should the macro be defined in <charconv>
instead of <utility>
?
[Marshall provides P/R and context]
std::to_chars
, etc were originally proposed for the header <utility>
and SD-6 reflected that. Somewhere along the way, they was put into their own header
<charconv>
, but the document was never updated to reflect that.
When these macros were added to the standard, the (now incorrect) value was copied as well.
[2018-07-20 Status to Tentatively Ready after five positive votes on the reflector.]
[2018-11, Adopted in San Diego]
Proposed resolution:
This wording is relative to (the Rapperswil post-mailing standard).
Change 17.3.1 [support.limits.general] (Table 35) as indicated:
Macro name | Value | Header(s) |
---|---|---|
__cpp_lib_to_chars | 201611L | <charconv |
COMMON_REF
is unimplementable as specifiedSection: 21.3.8.7 [meta.trans.other] Status: C++20 Submitter: Casey Carter Opened: 2018-07-07 Last modified: 2021-02-25
Priority: 0
View all other issues in [meta.trans.other].
View all issues with C++20 status.
Discussion:
21.3.8.7 [meta.trans.other]/3.2 states:
[Let]which is nonsensical: a specialization of a class template cannot possibly be a cv-qualified type or reference type.XREF(A)
denote a unary class templateT
such thatT<U>
denotes the same type asU
with the addition ofA
’s cv and reference qualifiers, for a non-reference cv-unqualified typeU
,
XREF(A)
must be a unary alias template.
[2018-09-11; Status set to Tentatively Ready after five positive votes on the reflector]
[2018-11, Adopted in San Diego]
Proposed resolution:
This wording is relative to N4762.
Change 21.3.8.7 [meta.trans.other] as indicated:
-3- Let:
(3.1) —
CREF(A)
beadd_lvalue_reference_t<const remove_reference_t<A>>
,(3.2) —
XREF(A)
denote a unaryclassalias templateT
such thatT<U>
denotes the same type asU
with the addition ofA
’s cv and reference qualifiers, for a non-reference cv-unqualified typeU
,(3.3) —
COPYCV(FROM, TO)
be an alias for typeTO
with the addition ofFROM
’s top-level cv-qualifiers. [Example:COPYCV(const int, volatile short)
is an alias forconst volatile short
. —end example]
CopyConstructible
doesn't preserve source valuesSection: 18.6 [concepts.object] Status: C++20 Submitter: Casey Carter Opened: 2018-07-07 Last modified: 2021-02-25
Priority: 2
View all issues with C++20 status.
Discussion:
The design intent of the CopyConstructible
concept has always been that the source of the
constructions it requires be left unmodified. Before P0541R1
reformulated the Constructible
concept in terms of is_constructible
, the wording
which is now in 18.2 [concepts.equality]/5:
This document uses a notational convention to specify which expressions declared in a requires-expression modify which inputs: except where otherwise specified, […] Operands that are constant lvalues or rvalues are required to not be modified.indirectly enforced that requirement. Unfortunately, nothing in the current wording in 18.4.14 [concept.copyconstructible] enforces that requirement.
[2018-11 Reflector prioritization]
Set Priority to 2
Previous resolution: [SUPERSEDED]This wording is relative to N4762.
Change 18.4.14 [concept.copyconstructible] as indicated:
template<class T> concept CopyConstructible = MoveConstructible<T> && Constructible<T, T&> && ConvertibleTo<T&, T> && Constructible<T, const T&> && ConvertibleTo<const T&, T> && Constructible<T, const T> && ConvertibleTo<const T, T>;-1- If
T
is an object type, then letv
be an lvalue of type (possiblyconst
)T
or an rvalue of typeconst T
.CopyConstructible<T>
is satisfied only if(1.1) — After the definition
T u = v;
,u
is equal tov
andv
is unmodified.(1.2) —
T(v)
is equal tov
and does not modifyv
.
[2020-02-13 Per LWG review, Casey provides updated P/R wording.]
[2020-02 Status to Immediate on Thursday night in Prague.]
Proposed resolution:
This wording is relative to N4849.
Change 18.4.14 [concept.copyconstructible] as indicated:
template<class T> concept copy_constructible = move_constructible<T> && constructible_from<T, T&> && convertible_to<T&, T> && constructible_from<T, const T&> && convertible_to<const T&, T> && constructible_from<T, const T> && convertible_to<const T, T>;-1- If
T
is an object type, then letv
be an lvalue of type (possiblyconst
)T
or an rvalue of typeconst T
.T
modelscopy_constructible
only if(1.1) — After the definition
T u = v;
,u
is equal tov
(18.2 [concepts.equality]) andv
is not modified.(1.2) —
T(v)
is equal tov
and does not modifyv
.
monotonic_buffer_resource
growth policy is unclearSection: 20.5.6 [mem.res.monotonic.buffer] Status: C++23 Submitter: Jonathan Wakely Opened: 2018-07-20 Last modified: 2023-11-22
Priority: 2
View all issues with C++23 status.
Discussion:
During the discussion of LWG 3120(i) it was pointed out that the current
wording in 20.5.6 [mem.res.monotonic.buffer] is contradictory. The introductory text for
the class says "Each additional buffer is larger than the previous one, following
a geometric progression" but the spec for do_allocate
doesn't agree.
N
times bigger than the previous buffer, the next buffer will be at
least N
times larger than the previous one. If N
is larger than the
implementation-defined growth factor it's not a geometric progression.
Secondly, it's not even clear that each additional buffer will be larger than the previous one.
Given a monotonic_buffer_resource
object with little remaining space in current_buffer
,
a request to allocate 10*next_buffer_size
will:
"set current_buffer
to upstream_rsrc->allocate(n, m)
, where n
is not
less than max(bytes, next_buffer_size)
and m
is not less than alignment
,
and increase next_buffer_size
by an implementation-defined growth factor (which need not
be integral), then allocate the return block from the newly-allocated current_buffer
."
The effects are to allocate a new buffer of at least max(10*next_buffer_size, next_buffer_size)
bytes, and then do next_buffer_size *= growth_factor
. If growth_factor < 10
then
the next allocated buffer might be smaller than the last one. This means that although
next_buffer_size
itself follows a geometric progression, the actual size of any single
allocated buffer can be much larger than next_buffer_size
. A graph of the allocated sizes looks
like a geometric progression with spikes where an allocation size is larger than next_buffer_size
.
If the intention is to set next_buffer_size = max(n, next_buffer_size * growth_factor)
so
that every allocation from upstream is larger than the previous one, then we need a change to the
Effects: to actually say that. Rather than a geometric progression with anomalous spikes,
this would produce a number of different geometric progressions with discontinuous jumps between them.
If the spiky interpretation is right then we need to weaken the "Each additional buffer is larger"
statement. Either way, we need to add a caveat to the "following a geometric progression" text
because that isn't true for the spiky interpretation or the jumpy interpretation.
Thirdly, the Effects: says that the size of the allocated block, n
, is not less than
max(bytes, next_buffer_size)
. This seems to allow an implementation to choose to do
n = ceil2(max(bytes, next_buffer_size))
if it wishes (maybe because allocating sizes that
are a power of 2 simplifies the monotonic_buffer_resource
implementation, or allows reducing
the bookkeeping overhead). This still results in an approximate geometric progression (under either
the spiky or jumpy interpretation) but the graph has steps rather than being a smooth curve (but
always above the curve). This is another way that "Each additional buffer is larger than the previous
one" is not guaranteed. Even if max(bytes, next_buffer_size)
is greater on every call, for a
growth factor between 1.0
and 2.0
the result of ceil2
might be the same
for two successive buffers. I see no reason to forbid this, but Pablo suggested it's not allowed
because it doesn't result in exponential growth (which I disagree with). If this is supposed to be
forbidden, the wording needs to be fixed to forbid it.
[2019-01-20 Reflector prioritization]
Set Priority to 2
[2020-02-13, Prague]
LWG looked at the issue and a suggestion was presented to eliminate most of 20.5.6 [mem.res.monotonic.buffer] to solve the problem the current requirements impose.
[2020-02-16; Prague]
Reviewed revised wording and moved to Ready for Varna.
[2020-11-09 Approved In November virtual meeting. Status changed: Ready → WP.]
Proposed resolution:
This wording is relative to N4849.
Modify 20.5.6 [mem.res.monotonic.buffer], as indicated:
-1- A
monotonic_buffer_resource
is a special-purpose memory resource intended for very fast memory allocations in situations where memory is used to build up a few objects and then is released all at once when the memory resource object is destroyed.It has the following qualities:
(1.1) — A call todeallocate
has no effect, thus the amount of memory consumed increases monotonically until the resource is destroyed.
(1.2) — The program can supply an initial buffer, which the allocator uses to satisfy memory requests.
(1.3) — When the initial buffer (if any) is exhausted, it obtains additional buffers from an upstream memory resource supplied at construction. Each additional buffer is larger than the previous one, following a geometric progression.
(1.4) — It is intended for access from one thread of control at a time. Specifically, calls toallocate
anddeallocate
do not synchronize with one another.
(1.5) — It frees the allocated memory on destruction, even ifdeallocate
has not been called for some of the allocated blocks.
span
does not have a const_pointer
typedefSection: 23.7.2.2.1 [span.overview] Status: C++20 Submitter: Louis Dionne Opened: 2018-07-23 Last modified: 2021-02-25
Priority: 0
View all other issues in [span.overview].
View all issues with C++20 status.
Discussion:
std::span
does not have a typedef for const_pointer
and const_reference
. According to
Marshall Clow, this is merely an oversight.
[2019-01-20 Reflector prioritization]
Set Priority to 0 and status to Tentatively Ready
Proposed resolution:
This wording is relative to N4750.
Change 23.7.2.2.1 [span.overview], class template span
synopsis, as indicated:
namespace std { template<class ElementType, ptrdiff_t Extent = dynamic_extent> class span { public: // constants and types using element_type = ElementType; using value_type = remove_cv_t<ElementType>; using index_type = ptrdiff_t; using difference_type = ptrdiff_t; using pointer = element_type*; using const_pointer = const element_type*; using reference = element_type&; using const_reference = const element_type&; using iterator = implementation-defined; using const_iterator = implementation-defined; using reverse_iterator = reverse_iterator<iterator>; using const_reverse_iterator = reverse_iterator<const_iterator>; static constexpr index_type extent = Extent; […] }; […] }
file_clock
breaks ABI for C++17 implementationsSection: 30.7.6 [time.clock.file] Status: C++20 Submitter: Billy Robert O'Neal III Opened: 2018-07-26 Last modified: 2021-02-25
Priority: 1
View all issues with C++20 status.
Discussion:
It was pointed out in one of Eric's changes to libc++ here that
P0355 adds file_clock
, which is intended to be the clock used for
std::filesystem::file_time_type
's clock.
std::file_clock
. For example, MSVC++'s is called std::filesystem::_File_time_clock
.
We can keep much the same interface of P0355 by making file_clock
a typedef for an unspecified type. This technically changes the associated namespaces for expressions using that
clock for the sake of ADL, but I can't imagine a user who cares, as clocks aren't generally called in ADL-able
expressions, duration
s and time_point
s are.
Previous resolution [SUPERSEDED]:
This wording is relative to N4750.
Change 30.2 [time.syn], header
<chrono>
synopsis, as indicated:[…] // 30.7.6 [time.clock.file],classtype file_clockclassusing file_clock = unspecified; […]Change 30.7.6 [time.clock.file] as indicated:
23.17.7.5
ClassTypefile_clock
[time.clock.file]Change 30.7.6.1 [time.clock.file.overview], class
file_clock
synopsis, as indicated:namespace std::chrono { using file_clock = see below;class file_clock { public: using rep = a signed arithmetic type; using period = ratio<unspecified, unspecified>; using duration = chrono::duration<rep, period>; using time_point = chrono::time_point<file_clock>; static constexpr bool is_steady = unspecified; static time_point now() noexcept; // Conversion functions, see below };}-1-
The clockfile_clock
is an alias for a type meeting theTrivialClock
requirements (30.3 [time.clock.req]), uses a signed arithmetic type forfile_clock::rep
, and is used to create thetime_point
system used forfile_time_type
(31.12 [filesystems]). Its epoch is unspecified. [Note: The typefile_clock
denotes may be in a different namespace thanstd::chrono
, such asstd::filesystem
. — end note]Change 30.7.6.2 [time.clock.file.members] as indicated:
static time_point now();-2- The class
-1- Returns: Afile_clock::time_point
indicating the current time.type denoted byfile_clock
shallfile_clock
provides precisely one of the following two sets of static member functions: […]
[2018-07-30 Priority set to 1 after reflector discussion; wording updates based on several discussion contributions.]
Previous resolution [SUPERSEDED]:
This wording is relative to N4762.
Change 30.2 [time.syn], header
<chrono>
synopsis, as indicated:[…] // 30.7.6 [time.clock.file],classtype file_clockclassusing file_clock = see below; […]Change 30.7.6 [time.clock.file] as indicated:
23.17.7.5
ClassTypefile_clock
[time.clock.file]Change 30.7.6.1 [time.clock.file.overview], class
file_clock
synopsis, as indicated:namespace std::chrono { using file_clock = see below;class file_clock { public: using rep = a signed arithmetic type; using period = ratio<unspecified, unspecified>; using duration = chrono::duration<rep, period>; using time_point = chrono::time_point<file_clock>; static constexpr bool is_steady = unspecified; static time_point now() noexcept; // Conversion functions, see below };}-1-
The clockfile_clock
is an alias for a type meeting theTrivialClock
requirements (30.3 [time.clock.req]), which uses a signed arithmetic type forfile_clock::rep
.file_clock
is used to create thetime_point
system used forfile_time_type
(31.12 [filesystems]). Its epoch is unspecified, andnoexcept(file_clock::now())
istrue
. [Note: The typefile_clock
denotes may be in a different namespace thanstd::chrono
, such asstd::filesystem
. — end note]Change 30.7.6.2 [time.clock.file.members] as indicated:
static time_point now();-2- The class
-1- Returns: Afile_clock::time_point
indicating the current time.type denoted byfile_clock
shallfile_clock
provides precisely one of the following two sets of static member functions: […]
[2018-08-23 Batavia Issues processing: Minor wording changes, and status to "Tentatively Ready".]
[2018-11, Adopted in San Diego]
Proposed resolution:
This wording is relative to N4762.
Change 30.2 [time.syn], header <chrono>
synopsis, as indicated:
[…] // 30.7.6 [time.clock.file],classtype file_clockclassusing file_clock = see below; […]
Change 30.7.6 [time.clock.file] as indicated:
23.17.7.5
ClassTypefile_clock
[time.clock.file]
Change 30.7.6.1 [time.clock.file.overview], class file_clock
synopsis, as indicated:
namespace std::chrono { using file_clock = see below;class file_clock { public: using rep = a signed arithmetic type; using period = ratio<unspecified, unspecified>; using duration = chrono::duration<rep, period>; using time_point = chrono::time_point<file_clock>; static constexpr bool is_steady = unspecified; static time_point now() noexcept; // Conversion functions, see below };}-1-
The clockfile_clock
is an alias for a type meeting theCpp17TrivialClock
requirements (30.3 [time.clock.req]), and using a signed arithmetic type forfile_clock::rep
.file_clock
is used to create thetime_point
system used forfile_time_type
(31.12 [filesystems]). Its epoch is unspecified, andnoexcept(file_clock::now())
istrue
. [Note: The type thatfile_clock
denotes may be in a different namespace thanstd::chrono
, such asstd::filesystem
. — end note]
Change 30.7.6.2 [time.clock.file.members] as indicated:
static time_point now();-2- The type
-1- Returns: Afile_clock::time_point
indicating the current time.type denoted byfile_clock
shallfile_clock
provides precisely one of the following two sets of static member functions: […]
std::ref
/cref
Section: 22.10.6.7 [refwrap.helpers] Status: C++23 Submitter: Agustín K-ballo Bergé Opened: 2018-07-10 Last modified: 2023-11-22
Priority: 3
View all issues with C++23 status.
Discussion:
The overloads of std::ref
/cref
that take a reference_wrapper
as argument are
defined as calling std::ref
/cref
recursively, whereas the return type is defined as
unwrapping just one level. Calling these functions with arguments of multiple level of wrapping leads
to ill-formed programs:
int i = 0; std::reference_wrapper<int> ri(i); std::reference_wrapper<std::reference_wrapper<int>> rri(ri); std::ref(rri); // error within 'std::ref'
[Note: these overloads were added by issue resolution 10.29 for TR1, which can be found at N1688, at Redmond 2004]
[2018-08-20 Priority set to 3 after reflector discussion]
[2021-05-22 Tim syncs wording to the current working draft]
[2021-08-20; LWG telecon]
Set status to Tentatively Ready after telecon review.
[2021-10-14 Approved at October 2021 virtual plenary. Status changed: Voting → WP.]
Proposed resolution:
This wording is relative to N4885.
Change 22.10.6.7 [refwrap.helpers] as indicated:
template<class T> constexpr reference_wrapper<T> ref(reference_wrapper<T> t) noexcept;[…]-3- Returns:
.
ref(t.get())ttemplate<class T> constexpr reference_wrapper<const T> cref(reference_wrapper<T> t) noexcept;-5- Returns:
.
cref(t.get())t
Section: 16.4.5.3.3 [macro.names] Status: C++20 Submitter: Casey Carter Opened: 2018-08-01 Last modified: 2021-02-25
Priority: 0
View other active issues in [macro.names].
View all other issues in [macro.names].
View all issues with C++20 status.
Discussion:
16.4.5.3.3 [macro.names]/2 forbids a translation unit to define names "lexically identical to […] the
attribute-tokens described in 9.13 [dcl.attr]." We recently added the attribute-tokens likely
and unlikely
(9.13.7 [dcl.attr.likelihood]). These names are in extremely wide use as function-like
macros in the open source community, forbidding users to define them breaks large amounts of code. (Reportedly
Chromium contains 19 definitions each of "likely" and "unlikely" as function-like macros.)
[[likely]]
and
[[unlikely]]
attributes.
[2018-08-20 Status to Tentatively Ready after five positive votes on the reflector.]
[2018-11, Adopted in San Diego]
Proposed resolution:
This wording is relative to N4762.
Change 16.4.5.3.3 [macro.names] as indicated:
[Drafting Note: If both this proposed resolution and the proposed resolution of LWG 3132(i) are accepted, the text inserted by LWG 3132(i) should precede the text added here.]
-2- A translation unit shall not
#define
or#undef
names lexically identical to keywords, to the identifiers listed in Table 4, or to the attribute-tokens described in 9.13 [dcl.attr], except that the nameslikely
andunlikely
may be defined as function-like macros (15.7 [cpp.replace]).
<concepts>
should be freestandingSection: 16.4.2.5 [compliance] Status: C++20 Submitter: Casey Carter Opened: 2018-08-09 Last modified: 2021-02-25
Priority: 0
View all other issues in [compliance].
View all issues with C++20 status.
Discussion:
The design intent of the <concepts>
header is that it contains
only fundamental concept definitions and implementations of customization points
that are used by those concept definitions. There should never be components in
the header that require operating system support. Consequently, freestanding
implementations can and should provide it. It is an oversight on the
part of LWG - and in particular the author of
P0898R3 "Standard Libary Concepts" - that the <concepts>
header is not required to be provided by freestanding implementations.
[2018-08 Batavia Monday issue prioritization]
Priority set to 0, status to 'Tentatively Ready'
[2018-11, Adopted in San Diego]
Proposed resolution:
This wording is relative to N4762.
In 16.4.2.5 [compliance], add a new row to Table 21:
Table 21 — C++ headers for freestanding implementations Subclause Header(s) […] […] […] 17.14 [support.runtime] Other runtime support <cstdarg>
18 [concepts] Concepts library <concepts>
21 [meta] Type traits <type_traits>
[…] […] […]
DefaultConstructible
should require default initializationSection: 18.4.12 [concept.default.init] Status: C++20 Submitter: Casey Carter Opened: 2018-08-09 Last modified: 2021-06-06
Priority: 2
View all other issues in [concept.default.init].
View all issues with C++20 status.
Discussion:
DefaultConstructible<T>
is equivalent to
Constructible<T>
(18.4.11 [concept.constructible]), which
is equivalent to is_constructible_v<T>
(21.3.5.4 [meta.unary.prop]). Per 21.3.5.4 [meta.unary.prop]
paragraph 8:
The predicate condition for a template specialization
is_constructible<T, Args...>
shall be satisfied if and only if the following variable definition would be well-formed for some invented variablet
:T t(declval<Args>()...);
DefaultConstructible<T>
requires that objects of type T
can be
value-initialized,
rather than
default-initialized
as intended.
The library needs a constraint that requires object types to be
default-initializable: the "rangified" versions of the algorithms in
26.11.3 [uninitialized.construct.default] proposed in
P0896 "The One Ranges Proposal", for
example. Users will also want a mechanism to provide such a constraint, and
they're likely to choose DefaultConstructible
despite its subtle
unsuitability.
There are two alternative solutions: (1) change DefaultConstructible
to require default-initialization, (2) change
is_default_constructible_v
to require default-initializaton and specify
the concept in terms of the trait. (2) is probably too breaking a change to be
feasible.
[2018-08-20 Priority set to 2 after reflector discussion]
Previous resolution [SUPERSEDED]:
Modify [concept.defaultconstructible] as follows:
template<class T> concept DefaultConstructible = Constructible<T> && see below;-?- Type
T
modelsDefaultConstructible
only if the variable definitionis well-formed for some invented variableT t;t
. Access checking is performed as if in a context unrelated toT
. Only the validity of the immediate context of the variable initialization is considered.
[2018-08-23 Tim provides updated P/R based on Batavia discussion]
[2018-10-28 Casey expands the problem statement and the P/R]
During Batavia review of P0896R3, Tim
Song noted that {}
is not necessarily a valid initializer for a
DefaultConstructible
type. In this sample program
(see Compiler Explorer):
struct S0 { explicit S0() = default; }; struct S1 { S0 x; }; // Note: aggregate S1 x; // Ok S1 y{}; // ill-formed; copy-list-initializes x from {}
S1
can be default-initialized, but not list-initialized from an empty
braced-init-list. The consensus among those present was that
DefaultConstructible
should prohibit this class of pathological types
by requiring that initialization form to be valid.
[2019 Cologne Wednesday night]
Status to Ready
[2019-10-07 Casey rebases P/R onto N4830 and incorporates WG21-approved changes from P1754R1]
Previous resolution [SUPERSEDED]:
This wording is relative to N4762.
Modify [concept.defaultconstructible] as follows:
template<class T> inline constexpr bool is-default-initializable = see below; // exposition only template<class T> concept DefaultConstructible = Constructible<T> && requires { T{}; } && is-default-initializable<T>;-?- For a type
T
,is-default-initializable<T>
istrue
if and only if the variable definitionis well-formed for some invented variableT t;t
; otherwise it isfalse
. Access checking is performed as if in a context unrelated toT
. Only the validity of the immediate context of the variable initialization is considered.
P1754R1 "Rename concepts to standard_case
for
C++20" - as approved by both LEWG and LWG in Cologne - contained instructions to rename the
DefaultConstructible
concept to default_initializable
"If LWG 3151 is accepted."
3151(i) is the unrelated "ConvertibleTo
rejects conversion from array and
function types"; this issue is intended by P1754R1. Since P1754R1 was applied to the working draft
in Cologne, whereas this issue was only made Ready, we should apply the desired renaming to the P/R
of this issue.
Previous resolution [SUPERSEDED]:
This wording is relative to N4830.
Modify 18.3 [concepts.syn], header
<concepts>
synopsis, as indicated:[…] // [concept.defaultconstructible], concept default_constructibleinitializable template<class T> concept default_constructibleinitializable = see below; […]Modify [concept.defaultconstructible] as indicated:
18.4.12 Concept
default_
[concept.defaultconstructibleinitializableconstructibleinitializable]template<class T> inline constexpr bool is-default-initializable = see below; // exposition only template<class T> concept default_constructibleinitializable = constructible_from<T> && requires { T{}; } && is-default-initializable<T>;-?- For a type
T
,is-default-initializable<T>
istrue
if and only if the variable definitionis well-formed for some invented variableT t;t
; otherwise it isfalse
. Access checking is performed as if in a context unrelated toT
. Only the validity of the immediate context of the variable initialization is considered.Modify 18.6 [concepts.object] as indicated:
-1- This subclause describes concepts that specify the basis of the value-oriented programming style on which the library is based.
template<class T> concept movable = is_object_v<T> && move_constructible<T> && […] template<class T> concept semiregular = copyable<T> && default_constructibleinitializable<T>; […]Modify 20.2.2 [memory.syn], header
<memory>
synopsis, as indicated:[…] namespace ranges { template<no-throw-forward-iterator I, no-throw-sentinel<I> S> requires default_constructibleinitializable<iter_value_t<I>> I uninitialized_default_construct(I first, S last); template<no-throw-forward-range R> requires default_constructibleinitializable<range_value_t<R>> safe_iterator_t<R> uninitialized_default_construct(R&& r); template<no-throw-forward-iterator I> requires default_constructibleinitializable<iter_value_t<I>> I uninitialized_default_construct_n(I first, iter_difference_t<I> n); } […] namespace ranges { template<no-throw-forward-iterator I, no-throw-sentinel<I> S> requires default_constructibleinitializable<iter_value_t<I>> I uninitialized_value_construct(I first, S last); template<no-throw-forward-range R> requires default_constructibleinitializable<range_value_t<R>> safe_iterator_t<R> uninitialized_value_construct(R&& r); template<no-throw-forward-iterator I> requires default_constructibleinitializable<iter_value_t<I>> I uninitialized_value_construct_n(I first, iter_difference_t<I> n); } […]Modify 26.11.3 [uninitialized.construct.default] as indicated:
namespace ranges { template<no-throw-forward-iterator I, no-throw-sentinel<I> S> requires default_constructibleinitializable<iter_value_t<I>> I uninitialized_default_construct(I first, S last); template<no-throw-forward-range R> requires default_constructibleinitializable<range_value_t<R>> safe_iterator_t<R> uninitialized_default_construct(Ramp;& r); }-2- Effects: Equivalent to:
[…]namespace ranges { template<no-throw-forward-iterator I> requires default_constructibleinitializable<iter_value_t<I>> I uninitialized_default_construct_n(I first, iter_difference_t<I> n); }-4- Effects: Equivalent to:
[…]Modify 26.11.4 [uninitialized.construct.value] as indicated:
namespace ranges { template<no-throw-forward-iterator I, no-throw-sentinel<I> S> requires default_constructibleinitializable<iter_value_t<I>> I uninitialized_value_construct(I first, S last); template<no-throw-forward-range R> requires default_constructibleinitializable<range_value_t<R>> safe_iterator_t<R> uninitialized_value_construct(R&& r); }-2- Effects: Equivalent to:
[…]namespace ranges { template<no-throw-forward-iterator I> requires default_constructibleinitializable<iter_value_t<I>> I uninitialized_value_construct_n(I first, iter_difference_t<I> n); }-4- Effects: Equivalent to:
[…]Modify [range.semi.wrap] as indicated:
-1- Many types in this subclause are specified in terms of an exposition-only class template
semiregular-box
.semiregular-box<T>
behaves exactly likeoptional<T>
with the following differences:
(1.1) — […]
(1.2) — If
T
modelsdefault_
, the default constructor ofconstructibleinitializablesemiregular-box<T>
is equivalent to:constexpr semiregular-box() noexcept(is_nothrow_default_constructible_v<T>) : semiregular-box{in_place} { }(1.3) — […]
Modify 25.6.6.2 [range.istream.view], Class template
basic_istream_view
synopsis, as indicated:namespace std::ranges { […] template<movable Val, class CharT, class Traits> requires default_constructibleinitializable<Val> && stream-extractable<Val, CharT, Traits> class basic_istream_view : public view_interface<basic_istream_view<Val, CharT, Traits>> { […] } […] }
[2019-11-17; Daniel comments and restores wording]
During the Belfast 2019 meeting the concept renaming was not voted in by this issue, but separately, the accepted wording can be found in P1917R0#3149. To prevent confusion, the here presented proposed wording has been synchronized with that of the voted in document.
Proposed resolution:
This wording is relative to N4762.
Modify [concept.defaultconstructible] as follows:
template<class T> inline constexpr bool is-default-initializable = see below; // exposition only template<class T> concept DefaultConstructible = Constructible<T> && requires { T{}; } && is-default-initializable<T>;-?- For a type
T
,is-default-initializable<T>
istrue
if and only if the variable definitionis well-formed for some invented variableT t;t
; otherwise it isfalse
. Access checking is performed as if in a context unrelated toT
. Only the validity of the immediate context of the variable initialization is considered.
UniformRandomBitGenerator
should validate min
and max
Section: 29.5.3.3 [rand.req.urng] Status: C++20 Submitter: Casey Carter Opened: 2018-08-09 Last modified: 2021-02-25
Priority: 3
View all other issues in [rand.req.urng].
View all issues with C++20 status.
Discussion:
29.5.3.3 [rand.req.urng]
paragraph 2 specifies axioms for
the UniformRandomBitGenerator
concept:
2 Let
g
be an object of typeG
.G
modelsUniformRandomBitGenerator
only if(2.1) — both
G::min()
andG::max()
are constant expressions (7.7 [expr.const]),(2.2) —
G::min() < G::max()
,(2.3) —
G::min() <= g()
,(2.4) —
g() <= G::max()
, and(2.5) —
g()
has amortized constant complexity.
Bullets 2.1 and 2.2 are both compile-time requirements that ought to be validated by the concept.
[2018-08-20 Priority set to 3 after reflector discussion]
Previous resolution [SUPERSEDED]:This wording is relative to N4791.
Modify 29.5.3.3 [rand.req.urng] as follows:
1 A uniform random bit generator
g
of typeG
is a function object returning unsigned integer values such that each value in the range of possible results has (ideally) equal probability of being returned. [Note: The degree to whichg
's results approximate the ideal is often determined statistically.—end note]template<auto> struct require-constant; // exposition-only template<class G> concept UniformRandomBitGenerator = Invocable<G&> && UnsignedIntegral<invoke_result_t<G&>> && requires { { G::min() } -> Same<invoke_result_t<G&>>; { G::max() } -> Same<invoke_result_t<G&>>; typename require-constant<G::min()>; typename require-constant<G::max()>; requires G::min() < G::max(); };2 Let
g
be an object of typeG
.G
modelsUniformRandomBitGenerator
only if
(2.1) — bothG::min()
andG::max()
are constant expressions (7.7 [expr.const]),
(2.2) —G::min() < G::max()
,(2.3) —
G::min() <= g()
,(2.4) —
g() <= G::max()
, and(2.5) —
g()
has amortized constant complexity.3 A class
G
meets the uniform random bit generator requirements ifG
modelsUniformRandomBitGenerator
,invoke_result_t<G&>
is an unsigned integer type (6.8.2 [basic.fundamental]), andG
provides a nested typedef-nameresult_type
that denotes the same type asinvoke_result_t<G&>
.
[2020-02-13; Prague]
LWG provided some improved wording.
[2020-02 Status to Immediate on Thursday night in Prague.]
Proposed resolution:
This wording is relative to N4849.
Modify 29.5.3.3 [rand.req.urng] as follows:
1 A uniform random bit generator
g
of typeG
is a function object returning unsigned integer values such that each value in the range of possible results has (ideally) equal probability of being returned. [Note: The degree to whichg
's results approximate the ideal is often determined statistically.—end note]template<class G> concept uniform_random_bit_generator = invocable<G&> && unsigned_integral<invoke_result_t<G&>> && requires { { G::min() } -> same_as<invoke_result_t<G&>>; { G::max() } -> same_as<invoke_result_t<G&>>; requires bool_constant<(G::min() < G::max())>::value; };-2- Let
g
be an object of typeG
.G
modelsuniform_random_bit_generator
only if
(2.1) — bothG::min()
andG::max()
are constant expressions (7.7 [expr.const]),
(2.2) —G::min() < G::max()
,(2.3) —
G::min() <= g()
,(2.4) —
g() <= G::max()
, and(2.5) —
g()
has amortized constant complexity.3 A class
G
meets the uniform random bit generator requirements ifG
modelsuniform_random_bit_generator
,invoke_result_t<G&>
is an unsigned integer type (6.8.2 [basic.fundamental]), andG
provides a nested typedef-nameresult_type
that denotes the same type asinvoke_result_t<G&>
.
ConvertibleTo
rejects conversions from array and function typesSection: 18.4.4 [concept.convertible] Status: Resolved Submitter: Casey Carter Opened: 2018-08-09 Last modified: 2020-11-09
Priority: 3
View other active issues in [concept.convertible].
View all other issues in [concept.convertible].
View all issues with Resolved status.
Discussion:
In the definition of ConvertibleTo
in 18.4.4 [concept.convertible]:
template<class From, class To> concept ConvertibleTo = is_convertible_v<From, To> && requires(From (&f)()) { static_cast<To>(f()); };
f
is an arbitrary function that returns type From
. Since
functions cannot return array or function types (9.3.4.6 [dcl.fct]
paragraph 11), ConvertibleTo
cannot be satisfied when From
is an array or function type regardless
of the type of To
. This is incompatibility with is_convertible_v
was not an intentional design feature, so it should be corrected. (Note that any
change made here must take care to avoid breaking the
ConvertibleTo<T, void>
cases.)
[2018-08-20 Priority set to 3 after reflector discussion]
Previous resolution [SUPERSEDED]:
[Drafting Note: I've used
declval
here, despite that "Concepts mean we never have to usedeclval
again!" because the alternative is less readable:]requires(add_rvalue_reference_t<From> (&f)()) { static_cast<To>(f()); };This wording is relative to N4762.
Modify 18.4.4 [concept.convertible] as follows:
template<class From, class To> concept ConvertibleTo = is_convertible_v<From, To> && requires(From (&f)()){ static_cast<To>(f()declval<From>()); };
[2020-11-09 Resolved for C++20. Status changed: Tentatively Resolved → Resolved.]
Proposed resolution:
This issue is resolved by the resolution of issue 3194(i).
common_type
and common_reference
have flaws in commonSection: 21.3.8.7 [meta.trans.other] Status: C++23 Submitter: Casey Carter Opened: 2018-08-10 Last modified: 2023-11-22
Priority: 3
View all other issues in [meta.trans.other].
View all issues with C++23 status.
Discussion:
21.3.8.7 [meta.trans.other] p5 characterizes the requirements for
program-defined specializations of common_type
with the sentence:
Such a specialization need not have a member namedtype
, but if it does, that member shall be a typedef-name for an accessible and unambiguous cv-unqualified non-reference typeC
to which each of the typesT1
andT2
is explicitly convertible.
This sentence - which 21.3.8.7 [meta.trans.other] p7 largely duplicates to
specify requirements on program-defined specializations of
basic_common_reference
- has two problems:
The grammar term "typedef-name" is overconstraining; there's no reason to prefer a typedef-name here to an actual type, and
"accessible" and "unambiguous" are not properties of types, they are properties of names and base classes.
While we're here, we may as well strike the unused name C
which
both Note B and Note D define for the type denoted by type
.
[2018-08 Batavia Monday issue prioritization]
Priority set to 3
[2021-06-23; Reflector poll]
Set status to Tentatively Ready after seven votes in favour during reflector poll.
[2021-10-14 Approved at October 2021 virtual plenary. Status changed: Voting → WP.]
Proposed resolution:
This wording is relative to N4762.
Modify 21.3.8.7 [meta.trans.other] p5 as follows:
-5- Note B: Notwithstanding the provisions of 21.3.3 [meta.type.synop], and pursuant to 16.4.5.2.1 [namespace.std], a program may specialize
common_type<T1, T2>
for typesT1
andT2
such thatis_same_v<T1, decay_t<T1>>
andis_same_v<T2, decay_t<T2>>
are each true. [Note: …] Such a specialization need not have a member namedtype
, but if it does,that member shall be a typedef-name for an accessible and unambiguousthe qualified-idcommon_type<T1, T2>::type
shall denote a cv-unqualified non-reference typeto which each of the typesC
T1
andT2
is explicitly convertible. Moreover, […]
Modify 21.3.8.7 [meta.trans.other] p7 similarly:
-7- Note D: Notwithstanding the provisions of 21.3.3 [meta.type.synop], and pursuant to 16.4.5.2.1 [namespace.std], a program may partially specialize
basic_common_reference<T, U, TQual, UQual>
for typesT
andU
such thatis_same_v<T, decay_t<T>>
andis_same_v<U, decay_t<U>>
are each true. [Note: …] Such a specialization need not have a member namedtype
, but if it does,that member shall be a typedef-name for an accessible and unambiguousthe qualified-idbasic_common_reference<T, U, TQual, UQual>::type
shall denote a typeto which each of the typesC
TQual<T>
andUQual<U>
is convertible. Moreover, […]
Common
and common_type
have too little in commonSection: 18.4.6 [concept.common] Status: C++20 Submitter: Casey Carter Opened: 2018-08-10 Last modified: 2021-02-25
Priority: 0
View all other issues in [concept.common].
View all issues with C++20 status.
Discussion:
The Common
concept when applied to types T
and U
requires that T
and U
are each ConvertibleTo
( [concept.convertibleto]) their common type
common_type_t<T, U>
. ConvertibleTo
requires both
implicit and explicit conversions with equivalent results. The requirement for
implicit conversion is notably not a requirement for specializing
common_type
as detailed in 21.3.8.7 [meta.trans.other]:
-5- Such a specialization need not have a member namedtype
, but if it does, that member shall be a typedef-name for an accessible and unambiguous cv-unqualified non-reference typeC
to which each of the typesT1
andT2
is explicitly convertible.
which only requires explicit conversion to be valid. While it's not
inconsistent that the Common
concept's requirements are a refinement of
the requirements for common_type
, there's no good reason for this
additional requirement. The stated design intent is to enable writing monomorphic
predicates that can compare T
s with U
s (and vice versa) by
accepting two arguments of type common_type_t<T, U>
, but this
role has been superseded by the addition of CommonReference
and
common_reference_t
to the ranges design. The existence of pairs of
types that are only explicitly convertible to their common type suggests that
using Common
in this way would never be a fully generic solution in any
case.
The only existing use of the Common
concept in
either the working draft or the Ranges proposal is as a soundness check on the
comparison
and
difference
operators of counted_iterator
, none of which actually convert any
argument to the common type in their normal operation. It would seem that we
could strike the additional requirement without impacting the Ranges design,
which would allow for future uses of the Common
concept with types
like chrono::duration
(30.5 [time.duration]) which sometimes
provide only explicit conversion to a common type.
Notably, removing the requirement for implicit conversion will also make the
Common
concept consistent with the description in
18.4.6 [concept.common] p1: "If T
and U
can both be
explicitly converted to some third type, C
, then T
and
U
share a common type, C
."
[2018-08 Batavia Monday issue prioritization]
P0; Status to 'Tentatively Ready' after adding two semicolons to the P/R.
[2018-11, Adopted in San Diego]
Proposed resolution:
This wording is relative to N4762.
Modify the definition of Common
in 18.4.6 [concept.common]
as follows:
template<class T, class U> concept Common = Same<common_type_t<T, U>, common_type_t<U, T>> &&ConvertibleTo<T, common_type_t<T, U>> &&ConvertibleTo<U, common_type_t<T, U>> &&requires { static_cast<common_type_t<T, U>>(declval<T>()); static_cast<common_type_t<T, U>>(declval<U>()); } && CommonReference< add_lvalue_reference_t<const T>, add_lvalue_reference_t<const U>> && CommonReference< add_lvalue_reference_t<common_type_t<T, U>>, common_reference_t< add_lvalue_reference_t<const T>, add_lvalue_reference_t<const U>>>;
Common
and CommonReference
have a common defectSection: 18.4.6 [concept.common] Status: C++20 Submitter: Casey Carter Opened: 2018-08-10 Last modified: 2021-02-25
Priority: 0
View all other issues in [concept.common].
View all issues with C++20 status.
Discussion:
The semantic requirements of both Common
(18.4.6 [concept.common]):
-2- Let
C
becommon_type_t<T, U>
. Lett
be a function whose return type isT
, and letu
be a function whose return type isU
.Common<T, U>
is satisfied only if:(2.1) —
C(t())
equalsC(t())
if and only ift()
is an equality-preserving expression (18.2 [concepts.equality]).(2.2) —
C(u())
equalsC(u())
if and only ifu()
is an equality-preserving expression (18.2 [concepts.equality]).
and similarly CommonReference
( [concept.commonreference]):
-2- Let
C
becommon_reference_t<T, U>
. Lett
be a function whose return type isT
, and letu
be a function whose return type isU
.CommonReference<T, U>
is satisfied only if:(2.1) —
C(t())
equalsC(t())
if and only ift()
is an equality-preserving expression (18.2 [concepts.equality]).(2.2) —
C(u())
equalsC(u())
if and only ifu()
is an equality-preserving expression.
don't properly reflect the intended design that conversions to the common type /
common reference type are identity-preserving: in other words, that converting
two values to the common type produces equal results if and only if the values
were initially equal. The phrasing "C(E)
equals C(E)
if and only if E
is an equality-preserving expression" is also clearly
defective regardless of the intended design: the assertion "E
is not
equality-preserving" does not imply that every evaluation of E
produces
different results.
[2018-08 Batavia Monday issue prioritization]
Priority set to 0, status to 'Tentatively Ready'
[2018-11, Adopted in San Diego]
Proposed resolution:
This wording is relative to N4762.
Modify 18.4.5 [concept.commonref] p2 as follows:
-2- Let
C
becommon_reference_t<T, U>
. Lett
be a function whose return type ist1
andt2
be equality-preserving expressions (18.2 [concepts.equality]) such thatdecltype((t1))
anddecltype((t2))
are eachT
, and letu
be a function whose return type isu1
andu2
be equality-preserving expressions such thatdecltype((u1))
anddecltype((u2))
are eachU
.T
andU
modelCommonReference<T, U>
is satisfiedonly if:(2.1) —
C(t1
equals())C(t2
if and only if())t1
equals()t2
, andis an equality-preserving expression (18.2 [concepts.equality]).(2.2) —
C(u1
equals())C(u2
if and only if())u1
equals()u2
is an equality-preserving expression.
Modify 18.4.6 [concept.common] p2 similarly:
-2- Let
C
becommon_type_t<T, U>
. Lett
be a function whose return type ist1
andt2
be equality-preserving expressions (18.2 [concepts.equality]) such thatdecltype((t1))
anddecltype((t2))
are eachT
, and letu
be a function whose return type isu1
andu2
be equality-preserving expressions such thatdecltype((u1))
anddecltype((u2))
are eachU
.T
andU
modelCommon<T, U>
is satisfiedonly if:(2.1) —
C(t1
equals())C(t2
if and only if())t1
equals()t2
, andis an equality-preserving expression (18.2 [concepts.equality]).(2.2) —
C(u1
equals())C(u2
if and only if())u1
equals()u2
is an equality-preserving expression (18.2 [concepts.equality]).
tuple<any, any>{allocator_arg_t, an_allocator}
Section: 22.4.4.2 [tuple.cnstr] Status: Resolved Submitter: Jonathan Wakely Opened: 2018-08-18 Last modified: 2021-10-23
Priority: 3
View other active issues in [tuple.cnstr].
View all other issues in [tuple.cnstr].
View all issues with Resolved status.
Discussion:
For a 2-element std::tuple
, attempting to call the "allocator-extended default constructor" might actually
pass the allocator_arg
tag and the allocator to the tuple element constructors:
tuple<any, any> t{allocator_arg, allocator<int>{}}; assert(std::get<0>(t).has_value());
This assertion should pass according to the standard, but users might expect the elements to be default constructed. If you really wanted to construct the elements with the tag and the allocator, you could do:
tuple<any, any> t{{allocator_arg}, {allocator<int>{}}};
or
tuple<any, any> t{tuple<allocator_arg_t, allocator<int>>{allocator_arg, allocator<int>{}}};
The deduction guides for std::tuple
always treat {allocator_arg_t, an_allocator}
as the
allocator-extended default constructor, so this creates an empty tuple:
tuple t{allocator_arg, allocator<int>{}};
And this is needed to create tuple<any, any>
:
tuple t{allocator_arg, allocator<int>{}, any{}, any{}};
The proposed resolution seems consistent with that, always calling an allocator-extended constructor for
{allocator_arg_t, a}
, instead of the tuple(UTypes&&...)
constructor.
[2018-08-20, Daniel comments]
The wording changes by this issue are very near to those suggested for LWG 3121(i).
[2018-08 Batavia Monday issue prioritization]
Priority set to 0, status to 'Tentatively Ready'. Alisdair to write a paper about SFINAE constraints on the Allocator-aware tuple constructors.
[2018-08 Batavia Friday]
Tim Song found a 3-element case of this issue. Status back to 'Open'
tuple<any,any,any>(allocator_arg_t, a, tuple)
[2018-09 Reflector prioritization]
Set Priority to 3
Previous resolution [SUPERSEDED]:
This wording is relative to N4762.
Modify 22.4.4.2 [tuple.cnstr] as indicated:
template<class... UTypes> explicit(see below) constexpr tuple(UTypes&&... u);-9- Effects: Initializes the elements in the tuple with the corresponding value in
-10- Remarks: This constructor shall not participate in overload resolution unlessstd::forward<UTypes>(u)
.sizeof...(Types) == sizeof...(UTypes)
andsizeof...(Types) >= 1
andis_constructible_v<Ti, Ui&&>
istrue
for alli
and(sizeof...(Types) != 2 || !is_same_v<remove_cvref_t<U0>, allocator_arg_t>)
. The expression insideexplicit
is equivalent to:!conjunction_v<is_convertible<UTypes, Types>...>
[2021-08-20; LWG telecon]
Status changed to Tentatively Resolved, by 3121(i).
[2021-10-23 Resolved by 3121(i), approved at October 2021 virtual plenary. Status changed: Tentatively Resolved → Resolved.]
Proposed resolution:
This issue is resolved by the resolution of issue 3121(i).
ForwardIterator
should only mean forward iteratorSection: 26.11 [specialized.algorithms] Status: Resolved Submitter: Casey Carter Opened: 2018-09-06 Last modified: 2020-05-02
Priority: 3
View other active issues in [specialized.algorithms].
View all other issues in [specialized.algorithms].
View all issues with Resolved status.
Discussion:
26.11 [specialized.algorithms] para 1.2 describes how the specialized
algorithms with a template parameter named ForwardIterator
impose
requirements on the type passed as argument for that parameter: it must meet
the Cpp17ForwardIterator
requirements, which is consistent with
how the rest of the Library uses the template parameter name
ForwardIterator
, and many of the required operations on that type must
not throw exceptions, which is not consistent with how the rest of the
Library uses that name.
To avoid confusion and keep the meaning of requirements imposed by template parameter names crisp, the specialized memory algorithms should use a different template parameter name for this different set of requirements.
Note that the proposed change has no normative effect; it's simply a clarification of the existing wording.
[2018-09 Reflector prioritization]
Set Priority to 3
Previous resolution [SUPERSEDED]:
This wording is relative to N4762.
Modify 20.2.2 [memory.syn] as indicated:
[…] // 26.11 [specialized.algorithms], specialized algorithms template<class T> constexpr T* addressof(T& r) noexcept; template<class T> const T* addressof(const T&&) = delete; template<class NoThrowForwardIterator> void uninitialized_default_construct(NoThrowForwardIterator first, NoThrowForwardIterator last); template<class ExecutionPolicy, class NoThrowForwardIterator> void uninitialized_default_construct(ExecutionPolicy&& exec, // see 26.3.5 [algorithms.parallel.overloads] NoThrowForwardIterator first, NoThrowForwardIterator last); template<class NoThrowForwardIterator, class Size> NoThrowForwardIterator uninitialized_default_construct_n(NoThrowForwardIterator first, Size n); template<class ExecutionPolicy, class NoThrowForwardIterator, class Size> NoThrowForwardIterator uninitialized_default_construct_n(ExecutionPolicy&& exec, // see 26.3.5 [algorithms.parallel.overloads] NoThrowForwardIterator first, Size n); template<class NoThrowForwardIterator> void uninitialized_value_construct(NoThrowForwardIterator first, NoThrowForwardIterator last); template<class ExecutionPolicy, class NoThrowForwardIterator> void uninitialized_value_construct(ExecutionPolicy&& exec, // see 26.3.5 [algorithms.parallel.overloads] NoThrowForwardIterator first, NoThrowForwardIterator last); template<class NoThrowForwardIterator, class Size> NoThrowForwardIterator uninitialized_value_construct_n(NoThrowForwardIterator first, Size n); template<class ExecutionPolicy, class NoThrowForwardIterator, class Size> NoThrowForwardIterator uninitialized_value_construct_n(ExecutionPolicy&& exec, // see 26.3.5 [algorithms.parallel.overloads] NoThrowForwardIterator first, Size n); template<class InputIterator, class NoThrowForwardIterator> NoThrowForwardIterator uninitialized_copy(InputIterator first, InputIterator last, NoThrowForwardIterator result); template<class ExecutionPolicy, class InputIterator, class NoThrowForwardIterator> NoThrowForwardIterator uninitialized_copy(ExecutionPolicy&& exec, // see 26.3.5 [algorithms.parallel.overloads] InputIterator first, InputIterator last, NoThrowForwardIterator result); template<class InputIterator, class Size, class NoThrowForwardIterator> NoThrowForwardIterator uninitialized_copy_n(InputIterator first, Size n, NoThrowForwardIterator result); template<class ExecutionPolicy, class InputIterator, class Size, class NoThrowForwardIterator> NoThrowForwardIterator uninitialized_copy_n(ExecutionPolicy&& exec, // see 26.3.5 [algorithms.parallel.overloads] InputIterator first, Size n, NoThrowForwardIterator result); template<class InputIterator, class NoThrowForwardIterator> NoThrowForwardIterator uninitialized_move(InputIterator first, InputIterator last, NoThrowForwardIterator result); template<class ExecutionPolicy, class InputIterator, class NoThrowForwardIterator> NoThrowForwardIterator uninitialized_move(ExecutionPolicy&& exec, // see 26.3.5 [algorithms.parallel.overloads] InputIterator first, InputIterator last, NoThrowForwardIterator result); template<class InputIterator, class Size, class NoThrowForwardIterator> pair<InputIterator, NoThrowForwardIterator> uninitialized_move_n(InputIterator first, Size n, NoThrowForwardIterator result); template<class ExecutionPolicy, class InputIterator, class Size, class NoThrowForwardIterator> pair<InputIterator, NoThrowForwardIterator> uninitialized_move_n(ExecutionPolicy&& exec, // see 26.3.5 [algorithms.parallel.overloads] InputIterator first, Size n, NoThrowForwardIterator result); template<class NoThrowForwardIterator, class T> void uninitialized_fill(NoThrowForwardIterator first, NoThrowForwardIterator last, const T& x); template<class ExecutionPolicy, class NoThrowForwardIterator, class T> void uninitialized_fill(ExecutionPolicy&& exec, // see 26.3.5 [algorithms.parallel.overloads] NoThrowForwardIterator first, NoThrowForwardIterator last, const T& x); template<class NoThrowForwardIterator, class Size, class T> NoThrowForwardIterator uninitialized_fill_n(NoThrowForwardIterator first, Size n, const T& x); template<class ExecutionPolicy, class NoThrowForwardIterator, class Size, class T> NoThrowForwardIterator uninitialized_fill_n(ExecutionPolicy&& exec, // see 26.3.5 [algorithms.parallel.overloads] NoThrowForwardIterator first, Size n, const T& x); template<class T> void destroy_at(T* location); template<class NoThrowForwardIterator> void destroy(NoThrowForwardIterator first, NoThrowForwardIterator last); template<class ExecutionPolicy, class NoThrowForwardIterator> void destroy(ExecutionPolicy&& exec, // see 26.3.5 [algorithms.parallel.overloads] NoThrowForwardIterator first, NoThrowForwardIterator last); template<class NoThrowForwardIterator, class Size> NoThrowForwardIterator destroy_n(NoThrowForwardIterator first, Size n); template<class ExecutionPolicy, class NoThrowForwardIterator, class Size> NoThrowForwardIterator destroy_n(ExecutionPolicy&& exec, // see 26.3.5 [algorithms.parallel.overloads] NoThrowForwardIterator first, Size n); // 20.3.1 [unique.ptr], class template unique_ptr […]Modify 26.11 [specialized.algorithms] as indicated:
[…]
(1.2) — If an algorithm's template parameter is named
NoThrowForwardIterator
, the template argument shall satisfy theCpp17ForwardIterator
requirements (24.3.5.5 [forward.iterators]), and is required to have the property that no exceptions are thrown from increment, assignment, comparison, or indirection through valid iterators.[…]
Modify the declarations of the specialized algorithms in the remainder of 26.11 [specialized.algorithms] to agree with the proposed changes to 20.2.2 [memory.syn] above.
[2020-05-02; Reflector discussions]
The issue has been resolved by accepting P1963R0 in Prague 2020.
Proposed resolution:
Resolved by P1963R0.
tuple(allocator_arg_t, const Alloc&)
should be conditionally explicitSection: 22.4.4.2 [tuple.cnstr] Status: C++20 Submitter: Jonathan Wakely Opened: 2018-08-21 Last modified: 2021-02-25
Priority: 3
View other active issues in [tuple.cnstr].
View all other issues in [tuple.cnstr].
View all issues with C++20 status.
Discussion:
std::tuple
's allocator-extended constructors say "Effects: Equivalent to the preceding constructors except
that each element is constructed with uses-allocator construction". That's not true for the first one, as shown by:
#include <tuple> struct X { explicit X() { } }; std::tuple<X> f() { return {}; } std::tuple<X> g() { return { std::allocator_arg, std::allocator<int>{} }; }
The function f()
doesn't compile because of the explicit constructor, but g()
does, despite using the
same constructor for X
. The conditional explicit-ness is not equivalent.
[2018-09 Reflector prioritization]
Set Priority to 3
[2019-02; Kona Wednesday night issue processing]
Status to Ready
Proposed resolution:
This wording is relative to N4762.
Modify 22.4.4 [tuple.tuple], class template tuple
synopsis, as indicated:
[…] // allocator-extended constructors template<class Alloc> explicit(see below) tuple(allocator_arg_t, const Alloc& a); template<class Alloc> explicit(see below) tuple(allocator_arg_t, const Alloc& a, const Types&...); […]
Modify 22.4.4.2 [tuple.cnstr], as indicated:
explicit(see below) constexpr tuple();-5- Effects: Value-initializes each element.
-6- Remarks: This constructor shall not participate in overload resolution unlessis_default_constructible_v<Ti>
istrue
for alli
. [Note: This behavior can be implemented by a constructor template with default template arguments. — end note] The expression insideexplicit
evaluates totrue
if and only ifTi
is notimplicitly default-constructiblecopy-list-initializable from an empty list for at least onei
. [Note: This behavior can be implemented with a trait that checks whether aconst Ti&
can be initialized with{}
. — end note][…]
template<class Alloc> explicit(see below) tuple(allocator_arg_t, const Alloc& a); template<class Alloc> explicit(see below) tuple(allocator_arg_t, const Alloc& a, const Types&...); […] template<class Alloc, class U1, class U2> explicit(see below) tuple(allocator_arg_t, const Alloc& a, pair<U1, U2>&&);-25- Requires:
[…]Alloc
shall satisfy theCpp17Allocator
requirements (Table 33).
atomic_ref() = delete;
should be deletedSection: 32.5.7 [atomics.ref.generic] Status: C++20 Submitter: Tim Song Opened: 2018-10-01 Last modified: 2021-02-25
Priority: 0
View all other issues in [atomics.ref.generic].
View all issues with C++20 status.
Discussion:
atomic_ref
has a deleted default constructor, which causes pointless ambiguities in cases like:
void meow(atomic_ref<int>); void meow(some_default_constructible_struct); meow({});
It should have no default constructor rather than a deleted one. (Note that it has other user-defined constructors and so cannot be an aggregate under any definition.)
[2018-10-06 Status to Tentatively Ready after seven positive votes on the reflector.]
[2018-11, Adopted in San Diego]
Proposed resolution:
This wording is relative to N4762.
Edit 32.5.7 [atomics.ref.generic], class template atomic_ref
synopsis, as indicated:
[…]atomic_ref() = delete;atomic_ref& operator=(const atomic_ref&) = delete; […]
Edit 32.5.7.3 [atomics.ref.int], class template specialization atomic_ref<integral>
synopsis, as indicated:
[…]atomic_ref() = delete;atomic_ref& operator=(const atomic_ref&) = delete; […]
Edit 32.5.7.4 [atomics.ref.float], class template specialization atomic_ref<floating-point>
synopsis, as indicated:
[…]atomic_ref() = delete;atomic_ref& operator=(const atomic_ref&) = delete; […]
Edit 32.5.7.5 [atomics.ref.pointer], class template specialization atomic_ref<T*>
synopsis, as indicated:
[…]atomic_ref() = delete;atomic_ref& operator=(const atomic_ref&) = delete; […]
Section: 16.3.2.4 [structure.specifications], 99 [res.on.required] Status: Resolved Submitter: Geoffrey Romer Opened: 2018-11-21 Last modified: 2020-07-17
Priority: 2
View other active issues in [structure.specifications].
View all other issues in [structure.specifications].
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Discussion:
16.3.2.4 [structure.specifications]/p3.4 specifies the Expects: element as "the conditions (sometimes termed preconditions) that the function assumes to hold whenever it is called". This is nonsensical (functions can't "assume" things because they're not sentient), and more to the point it says nothing about what happens if those conditions don't hold. This is a serious problem because the whole point of introducing Expects: was to correct the vagueness of Requires: on exactly that point.
99 [res.on.required]/p2 is more explicit: "Violation of any preconditions specified in a function's Expects: element results in undefined behavior." However, I think putting the actual meaning of the Expects: element in a subclause called "Requires paragraph", 21 pages away from where Expects: is nominally specified, is asking too much of the reader. Splitting the specification of Requires: into two places 21 pages apart also seems needlessly obtuse, but that's less pressing since Requires: should be going away soon.[2018-11 Reflector prioritization]
Set Priority to 2
[2019-02; Kona Wednesday night issue processing]
Alternate wording discussed; Marshall to check with Geoffrey and vote on reflector.
[2019 Cologne Wednesday night]
Revisit once the appropriate "Mandating" paper has landed
[2020-06; telecon and reflector discussion]
Changing Expects: to Preconditions: doesn't change the fact that the specification is split across two subclauses. Resolved editorially by integrating [res.on.expects] into [structure.specifications].Proposed resolution:
This wording is relative to N4778.
[Drafting Note: I have prepared two mutually exclusive options, depicted below by Option A and Option B, respectively. In case the committee would prefer to leave "Requires" alone, Option B describes just the "Expects" edits, as an alternate P/R]
Option A
Change 16.3.2.4 [structure.specifications] as indicated:
-3- Descriptions of function semantics contain the following elements (as appropriate):(footnote)
(3.1) — Requires:
the preconditions for calling the function.the conditions that are required to hold when the function is called in order for the call to successfully complete. [Note: When these conditions are violated, the function's Throws: element may specify throwing an exception. Otherwise, the behavior is undefined. — end note](3.2) — Constraints: […]
(3.3) — Mandates: […]
(3.4) — Expects: the conditions (sometimes termed preconditions) that
the function assumes to hold whenever it is calledare required to hold when the function is called in order for the call to have well-defined behavior. [Example: An implementation might express such conditions via an attribute such as[[expects]]
( [dcl.attr.contract]). However, some such conditions might not lend themselves to expression via code. — end example]
Delete 99 [res.on.required] in it's entirety as indicated:
15.5.4.11 Requires paragraph [res.on.required]-1- Violation of any preconditions specified in a function's Requires: element results in undefined behavior unless the function's Throws: element specifies throwing an exception when the precondition is violated.-2- Violation of any preconditions specified in a function's Expects: element results in undefined behavior.
Option B
Change 16.3.2.4 [structure.specifications] as indicated:
-3- Descriptions of function semantics contain the following elements (as appropriate):(footnote)
(3.1) — Requires: the preconditions for calling the function.
(3.2) — Constraints: […]
(3.3) — Mandates: […]
(3.4) — Expects: the conditions (sometimes termed preconditions) that
the function assumes to hold whenever it is calledare required to hold when the function is called in order for the call to have well-defined behavior. [Example: An implementation might express such conditions via an attribute such as[[expects]]
( [dcl.attr.contract]). However, some such conditions might not lend themselves to expression via code. — end example]
Change 99 [res.on.required] as indicated:
-1- Violation of any preconditions specified in a function's Requires: element results in undefined behavior unless the function's Throws: element specifies throwing an exception when the precondition is violated.
-2- Violation of any preconditions specified in a function's Expects: element results in undefined behavior.
ranges
permutation generators discard useful informationSection: 26.8.13 [alg.permutation.generators] Status: C++20 Submitter: Casey Carter Opened: 2018-11-26 Last modified: 2021-02-25
Priority: 0
View all issues with C++20 status.
Discussion:
In the Ranges design, algorithms that necessarily traverse an entire range -
consequently discovering the end iterator value - return that iterator value
unless the algorithm's sole purpose is to return a derived value,
for example, ranges::count
.
ranges::next_permutation
and ranges::prev_permutation
necessarily traverse the entirety of their range argument, but are currently
specified to discard the end iterator value and return only a bool
indicating whether they found a next (respectively previous) permutation or
"reset" the range to the first (respectively last) permutation.
They should instead return an aggregate composed of both
that bool
and the end iterator value to be consistent with the other
range
algorithms.
[2019-01-22; Daniel comments and updates wording]
During the reflector discussion it had been noticed that an additional update of
26.2 [algorithms.requirements] p.16 is necessary for the new type
next_permutation_result
and two missing occurrences of iterator_t<>
where
added. The proposed wording has been updated.
[2019-02-02 Priority to 0 and Status to Tentatively Ready after five positive votes on the reflector.]
Previous resolution [SUPERSEDED]:
Modify 26.2 [algorithms.requirements] as follows:
-16- The class templates
binary_transform_result
,for_each_result
,minmax_result
,mismatch_result
,next_permutation_result
,copy_result
, andpartition_copy_result
have the template parameters, data members, and special members specified above. They have no base classes or members other than those specified.Modify 26.4 [algorithm.syn] as follows:
// 26.8.13 [alg.permutation.generators], permutations template<class BidirectionalIterator> constexpr bool next_permutation(BidirectionalIterator first, BidirectionalIterator last); template<class BidirectionalIterator, class Compare> constexpr bool next_permutation(BidirectionalIterator first, BidirectionalIterator last, Compare comp); namespace ranges { template<class I> struct next_permutation_result { bool found; I in; }; template<BidirectionalIterator I, Sentinel<I> S, class Comp = ranges::less<>, class Proj = identity> requires Sortable<I, Comp, Proj> constexprboolnext_permutation_result<I> next_permutation(I first, S last, Comp comp = {}, Proj proj = {}); template<BidirectionalRange R, class Comp = ranges::less<>, class Proj = identity> requires Sortable<iterator_t<R>, Comp, Proj> constexprboolnext_permutation_result<iterator_t<R>> next_permutation(R&& r, Comp comp = {}, Proj proj = {}); } template<class BidirectionalIterator> constexpr bool prev_permutation(BidirectionalIterator first, BidirectionalIterator last); template<class BidirectionalIterator, class Compare> constexpr bool prev_permutation(BidirectionalIterator first, BidirectionalIterator last, Compare comp); namespace ranges { template<class I> using prev_permutation_result = next_permutation_result<I>; template<BidirectionalIterator I, Sentinel<I> S, class Comp = ranges::less<>, class Proj = identity> requires Sortable<I, Comp, Proj> constexprboolprev_permutation_result<I> prev_permutation(I first, S last, Comp comp = {}, Proj proj = {}); template<BidirectionalRange R, class Comp = ranges::less<>, class Proj = identity> requires Sortable<iterator_t<R>, Comp, Proj> constexprboolprev_permutation_result<iterator_t<R>> prev_permutation(R&& r, Comp comp = {}, Proj proj = {}); } }Modify 26.8.13 [alg.permutation.generators] as follows:
[…]template<class BidirectionalIterator> constexpr bool next_permutation(BidirectionalIterator first, BidirectionalIterator last); template<class BidirectionalIterator, class Compare> constexpr bool next_permutation(BidirectionalIterator first, BidirectionalIterator last, Compare comp); namespace ranges { template<BidirectionalIterator I, Sentinel<I> S, class Comp = ranges::less<>, class Proj = identity> requires Sortable<I, Comp, Proj> constexprboolnext_permutation_result<I> next_permutation(I first, S last, Comp comp = {}, Proj proj = {}); template<BidirectionalRange R, class Comp = ranges::less<>, class Proj = identity> requires Sortable<iterator_t<R>, Comp, Proj> constexprboolnext_permutation_result<iterator_t<R>> next_permutation(R&& r, Comp comp = {}, Proj proj = {}); }-4- Returns: Let
B
betrue
ifand only ifa next permutation was found and otherwisefalse
. Returns:
B
for the overloads in namespacestd
, or
{ B, last }
for the overloads in namespaceranges
.-5- Complexity: […]
[…]template<class BidirectionalIterator> constexpr bool prev_permutation(BidirectionalIterator first, BidirectionalIterator last); template<class BidirectionalIterator, class Compare> constexpr bool prev_permutation(BidirectionalIterator first, BidirectionalIterator last, Compare comp); namespace ranges { template<BidirectionalIterator I, Sentinel<I> S, class Comp = ranges::less<>, class Proj = identity> requires Sortable<I, Comp, Proj> constexprboolprev_permutation_result<I> prev_permutation(I first, S last, Comp comp = {}, Proj proj = {}); template<BidirectionalRange R, class Comp = ranges::less<>, class Proj = identity> requires Sortable<iterator_t<R>, Comp, Proj> constexprboolprev_permutation_result<iterator_t<R>> prev_permutation(R&& r, Comp comp = {}, Proj proj = {}); }-9- Returns: Let
B
betrue
ifand only ifa previous permutation was found and otherwisefalse
. Returns:
B
for the overloads in namespacestd
, or
{ B, last }
for the overloads in namespaceranges
.-10- Complexity: […]
[2019-02-10 Tomasz comments; Casey updates the P/R and resets status to "Review."]
Shouldn't the range overloads for an algorithms returnsafe_iterator_t<R>
instead of iterator_t<R>
?
Other algorithms are consistently returning the
safe_iterator_t
/safe_subrange_t
in situation, when range
argument is an rvalue (temporary) and returned iterator may be dangling.
[2019-02; Kona Wednesday night issue processing]
Status to Ready
Proposed resolution:
This wording is relative to N4800.
Modify 26.2 [algorithms.requirements] as follows:
-16- The class templates
binary_transform_result
,for_each_result
,minmax_result
,mismatch_result
,next_permutation_result
,copy_result
, andpartition_copy_result
have the template parameters, data members, and special members specified above. They have no base classes or members other than those specified.
Modify 26.4 [algorithm.syn] as follows:
// 26.8.13 [alg.permutation.generators], permutations template<class BidirectionalIterator> constexpr bool next_permutation(BidirectionalIterator first, BidirectionalIterator last); template<class BidirectionalIterator, class Compare> constexpr bool next_permutation(BidirectionalIterator first, BidirectionalIterator last, Compare comp); namespace ranges { template<class I> struct next_permutation_result { bool found; I in; }; template<BidirectionalIterator I, Sentinel<I> S, class Comp = ranges::less<>, class Proj = identity> requires Sortable<I, Comp, Proj> constexprboolnext_permutation_result<I> next_permutation(I first, S last, Comp comp = {}, Proj proj = {}); template<BidirectionalRange R, class Comp = ranges::less<>, class Proj = identity> requires Sortable<iterator_t<R>, Comp, Proj> constexprboolnext_permutation_result<safe_iterator_t<R>> next_permutation(R&& r, Comp comp = {}, Proj proj = {}); } template<class BidirectionalIterator> constexpr bool prev_permutation(BidirectionalIterator first, BidirectionalIterator last); template<class BidirectionalIterator, class Compare> constexpr bool prev_permutation(BidirectionalIterator first, BidirectionalIterator last, Compare comp); namespace ranges { template<class I> using prev_permutation_result = next_permutation_result<I>; template<BidirectionalIterator I, Sentinel<I> S, class Comp = ranges::less<>, class Proj = identity> requires Sortable<I, Comp, Proj> constexprboolprev_permutation_result<I> prev_permutation(I first, S last, Comp comp = {}, Proj proj = {}); template<BidirectionalRange R, class Comp = ranges::less<>, class Proj = identity> requires Sortable<iterator_t<R>, Comp, Proj> constexprboolprev_permutation_result<safe_iterator_t<R>> prev_permutation(R&& r, Comp comp = {}, Proj proj = {}); } }
Modify 26.8.13 [alg.permutation.generators] as follows:
[…]template<class BidirectionalIterator> constexpr bool next_permutation(BidirectionalIterator first, BidirectionalIterator last); template<class BidirectionalIterator, class Compare> constexpr bool next_permutation(BidirectionalIterator first, BidirectionalIterator last, Compare comp); namespace ranges { template<BidirectionalIterator I, Sentinel<I> S, class Comp = ranges::less<>, class Proj = identity> requires Sortable<I, Comp, Proj> constexprboolnext_permutation_result<I> next_permutation(I first, S last, Comp comp = {}, Proj proj = {}); template<BidirectionalRange R, class Comp = ranges::less<>, class Proj = identity> requires Sortable<iterator_t<R>, Comp, Proj> constexprboolnext_permutation_result<safe_iterator_t<R>> next_permutation(R&& r, Comp comp = {}, Proj proj = {}); }-4- Returns: Let
B
betrue
ifand only ifa next permutation was found and otherwisefalse
. Returns:
B
for the overloads in namespacestd
, or
{ B, last }
for the overloads in namespaceranges
.-5- Complexity: […]
[…]template<class BidirectionalIterator> constexpr bool prev_permutation(BidirectionalIterator first, BidirectionalIterator last); template<class BidirectionalIterator, class Compare> constexpr bool prev_permutation(BidirectionalIterator first, BidirectionalIterator last, Compare comp); namespace ranges { template<BidirectionalIterator I, Sentinel<I> S, class Comp = ranges::less<>, class Proj = identity> requires Sortable<I, Comp, Proj> constexprboolprev_permutation_result<I> prev_permutation(I first, S last, Comp comp = {}, Proj proj = {}); template<BidirectionalRange R, class Comp = ranges::less<>, class Proj = identity> requires Sortable<iterator_t<R>, Comp, Proj> constexprboolprev_permutation_result<safe_iterator_t<R>> prev_permutation(R&& r, Comp comp = {}, Proj proj = {}); }-9- Returns: Let
B
betrue
ifand only ifa previous permutation was found and otherwisefalse
. Returns:
B
for the overloads in namespacestd
, or
{ B, last }
for the overloads in namespaceranges
.-10- Complexity: […]
is_always_equal
added to std::allocator
makes the standard library treat
derived types as always equalSection: 20.2.10 [default.allocator] Status: C++23 Submitter: Billy O'Neal III Opened: 2018-11-29 Last modified: 2023-11-22
Priority: 2
View other active issues in [default.allocator].
View all other issues in [default.allocator].
View all issues with C++23 status.
Discussion:
I (Billy O'Neal) attempted to change MSVC++'s standard library to avoid instantiating allocators' operator==
for allocators that are declared is_always_equal
to reduce the number of template instantiations emitted into .objs.
std::allocator
's
operator==
and operator!=
which don't compare the root member. However, if this had been a conforming C++14
allocator with its own ==
and !=
we would still be treating it as is_always_equal
, as it picks that
up by deriving from std::allocator
.
std::allocator
doesn't actually need is_always_equal
because the defaults provided by allocator_traits
will say true_type
for it, since implementers don't make std::allocator
stateful.
Billy O'Neal thinks this is NAD on the grounds that we need to be able to add things or change the behavior of standard library types.
Stephan T Lavavej thinks we should resolve this anyway because we don't know of an implementation for which this would change
the default answer provided by allocator_traits
.
[2019-02 Priority set to 2 after reflector discussion]
Previous resolution [SUPERSEDED]:This wording is relative to N4778.
Modify 20.2.10 [default.allocator] as follows:
-1- All specializations of the default allocator satisfy the allocator completeness requirements (16.4.4.6.2 [allocator.requirements.completeness]).
namespace std { template<class T> class allocator { public: using value_type = T; using size_type = size_t; using difference_type = ptrdiff_t; using propagate_on_container_move_assignment = true_type;using is_always_equal = true_type;constexpr allocator() noexcept; constexpr allocator(const allocator&) noexcept; template<class U> constexpr allocator(const allocator<U>&) noexcept; ~allocator(); allocator& operator=(const allocator&) = default; [[nodiscard]] T* allocate(size_t n); void deallocate(T* p, size_t n); }; }-?-
allocator_traits<allocator<T>>::is_always_equal::value
istrue
for anyT
.
[2019-07 Cologne]
Jonathan provides updated wording.
[2020-10-02; Issue processing telecon: Moved to Tentatively Ready.]
[2020-11-09 Approved In November virtual meeting. Status changed: Tentatively Ready → WP.]
Proposed resolution:
This wording is relative to N4820.
Modify 20.2.10 [default.allocator] as follows:
-1- All specializations of the default allocator satisfy the allocator completeness requirements (16.4.4.6.2 [allocator.requirements.completeness]).
namespace std { template<class T> class allocator { public: using value_type = T; using size_type = size_t; using difference_type = ptrdiff_t; using propagate_on_container_move_assignment = true_type;using is_always_equal = true_type;constexpr allocator() noexcept; constexpr allocator(const allocator&) noexcept; template<class U> constexpr allocator(const allocator<U>&) noexcept; ~allocator(); allocator& operator=(const allocator&) = default; [[nodiscard]] T* allocate(size_t n); void deallocate(T* p, size_t n); }; }-?-
allocator_traits<allocator<T>>::is_always_equal::value
istrue
for anyT
.
Add a new subclause in Annex D after 99 [depr.str.strstreams]:
D.? The default allocator [depr.default.allocator]
-?- The following member is defined in addition to those specified in 20.2.10 [default.allocator]:namespace std { template <class T> class allocator { public: using is_always_equal = true_type; }; }
directory_entry
stream insertionSection: 31.12.10 [fs.class.directory.entry] Status: C++23 Submitter: Tim Song Opened: 2018-12-03 Last modified: 2023-11-22
Priority: 2
View all other issues in [fs.class.directory.entry].
View all issues with C++23 status.
Discussion:
directory_entry
has a conversion function to const path&
and depends on path
's stream insertion
operator for stream insertion support, which is now broken after LWG 2989(i) made it a hidden friend.
[2018-12-21 Reflector prioritization]
Set Priority to 2
[2019-02; Kona Wednesday night issue processing]
Status to Open; Marshall to move definition inline and re-vote on reflector.
Jonathan to write a paper about how to specify "hidden friends".
Previous resolution [SUPERSEDED]:
This wording is relative to N4778.
Modify [fs.class.directory_entry], class
directory_entry
synopsis, as follows:namespace std::filesystem { class directory_entry { public: […] private: filesystem::path pathobject; // exposition only friend class directory_iterator; // exposition only template<class charT, class traits> friend basic_ostream<charT, traits>& operator<<(basic_ostream<charT, traits>& os, const directory_entry& d); }; }Add a new subclause at the end of [fs.class.directory_entry], as follows:
28.11.11.4 Inserter [fs.dir.entry.io]
template<class charT, class traits> friend basic_ostream<charT, traits>& operator<<(basic_ostream<charT, traits>& os, const directory_entry& d);-1- Effects: Equivalent to:
return os << d.path();
[2020-05-02; Daniel resyncs wording with recent working draft and comments]
We have now the paper P1965R0, which introduced a specification of what friend functions in the library specification (see 16.4.6.6 [hidden.friends]) are supposed to mean, there is no longer an inline definition needed to clarify the meaning. In addition to updating the change of section names the provided wording has moved the friend declaration into the public part of the class definition as have done in all other cases where we take advantage of "hidden friends" declarations.
[2020-08-21 Issue processing telecon: moved to Tentatively Ready]
[2020-11-09 Approved In November virtual meeting. Status changed: Tentatively Ready → WP.]
Proposed resolution:
This wording is relative to N4861.
Modify 31.12.10 [fs.class.directory.entry], class directory_entry
synopsis, as follows:
namespace std::filesystem { class directory_entry { public: […] bool operator==(const directory_entry& rhs) const noexcept; strong_ordering operator<=>(const directory_entry& rhs) const noexcept; // 29.11.11.? [fs.dir.entry.io], inserter template<class charT, class traits> friend basic_ostream<charT, traits>& operator<<(basic_ostream<charT, traits>& os, const directory_entry& d); private: […] }; }
Add a new subclause at the end of 31.12.10 [fs.class.directory.entry], as indicated:
29.11.11.? Inserter [fs.dir.entry.io]
template<class charT, class traits> friend basic_ostream<charT, traits>& operator<<(basic_ostream<charT, traits>& os, const directory_entry& d);-?- Effects: Equivalent to:
return os << d.path();
ref-view
Section: 25.7.6.2 [range.ref.view] Status: C++20 Submitter: Casey Carter Opened: 2018-12-09 Last modified: 2021-06-06
Priority: 0
View all issues with C++20 status.
Discussion:
In the specification of view::all
in 25.7.6 [range.all],
paragraph 2.2 states that
view::all(E)
is sometimes expression-equivalent to
"ref-view{E}
if that expression is well-formed". Unfortunately,
the expression ref-view{E}
is never well-formed:
ref-view
's only non-default constructor is a
perfect-forwarding-ish constructor template that accepts only arguments that
convert to lvalues of the ref-view
's template argument type, and
either do not convert to rvalues or have a better lvalue conversion (similar to
the reference_wrapper
converting constructor
(22.10.6.2 [refwrap.const]) after issue 2993(i)).
Presumably this breakage was not intentional, and we should add a deduction guide to enable class template argument deduction to function as intended by paragraph 2.2.
[2018-12-16 Status to Tentatively Ready after six positive votes on the reflector.]
Proposed resolution:
This wording is relative to N4791.
Modify the ref-view
class synopsis
in [ranges.view.ref] as follows:
namespace std::ranges { template<Range R> requires is_object_v<R> class ref-view : public view_interface<ref-view<R>> { […] }; template<class R> ref_view(R&) -> ref_view<R>; }
CommonReference
requirement of concept SwappableWith
is not satisfied in the exampleSection: 18.4.9 [concept.swappable] Status: C++20 Submitter: Kostas Kyrimis Opened: 2018-12-14 Last modified: 2021-02-25
Priority: 1
View other active issues in [concept.swappable].
View all other issues in [concept.swappable].
View all issues with C++20 status.
Discussion:
The defect stems from the example found in sub-clause 18.4.9 [concept.swappable] p5:
[…] template<class T, std::SwappableWith<T> U> void value_swap(T&& t, U&& u) { ranges::swap(std::forward<T>(t), std::forward<U>(u)); } […] namespace N { struct A { int m; }; struct Proxy { A* a; }; Proxy proxy(A& a) { return Proxy{ &a }; } void swap(A& x, Proxy p) { ranges::swap(x.m, p.a->m); } void swap(Proxy p, A& x) { swap(x, p); } // satisfy symmetry requirement } int main() { […] N::A a1 = { 5 }, a2 = { -5 }; value_swap(a1, proxy(a2)); // diagnostic manifests here(#1) assert(a1.m == -5 && a2.m == 5); }
The call to value_swap(a1, proxy(a2))
resolves to [T
= N::A&
, U
= N::Proxy
]
The compiler will issue a diagnostic for #1 because:
rvalue proxy(a2)
is not swappable
concept SwappableWith<T, U>
requires N::A
and Proxy
to model
CommonReference<const remove_reference_t<T>&, const remove_reference_t<U>&>
It follows
from the example that there is no common reference for [T
= N::A&
, U
= N::Proxy
]
[2019-06-20; Casey Carter comments and provides improved wording]
The purpose of the CommonReference
requirements in the cross-type concepts is to provide a
sanity check. The fact that two types satisfy a single-type concept, have a common reference type
that satisfies that concept, and implement cross-type operations required by the cross-type flavor
of that concept very strongly suggests the programmer intends them to model the cross-type concept.
It's an opt-in that requires some actual work, so it's unlikely to be inadvertent.
CommonReference<const T&, const U&>
pattern makes sense for the comparison
concepts which require that all variations of const
and value category be comparable: we
use const
lvalues to trigger the "implicit expression variation" wording in
18.2 [concepts.equality]. SwappableWith
, however, doesn't care about implicit expression
variations: it only needs to witness that we can exchange the values denoted by two reference-y
expressions E1
and E2
. This suggests that CommonReference<decltype((E1)),
decltype((E2))>
is a more appropriate requirement than the current
CommonReference<const remove_reference_t<…>
mess that was blindly copied
from the comparison concepts.
We must change the definition of "exchange the values" in 18.4.9 [concept.swappable] —
which refers to the common reference type — consistently.
Previous resolution [SUPERSEDED]:
This wording is relative to N4791.
Change 18.4.9 [concept.swappable] as indicated:
-3- […]
template<class T> concept Swappable = requires(T& a, T& b) { ranges::swap(a, b); }; template<class T, class U> concept SwappableWith = CommonReference<T, Uconst remove_reference_t<T>&, const remove_reference_t<U>&> && requires(T&& t, U&& u) { ranges::swap(std::forward<T>(t), std::forward<T>(t)); ranges::swap(std::forward<U>(u), std::forward<U>(u)); ranges::swap(std::forward<T>(t), std::forward<U>(u)); ranges::swap(std::forward<U>(u), std::forward<T>(t)); };-4- […]
-5- [Example: User code can ensure that the evaluation ofswap
calls is performed in an appropriate context under the various conditions as follows:#include <cassert> #include <concepts> #include <utility> namespace ranges = std::ranges; template<class T, std::SwappableWith<T> U> void value_swap(T&& t, U&& u) { ranges::swap(std::forward<T>(t), std::forward<U>(u)); } template<std::Swappable T> void lv_swap(T& t1, T& t2) { ranges::swap(t1, t2); } namespace N { struct A { int m; }; struct Proxy { A* a; Proxy(A& a) : a{&a} {} friend void swap(Proxy&& x, Proxy&& y) { ranges::swap(x.a, y.a); } }; Proxy proxy(A& a) { return Proxy{ &a }; } void swap(A& x, Proxy p) { ranges::swap(x.m, p.a->m); } void swap(Proxy p, A& x) { swap(x, p); } // satisfy symmetry requirement } int main() { int i = 1, j = 2; lv_swap(i, j); assert(i == 2 && j == 1); N::A a1 = { 5 }, a2 = { -5 }; value_swap(a1, proxy(a2)); assert(a1.m == -5 && a2.m == 5); }
[2020-01-16 Priority set to 1 after discussion on the reflector.]
[2020-02-10 Move to Immediate Monday afternoon in Prague]
Proposed resolution:
This wording is relative to N4820.
Change 18.4.9 [concept.swappable] as indicated:
-1- Let
t1
andt2
be equality-preserving expressions that denote distinct equal objects of typeT
, and letu1
andu2
similarly denote distinct equal objects of typeU
. [Note:t1
andu1
can denote distinct objects, or the same object. — end note] An operation exchanges the values denoted byt1
andu1
if and only if the operation modifies neithert2
noru2
and:
(1.1) — If
T
andU
are the same type, the result of the operation is thatt1
equalsu2
andu1
equalst2
.(1.2) — If
T
andU
are different typesthat modelandCommonReference<const T&, const U&>
CommonReference<decltype((t1)), decltype((u1))>
is modeled, the result of the operation is thatC(t1)
equalsC(u2)
andC(u1)
equalsC(t2)
whereC
iscommon_reference_t<
.const T&, const U&decltype((t1)), decltype((u1))>-2- […]
-3- […]template<class T> concept Swappable = requires(T& a, T& b) { ranges::swap(a, b); }; template<class T, class U> concept SwappableWith = CommonReference<T, Uconst remove_reference_t<T>&, const remove_reference_t<U>&> && requires(T&& t, U&& u) { ranges::swap(std::forward<T>(t), std::forward<T>(t)); ranges::swap(std::forward<U>(u), std::forward<U>(u)); ranges::swap(std::forward<T>(t), std::forward<U>(u)); ranges::swap(std::forward<U>(u), std::forward<T>(t)); };-4- […]
-5- [Example: User code can ensure that the evaluation ofswap
calls is performed in an appropriate context under the various conditions as follows:#include <cassert> #include <concepts> #include <utility> namespace ranges = std::ranges; template<class T, std::SwappableWith<T> U> void value_swap(T&& t, U&& u) { ranges::swap(std::forward<T>(t), std::forward<U>(u)); } template<std::Swappable T> void lv_swap(T& t1, T& t2) { ranges::swap(t1, t2); } namespace N { struct A { int m; }; struct Proxy { A* a; Proxy(A& a) : a{&a} {} friend void swap(Proxy x, Proxy y) { ranges::swap(*x.a, *y.a); } }; Proxy proxy(A& a) { return Proxy{&a }; }void swap(A& x, Proxy p) { ranges::swap(x.m, p.a->m); } void swap(Proxy p, A& x) { swap(x, p); } // satisfy symmetry requirement} int main() { int i = 1, j = 2; lv_swap(i, j); assert(i == 2 && j == 1); N::A a1 = { 5 }, a2 = { -5 }; value_swap(a1, proxy(a2)); assert(a1.m == -5 && a2.m == 5); }
Container::key_equal
differs from Pred
Section: 23.2.8 [unord.req] Status: Resolved Submitter: S. B. Tam Opened: 2018-11-27 Last modified: 2020-01-13
Priority: 2
View other active issues in [unord.req].
View all other issues in [unord.req].
View all issues with Resolved status.
Discussion:
After acceptance of P0919R3 into the new working draft (N4791),
it is possible that an unordered container's member type key_equal
is different from its template argument for Pred
(the former being Hash::transparent_key_equal
while the latter being std::equal_to<Key>
). However, it is
unclear which is stored in the container and used as the predicate in this case.
[…] The container's object of type
Pred
— denoted bypred
— is called the key equality predicate of the container.
In Table 70, the row for X::key_equal
places requirements to Pred
and not to Hash::transparent_key_equal
:
Requires:
Pred
is Cpp17CopyConstructible.Pred
shall be a binary predicate that takes two arguments of typeKey
.Pred
is an equivalence relation.
The specification of operator==
and operator!=
in [unord.req]/12 uses Pred
:
[…] The behavior of a program that uses
operator==
or operator!= on unordered containers is undefined unless thePred
function object has the same behavior for both containers and the equality comparison function forKey
is a refinement(footnote 227) of the partition into equivalent-key groups produced byPred
.
The exception safety guarantees in [unord.req.except] mentions "the container's Pred
object" twice.
swap
member function are all in
terms of Pred
.
I think the intent is to make Hash::transparent_key_equal
override Pred
. If that's true, then all the
abovementioned uses of Pred
in the specification should probably be changed to uses key_equal
.
[2018-12-21 Reflector prioritization]
Set Priority to 2
[2020-01 Resolved by the adoption of P1690 in Belfast.]
Proposed resolution:
Section: 16.4.5.2.1 [namespace.std] Status: C++23 Submitter: Johel Ernesto Guerrero Peña Opened: 2018-12-11 Last modified: 2023-11-22
Priority: 3
View other active issues in [namespace.std].
View all other issues in [namespace.std].
View all issues with C++23 status.
Discussion:
The permission denoted by [namespace.std]/3 should be limited to program-defined types.
[2018-12-21 Reflector prioritization]
Set Priority to 3
Previous resolution [SUPERSEDED]:
This wording is relative to N4791.
Change 16.4.5.2.1 [namespace.std] as indicated:
-2- Unless explicitly prohibited, a program may add a template specialization for any standard library class template to namespace
-3- The behavior of a C++ program is undefined if it declares an explicit or partial specialization of any standard library variable template, except where explicitly permitted by the specification of that variable template, provided that the added declaration depends on at least one program-defined type.std
provided that (a) the added declaration depends on at least one program-defined type and (b) the specialization meets the standard library requirements for the original template.(footnote 174)
[2022-08-24; LWG telecon]
Each variable template that grants permission to specialize already
states requirements more precisely than proposed here anyway.
For example, disable_sized_range
only allows it for
cv-unqualified program-defined types.
Adding less precise wording here wouldn't be an improvement.
Add a note to make it clear we didn't just forget to say something here,
and to remind us to state requirements for each variable template in future.
[2022-08-25; Jonathan Wakely provides improved wording]
[2022-09-28; Reflector poll]
Set status to Tentatively Ready after seven votes in favour during reflector poll.
[2022-11-12 Approved at November 2022 meeting in Kona. Status changed: Voting → WP.]
Proposed resolution:
This wording is relative to N4910.
Change 16.4.5.2.1 [namespace.std] as indicated:
-2- Unless explicitly prohibited, a program may add a template specialization for any standard library class template to namespace
std
provided that (a) the added declaration depends on at least one program-defined type and (b) the specialization meets the standard library requirements for the original template.(footnote 163)-3- The behavior of a C++ program is undefined if it declares an explicit or partial specialization of any standard library variable template, except where explicitly permitted by the specification of that variable template.
[Note 1: The requirements on an explicit or partial specialization are stated by each variable template that grants such permission. — end note]
std::mismatch
is missing an upper boundSection: 99 [mismatch] Status: Resolved Submitter: Geoffrey Romer Opened: 2018-12-20 Last modified: 2020-05-02
Priority: 0
View all other issues in [mismatch].
View all issues with Resolved status.
Discussion:
Consider the following code:
std::vector<int> v1 = {1, 2, 3, 4}; std::vector<int> v2 = {1, 2, 3, 5}; auto result = std::mismatch(v1.begin(), v1.begin() + 2, v2.begin(), v2.begin() + 2);
The current wording of [mismatch] seems to require result
to be {v1.begin() + 3, v2.begin() + 3}
, because 3
is the smallest integer n
such that *(v1.begin() + n) != *(v2.begin + n)
. In other words, if there's a
mismatch that's reachable from first1
and first2
, then std::mismatch
must find and return it,
even if it's beyond the end iterators passed by the user.
[2019-01-26 Priority to 0 and Status to Tentatively Ready after discussions on the reflector]
During that reflector discussion several contributers argued in favour for changing the current wording in 99 [mismatch] p3 from "smallest integer" to "smallest nonnegative integer". This minor wording delta has also been added to the original proposed wording.
Previous resolution [SUPERSEDED]:
This wording is relative to N4791.
Change 99 [mismatch] as indicated:
template<class InputIterator1, class InputIterator2> constexpr pair<InputIterator1, InputIterator2> mismatch(InputIterator1 first1, InputIterator1 last1, InputIterator2 first2); […] namespace ranges { template<InputIterator I1, Sentinel<I1> S1, InputIterator I2, Sentinel<I2> S2, class Proj1 = identity, class Proj2 = identity, IndirectRelation<projected<I1, Proj1>, projected<I2, Proj2>> Pred = ranges::equal_to<>> constexpr mismatch_result<I1, I2> mismatch(I1 first1, S1 last1, I2 first2, S2 last2, Pred pred = {}, Proj1 proj1 = {}, Proj2 proj2 = {}); template<InputRange R1, InputRange R2, class Proj1 = identity, class Proj2 = identity, IndirectRelation<projected<iterator_t<R1>, Proj1>, projected<iterator_t<R2>, Proj2>> Pred = ranges::equal_to<>> constexpr mismatch_result<safe_iterator_t<R1>, safe_iterator_t<R2>> mismatch(R1&& r1, R2&& r2, Pred pred = {}, Proj1 proj1 = {}, Proj2 proj2 = {}); }-1- Let
-2- Letlast2
befirst2 + (last1 - first1)
for the overloads with no parameterlast2
orr2
.E
be:-?- Let
(2.1) —
!(*(first1 + n) == *(first2 + n))
for the overloads with no parameterpred
,(2.2) —
pred(*(first1 + n), *(first2 + n)) == false
for the overloads with a parameterpred
and no parameterproj1
,(2.3) —
!invoke(pred, invoke(proj1, *(first1 + n)), invoke(proj2, *(first2 + n)))
for the overloads with both parameterspred
andproj1
.N
bemin(last1 - first1, last2 - first2)
. -3- Returns:{ first1 + n, first2 + n }
, wheren
is the smallest nonnegative integer such thatE
holds, orif no such integer less than
min(last1 - first1, last2 - first2)NN
exists. -4- Complexity: At mostapplications of the corresponding predicate and any projections.
min(last1 - first1, last2 - first2)N
[2019-03-15; Daniel comments]
The editorial issue #2611 had been resolved via this pull request #2613. The editorial changes should make the suggested wording changes obsolete and I recommend to close this issue as Resolved.
[2020-05-02; Reflector discussions]
It seems that the editorial change has fixed the issue already. If the issue author objects, we can reopen it. Therefore:
Resolved by editorial pull request #2613.Rationale:
Resolved by editorial pull request #2613.Proposed resolution:
subrange
should always model Range
Section: 25.5.4.2 [range.subrange.ctor] Status: C++20 Submitter: Casey Carter Opened: 2018-12-21 Last modified: 2021-02-25
Priority: 0
View all issues with C++20 status.
Discussion:
The constructors of subrange
place no requirements on the iterator and
sentinel values from which a subrange
is constructed. They allow
constructions like subrange{myvec.end(), myvec.begin()}
in which the
resulting subrange
isn't in the domain of the Range
concept
which requires that "[ranges::begin(r), ranges::end(r))
denotes a
range" ([range.range]/3.1).
We should forbid the construction of abominable values like this to enforce the
useful semantic that values of subrange
are always in the domain of
Range
.
(From ericniebler/stl2#597.)
[2019-01-20 Reflector prioritization]
Set Priority to 0 and status to Tentatively Ready
Proposed resolution:
This wording is relative to N4791.
Change 25.5.4.2 [range.subrange.ctor] as indicated:
constexpr subrange(I i, S s) requires (!StoreSize);-?- Expects:
[i, s)
is a valid range.-1- Effects: Initializes
begin_
withi
andend_
withs
.constexpr subrange(I i, S s, iter_difference_t<I> n) requires (K == subrange_kind::sized);-2- Expects:
[i, s)
is a valid range, andn == ranges::distance(i, s)
.
ranges::minmax_element
Section: 26.4 [algorithm.syn] Status: C++20 Submitter: Casey Carter Opened: 2018-12-21 Last modified: 2021-02-25
Priority: 0
View all other issues in [algorithm.syn].
View all issues with C++20 status.
Discussion:
The overloads of std::ranges::minmax_element
are specified to return
std::ranges::minmax_result
, which is inconsistent with the intended
design. When an algorithm foo
returns an aggregate of multiple results,
the return type should be named foo_result
.
The spec should introduce an alias minmax_element_result
for
minmax_result
and use that alias as the return type of the
std::ranges::minmax_element
overloads.
[2019-01-11 Status to Tentatively Ready after five positive votes on the reflector.]
During that reflector discussion several contributers questioned the choice of alias templates to denote algorithm result types or particular aspects of it. Since this approach had been approved by LEWG before, it was suggested to those opponents to instead write a paper, because changing this as part of this issue would be a design change that would require a more global fixing approach.
Proposed resolution:
This wording is relative to N4791.
Change 26.4 [algorithm.syn] as indicated, and adjust the
declarations of std::ranges::minmax_element
in 26.8.9 [alg.min.max] to agree:
[…] template<class ExecutionPolicy, class ForwardIterator, class Compare> pair<ForwardIterator, ForwardIterator> minmax_element(ExecutionPolicy&& exec, // see 26.3.5 [algorithms.parallel.overloads] ForwardIterator first, ForwardIterator last, Compare comp); namespace ranges { template<class I> using minmax_element_result = minmax_result<I>; template<ForwardIterator I, Sentinel<I> S, class Proj = identity, IndirectStrictWeakOrder<projected<I, Proj>> Comp = ranges::less<>> constexpr minmax_element_result<I> minmax_element(I first, S last, Comp comp = {}, Proj proj = {}); template<ForwardRange R, class Proj = identity, IndirectStrictWeakOrder<projected<iterator_t<R>, Proj>> Comp = ranges::less<>> constexpr minmax_element_result<safe_iterator_t<R>> minmax_element(R&& r, Comp comp = {}, Proj proj = {}); } // 26.8.10 [alg.clamp], bounded value […]
Same
could be clearerSection: 18.4.2 [concept.same] Status: C++20 Submitter: Casey Carter Opened: 2019-01-05 Last modified: 2021-02-25
Priority: 0
View all issues with C++20 status.
Discussion:
The specification of the Same
concept in 18.4.2 [concept.same]:
template<class T, class U> concept Same = is_same_v<T, U>;-1-
Same<T, U>
subsumesSame<U, T>
and vice versa.
seems contradictory. From the concept definition alone, it is not the
case that Same<T, U>
subsumes Same<U, T>
nor vice
versa. Paragraph 1 is trying to tell us that there's some magic that provides
the stated subsumption relationship, but to a casual reader it appears to be a
mis-annotated note. We should either add a note to explain what's actually
happening here, or define the concept in such a way that it naturally
provides the specified subsumption relationship.
Given that there's a straightforward library implementation of the symmetric subsumption idiom, the latter option seems preferable.
[2019-01-20 Reflector prioritization]
Set Priority to and status to Tentatively Ready
Proposed resolution:
This wording is relative to N4791.
Change 18.4.2 [concept.same] as follows:
template<class T, class U> concept same-impl = // exposition only is_same_v<T, U>; template<class T, class U> concept Same =is_same_v<T, U>same-impl<T, U> && same-impl<U, T>;-1- [Note:
Same<T, U>
subsumesSame<U, T>
and vice versa.—end note]
Section: 24.3.4.8 [iterator.concept.sizedsentinel] Status: C++20 Submitter: Casey Carter Opened: 2019-01-14 Last modified: 2021-02-25
Priority: 0
View all issues with C++20 status.
Discussion:
P0896R4 "The One Ranges Proposal" added
boolean variable templates std::disable_sized_sentinel
and
std::ranges::disable_sized_range
which users are intended to specialize
to false
for program-defined Iterator
-Sentinel
pairs
/ Range
types which meet the syntax but do not model
the semantics of the SizedSentinel
/ SizedRange
concepts,
respectively. Specializing these traits allows the use of such types with the
library which would otherwise treat them as if they model SizedSentinel
/ SizedRange
. The wording in P0896R4 failed, however, to provide
normative permission to specialize these variable templates as is required by
16.4.5.2.1 [namespace.std] after the application of
P0551R3.
Furthermore, 16.4.5.2.1 [namespace.std] notably does not require that
program-defined specializations of standard library variable templates meet the
requirements on the primary template (as is the case for class templates) or
indeed any requirements. P0896R4 also added the enable_view
variable
template which is used to explicitly opt in or out of the View
concept
25.4.4 [range.view] when the default chosen by the heuristic is
incorrect. P0896R4 did include normative permission to specialize
enable_view
, but the wording does not place sufficient requirements on
such user specializations so as to make them usable by the View
concept
definition. Specializations must be required to be usable as constant
expressions of type bool
to avoid hard errors in the concept.
[2019-02-03 Priority to 0 and Status to Tentatively Ready after five positive votes on the reflector.]
Proposed resolution:
This wording is relative to N4791.
[Drafting Note: This wording uses the recently-defined core language term "usable in constant expressions" from 7.7 [expr.const] paragraph 3 which may be unfamiliar to reviewers.]
Change 24.3.4.8 [iterator.concept.sizedsentinel] as follows:
[…]
(2.2) — If
−N
is representable byiter_difference_t<I>
, theni - s
is well-defined and equals−N
.-?- Pursuant to 16.4.5.2.1 [namespace.std], users may specialize
disable_sized_sentinel
for cv-unqualified non-array object typesS
andI
at least one of which is a program-defined type. Such specializations shall be usable in constant expressions (7.7 [expr.const]) and have typeconst bool
.3 [Note:
disable_sized_sentinel
allows use of sentinels and iterators with the library that satisfy but do not in fact modelSizedSentinel
.—end note][…]
Add an index entry for disable_sized_sentinel
that points to
[iterator.concepts.sizedsentinel].
Change 25.4.3 [range.sized] as follows:
[…]
3 [Note: The complexity requirement for the evaluation of
ranges::size
is non-amortized, unlike the case for the complexity of the evaluations ofranges::begin
andranges::end
in theRange
concept.—end note]-?- Pursuant to 16.4.5.2.1 [namespace.std], users may specialize
disable_sized_range
for cv-unqualified program-defined types. Such specializations shall be usable in constant expressions (7.7 [expr.const]) and have typeconst bool
.4 [Note:
disable_sized_range
allows use of range types with the library that satisfy but do not in fact modelSizedRange
.—end note]
Add an index entry for disable_sized_range
that points to
25.4.3 [range.sized].
Change 25.4.4 [range.view] as follows:
[…]
5 Pursuant to 16.4.5.2.1 [namespace.std], users may specialize
enable_view
totrue
for cv-unqualified program-defined types which modelView
, andfalse
for types which do not. Such specializations shall be usable in constant expressions (7.7 [expr.const]) and have typeconst bool
.
bind_front
wordingSection: 22.10.14 [func.bind.partial] Status: C++20 Submitter: Tomasz Kamiński Opened: 2019-01-16 Last modified: 2023-02-07
Priority: 0
View all issues with C++20 status.
Discussion:
During the merge of the P0356R5, following "oddities" of the new wording was pointed out by Jens Maurer:
The initialization of the state entities of the bind_front/not_fn
is specified using formulation "xx
initialized with the initializer (yyy)
". Per author knowledge this specification is correct, however
inconsistent with the other parts of the of the standard, that direct-non-list-initialization term in such context.
The specification of the Mandates element for bind_front
uses conjunction_v
to specify
conjunction of the requirements, while corresponding element of the not_fn
specifies it using &&
.
As conjuction_v
implies order of evaluation that is not necessary in this case (for every valid program, all
provided traits must evaluate to true), it may be replaced with usage of fold expression with operator &&
.
[2019-01-26 Priority to 0 and Status to Tentatively Ready after five positive votes on the reflector.]
Proposed resolution:
This wording is relative to N4791.
Change [func.not_fn] as indicated:
template<class F> unspecified not_fn(F&& f);-1- In the text that follows:
(1.1) — […]
(1.2) — […]
(1.3) —
fd
is the target object ofg
(22.10.3 [func.def]) of typeFD
direct-non-list-initialized withinitialized with the initializer
(std::forward<F>(f))(9.5 [dcl.init]),(1.4) — […]
Change [func.bind_front] as indicated:
template <class F, class... Args> unspecified bind_front(F&& f, Args&&... args);-1- In the text that follows:
(1.1) — […]
(1.2) — […]
(1.3) —
fd
is the target object ofg
(22.10.3 [func.def]) of typeFD
direct-non-list-initialized withinitialized with the initializer
(std::forward<F>(f))(9.5 [dcl.init]),(1.4) — […]
(1.5) —
bound_args
is a pack of bound argument entities ofg
(22.10.3 [func.def]) of typesBoundArgs...
, direct-non-list-initialized withinitialized with initializers, respectively, and
(std::forward<Args>(args))...(1.6) — […]
-2- Mandates:
is_constructible_v<FD, F> && is_move_constructible_v<FD> && (is_constructible_v<BoundArgs, Args> && ...) && (is_move_constructible_v<BoundArgs> && ...)conjunction_v<is_constructible<FD, F>, is_move_constructible<FD>, is_constructible<BoundArgs, Args>..., is_move_constructible<BoundArgs>...>shall be true.
constexpr
and noexcept
Section: 20.2.8.2 [allocator.uses.construction] Status: C++20 Submitter: Pablo Halpern Opened: 2019-01-29 Last modified: 2021-02-25
Priority: 0
View all other issues in [allocator.uses.construction].
View all issues with C++20 status.
Discussion:
The uses-allocator construction functions introduced into WP when P0591r4
was accepted (Nov 2018, San Diego) should all be constexpr
. All but two should also be noexcept
.
Getting this right is an important part of correctly adding constexpr
memory allocation into the WP.
constexpr
to all of the new functions except
uninitialized_construct_using_allocator
and noexcept
to all
of the overloads of uses_allocator_construction_args
. Optionally, we could consider adding conditional
noexcept
to the remaining two functions. If p0784 is accepted,
then also add constexpr
to uninitialized_construct_using_allocator
.
[2019-02-12 Priority to 0 and Status to Tentatively Ready after six positive votes on the reflector.]
Proposed resolution:
This wording is relative to N4800.
Change header <memory>
synopsis, 20.2.2 [memory.syn], as indicated:
[…] // 20.2.8.2 [allocator.uses.construction], uses-allocator construction template <class T, class Alloc, class... Args> constexpr auto uses_allocator_construction_args(const Alloc& alloc, Args&&... args) noexcept -> see below; template <class T, class Alloc, class Tuple1, class Tuple2> constexpr auto uses_allocator_construction_args(const Alloc& alloc, piecewise_construct_t, Tuple1&& x, Tuple2&& y) noexcept -> see below; template <class T, class Alloc> constexpr auto uses_allocator_construction_args(const Alloc& alloc) noexcept -> see below; template <class T, class Alloc, class U, class V> constexpr auto uses_allocator_construction_args(const Alloc& alloc, U&& u, V&& v) noexcept -> see below; template <class T, class Alloc, class U, class V> constexpr auto uses_allocator_construction_args(const Alloc& alloc, const pair<U,V>& pr) noexcept -> see below; template <class T, class Alloc, class U, class V> constexpr auto uses_allocator_construction_args(const Alloc& alloc, pair<U,V>&& pr) noexcept -> see below; template <class T, class Alloc, class... Args> constexpr T make_obj_using_allocator(const Alloc& alloc, Args&&... args); template <class T, class Alloc, class... Args> T* uninitialized_construct_using_allocator(T* p, const Alloc& alloc, Args&&... args); […]
Change 20.2.8.2 [allocator.uses.construction] as indicated:
template <class T, class Alloc, class... Args> constexpr auto uses_allocator_construction_args(const Alloc& alloc, Args&&... args) noexcept -> see below;[…]
template <class T, class Alloc, class Tuple1, class Tuple2> constexpr auto uses_allocator_construction_args(const Alloc& alloc, piecewise_construct_t, Tuple1&& x, Tuple2&& y) noexcept -> see below;[…]
template <class T, class Alloc> constexpr auto uses_allocator_construction_args(const Alloc& alloc) noexcept -> see below;[…]
template <class T, class Alloc, class U, class V> constexpr auto uses_allocator_construction_args(const Alloc& alloc, U&& u, V&& v) noexcept -> see below;[…]
template <class T, class Alloc, class U, class V> constexpr auto uses_allocator_construction_args(const Alloc& alloc, const pair<U,V>& pr) noexcept -> see below;[…]
template <class T, class Alloc, class U, class V> constexpr auto uses_allocator_construction_args(const Alloc& alloc, pair<U,V>&& pr) noexcept -> see below;[…]
template <class T, class Alloc, class... Args> constexpr T make_obj_using_allocator(const Alloc& alloc, Args&&... args);[…]
template <class T, class Alloc, class... Args> T* uninitialized_construct_using_allocator(T* p, const Alloc& alloc, Args&&... args);[…]
ranges
removal, partition, and partial_sort_copy
algorithms discard useful informationSection: 26.7.8 [alg.remove], 26.7.9 [alg.unique], 26.8.2.4 [partial.sort.copy], 26.8.5 [alg.partitions] Status: C++20 Submitter: Tomasz Kamiński Opened: 2019-02-05 Last modified: 2021-02-25
Priority: 1
View all other issues in [alg.remove].
View all issues with C++20 status.
Discussion:
This is direct follow-up on the LWG issue 3169(i), that proposed to change additional algorithms that drop the
iterator value equal to sentinel, that needs to be always computed. These set include removal (remove
,
remove_if
, and unique
), partition (partition
, stable_partition
),
and partial_sort_copy
.
subrange
.
For partition algorithms, the end of "true" object, and the "end-of-range" iterator forms a valid range of objects for
which predicate returns "false", thus we propose to return subrange
.
For partial_sort_copy
we propose to return partial_sort_copy_result
as an alias to
copy_result
to match other copy algorithms.
[2019-02-12; Tomasz comments and improves proposed wording]
Proposed wording is updated to incorporate wording comments from Casey Carter:
We don't need to repeat the definition of partial_sort_copy_result
in 26.8.2.4 [partial.sort.copy];
the single definition in the synopsis is sufficient.
e
is a potentially confusing choice of placeholder name for the end of the output range, given that
we use a placeholder E
for predicates in the algorithm specifications.
The placeholder e
is replaced with j
that seems not to be used in the specification of
above algorithms.
[2019-02 Priority set to 1 after reflector discussion]
[2019-02; Kona Wednesday night issue processing]
Status to Ready
Proposed resolution:
This wording is relative to N4800.
Change header <algorithm>
synopsis, 26.4 [algorithm.syn], as indicated:
[…] //26.7.8 [alg.remove], remove […] namespace ranges { template<Permutable I, Sentinel<I> S, class T, class Proj = identity> requires IndirectRelation<ranges::equal_to<>, projected<I, Proj>, const T*> constexpr subrange<I> remove(I first, S last, const T& value, Proj proj = {}); template<ForwardRange R, class T, class Proj = identity> requires Permutable<iterator_t<R>> && IndirectRelation<ranges::equal_to<>, projected<iterator_t<R>, Proj>, const T*> constexpr safe_subrangeiterator_t<R> remove(R&& r, const T& value, Proj proj = {}); template<Permutable I, Sentinel<I> S, class Proj = identity, IndirectUnaryPredicate<projected<I, Proj>> Pred> constexpr subrange<I> remove_if(I first, S last, Pred pred, Proj proj = {}); template<ForwardRange R, class Proj = identity, IndirectUnaryPredicate<projected<iterator_t<R>, Proj>> Pred> requires Permutable<iterator_t<R>> constexpr safe_subrangeiterator_t<R> remove_if(R&& r, Pred pred, Proj proj = {}); } […] // 26.7.9 [alg.unique], unique […] namespace ranges { template<Permutable I, Sentinel<I> S, class Proj = identity, IndirectRelation<projected<I, Proj>> C = ranges::equal_to<>> constexpr subrange<I> unique(I first, S last, C comp = {}, Proj proj = {}); template<ForwardRange R, class Proj = identity, IndirectRelation<projected<iterator_t<R>, Proj>> C = ranges::equal_to<>> requires Permutable<iterator_t<R>> constexpr safe_subrangeiterator_t<R> unique(R&& r, C comp = {}, Proj proj = {}); } […] // 26.8.2 [alg.sort], sorting […] namespace ranges { template<class I, class O> using partial_sort_copy_result = copy_result<I, O>; template<InputIterator I1, Sentinel<I1> S1, RandomAccessIterator I2, Sentinel<I2> S2, class Comp = ranges::less<>, class Proj1 = identity, class Proj2 = identity> requires IndirectlyCopyable<I1, I2> && Sortable<I2, Comp, Proj2> && IndirectStrictWeakOrder<Comp, projected<I1, Proj1>, projected<I2, Proj2>> constexpr partial_sort_copy_result<I1, I2> partial_sort_copy(I1 first, S1 last, I2 result_first, S2 result_last, Comp comp = {}, Proj1 proj1 = {}, Proj2 proj2 = {}); template<InputRange R1, RandomAccessRange R2, class Comp = ranges::less<>, class Proj1 = identity, class Proj2 = identity> requires IndirectlyCopyable<iterator_t<R1>, iterator_t<R2>> && Sortable<iterator_t<R2>, Comp, Proj2> && IndirectStrictWeakOrder<Comp, projected<iterator_t<R1>, Proj1>, projected<iterator_t<R2>, Proj2>> constexpr partial_sort_copy_result<safe_iterator_t<R1>, safe_iterator_t<R2>> partial_sort_copy(R1&& r, R2&& result_r, Comp comp = {}, Proj1 proj1 = {}, Proj2 proj2 = {}); } […] // 26.8.5 [alg.partitions], partitions […] namespace ranges { template<Permutable I, Sentinel<I> S, class Proj = identity, IndirectUnaryPredicate<projected<I, Proj>> Pred> constexpr subrange<I> partition(I first, S last, Pred pred, Proj proj = {}); template<ForwardRange R, class Proj = identity, IndirectUnaryPredicate<projected<iterator_t<R>, Proj>> Pred> requires Permutable<iterator_t<R>> constexpr safe_subrangeiterator_t<R> partition(R&& r, Pred pred, Proj proj = {}); } […] namespace ranges { template<BidirectionalIterator I, Sentinel<I> S, class Proj = identity, IndirectUnaryPredicate<projected<I, Proj>> Pred> requires Permutable<I> subrange<I> stable_partition(I first, S last, Pred pred, Proj proj = {}); template<BidirectionalRange R, class Proj = identity, IndirectUnaryPredicate<projected<iterator_t<R>, Proj>> Pred> requires Permutable<iterator_t<R>> safe_subrangeiterator_t<R> stable_partition(R&& r, Pred pred, Proj proj = {}); } […]
Change 26.7.8 [alg.remove] as indicated:
[…] namespace ranges { template<Permutable I, Sentinel<I> S, class T, class Proj = identity> requires IndirectRelation<ranges::equal_to<>, projected<I, Proj>, const T*> constexpr subrange<I> remove(I first, S last, const T& value, Proj proj = {}); template<ForwardRange R, class T, class Proj = identity> requires Permutable<iterator_t<R>> && IndirectRelation<ranges::equal_to<>, projected<iterator_t<R>, Proj>, const T*> constexpr safe_subrangeiterator_t<R> remove(R&& r, const T& value, Proj proj = {}); template<Permutable I, Sentinel<I> S, class Proj = identity, IndirectUnaryPredicate<projected<I, Proj>> Pred> constexpr subrange<I> remove_if(I first, S last, Pred pred, Proj proj = {}); template<ForwardRange R, class Proj = identity, IndirectUnaryPredicate<projected<iterator_t<R>, Proj>> Pred> requires Permutable<iterator_t<R>> constexpr safe_subrangeiterator_t<R> remove_if(R&& r, Pred pred, Proj proj = {}); }[…]
-4- Returns: Letj
be tThe end of the resulting range. Returns:[…]
(4.?) —
j
for the overloads in namespacestd
, or(4.?) —
{j, last}
for the overloads in namespaceranges
.
Change 26.7.9 [alg.unique] as indicated:
[…] namespace ranges { template<Permutable I, Sentinel<I> S, class Proj = identity, IndirectRelation<projected<I, Proj>> C = ranges::equal_to<>> constexpr subrange<I> unique(I first, S last, C comp = {}, Proj proj = {}); template<ForwardRange R, class Proj = identity, IndirectRelation<projected<iterator_t<R>, Proj>> C = ranges::equal_to<>> requires Permutable<iterator_t<R>> constexpr safe_subrangeiterator_t<R> unique(R&& r, C comp = {}, Proj proj = {}); }[…]
-4- Returns: Letj
be tThe end of the resulting range. Returns:[…]
(4.?) —
j
for the overloads in namespacestd
, or(4.?) —
{j, last}
for the overloads in namespaceranges
.
Change 26.8.2.4 [partial.sort.copy] as indicated:
[…] namespace ranges { template<InputIterator I1, Sentinel<I1> S1, RandomAccessIterator I2, Sentinel<I2> S2, class Comp = ranges::less<>, class Proj1 = identity, class Proj2 = identity> requires IndirectlyCopyable<I1, I2> && Sortable<I2, Comp, Proj2> && IndirectStrictWeakOrder<Comp, projected<I1, Proj1>, projected<I2, Proj2>> constexpr partial_sort_copy_result<I1, I2> partial_sort_copy(I1 first, S1 last, I2 result_first, S2 result_last, Comp comp = {}, Proj1 proj1 = {}, Proj2 proj2 = {}); template<InputRange R1, RandomAccessRange R2, class Comp = ranges::less<>, class Proj1 = identity, class Proj2 = identity> requires IndirectlyCopyable<iterator_t<R1>, iterator_t<R2>> && Sortable<iterator_t<R2>, Comp, Proj2> && IndirectStrictWeakOrder<Comp, projected<iterator_t<R1>, Proj1>, projected<iterator_t<R2>, Proj2>> constexpr partial_sort_copy_result<safe_iterator_t<R1>, safe_iterator_t<R2>> partial_sort_copy(R1&& r, R2&& result_r, Comp comp = {}, Proj1 proj1 = {}, Proj2 proj2 = {}); }[…]
-4- Returns:[…]
(4.?) —
result_first + N
for the overloads in namespacestd
, or(4.?) —
{last, result_first + N}
for the overloads in namespaceranges
.
Change 26.8.5 [alg.partitions] as indicated:
[…] namespace ranges { template<Permutable I, Sentinel<I> S, class Proj = identity, IndirectUnaryPredicate<projected<I, Proj>> Pred> constexpr subrange<I> partition(I first, S last, Pred pred, Proj proj = {}); template<ForwardRange R, class Proj = identity, IndirectUnaryPredicate<projected<iterator_t<R>, Proj>> Pred> requires Permutable<iterator_t<R>> constexpr safe_subrangeiterator_t<R> partition(R&& r, Pred pred, Proj proj = {}); }[…]
-7- Returns: Leti
be aAn iteratorsuch thati
E(*j)
istrue
for every iteratorj
in[first, i)
andfalse
for every iteratorj
in[i, last)
. Returns:[…]
(7.?) —
i
for the overloads in namespacestd
, or(7.?) —
{i, last}
for the overloads in namespaceranges
.[…] namespace ranges { template<BidirectionalIterator I, Sentinel<I> S, class Proj = identity, IndirectUnaryPredicate<projected<I, Proj>> Pred> requires Permutable<I> subrange<I> stable_partition(I first, S last, Pred pred, Proj proj = {}); template<BidirectionalRange R, class Proj = identity, IndirectUnaryPredicate<projected<iterator_t<R>, Proj>> Pred> requires Permutable<iterator_t<R>> safe_subrangeiterator_t<R> stable_partition(R&& r, Pred pred, Proj proj = {}); }[…]
-11- Returns: Leti
be aAn iteratorsuch that for every iteratori
j
in[first, i)
,E(*j)
istrue
, and for every iteratorj
in the range[i, last)
,E(*j)
isfalse
,. Returns:[…]
(11.?) —
i
for the overloads in namespacestd
, or(11.?) —
{i, last}
for the overloads in namespaceranges
.
scoped_allocator_adaptor::construct()
Section: 20.2.8.2 [allocator.uses.construction] Status: C++20 Submitter: Jonathan Wakely Opened: 2019-02-14 Last modified: 2021-02-25
Priority: Not Prioritized
View all other issues in [allocator.uses.construction].
View all issues with C++20 status.
Discussion:
2586(i) fixed the value category in the uses-allocator checks done by scoped_allocator_adaptor
.
When we made that use uses_allocator_construction_args
we reintroduced the problem, because that
function has the same bug.
#include <memory> struct X { using allocator_type = std::allocator<X>; X(std::allocator_arg_t, allocator_type&&) { } X(const allocator_type&) { } }; int main() { std::allocator<X> a; std::make_obj_using_allocator<X>(a); }
This will fail to compile, because uses_allocator_construction_args
will check is_constructible
using an rvalue allocator, but then return tuple<allocator_arg_t, const allocator<X>&>
containing an lvalue allocator. Those args cannot be used to construct an X
.
[2019-02; Kona Wednesday night issue processing]
Status to Ready
Proposed resolution:
This wording is relative to N4800.
Change 20.2.8.2 [allocator.uses.construction] as indicated:
[Drafting Note: Arguably the
uses_allocator
specialization should also useconst Alloc&
but in practice that doesn't matter, except for even more contrived cases than the very contrived example above.]
template <class T, class Alloc, class... Args> auto uses_allocator_construction_args(const Alloc& alloc, Args&&... args) -> see below;[…]
-5- Returns: Atuple
value determined as follows:[…]
(5.1) — If
uses_allocator_v<T, Alloc>
isfalse
andis_constructible_v<T, Args...>
istrue
, returnforward_as_tuple(std::forward<Args>(args)...)
.(5.2) — Otherwise, if
uses_allocator_v<T, Alloc>
istrue
andis_constructible_v<T, allocator_arg_t, const Alloc&, Args...>
istrue
, returntuple<allocator_arg_t, const Alloc&, Args&&...>( allocator_arg, alloc, std::forward<Args>(args)...)(5.3) — Otherwise, if
uses_allocator_v<T, Alloc>
istrue
andis_constructible_v<T, Args..., const Alloc&>
istrue
, returnforward_as_tuple(std::forward<Args>(args)..., alloc)
.(5.4) — Otherwise, the program is ill-formed.
std::allocator::allocate
sometimes returns too little storageSection: 20.2.10.2 [allocator.members] Status: C++20 Submitter: Casey Carter Opened: 2019-02-20 Last modified: 2021-02-25
Priority: 3
View all other issues in [allocator.members].
View all issues with C++20 status.
Discussion:
20.2.10.2 [allocator.members]/2 says:
-2- Returns: A pointer to the initial element of an array of storage of size
n * sizeof(T)
, aligned appropriately for objects of typeT
.
As in LWG 3038(i), we should not return too little storage for n objects of size sizeof(T)
, e.g.
when n
is SIZE_MAX / 2
and T
is short
.
[2019-03-05 Priority set to 3 after reflector discussion]
[2019 Cologne Wednesday night]
Status to Ready; will open additional issue to reconcile this and 3038(i)
Proposed resolution:
This wording is relative to N4800.
Change 20.2.10.2 [allocator.members] as indicated:
[[nodiscard]] T* allocate(size_t n);[…]
-4- Throws:bad_array_new_length
ifSIZE_MAX / sizeof(T) < n
, orbad_alloc
if the storage cannot be obtained.
std::ranges::shuffle
synopsis does not match algorithm definitionSection: 26.7.13 [alg.random.shuffle] Status: C++20 Submitter: Christopher Di Bella Opened: 2019-03-02 Last modified: 2021-02-25
Priority: 1
View all other issues in [alg.random.shuffle].
View all issues with C++20 status.
Discussion:
26.4 [algorithm.syn] declares std::ranges::shuffle
like so:
namespace ranges { template<RandomAccessIterator I, Sentinel<I> S, class Gen> requires Permutable<I> && UniformRandomBitGenerator<remove_reference_t<Gen>> && ConvertibleTo<invoke_result_t<Gen&>, iter_difference_t<I>> I shuffle(I first, S last, Gen&& g); template<RandomAccessRange R, class Gen> requires Permutable<iterator_t<R> && UniformRandomBitGenerator<remove_reference_t<Gen>> && ConvertibleTo<invoke_result_t<Gen&>, iter_difference_t<iterator_t<R>>> safe_iterator_t<R> shuffle(R&& r, Gen&& g); }
26.7.13 [alg.random.shuffle] defines the algorithm like so:
namespace ranges { template<RandomAccessIterator I, Sentinel<I> S, class Gen> requires Permutable<I> && UniformRandomBitGenerator<remove_reference_t<Gen>> I shuffle(I first, S last, Gen&& g); template<RandomAccessRange R, class Gen> requires Permutable<iterator_t<R>> && UniformRandomBitGenerator<remove_reference_t<Gen>> safe_iterator_t<R> shuffle(R&& r, Gen&& g); }
Notice the missing ConvertibleTo
requirements in the latter. Looking at the
Ranges TS, [alg.random.shuffle] includes
this requirement, albeit in the Ranges TS-format.
[2019-03-03; Daniel comments]
Given that the accepted proposal P0896R4 voted in San Diego did contain the same error I decided to open this issue instead of submitting an editorial change request.
[2019-03-05 Priority set to 1 after reflector discussion]
Casey: The correct fix here is to remove the ConvertibleTo
requirement from the header
synopsis. UniformRandomBitGenerator
s have integral result types, and the core language guarantees
that all integral types are convertible to all other integral types. We don't need to validate the core
language in the associated constraints of ranges::shuffle
.
Previous resolution [SUPERSEDED]:
This wording is relative to N4800.
Change 26.7.13 [alg.random.shuffle] as indicated:
[…] namespace ranges { template<RandomAccessIterator I, Sentinel<I> S, class Gen> requires Permutable<I> && UniformRandomBitGenerator<remove_reference_t<Gen>> && ConvertibleTo<invoke_result_t<Gen&>, iter_difference_t<I>> I shuffle(I first, S last, Gen&& g); template<RandomAccessRange R, class Gen> requires Permutable<iterator_t<R>> && UniformRandomBitGenerator<remove_reference_t<Gen>> && ConvertibleTo<invoke_result_t<Gen&>, iter_difference_t<iterator_t<R>>> safe_iterator_t<R> shuffle(R&& r, Gen&& g); }
[2019-03-05 Updated proposed wording according to Casey's suggestion]
[2019-06-16 Set to "Tentatively Ready" after five positive votes on the reflector.]
Proposed resolution:
This wording is relative to N4800.
Change 26.4 [algorithm.syn] as indicated:
// 26.7.13 [alg.random.shuffle], shuffle […] namespace ranges { template<RandomAccessIterator I, Sentinel<I> S, class Gen> requires Permutable<I> && UniformRandomBitGenerator<remove_reference_t<Gen>>&& ConvertibleTo<invoke_result_t<Gen&>, iter_difference_t<I>>I shuffle(I first, S last, Gen&& g); template<RandomAccessRange R, class Gen> requires Permutable<iterator_t<R> && UniformRandomBitGenerator<remove_reference_t<Gen>>&& ConvertibleTo<invoke_result_t<Gen&>, iter_difference_t<iterator_t<R>>>safe_iterator_t<R> shuffle(R&& r, Gen&& g); } […]
ConvertibleTo
prose does not match codeSection: 18.4.4 [concept.convertible] Status: C++20 Submitter: Hubert Tong Opened: 2019-03-05 Last modified: 2021-02-25
Priority: 1
View other active issues in [concept.convertible].
View all other issues in [concept.convertible].
View all issues with C++20 status.
Discussion:
The prose in N4800 subclause [concept.convertibleto] indicates that the requirement is for an expression of a particular type and value category to be both implicitly and explicitly convertible to some other type. However, for a type
struct A { A(const A&) = delete; };
ConvertibleTo<const A, A>
would be false
despite the following being okay:
const A f(); A test() { static_cast<A>(f()); return f(); }
[2019-03-15 Priority set to 1 after reflector discussion]
[2019-07-14 Tim adds PR based on discussion in 2019-07-09 LWG telecon]
Previous resolution [SUPERSEDED]:This wording is relative to N4820, and also resolves LWG 3151(i).
Modify [concept.convertibleto] as indicated:
-1- The
ConvertibleTo
concept requiresana glvalue expression of a particular type and value category to be both implicitly and explicitly convertible to some other type. The implicit and explicit conversions are required to produce equal results.template<class From, class To> concept ConvertibleTo = is_convertible_v<From, To> && requires(add_rvalue_reference_t<From> (&f)()) { static_cast<To>(f()); };-2- Let
test
be the invented function:To test(add_rvalue_reference_t<From> (&f)()) { return f(); }for some types
From
andTo
, and letf
be a function with no arguments and return typeadd_rvalue_reference_t<From>
such thatf()
is equality-preserving.From
andTo
modelConvertibleTo<From, To>
only if:
(2.1) —
To
is not an object or reference-to-object type, orstatic_cast<To>(f())
is equal totest(f)
.(2.2) —
add_rvalue_reference_t<From>
is not a reference-to-object type, or
(2.2.1) — If
add_rvalue_reference_t<From>
is an rvalue reference to a non const-qualified type, the resulting state of the object referenced byf()
after either above expression is valid but unspecified (16.4.6.17 [lib.types.movedfrom]).(2.2.2) — Otherwise, the object referred to by
f()
is not modified by either above expression.
[2019-09-23; Daniel adjusts wording to working draft changes]
Due to the concept renaming caused by P1754R1 the proposed wording is outdated and needs adjustments.
Previous resolution [SUPERSEDED]:This wording is relative to N4830, and also resolves LWG 3151(i).
Modify 18.4.4 [concept.convertible] as indicated:
-1- The
convertible_to
concept requiresana glvalue expression of a particular type and value category to be both implicitly and explicitly convertible to some other type. The implicit and explicit conversions are required to produce equal results.template<class From, class To> concept convertible_to = is_convertible_v<From, To> && requires(add_rvalue_reference_t<From> (&f)()) { static_cast<To>(f()); };-2- Let
test
be the invented function:To test(add_rvalue_reference_t<From> (&f)()) { return f(); }for some types
From
andTo
, and letf
be a function with no arguments and return typeadd_rvalue_reference_t<From>
such thatf()
is equality-preserving.From
andTo
modelconvertible_to<From, To>
only if:
(2.1) —
To
is not an object or reference-to-object type, orstatic_cast<To>(f())
is equal totest(f)
.(2.2) —
add_rvalue_reference_t<From>
is not a reference-to-object type, or
(2.2.1) — If
add_rvalue_reference_t<From>
is an rvalue reference to a non const-qualified type, the resulting state of the object referenced byf()
after either above expression is valid but unspecified (16.4.6.17 [lib.types.movedfrom]).(2.2.2) — Otherwise, the object referred to by
f()
is not modified by either above expression.
[2019-11-06 Tim updates PR based on discussion in Belfast LWG evening session]
"glvalue" is incorrect because we want to allow testing convertible_to<void, void>
. It's also less than clear
how the "expression" and "a particular type" in the first sentence correspond to the parameters of the concept.
This wording is relative to N4835, and also resolves LWG 3151(i).
Modify 18.4.4 [concept.convertible] as indicated:
-1- The
convertible_to
concept for typesFrom
andTo
requires an expressionE
such thatdecltype((E))
isadd_rvalue_reference_t<From>
of a particular type and value categoryto be both implicitly and explicitly convertible tosome other typeTo
. The implicit and explicit conversions are required to produce equal results.template<class From, class To> concept convertible_to = is_convertible_v<From, To> && requires(add_rvalue_reference_t<From> (&f)()) { static_cast<To>(f()); };-2- Let
FromR
beadd_rvalue_reference_t<From>
andtest
be the invented function:To test(FromR (&f)()) { return f(); }for some types
From
andTo
, and letf
be a function with no arguments and return typeFromR
such thatf()
is equality-preserving.From
andTo
modelconvertible_to<From, To>
only if:
(2.1) —
To
is not an object or reference-to-object type, orstatic_cast<To>(f())
is equal totest(f)
.(2.2) —
FromR
is not a reference-to-object type, or
(2.2.1) — If
FromR
is an rvalue reference to a non const-qualified type, the resulting state of the object referenced byf()
after either above expression is valid but unspecified (16.4.6.17 [lib.types.movedfrom]).(2.2.2) — Otherwise, the object referred to by
f()
is not modified by either above expression.
[2019-11-09 Tim rephrased first sentence based on discussion in Belfast LWG Saturday session]
[Status to Tentatively ready after Belfast LWG Saturday session]
Proposed resolution:
This wording is relative to N4835, and also resolves LWG 3151(i).
Modify 18.4.4 [concept.convertible] as indicated:
-1- Given types
From
andTo
and an expressionE
such thatdecltype((E))
isadd_rvalue_reference_t<From>
,convertible_to<From, To>
Therequiresconvertible_to
conceptE
an expression of a particular type and value categoryto be both implicitly and explicitly convertible tosome othertypeTo
. The implicit and explicit conversions are required to produce equal results.template<class From, class To> concept convertible_to = is_convertible_v<From, To> && requires(add_rvalue_reference_t<From> (&f)()) { static_cast<To>(f()); };-2- Let
FromR
beadd_rvalue_reference_t<From>
andtest
be the invented function:To test(FromR (&f)()) { return f(); }for some types
From
andTo
, and letf
be a function with no arguments and return typeFromR
such thatf()
is equality-preserving.From
andTo
modelconvertible_to<From, To>
only if:
(2.1) —
To
is not an object or reference-to-object type, orstatic_cast<To>(f())
is equal totest(f)
.(2.2) —
FromR
is not a reference-to-object type, or
(2.2.1) — If
FromR
is an rvalue reference to a non const-qualified type, the resulting state of the object referenced byf()
after either above expression is valid but unspecified (16.4.6.17 [lib.types.movedfrom]).(2.2.2) — Otherwise, the object referred to by
f()
is not modified by either above expression.
weak_ptr
?Section: 20.3.2.3.2 [util.smartptr.weak.const] Status: C++23 Submitter: Casey Carter Opened: 2019-03-15 Last modified: 2023-11-22
Priority: 2
View all issues with C++23 status.
Discussion:
20.3.2.3.2 [util.smartptr.weak.const] specifies weak_ptr
's default constructor:
constexpr weak_ptr() noexcept;1 Effects: Constructs an empty
2 Ensures:weak_ptr
object.use_count() == 0
.
and shared_ptr
converting constructor template:
weak_ptr(const weak_ptr& r) noexcept; template<class Y> weak_ptr(const weak_ptr<Y>& r) noexcept; template<class Y> weak_ptr(const shared_ptr<Y>& r) noexcept;3 Remarks: The second and third constructors shall not participate in overload resolution unless
4 Effects: IfY*
is compatible withT*
.r
is empty, constructs an emptyweak_ptr
object; otherwise, constructs aweak_ptr
object that shares ownership withr
and stores a copy of the pointer stored inr
. 5 Ensures:use_count() == r.use_count()
.
Note that neither specifies the value of the stored pointer when the resulting weak_ptr
is empty.
This didn't matter — the stored pointer value was unobservable for an empty weak_ptr
—
until we added atomic<weak_ptr>
. 32.5.8.7.3 [util.smartptr.atomic.weak]/15 says:
Remarks: Two
weak_ptr
objects are equivalent if they store the same pointer value and either share ownership, or both are empty. The weak form may fail spuriously. See 32.5.8.2 [atomics.types.operations].
Two empty weak_ptr
objects that store different pointer values are not equivalent. We could
correct this by changing 32.5.8.7.3 [util.smartptr.atomic.weak]/15 to "Two weak_ptr
objects are
equivalent if they are both empty, or if they share ownership and store the same pointer value." In practice,
an implementation of atomic<weak_ptr>
will CAS on both the ownership (control block pointer)
and stored pointer value, so it seems cleaner to pin down the stored pointer value of an empty weak_ptr
.
[2019-06-09 Priority set to 2 after reflector discussion]
Previous resolution [SUPERSEDED]
This wording is relative to N4810.
Modify 20.3.2.3.2 [util.smartptr.weak.const] as indicated (note the drive-by edit to cleanup the occurrences of "constructs an object of class foo"):
constexpr weak_ptr() noexcept;-1- Effects: Constructs an empty
-2- Ensures:object that stores a null pointer value.weak_ptr
use_count() == 0
.weak_ptr(const weak_ptr& r) noexcept; template<class Y> weak_ptr(const weak_ptr<Y>& r) noexcept; template<class Y> weak_ptr(const shared_ptr<Y>& r) noexcept;-3- Remarks: The second and third constructors shall not participate in overload resolution unless
-4- Effects: IfY*
is compatible withT*
.r
is empty, constructs an emptyobject that stores a null pointer value; otherwise, constructs aweak_ptr
weak_ptr
object that shares ownership withr
and stores a copy of the pointer stored inr
. -5- Ensures:use_count() == r.use_count()
.
[2020-02-14 Casey updates P/R per LWG instruction]
While reviewing the P/R in Prague, Tim Song noticed that the stored pointer value of a moved-fromweak_ptr
must also be specified.
[2020-02-16; Prague]
Reviewed revised wording and moved to Ready for Varna.
[2020-11-09 Approved In November virtual meeting. Status changed: Ready → WP.]
Proposed resolution:
This wording is relative to N4849.
Modify 20.3.2.3.2 [util.smartptr.weak.const] as indicated:
constexpr weak_ptr() noexcept;-1- Effects: Constructs an empty
-2- Postconditions:weak_ptr
object that stores a null pointer value.use_count() == 0
.weak_ptr(const weak_ptr& r) noexcept; template<class Y> weak_ptr(const weak_ptr<Y>& r) noexcept; template<class Y> weak_ptr(const shared_ptr<Y>& r) noexcept;-3- Remarks: The second and third constructors shall not participate in overload resolution unless
-4- Effects: IfY*
is compatible withT*
.r
is empty, constructs an emptyweak_ptr
object that stores a null pointer value; otherwise, constructs aweak_ptr
object that shares ownership withr
and stores a copy of the pointer stored inr
. -5- Postconditions:use_count() == r.use_count()
.weak_ptr(weak_ptr&& r) noexcept; template<class Y> weak_ptr(weak_ptr<Y>&& r) noexcept;-6- Remarks: The second constructor shall not participate in overload resolution unless
-7- Effects: Move constructs aY*
is compatible withT*
.weak_ptr
instance fromr
. -8- Postconditions:*this
shall containcontains the old value ofr
.r
shall beis empty., stores a null pointer value, andr.use_count() == 0
.
std::optional<T>
is ill-formed is T
is an arraySection: 22.5.3 [optional.optional] Status: C++20 Submitter: Jonathan Wakely Opened: 2019-03-18 Last modified: 2021-02-25
Priority: 0
View all other issues in [optional.optional].
View all issues with C++20 status.
Discussion:
22.5.3 [optional.optional] appears to allow arrays:
T
shall be an object type other than cvin_place_t
or cvnullopt_t
and shall satisfy the Cpp17Destructible requirements (Table 30).
But instantiating it fails, because value_or
attempts to return T
by value, which isn't possible for an array type.
value_or
,
and MSVC fails a static assert saying the type needs to be destructible (which is misleading, because int[2]
is destructible, but either way it's ill-formed).
Previous resolution [SUPERSEDED]:
This wording is relative to N4810.
Modify 22.5.3 [optional.optional] as indicated:
-3-
T
shall be annon-array object type other than cvin_place_t
or cvnullopt_t
and shall satisfy the Cpp17Destructible requirements (Table 30).
[2019-03-26 Marshall provides updated resolution based on reflector discussion]
[2019-06-16 Moved to "Tentatively Ready" based on five positive votes on the reflector]
Proposed resolution:
This wording is relative to N4810.
In 16.4.4.2 [utility.arg.requirements], edit Table 30 — "Cpp17Destructible requirements" as indicated:
Table 30 — Cpp17Destructible requirements Expression Post-condition u.~T()
All resources owned by u
are reclaimed, no exception is propagated.[Note: Array types and non-object types are not Cpp17Destructible. — end note]
Modify 22.5.3 [optional.optional] as indicated:
-3-
T
shall be aobjecttype other than cvin_place_t
or cvnullopt_t
and shall satisfythat meets the Cpp17Destructible requirements (Table 30).
Modify 22.6.3 [variant.variant] as indicated:
-2-All types in
Types
shallbe (possibly cv-qualified) object types that are not arraysmeet the Cpp17Destructible requirements (Table 30).
std::span::span()
Section: 23.7.2.2.2 [span.cons] Status: C++20 Submitter: Lars Gullik Bjønnes Opened: 2019-04-03 Last modified: 2021-02-25
Priority: Not Prioritized
View all other issues in [span.cons].
View all issues with C++20 status.
Discussion:
It seems that this was left out of P1089. The constraint on
span()
(23.7.2.2.2 [span.cons]) in the current draft is:
Constraints:
Extent <= 0
istrue
.
This does not seem to make much sense.
The proposal is to change the constraint to be:[2019-06-09; Priority to 0 and Status to Tentatively Ready after five positive votes on the reflector.]Constraints:
Extent == dynamic_extent || Extent == 0
istrue
.
Proposed resolution:
This wording is relative to N4810.
Modify 23.7.2.2.2 [span.cons] as indicated:
constexpr span() noexcept;-1- Constraints:
Extent
is<== dynamic_extent || Extent == 0true
.
istream >> bitset<0>
failsSection: 22.9.4 [bitset.operators] Status: C++20 Submitter: Davis Herring Opened: 2019-04-05 Last modified: 2021-02-25
Priority: Not Prioritized
View all other issues in [bitset.operators].
View all issues with C++20 status.
Discussion:
From a StackOverflow question:
[bitset.operators]/5.1 says that extracting a bitset<0>
stops after reading 0 characters.
/6 then says that, since no characters were stored, failbit
is set.
Proposed resolution:
This wording is relative to N4810.
Modify 22.9.4 [bitset.operators] as indicated:
template<class charT, class traits, size_t N> basic_istream<charT, traits>& operator>>(basic_istream<charT, traits>& is, bitset<N>& x);[…]
-6- IfN > 0
and no characters are stored instr
, callsis.setstate(ios_base::failbit)
(which may throwios_base::failure
(31.5.4.4 [iostate.flags])).
midpoint
should not constrain T
is completeSection: 26.10.16 [numeric.ops.midpoint] Status: C++20 Submitter: Paolo Torres Opened: 2019-04-10 Last modified: 2021-02-25
Priority: 2
View all other issues in [numeric.ops.midpoint].
View all issues with C++20 status.
Discussion:
The constraint of the midpoint
overload in 26.10.16 [numeric.ops.midpoint]:
template<class T> constexpr T* midpoint(T* a, T* b);-4- Constraints:
T
is a complete object type.
is incorrect. Paragraph 4 states T
is constrained to be a complete object type, however it does not seem to
be possible to implement T
is complete. This means behavior is conditioned dependent on whether a type is
complete, and this seems inconsistent with the library precedent.
[2019-06-12 Priority set to 2 after reflector discussion]
[2020-02 Status to Immediate on Thursday night in Prague.]
Proposed resolution:
This wording is relative to N4810.
Modify 26.10.16 [numeric.ops.midpoint] as indicated:
template<class T> constexpr T* midpoint(T* a, T* b);-4- Constraints:
-?- Mandates:T
is ancompleteobject type.T
is a complete type. […]
lerp
should be marked as noexcept
Section: 29.7.4 [c.math.lerp] Status: C++20 Submitter: Paolo Torres Opened: 2019-04-10 Last modified: 2021-02-25
Priority: 2
View all issues with C++20 status.
Discussion:
The overloads of lerp
should be marked as noexcept
, and this can be explained through the Lakos Rule.
This function does not specify any undefined behaviour, and as such has no preconditions. This implies it has a wide
contract, meaning it cannot throw, and thus can be marked as noexcept
.
[2020-02 Moved to Immediate on Thursday afternoon in Prague.]
Proposed resolution:
This wording is relative to N4810.
Modify 29.7.1 [cmath.syn], header <cmath>
synopsis, as indicated:
// 29.7.4 [c.math.lerp], linear interpolation constexpr float lerp(float a, float b, float t) noexcept; constexpr double lerp(double a, double b, double t) noexcept; constexpr long double lerp(long double a, long double b, long double t) noexcept;
Modify 29.7.4 [c.math.lerp] as indicated:
constexpr float lerp(float a, float b, float t) noexcept; constexpr double lerp(double a, double b, double t) noexcept; constexpr long double lerp(long double a, long double b, long double t) noexcept;
Section: 22.10.2 [functional.syn] Status: C++20 Submitter: Jonathan Wakely Opened: 2019-04-23 Last modified: 2021-02-25
Priority: 0
View all issues with C++20 status.
Discussion:
P0318R1 was discussed in Batavia and the requested changes were made in D0318R2. In San Diego the R1 paper was voted into the WP, despite not having the requested changes. There were also changes to D0318R2 suggested on the reflector, which are incorporated below.
[2019-04-30 Priority to 0 and Status to Tentatively Ready after six positive votes on the reflector.]
Proposed resolution:
This wording is relative to N4810.
Modify 22.10.2 [functional.syn], header <functional>
synopsis, as indicated:
template<class T> struct unwrap_reference; template<class T> using unwrap_reference_t = typename unwrap_reference<T>::type; template<class T> struct unwrap_ref_decay: unwrap_reference<decay_t<T>> {}; template<class T> using unwrap_ref_decay_t = typename unwrap_ref_decay<T>::type;
Modify 99 [refwrap.unwrapref] as indicated:
template<class T> struct unwrap_reference;-1- If
T
is a specialization […]template<class T> struct unwrap_ref_decay;-?- The member typedef
type
ofunwrap_ref_decay<T>
denotes the typeunwrap_reference_t<decay_t<T>>
.
span
element access invalidationSection: 23.7.2.2.1 [span.overview] Status: WP Submitter: Johel Ernesto Guerrero Peña Opened: 2019-05-04 Last modified: 2023-11-22
Priority: 2
View all other issues in [span.overview].
View all issues with WP status.
Discussion:
span
doesn't explicitly point out when its accessed elements are invalidated like string_view
does in 27.3.3.4 [string.view.iterators] p2.
[2019-06-12 Priority set to 2 after reflector discussion]
Previous resolution [SUPERSEDED]:
This wording is relative to N4910.
Modify 23.7.2.2.1 [span.overview] as indicated:
-4-
-?- For aElementType
is required to be a complete object type that is not an abstract class type.span s
, any operation that invalidates a pointer in the range[s.data(), s.data() + s.size())
invalidates pointers, iterators, and references other than*this
returned froms
's member functions.
[2023-06-13; Varna]
The reference to N4910 27.3.3.4 [string.view.iterators] p2 is located now in N4950 27.3.3.1 [string.view.template.general] p2 where its says:
For a
basic_string_view str
, any operation that invalidates a pointer in the range[str.data(), str.data() + str.size())invalidates pointers, iterators, and references returned from
str
's member functions.
The group also suggested to adjust 27.3.3.1 [string.view.template.general] p2 similarly.
[2023-06-14 Varna; Move to Ready]
[2023-11-11 Approved at November 2023 meeting in Kona. Status changed: Voting → WP.]
Proposed resolution:
This wording is relative to N4950.
Modify 27.3.3.1 [string.view.template.general] as indicated:
[Drafting note: The proposed wording also removes the extra code block that previously defined the range]
-2- For a
basic_string_view str
, any operation that invalidates a pointer in the range[str.data(), str.data() + str.size())
invalidates pointers, iterators, and references to elements ofstr
returned from.str
's member functions
Modify 23.7.2.2.1 [span.overview] as indicated:
-4-
-?- For aElementType
is required to be a complete object type that is not an abstract class type.span s
, any operation that invalidates a pointer in the range[s.data(), s.data() + s.size())
invalidates pointers, iterators, and references to elements ofs
.
sub_match::swap
only swaps the base classSection: 28.6.8 [re.submatch] Status: C++23 Submitter: Jonathan Wakely Opened: 2019-05-07 Last modified: 2023-11-22
Priority: 3
View all other issues in [re.submatch].
View all issues with C++23 status.
Discussion:
sub_match<I>
derives publicly from pair<I,I>
, and so inherits
pair::swap(pair&)
. This means that the following program fails:
#include <regex> #include <cassert> int main() { std::sub_match<const char*> a, b; a.matched = true; a.swap(b); assert(b.matched); }
The pair::swap(pair&)
member should be hidden by a sub_match::swap(sub_match&)
member that does the right thing.
[2019-06-12 Priority set to 3 after reflector discussion]
[2020-05-01; Daniel adjusts wording to recent working draft]
[2022-04-25; Daniel adjusts wording to recent working draft]
In addition the revised wording uses the new standard phrase "The exception specification is equivalent to"
Previous resolution [SUPERSEDED]:
This wording is relative to N4910.
Modify 28.6.8 [re.submatch], class template
sub_match
synopsis, as indicated:template<class BidirectionalIterator> class sub_match : public pair<BidirectionalIterator, BidirectionalIterator> { public: […] int compare(const sub_match& s) const; int compare(const string_type& s) const; int compare(const value_type* s) const; void swap(sub_match& s) noexcept(see below); };Modify 28.6.8.2 [re.submatch.members] as indicated:
int compare(const value_type* s) const;[…]
void swap(sub_match& s) noexcept(see below);[Drafting note: The swappable requirement should really be unnecessary because Cpp17Iterator requires it, but there is no wording that requiresBidirectionalIterator
in Clause 28.6 [re] in general meets the bidirectional iterator requirements. Note that the definition found in 26.2 [algorithms.requirements] does not extend to 28.6 [re] normatively. — end drafting note]-?- Preconditions: Lvalues of type
-?- Effects: Equivalent to:BidirectionalIterator
are swappable (16.4.4.3 [swappable.requirements]).this->pair<BidirectionalIterator, BidirectionalIterator>::swap(s); std::swap(matched, s.matched);-?- Remarks: The exception specification is equivalent to
is_nothrow_swappable_v<BidirectionalIterator>
.
[2022-11-06; Daniel comments and improves wording]
With the informal acceptance of P2696R0 by LWG during a pre-Kona telecon, we should use the new requirement set Cpp17Swappable instead of the "LValues are swappable" requirements.
[2022-11-30; Reflector poll]
Set status to Tentatively Ready after six votes in favour during reflector poll.
[2023-02-13 Approved at February 2023 meeting in Issaquah. Status changed: Voting → WP.]
Proposed resolution:
This wording is relative to N4917 and assumes the acceptance of P2696R0.
Modify 28.6.8 [re.submatch], class template sub_match
synopsis, as indicated:
template<class BidirectionalIterator> class sub_match : public pair<BidirectionalIterator, BidirectionalIterator> { public: […] int compare(const sub_match& s) const; int compare(const string_type& s) const; int compare(const value_type* s) const; void swap(sub_match& s) noexcept(see below); };
Modify 28.6.8.2 [re.submatch.members] as indicated:
int compare(const value_type* s) const;[…]
void swap(sub_match& s) noexcept(see below);[Drafting note: The Cpp17Swappable requirement should really be unnecessary because Cpp17Iterator requires it, but there is no wording that requiresBidirectionalIterator
in Clause 28.6 [re] in general meets the bidirectional iterator requirements. Note that the definition found in 26.2 [algorithms.requirements] does not extend to 28.6 [re] normatively. — end drafting note]-?- Preconditions:
-?- Effects: Equivalent to:BidirectionalIterator
meets the Cpp17Swappable requirements (16.4.4.3 [swappable.requirements]).this->pair<BidirectionalIterator, BidirectionalIterator>::swap(s); std::swap(matched, s.matched);-?- Remarks: The exception specification is equivalent to
is_nothrow_swappable_v<BidirectionalIterator>
.
year_month_day
conversion to sys_days
uses not-existing member functionSection: 30.8.14.2 [time.cal.ymd.members] Status: C++20 Submitter: Tomasz Kamiński Opened: 2019-05-19 Last modified: 2021-02-25
Priority: 0
View all issues with C++20 status.
Discussion:
The current specification of the year_month_day
conversion function to sys_days
,
uses the day
member function on the sys_days
(a.k.a.
time_point<system_clock, days>
), that does not exist.
sys_days{y_/m_/last}.day()
is
ill-formed:
[…] Otherwise, if
y_.ok() && m_.ok()
istrue
, returns asys_days
which is offset fromsys_days{y_/m_/last}
by the number of daysd_
is offset fromsys_days{y_/m_/last}.day()
. Otherwise the value returned is unspecified.
[2019-06-08; Priority to 0 and Status to Tentatively Ready after six positive votes on the reflector.]
Proposed resolution:
This wording is relative to N4810.
Modify 30.8.14.2 [time.cal.ymd.members] as indicated:
constexpr operator sys_days() const noexcept;-18- Returns: If
ok()
, returns asys_days
holding a count of days from thesys_days
epoch to*this
(a negative value if*this
represents a date prior to thesys_days
epoch). Otherwise, ify_.ok() && m_.ok()
istrue
, returnssys_days{y_/m_/1d} + (d_ - 1d)
a. Otherwise the value returned is unspecified.sys_days
which is offset fromsys_days{y_/m_/last}
by the number of daysd_
is offset fromsys_days{y_/m_/last}.day()
Boolean
's expression requirements are ordered inconsistentlySection: 24.7 [iterator.range] Status: C++20 Submitter: Casey Carter Opened: 2019-06-06 Last modified: 2021-02-25
Priority: 0
View other active issues in [iterator.range].
View all other issues in [iterator.range].
View all issues with C++20 status.
Discussion:
For consistency of presentation, we should group and order the
&&
and ||
expression requirements similarly to the
==
and !=
expression requirements. Note that the suggested
change is not quite editorial: evaluation of requirements for satisfaction has
short-circuiting behavior, so the declaration order of requirements is
normatively significant in general.
[2019-06-13; Priority to 0 and Status to Tentatively Ready after seven positive votes on the reflector.]
Proposed resolution:
This wording is relative to N4810.
Modify [concept.boolean] as indicated:
[…] { b1 } -> ConvertibleTo<bool>; { !b1 } -> ConvertibleTo<bool>;{ b1 && a } -> Same<bool>;{ b1 || a } -> Same<bool>;{ b1 && b2 } -> Same<bool>; { b1 && a } -> Same<bool>; { a && b2 } -> Same<bool>; { b1 || b2 } -> Same<bool>; { b1 || a } -> Same<bool>; { a || b2 } -> Same<bool>; { b1 == b2 } -> ConvertibleTo<bool>; { b1 == a } -> ConvertibleTo<bool>; { a == b2 } -> ConvertibleTo<bool>; { b1 != b2 } -> ConvertibleTo<bool>; { b1 != a } -> ConvertibleTo<bool>; { a != b2 } -> ConvertibleTo<bool>; };
year::ok()
returns clause is ill-formedSection: 30.8.5.2 [time.cal.year.members] Status: C++20 Submitter: Tomasz Kamiński Opened: 2019-05-28 Last modified: 2021-02-25
Priority: 0
View all issues with C++20 status.
Discussion:
The expression in the Returns element of year::ok
in [time.cal.year.members] p18:
min() <= y_ && y_ <= max()
is ill-formed, as it attempts to compare type short
(type of y_
member) with type year
(type of year::min()
and year::max()
).
[2019-06-13; Priority to 0 and Status to Tentatively Ready after seven positive votes on the reflector.]
Proposed resolution:
This wording is relative to N4810.
Modify 30.8.5.2 [time.cal.year.members] as indicated:
constexpr bool ok() const noexcept;-18- Returns:
min().y_ <= y_ && y_ <= max().y_
.
std::tuple<>
should be trivially constructibleSection: 22.4.4.2 [tuple.cnstr] Status: C++23 Submitter: Louis Dionne Opened: 2019-05-29 Last modified: 2023-11-22
Priority: 3
View other active issues in [tuple.cnstr].
View all other issues in [tuple.cnstr].
View all issues with C++23 status.
Discussion:
That requirement is really easy to enforce, and it has been requested by users (e.g. libc++ bug 41714).
Previous resolution [SUPERSEDED]:This wording is relative to N4810.
Modify 22.4.4.2 [tuple.cnstr] as indicated:
-4- If
is_trivially_destructible_v<Ti>
istrue
for allTi
, then the destructor oftuple
is trivial. The default constructor oftuple<>
is trivial.
[2020-02-13, Prague]
LWG discussion revealed that all where happy that we want this, except that the new wording should become a separate paragraph.
[2020-11-09 Approved In November virtual meeting. Status changed: Ready → WP.]
Proposed resolution:
This wording is relative to N4849.
Modify 22.4.4.2 [tuple.cnstr] as indicated:
-4- If
-?- The default constructor ofis_trivially_destructible_v<Ti>
istrue
for allTi
, then the destructor oftuple
is trivial.tuple<>
is trivial.
tuple_element_t<1, const span<int, 42>>
is const int
Section: 99 [span.tuple] Status: Resolved Submitter: Tim Song Opened: 2019-05-31 Last modified: 2020-09-06
Priority: 2
View all issues with Resolved status.
Discussion:
As currently specified, it uses the cv-qualified partial specialization, which incorrectly adds cv-qualification to the element type.
Previous resolution [SUPERSEDED]:This wording is relative to N4810.
Modify 23.7.2.1 [span.syn], header
<span>
synopsis, as indicated:[…] // 99 [span.tuple], tuple interface template<class T> class tuple_size; template<size_t I, class T> class tuple_element; template<class ElementType, size_t Extent> struct tuple_size<span<ElementType, Extent>>; template<class ElementType> struct tuple_size<span<ElementType, dynamic_extent>>; // not defined template<size_t I, class ElementType, size_t Extent> struct tuple_element<I, span<ElementType, Extent>>; template<size_t I, class ElementType, size_t Extent> struct tuple_element<I, const span<ElementType, Extent>>; template<size_t I, class ElementType, size_t Extent> struct tuple_element<I, volatile span<ElementType, Extent>>; template<size_t I, class ElementType, size_t Extent> struct tuple_element<I, const volatile span<ElementType, Extent>>; […]Modify 99 [span.tuple] before p1 as indicated:
[Drafting note: The representation style for the newly added wording follows the new style for
tuple_element<I, span<ElementType, Extent>>::type
, which had recently been changed editorially.]template<class ElementType, size_t Extent> struct tuple_size<span<ElementType, Extent>> : integral_constant<size_t, Extent> { }; tuple_element<I, span<ElementType, Extent>>::type template<size_t I, class ElementType, size_t Extent> struct tuple_element<I, const span<ElementType, Extent>> { using type = ElementType; }; template<size_t I, class ElementType, size_t Extent> struct tuple_element<I, volatile span<ElementType, Extent>> { using type = ElementType; }; template<size_t I, class ElementType, size_t Extent> struct tuple_element<I, const volatile span<ElementType, Extent>> { using type = ElementType; };-1- Mandates:
Extent != dynamic_extent && I < Extent
istrue
.
[2020-02-13, Prague]
Wording needs to be adjusted, because we accepted P1831R1.
Previous resolution [SUPERSEDED]:This wording is relative to N4849.
Modify 23.7.2.1 [span.syn], header
<span>
synopsis, as indicated:[…] // 99 [span.tuple], tuple interface template<class T> class tuple_size; template<size_t I, class T> class tuple_element; template<class ElementType, size_t Extent> struct tuple_size<span<ElementType, Extent>>; template<class ElementType> struct tuple_size<span<ElementType, dynamic_extent>>; // not defined template<size_t I, class ElementType, size_t Extent> struct tuple_element<I, span<ElementType, Extent>>; template<size_t I, class ElementType, size_t Extent> struct tuple_element<I, const span<ElementType, Extent>>; […]Modify 99 [span.tuple] before p1 as indicated:
template<class ElementType, size_t Extent> struct tuple_size<span<ElementType, Extent>> : integral_constant<size_t, Extent> { }; template<size_t I, class ElementType, size_t Extent> struct tuple_element<I, span<ElementType, Extent>> { using type = ElementType; }; template<size_t I, class ElementType, size_t Extent> struct tuple_element<I, const span<ElementType, Extent>> { using type = ElementType; };-1- Mandates:
Extent != dynamic_extent && I < Extent
istrue
.Add the following wording to Annex D:
D.? Deprecated
1 The headerspan
tuple interface [depr.span.syn]<span>
(23.7.2.1 [span.syn]) has the following additions:namespace std { template<size_t I, class ElementType, size_t Extent> struct tuple_element<I, volatile span<ElementType, Extent>>; template<size_t I, class ElementType, size_t Extent> struct tuple_element<I, const volatile span<ElementType, Extent>>; }D.1.? tuple interface [depr.span.tuple]
template<size_t I, class ElementType, size_t Extent> struct tuple_element<I, volatile span<ElementType, Extent>> { using type = ElementType; }; template<size_t I, class ElementType, size_t Extent> struct tuple_element<I, const volatile span<ElementType, Extent>> { using type = ElementType; };-?- Mandates:
Extent != dynamic_extent && I < Extent
istrue
.
[2020-02-14, Prague]
Lengthy discussion about the intended design of span
's tuple interface. Has been send to LEWG, who
made several polls and concluded to remove the tuple interface for span
for C++20.
[2020-05-01; reflector discussions]
Resolved by P2116R0 voted in in Prague.
Proposed resolution:
Resolved by P2116R0.
for_each_n
and copy_n
missing requirements for Size
Section: 26.6.5 [alg.foreach], 26.7.1 [alg.copy] Status: Resolved Submitter: Jonathan Wakely Opened: 2019-05-31 Last modified: 2020-11-09
Priority: 3
View other active issues in [alg.foreach].
View all other issues in [alg.foreach].
View all issues with Resolved status.
Discussion:
search_n
and fill_n
and generate_n
require that "The type Size
shall be
convertible to integral type", but for_each_n
and copy_n
have no requirements on
Size
. Presumably it should be convertible to an integral type.
[2019-07 Issue Prioritization]
Priority to 3 after discussion on the reflector.
Previous resolution [SUPERSEDED]:This wording is relative to N4810.
[Drafting note: Clause [algorithms] has not yet gone through a "Mandating" cleanup of our new wording style for requirements expressed in Requires and Expects elements. The below wording changes perform this fix for the touched paragraphs and also replaces here Requires by Expects to prevent papers that address such wording changes to keep tracking of additional wording changes caused by proposed wording of issues.]
Modify 26.6.5 [alg.foreach] as indicated:
template<class InputIterator, class Size, class Function> constexpr InputIterator for_each_n(InputIterator first, Size n, Function f);-16- Requires: shall satisfy the Cpp17MoveConstructible requirements […]
-17-RequiresExpects: The typeSize
is convertible to integral type (7.3.9 [conv.integral], 11.4.8 [class.conv]).n >= 0
. […]template<class ExecutionPolicy, class ForwardIterator, class Size, class Function> ForwardIterator for_each_n(ExecutionPolicy&& exec, ForwardIterator first, Size n, Function f);-21- Requires: shall satisfy the Cpp17CopyConstructible requirements […]
-22-RequiresExpects: The typeSize
is convertible to integral type (7.3.9 [conv.integral], 11.4.8 [class.conv]).n >= 0
. […]Modify 26.7.1 [alg.copy] as indicated:
template<class InputIterator, class Size, class OutputIterator> constexpr OutputIterator copy_n(InputIterator first, Size n, OutputIterator result); template<class ExecutionPolicy, class ForwardIterator1, class Size, class ForwardIterator2> ForwardIterator2 copy_n(ExecutionPolicy&& exec, ForwardIterator1 first, Size n, ForwardIterator2 result); template<InputIterator I, WeaklyIncrementable O> requires IndirectlyCopyable<I, O> constexpr ranges::copy_n_result<I, O> ranges::copy_n(I first, iter_difference_t<I> n, O result);-10- Let
-?- Expects: The typeM
bemax(n, 0)
.Size
is convertible to integral type (7.3.9 [conv.integral], 11.4.8 [class.conv]). […]
[2020-05-01; Reflector discussions]
There was consensus that this issue has been resolved by P1718R2, voted in in Belfast 2019.
[2020-11-09 Resolved for C++20. Status changed: Tentatively Resolved → Resolved.]
Proposed resolution:
Resolved by P1718R2
construct
/destroy
in allocate_shared
Section: 20.3.2.2.7 [util.smartptr.shared.create] Status: WP Submitter: Billy O'Neal III Opened: 2019-06-11 Last modified: 2024-11-28
Priority: 3
View all other issues in [util.smartptr.shared.create].
View all issues with WP status.
Discussion:
The new allocate_shared
wording says we need to rebind the allocator back to T
's
type before we can call construct
or destroy
, but this is suboptimal (might make
extra unnecessary allocator copies), and is inconsistent with the containers' behavior, which call
allocator construct
on whatever T
they want. (For example,
std::list<T, alloc<T>>
rebinds to alloc<_ListNode<T>>
,
but calls construct(T*)
without rebinding back)
[2019-07 Issue Prioritization]
Priority to 3 after discussion on the reflector.
Previous resolution [SUPERSEDED]:
This wording is relative to N4810.
Modify 20.3.2.2.7 [util.smartptr.shared.create] as indicated:
[Drafting note: The edits to change
pv
topu
were suggested by Jonathan Wakely (thanks!). This wording also has theremove_cv_t
fixes specified by LWG 3210(i) — if that change is rejected some of those have to be stripped here.]template<class T, ...> shared_ptr<T> make_shared(args); template<class T, class A, ...> shared_ptr<T> allocate_shared(const A& a, args); template<class T, ...> shared_ptr<T> make_shared_default_init(args); template<class T, class A, ...> shared_ptr<T> allocate_shared_default_init(const A& a, args);-2- Requires: […]
[…] -7- Remarks:
(7.1) — […]
[…]
(7.5) — When a (sub)object of a non-array type
U
is specified to have an initial value ofv
, orU(l...)
, wherel...
is a list of constructor arguments,allocate_shared
shall initialize this (sub)object via the expression
(7.5.1) —
allocator_traits<A2>::construct(a2, p
orvu, v)(7.5.2) —
allocator_traits<A2>::construct(a2, p
vu, l...)respectively, where
p
is a pointer of typevuremove_cv_t<U>*
pointsing to storage suitable to hold an object of typeremove_cv_t<U>
anda2
of typeA2
is a potentially rebound copy of the allocatora
passed toallocate_shared
such that its.value_type
isremove_cv_t<U>
(7.6) — […]
(7.7) — When a (sub)object of non-array type
U
is specified to have a default initial value,allocate_shared
shallinitializes this (sub)object via the expressionallocator_traits<A2>::construct(a2, p
, wherevu)p
is a pointer of typevuremove_cv_t<U>*
pointsing to storage suitable to hold an object of typeremove_cv_t<U>
anda2
of typeA2
is a potentially rebound copy of the allocatora
passed toallocate_shared
such that its.value_type
isremove_cv_t<U>
[…]
(7.12) — When a (sub)object of non-array type
U
that was initialized byallocate_shared
is to be destroyed, it is destroyed via the expressionallocator_traits<A2>::destroy(a2, p
wherevu)p
is a pointer of typevuremove_cv_t<U>*
pointsing to that object of typeremove_cv_t<U>
anda2
of typeA2
is a potentially rebound copy of the allocatora
passed toallocate_shared
such that its.value_type
isremove_cv_t<U>
[2024-08-23; Jonathan provides updated wording]
make_shared_default_init
and allocate_shared_default_init
were renamed
by P1973R1 so this needs a rebase.
The edit to (7.11) is just for consistency, so that pv
is always void*
and pu
is remove_cv_t<U>*
.
Accepting this proposed resolution would also resolve issue 3210(i).
[2024-10-02; Reflector poll]
Set status to Tentatively Ready after six votes in favour during reflector poll.
[Wrocław 2024-11-23; Status changed: Voting → WP.]
Proposed resolution:
This wording is relative to N4988.
Modify 20.3.2.2.7 [util.smartptr.shared.create] as indicated:
template<class T, ...> shared_ptr<T> make_shared(args); template<class T, class A, ...> shared_ptr<T> allocate_shared(const A& a, args); template<class T, ...> shared_ptr<T> make_shared_for_overwrite(args); template<class T, class A, ...> shared_ptr<T> allocate_shared_for_overwrite(const A& a, args);-2- Preconditions: […]
[…] -7- Remarks:
(7.1) — […]
[…]
(7.5) — When a (sub)object of a non-array type
U
is specified to have an initial value ofv
, orU(l...)
, wherel...
is a list of constructor arguments,allocate_shared
shall initialize this (sub)object via the expression
(7.5.1) —
allocator_traits<A2>::construct(a2, p
orvu, v)(7.5.2) —
allocator_traits<A2>::construct(a2, p
vu, l...)respectively, where
p
is a pointer of typevuremove_cv_t<U>*
pointsing to storage suitable to hold an object of typeremove_cv_t<U>
anda2
of typeA2
is a potentially rebound copy of the allocatora
passed toallocate_shared
such that its.value_type
isremove_cv_t<U>
(7.6) — […]
(7.7) — When a (sub)object of non-array type
U
is specified to have a default initial value,allocate_shared
shallinitializes this (sub)object via the expressionallocator_traits<A2>::construct(a2, p
, wherevu)p
is a pointer of typevuremove_cv_t<U>*
pointsing to storage suitable to hold an object of typeremove_cv_t<U>
anda2
of typeA2
is a potentially rebound copy of the allocatora
passed toallocate_shared
such that its.value_type
isremove_cv_t<U>
[…]
[Drafting note: Issue 4024(i) will add
make_shared_for_overwrite
andallocate_shared_for_overwrite
to (7.11) but that doesn't conflict with this next edit.](7.11) — When a (sub)object of non-array type
U
that was initialized bymake_shared
is to be destroyed, it is destroyed via the expressionp
wherevu->~U()p
points to that object of typevuU
.(7.12) — When a (sub)object of non-array type
U
that was initialized byallocate_shared
is to be destroyed, it is destroyed via the expressionallocator_traits<A2>::destroy(a2, p
wherevu)p
is a pointer of typevuremove_cv_t<U>*
pointsing to that object of typeremove_cv_t<U>
anda2
of typeA2
is a potentially rebound copy of the allocatora
passed toallocate_shared
such that its.value_type
isremove_cv_t<U>
%d
parse flag does not match POSIX and format
specificationSection: 30.13 [time.parse] Status: C++20 Submitter: Tomasz Kamiński Opened: 2019-06-13 Last modified: 2021-02-25
Priority: 0
View other active issues in [time.parse].
View all other issues in [time.parse].
View all issues with C++20 status.
Discussion:
Currently, the '%d
' parse flags accepts the 'E
' modifier to parse the
locale's alternative representation, see Table 88 —
"Meaning of parse
flags":
The modified command
%Ed
interprets the locale's alternative representation of the day of the month.
This is inconsistent with the
POSIX
strftime
specification and the format
functions, that uses 'O
'
to output alternate locale representation. Per Table 87 —
"Meaning of format
conversion specifiers":
The modified command
%Od
produces the locale's alternative representation.
[2019-06-24; Howard comments]
This was simply a type-o in my documentation that infected the proposal and subsequently the C++ working draft.
None ofstd::time_put
, C's strftime
, or POSIX's strftime
support
%Ed
but all support %Od
. Furthermore the existing example implementation
supports %Od
but not %Ed
. And all the existing example implementation
does is forward to std::time_put
.
[2019-07 Issue Prioritization]
Status to Tentatively Ready after five positive votes on the reflector.
Proposed resolution:
This wording is relative to N4810.
Modify 30.13 [time.parse], Table 88 — "Meaning of parse
flags",
as indicated:
Table 88 — Meaning of parse
flagsFlag Parsed value […]
%d
The day of the month as a decimal number. The modified command %Nd
specifies the maximum number of characters to read. IfN
is not specified, the default is 2. Leading zeroes are permitted but not required. The modified command%
interprets the locale's alternative representation of the day of the month.EOd[…]
year_month
arithmetic with months
is ambiguousSection: 30.8.13.3 [time.cal.ym.nonmembers] Status: C++20 Submitter: Tomasz Kamiński Opened: 2019-06-16 Last modified: 2021-02-25
Priority: 0
View all issues with C++20 status.
Discussion:
The current specification of the addition of year_month
and
months
does not define a unique result value.
year(2019)/month(1)
and year(2018)/month(13)
are valid results of year(2018)/month(12) + months(1)
addition,
according to the spec in 30.8.13.3 [time.cal.ym.nonmembers].
[2019-06-24; LWG discussion]
During discussions on the LWG reflector there was a preference to add "is true
" at the
end of the modified Returns: element. This additional edit has been applied to Tomasz'
original wording below.
[2019-07 Issue Prioritization]
Status to Tentatively Ready after five positive votes on the reflector.
Proposed resolution:
This wording is relative to N4810.
Modify 30.8.13.3 [time.cal.ym.nonmembers] as indicated:
constexpr year_month operator+(const year_month& ym, const months& dm) noexcept;-3- Returns: A
Complexity:year_month
valuez
such thatz.ok() && z - ym == dm
istrue
.𝒪(1)
with respect to the value ofdm
.
Section: 26.10.9 [inclusive.scan], 26.10.11 [transform.inclusive.scan] Status: C++20 Submitter: Jonathan Wakely Opened: 2019-06-18 Last modified: 2021-02-25
Priority: 0
View all issues with C++20 status.
Discussion:
after applying P0574R1 to the working draft,
26.10.11 [transform.inclusive.scan] bullet 1.1 says "If init
is provided, […];
otherwise, ForwardIterator1
's value type shall be […]."
26.10.11 [transform.inclusive.scan] bullet 1.2 says "If init
is provided, […];
otherwise, […] shall be convertible to ForwardIterator1
's value type."
init
is not provided, but there is no ForwardIterator1
, so
these requirements cannot be met. The requirements for the first overload need to be stated in terms
of InputIterator
's value type, not ForwardIterator1
's value type.
The same problem exists in 26.10.9 [inclusive.scan]. 26.10.12 [adjacent.difference] solves
this problem by saying "Let T
be the value type of decltype(first)
".
[2019-07 Issue Prioritization]
Status to Tentatively Ready after five positive votes on the reflector.
Proposed resolution:
This wording is relative to N4820.
Modify 26.10.9 [inclusive.scan] as indicated:
template<class InputIterator, class OutputIterator, class BinaryOperation> OutputIterator inclusive_scan(InputIterator first, InputIterator last, OutputIterator result, BinaryOperation binary_op); template<class ExecutionPolicy, class ForwardIterator1, class ForwardIterator2, class BinaryOperation> ForwardIterator2 inclusive_scan(ExecutionPolicy&& exec, ForwardIterator1 first, ForwardIterator1 last, ForwardIterator2 result, BinaryOperation binary_op); template<class InputIterator, class OutputIterator, class BinaryOperation, class T> OutputIterator inclusive_scan(InputIterator first, InputIterator last, OutputIterator result, BinaryOperation binary_op, T init); template<class ExecutionPolicy, class ForwardIterator1, class ForwardIterator2, class BinaryOperation, class T> ForwardIterator2 inclusive_scan(ExecutionPolicy&& exec, ForwardIterator1 first, ForwardIterator1 last, ForwardIterator2 result, BinaryOperation binary_op, T init);-?- Let
-3- Requires:U
be the value type ofdecltype(first)
.
(3.1) — If
init
is provided,T
shall be Cpp17MoveConstructible (Table 26); otherwise,ForwardIterator1
's value typeU
shall be Cpp17MoveConstructible.(3.2) — If
init
is provided, all ofbinary_op(init, init)
,binary_op(init, *first)
, andbinary_op(*first, *first)
shall be convertible toT
; otherwise,binary_op(*first, *first)
shall be convertible toForwardIterator1
's value typeU
.(3.3) —
binary_op
shall neither invalidate iterators or subranges, nor modify elements in the ranges[first, last]
or[result, result + (last - first)]
.
Modify 26.10.11 [transform.inclusive.scan] as indicated:
template<class InputIterator, class OutputIterator, class BinaryOperation, class UnaryOperation> OutputIterator transform_inclusive_scan(InputIterator first, InputIterator last, OutputIterator result, BinaryOperation binary_op, UnaryOperation unary_op); template<class ExecutionPolicy, class ForwardIterator1, class ForwardIterator2, class BinaryOperation, class UnaryOperation> ForwardIterator2 transform_inclusive_scan(ExecutionPolicy&& exec, ForwardIterator1 first, ForwardIterator1 last, ForwardIterator2 result, BinaryOperation binary_op, UnaryOperation unary_op); template<class InputIterator, class OutputIterator, class BinaryOperation, class UnaryOperation, class T> OutputIterator transform_inclusive_scan(InputIterator first, InputIterator last, OutputIterator result, BinaryOperation binary_op, UnaryOperation unary_op, T init); template<class ExecutionPolicy, class ForwardIterator1, class ForwardIterator2, class BinaryOperation, class UnaryOperation, class T> ForwardIterator2 transform_inclusive_scan(ExecutionPolicy&& exec, ForwardIterator1 first, ForwardIterator1 last, ForwardIterator2 result, BinaryOperation binary_op, UnaryOperation unary_op, T init);-?- Let
-1- Requires:U
be the value type ofdecltype(first)
.
(1.1) — If
init
is provided,T
shall be Cpp17MoveConstructible (Table 26); otherwise,ForwardIterator1
's value typeU
shall be Cpp17MoveConstructible.(1.2) — If
init
is provided, all of
(1.2.1) —
binary_op(init, init)
,(1.2.2) —
binary_op(init, unary_op(*first))
, and(1.2.3) —
binary_op(unary_op(*first), unary_op(*first))
shall be convertible to
T
; otherwise,binary_op(unary_op(*first), unary_op(*first))
shall be convertible toForwardIterator1
's value typeU
.(1.3) — Neither
unary_op
norbinary_op
shall invalidate iterators or subranges, nor modify elements in the ranges[first, last]
or[result, result + (last - first)]
.
lerp
should not add the "sufficient additional overloads"Section: 29.7.1 [cmath.syn] Status: Resolved Submitter: Billy O'Neal III Opened: 2019-06-20 Last modified: 2023-02-07
Priority: 2
View other active issues in [cmath.syn].
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Discussion:
It seems that lerp
can't give a sensible answer for integer inputs. It's new machinery,
not the legacy C stuff extended with extra overloads.
lerp
in
29.7.1 [cmath.syn] p2.
[2019-07 Issue Prioritization]
Priority to 2 after discussion on the reflector.
[2023-02-07 Status changed: New → Resolved.]
Resolved by the application of P1467R9.
It's now clear that lerp
should have
"sufficient additional overloads".
Proposed resolution:
zoned_time
constructor from TimeZonePtr
does not specify initialization of
tp_
Section: 30.11.7.2 [time.zone.zonedtime.ctor] Status: C++20 Submitter: Tomasz Kamiński Opened: 2019-06-20 Last modified: 2021-02-25
Priority: 0
View all other issues in [time.zone.zonedtime.ctor].
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Discussion:
The zoned_time(TimeZonePtr z)
does not specify how the tp_
sub-element is initialized. It should be consistent with zoned_time(string_view)
that default initializes tp_
.
[2019-07 Issue Prioritization]
Status to Tentatively Ready after five positive votes on the reflector.
Proposed resolution:
This wording is relative to N4820.
Modify 30.11.7.2 [time.zone.zonedtime.ctor] as indicated:
explicit zoned_time(TimeZonePtr z);-5- Requires:
-6- Effects: Constructs az
refers to a time zone.zoned_time
by initializingzone_
withstd::move(z)
and default constructingtp_
.
zoned_time
converting constructor shall not be noexcept
Section: 30.11.7.2 [time.zone.zonedtime.ctor] Status: C++20 Submitter: Tomasz Kamiński Opened: 2019-06-20 Last modified: 2021-02-25
Priority: 0
View all other issues in [time.zone.zonedtime.ctor].
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Discussion:
The zoned_time
constructor from zoned_time<Duration2,
TimeZonePtr>
(preserving same time zone, different precision of
representation) is currently marked noexcept
. Given that the exposition-only
member tp_
of type sys_time<duration>
has essentially type
time_point<system_clock, Duration>
, this is incompatible with the
invoked time_point
constructor, which is not marked as noexcept
.
[2019-07 Issue Prioritization]
Status to Tentatively Ready after five positive votes on the reflector.
Proposed resolution:
This wording is relative to N4820.
Modify 30.11.7.1 [time.zone.zonedtime.overview], class template zoned_time
synopsis,
as indicated:
template<class Duration2> zoned_time(const zoned_time<Duration2, TimeZonePtr>& zt)noexcept;
Modify 30.11.7.2 [time.zone.zonedtime.ctor] as indicated:
template<class Duration2> zoned_time(const zoned_time<Duration2, TimeZonePtr>& y)noexcept;-9- Requires: Does not participate in overload resolution unless
-10- Effects: Constructs asys_time<Duration2>
is implicitly convertible tosys_time<Duration>
.zoned_time
by initializingzone_
withy.zone_
andtp_
withy.tp_
.
zoned_time
constructor from string_view
should accept
zoned_time<Duration2, TimeZonePtr2>
Section: 30.11.7.2 [time.zone.zonedtime.ctor] Status: C++20 Submitter: Tomasz Kamiński Opened: 2019-06-23 Last modified: 2021-02-25
Priority: 2
View all other issues in [time.zone.zonedtime.ctor].
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Discussion:
zoned_time
constructors with string_view
and
another zoned_time
are not accepting zoned_time
instances that use a different time zone representation (TimeZonePtr
parameter):
zoned_time(string_view name, const zoned_time<Duration>& zt); zoned_time(string_view name, const zoned_time<Duration>& zt, choose);
This makes them inconsistent with the constructors from TimeZonePtr
and zoned_time
, that they delegate to:
template<class Duration2, class TimeZonePtr2> zoned_time(TimeZonePtr z, const zoned_time<Duration2, TimeZonePtr2>& zt); template<class Duration2, class TimeZonePtr2> zoned_time(TimeZonePtr z, const zoned_time<Duration2, TimeZonePtr2>& zt, choose);
Furthermore it forces the creation of a temporary zoned_time
object in
case when the source uses the same TimeZonePtr
, but different Duration
.
This wording is relative to N4820.
Modify 30.11.7.1 [time.zone.zonedtime.overview], class template
zoned_time
synopsis, as indicated:template<class Duration2, class TimeZonePtr2> zoned_time(string_view name, const zoned_time<Duration2, TimeZonePtr2>& zt); template<class Duration2, class TimeZonePtr2> zoned_time(string_view name, const zoned_time<Duration2, TimeZonePtr2>& zt, choose);Modify 30.11.7.2 [time.zone.zonedtime.ctor] as indicated:
template<class Duration2, class TimeZonePtr2> zoned_time(string_view name, const zoned_time<Duration2, TimeZonePtr2>& y);-32- Remarks: This constructor does not participate in overload resolution unless
-33- Effects: Equivalent to construction withzoned_time
is constructible from the return type oftraits::locate_zone(name)
andzoned_time<Duration2, TimeZonePtr2>
.{traits::locate_zone(name), y}
.template<class Duration2, class TimeZonePtr2> zoned_time(string_view name, const zoned_time<Duration2, TimeZonePtr2>& y, choose c););-34- Remarks: This constructor does not participate in overload resolution unless
-35- Effects: Equivalent to construction withzoned_time
is constructible from the return type oftraits::locate_zone(name)
,zoned_time<Duration2, TimeZonePtr2>
, andchoose
.{traits::locate_zone(name), y, c}
. -36- [Note: Thechoose
parameter has no effect. — end note]
[2020-02-13, Prague]
During LWG discussions it was suggested to rebase the wording to reduce the chances for confusion.
[2020-02 Status to Immediate on Thursday morning in Prague.]
Proposed resolution:
This wording is relative to N4849.
Modify 30.11.7.1 [time.zone.zonedtime.overview], class template zoned_time
synopsis,
as indicated:
template<class Duration2, class TimeZonePtr2> zoned_time(string_view name, const zoned_time<Duration2, TimeZonePtr2>& zt); template<class Duration2, class TimeZonePtr2> zoned_time(string_view name, const zoned_time<Duration2, TimeZonePtr2>& zt, choose);
Modify 30.11.7.2 [time.zone.zonedtime.ctor] as indicated:
template<class Duration2, class TimeZonePtr2> zoned_time(string_view name, const zoned_time<Duration2, TimeZonePtr2>& y);-32- Constraints:
-33- Effects: Equivalent to construction withzoned_time
is constructible from the return type oftraits::locate_zone(name)
andzoned_time<Duration2, TimeZonePtr2>
.{traits::locate_zone(name), y}
.template<class Duration2, class TimeZonePtr2> zoned_time(string_view name, const zoned_time<Duration2, TimeZonePtr2>& y, choose c););-34- Constraints:
-35- Effects: Equivalent to construction withzoned_time
is constructible from the return type oftraits::locate_zone(name)
,zoned_time<Duration2, TimeZonePtr2>
, andchoose
.{traits::locate_zone(name), y, c}
. -36- [Note: Thechoose
parameter has no effect. — end note]
variant
constructionSection: 22.6.3.2 [variant.ctor] Status: Resolved Submitter: Barry Revzin Opened: 2019-06-25 Last modified: 2020-11-09
Priority: 2
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Discussion:
User mcencora on reddit today posted this example:
#include <variant> struct ConvertibleToBool { constexpr operator bool() const { return true; } }; static_assert(std::holds_alternative<bool>(std::variant<int, bool>(ConvertibleToBool{})));
Before P0608, the variant
holds bool
. After P0608,
the variant
holds int
so the static assertion fires.
bool
and (c)
hold int
is, but I think (a) and (b) are better options than (c).
[2019-07 Issue Prioritization]
Priority to 2 after discussion on the reflector.
[This is US212; status set to "LEWG" for guidance on desired behavior.]
[2020-05-28; LEWG issue reviewing]
P1957R2 was accepted in Prague as CWG motion 5 and resolves LWG 3228.
[2020-11-09 Resolved for C++20. Status changed: Tentatively Resolved → Resolved.]
Rationale:
Resolved by P1957R2.Proposed resolution:
%y/%Y
is missing locale alternative versionsSection: 30.13 [time.parse] Status: C++20 Submitter: Tomasz Kamiński Opened: 2019-06-29 Last modified: 2021-02-25
Priority: 0
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Discussion:
The year format specifier ('y', 'Y')
are missing the locale alternative
version ('%EY', '%Ey' and '%Oy')
. That makes it inconsistent with the
POSIX
strftime
specification:
%Ey
Replaced by the offset from %EC
(year only) in the locale's
alternative representation.
%EY
Replaced by the full alternative year representation.
%Oy
Replaced by the year (offset from %C
) using the locale's
alternative numeric symbols.
and parse
specifiers 30.13 [time.parse] that accepts these modified command.
[2019-07 Issue Prioritization]
Status to Tentatively Ready after five positive votes on the reflector.
Proposed resolution:
This wording is relative to N4820.
[Drafting note: For the
'%Oy'
specifier we preserve consistency with the current specification for'%Od'
and'%Oe'
from Table 87 "Meaning offormat
conversion specifier" [tab:time.format.spec]:
%d
[…] The modified command%Od
produces the locale's alternative representation.
%e
[…] The modified command%Oe
produces the locale's alternative representation.as their corresponding POSIX specification is matching one for
'%Oy'
:
%Od
Replaced by the day of the month, using the locale's alternative numeric symbols, filled as needed with leading zeros if there is any alternative symbol for zero; otherwise, with leading<space>
characters.
%Oe
Replaced by the day of the month, using the locale's alternative numeric symbols, filled as needed with leading<space>
characters.]
Modify "Table 87 — Meaning of format
conversion specifiers"
[tab:time.format.spec] as indicated:
Table 87 — Meaning of format
conversion specifiers [tab:time.format.spec]Specifier Replacement […]
%y
The last two decimal digits of the year. If the result is a single digit it is prefixed by 0
. The modified command%Oy
produces the locale's alternative representation. The modified command%Ey
produces the locale's alternative representation of offset from%EC
(year only).%Y
The year as a decimal number. If the result is less than four digits it is left-padded with 0
to four digits. The modified command%EY
produces the locale's alternative full year representation.[…]
year_month_day_last::day
specification does not cover !ok()
valuesSection: 30.8.15.2 [time.cal.ymdlast.members] Status: C++20 Submitter: Tomasz Kamiński Opened: 2019-06-29 Last modified: 2021-02-25
Priority: 0
View all issues with C++20 status.
Discussion:
The current specification of the year_month_day_last::day
function does not cover the behaviour in the situation when year_month_day_last
value is not ok()
. To illustrate the sentence from
30.8.15.2 [time.cal.ymdlast.members] p13:
A
day
representing the last day of the (year
,month
) pair represented by*this
.
is unclear in the situation when month
member has
!ok
value, e.g. what is last day of 14th month of 2019.
ymdl.day()
(and by
consequence conversion to sys_days
/local_days
)
unspecified if ymdl.ok()
is false
. This make is
consistent with rest of the library, that produces unspecified values in
similiar situation, e.g.: 30.8.14.2 [time.cal.ymd.members] p18,
30.8.16.2 [time.cal.ymwd.members] p19,
30.8.17.2 [time.cal.ymwdlast.members] p14.
[2019-07 Issue Prioritization]
Status to Tentatively Ready after five positive votes on the reflector.
Proposed resolution:
This wording is relative to N4820.
Modify 30.8.15.2 [time.cal.ymdlast.members] as indicated:
constexpr chrono::day day() const noexcept;-13- Returns: If
-14- [Note: This value may be computed on demand. — end note]ok()
istrue
, returns aAday
representing the last day of the (year, month
) pair represented by*this
. Otherwise the returned value is unspecified.
zoned_time
deduction guidesSection: 30.11.7.1 [time.zone.zonedtime.overview] Status: C++20 Submitter: Tomasz Kamiński Opened: 2019-06-30 Last modified: 2021-02-25
Priority: 0
View all other issues in [time.zone.zonedtime.overview].
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Discussion:
Currently, the time_zone
is always storing the time with
the precision not coarser that seconds
, as consequence any
instance with the duration with coarser precision with seconds
,
is de-facto equivalent to one instantiated to the duration of seconds.
This is caused by the fact, that time zone offset can be defined up to
seconds precision. To illustrate both of following specialization has
the same behavior (keep seconds
):
zoned_time<minutes> zt(current_zone(), floor<minutes>(system_clock::now()); zoned_time<hours> zt(current_zone(), floor<hours>(system_clock::now()); zoned_time<seconds> zt(current_zone(), floor<seconds>(system_clock::now());
To avoid unnecessary code bloat caused by above, most deduction guides
are instantiating zoned_time
with at least seconds
precision (i.e. zoned_time<seconds>
will be deduced
for all of above):
template<class TimeZonePtr, class Duration> zoned_time(TimeZonePtr, zoned_time<Duration>, choose = choose::earliest) -> zoned_time<common_type_t<Duration, seconds>, TimeZonePtr>;
However there is single exception:
template<class Duration, class TimeZonePtr, class TimeZonePtr2> zoned_time(TimeZonePtr, zoned_time<Duration, TimeZonePtr2>, choose = choose::earliest) -> zoned_time<Duration, TimeZonePtr>;
This deduction guide should be updated to preserve the consistency of design.
[2019-07 Issue Prioritization]
Status to Tentatively Ready after five positive votes on the reflector.
Proposed resolution:
This wording is relative to N4820.
Modify 30.11.7.1 [time.zone.zonedtime.overview] as indicated:
[…] template<class Duration, class TimeZonePtr, class TimeZonePtr2> zoned_time(TimeZonePtr, zoned_time<Duration, TimeZonePtr2>, choose = choose::earliest) -> zoned_time<common_type_t<Duration, seconds>, TimeZonePtr>; […]
shared_ptr
converting constructorsSection: 20.3.2.2.2 [util.smartptr.shared.const] Status: C++20 Submitter: Casey Carter Opened: 2019-07-10 Last modified: 2021-02-25
Priority: 0
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Discussion:
Issue 2875(i) added 20.3.2.2.2 [util.smartptr.shared.const] paragraph 13:
Remarks: Whenwhich pertains to the four constructor overloads:T
is an array type, this constructor shall not participate in overload resolution unlessis_move_constructible_v<D>
istrue
, the expressiond(p)
is well-formed, and eitherT
isU[N]
andY(*)[N]
is convertible toT*
, orT
isU[]
andY(*)[]
is convertible toT*
. WhenT
is not an array type, this constructor shall not participate in overload resolution unlessis_move_constructible_v<D>
istrue
, the expressiond(p)
is well-formed, andY*
is convertible toT*
.
which is fine (ignoring for now that two occurrences of "this constructor" should read "these constructors") for the two overloads with a template parametertemplate<class Y, class D> shared_ptr(Y* p, D d); template<class Y, class D, class A> shared_ptr(Y* p, D d, A a); template<class D> shared_ptr(nullptr_t p, D d); template<class D, class A> shared_ptr(nullptr_t p, D d, A a);
Y
, but not so much for the
two with no such template parameter.
MSVC ignores the constraints on Y
for the overloads with no such template parameter,
whereas libstdc++ and libc++ seem to ignore all of the constraints for those overloads (See
Compiler Explorer). We should fix the broken wording,
ideally by requiring the MSVC interpretation - the nullptr_t
constructors participate in
overload resolution only when is_movable_v<D>
is true
and d(p)
is
well-formed - so concepts and traits that check constructibility are correct.
[2019-11-17 Issue Prioritization]
Status to Tentatively Ready and priority to 0 after six positive votes on the reflector.
Proposed resolution:
This wording is relative to N4835.
Modify 20.3.2.2.2 [util.smartptr.shared.const] as indicated:
template<class Y, class D> shared_ptr(Y* p, D d); template<class Y, class D, class A> shared_ptr(Y* p, D d, A a); template<class D> shared_ptr(nullptr_t p, D d); template<class D, class A> shared_ptr(nullptr_t p, D d, A a);-?- Constraints:
is_move_constructible_v<D>
istrue
, andd(p)
is a well-formed expression. For the first two overloads:
If
T
is an array type, then eitherT
isU[N]
andY(*)[N]
is convertible toT*
, orT
isU[]
andY(*)[]
is convertible toT*
.If
T
is not an array type, thenY*
is convertible toT*
.-9-
RequiresExpects: Construction ofd
and a deleter of type […][…]
-13- Remarks: WhenT
is an array type, this constructor shall not participate in overload resolution unlessis_move_constructible_v<D>
istrue
, the expressiond(p)
is well-formed, and eitherT
isU[N]
andY(*)[N]
is convertible toT*
, orT
isU[]
andY(*)[]
is convertible toT*
. WhenT
is not an array type, this constructor shall not participate in overload resolution unlessis_move_constructible_v<D>
istrue
, the expressiond(p)
is well-formed, andY*
is convertible toT*
.
Section: 29.7.1 [cmath.syn] Status: Resolved Submitter: Casey Carter Opened: 2019-07-11 Last modified: 2023-02-07
Priority: 3
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Discussion:
The "sufficient additional overloads" wording in 29.7.1 [cmath.syn] paragraph 2 does not apply to the special math functions, since they are not "overloaded functions". The lack of "sufficient additional overloads" doesn't agree with N3060 (the final draft of ISO/IEC 29124 which standardized the mathematical special functions) [sf.cmath] paragraphs 3 and 4:
-3- Each of the functions specified above that has one or more
double
parameters (thedouble
version) shall have two additional overloads:
a version with each
double
parameter replaced with afloat
parameter (thefloat
version), anda version with each
double
parameter replaced with along double
parameter (thelong double
version).The return type of each such
-4- Moreover, eachfloat
version shall befloat
, and the return type of each suchlong double
version shall belong double
.double
version shall have sufficient additional overloads to determine which of the above three versions to actually call, by the following ordered set of rules:
First, if any argument corresponding to a
double
parameter in thedouble
version has typelong double
, thelong double
version is called.Otherwise, if any argument corresponding to a
double
parameter in thedouble
version has typedouble
or has an integer type, thedouble
version is called.Otherwise, the
float
version is called.
P226R1 "Mathematical Special Functions for C++17" notably
states: "At the level of and following [c.math], create a new subclause with heading and initial
content the same as IS 29124:2010's clause [sf.cmath], 'Additions to header <cmath>
,'
renumbering as appropriate to the new context." which suggests the change between 29124 and C++17
was an oversight and not intentional.
std::sph_neumann({}, {})
is
well-formed.
[2020-04-07 Issue Prioritization]
Priority to 3 after reflector discussion.
[2023-02-07 Status changed: New → Resolved.]
Resolved by the application of P1467R9. It's now clear that the special functions should have "sufficient additional overloads".
Proposed resolution:
parse
manipulator without abbreviation is not callableSection: 30.13 [time.parse] Status: C++20 Submitter: Tomasz Kamiński Opened: 2019-07-10 Last modified: 2021-02-25
Priority: 0
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Discussion:
The parse
overload that does not accept the abbreviation but does accept an offset,
because the expression in the Remarks: clause:
from_stream(declval<basic_istream<charT, traits>*>(), fmt.c_str(), tp, nullptr, &offset)
is not valid. This is caused by deduction failure for the basic_string<charT, traits, Alloc>*
from nullptr
(see this link):
[2019-08-17 Issue Prioritization]
Status to Tentatively Ready and priority to 0 after six positive votes on the reflector.
Proposed resolution:
This wording is relative to N4830.
[Drafting note: As a drive-by fix the Remarks element is also converted to a Constraints element.]
Modify 30.13 [time.parse] as indicated:
template<class charT, class traits, class Alloc, class Parsable> unspecified parse(const basic_string<charT, traits, Alloc>& fmt, Parsable& tp, minutes& offset);-6-
RemarksConstraints:This function shall not participate in overload resolution unlessThe expressionfrom_stream(declval<basic_istream<charT, traits>&>(), fmt.c_str(), tp, declval<basic_string<charT, traits, Alloc>*>()nullptr, &offset)is
-7- Returns: A manipulator that, when extracted from aa validwell-formedexpressionwhen treated as an unevaluated operand.basic_istream<charT, traits> is
, callsfrom_stream(is, fmt.c_str(), tp, static_cast<basic_string<charT, traits, Alloc>*>(nullptr), &offset)
.
Section: 24.3.5.7 [random.access.iterators] Status: C++23 Submitter: Peter Sommerlad Opened: 2019-07-15 Last modified: 2023-11-22
Priority: 3
View all other issues in [random.access.iterators].
View all issues with C++23 status.
Discussion:
For forward iterators we have very clear wording regarding the restricted domain of operator==
in 24.3.5.5 [forward.iterators] p2:
The domain of
==
for forward iterators is that of iterators over the same underlying sequence. However, value-initialized iterators may be compared and shall compare equal to other value-initialized iterators of the same type. [Note: Value-initialized iterators behave as if they refer past the end of the same empty sequence. — end note]
But for the relational operators of random access iterators specified in
24.3.5.7 [random.access.iterators], Table [tab:randomaccessiterator],
no such domain constraints are clearly defined, except that we can infer that they are
similarly constrained as the difference of the compared iterators by means of the
operational semantics of operator<
.
[2019-07-29; Casey comments and provides wording]
Change the "Operational Semantics" column of the "a < b
" row of [tab:randomaccessiterator] to
"Effects: Equivalent to: return
b - a > 0
;
a < b
is required to be well-defined over the domain for which
b - a
is required to be well-defined, which is the set of pairs (x, y)
such that
there exists a value n
of type difference_type such that x + n == b
.
[2020-02-13, Prague]
P3, but some hesitation to make it Immediate, therefore moving to Ready.
[2020-11-09 Approved In November virtual meeting. Status changed: Ready → WP.]
Proposed resolution:
This wording is relative to N4849.
Modify 24.3.5.7 [random.access.iterators] as indicated:
Table 87: Cpp17RandomAccessIterator requirements (in addition to Cpp17BidirectionalIterator) [tab:randomaccessiterator] Expression Return type Operational semantics Assertion/note
pre-/post-condition[…]
a < b
contextually convertible to bool
Effects: Equivalent to: return
b - a > 0;
<
is a total ordering relation
Section: 20.5.3.3 [mem.poly.allocator.mem] Status: C++20 Submitter: Casey Carter Opened: 2019-07-18 Last modified: 2021-02-25
Priority: 2
View all other issues in [mem.poly.allocator.mem].
View all issues with C++20 status.
Discussion:
Both LWG 3038(i) and LWG 3190(i) deal with how to respond to requests to allocate
"n * sizeof(T)
" bytes of memory when n * sizeof(T)
is not sufficient storage for
n
objects of type T
, i.e., when n > SIZE_MAX / sizeof(T)
. LWG
3038(i) changed polymorphic_allocator::allocate
to throw length_error
upon
detecting this condition, whereas LWG 3190(i) changed allocator::allocate
to
throw bad_array_new_length
. It's peculiar that two standard library components which allocate
memory both detect this condition but handle it by throwing different exception types; for consistency,
the two should be harmonized.
bad_array_new_length
was
the better option. Unlike length_error
, bad_array_new_length
derives from
bad_alloc
so we can make this change without altering the invariant that allocation functions
either succeed or throw an exception derived from bad_alloc
.
Further, P0339R6 "polymorphic_allocator<>
as a
vocabulary type" recently added the function template "template<class T> T*
allocate_object(size_t n = 1);
" to std::pmr::polymorphic_allocator
, which is another
instance of the "allocate memory for n
objects of type T
" pattern.
20.5.3.3 [mem.poly.allocator.mem] paragraph 8.1 specifies that allocate_object
throws
length_error
when SIZE_MAX / sizeof(T) < n
, presumably for consistency with std::pmr::polymorphic_allocator::allocate
specified in paragraph 1. allocate_object
's
behavior should be consistent with allocator::allocate
and
polymorphic_allocator::allocate
so we have a single means of communicating "request for
allocation of unrepresentable size" errors in the Standard Library.
[2020-02 Moved to Immediate on Thursday afternoon in Prague.]
Proposed resolution:
This wording is relative to N4820.
Modify 20.5.3.3 [mem.poly.allocator.mem] as indicated:
[[nodiscard]] Tp* allocate(size_t n);[…]-1- Effects: If
SIZE_MAX / sizeof(Tp) < n
, throws. Otherwise equivalent to:
length_errorbad_array_new_lengthreturn static_cast<Tp*>(memory_rsrc->allocate(n * sizeof(Tp), alignof(Tp)));template<class T> T* allocate_object(size_t n = 1);-8- Effects: Allocates memory suitable for holding an array of
n
objects of typeT
, as follows:
(8.1) — if
SIZE_MAX / sizeof(T) < n
, throws,
length_errorbad_array_new_length(8.2) — otherwise equivalent to:
return static_cast<T*>(allocate_bytes(n*sizeof(T), alignof(T)));
std::function
deduction guidesSection: 22.10.17.3.2 [func.wrap.func.con] Status: C++20 Submitter: Louis Dionne Opened: 2019-07-17 Last modified: 2021-02-25
Priority: Not Prioritized
View all other issues in [func.wrap.func.con].
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Discussion:
The following code is currently undefined behavior:
#include <functional> struct R { }; struct f0 { R operator()() && { return {}; } }; int main() { std::function f = f0{}; }
The reason is that 22.10.17.3.2 [func.wrap.func.con]/12 says:
This deduction guide participates in overload resolution only if
&F::operator()
is well-formed when treated as an unevaluated operand. In that case, ifdecltype(&F::operator())
is of the formR(G::*)(A...) cv &opt noexceptopt
for a class typeG
, then the deduced type isfunction<R(A...)>
.
However, it does not define the behavior when &F::operator()
is well-formed but not
of the required form (in the above example it's of the form R(G::*)(A...) &&
, which is
rvalue-reference qualified instead of optionally-lvalue-reference qualified). libc++'s implementation
of the deduction guide SFINAE's out when either &F::operator()
is not well-formed, or it
is not of the required form. It seems like mandating that behavior in the Standard is the way to go.
Previous resolution [SUPERSEDED]:
This wording is relative to N4820.
Modify 22.10.17.3.2 [func.wrap.func.con] as indicated:
template<class F> function(F) -> function<see below>;-12- Remarks: This deduction guide participates in overload resolution only if
&F::operator()
is well-formed when treated as an unevaluated operand, and its type is of the formR(G::*)(A...) cv &opt noexceptopt
for a class typeG
and a sequence of typesA...
. In that case,ifthe deduced type isdecltype(&F::operator())
is of the formR(G::*)(A...) cv &opt noexceptopt
for a class typeG
, thenfunction<R(A...)>
.
[2020-02-13; Prague]
LWG improves wording matching Marshall's Mandating paper.
[Status to Immediate on Friday in Prague.]
Proposed resolution:
This wording is relative to N4849.
Modify 22.10.17.3.2 [func.wrap.func.con] as indicated:
[Drafting note: This edit should be used instead of the corresponding edit in P1460]
template<class F> function(F) -> function<see below>;-?- Constraints:
-12- Remarks:&F::operator()
is well-formed when treated as an unevaluated operand anddecltype(&F::operator())
is of the formR(G::*)(A...) cv &opt noexceptopt
for a class typeG
.This deduction guide participates in overload resolution only ifThe deduced type is&F::operator()
is well-formed when treated as an unevaluated operand. In that case, ifdecltype(&F::operator())
is of the formR(G::*)(A...) cv &opt noexceptopt
for a class typeG
, then tfunction<R(A...)>
.
Section: 17.12.2.2 [cmp.partialord], 17.12.2.3 [cmp.weakord], 17.12.2.4 [cmp.strongord], 31.12.6 [fs.class.path], 23.2.5.1 [container.node.overview] Status: Resolved Submitter: Daniel Sunderland Opened: 2019-07-19 Last modified: 2021-06-06
Priority: Not Prioritized
View all issues with Resolved status.
Discussion:
LWG has been moving papers which use hidden friends to restrict an entity's lookup to be via ADL only. However, the current wording does not prevent an implementation from making these entities from also being available via (un)qualified lookup.
Walter Brown and I have an unreviewed paper (P1601R0) in the Post Kona mailing specifying that entities which are intended to be found via ADL only include a Remarks element which states something equivalent to the following: "Remarks: This function is to be found via argument-dependent lookup only." Adding this element after the draft ships will be a semantic change. Marshall suggested that I file an LWG issue to add/modify Remarks elements where needed to prevent (un)qualified lookup. The following stable names are places in the pre Cologne draft (N4820) which potentially use hidden friends. Furthermore, there are papers that LWG added to the straw polls which also potentially use hidden friends. LWG should review each of these subsections/papers to determine if they should include the text equivalent to the Remarks above. [Note: Not all these subsections should restrict lookup to ADL only. It is very likely that I missed a paper or subsection — end note].[cmp.weakeq]: Comparisons
[cmp.strongeq]: Comparisons
17.12.2.2 [cmp.partialord]: Comparisons
17.12.2.3 [cmp.weakord]: Comparisons
17.12.2.4 [cmp.strongord]: Comparisons
31.12.6 [fs.class.path]: operator<<
, operator>>
23.2.5.1 [container.node.overview]: swap
P0660R10 (Stop Token and Joining Thread):
Comparisons, swap
P1614R2 (The Mothership has Landed): Comparisons
[2019-11-17; Daniel comments]
The acceptance of P1965R0 at the Belfast 2019 meeting should resolve this issue.
[2019-11; Resolved by the adoption of P1965 in Belfast]
Proposed resolution:
chrono-spec
grammar ambiguity in §[time.format]Section: 30.12 [time.format] Status: C++20 Submitter: Victor Zverovich Opened: 2019-07-24 Last modified: 2021-02-25
Priority: 0
View other active issues in [time.format].
View all other issues in [time.format].
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Discussion:
The chrono-spec
grammar introduced by P1361R2 "Integration
of chrono with text formatting" in section [time.format] is ambiguous because '%'
can be
interpreted as either literal-char
or conversion-spec
:
chrono-spec ::= literal-char | conversion-spec literal-char ::= <a character other than '{' or '}'> conversion-spec ::= '%' [modifier] type
[2019-08-17 Issue Prioritization]
Status to Tentatively Ready and priority to 0 after five positive votes on the reflector.
Proposed resolution:
This wording is relative to N4830.
Modify the literal-char
grammar in 30.12 [time.format] as indicated:
literal-char: any character other than{
,or}
, or %
std::format
: missing rules for arg-id
in width
and precision
Section: 28.5.2.2 [format.string.std] Status: C++20 Submitter: Richard Smith Opened: 2019-07-31 Last modified: 2021-02-25
Priority: 1
View other active issues in [format.string.std].
View all other issues in [format.string.std].
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Discussion:
According to 28.5.2 [format.string] we have:
If the numeric
arg-id
s in a format string are0, 1, 2, ...
in sequence, they can all be omitted (not just some) and the numbers0, 1, 2, ...
will be automatically used in that order. A format string does not contain a mixture of automatic and manual indexing.
… but what does that mean in the presence of arg-id
in width
and precision
?
Can one replace
"{0:{1}} {2}"
with
"{:{}} {}"
? The grammar says the answer is no, because the arg-id
in width
is not optional, but
the normative wording says the answer is yes.
arg-id
should be optional in width
and precision
:
width ::= nonzero-digit [integer] | '{' [arg-id] '}' precision ::= integer | '{' [arg-id] '}'
[2020-02 Status to Immediate on Thursday morning in Prague.]
Proposed resolution:
This wording is relative to N4830.
Modify the width
and precision
grammar in the syntax
of format specifications of 28.5.2.2 [format.string.std] as indicated:
[…] width: positive-integer { arg-idopt } precision: . nonnegative-integer . { arg-idopt } […]
Modify 28.5.2.2 [format.string.std] as indicated:
-6- The # option causes […]
-?- If{ arg-idopt }
is used in a width or precision, the value of the corresponding formatting argument is used in its place. If the corresponding formatting argument is not of integral type, or its value is negative for precision or non-positive for width, an exception of typeformat_error
is thrown. -7- The positive-integer in width is a decimal integer defining the minimum field width. If width is not specified, there is no minimum field width, and the field width is determined based on the content of the field.
std::format
and negative zeroesSection: 28.5.2 [format.string] Status: C++20 Submitter: Richard Smith Opened: 2019-07-31 Last modified: 2021-02-25
Priority: 2
View all other issues in [format.string].
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Discussion:
What are these:
std::format("{}", -0.0); std::format("{:+}", -0.0); std::format("{:-}", -0.0); std::format("{: }", -0.0);
with
"{:{}} {}"
A negative zero is not a negative number, so I think the answer for an implementation that supports
signed zeroes is "0"
, "-0"
, "0"
, " 0"
. Is that the intent? (Note
that this doesn't satisfy to_chars
' round-trip guarantee.)
"+"
means "insert a leading +
if to_chars
'
output does not start with -"
and " "
actually means "insert a leading space if
to_chars
' output does not start with -"
(that is, the same as "%+f"
and "% f"
)
— so that the result of all four calls is "-0"
?
Victor Zverovich:
The latter. std::format
is specified in terms of to_chars
and the intent is to have similar
round trip guarantee here, so the result should be "-0"
, "-0"
, "-0"
, "-0"
.
This has also been extensively discussed in LEWG and the outcome of that discussion was to print '-'
for negative zeros.
So I think it should be clarified that '-'
and space apply to negative numbers and negative zero.
[2019-08-17 Priority set to 2 based on reflector discussion]
Previous resolution [SUPERSEDED]:This wording is relative to N4830.
Modify the sign options Table [tab:format.sign] in 28.5.2.2 [format.string.std] as indicated:
Table 59: Meaning of sign options [tab:format.sign] Option Meaning '+'
Indicates that a sign should be used for both non-negative and negative numbers. '-'
Indicates that a sign should be used only for negative numbers and negative zero (this is the default behavior). space Indicates that a leading space should be used for non-negative numbers other than negative zero, and a minus sign for negative numbers and negative zero.
[2020-02-13, Prague]
Rebased and some wording finetuning by LWG.
[2020-02 Status to Immediate on Thursday night in Prague.]
Proposed resolution:
This wording is relative to N4849.
Modify the sign options Table [tab:format.sign] in 28.5.2.2 [format.string.std] as indicated:
Table 58: Meaning of sign options [tab:format.sign] Option Meaning '+'
Indicates that a sign should be used for both non-negative and negative numbers. '-'
Indicates that a sign should be used onlyfor negative numbers and negative zero only (this is the default behavior).space Indicates that a leading space should be used for non-negative numbers other than negative zero, and a minus sign for negative numbers and negative zero.
Source
in §[fs.path.req] insufficiently constraintySection: 31.12.6.4 [fs.path.req] Status: C++20 Submitter: Casey Carter Opened: 2019-08-02 Last modified: 2021-02-25
Priority: 0
View all issues with C++20 status.
Discussion:
std::filesystem::path
has a number of functions - notably including a conversion
constructor template (31.12.6.5.1 [fs.path.construct]) and assignment operator template
(31.12.6.5.2 [fs.path.assign]) - that accept const Source&
. Per
31.12.6.4 [fs.path.req] paragraph 2:
-2- Functions taking template parameters named
Source
shall not participate in overload resolution unless either(2.1) —
Source
is a specialization ofbasic_string
orbasic_string_view
, or(2.2) — the qualified-id
iterator_traits<decay_t<Source>>::value_type
is valid and denotes a possiblyconst
encoded character type.
iterator_traits<decay_t<path>>::value_type
is not valid in C++17, so this
specification was sufficient to guard against the conversion constructor template (respectively
assignment operator template) "pretending" to be copy constructor (respectively copy assignment
operator). P0896R4 "The One Ranges Proposal", however,
altered the definition of iterator_traits
in the working draft. It now has some convenient
default behaviors for types that meet (roughly) the syntax of the Cpp17InputIterator
requirements. Notably those requirements include copy construction and copy assignment.
In the working draft, to determine the copyability of std::filesystem::path
we must perform
overload resolution to determine if we can initialize a path
from a constant lvalue of type
path
. The conversion constructor template that accepts const Source&
is a
candidate, since its second argument is defaulted, so
we must perform template argument deduction to see if this constructor is viable. Source
is
deduced to path
and we then must check the constraint from 31.12.6.4 [fs.path.req]
paragraph 2.2 (above). Checking the constraint requires us to specialize
iterator_traits<path>
, which (per 24.3.2.3 [iterator.traits] paragraph 3.2)
requires us to determine if path
satisfies the exposition-only
cpp17-input-iterator
concept, which requires path
to be copyable.
We've completed a cycle: determining if path
is copyable requires us to first determine if
path
is copyable. This unfortunate constraint recursion can be broken by explicitly
specifying that path
is not a valid Source
.
[2019-08-17 Issue Prioritization]
Status to Tentatively Ready and priority to 0 after seven positive votes on the reflector.
Proposed resolution:
This wording is relative to N4830.
Modify 31.12.6.4 [fs.path.req] as indicated:
-2- Functions taking template parameters named
Source
shall not participate in overload resolution unlessSource
denotes a type other thanpath
, and either[…]
'%p'
parse specifierSection: 30.13 [time.parse] Status: C++20 Submitter: Tomasz Kamiński Opened: 2019-07-31 Last modified: 2021-02-25
Priority: 0
View other active issues in [time.parse].
View all other issues in [time.parse].
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Discussion:
The current specification for the '%p'
flag in "[tab:time.parse.spec]
Meaning of parse
flags" places a restriction of it's
placement with regards to the '%I'
command:
The locale's equivalent of the AM/PM designations associated with a 12-hour clock. The command
%I
must precede%p
in the format string.
This restriction makes the migration to new API more difficult, as it is
not present for the POSIX
strptime
nor in the example implementation of the
library. Per Howard's comment:
Actually this is an obsolete requirement and it should be struck. The first time I implemented this I didn't know how to do it without this requirement. I've since reimplemented it without needing this.
[2019-08-17 Issue Prioritization]
Status to Tentatively Ready and priority to 0 after eight positive votes on the reflector.
Proposed resolution:
This wording is relative to N4830.
Modify Table 99 "Meaning of parse
flags [tab:time.parse.spec]" in
30.13 [time.parse] as indicated:
Table 99: Meaning of parse
flags [tab:time.parse.spec]Flag Parsed value […]
%p
The locale's equivalent of the AM/PM designations associated with a 12-hour clock. The command%I
must precede%p
in the format string.[…]
basic_format_arg
?Section: 28.5.8.1 [format.arg] Status: C++20 Submitter: Richard Smith Opened: 2019-08-01 Last modified: 2021-02-25
Priority: 0
View all other issues in [format.arg].
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Discussion:
In P0645R10 20.?.5.1/ we find:
Constraints:
typename Context::template formatter_type<T>
is enabled.
… which doesn't mean anything, because that is an arbitrary type. Presumably the intent
is that that type will always be a specialization of formatter, but there appear to be
no constraints whatsoever on the Context
template parameter, so there seems to be no
requirement that that is the case.
basic_format_arg
place some constraints on its Context
template parameter?
E.g., should it be required to be a specialization of basic_format_context
?
Victor Zverovich:
The intent here is to allow different context types provide their own formatter extension types.
The default formatter context and extension are basic_format_context
and formatter
respectively, but it's possible to have other. For example, in the fmt
library there
is a formatter context that supports printf
formatting for legacy code. It cannot use
the default formatter specializations because of the different syntax (%...
vs
{...})
.
Richard Smith:
In either case, the specification here seems to be missing the rules for what is a valid
Context
parameter.
I'm happy to editorially change "is enabled" to "is an enabled specialization of formatter",
since there's nothing else that this could mean, but we still need a wording fix for the
broader issue here. Here's what I have so far for this wording:
Constraints: The template specialization
typename Context::template formatter_type<T>
is an enabled specialization of formatter ([formatter.requirements]).
Tim Song:
I think what we actually want here is "typename Context::template formatter_type<T>
meets the Formatter requirements".
[2019-08-17 Issue Prioritization]
Status to Tentatively Ready and priority to 0 after six positive votes on the reflector.
Proposed resolution:
This wording is relative to N4830.
Modify 28.5.8.1 [format.arg] as indicated:
template<class T> explicit basic_format_arg(const T& v) noexcept;-4- Constraints: The template specialization
typename Context::template formatter_type<T>
is an enabled specialization ofmeets the Formatter requirements (28.5.6 [format.formatter]). The extent to which an implementation determines that the specializationformatter
is enabledmeets the Formatter requirements is unspecified, except that as a minimum the expressiontypename Context::template formatter_type<T>() .format(declval<const T&>(), declval<Context&>())shall be well-formed when treated as an unevaluated operand.
ranges::iter_move
should perform ADL-only lookup of iter_move
Section: 24.3.3.1 [iterator.cust.move] Status: C++20 Submitter: Casey Carter Opened: 2019-07-29 Last modified: 2021-02-25
Priority: 2
View all other issues in [iterator.cust.move].
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Discussion:
The specification of the ranges::iter_move
customization point in [iterator.cust.move]
doesn't include a deleted poison pill overload. There is no std::iter_move
to avoid,
so such a poison pill is not needed. This is fine, except that it suggests that unqualified
lookup for iter_move
in the iter_move(E)
expression in paragraph 1.1 should
find declarations of iter_move in the global namespace via normal unqualified lookup, when the
design intent is that only ADL be used to find overloads of iter_move
.
void iter_move();
" which has the desired effect.
[2020-02 Status to Immediate on Thursday morning in Prague.]
Proposed resolution:
This wording is relative to N4820.
[Drafting Note: There's a drive-by fix here to change "valid" — which suggests runtime behavior which cannot be validated at compile time — to "well-formed".]
Modify 24.3.3.1 [iterator.cust.move] as indicated:
-1- The name
ranges::iter_move
denotes a customization point object (16.3.3.3.5 [customization.point.object]). The expressionranges::iter_move(E)
for some subexpressionE
is expression-equivalent to:
(1.1) —
iter_move(E)
, if that expression isvalidwell-formed when treated as an unevaluated operand, with overload resolution performed in a context that does not include a declaration ofranges::iter_move
.but does include the declaration:void iter_move();[…]
std::format
#b
, #B
, #o
, #x
, and #X
presentation types misformat negative numbersSection: 28.5.2.2 [format.string.std] Status: C++20 Submitter: Richard Smith Opened: 2019-08-01 Last modified: 2021-02-25
Priority: 2
View other active issues in [format.string.std].
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Discussion:
According to both the specification for '#'
and the presentation types b
, B
,
o
, x
, and X
(which redundantly duplicate the rule for the relevant prefixes),
the string 0b
, 0B
, 0
, 0x
, or 0X
is prefixed to the result
of to_chars
. That means:
std::format("{0:#b} {0:#B} {0:#o} {0:#x} {0:#X}", -1)
produces
"0b-1 0B-1 0-1 0x-1 0X-1"
I assume that's a bug?
(Additionally, if the+
specifier is used, there appears to be no specification of where
the sign is inserted into the output.)
Victor Zverovich:
Yes. I suggest replacing
adds the respective prefix
"0b"
("0B"
),"0"
, or"0x"
("0X"
) to the output value
with something like
inserts the respective prefix
"0b"
("0B"
),"0"
, or"0x"
("0X"
) to the output value after the sign, if any,
I think the duplicate wording in the presentation types b
, B
, o
, x
, and
X
can be dropped.
+
specifier problem: How about adding
The sign is inserted before the output of
to_chars
.
?
[2019-08-21 Priority set to 2 based on reflector discussion]
Previous resolution [SUPERSEDED]:This wording is relative to N4830.
Modify the sign options Table [tab:format.sign] in 28.5.2.2 [format.string.std] as indicated:
Table 59: Meaning of sign options [tab:format.sign] Option Meaning '+'
Indicates that a sign should be used for both non-negative and negative numbers. The sign is inserted before the output of to_chars
.'-'
Indicates that a sign should be used only for negative numbers (this is the default behavior). space Indicates that a leading space should be used for non-negative numbers, and a minus sign for negative numbers. Modify [format.string] as indicated:
-6 The
#
option causes the alternate form to be used for the conversion. This option is only valid for arithmetic types other thancharT
andbool
or when an integer presentation type is specified. For integral types, the alternate formaddsinserts the base prefix (if any) specified in Table [tab:format.type.int] to the output value after the sign, if any. […]
[2019-08-21; Victor Zverovich provides improved wording]
Previous resolution [SUPERSEDED]:This wording is relative to N4830.
Modify the sign options Table [tab:format.sign] in 28.5.2.2 [format.string.std] as indicated:
Table 59: Meaning of sign options [tab:format.sign] Option Meaning '+'
Indicates that a sign should be used for both non-negative and negative numbers. The +
sign is inserted before the output ofto_chars
for non-negative numbers other than the negative zero. [Note: For negative numbers and the negative zero the output ofto_chars
will already contain the sign so no additional transformation is performed. — end note]'-'
Indicates that a sign should be used only for negative numbers (this is the default behavior). space Indicates that a leading space should be used for non-negative numbers, and a minus sign for negative numbers. Modify [format.string] as indicated:
-6- The
#
option causes the alternate form to be used for the conversion. This option is only valid for arithmetic types other thancharT
andbool
or when an integer presentation type is specified. For integral types, the alternate formaddsinserts the base prefix (if any) specified in Table [tab:format.type.int] to the output value after the sign character (possibly space) if there is one, or before the output ofto_chars
otherwise. […]
[2020-02-13, Prague]
LWG made some improvements to the wording.
[2020-02 Status to Immediate on Thursday night in Prague.]
Proposed resolution:
This wording is relative to N4849.
Modify the sign options Table [tab:format.sign] in 28.5.2.2 [format.string.std] as indicated:
Table 58: Meaning of sign options [tab:format.sign] Option Meaning '+'
Indicates that a sign should be used for both non-negative and negative numbers. The +
sign is inserted before the output ofto_chars
for non-negative numbers other than negative zero. [Note: For negative numbers and negative zero the output ofto_chars
will already contain the sign so no additional transformation is performed. — end note]'-'
Indicates that a sign should be used only for negative numbers (this is the default behavior). space Indicates that a leading space should be used for non-negative numbers, and a minus sign for negative numbers.
Modify [format.string] as indicated:
-6- The
#
option causes the alternate form to be used for the conversion. This option isonlyvalid for arithmetic types other thancharT
andbool
or when an integer presentation type is specified, and not otherwise. For integral types, the alternate formaddsinserts the base prefix (if any) specified in Table [tab:format.type.int]tointo the outputvalueafter the sign character (possibly space) if there is one, or before the output ofto_chars
otherwise. […]
Section: 32.5.5 [atomics.lockfree] Status: C++23 Submitter: Billy O'Neal III Opened: 2019-08-03 Last modified: 2023-11-22
Priority: 4
View all other issues in [atomics.lockfree].
View all issues with C++23 status.
Discussion:
According to SG1 experts, the requirement in [atomics.lockfree]/2 is intended to require that
the answer for is_lock_free()
be the same for a given T
for a given run of
the program. The wording does not achieve that because it's described in terms of 'pointers',
but there are no pointers in an atomic<int>
.
[2020-02 Status to Ready on Thursday morning in Prague.]
[2020-11-09 Approved In November virtual meeting. Status changed: Ready → WP.]
Proposed resolution:
This wording is relative to N4830.
Modify 32.5.5 [atomics.lockfree] as indicated:
-2- The functions
atomic<T>::is_lock_free
, andatomic_is_lock_free
(32.5.8.2 [atomics.types.operations]) indicateswhether the object is lock-free. In any given program execution, the result of the lock-free query is the same for all atomic objectsshall be consistent for all pointersof the same type.
std::format
: #
(alternate form) for NaN and infSection: 28.5.2.2 [format.string.std] Status: C++20 Submitter: Richard Smith Opened: 2019-08-05 Last modified: 2021-02-25
Priority: 0
View other active issues in [format.string.std].
View all other issues in [format.string.std].
View all issues with C++20 status.
Discussion:
We have:
"For floating-point numbers, the alternate form causes the result of the conversion to always contain a decimal-point character, even if no digits follow it."
So does that mean that infinity is formatted as "inf."
and NaN as "nan."
? (Or
something like that? Where exactly do we add the decimal point in this case?) Or does this
affect infinity but not NaN (because we can handwave that a NaN value is not a
"floating-point number")?
"inf"
and "nan"
without a decimal point.
[2020-02 Status to Immediate on Thursday morning in Prague.]
Proposed resolution:
This wording is relative to N4830.
Modify 28.5.2.2 [format.string.std] as indicated:
-6- […] For floating-point types, the alternate form causes the result of the conversion of finite values to always contain a decimal-point character, even if no digits follow it. Normally, a decimal-point character appears in the result of these conversions only if a digit follows it. In addition, for
g
andG
conversions, trailing zeros are not removed from the result.
std::format
alignment specifiers applied to string arguments?Section: 28.5.2 [format.string] Status: C++20 Submitter: Richard Smith Opened: 2019-08-02 Last modified: 2021-02-25
Priority: 2
View all other issues in [format.string].
View all issues with C++20 status.
Discussion:
We are told:
Formatting of objects of arithmetic types and
const void*
is done as if by callingto_chars
(unless otherwise specified) and copying the output through the output iterator of the format context with additional padding and adjustments as specified by the format specifiers.
… but there is no corresponding rule for strings. Is an alignment specifier intended to be applied to strings or not? The wording as-is is ambiguous.
(The above also doesn't cover formattingvoid*
or std::nullptr_t
.
Presumably at least those two should have the relevant adjustments applied to them!)
The wording never actually anywhere says that the basic_format_args
are in
any way involved in the formatting process, or how formatting actually happens.
(The wording doesn't say that basic_format_arg::handle::format
is ever
called, for example.)
Victor Zverovich:
An alignment specifier is intended to be applied to strings as well, void*
and
std::nullptr_t
are converted into const void*
when constructing
basic_format_arg
.
The wording for vformat
and similar functions says that basic_format_args
is involved:
Returns: A string object holding the character representation of formatting arguments provided by
args
formatted according to specifications given infmt
.
but I admit that it is hand-wavy. Perhaps we could add something along the lines of
For each replacement field referring to the argument with index
(arg-id) i
, thebasic_format_arg
object referring to the argument is obtained viaargs.get(i)
and theparse
andformat
functions of theformatter
specialization for the underlying argument type are called to parse the format specification and format the value.
to clarify how we format args
(basic_format_args
).
[2019-08-21 Priority set to 2 based on reflector discussion]
[2019-08-21; Victor Zverovich suggests wording]
[2020-02 Status to Immediate on Thursday night in Prague.]
Proposed resolution:
This wording is relative to N4830.
Modify 28.5.2.2 [format.string.std] as indicated:
-3- The align specifier applies to all argument types. The meaning of the various alignment options is as specified in Table [tab:format.align]. [Example: […
Section: 30.13 [time.parse] Status: C++20 Submitter: Tomasz Kamiński Opened: 2019-08-06 Last modified: 2021-02-25
Priority: 2
View other active issues in [time.parse].
View all other issues in [time.parse].
View all issues with C++20 status.
Discussion:
The current specification of the locale modifiers for the parse flags in
"[tab:time.parse.spec] Meaning of parse
flags" is
inconsistent with the
POSIX strptime
specification:
New flags %OC
, %Ou
are added
Flags %OI
, %OU
, %OW
are missing
Per Howard's comment:
I only invented in a couple places for these flags, and none of them involved locale-dependent stuff. If we are inconsistent with POSIX/C on this, it's a bug. Rationale,std::get_time
is already specified in terms ofstrptime
, and we can't afford to be inventive with locale-dependent things.
Note that, due above, the inconsistency between
POSIX strftime
specification that supports %Ou
and %OV
that are not handled by
strptime
should be (by design) reflected in "[tab:time.format.spec]
Meaning of conversion specifiers" and "[tab:time.parse.spec] Meaning of
parse
flags" tables.
%d
modifier was addressed by LWG 3218(i).
[2020-02 Status to Immediate on Thursday morning in Prague.]
Proposed resolution:
This wording is relative to N4830.
Modify Table 99 "Meaning of parse
flags [tab:time.parse.spec]" in
30.13 [time.parse] as indicated:
Table 99: Meaning of parse
flags [tab:time.parse.spec]Flag Parsed value […]
%C
The century as a decimal number. The modified command %NC
specifies the maximum number of characters to read. IfN
is not specified, the default is2
. Leading zeroes are permitted but not required. The modified commands%EC
andinterprets the locale's alternative representation of the century.%OC
[…]
%I
The hour (12-hour clock) as a decimal number. The modified command %NI
specifies the maximum number of characters to read. IfN
is not specified, the default is2
. Leading zeroes are permitted but not required. The modified command%OI
interprets the locale's alternative representation.[…]
%u
The ISO weekday as a decimal number ( 1-7
), where Monday is1
. The modified command%Nu
specifies the maximum number of characters to read. IfN
is not specified, the default is1
. Leading zeroes are permitted but not required.The modified command%Ou
interprets the locale's alternative representation.%U
The week number of the year as a decimal number. The first Sunday of the year is the first day of week 01
. Days of the same year prior to that are in week00
. The modified command%NU
specifies the maximum number of characters to read. IfN
is not specified, the default is2
. Leading zeroes are permitted but not required. The modified command%OU
interprets the locale's alternative representation.[…]
%W
The week number of the year as a decimal number. The first Monday of the year is the first day of week 01
. Days of the same year prior to that are in week00
. The modified command%NW
specifies the maximum number of characters to read. IfN
is not specified, the default is2
. Leading zeroes are permitted but not required. The modified command%OW
interprets the locale's alternative representation.[…]
basic_syncbuf::basic_syncbuf()
should not be explicitSection: 31.11.2.1 [syncstream.syncbuf.overview] Status: C++20 Submitter: Nevin Liber Opened: 2019-08-06 Last modified: 2021-02-25
Priority: 0
View all other issues in [syncstream.syncbuf.overview].
View all issues with C++20 status.
Discussion:
When P0935 "Eradicating unnecessarily explicit default
constructors from the standard library" was applied, basic_syncbuf
was not in the
working paper. basic_syncbuf
should not have an explicit default constructor.
[2019-09-02 Issue Prioritization]
Status to Tentatively Ready and priority to 0 after seven positive votes on the reflector.
Proposed resolution:
This wording is relative to N4830.
Modify 31.11.2.1 [syncstream.syncbuf.overview], class template basic_syncbuf
synopsis, as indicated:
template<class charT, class traits, class Allocator> class basic_syncbuf : public basic_streambuf<charT, traits> { public: […] // 31.11.2.2 [syncstream.syncbuf.cons], construction and destruction basic_syncbuf() : basic_syncbuf(nullptr) {} explicit basic_syncbuf(streambuf_type* obuf= nullptr) : basic_syncbuf(obuf, Allocator()) {} […] };
stop_token
's operator!=
Section: 32.3.4 [stoptoken] Status: C++20 Submitter: Casey Carter Opened: 2019-08-06 Last modified: 2021-02-25
Priority: 0
View all issues with C++20 status.
Discussion:
In the shiny new world of the future, we need not declare overloads of operator!=
that
return !(x == y);
the rewrite rule in 12.2.2.3 [over.match.oper] para 3.4.3
achieves the same effect. Consequently, we should not declare such unnecessary
operator!=
overloads. The synopsis of class stop_token
in 32.3.4 [stoptoken]
declares exactly such an operator!=
friend which should be struck.
[01-2020 Moved to Tentatively Ready after 5 positive votes on the reflector.]
Proposed resolution:
This wording is relative to N4830.
Modify 32.3.4 [stoptoken], class stop_token
synopsis, as indicated:
namespace std { class stop_token { public: […] [[nodiscard]] friend bool operator==(const stop_token& lhs, const stop_token& rhs) noexcept;[[nodiscard]] friend bool operator!=(const stop_token& lhs, const stop_token& rhs) noexcept;friend void swap(stop_token& lhs, stop_token& rhs) noexcept; }; }
Modify [stoptoken.cmp] and [stoptoken.special] as indicated:
32.3.3.3 Non-member functions
Comparisons[stoptoken.nonmemberscmp][[nodiscard]] bool operator==(const stop_token& lhs, const stop_token& rhs) noexcept;-1- Returns:
true
iflhs
andrhs
have ownership of the same stop state or if bothlhs
andrhs
do not have ownership of a stop state; otherwisefalse
.[[nodiscard]] bool operator!=(const stop_token& lhs, const stop_token& rhs) noexcept;
-2- Returns:!(lhs==rhs)
.
32.3.3.4 Specialized algorithms [stoptoken.special]friend void swap(stop_token& x, stop_token& y) noexcept;-1- Effects: Equivalent to:
x.swap(y)
.
span
's array
constructor is too strictSection: 23.7.2.2.2 [span.cons] Status: C++20 Submitter: Jean Guegant & Barry Revzin Opened: 2019-08-10 Last modified: 2021-02-25
Priority: 2
View all other issues in [span.cons].
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Discussion:
Barry Revzin:
From StackOverflow: This compiles:std::vector<int*> v = {nullptr, nullptr}; std::span<const int* const> s{v};
This does not:
std::array<int*, 2> a = {nullptr, nullptr}; std::span<const int* const> s{a};
The problem is that span
's constructors include
A constructor template that takes any Container
that is neither a raw array nor a std::array
A constructor template that takes an array<value_type, N>&
A constructor template that takes a const array<value_type, N>&
So the first is excluded, and the other two don't match. We can change the array constructor templates to take an
array<T, N>
with the requirement that T(*)[]
is convertible to ElementType(*)[]
?
std::span
from a std::array<const T, X>
given the current
set of constructors of std::span
(23.7.2.2.2 [span.cons]):
std::array<const int, 4> a = {1, 2, 3, 4}; std::span<const int> s{a}; // No overload can be found. std::span s{a}; // CTAD doesn't help either.
Both constructors accepting a std::array
(23.7.2.2.2 [span.cons] p11) require the first template
parameter of the std::array
parameter to be value_type
:
template<size_t N> constexpr span(array<value_type, N>& arr) noexcept; template<size_t N> constexpr span(const array<value_type, N>& arr) noexcept;
value_type
being defined as remove_cv_t<ElementType>
— this constrains the first
template parameter not to be const
.
Container
(23.7.2.2.2 [span.cons] p14) have a
constraint — (p14.3) Container
is not a specialization of array
—
rejecting std::array
.
While you can call std::array<const T, X>::data
and std::array<const T, X>::size
to manually create a std::span
, we should, in my opinion, offer a proper overload for this scenario.
Two reasons came to my mind:
std::span
handles C-arrays and std::arrays
in an asymmetric way. The constructor
taking a C-array (23.7.2.2.2 [span.cons] p11) is using element_type
and as such can work with
const T
:
const int a[] = {1, 2, 3, 4}; std::span<const int> s{a}; // It works
If a user upgrades her/his code from C-arrays to a std::array
s and literally take the type
const T
and use it as the first parameter of std::array
, he/she will face an error.
Even if the user is aware that const std::array<T, X>
is more idiomatic than
std::array<const T, X>
, the second form may appear in the context of template
instantiation.
At the time this issue is written gls::span
, from which std::span
is partly based on,
does not suffer from the same issue: Its constructor taking a generic const Container&
does
not constraint the Container
not to be a std::array
(although its constructor taking
a generic Container&
does). For the users willing to upgrade from gsl::span
to
std::span
, this could be a breaking change.
[2019-09-01 Priority set to 2 based on reflector discussion]
[2020-02-13 Tim updates PR]
The previous PR's change to the raw array constructor is both 1) unnecessary and 2) incorrect; it prevents
span<const int>
from being initialized with an int[42]
xvalue.
This wording is relative to N4830.
The only change is to make the constructors templated on the element type of the array as well. We already have the right constraints in place. It's just that the 2nd constraint is trivially satisfied today by the raw array constructor and either always or never satisfied by the
std::array
one.
Modify 23.7.2.2.1 [span.overview], class template
span
synopsis, as indicated:template<class ElementType, size_t Extent = dynamic_extent> class span { public: […] // 23.7.2.2.2 [span.cons], constructors, copy, and assignment constexpr span() noexcept; constexpr span(pointer ptr, index_type count); constexpr span(pointer first, pointer last); template<class T, size_t N> constexpr span(Telement_type(&arr)[N]) noexcept; template<class T, size_t N> constexpr span(array<Tvalue_type, N>& arr) noexcept; template<class T, size_t N> constexpr span(const array<Tvalue_type, N>& arr) noexcept; […] };Modify 23.7.2.2.2 [span.cons] as indicated:
template<class T, size_t N> constexpr span(Telement_type(&arr)[N]) noexcept; template<class T, size_t N> constexpr span(array<Tvalue_type, N>& arr) noexcept; template<class T, size_t N> constexpr span(const array<Tvalue_type, N>& arr) noexcept;-11- Constraints:
(11.1) —
extent == dynamic_extent || N == extent
istrue
, and(11.2) —
remove_pointer_t<decltype(data(arr))>(*)[]
is convertible toElementType(*)[]
.-12- Effects: Constructs a
-13- Ensures:span
that is a view over the supplied array.size() == N && data() == data(arr)
istrue
.
[2020-02 Status to Immediate on Thursday night in Prague.]
Proposed resolution:
This wording is relative to N4849.
Modify 23.7.2.2.1 [span.overview], class template span
synopsis, as indicated:
template<class ElementType, size_t Extent = dynamic_extent> class span { public: […] // 23.7.2.2.2 [span.cons], constructors, copy, and assignment constexpr span() noexcept; […] template<size_t N> constexpr span(element_type (&arr)[N]) noexcept; template<class T, size_t N> constexpr span(array<Tvalue_type, N>& arr) noexcept; template<class T, size_t N> constexpr span(const array<Tvalue_type, N>& arr) noexcept; […] };
Modify 23.7.2.2.2 [span.cons] as indicated:
template<size_t N> constexpr span(element_type (&arr)[N]) noexcept; template<class T, size_t N> constexpr span(array<Tvalue_type, N>& arr) noexcept; template<class T, size_t N> constexpr span(const array<Tvalue_type, N>& arr) noexcept;-11- Constraints:
(11.1) —
extent == dynamic_extent || N == extent
istrue
, and(11.2) —
remove_pointer_t<decltype(data(arr))>(*)[]
is convertible toElementType(*)[]
.-12- Effects: Constructs a
-13- Postconditions:span
that is a view over the supplied array.size() == N && data() == data(arr)
istrue
.
constexpr
algorithmsSection: 17.3.1 [support.limits.general] Status: C++20 Submitter: Antony Polukhin Opened: 2019-08-14 Last modified: 2021-02-25
Priority: 0
View all other issues in [support.limits.general].
View all issues with C++20 status.
Discussion:
Feature testing macro from P0202 "Add Constexpr Modifiers to Functions
in <algorithm>
and <utility>
Headers" is missing in the WD.
__cpp_lib_constexpr_algorithms
.
So remove __cpp_lib_constexpr_swap_algorithms
, define __cpp_lib_constexpr_algorithms
to
201703L
if P0202 is implemented, to 201806L
if
P0202+P0879 are implemented.
[2019-09-02 Issue Prioritization]
Status to Tentatively Ready and priority to 0 after five positive votes on the reflector.
Proposed resolution:
This wording is relative to N4830.
Modify the Table 36 "Standard library feature-test macros" [tab:support.ft] in 17.3.1 [support.limits.general] as indicated:
Table 36: Standard library feature-test macros [tab:support.ft] Macro name Value Header(s) […]
__cpp_lib_constexpr_
swap_algorithms201806L
<algorithm>
[…]
Section: 17.3.1 [support.limits.general] Status: C++20 Submitter: Antony Polukhin Opened: 2019-08-14 Last modified: 2021-02-25
Priority: 0
View all other issues in [support.limits.general].
View all issues with C++20 status.
Discussion:
P0858 "Constexpr iterator requirements" suggested to update
the feature-testing macros __cpp_lib_string_view
and __cpp_lib_array_constexpr
to the date of adoption.
[2019-09-02 Issue Prioritization]
Status to Tentatively Ready and priority to 0 after five positive votes on the reflector.
Proposed resolution:
This wording is relative to N4830.
Modify the Table 36 "Standard library feature-test macros" [tab:support.ft] in 17.3.1 [support.limits.general] as indicated:
Table 36: Standard library feature-test macros [tab:support.ft] Macro name Value Header(s) […]
__cpp_lib_array_constexpr
201
6803L<iterator> <array>
[…]
__cpp_lib_string_view
201
606803L<string> <string_view>
[…]
initializer_list
Section: 25.3.2 [range.access.begin], 25.3.3 [range.access.end], 25.3.6 [range.access.rbegin], 25.3.7 [range.access.rend] Status: Resolved Submitter: Casey Carter Opened: 2019-08-15 Last modified: 2021-06-23
Priority: 3
View all issues with Resolved status.
Discussion:
The specification of ranges::begin
in 25.3.2 [range.access.begin] includes a
"poison pill" overload:
which exists to create an ambiguity with the non-membertemplate<class T> void begin(initializer_list<T>&&) = delete;
initializer_list
overload of
begin
in namespace std
(17.11.2 [initializer.list.syn]) when performing
unqualified lookup, since specializations of initializer_list
should not satisfy
forwarding-range
(25.4.2 [range.range]). The design intent is that
const
specializations of initializer_list
should also not satisfy
forwarding-range
, although they are rare enough beasts that they were overlooked
when this wording is written.
ranges::end
(25.3.3 [range.access.end]) has a similar poison pill for
initializer_list
, which should be changed consistently.
Notably ranges::rbegin
(25.3.6 [range.access.rbegin]) and ranges::rend
(25.3.6 [range.access.rbegin]) as currently specified accept rvalue
initializer_list
arguments; they find the initializer_list
overloads of
std::rbegin
and std::rend
(24.7 [iterator.range]) via ADL. While I can't
put my finger on anything in particular that's broken by this behavior, it seems wise to make
rbegin
and rend
consistent with begin
and end
for
initializer_list
until and unless we discover a reason to do otherwise.
[2019-10 Priority set to 3 after reflector discussion]
[2021-06-23 Resolved by adoption of P2091R0 in Prague. Status changed: New → Resolved.]
Proposed resolution:
This wording is relative to N4830.
Modify 25.3.2 [range.access.begin] as follows:
-1- The name
ranges::begin
denotes a customization point object (16.3.3.3.5 [customization.point.object]). The expressionranges::begin(E)
for some subexpressionE
is expression-equivalent to:(1.1) —
E + 0
ifE
is an lvalue of array type (6.8.4 [basic.compound]).(1.2) — Otherwise, if
E
is an lvalue,decay-copy(E.begin())
if it is a valid expression and its typeI
modelsinput_or_output_iterator
.(1.3) — Otherwise,
decay-copy(begin(E))
if it is a valid expression and its typeI
modelsinput_or_output_iterator
with overload resolution performed in a context that includes the declarations:and does not include a declaration oftemplate<class T> void begin(T&&) = delete; template<class T> void begin(initializer_list<T>&&) = delete;ranges::begin
.[…]
Modify 25.3.3 [range.access.end] as follows:
-1- The name
ranges::end
denotes a customization point object (16.3.3.3.5 [customization.point.object]). The expressionranges::end(E)
for some subexpressionE
is expression-equivalent to:(1.1) —
E + extent_v<T>
ifE
is an lvalue of array type (6.8.4 [basic.compound])T
.(1.2) — Otherwise, if
E
is an lvalue,decay-copy(E.end())
if it is a valid expression and its typeS
modelssentinel_for<decltype(ranges::begin(E))>
(1.3) — Otherwise,
decay-copy(end(E))
if it is a valid expression and its typeS
modelssentinel_for<decltype(ranges::begin(E))>
with overload resolution performed in a context that includes the declarations:and does not include a declaration oftemplate<class T> void end(T&&) = delete; template<class T> void end(initializer_list<T>&&) = delete;ranges::end
.[…]
Modify 25.3.6 [range.access.rbegin] as follows:
-1- The name
ranges::rbegin
denotes a customization point object (16.3.3.3.5 [customization.point.object]). The expressionranges::rbegin(E)
for some subexpressionE
is expression-equivalent to:(1.1) — If
E
is an lvalue,decay-copy(E.rbegin())
if it is a valid expression and its typeI
modelsinput_or_output_iterator
.(1.2) — Otherwise,
decay-copy(rbegin(E))
if it is a valid expression and its typeI
modelsinput_or_output_iterator
with overload resolution performed in a context that includes the declarations:and does not include a declaration oftemplate<class T> void rbegin(T&&) = delete; template<class T> void rbegin(initializer_list<T>) = delete;ranges::rbegin
.[…]
Modify 25.3.7 [range.access.rend] as follows:
-1- The name
ranges::rend
denotes a customization point object (16.3.3.3.5 [customization.point.object]). The expressionranges::rend(E)
for some subexpressionE
is expression-equivalent to:(1.1) — If
E
is an lvalue,decay-copy(E.rend())
if it is a valid expression and its typeS
modelssentinel_for<decltype(ranges::rbegin(E))>(1.2) — Otherwise,
decay-copy(rend(E))
if it is a valid expression and its typeS
modelswith overload resolution performed in a context that includes the declarations:sentinel_for<decltype(ranges::rbegin(E))>and does not include a declaration oftemplate<class T> void rend(T&&) = delete; template<class T> void rend(initializer_list<T>) = delete;ranges::rend
.[…]
Section: 24.3.1 [iterator.requirements.general] Status: C++20 Submitter: Daniel Krügler Opened: 2019-08-18 Last modified: 2021-02-25
Priority: 0
View all other issues in [iterator.requirements.general].
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Discussion:
The current definition of constexpr iterators is specified in 24.3.1 [iterator.requirements.general] p16 as follows:
Iterators are called constexpr iterators if all operations provided to meet iterator category requirements are constexpr functions, except for
(16.1) — a pseudo-destructor call (7.5.5.5 [expr.prim.id.dtor]), and
(16.2) — the construction of an iterator with a singular value.
With the acceptance of some proposals during the Cologne 2019 meeting, these additional requirements become mostly obsolete, as it had already been pointed out during that meeting:
With the acceptance of P0784R7, destructors can be declaredconstexpr
and it is possible to perform a pseudo-destructor call within a
constant expression, so bullet (16.1) is no longer a necessary requirement.
With the acceptance of P1331R2, trivial default
initialization in constexpr contexts is now possible, and there is no longer a requirement
to initialize all sub-objects of a class object within a constant expression.
It seems to me that we should simply strike the above two constraining requirements of the
definition of constexpr iterators for C++20.
[2019-09-14 Issue Prioritization]
Status to Tentatively Ready and priority to 0 after five positive votes on the reflector.
Proposed resolution:
This wording is relative to N4830.
Modify 24.3.1 [iterator.requirements.general] as indicated:
-16- Iterators are called constexpr iterators if all operations provided to meet iterator category requirements are constexpr functions.
, except for
(16.1) — a pseudo-destructor call (7.5.5.5 [expr.prim.id.dtor]), and
(16.2) — the construction of an iterator with a singular value.
year_month*
arithmetic rejects durations convertible to yearsSection: 30.8 [time.cal] Status: C++20 Submitter: Tomasz Kamiński Opened: 2019-08-15 Last modified: 2021-02-25
Priority: 2
View all issues with C++20 status.
Discussion:
Currently, the year_month*
types (year_month
,
year_month_day
) provide separate arithmetic operators with
duration type of years
or months
. This is an intentional
optimization that avoids performing modulo arithmetic in the former case.
year_month*
types
with durations that are convertible to years
(and by
consequence to months
) ambiguous. For example, the following
code is ambiguous:
using decades = duration<int, ratio_multiply<ratio<10>, years::period>>;
auto ymd = 2001y/January/1d;
ymd += decades(1); // error, ambiguous
while less usual durations that are only convertible to months
work correctly:
using decamonths = duration<int, ratio_multiply<ratio<10>, months::period>>; auto ymd = 2001y/January/1d; ymd += decamonths(1);
The example implementation resolves the issues by making sure that the
years
overload will be preferred in case of ambiguity, by declaring the
months
overload a function template with a default argument for its parameter
(suggested by Tim Song):
template<class = unspecified> constexpr year_month_weekday& operator+=(const months& m) noexcept; constexpr year_month_weekday& operator+=(const years& m) noexcept;
[2019-09-14 Priority set to 2 based on reflector discussion]
[2019-09-14; Tomasz and Howard provide concrete wording]
Previous resolution [SUPERSEDED]:
This wording is relative to N4830.
[Drafting note: Suggested wording below assumes that we can add a Constraints: to a signature where the constraint does not apply to a deduced template. We have examples of such constraints in other parts of the WD (e.g. 20.3.1.3.2 [unique.ptr.single.ctor]/p15, 20.3.1.3.4 [unique.ptr.single.asgn]/p1). And we have the old form "does not participate …" being used for non-deduced templates in several places as well (e.g. 22.3.2 [pairs.pair]/p5).
There are several ways of implementing such a constraint, such as adding a gratuitous template parameter.]
Modify 30.8.13.2 [time.cal.ym.members] as indicated:
constexpr year_month& operator+=(const months& dm) noexcept;-?- Constraints: The argument supplied by the caller for the
-4- Effects:months
parameter is not implicitly convertible toyears
.*this = *this + dm
. […]constexpr year_month& operator-=(const months& dm) noexcept;-?- Constraints: The argument supplied by the caller for the
-6- Effects:months
parameter is not implicitly convertible toyears
.*this = *this - dm
. […]Modify 30.8.13.3 [time.cal.ym.nonmembers] as indicated:
constexpr year_month operator+(const year_month& ym, const months& dm) noexcept;-?- Constraints: The argument supplied by the caller for the
-3- Returns: Amonths
parameter is not implicitly convertible toyears
.year_month
valuez
such thatz - ym == dm
. […]constexpr year_month operator+(const months& dm, const year_month& ym) noexcept;-?- Constraints: The argument supplied by the caller for the
-5- Returns:months
parameter is not implicitly convertible toyears
.ym + dm
.constexpr year_month operator-(const year_month& ym, const months& dm) noexcept;-?- Constraints: The argument supplied by the caller for the
-6- Returns:months
parameter is not implicitly convertible toyears
.ym + -dm
.Modify 30.8.14.2 [time.cal.ymd.members] as indicated:
constexpr year_month_day& operator+=(const months& m) noexcept;-?- Constraints: The argument supplied by the caller for the
-7- Effects:months
parameter is not implicitly convertible toyears
.*this = *this + m
. […]constexpr year_month_day& operator-=(const months& m) noexcept;-?- Constraints: The argument supplied by the caller for the
-9- Effects:months
parameter is not implicitly convertible toyears
.*this = *this - m
. […]Modify 30.8.14.3 [time.cal.ymd.nonmembers] as indicated:
constexpr year_month_day operator+(const year_month_day& ymd, const months& dm) noexcept;-?- Constraints: The argument supplied by the caller for the
-3- Returns:months
parameter is not implicitly convertible toyears
.(ymd.year() / ymd.month() + dm) / ymd.day()
. […]constexpr year_month_day operator+(const months& dm, const year_month_day& ymd) noexcept;-?- Constraints: The argument supplied by the caller for the
-5- Returns:months
parameter is not implicitly convertible toyears
.ymd + dm
.constexpr year_month_day operator+(const months& dm, const year_month_day& ymd) noexcept;-?- Constraints: The argument supplied by the caller for the
-6- Returns:months
parameter is not implicitly convertible toyears
.ymd + (-dm)
.Modify 30.8.15.2 [time.cal.ymdlast.members] as indicated:
constexpr year_month_day_last& operator+=(const months& m) noexcept;-?- Constraints: The argument supplied by the caller for the
-2- Effects:months
parameter is not implicitly convertible toyears
.*this = *this + m
. […]constexpr year_month_day_last& operator-=(const months& m) noexcept;-?- Constraints: The argument supplied by the caller for the
-4- Effects:months
parameter is not implicitly convertible toyears
.*this = *this - m
. […]Modify 30.8.15.3 [time.cal.ymdlast.nonmembers] as indicated:
constexpr year_month_day_last operator+(const year_month_day_last& ymdl, const months& dm) noexcept;-?- Constraints: The argument supplied by the caller for the
-3- Returns:months
parameter is not implicitly convertible toyears
.(ymdl.year() / ymdl.month() + dm) / last
.constexpr year_month_day_last operator+(const months& dm, const year_month_day_last& ymdl) noexcept;-?- Constraints: The argument supplied by the caller for the
-4- Returns:months
parameter is not implicitly convertible toyears
.ymdl + dm
.constexpr year_month_day_last operator-(const year_month_day_last& ymdl, const months& dm) noexcept;-?- Constraints: The argument supplied by the caller for the
-5- Returns:months
parameter is not implicitly convertible toyears
.ymdl + (-dm)
.Modify 30.8.16.2 [time.cal.ymwd.members] as indicated:
constexpr year_month_weekday& operator+=(const months& m) noexcept;-?- Constraints: The argument supplied by the caller for the
-6- Effects:months
parameter is not implicitly convertible toyears
.*this = *this + m
. […]constexpr year_month_weekday& operator-=(const months& m) noexcept;-?- Constraints: The argument supplied by the caller for the
-8- Effects:months
parameter is not implicitly convertible toyears
.*this = *this - m
. […]Modify 30.8.16.3 [time.cal.ymwd.nonmembers] as indicated:
constexpr year_month_weekday operator+(const year_month_weekday& ymwd, const months& dm) noexcept;-?- Constraints: The argument supplied by the caller for the
-2- Returns:months
parameter is not implicitly convertible toyears
.(ymwd.year() / ymwd.month() + dm) / ymwd.weekday_indexed()
.constexpr year_month_weekday operator+(const months& dm, const year_month_weekday& ymwd) noexcept;-?- Constraints: The argument supplied by the caller for the
-3- Returns:months
parameter is not implicitly convertible toyears
.ymwd + dm
.constexpr year_month_weekday operator-(const year_month_weekday& ymwd, const months& dm) noexcept;-?- Constraints: The argument supplied by the caller for the
-4- Returns:months
parameter is not implicitly convertible toyears
.ymwd + (-dm)
.Modify 30.8.17.2 [time.cal.ymwdlast.members] as indicated:
constexpr year_month_weekday_last& operator+=(const months& m) noexcept;-?- Constraints: The argument supplied by the caller for the
-2- Effects:months
parameter is not implicitly convertible toyears
.*this = *this + m
. […]constexpr year_month_weekday_last& operator-=(const months& m) noexcept;-?- Constraints: The argument supplied by the caller for the
-4- Effects:months
parameter is not implicitly convertible toyears
.*this = *this - m
. […]Modify 30.8.17.3 [time.cal.ymwdlast.nonmembers] as indicated:
constexpr year_month_weekday_last operator+(const year_month_weekday_last& ymwdl, const months& dm) noexcept;-?- Constraints: The argument supplied by the caller for the
-2- Returns:months
parameter is not implicitly convertible toyears
.(ymwdl.year() / ymwdl.month() + dm) / ymwdl.weekday_last()
.constexpr year_month_weekday_last operator+(const months& dm, const year_month_weekday_last& ymwdl) noexcept;-?- Constraints: The argument supplied by the caller for the
-3- Returns:months
parameter is not implicitly convertible toyears
.ymwdl + dm
.constexpr year_month_weekday_last operator-(const year_month_weekday_last& ymwdl, const months& dm) noexcept;-?- Constraints: The argument supplied by the caller for the
-4- Returns:months
parameter is not implicitly convertible toyears
.ymwdl + (-dm)
.
[2020-02-13, Prague]
Tim Song found a wording problem that we would like to resolve:
Given a class likestruct C : months { operator years(); };
The previous wording requires calls with a C
argument to use the years
overload, which
would require implementation heroics since its conversion sequence to months
is better than years
.
[2020-02 Status to Immediate on Friday morning in Prague.]
Proposed resolution:
This wording is relative to N4849.
[Drafting note: Suggested wording below assumes that we can add a Constraints: to a signature where the constraint does not apply to a deduced template. We have examples of such constraints in other parts of the WD (e.g. 20.3.1.3.2 [unique.ptr.single.ctor]/p15, 20.3.1.3.4 [unique.ptr.single.asgn]/p1). And we have the old form "does not participate …" being used for non-deduced templates in several places as well (e.g. 22.3.2 [pairs.pair]/p5).
There are several ways of implementing such a constraint, such as adding a gratuitous template parameter.]
Modify 30.8.13.2 [time.cal.ym.members] as indicated:
constexpr year_month& operator+=(const months& dm) noexcept;-?- Constraints: If the argument supplied by the caller for the
-4- Effects:months
parameter is convertible toyears
, its implicit conversion sequence toyears
is worse than its implicit conversion sequence tomonths
(12.2.4.3 [over.ics.rank]).*this = *this + dm
. […]constexpr year_month& operator-=(const months& dm) noexcept;-?- Constraints: If the argument supplied by the caller for the
-6- Effects:months
parameter is convertible toyears
, its implicit conversion sequence toyears
is worse than its implicit conversion sequence tomonths
(12.2.4.3 [over.ics.rank]).*this = *this - dm
. […]
Modify 30.8.13.3 [time.cal.ym.nonmembers] as indicated:
constexpr year_month operator+(const year_month& ym, const months& dm) noexcept;-?- Constraints: If the argument supplied by the caller for the
-3- Returns: Amonths
parameter is convertible toyears
, its implicit conversion sequence toyears
is worse than its implicit conversion sequence tomonths
(12.2.4.3 [over.ics.rank]).year_month
valuez
such thatz - ym == dm
. […]constexpr year_month operator+(const months& dm, const year_month& ym) noexcept;-?- Constraints: If the argument supplied by the caller for the
-5- Returns:months
parameter is convertible toyears
, its implicit conversion sequence toyears
is worse than its implicit conversion sequence tomonths
(12.2.4.3 [over.ics.rank]).ym + dm
.constexpr year_month operator-(const year_month& ym, const months& dm) noexcept;-?- Constraints: If the argument supplied by the caller for the
-6- Returns:months
parameter is convertible toyears
, its implicit conversion sequence toyears
is worse than its implicit conversion sequence tomonths
(12.2.4.3 [over.ics.rank]).ym + -dm
.
Modify 30.8.14.2 [time.cal.ymd.members] as indicated:
constexpr year_month_day& operator+=(const months& m) noexcept;-?- Constraints: If the argument supplied by the caller for the
-7- Effects:months
parameter is convertible toyears
, its implicit conversion sequence toyears
is worse than its implicit conversion sequence tomonths
(12.2.4.3 [over.ics.rank]).*this = *this + m
. […]constexpr year_month_day& operator-=(const months& m) noexcept;-?- Constraints: If the argument supplied by the caller for the
-9- Effects:months
parameter is convertible toyears
, its implicit conversion sequence toyears
is worse than its implicit conversion sequence tomonths
(12.2.4.3 [over.ics.rank]).*this = *this - m
. […]
Modify 30.8.14.3 [time.cal.ymd.nonmembers] as indicated:
constexpr year_month_day operator+(const year_month_day& ymd, const months& dm) noexcept;-?- Constraints: If the argument supplied by the caller for the
-3- Returns:months
parameter is convertible toyears
, its implicit conversion sequence toyears
is worse than its implicit conversion sequence tomonths
(12.2.4.3 [over.ics.rank]).(ymd.year() / ymd.month() + dm) / ymd.day()
. […]constexpr year_month_day operator+(const months& dm, const year_month_day& ymd) noexcept;-?- Constraints: If the argument supplied by the caller for the
-5- Returns:months
parameter is convertible toyears
, its implicit conversion sequence toyears
is worse than its implicit conversion sequence tomonths
(12.2.4.3 [over.ics.rank]).ymd + dm
.constexpr year_month_day operator+(const months& dm, const year_month_day& ymd) noexcept;-?- Constraints: If the argument supplied by the caller for the
-6- Returns:months
parameter is convertible toyears
, its implicit conversion sequence toyears
is worse than its implicit conversion sequence tomonths
(12.2.4.3 [over.ics.rank]).ymd + (-dm)
.
Modify 30.8.15.2 [time.cal.ymdlast.members] as indicated:
constexpr year_month_day_last& operator+=(const months& m) noexcept;-?- Constraints: If the argument supplied by the caller for the
-2- Effects:months
parameter is convertible toyears
, its implicit conversion sequence toyears
is worse than its implicit conversion sequence tomonths
(12.2.4.3 [over.ics.rank]).*this = *this + m
. […]constexpr year_month_day_last& operator-=(const months& m) noexcept;-?- Constraints: If the argument supplied by the caller for the
-4- Effects:months
parameter is convertible toyears
, its implicit conversion sequence toyears
is worse than its implicit conversion sequence tomonths
(12.2.4.3 [over.ics.rank]).*this = *this - m
. […]
Modify 30.8.15.3 [time.cal.ymdlast.nonmembers] as indicated:
constexpr year_month_day_last operator+(const year_month_day_last& ymdl, const months& dm) noexcept;-?- Constraints: If the argument supplied by the caller for the
-3- Returns:months
parameter is convertible toyears
, its implicit conversion sequence toyears
is worse than its implicit conversion sequence tomonths
(12.2.4.3 [over.ics.rank]).(ymdl.year() / ymdl.month() + dm) / last
.constexpr year_month_day_last operator+(const months& dm, const year_month_day_last& ymdl) noexcept;-?- Constraints: If the argument supplied by the caller for the
-4- Returns:months
parameter is convertible toyears
, its implicit conversion sequence toyears
is worse than its implicit conversion sequence tomonths
(12.2.4.3 [over.ics.rank]).ymdl + dm
.constexpr year_month_day_last operator-(const year_month_day_last& ymdl, const months& dm) noexcept;-?- Constraints: If the argument supplied by the caller for the
-5- Returns:months
parameter is convertible toyears
, its implicit conversion sequence toyears
is worse than its implicit conversion sequence tomonths
(12.2.4.3 [over.ics.rank]).ymdl + (-dm)
.
Modify 30.8.16.2 [time.cal.ymwd.members] as indicated:
constexpr year_month_weekday& operator+=(const months& m) noexcept;-?- Constraints: If the argument supplied by the caller for the
-6- Effects:months
parameter is convertible toyears
, its implicit conversion sequence toyears
is worse than its implicit conversion sequence tomonths
(12.2.4.3 [over.ics.rank]).*this = *this + m
. […]constexpr year_month_weekday& operator-=(const months& m) noexcept;-?- Constraints: If the argument supplied by the caller for the
-8- Effects:months
parameter is convertible toyears
, its implicit conversion sequence toyears
is worse than its implicit conversion sequence tomonths
(12.2.4.3 [over.ics.rank]).*this = *this - m
. […]
Modify 30.8.16.3 [time.cal.ymwd.nonmembers] as indicated:
constexpr year_month_weekday operator+(const year_month_weekday& ymwd, const months& dm) noexcept;-?- Constraints: If the argument supplied by the caller for the
-2- Returns:months
parameter is convertible toyears
, its implicit conversion sequence toyears
is worse than its implicit conversion sequence tomonths
(12.2.4.3 [over.ics.rank]).(ymwd.year() / ymwd.month() + dm) / ymwd.weekday_indexed()
.constexpr year_month_weekday operator+(const months& dm, const year_month_weekday& ymwd) noexcept;-?- Constraints: If the argument supplied by the caller for the
-3- Returns:months
parameter is convertible toyears
, its implicit conversion sequence toyears
is worse than its implicit conversion sequence tomonths
(12.2.4.3 [over.ics.rank]).ymwd + dm
.constexpr year_month_weekday operator-(const year_month_weekday& ymwd, const months& dm) noexcept;-?- Constraints: If the argument supplied by the caller for the
-4- Returns:months
parameter is convertible toyears
, its implicit conversion sequence toyears
is worse than its implicit conversion sequence tomonths
(12.2.4.3 [over.ics.rank]).ymwd + (-dm)
.
Modify 30.8.17.2 [time.cal.ymwdlast.members] as indicated:
constexpr year_month_weekday_last& operator+=(const months& m) noexcept;-?- Constraints: If the argument supplied by the caller for the
-2- Effects:months
parameter is convertible toyears
, its implicit conversion sequence toyears
is worse than its implicit conversion sequence tomonths
(12.2.4.3 [over.ics.rank]).*this = *this + m
. […]constexpr year_month_weekday_last& operator-=(const months& m) noexcept;-?- Constraints: If the argument supplied by the caller for the
-4- Effects:months
parameter is convertible toyears
, its implicit conversion sequence toyears
is worse than its implicit conversion sequence tomonths
(12.2.4.3 [over.ics.rank]).*this = *this - m
. […]
Modify 30.8.17.3 [time.cal.ymwdlast.nonmembers] as indicated:
constexpr year_month_weekday_last operator+(const year_month_weekday_last& ymwdl, const months& dm) noexcept;-?- Constraints: If the argument supplied by the caller for the
-2- Returns:months
parameter is convertible toyears
, its implicit conversion sequence toyears
is worse than its implicit conversion sequence tomonths
(12.2.4.3 [over.ics.rank]).(ymwdl.year() / ymwdl.month() + dm) / ymwdl.weekday_last()
.constexpr year_month_weekday_last operator+(const months& dm, const year_month_weekday_last& ymwdl) noexcept;-?- Constraints: If the argument supplied by the caller for the
-3- Returns:months
parameter is convertible toyears
, its implicit conversion sequence toyears
is worse than its implicit conversion sequence tomonths
(12.2.4.3 [over.ics.rank]).ymwdl + dm
.constexpr year_month_weekday_last operator-(const year_month_weekday_last& ymwdl, const months& dm) noexcept;-?- Constraints: If the argument supplied by the caller for the
-4- Returns:months
parameter is convertible toyears
, its implicit conversion sequence toyears
is worse than its implicit conversion sequence tomonths
(12.2.4.3 [over.ics.rank]).ymwdl + (-dm)
.
Section: 30.12 [time.format] Status: C++20 Submitter: Tomasz Kamiński Opened: 2019-08-18 Last modified: 2021-02-25
Priority: 2
View other active issues in [time.format].
View all other issues in [time.format].
View all issues with C++20 status.
Discussion:
The current specification of the formatting for std::chrono::duration
and
std::hh_mm_ss
types is unclear in regards the the handling of negative values.
To illustrate:
std::cout << std::format("%H:%M:%S", -10'000s); // prints either -02:46:40 or -02:-46:-40
The indented behavior (and currently implemented, see here) is to apply the sign once, before the leftmost converted field.
[2019-09-14 Priority set to 2 based on reflector discussion]
Previous resolution [SUPERSEDED]:This wording is relative to N4830.
[Drafting note: With the above clarification, the specification of the
operator<<
forhh_mm_ss
may be simplified toformat("{:%T}", hms)
.]
Modify 30.12 [time.format] as indicated:
-2- Each conversion specifier conversion-spec is replaced by appropriate characters as described in Table [tab:time.format.spec]. Some of the conversion specifiers depend on the locale that is passed to the formatting function if the latter takes one, or the global locale otherwise. If the formatted object does not contain the information the conversion specifier refers to, an exception of type
-?- The result of formatting aformat_error
is thrown.std::chrono::duration
instance holding a negative value, or of anhh_mm_ss
objecth
for whichh.is_negative()
istrue
, is equivalent to the output of the corresponding positive value, with a-
character placed before the replacement of the leftmost conversion specifier. [Example:cout << format("%T", -10'000s); // prints: -02:46:40 cout << format("%H:%M:%S", -10'000s); // prints: -02:46:40 cout << format("minutes %M, hours %H, seconds %S", -10'000s); // prints: minutes -46, hours 02, seconds 40— end example]
-3- Unless explicitly requested, […]Modify 30.9.3 [time.hms.nonmembers] as indicated:
template<class charT, class traits, class Duration> basic_ostream<charT, traits>& operator<<(basic_ostream<charT, traits>& os, const hh_mm_ss<Duration>& hms);-1- Effects: Equivalent to:
return os << format(os.getloc(),hms.is_negative() ? STATICALLY-WIDEN<charT>("-{:%T}") :STATICALLY-WIDEN<charT>("{:%T}"),abs(hms.to_duration()));
[2019-09-14; Howard improves wording]
[2020-02; Status set to Immediate after LWG discussion Thursday in Prague. (Minor example wording cleanup)]
Proposed resolution:
This wording is relative to N4830.
[Drafting note: With the above clarification, the specification of the
operator<<
forhh_mm_ss
may be simplified toformat("{:%T}", hms)
.]
Modify 30.12 [time.format] as indicated:
-2- Each conversion specifier conversion-spec is replaced by appropriate characters as described in Table [tab:time.format.spec]. Some of the conversion specifiers depend on the locale that is passed to the formatting function if the latter takes one, or the global locale otherwise. If the formatted object does not contain the information the conversion specifier refers to, an exception of type
-?- The result of formatting aformat_error
is thrown.std::chrono::duration
instance holding a negative value, or of anhh_mm_ss
objecth
for whichh.is_negative()
istrue
, is equivalent to the output of the corresponding positive value, with aSTATICALLY-WIDEN<charT>("-")
character sequence placed before the replacement of the leftmost conversion specifier. [Example:cout << format("{%:T}", -10'000s); // prints: -02:46:40 cout << format("{:%H:%M:%S}", -10'000s); // prints: -02:46:40 cout << format("{:minutes %M, hours %H, seconds %S}", -10'000s); // prints: minutes -46, hours 02, seconds 40— end example]
-3- Unless explicitly requested, […]
Modify 30.9.3 [time.hms.nonmembers] as indicated:
template<class charT, class traits, class Duration> basic_ostream<charT, traits>& operator<<(basic_ostream<charT, traits>& os, const hh_mm_ss<Duration>& hms);-1- Effects: Equivalent to:
return os << format(os.getloc(),hms.is_negative() ? STATICALLY-WIDEN<charT>("-{:%T}") :STATICALLY-WIDEN<charT>("{:%T}"),abs(hms.to_duration()));
sized_range
and ranges::size
redundantly use disable_sized_range
Section: 25.4.3 [range.sized] Status: C++20 Submitter: Casey Carter Opened: 2019-08-26 Last modified: 2021-02-25
Priority: 1
View all other issues in [range.sized].
View all issues with C++20 status.
Discussion:
disable_sized_range
(25.4.3 [range.sized]) is an opt-out trait that users may
specialize when their range
type conforms to the syntax of sized_range
but not its
semantics, or when the type is so poorly suited to the Standard Library that even testing the
validity of the expressions r.size()
or size(r)
for a range r
is
impossible. The library inspects disable_sized_range
in two places. (1) In the definition of
the sized_range
concept:
If the pertinent specialization oftemplate<class T> concept sized_range = range<T> && !disable_sized_range<remove_cvref_t<T>> && requires(T& t) { ranges::size(t); };
disable_sized_range
is true
, we avoid checking
the validity of the expression ranges::size(t)
in the requires-expression.
(2) In the definition of the ranges::size
CPO itself (25.3.10 [range.prim.size]),
the validity of the expressions decay-copy(E.size())
and
decay-copy(size(E))
is not checked if the pertinent specialization of
disable_sized_range
is true
.
disable_sized_range
is effectively checked twice when evaluating
sized_range
. This redundancy could be forgiven, if it did not permit the existence of
non-sized_range
s for which ranges::size
returns a valid size:
We should remove this gap betweenstruct mytype {}; using A = mytype[42]; template <> constexpr bool std::ranges::disable_sized_range<A> = true; static_assert(std::ranges::range<A>); static_assert(!std::ranges::sized_range<A>); static_assert(std::ranges::size(A{}) == 42); struct myrange { constexpr int* begin() const { return nullptr; } constexpr int* end() const { return nullptr; } }; template <> constexpr bool std::ranges::disable_sized_range<myrange> = true; static_assert(std::ranges::range<myrange>); static_assert(!std::ranges::sized_range<myrange>); static_assert(std::ranges::size(myrange{}) == 0);
ranges::size
and sized_range
by checking
disable_sized_range
only in the definition of ranges::size
, and continuing to rely
on the validity of ranges::size
in the sized_range
concept.
[2019-09-14 Priority set to 1 based on reflector discussion]
[2019-11 Wednesday night issue processing in Belfast.]
Status to Ready
Proposed resolution:
This wording is relative to N4830.
Modify 25.4.3 [range.sized] as follows:
template<class T> concept sized_range = range<T> &&!disable_sized_range<remove_cvref_t<T>> &&requires(T& t) { ranges::size(t); };
move_iterator
's conversions are more broken after P1207Section: 24.5.4.4 [move.iter.cons] Status: C++23 Submitter: Casey Carter Opened: 2019-08-23 Last modified: 2023-11-22
Priority: 2
View all other issues in [move.iter.cons].
View all issues with C++23 status.
Discussion:
The converting constructor and assignment operator specified in 24.5.4.4 [move.iter.cons] were technically broken before P1207:
24.5.4.4 [move.iter.cons] para 3 (and 5 for that matter) is an instance of LWG
3105(i); it should instead mandate that u.base()
is convertible to Iterator
.
24.5.4.4 [move.iter.cons] para 5 uses "is convertible" to guard an assignment operation instead of a conversion; it should instead mandate.
After applying P1207R4 "Movability of Single-pass Iterators",
u.base()
is not always well-formed, exacerbating the problem. These operations must ensure
that u.base()
is well-formed.
Let's burninate "Constructs a move_iterator
" while we're touching this subclause.
We'll also burninate "Iterator operations applied..." since the requirement it wants to impose is covered (and indeed must be covered) by the specifications of those other operations.
[2019-09-14 Priority set to 2 based on reflector discussion]
Previous resolution [SUPERSEDED]:This wording is relative to N4830.
Modify 24.5.4.4 [move.iter.cons] as indicated:
constexpr move_iterator();-1- Effects:
Constructs aValue-initializesmove_iterator
, vingcurrent
.Iterator operations applied to the resulting iterator have defined behavior if and only if the corresponding operations are defined on a value-initialized iterator of typeIterator
.constexpr explicit move_iterator(Iterator i);-2- Effects:
Constructs a move_iterator, iInitializesingcurrent
withstd::move(i)
.template<class U> constexpr move_iterator(const move_iterator<U>& u);-3- Mandates:
-4- Effects:is well-formed and convertible to
Uu.base()Iterator
.Constructs aInitializesmove_iterator
, iingcurrent
withu.base()
.template<class U> constexpr move_iterator& operator=(const move_iterator<U>& u);-5- Mandates:
-6- Effects: AssignsU
is convertible toIterator
u.base()
is well-formed andis_assignable_v<Iterator&, const U&>
istrue
.u.base()
tocurrent
.
[2020-02-14; Prague]
LWG Review. Some wording improvements have been made and lead to revised wording.
[2020-02-16; Prague]
Reviewed revised wording and moved to Ready for Varna.
[2020-07-17; superseded by 3435(i)]
[2020-11-09 Approved In November virtual meeting. Status changed: Ready → WP.]
Proposed resolution:
This wording is relative to N4849.
Modify 24.5.4.4 [move.iter.cons] as indicated:
constexpr move_iterator();-1- Effects:
Constructs aValue-initializesmove_iterator
, vingcurrent
.Iterator operations applied to the resulting iterator have defined behavior if and only if the corresponding operations are defined on a value-initialized iterator of typeIterator
.constexpr explicit move_iterator(Iterator i);-2- Effects:
Constructs a move_iterator, iInitializesingcurrent
withstd::move(i)
.template<class U> constexpr move_iterator(const move_iterator<U>& u);-3- Mandates:
-4- Effects:is well-formed and convertible to
Uu.base()Iterator
.Constructs aInitializesmove_iterator
, iingcurrent
withu.base()
.template<class U> constexpr move_iterator& operator=(const move_iterator<U>& u);-5- Mandates:
-6- Effects: AssignsU
is convertible toIterator
u.base()
is well-formed andis_assignable_v<Iterator&, U>
istrue
.u.base()
tocurrent
.
to_chars(bool)
should be deletedSection: 28.2.1 [charconv.syn] Status: C++20 Submitter: Jens Maurer Opened: 2019-08-23 Last modified: 2021-02-25
Priority: 0
View all other issues in [charconv.syn].
View all issues with C++20 status.
Discussion:
28.2.2 [charconv.to.chars] does not present an overload for bool
(because it is neither a signed nor unsigned integer type), so an attempt to call
to_chars
with a bool argument would promote it to int
and
unambiguously call the int
overload of to_chars
.
bool
is 0/1
(as opposed to, say, "true"/"false"
).
The user should cast explicitly if he wants the 0/1
behavior. (Correspondingly,
there is no bool
overload for from_chars
in the status quo, and
conversions do not apply there because of the reference parameter.)
[2019-09-14 Issue Prioritization]
Status to Tentatively Ready and priority to 0 after eight positive votes on the reflector.
Proposed resolution:
This wording is relative to N4830.
Modify 28.2.1 [charconv.syn], header <charconv>
synopsis,
as indicated:
[…] // 28.2.2 [charconv.to.chars], primitive numerical output conversion struct to_chars_result { char* ptr; errc ec; friend bool operator==(const to_chars_result&, const to_chars_result&) = default; }; to_chars_result to_chars(char* first, char* last, see below value, int base = 10); to_chars_result to_chars(char* first, char* last, bool value, int base = 10) = delete; to_chars_result to_chars(char* first, char* last, float value); […]
Section: 30.13 [time.parse] Status: C++20 Submitter: Tomasz Kamiński Opened: 2019-09-01 Last modified: 2021-02-25
Priority: 2
View other active issues in [time.parse].
View all other issues in [time.parse].
View all issues with C++20 status.
Discussion:
None of the parse
manipulators for the chrono
types
specifies the result of the extraction from the stream, as consequence
they cannot be chained with the other read operations (at least portably).
For example the following code is not required to work:
std::chrono::sys_stime s; int x; std::cin >> std::chrono::parse("%X", s) >> x;
[2019-10 Priority set to 2 after reflector discussion]
Previous resolution [SUPERSEDED]:This wording is relative to N4830.
[Drafting note: As a drive-by fix the Remarks element is also converted to a Constraints element. The wording integrates the resolution for LWG 3235(i).
Modify 30.13 [time.parse] as indicated:
-1- Each parse overload specified in this subclause calls
from_stream
unqualified, so as to enable argument dependent lookup (6.5.4 [basic.lookup.argdep]). In the following paragraphs, letis
denote an object of typebasic_istream<charT, traits>
and letI
bebasic_istream<charT, traits>&
, wherecharT
andtraits
are template parameters in that context.template<class charT, class traits, class Alloc, class Parsable> unspecified parse(const basic_string<charT, traits, Alloc>& fmt, Parsable& tp);-2-
RemarksConstraints:This function shall not participate in overload resolution unlessThe expressionisfrom_stream(declval<basic_istream<charT, traits>&>(), fmt.c_str(), tp)a valid expressionwell-formed when treated as an unevaluated operand. -3- Returns: A manipulator such that, when extracted from athe expressionbasic_istream<charT, traits> is
,is >> parse(fmt, tp)
has typeI
, valueis
, and callsfrom_stream(is, fmt.c_str(), tp)
.template<class charT, class traits, class Alloc, class Parsable> unspecified parse(const basic_string<charT, traits, Alloc>& fmt, Parsable& tp, basic_string<charT, traits, Alloc>& abbrev);-4-
RemarksConstraints:This function shall not participate in overload resolution unlessThe expressionisfrom_stream(declval<basic_istream<charT, traits>&>(), fmt.c_str(), tp, addressof(abbrev))a valid expressionwell-formed when treated as an unevaluated operand. -5- Returns: A manipulator such that, when extracted from athe expressionbasic_istream<charT, traits> is
,is >> parse(fmt, tp, abbrev)
has typeI
, valueis
, and callsfrom_stream(is, fmt.c_str(), tp, addressof(abbrev))
.template<class charT, class traits, class Alloc, class Parsable> unspecified parse(const basic_string<charT, traits, Alloc>& fmt, Parsable& tp, minutes& offset);-6-
RemarksConstraints:This function shall not participate in overload resolution unlessThe expressionisfrom_stream(declval<basic_istream<charT, traits>&>(), fmt.c_str(), tp, declval<basic_string<charT, traits, Alloc>*>()nullptr, &offset)a valid expressionwell-formed when treated as an unevaluated operand. -7- Returns: A manipulator such that, when extracted from athe expressionbasic_istream<charT, traits> is
,is >> parse(fmt, tp, offset)
has typeI
, valueis
, and callsfrom_stream(is, fmt.c_str(), tp, static_cast<basic_string<charT, traits, Alloc>*>(nullptr), &offset)
.template<class charT, class traits, class Alloc, class Parsable> unspecified parse(const basic_string<charT, traits, Alloc>& fmt, Parsable& tp, basic_string<charT, traits, Alloc>& abbrev, minutes& offset);-8-
RemarksConstraints:This function shall not participate in overload resolution unlessThe expressionisfrom_stream(declval<basic_istream<charT, traits>&>(), fmt.c_str(), tp, addressof(abbrev), &offset)a valid expressionwell-formed when treated as an unevaluated operand. -9- Returns: A manipulator such that, when extracted from athe expressionbasic_istream<charT, traits> is
,is >> parse(fmt, tp, abbrev, offset)
has typeI
, valueis
, and callsfrom_stream(is, fmt.c_str(), tp, addressof(abbrev), &offset)
.
[2020-02-13, Prague]
Issue wording has been rebased.
[2020-02 Status to Immediate on Friday morning in Prague.]
Proposed resolution:
This wording is relative to N4849.
Modify 30.13 [time.parse] as indicated:
-1- Each
parse
overload specified in this subclause callsfrom_stream
unqualified, so as to enable argument dependent lookup (6.5.4 [basic.lookup.argdep]). In the following paragraphs, letis
denote an object of typebasic_istream<charT, traits>
and letI
bebasic_istream<charT, traits>&
, wherecharT
andtraits
are template parameters in that context.template<class charT, class traits, class Alloc, class Parsable> unspecified parse(const basic_string<charT, traits, Alloc>& fmt, Parsable& tp);-2- Constraints: The expression
isfrom_stream(declval<basic_istream<charT, traits>&>(), fmt.c_str(), tp)a valid expressionwell-formed when treated as an unevaluated operand. -3- Returns: A manipulator such that, when extracted from athe expressionbasic_istream<charT, traits> is
,is >> parse(fmt, tp)
has typeI
, valueis
, and callsfrom_stream(is, fmt.c_str(), tp)
.template<class charT, class traits, class Alloc, class Parsable> unspecified parse(const basic_string<charT, traits, Alloc>& fmt, Parsable& tp, basic_string<charT, traits, Alloc>& abbrev);-4- Constraints: The expression
isfrom_stream(declval<basic_istream<charT, traits>&>(), fmt.c_str(), tp, addressof(abbrev))a valid expressionwell-formed when treated as an unevaluated operand. -5- Returns: A manipulator such that, when extracted from athe expressionbasic_istream<charT, traits> is
,is >> parse(fmt, tp, abbrev)
has typeI
, valueis
, and callsfrom_stream(is, fmt.c_str(), tp, addressof(abbrev))
.template<class charT, class traits, class Alloc, class Parsable> unspecified parse(const basic_string<charT, traits, Alloc>& fmt, Parsable& tp, minutes& offset);-6- Constraints: The expression
is well-formed when treated as an unevaluated operand. -7- Returns: A manipulator such thatfrom_stream(declval<basic_istream<charT, traits>&>(), fmt.c_str(), tp, declval<basic_string<charT, traits, Alloc>*>(), &offset), when extracted from athe expressionbasic_istream<charT, traits> is
,is >> parse(fmt, tp, offset)
has typeI
, valueis
, and callsfrom_stream(is, fmt.c_str(), tp, static_cast<basic_string<charT, traits, Alloc>*>(nullptr), &offset)template<class charT, class traits, class Alloc, class Parsable> unspecified parse(const basic_string<charT, traits, Alloc>& fmt, Parsable& tp, basic_string<charT, traits, Alloc>& abbrev, minutes& offset);-8- Constraints: The expression
isfrom_stream(declval<basic_istream<charT, traits>&>(), fmt.c_str(), tp, addressof(abbrev), &offset)a valid expressionwell-formed when treated as an unevaluated operand. -9- Returns: A manipulator such that, when extracted from athe expressionbasic_istream<charT, traits> is
,is >> parse(fmt, tp, abbrev, offset)
has typeI
, valueis
, and callsfrom_stream(is, fmt.c_str(), tp, addressof(abbrev), &offset)
.
%j
with duration
sSection: 30.12 [time.format], 30.13 [time.parse] Status: C++20 Submitter: Howard Hinnant Opened: 2019-09-02 Last modified: 2021-02-25
Priority: 2
View other active issues in [time.format].
View all other issues in [time.format].
View all issues with C++20 status.
Discussion:
%j
represents the day number of the year when formatting and parsing time_point
s.
It is also handy to interpret this flag consistently when formatting and parsing durations. That
is if there is not enough information in the stream to represent a time_point
, and if the
target of the format/parse is a duration
, %j
represents a number of days.
#include <chrono> #include <iostream> #include <sstream> #include <string> int main() { using namespace std; using namespace std::chrono; // Parse %j as number of days into a duration istringstream in{"222"}; hours d; in >> parse("%j", d); cout << d << '\n'; cout << format("{:%j}", d) << '\n'; }
Output:
5328h 222
[2019-10 Priority set to 2 after reflector discussion]
Previous resolution [SUPERSEDED]:
This wording is relative to N4830.
Modify "Table 98 — Meaning of conversion specifiers" [tab:time.format.spec] as indicated:
Table 98 — Meaning of conversion specifiers [tab:time.format.spec] Specifier Replacement […]
%j
The day of the year as a decimal number. Jan 1 is 001
. If the result is less than three
digits, it is left-padded with0
to three digits. If the type being formatted is a
specialization ofduration
, it is formatted as a decimal number ofdays
.[…]
Modify "Table 99 — Meaning of
parse
flags" [tab:time.parse.spec] as indicated:
Table 99 — Meaning of parse
flags [tab:time.parse.spec]Flag Parsed value […]
%j
The day of the year as a decimal number. Jan 1 is 1
. The modified command%Nj
specifies the maximum number of characters to read. IfN
is not specified, the default
is3
. Leading zeroes are permitted but not required. If the type being parsed is a
specialization ofduration
, it is parsed as a decimal number ofdays
.[…]
[2020-02-13 After Thursday afternoon discussion in Prague, Marshall provides updated wording.]
[2020-02 Status to Immediate on Thursday night in Prague.]
Proposed resolution:
This wording is relative to N4830.
Modify "Table 98 — Meaning of conversion specifiers" [tab:time.format.spec] as indicated:
Table 98 — Meaning of conversion specifiers [tab:time.format.spec] Specifier Replacement […]
%j
If the type being formatted is a specialization of duration
, the decimal number ofdays
without padding. Otherwise, theTheday of the year as a decimal number.
Jan 1 is001
. If the result is less than three digits, it is left-padded with0
to three digits.[…]
Modify "Table 99 — Meaning of parse
flags"
[tab:time.parse.spec] as indicated:
Table 99 — Meaning of parse
flags [tab:time.parse.spec]Flag Parsed value […]
%j
If the type being parsed is a specialization of duration
,
a decimal number ofdays
. Otherwise, theTheday of the year as a decimal number. Jan 1 is1
.
In either case, theThemodified command%Nj
specifies the maximum number of characters to read.
IfN
is not specified, the default is3
. Leading zeroes are permitted but not required.[…]
%I%p
should parse/format duration
since midnightSection: 30.12 [time.format], 30.13 [time.parse] Status: C++20 Submitter: Howard Hinnant Opened: 2019-09-02 Last modified: 2021-02-25
Priority: 0
View other active issues in [time.format].
View all other issues in [time.format].
View all issues with C++20 status.
Discussion:
It is clear how "%I%p"
parses and formats time points. What is not clear is how these
flags interact with durations. We should treat durations as "elapsed time since midnight". For example:
#include <chrono> #include <iostream> #include <string> #include <sstream> int main() { using namespace std; using namespace std::chrono; // Format { // format time_point with %I%p cout << format("{:%F %I%p}", sys_days{2019_y/August/10}+14h) << '\n'; } { // format duration with %I%p cout << format("{:%I%p}", 14h) << '\n'; } // Parse { // Parse %I%p as day-of-year combined with an hour into a time_point istringstream in{"2019-08-10 2pm"}; system_clock::time_point tp; in >> parse("%F %I%p", tp); cout << tp << '\n'; } { // Parse %I%p as number of hours into a duration istringstream in{"2pm"}; hours d; in >> parse("%I%p", d); cout << d << '\n'; } }
Output:
2019-08-10 02PM 02PM 2019-08-10 14:00:00.000000 14h
[2019-10 Status set to 'Tentatively Ready' after reflector discussion]
Proposed resolution:
This wording is relative to N4830.
Modify 30.12 [time.format] as indicated:
-3- Unless explicitly requested, the result of formatting a chrono type does not contain time zone abbreviation and time zone offset information. If the information is available, the conversion specifiers
-?- If the type being formatted does not contain the information that the format flag needs, an exception of type%Z
and%z
will format this information (respectively). [Note: If the information is not available and a%Z
or%z
conversion specifier appears in the chrono-format-spec, an exception of typeformat_error
is thrown, as described above. — end note]format_error
is thrown. [Example: Aduration
does not contain enough information to format as aweekday
— end example]. However if a flag refers to a "time of day" (e.g.%H
,%I
,%p
, etc.), then a specialization ofduration
is interpreted as the time of day elapsed since midnight.
Modify 30.13 [time.parse] as indicated:
[Drafting note: The modification of 30.13 [time.parse] p1 is intended to be non-conflictingly mergeable with the change suggested by LWG 3269(i) and LWG 3271(i) at the same paragraph.]
-1- Each parse overload specified in this subclause calls
[…] -10- Allfrom_stream
unqualified, so as to enable argument dependent lookup (6.5.4 [basic.lookup.argdep]). In the following paragraphs, letis
denote an object of typebasic_istream<charT, traits>
, wherecharT
andtraits
are template parameters in that context.from_stream
overloads behave as unformatted input functions, except that they have an unspecified effect on the value returned by subsequent calls tobasic_istream<>::gcount()
. Each overload takes a format string containing ordinary characters and flags which have special meaning. Each flag begins with a%
. Some flags can be modified byE
orO
. During parsing each flag interprets characters as parts of date and time types according to Table [tab:time.parse.spec]. Some flags can be modified by a width parameter given as a positive decimal integer called out asN
below which governs how many characters are parsed from the stream in interpreting the flag. All characters in the format string that are not represented in Table [tab:time.parse.spec], except for white space, are parsed unchanged from the stream. A white space character matches zero or more white space characters in the input stream. -?- If the type being parsed can not represent the information that the format flag refers to,is.setstate(ios_base::failbit)
is called. [Example: Aduration
cannot represent aweekday
— end example]. However if a flag refers to a "time of day" (e.g.%H
,%I
,%p
, etc.), then a specialization ofduration
is parsed as the time of day elapsed since midnight.
weekday_indexed
to range of [0, 7]
Section: 30.8.7.2 [time.cal.wdidx.members] Status: C++20 Submitter: Howard Hinnant Opened: 2019-09-02 Last modified: 2021-02-25
Priority: 0
View all issues with C++20 status.
Discussion:
On one hand, we say that if you try to construct a weekday_indexed
with index 0,
you get an unspecified index instead (30.8.7.2 [time.cal.wdidx.members]/p1),:
The values held are unspecified if
!wd.ok()
or index is not in the range[1, 5]
.
OTOH, we take pains to pin down year_month_weekday
's conversion to sys_days
when the index is zero (30.8.7.2 [time.cal.wdidx.members]/p19):
If
index()
is0
the returnedsys_days
represents the date7
days prior to the firstweekday()
ofyear()/month()
.
This is inconsistent. We should allow a slightly wider range (say, [0, 7]
, since you need
at least 3 bits anyway to represent the 5 distinct valid values, and the 0
value referred
to by 30.8.7.2 [time.cal.wdidx.members]/p19.
[2019-09-24 Issue Prioritization]
Status to Tentatively Ready and priority to 0 after six positive votes on the reflector.
Proposed resolution:
This wording is relative to N4830.
Modify 30.8.7.2 [time.cal.wdidx.members] as indicated:
[Drafting note: As a drive-by fix a cleanup of "Constructs an object of type
weekday_indexed
" wording has been applied as well. ]
constexpr weekday_indexed(const chrono::weekday& wd, unsigned index) noexcept;-1- Effects:
Constructs an object of typeInitializesweekday_indexed
by initializingwd_
withwd
andindex_
withindex
. The values held are unspecified if!wd.ok()
orindex
is not in the range[
.10,57]
<span>
Section: 17.3.1 [support.limits.general] Status: C++20 Submitter: Jonathan Wakely Opened: 2019-09-05 Last modified: 2021-02-25
Priority: 0
View all other issues in [support.limits.general].
View all issues with C++20 status.
Discussion:
There is no feature test macro for std::span
.
201803L
for the original addition of
<span>
by P0122R7 (Jacksonville, 2018) and then
201902L
for the API changes from P1024R3 (Kona, 2019).
The C++ working draft only needs the newer value.
[2019-09-24 Issue Prioritization]
Status to Tentatively Ready and priority to 0 after eight positive votes on the reflector.
Proposed resolution:
This wording is relative to N4830.
Modify the Table 36 "Standard library feature-test macros" [tab:support.ft] in 17.3.1 [support.limits.general] as indicated:
Table 36: Standard library feature-test macros [tab:support.ft] Macro name Value Header(s) […]
__cpp_lib_span
201902L
<span>
[…]
split_view::outer_iterator::value_type
should inherit from view_interface
Section: 25.7.16.4 [range.lazy.split.outer.value] Status: C++20 Submitter: Eric Niebler Opened: 2019-09-09 Last modified: 2023-02-07
Priority: 0
View all other issues in [range.lazy.split.outer.value].
View all issues with C++20 status.
Discussion:
It is a view. It should have all the view goodies. Suggested priority P1 because it affects ABI.
The proposed change has been implemented and tested in range-v3.[2019-09-24 Issue Prioritization]
Status to Tentatively Ready and priority to 0 after six positive votes on the reflector.
Proposed resolution:
This wording is relative to N4830.
Modify [range.split.outer.value], class split_view::outer_iterator::value_type
synopsis, as indicated:
namespace std::ranges { template<class V, class Pattern> template<bool Const> struct split_view<V, Pattern>::outer_iterator<Const>::value_type : view_interface<value_type> { private: outer_iterator i_ = outer_iterator(); // exposition only public: value_type() = default; constexpr explicit value_type(outer_iterator i); constexpr inner_iterator<Const> begin() const; constexpr default_sentinel_t end() const; }; }
weakly_incrementable
Section: 24.3.4.13 [iterator.concept.random.access] Status: C++20 Submitter: Eric Niebler Opened: 2019-09-09 Last modified: 2021-02-25
Priority: 0
View all other issues in [iterator.concept.random.access].
View all issues with C++20 status.
Discussion:
See 24.3.4.13 [iterator.concept.random.access]/2.6, which shows ++
being applied to a prvalue iterator.
[2019-09-24 Issue Prioritization]
Status to Tentatively Ready and priority to 0 after six positive votes on the reflector.
Proposed resolution:
This wording is relative to N4830.
Modify 24.3.4.13 [iterator.concept.random.access] as indicated:
-2- Let
a
andb
be valid iterators of typeI
such thatb
is reachable froma
aftern
applications of++a
, letD
beiter_difference_t<I>
, and letn
denote a value of typeD
.I
modelsrandom_access_iterator
only if
(2.1) —
(a += n)
is equal tob
.[…]
(2.6) — If
(a + D(n - 1))
is valid, then(a + n)
is equal to.
++[](I c){ return ++c; }(a + D(n - 1))[…]
join_view<V>::iterator<true>
tries to write through const join_view
ptrSection: 25.7.14.2 [range.join.view] Status: Resolved Submitter: Eric Niebler Opened: 2019-09-09 Last modified: 2020-09-06
Priority: 2
View all other issues in [range.join.view].
View all issues with Resolved status.
Discussion:
The non-const
join_view::begin()
returns iterator<simple-view<V>>
.
If simple-view<V>
is true
, then the iterator stores a const join_view*
named parent_
. iterator::satisfy()
will try to write to parent_->inner_
if
ref_is_glvalue
is false
. That doesn't work because the inner_
field is not marked
mutable.
[2019-10 Priority set to 2 after reflector discussion]
[2020-02-10, Prague]
Would be resolved by P1983R0.
[2020-08-21 Issue processing telecon: resolved by P1983R0 §2.4. Status changed: New → Resolved.]
Proposed resolution:
This wording is relative to N4830.
Modify 25.7.14.2 [range.join.view], class template join_view
synopsis, as indicated:
[Drafting note: Changing the
join_view<V>::inner_
member to be mutable is safe because this exposition-only member is only used when thejoin_view
is single-pass and only modified by operations that invalidate other iterators]
namespace std::ranges { template<input_range V> requires view<V> && input_range<range_reference_t<V>>> && (is_reference_v<range_reference_t<V>> || view<range_value_t<V>>) class join_view : public view_interface<join_view<V>> { private: […] V base_ = V(); // exposition only mutable all_view<InnerRng> inner_ = // exposition only, present only when // !is_reference_v<InnerRng> all_view<InnerRng>(); public: […] }; }
shared_ptr<int>&
does not not satisfy readable
Section: 24.3.4.2 [iterator.concept.readable] Status: Resolved Submitter: Eric Niebler Opened: 2019-09-09 Last modified: 2020-09-06
Priority: 1
View all issues with Resolved status.
Discussion:
In the current spec, shared_ptr<int>
is readable
, but shared_ptr<int>&
is not. That is because readable_traits
is not stripping top-level references before testing
for nested typedefs.
readable_traits
should see through cv- and ref-qualifiers, or else the
readable
concept should strip top-level references when building the iter_value_t
associated type (e.g., iter_value_t<remove_reference_t<In>>
).
Suggest priority P1 because it effects the definition of a concept which cannot change after C++20.
[2019-10 Priority set to 1 after reflector discussion]
[2019-11 This should be resolved by P1878]
Previous resolution [SUPERSEDED]:
This wording is relative to N4830.
Modify 24.3.4.2 [iterator.concept.readable], concept
readable
synopsis, as indicated:template<class In> concept readable = requires { typename iter_value_t<remove_reference_t<In>>; typename iter_reference_t<In>; typename iter_rvalue_reference_t<In>; } && common_reference_with<iter_reference_t<In>&&, iter_value_t<remove_reference_t<In>>&> && common_reference_with<iter_reference_t<In>&&, iter_rvalue_reference_t<In>&&> && common_reference_with<iter_rvalue_reference_t<In>&&, const iter_value_t<remove_reference_t<In>>&>;
[2019-11; Resolved by the adoption of P1878 in Belfast]
Proposed resolution:
Resolved by P1878.
Section: 25.7.8.2 [range.filter.view], 25.7.9.2 [range.transform.view], 25.7.10.2 [range.take.view], 25.7.14.2 [range.join.view], 25.7.17.2 [range.split.view], 25.7.21.2 [range.reverse.view] Status: C++20 Submitter: Eric Niebler Opened: 2019-09-09 Last modified: 2021-02-25
Priority: 1
View all issues with C++20 status.
Discussion:
The following program fails to compile:
#include <ranges> int main() { namespace ranges = std::ranges; int a[] = {1, 7, 3, 6, 5, 2, 4, 8}; auto r0 = ranges::view::reverse(a); auto is_even = [](int i) { return i % 2 == 0; }; auto r1 = ranges::view::filter(r0, is_even); int sum = 0; for (auto i : r1) { sum += i; } return sum - 20; }
The problem comes from constraint recursion, caused by the following constructor:
template<viewable_range R> requires bidirectional_range<R> && constructible_from<V, all_view<R>> constexpr explicit reverse_view(R&& r);
This constructor owes its existence to class template argument deduction; it is the constructor
we intend to use to resolve reverse_view{r}
, which (in accordance to the deduction guide)
will construct an object of type reverse_view<all_view<decltype(r)>>
.
all_view<R>
is always one of:
decay_t<R>
ref_view<remove_reference_t<R>>
subrange<iterator_t<R>, sentinel_t<R>, [sized?]>
In all cases, there is a conversion from r
to the destination type. As a result, the
following non-template reverse_view
constructor can fulfill the duty that the above
constructor was meant to fulfill, and does not cause constraint recursion:
constexpr explicit reverse_view(V r);
In short, the problematic constructor can simply be removed with no negative impact on the design. And the similar constructors from the other range adaptors should similarly be stricken.
Suggested priority P1. The view types are unusable without this change. This proposed resolution has been implemented in range-v3 and has been shipping for some time.[2019-10 Priority set to 1 after reflector discussion]
[2019-10 Status set to ready Wednesday night discussion in Belfast.]
Proposed resolution:
This wording is relative to N4830.
Modify 25.7.8.2 [range.filter.view] as indicated:
namespace std::ranges { template<input_range V, indirect_unary_predicate<iterator_t<V>> Pred> requires view<V> && is_object_v<Pred> class filter_view : public view_interface<filter_view<V, Pred>> { […] public: filter_view() = default; constexpr filter_view(V base, Pred pred);[…]template<input_range R> requires viewable_range<R> && constructible_from<V, all_view<R>> constexpr filter_view(R&& r, Pred pred);[…] }; […] }template<input_range R> requires viewable_range<R> && constructible_from<V, all_view<R>> constexpr filter_view(R&& r, Pred pred);
-2- Effects: Initializesbase_
withviews::all(std::forward<R>(r))
and initializespred_
withstd::move(pred)
.
Modify 25.7.9.2 [range.transform.view] as indicated:
namespace std::ranges { template<input_range V, copy_constructible F> requires view<V> && is_object_v<F> && regular_invocable<F&, range_reference_t<V>> class transform_view : public view_interface<transform_view<V, F>> { private: […] public: transform_view() = default; constexpr transform_view(V base, F fun);[…]template<input_range R> requires viewable_range<R> && constructible_from<V, all_view<R>> constexpr transform_view(R&& r, F fun);[…] }; […] }template<input_range R> requires viewable_range<R> && constructible_from<V, all_view<R>> constexpr transform_view(R&& r, F fun);
-2- Effects: Initializesbase_
withviews::all(std::forward<R>(r))
andfun_
withstd::move(fun)
.
Modify 25.7.10.2 [range.take.view] as indicated:
namespace std::ranges { template<view V> class take_view : public view_interface<take_view<V>> { private: […] public: take_view() = default; constexpr take_view(V base, range_difference_t<V> count);[…]template<viewable_range R> requires constructible_from<V, all_view<R>> constexpr take_view(R&& r, range_difference_t<V> count);[…] }; […] }template<viewable_range R> requires constructible_from<V, all_view<R>> constexpr take_view(R&& r, range_difference_t<V> count);
-2- Effects: Initializesbase_
withviews::all(std::forward<R>(r))
andcount_
withcount
.
Modify 25.7.14.2 [range.join.view] as indicated:
namespace std::ranges { template<input_range V> requires view<V> && input_range<range_reference_t<V>> && (is_reference_v<range_reference_t<V>> || view<range_value_t<V>>) class join_view : public view_interface<join_view<V>> { private: […] public: join_view() = default; constexpr explicit join_view(V base);[…]template<input_range R> requires viewable_range<R> && constructible_from<V, all_view<R>> constexpr explicit join_view(R&& r);[…] }; […] }template<input_range R> requires viewable_range<R> && constructible_from<V, all_view<R>> constexpr explicit join_view(R&& r);
-2- Effects: Initializesbase_
withviews::all(std::forward<R>(r))
.
Modify 25.7.17.2 [range.split.view] as indicated:
namespace std::ranges { […] template<input_range V, forward_range Pattern> requires view<V> && view<Pattern> && indirectly_comparable<iterator_t<V>, iterator_t<Pattern>, ranges::equal_to> && (forward_range<V> || tiny-range<Pattern>) class split_view : public view_interface<split_view<V, Pattern>> { private: […] public: split_view() = default; constexpr split_view(V base, Pattern pattern);[…]template<input_range R, forward_range P> requires constructible_from<V, all_view<R>> && constructible_from<Pattern, all_view<P>> constexpr split_view(R&& r, P&& p);[…] }; […] }template<input_range R, forward_range P> requires constructible_from<V, all_view<R>> && constructible_from<Pattern, all_view<P>> constexpr split_view(R&& r, P&& p);
-2- Effects: Initializesbase_
withviews::all(std::forward<R>(r))
, andpattern_
withviews::all(std::forward<P>(p))
.
Modify 25.7.21.2 [range.reverse.view] as indicated:
namespace std::ranges { template<view V> requires bidirectional_range<V> class reverse_view : public view_interface<reverse_view<V>> { private: […] public: reverse_view() = default; constexpr explicit reverse_view(V r);[…]template<viewable_range R> requires bidirectional_range<R> && constructible_from<V, all_view<R>> constexpr explicit reverse_view(R&& r);[…] }; […] }template<viewable_range R> requires bidirectional_range<R> && constructible_from<V, all_view<R>> constexpr explicit reverse_view(R&& r);
-2- Effects: Initializesbase_
withviews::all(std::forward<R>(r))
.
pair-like
types to subrange
is a silent semantic promotionSection: 25.5.4 [range.subrange] Status: C++20 Submitter: Eric Niebler Opened: 2019-09-10 Last modified: 2021-02-25
Priority: 1
View all other issues in [range.subrange].
View all issues with C++20 status.
Discussion:
Just because a pair
is holding two iterators, it doesn't mean those two iterators denote a
valid range. Implicitly converting such pair-like
types to a subrange
is
dangerous and should be disallowed.
[2019-10 Priority set to 1 and status to LEWG after reflector discussion]
[2019-11 Status to Ready after LWG discussion Friday in Belfast.]
Proposed resolution:
This wording is relative to N4830.
Modify 25.5.4 [range.subrange] as indicated:
namespace std::ranges { […]template<class T, class U, class V> concept pair-like-convertible-to = // exposition only !range<T> && pair-like<remove_reference_t<T>> && requires(T&& t) { { get<0>(std::forward<T>(t)) } -> convertible_to<U>; { get<1>(std::forward<T>(t)) } -> convertible_to<V>; };[…] template<input_or_output_iterator I, sentinel_for<I> S = I, subrange_kind K = sized_sentinel_for<S, I> ? subrange_kind::sized : subrange_kind::unsized> requires (K == subrange_kind::sized || !sized_sentinel_for<S, I>) class subrange : public view_interface<subrange<I, S, K>> { private: […] public: […]template<not-same-as<subrange> PairLike> requires pair-like-convertible-to<PairLike, I, S> constexpr subrange(PairLike&& r) requires (!StoreSize) : subrange{std::get<0>(std::forward<PairLike>(r)), std::get<1>(std::forward<PairLike>(r))} {} template<pair-like-convertible-to<I, S> PairLike> constexpr subrange(PairLike&& r, make-unsigned-like-t(iter_difference_t<I>) n) requires (K == subrange_kind::sized) : subrange{std::get<0>(std::forward<PairLike>(r)), std::get<1>(std::forward<PairLike>(r)), n} {}[…] }; […] }
subrange
converting constructor should disallow derived to base conversionsSection: 25.5.4 [range.subrange] Status: C++20 Submitter: Eric Niebler Opened: 2019-09-10 Last modified: 2021-02-25
Priority: 1
View all other issues in [range.subrange].
View all issues with C++20 status.
Discussion:
The following code leads to slicing and general badness:
struct Base {}; struct Derived : Base {}; subrange<Derived*> sd; subrange<Base*> sb = sd;
Traversal operations on iterators that are pointers do pointer arithmetic. If a Base*
is
actually pointing to a Derived*
, then pointer arithmetic is invalid. subrange
's
constructors can easily flag this invalid code, and probably should.
[2019-10 Priority set to 1 and status to LEWG after reflector discussion]
[2019-10; Marshall comments]
This issue would resolve US-285.
[2019-11 LEWG says OK; Status to Open. Friday PM discussion in Belfast. Casey to investigate and report back.]
Previous resolution [SUPERSEDED]:
This wording is relative to N4830.
Modify 25.5.4 [range.subrange] as indicated:
namespace std::ranges { template<class From, class To> concept convertible-to-non-slicing = // exposition only convertible_to<From, To> && !(is_pointer_v<decay_t<From>> && is_pointer_v<decay_t<To>> && not-same-as<remove_pointer_t<decay_t<From>>, remove_pointer_t<decay_t<To>>>); template<class T> concept pair-like = // exposition only […]template<class T, class U, class V> concept pair-like-convertible-to = // exposition only !range<T> && pair-like<remove_reference_t<T>> && requires(T&& t) { { get<0>(std::forward<T>(t)) } -> convertible_to<U>; { get<1>(std::forward<T>(t)) } -> convertible_to<V>; };template<class T, class U, class V> concept pair-like-convertible-from = // exposition only !range<T> && pair-like<T> && constructible_from<T, U, V> && convertible-to-non-slicing<U, tuple_element_t<0, T>> && convertible_to<V, tuple_element_t<1, T>>; […] […] template<input_or_output_iterator I, sentinel_for<I> S = I, subrange_kind K = sized_sentinel_for<S, I> ? subrange_kind::sized : subrange_kind::unsized> requires (K == subrange_kind::sized || !sized_sentinel_for<S, I>) class subrange : public view_interface<subrange<I, S, K>> { private: […] public: subrange() = default; constexpr subrange(convertible-to-non-slicing<I> auto i, S s) requires (!StoreSize); constexpr subrange(convertible-to-non-slicing<I> auto i, S s, make-unsigned-like-t(iter_difference_t<I>) n) requires (K == subrange_kind::sized); template<not-same-as<subrange> R> requires forwarding-range<R> &&convertible_toconvertible-to-non-slicing<iterator_t<R>, I> && convertible_to<sentinel_t<R>, S> constexpr subrange(R&& r) requires (!StoreSize || sized_range<R>); template<forwarding-range R> requires convertible_to<iterator_t<R>, I> && convertible_to<sentinel_t<R>, S> constexpr subrange(R&& r, make-unsigned-like-t(iter_difference_t<I>) n) requires (K == subrange_kind::sized) : subrange{ranges::begin(r), ranges::end(r), n} {}template<not-same-as<subrange> PairLike> requires pair-like-convertible-to<PairLike, I, S> constexpr subrange(PairLike&& r) requires (!StoreSize) : subrange{std::get<0>(std::forward<PairLike>(r)), std::get<1>(std::forward<PairLike>(r))} {} template<pair-like-convertible-to<I, S> PairLike> constexpr subrange(PairLike&& r, make-unsigned-like-t(iter_difference_t<I>) n) requires (K == subrange_kind::sized) : subrange{std::get<0>(std::forward<PairLike>(r)), std::get<1>(std::forward<PairLike>(r)), n} {}[…] }; template<input_or_output_iterator I, sentinel_for<I> S> subrange(I, S) -> subrange<I, S>; […] }Modify 25.5.4.2 [range.subrange.ctor] as indicated:
constexpr subrange(convertible-to-non-slicing<I> auto i, S s) requires (!StoreSize);-1- Expects: […]
[…]constexpr subrange(convertible-to-non-slicing<I> auto i, S s, make-unsigned-like-t(iter_difference_t<I>) n) requires (K == subrange_kind::sized);-2- Expects: […]
[…]template<not-same-as<subrange> R> requires forwarding-range<R> &&convertible_toconvertible-to-non-slicing<iterator_t<R>, I> && convertible_to<sentinel_t<R>, S> constexpr subrange(R&& r) requires (!StoreSize || sized_range<R>);-6- Effects: […]
[…]
[2020-02-10; Prague]
The group identified minor problems that have been fixed in the revised wording.
[2020-02-10 Move to Immediate Monday afternoon in Prague]
Proposed resolution:
This wording is relative to N4830.
Modify 25.5.4 [range.subrange] as indicated:
namespace std::ranges { template<class From, class To> concept convertible-to-non-slicing = // exposition only convertible_to<From, To> && !(is_pointer_v<decay_t<From>> && is_pointer_v<decay_t<To>> && not-same-as<remove_pointer_t<decay_t<From>>, remove_pointer_t<decay_t<To>>>); template<class T> concept pair-like = // exposition only […]template<class T, class U, class V> concept pair-like-convertible-to = // exposition only !range<T> && pair-like<remove_reference_t<T>> && requires(T&& t) { { get<0>(std::forward<T>(t)) } -> convertible_to<U>; { get<1>(std::forward<T>(t)) } -> convertible_to<V>; };template<class T, class U, class V> concept pair-like-convertible-from = // exposition only !range<T> && pair-like<T> && constructible_from<T, U, V> && convertible-to-non-slicing<U, tuple_element_t<0, T>> && convertible_to<V, tuple_element_t<1, T>>; […] […] template<input_or_output_iterator I, sentinel_for<I> S = I, subrange_kind K = sized_sentinel_for<S, I> ? subrange_kind::sized : subrange_kind::unsized> requires (K == subrange_kind::sized || !sized_sentinel_for<S, I>) class subrange : public view_interface<subrange<I, S, K>> { private: […] public: subrange() = default; constexpr subrange(convertible-to-non-slicing<I> auto i, S s) requires (!StoreSize); constexpr subrange(convertible-to-non-slicing<I> auto i, S s, make-unsigned-like-t(iter_difference_t<I>) n) requires (K == subrange_kind::sized); template<not-same-as<subrange> R> requires forwarding-range<R> &&convertible_toconvertible-to-non-slicing<iterator_t<R>, I> && convertible_to<sentinel_t<R>, S> constexpr subrange(R&& r) requires (!StoreSize || sized_range<R>); template<forwarding-range R> requiresconvertible_toconvertible-to-non-slicing<iterator_t<R>, I> && convertible_to<sentinel_t<R>, S> constexpr subrange(R&& r, make-unsigned-like-t(iter_difference_t<I>) n) requires (K == subrange_kind::sized) : subrange{ranges::begin(r), ranges::end(r), n} {}template<not-same-as<subrange> PairLike> requires pair-like-convertible-to<PairLike, I, S> constexpr subrange(PairLike&& r) requires (!StoreSize) : subrange{std::get<0>(std::forward<PairLike>(r)), std::get<1>(std::forward<PairLike>(r))} {} template<pair-like-convertible-to<I, S> PairLike> constexpr subrange(PairLike&& r, make-unsigned-like-t(iter_difference_t<I>) n) requires (K == subrange_kind::sized) : subrange{std::get<0>(std::forward<PairLike>(r)), std::get<1>(std::forward<PairLike>(r)), n} {}[…] }; template<input_or_output_iterator I, sentinel_for<I> S> subrange(I, S) -> subrange<I, S>; […] }
Modify 25.5.4.2 [range.subrange.ctor] as indicated:
constexpr subrange(convertible-to-non-slicing<I> auto i, S s) requires (!StoreSize);-1- Expects: […]
[…]constexpr subrange(convertible-to-non-slicing<I> auto i, S s, make-unsigned-like-t(iter_difference_t<I>) n) requires (K == subrange_kind::sized);-2- Expects: […]
[…]template<not-same-as<subrange> R> requires forwarding-range<R> &&convertible_toconvertible-to-non-slicing<iterator_t<R>, I> && convertible_to<sentinel_t<R>, S> constexpr subrange(R&& r) requires (!StoreSize || sized_range<R>);-6- Effects: […]
[…]
input_iterator
but not equality_comparable
look like
C++17 output iteratorsSection: 24.3.2.3 [iterator.traits] Status: Resolved Submitter: Eric Niebler Opened: 2019-09-10 Last modified: 2021-05-18
Priority: 2
View all other issues in [iterator.traits].
View all issues with Resolved status.
Discussion:
In C++20, if an iterator doesn't define all of the associated iterator types (value,
category, reference, and difference), the primary std::iterator_traits
template picks
a category based on structural conformance to a set of implementation-defined concepts that
capture the old iterator requirements tables. (See 24.3.2.3 [iterator.traits].) In C++17,
input iterators were required to be equality-comparable with themselves. In C++20 that is not
the case, so such iterators must not be given intput_iterator_tag
as a category.
They don't, so that's all well and good.
std::output_iterator_tag
. It does this even if there is a nested
iterator_category
typedef declaring the iterator to be input. (This will happen
frequently as C++20 iterators don't require iterators to declare their reference type, for
instance.) This will be extremely confusing to users who, understandably, will be at a loss to
understand why the legacy STL algorithms think their iterator is an output iterator when they
have clearly stated that the iterator is input!
The fix is to tweak the specification such that the output category is assigned to an iterator
only (a) if it declares its category to be output, or (b) it doesn't specify a category at all.
The result, for the user, is that their iterator simply won't look like a C++17 iterator at all,
because it isn't!
Suggested priority: P1. We can't make this change after C++20 because it would be an observable change.
This fix has been implemented in range-v3.
[2019-10-12 Priority set to 1 after reflector discussion]
[2019-11 Wednesday night Issue processing in Belfast]
Much discussion along with 3289(i). CC to write rationale for NAD.
[2020-02-13, Prague; Priority reduced to 2 after LWG discussion]
[2021-05-18 Resolved by the adoption of P2259R1 at the February 2021 plenary. Status changed: New → Resolved.]
Proposed resolution:
This wording is relative to N4830.
Modify 24.3.2.3 [iterator.traits] as indicated:
-3- The members of a specialization
iterator_traits<I>
generated from theiterator_traits
primary template are computed as follows:
(3.1) — If
I
has valid […][…]
(3.3) — Otherwise, if
I
satisfies the exposition-only conceptcpp17-iterator
and either
I::iterator_category
is valid and denotesoutput_iterator_tag
or a type publicly and unambiguously derived fromoutput_iterator_tag
, orthere is no type
I::iterator_category
then
iterator_traits<I>
has the following publicly accessible members:[…]
random_access_iterator
semantic constraints accidentally promote difference type
using unary negateSection: 24.3.4.13 [iterator.concept.random.access] Status: C++20 Submitter: Eric Niebler Opened: 2019-09-10 Last modified: 2021-02-25
Priority: 0
View all other issues in [iterator.concept.random.access].
View all issues with C++20 status.
Discussion:
24.3.4.13 [iterator.concept.random.access]/p2.7 says:
(b += -n)
is equal toa
Unary minus can do integer promotion. That is not the intent here.
[2019-10-12 Issue Prioritization]
Status to Tentatively Ready and priority to 0 after five positive votes on the reflector.
Proposed resolution:
This wording is relative to N4830.
Modify 24.3.4.13 [iterator.concept.random.access] as indicated:
-2- Let
a
andb
be valid iterators of typeI
such thatb
is reachable froma
aftern
applications of++a
, letD
beiter_difference_t<I>
, and letn
denote a value of typeD
.I
modelsrandom_access_iterator
only if:
(2.1) —
(a += n)
is equal tob
.[…]
(2.7) —
(b += D(-n))
is equal toa
.[…]
semiregular
Section: 16.3.3.3.5 [customization.point.object] Status: C++20 Submitter: Eric Niebler Opened: 2019-09-10 Last modified: 2021-02-25
Priority: 0
View all other issues in [customization.point.object].
View all issues with C++20 status.
Discussion:
We should be testing the un-cv-qualified type of a customization point object.
[2019-10-12 Issue Prioritization]
Status to Tentatively Ready and priority to 0 after seven positive votes on the reflector.
Proposed resolution:
This wording is relative to N4830.
Modify 16.3.3.3.5 [customization.point.object] as indicated:
-2- The type of a customization point object ignoring cv-qualifiers shall model
semiregular
(18.6 [concepts.object]).
ranges::size
is not required to be valid after a call to ranges::begin
on an input rangeSection: 25.7.10.2 [range.take.view], 25.5.4.2 [range.subrange.ctor] Status: C++20 Submitter: Eric Niebler Opened: 2019-09-10 Last modified: 2021-02-25
Priority: 0
View all other issues in [range.take.view].
View all issues with C++20 status.
Discussion:
On an input (but not forward) range, begin(rng)
is not required to be an equality-preserving
expression (25.4.2 [range.range]/3.3). If the range is also sized, then it is not valid
to call size(rng)
after begin(rng)
(25.4.3 [range.sized]/2.2). In several
places in the ranges clause, this precondition is violated. A trivial re-expression of the effects
clause fixes the problem.
[2019-10-12 Issue Prioritization]
Status to Tentatively Ready and priority to 0 after seven positive votes on the reflector.
Proposed resolution:
This wording is relative to N4830.
Modify 25.7.10.2 [range.take.view], class template take_view
synopsis, as indicated:
namespace std::ranges { template<view V> class take_view : public view_interface<take_view<V>> { private: […] public: […] constexpr auto begin() requires (!simple-view<V>) { if constexpr (sized_range<V>) { if constexpr (random_access_range<V>) return ranges::begin(base_); else { auto sz = size(); return counted_iterator{ranges::begin(base_),size()sz}; } } else return counted_iterator{ranges::begin(base_), count_}; } constexpr auto begin() const requires range<const V> { if constexpr (sized_range<const V>) { if constexpr (random_access_range<const V>) return ranges::begin(base_); else { auto sz = size(); return counted_iterator{ranges::begin(base_),size()sz}; } } else return counted_iterator{ranges::begin(base_), count_}; } […] }; […] }
Modify 25.5.4.2 [range.subrange.ctor] as indicated:
template<not-same-as<subrange> R> requires forwarding-range<R> && convertible_to<iterator_t<R>, I> && convertible_to<sentinel_t<R>, S> constexpr subrange(R&& r) requires (!StoreSize || sized_range<R>);-6- Effects: Equivalent to:
(6.1) — If
StoreSize
istrue
,subrange{
.ranges::begin(r), ranges::end(r)r, ranges::size(r)}(6.2) — Otherwise,
subrange{ranges::begin(r), ranges::end(r)}
.
Section: 24.3.2.3 [iterator.traits], 24.5.5.2 [common.iter.types] Status: Resolved Submitter: Eric Niebler Opened: 2019-09-11 Last modified: 2021-05-18
Priority: 2
View all other issues in [iterator.traits].
View all issues with Resolved status.
Discussion:
The way to non-intrusively say that a type doesn't satisfy the C++17 iterator requirements is to
specialize std::iterator_traits
and not provide the nested typedefs. However, if a user
were to do that, she would also be saying that the type is not a C++20 iterator. That is
because readable
and weakly_incrementable
are specified in terms of
iter_value_t<I>
and iter_difference_t<I>
. Those aliases check to
see if std::iterator_traits<I>
has been specialized (answer: yes), and if so
resolve to std::iterator_traits<I>::value_type
and
std::iterator_traits<I>::difference_type
respectively.
std::iterator_traits
and specify all the nested typedefs except
::iterator_category
. That's a bit weird and may throw off code that is expecting all the
typedefs to be there, or none of them, so instead we can suggest users to set the iterator_category
typedef to denote output_iterator_tag
, which is a harmless lie; generic C++17 code will get
the message: this iterator is not a c++17 input iterator, which is the salient bit.
We then must fix up all the places in the Ranges clause that make faulty assumptions about an
iterator's iterator_category
typedef (as distinct from the iterator concept that it models).
[2019-10-19 Issue Prioritization]
Priority to 1 after reflector discussion.
[2019-11 Wednesday night Issue processing in Belfast]
Much discussion; no consensus that this is a good approach. Need to coordinate between this and 3283(i)
Previous resolution [SUPERSEDED]:This wording is relative to N4830.
Modify 24.3.2.3 [iterator.traits] as indicated:
-4- Explicit or partial specializations of
iterator_traits
may have a member typeiterator_concept
that is used to indicate conformance to the iterator concepts (24.3.4 [iterator.concepts]). [Example: To indicate conformance to theinput_iterator
concept but a lack of conformance to the Cpp17InputIterator requirements (24.3.5.3 [input.iterators]), aniterator_traits
specialization might haveiterator_concept
denoteinput_iterator_tag
anditerator_category
denoteoutput_iterator_tag
. — end example]Modify 24.5.5.2 [common.iter.types] as indicated:
-1- The nested typedef-names of the specialization of
iterator_traits
forcommon_iterator<I, S>
are defined as follows.
(1.1) —
iterator_concept
denotesforward_iterator_tag
ifI
modelsforward_iterator
; otherwise it denotesinput_iterator_tag
.(1.2) —
iterator_category
denotesforward_iterator_tag
ifiterator_traits<I>::iterator_category
modelsderived_from<forward_iterator_tag>
; otherwise it denotes.
input_iterator_tagiterator_traits<I>::iterator_category(1.3) — If the expression
a.operator->()
is well-formed, wherea
is an lvalue of typeconst common_iterator<I, S>
, thenpointer
denotes the type of that expression. Otherwise, pointer denotesvoid
.Modify 25.7.8.3 [range.filter.iterator] as indicated:
-3-
iterator::iterator_category
is defined as follows:
(3.1) — Let
C
denote the typeiterator_traits<iterator_t<V>>::iterator_category
.(3.2) — If
C
modelsderived_from<bidirectional_iterator_tag>
, theniterator_category
denotesbidirectional_iterator_tag
.
(3.3) — Otherwise, ifC
modelsderived_from<forward_iterator_tag>
, theniterator_category
denotesforward_iterator_tag
.(3.4) — Otherwise,
iterator_category
denotes.
input_iterator_tagCModify 25.7.14.3 [range.join.iterator] as indicated:
-3-
iterator::iterator_category
is defined as follows:
(3.1) — Let
OUTERC
denoteiterator_traits<iterator_t<Base>>::iterator_category
, and letINNERC
denoteiterator_traits<iterator_t<range_reference_t<Base>>>::iterator_category
.(3.?) — If
OUTERC
does not modelderived_from<input_iterator_tag>
,iterator_category
denotesOUTERC
.(3.?) — Otherwise, if
INNERC
does not modelderived_from<input_iterator_tag>
,iterator_category
denotesINNERC
.(3.2) — Otherwise, i
Ifref_is_glvalue
istrue
,
(3.2.1) — If
OUTERC
andINNERC
each modelderived_from<bidirectional_iterator_tag>
,iterator_category
denotesbidirectional_iterator_tag
.(3.2.2) — Otherwise, if
OUTERC
andINNERC
each modelderived_from<forward_iterator_tag>
,iterator_category
denotesforward_iterator_tag
.(3.3) — Otherwise,
iterator_category
denotesinput_iterator_tag
.Modify [range.split.outer] as indicated:
namespace std::ranges { template<class V, class Pattern> template<bool Const> struct split_view<V, Pattern>::outer_iterator { private: […] public: using iterator_concept = conditional_t<forward_range<Base>, forward_iterator_tag, input_iterator_tag>; using iterator_category = see belowinput_iterator_tag; […] }; […] }-?- The typedef-name
-1- Many of the following specifications refer to the notional memberiterator_category
denotesinput_iterator_tag
ifiterator_traits<iterator_t<Base>>::iterator_category
modelsderived_from<input_iterator_tag>
, anditerator_traits<iterator_t<Base>>::iterator_category
otherwise.current
ofouter_iterator
.current
is equivalent tocurrent_
ifV
modelsforward_range
, andparent_->current_
otherwise.Modify [range.split.inner] as indicated:
-1- The typedef-name
iterator_category
denotesforward_iterator_tag
ifiterator_traits<iterator_t<Base>>::iterator_category
modelsderived_from<forward_iterator_tag>
, andotherwise.
input_iterator_tagiterator_traits<iterator_t<Base>>::iterator_category
[2020-02-10, Prague]
The issue is out of sync with the current working draft, Daniel provides a synchronized merge.
[2020-02-13, Prague; Priority reduced to 2 after LWG discussion]
[2021-05-18 Resolved by the adoption of P2259R1 at the February 2021 plenary. Status changed: New → Resolved.]
Proposed resolution:
This wording is relative to N4849.
Modify 24.3.2.3 [iterator.traits] as indicated:
-4- Explicit or partial specializations of
iterator_traits
may have a member typeiterator_concept
that is used to indicate conformance to the iterator concepts (24.3.4 [iterator.concepts]). [Example: To indicate conformance to theinput_iterator
concept but a lack of conformance to the Cpp17InputIterator requirements (24.3.5.3 [input.iterators]), aniterator_traits
specialization might haveiterator_concept
denoteinput_iterator_tag
anditerator_category
denoteoutput_iterator_tag
. — end example]
Modify 24.5.5.2 [common.iter.types] as indicated:
-1- The nested typedef-names of the specialization of
iterator_traits
forcommon_iterator<I, S>
are defined as follows.
(1.1) —
iterator_concept
denotesforward_iterator_tag
ifI
modelsforward_iterator
; otherwise it denotesinput_iterator_tag
.(1.2) —
iterator_category
denotesforward_iterator_tag
ifiterator_traits<I>::iterator_category
modelsderived_from<forward_iterator_tag>
; otherwise it denotes.
input_iterator_tagiterator_traits<I>::iterator_category(1.3) — If the expression
a.operator->()
is well-formed, wherea
is an lvalue of typeconst common_iterator<I, S>
, thenpointer
denotes the type of that expression. Otherwise, pointer denotesvoid
.
Modify 25.7.8.3 [range.filter.iterator] as indicated:
-3-
iterator::iterator_category
is defined as follows:
(3.1) — Let
C
denote the typeiterator_traits<iterator_t<V>>::iterator_category
.(3.2) — If
C
modelsderived_from<bidirectional_iterator_tag>
, theniterator_category
denotesbidirectional_iterator_tag
.
(3.3) — Otherwise, ifC
modelsderived_from<forward_iterator_tag>
, theniterator_category
denotesforward_iterator_tag
.(3.4) — Otherwise,
iterator_category
denotesC
.
Modify 25.7.14.3 [range.join.iterator] as indicated:
-2-
iterator::iterator_category
is defined as follows:
(2.1) — Let
OUTERC
denoteiterator_traits<iterator_t<Base>>::iterator_category
, and letINNERC
denoteiterator_traits<iterator_t<range_reference_t<Base>>>::iterator_category
.(2.?) — If
OUTERC
does not modelderived_from<input_iterator_tag>
,iterator_category
denotesOUTERC
.(2.?) — Otherwise, if
INNERC
does not modelderived_from<input_iterator_tag>
,iterator_category
denotesINNERC
.(2.2) — Otherwise, i
Ifref-is-glvalue
istrue
andOUTERC
andINNERC
each modelderived_from<bidirectional_iterator_tag>
,iterator_category
denotesbidirectional_iterator_tag
.(2.3) — Otherwise, if
ref-is-glvalue
istrue
andOUTERC
andINNERC
each modelderived_from<forward_iterator_tag>
,iterator_category
denotesforward_iterator_tag
.(2.4) — Otherwise, if
OUTERC
andINNERC
each modelderived_from<input_iterator_tag>
,iterator_category
denotesinput_iterator_tag
.(2.5) — Otherwise,
iterator_category
denotesoutput_iterator_tag
.
Modify [range.split.outer] as indicated:
[Drafting note: The previous wording change has been adjusted to follow the pattern used in [range.split.inner] p1.]
namespace std::ranges { template<class V, class Pattern> template<bool Const> struct split_view<V, Pattern>::outer_iterator { private: […] public: using iterator_concept = conditional_t<forward_range<Base>, forward_iterator_tag, input_iterator_tag>; using iterator_category = see belowinput_iterator_tag; […] }; […] }-?- The typedef-name
iterator_category
denotes:
(?.?) —
input_iterator_tag
ifiterator_traits<iterator_t<Base>>::iterator_category
modelsderived_from<input_iterator_tag>
;(?.?) — otherwise,
iterator_traits<iterator_t<Base>>::iterator_category
.-1- Many of the following specifications refer to the notional member
current
ofouter-iterator
.current
is equivalent tocurrent_
ifV
modelsforward_range
, andparent_->current_
otherwise.
std::format
field widths code units, code points, or something else?Section: 28.5.2.2 [format.string.std] Status: C++20 Submitter: Tom Honermann Opened: 2019-09-08 Last modified: 2021-02-25
Priority: Not Prioritized
View other active issues in [format.string.std].
View all other issues in [format.string.std].
View all issues with C++20 status.
Discussion:
28.5.2.2 [format.string.std] p7 states:
The positive-integer in width is a decimal integer defining the minimum field width. If width is not specified, there is no minimum field width, and the field width is determined based on the content of the field.
Is field width measured in code units, code points, or something else?
Consider the following example assuming a UTF-8 locale:std::format("{}", "\xC3\x81"); // U+00C1 { LATIN CAPITAL LETTER A WITH ACUTE } std::format("{}", "\x41\xCC\x81"); // U+0041 U+0301 { LATIN CAPITAL LETTER A } { COMBINING ACUTE ACCENT }
In both cases, the arguments encode the same user-perceived character (Á). The first uses two UTF-8 code units to encode a single code point that represents a single glyph using a composed Unicode normalization form. The second uses three code units to encode two code points that represent the same glyph using a decomposed Unicode normalization form.
How is the field width determined? If measured in code units, the first has a width of 2 and the second of 3. If measured in code points, the first has a width of 1 and the second of 2. If measured in grapheme clusters, both have a width of 1. Is the determination locale dependent? Previous resolution [SUPERSEDED]:This wording is relative to N4830.
Modify 28.5.2.2 [format.string.std] as indicated:
-7- The positive-integer in width is a decimal integer defining the minimum field width. If width is not specified, there is no minimum field width, and the field width is determined based on the content of the field. Field width is measured in code units. Each byte of a multibyte character contributes to the field width.
[2020-02-13, Prague]
Resolved by P1868R2
[2020-04-07 Voted into the WP in Prague. Status changed: New → WP.]
Proposed resolution:
iota_view::iterator
has the wrong iterator_category
Section: 25.6.4.3 [range.iota.iterator] Status: C++20 Submitter: Eric Niebler Opened: 2019-09-13 Last modified: 2021-02-25
Priority: 0
View other active issues in [range.iota.iterator].
View all other issues in [range.iota.iterator].
View all issues with C++20 status.
Discussion:
In the old way of looking at the world, forward iterators need to return real references. Since
dereferencing iota_view
's iterators returns by value, it cannot be a C++17 forward
iterator. (It can, however, be a C++20 forward_iterator
.) However, iota_view
's
iterator has an iterator_category
that (sometimes) falsely claims that it is forward or
better (depending on the properties of the weakly_incrementable
type it wraps).
[2019-10-19 Issue Prioritization]
Status to Tentatively Ready and priority to 0 after eight positive votes on the reflector.
Proposed resolution:
This wording is relative to N4830.
Modify 25.6.4.3 [range.iota.iterator] as indicated:
namespace std::ranges { template<class W, class Bound> struct iota_view<W, Bound>::iterator { private: […] public: using iterator_conceptategory= see below; using iterator_category = input_iterator_tag; using value_type = W; using difference_type = IOTA-DIFF-T(W); […] }; […] }-1-
iterator::iterator_concept
is defined as follows:ategory
(1.1) — If
W
modelsadvanceable
, theniterator_concept
isategoryrandom_access_iterator_tag
.(1.2) — Otherwise, if
W
modelsdecrementable
, theniterator_concept
isategorybidirectional_iterator_tag
.(1.3) — Otherwise, if
W
modelsincrementable
, theniterator_concept
isategoryforward_iterator_tag
.(1.4) — Otherwise,
iterator_concept
isategoryinput_iterator_tag
.
iota_view
is under-constrainedSection: 25.6.4.2 [range.iota.view] Status: C++20 Submitter: Barry Revzin Opened: 2019-09-13 Last modified: 2021-02-25
Priority: Not Prioritized
View all other issues in [range.iota.view].
View all issues with C++20 status.
Discussion:
P1207R4 changed weakly_incrementable
from requiring
semiregular
to requiring default_constructible && movable
.
iota_view
currently is specified to require that W
be just weakly_incrementable
.
But we have to copy the W
from the view into its iterator and also in operator*()
to
return a W
.
The shortest resolution is just to add " && semiregular<W>
" to the class
constraints.
[Status to ready after discussion Friday morning in Belfast]
Proposed resolution:
This wording is relative to N4830.
Modify 25.6.4.2 [range.iota.view] as indicated:
namespace std::ranges { […] template<weakly_incrementable W, semiregular Bound = unreachable_sentinel_t> requires weakly-equality-comparable-with<W, Bound> && semiregular<W> class iota_view : public view_interface<iota_view<W, Bound> { […] }; […] }
move_iterator operator+()
has incorrect constraintsSection: 24.5.4.9 [move.iter.nonmember] Status: C++23 Submitter: Bo Persson Opened: 2019-09-13 Last modified: 2023-11-22
Priority: 3
View all other issues in [move.iter.nonmember].
View all issues with C++23 status.
Discussion:
Section 24.5.4.9 [move.iter.nonmember]/2-3 says:
template<class Iterator> constexpr move_iterator<Iterator> operator+(iter_difference_t<Iterator> n, const move_iterator<Iterator>& x);Constraints:
Returns:x + n
is well-formed and has typeIterator
.x + n
.
However, the return type of this operator is move_iterator<Iterator>
, so
the expression x + n
ought to have that type. Also, there is no operator+
that matches the constraints, so it effectively disables the addition.
[2019-10-31 Issue Prioritization]
Priority to 3 after reflector discussion.
Previous resolution [SUPERSEDED]:This wording is relative to N4830.
Modify 24.5.4.9 [move.iter.nonmember] as indicated:
template<class Iterator> constexpr move_iterator<Iterator> operator+(iter_difference_t<Iterator> n, const move_iterator<Iterator>& x);-2- Constraints:
-3- Returns:x + n
is well-formed and has typemove_iterator<Iterator>
.x + n
.
[2019-11-04; Casey comments and provides revised wording]
After applying the P/R the Constraint element requires x + n
to be well-formed (it always is,
since that operation is unconstrained) and requires x + n
to have type
move_iterator<Iterator>
(which it always does). Consequently, this Constraint
is always satisfied and it has no normative effect. The intent of the change in
P0896R4 was that this operator be constrained to require
addition on the base iterator to be well-formed and have type Iterator
, which ensures that
the other semantics of this operation are implementable.
[2021-06-23; Reflector poll]
Set status to Tentatively Ready after six votes in favour during reflector poll.
[2021-10-14 Approved at October 2021 virtual plenary. Status changed: Voting → WP.]
Proposed resolution:
This wording is relative to N4835.
Modify 24.5.4.9 [move.iter.nonmember] as indicated:
template<class Iterator> constexpr move_iterator<Iterator> operator+(iter_difference_t<Iterator> n, const move_iterator<Iterator>& x);-2- Constraints:
-3- Returns:x.base() + n
is well-formed and has typeIterator
.x + n
.
zoned_time
deduction guides misinterprets string
/char*
Section: 30.11.7.1 [time.zone.zonedtime.overview] Status: C++20 Submitter: Tomasz Kamiński Opened: 2019-09-14 Last modified: 2021-02-25
Priority: 0
View all other issues in [time.zone.zonedtime.overview].
View all issues with C++20 status.
Discussion:
The current deduction guide for zoned_time
for the following declarations
zoned_time zpc("America/New_York", std::chrono::system_clock::now()); zoned_time zps(std::string("America/New_York"), std::chrono::system_clock::now());
will attempt to produce a zoned_time
instance with const char*
(for zpc
) and with std::string
(for zps
), respectively,
as the deduced type for the TimeZonePtr
template parameter. This is caused by
the fact that the unconstrained TimeZonePtr
deduction guide template will
produce better candidates and will be selected by overload resolution.
std::string_view
/TimeZonePtr
deduction guides
into one guide, that deduces const time_zone*
for any type
convertible to string_view
. This is necessary to override
the deduction from TimeZonePtr
constructor candidates.
In addition, we disable the deduction from string_view
constructors, that would produce better candidates than the deduction guides
and create zoned_time
instances with durations coarser than
seconds
(causing similar issue as LWG 3232(i)):
std::chrono::local_time<hours> lh(10h); std::chrono::zoned_time zt1("Europe/Helsinki", lh); std::chrono::zoned_time zt2(std::string("Europe/Helsinki"), lh); std::chrono::zoned_time zt3(std::string_view("Europe/Helsinki"), lh);
Without disabling the deduction from the string_view
constructor, the type of the zt3
variable would be deduced to
zoned_time<hours>
, with the proposed change the types
of the variables zt1
, zt2
, and zt3
are consistently deduced as zoned_time<seconds>
.
zoned_time<Duration>
guide (covered by zoned_time<Duration, TimeZonePtr2>
).
The change was implemented in the example implementation. The dedicated
test can be found
here.
[2019-10-31 Issue Prioritization]
Status to Tentatively Ready and priority to 0 after five positive votes on the reflector.
Proposed resolution:
This wording is relative to N4830.
Modify 30.11.7.1 [time.zone.zonedtime.overview], class template zoned_time
synopsis, as indicated:
namespace std::chrono { […] zoned_time() -> zoned_time<seconds>; template<class Duration> zoned_time(sys_time<Duration>) -> zoned_time<common_type_t<Duration, seconds>>; template<class TimeZonePtrOrName> using time-zone-representation = conditional_t<is_convertible_v<TimeZonePtrOrName, string_view>, const time_zone*, remove_cv_ref<TimeZonePtrOrName>>; // exposition only template<class TimeZonePtrOrName> zoned_time(TimeZonePtrOrName&&) -> zoned_time<seconds, time-zone-representation<TimeZonePtr>>; template<class TimeZonePtrOrName, class Duration> zoned_time(TimeZonePtrOrName&&, sys_time<Duration>) -> zoned_time<common_type_t<Duration, seconds>, time-zone-representation<TimeZonePtrOrName>>; template<class TimeZonePtrOrName, class Duration> zoned_time(TimeZonePtrOrName&&, local_time<Duration>, choose = choose::earliest) -> zoned_time<common_type_t<Duration, seconds>, time-zone-representation<TimeZonePtrOrName>>;template<class TimeZonePtr, class Duration> zoned_time(TimeZonePtr, zoned_time<Duration>, choose = choose::earliest) ->> zoned_time<common_type_t<Duration, seconds>, TimeZonePtr>; zoned_time(string_view) -> zoned_time<seconds>; template<class Duration> zoned_time(string_view, sys_time<Duration>) -> zoned_time<common_type_t<Duration, seconds>>; template<class Duration> zoned_time(string_view, local_time<Duration>, choose = choose::earliest) -> zoned_time<common_type_t<Duration, seconds>>;template<class Duration, class TimeZonePtrOrName, class TimeZonePtr2> zoned_time(TimeZonePtrOrName&&, zoned_time<Duration, TimeZonePtr2>, choose = choose::earliest) -> zoned_time<Duration, time-zone-representation<TimeZonePtrOrName>>; }-1-
-2- Ifzoned_time
represents a logical pairing of atime_zone
and atime_point
with precisionDuration
.zoned_time<Duration>
maintains the invariant that it always refers to a valid time zone and represents a point in time that exists and is not ambiguous in that time zone.Duration
is not a specialization ofchrono::duration
, the program is ill-formed. -?- Every constructor ofzoned_time
that accepts astring_view
as first parameter does not participate in class template argument deduction (12.2.2.9 [over.match.class.deduct]).
operator==
are mis-specifiedSection: 17.12.2 [cmp.categories] Status: Resolved Submitter: Barry Revzin Opened: 2019-09-14 Last modified: 2019-12-29
Priority: 1
View all other issues in [cmp.categories].
View all issues with Resolved status.
Discussion:
All the defaulted operator==
s in 17.12.2 [cmp.categories] are currently specified as:
friend constexpr bool operator==(strong_ordering v, strong_ordering w) noexcept = default;
But the rule for defaulting operator==
requires that the arguments be const&
. All
five should all look like:
friend constexpr bool operator==(const strong_ordering& v, const strong_ordering& w) noexcept = default;
[2019-10-31 Issue Prioritization]
Priority to 1 after reflector discussion.
[2019-11 Wednesday night issue processing - status to Open]
Our preference is for CWG to fix this. JW to provide wording in case CWG cannot.
[Resolved by the adoption of P1946R0 in Belfast]
Proposed resolution:
basic_regex<>::assign
Section: 28.6.7 [re.regex] Status: C++20 Submitter: Mark de Wever Opened: 2019-09-16 Last modified: 2021-02-25
Priority: 0
View all other issues in [re.regex].
View all issues with C++20 status.
Discussion:
The declaration of the overload of basic_regex<>::assign(const charT* p, size_t len, flag_type f)
has an inconsistent default argument for the flag_type f
parameter.
basic_regex& assign(const charT* p, size_t len, flag_type f);
28.6.7.3 [re.regex.assign] before p12:
basic_regex& assign(const charT* ptr, size_t len, flag_type f = regex_constants::ECMAScript);
Since all other overloads have a default argument in both 28.6.7 [re.regex] and 28.6.7.3 [re.regex.assign] I propose to add a default argument for this overload in the declaration in 28.6.7 [re.regex].
It should be pointed out that there exists implementation divergence due to the current wording state: libc++ and libstdc++ do not implement the default argument. The MS STL library does have the default argument.[2019-10-31 Issue Prioritization]
Status to Tentatively Ready and priority to 0 after six positive votes on the reflector.
Proposed resolution:
This wording is relative to N4830.
Modify 28.6.7 [re.regex], class template basic_regex
synopsis,
as indicated:
[…] // 28.6.7.3 [re.regex.assign], assign […] basic_regex& assign(const charT* ptr, flag_type f = regex_constants::ECMAScript); basic_regex& assign(const charT* p, size_t len, flag_type f = regex_constants::ECMAScript); template<class string_traits, class A> basic_regex& assign(const basic_string<charT, string_traits, A>& s, flag_type f = regex_constants::ECMAScript); template<class InputIterator> basic_regex& assign(InputIterator first, InputIterator last, flag_type f = regex_constants::ECMAScript); basic_regex& assign(initializer_list<charT>, flag_type = regex_constants::ECMAScript); […]
Section: 24.3.3.1 [iterator.cust.move], 24.3.3.2 [iterator.cust.swap] Status: C++20 Submitter: Casey Carter Opened: 2019-10-07 Last modified: 2021-02-25
Priority: 0
View all other issues in [iterator.cust.move].
View all issues with C++20 status.
Discussion:
It is not intentional design that users may customize the behavior of
ranges::iter_move
(24.3.3.1 [iterator.cust.move]) and
ranges::iter_swap
(24.3.3.2 [iterator.cust.swap]) for pointers to program-defined
type by defining e.g. iter_move(my_type*)
or iter_swap(my_type*, my_type*)
in a
namespace associated with my_type
. The intent of customization points is that users may
define behavior for types they define, not that users may mess with the well-defined semantics for
existing types like pointers.
We should forbid such silliness by constraining the "finds an overload via ADL" cases for
customization points to only trigger with argument expressions of class or enumeration type. Note
that WG21 made a similar change to ranges::swap
shortly before merging it into the working
draft to forbid users customizing behavior for pointers to program-defined types or arrays of
program-defined types.
[2019-11-16 Issue Prioritization]
Status to Tentatively Ready and priority to 0 after seven positive votes on the reflector.
Proposed resolution:
This wording is relative to N4830.
[Drafting note: 3247(i) touches the same wording in 24.3.3.1 [iterator.cust.move]; if both are resolved simultaneously the changes should be reconciled before passing them on to the Editor.]
Modify 24.3.3.1 [iterator.cust.move] as follows:
(1.1) —
iter_move(E)
, ifthat expression is valid,E
has class or enumeration type anditer_move(E)
is a well-formed expression with overload resolution performed in a context that does not include a declaration ofranges::iter_move
.
Modify 24.3.3.2 [iterator.cust.swap] as follows:
(4.1) —
(void)iter_swap(E1, E2)
, ifthat expression is valid,eitherE1
orE2
has class or enumeration type anditer_swap(E1, E2)
is a well-formed expression with overload resolution performed in a context that includes the declarationand does not include a declaration oftemplate<class I1, class I2> void iter_swap(I1, I2) = delete;ranges::iter_swap
. If the function selected by overload resolution does not exchange the values denoted byE1
andE2
, the program is ill-formed with no diagnostic required.
ssize
overload is underconstrainedSection: 24.7 [iterator.range] Status: C++20 Submitter: Casey Carter Opened: 2019-09-27 Last modified: 2021-02-25
Priority: 3
View other active issues in [iterator.range].
View all other issues in [iterator.range].
View all issues with C++20 status.
Discussion:
The overload of ssize
specified in 24.7 [iterator.range]/18 has no constraints,
yet it specializes make_signed_t
which has a precondition that its type parameter is an
integral type or enumeration but not bool
(21.3.8.4 [meta.trans.sign]). This
precondition needs to be propagated to ssize
as "Mandates [or Constraints]:
decltype(c.size())
[meets the requirements for the type argument to make_signed
]".
"Mandates" seems to be more in line with LWG guidance since there are no traits nor concepts
that observe ssize
.
[2019-11-16 Issue Prioritization]
Priority to 3 after reflector discussion.
Previous resolution [SUPERSEDED]:
This wording is relative to N4830.
Modify 24.7 [iterator.range] as indicated:
template<class C> constexpr auto ssize(const C& c) -> common_type_t<ptrdiff_t, make_signed_t<decltype(c.size())>>;-?- Mandates:
-18- Returns:decltype(c.size())
is a (possibly cv-qualified) integral or enumeration type but not abool
type.static_cast<common_type_t<ptrdiff_t, make_signed_t<decltype(c.size())>>>(c.size())
[2019-10-28; Tim provides improved wording]
Previous resolution [SUPERSEDED]:
This wording is relative to N4835.
Modify 24.7 [iterator.range] as indicated:
template<class C> constexpr auto ssize(const C& c) -> common_type_t<ptrdiff_t, make_signed_t<decltype(c.size())>>;-?- Mandates:
-18- Returns:decltype(c.size())
is an integral or enumeration type other thanbool
.static_cast<common_type_t<ptrdiff_t, make_signed_t<decltype(c.size())>>>(c.size())
[2019-11-18; Casey comments and improves wording]
It would be better to provided the Mandates: guarantee in [tab:meta.trans.sign] instead of
one special place where the make_signed
template is used. The wording below attempts to
realize that.
[2019-11-23 Issue Prioritization]
Status to Tentatively Ready after five positive votes on the reflector.
Proposed resolution:
This wording is relative to N4835.
Change Table 52 — "Sign modifications" in [tab:meta.trans.sign] as indicated:
Template | Comments |
---|---|
template <class T>
|
If T names a (possibly cv-qualified) signed integer type (6.8.2 [basic.fundamental]) thenthe member typedef type names the type T ; otherwise, if T names a(possibly cv-qualified) unsigned integer type then type names thecorresponding signed integer type, with the same cv-qualifiers as T ;otherwise, type names the signed integer type with smallestrank (6.8.6 [conv.rank]) for which sizeof(T) == sizeof(type) , with the samecv-qualifiers as T .T bool |
template <class T>
|
If T names a (possibly cv-qualified) unsigned integer type (6.8.2 [basic.fundamental]) thenthe member typedef type names the type T ; otherwise, if T names a(possibly cv-qualified) signed integer type then type names thecorresponding unsigned integer type, with the same cv-qualifiers as T ;otherwise, type names the unsigned integer type with smallestrank (6.8.6 [conv.rank]) for which sizeof(T) == sizeof(type) , with the samecv-qualifiers as T .T bool |
Change 24.7 [iterator.range] as indicated:
template<class C> constexpr auto ssize(const C& c) -> common_type_t<ptrdiff_t, make_signed_t<decltype(c.size())>>;-18-
ReturnsEffects: Equivalent to:return static_cast<common_type_t<ptrdiff_t, make_signed_t<decltype(c.size())>>>(c.size());
transform_view::iterator
has incorrect iterator_category
Section: 25.7.9.3 [range.transform.iterator] Status: C++20 Submitter: Michel Morin Opened: 2019-10-03 Last modified: 2021-02-25
Priority: 1
View all other issues in [range.transform.iterator].
View all issues with C++20 status.
Discussion:
When the transformation function returns an rvalue, transform_view::iterator
cannot model cpp17-forward-iterator
. However, similar to LWG 3291(i),
the current wording on transform_view::iterator::iterator_category
does not
consider this.
input_iterator
that is not
cpp17-input-iterator
(this problem is not specific to the PR; it's pervasive
in adapted iterators) and concepts-based determination would be a better fix for issues around
iterator_category
. But anyway, I consider this PR as a minimal fix at the moment.
[2019-10-31 Issue Prioritization]
Priority to 1 after reflector discussion.
Previous resolution [SUPERSEDED]:This wording is relative to N4830.
Modify 25.7.9.3 [range.transform.iterator] as indicated:
-2-
iterator::iterator_category
is defined as follows: LetC
denote the typeiterator_traits<iterator_t<Base>>::iterator_category
.
(2.?) — If
is_lvalue_reference_v<iter_reference_t<iterator_t<Base>>>
istrue
,
(2.?.?) — If
C
modelsderived_from<contiguous_iterator_tag>
, theniterator_category
denotesrandom_access_iterator_tag
;(2.?.?) — O
otherwise,iterator_category
denotesC
.(2.?) — Otherwise,
iterator_category
denotesinput_iterator_tag
.
[2019-11-06, Tim updates P/R based on Belfast LWG evening session discussion]
The check in the original P/R is incorrect; we want to check the transformation's result, not the base iterator.
[2020-02-10 Move to Immediate Monday afternoon in Prague]
Proposed resolution:
This wording is relative to N4830.
Modify 25.7.9.3 [range.transform.iterator] as indicated:
-2-
iterator::iterator_category
is defined as follows: LetC
denote the typeiterator_traits<iterator_t<Base>>::iterator_category
.
(2.?) — If
is_lvalue_reference_v<invoke_result_t<F&, range_reference_t<Base>>>
istrue
,
(2.?.?) — If
C
modelsderived_from<contiguous_iterator_tag>
, theniterator_category
denotesrandom_access_iterator_tag
;(2.?.?) — O
otherwise,iterator_category
denotesC
.(2.?) — Otherwise,
iterator_category
denotesinput_iterator_tag
.
keys
and values
are unspecifiedSection: 25.2 [ranges.syn] Status: C++20 Submitter: Michel Morin Opened: 2019-10-04 Last modified: 2021-02-25
Priority: 1
View all other issues in [ranges.syn].
View all issues with C++20 status.
Discussion:
This issue was submitted as editorial issue cplusplus/draft#3231 but had been classified as non-editorial.
keys
and values
are listed in 25.2 [ranges.syn], but not specified.
It seems that P1035R7 forgot to specify them (as
elements<0>
and elements<1>
).
[2019-10-31 Issue Prioritization]
Priority to 1 after reflector discussion.
[2019-11 Wednesday night issue processing in Belfast.]
Status to Ready.
Proposed resolution:
This wording is relative to N4830.
Modify 25.2 [ranges.syn], header <ranges>
synopsis, as indicated:
namespace std::ranges { […] template<class R> using keys_view = elements_view<all_view<R>, 0>; template<class R> using values_view = elements_view<all_view<R>, 1>; namespace views { template<size_t N> inline constexpr unspecified elements = unspecified; inline constexpr autounspecifiedkeys = elements<0>unspecified; inline constexpr autounspecifiedvalues = elements<1>unspecified; } } […]
constexpr
" marker for destroy/destroy_n
Section: 20.2.2 [memory.syn] Status: C++20 Submitter: Jens Maurer Opened: 2019-10-10 Last modified: 2021-02-25
Priority: 0
View all other issues in [memory.syn].
View all issues with C++20 status.
Discussion:
This issue was submitted as editorial issue cplusplus/draft#3181 but is considered non-editorial.
P0784R7, approved in Cologne, added "constexpr
" markers to
the overloads of destroy
and destroy_n
taking an ExecutionPolicy
parameter. This seems to be in error; parallel algorithms should not be marked "constexpr
".
(None of the parallel algorithms in <algorithm>
is marked "constexpr
".)
[2019-11 Marked as 'Ready' during Monday issue prioritization in Belfast]
Proposed resolution:
This wording is relative to N4830.
Modify 20.2.2 [memory.syn], header <memory>
synopsis, as indicated:
namespace std { […] // 26.11.9 [specialized.destroy], destroy template<class T> constexpr void destroy_at(T* location); template<class ForwardIterator> constexpr void destroy(ForwardIterator first, ForwardIterator last); template<class ExecutionPolicy, class ForwardIterator>constexprvoid destroy(ExecutionPolicy&& exec, // see 26.3.5 [algorithms.parallel.overloads] ForwardIterator first, ForwardIterator last); template<class ForwardIterator, class Size> constexpr ForwardIterator destroy_n(ForwardIterator first, Size n); template<class ExecutionPolicy, class ForwardIterator, class Size>constexprForwardIterator destroy_n(ExecutionPolicy&& exec, // see 26.3.5 [algorithms.parallel.overloads] ForwardIterator first, Size n); […] }
std::polymorphic_allocator
should require [[nodiscard]]
Section: 20.5.3 [mem.poly.allocator.class] Status: C++20 Submitter: Hiroaki Ando Opened: 2019-10-16 Last modified: 2021-02-25
Priority: 3
View all other issues in [mem.poly.allocator.class].
View all issues with C++20 status.
Discussion:
[[nodiscard]]
is specified for std::polymorphic_allocator<>::allocate()
.
[[nodiscard]]
necessary for these functions?
[2019-11 Priority to 3 during Monday issue prioritization in Belfast]
[2019-11 After discussion with LEWG, assigning to LEWG]
[2019-11-4; Daniel comments]
This issue is related to LWG 3312(i).
[2019-11; Friday AM in Belfast. Status changed to "Ready"]
Proposed resolution:
This wording is relative to N4835.
Modify 20.5.3 [mem.poly.allocator.class], class template polymorphic_allocator
synopsis, as indicated:
namespace std::pmr { template<class Tp = byte> class polymorphic_allocator { […] // 20.5.3.3 [mem.poly.allocator.mem], member functions [[nodiscard]] Tp* allocate(size_t n); void deallocate(Tp* p, size_t n); [[nodiscard]] void* allocate_bytes(size_t nbytes, size_t alignment = alignof(max_align_t)); void deallocate_bytes(void* p, size_t nbytes, size_t alignment = alignof(max_align_t)); template<class T> [[nodiscard]] T* allocate_object(size_t n = 1); template<class T> void deallocate_object(T* p, size_t n = 1); template<class T, class... CtorArgs> [[nodiscard]] T* new_object(CtorArgs&&... ctor_args); template<class T> void delete_object(T* p); […] }; }
Modify 20.5.3.3 [mem.poly.allocator.mem] as indicated:
[[nodiscard]] void* allocate_bytes(size_t nbytes, size_t alignment = alignof(max_align_t));[…]-5- Effects: Equivalent to:
[…]return memory_rsrc->allocate(nbytes, alignment);
template<class T> [[nodiscard]] T* allocate_object(size_t n = 1);-8- Effects: Allocates memory suitable for holding an array of
n
objects of typeT
, as follows:[…]
(8.1) — if
SIZE_MAX / sizeof(T) < n
, throwslength_error
,(8.2) — otherwise equivalent to:
return static_cast<T*>(allocate_bytes(n*sizeof(T), alignof(T)));template<class T, class CtorArgs...> [[nodiscard]] T* new_object(CtorArgs&&... ctor_args);-11- Effects: Allocates and constructs an object of type
T
, as follows. Equivalent to:[…]T* p = allocate_object<T>(); try { construct(p, std::forward<CtorArgs>(ctor_args)...); } catch (...) { deallocate_object(p); throw; } return p;
any_cast<void>
Section: 22.7.5 [any.nonmembers] Status: WP Submitter: John Shaw Opened: 2019-10-16 Last modified: 2023-11-22
Priority: 2
View all other issues in [any.nonmembers].
View all issues with WP status.
Discussion:
any foo; void* p = any_cast<void>(&foo);
Per 22.7.5 [any.nonmembers]/9, since the operand isn't nullptr
and
operand->type() == typeid(T)
(because T = void
in this case), we should
return a pointer to the object contained by operand
. But there is no such object.
T = void
case, probably by just explicitly returning nullptr
.
[2019-11 Priority to 2 during Monday issue prioritization in Belfast. There is implementation divergence here.]
[2020-02 LWG discussion in Prague did not reach consensus. Status to Open.]
There was discussion about whether or not any_cast<void>(a)
should be ill-formed, or return nullptr
.
Poll "should it return nullptr" was 0-4-5-5-1.
[2022-02 Currently ill-formed in MSVC ("error C2338: std::any cannot contain void") and returns null pointer in libstdc++ and libc++.]
Previous resolution [SUPERSEDED]:
This wording is relative to N4835.
Modify 22.7.5 [any.nonmembers] as indicated:
template<class T> const T* any_cast(const any* operand) noexcept; template<class T> T* any_cast(any* operand) noexcept;-9- Returns: If
[…]operand != nullptr && operand->type() == typeid(T) && is_object_v<T>
, a pointer to the object contained byoperand
; otherwise,nullptr
.
[2023-06-14 Varna; Jonathan provides improved wording]
[2023-06-14 Varna; Move to Ready]
Poll: 7-0-1
[2023-11-11 Approved at November 2023 meeting in Kona. Status changed: Voting → WP.]
Proposed resolution:
This wording is relative to N4950.
Modify 22.7.5 [any.nonmembers] as indicated:
template<class T> const T* any_cast(const any* operand) noexcept; template<class T> T* any_cast(any* operand) noexcept;-8- Mandates:
is_void_v<T>
isfalse
.-9- Returns: If
[…]operand != nullptr && operand->type() == typeid(T)
istrue
, a pointer to the object contained byoperand
; otherwise,nullptr
.
ranges::advance
violates its preconditionsSection: 24.4.4.2 [range.iter.op.advance] Status: C++23 Submitter: Casey Carter Opened: 2019-10-27 Last modified: 2023-11-22
Priority: 2
View all other issues in [range.iter.op.advance].
View all issues with C++23 status.
Discussion:
Recall that "[i, s)
denotes a range" for an iterator i
and sentinel s
means that either i == s
holds, or i
is dereferenceable and [++i, s)
denotes a range ( [iterator.requirements.genera]).
The three-argument overload ranges::advance(i, n, bound)
is specified in
24.4.4.2 [range.iter.op.advance] paragraphs 5 through 7. Para 5 establishes a precondition that
[bound, i)
denotes a range when n < 0
(both bound
and i
must
have the same type in this case). When sized_sentinel_for<S, I>
holds and
n < bound - i
, para 6.1.1 says that ranges::advance(i, n, bound)
is equivalent
to ranges::advance(i, bound)
. Para 3, however, establishes a precondition for
ranges::advance(i, bound)
that [i, bound)
denotes a range. [bound, i)
and
[i, bound)
cannot both denote ranges unless i == bound
, which is not the case for
all calls that reach 6.1.1.
The call in para 6.1.1 wants the effects of either 4.1 - which really has no preconditions - or 4.2,
which is well-defined if either [i, bound)
or [bound, i)
denotes a range. Para 3's
stronger precondition is actually only required by Para 4.3, which increments i
blindly
looking for bound
. The straight-forward fix here seems to be to relax para 3's precondition
to only apply when 4.3 will be reached.
[2019-11 Priority to 2 during Monday issue prioritization in Belfast]
[2020-08-21 Issue processing telecon: moved to Tentatively Ready]
[2020-11-09 Approved In November virtual meeting. Status changed: Tentatively Ready → WP.]
Proposed resolution:
This wording is relative to N4835.
Modify 24.4.4.2 [range.iter.op.advance] as indicated:
template<input_or_output_iterator I, sentinel_for<I> S> constexpr void ranges::advance(I& i, S bound);-3- Expects: Either
assignable_from<I&, S> || sized_sentinel_for<S, I>
is modeled, or[i, bound)
denotes a range.-4- Effects:
(4.1) — If
I
andS
modelassignable_from<I&, S>
, equivalent toi = std::move(bound)
.(4.2) — Otherwise, if
S
andI
modelsized_sentinel_for<S, I>
, equivalent toranges::advance(i, bound - i)
.(4.3) — Otherwise, while
bool(i != bound)
istrue
, incrementsi
.
std::allocator<void>().allocate(n)
Section: 20.2.10 [default.allocator] Status: C++20 Submitter: Jonathan Wakely Opened: 2019-10-25 Last modified: 2021-02-25
Priority: 0
View other active issues in [default.allocator].
View all other issues in [default.allocator].
View all issues with C++20 status.
Discussion:
In C++20 the std::allocator<void>
explicit specialization is gone, which means it uses the
primary template, which has allocate and deallocate members.
sizeof(T)
, std::allocator<T>::allocate
doesn't have an explicit precondition that the value type is complete.
[2019-11 Status to 'Ready' in Monday issue prioritization in Belfast]
Proposed resolution:
This wording is relative to N4835.
Modify 20.2.10.2 [allocator.members] as indicated:
[[nodiscard]] constexpr T* allocate(size_t n);-?- Mandates:
-2- Returns: A pointer to the initial element of an array of storage of sizeT
is not an incomplete type (6.8 [basic.types]).n * sizeof(T)
, aligned appropriately for objects of typeT
. […]
SIZE_MAX
with numeric_limits<size_t>::max()
Section: 20.5.3.3 [mem.poly.allocator.mem] Status: C++20 Submitter: Japan Opened: 2019-11-04 Last modified: 2021-02-25
Priority: 0
View all other issues in [mem.poly.allocator.mem].
View all issues with C++20 status.
Discussion:
Addresses JP 218/219
It's better to use a C++ property than C standard library macro, SIZE_MAX
.
[2019-11 Status to Ready during Tuesday morning issue processing in Belfast.]
Proposed resolution:
This wording is relative to N4835.
Modify 20.5.3.3 [mem.poly.allocator.mem] as indicated:
[[nodiscard]] Tp* allocate(size_t n);[…]-1- Effects: If
, throws
SIZE_MAXnumeric_limits<size_t>::max() / sizeof(Tp) < nlength_error
. […]template<class T> T* allocate_object(size_t n = 1);-8- Effects: Allocates memory suitable for holding an array of
n
objects of typeT
, as follows:
(8.1) — if
, throws
SIZE_MAXnumeric_limits<size_t>::max() / sizeof(T) < nlength_error
,(8.2) — otherwise equivalent to:
return static_cast<T*>(allocate_bytes(n*sizeof(T), alignof(T)));
join_view::iterator::operator--
is incorrectly constrainedSection: 25.7.14.3 [range.join.iterator] Status: C++20 Submitter: United States Opened: 2019-11-04 Last modified: 2021-02-25
Priority: 0
View all other issues in [range.join.iterator].
View all issues with C++20 status.
Discussion:
Addresses US 294
join_view::iterator::operator--
is improperly constrained. In the Effects: clause in
paragraph 14, we see the statement:
inner_ = ranges::end(*--outer_);
However, this only well-formed when end
returns an iterator, not a sentinel. This
requirement is not reflected in the constraints of the function(s).
join_view::iterator::operator--
is specified as:
constexpr iterator& operator--() requires ref_is_glvalue && bidirectional_range<Base> && bidirectional_range<range_reference_t<Base>>;-14- Effects: Equivalent to:if (outer_ == ranges::end(parent_->base_)) inner_ = ranges::end(*--outer_); while (inner_ == ranges::begin(*outer_)) inner_ = ranges::end(*--outer_); --inner_; return *this;
The trouble is from the lines that do:
inner_ = ranges::end(*--outer_);
Clearly this will only compile when *--outer
returns a common_range
, but
nowhere is that requirement stated.
[2019-11 Status to Ready during Tuesday morning issue processing in Belfast.]
Proposed resolution:
This wording is relative to N4835.
Modify 25.7.14.3 [range.join.iterator], class template join_view::iterator
synopsis,
as indicated:
constexpr iterator& operator--() requires ref_is_glvalue && bidirectional_range<Base> && bidirectional_range<range_reference_t<Base>> && common_range<range_reference_t<Base>>; constexpr iterator operator--(int) requires ref_is_glvalue && bidirectional_range<Base> && bidirectional_range<range_reference_t<Base>> && common_range<range_reference_t<Base>>;
Modify 25.7.14.3 [range.join.iterator] as indicated:
constexpr iterator& operator--() requires ref_is_glvalue && bidirectional_range<Base> && bidirectional_range<range_reference_t<Base>> && common_range<range_reference_t<Base>>;-14- Effects: Equivalent to:
if (outer_ == ranges::end(parent_->base_)) inner_ = ranges::end(*--outer_); while (inner_ == ranges::begin(*outer_)) inner_ = ranges::end(*--outer_); --inner_; return *this;constexpr iterator operator--(int) requires ref_is_glvalue && bidirectional_range<Base> && bidirectional_range<range_reference_t<Base>> && common_range<range_reference_t<Base>>;-15- Effects: Equivalent to:
auto tmp = *this; --*this; return tmp;
Period::type
is micro
?Section: 30.5.11 [time.duration.io] Status: C++20 Submitter: Tom Honermann Opened: 2019-11-04 Last modified: 2021-02-25
Priority: 2
View all other issues in [time.duration.io].
View all issues with C++20 status.
Discussion:
30.5.11 [time.duration.io] states:
template<class charT, class traits, class Rep, class Period> basic_ostream<charT, traits>& operator<<(basic_ostream<charT, traits>& os, const duration<Rep, Period>& d);[…]
-3- The units suffix depends on the typePeriod::type
as follows:[…] -4- If
[…]
(3.5) — Otherwise, if
Period::type
ismicro
, the suffix is"µs"
("\u00b5\u0073"
).[…]
Period::type
ismicro
, but the character U+00B5 cannot be represented in the encoding used forcharT
, the unit suffix"us"
is used instead of"µs"
. […]
Which encoding is intended by "the encoding used for charT
"? There are two candidates:
The associated execution character set as defined by 5.3.1 [lex.charset] p3 used to encode
character and string literals (e.g., the "execution wide-character set" for wchar_t
).
The locale dependent character set used by the std::locale ctype
and codecvt
facets as specified in 28.3.4.2 [category.ctype], sometimes referred to as the
"native character set".
The behavior should not be dependent on locale
and should therefore be specified in terms of
the execution character sets.
/execution-charset
compiler
option. The Microsoft compiler might therefore use "us"
by default, but "µs"
when invoked with the /execution-charset:utf-8
or /execution-charset:.437
options.
In the latter two cases, the string contents would contain "\xb5\x73"
and "\xe6\x73"
respectively (Unicode and Windows code page 437 map µ (U+00B5, MICRO SIGN) to different code points).
This resolution relies on the character set for the locale used at run-time being compatible with the
execution character set if the produced string is to be displayed correctly when written to a terminal
or console. This is a typical requirement for character and string literals but is more strongly
relevant for this issue since µ lacks representation in many character sets. Additionally, if the
stream is imbued with a std::codecvt
facet, the facet must provide appropriate conversion
support for behavior to be well defined.
[2019-11 Priority to 2 during Tuesday morning issue processing in Belfast.]
Previous resolution [SUPERSEDED]:
This wording is relative to N4835.
Modify 30.5.11 [time.duration.io] as indicated:
[Drafting note: "implementation's native character set" is used in 28.3.4.2.2 [locale.ctype] and 28.3.4.2.5 [locale.codecvt] to refer to the locale dependent character encoding.]
template<class charT, class traits, class Rep, class Period> basic_ostream<charT, traits>& operator<<(basic_ostream<charT, traits>& os, const duration<Rep, Period>& d);[…]
-3- The units suffix depends on the typePeriod::type
as follows:[…] -4- If
[…]
(3.5) — Otherwise, if
Period::type
ismicro
, the suffix is"µs"
("\u00b5\u0073"
).[…]
Period::type
ismicro
, but the character U+00B5cannot be represented in the encoding usedlacks representation in the execution character set forcharT
, the unit suffix"us"
is used instead of"µs"
. If"µs"
is used but the implementation's native character set lacks representation for U+00B5 and the stream is associated with a terminal or console, or if the stream is imbued with astd::codecvt
facet that lacks conversion support for the character, then the result is unspecified. […]
[2019-11-12; Tom Honermann improves wording]
[2020-02 Status to Immediate on Thursday night in Prague.]
Proposed resolution:
This wording is relative to N4835.
Modify 30.5.11 [time.duration.io] as indicated:
template<class charT, class traits, class Rep, class Period> basic_ostream<charT, traits>& operator<<(basic_ostream<charT, traits>& os, const duration<Rep, Period>& d);[…]
-3- The units suffix depends on the typePeriod::type
as follows:[…]
[…]
(3.5) — Otherwise, if
Period::type
ismicro
, it is implementation-defined whether the suffix is"µs"
("\u00b5\u0073"
) or"us"
.[…]
-4- If[…]Period::type
ismicro
, but the character U+00B5 cannot be represented in the encoding used forcharT
, the unit suffix"us"
is used instead of"µs"
.
Section: 16.4.4.6 [allocator.requirements] Status: C++20 Submitter: United States Opened: 2019-11-04 Last modified: 2021-02-25
Priority: 0
View other active issues in [allocator.requirements].
View all other issues in [allocator.requirements].
View all issues with C++20 status.
Discussion:
Addresses US 162/US 163
The default behavior fora.destroy
is now to call destroy_at
Proposed change:
Replace "default" entry with: destroy_at(c)
The default behavior for a.construct
is now to call construct_at
Proposed change:
Replace "default" entry with: construct_at(c, std::forward<Args>(args)...)
Dietmar Kühl:
In Table 34 [tab:cpp17.allocator] the behavior of a.construct(c, args)
and
a.destroy(c)
are described to have a default behavior of
::new ((void*)c) C(forward<Args>(args))
and c->~C()
, respectively.
However, this table doesn't actually define what is happening if these operations are omitted:
The behavior is provided when using an allocator is used via std::allocator_traits
and is, thus, defined by the corresponding std::allocator_traits
functions. These
functions are specified in 20.2.9.3 [allocator.traits.members] paragraphs 5 and 6 to call
construct_at(c, std::forward<Args>(args)
and destroy_at(p)
, respectively.
The text in the table should be updated to match the actual behavior.
[2019-11 Status to Ready during Wednesday morning issue processing in Belfast.]
Proposed resolution:
This wording is relative to N4835.
Modify 16.4.4.6 [allocator.requirements], Table [tab:cpp17.allocator]
"Cpp17Allocator
requirements" as indicated:
Table 34 — Cpp17Allocator
requirements [tab:cpp17.allocator]Expression Return type Assertion/note
pre-/post-conditionDefault …
a.construct(c, args)
(not used) Effects: Constructs an object of type C
atc
::new ((void*)c) C(construct_at(c, std::forward<Args>(args)...)a.destroy(c)
(not used) Effects: Destroys the object at c
c->~C()destroy_at(c)…
utc_clock
/ utc_timepoint
Section: 30.7.3.1 [time.clock.utc.overview] Status: C++20 Submitter: Great Britain Opened: 2019-11-05 Last modified: 2021-02-25
Priority: 0
View all issues with C++20 status.
Discussion:
Addresses GB 333
UTC epoch is not correctly defined UTC has an officially recorded epoch of 1/1/1972 00:00:00 and is 10 seconds behind TAI. This can be confirmed through reference to the BIPM (the body that oversees international metrology)
"The defining epoch of 1 January 1972,
0 h 0m 0 s
UTC was set 10 s behind TAI, which was the approximate accumulated difference between TAI and UT1 since the inception of TAI in 1958, and a unique fraction of a second adjustment was applied so that UTC would differ from TAI by an integral number of seconds. The recommended maximum departure of UTC from UT1 was 0.7 s. The term "leap second" was introduced for the stepped second."
Proposed change:
utc_clock
and utc_timepoint
should correctly report relative to the official
UTC epoch. 27.2.2.1 footnote 1 should read:
In contrast to
sys_time
, which does not take leap seconds into account,utc_clock
and its associatedtime_point
,utc_time
, count time, including leap seconds, since 1972-01-01 00:00:00 UTC. [Example:clock_cast<utc_clock>(sys_seconds{sys_days{1972y/January/1}}).time_since_epoch()
is0s
.clock_cast<utc_clock>(sys_seconds{sys_days{2000y/January/1}}).time_since_epoch()
is883'612'822
, which is10'197 * 86'400s + 22s
. — end example]
Howard Hinnant:
Clarify that the epoch ofutc_clock
is intended to be 1970-01-01
.
Rationale: The main use case of utc_clock
is to get the correct number of seconds
when subtracting time points straddling a leap second insertion point, and this computation
is independent of the epoch. Furthermore learning/teaching that utc_clock
is
system_clock
except that utc_clock
includes leap seconds is easier.
And this fact is more easily understood when comparing the underlying .time_since_epoch()
of equivalent time points from each clock.
[2019-11 Status to Ready during Wednesday morning issue processing in Belfast.]
Proposed resolution:
This wording is relative to N4835.
Modify 30.7.3.1 [time.clock.utc.overview] as indicated:
-1- In contrast to
sys_time
, which does not take leap seconds into account,utc_clock
and its associatedtime_point
,utc_time
, count time, including leap seconds, since 1970-01-01 00:00:00 UTC. [Note: The UTC time standard began on 1972-01-01 00:00:10 TAI. To measure time since this epoch instead, one can add/subtract the constantsys_days{1972y/1/1} - sys_days{1970y/1/1}
(63'072'000s
) from theutc_time
— end note] [Example:clock_cast<utc_clock>(sys_seconds{sys_days{1970y/January/1}}).time_since_epoch()
is0s
.clock_cast<utc_clock>(sys_seconds{sys_days{2000y/January/1}}).time_since_epoch()
is946'684'822s
, which is10'957 * 86'400s + 22s
. — end example]
operator<<
for floating-point durationsSection: 30.5.11 [time.duration.io] Status: C++20 Submitter: United States Opened: 2019-11-05 Last modified: 2021-02-25
Priority: 0
View all other issues in [time.duration.io].
View all issues with C++20 status.
Discussion:
Addresses US 334
operator<<
for floating-point durations always produces output with six digits
after the decimal point, and doesn't use the stream's locale either.
Proposed change:
Rewrite the specification to not rely onto_string()
for floating-point formatting.
[2019-11 Status to Ready during Wednesday morning issue processing in Belfast.]
Proposed resolution:
This wording is relative to N4835.
Modify 30.5.11 [time.duration.io] as indicated:
template<class charT, class traits, class Rep, class Period> basic_ostream<charT, traits>& operator<<(basic_ostream<charT, traits>& os, const duration<Rep, Period>& d);-2- Effects:
-1- Requires:Rep
is an integral type whose integer conversion rank (6.8.6 [conv.rank]) is greater than or equal to that ofshort
, or a floating-point type.charT
ischar
orwchar_t
.Forms aInserts the durationbasic_string<charT, traits>
fromd.count()
usingto_string
ifcharT
ischar
, orto_wstring
ifcharT
iswchar_t
. Appends the units suffix described below to thebasic_string
. Inserts the resultingbasic_string
intoos
. [Note: This specification ensures that the result of this streaming operation will obey the width and alignment properties of the stream. — end note]d
onto the streamos
as if it were implemented as follows:-3-basic_ostringstream<charT, traits> s; s.flags(os.flags()); s.imbue(os.getloc()); s.precision(os.precision()); s << d.count() << units_suffix; return os << s.str();The units suffixunits_suffix
depends on the typePeriod::type
as follows:In the list above the use of
(3.1) — If
Period::type
isatto
,the suffixunits_suffix
is"as"
.(3.2) — Otherwise, if
Period::type
isfemto
,the suffixunits_suffix
is"fs"
.(3.3) — Otherwise, if
Period::type
ispico
,the suffixunits_suffix
is"ps"
.(3.4) — Otherwise, if
Period::type
isnano
,the suffixunits_suffix
is"ns"
.(3.5) — Otherwise, if
Period::type
ismicro
,the suffixunits_suffix
is"µs"
("\u00b5\u0073"
).(3.6) — Otherwise, if
Period::type
ismilli
,the suffixunits_suffix
is"ms"
.(3.7) — Otherwise, if
Period::type
iscenti
,the suffixunits_suffix
is"cs"
.(3.8) — Otherwise, if
Period::type
isdeci
,the suffixunits_suffix
is"ds"
.(3.9) — Otherwise, if
Period::type
isratio<1>
,the suffixunits_suffix
is"s"
.(3.10) — Otherwise, if
Period::type
isdeca
,the suffixunits_suffix
is"das"
.(3.11) — Otherwise, if
Period::type
ishecto
,the suffixunits_suffix
is"hs"
.(3.12) — Otherwise, if
Period::type
iskilo
,the suffixunits_suffix
is"ks"
.(3.13) — Otherwise, if
Period::type
ismega
,the suffixunits_suffix
is"Ms"
.(3.14) — Otherwise, if
Period::type
isgiga
,the suffixunits_suffix
is"Gs"
.(3.15) — Otherwise, if
Period::type
istera
,the suffixunits_suffix
is"Ts"
.(3.16) — Otherwise, if
Period::type
ispeta
,the suffixunits_suffix
is"Ps"
.(3.17) — Otherwise, if
Period::type
isexa
,the suffixunits_suffix
is"Es"
.(3.18) — Otherwise, if
Period::type
isratio<60>
,the suffixunits_suffix
is"min"
.(3.19) — Otherwise, if
Period::type
isratio<3600>
,the suffixunits_suffix
is"h"
.(3.20) — Otherwise, if
Period::type
isratio<86400>
,the suffixunits_suffix
is"d"
.(3.21) — Otherwise, if
Period::type::den == 1
,the suffixunits_suffix
is"[num]s"
.(3.22) — Otherwise,
the suffixunits_suffix
is"[num/den]s"
.num
andden
refer to the static data members ofPeriod::type
, which are converted to arrays ofcharT
using a decimal conversion with no leading zeroes. -4- IfPeriod::type
ismicro
, but the character U+00B5 cannot be represented in the encoding used forcharT
,the unit suffixunits_suffix
"us"
is used instead of"µs"
. -5- Returns:os
.
Section: 30.7.2.1 [time.clock.system.overview] Status: C++20 Submitter: Great Britain Opened: 2019-11-05 Last modified: 2021-02-25
Priority: 0
View all issues with C++20 status.
Discussion:
Addresses GB 335
Wording for clocks should be unified unless they are intended to behave differently
In 27.7.1.1 note 1 for system_clock
it is stated:
"Objects of type
system_clock
represent wall clock time from the system-wide realtime clock. Objects of typesys_time<Duration>
measure time since (and before) 1970-01-01 00:00:00 UTC"
The express statement of "since (and before)" is important given the time epoch of these clocks. If all the clocks support time prior to their zero-time then this should be stated explicitly. If not then likewise that should be noted. No change is proposed yet, clarification required over the intended behaviour when using values prior to a given clock's epoch is needed before the appropriate change can be suggested.
Proposed change: Unify the wording.Howard Hinnant:
The clocks that are specified to have a signedrep
imply that they will support
negative time points, but not how negative. For example if system_clock::duration
is nanoseconds
represented with 64 bits, then system_clock::time_point
can't possibly represent dates prior to 1677-09-21 00:12:43.145224192. This is a negative
time_point
since it is prior to 1970-01-01 00:00:00. But it is not very negative
compared to (for example) sys_time<microseconds>::min()
.
Those clocks with a signed rep
are:
system_clock
utc_clock
tai_clock
gps_clock
file_clock
Those clocks where the signed-ness of rep
is unspecified are:
steady_clock
high_resolution_clock
Therefore this response emphasizes the "Unify the wording" part of this NB comment.
[2019-11 Status to Ready during Wednesday morning issue processing in Belfast.]
Proposed resolution:
This wording is relative to N4835.
Modify 30.7.2.1 [time.clock.system.overview] as indicated:
-1- Objects of type
system_clock
represent wall clock time from the system-wide realtime clock. Objects of typesys_time<Duration>
measure time since(and before)1970-01-01 00:00:00 UTC excluding leap seconds. This measure is commonly referred to as Unix time. This measure facilitates an efficient mapping betweensys_time
and calendar types (30.8 [time.cal]). [Example:sys_seconds{sys_days{1970y/January/1}}.time_since_epoch()
is0s
.sys_seconds{sys_days{2000y/January/1}}.time_since_epoch()
is946'684'800s
, which is10'957 * 86'400s
. — end example]
Section: 30.11.1 [time.zone.general] Status: C++20 Submitter: Germany Opened: 2019-11-05 Last modified: 2021-02-25
Priority: 0
View all issues with C++20 status.
Discussion:
Addresses DE 344
This paragraph says
"27.11 describes an interface for accessing the IANA Time Zone database described in RFC 6557, …"
However, RFC 6557 does not describe the database itself; it only describes the maintenance procedures for that database, as its title implies (quoted in clause 2).
Proposed change: Add a reference to a specification of the database itself, or excise all references to the IANA time zone database.Howard Hinnant:
We can not entirely remove the reference to IANA because we need portabletime_zone
names (e.g. "America/New_York") and definitions. However the NB comment is quite accurate and
fixed with the proposed resolution.
[2019-11 Status to Ready during Wednesday morning issue processing in Belfast.]
Proposed resolution:
This wording is relative to N4835.
Modify 2 [intro.refs] as indicated:
-1- The following documents are referred to in the text in such a way that some or all of their content constitutes requirements of this document. For dated references, only the edition cited applies. For undated references, the latest edition of the referenced document (including any amendments) applies.
[…]
(1.2) — INTERNET ENGINEERING TASK FORCE (IETF). RFC 6557: Procedures for Maintaining the Time Zone Database [online]. Edited by E. Lear, P. Eggert. February 2012 [viewed 2018-03-26]. Available at https://www.ietf.org/rfc/rfc6557.txt[…]
Modify 30.11.1 [time.zone.general] as indicated:
-1- 30.11 [time.zone] describes an interface for accessing the IANA Time Zone
dDatabasedescribed in RFC 6557,that interoperates withsys_time
andlocal_time
. This interface provides time zone support to both the civil calendar types (30.8 [time.cal]) and to user-defined calendars.
Modify section "Bibliography" as indicated:
The following documents are cited informatively in this document.
— IANA Time Zone Database. Available at https://www.iana.org/time-zones
— ISO/IEC 10967-1:2012, Information technology — Language independent arithmetic — Part 1: Integer and floating point arithmetic
[…]
span::cbegin/cend
methods produce different results than std::[ranges::]cbegin/cend
Section: 23.7.2.2.7 [span.iterators] Status: C++20 Submitter: Poland Opened: 2019-11-06 Last modified: 2021-02-25
Priority: 0
View all issues with C++20 status.
Discussion:
Addresses PL 247
span<T>
provides a const
-qualified begin()
method and cbegin()
method that produces a different result if T
is not const
-qualifed:
begin()
produces mutable iterator over T
(as if T*
)
cbegin()
produces const
iterator over T
(as if T const*
)
As consequence for the object s
of type span<T>
, the call to the
std::cbegin(s)/std::ranges::cbegin(s)
produces different result than s.cbegin()
.
span<T>
members cbegin()/cend()/crbegin()/crend()/const_iterator
to be equivalent to begin()/end()/rbegin()/rend()/iterator
respectively.
Tomasz Kamiński:
Per LEWG discussion in Belfast these methods and related typedefs should be removed.[2019-11 Status to Ready during Wednesday night issue processing in Belfast.]
Proposed resolution:
This wording is relative to N4835.
Modify 23.7.2.2.1 [span.overview], class template span
synopsis, as indicated:
namespace std { template<class ElementType, size_t Extent = dynamic_extent> class span { public: // constants and types using element_type = ElementType; using value_type = remove_cv_t<ElementType>; using index_type = size_t; using difference_type = ptrdiff_t; using pointer = element_type*; using const_pointer = const element_type*; using reference = element_type&; using const_reference = const element_type&; using iterator = implementation-defined; // see 23.7.2.2.7 [span.iterators]using const_iterator = implementation-defined;using reverse_iterator = std::reverse_iterator<iterator>;using const_reverse_iterator = std::reverse_iterator<const_iterator>;static constexpr index_type extent = Extent; […] // 23.7.2.2.7 [span.iterators], iterator support constexpr iterator begin() const noexcept; constexpr iterator end() const noexcept;constexpr const_iterator cbegin() const noexcept; constexpr const_iterator cend() const noexcept;constexpr reverse_iterator rbegin() const noexcept; constexpr reverse_iterator rend() const noexcept;constexpr const_reverse_iterator crbegin() const noexcept; constexpr const_reverse_iterator crend() const noexcept;friend constexpr iterator begin(span s) noexcept { return s.begin(); } friend constexpr iterator end(span s) noexcept { return s.end(); } […] }; […] }
Modify 23.7.2.2.7 [span.iterators] as indicated:
using iterator = implementation-defined;using const_iterator = implementation-defined;[…]-1- The type
smodelscontiguous_iterator
(24.3.4.14 [iterator.concept.contiguous]), meets the Cpp17RandomAccessIterator requirements (24.3.5.7 [random.access.iterators]), and meets the requirements for constexpr iterators (24.3.1 [iterator.requirements.general]). All requirements on container iterators (23.2 [container.requirements]) apply tospan::iterator
andas well.span::const_iterator
constexpr const_iterator cbegin() const noexcept;
-6- Returns: A constant iterator referring to the first element in the span. Ifempty()
istrue
, then it returns the same value ascend()
.constexpr const_iterator cend() const noexcept;
-7- Returns: A constant iterator which is the past-the-end value.constexpr const_reverse_iterator crbegin() const noexcept;
-8- Effects: Equivalent to:return const_reverse_iterator(cend());
constexpr const_reverse_iterator crend() const noexcept;
-9- Effects: Equivalent to:return const_reverse_iterator(cbegin());
uninitialized_construct_using_allocator
should use construct_at
Section: 20.2.8.2 [allocator.uses.construction] Status: C++20 Submitter: United States Opened: 2019-11-06 Last modified: 2021-02-25
Priority: 0
View all other issues in [allocator.uses.construction].
View all issues with C++20 status.
Discussion:
Addresses US 213
uninitialized_construct_using_allocator
should use construct_at
instead
of operator new
Effects: Equivalent to:
return::new(static_cast<void*>(p))construct_at(p,T(make_obj_using_allocator<T>(alloc, std::forward<Args>(args)...)));
Tim Song:
The proposed wording in the NB comment is incorrect, because it prevents guaranteed elision.[2019-11 Status to Ready during Wednesday night issue processing in Belfast.]
Proposed resolution:
This wording is relative to N4835.
Modify 20.2.8.2 [allocator.uses.construction] as indicated:
template<class T, class Alloc, class... Args> constexpr T* uninitialized_construct_using_allocator(T* p, const Alloc& alloc, Args&&... args);-17- Effects: Equivalent to:
return::new(static_cast<void*>(p)) T(make_obj_using_allocator<T>(apply([&](auto&&...xs) { return construct_at(p, std::forward<decltype(xs)>(xs)...); }, uses_allocator_construction_args<T>(alloc, std::forward<Args>(args)...));
join_view::base()
member functionSection: 25.7.14.2 [range.join.view] Status: Resolved Submitter: United States Opened: 2019-11-06 Last modified: 2020-11-09
Priority: 0
View all other issues in [range.join.view].
View all issues with Resolved status.
Discussion:
Addresses US 293
join_view
is missing a base()
member for returning the underlying view. All the other
range adaptors provide this.
To the join_view
class template add the member:
constexpr V base() const { return base_; }
Jonathan Wakely:
The NB comment says "join_view
is missing a base()
member for returning the
underlying view. All the other range adaptors provide this."
In fact, split_view
and istream_view
do not provide base()
either. Of the
views that do define base()
, all except all_view
do so out-of-line, so the proposed
resolution adds it out-of-line too.
[2019-11 Status to Ready during Wednesday night issue processing in Belfast.]
[2019-12-16; Casey comments]
This issue has been resolved by P1456R1 "Move-only views", which
added no less than two member functions named "base" to join_view
.
Previous resolution [SUPERSEDED]:
This wording is relative to N4835.
Modify 25.7.14.2 [range.join.view], class template
join_view
synopsis, as indicated:[…] template<input_range R> requires viewable_range<R> && constructible_from<V, all_view<R>> constexpr explicit join_view(R&& r); constexpr V base() const; constexpr auto begin() { return iterator<simple-view<V>>{*this, ranges::begin(base_)}; } […]Modify 25.7.14.2 [range.join.view] as indicated:
template<input_range R> requires viewable_range<R> && constructible_from<V, all_view<R>> constexpr explicit join_view(R&& r);-2- Effects: […]
constexpr V base() const;-?- Effects: Equivalent to:
return base_;
[2020-11-09 Resolved for C++20. Status changed: Tentatively Resolved → Resolved.]
Proposed resolution:
Resolved by accepting P1456R1.
has-tuple-element
helper concept needs convertible_to
Section: 25.7.23.2 [range.elements.view] Status: C++20 Submitter: Great Britain Opened: 2019-11-06 Last modified: 2021-02-25
Priority: 0
View all other issues in [range.elements.view].
View all issues with C++20 status.
Discussion:
Addresses GB 299
has-tuple-element
helper concept needs convertible_to
has-tuple-element
concept (for elements_view
) is defined as
template<class T, size_t N> concept has-tuple-element = exposition only requires(T t) { typename tuple_size<T>::type; requires N < tuple_size_v<T>; typename tuple_element_t<N, T>; { get<N>(t) } -> const tuple_element_t<N, T>&; };
However, the return type constraint for { get<N>(t) }
is no longer valid
under the latest concepts changes
Change to:
template<class T, size_t N> concept has-tuple-element = exposition only requires(T t) { typename tuple_size<T>::type; requires N < tuple_size_v<T>; typename tuple_element_t<N, T>; { get<N>(t) } -> convertible_to<const tuple_element_t<N, T>&>; };
Jonathan Wakely:
The NB comment says "The return type constraint for{ get(t) }
is no longer valid under
the latest concepts changes." The changes referred to are those in P1452R2.
[2019-11 Status to Ready during Wednesday night issue processing in Belfast.]
Proposed resolution:
This wording is relative to N4835.
Modify 25.7.23.2 [range.elements.view], class template elements_view
synopsis,
as indicated:
namespace std::ranges { template<class T, size_t N> concept has-tuple-element = // exposition only requires(T t) { typename tuple_size<T>::type; requires N < tuple_size_v<T>; typename tuple_element_t<N, T>; { get<N>(t) } -> convertible_to<const tuple_element_t<N, T>&>; }; […] }
std::strong/weak/partial_order
for pointersSection: 17.12.6 [cmp.alg] Status: C++20 Submitter: Canada Opened: 2019-11-06 Last modified: 2021-02-25
Priority: 0
View other active issues in [cmp.alg].
View all other issues in [cmp.alg].
View all issues with C++20 status.
Discussion:
Addresses CA 178
std::strong_order
, weak_order
, and partial_order
have special cases
for floating point, but are missing special casing for pointers. compare_three_way
and
std::less
have the special casing for pointers.
strong_ordering(E <=> F)
if it is a well-formed expression."
strong_ordering(compare_three_way()(E, F))
if it is a well-formed expression."
Change [cmp.alg] bullet 2.4 from
weak_ordering(E <=> F)
if it is a well-formed expression."
weak_ordering(compare_three_way()(E, F))
if it is a well-formed expression."
partial_ordering(E <=> F)
if it is a well-formed expression."
partial_ordering(compare_three_way()(E, F))
if it is a well-formed expression."
Dietmar Kühl:
Usecompare_three_way
instead of <=>
for the various comparison algorithms.
[2019-11 Status to Ready during Wednesday night issue processing in Belfast.]
Proposed resolution:
This wording is relative to N4835.
Change 17.12.6 [cmp.alg] as indicated:
-1- The name
strong_order
[…]
[…]
(1.4) — Otherwise,
strong_ordering(
if it is a well-formed expression.E <=> Fcompare_three_way()(E, F))[…]
-2- The name
weak_order
[…]
[…]
(2.4) — Otherwise,
weak_ordering(
if it is a well-formed expression.E <=> Fcompare_three_way()(E, F))[…]
-3- The name
partial_order
[…]
[…]
(3.3) — Otherwise,
partial_ordering(
if it is a well-formed expression.E <=> Fcompare_three_way()(E, F))[…]
transform_view
Section: 25.7.9.2 [range.transform.view] Status: C++20 Submitter: United States Opened: 2019-11-06 Last modified: 2022-01-15
Priority: 0
View all issues with C++20 status.
Discussion:
Addresses US 303
The transform_view
does not constrain the return type of the transformation function. It
is invalid to pass a void
-returning transformation function to the transform_view
,
which would cause its iterator
s' operator*
member to return void
.
Change the constraints on transform_view
to the following:
template<input_range V, copy_constructible F> requires view<V> && is_object_v<F> && regular_invocable<F&, range_reference_t<V>> && can-reference<invoke_result_t<F&, range_reference_t<V>>> class transform_view;
Jonathan Wakely:
The NB comment says "Thetransform_view
does not constrain the return type of the transformation
function. It is invalid to pass a void
-returning transformation function to the transform_view
,
which would cause its iterator
s' operator*
member to return void
."
[2019-11 Status to Ready during Wednesday night issue processing in Belfast.]
Proposed resolution:
This wording is relative to N4835.
Modify 25.7.9.2 [range.transform.view], class template transform_view
synopsis,
as indicated:
namespace std::ranges { template<input_range V, copy_constructible F> requires view<V> && is_object_v<F> && regular_invocable<F&, range_reference_t<V>> && can-reference<invoke_result_t<F&, range_reference_t<V>>> class transform_view : public view_interface<transform_view<V, F>> { […] }; […] }
enable_view
has false positivesSection: 25.4.4 [range.view] Status: C++20 Submitter: Germany Opened: 2019-11-06 Last modified: 2021-02-25
Priority: 0
View all other issues in [range.view].
View all issues with C++20 status.
Discussion:
Addresses DE 282
"Since the difference between range and view is largely semantic, the
two are differentiated with the help of enable_view
." (§3)
enable_view
is designed as on opt-in trait to specify that a type is a
view. It defaults to true for types derived from view_base
(§4.2) which
is clearly a form of opt-in. But it also employs a heuristic assuming that anything with
iterator == const_iterator
is also view (§4.3).
This is a very poor heuristic, the same paragraph already needs to define six
exceptions from this rule for standard library types (§4.2).
Experience in working with range-v3 has revealed multiple of our own library types as being
affected from needing to opt-out from the "auto-opt-in", as well. This is counter-intuitive:
something that was never designed to be a view shouldn't go through hoops so that it isn't
treated as a view.
Proposed change:
Make enable_view
truly be opt-in by relying only on explicit specialisation or
inheritance from view_base
. This means removing 24.4.4 §4.2 - §4.4 and
introducing new §4.2 "Otherwise, false".
basic_string_view
and span
need to opt-in to being a view now.
Casey Carter:
enable_view
(25.4.4 [range.view]) is designed as on opt-in trait to specify that a type
is a view. It defaults to true
for types derived from view_base
— which is a
form of opt-in — and it also employs a heuristic. Unfortunately, the heuristic has false positives.
The working draft itself includes six exceptions to the heuristic for standard library types. Since
false positives are much more problematic for users than false negatives, we should eliminate the heuristic.
[2019-11 Status to Ready during Wednesday night issue processing in Belfast.]
Proposed resolution:
This wording is relative to N4835.
Modify 25.4.4 [range.view] as indicated:
template<class T> inline constexpr bool enable_view =see belowderived_from<T, view_base>;
-4- Remarks: For a typeT
, the default value ofenable_view<T>
is:
(4.1) — Ifderived_from<T, view_base>
istrue
,true
.
(4.2) — Otherwise, ifT
is a specialization of class templateinitializer_list
(17.11 [support.initlist]),set
(23.4.6 [set]),multiset
(23.4.7 [multiset]),unordered_set
(23.5.6 [unord.set]),unordered_multiset
(23.5.7 [unord.multiset]), ormatch_results
(28.6.9 [re.results]),false
.
(4.3) — Otherwise, if bothT
andconst T
modelrange
andrange_reference_t<T>
is not the same type asrange_reference_t<const T>
,false
. [Note: Deepconst
-ness implies element ownership, whereas shallow const-ness implies reference semantics. — end note]
(4.4) — Otherwise, true.
Modify 27.3.2 [string.view.synop], header <string_view>
synopsis, as indicated:
namespace std { // 27.3.3 [string.view.template], class template basic_string_view template<class charT, class traits = char_traits<charT>> class basic_string_view; template<class charT, class traits> inline constexpr bool ranges::enable_view<basic_string_view<charT, traits>> = true; […] }
Modify 23.7.2.1 [span.syn], header <span>
synopsis, as indicated:
namespace std { // constants inline constexpr size_t dynamic_extent = numeric_limits<size_t>::max(); // 23.7.2.2 [views.span], class template span template<class ElementType, size_t Extent = dynamic_extent> class span; template<class ElementType, size_t Extent> inline constexpr bool ranges::enable_view<span<ElementType, Extent>> = Extent == 0 || Extent == dynamic_extent; […] }
Modify [range.split.outer.value], class split_view::outer_iterator::value_type
synopsis, as indicated:
[Drafting note: The following applies the proposed wording for LWG 3276(i)]
namespace std::ranges { template<class V, class Pattern> template<bool Const> struct split_view<V, Pattern>::outer_iterator<Const>::value_type : view_interface<value_type> { private: outer_iterator i_ = outer_iterator(); // exposition only public: value_type() = default; constexpr explicit value_type(outer_iterator i); constexpr inner_iterator<Const> begin() const; constexpr default_sentinel_t end() const; }; }
Section: 28.5.2.2 [format.string.std] Status: C++20 Submitter: Great Britain Opened: 2019-11-07 Last modified: 2021-02-25
Priority: 0
View other active issues in [format.string.std].
View all other issues in [format.string.std].
View all issues with C++20 status.
Discussion:
Addresses GB 225
std::format()
alignment specifiers should be independent of text direction
The align specifiers for formatting standard integer and string types are expressed in
terms of "left" and "right". However, "left alignment" as currently defined in the
format()
specification might end up being right-aligned when the resulting
string is displayed in a RTL or bidirectional locale. This ambiguity can be resolved
by removing "left" and "right" and replacing with "start" and "end", without changing
any existing implementation and without changing the intent of the feature.
In [tab:format.align]:
Forces the field to be left-aligned within aligned to the start of
the available space and Forces the field to be right-aligned within
aligned to the end of the available space
Jeff Garland:
Wiki notes from Belfast Wed:# GB225
JG: SG16 approved this. JG: If you scroll back up, you'll see see it's very tiny. Two line change. JG: I'm willing to submit an LWG issue to suggest we make a wording change to take it off our plate. CC: Is this the one that changes left/right to beginning/end? Some people: yes MC: Any problem with Jeff's proposed direction and this proposed fix? MC: I hear none.
[2019-11 Moved to Ready on Friday AM in Belfast]
Proposed resolution:
This wording is relative to N4835.
Modify "Table 57 — Meaning of align options" [tab:format.align] as indicated:
Table 57 — Meaning of align options [tab:format.align] Option Meaning <
Forces the field to be left-aligned withinaligned to the start of the available space. This is the default for non-arithmetic types,charT
, andbool
, unless an integer presentation type is specified.>
Forces the field to be right-aligned withinaligned to the end of the available space. This is the default for arithmetic types other thancharT
andbool
or when an integer presentation type is specified.[…]
std::string
is not good for UTF-8Section: D.21 [depr.fs.path.factory] Status: C++20 Submitter: The Netherlands Opened: 2019-11-07 Last modified: 2021-02-25
Priority: 0
View all other issues in [depr.fs.path.factory].
View all issues with C++20 status.
Discussion:
Addresses NL 375
Example in deprecated section implies that std::string
is the type to use for utf8 strings.
[Example: A string is to be read from a database that is encoded in UTF-8, and used to create a directory using the native encoding for filenames:
namespace fs = std::filesystem; std::string utf8_string = read_utf8_data(); fs::create_directory(fs::u8path(utf8_string));
Proposed change:
Add clarification that std::string
is the wrong type for utf8 strings
Jeff Garland:
SG16 in Belfast: Recommend to accept with a modification to update the example in D.21 [depr.fs.path.factory] p4 to state thatstd::u8string
should
be preferred for UTF-8 data.
Rationale: The example code is representative of historic use of std::filesystem::u8path
and should not be changed to use std::u8string
. The recommended change is to a
non-normative example and may therefore be considered editorial.
Previous resolution [SUPERSEDED]:
This wording is relative to N4835.
Modify D.21 [depr.fs.path.factory] as indicated:
-4- [Example: A string is to be read from a database that is encoded in UTF-8, and used to create a directory using the native encoding for filenames:
For POSIX-based operating systems with the native narrow encoding set to UTF-8, no encoding or type conversion occurs. For POSIX-based operating systems with the native narrow encoding not set to UTF-8, a conversion to UTF-32 occurs, followed by a conversion to the current native narrow encoding. Some Unicode characters may have no native character set representation. For Windows-based operating systems a conversion from UTF-8 to UTF-16 occurs. — end example] [Note: The example above is representative of historic use ofnamespace fs = std::filesystem; std::string utf8_string = read_utf8_data(); fs::create_directory(fs::u8path(utf8_string));filesystem
u8path
. New code should usestd::u8string
in place ofstd::string
. — end note]
LWG Belfast Friday Morning
Requested changes:
[2020-02 Moved to Immediate on Tuesday in Prague.]
Proposed resolution:
This wording is relative to N4835.
Modify D.21 [depr.fs.path.factory] as indicated:
-4- [Example: A string is to be read from a database that is encoded in UTF-8, and used to create a directory using the native encoding for filenames:
For POSIX-based operating systems with the native narrow encoding set to UTF-8, no encoding or type conversion occurs. For POSIX-based operating systems with the native narrow encoding not set to UTF-8, a conversion to UTF-32 occurs, followed by a conversion to the current native narrow encoding. Some Unicode characters may have no native character set representation. For Windows-based operating systems a conversion from UTF-8 to UTF-16 occurs. — end example] [Note: The example above is representative of a historical use ofnamespace fs = std::filesystem; std::string utf8_string = read_utf8_data(); fs::create_directory(fs::u8path(utf8_string));filesystem::u8path
. Passing astd::u8string
topath
's constructor is preferred for an indication of UTF-8 encoding more consistent withpath
's handling of other encodings. — end note]
totally_ordered_with
both directly and indirectly requires common_reference_with
Section: 18.5.5 [concept.totallyordered] Status: C++20 Submitter: United States Opened: 2019-11-07 Last modified: 2021-02-25
Priority: 0
View all other issues in [concept.totallyordered].
View all issues with C++20 status.
Discussion:
Addresses US 201
The totally_ordered_with<T, U>
redundantly requires both
common_reference_with<const remove_reference_t&, const
remove_reference_t&>
and equality_comparable_with<T,
U>
(which also has the common_reference_with
requirement). The
redundant requirement should be removed.
totally_ordered_with
to:
template<class T, class U> concept totally_ordered_with = totally_ordered<T> && totally_ordered<U> && equality_comparable_with<T, U> && totally_ordered< common_reference_t< const remove_reference_t<T>&, const remove_reference_t<U<&>> && requires(const remove_reference_t<T<& t, const remove_reference_t>U>& u) { [… as before …]
[2019-11 Moved to Ready on Friday AM in Belfast]
Proposed resolution:
This wording is relative to N4835.
Change 18.5.5 [concept.totallyordered] as indicated:
For some type
T
, leta
,b
, andc
be lvalues of typeconst remove_reference_t<T>
.T
modelstotally_ordered
only if
(1.1) — Exactly one of
bool(a < b)
,bool(a > b)
, orbool(a == b)
istrue
.(1.2) — If
bool(a < b)
andbool(b < c)
, thenbool(a < c)
.(1.3) —
bool(a > b) == bool(b < a)
.(1.4) —
bool(a <= b) == !bool(b < a)
.(1.5) —
bool(a >= b) == !bool(a < b)
.template<class T, class U> concept totally_ordered_with = totally_ordered<T> && totally_ordered<U> &&common_reference_with<const remove_reference_t<T>&, const remove_reference_t<U>&> &&equality_comparable_with<T, U> && totally_ordered< common_reference_t< const remove_reference_t<T>&, const remove_reference_t<U>&>> &&equality_comparable_with<T, U> &&requires(const remove_reference_t<T>& t, const remove_reference_t<U>& u) { { t < u } -> boolean; { t > u } -> boolean; { t <= u } -> boolean; { t >= u } -> boolean; { u < t } -> boolean; { u > t } -> boolean; { u <= t } -> boolean; { u >= t } -> boolean; };
<compare>
from most library headersSection: 17.13.2 [coroutine.syn], 19.5.2 [system.error.syn], 22.2.1 [utility.syn], 22.4.2 [tuple.syn], 22.5.2 [optional.syn], 22.6.2 [variant.syn], 20.2.2 [memory.syn], 17.7.6 [type.index.synopsis], 27.4.2 [string.syn], 27.3.2 [string.view.synop], 23.3.2 [array.syn], 23.3.4 [deque.syn], 23.3.6 [forward.list.syn], 23.3.10 [list.syn], 23.3.12 [vector.syn], 23.4.2 [associative.map.syn], 23.4.5 [associative.set.syn], 23.5.2 [unord.map.syn], 23.5.5 [unord.set.syn], 23.6.2 [queue.syn], 23.6.5 [stack.syn], 24.2 [iterator.synopsis], 25.2 [ranges.syn], 30.2 [time.syn], 31.12.4 [fs.filesystem.syn], 28.6.3 [re.syn], 32.4.2 [thread.syn] Status: C++20 Submitter: United States Opened: 2019-11-07 Last modified: 2021-02-25
Priority: 0
View all other issues in [coroutine.syn].
View all issues with C++20 status.
Discussion:
Addresses US 181
The spaceship operator<=>
is typically not usable unless the library
header <compare>
is directly included by the user. Many standard
library headers provide overloads for this operator. Worse, several standard classes
have replaced their existing definition for comparison operators with a reliance on the
spaceship operator, and existing code will break if the necessary header is not
(transitively) included. In a manner similar to the mandated library headers
transitively #include
-ing <initializer_list>
in C++11,
these headers should mandate a transitive #include <compare>
.
#include <compare>
to the header synopsis for each of the following headers:
<array> <chrono> <coroutine> <deque> <forward_list> <filesystem> <iterator> <list> <map> <memory> <optional> <queue> <ranges> <regex> <set> <stack> <string> <string_view> <system_error> <thread> <tuple> <type_index> <unordered_map> <unordered_set> <utility> <variant> <vector>
[2019-11 Moved to Ready on Friday AM in Belfast]
Proposed resolution:
This wording is relative to N4835.
Add
#include <compare>
to the following header synopses:
totally_ordered/_with
in terms of partially-ordered-with
Section: 18.5.5 [concept.totallyordered] Status: C++20 Submitter: Great Britain Opened: 2019-11-08 Last modified: 2021-02-25
Priority: 0
View all other issues in [concept.totallyordered].
View all issues with C++20 status.
Discussion:
Addresses GB 202
Define totally_ordered[_with]
in terms of partially-ordered-with
.
This will simplify the definition of both concepts
(particularly totally_ordered_with
), and
make them in-line with equality_comparable[_with]
.
Now that we've defined partially-ordered-with
for 17.12.4 [cmp.concept], we should consider
utilising it in as many locations as possible.
Proposed change:
template<class T> concept totally_ordered = equality_comparable<T> && partially-ordered-with<T, T>; template<class T, class U> concept totally_ordered_with = totally_ordered<T> && totally_ordered<U> && common_reference_with< const remove_reference_t<T>&, const remove_reference_t<U>&> && totally_ordered< common_reference_t< const remove_reference_t<T>&, const remove_reference_t<U>&>> && equality_comparable_with<T, U> && partially-ordered-with<T, U>;
LWG discussion in Belfast notes that 3329(i)
also touches the definition of totally_ordered_with
;
the two sets of changes are consistent.
[2019-11 Status to Ready Friday afternoon LWG in Belfast]
Proposed resolution:
This wording is relative to N4835.
Change 18.5.5 [concept.totallyordered] as follows:
template<class T> concept totally_ordered = equality_comparable<T> && partially-ordered-with<T, T>;requires(const remove_reference_t<T>& a,const remove_reference_t<T>& b) {{ a < b } -> boolean;{ a > b } -> boolean;{ a <= b } -> boolean;{ a >= b } -> boolean;};-1- For some type
T
, leta
,b
, andc
be lvalues of typeconst remove_reference_t<T>
.T
modelstotally_ordered
only if(1.1) — Exactly one of
bool(a < b)
,bool(a > b)
, orbool(a == b)
istrue
.(1.2) — If
bool(a < b)
andbool(b < c)
, thenbool(a < c)
.
(1.3) —bool(a > b) == bool(b < a)
.(1.4) —
bool(a <= b) == !bool(b < a)
.(1.5) —
bool(a >= b) == !bool(a < b)
.template<class T, class U> concept totally_ordered_with = totally_ordered<T> && totally_ordered<U> && common_reference_with<const remove_reference_t<T>&, const remove_reference_t<U>&> && totally_ordered< common_reference_t< const remove_reference_t<T>&, const remove_reference_t<U>&>> && equality_comparable_with<T, U> && partially-ordered-with<T, U>;requires(const remove_reference_t<T>& t,const remove_reference_t<U>& u) {{ t < u } -> boolean;{ t > u } -> boolean;{ t <= u } -> boolean;{ t >= u } -> boolean;{ u < t } -> boolean;{ u > t } -> boolean;{ u <= t } -> boolean;{ u >= t } -> boolean;};[…]
Section: 30.12 [time.format] Status: C++20 Submitter: Mateusz Pusz Opened: 2019-11-05 Last modified: 2021-02-25
Priority: 0
View other active issues in [time.format].
View all other issues in [time.format].
View all issues with C++20 status.
Discussion:
Table 97 [tab:time.format.spec] enumerates 'q'
and 'Q'
but those are not specified in
chrono-format-spec provided in paragraph 1 of sub-clause 30.12 [time.format].
[2019-11-23 Issue Prioritization]
Status to Tentatively Ready and priority to 0 after eight positive votes on the reflector.
Proposed resolution:
This wording is relative to N4835.
Change the type grammar element in 30.12 [time.format] p1 as indicated:
type: one of a A b B c C d D e F g G h H I j m M n p q Q r R S t T u U V w W x X y Y z Z %
basic_osyncstream
move assignment and destruction calls basic_syncbuf::emit()
twiceSection: 31.11.3 [syncstream.osyncstream] Status: C++20 Submitter: Tim Song Opened: 2019-11-06 Last modified: 2021-02-25
Priority: 3
View all issues with C++20 status.
Discussion:
These functions are specified to call emit()
, which calls emit()
on
the basic_syncbuf
and sets badbit
if it fails. Then, the move
assignment is specified to move-assign the basic_syncbuf
, while the destructor
implicitly needs to destroy the basic_syncbuf
data member. This calls emit()
on the basic_syncbuf
again.
[2020-02-13 Tim adds wording after discussion with Peter]
[2020-02 Status to Immediate Thursday afternoon in Prague.]
Proposed resolution:
This wording is relative to N4849.
[Drafting note: There is no need to explicitly call
emit
at all in these functions; memberwise move-assignment/destruction is sufficient, so we can strike the specification entirely and rely on the wording in 16.3.3.5 [functions.within.classes]. — end drafting note]
Edit 31.11.3.2 [syncstream.osyncstream.cons] as indicated:
~basic_osyncstream();
-6- Effects: Callsemit()
. If an exception is thrown fromemit()
, that exception is caught and ignored.
Strike [syncstream.osyncstream.assign]:
basic_osyncstream& operator=(basic_osyncstream&& rhs) noexcept;
-1- Effects: First, callsemit()
. If an exception is thrown fromemit()
, that exception is caught and ignored. Move assignssb
fromrhs.sb
. [ Note: This disassociatesrhs
from its wrapped stream buffer ensuring destruction ofrhs
produces no output. — end note ]-2- Postconditions:nullptr == rhs.get_wrapped()
istrue
.get_wrapped()
returns the value previously returned byrhs.get_wrapped()
.
Section: 25.2 [ranges.syn] Status: C++20 Submitter: United States/Great Britain Opened: 2019-11-08 Last modified: 2021-02-25
Priority: 1
View all other issues in [ranges.syn].
View all issues with C++20 status.
Discussion:
all_view
is not a view like the others. For the other view types, foo_view{args...}
is a valid
way to construct an instance of type foo_view. However, all_view
is just an alias to the type of
view::all(arg)
, which could be one of several different types. all_view
feels like the wrong name.
Proposed change:
Suggest renaming all_view
to all_t
and
moving it into the views::
namespace.
Add range_size_t
.
LEWG asked that range_size_t
be removed from P1035,
as they were doing a good job of being neutral w.r.t whether or not size-types were signed or
unsigned at the time. Now that we've got a policy on what size-types are, and that
P1522 and P1523
have been adopted, it makes sense for there to be a range_size_t
.
Proposed change:
Add to [ranges.syn]:
template<range R> using range_difference_t = iter_difference_t<iterator_t<R>>; template<sized_range R> using range_size_t = decltype(ranges::size(declval<R&>()));
David Olsen:
The proposed wording has been approved by LEWG and LWG in Belfast.[2019-11-23 Issue Prioritization]
Priority to 1 after reflector discussion.
[2020-02-10 Move to Immediate Monday afternoon in Prague]
Proposed resolution:
This wording is relative to N4835.
Change 25.2 [ranges.syn], header <ranges>
synopsis, as indicated:
#include <initializer_list> #include <iterator> namespace std::ranges { […] // 25.4.2 [range.range], ranges template<class T> concept range = see below; […] template<range R> using range_difference_t = iter_difference_t<iterator_t<R>>; template<sized_range R> using range_size_t = decltype(ranges::size(declval<R&>())); template<range R> using range_value_t = iter_value_t<iterator_t<R>>; […] // 25.7.6.2 [range.ref.view], all view namespace views {inline constexpr unspecified all = unspecified; }inline constexpr unspecified all = unspecified; template<viewable_range R> using all_tview= decltype(views::all(declval<R>())); } […] }
Globally replace all occurrences of all_view
with views::all_t
. There are 36
occurrences in addition to the definition in the <ranges>
synopsis that was changed above.
std::vformat
handle exception thrown by formatters?Section: 28.5.5 [format.functions] Status: Resolved Submitter: Tam S. B. Opened: 2019-11-11 Last modified: 2020-09-06
Priority: 2
View all other issues in [format.functions].
View all issues with Resolved status.
Discussion:
The specification for std::vformat
in 28.5.5 [format.functions]/7 says
Throws:
format_error
iffmt
is not a format string.
It seems unclear whether vformat
throws when an exception is thrown by the formatter
,
e.g. when the format string is valid, but the corresponding argument cannot be formatted with the given
format string.
"c"
format specifier is specified to throw format_error
if the
corresponding argument is not in the range of representable values for charT
(Table 60 [tab:format.type.int]). It seems unclear whether vformat
propagates this exception.
It also appears unclear whether vformat
may throw other types of exception (e.g. bad_alloc
)
when the formatter or the constructor of std::string
result throws.
[2019-11-20 Issue Prioritization]
Priority to 2 after reflector discussion.
[2019-11-20; Victor Zverovich provides initial wording]
Previous resolution [SUPERSEDED]:This wording is relative to N4835.
[Drafting Note: LWG 3340(i) has considerable wording overlap with this issue. If LWG 3336(i) is applied at the same meeting or later than LWG 3340(i), please refer to Option A in LWG 3340(i) as a guideline how to apply the merge.]
Before 28.5.5 [format.functions] insert a new sub-clause as indicated:
20.20.? Error reporting [format.err.report]
-?- Formatting functions throw exceptions to report formatting and other errors. They throwformat_error
if an argumentfmt
is passed that is not a format string and propagate exceptions thrown byformatter
specializations and iterator operations. Failure to allocate storage is reported by throwing an exception as described in 16.4.6.14 [res.on.exception.handling].Modify 28.5.5 [format.functions] as indicated:
string vformat(string_view fmt, format_args args); wstring vformat(wstring_view fmt, wformat_args args); string vformat(const locale& loc, string_view fmt, format_args args); wstring vformat(const locale& loc, wstring_view fmt, wformat_args args);[…]-6- […]
-7- Throws:As specified in 28.5.3 [format.err.report].format_error
iffmt
is not a format stringtemplate<class Out> Out vformat_to(Out out, string_view fmt, format_args_t<Out, char> args); template<class Out> Out vformat_to(Out out, wstring_view fmt, format_args_t<Out, wchar_t> args); template<class Out> Out vformat_to(Out out, const locale& loc, string_view fmt, format_args_t<Out, char> args); template<class Out> Out vformat_to(Out out, const locale& loc, wstring_view fmt, format_args_t<Out, wchar_t> args);[…][…]
-15- Throws:As specified in 28.5.3 [format.err.report].format_error
iffmt
is not a format stringtemplate<class Out, class... Args> format_to_n_result<Out> format_to_n(Out out, iter_difference_t<Out> n, string_view fmt, const Args&... args); template<class Out, class... Args> format_to_n_result<Out> format_to_n(Out out, iter_difference_t<Out> n, wstring_view fmt, const Args&... args); template<class Out, class... Args> format_to_n_result<Out> format_to_n(Out out, iter_difference_t<Out> n, const locale& loc, string_view fmt, const Args&... args); template<class Out, class... Args> format_to_n_result<Out> format_to_n(Out out, iter_difference_t<Out> n, const locale& loc, wstring_view fmt, const Args&... args);[…][…]
-21- Throws:As specified in 28.5.3 [format.err.report].format_error
iffmt
is not a format stringtemplate<class... Args> size_t formatted_size(string_view fmt, const Args&... args); template<class... Args> size_t formatted_size(wstring_view fmt, const Args&... args); template<class... Args> size_t formatted_size(const locale& loc, string_view fmt, const Args&... args); template<class... Args> size_t formatted_size(const locale& loc, wstring_view fmt, const Args&... args);[…]
-25- Throws:As specified in 28.5.3 [format.err.report].format_error
iffmt
is not a format string
[2020-02-12, Prague; LWG discussion]
Proposed resolution:
default_constructible
to default_initializable
Section: 18.4.12 [concept.default.init] Status: C++20 Submitter: Casey Carter Opened: 2019-11-18 Last modified: 2021-06-06
Priority: 0
View all other issues in [concept.default.init].
View all issues with C++20 status.
Discussion:
WG21 merged P1754R1 "Rename concepts to standard_case
for C++20"
into the working draft as LWG Motion 11 in 2019 Cologne. That proposal contains editorial instructions to
rename what was the DefaultConstructible
concept:
IF LWG3151 ACCEPTED: default_initializable ELSE default_constructible
Notably LWG 3151(i) "ConvertibleTo
rejects conversions from array and function types" is
not the intended issue number, LWG 3149(i) "DefaultConstructible
should require default
initialization" is. It was made clear during discussion in LEWG that 3149 would change the concept to require default-initialization to be valid rather than value-initialization which the is_default_constructible
trait requires. LEWG agreed that it would be confusing to have a trait and concept with very similar names
yet slightly different meanings, and approved P1754R1's proposed renaming.
[2019-11-30 Issue Prioritization]
Status to Tentatively Ready and priority to 0 after eight positive votes on the reflector.
Proposed resolution:
This wording is relative to N4835.
Change the stable name "[concept.defaultconstructible]" to "[concept.default.init]" and retitle
"Concept default_constructible
" to "Concept default_initializable
". Replace all references
to the name default_constructible
with default_initializable
(There are 20 occurrences).
Section: 28.5.5 [format.functions] Status: C++20 Submitter: Great Britain Opened: 2019-11-17 Last modified: 2021-02-25
Priority: Not Prioritized
View all other issues in [format.functions].
View all issues with C++20 status.
Discussion:
Addresses GB 229
Formatting functions don't allow throwing on incorrect arguments.
std::format
is only allowed to throw if fmt
is not a format string, but the
intention is it also throws for errors during formatting, e.g. there are fewer arguments than
required by the format string.
Proposed change:
Allow exceptions even when the format string is valid. Possibly state the Effects: more precisely.
Victor Zverovich:
LEWG approved resolution of this NB comment as an LWG issue. Previous resolution [SUPERSEDED]:This wording is relative to N4835.
[Drafting Note: Depending on whether LWG 3336(i)'s wording has been accepted when this issue's wording has been accepted, two mutually exclusive options are prepared, depicted below by Option A and Option B, respectively.]
Option A (LWG 3336(i) has been accepted)
Change 28.5.2.1 [format.string.general] as follows:
-1- A format string for arguments
-2- The arg-id field specifies the index of the argument inargs
is a (possibly empty) sequence of replacement fields, escape sequences, and characters other than{
and}
. […]args
whose value is to be formatted and inserted into the output instead of the replacement field. If there is no argument with the index arg-id inargs
, the string is not a format string. The optional format-specifier field explicitly specifies a format for the replacement value. […] -5- The format-spec field contains format specifications that define how the value should be presented. Each type can define its own interpretation of the format-spec field. If format-spec doesn't conform to the format specifications for the argument inargs
referred to by arg-id, the string is not a format string. […]Before 28.5.5 [format.functions] insert a new sub-clause as indicated:
20.20.? Error reporting [format.err.report]
-?- Formatting functions throw exceptions to report formatting and other errors. They throwformat_error
if an argumentfmt
is passed that is not a format string for argumentsargs
and propagate exceptions thrown byformatter
specializations and iterator operations. Failure to allocate storage is reported by throwing an exception as described in 16.4.6.14 [res.on.exception.handling].Modify 28.5.5 [format.functions] as indicated:
string vformat(string_view fmt, format_args args); wstring vformat(wstring_view fmt, wformat_args args); string vformat(const locale& loc, string_view fmt, format_args args); wstring vformat(const locale& loc, wstring_view fmt, wformat_args args);[…]-6- […]
-7- Throws:As specified in 28.5.3 [format.err.report].format_error
iffmt
is not a format stringtemplate<class Out> Out vformat_to(Out out, string_view fmt, format_args_t<Out, char> args); template<class Out> Out vformat_to(Out out, wstring_view fmt, format_args_t<Out, wchar_t> args); template<class Out> Out vformat_to(Out out, const locale& loc, string_view fmt, format_args_t<Out, char> args); template<class Out> Out vformat_to(Out out, const locale& loc, wstring_view fmt, format_args_t<Out, wchar_t> args);[…][…]
-15- Throws:As specified in 28.5.3 [format.err.report].format_error
iffmt
is not a format stringtemplate<class Out, class... Args> format_to_n_result<Out> format_to_n(Out out, iter_difference_t<Out> n, string_view fmt, const Args&... args); template<class Out, class... Args> format_to_n_result<Out> format_to_n(Out out, iter_difference_t<Out> n, wstring_view fmt, const Args&... args); template<class Out, class... Args> format_to_n_result<Out> format_to_n(Out out, iter_difference_t<Out> n, const locale& loc, string_view fmt, const Args&... args); template<class Out, class... Args> format_to_n_result<Out> format_to_n(Out out, iter_difference_t<Out> n, const locale& loc, wstring_view fmt, const Args&... args);[…][…]
-21- Throws:As specified in 28.5.3 [format.err.report].format_error
iffmt
is not a format stringtemplate<class... Args> size_t formatted_size(string_view fmt, const Args&... args); template<class... Args> size_t formatted_size(wstring_view fmt, const Args&... args); template<class... Args> size_t formatted_size(const locale& loc, string_view fmt, const Args&... args); template<class... Args> size_t formatted_size(const locale& loc, wstring_view fmt, const Args&... args);[…]
-25- Throws:As specified in 28.5.3 [format.err.report].format_error
iffmt
is not a format stringOption B (LWG 3336(i) has not been accepted)
Change 28.5.2.1 [format.string.general] as follows:
-1- A format string for arguments
-2- The arg-id field specifies the index of the argument inargs
is a (possibly empty) sequence of replacement fields, escape sequences, and characters other than{
and}
. […]args
whose value is to be formatted and inserted into the output instead of the replacement field. If there is no argument with the index arg-id inargs
, the string is not a format string. The optional format-specifier field explicitly specifies a format for the replacement value. […] -5- The format-spec field contains format specifications that define how the value should be presented. Each type can define its own interpretation of the format-spec field. If format-spec doesn't conform to the format specifications for the argument inargs
referred to by arg-id, the string is not a format string. […]Modify 28.5.5 [format.functions] as indicated:
string vformat(string_view fmt, format_args args); wstring vformat(wstring_view fmt, wformat_args args); string vformat(const locale& loc, string_view fmt, format_args args); wstring vformat(const locale& loc, wstring_view fmt, wformat_args args);[…]-6- […]
-7- Throws:format_error
iffmt
is not a format string forargs
.template<class Out> Out vformat_to(Out out, string_view fmt, format_args_t<Out, char> args); template<class Out> Out vformat_to(Out out, wstring_view fmt, format_args_t<Out, wchar_t> args); template<class Out> Out vformat_to(Out out, const locale& loc, string_view fmt, format_args_t<Out, char> args); template<class Out> Out vformat_to(Out out, const locale& loc, wstring_view fmt, format_args_t<Out, wchar_t> args);[…][…]
-15- Throws:format_error
iffmt
is not a format string forargs
.template<class Out, class... Args> format_to_n_result<Out> format_to_n(Out out, iter_difference_t<Out> n, string_view fmt, const Args&... args); template<class Out, class... Args> format_to_n_result<Out> format_to_n(Out out, iter_difference_t<Out> n, wstring_view fmt, const Args&... args); template<class Out, class... Args> format_to_n_result<Out> format_to_n(Out out, iter_difference_t<Out> n, const locale& loc, string_view fmt, const Args&... args); template<class Out, class... Args> format_to_n_result<Out> format_to_n(Out out, iter_difference_t<Out> n, const locale& loc, wstring_view fmt, const Args&... args);[…][…]
-21- Throws:format_error
iffmt
is not a format string forargs
.template<class... Args> size_t formatted_size(string_view fmt, const Args&... args); template<class... Args> size_t formatted_size(wstring_view fmt, const Args&... args); template<class... Args> size_t formatted_size(const locale& loc, string_view fmt, const Args&... args); template<class... Args> size_t formatted_size(const locale& loc, wstring_view fmt, const Args&... args);[…]
-25- Throws:format_error
iffmt
is not a format string forargs
.
[2020-02-12, Prague; LWG discussion]
Option A is the only one we look at to resolve LWG 3336(i) as well. During the discussions some wording refinements have been suggested that are integrated below.
Proposed resolution:
This wording is relative to N4849.
Change 28.5.2.1 [format.string.general] as follows:
-1- A format string for arguments
-2- The arg-id field specifies the index of the argument inargs
is a (possibly empty) sequence of replacement fields, escape sequences, and characters other than{
and}
. […]args
whose value is to be formatted and inserted into the output instead of the replacement field. If there is no argument with the index arg-id inargs
, the string is not a format string forargs
. The optional format-specifier field explicitly specifies a format for the replacement value. […] -5- The format-spec field contains format specifications that define how the value should be presented. Each type can define its own interpretation of the format-spec field. If format-spec does not conform to the format specifications for the argument type referred to by arg-id, the string is not a format string forargs
. […]
Before 28.5.5 [format.functions] insert a new sub-clause as indicated:
20.20.? Error reporting [format.err.report]
-?- Formatting functions throwformat_error
if an argumentfmt
is passed that is not a format string forargs
. They propagate exceptions thrown by operations offormatter
specializations and iterators. Failure to allocate storage is reported by throwing an exception as described in 16.4.6.14 [res.on.exception.handling].
Modify 28.5.5 [format.functions] as indicated:
string vformat(string_view fmt, format_args args); wstring vformat(wstring_view fmt, wformat_args args); string vformat(const locale& loc, string_view fmt, format_args args); wstring vformat(const locale& loc, wstring_view fmt, wformat_args args);[…]-6- […]
-7- Throws:As specified in 28.5.3 [format.err.report].format_error
iffmt
is not a format stringtemplate<class Out> Out vformat_to(Out out, string_view fmt, format_args_t<Out, char> args); template<class Out> Out vformat_to(Out out, wstring_view fmt, format_args_t<Out, wchar_t> args); template<class Out> Out vformat_to(Out out, const locale& loc, string_view fmt, format_args_t<Out, char> args); template<class Out> Out vformat_to(Out out, const locale& loc, wstring_view fmt, format_args_t<Out, wchar_t> args);[…][…]
-15- Throws:As specified in 28.5.3 [format.err.report].format_error
iffmt
is not a format stringtemplate<class Out, class... Args> format_to_n_result<Out> format_to_n(Out out, iter_difference_t<Out> n, string_view fmt, const Args&... args); template<class Out, class... Args> format_to_n_result<Out> format_to_n(Out out, iter_difference_t<Out> n, wstring_view fmt, const Args&... args); template<class Out, class... Args> format_to_n_result<Out> format_to_n(Out out, iter_difference_t<Out> n, const locale& loc, string_view fmt, const Args&... args); template<class Out, class... Args> format_to_n_result<Out> format_to_n(Out out, iter_difference_t<Out> n, const locale& loc, wstring_view fmt, const Args&... args);[…][…]
-21- Throws:As specified in 28.5.3 [format.err.report].format_error
iffmt
is not a format stringtemplate<class... Args> size_t formatted_size(string_view fmt, const Args&... args); template<class... Args> size_t formatted_size(wstring_view fmt, const Args&... args); template<class... Args> size_t formatted_size(const locale& loc, string_view fmt, const Args&... args); template<class... Args> size_t formatted_size(const locale& loc, wstring_view fmt, const Args&... args);[…]
-25- Throws:As specified in 28.5.3 [format.err.report].format_error
iffmt
is not a format string
Section: 18.4.9 [concept.swappable], 24.3.3.2 [iterator.cust.swap], 24.4.4.2 [range.iter.op.advance], 24.4.4.3 [range.iter.op.distance], 24.5.1.2 [reverse.iterator], 24.5.4.2 [move.iterator], 24.5.4.7 [move.iter.nav], 24.5.5.2 [common.iter.types], 24.5.5.5 [common.iter.nav], 25.3 [range.access], 25.6.4.3 [range.iota.iterator], 25.7 [range.adaptors], 26 [algorithms] Status: Resolved Submitter: Daniel Krügler Opened: 2019-11-23 Last modified: 2020-05-01
Priority: 2
View other active issues in [concept.swappable].
View all other issues in [concept.swappable].
View all issues with Resolved status.
Discussion:
The current working draft uses at several places within normative wording the term "models" instead of "satisfies" even though it is clear from the context that these are conditions to be tested by the implementation. Since "models" includes both syntactic requirements as well as semantic requirements, such wording would require (at least in general) heroic efforts for an implementation. Just to name a few examples for such misusage:
The specification of the customization objects in 18.4.9 [concept.swappable], 24.3.3.2 [iterator.cust.swap], 25.3 [range.access]
The algorithmic decision logic for the effects of the functions specified in 24.4.4.2 [range.iter.op.advance], 24.4.4.3 [range.iter.op.distance]
The correct way to fix these presumably unintended extra requirements is to use the term "satisfies" at the places where it is clear that an implementation has to test them.
Note: There exists similar misusage in regard to wording for types that "meet Cpp17XX requirements, but those are not meant to be covered as part of this issue. This additional wording problem should be handled by a separate issue.[2019-12-08 Issue Prioritization]
Priority to 2 after reflector discussion.
[2019-12-15; Daniel synchronizes wording with N4842]
Previous resolution [SUPERSEDED]:This wording is relative to N4842.
[Drafting note: The proposed wording does intentionally not touch the definition of
enable_view
, whose definition is radically changed by LWG 3326(i) in a manner that does no longer need similar adjustments.]
Modify 18.4.9 [concept.swappable] as indicated:
-2- The name
ranges::swap
denotes a customization point object (16.3.3.3.5 [customization.point.object]). The expressionranges::swap(E1, E2)
for some subexpressionsE1
andE2
is expression-equivalent to an expressionS
determined as follows:
(2.1) — […]
(2.2) — […]
(2.3) — Otherwise, if
E1
andE2
are lvalues of the same typeT
thatmodelssatisfiesmove_constructible<T>
andassignable_from<T&, T>
,S
is an expression that exchanges the denoted values.S
is a constant expression if […](2.4) — […]
Modify 24.3.3.2 [iterator.cust.swap] as indicated:
-4- The expression
ranges::iter_swap(E1, E2)
for some subexpressionsE1
andE2
is expression-equivalent to:
(4.1) — […]
(4.2) — Otherwise, if the types of
E1
andE2
eachmodelsatisfyindirectly_readable
, and if the reference types ofE1
andE2
modelsatisfyswappable_with
(18.4.9 [concept.swappable]), thenranges::swap(*E1, *E2)
.(4.3) — Otherwise, if the types
T1
andT2
ofE1
andE2
modelsatisfyindirectly_movable_storable<T1, T2>
andindirectly_movable_storable<T2, T1>
, then(void)(*E1 = iter-exchange-move(E2, E1))
, except thatE1
is evaluated only once.(4.4) — […]
Modify 24.4.4 [range.iter.ops] as indicated:
-1- The library includes the function templates
ranges::advance
,ranges::distance
,ranges::next
, andranges::prev
to manipulate iterators. These operations adapt to the set of operators provided by each iterator category to provide the most efficient implementation possible for a concrete iterator type. [Example:ranges::advance
uses the+
operator to move arandom_access_iterator
forwardn
steps in constant time. For an iterator type that does notmodelsatisfyrandom_access_iterator
,ranges::advance
instead performsn
individual increments with the++
operator. — end example]Modify 24.4.4.2 [range.iter.op.advance] as indicated:
template<input_or_output_iterator I> constexpr void ranges::advance(I& i, iter_difference_t<I> n);-1- Preconditions: […]
-2- Effects:
(2.1) — If
I
modelssatisfiesrandom_access_iterator
, equivalent toi += n
.(2.2) — […]
(2.3) — […]
template<input_or_output_iterator I, sentinel_for<I> S> constexpr void ranges::advance(I& i, S bound);-3- Preconditions: […]
-4- Effects:
(4.1) — If
I
andS
modelsatisfyassignable_from<I&, S>
, equivalent toi = std::move(bound)
.(4.2) — Otherwise, if
S
andI
modelsatisfysized_sentinel_for<S, I>
, equivalent toranges::advance(i, bound - i)
.(4.3) — […]
template<input_or_output_iterator I, sentinel_for<I> S> constexpr iter_difference_t<I> ranges::advance(I& i, iter_difference_t<I> n, S bound);-5- Preconditions: […]
-6- Effects:
(6.1) — If
S
andI
modelsatisfysized_sentinel_for<S, I>
: […](6.2) — […]
Modify 24.4.4.3 [range.iter.op.distance] as indicated:
template<input_or_output_iterator I, sentinel_for<I> S> constexpr iter_difference_t<I> ranges::distance(I first, S last);-1- Preconditions: […]
-2- Effects: IfS
andI
modelsatisfysized_sentinel_for<S, I>
, returns(last - first)
; otherwise, returns the number of increments needed to get fromfirst
tolast
.template<range R> range_difference_t<R> ranges::distance(R&& r);-3- Effects: If
R
modelssatisfiessized_range
, equivalent to:return static_cast<range_difference_t<R>>(ranges::size(r)); // 25.3.10 [range.prim.size]Otherwise, equivalent to:
return ranges::distance(ranges::begin(r), ranges::end(r)); // 25.3 [range.access]Modify 24.5.1.2 [reverse.iterator] as indicated:
-1- The member typedef-name
iterator_concept
denotes-2- The member typedef-name
(1.1) —
random_access_iterator_tag
ifIterator
modelssatisfiesrandom_access_iterator
, and(1.2) —
bidirectional_iterator_tag
otherwise.iterator_category
denotes
(2.1) —
random_access_iterator_tag
if the typeiterator_traits<Iterator>::iterator_category
modelssatisfiesderived_from<random_access_iterator_tag>
, and(2.2) —
iterator_traits<Iterator>::iterator_category
otherwise.Modify 24.5.4.2 [move.iterator] as indicated:
-1- The member typedef-name
iterator_category
denotes
(1.1) —
random_access_iterator_tag
if the typeiterator_traits<Iterator>::iterator_category
modelssatisfiesderived_from<random_access_iterator_tag>
, and(1.2) —
iterator_traits<Iterator>::iterator_category
otherwise.Modify 24.5.4.7 [move.iter.nav] as indicated:
constexpr auto operator++(int);-3- Effects:
Iterator
modelssatisfiesforward_iterator
, equivalent to:move_iterator tmp = *this; ++current; return tmp;Otherwise, equivalent to
++current
.Modify 24.5.5.2 [common.iter.types] as indicated:
-1- The nested typedef-names of the specialization of
iterator_traits
forcommon_iterator<I, S>
are defined as follows.
(1.1) —
iterator_concept
denotesforward_iterator_tag
ifI
modelssatisfiesforward_iterator
; otherwise it denotesinput_iterator_tag
.(1.2) —
iterator_category
denotesforward_iterator_tag
ifiterator_traits<I>::iterator_category
modelssatisfiesderived_from<forward_iterator_tag>
; otherwise it denotesinput_iterator_tag
.(1.3) — […]
Modify 24.5.5.5 [common.iter.nav] as indicated:
decltype(auto) operator++(int);-4- Preconditions: […]
-5- Effects: IfI
modelssatisfiesforward_iterator
, equivalent to:common_iterator tmp = *this; ++*this; return tmp;Otherwise, equivalent to
return get<I>(v_)++;
Modify 25.3.2 [range.access.begin] as indicated:
-1- The name
ranges::begin
denotes a customization point object (16.3.3.3.5 [customization.point.object]). Given a subexpressionE
and an lvaluet
that denotes the same object asE
, ifE
is an rvalue andenable_safe_range<remove_cvref_t<decltype((E))>>
isfalse
,ranges::begin(E)
is ill-formed. Otherwise,ranges::begin(E)
is expression-equivalent to:
(1.1) — […]
(1.2) — Otherwise,
decay-copy(t.begin())
if it is a valid expression and its typeI
modelssatisfiesinput_or_output_iterator
.(1.3) — Otherwise,
decay-copy(begin(t))
if it is a valid expression and its typeI
modelssatisfiesinput_or_output_iterator
with overload resolution performed in a context that includes the declarations:template<class T> void begin(T&&) = delete; template<class T> void begin(initializer_list<T>&&) = delete;and does not include a declaration of
ranges::begin
.(1.4) — […]
Modify 25.3.3 [range.access.end] as indicated:
-1- The name
ranges::end
denotes a customization point object (16.3.3.3.5 [customization.point.object]). Given a subexpressionE
and an lvaluet
that denotes the same object asE
, ifE
is an rvalue andenable_safe_range<remove_cvref_t<decltype((E))>>
isfalse
,ranges::end(E)
is ill-formed. Otherwise,ranges::end(E)
is expression-equivalent to:
(1.1) — […]
(1.2) — Otherwise,
decay-copy(t.end())
if it is a valid expression and its typeS
modelssatisfiessentinel_for<decltype(ranges::begin(E))>
.(1.3) — Otherwise,
decay-copy(end(t))
if it is a valid expression and its typeS
modelssatisfiessentinel_for<decltype(ranges::begin(E))>
with overload resolution performed in a context that includes the declarations:and does not include a declaration oftemplate<class T> void end(T&&) = delete; template<class T> void end(initializer_list<T>&&) = delete;ranges::end
.(1.4) — […]
Modify 25.3.6 [range.access.rbegin] as indicated:
-1- The name
ranges::rbegin
denotes a customization point object (16.3.3.3.5 [customization.point.object]). Given a subexpressionE
and an lvaluet
that denotes the same object asE
, ifE
is an rvalue andenable_safe_range<remove_cvref_t<decltype((E))>>
isfalse
,ranges::rbegin(E)
is ill-formed. Otherwise,ranges::rbegin(E)
is expression-equivalent to:
(1.1) —
decay-copy(t.rbegin())
if it is a valid expression and its typeI
modelssatisfiesinput_or_output_iterator
.(1.2) — Otherwise,
decay-copy(rbegin(t))
if it is a valid expression and its typeI
modelssatisfiesinput_or_output_iterator
with overload resolution performed in a context that includes the declaration:and does not include a declaration oftemplate<class T> void rbegin(T&&) = delete;ranges::rbegin
.(1.3) — Otherwise,
make_reverse_iterator(ranges::end(t))
if bothranges::begin(t)
andranges::end(t)
are valid expressions of the same typeI
whichmodelssatisfiesbidirectional_iterator
(24.3.4.12 [iterator.concept.bidir]).(1.4) — […]
Modify 25.3.7 [range.access.rend] as indicated:
-1- The name
ranges::rend
denotes a customization point object (16.3.3.3.5 [customization.point.object]). Given a subexpressionE
and an lvaluet
that denotes the same object asE
, ifE
is an rvalue andenable_safe_range<remove_cvref_t<decltype((E))>>
isfalse
,ranges::rend(E)
is ill-formed. Otherwise,ranges::rend(E)
is expression-equivalent to:
(1.1) —
decay-copy(t.rend())
if it is a valid expression and its typeS
modelssatisfiessentinel_for<decltype(ranges::rbegin(E))>
.(1.2) — Otherwise,
decay-copy(rend(t))
if it is a valid expression and its typeS
modelssatisfiessentinel_for<decltype(ranges::rbegin(E))>
with overload resolution performed in a context that includes the declaration:and does not include a declaration oftemplate<class T> void rend(T&&) = delete;ranges::rend
.(1.3) — Otherwise,
make_reverse_iterator(ranges::begin(t))
if bothranges::begin(t)
andranges::end(t)
are valid expressions of the same typeI
whichmodelssatisfiesbidirectional_iterator
(24.3.4.12 [iterator.concept.bidir]).(1.4) — […]
Modify 25.3.10 [range.prim.size] as indicated:
[Drafting note: The term "is integer-like" is very specifically defined to be related to semantic requirements as well, and the term "satisfy integer-like" seems to be undefined. Fortunately, 24.3.4.4 [iterator.concept.winc] also introduces the exposition-only variable template
is-integer-like
which we use as predicate below to describe the syntactic constraints of "integer-like" alone. This wording change regarding "is integer-like" is strictly speaking not required because of the "if and only if" definition provided in 24.3.4.4 [iterator.concept.winc] p12, but it has been made nonetheless to improve the consistency between discriminating compile-time tests from potentially semantic requirements]-1- The name
size
denotes a customization point object (16.3.3.3.5 [customization.point.object]). The expressionranges::size(E)
for some subexpressionE
with typeT
is expression-equivalent to:
(1.1) — […]
(1.2) — Otherwise, if
disable_sized_range<remove_cv_t<T>>
(25.4.3 [range.sized]) isfalse
:
(1.2.1) —
decay-copy(E.size())
if it is a valid expression andits typeI
is integer-likeis-integer-like<I>
istrue
(24.3.4.4 [iterator.concept.winc]).(1.2.2) — Otherwise,
decay-copy(size(E))
if it is a valid expression andits typeI
is integer-likeis-integer-like<I>
istrue
with overload resolution performed in a context that includes the declaration:and does not include a declaration oftemplate<class T> void size(T&&) = delete;ranges::size
.(1.3) — Otherwise,
make-unsigned-like(ranges::end(E) - ranges::begin(E))
(25.5.4 [range.subrange]) if it is a valid expression and the typesI
andS
ofranges::begin(E)
andranges::end(E)
(respectively)modelsatisfy bothsized_sentinel_for<S, I>
(24.3.4.8 [iterator.concept.sizedsentinel]) andforward_iterator<I>
. However,E
is evaluated only once.(1.4) — […]
Modify 25.3.14 [range.prim.empty] as indicated:
-1- The name
empty
denotes a customization point object (16.3.3.3.5 [customization.point.object]). The expressionranges::empty(E)
for some subexpressionE
is expression-equivalent to:
(1.1) — […]
(1.2) — […]
(1.3) — Otherwise,
EQ
, whereEQ
isbool(ranges::begin(E) == ranges::end(E))
except thatE
is only evaluated once, ifEQ
is a valid expression and the type ofranges::begin(E)
modelssatisfiesforward_iterator
.(1.4) — […]
Modify 25.3.15 [range.prim.data] as indicated:
-1- The name
data
denotes a customization point object (16.3.3.3.5 [customization.point.object]). The expressionranges::data(E)
for some subexpressionE
is expression-equivalent to:
(1.1) — […]
(1.2) — Otherwise, if
ranges::begin(E)
is a valid expression whose typemodelssatisfiescontiguous_iterator
,to_address(ranges::begin(E))
.(1.3) — […]
Modify 25.5.5 [range.dangling] as indicated:
-1- The tag type
[…]dangling
is used together with the template aliasessafe_iterator_t
andsafe_subrange_t
to indicate that an algorithm that typically returns an iterator into or subrange of a range argument does not return an iterator or subrange which could potentially reference a range whose lifetime has ended for a particular rvalue range argument which does notmodelsatisfyforwarding-range
(25.4.2 [range.range]).-2- [Example:
vector<int> f(); auto result1 = ranges::find(f(), 42); // #1 static_assert(same_as<decltype(result1), ranges::dangling>); auto vec = f(); auto result2 = ranges::find(vec, 42); // #2 static_assert(same_as<decltype(result2), vector<int>::iterator>); auto result3 = ranges::find(subrange{vec}, 42); // #3 static_assert(same_as<decltype(result3), vector<int>::iterator>);The call to
ranges::find
at #1 returnsranges::dangling
sincef()
is an rvaluevector
; thevector
could potentially be destroyed before a returned iterator is dereferenced. However, the calls at #2 and #3 both return iterators since the lvaluevec
and specializations of subrangemodelsatisfyforwarding-range
. — end example]Modify 25.6.4.3 [range.iota.iterator] as indicated:
-1-
iterator::iterator_category
is defined as follows:
(1.1) — If
W
modelssatisfiesadvanceable
, theniterator_category
israndom_access_iterator_tag
.(1.2) — Otherwise, if
W
modelssatisfiesdecrementable
, theniterator_category
isbidirectional_iterator_tag
.(1.3) — Otherwise, if
W
modelssatisfiesincrementable
, theniterator_category
isforward_iterator_tag
.(1.4) — Otherwise,
iterator_category
isinput_iterator_tag
.Modify [range.semi.wrap] as indicated:
-1- Many types in this subclause are specified in terms of an exposition-only class template
semiregular-box
.semiregular-box<T>
behaves exactly likeoptional<T>
with the following differences:
(1.1) — […]
(1.2) — If
T
modelssatisfiesdefault_constructible
, the default constructor ofsemiregular-box<T>
is equivalent to: […](1.3) — If
assignable_from<T&, const T&>
is notmodeledsatisfied, the copy assignment operator is equivalent to: […](1.4) — If
assignable_from<T&, T>
is notmodeledsatisfied, the move assignment operator is equivalent to: […]Modify 25.7.6 [range.all] as indicated:
-2- The name
views::all
denotes a range adaptor object (25.7.2 [range.adaptor.object]). For some subexpressionE
, the expressionviews::all(E)
is expression-equivalent to:
(2.1) —
decay-copy(E)
if the decayed type ofE
modelssatisfiesview
.(2.2) — […]
(2.3) — […]
Modify 25.7.8.3 [range.filter.iterator] as indicated:
-2-
iterator::iterator_concept
is defined as follows:-3-
(2.1) — If
V
modelssatisfiesbidirectional_range
, theniterator_concept
denotesbidirectional_iterator_tag
.(2.2) — Otherwise, if
V
modelssatisfiesforward_range
, theniterator_concept
denotesforward_iterator_tag
.(2.3) — […]
iterator::iterator_category
is defined as follows:
(3.1) — Let
C
denote the typeiterator_traits<iterator_t<V>>::iterator_category
.(3.2) — If
C
modelssatisfiesderived_from<bidirectional_iterator_tag>
, theniterator_category
denotesbidirectional_iterator_tag
.(3.3) — Otherwise, if
C
modelssatisfiesderived_from<forward_iterator_tag>
, theniterator_category
denotesforward_iterator_tag
.(3.4) — […]
Modify 25.7.9.3 [range.transform.iterator] as indicated:
-1-
iterator::iterator_concept
is defined as follows:-2- Let
(1.1) — If
V
modelssatisfiesrandom_access_range
, theniterator_concept
denotesrandom_access_iterator_tag
.(1.2) — Otherwise, if
V
modelssatisfiesbidirectional_range
, theniterator_concept
denotesbidirectional_iterator_tag
.(1.3) — Otherwise, if
V
modelssatisfiesforward_range
, theniterator_concept
denotesforward_iterator_tag
.(1.4) — […]
C
denote the typeiterator_traits<iterator_t<Base>>::iterator_category
. IfC
modelssatisfiesderived_from<contiguous_iterator_tag>
, theniterator_category
denotesrandom_access_iterator_tag
; otherwise,iterator_category
denotesC
.Modify 25.7.14.3 [range.join.iterator] as indicated:
-1-
iterator::iterator_concept
is defined as follows:-2-
(1.1) — If
ref_is_glvalue
istrue
andBase
andrange_reference_t<Base>
eachmodelsatisfybidirectional_range
, theniterator_concept
denotesbidirectional_iterator_tag
.(1.2) — Otherwise, if
ref_is_glvalue
istrue
andBase
andrange_reference_t<Base>
eachmodelsatisfyforward_range
, theniterator_concept
denotesforward_iterator_tag
.(1.3) — […]
iterator::iterator_category
is defined as follows:
(2.1) — Let […]
(2.2) — If
ref_is_glvalue
istrue
andOUTERC
andINNERC
eachmodelsatisfyderived_from<bidirectional_iterator_tag>
,iterator_category
denotesbidirectional_iterator_tag
.(2.3) — Otherwise, if
ref_is_glvalue
istrue
andOUTERC
andINNERC
eachmodelsatisfyderived_from<forward_iterator_tag>
,iterator_category
denotesforward_iterator_tag
.(2.4) — Otherwise, if
OUTERC
andINNERC
eachmodelsatisfyderived_from<input_iterator_tag>
,iterator_category
denotesinput_iterator_tag
.(2.5) — […]
Modify [range.split.outer] as indicated:
[…] Parent* parent_ = nullptr; // exposition only iterator_t<Base> current_ = // exposition only, present only ifV
modelssatisfiesforward_range
iterator_t<Base>(); […]-1- Many of the following specifications refer to the notional member
current
ofouter_iterator
.current
is equivalent tocurrent_
ifV
modelssatisfiesforward_range
, andparent_->current_
otherwise.Modify [range.split.inner] as indicated:
-1- The typedef-name
iterator_category
denotes
(1.1) —
forward_iterator_tag
ifiterator_traits<iterator_t<Base>>::iterator_category
modelssatisfiesderived_from<forward_iterator_tag>
;(1.2) — otherwise,
iterator_traits<iterator_t<Base>>::iterator_category
.Modify 25.7.19 [range.counted] as indicated:
-2- The name
views::counted
denotes a customization point object (16.3.3.3.5 [customization.point.object]). LetE
andF
be expressions, and letT
bedecay_t<decltype((E))>
. Then the expressionviews::counted(E, F)
is expression-equivalent to:
(2.1) — If
T
modelssatisfiesinput_or_output_iterator
anddecltype((F))
modelssatisfiesconvertible_to<iter_difference_t<T>>
,
(2.1.1) —
subrange{E, E + static_cast<iter_difference_t<T>>(F)}
ifT
modelssatisfiesrandom_access_iterator
.(2.1.2) — […]
(2.2) — […]
Modify [range.common.adaptor] as indicated:
-1- The name
views::common denotes
a range adaptor object (25.7.2 [range.adaptor.object]). For some subexpressionE
, the expressionviews::common(E)
is expression-equivalent to:
(1.1) —
views::all(E)
, ifdecltype((E))
modelssatisfiescommon_range
andviews::all(E)
is a well-formed expression.(1.2) — […]
Modify 26.6.13 [alg.equal] as indicated:
template<class InputIterator1, class InputIterator2> constexpr bool equal(InputIterator1 first1, InputIterator1 last1, InputIterator2 first2); […] template<input_range R1, input_range R2, class Pred = ranges::equal_to, class Proj1 = identity, class Proj2 = identity> requires indirectly_comparable<iterator_t<R1>, iterator_t<R2>, Pred, Proj1, Proj2> constexpr bool ranges::equal(R1&& r1, R2&& r2, Pred pred = {}, Proj1 proj1 = {}, Proj2 proj2 = {});[…]
-3- Complexity: If the types offirst1
,last1
,first2
, andlast2
:
(3.1) — meet the Cpp17RandomAccessIterator requirements (24.3.5.7 [random.access.iterators]) for the overloads in namespace
std
, or(3.2) — pairwise
modelsatisfysized_sentinel_for
(24.3.4.8 [iterator.concept.sizedsentinel]) for the overloads in namespaceranges
, andlast1 - first1 != last2 - first2
, then no applications of the corresponding predicate and each projection; otherwise,(3.3) — […]
(3.4) — […]
Modify 26.6.14 [alg.is.permutation] as indicated:
template<forward_iterator I1, sentinel_for<I1> S1, forward_iterator I2, sentinel_for<I2> S2, class Pred = ranges::equal_to, class Proj1 = identity, class Proj2 = identity> requires indirectly_comparable<I1, I2, Pred, Proj1, Proj2> constexpr bool ranges::is_permutation(I1 first1, S1 last1, I2 first2, S2 last2, Pred pred = {}, Proj1 proj1 = {}, Proj2 proj2 = {}); template<forward_range R1, forward_range R2, class Pred = ranges::equal_to, class Proj1 = identity, class Proj2 = identity> requires indirectly_comparable<iterator_t<R1>, iterator_t<R2>, Pred, Proj1, Proj2> constexpr bool ranges::is_permutation(R1&& r1, R2&& r2, Pred pred = {}, Proj1 proj1 = {}, Proj2 proj2 = {});[…]
-7- Complexity: No applications of the corresponding predicate and projections if:
(7.1) —
S1
andI1
modelsatisfysized_sentinel_for<S1, I1>
,(7.2) —
S2
andI2
modelsatisfysized_sentinel_for<S2, I2>
, and(7.3) — […]
Modify 26.8.5 [alg.partitions] as indicated:
template<class ForwardIterator, class Predicate> constexpr ForwardIterator partition(ForwardIterator first, ForwardIterator last, Predicate pred); […] template<forward_range R, class Proj = identity, indirect_unary_predicate<projected<iterator_t<R>, Proj>> Pred> requires permutable<iterator_t<R>> constexpr safe_subrange_t<R> ranges::partition(R&& r, Pred pred, Proj proj = {});[…]
-8- Complexity: LetN = last - first
:
(8.1) — For the overload with no
ExecutionPolicy
, exactlyN
applications of the predicate and projection. At mostN/2
swaps if the type offirst
meets the Cpp17BidirectionalIterator requirements for the overloads in namespacestd
ormodelssatisfiesbidirectional_iterator
for the overloads in namespaceranges
, and at mostN
swaps otherwise.(8.2) […]
[2020-02-10, Prague; Daniel comments]
I expect that P2101R0 (See D2101R0) is going to resolve this issue.
This proposal should also resolve the corresponding NB comments US-298 and US-300.[2020-05-01; reflector discussions]
Resolved by P2101R0 voted in in Prague.
Proposed resolution:
Resolved by P2101R0.
pair
and tuple
copy and move constructor have backwards specificationSection: 22.3.2 [pairs.pair], 22.4.4.2 [tuple.cnstr] Status: C++20 Submitter: Richard Smith Opened: 2019-11-26 Last modified: 2021-02-25
Priority: 0
View other active issues in [pairs.pair].
View all other issues in [pairs.pair].
View all issues with C++20 status.
Discussion:
22.3.2 [pairs.pair] p2 and 22.4.4.2 [tuple.cnstr] p3 say:
The defaulted move and copy constructor, respectively, of {
pair
,tuple
} is a constexpr function if and only if all required element-wise initializations for copy and move, respectively, would satisfy the requirements for a constexpr function.
Note that we specify the copy constructor in terms of element move operations and the move constructor in terms of element copy operations. Is that really the intent? This appears to be how this was originally specified when the wording was added by N3471.
[2019-12-01; Daniel comments and provides wording]
These inverted wording effects are an unintended oversight caused by N3471.
[2019-12-08 Issue Prioritization]
Status to Tentatively Ready and priority to 0 after ten positive votes on the reflector.
Proposed resolution:
This wording is relative to N4835.
Modify 22.3.2 [pairs.pair] as indicated:
-2- The defaulted move and copy constructor, respectively, of
pair
shall be a constexpr function if and only if all required element-wise initializations forcopymove andmovecopy, respectively, would satisfy the requirements for a constexpr function.
Modify 22.4.4.2 [tuple.cnstr] as indicated:
-3- The defaulted move and copy constructor, respectively, of
tuple
shall be a constexpr function if and only if all required element-wise initializations forcopymove andmovecopy, respectively, would satisfy the requirements for a constexpr function. The defaulted move and copy constructor oftuple<>
shall be constexpr functions.
std::pair<T, U>
now requires T
and U
to be less-than-comparableSection: 22.3.3 [pairs.spec], 23.3.3.1 [array.overview] Status: C++20 Submitter: Jonathan Wakely Opened: 2019-12-03 Last modified: 2021-02-25
Priority: 1
View all other issues in [pairs.spec].
View all issues with C++20 status.
Discussion:
P1614R2 added operator<=>
as a hidden friend to
std::pair
:
friend constexpr common_comparison_category_t<synth-three-way-result<T1>, synth-three-way-result<T2>> operator<=>(const pair& x, const pair& y) { see below }
That is not a function template, so is not a SFINAE context. If one or both of
synth-three-way-result<T1>
or synth-three-way-result<T2>
is an
invalid type then the declaration of operator<=>
is ill-formed, and so the specialization
std::pair<T1, T2>
is ill-formed.
std::array
.
There are at least two ways to fix this:
Constrain the function and delay the use of synth-three-way-result until we know it's valid.
Replace the hidden friend with a namespace-scope function template, so invalid
synth-three-way-result
types cause substitution failure.
The first option is somewhat hard to specify, because current policy is to avoid the use of requires-clauses
in most of the library clauses. Even with a requires-clause, the potentially-invalid
synth-three-way-result
types cannot be used in the function declarator. Furthermore, the
operator<=>
for std::array
is currently specified in Table [tab:container.opt] and
so there's nowhere to add a Constraints: element.
std::pair
and std::array
and bring them closer to what was in C++17. The main motivation
for making operator==
a hidden friend was to allow it to be defaulted, so that std::pair
and std::array
would be usable as non-type template parameters. Following the acceptance of
P1907 in Belfast it isn't necessary to default it, so we can go back
to what was in C++17.
[2019-12-12 Issue Prioritization]
Priority to 1 after reflector discussion.
[2020-02-10 Move to Immediate Monday afternoon in Prague]
Proposed resolution:
This wording is relative to N4842.
Modify 22.2.1 [utility.syn] as indicated:
[Drafting note: This restores the pre-P1614R2
operator==
and usesoperator<=>
as replacement foroperator<
,operator<=
,operator>
,operator>=
.]
[…] // 22.3 [pairs], class template pair template<class T1, class T2> struct pair; // 22.3.3 [pairs.spec], pair specialized algorithms template<class T1, class T2> constexpr bool operator==(const pair<T1, T2>&, const pair<T1, T2>&); template<class T1, class T2> constexpr common_comparison_category_t<synth-three-way-result<T1>, synth-three-way-result<T2>> operator<=>(const pair<T1, T2>&, const pair<T1, T2>&); template<class T1, class T2> constexpr void swap(pair<T1, T2>& x, pair<T1, T2>& y) noexcept(noexcept(x.swap(y))); […]
Modify 22.3.2 [pairs.pair] as indicated:
[…] constexpr void swap(pair& p) noexcept(see below);// 22.3.3 [pairs.spec], pair specialized algorithms friend constexpr bool operator==(const pair&, const pair&) = default; friend constexpr bool operator==(const pair& x, const pair& y) requires (is_reference_v<T1> || is_reference_v<T2>) { return x.first == y.first && x.second == y.second; } friend constexpr common_comparison_category_t<synth-three-way-result<T1>, synth-three-way-result<T2>> operator<=>(const pair& x, const pair& y) { see below }}; template<class T1, class T2> pair(T1, T2) -> pair<T1, T2>; […]
Modify 22.3.3 [pairs.spec] as indicated:
20.4.3 Specialized algorithms [pairs.spec]
template<class T1, class T2> constexpr bool operator==(const pair<T1, T2>& x, const pair<T1, T2>& y);-?- Returns:
x.first == y.first && x.second == y.second
.template<class T1, class T2>friendconstexpr common_comparison_category_t<synth-three-way-result<T1>, synth-three-way-result<T2>> operator<=>(const pair<T1, T2>& x, const pair<T1, T2>& y);-1- Effects: Equivalent to:
if (auto c = synth-three-way(x.first, y.first); c != 0) return c; return synth-three-way(x.second, y.second);
Modify 23.3.2 [array.syn] as indicated:
[Drafting note: This restores the pre-P1614R2
operator==
and usesoperator<=>
as replacement foroperator<
,operator<=
,operator>
,operator>=
.]
namespace std { // 23.3.3 [array], class template array template<class T, size_t N> struct array; template<class T, size_t N> constexpr bool operator==(const array<T, N>& x, const array<T, N>& y); template<class T, size_t N> constexpr synth-three-way-result<T> operator<=>(const array<T, N>& x, const array<T, N>& y); template<class T, size_t N> constexpr void swap(array<T, N>& x, array<T, N>& y) noexcept(noexcept(x.swap(y))); […]
Modify 23.3.3.1 [array.overview] as indicated:
[Drafting note: there is no need to add definitions of
operator==
andoperator<=>
to [array.spec] because they are defined by Table 71: Container requirements [tab:container.req] and Table 73: Optional container operations [tab:container.opt] respectively.]
[…] constexpr T * data() noexcept; constexpr const T * data() const noexcept;friend constexpr bool operator==(const array&, const array&) = default; friend constexpr synth-three-way-result<value_type> operator<=>(const array&, const array&);}; template<class T, class... U> array(T, U...) -> array<T, 1 + sizeof...(U)>; […]
__cpp_lib_unwrap_ref
in wrong headerSection: 17.3.2 [version.syn], 99 [refwrap.unwrapref] Status: C++20 Submitter: Barry Revzin Opened: 2019-12-03 Last modified: 2021-02-25
Priority: 2
View all other issues in [version.syn].
View all issues with C++20 status.
Discussion:
cpplearner points out in this github comment that:
Since
unwrap_reference
andunwrap_ref_decay
are defined in<functional>
([functional.syn]), their feature test macro should also be defined there.
P1902R1 adds this feature test macro in <type_traits>
instead.
The feature test macro and the type traits should go into the same header: either both in <functional>
or both in <type_traits>
.
<functional>
.
[2019-12-12 Issue Prioritization]
Priority to 2 after reflector discussion.
Previous resolution [SUPERSEDED]:This wording is relative to N4842.
Modify 17.3.2 [version.syn] p2 as indicated:
[…] #define __cpp_lib_unordered_map_try_emplace 201411L // also in <unordered_map> #define __cpp_lib_unwrap_ref 201811L // also in <type_traitsfunctional> #define __cpp_lib_variant 201606L // also in <variant> […]
[2020-02-13, Prague]
During LWG discussions it had been suggested that they considered it is an improvement to move the definitions of
unwrap_reference
and unwrap_ref_decay
from <functional>
to <type_traits>
.
This is what the alternative wording tries to accomplish.
[Status to Immediate on Thursday night in Prague.]
Proposed resolution:
This wording is relative to N4849.
Modify 22.10.2 [functional.syn], header <functional>
synopsis, as indicated:
namespace std { […]template<class T> struct unwrap_reference; template<class T> using unwrap_reference_t = typename unwrap_reference<T>::type; template<class T> struct unwrap_ref_decay; template<class T> using unwrap_ref_decay_t = typename unwrap_ref_decay<T>::type;[…] }
Delete sub-clause 99 [refwrap.unwrapref] completely, as indicated:
20.14.5.6 Transformation type traitunwrap_reference
[refwrap.unwrapref]template<class T> struct unwrap_reference;
-1- IfT
is a specializationreference_wrapper<X>
for some typeX
, the member typedeftype
ofunwrap_reference<T>
isX&
, otherwise it isT
.template<class T> struct unwrap_ref_decay;
-2- The member typedeftype
ofunwrap_ref_decay<T>
denotes the typeunwrap_reference_t<decay_t<T>>
.
Modify 21.3.3 [meta.type.synop], header <type_traits>
synopsis, as indicated:
namespace std { […] // 21.3.8.7 [meta.trans.other], other transformations […] template<class T> struct underlying_type; template<class Fn, class... ArgTypes> struct invoke_result; template<class T> struct unwrap_reference; template<class T> struct unwrap_ref_decay; template<class T> using type_identity_t = typename type_identity<T>::type; […] template<class Fn, class... ArgTypes> using invoke_result_t = typename invoke_result<Fn, ArgTypes...>::type; template<class T> using unwrap_reference_t = typename unwrap_reference<T>::type; template<class T> using unwrap_ref_decay_t = typename unwrap_ref_decay<T>::type; template<class...> using void_t = void; […] }
Modify 21.3.8.7 [meta.trans.other], Table 55 — "Sign modifications" in [tab:meta.trans.sign] as indicated:
Template | Comments |
---|---|
[…]
|
|
template <class T>
|
If T is a specialization reference_wrapper<X> for some type X , the member typedef
type of unwrap_reference<T> is X& , otherwise it is T .
|
template <class T>
|
The member typedef type of unwrap_ref_decay<T> denotes the type
unwrap_reference_t<decay_t<T>> .
|
Insert between 21.3.8.7 [meta.trans.other] p1 and p2 as indicated:
In addition to being available via inclusion of the
<type_traits>
header, the templatesunwrap_reference
,unwrap_ref_decay
,unwrap_reference_t
, andunwrap_ref_decay_t
are available when the header<functional>
(22.10.2 [functional.syn]) is included.
__cpp_lib_constexpr_complex
for P0415R1Section: 17.3.2 [version.syn] Status: C++20 Submitter: Stephan T. Lavavej Opened: 2019-12-04 Last modified: 2021-02-25
Priority: 0
View all other issues in [version.syn].
View all issues with C++20 status.
Discussion:
P1902R1 "Missing feature-test macros 2017-2019", accepted in Belfast, said:
[P0415R1] (Constexpr for
std::complex
): this paper proposes to introduce the macro__cpp_lib_constexpr_complex
. That is, introducing a new macro for this header.
However, __cpp_lib_constexpr_complex
wasn't mentioned in the Wording section, and doesn't appear
in the latest WP N4842.
__cpp_lib_constexpr_complex
.",
so this has been accepted and then overlooked by WG21 twice.
[2019-12-12 Issue Prioritization]
Status to Tentatively Ready and priority to 0 after twelve positive votes on the reflector.
Proposed resolution:
This wording is relative to N4842.
Modify 17.3.2 [version.syn] p2 as indicated:
[…] #define __cpp_lib_constexpr_algorithms 201806L // also in <algorithm> #define __cpp_lib_constexpr_complex 201711L // also in <complex> #define __cpp_lib_constexpr_dynamic_alloc 201907L // also in <memory> […]
lexicographical_compare_three_way
Section: 26.8.12 [alg.three.way] Status: C++20 Submitter: Jonathan Wakely Opened: 2019-12-04 Last modified: 2021-02-25
Priority: 0
View all other issues in [alg.three.way].
View all issues with C++20 status.
Discussion:
The current return type is:
common_comparison_category_t<decltype(comp(*b1, *b2)), strong_ordering>
Finding the common category with strong_ordering
doesn't do anything. The common category of X
and strong_ordering
is always X
, so we can simplify it to:
common_comparison_category_t<decltype(comp(*b1, *b2))>
This can further be simplified, because the common category of any comparison category type is just that type.
If it's not a comparison category then the result would be void
, but the function would be ill-formed
in that case anyway, as we have:
Mandates:
decltype(comp(*b1, *b2))
is a comparison category type.
So the only effect of the complicated return type seems to be to cause the return type to be deduced as
void
for specializations of the function template that are ill-formed if called. That doesn't seem useful.
[2019-12-12 Issue Prioritization]
Status to Tentatively Ready and priority to 0 after seven positive votes on the reflector.
Proposed resolution:
This wording is relative to N4842.
Modify 26.4 [algorithm.syn] as indicated:
[…] // 26.8.12 [alg.three.way], three-way comparison algorithms template<class InputIterator1, class InputIterator2, class Cmp> constexpr auto lexicographical_compare_three_way(InputIterator1 b1, InputIterator1 e1, InputIterator2 b2, InputIterator2 e2, Cmp comp) ->common_comparison_category_t<decltype(comp(*b1, *b2)), strong_ordering>; template<class InputIterator1, class InputIterator2> constexpr auto lexicographical_compare_three_way(InputIterator1 b1, InputIterator1 e1, InputIterator2 b2, InputIterator2 e2); […]
Modify 26.8.12 [alg.three.way] as indicated:
template<class InputIterator1, class InputIterator2, class Cmp> constexpr auto lexicographical_compare_three_way(InputIterator1 b1, InputIterator1 e1, InputIterator2 b2, InputIterator2 e2, Cmp comp) ->common_comparison_category_t<decltype(comp(*b1, *b2)), strong_ordering>;-1- Mandates:
[…]decltype(comp(*b1, *b2))
is a comparison category type.
ranges::enable_safe_range
should not be constrainedSection: 25.2 [ranges.syn] Status: C++20 Submitter: Jonathan Wakely Opened: 2019-12-05 Last modified: 2021-02-25
Priority: 0
View all other issues in [ranges.syn].
View all issues with C++20 status.
Discussion:
Currently ranges::enable_safe_range
is constrained with ranges::range
, which not only
forces the compiler to do unnecessary satisfaction checking when it's used, but also creates a tricky
dependency cycle (ranges::range
depends on ranges::begin
which depends on
ranges::enable_safe_range
which depends on ranges::range
).
ranges::safe_range
concept,
which already checks range<T>
before using enable_safe_range<T>
anyway.
The constraint serves no purpose and should be removed.
[2019-12-12 Issue Prioritization]
Status to Tentatively Ready and priority to 0 after eight positive votes on the reflector.
Proposed resolution:
This wording is relative to N4842.
Modify 25.2 [ranges.syn] as indicated:
[Drafting note: The definition in 25.4.2 [range.range] p7 is already unconstrained, which contradicts the synopsis.]
[…] // 25.4.2 [range.range], ranges template<class T> concept range = see below; template<rangeclass T> inline constexpr bool enable_safe_range = false; template<class T> concept safe_range = see below; […]
strong_equality
isn't a thingSection: 23.2.2 [container.requirements.general] Status: C++20 Submitter: Casey Carter Opened: 2019-12-06 Last modified: 2021-02-25
Priority: 1
View other active issues in [container.requirements.general].
View all other issues in [container.requirements.general].
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Discussion:
[tab:container.req] includes the row:
Expression:
Return Type:i <=> j
strong_ordering
ifX::iterator
meets the random access iterator requirements, otherwisestrong_equality
. Complexity: constant
"strong_equality
" is (now) a meaningless term that appears nowhere else in the working draft. Presumably
we want to make the Return Type unconditionally strong_ordering
, and require this expression to be
valid only when i
and j
are random access iterators. It's not clear to me if the best way to
do so would be to add a "Constraints" to the "Assertion/note/pre-/post-condition" column of this table row,
or if we should pull the row out into yet another "conditionally-supported iterator requirements" table.
[2019-12-21 Issue Prioritization]
Priority to 1 after reflector discussion.
[2020-02-10, Prague; David Olsen provides new wording based upon Tim Songs suggestion]
[2020-02 Moved to Immediate on Tuesday in Prague.]
Proposed resolution:
Table 71 — Container requirements [tab:container.req] Expression Return type Operational
semanticsAssertion/note
pre/post-conditionComplexity […]
i <=> j
strong_ordering
if
X::iterator
meets the
random access iterator
requirements, otherwise
strong_equality
Constraints: X::iterator
meets the random access iterator requirements.constant […]
has_strong_structural_equality
has a meaningless definitionSection: 21.3.5.4 [meta.unary.prop], 17.3.2 [version.syn] Status: C++20 Submitter: Daniel Krügler Opened: 2019-12-08 Last modified: 2021-02-25
Priority: 1
View other active issues in [meta.unary.prop].
View all other issues in [meta.unary.prop].
View all issues with C++20 status.
Discussion:
After the Belfast 2019 meeting with the acceptance of P1907R1
the core language term "strong structural equality" has been removed and instead a more general
definition of a "structural type" has been introduced that is suitable to be used as non-type template
parameter. These changes have caused the current definition of the has_strong_structural_equality
type trait (which had originally been introduced by P1614R2
during the Cologne 2019 meeting) to become meaningless since it is currently defined as follows:
The type
T
has strong structural equality (11.10.1 [class.compare.default]).
Besides the now undefined term "strong structural equality", the reference to 11.10.1 [class.compare.default] doesn't make sense anymore, assuming that the trait definition is supposed to refer now to a type that can be used as non-type template parameter.
During library reflector discussions several informal naming suggestions has been mentioned, such asis_structural[_type]
, can_be_nttp
, is_nontype_template_parameter_type
.
Albeit is_structural_type
would come very near to the current core terminology, core experts
have argued that the term "structural type" should be considered as a "core-internal" term that may
easily be replaced post-C++20.
In addition to that definition and naming question of that type trait it should be discussed whether
there should exist a specific feature macro for just this type trait, similar to the reason why we
introduced the __cpp_lib_has_unique_object_representations
test macro for the
has_unique_object_representations
type trait while voting in
P0258R2. The submitter of this issue believes that such a feature
macro should be added and given that its final definition should be related to the date of the
acceptance of this issue (and should not be related to any historically accepted papers such as
P1614R2).
[2020-01 Priority set to 1 and questions to LEWG after review on the reflector.]
Previous resolution [SUPERSEDED]:This wording is relative to N4842.
Modify 17.3.2 [version.syn], header
<version>
synopsis, as indicated (The symbolic??????L
represents the date to be defined by the project editor):[…] #define __cpp_lib_is_swappable 201603L // also in <type_traits> #define __cpp_lib_is_template_parameter_type ??????L // also in <type_traits> #define __cpp_lib_jthread 201911L // also in <stop_token>, <thread> […]Modify 21.3.3 [meta.type.synop], header
<type_traits>
synopsis, as indicated:[…] template<class T> struct has_unique_object_representations; template<class T> structhas_strong_structural_equalityis_template_parameter_type; // 21.3.6 [meta.unary.prop.query], type property queries […] template<class T> inline constexpr bool has_unique_object_representations_v = has_unique_object_representations<T>::value; template<class T> inline constexpr boolhas_strong_structural_equality_vis_template_parameter_type_v =has_strong_structural_equalityis_template_parameter_type<T>::value; // 21.3.6 [meta.unary.prop.query], type property queries […]Modify 21.3.3 [meta.type.synop], Table 47 ([tab:meta.unary.prop]) — "Type property predicates" — as indicated:
Table 47: Type property predicates [tab:meta.unary.prop] Template Condition Preconditions …
template<class T>
struct
has_strong_structural_equalityis_template_parameter_type;The type T
has strong
structural
equality (11.10.1 [class.compare.default])
can be used as non-type
template-parameter (13.2 [temp.param]).T
shall be a complete type,
cvvoid
, or an array of
unknown bound.
IfT
is a class type,T
shall
be a complete type.…
[2020-02, Prague]
LEWG looked at this and suggested to remove the existing trait has_strong_structural_equality
for now until someone demonstrates which usecases exist that would justify its existence.
[2020-02-13, Prague]
Set to Immediate.
Proposed resolution:
This wording is relative to N4849.
Modify 21.3.3 [meta.type.synop], header <type_traits>
synopsis, as indicated:
[…] template<class T> struct has_unique_object_representations;template<class T> struct has_strong_structural_equality;// 21.3.6 [meta.unary.prop.query], type property queries […] template<class T> inline constexpr bool has_unique_object_representations_v = has_unique_object_representations<T>::value;template<class T> inline constexpr bool has_strong_structural_equality_v = has_strong_structural_equality<T>::value;// 21.3.6 [meta.unary.prop.query], type property queries […]
Modify 21.3.3 [meta.type.synop], Table 47 ([tab:meta.unary.prop]) — "Type property predicates" — as indicated:
Table 47: Type property predicates [tab:meta.unary.prop] Template Condition Preconditions …
template<class T>
struct
has_strong_structural_equality;The typeT
has strong
structural
equality (11.10.1 [class.compare.default]).T
shall be a complete type,
cvvoid
, or an array of
unknown bound.
Section: 26.11.5 [uninitialized.copy], 26.11.6 [uninitialized.move] Status: C++20 Submitter: Corentin Jabot Opened: 2019-11-12 Last modified: 2021-02-25
Priority: 2
View all other issues in [uninitialized.copy].
View all issues with C++20 status.
Discussion:
P1207 introduced move-only input iterators but did not modify the specialized memory algorithms to support them.
[2020-01 Priority set to 2 after review on the reflector.]
[2020-02 Status to Immediate on Friday morning in Prague.]
Proposed resolution:
This wording is relative to N4842.
Modify 26.11.5 [uninitialized.copy] as indicated:
namespace ranges { template<input_iterator I, sentinel_for<I> S1, no-throw-forward-iterator O, no-throw-sentinel<O> S2> requires constructible_from<iter_value_t<O>, iter_reference_t<I>> uninitialized_copy_result<I, O> uninitialized_copy(I ifirst, S1 ilast, O ofirst, S2 olast); template<input_range IR, no-throw-forward-range OR> requires constructible_from<range_value_t<OR>, range_reference_t<IR>> uninitialized_copy_result<safe_iterator_t<IR>, safe_iterator_t<OR>> uninitialized_copy(IR&& in_range, OR&& out_range); }[…]-4- Preconditions:
-5- Effects: Equivalent to:[ofirst, olast)
shall not overlap with[ifirst, ilast)
.for (; ifirst != ilast && ofirst != olast; ++ofirst, (void)++ifirst) { ::new (voidify(*ofirst)) remove_reference_t<iter_reference_t<O>>(*ifirst); } return {std::move(ifirst), ofirst};namespace ranges { template<input_iterator I, no-throw-forward-iterator O, no-throw-sentinel<O> S> requires constructible_from<iter_value_t<O>, iter_reference_t<I>> uninitialized_copy_n_result<I, O> uninitialized_copy_n(I ifirst, iter_difference_t<I> n, O ofirst, S olast); }-9- Preconditions:
-10- Effects: Equivalent to:[ofirst, olast)
shall not overlap with[ifirst, n)
.auto t = uninitialized_copy(counted_iterator(ifirst, n), default_sentinel, ofirst, olast); return {std::move(t.in).base(), t.out};
Modify 26.11.6 [uninitialized.move] as indicated:
namespace ranges { template<input_iterator I, sentinel_for<I> S1, no-throw-forward-iterator O, no-throw-sentinel<O> S2> requires constructible_from<iter_value_t<O>, iter_rvalue_reference_t<I>> uninitialized_move_result<I, O> uninitialized_move(I ifirst, S1 ilast, O ofirst, S2 olast); template<input_range IR, no-throw-forward-range OR> requires constructible_from<range_value_t<OR>, range_rvalue_reference_t<IR>> uninitialized_move_result<safe_iterator_t<IR>, safe_iterator_t<OR>> uninitialized_move(IR&& in_range, OR&& out_range); }[…]-3- Preconditions:
-4- Effects: Equivalent to:[ofirst, olast)
shall not overlap with[ifirst, ilast)
.for (; ifirst != ilast && ofirst != olast; ++ofirst, (void)++ifirst) { ::new (voidify(*ofirst)) remove_reference_t<iter_reference_t<O>>(ranges::iter_move(*ifirst)); } return {std::move(ifirst), ofirst};namespace ranges { template<input_iterator I, no-throw-forward-iterator O, no-throw-sentinel<O> S> requires constructible_from<iter_value_t<O>, iter_rvalue_reference_t<I>> uninitialized_move_n_result<I, O> uninitialized_move_n(I ifirst, iter_difference_t<I> n, O ofirst, S olast); }-8- Preconditions:
-9- Effects: Equivalent to:[ofirst, olast)
shall not overlap with[ifirst, n)
.auto t = uninitialized_move(counted_iterator(ifirst, n), default_sentinel, ofirst, olast); return {std::move(t.in).base(), t.out};
__cpp_lib_nothrow_convertible
should be __cpp_lib_is_nothrow_convertible
Section: 17.3.2 [version.syn] Status: C++20 Submitter: Barry Revzin Opened: 2019-12-09 Last modified: 2021-02-25
Priority: 0
View all other issues in [version.syn].
View all issues with C++20 status.
Discussion:
P1902R1 introduced the feature test macro
__cpp_lib_nothrow_convertible
, but every other example in SD-6 of a feature test macro
testing for the presence of a single type trait FOO is named __cpp_lib_FOO
.
This macro should be renamed __cpp_lib_is_nothrow_convertible
.
[2019-12-21 Issue Prioritization]
Status to Tentatively Ready and priority to 0 after seven positive votes on the reflector. A convincing argument was that currently no vendor had published a release with the previous feature macro.
Proposed resolution:
This wording is relative to N4842.
Modify 17.3.2 [version.syn], header <version>
synopsis, as indicated:
[…] #define __cpp_lib_not_fn 201603L // also in <functional> #define __cpp_lib_is_nothrow_convertible 201806L // also in <type_traits> #define __cpp_lib_null_iterators 201304L // also in <iterator> […]
to_address
can throwSection: 23.7.2.2.2 [span.cons] Status: C++20 Submitter: Casey Carter Opened: 2019-12-10 Last modified: 2021-02-25
Priority: 0
View all other issues in [span.cons].
View all issues with C++20 status.
Discussion:
[span.cons] paragraphs 6 and 9:
Throws: When and what
to_address(first)
throws.
could equivalently be "Throws: Nothing." since all overloads of std::to_address
are
noexcept
. However, paragraph 9 fails to account for the fact that paragraph 8:
Effects: Initializes
data_
withto_address(first)
andsize_
withlast - first
.
must evaluate last - first
.
[2020-01-14 Status set to Tentatively Ready after ten positive votes on the reflector.]
[2020-01-14; Daniel comments]
The fixed wording in 23.7.2.2.2 [span.cons] p9 depends on the no-throw-guarantee of integer-class conversions to integral types. This guarantee is specified by LWG 3367(i).
Proposed resolution:
This wording is relative to N4842.
Modify 23.7.2.2.2 [span.cons] paragraphs 6 and 9 as indicated:
[Drafting note:
]
The missing paragraph number of the Preconditions element at p7/p8 has already been reported as editorial issue
The effective change to "Throws: Nothing." in p6 has already been applied editorially.
template<class It> constexpr span(It first, size_type count);[…]
-4- Preconditions: […] -5- Effects: Initializesdata_
withto_address(first)
andsize_
withcount
. -6- Throws:When and whatNothing.to_address(first)
throwstemplate<class It, class End> constexpr span(It first, End last);[…]
-?- Preconditions: […] -8- Effects: Initializesdata_
withto_address(first)
andsize_
withlast - first
. -9- Throws: When and whatthrows.
to_address(first)last - first
<chrono>
leap second support should allow for negative leap secondsSection: 30.11.8 [time.zone.leap] Status: C++20 Submitter: Asher Dunn Opened: 2019-12-16 Last modified: 2021-02-25
Priority: 3
View all other issues in [time.zone.leap].
View all issues with C++20 status.
Discussion:
class leap
(which is expected to be renamed by P1981R0 to
leap_second
) defined in 30.11.8 [time.zone.leap] should include support for both
positive leap seconds (23:59:60
added to UTC at a specified time) and negative leap seconds
(23:59:59
removed from UTC at a specified time). While only positive leap seconds have been
inserted to date, the definition of UTC allows for both.
[2020-01 Priority set to 3 after review on the reflector.]
Previous resolution [SUPERSEDED]:This wording is relative to N4842.
Modify 30.7.3.2 [time.clock.utc.members] as indicated:
template<class Duration> static sys_time<common_type_t<Duration, seconds>> to_sys(const utc_time<Duration>& u);-2- Returns: A
sys_time t
, such thatfrom_sys(t) == u
if such a mapping exists. Otherwiseu
represents atime_point
during a positive leap second insertion, the conversion counts that leap second as not inserted, and the last representable value ofsys_time
prior to the insertion of the leap second is returned.template<class Duration> static utc_time<common_type_t<Duration, seconds>> from_sys(const sys_time<Duration>& t);-3- Returns: A
[…]utc_time u
, such thatu.time_since_epoch() - t.time_since_epoch()
is equal to thenumbersum of leap seconds that were inserted betweent
and 1970-01-01. Ift
is exactly the date of leap second insertion, then the conversion counts that leap second as inserted.Modify 30.7.3.3 [time.clock.utc.nonmembers] as indicated:
template<class Duration> leap_second_info get_leap_second_info(const utc_time<Duration>& ut);-6- Returns: A
leap_second_info
whereis_leap_second
istrue
ifut
is during a positive leap second insertion, and otherwisefalse
.elapsed
is thenumbersum of leap seconds between 1970-01-01 andut
. Ifis_leap_second
istrue
, the leap second referred to byut
is included in the count.Modify 30.7.4.1 [time.clock.tai.overview] as indicated:
-1- The clock
tai_clock
measures seconds since 1958-01-01 00:00:00 and is offset 10s ahead of UTC at this date. That is, 1958-01-01 00:00:00 TAI is equivalent to 1957-12-31 23:59:50 UTC. Leap seconds are not inserted into TAI. Therefore every time a leap second is inserted into UTC, UTCfalls another second behindshifts another second with respect to TAI. For example by 2000-01-01 there had been 22 positive and 0 negative leap seconds inserted so 2000-01-01 00:00:00 UTC is equivalent to 2000-01-01 00:00:32 TAI (22s plus the initial 10s offset).Modify 30.7.5.1 [time.clock.gps.overview] as indicated:
-1- The clock
gps_clock
measures seconds since the first Sunday of January, 1980 00:00:00 UTC. Leap seconds are not inserted into GPS. Therefore every time a leap second is inserted into UTC, UTCfalls another second behindshifts another second with respect to GPS. Aside from the offset from1958y/January/1
to1980y/January/Sunday[1]
, GPS is behind TAI by 19s due to the 10s offset between 1958 and 1970 and the additional 9 leap seconds inserted between 1970 and 1980.Modify 30.11.8.1 [time.zone.leap.overview] as indicated:
namespace std::chrono { class leap { public: leap(const leap&) = default; leap& operator=(const leap&) = default; // unspecified additional constructors constexpr sys_seconds date() const noexcept; constexpr seconds value() const noexcept; }; }-1- Objects of type
-2- [Example:leap
representing the date and value of the leap second insertions are constructed and stored in the time zone database when initialized.for (auto& l : get_tzdb().leaps) if (l <= 2018y/March/17d) cout << l.date() << ": " << l.value() << '\n';Produces the output:
1972-07-01 00:00:00: 1s 1973-01-01 00:00:00: 1s 1974-01-01 00:00:00: 1s 1975-01-01 00:00:00: 1s 1976-01-01 00:00:00: 1s 1977-01-01 00:00:00: 1s 1978-01-01 00:00:00: 1s 1979-01-01 00:00:00: 1s 1980-01-01 00:00:00: 1s 1981-07-01 00:00:00: 1s 1982-07-01 00:00:00: 1s 1983-07-01 00:00:00: 1s 1985-07-01 00:00:00: 1s 1988-01-01 00:00:00: 1s 1990-01-01 00:00:00: 1s 1991-01-01 00:00:00: 1s 1992-07-01 00:00:00: 1s 1993-07-01 00:00:00: 1s 1994-07-01 00:00:00: 1s 1996-01-01 00:00:00: 1s 1997-07-01 00:00:00: 1s 1999-01-01 00:00:00: 1s 2006-01-01 00:00:00: 1s 2009-01-01 00:00:00: 1s 2012-07-01 00:00:00: 1s 2015-07-01 00:00:00: 1s 2017-01-01 00:00:00: 1s— end example]
Modify 30.11.8.2 [time.zone.leap.members] as indicated:
constexpr sys_seconds date() const noexcept;-1- Returns: The date and time at which the leap second was inserted.
constexpr seconds value() const noexcept;-?- Returns: The value of the leap second. Always
+1s
to indicate a positive leap second or-1s
to indicate a negative leap second. All leap seconds inserted up through 2017 were positive leap seconds.Modify 30.12 [time.format] as indicated:
template<class Duration, class charT> struct formatter<chrono::utc_time<Duration>, charT>;-7- Remarks: If
%Z
is used, it is replaced withSTATICALLY-WIDEN<charT>("UTC")
. If%z
(or a modified variant of%z
) is used, an offset of0min
is formatted. If the argument represents a time during a positive leap second insertion, and if a seconds field is formatted, the integral portion of that format isSTATICALLY-WIDEN<charT>("60")
.
[2020-02-14; Prague]
LWG Review. Some wording improvements have been made and lead to revised wording.
Proposed resolution:
This wording is relative to N4849.
Modify 30.7.3.2 [time.clock.utc.members] as indicated:
template<class Duration> static sys_time<common_type_t<Duration, seconds>> to_sys(const utc_time<Duration>& u);-2- Returns: A
sys_time t
, such thatfrom_sys(t) == u
if such a mapping exists. Otherwiseu
represents atime_point
during a positive leap second insertion, the conversion counts that leap second as not inserted, and the last representable value ofsys_time
prior to the insertion of the leap second is returned.template<class Duration> static utc_time<common_type_t<Duration, seconds>> from_sys(const sys_time<Duration>& t);-3- Returns: A
[…]utc_time u
, such thatu.time_since_epoch() - t.time_since_epoch()
is equal to thenumbersum of leap seconds that were inserted betweent
and 1970-01-01. Ift
is exactly the date of leap second insertion, then the conversion counts that leap second as inserted.
Modify 30.7.3.3 [time.clock.utc.nonmembers] as indicated:
template<class Duration> leap_second_info get_leap_second_info(const utc_time<Duration>& ut);-6- Returns: A
leap_second_info
,lsi
, wherelsi.is_leap_second
istrue
ifut
is during a positive leap second insertion, and otherwisefalse
.lsi.elapsed
is thenumbersum of leap seconds between 1970-01-01 andut
. Iflsi.is_leap_second
istrue
, the leap second referred to byut
is included in thecountsum.
Modify 30.7.4.1 [time.clock.tai.overview] as indicated:
-1- The clock
tai_clock
measures seconds since 1958-01-01 00:00:00 and is offset 10s ahead of UTC at this date. That is, 1958-01-01 00:00:00 TAI is equivalent to 1957-12-31 23:59:50 UTC. Leap seconds are not inserted into TAI. Therefore every time a leap second is inserted into UTC, UTCfalls another second behindshifts another second with respect to TAI. For example by 2000-01-01 there had been 22 positive and 0 negative leap seconds inserted so 2000-01-01 00:00:00 UTC is equivalent to 2000-01-01 00:00:32 TAI (22s plus the initial 10s offset).
Modify 30.7.5.1 [time.clock.gps.overview] as indicated:
-1- The clock
gps_clock
measures seconds since the first Sunday of January, 1980 00:00:00 UTC. Leap seconds are not inserted into GPS. Therefore every time a leap second is inserted into UTC, UTCfalls another second behindshifts another second with respect to GPS. Aside from the offset from1958y/January/1
to1980y/January/Sunday[1]
, GPS is behind TAI by 19s due to the 10s offset between 1958 and 1970 and the additional 9 leap seconds inserted between 1970 and 1980.
Modify 30.11.8.1 [time.zone.leap.overview] as indicated:
namespace std::chrono { class leap { public: leap(const leap&) = default; leap& operator=(const leap&) = default; // unspecified additional constructors constexpr sys_seconds date() const noexcept; constexpr seconds value() const noexcept; }; }-1- Objects of type
-2- [Example:leap
representing the date and value of the leap second insertions are constructed and stored in the time zone database when initialized.for (auto& l : get_tzdb().leaps) if (l <= 2018y/March/17d) cout << l.date() << ": " << l.value() << '\n';Produces the output:
1972-07-01 00:00:00: 1s 1973-01-01 00:00:00: 1s 1974-01-01 00:00:00: 1s 1975-01-01 00:00:00: 1s 1976-01-01 00:00:00: 1s 1977-01-01 00:00:00: 1s 1978-01-01 00:00:00: 1s 1979-01-01 00:00:00: 1s 1980-01-01 00:00:00: 1s 1981-07-01 00:00:00: 1s 1982-07-01 00:00:00: 1s 1983-07-01 00:00:00: 1s 1985-07-01 00:00:00: 1s 1988-01-01 00:00:00: 1s 1990-01-01 00:00:00: 1s 1991-01-01 00:00:00: 1s 1992-07-01 00:00:00: 1s 1993-07-01 00:00:00: 1s 1994-07-01 00:00:00: 1s 1996-01-01 00:00:00: 1s 1997-07-01 00:00:00: 1s 1999-01-01 00:00:00: 1s 2006-01-01 00:00:00: 1s 2009-01-01 00:00:00: 1s 2012-07-01 00:00:00: 1s 2015-07-01 00:00:00: 1s 2017-01-01 00:00:00: 1s— end example]
Modify 30.11.8.2 [time.zone.leap.members] as indicated:
constexpr sys_seconds date() const noexcept;-1- Returns: The date and time at which the leap second was inserted.
constexpr seconds value() const noexcept;-?- Returns:
+1s
to indicate a positive leap second or-1s
to indicate a negative leap second. [Note: All leap seconds inserted up through 2019 were positive leap seconds. — end note]
Modify 30.12 [time.format] as indicated:
template<class Duration, class charT> struct formatter<chrono::utc_time<Duration>, charT>;-7- Remarks: If
%Z
is used, it is replaced withSTATICALLY-WIDEN<charT>("UTC")
. If%z
(or a modified variant of%z
) is used, an offset of0min
is formatted. If the argument represents a time during a positive leap second insertion, and if a seconds field is formatted, the integral portion of that format isSTATICALLY-WIDEN<charT>("60")
.
three_way_comparable_with
is inconsistent with similar conceptsSection: 17.12.4 [cmp.concept] Status: C++20 Submitter: Casey Carter Opened: 2019-12-18 Last modified: 2021-02-25
Priority: 0
View all other issues in [cmp.concept].
View all issues with C++20 status.
Discussion:
The concept three_way_comparable_with
is defined in 17.12.4 [cmp.concept] as:
template<class T, class U, class Cat = partial_ordering> concept three_way_comparable_with = weakly-equality-comparable-with<T, U> && partially-ordered-with<T, U> && three_way_comparable<T, Cat> && three_way_comparable<U, Cat> && common_reference_with<const remove_reference_t<T>&, const remove_reference_t<U>&> && three_way_comparable< common_reference_t<const remove_reference_t<T>&, const remove_reference_t<U>&>, Cat> && requires(const remove_reference_t<T>& t, const remove_reference_t<U>& u) { { t <=> u } -> compares-as<Cat>; { u <=> t } -> compares-as<Cat>; };
Which notably doesn't follow the requirement ordering:
same-type requirements on T
same-type requirements on U
common_reference_with
requirement
same-type requirements on common_reference_t<T, U>
cross-type requirements on T
and U
that the other cross-type comparison concepts (18.5.4 [concept.equalitycomparable], 18.5.5 [concept.totallyordered]) use. There were some motivating reasons for that ordering:
The existence of a common reference type is effectively an opt-in to cross-type concepts. Avoiding checking cross-type expressions when no common reference type exists can enable the concepts to work even in the presence of poorly-constrained "accidental" cross-type operator templates which could otherwise produce compile errors instead of dissatisfied concepts.
Putting the simpler same-type requirements first can help produce simpler error messages
when applying the wrong concept to a pair of types, or the right concept to the wrong pair of types.
"Frobnozzle <=> Frobnozzle
is not a valid expression" is more easily deciphered than
is "std::common_reference<int, FrobNozzle>
has no member named type".
three_way_comparable_with
should be made consistent with equality_comparable_with
and
totally_ordered_with
for the above reasons and to make it easier to reason about comparison concepts
in general.
[2020-01 Status set to Tentatively Ready after five positive votes on the reflector.]
Proposed resolution:
This wording is relative to N4842.
Modify 17.12.4 [cmp.concept] as indicated:
template<class T, class U, class Cat = partial_ordering> concept three_way_comparable_with =weakly-equality-comparable-with<T, U> &&partially-ordered-with<T, U> &&three_way_comparable<T, Cat> && three_way_comparable<U, Cat> && common_reference_with<const remove_reference_t<T>&, const remove_reference_t<U>&> && three_way_comparable< common_reference_t<const remove_reference_t<T>&, const remove_reference_t<U>&>, Cat> && weakly-equality-comparable-with<T, U> && partially-ordered-with<T, U> && requires(const remove_reference_t<T>& t, const remove_reference_t<U>& u) { { t <=> u } -> compares-as<Cat>; { u <=> t } -> compares-as<Cat>; };
safe_range<SomeRange&>
caseSection: 25.4.2 [range.range] Status: C++23 Submitter: Johel Ernesto Guerrero Peña Opened: 2019-12-19 Last modified: 2023-11-22
Priority: 3
View all other issues in [range.range].
View all issues with C++23 status.
Discussion:
25.5.5 [range.dangling] p2 hints at how safe_range
should allow lvalue ranges to model it.
However, its wording doesn't take into account that case.
[2020-01 Priority set to 3 after review on the reflector.]
Previous resolution [SUPERSEDED]:
This wording is relative to N4842.
Modify 25.4.2 [range.range] as indicated:
template<class T> concept safe_range = range<T> && (is_lvalue_reference_v<T> || enable_safe_range<remove_cvref_t<T>>);-5-
Given an expressionA typeE
such thatdecltype((E))
isT
,T
modelssafe_range
onlyif:-6- [Note:
(5.1) —
is_lvalue_reference_v<T>
istrue
, or(5.2) — given an expression
E
such thatdecltype((E))
isT
,ifthe validity of iterators obtained from the object denoted byE
is not tied to the lifetime of that object.Since the validity of iterators is not tied to the lifetime of an object whose type modelsA function can accept arguments ofsafe_range
, asucha type that modelssafe_range
by valueand return iterators obtained from it without danger of dangling. — end note]
[2021-05-19 Tim updates wording]
The new wording below attempts to keep the "borrowed" property generally
applicable to all models of borrowed_range
, instead of bluntly carving
out lvalue reference types.
[2021-09-20; Reflector poll]
Set status to Tentatively Ready after five votes in favour during reflector poll in June.
[2021-10-14 Approved at October 2021 virtual plenary. Status changed: Voting → WP.]
Proposed resolution:
This wording is relative to N4885.
Modify 25.4.2 [range.range] as indicated:
template<class T> concept borrowed_range = range<T> && (is_lvalue_reference_v<T> || enable_borrowed_range<remove_cvref_t<T>>);-5- Let
-6- [Note: Since the validity of iterators is not tied to the lifetime ofU
beremove_reference_t<T>
ifT
is an rvalue reference type, andT
otherwise. Givenan expressiona variableE
such thatdecltype((E))
isT
u
of typeU
,T
modelsborrowed_range
only if the validity of iterators obtained fromthe object denoted byu is not tied to the lifetime of thatE
objectvariable.an objecta variable whose type modelsborrowed_range
, a functioncan accept arguments ofwith a parameter of such a typeby value andcan return iterators obtained from it without danger of dangling. — end note]
stop_source
's operator!=
Section: 32.3.5 [stopsource] Status: C++20 Submitter: Tim Song Opened: 2020-01-03 Last modified: 2021-02-25
Priority: 0
View all issues with C++20 status.
Discussion:
Just like stop_token
(see LWG 3254(i)), stop_source
in 32.3.5 [stopsource]
declares an operator!=
friend that is unnecessary in light of the new core language rules and
should be struck.
[2020-01-14 Status set to Tentatively Ready after six positive votes on the reflector.]
Proposed resolution:
This wording is relative to N4842.
Modify 32.3.5 [stopsource], class stop_source
synopsis, as indicated:
namespace std { […] class stop_source { public: […] [[nodiscard]] friend bool operator==(const stop_source& lhs, const stop_source& rhs) noexcept;[[nodiscard]] friend bool operator!=(const stop_source& lhs, const stop_source& rhs) noexcept;friend void swap(stop_source& lhs, stop_source& rhs) noexcept; }; }
Modify [stopsource.cmp] and [stopsource.special] as indicated:
32.3.4.3 Non-member functions
Comparisons[stopsource.nonmemberscmp][[nodiscard]] bool operator==(const stop_source& lhs, const stop_source& rhs) noexcept;-1- Returns:
true
iflhs
andrhs
have ownership of the same stop state or if bothlhs
andrhs
do not have ownership of a stop state; otherwisefalse
.[[nodiscard]] bool operator!=(const stop_source& lhs, const stop_source& rhs) noexcept;
-2- Returns:!(lhs==rhs)
.
32.3.4.4 Specialized algorithms [stopsource.special]friend void swap(stop_source& x, stop_source& y) noexcept;-1- Effects: Equivalent to:
x.swap(y)
.
drop_while_view
should opt-out of sized_range
Section: 25.7.13.2 [range.drop.while.view] Status: C++20 Submitter: Johel Ernesto Guerrero Peña Opened: 2020-01-07 Last modified: 2021-02-25
Priority: 1
View all other issues in [range.drop.while.view].
View all issues with C++20 status.
Discussion:
If drop_while_view
's iterator_t
and sentinel_t
model forward_iterator
and sized_sentinel_for
, it will
incorrectly satisfy sized_range
thanks to the size member function
inherited from view_interface
.
Because it has to compute its begin()
, it can never model
sized_range
due to not meeting its non-amortized O(1) requirement.
[2020-01-16 Priority set to 1 after discussion on the reflector.]
[2020-02-10; Prague 2020; Casey comments and provides alternative wording]
The fundamental issue here is that both ranges::size
and view_interface::size
(it should be unsurprising that many of the "default" implementations of member meow
in
view_interface
look just like fallback cases of the ranges::meow
CPO) have
a case that returns the difference of ranges::end
and ranges::begin
. If
begin
and end
are amortized* O(1) but not "true" O(1), then the resulting
size
operation is amortized O(1) and not "true" O(1) as required by the
sized_range
concept. I don't believe we can or should fix this on a case by case basis,
but we should instead relax the complexity requirement for sized_range
to be
handwavy-amortized O(1).
Previous resolution [SUPERSEDED]:
This wording is relative to N4849.
Add the following specialization to 25.2 [ranges.syn]:
// [drop.while.view], drop while view template<view V, class Pred> requires input_range<V> && is_object_v<Pred> && indirect_unary_predicate<const Pred, iterator_t<V>> class drop_while_view; template<view V, class Pred> inline constexpr bool disable_sized_range<drop_while_view<V, Pred>> = true; namespace views { inline constexpr unspecified drop_while = unspecified; }
[2020-02 Moved to Immediate on Tuesday in Prague.]
Proposed resolution:
This wording is relative to N4849.
Modify 25.4.3 [range.sized] as indicated:
-2- Given an lvalue
t
of typeremove_reference_t<T>
,T
modelssized_range
only if
(2.1) —
ranges::size(t)
is amortized 𝒪(1
), does not modifyt
, and is equal toranges::distance(t)
, and[…]
-3- [Note: The complexity requirement for the evaluation ofranges::size
is non-amortized, unlike the case for the complexity of the evaluations ofranges::begin
andranges::end
in therange
concept. — end note]
Section: 25.7.11.2 [range.take.while.view], 25.7.12.2 [range.drop.view], 25.7.13.2 [range.drop.while.view], 25.7.23.3 [range.elements.iterator] Status: C++20 Submitter: Johel Ernesto Guerrero Peña Opened: 2020-01-07 Last modified: 2021-02-25
Priority: 0
View all other issues in [range.take.while.view].
View all issues with C++20 status.
Discussion:
Before P1035 was accepted, no data member in [ranges] whose type could potentially be an aggregate or fundamental type was left without initializer. P1035 left some such data members without initializer, so it is possible to have them have indeterminate values. We propose restoring consistency.
[2020-01-14 Status set to Tentatively Ready after five positive votes on the reflector.]
Proposed resolution:
This wording is relative to N4842.
Modify 25.7.11.2 [range.take.while.view] as follows:
class take_while_view : public view_interface<take_while_view<V, Pred>> { template<bool> class sentinel; // exposition only V base_ = V(); // exposition only semiregular-box<Pred> pred_; // exposition only public:
Modify 25.7.12.2 [range.drop.view] as follows:
private: V base_ = V(); // exposition only range_difference_t<V> count_ = 0; // exposition only };
Modify 25.7.13.2 [range.drop.while.view] as follows:
private: V base_ = V(); // exposition only semiregular-box<Pred> pred_; // exposition only };
Modify 25.7.23.3 [range.elements.iterator] as follows:
class elements_view<V, N>::iterator { // exposition only using base-t = conditional_t<Const, const V, V>; friend iterator<!Const>; iterator_t<base-t> current_ = iterator_t<base-t>(); public:
Section: 24.3.4.4 [iterator.concept.winc] Status: Resolved Submitter: Casey Carter Opened: 2020-01-07 Last modified: 2021-10-23
Priority: 3
View all other issues in [iterator.concept.winc].
View all issues with Resolved status.
Discussion:
The working draft ignores the possibility that:
the value of an expression of integer-class type might not be representable by the target integer type of a conversion, and
the value of an expression of integer type might not be representable by the target integer-class type of a conversion.
Presumably the behavior of these cases is undefined by omission; is this actually the intent?
Notably (2) could be specified away by mandating that all integer-class types are capable of representing the value range of all integer types of the same signedness.[2020-01-25 Issue Prioritization]
Priority to 3 after reflector discussion.
[2021-10-23 Resolved by the adoption of P2393R1 at the October 2021 plenary. Status changed: New → Resolved.]
Proposed resolution:
Section: 24.3.4.4 [iterator.concept.winc] Status: C++20 Submitter: Casey Carter Opened: 2020-01-07 Last modified: 2021-02-25
Priority: 0
View all other issues in [iterator.concept.winc].
View all issues with C++20 status.
Discussion:
It's widely established that neither conversions of integral types to bool
nor conversions
between different integral types throw exceptions. These properties are crucial to supporting exception
guarantees in algorithms, containers, and other uses of iterators and their difference types.
Integer-class types must provide the same guarantees to support the same use cases as do integer types.
[2020-01-14; Daniel comments]
We probably need to think about providing the stronger guarantee that all integer-class operations
are also noexcept
in addition to the guarantee that they do not throw any exceptions.
The fixed wording in LWG 3358(i), 23.7.2.2.2 [span.cons] p9 depends on the no-throw-guarantee of integer-class conversions to integral types.
[2020-01-25 Status set to Tentatively Ready after five positive votes on the reflector.]
Proposed resolution:
This wording is relative to N4842.
Modify 24.3.4.4 [iterator.concept.winc] as indicated:
[Drafting note: There's a bit of drive-by editing here to change occurrences of the meaningless "type is convertible to type" to "expression is convertible to type". Paragraph 7 only has drive-by edits. ]
-6-
-7-AllExpressions of integer-class typesare explicitly convertible toallany integral types and. Expressions of integral type are both implicitly and explicitly convertiblefrom all integral typesto any integer-class type. Conversions between integral and integer-class types do not exit via an exception.AllExpressionsE
of integer-class typesI
are contextually convertible tobool
as if bybool(
aE != I(0)), where.a
is an instance of the integral-class typeI
size
return end - begin
?Section: 25.3.10 [range.prim.size] Status: Resolved Submitter: Casey Carter Opened: 2020-01-07 Last modified: 2021-02-25
Priority: 0
View all issues with Resolved status.
Discussion:
The specification of ranges::size
in 25.3.10 [range.prim.size] suggests that bullet 1.3
("Otherwise, make-unsigned-like(ranges::end(E) - ranges::begin(E)) ...")
only applies
when disable_sized_range<remove_cv_t<T>>
is true
. This is not the
design intent, but the result of an erroneous attempt to factor out the common
"disable_sized_range
is false
" requirement from the member and non-member size
cases in bullets 1.2.1 and 1.2.2 that occurred between P0896R3 and
P0896R4. The intended design has always been that a range with
member or non-member size
with the same syntax but different semantics may opt-out of being
sized by specializing disable_sized_range
. It has never been intended that arrays or ranges
whose iterator and sentinel model sized_sentinel_for
be able to opt out of being sized via
disable_sized_range
. disable_sized_sentinel_for
can/must be used to opt out in the
latter case so that library functions oblivious to the range type that operate on the iterator and
sentinel of such a range will avoid subtraction.
[2020-01-25 Status set to Tentatively Ready after six positive votes on the reflector.]
[2020-11-09 Approved In November virtual meeting. Status changed: Tentatively Ready → WP.]
[2020-11-18 This was actually resolved by P2091R0 in Prague. Status changed: WP → Resolved.]
Proposed resolution:
This wording is relative to N4842.
Modify 25.3.10 [range.prim.size] as indicated:
[Drafting note: There are drive-by changes here to (1) avoid introducing unused type placeholders, (2) avoid reusing "
T
" as both the type of the subexpression and the template parameter of the poison pill, and (3) fix the cross-reference formake-unsigned-like
which is defined in [ranges.syn]/1, not in [range.subrange].]
-1- The name
size
denotes a customization point object (16.3.3.3.5 [customization.point.object]). The expressionranges::size(E)
for some subexpressionE
with typeT
is expression-equivalent to:
(1.1) —
decay-copy(extent_v<T>)
ifT
is an array type (6.8.4 [basic.compound]).
(1.2) — Otherwise, ifdisable_sized_range<remove_cv_t<T>>
(25.4.3 [range.sized]) isfalse
:(1.?
2.1) — Otherwise, ifdisable_sized_range<remove_cv_t<T>>
(25.4.3 [range.sized]) isfalse
anddecay-copy(E.size())
if itis a valid expressionand its typeof integer-like type (24.3.4.4 [iterator.concept.winc]),I
isdecay-copy(E.size())
.(1.?
2.2) — Otherwise, ifdisable_sized_range<remove_cv_t<T>>
isfalse
anddecay-copy(size(E))
if itis a valid expressionand its typeof integer-like type with overload resolution performed in a context that includes the declaration:I
isand does not include a declaration oftemplate<class T>void size(Tauto&&) = delete;ranges::size
,decay-copy(size(E))
.
(1.3) — Otherwise, make-unsigned-like(ranges::end(E) - ranges::begin(E))
(25.5.4 [range.subrange]25.2 [ranges.syn]) if it is a valid
expression and the types I
and S
of
ranges::begin(E)
and ranges::end(E)
(respectively) model both
sized_sentinel_for<S, I>
(24.3.4.8 [iterator.concept.sizedsentinel]) and
forward_iterator<I>
. However, E
is evaluated only once.
(1.4) — […]
span
's deduction-guide for built-in arrays doesn't workSection: 23.7.2.2.1 [span.overview] Status: C++20 Submitter: Stephan T. Lavavej Opened: 2020-01-08 Last modified: 2021-02-25
Priority: 0
View all other issues in [span.overview].
View all issues with C++20 status.
Discussion:
N4842 22.7.3.1 [span.overview] depicts:
template<class T, size_t N> span(T (&)[N]) -> span<T, N>;
This isn't constrained by 22.7.3.3 [span.deduct]. Then, 22.7.3.2 [span.cons]/10 specifies:
template<size_t N> constexpr span(element_type (&arr)[N]) noexcept; template<size_t N> constexpr span(array<value_type, N>& arr) noexcept; template<size_t N> constexpr span(const array<value_type, N>& arr) noexcept;Constraints:
extent == dynamic_extent || N == extent
istrue
, and
remove_pointer_t<decltype(data(arr))>(*)[]
is convertible toElementType(*)[]
.
Together, these cause CTAD to behave unexpectedly. Here's a minimal test case, reduced from libcxx's test suite:
C:\Temp>type span_ctad.cpp #include <stddef.h> #include <type_traits> inline constexpr size_t dynamic_extent = static_cast<size_t>(-1); template <typename T, size_t Extent = dynamic_extent> struct span { template <size_t Size> requires (Extent == dynamic_extent || Extent == Size) #ifdef WORKAROUND_WITH_TYPE_IDENTITY_T span(std::type_identity_t<T> (&)[Size]) {} #else span(T (&)[Size]) {} #endif }; template <typename T, size_t Extent> #ifdef WORKAROUND_WITH_REQUIRES_TRUE requires (true) #endif span(T (&)[Extent]) -> span<T, Extent>; int main() { int arr[] = {1,2,3}; span s{arr}; static_assert(std::is_same_v<decltype(s), span<int, 3>>, "CTAD should deduce span<int, 3>."); } C:\Temp>cl /EHsc /nologo /W4 /std:c++latest span_ctad.cpp span_ctad.cpp span_ctad.cpp(26): error C2338: CTAD should deduce span<int, 3>. C:\Temp>cl /EHsc /nologo /W4 /std:c++latest /DWORKAROUND_WITH_TYPE_IDENTITY_T span_ctad.cpp span_ctad.cpp C:\Temp>cl /EHsc /nologo /W4 /std:c++latest /DWORKAROUND_WITH_REQUIRES_TRUE span_ctad.cpp span_ctad.cpp C:\Temp>
(MSVC and GCC 10 demonstrate this behavior. Clang is currently affected by LLVM#44484.)
Usually, when there's an explicit deduction-guide, we can ignore any corresponding constructor, because the overload resolution tiebreaker 12.4.3 [over.match.best]/2.10 prefers deduction-guides. However, this is a mental shortcut only, and it's possible for guides generated from constructors to out-compete deduction-guides during CTAD. That's what's happening here. Specifically, the constructor is constrained, while the deduction-guide is not constrained. This activates the "more specialized" tiebreaker first (12.4.3 [over.match.best]/2.5 is considered before /2.10 for deduction-guides). That goes through 13.7.6.2 [temp.func.order]/2 and 13.5.4 [temp.constr.order] to prefer the more constrained overload. (In the test case, this results inspan<int, dynamic_extent>
being deduced. That's
because the constructor allows T
to be deduced to be int
. The constructor's Size
template parameter is deduced to be 3, but that's unrelated to the class's Extent
parameter.
Because Extent
has a default argument of dynamic_extent
, CTAD succeeds and deduces
span<int, dynamic_extent>
.)
There are at least two possible workarounds: we could alter the constructor to prevent it from
participating in CTAD, or we could constrain the deduction-guide, as depicted in the test case. Either
way, we should probably include a Note, following the precedent of 21.3.2.2 [string.cons]/12.
Note that there are also deduction-guides for span from std::array
. However, the constructors
take array<value_type, N>
with using value_type = remove_cv_t<ElementType>;
so that prevents the constructors from interfering with CTAD.
I'm currently proposing to alter the constructor from built-in arrays. An alternative resolution to
constrain the deduction-guide would look like: "Constraints: true
. [Note: This
affects class template argument deduction. — end note]"
[2020-01-25 Status set to Tentatively Ready after seven positive votes on the reflector.]
Proposed resolution:
This wording is relative to N4842.
Modify 23.7.2.2.1 [span.overview], class template span
synopsis, as indicated:
namespace std { template<class ElementType, size_t Extent = dynamic_extent> class span { public: […] // 23.7.2.2.2 [span.cons], constructors, copy, and assignment constexpr span() noexcept; […] template<size_t N> constexpr span(type_identity_t<element_type> (&arr)[N]) noexcept; […] }; […]
Modify 23.7.2.2.2 [span.cons] as indicated:
template<size_t N> constexpr span(type_identity_t<element_type> (&arr)[N]) noexcept; template<size_t N> constexpr span(array<value_type, N>& arr) noexcept; template<size_t N> constexpr span(const array<value_type, N>& arr) noexcept;-10- Constraints:
-11- Effects: Constructs a
(10.1) —
extent == dynamic_extent || N == extent
istrue
, and(10.2) —
remove_pointer_t<decltype(data(arr))>(*)[]
is convertible toElementType(*)[]
.span
that is a view over the supplied array. [Note:type_identity_t
affects class template argument deduction. — end note] -12- Postconditions:size() == N && data() == data(arr)
.
visit_format_arg
and make_format_args
are not hidden friendsSection: 28.5.8.1 [format.arg] Status: C++20 Submitter: Tim Song Opened: 2020-01-16 Last modified: 2021-02-25
Priority: 0
View all other issues in [format.arg].
View all issues with C++20 status.
Discussion:
After P1965R0, friend function and function template
declarations always introduce hidden friends under the new blanket wording in
16.4.6.6 [hidden.friends]. However, 28.5.8.1 [format.arg] contains
"exposition only" friend declarations of visit_format_arg
and make_format_args
,
and those are not intended to be hidden. The only reason to have these declarations in the first
place is because these function templates are specified using the exposition-only private data
members of basic_format_arg
, but that's unnecessary — for example,
shared_ptr
's constructors are not exposition-only friends of enable_shared_from_this
,
even though the former are shown as assigning to the latter's exposition-only weak_this
private data member (see 20.3.2.2.2 [util.smartptr.shared.const]p1).
[2020-02-01 Status set to Tentatively Ready after five positive votes on the reflector.]
Proposed resolution:
This wording is relative to N4842.
Edit 28.5.8.1 [format.arg], class template basic_format_arg
synopsis, as indicated:
namespace std { template<class Context> class basic_format_arg { […]template<class Visitor, class Ctx> friend auto visit_format_arg(Visitor&& vis, basic_format_arg<Ctx> arg); // exposition only template<class Ctx, class... Args> friend format-arg-store<Ctx, Args...> make_format_args(const Args&... args); // exposition only[…] }; }
vformat_to
should not try to deduce Out
twiceSection: 28.5.5 [format.functions] Status: C++20 Submitter: Tim Song Opened: 2020-01-16 Last modified: 2021-02-25
Priority: 0
View all other issues in [format.functions].
View all issues with C++20 status.
Discussion:
vformat_to
currently deduces Out
from its first and last arguments.
This requires its last argument's type to be a specialization of basic_format_args
,
which notably prevents the use of format-arg-store
arguments directly.
This is unnecessary: we should only deduce from the first argument.
[2020-02-01 Status set to Tentatively Ready after six positive votes on the reflector.]
Proposed resolution:
This wording is relative to N4842.
Edit 28.5.1 [format.syn], header <format>
synopsis, as indicated:
namespace std { […] template<class Out> Out vformat_to(Out out, string_view fmt, format_args_t<type_identity_t<Out>, char> args); template<class Out> Out vformat_to(Out out, wstring_view fmt, format_args_t<type_identity_t<Out>, wchar_t> args); template<class Out> Out vformat_to(Out out, const locale& loc, string_view fmt, format_args_t<type_identity_t<Out>, char> args); template<class Out> Out vformat_to(Out out, const locale& loc, wstring_view fmt, format_args_t<type_identity_t<Out>, wchar_t> args); […] }
Edit 28.5.5 [format.functions] p8 through p10 as indicated:
template<class Out, class... Args> Out format_to(Out out, string_view fmt, const Args&... args); template<class Out, class... Args> Out format_to(Out out, wstring_view fmt, const Args&... args);-8- Effects: Equivalent to:
using context = basic_format_context<Out, decltype(fmt)::value_type>; return vformat_to(out, fmt,{make_format_args<context>(args...)});template<class Out, class... Args> Out format_to(Out out, const locale& loc, string_view fmt, const Args&... args); template<class Out, class... Args> Out format_to(Out out, const locale& loc, wstring_view fmt, const Args&... args);-9- Effects: Equivalent to:
using context = basic_format_context<Out, decltype(fmt)::value_type>; return vformat_to(out, loc, fmt,{make_format_args<context>(args...)});template<class Out> Out vformat_to(Out out, string_view fmt, format_args_t<type_identity_t<Out>, char> args); template<class Out> Out vformat_to(Out out, wstring_view fmt, format_args_t<type_identity_t<Out>, wchar_t> args); template<class Out> Out vformat_to(Out out, const locale& loc, string_view fmt, format_args_t<type_identity_t<Out>, char> args); template<class Out> Out vformat_to(Out out, const locale& loc, wstring_view fmt, format_args_t<type_identity_t<Out>, wchar_t> args);-10- Let
charT
bedecltype(fmt)::value_type
.[…]
{to,from}_chars_result
and format_to_n_result
need the
"we really mean what we say" wordingSection: 28.2.1 [charconv.syn], 28.5.1 [format.syn] Status: C++20 Submitter: Tim Song Opened: 2020-01-16 Last modified: 2021-02-25
Priority: 0
View all other issues in [charconv.syn].
View all issues with C++20 status.
Discussion:
To ensure that to_chars_result
, from_chars_result
, and
format_to_n_result
can be used in structured bindings, they need the
special wording we use to negate the general library permission to add private
data members and bases.
[2020-02-01 Status set to Tentatively Ready after six positive votes on the reflector.]
Proposed resolution:
This wording is relative to N4842.
Add a paragraph at the end of 28.2.1 [charconv.syn] as follows:
-?- The types
to_chars_result
andfrom_chars_result
have the data members and special members specified above. They have no base classes or members other than those specified.
Add a paragraph at the end of 28.5.1 [format.syn] as follows:
-1- The class template
format_to_n_result
has the template parameters, data members, and special members specified above. It has no base classes or members other than those specified.
std::to_address
overload constexpr
Section: 20.2.4 [pointer.conversion] Status: C++20 Submitter: Billy O'Neal III Opened: 2020-01-14 Last modified: 2021-02-25
Priority: 0
View all other issues in [pointer.conversion].
View all issues with C++20 status.
Discussion:
While reviewing some interactions with P0653 +
P1006, Billy discovered that one of the overloads was
missing the constexpr
tag. This might be a typo or a missed merge interaction between
P0653 (which adds to_address
with the pointer overload being constexpr
) and
P1006 (which makes pointer_traits::pointer_to constexpr
). Mail was sent the LWG reflector,
and Glen Fernandes, the author of P0653, indicates that this might have been an oversight.
[2020-02-01 Status set to Tentatively Ready after seven positive votes on the reflector.]
Proposed resolution:
This wording is relative to N4842.
Modify 20.2.2 [memory.syn], header <memory>
synopsis, as indicated:
[…] // 20.2.4 [pointer.conversion], pointer conversion template<class T> constexpr T* to_address(T* p) noexcept; template<class Ptr> constexpr auto to_address(const Ptr& p) noexcept; […]
Modify 20.2.4 [pointer.conversion] as indicated:
template<class Ptr> constexpr auto to_address(const Ptr& p) noexcept;-3- Returns:
pointer_traits<Ptr>::to_address(p)
if that expression is well-formed (see 20.2.3.4 [pointer.traits.optmem]), otherwiseto_address(p.operator->())
.
decay
in viewable_range
should be remove_cvref
Section: 25.4.5 [range.refinements] Status: C++20 Submitter: Casey Carter Opened: 2020-01-14 Last modified: 2021-02-25
Priority: 0
View all other issues in [range.refinements].
View all issues with C++20 status.
Discussion:
The viewable_range concept is defined in 25.4.5 [range.refinements] as:
template<class T> concept viewable_range = range<T> && (safe_range<T> || view<decay_t<T>>);
Since neither pointer types, array types, nor function types model view
,
view<decay_t<T>>
here could simplified to view<remove_cvref_t<T>>
.
The use of decay_t
is an artifact of the Ranges TS being based on C++14 which didn't have
remove_cvref_t
. [Note that the proposed change is not purely editorial since the difference
is observable to subsumption.]
[2020-02-01 Status set to Tentatively Ready after five positive votes on the reflector.]
Previous resolution [SUPERSEDED]:
This wording is relative to N4842.
Modify 25.4.5 [range.refinements] as indicated:
-4- The
viewable_range
concept specifies the requirements of arange
type that can be converted to aview
safely.template<class T> concept viewable_range = range<T> && (safe_range<T> || view<decay_tremove_cvref<T>>);
[2020-02-06 Casey provides a corrected P/R]
... in response to Jonathan's observation that remove_cvref<T>
is both the wrong type and not what the
discussion argues for.
[2020-02 Status to Immediate on Thursday morning in Prague.]
Proposed resolution:
This wording is relative to N4849.
Modify 25.4.5 [range.refinements] as indicated:
-4- The
viewable_range
concept specifies the requirements of arange
type that can be converted to aview
safely.template<class T> concept viewable_range = range<T> && (safe_range<T> || view<decay_tremove_cvref_t<T>>);
Section: 24.3.4.4 [iterator.concept.winc] Status: Resolved Submitter: Jonathan Wakely Opened: 2020-01-16 Last modified: 2021-10-23
Priority: 3
View all other issues in [iterator.concept.winc].
View all issues with Resolved status.
Discussion:
24.3.4.4 [iterator.concept.winc] says:
A type
I
is an integer-class type if it is in a set of implementation-defined class types that behave as integer types do, as defined in below.
and
A type
I
is integer-like if it modelsintegral<I>
or if it is an integer-class type.
Some implementations support built-in integer types that do not necessarily model std::integral
,
e.g. with libstdc++ whether std::is_integral_v<__int128>
is true
depends whether
"strict" or "extensions" mode is in use. Because __int128
is not a class type, it can't be used
as an integer-like type in strict mode (which effectively means it can't be used at all, to avoid unwanted
ABI differences between modes).
[2020-02-08 Issue Prioritization]
Priority to 3 after reflector discussion.
[2021-10-23 Resolved by the adoption of P2393R1 at the October 2021 plenary. Status changed: New → Resolved.]
Proposed resolution:
elements_view::iterator
befriends a specialization of itselfSection: 25.7.23.3 [range.elements.iterator] Status: C++20 Submitter: Casey Carter Opened: 2020-01-18 Last modified: 2021-02-25
Priority: 0
View other active issues in [range.elements.iterator].
View all other issues in [range.elements.iterator].
View all issues with C++20 status.
Discussion:
The synopsis of the exposition-only class template elements_view::iterator
in
25.7.23.3 [range.elements.iterator] includes the declaration "friend iterator<!Const>;
".
We typically don't depict such friend relationships in the Library specification, leaving the
choice of how to implement access to private data from external sources to implementer magic.
For consistency, we should strike this occurrence from elements_view::iterator
.
[2020-02-08 Status set to Tentatively Ready after ten positive votes on the reflector.]
Proposed resolution:
This wording is relative to N4842.
Modify 25.7.23.3 [range.elements.iterator], class template elements_view::iterator
synopsis, as indicated:
namespace std::ranges { template<class V, size_t N> template<bool Const> class elements_view<V, N>::iterator { // exposition only using base_t = conditional_t<Const, const V, V>;friend iterator<!Const>;iterator_t<base_t> current_; public: […] }; […] }
safe
" in several library names is misleadingSection: 25.2 [ranges.syn] Status: C++20 Submitter: Casey Carter Opened: 2020-01-21 Last modified: 2021-02-25
Priority: 1
View all other issues in [ranges.syn].
View all issues with C++20 status.
Discussion:
Various WG21 members have commented that the use of "safe" in the names safe_range
,
enable_safe_range
, safe_iterator_t
, and safe_subrange_t
to mean "fairly
unlikely to produce dangling iterators" is inappropriate. The term "safe" has very strong
connotations in some application domains, and these names don't intend such connotations. We
could avoid confusion by changing these names to avoid the use of "safe". The Ranges Illuminati
has deemed "borrowed" to be more appropriate: it reflects the fact that such ranges often
"borrow" iterators from an adapted range which allows them to not be lifetime-bound to the
adapting range.
[2020-02-08 Issue Prioritization]
Priority to 1 after reflector discussion. This issue needs to be looked at by LEWG.
[2020-02-13, Prague]
Set to Immediate.
Proposed resolution:
This wording is relative to N4849.
Replace all occurrences of safe_range
, enable_safe_range
, safe_iterator_t
,
and safe_subrange_t
in the working draft with borrowed_range
, enable_borrowed_range
, borrowed_iterator_t
, and borrowed_subrange_t
.
common_type
and comparison categoriesSection: 21.3.8.7 [meta.trans.other] Status: C++20 Submitter: Casey Carter Opened: 2020-01-23 Last modified: 2021-02-25
Priority: 0
View all other issues in [meta.trans.other].
View all issues with C++20 status.
Discussion:
There are two paragraphs in the the definition of common_type
:
Otherwise, if both D1
and D2
denote comparison category types
(17.12.2.1 [cmp.categories.pre]), let C
denote the common comparison type
(11.10.3 [class.spaceship]) of D1
and D2
.
Otherwise, if decay_t<decltype(false ? declval<D1>() : declval<D2>())>
denotes a valid type, let C
denote that type.
P1614R2 added the first bullet so that
common_type_t<strong_equality, T>
would be the same type as
common_comparison_category_t<strong_equality, T>
; other cases are correctly handled
by the second (pre-existing) bullet. After application of P1959R0
in Belfast, std::strong_equality
is no more. We can now strike the first bullet without
changing the behavior of common_type
.
[2020-02-08 Status set to Tentatively Ready after seven positive votes on the reflector.]
Proposed resolution:
This wording is relative to N4849.
Modify 21.3.8.7 [meta.trans.other] as indicated:
-3- Note A: For the
common_type
trait applied to a template parameter packT
of types, the membertype
shall be either defined or not present as follows:
(3.1) — […]
[…]
(3.3) — If
sizeof...(T)
is two, let the first and second types constitutingT
be denoted byT1
andT2
, respectively, and letD1
andD2
denote the same types asdecay_t<T1>
anddecay_t<T2>
, respectively.
(3.3.1) — If
is_same_v<T1, D1>
isfalse
oris_same_v<T2, D2>
isfalse
, letC
denote the same type, if any, ascommon_type_t<D1, D2>
.(3.3.2) — [Note: None of the following will apply if there is a specialization
common_type<D1, D2>
. — end note]
(3.3.3) — Otherwise, if bothD1
andD2
denote comparison category types (17.12.2.1 [cmp.categories.pre]), letC
denote the common comparison type (11.10.3 [class.spaceship]) ofD1
andD2
.(3.3.4) — Otherwise, if
decay_t<decltype(false ? declval<D1>() : declval<D2>())>denotes a valid type, let
C
denote that type.(3.3.5) — […]
[…]
begin
and data
must agree for contiguous_range
Section: 25.4.5 [range.refinements] Status: C++20 Submitter: Casey Carter Opened: 2020-01-25 Last modified: 2021-02-25
Priority: 0
View all other issues in [range.refinements].
View all issues with C++20 status.
Discussion:
The definition of the contiguous_range
concept in 25.4.5 [range.refinements]/2
requires that ranges::data(r)
be valid for a contiguous_range r
, but fails to
impose the obvious semantic requirement that to_address(ranges::begin(r)) == ranges::data(r)
.
In other words, data
and begin
must agree so that [begin(r), end(r))
and
the counted range data(r) + [0, size(r))
(this is the new "counted range" specification
syntax per working draft issue 2932) denote
the same sequence of elements.
[2020-02 Prioritized as IMMEDIATE Monday morning in Prague]
Proposed resolution:
This wording is relative to N4849.
Modify 25.4.5 [range.refinements] as indicated:
-2-
contiguous_range
additionally requires that theranges::data
customization point (25.3.15 [range.prim.data]) is usable with the range.template<class T> concept contiguous_range = random_access_range<T> && contiguous_iterator<iterator_t<T>> && requires(T& t) { { ranges::data(t) } -> same_as<add_pointer_t<range_reference_t<T>>>; };-?- Given an expression
-3- Thet
such thatdecltype((t))
isT&
,T
modelscontiguous_range
only if(to_address(ranges::begin(t)) == ranges::data(t))
.common_range
concept […]
pair
and array
Section: 22.3.2 [pairs.pair], 23.3.3 [array] Status: C++20 Submitter: Barry Revzin Opened: 2020-01-27 Last modified: 2021-02-25
Priority: 2
View other active issues in [pairs.pair].
View all other issues in [pairs.pair].
View all issues with C++20 status.
Discussion:
We had this NB ballot issue, to ensure that
std::array
could be a NTTP. But after P1907, we still need
some kind of wording to ensure that std::array
(and also std::pair
) have no extra
private members or base classes.
The class template
pair
/array
has the data members specified above. It has no base classes or data members other than those specified.
[2020-02 Prioritized as P2 Monday morning in Prague]
Previous resolution [SUPERSEDED]:
This wording is relative to N4849.
Modify 22.3.2 [pairs.pair] as indicated:
-1- Constructors and member functions of
-2- The defaulted move and copy constructor, respectively, ofpair
do not throw exceptions unless one of the element-wise operations specified to be called for that operation throws an exception.pair
is a constexpr function if and only if all required element-wise initializations for copy and move, respectively, would satisfy the requirements for a constexpr function. -3- If(is_trivially_destructible_v<T1> && is_trivially_destructible_v<T2>)
istrue
, then the destructor ofpair
is trivial. -?- The class templatepair
has the data members specified above. It has no base classes or data members other than those specified.Modify 23.3.3.1 [array.overview] as indicated:
-1- The header
-2- An<array>
defines a class template for storing fixed-size sequences of objects. Anarray
is a contiguous container (23.2.2 [container.requirements.general]). An instance ofarray<T, N>
storesN
elements of typeT
, so thatsize() == N
is an invariant.array
is an aggregate (9.5.2 [dcl.init.aggr]) that can be list-initialized with up toN
elements whose types are convertible toT
. -3- Anarray
meets all of the requirements of a container and of a reversible container (23.2 [container.requirements]), except that a default constructedarray
object is not empty and thatswap
does not have constant complexity. Anarray
meets some of the requirements of a sequence container (23.2.4 [sequence.reqmts]). Descriptions are provided here only for operations onarray
that are not described in one of these tables and for operations where there is additional semantic information. -?- The class templatearray
has the data members specified in subclauses 23.3.3.1 [array.overview] and 23.3.3.5 [array.zero]. It has no base classes or data members other than those specified. -4- […]
[2020-02-13, Prague]
Tim Song and Tomasz were trying to come up with general wording that could be reused for both pair
and
array
(and other types). They suggest that if it should be in scope for C++20, it would be better to
provide non-general wording for pair
and array
(that is easier to get right).
The type
T
is structurally compatible withsubs
, if for the valuest1
andt2
of typeT
:
T
is a structural type (13.2 [temp.param]) if the types of subobjects oft1
designated bysubs
are all structural types.
t1
is template-argument-equivalent (13.6 [temp.type]) tot2
, if and only if, for each subject designed bysubs
, the value of subobject oft1
is template-argument-equivalent to the value of the correponding subobject oft2
.
Then changes for array
/pair
would then look like:
pair<T, U>
is structurally compatible (<some-reference>) withfirst
andsecond
.array<T, N>
is structurally compatible with its elements (if any).
[2020-02 Status to Immediate on Friday morning in Prague.]
Proposed resolution:
This wording is relative to N4849.
Modify 22.3.2 [pairs.pair] as indicated:
-1- Constructors and member functions of
-2- The defaulted move and copy constructor, respectively, ofpair
do not throw exceptions unless one of the element-wise operations specified to be called for that operation throws an exception.pair
is a constexpr function if and only if all required element-wise initializations for copy and move, respectively, would satisfy the requirements for a constexpr function. -3- If(is_trivially_destructible_v<T1> && is_trivially_destructible_v<T2>)
istrue
, then the destructor ofpair
is trivial. -?-pair<T, U>
is a structural type (13.2 [temp.param]) ifT
andU
are both structural types. Two valuesp1
andp2
of typepair<T, U>
are template-argument-equivalent (13.6 [temp.type]) if and only ifp1.first
andp2.first
are template-argument-equivalent andp1.second
andp2.second
are template-argument-equivalent.
Modify 23.3.3.1 [array.overview] as indicated:
-1- The header
-2- An<array>
defines a class template for storing fixed-size sequences of objects. Anarray
is a contiguous container (23.2.2 [container.requirements.general]). An instance ofarray<T, N>
storesN
elements of typeT
, so thatsize() == N
is an invariant.array
is an aggregate (9.5.2 [dcl.init.aggr]) that can be list-initialized with up toN
elements whose types are convertible toT
. -3- Anarray
meets all of the requirements of a container and of a reversible container (23.2 [container.requirements]), except that a default constructedarray
object is not empty and thatswap
does not have constant complexity. Anarray
meets some of the requirements of a sequence container (23.2.4 [sequence.reqmts]). Descriptions are provided here only for operations onarray
that are not described in one of these tables and for operations where there is additional semantic information. -?-array<T, N>
is a structural type (13.2 [temp.param]) ifT
is a structural type. Two valuesa1
anda2
of typearray<T, N>
are template-argument-equivalent (13.6 [temp.type]) if and only if each pair of corresponding elements ina1
anda2
are template-argument-equivalent. -4- […]
sys_seconds
should be replaced with seconds
Section: 30.11.8.3 [time.zone.leap.nonmembers] Status: C++20 Submitter: Jiang An Opened: 2020-01-30 Last modified: 2021-02-25
Priority: 1
View all issues with C++20 status.
Discussion:
In N4849 30.11.8.3 [time.zone.leap.nonmembers]/12, the type
template parameter Duration
is constrained by three_way_comparable_with<sys_seconds>
.
However, since std::chrono::sys_seconds
is a time point type and Duration
must be a
duration type, they can never be compared directly via operator<=>
.
Duration
with the duration type of
std::chrono::sys_seconds
, i.e. std::chrono::seconds
. And thus sys_seconds
should be replaced with seconds
here.
[2020-02 Prioritized as P1 Monday morning in Prague]
Previous resolution [SUPERSEDED]:This wording is relative to N4849.
Modify 30.2 [time.syn], header
<chrono>
synopsis, as indicated:namespace std { […] namespace chrono { […] template<three_way_comparable_with<sys_seconds> Duration> auto operator<=>(const leap& x, const sys_time<Duration>& y); […] } […] }Modify 30.11.8.3 [time.zone.leap.nonmembers] as indicated:
template<three_way_comparable_with<sys_seconds> Duration> constexpr auto operator<=>(const leap& x, const sys_time<Duration>& y) noexcept;-12- Returns:
x.date() <=> y
.
[2020-02-10, Prague; Howard suggests alternative wording]
The below shown alternative wording does more directly describe the constrained code (by comparing
time_point
s), even though in the very end the code specifying the effects finally goes down
to actually return x.date().time_since_epoch() <=> y.time_since_epoch()
(thus comparing
duration
s).
This wording is relative to N4849.
Modify 30.2 [time.syn], header
<chrono>
synopsis, as indicated:The synopsis does provide an additional drive-by fix to eliminate the mismatch of the
constexpr
andnoexcept
in declaration and prototype specification.namespace std { […] namespace chrono { […] template<three_way_comparable_with<sys_seconds>class Duration> requires three_way_comparable_with<sys_seconds, sys_time<Duration>> constexpr auto operator<=>(const leap& x, const sys_time<Duration>& y) noexcept; […] } […] }Modify 30.11.8.3 [time.zone.leap.nonmembers] as indicated:
template<three_way_comparable_with<sys_seconds>class Duration> requires three_way_comparable_with<sys_seconds, sys_time<Duration>> constexpr auto operator<=>(const leap& x, const sys_time<Duration>& y) noexcept;-12- Returns:
x.date() <=> y
.
[2020-02-11, Prague; Daniel suggests alternative wording]
During today's LWG discussion of this issue the observation was made that there also exists a
mismatch regarding the noexcept
specifier for both declarations, but for this second deviation
a corresponding change does not seem to be a good drive-by fix candidate, because we have a
function template here that allows supporting user-defined types, whose comparison may throw (Note
that the corresponding operator<=>
or other comparison function declarations of the
duration
and time_point
templates are not specified as noexcept
function templates). The revised wording below does therefore intentionally not change the
currently existing noexcept
-specifier mismatch, but a separate issue should instead be
opened for the general noexcept
-specifier mismatches for all comparison function templates
of std::chrono::leap
. Daniel has volunteered to take care for this issue.
[2020-02 Moved to Immediate on Tuesday in Prague.]
Proposed resolution:
This wording is relative to N4849.
Modify 30.2 [time.syn], header <chrono>
synopsis, as indicated:
[Drafting note: The synopsis does provide an additional drive-by fix to eliminate the mismatch of the
constexpr
in declaration and prototype specification, but does not so for a similar mismatch of the exception-specifications of both declarations.]
namespace std { […] namespace chrono { […] template<three_way_comparable_with<sys_seconds>class Duration> requires three_way_comparable_with<sys_seconds, sys_time<Duration>> constexpr auto operator<=>(const leap& x, const sys_time<Duration>& y); […] } […] }
Modify 30.11.8.3 [time.zone.leap.nonmembers] as indicated:
template<three_way_comparable_with<sys_seconds>class Duration> requires three_way_comparable_with<sys_seconds, sys_time<Duration>> constexpr auto operator<=>(const leap& x, const sys_time<Duration>& y) noexcept;-12- Returns:
x.date() <=> y
.
transform_view::sentinel
has an incorrect operator-
Section: 25.7.9.4 [range.transform.sentinel] Status: C++20 Submitter: Ville Voutilainen Opened: 2020-01-31 Last modified: 2021-02-25
Priority: 0
View all other issues in [range.transform.sentinel].
View all issues with C++20 status.
Discussion:
transform_view::iterator
has an exposition-only member current_
(25.7.9.3 [range.transform.iterator])
transform_view::sentinel
has an exposition-only member end_
(25.7.9.4 [range.transform.sentinel])
at 25.7.9.4 [range.transform.sentinel]/6 we have:
friend constexpr range_difference_t<Base> operator-(const sentinel& y, const iterator<Const>& x) requires sized_sentinel_for<sentinel_t<Base>, iterator_t<Base>>;
Effects: Equivalent to:
return x.end_ - y.current_;
x
is an iterator
, so it has current_
, not end_
.
y
is a sentinel
, so it has end_
, not current_
.
[2020-02 Prioritized as IMMEDIATE Monday morning in Prague]
Proposed resolution:
This wording is relative to N4849.
Modify 25.7.9.4 [range.transform.sentinel] as indicated:
friend constexpr range_difference_t<Base> operator-(const sentinel& y, const iterator<Const>& x) requires sized_sentinel_for<sentinel_t<Base>, iterator_t<Base>>;-6- Effects: Equivalent to:
return
xy.end_ -yx.current_;
common_iterator
is not sufficiently constrained for non-copyable iteratorsSection: 24.5.5.1 [common.iterator] Status: C++20 Submitter: Corentin Jabot Opened: 2020-01-31 Last modified: 2021-02-25
Priority: 0
View other active issues in [common.iterator].
View all other issues in [common.iterator].
View all issues with C++20 status.
Discussion:
We don't actually specify anywhere that non-copyable iterators do not have a common_iterator
(and it would make little sense for them to as algorithms dealing with C++17 iterators are not expecting
non-copyable things) As it stands common_iterator
can be created from move only iterator but
are then non-copyable themselves. P1862 already constrains
common_view
in a similar fashion
[2020-02 Prioritized as IMMEDIATE Monday morning in Prague]
Proposed resolution:
This wording is relative to N4849.
Modify 24.2 [iterator.synopsis], header <iterator>
synopsis, as indicated:
namespace std { […] // 24.5.5 [iterators.common], common iterators template<input_or_output_iterator I, sentinel_for<I> S> requires (!same_as<I, S> && copyable<I>) class common_iterator; […] }
Modify 24.5.5.1 [common.iterator], class template common_iterator
synopsis, as indicated:
namespace std { template<input_or_output_iterator I, sentinel_for<I> S> requires (!same_as<I, S> && copyable<I>) class common_iterator { public: […] }; […] }
elements_view
needs its own sentinel
typeSection: 25.7.23 [range.elements] Status: C++20 Submitter: Tim Song Opened: 2020-02-07 Last modified: 2021-02-25
Priority: 1
View all issues with C++20 status.
Discussion:
elements_view
is effectively a specialized version of transform_view
.
The latter has a custom sentinel
type, and so should elements_view
.
[i, s)
whose value_type
is pair<array<int, 2>, long>
,
where s
is a generic sentinel that checks if the second element (for this range in particular,
the long
) is zero:
struct S { friend bool operator==(input_iterator auto const& i, S) /* additional constraints */ { return get<1>(*i) == 0; } };
If we adapt [i, s)
with views::keys
, then the resulting adapted range would have
surprising behavior when used with S{}
: even though it's nominally a range of
array<int, 2>
, when its iterator is used with the sentinel S{}
it doesn't actually check the second element of the array, but the long
that's not even
part of the value_type
:
void algo(input_range auto&& r) /* constraints */{ // We want to stop at the first element of the range r whose second element is zero. for (auto&& x : subrange{ranges::begin(r), S{}}) { std::cout << get<0>(x); } } using P = pair<array<int, 2>, long>; vector<P> vec = { { {0, 1}, 1L }, { {1, 0}, 1L }, { {2, 2}, 0L } }; subrange r{vec.begin(), S{}}; // range with two elements: {0, 1}, {1, 0} algo(r | views::keys); // checks the long, prints '01' algo(r | views::transform(&P::first)); // checks the second element of the array, prints '0'
This is an API break since it changes the return type of end()
, so it should be
fixed before we ship C++20.
[2020-02 Prioritized as P1 Monday morning in Prague]
[2020-06-11 Voted into the WP in Prague. Status changed: New → WP.]
Proposed resolution:
The proposed wording is contained in P1994R0.
reverse_view<V>
unintentionally requires range<const V>
Section: 25.7.21.2 [range.reverse.view] Status: C++20 Submitter: Patrick Palka Opened: 2020-02-04 Last modified: 2021-02-25
Priority: 0
View all other issues in [range.reverse.view].
View all issues with C++20 status.
Discussion:
reverse_view<V>
requires bidirectional_range<V>
, but not range<const V>
,
which means that iterator_t<const V>
might be an invalid type. The return types of the
begin() const
and end() const
overloads make use of iterator_t<const V>
in a non-SFINAE context, which means that instantiating reverse_view<X>
is ill-formed unless
range<const X>
is satisfied.
x | views::filter(p) | views::reverse
fails to compile because
const filter_view<…>
does not model range
, so
iterator_t<const filter_view<…>>
is invalid.
Either range<const V>
needs to be in the class' requires
-clause, or the return types
of the const
-qualified begin()
and end()
need to delay use of
iterator_t<const V>
until range<const V>
is known to be satisfied.
Giving these overloads an auto
return type means the type is determined when the member is called.
The constraint common_range<const V>
appropriately restricts the selection of these overloads,
so they can only be called when the type is valid. This is what cmcstl2
does. range-v3
makes the begin() const
and end() const
members into function templates, so that they
are SFINAE contexts.
This is related to 3347(i).
[2020-02 Prioritized as IMMEDIATE Monday morning in Prague]
Proposed resolution:
This wording is relative to N4849.
Modify 25.7.21.2 [range.reverse.view] as indicated:
namespace std::ranges { template<view V> requires bidirectional_range<V> class reverse_view : public view_interface<reverse_view<V>> { […] constexpr reverse_iterator<iterator_t<V>> begin(); constexpr reverse_iterator<iterator_t<V>> begin() requires common_range<V>; constexpr[…]reverse_iterator<iterator_t<const V>>auto begin() const requires common_range<const V>; constexpr reverse_iterator<iterator_t<V>> end(); constexprreverse_iterator<iterator_t<const V>>auto end() const requires common_range<const V>; […] }; […] }constexpr reverse_iterator<iterator_t<V>> begin() requires common_range<V>; constexprreverse_iterator<iterator_t<const V>>auto begin() const requires common_range<const V>;-5- Effects: Equivalent to:
return make_reverse_iterator(ranges::end(base_));
constexpr reverse_iterator<iterator_t<V>> end(); constexprreverse_iterator<iterator_t<const V>>auto end() const requires common_range<const V>;-6- Effects: Equivalent to:
return make_reverse_iterator(ranges::begin(base_));
view
iterator types have ill-formed <=>
operatorsSection: 25.6.4.3 [range.iota.iterator], 25.7.9.3 [range.transform.iterator], 25.7.23.3 [range.elements.iterator] Status: C++20 Submitter: Jonathan Wakely Opened: 2020-02-07 Last modified: 2021-02-25
Priority: 0
View other active issues in [range.iota.iterator].
View all other issues in [range.iota.iterator].
View all issues with C++20 status.
Discussion:
25.6.4.3 [range.iota.iterator] and 25.7.9.3 [range.transform.iterator] and
25.7.23.3 [range.elements.iterator] all declare operator<=>
similar to this:
friend constexpr compare_three_way_result_t<W> operator<=>( const iterator& x, const iterator& y) requires totally_ordered<W> && three_way_comparable<W>;
Similar to issue 3347(i) and issue 3387(i), this is ill-formed if
three_way_comparable<W>
is not satisfied, because compare_three_way_result_t<W>
is invalid. This declaration is instantiated when the enclosing iterator type is instantiated, making
any use of iota_view<W, B>::iterator
ill-formed when three_way_comparable<W>
is not satisfied.
safe-compare-three-way-result-t
alias that denotes
void
or std::nonesuch
for spaceship-less types, so the declaration is valid (and then
disabled by the constraints), or simply make them return auto
.
[2020-02 Prioritized as IMMEDIATE Monday morning in Prague]
Proposed resolution:
This wording is relative to N4849.
Modify 25.6.4.3 [range.iota.iterator] as indicated:
namespace std::ranges { template<class W, class Bound> struct iota_view<W, Bound>::iterator { […] friend constexpr[…]compare_three_way_result_t<W>auto operator<=>( const iterator& x, const iterator& y) requires totally_ordered<W> && three_way_comparable<W>; […] }; […] }friend constexprcompare_three_way_result_t<W>auto operator<=>(const iterator& x, const iterator& y) requires totally_ordered<W> && three_way_comparable<W>;-19- Effects: Equivalent to:
return x.value_ <=> y.value_;
Modify 25.7.9.3 [range.transform.iterator] as indicated:
namespace std::ranges { template<class V, class F> template<bool Const> class transform_view<V, F>::iterator { […] friend constexpr[…]compare_three_way_result_t<iterator_t<Base>>auto operator<=>(const iterator& x, const iterator& y) requires random_access_range<Base> && three_way_comparable<iterator_t<Base>>; […] }; […] }friend constexprcompare_three_way_result_t<iterator_t<Base>>auto operator<=>(const iterator& x, const iterator& y) requires random_access_range<Base> && three_way_comparable<iterator_t<Base>>;-19- Effects: Equivalent to:
return x.current_ <=> y.current_;
Modify 25.7.23.3 [range.elements.iterator] as indicated:
namespace std::ranges { template<class V, size_t N> template<bool Const> class elements_view<V, N>::iterator { […] friend constexpr[…]compare_three_way_result_t<iterator_t<base-t>>auto operator<=>(const iterator& x, const iterator& y) requires random_access_range<base-t> && three_way_comparable<iterator_t<base-t>>; […] }; […] }friend constexprcompare_three_way_result_t<iterator_t<base-t>>auto operator<=>(const iterator& x, const iterator& y) requires random_access_range<base-t> && three_way_comparable<iterator_t<base-t>>;-18- Effects: Equivalent to:
return x.current_ <=> y.current_;
counted_iterator
Section: 24.5.7.2 [counted.iter.const] Status: C++20 Submitter: Patrick Palka Opened: 2020-02-07 Last modified: 2021-02-25
Priority: 0
View all issues with C++20 status.
Discussion:
P1207R4 ("Movability of single-pass iterators")
introduces the notion of a move-only non-forward iterator and makes some changes to
the iterator adaptor counted_iterator
in order to support move-only iterators.
counted_iterator
(24.5.7.2 [counted.iter.const] p2)
accepting such an iterator is specified as "Initializes current
with i
"
which would attempt copy-constructing current
from i
instead of move-constructing it.
[2020-02 Prioritized as IMMEDIATE Monday morning in Prague]
Proposed resolution:
This wording is relative to N4849.
Modify 24.5.7.2 [counted.iter.const] as indicated:
constexpr counted_iterator(I i, iter_difference_t<I> n);-1- Preconditions:
-2- Effects: Initializesn >= 0
.current
withstd::move(i)
andlength
withn
.
make_move_iterator()
cannot be used to construct a move_iterator
for a
move-only iteratorSection: 24.5.4.9 [move.iter.nonmember] Status: C++20 Submitter: Patrick Palka Opened: 2020-02-07 Last modified: 2021-02-25
Priority: 0
View all other issues in [move.iter.nonmember].
View all issues with C++20 status.
Discussion:
P1207R4 ("Movability of single-pass iterators") introduces the notion of a move-only non-forward iterator and makes some changes to the existing specification to realize that support.
The problem is that the specification ofmake_move_iterator()
provided in
24.5.4.9 [move.iter.nonmember] p6 does attempt to construct a
move_iterator<Iterator>
with an lvalue of i
instead
of an rvalue, having the effect of copying it instead of moving it, thus preventing
to accept move-only iterators.
[2020-02 Prioritized as IMMEDIATE Monday morning in Prague]
Proposed resolution:
This wording is relative to N4849.
Modify 24.5.4.9 [move.iter.nonmember] as indicated:
template<class Iterator> constexpr move_iterator<Iterator> make_move_iterator(Iterator i);-6- Returns:
move_iterator<Iterator>(std::move(i))
.
counted_iterator/move_iterator::base() const &
Section: 24.5.4 [move.iterators], 24.5.7 [iterators.counted] Status: C++23 Submitter: Patrick Palka Opened: 2020-02-07 Last modified: 2023-11-22
Priority: 2
View all other issues in [move.iterators].
View all issues with C++23 status.
Discussion:
It is not possible to use the const &
overloads of counted_iterator::base()
or
move_iterator::base()
to get at an underlying move-only iterator in order to compare it
against a sentinel.
auto v = r | views::take(5); ranges::begin(v) == ranges::end(v);
is invalid when r
is a view
whose begin()
is a move-only input iterator.
The code is invalid because ranges::take_view::sentinel::operator==()
must call
counted_iterator::base()
to compare the underlying iterator against its sentinel, and
therefore this operator==()
requires that the underlying iterator is copy_constructible
.
const & base()
overloads return the underlying iterator by const
reference.
Remove the copy_constructible
constraint on these overloads. Perhaps the base()
overloads for the iterator wrappers in 25.7 [range.adaptors] could use the same treatment?
[2020-02 Prioritized as P2 Monday morning in Prague]
[2021-01-28; Reflector poll]
Set status to Tentatively Ready after five votes in favour during reflector poll.
[2021-02-26 Approved at February 2021 virtual plenary. Status changed: Tentatively Ready → WP.]
Proposed resolution:
This wording is relative to N4849.
Modify 24.5.4.2 [move.iterator], class template move_iterator
synopsis, as indicated:
namespace std { template<class Iterator> class move_iterator { public: using iterator_type = Iterator; […] constexpr const iterator_type& base() const &; constexpr iterator_type base() &&; […] }; }
Modify 24.5.4.5 [move.iter.op.conv] as indicated:
constexpr const Iterator& base() const &;
-1- Constraints:Iterator
satisfiescopy_constructible
.-2- Preconditions:-3- Returns:Iterator
modelscopy_constructible
.current
.
Modify 24.5.7.1 [counted.iterator], class template counted_iterator
synopsis, as indicated:
namespace std { template<input_or_output_iterator I> class counted_iterator { public: using iterator_type = I; […] constexpr const I& base() const &requires copy_constructible<I>; constexpr I base() &&; […] }; }
Modify 24.5.7.3 [counted.iter.access] as indicated:
constexpr const I& base() const &requires copy_constructible<I>;-1- Effects: Equivalent to:
return current;
ranges::distance()
cannot be used on a move-only iterator with a sized sentinelSection: 24.4.4.3 [range.iter.op.distance] Status: C++23 Submitter: Patrick Palka Opened: 2020-02-07 Last modified: 2023-11-22
Priority: 3
View all other issues in [range.iter.op.distance].
View all issues with C++23 status.
Discussion:
One cannot use ranges::distance(I, S)
to compute the distance between a
move-only counted_iterator
and the default_sentinel
. In other words,
the following is invalid
// iter is a counted_iterator with an move-only underlying iterator ranges::distance(iter, default_sentinel);
and yet
(default_sentinel - iter);
is valid. The first example is invalid because ranges::distance()
takes
its first argument by value so when invoking it with an iterator lvalue
argument the iterator must be copyable, which a move-only iterator is
not. The second example is valid because counted_iterator::operator-()
takes its iterator argument by const
reference, so it doesn't require
copyability of the counted_iterator
.
ranges::distance()
to efficiently compute the distance between two
iterators or between an iterator-sentinel pair. Although it's a bit of
an edge case, it would be good if ranges::distance()
does the right
thing when the iterator is a move-only lvalue with a sized sentinel.
If this is worth fixing, one solution might be to define a separate
overload of ranges::distance(I, S)
that takes its arguments by const
reference, as follows.
[2020-02 Prioritized as P3 and LEWG Monday morning in Prague]
[2020-05-28; LEWG issue reviewing]
LEWG issue processing voted to accept the direction of 3392. Status change to Open.
Accept the direction of LWG3392 SF F N A SA 14 6 0 0 0
Previous resolution [SUPERSEDED]:
Modify 24.2 [iterator.synopsis], header
<iterator>
synopsis, as indicated:#include <concepts> namespace std { […] // 24.4.4 [range.iter.ops], range iterator operations namespace ranges { […] // 24.4.4.3 [range.iter.op.distance], ranges::distance template<input_or_output_iterator I, sentinel_for<I> S> requires (!sized_sentinel_for<S, I>) constexpr iter_difference_t<I> distance(I first, S last); template<input_or_output_iterator I, sized_sentinel_for<I> S> constexpr iter_difference_t<I> distance(const I& first, const S& last); template<range R> constexpr range_difference_t<R> distance(R&& r); […] } […] }Modify 24.4.4.3 [range.iter.op.distance] as indicated:
template<input_or_output_iterator I, sentinel_for<I> S> requires (!sized_sentinel_for<S, I>) constexpr iter_difference_t<I> ranges::distance(I first, S last);-1- Preconditions:
-2- Effects:[first, last)
denotes a range, or.[last, first)
denotes a range andS
andI
modelsame_as<S, I> && sized_sentinel_for<S, I>
IfReturns the number of increments needed to get fromS
andI
modelsized_sentinel_for<S, I>
, returns(last - first)
; otherwise, rfirst
tolast
.template<input_or_output_iterator I, sized_sentinel_for<I> S> constexpr iter_difference_t<I> ranges::distance(const I& first, const S& last);-?- Preconditions:
S
andI
modelsized_sentinel_for<S, I>
and either:-? Effects: Returns
(?.1) —
[first, last)
denotes a range, or(?.2) —
[last, first)
denotes a range andS
andI
modelsame_as<S, I>
.(last - first)
;
[2021-05-19 Tim updates wording]
The wording below removes the explicit precondition on the sized_sentinel_for
overload of distance
, relying instead on the semantic requirements of
that concept and the "Effects: Equivalent to:" word of power. This also removes
the potentially surprising inconsistency that given a non-empty
std::vector<int> v
,
ranges::distance(v.begin(), v.cend())
is well-defined but
ranges::distance(v.cend(), v.begin())
is currently undefined.
[2021-06-23; Reflector poll]
Set status to Tentatively Ready after seven votes in favour during reflector poll.
[2021-10-14 Approved at October 2021 virtual plenary. Status changed: Voting → WP.]
Proposed resolution:
This wording is relative to N4885.
Modify 24.2 [iterator.synopsis], header <iterator>
synopsis, as indicated:
[…] namespace std { […] // 24.4.4 [range.iter.ops], range iterator operations namespace ranges { […] // 24.4.4.3 [range.iter.op.distance], ranges::distance template<input_or_output_iterator I, sentinel_for<I> S> requires (!sized_sentinel_for<S, I>) constexpr iter_difference_t<I> distance(I first, S last); template<input_or_output_iterator I, sized_sentinel_for<I> S> constexpr iter_difference_t<I> distance(const I& first, const S& last); template<range R> constexpr range_difference_t<R> distance(R&& r); […] } […] }
Modify 24.4.4.3 [range.iter.op.distance] as indicated:
template<input_or_output_iterator I, sentinel_for<I> S> requires (!sized_sentinel_for<S, I>) constexpr iter_difference_t<I> ranges::distance(I first, S last);-1- Preconditions:
-2-[first, last)
denotes a range, or.[last, first)
denotes a range andS
andI
modelsame_as<S, I> && sized_sentinel_for<S, I>
Effects: IfReturns: The number of increments needed to get fromS
andI
modelsized_sentinel_for<S, I>
, returns(last - first)
; otherwise, returns thefirst
tolast
.template<input_or_output_iterator I, sized_sentinel_for<I> S> constexpr iter_difference_t<I> ranges::distance(const I& first, const S& last);-?- Effects: Equivalent to
return last - first;
Section: 17.3.2 [version.syn] Status: C++20 Submitter: Barry Revzin Opened: 2020-01-25 Last modified: 2021-02-25
Priority: 0
View all other issues in [version.syn].
View all issues with C++20 status.
Discussion:
We have a policy, established in P1353 (and needing to be added to SD-6):
In some cases a feature requires two macros, one for the language and one for the library. For example, the library does not want to define its three-way comparison operations unless the compiler supports the feature. For end-users, it is suggested that they test only the library macro, as that will only be true if the language macro is also true. As a result, the language macros contain "impl" to distinguish them from the more general version that is expected to be set by the library. That paper added two papers of macros: one for<=>
and one for destroying delete
.
We have a third such example in coroutines: there is library machinery that needs to be provided only when
the compiler has language support for it, and the end user should just check the library macro.
Previous resolution [SUPERSEDED]:
This wording is relative to N4849.
Modify 15.12 [cpp.predefined], Table [tab:cpp.predefined.ft], as indicated:
Table 18: Feature-test macros [tab:cpp.predefined.ft] Macro name Value […]
__cpp_impl_coroutines
201902L
[…]
Modify 17.3.2 [version.syn], header
<version>
synopsis, as indicated:[…] #define __cpp_lib_constexpr_vector 201907L // also in <vector> #define __cpp_lib_coroutines 201902L // also in <coroutine> #define __cpp_lib_destroying_delete 201806L // also in <new> […]
[2020-02-11, Prague]
LWG observed that the naming suggestion didn't follow naming conventions of SG-10 because of the plural form
corountines
. The submitter agreed with that complaint, so the revised wording uses now the singular
form.
[2020-02 Moved to Immediate on Tuesday in Prague.]
Proposed resolution:
This wording is relative to N4849.
Modify 15.12 [cpp.predefined], Table [tab:cpp.predefined.ft], as indicated:
Table 18: Feature-test macros [tab:cpp.predefined.ft] Macro name Value […]
__cpp_impl_coroutine
s201902L
[…]
Modify 17.3.2 [version.syn], header <version>
synopsis, as indicated:
[…] #define __cpp_lib_constexpr_vector 201907L // also in <vector> #define __cpp_lib_coroutine 201902L // also in <coroutine> #define __cpp_lib_destroying_delete 201806L // also in <new> […]
Section: 99 [defns.comparison] Status: C++20 Submitter: Jeff Garland Opened: 2020-02-10 Last modified: 2021-02-25
Priority: 1
View all issues with C++20 status.
Discussion:
Addresses US 152
This definition in 99 [defns.comparison] should be updated to accommodate the new 3-way comparison operator (7.6.8 [expr.spaceship]) as well.
[2020-02 Moved to Immediate on Tuesday in Prague.]
Proposed resolution:
This wording is relative to N4849.
Modify 99 [defns.comparison] as indicated:
comparison function
operator function (12.4 [over.oper]) for any of the equality (7.6.10 [expr.eq]),orrelational (7.6.9 [expr.rel]), or three-way comparison (7.6.8 [expr.spaceship]) operators
source_location::current()
(DE 169)Section: 17.8.2.2 [support.srcloc.cons] Status: C++20 Submitter: Jens Maurer Opened: 2020-02-13 Last modified: 2021-02-25
Priority: 2
View all other issues in [support.srcloc.cons].
View all issues with C++20 status.
Discussion:
Addresses DE 169
The expectation of the note that a default argument expression involving current() causes a source_location to be constructed that refers to the site of a function call where that default argument is needed has no basis in normative text. In particular, 9.2.3.6 paragraph 5 seems to imply that the name "current" and its semantics are bound where it appears lexically in the function declaration.
Proposed change: Add normative text to express the desired semantics.[2020-02 Moved to Immediate on Thursday afternoon in Prague.]
Proposed resolution:
This wording is relative to N4849.
Modify 17.8.2.2 [support.srcloc.cons] as indicated:
static consteval source_location current() noexcept;-1- […]
-2- Remarks:When a default member initializer is used to initialize a non-static data member, any calls toAny call tocurrent
current
that appears as a default member initializer (11.4 [class.mem]), or as a subexpression thereof, should correspond to the location of the constructor definition or aggregate initialization thatinitializes the memberuses the default member initializer. Any call tocurrent
that appears as a default argument (9.3.4.7 [dcl.fct.default]), or as a subexpression thereof, should correspond to the location of the invocation of the function that uses the default argument (7.6.1.3 [expr.call]).-3- [Note: When used as a default argument (9.3.4.7 [dcl.fct.default]), the value of thesource_location
will be the location of the call tocurrent
at the call site. — end note]
ranges::basic_istream_view::iterator
should not provide iterator_category
Section: 25.6.6.3 [range.istream.iterator] Status: C++20 Submitter: Tomasz Kamiński Opened: 2020-02-13 Last modified: 2021-02-25
Priority: 1
View all other issues in [range.istream.iterator].
View all issues with C++20 status.
Discussion:
The ranges::basic_istream_view::iterator
is a move-only type, and as such it does not meets the
Cpp17 iterator requirements, yet it does provides iterator_category
(intended to be used for
Cpp17 iterators). We should provide iterator_concept
instead.
[2020-02 Status to Immediate on Thursday night in Prague.]
Proposed resolution:
This wording is relative to N4849.
Modify 25.6.6.3 [range.istream.iterator] as indicated:
namespace std::ranges { template<class Val, class CharT, class Traits> class basic_istream_view<Val, CharT, Traits>::iterator { // exposition only public: usingiterator_categoryiterator_concept = input_iterator_tag; using difference_type = ptrdiff_t; using value_type = Val; iterator() = default; […] }; }
tuple_element_t
is also wrong for const subrange
Section: 25.2 [ranges.syn] Status: C++20 Submitter: Casey Carter Opened: 2019-02-13 Last modified: 2021-02-25
Priority: 0
View all other issues in [ranges.syn].
View all issues with C++20 status.
Discussion:
As currently specified, it uses the cv-qualified partial specialization, which incorrectly adds cv-qualification to the element type.
Previous resolution [SUPERSEDED]:This wording is relative to N4849.
Modify 25.2 [ranges.syn], header
<ranges>
synopsis, as indicated:[…] namespace std { namespace views = ranges::views; template<class I, class S, ranges::subrange_kind K> struct tuple_size<ranges::subrange<I, S, K>> : integral_constant<size_t, 2> {}; template<class I, class S, ranges::subrange_kind K> struct tuple_element<0, ranges::subrange<I, S, K>> { using type = I; }; template<class I, class S, ranges::subrange_kind K> struct tuple_element<1, ranges::subrange<I, S, K>> { using type = S; }; template<class I, class S, ranges::subrange_kind K> struct tuple_element<0, const ranges::subrange<I, S, K>> { using type = I; }; template<class I, class S, ranges::subrange_kind K> struct tuple_element<1, const ranges::subrange<I, S, K>> { using type = S; }; }Add the following wording to Annex D:
D.? Deprecated
1 The headersubrange
tuple interface [depr.ranges.syn]<ranges>
(25.2 [ranges.syn]) has the following additions:namespace std { template<size_t X, class I, class S, ranges::subrange_kind K> struct tuple_element<X, volatile ranges::subrange<I, S, K>> {}; template<size_t X, class I, class S, ranges::subrange_kind K> struct tuple_element<X, const volatile ranges::subrange<I, S, K>> {}; }
[2020-02-14, Prague]
LWG decided to remove the volatile support, we shouldn't give the impression to support an idiom that wouldn't work.
Proposed resolution:
This wording is relative to N4849.
Modify 25.2 [ranges.syn], header <ranges>
synopsis, as indicated:
[…] namespace std { namespace views = ranges::views; template<class I, class S, ranges::subrange_kind K> struct tuple_size<ranges::subrange<I, S, K>> : integral_constant<size_t, 2> {}; template<class I, class S, ranges::subrange_kind K> struct tuple_element<0, ranges::subrange<I, S, K>> { using type = I; }; template<class I, class S, ranges::subrange_kind K> struct tuple_element<1, ranges::subrange<I, S, K>> { using type = S; }; template<class I, class S, ranges::subrange_kind K> struct tuple_element<0, const ranges::subrange<I, S, K>> { using type = I; }; template<class I, class S, ranges::subrange_kind K> struct tuple_element<1, const ranges::subrange<I, S, K>> { using type = S; }; }
ranges::ssize(E)
doesn't match ranges::size(E)
Section: 25.3.11 [range.prim.ssize] Status: C++23 Submitter: Jonathan Wakely Opened: 2020-02-19 Last modified: 2023-11-22
Priority: 2
View all issues with C++23 status.
Discussion:
ranges::size(E)
works with a non-range for which E.size()
or size(E)
is valid.
But ranges::ssize(E)
requires the type range_difference_t
which requires
ranges::begin(E)
to be valid. This means there are types for which ranges::size(E)
is valid but ranges::ssize(E)
is not.
ranges::ssize
to work with any argument that ranges::size
accepts.
That suggest to me that we're going to need make-signed-like-t<T>
after all,
so we can "Let E
be an expression, and let D
be the wider of ptrdiff_t
or decltype(ranges::size(E))
. Then ranges::ssize(E)
is expression-equivalent to
static_cast<make-signed-like-t<D>>(ranges::size(E))
." Although this wording
is still slightly icky since D
isn't a valid type when ranges::size(E)
isn't a valid
expression, I think it's an improvement?
[2020-03-11 Issue Prioritization]
Priority to 2 after reflector discussion.
[2020-07-22 Casey provides wording]
Previous resolution [SUPERSEDED]:
This wording is relative to N4861.
Add a new paragraph after paragraph 1 in 25.2 [ranges.syn]:
-?- Also within this clause,make-signed-like-t<X>
for an integer-like typeX
denotesmake_signed_t<X>
ifX
is an integer type; otherwise, it denotes a corresponding unspecified signed-integer-like type of the same width asX
.Modify 25.3.11 [range.prim.ssize] as indicated:
-1- The name
ranges::ssize
denotes a customization point object (16.3.3.3.5 [customization.point.object]).The expressionranges::ssize(E)
for a subexpressionE
of typeT
is expression-equivalent to:
(1.1) — Ifrange_difference_t<T>
has width less thanptrdiff_t
,static_cast<ptrdiff_t>(ranges::size(E))
.
(1.2) — Otherwise,static_cast<range_difference_t<T>>(ranges::size(E))
.-?- Given a subexpression
E
with typeT
, lett
be an lvalue that denotes the reified object forE
. Ifranges::size(t)
is ill-formed,ranges::ssize(E)
is ill-formed. Otherwise, letD
be the wider ofptrdiff_t
ordecltype(ranges::size(t))
;ranges::ssize(E)
is expression-equivalent tostatic_cast<make-signed-like-t<D>>(ranges::size(t))
.
[2020-07-31 Casey provides updated wording]
Per discussion on the reflector.[2020-08-21; Issue processing telecon: Tentatively Ready]
[2020-11-09 Approved In November virtual meeting. Status changed: Tentatively Ready → WP.]
Proposed resolution:
This wording is relative to N4861.
Add a new paragraph after paragraph 1 in 25.2 [ranges.syn]:
[Drafting note: The following does not define an analog
to-signed-like
forto-unsigned-like
since we don't need it at this time.]
-?- Also within this Clause,make-signed-like-t<X>
for an integer-like typeX
denotesmake_signed_t<X>
ifX
is an integer type; otherwise, it denotes a corresponding unspecified signed-integer-like type of the same width asX
.
Modify 25.3.11 [range.prim.ssize] as indicated:
-1- The name
ranges::ssize
denotes a customization point object (16.3.3.3.5 [customization.point.object]).The expressionranges::ssize(E)
for a subexpressionE
of typeT
is expression-equivalent to:
(1.1) — Ifrange_difference_t<T>
has width less thanptrdiff_t
,static_cast<ptrdiff_t>(ranges::size(E))
.
(1.2) — Otherwise,static_cast<range_difference_t<T>>(ranges::size(E))
.-?- Given a subexpression
E
with typeT
, lett
be an lvalue that denotes the reified object forE
. Ifranges::size(t)
is ill-formed,ranges::ssize(E)
is ill-formed. Otherwise letD
bemake-signed-like-t<decltype(ranges::size(t))>
, orptrdiff_t
if it is wider than that type;ranges::ssize(E)
is expression-equivalent tostatic_cast<D>(ranges::size(t))
.
subrange
's conversions from pair-like
Section: 25.5.4 [range.subrange] Status: C++23 Submitter: Casey Carter Opened: 2020-02-20 Last modified: 2023-11-22
Priority: 0
View all other issues in [range.subrange].
View all issues with C++23 status.
Discussion:
Both LWG 3281(i) "Conversion from pair-like
types to subrange
is a
silent semantic promotion" and LWG 3282(i) "subrange
converting constructor should
disallow derived to base conversions" removed subrange
's hated implicit conversions from
pair-like
types. Notably, neither issue removed the two "iterator-sentinel-pair
"
deduction guides which target the removed constructors nor the exposition-only iterator-sentinel-pair
concept itself, all of which are now useless.
[2020-03-11 Issue Prioritization]
Status set to Tentatively Ready after seven positive votes on the reflector.
[2020-11-09 Approved In November virtual meeting. Status changed: Tentatively Ready → WP.]
Proposed resolution:
This wording is relative to N4849.
Modify 25.5.4 [range.subrange] as indicated:
[…] template<class T, class U, class V> concept pair-like-convertible-from = // exposition only !range<T> && pair-like<T> && constructible_from<T, U, V>;template<class T> concept iterator-sentinel-pair = // exposition only !range<T> && pair-like<T> && sentinel_for<tuple_element_t<1, T>, tuple_element_t<0, T>>;[…]template<iterator-sentinel-pair P> subrange(P) -> subrange<tuple_element_t<0, P>, tuple_element_t<1, P>>;template<iterator-sentinel-pair P> subrange(P, make-unsigned-like-t(iter_difference_t<tuple_element_t<0, P>>)) -> subrange<uple_element_t<0, P>, tuple_element_t<1, P>, subrange_kind::sized>;[…]
common_view
's converting constructor is bad, tooSection: 25.7.20.2 [range.common.view] Status: C++23 Submitter: Casey Carter Opened: 2020-02-20 Last modified: 2023-11-22
Priority: 0
View all other issues in [range.common.view].
View all issues with C++23 status.
Discussion:
LWG 3280(i) struck the problematic/extraneous converting constructor templates from the
meow_view
range adaptor types in the standard library with the exception of common_view
.
The omission of common_view
seems to have been simply an oversight: its converting constructor
template is no less problematic or extraneous. We should remove common_view
's converting
constructor template as well to finish the task. Both cmcstl2
and range-v3
removed
the converting constructor template from common_view
when removing the other converting
constructor templates, so we have implementation experience that this change is good as well as
consistent with the general thrust of LWG 3280(i).
[2020-03-11 Issue Prioritization]
Status set to Tentatively Ready after seven positive votes on the reflector.
[2020-11-09 Approved In November virtual meeting. Status changed: Tentatively Ready → WP.]
Proposed resolution:
This wording is relative to N4849.
Modify 25.7.20.2 [range.common.view], class template common_view
synopsis, as indicated:
[…] constexpr explicit common_view(V r);[…]template<viewable_range R> requires (!common_range<R> && constructible_from<V, all_view<R>>) constexpr explicit common_view(R&& r);constexpr V base() const& requires copy_constructible<V> { return base_; } […]template<viewable_range R> requires (!common_range<R> && constructible_from<V, all_view<R>>) constexpr explicit common_view(R&& r);
-2- Effects: Initializesbase_
withviews::all(std::forward<R>(r))
.
elements_view::begin()
and elements_view::end()
have incompatible
constraintsSection: 25.7.23.2 [range.elements.view] Status: C++23 Submitter: Patrick Palka Opened: 2020-02-21 Last modified: 2023-11-22
Priority: 1
View all other issues in [range.elements.view].
View all issues with C++23 status.
Discussion:
P1994R1 (elements_view
needs its own
sentinel
) introduces a distinct sentinel
type for elements_view
.
In doing so, it replaces the two existing overloads of elements_view::end()
with four new ones:
- constexpr auto end() requires (!simple-view<V>) - { return ranges::end(base_); } - - constexpr auto end() const requires simple-view<V> - { return ranges::end(base_); }+ constexpr auto end() + { return sentinel<false>{ranges::end(base_)}; } + + constexpr auto end() requires common_range<V> + { return iterator<false>{ranges::end(base_)}; } + + constexpr auto end() const + requires range<const V> + { return sentinel<true>{ranges::end(base_)}; } + + constexpr auto end() const + requires common_range<const V> + { return iterator<true>{ranges::end(base_)}; }
But now these new overloads of elements_view::end()
have constraints
that are no longer consistent with the constraints of elements_view::begin()
:
constexpr auto begin() requires (!simple-view<V>) { return iterator<false>(ranges::begin(base_)); } constexpr auto begin() const requires simple-view<V> { return iterator<true>(ranges::begin(base_)); }
This inconsistency means that we can easily come up with a view V
for
which elements_view<V>::begin()
returns an iterator<true>
and elements_view<V>::end()
returns an sentinel<false>
,
i.e. incomparable things of opposite constness. For example:
tuple<int, int> x[] = {{0,0}}; ranges::subrange r = {counted_iterator(x, 1), default_sentinel}; auto v = r | views::elements<0>; v.begin() == v.end(); // ill-formed
Here, overload resolution for begin()
selects the const
overload because
the subrange r
models simple-view
. But overload resolution for
end()
selects the non-const
non-common_range
overload. Hence
the last line of this snippet is ill-formed because it is comparing an iterator and
sentinel of opposite constness, for which we have no matching operator==
overload. So in this example v
does not even model range because its begin()
and end()
are incomparable.
elements_view::begin()
and on elements_view::end()
are consistent and compatible. The following proposed
resolution seems to be one way to achieve that and takes inspiration from the design of
transform_view
.
[2020-04-04 Issue Prioritization]
Priority to 1 after reflector discussion.
[2020-07-17; telecon]
Should be considered together with 3448(i) and 3449(i).
Previous resolution [SUPERSEDED]:
This wording is relative to N4849 after application of P1994R1.
Modify 25.7.23.2 [range.elements.view], class template
elements_view
synopsis, as indicated:namespace std::ranges { […] template<input_range V, size_t N> requires view<V> && has-tuple-element<range_value_t<V>, N> && has-tuple-element<remove_reference_t<range_reference_t<V>>, N> class elements_view : public view_interface<elements_view<V, N>> { public: […] constexpr V base() && { return std::move(base_); } constexpr auto begin()requires (!simple-view<V>){ return iterator<false>(ranges::begin(base_)); } constexpr auto begin() const requiressimple-view<V>range<const V> { return iterator<true>(ranges::begin(base_)); } […] }; }
[2020-06-05 Tim updates P/R in light of reflector discussions and LWG 3448(i) and comments]
The fact that, as currently specified, sentinel<false>
is not
comparable with iterator<true>
is a problem with the specification
of this comparison, as noted in LWG 3448(i). The P/R below repairs
this problem along the lines suggested in that issue. The constraint mismatch does
make this problem easier to observe for elements_view
, but the mismatch
is not independently a problem: since begin
can only add constness on
simple-view
s for which constness is immaterial, whether
end
also adds constness or not ought not to matter.
begin
overload set: if const V
is a range,
but V
is not a simple-view
, then a const elements_view<V, N>
has no viable begin
at all (the simplest example of such non-simple-views
is probably single_view
). That's simply broken; the fix is to constrain
the const
overload of begin
with just range<const V>
instead of simple-view<V>
. Notably, this is how many other
uses of simple-view
work already (see, e.g., take_view
in
25.7.10.2 [range.take.view]).
The previous simple-view
constraint on end
served a
useful purpose (when done correctly): it reduces template instantiations if
the underlying view is const-agnostic. This was lost in P1994 because that paper
modeled the overload set on transform_view
; however, discussion with
Eric Niebler confirmed that the reason transform_view
doesn't have the
simple-view
optimization is because it would add constness to
the callable object as well, which can make a material difference in the result.
Such concerns are not present in elements_view
where the "callable
object" is effectively hard-coded into the type and unaffected by
const-qualification. The revised P/R below therefore restores the
simple-view
optimization.
I have implemented this P/R together with P1994 on top of libstdc++ trunk and
can confirm that no existing test cases were broken by these changes, and that
it fixes the issue reported here as well as in LWG 3448 (as it relates to
elements_view
).
[2020-10-02; Status to Tentatively Ready after five positive votes on the reflector]
[2020-11-09 Approved In November virtual meeting. Status changed: Tentatively Ready → WP.]
Proposed resolution:
This wording is relative to N4861. It assumes
the maybe-const
helper introduced by the P/R of LWG 3448(i).
Modify 25.7.23.2 [range.elements.view], class template elements_view
synopsis, as indicated:
namespace std::ranges { […] template<input_range V, size_t N> requires view<V> && has-tuple-element<range_value_t<V>, N> && has-tuple-element<remove_reference_t<range_reference_t<V>>, N> class elements_view : public view_interface<elements_view<V, N>> { public: […] constexpr V base() && { return std::move(base_); } constexpr auto begin() requires (!simple-view<V>) { return iterator<false>(ranges::begin(base_)); } constexpr auto begin() const requiressimple-view<V>range<const V> { return iterator<true>(ranges::begin(base_)); } constexpr auto end() requires (!simple-view<V> && !common_range<V>) { return sentinel<false>{ranges::end(base_)}; } constexpr auto end() requires (!simple-view<V> && common_range<V>) { return iterator<false>{ranges::end(base_)}; } constexpr auto end() const requires range<const V> { return sentinel<true>{ranges::end(base_)}; } constexpr auto end() const requires common_range<const V> { return iterator<true>{ranges::end(base_)}; } […] }; }
Modify 25.7.23.4 [range.elements.sentinel] as indicated:
[Drafting note: Unlike the current P/R of LWG 3448(i), this P/R also changes the return type of
operator-
to depend on the constness of iterator rather than that of the sentinel. This is consistent withsized_sentinel_for<S, I>
(24.3.4.8 [iterator.concept.sizedsentinel]), which requiresdecltype(i - s)
to beiter_difference_t<I>
.]
[…]namespace std::ranges { template<input_range V, size_t N> requires view<V> && has-tuple-element<range_value_t<V>, N> && has-tuple-element<remove_reference_t<range_reference_t<V>>, N> template<bool Const> class elements_view<V, F>::sentinel { […] constexpr sentinel_t<Base> base() const; template<bool OtherConst> requires sentinel_for<sentinel_t<Base>, iterator_t<maybe-const<OtherConst, V>>> friend constexpr bool operator==(const iterator<OtherConst>& x, const sentinel& y); template<bool OtherConst> requires sized_sentinel_for<sentinel_t<Base>, iterator_t<maybe-const<OtherConst, V>>> friend constexpr range_difference_t<Basemaybe-const<OtherConst, V>> operator-(const iterator<OtherConst>& x, const sentinel& y)requires sized_sentinel_for<sentinel_t<Base>, iterator_t<Base>>; template<bool OtherConst> requires sized_sentinel_for<sentinel_t<Base>, iterator_t<maybe-const<OtherConst, V>>> friend constexpr range_difference_t<Basemaybe-const<OtherConst, V>> operator-(const sentinel& x, const iterator<OtherConst>& y)requires sized_sentinel_for<sentinel_t<Base>, iterator_t<Base>>; }; }template<bool OtherConst> requires sentinel_for<sentinel_t<Base>, iterator_t<maybe-const<OtherConst, V>>> friend constexpr bool operator==(const iterator<OtherConst>& x, const sentinel& y);-4- Effects: Equivalent to:
return x.current_ == y.end_;
template<bool OtherConst> requires sized_sentinel_for<sentinel_t<Base>, iterator_t<maybe-const<OtherConst, V>>> friend constexpr range_difference_t<Basemaybe-const<OtherConst, V>> operator-(const iterator<OtherConst>& x, const sentinel& y)requires sized_sentinel_for<sentinel_t<Base>, iterator_t<Base>>;-5- Effects: Equivalent to:
return x.current_ - y.end_;
template<bool OtherConst> requires sized_sentinel_for<sentinel_t<Base>, iterator_t<maybe-const<OtherConst, V>>> friend constexpr range_difference_t<Basemaybe-const<OtherConst, V>> operator-(const sentinel& x, const iterator<OtherConst>& y)requires sized_sentinel_for<sentinel_t<Base>, iterator_t<Base>>;-6- Effects: Equivalent to:
return x.end_ - y.current_;
Section: 25.7.10.1 [range.take.overview], 25.6.4 [range.iota] Status: C++23 Submitter: Patrick Palka Opened: 2020-02-21 Last modified: 2023-11-22
Priority: 2
View other active issues in [range.take.overview].
View all other issues in [range.take.overview].
View all issues with C++23 status.
Discussion:
Section 6.1 of P1739R4 changes the specification of
views::take
as follows:
-2- The name
views::take
denotes a range adaptor object (25.7.2 [range.adaptor.object]).Given subexpressionsLetE
andF
, the expressionviews::take(E, F)
is expression-equivalent totake_view{E, F}
.E
andF
be expressions, letT
beremove_cvref_t<decltype((E))>
, and letD
berange_difference_t<decltype((E))>
. Ifdecltype((F))
does not modelconvertible_to<D>
,views::take(E, F)
is ill-formed. Otherwise, the expressionviews::take(E, F)
is expression-equivalent to:
— if
T
is a specialization ofranges::empty_view
(25.6.2.2 [range.empty.view]), then((void) F, decay-copy(E))
;— otherwise, if
T
modelsrandom_access_range
andsized_range
and is
— a specialization of
span
(23.7.2.2 [views.span]) whereT::extent == dynamic_extent
,— a specialization of
basic_string_view
(27.3 [string.view]),— a specialization of
ranges::iota_view
(25.6.4.2 [range.iota.view]), or— a specialization of
ranges::subrange
(25.5.4 [range.subrange]),then
T{ranges::begin(E), ranges::begin(E) + min<D>(ranges::size(E), F)}
, except thatE
is evaluated only once;— otherwise,
ranges::take_view{E, F}
.
Consider the case when T = subrange<counted_iterator<int>, default_sentinel_t>
.
Then according to the above wording, views::take(E, F)
is expression-equivalent to
T{ranges::begin(E), ranges:begin(E) + min<D>(ranges::size(E), F)}; (*)
But this expression is ill-formed for the T
we chose because
subrange<counted_iterator<int>, default_sentinel_t>
has no matching
constructor that takes an iterator-iterator
pair.
T
is a specialization of
subrange
that does not model common_range
. But a similar issue also
exists when T
is a specialization of iota_view
whose value type differs
from its bound type. In this case yet another issue arises: In order for the expression
(*)
to be well-formed when T
is a specialization of iota_view
,
we need to be able to construct an iota_view
out of an iterator-iterator
pair,
and for that it seems we need to add another constructor to iota_view
.
[2020-02-24, Casey comments]
Furthermore, the pertinent subrange constructor is only available when subrange::StoreSize
is
false
(i.e., when either the subrange
specialization's third template argument is
not subrange_kind::sized
or its iterator and sentinel types I
and S
model
sized_sentinel_for<S, I>
).
[2020-03-16, Tomasz comments]
A similar problem occurs for the views::drop
for the subrange<I, S, subrange_kind::sized>
,
that explicitly stores size (i.e. sized_sentinel_for<I, S>
is false
). In such case,
the (iterator, sentinel)
constructor that views::drop
will be expression-equivalent is
not available.
[2020-03-29 Issue Prioritization]
Priority to 2 after reflector discussion.
[2021-05-18 Tim adds wording]
The proposed resolution below is based on the MSVC implementation, with one caveat:
the MSVC implementation uses a SCARY iterator type for iota_view
and therefore
its iota_view
case for take
is able to directly construct the new view
from the iterator type of the original. This is not required to work, so the wording below
constructs the iota_view
from the result of dereferencing the iterators instead
in this case.
[2021-06-18 Tim syncs wording to the current working draft]
The wording below also corrects the size calculation in the presence of integer-class types.
[2021-08-20; LWG telecon]
Set status to Tentatively Ready after telecon review.
[2021-10-14 Approved at October 2021 virtual plenary. Status changed: Voting → WP.]
Proposed resolution:
This wording is relative to N4892.
Edit 25.7.10.1 [range.take.overview] as indicated:
-2- The name
views::take
denotes a range adaptor object (25.7.2 [range.adaptor.object]). LetE
andF
be expressions, letT
beremove_cvref_t<decltype((E))>
, and letD
berange_difference_t<decltype((E))>
. Ifdecltype((F))
does not modelconvertible_to<D>
,views::take(E, F)
is ill-formed. Otherwise, the expressionviews::take(E, F)
is expression-equivalent to:
(2.1) — if
T
is a specialization ofranges::empty_view
(25.6.2.2 [range.empty.view]), then((void) F, decay-copy(E))
, except that the evaluations ofE
andF
are indeterminately sequenced;(2.2) — otherwise, if
T
modelsrandom_access_range
andsized_range
, and is a specialization ofspan
(23.7.2.2 [views.span]),basic_string_view
(27.3 [string.view]), orranges::subrange
(25.5.4 [range.subrange]), thenU(ranges::begin(E), ranges::begin(E) + std::min<D>(ranges::distance(E), F))
, except thatE
is evaluated only once, whereU
is a type determined as follows:
(2.2.1) — if
T
is a specialization ofspan
(23.7.2.2 [views.span]) where, thenT::extent == dynamic_extent
U
isspan<typename T::element_type>
;(2.2.2) — otherwise, if
T
is a specialization ofbasic_string_view
(27.3 [string.view]), thenU
isT
;
(2.2.3) — a specialization ofranges::iota_view
(25.6.4.2 [range.iota.view]), or(2.2.4) — otherwise,
T
is a specialization ofranges::subrange
(25.5.4 [range.subrange]), andU
isranges::subrange<iterator_t<T>>
;
thenT{ranges::begin(E), ranges::begin(E) + min<D>(ranges::size(E), F)}
, except thatE
is evaluated only once;(2.?) — otherwise, if
T
is a specialization ofranges::iota_view
(25.6.4.2 [range.iota.view]) that modelsrandom_access_range
andsized_range
, thenranges::iota_view(*ranges::begin(E), *(ranges::begin(E) + std::min<D>(ranges::distance(E), F)))
, except thatE
andF
are each evaluated only once;(2.3) — otherwise,
ranges::take_view(E, F)
.
Edit 25.7.12.1 [range.drop.overview] as indicated:
-2- The name
views::drop
denotes a range adaptor object (25.7.2 [range.adaptor.object]). LetE
andF
be expressions, letT
beremove_cvref_t<decltype((E))>
, and letD
berange_difference_t<decltype((E))>
. Ifdecltype((F))
does not modelconvertible_to<D>
,views::drop(E, F)
is ill-formed. Otherwise, the expressionviews::drop(E, F)
is expression-equivalent to:
(2.1) — if
T
is a specialization ofranges::empty_view
(25.6.2.2 [range.empty.view]), then((void) F, decay-copy(E))
, except that the evaluations ofE
andF
are indeterminately sequenced;(2.2) — otherwise, if
T
modelsrandom_access_range
andsized_range
, and is
(2.2.1) —a specialization of
span
(23.7.2.2 [views.span])where,T::extent == dynamic_extent
(2.2.2) — a specialization of
basic_string_view
(27.3 [string.view]),(2.2.3) — a specialization of
ranges::iota_view
(25.6.4.2 [range.iota.view]), or(2.2.4) — a specialization of
ranges::subrange
(25.5.4 [range.subrange]) whereT::StoreSize
isfalse
,then
, except that
TU(ranges::begin(E) + std::min<D>(ranges::sizedistance(E), F), ranges::end(E))E
is evaluated only once, whereU
isspan<typename T::element_type>
ifT
is a specialization ofspan
andT
otherwise;(2.?) — otherwise, if
T
is a specialization ofranges::subrange
(25.5.4 [range.subrange]) that modelsrandom_access_range
andsized_range
, thenT(ranges::begin(E) + std::min<D>(ranges::distance(E), F), ranges::end(E), to-unsigned-like(ranges::distance(E) - std::min<D>(ranges::distance(E), F)))
, except thatE
andF
are each evaluated only once;(2.3) — otherwise,
ranges::drop_view(E, F)
.
counted_iterator
Section: 24.5.7.1 [counted.iterator] Status: Resolved Submitter: Patrick Palka Opened: 2020-02-25 Last modified: 2021-05-18
Priority: 2
View all other issues in [counted.iterator].
View all issues with Resolved status.
Discussion:
LWG 3291(i) makes changes to iota_view
so that it no longer falsely claims
it's a C++17 forward iterator. A side effect of this change is that the counted_iterator
of an iota_view
can no longer model forward_iterator
, no matter what the
properties of the underlying weakly_incrementable
type are. For example, the following
snippet is ill-formed after LWG 3291:
auto v = views::iota(0); auto i = counted_iterator{v.begin(), 5}; static_assert(random_access_iterator<decltype(i)>); // fails after LWG 3291
The problem seems to be that counted_iterator
populates its iterator_traits
but it does not specify its iterator_concept
, so we fall back to looking at its
iterator_category
to determine its C++20 iterator-ness. It seems
counted_iterator
should specify its iterator_concept
appropriately based on
the iterator_concept
of the iterator it wraps.
[2020-03-29 Issue Prioritization]
Priority to 2 after reflector discussion.
[2021-05-18 Resolved by the adoption of P2259R1 at the February 2021 plenary. Status changed: New → Resolved.]
Proposed resolution:
lexicographical_compare_three_way
is overspecifiedSection: 26.8.12 [alg.three.way] Status: C++23 Submitter: Casey Carter Opened: 2020-02-27 Last modified: 2023-11-22
Priority: 3
View all other issues in [alg.three.way].
View all issues with C++23 status.
Discussion:
26.8.12 [alg.three.way]/2 specifies the effects of the lexicographical_compare_three_way
algorithm via a large "equivalent to" codeblock. This codeblock specifies one more iterator comparison
than necessary when the first input sequence is greater than the second, and two more than necessary
in other cases. Requiring unnecessary work is the antithesis of C++.
[2020-03-29 Issue Prioritization]
Priority to 3 after reflector discussion.
[2021-05-19 Tim adds wording]
The wording below simply respecifies the algorithm in words. It seems pointless
to try to optimize the code when the reading in the discussion above would
entirely rule out things like memcmp
optimizations for arbitrary
contiguous iterators of bytes.
[2021-05-26; Reflector poll]
Set status to Tentatively Ready after five votes in favour during reflector poll.
[2021-06-07 Approved at June 2021 virtual plenary. Status changed: Voting → WP.]
Proposed resolution:
This wording is relative to N4885.
Modify 26.8.12 [alg.three.way] as indicated:
template<class InputIterator1, class InputIterator2, class Cmp> constexpr auto lexicographical_compare_three_way(InputIterator1 b1, InputIterator1 e1, InputIterator2 b2, InputIterator2 e2, Cmp comp) -> decltype(comp(*b1, *b2));-?- Let N be min(
-1- Mandates:e1 - b1, e2 - b2
). Let E(n) becomp(*(b1 + n), *(b2 + n))
.decltype(comp(*b1, *b2))
is a comparison category type. -?- Returns: E(i), where i is the smallest integer in [0, N) such that E(i)!= 0
istrue
, or(e1 - b1) <=> (e2 - b2)
if no such integer exists. -?- Complexity: At most N applications ofcomp
.-2- Effects: Lexicographically compares two ranges and produces a result of the strongest applicable comparison category type. Equivalent to:for ( ; b1 != e1 && b2 != e2; void(++b1), void(++b2) ) if (auto cmp = comp(*b1,*b2); cmp != 0) return cmp; return b1 != e1 ? strong_ordering::greater : b2 != e2 ? strong_ordering::less : strong_ordering::equal;
Section: 8.3 [fund.ts.v3::memory.resource.syn] Status: C++23 Submitter: Thomas Köppe Opened: 2020-02-28 Last modified: 2023-11-22
Priority: 3
View all issues with C++23 status.
Discussion:
Addresses: fund.ts.v3
The Library Fundamentals TS, N4840, contains a rule about the use of namespaces (paragraph 1), with the consequence:
"This TS does not define
std::experimental::fundamentals_v3::pmr
"
However, the TS then goes on to define exactly that namespace.
At the time when the subclause memory.resource.syn was added, the IS didn't use to contain a namespacepmr
. When the IS adopted that namespace and the TS was rebased, the
namespace rule started conflicting with the material in the TS.
I do not have a workable proposed resolution at this point.
[2020-04-07 Issue Prioritization]
Priority to 3 after reflector discussion.
[2021-11-17; Jiang An comments and provides wording]
Given namespaces std::chrono::experimental::fundamentals_v2
and std::experimental::fundamentals_v2::pmr
are used in LFTS v2, I think that the intent is that
the prefix experimental::fundamentals_vN
is only added to a namespace specified by the
standard, and
every name specified by the standard in the outer namespace of that of the related component in the TS.
If we follow the convention, perhaps we should relocate resource_adaptor
from
std::experimental::fundamentals_v3::pmr
to std::pmr::experimental::fundamentals_v3
in LFTS v3.
If it's decided that resource_adaptor
shouldn't be relocated, I suppose that LWG 3411 can be by striking
the wrong wording in 4.1 [fund.ts.v3::general.namespaces] and using qualified std::pmr::memory_resource
when needed.
Previous resolution [SUPERSEDED]:
This wording is relative to N4853.
[Drafting Note: Two mutually exclusive options are prepared, depicted below by Option A and Option B, respectively.]
Option A:
Modify 4.1 [fund.ts.v3::general.namespaces] as indicated:
-2- Each header described in this technical specification shall import the contents of
outer-namespace
intostd::experimental::fundamentals_v3outer-namespace
as if bystd::experimentalnamespace outer-namespacestd::experimental::inline fundamentals_v3 {}where
outer-namespace
is a namespace defined in the C++ Standard Library.Modify [fund.ts.v3::memory.type.erased.allocator], Table 5, as indicated:
Table 5 — Computed memory_resource
for type-erased allocatorIf the type of alloc
isthen the value of rptr
is[…]
any other type meeting the requirements (C++20 ¶16.5.3.5) a pointer to a value of type pmr::experimental::resource_adaptor<A>
whereA
is the type ofalloc
.rptr
remains valid only for the lifetime ofX
.[…]
Modify 8.3 [fund.ts.v3::memory.resource.syn], header
<experimental/memory_resource>
synopsis, as indicated:namespace std::pmr::experimental::inline fundamentals_v3::pmr{ […] } // namespace std::pmr::experimental::inline fundamentals_v3::pmrOption B:
Modify 4.1 [fund.ts.v3::general.namespaces] as indicated:
-1- Since the extensions described in this technical specification are experimental and not part of the C++ standard library, they should not be declared directly within namespace
std
. Unless otherwise specified, all components described in this technical specification either:
— modify an existing interface in the C++ Standard Library in-place,
— are declared in a namespace whose name appends
::experimental::fundamentals_v3
to a namespace defined in the C++ Standard Library, such asstd
orstd::chrono
, or— are declared in a subnamespace of a namespace described in the previous bullet
, whose name is not the same as an existing subnamespace of namespace.std
[Example: This TS does not definestd::experimental::fundamentals_v3::pmr
because the C++ Standard Library definesstd::pmr
. — end example]Modify 7.2 [fund.ts.v3::func.wrap.func], class template
function
synopsis, as indicated:namespace std { namespace experimental::inline fundamentals_v3 { template<class> class function; // undefined template<class R, class... ArgTypes> class function<R(ArgTypes...)> { public: […] std::pmr::memory_resource* get_memory_resource() const noexcept; }; […] } // namespace experimental::inline fundamentals_v3 […] } // namespace stdModify [fund.ts.v3::memory.type.erased.allocator], Table 5, as indicated:
Table 5 — Computed memory_resource
for type-erased allocatorIf the type of alloc
isthen the value of rptr
is[…]
any other type meeting the requirements (C++20 ¶16.5.3.5) a pointer to a value of type experimental::pmr::resource_adaptor<A>
whereA
is the type ofalloc
.rptr
remains valid only for the lifetime ofX
.[…]
Modify 8.4.1 [fund.ts.v3::memory.resource.adaptor.overview] as indicated:
-1- An instance of
resource_adaptor<Allocator>
is an adaptor that wraps astd::pmr::memory_resource
interface aroundAllocator
. […]// The nameresource_adaptor_imp
is for exposition only. template<class Allocator> class resource_adaptor_imp : public std::pmr::memory_resource { public: […] virtual bool do_is_equal(const std::pmr::memory_resource& other) const noexcept; };Modify 8.4.3 [fund.ts.v3::memory.resource.adaptor.mem] as indicated:
-6- bool do_is_equal(const std::pmr::memory_resource& other) const noexcept;[…]
Modify 32.10.6 [futures.promise], class template
promise
synopsis, as indicated:namespace std { namespace experimental::inline fundamentals_v3 { template<class R> class promise { public: […] std::pmr::memory_resource* get_memory_resource() const noexcept; }; […] } // namespace experimental::inline fundamentals_v3 […] } // namespace stdModify 32.10.10 [futures.task], class template
packaged_task
synopsis, as indicated:namespace std { namespace experimental::inline fundamentals_v3 { template<class R, class... ArgTypes> class packaged_task<R(ArgTypes...)> { public: […] std::pmr::memory_resource* get_memory_resource() const noexcept; }; […] } // namespace experimental::inline fundamentals_v3 […] } // namespace std
[2022-10-12; Jonathan provides updated wording]
The LWG telecon decided on a simpler form of Option A.
The changes to 4.1 [fund.ts.v3::general.namespaces] generated some questions
and disagreement, but it was decided that they are not needed anyway.
The normative synopses already depict the use of inline namespaces with the
stated effects. That paragraph seems more informative than normative, and there
were suggestions to strike it entirely. It was decided to keep it but without
making the edits. As such, it remains correct for the contents of
std::experimental::fundamentals_v3
. It doesn't apply to
pmr::resource_adaptor
, but is not incorrect for that either.
The rest of the proposed resolution fully specifies the pmr
parts.
[2022-10-19; Reflector poll]
Set status to "Tentatively Ready" after eight votes in favour in reflector poll.
[2022-11-12 Approved at November 2022 meeting in Kona. Status changed: Voting → WP.]
Proposed resolution:
This wording is relative to N4920.
Modify [fund.ts.v3::memory.type.erased.allocator], Table 5, as indicated:
Table 5 — Computed memory_resource
for type-erased allocatorIf the type of alloc
isthen the value of rptr
is[…]
any other type meeting the requirements (C++20 ¶16.5.3.5) a pointer to a value of type pmr::experimental::resource_adaptor<A>
whereA
is the type ofalloc
.rptr
remains valid only for the lifetime ofX
.[…]
Modify 8.3 [fund.ts.v3::memory.resource.syn], header <experimental/memory_resource>
synopsis, as indicated:
namespace std::pmr::experimental::inline fundamentals_v3::pmr{ […] } // namespace std::pmr::experimental::inline fundamentals_v3::pmr
Section: 28.5.2.2 [format.string.std] Status: Resolved Submitter: Hubert Tong Opened: 2020-02-29 Last modified: 2023-03-23
Priority: 3
View other active issues in [format.string.std].
View all other issues in [format.string.std].
View all issues with Resolved status.
Discussion:
In 28.5.2.2 [format.string.std], the meaning of "Unicode encoding" in the text added by P1868R2 (the "Unicorn width" paper) is unclear.
One interpretation of what is meant by "Unicode encoding" is "UCS encoding scheme" (as defined by ISO/IEC 10646). Another interpretation is an encoding scheme capable of encoding all UCS code points that have been assigned to characters. Yet another interpretation is an encoding scheme capable of encoding all UCS scalar values. SG16 reflector discussion (with the LWG reflector on CC) indicates that the third option above is the closest to the intent of the study group. The situation of the current wording, which readers can easily read as indicating the first option above, is undesirable.[2020-07-17; Priority set to 3 in telecon]
[2023-03-22 Resolved by the adoption of P2736R2 in Issaquah. Status changed: New → Resolved.]
Proposed resolution:
propagate_const
's swap
's noexcept
specification needs to
be constrained and use a traitSection: 6.1.2.8 [fund.ts.v3::propagate_const.modifiers], 6.1.2.10 [fund.ts.v3::propagate_const.algorithms] Status: C++23 Submitter: Thomas Köppe Opened: 2020-02-29 Last modified: 2023-11-22
Priority: 0
View all issues with C++23 status.
Discussion:
Addresses: fund.ts.v3
In the Fundamentals TS, the noexcept
specifications of both the member and non-member swap
functions for propagate_const
are using the old, ill-formed pattern of attempting to use
"noexcept(swap(...))
" as the boolean predicate. According to LWG 2456(i), this is
ill-formed, and a resolution such as in P0185R1 is required.
This wording is relative to N4840.
Modify 6.1.1 [fund.ts.v3::propagate_const.syn], header
<experimental/propagate_const>
synopsis, as indicated:// 6.1.2.10 [fund.ts.v3::propagate_const.algorithms], propagate_const specialized algorithms template <class T> constexpr void swap(propagate_const<T>& pt, propagate_const<T>& pt2) noexcept(see belowis_nothrow_swappable_v<T>);Modify 6.1.2.1 [fund.ts.v3::propagate_const.overview], class template
propagate_const
synopsis, as indicated:// 6.1.2.8 [fund.ts.v3::propagate_const.modifiers], propagate_const modifiers constexpr void swap(propagate_const& pt) noexcept(see belowis_nothrow_swappable_v<T>);Modify 6.1.2.8 [fund.ts.v3::propagate_const.modifiers] as indicated:
constexpr void swap(propagate_const& pt) noexcept(see belowis_nothrow_swappable_v<T>);-3- Effects:
-2- The constant-expression in the exception-specification isnoexcept(swap(t_, pt.t_))
.swap(t_, pt.t_)
.Modify 6.1.2.10 [fund.ts.v3::propagate_const.algorithms] as indicated:
template <class T> constexpr void swap(propagate_const<T>& pt1, propagate_const<T>& pt2) noexcept(see belowis_nothrow_swappable_v<T>);-3- Effects:
-2- The constant-expression in the exception-specification isnoexcept(pt1.swap(pt2))
.pt1.swap(pt2)
.
[2020-03-30; Reflector discussion]
This issue has very much overlap with LWG 2561(i), especially now that the library fundamentals has been rebased
to C++20 the there reported problem for the corresponding swap
problem for optional
is now moot. During the
reflector discussion of this issue here it was also observed that the free swap
template for propagate_const
needs to be constrained. This has been done below in the revised wording which also attempts to use a similar style as
the IS.
This wording is relative to N4840.
Modify 6.1.2.1 [fund.ts.v3::propagate_const.overview], class template
propagate_const
synopsis, as indicated:// 6.1.2.8 [fund.ts.v3::propagate_const.modifiers], propagate_const modifiers constexpr void swap(propagate_const& pt) noexcept(see belowis_nothrow_swappable_v<T>);Modify 6.1.2.8 [fund.ts.v3::propagate_const.modifiers] as indicated:
constexpr void swap(propagate_const& pt) noexcept(see belowis_nothrow_swappable_v<T>);-3- Effects:
-2- The constant-expression in the exception-specification isnoexcept(swap(t_, pt.t_))
.swap(t_, pt.t_)
.Modify 6.1.2.10 [fund.ts.v3::propagate_const.algorithms] as indicated:
template <class T> constexpr void swap(propagate_const<T>& pt1, propagate_const<T>& pt2) noexcept(see below);-?- Constraints:
is_swappable_v<T>
istrue
.-2- The constant-expression in the exception-specification is-3- Effects:noexcept(pt1.swap(pt2))
.pt1.swap(pt2)
. -?- Remarks: The expression insidenoexcept
is equivalent to:noexcept(pt1.swap(pt2))
[2020-04-06; Wording update upon reflector discussions]
[2020-05-03 Issue Prioritization]
Status set to Tentatively Ready after five positive votes on the reflector.
[2020-11-09 Approved In November virtual meeting. Status changed: Tentatively Ready → WP.]
Proposed resolution:
This wording is relative to N4840.
Modify 6.1.2.1 [fund.ts.v3::propagate_const.overview], class template propagate_const
synopsis,
as indicated:
// 6.1.2.8 [fund.ts.v3::propagate_const.modifiers], propagate_const modifiers constexpr void swap(propagate_const& pt) noexcept(see belowis_nothrow_swappable_v<T>);
Modify 6.1.2.8 [fund.ts.v3::propagate_const.modifiers] as indicated:
constexpr void swap(propagate_const& pt) noexcept(see belowis_nothrow_swappable_v<T>);-?- Preconditions: Lvalues of type
T
are swappable (C++17 §20.5.3.2).-2- The constant-expression in the exception-specification is-3- Effects:noexcept(swap(t_, pt.t_))
.swap(t_, pt.t_)
.
Modify 6.1.2.10 [fund.ts.v3::propagate_const.algorithms] as indicated:
template <class T> constexpr void swap(propagate_const<T>& pt1, propagate_const<T>& pt2) noexcept(see below);-?- Constraints:
is_swappable_v<T>
istrue
.-2- The constant-expression in the exception-specification is-3- Effects: Equivalent to:noexcept(pt1.swap(pt2))
.pt1.swap(pt2)
. -?- Remarks: The expression insidenoexcept
is equivalent to:noexcept(pt1.swap(pt2))
service_already_exists
has no usable constructorsSection: 13.7 [networking.ts::async.exec.ctx] Status: C++23 Submitter: Jonathan Wakely Opened: 2020-03-17 Last modified: 2023-11-22
Priority: 0
View all issues with C++23 status.
Discussion:
Addresses: networking.ts
In the Networking TS, the service_already_exists
exception type
has no constructors declared. The logic_error
base class is not
default constructible, so service_already_exists
's implicit default
constructor is defined as deleted.
Implementations can add one or more private constructors that can be used
by make_service
, but there seems to be little benefit to that.
The Boost.Asio type of the same name has a public default constructor.
[2020-04-18 Issue Prioritization]
Status set to Tentatively Ready after six positive votes on the reflector.
[2020-11-09 Approved In November virtual meeting. Status changed: Tentatively Ready → WP.]
Proposed resolution:
This wording is relative to N4734.
Modify 13.7 [networking.ts::async.exec.ctx] p1, as indicated:
// service access template<class Service> typename Service::key_type& use_service(execution_context& ctx); template<class Service, class... Args> Service& make_service(execution_context& ctx, Args&&... args); template<class Service> bool has_service(const execution_context& ctx) noexcept; class service_already_exists : public logic_error{ };{ public: service_already_exists(); };
Add a new subclause after [async.exec.ctx.globals]:
13.7.6 Class
service_already_exists
[async.exec.ctx.except]-1- The class
service_already_exists
defines the type of objects thrown as exceptions to report an attempt to add an existing service to anexecution_context
.service_already_exists();-2- Postconditions:
what()
returns an implementation-defined NTBS.
Section: 26.2 [algorithms.requirements] Status: C++23 Submitter: Richard Smith Opened: 2020-03-24 Last modified: 2023-11-22
Priority: 0
View all other issues in [algorithms.requirements].
View all issues with C++23 status.
Discussion:
26.2 [algorithms.requirements]/15 says:
"The number and order of deducible template parameters for algorithm declarations are unspecified, except where explicitly stated otherwise. [Note: Consequently, the algorithms may not be called with explicitly-specified template argument lists. — end note]"
But the note doesn't follow from the normative rule. For example, we felt the need to explicitly allow
deduction for min
's template parameter:
template<typename T> const T& min(const T&, const T&);
… but if only the order and number of deducible template parameters is permitted to vary, then because of the required deduction behavior of this function template, there are only three possible valid declarations:
template<typename T> ??? min(const T&, const T&); template<typename T, typename U> ??? min(const T&, const U&); template<typename T, typename U> ??? min(const U&, const T&);
(up to minor differences in the parameter type). This doesn't prohibit calls with an explicitly-specified
template argument list, contrary to the claim in the note. (Indeed, because a call such as min(1, {})
is valid, either the first of the above three overloads must be present or there must be a default template
argument typename U = T
, which further adds to the fact that there may be valid calls with an
explicitly-specified template argument list.)
T
of the overloads in namespace std
." which
doesn't "specify otherwise" the normative rule, but does "specify otherwise" the claim in the note.
All this leads me to believe that [algorithms.requirements]/15 is backwards: the normative rule should be a
note and the note should be the normative rule.
[2020-04-04 Issue Prioritization]
Status set to Tentatively Ready after six positive votes on the reflector.
[2020-11-09 Approved In November virtual meeting. Status changed: Tentatively Ready → WP.]
Proposed resolution:
This wording is relative to N4861.
Modify 26.2 [algorithms.requirements] as indicated:
-15- The well-formedness and behavior of a call to an algorithm with an explicitly-specified template argument list is
number and order of deducible template parameters for algorithm declarations areunspecified, except where explicitly stated otherwise. [Note: Consequently, an implementation can declare an algorithm with different template parameters than those presentedthe algorithms may not be called with explicitly-specified template argument lists. — end note]
cpp17-iterator
should check that the type looks like an iterator firstSection: 24.3.2.3 [iterator.traits] Status: C++23 Submitter: Tim Song Opened: 2020-02-29 Last modified: 2023-11-22
Priority: 0
View all other issues in [iterator.traits].
View all issues with C++23 status.
Discussion:
It is common in pre-C++20 code to rely on SFINAE-friendly iterator_traits
to rule out non-iterators in template constraints (std::filesystem::path
is one example in the standard library).
iterator_traits
to automatically detect its members in
some cases, and this detection can cause constraint recursion. LWG 3244(i)
tries to fix this for path
by short-circuiting the check when the source type is
path
itself, but this isn't sufficient:
struct Foo { Foo(const std::filesystem::path&); }; static_assert(std::copyable<Foo>);
Here the copyability determination will ask whether a path
can be
constructed from a Foo
, which asks whether Foo
is an iterator, which
checks whether Foo
is copyable
[…].
cpp17-iterator
so that it does not ask about copyability until the type is known to resemble an iterator.
[2020-04-04 Issue Prioritization]
Status set to Tentatively Ready after six positive votes on the reflector.
[2020-11-09 Approved In November virtual meeting. Status changed: Tentatively Ready → WP.]
Proposed resolution:
This wording is relative to N4861.
Modify 24.3.2.3 [iterator.traits] as indicated:
-2- The definitions in this subclause make use of the following exposition-only concepts:
template<class I> concept cpp17-iterator =copyable<I> &&requires(I i) { { *i } -> can-reference; { ++i } -> same_as<I&>; { *i++ } -> can-reference; } && copyable<I>; […]
boolean-testable
Section: 18.5.2 [concept.booleantestable] Status: C++23 Submitter: Davis Herring Opened: 2020-03-24 Last modified: 2023-11-22
Priority: 0
View all issues with C++23 status.
Discussion:
18.5.2 [concept.booleantestable]/4 checks for "a specialization of a class template that is a member of
the same namespace as D
", which ignores the possibility of inline namespaces.
[2020-04-18 Issue Prioritization]
Status set to Tentatively Ready after six positive votes on the reflector.
[2020-11-09 Approved In November virtual meeting. Status changed: Tentatively Ready → WP.]
Proposed resolution:
This wording is relative to N4861.
Modify 18.5.2 [concept.booleantestable] as indicated:
-4- A key parameter of a function template
D
is a function parameter of type cvX
or reference thereto, whereX
names a specialization of a class template that has the same innermost enclosing non-inlineis a member of the samenamespace asD
, andX
contains at least one template parameter that participates in template argument deduction. […]
seed_seq
's constructorsSection: 29.5.8.1 [rand.util.seedseq] Status: C++23 Submitter: Jiang An Opened: 2020-03-25 Last modified: 2023-11-22
Priority: 3
View all other issues in [rand.util.seedseq].
View all issues with C++23 status.
Discussion:
29.5.8.1 [rand.util.seedseq] says that std::seed_seq
has following 3 constructors:
#1: seed_seq()
#2: template<class T> seed_seq(initializer_list<T> il)
#3: template<class InputIterator> seed_seq(InputIterator begin, InputIterator end)
vector<result_type>
's default
constructor is already noexcept
since C++17, so #1 should also be noexcept
.
Despite that the vector<result_type>
member is exposition-only, current implementations (at least
libc++,
libstdc++ and
MSVC STL) all hold it as the only
data member of seed_seq
, even with different names. And #1 is already noexcept
in libc++ and libstdc++.
These constructors are not constrained, so #3 would never be matched in list-initialization. Consider following code:
#include <random> #include <vector> int main() { std::vector<int> v(32); std::seed_seq{std::begin(v), std::end(v)}; // error: #2 matched and T is not an integer type std::seed_seq(std::begin(v), std::end(v)); // OK }
#3 should be made available in list-initialization by changing Mandates in 29.5.8.1 [rand.util.seedseq]/3 to Constraints IMO.
[2020-04-18 Issue Prioritization]
Priority to 3 after reflector discussion.
[2021-06-23; Reflector poll]
Set status to Tentatively Ready after five votes in favour during reflector poll.
[2021-10-14 Approved at October 2021 virtual plenary. Status changed: Voting → WP.]
Proposed resolution:
This wording is relative to N4861.
Modify 29.5.8.1 [rand.util.seedseq] as indicated:
class seed_seq { public: // types using result_type = uint_least32_t; // constructors seed_seq() noexcept; […] };
seed_seq() noexcept;-1- Postconditions:
v.empty()
istrue
.-2- Throws: Nothing.template<class T> seed_seq(initializer_list<T> il);-3- Constraints
-4- Effects: Same asMandates:T
is an integer type.seed_seq(il.begin(), il.end())
.
condition_variable_any
fails to constrain its Lock
parametersSection: 32.7.5 [thread.condition.condvarany] Status: C++23 Submitter: Casey Carter Opened: 2020-04-04 Last modified: 2023-11-22
Priority: 0
View all other issues in [thread.condition.condvarany].
View all issues with C++23 status.
Discussion:
32.7.5 [thread.condition.condvarany]/1 says "A Lock
type shall meet the Cpp17BasicLockable
requirements (32.2.5.2 [thread.req.lockable.basic]).", which is fine, but it notably doesn't require anything
to be a Lock
type or meet the requirements of a Lock
type. Given that every member template of
condition_variable_any
has a template parameter named Lock
, the intent is clearly to impose a
requirement on the template arguments supplied for those parameters but the use of code font for "Lock
"
in the definition of "Lock
type" is a bit subtle to establish that connection. We should specify this
more clearly.
This wording is relative to N4861.
Modify 32.7.5 [thread.condition.condvarany] as indicated:
-1-
ATemplate arguments for template parameters of member templates ofLock
typeconditional_variable_any
namedLock
shall meet the Cpp17BasicLockable requirements (32.2.5.2 [thread.req.lockable.basic]). [Note: All of the standard mutex types meet this requirement. If atype other than one of the standard mutex types or aLock
unique_lock
wrapper for a standard mutex type is used withcondition_variable_any
, the user should ensure that any necessary synchronization is in place with respect to the predicate associated with thecondition_variable_any
instance. — end note]
[2020-04-06; Tim improves wording]
[2020-04-11 Issue Prioritization]
Status set to Tentatively Ready after seven positive votes on the reflector.
[2020-11-09 Approved In November virtual meeting. Status changed: Tentatively Ready → WP.]
Proposed resolution:
This wording is relative to N4861.
Modify 32.7.5 [thread.condition.condvarany] as indicated:
-1-
AIn this subclause 32.7.5 [thread.condition.condvarany], template arguments for template parameters namedLock
typeLock
shall meet the Cpp17BasicLockable requirements (32.2.5.2 [thread.req.lockable.basic]). [Note: All of the standard mutex types meet this requirement. If atype other than one of the standard mutex types or aLock
unique_lock
wrapper for a standard mutex type is used withcondition_variable_any
, the user should ensure that any necessary synchronization is in place with respect to the predicate associated with thecondition_variable_any
instance. — end note]
operator<=>(const unique_ptr<T, D>&, nullptr_t)
can't get no satisfactionSection: 20.3.1.6 [unique.ptr.special] Status: C++23 Submitter: Jonathan Wakely Opened: 2020-04-09 Last modified: 2023-11-22
Priority: 0
View all other issues in [unique.ptr.special].
View all issues with C++23 status.
Discussion:
The constraint for operator<=>(const unique_ptr<T, D>&, nullptr_t)
cannot be satisfied, because std::three_way_comparable<nullptr_t>
is false
,
because nullptr <=> nullptr
is ill-formed.
x.get()
to pointer(nullptr)
instead of to nullptr
directly.
[2020-04-14; Replacing the functional cast by a static_cast
as result of reflector discussion]
[2020-04-18 Issue Prioritization]
Status set to Tentatively Ready after seven positive votes on the reflector.
[2020-11-09 Approved In November virtual meeting. Status changed: Tentatively Ready → WP.]
Proposed resolution:
This wording is relative to N4861.
Modify 20.2.2 [memory.syn] as indicated:
[…] template<class T, class D> bool operator>=(nullptr_t, const unique_ptr<T, D>& y); template<class T, class D> requires three_way_comparable_with<typename unique_ptr<T, D>::pointer, nullptr_t> compare_three_way_result_t<typename unique_ptr<T, D>::pointer, nullptr_t> operator<=>(const unique_ptr<T, D>& x, nullptr_t); […]
Modify 20.3.1.6 [unique.ptr.special] as indicated:
template<class T, class D> requires three_way_comparable_with<typename unique_ptr<T, D>::pointer, nullptr_t> compare_three_way_result_t<typename unique_ptr<T, D>::pointer, nullptr_t> operator<=>(const unique_ptr<T, D>& x, nullptr_t);-18- Returns:
compare_three_way()(x.get(), static_cast<typename unique_ptr<T, D>::pointer>(nullptr))
.
operator<=>(const shared_ptr<T>&, nullptr_t)
definition ill-formedSection: 20.3.2.2.8 [util.smartptr.shared.cmp] Status: C++23 Submitter: Daniel Krügler Opened: 2020-04-11 Last modified: 2023-11-22
Priority: 0
View all other issues in [util.smartptr.shared.cmp].
View all issues with C++23 status.
Discussion:
This is similar to LWG 3426(i): This time the definition of
operator<=>(const shared_ptr<T>&, nullptr_t)
is ill-formed, whose effects are essentially
specified as calling:
compare_three_way()(a.get(), nullptr)
This call will be ill-formed by constraint-violation, because both nullptr <=> nullptr
as well as
((T*) 0) <=> nullptr
are ill-formed.
a.get()
to (element_type*) nullptr
instead of to nullptr
directly.
As a long-term solution we should at least consider to deprecate the mixed relational operators as well as the
mixed three-way comparison operator of all our smart-pointers with std::nullptr_t
since the core language
has eliminated relational comparisons of pointers with std::nullptr_t
with
N3624 four years after they had been originally accepted by
CWG 879. Consequently, for C++20, the mixed three-way comparison between
pointers and nullptr
is not supported either. For this long-term solution I'm suggesting to handle this
via a proposal.
[2020-05-16 Reflector discussions]
Status to Tentatively Ready and priority to 0 after five positive votes on the reflector.
[2020-11-09 Approved In November virtual meeting. Status changed: Tentatively Ready → WP.]
Proposed resolution:
This wording is relative to N4861.
Modify 20.3.2.2.8 [util.smartptr.shared.cmp] as indicated:
template<class T> strong_ordering operator<=>(const shared_ptr<T>& a, nullptr_t) noexcept;-5- Returns:
compare_three_way()(a.get(), static_cast<typename shared_ptr<T>::element_type*>(nullptr))
.
single_view
's in place constructor should be explicitSection: 25.6.3.2 [range.single.view] Status: C++23 Submitter: Tim Song Opened: 2020-04-07 Last modified: 2023-11-22
Priority: 0
View all other issues in [range.single.view].
View all issues with C++23 status.
Discussion:
The in_place_t
constructor template of single_view
is not explicit:
template<class... Args> requires constructible_from<T, Args...> constexpr single_view(in_place_t, Args&&... args);
so it defines an implicit conversion from std::in_place_t
to
single_view<T>
whenever constructible_from<T>
is modeled,
which seems unlikely to be the intent.
[2020-04-18 Issue Prioritization]
Status set to Tentatively Ready after six positive votes on the reflector.
[2020-11-09 Approved In November virtual meeting. Status changed: Tentatively Ready → WP.]
Proposed resolution:
This wording is relative to N4861.
Modify 25.6.3.2 [range.single.view] as indicated:
[…]namespace std::ranges { template<copy_constructible T> requires is_object_v<T> class single_view : public view_interface<single_view<T>> { […] public: […] template<class... Args> requires constructible_from<T, Args...> constexpr explicit single_view(in_place_t, Args&&... args); […] }; }template<class... Args> constexpr explicit single_view(in_place_t, Args&&... args);-3- Effects: Initializes
value_
as if byvalue_{in_place, std::forward<Args>(args)...}
.
std::fstream
& co. should be constructible from string_view
Section: 31.10.1 [fstream.syn] Status: C++23 Submitter: Jonathan Wakely Opened: 2020-04-15 Last modified: 2023-11-22
Priority: 3
View all issues with C++23 status.
Discussion:
We have:
basic_fstream(const char*, openmode); basic_fstream(const filesystem::path::value_type*, openmode); // wide systems only basic_fstream(const string&, openmode); basic_fstream(const filesystem::path&, openmode);
I think the omission of a string_view
overload was intentional, because the underlying OS call
(such as fopen
) needs a NTBS. We wanted the allocation required to turn a string_view
into an NTBS to be explicitly requested by the user. But then we added the path
overload,
which is callable with a string_view
. Converting to a path
is more expensive than
converting to std::string
, because a path has to at least construct a basic_string
,
and potentially also does an encoding conversion, parses the path, and potentially allocates a sequence
of path
objects for the path components.
string_view sv = "foo.txt"; fstream f1(sv); // bad fstream f2(string(sv)); // good
We should just allow passing a string_view
directly, since it already compiles but
doesn't do what anybody expects or wants.
string_view
overload, passing types like const char16_t*
or u32string_view
will still implicitly convert to filesystem::path
, but that seems reasonable. In those cases the
encoding conversion is necessary. For Windows we support construction from const wchar_t*
but not
from wstring
or wstring_view
, which means those types will convert to filesystem::path
.
That seems suboptimal, so we might also want to add wstring
and wstring_view
overloads for
"wide systems only", as per 31.10.1 [fstream.syn] p3.
Daniel:
LWG 2883(i) has a more general view on that but does not consider potential cost differences
in the presence of path
overloads (Which didn't exist at this point yet).
[2020-05-09; Reflector prioritization]
Set priority to 3 after reflector discussions.
[2020-08-10; Jonathan comments]
An alternative fix would be to retain the original design and not allow
construction from a string_view
. The path
parameters
could be changed to template parameters which are constrained to be exactly
path
, and not things like string_view
which can convert
to path
.
[2020-08-21; Issue processing telecon: send to LEWG]
Just adding support for string_view
doesn't prevent expensive
conversions from other types that convert to path
.
Preference for avoiding all expensive implicit conversions to path
,
maybe via abbreviated function templates:
basic_fstream(same_as<filesystem::path> auto const&, openmode);
It's possible path_view
will provide a better option at some point.
It was noted that 2676(i) did intentionally allow conversions
from "strings of character types wchar_t
, char16_t
,
and char32_t
". Those conversions don't need to be implicit
for that to be supported.
[2020-09-11; Tomasz comments and provides wording]
During the LEWG 2020-08-24 telecon the LEWG provided following guidance on the issue:
We took one poll (exact wording in the notes) to constrain the constructor which takes
filesystem::path
to only takefilesystem::path
and not things convertible to it, but only 9 out of 26 people present actually voted. Our interpretation: LWG should go ahead with making this change. There is still plenty of time for someone who hasn't yet commented on this to bring it up even if it is in a tentatively ready state. It would be nice to see a paper to address the problem of the templated path constructor, but no one has yet volunteered to do so. Note: the issue description is now a misnomer, as adding astring_view
constructor is no longer being considered at this time.
The proposed P/R follows original LWG proposal and makes the path
constructor of the
basic_*fstreams
"explicit
". To adhere to current policy, we refrain from use of
requires clauses and abbreviated function syntax, and introduce a Constraints element.
[2021-05-21; Reflector poll]
Set status to Tentatively Ready after seven votes in favour during reflector poll.
[2021-06-07 Approved at June 2021 virtual plenary. Status changed: Voting → WP.]
Proposed resolution:
This wording is relative to N4861.
Modify 31.10.4 [ifstream], class template basic_ifstream
synopsis, as indicated:
[…] explicit basic_ifstream(const string& s, ios_base::openmode mode = ios_base::in); template<class T> explicit basic_ifstream(constfilesystem::pathT& s, ios_base::openmode mode = ios_base::in); […]
Modify 31.10.4.2 [ifstream.cons] as indicated:
explicit basic_ifstream(const string& s, ios_base::openmode mode = ios_base::in);-?- Effects: Equivalent to:
basic_ifstream(s.c_str(), mode)
.template<class T> explicit basic_ifstream(constfilesystem::pathT& s, ios_base::openmode mode = ios_base::in);-?- Constraints:
-3- Effects: Equivalent to:is_same_v<T, filesystem::path>
istrue
.basic_ifstream(s.c_str(), mode)
.
Modify 31.10.5 [ofstream], class template basic_ofstream
synopsis, as indicated:
[…] explicit basic_ofstream(const string& s, ios_base::openmode mode = ios_base::out); template<class T> explicit basic_ofstream(constfilesystem::pathT& s, ios_base::openmode mode = ios_base::out); […]
Modify 31.10.5.2 [ofstream.cons] as indicated:
explicit basic_ofstream(const string& s, ios_base::openmode mode = ios_base::out);-?- Effects: Equivalent to:
basic_ofstream(s.c_str(), mode)
.template<class T> explicit basic_ofstream(constfilesystem::pathT& s, ios_base::openmode mode = ios_base::out);-?- Constraints:
-3- Effects: Equivalent to:is_same_v<T, filesystem::path>
istrue
.basic_ofstream(s.c_str(), mode)
.
Modify 31.10.6 [fstream], class template basic_fstream
synopsis, as indicated:
[…] explicit basic_fstream( const string& s, ios_base::openmode mode = ios_base::in | ios_base::out); template<class T> explicit basic_fstream( constfilesystem::pathT& s, ios_base::openmode mode = ios_base::in | ios_base::out); […]
Modify 31.10.6.2 [fstream.cons] as indicated:
explicit basic_fstream( const string& s, ios_base::openmode mode = ios_base::in | ios_base::out);-?- Effects: Equivalent to:
basic_fstream(s.c_str(), mode)
.template<class T> explicit basic_fstream( constfilesystem::pathT& s, ios_base::openmode mode = ios_base::in | ios_base::out);-?- Constraints:
-3- Effects: Equivalent to:is_same_v<T, filesystem::path>
istrue
.basic_fstream(s.c_str(), mode)
.
<=>
for containers should require three_way_comparable<T>
instead of <=>
Section: 23.2.2.4 [container.opt.reqmts] Status: WP Submitter: Jonathan Wakely Opened: 2020-04-17 Last modified: 2023-11-22
Priority: 2
View all issues with WP status.
Discussion:
The precondition for <=>
on containers is:
<=>
is defined for values of type (possibly const
) T
,
or <
is defined for values of type (possibly const
) T
and
<
is a total ordering relationship."
I don't think <=>
is sufficient, because synth-three-way
won't
use <=>
unless three_way_comparable<T>
is satisfied, which requires
weakly-equality-comparable-with<T, T>
as well as <=>
.
So to use <=>
I think the type also requires ==
, or more precisely, it
must satisfy three_way_comparable
.
The problem becomes clearer with the following example:
#include <compare> #include <vector> struct X { friend std::strong_ordering operator<=>(X, X) { return std::strong_ordering::equal; } }; std::vector<X> v(1); std::strong_ordering c = v <=> v;
This doesn't compile, because despite X
meeting the preconditions for <=>
in
[tab:container.opt], synth-three-way
will return std::weak_ordering
.
#include <compare> #include <vector> struct X { friend bool operator<(X, X) { return true; } // The return value is intentional, see below friend std::strong_ordering operator<=>(X, X) { return std::strong_ordering::equal; } }; std::vector<X> v(1); std::weak_ordering c = v <=> v;
This meets the precondition because it defines <=>
, but the result of <=>
on vector<X>
will be nonsense, because synth-three-way
will use
operator<
not operator<=>
and that defines a broken ordering.
synth-three-way
actually does.
[2020-04-25 Issue Prioritization]
Priority to 2 after reflector discussion.
Previous resolution [SUPERSEDED]:
This wording is relative to N4861.
Modify 23.2.2 [container.requirements.general], Table [tab:container.opt], as indicated:
Table 75: Optional container operations [tab:container.opt] Expression Return type Operational
semanticsAssertion/note
pre-/post-conditionComplexity …
a <=> b
synth-three-way-result<value_type>
lexicographical_compare_three_way(
a.begin(), a.end(), b.begin(), b.end(),
synth-three-way)Preconditions: Either <=>
is defined for
values of type (possiblyconst
)
T
satisfiesthree_way_comparable
,
or<
is defined for values of type
(possiblyconst
)T
and
<
is a total ordering relationship.linear …
[2022-04-24; Daniel rebases wording on N4910]
Previous resolution [SUPERSEDED]:
This wording is relative to N4910.
Modify 23.2.2.4 [container.opt.reqmts] as indicated:
a <=> b-2- Result:
-3- Preconditions: Eithersynth-three-way-result<X::value_type>
.<=>
is defined for values of type (possiblyconst
)T
satisfiesthree_way_comparable
, or<
is defined for values of type (possiblyconst
)T
and<
is a total ordering relationship. -4- Returns:lexicographical_compare_three_way(a.begin(), a.end(), b.begin(), b.end(), synth-three-way)
[Note 1: The algorithmlexicographical_compare_three_way
is defined in Clause 27. — end note] -5- Complexity: Linear.
[2023-06-13; Varna]
The group liked the previously suggested wording but would prefer to say "models" instead of "satisfies" in preconditions.
[2023-06-14 Varna; Move to Ready]
[2023-11-11 Approved at November 2023 meeting in Kona. Status changed: Voting → WP.]
Proposed resolution:
This wording is relative to N4950.
Modify 23.2.2.4 [container.opt.reqmts] as indicated:
a <=> b-2- Result:
-3- Preconditions: Eithersynth-three-way-result<X::value_type>
.<=>
is defined for values of type (possiblyconst
)T
modelsthree_way_comparable
, or<
is defined for values of type (possiblyconst
)T
and<
is a total ordering relationship. -4- Returns:lexicographical_compare_three_way(a.begin(), a.end(), b.begin(), b.end(), synth-three-way)
[Note 1: The algorithmlexicographical_compare_three_way
is defined in Clause 27. — end note] -5- Complexity: Linear.
comparison_category
Section: 27.3.4 [string.view.comparison] Status: C++23 Submitter: Jonathan Wakely Opened: 2020-04-19 Last modified: 2023-11-22
Priority: 0
View all issues with C++23 status.
Discussion:
It's not clear what happens if a program-defined character traits type
defines comparison_category
as a synonym for void
,
or some other bogus type.
Discussion on the LWG reflector settled on making it ill-formed at the point of use.
[2020-07-17; Moved to Ready in telecon]
[2020-11-09 Approved In November virtual meeting. Status changed: Ready → WP.]
Proposed resolution:
This wording is relative to N4861.
Modify 27.3.4 [string.view.comparison] by adding a new paragraph after p3:
As result of reflector discussion we decided to make a drive-by fix in p3 below.
template<class charT, class traits> constexpr see below operator<=>(basic_string_view<charT, traits> lhs, basic_string_view<charT, traits> rhs) noexcept;-3- Let
R
denote the typetraits::comparison_category
if that qualified-id is valid and denotes a type (13.10.3 [temp.deduct])it exists, otherwiseR
isweak_ordering
.-?- Mandates:
R
denotes a comparison category type (17.12.2 [cmp.categories]).-4- Returns:
static_cast<R>(lhs.compare(rhs) <=> 0)
.
subrange::advance(n)
has UB when n < 0
Section: 25.5.4.3 [range.subrange.access] Status: C++23 Submitter: Casey Carter Opened: 2020-04-21 Last modified: 2023-11-22
Priority: 2
View all other issues in [range.subrange.access].
View all issues with C++23 status.
Discussion:
subrange::advance(n)
is specified to call ranges::advance(begin_, n, end_)
in both StoreSize
and !StoreSize
cases (25.5.4.3 [range.subrange.access]/9).
Unfortunately, ranges::advance(begin_, n, end_)
has undefined behavior when
n < 0
unless [end_, begin_)
denotes a valid range
(24.4.4.2 [range.iter.op.advance]/5). This would all be perfectly fine — the UB is exposed to
the caller via effects-equivalent-to — were it not the design intent that subrange::advance(-n)
be usable to reverse the effects of subrange::advance(n)
when the subrange
has a
bidirectional iterator type. That is, however, clearly the design intent: subrange::prev()
,
for example, is equivalent to subrange::advance(-1)
.
[2020-05-11; Reflector prioritization]
Set priority to 2 after reflector discussions.
[2021-01-15; Issue processing telecon]
Set status to Tentatively Ready after discussion and poll.
F | A | N |
---|---|---|
7 | 0 | 3 |
[2021-02-26 Approved at February 2021 virtual plenary. Status changed: Tentatively Ready → WP.]
Proposed resolution:
This wording is relative to N4861.
Modify 25.5.4.3 [range.subrange.access] as indicated:
constexpr subrange& advance(iter_difference_t<I> n);-9- Effects: Equivalent to:
(9.1) — IfStoreSize
istrue
,auto d = n - ranges::advance(begin_, n, end_); if (d >= 0) size_ -= to-unsigned-like(d); else size_ += to-unsigned-like(-d); return *this;
(9.2) — Otherwise,ranges::advance(begin_, n, end_); return *this;if constexpr (bidirectional_iterator<I>) { if (n < 0) { ranges::advance(begin_, n); if constexpr (StoreSize) size_ += to-unsigned-like(-n); return *this; } } auto d = n - ranges::advance(begin_, n, end_); if constexpr (StoreSize) size_ -= to-unsigned-like(d); return *this;
ios_base
never reclaims memory for iarray
and parray
Section: 31.5.2.8 [ios.base.cons] Status: C++23 Submitter: Alisdair Meredith Opened: 2020-04-26 Last modified: 2023-11-22
Priority: Not Prioritized
View all other issues in [ios.base.cons].
View all issues with C++23 status.
Discussion:
According to 31.5.2.6 [ios.base.storage] the class ios_base
allocates memory,
represented by two exposition-only pointers, iarray
and parray
in response
to calls to iword
and pword
. However, the specification for the destructor
in 31.5.2.8 [ios.base.cons] says nothing about reclaiming any allocated memory.
[2020-07-17; Reflector prioritization]
Set priority to 0 and status to Tentatively Ready after six votes in favour during reflector discussions.
[2020-11-09 Approved In November virtual meeting. Status changed: Tentatively Ready → WP.]
Proposed resolution:
This wording is relative to N4861.
Modify 31.5.2.8 [ios.base.cons] as indicated:
[Drafting note: Wording modeled on container requirements]
~ios_base();-2- Effects: Calls each registered callback pair
(fn, idx)
(31.5.2.7 [ios.base.callback]) as(*fn)(erase_event, *this, idx)
at such time that anyios_base
member function called from withinfn
has well-defined results. Then, any memory obtained is deallocated.
three_way_comparable_with<reverse_iterator<int*>, reverse_iterator<const int*>>
Section: 24.5.4.4 [move.iter.cons], 24.5.1.4 [reverse.iter.cons] Status: C++23 Submitter: Casey Carter Opened: 2020-04-26 Last modified: 2023-11-22
Priority: 2
View all other issues in [move.iter.cons].
View all issues with C++23 status.
Discussion:
Despite that reverse_iterator<int*>
and reverse_iterator<const int*>
are comparable with
<=>
, three_way_comparable_with<reverse_iterator<int*>, reverse_iterator<const int*>>
is false
. This unfortunate state of affairs results from the absence of constraints on reverse_iterator
's
converting constructor: both convertible_to<reverse_iterator<int*>, reverse_iterator<const int*>>
and convertible_to<reverse_iterator<const int*>, reverse_iterator<int*>>
evaluate to true
,
despite that reverse_iterator<int*>
's converting constructor template is ill-formed when instantiated
for reverse_iterator<const int*>
. This apparent bi-convertibility results in ambiguity when trying to
determine common_reference_t<const reverse_iterator<int*>&, const reverse_iterator<const int*>&>
,
causing the common_reference
requirement in three_way_comparable_with
to fail.
reverse_iterator
's conversion constructor (and converting
assignment operator, while we're here) correctly so we can use the concept to determine when it's ok to compare
specializations of reverse_iterator
with <=>
.
move_iterator
has similar issues due to its similarly unconstrained conversions. We should fix both similarly.
[2020-07-17; Priority set to 2 in telecon]
[2020-07-26; Reflector poll]
Set status to Tentatively Ready after five votes in favour during reflector discussions.
[2020-11-09 Approved In November virtual meeting. Status changed: Tentatively Ready → WP.]
Proposed resolution:
This wording is relative to N4861.
Modify 24.5.4.4 [move.iter.cons] as indicated:
[Drafting note: this incorporates and supercedes the P/R of LWG 3265(i).]
constexpr move_iterator();-1- Effects:
Constructs aValue-initializesmove_iterator
, vingcurrent
.Iterator operations applied to the resulting iterator have defined behavior if and only if the corresponding operations are defined on a value-initialized iterator of typeIterator
.constexpr explicit move_iterator(Iterator i);-2- Effects:
Constructs aInitializesmove_iterator
, iingcurrent
withstd::move(i)
.template<class U> constexpr move_iterator(const move_iterator<U>& u);-3-
-4- Effects:Mandates:Constraints:U
is convertible toIterator
is_same_v<U, Iterator>
isfalse
andconst U&
modelsconvertible_to<Iterator>
.Constructs aInitializesmove_iterator
, iingcurrent
withu.current
.base()template<class U> constexpr move_iterator& operator=(const move_iterator<U>& u);-5-
-6- Effects: AssignsMandates:Constraints:U
is convertible toIterator
is_same_v<U, Iterator>
isfalse
,const U&
modelsconvertible_to<Iterator>
, andassignable_from<Iterator&, const U&>
is modeled.u.current
tobase()current
.
Modify 24.5.1.4 [reverse.iter.cons] as indicated:
template<class U> constexpr reverse_iterator(const reverse_iterator<U>& u);-?- Constraints:
-3- Effects: Initializesis_same_v<U, Iterator>
isfalse
andconst U&
modelsconvertible_to<Iterator>
.current
withu.current
.template<class U> constexpr reverse_iterator& operator=(const reverse_iterator<U>& u);-?- Constraints:
-4- Effects: Assignsis_same_v<U, Iterator>
isfalse
,const U&
modelsconvertible_to<Iterator>
, andassignable_from<Iterator&, const U&>
is modeled.u.current
tobase()current
. -5- Returns:*this
.
std::construct_at
should support arraysSection: 26.11.8 [specialized.construct] Status: WP Submitter: Jonathan Wakely Opened: 2020-04-29 Last modified: 2024-11-28
Priority: 2
View all other issues in [specialized.construct].
View all issues with WP status.
Discussion:
std::construct_at
is ill-formed for array types, because the type of the new
-expression is T
not T*
so it cannot be converted to the return type.
allocator_traits::construct
did work for arrays, because it returns void
so there is no
ill-formed conversion. On the other hand, in C++17 allocator_traits::destroy
didn't work for arrays,
because p->~T()
isn't valid.
In C++20 allocator_traits::destroy
does work, because std::destroy_at
treats arrays specially,
but allocator_traits::construct
no longer works because it uses std::construct_at
.
It seems unnecessary and/or confusing to remove support for arrays in construct
when we're adding it in destroy
.
I suggest that std::construct_at
should also handle arrays. It might be reasonable to restrict that
support to the case where sizeof...(Args) == 0
, if supporting parenthesized aggregate-initialization
is not desirable in std::construct_at
.
[2020-05-09; Reflector prioritization]
Set priority to 2 after reflector discussions.
[2021-01-16; Zhihao Yuan provides wording]
Previous resolution [SUPERSEDED]:
This wording is relative to N4878.
Modify 26.11.8 [specialized.construct] as indicated:
template<class T, class... Args> constexpr T* construct_at(T* location, Args&&... args); namespace ranges { template<class T, class... Args> constexpr T* construct_at(T* location, Args&&... args); }-1- Constraints: The expression
-2- Effects: Equivalent to:::new (declval<void*>()) T(declval<Args>()...)
is well-formed when treated as an unevaluated operand.returnauto ptr = ::new (voidify(*location)) T(std::forward<Args>(args)...); if constexpr (is_array_v<T>) return launder(location); else return ptr;
[2021-12-07; Zhihao Yuan comments and provides improved wording]
The previous PR allows constructing arbitrary number of elements when
T
is an array of unknown bound:
extern int a[]; std::construct_at(&a, 0, 1, 2);
and leads to a UB.
Previous resolution [SUPERSEDED]:
This wording is relative to N4901.
Modify 26.11.8 [specialized.construct] as indicated:
template<class T, class... Args> constexpr T* construct_at(T* location, Args&&... args); namespace ranges { template<class T, class... Args> constexpr T* construct_at(T* location, Args&&... args); }-1- Constraints: The expression
-2- Effects: Equivalent to:::new (declval<void*>()) T(declval<Args>()...)
is well-formed when treated as an unevaluated operand (7.2.3 [expr.context]) andis_unbounded_array_v<T>
isfalse
.returnauto ptr = ::new (voidify(*location)) T(std::forward<Args>(args)...); if constexpr (is_array_v<T>) return launder(location); else return ptr;
[2024-03-18; Jonathan provides new wording]
During Core review in Varna, Hubert suggested creating T[1]
for the array case.
Previous resolution [SUPERSEDED]:
This wording is relative to N4971.
Modify 26.11.8 [specialized.construct] as indicated:
template<class T, class... Args> constexpr T* construct_at(T* location, Args&&... args); namespace ranges { template<class T, class... Args> constexpr T* construct_at(T* location, Args&&... args); }-1- Constraints:
-2- Effects: Equivalent to:is_unbounded_array_v<T>
isfalse
. The expression::new (declval<void*>()) T(declval<Args>()...)
is well-formed when treated as an unevaluated operand (7.2.3 [expr.context]).if constexpr (is_array_v<T>) return ::new (voidify(*location)) T[1]{{std::forward<Args>(args)...}}; else return ::new (voidify(*location)) T(std::forward<Args>(args)...);
[St. Louis 2024-06-24; Jonathan provides improved wording]
Why not support unbounded arrays, deducing the bound from sizeof...(Args)
?
JW: There's no motivation to support that here in construct_at
.
It isn't possible to create unbounded arrays via allocators,
nor via any of the uninitialized_xxx
algorithms. Extending construct_at
that way seems like a design change, not restoring support for something
that used to work with allocators and then got broken in C++20.
Tim observed that the proposed resolution is ill-formed if T
has an
explicit default constructor. Value-initialization would work for that case,
and there seems to be little motivation for supplying arguments to
initialize the array. In C++17 the allocator_traits::construct
case only
supported value-initialization.
[St. Louis 2024-06-24; move to Ready.]
[Wrocław 2024-11-23; Status changed: Voting → WP.]
Proposed resolution:
This wording is relative to N4981.
Modify 26.11.8 [specialized.construct] as indicated:
template<class T, class... Args> constexpr T* construct_at(T* location, Args&&... args); namespace ranges { template<class T, class... Args> constexpr T* construct_at(T* location, Args&&... args); }-1- Constraints:
is_unbounded_array_v<T>
isfalse
. The expression::new (declval<void*>()) T(declval<Args>()...)
is well-formed when treated as an unevaluated operand (7.2.3 [expr.context]).-?- Mandates: If
is_array_v<T>
istrue
,sizeof...(Args)
is zero.-2- Effects: Equivalent to:
if constexpr (is_array_v<T>) return ::new (voidify(*location)) T[1](); else return ::new (voidify(*location)) T(std::forward<Args>(args)...);
__cpp_lib_polymorphic_allocator
is in the wrong headerSection: 17.3.2 [version.syn] Status: C++23 Submitter: Jonathan Wakely Opened: 2020-04-29 Last modified: 2023-11-22
Priority: 0
View all other issues in [version.syn].
View all issues with C++23 status.
Discussion:
17.3.2 [version.syn] says that __cpp_lib_polymorphic_allocator
is also defined in <memory>
,
but std::polymorphic_allocator
is defined in <memory_resource>
. This seems like an error in
P1902R1. The macro should be in the same header as the feature it relates to.
[2020-05-09; Reflector prioritization]
Set status to Tentatively Ready after six votes in favour during reflector discussions.
[2020-11-09 Approved In November virtual meeting. Status changed: Tentatively Ready → WP.]
Proposed resolution:
This wording is relative to N4861.
Modify 17.3.2 [version.syn] as indicated:
[…] #define __cpp_lib_parallel_algorithm 201603L // also in <algorithm>, <numeric> #define __cpp_lib_polymorphic_allocator 201902L // also in <memorymemory_resource> #define __cpp_lib_quoted_string_io 201304L // also in <iomanip> […]
Section: 16.4.5.2.1 [namespace.std] Status: C++23 Submitter: Michael Park Opened: 2020-05-08 Last modified: 2023-11-22
Priority: 1
View other active issues in [namespace.std].
View all other issues in [namespace.std].
View all issues with C++23 status.
Discussion:
P0551 (Thou Shalt Not Specialize std
Function Templates!)
added a clause in [namespace.std]/7:
Other than in namespace
std
or in a namespace within namespacestd
, a program may provide an overload for any library function template designated as a customization point, provided that (a) the overload's declaration depends on at least one user-defined type and (b) the overload meets the standard library requirements for the customization point. (footnote 174) [Note: This permits a (qualified or unqualified) call to the customization point to invoke the most appropriate overload for the given arguments. — end note]
Given that std::swap
is a designated customization point, the note seems to suggest the following:
namespace N {
struct X {};
void swap(X&, X&) {}
}
N::X a, b;
std::swap(a, b); // qualified call to customization point finds N::swap?
This is not what happens, as the call to std::swap
does not find N::swap
.
[2020-07-17; Priority set to 1 in telecon]
[2020-09-11; discussed during telecon]
The note is simply bogus, not backed up by anything normative.
The normative part of the paragraph is an unacceptable landgrab on those
identifiers.
We have no right telling users they can't use the names data
and size
unless they do exactly what we say std::data
and std::size
do.
The library only ever uses swap
unqualified, so the effect of
declaring the others as designated customization points is unclear.
The rule only needs to apply to such overloads when actually found by overload resolution in a context that expects the semantics of the customization point.
Frank: do we need to designate operator<<
as a
customization point? Users overload that in their own namespaces all the time.
Walter: This clearly needs a paper.
[2020-10-02; status to Open]
Previous resolution [SUPERSEDED]:
This wording is relative to N4861.
Modify 16.4.5.2.1 [namespace.std] as indicated:
-7- Other than in namespace
std
or in a namespace within namespacestd
, a program may provide an overload for any library function template designated as a customization point, provided that (a) the overload's declaration depends on at least one user-defined type and (b) the overload meets the standard library requirements for the customization point. (footnote 173)[Note: This permits a (qualified or unqualified) call to the customization point to invoke the most appropriate overload for the given arguments. — end note]
[Issaquah 2023-02-09; Jonathan provides improved wording]
The normative part of 16.4.5.2.1 [namespace.std] paragraph 7
(i.e. not the note and the footnote) does not actually do anything
except tell users they're not allowed to use the names
begin
, size
etc. unless they conform to the
provisions of paragraph 7. This means a program that contains the
following declaration might have undefined behaviour:
namespace user { int data(int, int); }
This is not OK! It's not even clear what "the requirements for
the customization point" are, if users wanted to meet them.
It's not even clear what "provide an overload" means.
In particular, paragraph 7 does not give permission for the
designated customization points to actually use such
overloads. Just by forbidding users from using data
for their own functions isn't sufficient to allow the library to
use ADL to call data
, and it's unclear why we'd want
that anyway (what problem with std::data
are we trying
to solve that way?). As shown in LWG 3442(i), if
std::data
became a customization point that would be
a backwards-incompatible change, and create a portability problem
if some implementations did that and others didn't.
So the non-normative note and footnote make claims that do not follow from the normative wording, and should be removed.
The only one of the designated customization points that
is actually looked up using ADL is std::swap
, but how that
works is fully specified by 16.4.2.2 [contents] and
16.4.4.3 [swappable.requirements]. The additional specification
that it's a designated customization point serves no benefit.
In particular, the permission that the note and footnote claim
exists is not needed. We specify precisely how swap
calls are performed.
We don't even need to say that user overloads of swap
in their own namespaces must meet the library requirements,
because the "swappable with" and Cpp17Swappable requirements
state the required semantics, and functions that use swap
have preconditions that the types are swappable. So we correctly
impose preconditions in the places that actually call swap,
and don't need to globally make it undefined for any function
called swap
to not meet the requirements, even if that
function is never found by ADL by the library (e.g. because it's
in a namespace that is never an associated namespace of any types
used with library components that require swappable types).
Paragraph 7 and its accompanying notes should go.
[Issaquah 2023-02-09; LWG]
Move to Immediate for C++23
[2023-02-13 Approved at February 2023 meeting in Issaquah. Status changed: Immediate → WP.]
Proposed resolution:
This wording is relative to N4928.
Modify 16.4.5.2.1 [namespace.std] as indicated:
[Drafting note: This should also remove the only index entry for "customization point". ]
-7- Other than in namespacestd
or in a namespace within namespacestd
, a program may provide an overload for any library function template designated as a customization point, provided that (a) the overload's declaration depends on at least one user-defined type and (b) the overload meets the standard library requirements for the customization point. 163
[Note 3: This permits a (qualified or unqualified) call to the customization point to invoke the most appropriate overload for the given arguments. — end note]
163) Any library customization point must be prepared to work adequately with any user-defined overload that meets the minimum requirements of this document. Therefore an implementation can elect, under the as-if rule (6.9.1 [intro.execution]), to provide any customization point in the form of an instantiated function object (22.10 [function.objects]) even though the customization point's specification is in the form of a function template. The template parameters of each such function object and the function parameters and return type of the object'soperator()
must match those of the corresponding customization point's specification.
Modify 22.2.2 [utility.swap] as indicated:
template<class T> constexpr void swap(T& a, T& b) noexcept(see below);
-1- Constraints:
is_move_constructible_v<T>
istrue
andis_move_assignable_v<T>
istrue
.-2- Preconditions: Type
T
meets the Cpp17MoveConstructible (Table 32) and Cpp17MoveAssignable (Table 34) requirements.-3- Effects: Exchanges values stored in two locations.
-4- Remarks:
This function is a designated customization point (16.4.5.2.1 [namespace.std]).The exception specification is equivalent to:is_nothrow_move_constructible_v<T> && is_nothrow_move_assignable_v<T>
Modify 24.7 [iterator.range] as indicated:
-1- In addition to being available via inclusion of the <iterator> header, the function templates in 24.7 [iterator.range] are available when any of the following headers are included: <array> (23.3.2 [array.syn]), <deque> (23.3.4 [deque.syn]), <forward_list> (23.3.6 [forward.list.syn]), <list> (23.3.10 [list.syn]), <map> (23.4.2 [associative.map.syn]), <regex> (28.6.3 [re.syn]), <set> (23.4.5 [associative.set.syn]), <span> (23.7.2.1 [span.syn]), <string> (27.4.2 [string.syn]), <string_view> ( [string.view.syn]), <unordered_map> (23.5.2 [unord.map.syn]), <unordered_set> (23.5.5 [unord.set.syn]), and <vector> (23.3.12 [vector.syn]).
Each of these templates is a designated customization point (16.4.5.2.1 [namespace.std]).
Section: 16.4.5.2.1 [namespace.std] Status: Resolved Submitter: Michael Park Opened: 2020-05-08 Last modified: 2023-03-22
Priority: 1
View other active issues in [namespace.std].
View all other issues in [namespace.std].
View all issues with Resolved status.
Discussion:
Footnote 173 under [namespace.std]/7 reads (emphasis mine):
Any library customization point must be prepared to work adequately with any user-defined overload that meets the minimum requirements of this document. Therefore an implementation may elect, under the as-if rule (6.9.1 [intro.execution]), to provide any customization point in the form of an instantiated function object (22.10 [function.objects]) even though the customization point's specification is in the form of a function template. The template parameters of each such function object and the function parameters and return type of the object's
operator()
must match those of the corresponding customization point's specification.
This implementation suggestion doesn't seem to be satisfiable with the as-if rule.
In order to maintain as-if rule for qualified calls to std::swap
, std::swap
cannot perform ADL look-up like std::ranges::swap
does.
But then we cannot maintain as-if rule for two-step calls to std::swap
, since ADL
is turned off when function objects are involved.
#include <iostream> namespace S { #ifdef CPO static constexpr auto swap = [](auto&, auto&) { std::cout << "std"; }; #else template<typename T> swap(T&, T&) { std::cout << "std"; } #endif } namespace N { struct X {}; void swap(X&, X&) { std::cout << "mine"; } } int main() { N::X a, b; S::swap(a, b); // (1) printsstd
in both cases using S::swap; swap(a, b); // (2) printsstd
with-DCPO
, andmine
without. }
We can try to satisfy the as-if rule for (2) by having std::swap
perform ADL like
std::ranges::swap
, but then that would break (1) since qualified calls to std::swap
would also ADL when it did not do so before.
[2020-07-17; Priority set to 1 in telecon]
[2020-10-02; status to Open]
[Issaquah 2023-02-09; Jonathan adds note]
This would be resolved by the new proposed resolution of LWG 3441(i).
[2023-03-22 LWG 3441 was approved in Issaquah. Status changed: Open → Resolved.]
Proposed resolution:
This wording is relative to N4861.
Modify 16.4.5.2.1 [namespace.std], footnote 173, as indicated:
footnote 173) Any library customization point must be prepared to work adequately with any user-defined overload that meets the minimum requirements of this document.
Therefore an implementation may elect, under the as-if rule (6.9.1 [intro.execution]), to provide any customization point in the form of an instantiated function object (22.10 [function.objects]) even though the customization point's specification is in the form of a function template. The template parameters of each such function object and the function parameters and return type of the object'soperator()
must match those of the corresponding customization point's specification.
net::basic_socket_iostream
should use addressof
Section: 19.2.1 [networking.ts::socket.iostream.cons] Status: C++23 Submitter: Jonathan Wakely Opened: 2020-05-14 Last modified: 2023-11-22
Priority: 0
View all other issues in [networking.ts::socket.iostream.cons].
View all issues with C++23 status.
Discussion:
Addresses: networking.ts
19.2.1 [networking.ts::socket.iostream.cons] uses &sb_
which could find something bad via ADL.
[2020-07-17; Moved to Ready in telecon]
Jens suggested we should have blanket wording saying that when the library
uses the &
operator, it means std::addressof
.
[2020-11-09 Approved In November virtual meeting. Status changed: Ready → WP.]
Proposed resolution:
This wording is relative to N4771.
Modify 19.2.1 [networking.ts::socket.iostream.cons] as indicated:
basic_socket_iostream();-1- Effects: Initializes the base class as
basic_iostream<char>(
, value-initializes&addressof(sb_))sb_
, and performssetf(std::ios_base::unitbuf)
.explicit basic_socket_iostream(basic_stream_socket<protocol_type> s);-2- Effects: Initializes the base class as
basic_iostream<char>(
, initializes&addressof(sb_))sb_
withstd::move(s)
, and performssetf(std::ios_base::unitbuf)
.basic_socket_iostream(basic_socket_iostream&& rhs);-3- Effects: Move constructs from the rvalue
rhs
. This is accomplished by move constructing the base class, and the containedbasic_socket_streambuf
. Nextbasic_iostream<char>::set_rdbuf(
is called to install the contained&addressof(sb_))basic_socket_streambuf
.template<class... Args> explicit basic_socket_iostream(Args&&... args);-4- Effects: Initializes the base class as
basic_iostream<char>(
, value-initializes&addressof(sb_))sb_
, and performssetf(std::ios_base::unitbuf)
. Then callsrdbuf()->connect(forward<Args>(args)...)
. If that function returns a null pointer, callssetstate(failbit)
.
indirectly_readable_traits
ambiguity for types with both value_type
and element_type
Section: 24.3.2.2 [readable.traits] Status: C++23 Submitter: Casey Carter Opened: 2020-05-15 Last modified: 2023-11-22
Priority: 2
View all other issues in [readable.traits].
View all issues with C++23 status.
Discussion:
Per 24.3.2.2 [readable.traits], indirectly_readable_traits<T>::value_type
is the same type as
remove_cv_t<T::value_type>
if it denotes an object type, or remove_cv_t<T::element_type>
if it denotes an object type. If both T::value_type
and T::element_type
denote types,
indirectly_readable_traits<T>::value_type
is ill-formed. This was perhaps not the best design,
given that there are iterators in the wild (Boost's unordered containers) that define both nested types.
indirectly_readable_traits
should tolerate iterators that define both nested types consistently.
[2020-07-17; Priority set to 2 in telecon]
Previous resolution [SUPERSEDED]:This wording is relative to N4861.
Modify 24.3.2.2 [readable.traits] as indicated:
[…] template<class> struct cond-value-type { }; // exposition only template<class T> requires is_object_v<T> struct cond-value-type<T> { using value_type = remove_cv_t<T>; }; template<class> struct indirectly_readable_traits { }; […] template<class T> requires requires { typename T::value_type; } struct indirectly_readable_traits<T> : cond-value-type<typename T::value_type> { }; template<class T> requires requires { typename T::element_type; } struct indirectly_readable_traits<T> : cond-value-type<typename T::element_type> { }; template<class T> requires requires { typename T::element_type; typename T::value_type; requires same_as< remove_cv_t<typename T::element_type>, remove_cv_t<typename T::value_type>>; } struct indirectly_readable_traits<T> : cond-value-type<typename T::value_type> { }; […]
[2020-07-23; Casey improves wording per reflector discussion]
[2020-08-21; moved to Tentatively Ready after five votes in favour in reflector poll]
[2020-11-09 Approved In November virtual meeting. Status changed: Tentatively Ready → WP.]
Proposed resolution:
This wording is relative to N4861.
Modify 24.3.2.2 [readable.traits] as indicated:
[…] template<class> struct cond-value-type { }; // exposition only template<class T> requires is_object_v<T> struct cond-value-type<T> { using value_type = remove_cv_t<T>; }; template<class T> concept has-member-value-type = requires { typename T::value_type; }; // exposition only template<class T> concept has-member-element-type = requires { typename T::element_type; }; // exposition only template<class> struct indirectly_readable_traits { }; […] template<classhas-member-value-type T>requires requires { typename T::value_type; }struct indirectly_readable_traits<T> : cond-value-type<typename T::value_type> { }; template<classhas-member-element-type T>requires requires { typename T::element_type; }struct indirectly_readable_traits<T> : cond-value-type<typename T::element_type> { }; template<classhas-member-value-type T> requires has-member-element-type<T> && same_as<remove_cv_t<typename T::element_type>, remove_cv_t<typename T::value_type>> struct indirectly_readable_traits<T> : cond-value-type<typename T::value_type> { }; […]
take_view
and drop_view
have different constraintsSection: 25.7.10.2 [range.take.view] Status: C++23 Submitter: Jens Maurer Opened: 2020-05-15 Last modified: 2023-11-22
Priority: 0
View all other issues in [range.take.view].
View all issues with C++23 status.
Discussion:
From this editorial issue request:
(Note "range R
" vs "class R
".)
In 25.7.10.2 [range.take.view], the deduction guide for take_view
is declared as:
template<range R> take_view(R&&, range_difference_t<R>) -> take_view<views::all_t<R>>;
In 25.7.12.2 [range.drop.view], the deduction guide for drop_view
is declared as:
template<class R> drop_view(R&&, range_difference_t<R>) -> drop_view<views::all_t<R>>;
Note the difference between their template parameter lists.
Suggested resolution: Change the deduction guide oftake_view
from
template<range R>
to
template<class R>
[2020-07-17; Moved to Ready in telecon]
[2020-11-09 Approved In November virtual meeting. Status changed: Ready → WP.]
Proposed resolution:
This wording is relative to N4861.
Modify 25.7.10.2 [range.take.view], class template take_view
synopsis, as indicated:
[…] template<rangeclass R> take_view(R&&, range_difference_t<R>) -> take_view<views::all_t<R>>; […]
transform_view
's sentinel<false>
not comparable with iterator<true>
Section: 25.7.9.4 [range.transform.sentinel], 25.7.14.4 [range.join.sentinel] Status: C++23 Submitter: Jonathan Wakely Opened: 2020-05-26 Last modified: 2023-11-22
Priority: 1
View all other issues in [range.transform.sentinel].
View all issues with C++23 status.
Discussion:
A user reported that this doesn't compile:
#include <list> #include <ranges> std::list v{1, 2}; // works if std::vector auto view1 = v | std::views::take(2); auto view2 = view1 | std::views::transform([] (int i) { return i; }); bool b = std::ranges::cbegin(view2) == std::ranges::end(view2);
The comparison is supposed to use operator==(iterator<Const>, sentinel<Const>)
after converting sentinel<false>
to sentinel<true>
. However, the
operator==
is a hidden friend so is not a candidate when comparing iterator<true>
with sentinel<false>
. The required conversion would only happen if we'd found the operator,
but we can't find the operator until after the conversion happens.
join_view
sentinel has a similar problem.
The proposed wording shown below has been suggested by Casey and has been implemented and tested in GCC's libstdc++.
[2020-07-17; Priority set to 1 in telecon]
Should be considered together with 3406(i) and 3449(i).
Previous resolution [SUPERSEDED]:
This wording is relative to N4861.
Modify 25.2 [ranges.syn], header
<ranges>
synopsis, as indicated:[Drafting note: The project editor is kindly asked to consider replacing editorially all of the
"using Base = conditional_t<Const, const V, V>;
" occurrences by "using Base = maybe-const<Const, V>;
" ][…] namespace std::ranges { […] namespace views { inline constexpr unspecified filter = unspecified; } template<bool Const, class T> using maybe-const = conditional_t<Const, const T, T>; // exposition-only // 25.7.9 [range.transform], transform view template<input_range V, copy_constructible F> requires view<V> && is_object_v<F> && regular_invocable<F&, range_reference_t<V>> class transform_view; […] }Modify 25.7.9.4 [range.transform.sentinel], class template
transform_view::sentinel
synopsis, as indicated:[…]namespace std::ranges { template<input_range V, copy_constructible F> requires view<V> && is_object_v<F> && regular_invocable<F&, range_reference_t<V>> && can-reference<invoke_result_t<F&, range_reference_t<V>>> template<bool Const> class transform_view<V, F>::sentinel { […] constexpr sentinel_t<Base> base() const; template<bool OtherConst> requires sentinel_for<sentinel_t<Base>, iterator_t<maybe-const<OtherConst, V>>> friend constexpr bool operator==(const iterator<OtherConst>& x, const sentinel& y); template<bool OtherConst> requires sized_sentinel_for<sentinel_t<Base>, iterator_t<maybe-const<OtherConst, V>>> friend constexpr range_difference_t<Base> operator-(const iterator<OtherConst>& x, const sentinel& y)requires sized_sentinel_for<sentinel_t<Base>, iterator_t<Base>>; template<bool OtherConst> requires sized_sentinel_for<sentinel_t<Base>, iterator_t<maybe-const<OtherConst, V>>> friend constexpr range_difference_t<Base> operator-(const sentinel& y, const iterator<OtherConst>& x)requires sized_sentinel_for<sentinel_t<Base>, iterator_t<Base>>; }; }template<bool OtherConst> requires sentinel_for<sentinel_t<Base>, iterator_t<maybe-const<OtherConst, V>>> friend constexpr bool operator==(const iterator<OtherConst>& x, const sentinel& y);-4- Effects: Equivalent to:
return x.current_ == y.end_;
template<bool OtherConst> requires sized_sentinel_for<sentinel_t<Base>, iterator_t<maybe-const<OtherConst, V>>> friend constexpr range_difference_t<Base> operator-(const iterator<OtherConst>& x, const sentinel& y)requires sized_sentinel_for<sentinel_t<Base>, iterator_t<Base>>;-5- Effects: Equivalent to:
return x.current_ - y.end_;
template<bool OtherConst> requires sized_sentinel_for<sentinel_t<Base>, iterator_t<maybe-const<OtherConst, V>>> friend constexpr range_difference_t<Base> operator-(const sentinel& y, const iterator<OtherConst>& x)requires sized_sentinel_for<sentinel_t<Base>, iterator_t<Base>>;-6- Effects: Equivalent to:
return y.end_ - x.current_;
Modify 25.7.14.4 [range.join.sentinel], class template
join_view::sentinel
synopsis, as indicated:[…]namespace std::ranges { template<input_range V> requires view<V> && input_range<range_reference_t<V>> && (is_reference_v<range_reference_t<V>> || view<range_value_t<V>>) template<bool Const> class join_view<V>::sentinel { […] template<bool OtherConst> requires sentinel_for<sentinel_t<Base>, iterator_t<maybe-const<OtherConst, V>>> friend constexpr bool operator==(const iterator<OtherConst>& x, const sentinel& y); }; }template<bool OtherConst> requires sentinel_for<sentinel_t<Base>, iterator_t<maybe-const<OtherConst, V>>> friend constexpr bool operator==(const iterator<OtherConst>& x, const sentinel& y);-3- Effects: Equivalent to:
return x.outer_ == y.end_;
[2020-08-21 Tim updates PR per telecon discussion]
As noted in the PR of LWG 3406(i), the return type of operator-
should be based on the constness of the iterator rather than that of the sentinel, as
sized_sentinel_for<S, I>
(24.3.4.8 [iterator.concept.sizedsentinel])
requires decltype(i - s)
to be iter_difference_t<I>
.
[2020-10-02; Status to Tentatively Ready after five positive votes on the reflector]
[2020-11-09 Approved In November virtual meeting. Status changed: Tentatively Ready → WP.]
Proposed resolution:
This wording is relative to N4861.
Modify 25.2 [ranges.syn], header <ranges>
synopsis, as indicated:
[Drafting note: The project editor is kindly asked to consider replacing editorially all of the
"using Base = conditional_t<Const, const V, V>;
" occurrences by "using Base = maybe-const<Const, V>;
" ]
[…] namespace std::ranges { […] namespace views { inline constexpr unspecified filter = unspecified; } template<bool Const, class T> using maybe-const = conditional_t<Const, const T, T>; // exposition-only // 25.7.9 [range.transform], transform view template<input_range V, copy_constructible F> requires view<V> && is_object_v<F> && regular_invocable<F&, range_reference_t<V>> class transform_view; […] }
Modify 25.7.9.4 [range.transform.sentinel], class template transform_view::sentinel
synopsis, as indicated:
[…]namespace std::ranges { template<input_range V, copy_constructible F> requires view<V> && is_object_v<F> && regular_invocable<F&, range_reference_t<V>> && can-reference<invoke_result_t<F&, range_reference_t<V>>> template<bool Const> class transform_view<V, F>::sentinel { […] constexpr sentinel_t<Base> base() const; template<bool OtherConst> requires sentinel_for<sentinel_t<Base>, iterator_t<maybe-const<OtherConst, V>>> friend constexpr bool operator==(const iterator<OtherConst>& x, const sentinel& y); template<bool OtherConst> requires sized_sentinel_for<sentinel_t<Base>, iterator_t<maybe-const<OtherConst, V>>> friend constexpr range_difference_t<Basemaybe-const<OtherConst, V>> operator-(const iterator<OtherConst>& x, const sentinel& y)requires sized_sentinel_for<sentinel_t<Base>, iterator_t<Base>>; template<bool OtherConst> requires sized_sentinel_for<sentinel_t<Base>, iterator_t<maybe-const<OtherConst, V>>> friend constexpr range_difference_t<Basemaybe-const<OtherConst, V>> operator-(const sentinel& y, const iterator<OtherConst>& x)requires sized_sentinel_for<sentinel_t<Base>, iterator_t<Base>>; }; }template<bool OtherConst> requires sentinel_for<sentinel_t<Base>, iterator_t<maybe-const<OtherConst, V>>> friend constexpr bool operator==(const iterator<OtherConst>& x, const sentinel& y);-4- Effects: Equivalent to:
return x.current_ == y.end_;
template<bool OtherConst> requires sized_sentinel_for<sentinel_t<Base>, iterator_t<maybe-const<OtherConst, V>>> friend constexpr range_difference_t<Basemaybe-const<OtherConst, V>> operator-(const iterator<OtherConst>& x, const sentinel& y)requires sized_sentinel_for<sentinel_t<Base>, iterator_t<Base>>;-5- Effects: Equivalent to:
return x.current_ - y.end_;
template<bool OtherConst> requires sized_sentinel_for<sentinel_t<Base>, iterator_t<maybe-const<OtherConst, V>>> friend constexpr range_difference_t<Basemaybe-const<OtherConst, V>> operator-(const sentinel& y, const iterator<OtherConst>& x)requires sized_sentinel_for<sentinel_t<Base>, iterator_t<Base>>;-6- Effects: Equivalent to:
return y.end_ - x.current_;
Modify 25.7.14.4 [range.join.sentinel], class template join_view::sentinel
synopsis, as indicated:
[…]namespace std::ranges { template<input_range V> requires view<V> && input_range<range_reference_t<V>> && (is_reference_v<range_reference_t<V>> || view<range_value_t<V>>) template<bool Const> class join_view<V>::sentinel { […] template<bool OtherConst> requires sentinel_for<sentinel_t<Base>, iterator_t<maybe-const<OtherConst, V>>> friend constexpr bool operator==(const iterator<OtherConst>& x, const sentinel& y); }; }template<bool OtherConst> requires sentinel_for<sentinel_t<Base>, iterator_t<maybe-const<OtherConst, V>>> friend constexpr bool operator==(const iterator<OtherConst>& x, const sentinel& y);-3- Effects: Equivalent to:
return x.outer_ == y.end_;
take_view
and take_while_view
's sentinel<false>
not comparable with their const iteratorSection: 25.7.10.3 [range.take.sentinel], 25.7.11.3 [range.take.while.sentinel] Status: C++23 Submitter: Tim Song Opened: 2020-06-06 Last modified: 2023-11-22
Priority: 1
View all other issues in [range.take.sentinel].
View all issues with C++23 status.
Discussion:
During reflector discussion it was noted that the issue observed in
LWG 3448(i) applies to take_view::sentinel
and take_while_view::sentinel
as well.
[2020-06-15 Tim corrects a typo in the PR]
[2020-10-02; Priority set to 1 in telecon]
Should be considered together with 3406(i) and 3448(i).
[2020-10-02; Status to Tentatively Ready after five positive votes on the reflector]
[2020-11-09 Approved In November virtual meeting. Status changed: Tentatively Ready → WP.]
Proposed resolution:
This wording is relative to N4861. It assumes
the maybe-const
helper introduced by the P/R of LWG 3448(i).
Modify 25.7.10.3 [range.take.sentinel] as indicated:
[Drafting note: Unlike LWG 3448(i), these operators can't be easily templatized, so the approach taken here is to add an overload for the opposite constness instead. However, because
const V
may not even be a range (and thereforeiterator_t<const V>
may not be well-formed), we need to defer instantiating the signature of the new overload until the point of use. ]
[…]namespace std::ranges { template<view V> template<bool Const> class take_view<V>::sentinel { private: using Base = conditional_t<Const, const V, V>; // exposition only template<bool OtherConst> using CI = counted_iterator<iterator_t<Basemaybe-const<OtherConst, V>>>; // exposition only […] constexpr sentinel_t<Base> base() const; friend constexpr bool operator==(const CI<Const>& y, const sentinel& x); template<bool OtherConst = !Const> requires sentinel_for<sentinel_t<Base>, iterator_t<maybe-const<OtherConst, V>>> friend constexpr bool operator==(const CI<OtherConst>& y, const sentinel& x); }; }friend constexpr bool operator==(const CI<Const>& y, const sentinel& x); template<bool OtherConst = !Const> requires sentinel_for<sentinel_t<Base>, iterator_t<maybe-const<OtherConst, V>>> friend constexpr bool operator==(const CI<OtherConst>& y, const sentinel& x);-4- Effects: Equivalent to:
return y.count() == 0 || y.base() == x.end_;
Modify 25.7.11.3 [range.take.while.sentinel] as indicated:
[…]namespace std::ranges { template<view V, class Pred> requires input_range<V> && is_object_v<Pred> && indirect_unary_predicate<const Pred, iterator_t<V>> template<bool Const> class take_while_view<V>::sentinel { […] constexpr sentinel_t<Base> base() const { return end_; } friend constexpr bool operator==(const iterator_t<Base>& x, const sentinel& y); template<bool OtherConst = !Const> requires sentinel_for<sentinel_t<Base>, iterator_t<maybe-const<OtherConst, V>>> friend constexpr bool operator==(const iterator_t<maybe-const<OtherConst, V>>& x, const sentinel& y); }; }friend constexpr bool operator==(const iterator_t<Base>& x, const sentinel& y); template<bool OtherConst = !Const> requires sentinel_for<sentinel_t<Base>, iterator_t<maybe-const<OtherConst, V>>> friend constexpr bool operator==(const iterator_t<maybe-const<OtherConst, V>>& x, const sentinel& y);-3- Effects: Equivalent to:
return y.end_ == x || !invoke(*y.pred_, *x);
const
overloads of take_while_view::begin/end
are underconstrainedSection: 25.7.11.2 [range.take.while.view] Status: C++23 Submitter: Tim Song Opened: 2020-06-06 Last modified: 2023-11-22
Priority: 0
View all other issues in [range.take.while.view].
View all issues with C++23 status.
Discussion:
The const
overloads of take_while_view::begin
and take_while_view::end
are underconstrained: they should require
the predicate to model indirect_unary_predicate<iterator_t<const V>>
.
A simple example is
#include <ranges> auto v = std::views::single(1) | std::views::take_while([](int& x) { return true;}); static_assert(std::ranges::range<decltype(v)>); static_assert(std::ranges::range<decltype(v) const>); bool b = std::ranges::cbegin(v) == std::ranges::cend(v);
Here, the static_assert
s pass but the comparison fails to compile. The other
views using indirect_unary_predicate
(filter_view
and
drop_while_view
) do not have this problem because they do not support
const-iteration.
[2020-07-17; Moved to Ready in telecon]
[2020-11-09 Approved In November virtual meeting. Status changed: Ready → WP.]
Proposed resolution:
This wording is relative to N4861.
Modify 25.7.11.2 [range.take.while.view], class template take_while_view
synopsis, as indicated:
namespace std::ranges { template<view V, class Pred> requires input_range<V> && is_object_v<Pred> && indirect_unary_predicate<const Pred, iterator_t<V>> class take_while_view : public view_interface<take_while_view<V, Pred>> { […] constexpr auto begin() requires (!simple-view<V>) { return ranges::begin(base_); } constexpr auto begin() const requires range<const V> && indirect_unary_predicate<const Pred, iterator_t<const V>> { return ranges::begin(base_); } constexpr auto end() requires (!simple-view<V>) { return sentinel<false>(ranges::end(base_), addressof(*pred_)); } constexpr auto end() const requires range<const V> && indirect_unary_predicate<const Pred, iterator_t<const V>> { return sentinel<true>(ranges::end(base_), addressof(*pred_)); } }; }
𝒪(1)
destruction?Section: 25.4.4 [range.view] Status: Resolved Submitter: Mathias Stearn Opened: 2020-06-16 Last modified: 2021-10-23
Priority: 2
View all other issues in [range.view].
View all issues with Resolved status.
Discussion:
The second bullet of 25.4.4 [range.view] paragraph 3 says
"Examples of views are:[…]
A range type that holds its elements by shared_ptr
and shares ownership with all its copies".
That clearly does not have 𝒪(1)
destruction in all cases.
However, that does seem like a useful type of view,
and is related to the discussions around the proposed std::generator
.
What is the purpose of requiring 𝒪(1)
destruction?
Would it still be achieved by weakening it slightly to something like
"If N copies and/or moves are made from a view that yields M values,
destroying all of them takes time proportional to at worst 𝒪(N+M)
"?
This in particular prevents the 𝒪(N*M)
case that I think the rules
are trying to prevent, while still allowing some more interesting types of views.
If instead we actually really do want strict 𝒪(1)
destruction,
then the example needs to be fixed.
[2020-06-26; Reflector prioritization]
Set priority to 2 after reflector discussions.
[2021-01-15; Telecon discussion]
Set status to LEWG for guidance on the issue of what it means to model view.
[2021-02-16; Library Evolution Telecon Minutes]
Poll: The shared_ptr
part of the example mentioned in LWG3452 should be removed.
SF F N A SA 4 6 3 1 0
Attendance: 25
Outcome: Remove theshared_ptr
part.
Previous resolution [SUPERSEDED]:
This wording is relative to N4878.
Modify 25.4.4 [range.view] as indicated:
template<class T> concept view = range<T> && movable<T> && default_initializable<T> && enable_view<T>;[…]-3- [Example 1: Examples of
view
s are:
(3.1) — A
range
type that wraps a pair of iterators.
(3.2) — Arange
type that holds its elements by shared_ptr and shares ownership with all its copies.(3.3) — A
range
type that generates its elements on demand.Most containers (Clause 22) are not views since destruction of the container destroys the elements, which cannot be done in constant time. — end example]
[2021-10-23 Resolved by the adoption of P2415R2 at the October 2021 plenary. Status changed: LEWG → Resolved.]
Proposed resolution:
ranges::advance(i, s)
Section: 24.4.4.2 [range.iter.op.advance], 24.3.4.7 [iterator.concept.sentinel] Status: C++23 Submitter: Casey Carter Opened: 2020-06-18 Last modified: 2023-11-22
Priority: 2
View all other issues in [range.iter.op.advance].
View all issues with C++23 status.
Discussion:
The specification of the iterator & sentinel overload of ranges::advance
in 24.4.4.2 [range.iter.op.advance] reads:
template<input_or_output_iterator I, sentinel_for<I> S> constexpr void ranges::advance(I& i, S bound);-3- Preconditions:
-4- Effects:[i, bound)
denotes a range.
(4.1) — If
I
andS
modelassignable_from<I&, S>
, equivalent toi = std::move(bound)
.(4.2) — […]
The assignment optimization in bullet 4.1 is just fine for callers with concrete types who can decide whether
or not to call advance
depending on the semantics of the assignment performed. However, since this assignment
operation isn't part of the input_or_output_iterator
or sentinel_for
requirements its semantics
are unknown for arbitrary types. Effectively, generic code is forbidden to call this overload of advance
when
assignable_from<I&, S>
is satisfied and non-generic code must tread lightly. This seems to
make the library dangerously unusable.
We can correct this problem by either:
Making the assignment operation in question an optional part of the sentinel_for
concept with well-defined
semantics. This concept change should be relatively safe given that assignable_from<I&, S>
requires common_reference_with<const I&, const S&>
, which is very rarely satisfied
inadvertently.
Requiring instead same_as<I, S>
to trigger the assignment optimization in bullet 4.1 above.
S
is semiregular
, so i = std::move(s)
is certainly well-formed (and has well-defined
semantics thanks to semiregular
) when I
and S
are the same type. The optimization will
not apply in as many cases, but we don't need to make a scary concept change, either.
[2020-06-26; Reflector prioritization]
Set priority to 2 after reflector discussions.
[2020-08-21; Issue processing telecon: Option A is Tentatively Ready]
Previous resolution [SUPERSEDED]:
This wording is relative to N4861.
Wording for both Option A and Option B are provided.
Option A:
Modify 24.3.4.7 [iterator.concept.sentinel] as indicated:
template<class S, class I> concept sentinel_for = semiregular<S> && input_or_output_iterator<I> && weakly-equality-comparable-with<S, I>; // See 18.5.4 [concept.equalitycomparable]-2- Let
s
andi
be values of typeS
andI
such that[i, s)
denotes a range. TypesS
andI
modelsentinel_for<S, I>
only if
(2.1) —
i == s
is well-defined.(2.2) — If
bool(i != s)
theni
is dereferenceable and[++i, s)
denotes a range.(2.?) —
assignable_from<I&, S>
is either modeled or not satisfied.Option B:
Modify 24.4.4.2 [range.iter.op.advance] as indicated:
template<input_or_output_iterator I, sentinel_for<I> S> constexpr void ranges::advance(I& i, S bound);-3- Preconditions:
-4- Effects:[i, bound)
denotes a range.
(4.1) — If
I
andS
model, equivalent to
assignable_from<I&, S>same_as<I, S>i = std::move(bound)
.(4.2) — […]
[2020-11-09 Approved In November virtual meeting. Status changed: Tentatively Ready → WP.]
Proposed resolution:
This wording is relative to N4861.
Modify 24.3.4.7 [iterator.concept.sentinel] as indicated:
template<class S, class I> concept sentinel_for = semiregular<S> && input_or_output_iterator<I> && weakly-equality-comparable-with<S, I>; // See 18.5.4 [concept.equalitycomparable]-2- Let
s
andi
be values of typeS
andI
such that[i, s)
denotes a range. TypesS
andI
modelsentinel_for<S, I>
only if
(2.1) —
i == s
is well-defined.(2.2) — If
bool(i != s)
theni
is dereferenceable and[++i, s)
denotes a range.(2.?) —
assignable_from<I&, S>
is either modeled or not satisfied.
unique_ptr
move assignmentSection: 20.3.1.3.4 [unique.ptr.single.asgn] Status: C++23 Submitter: Howard Hinnant Opened: 2020-06-22 Last modified: 2023-11-22
Priority: 0
View all other issues in [unique.ptr.single.asgn].
View all issues with C++23 status.
Discussion:
20.3.1.3.4 [unique.ptr.single.asgn]/p5 says this about the unique_ptr
move assignment operator:
Postconditions:
u.get() == nullptr
.
But this is only true if this != &u
. For example:
#include <iostream> #include <memory> int main() { auto x = std::unique_ptr<int>(new int{3}); x = std::move(x); if (x) std::cout << *x << '\n'; else std::cout << "nullptr\n"; }
Output:
3
An alternative resolution to that proposed below is to just delete the Postcondition altogether as the Effects element completely specifies everything. If we do that, then we should also remove p10, the Postconditions element for the converting move assignment operator. I have a slight preference for the proposed change below as it is more informative, at the expense of being a little more repetitive.
[2020-06-26; Reflector prioritization]
Set priority to 0 and status to Tentatively Ready after seven votes in favor during reflector discussions.
[2020-11-09 Approved In November virtual meeting. Status changed: Tentatively Ready → WP.]
Proposed resolution:
This wording is relative to N4861.
Modify 20.3.1.3.4 [unique.ptr.single.asgn] as indicated:
unique_ptr& operator=(unique_ptr&& u) noexcept;-1- Constraints:
-2- Preconditions: Ifis_move_assignable_v<D>
istrue
.D
is not a reference type,D
meets the Cpp17MoveAssignable requirements (Table [tab:cpp17.moveassignable]) and assignment of the deleter from an rvalue of typeD
does not throw an exception. Otherwise,D
is a reference type;remove_reference_t<D>
meets the Cpp17CopyAssignable requirements and assignment of the deleter from an lvalue of typeD
does not throw an exception. -3- Effects: Callsreset(u.release())
followed byget_deleter() = std::forward<D>(u.get_deleter())
. -4- Returns:*this
. -5- Postconditions: Ifthis != addressof(u)
,u.get() == nullptr
, otherwiseu.get()
is unchanged.
shared_future
intended to work with arrays or function types?Section: 32.10.7 [futures.unique.future], 32.10.8 [futures.shared.future] Status: Resolved Submitter: Tomasz Kamiński Opened: 2020-06-28 Last modified: 2020-11-09
Priority: 0
View all other issues in [futures.unique.future].
View all issues with Resolved status.
Discussion:
Currently there is no prohibition of creating shared_future<T>
where T
is
either an array type or a function type. However such objects cannot be constructed, as the corresponding
future<T>
, is ill-formed due the signature get()
method being ill-formed —
it will be declared as function returning an array or function type, respectively. [Note: For
shared_future
get
always returns an reference, so it is well-formed for array or function
types.]
[2020-07-17; Reflector prioritization]
Set priority to 0 and status to Tentatively Ready after six votes in favour during reflector discussions.
Previous resolution [SUPERSEDED]:
This wording is relative to N4861.
Ideally the wording below would use a Mandates: element, but due to the still open issue LWG 3193(i) the wording below uses instead the more general "ill-formed" vocabulary.
Modify 32.10.7 [futures.unique.future] as indicated:
namespace std { template<class R> class future { […] }; }-?- If
-4- The implementation provides the templateis_array_v<R>
istrue
oris_function_v<R>
istrue
, the program is ill-formed.future
and two specializations,future<R&>
andfuture<void>
. These differ only in the return type and return value of the member functionget
, as set out in its description, below.Modify 32.10.8 [futures.shared.future] as indicated:
namespace std { template<class R> class shared_future { […] }; }-?- If
-4- The implementation provides the templateis_array_v<R>
istrue
oris_function_v<R>
istrue
, the program is ill-formed.shared_future
and two specializations,shared_future<R&>
andshared_future<void>
. These differ only in the return type and return value of the member functionget
, as set out in its description, below.
[2020-08-02; Daniel comments]
I'm request to reopen the issue and to follow the recent wording update of LWG 3466(i) instead.
[2020-11-09 Resolved by acceptance of 3466(i). Status changed: Tentatively Resolved → Resolved.]
Proposed resolution:
noop_coroutine_handle
guaranteesSection: 17.13.5.2.4 [coroutine.handle.noop.resumption] Status: C++23 Submitter: Casey Carter Opened: 2020-07-01 Last modified: 2023-11-22
Priority: 2
View all issues with C++23 status.
Discussion:
17.13.5.2.4 [coroutine.handle.noop.resumption]/2 states "Remarks: If noop_coroutine_handle
is converted to coroutine_handle<>
, calls to operator()
, resume
and
destroy
on that handle will also have no observable effects." This suggests that e.g. in this function:
void f(coroutine_handle<> meow) { auto woof = noop_coroutine(); static_cast<coroutine_handle<>&>(woof) = meow; static_cast<coroutine_handle<>&>(woof).resume(); }
the final call to coroutine_handle<>::resume
must have no effect regardless of what
coroutine (if any) meow
refers to, contradicting the specification of
coroutine_handle<>::resume
. Even absent this contradiction, implementing the specification
requires coroutine_handle<>::resume
to determine if *this
is a base subobject of a
noop_coroutine_handle
, which seems pointlessly expensive to implement.
noop_coroutine_handle
's
ptr
is always a non-null pointer value." Similar to the above case, a slicing assignment of a
default-initialized coroutine_handle<>
to a noop_coroutine_handle
must result in
ptr
having a null pointer value — another contradiction between the requirements of
noop_coroutine_handle
and coroutine_handle<>
.
[2020-07-12; Reflector prioritization]
Set priority to 2 after reflector discussions.
[2020-07-29 Tim adds PR and comments]
The root cause for this issue as well as issue 3469(i) is the unnecessary
public derivation from coroutine_handle<void>
. The proposed resolution below
replaces the derivation with a conversion function and adds explicit declarations for
members that were previously inherited. It also modifies the preconditions
on from_address
with goal of making it impossible to obtain a coroutine_handle<P>
to a coroutine whose promise type is not P
in well-defined code.
[2020-08-21; Issue processing telecon: moved to Tentatively Ready]
[2020-11-09 Approved In November virtual meeting. Status changed: Tentatively Ready → WP.]
Proposed resolution:
This wording is relative to N4861 and also resolves LWG issue 3469(i).
Edit 17.13.4 [coroutine.handle] as indicated:
namespace std { […] template<class Promise> struct coroutine_handle: coroutine_handle<>{ // [coroutine.handle.con], construct/resetusing coroutine_handle<>::coroutine_handle;constexpr coroutine_handle() noexcept; constexpr coroutine_handle(nullptr_t) noexcept; static coroutine_handle from_promise(Promise&); coroutine_handle& operator=(nullptr_t) noexcept; // [coroutine.handle.export.import], export/import constexpr void* address() const noexcept; static constexpr coroutine_handle from_address(void* addr); // [coroutine.handle.conv], conversion constexpr operator coroutine_handle<>() const noexcept; // [coroutine.handle.observers], observers constexpr explicit operator bool() const noexcept; bool done() const; // [coroutine.handle.resumption], resumption void operator()() const; void resume() const; void destroy() const; // [coroutine.handle.promise], promise access Promise& promise() const; private: void* ptr; // exposition only }; }-1- An object of type
coroutine_handle<T>
is called a coroutine handle and can be used to refer to a suspended or executing coroutine. Adefault-constructedcoroutine_handle
object whose memberaddress()
returns a null pointer value does not refer to any coroutine. Twocoroutine_handle
objects refer to the same coroutine if and only if their memberaddress()
returns the same value.
Add the following subclause under 17.13.4 [coroutine.handle], immediately after 17.13.4.2 [coroutine.handle.con]:
?.?.?.? Conversion [coroutine.handle.conv]
constexpr operator coroutine_handle<>() const noexcept;-1- Effects: Equivalent to:
return coroutine_handle<>::from_address(address());
.
Edit 17.13.4.4 [coroutine.handle.export.import] as indicated, splitting the two versions:
static constexpr coroutine_handle<> coroutine_handle<>::from_address(void* addr);-?- Preconditions:
-?- Postconditions:addr
was obtained via a prior call toaddress
on an object whose type is a specialization ofcoroutine_handle
.from_address(address()) == *this
.static constexpr coroutine_handle<Promise> coroutine_handle<Promise>::from_address(void* addr);-2- Preconditions:
-3- Postconditions:addr
was obtained via a prior call toaddress
on an object of type cvcoroutine_handle<Promise>
.from_address(address()) == *this
.
Edit 17.13.5.2 [coroutine.handle.noop] as indicated:
namespace std { template<> struct coroutine_handle<noop_coroutine_promise>: coroutine_handle<>{ // [coroutine.handle.noop.conv], conversion constexpr operator coroutine_handle<>() const noexcept; // [coroutine.handle.noop.observers], observers constexpr explicit operator bool() const noexcept; constexpr bool done() const noexcept; // [coroutine.handle.noop.resumption], resumption constexpr void operator()() const noexcept; constexpr void resume() const noexcept; constexpr void destroy() const noexcept; // [coroutine.handle.noop.promise], promise access noop_coroutine_promise& promise() const noexcept; // [coroutine.handle.noop.address], address constexpr void* address() const noexcept; private: coroutine_handle(unspecified); void* ptr; // exposition only }; }
Add the following subclause under 17.13.5.2 [coroutine.handle.noop], immediately before 17.13.5.2.3 [coroutine.handle.noop.observers]:
?.?.?.?.? Conversion [coroutine.handle.noop.conv]
constexpr operator coroutine_handle<>() const noexcept;-1- Effects: Equivalent to:
return coroutine_handle<>::from_address(address());
.
convertible_to
's description mishandles cv-qualified void
Section: 18.4.4 [concept.convertible] Status: C++23 Submitter: Tim Song Opened: 2020-07-03 Last modified: 2023-11-22
Priority: 0
View other active issues in [concept.convertible].
View all other issues in [concept.convertible].
View all issues with C++23 status.
Discussion:
There are no expressions of type cv-qualified void
because any such
expression must be prvalues and 7.2.2 [expr.type]/2 states:
If a prvalue initially has the type "cv
T
", whereT
is a cv-unqualified non-class, non-array type, the type of the expression is adjusted toT
prior to any further analysis.
However, 18.4.4 [concept.convertible] p1 states:
Given types
From
andTo
and an expressionE
such thatdecltype((E))
isadd_rvalue_reference_t<From>
,convertible_to<From, To>
requiresE
to be both implicitly and explicitly convertible to typeTo
.
When From
is cv-qualified void
, E
does not exist, yet we do want
convertible_to<const void, void>
to be modeled.
[2020-07-12; Reflector prioritization]
Set priority to 0 and status to Tentatively Ready after five votes in favour during reflector discussions.
[2020-11-09 Approved In November virtual meeting. Status changed: Tentatively Ready → WP.]
Proposed resolution:
This wording is relative to N4861.
Modify 18.4.4 [concept.convertible] as indicated:
-1- Given types
From
andTo
and an expressionE
such thatwhose type and value category are the same as those ofdecltype((E))
isadd_rvalue_reference_t<From>
declval<From>()
,convertible_to<From, To>
requiresE
to be both implicitly and explicitly convertible to typeTo
. The implicit and explicit conversions are required to produce equal results.
fc.arg()
Section: 28.5.6.1 [formatter.requirements] Status: C++23 Submitter: Alberto Barbati Opened: 2020-06-30 Last modified: 2023-11-22
Priority: 3
View all other issues in [formatter.requirements].
View all issues with C++23 status.
Discussion:
The requirements on the expression f.format(t, fc)
in [tab:formatter] say
Formats
t
according to the specifiers stored in*this
, writes the output tofc.out()
and returns an iterator past the end of the output range. The output shall only depend ont
,fc.locale()
, and the range[pc.begin(), pc.end())
from the last call tof.parse(pc)
.
Strictly speaking, this wording effectively forbids f.format(t, fc)
from calling fc.arg(n)
,
whose motivation is precisely to allow a formatter to rely on arguments different from t
. According to this
interpretation, there's no conforming way to implement the "{ arg-id }"
form of the width and
precision fields of standard format specifiers. Moreover, the formatter described in the example if paragraph
28.5.6.7 [format.context]/8 would also be non-conforming.
[2020-07-12; Reflector prioritization]
Set priority to 3 after reflector discussions.
Previous resolution [SUPERSEDED]:
This wording is relative to N4861.
Modify 28.5.6.1 [formatter.requirements], Table [tab:formatter], as indicated:
Table 67: Formatter requirements [tab:formatter] Expression Return type Requirement …
f.format(t, fc)
FC::iterator
Formats t
according to the specifiers stored in*this
, writes the output tofc.out()
and returns an iterator past the end of the output range. The output shall only depend ont
,fc.locale()
,andthe range[pc.begin(), pc.end())
from the last call tof.parse(pc)
, andfc.arg(n)
, wheren
is asize_t
index value that has been validated with a call topc.check_arg_id(n)
in the last call tof.parse(pc)
.…
[2021-05-20 Tim comments and updates wording]
During reflector discussion Victor said that the formatter requirements should allow dependency on any of the format arguments in the context. The wording below reflects that.
[2021-05-24; Reflector poll]
Set status to Tentatively Ready after six votes in favour during reflector poll.
[2021-06-07 Approved at June 2021 virtual plenary. Status changed: Voting → WP.]
Proposed resolution:
This wording is relative to N4885.
Modify 28.5.6.1 [formatter.requirements], Table [tab:formatter], as indicated:
Table 67: Formatter requirements [tab:formatter] Expression Return type Requirement …
f.format(t, fc)
FC::iterator
Formats t
according to the specifiers stored in*this
, writes the output tofc.out()
and returns an iterator past the end of the output range. The output shall only depend ont
,fc.locale()
,fc.arg(n)
for any valuen
of typesize_t
, and the range[pc.begin(), pc.end())
from the last call tof.parse(pc)
.…
istream::gcount()
can overflowSection: 31.7.5.4 [istream.unformatted] Status: C++23 Submitter: Jonathan Wakely Opened: 2020-07-10 Last modified: 2023-11-22
Priority: 0
View all other issues in [istream.unformatted].
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Discussion:
The standard doesn't say what gcount()
should return if the last unformatted input
operation extracted more than numeric_limits<streamsize>::max()
characters.
This is possible when using istream::ignore(numeric_limits<streamsize>::max(), delim)
,
which will keep extracting characters until the delimiter is found. On a 32-bit platform files
larger than 2GB can overflow the counter, so can a streambuf reading from a network socket,
or producing random characters.
istream::ignore
, so that gcount()
returns
numeric_limits<streamsize>::max()
. Libc++ results in an integer overflow.
As far as I'm aware, only istream::ignore
can extract more than
numeric_limits<streamsize>::max()
characters at once. We could either fix it
in the specification of ignore
, or in gcount
.
Previous resolution [SUPERSEDED]:
This wording is relative to N4861.
Option A:Option B:
Modify 31.7.5.4 [istream.unformatted] as indicated:
streamsize gcount() const;-2- Effects: None. This member function does not behave as an unformatted input function (as described above).
-3- Returns: The number of characters extracted by the last unformatted input member function called for the object. If the number cannot be represented, returnsnumeric_limits<streamsize>::max()
.
Modify 31.7.5.4 [istream.unformatted] as indicated:
basic_istream<charT, traits>& ignore(streamsize n = 1, int_type delim = traits::eof());-25- Effects: Behaves as an unformatted input function (as described above). After constructing a sentry object, extracts characters and discards them. Characters are extracted until any of the following occurs:
(25.1) —
n != numeric_limits<streamsize>::max()
(17.3.5 [numeric.limits]) andn
characters have been extracted so far(25.2) — end-of-file occurs on the input sequence (in which case the function calls
setstate(eofbit)
, which may throwios_base::failure
(31.5.4.4 [iostate.flags]));(25.3) —
traits::eq_int_type(traits::to_int_type(c), delim)
for the next available input characterc
(in which casec
is extracted).-?- If the number of characters extracted is greater than
-26- Remarks: The last condition will never occur ifnumeric_limits<streamsize>::max()
then for the purposes ofgcount()
the number is treated asnumeric_limits<streamsize>::max()
.traits::eq_int_type(delim, traits::eof())
. -27- Returns:*this
.
[2020-07-17; Moved to Ready in telecon]
On the reflector Davis pointed out that there are other
members which can cause gcount()
to overflow.
There was unanimous agreement on the reflector and the telecon
that Option A is better.
[2020-11-09 Approved In November virtual meeting. Status changed: Ready → WP.]
Proposed resolution:
This wording is relative to N4861.
Modify 31.7.5.4 [istream.unformatted] as indicated:
streamsize gcount() const;-2- Effects: None. This member function does not behave as an unformatted input function (as described above).
-3- Returns: The number of characters extracted by the last unformatted input member function called for the object. If the number cannot be represented, returnsnumeric_limits<streamsize>::max()
.
compare_partial_order_fallback
requires F < E
Section: 17.12.6 [cmp.alg] Status: C++23 Submitter: Stephan T. Lavavej Opened: 2020-07-18 Last modified: 2023-11-22
Priority: 0
View other active issues in [cmp.alg].
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Discussion:
compare_partial_order_fallback
uses three expressions, but requires only two. The decayed types
of E
and F
are required to be identical, but variations in constness might make a difference.
[2020-07-26; Reflector prioritization]
Set priority to 0 and status to Tentatively Ready after seven votes in favour during reflector discussions.
[2020-11-09 Approved In November virtual meeting. Status changed: Tentatively Ready → WP.]
Proposed resolution:
This wording is relative to N4861.
Modify 17.12.6 [cmp.alg] as indicated:
-6- The name
compare_partial_order_fallback
denotes a customization point object (16.3.3.3.5 [customization.point.object]). Given subexpressionsE
andF
, the expressioncompare_partial_order_fallback(E, F)
is expression-equivalent ( [defns.expression-equivalent]) to:
(6.1) — If the decayed types of
E
andF
differ,compare_partial_order_fallback(E, F)
is ill-formed.(6.2) — Otherwise,
partial_order(E, F)
if it is a well-formed expression.(6.3) — Otherwise, if the expressions
E == F
,andE < F
, andF < E
are allbothwell-formed and convertible tobool
,E == F ? partial_ordering::equivalent : E < F ? partial_ordering::less : F < E ? partial_ordering::greater : partial_ordering::unorderedexcept that
E
andF
are evaluated only once.(6.4) — Otherwise,
compare_partial_order_fallback(E, F)
is ill-formed.
promise
/future
/shared_future
consistentlySection: 32.10.6 [futures.promise] Status: C++23 Submitter: Tomasz Kamiński Opened: 2020-07-18 Last modified: 2023-11-22
Priority: 3
View other active issues in [futures.promise].
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Discussion:
The resolution of the LWG 3458(i) clearly specified the requirement that
future
/shared_future
are ill-formed in situations when T
is native array
or function type. This requirement was not strictly necessary for future<T>
as it was already ill-formed due the signature of the get
function (that would be
ill-formed in such case), however it was still added for consistency of specification.
Similar, requirement should be introduced for the promise<T>
, for which
any call to get_future()
would be ill-formed, if T
is of array or function type.
promise<int[10]>
is ill-formed for libstdc++ and libc++, see
this code]
[2020-07-26; Reflector prioritization]
Set priority to 3 after reflector discussions. Tim Song made the suggestion to replace the P/R wording by the following alternative wording:
For the primary template,
R
shall be an object type that meets the Cpp17Destructible requirements.
Previous resolution [SUPERSEDED]:
This wording is relative to N4861.
Ideally the wording below would use a Mandates: element, but due to the still open issue LWG 3193(i) the wording below uses instead the more general "ill-formed" vocabulary.
Modify 32.10.6 [futures.promise] as indicated:
namespace std { template<class R> class promise { […] }; […] }-?- If
is_array_v<R>
istrue
oris_function_v<R>
istrue
, the program is ill-formed.
[2020-08-02; Daniel comments and provides alternative wording]
Following the suggestion of Tim Song a revised wording is provided which is intended to replace the currently agreed on wording for LWG 3458(i).
[2020-08-21; Issue processing telecon: Tentatively Ready]
Discussed a note clarifying that Cpp17Destructible disallows arrays (as well as types without accessible destructors). Can be added editorially.
[2020-11-09 Approved In November virtual meeting. Status changed: Tentatively Ready → WP.]
Proposed resolution:
This wording is relative to N4861.
Modify 32.10.6 [futures.promise] as indicated:
namespace std { template<class R> class promise { […] }; […] }-?- For the primary template,
-1- The implementation provides the templateR
shall be an object type that meets the Cpp17Destructible requirements.promise
and two specializations,promise<R&>
andpromise<void>
. These differ only in the argument type of the member functionsset_value
andset_value_at_thread_exit
, as set out in their descriptions, below.
Modify 32.10.7 [futures.unique.future] as indicated:
namespace std { template<class R> class future { […] }; }-?- For the primary template,
-4- The implementation provides the templateR
shall be an object type that meets the Cpp17Destructible requirements.future
and two specializations,future<R&>
andfuture<void>
. These differ only in the return type and return value of the member functionget
, as set out in its description, below.
Modify 32.10.8 [futures.shared.future] as indicated:
namespace std { template<class R> class shared_future { […] }; }-?- For the primary template,
-4- The implementation provides the templateR
shall be an object type that meets the Cpp17Destructible requirements.shared_future
and two specializations,shared_future<R&>
andshared_future<void>
. These differ only in the return type and return value of the member functionget
, as set out in its description, below.
bool
can't be an integer-like typeSection: 24.3.4.4 [iterator.concept.winc] Status: C++23 Submitter: Casey Carter Opened: 2020-07-23 Last modified: 2023-11-22
Priority: 0
View all other issues in [iterator.concept.winc].
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Discussion:
Per 25.2 [ranges.syn]/1, the Standard Library believes it can convert an integer-like type X
to an unsigned integer-like type with the exposition-only type alias make-unsigned-like-t
.
make-unsigned-like-t<X>
is specified as being equivalent to
make_unsigned_t<X>
when X
is an integral type. However, despite being an integral type,
bool
is not a valid template type argument for make_unsigned_t
per [tab:meta.trans.sign].
bool
was an oversight when we added support for integer-like types: it was certainly
not the design intent to allow ranges::size(r)
to return false
! While we could devise some
more-complicated metaprogramming to allow use of bool
, it seems easier — and consistent with the
design intent — to simply exclude bool
from the set of integer-like types.
[2020-08-02; Reflector prioritization]
Set priority to 0 and status to Tentatively Ready after six votes in favour during reflector discussions.
[2020-11-09 Approved In November virtual meeting. Status changed: Tentatively Ready → WP.]
Proposed resolution:
This wording is relative to N4861.
Modify 24.3.4.4 [iterator.concept.winc] as indicated:
-11- A type
I
other thancv bool
is integer-like if it modelsintegral<I>
or if it is an integer-class type. An integer-like typeI
is signed-integer-like if it modelssigned_integral<I>
or if it is a signed-integer-class type. An integer-like typeI
is unsigned-integer-like if it modelsunsigned_integral<I>
or if it is an unsigned-integer-class type.
coroutine_handle::promise
may be insufficientSection: 17.13.4.7 [coroutine.handle.promise] Status: Resolved Submitter: Jiang An Opened: 2020-07-25 Last modified: 2020-11-09
Priority: 2
View all issues with Resolved status.
Discussion:
The issue is related to LWG 3460(i).
Because thecoroutine_handle<>
base subobject of a coroutine_handle<P1>
can be assigned from the one of a coroutine_handle<P2>
, a coroutine_handle<P1>
may refer to a coroutine whose promise type is P2
. If a coroutine_handle<P>
refers
to a coroutine with difference, a call to promise()
should result in undefined behavior IMO.
I think that 17.13.4.7 [coroutine.handle.promise]/1 should be changed to: "Preconditions:
*this
refers to a coroutine whose promise type is Promise
.", and the same precondition
should be added to 17.13.5.2.5 [coroutine.handle.noop.promise], and hence noexcept
should be
removed from coroutine_handle<noop_coroutine_promise>::promise
.
[2020-08-21; Reflector prioritization]
Set priority to 2 after reflector discussions.
Previous resolution [SUPERSEDED]:
This wording is relative to N4861.
Modify 17.13.4.7 [coroutine.handle.promise] as indicated:
Promise& promise() const;-1- Preconditions:
-2- Returns: A reference to the promise of the coroutine.*this
refers to a coroutine whose promise type isPromise
.Modify 17.13.5.2 [coroutine.handle.noop], class
coroutine_handle<noop_coroutine_promise>
synopsis, as indicated:[…] // 17.13.5.2.5 [coroutine.handle.noop.promise], promise access noop_coroutine_promise& promise() constnoexcept; […]Modify 17.13.5.2.5 [coroutine.handle.noop.promise] as indicated:
noop_coroutine_promise& promise() constnoexcept;-?- Preconditions:
-1- Returns: A reference to the promise object associated with this coroutine handle.*this
refers to a coroutine whose promise type isnoop_coroutine_promise
.
[2020-11-09 Resolved by acceptance of 3460(i). Status changed: Tentatively Resolved → Resolved.]
Proposed resolution:
This issue is resolved by the resolution of issue 3460(i).
convertible-to-non-slicing
seems to reject valid caseSection: 25.5.4 [range.subrange] Status: C++23 Submitter: S. B. Tam Opened: 2020-07-26 Last modified: 2023-11-22
Priority: 3
View all other issues in [range.subrange].
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Discussion:
Consider
#include <ranges> int main() { int a[3] = { 1, 2, 3 }; int* b[3] = { &a[2], &a[0], &a[1] }; auto c = std::ranges::subrange<const int*const*>(b); }
The construction of c
is ill-formed because convertible-to-non-slicing<int**, const int*const*>
is false
, although the conversion does not involve object slicing.
subrange
should allow such qualification conversion, just like unique_ptr<T[]>
already does.
(Given that this constraint is useful in more than one context, maybe it deserves a named type trait?)
[2020-08-21; Reflector prioritization]
Set priority to 3 after reflector discussions.
[2021-05-19 Tim adds wording]
The wording below, which has been implemented and tested on top of libstdc++,
uses the same technique we use for unique_ptr
, shared_ptr
,
and span
. It seems especially appropriate to have feature parity between
subrange
and span
in this respect.
[2021-06-23; Reflector poll]
Set status to Tentatively Ready after five votes in favour during reflector poll.
[2021-10-14 Approved at October 2021 virtual plenary. Status changed: Voting → WP.]
Proposed resolution:
This wording is relative to N4885.
Modify 25.5.4 [range.subrange] as indicated:
namespace std::ranges { template<class From, class To> concept uses-nonqualification-pointer-conversion = // exposition only is_pointer_v<From> && is_pointer_v<To> && !convertible_to<remove_pointer_t<From>(*)[], remove_pointer_t<To>(*)[]>; template<class From, class To> concept convertible-to-non-slicing = // exposition only convertible_to<From, To> && !uses-nonqualification-pointer-conversion<decay_t<From>, decay_t<To>>;!(is_pointer_v<decay_t<From>> && is_pointer_v<decay_t<To>> && not-same-as<remove_pointer_t<decay_t<From>>, remove_pointer_t<decay_t<To>>> );[…] } […]
polymorphic_allocator::allocate
does not satisfy Cpp17Allocator requirementsSection: 20.5 [mem.res] Status: C++23 Submitter: Alisdair Meredith Opened: 2020-07-27 Last modified: 2023-11-22
Priority: 3
View all other issues in [mem.res].
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Discussion:
With the adoption of P0593R6 in Prague,
std::ptr::polymorphic_allocator
no longer satisfies the of Cpp17Allocator
requirements. Specifically, all calls to allocate(n)
need to create an object
for an array of n T
s (but not initialize any of those elements).
std::pmr::polymorphic_allocator
calls its underlying memory resource to allocate
sufficient bytes of storage, but it does not create (and start the lifetime of) the array
object within that storage.
[2020-08-03; Billy comments]
It's worth noting that the resolution of CWG 2382 has impact on implementors for this issue.
[2020-08-21; Reflector prioritization]
Set priority to 3 after reflector discussions.
Previous resolution [SUPERSEDED]:
This wording is relative to N4861.
[Drafting note: The proposed wording is inspired by the example given in the "Assertion/note" column of the expression
a.allocate(n)
in table [tab:cpp17.allocator].]
Modify 20.5.2.2 [mem.res.public] as indicated:
[[nodiscard]] void* allocate(size_t bytes, size_t alignment = max_align);-2- Effects: Equivalent to:
return do_allocate(bytes, alignment);
void* p = do_allocate(bytes, alignment); return launder(new (p) byte[bytes]);
[2021-05-20 Tim comments and updates wording]
memory_resource::allocate
is the PMR equivalent of malloc
and operator new
. It therefore needs the "suitable created object"
wording (see 6.7.2 [intro.object], 20.2.12 [c.malloc]).
This ensures that it creates the requisite array object automatically for all
the allocation functions of polymorphic_allocator
,
and returns the correct pointer value in all cases.
[2022-01-31; Reflector poll]
Set status to Tentatively Ready after seven votes in favour during reflector poll.
[2022-02-10 Approved at February 2022 virtual plenary. Status changed: Tentatively Ready → WP.]
Proposed resolution:
This wording is relative to N4885.
Modify 20.5.2.2 [mem.res.public] as indicated:
[[nodiscard]] void* allocate(size_t bytes, size_t alignment = max_align);-2- Effects:
-?- Returns: A pointer to a suitable created object (6.7.2 [intro.object]) in the allocated region of storage. -?- Throws: What and when the call toEquivalent to:Allocates storage by callingreturn
do_allocate(bytes, alignment)
and implicitly creates objects within the allocated region of storage.;
do_allocate
throws.
counted_iterator
is missing preconditionsSection: 24.5.7 [iterators.counted] Status: C++23 Submitter: Michael Schellenberger Costa Opened: 2020-07-29 Last modified: 2023-11-22
Priority: Not Prioritized
View all issues with C++23 status.
Discussion:
C++20 introduces a new iterator counted_iterator
that keeps track of the end of its range via an
additional exposition only member length
.
counted_iterator
, but
it seems some are missing:
operator*
length
. However, given that length
denotes the
distance to the end of the range it should be invalid to dereference a counted_iterator
with
length 0
.
Moreover, operator[]
has a precondition of "n < length
". Consider the following code snippet:
int some_ints[] = {0,1,2}; counted_iterator<int*> i{some_ints, 0};
Here "i[0]
" would be invalid due to the precondition "n < length
". However, "*i
"
would be a valid expression. This violates the definition of operator[]
which states according to
7.6.1.2 [expr.sub] p1:
[…] The expression
E1[E2]
is identical (by definition) to*((E1)+(E2))
[…]
Substituting E2->0
we get
[…] The expression
E1[0]
is identical (by definition) to*(E1)
[…]
With the current wording counted_iterator
violates that definition and we should add to operator*
:
Preconditions:
length > 0
.
iter_move
Effects: Equivalent to:
return ranges::iter_move(i.current);
However, looking at the requirements of ranges::iter_move
we have in 24.3.3.1 [iterator.cust.move] p2:
If
ranges::iter_move(E)
is not equal to*E
, the program is ill-formed, no diagnostic required.
This clearly requires that for counted_iterator::iter_move
to be well-formed, we need
counted_iterator::operator*
to be well formed. Consequently we should also add the same precondition to
counted_iterator::iter_move
:
Preconditions:
length > 0
.
iter_swap
counted_iterator::iter_move
. The essential observation is that
ranges::iter_swap
is defined in terms of ranges::iter_move
(see 24.3.3.2 [iterator.cust.swap])
so it must have the same preconditions and we should add:
Preconditions:
length > 0
.
[2020-08-21 Issue processing telecon: moved to Tentatively Ready.]
[2020-11-09 Approved In November virtual meeting. Status changed: Tentatively Ready → WP.]
Proposed resolution:
This wording is relative to N4861.
Modify 24.5.7.4 [counted.iter.elem] as indicated:
constexpr decltype(auto) operator*(); constexpr decltype(auto) operator*() const requires dereferenceable<const I>;-?- Preconditions:
-1- Effects: Equivalent to:length > 0
.return *current;
Modify 24.5.7.7 [counted.iter.cust] as indicated:
friend constexpr iter_rvalue_reference_t<I> iter_move(const counted_iterator& i) noexcept(noexcept(ranges::iter_move(i.current))) requires input_iterator<I>;-?- Preconditions:
-1- Effects: Equivalent to:i.length > 0
.return ranges::iter_move(i.current);
template<indirectly_swappable<I> I2> friend constexpr void iter_swap(const counted_iterator& x, const counted_iterator<I2>& y) noexcept(noexcept(ranges::iter_swap(x.current, y.current)));-?- Preconditions:
-1- Effects: Equivalent to:x.length > 0
andy.length > 0
.return ranges::iter_swap(x.current, y.current);
Section: 28.5.8.3 [format.args] Status: C++23 Submitter: Jonathan Wakely Opened: 2020-07-31 Last modified: 2023-11-22
Priority: 0
View all other issues in [format.args].
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Discussion:
The note in the final paragraph of 28.5.8.3 [format.args] gives encouragement to implementations, which is not allowed in a note.
It needs to be normative text, possibly using "should", or if left as a note could be phrased as "Implementations can optimize the representation […]".[2020-08-09; Reflector prioritization]
Set priority to 0 and status to Tentatively Ready after six votes in favour during reflector discussions.
[2020-11-09 Approved In November virtual meeting. Status changed: Tentatively Ready → WP.]
Proposed resolution:
This wording is relative to N4861.
Modify 28.5.8.3 [format.args] as indicated:
-1- An instance of
basic_format_args
provides access to formatting arguments. Implementations should optimize the representation ofbasic_format_args
for a small number of formatting arguments. [Note: For example, by storing indices of type alternatives separately from values and packing the former. — end note]basic_format_args() noexcept;[…]-2- Effects: Initializes
size_
with0
.basic_format_arg<Context> get(size_t i) const noexcept;-4- Returns:
i < size_ ? data_[i] : basic_format_arg<Context>()
.
[Note: Implementations are encouraged to optimize the representation ofbasic_format_args
for small number of formatting arguments by storing indices of type alternatives separately from values and packing the former. — end note]
join_view
s is broken because of CTADSection: 25.7.14 [range.join] Status: C++23 Submitter: Barry Revzin Opened: 2020-08-04 Last modified: 2023-11-22
Priority: Not Prioritized
View all other issues in [range.join].
View all issues with C++23 status.
Discussion:
Let's say I had a range of range of ranges and I wanted to recursively flatten it. That would involve
repeated invocations of join
. But this doesn't work:
std::vector<std::vector<std::vector<int>>> nested_vectors = { {{1, 2, 3}, {4, 5}, {6}}, {{7}, {8, 9}, {10, 11, 12}}, {{13}} }; auto joined = nested_vectors | std::views::join | std::views::join;
The expectation here is that the value_type
of joined is int
, but it's actually
vector<int>
— because the 2nd invocation of join
ends up just copying
the first. This is because join
is specified to do:
The name
views::join
denotes a range adaptor object (25.7.2 [range.adaptor.object]). Given a subexpressionE
, the expressionviews::join(E)
is expression-equivalent tojoin_view{E}
.
And join_view{E}
for an E
that's already a specialization of a join_view
just gives you the same join_view
back. Yay CTAD. We need to do the same thing with join
that we did with reverse
in P1252. We can do that either in
exposition (Option A) my modifying 25.7.14.1 [range.join.overview] p2
The name
views::join
denotes a range adaptor object (25.7.2 [range.adaptor.object]). Given a subexpressionE
, the expressionviews::join(E)
is expression-equivalent tojoin_view<views::all_t<decltype((E))>>{E}
.
Or in code (Option B) add a deduction guide to 25.7.14.2 [range.join.view]:
template<class R> explicit join_view(R&&) -> join_view<views::all_t<R>>; template<class V> explicit join_view(join_view<V>) -> join_view<join_view<V>>;
[2020-08-21; Issue processing telecon: Option A is Tentatively Ready]
Previous resolution [SUPERSEDED]:
This wording is relative to N4861.
[Drafting Note: Two mutually exclusive options are prepared, depicted below by Option A and Option B, respectively.]
Option A:
Modify 25.7.14.1 [range.join.overview] as indicated:
-2-
The name views::join
denotes a range adaptor object (25.7.2 [range.adaptor.object]). Given a subexpressionE
, the expressionviews::join(E)
is expression-equivalent tojoin_view<views::all_t<decltype((E))>>{E}
.Option B:
Modify 25.7.14.2 [range.join.view] as indicated:
namespace std::ranges { […] template<class R> explicit join_view(R&&) -> join_view<views::all_t<R>>; template<class V> explicit join_view(join_view<V>) -> join_view<join_view<V>>; }
[2020-11-09 Approved In November virtual meeting. Status changed: Tentatively Ready → WP.]
Proposed resolution:
This wording is relative to N4861.
Modify 25.7.14.1 [range.join.overview] as indicated:
-2-
The name views::join
denotes a range adaptor object (25.7.2 [range.adaptor.object]). Given a subexpressionE
, the expressionviews::join(E)
is expression-equivalent tojoin_view<views::all_t<decltype((E))>>{E}
.
thread
and jthread
constructors require that the parameters be move-constructible
but never move construct the parametersSection: 32.4.3.3 [thread.thread.constr], 32.4.4.2 [thread.jthread.cons], 32.10.9 [futures.async] Status: C++23 Submitter: Billy O'Neal III Opened: 2020-08-18 Last modified: 2023-11-22
Priority: 0
View all other issues in [thread.thread.constr].
View all issues with C++23 status.
Discussion:
I think this was upgraded to Mandates because C++17 and earlier had "F
and each Ti
in Args
shall satisfy the Cpp17MoveConstructible requirements." And for those, I think the requirement
was attempting to make the subsequent decay-copy
valid. However, the 'Mandating the standard library'
papers added is_constructible
requirements which already serve that purpose; std::(j)thread
has no
reason to move the elements after they have been decay-copy
'd to transfer to the launched thread.
[2020-08-26; Reflector discussion]
Jonathan noticed that the wording for std::async
is affected by exactly the same unnecessary move-constructible
requirements. The proposed wording has been updated to cope for that as well.
[2020-09-02; Reflector prioritization]
Set priority to 0 and status to Tentatively Ready after five votes in favour during reflector discussions.
[2020-11-09 Approved In November virtual meeting. Status changed: Tentatively Ready → WP.]
Proposed resolution:
This wording is relative to N4861.
Modify 32.4.3.3 [thread.thread.constr] as indicated:
template<class F, class... Args> explicit thread(F&& f, Args&&... args);-3- Constraints:
-4- Mandates: The following are allremove_cvref_t<F>
is not the same type asthread
.true
:
(4.1) —
is_constructible_v<decay_t<F>, F>
,(4.2) —
(is_constructible_v<decay_t<Args>, Args> && ...)
, and
(4.3) —is_move_constructible_v<decay_t<F>>
,
(4.4) —(is_move_constructible_v<decay_t<Args>> && ...)
, and(4.5) —
is_invocable_v<decay_t<F>, decay_t<Args>...>
.
-5- Preconditions:decay_t<F>
and each type indecay_t<Args>
meet the Cpp17MoveConstructible requirements.
Modify 32.4.4.2 [thread.jthread.cons] as indicated:
template<class F, class... Args> explicit jthread(F&& f, Args&&... args);-3- Constraints:
-4- Mandates: The following are allremove_cvref_t<F>
is not the same type asjthread
.true
:
(4.1) —
is_constructible_v<decay_t<F>, F>
,(4.2) —
(is_constructible_v<decay_t<Args>, Args> && ...)
, and
(4.3) —is_move_constructible_v<decay_t<F>>
,
(4.4) —(is_move_constructible_v<decay_t<Args>> && ...)
, and(4.5) —
is_invocable_v<decay_t<F>, decay_t<Args>...> || is_invocable_v<decay_t<F>, stop_token, decay_t<Args>...>
.
-5- Preconditions:decay_t<F>
and each type indecay_t<Args>
meet the Cpp17MoveConstructible requirements.
Modify 32.10.9 [futures.async] as indicated:
template<class F, class... Args> [[nodiscard]] future<invoke_result_t<decay_t<F>, decay_t<Args>...>> async(F&& f, Args&&... args); template<class F, class... Args> [[nodiscard]] future<invoke_result_t<decay_t<F>, decay_t<Args>...>> async(launch policy, F&& f, Args&&... args);-2- Mandates: The following are all
true
:
(2.1) —
is_constructible_v<decay_t<F>, F>
,(2.2) —
(is_constructible_v<decay_t<Args>, Args> && ...)
, and
(2.3) —is_move_constructible_v<decay_t<F>>
,
(2.4) —(is_move_constructible_v<decay_t<Args>> && ...)
, and(2.5) —
is_invocable_v<decay_t<F>, decay_t<Args>...>
.
-3- Preconditions:decay_t<F>
and each type indecay_t<Args>
meet the Cpp17MoveConstructible requirements.
semiregular-box
Section: 25.7.3 [range.move.wrap] Status: C++23 Submitter: Casey Carter Opened: 2020-08-19 Last modified: 2023-11-22
Priority: 0
View all other issues in [range.move.wrap].
View all issues with C++23 status.
Discussion:
The exposition-only semiregular-box
class template specified in [range.semi.wrap]
implements a default constructor, copy assignment operator, and move assignment operator atop the facilities
provided by std::optional
when the wrapped type is not default constructible, copy assignable, or
move assignable (respectively). The constraints on the copy and move assignment operator implementations go
out of their way to be unnecessarily minimal. The meaning of the constraint on the copy assignment operator
— !assignable<T, const T&>
— has even changed since this wording was written
as a result of LWG reformulating the implicit expression variations wording in 18.2 [concepts.equality].
!movable<T>
and !copyable<T>
.
[2020-09-03; Reflector prioritization]
Set priority to 0 and status to Tentatively Ready after six votes in favour during reflector discussions.
[2020-11-09 Approved In November virtual meeting. Status changed: Tentatively Ready → WP.]
Proposed resolution:
This wording is relative to N4861.
Modify [range.semi.wrap] as indicated:
-1- Many types in this subclause are specified in terms of an exposition-only class template
semiregular-box
.semiregular-box<T>
behaves exactly likeoptional<T>
with the following differences:
(1.1) — […]
(1.2) — […]
(1.3) — If
is not modeled, the copy assignment operator is equivalent to:
assignable_from<T&, const T&>copyable<T>semiregular-box& operator=(const semiregular-box& that) noexcept(is_nothrow_copy_constructible_v<T>) { if (that) emplace(*that); else reset(); return *this; }(1.4) — If
is not modeled, the move assignment operator is equivalent to:
assignable_from<T&, T>movable<T>semiregular-box& operator=(semiregular-box&& that) noexcept(is_nothrow_move_constructible_v<T>) { if (that) emplace(std::move(*that)); else reset(); return *this; }
views::split
drops trailing empty rangeSection: 25.7.17 [range.split] Status: Resolved Submitter: Barry Revzin Opened: 2020-08-20 Last modified: 2021-06-14
Priority: 2
View all issues with Resolved status.
Discussion:
From StackOverflow, the program:
#include <iostream> #include <string> #include <ranges> int main() { std::string s = " text "; auto sv = std::ranges::views::split(s, ' '); std::cout << std::ranges::distance(sv.begin(), sv.end()); }
prints 2 (as specified), but it really should print 3. If a range has N
delimiters in it,
splitting should produce N+1
pieces. If the N
th delimiter is the last
element in the input range, views::split
produces only N
pieces — it doesn't
emit a trailing empty range.
Rust, Python, Javascript, Go, Kotlin, Haskell's "splitOn"
all provide N+1
parts
if there were N
delimiters.
APL, D, Elixir, Haskell's "words"
, Ruby, and Clojure all compress all empty words.
Splitting " x "
on " "
would give ["x"]
here, whereas the languages in the
above group would give ["", "x", ""]
Java is distinct from both groups in that it is mostly a first category language, except that by default it removes all trailing empty strings (but it keeps all leading and intermediate empty strings, unlike the second category languages) — although it has a parameter that lets you keep the trailing ones too.
C++20's behavior is closest to Java's default, except that it only removes one trailing empty string instead of every trailing empty string — and this behavior is not parameterizeable. But I think the intent is to be squarely in the first category, so I think the current behavior is just a specification error. Many of these languages also provide an additional extra parameter to limit how many splits happen (e.g. Java, Kotlin, Python, Rust, JavaScript), but that's a separate design question.[2020-09-02; Reflector prioritization]
Set priority to 2 as result of reflector discussions.
[2021-06-13 Resolved by the adoption of P2210R2 at the June 2021 plenary. Status changed: New → Resolved.]
Proposed resolution:
semiregular-box
mishandles self-assignmentSection: 25.7.3 [range.move.wrap] Status: Resolved Submitter: Casey Carter Opened: 2020-08-25 Last modified: 2023-02-07
Priority: 3
View all other issues in [range.move.wrap].
View all issues with Resolved status.
Discussion:
The exposition-only wrapper type semiregular-box
— specified in [range.semi.wrap]
— layers behaviors onto std::optional
so semiregular-box<T>
is
semiregular even when T
is only copy constructible. It provides copy and move assignment
operators when optional<T>
's are deleted:
(1.1) — […]
(1.2) — […]
(1.3) — If
assignable_from<T&, const T&>
is not modeled, the copy assignment operator is equivalent to:semiregular-box& operator=(const semiregular-box& that) noexcept(is_nothrow_copy_constructible_v<T>) { if (that) emplace(*that); else reset(); return *this; }(1.4) — If
assignable_from<T&, T>
is not modeled, the move assignment operator is equivalent to:semiregular-box& operator=(semiregular-box&& that) noexcept(is_nothrow_move_constructible_v<T>) { if (that) emplace(std::move(*that)); else reset(); return *this; }
How do these assignment operators handle self-assignment? When *this
is empty, that
will test as false
and reset()
has no effect, so the result state of the object is
the same. No problems so far. When *this
isn't empty, that
will test as true
,
and we evaluate optional::emplace(**this)
(resp. optional::emplace(std::move(**this)))
.
This outcome is not as pretty: emplace
is specified in 22.5.3.4 [optional.assign]/30:
"Effects: Calls *this = nullopt
. Then initializes the contained value as if
direct-non-list-initializing an object of type T
with the arguments
std::forward<Args>(args)...
." When the sole argument is an lvalue (resp. xvalue) of type
T
that denotes the optional
's stored value, emplace
will destroy that
stored value and then try to copy/move construct a new object at the same address from the dead object
that used to live there resulting in undefined behavior. Mandatory undefined behavior does not
meet the semantic requirements for the copyable
or movable
concepts, we should do better.
[2020-09-13; Reflector prioritization]
Set priority to 3 during reflector discussions.
Previous resolution [SUPERSEDED]:
This wording is relative to N4861.
Modify [range.semi.wrap] as indicated:
-1- Many types in this subclause are specified in terms of an exposition-only class template
semiregular-box
.semiregular-box<T>
behaves exactly likeoptional<T>
with the following differences:
(1.1) — […]
(1.2) — […]
(1.3) — If
assignable_from<T&, const T&>
is not modeled, the copy assignment operator is equivalent to:semiregular-box& operator=(const semiregular-box& that) noexcept(is_nothrow_copy_constructible_v<T>) { if (this != addressof(that)) { if (that) emplace(*that); else reset(); } return *this; }(1.4) — If
assignable_from<T&, T>
is not modeled, the move assignment operator is equivalent to:semiregular-box& operator=(semiregular-box&& that) noexcept(is_nothrow_move_constructible_v<T>) { reset(); if (that) emplace(std::move(*that));else reset();return *this; }
[2021-06-13 Resolved by the adoption of P2325R3 at the June 2021 plenary. Status changed: New → Resolved.]
Proposed resolution:
directory_iterator
and recursive_directory_iterator
are not C++20 rangesSection: 31.12.11 [fs.class.directory.iterator], 31.12.12 [fs.class.rec.dir.itr] Status: C++23 Submitter: Barry Revzin Opened: 2020-08-27 Last modified: 2023-11-22
Priority: 3
View all other issues in [fs.class.directory.iterator].
View all issues with C++23 status.
Discussion:
std::filesystem::directory_iterator
and std::filesystem::recursive_directory_iterator
are
intended to be ranges, but both fail to satisfy the concept std::ranges::range
.
directory_iterator begin(directory_iterator iter) noexcept; directory_iterator end(const directory_iterator&) noexcept; recursive_directory_iterator begin(recursive_directory_iterator iter) noexcept; recursive_directory_iterator end(const recursive_directory_iterator&) noexcept;
This is good enough for a range-based for statement, but for the range
concept, non-member
end
is looked up in a context that includes (25.3.3 [range.access.end]/2.6) the declarations:
void end(auto&) = delete; void end(const auto&) = delete;
Which means that non-const
directory_iterator
and non-const
recursive_directory_iterator
, the void end(auto&)
overload ends up being
a better match and thus the CPO ranges::end
doesn't find a candidate. Which means that
{recursive_,}directory_iterator
is not a range, even though const {recursive_,}directory_iterator
is a range.
end
for both of these types just take by value
(as libstdc++ currently does anyway) or by adding member functions begin() const
and end() const
.
A broader direction would be to consider removing the poison pill overloads. Their motivation from
P0970 was to support what are now called borrowed ranges — but
that design now is based on specializing a variable template instead of providing a non-member begin
that takes an rvalue, so the initial motivation simply no longer exists. And, in this particular case,
causes harm.
[2020-09-06; Reflector prioritization]
Set priority to 3 during reflector discussions.
[2021-02-22, Barry Revzin comments]
When we do make whichever of the alternative adjustments necessary such that
range<directory_iterator>
is true
, we should also remember
to specialize enable_borrowed_range
for both types to be true
(since
the iterator is the range, this is kind of trivially true).
[2021-05-17, Tim provides wording]
Both MSVC and libstdc++'s end
already take its argument by value, so
the wording below just does that. Any discussion about changing or removing the
poison pills is probably better suited for a paper.
[2021-06-23; Reflector poll]
Set status to Tentatively Ready after seven votes in favour during reflector poll.
[2021-10-14 Approved at October 2021 virtual plenary. Status changed: Voting → WP.]
Proposed resolution:
This wording is relative to N4885.
Edit 31.12.4 [fs.filesystem.syn], header <filesystem>
synopsis, as indicated:
[…] namespace std::filesystem { […] // 31.12.11.3 [fs.dir.itr.nonmembers], range access for directory iterators directory_iterator begin(directory_iterator iter) noexcept; directory_iterator end(constdirectory_iterator&) noexcept; […] // 31.12.12.3 [fs.rec.dir.itr.nonmembers], range access for recursive directory iterators recursive_directory_iterator begin(recursive_directory_iterator iter) noexcept; recursive_directory_iterator end(constrecursive_directory_iterator&) noexcept; […] } namespace std::ranges { template<> inline constexpr bool enable_borrowed_range<filesystem::directory_iterator> = true; template<> inline constexpr bool enable_borrowed_range<filesystem::recursive_directory_iterator> = true; template<> inline constexpr bool enable_view<filesystem::directory_iterator> = true; template<> inline constexpr bool enable_view<filesystem::recursive_directory_iterator> = true; }
Edit 31.12.11.3 [fs.dir.itr.nonmembers] as indicated:
-1- These functions enable range access for
directory_iterator
.directory_iterator begin(directory_iterator iter) noexcept;-2- Returns:
iter
.directory_iterator end(constdirectory_iterator&) noexcept;-3- Returns:
directory_iterator()
.
Edit 31.12.12.3 [fs.rec.dir.itr.nonmembers] as indicated:
-1- These functions enable use of
recursive_directory_iterator
with range-based for statements.recursive_directory_iterator begin(recursive_directory_iterator iter) noexcept;-2- Returns:
iter
.recursive_directory_iterator end(constrecursive_directory_iterator&) noexcept;-3- Returns:
recursive_directory_iterator()
.
viewable_range
mishandles lvalue move-only viewsSection: 25.4.5 [range.refinements] Status: C++23 Submitter: Casey Carter Opened: 2020-08-29 Last modified: 2023-11-22
Priority: 2
View all other issues in [range.refinements].
View all issues with C++23 status.
Discussion:
The viewable_range concept
(25.4.5 [range.refinements]) and the views:all
range adaptor (25.7.6 [range.all]) are duals: viewable_range
is intended to admit
exactly types T
for which views::all(declval<T>())
is well-formed. (Recall
that views::all(meow)
is a prvalue whose type models view
when it is
well-formed.) Before the addition of move-only view types to the design, this relationship was in
place (modulo an incredibly pathological case: a volatile
value of a view type with
volatile
-qualified begin
and end
models viewable_range
but is
rejected by views::all
unless it also has a volatile
-qualified copy constructor
and copy assignment operator). Adding move-only views to the design punches a bigger hole, however:
viewable_range
admits lvalues of move-only view types for which views::all
is
ill-formed because these lvalues cannot be decay-copied.
viewable_range
and views::all
so that instantiations of components constrained with viewable_range
(which often appears
indirectly as views::all_t<R>
in deduction guides) continue to be well-formed when
the constraints are satisfied.
[2020-09-06; Reflector prioritization]
Set priority to 2 during reflector discussions.
[2021-05-24; Reflector poll]
Set status to Tentatively Ready after five votes in favour during reflector poll.
[2021-06-07 Approved at June 2021 virtual plenary. Status changed: Voting → WP.]
Proposed resolution:
This wording is relative to N4861.
Modify 25.4.5 [range.refinements] as indicated:
-5- The
viewable_range
concept specifies the requirements of arange
type that can be converted to aview
safely.template<class T> concept viewable_range = range<T> &&(borrowed_range<T> || view<remove_cvref_t<T>>);((view<remove_cvref_t<T>> && constructible_from<remove_cvref_t<T>, T>) || (!view<remove_cvref_t<T>> && borrowed_range<T>));
drop_view
's const begin
should additionally require sized_range
Section: 25.7.12.2 [range.drop.view] Status: C++23 Submitter: Casey Carter Opened: 2020-08-31 Last modified: 2023-11-22
Priority: 0
View other active issues in [range.drop.view].
View all other issues in [range.drop.view].
View all issues with C++23 status.
Discussion:
When the underlying range models both random_access_range
and sized_range
, a
drop_view
can easily calculate its first iterator in 𝒪
(1) as by the
underlying range's first iterator plus the minimum of the number of elements to drop and the
size of the underlying range. In this case drop_view::begin
need not "cache the result
within the drop_view
for use on subsequent calls" "in order to provide the amortized
constant-time complexity required by the range
concept" (25.7.12.2 [range.drop.view]/4).
However, drop_view::begin() const
does not require sized_range
, it requires
only random_access_range
. There's no way to implementing what amounts to a requirement
that calls to begin
after the first must be 𝒪
(1) without memoization.
const
member function in a manner consistent with
16.4.6.10 [res.on.data.races] is impossible without some kind of thread synchronization. It
is not the intended design for anything in current Range library to require such implementation
heroics, we typically fall back to mutable-only iteration to avoid thread synchronization concerns.
(Note that both range-v3
and cmcstl2
handle drop_view::begin() const
incorrectly by performing 𝒪
(N
) lookup of the first iterator on each call to
begin
, which is consistent with 16.4.6.10 [res.on.data.races] but fails to meet the
complexity requirements imposed by the range
concept.) We should fall back to mutable-only
iteration here as well when the underlying range is not a sized_range
.
For drop_view
, changing the constraints on the const
overload of begin
also requires changing the constraints on the non-const
overload. The non-const begin
tries to constrain itself out of overload resolution when the const
overload would be valid
if the underlying range models the exposition-only simple-view
concept. (Recall that
T
models simple-view iff T
models view
, const T
models range
,
and T
and const T
have the same iterator and sentinel types.) Effectively this means
the constraints on the non-const
overload must require either that the underlying range fails
to model simple-view
or that the constraints on the const
overload would not
be satisfied. So when we add a new sized_range
requirement to the const
overload,
we must also add its negation to the mutable overload. (The current form of the constraint on the
mutable begin
overload is !(simple-view<V> && random_access_range<V>)
instead of !(simple-view<V> && random_access_range<const V>)
because
of an unstated premise that V
and const V
should both have the same category when
both are ranges. Avoiding this unstated premise would make it easier for future readers to grasp what's
happening here; we should formulate our new constraints in terms of const V
instead of V
.)
[2020-09-29; Reflector discussions]
Status to Tentatively Ready and priority to 0 after five positive votes on the reflector.
[2020-11-09 Approved In November virtual meeting. Status changed: Tentatively Ready → WP.]
Proposed resolution:
This wording is relative to N4861.
Modify 25.7.12.2 [range.drop.view] as indicated:
[…]namespace std::ranges { template<view V> class drop_view : public view_interface<drop_view<V>> { public: […] constexpr auto begin() requires (!(simple-view<V> && random_access_range<const V> && sized_range<const V>)); constexpr auto begin() const requires random_access_range<const V> && sized_range<const V>; […] }; }constexpr auto begin() requires (!(simple-view<V> && random_access_range<const V> && sized_range<const V>)); constexpr auto begin() const requires random_access_range<const V> && sized_range<const V>;-3- Returns:
-4- Remarks: In order to provide the amortized constant-time complexity required by theranges::next(ranges::begin(base_), count_, ranges::end(base_))
.range
concept whendrop_view
modelsforward_range
, the first overload caches the result within thedrop_view
for use on subsequent calls. [Note: Without this, applying areverse_view
over adrop_view
would have quadratic iteration complexity. — end note]
transform_view::iterator
's difference is overconstrainedSection: 25.7.9.3 [range.transform.iterator], 25.7.23.3 [range.elements.iterator] Status: C++23 Submitter: Casey Carter Opened: 2020-09-04 Last modified: 2023-11-22
Priority: 0
View all other issues in [range.transform.iterator].
View all issues with C++23 status.
Discussion:
The difference operation for transform_view::iterator
is specified in
25.7.9.3 [range.transform.iterator] as:
friend constexpr difference_type operator-(const iterator& x, const iterator& y) requires random_access_range<Base>;-22- Effects: Equivalent to:
return x.current_ - y.current_;
The member current_
is an iterator of type iterator_t<Base>
, where
Base
is V
for transform_view<V, F>::iterator<false>
and const V
for transform_view<V, F>::iterator<true>
. The difference
of iterators that appears in the above Effects: element is notably well-defined if their type models
sized_sentinel_for<iterator_t<Base>, iterator_t<Base>>
which
random_access_range<Base>
refines. This overstrong requirement seems to be simply the
result of an oversight; it has been present since P0789R0, without
— to my recollection — ever having been discussed. We should relax this requirement to provide
difference capability for transform_view
's iterators whenever the underlying iterators do.
[2020-09-08; Reflector discussion]
During reflector discussions it was observed that elements_view::iterator
has the same issue
and the proposed wording has been extended to cover this template as well.
[2020-09-13; Reflector prioritization]
Set priority to 0 and status to Tentatively Ready after seven votes in favour during reflector discussions.
[2020-11-09 Approved In November virtual meeting. Status changed: Tentatively Ready → WP.]
Proposed resolution:
This wording is relative to N4861.
Modify 25.7.9.3 [range.transform.iterator] as indicated:
[…]namespace std::ranges { template<input_range V, copy_constructible F> requires view<V> && is_object_v<F> && regular_invocable<F&, range_reference_t<V>> && can-reference<invoke_result_t<F&, range_reference_t<V>>> template<bool Const> class transform_view<V, F>::iterator { public: […] friend constexpr iterator operator-(iterator i, difference_type n) requires random_access_range<Base>; friend constexpr difference_type operator-(const iterator& x, const iterator& y) requiresrandom_access_range<Base>sized_sentinel_for<iterator_t<Base>, iterator_t<Base>>; […] }; }friend constexpr difference_type operator-(const iterator& x, const iterator& y) requiresrandom_access_range<Base>sized_sentinel_for<iterator_t<Base>, iterator_t<Base>>;-22- Effects:
return x.current_ - y.current_;
Modify 25.7.23.3 [range.elements.iterator] as indicated:
[…]namespace std::ranges { template<input_range V, size_t N> requires view<V> && has-tuple-element<range_value_t<V>, N> && has-tuple-element<remove_reference_t<range_reference_t<V>>, N> template<bool Const> class elements_view<V, N>::iterator { // exposition only […] friend constexpr iterator operator-(iterator x, difference_type y) requires random_access_range<Base>; friend constexpr difference_type operator-(const iterator& x, const iterator& y) requiresrandom_access_range<Base>sized_sentinel_for<iterator_t<Base>, iterator_t<Base>>; }; }constexpr difference_type operator-(const iterator& x, const iterator& y) requiresrandom_access_range<Base>sized_sentinel_for<iterator_t<Base>, iterator_t<Base>>;-21- Effects:
return x.current_ - y.current_;
ranges::drop_while_view::begin()
is missing a preconditionSection: 25.7.13.2 [range.drop.while.view] Status: C++23 Submitter: Michael Schellenberger Costa Opened: 2020-10-13 Last modified: 2023-11-22
Priority: 0
View all other issues in [range.drop.while.view].
View all issues with C++23 status.
Discussion:
Similar to ranges::filter_view
25.7.8.2 [range.filter.view] p3, ranges::drop_while_view
should have a precondition on its begin()
method that the predicate is set.
Preconditions:pred_.has_value()
.
[2020-11-07; Reflector prioritization]
Set priority to 0 and status to Tentatively Ready after six votes in favour during reflector discussions.
[2021-02-26 Approved at February 2021 virtual plenary. Status changed: Tentatively Ready → WP.]
Proposed resolution:
This wording is relative to N4868.
Modify 25.7.13.2 [range.drop.while.view] as indicated:
Since we usually don't rely on implicit
bool
conversion in Preconditions: elements an explicit "istrue
" has been added. Editorial fixes of the referenced paragraph 25.7.13.2 [range.drop.while.view] p3 and similar places have been requested separately.
constexpr auto begin();-?- Preconditions:
-3- Returns:pred_.has_value()
istrue
.ranges::find_if_not(base_, cref(*pred_))
. -4- […]
elements_view::iterator
Section: 25.7.23.3 [range.elements.iterator] Status: C++23 Submitter: Michael Schellenberger Costa Opened: 2020-10-28 Last modified: 2023-11-22
Priority: 0
View other active issues in [range.elements.iterator].
View all other issues in [range.elements.iterator].
View all issues with C++23 status.
Discussion:
During code review of elements_view
for MSVC-STL we found two issues that should be easily addressed:
elements_view::iterator
constraints both operator++(int)
member functions
constexpr void operator++(int) requires (!forward_range<Base>); constexpr iterator operator++(int) requires forward_range<Base>;
However, given that a constrained method would be preferred we only need to constrain one of those.
The proposal would be to remove the constraint from the void
returning overload and change
the declaration to
constexpr void operator++(int); constexpr iterator operator++(int) requires forward_range<Base>;
elements_view::iterator operator-
is constrained as follows:
friend constexpr difference_type operator-(const iterator& x, const iterator& y) requires random_access_range<Base>;
However, that requires its base to have operator-
defined. We should change the constraint to
sized_sentinel_for<iterator_t<Base>, iterator_t<Base>>
:
friend constexpr difference_type operator-(const iterator& x, const iterator& y) requires sized_sentinel_for<iterator_t<Base>, iterator_t<Base>>;
[2020-11-01; Daniel comments]
Bullet (2) of the discussion has already been resolved by LWG 3483(i), it has therefore been omitted from the proposed wording below.
[2020-11-15; Reflector prioritization]
Set priority to 0 and status to Tentatively Ready after five votes in favour during reflector discussions.
[2021-02-26 Approved at February 2021 virtual plenary. Status changed: Tentatively Ready → WP.]
Proposed resolution:
This wording is relative to N4868.
This wording intentionally only touches
operator++(int)
and notoperator-
, see the 2020-11-01 comment for the reason why.
Modify 25.7.23.3 [range.elements.iterator], class template elements_view::iterator
synopsis,
as indicated:
[…][…] constexpr iterator& operator++(); constexpr void operator++(int)requires (!forward_range<Base>); constexpr iterator operator++(int) requires forward_range<Base>; […]constexpr void operator++(int)requires (!forward_range<Base>);-6- Effects: Equivalent to:
++current_
.constexpr iterator operator++(int) requires forward_range<Base>;[…]
Section: 25.7.21 [range.reverse], 25.7.10 [range.take], 25.7.12 [range.drop], 25.7.13 [range.drop.while], 25.7.20 [range.common], 25.7.13 [range.drop.while], 25.7.23 [range.elements] Status: C++23 Submitter: Barry Revzin Opened: 2020-11-01 Last modified: 2023-11-22
Priority: Not Prioritized
View all other issues in [range.reverse].
View all issues with C++23 status.
Discussion:
Consider the following approach to trimming a std::string
:
auto trim(std::string const& s) { auto isalpha = [](unsigned char c){ return std::isalpha(c); }; auto b = ranges::find_if(s, isalpha); auto e = ranges::find_if(s | views::reverse, isalpha).base(); return subrange(b, e); }
This is a fairly nice and, importantly, safe way to implement trim
. The iterators b
and e
returned from find_if
will not dangle, since they point into the string s
whose lifetime outlives the function. But the status quo in C++20 is that s | views::reverse
is not a borrowed range (because reverse_view<V>
is never a borrowed range for any V
).
As a result, find_if(s | views::reverse, isalpha)
returns dangling rather than
a real iterator.
auto trim(std::string const& s) { auto isalpha = [](unsigned char c){ return std::isalpha(c); }; auto b = ranges::find_if(s, isalpha); auto reversed = s | views::reverse; auto e = ranges::find_if(reversed, isalpha).base(); return subrange(b, e); }
But borrowed range can be a transitive property. s
itself is a borrowed range (as all
lvalue references are) so s | views::reverse
could be made to be too, which would allow
the first example above to work with really no downside. We know such an iterator would not dangle,
we just need to teach the library this.
reverse_view<V>
a borrowed range when V
is a borrowed range (and likewise several other range adapters).
[2021-01-15; Telecon prioritization]
Set status to Tentatively Ready after five P0 votes in reflector discussion.
[2021-02-26 Approved at February 2021 virtual plenary. Status changed: Tentatively Ready → WP.]
Rationale:
Resolved by P2017R1.Proposed resolution:
constexpr launder
makes pointers to inactive members of unions usableSection: 17.6.5 [ptr.launder] Status: C++23 Submitter: Hubert Tong Opened: 2020-11-10 Last modified: 2023-11-22
Priority: 3
View all other issues in [ptr.launder].
View all issues with C++23 status.
Discussion:
The wording in 17.6.5 [ptr.launder] paragraph 4:
An invocation of this function may be used in a core constant expression whenever the value of its argument may be used in a core constant expression.
can be taken to mean that the invocation may be used only when the value of its argument can be used in place of the invocation itself.
That interpretation is not particularly obvious, but based on comments on the CWG reflector (see here), that is the interpretation that matches the design intent. Consider:#include <new> struct A { int x; int y; }; struct B { float x; int y; }; union U { A a; B b; }; constexpr A foo() { U u; int* byp = &u.b.y; static_assert(&u.b.y == static_cast<void*>(&u.a.y)); u.a.y = 42; *std::launder(byp) = 13; return u.a; } extern constexpr A globA = foo();
If the static_assert
succeeds, then a possible interpretation is that the source file
above compiles because the call to std::launder
produces a pointer to u.a.y
.
That interpretation is apparently not desirable.
[2020-11-21; Reflector prioritization]
Set priority to 3 during reflector discussions.
[2020-12-07; Davis Herring comments]
This issue is related to CWG 2464.
[2021-02-08; Reflector poll]
Set status to Tentatively Ready after five votes in favour during reflector poll.
[2021-02-26 Approved at February 2021 virtual plenary. Status changed: Tentatively Ready → WP.]
Proposed resolution:
This wording is relative to N4868.
Modify 17.6.5 [ptr.launder] as indicated:
template<class T> [[nodiscard]] constexpr T* launder(T* p) noexcept;[…]
-4- Remarks: An invocation of this function may be used in a core constant expressionwhenever theif and only if the (converted) value of its argument may be used ina core constant expressionplace of the function invocation. A byte of storageb
is reachable through a pointer value that points to an objectY
if there is an objectZ
, pointer-interconvertible withY
, such thatb
is within the storage occupied byZ
, or the immediately-enclosing array object ifZ
is an array element. […]
noexcept
-specifiers for basic_syncbuf
Section: 31.11.2.1 [syncstream.syncbuf.overview], 31.11.2.3 [syncstream.syncbuf.assign] Status: C++23 Submitter: Jonathan Wakely Opened: 2020-11-10 Last modified: 2023-11-22
Priority: 3
View all other issues in [syncstream.syncbuf.overview].
View all issues with C++23 status.
Discussion:
The synopsis in 31.11.2.1 [syncstream.syncbuf.overview] shows the move assignment operator and
swap
member as potentially throwing. The detailed descriptions in
31.11.2.3 [syncstream.syncbuf.assign] are noexcept
.
[2020-11-21; Reflector prioritization]
Set priority to 3 during reflector discussions.
[2021-05-22 Tim adds PR]
The move assignment is specified to call emit()
which can throw,
and there's nothing in the wording providing for catching/ignoring the exception,
so it can't be noexcept
. The swap
needs
to call basic_streambuf::swap
, which isn't noexcept
,
so it shouldn't be noexcept
either.
[2021-06-23; Reflector poll]
Set status to Tentatively Ready after eight votes in favour during reflector poll.
[2021-10-14 Approved at October 2021 virtual plenary. Status changed: Voting → WP.]
Proposed resolution:
This wording is relative to N4885.
Modify 31.11.2.3 [syncstream.syncbuf.assign] as indicated:
basic_syncbuf& operator=(basic_syncbuf&& rhs)noexcept;-1- Effects: […]
-2- Postconditions: […] -3- Returns: […] -4- Remarks: […]void swap(basic_syncbuf& other)noexcept;-5- Preconditions: […]
-6- Effects: […]
join_view::iterator::operator->()
is bogusSection: 25.7.14.3 [range.join.iterator] Status: C++23 Submitter: Michael Schellenberger Costa Opened: 2020-11-15 Last modified: 2023-11-22
Priority: 0
View all other issues in [range.join.iterator].
View all issues with C++23 status.
Discussion:
There seems to be a copy & paste error in the join_view::iterator::operator->()
specification. According to 25.7.14.3 [range.join.iterator] p8 it is specified as:
constexpr iterator_t<Base> operator->() const requires has-arrow<iterator_t<Base>> && copyable<iterator_t<Base>>;-8- Effects: Equivalent to
return inner_;
Now inner_
is of type iterator_t<range_reference_t<Base>>
. So it
is unclear how that should be converted to iterator_t<Base>
, or why the
constraints concern the outer iterator type iterator_t<Base>
. On the other hand
returning outer_
would not make any sense here.
iterator_t<Base>
with
iterator_t<range_reference_t<Base>>
so that the new specification would
read
constexpr iterator_t<range_reference_t<Base>> operator->() const requires has-arrow<iterator_t<range_reference_t<Base>>> && copyable<iterator_t<range_reference_t<Base>>>;-8- Effects: Equivalent to
return inner_;
Generally it would help readability of the specification a lot if we would introduce some exposition only aliases:
using OuterIter = iterator_t<Base>; //exposition-only using InnerIter = iterator_t<range_reference_t<Base>> //exposition-only
and use them throughout join_view::iterator
.
[2020-11-21; Reflector prioritization]
Set priority to 0 and status to Tentatively Ready after six votes in favour during reflector discussions.
[2021-02-26 Approved at February 2021 virtual plenary. Status changed: Tentatively Ready → WP.]
Proposed resolution:
This wording is relative to N4868.
Modify 25.7.14.3 [range.join.iterator], class template join_view::iterator
synopsis,
as indicated:
template<input_range V> requires view<V> && input_range<range_reference_t<V>> && (is_reference_v<range_reference_t<V>> || view<range_value_t<V>>) struct join_view<V>::iterator { private: using Parent = // exposition only conditional_t<Const, const join_view, join_view>; using Base = conditional_t<Const, const V, V>; // exposition only using OuterIter = iterator_t<Base>; //exposition-only using InnerIter = iterator_t<range_reference_t<Base>> //exposition-only static constexpr bool ref-is-glvalue = // exposition only is_reference_v<range_reference_t<Base>>; OuterIteriterator_t<Base>outer_ = OuterIteriterator_t<Base>(); // exposition only InnerIteriterator_t<range_reference_t<Base>>inner_ = // exposition only InnerIteriterator_t<range_reference_t<Base>>(); Parent* parent_ = nullptr; // exposition only constexpr void satisfy(); // exposition only public: […] iterator() = default; constexpr iterator(Parent& parent, OuterIteriterator_t<Base>outer); constexpr iterator(iterator<!Const> i) requires Const && convertible_to<iterator_t<V>, OuterIteriterator_t<Base>> && convertible_to<iterator_t<InnerRng>, InnerIteriterator_t<range_reference_t<Base>>>; constexpr decltype(auto) operator*() const { return *inner_; } constexpr InnerIteriterator_t<Base>operator->() const requires has-arrow<InnerIteriterator_t<Base>> && copyable<InnerIteriterator_t<Base>>; constexpr iterator& operator++(); […][…]
constexpr void satisfy(); // exposition only-5- Effects: Equivalent to:
[…] if constexpr (ref-is-glvalue) inner_ = InnerIteriterator_t<range_reference_t<Base>>();constexpr iterator(Parent& parent, OuterIteriterator_t<Base>outer);[…]
constexpr iterator(iterator<!Const> i) requires Const && convertible_to<iterator_t<V>, OuterIteriterator_t<Base>> && convertible_to<iterator_t<InnerRng>, InnerIteriterator_t<range_reference_t<Base>>>;[…]
constexpr InnerIteriterator_t<Base>operator->() const requires has-arrow<InnerIteriterator_t<Base>> && copyable<InnerIteriterator_t<Base>>;-8- Effects: Equivalent to
return inner_;
elements_view
should not be allowed to return dangling referencesSection: 25.7.23.3 [range.elements.iterator] Status: C++23 Submitter: Tim Song Opened: 2020-11-18 Last modified: 2023-11-22
Priority: 2
View other active issues in [range.elements.iterator].
View all other issues in [range.elements.iterator].
View all issues with C++23 status.
Discussion:
This compiles but the resulting view is full of dangling references:
std::vector<int> vec = {42}; auto r = vec | std::views::transform([](auto c) { return std::make_tuple(c, c); }) | std::views::keys;
This is because elements_view::iterator::operator*
is specified as
constexpr decltype(auto) operator*() const { return get<N>(*current_); }
Here *current_
is a prvalue, and so the get<N>
produces a reference
into the materialized temporary that becomes dangling as soon as operator*
returns.
operator*
(and operator[]
) return by
value when *current_
is a prvalue and the corresponding tuple element is not a reference
(since this get
is std::get
, we need not worry about weird user-defined overloads.)
[2020-11-29; Reflector prioritization]
Set priority to 2 during reflector discussions.
[2021-01-31 Tim adds PR]
[2021-02-08; Reflector poll]
Set status to Tentatively Ready after six votes in favour during reflector poll.
[2021-02-26 Approved at February 2021 virtual plenary. Status changed: Tentatively Ready → WP.]
Proposed resolution:
This wording is relative to N4878.
Modify 25.7.23.2 [range.elements.view], as indicated:
namespace std::ranges { template<class T, size_t N> concept has-tuple-element = // exposition only requires(T t) { typename tuple_size<T>::type; requires N < tuple_size_v<T>; typename tuple_element_t<N, T>; { get<N>(t) } -> convertible_to<const tuple_element_t<N, T>&>; }; template<class T, size_t N> concept returnable-element = is_reference_v<T> || move_constructible<tuple_element_t<N, T>>; template<input_range V, size_t N> requires view<V> && has-tuple-element<range_value_t<V>, N> && has-tuple-element<remove_reference_t<range_reference_t<V>>, N> && returnable-element<range_reference_t<V>, N> class elements_view : public view_interface<elements_view<V, N>> { […] }; }
Modify 25.7.23.3 [range.elements.iterator] as indicated:
namespace std::ranges { template<input_range V, size_t N> requires view<V> && has-tuple-element<range_value_t<V>, N> && has-tuple-element<remove_reference_t<range_reference_t<V>>, N> && returnable-element<range_reference_t<V>, N> template<bool Const> class elements_view<V, N>::iterator { // exposition only using Base = maybe-const<Const, V>; // exposition only iterator_t<Base> current_ = iterator_t<Base>(); // exposition only static constexpr decltype(auto) get-element(const iterator_t<Base>& i); // exposition only public: […] constexpr decltype(auto) operator*() const { returnget<N>get-element(*current_); } […] constexpr decltype(auto) operator[](difference_type n) const requires random_access_range<Base> { returnget<N>get-element(*(current_ + n)); } }; }static constexpr decltype(auto) get-element(const iterator_t<Base>& i); // exposition only-?- Effects: Equivalent to:
if constexpr (is_reference_v<range_reference_t<Base>>) { return get<N>(*i); } else { using E = remove_cv_t<tuple_element_t<N, range_reference_t<Base>>>; return static_cast<E>(get<N>(*i)); }
Modify 25.7.23.4 [range.elements.sentinel] as indicated:
namespace std::ranges { template<input_range V, size_t N> requires view<V> && has-tuple-element<range_value_t<V>, N> && has-tuple-element<remove_reference_t<range_reference_t<V>>, N> && returnable-element<range_reference_t<V>, N> template<bool Const> class elements_view<V, N>::sentinel { // exposition only […] }; }
split_view::outer-iterator::operator++
misspecifiedSection: 25.7.16.3 [range.lazy.split.outer] Status: C++23 Submitter: Tim Song Opened: 2020-11-20 Last modified: 2023-11-22
Priority: 2
View other active issues in [range.lazy.split.outer].
View all other issues in [range.lazy.split.outer].
View all issues with C++23 status.
Discussion:
Prior to the application of P1862R1, the part of
split_view::outer-iterator::operator++
that searches for the pattern
is specified as:
do { const auto [b, p] = ranges::mismatch(current, end, pbegin, pend); if (p == pend) { current = b; // The pattern matched; skip it break; } } while (++current != end);
P1862R1, trying to accommodate move-only iterators, respecified this as
do { auto [b, p] = ranges::mismatch(std::move(current), end, pbegin, pend); current = std::move(b); if (p == pend) { break; // The pattern matched; skip it } } while (++current != end);
but this is not correct, because if the pattern didn't match, it advances current
to the point of first mismatch, skipping elements before that point. This is totally wrong if the
pattern contains more than one element.
std::views::split("xxyx"sv, "xy"sv)
:
at the beginning, current
points to the first 'x'
ranges::mismatch
produces [
iterator to second 'x'
,
iterator to 'y'
in pattern]
current
now points to second 'x'
we increment current
in the condition, so it now points to 'y'
At this point there's no way we can find the "xy"
in the middle. In fact,
in this particular example, we'll increment past the end of the source range at the
end of the third iteration.
[2020-11-29; Reflector prioritization]
Set priority to 2 during reflector discussions.
[2021-02-08; Reflector poll]
Set status to Tentatively Ready after six votes in favour during reflector poll.
[2021-02-26 Approved at February 2021 virtual plenary. Status changed: Tentatively Ready → WP.]
Proposed resolution:
This wording is relative to N4868.
Modify [range.split.outer] as indicated:
constexpr outer-iterator& operator++();-6- Effects: Equivalent to:
const auto end = ranges::end(parent_->base_); if (current == end) return*this
; const auto [pbegin, pend] = subrange{parent_->pattern_}; if (pbegin == pend) ++current; else if constexpr (tiny-range<Pattern>) { current = ranges::find(std::move(current), end, *pbegin); if (current != end) { ++current; } } else { do { auto [b, p] = ranges::mismatch(std::move(current), end, pbegin, pend);current = std::move(b);if (p == pend) { current = b; break; // The pattern matched; skip it } } while (++current != end); } return *this;
priority_queue
Section: 23.6.4 [priority.queue] Status: C++23 Submitter: Tim Song Opened: 2020-11-21 Last modified: 2023-11-22
Priority: 3
View all other issues in [priority.queue].
View all issues with C++23 status.
Discussion:
priority_queue
has two constructor templates taking a pair of input
iterators in addition to a comparator and a container, but it does not have
allocator-extended constructors corresponding to these constructor templates:
template<class InputIterator> priority_queue(InputIterator first, InputIterator last, const Compare& x, const Container&); template<class InputIterator> priority_queue(InputIterator first, InputIterator last, const Compare& x = Compare(), Container&& = Container());
[2020-11-29; Reflector prioritization]
Set priority to 3 during reflector discussions. It has been pointed out that this issue is related to LWG 1199(i), LWG 2210(i), and LWG 2713(i).
[2021-02-17 Tim adds PR]
[2021-02-26; LWG telecon]
Set status to Tentatively Ready after discussion and poll.
F | A | N |
---|---|---|
11 | 0 | 0 |
[2021-06-07 Approved at June 2021 virtual plenary. Status changed: Voting → WP.]
Proposed resolution:
This wording is relative to N4878.
Add the following paragraph at the end of 23.6.1 [container.adaptors.general]:
-6- The exposition-only alias template
iter-value-type
defined in 23.3.1 [sequences.general] may appear in deduction guides for container adaptors.
Modify 23.6.4 [priority.queue], class template priority_queue
synopsis, as indicated:
namespace std { template<class T, class Container = vector<T>, class Compare = less<typename Container::value_type>> class priority_queue { // […] public: priority_queue() : priority_queue(Compare()) {} explicit priority_queue(const Compare& x) : priority_queue(x, Container()) {} priority_queue(const Compare& x, const Container&); priority_queue(const Compare& x, Container&&); template<class InputIterator> priority_queue(InputIterator first, InputIterator last, const Compare& x, const Container&); template<class InputIterator> priority_queue(InputIterator first, InputIterator last, const Compare& x = Compare(), Container&& = Container()); template<class Alloc> explicit priority_queue(const Alloc&); template<class Alloc> priority_queue(const Compare&, const Alloc&); template<class Alloc> priority_queue(const Compare&, const Container&, const Alloc&); template<class Alloc> priority_queue(const Compare&, Container&&, const Alloc&); template<class Alloc> priority_queue(const priority_queue&, const Alloc&); template<class Alloc> priority_queue(priority_queue&&, const Alloc&); template<class InputIterator, class Alloc> priority_queue(InputIterator, InputIterator, const Alloc&); template<class InputIterator, class Alloc> priority_queue(InputIterator, InputIterator, const Compare&, const Alloc&); template<class InputIterator, class Alloc> priority_queue(InputIterator, InputIterator, const Compare&, const Container&, const Alloc&); template<class InputIterator, class Alloc> priority_queue(InputIterator, InputIterator, const Compare&, Container&&, const Alloc&); // […] }; template<class Compare, class Container> priority_queue(Compare, Container) -> priority_queue<typename Container::value_type, Container, Compare>; template<class InputIterator, class Compare = less<typename iterator_traitsiter-value-type<InputIterator>::value_type>, class Container = vector<typename iterator_traitsiter-value-type<InputIterator>::value_type>> priority_queue(InputIterator, InputIterator, Compare = Compare(), Container = Container()) -> priority_queue<typename iterator_traitsiter-value-type<InputIterator>::value_type, Container, Compare>; template<class Compare, class Container, class Allocator> priority_queue(Compare, Container, Allocator) -> priority_queue<typename Container::value_type, Container, Compare>; template<class InputIterator, class Allocator> priority_queue(InputIterator, InputIterator, Allocator) -> priority_queue<iter-value-type<InputIterator>, vector<iter-value-type<InputIterator>, Allocator>, less<iter-value-type<InputIterator>>>; template<class InputIterator, class Compare, class Allocator> priority_queue(InputIterator, InputIterator, Compare, Allocator) -> priority_queue<iter-value-type<InputIterator>, vector<iter-value-type<InputIterator>, Allocator>, Compare>; template<class InputIterator, class Compare, class Container, class Allocator> priority_queue(InputIterator, InputIterator, Compare, Container, Allocator) -> priority_queue<typename Container::value_type, Container, Compare>; // […] }
Add the following paragraphs to 23.6.4.3 [priqueue.cons.alloc]:
template<class InputIterator, class Alloc> priority_queue(InputIterator first, InputIterator last, const Alloc& a);-?- Effects: Initializes
c
withfirst
as the first argument,last
as the second argument, anda
as the third argument, and value-initializescomp
; callsmake_heap(c.begin(), c.end(), comp)
.template<class InputIterator, class Alloc> priority_queue(InputIterator first, InputIterator last, const Compare& compare, const Alloc& a);-?- Effects: Initializes
c
withfirst
as the first argument,last
as the second argument, anda
as the third argument, and initializescomp
withcompare
; callsmake_heap(c.begin(), c.end(), comp)
.template<class InputIterator, class Alloc> priority_queue(InputIterator first, InputIterator last, const Compare& compare, const Container& cont, const Alloc& a);-?- Effects: Initializes
c
withcont
as the first argument anda
as the second argument, and initializescomp
withcompare
; callsc.insert(c.end(), first, last)
; and finally callsmake_heap(c.begin(), c.end(), comp)
.template<class InputIterator, class Alloc> priority_queue(InputIterator first, InputIterator last, const Compare& compare, Container&& cont, const Alloc& a);-?- Effects: Initializes
c
withstd::move(cont)
as the first argument anda
as the second argument, and initializescomp
withcompare
; callsc.insert(c.end(), first, last)
; and finally callsmake_heap(c.begin(), c.end(), comp)
.
atomic_ref<cv T>
is not well-specifiedSection: 32.5.7.1 [atomics.ref.generic.general] Status: Resolved Submitter: Casey Carter Opened: 2020-12-02 Last modified: 2025-02-07
Priority: 2
View all issues with Resolved status.
Discussion:
atomic_ref<T>
requires only that its template parameter T
is trivially copyable,
which is not sufficient to implement many of the class template's member functions. Consider, for example:
int main() { static const int x = 42; std::atomic_ref<const int> meow{x}; meow.store(0); return meow.load(); }
Since const int
is trivially copyable, this is a well-formed C++20 program that returns 0
.
That said, the store
call that mutates the value of a constant object is (a) not implementable with
library technology, and (b) pure madness that violates the language semantics. atomic_ref::store
should presumably require is_copy_assignable_v<T>
, and other operations need to have their
requirements audited as well. (Related: LWG 3012(i) made similar changes to atomic
.)
volatile atomic<T>
recently had its member functions deprecated when they
are not lock-free. Presumably atomic_ref<volatile T>
should require that atomic operations on
T
be lock-free for consistency.
Jonathan:
The last point is related to LWG 3417(i) (another case where we screwed up the volatile
deprecations).
[2020-12-19; Reflector prioritization]
Set priority to 2 during reflector discussions.
[2024-06; Related to issue 4069(i).]
[St. Louis 2024-06-28; SG1 feedback]
SG1 forwarded P3323R0 to LEWG to resolve LWG issues 3508(i) and 4069(i).
[2025-02-07 Status changed: Open → Resolved.]
Resolved by P3323R0 in Wrocław.
Proposed resolution:
Section: 25.7.2 [range.adaptor.object] Status: Resolved Submitter: Tim Song Opened: 2020-12-15 Last modified: 2021-06-14
Priority: 2
View other active issues in [range.adaptor.object].
View all other issues in [range.adaptor.object].
View all issues with Resolved status.
Discussion:
There is implementation divergence in the handling of this example:
template<class F> auto filter(F f) { return std::views::filter(f); } std::vector<int> v = {1, 2, 3, 4}; auto f = filter([i = std::vector{4}] (auto x) { return x == i[0]; }); auto x = v | f;
libstdc++'s views::filter
stores a reference to the argument if it is
passed as an lvalue, so f
contains a dangling reference in the above
example. MSVC's views::filter
always stores a copy, and forwards that
as either an lvalue or an rvalue depending on the range adaptor closure object's
value category.
adaptor(args...)
case; arguably, the requirement that adaptor(range, args...)
be
equivalent to adaptor(args...)(range)
may even rule out the ability to
make an extra copy. Certainly nothing in the wording guarantees that
it is safe to reuse lvalue range pipelines (that is, v | f
doesn't
move from f
).
[2021-01-15; Telecon prioritization]
Set priority to 2 following reflector and telecon discussions.
[2021-06-13 Resolved by the adoption of P2281R1 at the June 2021 plenary. Status changed: New → Resolved.]
Proposed resolution:
const
tooSection: 16.3.3.3.5 [customization.point.object] Status: Resolved Submitter: Tim Song Opened: 2020-12-15 Last modified: 2021-06-14
Priority: 3
View all other issues in [customization.point.object].
View all issues with Resolved status.
Discussion:
16.3.3.3.5 [customization.point.object] promises that customization point objects
are semiregular and that all copies are equal, which permits copies to
be freely made. In C++, making copies tends to drop cv-qualification,
but there does not appear to be anything that guarantees that you can
invoke these objects as non-const
(or as rvalues, for that matter).
invocable<const F&, Args...>
here was
meant to bring in implicit expression variations, but P2102R0
has since clarified that this formulation doesn't do that.
[2021-01-15; Telecon prioritization]
Set priority to 3 following reflector and telecon discussions.
[2021-06-13 Resolved by the adoption of P2281R1 at the June 2021 plenary. Status changed: New → Resolved.]
Proposed resolution:
stacktrace
should add type alias pmr::stacktrace
Section: 19.6.2 [stacktrace.syn] Status: Resolved Submitter: Hiroaki Ando Opened: 2021-01-11 Last modified: 2021-10-23
Priority: 3
View all other issues in [stacktrace.syn].
View all issues with Resolved status.
Discussion:
std::stacktrace
is almost std::vector<stacktrace_entry>
. This
makes it an obvious candidate to define an alias for std::polymorphic_allocator
.
namespace pmr { template<class T> using basic_stacktrace = std::basic_stacktrace<polymorphic_allocator<T>>; }
— albeit it would seem a natural choice when comparing with existing pmr
typedef
additions — would not provide any advantage for the user, because template parameter
T
would essentially need to be constrained to be equal to std::stacktrace_entry
.
[2021-03-12; Reflector poll]
Set priority to 3 and status to LEWG following reflector poll.
P2301 would resolve this.
[2021-10-23 Resolved by the adoption of P2301R1 at the October 2021 plenary. Status changed: Tentatively Resolved → Resolved.]
Proposed resolution:
This wording is relative to N4878.
Modify 19.6.2 [stacktrace.syn], header <stacktrace>
synopsis, as indicated:
namespace std { // 19.6.3 [stacktrace.entry], class stacktrace_entry class stacktrace_entry; // 19.6.4 [stacktrace.basic], class template basic_stacktrace template<class Allocator> class basic_stacktrace; // basic_stacktrace typedef names using stacktrace = basic_stacktrace<allocator<stacktrace_entry>>; […] // 19.6.6 [stacktrace.basic.hash], hash support template<class T> struct hash; template<> struct hash<stacktrace_entry>; template<class Allocator> struct hash<basic_stacktrace<Allocator>>; namespace pmr { using stacktrace = std::basic_stacktrace<polymorphic_allocator<stacktrace_entry>>; } }
operator<<
should be less templatizedSection: 19.6.2 [stacktrace.syn], 19.6.4.6 [stacktrace.basic.nonmem] Status: C++23 Submitter: Jiang An Opened: 2021-01-25 Last modified: 2023-11-22
Priority: 2
View all other issues in [stacktrace.syn].
View all issues with C++23 status.
Discussion:
According to 27.4.4.4 [string.io], the operator<<
overloads in
19.6.4.6 [stacktrace.basic.nonmem] are well-formed only if the template parameters
charT
and traits
are char
and std::char_traits<char>
(that of std::string
) respectively, because it is required in Effects: that
these overloads behave as-if insert a std::string
.
ostream& operator<<(ostream& os, const stacktrace_entry& f); template<class Allocator> ostream& operator<<(ostream& os, const basic_stacktrace<Allocator>& st);
[2021-03-12; Reflector poll]
Set priority to 2 and status to LEWG following reflector poll.
[2022-11-07; Kona]
Move to Immediate.
[2022-11-12 Approved at November 2022 meeting in Kona. Status changed: Immediate → WP.]
Proposed resolution:
This wording is relative to N4878.
Modify 19.6.2 [stacktrace.syn], header <stacktrace>
synopsis, as indicated:
namespace std { // 19.6.3 [stacktrace.entry], class stacktrace_entry class stacktrace_entry; // 19.6.4 [stacktrace.basic], class template basic_stacktrace template<class Allocator> class basic_stacktrace; […] // 19.6.4.6 [stacktrace.basic.nonmem], non-member functions […] string to_string(const stacktrace_entry& f); template<class Allocator> string to_string(const basic_stacktrace<Allocator>& st);template<class charT, class traits> basic_ostream<charT, traits>& operator<<(basic_ostream<charT, traits>& os, const stacktrace_entry& f); template<class charT, class traits,class Allocator>basic_ostream<charT, traits>& operator<<(basic_ostream<charT, traits>& os, const basic_stacktrace<Allocator>& st); […] }
Modify 19.6.4.6 [stacktrace.basic.nonmem] as indicated:
template<class charT, class traits> basic_ostream<charT, traits>& operator<<(basic_ostream<charT, traits>& os, const stacktrace_entry& f);-4- Effects: Equivalent to:
return os << to_string(f);
template<class charT, class traits,class Allocator>basic_ostream<charT, traits>& operator<<(basic_ostream<charT, traits>& os, const basic_stacktrace<Allocator>& st);-5- Effects: Equivalent to:
return os << to_string(st);
join_view::iterator
's iter_swap
is underconstrainedSection: 25.7.14.3 [range.join.iterator] Status: C++23 Submitter: Casey Carter Opened: 2021-01-28 Last modified: 2023-11-22
Priority: Not Prioritized
View all other issues in [range.join.iterator].
View all issues with C++23 status.
Discussion:
std::ranges::join_view::iterator
's hidden friend iter_swap
is specified in
25.7.14.3 [range.join.iterator]/16 as:
friend constexpr void iter_swap(const iterator& x, const iterator& y) noexcept(noexcept(ranges::iter_swap(x.inner_, y.inner_)));-16- Effects: Equivalent to:
return ranges::iter_swap(x.inner_, y.inner_);
Notably, the expression ranges::iter_swap(meow, woof)
is not valid for all
iterators meow
and woof
, or even all input iterators of the same
type as is the case here. This iter_swap
overload should be constrained to require the
type of iterator::inner_
(iterator_t<range_reference_t<maybe-const<Const, V>>>)
to satisfy indirectly_swappable
. Notably this is already the case for iter_swap
friends of every other iterator adaptor in the Standard Library (reverse_iterator
, move_iterator
,
common_iterator
, counted_iterator
, filter_view::iterator
, transform_view::iterator
, and split_view::inner-iterator
). The omission for join_view::iterator
seems to
have simply been an oversight.
[2021-03-12; Reflector poll]
Set status to Tentatively Ready after six votes in favour during reflector poll.
[2021-06-07 Approved at June 2021 virtual plenary. Status changed: Voting → WP.]
Proposed resolution:
This wording is relative to N4878.
Modify 25.7.14.3 [range.join.iterator] as indicated:
[Drafting note: If 3500(i) is accepted before this issue, it is kindly suggested to the Project Editor to apply the equivalent replacement of "
iterator_t<range_reference_t<Base>>
" by "InnerIter
" to the newly insertedrequires
.
[…]namespace std::ranges { template<input_range V> requires view<V> && input_range<range_reference_t<V>> && (is_reference_v<range_reference_t<V>> || view<range_value_t<V>>) template<bool Const> struct join_view<V>::iterator { […] friend constexpr void iter_swap(const iterator& x, const iterator& y) noexcept(noexcept(ranges::iter_swap(x.inner_, y.inner_))) requires indirectly_swappable<iterator_t<range_reference_t<Base>>>; }; }
friend constexpr void iter_swap(const iterator& x, const iterator& y) noexcept(noexcept(ranges::iter_swap(x.inner_, y.inner_))) requires indirectly_swappable<iterator_t<range_reference_t<Base>>>;-16- Effects: Equivalent to:
return ranges::iter_swap(x.inner_, y.inner_);
Section: 27.2.2 [char.traits.require] Status: C++23 Submitter: Zoe Carver Opened: 2021-02-01 Last modified: 2023-11-22
Priority: Not Prioritized
View other active issues in [char.traits.require].
View all other issues in [char.traits.require].
View all issues with C++23 status.
Discussion:
27.2.2 [char.traits.require] p1 says:
X
denotes a traits class defining types and functions for the character container typeC
[…] Operations onX
shall not throw exceptions.
It should be clarified what "operations on X
" means. For example, in this
patch, there was some confusion around the exact meaning of "operations on X
". If it refers to
the expressions specified in [tab:char.traits.req] or if it refers to all member functions of X
,
this should be worded in some clearer way.
[2021-03-12; Reflector poll]
Set status to Tentatively Ready after six votes in favour during reflector poll.
[2021-06-07 Approved at June 2021 virtual plenary. Status changed: Voting → WP.]
Proposed resolution:
This wording is relative to N4878.
Modify 27.2.2 [char.traits.require] as indicated:
-1- In Table [tab:char.traits.req],
X
denotes a traits class defining types and functions for the character container typeC
; […]Operations onNo expression which is part of the character traits requirements specified in this subclause 27.2.2 [char.traits.require] shall exit via an exception.X
shall not throw exceptions
<random>
classesSection: 29.5.4 [rand.eng], 29.5.5 [rand.adapt], 29.5.9 [rand.dist] Status: C++23 Submitter: Jens Maurer Opened: 2021-02-02 Last modified: 2023-11-22
Priority: 3
View all other issues in [rand.eng].
View all issues with C++23 status.
Discussion:
The synopses for the engine and distribution classes in <random>
do
not show declarations operator==
, operator!=
, operator<<
,
and operator>>
, although they are part of the engine and distribution requirements.
[2021-02-07: Daniel provides concrete wording]
The proposed wording attempts to use a conservative approach in regard to exception specifications. Albeit
29.5.4.1 [rand.eng.general] p3 says that "no function described in 29.5.4 [rand.eng] throws
an exception" (unless specified otherwise), not all implementations have marked the equality operators
as noexcept
(But some do for their engines, such as VS 2019). [No implementations marks the IO
stream operators as noexcept
of-course, because these cannot be prevented to throw exceptions in
general by design].
operator!=
, because the auto-generated form already has
the right semantics. The wording also covers the engine adaptor class templates because similar requirements
apply to them.
[2021-03-12; Reflector poll]
Set priority to 3 following reflector poll.
[2021-03-12; LWG telecon]
Set status to Tentatively Ready after discussion and poll.
F | A | N |
---|---|---|
10 | 0 | 0 |
[2021-06-07 Approved at June 2021 virtual plenary. Status changed: Voting → WP.]
Proposed resolution:
This wording is relative to N4878.
Modify 29.5.4.2 [rand.eng.lcong] as indicated:
template<class UIntType, UIntType a, UIntType c, UIntType m> class linear_congruential_engine { […] // constructors and seeding functions […] // equality operators friend bool operator==(const linear_congruential_engine& x, const linear_congruential_engine& y); // generating functions […] // inserters and extractors template<class charT, class traits> friend basic_ostream<charT, traits>& operator<<(basic_ostream<charT, traits>& os, const linear_congruential_engine& x); template<class charT, class traits> friend basic_istream<charT, traits>& operator>>(basic_istream<charT, traits>& is, linear_congruential_engine& x); };
Modify 29.5.4.3 [rand.eng.mers] as indicated:
template<class UIntType, size_t w, size_t n, size_t m, size_t r, UIntType a, size_t u, UIntType d, size_t s, UIntType b, size_t t, UIntType c, size_t l, UIntType f> class mersenne_twister_engine { […] // constructors and seeding functions […] // equality operators friend bool operator==(const mersenne_twister_engine& x, const mersenne_twister_engine& y); // generating functions […] // inserters and extractors template<class charT, class traits> friend basic_ostream<charT, traits>& operator<<(basic_ostream<charT, traits>& os, const mersenne_twister_engine& x); template<class charT, class traits> friend basic_istream<charT, traits>& operator>>(basic_istream<charT, traits>& is, mersenne_twister_engine& x); };
Modify 29.5.4.4 [rand.eng.sub] as indicated:
template<class UIntType, size_t w, size_t s, size_t r> class subtract_with_carry_engine { […] // constructors and seeding functions […] // equality operators friend bool operator==(const subtract_with_carry_engine& x, const subtract_with_carry_engine& y); // generating functions […] // inserters and extractors template<class charT, class traits> friend basic_ostream<charT, traits>& operator<<(basic_ostream<charT, traits>& os, const subtract_with_carry_engine& x); template<class charT, class traits> friend basic_istream<charT, traits>& operator>>(basic_istream<charT, traits>& is, subtract_with_carry_engine& x); };
Modify 29.5.5.2 [rand.adapt.disc] as indicated:
template<class Engine, size_t p, size_t r> class discard_block_engine { […] // constructors and seeding functions […] // equality operators friend bool operator==(const discard_block_engine& x, const discard_block_engine& y); // generating functions […] // inserters and extractors template<class charT, class traits> friend basic_ostream<charT, traits>& operator<<(basic_ostream<charT, traits>& os, const discard_block_engine& x); template<class charT, class traits> friend basic_istream<charT, traits>& operator>>(basic_istream<charT, traits>& is, discard_block_engine& x); };
Modify 29.5.5.3 [rand.adapt.ibits] as indicated:
template<class Engine, size_t w, class UIntType> class independent_bits_engine { […] // constructors and seeding functions […] // equality operators friend bool operator==(const independent_bits_engine& x, const independent_bits_engine& y); // generating functions […] // inserters and extractors template<class charT, class traits> friend basic_ostream<charT, traits>& operator<<(basic_ostream<charT, traits>& os, const independent_bits_engine& x); template<class charT, class traits> friend basic_istream<charT, traits>& operator>>(basic_istream<charT, traits>& is, independent_bits_engine& x); };
Modify 29.5.5.4 [rand.adapt.shuf] as indicated:
template<class Engine, size_t k> class shuffle_order_engine { […] // constructors and seeding functions […] // equality operators friend bool operator==(const shuffle_order_engine& x, const shuffle_order_engine& y); // generating functions […] // inserters and extractors template<class charT, class traits> friend basic_ostream<charT, traits>& operator<<(basic_ostream<charT, traits>& os, const shuffle_order_engine& x); template<class charT, class traits> friend basic_istream<charT, traits>& operator>>(basic_istream<charT, traits>& is, shuffle_order_engine& x); };
Modify 29.5.9.2.1 [rand.dist.uni.int] as indicated:
template<class IntType = int> class uniform_int_distribution { […] // constructors and reset functions […] // equality operators friend bool operator==(const uniform_int_distribution& x, const uniform_int_distribution& y); // generating functions […] // property functions […] // inserters and extractors template<class charT, class traits> friend basic_ostream<charT, traits>& operator<<(basic_ostream<charT, traits>& os, const uniform_int_distribution& x); template<class charT, class traits> friend basic_istream<charT, traits>& operator>>(basic_istream<charT, traits>& is, uniform_int_distribution& x); };
Modify 29.5.9.2.2 [rand.dist.uni.real] as indicated:
template<class RealType = double> class uniform_real_distribution { […] // constructors and reset functions […] // equality operators friend bool operator==(const uniform_real_distribution& x, const uniform_real_distribution& y); // generating functions […] // property functions […] // inserters and extractors template<class charT, class traits> friend basic_ostream<charT, traits>& operator<<(basic_ostream<charT, traits>& os, const uniform_real_distribution& x); template<class charT, class traits> friend basic_istream<charT, traits>& operator>>(basic_istream<charT, traits>& is, uniform_real_distribution& x); };
Modify 29.5.9.3.1 [rand.dist.bern.bernoulli] as indicated:
class bernoulli_distribution { […] // constructors and reset functions […] // equality operators friend bool operator==(const bernoulli_distribution& x, const bernoulli_distribution& y); // generating functions […] // property functions […] // inserters and extractors template<class charT, class traits> friend basic_ostream<charT, traits>& operator<<(basic_ostream<charT, traits>& os, const bernoulli_distribution& x); template<class charT, class traits> friend basic_istream<charT, traits>& operator>>(basic_istream<charT, traits>& is, bernoulli_distribution& x); };
Modify 29.5.9.3.2 [rand.dist.bern.bin] as indicated:
template<class IntType = int> class binomial_distribution { […] // constructors and reset functions […] // equality operators friend bool operator==(const binomial_distribution& x, const binomial_distribution& y); // generating functions […] // property functions […] // inserters and extractors template<class charT, class traits> friend basic_ostream<charT, traits>& operator<<(basic_ostream<charT, traits>& os, const binomial_distribution& x); template<class charT, class traits> friend basic_istream<charT, traits>& operator>>(basic_istream<charT, traits>& is, binomial_distribution& x); };
Modify 29.5.9.3.3 [rand.dist.bern.geo] as indicated:
template<class IntType = int> class geometric_distribution { […] // constructors and reset functions […] // equality operators friend bool operator==(const geometric_distribution& x, const geometric_distribution& y); // generating functions […] // property functions […] // inserters and extractors template<class charT, class traits> friend basic_ostream<charT, traits>& operator<<(basic_ostream<charT, traits>& os, const geometric_distribution& x); template<class charT, class traits> friend basic_istream<charT, traits>& operator>>(basic_istream<charT, traits>& is, geometric_distribution& x); };
Modify 29.5.9.3.4 [rand.dist.bern.negbin] as indicated:
template<class IntType = int> class negative_binomial_distribution { […] // constructors and reset functions […] // equality operators friend bool operator==(const negative_binomial_distribution& x, const negative_binomial_distribution& y); // generating functions […] // property functions […] // inserters and extractors template<class charT, class traits> friend basic_ostream<charT, traits>& operator<<(basic_ostream<charT, traits>& os, const negative_binomial_distribution& x); template<class charT, class traits> friend basic_istream<charT, traits>& operator>>(basic_istream<charT, traits>& is, negative_binomial_distribution& x); };
Modify 29.5.9.4.1 [rand.dist.pois.poisson] as indicated:
template<class IntType = int> class poisson_distribution { […] // constructors and reset functions […] // equality operators friend bool operator==(const poisson_distribution& x, const poisson_distribution& y); // generating functions […] // property functions […] // inserters and extractors template<class charT, class traits> friend basic_ostream<charT, traits>& operator<<(basic_ostream<charT, traits>& os, const poisson_distribution& x); template<class charT, class traits> friend basic_istream<charT, traits>& operator>>(basic_istream<charT, traits>& is, poisson_distribution& x); };
Modify 29.5.9.4.2 [rand.dist.pois.exp] as indicated:
template<class RealType = double> class exponential_distribution { […] // constructors and reset functions […] // equality operators friend bool operator==(const exponential_distribution& x, const exponential_distribution& y); // generating functions […] // property functions […] // inserters and extractors template<class charT, class traits> friend basic_ostream<charT, traits>& operator<<(basic_ostream<charT, traits>& os, const exponential_distribution& x); template<class charT, class traits> friend basic_istream<charT, traits>& operator>>(basic_istream<charT, traits>& is, exponential_distribution& x); };
Modify 29.5.9.4.3 [rand.dist.pois.gamma] as indicated:
template<class RealType = double> class gamma_distribution { […] // constructors and reset functions […] // equality operators friend bool operator==(const gamma_distribution& x, const gamma_distribution& y); // generating functions […] // property functions […] // inserters and extractors template<class charT, class traits> friend basic_ostream<charT, traits>& operator<<(basic_ostream<charT, traits>& os, const gamma_distribution& x); template<class charT, class traits> friend basic_istream<charT, traits>& operator>>(basic_istream<charT, traits>& is, gamma_distribution& x); };
Modify 29.5.9.4.4 [rand.dist.pois.weibull] as indicated:
template<class RealType = double> class weibull_distribution { […] // constructors and reset functions […] // equality operators friend bool operator==(const weibull_distribution& x, const weibull_distribution& y); // generating functions […] // property functions […] // inserters and extractors template<class charT, class traits> friend basic_ostream<charT, traits>& operator<<(basic_ostream<charT, traits>& os, const weibull_distribution& x); template<class charT, class traits> friend basic_istream<charT, traits>& operator>>(basic_istream<charT, traits>& is, weibull_distribution& x); };
Modify 29.5.9.4.5 [rand.dist.pois.extreme] as indicated:
template<class RealType = double> class extreme_value_distribution { […] // constructors and reset functions […] // equality operators friend bool operator==(const extreme_value_distribution& x, const extreme_value_distribution& y); // generating functions […] // property functions […] // inserters and extractors template<class charT, class traits> friend basic_ostream<charT, traits>& operator<<(basic_ostream<charT, traits>& os, const extreme_value_distribution& x); template<class charT, class traits> friend basic_istream<charT, traits>& operator>>(basic_istream<charT, traits>& is, extreme_value_distribution& x); };
Modify 29.5.9.5.1 [rand.dist.norm.normal] as indicated:
template<class RealType = double> class normal_distribution { […] // constructors and reset functions […] // equality operators friend bool operator==(const normal_distribution& x, const normal_distribution& y); // generating functions […] // property functions […] // inserters and extractors template<class charT, class traits> friend basic_ostream<charT, traits>& operator<<(basic_ostream<charT, traits>& os, const normal_distribution& x); template<class charT, class traits> friend basic_istream<charT, traits>& operator>>(basic_istream<charT, traits>& is, normal_distribution& x); };
Modify 29.5.9.5.2 [rand.dist.norm.lognormal] as indicated:
template<class RealType = double> class lognormal_distribution { […] // constructors and reset functions […] // equality operators friend bool operator==(const lognormal_distribution& x, const lognormal_distribution& y); // generating functions […] // property functions […] // inserters and extractors template<class charT, class traits> friend basic_ostream<charT, traits>& operator<<(basic_ostream<charT, traits>& os, const lognormal_distribution& x); template<class charT, class traits> friend basic_istream<charT, traits>& operator>>(basic_istream<charT, traits>& is, lognormal_distribution& x); };
Modify 29.5.9.5.3 [rand.dist.norm.chisq] as indicated:
template<class RealType = double> class chi_squared_distribution { […] // constructors and reset functions […] // equality operators friend bool operator==(const chi_squared_distribution& x, const chi_squared_distribution& y); // generating functions […] // property functions […] // inserters and extractors template<class charT, class traits> friend basic_ostream<charT, traits>& operator<<(basic_ostream<charT, traits>& os, const chi_squared_distribution& x); template<class charT, class traits> friend basic_istream<charT, traits>& operator>>(basic_istream<charT, traits>& is, chi_squared_distribution& x); };
Modify 29.5.9.5.4 [rand.dist.norm.cauchy] as indicated:
template<class RealType = double> class cauchy_distribution { […] // constructors and reset functions […] // equality operators friend bool operator==(const cauchy_distribution& x, const cauchy_distribution& y); // generating functions […] // property functions […] // inserters and extractors template<class charT, class traits> friend basic_ostream<charT, traits>& operator<<(basic_ostream<charT, traits>& os, const cauchy_distribution& x); template<class charT, class traits> friend basic_istream<charT, traits>& operator>>(basic_istream<charT, traits>& is, cauchy_distribution& x); };
Modify 29.5.9.5.5 [rand.dist.norm.f] as indicated:
template<class RealType = double> class fisher_f_distribution { […] // constructors and reset functions […] // equality operators friend bool operator==(const fisher_f_distribution& x, const fisher_f_distribution& y); // generating functions […] // property functions […] // inserters and extractors template<class charT, class traits> friend basic_ostream<charT, traits>& operator<<(basic_ostream<charT, traits>& os, const fisher_f_distribution& x); template<class charT, class traits> friend basic_istream<charT, traits>& operator>>(basic_istream<charT, traits>& is, fisher_f_distribution& x); };
Modify 29.5.9.5.6 [rand.dist.norm.t] as indicated:
template<class RealType = double> class student_t_distribution { […] // constructors and reset functions […] // equality operators friend bool operator==(const student_t_distribution& x, const student_t_distribution& y); // generating functions […] // property functions […] // inserters and extractors template<class charT, class traits> friend basic_ostream<charT, traits>& operator<<(basic_ostream<charT, traits>& os, const student_t_distribution& x); template<class charT, class traits> friend basic_istream<charT, traits>& operator>>(basic_istream<charT, traits>& is, student_t_distribution& x); };
Modify 29.5.9.6.1 [rand.dist.samp.discrete] as indicated:
template<class IntType = int> class discrete_distribution { […] // constructors and reset functions […] // equality operators friend bool operator==(const discrete_distribution& x, const discrete_distribution& y); // generating functions […] // property functions […] // inserters and extractors template<class charT, class traits> friend basic_ostream<charT, traits>& operator<<(basic_ostream<charT, traits>& os, const discrete_distribution& x); template<class charT, class traits> friend basic_istream<charT, traits>& operator>>(basic_istream<charT, traits>& is, discrete_distribution& x); };
Modify 29.5.9.6.2 [rand.dist.samp.pconst] as indicated:
template<class RealType = double> class piecewise_constant_distribution { […] // constructors and reset functions […] // equality operators friend bool operator==(const piecewise_constant_distribution& x, const piecewise_constant_distribution& y); // generating functions […] // property functions […] // inserters and extractors template<class charT, class traits> friend basic_ostream<charT, traits>& operator<<(basic_ostream<charT, traits>& os, const piecewise_constant_distribution& x); template<class charT, class traits> friend basic_istream<charT, traits>& operator>>(basic_istream<charT, traits>& is, piecewise_constant_distribution& x); };
Modify 29.5.9.6.3 [rand.dist.samp.plinear] as indicated:
template<class RealType = double> class piecewise_linear_distribution { […] // constructors and reset functions […] // equality operators friend bool operator==(const piecewise_linear_distribution& x, const piecewise_linear_distribution& y); // generating functions […] // property functions […] // inserters and extractors template<class charT, class traits> friend basic_ostream<charT, traits>& operator<<(basic_ostream<charT, traits>& os, const piecewise_linear_distribution& x); template<class charT, class traits> friend basic_istream<charT, traits>& operator>>(basic_istream<charT, traits>& is, piecewise_linear_distribution& x); };
iter_move
and iter_swap
are inconsistent for transform_view::iterator
Section: 25.7.9.3 [range.transform.iterator] Status: C++23 Submitter: Tim Song Opened: 2021-02-03 Last modified: 2023-11-22
Priority: 2
View all other issues in [range.transform.iterator].
View all issues with C++23 status.
Discussion:
For transform_view::iterator
, iter_move
is specified to operate on the
transformed value but iter_swap
is specified to operate on the underlying iterator.
struct X { int x; int y; }; std::vector<X> v = {...}; auto t = v | views::transform(&X::x); ranges::sort(t);
iter_swap
on t
's iterators would swap the whole X
, including the
y
part, but iter_move
will only move the x
data member and leave
the y
part intact. Meanwhile, ranges::sort
can use both iter_move
and
iter_swap
, and does so in at least one implementation. The mixed behavior means that we
get neither "sort X
s by their x
data member" (as ranges::sort(v, {}, &X::x)
would do), nor "sort the x
data member of these X
s and leave the rest unchanged",
as one might expect, but instead some arbitrary permutation of y
. This seems like a
questionable state of affairs.
[2021-03-12; Reflector poll]
Set priority to 2 following reflector poll.
[2021-03-12; LWG telecon]
Set status to Tentatively Ready after discussion and poll.
F | A | N |
---|---|---|
9 | 0 | 0 |
[2021-06-07 Approved at June 2021 virtual plenary. Status changed: Voting → WP.]
Proposed resolution:
This wording is relative to N4878.
Modify 25.7.9.3 [range.transform.iterator] as indicated:
[…]namespace std::ranges { template<input_range V, copy_constructible F> requires view<V> && is_object_v<F> && regular_invocable<F&, range_reference_t<V>> && can-reference<invoke_result_t<F&, range_reference_t<V>>> template<bool Const> class transform_view<V, F>::iterator { […]friend constexpr void iter_swap(const iterator& x, const iterator& y) noexcept(noexcept(ranges::iter_swap(x.current_, y.current_))) requires indirectly_swappable<iterator_t<Base>>;}; }friend constexpr void iter_swap(const iterator& x, const iterator& y) noexcept(noexcept(ranges::iter_swap(x.current_, y.current_))) requires indirectly_swappable<iterator_t<Base>>;
-23- Effects: Equivalent toranges::iter_swap(x.current_, y.current_)
.
qsort
and bsearch
Section: 26.13 [alg.c.library] Status: C++23 Submitter: Richard Smith Opened: 2021-02-02 Last modified: 2023-11-22
Priority: Not Prioritized
View all other issues in [alg.c.library].
View all issues with C++23 status.
Discussion:
Per 26.13 [alg.c.library]/2, for qsort
and bsearch
, we have:
Preconditions: The objects in the array pointed to by
base
are of trivial type.
This seems like an unnecessarily strict requirement. qsort
only needs the objects to be of a
trivially-copyable type (because it will use memcpy
or equivalent to relocate them), and
bsearch
doesn't need any particular properties of the array element type. Presumably it
would be in improvement to specify the more-precise requirements instead.
is_trivially_copyable
plus
is_trivially_default_constructible
instead, or perhaps is_trivially_copy_constructible
and is_trivially_move_constructible
and so on.
Other than qsort
and bsearch
, the only uses of this type property in the standard are
to constrain max_align_t
, aligned_storage
, aligned_union
, and the element type
of basic_string
(and in the definition of the deprecated is_pod
trait), all of which
(other than is_pod
) I think really mean "is trivially default constructible", not "has at least
one eligible default constructor and all eligible default constructors are trivial". And in fact I think
the alignment types are underspecified — we don't want to require merely that they be
trivially-copyable, since that doesn't require any particular operation on them to actually be valid —
we also want to require that they actually model semiregular
.
[2021-02-23; Casey Carter provides concrete wording]
[2021-03-12; Reflector poll]
Set status to Tentatively Ready after five votes in favour during reflector poll.
[2021-06-07 Approved at June 2021 virtual plenary. Status changed: Voting → WP.]
Proposed resolution:
This wording is relative to N4878.
Modify 26.13 [alg.c.library] as indicated:
void* bsearch(const void* key, const void* base, size_t nmemb, size_t size, c-compare-pred* compar); void* bsearch(const void* key, const void* base, size_t nmemb, size_t size, compare-pred* compar); void qsort(void* base, size_t nmemb, size_t size, c-compare-pred* compar); void qsort(void* base, size_t nmemb, size_t size, compare-pred* compar);-2- Preconditions: For
qsort
, tThe objects in the array pointed to by base are oftrivialtrivially copyable type.
InputIterator
template parameter for priority_queue
constructorsSection: 23.6.4 [priority.queue] Status: C++23 Submitter: Tim Song Opened: 2021-02-17 Last modified: 2023-11-22
Priority: Not Prioritized
View all other issues in [priority.queue].
View all issues with C++23 status.
Discussion:
There is nothing in 23.6.4 [priority.queue] or more generally 23.6 [container.adaptors]
saying that InputIterator
in the following constructor templates has to be an input iterator.
template<class InputIterator> priority_queue(InputIterator first, InputIterator last, const Compare& x, const Container&); template<class InputIterator> priority_queue(InputIterator first, InputIterator last, const Compare& x = Compare(), Container&& = Container());
The second constructor template above therefore accepts
std::priority_queue<int> x = {1, 2};
to produce a priority_queue
that contains a single element 2
. This behavior seems extremely questionable.
[2021-02-26; LWG telecon]
Set status to Tentatively Ready after discussion and poll.
F | A | N |
---|---|---|
11 | 0 | 0 |
[2021-06-07 Approved at June 2021 virtual plenary. Status changed: Voting → WP.]
Proposed resolution:
[Drafting note: Because an upcoming paper provides iterator-pair constructors for other container adaptors, the wording below adds the restriction to 23.6.1 [container.adaptors.general] so that it also covers the constructors that will be added by that paper. — end drafting note]
This wording is relative to N4878.
Add the following paragraph to 23.6.1 [container.adaptors.general] after p3:
-?- A constructor template of a container adaptor shall not participate in overload resolution if it has an
InputIterator
template parameter and a type that does not qualify as an input iterator is deduced for that parameter.-4- A deduction guide for a container adaptor shall not participate in overload resolution if any of the following are true:
(4.1) — It has an
InputIterator
template parameter and a type that does not qualify as an input iterator is deduced for that parameter.[…]
iota_view::sentinel
is not always iota_view
's sentinelSection: 25.6.4.2 [range.iota.view] Status: C++23 Submitter: Tim Song Opened: 2021-02-17 Last modified: 2023-11-22
Priority: Not Prioritized
View all other issues in [range.iota.view].
View all issues with C++23 status.
Discussion:
P1739R4 added the following constructor to
iota_view
:
constexpr iota_view(iterator first, sentinel last) : iota_view(*first, last.bound_) {}
However, while iota_view
's iterator type is always iota_view::iterator
, its sentinel type
is not always iota_view::sentinel
. First, if Bound
is unreachable_sentinel_t
, then
the sentinel type is unreachable_sentinel_t
too - we don't add an unnecessary level of wrapping
on top. Second, when W
and Bound
are the same type, iota_view
models common_range
, and
the sentinel type is the same as the iterator type - that is, iterator
, not sentinel
.
Presumably the intent is to use the view's actual sentinel type, rather than always use the
sentinel
type.
[2021-03-12; Reflector poll]
Set status to Tentatively Ready after five votes in favour during reflector poll.
[2021-06-07 Approved at June 2021 virtual plenary. Status changed: Voting → WP.]
Proposed resolution:
This wording is relative to N4878.
Edit 25.6.4.2 [range.iota.view], as indicated:
namespace std::ranges { // [...] template<weakly_incrementable W, semiregular Bound = unreachable_sentinel_t> requires weakly-equality-comparable-with<W, Bound> && semiregular<W> class iota_view : public view_interface<iota_view<W, Bound>> { private: // [range.iota.iterator], class iota_view::iterator struct iterator; // exposition only // [range.iota.sentinel], class iota_view::sentinel struct sentinel; // exposition only W value_ = W(); // exposition only Bound bound_ = Bound(); // exposition only public: iota_view() = default; constexpr explicit iota_view(W value); constexpr iota_view(type_identity_t<W> value, type_identity_t<Bound> bound); constexpr iota_view(iterator first,sentinelsee below last);: iota_view(*first, last.bound_) {}constexpr iterator begin() const; constexpr auto end() const; constexpr iterator end() const requires same_as<W, Bound>; constexpr auto size() const requires see below; }; template<class W, class Bound> requires (!is-integer-like<W> || !is-integer-like<Bound> || (is-signed-integer-like<W> == is-signed-integer-like<Bound>)) iota_view(W, Bound) -> iota_view<W, Bound>; }[...]
constexpr iota_view(type_identity_t<W> value, type_identity_t<Bound> bound);-8- Preconditions:
Bound
denotesunreachable_sentinel_t
orbound
is reachable fromvalue
. WhenW
andBound
modeltotally_ordered_with
, thenbool(value <= bound)
is true.-9- Effects: Initializes
value_
withvalue
andbound_
withbound
.constexpr iota_view(iterator first, see below last);-?- Effects: Equivalent to:
(?.1) — If
same_as<W, Bound>
istrue
,iota_view(first.value_, last.value_)
.(?.2) — Otherwise, if
Bound
denotesunreachable_sentinel_t
,iota_view(first.value_, last)
.(?.3) — Otherwise,
iota_view(first.value_, last.bound_)
.-?- Remarks: The type of
last
is:
(?.1) — If
same_as<W, Bound>
istrue
,iterator
.(?.2) — Otherwise, if
Bound
denotesunreachable_sentinel_t
,Bound
.(?.3) — Otherwise,
sentinel
.
Section: 25.7 [range.adaptors] Status: Resolved Submitter: Tim Song Opened: 2021-02-19 Last modified: 2021-06-14
Priority: 3
View all issues with Resolved status.
Discussion:
The specification of various range factory and adaptor objects generally says that some function call
expression on them is expression-equivalent to an expression that performs list-initialization or in
some cases a comma expression. This imposes evaluation order requirements that are unlikely to be
intended and sometimes outright contradictory. For example, 25.7.12.1 [range.drop.overview] says
that views::drop(E, F)
is expression-equivalent to "((void) F, decay-copy(E))
" in
one case, and drop_view{E, F}
in another. The first expression requires F
to be
sequenced before E
, while the second expression requires E
to be sequenced before
F
. They can't both hold in the absence of high levels of compiler magic.
E
whose difference_type
is int32_t
,
views::drop(E, int64_t())
is required to work, but int64_t l = 0; views::drop(E, l)
is required to be ill-formed. This seems unlikely to be the intent either.
[2021-03-12; Reflector poll]
Set priority to 3 following reflector poll.
[2021-06-13 Resolved by the adoption of P2367R0 at the June 2021 plenary. Status changed: New → Resolved.]
Proposed resolution:
uses_allocator_construction_args
fails to handle types convertible to pair
Section: 20.2.8.2 [allocator.uses.construction] Status: C++23 Submitter: Tim Song Opened: 2021-02-23 Last modified: 2023-11-22
Priority: 3
View all other issues in [allocator.uses.construction].
View all issues with C++23 status.
Discussion:
As currently specified, the following program is ill-formed (and appears to have been since LWG 2975(i)):
struct S { operator std::pair<const int, int>() const { return {}; } }; void f() { std::pmr::map<int, int> s; s.emplace(S{}); }
There's no matching overload for uses_allocator_construction_args<pair<const int, int>>(alloc, S&&)
,
since S
is not a pair
and every overload for constructing pair
s that takes one
non-allocator argument expects a pair
from which template arguments can be deduced.
[2021-02-27 Tim adds PR and comments]
The superseded resolution below attempts to solve this issue by adding two
additional overloads of uses_allocator_construction_args
to
handle this case. However, the new overloads forces implicit conversions
at the call to uses_allocator_construction_args
, which requires
the result to be consumed within the same full-expression
before any temporary created from the conversion is destroyed.
This is not the case for the piecewise_construct
overload of
uses_allocator_construction_args
, which recursively calls
uses_allocator_construction_args
for the two elements of the pair, which
might themselves be pair
s.
The approach taken in the revised PR is to produce an exposition-only
pair-constructor
object instead. The object holds the allocator
and the argument by reference, implicitly converts to the specified specialization of pair
,
and when so converted return a pair
that is constructed by uses-allocator
construction with the converted value of the original argument.
This maintains the existing design that pair
itself doesn't know
anything about allocator construction.
Previous resolution [SUPERSEDED]:
This wording is relative to N4878.
Edit 20.2.2 [memory.syn], header
<memory>
synopsis, as indicated:namespace std { […] // 20.2.8.2 [allocator.uses.construction], uses-allocator construction […] template<class T, class Alloc> constexpr auto uses_allocator_construction_args(const Alloc& alloc, const remove_cv_t<T>& pr) noexcept; template<class T, class Alloc, class U, class V> constexpr auto uses_allocator_construction_args(const Alloc& alloc, const pair<U, V>& pr) noexcept -> see below; template<class T, class Alloc> constexpr auto uses_allocator_construction_args(const Alloc& alloc, remove_cv_t<T>&& pr) noexcept; template<class T, class Alloc, class U, class V> constexpr auto uses_allocator_construction_args(const Alloc& alloc, pair<U, V>&& pr) noexcept -> see below; […] }Edit 20.2.8.2 [allocator.uses.construction] as indicated:
template<class T, class Alloc> constexpr auto uses_allocator_construction_args(const Alloc& alloc, const remove_cv_t<T>& pr) noexcept; template<class T, class Alloc, class U, class V> constexpr auto uses_allocator_construction_args(const Alloc& alloc, const pair<U, V>& pr) noexcept -> see below;-12- Constraints:
-13- Effects: Equivalent to:T
is a specialization ofpair
. For the second overload,is_same_v<pair<U, V>, remove_cv_t<T>>
isfalse
.return uses_allocator_construction_args<T>(alloc, piecewise_construct, forward_as_tuple(pr.first), forward_as_tuple(pr.second));template<class T, class Alloc> constexpr auto uses_allocator_construction_args(const Alloc& alloc, remove_cv_t<T>&& pr) noexcept; template<class T, class Alloc, class U, class V> constexpr auto uses_allocator_construction_args(const Alloc& alloc, pair<U, V>&& pr) noexcept -> see below;-14- Constraints:
-15- Effects: Equivalent to:T
is a specialization ofpair
. For the second overload,is_same_v<pair<U, V>, remove_cv_t<T>>
isfalse
.return uses_allocator_construction_args<T>(alloc, piecewise_construct, forward_as_tuple(std::move(pr).first), forward_as_tuple(std::move(pr).second));
[2021-03-12; Reflector poll]
Set priority to 3 following reflector poll.
Previous resolution [SUPERSEDED]:
This wording is relative to N4878.
Edit 20.2.2 [memory.syn], header
<memory>
synopsis, as indicated:namespace std { […] // 20.2.8.2 [allocator.uses.construction], uses-allocator construction […] template<class T, class Alloc, class U, class V> constexpr auto uses_allocator_construction_args(const Alloc& alloc, const pair<U, V>& pr) noexcept -> see below; template<class T, class Alloc, class U, class V> constexpr auto uses_allocator_construction_args(const Alloc& alloc, pair<U, V>&& pr) noexcept -> see below; template<class T, class Alloc, class U> constexpr auto uses_allocator_construction_args(const Alloc& alloc, U&& u) noexcept; […] }Add the following to 20.2.8.2 [allocator.uses.construction]:
template<class T, class Alloc, class U> constexpr auto uses_allocator_construction_args(const Alloc& alloc, U&& u) noexcept;-?- Let
FUN
be the function template:template<class A, class B> void FUN(const pair<A, B>&);-?- Constraints:
-?- Effects: Equivalent to:T
is a specialization ofpair
, and the expressionFUN(u)
is not well-formed when considered as an unevaluated operand.return make_tuple(pair-constructor{alloc, u});where
pair-constructor
is an exposition-only class defined as follows:struct pair-constructor { using pair-type = remove_cv_t<T>; // exposition only constexpr operator pair-type() const { return do-construct(std::forward<U>(u)); } constexpr auto do-construct(const pair-type& p) const { // exposition only return make_obj_using_allocator<pair-type>(alloc, p); } constexpr auto do-construct(pair-type&& p) const { // exposition only return make_obj_using_allocator<pair-type>(alloc, std::move(p)); } const Alloc& alloc; // exposition only U& u; // exposition only };
[2021-12-02 Tim updates PR to avoid public exposition-only members]
[2022-01-31; Reflector poll]
Set status to Tentatively Ready after seven votes in favour during reflector poll.
[2022-02-10 Approved at February 2022 virtual plenary. Status changed: Tentatively Ready → WP.]
Proposed resolution:
This wording is relative to N4901.
Edit 20.2.2 [memory.syn], header <memory>
synopsis, as indicated:
namespace std { […] // 20.2.8.2 [allocator.uses.construction], uses-allocator construction […] template<class T, class Alloc, class U, class V> constexpr auto uses_allocator_construction_args(const Alloc& alloc, pair<U, V>& pr) noexcept; template<class T, class Alloc, class U, class V> constexpr auto uses_allocator_construction_args(const Alloc& alloc, const pair<U, V>& pr) noexcept; template<class T, class Alloc, class U, class V> constexpr auto uses_allocator_construction_args(const Alloc& alloc, pair<U, V>&& pr) noexcept; template<class T, class Alloc, class U, class V> constexpr auto uses_allocator_construction_args(const Alloc& alloc, const pair<U, V>&& pr) noexcept; template<class T, class Alloc, class U> constexpr auto uses_allocator_construction_args(const Alloc& alloc, U&& u) noexcept; […] }
Add the following to 20.2.8.2 [allocator.uses.construction]:
template<class T, class Alloc, class U> constexpr auto uses_allocator_construction_args(const Alloc& alloc, U&& u) noexcept;-?- Let
FUN
be the function template:template<class A, class B> void FUN(const pair<A, B>&);-?- Constraints:
-?- LetT
is a specialization ofpair
, and the expressionFUN(u)
is not well-formed when considered as an unevaluated operand.pair-constructor
be an exposition-only class defined as follows:class pair-constructor { using pair-type = remove_cv_t<T>; // exposition only constexpr auto do-construct(const pair-type& p) const { // exposition only return make_obj_using_allocator<pair-type>(alloc_, p); } constexpr auto do-construct(pair-type&& p) const { // exposition only return make_obj_using_allocator<pair-type>(alloc_, std::move(p)); } const Alloc& alloc_; // exposition only U& u_; // exposition only public: constexpr operator pair-type() const { return do-construct(std::forward<U>(u_)); } };-?- Returns:
make_tuple(pc)
, wherepc
is apair-constructor
object whosealloc_
member is initialized withalloc
and whoseu_
member is initialized withu
.
uses_allocator_construction_args
unspecifiedSection: 20.2.8.2 [allocator.uses.construction] Status: C++23 Submitter: Casey Carter Opened: 2021-02-25 Last modified: 2023-11-22
Priority: 3
View all other issues in [allocator.uses.construction].
View all issues with C++23 status.
Discussion:
The synopsis of <memory>
in 20.2.2 [memory.syn] declares six overloads of
uses_allocator_construction_args
with return types "see below":
template<class T, class Alloc, class... Args> constexpr auto uses_allocator_construction_args(const Alloc& alloc, Args&&... args) noexcept -> see below; template<class T, class Alloc, class Tuple1, class Tuple2>> constexpr auto uses_allocator_construction_args(const Alloc& alloc, piecewise_construct_t, Tuple1&& x, Tuple2&& y) noexcept -> see below; template<class T, class Alloc> constexpr auto uses_allocator_construction_args(const Alloc& alloc) noexcept -> see below; template<class T, class Alloc, class U, class V> constexpr auto uses_allocator_construction_args(const Alloc& alloc, U&& u, V&& v) noexcept -> see below; template<class T, class Alloc, class U, class V> constexpr auto uses_allocator_construction_args(const Alloc& alloc, const pair<U, V>& pr) noexcept -> see below; template<class T, class Alloc, class U, class V> constexpr auto uses_allocator_construction_args(const Alloc& alloc, pair<U, V>&& pr) noexcept -> see below;
The "see belows" also appear in the detailed specification of these overloaded function templates in 20.2.8.2 [allocator.uses.construction] para 4 through 15. Despite that the values these function templates return are specified therein, the return types are not. Presumably LWG wanted to specify deduced return types, but couldn't figure out how to do so, and just gave up?
[2021-02-27; Daniel comments and provides wording]
My interpretation is that the appearance of the trailing-return-type was actually unintended and that
these functions where supposed to use the return type placeholder to signal the intention that the actual
return type is deduced by the consistent sum of all return statements as they appear in the prototype specifications.
Given that at least one implementation has indeed realized this form, I suggest to simply adjust the specification
to remove the trailing-return-type. Specification-wise we have already existing practice for this
approach (See e.g. to_address
).
[2021-03-12; Reflector poll]
Set priority to 3 following reflector poll.
[2021-05-26; Reflector poll]
Set status to Tentatively Ready after nine votes in favour during reflector poll.
[2021-06-07 Approved at June 2021 virtual plenary. Status changed: Voting → WP.]
Proposed resolution:
This wording is relative to N4878.
Edit 20.2.2 [memory.syn], header <memory>
synopsis, as indicated:
namespace std { […] // 20.2.8.2 [allocator.uses.construction], uses-allocator construction template<class T, class Alloc, class... Args> constexpr auto uses_allocator_construction_args(const Alloc& alloc, Args&&... args) noexcept-> see below; template<class T, class Alloc, class Tuple1, class Tuple2> constexpr auto uses_allocator_construction_args(const Alloc& alloc, piecewise_construct_t, Tuple1&& x, Tuple2&& y) noexcept-> see below; template<class T, class Alloc> constexpr auto uses_allocator_construction_args(const Alloc& alloc) noexcept-> see below; template<class T, class Alloc, class U, class V> constexpr auto uses_allocator_construction_args(const Alloc& alloc, U&& u, V&& v) noexcept-> see below; template<class T, class Alloc, class U, class V> constexpr auto uses_allocator_construction_args(const Alloc& alloc, const pair<U, V>& pr) noexcept-> see below; template<class T, class Alloc, class U, class V> constexpr auto uses_allocator_construction_args(const Alloc& alloc, pair<U, V>&& pr) noexcept-> see below; […] }
Edit 20.2.8.2 [allocator.uses.construction] as indicated:
template<class T, class Alloc, class... Args> constexpr auto uses_allocator_construction_args(const Alloc& alloc, Args&&... args) noexcept-> see below;[…]
-5- Returns: Atuple
value determined as follows:
(5.1) — if […], return
forward_as_tuple(std::forward<Args>(args)...)
.(5.2) — Otherwise, if […], return
tuple<allocator_arg_t, const Alloc&, Args&&...>( allocator_arg, alloc, std::forward<Args>(args)...)(5.3) — Otherwise, if […], return
forward_as_tuple(std::forward<Args>(args)..., alloc)
.(5.4) — Otherwise, the program is ill-formed.
template<class T, class Alloc, class Tuple1, class Tuple2> constexpr auto uses_allocator_construction_args(const Alloc& alloc, piecewise_construct_t, Tuple1&& x, Tuple2&& y) noexcept-> see below;[…]
-7- Effects: ForT
specified aspair<T1, T2>
, equivalent to:return make_tuple( piecewise_construct, apply([&alloc](auto&&... args1) { return uses_allocator_construction_args<T1>( alloc, std::forward<decltype(args1)>(args1)...); }, std::forward<Tuple1>(x)), apply([&alloc](auto&&... args2) { return uses_allocator_construction_args<T2>( alloc, std::forward<decltype(args2)>(args2)...); }, std::forward<Tuple2>(y)));template<class T, class Alloc> constexpr auto uses_allocator_construction_args(const Alloc& alloc) noexcept-> see below;[…]
-9- Effects: Equivalent to:return uses_allocator_construction_args<T>(alloc, piecewise_construct, tuple<>{}, tuple<>{});template<class T, class Alloc, class U, class V> constexpr auto uses_allocator_construction_args(const Alloc& alloc, U&& u, V&& v) noexcept-> see below;[…]
-11- Effects: Equivalent to:return uses_allocator_construction_args<T>(alloc, piecewise_construct, forward_as_tuple(std::forward<U>(u)), forward_as_tuple(std::forward<V>(v));template<class T, class Alloc, class U, class V> constexpr auto uses_allocator_construction_args(const Alloc& alloc, const pair<U, V>& pr) noexcept-> see below;[…]
-13- Effects: Equivalent to:return uses_allocator_construction_args<T>(alloc, piecewise_construct, forward_as_tuple(pr.first), forward_as_tuple(pr.second));template<class T, class Alloc, class U, class V> constexpr auto uses_allocator_construction_args(const Alloc& alloc, pair<U, V>&& pr) noexcept-> see below;[…]
-15- Effects: Equivalent to:return uses_allocator_construction_args<T>(alloc, piecewise_construct, forward_as_tuple(std::move(pr).first), forward_as_tuple(std::move(pr).second));
uses_allocator_construction_args
handles rvalue pairs of rvalue references incorrectlySection: 20.2.8.2 [allocator.uses.construction] Status: C++23 Submitter: Tim Song Opened: 2021-02-27 Last modified: 2023-11-22
Priority: Not Prioritized
View all other issues in [allocator.uses.construction].
View all issues with C++23 status.
Discussion:
For an rvalue pair pr
, uses_allocator_construction_args
is specified to forward
std::move(pr).first
and std::move(pr).second
. This is correct
for non-references and lvalue references, but wrong for rvalue refrences because
the class member access produces an lvalue (see 7.6.1.5 [expr.ref]/6).
get
produces an xvalue, which is what is desired here.
[2021-03-12; Reflector poll]
Set status to Tentatively Ready after five votes in favour during reflector poll.
[2021-06-07 Approved at June 2021 virtual plenary. Status changed: Voting → WP.]
Proposed resolution:
This wording is relative to N4878.
Edit 20.2.8.2 [allocator.uses.construction] as indicated:
template<class T, class Alloc, class U, class V> constexpr auto uses_allocator_construction_args(const Alloc& alloc, pair<U, V>&& pr) noexcept -> see below;[…]
-15- Effects: Equivalent to:return uses_allocator_construction_args<T>(alloc, piecewise_construct, forward_as_tuple(std::move(pr).firstget<0>(std::move(pr))), forward_as_tuple(std::move(pr).secondget<1>(std::move(pr))));
make_from_tuple
can perform (the equivalent of) a C-style castSection: 22.4.6 [tuple.apply] Status: C++23 Submitter: Tim Song Opened: 2021-02-28 Last modified: 2023-11-22
Priority: 3
View all issues with C++23 status.
Discussion:
make_from_tuple
is specified to return T(get<I>(std::forward<Tuple>(t))...)
.
When there is only a single tuple element, this is equivalent to a C-style cast
that may be a reinterpret_cast
, a const_cast
,
or an access-bypassing static_cast
.
[2021-03-12; Reflector poll]
Set priority to 3 following reflector poll. Set status to Tentatively Ready after five votes in favour during reflector poll.
[2021-06-07 Approved at June 2021 virtual plenary. Status changed: Voting → WP.]
Proposed resolution:
This wording is relative to N4878.
Edit 22.4.6 [tuple.apply] as indicated:
template<class T, class Tuple> constexpr T make_from_tuple(Tuple&& t);[…]
-2- Effects: Given the exposition-only function:template<class T, class Tuple, size_t... I> requires is_constructible_v<T, decltype(get<I>(declval<Tuple>()))...> constexpr T make-from-tuple-impl(Tuple&& t, index_sequence<I...>) { // exposition only return T(get<I>(std::forward<Tuple>(t))...); }Equivalent to:
return make-from-tuple-impl<T>( std::forward<Tuple>(t), make_index_sequence<tuple_size_v<remove_reference_t<Tuple>>>{});[Note 1: The type of T must be supplied as an explicit template parameter, as it cannot be deduced from the argument list. — end note]
priority_queue(first, last)
should construct c
with (first, last)
Section: 23.6.4 [priority.queue] Status: C++23 Submitter: Arthur O'Dwyer Opened: 2021-03-01 Last modified: 2023-11-22
Priority: Not Prioritized
View all other issues in [priority.queue].
View all issues with C++23 status.
Discussion:
Tim's new constructors for priority_queue
(LWG 3506(i))
are specified so that when you construct
auto pq = PQ(first, last, a);
it calls this new-in-LWG3506 constructor:
template<class InputIterator, class Alloc> priority_queue(InputIterator first, InputIterator last, const Alloc& a);Effects: Initializes
c
withfirst
as the first argument,last
as the second argument, anda
as the third argument, and value-initializescomp
; callsmake_heap(c.begin(), c.end(), comp)
.
But the pre-existing constructors are specified so that when you construct
auto pq = PQ(first, last);
it calls this pre-existing constructor:
template<class InputIterator> priority_queue(InputIterator first, InputIterator last, const Compare& x = Compare(), Container&& y = Container());Preconditions:
x
defines a strict weak ordering ([alg.sorting]).Effects: Initializes
comp
withx
andc
withy
(copy constructing or move constructing as appropriate); callsc.insert(c.end(), first, last)
; and finally callsmake_heap(c.begin(), c.end(), comp)
.
In other words,
auto pq = PQ(first, last);
will default-construct a Container
,
then move-construct c
from that object,
then c.insert(first, last)
,
and finally make_heap
.
But our new
auto pq = PQ(first, last, a);
will simply construct c
with (first, last)
,
then make_heap
.
The latter is obviously better.
Also, Corentin's P1425R3
specifies the new iterator-pair constructors for
stack
and queue
to construct c
from (first, last)
. Good.
LWG should refactor the existing constructor overload set so that
the existing non-allocator-taking constructors simply construct c
from (first, last)
.
This will improve consistency with the resolutions of LWG3506 and P1425,
and reduce the surprise factor for users.
[2021-03-12; Reflector poll]
Set status to Tentatively Ready after five votes in favour during reflector poll.
[2021-06-07 Approved at June 2021 virtual plenary. Status changed: Voting → WP.]
Proposed resolution:
This wording is relative to N4878.
Edit 23.6.4.1 [priqueue.overview] as indicated:
template<class InputIterator> priority_queue(InputIterator first, InputIterator last, const Compare& x = Compare()); template<class InputIterator> priority_queue(InputIterator first, InputIterator last, const Compare& x, const Container& y); template<class InputIterator> priority_queue(InputIterator first, InputIterator last, const Compare& x, Container&& y= Compare()= Container());
Edit 23.6.4.2 [priqueue.cons] as indicated:
template<class InputIterator> priority_queue(InputIterator first, InputIterator last, const Compare& x = Compare());Preconditions:
x
defines a strict weak ordering ([alg.sorting]).Effects: Initializes
c
withfirst
as the first argument andlast
as the second argument, and initializescomp
withx
; then callsmake_heap(c.begin(), c.end(), comp)
.template<class InputIterator> priority_queue(InputIterator first, InputIterator last, const Compare& x, const Container& y); template<class InputIterator> priority_queue(InputIterator first, InputIterator last, const Compare& x= Compare(), Container&& y= Container());Preconditions:
x
defines a strict weak ordering ([alg.sorting]).Effects: Initializes
comp
withx
andc
withy
(copy constructing or move constructing as appropriate); callsc.insert(c.end(), first, last)
; and finally callsmake_heap(c.begin(), c.end(), comp)
.
BUILTIN-PTR-MEOW
should not opt the type out of syntactic checksSection: 22.10.8.8 [comparisons.three.way], 22.10.9 [range.cmp] Status: C++23 Submitter: Tim Song Opened: 2021-03-04 Last modified: 2023-11-22
Priority: Not Prioritized
View all issues with C++23 status.
Discussion:
The use of BUILTIN-PTR-MEOW
for the constrained comparison
function objects was needed to disable the semantic requirements on the
associated concepts when the comparison resolves to a built-in operator
comparing pointers: the comparison object is adding special handling for this
case to produce a total order despite the core language saying otherwise,
so requiring the built-in operator to then produce a total order as part
of the semantic requirements doesn't make sense.
ranges::less
requires all six comparison operators
(because of totally_ordered_with
) to be present … except when
operator<
on the arguments resolves to a built-in operator comparing
pointers, in which case it just requires operator<
and operator==
(except that the latter isn't even required to be checked — it comes from the use
of ranges::equal_to
in the precondition of ranges::less
).
This seems entirely arbitrary.
[2021-03-12; Reflector poll]
Set status to Tentatively Ready after five votes in favour during reflector poll.
[2021-06-07 Approved at June 2021 virtual plenary. Status changed: Voting → WP.]
Proposed resolution:
This wording is relative to N4878.
Edit 22.10.8.8 [comparisons.three.way] as indicated:
-1- In this subclause,BUILTIN-PTR-THREE-WAY(T, U)
for typesT
andU
is a boolean constant expression.BUILTIN-PTR-THREE-WAY(T, U)
is true if and only if<=>
in the expressiondeclval<T>() <=> declval<U>()
resolves to a built-in operator comparing pointers.struct compare_three_way { template<class T, class U>requires three_way_comparable_with<T, U> || BUILTIN-PTR-THREE-WAY(T, U)constexpr auto operator()(T&& t, U&& u) const; using is_transparent = unspecified; };template<class T, class U>requires three_way_comparable_with<T, U> || BUILTIN-PTR-THREE-WAY(T, U)constexpr auto operator()(T&& t, U&& u) const;-?- Constraints:
-2- Preconditions: If the expressionT
andU
satisfythree_way_comparable_with
.std::forward<T>(t) <=> std::forward<U>(u)
results in a call to a built-in operator<=>
comparing pointers of typeP
, the conversion sequences from bothT
andU
toP
are equality-preserving (18.2 [concepts.equality]); otherwise,T
andU
modelthree_way_comparable_with
. -3- Effects:
(3.1) — If the expression
std::forward<T>(t) <=> std::forward<U>(u)
results in a call to a built-in operator<=>
comparing pointers of typeP
, returnsstrong_ordering::less
if (the converted value of)t
precedesu
in the implementation-defined strict total order over pointers (3.28 [defns.order.ptr]),strong_ordering::greater
ifu
precedest
, and otherwisestrong_ordering::equal
.(3.2) — Otherwise, equivalent to:
return std::forward<T>(t) <=> std::forward<U>(u);
Edit 22.10.9 [range.cmp] as indicated:
-1- In this subclause,BUILTIN-PTR-CMP(T,
op, U)
for typesT
andU
and where op is an equality (7.6.10 [expr.eq]) or relational operator (7.6.9 [expr.rel]) is a boolean constant expression.BUILTIN-PTR-CMP(T,
op, U)
is true if and only if op in the expressiondeclval<T>()
opdeclval<U>()
resolves to a built-in operator comparing pointers.struct ranges::equal_to { template<class T, class U>requires equality_comparable_with<T, U> || BUILTIN-PTR-CMP(T, ==, U)constexpr bool operator()(T&& t, U&& u) const; using is_transparent = unspecified; };template<class T, class U>requires equality_comparable_with<T, U> || BUILTIN-PTR-CMP(T, ==, U)constexpr bool operator()(T&& t, U&& u) const;-?- Constraints:
-2- Preconditions: If the expressionT
andU
satisfyequality_comparable_with
.std::forward<T>(t) == std::forward<U>(u)
results in a call to a built-in operator==
comparing pointers of typeP
, the conversion sequences from bothT
andU
toP
are equality-preserving (18.2 [concepts.equality]); otherwise,T
andU
modelequality_comparable_with
. -3- Effects:
(3.1) — If the expression
std::forward<T>(t) == std::forward<U>(u)
results in a call to a built-in operator==
comparing pointers of typeP
, returnsfalse
if either (the converted value of)t
precedesu
oru
precedest
in the implementation-defined strict total order over pointers (3.28 [defns.order.ptr]) and otherwisetrue
.(3.2) — Otherwise, equivalent to:
return std::forward<T>(t) == std::forward<U>(u);
struct ranges::not_equal_to { template<class T, class U>requires equality_comparable_with<T, U> || BUILTIN-PTR-CMP(T, ==, U)constexpr bool operator()(T&& t, U&& u) const; using is_transparent = unspecified; };template<class T, class U> constexpr bool operator()(T&& t, U&& u) const;-?- Constraints:
-4-T
andU
satisfyequality_comparable_with
.Effects: Equivalent to:operator()
has effects ereturn !ranges::equal_to{}(std::forward<T>(t), std::forward<U>(u));struct ranges::greater { template<class T, class U>requires totally_ordered_with<T, U> || BUILTIN-PTR-CMP(T, <, U)constexpr bool operator()(T&& t, U&& u) const; using is_transparent = unspecified; };template<class T, class U> constexpr bool operator()(T&& t, U&& u) const;-?- Constraints:
-5-T
andU
satisfytotally_ordered_with
.Effects: Equivalent to:operator()
has effects ereturn ranges::less{}(std::forward<U>(u), std::forward<T>(t));struct ranges::less { template<class T, class U>requires totally_ordered_with<T, U> || BUILTIN-PTR-CMP(T, <, U)constexpr bool operator()(T&& t, U&& u) const; using is_transparent = unspecified; };template<class T, class U>requires totally_ordered_with<T, U> || BUILTIN-PTR-CMP(T, <, U)constexpr bool operator()(T&& t, U&& u) const;-?- Constraints:
-6- Preconditions: If the expressionT
andU
satisfytotally_ordered_with
.std::forward<T>(t) < std::forward<U>(u)
results in a call to a built-in operator<
comparing pointers of typeP
, the conversion sequences from bothT
andU
toP
are equality-preserving (18.2 [concepts.equality]); otherwise,T
andU
modeltotally_ordered_with
. For any expressionsET
andEU
such thatdecltype((ET))
isT
anddecltype((EU))
isU
, exactly one ofranges::less{}(ET, EU)
,ranges::less{}(EU, ET)
, orranges::equal_to{}(ET, EU)
istrue
. -7- Effects:
(7.1) — If the expression
std::forward<T>(t) < std::forward<U>(u)
results in a call to a built-in operator<
comparing pointers of typeP
, returnstrue
if (the converted value of)t
precedesu
in the implementation-defined strict total order over pointers (3.28 [defns.order.ptr]) and otherwisefalse
.(7.2) — Otherwise, equivalent to:
return std::forward<T>(t) < std::forward<U>(u);
struct ranges::greater_equal { template<class T, class U>requires totally_ordered_with<T, U> || BUILTIN-PTR-CMP(T, <, U)constexpr bool operator()(T&& t, U&& u) const; using is_transparent = unspecified; };template<class T, class U> constexpr bool operator()(T&& t, U&& u) const;-?- Constraints:
-8-T
andU
satisfytotally_ordered_with
.Effects: Equivalent to:operator()
has effects ereturn !ranges::less{}(std::forward<T>(t), std::forward<U>(u));struct ranges::less_equal { template<class T, class U>requires totally_ordered_with<T, U> || BUILTIN-PTR-CMP(T, <, U)constexpr bool operator()(T&& t, U&& u) const; using is_transparent = unspecified; };template<class T, class U> constexpr bool operator()(T&& t, U&& u) const;-?- Constraints:
-9-T
andU
satisfytotally_ordered_with
.Effects: Equivalent to:operator()
has effects ereturn !ranges::less{}(std::forward<U>(u), std::forward<T>(t));
split_view<V, P>::inner-iterator<true>::operator++(int)
should depend on Base
Section: 25.7.16.5 [range.lazy.split.inner] Status: C++23 Submitter: Casey Carter Opened: 2021-03-11 Last modified: 2023-11-22
Priority: Not Prioritized
View all other issues in [range.lazy.split.inner].
View all issues with C++23 status.
Discussion:
split_view<V, P>::inner-iterator<Const>::operator++(int)
is specified directly in the
synopsis in [range.split.inner] as:
constexpr decltype(auto) operator++(int) { if constexpr (forward_range<V>) { auto tmp = *this; ++*this; return tmp; } else ++*this; }
The dependency on the properties of V
here is odd given that we are wrapping an iterator obtained from a
maybe-const<Const, V>
(aka Base
). It seems like this function should instead
be concerned with forward_range<Base>
.
[2021-04-20; Reflector poll]
Set status to Tentatively Ready after five votes in favour during reflector poll.
[2021-06-07 Approved at June 2021 virtual plenary. Status changed: Voting → WP.]
Proposed resolution:
This wording is relative to N4878.
Modify [range.split.inner] as indicated:
constexpr decltype(auto) operator++(int) { if constexpr (forward_range<VBase>) { auto tmp = *this; ++*this; return tmp; } else ++*this; }
base() const &
consistent across iterator wrappers that supports input_iterator
sSection: 25.7.8.3 [range.filter.iterator], 25.7.9.3 [range.transform.iterator], 25.7.23.3 [range.elements.iterator] Status: C++23 Submitter: Tomasz Kamiński Opened: 2021-03-14 Last modified: 2023-11-22
Priority: Not Prioritized
View all issues with C++23 status.
Discussion:
The resolution of LWG issue 3391(i) changed the base()
function for the
counted_iterator
/move_iterator
to return const &
, because the previous
specification prevented non-mutating uses of the base iterator (comparing against sentinel) and made
take_view
unimplementable. However, this change was not applied for all other iterators wrappers,
that may wrap move-only input iterators. As consequence, we end-up with inconsistency where a user can
perform the following operations on some adapters, but not on others (e. g. take_view
uses
counted_iterator
so it supports them).
read the original value of the base iterator, by calling operator*
find position of an element in the underlying iterator, when sentinel is sized by calling operator-
(e.g. all input iterators wrapped into counted_iterator
).
To fix above, the proposed wording below proposes to modify the signature of iterator::base() const &
member function for all iterators adapters that support input iterator. These include:
filter_view::iterator
(uses case B)
transform_view::iterator
(uses case A)
elements_view::iterator
(uses case B)
lazy_split_view<V, Pattern>::inner-iterator
(uses case B) if
P2210 is accepted
Note: common_iterator
does not expose the base()
function (because it can either point to
iterator or sentinel), so changes to above are not proposed. However, both (A) and (B) use cases are supported.
[2021-04-20; Reflector poll]
Set status to Tentatively Ready after five votes in favour during reflector poll.
[2021-06-07 Approved at June 2021 virtual plenary. Status changed: Voting → WP.]
Proposed resolution:
This wording is relative to N4878.
If P2210 would become accepted, the corresponding subclause [range.lazy.split.inner] (?) for
lazy_split_view::inner-iterator
would require a similar change.
Modify 25.7.8.3 [range.filter.iterator] as indicated:
[…][…] constexpr const iterator_t<V>& base() const &requires copyable<iterator_t<V>>; […]
constexpr const iterator_t<V>& base() const &requires copyable<iterator_t<V>>;-5- Effects: Equivalent to:
return current_;
Modify 25.7.9.3 [range.transform.iterator] as indicated:
[…][…] constexpr const iterator_t<Base>& base() const &requires copyable<iterator_t<Base>>; […]
constexpr const iterator_t<Base>& base() const &requires copyable<iterator_t<Base>>;-5- Effects: Equivalent to:
return current_;
Modify 25.7.23.3 [range.elements.iterator] as indicated:
[…][…] constexpr const iterator_t<Base>& base() const &requires copyable<iterator_t<Base>>; […]
constexpr const iterator_t<Base>& base() const &requires copyable<iterator_t<Base>>;-3- Effects: Equivalent to:
return current_;
join_view::iterator::iterator_category
and ::iterator_concept
lieSection: 25.7.14.3 [range.join.iterator] Status: C++23 Submitter: Casey Carter Opened: 2021-03-16 Last modified: 2023-11-22
Priority: 2
View all other issues in [range.join.iterator].
View all issues with C++23 status.
Discussion:
Per 25.7.14.3 [range.join.iterator]/1, join_view::iterator::iterator_concept
denotes
bidirectional_iterator_tag
if ref-is-glvalue
is true
and Base
and
range_reference_t<Base>
each model bidirectional_range
. Similarly, paragraph 2 says that
join_view::iterator::iterator_category
is present if ref-is-glvalue
is true
,
and denotes bidirectional_iterator_tag
if the categories of the iterators of both Base
and
range_reference_t<Base>
derive from bidirectional_iterator_tag
.
operator--
and operator--(int)
in the synopsis
that immediately precedes paragraph 1 disagree. Certainly they also consistently require ref-is-glvalue
&& bidirectional_range<Base> && bidirectional_range<range_reference_t<Base>>
,
but they additionally require common_range<range_reference_t<Base>>
. So as currently specified,
this iterator sometimes declares itself to be bidirectional despite not implementing --
. This is not
incorrect for iterator_concept — recall that iterator_concept
is effectively an upper bound since
the concepts require substantial syntax — but slightly misleading. It is, however, very much incorrect for
iterator_category
which must not denote a type derived from a tag that corresponds to a stronger category
than that iterator implements.
It's worth pointing out, that LWG 3313(i) fixed the constraints on operator--()
and
operator--(int)
by adding the common_range
requirements, but failed to make a consistent change
to the definitions of iterator_concept
and iterator_category
.
[2021-04-04; Daniel comments]
The below proposed wording can be compared as being based on N4885.
[2021-04-20; Reflector poll]
Priority set to 2.
[2021-06-23; Reflector poll]
Set status to Tentatively Ready after six votes in favour during reflector poll.
[2021-10-14 Approved at October 2021 virtual plenary. Status changed: Voting → WP.]
Proposed resolution:
This wording is relative to the post-2021-February-virtual-meeting working draft.
Modify 25.7.14.3 [range.join.iterator] as indicated:
-1-
iterator::iterator_concept
is defined as follows:
(1.1) — If
ref-is-glvalue
istrue
,andBase
andmodelsrange_reference_t<Base>
eachbidirectional_range
, andrange_reference_t<Base>
models bothbidirectional_range
andcommon_range
, theniterator_concept
denotesbidirectional_iterator_tag
.[…]
[…]
-2- The member typedef-nameiterator_category
is defined if and only ifref-is-glvalue
istrue
,Base
modelsforward_range
, andrange_reference_t<Base>
modelsforward_range
. In that case,iterator::iterator_category
is defined as follows:
(2.1) — Let
OUTERC
denoteiterator_traits<iterator_t<Base>>::iterator_category
, and letINNERC
denoteiterator_traits<iterator_t<range_reference_t<Base>>>::iterator_category
.(2.2) — If
OUTERC
andINNERC
each modelderived_from<bidirectional_iterator_tag>
, andrange_reference_t<Base>
modelscommon_range
,iterator_category
denotesbidirectional_iterator_tag
.[…]
chrono::from_stream()
assign zero to duration
for failure?Section: 30.5.11 [time.duration.io] Status: C++23 Submitter: Matt Stephanson Opened: 2021-03-19 Last modified: 2023-11-22
Priority: Not Prioritized
View all other issues in [time.duration.io].
View all issues with C++23 status.
Discussion:
The duration
specialization of from_stream
says in N4878
30.5.11 [time.duration.io]/3:
If the parse parses everything specified by the parsing format flags without error, and yet none of the flags impacts a duration,
d
will be assigned a zero value.
This is in contrast to the other specializations that say, for example, 30.8.3.3 [time.cal.day.nonmembers]/8:
If the parse fails to decode a valid day,
is.setstate(ios_base::failbit)
is called andd
is not modified.
The wording ("none of the flags impacts a duration" vs. "parse fails to decode a valid [meow]") and semantics
("assigned a zero value" vs. "not modified") are different, and it's not clear why that should be so. It also
leaves unspecified what should be done in case of a parse failure, for example parsing "%j"
from a
stream containing "meow"
. 30.13 [time.parse]/12 says that failbit
should be set,
but neither it nor 30.5.11 [time.duration.io]/3 mention the duration
result if parsing fails.
This looks like a bug in the standard to me, due to two errors on my part.
I believe that the standard should clearly say that iffailbit
is set, the parsed variable (duration
,time_point
, whatever) is not modified. I mistakenly believed that the definition of unformatted input function covered this behavior. But after review, I don't believe it does. Instead each extraction operator seems to say this separately. I also at first did not have my example implementation coded to leave the duration unchanged. So that's how the wording got in in the first place. Here's the commit where I fixed my implementation: HowardHinnant/date@d53db7a. And I failed to propagate that fix into the proposal/standard.
It would be clearer and simpler for users if the from_stream
specializations were consistent in wording
and behavior.
[2021-04-20; Reflector poll]
Set status to Tentatively Ready after eight votes in favour during reflector poll.
[2021-06-07 Approved at June 2021 virtual plenary. Status changed: Voting → WP.]
Proposed resolution:
This wording is relative to N4878.
Modify 30.5.11 [time.duration.io] as indicated:
template<class charT, class traits, class Rep, class Period, class Alloc = allocator<charT>> basic_istream<charT, traits>& from_stream(basic_istream<charT, traits>& is, const charT* fmt, duration<Rep, Period>& d, basic_string<charT, traits, Alloc>* abbrev = nullptr, minutes* offset = nullptr);-3- Effects: Attempts to parse the input stream
is
into the durationd
using the format flags given in the NTCTSfmt
as specified in 30.13 [time.parse].If the parse parses everything specified by the parsing format flags without error, and yet none of the flags impacts a duration,If the parse fails to decode a valid duration,d
will be assigned a zero valueis.setstate(ios_base::failbit)
is called andd
is not modified. If%Z
is used and successfully parsed, that value will be assigned to*abbrev
ifabbrev
is non-null. If%z
(or a modified variant) is used and successfully parsed, that value will be assigned to*offset
ifoffset
is non-null.
format_to
must not copy models of output_iterator<const charT&>
Section: 28.5.5 [format.functions] Status: C++23 Submitter: Casey Carter Opened: 2021-03-31 Last modified: 2023-11-22
Priority: Not Prioritized
View all other issues in [format.functions].
View all issues with C++23 status.
Discussion:
28.5.5 [format.functions] specifies the overloads of format_to
as:
template<class Out, class... Args> Out format_to(Out out, string_view fmt, const Args&... args); template<class Out, class... Args> Out format_to(Out out, wstring_view fmt, const Args&... args);-8- Effects: Equivalent to:
using context = basic_format_context<Out, decltype(fmt)::value_type>; return vformat_to(out, fmt, make_format_args<context>(args...));template<class Out, class... Args> Out format_to(Out out, const locale& loc, string_view fmt, const Args&... args); template<class Out, class... Args> Out format_to(Out out, const locale& loc, wstring_view fmt, const Args&... args); Out format_to(Out out, wstring_view fmt, const Args&... args);-9- Effects: Equivalent to:
using context = basic_format_context<Out, decltype(fmt)::value_type>; return vformat_to(out, loc, fmt, make_format_args<context>(args...));
but the overloads of vformat_to
take their first argument by value (from the same subclause):
template<class Out> Out vformat_to(Out out, string_view fmt, format_args_t<type_identity_t<Out>, char> args); template<class Out> Out vformat_to(Out out, wstring_view fmt, format_args_t<type_identity_t<Out>, wchar_t> args); template<class Out> Out vformat_to(Out out, const locale& loc, string_view fmt, format_args_t<type_identity_t<Out>, char> args); template<class Out> Out vformat_to(Out out, const locale& loc, wstring_view fmt, format_args_t<type_identity_t<Out>, wchar_t> args);-10- Let
-11- Constraints:charT
bedecltype(fmt)::value_type
.Out
satisfiesoutput_iterator<const charT&>
. -12- Preconditions:Out
modelsoutput_iterator<const charT&>
.
and require its type to model output_iterator<const charT&>
. output_iterator<T, U>
refines input_or_output_iterator<T>
which refines movable<T>
, but it notably does not
refine copyable<T>
. Consequently, the "Equivalent to" code for the format_to
overloads is
copying an iterator that could be move-only. I suspect it is not the intent that calls to format_to
with
move-only iterators be ill-formed, but that it was simply an oversight that this wording needs updating to be
consistent with the change to allow move-only single-pass iterators in C++20.
[2021-04-20; Reflector poll]
Set status to Tentatively Ready after seven votes in favour during reflector poll.
[2021-06-07 Approved at June 2021 virtual plenary. Status changed: Voting → WP.]
Proposed resolution:
This wording is relative to N4885.
Modify 28.5.5 [format.functions] as indicated:
template<class Out, class... Args> Out format_to(Out out, string_view fmt, const Args&... args); template<class Out, class... Args> Out format_to(Out out, wstring_view fmt, const Args&... args);-8- Effects: Equivalent to:
using context = basic_format_context<Out, decltype(fmt)::value_type>; return vformat_to(std::move(out), fmt, make_format_args<context>(args...));template<class Out, class... Args> Out format_to(Out out, const locale& loc, string_view fmt, const Args&... args); template<class Out, class... Args> Out format_to(Out out, const locale& loc, wstring_view fmt, const Args&... args);-9- Effects: Equivalent to:
using context = basic_format_context<Out, decltype(fmt)::value_type>; return vformat_to(std::move(out), loc, fmt, make_format_args<context>(args...));
const
in basic_format_arg(const T* p)
Section: 28.5.8.1 [format.arg] Status: C++23 Submitter: S. B. Tam Opened: 2021-04-07 Last modified: 2023-11-22
Priority: Not Prioritized
View all other issues in [format.arg].
View all issues with C++23 status.
Discussion:
When P0645R10 "Text formatting" was merged into the draft standard, there was
a typo: an exposition-only constructor of basic_format_arg
is declared as accepting const T*
in the
class synopsis, but is later declared to accept T*
. This was
editorial issue 3461 and was resolved by adding
const
to the redeclaration.
basic_format_arg
from void*
will select
template<class T> explicit basic_format_arg(const T& v)
and store a
basic_format_arg::handle
instead of select template<class T> basic_format_arg(const T*)
and
store a const void*
, because void*
→ void*const&
is identity conversion, while
void*
→ const void*
is qualification conversion.
While this technically works, it seems that storing a const void*
would be more intuitive.
Hence, I think const
should be removed from both declarations of basic_format_arg(const T*)
,
so that construction from void*
will select this constructor, resulting in more intuitive behavior.
[2021-04-20; Reflector poll]
Set status to Tentatively Ready after five votes in favour during reflector poll.
[2021-06-07 Approved at June 2021 virtual plenary. Status changed: Voting → WP.]
Proposed resolution:
This wording is relative to N4885.
Modify 28.5.8.1 [format.arg] as indicated:
namespace std { template<class Context> class basic_format_arg { public: class handle; private: using char_type = typename Context::char_type; // exposition only variant<monostate, bool, char_type, int, unsigned int, long long int, unsigned long long int, float, double, long double, const char_type*, basic_string_view<char_type>, const void*, handle> value; // exposition only template<class T> explicit basic_format_arg(const T& v) noexcept; // exposition only explicit basic_format_arg(float n) noexcept; // exposition only explicit basic_format_arg(double n) noexcept; // exposition only explicit basic_format_arg(long double n) noexcept; // exposition only explicit basic_format_arg(const char_type* s); // exposition only template<class traits> explicit basic_format_arg( basic_string_view<char_type, traits> s) noexcept; // exposition only template<class traits, class Allocator> explicit basic_format_arg( const basic_string<char_type, traits, Allocator>& s) noexcept; // exposition only explicit basic_format_arg(nullptr_t) noexcept; // exposition only template<class T> explicit basic_format_arg(constT* p) noexcept; // exposition only public: basic_format_arg() noexcept; explicit operator bool() const noexcept; }; }[…]
template<class T> explicit basic_format_arg(constT* p) noexcept;-12- Constraints:
-13- Effects: Initializesis_void_v<T>
istrue
.value
withp
. -14- [Note 1: Constructingbasic_format_arg
from a pointer to a member is ill-formed unless the user provides an enabled specialization of formatter for that pointer to member type. — end note]
indirectly_readable_traits
should be SFINAE-friendly for all typesSection: 24.3.2.2 [readable.traits] Status: C++23 Submitter: Christopher Di Bella Opened: 2021-04-08 Last modified: 2023-11-22
Priority: Not Prioritized
View all other issues in [readable.traits].
View all issues with C++23 status.
Discussion:
Following the outcome of LWG 3446(i), a strict implementation of std::indirectly_readable_traits
isn't SFINAE-friendly for types that have different value_type
and element_type
members due to
the ambiguity between the has-member-value-type
and has-member-element-type
specialisations. Other traits types are empty when requirements aren't met; we should follow suit here.
[2021-04-20; Reflector poll]
Set status to Tentatively Ready after seven votes in favour during reflector poll.
[2021-06-07 Approved at June 2021 virtual plenary. Status changed: Voting → WP.]
Proposed resolution:
This wording is relative to N4885.
Modify 24.3.2.2 [readable.traits] p1 as indicated:
[…] template<class T> concept has-member-value-type = requires { typename T::value_type; }; // exposition only template<class T> concept has-member-element-type = requires { typename T::element_type; }; // exposition only template<class> struct indirectly_readable_traits { }; […] template<has-member-value-type T> struct indirectly_readable_traits<T> : cond-value-type<typename T::value_type> { }; template<has-member-element-type T> struct indirectly_readable_traits<T> : cond-value-type<typename T::element_type> { }; template<has-member-value-type T> requires has-member-element-type<T> struct indirectly_readable_traits<T> { }; template<has-member-value-type T> requires has-member-element-type<T> && same_as<remove_cv_t<typename T::element_type>, remove_cv_t<typename T::value_type>> struct indirectly_readable_traits<T> : cond-value-type<typename T::value_type> { }; […]
basic_format_arg
mis-handles basic_string_view
with custom traitsSection: 28.5.8.1 [format.arg] Status: C++23 Submitter: Casey Carter Opened: 2021-04-20 Last modified: 2023-11-22
Priority: Not Prioritized
View all other issues in [format.arg].
View all issues with C++23 status.
Discussion:
basic_format_arg
has a constructor that accepts a basic_string_view
of an appropriate character type,
with any traits
type. The constructor is specified in 28.5.8.1 [format.arg] as:
template<class traits> explicit basic_format_arg(basic_string_view<char_type, traits> s) noexcept;-9- Effects: Initializes
value
withs
.
Recall that value
is a variant<monostate, bool, char_type, int, unsigned int, long long int,
unsigned long long int, float, double, long double, const char_type*, basic_string_view<char_type>, const void*, handle>
as specified earlier in the subclause. Since basic_string_view<meow, woof>
cannot be
initialized with an lvalue basic_string_view<meow, quack>
— and certainly none
of the other alternative types can be initialized by such an lvalue — the effects of this constructor are
ill-formed when traits
is not std::char_traits<char_type>
.
basic_string
constructor deals with this same issue by ignoring the deduced traits type when initializing
value
's basic_string_view
member:
template<class traits, class Allocator> explicit basic_format_arg( const basic_string<char_type, traits, Allocator>& s) noexcept;-10- Effects: Initializes
value
withbasic_string_view<char_type>(s.data(), s.size())
.
which immediately begs the question of "why doesn't the basic_string_view
constructor do the same?"
[2021-05-10; Reflector poll]
Set status to Tentatively Ready after eight votes in favour during reflector poll.
[2021-06-07 Approved at June 2021 virtual plenary. Status changed: Voting → WP.]
Proposed resolution:
This wording is relative to N4885.
Modify 28.5.8.1 [format.arg] as indicated:
template<class traits> explicit basic_format_arg(basic_string_view<char_type, traits> s) noexcept;-9- Effects: Initializes
value
with.
sbasic_string_view<char_type>(s.data(), s.size())
counted_iterators
refer to the same sequence isn't quite rightSection: 24.5.7.1 [counted.iterator] Status: C++23 Submitter: Tim Song Opened: 2021-04-21 Last modified: 2023-11-22
Priority: Not Prioritized
View all other issues in [counted.iterator].
View all issues with C++23 status.
Discussion:
24.5.7.1 [counted.iterator]/3 says:
Two values
i1
andi2
of typescounted_iterator<I1>
andcounted_iterator<I2>
refer to elements of the same sequence if and only ifnext(i1.base(), i1.count())
andnext(i2.base(), i2.count())
refer to the same (possibly past-the-end) element.
However, some users of counted_iterator
(such as take_view
) don't guarantee that
there are count()
elements past base()
. It seems that we need to rephrase this
definition to account for such uses.
[2021-05-10; Reflector poll]
Set status to Tentatively Ready after five votes in favour during reflector poll.
[2021-06-07 Approved at June 2021 virtual plenary. Status changed: Voting → WP.]
Proposed resolution:
This wording is relative to N4885.
Modify 24.5.7.1 [counted.iterator] as indicated:
-3- Two values
i1
andi2
of typescounted_iterator<I1>
andcounted_iterator<I2>
refer to elements of the same sequence if and only if there exists some integern
such thatnext(i1.base(), i1.count() + n)
andnext(i2.base(), i2.count() + n)
refer to the same (possibly past-the-end) element.
format-arg-store::args
is unintentionally not exposition-onlySection: 28.5.8.2 [format.arg.store] Status: C++23 Submitter: Casey Carter Opened: 2021-04-22 Last modified: 2023-11-22
Priority: 3
View all issues with C++23 status.
Discussion:
Despite the statement in 28.5.8.3 [format.args]/1:
An instance of
basic_format_args
provides access to formatting arguments. Implementations should optimize the representation ofbasic_format_args
for a small number of formatting arguments. [Note 1: For example, by storing indices of type alternatives separately from values and packing the former. — end note]
make_format_args
and make_wformat_args
are specified to return an object whose type is
a specialization of the exposition-only class template format-arg-store
which has a public
non-static data member that is an array
of basic_format_arg
. In order to actually
"optimize the representation of basic_format_args
" an implementation must internally avoid using
make_format_args
(and make_wformat_args
) and instead use a different mechanism to type-erase
arguments. basic_format_args
must still be convertible from format-arg-store
as
specified, however, so internally basic_format_args
must support both the bad/slow standard mechanism
and a good/fast internal-only mechanism for argument storage.
<format>
with no commensurate benefit. Indeed, naive users may make the mistake of thinking
that e.g. vformat(fmt, make_format_args(args...))
is as efficient as format(fmt, args...)
—
that's what the "Effects: Equivalent to" in 28.5.5 [format.functions]/2 implies — and
inadvertently introduce performance regressions. It would be better for both implementers and users if
format-arg-store
had no public data members and its member args
were made exposition-only.
[2021-05-10; Reflector poll]
Priority set to 3.
Tim: "The current specification of make_format_args
depends on
format-arg-store
being an aggregate,
which is no longer true with this PR."
Previous resolution [SUPERSEDED]:
This wording is relative to N4885.
Modify 28.5.1 [format.syn], header
<format>
synopsis, as indicated:[…] // 28.5.8.2 [format.arg.store], class template format-arg-store template<class Context, class... Args>structclass format-arg-store; // exposition only […]Modify 28.5.8.2 [format.arg.store] as indicated:
namespace std { template<class Context, class... Args>structclass format-arg-store { // exposition only array<basic_format_arg<Context>, sizeof...(Args)> args; // exposition only }; }
[2021-05-18; Tim updates wording.]
[2021-05-20; Reflector poll]
Set status to Tentatively Ready after six votes in favour during reflector poll.
[2021-06-07 Approved at June 2021 virtual plenary. Status changed: Voting → WP.]
Proposed resolution:
This wording is relative to N4885.
Modify 28.5.1 [format.syn], header <format>
synopsis, as indicated:
[…] // 28.5.8.2 [format.arg.store], class template format-arg-store template<class Context, class... Args>structclass format-arg-store; // exposition only template<class Context = format_context, class... Args> format-arg-store<Context, Args...> make_format_args(const Args&... fmt_args); […]
Modify 28.5.8.2 [format.arg.store] as indicated:
namespace std { template<class Context, class... Args>structclass format-arg-store { // exposition only array<basic_format_arg<Context>, sizeof...(Args)> args; // exposition only }; }-1- An instance of
format-arg-store
stores formatting arguments.template<class Context = format_context, class... Args> format-arg-store<Context, Args...> make_format_args(const Args&... fmt_args);-2- Preconditions: The type
-3- Returns:typename Context::template formatter_type<Ti>
meets the Formatter requirements (28.5.6.1 [formatter.requirements]) for eachTi
inArgs
.An object of typeformat-arg-store<Context, Args...>
whoseargs
data member is initialized with{basic_format_arg<Context>(fmt_args)...}
.
std::pointer_traits
should be SFINAE-friendlySection: 20.2.3 [pointer.traits] Status: C++23 Submitter: Glen Joseph Fernandes Opened: 2021-04-20 Last modified: 2023-11-22
Priority: 2
View all other issues in [pointer.traits].
View all issues with C++23 status.
Discussion:
P1474R1 chose to use std::to_address
(a mechanism of converting pointer-like types to raw pointers) for contiguous iterators.
std::to_address
provides an optional customization point via an optional member in
std::pointer_traits
. However all iterators are not pointers, and the primary
template of std::pointer_traits<Ptr>
requires that either
Ptr::element_type
is valid or Ptr
is of the form
template<T, Args...>
or the pointer_traits
specialization is
ill-formed. This requires specializing pointer_traits
for those contiguous iterator
types which is inconvenient for users. P1474
should have also made pointer_traits
SFINAE friendly.
[2021-05-10; Reflector poll]
Priority set to 2. Send to LEWG.
Daniel: "there is no similar treatment for the rebind
member
template and I think it should be clarified whether pointer_to
's
signature should exist and in which form in the offending case."
[2022-01-29; Daniel comments]
This issue has some overlap with LWG 3665(i) in regard to the question how we should handle
the rebind_alloc
member template of the allocator_traits
template as specified by
20.2.9.2 [allocator.traits.types]/11. It would seem preferable to decide for the same approach in both
cases.
[2022-02-22 LEWG telecon; Status changed: LEWG → Open]
No objection to unanimous consent for Jonathan's suggestion to make
pointer_traits
an empty class when there is no
element_type
. Jonathan to provide a paper.
Previous resolution [SUPERSEDED]:
This wording is relative to N4885.
Modify 20.2.3.2 [pointer.traits.types] as indicated:
As additional drive-by fix the improper usage of the term "instantiation" has been corrected.
using element_type = see below;-1- Type:
Ptr::element_type
if the qualified-idPtr::element_type
is valid and denotes a type (13.10.3 [temp.deduct]); otherwise,T
ifPtr
is a class templateinstantiationspecialization of the formSomePointer<T, Args>
, whereArgs
is zero or more type arguments; otherwise,the specialization is ill-formedpointer_traits
has no memberelement_type
.
[2022-09-27; Jonathan provides new wording]
Previous resolution [SUPERSEDED]:
This wording is relative to N4917.
Modify 20.2.3.1 [pointer.traits.general] as indicated:
-1- The class template
pointer_traits
supplies a uniform interface to certain attributes of pointer-like types.namespace std { template<class Ptr> struct pointer_traits {using pointer = Ptr;using element_type = see below;using difference_type = see below;template<class U> using rebind = see below;static pointer pointer_to(see below r);see below; }; template<class T> struct pointer_traits<T*> { using pointer = T*; using element_type = T; using difference_type = ptrdiff_t; template<class U> using rebind = U*; static constexpr pointer pointer_to(see below r) noexcept; }; }Modify 20.2.3.2 [pointer.traits.types] as indicated:
-?- The definitions in this subclause make use of the following exposition-only class template and concept:
template<class T> struct ptr-traits-elem // exposition only { }; template<class T> requires requires { typename T::element_type; } struct ptr-traits-elem<T> { using type = typename T::element_type; }; template<template<class...> class SomePointer, class T, class... Args> requires (!requires { typename SomePointer<T, Args...>::element_type; }) struct ptr-traits-elem<SomePointer<T, Args...>> { using type = T; }; template<class Ptr> concept has-elem-type = // exposition only requires { typename ptr-traits-elem<Ptr>::type; }-?- If
Ptr
satisfieshas-elem-type
, a specializationpointer_traits<Ptr>
generated from thepointer_traits
primary template has the members described in 20.2.3.2 [pointer.traits.types] and 20.2.3.3 [pointer.traits.functions]; otherwise, such a specialization has no members by any of the names described in those subclauses or in 20.2.3.4 [pointer.traits.optmem].using pointer = Ptr;using element_type =see belowtypename ptr-traits-elem<Ptr>::type;
-1- Type:Ptr::element_type
if the qualified-idPtr::element_type
is valid and denotes a type (13.10.3 [temp.deduct]); otherwise,T
ifPtr
is a class template instantiation of the formSomePointer<T, Args>
, whereArgs
is zero or more type arguments; otherwise, the specialization is ill-formed.using difference_type = see below;-2- Type:
Ptr::difference_type
if the qualified-idPtr::difference_type
is valid and denotes a type (13.10.3 [temp.deduct]); otherwise,ptrdiff_t
.template<class U> using rebind = see below;-3- Alias template:
Ptr::rebind<U>
if the qualified-idPtr::rebind<U>
is valid and denotes a type (13.10.3 [temp.deduct]); otherwise,SomePointer<U, Args>
ifPtr
is a class template instantiation of the formSomePointer<T, Args>
, whereArgs
is zero or more type arguments; otherwise, the instantiation ofrebind
is ill-formed.
[2022-10-11; Jonathan provides improved wording]
[2022-10-19; Reflector poll]
Set status to "Tentatively Ready" after six votes in favour in reflector poll.
[2022-11-12 Approved at November 2022 meeting in Kona. Status changed: Voting → WP.]
Proposed resolution:
This wording is relative to N4917.
Modify 20.2.3.1 [pointer.traits.general] as indicated:
-1- The class template
pointer_traits
supplies a uniform interface to certain attributes of pointer-like types.namespace std { template<class Ptr> struct pointer_traits {using pointer = Ptr;using element_type = see below;using difference_type = see below;template<class U> using rebind = see below;static pointer pointer_to(see below r);see below; }; template<class T> struct pointer_traits<T*> { using pointer = T*; using element_type = T; using difference_type = ptrdiff_t; template<class U> using rebind = U*; static constexpr pointer pointer_to(see below r) noexcept; }; }
Modify 20.2.3.2 [pointer.traits.types] as indicated:
-?- The definitions in this subclause make use of the following exposition-only class template and concept:
template<class T> struct ptr-traits-elem // exposition only { }; template<class T> requires requires { typename T::element_type; } struct ptr-traits-elem<T> { using type = typename T::element_type; }; template<template<class...> class SomePointer, class T, class... Args> requires (!requires { typename SomePointer<T, Args...>::element_type; }) struct ptr-traits-elem<SomePointer<T, Args...>> { using type = T; }; template<class Ptr> concept has-elem-type = // exposition only requires { typename ptr-traits-elem<Ptr>::type; }-?- If
Ptr
satisfieshas-elem-type
, a specializationpointer_traits<Ptr>
generated from thepointer_traits
primary template has the following members as well as those described in 20.2.3.3 [pointer.traits.functions]; otherwise, such a specialization has no members by any of those names.using pointer = see below;-?- Type:
Ptr
.using element_type = see below;-1- Type:
typename ptr-traits-elem<Ptr>::type
.Ptr::element_type
if the qualified-idPtr::element_type
is valid and denotes a type (13.10.3 [temp.deduct]); otherwise,T
ifPtr
is a class template instantiation of the formSomePointer<T, Args>
, whereArgs
is zero or more type arguments; otherwise, the specialization is ill-formed.using difference_type = see below;-2- Type:
Ptr::difference_type
if the qualified-idPtr::difference_type
is valid and denotes a type (13.10.3 [temp.deduct]); otherwise,ptrdiff_t
.template<class U> using rebind = see below;-3- Alias template:
Ptr::rebind<U>
if the qualified-idPtr::rebind<U>
is valid and denotes a type (13.10.3 [temp.deduct]); otherwise,SomePointer<U, Args>
ifPtr
is a class template instantiation of the formSomePointer<T, Args>
, whereArgs
is zero or more type arguments; otherwise, the instantiation ofrebind
is ill-formed.
Modify 20.2.3.4 [pointer.traits.optmem] as indicated:
-1- Specializations of
pointer_traits
may define the member declared in this subclause to customize the behavior of the standard library. A specialization generated from thepointer_traits
primary template has no member by this name.static element_type* to_address(pointer p) noexcept;-1- Returns: A pointer of type
element_type*
that references the same location as the argumentp
.
common_iterator
's postfix-proxy
is not quite rightSection: 24.5.5.5 [common.iter.nav] Status: C++23 Submitter: Tim Song Opened: 2021-04-23 Last modified: 2023-11-22
Priority: Not Prioritized
View all other issues in [common.iter.nav].
View all issues with C++23 status.
Discussion:
P2259R1 modeled common_iterator::operator++(int)
's
postfix-proxy
class on the existing proxy class used by common_iterator::operator->
,
but in doing so it overlooked two differences:
operator->
's proxy is only used when iter_reference_t<I>
is not a
reference type; this is not the case for postfix-proxy
;
operator->
returns a prvalue proxy, while operator++
's postfix-proxy
is returned by (elidable) move, so the latter needs to require iter_value_t<I>
to be
move_constructible
.
The proposed wording has been implemented and tested.
[2021-05-10; Reflector poll]
Set status to Tentatively Ready after five votes in favour during reflector poll.
[2021-05-17; Reflector poll]
Set status to Tentatively Ready after five votes in favour during reflector poll.
[2021-06-07 Approved at June 2021 virtual plenary. Status changed: Voting → WP.]
Proposed resolution:
This wording is relative to N4885.
Modify 24.5.5.5 [common.iter.nav] as indicated:
decltype(auto) operator++(int);-4- Preconditions:
-5- Effects: Ifholds_alternative<I>(v_)
istrue
.I
modelsforward_iterator
, equivalent to:common_iterator tmp = *this; ++*this; return tmp;Otherwise, if
requires (I& i) { { *i++ } -> can-reference; }
istrue
orconstructible_from<iter_value_t<I>, iter_reference_t<I>> && move_constructible<iter_value_t<I>>
isfalse
, equivalent to:return get<I>(v_)++;Otherwise, equivalent to:
postfix-proxy p(**this); ++*this; return p;where
postfix-proxy
is the exposition-only class:class postfix-proxy { iter_value_t<I> keep_; postfix-proxy(iter_reference_t<I>&& x) : keep_(std::moveforward<iter_reference_t<I>>(x)) {} public: const iter_value_t<I>& operator*() const { return keep_; } };
Section: 30.12 [time.format] Status: Resolved Submitter: Corentin Jabot Opened: 2021-04-27 Last modified: 2021-10-23
Priority: 2
View other active issues in [time.format].
View all other issues in [time.format].
View all issues with Resolved status.
Discussion:
In 30.12 [time.format] it is specified:
Some of the conversion specifiers depend on the locale that is passed to the formatting function if the latter takes one, or the global locale otherwise.
This is not consistent with the format design after the adoption of P1892. In 28.5.2.2 [format.string.std] we say:
When the
L
option is used, the form used for the conversion is called the locale-specific form. TheL
option is only valid for arithmetic types, and its effect depends upon the type.
This has two issues: First, it is inconsistent.
format("{}, 0.0"); // locale independent format("{:L}", 0.0); // use locale format("{:%r}, some_time); // use globale locale format("{:%rL}, some_time); // error
And second it perpetuates the issues P1892 intended to solve. It is likely that this inconsistency resulted from both papers being in flight around the same time.
TheL
option should be used and consistent with floating point. We suggest using the C locale
which is the non-locale locale, see also
here.
[2021-05-17; Reflector poll]
Priority set to 2. This will be resolved by P2372. Tim noted: "P1892 didn't change format's ignore-locale-by-default design, so that paper being in flight at the same time as P1361 cannot explain why the latter is locale-sensitive-by-default."
[2021-10-23 Resolved by the adoption of P2372R3 at the October 2021 plenary. Status changed: New → Resolved.]
Proposed resolution:
This wording is relative to N4885.
Modify 30.12 [time.format] as indicated:
chrono-format-spec: fill-and-alignopt widthopt precisionopt Lopt chrono-specsopt […]-1- […]
-2- Each conversion specifier conversion-spec is replaced by appropriate characters as described in Table [tab:time.format.spec]; the formats specified in ISO 8601:2004 shall be used where so described. Some of the conversion specifiers depend onthe locale that is passed to the formatting function if the latter takes one, or the global locale otherwisea locale. If theL
option is used, that locale is the locale that is passed to the formatting function if the latter takes one, or the global locale otherwise. If theL
option is not used, that locale is the"C"
locale. If the formatted object does not contain the information the conversion specifier refers to, an exception of typeformat_error
is thrown.
shared_ptr
construction from unique_ptr
should move (not copy) the deleterSection: 20.3.2.2.2 [util.smartptr.shared.const] Status: C++23 Submitter: Thomas Köppe Opened: 2021-05-04 Last modified: 2023-11-22
Priority: Not Prioritized
View other active issues in [util.smartptr.shared.const].
View all other issues in [util.smartptr.shared.const].
View all issues with C++23 status.
Discussion:
The construction of a shared_ptr
from a suitable unique_ptr
rvalue r
(which was last modified by LWG 2415(i), but not in ways relevant to this issue) calls
for shared_ptr(r.release(), r.get_deleter())
in the case where the deleter is not a reference.
unique_ptr(unique_ptr&& u)
only requires u
's deleter
to be Cpp17MoveConstructible. By analogy, the constructor shared_ptr(unique_ptr<Y, D>&&)
should also require D
to be only move-, not copy-constructible.
[2021-05-17; Reflector poll]
Set status to Tentatively Ready after eight votes in favour during reflector poll.
[2021-06-07 Approved at June 2021 virtual plenary. Status changed: Voting → WP.]
Proposed resolution:
This wording is relative to N4885.
Modify 20.3.2.2.2 [util.smartptr.shared.const] as indicated:
template<class Y, class D> shared_ptr(unique_ptr<Y, D>&& r);-28- Constraints:
-29- Effects: IfY*
is compatible withT*
andunique_ptr<Y, D>::pointer
is convertible toelement_type*
.r.get() == nullptr
, equivalent toshared_ptr()
. Otherwise, ifD
is not a reference type, equivalent toshared_ptr(r.release(), std::move(r.get_deleter()))
. Otherwise, equivalent toshared_ptr(r.release(), ref(r.get_deleter()))
. If an exception is thrown, the constructor has no effect.
view_interface
is overspecified to derive from view_base
Section: 25.5.3 [view.interface], 25.4.4 [range.view] Status: C++23 Submitter: Tim Song Opened: 2021-05-09 Last modified: 2023-11-22
Priority: 2
View all other issues in [view.interface].
View all issues with C++23 status.
Discussion:
view_interface<D>
is currently specified to publicly derive from
view_base
. This derivation requires unnecessary padding in range adaptors
like reverse_view<V>
: the view_base
subobject of the
reverse_view
is required to reside at a different address than the
view_base
subobject of V
(assuming that V
similarly
opted into being a view through derivation from view_interface
or
view_base
).
view_interface
to make the class a view
; the exact
mechanism of how that opt-in is accomplished should have been left as an
implementation detail.
The proposed resolution below has been implemented on top of libstdc++ master
and passes its ranges testsuite, with the exception of a test that checks for the size of
various range adaptors (and therefore asserts the existence of
view_base
-induced padding).
[2021-05-17; Reflector poll]
Priority set to 2.
[2021-05-26; Reflector poll]
Set status to Tentatively Ready after five votes in favour during reflector poll.
[2021-06-07 Approved at June 2021 virtual plenary. Status changed: Voting → WP.]
Proposed resolution:
This wording is relative to N4885.
Modify 25.4.4 [range.view] as indicated:
template<class T> inline constexpr bool is-derived-from-view-interface = see below; // exposition only template<class T> inline constexpr bool enable_view = derived_from<T, view_base> || is-derived-from-view-interface<T>;-?- For a type
-5- Remarks: Pursuant to 16.4.5.2.1 [namespace.std], users may specializeT
,is-derived-from-view-interface<T>
istrue
if and only ifT
has exactly one public base classview_interface<U>
for some typeU
andT
has no base classes of typeview_interface<V>
for any other typeV
.enable_view
totrue
for cv-unqualified program-defined types which modelview
, andfalse
for types which do not. Such specializations shall be usable in constant expressions (7.7 [expr.const]) and have typeconst bool
.
Modify 25.5.3.1 [view.interface.general] as indicated:
namespace std::ranges { template<class D> requires is_class_v<D> && same_as<D, remove_cv_t<D>> class view_interface: public view_base{ […] }; }
borrowed_{iterator,subrange}_t
are overspecifiedSection: 25.5.5 [range.dangling] Status: C++23 Submitter: Tim Song Opened: 2021-05-12 Last modified: 2023-11-22
Priority: Not Prioritized
View all issues with C++23 status.
Discussion:
As discussed in P1715R0,
there are ways to implement something equivalent to std::conditional_t
that are better for compile times. However, borrowed_{iterator,subrange}_t
are currently specified to use conditional_t
, and this appears to be
user-observable due to the transparency of alias templates. We should simply
specify the desired result and leave the actual definition to the implementation.
[2021-05-20; Reflector poll]
Set status to Tentatively Ready after seven votes in favour during reflector poll.
[2021-06-07 Approved at June 2021 virtual plenary. Status changed: Voting → WP.]
Proposed resolution:
This wording is relative to N4885.
Modify 25.2 [ranges.syn], header <ranges>
synopsis, as indicated:
[…] namespace std::ranges { […] // 25.5.5 [range.dangling], dangling iterator handling struct dangling; template<range R> using borrowed_iterator_t = see belowconditional_t<borrowed_range<R>, iterator_t<R>, dangling>; template<range R> using borrowed_subrange_t = see belowconditional_t<borrowed_range<R>, subrange<iterator_t<R>>, dangling>; […] }
Modify 25.5.5 [range.dangling] as indicated:
-1- The tag type
dangling
is used together with the template aliasesborrowed_iterator_t
andborrowed_subrange_t
. When an algorithm that typically returns an iterator into, or a subrange of, a range argument is called with an rvalue range argument that does not modelborrowed_range
(25.4.2 [range.range]), the return value possibly refers to a range whose lifetime has ended. In such cases, the tag typedangling
is returned instead of an iterator or subrange.namespace std::ranges { struct dangling { […] }; }-2- [Example 1: […] — end example]
-?- For a typeR
that modelsrange
:
(?.1) — if
R
modelsborrowed_range
, thenborrowed_iterator_t<R>
denotesiterator_t<R>
, andborrowed_subrange_t<R>
denotessubrange<iterator_t<R>>
;(?.2) — otherwise, both
borrowed_iterator_t<R>
andborrowed_subrange_t<R>
denotedangling
.
Section: 20.2.2 [memory.syn] Status: C++23 Submitter: Tim Song Opened: 2021-05-16 Last modified: 2023-11-22
Priority: Not Prioritized
View all other issues in [memory.syn].
View all issues with C++23 status.
Discussion:
The parallel versions of uninitialized_{copy,move}{,_n}
are currently
depicted as accepting input iterators for their source range. Similar to the
non-uninitialized versions, they should require the source range to be at least forward.
[2021-05-20; Reflector poll]
Set status to Tentatively Ready after six votes in favour during reflector poll.
[2021-06-07 Approved at June 2021 virtual plenary. Status changed: Voting → WP.]
Proposed resolution:
This wording is relative to N4885.
Modify 20.2.2 [memory.syn], header <memory>
synopsis, as indicated:
[…] namespace std { […] template<class InputIterator, class NoThrowForwardIterator> NoThrowForwardIterator uninitialized_copy(InputIterator first, InputIterator last, NoThrowForwardIterator result); template<class ExecutionPolicy, classInputForwardIterator, class NoThrowForwardIterator> NoThrowForwardIterator uninitialized_copy(ExecutionPolicy&& exec, // see 26.3.5 [algorithms.parallel.overloads]InputForwardIterator first,InputForwardIterator last, NoThrowForwardIterator result); template<class InputIterator, class Size, class NoThrowForwardIterator> NoThrowForwardIterator uninitialized_copy_n(InputIterator first, Size n, NoThrowForwardIterator result); template<class ExecutionPolicy, classInputForwardIterator, class Size, class NoThrowForwardIterator> NoThrowForwardIterator uninitialized_copy_n(ExecutionPolicy&& exec, // see 26.3.5 [algorithms.parallel.overloads]InputForwardIterator first, Size n, NoThrowForwardIterator result); […] template<class InputIterator, class NoThrowForwardIterator> NoThrowForwardIterator uninitialized_move(InputIterator first, InputIterator last, NoThrowForwardIterator result); template<class ExecutionPolicy, classInputForwardIterator, class NoThrowForwardIterator> NoThrowForwardIterator uninitialized_move(ExecutionPolicy&& exec, // see 26.3.5 [algorithms.parallel.overloads]InputForwardIterator first,InputForwardIterator last, NoThrowForwardIterator result); template<class InputIterator, class Size, class NoThrowForwardIterator> pair<InputIterator, NoThrowForwardIterator> uninitialized_move_n(InputIterator first, Size n, NoThrowForwardIterator result); template<class ExecutionPolicy, classInputForwardIterator, class Size, class NoThrowForwardIterator> pair<InputForwardIterator, NoThrowForwardIterator> uninitialized_move_n(ExecutionPolicy&& exec, // see 26.3.5 [algorithms.parallel.overloads]InputForwardIterator first, Size n, NoThrowForwardIterator result); […] }
split_view::outer-iterator::value_type::begin()
Section: 25.7.16.4 [range.lazy.split.outer.value] Status: C++23 Submitter: Hewill Kang Opened: 2021-05-18 Last modified: 2023-11-22
Priority: Not Prioritized
View all other issues in [range.lazy.split.outer.value].
View all issues with C++23 status.
Discussion:
The copyable
constraint in split_view::outer-iterator::value_type::begin()
is useless because outer-iterator
always satisfies copyable
.
V
does not model forward_range
, the outer-iterator
only contains a pointer, so it
is obviously copyable; When V
is a forward_range
, the iterator_t<Base>
is a
forward_iterator
, so it is copyable, which makes outer-iterator
also copyable.
Suggested resolution: Remove the no-const begin()
and useless copyable
constraint in
[range.split.outer.value].
[2021-05-26; Reflector poll]
Set status to Tentatively Ready after five votes in favour during reflector poll.
[2021-06-07 Approved at June 2021 virtual plenary. Status changed: Voting → WP.]
Proposed resolution:
This wording is relative to N4885.
Modify [range.split.outer.value], class split_view::outer-iterator::value_type
synopsis,
as indicated:
[…]namespace std::ranges { template<input_range V, forward_range Pattern> requires view<V> && view<Pattern> && indirectly_comparable<iterator_t<V>, iterator_t<Pattern>, ranges::equal_to> && (forward_range<V> || tiny-range<Pattern>) template<bool Const> struct split_view<V, Pattern>::outer-iterator<Const>::value_type : view_interface<value_type> { private: outer-iterator i_ = outer-iterator(); // exposition only public: value_type() = default; constexpr explicit value_type(outer-iterator i); constexpr inner-iterator<Const> begin() constrequires copyable<outer-iterator>;constexpr inner-iterator<Const> begin() requires (!copyable<outer-iterator>);constexpr default_sentinel_t end() const; }; }constexpr inner-iterator<Const> begin() constrequires copyable<outer-iterator>;-2- Effects: Equivalent to:
return inner-iterator<Const>{i_};
constexpr inner-iterator<Const> begin() requires (!copyable<outer-iterator>);[…]
-3- Effects: Equivalent to:return inner-iterator<Const>{std::move(i_)};
chrono::parse
needs const charT*
and basic_string_view<charT>
overloadsSection: 30.13 [time.parse] Status: C++23 Submitter: Howard Hinnant Opened: 2021-05-22 Last modified: 2023-11-22
Priority: 2
View other active issues in [time.parse].
View all other issues in [time.parse].
View all issues with C++23 status.
Discussion:
The chrono::parse
functions take const basic_string<charT, traits, Alloc>&
parameters
to specify the format strings for the parse. Due to an oversight on my part in the proposal, overloads taking
const charT*
and basic_string_view<charT, traits>
were omitted. These are necessary when
the supplied arguments is a string literal or string_view
respectively:
in >> parse("%F %T", tp);
These overloads have been implemented in the example implementation.
[2021-05-26; Reflector poll]
Set priority to 2 after reflector poll.
Previous resolution [SUPERSEDED]:
This wording is relative to N4885.
Modify 30.2 [time.syn], header
<chrono>
synopsis, as indicated:[…] namespace chrono { // 30.13 [time.parse], parsing template<class charT, class Parsable> unspecified parse(const charT* fmt, Parsable& tp); template<class charT, class traits, class Parsable> unspecified parse(basic_string_view<charT, traits> fmt, Parsable& tp); template<class charT, class traits, class Alloc, class Parsable> unspecified parse(const charT* fmt, Parsable& tp, basic_string<charT, traits, Alloc>& abbrev); template<class charT, class traits, class Alloc, class Parsable> unspecified parse(basic_string_view<charT, traits> fmt, Parsable& tp, basic_string<charT, traits, Alloc>& abbrev); template<class charT, class Parsable> unspecified parse(const charT* fmt, Parsable& tp, minutes& offset); template<class charT, class traits, class Parsable> unspecified parse(basic_string_view<charT, traits> fmt, Parsable& tp, minutes& offset); template<class charT, class traits, class Alloc, class Parsable> unspecified parse(const charT* fmt, Parsable& tp, basic_string<charT, traits, Alloc>& abbrev, minutes& offset); template<class charT, class traits, class Alloc, class Parsable> unspecified parse(basic_string_view<charT, traits> fmt, Parsable& tp, basic_string<charT, traits, Alloc>& abbrev, minutes& offset); template<class charT, class traits, class Alloc, class Parsable> unspecified parse(const basic_string<charT, traits, Alloc>& format, Parsable& tp); template<class charT, class traits, class Alloc, class Parsable> unspecified parse(const basic_string<charT, traits, Alloc>& format, Parsable& tp, basic_string<charT, traits, Alloc>& abbrev); template<class charT, class traits, class Alloc, class Parsable> unspecified parse(const basic_string<charT, traits, Alloc>& format, Parsable& tp, minutes& offset); template<class charT, class traits, class Alloc, class Parsable> unspecified parse(const basic_string<charT, traits, Alloc>& format, Parsable& tp, basic_string<charT, traits, Alloc>& abbrev, minutes& offset); […] } […]Modify 30.13 [time.parse] as indicated:
template<class charT, class Parsable> unspecified parse(const charT* fmt, Parsable& tp);-?- Constraints: The expression
from_stream(declval<basic_istream<charT>&>(), fmt, tp)is well-formed when treated as an unevaluated operand.
-?- Effects: Equivalent toreturn parse(basic_string<charT>{fmt}, tp);
template<class charT, class traits, class Parsable> unspecified parse(basic_string_view<charT, traits> fmt, Parsable& tp);-?- Constraints: The expression
from_stream(declval<basic_istream<charT, traits>&>(), fmt.data(), tp)is well-formed when treated as an unevaluated operand.
-?- Effects: Equivalent toreturn parse(basic_string<charT, traits>{fmt}, tp);
template<class charT, class traits, class Alloc, class Parsable> unspecified parse(const charT* fmt, Parsable& tp, basic_string<charT, traits, Alloc>& abbrev);-?- Constraints: The expression
from_stream(declval<basic_istream<charT, traits>&>(), fmt, tp, addressof(abbrev))is well-formed when treated as an unevaluated operand.
-?- Effects: Equivalent toreturn parse(basic_string<charT, traits, Alloc>{fmt}, tp, abbrev);
template<class charT, class traits, class Alloc, class Parsable> unspecified parse(basic_string_view<charT, traits> fmt, Parsable& tp, basic_string<charT, traits, Alloc>& abbrev);-?- Constraints: The expression
from_stream(declval<basic_istream<charT, traits>&>(), fmt.data(), tp, addressof(abbrev))is well-formed when treated as an unevaluated operand.
-?- Effects: Equivalent toreturn parse(basic_string<charT, traits, Alloc>{fmt}, tp, abbrev);
template<class charT, class Parsable> unspecified parse(const charT* fmt, Parsable& tp, minutes& offset);-?- Constraints: The expression
from_stream(declval<basic_istream<charT>&>(), fmt, tp, declval<basic_string<charT>*>(), &offset)is well-formed when treated as an unevaluated operand.
-?- Effects: Equivalent toreturn parse(basic_string<charT>{fmt}, tp, offset);
template<class charT, class traits, class Parsable> unspecified parse(basic_string_view<charT, traits> fmt, Parsable& tp, minutes& offset);-?- Constraints: The expression
from_stream(declval<basic_istream<charT, traits>&>(), fmt.data(), tp, declval<basic_string<charT, traits>*>(), &offset)is well-formed when treated as an unevaluated operand.
-?- Effects: Equivalent toreturn parse(basic_string<charT, traits>{fmt}, tp, offset);
template<class charT, class traits, class Alloc, class Parsable> unspecified parse(const charT* fmt, Parsable& tp, basic_string<charT, traits, Alloc>& abbrev, minutes& offset);-?- Constraints: The expression
from_stream(declval<basic_istream<charT, traits>&>(), fmt, tp, addressof(abbrev), &offset)is well-formed when treated as an unevaluated operand.
-?- Effects: Equivalent toreturn parse(basic_string<charT, traits, Alloc>{fmt}, tp, abbrev, offset);
template<class charT, class traits, class Alloc, class Parsable> unspecified parse(basic_string_view<charT, traits> fmt, Parsable& tp, basic_string<charT, traits, Alloc>& abbrev, minutes& offset);-?- Constraints: The expression
from_stream(declval<basic_istream<charT, traits>&>(), fmt.data(), tp, addressof(abbrev), &offset)is well-formed when treated as an unevaluated operand.
-?- Effects: Equivalent toreturn parse(basic_string<charT, traits, Alloc>{fmt}, tp, abbrev, offset);
template<class charT, class traits, class Alloc, class Parsable> unspecified parse(const basic_string<charT, traits, Alloc>& format, Parsable& tp);-2- Constraints: The expression
from_stream(declval<basic_istream<charT, traits>&>(), fmt.c_str(), tp)is well-formed when treated as an unevaluated operand.
-3- Returns: A manipulator such that the expressionis >> parse(fmt, tp)
has typeI
, has valueis
, and callsfrom_stream(is, fmt.c_str(), tp)
.
[2021-06-02 Tim comments and provides updated wording]
The current specification suggests that parse
takes a reference to
the format string (stream extraction on the resulting manipulator is specified
to call from_stream
on fmt.c_str()
, not some copy), so we
can't call it with a temporary string.
from_stream
) requires a
null-terminated string, the usual practice is to avoid providing a
string_view
overload (see, e.g., LWG 3430(i)). The wording
below therefore only provides const charT*
overloads. It does not use
"Equivalent to" to avoid requiring that the basic_string
overload
and the const charT*
overload return the same type. As the manipulator
is intended to be immediately consumed, the wording also adds normative
encouragement to make misuse more difficult.
[2021-06-23; Reflector poll]
Set status to Tentatively Ready after six votes in favour during reflector poll.
[2021-10-14 Approved at October 2021 virtual plenary. Status changed: Voting → WP.]
Proposed resolution:
This wording is relative to N4885.
Modify 30.2 [time.syn], header <chrono>
synopsis, as indicated:
[…] namespace chrono { // 30.13 [time.parse], parsing template<class charT, class Parsable> unspecified parse(const charT* fmt, Parsable& tp); template<class charT, class traits, class Alloc, class Parsable> unspecified parse(const basic_string<charT, traits, Alloc>&formatfmt, Parsable& tp); template<class charT, class traits, class Alloc, class Parsable> unspecified parse(const charT* fmt, Parsable& tp, basic_string<charT, traits, Alloc>& abbrev); template<class charT, class traits, class Alloc, class Parsable> unspecified parse(const basic_string<charT, traits, Alloc>&formatfmt, Parsable& tp, basic_string<charT, traits, Alloc>& abbrev); template<class charT, class Parsable> unspecified parse(const charT* fmt, Parsable& tp, minutes& offset); template<class charT, class traits, class Alloc, class Parsable> unspecified parse(const basic_string<charT, traits, Alloc>&formatfmt, Parsable& tp, minutes& offset); template<class charT, class traits, class Alloc, class Parsable> unspecified parse(const charT* fmt, Parsable& tp, basic_string<charT, traits, Alloc>& abbrev, minutes& offset); template<class charT, class traits, class Alloc, class Parsable> unspecified parse(const basic_string<charT, traits, Alloc>&formatfmt, Parsable& tp, basic_string<charT, traits, Alloc>& abbrev, minutes& offset); […] } […]
Modify 30.13 [time.parse] as indicated:
-1- Each parse
overload specified in this subclause calls
from_stream
unqualified, so as to enable argument dependent lookup
(6.5.4 [basic.lookup.argdep]). In the following paragraphs,
let is
denote an object of type basic_istream<charT, traits>
and let I
be basic_istream<charT, traits>&
,
where charT
and traits
are template parameters in that context.
parse
immovable and preventing stream extraction into an lvalue of such a manipulator
type.
template<class charT, class Parsable> unspecified parse(const charT* fmt, Parsable& tp); template<class charT, class traits, class Alloc, class Parsable> unspecified parse(const basic_string<charT, traits, Alloc>& fmt, Parsable& tp);-?- Let F be
-2- Constraints: The expressionfmt
for the first overload andfmt.c_str()
for the second overload. Lettraits
bechar_traits<charT>
for the first overload.from_stream(declval<basic_istream<charT, traits>&>(),fmt.c_str()F, tp)is well-formed when treated as an unevaluated operand.
-3- Returns: A manipulator such that the expressionis >> parse(fmt, tp)
has typeI
, has valueis
, and callsfrom_stream(is,
.fmt.c_str()F, tp)template<class charT, class traits, class Alloc, class Parsable> unspecified parse(const charT* fmt, Parsable& tp, basic_string<charT, traits, Alloc>& abbrev); template<class charT, class traits, class Alloc, class Parsable> unspecified parse(const basic_string<charT, traits, Alloc>& fmt, Parsable& tp, basic_string<charT, traits, Alloc>& abbrev);-?- Let F be
-4- Constraints: The expressionfmt
for the first overload andfmt.c_str()
for the second overload.from_stream(declval<basic_istream<charT, traits>&>(),fmt.c_str()F, tp, addressof(abbrev))is well-formed when treated as an unevaluated operand.
-5- Returns: A manipulator such that the expressionis >> parse(fmt, tp, abbrev)
has typeI
, has valueis
, and callsfrom_stream(is,
.fmt.c_str()F, tp, addressof(abbrev))template<class charT, class Parsable> unspecified parse(const charT* fmt, Parsable& tp, minutes& offset); template<class charT, class traits, class Alloc, class Parsable> unspecified parse(const basic_string<charT, traits, Alloc>& fmt, Parsable& tp, minutes& offset);-?- Let F be
-6- Constraints: The expressionfmt
for the first overload andfmt.c_str()
for the second overload. Lettraits
bechar_traits<charT>
andAlloc
beallocator<charT>
for the first overload.from_stream(declval<basic_istream<charT, traits>&>(),fmt.c_str()F, tp, declval<basic_string<charT, traits, Alloc>*>(), &offset)is well-formed when treated as an unevaluated operand.
-7- Returns: A manipulator such that the expressionis >> parse(fmt, tp, offset)
has typeI
, has valueis
, and calls:from_stream(is,fmt.c_str()F, tp, static_cast<basic_string<charT, traits, Alloc>*>(nullptr), &offset)template<class charT, class traits, class Alloc, class Parsable> unspecified parse(const charT* fmt, Parsable& tp, basic_string<charT, traits, Alloc>& abbrev, minutes& offset); template<class charT, class traits, class Alloc, class Parsable> unspecified parse(const basic_string<charT, traits, Alloc>& fmt, Parsable& tp, basic_string<charT, traits, Alloc>& abbrev, minutes& offset);-?- Let F be
-8- Constraints: The expressionfmt
for the first overload andfmt.c_str()
for the second overload.from_stream(declval<basic_istream<charT, traits>&>(),fmt.c_str()F, tp, addressof(abbrev), &offset)is well-formed when treated as an unevaluated operand.
-9- Returns: A manipulator such that the expressionis >> parse(fmt, tp, abbrev, offset)
has typeI
, has valueis
, and callsfrom_stream(is,
.fmt.c_str()F, tp, addressof(abbrev), &offset)
{transform,elements}_view::iterator::iterator_concept
should consider const-qualification of the underlying rangeSection: 25.7.9.3 [range.transform.iterator], 25.7.23.3 [range.elements.iterator] Status: C++23 Submitter: Tim Song Opened: 2021-05-23 Last modified: 2023-11-22
Priority: Not Prioritized
View all other issues in [range.transform.iterator].
View all issues with C++23 status.
Discussion:
transform_view::iterator<true>::iterator_concept
and
elements_view::iterator<true>::iterator_concept
(i.e.,
the const versions) are currently specified as looking at the properties of V
(i.e., the underlying view without const), while the actual iterator operations
are all correctly specified as using Base
(which includes
the const). iterator_concept
should do so too.
[2021-05-26; Reflector poll]
Set status to Tentatively Ready after six votes in favour during reflector poll.
[2021-06-07 Approved at June 2021 virtual plenary. Status changed: Voting → WP.]
Proposed resolution:
This wording is relative to N4885.
Modify 25.7.9.3 [range.transform.iterator] as indicated:
-1-
iterator::iterator_concept
is defined as follows:
(1.1) — If
models
VBaserandom_access_range
, theniterator_concept
denotesrandom_access_iterator_tag
.(1.2) — Otherwise, if
models
VBasebidirectional_range
, theniterator_concept
denotesbidirectional_iterator_tag
.(1.3) — Otherwise, if
models
VBaseforward_range
, theniterator_concept
denotesforward_iterator_tag
.(1.4) — Otherwise,
iterator_concept
denotesinput_iterator_tag
.
Modify 25.7.23.3 [range.elements.iterator] as indicated:
-1- The member typedef-name
iterator_concept
is defined as follows:
(1.1) — If
models
VBaserandom_access_range
, theniterator_concept
denotesrandom_access_iterator_tag
.(1.2) — Otherwise, if
models
VBasebidirectional_range
, theniterator_concept
denotesbidirectional_iterator_tag
.(1.3) — Otherwise, if
models
VBaseforward_range
, theniterator_concept
denotesforward_iterator_tag
.(1.4) — Otherwise,
iterator_concept
denotesinput_iterator_tag
.
static_cast
expression in convertible_to
has the wrong operandSection: 18.4.4 [concept.convertible] Status: C++23 Submitter: Tim Song Opened: 2021-05-26 Last modified: 2023-11-22
Priority: Not Prioritized
View other active issues in [concept.convertible].
View all other issues in [concept.convertible].
View all issues with C++23 status.
Discussion:
The specification of convertible_to
implicitly requires static_cast<To>(f())
to be equality-preserving. Under 18.2 [concepts.equality] p1, the operand of this expression
is f
, but what we really want is for f()
to be treated as the operand. We should just
use declval
(which is treated specially by the definition of "operand" for this purpose) instead
of reinventing the wheel.
[2021-06-07; Reflector poll]
Set status to Tentatively Ready after nine votes in favour during reflector poll.
[2021-10-14 Approved at October 2021 virtual plenary. Status changed: Voting → WP.]
Proposed resolution:
This wording is relative to N4885.
Modify 18.4.4 [concept.convertible] as indicated:
template<class From, class To> concept convertible_to = is_convertible_v<From, To> && requires(add_rvalue_reference_t<From> (&f)()){ static_cast<To>(fdeclval<From>()); };
sized_range
is circularSection: 25.4.3 [range.sized] Status: C++23 Submitter: Tim Song Opened: 2021-06-03 Last modified: 2023-11-22
Priority: Not Prioritized
View all other issues in [range.sized].
View all issues with C++23 status.
Discussion:
25.4.3 [range.sized] p2.1 requires that for a sized_range t
, ranges::size(t)
is equal to ranges::distance(t)
. But for a sized_range
, ranges::distance(t)
is simply ranges::size(t)
cast to the difference type.
distance(begin, end)
— the actual distance you get
from traversing the range — is equal to what ranges::size
produces, and not merely that
casting from the size type to difference type is value-preserving. Otherwise, sized_range
would
be more or less meaningless.
[2021-06-14; Reflector poll]
Set status to Tentatively Ready after seven votes in favour during reflector poll.
[2021-10-14 Approved at October 2021 virtual plenary. Status changed: Voting → WP.]
Proposed resolution:
This wording is relative to N4885.
Modify 25.4.3 [range.sized] as indicated:
template<class T> concept sized_range = range<T> && requires(T& t) { ranges::size(t); };-2- Given an lvalue
t
of typeremove_reference_t<T>
,T
modelssized_range
only if
(2.1) —
ranges::size(t)
is amortized 𝒪(1), does not modifyt
, and is equal toranges::distance(ranges::begin(t), ranges::end(t)
, andt)(2.2) — if
iterator_t<T>
modelsforward_iterator
,ranges::size(t)
is well-defined regardless of the evaluation ofranges::begin(t)
.
ranges::equal
and ranges::is_permutation
should short-circuit for sized_ranges
Section: 26.6.13 [alg.equal], 26.6.14 [alg.is.permutation] Status: C++23 Submitter: Tim Song Opened: 2021-06-03 Last modified: 2023-11-22
Priority: Not Prioritized
View all other issues in [alg.equal].
View all issues with C++23 status.
Discussion:
ranges::equal
and ranges::is_permutation
are currently only required
to short-circuit on different sizes if the iterator and sentinel of the two ranges
pairwise model sized_sentinel_for
. They should also short-circuit if the ranges are sized.
[2021-06-23; Reflector poll]
Set status to Tentatively Ready after five votes in favour during reflector poll.
[2021-10-14 Approved at October 2021 virtual plenary. Status changed: Voting → WP.]
Proposed resolution:
This wording is relative to N4885.
Modify 26.6.13 [alg.equal] as indicated:
template<class InputIterator1, class InputIterator2> constexpr bool equal(InputIterator1 first1, InputIterator1 last1, InputIterator2 first2); […] template<class InputIterator1, class InputIterator2, class BinaryPredicate> constexpr bool equal(InputIterator1 first1, InputIterator1 last1, InputIterator2 first2, InputIterator2 last2, BinaryPredicate pred); […] template<input_iterator I1, sentinel_for<I1> S1, input_iterator I2, sentinel_for<I2> S2, class Pred = ranges::equal_to, class Proj1 = identity, class Proj2 = identity> requires indirectly_comparable<I1, I2, Pred, Proj1, Proj2> constexpr bool ranges::equal(I1 first1, S1 last1, I2 first2, S2 last2, Pred pred = {}, Proj1 proj1 = {}, Proj2 proj2 = {}); template<input_range R1, input_range R2, class Pred = ranges::equal_to, class Proj1 = identity, class Proj2 = identity> requires indirectly_comparable<iterator_t<R1>, iterator_t<R2>, Pred, Proj1, Proj2> constexpr bool ranges::equal(R1&& r1, R2&& r2, Pred pred = {}, Proj1 proj1 = {}, Proj2 proj2 = {});[…]
-3- Complexity: Ifthe types of:first1
,last1
,first2
, andlast2
(3.1) — the types of
first1
,last1
,first2
, andlast2
meet the Cpp17RandomAccessIterator requirements (24.3.5.7 [random.access.iterators]) andlast1 - first1 != last2 - first2
for the overloads in namespacestd
;(3.2) — the types of
first1
,last1
,first2
, andlast2
pairwise modelsized_sentinel_for
(24.3.4.8 [iterator.concept.sizedsentinel]) andlast1 - first1 != last2 - first2
for the first overloadsin namespaceranges
,(3.3) —
R1
andR2
each modelsized_range
andranges::distance(r1) != ranges::distance(r2)
for the second overload in namespaceranges
,
andthen no applications of the corresponding predicate and each projection; otherwise, […]last1 - first1 != last2 - first2
,
Modify 26.6.14 [alg.is.permutation] as indicated:
template<forward_iterator I1, sentinel_for<I1> S1, forward_iterator I2, sentinel_for<I2> S2, class Proj1 = identity, class Proj2 = identity, indirect_equivalence_relation<projected<I1, Proj1>, projected<I2, Proj2>> Pred = ranges::equal_to> constexpr bool ranges::is_permutation(I1 first1, S1 last1, I2 first2, S2 last2, Pred pred = {}, Proj1 proj1 = {}, Proj2 proj2 = {}); template<forward_range R1, forward_range R2, class Proj1 = identity, class Proj2 = identity, indirect_equivalence_relation<projected<iterator_t<R1>, Proj1>, projected<iterator_t<R2>, Proj2>> Pred = ranges::equal_to> constexpr bool ranges::is_permutation(R1&& r1, R2&& r2, Pred pred = {}, Proj1 proj1 = {}, Proj2 proj2 = {});[…]
-7- Complexity: No applications of the corresponding predicate and projections if:
(7.1) — for the first overload,
(7.1.1) —
S1
andI1
modelsized_sentinel_for<S1, I1>
,(7.1.2) —
S2
andI2
modelsized_sentinel_for<S2, I2>
, and(7.1.3) —
last1 - first1 != last2 - first2
.;(7.2) — for the second overload,
R1
andR2
each modelsized_range
, andranges::distance(r1) != ranges::distance(r2)
.Otherwise, exactly
last1 - first1
applications of the corresponding predicate and projections ifranges::equal(first1, last1, first2, last2, pred, proj1, proj2)
would returntrue
; otherwise, at worst 𝒪(N2
), whereN
has the valuelast1 - first1
.
discard_block_engine
Section: 29.5.5.2 [rand.adapt.disc] Status: C++23 Submitter: Ilya Burylov Opened: 2021-06-03 Last modified: 2023-11-22
Priority: Not Prioritized
View all other issues in [rand.adapt.disc].
View all issues with C++23 status.
Discussion:
A discard_block_engine
engine adaptor is described in 29.5.5.2 [rand.adapt.disc]. It produces
random numbers from the underlying engine discarding (p - r)
last values of p - size
block,
where r
and p
are template parameters of std::size_t
type:
template<class Engine, size_t p, size_t r> class discard_block_engine;
The transition algorithm for discard_block_engine
is described as follows (paragraph 2):
The transition algorithm discards all but
r > 0
values from each block ofp = r
values delivered byℯ
. The state transition is performed as follows: Ifn = r
, advance the state ofe
fromei
toei+p-r
and setn
to0
. In any case, then incrementn
and advancee
's then-current stateej
toej+1
.
Where n
is of integer type. In the API of discard block engine, n
is represented
in the following way:
[…] int n; // exposition only
In cases where int
is equal to int32_t
, overflow is possible for n
that leads
to undefined behavior. Such situation can happen when the p
and r
template parameters exceed
INT_MAX
.
GNU Libstdc++ uses size_t
for n
— in this case no overflow happened even if template
parameters exceed INT_MAX
LLVM Libcxx uses int
for n
together with a static_assert
that is checking that
p
and r
values are <= INT_MAX
Such difference in implementation makes code not portable and may potentially breaks random number sequence consistency between different implementors of C++ std lib.
The problematic use case is the following one:#include <iostream> #include <random> #include <limits> int main() { std::minstd_rand0 generator; constexpr std::size_t skipped_size = static_cast<std::size_t>(std::numeric_limits<int>::max()); constexpr std::size_t block_size = skipped_size + 5; constexpr std::size_t used_block = skipped_size; std::cout << "block_size = " << block_size << std::endl; std::cout << "used_block = " << used_block << "\n" << std::endl; std::discard_block_engine<std::minstd_rand0, block_size, used_block> discard_generator; // Call discard procedures discard_generator.discard(used_block); generator.discard(block_size); // Call generation. Results should be equal for (std::int32_t i = 0; i < 10; ++i) { std::cout << discard_generator() << " should be equal " << generator() << std::endl; } }
We see no solid reason for n
to be an int
, given that the relevant template parameters are
std::size_t
. It seems like a perfectly fine use case to generate random numbers in amounts larger than
INT_MAX
.
std::size_t
:
size_t n; // exposition only
It will not mandate the existing libraries to break ABI, but at least guide for better implementation.
[2021-06-14; Reflector poll]
Set status to Tentatively Ready after six votes in favour during reflector poll.
[2021-10-14 Approved at October 2021 virtual plenary. Status changed: Voting → WP.]
Proposed resolution:
This wording is relative to N4885.
Modify 29.5.5.2 [rand.adapt.disc], class template discard_block_engine
synopsis, as indicated:
[…] // generating functions result_type operator()(); void discard(unsigned long long z); // property functions const Engine& base() const noexcept { return e; }; private: Engine e; // exposition onlyintsize_t n; // exposition only };
keys_view
example is brokenSection: 25.7.23.1 [range.elements.overview] Status: C++23 Submitter: Barry Revzin Opened: 2021-05-29 Last modified: 2023-11-22
Priority: 3
View all issues with C++23 status.
Discussion:
In 25.7.23.1 [range.elements.overview] we have:
[Example 2:
keys_view
is an alias forelements_view<views::all_t<R>, 0>
, and is useful for extracting keys from associative containers.auto names = keys_view{historical_figures}; for (auto&& name : names) { cout << name << ' '; // prints Babbage Hamilton Lovelace Turing }— end example]
Where keys_view
is defined as:
template<class R> using keys_view = elements_view<views::all_t<R>, 0>;
There's a similar example for values_view
.
keys_view(r)
cannot deduce R
—
it's a non-deduced context. At the very least, the example is wrong and should be rewritten as
views::keys(historical_figures)
. Or, I guess, as
keys_view<decltype(historical_figures)>(historical_figures)>
(but really, not).
[2021-05-29 Tim comments and provides wording]
I have some ideas about making CTAD work for keys_view
and
values_view
, but current implementations of alias template CTAD are
not yet in a shape to permit those ideas to be tested.
What is clear, though, is that the current definitions of keys_view
and values_view
can't possibly ever work for CTAD, because R
is only
used in a non-deduced context and so they can never meet the "deducible" constraint in
12.2.2.9 [over.match.class.deduct] p2.3.
views::all_t
transformation
in those alias templates. This makes these aliases transparent enough to support CTAD out
of the box when a view is used, and any questions about deduction guides on
elements_view
can be addressed later once the implementations become more mature.
This also makes them consistent with all the other views: you can't write
elements_view<map<int, int>&, 0>
or
reverse_view<map<int, int>&>
, so why do we allow
keys_view<map<int, int>&>
? It is, however, a breaking change,
so it is important that we get this in sooner rather than later.
The proposed resolution has been implemented and tested.
[2021-06-14; Reflector poll]
Set priority to 3 after reflector poll in August. Partially addressed by an editorial change.
Previous resolution [SUPERSEDED]:
This wording is relative to N4885.
Modify 25.2 [ranges.syn], header
<ranges>
synopsis, as indicated:[…] namespace std::ranges { […] // 25.7.23 [range.elements], elements view template<input_range V, size_t N> requires see below class elements_view; template<class T, size_t N> inline constexpr bool enable_borrowed_range<elements_view<T, N>> = enable_borrowed_range<T>; template<class R> using keys_view = elements_view<views::all_t<R>, 0>; template<class R> using values_view = elements_view<views::all_t<R>, 1>; […] }Modify 25.7.23.1 [range.elements.overview] as indicated:
-3-
[Example 2:keys_view
is an alias forelements_view<
, and is useful for extracting keys from associative containers.views::all_t<R>, 0>auto names = keys_view{views::all(historical_figures)}; for (auto&& name : names) { cout << name << ' '; // prints Babbage Hamilton Lovelace Turing }— end example]
-4-values_view
is an alias forelements_view<
, and is useful for extracting values from associative containers. [Example 3:views::all_t<R>, 1>auto is_even = [](const auto x) { return x % 2 == 0; }; cout << ranges::count_if(values_view{views::all(historical_figures)}, is_even); // prints 2— end example]
[2021-06-18 Tim syncs wording to latest working draft]
The examples have been editorially corrected, so the new wording simply changes the
definition of keys_view
and values_view
.
[2021-09-20; Reflector poll]
Set status to Tentatively Ready after six votes in favour during reflector poll.
[2021-10-14 Approved at October 2021 virtual plenary. Status changed: Voting → WP.]
Proposed resolution:
This wording is relative to N4892.
Modify 25.2 [ranges.syn], header <ranges>
synopsis, as indicated:
[…] namespace std::ranges { […] // 25.7.23 [range.elements], elements view template<input_range V, size_t N> requires see below class elements_view; template<class T, size_t N> inline constexpr bool enable_borrowed_range<elements_view<T, N>> = enable_borrowed_range<T>; template<class R> using keys_view = elements_view<views::all_t<R>, 0>; template<class R> using values_view = elements_view<views::all_t<R>, 1>; […] }
Modify 25.7.23.1 [range.elements.overview] as indicated:
-3-
[Example 2: … — end example] -4-keys_view
is an alias forelements_view<
, and is useful for extracting keys from associative containers.views::all_t<R>, 0>values_view
is an alias forelements_view<
, and is useful for extracting values from associative containers. [Example 3: … — end example]views::all_t<R>, 1>
transform_view::iterator<true>::value_type
and iterator_category
should
use const F&
Section: 25.7.9.3 [range.transform.iterator] Status: C++23 Submitter: Tim Song Opened: 2021-06-06 Last modified: 2023-11-22
Priority: 2
View all other issues in [range.transform.iterator].
View all issues with C++23 status.
Discussion:
Iterators obtained from a const transform_view
invoke the
transformation function as const
, but the value_type
and
iterator_category
determination uses plain F&
, i.e., non-const
.
[2021-06-14; Reflector poll]
Set priority to 2 after reflector poll, send to SG9 for design clarification.
Should r
and as_const(r)
guarantee same elements?
[2022-07-08; Reflector poll]
SG9 has decided to proceed with this PR in its 2021-09-13 telecon and not block the issue for a paper on the more general const/non-const problem.
Set status to Tentatively Ready after five votes in favour during reflector poll.
[2022-07-15; LWG telecon: move to Ready]
[2022-07-25 Approved at July 2022 virtual plenary. Status changed: Ready → WP.]
Proposed resolution:
This wording is relative to N4885.
Modify 25.7.9.3 [range.transform.iterator] as indicated:
namespace std::ranges { template<input_range V, copy_constructible F> requires view<V> && is_object_v<F> && regular_invocable<F&, range_reference_t<V>> && can-reference<invoke_result_t<F&, range_reference_t<V>>> template<bool Const> class transform_view<V, F>::iterator { private: […] public: using iterator_concept = see below; using iterator_category = see below; // not always present using value_type = remove_cvref_t<invoke_result_t<maybe-const<Const, F>&, range_reference_t<Base>>>; using difference_type = range_difference_t<Base>; […] }; }-1- […]
-2- The member typedef-nameiterator_category
is defined if and only ifBase
modelsforward_range
. In that case,iterator::iterator_category
is defined as follows: LetC
denote the typeiterator_traits<iterator_t<Base>>::iterator_category
.
(2.1) — If
is_lvalue_reference_v<invoke_result_t<maybe-const<Const, F>&, range_reference_t<Base>>>
istrue
, then
(2.1.1) — […]
(2.1.2) — […]
(2.2) — Otherwise,
iterator_category
denotesinput_iterator_tag
.
chrono
types is underspecifiedSection: 30.12 [time.format] Status: Resolved Submitter: Victor Zverovich Opened: 2021-05-31 Last modified: 2023-03-23
Priority: 2
View other active issues in [time.format].
View all other issues in [time.format].
View all issues with Resolved status.
Discussion:
When formatting chrono types using a locale the result is underspecified, possibly a mix of the literal and locale encodings. For example:
std::locale::global(std::locale("Russian.1251")); auto s = std::format("День недели: {}", std::chrono::Monday);
(Note that "{}
" should be replaced with "{:L}
" if P2372 is adopted but that's non-essential.)
"Russian.1251"
locale exists we have a mismatch between encodings.
As far as I can see the standard doesn't specify what happens in this case.
One possible and undesirable result is
"День недели: \xcf\xed"
where "\xcf\xed"
is "Пн"
(Mon in Russian) in CP1251 and is not valid UTF-8.
"День недели: Пн"
where everything is in one encoding (UTF-8).
This issue is not resolved by LWG 3547(i) / P2372 but the resolution proposed here is compatible with P2372 and can be rebased onto its wording if the paper is adopted.[2021-06-14; Reflector poll]
Set priority to 2 after reflector poll. Send to SG16.
Previous resolution [SUPERSEDED]:
This wording is relative to N4885.
Modify 30.12 [time.format] as indicated:
-2- Each conversion specifier conversion-spec is replaced by appropriate characters as described in Table [tab:time.format.spec]; the formats specified in ISO 8601:2004 shall be used where so described. Some of the conversion specifiers depend on the locale that is passed to the formatting function if the latter takes one, or the global locale otherwise. If the string literal encoding is UTF-8 the replacement of a conversion specifier that depends on the locale is transcoded to UTF-8 for narrow strings, otherwise the replacement is taken as is. If the formatted object does not contain the information the conversion specifier refers to, an exception of type
format_error
is thrown.
[2023-03-22 Resolved by the adoption of P2419R2 in the July 2022 virtual plenary. Status changed: SG16 → Resolved.]
Proposed resolution:
operator<=>(optional<T>, U)
Section: 22.5.8 [optional.comp.with.t] Status: C++23 Submitter: Casey Carter Opened: 2021-06-07 Last modified: 2023-11-22
Priority: Not Prioritized
View all other issues in [optional.comp.with.t].
View all issues with C++23 status.
Discussion:
The three-way-comparison operator for optionals and non-optionals is defined in [optional.comp.with.t] paragraph 25:
template<class T, three_way_comparable_with<T> U> constexpr compare_three_way_result_t<T, U> operator<=>(const optional<T>& x, const U& v);-25- Effects: Equivalent to:
return bool(x) ? *x <=> v : strong_ordering::less;
Checking three_way_comparable_with
in particular requires checking that x < v
is well-formed
for an lvalue const optional<T>
and lvalue const U
. x < v
can be rewritten as
(v <=> x) > 0
. If U
is a specialization of optional
, this overload could be used for
v <=> x
, but first we must check the constraints…
The straightforward fix for this recursion seems to be to refuse to check three_way_comparable_with<T>
when U
is a specialization of optional
. MSVC has been shipping such a fix; our initial tests for
this new operator triggered the recursion.
Previous resolution [SUPERSEDED]:
This wording is relative to N4885.
Modify 22.5.2 [optional.syn], header
<optional>
synopsis, as indicated:[…] // 22.5.8 [optional.comp.with.t], comparison with T […] template<class T, class U> constexpr bool operator>=(const optional<T>&, const U&); template<class T, class U> constexpr bool operator>=(const T&, const optional<U>&); template<class T,three_way_comparable_with<T>class U> requires see below constexpr compare_three_way_result_t<T, U> operator<=>(const optional<T>&, const U&); […]Modify 22.5.8 [optional.comp.with.t] as indicated:
template<class T,three_way_comparable_with<T>class U> requires (!is-specialization-of<U, optional>) && three_way_comparable_with<T, U> constexpr compare_three_way_result_t<T, U> operator<=>(const optional<T>&, const U&);-?- The exposition-only trait template
-25- Effects: Equivalent to:is-specialization-of<A, B>
is a constant expression with valuetrue
whenA
denotes a specialization of the class templateB
, andfalse
otherwise.return bool(x) ? *x <=> v : strong_ordering::less;
[2021-06-14; Improved proposed wording based on reflector discussion]
[2021-06-23; Reflector poll]
Set status to Tentatively Ready after five votes in favour during reflector poll.
[2021-10-14 Approved at October 2021 virtual plenary. Status changed: Voting → WP.]
Proposed resolution:
This wording is relative to N4885.
Modify 22.5.2 [optional.syn], header <optional>
synopsis, as indicated:
[…] // 22.5.3 [optional.optional], class template optional template<class T> class optional; template<class T> constexpr bool is-optional = false; // exposition only template<class T> constexpr bool is-optional<optional<T>> = true; // exposition only […] // 22.5.8 [optional.comp.with.t], comparison with T […] template<class T, class U> constexpr bool operator>=(const optional<T>&, const U&); template<class T, class U> constexpr bool operator>=(const T&, const optional<U>&); template<class T,three_way_comparable_with<T>class U> requires (!is-optional<U>) && three_way_comparable_with<T, U> constexpr compare_three_way_result_t<T, U> operator<=>(const optional<T>&, const U&); […]
Modify 22.5.8 [optional.comp.with.t] as indicated:
template<class T,three_way_comparable_with<T>class U> requires (!is-optional<U>) && three_way_comparable_with<T, U> constexpr compare_three_way_result_t<T, U> operator<=>(const optional<T>& x, const U& v);-25- Effects: Equivalent to:
return bool(x) ? *x <=> v : strong_ordering::less;
Section: 28.5.6.7 [format.context] Status: C++23 Submitter: Casey Carter Opened: 2021-06-08 Last modified: 2023-11-22
Priority: Not Prioritized
View all other issues in [format.context].
View all issues with C++23 status.
Discussion:
LWG 3539(i) fixed copies of potentially-move-only iterators in the format_to
and
vformat_to
overloads, but missed the fact that member functions of basic_format_context
are specified to copy iterators as well. In particular, 28.5.6.7 [format.context] states:
iterator out();-7- Returns:
out_
.void advance_to(iterator it);-8- Effects: Equivalent to:
out_ = it;
both of which appear to require copyability.
[2021-06-23; Reflector poll]
Set status to Tentatively Ready after six votes in favour during reflector poll.
[2021-10-14 Approved at October 2021 virtual plenary. Status changed: Voting → WP.]
Proposed resolution:
This wording is relative to N4885.
Modify 28.5.6.7 [format.context] as indicated:
iterator out();-7-
Returns:Effects: Equivalent to:out_
.return std::move(out_);
void advance_to(iterator it);-8- Effects: Equivalent to:
out_ = std::move(it);
basic_istream_view
needs to initialize value_
Section: 25.6.6.2 [range.istream.view] Status: C++23 Submitter: Casey Carter Opened: 2021-06-15 Last modified: 2023-11-22
Priority: Not Prioritized
View all issues with C++23 status.
Discussion:
P2325R3 removes the default member initializers for basic_istream_view
's exposition-only
stream_
and value_
members. Consequently, copying a basic_istream_view
before the first call to begin
may produce undefined behavior since doing so necessarily copies the
uninitialized value_
. We should restore value_
's default member initializer and
remove this particular footgun.
[2021-06-23; Reflector poll]
Set status to Tentatively Ready after seven votes in favour during reflector poll.
[2021-10-14 Approved at October 2021 virtual plenary. Status changed: Voting → WP.]
Proposed resolution:
Wording relative to the post-202106 working draft (as near as possible). This PR is currently being implemented in MSVC.
Modify 25.6.6.2 [range.istream.view] as indicated:
namespace std::ranges { […] template<movable Val, class CharT, class Traits> requires default_initializable<Val> && stream-extractable<Val, CharT, Traits> class basic_istream_view : public view_interface<basic_istream_view<Val, CharT, Traits>> { […] Val value_ = Val(); // exposition only }; }
join_view
fails to support ranges of ranges with non-default_initializable
iteratorsSection: 25.7.14.3 [range.join.iterator] Status: C++23 Submitter: Casey Carter Opened: 2021-06-16 Last modified: 2023-11-22
Priority: 3
View all other issues in [range.join.iterator].
View all issues with C++23 status.
Discussion:
join_view::iterator
has exposition-only members outer_
— which holds an
iterator into the adapted range — and inner_
— which holds an iterator into the
range denoted by outer_
. After application of P2325R3 "Views should not be
required to be default constructible" to the working draft, single-pass iterators can be
non-default_initializable
. P2325R3 constrains join_view::iterator
's default constructor
to require that the types of both outer_
and inner_
are default_initializable
,
indicating an intent to support such iterator types. However, the effect of the non-default constructor specified
in 25.7.14.3 [range.join.iterator] paragraph 6 is to default-initialize inner_
, which is
ill-formed if its type is not default_initializable
.
[2021-06-23; Reflector poll]
Set priority to 3 after reflector poll.
Previous resolution [SUPERSEDED]:
Wording relative to the post 2021-06 virtual plenary working draft. This PR is currently being implemented in MSVC.
Modify 25.7.14.3 [range.join.iterator] as indicated:
[…]namespace std::ranges { template<input_range V> requires view<V> && input_range<range_reference_t<V>> && (is_reference_v<range_reference_t<V>> || view<range_value_t<V>>) template<bool Const> struct join_view<V>::iterator { […] optional<InnerIter> inner_= InnerIter(); […] constexpr decltype(auto) operator*() const { return **inner_; } […] friend constexpr decltype(auto) iter_move(const iterator& i) noexcept(noexcept(ranges::iter_move(*i.inner_))) { return ranges::iter_move(*i.inner_); } friend constexpr void iter_swap(const iterator& x, const iterator& y) noexcept(noexcept(ranges::iter_swap(*x.inner_, *y.inner_))) requires indirectly_swappable<InnerIter>; }; }constexpr void satisfy(); // exposition only[…]-5- Effects: Equivalent to:
auto update_inner = [this](const iterator_t<Base>& x) -> auto&& { […] }; for (; outer_ != ranges::end(parent_->base_); ++outer_) { auto&& inner = update_inner(*outer_); inner_ = ranges::begin(inner); if (*inner_ != ranges::end(inner)) return; } if constexpr (ref-is-glvalue) inner_.reset()= InnerIter();constexpr InnerIter operator->() const requires has-arrow<InnerIter> && copyable<InnerIter>;-8- Effects: Equivalent to:
return *inner_;
constexpr iterator& operator++();[…]-9- Let inner-range be:
[…] -10- Effects: Equivalent to:auto&& inner_rng = inner-range; if (++*inner_ == ranges::end(inner_rng)) { ++outer_; satisfy(); } return *this;constexpr iterator& operator--() requires ref-is-glvalue && bidirectional_range<Base> && bidirectional_range<range_reference_t<Base>> && common_range<range_reference_t<Base>>;[…]-13- Effects: Equivalent to:
if (outer_ == ranges::end(parent_->base_)) inner_ = ranges::end(*--outer_); while (*inner_ == ranges::begin(*outer_)) *inner_ = ranges::end(*--outer_); --*inner_; return *this;friend constexpr void iter_swap(const iterator& x, const iterator& y) noexcept(noexcept(ranges::iter_swap(*x.inner_, *y.inner_))) requires indirectly_swappable<InnerIter>;-16- Effects: Equivalent to: return
ranges::iter_swap(*x.inner_, *y.inner_);
[2021-08-23; Louis Dionne comments and provides improved wording]
I believe the currently proposed resolution is missing the removal of the default_initializable<InnerIter>
constraint on join_view::iterator
's default constructor in 25.7.14.3 [range.join.iterator]. Indeed,
after the currently-proposed resolution, join_view::iterator
reads like:
template<input_range V> requires […] struct join_view<V>::iterator { private: optional<InnerIter> inner_; // exposition only […] public: iterator() requires default_initializable<OuterIter> && default_initializable<InnerIter> = default; […] };
I believe we should drop the default_initializable<InnerIter>
constraint from the default constructor
(that seems like an oversight unless I missed something):
template<input_range V> requires […] struct join_view<V>::iterator { private: optional<InnerIter> inner_; // exposition only […] public: iterator() requires default_initializable<OuterIter> = default; […] };
[Kona 2022-11-08; Accepted at joint LWG/SG9 session. Move to Immediate]
[2022-11-12 Approved at November 2022 meeting in Kona. Status changed: Immediate → WP.]
Proposed resolution:
This wording is relative to N4892.
Modify 25.7.14.3 [range.join.iterator] as indicated:
[…]namespace std::ranges { template<input_range V> requires view<V> && input_range<range_reference_t<V>> && (is_reference_v<range_reference_t<V>> || view<range_value_t<V>>) template<bool Const> struct join_view<V>::iterator { […] optional<InnerIter> inner_= InnerIter(); […] iterator() requires default_initializable<OuterIter>&& default_initializable<InnerIter>= default; […] constexpr decltype(auto) operator*() const { return **inner_; } […] friend constexpr decltype(auto) iter_move(const iterator& i) noexcept(noexcept(ranges::iter_move(*i.inner_))) { return ranges::iter_move(*i.inner_); } friend constexpr void iter_swap(const iterator& x, const iterator& y) noexcept(noexcept(ranges::iter_swap(*x.inner_, *y.inner_))) requires indirectly_swappable<InnerIter>; }; }constexpr void satisfy(); // exposition only[…]-5- Effects: Equivalent to:
auto update_inner = [this](const iterator_t<Base>& x) -> auto&& { […] }; for (; outer_ != ranges::end(parent_->base_); ++outer_) { auto&& inner = update_inner(*outer_); inner_ = ranges::begin(inner); if (*inner_ != ranges::end(inner)) return; } if constexpr (ref-is-glvalue) inner_.reset()= InnerIter();constexpr InnerIter operator->() const requires has-arrow<InnerIter> && copyable<InnerIter>;-8- Effects: Equivalent to:
return *inner_;
constexpr iterator& operator++();[…]-9- Let inner-range be:
[…] -10- Effects: Equivalent to:auto&& inner_rng = inner-range; if (++*inner_ == ranges::end(inner_rng)) { ++outer_; satisfy(); } return *this;constexpr iterator& operator--() requires ref-is-glvalue && bidirectional_range<Base> && bidirectional_range<range_reference_t<Base>> && common_range<range_reference_t<Base>>;[…]-13- Effects: Equivalent to:
if (outer_ == ranges::end(parent_->base_)) inner_ = ranges::end(*--outer_); while (*inner_ == ranges::begin(*outer_)) *inner_ = ranges::end(*--outer_); --*inner_; return *this;friend constexpr void iter_swap(const iterator& x, const iterator& y) noexcept(noexcept(ranges::iter_swap(*x.inner_, *y.inner_))) requires indirectly_swappable<InnerIter>;-16- Effects: Equivalent to: return
ranges::iter_swap(*x.inner_, *y.inner_);
basic_osyncstream::emit
should be an unformatted output functionSection: 31.11.3.3 [syncstream.osyncstream.members] Status: C++23 Submitter: Tim Song Opened: 2021-06-19 Last modified: 2023-11-22
Priority: Not Prioritized
View all issues with C++23 status.
Discussion:
basic_osyncstream::emit
seems rather similar to basic_ostream::flush
—
both are "flushing" operations that forward to member functions of the
streambuf
and set badbit
if those functions fail. But the former isn't
currently specified as an unformatted output function, so it a) doesn't construct or check a
sentry
and b) doesn't set badbit
if emit
exits via an exception. At
least the latter appears to be clearly desirable — a streambuf
operation that throws
ordinarily results in badbit
being set.
sentry
and only
calls emit
on the streambuf
if the sentry
converts to true
,
so this seems to be an accidental omission in the wording.
[2021-06-23; Reflector poll]
Set status to Tentatively Ready after five votes in favour during reflector poll.
[2021-10-14 Approved at October 2021 virtual plenary. Status changed: Voting → WP.]
Proposed resolution:
This wording is relative to N4885.
Modify 31.11.3.3 [syncstream.osyncstream.members] as indicated:
void emit();-1- Effects: Behaves as an unformatted output function (31.7.6.4 [ostream.unformatted]). After constructing a
sentry
object, cCallssb.emit()
. If that call returnsfalse
, callssetstate(ios_base::badbit)
.
flush_emit
should set badbit
if the emit
call failsSection: 31.7.6.5 [ostream.manip] Status: C++23 Submitter: Tim Song Opened: 2021-06-19 Last modified: 2023-11-22
Priority: Not Prioritized
View all other issues in [ostream.manip].
View all issues with C++23 status.
Discussion:
basic_osyncstream::emit
is specified to set badbit
if it fails
(31.11.3.3 [syncstream.osyncstream.members] p1), but the emit
part of the
flush_emit
manipulator isn't, even though the flush
part does set
badbit
if it fails.
osyncstream s
, s << flush_emit;
should
probably have the same behavior as s.flush(); s.emit()
.
The reference implementation linked in P0753R2 does set badbit
on
failure, so at least this part appears to be an oversight. As
discussed in LWG 3570(i), basic_osyncstream::emit
should probably be an
unformatted output function, so the emit
part of flush_emit
should do so too.
[2021-06-23; Reflector poll]
Set status to Tentatively Ready after six votes in favour during reflector poll.
[2021-10-14 Approved at October 2021 virtual plenary. Status changed: Voting → WP.]
Proposed resolution:
This wording is relative to N4885.
Modify 31.7.6.5 [ostream.manip] as indicated:
template<class charT, class traits> basic_ostream<charT, traits>& flush_emit(basic_ostream<charT, traits>& os);-12- Effects: Calls
os.flush()
. Then, ifos.rdbuf()
is abasic_syncbuf<charT, traits, Allocator>*
, calledbuf
for the purpose of exposition, behaves as an unformatted output function (31.7.6.4 [ostream.unformatted]) ofos
. After constructing asentry
object, callsbuf->emit()
. If that call returnsfalse
, callsos.setstate(ios_base::badbit)
.
copyable-box
should be fully constexpr
Section: 25.7.3 [range.move.wrap] Status: C++23 Submitter: Tim Song Opened: 2021-06-19 Last modified: 2023-11-22
Priority: Not Prioritized
View all other issues in [range.move.wrap].
View all issues with C++23 status.
Discussion:
P2231R1 made optional
fully constexpr
, but missed its cousin
semiregular-box
(which was renamed to copyable-box
by
P2325R3). Most operations of copyable-box
are already
constexpr
simply because copyable-box
is specified in terms of optional
;
the only missing ones are the assignment operators layered on top when the wrapped type
isn't assignable itself.
[2021-06-23; Reflector poll]
Set status to Tentatively Ready after eight votes in favour during reflector poll.
[2021-10-14 Approved at October 2021 virtual plenary. Status changed: Voting → WP.]
Proposed resolution:
This wording is relative to N4892.
Modify [range.copy.wrap] as indicated:
-1- Many types in this subclause are specified in terms of an exposition-only class template
copyable-box
.copyable-box<T>
behaves exactly likeoptional<T>
with the following differences:
(1.1) — […]
(1.2) — […]
(1.3) — If
copyable<T>
is not modeled, the copy assignment operator is equivalent to:constexpr copyable-box& operator=(const copyable-box& that) noexcept(is_nothrow_copy_constructible_v<T>) { if (this != addressof(that)) { if (that) emplace(*that); else reset(); } return *this; }(1.4) — If
movable<T>
is not modeled, the move assignment operator is equivalent to:constexpr copyable-box& operator=(copyable-box&& that) noexcept(is_nothrow_move_constructible_v<T>) { if (this != addressof(that)) { if (that) emplace(std::move(*that)); else reset(); } return *this; }
basic_string_view(It begin, End end)
Section: 27.3.3.2 [string.view.cons] Status: C++23 Submitter: Hewill Kang Opened: 2021-07-13 Last modified: 2023-11-22
Priority: Not Prioritized
View all other issues in [string.view.cons].
View all issues with C++23 status.
Discussion:
The standard does not specify the exceptions of this constructor, but since std::to_address
is a
noexcept
function, this constructor throws if and only when end - begin
throws, we should
add a Throws element for it.
[2021-08-20; Reflector poll]
Set status to Tentatively Ready after eight votes in favour during reflector poll.
[2021-10-14 Approved at October 2021 virtual plenary. Status changed: Voting → WP.]
Proposed resolution:
This wording is relative to N4892.
Modify 27.3.3.2 [string.view.cons] as indicated:
template<class It, class End> constexpr basic_string_view(It begin, End end);-7- Constraints:
[…] -8- Preconditions: […] -9- Effects: Initializesdata_
withto_address(begin)
and initializessize_
withend - begin
. -?- Throws: When and whatend - begin
throws.
common_iterator
should be completely constexpr
-ableSection: 24.5.5.1 [common.iterator] Status: C++23 Submitter: Hewill Kang Opened: 2021-07-21 Last modified: 2023-11-22
Priority: 3
View other active issues in [common.iterator].
View all other issues in [common.iterator].
View all issues with C++23 status.
Discussion:
After P2231R1, variant
becomes completely constexpr
-able, which means that
common_iterator
can also be completely constexpr
-able.
[2021-08-20; Reflector poll]
Set priority to 3 after reflector poll.
[2021-08-23; Reflector poll]
Set status to Tentatively Ready after five votes in favour during reflector poll.
[2021-10-14 Approved at October 2021 virtual plenary. Status changed: Voting → WP.]
Proposed resolution:
This wording is relative to N4892.
Modify 24.5.5.1 [common.iterator], class template common_iterator
synopsis, as indicated:
namespace std { template<input_or_output_iterator I, sentinel_for<I> S> requires (!same_as<I, S> && copyable<I>) class common_iterator { public: constexpr common_iterator() requires default_initializable<I> = default; constexpr common_iterator(I i); constexpr common_iterator(S s); template<class I2, class S2> requires convertible_to<const I2&, I> && convertible_to<const S2&, S> constexpr common_iterator(const common_iterator<I2, S2>& x); template<class I2, class S2> requires convertible_to<const I2&, I> && convertible_to<const S2&, S> && assignable_from<I&, const I2&> && assignable_from<S&, const S2&> constexpr common_iterator& operator=(const common_iterator<I2, S2>& x); constexpr decltype(auto) operator*(); constexpr decltype(auto) operator*() const requires dereferenceable<const I>; constexpr decltype(auto) operator->() const requires see below; constexpr common_iterator& operator++(); constexpr decltype(auto) operator++(int); template<class I2, sentinel_for<I> S2> requires sentinel_for<S, I2> friend constexpr bool operator==( const common_iterator& x, const common_iterator<I2, S2>& y); template<class I2, sentinel_for<I> S2> requires sentinel_for<S, I2> && equality_comparable_with<I, I2> friend constexpr bool operator==( const common_iterator& x, const common_iterator<I2, S2>& y); template<sized_sentinel_for<I> I2, sized_sentinel_for<I> S2> requires sized_sentinel_for<S, I2> friend constexpr iter_difference_t<I2> operator-( const common_iterator& x, const common_iterator<I2, S2>& y); friend constexpr iter_rvalue_reference_t<I> iter_move(const common_iterator& i) noexcept(noexcept(ranges::iter_move(declval<const I&>()))) requires input_iterator<I>; template<indirectly_swappable<I> I2, class S2> friend constexpr void iter_swap(const common_iterator& x, const common_iterator<I2, S2>& y) noexcept(noexcept(ranges::iter_swap(declval<const I&>(), declval<const I2&>()))); private: variant<I, S> v_; // exposition only }; […] }
Modify 24.5.5.3 [common.iter.const] as indicated:
template<class I2, class S2> requires convertible_to<const I2&, I> && convertible_to<const S2&, S> && assignable_from<I&, const I2&> && assignable_from<S&, const S2&> constexpr common_iterator& operator=(const common_iterator<I2, S2>& x);-5- Preconditions:
[…]x.v_.valueless_by_exception()
isfalse
.
Modify 24.5.5.4 [common.iter.access] as indicated:
constexpr decltype(auto) operator*(); constexpr decltype(auto) operator*() const requires dereferenceable<const I>;-1- Preconditions:
[…]holds_alternative<I>(v_)
istrue
.constexpr decltype(auto) operator->() const requires see below;-3- The expression in the requires-clause is equivalent to:
[…]
Modify 24.5.5.5 [common.iter.nav] as indicated:
constexpr common_iterator& operator++();-1- Preconditions:
[…]holds_alternative<I>(v_)
istrue
.constexpr decltype(auto) operator++(int);-4- Preconditions:
[…]holds_alternative<I>(v_)
istrue
.
Modify 24.5.5.6 [common.iter.cmp] as indicated:
template<class I2, sentinel_for<I> S2> requires sentinel_for<S, I2> friend constexpr bool operator==( const common_iterator& x, const common_iterator<I2, S2>& y);-1- Preconditions: […]
template<class I2, sentinel_for<I> S2> requires sentinel_for<S, I2> && equality_comparable_with<I, I2> friend constexpr bool operator==( const common_iterator& x, const common_iterator<I2, S2>& y);-3- Preconditions: […]
template<sized_sentinel_for<I> I2, sized_sentinel_for<I> S2> requires sized_sentinel_for<S, I2> friend constexpr iter_difference_t<I2> operator-( const common_iterator& x, const common_iterator<I2, S2>& y);-5- Preconditions: […]
Modify 24.5.5.7 [common.iter.cust] as indicated:
friend constexpr iter_rvalue_reference_t<I> iter_move(const common_iterator& i) noexcept(noexcept(ranges::iter_move(declval<const I&>()))) requires input_iterator<I>;-1- Preconditions: […]
template<indirectly_swappable<I> I2, class S2> friend constexpr void iter_swap(const common_iterator& x, const common_iterator<I2, S2>& y) noexcept(noexcept(ranges::iter_swap(declval<const I&>(), declval<const I2&>())));-3- Preconditions: […]
<=>
for integer-class types isn't consistently specifiedSection: 24.3.4.4 [iterator.concept.winc] Status: Resolved Submitter: Jiang An Opened: 2021-07-25 Last modified: 2021-10-23
Priority: 3
View all other issues in [iterator.concept.winc].
View all issues with Resolved status.
Discussion:
It seems that the return type of <=>
for integer-class types is not specified consistently with
other comparison operators. Even P2393R0 has ignored it.
strong_ordering
should be added to 24.3.4.4 [iterator.concept.winc]/(5.3), and
three_way_comparable<strong_ordering>
should be added to 24.3.4.4 [iterator.concept.winc]/8.
[2021-07-31, Daniel comments]
The upcoming revision P2393R1 will provide additional wording to solve this issue.
[2021-08-20; Reflector poll]
Set priority to 3 after reflector poll. Tentatively Resolved by P2393R1 which has been approved by LWG.
[2021-10-23 Resolved by the adoption of P2393R1 at the October 2021 plenary. Status changed: Tentatively Resolved → Resolved.]
Proposed resolution:
std::format
Section: 28.5.2.2 [format.string.std] Status: Resolved Submitter: Mark de Wever Opened: 2021-08-01 Last modified: 2023-03-23
Priority: 2
View other active issues in [format.string.std].
View all other issues in [format.string.std].
View all issues with Resolved status.
Discussion:
The paper P1868 "width: clarifying units of width and precision in
std::format
" added optional Unicode support to the format
header.
This paper didn't update the definition of the fill character, which is defined as
"The fill character can be any character other than
{
or}
."
This wording means the fill is a character and not a Unicode grapheme cluster. Based
on the current wording the range of available fill characters depends on the
char_type
of the format string. After P1868 the determination of the required
padding size is Unicode aware, but it's not possible to use a Unicode grapheme clusters
as padding. This looks odd from a user's perspective and already lead to implementation
divergence between libc++ and MSVC STL:
The WIP libc++ implementation stores one char_type
, strictly adhering
to the wording of the Standard.
MSVC STL stores one code point, regardless of the char_type
used. This
is already better from a user's perspective; all 1 code point grapheme clusters are
properly handled.
For the width calculation the width of a Unicode grapheme cluster is estimated to be 1 or 2. Since padding with a 2 column width can't properly pad an odd number of columns the grapheme cluster used should always have a column width of 1.
The responsibility for precondition can be either be validated in the library or by the user. It would be possible to do the validation compile time and make the code ill-formed when the precondition is violated. For the following reason I think it's better to not validate the width:P1868 14. Implementation
"More importantly, our approach permits refining the definition in the future if there is interest in doing so. It will mostly require researching the status of Unicode support on terminals and minimal or no changes to the implementation."
When an estimated width of 1 is required it means that improving the Standard may make previously valid code ill-formed after the improvement.
P1868 13. Examples
The example of the family grapheme cluster is only rendered properly on the MacOS terminal. So even when the library does a proper validation it's not certain the output will be rendered properly.
Changing the fill type changes the size of the std::formatter
and thus will be an ABI break.
Previous resolution [SUPERSEDED]:
This wording is relative to N4892.
Modify 28.5.2.2 [format.string.std] as indicated:
-2- [Note 2:
The fill character can be any character other thanFor a string in a Unicode encoding, the fill character can be any Unicode grapheme cluster other than{
or}
.{
or}
. For a string in a non-Unicode encoding, the fill character can be any character other than{
or}
. The output width of the fill character is always assumed to be one column. The presence of a fill character is signaled by the character following it, which must be one of the alignment options. If the second character of std-format-spec is not a valid alignment option, then it is assumed that both the fill character and the alignment option are absent. — end note]
[2021-08-09; Mark de Wever provides improved wording]
[2021-08-20; Reflector poll]
Set priority to 2 and status to "SG16" after reflector poll.
Previous resolution [SUPERSEDED]:
This wording is relative to N4892.
Modify 28.5.2.2 [format.string.std] as indicated:
-1- […] The syntax of format specifications is as follows:
[…] fill: any Unicode grapheme cluster or character other than{
or}
[…]-2-
[Note 2: The presence of a fill character is signaled by the character following it, which must be one of the alignment options. If the second character of std-format-spec is not a valid alignment option, then it is assumed that both the fill character and the alignment option are absent. — end note][Note 2: The fill character can be any character other thanFor a string in a Unicode encoding, the fill character can be any Unicode grapheme cluster other than{
or}
.{
or}
. For a string in a non-Unicode encoding, the fill character can be any character other than{
or}
. The output width of the fill character is always assumed to be one column.
[2021-08-26; SG16 reviewed and provides alternative wording]
[2023-01-11; LWG telecon]
P2572 would resolve this issue and LWG 3639(i).
Previous resolution [SUPERSEDED]:
This wording is relative to N4892.
Modify 28.5.2.2 [format.string.std] as indicated:
-1- […] The syntax of format specifications is as follows:
[…] fill: anycharactercodepoint of the literal encoding other than{
or}
[…]-2- [Note 2: The fill character can be any
charactercodepoint other than{
or}
. The presence of a fill character is signaled by the character following it, which must be one of the alignment options. If the second character of std-format-spec is not a valid alignment option, then it is assumed that both the fill character and the alignment option are absent. — end note]
[2023-03-22 Resolved by the adoption of P2572R1 in Issaquah. Status changed: SG16 → Resolved.]
Proposed resolution:
Section: 23.2.7.1 [associative.reqmts.general] Status: WP Submitter: Joaquín M López Muñoz Opened: 2021-08-04 Last modified: 2025-02-16
Priority: 3
View other active issues in [associative.reqmts.general].
View all other issues in [associative.reqmts.general].
View all issues with WP status.
Discussion:
For the expression a.merge(a2)
, postconditions say that "iterators referring to the transferred elements
[…] now behave as iterators into a
[…]". When a
and a2
are of different
types, this seems to imply, under the widest interpretation for "behaving as", that a
-iterators and
a2
-iterators are actually of the same type, that is, that associative containers have SCARY iterators,
which is, to the best of my knowledge, not currently mandated by the standard.
Indicate that "behaving as" only applies to the case where a
and a2
are of the same type.
Clarify what "behaving as" means. A non-SCARY interpretation is that an a2
-iterator to a transferred
element can still be dereferenced, incremented (if not past the last element of a
) and decremented (if
not pointing to the first element of a
), while comparison with a
-iterators and use in the
interface of a
is not guaranteed.
Mandate SCARY iterators by, for instance, requiring that associative containers with compatible nodes (23.2.5.1 [container.node.overview]/1) have the same iterator types.
Note that this issue does not extend to unordered associative containers, as there (23.2.8.1 [unord.req.general]) iterators to transferred elements are invalidated, which makes the point of SCARYness irrelevant. That said, if SCARY iterators are finally required for associative containers, it makes much sense to extend the requirement to unordered associative containers as well.
[2021-08-20; Reflector poll]
Set priority to 3 after reflector poll.
[2024-12-04; Jonathan provides wording]
If we want to require SCARY iterators then that should be a proposal that goes through LEWG design review. I propose an almost minimal change to make the spec consistent without imposing any new requirements on implementations.
The minimal change would be to say that iterators remain valid if a
and a2
have the same type, which is the minimum portable guarantee that always holds.
However what matters in practice is whether the iterator types are the same.
That would not be a portable guarantee, because it depends on whether the
implementation uses SCARY iterators for its maps and sets, so users could
write code that works on one implementation and fails to compile when moved
to a different implementation. But that portability trap would be present
even if we only say iterators remain valid when a
and a2
are the same type.
If the code compiles and works on an implementation with SCARY iterators,
then users will rely on that, even if unintentionally. Leaving that case
unspecified or undefined in the standard doesn't prevent users from relying
on it. It doesn't seem to serve any benefit to pretend it doesn't work when
it actually does on some implementations.
N.B. Libstdc++ associative container iterators are SCARY by default,
but non-SCARY when -D_GLIBCXX_DEBUG
is used to enable Debug Mode
(see Bug 62169).
I believe libc++ iterators are SCARY even when
-D_LIBCPP_HARDENING_MODE=_LIBCPP_HARDENING_MODE_DEBUG
is used,
and MSVC STL iterators are SCARY even when /D_ITERATOR_DEBUG_LEVEL
is used.
[2024-12-09; Reflector poll]
Set status to Tentatively Ready after eight votes in favour during reflector poll.
[Hagenberg 2025-02-16; Status changed: Voting → WP.]
Proposed resolution:
This wording is relative to N4993.
Modify 23.2.7.1 [associative.reqmts.general] as indicated:
a.merge(a2)
-112- Result:
void
-113- Preconditions:
a.get_allocator() == a2.get_allocator()
istrue
.-114- Effects: Attempts to extract each element in
a2
and insert it intoa
using the comparison object ofa
. In containers with unique keys, if there is an element ina
with key equivalent to the key of an element froma2
, then that element is not extracted froma2
.-115- Postconditions: Pointers and references to the transferred elements of
a2
refer to those same elements but as members ofa
. Ifa.begin()
anda2.begin()
have the same type, iteratorsIteratorsreferring to the transferred elements will continue to refer to their elements, but they now behave as iterators intoa
, not intoa2
.-116- Throws: Nothing unless the comparison objects throws.
-117- Complexity: N log(
a.size()
+N), where N has the valuea2.size()
.
iota_view's iterator
's binary operator+
should be improvedSection: 25.6.4.3 [range.iota.iterator] Status: C++23 Submitter: Zoe Carver Opened: 2021-08-14 Last modified: 2023-11-22
Priority: Not Prioritized
View other active issues in [range.iota.iterator].
View all other issues in [range.iota.iterator].
View all issues with C++23 status.
Discussion:
iota_view
's iterator's operator+
could avoid a copy construct by doing
"i += n; return i
" rather than "return i += n;
" as seen in 25.6.4.3 [range.iota.iterator].
[2021-08-20; Reflector poll]
Set status to Tentatively Ready after six votes in favour during reflector poll.
[2021-10-14 Approved at October 2021 virtual plenary. Status changed: Voting → WP.]
Proposed resolution:
This wording is relative to N4892.
Modify 25.6.4.3 [range.iota.iterator] as indicated:
friend constexpr iterator operator+(iterator i, difference_type n) requires advanceable<W>;-20- Effects: Equivalent to:
returni += n; return i;friend constexpr iterator operator+(difference_type n, iterator i) requires advanceable<W>;-21- Effects: Equivalent to:
return i + n;
friend constexpr iterator operator-(iterator i, difference_type n) requires advanceable<W>;-22- Effects: Equivalent to:
returni -= n; return i;
basic_string_view
not trivially move constructibleSection: 27.3.3.2 [string.view.cons] Status: C++23 Submitter: Jiang An, Casey Carter Opened: 2021-08-16 Last modified: 2023-11-22
Priority: Not Prioritized
View all other issues in [string.view.cons].
View all issues with C++23 status.
Discussion:
Jiang An:
The issue is found in this Microsoft STL pull request.
I think an additional constraint should be added to 27.3.3.2 [string.view.cons]/11 (a similar constraint is already present forreference_wrapper
):
(11.?) —
is_same_v<remove_cvref_t<R>, basic_string_view>
isfalse
.
Casey Carter:
P1989R2 "Range constructor for std::string_view
2: Constrain Harder"
added a converting constructor to basic_string_view
that accepts (by forwarding reference) any sized
contiguous range with a compatible element type. This constructor unfortunately intercepts attempts at move
construction which previously would have resulted in a call to the copy constructor. (I suspect the authors of
P1989 were under the mistaken impression that basic_string_view
had a defaulted move constructor, which
would sidestep this issue by being chosen by overload resolution via the "non-template vs. template" tiebreaker.)
basic_string_view
, but it would be awkward to add text to specify that these moves always leave the
source intact. Presumably this is why the move operations were omitted in the first place. We therefore recommend
constraining the conversion constructor template to reject arguments whose decayed type is basic_string_view
(which we should probably do for all conversion constructor templates in the Standard Library).
Implementation experience: MSVC STL is in the process of implementing this fix. Per Jonathan Wakely, libstdc++ has a
similar constraint.
[2021-09-20; Reflector poll]
Set status to Tentatively Ready after nine votes in favour during reflector poll.
[2021-10-14 Approved at October 2021 virtual plenary. Status changed: Voting → WP.]
Proposed resolution:
This wording is relative to N4892.
Modify 27.3.3.2 [string.view.cons] as indicated:
template<class R> constexpr basic_string_view(R&& r);-10- Let
-11- Constraints:d
be an lvalue of typeremove_cvref_t<R>
.
(11.?) —
remove_cvref_t<R>
is not the same type asbasic_string_view
,(11.1) —
R
modelsranges::contiguous_range
andranges::sized_range
,(11.2) —
is_same_v<ranges::range_value_t<R>, charT>
istrue
,[…]
Section: 22.6.3.4 [variant.assign] Status: C++23 Submitter: Barry Revzin Opened: 2021-09-01 Last modified: 2023-11-22
Priority: Not Prioritized
View other active issues in [variant.assign].
View all other issues in [variant.assign].
View all issues with C++23 status.
Discussion:
Example originally from StackOverflow but with a more reasonable example from Tim Song:
#include <variant> #include <string> struct A { A() = default; A(A&&) = delete; }; int main() { std::variant<A, std::string> v; v = "hello"; }
There is implementation divergence here: libstdc++ rejects, libc++ and msvc accept.
22.6.3.4 [variant.assign] bullet (13.3) says that if we're changing the alternative in assignment and it is not the case that the converting construction won't throw (as in the above), then "Otherwise, equivalent tooperator=(variant(std::forward<T>(t)))
." That is, we defer to move assignment.
variant<A, string>
isn't move-assignable (because A
isn't move constructible). Per
the wording in the standard, we have to reject this. libstdc++ follows the wording of the standard.
But we don't actually have to do a full move assignment here, since we know the situation we're in is
changing the alternative, so the fact that A
isn't move-assignable shouldn't matter. libc++ and
msvc instead do something more direct, allowing the above program.
[2021-09-20; Reflector poll]
Set status to Tentatively Ready after seven votes in favour during reflector poll.
[2021-10-14 Approved at October 2021 virtual plenary. Status changed: Voting → WP.]
Proposed resolution:
This wording is relative to N4892.
Modify 22.6.3.4 [variant.assign] as indicated:
[Drafting note: This should cover the case that we want to cover: that if construction of
Tj
fromT
throws, we haven't yet changed any state in the variant.]
template<class T> constexpr variant& operator=(T&& t) noexcept(see below);-11- Let
-12- Constraints: […] -13- Effects:Tj
be a type that is determined as follows: build an imaginary functionFUN(Ti)
for each alternative typeTi
for whichTi x[] = {std::forward<T>(t)};
is well-formed for some invented variablex
. The overloadFUN(Tj)
selected by overload resolution for the expressionFUN(std::forward<T>(t))
defines the alternativeTj
which is the type of the contained value after assignment.
(13.1) — If
*this
holds aTj
, assignsstd::forward<T>(t)
to the value contained in*this
.(13.2) — Otherwise, if
is_nothrow_constructible_v<Tj, T> || !is_nothrow_move_constructible_v<Tj>
istrue
, equivalent toemplace<j>(std::forward<T>(t))
.(13.3) — Otherwise, equivalent to
.
operator=(variant(std::forward<T>(t)))emplace<j>(Tj(std::forward<T>(t)))
const
lvalue reference overload of get
for subrange
does not constrain I
to be copyable
when N == 0
Section: 25.5.4 [range.subrange] Status: C++23 Submitter: Hewill Kang Opened: 2021-09-08 Last modified: 2023-11-22
Priority: 3
View all other issues in [range.subrange].
View all issues with C++23 status.
Discussion:
The const
lvalue reference overload of get
used for subrange
only constraint that N < 2
, this will cause the "discards qualifiers" error
inside the function body when applying get
to the lvalue subrange
that
stores a non-copyable
iterator since its begin()
is non-const
qualified, we probably need to add a constraint for it.
[2021-09-20; Reflector poll]
Set priority to 3 after reflector poll.
[2021-09-20; Reflector poll]
Set status to Tentatively Ready after six votes in favour during reflector poll.
[2021-10-14 Approved at October 2021 virtual plenary. Status changed: Voting → WP.]
Proposed resolution:
This wording is relative to N4892.
Modify 25.5.4 [range.subrange] as indicated:
namespace std::ranges { […] template<size_t N, class I, class S, subrange_kind K> requires ((N< 2== 0 && copyable<I>) || N == 1) constexpr auto get(const subrange<I, S, K>& r); template<size_t N, class I, class S, subrange_kind K> requires (N < 2) constexpr auto get(subrange<I, S, K>&& r); }
Modify 25.5.4.3 [range.subrange.access] as indicated:
[…] template<size_t N, class I, class S, subrange_kind K> requires ((N< 2== 0 && copyable<I>) || N == 1) constexpr auto get(const subrange<I, S, K>& r); template<size_t N, class I, class S, subrange_kind K> requires (N < 2) constexpr auto get(subrange<I, S, K>&& r);-10- Effects: Equivalent to:
if constexpr (N == 0) return r.begin(); else return r.end();
split_view::base() const &
is overconstrainedSection: 25.7.17.2 [range.split.view] Status: C++23 Submitter: Tim Song Opened: 2021-09-13 Last modified: 2023-11-22
Priority: Not Prioritized
View all issues with C++23 status.
Discussion:
Unlike every other range adaptor, split_view::base() const &
is
constrained on copyable<V>
instead of copy_constructible<V>
.
copyable
— copy_constructible
is sufficient.
[2021-09-24; Reflector poll]
Set status to Tentatively Ready after eight votes in favour during reflector poll.
[2021-10-14 Approved at October 2021 virtual plenary. Status changed: Voting → WP.]
Proposed resolution:
This wording is relative to N4892.
Modify 25.7.17.2 [range.split.view], class template split_view
synopsis, as indicated:
namespace std::ranges { template<forward_range V, forward_range Pattern> requires view<V> && view<Pattern> && indirectly_comparable<iterator_t<V>, iterator_t<Pattern>, ranges::equal_to> class split_view : public view_interface<split_view<V, Pattern>> { private: […] public: […] constexpr V base() const& requirescopyablecopy_constructible<V> { return base_; } constexpr V base() && { return std::move(base_); } […] };
lazy_split_view<input_view>::inner-iterator::base() &&
invalidates outer iteratorsSection: 25.7.16.5 [range.lazy.split.inner] Status: C++23 Submitter: Tim Song Opened: 2021-09-13 Last modified: 2023-11-22
Priority: Not Prioritized
View all other issues in [range.lazy.split.inner].
View all issues with C++23 status.
Discussion:
The base() &&
members of iterator adaptors (and iterators of range
adaptors) invalidate the adaptor itself by moving from the contained iterator. This
is generally unobjectionable — since you are calling base()
on an rvalue,
it is expected that you won't be using it afterwards.
lazy_split_view<input_view>::inner-iterator::base() &&
is special: the iterator being moved from is stored in the lazy_split_view
itself
and shared between the inner and outer iterators, so the operation invalidates not just the
inner-iterator
on which it is called, but also the outer-iterator
from which the inner-iterator
was obtained.
This spooky-action-at-a-distance behavior can be surprising, and the value category of the
inner iterator seems to be too subtle to base it upon.
The PR below constrains this overload to forward ranges. Forward iterators are copyable anyway,
but moving could potentially be more efficient.
[2021-09-24; Reflector poll]
Set status to Tentatively Ready after six votes in favour during reflector poll.
[2021-10-14 Approved at October 2021 virtual plenary. Status changed: Voting → WP.]
Proposed resolution:
This wording is relative to N4892.
Modify 25.7.16.5 [range.lazy.split.inner] as indicated:
[Drafting note: The constraint uses
forward_range<V>
since that's the condition for caching inlazy_split_view
.]
[…]namespace std::ranges { template<input_range V, forward_range Pattern> requires view<V> && view<Pattern> && indirectly_comparable<iterator_t<V>, iterator_t<Pattern>, ranges::equal_to> && (forward_range<V> || tiny-range<Pattern>) template<bool Const> struct lazy_split_view<V, Pattern>::inner-iterator { private: […] public: […] constexpr const iterator_t<Base>& base() const &; constexpr iterator_t<Base> base() && requires forward_range<V>; […] };constexpr iterator_t<Base> base() && requires forward_range<V>;-4- Effects: Equivalent to:
return std::move(i_.current);
lazy_split_view
needs to check the simpleness of Pattern
Section: 25.7.16.2 [range.lazy.split.view] Status: C++23 Submitter: Tim Song Opened: 2021-09-15 Last modified: 2023-11-22
Priority: Not Prioritized
View other active issues in [range.lazy.split.view].
View all other issues in [range.lazy.split.view].
View all issues with C++23 status.
Discussion:
For forward ranges, the non-const
versions of lazy_split_view::begin()
and lazy_split_view::end()
returns an outer-iterator<simple-view<V>>
,
promoting to const
if V
models simple-view
. However, this failed to
take Pattern
into account, even though that will also be accessed as const
if
const
-promotion takes place. There is no requirement that const Pattern
is a range,
however.
[2021-09-24; Reflector poll]
Set status to Tentatively Ready after six votes in favour during reflector poll.
[2021-10-14 Approved at October 2021 virtual plenary. Status changed: Voting → WP.]
Proposed resolution:
This wording is relative to N4892.
Modify 25.7.16.2 [range.lazy.split.view], class template lazy_split_view
synopsis, as indicated:
namespace std::ranges { […] template<input_range V, forward_range Pattern> requires view<V> && view<Pattern> && indirectly_comparable<iterator_t<V>, iterator_t<Pattern>, ranges::equal_to> && (forward_range<V> || tiny-range<Pattern>) class lazy_split_view : public view_interface<lazy_split_view<V, Pattern>> { private: […] public: […] constexpr auto begin() { if constexpr (forward_range<V>) return outer-iterator<simple-view<V> && simple-view<Pattern>>{*this, ranges::begin(base_)}; else { current_ = ranges::begin(base_); return outer-iterator<false>{*this}; } } […] constexpr auto end() requires forward_range<V> && common_range<V> { return outer-iterator<simple-view<V> && simple-view<Pattern>>{*this, ranges::end(base_)}; } […] }; […] }
base() const &
and lazy_split_view::outer-iterator::value_type::end()
missing noexcept
Section: 24.5.4 [move.iterators], 24.5.7 [iterators.counted], 25.7.8.3 [range.filter.iterator], 25.7.9.3 [range.transform.iterator], 25.7.16.4 [range.lazy.split.outer.value], 25.7.16.5 [range.lazy.split.inner], 25.7.23.3 [range.elements.iterator] Status: C++23 Submitter: Hewill Kang Opened: 2021-09-14 Last modified: 2023-11-22
Priority: Not Prioritized
View all other issues in [move.iterators].
View all issues with C++23 status.
Discussion:
LWG 3391(i) and 3533(i) changed some iterators' base() const &
from returning value to
returning const
reference, which also prevents them from throwing exceptions, we should add noexcept
for them.
Also, lazy_split_view::outer-iterator::value_type::end()
can be noexcept
since it only returns
default_sentinel
.
[2021-09-24; Reflector poll]
Set status to Tentatively Ready after seven votes in favour during reflector poll.
[2021-10-14 Approved at October 2021 virtual plenary. Status changed: Voting → WP.]
Proposed resolution:
This wording is relative to N4892.
Modify 24.5.4 [move.iterators], class template move_iterator
synopsis, as indicated:
[…] constexpr const iterator_type& base() const & noexcept; constexpr iterator_type base() &&; […]
Modify 24.5.4.5 [move.iter.op.conv] as indicated:
constexpr const Iterator& base() const & noexcept;-1- Returns:
current
.
Modify 24.5.7.1 [counted.iterator], class template counted_iterator
synopsis, as indicated:
[…] constexpr const I& base() const & noexcept; constexpr I base() &&; […]
Modify 24.5.7.3 [counted.iter.access] as indicated:
constexpr const I& base() const & noexcept;-1- Effects: Equivalent to:
return current;
Modify 25.7.8.3 [range.filter.iterator] as indicated:
[…][…] constexpr const iterator_t<V>& base() const & noexcept; constexpr iterator_t<V> base() &&; […]constexpr const iterator_t<V>& base() const & noexcept;-5- Effects: Equivalent to:
return current_;
Modify 25.7.9.3 [range.transform.iterator] as indicated:
[…][…] constexpr const iterator_t<Base>& base() const & noexcept; constexpr iterator_t<Base> base() &&; […]constexpr const iterator_t<Base>& base() const & noexcept;-5- Effects: Equivalent to:
return current_;
Modify 25.7.16.4 [range.lazy.split.outer.value] as indicated:
[…][…] constexpr inner-iterator<Const> begin() const; constexpr default_sentinel_t end() const noexcept; […]constexpr default_sentinel_t end() const noexcept;-3- Effects: Equivalent to:
return default_sentinel;
Modify 25.7.16.5 [range.lazy.split.inner] as indicated:
[…][…] constexpr const iterator_t<Base>& base() const & noexcept; constexpr iterator_t<Base> base() &&; […]constexpr const iterator_t<Base>& base() const & noexcept;-3- Effects: Equivalent to:
return i_.current;
Modify 25.7.23.3 [range.elements.iterator] as indicated:
[…][…] constexpr const iterator_t<Base>& base() const& noexcept; constexpr iterator_t<Base> base() &&; […]constexpr const iterator_t<Base>& base() const& noexcept;-6- Effects: Equivalent to:
return current_;
inout_ptr
— inconsistent release()
in destructorSection: 20.3.4.3 [inout.ptr.t] Status: C++23 Submitter: JeanHeyd Meneide Opened: 2021-09-16 Last modified: 2023-11-22
Priority: 1
View all other issues in [inout.ptr.t].
View all issues with C++23 status.
Discussion:
More succinctly, the model for std::out_ptr_t
and std::inout_ptr_t
both have a conditional release in their destructors as part of their
specification (20.3.4.1 [out.ptr.t] p8) (Option #2 below).
But, if the wording is followed, they have an unconditional release in
their constructor (Option #1 below). This is not exactly correct and
can cause issues with double-free in inout_ptr
in particular.
MyFunc
that sets rawPtr
to nullptr
when freeing
an old value and deciding not to produce a new value, as shown below:
// Option #1: auto uptr = std::make_unique<BYTE[]>(25); auto rawPtr = uptr.get(); uptr.release(); // UNCONDITIONAL MyFunc(&rawPtr); If (rawPtr) { uptr.reset(rawPtr); } // Option #2: auto uptr = std::make_unique<BYTE[]>(25); auto rawPtr = uptr.get(); MyFunc(&rawPtr); If (rawPtr) { uptr.release(); // CONDITIONAL uptr.reset(rawPtr); }
This is no problem if the implementation selected Option #1 (release in the constructor), but results in double-free if the implementation selected option #2 (release in the destructor).
As the paper author and after conferring with others, the intent was that the behavior was identical and whether a choice between the constructor or destructor is made. The reset should be unconditional, at least forinout_ptr_t
. Suggested change for the ~inout_ptr_t
destructor text is to remove the "if (p) { ... }
" wrapper from around
the code in 20.3.4.3 [inout.ptr.t] p11.
[2021-09-24; Reflector poll]
Set priority to 1 after reflector poll.
Previous resolution [SUPERSEDED]:
This wording is relative to N4892.
Modify 20.3.4.3 [inout.ptr.t] as indicated:
~inout_ptr_t();-9- Let
-10- LetSP
bePOINTER_OF_OR(Smart, Pointer)
(20.2.1 [memory.general]).release-statement
bes.release()
; if an implementation does not calls.release()
in the constructor. Otherwise, it is empty. -11- Effects: Equivalent to:
(11.1) —
if (p) {apply([&](auto&&... args) { s = Smart( static_cast<SP>(p), std::forward<Args>(args)...); }, std::move(a));}if
is_pointer_v<Smart>
istrue
;(11.2) — otherwise,
if (p) {apply([&](auto&&... args) { release-statement; s.reset(static_cast<SP>(p), std::forward<Args>(args)...); }, std::move(a));}if the expression
s.reset(static_cast<SP>(p), std::forward<Args>(args)...)
is well-formed;(11.3) — otherwise,
if (p) {apply([&](auto&&... args) { release-statement; s = Smart(static_cast<SP>(p), std::forward<Args>(args)...); }, std::move(a));}if
is_constructible_v<Smart, SP, Args...>
istrue
;(11.4) — otherwise, the program is ill-formed.
[2021-10-28; JeanHeyd Meneide provides improved wording]
[2022-08-24 Approved unanimously in LWG telecon.]
[2022-11-12 Approved at November 2022 meeting in Kona. Status changed: Voting → WP.]
Proposed resolution:
This wording is relative to N4901.
Modify 20.3.4.3 [inout.ptr.t] as indicated:
~inout_ptr_t();-9- Let
-10- LetSP
bePOINTER_OF_OR(Smart, Pointer)
(20.2.1 [memory.general]).release-statement
bes.release()
; if an implementation does not calls.release()
in the constructor. Otherwise, it is empty. -11- Effects: Equivalent to:
(11.1) —
if (p) { apply([&](auto&&... args) { s = Smart( static_cast<SP>(p), std::forward<Args>(args)...); }, std::move(a)); }if
is_pointer_v<Smart>
istrue
;(11.2) — otherwise,
release-statement; if (p) { apply([&](auto&&... args) {release-statement;s.reset(static_cast<SP>(p), std::forward<Args>(args)...); }, std::move(a)); }if the expression
s.reset(static_cast<SP>(p), std::forward<Args>(args)...)
is well-formed;(11.3) — otherwise,
release-statement; if (p) { apply([&](auto&&... args) {release-statement;s = Smart(static_cast<SP>(p), std::forward<Args>(args)...); }, std::move(a)); }if
is_constructible_v<Smart, SP, Args...>
istrue
;(11.4) — otherwise, the program is ill-formed.
proxy
and postfix-proxy
for common_iterator
should be fully constexpr
Section: 24.5.5.4 [common.iter.access], 24.5.5.5 [common.iter.nav] Status: C++23 Submitter: Hewill Kang Opened: 2021-09-18 Last modified: 2023-11-22
Priority: Not Prioritized
View all other issues in [common.iter.access].
View all issues with C++23 status.
Discussion:
LWG 3574(i) added constexpr
to all member functions and friends of common_iterator
to make it
fully constexpr
, but accidentally omitted the exposition-only classes proxy
and
postfix-proxy
defined for some member functions. We should make these two classes fully constexpr
, too.
[2021-09-24; Reflector poll]
Set status to Tentatively Ready after seven votes in favour during reflector poll.
[2021-10-14 Approved at October 2021 virtual plenary. Status changed: Voting → WP.]
Proposed resolution:
This wording is relative to N4892.
Modify 24.5.5.4 [common.iter.access] as indicated:
decltype(auto) operator->() const requires see below;-3- […]
-4- […] -5- Effects:
(5.1) — […]
(5.2) — […]
(5.3) — Otherwise, equivalent to:
return proxy(*get<I>(v_));
whereproxy
is the exposition-only class:class proxy { iter_value_t<I> keep_; constexpr proxy(iter_reference_t<I>&& x) : keep_(std::move(x)) {} public: constexpr const iter_value_t<I>* operator->() const noexcept { return addressof(keep_); } };
Modify 24.5.5.5 [common.iter.nav] as indicated:
decltype(auto) operator++(int);-4- […]
-5- Effects: If I models forward_iterator, equivalent to: […] Otherwise, if […] […] Otherwise, equivalent to:postfix-proxy p(**this); ++*this; return p;where
postfix-proxy
is the exposition-only class:class postfix-proxy { iter_value_t<I> keep_; constexpr postfix-proxy(iter_reference_t<I>&& x) : keep_(std::forward<iter_reference_t<I>>(x)) {} public: constexpr const iter_value_t<I>& operator*() const noexcept { return keep_; } };
advanceable
Section: 25.6.4.2 [range.iota.view] Status: C++23 Submitter: Jiang An Opened: 2021-09-23 Last modified: 2023-11-22
Priority: 3
View all other issues in [range.iota.view].
View all issues with C++23 status.
Discussion:
Unsigned integer types satisfy advanceable
, but don't model it, since
every two values of an unsigned integer type are reachable from each other, and modular arithmetic is performed on unsigned integer types,
which makes the last three bullets of the semantic requirements of advanceable
(25.6.4.2 [range.iota.view]/5) can't be satisfied, and some (if not all) uses of
iota_view
s of unsigned integer types ill-formed, no diagnostic required.
advanceable
behave incorrectly for unsigned integer types. E.g. according to 25.6.4.2 [range.iota.view]/6
and 25.6.4.2 [range.iota.view]/16, std::ranges::iota_view<std::uint8_t, std::uint8_t>(std::uint8_t(1)).size()
is well-defined IMO, because
Bound()
isstd::uint8_t(0)
, which is reachable fromstd::uint8_t(1)
, and not modelingadvanceable
shouldn't affect the validity, as bothW
andBound
are integer types.
However, it returns unsigned(-1)
on common implementations (where sizeof(int) > sizeof(std::uint8_t))
,
which is wrong.
advanceable
should be adjusted,
and a refined definition of reachability in 25.6.4 [range.iota] is needed to avoid reaching
a
from b
when a > b
(the iterator type is also affected).
[2021-10-14; Reflector poll]
Set priority to 3 after reflector poll.
[Tim Song commented:]
The advanceable part of the issue is NAD.
This is no different from NaN and totally_ordered
,
see 16.3.2.3 [structure.requirements]/8.
The part about iota_view<uint8_t, uint8_t>(1)
is simply this:
when we added "When W
and Bound
model ..." to
25.6.4.2 [range.iota.view]/8,
we forgot to add its equivalent to the single-argument constructor.
We should do that.
[2022-10-12; Jonathan provides wording]
[2022-10-19; Reflector poll]
Set status to "Tentatively Ready" after seven votes in favour in reflector poll.
[2022-11-12 Approved at November 2022 meeting in Kona. Status changed: Voting → WP.]
Proposed resolution:
This wording is relative to N4917.
Modify 25.6.4.2 [range.iota.view] as indicated:
constexpr explicit iota_view(W value);-6- Preconditions:
Bound
denotesunreachable_sentinel_t
orBound()
is reachable fromvalue
. WhenW
andBound
modeltotally_ordered_with
, thenbool(value <= Bound())
istrue
.-7- Effects: Initializes
value_
withvalue
.constexpr iota_view(type_identity_t<W> value, type_identity_t<Bound> bound);-8- Preconditions:
Bound
denotesunreachable_sentinel_t
orbound
is reachable fromvalue
. WhenW
andBound
modeltotally_ordered_with
, thenbool(value <= bound)
istrue
.-9- Effects: Initializes
value_
withvalue
andbound_
withbound
.
system_category().default_error_condition(0)
is underspecifiedSection: 19.5.3.5 [syserr.errcat.objects] Status: C++23 Submitter: Jonathan Wakely Opened: 2021-09-23 Last modified: 2023-11-22
Priority: Not Prioritized
View all other issues in [syserr.errcat.objects].
View all issues with C++23 status.
Discussion:
19.5.3.5 [syserr.errcat.objects] says:
If the argument
ev
corresponds to a POSIXerrno
valueposv
, the function shall returnerror_condition(posv, generic_category())
. Otherwise, the function shall returnerror_condition(ev, system_category())
. What constitutes correspondence for any given operating system is unspecified.
What is "a POSIX errno
value"? Does it mean a value equal to one of the <cerrno>
Exxx
macros? Because in that case, the value 0
is not "a POSIX errno
value".
So arguably system_category().default_error_condition(0)
is required to return an
error_condition
using the "system" category, which means that error_code{} == error_condition{}
is required to be false
. This seems wrong.
0
should definitely correspond to the generic category.
Arguably that needs to be true
for all systems, because the std::error_code
API
strongly encourages a model where zero means "no error".
The proposed resolution has been implemented in libstdc++. Libc++ has always treated system error code
0
as corresponding to generic error code 0
.
[2021-10-14; Reflector poll]
Set status to Tentatively Ready after six votes in favour during reflector poll.
[2022-02-10 Approved at February 2022 virtual plenary. Status changed: Tentatively Ready → WP.]
Proposed resolution:
This wording is relative to N4892.
Modify 19.5.3.5 [syserr.errcat.objects] as indicated:
const error_category& system_category() noexcept;-3- Returns: […]
-4- Remarks: The object'sequivalent
virtual functions shall behave as specified for classerror_category
. The object'sname
virtual function shall return a pointer to the string"system"
. The object'sdefault_error_condition
virtual function shall behave as follows: If the argumentev
is equal to0
, the function returnserror_condition(0, generic_category())
. Otherwise, ifev
corresponds to a POSIXerrno
valueposv
, the functionshall returnreturnserror_condition(posv, generic_category())
. Otherwise, the functionshall returnreturnserror_condition(ev, system_category())
. What constitutes correspondence for any given operating system is unspecified. [Note 1: The number of potential system error codes is large and unbounded, and some might not correspond to any POSIXerrno
value. Thus implementations are given latitude in determining correspondence. — end note]
istream_iterator
copy constructor trivial is an ABI breakSection: 24.6.2.2 [istream.iterator.cons] Status: C++23 Submitter: Jonathan Wakely Opened: 2021-09-23 Last modified: 2023-11-22
Priority: 3
View all other issues in [istream.iterator.cons].
View all issues with C++23 status.
Discussion:
Libstdc++ never implemented this change made between C++03 and C++11 (by N2994):
24.6.1.1 [istream.iterator.cons] p3:istream_iterator(const istream_iterator<T,charT,traits,Distance>& x) = default;-3- Effects: Constructs a copy of
x
. IfT
is a literal type, then this constructor shall be a trivial copy constructor.
This breaks our ABI, as it changes the argument passing convention for the type, meaning this function segfaults if compiled with today's libstdc++ and called from one that makes the triviality change:
#include <iterator> #include <istream> int f(std::istream_iterator<int> i) { return *i++; }
As a result, it's likely that libstdc++ will never implement the change.
There is no reason to require this constructor to be trivial. It was required for C++0x at one point, so the type could be literal, but that is not true in the current language. We should strike the requirement, to reflect reality. MSVC and libc++ are free to continue to define it as defaulted (and so trivial when appropriate) but we should not require it from libstdc++. The cost of an ABI break is not worth the negligible benefit from making it trivial.Previous resolution [SUPERSEDED]:
This wording is relative to N4892.
Modify 24.6.2.2 [istream.iterator.cons] as indicated:
istream_iterator(const istream_iterator& x) = default;-5- Postconditions:
in_stream == x.in_stream
istrue
.-6- Remarks: Ifis_trivially_copy_constructible_v<T>
istrue
, then this constructor is trivial.
[2021-09-30; Jonathan revises wording after reflector discussion]
A benefit of triviality is that it is constexpr, want to preserve that.
[2021-10-14; Reflector poll]
Set priority to 3 after reflector poll.
Previous resolution [SUPERSEDED]:
This wording is relative to N4892.
Modify the class synopsis in 24.6.2.1 [istream.iterator.general] as indicated:
constexpr istream_iterator(); constexpr istream_iterator(default_sentinel_t); istream_iterator(istream_type& s); constexpr istream_iterator(const istream_iterator& x)= default; ~istream_iterator() = default; istream_iterator& operator=(const istream_iterator&) = default;Modify 24.6.2.2 [istream.iterator.cons] as indicated:
constexpr istream_iterator(const istream_iterator& x)= default;-5- Postconditions:
in_stream == x.in_stream
istrue
.-6- Remarks:
IfIf the initializeris_trivially_copy_constructible_v<T>
istrue
, then this constructor is trivial.T(x.value)
in the declarationauto val = T(x.value);
is a constant initializer ([expr.const]), then this constructor is a constexpr constructor.
[2022-10-12; Jonathan provides improved wording]
Discussed on the reflector September 2021.
[2022-10-13; Jonathan revises wording to add a noexcept-specifier]
[2022-11-07; Reflector poll]
Set status to Tentatively Ready after six votes in favour during reflector poll.
[2022-11-12 Approved at November 2022 meeting in Kona. Status changed: Voting → WP.]
Proposed resolution:
This wording is relative to N4917.
Modify the class synopsis in 24.6.2.1 [istream.iterator.general] as indicated:
constexpr istream_iterator(); constexpr istream_iterator(default_sentinel_t); istream_iterator(istream_type& s); constexpr istream_iterator(const istream_iterator& x) noexcept(see below)= default; ~istream_iterator() = default; istream_iterator& operator=(const istream_iterator&) = default;
Modify 24.6.2.2 [istream.iterator.cons] as indicated:
constexpr istream_iterator(const istream_iterator& x) noexcept(see below)= default;
-5- Postconditions:in_stream == x.in_stream
istrue
.-?- Effects: Initializes
in_stream
withx.in_stream
and initializesvalue
withx.value
.-6- Remarks:
IfAn invocation of this constructor may be used in a core constant expression if and only if the initialization ofis_trivially_copy_constructible_v<T>
istrue
, then this constructor is trivial.value
fromx.value
is a constant subexpression ([defns.const.subexpr]). The exception specification is equivalent tois_nothrow_copy_constructible_v<T>
.
common_iterator
's postfix-proxy
needs indirectly_readable
Section: 24.5.5.5 [common.iter.nav] Status: C++23 Submitter: Casey Carter Opened: 2021-09-24 Last modified: 2023-11-22
Priority: Not Prioritized
View all other issues in [common.iter.nav].
View all issues with C++23 status.
Discussion:
It would appear that when P2259R1 added postfix-proxy
to
common_iterator::operator++(int)
LWG missed a crucial difference between operator++(int)
and
operator->
which uses a similar proxy: operator->
requires the wrapped type to be
indirectly_readable
, but operator++(int)
does not. Consequently, operations that read from the
wrapped type for the postfix-proxy
case in operator++(int)
are not properly constrained
to be valid.
[2021-10-14; Reflector poll]
Set status to Tentatively Ready after six votes in favour during reflector poll.
[2022-02-10 Approved at February 2022 virtual plenary. Status changed: Tentatively Ready → WP.]
Proposed resolution:
This wording is relative to N4892.
Modify 24.5.5.5 [common.iter.nav] as indicated:
decltype(auto) operator++(int);-4- Preconditions:
-5- Effects: Ifholds_alternative<I>(v_)
istrue
.I
modelsforward_iterator
, equivalent to:common_iterator tmp = *this; ++*this; return tmp;Otherwise, if
requires (I& i) { { *i++ } -> can-reference; }
istrue
orindirectly_readable<I> && constructible_from<iter_value_t<I>, iter_reference_t<I>> && move_constructible<iter_value_t<I>>
isfalse
, equivalent to:return get<I>(v_)++;Otherwise, equivalent to: […]
contiguous_iterator
should not be allowed to have custom iter_move
and iter_swap
behaviorSection: 24.3.4.14 [iterator.concept.contiguous] Status: C++23 Submitter: Tim Song Opened: 2021-09-28 Last modified: 2023-11-22
Priority: Not Prioritized
View all other issues in [iterator.concept.contiguous].
View all issues with C++23 status.
Discussion:
The iter_move
and iter_swap
customization points were introduced
primarily for proxy iterators. Whatever their application to non-proxy
iterators in general, they should not be allowed to have custom
behavior for contiguous iterators — this new iterator category was
introduced in large part to permit better optimizations, and allowing
custom iter_move/iter_swap
prevents such optimizations for a wide
variety of algorithms that are specified to call them.
[2021-10-14; Reflector poll]
Set status to Tentatively Ready after seven votes in favour during reflector poll.
[2022-02-10 Approved at February 2022 virtual plenary. Status changed: Tentatively Ready → WP.]
Proposed resolution:
This wording is relative to N4892.
Modify 24.3.4.14 [iterator.concept.contiguous] as indicated:
template<class I> concept contiguous_iterator = random_access_iterator<I> && derived_from<ITER_CONCEPT(I), contiguous_iterator_tag> && is_lvalue_reference_v<iter_reference_t<I>> && same_as<iter_value_t<I>, remove_cvref_t<iter_reference_t<I>>> && requires(const I& i) { { to_address(i) } -> same_as<add_pointer_t<iter_reference_t<I>>>; };-2- Let
a
andb
be dereferenceable iterators andc
be a non-dereferenceable iterator of typeI
such thatb
is reachable froma
andc
is reachable fromb
, and letD
beiter_difference_t<I>
. The typeI
modelscontiguous_iterator
only if
(2.1) —
to_address(a) == addressof(*a)
,(2.2) —
to_address(b) == to_address(a) + D(b - a)
,and(2.3) —
to_address(c) == to_address(a) + D(c - a)
.,(2.?) —
ranges::iter_move(a)
has the same type, value category, and effects asstd::move(*a)
, and(2.?) — if
ranges::iter_swap(a, b)
is well-formed, it has effects equivalent toranges::swap(*a, *b)
.
iota_view::size
sometimes rejects integer-class typesSection: 25.6.4.2 [range.iota.view] Status: C++23 Submitter: Jiang An Opened: 2021-09-29 Last modified: 2023-11-22
Priority: Not Prioritized
View all other issues in [range.iota.view].
View all issues with C++23 status.
Discussion:
It seems that the iota_view
tends to accept integer-class types as its value types, by using
is-integer-like
or is-signed-integer-like
through the specification,
although it's unspecified whether any of them satisfies weakly_incrementable
. However, the
requires-clause of iota_view::size
(25.6.4.2 [range.iota.view] p16) uses
(integral<W> && integral<Bound>)
, which sometimes rejects integer-class types.
(is-integer-like<W> &&
is-integer-like<Bound>)
?
[2021-10-14; Reflector poll]
Set status to Tentatively Ready after six votes in favour during reflector poll.
[2022-02-10 Approved at February 2022 virtual plenary. Status changed: Tentatively Ready → WP.]
Proposed resolution:
This wording is relative to N4892.
Modify 25.6.4.2 [range.iota.view] as indicated:
constexpr auto size() const requires see below;-15- Effects: Equivalent to:
if constexpr (is-integer-like<W> && is-integer-like<Bound>) return (value_ < 0) ? ((bound_ < 0) ? to-unsigned-like(-value_) - to-unsigned-like(-bound_) : to-unsigned-like(bound_) + to-unsigned-like(-value_)) : to-unsigned-like(bound_) - to-unsigned-like(value_); else return to-unsigned-like(bound_ - value_);-16- Remarks: The expression in the requires-clause is equivalent to:
(same_as<W, Bound> && advanceable<W>) || (integralis-integer-like<W> &&integralis-integer-like<Bound>) || sized_sentinel_for<Bound, W>
std::format
Section: 28.5.2.2 [format.string.std] Status: C++23 Submitter: Victor Zverovich Opened: 2021-10-02 Last modified: 2023-11-22
Priority: Not Prioritized
View other active issues in [format.string.std].
View all other issues in [format.string.std].
View all issues with C++23 status.
Discussion:
According to [tab:format.type.ptr] pointers are formatted as hexadecimal integers (at least in the
common case when uintptr_t
is available). However, it appears that they have left alignment
by default according to [tab:format.align]:
Forces the field to be aligned to the start of the available space. This is the default for non-arithmetic types,
charT
, andbool
, unless an integer presentation type is specified.
because pointers are not arithmetic types.
For example:void* p = … std::format("{:#16x}", std::bit_cast<uintptr_t>(p)); std::format("{:16}", p);
may produce " 0x7fff88716c84"
and "0x7fff88716c84 "
(the actual output depends
on the value of p
).
[2021-10-14; Reflector poll]
Set status to Tentatively Ready after seven votes in favour during reflector poll.
[2022-02-10 Approved at February 2022 virtual plenary. Status changed: Tentatively Ready → WP.]
Proposed resolution:
This wording is relative to N4892.
Modify 28.5.2.2 [format.string.std], Table [tab:format.align], as indicated:
Table 59 — Meaning of align options [tab:format.align] Option Meaning <
Forces the field to be aligned to the start of the available space. This is the default for non-arithmetic non-pointer types, charT
, andbool
, unless an integer presentation type is specified.>
Forces the field to be aligned to the end of the available space. This is the default for arithmetic types other than charT
andbool
, pointer types or when an integer presentation type is specified.[…]
[Drafting note: The wording above touches a similar area as LWG 3586(i). To help solving the merge conflict the following shows the delta of this proposed wording on top of the LWG 3586(i) merge result]
Table 59 — Meaning of align options [tab:format.align] Option Meaning <
Forces the field to be aligned to the start of the available space. This is the default when the presentation type is a non-arithmetic non-pointer type. >
Forces the field to be aligned to the end of the available space. This is the default when the presentation type is an arithmetic or pointer type. […]
swap
for basic_syncbuf
Section: 31.11.2.6 [syncstream.syncbuf.special] Status: C++23 Submitter: S. B. Tam Opened: 2021-10-07 Last modified: 2023-11-22
Priority: Not Prioritized
View all issues with C++23 status.
Discussion:
LWG 3498(i) fixes the inconsistent noexcept
-specifiers for member functions of basic_syncbuf
,
but the proposed resolution in LWG 3498(i) seems to miss the non-member swap
, which also has
inconsistent noexcept
-specifier: 31.11.2.6 [syncstream.syncbuf.special] says it's noexcept
,
while 31.11.1 [syncstream.syn] says it's not.
swap
and the member swap
have equivalent effect, and LWG 3498 removes
noexcept
from the latter, I think it's pretty clear that the former should not be noexcept
.
[2021-10-14; Reflector poll]
Set status to Tentatively Ready after seven votes in favour during reflector poll.
[2022-02-10 Approved at February 2022 virtual plenary. Status changed: Tentatively Ready → WP.]
[2022-02-10; Jens comments]
The desired fix was applied editorially while applying
P2450 C++ Standard Library Issues to be moved in Virtual Plenary, Oct. 2021Proposed resolution:
This wording is relative to N4892.
Modify 31.11.2.6 [syncstream.syncbuf.special] as indicated:
template<class charT, class traits, class Allocator> void swap(basic_syncbuf<charT, traits, Allocator>& a, basic_syncbuf<charT, traits, Allocator>& b)noexcept;-1- Effects: Equivalent to
a.swap(b)
.
function
/packaged_task
deduction guides and deducing this
Section: 22.10.17.3.2 [func.wrap.func.con], 32.10.10.2 [futures.task.members] Status: C++23 Submitter: Barry Revzin Opened: 2021-10-09 Last modified: 2023-11-22
Priority: 2
View all other issues in [func.wrap.func.con].
View all issues with C++23 status.
Discussion:
With the adoption of deducing this
(P0847), we can now create
types whose call operator is an explicit object member function, which means that decltype(&F::operator())
could have pointer-to-function type rather than pointer-to-member-function type. This means that the deduction
guides for std::function
(22.10.17.3.2 [func.wrap.func.con]) and std::packaged_task
(32.10.10.2 [futures.task.members]) will simply fail:
struct F { int operator()(this const F&) { return 42; } }; std::function g = F{}; // error: decltype(&F::operator()) is not of the form R(G::*)(A...) cv &opt noexceptopt
We should update the deduction guides to keep them in line with the core language.
[2021-10-14; Reflector poll]
Set priority to 2 after reflector poll.
Previous resolution [SUPERSEDED]:
This wording is relative to N4892.
Modify 22.10.17.3.2 [func.wrap.func.con] as indicated:
template<class F> function(F) -> function<see below>;-16- Constraints:
-17- Remarks: The deduced type is&F::operator()
is well-formed when treated as an unevaluated operand anddecltype(&F::operator())
is either of the formR(G::*)(A...) cv &opt noexceptopt
or of the formR(*)(G cv &opt, A...) noexceptopt
for aclasstypeG
.function<R(A...)>
.Modify 32.10.10.2 [futures.task.members] as indicated:
template<class F> packaged_task(F) -> packaged_task<see below>;-7- Constraints:
-8- Remarks: The deduced type is&F::operator()
is well-formed when treated as an unevaluated operand anddecltype(&F::operator())
is either of the formR(G::*)(A...) cv &opt noexceptopt
or of the formR(*)(G cv &opt, A...) noexceptopt
for aclasstypeG
.packaged_task<R(A...)>
.
[2021-10-17; Improved wording based on Tim Song's suggestion]
[2022-07-01; Reflector poll]
Set status to Tentatively Ready after nine votes in favour during reflector poll.
[2022-07-15; LWG telecon: move to Ready]
[2022-07-25 Approved at July 2022 virtual plenary. Status changed: Ready → WP.]
Proposed resolution:
This wording is relative to N4892.
Modify 22.10.17.3.2 [func.wrap.func.con] as indicated:
template<class F> function(F) -> function<see below>;-16- Constraints:
-17- Remarks: The deduced type is&F::operator()
is well-formed when treated as an unevaluated operand anddecltype(&F::operator())
is either of the formR(G::*)(A...) cv &opt noexceptopt
or of the formR(*)(G, A...) noexceptopt
for aclasstypeG
.function<R(A...)>
.
Modify 32.10.10.2 [futures.task.members] as indicated:
template<class F> packaged_task(F) -> packaged_task<see below>;-7- Constraints:
-8- Remarks: The deduced type is&F::operator()
is well-formed when treated as an unevaluated operand anddecltype(&F::operator())
is either of the formR(G::*)(A...) cv &opt noexceptopt
or of the formR(*)(G, A...) noexceptopt
for aclasstypeG
.packaged_task<R(A...)>
.
iter_move
for transform_view::iterator
Section: 25.7.9.3 [range.transform.iterator] Status: C++23 Submitter: Barry Revzin Opened: 2021-10-12 Last modified: 2023-11-22
Priority: Not Prioritized
View all other issues in [range.transform.iterator].
View all issues with C++23 status.
Discussion:
transform_view
's iterator currently specifies a customization point for iter_move
.
This customization point does the same thing that the default implementation would do —
std::move(*E)
if *E
is an lvalue, else *E
— except that it is only
there to provide the correct conditional noexcept
specification. But for iota_view
,
we instead provide a noexcept
-specifier on the iterator's operator*
. We should do
the same thing for both, and the simplest thing would be:
noexcept
:
constexpr decltype(auto) operator*() const noexcept(noexcept(invoke(*parent_->fun_, *current_))) { return invoke(*parent_->fun_, *current_); }
And to remove this:
friend constexpr decltype(auto) iter_move(const iterator& i) noexcept(noexcept(invoke(*i.parent_->fun_, *i.current_))) { if constexpr (is_lvalue_reference_v<decltype(*i)>) return std::move(*i); else return *i; }
[2022-01-29; Reflector poll]
Set status to Tentatively Ready after seven votes in favour during reflector poll.
[2022-02-10 Approved at February 2022 virtual plenary. Status changed: Tentatively Ready → WP.]
Proposed resolution:
This wording is relative to N4892.
Modify 25.7.9.3 [range.transform.iterator], class template transform_view::iterator
,
as indicated:
[…] constexpr decltype(auto) operator*() const noexcept(noexcept(invoke(*parent_->fun_, *current_))) { return invoke(*parent_->fun_, *current_); } […]friend constexpr decltype(auto) iter_move(const iterator& i) noexcept(noexcept(invoke(*i.parent_->fun_, *i.current_))) { if constexpr (is_lvalue_reference_v<decltype(*i)>) return std::move(*i); else return *i; }[…]
vformat_to
contains ill-formed formatted_size
callsSection: 28.5.5 [format.functions] Status: C++23 Submitter: Tim Song Opened: 2021-10-17 Last modified: 2023-11-22
Priority: Not Prioritized
View all other issues in [format.functions].
View all issues with C++23 status.
Discussion:
The specification of vformat_to
says that it formats "into the range
[out, out + N)
, where N
is formatted_size(fmt, args...)
for the
functions without a loc
parameter and formatted_size(loc, fmt, args...)
for the functions with a loc
parameter".
First, args
is a (w)format_args
, not a pack, so it doesn't make
sense to use ...
to expand it.
Second, fmt
is a (w)string_view
parameter, and it is never a
constant expression, and so the call is ill-formed after P2216 added
compile-time format string checking.
[2022-01-29; Reflector poll]
Set status to Tentatively Ready after six votes in favour during reflector poll.
[2022-02-10 Approved at February 2022 virtual plenary. Status changed: Tentatively Ready → WP.]
Proposed resolution:
This wording is relative to N4892.
Modify 28.5.5 [format.functions] as indicated:
template<class Out> Out vformat_to(Out out, string_view fmt, format_args args); template<class Out> Out vformat_to(Out out, wstring_view fmt, wformat_args args); template<class Out> Out vformat_to(Out out, const locale& loc, string_view fmt, format_args args); template<class Out> Out vformat_to(Out out, const locale& loc, wstring_view fmt, wformat_args args);-12- Let
-13- Constraints:charT
bedecltype(fmt)::value_type
.Out
satisfiesoutput_iterator<const charT&>
. -14- Preconditions:Out
modelsoutput_iterator<const charT&>
. -15- Effects: Places the character representation of formatting the arguments provided byargs
, formatted according to the specifications given infmt
, into the range[out, out + N)
, whereN
isthe number of characters in that character representation. If present,formatted_size(fmt, args...)
for the functions without aloc
parameter andformatted_size(loc, fmt, args...)
for the functions with aloc
parameterloc
is used for locale-specific formatting. -16- Returns:out + N
. -17- […]
__cpp_lib_monadic_optional
Section: 17.3.2 [version.syn] Status: C++23 Submitter: Jens Maurer Opened: 2021-10-18 Last modified: 2023-11-22
Priority: Not Prioritized
View all other issues in [version.syn].
View all issues with C++23 status.
Discussion:
P0798R8 "Monadic operations for std::optional
"
created a new feature-test macro __cpp_lib_monadic_optional
for a relatively minor enhancement.
__cpp_lib_optional
.
[2022-01-29; Reflector poll]
Set status to Tentatively Ready after eight votes in favour during reflector poll.
[2022-02-10 Approved at February 2022 virtual plenary. Status changed: Tentatively Ready → WP.]
Proposed resolution:
This wording is relative to N4901.
Modify 17.3.2 [version.syn] as indicated:
[…]#define __cpp_lib_monadic_optional 202110L // also in <optional>[…] #define __cpp_lib_optional202106L202110L // also in <optional> […]
Section: 23.2.8.1 [unord.req.general] Status: C++23 Submitter: Thomas Köppe Opened: 2021-10-20 Last modified: 2023-11-22
Priority: 2
View all issues with C++23 status.
Discussion:
The paper P2077R3 ("Heterogeneous erasure overloads for associative containers")
adds a new variable kx
with specific meaning for use in the Table of Unordered Associative
Container Requirements, 23.2.8.1 [unord.req.general] p11, which is meant to stand for an
equivalence class of heterogeneous values that can be compared with container keys.
kx
is transitivity of equality/equivalence, but this is currently specified as:
"
kx
is a value such that […](eq(r1, kx) && eq(r1, r2)) == eq(r2, kx)
[…], wherer1
andr2
are [any] keys".
But this doesn't seem right. Transitivity means that eq(r1, kx) && eq(r1, r2)
being
true implies eq(r2, kx)
being true, but it does not imply that both sides are equal
in general. In particular, eq(r2, kx)
can be true even when eq(r1, kx) && eq(r1, r2)
is false.
ke
", which
suffers from the same problem, and so we propose to fix both cases.
[2022-01-29; Reflector poll]
Set priority to 2 after reflector poll.
Previous resolution [SUPERSEDED]:
This wording is relative to N4901.
Modify 23.2.8.1 [unord.req.general] as indicated:
[…]
(11.19) —
ke
is a value such that
(11.19.1) —
eq(r1, ke) == eq(ke, r1)
,(11.19.2) —
hf(r1) == hf(ke)
ifeq(r1, ke)
istrue
, and(11.19.3) —
(eq(r1, ke) && eq(r1, r2)) == eq(r2, ke)
eq(ke, r2)
istrue
ifeq(ke, r1) && eq(r1, r2)
istrue
,where
r1
andr2
are keys of elements ina_tran
,(11.20) —
kx
is a value such that
(11.20.1) —
eq(r1, kx) == eq(kx, r1)
,(11.20.2) —
hf(r1) == hf(kx)
ifeq(r1, kx)
istrue
,(11.20.3) —
(eq(r1, kx) && eq(r1, r2)) == eq(r2, kx)
eq(kx, r2)
istrue
ifeq(kx, r1) && eq(r1, r2)
istrue
, and(11.20.4) —
kx
is not convertible to eitheriterator
orconst_iterator
,where
r1
andr2
are keys of elements ina_tran
,[…]
[2022-02-07 Tim comments and provides updated wording]
For heterogeneous lookup on unordered containers to work properly, we
need all keys comparing equal to the transparent key to be grouped
together. Since the only keys guaranteed to be so grouped are the ones
that are equal according to eq
, we cannot allow
eq(r1, r2) == false
but eq(r1, ke) == true &&
eq(r2, ke) == true
. The one-way
transitivity of equality is insufficient.
eq(r1, ke)
is true
and eq(r1, r2)
is true
then eq(r2, ke)
is true
.
eq(r1, ke)
is true
and eq(r2, ke)
is true
then eq(r1, r2)
is true
In a table:
eq(r1, ke) | eq(r1, r2) | eq(r2, ke) | OK? |
---|---|---|---|
T | T | T | Y |
T | T | F | N |
T | F | T | N |
T | F | F | Y |
F | T | T | N |
F | T | F | Y |
F | F | T | Y |
F | F | F | Y |
[Issaquah 2023-02-08; LWG]
Move to Immediate for C++23
[2023-02-13 Approved at February 2023 meeting in Issaquah. Status changed: Immediate → WP.]
Proposed resolution:
This wording is relative to N4928.
Modify 23.2.8.1 [unord.req.general] as indicated:
[…]
(10.20) —
ke
is a value such that
(10.20.1) —
eq(r1, ke) == eq(ke, r1)
,(10.20.2) —
hf(r1) == hf(ke)
ifeq(r1, ke)
istrue
, and(10.20.3) —
if any two of(eq(r1, ke) && eq(r1, r2)) == eq(r2, ke)
eq(r1, ke)
,eq(r2, ke)
andeq(r1, r2)
aretrue
, then all three aretrue
,where
r1
andr2
are keys of elements ina_tran
,(10.21) —
kx
is a value such that
(10.21.1) —
eq(r1, kx) == eq(kx, r1)
,(10.21.2) —
hf(r1) == hf(kx)
ifeq(r1, kx)
istrue
,(10.21.3) —
if any two of(eq(r1, kx) && eq(r1, r2)) == eq(r2, kx)
eq(r1, kx)
,eq(r2, kx)
andeq(r1, r2)
aretrue
, then all three aretrue
, and(10.21.4) —
kx
is not convertible to eitheriterator
orconst_iterator
,where
r1
andr2
are keys of elements ina_tran
,[…]
make_error_code
and make_error_condition
are customization pointsSection: 19.5 [syserr] Status: C++23 Submitter: Jonathan Wakely Opened: 2021-10-31 Last modified: 2023-11-22
Priority: 2
View all other issues in [syserr].
View all issues with C++23 status.
Discussion:
The rule in 16.4.2.2 [contents] means that the calls to
make_error_code
in 19.5.4.2 [syserr.errcode.constructors]
and 19.5.4.3 [syserr.errcode.modifiers] are required to call
std::make_error_code
,
which means program-defined error codes do not work.
The same applies to the make_error_condition
calls in
19.5.5.2 [syserr.errcondition.constructors] and
19.5.5.3 [syserr.errcondition.modifiers].
They need to use ADL. This is what all known implementations (including Boost.System) do.
[2022-01-29; Reflector poll]
Set priority to 2 after reflector poll.
Previous resolution [SUPERSEDED]:
This wording is relative to N4901.
Modify 19.5.2 [system.error.syn] as indicated:
-1- The value of each
enum errc
constant shall be the same as the value of the<cerrno>
macro shown in the above synopsis. Whether or not the<system_error>
implementation exposes the<cerrno>
macros is unspecified.-?- Invocations of
make_error_code
andmake_error_condition
shown in subclause 19.5 [syserr] select a function to call via overload resolution (12.2 [over.match]) on a candidate set that includes the lookup set found by argument dependent lookup (6.5.4 [basic.lookup.argdep]).-2- The
is_error_code_enum
andis_error_condition_enum
templates may be specialized for program-defined types to indicate that such types are eligible forclass error_code
andclass error_condition
implicit conversions, respectively.[Note 1: Conversions from such types are done by program-defined overloads of
make_error_code
andmake_error_condition
, found by ADL. —end note]
[2022-08-25; Jonathan Wakely provides improved wording]
Discussed in LWG telecon and decided on new direction:
make_error_code
and make_error_condition
to 16.4.2.2 [contents] as done for swap
.
Describe form of lookup used for them.error_code
and error_condition
constructors in terms of "Effects: Equivalent to" so that the
requirements on program-defined overloads found by ADL are implied by those
effects.[2022-09-07; Jonathan Wakely revises wording]
Discussed in LWG telecon. Decided to change "established as-if by performing unqualified name lookup and argument-dependent lookup" to simply "established as-if by performing argument-dependent lookup".
This resolves the question of whether std::make_error_code(errc)
,
std::make_error_code(io_errc)
, etc. should be visible to the
unqualified name lookup. This affects whether a program-defined type that
specializes is_error_code_enum
but doesn't provide an overload of
make_error_code
should find the overloads in namespace std
and consider them for overload resolution, via implicit conversion to
std::errc
, std::io_errc
, etc.
[2022-09-23; Reflector poll]
Set status to Tentatively Ready after seven votes in favour during reflector poll.
[2022-11-12 Approved at November 2022 meeting in Kona. Status changed: Voting → WP.]
Proposed resolution:
This wording is relative to N4910.
Modify 16.4.2.2 [contents] as indicated:
-3- Whenever an unqualified name other than
swap
,make_error_code
, ormake_error_condition
is used in the specification of a declarationD
in 17 [support] through 32 [thread] or D [depr], its meaning is established as-if by performing unqualified name lookup (6.5.3 [basic.lookup.unqual]) in the context ofD
.[Note 1: Argument-dependent lookup is not performed. — end note]
Similarly, the meaning of a qualified-id is established as-if by performing qualified name lookup (6.5.5 [basic.lookup.qual]) in the context of
D
.[Example 1: The reference to
is_array_v
in the specification ofstd::to_array
(23.3.3.6 [array.creation]) refers to::std::is_array_v
. — end example][Note 2: Operators in expressions (12.2.2.3 [over.match.oper]) are not so constrained; see 16.4.6.4 [global.functions]. — end note]
The meaning of the unqualified name
swap
is established in an overload resolution context for swappable values (16.4.4.3 [swappable.requirements]). The meanings of the unqualified namesmake_error_code
andmake_error_condition
are established as-if by performing argument-dependent lookup (6.5.4 [basic.lookup.argdep]).
Modify 19.5.4.2 [syserr.errcode.constructors] as indicated:
error_code() noexcept;
-1- Postconditions:val_ == 0
andcat_ == &system_category()
.
-1- Effects: Initializesval_
with0
andcat_
with&system_category()
.error_code(int val, const error_category& cat) noexcept;
-2- Postconditions:val_ == val
andcat_ == &cat
.
-2- Effects: Initializesval_
withval
andcat_
with&cat
.template<class ErrorCodeEnum> error_code(ErrorCodeEnum e) noexcept;-3- Constraints:
is_error_code_enum_v<ErrorCodeEnum>
istrue
.
-4- Postconditions:*this == make_error_code(e)
.
-4- Effects: Equivalent to:error_code ec = make_error_code(e); assign(ec.value(), ec.category());
Modify 19.5.4.3 [syserr.errcode.modifiers] as indicated:
template<class ErrorCodeEnum> error_code& operator=(ErrorCodeEnum e) noexcept;-2- Constraints:
is_error_code_enum_v<ErrorCodeEnum>
istrue
.
-3- Postconditions:*this == make_error_code(e)
.
-3- Effects: Equivalent to:error_code ec = make_error_code(e); assign(ec.value(), ec.category());-4- Returns:
*this
.
Modify 19.5.5.2 [syserr.errcondition.constructors] as indicated:
error_condition() noexcept;
-1- Postconditions:val_ == 0
andcat_ == &generic_category()
.
-1- Effects: Initializesval_
with0
andcat_
with&generic_category()
.error_condition(int val, const error_category& cat) noexcept;
-2- Postconditions:val_ == val
andcat_ == &cat
.
-2- Effects: Initializesval_
withval
andcat_
with&cat
.template<class ErrorConditionEnum> error_condition(ErrorConditionEnum e) noexcept;-3- Constraints:
is_error_condition_enum_v<ErrorConditionEnum>
istrue
.
-4- Postconditions:*this == make_error_condition(e)
.
-4- Effects: Equivalent to:error_condition ec = make_error_condition(e); assign(ec.value(), ec.category());
Modify 19.5.5.3 [syserr.errcondition.modifiers] as indicated:
template<class ErrorConditionEnum> error_condition& operator=(ErrorConditionEnum e) noexcept;-2- Constraints:
is_error_condition_enum_v<ErrorConditionEnum>
istrue
.
-3- Postconditions:*this == make_error_condition(e)
.
-3- Effects: Equivalent to:error_condition ec = make_error_condition(e); assign(ec.value(), ec.category());-4- Returns:
*this
.
basic_format_arg(T&&)
should use remove_cvref_t<T>
throughoutSection: 28.5.8.1 [format.arg] Status: C++23 Submitter: Tim Song Opened: 2021-11-03 Last modified: 2023-11-22
Priority: 3
View all other issues in [format.arg].
View all issues with C++23 status.
Discussion:
P2418R2 changed basic_format_arg
's constructor to take a forwarding
reference but didn't change 28.5.8.1 [format.arg]/5 which inspects various
properties of T
. Now that the deduced type can be cvref-qualified,
they need to be removed before the checks.
[2022-01-29; Reflector poll]
Set priority to 3 after reflector poll.
Two suggestions to just change it to be T&
because we don't
need forwarding references here, and only accepting lvalues prevents
forming dangling references.
Previous resolution [SUPERSEDED]:
This wording is relative to N4901.
Modify 28.5.8.1 [format.arg] as indicated:
template<class T> explicit basic_format_arg(T&& v) noexcept;-?- Let
-4- Constraints: The template specializationTD
beremove_cvref_t<T>
.typename Context::template formatter_type<remove_cvref_t<T>TD>meets the BasicFormatter requirements (28.5.6.1 [formatter.requirements]). The extent to which an implementation determines that the specialization meets the BasicFormatter requirements is unspecified, except that as a minimum the expression
typename Context::template formatter_type<remove_cvref_t<T>TD>() .format(declval<T&>(), declval<Context&>())shall be well-formed when treated as an unevaluated operand (7.2.3 [expr.context]).
-5- Effects:
(5.1) — if
TD
isbool
orchar_type
, initializesvalue
withv
;(5.2) — otherwise, if
TD
ischar
andchar_type
iswchar_t
, initializesvalue
withstatic_cast<wchar_t>(v)
;(5.3) — otherwise, if
TD
is a signed integer type (6.8.2 [basic.fundamental]) andsizeof(TD) <= sizeof(int)
, initializesvalue
withstatic_cast<int>(v)
;(5.4) — otherwise, if
TD
is an unsigned integer type andsizeof(TD) <= sizeof(unsigned int)
, initializesvalue
withstatic_cast<unsigned int>(v)
;(5.5) — otherwise, if
TD
is a signed integer type andsizeof(TD) <= sizeof(long long int)
, initializesvalue
withstatic_cast<long long int>(v)
;(5.6) — otherwise, if
TD
is an unsigned integer type andsizeof(TD) <= sizeof(unsigned long long int)
, initializesvalue
withstatic_cast<unsigned long long int>(v)
;(5.7) — otherwise, initializes
value
withhandle(v)
.
[2022-10-19; Jonathan provides improved wording]
During reflector prioritization it was pointed out that forwarding references
are unnecessary (arguments are always lvalues), and so using T&
would be simpler.
In order to resolve the overload ambiguities discussed in 3718(i) replace all unary constructor overloads with a single constructor that works for any formattable type.
Previous resolution [SUPERSEDED]:
This wording is relative to N4917.
Modify 28.5.8.1 [format.arg] as indicated:
template<class T> explicit basic_format_arg(T&&v) noexcept;-4- Constraints:
T
satisfiesformattable<char_type>
.The template specializationtypename Context::template formatter_type<remove_cvref_t<T>>
meets the BasicFormatter requirements (28.5.6.1 [formatter.requirements]). The extent to which an implementation determines that the specialization meets the BasicFormatter requirements is unspecified, except that as a minimum the expressiontypename Context::template formatter_type<remove_cvref_t<T>>() .format(declval<T&>(), declval<Context&>())
shall be well-formed when treated as an unevaluated operand (7.2.3 [expr.context]).-?- Preconditions: If
decay_t<T>
ischar_type*
orconst char_type*
,static_cast<const char_type*>(v)
points to a NTCTS (3.36 [defns.ntcts]).-5- Effects: Let
TD
beremove_const_t<T>
.
(5.1) — if
TD
isbool
orchar_type
, initializesvalue
withv
;(5.2) — otherwise, if
TD
ischar
andchar_type
iswchar_t
, initializesvalue
withstatic_cast<wchar_t>(v)
;(5.3) — otherwise, if
TD
is a signed integer type (6.8.2 [basic.fundamental]) andsizeof(TD) <= sizeof(int)
, initializesvalue
withstatic_cast<int>(v)
;(5.4) — otherwise, if
TD
is an unsigned integer type andsizeof(TD) <= sizeof(unsigned int)
, initializesvalue
withstatic_cast<unsigned int>(v)
;(5.5) — otherwise, if
TD
is a signed integer type andsizeof(TD) <= sizeof(long long int)
, initializesvalue
withstatic_cast<long long int>(v)
;(5.6) — otherwise, if
TD
is an unsigned integer type andsizeof(TD) <= sizeof(unsigned long long int)
, initializesvalue
withstatic_cast<unsigned long long int>(v)
;(5.?) — otherwise, if
TD
is a standard floating-point type, initializesvalue
withv
;(5.?) — otherwise, if
TD
is a specialization ofbasic_string_view
orbasic_string
andTD::value_type
ischar_type
, initializesvalue
withbasic_string_view<char_type>(v.data(), v.size())
;(5.?) — otherwise, if
decay_t<TD>
ischar_type*
orconst char_type*
, initializesvalue
withstatic_cast<const char_type*>(v)
;(5.?) — otherwise, if
is_void_v<remove_pointer_t<TD>>
istrue
oris_null_pointer_v<TD>
istrue
, initializesvalue
withstatic_cast<const void*>(v)
;(5.7) — otherwise, initializes
value
withhandle(v)
.-?- [Note: Constructing
basic_format_arg
from a pointer to a member is ill-formed unless the user provides an enabled specialization offormatter
for that pointer to member type. — end note]explicit basic_format_arg(float n) noexcept; explicit basic_format_arg(double n) noexcept; explicit basic_format_arg(long double n) noexcept;
-6- Effects: Initializesvalue
withn
.explicit basic_format_arg(const char_type* s) noexcept;
-7- Preconditions:s
points to a NTCTS (3.36 [defns.ntcts]).
-8- Effects: Initializesvalue
withs
.template<class traits>> explicit basic_format_arg(basic_string_view<char_type, traits> s) noexcept;
-9- Effects: Initializesvalue
withbasic_string_view<char_type>(s.data(), s.size())
.template<class traits, class Allocator>> explicit basic_format_arg( const basic_string<char_type, traits, Allocator>& s) noexcept;
-10- Effects: Initializesvalue
withbasic_string_view<char_type>(s.data(), s.size())
.explicit basic_format_arg(nullptr_t) noexcept;
-11- Effects: Initializesvalue
withstatic_cast<const void*>(nullptr)
.template<class T> explicit basic_format_arg(T* p) noexcept;
-12- Constraints:is_void_v<T>
istrue
.
-13- Effects: Initializesvalue
withp
.
-14- [Note: Constructingbasic_format_arg
from a pointer to a member is ill-formed unless the user provides an enabled specialization offormatter
for that pointer to member type. — end note]Modify 28.5.8.1 [format.arg] p17 and p18 as indicated:
template<class T> explicit handle(T&&v) noexcept;-17- Let
— (17.1)
TD
beremove_
,cvrefconst_t<T>
— (17.2)const-formattable
betrue
iftypename Context::template formatter_type<TD>().format(declval<const TD&>(), declval<Context&>())
is well-formed, otherwisefalse
,— (17.3)
TQ
beconst TD
ifconst-formattable
istrue
const TD
satisfiesformattable<char_type>
andTD
otherwise.-18- Mandates:
is
const-formattableformattable<const TD, char_type>
|| !is_const_v<remove_reference_t<T>true
.-19- Effects: Initializes
ptr_
withaddressof(val)
andformat_
with[](basic_format_parse_context<char_type>& parse_ctx, Context& format_ctx, const void* ptr) { typename Context::template formatter_type<TD> f; parse_ctx.advance_to(f.parse(parse_ctx)); format_ctx.advance_to(f.format(*const_cast<TQ*>(static_cast<const TD*>(ptr)), format_ctx)); }
[2022-11-10; Jonathan provides improved wording]
[2022-11-29; Casey expands the issue to also cover make_format_args
]
[2023-01-11; Jonathan adds the missing edits to the class synopsis]
[Issaquah 2023-02-07; LWG]
Edited proposed resolution to
remove extra =
in concept definition
and capitialize start of (5.1).
Move to Immediate for C++23
[2023-02-13 Approved at February 2023 meeting in Issaquah. Status changed: Immediate → WP.]
Proposed resolution:
This wording is relative to N4917.
Modify 28.5.6.3 [format.formattable] as indicated:
-1- Let fmt-iter-for<charT>
be an unspecified type that models
output_iterator<const charT&>
(24.3.4.10 [iterator.concept.output]).
template<class T, class Context, class Formatter = typename Context::template formatter_type<remove_const_t<T>>> concept formattable-with = // exposition only semiregular<Formatter> && requires (Formatter& f, const Formatter& cf, T&& t, Context fc, basic_format_parse_context<typename Context::char_type> pc) { { f.parse(pc) } -> same_as<typename decltype(pc)::iterator>; { cf.format(t, fc) } -> same_as<typename Context::iterator>; }; template<class T, class charT> concept formattable =semiregular<formatter<remove_cvref_t<T>, charT>> && requires(formatter<remove_cvref_t<T>, charT> f, const formatter<remove_cvref_t<T>, charT> cf, T t, basic_format_context<fmt-iter-for<charT>, charT> fc, basic_format_parse_context<charT> pc) { { f.parse(pc) } -> same_as<basic_format_parse_context<charT>::iterator>; { cf.format(t, fc) } -> same_as<fmt-iter-for<charT>>; };formattable-with<remove_reference_t<T>, basic_format_context<fmt-iter-for<charT>>>;
Modify 28.5.8.1 [format.arg] as indicated:
namespace std { template<class Context> class basic_format_arg { public: class handle; private: using char_type = typename Context::char_type; // exposition only variant<monostate, bool, char_type, int, unsigned int, long long int, unsigned long long int, float, double, long double, const char_type*, basic_string_view<char_type>, const void*, handle> value; // exposition only template<class T> explicit basic_format_arg(T&…&v) noexcept; // exposition onlyexplicit basic_format_arg(float n) noexcept; // exposition onlyexplicit basic_format_arg(double n) noexcept; // exposition onlyexplicit basic_format_arg(long double n) noexcept; // exposition onlyexplicit basic_format_arg(const char_type* s); // exposition onlytemplate<class traits>explicit basic_format_arg(basic_string_view<char_type, traits> s) noexcept; // exposition onlytemplate<class traits, class Allocator>explicit basic_format_arg(const basic_string<char_type, traits, Allocator>& s) noexcept; // exposition onlyexplicit basic_format_arg(nullptr_t) noexcept; // exposition onlytemplate<class T>explicit basic_format_arg(T* p) noexcept; // exposition only
template<class T> explicit basic_format_arg(T&&v) noexcept;-4- Constraints:
T
satisfiesformattable-with<Context>
.The template specializationtypename Context::template formatter_type<remove_cvref_t<T>>
meets the BasicFormatter requirements (28.5.6.1 [formatter.requirements]). The extent to which an implementation determines that the specialization meets the BasicFormatter requirements is unspecified, except that as a minimum the expressiontypename Context::template formatter_type<remove_cvref_t<T>>() .format(declval<T&>(), declval<Context&>())
shall be well-formed when treated as an unevaluated operand (7.2.3 [expr.context]).-?- Preconditions: If
decay_t<T>
ischar_type*
orconst char_type*
,static_cast<const char_type*>(v)
points to a NTCTS (3.36 [defns.ntcts]).-5- Effects: Let
TD
beremove_const_t<T>
.
(5.1) — If
ifTD
isbool
orchar_type
, initializesvalue
withv
;(5.2) — otherwise, if
TD
ischar
andchar_type
iswchar_t
, initializesvalue
withstatic_cast<wchar_t>(v)
;(5.3) — otherwise, if
TD
is a signed integer type (6.8.2 [basic.fundamental]) andsizeof(TD) <= sizeof(int)
, initializesvalue
withstatic_cast<int>(v)
;(5.4) — otherwise, if
TD
is an unsigned integer type andsizeof(TD) <= sizeof(unsigned int)
, initializesvalue
withstatic_cast<unsigned int>(v)
;(5.5) — otherwise, if
TD
is a signed integer type andsizeof(TD) <= sizeof(long long int)
, initializesvalue
withstatic_cast<long long int>(v)
;(5.6) — otherwise, if
TD
is an unsigned integer type andsizeof(TD) <= sizeof(unsigned long long int)
, initializesvalue
withstatic_cast<unsigned long long int>(v)
;(5.?) — otherwise, if
TD
is a standard floating-point type, initializesvalue
withv
;(5.?) — otherwise, if
TD
is a specialization ofbasic_string_view
orbasic_string
andTD::value_type
ischar_type
, initializesvalue
withbasic_string_view<char_type>(v.data(), v.size())
;(5.?) — otherwise, if
decay_t<TD>
ischar_type*
orconst char_type*
, initializesvalue
withstatic_cast<const char_type*>(v)
;(5.?) — otherwise, if
is_void_v<remove_pointer_t<TD>>
istrue
oris_null_pointer_v<TD>
istrue
, initializesvalue
withstatic_cast<const void*>(v)
;(5.7) — otherwise, initializes
value
withhandle(v)
.-?- [Note: Constructing
basic_format_arg
from a pointer to a member is ill-formed unless the user provides an enabled specialization offormatter
for that pointer to member type. — end note]explicit basic_format_arg(float n) noexcept; explicit basic_format_arg(double n) noexcept; explicit basic_format_arg(long double n) noexcept;
-6- Effects: Initializesvalue
withn
.explicit basic_format_arg(const char_type* s) noexcept;
-7- Preconditions:s
points to a NTCTS (3.36 [defns.ntcts]).
-8- Effects: Initializesvalue
withs
.template<class traits>> explicit basic_format_arg(basic_string_view<char_type, traits> s) noexcept;
-9- Effects: Initializesvalue
withbasic_string_view<char_type>(s.data(), s.size())
.template<class traits, class Allocator>> explicit basic_format_arg( const basic_string<char_type, traits, Allocator>& s) noexcept;
-10- Effects: Initializesvalue
withbasic_string_view<char_type>(s.data(), s.size())
.explicit basic_format_arg(nullptr_t) noexcept;
-11- Effects: Initializesvalue
withstatic_cast<const void*>(nullptr)
.template<class T> explicit basic_format_arg(T* p) noexcept;
-12- Constraints:is_void_v<T>
istrue
.
-13- Effects: Initializesvalue
withp
.
-14- [Note: Constructingbasic_format_arg
from a pointer to a member is ill-formed unless the user provides an enabled specialization offormatter
for that pointer to member type. — end note]
Modify 28.5.8.1 [format.arg] p17 and p18 as indicated:
template<class T> explicit handle(T&&v) noexcept;-17- Let
— (17.1)
TD
beremove_
,cvrefconst_t<T>
— (17.2)const-formattable
betrue
iftypename Context::template formatter_type<TD>().format(declval<const TD&>(), declval<Context&>())
is well-formed, otherwisefalse
,— (17.3)
TQ
beconst TD
ifconst-formattable
istrue
const TD
satisfiesformattable-with<Context>
andTD
otherwise.-18- Mandates:
const-formattable || !is_const_v<remove_reference_t<T>>
istrue
.TQ
satisfiesformattable-with<Context>
.-19- Effects: Initializes
ptr_
withaddressof(val)
andformat_
with[](basic_format_parse_context<char_type>& parse_ctx, Context& format_ctx, const void* ptr) { typename Context::template formatter_type<TD> f; parse_ctx.advance_to(f.parse(parse_ctx)); format_ctx.advance_to(f.format(*const_cast<TQ*>(static_cast<const TD*>(ptr)), format_ctx)); }
Modify 28.5.8.2 [format.arg.store] p2 as indicated:
template<class Context = format_context, class... Args> format-arg-store<Context, Args...> make_format_args(Args&&... fmt_args);-2- Preconditions: The type
typename Context::template formatter_type<remove_cvref_t<T
i>>
meets the BasicFormatter requirements (28.5.6.1 [formatter.requirements]) for eachT
i inArgs
.
unique_ptr
"Mandates: This constructor is not selected by class template argument deduction"Section: 20.3.1.3.2 [unique.ptr.single.ctor] Status: C++23 Submitter: Arthur O'Dwyer Opened: 2021-11-03 Last modified: 2023-11-22
Priority: Not Prioritized
View all other issues in [unique.ptr.single.ctor].
View all issues with C++23 status.
Discussion:
P1460R1 changed the wording for unique_ptr
's constructor unique_ptr(pointer)
.
In C++17, it said in [unique.ptr.single.ctor] p5:
explicit unique_ptr(pointer p) noexcept;Preconditions: […]
Effects: […] Postconditions: […] Remarks: Ifis_pointer_v<deleter_type>
istrue
oris_default_constructible_v<deleter_type>
isfalse
, this constructor shall not participate in overload resolution. If class template argument deduction would select the function template corresponding to this constructor, then the program is ill-formed.
In C++20, it says in [unique.ptr.single.ctor] p5:
explicit unique_ptr(pointer p) noexcept;Constraints:
Mandates: This constructor is not selected by class template argument deduction. Preconditions: […] Effects: […] Postconditions: […]is_pointer_v<deleter_type>
isfalse
andis_default_constructible_v<deleter_type>
istrue
.
Normally, we use "Mandates:" for static_assert
-like stuff, not just to indicate that
some constructor doesn't contribute to CTAD. Both libstdc++ and Microsoft (and soon libc++, see
LLVM issue) seem to agree about the intent of this wording:
It's basically asking for the constructor to be implemented with a CTAD firewall, as
explicit unique_ptr(type_identity_t<pointer> p) noexcept;
and there is no actual static_assert
corresponding to this "Mandates:" element. In particular,
the following program is well-formed on all vendors:
// godbolt link template<class T> auto f(T p) -> decltype(std::unique_ptr(p)); template<class T> constexpr bool f(T p) { return true; } static_assert(f((int*)nullptr));
I claim that this is a confusing and/or wrong use of "Mandates:". My proposed resolution is simply to respecify the constructor as
explicit unique_ptr(type_identity_t<pointer> p) noexcept;Constraints:
Preconditions: […] Effects: […] Postconditions: […]is_pointer_v<deleter_type>
isfalse
andis_default_constructible_v<deleter_type>
istrue
.
with no Mandates: or Remarks: elements at all.
[2022-01-29; Reflector poll]
Set status to Tentatively Ready after five votes in favour during reflector poll.
[2022-02-10 Approved at February 2022 virtual plenary. Status changed: Tentatively Ready → WP.]
Proposed resolution:
This wording is relative to N4901.
Modify 20.3.1.3.1 [unique.ptr.single.general], class template unique_ptr
synopsis, as indicated:
[…] // 20.3.1.3.2 [unique.ptr.single.ctor], constructors constexpr unique_ptr() noexcept; explicit unique_ptr(type_identity_t<pointer> p) noexcept; unique_ptr(type_identity_t<pointer> p, see below d1) noexcept; unique_ptr(type_identity_t<pointer> p, see below d2) noexcept; […]
Modify 20.3.1.3.2 [unique.ptr.single.ctor] as indicated:
explicit unique_ptr(type_identity_t<pointer> p) noexcept;-5- Constraints:
is_pointer_v<deleter_type>
isfalse
andis_default_constructible_v<deleter_type>
istrue
.-6- Mandates: This constructor is not selected by class template argument deduction (12.2.2.9 [over.match.class.deduct]).-7- Preconditions: […] -8- Effects: […] -9- Postconditions: […]unique_ptr(type_identity_t<pointer> p, const D& d) noexcept; unique_ptr(type_identity_t<pointer> p, remove_reference_t<D>&& d) noexcept;-10- Constraints:
is_constructible_v<D, decltype(d)>
istrue
.-11- Mandates: These constructors are not selected by class template argument deduction (12.2.2.9 [over.match.class.deduct]).-12- Preconditions: […] -13- Effects: […] -14- Postconditions: […] -15- Remarks: IfD
is a reference type, the second constructor is defined as deleted. -16- [Example 1: […] — end example]
formatter<T>::format
should be const
-qualifiedSection: 28.5.6.1 [formatter.requirements] Status: C++23 Submitter: Arthur O'Dwyer Opened: 2021-11-11 Last modified: 2023-11-22
Priority: 1
View all other issues in [formatter.requirements].
View all issues with C++23 status.
Discussion:
In libc++ review, we've noticed that we don't understand the implications of 28.5.6.1 [formatter.requirements] bullet 3.1 and Table [tab:formatter.basic]: (emphasize mine):
(3.1) —
[…] Table 70: BasicFormatter requirements [tab:formatter.basic] […]f
is a value of typeF
,f.parse(pc)
[must compile] […]f.format(u, fc)
[must compile] […]
According to Victor Zverovich, his intent was that f.parse(pc)
should modify the
state of f
, but f.format(u, fc)
should merely read f
's state to
support format string compilation where formatter objects are immutable and therefore the
format
function must be const
-qualified.
struct WidgetFormatter { auto parse(std::format_parse_context&) -> std::format_parse_context::iterator; auto format(const Widget&, std::format_context&) const -> std::format_context::iterator; };
However, this is not reflected in the wording, which treats parse
and format
symmetrically.
Also, there is at least one example that shows a non-const format
method:
template<> struct std::formatter<color> : std::formatter<const char*> { auto format(color c, format_context& ctx) { return formatter<const char*>::format(color_names[c], ctx); } };
Victor writes:
Maybe we should […] open an LWG issue clarifying that all standard formatters have a
const
format function.
I'd like to be even more assertive: Let's open an LWG issue clarifying that all formatters must have a
const
format function!
[2022-01-30; Reflector poll]
Set priority to 1 after reflector poll.
[2022-08-24 Approved unanimously in LWG telecon.]
[2022-11-12 Approved at November 2022 meeting in Kona. Status changed: Voting → WP.]
Proposed resolution:
This wording is relative to N4901.
Modify 28.5.6.1 [formatter.requirements] as indicated:
[Drafting note: It might also be reasonable to do a drive-by clarification that when the Table 70 says "Stores the parsed format specifiers in
*this
," what it actually means is "Stores the parsed format specifiers ing
." (But I don't think anyone's seriously confused by that wording.)
-3- Given character type
charT
, output iterator typeOut
, and formatting argument typeT
, in Table 70 and Table 71:
(3.1) —
f
is a value of type (possiblyconst
)F
,(3.?) —
g
is an lvalue of typeF
,(3.2) —
u
is an lvalue of typeT
,(3.3) —
t
is a value of a type convertible to (possiblyconst
)T
,[…]
[…]
Table 70: Formatter requirements [tab:formatter] Expression Return type Requirement
fg.parse(pc)PC::iterator
[…]
Stores the parsed format specifiers in*this
and returns an iterator past the end of the parsed range.…
Modify 28.5.6.4 [format.formatter.spec] as indicated:
-6- An enabled specialization
[Example 1:formatter<T, charT>
meets the BasicFormatter requirements (28.5.6.1 [formatter.requirements]).#include <format> enum color { red, green, blue }; const char* color_names[] = { "red", "green", "blue" }; template<> struct std::formatter<color> : std::formatter<const char*> { auto format(color c, format_context& ctx) const { return formatter<const char*>::format(color_names[c], ctx); } }; […]— end example]
Modify 28.5.6.7 [format.context] as indicated:
void advance_to(iterator it);-8- Effects: Equivalent to:
[Example 1:out_ = std::move(it);
struct S { int value; }; template<> struct std::formatter<S> { size_t width_arg_id = 0; // Parses a width argument id in the format { digit }. constexpr auto parse(format_parse_context& ctx) { […] } // Formats an S with width given by the argument width_arg_id. auto format(S s, format_context& ctx) const { int width = visit_format_arg([](auto value) -> int { if constexpr (!is_integral_v<decltype(value)>) throw format_error("width is not integral"); else if (value < 0 || value > numeric_limits<int>::max()) throw format_error("invalid width"); else return value; }, ctx.arg(width_arg_id)); return format_to(ctx.out(), "{0:x<{1}}", s.value, width); } }; […]— end example]
Modify 30.12 [time.format] as indicated:
template<class Duration, class charT> struct formatter<chrono::local-time-format-t<Duration>, charT>;-15- Let
-16- Remarks: […]f
be […]template<class Duration, class TimeZonePtr, class charT> struct formatter<chrono::zoned_time<Duration, TimeZonePtr>, charT> : formatter<chrono::local-time-format-t<Duration>, charT> { template<class FormatContext> typename FormatContext::iterator format(const chrono::zoned_time<Duration, TimeZonePtr>& tp, FormatContext& ctx) const; };template<class FormatContext> typename FormatContext::iterator format(const chrono::zoned_time<Duration, TimeZonePtr>& tp, FormatContext& ctx) const;-17- Effects: Equivalent to:
sys_info info = tp.get_info(); return formatter<chrono::local-time-format-t<Duration>, charT>:: format({tp.get_local_time(), &info.abbrev, &info.offset}, ctx);
std::format
Section: 28.5.2.2 [format.string.std] Status: Resolved Submitter: Victor Zverovich Opened: 2021-11-13 Last modified: 2023-03-23
Priority: 3
View other active issues in [format.string.std].
View all other issues in [format.string.std].
View all issues with Resolved status.
Discussion:
28.5.2.2 [format.string.std] doesn't specify if implementations should consider the estimated width of the fill character when substituting it into the formatted results.
For example:auto s = std::format("{:🤡>10}", 42);
"🤡"
(U+1F921) is a single code point but its estimated display width is two.
s == "🤡🤡🤡🤡42"
: use the estimated display width,
correctly displayed on compatible terminals.
s == "🤡🤡🤡🤡🤡🤡🤡🤡42"
:
assume the display width of 1, incorrectly displayed.
Require the fill character to have the estimated width of 1.
[2021-11-14; Daniel comments]
Resolving this issue should be harmonized with resolving LWG 3576(i).
[2022-01-30; Reflector poll]
Set priority to 3 after reflector poll. Sent to SG16.
[2023-01-11; LWG telecon]
P2572 would resolve this issue and LWG 3576(i).
[2023-03-22 Resolved by the adoption of P2572R1 in Issaquah. Status changed: SG16 → Resolved.]
Proposed resolution:
constexpr
in std::counted_iterator
Section: 24.5.7.5 [counted.iter.nav] Status: C++23 Submitter: Jiang An Opened: 2021-11-21 Last modified: 2023-11-22
Priority: Not Prioritized
View all issues with C++23 status.
Discussion:
One overload of std::counted_operator::operator++
is not constexpr
currently,
which is seemly because of that a try-block (specified in 24.5.7.5 [counted.iter.nav]/4)
is not allowed in a constexpr function until C++20. Given a try-block is allowed in a constexpr
function in C++20, IMO this overload should also be constexpr
.
constexpr
at first.
The situation of this overload is originally found by Casey Carter, but no LWG issue has been submitted.
[2022-01-30; Reflector poll]
Set status to Tentatively Ready after nine votes in favour during reflector poll.
[2022-02-10 Approved at February 2022 virtual plenary. Status changed: Tentatively Ready → WP.]
Proposed resolution:
This wording is relative to N4901.
Modify 24.5.7.1 [counted.iterator], class template counted_iterator
synopsis, as indicated:
[…] constexpr counted_iterator& operator++(); constexpr decltype(auto) operator++(int); constexpr counted_iterator operator++(int) requires forward_iterator<I>; constexpr counted_iterator& operator--() requires bidirectional_iterator<I>; constexpr counted_iterator operator--(int) requires bidirectional_iterator<I>; […]
Modify 24.5.7.5 [counted.iter.nav] as indicated:
constexpr decltype(auto) operator++(int);-3- Preconditions:
-4- Effects: Equivalent to:length > 0
.--length; try { return current++; } catch(...) { ++length; throw; }
resize_and_overwrite
is overspecified to call its callback with lvaluesSection: 27.4.3.5 [string.capacity] Status: C++23 Submitter: Arthur O'Dwyer Opened: 2021-11-28 Last modified: 2023-11-22
Priority: 2
View all other issues in [string.capacity].
View all issues with C++23 status.
Discussion:
27.4.3.5 [string.capacity] p7 says:
[Let] OP
be the expression std::move(op)(p, n)
.
[Precondition:] OP
does not throw an exception or modify p
or n
.
Notice that p
and n
above are lvalue expressions.
s.resize_and_overwrite(100, [](char*&&, size_t&&){ return 0; });
which is surprising.
B. This wording requires vendors to accepts.resize_and_overwrite(100, [](char*&, size_t&){ return 0; });
which is even more surprising, and also threatens to allow the user to corrupt the internal state (which is why we need to specify the Precondition above).
C. A user who writess.resize_and_overwrite(100, [](auto&&, auto&&){ return 0; });
can detect that they're being passed lvalues instead of rvalues. If we change the wording to permit implementations to pass either lvalues or rvalues (their choice), then this will be detectable by the user, so we don't want that if we can help it.
X. We want to enable implementations to say move(op)(__p, __n)
and then use __p
and __n
.
Y. We have one implementation which wants to say move(op)(data(), __n)
,
which is not currently allowed, but arguably should be.
Z. We have to do or say something about disallowing writes to any
internal state to which Op
might get a reference.
Given all of this, Mark and Arthur think that the simplest way out is to say that the arguments are prvalues. It prevents X, but fixes the surprises in A, B, Y, Z. We could do this in the Let bullets. Either like so:
[Let] p
be a prvalue of type charT*
…
m
be a prvalue of type size_type
equal to n
,
OP
be the expression std::move(op)(p, m)
.
or (Arthur's preference) by specifying prvalues in the expression OP
itself:
[Let] OP
be the expression std::move(op)(auto(p), auto(n))
.
No matter which specification approach we adopt, we can also simplify the Preconditions bullet to:
[Precondition:] OP
does not throw an exception.
because once the user is receiving prvalue copies, it will no longer be physically possible for the
user to modify the library's original variables p
and n
.
[2021-11-29; Arthur O'Dwyer provides wording]
[2022-01-30; Reflector poll]
Set priority to 2 after reflector poll.
Previous resolution [SUPERSEDED]:
This wording is relative to N4901.
Modify 27.4.3.5 [string.capacity] as indicated:
template<class Operation> constexpr void resize_and_overwrite(size_type n, Operation op);-7- Let
(7.1) —
o = size()
before the call toresize_and_overwrite
.(7.2) —
k
bemin(o, n)
.(7.3) —
p
be acharT*
, such that the range[p, p + n]
is valid andthis->compare(0, k, p, k) == 0
istrue
before the call. The values in the range[p + k, p + n]
may be indeterminate (6.7.5 [basic.indet]).(7.4) —
OP
be the expressionstd::move(op)(auto(p), auto(n))
.(7.5) —
r = OP
.-8- Mandates:
-9- Preconditions:OP
has an integer-like type (24.3.4.4 [iterator.concept.winc]).
(9.1) —
OP
does not throw an exceptionor modify.p
orn
(9.2) —
r ≥ 0
.(9.3) —
r ≤ n
.(9.4) — After evaluating
OP
there are no indeterminate values in the range[p, p + r)
.-10- Effects: Evaluates
-11- Recommended practice: Implementations should avoid unnecessary copies and allocations by, for example, makingOP
, replaces the contents of*this
with[p, p + r)
, and invalidates all pointers and references to the range[p, p + n]
.p
a pointer into internal storage and by restoring*(p + r)
tocharT()
after evaluatingOP
.
[2023-01-11; Jonathan Wakely provides new wording requested by LWG]
[Issaquah 2023-02-07; LWG]
Move to Immediate for C++23
[2023-02-13 Approved at February 2023 meeting in Issaquah. Status changed: Immediate → WP.]
Proposed resolution:
This wording is relative to N4917.
Modify 27.4.3.5 [string.capacity] as indicated:
template<class Operation> constexpr void resize_and_overwrite(size_type n, Operation op);-7- Let
(7.1) —
o = size()
before the call toresize_and_overwrite
.(7.2) —
k
bemin(o, n)
.(7.3) —
p
be a value of typecharT*
orcharT* const
, such that the range[p, p + n]
is valid andthis->compare(0, k, p, k) == 0
istrue
before the call. The values in the range[p + k, p + n]
may be indeterminate (6.7.5 [basic.indet]).(7.?) —
m
be a value of typesize_type
orconst size_type
equal ton
.(7.4) —
OP
be the expressionstd::move(op)(p,
.nm)(7.5) —
r = OP
.-8- Mandates:
-9- Preconditions:OP
has an integer-like type (24.3.4.4 [iterator.concept.winc]).
(9.1) —
OP
does not throw an exception or modifyp
or.
nm(9.2) —
r ≥ 0
.(9.3) —
r ≤
.nm(9.4) — After evaluating
OP
there are no indeterminate values in the range[p, p + r)
.-10- Effects: Evaluates
-11- Recommended practice: Implementations should avoid unnecessary copies and allocations by, for example, makingOP
, replaces the contents of*this
with[p, p + r)
, and invalidates all pointers and references to the range[p, p + n]
.p
a pointer into internal storage and by restoring*(p + r)
tocharT()
after evaluatingOP
.
std::ranges::view_interface::size
returns a signed typeSection: 25.5.3.1 [view.interface.general] Status: C++23 Submitter: Jiang An Opened: 2021-11-29 Last modified: 2023-11-22
Priority: 3
View all other issues in [view.interface.general].
View all issues with C++23 status.
Discussion:
According to 25.5.3.1 [view.interface.general], view_interface::size
returns
the difference between the sentinel and the beginning iterator, which always has a
signed-integer-like type. However, IIUC the decision that a size
member function
should return an unsigned type by default was made when adopting P1227R2,
and the relative changes of the ranges library were done in P1523R1.
I don't know why view_interface::size
was unchanged, while ranges::size
returns an unsigned type in similar situations (25.3.10 [range.prim.size] (2.5)).
views_interface::size
to return an unsigned type, the both
overloads should be changed as below:
constexpr auto size() requires forward_range<D> && sized_sentinel_for<sentinel_t<D>, iterator_t<D>> { return to-unsigned-like(ranges::end(derived()) - ranges::begin(derived())); } constexpr auto size() const requires forward_range<const D> && sized_sentinel_for<sentinel_t<const D>, iterator_t<const D>> { return to-unsigned-like(ranges::end(derived()) - ranges::begin(derived())); }
[2022-01-30; Reflector poll]
Set priority to 3 after reflector poll.
[2022-06-22; Reflector poll]
LEWG poll approved the proposed resolution
[2022-09-23; Reflector poll]
Set status to Tentatively Ready after five votes in favour during reflector poll in July 2022.
[2022-11-12 Approved at November 2022 meeting in Kona. Status changed: Voting → WP.]
Proposed resolution:
This wording is relative to N4901.
Modify 25.5.3.1 [view.interface.general], class template view_interface
synopsis, as indicated:
[…] constexpr auto size() requires forward_range<D> && sized_sentinel_for<sentinel_t<D>, iterator_t<D>> { return to-unsigned-like(ranges::end(derived()) - ranges::begin(derived())); } constexpr auto size() const requires forward_range<const D> && sized_sentinel_for<sentinel_t<const D>, iterator_t<const D>> { return to-unsigned-like(ranges::end(derived()) - ranges::begin(derived())); } […]
format
should not print bool
with 'c'
Section: 28.5.2.2 [format.string.std] Status: C++23 Submitter: Zhihao Yuan Opened: 2021-11-30 Last modified: 2023-11-22
Priority: Not Prioritized
View other active issues in [format.string.std].
View all other issues in [format.string.std].
View all issues with C++23 status.
Discussion:
P1652R1 prints integral inputs as characters with 'c'
and preserves the
wording to treat bool
as a one-byte unsigned integer; this accidentally asks the
implementation to cast bool
into a 1-bit character if a user asks for the 'c'
presentation type.
[2021-12-04; Daniel comments]
This issue relates to LWG 3644(i).
[2022-01-30; Reflector poll]
Set status to Tentatively Ready after five votes in favour during reflector poll.
[2022-02-10 Approved at February 2022 virtual plenary. Status changed: Tentatively Ready → WP.]
Proposed resolution:
This wording is relative to N4901.
Modify 28.5.2.2 [format.string.std], Table 67 [tab:format.type.bool], as indicated:
Table 67 — Meaning of type options for bool
[tab:format.type.bool]Type Meaning none, s
Copies textual representation, either true
orfalse
, to the output.b
,B
,c
,d
,o
,x
,X
As specified in Table 65 [tab:format.type.int] for the value static_cast<unsigned char>(value)
.
__cpp_lib_experimental_memory_resource
feature test macroSection: 4.2 [fund.ts.v3::general.feature.test] Status: TS Submitter: Thomas Köppe Opened: 2021-12-03 Last modified: 2022-07-11
Priority: Not Prioritized
View all issues with TS status.
Discussion:
Addresses: fund.ts.v3
The rebase on C++17 in P0996 had the effect of deleting the feature test macro
__cpp_lib_experimental_memory_resource
: the macro was ostensibly tied to the
memory_resource
facility that had become part of C++17, but we overlooked that there
was a residual piece that has not been adopted in the IS, namely the resource_adaptor
class template.
resource_adaptor
, so we should
reinstate the feature test macro and bump its value.
[2022-01-30; Reflector poll]
Set status to Tentatively Ready after six votes in favour during reflector poll.
[2022-07-11 Approved at February 2022 virtual plenary. Status changed: Tentatively Ready → TS.]
Proposed resolution:
This wording is relative to N4853.
Modify 4.2 [fund.ts.v3::general.feature.test], Table 2, as indicated:
Table 2 — Significant features in this technical specification Doc. No. Title Primary
SectionMacro Name Suffix Value Header …
N3916 Type-erased allocator for std::function 4.2 function_erased_allocator
201406
<experimental/functional>
N3916 Polymorphic Memory Resources 8.3 [fund.ts.v3::memory.resource.syn] memory_resources
[new value] <experimental/memory_resource>
N4282 The World's Dumbest Smart Pointer 8.12 observer_ptr
201411
<experimental/memory>
…
std::basic_string
's iterator
and const_iterator
constexpr iterators?Section: 27.4.3.1 [basic.string.general] Status: C++23 Submitter: Jiang An Opened: 2021-12-04 Last modified: 2023-11-22
Priority: Not Prioritized
View all other issues in [basic.string.general].
View all issues with C++23 status.
Discussion:
std::vector
's iterator
and const_iterator
are required to meet constexpr iterator
requirements in C++20 per P1004R2, but it seems that the similar wording is missing for
std::basic_string
in both P0980R1 and the current working draft.
iterator
and const_iterator
meet the constexpr
iterator requirements (24.3.1 [iterator.requirements.general])." to 27.4.3.1 [basic.string.general].
[2022-01-30; Reflector poll]
Set status to Tentatively Ready after six votes in favour during reflector poll.
[2022-02-10 Approved at February 2022 virtual plenary. Status changed: Tentatively Ready → WP.]
Proposed resolution:
This wording is relative to N4901.
Modify 27.4.3.1 [basic.string.general], as indicated:
-3- In all cases,
-4- A[data(), data() + size()]
is a valid range,data() + size()
points at an object with valuecharT()
(a "null terminator"), andsize() <= capacity()
istrue
.size_type
parameter type in abasic_string
deduction guide refers to thesize_type
member type of the type deduced by the deduction guide. -?- The typesiterator
andconst_iterator
meet the constexpr iterator requirements (24.3.1 [iterator.requirements.general]).
basic_format_context::arg(size_t)
should be noexcept
Section: 28.5.6.7 [format.context] Status: C++23 Submitter: Hewill Kang Opened: 2021-12-26 Last modified: 2023-11-22
Priority: Not Prioritized
View all other issues in [format.context].
View all issues with C++23 status.
Discussion:
basic_format_context::arg(size_t)
simply returns args_.get(id)
to get the elements of
args_
, where the type of args_
is basic_format_args<basic_format_context>
.
Since basic_format_args
's get(size_t)
is noexcept
, this function can also be
noexcept
.
[2022-01-30; Reflector poll]
Set status to Tentatively Ready after six votes in favour during reflector poll.
[2022-02-10 Approved at February 2022 virtual plenary. Status changed: Tentatively Ready → WP.]
Proposed resolution:
This wording is relative to N4901.
Modify 28.5.6.7 [format.context] as indicated:
namespace std { template<class Out, class charT> class basic_format_context { basic_format_args<basic_format_context> args_; // exposition only Out out_; // exposition only public: using iterator = Out; using char_type = charT; template<class T> using formatter_type = formatter<T, charT>; basic_format_arg<basic_format_context> arg(size_t id) const noexcept; std::locale locale(); iterator out(); void advance_to(iterator it); }; }[…]
basic_format_arg<basic_format_context> arg(size_t id) const noexcept;-5- Returns:
args_.get(id)
.
INVOKE
operation and union
typesSection: 22.10.4 [func.require] Status: C++23 Submitter: Jiang An Opened: 2021-12-29 Last modified: 2023-11-22
Priority: 3
View other active issues in [func.require].
View all other issues in [func.require].
View all issues with C++23 status.
Discussion:
There are two cases of the INVOKE
operation specified with std::is_base_of_v
(22.10.4 [func.require] (1.1), (1,4)), which means the following code snippet is ill-formed, as
std::is_base_of_v<B, D>
is false
when either B
or D
is a
union
type.
union Foo { int x; }; static_assert(std::is_invocable_v<int Foo::*, Foo&>);
Currently libstdc++ accepts this code, because it uses slightly different conditions that handle union
types. libc++ and MSVC STL reject this code as specified in 22.10.4 [func.require].
union
types?
[2022-01-30; Reflector poll]
Set priority to 3 after reflector poll.
[2023-02-07; Jonathan adds wording]
This is a regression introduced by LWG 2219(i).
In C++14 std::result_of<int Foo::*(Foo&)>::type
was valid, because the INVOKE
wording used to say
"f
is a pointer to member data of a class T
and t1
is an object of type T
or a reference to an object of type T
or a reference to an object of a type derived from T
".
Since LWG 2219 we use is_base_of
which is always false for
union types.
I don't think LWG 2219 intended to break this case, so we should fix it.
Previous resolution [SUPERSEDED]:
This wording is relative to N4928.
Modify 22.10.4 [func.require] as indicated:
-1- Define
INVOKE(f, t1, t2, …, tN)
as follows:
- (1.1) —
(t1.*f)(t2, …, tN)
whenf
is a pointer to a member function of a classT
andis_same_v<T, remove_cvref_t<decltype(t1)>> || is_base_of_v<T, remove_reference_t<decltype(t1)>>
istrue
;- (1.2) —
(t1.get().*f)(t2, …, tN)
whenf
is a pointer to a member function of a classT
andremove_cvref_t<decltype(t1)>
is a specialization ofreference_wrapper
;- (1.3) —
((*t1).*f)(t2, …, tN)
whenf
is a pointer to a member function of a classT
andt1
does not satisfy the previous two items;- (1.4) —
t1.*f
whenN == 1
andf
is a pointer to data member of a classT
andis_same_v<T, remove_cvref_t<decltype(t1)>> || is_base_of_v<T, remove_reference_t<decltype(t1)>>
istrue
;- (1.5) —
t1.get().*f
whenN == 1
andf
is a pointer to data member of a classT
andremove_cvref_t<decltype(t1)>
is a specialization ofreference_wrapper
;- (1.6) —
(*t1).*f
whenN == 1
andf
is a pointer to data member of a classT
andt1
does not satisfy the previous two items;- (1.7) —
f(t1, t2, …, tN)
in all other cases.
[2023-02-07; Jonathan provides wording change requested by LWG]
Change remove_reference_t
to remove_cvref_t
.
is_base_of
ignores cv-qualifiers, so this isn't necessary,
but just using the same transformation in both cases seems simpler to grok.
[Issaquah 2023-02-07; LWG]
Move to Immediate for C++23
[2023-02-13 Approved at February 2023 meeting in Issaquah. Status changed: Immediate → WP.]
Proposed resolution:
This wording is relative to N4928.
Modify 22.10.4 [func.require] as indicated:
-1- Define
INVOKE(f, t1, t2, …, tN)
as follows:
- (1.1) —
(t1.*f)(t2, …, tN)
whenf
is a pointer to a member function of a classT
andis_same_v<T, remove_cvref_t<decltype(t1)>> || is_base_of_v<T, remove_
isreferencecvref_t<decltype(t1)>>true
;- (1.2) —
(t1.get().*f)(t2, …, tN)
whenf
is a pointer to a member function of a classT
andremove_cvref_t<decltype(t1)>
is a specialization ofreference_wrapper
;- (1.3) —
((*t1).*f)(t2, …, tN)
whenf
is a pointer to a member function of a classT
andt1
does not satisfy the previous two items;- (1.4) —
t1.*f
whenN == 1
andf
is a pointer to data member of a classT
andis_same_v<T, remove_cvref_t<decltype(t1)>> || is_base_of_v<T, remove_
isreferencecvref_t<decltype(t1)>>true
;- (1.5) —
t1.get().*f
whenN == 1
andf
is a pointer to data member of a classT
andremove_cvref_t<decltype(t1)>
is a specialization ofreference_wrapper
;- (1.6) —
(*t1).*f
whenN == 1
andf
is a pointer to data member of a classT
andt1
does not satisfy the previous two items;- (1.7) —
f(t1, t2, …, tN)
in all other cases.
Section: 22.11.5 [bit.pow.two] Status: C++23 Submitter: Nicolai Josuttis Opened: 2021-12-30 Last modified: 2023-11-22
Priority: 3
View all issues with C++23 status.
Discussion:
Among the bit operations returning a count, bit_width()
is the only one not returning an int
.
std::uint64_t b64 = 1; b64 = std::rotr(b64, 1); int count1 = std::popcount(b64); // OK int count2 = std::countl_zero(b64); // OK int count3 = std::bit_width(b64); // OOPS
The last line may result in a warning such as:
Warning: conversion from
long long unsigned
toint
may change value
You have to use a static_cast
to avoid the warning.
The counting operations return "
int
" quantities, consistent with the rule "use anint
unless you need something else". This choice does not reflect, in the type, the fact that counts are always non-negative.
[2022-01-30; Reflector poll]
Set priority to 3 after reflector poll. Eight votes for P0, but request LEWG confirmation before setting it to Tentatively Ready.
[2022-02-22 LEWG telecon; Status changed: LEWG → Open]
No objection to unanimous consent to send the proposed resolution for LWG3656 to LWG for C++23. The changes in P1956 changed the functions to be more counting than mathematical.
[2022-07-08; Reflector poll]
Set status to Tentatively Ready after ten votes in favour during reflector poll.
[2022-07-15; LWG telecon: move to Ready]
[2022-07-25 Approved at July 2022 virtual plenary. Status changed: Ready → WP.]
Proposed resolution:
This wording is relative to N4901.
Modify 22.11.2 [bit.syn], header <bit>
synopsis, as indicated:
[…] template<class T> constexprTint bit_width(T x) noexcept; […]
Modify 22.11.5 [bit.pow.two], as indicated:
template<class T> constexprTint bit_width(T x) noexcept;-11- Constraints:
-12- Returns: IfT
is an unsigned integer type (6.8.2 [basic.fundamental]).x == 0
,0
; otherwise one plus the base-2 logarithm ofx
, with any fractional part discarded.
std::hash<std::filesystem::path>
is not enabledSection: 31.12.6 [fs.class.path] Status: C++23 Submitter: Jiang An Opened: 2022-01-02 Last modified: 2023-11-22
Priority: Not Prioritized
View all other issues in [fs.class.path].
View all issues with C++23 status.
Discussion:
The hash support of std::filesystem::path
is provided by std::filesystem::hash_value
,
but the specialization std::hash<std::filesystem::path>
is currently disabled. IMO the
specialization should be enabled, and its operator()
should return the same value as hash_value
.
[2022-01-15; Daniel provides wording]
Previous resolution [SUPERSEDED]:
This wording is relative to N4901.
Modify 31.12.4 [fs.filesystem.syn], header
<filesystem>
synopsis, as indicated:#include <compare> // see 17.12.1 [compare.syn] namespace std::filesystem { // 31.12.6 [fs.class.path], paths class path; // 31.12.6.8 [fs.path.nonmember], path non-member functions void swap(path& lhs, path& rhs) noexcept; size_t hash_value(const path& p) noexcept; […] } // [fs.path.hash], hash support namespace std { template<class T> struct hash; template<> struct hash<filesystem::path>; }Following subclause 31.12.6.8 [fs.path.nonmember], introduce a new subclause [fs.path.hash], as indicated:
29.12.6.? Hash support [fs.path.hash]
template<> struct hash<filesystem::path>;-?- For an object
p
of typefilesystem::path
,hash<filesystem::path>()(p)
shall evaluate to the same result ashash_value(p)
.
[2022-01-18; Daniel improves wording based on reflector discussion feedback]
[2022-01-30; Reflector poll]
Set status to Tentatively Ready after five votes in favour during reflector poll.
[2022-02-10 Approved at February 2022 virtual plenary. Status changed: Tentatively Ready → WP.]
Proposed resolution:
This wording is relative to N4901.
Modify 31.12.4 [fs.filesystem.syn], header <filesystem>
synopsis, as indicated:
#include <compare> // see 17.12.1 [compare.syn] namespace std::filesystem { // 31.12.6 [fs.class.path], paths class path; // 31.12.6.8 [fs.path.nonmember], path non-member functions void swap(path& lhs, path& rhs) noexcept; size_t hash_value(const path& p) noexcept; […] } // [fs.path.hash], hash support namespace std { template<class T> struct hash; template<> struct hash<filesystem::path>; }
Following subclause 31.12.6.8 [fs.path.nonmember], introduce a new subclause [fs.path.hash], as indicated:
29.12.6.? Hash support [fs.path.hash]
template<> struct hash<filesystem::path>;-?- For an object
p
of typefilesystem::path
,hash<filesystem::path>()(p)
evaluates to the same result asfilesystem::hash_value(p)
.
ATOMIC_FLAG_INIT
undeprecationSection: 32.5.10 [atomics.flag] Status: C++23 Submitter: Aaron Ballman Opened: 2022-01-18 Last modified: 2023-11-22
Priority: 3
View all other issues in [atomics.flag].
View all issues with C++23 status.
Discussion:
P0883R2 deprecated ATOMTIC_VAR_INIT
and
ATOMIC_FLAG_INIT
largely based on rationale from WG14. However, WG14
only deprecated ATOMIC_VAR_INIT
because we were told by implementers
that ATOMIC_FLAG_INIT
is still necessary for some platforms (platforms
for which "clear" is actually not all zero bits, which I guess exist).
ATOMIC_FLAG_INIT
as there was no motivation that I could find in the paper on the topic
or in the discussion at
P0883R0 [Jacksonville 2018].
One possible approach would be to undeprecate it from <stdatomic.h>
only (C++
can still use the constructors from <atomic>
and shared code can use the macros
from <stdatomic.h>
).
[2022-01-30; Reflector poll]
Set priority to 3 after reflector poll. Send to SG1.
[2022-07-06; SG1 confirm the direction, Jonathan adds wording]
"In response to LWG 3659, add ATOMIC_FLAG_INIT to <stdatomic.h> as undeprecated."
SF | F | N | A | SA |
3 | 0 | 0 | 0 | 0 |
"In response to LWG 3659, undeprecate ATOMIC_FLAG_INIT in <atomic>."
SF | F | N | A | SA |
2 | 3 | 1 | 0 | 0 |
[2022-07-11; Reflector poll]
Set status to Tentatively Ready after six votes in favour during reflector poll.
[2022-07-15; LWG telecon: move to Ready]
[2022-07-25 Approved at July 2022 virtual plenary. Status changed: Ready → WP.]
Proposed resolution:
This wording is relative to N4910.
Modify 32.5.2 [atomics.syn], Header <atomic>
synopsis,
as indicated:
void atomic_flag_notify_all(volatile atomic_flag*) noexcept; void atomic_flag_notify_all(atomic_flag*) noexcept; #define ATOMIC_FLAG_INIT see below // [atomics.fences], fences extern "C" void atomic_thread_fence(memory_order) noexcept; extern "C" void atomic_signal_fence(memory_order) noexcept;
Move the content of [depr.atomics.flag] from Annex D to the end of 32.5.10 [atomics.flag].
#define ATOMIC_FLAG_INIT see belowRemarks: The macro
ATOMIC_FLAG_INIT
is defined in such a way that it can be used to initialize an object of typeatomic_flag
to the clear state. The macro can be used in the form:atomic_flag guard = ATOMIC_FLAG_INIT;It is unspecified whether the macro can be used in other initialization contexts. For a complete static-duration object, that initialization shall be static.
Modify 32.5.12 [stdatomic.h.syn] C compatibility, as indicated:
using std::atomic_flag_clear; // see below using std::atomic_flag_clear_explicit; // see below #define ATOMIC_FLAG_INIT see below using std::atomic_thread_fence; // see below using std::atomic_signal_fence; // see below
Modify D.22.1 [depr.atomics.general] in Annex D as indicated:
#define ATOMIC_VAR_INIT(value) see below#define ATOMIC_FLAG_INIT see below}
iterator_traits<common_iterator>::pointer
should conform to §[iterator.traits]Section: 24.5.5.2 [common.iter.types] Status: C++23 Submitter: Casey Carter Opened: 2022-01-20 Last modified: 2023-11-22
Priority: Not Prioritized
View all issues with C++23 status.
Discussion:
24.3.2.3 [iterator.traits]/1 says:
[…] In addition, the types
iterator_traits<I>::pointer iterator_traits<I>::referenceshall be defined as the iterator's pointer and reference types; that is, for an iterator object
a
of class type, the same type asdecltype(a.operator->())
anddecltype(*a)
, respectively. The typeiterator_traits<I>::pointer
shall bevoid
for an iterator of class typeI
that does not supportoperator->
. […]
24.5.5.2 [common.iter.types]/1 slightly contradicts this:
The nested typedef-names of the specialization of
iterator_traits
forcommon_iterator<I, S>
are defined as follows.
[…]
(1.3) — If the expression
a.operator->()
is well-formed, wherea
is an lvalue of typeconst common_iterator<I, S>
, thenpointer
denotes the type of that expression. Otherwise,pointer
denotesvoid
.
"The type of a.operator->()
" is not necessarily the same as decltype(a.operator->())
:
when the expression is an lvalue or xvalue of type T
, "the type of a.operator->()
" is T
but decltype(a.operator->())
is either T&
or T&&
. An implementation
therefore cannot conform to the requirements of both cited paragraphs for some specializations of common_iterator
.
a.operator->()
" was not cognizant of the difference in meaning and intended to actually write
decltype(a.operator->())
.
[2022-01-30; Reflector poll]
Set status to Tentatively Ready after five votes in favour during reflector poll.
[2022-02-10 Approved at February 2022 virtual plenary. Status changed: Tentatively Ready → WP.]
Proposed resolution:
This wording is relative to N4901.
[Drafting Note: The wording change below includes an additional drive-by fix that ensures that the last sentence "Otherwise,
pointer
denotesvoid
" cannot be misinterpreted (due to the leading "If") to apply also for a situation when the outcome of the expressiona.operator->()
for a non-const
common_iterator<I, S>
is reflected upon.]
Modify 24.5.5.2 [common.iter.types] as indicated:
-1- The nested typedef-names of the specialization of
iterator_traits
forcommon_iterator<I, S>
are defined as follows.
[…]
(1.3) — Let
a
denote an lvalue of typeconst common_iterator<I, S>
. If the expressiona.operator->()
is well-formed,wherethena
is an lvalue of typeconst common_iterator<I, S>
,pointer
denotesdecltype(a.operator->())
the type of that expression. Otherwise,pointer
denotesvoid
.
constinit atomic<shared_ptr<T>> a(nullptr);
should workSection: 32.5.8.7.2 [util.smartptr.atomic.shared] Status: C++23 Submitter: Jonathan Wakely Opened: 2022-01-21 Last modified: 2023-11-22
Priority: Not Prioritized
View all other issues in [util.smartptr.atomic.shared].
View all issues with C++23 status.
Discussion:
All the following are valid except for the last line:
constinit int i1{};
constinit std::atomic<int> a1{};
constinit int i2{0};
constinit std::atomic<int> a2{0};
constinit std::shared_ptr<int> i3{};
constinit std::atomic<std::shared_ptr<int>> a3{};
constinit std::shared_ptr<int> i4{nullptr};
constinit std::atomic<std::shared_ptr<int>> a4{nullptr}; // error
The initializer for a4
will create a shared_ptr<int>
temporary (using the same constructor
as i4
) but then try to use atomic(shared_ptr<int>)
which is not constexpr
.
atomic<shared_ptr<T>>
that can
easily be fixed. The proposed resolution has been implemented in libstdc++.
There is no need to also change atomic<weak_ptr<T>>
because weak_ptr
doesn't have a
constructor taking nullptr
.
[2022-01-30; Reflector poll]
Set status to Tentatively Ready after seven votes in favour during reflector poll.
[2022-02-10 Approved at February 2022 virtual plenary. Status changed: Tentatively Ready → WP.]
Proposed resolution:
This wording is relative to N4901.
Modify 32.5.8.7.2 [util.smartptr.atomic.shared], class template partial specialization
atomic<shared_ptr<T>>
synopsis, as indicated:
[…] constexpr atomic() noexcept; constexpr atomic(nullptr_t) noexcept : atomic() { } atomic(shared_ptr<T> desired) noexcept; atomic(const atomic&) = delete; void operator=(const atomic&) = delete; […]
std::ranges::distance(a, a+3)
Section: 24.4.4.3 [range.iter.op.distance] Status: C++23 Submitter: Arthur O'Dwyer Opened: 2022-01-23 Last modified: 2023-11-22
Priority: 2
View all other issues in [range.iter.op.distance].
View all issues with C++23 status.
Discussion:
Consider the use of std::ranges::distance(first, last)
on a simple C array.
This works fine with std::distance
, but currently does not work with
std::ranges::distance
.
// godbolt link #include <ranges> #include <cassert> int main() { int a[] = {1, 2, 3}; assert(std::ranges::distance(a, a+3) == 3); assert(std::ranges::distance(a, a) == 0); assert(std::ranges::distance(a+3, a) == -3); }
Before LWG 3392(i), we had a single iterator-pair overload:
template<input_or_output_iterator I, sentinel_for<I> S> constexpr iter_difference_t<I> distance(I first, S last);
which works fine for C pointers. After LWG 3392(i), we have two iterator-pair overloads:
template<input_or_output_iterator I, sentinel_for<I> S> requires (!sized_sentinel_for<S, I>) constexpr iter_difference_t<I> distance(I first, S last); template<input_or_output_iterator I, sized_sentinel_for<I> S> constexpr iter_difference_t<I> distance(const I& first, const S& last);
and unfortunately the one we want — distance(I first, S last)
—
is no longer viable because [with I=int*
, S=int*
], we have
sized_sentinel_for<S, I>
and so its constraints aren't satisfied.
So we look at the other overload [with I=int[3]
, S=int[3]
], but
unfortunately its constraints aren't satisfied either, because int[3]
is not an input_or_output_iterator
.
[2022-01-30; Reflector poll]
Set priority to 2 after reflector poll.
Previous resolution [SUPERSEDED]:
This wording is relative to N4901.
[Drafting Note: Thanks to Casey Carter. Notice that
sentinel_for<S, I>
already implies and subsumesinput_or_output_iterator<I>
, so that constraint wasn't doing anything; personally I'd prefer to remove it for symmetry (and to save the environment). Otherwise you'll have people asking why one of theI
's is constrained and the other isn't.]
Modify 24.2 [iterator.synopsis], header
<iterator>
synopsis, as indicated:[…] // 24.4.4.3 [range.iter.op.distance], ranges::distance template<classinput_or_output_iteratorI, sentinel_for<I> S> requires (!sized_sentinel_for<S, I>) constexpr iter_difference_t<I> distance(I first, S last); template<classinput_or_output_iteratorI, sized_sentinel_for<decay_t<I>> S> constexpr iter_difference_t<I> distance(const I& first,constS&last); […]Modify 24.4.4.3 [range.iter.op.distance] as indicated:
template<classinput_or_output_iteratorI, sentinel_for<I> S> requires (!sized_sentinel_for<S, I>) constexpr iter_difference_t<I> ranges::distance(I first, S last);-1- Preconditions:
-2- Effects: Increments[first, last)
denotes a range.first
untillast
is reached and returns the number of increments.template<classinput_or_output_iteratorI, sized_sentinel_for<decay_t<I>> S> constexpr iter_difference_t<I> ranges::distance(const I& first,constS&last);-3- Effects: Equivalent to:
return last - first;
[2022-02-16; Arthur and Casey provide improved wording]
[Kona 2022-11-08; Move to Ready]
[2023-02-13 Approved at February 2023 meeting in Issaquah. Status changed: Voting → WP.]
Proposed resolution:
This wording is relative to N4901.
[Drafting Note: Arthur thinks it's a bit "cute" of the Effects: element to
static_cast
fromT(&)[N]
toT* const&
in the array case, but it does seem to do the right thing in all cases, and it saves us from having to use anif constexpr (is_array_v...)
or something like that.]
Modify 24.2 [iterator.synopsis], header <iterator>
synopsis, as indicated:
[…] // 24.4.4.3 [range.iter.op.distance], ranges::distance template<classinput_or_output_iteratorI, sentinel_for<I> S> requires (!sized_sentinel_for<S, I>) constexpr iter_difference_t<I> distance(I first, S last); template<classinput_or_output_iteratorI, sized_sentinel_for<decay_t<I>> S> constexpr iter_difference_t<decay_t<I>> distance(constI&& first,constS&last); […]
Modify 24.4.4.3 [range.iter.op.distance] as indicated:
template<classinput_or_output_iteratorI, sentinel_for<I> S> requires (!sized_sentinel_for<S, I>) constexpr iter_difference_t<I> ranges::distance(I first, S last);-1- Preconditions:
-2- Effects: Increments[first, last)
denotes a range.first
untillast
is reached and returns the number of increments.template<classinput_or_output_iteratorI, sized_sentinel_for<decay_t<I>> S> constexpr iter_difference_t<decay_t<I>> ranges::distance(constI&& first,constS&last);-3- Effects: Equivalent to:
return last - static_cast<const decay_t<I>&>(first);
Section: 25.6.4.3 [range.iota.iterator] Status: C++23 Submitter: Casey Carter Opened: 2022-02-04 Last modified: 2023-11-22
Priority: Not Prioritized
View other active issues in [range.iota.iterator].
View all other issues in [range.iota.iterator].
View all issues with C++23 status.
Discussion:
25.6.4.3 [range.iota.iterator] defines
using iterator_category = input_iterator_tag; // present only if W models incrementable
but when difference_type
is an integer-class type the iterator does not meet the
Cpp17InputIterator requirements.
[2022-02-07; Daniel comments]
As requested by LWG 3376(i) and wording implemented by P2393R1, integer-class types are no longer required to have class type.
Previous resolution [SUPERSEDED]:
This wording is relative to N4901.
Modify 25.6.4.3 [range.iota.iterator], class
iota_view::iterator
synopsis, as indicated:namespace std::ranges { template<weakly_incrementable W, semiregular Bound> requires weakly-equality-comparable-with<W, Bound> && copyable<W> struct iota_view<W, Bound>::iterator { […] using iterator_category = input_iterator_tag; // present only if W models incrementable and // IOTA-DIFF-T(W) is not a class type using value_type = W; using difference_type = IOTA-DIFF-T(W); […] }; }
[2022-02-07; Casey Carter provides improved wording]
[2022-03-04; Reflector poll]
Set status to Tentatively Ready after seven votes in favour during reflector poll.
[2022-07-15; LWG telecon: move to Ready]
[2022-07-25 Approved at July 2022 virtual plenary. Status changed: Ready → WP.]
Proposed resolution:
This wording is relative to N4901.
Modify 25.6.4.3 [range.iota.iterator], class iota_view::iterator
synopsis, as indicated:
namespace std::ranges { template<weakly_incrementable W, semiregular Bound> requires weakly-equality-comparable-with<W, Bound> && copyable<W> struct iota_view<W, Bound>::iterator { […] using iterator_category = input_iterator_tag; // present only if W models incrementable and // IOTA-DIFF-T(W) is an integral type using value_type = W; using difference_type = IOTA-DIFF-T(W); […] }; }
atomic_fetch_xor
missing from stdatomic.h
Section: 32.5.12 [stdatomic.h.syn] Status: C++23 Submitter: Hubert Tong Opened: 2022-02-09 Last modified: 2023-11-22
Priority: Not Prioritized
View all issues with C++23 status.
Discussion:
C17 subclause 7.17.7.5 provides atomic_fetch_xor
and atomic_fetch_xor_explicit
.
stdatomic.h
in the working draft (N4901) does not.
[2022-02-09; Jonathan comments and provides wording]
C++20 32.5.2 [atomics.syn] has both of them, too, so it should definitely be in the common subset.
[2022-03-04; Reflector poll]
Set status to Tentatively Ready after eight votes in favour during reflector poll.
[2022-07-15; LWG telecon: move to Ready]
[2022-07-25 Approved at July 2022 virtual plenary. Status changed: Ready → WP.]
Proposed resolution:
This wording is relative to N4901.
Modify 32.5.12 [stdatomic.h.syn], header <stdatomic.h>
synopsis, as indicated:
[Drafting Note: The editor is kindly requested to reorder these "
atomic_fetch_KEY
" declarations to match the other synopses in Clause 32.5 [atomics]:add
,sub
,and
,or
,xor
.]
[…] using std::atomic_fetch_or; // see below using std::atomic_fetch_or_explicit; // see below using std::atomic_fetch_xor; // see below using std::atomic_fetch_xor_explicit; // see below using std::atomic_fetch_and; // see below using std::atomic_fetch_and_explicit; // see below […]
common_iterator::operator->()
should return by valueSection: 24.5.5.4 [common.iter.access] Status: C++23 Submitter: Jonathan Wakely Opened: 2022-02-12 Last modified: 2023-11-22
Priority: Not Prioritized
View all other issues in [common.iter.access].
View all issues with C++23 status.
Discussion:
24.5.5.4 [common.iter.access] p5 (5.1) says that common_iterator<T*, S>::operator->()
returns std::get<T*>(v_)
which has type T* const&
. That means that
iterator_traits::pointer
is T* const&
as well (this was recently clarified by LWG
3660(i)). We have an actual pointer here, why are we returning it by reference?
decltype(auto)
is equivalent to auto
. For the first bullet, it would make a lot
more sense for raw pointers to be returned by value. That leaves the case where the iterator has an
operator->()
member, which could potentially benefit from returning by reference. But it must
return something that is iterator-like or pointer-like, which we usually just pass by value. Casey suggested
we should just change common_iterator<I, S>::operator->()
to return by value in all cases.
Libstdc++ has always returned by value, as an unintended consequence of using a union instead of
std::variant<I, S>
, so that it doesn't use std::get<I>
to return the member.
[2022-03-04; Reflector poll]
Set status to Tentatively Ready after six votes in favour during reflector poll.
[2022-07-15; LWG telecon: move to Ready]
[2022-07-25 Approved at July 2022 virtual plenary. Status changed: Ready → WP.]
Proposed resolution:
This wording is relative to N4901.
Modify 24.5.5.1 [common.iterator], class template common_iterator
synopsis, as indicated:
[…] constexpr decltype(auto) operator*(); constexpr decltype(auto) operator*() const requires dereferenceable<const I>; constexprdecltype(auto)operator->() const requires see below; […]
Modify 24.5.5.4 [common.iter.access] as indicated:
constexprdecltype(auto)operator->() const requires see below;-3- The expression in the requires-clause is equivalent to: […]
-4- Preconditions:holds_alternative<I>(v_)
istrue
. -5- Effects:
(5.1) — If
I
is a pointer type or if the expressionget<I>(v_).operator->()
is well-formed, equivalent to:return get<I>(v_);
(5.2) — Otherwise, if
iter_reference_t<I>
is a reference type, equivalent to:auto&& tmp = *get<I> (v_); return addressof(tmp);(5.3) — Otherwise, equivalent to:
return proxy(*get<I>(v_));
whereproxy
is the exposition-only class:class proxy { iter_value_t<I> keep_; constexpr proxy(iter_reference_t<I>&& x) : keep_(std::move(x)) {} public: constexpr const iter_value_t<I>* operator->() const noexcept { return addressof(keep_); } };
Section: 28.3.3.1.3 [locale.cons] Status: Resolved Submitter: Hubert Tong Opened: 2022-02-12 Last modified: 2023-03-22
Priority: 3
View all other issues in [locale.cons].
View all issues with Resolved status.
Discussion:
locale(const locale& other, const char* std_name, category);
has
Throws:
runtime_error
if the argument is not valid, or is null.
There being three arguments, the statement is rather problematic. It looks like a copy/paste from
explicit locale(const char* std_name);
The conclusion, assuming that "the argument" is also std_name
in the problem case, seems to be
that the statement should be changed to read:
Throws:
runtime_error
ifstd_name
is not valid, or is null.
However there is implementation divergence over whether or not values for the category argument not
explicitly described as valid by 28.3.3.1.2.1 [locale.category] result in runtime_error
.
#include <locale> #include <stdio.h> #include <exception> int main(void) { std::locale Generic("C"); try { std::locale Abomination(Generic, Generic, 0x7fff'ffff); } catch (std::runtime_error&) { fprintf(stderr, "Threw\n"); } }
[2022-03-04; Reflector poll]
Set priority to 3 after reflector poll.
[2022-11-01; Jonathan comments]
The proposed resolution of 2295(i) would resolve this too.
The implementation divergence is not a problem.
28.3.3.1.2.1 [locale.category] p2 makes invalid category
values
undefined, so silently ignoring them or throwing exceptions are both valid.
[2023-03-22 LWG 2295 was approved in Issaquah. Status changed: New → Resolved.]
Proposed resolution:
Preconditions: The
category
argument is a valid value 28.3.3.1.2.1 [locale.category].Throws:
runtime_error
ifthe argumentstd_name
is not valid, or is null.
std::locale::none
Section: 28.3.3.1.3 [locale.cons] Status: Resolved Submitter: Hubert Tong Opened: 2022-02-14 Last modified: 2023-03-22
Priority: 3
View all other issues in [locale.cons].
View all issues with Resolved status.
Discussion:
For
locale(const locale& other, const locale& one, category cats);
the Remarks say:
The resulting locale has a name if and only if the first two arguments have names.
The case where cats
is locale::none
seems very similar to the issue that
LWG 2295(i) reports with the case where the provided facet pointer is nullptr
.
That is, if no composition is actually occurring, then using the name of other
seems to be reasonable.
locale::all
case should not imply using the name of one
.
locale::all
does not necessarily cover all of the C locale categories on a platform.
[2022-03-04; Reflector poll]
Set priority to 3 after reflector poll.
[2022-11-01; Jonathan comments]
The proposed resolution of 2295(i) would resolve this too.
[2023-03-22 LWG 2295 was approved in Issaquah. Status changed: New → Resolved.]
Proposed resolution:
pair
specially handled in uses-allocator construction?Section: 20.2.8.2 [allocator.uses.construction] Status: C++23 Submitter: Jiang An Opened: 2022-02-16 Last modified: 2023-11-22
Priority: 2
View all other issues in [allocator.uses.construction].
View all issues with C++23 status.
Discussion:
It seems unclear whether cv-qualified pair
specializations are considered as
specializations of pair
in 20.2.8.2 [allocator.uses.construction].
const
-qualified pair types as
such specializations. The resolution of LWG 3525(i) uses remove_cv_t
,
which possibly imply that the specialization of pair may be cv-qualified.
The difference can be observed via the following program:
#include <utility>
#include <memory>
#include <vector>
#include <cassert>
template<class T>
class payload_ator {
int payload{};
public:
payload_ator() = default;
constexpr explicit payload_ator(int n) noexcept : payload{n} {}
template<class U>
constexpr explicit payload_ator(payload_ator<U> a) noexcept : payload{a.payload} {}
friend bool operator==(payload_ator, payload_ator) = default;
template<class U>
friend constexpr bool operator==(payload_ator x, payload_ator<U> y) noexcept
{
return x.payload == y.payload;
}
using value_type = T;
constexpr T* allocate(std::size_t n) { return std::allocator<T>{}.allocate(n); }
constexpr void deallocate(T* p, std::size_t n) { return std::allocator<T>{}.deallocate(p, n); }
constexpr int get_payload() const noexcept { return payload; }
};
bool test()
{
constexpr int in_v = 42;
using my_pair_t = std::pair<int, std::vector<int, payload_ator<int>>>;
auto out_v = std::make_obj_using_allocator<const my_pair_t>(payload_ator<int>{in_v}).second.get_allocator().get_payload();
return in_v == out_v;
}
int main()
{
assert(test()); // passes only if a const-qualified pair specialization is considered as a pair specialization
}
[2022-03-04; Reflector poll]
Set priority to 2 after reflector poll.
[2022-08-24; LWG telecon]
Change every T
to remove_cv_t<T>
.
[2022-08-25; Jonathan Wakely provides wording]
Previous resolution [SUPERSEDED]:
This wording is relative to N4910.
Modify 20.2.8.2 [allocator.uses.construction] as indicated, using
remove_cv_t
in every Constraints: element:Constraints:
remove_cv_t<T>
[is|is not] a specialization ofpair
[2022-09-23; Jonathan provides improved wording]
[2022-09-30; moved to Tentatively Ready after seven votes in reflector poll]
[2022-11-12 Approved at November 2022 meeting in Kona. Status changed: Voting → WP.]
Proposed resolution:
This wording is relative to N4917.
Modify 20.2.8.2 [allocator.uses.construction] as indicated,
using remove_cv_t
in every Constraints: element (paragraphs 4, 6, 8, 10, 12, 14, 17):
Constraints:
remove_cv_t<T>
[is|is not] a specialization ofpair
Add remove_cv_t
in paragraph 5:
-5- Returns: A
tuple
value determined as follows:(5.1) — If
uses_allocator_v<remove_cv_t<T>, Alloc>
isfalse
andis_constructible_v<T, Args...>
istrue
, returnforward_as_tuple(std::forward<Args>(args)...)
.(5.2) — Otherwise, if
uses_allocator_v<remove_cv_t<T>, Alloc>
istrue
andis_constructible_v<T, allocator_arg_t, const Alloc&, Args...>
istrue
, returntuple<allocator_arg_t, const Alloc&, Args&&...>( allocator_arg, alloc, std::forward<Args>(args)...)
(5.3) — Otherwise, if
uses_allocator_v<remove_cv_t<T>, Alloc>
istrue
andis_constructible_v<T, Args..., const Alloc&>
istrue
, returnforward_as_tuple(std::forward<Args>(args)..., alloc)
.(5.4) — Otherwise, the program is ill-formed.
Rephrase paragraph 7 in terms of the pair member types:
-?- Let
T1
beT::first_type
. LetT2
beT::second_type
.-6- Constraints:
remove_cv_t<T>
is a specialization ofpair
-7- Effects::
ForEquivalent to:T
specified aspair<T1, T2>
, equivalent
operator==
for polymorphic_allocator
cannot deduce template argument in common casesSection: 20.5.3 [mem.poly.allocator.class] Status: C++23 Submitter: Pablo Halpern Opened: 2022-03-18 Last modified: 2023-11-22
Priority: Not Prioritized
View all other issues in [mem.poly.allocator.class].
View all issues with C++23 status.
Discussion:
In <memory_resource>
, the equality comparison operator for pmr::polymorphic_allocator
is declared in namespace scope as:
template<class T1, class T2> bool operator==(const polymorphic_allocator<T1>& a, const polymorphic_allocator<T2>& b) noexcept;
Since polymorphic_allocator
is implicitly convertible from memory_resource*
,
one would naively expect — and the author of polymorphic_allocator
intended
— the following code to work:
std::pmr::unsynchronized_pool_resource pool_rsrc; std::pmr::vector<int> vec(&pool_rsrc); // Converts to std::pmr::polymorphic_allocator<int> […] assert(vec.get_allocator() == &pool_rsrc); // (1) Compare polymorphic_allocator to memory_resource*
Unfortunately, the line labeled (1) is ill-formed because the type T2
in operator==
cannot be deduced.
operator==
, overloaded for comparison to
memory_resource*
:
template<class T1, class T2> bool operator==(const polymorphic_allocator<T1>& a, const polymorphic_allocator<T2>& b) noexcept; template<class T> bool operator==(const polymorphic_allocator<T>& a, memory_resource* b) noexcept;
The rules for implicitly defined spaceship and comparison operators obviates defining operator!=
or operator==(b, a)
. This PR would allow polymorphic_allocator
to be compared for equality
with memory_resource*
, but not with any other type that is convertible to polymorphic_allocator
.
operator==
with a homogeneous version where type deduction
occurs only for one template parameter:
template<class T1, class T2> bool operator==(const polymorphic_allocator<T1>& a, const polymorphic_allocator<T2>& b) noexcept;template<class T> bool operator==(const polymorphic_allocator<T>& a, const type_identity_t<polymorphic_allocator<T>>& b) noexcept;
This version will work with any type that is convertible to polymorphic_allocator
.
polymorphic_allocator
and the other argument is convertible to polymorphic_allocator
. As with PR2, this PR will work
with any type that is convertible to polymorphic_allocator
.
Note to reader: Proof of concept for the three possible resolutions can be seen at this godbolt link. Uncomment one of PR1, PR2, or PR3 macros to see the effects of each PR.
[2022-05-17; Reflector poll]
Set status to Tentatively Ready after six votes in favour during reflector poll.
[2022-07-15; LWG telecon: move to Ready]
[2022-07-25 Approved at July 2022 virtual plenary. Status changed: Ready → WP.]
Proposed resolution:
This wording is relative to N4901.
Modify 20.5.3 [mem.poly.allocator.class], class template polymorphic_allocator
synopsis, as indicated:
namespace std::pmr { template<class Tp = byte> class polymorphic_allocator { memory_resource* memory_rsrc; // exposition only public: using value_type = Tp; […] memory_resource* resource() const; // friends friend bool operator==(const polymorphic_allocator& a, const polymorphic_allocator& b) noexcept { return *a.resource() == *b.resource(); } }; }
expected<cv void, E>
move constructor should moveSection: 22.8.7.4 [expected.void.assign] Status: C++23 Submitter: Jonathan Wakely Opened: 2022-03-23 Last modified: 2023-11-22
Priority: Not Prioritized
View all other issues in [expected.void.assign].
View all issues with C++23 status.
Discussion:
For expected<cv void>::operator=(expected&&)
we have this
in the last bullet of the Effects element:
Otherwise, equivalent to
unex = rhs.error()
.
That should be a move assignment, not a copy assignment.
[2022-05-17; Reflector poll]
Set status to Tentatively Ready after eight votes in favour during reflector poll.
[2022-07-15; LWG telecon: move to Ready]
[2022-07-25 Approved at July 2022 virtual plenary. Status changed: Ready → WP.]
Proposed resolution:
This wording is relative to N4910.
Modify 22.8.7.4 [expected.void.assign] as indicated:
constexpr expected& operator=(expected&& rhs) noexcept(see below);-4- Effects:
(4.1) — If
this->has_value() && rhs.has_value()
istrue
, no effects.(4.2) — Otherwise, if
this->has_value()
istrue
, equivalent to:construct_at(addressof(unex), std::move(rhs.unex)); has_val = false;(4.3) — Otherwise, if
rhs.has_value()
istrue
, destroysunex
and setshas_val
totrue
.(4.4) — Otherwise, equivalent to
unex = std::move(rhs.error())
.
zip_view::iterator
's operator<=>
is overconstrainedSection: 25.7.25.3 [range.zip.iterator] Status: C++23 Submitter: S. B. Tam Opened: 2022-04-21 Last modified: 2023-11-22
Priority: Not Prioritized
View all issues with C++23 status.
Discussion:
zip_view::iterator
's operator<=>
is constrained with
(three_way_comparable<iterator_t<maybe-const<Const, Views>>> && ...)
.
This is unnecessary, because the comparison is performed on the stored tuple-or-pair
,
and both std::tuple
and std::pair
provide operator<=>
regardless of
whether the elements are three-way comparable.
std::tuple
nor std::pair
provides operator<
since C++20, comparing two zip::iterator
s with operator<
(which is specified
to return x.current_ < y.current_
, where current_
is a
tuple-or-pair
) eventually uses tuple
or pair
's operator<=>
anyway.
Thus, I think it's possible to make operator<=>
not require three_way_comparable
.
This also makes it possible to remove the operator functions for <
, >
, <=
,
>=
and rely on the operators synthesized from operator<=>
.
[2022-04-24; Daniel comments and provides wording]
It should be pointed out that by still constraining operator<=>
with
all-random-access<Const, Views...>
we also constrain by
random_access_iterator<iterator_t<maybe-const<Const, Views>>>
which again means that we constrain by
totally_ordered<iterator_t<maybe-const<Const, Views>>>
, so
this operator will only be satisfied with iterators I
that satisfy
partially-ordered-with<I, I>
. Based on this argument the delegation to
tuple-or-pair
's operator<=>
that solely depends on
synth-three-way
should be appropriate.
[2022-05-17; Reflector poll]
Set status to Tentatively Ready after six votes in favour during reflector poll.
[2022-07-15; LWG telecon: move to Ready]
[2022-07-25 Approved at July 2022 virtual plenary. Status changed: Ready → WP.]
Proposed resolution:
This wording is relative to N4910.
Modify 25.7.25.3 [range.zip.iterator] as indicated:
[…]namespace std::ranges { […] template<input_range... Views> requires (view<Views> && ...) && (sizeof...(Views) > 0) template<bool Const> class zip_view<Views...>::iterator { tuple-or-pair<iterator_t<maybe-const<Const, Views>>...> current_; // exposition only constexpr explicit iterator(tuple-or-pair<iterator_t<maybe-const<Const, Views>>...>); // exposition only public: […] friend constexpr bool operator==(const iterator& x, const iterator& y) requires (equality_comparable<iterator_t<maybe-const<Const, Views>>> && ...);friend constexpr bool operator<(const iterator& x, const iterator& y) requires all-random-access<Const, Views...>; friend constexpr bool operator>(const iterator& x, const iterator& y) requires all-random-access<Const, Views...>; friend constexpr bool operator<=(const iterator& x, const iterator& y) requires all-random-access<Const, Views...>; friend constexpr bool operator>=(const iterator& x, const iterator& y) requires all-random-access<Const, Views...>;friend constexpr auto operator<=>(const iterator& x, const iterator& y) requires all-random-access<Const, Views...>&& three_way_comparable<iterator_t<maybe-const<Const, Views>>> && ...); […] }; […] }friend constexpr bool operator<(const iterator& x, const iterator& y) requires all-random-access<Const, Views...>;
-16- Returns:x.current_ < y.current_
.friend constexpr bool operator>(const iterator& x, const iterator& y) requires all-random-access<Const, Views...>;
-17- Effects: Equivalent to:return y < x;
friend constexpr bool operator<=(const iterator& x, const iterator& y) requires all-random-access<Const, Views...>;
-18- Effects: Equivalent to:return !(y < x);
friend constexpr bool operator>=(const iterator& x, const iterator& y) requires all-random-access<Const, Views...>;
-19- Effects: Equivalent to:return !(x < y);
friend constexpr auto operator<=>(const iterator& x, const iterator& y) requires all-random-access<Const, Views...>&& (three_way_comparable<iterator_t<maybe-const<Const, Views>>> && ...);-20- Returns:
x.current_ <=> y.current_
.
regex_iterator
and join_view
don't work together very wellSection: 28.6.11 [re.iter], 25.7.14 [range.join] Status: Resolved Submitter: Barry Revzin Opened: 2022-05-12 Last modified: 2023-03-23
Priority: 2
View all other issues in [re.iter].
View all issues with Resolved status.
Discussion:
Consider this example (from StackOverflow):
#include <ranges> #include <regex> #include <iostream> int main() { char const text[] = "Hello"; std::regex regex{"[a-z]"}; auto lower = std::ranges::subrange( std::cregex_iterator( std::ranges::begin(text), std::ranges::end(text), regex), std::cregex_iterator{} ) | std::views::join | std::views::transform([](auto const& sm) { return std::string_view(sm.first, sm.second); }); for (auto const& sv : lower) { std::cout << sv << '\n'; } }
This example seems sound, having lower
be a range of string_view
that should refer
back into text
, which is in scope for all this time. The std::regex
object is also
in scope for all this time.
transform_view
's iterator with heap-use-after-free.
The problem here is ultimately that regex_iterator
is a stashing iterator (it has a member
match_results
) yet advertises itself as a forward_iterator
(despite violating
24.3.5.5 [forward.iterators] p6 and 24.3.4.11 [iterator.concept.forward] p3.
Then, join_view
's iterator stores an outer iterator (the regex_iterator
) and an
inner_iterator
(an iterator into the container that the regex_iterator
stashes).
Copying that iterator effectively invalidates it — since the new iterator's inner iterator will
refer to the old iterator's outer iterator's container. These aren't (and can't be) independent copies.
In this particular example, join_view
's begin
iterator is copied into the
transform_view
's iterator, and then the original is destroyed (which owns the container that
the new inner iterator still points to), which causes us to have a dangling iterator.
Note that the example is well-formed in libc++ because libc++ moves instead of copying an iterator,
which happens to work. But I can produce other non-transform-view related examples that fail.
This is actually two different problems:
regex_iterator
is really an input iterator, not a forward iterator. It does not meet either
the C++17 or the C++20 forward iterator requirements.
join_view
can't handle stashing iterators, and would need to additionally store the outer
iterator in a non-propagating-cache for input ranges (similar to how it already potentially stores the
inner iterator in a non-propagating-cache).
(So potentially this could be two different LWG issues, but it seems nicer to think of them together.)
[2022-05-17; Reflector poll]
Set priority to 2 after reflector poll.
[Kona 2022-11-08; Move to Open]
Tim to write a paper
[2023-01-16; Tim comments]
The paper P2770R0 is provided with proposed wording.
[2023-03-22 Resolved by the adoption of P2770R0 in Issaquah. Status changed: Open → Resolved.]
Proposed resolution:
const begin
of the join_view
family does not require InnerRng
to be a rangeSection: 25.7.14.2 [range.join.view], 25.7.15.2 [range.join.with.view] Status: Resolved Submitter: Hewill Kang Opened: 2022-05-17 Last modified: 2023-03-23
Priority: 3
View all other issues in [range.join.view].
View all issues with Resolved status.
Discussion:
This is a follow-up of LWG 3599(i).
Currently, the const
version of join_view::begin
has the following constraints
constexpr auto begin() const requires input_range<const V> && is_reference_v<range_reference_t<const V>> { return iterator<true>{*this, ranges::begin(base_)}; }
which only requires InnerRng
to be a reference type, but in some cases, InnerRng
may not be a range
. Consider
#include <ranges> int main() { auto r = std::views::iota(0, 5) | std::views::split(1); auto s = std::views::single(r); auto j = s | std::views::join; auto f = j.front(); }
The reference type of single_view
is const split_view&
, which only has the non-const
version of begin
, which will cause view_interface
's const front
to be incorrectly
instantiated, making r.front()
unnecessarily ill-formed.
We should add this check for join_view
's const begin
, as well as join_with_view
.
[2022-06-21; Reflector poll]
Set priority to 3 after reflector poll.
Previous resolution [SUPERSEDED]:
This wording is relative to N4910.
Modify 25.7.14.2 [range.join.view], class template
join_view
synopsis, as indicated:namespace std::ranges { […] template<input_range V> requires view<V> && input_range<range_reference_t<V>> class join_view : public view_interface<join_view<V>> { private: […] public: […] constexpr auto begin() { […] } constexpr auto begin() const requires input_range<const V> && input_range<range_reference_t<const V>> && is_reference_v<range_reference_t<const V>> { return iterator<true>{*this, ranges::begin(base_)}; } constexpr auto end() { […] } constexpr auto end() const requires input_range<const V> && input_range<range_reference_t<const V>> && is_reference_v<range_reference_t<const V>> { if constexpr (forward_range<const V> && forward_range<range_reference_t<const V>> && common_range<const V> && common_range<range_reference_t<const V>>) return iterator<true>{*this, ranges::end(base_)}; else return sentinel<true>{*this}; } }; […] }Modify 25.7.15.2 [range.join.with.view], class template
join_with_view
synopsis, as indicated:namespace std::ranges { […] template<input_range V, forward_range Pattern> requires view<V> && input_range<range_reference_t<V>> && view<Pattern> && compatible-joinable-ranges<range_reference_t<V>, Pattern> class join_with_view : public view_interface<join_with_view<V, Pattern>> { private: […] public: […] constexpr auto begin() { […] } constexpr auto begin() const requires input_range<const V> && forward_range<const Pattern> && input_range<range_reference_t<const V>> && is_reference_v<range_reference_t<const V>> { return iterator<true>{*this, ranges::begin(base_)}; } constexpr auto end() { […] } constexpr auto end() const requires input_range<const V> && forward_range<const Pattern> && input_range<range_reference_t<const V>> && is_reference_v<range_reference_t<const V>> { using InnerConstRng = range_reference_t<const V>; if constexpr (forward_range<const V> && forward_range<InnerConstRng> && common_range<const V> && common_range<InnerConstRng>) return iterator<true>{*this, ranges::end(base_)}; else return sentinel<true>{*this}; } }; […] }
[2023-03-22 Resolved by the adoption of P2770R0 in Issaquah. Status changed: New → Resolved.]
Proposed resolution:
formatter<remove_cvref_t<const charT[N]>, charT>
requirement explicitSection: 28.5.6.4 [format.formatter.spec] Status: C++23 Submitter: Mark de Wever Opened: 2022-05-17 Last modified: 2023-11-22
Priority: Not Prioritized
View other active issues in [format.formatter.spec].
View all other issues in [format.formatter.spec].
View all issues with C++23 status.
Discussion:
The wording in 28.5.5 [format.functions]/20 and 28.5.5 [format.functions]/25 both contain
formatter<remove_cvref_t<Ti>, charT>
meets the BasicFormatter requirements (28.5.6.1 [formatter.requirements]) for eachTi
inArgs
.
The issue is that remove_cvref_t<const charT[N]>
becomes charT[N]
.
28.5.6.4 [format.formatter.spec]/2.2 requires a specialization for
template<size_t N> struct formatter<const charT[N], charT>;
but there's no requirement to provide
template<size_t N> struct formatter<charT[N], charT>;
There's no wording preventing library vendors from providing additional specializations.
So it's possible to implement the current specification but the indirect requirement is odd.
I noticed this while implementing a formattable
concept. The concept is based on the
formattable
concept of P2286 "Formatting Ranges" (This paper is targeting C++23.)
template<size_t N> struct formatter<const charT[N], charT>
is not needed and should be removed from the Standard. This will be an API break. Vendors can decide to keep the no longer required specialization as an extension; which would lead to implementation divergence. Microsoft is already shipping this specialization as stable and Victor doesn't like the removal too.
Therefore I only propose to add the requiredformatter
specialization.
[2022-06-21; Reflector poll]
Set status to Tentatively Ready after seven votes in favour during reflector poll.
[2022-07-15; LWG telecon: move to Ready]
[2022-07-25 Approved at July 2022 virtual plenary. Status changed: Ready → WP.]
Proposed resolution:
This wording is relative to N4910.
Modify 28.5.6.4 [format.formatter.spec] as indicated:
-2- Let
charT
be eitherchar
orwchar_t
. Each specialization offormatter
is either enabled or disabled, as described below. Each header that declares the templateformatter
provides the following enabled specializations:
(2.1) — The specializations […]
(2.2) — For each
charT
, the string type specializationstemplate<> struct formatter<charT*, charT>; template<> struct formatter<const charT*, charT>; template<size_t N> struct formatter<charT[N], charT>; template<size_t N> struct formatter<const charT[N], charT>; template<class traits, class Allocator> struct formatter<basic_string<charT, traits, Allocator>, charT>; template<class traits> struct formatter<basic_string_view<charT, traits>, charT>;(2.3) — […]
(2.4) — […]
zip_transform_view::iterator
remove operator<
?Section: 25.7.26.3 [range.zip.transform.iterator] Status: C++23 Submitter: Hewill Kang Opened: 2022-05-21 Last modified: 2023-11-22
Priority: Not Prioritized
View all issues with C++23 status.
Discussion:
After LWG 3692(i), zip_view::iterator
only provides operator<=>
.
Since the comparison of zip_transform_view::iterator
uses zip_view::iterator
's
operator<=>
, it is possible to remove zip_transform_view::iterator
's
operator<
, >
, <=
, >=
and just detect if ziperator
's
operator<=>
is available.
ziperator
's operator<=>
is valid only when zip_view
is a
random_access_range
, we don't need to additionally constrain the ziperator
to be
three_way_comparable
.
[2022-06-21; Reflector poll]
Set status to Tentatively Ready after five votes in favour during reflector poll.
[2022-07-15; LWG telecon: move to Ready]
[2022-07-25 Approved at July 2022 virtual plenary. Status changed: Ready → WP.]
Proposed resolution:
This wording is relative to N4910.
Modify 25.7.26.3 [range.zip.transform.iterator] as indicated:
[…]namespace std::ranges { […] template<copy_constructible F, input_range... Views> requires (view<Views> && ...) && (sizeof...(Views) > 0) && is_object_v<F> && regular_invocable<F&, range_reference_t<Views>...> && can-reference<invoke_result_t<F&, range_reference_t<Views>...>> template<bool Const> class zip_transform_view<F, Views...>::iterator { using Parent = maybe-const<Const, zip_transform_view>; // exposition only using Base = maybe-const<Const, InnerView>; // exposition only Parent* parent_ = nullptr; // exposition only ziperator<Const> inner_; // exposition only constexpr iterator(Parent& parent, ziperator<Const> inner); // exposition only public: […] friend constexpr bool operator==(const iterator& x, const iterator& y) requires equality_comparable<ziperator<Const>>;friend constexpr bool operator<(const iterator& x, const iterator& y) requires random_access_range<Base>; friend constexpr bool operator>(const iterator& x, const iterator& y) requires random_access_range<Base>; friend constexpr bool operator<=(const iterator& x, const iterator& y) requires random_access_range<Base>; friend constexpr bool operator>=(const iterator& x, const iterator& y) requires random_access_range<Base>;friend constexpr auto operator<=>(const iterator& x, const iterator& y) requires random_access_range<Base>&& three_way_comparable<ziperator<Const>>; […] }; […] }friend constexpr bool operator==(const iterator& x, const iterator& y) requires equality_comparable<ziperator<Const>>;friend constexpr bool operator<(const iterator& x, const iterator& y) requires random_access_range<Base>; friend constexpr bool operator>(const iterator& x, const iterator& y) requires random_access_range<Base>; friend constexpr bool operator<=(const iterator& x, const iterator& y) requires random_access_range<Base>; friend constexpr bool operator>=(const iterator& x, const iterator& y) requires random_access_range<Base>;friend constexpr auto operator<=>(const iterator& x, const iterator& y) requires random_access_range<Base>&& three_way_comparable<ziperator<Const>>;-14- Let
op
be the operator.-15- Effects: Equivalent to:
return x.inner_ op y.inner_;
expected<T, E> requires is_void<T>
Section: 22.8.7.1 [expected.void.general] Status: C++23 Submitter: Casey Carter Opened: 2022-05-24 Last modified: 2023-11-22
Priority: 2
View all issues with C++23 status.
Discussion:
The partial specialization expected<T, E> requires is_void<T>
specified in
22.8.7.1 [expected.void.general] is missing some template parameter requirements that should
have been copied from 22.8.6.1 [expected.object.general]. We should copy the pertinent
requirements from the first two paragraphs of the latter subclause into new paragraphs in the first
subclause (the pertinent requirement from the third paragraph is already present in
22.8.7.1 [expected.void.general]).
[2022-06-21; Jonathan adds "Member" before "has_val
"]
[2022-06-21; Reflector poll]
Set priority to 2 after reflector poll.
[2022-06-21; Reflector poll]
Set status to Tentatively Ready after five votes in favour during reflector poll.
[2022-07-15; LWG telecon: move to Ready]
[2022-07-25 Approved at July 2022 virtual plenary. Status changed: Ready → WP.]
Proposed resolution:
This wording is relative to N4910.
[Drafting note: There is some drive-by cleanup that I couldn't resist while touching this wording: (1) strike the redundant "suitably aligned" guarantee, (2) Don't repeat in prose that the exposition-only members are exposition-only.]
Modify 22.8.6.1 [expected.object.general] as indicated:
-1- Any object of type
expected<T, E>
either contains a value of typeT
or a value of typeE
within its own storage. Implementations are not permitted to use additional storage, such as dynamic memory, to allocate the object of typeT
or the object of typeE
.These objects are allocated in a region of theMemberexpected<T, E>
storage suitably aligned for the typesT
andE
. Membershas_val
,val
, andunex
are provided for exposition only.has_val
indicates whether theexpected<T, E>
object contains an object of typeT
.
Modify 22.8.7.1 [expected.void.general] as indicated:
-?- Any object of type
-?- A program that instantiates the definition of the templateexpected<T, E>
either represents a value of typeT
, or contains a value of typeE
within its own storage. Implementations are not permitted to use additional storage, such as dynamic memory, to allocate the object of typeE
. Memberhas_val
indicates whether theexpected<T, E>
object represents a value of typeT
.expected<T, E>
with a type for theE
parameter that is not a valid template argument forunexpected
is ill-formed. -1-E
shall meet the requirements of Cpp17Destructible (Table [tab:cpp17.destructible]).
Section: 23.4.6.1 [set.overview], 23.4.7.1 [multiset.overview], 23.5.6.1 [unord.set.overview], 23.5.7.1 [unord.multiset.overview] Status: C++23 Submitter: Jonathan Wakely Opened: 2022-05-25 Last modified: 2023-11-22
Priority: Not Prioritized
View all issues with C++23 status.
Discussion:
The restored erase(iterator)
overloads introduced by LWG 2059(i) would be duplicates
of the erase(const_iterator)
ones if iterator
and const_iterator
are the same
type, which is allowed for sets.
erase(iterator)
overloads are only present
when the iterator types are distinct.
This applies to set
, multiset
, unordered_set
, unordered_multiset
(and flat_set
and flat_multiset
).
[2022-06-21; Reflector poll]
Set status to Tentatively Ready after seven votes in favour during reflector poll.
[2022-07-15; LWG telecon: move to Ready]
[2022-07-25 Approved at July 2022 virtual plenary. Status changed: Ready → WP.]
Proposed resolution:
This wording is relative to N4910.
Modify 23.4.6.1 [set.overview], class template set
synopsis, as indicated:
iterator erase(iterator position) requires (!same_as<iterator, const_iterator>); iterator erase(const_iterator position);
Modify 23.4.7.1 [multiset.overview], class template multiset
synopsis, as indicated:
iterator erase(iterator position) requires (!same_as<iterator, const_iterator>); iterator erase(const_iterator position);
Modify 23.5.6.1 [unord.set.overview], class template unordered_set
synopsis, as indicated:
iterator erase(iterator position) requires (!same_as<iterator, const_iterator>); iterator erase(const_iterator position);
Modify 23.5.7.1 [unord.multiset.overview], class template unordered_multiset
synopsis, as indicated:
iterator erase(iterator position) requires (!same_as<iterator, const_iterator>); iterator erase(const_iterator position);
basic_string
's allocatorSection: 27.4.6 [basic.string.hash] Status: C++23 Submitter: Casey Carter Opened: 2022-05-26 Last modified: 2023-11-22
Priority: Not Prioritized
View all other issues in [basic.string.hash].
View all issues with C++23 status.
Discussion:
27.4.6 [basic.string.hash] says:
template<> struct hash<string>; template<> struct hash<u8string>; template<> struct hash<u16string>; template<> struct hash<u32string>; template<> struct hash<wstring>; template<> struct hash<pmr::string>; template<> struct hash<pmr::u8string>; template<> struct hash<pmr::u16string>; template<> struct hash<pmr::u32string>; template<> struct hash<pmr::wstring>;-1- If
S
is one of these string types,SV
is the corresponding string view type, ands
is an object of typeS
, thenhash<S>()(s) == hash<SV>()(SV(s))
Despite that the hash value of a basic_string
object is equivalent to the hash value of a
corresponding basic_string_view
object, which has no allocator, the capability to hash a
basic_string
depends on its allocator. All of the enabled specializations have specific
allocators, which fact becomes more clear if we expand the type aliases:
template<> struct hash<basic_string<char, char_traits<char>, allocator<char>>; template<> struct hash<basic_string<char8_t, char_traits<char8_t>, allocator<char8_t>>; template<> struct hash<basic_string<char16_t, char_traits<char16_t>, allocator<char16_t>>; template<> struct hash<basic_string<char32_t, char_traits<char32_t>, allocator<char32_t>>; template<> struct hash<basic_string<wchar_t, char_traits<wchar_t>, allocator<wchar_t>>; template<> struct hash<basic_string<char, char_traits<char>, pmr::polymorphic_allocator<char>>; template<> struct hash<basic_string<char8_t, char_traits<char8_t>, pmr::polymorphic_allocator<char8_t>>; template<> struct hash<basic_string<char16_t, char_traits<char16_t>, pmr::polymorphic_allocator<char16_t>>; template<> struct hash<basic_string<char32_t, char_traits<char32_t>, pmr::polymorphic_allocator<char32_t>>; template<> struct hash<basic_string<wchar_t, char_traits<wchar_t>, pmr::polymorphic_allocator<wchar_t>>;
If the hash value doesn't depend on the allocator type, why should we care about the allocator type?
I posit that we should not, and that these ten explicit specializations should be replaced by 5 partial
specializations that enable hashing basic_string
specializations using these combinations of
character type and traits type with any allocator type.
[2022-06-21; Reflector poll]
Set status to Tentatively Ready after seven votes in favour during reflector poll.
[2022-07-15; LWG telecon: move to Ready]
[2022-07-25 Approved at July 2022 virtual plenary. Status changed: Ready → WP.]
Proposed resolution:
This wording is relative to N4910.
Modify 27.4.2 [string.syn], header <string>
synopsis, as indicated:
[…] // 27.4.6 [basic.string.hash], hash support template<class T> struct hash;template<> struct hash<string>; template<> struct hash<u8string>; template<> struct hash<u16string>; template<> struct hash<u32string>; template<> struct hash<wstring>; template<> struct hash<pmr::string>; template<> struct hash<pmr::u8string>; template<> struct hash<pmr::u16string>; template<> struct hash<pmr::u32string>; template<> struct hash<pmr::wstring>;template<class A> struct hash<basic_string<char, char_traits<char>, A>>; template<class A> struct hash<basic_string<char8_t, char_traits<char8_t>, A>>; template<class A> struct hash<basic_string<char16_t, char_traits<char16_t>, A>>; template<class A> struct hash<basic_string<char32_t, char_traits<char32_t>, A>>; template<class A> struct hash<basic_string<wchar_t, char_traits<wchar_t>, A>>; […]
Modify 27.4.6 [basic.string.hash] as indicated:
template<> struct hash<string>; template<> struct hash<u8string>; template<> struct hash<u16string>; template<> struct hash<u32string>; template<> struct hash<wstring>; template<> struct hash<pmr::string>; template<> struct hash<pmr::u8string>; template<> struct hash<pmr::u16string>; template<> struct hash<pmr::u32string>; template<> struct hash<pmr::wstring>;template<class A> struct hash<basic_string<char, char_traits<char>, A>>; template<class A> struct hash<basic_string<char8_t, char_traits<char8_t>, A>>; template<class A> struct hash<basic_string<char16_t, char_traits<char16_t>, A>>; template<class A> struct hash<basic_string<char32_t, char_traits<char32_t>, A>>; template<class A> struct hash<basic_string<wchar_t, char_traits<wchar_t>, A>>;-1- If
S
is one of these string types,SV
is the corresponding string view type, ands
is an object of typeS
, thenhash<S>()(s) == hash<SV>()(SV(s))
chunk_view::outer-iterator::value_type::size
should return unsigned typeSection: 25.7.29.4 [range.chunk.outer.value] Status: C++23 Submitter: Hewill Kang Opened: 2022-06-01 Last modified: 2023-11-22
Priority: Not Prioritized
View other active issues in [range.chunk.outer.value].
View all other issues in [range.chunk.outer.value].
View all issues with C++23 status.
Discussion:
Currently, the size
function of chunk_view::outer-iterator::value_type
returns
the result of ranges::min
, since the operands are of type range_difference_t<V>
,
this will return a signed type, which is inconsistent with the return type of size
of the
forward
-version of chunk_view::iterator::value_type
(25.7.29.7 [range.chunk.fwd.iter]),
which always returns an unsigned type.
I think it's more reasonable to return an unsigned type, since this is intentional behavior and
doesn't fall back to using the default view_interface::size
.
And if LWG 3646(i) is eventually adopted, there's no reason why it shouldn't be made
unsigned.
[2022-06-21; Reflector poll]
Set status to Tentatively Ready after five votes in favour during reflector poll.
[2022-07-15; LWG telecon: move to Ready]
[2022-07-25 Approved at July 2022 virtual plenary. Status changed: Ready → WP.]
Proposed resolution:
This wording is relative to N4910.
Modify 25.7.29.4 [range.chunk.outer.value] as indicated:
constexpr auto size() const requires sized_sentinel_for<sentinel_t<V>, iterator_t<V>>;-4- Effects: Equivalent to:
return to-unsigned-like(ranges::min(parent_->remainder_, ranges::end(parent_->base_) - *parent_->current_));
take_while_view::sentinel
's conversion constructor should moveSection: 25.7.11.3 [range.take.while.sentinel] Status: C++23 Submitter: Hewill Kang Opened: 2022-06-03 Last modified: 2023-11-22
Priority: Not Prioritized
View all issues with C++23 status.
Discussion:
The conversion constructor of take_while_view::sentinel
requires
sentinel_t<V>
must satisfy convertible_to<sentinel_t<Base>>
,
which indicates that the rvalue reference of sentinel_t<V>
can be converted to
sentinel_t<Base>
, but in the Effects element, we assign the lvalue
s.end_
to end_
.
[2022-06-21; Reflector poll]
Set status to Tentatively Ready after seven votes in favour during reflector poll.
[2022-07-15; LWG telecon: move to Ready]
[2022-07-25 Approved at July 2022 virtual plenary. Status changed: Ready → WP.]
Proposed resolution:
This wording is relative to N4910.
Modify 25.7.11.3 [range.take.while.sentinel] as indicated:
constexpr sentinel(sentinel<!Const> s) requires Const && convertible_to<sentinel_t<V>, sentinel_t<Base>>;-2- Effects: Initializes
end_
withstd::move(s.end_)
andpred_
withs.pred_
.
Section: 22.5.3.1 [optional.optional.general], 22.6.3.1 [variant.variant.general] Status: C++23 Submitter: Casey Carter Opened: 2022-06-08 Last modified: 2023-11-22
Priority: Not Prioritized
View all other issues in [optional.optional.general].
View all issues with C++23 status.
Discussion:
LWG 3703(i) struck redundant language from 22.8.7.1 [expected.void.general] specifying that
(1) space allocated for an object is suitably aligned, and (2) a member annotated // exposition only
in a class synopsis "is provided for exposition only."
optional
and variant
.
[2022-06-21; Reflector poll]
Set status to Tentatively Ready after nine votes in favour during reflector poll.
[2022-07-15; LWG telecon: move to Ready]
[2022-07-25 Approved at July 2022 virtual plenary. Status changed: Ready → WP.]
Proposed resolution:
This wording is relative to N4910.
Modify 22.5.3.1 [optional.optional.general] as indicated:
-1- Any instance of
-2-optional<T>
at any given time either contains a value or does not contain a value. When an instance ofoptional<T>
contains a value, it means that an object of typeT
, referred to as the optional object's contained value, is allocated within the storage of the optional object. Implementations are not permitted to use additional storage, such as dynamic memory, to allocate its contained value.The contained value shall be allocated in a region of theWhen an object of typeoptional<T>
storage suitably aligned for the typeT
.optional<T>
is contextually converted tobool
, the conversion returnstrue
if the object contains a value; otherwise the conversion returnsfalse
.MemberWhen anval
is provided for exposition only.optional<T>
object contains a value, memberval
points to the contained value.
Modify 22.6.3.1 [variant.variant.general] as indicated:
-1- Any instance of
variant
at any given time either holds a value of one of its alternative types or holds no value. When an instance ofvariant
holds a value of alternative typeT
, it means that a value of typeT
, referred to as thevariant
object's contained value, is allocated within the storage of thevariant
object. Implementations are not permitted to use additional storage, such as dynamic memory, to allocate the contained value.The contained value shall be allocated in a region of the variant storage suitably aligned for all types inTypes
.
end
of chunk_view
for input ranges can be const
Section: 25.7.29.2 [range.chunk.view.input] Status: C++23 Submitter: Hewill Kang Opened: 2022-06-09 Last modified: 2023-11-22
Priority: Not Prioritized
View all other issues in [range.chunk.view.input].
View all issues with C++23 status.
Discussion:
The input range version of chunk_view
's end
is a very simple function that only returns
default_sentinel
, and simple end
s like this also appear in other range adaptors, such
as basic_istream_view
, lazy_split_view::outer-iterator::value_type
, and
chunk_view::outer-iterator::value_type
.
chunk_view
, their end
s all are const
-qualified,
which allows us to freely get default_sentinel
through the end
of these const
objects even though they may not themselves be range
s.
I think we should add const
to this chunk_view
's end
as
I don't see any harm in doing this, and in some cases, it may have a certain value. Also, this makes it
consistent with basic_istream_view
and the upcoming std::generator
, which, like it, only
has a non-const begin
.
[2022-06-21; Reflector poll]
Set status to Tentatively Ready after eight votes in favour during reflector poll.
[2022-07-15; LWG telecon: move to Ready]
[2022-07-25 Approved at July 2022 virtual plenary. Status changed: Ready → WP.]
Proposed resolution:
This wording is relative to N4910.
Modify 25.7.29.2 [range.chunk.view.input] as indicated:
[…]namespace std::ranges { […] template<view V> requires input_range<V> class chunk_view : public view_interface<chunk_view<V>> { V base_ = V(); // exposition only […] public: […] constexpr outer-iterator begin(); constexpr default_sentinel_t end() const noexcept; constexpr auto size() requires sized_range<V>; constexpr auto size() const requires sized_range<const V>; }; […] }constexpr default_sentinel_t end() const noexcept;-4- Returns:
default_sentinel
.
slide_view
constructorSection: 25.7.30.2 [range.slide.view] Status: C++23 Submitter: Hewill Kang Opened: 2022-06-10 Last modified: 2023-11-22
Priority: Not Prioritized
View all other issues in [range.slide.view].
View all issues with C++23 status.
Discussion:
It doesn't make sense for slide_view
when n
is not positive,
we should therefore add a precondition for its constructor just like we did for
chunk_view
.
N
is positive" in the design wording
but omitted to add a normative precondition in the proposed wording.
[2022-06-21; Reflector poll]
Set status to Tentatively Ready after six votes in favour during reflector poll.
[2022-07-15; LWG telecon: move to Ready]
[2022-07-25 Approved at July 2022 virtual plenary. Status changed: Ready → WP.]
Proposed resolution:
This wording is relative to N4910.
Modify 25.7.30.2 [range.slide.view] as indicated:
constexpr explicit slide_view(V base, range_difference_t<V> n);-?- Preconditions:
-1- Effects: Initializesn > 0
istrue
.base_
withstd::move(base)
andn_
withn
.
chunk_view
and slide_view
should not be default_initializable
Section: 25.7.29.2 [range.chunk.view.input], 25.7.29.6 [range.chunk.view.fwd], 25.7.30.2 [range.slide.view] Status: C++23 Submitter: Hewill Kang Opened: 2022-06-10 Last modified: 2023-11-22
Priority: Not Prioritized
View all other issues in [range.chunk.view.input].
View all issues with C++23 status.
Discussion:
Both chunk_view
and slide_view
have a precondition that N
must be positive,
but they are still default_initializable
when the underlying range is default_initializable
,
which makes the member variable n_
initialized with an invalid value 0
when they
are default-constructed, which produces the following unexpected result:
#include <ranges> using V = std::ranges::iota_view<int, int>; static_assert(std::ranges::slide_view<V>().empty()); // fails static_assert(std::ranges::chunk_view<V>().empty()); // division by zero is not a constant expression
Although we could provide a default positive value for n_
, I think a more appropriate solution
would be to not provide the default constructor, since default-constructed values for integer types will
never be valid.
[2022-06-21; Reflector poll]
Set status to Tentatively Ready after six votes in favour during reflector poll.
[2022-07-15; LWG telecon: move to Ready]
[2022-07-25 Approved at July 2022 virtual plenary. Status changed: Ready → WP.]
Proposed resolution:
This wording is relative to N4910.
Modify 25.7.29.2 [range.chunk.view.input] as indicated:
namespace std::ranges { […] template<view V> requires input_range<V> class chunk_view : public view_interface<chunk_view<V>> { V base_= V(); // exposition only range_difference_t<V> n_= 0; // exposition only range_difference_t<V> remainder_ = 0; // exposition only […] public:chunk_view() requires default_initializable<V> = default;constexpr explicit chunk_view(V base, range_difference_t<V> n); […] }; […] }
Modify 25.7.29.6 [range.chunk.view.fwd] as indicated:
namespace std::ranges { template<view V> requires forward_range<V> class chunk_view<V> : public view_interface<chunk_view<V>> { V base_= V(); // exposition only range_difference_t<V> n_= 0; // exposition only […] public:chunk_view() requires default_initializable<V> = default;constexpr explicit chunk_view(V base, range_difference_t<V> n); […] }; }
Modify 25.7.30.2 [range.slide.view] as indicated:
namespace std::ranges { […] template<forward_range V> requires view<V> class slide_view : public view_interface<slide_view<V>> { V base_= V(); // exposition only range_difference_t<V> n_= 0; // exposition only […] public:slide_view() requires default_initializable<V> = default;constexpr explicit slide_view(V base, range_difference_t<V> n); […] }; […] }
Section: 26.8.1 [alg.sorting.general] Status: C++23 Submitter: Casey Carter Opened: 2022-06-10 Last modified: 2023-11-22
Priority: Not Prioritized
View all issues with C++23 status.
Discussion:
P0896R4 changed the term of art "sorted with respect to comparator" defined in 26.8.1 [alg.sorting.general] paragraph 5 to "sorted with respect to comparator and projection." That proposal updated the algorithm specifications consistently. However, there were uses of the old term outside of 26 [algorithms] that are now without meaning. We should bring back the term "sorted with respect to comparator" to fix that lack.
[2022-06-21; Reflector poll]
Set status to Tentatively Ready after eight votes in favour during reflector poll.
[2022-07-15; LWG telecon: move to Ready]
[2022-07-25 Approved at July 2022 virtual plenary. Status changed: Ready → WP.]
Proposed resolution:
This wording is relative to N4910.
Modify 26.8.1 [alg.sorting.general] as indicated:
-5- A sequence is sorted with respect to a
comp
andproj
for a comparator and projectioncomp
andproj
if for every iteratori
pointing to the sequence and every non-negative integern
such thati + n
is a valid iterator pointing to an element of the sequence,bool(invoke(comp, invoke(proj, *(i + n)), invoke(proj, *i)))is
-?- A sequence is sorted with respect to a comparatorfalse
.comp
for a comparatorcomp
if it is sorted with respect tocomp
andidentity{}
(the identity projection).
view_interface::empty
is overconstrainedSection: 25.5.3.1 [view.interface.general] Status: C++23 Submitter: Hewill Kang Opened: 2022-06-12 Last modified: 2023-11-22
Priority: Not Prioritized
View all other issues in [view.interface.general].
View all issues with C++23 status.
Discussion:
Currently, view_interface::empty
has the following constraints
constexpr bool empty() requires forward_range<D> { return ranges::begin(derived()) == ranges::end(derived()); }
which seems reasonable, since we need to guarantee the equality preservation of the expression
ranges::begin(r)
.
D
models
sized_range
, we only need to determine whether the value of ranges::size
is 0
.
Since sized_range
and forward_range
are orthogonal to each other, this also
prevents any range that models sized_range
but not forward_range
.
Consider:
#include <iostream>
#include <ranges>
int main() {
auto f = std::views::iota(0, 5)
| std::views::filter([](int) { return true; });
auto r = std::views::counted(f.begin(), 4)
| std::views::slide(2);
std::cout << (r.size() == 0) << "\n"; // #1
std::cout << r.empty() << "\n"; // #2, calls r.begin() == r.end()
}
Since r
models sized_range
, #1
will invoke slide_view::size
,
which mainly invokes ranges::distance
; However, #2
invokes view_interface::empty
and evaluates r.begin() == r.end()
, which constructs the iterator, invokes ranges::next
,
and caches the result, which is unnecessary.
#include <iostream>
#include <ranges>
int main() {
auto i = std::views::istream<int>(std::cin);
auto r = std::views::counted(i.begin(), 4)
| std::views::chunk(2);
std::cout << (r.size() == 0) << "\n"; // #1
std::cout << !r << "\n"; // #2, equivalent to r.size() == 0
std::cout << r.empty() << "\n"; // #3, ill-formed
}
Since r
is still sized_range
, #1
will invoke chunk_view::size
.
#2
is also well-formed since view_interface::operator bool
only requires the
expression ranges::empty(r)
to be well-formed, which first determines the validity of
r.empty()
, and ends up evaluating #1
; However, #3
is ill-formed since
r
is not a forward_range
.
ranges::empty
to determine whether r
is empty, this
inconsistency of the validity of !r
and r.empty()
is quite unsatisfactory.
I see no reason to prevent view_interface::empty
when D
is sized_range
,
since checking whether ranges::size(r) == 0
is an intuitive way to check for empty, as
ranges::empty
does.
[2022-06-21; Reflector poll]
Set status to Tentatively Ready after six votes in favour during reflector poll.
[2022-07-15; LWG telecon: move to Ready]
[2022-07-25 Approved at July 2022 virtual plenary. Status changed: Ready → WP.]
Proposed resolution:
This wording is relative to N4910.
Modify 25.5.3.1 [view.interface.general] as indicated:
namespace std::ranges { template<class D> requires is_class_v<D> && same_as<D, remove_cv_t<D>> class view_interface { private: constexpr D& derived() noexcept { // exposition only return static_cast<D&>(*this); } constexpr const D& derived() const noexcept { // exposition only return static_cast<const D&>(*this); } public: constexpr bool empty() requires sized_range<D> || forward_range<D> { if constexpr (sized_range<D>) return ranges::size(derived()) == 0; else return ranges::begin(derived()) == ranges::end(derived()); } constexpr bool empty() const requires sized_range<const D> || forward_range<const D> { if constexpr (sized_range<const D>) return ranges::size(derived()) == 0; else return ranges::begin(derived()) == ranges::end(derived()); } […] }; }
common_view::end
should improve random_access_range
caseSection: 25.7.20.2 [range.common.view] Status: C++23 Submitter: Hewill Kang Opened: 2022-06-15 Last modified: 2023-11-22
Priority: 3
View all other issues in [range.common.view].
View all issues with C++23 status.
Discussion:
This issue is part of NB comment US 47-109 26 [ranges] Resolve open issues
In view of the fact that random access iterators are only required to work with its difference type,
P2393R1 improves the wording of take_view
, which first convert the integer type
to difference type and then operate with the iterator. However, the paper omits the handling of random
access iterators in common_view::end
, which directly operates on the return types of
ranges::begin
and ranges::size
. We should improve this, too.
[2022-07-06; Reflector poll]
Set priority to 3 after reflector poll.
Previous resolution [SUPERSEDED]:
This wording is relative to N4910.
Modify 25.7.20.2 [range.common.view] as indicated:
namespace std::ranges { template<view V> requires (!common_range<V> && copyable<iterator_t<V>>) class common_view : public view_interface<common_view<V>> { private: V base_ = V(); // exposition only public: […] constexpr auto begin() { if constexpr (random_access_range<V> && sized_range<V>) return ranges::begin(base_); else return common_iterator<iterator_t<V>, sentinel_t<V>>(ranges::begin(base_)); } constexpr auto begin() const requires range<const V> { if constexpr (random_access_range<const V> && sized_range<const V>) return ranges::begin(base_); else return common_iterator<iterator_t<const V>, sentinel_t<const V>>(ranges::begin(base_)); } constexpr auto end() { if constexpr (random_access_range<V> && sized_range<V>) return ranges::begin(base_) + range_difference_t<V>(size())ranges::size(base_); else return common_iterator<iterator_t<V>, sentinel_t<V>>(ranges::end(base_)); } constexpr auto end() const requires range<const V> { if constexpr (random_access_range<const V> && sized_range<const V>) return ranges::begin(base_) + range_difference_t<const V>(size())ranges::size(base_); else return common_iterator<iterator_t<const V>, sentinel_t<const V>>(ranges::end(base_)); } constexpr auto size() requires sized_range<V> { return ranges::size(base_); } constexpr auto size() const requires sized_range<const V> { return ranges::size(base_); } }; […] }
[Kona 2022-11-08; Discussed at joint LWG/SG9 session. Move to Open]
[2022-11-09 Tim updates wording per LWG discussion]
[Kona 2022-11-10; Move to Immediate]
[2022-11-12 Approved at November 2022 meeting in Kona. Status changed: Immediate → WP.]
Proposed resolution:
This wording is relative to N4917.
Modify 25.7.20.2 [range.common.view] as indicated:
namespace std::ranges { template<view V> requires (!common_range<V> && copyable<iterator_t<V>>) class common_view : public view_interface<common_view<V>> { private: V base_ = V(); // exposition only public: […] constexpr auto begin() { if constexpr (random_access_range<V> && sized_range<V>) return ranges::begin(base_); else return common_iterator<iterator_t<V>, sentinel_t<V>>(ranges::begin(base_)); } constexpr auto begin() const requires range<const V> { if constexpr (random_access_range<const V> && sized_range<const V>) return ranges::begin(base_); else return common_iterator<iterator_t<const V>, sentinel_t<const V>>(ranges::begin(base_)); } constexpr auto end() { if constexpr (random_access_range<V> && sized_range<V>) return ranges::begin(base_) + ranges::sizedistance(base_); else return common_iterator<iterator_t<V>, sentinel_t<V>>(ranges::end(base_)); } constexpr auto end() const requires range<const V> { if constexpr (random_access_range<const V> && sized_range<const V>) return ranges::begin(base_) + ranges::sizedistance(base_); else return common_iterator<iterator_t<const V>, sentinel_t<const V>>(ranges::end(base_)); } constexpr auto size() requires sized_range<V> { return ranges::size(base_); } constexpr auto size() const requires sized_range<const V> { return ranges::size(base_); } }; […] }
std::basic_format_arg
Section: 28.5.8.1 [format.arg] Status: Resolved Submitter: Jiang An Opened: 2022-06-17 Last modified: 2023-03-23
Priority: 2
View all other issues in [format.arg].
View all issues with Resolved status.
Discussion:
While correcting some bugs in MSVC STL,
it is found that
P2418R2 broke the overload resolution involving non-const lvalue: constructing
basic_format_arg
from a non-const basic_string
lvalue incorrectly selects the
T&&
overload and uses handle
, while the separated basic_string
overload should be selected (i.e. the old behavior before P2418R2 did the right thing). Currently
MSVC STL is using a workaround that treats basic_string
to be as if passed by value
during overload resolution.
T&&
overload should not interfere the old result of overload resolution,
which means that when a type is const
-formattable, the newly added non-const
mechanism shouldn't be considered.
[2022-07-06; Reflector poll]
Set priority to 2 after reflector poll.
[2022-10-19; Would be resolved by 3631(i)]
Previous resolution [SUPERSEDED]:
This wording is relative to N4910.
[Drafting note: The below presented wording adds back the
const T&
constructor and its specification that were present before P2418R2. Furthermore it adds an additional constraint to theT&&
constructor and simplifies the Effects of this constructors to thehandle
construction case.]
Modify 28.5.8.1 [format.arg] as indicated:
[…]namespace std { template<class Context> class basic_format_arg { public: class handle; private: […] template<class T> explicit basic_format_arg(T&& v) noexcept; // exposition only template<class T> explicit basic_format_arg(const T& v) noexcept; // exposition only […] }; }template<class T> explicit basic_format_arg(T&& v) noexcept;-4- Constraints: The template specialization
typename Context::template formatter_type<remove_cvref_t<T>>meets the BasicFormatter requirements (28.5.6.1 [formatter.requirements]). The extent to which an implementation determines that the specialization meets the BasicFormatter requirements is unspecified, except that as a minimum the expression
typename Context::template formatter_type<remove_cvref_t<T>>() .format(declval<T&>(), declval<Context&>())shall be well-formed when treated as an unevaluated operand (7.2.3 [expr.context]), and if this overload were not declared, the overload resolution would find no usable candidate or be ambiguous. [Note ?: This overload has no effect if the overload resolution among other overloads succeeds. — end note].
-5- Effects:
(5.1) — ifT
isbool
orchar_type
, initializesvalue
withv
;
(5.2) — otherwise, ifT
ischar
andchar_type
iswchar_t
, initializesvalue
withstatic_cast<wchar_t>(v)
;
(5.3) — otherwise, ifT
is a signed integer type (6.8.2 [basic.fundamental]) andsizeof(T) <= sizeof(int)
, initializesvalue
withstatic_cast<int>(v)
;
(5.4) — otherwise, ifT
is an unsigned integer type andsizeof(T) <= sizeof(unsigned int)
, initializesvalue
withstatic_cast<unsigned int>(v)
;
(5.5) — otherwise, ifT
is a signed integer type andsizeof(T) <= sizeof(long long int)
, initializesvalue
withstatic_cast<long long int>(v)
;
(5.6) — otherwise, ifT
is an unsigned integer type andsizeof(T) <= sizeof(unsigned long long int)
, initializesvalue
withstatic_cast<unsigned long long int>(v)
;
(5.7) — otherwise, iInitializesvalue
withhandle(v)
.template<class T> explicit basic_format_arg(const T& v) noexcept;-?- Constraints: The template specialization
typename Context::template formatter_type<T>meets the Formatter requirements (28.5.6.1 [formatter.requirements]). The extent to which an implementation determines that the specialization meets the Formatter requirements is unspecified, except that as a minimum the expression
typename Context::template formatter_type<T>() .format(declval<const T&>(), declval<Context&>())shall be well-formed when treated as an unevaluated operand (7.2.3 [expr.context]).
-?- Effects:
(?.1) — if
T
isbool
orchar_type
, initializesvalue
withv
;(?.2) — otherwise, if
T
ischar
andchar_type
iswchar_t
, initializesvalue
withstatic_cast<wchar_t>(v)
;(?.3) — otherwise, if
T
is a signed integer type (6.8.2 [basic.fundamental]) andsizeof(T) <= sizeof(int)
, initializesvalue
withstatic_cast<int>(v)
;(?.4) — otherwise, if
T
is an unsigned integer type andsizeof(T) <= sizeof(unsigned int)
, initializesvalue
withstatic_cast<unsigned int>(v)
;(?.5) — otherwise, if
T
is a signed integer type andsizeof(T) <= sizeof(long long int)
, initializesvalue
withstatic_cast<long long int>(v)
;(?.6) — otherwise, if
T
is an unsigned integer type andsizeof(T) <= sizeof(unsigned long long int)
, initializesvalue
withstatic_cast<unsigned long long int>(v)
;(?.7) — otherwise, initializes
value
withhandle(v)
.
[2023-03-22 Resolved by the adoption of 3631(i) in Issaquah. Status changed: New → Resolved.]
Proposed resolution:
Section: 31.12.11.1 [fs.class.directory.iterator.general], 31.12.12.1 [fs.class.rec.dir.itr.general] Status: C++23 Submitter: Jonathan Wakely Opened: 2022-06-17 Last modified: 2024-01-29
Priority: Not Prioritized
View all issues with C++23 status.
Discussion:
We added comparisons with default_sentinel_t
to the stream and streambuf iterators, because their
past-the-end iterator is just a default-constructed iterator. We didn't do the same for filesystem directory
iterators, but they also use a default-constructed value as the sentinel.
Previous resolution [SUPERSEDED]:
This wording is relative to N4910.
Modify 31.12.11.1 [fs.class.directory.iterator.general], class
directory_iterator
synopsis, as indicated:namespace std::filesystem { class directory_iterator { […] const directory_entry& operator*() const; const directory_entry* operator->() const; directory_iterator& operator++(); directory_iterator& increment(error_code& ec); friend bool operator==(const directory_iterator& lhs, default_sentinel_t) noexcept { return lhs == end(lhs); } // other members as required by 24.3.5.3 [input.iterators], input iterators }; }Modify 31.12.12.1 [fs.class.rec.dir.itr.general], class
recursive_directory_iterator
synopsis, as indicated:namespace std::filesystem { class recursive_directory_iterator { […] void pop(); void pop(error_code& ec); void disable_recursion_pending(); friend bool operator==(const recursive_directory_iterator& lhs, default_sentinel_t) noexcept { return lhs == end(lhs); } // other members as required by 24.3.5.3 [input.iterators], input iterators }; }
[2022-07-06; Jonathan Wakely revises proposed resolution and adds regex iterators as suggested on the reflector.]
[2022-07-11; Reflector poll]
Set status to Tentatively Ready after six votes in favour during reflector poll.
[2022-07-15; LWG telecon: move to Ready]
[2022-07-25 Approved at July 2022 virtual plenary. Status changed: Ready → WP.]
Proposed resolution:
This wording is relative to N4910.
Modify 31.12.11.1 [fs.class.directory.iterator.general], class directory_iterator
synopsis,
as indicated:
namespace std::filesystem { class directory_iterator { […] const directory_entry& operator*() const; const directory_entry* operator->() const; directory_iterator& operator++(); directory_iterator& increment(error_code& ec); bool operator==(default_sentinel_t) const noexcept { return *this == directory_iterator(); } // other members as required by 24.3.5.3 [input.iterators], input iterators }; }
Modify 31.12.12.1 [fs.class.rec.dir.itr.general], class recursive_directory_iterator
synopsis,
as indicated:
namespace std::filesystem { class recursive_directory_iterator { […] void pop(); void pop(error_code& ec); void disable_recursion_pending(); bool operator==(default_sentinel_t) const noexcept { return *this == recursive_directory_iterator(); } // other members as required by 24.3.5.3 [input.iterators], input iterators }; }
Modify 28.6.11.1.1 [re.regiter.general], regex_iterator
synopsis, as indicated:
namespace std { template<class BidirectionalIterator, class charT = typename iterator_traits<BidirectionalIterator>::value_type, class traits = regex_traits<charT>> class regex_iterator { […] regex_iterator& operator=(const regex_iterator&); bool operator==(const regex_iterator&) const; bool operator==(default_sentinel_t) const { return *this == regex_iterator(); } const value_type& operator*() const; const value_type* operator->() const;
Modify 28.6.11.2.1 [re.tokiter.general], regex_token_iterator
synopsis, as indicated:
namespace std { template<class BidirectionalIterator, class charT = typename iterator_traits<BidirectionalIterator>::value_type, class traits = regex_traits<charT>> class regex_token_iterator { […] regex_iterator& operator=(const regex_token_iterator&); bool operator==(const regex_token_iterator&) const; bool operator==(default_sentinel_t) const { return *this == regex_token_iterator(); } const value_type& operator*() const; const value_type* operator->() const;
Section: 28.5.2.2 [format.string.std] Status: C++23 Submitter: Mark de Wever Opened: 2022-06-19 Last modified: 2023-11-22
Priority: 2
View other active issues in [format.string.std].
View all other issues in [format.string.std].
View all issues with C++23 status.
Discussion:
Per 28.5.2.2 [format.string.std]/7
If
{ arg-idopt }
is used in a width or precision, the value of the corresponding formatting argument is used in its place. If the corresponding formatting argument is not of integral type, or its value is negative for precision or non-positive for width, an exception of typeformat_error
is thrown.
The issue is the integral type requirement. The following code is currently valid:
std::cout << std::format("{:*^{}}\n", 'a', '0'); std::cout << std::format("{:*^{}}\n", 'a', true);
The output of the first example depends on the value of '0'
in the implementation.
When a char
has signed char
as underlying type negative values are invalid,
while the same value would be valid when the underlying type is unsigned char
.
For the second example the range of a boolean is very small, so this seems not really useful.
std::cout << std::format("{:*^{}}\n", 'a', L'0'); std::cout << std::format("{:*^{}}\n", 'a', u'0'); std::cout << std::format("{:*^{}}\n", 'a', U'0'); std::cout << std::format("{:*^{}}\n", 'a', u8'0');
In order to accept these character types they need to meet the basic formatter requirements per 28.5.5 [format.functions]/20 and 28.5.5 [format.functions]/25
formatter<remove_cvref_t<Ti>, charT>
meets the BasicFormatter requirements (28.5.6.1 [formatter.requirements]) for eachTi
inArgs
.
which requires adding the following enabled formatter specializations to 28.5.6.4 [format.formatter.spec].
template<> struct formatter<wchar_t, char>; template<> struct formatter<char8_t, charT>; template<> struct formatter<char16_t, charT>; template<> struct formatter<char32_t, charT>;
Note, that the specialization template<> struct formatter<char, wchar_t>
is already required by the Standard.
charT
.
Instead of requiring these specializations, I propose to go the other
direction and limit the allowed types to signed and unsigned integers.
[2022-07-08; Reflector poll]
Set priority to 2 after reflector poll. Tim Song commented:
"This is technically a breaking change, so we should do it sooner rather than later.
"I don't agree with the second part of the argument though - I don't see how this wording requires adding those transcoding specializations. Nothing in this wording requires integral types that cannot be packed into basic_format_arg to be accepted.
"I also think we need to restrict this to signed or unsigned integer types with size no greater than sizeof(long long). Larger types get type-erased into a handle and the value isn't really recoverable without heroics."
Previous resolution [SUPERSEDED]:
This wording is relative to N4910.
Modify 28.5.2.2 [format.string.std] as indicated:
-7- If
{ arg-idopt }
is used in a width or precision, the value of the corresponding formatting argument is used in its place. If the corresponding formatting argument is not ofintegralsigned or unsigned integer type, or its value is negative for precision or non-positive for width, an exception of typeformat_error
is thrown.Add a new paragraph to C.2.10 [diff.cpp20.utilities] as indicated:
Affected subclause: 28.5 [format]
Change: Requirement changes of arg-id of the width and precision fields of std-format-spec. arg-id now requires a signed or unsigned integer type instead of an integral type. Rationale: Avoid types that are not useful and the need to specify enabled formatter specializations for all character types. Effect on original feature: Valid C++ 2020 code that passes a boolean or character type as arg-id becomes invalid. For example:std::format("{:*^{}}", "", true); // ill-formed, previously returned "*"
[2022-11-01; Jonathan provides improved wording]
Previous resolution [SUPERSEDED]:
This wording is relative to N4917.
Modify 28.5.2.2 [format.string.std] as indicated:
-8- If
{ arg-idopt }
is used in a width or precision, the value of the corresponding formatting argument is used in its place. If the corresponding formatting argument is not ofintegralstandard signed or unsigned integer type, or its value is negative, an exception of typeformat_error
is thrown.Add a new paragraph to C.2.10 [diff.cpp20.utilities] as indicated:
Affected subclause: 28.5.2.2 [format.string.std]
Change: Restrict types of formatting arguments used as width or precision in a std-format-spec. Rationale: Avoid types that are not useful or do not have portable semantics. Effect on original feature: Valid C++ 2020 code that passes a boolean or character type as arg-id becomes invalid. For example:std::format("{:*^{}}", "", true); // ill-formed, previously returned "*" std::format("{:*^{}}", "", '1'); // ill-formed, previously returned an implementation-defined number of '*' characters
[2022-11-10; Jonathan revises wording]
Improve Annex C entry.
[Kona 2022-11-10; Move to Ready]
[2023-02-13 Approved at February 2023 meeting in Issaquah. Status changed: Voting → WP.]
Proposed resolution:
This wording is relative to N4917.
Modify 28.5.2.2 [format.string.std] as indicated:
-8- If
{ arg-idopt }
is used in a width or precision, the value of the corresponding formatting argument is used in its place. If the corresponding formatting argument is not ofintegralstandard signed or unsigned integer type, or its value is negative, an exception of typeformat_error
is thrown.
Add a new paragraph to C.2.10 [diff.cpp20.utilities] as indicated:
Affected subclause: 28.5.2.2 [format.string.std]
Change: Restrict types of formatting arguments used as width or precision in a std-format-spec. Rationale: Disallow types that do not have useful or portable semantics as a formatting width or precision. Effect on original feature: Valid C++ 2020 code that passes a boolean or character type as arg-id becomes invalid. For example:std::format("{:*^{}}", "", true); // ill-formed, previously returned "*" std::format("{:*^{}}", "", '1'); // ill-formed, previously returned an implementation-defined number of '*' characters
Section: 28.5.2.2 [format.string.std] Status: C++23 Submitter: Mark de Wever Opened: 2022-06-19 Last modified: 2023-11-22
Priority: 3
View other active issues in [format.string.std].
View all other issues in [format.string.std].
View all issues with C++23 status.
Discussion:
Per 28.5.2.2 [format.string.std]/7
If
{ arg-idopt }
is used in a width or precision, the value of the corresponding formatting argument is used in its place. If the corresponding formatting argument is not of integral type, or its value is negative for precision or non-positive for width, an exception of typeformat_error
is thrown.
During a libc++ code review Victor mentioned it would be nice to allow zero as a valid value for the arg-id when used for the width. This would simplify the code by having the same requirements for the arg-id for the width and precision fields. A width of zero has no effect on the output.
In the std-format-spec the width is restricted to a positive-integer to avoid parsing ambiguity with the zero-padding option. This ambiguity doesn't happen using an arg-id with the value zero. Therefore I only propose to change the arg-id's requirement. Note the Standard doesn't specify the width field's effect on the output. Specifically [tab:format.align] doesn't refer to the width field. This is one of the items addressed by P2572. The proposed resolution works in combination with the wording of that paper.[2022-07-08; Reflector poll]
Set priority to 3 after reflector poll.
[2022-07-11; Reflector poll]
Set status to Tentatively Ready after seven votes in favour during reflector poll.
[2022-07-15; LWG telecon: move to Ready]
[2022-07-25 Approved at July 2022 virtual plenary. Status changed: Ready → WP.]
Proposed resolution:
This wording is relative to N4910.
Modify 28.5.2.2 [format.string.std] as indicated:
-7- If
{ arg-idopt }
is used in a width or precision, the value of the corresponding formatting argument is used in its place. If the corresponding formatting argument is not of integral type, or its value is negativefor precision or non-positive for width, an exception of typeformat_error
is thrown.
priority_queue::push_range
needs to append_range
Section: 23.6.4.4 [priqueue.members] Status: C++23 Submitter: Casey Carter Opened: 2022-06-20 Last modified: 2023-11-22
Priority: 2
View all issues with C++23 status.
Discussion:
The push_range
members of the queue
(23.6.3.4 [queue.mod]) and
stack
(23.6.6.5 [stack.mod]) container adaptors are both specified as
"Effects: Equivalent to c.append_range(std::forward<R>(rg))
if that
is a valid expression, otherwise ranges::copy(rg, back_inserter(c))
.". For
priority_queue
, however, we have instead (23.6.4.4 [priqueue.members]):
-3- Effects: Insert all elements of
-4- Postconditions:rg
inc
.is_heap(c.begin(), c.end(), comp)
istrue
.
Since append_range
isn't one of the operations required of the underlying container,
"Insert all elements of rg
" must be implemented via potentially less efficient means.
It would be nice if this push_back
could take advantage of append_range
when it's available just as do the other two overloads.
[2022-07-08; Reflector poll]
Set priority to 2 after reflector poll.
[Issaquah 2023-02-08; LWG]
Unanimous consent to move to Immediate.
[2023-02-13 Approved at February 2023 meeting in Issaquah. Status changed: Immediate → WP.]
Proposed resolution:
This wording is relative to N4910.
Modify 23.6.4.4 [priqueue.members] as indicated:
template<container-compatible-range<T> R> void push_range(R&& rg);-3- Effects: Inserts all elements of
-4- Postconditions:rg
inc
viac.append_range(std::forward<R>(rg))
if that is a valid expression, orranges::copy(rg, back_inserter(c))
otherwise. Then restores the heap property as if bymake_heap(c.begin(), c.end(), comp)
.is_heap(c.begin(), c.end(), comp)
istrue
.
decay-copy
should be constrainedSection: 16.3.3.2 [expos.only.entity] Status: C++23 Submitter: Hui Xie Opened: 2022-06-23 Last modified: 2023-11-22
Priority: 3
View all other issues in [expos.only.entity].
View all issues with C++23 status.
Discussion:
The spec of views::all
25.7.6.1 [range.all.general] p2 says:
Given a subexpression
E
, the expressionviews::all(E)
is expression-equivalent to:
(2.1) —
decay-copy(E)
if the decayed type of E modelsview
.[…]
If E
is an lvalue move-only view
, according to the spec, views::all(E)
would be expression-equivalent to decay-copy(E)
.
decay-copy
as follows
template<class T> constexpr decay_t<T> decay-copy(T&& v) noexcept(is_nothrow_convertible_v<T, decay_t<T>>) // exposition only { return std::forward<T>(v); }
It is unconstrained. As a result, for the above example, views::all(E)
is a well-formed
expression and it would only error on the template instantiation of the function body of
decay-copy
, because E
is an lvalue of move-only type and not copyable.
decay-copy(E)
ill-formed
if it is not copyable, so that views::all(E)
can be SFINAE friendly.
[2022-07-08; Reflector poll]
Set priority to 3 after reflector poll.
[2022-07-11; Reflector poll]
Set status to Tentatively Ready after seven votes in favour during reflector poll.
[2022-07-15; LWG telecon: move to Ready]
[2022-07-25 Approved at July 2022 virtual plenary. Status changed: Ready → WP.]
Proposed resolution:
This wording is relative to N4910.
Modify [expos.only.func] as indicated:
-2- The following are defined for exposition only to aid in the specification of the library:
namespace std { template<class T> requires convertible_to<T, decay_t<T>> constexpr decay_t<T> decay-copy(T&& v) noexcept(is_nothrow_convertible_v<T, decay_t<T>>) // exposition only { return std::forward<T>(v); } […] }
prepend_range
and append_range
can't be amortized constant timeSection: 23.2.4 [sequence.reqmts] Status: C++23 Submitter: Tim Song Opened: 2022-07-06 Last modified: 2023-11-22
Priority: Not Prioritized
View other active issues in [sequence.reqmts].
View all other issues in [sequence.reqmts].
View all issues with C++23 status.
Discussion:
23.2.4 [sequence.reqmts]/69 says "An implementation shall implement them so as to take amortized constant time."
followed by a list of operations that includes the newly added append_range
and prepend_range
.
Obviously these operations cannot be implemented in amortized constant time.
[2022-08-23; Reflector poll]
Set status to Tentatively Ready after six votes in favour during reflector poll.
[2022-11-12 Approved at November 2022 meeting in Kona. Status changed: Voting → WP.]
Proposed resolution:
This wording is relative to N4910.
Modify 23.2.4 [sequence.reqmts] as indicated:
-69- The following operations are provided for some types of sequence containers but not others.
An implementation shall implement themOperations other thanprepend_range
andappend_range
are implemented so as to take amortized constant time.
ranges::to
misuses cpp17-input-iterator
Section: 25.5.7.2 [range.utility.conv.to] Status: C++23 Submitter: S. B. Tam Opened: 2022-07-10 Last modified: 2024-01-29
Priority: 2
View other active issues in [range.utility.conv.to].
View all other issues in [range.utility.conv.to].
View all issues with C++23 status.
Discussion:
ranges::to
uses cpp17-input-iterator<iterator_t<R>>
to check
whether an iterator is a Cpp17InputIterator, which misbehaves if there is a
std::iterator_traits
specialization for that iterator (e.g. if the iterator is a
std::common_iterator
).
struct MyContainer {
template<class Iter>
MyContainer(Iter, Iter);
char* begin();
char* end();
};
auto nul_terminated = std::views::take_while([](char ch) { return ch != '\0'; });
auto c = nul_terminated("") | std::views::common | std::ranges::to<MyContainer>(); // error
I believe that ranges::to
should instead use
derived_from<typename iterator_traits<iterator_t<R>>::iterator_category, input_iterator_tag>
,
which correctly detects the iterator category of a std::common_iterator
.
[2022-08-23; Reflector poll]
Set priority to 2 after reflector poll. Set status to Tentatively Ready after five votes in favour during reflector poll.
Previous resolution [SUPERSEDED]:
This wording is relative to N4910.
Modify 25.5.7.2 [range.utility.conv.to] as indicated:
template<class C, input_range R, class... Args> requires (!view<C>) constexpr C to(R&& r, Args&&... args);-1- Returns: An object of type
C
constructed from the elements ofr
in the following manner:
(1.1) — If
convertible_to<range_reference_t<R>, range_value_t<C>>
istrue
:
(1.1.1) — If
constructible_from<C, R, Args...>
istrue
:C(std::forward<R>(r), std::forward<Args>(args)...)
(1.1.2) — Otherwise, if
constructible_from<C, from_range_t, R, Args...>
istrue
:C(from_range, std::forward<R>(r), std::forward<Args>(args)...)
(1.1.3) — Otherwise, if
(1.1.3.1) —
common_range<R>
istrue
,(1.1.3.2) —
is
cpp17-input-iteratorderived_from<typename iterator_traits<iterator_t<R>>::iterator_category, input_iterator_tag>true
, and(1.1.3.3) —
constructible_from<C, iterator_t<R>, sentinel_t<R>, Args...>
istrue
:C(ranges::begin(r), ranges::end(r), std::forward<Args>(args)...)
(1.1.4) — Otherwise, if
(1.1.4.1) —
constructible_from<C, Args...>
istrue
, and(1.1.4.2) —
container-insertable<C, range_reference_t<R>>
istrue
:[…](1.2) — Otherwise, if
input_range<range_reference_t<R>>
istrue
:to<C>(r | views::transform([](auto&& elem) { return to<range_value_t<C>>(std::forward<decltype(elem)>(elem)); }), std::forward<Args>(args)...);(1.3) — Otherwise, the program is ill-formed.
[2022-08-27; Hewill Kang reopens and suggests a different resolution]
This issue points out that the standard misuses cpp17-input-iterator
to check
whether the iterator meets the requirements of Cpp17InputIterator, and proposes to use
iterator_traits<I>::iterator_category
to check the iterator's category directly,
which may lead to the following potential problems:
common_range
and input_range
,
the expression iterator_traits<I>::iterator_category
may not be valid, consider
#include <ranges> struct I { using difference_type = int; using value_type = int; int operator*() const; I& operator++(); void operator++(int); bool operator==(const I&) const; bool operator==(std::default_sentinel_t) const; }; int main() { auto r = std::ranges::subrange(I{}, I{}); auto v = r | std::ranges::to<std::vector<int>>(0); }
Although I
can serve as its own sentinel, it does not model
cpp17-input-iterator
since postfix operator++
returns void
,
which causes iterator_traits<R>
to be an empty class, making the
expression derived_from<iterator_traits<I>::iterator_category, input_iterator_tag>
ill-formed.
common_iterator
, iterator_traits<I>::iterator_category
does not guarantee a strictly correct iterator category in the current standard.
For example, when the above I::operator*
returns a non-copyable
object that
can be converted to int
, this makes common_iterator<I, default_sentinel_t>
unable to synthesize a C++17-conforming postfix operator++
, however,
iterator_traits<common_iterator<I, S>>::iterator_category
will still
give input_iterator_tag
even though it's not even a C++17 iterator.
Another example is that for input_iterator
s with difference type of integer-class type,
the difference type of the common_iterator
wrapped on it is still of the integer-class type,
but the iterator_category
obtained by the iterator_traits
is input_iterator_tag
.
The proposed resolution only addresses the first issue since I believe that the problem with
common_iterator
requires a paper.
[2023-01-11; LWG telecon]
Set status to Tentatively Ready (poll results F6/A0/N1)
[2023-02-13 Approved at February 2023 meeting in Issaquah. Status changed: Voting → WP.]
Proposed resolution:
This wording is relative to N4917.
Modify 25.5.7.2 [range.utility.conv.to] as indicated:
template<class C, input_range R, class... Args> requires (!view<C>) constexpr C to(R&& r, Args&&... args);-1- Returns: An object of type
C
constructed from the elements ofr
in the following manner:
(1.1) — If
convertible_to<range_reference_t<R>, range_value_t<C>>
istrue
:
(1.1.1) — If
constructible_from<C, R, Args...>
istrue
:C(std::forward<R>(r), std::forward<Args>(args)...)
(1.1.2) — Otherwise, if
constructible_from<C, from_range_t, R, Args...>
istrue
:C(from_range, std::forward<R>(r), std::forward<Args>(args)...)
(1.1.3) — Otherwise, if
(1.1.3.1) —
common_range<R>
istrue
,(1.1.3.2) —
if the qualified-idcpp17-input-iterator
iterator_traits
<iterator_t<R>>
::iterator_category
isvalid and denotes a type that modelstrue
derived_from<input_iterator_tag>
, and(1.1.3.3) —
constructible_from<C, iterator_t<R>, sentinel_t<R>, Args...>
:C(ranges::begin(r), ranges::end(r), std::forward<Args>(args)...)
(1.1.4) — Otherwise, if
(1.1.4.1) —
constructible_from<C, Args...>
istrue
, and(1.1.4.2) —
container-insertable<C, range_reference_t<R>>
istrue
:[…](1.2) — Otherwise, if
input_range<range_reference_t<R>>
istrue
:to<C>(r | views::transform([](auto&& elem) { return to<range_value_t<C>>(std::forward<decltype(elem)>(elem)); }), std::forward<Args>(args)...);(1.3) — Otherwise, the program is ill-formed.
inout_ptr
and out_ptr
for empty caseSection: 20.3.4.1 [out.ptr.t] Status: C++23 Submitter: Doug Cook Opened: 2022-07-11 Last modified: 2023-11-22
Priority: 2
View all issues with C++23 status.
Discussion:
out_ptr
and inout_ptr
are inconsistent when a pointer-style function returns
nullptr
.
out_ptr
leaves the stale value in smart
(not the value returned by the pointer-style function).
inout_ptr
(as resolved by LWG 3594(i)) leaves nullptr
in smart
(the value
returned by the pointer-style function).
Assume we have the following pointer-style functions that return nullptr
in case of failure:
void ReplaceSomething(/*INOUT*/ int** pp) { delete *pp; *pp = nullptr; return; // Failure! } void GetSomething(/*OUT*/ int** pp) { *pp = nullptr; return; // Failure! }
In the scenario that led to the creation of issue LWG 3594(i):
// Before the call, inout contains a stale value. auto inout = std::make_unique<int>(1); ReplaceSomething(std::inout_ptr(inout)); // (1) If ReplaceSomething failed (returned nullptr), what does inout contain?
Assuming LWG 3594(i) is resolved as suggested, inout
will be empty.
(The original N4901 text allows inout
to be either empty or
to hold a pointer to already-deleted memory.) Using the resolution suggested by
LWG 3594(i), it expands to something like the following (simplified to
ignore exceptions and opting to perform the release()
before the
ReplaceSomething()
operation):
// Before the call, inout contains a stale value. auto inout = std::make_unique<int>(1); int* p = inout.release(); ReplaceSomething(&p); if (p) { inout.reset(p); } // (1) If ReplaceSomething failed (returned nullptr), inout contains nullptr.
This behavior seems reasonable.
Now consider the corresponding scenario without_ptr
:
// Before the call, out contains a stale value. auto out = std::make_unique<int>(2); GetSomething(std::out_ptr(out)); // (2) If GetSomething failed (returned nullptr), what does out contain?
Based on N4901, out
contains the stale value (from
make_unique
), not the nullptr
value returned by GetSomething()
.
The N4901 model (simplified to ignore exceptions) expands to the following:
// Before the call, out contains a stale value. auto out = std::make_unique<int>(2); int* p{}; GetSomething(&p); if (p) { out.reset(p); } // (2) If GetSomething failed (returned nullptr), out contains a pointer to "2".
This behavior seems incorrect to me. It is inconsistent with the behavior of inout_ptr
and it is inconsistent with my expectation that out
should contain the value returned
by GetSomething()
, even if that value is nullptr
. Intuitively, I expect it to
behave as if the out.reset(p)
were unconditional.
reset(p)
is conditional as an optimization for cases where reset
is
non-trivial. For example, shared_ptr
's reset(p)
requires the allocation of
a control block even if p
is nullptr
. As such, simply making the reset
unconditional may be sub-optimal.
I see two primary options for making out_ptr
's behavior consistent with inout_ptr
:
Perform an unconditional out.reset()
or out = Smart()
in the out_ptr_t
constructor.
Add an else clause to the if statement, containing out.reset()
or out = Smart()
.
I note that these solutions do not make use of the additional args...
, leaving the
out
pointer in an empty state. This is analogous to the corresponding state in the similar
inout
scenario where the inout
pointer is left empty as a result of the call to
smart.release()
.
out_ptr_t
constructor.
[2022-08-23; Reflector poll]
Set priority to 2 after reflector poll. "A bit like design."
[Issaquah 2023-02-07; LWG]
Move to Immediate for C++23
[2023-02-13 Approved at February 2023 meeting in Issaquah. Status changed: Immediate → WP.]
Proposed resolution:
This wording is relative to N4910.
Modify 20.3.4.1 [out.ptr.t] as indicated:
explicit out_ptr_t(Smart& smart, Args... args);-6- Effects: Initializes
s
withsmart
,a
withstd::forward<Args>(args)...
, and value-initializesp
. Then, equivalent to:
- (6.1) —
s.reset();if the expression
s.reset()
is well-formed;(6.2) — otherwise,
s = Smart();if
is_constructible_v<Smart>
istrue
;(6.3) — otherwise, the program is ill-formed.
-7- [Note 2: The constructor is not
noexcept
to allow for a variety of non-terminating and safe implementation strategies. For example, an implementation can allocate ashared_ptr
's internal node in the constructor and let implementation-defined exceptions escape safely. The destructor can then move the allocated control block in directly and avoid any other exceptions. — end note]
move_iterator
missing disable_sized_sentinel_for
specializationSection: 24.2 [iterator.synopsis] Status: C++23 Submitter: Hewill Kang Opened: 2022-07-14 Last modified: 2023-11-22
Priority: Not Prioritized
View all other issues in [iterator.synopsis].
View all issues with C++23 status.
Discussion:
Since reverse_iterator::operator-
is not constrained, the standard adds a
disable_sized_sentinel_for
specialization for it to avoid situations where the
underlying iterator can be subtracted making reverse_iterator
accidentally model
sized_sentinel_for
.
move_iterator::operator-
is also unconstrained and the standard
does not have the disable_sized_sentinel_for
specialization for it, this makes
subrange<move_iterator<I>, move_iterator<I>>
unexpectedly satisfy
sized_range
and incorrectly use I::operator-
to get size when I
can be subtracted but not modeled sized_sentinel_for<I>
.
In addition, since P2520 makes move_iterator
no longer just input_iterator
,
ranges::size
can get the size of the range by subtracting two move_iterator
pairs,
this also makes r | views::as_rvalue
may satisfy sized_range
when neither r
nor
r | views::reverse
is sized_range
.
We should add a move_iterator
version of the disable_sized_sentinel_for
specialization to the standard to avoid the above situation.
[2022-08-23; Reflector poll]
Set status to Tentatively Ready after six votes in favour during reflector poll.
"but I don't think the issue text is quite right - both
move_iterator
and reverse_iterator
's
operator-
are constrained on x.base() - y.base()
being valid."
Does anyone remember why we did this for reverse_iterator
and not move_iterator
?
[2022-11-12 Approved at November 2022 meeting in Kona. Status changed: Voting → WP.]
Proposed resolution:
This wording is relative to N4910.
Modify 24.2 [iterator.synopsis], header <iterator>
synopsis, as indicated:
namespace std::ranges { […] template<class Iterator> constexpr move_iterator<Iterator> make_move_iterator(Iterator i); template<class Iterator1, class Iterator2> requires (!sized_sentinel_for<Iterator1, Iterator2>) inline constexpr bool disable_sized_sentinel_for<move_iterator<Iterator1>, move_iterator<Iterator2>> = true; […] }
take_view::sentinel
should provide operator-
Section: 25.7.10.3 [range.take.sentinel] Status: C++23 Submitter: Hewill Kang Opened: 2022-07-15 Last modified: 2023-11-22
Priority: 3
View all other issues in [range.take.sentinel].
View all issues with C++23 status.
Discussion:
This issue is part of NB comment US 47-109 26 [ranges] Resolve open issues
When the underlying range is not a sized_range
, the begin
and end
functions
of take_view
return counted_iterator
and take_view::sentinel
respectively.
However, the sentinel type of the underlying range may still model sized_sentinel_for
for its
iterator type, and since take_view::sentinel
can only be compared to counted_iterator
,
this makes take_view
no longer able to compute the distance between its iterator and sentinel.
counted_iterator::count
and the difference between
the underlying iterator and sentinel, I think providing operator-
for
take_view::sentinel
does bring some value.
[2022-08-23; Reflector poll]
Set priority to 3 after reflector poll.
Some P0 votes, but with objections: "This seems like a) a feature not a bug - of fairly limited utility?, and b) I’d like to see an implementation (maybe it’s in MSVC?) to be sure there isn’t a negative interaction we’re not thinking of."
Previous resolution [SUPERSEDED]:
This wording is relative to N4910.
Modify 25.7.10.3 [range.take.sentinel], class template
take_view::sentinel
synopsis, as indicated:[…]namespace std::ranges { template<view V> template<bool Const> class take_view<V>::sentinel { private: using Base = maybe-const<Const, V>; // exposition only template<bool OtherConst> using CI = counted_iterator<iterator_t<maybe-const<OtherConst, V>>>; // exposition only sentinel_t<Base> end_ = sentinel_t<Base>(); // exposition only public: […] friend constexpr bool operator==(const CI<Const>& y, const sentinel& x); template<bool OtherConst = !Const> requires sentinel_for<sentinel_t<Base>, iterator_t<maybe-const<OtherConst, V>>> friend constexpr bool operator==(const CI<OtherConst>& y, const sentinel& x); friend constexpr range_difference_t<Base> operator-(const sentinel& x, const CI<Const>& y) requires sized_sentinel_for<sentinel_t<Base>, iterator_t<Base>>; template<bool OtherConst = !Const> requires sized_sentinel_for<sentinel_t<Base>, iterator_t<maybe-const<OtherConst, V>>> friend constexpr range_difference_t<maybe-const<OtherConst, V>> operator-(const sentinel& x, const CI<OtherConst>& y); friend constexpr range_difference_t<Base> operator-(const CI<Const>& x, const sentinel& y) requires sized_sentinel_for<sentinel_t<Base>, iterator_t<Base>>; template<bool OtherConst = !Const> requires sized_sentinel_for<sentinel_t<Base>, iterator_t<maybe-const<OtherConst, V>>> friend constexpr range_difference_t<maybe-const<OtherConst, V>> operator-(const CI<OtherConst>& x, const sentinel& y); }; }friend constexpr bool operator==(const CI<Const>& y, const sentinel& x); template<bool OtherConst = !Const> requires sentinel_for<sentinel_t<Base>, iterator_t<maybe-const<OtherConst, V>>> friend constexpr bool operator==(const CI<OtherConst>& y, const sentinel& x);-4- Effects: Equivalent to:
return y.count() == 0 || y.base() == x.end_;
friend constexpr range_difference_t<Base> operator-(const sentinel& x, const CI<Const>& y) requires sized_sentinel_for<sentinel_t<Base>, iterator_t<Base>>; template<bool OtherConst = !Const> requires sized_sentinel_for<sentinel_t<Base>, iterator_t<maybe-const<OtherConst, V>>> friend constexpr range_difference_t<maybe-const<OtherConst, V>> operator-(const sentinel& x, const CI<OtherConst>& y);-?- Effects: Equivalent to:
return ranges::min(y.count(), x.end_ - y.base());
friend constexpr range_difference_t<Base> operator-(const CI<Const>& x, const sentinel& y) requires sized_sentinel_for<sentinel_t<Base>, iterator_t<Base>>; template<bool OtherConst = !Const> requires sized_sentinel_for<sentinel_t<Base>, iterator_t<maybe-const<OtherConst, V>>> friend constexpr range_difference_t<maybe-const<OtherConst, V>> operator-(const CI<OtherConst>& x, const sentinel& y);-?- Effects: Equivalent to:
return -(y - x);
[Kona 2022-11-08; Discussed at joint LWG/SG9 session. Move to Open]
[2022-11-09 Tim updates wording following LWG discussion]
This case is only possible if the source view is not a sized_range
, yet its
iterator/sentinel types model sized_sentinel_for
(typically when source
is an input range). In such a case we should just have begin
compute
the correct size so that end
can just return default_sentinel
.
[Kona 2022-11-10; Move to Immediate]
[2022-11-12 Approved at November 2022 meeting in Kona. Status changed: Immediate → WP.]
Proposed resolution:
This wording is relative to N4917.
Modify 25.7.10.2 [range.take.view], class template take_view
synopsis, as indicated:
namespace std::ranges { template<view V> class take_view : public view_interface<take_view<V>> { private: […] public: […] constexpr auto begin() requires (!simple-view<V>) { if constexpr (sized_range<V>) { if constexpr (random_access_range<V>) { return ranges::begin(base_); } else { auto sz = range_difference_t<V>(size()); return counted_iterator(ranges::begin(base_), sz); } } else if constexpr (sized_sentinel_for<sentinel_t<V>, iterator_t<V>>) { auto it = ranges::begin(base_); auto sz = std::min(count_, ranges::end(base_) - it); return counted_iterator(std::move(it), sz); } else { return counted_iterator(ranges::begin(base_), count_); } } constexpr auto begin() const requires range<const V> { if constexpr (sized_range<const V>) { if constexpr (random_access_range<const V>) { return ranges::begin(base_); } else { auto sz = range_difference_t<const V>(size()); return counted_iterator(ranges::begin(base_), sz); } } else if constexpr (sized_sentinel_for<sentinel_t<const V>, iterator_t<const V>>) { auto it = ranges::begin(base_); auto sz = std::min(count_, ranges::end(base_) - it); return counted_iterator(std::move(it), sz); } else { return counted_iterator(ranges::begin(base_), count_); } } constexpr auto end() requires (!simple-view<V>) { if constexpr (sized_range<V>) { if constexpr (random_access_range<V>) return ranges::begin(base_) + range_difference_t<V>(size()); else return default_sentinel; } else if constexpr (sized_sentinel_for<sentinel_t<V>, iterator_t<V>>) { return default_sentinel; } else { return sentinel<false>{ranges::end(base_)}; } } constexpr auto end() const requires range<const V> { if constexpr (sized_range<const V>) { if constexpr (random_access_range<const V>) return ranges::begin(base_) + range_difference_t<const V>(size()); else return default_sentinel; } else if constexpr (sized_sentinel_for<sentinel_t<const V>, iterator_t<const V>>) { return default_sentinel; } else { return sentinel<true>{ranges::end(base_)}; } } […] }; […] }
take_view
constructorSection: 25.7.10.2 [range.take.view] Status: C++23 Submitter: Hewill Kang Opened: 2022-07-15 Last modified: 2023-11-22
Priority: Not Prioritized
View all other issues in [range.take.view].
View all issues with C++23 status.
Discussion:
When V
does not model sized_range
, take_view::begin
returns
counted_iterator(ranges::begin(base_), count_)
. Since the
counted_iterator
constructor (24.5.7.2 [counted.iter.const]) already has
a precondition that n >= 0
, we should add this to take_view
as well,
which is consistent with drop_view
.
[2022-08-23; Reflector poll]
Set status to Tentatively Ready after eight votes in favour during reflector poll.
[2022-11-12 Approved at November 2022 meeting in Kona. Status changed: Voting → WP.]
Proposed resolution:
This wording is relative to N4910.
Modify 25.7.10.2 [range.take.view] as indicated:
constexpr take_view(V base, range_difference_t<V> count);-?- Preconditions:
count >= 0
istrue
.-1- Effects: Initializes
base_
withstd::move(base)
andcount_
withcount
.
deque::prepend_range
needs to permuteSection: 23.2.4 [sequence.reqmts] Status: C++23 Submitter: Casey Carter Opened: 2022-07-16 Last modified: 2023-11-22
Priority: 2
View other active issues in [sequence.reqmts].
View all other issues in [sequence.reqmts].
View all issues with C++23 status.
Discussion:
When the range to be inserted is neither bidirectional nor sized, it's simpler to prepend
elements one at a time, and then reverse the prepended elements. When the range to be
inserted is neither forward nor sized, I believe this approach is necessary to implement
prepend_range
at all — there is no way to determine the length of the range
modulo the block size of the deque
ahead of time so as to insert the new elements
in the proper position.
prepend_range
to permute elements in a
deque
. I believe we must allow permutation when the range is neither
forward nor sized, and we should allow permutation when the range is not bidirectional
to allow implementations the freedom to make a single pass through the range.
[2022-07-17; Daniel comments]
The below suggested wording follows the existing style used in the specification of insert
and insert_range
, for example. Unfortunately, this existing practice violates the usual
wording style that a Cpp17XXX requirement shall be met and that we should better
say that "lvalues of type T
are swappable (16.4.4.3 [swappable.requirements])" to
be clearer about the specific swappable context. A separate editorial issue will be reported to take
care of this problem.
[2022-08-23; Reflector poll]
Set status to Tentatively Ready after five votes in favour during reflector poll.
Previous resolution [SUPERSEDED]:
This wording is relative to N4910.
Modify 23.2.4 [sequence.reqmts] as indicated:
a.prepend_range(rg)-94- Result:
-95- Preconditions:void
T
is Cpp17EmplaceConstructible intoX
from*ranges::begin(rg)
. Fordeque
,T
is alsoCpp17MoveInsertable
intoX
, Cpp17MoveConstructible, Cpp17MoveAssignable, and swappable (16.4.4.3 [swappable.requirements]). -96- Effects: Inserts copies of elements inrg
beforebegin()
. Each iterator in the rangerg
is dereferenced exactly once. [Note 3: The order of elements inrg
is not reversed. — end note] -97- Remarks: Required fordeque
,forward_list
, andlist
.
[2022-11-07; Daniel reopens and comments]
The proposed wording has two problems:
It still uses "swappable" instead of "swappable lvalues", which with the (informal) acceptance of P2696R0 should now become Cpp17Swappable.
It uses an atypical form to say "T
(is) Cpp17MoveConstructible,
Cpp17MoveAssignable" instead of the more correct form "T
meets the
Cpp17MoveConstructible and Cpp17MoveAssignable requirements". This
form was also corrected by P2696R0.
The revised wording uses the P2696R0 wording approach to fix both problems.
[Kona 2022-11-12; Set priority to 2]
[2022-11-30; Reflector poll]
Set status to Tentatively Ready after five votes in favour during reflector poll.
[2023-02-13 Approved at February 2023 meeting in Issaquah. Status changed: Voting → WP.]
Proposed resolution:
This wording is relative to N4917 assuming that P2696R0 has been accepted.
Modify 23.2.4 [sequence.reqmts] as indicated:
a.prepend_range(rg)-94- Result:
-95- Preconditions:void
T
is Cpp17EmplaceConstructible intoX
from*ranges::begin(rg)
. Fordeque
,T
is alsoCpp17MoveInsertable
intoX
, andT
meets the Cpp17MoveConstructible, Cpp17MoveAssignable, and Cpp17Swappable (16.4.4.3 [swappable.requirements]) requirements. -96- Effects: Inserts copies of elements inrg
beforebegin()
. Each iterator in the rangerg
is dereferenced exactly once. [Note 3: The order of elements inrg
is not reversed. — end note] -97- Remarks: Required fordeque
,forward_list
, andlist
.
ranges::to
's reserve
may be ill-formedSection: 25.5.7.2 [range.utility.conv.to] Status: C++23 Submitter: Hewill Kang Opened: 2022-07-21 Last modified: 2024-01-29
Priority: Not Prioritized
View other active issues in [range.utility.conv.to].
View all other issues in [range.utility.conv.to].
View all issues with C++23 status.
Discussion:
When the "reserve" branch is satisfied, ranges::to
directly passes the
result of ranges::size(r)
into the reserve
call. However, given
that the standard only guarantees that integer-class type can be explicitly converted
to any integer-like type (24.3.4.4 [iterator.concept.winc] p6), this makes
the call potentially ill-formed, since ranges::size(r)
may return an
integer-class type:
#include <ranges>
#include <vector>
int main() {
auto r = std::ranges::subrange(std::views::iota(0ULL) | std::views::take(5), 5);
auto v = r | std::ranges::to<std::vector<std::size_t>>(0); // cannot implicitly convert _Unsigned128 to size_t in MSVC-STL
}
We should do an explicit cast before calling reserve
.
[2022-08-23; Reflector poll]
Set status to Tentatively Ready after six votes in favour during reflector poll.
Are we all happy that the result of conversion to the container's size type may be less than the length of the source range, so the reservation is too small but we don't realize until pushing the max_size() + 1st element fails? I think it's acceptable that converting pathologically large ranges to containers fails kind of messily, but I could imagine throwing if the range length is greater than container's max_size().
[2022-11-12 Approved at November 2022 meeting in Kona. Status changed: Voting → WP.]
Proposed resolution:
This wording is relative to N4910.
Modify 25.5.7.2 [range.utility.conv.to] as indicated:
template<class C, input_range R, class... Args> requires (!view<C>) constexpr C to(R&& r, Args&&... args);-1- Returns: An object of type
C
constructed from the elements ofr
in the following manner:
(1.1) — If
convertible_to<range_reference_t<R>, range_value_t<C>>
istrue
:
(1.1.1) — If
constructible_from<C, R, Args...>
istrue
:C(std::forward<R>(r), std::forward<Args>(args)...)
(1.1.2) — Otherwise, if
constructible_from<C, from_range_t, R, Args...>
istrue
:C(from_range, std::forward<R>(r), std::forward<Args>(args)...)
(1.1.3) — Otherwise, if
(1.1.3.1) —
common_range<R>
istrue
,(1.1.3.2) —
cpp17-input-iterator<iterator_t<R>>
istrue
, and(1.1.3.3) —
constructible_from<C, iterator_t<R>, sentinel_t<R>, Args...>
istrue
:C(ranges::begin(r), ranges::end(r), std::forward<Args>(args)...)
(1.1.4) — Otherwise, if
(1.1.4.1) —
constructible_from<C, Args...>
istrue
, and(1.1.4.2) —
container-insertable<C, range_reference_t<R>>
istrue
:C c(std::forward<Args>(args)...); if constexpr (sized_range<R> && reservable-container<C>) c.reserve(static_cast<range_size_t<C>>(ranges::size(r))); ranges::copy(r, container-inserter<range_reference_t<R>>(c));(1.2) — Otherwise, if
input_range<range_reference_t<R>>
istrue
:to<C>(r | views::transform([](auto&& elem) { return to<range_value_t<C>>(std::forward<decltype(elem)>(elem)); }), std::forward<Args>(args)...);(1.3) — Otherwise, the program is ill-formed.
std::atomic_wait
and its friends lack noexcept
Section: 32.5.2 [atomics.syn] Status: C++23 Submitter: Jiang An Opened: 2022-07-25 Last modified: 2023-11-22
Priority: Not Prioritized
View other active issues in [atomics.syn].
View all other issues in [atomics.syn].
View all issues with C++23 status.
Discussion:
Currently function templates std::atomic_wait
, std::atomic_wait_explicit
,
std::atomic_notify_one
, and std::atomic_notify_all
are not noexcept
in the Working Draft, but the equivalent member functions are all noexcept
. I think
these function templates should be specified as noexcept
, in order to be consistent
with the std::atomic_flag_*
free functions, the corresponding member functions, and
other std::atomic_*
function templates.
noexcept
to them.
[2022-07-30; Daniel provides wording]
[2022-08-23; Reflector poll]
Set status to Tentatively Ready after six votes in favour during reflector poll.
"Technically there's a difference between these and the member functions - the pointer can be null - but we don't seem to have let that stop us from adding noexcept to the rest of these functions."
[2022-11-12 Approved at November 2022 meeting in Kona. Status changed: Voting → WP.]
Proposed resolution:
This wording is relative to N4910.
Modify 32.5.2 [atomics.syn], header <atomic>
synopsis, as indicated:
[…] template<class T> void atomic_wait(const volatile atomic<T>*, typename atomic<T>::value_type) noexcept; template<class T> void atomic_wait(const atomic<T>*, typename atomic<T>::value_type) noexcept; template<class T> void atomic_wait_explicit(const volatile atomic<T>*, typename atomic<T>::value_type, memory_order) noexcept; template<class T> void atomic_wait_explicit(const atomic<T>*, typename atomic<T>::value_type, memory_order) noexcept; template<class T> void atomic_notify_one(volatile atomic<T>*) noexcept; template<class T> void atomic_notify_one(atomic<T>*) noexcept; template<class T> void atomic_notify_all(volatile atomic<T>*) noexcept; template<class T> void atomic_notify_all(atomic<T>*) noexcept; […]
optional
's spaceship with U
with a type derived from optional
causes infinite constraint meta-recursionSection: 22.5.8 [optional.comp.with.t] Status: C++23 Submitter: Ville Voutilainen Opened: 2022-07-25 Last modified: 2023-11-22
Priority: Not Prioritized
View all other issues in [optional.comp.with.t].
View all issues with C++23 status.
Discussion:
What ends up happening is that the constraints of
operator<=>(const optional<T>&, const U&)
end up
in three_way_comparable_with
, and then in partially-ordered-with
,
and the expressions there end up performing a conversion from U
to an
optional
, and we end up instantiating the same operator<=>
again, evaluating its constraints again, until the compiler bails out.
U
so that U
is not publicly and unambiguously derived
from a specialization of optional, SFINAEing that candidate out, and letting
22.5.6 [optional.relops]/20 perform the comparison instead.
[2022-08-23; Reflector poll]
Set status to Tentatively Ready after five votes in favour during reflector poll.
[2022-11-12 Approved at November 2022 meeting in Kona. Status changed: Voting → WP.]
Proposed resolution:
This wording is relative to N4910.
[Drafting note: This proposed wording removes the only use of
is-optional
. ]
Modify 22.5.2 [optional.syn], header <optional>
synopsis, as indicated:
[…] namespace std { // 22.5.3 [optional.optional], class template optional template<class T> class optional;template<class T> constexpr bool is-optional = false; // exposition only template<class T> constexpr bool is-optional<optional<T>> = true; // exposition onlytemplate<class T> concept is-derived-from-optional = requires(const T& t) { // exposition only []<class U>(const optional<U>&){ }(t); }; […] // 22.5.8 [optional.comp.with.t], comparison with T […] template<class T, class U> constexpr bool operator>=(const T&, const optional<U>&); template<class T, class U> requires (!is-derived-from-optional<U>) && three_way_comparable_with<T, U> constexpr compare_three_way_result_t<T, U> operator<=>(const optional<T>&, const U&); […] }
Modify 22.5.8 [optional.comp.with.t] as indicated:
template<class T, class U> requires (!is-derived-from-optional<U>) && three_way_comparable_with<T, U> constexpr compare_three_way_result_t<T, U> operator<=>(const optional<T>&, const U&);-25- Effects: Equivalent to:
return x.has_value() ? *x <=> v : strong_ordering::less;
ranges::uninitialized_copy_n
, ranges::uninitialized_move_n
, and
ranges::destroy_n
should use std::move
Section: 26.11.5 [uninitialized.copy], 26.11.6 [uninitialized.move], 26.11.9 [specialized.destroy] Status: C++23 Submitter: Hewill Kang Opened: 2022-07-28 Last modified: 2023-11-22
Priority: Not Prioritized
View all other issues in [uninitialized.copy].
View all issues with C++23 status.
Discussion:
Currently, ranges::uninitialized_copy_n
has the following equivalent Effects:
auto t = uninitialized_copy(counted_iterator(ifirst, n), default_sentinel, ofirst, olast); return {std::move(t.in).base(), t.out};
Given that ifirst
is just an input_iterator
which is not guaranteed to be copyable
,
we should move it into counted_iterator
. The same goes for ranges::uninitialized_move_n
and ranges::destroy_n
.
[2022-08-23; Reflector poll]
Set status to Tentatively Ready after eight votes in favour during reflector poll.
[2022-11-12 Approved at November 2022 meeting in Kona. Status changed: Voting → WP.]
Proposed resolution:
This wording is relative to N4910.
Modify 26.11.5 [uninitialized.copy] as indicated:
namespace ranges { template<input_iterator I, nothrow-forward-iterator O, nothrow-sentinel-for<O> S> requires constructible_from<iter_value_t<O>, iter_reference_t<I>> uninitialized_copy_n_result<I, O> uninitialized_copy_n(I ifirst, iter_difference_t<I> n, O ofirst, S olast); }-9- Preconditions: [
ofirst, olast
) does not overlap withifirst +
[0, n
) .-10- Effects: Equivalent to:
auto t = uninitialized_copy(counted_iterator(std::move(ifirst), n), default_sentinel, ofirst, olast); return {std::move(t.in).base(), t.out};
Modify 26.11.6 [uninitialized.move] as indicated:
namespace ranges { template<input_iterator I, nothrow-forward-iterator O, nothrow-sentinel-for<O> S> requires constructible_from<iter_value_t<O>, iter_rvalue_reference_t<I>> uninitialized_move_n_result<I, O> uninitialized_move_n(I ifirst, iter_difference_t<I> n, O ofirst, S olast); }-8- Preconditions: [
ofirst, olast
) does not overlap withifirst +
[0, n
) .-9- Effects: Equivalent to:
auto t = uninitialized_move(counted_iterator(std::move(ifirst), n), default_sentinel, ofirst, olast); return {std::move(t.in).base(), t.out};
Modify 26.11.9 [specialized.destroy] as indicated:
namespace ranges { template<nothrow-input-iterator I> requires destructible<iter_value_t<I>> constexpr I destroy_n(I first, iter_difference_t<I> n) noexcept; }-5- Effects: Equivalent to:
return destroy(counted_iterator(std::move(first), n), default_sentinel).base();
common_iterator
should handle integer-class difference typesSection: 24.5.5 [iterators.common] Status: WP Submitter: Hewill Kang Opened: 2022-08-01 Last modified: 2023-11-22
Priority: 2
View all issues with WP status.
Discussion:
The partial specialization of iterator_traits
for common_iterator
is defined in
24.5.5.1 [common.iterator] as
template<input_iterator I, class S> struct iterator_traits<common_iterator<I, S>> { using iterator_concept = see below; using iterator_category = see below; using value_type = iter_value_t<I>; using difference_type = iter_difference_t<I>; using pointer = see below; using reference = iter_reference_t<I>; };
where difference_type
is defined as iter_difference_t<I>
and iterator_category
is defined as at least input_iterator_tag
. However, when difference_type
is an
integer-class type, common_iterator
does not satisfy Cpp17InputIterator, which makes
iterator_category
incorrectly defined as input_iterator_tag
.
Since the main purpose of common_iterator
is to be compatible with the legacy iterator system,
which is reflected in its efforts to try to provide the operations required by C++17 iterators even if
the underlying iterator does not support it. We should handle this case of difference type incompatibility
as well.
ptrdiff_t
.
Daniel:
The second part of this issue provides an alternative resolution for the first part of LWG 3748(i) and solves the casting problem mentioned in LWG 3748(i) as well.
[2022-08-23; Reflector poll]
Set priority to 2 after reflector poll.
"I think common_iterator
should reject iterators with
integer-class difference types since it can't possibly achieve the design intent
of adapting them to Cpp17Iterators."
"I'm not yet convinced that we need to outright reject such uses, but I'm pretty sure that we shouldn't mess with the difference type and that the PR is in the wrong direction."
Previous resolution [SUPERSEDED]:
This wording is relative to N4910.
Modify 24.5.5.1 [common.iterator] as indicated:
[Drafting note:
common_iterator
requires iterator typeI
must modelinput_or_output_iterator
which ensures thatiter_difference_t<I>
is a signed-integer-like type. The modification ofcommon_iterator::operator-
is to ensure that the pair ofcommon_iterator<I, S>
modelssized_sentinel_for
whensized_sentinel_for<I, S>
is modeled for iterator typeI
with an integer-class difference type and its sentinel typeS
.]namespace std { template<class D> requires is-signed-integer-like<D> using make-cpp17-diff-t = conditional_t<signed_integral<D>, D, ptrdiff_t>; // exposition only template<input_or_output_iterator I, sentinel_for<I> S> requires (!same_as<I, S> && copyable<I>) class common_iterator { public: […] template<sized_sentinel_for<I> I2, sized_sentinel_for<I> S2> requires sized_sentinel_for<S, I2> friend constexpr make-cpp17-diff-t<iter_difference_t<I2>> operator-( const common_iterator& x, const common_iterator<I2, S2>& y); […] }; template<class I, class S> struct incrementable_traits<common_iterator<I, S>> { using difference_type = make-cpp17-diff-t<iter_difference_t<I>>; }; template<input_iterator I, class S> struct iterator_traits<common_iterator<I, S>> { using iterator_concept = see below; using iterator_category = see below; using value_type = iter_value_t<I>; using difference_type = make-cpp17-diff-t<iter_difference_t<I>>; using pointer = see below; using reference = iter_reference_t<I>; }; }Modify 24.5.5.6 [common.iter.cmp] as indicated:
[Drafting note: If this issue is voted in at the same time as LWG 3748(i), the editor is kindly informed that the changes indicated below supersede those of the before mentioned issues first part.]
template<sized_sentinel_for<I> I2, sized_sentinel_for<I> S2> requires sized_sentinel_for<S, I2> friend constexpr make-cpp17-diff-t<iter_difference_t<I2>> operator-( const common_iterator& x, const common_iterator<I2, S2>& y);-5- Preconditions:
-6- Returns:x.v_.valueless_by_exception()
andy.v_.valueless_by_exception()
are eachfalse
.0
ifi
andj
are each1
, and otherwisestatic_cast<make-cpp17-diff-t<iter_difference_t<I2>>>(
get<i>(x.v_) - get<j>(y.v_)
)
, wherei
isx.v_.index()
andj
isy.v_.index()
.
[2023-06-13; Varna; Tomasz provides wording]
[2023-06-14 Varna; Move to Ready]
[2023-11-11 Approved at November 2023 meeting in Kona. Status changed: Voting → WP.]
Proposed resolution:
This wording is relative to N4950.
Modify 24.5.5.1 [common.iterator] as indicated:
namespace std { […] template<input_iterator I, class S> struct iterator_traits<common_iterator<I, S>> { using iterator_concept = see below; using iterator_category = see below; // not always present using value_type = iter_value_t<I>; using difference_type = iter_difference_t<I>; using pointer = see below; using reference = iter_reference_t<I>; }; }
Modify 24.5.5.2 [common.iter.types] as indicated:
-?- The nested typedef-name
iterator_category
of the specialization ofiterator_traits
forcommon_iterator<I, S>
is defined if and only ifiter_difference_t<I>
is an integral type. In that case,iterator_category
denotesforward_iterator_tag
if the qualified-iditerator_traits<I>::iterator_category
is valid and denotes a type that modelsderived_from<forward_iterator_tag>
; otherwise it denotesinput_iterator_tag
.-1- The remaining nested typedef-names of the specialization of
iterator_traits
forcommon_iterator<I, S>
are defined as follows.:
(1.1) —
iterator_concept
denotesforward_iterator_tag
ifI
modelsforward_iterator
; otherwise it denotesinput_iterator_tag
.
(1.2) —iterator_category
denotesforward_iterator_tag
if the qualified-iditerator_traits<I>::iterator_category
is valid and denotes a type that modelsderived_from<forward_iterator_tag>
; otherwise it denotesinput_iterator_tag
.(1.3) — Let
a
denote an lvalue of typeconst common_iterator<I, S>
. If the expressiona.operator->()
is well-formed, thenpointer
denotesdecltype(a.operator->())
. Otherwise,pointer
denotesvoid
.
__cpp_lib_format
Section: 17.3.2 [version.syn] Status: C++23 Submitter: Barry Revzin Opened: 2022-08-04 Last modified: 2023-11-22
Priority: Not Prioritized
View all other issues in [version.syn].
View all issues with C++23 status.
Discussion:
As pointed out by Casey Carter, four papers approved at the recent July 2022 plenary:
P2419R2 "Clarify handling of encodings in localized formatting of chrono types"
P2508R1 "Expose std::basic-format-string<charT, Args...>
"
P2286R8 "Formatting Ranges"
P2585R1 "Improve container default formatting"
all bump the value of __cpp_lib_format
. We never accounted for all of these papers
being moved at the same time, and these papers have fairly different implementation complexities.
__cpp_lib_format_ranges
(with value 202207L
) for the two formatting
ranges papers (P2286 and P2585, which should probably be implemented
concurrently anyway, since the latter modifies the former) and bump __cpp_lib_format
for
the other two, which are both minor changes.
We should do that.
[2022-08-23; Reflector poll]
Set status to Tentatively Ready after 12 votes in favour during reflector poll.
[2022-11-12 Approved at November 2022 meeting in Kona. Status changed: Voting → WP.]
Proposed resolution:
Modify 17.3.2 [version.syn] as indicated:
#define __cpp_lib_format 202207L // also in <format> #define __cpp_lib_format_ranges 202207L // also in <format>
flat_set
Section: 17.3.2 [version.syn] Status: C++23 Submitter: Barry Revzin Opened: 2022-08-04 Last modified: 2023-11-22
Priority: Not Prioritized
View all other issues in [version.syn].
View all issues with C++23 status.
Discussion:
As pointed out by Casey Carter,
while there is a feature macro for flat_map
in P0429, there is
no corresponding macro for flat_set in P1222. We should add one.
[2022-08-23; Reflector poll]
Set status to Tentatively Ready after 10 votes in favour during reflector poll.
[2022-11-12 Approved at November 2022 meeting in Kona. Status changed: Voting → WP.]
Proposed resolution:
Modify 17.3.2 [version.syn] as indicated:
#define __cpp_lib_flat_map 202207L // also in <flat_map> #define __cpp_lib_flat_set 202207L // also in <flat_set>
Section: 16.3.3.7 [freestanding.item] Status: C++23 Submitter: Ben Craig Opened: 2022-08-23 Last modified: 2023-11-22
Priority: 2
View all other issues in [freestanding.item].
View all issues with C++23 status.
Discussion:
This addresses NB comment GB-075 ( [freeestanding.entity] "Freestanding entities" are not entities)
[freestanding.entity] p1 defines a freestanding entity as a declaration or macro definition.
[freestanding.entity] p3 then says "entities followed with a comment […] are freestanding entities".
This is inconsistent, and breaks with macros, because macros are not entities, but they can be freestanding entities.
[2022-09-23; Reflector poll]
Set priority to 2 after reflector poll.
It's confusing for "freestanding entities" to be two things, neither of which are entities. Declarations may declare entities, they are not entities themselves. Given this definition, p6/7/8 makes no sense. A namespace can't be a freestanding entity since it's neither a declaration nor a macro definition.
"freestanding entities" is not best name, given the collision with core entity, but I think that this is separable issue.
[2022-09-28; Reflector poll]
Set status to Tentatively Ready after five votes in favour during reflector poll.
Previous resolution [SUPERSEDED]:
This wording is relative to the forthcoming C++23 CD.
Modify [freestanding.entity] as indicated:
-3- In a header synopsis,entitiesdeclarations and macro definitions followed with a comment that includes freestanding are freestanding entities.
[2022-11-06; Ben Craig provides new wording]
[2022-11-07; Kona - move to open]
New proposed resolution to be added.
Previous resolution [SUPERSEDED]:
This wording is relative to N4917.
Modify [freestanding.entity] as indicated:
[Drafting note: Replace the section name [freestanding.entity] by [freestanding.item] throughout the working draft]
16.3.3.6 Freestanding
-1- A freestandingentitiesitems [freestanding.itementity]entityitem isa declarationan entity or macro definition that is present in a freestanding implementation and a hosted implementation. -2- Unless otherwise specified, the requirements on freestandingentitiesitems on a freestanding implementation are the same as the corresponding requirements in a hosted implementation. -3- In a header synopsis, entities introduced by declarations followed with a comment that includes freestanding are freestandingentitiesitems. -?- In a header synopsis, macro definitions followed with a comment that includes freestanding are freestanding items. […] -4- If a header synopsis begins with a comment that includes all freestanding, then all of the entities introduced by declarationsand macro definitionsin the header synopsis are freestandingentitiesitems. -?- If a header synopsis begins with a comment that includes all freestanding, then all of the macro definitions in the header synopsis are freestanding items. […] -5- Deduction guides for freestandingentityitem class templates are freestandingentitiesitems. -6- Enclosing namespaces of freestandingentitiesitems are freestandingentitiesitems. -7- Friends of freestandingentitiesitems are freestandingentitiesitems. -8- Entities denoted by freestandingentityitem typedef-names and freestandingentityitem alias templates are freestandingentitiesitems.Modify 16.4.2.5 [compliance] as indicated:
-3- For each of the headers listed in Table 28, a freestanding implementation provides at least the freestanding
entitiesitems ( [freestanding.entity]) declared in the header.Modify 22.10.15.5 [func.bind.place] as indicated:
-3- Placeholders are freestanding
entitiesitems ( [freestanding.entity]).
[2022-11-08; Ben Craig provides improved wording]
This combined resolution addresses both 3753 and LWG 3815(i), and has already been reviewed by LWG.
This resolves ballot comment GB-75. It also partially addresses GB-130 (along with LWG 3814(i)).
Previous resolution [SUPERSEDED]:
This wording is relative to N4917.
Modify 16.3.3.6 [freestanding.entity] as indicated:
[Drafting note: Replace the section name [freestanding.entity] by [freestanding.item] throughout the working draft]
16.3.3.6 Freestanding
-1- A freestandingentitiesitems [freestanding.itementity]entityitem isa declarationan entity or macro definition that is present in a freestanding implementation and a hosted implementation. -2- Unless otherwise specified, the requirements on non-namespace freestandingentitiesitems on a freestanding implementation are the same as the corresponding requirements in a hosted implementation. [Note: Enumerators impose requirements on their enumerations. Freestanding item enumerations have the same enumerators on freestanding implementations and hosted implementations. Members and deduction guides impose requirements on their class types. Class types have the same deduction guides and members on freestanding implementations and hosted implementations. — end note] -3- In a header synopsis,entitieseach entity introduced by a declaration followedwithby a comment that includes freestandingareis a freestandingentitiesitem. -?- In a header synopsis, each macro definition followed by a comment that includes freestanding is a freestanding item. […] -4- If a header synopsis begins with a comment that includes all freestanding, thenall of the declarations and macro definitionseach entity introduced by a declaration in the header synopsisareis a freestandingentitiesitem. -?- If a header synopsis begins with a comment that includes all freestanding, then each macro definition in the header synopsis is a freestanding item. […]-5- Deduction guides for freestanding entity class templates are freestanding entities.-6-Enclosing namespaces of freestanding entities are freestanding entities.Each enclosing namespace of each freestanding item is a freestanding item. -7-Friends of freestanding entities are freestanding entities.Each friend of each freestanding item is a freestanding item. -8-Entities denoted by freestanding entity typedef-names and freestanding entity alias templates are freestanding entities.Each entity denoted by each freestanding item typedef-name and each freestanding item alias template is a freestanding item.Modify 16.4.2.5 [compliance] as indicated:
-3- For each of the headers listed in Table 28, a freestanding implementation provides at least the freestanding
entitiesitems (16.3.3.6 [freestanding.entityitem]) declared in the header.Modify 22.10.15.5 [func.bind.place] as indicated:
-3- Placeholders are freestanding
entitiesitems (16.3.3.6 [freestanding.entityitem]).
[2022-11-09; Ben Craig and Tomasz provide improved wording]
This new resolution merges definition of freestanding item for entity in macro into bullet lists. It still addresses both 3753 and LWG 3815(i).
This resolves ballot comment GB-75. It also partially addresses GB-130 (along with LWG 3814(i)).
Previous resolution [SUPERSEDED]:
This wording is relative to N4917.
Modify [freestanding.entity] as indicated:
[Drafting note: Replace the section name [freestanding.entity] by [freestanding.item] throughout the working draft]
16.3.3.6 Freestanding
-1- A freestandingentitiesitems [freestanding.itementity]entityitem isa declarationan entity or macro definition that is present in a freestanding implementation and a hosted implementation. -2- Unless otherwise specified, the requirements on freestandingentitiesitems, except namespaces,onfor a freestanding implementation are the same as the corresponding requirementsinfor a hosted implementation. [Note: This implies that freestanding item enumerations have the same enumerators on freestanding implementations and hosted implementations. Furthermore, class types have the same deduction guides and members on freestanding implementations and hosted implementations. — end note] -3- An entity is a freestanding item, if it is:-4- A macro definition is a freestanding item, if it is defined in the header synopsis and
(3.1) — introduced by a declaration in the header synopsis, and
(3.1.1) — the declaration is followed by a comment that includes freestanding, or
(3.1.2) — the header synopsis begins with a comment that includes all freestanding;
(3.2) — an enclosing namespace of a freestanding item,
(3.3) — a friend of a freestanding item,
(3.4) — denoted by a typedef-name, that is a freestanding item, or
(3.5) — denoted by a template alias, that is a freestanding item.
(4.1) — the definition is followed by a comment that includes freestanding in the header synopsis, or
(4.2) — the header synopsis begins with a comment that includes all freestanding.
-3- In a header synopsis, entities followed with a comment that includes freestanding are freestanding entities.[Example 1: … — end example]-4- If a header synopsis begins with a comment that includes all freestanding, then all of the declarations and macro definitions in the header synopsis are freestanding entities..[Example 2: … — end example]-5- Deduction guides for freestanding entity class templates are freestanding entities.-6- Enclosing namespaces of freestanding entities are freestanding entities.-7- Friends of freestanding entities are freestanding entities.-8- Entities denoted by freestanding entity typedef-names and freestanding entity alias templates are freestanding entities.Modify 16.4.2.5 [compliance] as indicated:
-3- For each of the headers listed in Table 28, a freestanding implementation provides at least the freestanding
entitiesitems (16.3.3.6 [freestanding.entityitem]) declared in the header.Modify 22.10.15.5 [func.bind.place] as indicated:
-3- Placeholders are freestanding
entitiesitems (16.3.3.6 [freestanding.entityitem]).
[2022-11-10; Tomasz provide improved wording]
Updated wording to support freestanding typedef-names and using declaration that are not entities. It still addresses both 3753 and LWG 3815(i).
This resolves ballot comment GB-75. It also partially addresses GB-130 (along with LWG 3814(i)).
[Kona 2022-11-10; Move to Immediate]
[2022-11-12 Approved at November 2022 meeting in Kona. Status changed: Immediate → WP.]
Proposed resolution:
This wording is relative to N4917.
Modify [freestanding.entity] as indicated:
[Drafting note: Replace the section name [freestanding.entity] by [freestanding.item] throughout the working draft]
16.3.3.6 Freestanding
-1- A freestandingentitiesitems [freestanding.itementity]entityitem is a declaration, entity, typedef-name, or macrodefinitionthat is required to be present in a freestanding implementation and a hosted implementation. -2- Unless otherwise specified, the requirements on freestandingentitiesitemsonfor a freestanding implementation are the same as the corresponding requirementsinfor a hosted implementation, except that not all of the members of the namespaces are required to be present. [Note: This implies that freestanding item enumerations have the same enumerators on freestanding implementations and hosted implementations. Furthermore, class types have the same members and class templates have the same deduction guides on freestanding implementations and hosted implementations. — end note] -3- A declaration in a header synopsis is a freestanding item if-4- An entity or typedef-name is a freestanding item if it is:
(3.1) — it is followed by a comment that includes freestanding, or
(3.1) — the header synopsis begins with a comment that includes all freestanding.
-5- A macro is a freestanding item if it is defined in a header synopsis and
(4.1) — introduced by a declaration that is a freestanding item,
(4.2) — an enclosing namespace of a freestanding item,
(4.3) — a friend of a freestanding item,
(4.4) — denoted by a typedef-name that is a freestanding item, or
(4.5) — denoted by an alias template that is a freestanding item.
(5.1) — the definition is followed by a comment that includes freestanding, or
(5.2) — the header synopsis begins with a comment that includes all freestanding.
-3- In a header synopsis, entities followed with a comment that includes freestanding are freestanding entities.[Example 1: … — end example]-4- If a header synopsis begins with a comment that includes all freestanding, then all of the declarations and macro definitions in the header synopsis are freestanding entities..[Example 2: … — end example]-5- Deduction guides for freestanding entity class templates are freestanding entities.-6- Enclosing namespaces of freestanding entities are freestanding entities.-7- Friends of freestanding entities are freestanding entities.-8- Entities denoted by freestanding entity typedef-names and freestanding entity alias templates are freestanding entities.
Modify 16.4.2.5 [compliance] as indicated:
-3- For each of the headers listed in Table 28, a freestanding implementation provides at least the freestanding
entitiesitems (16.3.3.6 [freestanding.entityitem]) declared in the header.
Modify 22.10.15.5 [func.bind.place] as indicated:
-3- Placeholders are freestanding
entitiesitems (16.3.3.6 [freestanding.entityitem]).
expected
synopsis contains declarations that do not match the detailed descriptionSection: 22.8.6.1 [expected.object.general] Status: C++23 Submitter: S. B. Tam Opened: 2022-08-23 Last modified: 2024-01-29
Priority: Not Prioritized
View all other issues in [expected.object.general].
View all issues with C++23 status.
Discussion:
22.8.6.1 [expected.object.general] declares the following constructors:
template<class G> constexpr expected(const unexpected<G>&); template<class G> constexpr expected(unexpected<G>&&);
But in [expected.object.ctor], these constructors are declared as:
template<class G> constexpr explicit(!is_convertible_v<const G&, E>) expected(const unexpected<G>& e); template<class G> constexpr explicit(!is_convertible_v<G, E>) expected(unexpected<G>&& e);
Note that they have no explicit-specifiers in 22.8.6.1 [expected.object.general], but are conditionally explicit in [expected.object.ctor].
I presume that 22.8.6.1 [expected.object.general]
is missing a few explicit(see below)
.
The same inconsistency exists in 22.8.7 [expected.void].
[2022-09-05; Jonathan Wakely provides wording]
In N4910 the expected
synopses had
explicit(see below)
on the copy and move constructors.
That was fixed editorially, but this other inconsistency was not noticed.
[2022-09-07; Moved to "Ready" at LWG telecon]
[2022-11-12 Approved at November 2022 meeting in Kona. Status changed: Voting → WP.]
Proposed resolution:
This wording is relative to N4917.
Change 22.8.6.1 [expected.object.general] as indicated:
// 22.8.6.2, constructors constexpr expected(); constexpr expected(const expected&); constexpr expected(expected&&) noexcept(see below); template<class U, class G> constexpr explicit(see below) expected(const expected<U, G>&); template<class U, class G> constexpr explicit(see below) expected(expected<U, G>&&); template<class U = T> constexpr explicit(see below) expected(U&& v); template<class G> constexpr explicit(see below) expected(const unexpected<G>&); template<class G> constexpr explicit(see below) expected(unexpected<G>&&); template<class... Args> constexpr explicit expected(in_place_t, Args&&...);
Change 22.8.7.1 [expected.void.general] as indicated:
// 22.8.7.2, constructors constexpr expected() noexcept; constexpr expected(const expected&); constexpr expected(expected&&) noexcept(see below); template<class U, class G> constexpr explicit(see below) expected(const expected<U, G>&&); template<class G> constexpr explicit(see below) expected(const unexpected<G>&); template<class G> constexpr explicit(see below) expected(unexpected<G>&&); constexpr explicit expected(in_place_t) noexcept;
tuple-for-each
can call user-defined operator,
Section: 25.7.25.2 [range.zip.view] Status: C++23 Submitter: Nicole Mazzuca Opened: 2022-08-26 Last modified: 2023-11-22
Priority: Not Prioritized
View all other issues in [range.zip.view].
View all issues with C++23 status.
Discussion:
The specification for tuple-for-each
is:
template<class F, class Tuple> constexpr auto tuple-for-each(F&& f, Tuple&& t) { // exposition only apply([&]<class... Ts>(Ts&&... elements) { (invoke(f, std::forward<Ts>(elements)), ...); }, std::forward<Tuple>(t)); }
Given
struct Evil { void operator,(Evil) { abort(); } };
and tuple<int, int> t
, then
tuple-for-each([](int) { return Evil{}; }, t)
,
the program will (unintentionally) abort.
It seems likely that our Evil
's operator,
should not be called.
[2022-09-23; Reflector poll]
Set status to Tentatively Ready after nine votes in favour during reflector poll.
Feedback from reviewers:
"NAD. This exposition-only facility is only used with things that return
void
. As far as I know, users can't defineoperator,
forvoid
." "If I see thevoid
cast, I don't need to audit the uses or be concerned that we'll add a broken use in the future."
[2022-11-12 Approved at November 2022 meeting in Kona. Status changed: Voting → WP.]
Proposed resolution:
This wording is relative to the forthcoming C++23 CD.
Modify [range.adaptor.tuple] as indicated:
template<class F, class Tuple> constexpr auto tuple-for-each(F&& f, Tuple&& t) { // exposition only apply([&]<class... Ts>(Ts&&... elements) { (static_cast<void>(invoke(f, std::forward<Ts>(elements))), ...); }, std::forward<Tuple>(t)); }
std::atomic_flag
class signal-safe?Section: 17.14.5 [support.signal], 32.5.10 [atomics.flag] Status: C++23 Submitter: Ruslan Baratov Opened: 2022-08-18 Last modified: 2023-11-22
Priority: 3
View all issues with C++23 status.
Discussion:
Following document number N4910 about signal-safe instructions
17.14.5 [support.signal] Signal handlers, and it's unclear whether
std::atomic_flag
is signal-safe.
f
is a non-static member function invoked on an object A
, such that
A.is_lock_free()
yields true
, or
(there is no
is_lock_free
method instd::atomic_flag
class)
f
is a non-member function, and for every pointer-to-atomic argument A
passed to f
, atomic_is_lock_free(A)
yields true
(
std::atomic_flag
object can't be passed toatomic_is_lock_free
as argument)
However, std::atomic_flag
seem to fit well here, it's atomic, and it's
always lock-free.
std::atomic_flag
is signal-safe, then it
should be explicitly mentioned in 17.14.5 [support.signal], e.g., the following lines
should be added:
f
is a non-static member function invoked on anatomic_flag
object, or
f
is a non-member function, and every pointer-to-atomic argument passed tof
isatomic_flag
, or
If the std::atomic_flag
is not signal-safe, the following note could be added:
[Note: Even though
atomic_flag
is atomic and lock-free, it's not signal-safe. — end note]
[2022-09-23; Reflector poll]
Set priority to 3 after reflector poll. Send to SG1.
Another way to fix this is to add is_always_lock_free (=true)
and
is_lock_free() { return true; }
to atomic_flag
.
[Kona 2022-11-10; SG1 yields a recommendation]
Poll: Adopt the proposed resolution for LWG3756
"f is a non-static member function invoked on an
atomic_flag object, or"
"f is a non-member function, and every pointer-to-
atomic argument passed to f is atomic_flag, or"
SF F N A SA 11 3 0 0 0
Unanimous consent
Previous resolution [SUPERSEDED]:
This wording is relative to N4917.
Modify 17.14.5 [support.signal] as indicated:
-1- A call to the function
signal
synchronizes with any resulting invocation of the signal handler so installed.-2- A plain lock-free atomic operation is an invocation of a function
f
from 32.5 [atomics], such that:
- (2.1) —
f
is the functionatomic_is_lock_free()
, or- (2.2) —
f
is the member functionis_lock_free()
, or- (2.?) —
f
is a non-static member function invoked on anatomic_flag
object, or- (2.?) —
f
is a non-member function, and every pointer-to-atomic argument passed tof
isatomic_flag
, or- (2.3) —
f
is a non-static member function invoked on an objectA
, such thatA.is_lock_free()
yieldstrue
, or- (2.4) —
f
is a non-member function, and for every pointer-to-atomic argumentA
passed tof
,atomic_is_lock_free(A)
yieldstrue
.-3- An evaluation is signal-safe unless it includes one of the following:
- (3.1) — a call to any standard library function, except for plain lock-free atomic operations and functions explicitly identified as signal-safe;
[Note 1: This implicitly excludes the use of
new
anddelete
expressions that rely on a library-provided memory allocator. — end note]- (3.2) — an access to an object with thread storage duration;
- (3.3) — a
dynamic_cast
expression;- (3.4) — throwing of an exception;
- (3.5) — control entering a try-block or function-try-block;
- (3.6) — initialization of a variable with static storage duration requiring dynamic initialization (6.9.3.3 [basic.start.dynamic], 8.9 [stmt.dcl])206; or
- (3.7) — waiting for the completion of the initialization of a variable with static storage duration (8.9 [stmt.dcl]).
A signal handler invocation has undefined behavior if it includes an evaluation that is not signal-safe.
[2022-11-11; Jonathan provides improved wording]
[Kona 2022-11-11; Move to Ready]
[2023-02-13 Approved at February 2023 meeting in Issaquah. Status changed: Voting → WP.]
Proposed resolution:
This wording is relative to N4917.
Modify 17.14.5 [support.signal] as indicated:
-1- A call to the function
signal
synchronizes with any resulting invocation of the signal handler so installed.-2- A plain lock-free atomic operation is an invocation of a function
f
from 32.5 [atomics], such that:
- (2.1) —
f
is the functionatomic_is_lock_free()
, or- (2.2) —
f
is the member functionis_lock_free()
, or- (2.?) —
f
is a non-static member function of classatomic_flag
, or- (2.?) —
f
is a non-member function, and the first parameter off
has type cvatomic_flag*
, or- (2.3) —
f
is a non-static member function invoked on an objectA
, such thatA.is_lock_free()
yieldstrue
, or- (2.4) —
f
is a non-member function, and for every pointer-to-atomic argumentA
passed tof
,atomic_is_lock_free(A)
yieldstrue
.-3- An evaluation is signal-safe unless it includes one of the following:
- (3.1) — a call to any standard library function, except for plain lock-free atomic operations and functions explicitly identified as signal-safe;
[Note 1: This implicitly excludes the use of
new
anddelete
expressions that rely on a library-provided memory allocator. — end note]- (3.2) — an access to an object with thread storage duration;
- (3.3) — a
dynamic_cast
expression;- (3.4) — throwing of an exception;
- (3.5) — control entering a try-block or function-try-block;
- (3.6) — initialization of a variable with static storage duration requiring dynamic initialization (6.9.3.3 [basic.start.dynamic], 8.9 [stmt.dcl])206; or
- (3.7) — waiting for the completion of the initialization of a variable with static storage duration (8.9 [stmt.dcl]).
A signal handler invocation has undefined behavior if it includes an evaluation that is not signal-safe.
std::forward_like<void>(x)
?Section: 22.2.4 [forward] Status: C++23 Submitter: Jiang An Opened: 2022-08-24 Last modified: 2023-11-22
Priority: Not Prioritized
View all other issues in [forward].
View all issues with C++23 status.
Discussion:
Currently the return type of std::forward_like
is specified by the following bullet:
— Let
V
beOVERRIDE_REF(T&&, COPY_CONST(remove_reference_t<T>, remove_reference_t<U>))
where T&&
is not always valid, e.g. it's invalid when T
is void
.
T
is not referenceable
(which is currently implemented in MSVC STL), but this seems not clarified. It is unclear to me whether
the intent is that hard error, substitution failure, or no error is caused when T&&
is invalid.
[2022-09-23; Reflector poll]
Set status to Tentatively Ready after seven votes in favour during reflector poll.
[2022-11-12 Approved at November 2022 meeting in Kona. Status changed: Voting → WP.]
Proposed resolution:
This wording is relative to N4917.
Modify 22.2.4 [forward] as indicated:
template<class T, class U> [[nodiscard]] constexpr auto forward_like(U&& x) noexcept -> see below;Mandates:
[…]T
is a referenceable type (3.45 [defns.referenceable]).
ranges::rotate_copy
should use std::move
Section: 26.7.11 [alg.rotate] Status: C++23 Submitter: Hewill Kang Opened: 2022-08-25 Last modified: 2023-11-22
Priority: Not Prioritized
View all other issues in [alg.rotate].
View all issues with C++23 status.
Discussion:
The range version of ranges::rotate_copy
directly passes the result
to the
iterator-pair version. Since the type of result
only models weakly_incrementable
and may not be copied, we should use std::move
here.
[2022-09-23; Reflector poll]
Set status to Tentatively Ready after seven votes in favour during reflector poll.
[2022-11-12 Approved at November 2022 meeting in Kona. Status changed: Voting → WP.]
Proposed resolution:
This wording is relative to N4910.
Modify 26.7.11 [alg.rotate] as indicated:
template<forward_range R, weakly_incrementable O> requires indirectly_copyable<iterator_t<R>, O> constexpr ranges::rotate_copy_result<borrowed_iterator_t<R>, O> ranges::rotate_copy(R&& r, iterator_t<R> middle, O result);-11- Effects: Equivalent to:
return ranges::rotate_copy(ranges::begin(r), middle, ranges::end(r), std::move(result));
cartesian_product_view::iterator
's parent_
is never validSection: 25.7.33 [range.cartesian] Status: C++23 Submitter: Hewill Kang Opened: 2022-08-27 Last modified: 2023-11-22
Priority: Not Prioritized
View all issues with C++23 status.
Discussion:
cartesian_product_view::iterator
has a pointer member parent_
that points
to cartesian_product_view
, but its constructor only accepts a tuple
of iterators, which
makes parent_
always default-initialized to nullptr
.
Parent
parameter to the constructor and
initialize parent_
with addressof
, as we usually do.
[2022-09-23; Reflector poll]
Set status to Tentatively Ready after eight votes in favour during reflector poll.
[2022-11-12 Approved at November 2022 meeting in Kona. Status changed: Voting → WP.]
Proposed resolution:
This wording is relative to N4917.
Modify 25.7.33.2 [range.cartesian.view] as indicated:
constexpr iterator<false> begin() requires (!simple-view<First> || ... || !simple-view<Vs>);-2- Effects: Equivalent to:
return iterator<false>(*this, tuple-transform(ranges::begin, bases_));
constexpr iterator<true> begin() const requires (range<const First> && ... && range<const Vs>);-3- Effects: Equivalent to:
return iterator<true>(*this, tuple-transform(ranges::begin, bases_));
constexpr iterator<false> end() requires ((!simple-view<First> || ... || !simple-view<Vs>) && cartesian-product-is-common<First, Vs...>); constexpr iterator<true> end() const requires cartesian-product-is-common<const First, const Vs...>;-4- Let:
(4.1) —
is-const
betrue
for the const-qualified overload, andfalse
otherwise;(4.2) —
is-empty
betrue
if the expressionranges::empty(rng)
istrue
for anyrng
among the underlying ranges except the first one andfalse
otherwise; and(4.3) —
begin-or-first-end(rng)
be expression-equivalent tois-empty ? ranges::begin(rng) : cartesian-common-arg-end(rng)
ifrng
is the first underlying range andranges::begin(rng)
otherwise.-5- Effects: Equivalent to:
iterator<is-const> it(*this, tuple-transform( [](auto& rng){ return begin-or-first-end(rng); }, bases_)); return it;
Modify [ranges.cartesian.iterator] as indicated:
namespace std::ranges { template<input_range First, forward_range... Vs> requires (view<First> && ... && view<Vs>) template<bool Const> class cartesian_product_view<First, Vs...>::iterator { public: […] private: using Parent = maybe-const<Const, cartesian_product_view>; // exposition only Parentmaybe-const<Const, cartesian_product_view>* parent_ = nullptr; // exposition only tuple<iterator_t<maybe-const<Const, First>>, iterator_t<maybe-const<Const, Vs>>...> current_; // exposition only template<size_t N = sizeof...(Vs)> constexpr void next(); // exposition only template<size_t N = sizeof...(Vs)> constexpr void prev(); // exposition only template<class Tuple> constexpr difference_type distance-from(Tuple t); // exposition only constexprexplicititerator(Parent& parent, tuple<iterator_t<maybe-const<Const, First>>, iterator_t<maybe-const<Const, Vs>>...> current); // exposition only }; }[…]
constexprexplicititerator(Parent& parent, tuple<iterator_t<maybe-const<Const, First>>, iterator_t<maybe-const<Const, Vs>>...> current);-10- Effects: Initializes
parent_
withaddressof(parent)
andcurrent_
withstd::move(current)
.constexpr iterator(iterator<!Const> i) requires Const && (convertible_to<iterator_t<First>, iterator_t<const First>> && ... && convertible_to<iterator_t<Vs>, iterator_t<const Vs>>);-11- Effects: Initializes
parent_
withi.parent_
andcurrent_
withstd::move(i.current_)
.
cartesian_product_view::iterator::operator-
should pass by referenceSection: 25.7.33.3 [range.cartesian.iterator] Status: C++23 Submitter: Hewill Kang Opened: 2022-08-27 Last modified: 2023-11-22
Priority: Not Prioritized
View all other issues in [range.cartesian.iterator].
View all issues with C++23 status.
Discussion:
There are two problems with cartesian_product_view::iterator::operator-
.
distance-from
is not const
-qualified, which would cause
the common version of operator-
to produce a hard error inside the function body
since it invokes distance-from
on a const iterator
object.
Second, the non-common version of operator-
passes iterator
by value,
which unnecessarily prohibits subtracting default_sentinel
from an lvalue
iterator
whose first underlying iterator is non-copyable
.
Even if we std::move
it into operator-
, the other overload will still be ill-formed
since it returns -(i - s)
which will calls i
's deleted copy constructor.
The proposed resolution is to add const
qualification to distance-from
and make the non-common version of iterator::operator-
pass by const reference.
Although the Tuple
parameter of distance-from
is guaranteed to be
copyable
, I think it would be more appropriate to pass it by reference.
[2022-09-08]
As suggested by Tim Song, the originally submitted proposed wording has been refined to use
const Tuple&
instead of Tuple&
.
[2022-09-23; Reflector poll]
Set status to Tentatively Ready after six votes in favour during reflector poll.
[2022-11-12 Approved at November 2022 meeting in Kona. Status changed: Voting → WP.]
Proposed resolution:
This wording is relative to N4917.
Modify [ranges.cartesian.iterator] as indicated:
namespace std::ranges { template<input_range First, forward_range... Vs> requires (view<First> && ... && view<Vs>) template<bool Const> class cartesian_product_view<First, Vs...>::iterator { public: […] friend constexpr difference_type operator-(const iterator& i, default_sentinel_t) requires cartesian-is-sized-sentinel<Const, sentinel_t, First, Vs...>; friend constexpr difference_type operator-(default_sentinel_t, const iterator& i) requires cartesian-is-sized-sentinel<Const, sentinel_t, First, Vs...>; […] private: […] template<class Tuple> constexpr difference_type distance-from(const Tuple& t) const; // exposition only […] }; }[…]
template<class Tuple> constexpr difference_type distance-from(const Tuple& t) const;-7- Let:
(7.1) —
scaled-size(N)
be the product ofstatic_cast<difference_type>(ranges::size(std::get<N>(parent_->bases_)))
andscaled-size(N + 1)
ifN < sizeof...(Vs)
, otherwisestatic_cast<difference_type>(1)
;(7.2) —
scaled-distance(N)
be the product ofstatic_cast<difference_type>(std::get<N>(current_) - std::get<N>(t))
andscaled-size(N + 1)
; and(7.3) —
scaled-sum
be the sum ofscaled-distance(N)
for every integer0 ≤ N ≤ sizeof...(Vs)
.-8- Preconditions:
scaled-sum
can be represented bydifference_type
.-9- Returns:
scaled-sum
.[…]
friend constexpr difference_type operator-(const iterator& i, default_sentinel_t) requires cartesian-is-sized-sentinel<Const, sentinel_t, First, Vs...>;-32- Let
end-tuple
be an object of a type that is a specialization oftuple
, such that:
(32.1) —
std::get<0>(end-tuple)
has the same value asranges::end(std::get<0>(i.parent_->bases_))
;(32.2) —
std::get<N>(end-tuple)
has the same value asranges::begin(std::get<N>(i.parent_->bases_))
for every integer1 ≤ N ≤ sizeof...(Vs)
.-33- Effects: Equivalent to:
return i.distance-from(end-tuple);
friend constexpr difference_type operator-(default_sentinel_t, const iterator& i) requires cartesian-is-sized-sentinel<Const, sentinel_t, First, Vs...>;-34- Effects: Equivalent to:
return -(i - s);
generator::iterator::operator==
should pass by referenceSection: 25.8.6 [coro.generator.iterator] Status: C++23 Submitter: Hewill Kang Opened: 2022-08-27 Last modified: 2023-11-22
Priority: Not Prioritized
View other active issues in [coro.generator.iterator].
View all other issues in [coro.generator.iterator].
View all issues with C++23 status.
Discussion:
Currently, generator::iterator::operator==
passes iterator
by value.
Given that iterator
is a move-only type, this makes it impossible for
generator
to model range
, since default_sentinel
cannot be compared
to the iterator
of const
lvalue and is not eligible to be its sentinel.
[2022-09-23; Reflector poll]
Set status to Tentatively Ready after six votes in favour during reflector poll.
[2022-11-12 Approved at November 2022 meeting in Kona. Status changed: Voting → WP.]
Proposed resolution:
This wording is relative to N4917.
Modify 25.8.6 [coro.generator.iterator] as indicated:
namespace std { template<class Ref, class V, class Allocator> class generator<Ref, V, Allocator>::iterator { public: using value_type = value; using difference_type = ptrdiff_t; iterator(iterator&& other) noexcept; iterator& operator=(iterator&& other) noexcept; reference operator*() const noexcept(is_nothrow_copy_constructible_v<reference>); iterator& operator++(); void operator++(int); friend bool operator==(const iterator& i, default_sentinel_t); private: coroutine_handle<promise_type> coroutine_; // exposition only }; }[…]
friend bool operator==(const iterator& i, default_sentinel_t);-10- Effects: Equivalent to:
return i.coroutine_.done();
reference_wrapper::operator()
should propagate noexcept
Section: 22.10.6.5 [refwrap.invoke] Status: C++23 Submitter: Zhihao Yuan Opened: 2022-08-31 Last modified: 2023-11-22
Priority: Not Prioritized
View all other issues in [refwrap.invoke].
View all issues with C++23 status.
Discussion:
The following assertion does not hold, making the type less capable than the function pointers.
void f() noexcept; std::reference_wrapper fn = f; static_assert(std::is_nothrow_invocable_v<decltype(fn)>);
[2022-09-23; Reflector poll]
Set status to Tentatively Ready after six votes in favour during reflector poll.
Already implemented in MSVC and libstdc++.
[2022-11-12 Approved at November 2022 meeting in Kona. Status changed: Voting → WP.]
Proposed resolution:
This wording is relative to N4910.
Modify 22.10.6.1 [refwrap.general], class template reference_wrapper
synopsis, as indicated:
// 22.10.6.5 [refwrap.invoke], invocation template<class... ArgTypes> constexpr invoke_result_t<T&, ArgTypes...> operator()(ArgTypes&&...) const noexcept(is_nothrow_invocable_v<T&, ArgTypes...>);
Modify 22.10.6.5 [refwrap.invoke] as indicated:
template<class... ArgTypes> constexpr invoke_result_t<T&, ArgTypes...> operator()(ArgTypes&&... args) const noexcept(is_nothrow_invocable_v<T&, ArgTypes...>);-1- Mandates:
-2- Returns:T
is a complete type.INVOKE(get(), std::forward<ArgTypes>(args)...)
. (22.10.4 [func.require])
const_sentinel
should be constrainedSection: 24.2 [iterator.synopsis], 24.5.3.2 [const.iterators.alias] Status: C++23 Submitter: Hewill Kang Opened: 2022-09-03 Last modified: 2023-11-22
Priority: Not Prioritized
View all other issues in [iterator.synopsis].
View all issues with C++23 status.
Discussion:
The current standard does not have any constraints on const_sentinel
template parameters,
which makes it possible to pass almost any type of object to make_const_sentinel
.
It would be more appropriate to constrain the type to meet the minimum requirements of the
sentinel type such as semiregular
as move_sentinel
does.
[2022-09-23; Reflector poll]
Set status to Tentatively Ready after six votes in favour during reflector poll.
[2022-11-12 Approved at November 2022 meeting in Kona. Status changed: Voting → WP.]
Proposed resolution:
This wording is relative to N4917.
Modify 24.2 [iterator.synopsis], header <iterator>
synopsis, as indicated:
#include <compare> // see 17.12.1 [compare.syn] #include <concepts> // see 18.3 [concepts.syn] namespace std { […] // 24.5.3 [const.iterators], constant iterators and sentinels // 24.5.3.2 [const.iterators.alias], alias templates […] template<input_iterator I> using const_iterator = see below; // freestanding template<semiregularclassS> using const_sentinel = see below; // freestanding […] template<input_iterator I> constexpr const_iterator<I> make_const_iterator(I it) { return it; } // freestanding template<semiregularclassS> constexpr const_sentinel<S> make_const_sentinel(S s) { return s; } // freestanding […] }
Modify 24.5.3.2 [const.iterators.alias] as indicated:
template<semiregularclassS> using const_sentinel = see below;-2- Result: If
S
modelsinput_iterator
,const_iterator<S>
. Otherwise,S
.
view_interface::cbegin
is underconstrainedSection: 25.5.3.1 [view.interface.general] Status: C++23 Submitter: Hewill Kang Opened: 2022-09-04 Last modified: 2023-11-22
Priority: 2
View all other issues in [view.interface.general].
View all issues with C++23 status.
Discussion:
Currently, view_interface::cbegin
simply returns ranges::cbegin(derived())
,
which returns the type alias const_iterator
for its iterator, which requires that the template
parameter I
must model the input_iterator
.
view_interface::cbegin
does not have any constraints, when D
models only
output_range
, calling its cbegin()
will result in a hard error inside the function body:
#include <ranges>
#include <vector>
int main() {
std::vector<int> v;
auto r = std::views::counted(std::back_inserter(v), 3);
auto b = r.cbegin(); // hard error
}
We should add a constraint for view_interface::cbegin
that D
must model input_range
.
[2022-09-23; Reflector poll]
Set priority to 2 after reflector poll.
This should be done for cend
too.
Previous resolution [SUPERSEDED]:
This wording is relative to N4917.
Modify 25.5.3.1 [view.interface.general] as indicated:
namespace std::ranges { template<class D> requires is_class_v<D> && same_as<D, remove_cv_t<D>> class view_interface { […] public: […] constexpr auto cbegin() requires input_range<D> { return ranges::cbegin(derived()); } constexpr auto cbegin() const requires input_range<const D> { return ranges::cbegin(derived()); } […] }; }
[2022-09-25; Hewill provides improved wording]
[Kona 2022-11-08; Accepted at joint LWG/SG9 session. Move to Immediate]
[2022-11-12 Approved at November 2022 meeting in Kona. Status changed: Immediate → WP.]
Proposed resolution:
This wording is relative to N4917.
Modify 25.5.3.1 [view.interface.general] as indicated:
namespace std::ranges { template<class D> requires is_class_v<D> && same_as<D, remove_cv_t<D>> class view_interface { […] public: […] constexpr auto cbegin() requires input_range<D> { return ranges::cbegin(derived()); } constexpr auto cbegin() const requires input_range<const D> { return ranges::cbegin(derived()); } constexpr auto cend() requires input_range<D> { return ranges::cend(derived()); } constexpr auto cend() const requires input_range<const D> { return ranges::cend(derived()); } […] }; }
codecvt<charN_t, char8_t, mbstate_t>
incorrectly added to localeSection: 28.3.3.1.2.1 [locale.category], 28.3.4.2.5.1 [locale.codecvt.general] Status: WP Submitter: Victor Zverovich Opened: 2022-09-05 Last modified: 2024-04-02
Priority: 3
View all other issues in [locale.category].
View all issues with WP status.
Discussion:
Table [tab:locale.category.facets] includes the following two facets:
codecvt<char16_t, char8_t, mbstate_t>
codecvt<char32_t, char8_t, mbstate_t>
However, neither of those actually has anything to do with a locale and therefore
it doesn't make sense to dynamically register them with std::locale
.
Instead they provide conversions between fixed encodings (UTF-8, UTF-16, UTF-32)
that are unrelated to locale encodings other than they may happen to coincide with
encodings of some locales by accident.
codecvt<char[16|32]_t, char, mbstate_t>
in
N2035 which gave no design rationale for using codecvt
in the first
place. Likely it was trying to do a minimal amount of changes and copied the wording for
codecvt<wchar_t, char, mbstate_t>
but unfortunately didn't consider encoding implications.
P0482 changed char
to char8_t
in these facets which
made the issue more glaring but unfortunately, despite the breaking change, it failed to address it.
Apart from an obvious design mistake this also adds a small overhead for every locale
construction because the implementation has to copy these pseudo-facets for no good
reason violating "don't pay for what you don't use" principle.
A simple fix is to remove the two facets from table [tab:locale.category.facets] and make them
directly constructible.
[2022-09-23; Reflector poll]
Set priority to 3 after reflector poll. Send to SG16 (then maybe LEWG).
[2022-09-28; SG16 responds]
SG16 agrees that the codecvt facets mentioned in LWG3767
"codecvt<charN_t, char8_t, mbstate_t>
incorrectly added to locale" are intended to be invariant
with respect to locale. Unanimously in favor.
[Issaquah 2023-02-10; LWG issue processing]
Removing these breaks most code using them today, because the most obvious
way to use them is via use_facet
on a locale, which would throw
if they're removed (and because they were guaranteed to be present, code
using them might have not bothered to check for them using has_facet
).
Instead of removing them, deprecate the guarantee that they're always present
(so move them to D.19 [depr.locale.category]).
Don't bother changing the destructor.
Victor to update wording.
Previous resolution [SUPERSEDED]:
This wording is relative to N4917.
Modify 28.3.3.1.2.1 [locale.category], Table 105 ([tab:locale.category.facets]) — "Locale category facets" — as indicated:
Table 105: Locale category facets [tab:locale.category.facets] Category Includes facets …
ctype ctype<char>, ctype<wchar_t>
codecvt<char, char, mbstate_t>
codecvt<char16_t, char8_t, mbstate_t>
codecvt<char32_t, char8_t, mbstate_t>
codecvt<wchar_t, char, mbstate_t>…
Modify 28.3.4.2.5.1 [locale.codecvt.general] as indicated:
namespace std { […] template<class internT, class externT, class stateT> class codecvt : public locale::facet, public codecvt_base { public: using intern_type = internT; using extern_type = externT; using state_type = stateT; explicit codecvt(size_t refs = 0); ~codecvt(); […] protected:~codecvt();[…] }; }[…]
-3- The specializations required in Table105 [tab:locale.category.facets]106 [tab:locale.spec] (28.3.3.1.2.1 [locale.category]) convert the implementation-defined native character set.codecvt<char, char, mbstate_t>
implements a degenerate conversion; it does not convert at all. The specializationcodecvt<char16_t, char8_t, mbstate_t>
converts between the UTF-16 and UTF-8 encoding forms, and the specializationcodecvt<char32_t, char8_t, mbstate_t>
converts between the UTF-32 and UTF-8 encoding forms.codecvt<wchar_t, char, mbstate_t>
converts between the native character sets for ordinary and wide characters. Specializations onmbstate_t
perform conversion between encodings known to the library implementer. Other encodings can be converted by specializing on a program-definedstateT
type. Objects of typestateT
can contain any state that is useful to communicate to or from the specializeddo_in
ordo_out
members.
[2023-02-10; Victor Zverovich comments and provides improved wording]
Per today's LWG discussion the following changes have been implemented in revised wording:
Deprecated the facets instead of removing them (also _byname
variants which were previously missed).
Removed the changes to facet dtor since with deprecation it's no longer critical to provide other ways to access them.
[Kona 2023-11-07; move to Ready]
[Tokyo 2024-03-23; Status changed: Voting → WP.]
Proposed resolution:
This wording is relative to N4928.
Modify 28.3.3.1.2.1 [locale.category], Table 105 ([tab:locale.category.facets]) — "Locale category facets" — and Table 106 ([tab:locale.spec]) "Required specializations" as indicated:
[…]
Table 105: Locale category facets [tab:locale.category.facets] Category Includes facets …
ctype ctype<char>, ctype<wchar_t>
codecvt<char, char, mbstate_t>
codecvt<char16_t, char8_t, mbstate_t>
codecvt<char32_t, char8_t, mbstate_t>
codecvt<wchar_t, char, mbstate_t>…
Table 106: Required specializations [tab:locale.spec] Category Includes facets …
ctype ctype_byname<char>, ctype_byname<wchar_t>
codecvt_byname<char, char, mbstate_t>
codecvt_byname<char16_t, char8_t, mbstate_t>
codecvt_byname<char32_t, char8_t, mbstate_t>
codecvt_byname<wchar_t, char, mbstate_t>…
Modify 28.3.4.2.5.1 [locale.codecvt.general] as indicated:
[…]
-3- The specializations required in Table 105 (28.3.3.1.2.1 [locale.category]) convert the implementation-defined native character set.codecvt<char, char, mbstate_t>
implements a degenerate conversion; it does not convert at all.The specializationcodecvt<char16_t, char8_t, mbstate_t>
converts between the UTF-16 and UTF-8 encoding forms, and the specializationcodecvt<char32_t, char8_t, mbstate_t>
converts between the UTF-32 and UTF-8 encoding forms.codecvt<wchar_t, char, mbstate_t>
converts between the native character sets for ordinary and wide characters. Specializations onmbstate_t
perform conversion between encodings known to the library implementer. Other encodings can be converted by specializing on a program-definedstateT
type. Objects of typestateT
can contain any state that is useful to communicate to or from the specializeddo_in
ordo_out
members.
Modify D.19 [depr.locale.category] (Deprecated locale category facets) in Annex D as indicated:
-1- The
ctype
locale category includes the following facets as if they were specified in table Table 105 [tab:locale.category.facets] of 28.3.4.2.5.1 [locale.codecvt.general].codecvt<char16_t, char, mbstate_t> codecvt<char32_t, char, mbstate_t> codecvt<char16_t, char8_t, mbstate_t> codecvt<char32_t, char8_t, mbstate_t>-1- The
ctype
locale category includes the following facets as if they were specified in table Table 106 [tab:locale.spec] of 28.3.4.2.5.1 [locale.codecvt.general].codecvt_byname<char16_t, char, mbstate_t> codecvt_byname<char32_t, char, mbstate_t> codecvt_byname<char16_t, char8_t, mbstate_t> codecvt_byname<char32_t, char8_t, mbstate_t>-3- The following class template specializations are required in addition to those specified in 28.3.4.2.5 [locale.codecvt]. The specializations
codecvt<char16_t, char, mbstate_t>
andcodecvt<char16_t, char8_t, mbstate_t>
convertsbetween the UTF-16 and UTF-8 encoding forms, and the specializationscodecvt<char32_t, char, mbstate_t>
andcodecvt<char32_t, char8_t, mbstate_t>
convertsbetween the UTF-32 and UTF-8 encoding forms.
basic_const_iterator::operator==
causes infinite constraint recursionSection: 24.5.3 [const.iterators] Status: C++23 Submitter: Hewill Kang Opened: 2022-09-05 Last modified: 2023-11-22
Priority: 1
View other active issues in [const.iterators].
View all other issues in [const.iterators].
View all issues with C++23 status.
Discussion:
Currently, basic_const_iterator::operator==
is defined as a friend
function:
template<sentinel_for<Iterator> S> friend constexpr bool operator==(const basic_const_iterator& x, const S& s);
which only requires S
to model sentinel_for<Iterator>
, and since
basic_const_iterator
has a conversion constructor that accepts I
, this will
result in infinite constraint checks when comparing basic_const_iterator<int*>
with int*
(online example):
#include <iterator>
template<std::input_iterator I>
struct basic_const_iterator {
basic_const_iterator() = default;
basic_const_iterator(I);
template<std::sentinel_for<I> S>
friend bool operator==(const basic_const_iterator&, const S&);
};
static_assert(std::sentinel_for<basic_const_iterator<int*>, int*>); // infinite meta-recursion
That is, sentinel_for
ends with weakly-equality-comparable-with
and instantiates operator==
, which in turn rechecks sentinel_for
and
instantiates the same operator==
, making the circle closed.
operator==
to be a member function so that
S
is no longer accidentally instantiated as basic_const_iterator
.
The same goes for basic_const_iterator::operator-
.
[2022-09-23; Reflector poll]
Set priority to 1 after reflector poll.
"Although I am not a big fan of member ==, the proposed solution seems to be simple."
"prefer if we would keep operator==
as non-member for consistency."
This wording is relative to N4917.
Modify 24.5.3.3 [const.iterators.iterator], class template
basic_const_iterator
synopsis, as indicated:namespace std { template<class I> concept not-a-const-iterator = see below; template<input_iterator Iterator> class basic_const_iterator { Iterator current_ = Iterator(); using reference = iter_const_reference_t<Iterator>; // exposition only public: […] template<sentinel_for<Iterator> S>friendconstexpr bool operator==(const basic_const_iterator& x,const S& s) const; […] template<sized_sentinel_for<Iterator> S>friendconstexpr difference_type operator-(const basic_const_iterator& x,const S& y) const; template<not-a-const-iteratorsized_sentinel_for<Iterator>S> requires sized_sentinel_fordifferent-from<S, Iteratorbasic_const_iterator> friend constexpr difference_type operator-(const S& x, const basic_const_iterator& y); }; }Modify 24.5.3.5 [const.iterators.ops] as indicated:
[…]
template<sentinel_for<Iterator> S>friendconstexpr bool operator==(const basic_const_iterator& x,const S& s) const;[…]-16- Effects: Equivalent to:
return
.x.current_ == s;template<sized_sentinel_for<Iterator> S>friendconstexpr difference_type operator-(const basic_const_iterator& x,const S& y) const;-24- Effects: Equivalent to:
return
.x.current_ - y;template<not-a-const-iteratorsized_sentinel_for<Iterator>S> requires sized_sentinel_fordifferent-from<S, Iteratorbasic_const_iterator> friend constexpr difference_type operator-(const S& x, const basic_const_iterator& y);-25- Effects: Equivalent to:
return x - y.current_;
.
[2022-11-04; Tomasz comments and improves proposed wording]
Initially, LWG requested an investigation of alternative resolutions that would avoid using member functions for the affected operators.
Later, it was found that in addition to ==
/-
, all comparison operators (<
, >
, <=
, >=
, <=>
) are affected by same problem for the calls
with basic_const_iterator<basic_const_iterator<int*>>
and int*
as arguments, i.e. totally_ordered_with<basic_const_iterator<basic_const_iterator<int*>>, int*>
causes infinite recursion in constraint checking.
The new resolution, change all of the friends overloads for operators ==
, <
, >
, <=
, >=
, <=>
and -
that accept basic_const_iterator
as lhs, to const
member functions.
This change is applied to homogeneous (basic_const_iterator, basic_const_iterator)
for consistency.
For the overload of <
, >
, <=
, >=
and -
that accepts (I, basic_const_iterator)
we declared them as friends and consistently constrain them with not-const-iterator
.
Finally, its put (now member) operator
in the block with other heterogeneous overloads in the synopsis.
<=>
(I)
The use of member functions addresses issues, because:
basic_const_iterator
in the left-hand side of op, i.e. eliminates issues for (sized_)sentinel_for<basic_const_iterator<int*>, int*>
and totally_ordered<basic_const_iterator<int*>, int*>
basic_const_iterator<basic_const_iterator<S>>
, so we address recursion for nested iterators[Kona 2022-11-08; Move to Ready]
[2023-02-13 Approved at February 2023 meeting in Issaquah. Status changed: Voting → WP.]
Proposed resolution:
This wording is relative to N4917.
Modify 24.5.3.3 [const.iterators.iterator], class template basic_const_iterator
synopsis, as indicated:
namespace std { template<class I> concept not-a-const-iterator = see below; template<input_iterator Iterator> class basic_const_iterator { Iterator current_ = Iterator(); using reference = iter_const_reference_t<Iterator>; // exposition only public: […] template<sentinel_for<Iterator> S>friendconstexpr bool operator==(const basic_const_iterator& x,const S& s) const;friendconstexpr bool operator<(const basic_const_iterator& x,const basic_const_iterator& y) const requires random_access_iterator<Iterator>;friendconstexpr bool operator>(const basic_const_iterator& x,const basic_const_iterator& y) const requires random_access_iterator<Iterator>;friendconstexpr bool operator<=(const basic_const_iterator& x,const basic_const_iterator& y) const requires random_access_iterator<Iterator>;friendconstexpr bool operator>=(const basic_const_iterator& x,const basic_const_iterator& y) const requires random_access_iterator<Iterator>;friendconstexpr auto operator<=>(const basic_const_iterator& x,const basic_const_iterator& y) const requires random_access_iterator<Iterator> && three_way_comparable<Iterator>; template<different-from<basic_const_iterator> I>friendconstexpr bool operator<(const basic_const_iterator& x,const I& y) const requires random_access_iterator<Iterator> && totally_ordered_with<Iterator, I>; template<different-from<basic_const_iterator> I>friendconstexpr bool operator>(const basic_const_iterator& x,const I& y) const requires random_access_iterator<Iterator> && totally_ordered_with<Iterator, I>; template<different-from<basic_const_iterator> I>friendconstexpr bool operator<=(const basic_const_iterator& x,const I& y) const requires random_access_iterator<Iterator> && totally_ordered_with<Iterator, I>; template<different-from<basic_const_iterator> I>friendconstexpr bool operator>=(const basic_const_iterator& x,const I& y) const requires random_access_iterator<Iterator> && totally_ordered_with<Iterator, I>; template<different-from<basic_const_iterator> I> constexpr auto operator<=>(const I& y) const requires random_access_iterator<Iterator> && totally_ordered_with<Iterator, I> && three_way_comparable_with<Iterator, I>; template<not-a-const-iterator I> friend constexpr bool operator<(const I& y, const basic_const_iterator& x) requires random_access_iterator<Iterator> && totally_ordered_with<Iterator, I>; template<not-a-const-iterator I> friend constexpr bool operator>(const I& y, const basic_const_iterator& x) requires random_access_iterator<Iterator> && totally_ordered_with<Iterator, I>; template<not-a-const-iterator I> friend constexpr bool operator<=(const I& y, const basic_const_iterator& x) requires random_access_iterator<Iterator> && totally_ordered_with<Iterator, I>; template<not-a-const-iterator I> friend constexpr bool operator>=(const I& y, const basic_const_iterator& x) requires random_access_iterator<Iterator> && totally_ordered_with<Iterator, I>;template<different-from<basic_const_iterator> I> friend constexpr auto operator<=>(const basic_const_iterator& x, const I& y) requires random_access_iterator<Iterator> && totally_ordered_with<Iterator, I> && three_way_comparable_with<Iterator, I>;[…] template<sized_sentinel_for<Iterator> S>friendconstexpr difference_type operator-(const basic_const_iterator& x,const S& y) const; template<not-a-const-iteratorsized_sentinel_for<Iterator>S> requires sized_sentinel_fordifferent-from<S, Iteratorbasic_const_iterator> friend constexpr difference_type operator-(const S& x, const basic_const_iterator& y); }; }
Modify 24.5.3.5 [const.iterators.ops] as indicated:
[…]
template<sentinel_for<Iterator> S>friendconstexpr bool operator==(const basic_const_iterator& x,const S& s) const;-16- Effects: Equivalent to:
return
x.current_ == s;friendconstexpr bool operator<(const basic_const_iterator& x,const basic_const_iterator& y) const requires random_access_iterator<Iterator>;friendconstexpr bool operator>(const basic_const_iterator& x,const basic_const_iterator& y) const requires random_access_iterator<Iterator>;friendconstexpr bool operator<=(const basic_const_iterator& x,const basic_const_iterator& y) const requires random_access_iterator<Iterator>;friendconstexpr bool operator>=(const basic_const_iterator& x,const basic_const_iterator& y) const requires random_access_iterator<Iterator>;friendconstexpr auto operator<=>(const basic_const_iterator& x,const basic_const_iterator& y) const requires random_access_iterator<Iterator> && three_way_comparable<Iterator>;-17- Let op be the operator.
-18- Effects: Equivalent to:
return
x.current_ op y.current_;template<different-from<basic_const_iterator> I>friendconstexpr bool operator<(const basic_const_iterator& x,const I& y) const requires random_access_iterator<Iterator> && totally_ordered_with<Iterator, I>; template<different-from<basic_const_iterator> I>friendconstexpr bool operator>(const basic_const_iterator& x,const I& y) const requires random_access_iterator<Iterator> && totally_ordered_with<Iterator, I>; template<different-from<basic_const_iterator> I>friendconstexpr bool operator<=(const basic_const_iterator& x,const I& y) const requires random_access_iterator<Iterator> && totally_ordered_with<Iterator, I>; template<different-from<basic_const_iterator> I>friendconstexpr bool operator>=(const basic_const_iterator& x,const I& y) const requires random_access_iterator<Iterator> && totally_ordered_with<Iterator, I>; template<different-from<basic_const_iterator> I>friendconstexpr auto operator<=>(const basic_const_iterator& x,const I& y) const requires random_access_iterator<Iterator> && totally_ordered_with<Iterator, I> && three_way_comparable_with<Iterator, I>;[…]-19- Let
op
be the operator.-20-
ReturnsEffects: Equivalent to:return
x.current_ op y;template<sized_sentinel_for<Iterator> S>friendconstexpr difference_type operator-(const basic_const_iterator& x,const S& y) const;-24- Effects: Equivalent to:
return
x.current_ - y;template<not-a-const-iteratorsized_sentinel_for<Iterator>S> requires sized_sentinel_fordifferent-from<S, Iteratorbasic_const_iterator> friend constexpr difference_type operator-(const S& x, const basic_const_iterator& y);-25- Effects: Equivalent to:
return x - y.current_;
const_sentinel_t
is missingSection: 25.2 [ranges.syn] Status: C++23 Submitter: Hewill Kang Opened: 2022-09-05 Last modified: 2023-11-22
Priority: 3
View all other issues in [ranges.syn].
View all issues with C++23 status.
Discussion:
The type alias pair const_iterator
and const_sentinel
and the factory function pair make_const_iterator
and
make_const_sentinel
are defined in <iterator>
,
however, only const_iterator_t
exists in <ranges>
,
which lacks const_sentinel_t
, we should add it to ensure consistency.
const_iterator_t
to input_range
to make it consistent with const_iterator
,
which already requires that its template parameter must model
input_iterator
.
[2022-09-23; Reflector poll]
Set priority to 3 after reflector poll.
Comment from a reviewer:"I don't see why we should redundantly constrain
const_iterator_t
withinput_range
; it's not as if we can make it more ill-formed or more SFINAE-friendly than it is now."
Previous resolution [SUPERSEDED]:
This wording is relative to N4917.
Modify 25.2 [ranges.syn], header
<ranges>
synopsis, as indicated:#include <compare> // see 17.12.1 [compare.syn] #include <initializer_list> // see 17.11.2 [initializer.list.syn] #include <iterator> // see 24.2 [iterator.synopsis] namespace std::ranges { […] template<class T> using iterator_t = decltype(ranges::begin(declval<T&>())); // freestanding template<range R> using sentinel_t = decltype(ranges::end(declval<R&>())); // freestanding template<input_range R> using const_iterator_t = const_iterator<iterator_t<R>>; // freestanding template<range R> using const_sentinel_t = const_sentinel<sentinel_t<R>>; // freestanding […] }
[2022-09-25; Daniel and Hewill provide alternative wording]
The alternative solution solely adds the suggested const_sentinel_t
alias template
and doesn't touch the const_iterator_t
constraints.
[2022-10-12; Reflector poll]
Set status to "Tentatively Ready" after seven votes in favour.
[2022-11-12 Approved at November 2022 meeting in Kona. Status changed: Voting → WP.]
Proposed resolution:
This wording is relative to N4917.
Modify 25.2 [ranges.syn], header <ranges>
synopsis, as indicated:
#include <compare> // see 17.12.1 [compare.syn] #include <initializer_list> // see 17.11.2 [initializer.list.syn] #include <iterator> // see 24.2 [iterator.synopsis] namespace std::ranges { […] template<class T> using iterator_t = decltype(ranges::begin(declval<T&>())); // freestanding template<range R> using sentinel_t = decltype(ranges::end(declval<R&>())); // freestanding template<range R> using const_iterator_t = const_iterator<iterator_t<R>>; // freestanding template<range R> using const_sentinel_t = const_sentinel<sentinel_t<R>>; // freestanding […] }
function
Section: 7.2.1 [fund.ts.v3::func.wrap.func.overview] Status: C++23 Submitter: Alisdair Meredith Opened: 2022-09-12 Last modified: 2023-11-22
Priority: Not Prioritized
View all issues with C++23 status.
Discussion:
Addresses: fund.ts.v3
The LFTSv3 bases its specification for experimental::function
on std::function
in the C++20 standard.
However, the wording was largely copied over from LFTSv2 which based its
wording on the C++14 standard.
For C++17, we removed the conditionally defined typedefs for the legacy binders
API, but this removal was not reflected in the TS. We are now left with a
specification referring to unknown types, T1
and T2
.
These typedefs should be excised to match the referenced standard.
[2022-09-23; Reflector poll]
Set status to Tentatively Ready after ten votes in favour during reflector poll.
[2022-11-12 Approved at November 2022 meeting in Kona. Status changed: Voting → WP.]
Proposed resolution:
This wording is relative to N4920.
Modify the synopsis in 7.2.1 [fund.ts.v3::func.wrap.func.overview] as indicated:
namespace std { namespace experimental::inline fundamentals_v3 { template<class> class function; // undefined template<class R, class... ArgTypes> class function<R(ArgTypes...)> { public: using result_type = R;using argument_type = T1;using first_argument_type T1;using second_argument_type = T2;using allocator_type = erased_type; // ... } }
repeat_view
's piecewise constructor is missing PostconditionsSection: 25.6.5.2 [range.repeat.view] Status: C++23 Submitter: Hewill Kang Opened: 2022-09-12 Last modified: 2023-11-22
Priority: 2
View all other issues in [range.repeat.view].
View all issues with C++23 status.
Discussion:
The first two value-bound pair constructors of repeat_view
have the
Preconditions that the integer-like object bound
must be
non-negative.
However, the piecewise constructor does not mention the valid values for
bound_args
.
It would be nice to add a Postconditions that the initialized
bound_
must be greater than or equal to 0
here.
[2022-09-23; Reflector poll]
Set priority to 2 after reflector poll.
This is trying to state a requirement on users, but that's not what
Postconditions: means. Should be something more like:
Precondition: If Bound
is not unreachable_sentinel_t
,
the bound_
≥ 0 after its initialization from bound_args
.
Previous resolution [SUPERSEDED]:
This wording is relative to N4917.
Modify 25.6.5.2 [range.repeat.view] as shown:
template<class... WArgs, class... BoundArgs> requires constructible_from<W, WArgs...> && constructible_from<Bound, BoundArgs...> constexpr explicit repeat_view(piecewise_construct_t, tuple<Wargs...> value_args, tuple<BoundArgs...> bound_args = tuple<>{});-5- Effects: Initializes
value_
with arguments of typesWArgs...
obtained by forwarding the elements ofvalue_args
and initializesbound_
with arguments of typesBoundArgs...
obtained by forwarding the elements ofbound_args
. (Here, forwarding an elementx
of typeU
within a tuple object means callingstd::forward<U>(x)
.)-?- Postconditions: If
Bound
is notunreachable_sentinel_t
,bound_
≥ 0.
[2022-09-23; Jonathan provides improved wording]
Previous resolution [SUPERSEDED]:
This wording is relative to N4917.
Modify 25.6.5.2 [range.repeat.view] as shown:
template<class... WArgs, class... BoundArgs> requires constructible_from<W, WArgs...> && constructible_from<Bound, BoundArgs...> constexpr explicit repeat_view(piecewise_construct_t, tuple<Wargs...> value_args, tuple<BoundArgs...> bound_args = tuple<>{});-?- Preconditions:
Bound
isunreachable_sentinel_t
, or the initialization ofbound_
yields a non-negative value.-5- Effects: Initializes
value_
with arguments of typesWArgs...
obtained by forwarding the elements ofvalue_args
and initializesbound_
with arguments of typesBoundArgs...
obtained by forwarding the elements ofbound_args
. (Here, forwarding an elementx
of typeU
within a tuple object means callingstd::forward<U>(x)
.)
[2023-01-11; Jonathan provides new wording requested by LWG]
[Issaquah 2023-02-07; LWG]
Move to Immediate for C++23
[2023-02-13 Approved at February 2023 meeting in Issaquah. Status changed: Immediate → WP.]
Proposed resolution:
This wording is relative to N4917.
Modify 25.6.5.2 [range.repeat.view] as shown:
template<class... TArgs, class... BoundArgs> requires constructible_from<T, TArgs...> && constructible_from<Bound, BoundArgs...> constexpr explicit repeat_view(piecewise_construct_t, tuple<Targs...> value_args, tuple<BoundArgs...> bound_args = tuple<>{});-5- Effects: Initializes
value_
witharguments of typesTArgs...
obtained by forwarding the elements ofvalue_args
make_from_tuple<T>(std::move(value_args))
and initializesbound_
witharguments of typesBoundArgs...
obtained by forwarding the elements ofbound_args
. (Here, forwarding an elementx
of typeU
within a tuple object means callingstd::forward<U>(x)
.)make_from_tuple<Bound>(std::move(bound_args))
. The behavior is undefined ifBound
is notunreachable_sentinel_t
andbound_
is negative.
views::zip_transform
still requires F
to be copy_constructible
when empty packSection: 25.7.26.1 [range.zip.transform.overview] Status: C++23 Submitter: Hewill Kang Opened: 2022-09-12 Last modified: 2024-01-29
Priority: Not Prioritized
View all issues with C++23 status.
Discussion:
After P2494R2, range adaptors only require callable to be
move_constructible
, however, for views::zip_transform
,
when empty pack, it still requires callable to be copy_constructible
.
We should relax this requirement, too.
[2022-09-23; Reflector poll]
Set status to Tentatively Ready after seven votes in favour during reflector poll.
[2022-11-12 Approved at November 2022 meeting in Kona. Status changed: Voting → WP.]
Proposed resolution:
This wording is relative to N4910.
Modify 25.7.26.1 [range.zip.transform.overview] as indicated:
-2- The name
views::zip_transform
denotes a customization point object (16.3.3.3.5 [customization.point.object]). LetF
be a subexpression, and letEs...
be a pack of subexpressions.
(2.1) — If
Es
is an empty pack, letFD
bedecay_t<decltype((F))>
.
(2.1.1) — If
move_constructible
iscopy_constructible<FD> && regular_invocable<FD&>false
, or ifdecay_t<invoke_result_t<FD&>>
is not an object type,views::zip_transform(F, Es...)
is ill-formed.(2.1.2) — Otherwise, the expression
views::zip_transform(F, Es...)
is expression-equivalent to((void)F, auto(views::empty<decay_t<invoke_result_t<FD&>>>))(2.2) — Otherwise, the expression
views::zip_transform(F, Es...)
is expression-equivalent tozip_transform_view(F, Es...)
.
<flat_set>
should include <compare>
Section: 23.6.10 [flat.set.syn] Status: C++23 Submitter: Jiang An Opened: 2022-09-12 Last modified: 2023-11-22
Priority: Not Prioritized
View all issues with C++23 status.
Discussion:
This was originally editorial PR #5789 which is considered not-editorial.
std::flat_set
and std::flat_multiset
have operator<=>
so <compare>
should be included in the corresponding header,
in the spirit of LWG 3330(i).
#include <compare>
is also missing in the adopted paper
P1222R4.
[2022-09-23; Reflector poll]
Set status to Tentatively Ready after eight votes in favour during reflector poll.
[2022-11-12 Approved at November 2022 meeting in Kona. Status changed: Voting → WP.]
Proposed resolution:
This wording is relative to N4917.
Modify the synopsis in 23.6.10 [flat.set.syn] as indicated:
#include <compare> // see 17.12.1 [compare.syn] #include <initializer_list> // see 17.11.2 [initializer.list.syn]
Section: 16.4.4.6.1 [allocator.requirements.general] Status: C++23 Submitter: Alisdair Meredith Opened: 2022-09-22 Last modified: 2023-11-22
Priority: Not Prioritized
View other active issues in [allocator.requirements.general].
View all other issues in [allocator.requirements.general].
View all issues with C++23 status.
Discussion:
This issue is extracted from P0177R2 as that paper stalled on the author's ability to update in time for C++17. While the issue was recorded and going to be resolved in the paper, we did not file an issue for the list when work on that paper stopped.
Many of the types and expressions in the Cpp17Allocator requirements are optional, and as such a default is provided that is exposed throughstd::allocator_traits
. However, some types and operations are
specified directly in terms of the allocator member, when really they should be
specified allowing for reliance on the default, obtained through
std::allocator_traits
. For example, X::pointer
is
an optional type and not required to exist; XX::pointer
is either
X::pointer
when it is present, or the default formula otherwise,
and so is guaranteed to always exist, and the intended interface for user code.
Observe that bullet list in p2, which acts as the key to the names in the
Cpp17Allocator requirements, gets this right, unlike most of the text
that follows.
This change corresponds to the known implementations, which meet the intended
contract rather than that currently specified. For example,
std::allocator
does not provide any of the pointer
related typedef members, so many of the default semantics indicated today would
be ill-formed if implementations were not already implementing the fix.
An alternative resolution might be to add wording around p1-3 to state that if
a name lookup fails then the default formula is used. However, it is simply
clearer to write the constraints as intended, in the form of code that users
can write, rather than hide behind a layer of indirect semantics that may be
interpreted as requiring another layer of SFINAE metaprogramming.
[2022-10-12; Reflector poll]
Set status to Tentatively Ready after eight votes in favour during reflector poll.
[2022-11-12 Approved at November 2022 meeting in Kona. Status changed: Voting → WP.]
Proposed resolution:
This wording is relative to N4917.
Modify 16.4.4.6.1 [allocator.requirements.general] as indicated:
typename X::pointer-4- Remarks: Default:
T*
typename X::const_pointer-5- Mandates:
-6- Remarks: Default:XX::pointer
is convertible toXX::const_pointer
.pointer_traits<XX::pointer>::rebind<const T>
typename X::void_pointer typename Y::void_pointer-7- Mandates:
-8- Remarks: Default:XX::pointer
is convertible toXX::void_pointer
.XX::void_pointer
andYY::void_pointer
are the same type.pointer_traits<XX::pointer>::rebind<void>
typename X::const_void_pointer typename Y::const_void_pointer-9- Mandates:
-10- Remarks: Default:XX::pointer
,XX::const_pointer
, andXX::void_pointer
are convertible toXX::const_void_pointer
.XX::const_void_pointer
andYY::const_void_pointer
are the same type.pointer_traits<XX::pointer>::rebind<const void>
typename X::value_type-11- Result: Identical to
T
.typename X::size_type-12- Result: An unsigned integer type that can represent the size of the largest object in the allocation model.
-13- Remarks: Default:make_unsigned_t<XX::difference_type>
typename X::difference_type-14- Result: A signed integer type that can represent the difference between any two pointers in the allocation model.
-15- Remarks: Default:pointer_traits<XX::pointer>::difference_type
typename X::template rebind<U>::other-16- Result:
-17- Postconditions: For allY
U
(includingT
),YY::
istemplate rebindrebind_alloc<T>::otherX
. -18- Remarks: IfAllocator
is a class template instantiation of the formSomeAllocator<T, Args>
, whereArgs
is zero or more type arguments, andAllocator
does not supply arebind
member template, the standardallocator_traits
template usesSomeAllocator<U, Args>
in place ofAllocator::rebind<U>::other
by default. For allocator types that are not template instantiations of the above form, no default is provided. -19- [Note 1: The member class templaterebind
ofX
is effectively a typedef template. In general, if the nameAllocator
is bound toSomeAllocator<T>
, thenAllocator::rebind<U>::other
is the same type asSomeAllocator<U>
, whereSomeAllocator<T>::value_type
isT
andSomeAllocator<U>::value_type
isU
. — end note][…]
static_cast<XX::pointer>(w)-29- Result:
-30- Postconditions:XX::pointer
static_cast<XX::pointer>(w) == p
.static_cast<XX::const_pointer>(x)-31- Result:
-32- Postconditions:XX::const_pointer
static_cast<XX::const_pointer>(x) == q
.pointer_traits<XX::pointer>::pointer_to(r)-33- Result:
-34- Postconditions: Same asXX::pointer
p
.a.allocate(n)-35- Result:
[…]XX::pointer
a.allocate(n, y)-40- Result:
[…]XX::pointer
a.allocate_at_least(n)[…]-43- Result:
-44- Returns:allocation_result<XX::pointer>
allocation_result<XX::pointer>{ptr, count}
whereptr
is memory allocated for an array of countT
and such an object is created but array elements are not constructed, such thatcount = n
. Ifn == 0
, the return value is unspecified. […]a.max_size()[…]-50- Result:
-51- Returns: The largest valueXX::size_type
n
that can meaningfully be passed to. -52- Remarks: Default:
X::a.allocate(n)numeric_limits<size_type>::max() / sizeof(value_type)
a == b[…]-59- Result:
-60- Returns:bool
a == YY::rebind_alloc<T>
.::other(b)-92- An allocator type
-93- LetX
shall meet the Cpp17CopyConstructible requirements (Table 33). TheXX::pointer
,XX::const_pointer
,XX::void_pointer
, andXX::const_void_pointer
types shall meet the Cpp17NullablePointer requirements (Table 37). No constructor, comparison operator function, copy operation, move operation, or swap operation on these pointer types shall exit via an exception.XX::pointer
andXX::const_pointer
shall also meet the requirements for a Cpp17RandomAccessIterator (25.3.5.7) and the additional requirement that, whenand
ap(
are dereferenceable pointer values for some integral valueap + n)n
,addressof(*(
isap + n)) == addressof(*ap) + ntrue
.x1
andx2
denote objects of (possibly different) typesXX::void_pointer
,XX::const_void_pointer
,XX::pointer
, orXX::const_pointer
. Then,x1
andx2
are equivalently-valued pointer values, if and only if bothx1
andx2
can be explicitly converted to the two corresponding objectspx1
andpx2
of typeXX::const_pointer
, using a sequence ofstatic_cast
s using only these four types, and the expressionpx1 == px2
evaluates totrue
. -94- Letw1
andw2
denote objects of typeXX::void_pointer
. Then for the expressionsw1 == w2 w1 != w2either or both objects may be replaced by an equivalently-valued object of type
-95- LetXX::const_void_pointer
with no change in semantics.p1
andp2
denote objects of typeXX::pointer
. Then for the expressionsp1 == p2 p1 != p2 p1 < p2 p1 <= p2 p1 >= p2 p1 > p2 p1 - p2either or both objects may be replaced by an equivalently-valued object of type
XX::const_pointer
with no change in semantics.
vector<bool>
missing exception specificationsSection: 23.3.14.1 [vector.bool.pspc] Status: C++23 Submitter: Alisdair Meredith Opened: 2022-09-13 Last modified: 2023-11-22
Priority: Not Prioritized
View all other issues in [vector.bool.pspc].
View all issues with C++23 status.
Discussion:
As noted back in P0177R2, the primary template for vector
has picked
up some exception specification, but the partial specialization for vector<bool>
has not been so updated.
vector
in the intervening years, but these
particular exception specifications have still not been updated. Note that the free-function
swap
in the header synopsis will automatically pick up this update once applied.
[2022-09-28; Reflector poll]
Set status to Tentatively Ready after seven votes in favour during reflector poll.
[2022-11-12 Approved at November 2022 meeting in Kona. Status changed: Voting → WP.]
Proposed resolution:
This wording is relative to N4917.
Modify 23.3.14.1 [vector.bool.pspc], partial class template
vector<bool, Allocator>
synopsis, as indicated:
namespace std { template<class Allocator> class vector<bool, Allocator> { public: […] // construct/copy/destroy constexpr vector() noexcept(noexcept(Allocator())) : vector(Allocator()) { } constexpr explicit vector(const Allocator&) noexcept; constexpr explicit vector(size_type n, const Allocator& = Allocator()); constexpr vector(size_type n, const bool& value, const Allocator& = Allocator()); template<class InputIterator> constexpr vector(InputIterator first, InputIterator last, const Allocator& = Allocator()); template<container-compatible-range <bool> R> constexpr vector(from_range_t, R&& rg, const Allocator& = Allocator()); constexpr vector(const vector& x); constexpr vector(vector&& x) noexcept; constexpr vector(const vector&, const type_identity_t<Allocator>&); constexpr vector(vector&&, const type_identity_t<Allocator>&); constexpr vector(initializer_list<bool>, const Allocator& = Allocator()); constexpr ~vector(); constexpr vector& operator=(const vector& x); constexpr vector& operator=(vector&& x) noexcept(allocator_traits<Allocator>::propagate_on_container_move_assignment::value || allocator_traits<Allocator>::is_always_equal::value); constexpr vector& operator=(initializer_list<bool>); template<class InputIterator> constexpr void assign(InputIterator first, InputIterator last); template<container-compatible-range <bool> R> constexpr void assign_range(R&& rg); constexpr void assign(size_type n, const bool& t); constexpr void assign(initializer_list<bool>); constexpr allocator_type get_allocator() const noexcept; […] constexpr iterator erase(const_iterator position); constexpr iterator erase(const_iterator first, const_iterator last); constexpr void swap(vector&) noexcept(allocator_traits<Allocator>::propagate_on_container_swap::value || allocator_traits<Allocator>::is_always_equal::value); constexpr static void swap(reference x, reference y) noexcept; constexpr void flip() noexcept; // flips all bits constexpr void clear() noexcept; }; }
format
's width estimation is too approximate and not forward compatibleSection: 28.5.2.2 [format.string.std] Status: Resolved Submitter: Corentin Jabot Opened: 2022-09-15 Last modified: 2023-03-23
Priority: 3
View other active issues in [format.string.std].
View all other issues in [format.string.std].
View all issues with Resolved status.
Discussion:
For the purpose of width estimation, format
considers ranges of codepoints initially
derived from an implementation of wcwidth
with modifications (see P1868R1).
From a reading of the spec, it is not clear how these ranges were selected.
Poor forward compatibility with future Unicode versions. The list will become less and less meaningful overtime or require active maintenance at each Unicode release (which we have not done for Unicode 14 already).
Some of these codepoints are unassigned or otherwise reserved, which is another forward compatibility concern.
Instead, we propose to
Rely on UAX-11 for most of the codepoints)
Grand-father specific and fully assigned, blocks of codepoints to support additional pictograms per the original intent of the paper and existing practices. We add the name of these blocks in the wording for clarity.
Note that per UAX-11
Most emojis are considered
East_Asian_Width="W"
By design,
East_Asian_Width="W"
includes specific unassigned ranges, which should always be treated as Wide.
This change:
Considers 8477 extra codepoints as having a width 2 (as of Unicode 15) (mostly Tangut Ideographs)
Change the width of 85 unassigned code points from 2 to 1
Change the width of 8 codepoints (in the range
U+3248 CIRCLED NUMBER TEN ON BLACK SQUARE
…U+324F CIRCLED NUMBER EIGHTY ON BLACK SQUARE
) from 2 to 1, because it seems questionable to make an exception for those without input from Unicode
For the following code points, the estimated width used to be 1, and is 2 after the suggested change:
U+231A WATCH .. U+231B HOURGLASS
U+23E9 BLACK RIGHT-POINTING DOUBLE TRIANGLE .. U+23EC BLACK DOWN-POINTING DOUBLE TRIANGLE
U+23F0 ALARM CLOCK
U+23F3 HOURGLASS WITH FLOWING SAND
U+25FD WHITE MEDIUM SMALL SQUARE .. U+25FE BLACK MEDIUM SMALL SQUARE
U+2614 UMBRELLA WITH RAIN DROPS .. U+2615 HOT BEVERAGE
U+2648 ARIES .. U+2653 PISCES
U+267F WHEELCHAIR SYMBOL
U+2693 ANCHOR
U+26A1 HIGH VOLTAGE SIGN
U+26AA MEDIUM WHITE CIRCLE .. U+26AB MEDIUM BLACK CIRCLE
U+26BD SOCCER BALL .. U+26BE BASEBALL
U+26C4 SNOWMAN WITHOUT SNOW .. U+26C5 SUN BEHIND CLOUD
U+26CE OPHIUCHUS
U+26D4 NO ENTRY
U+26EA CHURCH
U+26F2 FOUNTAIN .. U+26F3 FLAG IN HOLE
U+26F5 SAILBOAT
U+26FA TENT
U+26FD FUEL PUMP
U+2705 WHITE HEAVY CHECK MARK
U+270A RAISED FIST .. U+270B RAISED HAND
U+2728 SPARKLES
U+274C CROSS MARK
U+274E NEGATIVE SQUARED CROSS MARK
U+2753 BLACK QUESTION MARK ORNAMENT .. U+2755 WHITE EXCLAMATION MARK ORNAMENT
U+2757 HEAVY EXCLAMATION MARK SYMBOL
U+2795 HEAVY PLUS SIGN .. U+2797 HEAVY DIVISION SIGN
U+27B0 CURLY LOOP
U+27BF DOUBLE CURLY LOOP
U+2B1B BLACK LARGE SQUARE .. U+2B1C WHITE LARGE SQUARE
U+2B50 WHITE MEDIUM STAR
U+2B55 HEAVY LARGE CIRCLE
U+A960 HANGUL CHOSEONG TIKEUT-MIEUM .. U+A97C HANGUL CHOSEONG SSANGYEORINHIEUH
U+16FE0 TANGUT ITERATION MARK .. U+16FE4 KHITAN SMALL SCRIPT FILLER
U+16FF0 VIETNAMESE ALTERNATE READING MARK CA .. U+16FF1 VIETNAMESE ALTERNATE READING MARK NHAY
U+17000 TANGUT IDEOGRAPH-# .. U+187F7 TANGUT IDEOGRAPH-#
U+18800 TANGUT COMPONENT-001 .. U+18CD5 KHITAN SMALL SCRIPT CHARACTER-#
U+18D00 TANGUT IDEOGRAPH-# .. U+18D08 TANGUT IDEOGRAPH-#
U+1AFF0 KATAKANA LETTER MINNAN TONE-2 .. U+1AFF3 KATAKANA LETTER MINNAN TONE-5
U+1AFF5 KATAKANA LETTER MINNAN TONE-7 .. U+1AFFB KATAKANA LETTER MINNAN NASALIZED TONE-5
U+1AFFD KATAKANA LETTER MINNAN NASALIZED TONE-7 .. U+1AFFE KATAKANA LETTER MINNAN NASALIZED TONE-8
U+1B000 KATAKANA LETTER ARCHAIC E .. U+1B122 KATAKANA LETTER ARCHAIC WU
U+1B132 HIRAGANA LETTER SMALL KO
U+1B150 HIRAGANA LETTER SMALL WI .. U+1B152 HIRAGANA LETTER SMALL WO
U+1B155 KATAKANA LETTER SMALL KO
U+1B164 KATAKANA LETTER SMALL WI .. U+1B167 KATAKANA LETTER SMALL N
U+1B170 NUSHU CHARACTER-# .. U+1B2FB NUSHU CHARACTER-#
U+1F004 MAHJONG TILE RED DRAGON
U+1F0CF PLAYING CARD BLACK JOKER
U+1F18E NEGATIVE SQUARED AB
U+1F191 SQUARED CL .. U+1F19A SQUARED VS
U+1F200 SQUARE HIRAGANA HOKA .. U+1F202 SQUARED KATAKANA SA
U+1F210 SQUARED CJK UNIFIED IDEOGRAPH-624B .. U+1F23B SQUARED CJK UNIFIED IDEOGRAPH-914D
U+1F240 TORTOISE SHELL BRACKETED CJK UNIFIED IDEOGRAPH-672C .. U+1F248 TORTOISE SHELL BRACKETED CJK UNIFIED IDEOGRAPH-6557
U+1F250 CIRCLED IDEOGRAPH ADVANTAGE .. U+1F251 CIRCLED IDEOGRAPH ACCEPT
U+1F260 ROUNDED SYMBOL FOR FU .. U+1F265 ROUNDED SYMBOL FOR CAI
U+1F680 ROCKET .. U+1F6C5 LEFT LUGGAGE
U+1F6CC SLEEPING ACCOMMODATION
U+1F6D0 PLACE OF WORSHIP .. U+1F6D2 SHOPPING TROLLEY
U+1F6D5 HINDU TEMPLE .. U+1F6D7 ELEVATOR
U+1F6DC WIRELESS .. U+1F6DF RING BUOY
U+1F6EB AIRPLANE DEPARTURE .. U+1F6EC AIRPLANE ARRIVING
U+1F6F4 SCOOTER .. U+1F6FC ROLLER SKATE
U+1F7E0 LARGE ORANGE CIRCLE .. U+1F7EB LARGE BROWN SQUARE
U+1F7F0 HEAVY EQUALS SIGN
U+1FA70 BALLET SHOES .. U+1FA7C CRUTCH
U+1FA80 YO-YO .. U+1FA88 FLUTE
U+1FA90 RINGED PLANET .. U+1FABD WING
U+1FABF GOOSE .. U+1FAC5 PERSON WITH CROWN
U+1FACE MOOSE .. U+1FADB PEA POD
U+1FAE0 MELTING FACE .. U+1FAE8 SHAKING FACE
U+1FAF0 HAND WITH INDEX FINGER AND THUMB CROSSED .. U+1FAF8 RIGHTWARDS PUSHING HAND
For the following code points, the estimated width used to be 2, and is 1 after the suggested change:
U+2E9A
U+2EF4 .. U+2EFF
U+2FD6 .. U+2FEF
U+2FFC .. U+2FFF
U+3040
U+3097 .. U+3098
U+3100 .. U+3104
U+3130
U+318F
U+31E4 .. U+31EF
U+321F
U+A48D .. U+A48F
U+A4C7 .. U+A4CF
U+FE53
U+FE67
U+FE6C .. U+FE6F
U+FF00
U+3248 CIRCLED NUMBER TEN ON BLACK SQUARE .. U+324F CIRCLED NUMBER EIGHTY ON BLACK SQUARE
[2022-10-12; Reflector poll]
Set priority to 3 after reflector poll. Send to SG16.
Previous resolution [SUPERSEDED]:
This wording is relative to N4917.
Modify 28.5.2.2 [format.string.std] as indicated:
-12- For a string in a Unicode encoding, implementations should estimate the width of a string as the sum of estimated widths of the first code points in its extended grapheme clusters. The extended grapheme clusters of a string are defined by UAX #29. The estimated width of the following code points is 2:
(12.1) — U+1100 – U+115F
(12.2) — U+2329 – U+232A
(12.3) — U+2E80 – U+303E
(12.4) — U+3040 – U+A4CF
(12.5) — U+AC00 – U+D7A3
(12.6) — U+F900 – U+FAFF
(12.7) — U+FE10 – U+FE19
(12.8) — U+FE30 – U+FE6F
(12.9) — U+FF00 – U+FF60
(12.10) — U+FFE0 – U+FFE6
(12.11) — U+1F300 – U+1F64F
(12.12) — U+1F900 – U+1F9FF
(12.13) — U+20000 – U+2FFFD
(12.14) — U+30000 – U+3FFFD(?.1) — Any code point with the
East_Asian_Width="W"
orEast_Asian_Width="F"
Derived Extracted Property as described by UAX #44(?.2) — U+4DC0 – U+4DFF (Yijing Hexagram Symbols)
(?.3) — U+1F300 – U+1F5FF (Miscellaneous Symbols and Pictographs)
(?.4) — U+1F900 – U+1F9FF (Supplemental Symbols and Pictographs)
The estimated width of other code points is 1.
[2023-03-22 Resolved by the adoption of P2675R1 in Issaquah. Status changed: SG16 → Resolved.]
Proposed resolution:
cont-key-type
and cont-mapped-type
should be removedSection: 23.6.1 [container.adaptors.general] Status: C++23 Submitter: Hewill Kang Opened: 2022-09-16 Last modified: 2023-11-22
Priority: Not Prioritized
View all issues with C++23 status.
Discussion:
P0429R9 removed the flat_map
's single-range argument constructor and uses C++23-compatible
from_range_t
version constructor with alias templates range-key-type
and
range-mapped-type
to simplify the deduction guide.
cont-key-type
and cont-mapped-type
no longer useful, we should remove them.
[2022-10-12; Reflector poll]
Set status to Tentatively Ready after seven votes in favour during reflector poll.
[2022-11-12 Approved at November 2022 meeting in Kona. Status changed: Voting → WP.]
Proposed resolution:
This wording is relative to N4917.
Modify 23.6.1 [container.adaptors.general] as indicated:
-7- The exposition-only alias template
iter-value-type
defined in 23.3.1 [sequences.general] and the exposition-only alias templatesiter-key-type
anditer-mapped-type
defined in 23.4.1 [associative.general] may appear in deduction guides for container adaptors.
-8- The following exposition-only alias templates may appear in deduction guides for container adaptors:template<class Container> using cont-key-type = // exposition only remove_const_t<typename Container::value_type::first_type>; template<class Container> using cont-mapped-type = // exposition only typename Container::value_type::second_type;
<math.h>
declare ::lerp
?Section: 17.15.7 [support.c.headers.other] Status: C++23 Submitter: Jiang An Opened: 2022-09-17 Last modified: 2023-11-22
Priority: Not Prioritized
View other active issues in [support.c.headers.other].
View all other issues in [support.c.headers.other].
View all issues with C++23 status.
Discussion:
According to 17.15.7 [support.c.headers.other]/1, <math.h>
is required to
provide ::lerp
, despite that it is a C++-only component. In P0811R3, neither
<math.h>
nor C compatibility is mentioned, so it seems unintended not to list
lerp
as an exception in 17.15.7 [support.c.headers.other].
::lerp
in
<math.h>
but not in <cmath>
, while MSVC STL and libc++ don't provide
::lerp
in <math.h>
or <cmath>
at all.
I'm not sure whether this should be considered together with LWG 3484(i).
Since nullptr_t
has become a part of the C standard library as of C23
(see WG14-N3042 and
WG14-N3048),
I believe we should keep providing ::nullptr_t
in <stddef.h>
.
[2022-10-12; Reflector poll]
Set status to Tentatively Ready after six votes in favour during reflector poll.
[2022-11-12 Approved at November 2022 meeting in Kona. Status changed: Voting → WP.]
Proposed resolution:
This wording is relative to N4917.
Modify 17.15.7 [support.c.headers.other] as indicated:
-1- Every C header other than
<complex.h>
(17.15.2 [complex.h.syn]),<iso646.h>
(17.15.3 [iso646.h.syn]),<stdalign.h>
(17.15.4 [stdalign.h.syn]),<stdatomic.h>
(32.5.12 [stdatomic.h.syn]),<stdbool.h>
(17.15.5 [stdbool.h.syn]), and<tgmath.h>
(17.15.6 [tgmath.h.syn]), each of which has a name of the form<name.h>
, behaves as if each name placed in the standard library namespace by the corresponding<cname>
header is placed within the global namespace scope, except for the functions described in 29.7.6 [sf.cmath], thestd::lerp
function overloads (29.7.4 [c.math.lerp]), the declaration ofstd::byte
(17.2.1 [cstddef.syn]), and the functions and function templates described in 17.2.5 [support.types.byteops]. […]
std.compat
should not provide ::byte
and its friendsSection: 16.4.2.4 [std.modules] Status: C++23 Submitter: Jiang An Opened: 2022-09-19 Last modified: 2023-11-22
Priority: Not Prioritized
View all issues with C++23 status.
Discussion:
Currently 16.4.2.4 [std.modules]/3 seemly requires that the std.compat
module
has to provide byte
(via <cstddef>
), beta
(via <cmath>
)
etc. in the global namespace, which is defective to me as these components are C++-only,
and doing so would increase the risk of conflict.
std.compat
provide the same set of global declarations as
<xxx.h>
headers.
[2022-10-12; Reflector poll]
Set status to Tentatively Ready after nine votes in favour during reflector poll.
[2022-11-12 Approved at November 2022 meeting in Kona. Status changed: Voting → WP.]
Proposed resolution:
This wording is relative to N4917.
Modify 16.4.2.4 [std.modules] as indicated:
-3- The named module
std.compat
exports the same declarations as the named modulestd
, and additionally exports declarations in the global namespace corresponding to the declarations in namespacestd
that are provided by the C++ headers for C library facilities (Table 26), except the explicitly excluded declarations described in 17.15.7 [support.c.headers.other].
ranges::to
is over-constrained on the destination type being a rangeSection: 25.5.7.2 [range.utility.conv.to] Status: C++23 Submitter: Barry Revzin Opened: 2022-09-19 Last modified: 2024-01-29
Priority: Not Prioritized
View other active issues in [range.utility.conv.to].
View all other issues in [range.utility.conv.to].
View all issues with C++23 status.
Discussion:
The current wording in 25.5.7.2 [range.utility.conv.to] starts:
If
convertible_to<range_reference_t<R>, range_value_t<C>>
istrue
and then tries to do C(r, args...)
and then C(from_range, r, args...)
. The problem
is that C
might not be a range — indeed we explicitly removed that requirement from an
earlier revision of the paper — which makes this check ill-formed. One example use-case is using
ranges::to
to convert a range of expected<T, E>
into a
expected<vector<T>, E>
— expected
isn't any kind of range, but it
could support this operation, which is quite useful. Unfortunately, the wording explicitly rejects that.
This change happened between R6 and R7 of the paper and seems to have unintentionally rejected this use-case.
[2022-09-28; Reflector poll]
Set status to Tentatively Ready after five votes in favour during reflector poll.
During telecon review we agreed that supporting non-ranges was an intended
part of the original design, but was inadvertently broken when adding
range_value_t
for other reasons.
[2022-11-12 Approved at November 2022 meeting in Kona. Status changed: Voting → WP.]
Proposed resolution:
This wording is relative to N4917.
Modify 25.5.7.2 [range.utility.conv.to] as indicated:
[Drafting note: We need to be careful that this short-circuits, since if
C
does not satisfyinput_range
, thenrange_value_t<C>
will be ill-formed.]
template<class C, input_range R, class... Args> requires (!view<C>) constexpr C to(R&& r, Args&&... args);-1- Returns: An object of type
C
constructed from the elements ofr
in the following manner:
(1.1) — If
C
does not satisfyinput_range
orconvertible_to<range_reference_t<R>, range_value_t<C>>
istrue
:
(1.1.1) — If
constructible_from<C, R, Args...>
istrue
:C(std::forward<R>(r), std::forward<Args>(args)...)
(1.1.2) — Otherwise, if
constructible_from<C, from_range_t, R, Args...>
istrue
:C(from_range, std::forward<R>(r), std::forward<Args>(args)...)
(1.1.3) — Otherwise, if
(1.1.3.1) —
common_range<R>
istrue
,(1.1.3.2) —
cpp17-input-iterator<iterator_t<R>>
istrue
, and(1.1.3.3) —
constructible_from<C, iterator_t<R>, sentinel_t<R>, Args...>
istrue
:C(ranges::begin(r), ranges::end(r), std::forward<Args>(args)...)
(1.1.4) — Otherwise, if
(1.1.4.1) —
constructible_from<C, Args...>
istrue
, and(1.1.4.2) —
container-insertable<C, range_reference_t<R>>
istrue
:C c(std::forward<Args>(args)...); if constexpr (sized_range<R> && reservable-container<C>) c.reserve(ranges::size(r)); ranges::copy(r, container-inserter<range_reference_t<R>>(c));(1.2) — Otherwise, if
input_range<range_reference_t<R>>
istrue
:to<C>(r | views::transform([](auto&& elem) { return to<range_value_t<C>>(std::forward<decltype(elem)>(elem)); }), std::forward<Args>(args)...);(1.3) — Otherwise, the program is ill-formed.
Allocator
to be usefulSection: 23.6.8.2 [flat.map.defn], 23.6.9.2 [flat.multimap.defn] Status: C++23 Submitter: Johel Ernesto Guerrero Peña Opened: 2022-09-25 Last modified: 2023-11-22
Priority: 2
View all other issues in [flat.map.defn].
View all issues with C++23 status.
Discussion:
This originated from the editorial issue #5800.
P0429R9 added some deduction guides with a non-defaultedAllocator
template parameter and a corresponding function parameter that is defaulted. Since the
template parameter Allocator
is not defaulted, these deduction guides are never used.
[2022-09-28; LWG telecon]
We should not just ignore the allocator, it should be rebound and used for the two container types.
Previous resolution [SUPERSEDED]:
This wording is relative to N4917.
Modify 23.6.8.2 [flat.map.defn], class template
flat_map
synopsis, as indicated:[…] template<ranges::input_range R, class Compare = less<range-key-type<R>>, class Allocator = allocator<void>> flat_map(from_range_t, R&&, Compare = Compare(), Allocator = Allocator()) -> flat_map<range-key-type<R>, range-mapped-type<R>, Compare>; […]Modify 23.6.9.2 [flat.multimap.defn], class template
flat_multimap
synopsis, as indicated:[…] template<ranges::input_range R, class Compare = less<range-key-type<R>>, class Allocator = allocator<void>> flat_multimap(from_range_t, R&&, Compare = Compare(), Allocator = Allocator()) -> flat_multimap<range-key-type<R>, range-mapped-type<R>, Compare>; […]
[2022-10-19; Jonathan provides improved wording]
Previous resolution [SUPERSEDED]:
This wording is relative to N4917.
Modify 23.6.1 [container.adaptors.general] as indicated:
-8- The following exposition-only alias templates may appear in deduction guides for container adaptors:
template<class Container> using cont-key-type = // exposition only remove_const_t<typename Container::value_type::first_type>; template<class Container> using cont-mapped-type = // exposition only typename Container::value_type::second_type; template<class Allocator, class T> using alloc-rebind = // exposition only typename allocator_traits<Allocator>::template rebind_alloc<T>;Modify 23.6.8.2 [flat.map.defn], class template
flat_map
synopsis, as indicated:[…] template<ranges::input_range R, class Compare = less<range-key-type<R>>, class Allocator = allocator<void>> flat_map(from_range_t, R&&, Compare = Compare(), Allocator = Allocator()) -> flat_map<range-key-type<R>, range-mapped-type<R>, Compare, vector<range-key-type<R>, alloc-rebind<Allocator, range-key-type<R>>, vector<range-mapped-type<R>, alloc-rebind<Allocator, range-mapped-type<R>>>; template<ranges::input_range R, class Allocator> flat_map(from_range_t, R&&, Allocator) -> flat_map<range-key-type<R>, range-mapped-type<R>, less<range-key-type<R>>, vector<range-key-type<R>, alloc-rebind<Allocator, range-key-type<R>>, vector<range-mapped-type<R>, alloc-rebind<Allocator, range-mapped-type<R>>>; […]Modify 23.6.9.2 [flat.multimap.defn], class template
flat_multimap
synopsis, as indicated:[…] template<ranges::input_range R, class Compare = less<range-key-type<R>>, class Allocator = allocator<void>> flat_multimap(from_range_t, R&&, Compare = Compare(), Allocator = Allocator()) -> flat_multimap<range-key-type<R>, range-mapped-type<R>, Compare, vector<range-key-type<R>, alloc-rebind<Allocator, range-key-type<R>>, vector<range-mapped-type<R>, alloc-rebind<Allocator, range-mapped-type<R>>>; template<ranges::input_range R, class Allocator> flat_multimap(from_range_t, R&&, Allocator) -> flat_multimap<range-key-type<R>, range-mapped-type<R>, less<range-key-type<R>>, vector<range-key-type<R>, alloc-rebind<Allocator, range-key-type<R>>, vector<range-mapped-type<R>, alloc-rebind<Allocator, range-mapped-type<R>>>; […]Modify 23.6.11.2 [flat.set.defn], class template
flat_set
synopsis, as indicated:[…] template<ranges::input_range R, class Compare = less<ranges::range_value_t<R>>, class Allocator = allocator<ranges::range_value_t<R>>> flat_set(from_range_t, R&&, Compare = Compare(), Allocator = Allocator()) -> flat_set<ranges::range_value_t<R>, Compare, vector<ranges::range_value_t<R>, alloc-rebind<Allocator, ranges::range_value_t<R>>>>; template<ranges::input_range R, class Allocator> flat_set(from_range_t, R&&, Allocator) -> flat_set<ranges::range_value_t<R>, less<ranges::range_value_t<R>>, vector<ranges::range_value_t<R>, alloc-rebind<Allocator, ranges::range_value_t<R>>>>; […]Modify 23.6.12.2 [flat.multiset.defn], class template
flat_multiset
synopsis, as indicated:[…] template<ranges::input_range R, class Compare = less<ranges::range_value_t<R>>, class Allocator = allocator<ranges::range_value_t<R>>> flat_multiset(from_range_t, R&&, Compare = Compare(), Allocator = Allocator()) -> flat_multiset<ranges::range_value_t<R>, Compare, vector<ranges::range_value_t<R>, alloc-rebind<Allocator, ranges::range_value_t<R>>>>; template<ranges::input_range R, class Allocator> flat_multiset(from_range_t, R&&, Allocator) -> flat_multiset<ranges::range_value_t<R>, less<ranges::range_value_t<R>>, vector<ranges::range_value_t<R>, alloc-rebind<Allocator, ranges::range_value_t<R>>>>; […]
[2023-01-11; Jonathan Wakely provides improved wording]
During LWG telecon Tim pointed out that because
allocator<void>
does not meet the Cpp17Allocator
requirements, it might not "qualify as an allocator" and so would cause
the deduction guides to not participate in overload resolution, as per
23.6.1 [container.adaptors.general] p6 (6.4).
Use allocator<byte>
instead.
[Issaquah 2023-02-07; LWG]
Edited proposed resolution to fix missing >
in guides for maps.
Move to Immediate for C++23
[2023-02-13 Approved at February 2023 meeting in Issaquah. Status changed: Immediate → WP.]
Proposed resolution:
This wording is relative to N4917.
Modify 23.6.1 [container.adaptors.general] as indicated:
-8- The following exposition-only alias template may appear in deduction guides for container adaptors:
template<class Allocator, class T> using alloc-rebind = // exposition only typename allocator_traits<Allocator>::template rebind_alloc<T>;
Modify 23.6.8.2 [flat.map.defn], class template flat_map
synopsis, as indicated:
[…] template<ranges::input_range R, class Compare = less<range-key-type<R>>, class Allocator = allocator<byte>> flat_map(from_range_t, R&&, Compare = Compare(), Allocator = Allocator()) -> flat_map<range-key-type<R>, range-mapped-type<R>, Compare, vector<range-key-type<R>, alloc-rebind<Allocator, range-key-type<R>>>, vector<range-mapped-type<R>, alloc-rebind<Allocator, range-mapped-type<R>>>>; template<ranges::input_range R, class Allocator> flat_map(from_range_t, R&&, Allocator) -> flat_map<range-key-type<R>, range-mapped-type<R>, less<range-key-type<R>>, vector<range-key-type<R>, alloc-rebind<Allocator, range-key-type<R>>>, vector<range-mapped-type<R>, alloc-rebind<Allocator, range-mapped-type<R>>>>; […]
Modify 23.6.9.2 [flat.multimap.defn], class template flat_multimap
synopsis, as indicated:
[…] template<ranges::input_range R, class Compare = less<range-key-type<R>>, class Allocator = allocator<byte>> flat_multimap(from_range_t, R&&, Compare = Compare(), Allocator = Allocator()) -> flat_multimap<range-key-type<R>, range-mapped-type<R>, Compare, vector<range-key-type<R>, alloc-rebind<Allocator, range-key-type<R>>>, vector<range-mapped-type<R>, alloc-rebind<Allocator, range-mapped-type<R>>>>; template<ranges::input_range R, class Allocator> flat_multimap(from_range_t, R&&, Allocator) -> flat_multimap<range-key-type<R>, range-mapped-type<R>, less<range-key-type<R>>, vector<range-key-type<R>, alloc-rebind<Allocator, range-key-type<R>>>, vector<range-mapped-type<R>, alloc-rebind<Allocator, range-mapped-type<R>>>>; […]
Modify 23.6.11.2 [flat.set.defn], class template flat_set
synopsis, as indicated:
[…] template<ranges::input_range R, class Compare = less<ranges::range_value_t<R>>, class Allocator = allocator<ranges::range_value_t<R>>> flat_set(from_range_t, R&&, Compare = Compare(), Allocator = Allocator()) -> flat_set<ranges::range_value_t<R>, Compare, vector<ranges::range_value_t<R>, alloc-rebind<Allocator, ranges::range_value_t<R>>>>; template<ranges::input_range R, class Allocator> flat_set(from_range_t, R&&, Allocator) -> flat_set<ranges::range_value_t<R>, less<ranges::range_value_t<R>>, vector<ranges::range_value_t<R>, alloc-rebind<Allocator, ranges::range_value_t<R>>>>; […]
Modify 23.6.12.2 [flat.multiset.defn], class template flat_multiset
synopsis, as indicated:
[…] template<ranges::input_range R, class Compare = less<ranges::range_value_t<R>>, class Allocator = allocator<ranges::range_value_t<R>>> flat_multiset(from_range_t, R&&, Compare = Compare(), Allocator = Allocator()) -> flat_multiset<ranges::range_value_t<R>, Compare, vector<ranges::range_value_t<R>, alloc-rebind<Allocator, ranges::range_value_t<R>>>>; template<ranges::input_range R, class Allocator> flat_multiset(from_range_t, R&&, Allocator) -> flat_multiset<ranges::range_value_t<R>, less<ranges::range_value_t<R>>, vector<ranges::range_value_t<R>, alloc-rebind<Allocator, ranges::range_value_t<R>>>>; […]
ranges::to
's template parameter C
should not be a reference typeSection: 25.5.7.2 [range.utility.conv.to], 25.5.7.3 [range.utility.conv.adaptors] Status: Resolved Submitter: Hewill Kang Opened: 2022-09-25 Last modified: 2024-01-29
Priority: 3
View other active issues in [range.utility.conv.to].
View all other issues in [range.utility.conv.to].
View all issues with Resolved status.
Discussion:
It doesn't make sense for the template parameter C
of ranges::to
be a reference,
which allows us to write something like ranges::to<vector<int>&&>(std::move(v))
or
ranges::to<const vector<int>&&>(v)
, we should forbid this.
C
to be object type to conform to
its description: "The range conversion functions construct an object (usually a container) from a range".
[2022-09-28; Reflector poll]
Set priority to 3 after reflector poll. Some suggestions that it should be Mandates: not Constraints:, but no consensus.
[Issaquah 2023-02-08; LWG]
This would be resolved by LWG 3847(i).
[2023-03-22 LWG 3847 was approved in Issaquah. Status changed: New → Resolved.]
Proposed resolution:
This wording is relative to N4917.
Modify 25.2 [ranges.syn], header <ranges>
synopsis, as indicated:
#include <compare> // see 17.12.1 [compare.syn] #include <initializer_list> // see 17.11.2 [initializer.list.syn] #include <iterator> // see 24.2 [iterator.synopsis] namespace std::ranges { […] // 25.5.7 [range.utility.conv], range conversions template<class C, input_range R, class... Args> requires is_object_v<C> && (!view<C>) constexpr C to(R&& r, Args&&... args); // freestanding template<template<class...> class C, input_range R, class... Args> constexpr auto to(R&& r, Args&&... args); // freestanding template<class C, class... Args> requires is_object_v<C> && (!view<C>) constexpr auto to(Args&&... args); // freestanding template<template<class...> class C, class... Args> constexpr auto to(Args&&... args); // freestanding […] }
Modify 25.5.7.2 [range.utility.conv.to] as indicated:
template<class C, input_range R, class... Args> requires is_object_v<C> && (!view<C>) constexpr C to(R&& r, Args&&... args);-1- Returns: An object of type
[…]C
constructed from the elements ofr
in the following manner:
Modify 25.5.7.3 [range.utility.conv.adaptors] as indicated:
template<class C, class... Args> requires is_object_v<C> && (!view<C>) constexpr auto to(Args&&... args); template<template<class...> class C, class... Args> constexpr auto to(Args&&... args);-1- Returns: A range adaptor closure object (25.7.2 [range.adaptor.object])
[…]f
that is a perfect forwarding call wrapper (22.10.4 [func.require]) with the following properties:
jthread::operator=(jthread&&)
postconditions are unimplementable under self-assignmentSection: 32.4.4.2 [thread.jthread.cons] Status: C++23 Submitter: Nicole Mazzuca Opened: 2022-09-22 Last modified: 2023-11-22
Priority: 3
View all issues with C++23 status.
Discussion:
In the Postconditions element of jthread& jthread::operator=(jthread&&)
(32.4.4.2 [thread.jthread.cons] p13), we have the following:
Postconditions:
x.get_id() == id()
, andget_id()
returns the value ofx.get_id()
prior to the assignment.ssource
has the value ofx.ssource
prior to the assignment andx.ssource.stop_possible()
isfalse
.
Assume j
is a joinable jthread
. Then, j = std::move(j);
results in the following post-conditions:
Let old_id = j.get_id()
(and since j
is joinable, old_id != id()
)
Since x.get_id() == id()
, j.get_id() == id()
Since get_id() == old_id
, j.get_id() == old_id
Thus, id() == j.get_id() == old_id
, and old_id != id()
, which is a contradiction.
One can see that these postconditions are therefore unimplementable.
Two standard libraries – the MSVC STL and libstdc++ – currently implementjthread
.
The MSVC STL chooses to follow the letter of the Effects element, which results in unfortunate behavior.
It first request_stop()
s, then join()
s; then, it assigns the values over. This results in
j.get_id() == id()
– this means that std::swap(j, j)
stops and joins j
.
libstdc++ chooses instead to implement this move assignment operator via the move-swap idiom.
This results in j.get_id() == old_id
, and std::swap(j, j)
is a no-op.
It is the opinion of the issue writer that libstdc++'s choice is the correct one, and should be
taken into the standard.
[2022-09-28; Reflector poll]
Set priority to 3 after reflector poll.
[2022-09-28; Jonathan provides wording]
[2022-09-29; Reflector poll]
Set status to Tentatively Ready after five votes in favour during reflector poll.
[2022-11-12 Approved at November 2022 meeting in Kona. Status changed: Voting → WP.]
Proposed resolution:
This wording is relative to N4917.
Modify 32.4.4.2 [thread.jthread.cons] as indicated:
jthread& operator=(jthread&& x) noexcept;-12- Effects: If
&x == this
istrue
, there are no effects. Otherwise, ifjoinable()
istrue
, callsrequest_stop()
and thenjoin()
. Assigns, then assigns the state ofx
to*this
and setsx
to a default constructed state.-13- Postconditions:
x.get_id() == id()
andget_id()
returns the value ofx.get_id()
prior to the assignment.ssource
has the value ofx.ssource
prior to the assignmentand.x.ssource.stop_possible()
isfalse
-14- Returns:
*this
.
nexttoward
's signatureSection: 29.7.1 [cmath.syn] Status: C++23 Submitter: David Olsen Opened: 2022-09-30 Last modified: 2023-11-22
Priority: 1
View other active issues in [cmath.syn].
View all other issues in [cmath.syn].
View all issues with C++23 status.
Discussion:
P1467 (Extended floating-point types), which was adopted for C++23 at the July plenary,
has a typo (which is entirely my fault) that no one noticed during wording review. The changes to the
<cmath>
synopsis in the paper included changing this:
constexpr float nexttoward(float x, long double y); // see [library.c] constexpr double nexttoward(double x, long double y); constexpr long double nexttoward(long double x, long double y); // see [library.c]
to this:
constexpr floating-point-type nexttoward(floating-point-type x, floating-point-type y);
That changed the second parameter of nexttoward
from always being long double
to being
floating-point-type
, which matches the type of the first parameter.
<cmath>
was to add overloads
of the functions for extended floating-point types, not to change any existing signatures.
[2022-10-10; Reflector poll]
Set priority to 1 after reflector poll. Discussion during prioritization
revolved around whether to delete nexttoward
for new FP types
or just restore the C++20 signatures, which might accept the new types via
implicit conversions (and so return a different type, albeit with the same
representation and same set of values).
"When the first argument to nexttoward
is an extended
floating-point type that doesn't have the same representation as a standard
floating-point type, such as std::float16_t
,
std::bfloat16_t
, or std::float128_t
(on some systems),
the call to nexttoward
is ambiguous and ill-formed,
so the unexpected return type is not an issue.
Going through the extra effort of specifying '= delete' for
nexttoward
overloads that have extended floating-point arguments
is a solution for a problem that doesn't really exist."
Previous resolution [SUPERSEDED]:
This wording is relative to N4917.
Modify 29.7.1 [cmath.syn], header
<cmath>
synopsis, as indicated:[…] constexpr floating-point-type nexttoward(floating-point-type x,floating-point-typelong double y); constexpr float nexttowardf(float x, long double y); constexpr long double nexttowardl(long double x, long double y); […]
[2022-10-04; David Olsen comments and provides improved wording]
C23 specifies variants of most of the functions in <math.h>
for the _FloatN
types
(which are C23's equivalent of C++23's std::floatN_t
types). But it does not specify those variants
for nexttoward
.
nexttoward
's signature unchanged
from C++20. There would be no requirement to provide overloads for extended floating-point types, only for
the standard floating-point types. Instead of explicitly deleting the overloads with extended floating-point
types, we can just never declare them in the first place.
Previous resolution [SUPERSEDED]:
This wording is relative to N4917.
Modify 29.7.1 [cmath.syn], header
<cmath>
synopsis, as indicated:[…] constexpr float nexttoward(float x, long double y); constexpr double nexttoward(double x, long double y); constexpr long double nexttoward(long double x, long double y);constexpr floating-point-type nexttoward(floating-point-type x, floating-point-type y);constexpr float nexttowardf(float x, long double y); constexpr long double nexttowardl(long double x, long double y); […]
[2022-11-12; Tomasz comments and provides improved wording]
During 2022-10-26 LWG telecon we decided that we want to make the calls of the nexttoward
to be ill-formed (equivalent of Mandates) when the first argument is extended floating-point type.
[2023-01-11; Reflector poll]
Set status to Tentatively Ready after seven votes in favour during reflector poll.
[2023-02-13 Approved at February 2023 meeting in Issaquah. Status changed: Voting → WP.]
Proposed resolution:
This wording is relative to N4917.
Modify 29.7.1 [cmath.syn], header <cmath>
synopsis, as indicated:
[…] constexpr floating-point-type nexttoward(floating-point-type x,floating-point-typelong double y); constexpr float nexttowardf(float x, long double y); constexpr long double nexttowardl(long double x, long double y); […]
Add following paragraph at the end of 29.7.1 [cmath.syn], header <cmath>
synopsis:
-?- An invocation of nexttoward
is ill-formed if the argument corresponding to the floating-point-type
parameter has extended floating-point type.
join_view::iterator::operator--
may be ill-formedSection: 25.7.14.3 [range.join.iterator] Status: Resolved Submitter: Hewill Kang Opened: 2022-09-30 Last modified: 2023-03-23
Priority: 3
View all other issues in [range.join.iterator].
View all issues with Resolved status.
Discussion:
Currently, join_view::iterator::operator--
has the following Effects:
if (outer_ == ranges::end(parent_->base_)) inner_ = ranges::end(*--outer_); while (inner_ == ranges::begin(*outer_)) inner_ = ranges::end(*--outer_); --inner_; return *this;
which uses ranges::end(*--outer_)
to get the sentinel of the inner range.
However, *--outer_
may return an rvalue reference to a non-borrowed range,
in which case calling ranges::end
will be ill-formed,
for example:
#include <ranges>
#include <vector>
int main() {
std::vector<std::vector<int>> v;
auto r = v | std::views::transform([](auto& x) -> auto&& { return std::move(x); })
| std::views::join;
auto e = --r.end(); // hard error
}
The proposed resolution uses a temporary reference to bind *--outer_
, so that
ranges::end
is always invoked on an lvalue range, which is consistent with the behavior of
join_with_view::iterator::operator--
.
[2022-10-12; Reflector poll]
Set priority to 3 after reflector poll.
"Could introduce an as_lvalue
lambda (like
auto as_lvalue = []<class T>(T&& x) -> T& { return (T&)x; };
) and use it throughout."
Previous resolution [SUPERSEDED]:
This wording is relative to N4917.
Modify 25.7.14.3 [range.join.iterator] as indicated:
constexpr iterator& operator--() requires ref-is-glvalue && bidirectional_range<Base> && bidirectional_range<range_reference_t<Base>> && common_range<range_reference_t<Base>>;-13- Effects: Equivalent to:
if (outer_ == ranges::end(parent_->base_)) { auto&& inner = *--outer_; inner_ = ranges::end(inner*--outer_); } while (trueinner_ == ranges::begin(*outer_)) { if (auto&& tmp = *outer_; inner_ == ranges::begin(tmp)) { auto&& inner = *--outer_; inner_ = ranges::end(inner*--outer_); } else { break; } } --inner_; return *this;
[2023-03-22 Resolved by the adoption of P2770R0 in Issaquah. Status changed: New → Resolved.]
Proposed resolution:
__cpp_lib_constexpr_algorithms
should also be defined in <utility>
Section: 17.3.2 [version.syn] Status: C++23 Submitter: Marc Mutz Opened: 2022-10-05 Last modified: 2023-11-22
Priority: Not Prioritized
View all other issues in [version.syn].
View all issues with C++23 status.
Discussion:
17.3.2 [version.syn] says that __cpp_lib_constexpr_algorithms
is only defined
in <version>
and <algorithm>
, but the macro applies to std::exchange()
in <utility>
as well (via P0202R3), so it should also be available from
<utility>
.
[2022-10-12; Reflector poll]
Set status to Tentatively Ready after eight votes in favour during reflector poll.
[2022-11-12 Approved at November 2022 meeting in Kona. Status changed: Voting → WP.]
Proposed resolution:
This wording is relative to N4917.
Modify 17.3.2 [version.syn] as indicated (It is suggested to retroactively apply to C++20):
[…] #define __cpp_lib_concepts 202207L // also in <concepts>, <compare> #define __cpp_lib_constexpr_algorithms 201806L // also in <algorithm>, <utility> #define __cpp_lib_constexpr_bitset 202202L // also in <bitset> […]
std::future
and std::shared_future
have unimplementable postconditionsSection: 32.10.7 [futures.unique.future], 32.10.8 [futures.shared.future] Status: C++23 Submitter: Jiang An Opened: 2022-10-19 Last modified: 2023-11-22
Priority: 3
View all other issues in [futures.unique.future].
View all issues with C++23 status.
Discussion:
The move assignment operators of std::future
and std::shared_future
have their postconditions specified as below:
Postconditions:
valid()
returns the same value asrhs.valid()
returned prior to the assignment.
rhs.valid() == false
.
It can be found that when *this
and rhs
is the same object and this->valid()
is true
before the assignment, the postconditions can't be achieved.
[2022-11-01; Reflector poll]
Set priority to 3 after reflector poll.
[2022-11-01; Jonathan provides wording]
[2022-11-07; Reflector poll]
Set status to Tentatively Ready after six votes in favour during reflector poll.
[2022-11-12 Approved at November 2022 meeting in Kona. Status changed: Voting → WP.]
Proposed resolution:
This wording is relative to N4917.
Modify 32.10.7 [futures.unique.future] as indicated:
future& operator=(future&& rhs) noexcept;-11- Effects: If
addressof(rhs) == this
istrue
, there are no effects. Otherwise:
- (11.1) — Releases any shared state (32.10.5 [futures.state]).
- (11.2) — move assigns the contents of
rhs
to*this
.-12- Postconditions:
- (12.1) —
valid()
returns the same value asrhs.valid()
prior to the assignment.- (12.2) — If
addressof(rhs) == this
isfalse
,rhs.valid() == false
.
Modify 32.10.8 [futures.shared.future] as indicated:
shared_future& operator=(shared_future&& rhs) noexcept;-13- Effects: If
addressof(rhs) == this
istrue
, there are no effects. Otherwise:
- (13.1) — Releases any shared state (32.10.5 [futures.state]).
- (13.2) — move assigns the contents of
rhs
to*this
.-14- Postconditions:
- (14.1) —
valid()
returns the same value asrhs.valid()
returned prior to the assignment.- (14.2) — If
addressof(rhs) == this
isfalse
,rhs.valid() == false
.shared_future& operator=(const shared_future& rhs) noexcept;-15- Effects: If
addressof(rhs) == this
istrue
, there are no effects. Otherwise:
- (15.1) — Releases any shared state (32.10.5 [futures.state]).
- (15.2) — assigns the contents of
rhs
to*this
.[Note 3: As a result,
*this
refers to the same shared state asrhs
(if any). — end note]-16- Postconditions:
valid() == rhs.valid()
.
movable-box
as member should use default-initialization instead of copy-initializationSection: 25.6.5.2 [range.repeat.view], 25.7.31.2 [range.chunk.by.view] Status: C++23 Submitter: Hewill Kang Opened: 2022-10-20 Last modified: 2023-11-22
Priority: Not Prioritized
View all other issues in [range.repeat.view].
View all issues with C++23 status.
Discussion:
Currently, the member variable value_
of repeat_view
is initialized with the following expression:
movable-box<W> value_ = W();
which will result in the following unexpected error in libstdc++:
#include <ranges>
int main() {
std::ranges::repeat_view<int> r; // error: could not convert '0' from 'int' to 'std::ranges::__detail::__box<int>'
}
This is because the single-argument constructors of the simple version of movable-box
in libstdc++
are declared as explicit
, which is different from the conditional explicit
declared by the
optional
's constructor, resulting in no visible conversion between those two types.
movable-box
does not provide a single-argument constructor,
so this form of initialization will also produce a hard error.
This may be a bug of existing implementations, but we don't need such "copy-initialization", the default-constructed
movable-box
already value-initializes the underlying type.
[2022-11-01; Reflector poll]
Set status to Tentatively Ready after nine votes in favour during reflector poll.
[2022-11-12 Approved at November 2022 meeting in Kona. Status changed: Voting → WP.]
Proposed resolution:
This wording is relative to N4917.
Modify 25.6.5.2 [range.repeat.view] as indicated:
namespace std::ranges { template<move_constructible W, semiregular Bound = unreachable_sentinel_t> requires (is_object_v<W> && same_as<W, remove_cv_t<W>> && (is-integer-like<Bound> || same_as<Bound, unreachable_sentinel_t>)) class repeat_view : public view_interface<repeat_view<W, Bound>> { private: // 25.6.5.3 [range.repeat.iterator], class repeat_view::iterator struct iterator; // exposition only movable-box<W> value_= W(); // exposition only, see 25.7.3 [range.move.wrap] Bound bound_ = Bound(); // exposition only […] }; […] }
Modify 25.7.31.2 [range.chunk.by.view] as indicated:
namespace std::ranges { template<forward_range V, indirect_binary_predicate<iterator_t<V>, iterator_t<V>> Pred> requires view<V> && is_object_v<Pred> class chunk_by_view : public view_interface<chunk_by_view<V, Pred>> { V base_ = V(); // exposition only movable-box<Pred> pred_= Pred(); // exposition only // 25.7.31.3 [range.chunk.by.iter], class chunk_by_view::iterator class iterator; // exposition only […] }; […] }
iterator_category
Section: 24.3.2.3 [iterator.traits] Status: C++23 Submitter: Jiang An Opened: 2022-10-22 Last modified: 2023-11-22
Priority: Not Prioritized
View all other issues in [iterator.traits].
View all issues with C++23 status.
Discussion:
Since C++11, a forward iterator may have an rvalue reference as its reference
type.
I think this is an intentional change made by N3066. However, several components
added (or changed) in C++20/23 recognize an iterator as a Cpp17ForwardIterator (by using
forward_iterator_tag
or a stronger iterator category tag type as the iterator_category
type) only if the reference
type would be an lvalue reference type, which seems a regression.
[2022-11-01; Reflector poll]
Set status to Tentatively Ready after eight votes in favour during reflector poll.
[2022-11-12 Approved at November 2022 meeting in Kona. Status changed: Voting → WP.]
Proposed resolution:
This wording is relative to N4917.
Modify 24.3.2.3 [iterator.traits] as indicated:
-2- The definitions in this subclause make use of the following exposition-only concepts:
[…] template<class I> concept cpp17-forward-iterator = cpp17-input-iterator<I> && constructible_from<I> && is_lvalue_reference_v<iter_reference_t<I>> && same_as<remove_cvref_t<iter_reference_t<I>>, typename indirectly_readable_traits<I>::value_type> && requires(I i) { { i++ } -> convertible_to<const I&>; { *i++ } -> same_as<iter_reference_t<I>>; }; […]
Modify 25.7.9.3 [range.transform.iterator] as indicated:
-2- The member typedef-name
iterator_category
is defined if and only if Base modelsforward_range
. In that case,iterator::iterator_category
is defined as follows: LetC
denote the typeiterator_traits<iterator_t<Base>>::iterator_category
.
(2.1) — If
is_
islvalue_reference_v<invoke_result_t<maybe-const<Const, F>&, range_reference_t<Base>>>true
, then
(2.1.1) — if
C
modelsderived_from<contiguous_iterator_tag>
,iterator_category denotes random_access_iterator_tag
;(2.1.2) — otherwise,
iterator_category
denotesC
.(2.2) — Otherwise,
iterator_category
denotesinput_iterator_tag
.
Modify 25.7.15.3 [range.join.with.iterator] as indicated:
-2- The member typedef-name
iterator_category
is defined if and only ifref-is-glvalue
istrue
, andBase
andInnerBase
each modelforward_range
. In that case,iterator::iterator_category
is defined as follows:
(2.1) — […]
(2.2) — If
is_lvalue_reference_v<common_reference_t<iter_reference_t<InnerIter>, iter_reference_t<PatternIter>>>is
false
,iterator_category
denotesinput_iterator_tag
.(2.3) — […]
Modify 25.7.26.3 [range.zip.transform.iterator] as indicated:
-1- The member typedef-name
iterator::iterator_category
is defined if and only ifBase
modelsforward_range
. In that case,iterator::iterator_category
is defined as follows:
(1.1) — If
invoke_result_t<maybe-const<Const, F>&, range_reference_t<maybe-const<Const, Views>>...>is not a
n lvaluereference,iterator_category
denotesinput_iterator_tag
.(1.2) — […]
Modify 25.7.28.3 [range.adjacent.transform.iterator] as indicated:
-1- The member typedef-name
iterator::iterator_category
is defined as follows:
(1.1) — If
invoke_result_t<maybe-const<Const, F>&, REPEAT(range_reference_t<Base>, N)...>
is not an lvaluereference,iterator_category
denotesinput_iterator_tag
.(1.2) — […]
cartesian_product_view::iterator::distance-from
ignores the size of last underlying rangeSection: 25.7.33.3 [range.cartesian.iterator] Status: C++23 Submitter: Patrick Palka Opened: 2022-10-25 Last modified: 2023-11-22
Priority: Not Prioritized
View all other issues in [range.cartesian.iterator].
View all issues with C++23 status.
Discussion:
The helper scaled-size(N)
from the wording for P2374R4's
cartesian_product_view::iterator::distance-from
is recursively
specified as:
Let
scaled-size(N)
be the product ofstatic_cast<difference_type>(ranges::size(std::get<N>(parent_->bases_)))
andscaled-size(N + 1)
ifN < sizeof...(Vs)
, otherwisestatic_cast<difference_type>(1)
;
Intuitively, scaled-size(N)
is the size of the cartesian product of all
but the first N
underlying ranges. Thus scaled-size(sizeof...(Vs))
ought to just yield the size of the last underlying range (since there are
1 + sizeof...(Vs)
underlying ranges), but according to this definition it
yields 1
. Similarly at the other extreme, scaled-size(0)
should yield
the product of the sizes of all the underlying ranges, but it instead yields that of all but
the last underlying range.
cartesian_product_view
s of two or more underlying ranges, this
causes the relevant operator-
overloads to compute wrong distances, e.g.
int x[] = {1, 2, 3}; auto v = views::cartesian_product(x, x); auto i = v.begin() + 5; // *i == {2, 3} assert(*i == tuple{2, 3}); assert(i - v.begin() == 5); // fails, expects 3, because: // scaled-sum = scaled-distance(0) + scaled-distance(1) // = ((1 - 0) * scaled-size(1)) + ((2 - 0) * scaled-size(2)) // = 1 + 2 instead of 3 + 2
The recursive condition should probably use <=
instead of <
.
[2022-11-04; Reflector poll]
Set status to Tentatively Ready after seven votes in favour during reflector poll.
[2022-11-12 Approved at November 2022 meeting in Kona. Status changed: Voting → WP.]
Proposed resolution:
This wording is relative to N4917.
Modify [ranges.cartesian.iterator] as indicated:
template<class Tuple> constexpr difference_type distance-from(Tuple t);-7- Let:
(7.1) —
scaled-size(N)
be the product ofstatic_cast<difference_type>(ranges::size(std::get<N>(parent_->bases_)))
andscaled-size(N + 1)
ifN ≤
, otherwise<sizeof...(Vs)static_cast<difference_type>(1)
;(7.2) —
scaled-distance(N)
be the product ofstatic_cast<difference_type>(std::get<N>(current_) - std::get<N>(t))
andscaled-size(N + 1)
; and(7.3) —
scaled-sum
be the sum ofscaled-distance(N)
for every integer0 ≤ N ≤ sizeof...(Vs)
.
flat_foo
constructors taking KeyContainer
lack KeyCompare
parameterSection: 23.6.8 [flat.map], 23.6.9 [flat.multimap], 23.6.11 [flat.set], 23.6.12 [flat.multiset] Status: C++23 Submitter: Arthur O'Dwyer Opened: 2022-10-25 Last modified: 2023-11-22
Priority: 1
View other active issues in [flat.map].
View all other issues in [flat.map].
View all issues with C++23 status.
Discussion:
flat_set
's current constructor overload set has these two overloads:
explicit flat_set(container_type cont); template<class Allocator> flat_set(const container_type& cont, const Allocator& a);
I believe it should have these two in addition:
flat_set(const key_compare& comp, container_type cont); template<class Allocator> flat_set(const key_compare& comp, const container_type& cont, const Allocator& a);
with corresponding deduction guides. Similar wording changes would have to be made to all the
flat_foo
containers.
struct LessWhenDividedBy { int divisor_; bool operator()(int x, int y) const { return x/divisor_ < y/divisor_; } }; std::flat_set<int, LessWhenDividedBy> s(data.begin(), data.end(), LessWhenDividedBy(10)); // BEFORE AND AFTER: okay, but cumbersome std::flat_set<int, LessWhenDividedBy> s(data); // BEFORE AND AFTER: oops, this default-constructs the comparator std::flat_set<int, LessWhenDividedBy> s(LessWhenDividedBy(10), data); // BEFORE: fails to compile // AFTER: compiles successfully
[2022-11-04; Reflector poll]
Set priority to 1 after reflector poll.
[2023-02-09 Tim adds wording]
For each construction that takes containers, this wording allow a comparator to be specified as well.
Differing from the suggestion in the issue, the comparator goes last (but before the allocator),
for consistency with every other constructor of flat_meow
taking a comparator.
(This is inconsistent with priority_queue
, but consistency among the type's own
constructors seems more important.)
[Issaquah 2023-02-09; LWG]
Move to Immediate for C++23
[2023-02-13 Approved at February 2023 meeting in Issaquah. Status changed: Immediate → WP.]
Proposed resolution:
This wording is relative to N4928.
Add a new bullet to 23.6.1 [container.adaptors.general] p6 as indicated:
-6- A deduction guide for a container adaptor shall not participate in overload resolution if any of the following are true:
— (6.?) It has both
KeyContainer
andCompare
template parameters, andis_invocable_v<const Compare&, const typename KeyContainer::value_type&, const typename KeyContainer::value_type&>
is not a valid expression or isfalse
.
Modify 23.6.8.2 [flat.map.defn] as indicated:
namespace std { template<class Key, class T, class Compare = less<Key>, class KeyContainer = vector<Key>, class MappedContainer = vector<T>> class flat_map { public: […] // 23.6.8.3 [flat.map.cons], construct/copy/destroy flat_map() : flat_map(key_compare()) { } flat_map(key_container_type key_cont, mapped_container_type mapped_cont, const key_compare& comp = key_compare()); template<class Allocator> flat_map(const key_container_type& key_cont, const mapped_container_type& mapped_cont, const Allocator& a); template<class Allocator> flat_map(const key_container_type& key_cont, const mapped_container_type& mapped_cont, const key_compare& comp, const Allocator& a); flat_map(sorted_unique_t, key_container_type key_cont, mapped_container_type mapped_cont, const key_compare& comp = key_compare()); template<class Allocator> flat_map(sorted_unique_t, const key_container_type& key_cont, const mapped_container_type& mapped_cont, const Allocator& a); template<class Allocator> flat_map(sorted_unique_t, const key_container_type& key_cont, const mapped_container_type& mapped_cont, const key_compare& comp, const Allocator& a); […] }; template<class KeyContainer, class MappedContainer, class Compare = less<typename KeyContainer::value_type>> flat_map(KeyContainer, MappedContainer, Compare = Compare()) -> flat_map<typename KeyContainer::value_type, typename MappedContainer::value_type, Compareless<typename KeyContainer::value_type>, KeyContainer, MappedContainer>; template<class KeyContainer, class MappedContainer, class Allocator> flat_map(KeyContainer, MappedContainer, Allocator) -> flat_map<typename KeyContainer::value_type, typename MappedContainer::value_type, less<typename KeyContainer::value_type>, KeyContainer, MappedContainer>; template<class KeyContainer, class MappedContainer, class Compare, class Allocator> flat_map(KeyContainer, MappedContainer, Compare, Allocator) -> flat_map<typename KeyContainer::value_type, typename MappedContainer::value_type, Compare, KeyContainer, MappedContainer>; template<class KeyContainer, class MappedContainer, class Compare = less<typename KeyContainer::value_type>> flat_map(sorted_unique_t, KeyContainer, MappedContainer, Compare = Compare()) -> flat_map<typename KeyContainer::value_type, typename MappedContainer::value_type, Compareless<typename KeyContainer::value_type>, KeyContainer, MappedContainer>; template<class KeyContainer, class MappedContainer, class Allocator> flat_map(sorted_unique_t, KeyContainer, MappedContainer, Allocator) -> flat_map<typename KeyContainer::value_type, typename MappedContainer::value_type, less<typename KeyContainer::value_type>, KeyContainer, MappedContainer>; template<class KeyContainer, class MappedContainer, class Compare, class Allocator> flat_map(sorted_unique_t, KeyContainer, MappedContainer, Compare, Allocator) -> flat_map<typename KeyContainer::value_type, typename MappedContainer::value_type, Compare, KeyContainer, MappedContainer>; […] }
Modify 23.6.8.3 [flat.map.cons] as indicated:
flat_map(key_container_type key_cont, mapped_container_type mapped_cont, const key_compare& comp = key_compare());-1- Effects: Initializes
c.keys
withstd::move(key_cont)
,andc.values
withstd::move(mapped_cont)
, andcompare
withcomp
; value-initializes; sorts the rangecompare
[begin(), end())
with respect tovalue_comp()
; and finally erases the duplicate elements as if by:auto zv = ranges::zip_view(c.keys, c.values); auto it = ranges::unique(zv, key_equiv(compare)).begin(); auto dist = distance(zv.begin(), it); c.keys.erase(c.keys.begin() + dist, c.keys.end()); c.values.erase(c.values.begin() + dist, c.values.end());-2- Complexity: Linear in N if the container arguments are already sorted with respect to
value_comp()
and otherwise N log N, where N is the value ofkey_cont.size()
before this call.template<class Allocator> flat_map(const key_container_type& key_cont, const mapped_container_type& mapped_cont, const Allocator& a); template<class Allocator> flat_map(const key_container_type& key_cont, const mapped_container_type& mapped_cont, const key_compare& comp, const Allocator& a);-3- Constraints:
-4- Effects: Equivalent touses_allocator_v<key_container_type, Allocator>
istrue
anduses_allocator_v<mapped_container_type, Allocator>
istrue
.flat_map(key_cont, mapped_cont)
andflat_map(key_cont, mapped_cont, comp)
, respectively, except thatc.keys
andc.values
are constructed with uses-allocator construction (20.2.8.2 [allocator.uses.construction]). -5- Complexity: Same asflat_map(key_cont, mapped_cont)
andflat_map(key_cont, mapped_cont, comp)
, respectively.flat_map(sorted_unique_t, key_container_type key_cont, mapped_container_type mapped_cont, const key_compare& comp = key_compare());-6- Effects: Initializes
-7- Complexity: Constant.c.keys
withstd::move(key_cont)
,andc.values
withstd::move(mapped_cont)
, andcompare
withcomp
; value-initializes.compare
template<class Allocator> flat_map(sorted_unique_t s, const key_container_type& key_cont, const mapped_container_type& mapped_cont, const Allocator& a); template<class Allocator> flat_map(sorted_unique_t s, const key_container_type& key_cont, const mapped_container_type& mapped_cont, const key_compare& comp, const Allocator& a);-8- Constraints:
-9- Effects: Equivalent touses_allocator_v<key_container_type, Allocator>
istrue
anduses_allocator_v<mapped_container_type, Allocator>
istrue
.flat_map(s, key_cont, mapped_cont)
andflat_map(s, key_cont, mapped_cont, comp)
, respectively, except thatc.keys
andc.values
are constructed with uses-allocator construction (20.2.8.2 [allocator.uses.construction]). -10- Complexity: Linear.
Modify 23.6.9.2 [flat.multimap.defn] as indicated:
namespace std { template<class Key, class T, class Compare = less<Key>, class KeyContainer = vector<Key>, class MappedContainer = vector<T>> class flat_multimap { public: […] // 23.6.9.3 [flat.multimap.cons], construct/copy/destroy flat_multimap() : flat_multimap(key_compare()) { } flat_multimap(key_container_type key_cont, mapped_container_type mapped_cont, const key_compare& comp = key_compare()); template<class Allocator> flat_multimap(const key_container_type& key_cont, const mapped_container_type& mapped_cont, const Allocator& a); template<class Allocator> flat_multimap(const key_container_type& key_cont, const mapped_container_type& mapped_cont, const key_compare& comp, const Allocator& a); flat_multimap(sorted_equivalent_t, key_container_type key_cont, mapped_container_type mapped_cont, const key_compare& comp = key_compare()); template<class Allocator> flat_multimap(sorted_equivalent_t, const key_container_type& key_cont, const mapped_container_type& mapped_cont, const Allocator& a); template<class Allocator> flat_multimap(sorted_equivalent_t, const key_container_type& key_cont, const mapped_container_type& mapped_cont, const key_compare& comp, const Allocator& a); […] }; template<class KeyContainer, class MappedContainer, class Compare = less<typename KeyContainer::value_type>> flat_multimap(KeyContainer, MappedContainer, Compare = Compare()) -> flat_multimap<typename KeyContainer::value_type, typename MappedContainer::value_type, Compareless<typename KeyContainer::value_type>, KeyContainer, MappedContainer>; template<class KeyContainer, class MappedContainer, class Allocator> flat_multimap(KeyContainer, MappedContainer, Allocator) -> flat_multimap<typename KeyContainer::value_type, typename MappedContainer::value_type, less<typename KeyContainer::value_type>, KeyContainer, MappedContainer>; template<class KeyContainer, class MappedContainer, class Compare, class Allocator> flat_multimap(KeyContainer, MappedContainer, Compare, Allocator) -> flat_multimap<typename KeyContainer::value_type, typename MappedContainer::value_type, Compare, KeyContainer, MappedContainer>; template<class KeyContainer, class MappedContainer, class Compare = less<typename KeyContainer::value_type>> flat_multimap(sorted_equivalent_t, KeyContainer, MappedContainer, Compare = Compare()) -> flat_multimap<typename KeyContainer::value_type, typename MappedContainer::value_type, Compareless<typename KeyContainer::value_type>, KeyContainer, MappedContainer>; template<class KeyContainer, class MappedContainer, class Allocator> flat_multimap(sorted_equivalent_t, KeyContainer, MappedContainer, Allocator) -> flat_multimap<typename KeyContainer::value_type, typename MappedContainer::value_type, less<typename KeyContainer::value_type>, KeyContainer, MappedContainer>; template<class KeyContainer, class MappedContainer, class Compare, class Allocator> flat_multimap(sorted_equivalent_t, KeyContainer, MappedContainer, Compare, Allocator) -> flat_multimap<typename KeyContainer::value_type, typename MappedContainer::value_type, Compare, KeyContainer, MappedContainer>; […] }
Modify 23.6.9.3 [flat.multimap.cons] as indicated:
flat_multimap(key_container_type key_cont, mapped_container_type mapped_cont, const key_compare& comp = key_compare());-1- Effects: Initializes
c.keys
withstd::move(key_cont)
,andc.values
withstd::move(mapped_cont)
, andcompare
withcomp
; value-initializes; and sorts the rangecompare
[begin(), end())
with respect tovalue_comp()
.-2- Complexity: Linear in N if the container arguments are already sorted with respect to
value_comp()
and otherwise N log N, where N is the value ofkey_cont.size()
before this call.template<class Allocator> flat_multimap(const key_container_type& key_cont, const mapped_container_type& mapped_cont, const Allocator& a); template<class Allocator> flat_multimap(const key_container_type& key_cont, const mapped_container_type& mapped_cont, const key_compare& comp, const Allocator& a);-3- Constraints:
-4- Effects: Equivalent touses_allocator_v<key_container_type, Allocator>
istrue
anduses_allocator_v<mapped_container_type, Allocator>
istrue
.flat_multimap(key_cont, mapped_cont)
andflat_multimap(key_cont, mapped_cont, comp)
, respectively, except thatc.keys
andc.values
are constructed with uses-allocator construction (20.2.8.2 [allocator.uses.construction]). -5- Complexity: Same asflat_multimap(key_cont, mapped_cont)
andflat_multimap(key_cont, mapped_cont, comp)
, respectively.flat_multimap(sorted_equivalent_t, key_container_type key_cont, mapped_container_type mapped_cont, const key_compare& comp = key_compare());-6- Effects: Initializes
-7- Complexity: Constant.c.keys
withstd::move(key_cont)
,andc.values
withstd::move(mapped_cont)
, andcompare
withcomp
; value-initializes.compare
template<class Allocator> flat_multimap(sorted_equivalent_t s, const key_container_type& key_cont, const mapped_container_type& mapped_cont, const Allocator& a); template<class Allocator> flat_multimap(sorted_equivalent_t s, const key_container_type& key_cont, const mapped_container_type& mapped_cont, const key_compare& comp, const Allocator& a);-8- Constraints:
-9- Effects: Equivalent touses_allocator_v<key_container_type, Allocator>
istrue
anduses_allocator_v<mapped_container_type, Allocator>
istrue
.flat_multimap(s, key_cont, mapped_cont)
andflat_multimap(s, key_cont, mapped_cont, comp)
, respectively, except thatc.keys
andc.values
are constructed with uses-allocator construction (20.2.8.2 [allocator.uses.construction]). -10- Complexity: Linear.
Modify 23.6.11.2 [flat.set.defn] as indicated:
namespace std { template<class Key, class Compare = less<Key>, class KeyContainer = vector<Key>> class flat_set { public: […] // [flat.set.cons], constructors flat_set() : flat_set(key_compare()) { } explicit flat_set(container_type cont, const key_compare& comp = key_compare()); template<class Allocator> flat_set(const container_type& cont, const Allocator& a); template<class Allocator> flat_set(const container_type& cont, const key_compare& comp, const Allocator& a); flat_set(sorted_unique_t, container_type cont, const key_compare& comp = key_compare()) : c(std::move(cont)), compare(compkey_compare()) { } template<class Allocator> flat_set(sorted_unique_t, const container_type& cont, const Allocator& a); template<class Allocator> flat_set(sorted_unique_t, const container_type& cont, const key_compare& comp, const Allocator& a); […] }; template<class KeyContainer, class Compare = less<typename KeyContainer::value_type>> flat_set(KeyContainer, Compare = Compare()) -> flat_set<typename KeyContainer::value_type, Compare, KeyContainer>; template<class KeyContainer, class Allocator> flat_set(KeyContainer, Allocator) -> flat_set<typename KeyContainer::value_type, less<typename KeyContainer::value_type>, KeyContainer>; template<class KeyContainer, class Compare, class Allocator> flat_set(KeyContainer, Compare, Allocator) -> flat_set<typename KeyContainer::value_type, Compare, KeyContainer>; template<class KeyContainer, class Compare = less<typename KeyContainer::value_type>> flat_set(sorted_unique_t, KeyContainer, Compare = Compare()) -> flat_set<typename KeyContainer::value_type, Compare, KeyContainer>; template<class KeyContainer, class Allocator> flat_set(sorted_unique_t, KeyContainer, Allocator) -> flat_set<typename KeyContainer::value_type, less<typename KeyContainer::value_type>, KeyContainer>; template<class KeyContainer, class Compare, class Allocator> flat_set(sorted_unique_t, KeyContainer, Compare, Allocator) -> flat_set<typename KeyContainer::value_type, Compare, KeyContainer>; […] }
Modify 23.6.11.3 [flat.set.cons] as indicated:
flat_set(container_type cont, const key_compare& comp = key_compare());-1- Effects: Initializes
c
withstd::move(cont)
andcompare
withcomp
, value-initializes, sorts the rangecompare
[begin(), end())
with respect tocompare
, and finally erases all but the first element from each group of consecutive equivalent elements.-2- Complexity: Linear in N if
cont
is already sorted with respect tocompare
and otherwise N log N, where N is the value ofcont.size()
before this call.template<class Allocator> flat_set(const container_type& cont, const Allocator& a); template<class Allocator> flat_set(const container_type& cont, const key_compare& comp, const Allocator& a);-3- Constraints:
-4- Effects: Equivalent touses_allocator_v<container_type, Allocator>
istrue
.flat_set(cont)
andflat_set(cont, comp)
, respectively, except thatc
is constructed with uses-allocator construction (20.2.8.2 [allocator.uses.construction]). -5- Complexity: Same asflat_set(cont)
andflat_set(cont, comp)
, respectively.template<class Allocator> flat_set(sorted_unique_t s, const container_type& cont, const Allocator& a); template<class Allocator> flat_set(sorted_unique_t s, const container_type& cont, const key_compare& comp, const Allocator& a);-6- Constraints:
-7- Effects: Equivalent touses_allocator_v<container_type, Allocator>
istrue
.flat_set(s, cont)
andflat_set(s, cont, comp)
, respectively, except thatc
is constructed with uses-allocator construction (20.2.8.2 [allocator.uses.construction]). -8- Complexity: Linear.
Modify 23.6.12.2 [flat.multiset.defn] as indicated:
namespace std { template<class Key, class Compare = less<Key>, class KeyContainer = vector<Key>> class flat_multiset { public: […] // [flat.multiset.cons], constructors flat_multiset() : flat_multiset(key_compare()) { } explicit flat_multiset(container_type cont, const key_compare& comp = key_compare()); template<class Allocator> flat_multiset(const container_type& cont, const Allocator& a); template<class Allocator> flat_multiset(const container_type& cont, const key_compare& comp, const Allocator& a); flat_multiset(sorted_equivalent_t, container_type cont, const key_compare& comp = key_compare()) : c(std::move(cont)), compare(compkey_compare()) { } template<class Allocator> flat_multiset(sorted_equivalent_t, const container_type& cont, const Allocator& a); template<class Allocator> flat_multiset(sorted_equivalent_t, const container_type& cont, const key_compare& comp, const Allocator& a); […] }; template<class KeyContainer, class Compare = less<typename KeyContainer::value_type>> flat_multiset(KeyContainer, Compare = Compare()) -> flat_multiset<typename KeyContainer::value_type, Compare, KeyContainer>; template<class KeyContainer, class Allocator> flat_multiset(KeyContainer, Allocator) -> flat_multiset<typename KeyContainer::value_type, less<typename KeyContainer::value_type>, KeyContainer>; template<class KeyContainer, class Compare, class Allocator> flat_multiset(KeyContainer, Compare, Allocator) -> flat_multiset<typename KeyContainer::value_type, Compare, KeyContainer>; template<class KeyContainer, class Compare = less<typename KeyContainer::value_type>> flat_multiset(sorted_equivalent_t, KeyContainer, Compare = Compare()) -> flat_multiset<typename KeyContainer::value_type, Compare, KeyContainer>; template<class KeyContainer, class Allocator> flat_multiset(sorted_equivalent_t, KeyContainer, Allocator) -> flat_multiset<typename KeyContainer::value_type, less<typename KeyContainer::value_type>, KeyContainer>; template<class KeyContainer, class Compare, class Allocator> flat_multiset(sorted_equivalent_t, KeyContainer, Compare, Allocator) -> flat_multiset<typename KeyContainer::value_type, Compare, KeyContainer>; […] }
Modify 23.6.12.3 [flat.multiset.cons] as indicated:
flat_multiset(container_type cont, const key_compare& comp = key_compare());-1- Effects: Initializes
c
withstd::move(cont)
andcompare
withcomp
, value-initializes, and sorts the rangecompare
[begin(), end())
with respect tocompare
.-2- Complexity: Linear in N if
cont
is already sorted with respect tocompare
and otherwise N log N, where N is the value ofcont.size()
before this call.template<class Allocator> flat_multiset(const container_type& cont, const Allocator& a); template<class Allocator> flat_multiset(const container_type& cont, const key_compare& comp, const Allocator& a);-3- Constraints:
-4- Effects: Equivalent touses_allocator_v<container_type, Allocator>
istrue
.flat_multiset(cont)
andflat_multiset(cont, comp)
, respectively, except thatc
is constructed with uses-allocator construction (20.2.8.2 [allocator.uses.construction]). -5- Complexity: Same asflat_multiset(cont)
andflat_multiset(cont, comp)
, respectively.template<class Allocator> flat_multiset(sorted_equivalent_t s, const container_type& cont, const Allocator& a); template<class Allocator> flat_multiset(sorted_equivalent_t s, const container_type& cont, const key_compare& comp, const Allocator& a);-6- Constraints:
-7- Effects: Equivalent touses_allocator_v<container_type, Allocator>
istrue
.flat_multiset(s, cont)
andflat_multiset(s, cont, comp)
, respectively, except thatc
is constructed with uses-allocator construction (20.2.8.2 [allocator.uses.construction]). -8- Complexity: Linear.
ranges::find_last
should be renamedSection: 17.3.2 [version.syn] Status: C++23 Submitter: Daniel Marshall Opened: 2022-11-02 Last modified: 2023-11-22
Priority: Not Prioritized
View all other issues in [version.syn].
View all issues with C++23 status.
Discussion:
The current feature test macro is __cpp_lib_find_last
which is inconsistent with almost all
other ranges algorithms which are in the form __cpp_lib_ranges_xxx
.
__cpp_lib_ranges_find_last
.
[Kona 2022-11-12; Move to Ready]
[2023-02-13 Approved at February 2023 meeting in Issaquah. Status changed: Voting → WP.]
Proposed resolution:
This wording is relative to N4917.
Modify 17.3.2 [version.syn], header <version>
synopsis, as indicated:
[…] #define __cpp_lib_filesystem 201703L // also in <filesystem> #define __cpp_lib_ranges_find_last 202207L // also in <algorithm> #define __cpp_lib_flat_map 202207L // also in <flat_map> […]
std::subtract_with_carry_engine<uint16_t>
supposed to work?Section: 29.5.4.4 [rand.eng.sub] Status: WP Submitter: Jonathan Wakely Opened: 2022-11-02 Last modified: 2023-11-22
Priority: 3
View all other issues in [rand.eng.sub].
View all issues with WP status.
Discussion:
The standard requires subtract_with_carry_engine<T>
to use:
linear_congruential_engine<T, 40014u, 0u, 2147483563u>
where each of those values is converted to T
.
subtract_with_carry_engine
cannot be used with uint16_t
, because
2147483563u
cannot be converted to uint16_t
without narrowing.
What is the intention here? Should it be ill-formed? Should the seed engine be
linear_congruential_engine<uint_least32_t, …>
instead? The values from the
linear_congruential_engine
are used modulo 2^32 so getting 64-bit values from it is pointless,
and getting 16-bit values from it doesn't compile.
[Kona 2022-11-12; Set priority to 3]
[Kona 2022-11-25; Jonathan provides wording]
[2023-05; reflector poll]
Status to Tentatively Ready after six votes in favour.
[2023-11-11 Approved at November 2023 meeting in Kona. Status changed: Voting → WP.]
Proposed resolution:
This wording is relative to N4917.
Modify the class synopsis in 29.5.4.4 [rand.eng.sub] as indicated:
namespace std { template<class UIntType, size_t w, size_t s, size_t r> class subtract_with_carry_engine { public: // types using result_type = UIntType; // engine characteristics static constexpr size_t word_size = w; static constexpr size_t short_lag = s; static constexpr size_t long_lag = r; static constexpr result_type min() { return 0; } static constexpr result_type max() { return m − 1; } static constexprresult_typeuint_least32_t default_seed = 19780503u; // constructors and seeding functions subtract_with_carry_engine() : subtract_with_carry_engine(default_seed0u) {} explicit subtract_with_carry_engine(result_type value); template<class Sseq> explicit subtract_with_carry_engine(Sseq& q); void seed(result_type value =default_seed0u); template<class Sseq> void seed(Sseq& q);
Modify 29.5.4.4 [rand.eng.sub] p7 as indicated:
explicit subtract_with_carry_engine(result_type value);-7- Effects: Sets the values of , in that order, as specified below. If is then , sets to ; otherwise sets to .
To set the values , first construct
e
, alinear_congruential_engine
object, as if by the following definition:linear_congruential_engine<result_typeuint_least32_t, 40014u,0u,2147483563u> e(value == 0u ? default_seed : value);Then, to set each , obtain new values from successive invocations of
e
. Set to .
std::basic_format_args
Section: 28.5.8.3 [format.args] Status: C++23 Submitter: Jonathan Wakely Opened: 2022-11-03 Last modified: 2023-11-22
Priority: 3
View all other issues in [format.args].
View all issues with C++23 status.
Discussion:
It seems desirable for this should work:
auto args_store = std::make_format_args<C>(1,2,3); // … std::basic_format_args args = args_store;
i.e. CTAD should deduce the Context
argument from the fmt-store-args<C, int, int, int>
object returned by make_format_args
.
template<typename Context> void foo(basic_format_args<Context> c); foo(make_format_args<SomeContext>(…)); // won't work foo(basic_format_args(make_format_args<SomeContext>(…))); // should work
Since fmt-store-args
is exposition-only, it's not entirely clear that it must have
exactly the form shown in 28.5.8.2 [format.arg.store]. E.g. maybe it can have different template arguments,
or could be a nested type defined inside basic_format_args
. I don't know how much of the exposition-only
spec is actually required for conformance. If CTAD is already intended to be required, it's a bit subtle.
[Kona 2022-11-12; Set priority to 3, status to LEWG]
[2023-01-10; LEWG telecon]
Unanimous consensus in favor.
[Issaquah 2023-02-06; LWG]
Unanimous consent (9/0/0) to move to Immediate for C++23.
[2023-02-13 Approved at February 2023 meeting in Issaquah. Status changed: Immediate → WP.]
Proposed resolution:
This wording is relative to N4917.
Modify 28.5.8.3 [format.args] as indicated:
namespace std { template<class Context> class basic_format_args { size_t size_; // exposition only const basic_format_arg<Context>* data_; // exposition only public: basic_format_args() noexcept; template<class... Args> basic_format_args(const format-arg-store<Context, Args...>& store) noexcept; basic_format_arg<Context> get(size_t i) const noexcept; }; template<class Context, class... Args> basic_format_args(format-arg-store<Context, Args...>) -> basic_format_args<Context>; }
views::as_const
on ref_view<T>
should return ref_view<const T>
Section: 25.7.22.1 [range.as.const.overview] Status: C++23 Submitter: Tomasz Kamiński Opened: 2022-11-03 Last modified: 2023-11-22
Priority: Not Prioritized
View all other issues in [range.as.const.overview].
View all issues with C++23 status.
Discussion:
For v
being a non-const lvalue of type std::vector<int>
, views::as_const(v)
produces ref_view<std::vector<int> const>
. However, when v
is converted to
ref_view
by using views::all
, views::as_const(views::all(v))
produces
as_const_view<ref_view<std::vector<int>>>
.
views::as_const
on ref_view<T>
should produce ref_view<const T>
when
const T
models a constant range. This will reduce the number of instantiations, and make a behavior
of views::as_const
consistent on references and ref_view
to containers.
[Kona 2022-11-08; Move to Ready]
[2023-02-13 Approved at February 2023 meeting in Issaquah. Status changed: Voting → WP.]
Proposed resolution:
This wording is relative to N4917.
Modify 25.7.22.1 [range.as.const.overview] as indicated:
[Drafting note: If we have
ref_view<V>
, whenV
is constant propagating view (single_view
,owning_view
), we still can (and should) produceref_view<V const>
. This wording achieves that.]
-2- The name
views::as_const
denotes a range adaptor object (25.7.2 [range.adaptor.object]). LetE
be an expression, letT
bedecltype((E))
, and letU
beremove_cvref_t<T>
. The expressionviews::as_const(E)
is expression-equivalent to:
(2.1) — If
views::all_t<T>
modelsconstant_range
, thenviews::all(E)
.(2.2) — Otherwise, if
U
denotesspan<X, Extent>
for some typeX
and some extentExtent
, thenspan<const X, Extent>(E)
.(2.?) — Otherwise, if
U
denotesref_view<X>
for some typeX
andconst X
modelsconstant_range
, thenref_view(static_cast<const X&>(E.base()))
.(2.3) — Otherwise, if
E
is an lvalue,const U
modelsconstant_range
, andU
does not modelview
, thenref_view(static_cast<const U&>(E))
.(2.4) — Otherwise,
as_const_view(E)
.
Section: 20.2.2 [memory.syn], 25.2 [ranges.syn], 32.5.2 [atomics.syn] Status: C++23 Submitter: Ben Craig Opened: 2022-11-06 Last modified: 2023-11-22
Priority: Not Prioritized
View all other issues in [memory.syn].
View all issues with C++23 status.
Discussion:
This addresses the following NB comments:
GB-085 (20.2.2 [memory.syn] start_lifetime_as
and start_lifetime_as_array
should be freestanding)
GB-110 (25.2 [ranges.syn] New views should be freestanding (repeat
, stride
, cartesian_product
))
(partial) GB-130 (32.5.2 [atomics.syn] memory_order_acquire
etc should be freestanding)
The explicit lifetime management functions requested by GB-085 have not been reviewed by LEWG in the context of freestanding, but they seem non-controversial in that context. None of the requested lifetime management functions run any code. I believe these were missed in post-merge conflict searches because the papers weren't targeted to LEWG or LWG at the time of those searches.
The ranges facilities requested by GB-110 have been reviewed on the LEWG mailing list in the context of freestanding. P1642R11 mentions therepeat
, stride
, and cartesian_product
papers in "Potential Post-LEWG
merge conflicts". All were discussed in an April 2022 reflector discussion and received six votes in favor of allowing these papers
into freestanding, with no opposition.
The atomics facilities requested by GB-130 are essentially new names for existing facilities. Marking these as freestanding isn't
concerning. There are concerns in GB-130 dealing with the specification details of freestanding enums, but those concerns won't be
addressed in this issue.
[Kona 2022-11-07; Move to Immediate]
[2022-11-12 Approved at November 2022 meeting in Kona. Status changed: Immediate → WP.]
Proposed resolution:
This wording is relative to N4917.
Modify 20.2.2 [memory.syn], header <memory>
synopsis, as indicated:
[…] // 20.2.6 [obj.lifetime], explicit lifetime management template<class T> T* start_lifetime_as(void* p) noexcept; // freestanding template<class T> const T* start_lifetime_as(const void* p) noexcept; // freestanding template<class T> volatile T* start_lifetime_as(volatile void* p) noexcept; // freestanding template<class T> const volatile T* start_lifetime_as(const volatile void* p) noexcept; // freestanding template<class T> T* start_lifetime_as_array(void* p, size_t n) noexcept; // freestanding template<class T> const T* start_lifetime_as_array(const void* p, size_t n) noexcept; // freestanding template<class T> volatile T* start_lifetime_as_array(volatile void* p, size_t n) noexcept; // freestanding template<class T> const volatile T* start_lifetime_as_array(const volatile void* p, size_t n) noexcept; // freestanding […]
Modify 25.2 [ranges.syn], header <ranges>
synopsis, as indicated:
[…] // 25.6.5 [range.repeat], repeat view template<move_constructible W, semiregular Bound = unreachable_sentinel_t> requires (is_object_v<W> && same_as<W, remove_cv_t<W>> && (is-integer-like<Bound> || same_as<Bound, unreachable_sentinel_t>)) class repeat_view; // freestanding namespace views { inline constexpr unspecified repeat = unspecified; } // freestanding […] // 25.7.32 [range.stride], stride view template<input_range V> requires view<V> class stride_view; // freestanding template<class V> inline constexpr bool enable_borrowed_range<stride_view<V>> = enable_borrowed_range<V>; // freestanding namespace views { inline constexpr unspecified stride = unspecified; } // freestanding // 25.7.33 [range.cartesian], cartesian product view template<input_range First, forward_range... Vs> requires (view<First> && ... && view<Vs>) class cartesian_product_view; // freestanding namespace views { inline constexpr unspecified cartesian_product = unspecified; } // freestanding […]
Modify 32.5.2 [atomics.syn], header <atomic>
synopsis, as indicated:
namespace std { // 32.5.4 [atomics.order], order and consistency enum class memory_order : unspecified; // freestanding inline constexpr memory_order memory_order_relaxed = memory_order::relaxed; // freestanding inline constexpr memory_order memory_order_consume = memory_order::consume; // freestanding inline constexpr memory_order memory_order_acquire = memory_order::acquire; // freestanding inline constexpr memory_order memory_order_release = memory_order::release; // freestanding inline constexpr memory_order memory_order_acq_rel = memory_order::acq_rel; // freestanding inline constexpr memory_order memory_order_seq_cst = memory_order::seq_cst; // freestanding […] }
Section: 16.3.3.7 [freestanding.item], 32.5.2 [atomics.syn] Status: Resolved Submitter: Ben Craig Opened: 2022-11-06 Last modified: 2023-02-07
Priority: Not Prioritized
View all other issues in [freestanding.item].
View all issues with Resolved status.
Discussion:
This is a partial resolution of GB-130
(32.5.2 [atomics.syn] memory_order_acquire
etc should be freestanding).
std::memory_order
enum type are also freestanding,
as those enumerators aren't shown in the synopsis, only in 32.5.4 [atomics.order].
Previous resolution [SUPERSEDED]:
This wording is relative to N4917 assuming that LWG 3753(i) has been accepted (freestanding entity -> freestanding item).
[Drafting Note: Four mutually exclusive options are prepared, depicted below by Option A, Option B, Option C, and Option D, respectively.]
Option A: Not a defect; no change required. Enumerators of freestanding enums are already part of freestanding with the current wording. [freestanding.entity]#2 says the requirements of freestanding entities are the same as for hosted entities. The existence of the right enumerators (with the appropriate name scoping) are part of the requirements of the enum type.
Option B: Not a defect; provide a clarifying note. Same rationale as above, but we make this stance clear in [freestanding.entity]#2.
Modify [freestanding.entity] (Which has been renamed to [freestanding.item]) as indicated:
16.3.3.6 Freestanding items [freestanding.item]
[…] -2- Unless otherwise specified, the requirements on freestanding items on a freestanding implementation are the same as the corresponding requirements in a hosted implementation. [Note: An enumerator places requirements on its enumeration. Freestanding item enumerations have the same enumerators on freestanding implementations and hosted implementations. — end note] […]Option C: Add additional normative wording that makes enumerators freestanding if their enumeration types are freestanding.
Modify [freestanding.entity] (Which has been renamed to [freestanding.item]) as indicated:
16.3.3.6 Freestanding items [freestanding.item]
-1- A freestanding item is an entity or macro definition that is present in a freestanding implementation and a hosted implementation. -?- An enumerator of a freestanding item enumeration is a freestanding item. […]Option D: This is Option C, plus it handles class types as well. If enumerators aren't automatically included with the existing wording, then arguably, neither are class type members.
Modify [freestanding.entity] (Which has been renamed to [freestanding.item]) as indicated:
16.3.3.6 Freestanding items [freestanding.item]
-1- A freestanding item is an entity or macro definition that is present in a freestanding implementation and a hosted implementation. -?- An enumerator of a freestanding item enumeration is a freestanding item. -?- Members of freestanding item class types are freestanding items. […]
[2022-11-08; Kona - move to open]
This will be resolved by the combined resolution in 3753(i).
[2022-11-22 Resolved by 3753(i) accepted in Kona. Status changed: Open → Resolved.]
Proposed resolution:
flat_map
and flat_multimap
should impose sequence container requirementsSection: 23.6.8.1 [flat.map.overview], 23.6.9.1 [flat.multimap.overview] Status: C++23 Submitter: Tomasz Kamiński Opened: 2022-11-08 Last modified: 2023-11-22
Priority: Not Prioritized
View all other issues in [flat.map.overview].
View all issues with C++23 status.
Discussion:
This is resolution of US 42-103 (23.6.8.1 [flat.map.overview] p6 23.6.9.1 [flat.multimap.overview] p6 Clearing when restoring invariants).
Currently both 23.6.8.1 [flat.map.overview] p7 and 23.6.9.1 [flat.multimap.overview] p7
claims that flat_(multi)map
supports "Any sequence container (23.2.4 [sequence.reqmts])
C
supporting Cpp17RandomAccessIterator", which arguably includes std::array
(see LWG 617(i)).
This is incorrect as std::array
does not provide operations required to restored these adaptors invariant,
including clear
. We should require that C
meets sequence container requirements, and we
state that fact explicitly in 23.3.3.1 [array.overview] p3: "An array
meets some of the requirements
of a sequence container (23.2.4 [sequence.reqmts])".
[Kona 2022-11-08; Move to Immediate status]
[2022-11-12 Approved at November 2022 meeting in Kona. Status changed: Immediate → WP.]
Proposed resolution:
This wording is relative to N4917.
Modify 23.6.8.1 [flat.map.overview] as indicated:
-7- Any
sequence container (23.2.4 [sequence.reqmts])typeC
supporting Cpp17RandomAccessIteratorthat meets sequence container requirements (23.2.4 [sequence.reqmts]) can be used to instantiateflat_map
, as long asC::iterator
meets the Cpp17RandomAccessIterator requirements and invocations of member functionsC::size
andC::max_size
do not exit via an exception. In particular,vector
(23.3.13 [vector]) anddeque
(23.3.5 [deque]) can be used.
Modify 23.6.9.1 [flat.multimap.overview] as indicated:
-7- Any
sequence container (23.2.4 [sequence.reqmts])typeC
supporting Cpp17RandomAccessIteratorthat meets sequence container requirements (23.2.4 [sequence.reqmts]) can be used to instantiateflat_multimap
, as long asC::iterator
meets the Cpp17RandomAccessIterator requirements and invocations of member functionsC::size
andC::max_size
do not exit via an exception. In particular,vector
(23.3.13 [vector]) anddeque
(23.3.5 [deque]) can be used.
forward_list
modifiersSection: 23.3.7.5 [forward.list.modifiers] Status: C++23 Submitter: Tomasz Kamiński Opened: 2022-11-08 Last modified: 2023-11-22
Priority: Not Prioritized
View all other issues in [forward.list.modifiers].
View all issues with C++23 status.
Discussion:
This is resolution of GB-101 (23.3.7.5 [forward.list.modifiers] p12,15,20,21 Missing preconditions on forward_list modifiers).
Some of the modifiers to forward_list
are special to that container and accordingly are not described
in 23.2 [container.requirements]. Specifically, insert_after
(iterator overload),
insert_range_after
and emplace_after
do not verify that the value_type
is Cpp17EmplaceConstructible
from the appropriate argument(s).
Furthermore insert_after
(value overloads) are missing Cpp17CopyInsertable/Cpp17MoveInsertable
requirements.
[Kona 2022-11-08; Move to Ready]
[Kona 2022-11-12; Correct status to Immediate]
[2022-11-12 Approved at November 2022 meeting in Kona. Status changed: Immediate → WP.]
Proposed resolution:
This wording is relative to N4917.
Modify 23.3.7.5 [forward.list.modifiers] as indicated:
[Drafting note:
emplace_front
,push_front
, andprepend_range
are already covered by 23.2 [container.requirements]. ]
iterator insert_after(const_iterator position, const T& x);
-?- Preconditions:
T
is Cpp17CopyInsertable intoforward_list
.position
isbefore_begin()
or is a dereferenceable iterator in the range [begin()
,end()
).-?- Effects: Inserts a copy of
x
afterposition
.-?- Returns: An iterator pointing to the copy of
x
.
iterator insert_after(const_iterator position, T&& x);
-6- Preconditions:
T
is Cpp17MoveInsertable intoforward_list
.position
isbefore_begin()
or is a dereferenceable iterator in the range [begin()
,end()
).-7- Effects: Inserts a copy of
x
afterposition
.-8- Returns: An iterator pointing to the copy of
x
.
iterator insert_after(const_iterator position, size_type n, const T& x);
-9- Preconditions:
T
is Cpp17CopyInsertable intoforward_list
.position
isbefore_begin()
or is a dereferenceable iterator in the range [begin()
,end()
).-10- Effects: Inserts
n
copies ofx
afterposition
.-11- Returns: An iterator pointing to the last inserted copy of
x
, orposition
ifn == 0
istrue
.
template<class InputIterator> iterator insert_after(const_iterator position, InputIterator first, InputIterator last);
-12- Preconditions:
T
is Cpp17EmplaceConstructible intoforward_list
from*first
.position
isbefore_begin()
or is a dereferenceable iterator in the range [begin()
,end()
). Neitherfirst
norlast
are iterators in*this
.-13- Effects: Inserts copies of elements in [
first
,last
) afterposition
.-14- Returns: An iterator pointing to the last inserted element, or
position
iffirst == last
istrue
.
template<container-compatible-range<T> R> iterator insert_after(const_iterator position, R&& rg);
-15- Preconditions:
T
is Cpp17EmplaceConstructible intoforward_list
from*ranges::begin(rg)
.position
isbefore_begin()
or is a dereferenceable iterator in the range [begin()
,end()
).rg
and*this
do not overlap.-16- Effects: Inserts copies of elements in range
rg
afterposition
.-17- Returns: An iterator pointing to the last inserted element, or
position
ifrg
is empty.
iterator insert_after(const_iterator position, initializer_list<T> il);
-18- Effects: Equivalent to:
return
insert_after(position, il.begin(), il.end())
.;
-19- Returns: An iterator pointing to the last inserted element orposition
ifil
is empty.
template<class... Args> iterator emplace_after(const_iterator position, Args&&... args);
-20- Preconditions:
T
is Cpp17EmplaceConstructible intoforward_list
fromstd::forward<Args>(args)...
.position
isbefore_begin()
or is a dereferenceable iterator in the range [begin()
,end()
).-21- Effects: Inserts an object of type
value_type
constructeddirect-non-list-initialized withafter
value_type(std::forward<Args>(args)...)position
.-22- Returns: An iterator pointing to the new object.
Section: 16.3.3 [conventions] Status: C++23 Submitter: Tim Song Opened: 2022-11-08 Last modified: 2023-11-22
Priority: Not Prioritized
View all issues with C++23 status.
Discussion:
This is the resolution for GB-074.
The comment is:
[expos.only.func] introduces exposition-only function templates. [expos.only.types] introduces exposition-only types. 16.3.3.6 [objects.within.classes] introduces exposition-only private members.
There is nothing about exposition-only concepts, despite them being used extensively in the library clauses. The same problem exists for exposition-only variable templates.
[Kona 2022-11-08; Move to Immediate status]
Previous resolution [SUPERSEDED]:
Modify [expos.only.func] as indicated, changing the stable name:
16.3.3.2 Exposition-only
-1- Severalfunctionsentities [expos.only.funcentity]function templatesentities defined in 17 [support] through 32 [thread] and D [depr] are only defined for the purpose of exposition. The declaration of sucha functionan entity is followed by a comment ending in exposition only.Strike [expos.only.types] as redundant:
16.3.3.3.2 Exposition-only types [expos.only.types]-1- Several types defined in 17 [support] through 32 [thread] and D [depr] are defined for the purpose of exposition. The declaration of such a type is followed by a comment ending in exposition only.[Example 1:namespace std { extern "C" using some-handler = int(int, void*, double); // exposition only }
The type placeholdersome-handler
can now be used to specify a function that takes a callback parameter with C language linkage. — end example]
[2022-11-09 Tim reopens]
During LWG review of 3753(i), it was pointed out that typedef-names are not necessarily entities.
[Kona 2022-11-11; Move to Immediate]
[2022-11-12 Approved at November 2022 meeting in Kona. Status changed: Immediate → WP.]
Proposed resolution:
This wording is relative to N4917.
Modify [expos.only.func] as indicated, changing the stable name:
16.3.3.2 Exposition-only
-1- Severalfunctionsentities, etc. [expos.only.funcentity]function templatesentities and typedef-names defined in 17 [support] through 32 [thread] and D [depr] are only defined for the purpose of exposition. The declaration of sucha functionan entity or typedef-name is followed by a comment ending in exposition only.
Strike [expos.only.types] as redundant:
16.3.3.3.2 Exposition-only types [expos.only.types]-1- Several types defined in 17 [support] through 32 [thread] and D [depr] are defined for the purpose of exposition. The declaration of such a type is followed by a comment ending in exposition only.[Example 1:namespace std { extern "C" using some-handler = int(int, void*, double); // exposition only }
The type placeholdersome-handler
can now be used to specify a function that takes a callback parameter with C language linkage. — end example]
reference_meows_from_temporary
should not use is_meowible
Section: 21.3.5.4 [meta.unary.prop] Status: C++23 Submitter: Tim Song Opened: 2022-11-08 Last modified: 2023-11-22
Priority: Not Prioritized
View other active issues in [meta.unary.prop].
View all other issues in [meta.unary.prop].
View all issues with C++23 status.
Discussion:
The intent of P2255R2 is for the reference_meows_from_temporary
traits
to fully support cases where a prvalue is used as the source. Unfortunately the wording fails
to do so because it tries to use the is_meowible
traits to say
"the initialization is well-formed", but those traits only consider initialization from xvalues,
not prvalues. For example, given:
struct U { U(); U(U&&) = delete; }; struct T { T(U); };
reference_constructs_from_temporary_v<const T&, U>
should be true, but is currently defined as false
.
We need to spell out the "is well-formed" condition directly.
[Kona 2022-11-08; Move to Tentatively Ready]
[2023-02-13 Approved at February 2023 meeting in Issaquah. Status changed: Voting → WP.]
Proposed resolution:
This wording is relative to N4917.
[Drafting note: The note is already repeated every time we talk about "immediate context".]
Modify 21.3.3 [meta.type.synop], Table 46 ([tab:meta.unary.prop]) — "Type property predicates" — as indicated:
Table 46: Type property predicates [tab:meta.unary.prop] Template Condition Preconditions …
template<class T, class U>
struct reference_constructs_from_temporary;conjunction_v<is_reference<T>, is_constructible<T, U>>
istrue
T
is a reference type, and the initializationT t(VAL<U>);
is well-formed and bindst
to a temporary object whose lifetime is extended (6.7.7 [class.temporary]). Access checking is performed as if in a context unrelated toT
andU
. Only the validity of the immediate context of the variable initialization is considered. [Note ?: The initialization can result in effects such as the instantiation of class template specializations and function template specializations, the generation of implicitly-defined functions, and so on. Such effects are not in the "immediate context" and can result in the program being ill-formed. — end note]T
andU
shall be complete types, cvvoid
, or arrays of unknown bound.template<class T, class U>
struct reference_converts_from_temporary;conjunction_v<is_reference<T>, is_convertible<U, T>>
istrue
T
is a reference type, and the initializationT t = VAL<U>;
is well-formed and bindst
to a temporary object whose lifetime is extended (6.7.7 [class.temporary]). Access checking is performed as if in a context unrelated toT
andU
. Only the validity of the immediate context of the variable initialization is considered. [Note ?: The initialization can result in effects such as the instantiation of class template specializations and function template specializations, the generation of implicitly-defined functions, and so on. Such effects are not in the "immediate context" and can result in the program being ill-formed. — end note]T
andU
shall be complete types, cvvoid
, or arrays of unknown bound.
cartesian_product_view::iterator::prev
is not quite rightSection: 25.7.33.3 [range.cartesian.iterator] Status: C++23 Submitter: Hewill Kang Opened: 2022-11-08 Last modified: 2023-11-22
Priority: Not Prioritized
View all other issues in [range.cartesian.iterator].
View all issues with C++23 status.
Discussion:
Currently, cartesian_product_view::iterator::prev
has the following Effects:
auto& it = std::get<N>(current_); if (it == ranges::begin(std::get<N>(parent_->bases_))) { it = cartesian-common-arg-end(std::get<N>(parent_->bases_)); if constexpr (N > 0) { prev<N - 1>(); } } --it;
which decrements the underlying iterator one by one using recursion.
However, when N == 0
, it still detects if the first iterator has reached the beginning and assigns it to
the end, which is not only unnecessary, but also causes cartesian-common-arg-end
to be applied to
the first range, making it ill-formed in some cases, for example:
#include <ranges>
int main() {
auto r = std::views::cartesian_product(std::views::iota(0));
r.begin() += 3; // hard error
}
This is because, for the first range, cartesian_product_view::iterator::operator+=
only requires
it to model random_access_range
.
However, when x
is negative, this function will call prev
and indirectly calls
cartesian-common-arg-end
, since the latter constrains its argument to satisfy
cartesian-product-common-arg
, that is, common_range<R> || (sized_range<R> &&
random_access_range<R>)
, which is not the case for the unbounded iota_view
, resulting in a
hard error in prev
's function body.
if constexpr
so that we just decrement the first
iterator and nothing else.
[Kona 2022-11-08; Move to Ready]
[2023-02-13 Approved at February 2023 meeting in Issaquah. Status changed: Voting → WP.]
Proposed resolution:
This wording is relative to N4917.
Modify [ranges.cartesian.iterator] as indicated:
template<size_t N = sizeof...(Vs)> constexpr void prev();-6- Effects: Equivalent to:
auto& it = std::get<N>(current_); if constexpr (N > 0) { if (it == ranges::begin(std::get<N>(parent_->bases_))) { it = cartesian-common-arg-end(std::get<N>(parent_->bases_));if constexpr (N > 0) {prev<N - 1>();}} } --it;
uses_allocator_construction_args
should have overload for pair-like
Section: 20.2.8.2 [allocator.uses.construction] Status: C++23 Submitter: Tim Song Opened: 2022-11-08 Last modified: 2023-11-22
Priority: 2
View all other issues in [allocator.uses.construction].
View all issues with C++23 status.
Discussion:
P2165R4 added a pair-like
constructor to
std::pair
but didn't add a corresponding uses_allocator_construction_args
overload.
It was in P2165R3 but incorrectly removed during the small group review.
pair-like
into a
pmr::vector<pair>
to be outright ill-formed.
With that issue's resolution, in cases where the constructor is not explicit we would create a temporary pair
and then do uses-allocator construction using its pieces, and it still won't work when the constructor is explicit.
We should just do this properly.
[2022-11-09 Tim updates wording following LWG review]
During review of this issue LWG noticed that neither the constructor nor the new overload should accept subrange
.
remove_cv_t
in the new paragraph is added for consistency with LWG 3677(i).
[Kona 2022-11-12; Set priority to 2]
[2023-01-11; LWG telecon]
Replace P
with U
in p17 and
set status to Tentatively Ready (poll result: 8/0/0).
[2023-02-13 Approved at February 2023 meeting in Issaquah. Status changed: Voting → WP.]
Proposed resolution:
This wording is relative to N4917 after the application of LWG 3677(i).
Edit 22.3.2 [pairs.pair] as indicated:
template<class U1, class U2> constexpr explicit(see below) pair(pair<U1, U2>& p); template<class U1, class U2> constexpr explicit(see below) pair(const pair<U1, U2>& p); template<class U1, class U2> constexpr explicit(see below) pair(pair<U1, U2>&& p); template<class U1, class U2> constexpr explicit(see below) pair(const pair<U1, U2>&& p); template<pair-like P> constexpr explicit(see below) pair(P&& p);-14- Let
-15- Constraints:FWD(u)
bestatic_cast<decltype(u)>(u)
.
(15.?) — For the last overload,
remove_cvref_t<P>
is not a specialization ofranges::subrange
,(15.1) —
is_constructible_v<T1, decltype(get<0>(FWD(p)))>
istrue
and(15.2) —
is_constructible_v<T2, decltype(get<1>(FWD(p)))>
istrue
.-16- Effects: Initializes
first
withget<0>(FWD(p))
andsecond
withget<1>(FWD(p))
.
Edit 20.2.2 [memory.syn], header <memory>
synopsis, as indicated:
namespace std { […] // 20.2.8.2 [allocator.uses.construction], uses-allocator construction […] template<class T, class Alloc, class U, class V> constexpr auto uses_allocator_construction_args(const Alloc& alloc, pair<U, V>& pr) noexcept; template<class T, class Alloc, class U, class V> constexpr auto uses_allocator_construction_args(const Alloc& alloc, const pair<U, V>& pr) noexcept; template<class T, class Alloc, class U, class V> constexpr auto uses_allocator_construction_args(const Alloc& alloc, pair<U, V>&& pr) noexcept; template<class T, class Alloc, class U, class V> constexpr auto uses_allocator_construction_args(const Alloc& alloc, const pair<U, V>&& pr) noexcept; template<class T, class Alloc, pair-like P> constexpr auto uses_allocator_construction_args(const Alloc& alloc, P&& p) noexcept; template<class T, class Alloc, class U> constexpr auto uses_allocator_construction_args(const Alloc& alloc, U&& u) noexcept; […] }
Add the following to 20.2.8.2 [allocator.uses.construction]:
template<class T, class Alloc, pair-like P> constexpr auto uses_allocator_construction_args(const Alloc& alloc, P&& p) noexcept;-?- Constraints:
-?- Effects: Equivalent to:remove_cv_t<T>
is a specialization ofpair
andremove_cvref_t<P>
is not a specialization ofranges::subrange
.return uses_allocator_construction_args<T>(alloc, piecewise_construct, forward_as_tuple(get<0>(std::forward<P>(p))), forward_as_tuple(get<1>(std::forward<P>(p))));
Edit 20.2.8.2 [allocator.uses.construction] p17:
template<class T, class Alloc, class U> constexpr auto uses_allocator_construction_args(const Alloc& alloc, U&& u) noexcept;-16- Let
FUN
be the function template:template<class A, class B> void FUN(const pair<A, B>&);-17- Constraints:
remove_cv_t<T>
is a specialization ofpair
, and either:
(17.1) —
remove_cvref_t<U>
is a specialization ofranges::subrange
, or(17.2) —
U
does not satisfypair-like
and the expressionFUN(u)
is not well-formed when considered as an unevaluated operand..
filesystem::weakly_canonical
Section: 31.12.13.40 [fs.op.weakly.canonical] Status: C++23 Submitter: US Opened: 2022-11-08 Last modified: 2023-11-22
Priority: Not Prioritized
View all other issues in [fs.op.weakly.canonical].
View all issues with C++23 status.
Discussion:
This addresses NB comment US-60-125 (31.12.13.40 [fs.op.weakly.canonical] Avoiding normalization)
NB comment: "Implementations cannot avoid normalization because arbitrary file system changes may have occurred since any previous call. Proposed change: Remove the paragraph."
[Kona 2022-11-07; LWG review]
Discussion revolved around two different interpretations of the Remarks:
path
objects to cache some kind of
flag that indicates they are in a normalized form, which would be checked in
weakly_canonical
to avoid normalizing again. This is the interpretation
assumed by the NB comment, which correctly notes that such caching would be
unreliable.
"/a/b/c"
by incrementally building up canonicalized
components, if the entire path exists then the result will already have been
normalized and should not be normalized again explicitly.
For the first interpretation, the recommendation is a bad recommendation and should be removed as suggested by the comment. For the second interpretation, we don't need to give hints to implementors about not doing unnecessary work; they already know they shouldn't do that. Either way, it should go.
[Kona 2022-11-09; Move to Immediate]
[2022-11-12 Approved at November 2022 meeting in Kona. Status changed: Immediate → WP.]
Proposed resolution:
This wording is relative to N4917.
Modify 31.12.13.40 [fs.op.weakly.canonical] as indicated:
path filesystem::weakly_canonical(const path& p); path filesystem::weakly_canonical(const path& p, error_code& ec);-1- Effects: Using
status(p)
orstatus(p, ec)
, respectively, to determine existence, return a path composed byoperator/=
from the result of callingcanonical()
with a path argument composed of the leading elements ofp
that exist, if any, followed by the elements ofp
that do not exist, if any. For the first form,canonical()
is called without anerror_code
argument. For the second form,canonical()
is called withec
as anerror_code
argument, andpath()
is returned at the first error occurrence, if any.-2- Postconditions: The returned path is in normal form (31.12.6.2 [fs.path.generic]).
-3- Returns:
p
with symlinks resolved and the result normalized (31.12.6.2 [fs.path.generic]).-4- Throws: As specified in 31.12.5 [fs.err.report].
-5- Remarks: Implementations should avoid unnecessary normalization such as whencanonical
has already been called on the entirety ofp
.
is_aggregate
Section: 21.3.5.4 [meta.unary.prop] Status: C++23 Submitter: Tomasz Kamiński Opened: 2022-11-09 Last modified: 2023-11-22
Priority: Not Prioritized
View other active issues in [meta.unary.prop].
View all other issues in [meta.unary.prop].
View all issues with C++23 status.
Discussion:
This is resolution of GB-090
(21.3.5.4 [meta.unary.prop] Unnecessary precondition for is_aggregate
).
The precondition for is_aggregate
is "remove_all_extents_t<T>
shall be a complete type
or cv void
." This means that is_aggregate_v<Incomplete[2]>
is undefined, but
an array is always an aggregate, we don't need a complete element type to know that.
Historically the is_aggregate
was introduced by LWG 2911(i) as part of the resolution of the NB comments.
The comment proposed to introduce this trait with requirement "remove_all_extents_t<T>
shall
be a complete type, an array type, or (possibly cv-qualified) void
.",
that is close to resolution proposed in this issue.
According to notes this was simplified during review, after realizing that remove_all_extents_t<T>
is never an array type.
[Kona 2022-11-09; Move to Immediate]
[2022-11-12 Approved at November 2022 meeting in Kona. Status changed: Immediate → WP.]
Proposed resolution:
This wording is relative to N4917.
Modify 21.3.3 [meta.type.synop], Table 46 ([tab:meta.unary.prop]) — "Type property predicates" — as indicated:
Table 48: Type property predicates [tab:meta.unary.prop] Template Condition Preconditions …
template<class T, class U>
struct is_aggregate;T
is an aggregate type (9.5.2 [dcl.init.aggr])shall be an array type, a complete type, or cv
remove_all_extents_t<T>void
.
bind
placeholders is underspecifiedSection: 22.10.15.5 [func.bind.place] Status: C++23 Submitter: Tomasz Kamiński Opened: 2022-11-09 Last modified: 2023-11-22
Priority: Not Prioritized
View all other issues in [func.bind.place].
View all issues with C++23 status.
Discussion:
This is resolution of GB-95
(22.10.15.5 [func.bind.place] Number of bind
placeholders is underspecified).
22.10.2 [functional.syn] and 22.10.15.5 [func.bind.place] both contain a comment that says "M is the implementation-defined number of placeholders". There is no wording anywhere to say that there is any such number, only that comment.
[Kona 2022-11-09; Move to Immediate]
[2022-11-12 Approved at November 2022 meeting in Kona. Status changed: Immediate → WP.]
Proposed resolution:
This wording is relative to N4917.
Modify 22.10.15.5 [func.bind.place] as indicated:
namespace std::placeholders { // M is theimplementation-definednumber of placeholders see below _1; see below _2; . . . see below _M; }-?- The number
M
of placeholders is implementation-defined.-1- All placeholder types meet the Cpp17DefaultConstructible and Cpp17CopyConstructible requirements, and their default constructors and copy/move constructors are constexpr functions that do not throw exceptions. It is implementation-defined whether placeholder types meet the Cpp17CopyAssignable requirements, but if so, their copy assignment operators are constexpr functions that do not throw exceptions.
id
check in basic_format_parse_context::next_arg_id
Section: 28.5.6.6 [format.parse.ctx] Status: C++23 Submitter: Victor Zverovich Opened: 2022-11-09 Last modified: 2023-11-22
Priority: Not Prioritized
View all other issues in [format.parse.ctx].
View all issues with C++23 status.
Discussion:
The definition of check_arg_id
in 28.5.6.6 [format.parse.ctx] includes a (compile-time)
argument id
check in the Remarks element:
constexpr void check_arg_id(size_t id);[…]
Remarks: Call expressions whereid >= num_args_
are not core constant expressions (7.7 [expr.const]).
However, a similar check is missing from next_arg_id
which means that there is no argument id validation
in user-defined format specification parsing code that invokes this function (e.g. when parsing automatically
indexed dynamic width).
Previous resolution [SUPERSEDED]:
This wording is relative to N4917.
Modify 28.5.6.6 [format.parse.ctx] as indicated:
constexpr size_t next_arg_id();-7- Effects: If
indexing_ != manual
, equivalent to:if (indexing_ == unknown) indexing_ = automatic; return next_arg_id_++;-8- Throws:
-?- Remarks: Call expressions whereformat_error
ifindexing_ == manual
which indicates mixing of automatic and manual argument indexing.next_arg_id_ >= num_args_
are not core constant expressions (7.7 [expr.const]).
[2022-11-11; Tomasz provide improved wording; Move to Open]
Clarify that the value of next_arg_id_
is used, and add missing "is true
."
[Kona 2022-11-11; move to Ready]
[2023-02-13 Approved at February 2023 meeting in Issaquah. Status changed: Voting → WP.]
Proposed resolution:
This wording is relative to N4917.
Modify 28.5.6.6 [format.parse.ctx] as indicated:
constexpr size_t next_arg_id();-7- Effects: If
indexing_ != manual
istrue
, equivalent to:if (indexing_ == unknown) indexing_ = automatic; return next_arg_id_++;-8- Throws:
-?- Remarks: Let cur-arg-id be the value offormat_error
ifindexing_ == manual
istrue
which indicates mixing of automatic and manual argument indexing.next_arg_id_
prior to this call. Call expressions wherecur-arg-id >= num_args_
istrue
are not core constant expressions (7.7 [expr.const]).
constexpr size_t check_arg_id(size_t id);-9- Effects: If
indexing_ != automatic
istrue
, equivalent to:if (indexing_ == unknown) indexing_ = manual;-10- Throws:
-11- Remarks: Call expressions whereformat_error
ifindexing_ == automatic
istrue
which indicates mixing of automatic and manual argument indexing.id >= num_args_
istrue
are not core constant expressions (7.7 [expr.const]).
yield_value
]Section: 25.8.5 [coro.generator.promise] Status: C++23 Submitter: US Opened: 2022-11-10 Last modified: 2023-11-22
Priority: Not Prioritized
View all other issues in [coro.generator.promise].
View all issues with C++23 status.
Discussion:
This is in resolution of US 56-118 (25.8.5 [coro.generator.promise] Redundant specification).
[Paragraph 14] is redundant given [paragraphs] 13 and 12. Remove it.
Paragraphs 12 and 14 are identical: "Remarks: A yield-expression that calls this
function has type void
(7.6.17 [expr.yield])." Paragraph 13 states that the overload
of yield_value
that accepts ranges::elements_of
for arbitrary ranges has
"Effects: Equivalent to:" calling the overload of yield_value
that accepts
specializations of generator
, which paragraph 12 specifies. Per
16.3.2.4 [structure.specifications] paragraph 4, the former overload "inherits" the
Remarks of paragraph 12 making paragraph 14 redundant.
LWG is concerned that the redundancy is not immediately obvious — it depends on an understanding of how await expressions function — so we'd like to preserve comment despite that we agree that it is normatively redundant.
[2022-11-10 Casey provides wording]
[Kona 2022-11-11; Move to Immediate]
[2022-11-12 Approved at November 2022 meeting in Kona. Status changed: Immediate → WP.]
Proposed resolution:
This wording is relative to N4917.
Modify 25.8.5 [coro.generator.promise] as indicated:
template<ranges::input_range R, class Alloc> requires convertible_to<ranges::range_reference_t<R>, yielded> auto yield_value(ranges::elements_of<R, Alloc> r) noexcept;-13- Effects: Equivalent to: […]
-14-
Remarks:[Note 1: A yield-expression that calls this function has typevoid
(7.6.17 [expr.yield]). — end note]
<stdalign.h>
and <stdbool.h>
macrosSection: 17.15.4 [stdalign.h.syn], 17.15.5 [stdbool.h.syn] Status: C++23 Submitter: GB Opened: 2022-11-10 Last modified: 2024-01-29
Priority: Not Prioritized
View all issues with C++23 status.
Discussion:
This is the resolution for NB comments:
GB-081:
C2x defines alignas
as a keyword, so <stdalign.h>
is empty in C2x. C++23 should
deprecate the __alignas_is_defined
macro now, rather than wait until a future C++ standard is based on C2x.
That gives users longer to prepare for the removal of the macro.
Recommended change:
Deprecate __alignas_is_defined
and move it to Annex D.
Maybe keep a note in 17.15.4 [stdalign.h.syn]
that the macro is present but deprecated.
GB-082:
C2x supports bool
as a built-in type, and true
and false
as keywords.
Consequently, C2x marks the __bool_true_false_are_defined
as obsolescent. C++23 should
deprecate that attribute now, rather than wait until a future C++ standard is based on C2x. That
gives users longer to prepare for the removal of the macro.
Recommended change:
Deprecate __bool_true_false_are_defined
and move it to Annex D.
Maybe keep a note in 17.15.5 [stdbool.h.syn] that the macro is present but deprecated.
[Kona 2022-11-10; Jonathan provides wording]
[Kona 2022-11-10; Waiting for LEWG electronic polling]
[2022-11-08; Kona LEWG]
Strong consensus to accept GB-81. Strong consensus to accept GB-82.
Previous resolution [SUPERSEDED]:
This wording is relative to N4917.
Modify 17.15.4 [stdalign.h.syn] as indicated:
17.14.4 Header
<stdalign.h>
synopsis [stdalign.h.syn]#define __alignas_is_defined 1-1- The contents of the C++ header
<stdalign.h>
are the same as the C standard library header<stdalign.h>
, with the following changes: The header<stdalign.h>
does not define a macro namedalignas
. The macro__alignas_is_defined
(D.11 [depr.c.macros]) is deprecated.See also: ISO C 7.15
Modify 17.15.5 [stdbool.h.syn] as indicated:
17.14.5 Header
<stdbool.h>
synopsis [stdbool.h.syn]#define __bool_true_false_are_defined 1-1- The contents of the C++ header
<stdbool.h>
are the same as the C standard library header<stdbool.h>
, with the following changes: The header<stdbool.h>
does not define macros namedbool
,true
, orfalse
. The macro__bool_true_false_are_defined
(D.11 [depr.c.macros]) is deprecated.See also: ISO C 7.18
- Add a new subclause to Annex D [depr] between [depr.res.on.required] and D.14 [depr.relops], with this content:
D?? Deprecated C macros [depr.c.macros]
-1- The header
<stdalign.h>
has the following macro:#define __alignas_is_defined 1-2- The header
<stdbool.h>
has the following macro:#define __bool_true_false_are_defined 1
[Issaquah 2023-02-06; LWG]
Green text additions in Clause 17 was supposed to be adding notes. Drop them instead. Unanimous consent (14/0/0) to move to Immediate for C++23.
[2023-02-13 Approved at February 2023 meeting in Issaquah. Status changed: Immediate → WP.]
Proposed resolution:
This wording is relative to N4928.
Modify 17.15.4 [stdalign.h.syn] as indicated:
17.14.4 Header
<stdalign.h>
synopsis [stdalign.h.syn]#define __alignas_is_defined 1-1- The contents of the C++ header
<stdalign.h>
are the same as the C standard library header<stdalign.h>
, with the following changes: The header<stdalign.h>
does not define a macro namedalignas
.See also: ISO C 7.15
Modify 17.15.5 [stdbool.h.syn] as indicated:
17.14.5 Header
<stdbool.h>
synopsis [stdbool.h.syn]#define __bool_true_false_are_defined 1-1- The contents of the C++ header
<stdbool.h>
are the same as the C standard library header<stdbool.h>
, with the following changes: The header<stdbool.h>
does not define macros namedbool
,true
, orfalse
.See also: ISO C 7.18
D?? Deprecated C macros [depr.c.macros]
-1- The header
<stdalign.h>
has the following macro:#define __alignas_is_defined 1-2- The header
<stdbool.h>
has the following macro:#define __bool_true_false_are_defined 1
intmax_t
and uintmax_t
with C2xSection: 17.4.1 [cstdint.syn] Status: C++23 Submitter: GB Opened: 2022-11-10 Last modified: 2023-11-22
Priority: Not Prioritized
View all other issues in [cstdint.syn].
View all issues with C++23 status.
Discussion:
This is the resolution for NB comment GB-080
17.4.1 [cstdint.syn] Sync intmax_t
and uintmax_t
with C2x.
With the approval of WG14 N2888 the next C standard will resolve the long-standing issue that implementations cannot support 128-bit integer types properly without ABI breaks. C++ should adopt the same fix now, rather than waiting until a future C++ standard is rebased on C2x.
31.13.2 [cinttypes.syn] also mentions those types, but doesn't need
a change. The proposed change allows intmax_t
to be an
extended integer type of the same width as long long
,
in which case we'd still want those abs
overloads.
Recommended change:
Add to 31.13.2 [cinttypes.syn] p2 "except that intmax_t
is not required to be able to
represent all values of extended integer types wider than long long
, and uintmax_t
is not required
to be able to represent all values of extended integer types wider than unsigned long long
."
[Kona 2022-11-10; Waiting for LEWG electronic polling]
[2022-11; LEWG electronic polling]
Unanimous consensus in favor.
[Issaquah 2023-02-06; LWG]
Unanimous consent (13/0/0) to move to Immediate for C++23.
[2023-02-13 Approved at February 2023 meeting in Issaquah. Status changed: Immediate → WP.]
Proposed resolution:
This wording is relative to N4917.
Modify 31.13.2 [cinttypes.syn] as indicated:
-1- The contents and meaning of the header
<cinttypes>
are the same as the C standard library header<inttypes.h>
, with the following changes:
(1.1) — The header
<cinttypes>
includes the header<cstdint>
instead of<stdint.h>
, and(1.?) —
intmax_t
anduintmax_t
are not required to be able to represent all values of extended integer types wider thanlong long
andunsigned long long
respectively, and(1.2) — if and only if the type
intmax_t
designates an extended integer type, the following function signatures are added:intmax_t abs(intmax_t); imaxdiv_t div(intmax_t, intmax_t);which shall have the same semantics as the function signatures
intmax_t imaxabs(intmax_t)
andimaxdiv_t imaxdiv(intmax_t, intmax_t)
, respectively.See also: ISO C 7.8
template<size_t N> struct formatter<const charT[N], charT>
Section: 28.5.6.4 [format.formatter.spec] Status: C++23 Submitter: Mark de Wever Opened: 2022-11-27 Last modified: 2023-11-22
Priority: 2
View other active issues in [format.formatter.spec].
View all other issues in [format.formatter.spec].
View all issues with C++23 status.
Discussion:
In the past I discussed with Victor and Charlie to remove
template<size_t N> struct formatter<const charT[N], charT>;
Charlie disliked that since MSVC STL already shipped it and Victor
mentioned it was useful. Instead of proposing to remove the
specialization in LWG 3701(i) ("Make formatter<remove_cvref_t<const charT[N]>, charT>
requirement explicit") I proposed to keep it. It's unused but it doesn't hurt.
template<class R> constexpr unspecified format_kind = unspecified; template<ranges::input_range R> requires same_as<R, remove_cvref_t<R>> constexpr range_format format_kind<R> = see below;
combined with 28.5.7.1 [format.range.fmtkind] p1:
A program that instantiates the primary template of
format_kind
is ill-formed.
The issue is that const charT[N]
does not satisfy the requirement
same_as<R, remove_cvref_t<R>>
. So it tries to instantiate the primary
template, which is ill-formed.
Removing the specialization
template<size_t N> struct formatter<const charT[N], charT>;
Adding a specialization
template<class charT, size_t N> constexpr range_format format_kind<const charT[N]> = range_format::disabled;
I discussed this issue privately and got no objection for solution 1, therefore I propose to take that route. Implementations can still implement solution 2 as an extension until they are ready to ship an API/ABI break.
[2022-11-30; Reflector poll]
Set priority to 2 after reflector poll.
"Rationale is not really convincing why the first option is the right one."
"The point is not that we would need to add a format_kind specialization,
it is that the specialization is inconsistent with the design of formatter
,
which is supposed to be instantiated only for cv-unqualified non-reference types."
Previous resolution [SUPERSEDED]:
This wording is relative to N4917.
Modify 28.5.6.4 [format.formatter.spec] as indicated:
-2- Let
charT
be eitherchar
orwchar_t
. Each specialization offormatter
is either enabled or disabled, as described below. A debug-enabled specialization offormatter
additionally provides a public, constexpr, non-static member functionset_debug_format()
which modifies the state of theformatter
to be as if the type of the std-format-spec parsed by the last call toparse
were?
. Each header that declares the templateformatter
provides the following enabled specializations:
(2.1) — The debug-enabled specializations […]
(2.2) — For each
charT
, the debug-enabled string type specializationstemplate<> struct formatter<charT*, charT>; template<> struct formatter<const charT*, charT>; template<size_t N> struct formatter<charT[N], charT>;template<size_t N> struct formatter<const charT[N], charT>;template<class traits, class Allocator> struct formatter<basic_string<charT, traits, Allocator>, charT>; template<class traits> struct formatter<basic_string_view<charT, traits>, charT>;(2.3) — […]
(2.4) — […]
Add a new paragraph to C.2.10 [diff.cpp20.utilities] as indicated:
Affected subclause: 28.5.6.4 [format.formatter.spec]
Change: Remove theformatter
specializationtemplate<size_t N> struct formatter<const charT[N], charT>
. Rationale: Theformatter
specialization was not used in the Standard library. Keeping the specialization well-formed required an additionalformat_kind
specialization. Effect on original feature: Valid C++ 2020 code that instantiated the removed specialization is now ill-formed.
[2023-02-07 Tim provides updated wording]
[Issaquah 2023-02-08; LWG]
Unanimous consent to move to Immediate.
[2023-02-13 Approved at February 2023 meeting in Issaquah. Status changed: Immediate → WP.]
Proposed resolution:
This wording is relative to N4928.
Modify 28.5.6.4 [format.formatter.spec] as indicated:
-2- Let
charT
be eitherchar
orwchar_t
. Each specialization offormatter
is either enabled or disabled, as described below. A debug-enabled specialization offormatter
additionally provides a public, constexpr, non-static member functionset_debug_format()
which modifies the state of theformatter
to be as if the type of the std-format-spec parsed by the last call toparse
were?
. Each header that declares the templateformatter
provides the following enabled specializations:
(2.1) — The debug-enabled specializations […]
(2.2) — For each
charT
, the debug-enabled string type specializationstemplate<> struct formatter<charT*, charT>; template<> struct formatter<const charT*, charT>; template<size_t N> struct formatter<charT[N], charT>;template<size_t N> struct formatter<const charT[N], charT>;template<class traits, class Allocator> struct formatter<basic_string<charT, traits, Allocator>, charT>; template<class traits> struct formatter<basic_string_view<charT, traits>, charT>;(2.3) — […]
(2.4) — […]
Add a new paragraph to C.2.10 [diff.cpp20.utilities] as indicated:
Affected subclause: 28.5.6.4 [format.formatter.spec]
Change: Removed theformatter
specializationtemplate<size_t N> struct formatter<const charT[N], charT>
. Rationale: The specialization is inconsistent with the design offormatter
, which is intended to be instantiated only with cv-unqualified object types. Effect on original feature: Valid C++ 2020 code that instantiated the removed specialization can become ill-formed.
constexpr
for std::intmax_t
math functions in <cinttypes>
Section: 31.13.2 [cinttypes.syn] Status: C++23 Submitter: George Tokmaji Opened: 2022-11-27 Last modified: 2023-11-22
Priority: Not Prioritized
View all issues with C++23 status.
Discussion:
P0533R9 adds constexpr
to math functions in <cmath>
and <cstdlib>
, which includes std::abs
and std::div
. This
misses the overloads for std::intmax_t
in <cinttypes>
, as well as
std::imaxabs
and std::imaxdiv
, which seems like an oversight.
[2023-01-06; Reflector poll]
Set status to Tentatively Ready after six votes in favour during reflector poll.
[2023-02-13 Approved at February 2023 meeting in Issaquah. Status changed: Voting → WP.]
Proposed resolution:
This wording is relative to N4917.
Modify 31.13.2 [cinttypes.syn], header <cinttypes>
synopsis, as indicated:
[…] namespace std { using imaxdiv_t = see below; constexpr intmax_t imaxabs(intmax_t j); constexpr imaxdiv_t imaxdiv(intmax_t numer, intmax_t denom); intmax_t strtoimax(const char* nptr, char** endptr, int base); uintmax_t strtoumax(const char* nptr, char** endptr, int base); intmax_t wcstoimax(const wchar_t* nptr, wchar_t** endptr, int base); uintmax_t wcstoumax(const wchar_t* nptr, wchar_t** endptr, int base); constexpr intmax_t abs(intmax_t); // optional, see below constexpr imaxdiv_t div(intmax_t, intmax_t); // optional, see below […] } […]-1- The contents and meaning of the header
<cinttypes>
are the same as the C standard library header<inttypes.h>
, with the following changes:
(1.1) — The header
<cinttypes>
includes the header<cstdint>
(17.4.1 [cstdint.syn]) instead of<stdint.h>
, and(1.2) — if and only if the type
intmax_t
designates an extended integer type (6.8.2 [basic.fundamental]), the following function signatures are added:constexpr intmax_t abs(intmax_t); constexpr imaxdiv_t div(intmax_t, intmax_t);which shall have the same semantics as the function signatures
constexpr intmax_t imaxabs(intmax_t)
andconstexpr imaxdiv_t imaxdiv(intmax_t, intmax_t)
, respectively.
std::expected<bool, E1>
conversion constructor expected(const expected<U, G>&)
should take precedence over expected(U&&)
with operator bool
Section: 22.8.6.2 [expected.object.cons] Status: C++23 Submitter: Hui Xie Opened: 2022-11-30 Last modified: 2023-11-22
Priority: 1
View all other issues in [expected.object.cons].
View all issues with C++23 status.
Discussion:
The issue came up when implementing std::expected
in libc++. Given the following example:
struct BaseError{};
struct DerivedError : BaseError{};
std::expected<int, DerivedError> e1(5);
std::expected<int, BaseError> e2(e1); // e2 holds 5
In the above example, e2
is constructed with the conversion constructor
expected::expected(const expected<U, G>&)
and the value 5
is correctly copied into e2
as expected.
int
to bool
, the behaviour is very surprising.
std::expected<bool, DerivedError> e1(false);
std::expected<bool, BaseError> e2(e1); // e2 holds true
In this example e2
is constructed with
expected::expected(U&&)
together with
expected::operator bool() const
Instead of copying e1
's "false
" into e2
, it uses operator bool
, which returns
true
in this case and e2
would hold "true
" instead.
int
and bool
.
The reason why the second example uses a different overload is that the constructor
expected(const expected<U, G>& rhs);
has the following constraint
(22.8.6.2 [expected.object.cons] p17):
(17.3) —
is_constructible_v<T, expected<U, G>&>
isfalse
; and(17.4) —
is_constructible_v<T, expected<U, G>>
isfalse
; and(17.5) —
is_constructible_v<T, const expected<U, G>&>
isfalse
; and(17.6) —
is_constructible_v<T, const expected<U, G>>
isfalse
; and(17.7) —
is_convertible_v<expected<U, G>&, T>
isfalse
; and(17.8) —
is_convertible_v<expected<U, G>&&, T>
isfalse
; and(17.9) —
is_convertible_v<const expected<U, G>&, T>
isfalse
; and(17.10) —
is_convertible_v<const expected<U, G>&&, T>
isfalse
; and
Since T
is bool
in the second example, and bool
can be constructed from
std::expected
, this overload will be removed. and the overload that takes U&&
will be selected.
bool
, i.e.
(The above 8 constraints); or
is_same_v<remove_cv_t<T>, bool>
is true
And we need to make sure this overload and the overload that takes expected(U&&)
be mutually exclusive.
[2023-01-06; Reflector poll]
Set priority to 1 after reflector poll.
There was a mix of votes for P1 and P2 but also one for NAD
("The design of forward/repack construction for expected matches optional,
when if the stored value can be directly constructed, we use that.").
std::optional<bool>
is similarly affected.
Any change should consider the effects on
expected<expected<>>
use cases.
[Issaquah 2023-02-08; Jonathan provides wording]
[Issaquah 2023-02-09; LWG]
Move to Immediate for C++23
[2023-02-13 Approved at February 2023 meeting in Issaquah. Status changed: Immediate → WP.]
Proposed resolution:
This wording is relative to N4928.
Modify 22.5.3.2 [optional.ctor] as indicated:
template<class U = T> constexpr explicit(see below) optional(U&& v);
-23- Constraints:
[Drafting note: Change this paragraph to a bulleted list.]
- (23.1) —
is_constructible_v<T, U>
istrue
,- (23.2) —
is_same_v<remove_cvref_t<U>, in_place_t>
isfalse
,and- (23.3) —
is_same_v<remove_cvref_t<U>, optional>
isfalse
, and- (23.4) — if
T
is cvbool
,remove_cvref_t<U>
is not a specialization ofoptional
.-24- Effects: Direct-non-list-initializes the contained value with
std::forward>U>(v)
.-25- Postconditions:
*this
has a value.-26- Throws: Any exception thrown by the selection constructor of
T
.-27- Remarks: If
T
's selected constructor is a constexpr constructor, this constructor is a constexpr constructor. The expression insideexplicit
is equivalent to:
!is_convertible_v<U, T>template<class U> constexpr explicit(see below) optional(const optional<U>& rhs);
-28- Constraints:
- (28.1) —
is_constructible_v<T, const U&>
istrue
, and- (28.1) — if
T
is not cvbool
,converts-from-any-cvref<T, optional<U>>
isfalse
.-29- Effects: If
rhs
contains a value, direct-non-list-initializes the contained value with*rhs
.-30- Postconditions:
rhs.has_value() == this->has_value()
.-31- Throws: Any exception thrown by the selection constructor of
T
.-32- Remarks: The expression inside
explicit
is equivalent to:
!is_convertible_v<const U&, T>template<class U> constexpr explicit(see below) optional(optional<U>&& rhs);
-33- Constraints:
- (33.1) —
is_constructible_v<T, U>
istrue
, and- (33.1) — if
T
is not cvbool
,converts-from-any-cvref<T, optional<U>>
isfalse
.-34- Effects: If
rhs
contains a value, direct-non-list-initializes the contained value withstd::move(*rhs)
.rhs.has_value()
is unchanged.-35- Postconditions:
rhs.has_value() == this->has_value()
.-36- Throws: Any exception thrown by the selection constructor of
T
.-37- Remarks: The expression inside
explicit
is equivalent to:
!is_convertible_v<U, T>
Modify 22.8.6.2 [expected.object.cons] as indicated:
template<class U, class G> constexpr explicit(see below) expected(const expected<U, G>& rhs); template<class U, class G> constexpr explicit(see below) expected(expected<U, G>&& rhs);
-17- Let:
- (17.1) —
UF
beconst U&
for the first overload andU
for the second overload.- (17.2) —
GF
beconst G&
for the first overload andG
for the second overload.-18- Constraints:
- (18.1) —
is_constructible_v<T, UF>
istrue
; and- (18.2) —
is_constructible_v<E, GF>
istrue
; and- (18.3) — if
T
is not cvbool
,converts-from-any-cvref<T, expected<U, G>>
isfalse
; and- (18.4) —
is_constructible_v<unexpected<E>, expected<U, G>&>
isfalse
; and- (18.5) —
is_constructible_v<unexpected<E>, expected<U, G>>
isfalse
; and- (18.6) —
is_constructible_v<unexpected<E>, const expected<U, G>&>
isfalse
; and- (18.7) —
is_constructible_v<unexpected<E>, const expected<U, G>>
isfalse
.-19- Effects: If
rhs.has_value()
, direct-non-list-initializesval
withstd::forward>UF>(*rhs)
. Otherwise, direct-non-list-initializesunex
withstd::forward>GF>(rhs.error())
.-20- Postconditions:
rhs.has_value()
is unchanged;rhs.has_value() == this->has_value()
istrue
.-21- Throws: Any exception thrown by the initialization of
val
orunex
.-22- Remarks: The expression inside
explicit
is equivalent to!is_convertible_v<UF, T> || !is_convertible_v<GF, E>
.template<class U = T> constexpr explicit(!is_convertible_v<U, T>) expected(U&& v);
-23- Constraints:
- (23.1) —
is_same_v<remove_cvref_t<U>, in_place_t>
isfalse
; and- (23.2) —
is_same_v<expected, remove_cvref_t<U>>
isfalse
; and- (23.3) —
is_constructible_v<T, U>
istrue
; and- (23.4) —
remove_cvref_t<U>
is not a specialization ofunexpected
; and- (23.5) — if
T
is cvbool
,remove_cvref_t<U>
is not a specialization ofexpected
.-24- Effects: Direct-non-list-initializes
val
withstd::forward>U>(v)
.-25- Postconditions:
has_value()
istrue
.-26- Throws: Any exception thrown by the initialization of
val
.
range_formatter
's set_separator
, set_brackets
, and underlying
functions should be noexcept
Section: 28.5.7.2 [format.range.formatter], 28.5.7.3 [format.range.fmtdef], 28.5.9 [format.tuple] Status: C++23 Submitter: Hewill Kang Opened: 2022-12-11 Last modified: 2023-11-22
Priority: Not Prioritized
View all other issues in [format.range.formatter].
View all issues with C++23 status.
Discussion:
The set_separator
and set_brackets
of range_formatter
only invoke
basic_string_view
's assignment operator, which is noexcept
, we should add noexcept
specifications for them.
underlying
function returns a reference to the underlying formatter
,
which never throws, they should also be noexcept
.
Similar rules apply to range-default-formatter
and formatter
's tuple
specialization.
[2023-01-06; Reflector poll]
Set status to Tentatively Ready after eight votes in favour during reflector poll.
[2023-02-13 Approved at February 2023 meeting in Issaquah. Status changed: Voting → WP.]
Proposed resolution:
This wording is relative to N4917.
Modify 28.5.7.2 [format.range.formatter] as indicated:
[…]namespace std { template<class T, class charT = char> requires same_as<remove_cvref_t<T>, T> && formattable<T, charT> class range_formatter { formatter<T, charT> underlying_; // exposition only basic_string_view<charT> separator_ = STATICALLY-WIDEN<charT>(", "); // exposition only basic_string_view<charT> opening-bracket_ = STATICALLY-WIDEN<charT>("["); // exposition only basic_string_view<charT> closing-bracket_ = STATICALLY-WIDEN<charT>("]"); // exposition only public: constexpr void set_separator(basic_string_view<charT> sep) noexcept; constexpr void set_brackets(basic_string_view<charT> opening, basic_string_view<charT> closing) noexcept; constexpr formatter<T, charT>& underlying() noexcept { return underlying_; } constexpr const formatter<T, charT>& underlying() const noexcept { return underlying_; } […] }; }constexpr void set_separator(basic_string_view<charT> sep) noexcept;-7- Effects: Equivalent to:
separator_ = sep;
constexpr void set_brackets(basic_string_view<charT> opening, basic_string_view<charT> closing) noexcept;-8- Effects: Equivalent to:
opening-bracket_ = opening; closing-bracket_ = closing;
Modify 28.5.7.3 [format.range.fmtdef] as indicated:
namespace std { template<ranges::input_range R, class charT> struct range-default-formatter<range_format::sequence, R, charT> { // exposition only private: using maybe-const-r = fmt-maybe-const<R, charT>; // exposition only range_formatter<remove_cvref_t<ranges::range_reference_t<maybe-const-r>>, charT> underlying_; // exposition only public: constexpr void set_separator(basic_string_view<charT> sep) noexcept; constexpr void set_brackets(basic_string_view<charT> opening, basic_string_view<charT> closing) noexcept; […] }; }constexpr void set_separator(basic_string_view<charT> sep) noexcept;-1- Effects: Equivalent to:
underlying_.set_separator(sep);
.constexpr void set_brackets(basic_string_view<charT> opening, basic_string_view<charT> closing) noexcept;-2- Effects: Equivalent to:
underlying_.set_brackets(opening, closing);
.
Modify 28.5.9 [format.tuple] as indicated:
[…]namespace std { template<class charT, formattable<charT>... Ts> struct formatter<pair-or-tuple<Ts...>, charT> { private: tuple<formatter<remove_cvref_t<Ts>, charT>...> underlying_; // exposition only basic_string_view<charT> separator_ = STATICALLY-WIDEN<charT>(", "); // exposition only basic_string_view<charT> opening-bracket_ = STATICALLY-WIDEN<charT>("("); // exposition only basic_string_view<charT> closing-bracket_ = STATICALLY-WIDEN<charT>(")"); // exposition only public: constexpr void set_separator(basic_string_view<charT> sep) noexcept; constexpr void set_brackets(basic_string_view<charT> opening, basic_string_view<charT> closing) noexcept; […] }; }constexpr void set_separator(basic_string_view<charT> sep) noexcept;-5- Effects: Equivalent to:
separator_ = sep;
constexpr void set_brackets(basic_string_view<charT> opening, basic_string_view<charT> closing) noexcept;-6- Effects: Equivalent to:
opening-bracket_ = opening; closing-bracket_ = closing;
<version>
should not be "all freestanding"Section: 17.3.2 [version.syn] Status: C++23 Submitter: Jonathan Wakely Opened: 2022-12-14 Last modified: 2024-01-29
Priority: Not Prioritized
View all other issues in [version.syn].
View all issues with C++23 status.
Discussion:
It's reasonable for the <version>
header to be required for freestanding,
so that users can include it and see the "implementation-dependent information …
(e.g. version number and release date)", and also to ask which features are present
(which is the real intended purpose of <version>
).
It seems less reasonable to require every macro to be present on freestanding implementations,
even the ones that correspond to non-freestanding features.
[2023-01-06; Reflector poll]
Set status to Tentatively Ready after eight votes in favour during reflector poll.
[2023-02-13 Approved at February 2023 meeting in Issaquah. Status changed: Voting → WP.]
Proposed resolution:
This wording is relative to N4917.
Modify 17.3.2 [version.syn], header <version>
synopsis, as indicated:
-2- Each of the macros defined in
[Note 1: Future revisions of C++ might replace the values of these macros with greater values. — end note]<version>
is also defined after inclusion of any member of the set of library headers indicated in the corresponding comment in this synopsis.// all freestanding#define __cpp_lib_addressof_constexpr 201603L // also in <memory> […]
Section: 30.12 [time.format] Status: C++23 Submitter: Jonathan Wakely Opened: 2022-12-14 Last modified: 2024-01-29
Priority: Not Prioritized
View other active issues in [time.format].
View all other issues in [time.format].
View all issues with C++23 status.
Discussion:
30.12 [time.format] says:
[…] Giving a precision specification in the chrono-format-spec is valid only for
std::chrono::duration
types where the representation typeRep
is a floating-point type. For all otherRep
types, an exception of typeformat_error
is thrown if the chrono-format-spec contains a precision specification. […]
It's unclear whether the restriction in the first sentence applies to all types, or only duration types.
The second sentence seems to restrict the exceptional case to only types with a non-floating-point Rep
,
but what about types with no Rep
type at all?
sys_time<duration<float>>
? That is not a duration
type at all,
so does the restriction apply? What about hh_mm_ss<duration<int>>
? That's not a
duration
type, but it uses one, and its Rep
is not a floating-point type.
What about sys_info
? That's not a duration
and doesn't have any associated duration
,
or Rep
type.
What is the intention here?
Less importantly, I don't like the use of Rep
here. That's the template parameter of the
duration
class template, but that name isn't in scope here. Why aren't we talking about the
duration
type's rep
type, which is the public name for it? Or about a concrete
specialization duration<Rep, Period>
, instead of the class template?
The suggested change below would preserve the intended meaning, but with more … precision.
[2023-01-06; Reflector poll]
Set status to Tentatively Ready after eight votes in favour during reflector poll.
[2023-02-13 Approved at February 2023 meeting in Issaquah. Status changed: Voting → WP.]
Proposed resolution:
This wording is relative to N4917.
Modify 30.12 [time.format] as indicated:
-1- […]
The productions fill-and-align, width, and precision are described in 28.5.2 [format.string]. Giving a precision specification in the chrono-format-spec is valid only for types that are specializations ofstd::chrono::duration
types where the representation typefor which the nested typedef-nameRep
isrep
denotes a floating-point type. For all othertypes, an exception of typeRep
format_error
is thrown if the chrono-format-spec contains a precision specification. […]
std::expected<T,E>::value() &
assumes E
is copy constructibleSection: 22.8.6.6 [expected.object.obs] Status: C++23 Submitter: Jonathan Wakely Opened: 2022-12-20 Last modified: 2023-11-22
Priority: Not Prioritized
View all issues with C++23 status.
Discussion:
22.8.6.6 [expected.object.obs] p9 says:
Throws:bad_expected_access(error())
ifhas_value()
isfalse
.
But if error()
returns a reference to a move-only type
then it can't be copied and the function body is ill-formed.
Should it be constrained with is_copy_constructible_v<E>
?
Or just mandate it?
Similarly, the value()&&
and value() const&&
overloads require is_move_constructible_v<E>
to be true
for bad_expected_access(std::move(error()))
to be valid.
Casey Carter pointed out they also require it to be copyable so that the
exception can be thrown, as per 14.2 [except.throw] p5.
[Issaquah 2023-02-09; LWG]
Move to Immediate for C++23
[2023-02-13 Approved at February 2023 meeting in Issaquah. Status changed: Immediate → WP.]
Proposed resolution:
Modify 22.8.6.6 [expected.object.obs] as indicated:
constexpr const T& value() const &; constexpr T& value() &;-?- Mandates:
is_copy_constructible_v<E>
istrue
.-8- Returns:
val
, ifhas_value()
istrue
.-9- Throws:
bad_expected_access(as_const(error()))
ifhas_value()
isfalse
.constexpr T&& value() &&; constexpr const T&& value() const &&;-?- Mandates:
is_copy_constructible_v<E>
istrue
andis_constructible_v<E, decltype(std::move(error()))>
istrue
.-10- Returns:
std::move(val)
, ifhas_value()
istrue
.-11- Throws:
bad_expected_access(std::move(error()))
ifhas_value()
isfalse
.
ranges::to
can still return viewsSection: 25.5.7.2 [range.utility.conv.to], 25.5.7.3 [range.utility.conv.adaptors] Status: C++23 Submitter: Hewill Kang Opened: 2023-01-06 Last modified: 2024-01-29
Priority: 2
View other active issues in [range.utility.conv.to].
View all other issues in [range.utility.conv.to].
View all issues with C++23 status.
Discussion:
The intention of ranges::to
is to construct a non-view object,
which is reflected in its constraint that object type C
should not
model a view
.
The specification allows C
to be a const
-qualified type
which does not satisfy view
such as const string_view
,
making ranges::to
return a dangling view. We should ban such cases.
[2023-02-01; Reflector poll]
Set priority to 2 after reflector poll. Just require C to be a cv-unqualified object type.
Previous resolution [SUPERSEDED]:
This wording is relative to N4917.
Modify 25.2 [ranges.syn] as indicated:
#include <compare> // see 17.12.1 [compare.syn] #include <initializer_list> // see 17.11.2 [initializer.list.syn] #include <iterator> // see 24.2 [iterator.synopsis] namespace std::ranges { […] // 25.5.7 [range.utility.conv], range conversions template<class C, input_range R, class... Args> requires (!view<remove_cv_t<C>>) constexpr C to(R&& r, Args&&... args); // freestanding template<template<class...> class C, input_range R, class... Args> constexpr auto to(R&& r, Args&&... args); // freestanding template<class C, class... Args> requires (!view<remove_cv_t<C>>) constexpr auto to(Args&&... args); // freestanding template<template<class...> class C, class... Args> constexpr auto to(Args&&... args); // freestanding […] }Modify 25.5.7.2 [range.utility.conv.to] as indicated:
template<class C, input_range R, class... Args> requires (!view<remove_cv_t<C>>) constexpr C to(R&& r, Args&&... args);-1- Returns: An object of type
[…]C
constructed from the elements ofr
in the following manner:Modify 25.5.7.3 [range.utility.conv.adaptors] as indicated:
template<class C, class... Args> requires (!view<remove_cv_t<C>>) constexpr auto to(Args&&... args); template<template<class...> class C, class... Args> constexpr auto to(Args&&... args);-1- Returns: A range adaptor closure object (25.7.2 [range.adaptor.object])
[…]f
that is a perfect forwarding call wrapper (22.10.4 [func.require]) with the following properties:
[2023-02-02; Jonathan provides improved wording]
[Issaquah 2023-02-08; LWG]
Unanimous consent to move to Immediate. This also resolves LWG 3787(i).
[2023-02-13 Approved at February 2023 meeting in Issaquah. Status changed: Immediate → WP.]
Proposed resolution:
This wording is relative to N4928.
Modify 25.5.7.2 [range.utility.conv.to] as indicated:
template<class C, input_range R, class... Args> requires (!view<C>) constexpr C to(R&& r, Args&&... args);-?- Mandates:
C
is a cv-unqualified class type.-1- Returns: An object of type
[…]C
constructed from the elements ofr
in the following manner:
Modify 25.5.7.3 [range.utility.conv.adaptors] as indicated:
template<class C, class... Args> requires (!view<C>) constexpr auto to(Args&&... args); template<template<class...> class C, class... Args> constexpr auto to(Args&&... args);-?- Mandates: For the first overload,
C
is a cv-unqualified class type.-1- Returns: A range adaptor closure object (25.7.2 [range.adaptor.object])
[…]f
that is a perfect forwarding call wrapper (22.10.4 [func.require]) with the following properties:
adjacent_view
, adjacent_transform_view
and slide_view
missing base
accessorSection: 25.7.27.2 [range.adjacent.view], 25.7.28.2 [range.adjacent.transform.view], 25.7.30.2 [range.slide.view] Status: C++23 Submitter: Hewill Kang Opened: 2023-01-06 Last modified: 2023-11-22
Priority: Not Prioritized
View all issues with C++23 status.
Discussion:
Like most range adaptors, these three views accept a single range
and store it as a member variable.
However, they do not provide a base()
member function for accessing
the underlying range, which seems like an oversight.
[2023-02-01; Reflector poll]
Set status to Tentatively Ready after seven votes in favour during reflector poll.
[2023-02-13 Approved at February 2023 meeting in Issaquah. Status changed: Voting → WP.]
Proposed resolution:
This wording is relative to N4917.
Modify 25.7.27.2 [range.adjacent.view] as indicated:
namespace std::ranges { template<forward_range V, size_t N> requires view<V> && (N > 0) class adjacent_view : public view_interface<adjacent_view<V, N>> { V base_ = V(); // exposition only […] public: adjacent_view() requires default_initializable<V> = default; constexpr explicit adjacent_view(V base); constexpr V base() const & requires copy_constructible<V> { return base_; } constexpr V base() && { return std::move(base_); } […] }; }
Modify 25.7.28.2 [range.adjacent.transform.view] as indicated:
namespace std::ranges { template<forward_range V, move_constructible F, size_t N> requires view<V> && (N > 0) && is_object_v<F> && regular_invocable<F&, REPEAT(range_reference_t<V>, N)...> && can-reference<invoke_result_t<F&, REPEAT(range_reference_t<V>, N)...>> class adjacent_transform_view : public view_interface<adjacent_transform_view<V, F, N>> { movable-box<F> fun_; // exposition only adjacent_view<V, N> inner_; // exposition only using InnerView = adjacent_view<V, N>; // exposition only […] public: adjacent_transform_view() = default; constexpr explicit adjacent_transform_view(V base, F fun); constexpr V base() const & requires copy_constructible<InnerView> { return inner_.base(); } constexpr V base() && { return std::move(inner_).base(); } […] }; }
Modify 25.7.30.2 [range.slide.view] as indicated:
namespace std::ranges { […] template<forward_range V> requires view<V> class slide_view : public view_interface<slide_view<V>> { V base_; // exposition only range_difference_t<V> n_; // exposition only […] public: constexpr explicit slide_view(V base, range_difference_t<V> n); constexpr V base() const & requires copy_constructible<V> { return base_; } constexpr V base() && { return std::move(base_); } […] }; […] }
cartesian_product_view::iterator
's default constructor is overconstrainedSection: 25.7.33.3 [range.cartesian.iterator] Status: C++23 Submitter: Hewill Kang Opened: 2023-01-06 Last modified: 2023-11-22
Priority: Not Prioritized
View all other issues in [range.cartesian.iterator].
View all issues with C++23 status.
Discussion:
Currently, cartesian_product_view::iterator
only provides
the default constructor when the first range models forward_range
,
which seems too restrictive since several input iterators like
istream_iterator
are still default-constructible.
It would be more appropriate to constrain the default constructor
only by whether the underlying iterator satisfies
default_initializable
, as most other range adaptors do.
Since cartesian_product_view::iterator
contains
a tuple
member that already has a constrained default constructor,
the proposed resolution simply removes the constraint.
[2023-02-01; Reflector poll]
Set status to Tentatively Ready after nine votes in favour during reflector poll.
[2023-02-13 Approved at February 2023 meeting in Issaquah. Status changed: Voting → WP.]
Proposed resolution:
This wording is relative to N4917.
Modify [ranges.cartesian.iterator] as indicated:
namespace std::ranges { template<input_range First, forward_range... Vs> requires (view<First> && ... && view<Vs>) template<bool Const> class cartesian_product_view<First, Vs...>::iterator { public: […] iterator()requires forward_range<maybe-const<Const, First>>= default; […] private: using Parent = maybe-const<Const, cartesian_product_view>; // exposition only Parent* parent_ = nullptr; // exposition only tuple<iterator_t<maybe-const<Const, First>>, iterator_t<maybe-const<Const, Vs>>...> current_; // exposition only […] }; }
views::as_const
on empty_view<T>
should return empty_view<const T>
Section: 25.7.22.1 [range.as.const.overview] Status: C++23 Submitter: Hewill Kang Opened: 2023-01-06 Last modified: 2023-11-22
Priority: Not Prioritized
View all other issues in [range.as.const.overview].
View all issues with C++23 status.
Discussion:
Currently, applying views::as_const
to an empty_view<int>
will result in an as_const_view<empty_view<int>>
,
and its iterator type will be basic_const_iterator<int*>
.
This amount of instantiation is not desirable for such a simple view,
in which case simply returning empty_view<const int>
should be enough.
[2023-02-01; Reflector poll]
Set status to Tentatively Ready after eight votes in favour during reflector poll.
[2023-02-13 Approved at February 2023 meeting in Issaquah. Status changed: Voting → WP.]
Proposed resolution:
This wording is relative to N4917.
Modify 25.7.22.1 [range.as.const.overview] as indicated:
-2- The name
views::as_const
denotes a range adaptor object (25.7.2 [range.adaptor.object]). LetE
be an expression, letT
bedecltype((E))
, and letU
beremove_cvref_t<T>
. The expressionviews::as_const(E)
is expression-equivalent to:
(2.1) — If
views::all_t<T>
modelsconstant_range
, thenviews::all(E)
.(2.?) — Otherwise, if
U
denotesempty_view<X>
for some typeX
, thenauto(views::empty<const X>)
.(2.2) — Otherwise, if
U
denotesspan<X, Extent>
for some typeX
and some extentExtent
, thenspan<const X, Extent>(E)
.(2.3) — Otherwise, if
E
is an lvalue,const U
modelsconstant_range
, andU
does not modelview
, thenref_view(static_cast<const U&>(E))
.(2.4) — Otherwise,
as_const_view(E)
.
chunk_view::inner-iterator
missing custom iter_move
and iter_swap
Section: 25.7.29.5 [range.chunk.inner.iter] Status: C++23 Submitter: Hewill Kang Opened: 2023-01-06 Last modified: 2023-11-22
Priority: Not Prioritized
View all issues with C++23 status.
Discussion:
For the input version of chunk_view
,
its inner-iterator::operator*()
simply
dereferences the underlying input iterator.
However, there are no customized iter_move
and iter_swap
overloads for the inner-iterator
,
which prevents customized behavior when applying views::chunk
to a range whose iterator type is a proxy iterator,
for example:
#include <algorithm>
#include <cassert>
#include <ranges>
#include <sstream>
#include <vector>
int main() {
auto ints = std::istringstream{"0 1 2 3 4"};
std::vector<std::string> vs{"the", "quick", "brown", "fox"};
auto r = std::views::zip(vs, std::views::istream<int>(ints))
| std::views::chunk(2)
| std::views::join;
std::vector<std::tuple<std::string, int>> res;
std::ranges::copy(std::move_iterator(r.begin()), std::move_sentinel(r.end()),
std::back_inserter(res));
assert(vs.front().empty()); // assertion failed
}
zip
iterator has a customized iter_move
behavior,
but since there is no iter_move
specialization for
inner-iterator
,
when we try to move elements in chunk_view
,
move_iterator
will fallback to use the default implementation of
iter_move
, making string
s not moved as expected
from the original vector
but copied instead.
[2023-02-01; Reflector poll]
Set status to Tentatively Ready after five votes in favour during reflector poll.
[2023-02-13 Approved at February 2023 meeting in Issaquah. Status changed: Voting → WP.]
Proposed resolution:
This wording is relative to N4917.
Modify 25.7.29.5 [range.chunk.inner.iter] as indicated:
[…]namespace std::ranges { template<view V> requires input_range<V> class chunk_view<V>::inner-iterator { chunk_view* parent_; // exposition only constexpr explicit inner-iterator(chunk_view& parent) noexcept; // exposition only public: […] friend constexpr difference_type operator-(default_sentinel_t y, const inner-iterator& x) requires sized_sentinel_for<sentinel_t<V>, iterator_t<V>>; friend constexpr difference_type operator-(const inner-iterator& x, default_sentinel_t y) requires sized_sentinel_for<sentinel_t<V>, iterator_t<V>>; friend constexpr range_rvalue_reference_t<V> iter_move(const inner-iterator& i) noexcept(noexcept(ranges::iter_move(*i.parent_->current_))); friend constexpr void iter_swap(const inner-iterator& x, const inner-iterator& y) noexcept(noexcept(ranges::iter_swap(*x.parent_->current_, *y.parent_->current_))) requires indirectly_swappable<iterator_t<V>>; }; }friend constexpr range_rvalue_reference_t<V> iter_move(const inner-iterator& i) noexcept(noexcept(ranges::iter_move(*i.parent_->current_)));-?- Effects: Equivalent to:
return ranges::iter_move(*i.parent_->current_);
friend constexpr void iter_swap(const inner-iterator& x, const inner-iterator& y) noexcept(noexcept(ranges::iter_swap(*x.parent_->current_, *y.parent_->current_))) requires indirectly_swappable<iterator_t<V>>;-?- Effects: Equivalent to:
ranges::iter_swap(*x.parent_->current_, *y.parent_->current_)
.
basic_const_iterator<volatile int*>::operator->
is ill-formedSection: 24.5.3 [const.iterators] Status: C++23 Submitter: Hewill Kang Opened: 2023-01-06 Last modified: 2023-11-22
Priority: Not Prioritized
View other active issues in [const.iterators].
View all other issues in [const.iterators].
View all issues with C++23 status.
Discussion:
Currently, basic_const_iterator::operator->
constrains the value type
of the underlying iterator to be only the cv-unqualified type of its reference type,
which is true for raw pointers.
However, since it also explicitly specifies returning a pointer to a const
value type,
this will cause a hard error when the value type is actually volatile
-qualified:
std::basic_const_iterator<volatile int*> it;
auto* p = it.operator->(); // invalid conversion from 'volatile int*' to 'const int*'
The proposed resolution changes the return type from const value_type*
to const auto*
,
which makes it deduce the correct type in the above example,
i.e. const volatile int*
.
[2023-02-01; Reflector poll]
Set status to Tentatively Ready after six votes in favour during reflector poll.
[2023-02-13 Approved at February 2023 meeting in Issaquah. Status changed: Voting → WP.]
Proposed resolution:
This wording is relative to N4917.
Modify 24.5.3 [const.iterators] as indicated:
namespace std { template<class I> concept not-a-const-iterator = see below; template<input_iterator Iterator> class basic_const_iterator { Iterator current_ = Iterator(); // exposition only using reference = iter_const_reference_t<Iterator>; // exposition only public: […] constexpr reference operator*() const; constexpr const auto[…]value_type* operator->() const requires is_lvalue_reference_v<iter_reference_t<Iterator>> && same_as<remove_cvref_t<iter_reference_t<Iterator>>, value_type>; […] }; }constexpr const autovalue_type* operator->() const requires is_lvalue_reference_v<iter_reference_t<Iterator>> && same_as<remove_cvref_t<iter_reference_t<Iterator>>, value_type>;-7- Returns: If
Iterator
modelscontiguous_iterator
,to_address(current_)
; otherwise,addressof(*current_)
.
basic_string_view
should allow explicit conversion when only traits varySection: 27.3.3.2 [string.view.cons] Status: C++23 Submitter: Casey Carter Opened: 2023-01-10 Last modified: 2023-11-22
Priority: Not Prioritized
View all other issues in [string.view.cons].
View all issues with C++23 status.
Discussion:
basic_string_view
has a constructor that converts appropriate contiguous ranges to basic_string_view
.
This constructor accepts an argument by forwarding reference (R&&
), and has several constraints
including that specified in 27.3.3.2 [string.view.cons]/12.6:
if the qualified-id
remove_reference_t<R>::traits_type
is valid and denotes a type,is_same_v<remove_reference_t<R>::traits_type, traits>
istrue
.
This constraint prevents conversions from basic_string_view<C, T1>
and basic_string<C, T1, A>
to basic_string_view<C, T2>
. Preventing such seemingly semantic-affecting conversions from happening
implicitly was a good idea, but since the constructor was changed to be explicit it no longer seems necessary to
forbid these conversions. If a user wants to convert a basic_string_view<C, T2>
to
basic_string_view<C, T1>
with static_cast<basic_string_view<C, T1>>(meow)
instead of by writing out basic_string_view<C, T1>{meow.data(), meow.size()}
that seems fine to me.
Indeed, if we think conversions like this are so terribly dangerous we probably shouldn't be performing them ourselves
in 28.5.8.1 [format.arg]/9 and 28.5.8.1 [format.arg]/10.
[2023-02-01; Reflector poll]
Set status to Tentatively Ready after six votes in favour during reflector poll.
[2023-02-13 Approved at February 2023 meeting in Issaquah. Status changed: Voting → WP.]
Proposed resolution:
This wording is relative to N4917.
Modify 27.3.3.2 [string.view.cons] as indicated:
template<class R> constexpr explicit basic_string_view(R&& r);-11- Let
-12- Constraints:d
be an lvalue of typeremove_cvref_t<R>
.
(12.1) —
remove_cvref_t<R>
is not the same type asbasic_string_view
,(12.2) —
R
modelsranges::contiguous_range
andranges::sized_range
,(12.3) —
is_same_v<ranges::range_value_t<R>, charT>
istrue
,(12.4) —
is_convertible_v<R, const charT*>
isfalse
, and(12.5) —
d.operator ::std::basic_string_view<charT, traits>()
is not a valid expression, and.
(12.6) — if the qualified-idremove_reference_t<R>::traits_type
is valid and denotes a type,is_same_v<remove_reference_t<R>::traits_type, traits>
istrue
.
std::projected
cannot handle proxy iteratorSection: 24.3.6.4 [projected] Status: Resolved Submitter: Hewill Kang Opened: 2023-01-24 Last modified: 2023-03-23
Priority: 3
View all other issues in [projected].
View all issues with Resolved status.
Discussion:
Currently, std::projected
is heavily used in <algorithm>
to transform the original iterator into
a new readable type for concept checking, which has the following definition:
template<indirectly_readable I, indirectly_regular_unary_invocable<I> Proj> struct projected { using value_type = remove_cvref_t<indirect_result_t<Proj&, I>>; indirect_result_t<Proj&, I> operator*() const; // not defined };
It provides the member type value_type
, which is defined as the cvref-unqualified of a projection function
applied to the reference of the iterator, this seems reasonable since this is how iterators are usually defined for
the value type.
zip_view::iterator
, we cannot obtain
the tuple
of value by simply removing the cvref-qualifier of the tuple
of reference.
#include <algorithm>
#include <ranges>
#include <vector>
struct Cmp {
bool operator()(std::tuple<int&>, std::tuple<int&>) const;
bool operator()(auto, auto) const = delete;
};
int main() {
std::vector<int> v;
std::ranges::sort(std::views::zip(v), Cmp{}); // hard error
}
In the above example, the value type and reference of the original iterator I
are tuple<int>
and tuple<int&>
respectively, however, the value type and reference of projected<I, identity>
will be tuple<int&>
and tuple<int&>&&
, which makes the constraint only
require that the comparator can compare two tuple<int&>
s, resulting in a hard error in the implementation.
[2023-02-06; Reflector poll]
Set priority to 3 after reflector poll.
Previous resolution [SUPERSEDED]:
This wording is relative to N4928.
[Drafting note: The proposed resolution is to alias
projected
asI
when the projection function is exactlyidentity
. This form of type aliasing has similarities to the proposed wording of P2538R1, except for the nestedstruct type
. — end drafting note]
Modify 24.2 [iterator.synopsis], header
<iterator>
synopsis, as indicated:namespace std { […] // 24.3.6.4 [projected], projected template<indirectly_readable I, indirectly_regular_unary_invocable<I> Proj> usingstructprojected = see below; // freestandingtemplate<weakly_incrementable I, class Proj>struct incrementable_traits<projected<I, Proj>>; // freestanding[…] }Modify 24.3.6.4 [projected] as indicated:
-1- Class template
projected
is used to constrain algorithms that accept callable objects and projections (3.44 [defns.projection]). It combines aindirectly_readable
typeI
and a callable object typeProj
into a newindirectly_readable
type whosereference
type is the result of applyingProj
to theiter_reference_t
ofI
.namespace std { template<classindirectly_readableI, classindirectly_regular_unary_invocable<I>Proj> struct projected-implprojected{ // exposition only using value_type = remove_cvref_t<indirect_result_t<Proj&, I>>; using difference_type = iter_difference_t<I>; // present only if I models weakly_incrementable indirect_result_t<Proj&, I> operator*() const; // not defined };template<weakly_incrementable I, class Proj>struct incrementable_traits<projected<I, Proj>> {using difference_type = iter_difference_t<I>;};template<indirectly_readable I, indirectly_regular_unary_invocable<I> Proj> using projected = conditional_t<same_as<Proj, identity>, I, projected-impl<I, Proj>>; }
[2023-03-22 Resolved by the adoption of P2609R3 in Issaquah. Status changed: New → Resolved.]
Proposed resolution:
range_common_reference_t
is missingSection: 25.2 [ranges.syn] Status: C++23 Submitter: Hewill Kang Opened: 2023-01-24 Last modified: 2023-11-22
Priority: Not Prioritized
View all other issues in [ranges.syn].
View all issues with C++23 status.
Discussion:
For the alias template iter_meow_t
in <iterator>
, there are almost all corresponding
range_meow_t
in <ranges>
, except for iter_common_reference_t
, which is used to
calculate the common reference type shared by reference
and value_type
of the iterator.
iter_const_reference_t
, and the latter has a corresponding sibling,
I think we should add a range_common_reference_t
for <ranges>
.
This increases the consistency of the two libraries and simplifies the text of getting common reference from a range.
Since C++23 brings proxy iterators and tuple
enhancements, I believe such introduction can bring some value.
[2023-02-06; Reflector poll]
Set status to Tentatively Ready after seven votes in favour during reflector poll.
[2023-02-13 Approved at February 2023 meeting in Issaquah. Status changed: Voting → WP.]
Proposed resolution:
This wording is relative to N4928.
Modify 25.2 [ranges.syn], header <ranges>
synopsis, as indicated:
#include <compare> // see 17.12.1 [compare.syn] #include <initializer_list> // see 17.11.2 [initializer.list.syn] #include <iterator> // see 24.2 [iterator.synopsis] namespace std::ranges { […] template<range R> using range_reference_t = iter_reference_t<iterator_t<R>>; // freestanding template<range R> using range_const_reference_t = iter_const_reference_t<iterator_t<R>>; // freestanding template<range R> using range_rvalue_reference_t = iter_rvalue_reference_t<iterator_t<R>>; // freestanding template<range R> using range_common_reference_t = iter_common_reference_t<iterator_t<R>>; // freestanding […] }
mdspan
layout_stride::mapping
default constructor problemSection: 23.7.3.4.7 [mdspan.layout.stride] Status: Resolved Submitter: Christian Robert Trott Opened: 2023-01-25 Last modified: 2023-06-13
Priority: 1
View all other issues in [mdspan.layout.stride].
View all issues with Resolved status.
Discussion:
During work on some follow on proposals for C++26 (like the padded layouts and submdspan
) we stumbled over an
issue in the C++23 mdspan
definition.
layout_stride::mapping
with just static extents, its default constructor will produce an inconsistent mapping.
All its strides will be 0
but its extents are not. Thus the is_unique
property is violated. We wrote a paper to
fix this: P2763R0. We describe two options to fix it and state why we prefer one over the other, with proposed wording changes.
[2023-02-06; Reflector poll]
Set priority to 1 after reflector poll.
[2023-06-12 Status changed: New → Resolved.]
Resolved in Issaquah by P2763R0.
Proposed resolution:
basic_const_iterator
's common_type
specialization is underconstrainedSection: 24.2 [iterator.synopsis] Status: C++23 Submitter: Hewill Kang Opened: 2023-01-25 Last modified: 2023-11-22
Priority: Not Prioritized
View all other issues in [iterator.synopsis].
View all issues with C++23 status.
Discussion:
To make basic_const_iterator
compatible with its unwrapped iterators, the standard defines the following
common_type
specialization:
template<class T, common_with<T> U> struct common_type<basic_const_iterator<T>, U> { using type = basic_const_iterator<common_type_t<T, U>>; };
For type U
, when it shares a common type with the unwrapped type T
of basic_const_iterator
,
the common type of both is basic_const_iterator
of the common type of T
and U
.
U
and T
to have a common type,
this allows U
to be any type that satisfies such requirement, such as optional<T>
,
in which case computing the common type of both would produce a hard error inside the specialization,
because basic_const_iterator
requires the template parameter to be input_iterator
,
while optional
clearly isn't.
Previous resolution [SUPERSEDED]:
This wording is relative to N4928.
Modify 24.2 [iterator.synopsis], header
<iterator>
synopsis, as indicated:#include <compare> // see 17.12.1 [compare.syn] #include <concepts> // see 18.3 [concepts.syn] namespace std { […] template<class T, common_with<T> U> requires input_iterator<U> struct common_type<basic_const_iterator<T>, U> { // freestanding using type = basic_const_iterator<common_type_t<T, U>>; }; template<class T, common_with<T> U> requires input_iterator<U> struct common_type<U, basic_const_iterator<T>> { // freestanding using type = basic_const_iterator<common_type_t<T, U>>; }; […] }
[2023-02-06; Jonathan provides improved wording based on Casey's suggestion during the prioritization poll.]
[Issaquah 2023-02-07; LWG]
Move to Immediate for C++23
[2023-02-13 Approved at February 2023 meeting in Issaquah. Status changed: Immediate → WP.]
Proposed resolution:
This wording is relative to N4928.
Modify 24.2 [iterator.synopsis], header <iterator>
synopsis, as indicated:
#include <compare> // see 17.12.1 [compare.syn] #include <concepts> // see 18.3 [concepts.syn] namespace std { […] template<class T, common_with<T> U> requires input_iterator<common_type_t<T, U>> struct common_type<basic_const_iterator<T>, U> { // freestanding using type = basic_const_iterator<common_type_t<T, U>>; }; template<class T, common_with<T> U> requires input_iterator<common_type_t<T, U>> struct common_type<U, basic_const_iterator<T>> { // freestanding using type = basic_const_iterator<common_type_t<T, U>>; }; template<class T, common_with<T> U> requires input_iterator<common_type_t<T, U>> struct common_type<basic_const_iterator<T>, basic_const_iterator<U>> { // freestanding using type = basic_const_iterator<common_type_t<T, U>>; }; […] }
pair
sSection: 22.3.3 [pairs.spec] Status: C++23 Submitter: Barry Revzin Opened: 2023-01-28 Last modified: 2023-11-22
Priority: 2
View all other issues in [pairs.spec].
View all issues with C++23 status.
Discussion:
Consider this example:
#include <algorithm> #include <ranges> int main() { int a[3] = {1, 2, -1}; int b[3] = {1, 4, 1}; std::ranges::sort(std::views::zip(a, b)); }
This is currently valid C++23 code, but wasn't before P2165 (Compatibility between tuple
, pair
and tuple-like objects). Before P2165, zip(a, b)
returned a range whose reference was
std::pair<int&, int&>
and whose value_type
was std::pair<int, int>
and
std::pair
, unlike std::tuple
, does not have any heterogeneous comparisons — which is required to
satisfy the sortable
concept.
pair<T&, U&>
and whose value_type
is pair<T, U>
(which is now a valid range after the zip paper) and then discovering that this range isn't sortable, even though the
equivalent using tuple
is.
Suggested resolution:
Change pair
's comparison operators from comparing two arguments of type const pair<T1, T2>&
to instead comparing arguments of types const pair<T1, T2>&
and const pair<U1, U2>&
.
[2023-02-05; Barry provides wording]
[2023-02-06; Reflector poll]
Set priority to 2 after reflector poll.
[Issaquah 2023-02-07; LWG]
Move to Immediate for C++23
[2023-02-13 Approved at February 2023 meeting in Issaquah. Status changed: Immediate → WP.]
Proposed resolution:
This wording is relative to N4928.
Modify 22.2.1 [utility.syn], header <utility>
synopsis, as indicated:
[…] // 22.3.3 [pairs.spec], pair specialized algorithms template<class T1, class T2, class U1, class U2> constexpr bool operator==(const pair<T1, T2>&, const pair<TU1,TU2>&); template<class T1, class T2, class U1, class U2> constexpr common_comparison_category_t<synth-three-way-result<T1, U1>, synth-three-way-result<T2, U2>> operator<=>(const pair<T1, T2>&, const pair<TU1,TU2>&); […]
Modify 22.3.3 [pairs.spec] as indicated:
template<class T1, class T2, class U1, class U2> constexpr bool operator==(const pair<T1, T2>& x, const pair<TU1,TU2>& y);-1- […]
-2- […]template<class T1, class T2, class U1, class U2> constexpr common_comparison_category_t<synth-three-way-result<T1, U1>, synth-three-way-result<T2, U2>> operator<=>(const pair<T1, T2>& x, const pair<TU1,TU2>& y);-3- […]
expected::transform_error
overloadsSection: 22.8.6.7 [expected.object.monadic], 22.8.7.7 [expected.void.monadic] Status: C++23 Submitter: Casey Carter Opened: 2023-01-29 Last modified: 2023-11-22
Priority: Not Prioritized
View all other issues in [expected.object.monadic].
View all issues with C++23 status.
Discussion:
The overloads of expected::transform_error
mandate that "[type] G
is a valid value type for expected
"
(22.8.6.7 [expected.object.monadic]/27 and 31 as well as 22.8.7.7 [expected.void.monadic]/24 and 27)).
expected<T, G>
(for some type T
) which doesn't require
G
to be a valid value type for expected
(22.8.6.1 [expected.object.general]/2) but instead requires
that G
is "a valid template argument for unexpected
" (22.8.6.1 [expected.object.general]/2).
Comparing 22.8.6.1 [expected.object.general]/2 with 22.8.3.1 [expected.un.general]/2 it's clear that there are
types — const int
, for example — which are valid value types for expected
but not valid
template arguments for unexpected
. Presumably this unimplementable requirement is a typo, and the subject
paragraphs intended to require that G
be a valid template argument for unexpected
.
[2023-02-06; Reflector poll]
Set status to Tentatively Ready after five votes in favour during reflector poll.
[2023-02-13 Approved at February 2023 meeting in Issaquah. Status changed: Voting → WP.]
Proposed resolution:
This wording is relative to N4928.
Modify 22.8.6.7 [expected.object.monadic] as indicated:
template<class F> constexpr auto transform_error(F&& f) &; template<class F> constexpr auto transform_error(F&& f) const &;[…][…]
-27- Mandates:G
is a validvalue type fortemplate argument forexpected
unexpected
(22.8.3.1 [expected.un.general]) and the declarationG g(invoke(std::forward<F>(f), error()));is well-formed.
[…]template<class F> constexpr auto transform_error(F&& f) &&; template<class F> constexpr auto transform_error(F&& f) const &&;[…]
-31- Mandates:G
is a validvalue type fortemplate argument forexpected
unexpected
(22.8.3.1 [expected.un.general]) and the declarationG g(invoke(std::forward<F>(f), std::move(error())));is well-formed.
[…]
Modify 22.8.7.7 [expected.void.monadic] as indicated:
template<class F> constexpr auto transform_error(F&& f) &; template<class F> constexpr auto transform_error(F&& f) const &;[…][…]
-24- Mandates:G
is a validvalue type fortemplate argument forexpected
unexpected
(22.8.3.1 [expected.un.general]) and the declarationG g(invoke(std::forward<F>(f), error()));is well-formed.
[…]template<class F> constexpr auto transform_error(F&& f) &&; template<class F> constexpr auto transform_error(F&& f) const &&;[…]
-27- Mandates:G
is a validvalue type fortemplate argument forexpected
unexpected
(22.8.3.1 [expected.un.general]) and the declarationG g(invoke(std::forward<F>(f), std::move(error())));is well-formed.
[…]
std::basic_osyncstream
's move assignment operator be noexcept
?Section: 31.11.3.1 [syncstream.osyncstream.overview] Status: C++23 Submitter: Jiang An Opened: 2023-01-29 Last modified: 2023-11-22
Priority: Not Prioritized
View all other issues in [syncstream.osyncstream.overview].
View all issues with C++23 status.
Discussion:
The synopsis of std::basic_osyncstream
(31.11.3.1 [syncstream.osyncstream.overview]) indicates that it's
member functions behave as if it hold a std::basic_syncbuf
as its subobject, and according to
16.3.3.5 [functions.within.classes], std::basic_osyncstream
's move assignment operator should call
std::basic_syncbuf
's move assignment operator.
std::basic_osyncstream
's move assignment operator is noexcept
, while
std::basic_syncbuf
's is not. So when an exception is thrown from move assignment between std::basic_syncbuf
objects, std::terminate
should be called.
It's clarified in LWG 3498(i) that an exception can escape from std::basic_syncbuf
's move
assignment operator. Is there any reason that an exception shouldn't escape from std::basic_osyncstream
's
move assignment operator?
[2023-02-06; Reflector poll]
Set status to Tentatively Ready after seven votes in favour during reflector poll.
[2023-02-13 Approved at February 2023 meeting in Issaquah. Status changed: Voting → WP.]
Proposed resolution:
This wording is relative to N4928.
Modify 31.11.3.1 [syncstream.osyncstream.overview] as indicated:
namespace std { template<class charT, class traits = char_traits<charT>, class Allocator = allocator<charT>> class basic_osyncstream : public basic_ostream<charT, traits> { public: […] using syncbuf_type = basic_syncbuf<charT, traits, Allocator>; […] // assignment basic_osyncstream& operator=(basic_osyncstream&&)noexcept; […] private: syncbuf_type sb; // exposition only }; }
std::errc
constants related to UNIX STREAMSSection: 19.5.2 [system.error.syn] Status: C++23 Submitter: Jonathan Wakely Opened: 2023-01-30 Last modified: 2023-11-22
Priority: Not Prioritized
View all other issues in [system.error.syn].
View all issues with C++23 status.
Discussion:
This is the resolution for NB comment GB-084
The error numbers
ENODATA
, ENOSR
, ENOSTR
and ETIME
are all marked "obsolecent" in POSIX 2017 (the current normative reference for C++)
and they are absent in the current POSIX 202x draft.
They related to the obsolete STREAMS API,
which was optional and not required for conformance to the previous POSIX standard
(because popular unix-like systems refused to implement it).
C++11 added those error numbers to <errno.h>
and also defined corresponding errc
enumerators:
errc::no_message_available
,
errc::no_stream_resources
,
errc::not_a_stream
and
errc::stream_timeout
.
Given the obsolescent status of those constants in the current normative reference
and their absence from the next POSIX standard, WG21 should consider deprecating them now.
A deprecation period will allow removing them when C++ is eventually rebased to a new POSIX standard.
Otherwise C++ will be left with dangling references to
ENODATA
, ENOSR
, ENOSTR
and ETIME
that are not defined in the POSIX reference.
After a period of deprecation they can be removed from Annex D, and the names added to 16.4.5.3.2 [zombie.names] so that implementations can continue to define them if they need to.
[Issaquah 2023-02-06; LWG]
Unanimous consent (9/0/0) to move to Immediate for C++23.
[2023-02-13 Approved at February 2023 meeting in Issaquah. Status changed: Immediate → WP.]
Proposed resolution:
This wording is relative to N4928.
Modify 19.4.2 [cerrno.syn], header <cerrno>
synopsis, as indicated:
#define ENETUNREACH see below #define ENFILE see below #define ENOBUFS see below
#define ENODATA see below#define ENODEV see below #define ENOENT see below #define ENOEXEC see below #define ENOLCK see below #define ENOLINK see below #define ENOMEM see below #define ENOMSG see below #define ENOPROTOOPT see below #define ENOSPC see below#define ENOSR see below#define ENOSTR see below#define ENOSYS see below #define ENOTCONN see below #define ENOTDIR see below #define ENOTEMPTY see below ... #define EROFS see below #define ESPIPE see below #define ESRCH see below#define ETIME see below#define ETIMEDOUT see below #define ETXTBSY see below #define EWOULDBLOCK see below #define EXDEV see below-1- The meaning of the macros in this header is defined by the POSIX standard.
Modify 19.5.2 [system.error.syn], header <system_error>
synopsis, as indicated:
no_child_process, // ECHILD no_link, // ENOLINK no_lock_available, // ENOLCK
no_message_available, // ENODATAno_message, // ENOMSG no_protocol_option, // ENOPROTOOPT no_space_on_device, // ENOSPCno_stream_resources, // ENOSRno_such_device_or_address, // ENXIO no_such_device, // ENODEV no_such_file_or_directory, // ENOENT no_such_process, // ESRCH not_a_directory, // ENOTDIR not_a_socket, // ENOTSOCKnot_a_stream, // ENOSTRnot_connected, // ENOTCONN not_enough_memory, // ENOMEM ... result_out_of_range, // ERANGE state_not_recoverable, // ENOTRECOVERABLEstream_timeout, // ETIMEtext_file_busy, // ETXTBSY timed_out, // ETIMEDOUT
Modify D [depr], Annex D, Compatibility Features, by adding a new subclause before [depr.default.allocator]: :
D.?? Deprecated error numbers [depr.cerrno]
-1- The following macros are defined in addition to those specified in 19.4.2 [cerrno.syn]:
#define ENODATA see below #define ENOSR see below #define ENOSTR see below #define ETIME see below
-2- The meaning of these macros is defined by the POSIX standard.
-4- The following
enum errc
enumerators are defined in addition to those specified in 19.5.2 [system.error.syn]:no_message_available, // ENODATA no_stream_resources, // ENOSR not_a_stream, // ENOSTR stream_timeout, // ETIME
-4- The value of each
enum errc
enumerator above is the same as the value of the<cerrno>
macro shown in the above synopsis.
voidify
Section: 26.11.1 [specialized.algorithms.general] Status: C++23 Submitter: Jonathan Wakely Opened: 2023-01-30 Last modified: 2024-03-18
Priority: Not Prioritized
View all issues with C++23 status.
Discussion:
This is the resolution for NB comment GB-121
The voidify
helper breaks const-correctness, for no tangible benefit.
C++20 ballot comment US 215 also suggested removing it,
but failed to achieve consensus. That should be reconsidered.
The only claimed benefits are:
uninitialized_xxx
algorithms
to create objects in const storage
(including overwriting objects declared as const which is usually UB).
The caller should be responsible for using const_cast
if that's really desirable.
Implicitly removing 'const' is unsafe and unnecessary.
std::construct_at
.
This seems reasonable, but should be supported by adding a dedicated
function that doesn't conflate the type of the storage to write to
and the object to create, e.g. construct_at<const T>(ptr)
.
[Issaquah 2023-02-06; LWG]
Casey noted:
The claimed benefit is allowing the uninitialized_xxx
algorithms
to create objects of const and/or volatile type, which they cannot otherwise do
since they deduce the type of object to be created from the reference type
of the pertinent iterator. Creating const objects has some (marginal?) benefits
over using const pointers to mutable objects. For example, their non-mutable
members cannot be modified via casting away const without undefined behavior.
A unit test might take advantage of this behavior to force a compiler to
diagnose such undefined behavior in a constant expression.
The issue submitter was aware of this, but an open Core issue,
CWG 2514,
would invalidate that benefit. If accepted, objects with dynamic storage
duration (such as those created by std::construct_as
and the
std::uninitialized_xxx
algorithms) would never be const objects,
so casting away the const would not be undefined. So implicitly removing
const in voidify
would still allow modifying "truly const"
objects (resulting in undefined behaviour), without being able to create
"truly const" objects in locations where that actually is safe.
If CWG 2514 is accepted, the voidify
behaviour would be all
downside.
LWG requested removing the remaining casts from the proposed resolution,
relying on an implicit conversion to void*
instead.
Move to Immediate for C++23.
Previous resolution [SUPERSEDED]:
This wording is relative to N4928.
Modify 26.11.1 [specialized.algorithms.general], General, as indicated:
-4- Some algorithms specified in 26.11 [specialized.algorithms] make use of the exposition-only function
voidify
:template<class T> constexpr void* voidify(T& obj) noexcept { return
const_cast<void*>(static_cast<const volatilevoid*>(addressof(obj))); }
[2023-02-13 Approved at February 2023 meeting in Issaquah. Status changed: Immediate → WP.]
Proposed resolution:
This wording is relative to N4928.
Modify 26.11.1 [specialized.algorithms.general], General, as indicated:
-4- Some algorithms specified in 26.11 [specialized.algorithms] make use of the exposition-only function
voidify
:template<class T> constexpr void* voidify(T& obj) noexcept { return
const_cast<void*>(static_cast<const volatile void*>(addressof(obj))); }
terminate
Section: 16.4.2.5 [compliance] Status: C++23 Submitter: CA Opened: 2023-02-01 Last modified: 2023-11-22
Priority: Not Prioritized
View all other issues in [compliance].
View all issues with C++23 status.
Discussion:
This is the resolution for NB comment CA-076
16.4.2.5 [compliance] p4 has this note:
[Note 1: Throwing a standard library provided exception is not
observably different from terminate()
if the implementation
does not unwind the stack during exception handling
(14.4 [except.handle])
and the user's program contains no catch blocks. — end note]
Even under the conditions described by the note, a call to terminate()
is observably different from throwing an exception if the current
terminate_handler
function observes what would have been
the currently handled exception in the case where the exception was thrown.
The set of conditions should be extended to include something along the lines of
"and the current terminate_handler
function simply calls abort()
".
Previous resolution [SUPERSEDED]:
This wording is relative to N4928.
Modify 16.4.2.5 [compliance], "Freestanding implementations", as indicated:
-4- [Note 1: Throwing a standard library provided exception is not observably different fromterminate()
if the implementation does not unwind the stack during exception handling (14.4 [except.handle]) and the user's program contains no catch blocks and the currentterminate_handler
function simply callsabort()
. — end note]
[Issaquah 2023-02-06; LWG]
If the note isn't true then remove it.
Poll: keep note and change as proposed? 3/1/10.
Poll: drop the note entirely? 10/0/5.
Drop the note and move to Immediate for C++20: 9/0/2.
[2023-02-13 Approved at February 2023 meeting in Issaquah. Status changed: Immediate → WP.]
Proposed resolution:
This wording is relative to N4928.
Modify 16.4.2.5 [compliance], "Freestanding implementations", as indicated:
-4- [Note 1: Throwing a standard library provided exception is not observably different fromterminate()
if the implementation does not unwind the stack during exception handling (14.4 [except.handle]) and the user's program contains no catch blocks. — end note]
basic_const_iterator
should have custom iter_move
Section: 24.5.3 [const.iterators] Status: C++23 Submitter: Hewill Kang Opened: 2023-01-31 Last modified: 2023-11-22
Priority: 3
View other active issues in [const.iterators].
View all other issues in [const.iterators].
View all issues with C++23 status.
Discussion:
The standard does not currently customize iter_move
for basic_const_iterator
,
which means that applying iter_move
to basic_const_iterator
will invoke the default behavior.
Although the intent of such an operation is unpredictable, it does introduce some inconsistencies:
int x[] = {1, 2, 3};
using R1 = decltype( x | views::as_rvalue | views::as_const);
using R2 = decltype( x | views::as_const | views::as_rvalue);
using Z1 = decltype(views::zip(x) | views::as_rvalue | views::as_const);
using Z2 = decltype(views::zip(x) | views::as_const | views::as_rvalue);
static_assert(same_as<ranges::range_reference_t<R1>, const int&&>);
static_assert(same_as<ranges::range_reference_t<R2>, const int&&>);
static_assert(same_as<ranges::range_reference_t<Z1>, tuple<const int&&>>);
static_assert(same_as<ranges::range_reference_t<Z2>, tuple<const int&&>>); // failed
In the above example, views::zip(x) | views::as_const
will produce a range whose iterator type is
basic_const_iterator
with reference of tuple<const int&>
.
Since iter_move
adopts the default behavior, its rvalue reference will also be
tuple<const int&>
, so applying views::as_rvalue
to it won't have any effect.
iter_move
specialization for basic_const_iterator
and specifies
the return type as
common_reference_t<const iter_value_t<It>&&, iter_rvalue_reference_t<It>>
,
which is the type that input_iterator
is guaranteed to be valid. This is also in sync with the behavior
of range-v3.
[Issaquah 2023-02-10; LWG issue processing]
Move to Immediate for C++23
[2023-02-13 Approved at February 2023 meeting in Issaquah. Status changed: Immediate → WP.]
Proposed resolution:
This wording is relative to N4928.
Modify 24.5.3 [const.iterators] as indicated:
namespace std { template<class I> concept not-a-const-iterator = see below; template<indirectly_readable I> using iter-const-rvalue-reference-t = // exposition only common_reference_t<const iter_value_t<I>&&, iter_rvalue_reference_t<I>>; template<input_iterator Iterator> class basic_const_iterator { Iterator current_ = Iterator(); // exposition only using reference = iter_const_reference_t<Iterator>; // exposition only using rvalue-reference = iter-const-rvalue-reference-t<Iterator>; // exposition only public: […] template<sized_sentinel_for<Iterator> S> requires different-from<S, basic_const_iterator> friend constexpr difference_type operator-(const S& x, const basic_const_iterator& y); friend constexpr rvalue-reference iter_move(const basic_const_iterator& i) noexcept(noexcept(static_cast<rvalue-reference>(ranges::iter_move(i.current_)))) { return static_cast<rvalue-reference>(ranges::iter_move(i.current_)); } }; }
std::ranges::repeat_view<T, IntegerClass>::iterator
may be ill-formedSection: 25.6.5 [range.repeat] Status: C++23 Submitter: Jiang An Opened: 2023-02-05 Last modified: 2023-11-22
Priority: Not Prioritized
View all issues with C++23 status.
Discussion:
25.6.5.3 [range.repeat.iterator] specifies difference_type
as
using difference_type = conditional_t<is-signed-integer-like<index-type>, index-type, IOTA-DIFF-T(index-type)>;
which always instantiates IOTA-DIFF-T(index-type)
, and thus possibly makes the program ill-formed when
index-type
is an integer-class type (index-type
is same as Bound
in this case), because
IOTA-DIFF-T(index-type)
is specified to be iter_difference_t<index-type>
which may be ill-formed (25.6.4.2 [range.iota.view]/1.1).
index-type
as-is without instantiating IOTA-DIFF-T
when
is-signed-integer-like<index-type>
is true
.
However, when Bound
is an unsigned integer-class type, it's unclear which type should the difference type be,
or whether repeat_view
should be well-formed when the possibly intended IOTA-DIFF-T(Bound)
is ill-formed.
[2023-02-09 Tim adds wording]
We should reject types for which there is no usable difference type.
[Issaquah 2023-02-09; LWG]
Move to Immediate for C++23
[2023-02-13 Approved at February 2023 meeting in Issaquah. Status changed: Immediate → WP.]
Proposed resolution:
This wording is relative to N4928.
Modify 25.2 [ranges.syn], header <ranges>
synopsis, as indicated:
// […] namespace std::ranges { template<move_constructible W, semiregular Bound = unreachable_sentinel_t> requires see below(is_object_v<W> && same_as<W, remove_cv_t<W>> && (is-integer-like<Bound> || same_as<Bound, unreachable_sentinel_t>))class repeat_view;
Modify 25.6.5.2 [range.repeat.view] as indicated:
namespace std::ranges { template<class T> concept integer-like-with-usable-difference-type = //exposition only is-signed-integer-like<T> || (is-integer-like<T> && weakly_incrementable<T>); template<move_constructible W, semiregular Bound = unreachable_sentinel_t> requires (is_object_v<W> && same_as<W, remove_cv_t<W>> && (is-integer-likeinteger-like-with-usable-difference-type<Bound> || same_as<Bound, unreachable_sentinel_t>)) class repeat_view { […] }; }
Modify 25.6.5.3 [range.repeat.iterator] as indicated:
namespace std::ranges { template<move_constructible W, semiregular Bound= unreachable_sentinel_t> requires (is_object_v<W> && same_as<W, remove_cv_t<W>> && (is-integer-likeinteger-like-with-usable-difference-type<Bound> || same_as<Bound, unreachable_sentinel_t>)) class repeat_view<W, Bound>::iterator { private: using index-type = // exposition only conditional_t<same_as<Bound, unreachable_sentinel_t>, ptrdiff_t, Bound>; […] public: […] using difference_type = see belowconditional_t<is-signed-integer-like<index-type>, index-type, IOTA-DIFF-T(index-type)>; […] }; }
-?- If is-signed-integer-like<index-type>
is true
,
the member typedef-name difference_type
denotes index-type
.
Otherwise, it denotes IOTA-DIFF-T(index-type)
(25.6.4.2 [range.iota.view]).
std::layout_XX::mapping
misses preconditionSection: 23.7.3.4 [mdspan.layout] Status: C++23 Submitter: Christian Trott Opened: 2023-02-09 Last modified: 2023-11-22
Priority: Not Prioritized
View all issues with C++23 status.
Discussion:
As shortly discussed during the LWG review of a layout_stride
defect, there is currently no protection against
creating layout mappings with all static extents where the product of those extents exceeds the representable
value range of the index_type
.
layout_left::mapping<extents<int, 100000, 100000>> a{};
But a.required_span_size()
would overflow since the implied span size is 10B and thus exceeds what
int
can represent.
0
.
Hence we can check for this via a mandates check on the class.
The paper P2798R0 has been provided with the proposed wording as shown below.
[Issaquah 2023-02-10; LWG issue processing]
Move to Immediate for C++23
[2023-02-13 Approved at February 2023 meeting in Issaquah. Status changed: Immediate → WP.]
Proposed resolution:
This wording is relative to N4928.
Modify 23.7.3.4.5.1 [mdspan.layout.left.overview] as indicated:
-2- If
-3-Extents
is not a specialization ofextents
, then the program is ill-formed.layout_left::mapping<E>
is a trivially copyable type that modelsregular
for eachE
. -?- Mandates: IfExtents::rank_dynamic() == 0
istrue
, then the size of the multidimensional index spaceExtents()
is representable as a value of typetypename Extents::index_type
.
Modify 23.7.3.4.6.1 [mdspan.layout.right.overview] as indicated:
-2- If
-3-Extents
is not a specialization ofextents
, then the program is ill-formed.layout_right::mapping<E>
is a trivially copyable type that modelsregular
for eachE
. -?- Mandates: IfExtents::rank_dynamic() == 0
istrue
, then the size of the multidimensional index spaceExtents()
is representable as a value of typetypename Extents::index_type
.
Modify 23.7.3.4.7.1 [mdspan.layout.stride.overview] as indicated:
-2- If
-3-Extents
is not a specialization ofextents
, then the program is ill-formed.layout_stride::mapping<E>
is a trivially copyable type that modelsregular
for eachE
. -?- Mandates: IfExtents::rank_dynamic() == 0
istrue
, then the size of the multidimensional index spaceExtents()
is representable as a value of typetypename Extents::index_type
.
const
-qualified monadic overloads for std::expected
Section: 22.8.6.7 [expected.object.monadic], 22.8.7.7 [expected.void.monadic] Status: C++23 Submitter: Sy Brand Opened: 2023-02-09 Last modified: 2023-11-22
Priority: Not Prioritized
View all other issues in [expected.object.monadic].
View all issues with C++23 status.
Discussion:
The constraints for and_then
, transform
, transform_error
, and or_else
for std::expected
seem incorrect for const
overloads. E.g., from 22.8.6.7 [expected.object.monadic]
template<class F> constexpr auto transform(F&& f) &&; template<class F> constexpr auto transform(F&& f) const &&; […]Constraints:
is_move_constructible_v<E>
istrue
.
That constraint should likely be is_move_constructible_v<const E>
for the const
-qualified version.
Same for the lvalue overloads, and for the three other functions, including in the void
partial specialization.
For example, currently this code would result in a hard compiler error inside the body of transform
rather than
failing the constraint:
const std::expected<int, std::unique_ptr<int>> e; std::move(e).transform([](auto) { return 42; });
Previous resolution [SUPERSEDED]:
This wording is relative to N4928.
Modify 22.8.6.7 [expected.object.monadic] as indicated:
template<class F> constexpr auto and_then(F&& f) &; template<class F> constexpr auto and_then(F&& f) const &;[…]
-2- Constraints:is_copy_constructible_v<
isEdecltype((error()))>true
. […]template<class F> constexpr auto and_then(F&& f) &&; template<class F> constexpr auto and_then(F&& f) const &&;[…]
-6- Constraints:is_move_constructible_v<
isEdecltype((error()))>true
. […]template<class F> constexpr auto or_else(F&& f) &; template<class F> constexpr auto or_else(F&& f) const &;[…]
-10- Constraints:is_copy_constructible_v<
isTdecltype((value()))>true
. […]template<class F> constexpr auto or_else(F&& f) &&; template<class F> constexpr auto or_else(F&& f) const &&;[…]
-14- Constraints:is_move_constructible_v<
isTdecltype((value()))>true
. […]template<class F> constexpr auto transform(F&& f) &; template<class F> constexpr auto transform(F&& f) const &;[…]
-18- Constraints:is_copy_constructible_v<
isEdecltype((error()))>true
. […]template<class F> constexpr auto transform(F&& f) &&; template<class F> constexpr auto transform(F&& f) const &&;[…]
-22- Constraints:is_move_constructible_v<
isEdecltype((error()))>true
. […]template<class F> constexpr auto transform_error(F&& f) &; template<class F> constexpr auto transform_error(F&& f) const &;[…]
-26- Constraints:is_copy_constructible_v<
isTdecltype((value()))>true
. […]template<class F> constexpr auto transform_error(F&& f) &&; template<class F> constexpr auto transform_error(F&& f) const &&;[…]
-30- Constraints:is_move_constructible_v<
isTdecltype((value()))>true
.Modify 22.8.7.7 [expected.void.monadic] as indicated:
template<class F> constexpr auto and_then(F&& f) &; template<class F> constexpr auto and_then(F&& f) const &;[…]
-2- Constraints:is_copy_constructible_v<
isEdecltype((error()))>true
. […]template<class F> constexpr auto and_then(F&& f) &&; template<class F> constexpr auto and_then(F&& f) const &&;[…]
-6- Constraints:is_move_constructible_v<
isEdecltype((error()))>true
. […]template<class F> constexpr auto transform(F&& f) &; template<class F> constexpr auto transform(F&& f) const &;[…]
-16- Constraints:is_copy_constructible_v<
isEdecltype((error()))>true
. […]template<class F> constexpr auto transform(F&& f) &&; template<class F> constexpr auto transform(F&& f) const &&;[…]
-20- Constraints:is_move_constructible_v<
isEdecltype((error()))>true
. […]
[Issaquah 2023-02-09; Jonathan provides improved wording]
[Issaquah 2023-02-09; LWG]
Move to Immediate for C++23
[2023-02-13 Approved at February 2023 meeting in Issaquah. Status changed: Immediate → WP.]
Proposed resolution:
This wording is relative to N4928.
Modify 22.8.6.7 [expected.object.monadic] as indicated:
template<class F> constexpr auto and_then(F&& f) &; template<class F> constexpr auto and_then(F&& f) const &;[…]
-2- Constraints:is_
iscopy_constructible_v<E, decltype(error())>true
. […]template<class F> constexpr auto and_then(F&& f) &&; template<class F> constexpr auto and_then(F&& f) const &&;[…]
-6- Constraints:is_
iscopy_constructible_v<E, decltype(std::move(error()))>true
. […]template<class F> constexpr auto or_else(F&& f) &; template<class F> constexpr auto or_else(F&& f) const &;[…]
-10- Constraints:is_
iscopy_constructible_v<T, decltype(value())>true
. […]template<class F> constexpr auto or_else(F&& f) &&; template<class F> constexpr auto or_else(F&& f) const &&;[…]
-14- Constraints:is_
iscopy_constructible_v<T, decltype(std::move(value()))>true
. […]template<class F> constexpr auto transform(F&& f) &; template<class F> constexpr auto transform(F&& f) const &;[…]
-18- Constraints:is_
iscopy_constructible_v<E, decltype(error())>true
. […]template<class F> constexpr auto transform(F&& f) &&; template<class F> constexpr auto transform(F&& f) const &&;[…]
-22- Constraints:is_
iscopy_constructible_v<E, decltype(std::move(error()))>true
. […]template<class F> constexpr auto transform_error(F&& f) &; template<class F> constexpr auto transform_error(F&& f) const &;[…]
-26- Constraints:is_
iscopy_constructible_v<T, decltype(value())>true
. […]template<class F> constexpr auto transform_error(F&& f) &&; template<class F> constexpr auto transform_error(F&& f) const &&;[…]
-30- Constraints:is_
iscopy_constructible_v<T, decltype(std::move(value()))>true
.
Modify 22.8.7.7 [expected.void.monadic] as indicated:
template<class F> constexpr auto and_then(F&& f) &; template<class F> constexpr auto and_then(F&& f) const &;[…]
-2- Constraints:is_
iscopy_constructible_v<E, decltype(error())>true
. […]template<class F> constexpr auto and_then(F&& f) &&; template<class F> constexpr auto and_then(F&& f) const &&;[…]
-6- Constraints:is_
iscopy_constructible_v<E, decltype(std::move(error()))>true
. […]template<class F> constexpr auto transform(F&& f) &; template<class F> constexpr auto transform(F&& f) const &;[…]
-16- Constraints:is_
iscopy_constructible_v<E, decltype(error())>true
. […]template<class F> constexpr auto transform(F&& f) &&; template<class F> constexpr auto transform(F&& f) const &&;[…]
-20- Constraints:is_
iscopy_constructible_v<E, decltype(std::move(error()))>true
. […]
import std;
should guarantee initialization of standard iostreams objectsSection: 31.4.2 [iostream.objects.overview] Status: C++23 Submitter: Tim Song Opened: 2023-02-09 Last modified: 2024-01-29
Priority: Not Prioritized
View all other issues in [iostream.objects.overview].
View all issues with C++23 status.
Discussion:
In the old world, #include <iostream>
behaves as if it defined a static-storage-duration ios_base::Init
object,
which causes the standard iostreams objects to be initialized (if necessary)
on startup and flushed on shutdown.
import std;
, so we need separate wording to
provide this guarantee. The proposed resolution below was adapted from a suggestion by
Mathias Stearn on the reflector.
[2023-02-09 Tim updates wording following LWG discussion]
[Issaquah 2023-02-09; LWG]
Move to Immediate for C++23
[2023-02-13 Approved at February 2023 meeting in Issaquah. Status changed: Immediate → WP.]
Proposed resolution:
This wording is relative to N4928.
Modify 31.4.2 [iostream.objects.overview]p5 as indicated:
-5- The results of including <iostream>
in a translation unit shall be
as if <iostream>
defined an instance of ios_base::Init
with static
storage duration. Each C++ library module (16.4.2.4 [std.modules])
in a hosted implementation shall behave as if it contains an interface unit that defines
an unexported ios_base::Init
variable with ordered initialization
(6.9.3.3 [basic.start.dynamic]).
erase_if
for flat_{,multi}set
is incorrectly specifiedSection: 23.6.11.6 [flat.set.erasure], 23.6.12.6 [flat.multiset.erasure] Status: C++23 Submitter: Tim Song Opened: 2023-02-09 Last modified: 2023-11-22
Priority: Not Prioritized
View all issues with C++23 status.
Discussion:
The current specification of erase_if
for flat_{,multi}set
calls ranges::remove_if
on the set, which is obviously incorrect —
the set only present constant views of its elements.
[Issaquah 2023-02-09; LWG]
Move to Immediate for C++23
[2023-02-13 Approved at February 2023 meeting in Issaquah. Status changed: Immediate → WP.]
Proposed resolution:
This wording is relative to N4928.
Modify 23.6.11.6 [flat.set.erasure] as indicated:
template<class Key, class Compare, class KeyContainer, class Predicate> typename flat_set<Key, Compare, KeyContainer>::size_type erase_if(flat_set<Key, Compare, KeyContainer>& c, Predicate pred);
-1- Effects: Equivalent to:auto [erase_first, erase_last] = ranges::remove_if(c, pred); auto n = erase_last - erase_first; c.erase(erase_first, erase_last); return n;-1- Preconditions:
-2- Effects: Let E beKey
meets the Cpp17MoveAssignable requirements.bool(pred(as_const(e)))
. Erases all elementse
inc
for which E holds. -3- Returns: The number of elements erased. -4- Complexity: Exactlyc.size()
applications of the predicate. -5- Remarks: Stable (16.4.6.8 [algorithm.stable]). If an invocation oferase_if
exits via an exception,c
is in a valid but unspecified state (3.67 [defns.valid]). [Note 1:c
still meets its invariants, but can be empty. — end note]
Modify 23.6.12.6 [flat.multiset.erasure] as indicated:
template<class Key, class Compare, class KeyContainer, class Predicate> typename flat_multiset<Key, Compare, KeyContainer>::size_type erase_if(flat_multiset<Key, Compare, KeyContainer>& c, Predicate pred);
-1- Effects: Equivalent to:auto [erase_first, erase_last] = ranges::remove_if(c, pred); auto n = erase_last - erase_first; c.erase(erase_first, erase_last); return n;-1- Preconditions:
-2- Effects: Let E beKey
meets the Cpp17MoveAssignable requirements.bool(pred(as_const(e)))
. Erases all elementse
inc
for which E holds. -3- Returns: The number of elements erased. -4- Complexity: Exactlyc.size()
applications of the predicate. -5- Remarks: Stable (16.4.6.8 [algorithm.stable]). If an invocation oferase_if
exits via an exception,c
is in a valid but unspecified state (3.67 [defns.valid]). [Note 1:c
still meets its invariants, but can be empty. — end note]
operator+=
complexity for {chunk,stride}_view::iterator
Section: 25.7.29.7 [range.chunk.fwd.iter], 25.7.32.3 [range.stride.iterator] Status: C++23 Submitter: Tim Song Opened: 2023-02-09 Last modified: 2023-11-22
Priority: Not Prioritized
View all issues with C++23 status.
Discussion:
The intent was that the precondition allows the call to ranges::advance
,
which otherwise would have time linear in the argument of operator+=
,
to actually be implemented using operator+=
or equivalent for all but
the last step. This is at best very non-obvious and should be clarified.
[Issaquah 2023-02-10; LWG issue processing]
Move to Immediate for C++23
[2023-02-13 Approved at February 2023 meeting in Issaquah. Status changed: Immediate → WP.]
Proposed resolution:
This wording is relative to N4928.
Modify 25.7.29.7 [range.chunk.fwd.iter] p13 as indicated:
constexpr iterator& operator+=(difference_type x) requires random_access_range<Base>;-12- Preconditions: If
[Note 1: Ifx
is positive,ranges::distance(current_, end_) > n_ * (x - 1)
istrue
.x
is negative, the Effects paragraph implies a precondition. — end note] -13- Effects: Equivalent to:if (x > 0) { ranges::advance(current_, n_ * (x - 1)); missing_ = ranges::advance(current_, n_* x, end_); } else if (x < 0) { ranges::advance(current_, n_ * x + missing_); missing_ = 0; } return *this;
Modify 25.7.32.3 [range.stride.iterator] p14 as indicated:
constexpr iterator& operator+=(difference_type n) requires random_access_range<Base>;-13- Preconditions: If
[Note 1: Ifn
is positive,ranges::distance(current_, end_) > stride_ * (n - 1)
istrue
.n
is negative, the Effects paragraph implies a precondition. — end note] -14- Effects: Equivalent to:if (n > 0) { ranges::advance(current_, stride_ * (n - 1)); missing_ = ranges::advance(current_, stride_* n, end_); } else if (n < 0) { ranges::advance(current_, stride_ * n + missing_); missing_ = 0; } return *this;
std::string
Section: 23.6.13 [container.adaptors.format] Status: C++23 Submitter: Victor Zverovich Opened: 2023-02-10 Last modified: 2023-11-22
Priority: Not Prioritized
View all issues with C++23 status.
Discussion:
According to 23.6.13 [container.adaptors.format] container adapters such as std::stack
are
formatted by forwarding to the underlying container:
template<class FormatContext> typename FormatContext::iterator format(maybe-const-adaptor& r, FormatContext& ctx) const;Effects: Equivalent to:
return underlying_.format(r.c, ctx);
This gives expected results for std::stack<T>
and most types of underlying container:
auto s = std::format("{}", std::stack(std::deque{'a', 'b', 'c'})); // s == "['a', 'b', 'c']"
However, when the underlying container is std::string
the output is:
auto s = std::format("{}", std::stack{std::string{"abc"}}); // s == "abc"
This is clearly incorrect because std::stack
itself is not a string (it is only backed by a string)
and inconsistent with formatting of ranges where non-string range types are formatted as comma-separated values
delimited by '['
and ']'
. The correct output in this case would be ['a', 'b', 'c']
.
std::views::all(_t)
(https://godbolt.org/z/8MT1be838).
Previous resolution [SUPERSEDED]:
This wording is relative to N4928.
Modify 23.6.13 [container.adaptors.format] as indicated:
-1- For each of
queue
,priority_queue
, andstack
, the library provides the following formatter specialization whereadaptor-type
is the name of the template:[…]namespace std { template<class charT, class T, formattable<charT> Container, class... U> struct formatter<adaptor-type<T, Container, U...>, charT> { private: using maybe-const-adaptor = // exposition only fmt-maybe-const<adaptor-type<T, Container, U...>, charT>; formatter<views::all_t<const Container&>, charT> underlying_; // exposition only public: template<class ParseContext> constexpr typename ParseContext::iterator parse(ParseContext& ctx); template<class FormatContext> typename FormatContext::iterator format(maybe-const-adaptor& r, FormatContext& ctx) const; }; }template<class FormatContext> typename FormatContext::iterator format(maybe-const-adaptor& r, FormatContext& ctx) const;-3- Effects: Equivalent to:
return underlying_.format(views::all(r.c), ctx);
[2023-02-10 Tim provides updated wording]
The container elements may not be const-formattable so we cannot
use the const
formatter unconditionally. Also the current wording
is broken because an adaptor is not range and we cannot use
fmt-maybe-const
on the adaptor — only the underlying container.
[Issaquah 2023-02-10; LWG issue processing]
Move to Immediate for C++23
[2023-02-13 Approved at February 2023 meeting in Issaquah. Status changed: Immediate → WP.]
Proposed resolution:
This wording is relative to N4928.
Modify 23.6.13 [container.adaptors.format] as indicated:
-1- For each of
queue
,priority_queue
, andstack
, the library provides the following formatter specialization whereadaptor-type
is the name of the template:[…]namespace std { template<class charT, class T, formattable<charT> Container, class... U> struct formatter<adaptor-type<T, Container, U...>, charT> { private: using maybe-const-container = // exposition only fmt-maybe-const<Container, charT>; using maybe-const-adaptor = // exposition onlyfmt-maybe-const<is_const_v<maybe-const-container>, adaptor-type<T, Container, U...>, charT>; // see 25.2 [ranges.syn] formatter<ranges::ref_view<maybe-const-container>Container, charT> underlying_; // exposition only public: template<class ParseContext> constexpr typename ParseContext::iterator parse(ParseContext& ctx); template<class FormatContext> typename FormatContext::iterator format(maybe-const-adaptor& r, FormatContext& ctx) const; }; }template<class FormatContext> typename FormatContext::iterator format(maybe-const-adaptor& r, FormatContext& ctx) const;-3- Effects: Equivalent to:
return underlying_.format(r.c, ctx);
flat_foo
is missing allocator-extended copy/move constructorsSection: 23.6.8 [flat.map], 23.6.9 [flat.multimap], 23.6.11 [flat.set], 23.6.12 [flat.multiset] Status: WP Submitter: Arthur O'Dwyer Opened: 2023-02-06 Last modified: 2023-11-22
Priority: Not Prioritized
View other active issues in [flat.map].
View all other issues in [flat.map].
View all issues with WP status.
Discussion:
This issue is part of the "flat_foo
" sequence, LWG 3786(i), 3802(i),
3803(i), 3804(i). flat_set
, flat_multiset
, flat_map
, and
flat_multimap
all have implicitly defaulted copy and move constructors, but they lack
allocator-extended versions of those constructors. This means that the following code is broken:
// https://godbolt.org/z/qezv5rTrW #include <cassert> #include <flat_set> #include <memory_resource> #include <utility> #include <vector> template<class T, class Comp = std::less<T>> using pmr_flat_set = std::flat_set<T, Comp, std::pmr::vector<T>>; int main() { std::pmr::vector<pmr_flat_set<int>> vs = { {1,2,3}, {4,5,6}, }; std::pmr::vector<int> v = std::move(vs[0]).extract(); assert(v.get_allocator().resource() == std::pmr::get_default_resource()); }
pmr_flat_set<int>
advertises that it "uses_allocator
" std::pmr::polymorphic_allocator<int>
,
but in fact it lacks the allocator-extended constructor overload set necessary to interoperate with std::pmr::vector
.
[2023-03-22; Reflector poll]
Set status to Tentatively Ready after five votes in favour during reflector poll.
[2023-06-17 Approved at June 2023 meeting in Varna. Status changed: Voting → WP.]
Proposed resolution:
This wording is relative to N4928.
Modify 23.6.8.2 [flat.map.defn] as indicated:
namespace std { template<class Key, class T, class Compare = less<Key>, class KeyContainer = vector<Key>, class MappedContainer = vector<T>> class flat_map { public: […] // 23.6.8.3 [flat.map.cons], construct/copy/destroy flat_map() : flat_map(key_compare()) { } template<class Allocator> flat_map(const flat_map&, const Allocator& a); template<class Allocator> flat_map(flat_map&&, const Allocator& a); […] }; […] }
Modify 23.6.8.3 [flat.map.cons] as indicated:
[Drafting note: As a drive-by fix, a missing closing
>
is inserted in p11.]
template<class Allocator> flat_map(const flat_map&, const Allocator& a); template<class Allocator> flat_map(flat_map&&, const Allocator& a); template<class Allocator> flat_map(const key_compare& comp, const Allocator& a); template<class Allocator> explicit flat_map(const Allocator& a); template<class InputIterator, class Allocator> flat_map(InputIterator first, InputIterator last, const key_compare& comp, const Allocator& a); template<class InputIterator, class Allocator> flat_map(InputIterator first, InputIterator last, const Allocator& a); […]-11- Constraints:
-12- Effects: Equivalent to the corresponding non-allocator constructors except thatuses_allocator_v<key_container_type, Allocator>
istrue
anduses_allocator_v<mapped_container_type, Allocator>
istrue
.c.keys
andc.values
are constructed with uses-allocator construction (20.2.8.2 [allocator.uses.construction]).
Modify 23.6.9.2 [flat.multimap.defn] as indicated:
namespace std { template<class Key, class T, class Compare = less<Key>, class KeyContainer = vector<Key>, class MappedContainer = vector<T>> class flat_multimap { public: […] // 23.6.9.3 [flat.multimap.cons], construct/copy/destroy flat_multimap() : flat_multimap(key_compare()) { } template<class Allocator> flat_multimap(const flat_multimap&, const Allocator& a); template<class Allocator> flat_multimap(flat_multimap&&, const Allocator& a); […] }; […] }
Modify 23.6.9.3 [flat.multimap.cons] as indicated:
template<class Allocator> flat_multimap(const flat_multimap&, const Allocator& a); template<class Allocator> flat_multimap(flat_multimap&&, const Allocator& a); template<class Allocator> flat_multimap(const key_compare& comp, const Allocator& a); […]-11- Constraints:
-12- Effects: Equivalent to the corresponding non-allocator constructors except thatuses_allocator_v<key_container_type, Allocator>
istrue
anduses_allocator_v<mapped_container_type, Allocator>
istrue
.c.keys
andc.values
are constructed with uses-allocator construction (20.2.8.2 [allocator.uses.construction]).
Modify 23.6.11.2 [flat.set.defn] as indicated:
namespace std { template<class Key, class Compare = less<Key>, class KeyContainer = vector<Key>> class flat_set { public: […] // 23.6.11.3 [flat.set.cons], constructors flat_set() : flat_set(key_compare()) { } template<class Allocator> flat_set(const flat_set&, const Allocator& a); template<class Allocator> flat_set(flat_set&&, const Allocator& a); […] }; […] }
Modify 23.6.11.3 [flat.set.cons] as indicated:
template<class Allocator> flat_set(const flat_set&, const Allocator& a); template<class Allocator> flat_set(flat_set&&, const Allocator& a); template<class Allocator> flat_set(const key_compare& comp, const Allocator& a); […]-9- Constraints:
-10- Effects: Equivalent to the corresponding non-allocator constructors except thatuses_allocator_v<container_type, Allocator>
istrue
.c
is constructed with uses-allocator construction (20.2.8.2 [allocator.uses.construction]).
Modify 23.6.12.2 [flat.multiset.defn] as indicated:
namespace std { template<class Key, class Compare = less<Key>, class KeyContainer = vector<Key>> class flat_multiset { public: […] // 23.6.12.3 [flat.multiset.cons], constructors flat_multiset() : flat_multiset(key_compare()) { } template<class Allocator> flat_multiset(const flat_multiset&, const Allocator& a); template<class Allocator> flat_multiset(flat_multiset&&, const Allocator& a); […] }; […] }
Modify 23.6.12.3 [flat.multiset.cons] as indicated:
template<class Allocator> flat_multiset(const flat_multiset&, const Allocator& a); template<class Allocator> flat_multiset(flat_multiset&&, const Allocator& a); template<class Allocator> flat_multiset(const key_compare& comp, const Allocator& a); […]-9- Constraints:
-10- Effects: Equivalent to the corresponding non-allocator constructors except thatuses_allocator_v<container_type, Allocator>
istrue
.c
is constructed with uses-allocator construction (20.2.8.2 [allocator.uses.construction]).
op
' should be in [zombie.names]Section: 16.4.5.3.2 [zombie.names] Status: WP Submitter: Jonathan Wakely Opened: 2023-02-11 Last modified: 2023-11-22
Priority: Not Prioritized
View all issues with WP status.
Discussion:
C++98 and C++11 defined a protected member std::bind1st::op
so 'op
' should still be a reserved name,
just like 'bind1st
' itself.
[2023-03-22; Reflector poll]
Set status to Tentatively Ready after seven votes in favour during reflector poll.
[2023-06-17 Approved at June 2023 meeting in Varna. Status changed: Voting → WP.]
Proposed resolution:
This wording is relative to N4928.
Modify 16.4.5.3.2 [zombie.names] as indicated:
-2- The following names are reserved as members for previous standardization, and may not be used as a name for object-like macros in portable code:
(2.1) —
argument_type
,[…]
(2.3) —
io_state
,(2.?) —
op
,(2.4) —
open_mode
,[…]
Section: 22.5.3.1 [optional.optional.general], 22.8.6.1 [expected.object.general] Status: WP Submitter: Casey Carter Opened: 2023-02-13 Last modified: 2024-11-28
Priority: 3
View all other issues in [optional.optional.general].
View all issues with WP status.
Discussion:
While implementing P2505R5 "Monadic Functions for std::expected
" we found it odd that
the template type parameter for the assignment operator that accepts an argument by forwarding reference is
defaulted, but the template type parameter for value_or
is not. For consistency, it would seem that
meow.value_or(woof)
should accept the same arguments woof
as does
meow = woof
, even when those arguments are braced-initializers.
value_or
to T
instead of remove_cv_t<T>
. For expected<const vector<int>, int> meow{unexpect, 42};
,
for example, meow.value_or({1, 2, 3})
would create a temporary const vector<int>
for the argument and return a copy of that argument. Were the default template argument instead
remove_cv_t<T>
, meow.value_or({1, 2, 3})
could move construct its return value
from the argument vector<int>
. For the same reason, the constructor that accepts a forwarding
reference with a default template argument of T
should default that argument to remove_cv_t<T>
.
For consistency, it would be best to default the template argument of the perfect-forwarding construct,
perfect-forwarding assignment operator, and value_or
to remove_cv_t<T>
. Since all of
the arguments presented apply equally to optional
, we believe optional
should be changed
consistently with expected
. MSVCSTL has prototyped these changes successfully.
[2023-03-22; Reflector poll]
Set priority to 3 after reflector poll.
[2024-09-18; Reflector poll]
Set status to Tentatively Ready after six votes in favour during reflector poll.
[Wrocław 2024-11-23; Status changed: Voting → WP.]
Proposed resolution:
This wording is relative to N4928.
Modify 22.5.3.1 [optional.optional.general] as indicated:
namespace std { template<class T> class optional { public: […] template<class U = remove_cv_t<T>> constexpr explicit(see below) optional(U&&); […] template<class U = remove_cv_t<T>> constexpr optional& operator=(U&&); […] template<class U = remove_cv_t<T>> constexpr T value_or(U&&) const &; template<class U = remove_cv_t<T>> constexpr T value_or(U&&) &&; […] }; […] }
Modify 22.5.3.2 [optional.ctor] as indicated:
template<class U = remove_cv_t<T>> constexpr explicit(see below) optional(U&& v);-23- Constraints: […]
Modify 22.5.3.4 [optional.assign] as indicated:
template<class U = remove_cv_t<T>> constexpr optional& operator=(U&& v);-12- Constraints: […]
Modify 22.5.3.7 [optional.observe] as indicated:
template<class U = remove_cv_t<T>> constexpr T value_or(U&& v) const &;-15- Mandates: […]
[…]template<class U = remove_cv_t<T>> constexpr T value_or(U&& v) &&;-17- Mandates: […]
Modify 22.8.6.1 [expected.object.general] as indicated:
namespace std { template<class T, class E> class expected { public: […] template<class U = remove_cv_t<T>> constexpr explicit(see below) expected(U&& v); […] template<class U = remove_cv_t<T>> constexpr expected& operator=(U&&); […] template<class U = remove_cv_t<T>> constexpr T value_or(U&&) const &; template<class U = remove_cv_t<T>> constexpr T value_or(U&&) &&; […] }; […] }
Modify 22.8.6.2 [expected.object.cons] as indicated:
template<class U = remove_cv_t<T>> constexpr explicit(!is_convertible_v<U, T>) expected(U&& v);-23- Constraints: […]
Modify 22.8.6.4 [expected.object.assign] as indicated:
template<class U = remove_cv_t<T>> constexpr expected& operator=(U&& v);-9- Constraints: […]
Modify 22.8.6.6 [expected.object.obs] as indicated:
template<class U = remove_cv_t<T>> constexpr T value_or(U&& v) const &;-16- Mandates: […]
[…]template<class U = remove_cv_t<T>> constexpr T value_or(U&& v) &&;-18- Mandates: […]
allocate_at_least
Section: 17.3.2 [version.syn] Status: WP Submitter: Alisdair Meredith Opened: 2023-02-14 Last modified: 2023-11-22
Priority: Not Prioritized
View all other issues in [version.syn].
View all issues with WP status.
Discussion:
In Issaquah, we just adopted P2652R0, forbidding user specialization of allocator_traits
.
allocate_at_least
, which is covered by the macro
__cpp_lib_allocate_at_least
.
I believe we should have updated that macro for this significant change in how that function is accessed
(from free function to a member of allocator_traits
). Unfortunately, there are no more meetings
to process a comment to that effect, and I suspect this rises beyond the purview of an editorial change,
although I live in hope (as the original paper left the value of the macro to the editor, although we
approved existing working papers where that macro does have a value, i.e., status quo).
[2023-03-22; Reflector poll]
Set status to Tentatively Ready after seven votes in favour during reflector poll. But "if we don’t get the editors to do it for C++23 there doesn’t seem to be any point doing it."
[2023-06-17 Approved at June 2023 meeting in Varna. Status changed: Voting → WP.]
Proposed resolution:
This wording is relative to N4928.
Modify 17.3.2 [version.syn], header <version>
synopsis, as indicated and replace
the placeholder YYYYMM
by the year and month of adoption of P2652R0:
[…] #define __cpp_lib_algorithm_iterator_requirements 202207L // also in <algorithm>, <numeric>, <memory> #define __cpp_lib_allocate_at_least202106YYYYMML // also in <memory> #define __cpp_lib_allocator_traits_is_always_equal 201411L // also in […] […]
Section: 28.5.7.2 [format.range.formatter], 28.5.9 [format.tuple] Status: WP Submitter: Victor Zverovich Opened: 2023-02-20 Last modified: 2023-11-22
Priority: 2
View all other issues in [format.range.formatter].
View all issues with WP status.
Discussion:
formatter
specializations for ranges and tuples set debug format for underlying element formatters in their
parse
functions e.g. 28.5.7.2 [format.range.formatter] p9:
template<class ParseContext> constexpr typename ParseContext::iterator parse(ParseContext& ctx);Effects: Parses the format specifier as a range-format-spec and stores the parsed specifiers in
*this
. The values ofopening-bracket_
,closing-bracket_
, andseparator_
are modified if and only if required by the range-type or then
option, if present. If:
— the range-type is neither
s
nor?s
,—
underlying_.set_debug_format()
is a valid expression, and— there is no range-underlying-spec,
then calls
underlying_.set_debug_format()
.
However, they don't say anything about calling parse
functions of those formatters. As as result,
formatting of nested ranges can be incorrect, e.g.
std::string s = std::format("{}", std::vector<std::vector<std::string>>{{"a, b", "c"}});
With the current specification s
is [[a, b, c]]
instead of [["a, b", "c"]]
, i.e. strings
in the output are not correctly escaped. The same is true for nested tuples and combinations of tuples and ranges.
parse
for underlying formatter.
Additionally the standard should clarify that format-spec cannot start with '}'
because that's the
implicit assumption in range formatting and what happens when format-spec is not present.
[2023-03-22; Reflector poll]
Set priority to 2 after reflector poll.
Previous resolution [SUPERSEDED]:
This wording is relative to N4928.
Modify 28.5.2.1 [format.string.general] p1 as indicated:
- […]
- format-specifier:
- : format-spec
- format-spec:
- as specified by the
formatter
specialization for the argument type; cannot start with}
Modify 28.5.6.1 [formatter.requirements] as indicated:
-3- Given character type
[…]charT
, output iterator typeOut
, and formatting argument typeT
, in Table 74 [tab:formatter.basic] and Table 75 [tab:formatter]:pc.begin()
points to the beginning of the format-spec (28.5.2 [format.string]) of the replacement field being formatted in the format string. If format-spec is not present or empty then eitherpc.begin() == pc.end()
or*pc.begin() == '}'
.Modify 28.5.7.2 [format.range.formatter] as indicated:
template<class ParseContext> constexpr typename ParseContext::iterator parse(ParseContext& ctx);-9- Effects: Parses the format specifiers as a range-format-spec and stores the parsed specifiers in
*this
. Callsunderlying_.parse(ctx)
to parse format-spec in range-format-spec or, if the latter is not present, empty format-spec. The values ofopening-bracket_
,closing-bracket_
, andseparator_
are modified if and only if required by the range-type or then
option, if present. If:
(9.1) — the range-type is neither
s
nor?s
,(9.2) —
underlying_.set_debug_format()
is a valid expression, and(9.3) — there is no range-underlying-spec,
then calls
underlying_.set_debug_format()
.Modify 28.5.9 [format.tuple] as indicated:
template<class ParseContext> constexpr typename ParseContext::iterator parse(ParseContext& ctx);-7- Effects: Parses the format specifiers as a tuple-format-spec and stores the parsed specifiers in
*this
. The values ofopening-bracket_
,closing-bracket_
, andseparator_
are modified if and only if required by the tuple-type, if present. For each elemente
inunderlying_
, callse.parse(ctx)
to parse empty format-spec and, ife.set_debug_format()
is a valid expression, callse.set_debug_format()
.
[Varna 2023-06-16; Jonathan provides tweaked wording]
Add "an" in two places.
[Varna 2023-06-16; Move to Ready]
This would allow resolving LWG 3776(i) as NAD.
[2023-11-11 Approved at November 2023 meeting in Kona. Status changed: Voting → WP.]
Proposed resolution:
This wording is relative to N4928.
Modify 28.5.2.1 [format.string.general] p1 as indicated:
- […]
- format-specifier:
- : format-spec
- format-spec:
- as specified by the
formatter
specialization for the argument type; cannot start with}
Modify 28.5.6.1 [formatter.requirements] as indicated:
-3- Given character type
[…]charT
, output iterator typeOut
, and formatting argument typeT
, in Table 74 [tab:formatter.basic] and Table 75 [tab:formatter]:pc.begin()
points to the beginning of the format-spec (28.5.2 [format.string]) of the replacement field being formatted in the format string. If format-spec is not present or empty then eitherpc.begin() == pc.end()
or*pc.begin() == '}'
.
Modify 28.5.7.2 [format.range.formatter] as indicated:
template<class ParseContext> constexpr typename ParseContext::iterator parse(ParseContext& ctx);-9- Effects: Parses the format specifiers as a range-format-spec and stores the parsed specifiers in
*this
. Callsunderlying_.parse(ctx)
to parse format-spec in range-format-spec or, if the latter is not present, an empty format-spec. The values ofopening-bracket_
,closing-bracket_
, andseparator_
are modified if and only if required by the range-type or then
option, if present. If:
(9.1) — the range-type is neither
s
nor?s
,(9.2) —
underlying_.set_debug_format()
is a valid expression, and(9.3) — there is no range-underlying-spec,
then calls
underlying_.set_debug_format()
.
Modify 28.5.9 [format.tuple] as indicated:
template<class ParseContext> constexpr typename ParseContext::iterator parse(ParseContext& ctx);-7- Effects: Parses the format specifiers as a tuple-format-spec and stores the parsed specifiers in
*this
. The values ofopening-bracket_
,closing-bracket_
, andseparator_
are modified if and only if required by the tuple-type, if present. For each elemente
inunderlying_
, callse.parse(ctx)
to parse an empty format-spec and, ife.set_debug_format()
is a valid expression, callse.set_debug_format()
.
atomic<shared_ptr<T>> a; a = nullptr;
Section: 32.5.8.7.2 [util.smartptr.atomic.shared] Status: WP Submitter: Zachary Wassall Opened: 2023-02-22 Last modified: 2023-11-22
Priority: Not Prioritized
View all other issues in [util.smartptr.atomic.shared].
View all issues with WP status.
Discussion:
LWG 3661(i), "constinit atomic<shared_ptr<T>> a(nullptr);
should work"
added the following to atomic<shared_ptr<T>>
:
constexpr atomic(nullptr_t) noexcept : atomic() { }
I believe that doing so broke the following example:
atomic<shared_ptr<T>> a; a = nullptr;
For reference, atomic<shared_ptr<T>>
provides two assignment operator overloads:
void operator=(const atomic&) = delete; // #1 void operator=(shared_ptr<T> desired) noexcept; // #2
Prior to LWG 3661(i), the assignment in the example unambiguously matches #2. #1 is not viable because
nullptr_t
is not convertible to atomic<shared_ptr<T>>
. After LWG 3611, #1 is viable
and the assignment is ambiguous between #1 and #2.
nullptr_t
constructor:
void operator=(nullptr_t) noexcept;
[2023-02-25; Daniel comments and provides wording]
The suggested delegation below to store(nullptr)
is not ambiguous and calls the constructor
shared_ptr(nullptr_t)
, which is guaranteed to be no-throwing.
[2023-03-22; Reflector poll]
Set status to Tentatively Ready after six votes in favour during reflector poll.
[2023-06-17 Approved at June 2023 meeting in Varna. Status changed: Voting → WP.]
Proposed resolution:
This wording is relative to N4928.
Modify 32.5.8.7.2 [util.smartptr.atomic.shared] as indicated:
[…]namespace std { template<class T> struct atomic<shared_ptr<T>> { […] constexpr atomic() noexcept; constexpr atomic(nullptr_t) noexcept : atomic() { } atomic(shared_ptr<T> desired) noexcept; atomic(const atomic&) = delete; void operator=(const atomic&) = delete; […] void operator=(shared_ptr<T> desired) noexcept; void operator=(nullptr_t) noexcept; […] private: shared_ptr<T> p; // exposition only }; }void operator=(shared_ptr<T> desired) noexcept;-5- Effects: Equivalent to
store(desired)
.void operator=(nullptr_t) noexcept;-?- Effects: Equivalent to
store(nullptr)
.
generator::promise_type::yield_value(ranges::elements_of<Rng, Alloc>)
should not be noexcept
Section: 25.8.5 [coro.generator.promise] Status: WP Submitter: Tim Song Opened: 2023-02-25 Last modified: 2023-11-22
Priority: Not Prioritized
View all other issues in [coro.generator.promise].
View all issues with WP status.
Discussion:
The overload of yield_value
for yielding elements of arbitrary ranges does so by creating a nested generator,
but to do so it needs to:
call ranges::begin
/ranges::end
on the range
allocate a new coroutine frame (unless the allocation is elided by the compiler, which isn't guaranteed)
copy/move the iterator and sentinel into the coroutine frame
All of these are allowed to throw, so this overload should not be noexcept
.
[2023-03-22; Reflector poll]
Set status to Tentatively Ready after six votes in favour during reflector poll.
[2023-06-17 Approved at June 2023 meeting in Varna. Status changed: Voting → WP.]
Proposed resolution:
This wording is relative to N4928.
Modify 25.8.5 [coro.generator.promise] as indicated:
[…]namespace std { template<class Ref, class V, class Allocator> class generator<Ref, V, Allocator>::promise_type { public: […] auto yield_value(const remove_reference_t<yielded>& lval) requires is_rvalue_reference_v<yielded> && constructible_from<remove_cvref_t<yielded>, const remove_reference_t<yielded>&>; template<class R2, class V2, class Alloc2, class Unused> requires same_as<typename generator<R2, V2, Alloc2>::yielded, yielded> auto yield_value(ranges::elements_of<generator<R2, V2, Alloc2>&&, Unused> g) noexcept; template<ranges::input_range R, class Alloc> requires convertible_to<ranges::range_reference_t<R>, yielded> auto yield_value(ranges::elements_of<R, Alloc> r)noexcept; […] }; }template<ranges::input_range R, class Alloc> requires convertible_to<ranges::range_reference_t<R>, yielded> auto yield_value(ranges::elements_of<R, Alloc> r)noexcept;-13- Effects: Equivalent to:
[…]auto nested = [](allocator_arg_t, Alloc, ranges::iterator_t<R> i, ranges::sentinel_t<R> s) -> generator<yielded, ranges::range_value_t<R>, Alloc> { for (; i != s; ++i) { co_yield static_cast<yielded>(*i); } }; return yield_value(ranges::elements_of(nested( allocator_arg, r.allocator, ranges::begin(r.range), ranges::end(r.range))));
inout_ptr
will not update raw pointer to 0Section: 20.3.4.3 [inout.ptr.t] Status: WP Submitter: Doug Cook Opened: 2023-02-27 Last modified: 2023-11-22
Priority: 2
View all other issues in [inout.ptr.t].
View all issues with WP status.
Discussion:
inout_ptr
seems useful for two purposes:
Using smart pointers with C-style APIs.
Annotating raw pointers for use with C-style APIs.
Unfortunately, as presently specified, it is not safe for developers to use inout_ptr
for the second purpose.
It is not safe to change code from
void* raw_ptr1; InitSomething(&raw_ptr1); UpdateSomething(&raw_ptr1); // In some cases may set raw_ptr1 = nullptr. CleanupSomething(raw_ptr1);
to
void* raw_ptr2; InitSomething(std::out_ptr(raw_ptr2)); UpdateSomething(std::inout_ptr(raw_ptr2)); // May leave dangling pointer CleanupSomething(raw_ptr2); // Possible double-delete
In the case where UpdateSomething
would set raw_ptr1 = nullptr
, the currently-specified inout_ptr
implementation will leave raw_ptr2
at its old value. This would likely lead to a double-delete in CleanupSomething
.
inout_ptr
is specified as follows:
Constructor: If the user's pointer is a smart pointer, perform a "release
" operation.
(C-style API executes)
If the C-style API returns a non-NULL
pointer, propagate the returned value to the user's pointer.
If the user's pointer is not a smart pointer, no "release
" operation occurs, and if the C-style API returns a
NULL
pointer, no propagation of the NULL
occurs. We're left with a dangling raw pointer which is
different from the original behavior using &
.
Make the "release
" operation unconditional (i.e. it applies to both smart and raw pointers). For raw pointers,
define the "release
" operation as setting the raw pointer to nullptr
.
Make the return value propagation unconditional for raw pointers.
Solution #2 seems likely to lead to more optimal code as it avoids an unnecessary branch.
[2023-03-22; Reflector poll]
Set priority to 2 after reflector poll.
[Varna 2023-06-16; Move to Ready]
[2023-11-11 Approved at November 2023 meeting in Kona. Status changed: Voting → WP.]
Proposed resolution:
This wording is relative to N4928.
Modify 20.3.4.3 [inout.ptr.t] as indicated:
~inout_ptr_t();-9- Let
-10- Let release-statement beSP
bePOINTER_OF_OR(Smart, Pointer)
(20.2.1 [memory.general]).s.release();
if an implementation does not calls.release()
in the constructor. Otherwise, it is empty. -11- Effects: Equivalent to:
(11.1) —
if (p) {apply([&](auto&&... args) { s = Smart( static_cast<SP>(p), std::forward<Args>(args)...); }, std::move(a));}if
is_pointer_v<Smart>
istrue
;(11.2) — otherwise,
release-statement; if (p) { apply([&](auto&&... args) { s.reset(static_cast<SP>(p), std::forward<Args>(args)...); }, std::move(a)); }if the expression
s.reset(static_cast<SP>(p), std::forward<Args>(args)...)
is well-formed;(11.3) — otherwise,
release-statement; if (p) { apply([&](auto&&... args) { s = Smart(static_cast<SP>(p), std::forward<Args>(args)...); }, std::move(a)); }if
is_constructible_v<Smart, SP, Args...>
istrue
;(11.4) — otherwise, the program is ill-formed.
co_yield
ing elements of an lvalue generator
is unnecessarily inefficientSection: 25.8.5 [coro.generator.promise] Status: WP Submitter: Tim Song Opened: 2023-03-04 Last modified: 2024-11-28
Priority: 3
View all other issues in [coro.generator.promise].
View all issues with WP status.
Discussion:
Consider:
std::generator<int> f(); std::generator<int> g() { auto gen = f(); auto gen2 = f(); co_yield std::ranges::elements_of(std::move(gen)); // #1 co_yield std::ranges::elements_of(gen2); // #2 // other stuff }
Both #1 and #2 compile. The differences are:
#2 is significantly less efficient (it uses the general overload of yield_value
,
so it creates a new coroutine frame and doesn't do symmetric transfer into gen2
's coroutine)
the coroutine frame of gen
and gen2
are destroyed at different
times: gen
's frame is destroyed at the end of #1, but gen2
's is
not destroyed until the closing brace.
But as far as the user is concerned, neither gen
nor gen2
is
usable after the co_yield
. In both cases the only things you can do
with the objects are:
destroying them;
assigning to them;
call end()
on them to get a copy of default_sentinel
.
We could make #2 ill-formed, but that seems unnecessary: there is no meaningful
difference between generator
and any other single-pass input range
(or a generator
with a different yielded type that has to go through
the general overload) in this regard. We should just make #2 do the efficient
thing too.
[2023-03-22; Reflector poll]
Set priority to 3 after reflector poll.
[St. Louis 2024-06-28; move to Ready]
[Wrocław 2024-11-23; Status changed: Voting → WP.]
Proposed resolution:
This wording is relative to N4928.
Modify 25.8.5 [coro.generator.promise] as indicated:
[…]namespace std { template<class Ref, class V, class Allocator> class generator<Ref, V, Allocator>::promise_type { public: […] auto yield_value(const remove_reference_t<yielded>& lval) requires is_rvalue_reference_v<yielded> && constructible_from<remove_cvref_t<yielded>, const remove_reference_t<yielded>&>; template<class R2, class V2, class Alloc2, class Unused> requires same_as<typename generator<R2, V2, Alloc2>::yielded, yielded> auto yield_value(ranges::elements_of<generator<R2, V2, Alloc2>&&, Unused> g) noexcept; template<class R2, class V2, class Alloc2, class Unused> requires same_as<typename generator<R2, V2, Alloc2>::yielded, yielded> auto yield_value(ranges::elements_of<generator<R2, V2, Alloc2>&, Unused> g) noexcept; template<ranges::input_range R, class Alloc> requires convertible_to<ranges::range_reference_t<R>, yielded> auto yield_value(ranges::elements_of<R, Alloc> r) noexcept; […] }; }template<class R2, class V2, class Alloc2, class Unused> requires same_as<typename generator<R2, V2, Alloc2>::yielded, yielded> auto yield_value(ranges::elements_of<generator<R2, V2, Alloc2>&&, Unused> g) noexcept; template<class R2, class V2, class Alloc2, class Unused> requires same_as<typename generator<R2, V2, Alloc2>::yielded, yielded> auto yield_value(ranges::elements_of<generator<R2, V2, Alloc2>&, Unused> g) noexcept;-10- Preconditions: A handle referring to the coroutine whose promise object is
-11- Returns: An awaitable object of an unspecified type (7.6.2.4 [expr.await]) into which*this
is at the top of*active_
of some generator objectx
. The coroutine referred to byg.range.coroutine_
is suspended at its initial suspend point.g.range
is moved, whose memberawait_ready
returnsfalse
, whose memberawait_suspend
pushesg.range.coroutine_
into*x.active_
and resumes execution of the coroutine referred to byg.range.coroutine_
, and whose memberawait_resume
evaluatesrethrow_exception(except_)
ifbool(except_)
istrue
. Ifbool(except_)
isfalse
, theawait_resume
member has no effects. -12- Remarks: A yield-expression that callsthis functionone of these functions has typevoid
(7.6.17 [expr.yield]).
allocator_arg_t
overloads of generator::promise_type::operator new
should not be constrainedSection: 25.8.5 [coro.generator.promise] Status: WP Submitter: Tim Song Opened: 2023-03-04 Last modified: 2024-11-28
Priority: 3
View all other issues in [coro.generator.promise].
View all issues with WP status.
Discussion:
When the allocator is not type-erased, the allocator_arg_t
overloads of
generator::promise_type::operator new
are constrained on
convertible_to<const Alloc&, Allocator>
. As a result, if the
the allocator is default-constructible (like polymorphic_allocator
is)
but the user accidentally provided a wrong type (say, memory_resource&
instead of memory_resource*
), their code will silently fall back to
using a default-constructed allocator. It would seem better to take the tag
as definitive evidence of the user's intent to supply an allocator for the coroutine,
and error out if the supplied allocator cannot be used.
std::allocator_arg_t
tag) for their own use
inside the coroutine, but that sort of API seems fragile and confusing at best,
since the usual case is that allocators so passed will be used by
generator
.
[2023-03-22; Reflector poll]
Set priority to 3 after reflector poll.
[St. Louis 2024-06-28; move to Ready]
[Wrocław 2024-11-23; Status changed: Voting → WP.]
Proposed resolution:
This wording is relative to N4928.
Modify 25.8.5 [coro.generator.promise] as indicated:
[…]namespace std { template<class Ref, class V, class Allocator> class generator<Ref, V, Allocator>::promise_type { public: […] void* operator new(size_t size) requires same_as<Allocator, void> || default_initializable<Allocator>; template<class Alloc, class... Args>requires same_as<Allocator, void> || convertible_to<const Alloc&, Allocator>void* operator new(size_t size, allocator_arg_t, const Alloc& alloc, const Args&...); template<class This, class Alloc, class... Args>requires same_as<Allocator, void> || convertible_to<const Alloc&, Allocator>void* operator new(size_t size, const This&, allocator_arg_t, const Alloc& alloc, const Args&...); […] }; }void* operator new(size_t size) requires same_as<Allocator, void> || default_initializable<Allocator>; template<class Alloc, class... Args>requires same_as<Allocator, void> || convertible_to<const Alloc&, Allocator>void* operator new(size_t size, allocator_arg_t, const Alloc& alloc, const Args&...); template<class This, class Alloc, class... Args>requires same_as<Allocator, void> || convertible_to<const Alloc&, Allocator>void* operator new(size_t size, const This&, allocator_arg_t, const Alloc& alloc, const Args&...);-17- Let
A
be
(17.1) —
Allocator
, if it is notvoid
,(17.2) —
Alloc
for the overloads with a template parameterAlloc
, or(17.3) —
allocator<void>
otherwise.Let
-18- Mandates:B
beallocator_traits<A>::template rebind_alloc<U>
whereU
is an unspecified type whose size and alignment are both__STDCPP_DEFAULT_NEW_ALIGNMENT__
.allocator_traits<B>::pointer
is a pointer type. For the overloads with a template parameterAlloc
,same_as<Allocator, void> || convertible_to<const Alloc&, Allocator>
is modeled. -19- Effects: Initializes an allocatorb
of typeB
withA(alloc)
, for the overloads with a function parameteralloc
, and withA()
otherwise. Usesb
to allocate storage for the smallest array ofU
sufficient to provide storage for a coroutine state of sizesize
, and unspecified additional state necessary to ensure thatoperator delete
can later deallocate this memory block with an allocator equal tob
. -20- Returns: A pointer to the allocated storage.
span
destructor is redundantly noexcept
Section: 23.7.2.2.1 [span.overview] Status: WP Submitter: Ben Craig Opened: 2023-03-11 Last modified: 2023-11-22
Priority: Not Prioritized
View all other issues in [span.overview].
View all issues with WP status.
Discussion:
The span
class template synopsis in 23.7.2.2.1 [span.overview] has this declaration:
~span() noexcept = default;
The noexcept
is redundant, as ~span
is noexcept
automatically. I think the entire
declaration is unnecessary as well. There is no additional specification for ~span()
at all, much
less some that warrants inclusion in the class template synopsis.
~span() noexcept = default;
Alternative fix:
~span()noexcept= default;
[2023-03-22; Reflector poll]
Set status to Tentatively Ready after six votes in favour during reflector poll.
[2023-06-17 Approved at June 2023 meeting in Varna. Status changed: Voting → WP.]
Proposed resolution:
This wording is relative to N4928.
Modify 23.7.2.2.1 [span.overview], class template span
synopsis, as indicated:
[…]~span() noexcept = default;[…]
lazy_split_view::outer-iterator
's const
-converting constructor isn't setting trailing_empty_
Section: 25.7.16.3 [range.lazy.split.outer] Status: WP Submitter: Patrick Palka Opened: 2023-03-13 Last modified: 2023-11-22
Priority: Not Prioritized
View other active issues in [range.lazy.split.outer].
View all other issues in [range.lazy.split.outer].
View all issues with WP status.
Discussion:
It seems the const
-converting constructor for lazy_split_view
's iterator fails
to propagate trailing_empty_
from the argument, which effectively results in
the trailing empty range getting skipped over:
auto r = views::single(0) | views::lazy_split(0); // r is { {}, {} } auto i = r.begin(); ++i; // i.trailing_empty_ is correctly true decltype(std::as_const(r).begin()) j = i; // j.trailing_empty_ is incorrectly false auto k = r.end(); // k.trailing_empty_ is correctly false, and we wrongly have j == k
[2023-05-24; Reflector poll]
Set status to Tentatively Ready after five votes in favour during reflector poll.
[2023-06-17 Approved at June 2023 meeting in Varna. Status changed: Voting → WP.]
Proposed resolution:
This wording is relative to N4928.
Modify 25.7.16.3 [range.lazy.split.outer] as indicated:
constexpr outer-iterator(outer-iterator<!Const> i) requires Const && convertible_to<iterator_t<V>, iterator_t<Base>>;-4- Effects: Initializes
parent_
withi.parent_
,andcurrent_
withstd::move(i.current_)
, andtrailing_empty_
withi.trailing_empty_
.
std::fexcept_t
Section: 29.3.1 [cfenv.syn] Status: WP Submitter: Sam Elliott Opened: 2023-03-13 Last modified: 2023-11-22
Priority: Not Prioritized
View all issues with WP status.
Discussion:
<cfenv>
, as specified in 29.3.1 [cfenv.syn], requires fexcept_t
to be
an integer type:
using fexcept_t = integer type;
<cfenv>
was initially added to the (first) Technical Report on
C++ Library Extensions via N1568 and then integrated into the
C++ Working Draft N2009 in Berlin (April, 2006).
fexcept_t
is
an integer type, it only requires:
The type
fexcept_t
represents the floating-point status flags collectively, including any status the implementation associates with the flags.
Relaxing this requirement should not cause conforming C++ implementations to no longer be conforming.
In fact, this should enable conforming C implementations to become conforming C++ implementations without
an ABI break. The only incompatibility I foresee is where a user's program is initializing a std::fexcept_t
with an integer value, which would become invalid on some C++ implementations (but not those that were
previously conforming).
[2023-05-24; Reflector poll]
Set status to Tentatively Ready after six votes in favour during reflector poll.
[2023-06-17 Approved at June 2023 meeting in Varna. Status changed: Voting → WP.]
Proposed resolution:
This wording is relative to N4928.
Modify 29.3.1 [cfenv.syn], header <cfenv>
synopsis, as indicated:
[…] namespace std { // types using fenv_t = object type; using fexcept_t =integerobject type; […] }
enumerate_view::iterator::operator-
should be noexcept
Section: 25.7.24.3 [range.enumerate.iterator] Status: WP Submitter: Hewill Kang Opened: 2023-03-27 Last modified: 2023-11-22
Priority: Not Prioritized
View other active issues in [range.enumerate.iterator].
View all other issues in [range.enumerate.iterator].
View all issues with WP status.
Discussion:
The distance between two enumerate_view::iterator
is calculated by subtracting two integers, which never throws.
[2023-05-24; Reflector poll]
Set status to Tentatively Ready after five votes in favour during reflector poll.
[2023-06-17 Approved at June 2023 meeting in Varna. Status changed: Voting → WP.]
Proposed resolution:
This wording is relative to N4944.
Modify 25.7.24.3 [range.enumerate.iterator] as indicated:
[…]namespace std::ranges { template<view V> requires range-with-movable-references<V> template<bool Const> class enumerate_view<V>::iterator { […] public: […] friend constexpr difference_type operator-(const iterator& x, const iterator& y) noexcept; […] } […] }
friend constexpr difference_type operator-(const iterator& x, const iterator& y) noexcept;
-19- Effects: Equivalent to:
return x.pos_ - y.pos_;
ranges::enumerate_view
Section: 25.2 [ranges.syn], 25.7.24 [range.enumerate] Status: WP Submitter: Johel Ernesto Guerrero Peña Opened: 2023-03-30 Last modified: 2023-11-22
Priority: Not Prioritized
View all other issues in [ranges.syn].
View all issues with WP status.
Discussion:
Originally editorial issue Editorial issue #6151.
The template-head ofranges::enumerate_view
in the header synopsis
is different from those in the class synopses of itself and its iterator/sentinel pair.
[2023-05-24; Reflector poll]
Set status to Tentatively Ready after five votes in favour during reflector poll.
[2023-06-17 Approved at June 2023 meeting in Varna. Status changed: Voting → WP.]
Proposed resolution:
This wording is relative to N4944.
Modify 25.2 [ranges.syn] as indicated:
[…] // 25.7.24 [range.enumerate], enumerate view template<view Vinput_range View> requires see belowview<View>class enumerate_view; […]
Section: 25.4.2 [range.range] Status: WP Submitter: Johel Ernesto Guerrero Peña Opened: 2023-04-01 Last modified: 2023-11-22
Priority: Not Prioritized
View all other issues in [range.range].
View all issues with WP status.
Discussion:
Originally editorial issue Editorial issue #4431.
Expression variations kick in for "an expression that is non-modifying for some constant lvalue operand", butstd::ranges::range
's is an non-constant lvalue, so 25.4.2 [range.range] p2 is redundant.
I suppose that the change that clarified the template parameters' cv-qualification for purposes of
equality-preservation and requiring additional variations happened concurrently with the change of
std::ranges::range
's operand from a forwarding reference to a non-constant lvalue reference.
[2023-05-24; Reflector poll]
Set status to Tentatively Ready after six votes in favour during reflector poll.
[2023-06-17 Approved at June 2023 meeting in Varna. Status changed: Voting → WP.]
Proposed resolution:
This wording is relative to N4944.
Modify 25.2 [ranges.syn] as indicated:
-1- The
range
concept defines the requirements of a type that allows iteration over its elements by providing an iterator and sentinel that denote the elements of the range.template<class T> concept range = requires(T& t) { ranges::begin(t); // sometimes equality-preserving (see below) ranges::end(t); };-2- The required expressions-3- […]ranges::begin(t)
andranges::end(t)
of therange
concept do not require implicit expression variations (18.2 [concepts.equality]).
std::uninitialized_move/_n
and guaranteed copy elisionSection: 26.11.6 [uninitialized.move] Status: WP Submitter: Jiang An Opened: 2023-04-04 Last modified: 2024-11-28
Priority: 3
View all issues with WP status.
Discussion:
Currently std::move
is unconditionally used in std::uninitialized_move
and std::uninitialized_move_n
,
which may involve unnecessary move construction if dereferencing the input iterator yields a prvalue.
[2023-06-01; Reflector poll]
Set priority to 3 after reflector poll. Send to LEWG.
"P2283 wants to remove guaranteed elision here."
"Poorly motivated, not clear anybody is using these algos with proxy iterators."
"Consider using iter_move
in the move algos."
Previous resolution [SUPERSEDED]:
This wording is relative to N4944.
Modify 26.11.1 [specialized.algorithms.general] as indicated:
-3- Some algorithms specified in 26.11 [specialized.algorithms] make use of the following exposition-only functions
:voidify
template<class T> constexpr void* voidify(T& obj) noexcept { return addressof(obj); } template<class I> decltype(auto) deref-move(const I& it) { if constexpr (is_lvalue_reference_v<decltype(*it)>) return std::move(*it); else return *it; }Modify 26.11.6 [uninitialized.move] as indicated:
template<class InputIterator, class NoThrowForwardIterator> NoThrowForwardIterator uninitialized_move(InputIterator first, InputIterator last, NoThrowForwardIterator result);[…]-1- Preconditions:
-2- Effects: Equivalent to:result + [0, (last - first))
does not overlap with[first, last)
.for (; first != last; (void)++result, ++first) ::new (voidify(*result)) typename iterator_traits<NoThrowForwardIterator>::value_type(std::move(*deref-move(first)); return result;template<class InputIterator, class Size, class NoThrowForwardIterator> pair<InputIterator, NoThrowForwardIterator> uninitialized_move_n(InputIterator first, Size n, NoThrowForwardIterator result);-6- Preconditions:
-7- Effects: Equivalent to:result + [0, n)
does not overlap withfirst + [0, n)
.for (; n > 0; ++result,(void) ++first, --n) ::new (voidify(*result)) typename iterator_traits<NoThrowForwardIterator>::value_type(std::move(*deref-move(first)); return {first, result};
[2024-03-22; Tokyo: Jonathan updates wording after LEWG review]
LEWG agrees it would be good to do this.
Using iter_move
was discussed, but it was noted that the versions of these
algos in the ranges
namespace already use it and introducing
ranges::iter_move
into the non-ranges versions wasn't desirable.
It was observed that the proposed deref-move has a
const I&
parameter which would be ill-formed for any iterator
with a non-const operator*
member. Suggested removing the const and
recommended LWG to accept the proposed resolution.
Previous resolution [SUPERSEDED]:
This wording is relative to N4971.
Modify 26.11.1 [specialized.algorithms.general] as indicated:
-3- Some algorithms specified in 26.11 [specialized.algorithms] make use of the following exposition-only functions
:voidify
template<class T> constexpr void* voidify(T& obj) noexcept { return addressof(obj); } template<class I> decltype(auto) deref-move(I& it) { if constexpr (is_lvalue_reference_v<decltype(*it)>) return std::move(*it); else return *it; }Modify 26.11.6 [uninitialized.move] as indicated:
template<class InputIterator, class NoThrowForwardIterator> NoThrowForwardIterator uninitialized_move(InputIterator first, InputIterator last, NoThrowForwardIterator result);[…]-1- Preconditions:
-2- Effects: Equivalent to:result + [0, (last - first))
does not overlap with[first, last)
.for (; first != last; (void)++result, ++first) ::new (voidify(*result)) typename iterator_traits<NoThrowForwardIterator>::value_type(std::move(*deref-move(first)); return result;template<class InputIterator, class Size, class NoThrowForwardIterator> pair<InputIterator, NoThrowForwardIterator> uninitialized_move_n(InputIterator first, Size n, NoThrowForwardIterator result);-6- Preconditions:
-7- Effects: Equivalent to:result + [0, n)
does not overlap withfirst + [0, n)
.for (; n > 0; ++result,(void) ++first, --n) ::new (voidify(*result)) typename iterator_traits<NoThrowForwardIterator>::value_type(std::move(*deref-move(first)); return {first, result};
[St. Louis 2024-06-24; revert P/R and move to Ready]
Tim observed that the iterator requirements require all iterators to be const-dereferenceable, so there was no reason to remove the const. Restore the original resolution and move to Ready.
[Wrocław 2024-11-23; Status changed: Voting → WP.]
Proposed resolution:
This wording is relative to N4971.
Modify 26.11.1 [specialized.algorithms.general] as indicated:
-3- Some algorithms specified in 26.11 [specialized.algorithms] make use of the following exposition-only functions
:voidify
template<class T> constexpr void* voidify(T& obj) noexcept { return addressof(obj); } template<class I> decltype(auto) deref-move(I& it) { if constexpr (is_lvalue_reference_v<decltype(*it)>) return std::move(*it); else return *it; }
Modify 26.11.6 [uninitialized.move] as indicated:
template<class InputIterator, class NoThrowForwardIterator> NoThrowForwardIterator uninitialized_move(InputIterator first, InputIterator last, NoThrowForwardIterator result);[…]-1- Preconditions:
-2- Effects: Equivalent to:result + [0, (last - first))
does not overlap with[first, last)
.for (; first != last; (void)++result, ++first) ::new (voidify(*result)) typename iterator_traits<NoThrowForwardIterator>::value_type(std::move(*deref-move(first)); return result;template<class InputIterator, class Size, class NoThrowForwardIterator> pair<InputIterator, NoThrowForwardIterator> uninitialized_move_n(InputIterator first, Size n, NoThrowForwardIterator result);-6- Preconditions:
-7- Effects: Equivalent to:result + [0, n)
does not overlap withfirst + [0, n)
.for (; n > 0; ++result,(void) ++first, --n) ::new (voidify(*result)) typename iterator_traits<NoThrowForwardIterator>::value_type(std::move(*deref-move(first)); return {first, result};
enumerate_view
may invoke UB for sized common non-forward underlying rangesSection: 25.7.24 [range.enumerate] Status: WP Submitter: Patrick Palka Opened: 2023-04-07 Last modified: 2024-04-02
Priority: 3
View all issues with WP status.
Discussion:
For a sized common range, enumerate_view::end()
is specified to call
ranges::distance
. But ranges::distance
is not necessarily well-defined
for a sized non-forward range after calling ranges::begin
(according to
25.4.3 [range.sized]).
enumerate_view::begin()
followed by enumerate_view::end()
may invoke UB
and thus make enumerate_view
potentially unusable for such ranges.
I suppose we might need to instead call and cache the result of
ranges::distance
from enumerate_view::begin()
for such ranges.
[2022-04-12; Patrick Palka provides wording]
The proposed wording follows the suggestion provided by Tim Song, to simply make enumerate
non-common for this case.
[2023-05-24; Reflector poll]
Set priority to 3 after reflector poll.
[Kona 2023-11-10; move to Ready]
[Tokyo 2024-03-23; Status changed: Voting → WP.]
Proposed resolution:
This wording is relative to N4944.
Modify 25.7.24.2 [range.enumerate.view], class template class enumerate_view
synopsis, as indicated:
[…] constexpr auto end() requires (!simple-view<V>) { if constexpr (forward_range<V> && common_range<V> && sized_range<V>) return iterator<false>(ranges::end(base_), ranges::distance(base_)); else return sentinel<false>(ranges::end(base_)); } constexpr auto end() const requires range-with-movable-references<const V> { if constexpr (forward_range<const V> && common_range<const V> && sized_range<const V>) return iterator<true>(ranges::end(base_), ranges::distance(base_)); else return sentinel<true>(ranges::end(base_)); } […]
formattable
's definition is incorrectSection: 28.5.6.3 [format.formattable] Status: WP Submitter: Mark de Wever Opened: 2023-04-16 Last modified: 2023-11-22
Priority: Not Prioritized
View other active issues in [format.formattable].
View all other issues in [format.formattable].
View all issues with WP status.
Discussion:
LWG 3631(i) modified the formattable
concept. The new wording
contains a small issue: basic_format_context
requires two template arguments,
but only one is provided.
[2023-05-24; Reflector poll]
Set status to Tentatively Ready after ten votes in favour during reflector poll.
[2023-06-17 Approved at June 2023 meeting in Varna. Status changed: Voting → WP.]
Proposed resolution:
This wording is relative to N4944.
Modify 28.5.6.3 [format.formattable] as indicated:
-1- Let
fmt-iter-for<charT>
be an unspecified type that modelsoutput_iterator<const charT&>
(24.3.4.10 [iterator.concept.output]).[…] template<class T, class charT> concept formattable = formattable-with<remove_reference_t<T>, basic_format_context<fmt-iter-for<charT>, charT>>;
operator[]
for sequence containersSection: 23.2.4 [sequence.reqmts] Status: WP Submitter: Ville Voutilainen Opened: 2023-04-24 Last modified: 2023-11-22
Priority: Not Prioritized
View other active issues in [sequence.reqmts].
View all other issues in [sequence.reqmts].
View all issues with WP status.
Discussion:
In 23.2.4 [sequence.reqmts]/118, we specify what a[n]
means for a sequence
container a
, but we don't state that it actually has preconditions, other
than implied ones.
[2023-05-24; Reflector poll]
Set status to Tentatively Ready after six votes in favour during reflector poll.
[2023-06-17 Approved at June 2023 meeting in Varna. Status changed: Voting → WP.]
Proposed resolution:
This wording is relative to N4944.
Modify 23.2.4 [sequence.reqmts] as indicated:
a[n]-118- Result:
-119-reference
;const_reference
for constanta
ReturnsEffects: Equivalent to:return *(a.begin() + n);
-120- Remarks: Required forbasic_string
,array
,deque
, andvector
.
template<class X> constexpr complex& operator=(const complex<X>&)
has no specificationSection: 29.4.3 [complex] Status: WP Submitter: Daniel Krügler Opened: 2023-05-21 Last modified: 2023-11-22
Priority: Not Prioritized
View other active issues in [complex].
View all other issues in [complex].
View all issues with WP status.
Discussion:
The class template complex
synopsis in 29.4.3 [complex] shows the following member function template:
template<class X> constexpr complex& operator= (const complex<X>&);
but does not specify its semantics. This affects a code example such as the following one:
#include <complex> int main() { std::complex<double> zd(1, 1); std::complex<float> zf(2, 0); zd = zf; }
This problem exists since the 1998 version of the standard (at that time this was declared in subclause [lib.complex]), and even though this looks like a "copy-assignment-like" operation, its effects aren't implied by our general 16.3.3.5 [functions.within.classes] wording (a function template is never considered as copy assignment operator, see 11.4.6 [class.copy.assign]).
It should be point out that the refactoring done by P1467R9 had caused some other members to become finally specified that were not specified before, but in addition to LWG 3934(i)'s observation about the missing specification of the assignment operator taking a value type as parameter, this is now an additional currently unspecified member, but unaffected by any decision on LWG 3933(i).[2023-06-01; Reflector poll]
Set status to Tentatively Ready after eight votes in favour during reflector poll.
[2023-06-17 Approved at June 2023 meeting in Varna. Status changed: Voting → WP.]
Proposed resolution:
This wording is relative to N4950.
Add a new prototype specification between the current paragraphs 8 and 9 of 29.4.5 [complex.member.ops] as indicated:
[Drafting note: Similar to the proposed wording for LWG 3934(i), the wording form used below intentionally deviates from the rest of the [complex.member.ops] wording forms, because it seems much simpler and clearer to follow the wording forms used that specify the effects of
imag
andreal
functions plus borrowing relevant parts from 29.4.4 [complex.members] p2.]
constexpr complex& operator/=(const T& rhs);[…]
template<class X> constexpr complex& operator=(const complex<X>& rhs);-?- Effects: Assigns the value
-?- Returns:rhs.real()
to the real part and the valuerhs.imag()
to the imaginary part of the complex value*this
.*this
.template<class X> constexpr complex& operator+=(const complex<X>& rhs);[…]
std::expected
monadic ops with move-only error_type
Section: 22.8.6.7 [expected.object.monadic] Status: WP Submitter: Jonathan Wakely Opened: 2023-05-25 Last modified: 2023-11-22
Priority: Not Prioritized
View all other issues in [expected.object.monadic].
View all issues with WP status.
Discussion:
The monadic ops for std::expected
are specified in terms of calls
to value()
and error()
, but LWG 3843(i)
("std::expected<T,E>::value()&
assumes E
is copy constructible") added additional Mandates requirements to
value()
. This means that you can never call value()
for a move-only error_type
, even the overloads of
value()
with rvalue ref-qualifiers.
The changes to value()
are because it needs to be able to throw a
bad_expected_access<E>
which requires a copyable E
.
But in the monadic ops we know it can't throw, because we always check.
All the monadic ops are of the form:
if (has_value()) do something with value(); else do something with error();
We know that value()
won't throw here, but because we use
"Effects: Equivalent to ..." the requirement for E
to be copyable is inherited from value()
.
Should we have changed the monadic ops to use operator*()
instead of value()
?
For example, for the first and_then
overloads the change would be:
-4- Effects: Equivalent to:if (has_value()) return invoke(std::forward<F>(f),
value()**this); else return U(unexpect, error());
[2023-06-01; Reflector poll]
Set status to Tentatively Ready after seven votes in favour during reflector poll.
[2023-06-17 Approved at June 2023 meeting in Varna. Status changed: Voting → WP.]
Proposed resolution:
This wording is relative to N4950.
For each Effects: element in 22.8.6.7 [expected.object.monadic],
replace value()
with **this
as indicated:
template<class F> constexpr auto and_then(F&& f) &; template<class F> constexpr auto and_then(F&& f) const &;
-1- Let
U
beremove_cvref_t<invoke_result_t<F, decltype(
.value()**this)>>-2- Constraints:
is_constructible_v<E, decltype(error())>
istrue
.-3- Mandates:
U
is a specialization ofexpected
andis_same_v<U::error_type, E>
istrue
.-4- Effects: Equivalent to:
if (has_value()) return invoke(std::forward<F>(f),
value()**this); else return U(unexpect, error());template<class F> constexpr auto and_then(F&& f) &&; template<class F> constexpr auto and_then(F&& f) const &&;
-5- Let
U
beremove_cvref_t<invoke_result_t<F, decltype(std::move(
.value()**this))>>-6- Constraints:
is_constructible_v<E, decltype(std::move(error()))>
istrue
.-7- Mandates:
U
is a specialization ofexpected
andis_same_v<U::error_type, E>
istrue
.-8- Effects: Equivalent to:
if (has_value()) return invoke(std::forward<F>(f), std::move(
value()**this)); else return U(unexpect, std::move(error()));template<class F> constexpr auto or_else(F&& f) &; template<class F> constexpr auto or_else(F&& f) const &;
-9- Let
G
beremove_cvref_t<invoke_result_t<F, decltype(error())>>
.-10- Constraints:
is_constructible_v<T, decltype(
isvalue()**this)>true
.-11- Mandates:
G
is a specialization ofexpected
andis_same_v<G::value_type, T>
istrue
.-12- Effects: Equivalent to:
if (has_value()) return G(in_place,
value()**this); else return invoke(std::forward<F>(f), error());template<class F> constexpr auto or_else(F&& f) &&; template<class F> constexpr auto or_else(F&& f) const &&;
-13- Let
G
beremove_cvref_t<invoke_result_t<F, decltype(std::move(error()))>>
.-14- Constraints:
is_constructible_v<T, decltype(std::move(
isvalue()**this))>true
.-15- Mandates:
G
is a specialization ofexpected
andis_same_v<G::value_type, T>
istrue
.-16- Effects: Equivalent to:
if (has_value()) return G(in_place, std::move(
value()**this)); else return invoke(std::forward<F>(f), std::move(error()));template<class F> constexpr auto transform(F&& f) &; template<class F> constexpr auto transform(F&& f) const &;
-17- Let
U
beremove_cv_t<invoke_result_t<F, decltype(
.value()**this)>>-18- Constraints:
is_constructible_v<E, decltype(error())>
istrue
.-19- Mandates:
U
is a valid value type forexpected
. Ifis_void_v<U>
isfalse
, the declarationis well-formed.U u(invoke(std::forward<F>(f),
value()**this));-20- Effects:
- (20.1) — If
has_value()
isfalse
, returnsexpected<U, E>(unexpect, error())
.- (20.2) — Otherwise, if
is_void_v<U>
isfalse
, returns anexpected<U, E>
object whosehas_val
member istrue
andval
member is direct-non-list-initialized withinvoke(std::forward<F>(f),
.value()**this)- (20.3) — Otherwise, evaluates
invoke(std::forward<F>(f),
and then returnsvalue()**this)expected<U, E>()
.template<class F> constexpr auto transform(F&& f) &&; template<class F> constexpr auto transform(F&& f) const &&;
-21- Let
U
beremove_cv_t<invoke_result_t<F, decltype(std::move(
.value()**this))>>-22- Constraints:
is_constructible_v<E, decltype(std::move(error()))>
istrue
.-23- Mandates:
U
is a valid value type forexpected
. Ifis_void_v<U>
isfalse
, the declarationis well-formedU u(invoke(std::forward<F>(f), std::move(
value()**this)));for some invented variable.u
[Drafting Note: The removal of "for some invented variable u" in paragraph 23 is a drive-by fix for consistency with paragraphs 19, 27 and 31.]
-24- Effects:
- (24.1) — If
has_value()
isfalse
, returnsexpected<U, E>(unexpect, error())
.- (24.2) — Otherwise, if
is_void_v<U>
isfalse
, returns anexpected<U, E>
object whosehas_val
member istrue
andval
member is direct-non-list-initialized withinvoke(std::forward<F>(f), std::move(
.value()**this))- (24.3) — Otherwise, evaluates
invoke(std::forward<F>(f), std::move(
and then returnsvalue()**this))expected<U, E>()
.template<class F> constexpr auto transform_error(F&& f) &; template<class F> constexpr auto transform_error(F&& f) const &;
-25- Let
G
beremove_cv_t<invoke_result_t<F, decltype(error())>>
.-26- Constraints:
is_constructible_v<T, decltype(
isvalue()**this)>true
.-27- Mandates:
G
is a valie template argument forunexpected
( [unexpected.un.general]) and the declarationis well-formed.G g(invoke(std::forward<F>(f), error()));
-28- Returns: If
has_value()
istrue
,expected<T, G>(in_place,
; otherwise, anvalue()**this);expected<T, G>
object whosehas_val
member isfalse
andunex
member is direct-non-list-initialized withinvoke(std::forward<F>(f), error())
.template<class F> constexpr auto transform_error(F&& f) &&; template<class F> constexpr auto transform_error(F&& f) const &&;
-29- Let
G
beremove_cv_t<invoke_result_t<F, decltype(std::move(error()))>>
.-30- Constraints:
is_constructible_v<T, decltype(std::move(
isvalue()**this))>true
.-31- Mandates:
G
is a valie template argument forunexpected
( [unexpected.un.general]) and the declarationis well-formed.G g(invoke(std::forward<F>(f), std::move(error())));
-32- Returns: If
has_value()
istrue
,expected<T, G>(in_place, std::move(
; otherwise, anvalue()**this));expected<T, G>
object whosehas_val
member isfalse
andunex
member is direct-non-list-initialized withinvoke(std::forward<F>(f), std::move(error()))
.
std::expected<void, E>::value()
also needs E
to be copy constructibleSection: 22.8.7.6 [expected.void.obs] Status: WP Submitter: Jiang An Opened: 2023-05-26 Last modified: 2023-11-22
Priority: Not Prioritized
View all issues with WP status.
Discussion:
LWG 3843(i) added Mandates: to std::expected::value
, but the similar handling is
missing for expected<cv void, E>
.
[2023-06-01; Reflector poll]
Set status to Tentatively Ready after seven votes in favour during reflector poll.
[2023-06-17 Approved at June 2023 meeting in Varna. Status changed: Voting → WP.]
Proposed resolution:
This wording is relative to N4950.
Modify 22.8.7.6 [expected.void.obs] as indicated:
constexpr void value() const &;-?- Mandates:
-3- Throws:is_copy_constructible_v<E>
istrue
.bad_expected_access(error())
ifhas_value()
isfalse
.constexpr void value() &&;-?- Mandates:
-4- Throws:is_copy_constructible_v<E>
istrue
andis_move_constructible_v<E>
istrue
.bad_expected_access(std::move(error()))
ifhas_value()
isfalse
.
char
to sequences of wchar_t
Section: 28.5.6.4 [format.formatter.spec] Status: WP Submitter: Mark de Wever Opened: 2023-06-01 Last modified: 2024-07-08
Priority: 3
View other active issues in [format.formatter.spec].
View all other issues in [format.formatter.spec].
View all issues with WP status.
Discussion:
I noticed some interesting features introduced by the range based formatters in C++23
// Ill-formed in C++20 and C++23 const char* cstr = "hello"; char* str = const_cast<char*>(cstr); std::format(L"{}", str); std::format(L"{}",cstr); // Ill-formed in C++20 // In C++23 they give L"['h', 'e', 'l', 'l', 'o']" std::format(L"{}", "hello"); // A libc++ bug prevents this from working. std::format(L"{}", std::string_view("hello")); std::format(L"{}", std::string("hello")); std::format(L"{}", std::vector{'h', 'e', 'l', 'l', 'o'});
An example is shown here. This only shows libc++ since libstdc++ and MSVC STL have not implemented the formatting ranges papers (P2286R8 and P2585R0) yet.
The difference between C++20 and C++23 is the existence of range formatters. These formatters use the formatter specializationformatter<char, wchar_t>
which converts the sequence of char
s
to a sequence of wchar_t
s.
In this conversion same_as<char, charT>
is false
, thus the requirements
of the range-type s
and ?s
([tab:formatter.range.type]) aren't met. So
the following is ill-formed:
std::format(L"{:s}", std::string("hello")); // Not L"hello"
It is surprising that some string types can be formatted as a sequence
of wide-characters, but others not. A sequence of characters can be a
sequence UTF-8 code units. This is explicitly supported in the width
estimation of string types. The conversion of char
to wchar_t
will
convert the individual code units, which will give incorrect results for
multi-byte code points. It will not transcode UTF-8 to UTF-16/32. The
current behavior is not in line with the note in
28.5.6.4 [format.formatter.spec]/2
[Note 1: Specializations such as
formatter<wchar_t, char>
andformatter<const char*, wchar_t>
that would require implicit multibyte / wide string or character conversion are disabled. — end note]
Disabling this could be done by explicitly disabling the char
to wchar_t
sequence formatter. Something along the lines of
template<ranges::input_range R> requires(format_kind<R> == range_format::sequence && same_as<remove_cvref_t<ranges::range_reference_t<R>>, char>) struct formatter<R, wchar_t> : __disabled_formatter {};
where __disabled_formatter
satisfies 28.5.6.4 [format.formatter.spec]/5, would
do the trick. This disables the conversion for all sequences not only
the string types. So vector
, array
, span
, etc. would be disabled.
range_formatter
. This allows
users to explicitly opt in to this formatter for their own
specializations.
An alternative would be to only disable this conversion for string type
specializations (28.5.6.4 [format.formatter.spec]/2.2) where char
to
wchar_t
is used:
template<size_t N> struct formatter<charT[N], charT>; template<class traits, class Allocator> struct formatter<basic_string<charT, traits, Allocator>, charT>; template<class traits> struct formatter<basic_string_view<charT, traits>, charT>;
Disabling following the following two is not strictly required:
template<> struct formatter<char*, wchar_t>; template<> struct formatter<const char*, wchar_t>;
However, if (const
) char*
becomes an input_range
in a future version C++, these formatters would become enabled.
Disabling all five instead of the three required specializations seems like a
future proof solution.
template<> struct formatter<wchar_t, char>;
there are no issues for wchar_t
to char
conversions.
Do we want to allow string types of char
s to be formatted as
sequences of wchar_t
s?
Do we want to allow non string type sequences of char
s to be
formatted as sequences of wchar_t
s?
Should we disable char
to wchar_t
conversion in the range_formatter
?
SG16 has indicated they would like to discuss this issue during a telecon.
[2023-06-08; Reflector poll]
Set status to SG16 and priority to 3 after reflector poll.
[2023-07-26; Mark de Wever provides wording confirmed by SG16]
[2024-03-18; Tokyo: move to Ready]
[St. Louis 2024-06-29; Status changed: Voting → WP.]
Proposed resolution:
This wording is relative to N4950.
Modify 28.5.6.4 [format.formatter.spec] as indicated:
[Drafting note: The unwanted conversion happens due to the
formatter
base class specialization (28.5.7.3 [format.range.fmtdef])struct range-default-formatter<range_format::sequence, R, charT>which is defined the header
<format>
. Therefore the disabling is only needed in this header) — end drafting note]
-2- […]
Theparse
member functions of these formatters interpret the format specification as a std-format-spec as described in 28.5.2.2 [format.string.std]. [Note 1: Specializations such asformatter<wchar_t, char>
andthat would require implicit multibyte / wide string or character conversion are disabled. — end note] -?- The headerformatter<const char*, wchar_t>
<format>
provides the following disabled specializations:
(?.1) — The string type specializations
template<> struct formatter<char*, wchar_t>; template<> struct formatter<const char*, wchar_t>; template<size_t N> struct formatter<char[N], wchar_t>; template<class traits, class Allocator> struct formatter<basic_string<char, traits, Allocator>, wchar_t>; template<class traits> struct formatter<basic_string_view<char, traits>, wchar_t>;-3- For any types
T
andcharT
for which neither the library nor the user provides an explicit or partial specialization of the class templateformatter
,formatter<T, charT>
is disabled.
const_iterator_t
should be reworkedSection: 25.2 [ranges.syn] Status: WP Submitter: Christopher Di Bella Opened: 2023-06-13 Last modified: 2023-11-22
Priority: Not Prioritized
View all other issues in [ranges.syn].
View all issues with WP status.
Discussion:
During the reflector discussion of P2836, consensus was reached that const_iterator_t<R>
doesn't necessarily provide the same type as decltype(ranges::cbegin(r))
, and that it should be changed to
the proposed resolution below so that they're consistent.
[Varna 2023-06-14; Move to Ready]
[2023-11-11 Approved at November 2023 meeting in Kona. Status changed: Voting → WP.]
Proposed resolution:
This wording is relative to N4950.
Modify 25.2 [ranges.syn], header <ranges>
synopsis, as indicated:
[…] template<range R> using const_iterator_t = decltype(ranges::cbegin(declval<R&>()))const_iterator<iterator_t<R>>; // freestanding template<range R> using const_sentinel_t = decltype(ranges::cend(declval<R&>()))const_sentinel<sentinel_t<R>>; // freestanding […]
adjacent_transform_view::base()
Section: 25.7.28.2 [range.adjacent.transform.view] Status: WP Submitter: Bo Persson Opened: 2023-06-17 Last modified: 2023-11-22
Priority: Not Prioritized
View all issues with WP status.
Discussion:
In section 25.7.28.2 [range.adjacent.transform.view] the class
ranges::adjacent_transform_view
got two new base()
members from
3848(i).
constexpr V base() const & requires copy_constructible<InnerView> { return inner_.base(); }
Here the requirement is that InnerView
is copy constructible, when it in
fact returns an object of type V
. That seems odd.
copy_constructible<V>
.
[2023-10-27; Reflector poll]
Set status to Tentatively Ready after five votes in favour during reflector poll.
[2023-11-11 Approved at November 2023 meeting in Kona. Status changed: Voting → WP.]
Proposed resolution:
This wording is relative to N4950.
Modify 25.7.28.2 [range.adjacent.transform.view], class template
adjacent_transform_view
synopsis, as indicated:
namespace std::ranges { template<forward_range V, move_constructible F, size_t N> requires view<V> && (N > 0) && is_object_v<F> && regular_invocable<F&, REPEAT(range_reference_t<V>, N)...> && can-reference<invoke_result_t<F&, REPEAT(range_reference_t<V>, N)...>> class adjacent_transform_view : public view_interface<adjacent_transform_view<V, F, N>> { […] adjacent_view<V, N> inner_; // exposition only using InnerView = adjacent_view<V, N>; // exposition only […] public: […] constexpr V base() const & requires copy_constructible<VInnerView> { return inner_.base(); } constexpr V base() && { return std::move(inner_).base(); } […] }; }
possibly-const-range
and as-const-pointer
should be noexcept
Section: 25.2 [ranges.syn], 25.3.16 [range.prim.cdata] Status: WP Submitter: Jiang An Opened: 2023-06-20 Last modified: 2023-11-22
Priority: Not Prioritized
View all other issues in [ranges.syn].
View all issues with WP status.
Discussion:
As of P2278R4, several range access CPOs are specified with possibly-const-range
and as-const-pointer
. These helper functions never throw exceptions, but are not marked with
noexcept
. As a result, implementations are currently allowed to make a call to
ranges::ccpo
potentially throwing while the underlying ranges::cpo
call is
non-throwing, which doesn't seem to be intended.
[2023-10-27; Reflector poll]
Set status to Tentatively Ready after five votes in favour during reflector poll.
[2023-11-11 Approved at November 2023 meeting in Kona. Status changed: Voting → WP.]
Proposed resolution:
This wording is relative to N4950.
Modify 25.2 [ranges.syn], header <ranges>
synopsis, as indicated:
[…] // 25.7.22 [range.as.const], as const view template<input_range R> constexpr auto& possibly-const-range(R& r) noexcept { // exposition only if constexpr (constant_range<const R> && !constant_range<R>) { return const_cast<const R&>(r); } else { return r; } } […]
Modify 25.3.16 [range.prim.cdata] before p1 as indicated:
template<class T> constexpr auto as-const-pointer(const T* p) noexcept { return p; } // exposition only
std::atomic<bool>
's trivial destructor dropped in C++17 spec wordingSection: 32.5.8.1 [atomics.types.generic.general] Status: WP Submitter: Jeremy Hurwitz Opened: 2023-06-20 Last modified: 2023-11-22
Priority: Not Prioritized
View all issues with WP status.
Discussion:
std::atomic<bool>
was originally required to have a trivial default constructor and a trivial destructor
[C++11 N3337: Section [atomics.types.generic], Paragraph 5],
the same as the integral [C++11 N3337: Section [atomics.types.generic],
Paragraph 5] and pointer specializations [C++11 N3337:
Section [atomics.types.generic], Paragraph 6]. P0558 rearranged the text,
accidentally (as far as we can tell) removing the constructor and destructor requirements from std::atomic<bool>
,
which has the surprising effect that std::atomic<bool>
has no longer the same constructor/destructor
guarantees as std::atomic<int>
.
[2023-10-27; Reflector poll]
Set status to Tentatively Ready after eight votes in favour during reflector poll.
[2023-11-11 Approved at November 2023 meeting in Kona. Status changed: Voting → WP.]
Proposed resolution:
This wording is relative to N4950.
Modify 32.5.8.1 [atomics.types.generic.general] as indicated:
-1- […]
-2- The specializationatomic<bool>
is a standard-layout struct. It has a trivial destructor.
std::basic_string_view
comparison operators are overspecifiedSection: 27.3.2 [string.view.synop] Status: WP Submitter: Giuseppe D'Angelo Opened: 2023-06-21 Last modified: 2024-04-02
Priority: Not Prioritized
View all issues with WP status.
Discussion:
The <string_view>
synopsis in 27.3.2 [string.view.synop] has these signatures
for operator==
and operator<=>
:
// 27.3.4 [string.view.comparison], non-member comparison functions template<class charT, class traits> constexpr bool operator==(basic_string_view<charT, traits> x, basic_string_view<charT, traits> y) noexcept; template<class charT, class traits> constexpr see below operator<=>(basic_string_view<charT, traits> x, basic_string_view<charT, traits> y) noexcept; // see 27.3.4 [string.view.comparison], sufficient additional overloads of comparison functions
In 27.3.4 [string.view.comparison], paragraph 1 states that "Implementations
shall provide sufficient additional overloads" so that all comparisons
between a basic_string_view<C, T>
object and an object of a type
convertible to basic_string_view<C, T>
work (with the reasonable
semantics).
operator==
:
template<class charT, class traits> constexpr bool operator==(basic_string_view<charT, traits> lhs, basic_string_view<charT, traits> rhs) noexcept { return lhs.compare(rhs) == 0; } template<class charT, class traits> constexpr bool operator==(basic_string_view<charT, traits> lhs, type_identity_t<basic_string_view<charT, traits>> rhs) noexcept { return lhs.compare(rhs) == 0; }
With the current semantics of rewritten candidates for the comparison
operators, it is however superfluous to actually specify both overloads
(the same applies for operator<=>
).
type_identity_t
) is indeed necessary to
implement the "sufficient additional overloads" part of 27.3.4 [string.view.comparison],
but it is also sufficient, as all the following cases
sv == sv
sv == convertible_to_sv
convertible_to_sv == sv
can in fact use it (directly, or after being rewritten e.g. with the arguments swapped).
The reason why we still do have both operators seems to be historical; there is an explanation offered here by Barry Revzin. Basically, there were three overloads before a bunch of papers regardingoperator<=>
and operator==
were merged:
operator==(bsv, bsv)
to deal with sv == sv
;
operator==(bsv, type_identity_t<bsv>)
and
operator==(type_identity_t<bsv>, bsv)
to deal with
sv == convertible_to_sv
and vice versa.
Overload (1) was necessary because with only (2) and (3) a call like
sv == sv
would otherwise be ambiguous. With the adoption of the rewriting
rules, overload (3) has been dropped, without realizing that overload
(1) would then become redundant.
type_identity_t
.
[Kona 2023-11-10; move to Ready]
Editorial issue 6324 provides the changes as a pull request to the draft.
[Tokyo 2024-03-23; Status changed: Voting → WP.]
Proposed resolution:
This wording is relative to N4950.
Modify 27.3.2 [string.view.synop], header <string_view>
synopsis, as indicated:
[…] // 27.3.4 [string.view.comparison], non-member comparison functions template<class charT, class traits> constexpr bool operator==(basic_string_view<charT, traits> x, type_identity_t<basic_string_view<charT, traits>> y) noexcept; template<class charT, class traits> constexpr see below operator<=>(basic_string_view<charT, traits> x, type_identity_t<basic_string_view<charT, traits>> y) noexcept;// see 27.3.4 [string.view.comparison], sufficient additional overloads of comparison functions[…]
Modify 27.3.4 [string.view.comparison] as indicated:
-1- LetS
bebasic_string_view<charT, traits>
, andsv
be an instance ofS
. Implementations shall provide sufficient additional overloads markedconstexpr
andnoexcept
so that an objectt
with an implicit conversion toS
can be compared according to Table 81 [tab:string.view.comparison.overloads].
Table 81: Additionalbasic_string_view
comparison overloads [tab:string.view.comparison.overloads]ExpressionEquivalent tot == sv
S(t) == sv
sv == t
sv == S(t)
t != sv
S(t) != sv
sv != t
sv != S(t)
t < sv
S(t) < sv
sv < t
sv < S(t)
t > sv
S(t) > sv
sv > t
sv > S(t)
t <= sv
S(t) <= sv
sv <= t
sv <= S(t)
t >= sv
S(t) >= sv
sv >= t
sv >= S(t)
t <=> sv
S(t) <=> sv
sv <=> t
sv <=> S(t)
[Example 1: A sample conforming implementation foroperator==
would be:template<class charT, class traits> constexpr bool operator==(basic_string_view<charT, traits> lhs, basic_string_view<charT, traits> rhs) noexcept { return lhs.compare(rhs) == 0; } template<class charT, class traits> constexpr bool operator==(basic_string_view<charT, traits> lhs, type_identity_t<basic_string_view<charT, traits>> rhs) noexcept { return lhs.compare(rhs) == 0; }
— end example]template<class charT, class traits> constexpr bool operator==(basic_string_view<charT, traits> lhs, type_identity_t<basic_string_view<charT, traits>> rhs) noexcept;-2- Returns:
lhs.compare(rhs) == 0
.template<class charT, class traits> constexpr see below operator<=>(basic_string_view<charT, traits> lhs, type_identity_t<basic_string_view<charT, traits>> rhs) noexcept;-3- Let
-4- Mandates:R
denote the typetraits::comparison_category
if that qualified-id is valid and denotes a type (13.10.3 [temp.deduct]), otherwiseR
isweak_ordering
.R
denotes a comparison category type (17.12.2 [cmp.categories]). -5- Returns:static_cast<R>(lhs.compare(rhs) <=> 0)
. [Note: The usage oftype_identity_t
as parameter ensures that an object of typebasic_string_view<charT, traits>
can always be compared with an object of a typeT
with an implicit conversion tobasic_string_view<charT, traits>
, and vice versa, as per 12.2.2.3 [over.match.oper]. — end note]
value()
instead of has_value()
Section: 22.8.6.5 [expected.object.swap], 22.8.7.5 [expected.void.swap] Status: WP Submitter: Ben Craig Opened: 2023-06-25 Last modified: 2023-11-22
Priority: Not Prioritized
View all issues with WP status.
Discussion:
22.8.6.5 [expected.object.swap] p2 has the following text in it:
For the case where
rhs.value()
isfalse
andthis->has_value()
istrue
, equivalent to: […]
The table preceding that text is a table of this->has_value()
vs. rhs.has_value()
. The rhs.value()
in the text is almost certainly a typo, as a .value()
call here doesn't make any sense, especially if this is an
expected<non-bool, E>
.
[2023-10-27; Reflector poll]
Set status to Tentatively Ready after seven votes in favour during reflector poll.
[2023-11-11 Approved at November 2023 meeting in Kona. Status changed: Voting → WP.]
Proposed resolution:
This wording is relative to N4950.
Modify 22.8.6.5 [expected.object.swap] as indicated:
constexpr void swap(expected& rhs) noexcept(see below);-1- […]
-2- Effects: See Table 63 [tab:expected.object.swap]. For the case whererhs.has_value()
isfalse
andthis->has_value()
istrue
, equivalent to: […]
Modify 22.8.7.5 [expected.void.swap] as indicated:
constexpr void swap(expected& rhs) noexcept(see below);-1- […]
-2- Effects: See Table 64 [tab:expected.void.swap]. For the case whererhs.has_value()
isfalse
andthis->has_value()
istrue
, equivalent to: […]
iter_move
for common_iterator
and counted_iterator
should return decltype(auto)
Section: 24.5.5.7 [common.iter.cust], 24.5.7.7 [counted.iter.cust] Status: WP Submitter: Hewill Kang Opened: 2023-06-30 Last modified: 2023-11-22
Priority: Not Prioritized
View all issues with WP status.
Discussion:
Although the customized iter_move
of both requires the underlying iterator I
to be input_iterator
,
they still explicitly specify the return type as iter_rvalue_reference_t<I>
,
which makes it always instantiated.
From the point of view that its validity is only specified in the input_iterator
concept, it would be better to remove such
unnecessary type instantiation, which does not make much sense for an output_iterator
even if it is still valid.
[2023-10-27; Reflector poll]
Set status to Tentatively Ready after six votes in favour during reflector poll.
[2023-11-11 Approved at November 2023 meeting in Kona. Status changed: Voting → WP.]
Proposed resolution:
This wording is relative to N4950.
Modify 24.5.5.1 [common.iterator], class template common_iterator
synopsis, as indicated:
namespace std { template<input_or_output_iterator I, sentinel_for<I> S> requires (!same_as<I, S> && copyable<I>) class common_iterator { public: […] friend constexpriter_rvalue_reference_t<I>decltype(auto) iter_move(const common_iterator& i) noexcept(noexcept(ranges::iter_move(declval<const I&>()))) requires input_iterator<I>; […] }; […] }
Modify 24.5.5.7 [common.iter.cust] as indicated:
friend constexpriter_rvalue_reference_t<I>decltype(auto) iter_move(const common_iterator& i) noexcept(noexcept(ranges::iter_move(declval<const I&>()))) requires input_iterator<I>;-1- Preconditions: […]
-2- Effects: Equivalent to:return ranges::iter_move(get<I>(i.v_));
Modify 24.5.7.1 [counted.iterator], class template counted_iterator
synopsis, as indicated:
namespace std { template<input_or_output_iterator I> class counted_iterator { public: […] friend constexpriter_rvalue_reference_t<I>decltype(auto) iter_move(const counted_iterator& i) noexcept(noexcept(ranges::iter_move(i.current))) requires input_iterator<I>; […] }; […] }
Modify 24.5.7.7 [counted.iter.cust] as indicated:
friend constexpriter_rvalue_reference_t<I>decltype(auto) iter_move(const counted_iterator& i) noexcept(noexcept(ranges::iter_move(i.current))) requires input_iterator<I>;-1- Preconditions: […]
-2- Effects: Equivalent to:return ranges::iter_move(i.current);
chrono::parse
uses from_stream
as a customization pointSection: 30.13 [time.parse] Status: WP Submitter: Jonathan Wakely Opened: 2023-07-15 Last modified: 2025-02-16
Priority: 3
View other active issues in [time.parse].
View all other issues in [time.parse].
View all issues with WP status.
Discussion:
30.13 [time.parse] says: "Each parse
overload specified
in this subclause calls from_stream
unqualified,
so as to enable argument dependent lookup (6.5.4 [basic.lookup.argdep])."
That name should be added to 16.4.2.2 [contents] along with
swap
,
make_error_code
, and
make_error_condition
.
We should decide whether calls to from_stream
should use normal
lookup (i.e. unqualified lookup plus ADL) or just ADL, as was done for
make_error_code
and make_error_condition
(see LWG 3629(i)).
[2023-10-30; Reflector poll]
Set priority to 3 after reflector poll.
[2024-12-02; Jonathan provides wording]
I suggest that from_stream
should only be found via ADL,
not unqualified lookup. This is consistent with what we did for
make_error_code
and make_error_condition
, and more recently for
submdspan_mapping
. I see no reason to treat from_stream
differently.
This implies that implementations might need a poison poll in std::chrono
so that unqualified lookup stops as soon as those are found.
[2024-12-09; Reflector poll]
Set status to Tentatively Ready after six votes in favour during reflector poll.
[Hagenberg 2025-02-16; Status changed: Voting → WP.]
Proposed resolution:
This wording is relative to N4993.
Modify 16.4.2.2 [contents] as indicated:
-3- Whenever an unqualified name other than
swap
,make_error_code
,make_error_condition
,from_stream
, orsubmdspan_mapping
is used in the specification of a declarationD
in Clause 17 through Clause 33 or Annex D, its meaning is established as-if by performing unqualified name lookup (6.5.3 [basic.lookup.unqual]) in the context ofD
.[Note 1: Argument-dependent lookup is not performed. — end note]
Similarly, the meaning of a qualified-id is established as-if by performing qualified name lookup (6.5.5 [basic.lookup.qual]) in the context of
D
.[Example 1: The reference to
is_array_v
in the specification ofstd::to_array
(23.3.3.6 [array.creation]) refers to::std::is_array_v
. — end example][Note 2: Operators in expressions (12.2.2.3 [over.match.oper]) are not so constrained; see 16.4.6.4 [global.functions]. — end note]
The meaning of the unqualified name
swap
is established in an overload resolution context for swappable values (16.4.4.3 [swappable.requirements]). The meanings of the unqualified namesmake_error_code
,make_error_condition
,from_stream
, andsubmdspan_mapping
are established as-if by performing argument-dependent lookup (6.5.4 [basic.lookup.argdep]).
v
should be claimedSection: 23.2.2.5 [container.alloc.reqmts] Status: WP Submitter: jim x Opened: 2023-07-10 Last modified: 2023-11-22
Priority: Not Prioritized
View all other issues in [container.alloc.reqmts].
View all issues with WP status.
Discussion:
23.2.2.5 [container.alloc.reqmts] p2 says:
[…] an expression
v
of typeT
orconst T
, […]
Then 23.2.2.5 [container.alloc.reqmts] bullet (2.4) says:
T
is Cpp17CopyInsertable intoX
means that, in addition toT
being Cpp17MoveInsertable intoX
, the following expression is well-formed:allocator_traits<A>::construct(m, p, v)
So, what is the value category of the expression v
? We didn't explicitly phrase the wording.
The intent may be that the value category of v
is any defined value category in 7.2.1 [basic.lval],
however, the intent is not clear in the current wording. Maybe, we can say:
[…] the following expression is well-formed:
allocator_traits<A>::construct(m, p, v)for
v
of any value category.
which can make the intent meaning clearer.
[2023-10-27; Reflector poll]
Set status to Tentatively Ready after five votes in favour during reflector poll.
[2023-11-11 Approved at November 2023 meeting in Kona. Status changed: Voting → WP.]
Proposed resolution:
This wording is relative to N4950.
Modify 23.2.2.5 [container.alloc.reqmts] as indicated:
-2- Given an allocator type
A
and given a container typeX
having avalue_type
identical toT
and anallocator_type
identical toallocator_traits<A>::rebind_alloc<T>
and given an lvaluem
of typeA
, a pointerp
of typeT*
, an expressionv
that denotes an lvalue of typeT
orconst T
or an rvalue of typeconst T
, and an rvaluerv
of typeT
, the following terms are defined. […]
[…]
(2.3) —
T
is Cpp17MoveInsertable intoX
means that the following expression is well-formed:allocator_traits<A>::construct(m, p, rv)and its evaluation causes the following postcondition to hold: The value of
[Note 1:*p
is equivalent to the value ofrv
before the evaluation.rv
remains a valid object. Its state is unspecified — end note](2.4) —
T
is Cpp17CopyInsertable intoX
means that, in addition toT
being Cpp17MoveInsertable intoX
, the following expression is well-formed:allocator_traits<A>::construct(m, p, v)and its evaluation causes the following postcondition to hold: The value of
v
is unchanged and is equivalent to*p
.[…]
Section: 28.5.6.5 [format.string.escaped] Status: WP Submitter: Tom Honermann Opened: 2023-07-31 Last modified: 2023-11-22
Priority: Not Prioritized
View all issues with WP status.
Discussion:
The C++23 DIS contains the following example in 28.5.6.5 [format.string.escaped] p3. (This example does not appear in the most recent N4950 WP or on https://eel.is/c++draft because the project editor has not yet merged changes needed to support rendering of some of the characters involved).
string s6 = format("[{:?}]", "🤷♂️"); // s6 has value: ["🤷\u{200d}♂\u{fe0f}"]
The character to be formatted (🤷♂️) consists of the following sequence of code points in the order presented:
U+1F937 (SHRUG)
U+200D (ZERO WIDTH JOINER)
U+2642 (MALE SIGN)
U+FE0F (VARIATION SELECTOR-16)
28.5.6.5 [format.string.escaped] bullet 2.2.1 specifies which code points are to be formatted as a
\u{hex-digit-sequence}
escape sequence:
(2.2.1) — If X encodes a single character C, then:
(2.2.1.1) — If C is one of the characters in Table 75 [tab:format.escape.sequences], then the two characters shown as the corresponding escape sequence are appended to E.
(2.2.1.2) — Otherwise, if C is not U+0020 SPACE and
(2.2.1.2.1) — CE is UTF-8, UTF-16, or UTF-32 and C corresponds to a Unicode scalar value whose
Unicode property General_Category
has a value in the groups Separator
(Z
) or Other
(C
), as described by UAX #44 of the Unicode Standard, or
(2.2.1.2.2) — CE is UTF-8, UTF-16, or UTF-32 and C corresponds to a Unicode scalar value with
the Unicode property Grapheme_Extend=Yes
as described by UAX #44 of the Unicode
Standard and C is not immediately preceded in S by a character P appended to E without
translation to an escape sequence, or
(2.2.1.2.3) — CE is neither UTF-8, UTF-16, nor UTF-32 and C is one of an implementation-defined set of separator or non-printable characters
then the sequence \u{hex-digit-sequence}
is appended to E, where hex-digit-sequence
is the shortest hexadecimal representation of C using lower-case hexadecimal digits.
(2.2.1.3) — Otherwise, C is appended to E.
The example is not consistent with the above specification for the final code point.
U+FE0F is a single character,
is not one of the characters in Table 75, is not U+0020, has a General_Category
of Nonspacing Mark (Mn)
which is neither Z
nor C
, has Grapheme_Extend=Yes
but the prior character (U+2642) is not
formatted as an escape sequence, and is not one of an implementation-defined set of separator or non-printable characters
(for the purposes of this example; the example assumes a UTF-8 encoding). Thus, formatting for this character falls to
the last bullet point and the character should be appended as is (without translation to an escape sequence).
Since this character is a combining character, it should combine with the previous character and thus alter the
appearance of U+2642 (thus producing "♂️"
instead of "♂\u{fe0f}"
).
[2023-10-27; Reflector poll]
Set status to Tentatively Ready after six votes in favour during reflector poll.
[2023-11-11 Approved at November 2023 meeting in Kona. Status changed: Voting → WP.]
Proposed resolution:
This wording is relative to N4950 plus missing editorial pieces from P2286R8.
Modify the example following 28.5.6.5 [format.string.escaped] p3 as indicated:
[Drafting note: The presented example was voted in as part of P2286R8 during the July 2022 Virtual Meeting but is not yet accessible in the most recent working draft N4950.
Note that the final character (♂️) is composed from the two code points U+2642 and U+FE0F. ]
string s6 = format("[{:?}]", "🤷♂️"); // s6 has value:["🤷\u{200d}♂\u{fe0f}"]["🤷\u{200d}♂️"]
full_extent_t
and full_extent
Section: 23.7.3.2 [mdspan.syn] Status: WP Submitter: S. B. Tam Opened: 2023-08-16 Last modified: 2023-11-22
Priority: Not Prioritized
View all issues with WP status.
Discussion:
submdspan
uses a type called full_extent_t
, but there isn't a definition for that type.
full_extent_t
(along with full_extent
) was proposed in P0009
before submdspan
was moved into its own paper, and its definition failed to be included in
P2630 Submdspan.
[2023-10-27; Reflector poll]
Set status to Tentatively Ready after nine votes in favour during reflector poll.
[2023-11-11 Approved at November 2023 meeting in Kona. Status changed: Voting → WP.]
Proposed resolution:
This wording is relative to N4958.
Modify 23.7.3.2 [mdspan.syn] as indicated:
[…] // [mdspan.submdspan], submdspan creation template<class OffsetType, class LengthType, class StrideType> struct strided_slice; template<class LayoutMapping> struct submdspan_mapping_result; struct full_extent_t { explicit full_extent_t() = default; }; inline constexpr full_extent_t full_extent{}; […]
Section: 22.8.6.7 [expected.object.monadic], 22.5.3.8 [optional.monadic] Status: WP Submitter: Jiang An Opened: 2023-08-10 Last modified: 2023-11-22
Priority: Not Prioritized
View all other issues in [expected.object.monadic].
View all issues with WP status.
Discussion:
LWG 3938(i) switched to use **this
to access the value stored in std::expected
.
However, as shown in LWG 3969(i), **this
can trigger ADL and find an unwanted overload,
and thus may caused unintended behavior.
**this
, but use the name of the union member instead.
Moreover, P2407R5 will change the monadic operations of std::optional
to use **this
,
which is also problematic.
[2023-09-19; Wording update]
Several people preferred to replace operator*()
by the corresponding union members, so this part of the proposed wording
has been adjusted, which is a rather mechanical replacement.
[2023-10-30; Reflector poll]
Set status to Tentatively Ready after five votes in favour during reflector poll.
[2023-11-11 Approved at November 2023 meeting in Kona. Status changed: Voting → WP.]
Proposed resolution:
This wording is relative to N4958.
Modify 22.8.6.7 [expected.object.monadic] as indicated:
[Drafting note: Effectively replace all occurrences of
**this
byval
, except fordecltype
.]
template<class F> constexpr auto and_then(F&& f) &; template<class F> constexpr auto and_then(F&& f) const &;-1- Let
-2- […] -3- […] -4- Effects: Equivalent to:U
beremove_cvref_t<invoke_result_t<F, decltype(
.**this(val))>>if (has_value()) return invoke(std::forward<F>(f),**thisval); else return U(unexpect, error());template<class F> constexpr auto and_then(F&& f) &&; template<class F> constexpr auto and_then(F&& f) const &&;-5- Let
-6- […] -7- […] -8- Effects: Equivalent to:U
beremove_cvref_t<invoke_result_t<F, decltype((std::move(
.**thisval))>>if (has_value()) return invoke(std::forward<F>(f), std::move(**thisval)); else return U(unexpect, std::move(error()));template<class F> constexpr auto or_else(F&& f) &; template<class F> constexpr auto or_else(F&& f) const &;-9- Let
-10- Constraints:G
beremove_cvref_t<invoke_result_t<F, decltype(error())>>
.is_constructible_v<T, decltype(
is**this(val))>true
. -11- […] -12- Effects: Equivalent to:if (has_value()) return G(in_place,**thisval); else return invoke(std::forward<F>(f), error());template<class F> constexpr auto or_else(F&& f) &&; template<class F> constexpr auto or_else(F&& f) const &&;-13- Let
-14- Constraints:G
beremove_cvref_t<invoke_result_t<F, decltype(std::move(error()))>>
.is_constructible_v<T, decltype(std::move(
is**thisval))>true
. -15- […] -16- Effects: Equivalent to:if (has_value()) return G(in_place, std::move(**thisval)); else return invoke(std::forward<F>(f), std::move(error()));template<class F> constexpr auto transform(F&& f) &; template<class F> constexpr auto transform(F&& f) const &;-17- Let
-18- […] -19- Mandates:U
beremove_cvref_t<invoke_result_t<F, decltype(
.**this(val))>>U
is a valid value type forexpected
. Ifis_void_v<U>
isfalse
, the declarationU u(invoke(std::forward<F>(f),**thisval));is well-formed.
-20- Effects:
(20.1) — […]
(20.2) — Otherwise, if
is_void_v<U>
isfalse
, returns anexpected<U, E>
object whosehas_val
member istrue
andval
member is direct-non-list-initialized withinvoke(std::forward<F>(f),
.**thisval)(20.3) — Otherwise, evaluates
invoke(std::forward<F>(f),
and then returns**thisval)expected<U, E>()
.template<class F> constexpr auto transform(F&& f) &&; template<class F> constexpr auto transform(F&& f) const &&;-21- Let
-22- […] -23- Mandates:U
beremove_cvref_t<invoke_result_t<F, decltype(std::move(
.**thisval))>>U
is a valid value type forexpected
. Ifis_void_v<U>
isfalse
, the declarationU u(invoke(std::forward<F>(f), std::move(**thisval)));is well-formed.
-24- Effects:
(24.1) — […]
(24.2) — Otherwise, if
is_void_v<U>
isfalse
, returns anexpected<U, E>
object whosehas_val
member istrue
andval
member is direct-non-list-initialized withinvoke(std::forward<F>(f), std::move(
.**thisval))(24.3) — Otherwise, evaluates
invoke(std::forward<F>(f), std::move(
and then returns**thisval))expected<U, E>()
.template<class F> constexpr auto transform_error(F&& f) &; template<class F> constexpr auto transform_error(F&& f) const &;-25- Let
-26- Constraints:G
beremove_cvref_t<invoke_result_t<F, decltype(error())>>
.is_constructible_v<T, decltype(
is**this(val))>true
. -27- Mandates: […] -28- Returns: Ifhas_value()
istrue
,expected<T, G>(in_place,
; otherwise, an**thisval)expected<T, G>
object whosehas_val
member isfalse
andunex
member is direct-non-list-initialized withinvoke(std::forward<F>(f), error())
.template<class F> constexpr auto transform_error(F&& f) &&; template<class F> constexpr auto transform_error(F&& f) const &&;-29- Let
-30- Constraints:G
beremove_cvref_t<invoke_result_t<F, decltype(std::move(error()))>>
.is_constructible_v<T, decltype(std::move(
is**thisval))>true
. -31- Mandates: […] -32- Returns: Ifhas_value()
istrue
,expected<T, G>(in_place, std::move(
; otherwise, an**thisval))expected<T, G>
object whosehas_val
member isfalse
andunex
member is direct-non-list-initialized withinvoke(std::forward<F>(f), std::move(error()))
.
Modify 22.5.3.8 [optional.monadic] as indicated:
[Drafting note: Effectively replace all occurrences of
value()
by*val
.]
template<class F> constexpr auto and_then(F&& f) &; template<class F> constexpr auto and_then(F&& f) const &;-1- Let
-2- […] -3- Effects: Equivalent to:U
beinvoke_result_t<F, decltype(
.value()*val)>if (*this) { return invoke(std::forward<F>(f),value()*val); } else { return remove_cvref_t<U>(); }template<class F> constexpr auto and_then(F&& f) &&; template<class F> constexpr auto and_then(F&& f) const &&;-4- Let
-5- […] -6- Effects: Equivalent to:U
beinvoke_result_t<F, decltype(std::move(
.value()*val))>if (*this) { return invoke(std::forward<F>(f), std::move(value()*val)); } else { return remove_cvref_t<U>(); }template<class F> constexpr auto transform(F&& f) &; template<class F> constexpr auto transform(F&& f) const &;-7- Let
-8- Mandates:U
beremove_cv_t<invoke_result_t<F, decltype(
.value()*val)>>U
is a non-array object type other thanin_place_t
ornullopt_t
. The declarationU u(invoke(std::forward<F>(f),value()*val));is well-formed for some invented variable
[…] -9- Returns: Ifu
.*this
contains a value, anoptional<U>
object whose contained value is direct-non-list-initialized withinvoke(std::forward<F>(f),
; otherwise,value()*val)optional<U>()
.template<class F> constexpr auto transform(F&& f) &&; template<class F> constexpr auto transform(F&& f) const &&;-10- Let
-11- Mandates:U
beremove_cv_t<invoke_result_t<F, decltype(std::move(
.value()*val))>>U
is a non-array object type other thanin_place_t
ornullopt_t
. The declarationU u(invoke(std::forward<F>(f), std::move(value()*val)));is well-formed for some invented variable
[…] -12- Returns: Ifu
.*this
contains a value, anoptional<U>
object whose contained value is direct-non-list-initialized withinvoke(std::forward<F>(f), std::move(
; otherwise,value()*val))optional<U>()
.
mdspan::operator[]
should not copy OtherIndexTypes
Section: 23.7.3.6.3 [mdspan.mdspan.members] Status: WP Submitter: Casey Carter Opened: 2023-08-12 Last modified: 2023-11-22
Priority: Not Prioritized
View all issues with WP status.
Discussion:
The wording for mdspan
's operator[]
overloads that accept span
and array
is in
23.7.3.6.3 [mdspan.mdspan.members] paragraphs 5 and 6:
template<class OtherIndexType> constexpr reference operator[](span<OtherIndexType, rank()> indices) const; template<class OtherIndexType> constexpr reference operator[](const array<OtherIndexType, rank()>& indices) const;-5- Constraints:
(5.1) —
is_convertible_v<const OtherIndexType&, index_type>
istrue
, and(5.2) —
is_nothrow_constructible_v<index_type, const OtherIndexType&>
istrue
.-6- Effects: Let
P
be a parameter pack such thatis_same_v<make_index_sequence<rank()>, index_sequence<P...>>is
true
. Equivalent to:return operator[](as_const(indices[P])...);
The equivalent code calls the other operator[]
overload:
template<class... OtherIndexTypes> constexpr reference operator[](OtherIndexTypes... indices) const;
with a pack of const OtherIndexType
lvalues, but we notably haven't required OtherIndexTypes
to be copyable —
we only require that we can convert them to index_type
. While one could argue that the use in "Effects: equivalent to"
implies a requirement of copyability, it's odd that this implicit requirement would be the only requirement for copyable
OtherIndexTypes
in the spec. We could fix this by changing the operator[]
overload accepting OtherIndexTypes
to take them by const&
, but that would be inconsistent with virtually every other place in the spec where types
convertible to index_type
are taken by-value. I think the best localized fix is to perform the conversion to index_type
in the "Effects: equivalent to" code so the actual arguments have type index_type
which we know is copyable.
[2023-11-02; Reflector poll]
Set status to Tentatively Ready after eight votes in favour during reflector poll.
[2023-11-11 Approved at November 2023 meeting in Kona. Status changed: Voting → WP.]
Proposed resolution:
This wording is relative to N4958.
Modify 23.7.3.6.3 [mdspan.mdspan.members] as indicated:
template<class OtherIndexType> constexpr reference operator[](span<OtherIndexType, rank()> indices) const; template<class OtherIndexType> constexpr reference operator[](const array<OtherIndexType, rank()>& indices) const;-5- Constraints:
(5.1) —
is_convertible_v<const OtherIndexType&, index_type>
istrue
, and(5.2) —
is_nothrow_constructible_v<index_type, const OtherIndexType&>
istrue
.-6- Effects: Let
P
be a parameter pack such thatis_same_v<make_index_sequence<rank()>, index_sequence<P...>>is
true
. Equivalent to:return operator[](extents_type::index-cast(as_const(indices[P]))...);
basic_format_context
should not be permittedSection: 28.5.6.7 [format.context] Status: WP Submitter: Brian Bi Opened: 2023-08-13 Last modified: 2024-04-02
Priority: 3
View all other issues in [format.context].
View all issues with WP status.
Discussion:
The current wording allows users to specialize std::basic_format_context
. However, an implementation is not
likely to accept a program that uses the library in a way that would instantiate such a specialization, because
28.5.6.7 [format.context] does not provide a complete description of the interface that such a specialization
would need to have (e.g., it does not provide a means to initialize the exposition-only args_
member). Since the
library was not designed to be able to work with user specializations of std::basic_format_context
, declaring
such specializations should be explicitly disallowed.
Previous resolution [SUPERSEDED]:
This wording is relative to N4958.
Modify the 28.5.6.7 [format.context] as indicated:
-1- An instance of
-?- The behavior of a program that adds specializations ofbasic_format_context
holds formatting state consisting of the formatting arguments and the output iterator.basic_format_context
is undefined. -2-Out
shall modeloutput_iterator<const charT&>
.
[2023-09-23; Daniel comments and provides improved wording]
During the reflector discussion, Dietmar pointed out that the constraint can in principle be checked statically (e.g. when the
Library creates or refers to an instantiation of basic_format_context
), so we can reduce the rather draconian consequence of
"undefined behaviour" to "ill-formed, no diagnostics required". Furthermore, the new wording also adds the same constraint to
basic_format_parse_context
as suggested by Tim. This is needed, since only one public constructor is specified, but
that specification does not allow to construct an object a non-zero num_args_
or with the type information necessary
for the check_dynamic_spec*
functions, so the library has an unspecified way to realize this.
[2023-10-30; Reflector poll]
Set priority to 3 after reflector poll.
[Kona 2023-11-07; move to Ready]
[Tokyo 2024-03-23; Status changed: Voting → WP.]
Proposed resolution:
This wording is relative to N4958.
[Drafting note: The suggested wording form is borrowed from exactly the same wording form used for
allocator_traits
.]
Modify 28.5.6.7 [format.context] as indicated:
-1- An instance of
-?- If a program declares an explicit or partial specialization ofbasic_format_context
holds formatting state consisting of the formatting arguments and the output iterator.basic_format_context
, the program is ill-formed, no diagnostic required. -2-Out
shall modeloutput_iterator<const charT&>
.
Modify 28.5.6.6 [format.parse.ctx] as indicated:
-1- An instance of
-?- If a program declares an explicit or partial specialization ofbasic_format_parse_context
holds the format string parsing state consisting of the format string range being parsed and the argument counter for automatic indexing.basic_format_parse_context
, the program is ill-formed, no diagnostic required.
std::ignore
is unimplementable for volatile
bit-fieldsSection: 22.4.5 [tuple.creation] Status: Resolved Submitter: Jiang An Opened: 2023-08-19 Last modified: 2024-07-25
Priority: 4
View all other issues in [tuple.creation].
View all issues with Resolved status.
Discussion:
22.4.5 [tuple.creation]/5 currently says:
[…] When an argument in
t
isignore
, assigning any value to the corresponding tuple element has no effect.
which is unimplementable for volatile
-qualified bit-field glvalues.
operator=
function, a distinct object needs to be created and thus the read from
the volatile
glvalue, which is a side effect (6.9.1 [intro.execution]/7), is unavoidable.
P2968R0 addresses the impossibility of assignment from void
values, but doesn't talk about
volatile
bit-fields. Perhaps we should explicitly say that the program is ill-formed if a volatile
bit-field
value is assigned to std::ignore
(which is implemented in libstdc++ and MSVC STL, but not in libc++).
[2023-11-03; Reflector poll]
Set priority to 4 after reflector poll. "Specify it as code." "P2968 should fix this."
[2024-07-25 Status changed: New → Resolved.]
Resolved by P2968R2, approved in St. Louis.
Proposed resolution:
ranges::to
's recursion branch may be ill-formedSection: 25.5.7.2 [range.utility.conv.to] Status: WP Submitter: Hewill Kang Opened: 2023-08-23 Last modified: 2024-04-02
Priority: 3
View other active issues in [range.utility.conv.to].
View all other issues in [range.utility.conv.to].
View all issues with WP status.
Discussion:
When r
is a nested range, ranges::to
constructs the object recursively through r | views::transform(...)
.
r
does not model viewable_range
(demo):
#include <ranges>
#include <vector>
#include <list>
int main() {
std::vector<std::vector<int>> v;
auto r = std::views::all(std::move(v));
auto l = std::ranges::to<std::list<std::list<int>>>(r); // hard error in MSVC-STL and libc++
}
[2023-11-03; Reflector poll]
Set priority to 3 after reflector poll.
"Should be std::forward<R>(r)
instead?"
[Kona 2023-11-07; move to Ready]
[Tokyo 2024-03-23; Status changed: Voting → WP.]
Proposed resolution:
This wording is relative to N4958.
Modify 25.5.7.2 [range.utility.conv.to] as indicated:
template<class C, input_range R, class... Args> requires (!view<C>) constexpr C to(R&& r, Args&&... args);-1- Mandates:
C
is a cv-unqualified class type.-2- Returns: An object of type
C
constructed from the elements ofr
in the following manner:
(2.1) — If
C
does not satisfyinput_range
orconvertible_to<range_reference_t<R>, range_value_t<C>>
istrue
:
[…]
(2.2) — Otherwise, if
input_range<range_reference_t<R>>
istrue
:to<C>(ref_view(r) | views::transform([](auto&& elem) { return to<range_value_t<C>>(std::forward<decltype(elem)>(elem)); }), std::forward<Args>(args)...);(2.3) — Otherwise, the program is ill-formed.
<flat_foo>
doesn't provide std::begin/end
Section: 24.7 [iterator.range] Status: WP Submitter: Hewill Kang Opened: 2023-08-27 Last modified: 2023-11-22
Priority: Not Prioritized
View other active issues in [iterator.range].
View all other issues in [iterator.range].
View all issues with WP status.
Discussion:
It seems that 24.7 [iterator.range] should also add <flat_foo>
to the list as the
latter provides a series of range access member functions such as begin/end
.
[2023-11-02; Reflector poll]
Set status to Tentatively Ready after nine votes in favour during reflector poll.
[2023-11-11 Approved at November 2023 meeting in Kona. Status changed: Voting → WP.]
Proposed resolution:
This wording is relative to N4958.
Modify 24.7 [iterator.range] as indicated:
-1- In addition to being available via inclusion of the
<iterator>
header, the function templates in [iterator.range] are available when any of the following headers are included:<array>
,<deque>
,<flat_map>
,<flat_set>
,<forward_list>
,<list>
,<map>
,<regex>
,<set>
,<span>
,<string>
,<string_view>
,<unordered_map>
,<unordered_set>
, and<vector>
.
std::tuple
and std::variant
can't be properly supportedSection: 22.4.4 [tuple.tuple], 22.6.3.1 [variant.variant.general] Status: WP Submitter: Jiang An Opened: 2023-08-29 Last modified: 2023-11-22
Priority: Not Prioritized
View all other issues in [tuple.tuple].
View all issues with WP status.
Discussion:
Currently, program-defined specializations of std::tuple
and std::variant
are not explicitly disallowed.
However, they can't be properly supported by standard library implementations, because the corresponding std::get
function templates have to inspect the implementation details of these types, and users have no way to make std::get
behave correctly for a program-defined specializations.
std::tuple
and std::variant
.
[2023-11-02; Reflector poll]
Set status to Tentatively Ready after five votes in favour during reflector poll.
[2023-11-11 Approved at November 2023 meeting in Kona. Status changed: Voting → WP.]
Proposed resolution:
This wording is relative to N4958.
Modify 22.4.4 [tuple.tuple] as indicated:
namespace std { template<class... Types> class tuple { […] }; […] }
-?- If a program declares an explicit or partial specialization of
tuple
, the program is ill-formed, no diagnostic required.
Modify 22.6.3.1 [variant.variant.general] as indicated:
[…]
-3- A program that instantiates the definition ofvariant
with no template arguments is ill-formed. -?- If a program declares an explicit or partial specialization ofvariant
, the program is ill-formed, no diagnostic required.
iota_view
should provide empty
Section: 25.6.4.2 [range.iota.view] Status: WP Submitter: Hewill Kang Opened: 2023-10-27 Last modified: 2023-11-22
Priority: Not Prioritized
View all other issues in [range.iota.view].
View all issues with WP status.
Discussion:
When iota_view
's template parameter is not an integer type and does not model advanceable
,
its size
member will not be provided as constraints are not satisfied.
incrementable
, this results in its view_interface
base being
unable to synthesize a valid empty
member as iota_view
will just be an input_range
(demo):
#include <ranges>
#include <vector>
#include <iostream>
int main() {
std::vector<int> v;
auto it = std::back_inserter(v);
auto s = std::ranges::subrange(it, std::unreachable_sentinel);
auto r = std::views::iota(it);
std::cout << s.empty() << "\n"; // 0
std::cout << r.empty() << "\n"; // ill-formed
}
This seems to be an oversight. I don't see a reason why iota_view
doesn't provide empty
as it does store the start and end like subrange
, in which case it's easy to tell if it's empty
just by comparing the two.
[2023-11-02; Reflector poll]
Set status to Tentatively Ready after seven votes in favour during reflector poll.
[2023-11-11 Approved at November 2023 meeting in Kona. Status changed: Voting → WP.]
Proposed resolution:
This wording is relative to N4964.
Modify 25.6.4.2 [range.iota.view], class template iota_view
synopsis, as indicated:
namespace std::ranges { […] template<weakly_incrementable W, semiregular Bound = unreachable_sentinel_t> requires weakly-equality-comparable-with<W, Bound> && copyable<W> class iota_view : public view_interface<iota_view<W, Bound>> { private: […] W value_ = W(); // exposition only Bound bound_ = Bound(); // exposition only public: […] constexpr iterator begin() const; constexpr auto end() const; constexpr iterator end() const requires same_as<W, Bound>; constexpr bool empty() const; constexpr auto size() const requires see below; }; […] }[…]
constexpr iterator end() const requires same_as<W, Bound>;-14- Effects: Equivalent to:
return iterator{bound_};
constexpr bool empty() const;-?- Effects: Equivalent to:
[…]return value_ == bound_;
Section: 23.7.2.2.6 [span.elem] Status: WP Submitter: Arthur O'Dwyer Opened: 2023-11-09 Last modified: 2024-04-02
Priority: Not Prioritized
View all issues with WP status.
Discussion:
In reviewing the wording for P2821 span.at()
, it had been noticed that
23.7.2.2.6 [span.elem] uses a lot of "Effects: Equivalent to return […];" which
could be simply "Returns: […]".
[2024-03-11; Reflector poll]
Set status to Tentatively Ready after seven votes in favour during reflector poll.
[Tokyo 2024-03-23; Status changed: Voting → WP.]
Proposed resolution:
This wording is relative to N4964.
Modify 23.7.2.2.6 [span.elem] as indicated:
constexpr reference operator[](size_type idx) const;
-1- Preconditions:
-2-idx < size()
istrue
.EffectsReturns:Equivalent to:. -?- Throws: Nothing.
return*(data() + idx);
constexpr reference front() const;
-3- Preconditions:
-4-empty()
isfalse
.EffectsReturns:Equivalent to:. -?- Throws: Nothing.
return*data();
constexpr reference back() const;
-5- Preconditions:
-6-empty()
isfalse
.EffectsReturns:Equivalent to:. -?- Throws: Nothing.
return*(data() + (size() - 1));
constexpr pointer data() const noexcept;
-7-
EffectsReturns:Equivalent to:.
returndata_;
common_view::begin/end
are missing the simple-view
checkSection: 25.7.20.2 [range.common.view] Status: WP Submitter: Hewill Kang Opened: 2023-11-11 Last modified: 2024-04-02
Priority: Not Prioritized
View all other issues in [range.common.view].
View all issues with WP status.
Discussion:
common_view::begin/end
have exactly the same implementation as their corresponding const
versions,
which implies that when the underlying V
satisfies simple-view
, it is sufficient to
just provide const
-qualified members.
[2024-03-11; Reflector poll]
Set status to Tentatively Ready after six votes in favour during reflector poll.
[Tokyo 2024-03-23; Status changed: Voting → WP.]
Proposed resolution:
This wording is relative to N4964.
Modify 25.7.20.2 [range.common.view] as indicated:
namespace std::ranges { template<view V> requires (!common_range<V> && copyable<iterator_t<V>>) class common_view : public view_interface<common_view<V>> { private: V base_ = V(); // exposition only public: […] constexpr auto begin() requires (!simple-view<V>) { if constexpr (random_access_range<V> && sized_range<V>) return ranges::begin(base_); else return common_iterator<iterator_t<V>, sentinel_t<V>>(ranges::begin(base_)); } constexpr auto begin() const requires range<const V> { if constexpr (random_access_range<const V> && sized_range<const V>) return ranges::begin(base_); else return common_iterator<iterator_t<const V>, sentinel_t<const V>>(ranges::begin(base_)); } constexpr auto end() requires (!simple-view<V>) { if constexpr (random_access_range<V> && sized_range<V>) return ranges::begin(base_) + ranges::distance(base_); else return common_iterator<iterator_t<V>, sentinel_t<V>>(ranges::end(base_)); } constexpr auto end() const requires range<const V> { if constexpr (random_access_range<const V> && sized_range<const V>) return ranges::begin(base_) + ranges::distance(base_); else return common_iterator<iterator_t<const V>, sentinel_t<const V>>(ranges::end(base_)); } […] }; […] }
lazy_split_view::outer-iterator::value_type
should not provide default constructorSection: 25.7.16.4 [range.lazy.split.outer.value] Status: WP Submitter: Hewill Kang Opened: 2023-11-11 Last modified: 2024-04-02
Priority: Not Prioritized
View all other issues in [range.lazy.split.outer.value].
View all issues with WP status.
Discussion:
After P2325, there is no reason for lazy_split_view::outer-iterator::value_type
to provide a default constructor, which only leads to unexpected behavior:
#include <ranges>
constexpr int arr[] = {42};
constexpr auto split = arr | std::views::lazy_split(0);
static_assert(!std::ranges::range_value_t<decltype(split)>{}); // UB, dereferencing a null pointer
Also, the other constructor should be private because it makes no sense for the user to construct it arbitrarily, which is not the intention.
[2024-03-11; Reflector poll]
Set status to Tentatively Ready after six votes in favour during reflector poll.
[Tokyo 2024-03-23; Status changed: Voting → WP.]
Proposed resolution:
This wording is relative to N4964.
Modify 25.7.16.4 [range.lazy.split.outer.value], class split_view::outer-iterator::value_type
synopsis,
as indicated:
namespace std::ranges { template<input_range V, forward_range Pattern> requires view<V> && view<Pattern> && indirectly_comparable<iterator_t<V>, iterator_t<Pattern>, ranges::equal_to> && (forward_range<V> || tiny-range<Pattern>) template<bool Const> struct lazy_split_view<V, Pattern>::outer-iterator<Const>::value_type : view_interface<value_type> { private: outer-iterator i_ = outer-iterator(); // exposition only constexpr explicit value_type(outer-iterator i); // exposition only public:value_type() = default;constexpr explicit value_type(outer-iterator i);constexpr inner-iterator<Const> begin() const; constexpr default_sentinel_t end() const noexcept; }; }
std::subtract_with_carry_engine
codeSection: 29.5.4.4 [rand.eng.sub] Status: WP Submitter: Matt Stephanson Opened: 2023-11-15 Last modified: 2024-11-28
Priority: 2
View all other issues in [rand.eng.sub].
View all issues with WP status.
Discussion:
Issue 3809(i) pointed out that subtract_with_carry_engine<T>
can be seeded with values
from a linear_congruential_engine<T, 40014u, 0u, 2147483563u>
object, which results in narrowing
when T
is less than 32 bits. Part of the resolution was to modify the LCG seed sequence as follows:
explicit subtract_with_carry_engine(result_type value);-7- Effects: Sets the values of , in that order, as specified below. If is then , sets to ; otherwise sets to .
To set the values , first construct
e
, alinear_congruential_engine
object, as if by the following definition:linear_congruential_engine<result_typeuint_least32_t, 40014u,0u,2147483563u> e(value == 0u ? default_seed : value);Then, to set each , obtain new values from successive invocations of
e
. Set to .
Inside linear_congruential_engine
, the seed is reduced modulo 2147483563, so uint_least32_t
is fine from that point on. This resolution, however, forces value
, the user-provided seed, to be
truncated from result_type
to uint_least32_t
before the reduction, which generally will
change the result. It also breaks the existing behavior that two seeds are equivalent if they're in the same
congruence class modulo the divisor.
[2024-01-11; Reflector poll]
Set priority to 2 after reflector poll.
[2024-01-11; Jonathan comments]
More precisely, the resolution forces value
to be converted
to uint_least32_t
, which doesn't necessarily truncate, and if it
does truncate, it doesn't necessarily change the value.
But it will truncate whenever value_type
is wider than
uint_least32_t
,
e.g. for 32-bit uint_least32_t
you get a different result for
std::ranlux48_base(UINT_MAX + 1LL)()
.
The new proposed resolution below restores the old behaviour for that type.
[2024-10-09; LWG telecon: Move to Ready]
[Wrocław 2024-11-23; Status changed: Voting → WP.]
Proposed resolution:
This wording is relative to N4964 after the wording changes applied by LWG 3809(i), which had been accepted into the working paper during the Kona 2023-11 meeting.
Modify 29.5.4.4 [rand.eng.sub] as indicated:
explicit subtract_with_carry_engine(result_type value);-7- Effects: Sets the values of , in that order, as specified below. If is then , sets to ; otherwise sets to .
To set the values , first construct
e
, alinear_congruential_engine
object, as if by the following definition:linear_congruential_engine<uint_least32_t, 40014u,0u,2147483563u> e(value == 0u ? default_seed : static_cast<uint_least32_t>(value % 2147483563u));Then, to set each , obtain new values from successive invocations of
e
. Set to .
container-insertable
checks do not match what container-inserter
doesSection: 25.5.7 [range.utility.conv] Status: WP Submitter: Jonathan Wakely Opened: 2023-11-24 Last modified: 2024-04-02
Priority: Not Prioritized
View all issues with WP status.
Discussion:
The exposition-only helper container-inserter
uses either
std::back_inserter
or std::inserter
. Both
std::back_insert_iterator
and std::insert_iterator
require C::value_type
to be a valid type, and we do not check
for that in container-insertable
.
The insert iterators can also incur a conversion to construct a
C::value_type
which then gets moved into the container.
Using emplace instead of insert would avoid that temporary object.
It's also possible (although arguably not worth caring about) that
range_value_t<C>
is not the same type as
C::value_type
, and that conversion to C::value_type
could be ill-formed (we only check that conversion from
range_reference_t<R>
to range_value_t<C>
is well-formed).
It seems preferable to remove the use of insert iterators, so that we don't need to check their requirements at all.
[2023-1-26; Rename exposition-only concept and function after reflector discussion.]
[2024-03-11; Reflector poll]
Set status to Tentatively Ready after six votes in favour during reflector poll.
[Tokyo 2024-03-23; Status changed: Voting → WP.]
Proposed resolution:
This wording is relative to N4964.
Modify 25.5.7.1 [range.utility.conv.general] as indicated:
-4- Let
container-
be defined as follows:insertableappendabletemplate<class Container, class Ref> constexpr bool container-
insertableappendable = // exposition only requires(Container& c, Ref&& ref) { requires (requires { c.emplace_back(std::forward<Ref>(ref)); } || requires { c.push_back(std::forward<Ref>(ref)); } || requires { c.emplace(c.end(), std::forward<Ref>(ref)); } || requires { c.insert(c.end(), std::forward<Ref>(ref)); }); };-5- Let
container-
be defined as follows:inserterappendtemplate<class Container
, class Ref> constexpr auto container-inserterappend(Container& c) { // exposition onlyif constexpr (requires { c.push_back(declval<Ref>()); }) return back_inserter(c); else return inserter(c, c.end());return [&c]<class Ref>(Ref&& ref) { if constexpr (requires { c.emplace_back(declval<Ref>()); }) c.emplace_back(std::forward<Ref>(ref)); else if constexpr (requires { c.push_back(declval<Ref>()); }) c.push_back(std::forward<Ref>(ref)); else if constexpr (requires { c.emplace(c.end(), declval<Ref>()); }) c.emplace(c.end(), std::forward<Ref>(ref)); else c.insert(c.end(), std::forward<Ref>(ref)); }; };
Modify 25.5.7.2 [range.utility.conv.to] as indicated:
(2.1.4) Otherwise, if
- —
constructible_from<C, Args...>
istrue
, and- —
container-
isinsertableappendable<C, range_reference_t<R>>true
:C c(std::forward<Args>(args)...); if constexpr (sized_range<R> && reservable-container<C>) c.reserve(static_cast<range_size_t<C>>(ranges::size(r))); ranges::
copyfor_each(r, container-inserterappend<range_reference_t<R>>(c));
std::basic_streambuf::setg/setp
Section: 31.6.3.4 [streambuf.protected] Status: WP Submitter: Jiang An Opened: 2023-12-08 Last modified: 2024-04-02
Priority: Not Prioritized
View all issues with WP status.
Discussion:
It seems that operations of std::basic_streambuf
expect that
[eback(), egptr())
is a valid range and gptr()
points into that range, and
[pbase(), pptr())
is a valid range and epptr()
points into that range.
However, it is currently not specified for setg/setp
that such invariants need to be established.
[2024-03-11; Reflector poll]
Set status to Tentatively Ready after six votes in favour during reflector poll.
[Tokyo 2024-03-23; Status changed: Voting → WP.]
Proposed resolution:
This wording is relative to N4964.
Modify 31.6.3.4.2 [streambuf.get.area] as indicated:
void setg(char_type* gbeg, char_type* gnext, char_type* gend);-?- Preconditions:
-5- Postconditions:[gbeg, gnext)
,[gbeg, gend)
, and[gnext, gend)
are all valid ranges.gbeg == eback()
,gnext == gptr()
, andgend == egptr()
are alltrue
.
Modify 31.6.3.4.3 [streambuf.put.area] as indicated:
void setp(char_type* pbeg, char_type* pend);-?- Preconditions:
-5- Postconditions:[pbeg, pend)
is a valid range.pbeg == pbase()
,pbeg == pptr()
, andpend == epptr()
are alltrue
.
std::make_shared_for_overwrite/std::allocate_shared_for_overwrite
Section: 20.3.2.2.7 [util.smartptr.shared.create] Status: WP Submitter: Jiang An Opened: 2023-12-16 Last modified: 2024-11-28
Priority: 2
View all other issues in [util.smartptr.shared.create].
View all issues with WP status.
Discussion:
Currently, only destructions of non-array (sub)objects created in std::make_shared
and std::allocate_shared
are specified in 20.3.2.2.7 [util.smartptr.shared.create]. Presumably, objects created in
std::make_shared_for_overwrite
and std::allocate_shared_for_overwrite
should be destroyed by plain
destructor calls.
[2024-03-11; Reflector poll]
Set priority to 2 after reflector poll in December 2023.
This was the P1020R1 author's intent (see LWG reflector mail in November 2018) but it was never clarified in the wording. This fixes that.
[2024-08-21; Move to Ready at LWG telecon]
[Wrocław 2024-11-23; Status changed: Voting → WP.]
Proposed resolution:
This wording is relative to N4964.
Modify 20.3.2.2.7 [util.smartptr.shared.create] as indicated:
template<class T, ...> shared_ptr<T> make_shared(args); template<class T, class A, ...> shared_ptr<T> allocate_shared(const A& a, args); template<class T, ...> shared_ptr<T> make_shared_for_overwrite(args); template<class T, class A, ...> shared_ptr<T> allocate_shared_for_overwrite(const A& a, args);[…]
-7- Remarks:
[…]
(7.11) — When a (sub)object of non-array type
U
that was initialized bymake_shared
,make_shared_for_overwrite
, orallocate_shared_for_overwrite
is to be destroyed, it is destroyed via the expressionpv->~U()
wherepv
points to that object of typeU
.[…]
std::expected<cv void, E>
should not be conditionally deletedSection: 22.8.7.4 [expected.void.assign] Status: WP Submitter: Jiang An Opened: 2023-12-16 Last modified: 2024-04-02
Priority: Not Prioritized
View all other issues in [expected.void.assign].
View all issues with WP status.
Discussion:
It seems intended that copy functions of std::optional
, std::variant
, and std::expected
are conditionally deleted, while move functions are constrained. However, the move assignment operator of
std::expected<cv void, E>
is currently conditionally deleted, which is inconsistent.
[2024-03-11; Reflector poll]
Set status to Tentatively Ready after six votes in favour during reflector poll.
[Tokyo 2024-03-23; Status changed: Voting → WP.]
Proposed resolution:
This wording is relative to N4971.
Modify 22.8.7.4 [expected.void.assign] as indicated:
constexpr expected& operator=(expected&& rhs) noexcept(see below);-?- Constraints:
[…] -6- Remarks: The exception specification is equivalent tois_move_constructible_v<E>
istrue
andis_move_assignable_v<E>
istrue
.is_nothrow_move_constructible_v<E> && is_nothrow_move_assignable_v<E>
.-7- This operator is defined as deleted unlessis_move_constructible_v<E>
istrue
andis_move_assignable_v<E>
istrue
.
possibly-const-range
should prefer returning const R&
Section: 25.2 [ranges.syn] Status: WP Submitter: Hewill Kang Opened: 2023-12-17 Last modified: 2024-11-28
Priority: 2
View all other issues in [ranges.syn].
View all issues with WP status.
Discussion:
possibly-const-range
currently only returns const R&
when R
does not
satisfy constant_range
and const R
satisfies constant_range
.
std::cbegin
(demo):
#include <ranges>
int main() {
auto r = std::views::single(0)
| std::views::transform([](int) { return 0; });
using C1 = decltype(std::ranges::cbegin(r));
using C2 = decltype(std::cbegin(r));
static_assert(std::same_as<C1, C2>); // failed
}
Since R
itself is constant_range
, so possibly-const-range
, above just returns
R&
and C1
is transform_view::iterator<false>
; std::cbegin
specifies to return as_const(r).begin()
, which makes that C2
is
transform_view::iterator<true>
which is different from C1
.
const R&
should always be returned if it's a range, regardless of whether const R
or R
is a constant_range
, just as fmt-maybe-const
in format ranges always prefers
const R
over R
.
Although it is theoretically possible for R
to satisfy constant_range
and that const R
is a mutable range, such nonsense range type should not be of interest.
This relaxation of constraints allows for maximum consistency with std::cbegin
, and in some cases can
preserve constness to the greatest extent (demo):
#include <ranges>
int main() {
auto r = std::views::single(0) | std::views::lazy_split(0);
(*std::ranges::cbegin(r)).front() = 42; // ok
(*std::cbegin(r)).front() = 42; // not ok
}
Above, *std::ranges::cbegin
returns a range of type const lazy_split_view::outer-iterator<false>::value_type
,
which does not satisfy constant_range
because its reference type is int&
.
*std::cbegin(r)
returns lazy_split_view::outer-iterator<true>::value_type
whose reference type is const int&
and satisfies constant_range
.
[2024-03-11; Reflector poll]
Set priority to 2 after reflector poll. Send to SG9.
[St. Louis 2024-06-28; LWG and SG9 joint session: move to Ready]
[Wrocław 2024-11-23; Status changed: Voting → WP.]
Proposed resolution:
This wording is relative to N4971.
Modify 25.2 [ranges.syn], header <ranges>
synopsis, as indicated:
#include <compare> // see 17.12.1 [compare.syn] #include <initializer_list> // see 17.11.2 [initializer.list.syn] #include <iterator> // see 24.2 [iterator.synopsis] namespace std::ranges { […] // 25.7.22 [range.as.const], as const view template<input_range R> constexpr auto& possibly-const-range(R& r) noexcept { // exposition only if constexpr (inputconstant_range<const R>&& !constant_range<R>) { return const_cast<const R&>(r); } else { return r; } } […] }
Section: 26.10.17.1 [numeric.sat.func] Status: WP Submitter: Thomas Köppe Opened: 2023-12-18 Last modified: 2024-04-02
Priority: Not Prioritized
View all issues with WP status.
Discussion:
During the application of P0543R0, "Saturation arithmetic", it was pointed out that it
might not be entirely clear what we want something like "x
+ y
" to mean. The paper does not
suggest any formatting for those symbols, and a non-normative note explains that the intention is for
the expression to be considered mathematically.
$\tcode{x} + \tcode{y}$
throughout, i.e. the variables are in code font,
but the symbol is maths, not code. This is quite subtle. (See also
GitHub discussion.)
I think it would be an improvement if we simply made the note not be a note. It seems to contain entirely reasonable, mandatory content.
[2024-03-11; Reflector poll]
Set status to Tentatively Ready after six votes in favour during reflector poll.
[Tokyo 2024-03-23; Status changed: Voting → WP.]
Proposed resolution:
This wording is relative to N4971.
Modify 26.10.17.1 [numeric.sat.func] as indicated:
-1-
[Note 1:In the following descriptions, an arithmetic operation is performed as a mathematical operation with infinite range and then it is determined whether the mathematical result fits into the result type.— end note]
bad_expected_access<void>
member functions should be noexcept
Section: 22.8.5 [expected.bad.void] Status: WP Submitter: Cassio Neri Opened: 2023-12-24 Last modified: 2024-04-02
Priority: Not Prioritized
View all issues with WP status.
Discussion:
According to 17.9.3 [exception]/2:
Each standard library class
T
that derives from classexception
has the following publicly accessible member functions, each of them having a non-throwing exception specification (14.5):
(2.1) — default constructor (unless the class synopsis shows other constructors)
(2.2) — copy constructor
(2.3) — copy assignment operator
For good reasons, bad_expected_access<void>
overrules from this general rule by
protecting its special member functions. However, there's no reason these functions should not be
noexcept
.
[2024-03-12; Reflector poll]
Set status to Tentatively Ready after five votes in favour during reflector poll.
[Tokyo 2024-03-23; Status changed: Voting → WP.]
Proposed resolution:
This wording is relative to N4971.
Modify 22.8.5 [expected.bad.void] as indicated:
namespace std { template<> class bad_expected_access<void> : public exception { protected: bad_expected_access() noexcept; bad_expected_access(const bad_expected_access&) noexcept; bad_expected_access(bad_expected_access&&) noexcept; bad_expected_access& operator=(const bad_expected_access&) noexcept; bad_expected_access& operator=(bad_expected_access&&) noexcept; ~bad_expected_access(); public: const char* what() const noexcept override; }; }
single_view
should provide empty
Section: 25.6.3.2 [range.single.view] Status: WP Submitter: Hewill Kang Opened: 2023-12-31 Last modified: 2024-04-02
Priority: Not Prioritized
View all other issues in [range.single.view].
View all issues with WP status.
Discussion:
Although single_view::empty
can be synthesized through view_interface
,
it seems more worthwhile to provide a static empty
for it which eliminates the
need to pass in an object parameter, guarantees noexcept
-ness, and is consistent
with the design of empty_view
(demo):
#include <ranges>
auto empty = std::views::empty<int>;
static_assert(noexcept(empty.empty()));
static_assert(noexcept(empty.size()));
auto single = std::views::single(0);
static_assert(noexcept(single.empty())); // fire
static_assert(noexcept(single.size()));
[2024-03-12; Reflector poll]
Set status to Tentatively Ready after six votes in favour during reflector poll.
[Tokyo 2024-03-23; Status changed: Voting → WP.]
Proposed resolution:
This wording is relative to N4971.
Modify 25.6.3.2 [range.single.view] as indicated:
[…]namespace std::ranges { template<move_constructible T> requires is_object_v<T> class single_view : public view_interface<single_view<T>> { […] public: […] constexpr T* begin() noexcept; constexpr const T* begin() const noexcept; constexpr T* end() noexcept; constexpr const T* end() const noexcept; static constexpr bool empty() noexcept; static constexpr size_t size() noexcept; constexpr T* data() noexcept; constexpr const T* data() const noexcept; }; […] }constexpr T* end() noexcept; constexpr const T* end() const noexcept;-5- Effects: Equivalent to:
return data() + 1;
static constexpr bool empty() noexcept;-?- Effects: Equivalent to:
return false;
__alignof_is_defined
is only implicitly specified in C++ and not yet deprecatedSection: D.11 [depr.c.macros] Status: WP Submitter: Jiang An Opened: 2024-01-12 Last modified: 2024-11-01
Priority: Not Prioritized
View all issues with WP status.
Discussion:
Currently 17.15.4 [stdalign.h.syn] states
The contents of the C++ header
See also: ISO/IEC 9899:2018, 7.15<stdalign.h>
are the same as the C standard library header<stdalign.h>
, with the following changes: The header<stdalign.h>
does not define a macro namedalignas
.
which implicitly specifies that __alignof_is_defined
is also provided in C++, because C17
specified that the macro is provided in <stdaligh.h>
.
__alignof_is_defined
in the C++ standard wording.
And D.11 [depr.c.macros]/1 (added by LWG 3827(i)) seemingly contradicts with
17.15.4 [stdalign.h.syn] and only makes __alignas_is_defined
deprecated.
It seems that we should explicitly mention __alignof_is_defined
in D.11 [depr.c.macros]
at this moment.
[2024-03-12; Reflector poll]
Set status to Tentatively Ready after seven votes in favour during reflector poll.
[Tokyo 2024-03-23; Status changed: Voting → WP.]
[2024-10-22; Note: this should have been handled by LWG 2241(i) but that issue was closed as Resolved without properly resolving it.]
Proposed resolution:
This wording is relative to N4971.
Modify D.11 [depr.c.macros] as indicated:
-1- The header
<stdalign.h>
has the following macros:#define __alignas_is_defined 1 #define __alignof_is_defined 1
ctype_base
are not yet required to be usable in constant expressionsSection: 28.3.4.2.1 [category.ctype.general] Status: WP Submitter: Jiang An Opened: 2024-01-12 Last modified: 2024-04-02
Priority: Not Prioritized
View all issues with WP status.
Discussion:
It may be desired that static data members ctype_base
are "real constants", i.e. usable in constant expressions.
However, this is not strictly required because mask
is only required to be a bitmask type that can be a class type,
which makes the plain const
potentially insufficient.
[2024-03-12; Reflector poll]
Set status to Tentatively Ready after five votes in favour during reflector poll.
[Tokyo 2024-03-23; Status changed: Voting → WP.]
Proposed resolution:
This wording is relative to N4971.
Modify 28.3.4.2.1 [category.ctype.general] as indicated:
namespace std { class ctype_base { public: using mask = see below; // numeric values are for exposition only. static constexpr mask space = 1 << 0; static constexpr mask print = 1 << 1; static constexpr mask cntrl = 1 << 2; static constexpr mask upper = 1 << 3; static constexpr mask lower = 1 << 4; static constexpr mask alpha = 1 << 5; static constexpr mask digit = 1 << 6; static constexpr mask punct = 1 << 7; static constexpr mask xdigit = 1 << 8; static constexpr mask blank = 1 << 9; static constexpr mask alnum = alpha | digit; static constexpr mask graph = alnum | punct; }; }-1- The type
mask
is a bitmask type (16.3.3.3.3 [bitmask.types]).
std::text_encoding::aliases_view
should have constexpr iteratorsSection: 28.4.2.5 [text.encoding.aliases] Status: WP Submitter: Jonathan Wakely Opened: 2024-01-16 Last modified: 2024-04-02
Priority: Not Prioritized
View all issues with WP status.
Discussion:
aliases_view::begin()
and aliases_view::end()
are constexpr functions, but there is no requirement that you can use
the returned iterator and sentinel in constant expressions.
[2024-03-12; Reflector poll]
Set status to Tentatively Ready after seven votes in favour during reflector poll.
[Tokyo 2024-03-23; Status changed: Voting → WP.]
Proposed resolution:
This wording is relative to N4971.
Modify 28.4.2.5 [text.encoding.aliases] as indicated:
struct text_encoding::aliases_view : ranges::view_interface<text_encoding::aliases_view> { constexpr implementation-defined begin() const; constexpr implementation-defined end() const; };-1-
text_encoding::aliases_view
modelscopyable
,ranges::view
,ranges::random_access_range
, andranges::borrowed_range
.-2- Both
ranges::range_value_t<text_encoding::aliases_view>
andranges::range_reference_t<text_encoding::aliases_view>
denoteconst char*
.-?-
ranges::iterator_t<text_encoding::aliases_view>
is a constexpr iterator (24.3.1 [iterator.requirements.general]).
std::print
should permit an efficient implementationSection: 31.7.10 [print.fun] Status: Resolved Submitter: Victor Zverovich Opened: 2024-01-20 Last modified: 2025-03-10
Priority: 3
View all other issues in [print.fun].
View all issues with Resolved status.
Discussion:
std::print
/std::vprint*
is currently defined in terms of formatting into a temporary string, e.g.
31.7.10 [print.fun]:
void vprint_nonunicode(FILE* stream, string_view fmt, format_args args);Preconditions:
Effects: Writes the result ofstream
is a valid pointer to an output C stream.vformat(fmt, args)
tostream
. Throws: Any exception thrown by the call tovformat
(28.5.3 [format.err.report]).system_error
if writing tostream
fails. May throwbad_alloc
.
This is done to make it clear that noninterleaved output is desired while keeping specification simple and portable.
Unfortunately, the current specification seems to prohibit a more efficient implementation that performs formatting directly into a stream buffer under a lock (flockfile
/funlockfile
in POSIX) like printf
does.
The difference can be observable in case of an I/O error that occurs before a custom formatter is called. In the
(double buffered) implementation that directly follows the spec all formatters will be called, while in a more efficient
(locking) implementation subsequent formatters may not be called.
The easiest fix, given in the current proposed resolution, is to say that some arguments may not be formatted in
case of a write error. It might be a bit weird considering that the spec says that we construct a string first so an
alternative resolution is to replace vformat
with vformat_to
info some unspecified buffer iterator
and state noninterleaving requirement separately.
[2024-02-19; Feb 2024 mailing]
This would be resolved by P3107.
[2024-03-12; Reflector poll]
Set priority to 3 and status to LEWG after reflector poll in January 2024.
"This loses the guarantee that if the formatting throws then there's no output."
[2025-03-10 Status changed: LEWG → Resolved.]
Resolved by P3107R5, approved in Tokyo, March 2024.
Proposed resolution:
This wording is relative to N4971.
Modify 31.7.10 [print.fun] as indicated:
void vprint_unicode(FILE* stream, string_view fmt, format_args args);[…]-6- Preconditions:
-7- Effects: The function initializes an automatic variable viastream
is a valid pointer to an output C stream.string out = vformat(fmt, args);If
If writing to the terminal orstream
refers to a terminal capable of displaying Unicode, writes out to the terminal using the native Unicode API; if out contains invalid code units, the behavior is undefined and implementations are encouraged to diagnose it. Otherwise writesout
tostream
unchanged. If the native Unicode API is used, the function flushesstream
before writingout
.stream
fails, some arguments inargs
may not be formatted. […]void vprint_nonunicode(FILE* stream, string_view fmt, format_args args);-11- Preconditions:
-12- Effects: Writes the result ofstream
is a valid pointer to an output C stream.vformat(fmt, args)
tostream
. If writing tostream
fails, some arguments inargs
may not be formatted. -13- Throws: […].
"ASCII"
is not a registered character encodingSection: 28.4.2.2 [text.encoding.general] Status: WP Submitter: Jonathan Wakely Opened: 2024-01-23 Last modified: 2024-04-02
Priority: Not Prioritized
View all issues with WP status.
Discussion:
The IANA Charater Sets registry does not contain "ASCII" as an alias of the "US-ASCII" encoding. This is apparently for historical reasons, because there used to be some ambiguity about exactly what "ASCII" meant. I don't think those historical reasons are relevant to C++26, but the absence of "ASCII" in the IANA registry means that it's not a registered character encoding as defined by 28.4.2.2 [text.encoding.general].
This means that the encoding referred to by notes in the C++ standard
(31.12.6.2 [fs.path.generic], 28.3.4.4.1.3 [facet.numpunct.virtuals])
and by an example in the std::text_encoding
proposal
(P1885) isn't actually usable in portable code.
So std::text_encoding("ASCII")
creates an object with
mib() == std::text_encoding::other
, which is not the same
encoding as std::text_encoding("US-ASCII")
.
This seems surprising.
[2024-03-12; Reflector poll]
SG16 approved the proposed resolution. Set status to Tentatively Ready after seven votes in favour during reflector poll.
[Tokyo 2024-03-23; Status changed: Voting → WP.]
Proposed resolution:
This wording is relative to N4971.
Modify 28.4.2.2 [text.encoding.general] as indicated:
-1- A registered character encoding is a character encoding scheme in the IANA Character Sets registry.
[Note 1: The IANA Character Sets registry uses the term “character sets” to refer to character encodings. — end note]
The primary name of a registered character encoding is the name of that encoding specified in the IANA Character Sets registry.
-2- The set of known registered character encodings contains every registered character encoding specified in the IANA Character Sets registry except for the following:
- (2.1) – NATS-DANO (33)
- (2.2) – NATS-DANO-ADD (34)
-3- Each known registered character encoding is identified by an enumerator in
text_encoding::id
, and has a set of zero or more aliases.-4- The set of aliases of a known registered character encoding is an implementation-defined superset of the aliases specified in the IANA Character Sets registry. The set of aliases for US-ASCII includes
"ASCII"
. No two aliases or primary names of distinct registered character encodings are equivalent when compared bytext_encoding::comp-name
.
std::print
on POSIX platformsSection: 31.7.10 [print.fun] Status: WP Submitter: Jonathan Wakely Opened: 2024-01-24 Last modified: 2024-11-28
Priority: 3
View all other issues in [print.fun].
View all issues with WP status.
Discussion:
The effects for vprintf_unicode
say:
If
stream
refers to a terminal capable of displaying Unicode, writesout
to the terminal using the native Unicode API; ifout
contains invalid code units, the behavior is undefined and implementations are encouraged to diagnose it. Otherwise writesout
tostream
unchanged. If the native Unicode API is used, the function flushesstream
before writingout
.[Note 1: On POSIX and Windows,
stream
referring to a terminal means that, respectively,isatty(fileno(stream))
andGetConsoleMode(_get_osfhandle(_fileno(stream)), ...)
return nonzero. — end note][Note 2: On Windows, the native Unicode API is
WriteConsoleW
. — end note]-8- Throws: [...]
-9- Recommended practice: If invoking the native Unicode API requires transcoding, implementations should substitute invalid code units with u+fffd replacement character per the Unicode Standard, Chapter 3.9 u+fffd Substitution in Conversion.
The very explicit mention of isatty
for POSIX platforms has
confused at least two implementers into thinking that we're supposed to
use isatty
, and supposed to do something differently based
on what it returns. That seems consistent with the nearly identical wording
in 28.5.2.2 [format.string.std] paragraph 12, which says
"Implementations should use either UTF-8, UTF-16, or UTF-32,
on platforms capable of displaying Unicode text in a terminal"
and then has a note explicitly saying this is the case for Windows-based and
many POSIX-based operating systems. So it seems clear that POSIX platforms
are supposed to be considered to have "a terminal capable of displaying
Unicode text", and so std::print
should use isatty
and then use a native Unicode API, and diagnose invalid code units.
This is a problem however, because isatty
needs
to make a system call on Linux, adding 500ns to every std::print
call. This results in a 10x slowdown on Linux, where std::print
can take just 60ns without the isatty
check.
From discussions with Tom Honermann I learned that the "native Unicode API"
wording is only relevant on Windows. This makes sense, because for POSIX
platforms, writing to a terminal is done using the usual stdio functions,
so there's no need to treat a terminal differently to any other file stream.
And substitution of invalid code units with
u+fffd
is recommended for Windows because that's what typical modern terminals do on
POSIX platforms, so requiring the implementation to do that on Windows gives
consistent behaviour. But the implementation doesn't need to do anything to
make that happen with a POSIX terminal, it happens anyway.
So the isatty
check is unnecessary for POSIX platforms,
and the note mentioning it just causes confusion and has no benefit.
Secondly, there initially seems to be a contradiction between the "implementations are encouraged to diagnose it" wording and the later Recommended practice. In fact, there's no contradiction because the native Unicode API might accept UTF-8 and therefore require no transcoding, and so the Recommended practice wouldn't apply. The intention is that diagnosing invalid UTF-8 is still desirable in this case, but how should it be diagnosed? By writing an error to the terminal alongside the formatted string? Or by substituting u+fffd maybe? If the latter is the intention, why is one suggestion in the middle of the Effects, and one given as Recommended practice?
The proposed resolution attempts to clarify that a "native Unicode API" is only needed if that's how you display Unicode on the terminal. It also moves the flushing requirement to be adjacent to the other requirements for systems using a native Unicode API instead of on its own later in the paragraph. And the suggestion to diagnose invalid code units is moved into the Recommended practice and clarified that it's only relevant if using a native Unicode API. I'm still not entirely happy with encouragement to diagnose invalid code units without giving any clue as to how that should be done. What does it mean to diagnose something at runtime? That's novel for the C++ standard. The way it's currently phrased seems to imply something other than u+fffd substitution should be done, although that seems the most obvious implementation to me.
[2024-03-12; Reflector poll]
Set priority to 3 after reflector poll and send to SG16.
Previous resolution [SUPERSEDED]:
This wording is relative to N4971.
Modify 31.7.6.3.5 [ostream.formatted.print] as indicated:
void vprint_unicode(ostream& os, string_view fmt, format_args args); void vprint_nonunicode(ostream& os, string_view fmt, format_args args);-3- Effects: Behaves as a formatted output function (31.7.6.3.1 [ostream.formatted.reqmts]) of
os
, except that:
- (3.1) – failure to generate output is reported as specified below, and
- (3.2) – any exception thrown by the call to
vformat
is propagated without regard to the value ofos.exceptions()
and without turning onios_base::badbit
in the error state ofos
.After constructing a
sentry
object, the function initializes an automatic variable viaIf the function isstring out = vformat(os.getloc(), fmt, args);
vprint_unicode
andos
is a stream that refers to a terminal capable of displaying Unicode via a native Unicode API, which is determined in an implementation-defined manner, flushesos
and then writesout
to the terminal using the native Unicode API; ifout
contains invalid code units, the behavior is undefinedand implementations are encouraged to diagnose it.If the native Unicode API is used, the function flushesOtherwise, (ifos
before writingout
.os
is not such a stream or the function isvprint_nonunicode
), inserts the character sequence [out.begin()
,out.end()
) intoos
. If writing to the terminal or inserting intoos
fails, callsos.setstate(ios_base::badbit)
(which may throwios_base::failure
).-4- Recommended practice: For
vprint_unicode
, if invoking the native Unicode API requires transcoding, implementations should substitute invalid code units with u+fffd replacement character per the Unicode Standard, Chapter 3.9 u+fffd Substitution in Conversion. If invoking the native Unicode API does not require transcoding, implementations are encouraged to diagnose invalid code units.Modify 31.7.10 [print.fun] as indicated:
void vprint_unicode(FILE* stream, string_view fmt, format_args args);-6- Preconditions:
stream
is a valid pointer to an output C stream.-7- Effects: The function initializes an automatic variable via
Ifstring out = vformat(fmt, args);
stream
refers to a terminal capable of displaying Unicode via a native Unicode API, flushesstream
and then writesout
to the terminal using the native Unicode API; ifout
contains invalid code units, the behavior is undefinedand implementations are encouraged to diagnose it. Otherwise writesout
tostream
unchanged.If the native Unicode API is used, the function flushesstream
before writingout
.[Note 1: On
POSIX andWindows,the native Unicode API isWriteConsoleW
andstream
referring to a terminal means that, respectively,isatty(fileno(stream))
andGetConsoleMode(_get_osfhandle(_fileno(stream)), ...)
return nonzero. — end note]
[Note 2: On Windows, the native Unicode API isWriteConsoleW
. — end note]-8- Throws: [...]
-9- Recommended practice: If invoking the native Unicode API requires transcoding, implementations should substitute invalid code units with u+fffd replacement character per the Unicode Standard, Chapter 3.9 u+fffd Substitution in Conversion. If invoking the native Unicode API does not require transcoding, implementations are encouraged to diagnose invalid code units.
[2024-03-12; Jonathan updates wording based on SG16 feedback]
SG16 reviewed the issue and approved the proposed resolution with the wording about diagnosing invalid code units removed.
SG16 favors removing the following text (both occurrences) from the proposed wording. This is motivated by a lack of understanding regarding what it means to diagnose such invalid code unit sequences given that the input is likely provided at run-time.
If invoking the native Unicode API does not require transcoding, implementations are encouraged to diagnose invalid code units.
Some concern was expressed regarding how the current wording is structured. At present, the wording leads with a Windows centric perspective; if the stream refers to a terminal ... use the native Unicode API ... otherwise write code units to the stream. It might be an improvement to structure the wording such that use of the native Unicode API is presented as a fallback for implementations that require its use when writing directly to the stream is not sufficient to produce desired results. In other words, the wording should permit direct writing to the stream even when the stream is directed to a terminal and a native Unicode API is available when the implementation has reason to believe that doing so will produce the correct results. For example, Microsoft's HoloLens has a Windows based operating system, but it only supports use of UTF-8 as the system code page and therefore would not require the native Unicode API bypass; implementations for it could avoid the overhead of checking to see if the stream is directed to a console.
Previous resolution [SUPERSEDED]:
This wording is relative to N4971.
Modify 31.7.6.3.5 [ostream.formatted.print] as indicated:
void vprint_unicode(ostream& os, string_view fmt, format_args args); void vprint_nonunicode(ostream& os, string_view fmt, format_args args);-3- Effects: Behaves as a formatted output function (31.7.6.3.1 [ostream.formatted.reqmts]) of
os
, except that:
- (3.1) – failure to generate output is reported as specified below, and
- (3.2) – any exception thrown by the call to
vformat
is propagated without regard to the value ofos.exceptions()
and without turning onios_base::badbit
in the error state ofos
.After constructing a
sentry
object, the function initializes an automatic variable viaIf the function isstring out = vformat(os.getloc(), fmt, args);
vprint_unicode
andos
is a stream that refers to a terminal that is only capable of displaying Unicode via a native Unicode API, which is determined in an implementation-defined manner, flushesos
and then writesout
to the terminal using the native Unicode API; ifout
contains invalid code units, the behavior is undefinedand implementations are encouraged to diagnose it.If the native Unicode API is used, the function flushesOtherwise, (ifos
before writingout
.os
is not such a stream or the function isvprint_nonunicode
), inserts the character sequence [out.begin()
,out.end()
) intoos
. If writing to the terminal or inserting intoos
fails, callsos.setstate(ios_base::badbit)
(which may throwios_base::failure
).-4- Recommended practice: For
vprint_unicode
, if invoking the native Unicode API requires transcoding, implementations should substitute invalid code units with u+fffd replacement character per the Unicode Standard, Chapter 3.9 u+fffd Substitution in Conversion.Modify 31.7.10 [print.fun] as indicated:
void vprint_unicode(FILE* stream, string_view fmt, format_args args);-6- Preconditions:
stream
is a valid pointer to an output C stream.-7- Effects: The function initializes an automatic variable via
Ifstring out = vformat(fmt, args);
stream
refers to a terminal that is only capable of displaying Unicode via a native Unicode API, flushesstream
and then writesout
to the terminal using the native Unicode API; ifout
contains invalid code units, the behavior is undefinedand implementations are encouraged to diagnose it. Otherwise writesout
tostream
unchanged.If the native Unicode API is used, the function flushesstream
before writingout
.[Note 1: On
POSIX andWindows,the native Unicode API isWriteConsoleW
andstream
referring to a terminal means that, respectively,isatty(fileno(stream))
andGetConsoleMode(_get_osfhandle(_fileno(stream)), ...)
return nonzero. — end note]
[Note 2: On Windows, the native Unicode API isWriteConsoleW
. — end note]-8- Throws: [...]
-9- Recommended practice: If invoking the native Unicode API requires transcoding, implementations should substitute invalid code units with u+fffd replacement character per the Unicode Standard, Chapter 3.9 u+fffd Substitution in Conversion.
[2024-03-19; Tokyo: Jonathan updates wording after LWG review]
Split the Effects: into separate bullets for the "native Unicode API"
and "otherwise" cases. Remove the now-redundant "if os
is not such a stream"
parenthesis.
[St. Louis 2024-06-24; move to Ready.]
[Wrocław 2024-11-23; Status changed: Voting → WP.]
Proposed resolution:
This wording is relative to N4971.
Modify 31.7.6.3.5 [ostream.formatted.print] as indicated:
void vprint_unicode(ostream& os, string_view fmt, format_args args); void vprint_nonunicode(ostream& os, string_view fmt, format_args args);-3- Effects: Behaves as a formatted output function (31.7.6.3.1 [ostream.formatted.reqmts]) of
os
, except that:
- (3.1) – failure to generate output is reported as specified below, and
- (3.2) – any exception thrown by the call to
vformat
is propagated without regard to the value ofos.exceptions()
and without turning onios_base::badbit
in the error state ofos
.-?- After constructing a
sentry
object, the function initializes an automatic variable viastring out = vformat(os.getloc(), fmt, args);
- (?.1) – If the function is
vprint_unicode
andos
is a stream that refers to a terminal that is only capable of displaying Unicode via a native Unicode API, which is determined in an implementation-defined manner, flushesos
and then writesout
to the terminal using the native Unicode API; ifout
contains invalid code units, the behavior is undefinedand implementations are encouraged to diagnose it.If the native Unicode API is used, the function flushes.os
before writingout
- (?.2) – Otherwise,
(ifinserts the character sequence [os
is not such a stream or the function isvprint_nonunicode
),out.begin()
,out.end()
) intoos
.-?- If writing to the terminal or inserting into
os
fails, callsos.setstate(ios_base::badbit)
(which may throwios_base::failure
).-4- Recommended practice: For
vprint_unicode
, if invoking the native Unicode API requires transcoding, implementations should substitute invalid code units with u+fffd replacement character per the Unicode Standard, Chapter 3.9 u+fffd Substitution in Conversion.
Modify 31.7.10 [print.fun] as indicated:
void vprint_unicode(FILE* stream, string_view fmt, format_args args);-6- Preconditions:
stream
is a valid pointer to an output C stream.-7- Effects: The function initializes an automatic variable via
string out = vformat(fmt, args);
- (7.1) – If
stream
refers to a terminal that is only capable of displaying Unicode via a native Unicode API, flushesstream
and then writesout
to the terminal using the native Unicode API; ifout
contains invalid code units, the behavior is undefinedand implementations are encouraged to diagnose it.- (7.2) – Otherwise writes
out
tostream
unchanged.
If the native Unicode API is used, the function flushesstream
before writingout
.[Note 1: On
POSIX andWindows,the native Unicode API isWriteConsoleW
andstream
referring to a terminal means that, respectively,isatty(fileno(stream))
andGetConsoleMode(_get_osfhandle(_fileno(stream)), ...)
returns nonzero. — end note]
[Note 2: On Windows, the native Unicode API isWriteConsoleW
. — end note]-8- Throws: [...]
-9- Recommended practice: If invoking the native Unicode API requires transcoding, implementations should substitute invalid code units with u+fffd replacement character per the Unicode Standard, Chapter 3.9 u+fffd Substitution in Conversion.
tuple
can create dangling references from tuple-like
Section: 22.4.4.2 [tuple.cnstr] Status: WP Submitter: Jonathan Wakely Opened: 2024-01-24 Last modified: 2024-04-02
Priority: Not Prioritized
View other active issues in [tuple.cnstr].
View all other issues in [tuple.cnstr].
View all issues with WP status.
Discussion:
P2165R4
(Compatibility between tuple, pair and tuple-like objects)
added two new constructors to std::tuple
:
template<tuple-likeUTuple>
constexpr explicit(see below ) tuple(UTuple&& u);
and the allocator-extended equivalent. Unlike the existing constructors taking a single parameter of tuple type, these new constructors are not defined as deleted if they would create a dangling reference to a temporary. The existing constructors gained that restriction from P2255R2 (A type trait to detect reference binding to temporary) which was approved one meeting before P2165R4 so LWG seem to have missed the inconsistency.
The proposal also added a new constructor for std::pair
:
template<pair-like P> constexpr explicit(see below) pair(P&& p);
This is deleted if it would create a dangling reference, although that seems to be an almost accidental consequence of adding the new signature after existing ones which already have the Remarks: about being deleted.
[2024-03-12; Reflector poll]
Set status to Tentatively Ready after eleven votes in favour during reflector poll.
[Tokyo 2024-03-23; Status changed: Voting → WP.]
Proposed resolution:
This wording is relative to N4971.
Modify 22.4.4.2 [tuple.cnstr] as indicated:
template<tuple-like UTuple> constexpr explicit(see below) tuple(UTuple&& u);
-28- Let
I
be the pack0, 1, ..., (sizeof...(Types) - 1)
.-29- Constraints:
- (29.1) –
different-from<UTuple, tuple>
(25.5.2 [range.utility.helpers]) istrue
,- (29.2) –
remove_cvref_t<UTuple>
is not a specialization ofranges::subrange
,- (29.3) –
sizeof...(Types)
equalstuple_size_v<remove_cvref_t<UTuple>>
,- (29.4) –
(is_constructible_v<Types, decltype(get<I>(std::forward<UTuple>(u)))> && ...)
istrue
, and- (29.5) – either
sizeof...(Types)
is not 1, or (whenTypes...
expands toT
)is_convertible_v<UTuple, T>
andis_constructible_v<T, UTuple>
are bothfalse
.-30- Effects: For all i, initializes the ith element of
*this
withget<i>(std::forward<UTuple>(u))
.-31- Remarks: The expression inside
explicit
is equivalent to:!(is_convertible_v<decltype(get<I>(std::forward<UTuple>(u))), Types> && ...)
The constructor is defined as deleted if
(reference_constructs_from_temporary_v<Types, decltype(get<I>(std::forward<UTuple>(u)))> || ...)
is
true
.
std::views::repeat
does not decay the argumentSection: 25.6.5.2 [range.repeat.view] Status: WP Submitter: Jiang An Opened: 2024-02-05 Last modified: 2024-04-02
Priority: Not Prioritized
View all other issues in [range.repeat.view].
View all issues with WP status.
Discussion:
Currently, a binary call to std::views::repeat
decay the arguments due to the deduction guide,
but a unary call doesn't, which is inconsistent.
#include <concepts>
#include <ranges>
using RPV = std::ranges::repeat_view<const char*>;
static_assert(std::same_as<decltype(std::views::repeat("foo", std::unreachable_sentinel)), RPV>); // OK
static_assert(std::same_as<decltype(std::views::repeat(+"foo", std::unreachable_sentinel)), RPV>); // OK
static_assert(std::same_as<decltype(std::views::repeat("foo")), RPV>); // ill-formed
static_assert(std::same_as<decltype(std::views::repeat(+"foo")), RPV>); // OK
Presumably we should extend the deduction guide of std::ranges::repeat_view
to cover the unary cases.
[2024-03-12; Reflector poll]
Set status to Tentatively Ready after six votes in favour during reflector poll.
[Tokyo 2024-03-23; Status changed: Voting → WP.]
Proposed resolution:
This wording is relative to N4971.
Modify 25.6.5.2 [range.repeat.view], class template repeat_view
synopsis, as indicated:
[Drafting note: The proposed wording has been suggested by Casey Carter, see microsoft/STL#3576]
namespace std::ranges { […] template<class T, class Bound = unreachable_sentinel_t> repeat_view(T, Bound = Bound()) -> repeat_view<T, Bound>; }
repeat_view
should repeat the viewSection: 25.6.5.1 [range.repeat.overview] Status: WP Submitter: Tim Song Opened: 2024-02-12 Last modified: 2024-04-02
Priority: Not Prioritized
View all issues with WP status.
Discussion:
views::repeat(views::repeat(5))
should be a view of repeat_view
s, but it's currently a view of
ints
due to the use of CTAD in the specification of views::repeat
.
[2024-03-12; Reflector poll]
Set status to Tentatively Ready after seven votes in favour during reflector poll.
[Tokyo 2024-03-23; Status changed: Voting → WP.]
Proposed resolution:
This wording is relative to N4971.
Modify 25.6.5.1 [range.repeat.overview] as indicated:
-1-
-2- The namerepeat_view
generates a sequence of elements by repeatedly producing the same value.views::repeat
denotes a customization point object (16.3.3.3.5 [customization.point.object]). Given subexpressionsE
andF
, the expressionsviews::repeat(E)
andviews::repeat(E, F)
are expression-equivalent torepeat_view<decay_t<decltype((E))>>(E)
andrepeat_view(E, F)
, respectively.
submdspan
preconditions do not forbid creating invalid pointerSection: 23.7.3.7.7 [mdspan.sub.sub] Status: WP Submitter: Mark Hoemmen Opened: 2024-03-26 Last modified: 2024-07-08
Priority: Not Prioritized
View all issues with WP status.
Discussion:
Oliver Lee and Ryan Wooster pointed out to us that creating a submdspan
with zero-length
tuple-like
or strided_slice
slice specifiers at the upper extent can cause
the Standard submdspan_mapping
overloads to access the input mdspan
's mapping
out of bounds.
This happens in the following line of specification (23.7.3.7.6 [mdspan.sub.map] p8 in
N4971 moved to [mdspan.sub.map.common] p8 after the merge of
P2642R6).
Let
offset
be a value of typesize_t
equal to(*this)(first_<index_type, P>(slices...)...)
.
If data_handle_type
is a pointer to an array, then the resulting offset can be larger than
required_span_size()
, thus making the pointer invalid (not just one past the end). In a
constexpr context, the result is ill-formed. With the reference
mdspan
implementation, Clang can actually report a build error (e.g., for out-of-bounds access
to a std::array
). The contributed example illustrates this.
Example 1:
auto x = std::array<int, 3>{}; auto A = mdspan{x.data(), extents{3}}; auto B = submdspan(A, pair{3, 3});B is an
Example 2:mdspan
with zero elements.auto y = std::array<int, 9>{}; auto C = mdspan{y.data(), extents{3, 3}}; auto D = submdspan(C, pair{3, 3}, pair{3, 3});A precondition for each slice specifier is (23.7.3.7.5 [mdspan.sub.extents]):
0 ≤ first_<index_type, k>(slices...) ≤ last_<k>(src.extents(), slices...) ≤ src.extent(k).Our understanding is that precondition is satisfied. In the second example,
However, the submapping offset is defined asfirst_<0>
is 3 andfirst_<1>
is also 3.(*this)(first_<index_type, P>(slices...)...)
, which then can result in an invalid data handle of thesubmdspan
, even if the data handle is never accessed/dereferenced. godbolt demo
We expect this situation to come up in practice.
Suppose we have anN x N
mdspan representing a matrix A
, and we want to partition it
into a 2 x 2
"matrix of matrices" (also called a "block matrix"). This partitioning is a
common operation in linear algebra algorithms such as matrix factorizations.
Examples of this 2 x 2
partitioning appear in P2642 and P1673.
mdspan A{A_ptr, N, N}; size_t p = partition_point(N); // integer in 0, 1, …, N (inclusive) auto A_00 = submdspan(A, tuple{0, p}, tuple{0, p}); auto A_10 = submdspan(A, tuple{p, N}, tuple{0, 0}); auto A_01 = submdspan(A, tuple{0, p}, tuple{p, N}); auto A_11 = submdspan(A, tuple{p, N}, tuple{p, N});
Table illustrating the resulting 2 x 2
block matrix follows:
A_00 |
A_01 |
A_10 |
A_11 |
It's valid for p
to be 0
. That makes every block but A_11
have zero size.
Thus, it should also be valid for p
to be N
. That makes every block but
A_00
have zero size. However, that leads to the aforementioned UB.
first_
or last_
. The definitions of
first_
and last_
are meant to turn the slice specifier into a pair of bounds.
Since submdspan(A, tuple{p, N}, tuple{p, N})
is valid even if p
equals N
,
then that strongly suggests that first_<0>
and first_<1>
should always be p
, even if p
equals N
.
As a result, we find ourselves needing to change submdspan_mapping
. This will affect both
the Standard submdspan_mapping
overloads, and any custom (user-defined) overloads.
[2024-05-08; Reflector poll]
Set status to Tentatively Ready after five votes in favour during reflector poll.
[St. Louis 2024-06-29; Status changed: Voting → WP.]
Proposed resolution:
This wording is relative to N4971 after the merge of P2642R6.
Modify the new 23.7.3.7.6.1 [mdspan.sub.map.common] as indicated:
-8- If
first_<index_type, k>(slices...)
equalsextents().extent(k)
for any rank indexk
ofextents()
, then lLetoffset
be a value of typesize_t
equal to(*this).required_span_size()
. Otherwise, letoffset
be a value of typesize_t
equal to(*this)(first_<index_type, P>(slices...)...)
.
Modify 23.7.3.7.7 [mdspan.sub.sub] as indicated:
As a drive-by readability fix, we also propose changing a variable name in paragraph 6 as indicated below.
template<class ElementType, class Extents, class LayoutPolicy, class AccessorPolicy, class... SliceSpecifiers> constexpr auto submdspan( const mdspan<ElementType, Extents, LayoutPolicy, AccessorPolicy>& src, SliceSpecifiers... slices) -> see below;-1- Let
-2- Letindex_type
betypename Extents::index_type
.sub_map_offset
be the result ofsubmdspan_mapping(src.mapping(), slices...)
. […] -3- Constraints: […] -4- Mandates: […] -5-Preconditions: […] -6- Effects: Equivalent to:auto sub_map_resultoffset= submdspan_mapping(src.mapping(), slices...); return mdspan(src.accessor().offset(src.data(), sub_map_resultoffset.offset), sub_map_resultoffset.mapping, AccessorPolicy::offset_policy(src.accessor()));
std::basic_format_context
be default-constructible/copyable/movable?Section: 28.5.6.7 [format.context] Status: WP Submitter: Jiang An Opened: 2024-03-24 Last modified: 2024-07-08
Priority: Not Prioritized
View all other issues in [format.context].
View all issues with WP status.
Discussion:
Per 28.5.6.7 [format.context], it seems that std::basic_format_context
has a default
constructor that is effectively defaulted, which means that it is default constructible if and only
if OutIt
is default constructible. Currently only libstdc++ makes it conditionally default
constructible, while libc++ and MSVC STL (together with fmtlib) make it never default constructible.
basic_format_context
objects are supposed to be created by the implementation
in some internal way, and user codes are only supposed to modify existing basic_format_context
objects during formatting.
[2024-05-08; Reflector poll]
Set status to Tentatively Ready after six votes in favour during reflector poll.
[St. Louis 2024-06-29; Status changed: Voting → WP.]
Proposed resolution:
This wording is relative to N4971.
Modify 28.5.6.7 [format.context] as indicated:
namespace std { template<class Out, class charT> class basic_format_context { basic_format_args<basic_format_context> args_; // exposition only Out out_; // exposition only basic_format_context(const basic_format_context&) = delete; basic_format_context& operator=(const basic_format_context&) = delete; public: using iterator = Out; using char_type = charT; template<class T> using formatter_type = formatter<T, charT>; basic_format_arg<basic_format_context> arg(size_t id) const noexcept; std::locale locale(); iterator out(); void advance_to(iterator it); }; }
std::launder
is not needed when using the result of std::memcpy
Section: 27.5.1 [cstring.syn] Status: WP Submitter: Jan Schultke Opened: 2024-04-05 Last modified: 2024-11-28
Priority: 3
View all issues with WP status.
Discussion:
int x = 0; alignas(int) std::byte y[sizeof(int)]; int z = *static_cast<int*>(std::memcpy(y, &x, sizeof(int)));
This example should be well-defined, even without the use of std::launder
.
std::memcpy
implicitly creates an int
inside y
, and
https://www.iso-9899.info/n3047.html#7.26.2.1p3
states that
The
memcpy
function returns the value of [the destination operand].
In conjunction with 27.5.1 [cstring.syn] p3, this presumably means that std::memcpy
returns a pointer to the (first) implicitly-created object, and no use of std::launder
is necessary.
[2024-06-24; Reflector poll]
Set priority to 3 after reflector poll.
Previous resolution [SUPERSEDED]:
This wording is relative to N4971.
Modify 27.5.1 [cstring.syn] as indicated:
-3- The functions
memcpy
andmemmove
are signal-safe (17.14.5 [support.signal]). Both functions implicitly create objects (6.7.2 [intro.object]) in the destination region of storage immediately prior to copying the sequence of characters to the destination. Both functions return a pointer to a suitable created object.
[St. Louis 2024-06-26; CWG suggested improved wording]
[St. Louis 2024-06-28; LWG: move to Ready]
[Wrocław 2024-11-18; approved by Core (again)]
[Wrocław 2024-11-23; Status changed: Voting → WP.]
Proposed resolution:
This wording is relative to N4981.
Modify 27.5.1 [cstring.syn] as indicated:
-3- The functions
memcpy
andmemmove
are signal-safe (17.14.5 [support.signal]).BothEach of these functions implicitlycreatecreates objects (6.7.2 [intro.object]) in the destination region of storage immediately prior to copying the sequence of characters to the destination. Each of these functions returns a pointer to a suitable created object, if any, otherwise the value of the first parameter.
std::atomic<volatile T>
should be ill-formedSection: 32.5.8 [atomics.types.generic] Status: Resolved Submitter: Jonathan Wakely Opened: 2024-04-19 Last modified: 2025-02-07
Priority: 2
View all other issues in [atomics.types.generic].
View all issues with Resolved status.
Discussion:
As a result of Core DR 2094
(concerning triviality of volatile-qualified subobjects),
is_trivially_copyable_v<volatile int>
is now true,
which means that volatile int
is a valid type for std::atomic
.
Libstdc++ and libc++ can't actually compile that type though, and that seems very sensible to me.
Even worse is that std::atomic<volatile int>
will not
select the specialization for int
, because that is clearly specified by
32.5.8.3 [atomics.types.int] to only be for cv-unqualified types.
32.5.8.4 [atomics.types.float] also says it's only for cv-unqualified
floating-point types. And 32.5.8.5 [atomics.types.pointer] will only
match cv-unqualified pointer types.
This means that even when std::atomic<volatile int>
compiles (as with MSVC) you can't use members like fetch_add
because that
only exists on the specialization, not the primary template.
Should we add something to std::atomic
to make it not valid again,
as was the case (and presumably the original intent) before CWG DR 2094?
A similar question exists for std::atomic_ref<volatile int>
although there are apparently valid uses for that type. However, the
atomic_ref
specializations for integers, floats, and pointers are only
for cv-unqualified types, so it doesn't work usefully.
For atomic_ref
we might want to allow volatile
-qualified types and
make the specializations match them.
[2024-04-29; Reflector poll]
Set priority to 2 after reflector poll and send to SG1.
[2024-06; Related to issue 3508(i).]
[St. Louis 2024-06-28; SG1 feedback]
SG1 forwarded P3323R0 to LEWG to resolve LWG issues 3508(i) and 4069(i).
[2025-02-07 Status changed: Open → Resolved.]
Resolved by P3323R0 in Wrocław.
Proposed resolution:
reference_wrapper
comparisons are not SFINAE-friendlySection: 22.10.6.6 [refwrap.comparisons] Status: WP Submitter: Jonathan Wakely Opened: 2024-04-19 Last modified: 2024-07-08
Priority: Not Prioritized
View all issues with WP status.
Discussion:
P2944R3 added these hidden friends to reference_wrapper
:
friend constexpr synth-three-way-result<T> operator<=>(reference_wrapper, reference_wrapper);
friend constexpr synth-three-way-result<T> operator<=>(reference_wrapper, const T&);
friend constexpr synth-three-way-result<T> operator<=>(reference_wrapper, reference_wrapper<const T>);
These functions are not templates, and so their declarations are ill-formed for any type that does have any comparison operators, e.g.
struct A { } a;
std::reference_wrapper<A> r(a);
Instantiating reference_wrapper<A>
will instantiate
the declarations of the hidden friends, which will attempt to determine the
return types of the operator<=>
functions.
That fails because synth-three-way
is constrained
and can't be called with arguments of type A
.
This can be solved by changing those functions into templates, so they aren't instantiated eagerly, e.g.,
template<class U = T>
friend constexpr synth-three-way-result<TU> operator<=>(reference_wrapper, reference_wrapper);
or by giving them a deduced return type (so that it isn't instantiated eagerly)
and constraining them to only be callable when valid:
friend constexpr synth-three-way-result<T>auto operator<=>(reference_wrapper x, reference_wrapper y)
requires requires (const T t) { synth-three-way(t, t); }
The second alternative is used in the proposed resolution.
In practice the requires-clause can be implemented more simply (and efficiently) by checking the constraints of synth-three-way directly:
requires (const T t) { { t < t } -> boolean-testable; }
but when specified in prose in a Constraints: element it seems
clearer to just use synth-three-way(x.get(), y.get())
.
The proposed resolution has been committed to libstdc++'s master branch.
[2024-05-08; Reflector poll]
Set status to Tentatively Ready after eight votes in favour during reflector poll.
[St. Louis 2024-06-29; Status changed: Voting → WP.]
Proposed resolution:
This wording is relative to N4981.
Modify 22.10.6.1 [refwrap.general] as indicated:
// [refwrap.comparisons], comparisons friend constexpr bool operator==(reference_wrapper, reference_wrapper); friend constexpr bool operator==(reference_wrapper, const T&); friend constexpr bool operator==(reference_wrapper, reference_wrapper<const T>); friend constexpr
synth-three-way-result<T>auto operator<=>(reference_wrapper, reference_wrapper); friend constexprsynth-three-way-result<T>auto operator<=>(reference_wrapper, const T&); friend constexprsynth-three-way-result<T>auto operator<=>(reference_wrapper, reference_wrapper<const T>);
Modify 22.10.6.6 [refwrap.comparisons] as indicated:
friend constexpr
synth-three-way-result<T>auto operator<=>(reference_wrapper x, reference_wrapper y);-?- Constraints: The expression
synth-three-way(x.get(), y.get())
is well-formed.-7- Returns:
synth-three-way(x.get(), y.get())
.friend constexpr
synth-three-way-result<T>auto operator<=>(reference_wrapper x, const T& y);-?- Constraints: The expression
synth-three-way(x.get(), y)
is well-formed.-8- Returns:
synth-three-way(x.get(), y)
.friend constexpr
synth-three-way-result<T>auto operator<=>(reference_wrapper x, reference_wrapper<const T> y);-9- Constraints:
is_const_v<T>
isfalse
. The expressionsynth-three-way(x.get(), y.get())
is well-formed.-10- Returns:
synth-three-way(x.get(), y.get())
.
std::optional
comparisons: constrain harderSection: 22.5.8 [optional.comp.with.t] Status: WP Submitter: Jonathan Wakely Opened: 2024-04-19 Last modified: 2024-11-28
Priority: 1
View all other issues in [optional.comp.with.t].
View all issues with WP status.
Discussion:
P2944R3 added constraints to std::optional
's comparisons, e.g.
-1-template<class T, class U> constexpr bool operator==(const optional<T>& x, const optional<U>& y);
MandatesConstraints: The expression*x == *y
is well-formed and its result is convertible tobool
.…
-1-template<class T, class U> constexpr bool operator==(const optional<T>& x, const U& v);
MandatesConstraints: The expression*x == v
is well-formed and its result is convertible tobool
.
But I don't think the constraint on the second one (the "compare with value")
is correct. If we try to compare two optionals that can't be compared,
such as optional<void*>
and optional<int>
,
then the first overload is not valid due to the new constraints, and so does
not participate in overload resolution. But that means we now consider the
second overload, but that's ambiguous. We could either use
operator==<void*, optional<int>>
or we could use
operator==<optional<void*>, int>
with the arguments
reversed (using the C++20 default comparison rules).
We never even get as far as checking the new constraints on those overloads,
because they're simply ambiguous.
Before P2944R3 overload resolution always would have selected the first overload, for comparing two optionals. But because that is now constrained away, we consider an overload that should never be used for comparing two optionals. The solution is to add an additional constraint to the "compare with value" overloads so that they won't be used when the "value" is really another optional.
A similar change was made to optional
's operator<=>
by
LWG 3566(i), and modified by LWG 3746(i).
I haven't analyzed whether we need the modification here too.
The proposed resolution (without is-derived-from-optional
)
has been implemented and tested in libstdc++.
[2024-06-24; Reflector poll]
Set priority to 1 after reflector poll.
LWG 3746(i) changes might be needed here too,
i.e, consider types derived from optional
, not only optional
itself.
[2024-08-21; Move to Ready at LWG telecon]
[Wrocław 2024-11-23; Status changed: Voting → WP.]
Proposed resolution:
This wording is relative to N4981.
Modify 22.5.8 [optional.comp.with.t] as indicated:
template<class T, class U> constexpr bool operator==(const optional<T>& x, const U& v);
-1- Constraints:
U
is not a specialization ofoptional
. The expression*x == v
is well-formed and its result is convertible tobool
.template<class T, class U> constexpr bool operator==(const T& v, const optional<U>& x);
-3- Constraints:
T
is not a specialization ofoptional
. The expressionv == *x
is well-formed and its result is convertible tobool
.template<class T, class U> constexpr bool operator!=(const optional<T>& x, const U& v);
-5- Constraints:
U
is not a specialization ofoptional
. The expression*x != v
is well-formed and its result is convertible tobool
.template<class T, class U> constexpr bool operator!=(const T& v, const optional<U>& x);
-7- Constraints:
T
is not a specialization ofoptional
. The expressionv != *x
is well-formed and its result is convertible tobool
.template<class T, class U> constexpr bool operator<(const optional<T>& x, const U& v);
-9- Constraints:
U
is not a specialization ofoptional
. The expression*x < v
is well-formed and its result is convertible tobool
.template<class T, class U> constexpr bool operator<(const T& v, const optional<U>& x);
-11- Constraints:
T
is not a specialization ofoptional
. The expressionv < *x
is well-formed and its result is convertible tobool
.template<class T, class U> constexpr bool operator>(const optional<T>& x, const U& v);
-13- Constraints:
U
is not a specialization ofoptional
. The expression*x > v
is well-formed and its result is convertible tobool
.template<class T, class U> constexpr bool operator>(const T& v, const optional<U>& x);
-15- Constraints:
T
is not a specialization ofoptional
. The expressionv > *x
is well-formed and its result is convertible tobool
.template<class T, class U> constexpr bool operator<=(const optional<T>& x, const U& v);
-17- Constraints:
U
is not a specialization ofoptional
. The expression*x <= v
is well-formed and its result is convertible tobool
.template<class T, class U> constexpr bool operator<=(const T& v, const optional<U>& x);
-19- Constraints:
T
is not a specialization ofoptional
. The expressionv <= *x
is well-formed and its result is convertible tobool
.template<class T, class U> constexpr bool operator>=(const optional<T>& x, const U& v);
-21- Constraints:
U
is not a specialization ofoptional
. The expression*x >= v
is well-formed and its result is convertible tobool
.template<class T, class U> constexpr bool operator>=(const T& v, const optional<U>& x);
-23- Constraints:
T
is not a specialization ofoptional
. The expressionv >= *x
is well-formed and its result is convertible tobool
.
compatible-joinable-ranges
is underconstrainedSection: 25.7.15.2 [range.join.with.view] Status: WP Submitter: Hewill Kang Opened: 2024-04-21 Last modified: 2024-07-08
Priority: Not Prioritized
View all other issues in [range.join.with.view].
View all issues with WP status.
Discussion:
join_with_view
requires the value type, reference and rvalue reference of the inner range
and pattern range to share common (reference) types through compatible-joinable-ranges
.
concat_view
and generator
do, this concept only requires that
these three types be valid and does not further check the relationship between them to be compatible
with the indirectly_readable
requirement for input_iterator
.
This results in a validly-constructed join_with_view
that may not model input_range
,
which seems unintended.
The proposed resolution aliases compatible-joinable-ranges
to concatable
i.e. specialization for two ranges to fully constrain, and I believe this could also be a better fit for
LWG 3971(i).
Previous resolution [SUPERSEDED]:
This wording is relative to N4981.
Modify 25.7.15.2 [range.join.with.view] as indicated:
namespace std::ranges { template<class R, class P> concept compatible-joinable-ranges = concatable<R, P>; // exposition onlycommon_with<range_value_t<R>, range_value_t<P>> && common_reference_with<range_reference_t<R>, range_reference_t<P>> && common_reference_with<range_rvalue_reference_t<R>, range_rvalue_reference_t<P>>;[…] }
[2024-04-24; Hewill Kang provides improved wording]
[2024-05-08; Reflector poll]
Set status to Tentatively Ready after five votes in favour during reflector poll.
[St. Louis 2024-06-29; Status changed: Voting → WP.]
Proposed resolution:
This wording is relative to N4981.
Modify 25.2 [ranges.syn] as indicated:
#include <compare> // see 17.12.1 [compare.syn] #include <initializer_list> // see 17.11.2 [initializer.list.syn] #include <iterator> // see 24.2 [iterator.synopsis] namespace std::ranges { […] // 25.7.15 [range.join.with], join with viewtemplate<class R, class P> concept compatible-joinable-ranges = see below; // exposition onlytemplate<input_range V, forward_range Pattern> requiresview<V> && input_range<range_reference_t<V>> && view<Pattern> && compatible-joinable-ranges<range_reference_t<V>, Pattern>see below class join_with_view; // freestanding […] }
Modify 25.7.15.2 [range.join.with.view] as indicated:
namespace std::ranges {template<class R, class P> concept compatible-joinable-ranges = // exposition only common_with<range_value_t<R>, range_value_t<P>> && common_reference_with<range_reference_t<R>, range_reference_t<P>> && common_reference_with<range_rvalue_reference_t<R>, range_rvalue_reference_t<P>>;[…] template<input_range V, forward_range Pattern> requires view<V> && input_range<range_reference_t<V>> && view<Pattern> &&compatible-joinable-rangesconcatable<range_reference_t<V>, Pattern> class join_with_view : public view_interface<join_with_view<V, Pattern>> { […] constexpr auto begin() const requires forward_range<const V> && forward_range<const Pattern> && is_reference_v<range_reference_t<const V>> && input_range<range_reference_t<const V>> && concatable<range_reference_t<const V>, const Pattern> { return iterator<true>{*this, ranges::begin(base_)}; } […] constexpr auto end() const requires forward_range<const V> && forward_range<const Pattern> && is_reference_v<range_reference_t<const V>> && input_range<range_reference_t<const V>> && concatable<range_reference_t<const V>, const Pattern> { […] } }; }
Modify 25.7.15.3 [range.join.with.iterator] as indicated:
namespace std::ranges { template<input_range V, forward_range Pattern> requires view<V> && input_range<range_reference_t<V>> && view<Pattern> &&compatible-joinable-rangesconcatable<range_reference_t<V>, Pattern> template<bool Const> class join_with_view<V, Pattern>::iterator { […] }; }
Modify 25.7.15.4 [range.join.with.sentinel] as indicated:
namespace std::ranges { template<input_range V, forward_range Pattern> requires view<V> && input_range<range_reference_t<V>> && view<Pattern> &&compatible-joinable-rangesconcatable<range_reference_t<V>, Pattern> template<bool Const> class join_with_view<V, Pattern>::sentinel { […] }; }
concat_view
should be freestandingSection: 17.3.2 [version.syn] Status: WP Submitter: Hewill Kang Opened: 2024-04-21 Last modified: 2024-07-08
Priority: Not Prioritized
View all other issues in [version.syn].
View all issues with WP status.
Discussion:
concat_view
can be freestanding, but this never seems to come up in the discussion,
which seems to be an oversight.
[2024-04-21; Daniel comments]
The specification of some member functions of concat_view
seem to depend on freestanding-deleted
get
overloads for variant
, but so does join_with_view
, which is marked as freestanding,
so it does not seem to be a good reason to accept join_with_view
but not concat_view
as freestanding.
[2024-05-08; Reflector poll]
Set status to Tentatively Ready after seven votes in favour during reflector poll.
[St. Louis 2024-06-29; Status changed: Voting → WP.]
Proposed resolution:
This wording is relative to N4981.
Modify 17.3.2 [version.syn] as indicated:
#define __cpp_lib_ranges_concat 202403L // freestanding, also in <ranges>
Modify 25.2 [ranges.syn] as indicated:
#include <compare> // see 17.12.1 [compare.syn] #include <initializer_list> // see 17.11.2 [initializer.list.syn] #include <iterator> // see 24.2 [iterator.synopsis] namespace std::ranges { […] // 25.7.18 [range.concat], concat view template<input_range... Views> requires see below class concat_view; // freestanding namespace views { inline constexpr unspecified concat = unspecified; } // freestanding […] }
concat_view::iterator
's conversion constructorSection: 25.7.18.3 [range.concat.iterator] Status: WP Submitter: Hewill Kang Opened: 2024-04-26 Last modified: 2024-07-08
Priority: Not Prioritized
View other active issues in [range.concat.iterator].
View all other issues in [range.concat.iterator].
View all issues with WP status.
Discussion:
This conversion constructor obtains the alternative iterator of the argument
through std::get
, which will throw when the variant
is valueless.
We seem to be missing a Preconditions element here.
[2024-05-08; Reflector poll]
Set status to Tentatively Ready after seven votes in favour during reflector poll.
[St. Louis 2024-06-29; Status changed: Voting → WP.]
Proposed resolution:
This wording is relative to N4981.
Modify 25.7.18.3 [range.concat.iterator] as indicated:
constexpr iterator(iterator<!Const> it) requires Const && (convertible_to<iterator_t<Views>, iterator_t<const Views>> && ...);-?- Preconditions:
-8- Effects: Initializesit.it_.valueless_by_exception()
isfalse
.parent_
withit.parent_
, and leti
beit.it_.index()
, initializesit_
withbase-iter(in_place_index<i>, std::get<i>(std::move(it.it_)))
.
views::concat(r)
is well-formed when r
is an output_range
Section: 25.7.18.1 [range.concat.overview] Status: WP Submitter: Hewill Kang Opened: 2024-04-27 Last modified: 2024-07-08
Priority: Not Prioritized
View all issues with WP status.
Discussion:
Currently, views::concat
will return views::all(r)
when it takes only one argument,
which only requires that the type of r
models viewable_range
which includes output_range
:
std::vector<int> v;
auto r = std::views::counted(std::back_inserter(v), 3);
auto c = std::views::concat(r); // well-formed
Since concat_view
requires all ranges to be input_range
, this seems inconsistent.
We should reject the above just like views::zip_transform
still requires F
to be
move_constructible
in the case of an empty pack.
[2024-05-08; Reflector poll]
Set status to Tentatively Ready after seven votes in favour during reflector poll.
[St. Louis 2024-06-29; Status changed: Voting → WP.]
Proposed resolution:
This wording is relative to N4981.
Modify 25.7.18.1 [range.concat.overview] as indicated:
-2- The name
views::concat
denotes a customization point object (16.3.3.3.5 [customization.point.object]). Given a pack of subexpressionsEs...
, the expressionviews::concat(Es...)
is expression-equivalent to
(2.1) —
views::all(Es...)
ifEs
is a pack with only one element whose type modelsinput_range
,(2.2) — otherwise,
concat_view(Es...)
.
views::as_rvalue
should reject non-input rangesSection: 25.7.7.1 [range.as.rvalue.overview] Status: WP Submitter: Hewill Kang Opened: 2024-04-27 Last modified: 2024-07-08
Priority: Not Prioritized
View all other issues in [range.as.rvalue.overview].
View all issues with WP status.
Discussion:
views::as_rvalue(r)
equivalent to views::all(r)
when r
's reference and rvalue reference are
of the same type, which means that in this case we only need to check whether the type of r
models viewable_range
.
as_rvalue_view{r}
to be valid, which leads to
divergence when r
is not an input_range
(demo):
#include <ranges>
struct I {
int operator*();
using difference_type = int;
I& operator++();
void operator++(int);
};
std::ranges::range auto r = std::ranges::subrange{I{}, std::unreachable_sentinel}
| std::views::as_rvalue; // // well-formed in libc++/MSVC-STL, ill-formed in libstdc++
Although this is precisely a bug in libstdc++ that does not conform to the current wording, it is reasonable to
require r
to be an input_range
to be consistent with the constraints of as_rvalue_view
.
[2024-05-08; Reflector poll]
Set status to Tentatively Ready after seven votes in favour during reflector poll.
[St. Louis 2024-06-29; Status changed: Voting → WP.]
Proposed resolution:
This wording is relative to N4981.
Modify 25.7.7.1 [range.as.rvalue.overview] as indicated:
-2- The name
views::as_rvalue
denotes a range adaptor object (25.7.2 [range.adaptor.object]). LetE
be an expression and letT
bedecltype((E))
. The expressionviews::as_rvalue(E)
is expression-equivalent to:
(2.1) —
views::all(E)
ifT
modelsinput_range
andsame_as<range_rvalue_reference_t<T>, range_reference_t<T>>
istrue
.(2.2) — Otherwise,
as_rvalue_view(E)
.
std::fixed
ignores std::uppercase
Section: 28.3.4.3.3.3 [facet.num.put.virtuals] Status: WP Submitter: Jonathan Wakely Opened: 2024-04-30 Last modified: 2024-11-28
Priority: 3
View other active issues in [facet.num.put.virtuals].
View all other issues in [facet.num.put.virtuals].
View all issues with WP status.
Discussion:
In Table 114 – Floating-point conversions [tab:facet.num.put.fp]
we specify that a floating-point value should be printed as if by %f
when (flags & floatfield) == fixed
.
This ignores whether uppercase
is also set in flags
,
meaning there is no way to use the conversion specifier %F
that was added to printf
in C99.
That's fine for finite values, because 1.23 in fixed format has
no exponent character and no hex digits that would need to use uppercase.
But %f
and %F
are not equivalent for non-finite values,
because %F
prints "NAN"
and "INF"
(or "INFINITY"
).
It seems there is no way to print "NAN"
or "INF"
using std::num_put
.
Libstdc++ and MSVC print "inf"
for the following code,
but libc++ prints "INF"
which I think is non-conforming:
std::cout << std::uppercase << std::fixed << std::numeric_limits<double>::infinity();
The libc++ behaviour seems more useful and less surprising.
[2024-05-08; Reflector poll]
Set priority to 3 after reflector poll. Send to LEWG.
[2024-09-17; LEWG mailing list vote]
Set status to Open after LEWG approved the proposed change.
[2024-09-19; Reflector poll]
Set status to Tentatively Ready after eight votes in favour during reflector poll.
[Wrocław 2024-11-23; Status changed: Voting → WP.]
Proposed resolution:
This wording is relative to N4981.
Modify 28.3.4.3.3.3 [facet.num.put.virtuals] as indicated:
Table 114 – Floating-point conversions [tab:facet.num.put.fp] State stdio
equivalentfloatfield == ios_base::fixed
&& !uppercase
%f
floatfield == ios_base::fixed
%F
floatfield == ios_base::scientific && !uppercase
%e
floatfield == ios_base::scientific
%E
floatfield == (ios_base::fixed | ios_base::scientific)` && !uppercase
%a
floatfield == (ios_base::fixed | ios_base::scientific)
%A
!uppercase
%g
otherwise %G
ranges::generate_random
's helper lambda should specify the return typeSection: 26.12.2 [alg.rand.generate] Status: WP Submitter: Hewill Kang Opened: 2024-04-28 Last modified: 2024-11-28
Priority: 2
View all other issues in [alg.rand.generate].
View all issues with WP status.
Discussion:
The non-specialized case of generate_random(r, g, d)
would call
ranges::generate(r, [&d, &g] { return invoke(d, g); })
.
However, the lambda does not explicitly specify the return type, which leads
to a hard error when invoke
returns a reference to the object that is not copyable or
R
is not the output_range
for decay_t<invoke_result_t<D&, G&>>
.
Previous resolution [SUPERSEDED]:
This wording is relative to N4981.
Modify 26.12.2 [alg.rand.generate] as indicated:
template<class R, class G, class D> requires output_range<R, invoke_result_t<D&, G&>> && invocable<D&, G&> && uniform_random_bit_generator<remove_cvref_t<G>> constexpr borrowed_iterator_t<R> ranges::generate_random(R&& r, G&& g, D&& d);-5- Effects:
(5.1) — […]
(5.2) — […]
(5.3) — Otherwise, calls:
ranges::generate(std::forward<R>(r), [&d, &g] -> decltype(auto) { return invoke(d, g); });-6- Returns:
ranges::end(r)
-7- Remarks: The effects of
generate_random(r, g, d)
shall be equivalent toranges::generate(std::forward<R>(r), [&d, &g] -> decltype(auto) { return invoke(d, g); })
[2024-05-12, Hewill Kang provides improved wording]
[2024-08-02; Reflector poll]
Set priority to 2 after reflector poll.
[2024-10-09; LWG telecon: Move to Ready]
[Wrocław 2024-11-23; Status changed: Voting → WP.]
Proposed resolution:
This wording is relative to N4981.
Modify 29.5.2 [rand.synopsis], header <random>
synopsis, as indicated:
#include <initializer_list> // see 17.11.2 [initializer.list.syn] namespace std { […] namespace ranges { […] template<class R, class G, class D> requires output_range<R, invoke_result_t<D&, G&>> && invocable<D&, G&> && uniform_random_bit_generator<remove_cvref_t<G>> && is_arithmetic_v<invoke_result_t<D&, G&>> constexpr borrowed_iterator_t<R> generate_random(R&& r, G&& g, D&& d); template<class G, class D, output_iterator<invoke_result_t<D&, G&>> O, sentinel_for<O> S> requires invocable<D&, G&> && uniform_random_bit_generator<remove_cvref_t<G>> && is_arithmetic_v<invoke_result_t<D&, G&>> constexpr O generate_random(O first, S last, G&& g, D&& d); } […] }
Modify 26.12.2 [alg.rand.generate] as indicated:
template<class R, class G, class D> requires output_range<R, invoke_result_t<D&, G&>> && invocable<D&, G&> && uniform_random_bit_generator<remove_cvref_t<G>> && is_arithmetic_v<invoke_result_t<D&, G&>> constexpr borrowed_iterator_t<R> ranges::generate_random(R&& r, G&& g, D&& d);-5- Effects:
[…]template<class G, class D, output_iterator<invoke_result_t<D&, G&>> O, sentinel_for<O> S> requires invocable<D&, G&> && uniform_random_bit_generator<remove_cvref_t<G>> && is_arithmetic_v<invoke_result_t<D&, G&>> constexpr O ranges::generate_random(O first, S last, G&& g, D&& d);-8- Effects: Equivalent to:
[…]
println
ignores the locale imbued in std::ostream
Section: 31.7.6.3.5 [ostream.formatted.print] Status: WP Submitter: Jens Maurer Opened: 2024-04-30 Last modified: 2024-11-28
Priority: Not Prioritized
View all other issues in [ostream.formatted.print].
View all issues with WP status.
Discussion:
31.7.6.3.5 [ostream.formatted.print] specifies that std::print
uses the locale
imbued in the std::ostream&
argument for formatting, by using this equivalence:
vformat(os.getloc(), fmt, args);
(in the vformat_(non)unicode
delegation).
std::println
ignores the std::ostream
's locale
for its locale-dependent formatting:
print(os, "{}\n", format(fmt, std::forward<Args>(args)...));
This is inconsistent.
[2024-10-03; Reflector poll]
Set status to Tentatively Ready after seven votes in favour during reflector poll.
[Wrocław 2024-11-23; Status changed: Voting → WP.]
Proposed resolution:
This wording is relative to N4981.
Modify 31.7.6.3.5 [ostream.formatted.print] as indicated:
template<class... Args> void println(ostream& os, format_string<Args...> fmt, Args&&... args);-2- Effects: Equivalent to:
print(os, "{}\n", format(os.getloc(), fmt, std::forward<Args>(args)...));
views::iota(views::iota(0))
should be rejectedSection: 25.6.4.1 [range.iota.overview] Status: WP Submitter: Hewill Kang Opened: 2024-05-08 Last modified: 2024-07-08
Priority: Not Prioritized
View all issues with WP status.
Discussion:
views::iota(E)
literally means incrementing element E endlessly, but
views::iota(views::iota(0))
is currently well-formed due to CTAD,
rejecting such unreasonable spelling seems therefore reasonable.
[2024-06-24; Reflector poll]
Set status to Tentatively Ready after seven votes in favour during reflector poll.
[St. Louis 2024-06-29; Status changed: Voting → WP.]
Proposed resolution:
This wording is relative to N4981.
Modify 25.6.4.1 [range.iota.overview] as indicated:
-1-
-2- The nameiota_view
generates a sequence of elements by repeatedly incrementing an initial value.views::iota
denotes a customization point object (16.3.3.3.5 [customization.point.object]). Given subexpressionsE
andF
, the expressionsviews::iota(E)
andviews::iota(E, F)
are expression-equivalent toiota_view<decay_t<decltype((E))>>(E)
andiota_view(E, F)
, respectively.
views::adjacent<0>
should reject non-forward rangesSection: 25.7.27.1 [range.adjacent.overview], 25.7.28.1 [range.adjacent.transform.overview] Status: WP Submitter: Hewill Kang Opened: 2024-05-10 Last modified: 2024-07-08
Priority: Not Prioritized
View all other issues in [range.adjacent.overview].
View all issues with WP status.
Discussion:
Following-up LWG 4082(i) and LWG 4083(i), the current wording makes
views::adjacent<0>(r)
and views::adjacent_transform<0>(r, [] { return 0; })
well-formed even when r
is just an input range or an output range, which seems to be an oversight.
[2024-06-24; Reflector poll]
Set status to Tentatively Ready after six votes in favour during reflector poll.
[St. Louis 2024-06-29; Status changed: Voting → WP.]
Proposed resolution:
This wording is relative to N4981.
Modify 25.7.27.1 [range.adjacent.overview] as indicated:
-2- The name
views::adjacent<N>
denotes a range adaptor object (25.7.2 [range.adaptor.object]). Given a subexpressionE
and a constant expressionN
, the expressionviews::adjacent<N>(E)
is expression-equivalent to
(2.1) —
((void)E, auto(views::empty<tuple<>>))
ifN
is equal to0
anddecltype((E))
modelsforward_range
,(2.2) — otherwise,
adjacent_view<views::all_t<decltype((E))>, N>(E)
.
Modify 25.7.28.1 [range.adjacent.transform.overview] as indicated:
-2- The name
views::adjacent_transform<N>
denotes a range adaptor object (25.7.2 [range.adaptor.object]). Given subexpressionsE
andF
and a constant expressionN
:
(2.1) — If
N
is equal to0
anddecltype((E))
modelsforward_range
,views::adjacent_transform<N>(E, F)
is expression-equivalent to((void)E, views::zip_transform(F))
, except that the evaluations ofE
andF
are indeterminately sequenced.(2.2) — Otherwise, the expression
views::adjacent_transform<N>(E, F)
is expression-equivalent toadjacent_transform_view<views::all_t<decltype((E))>, decay_t<decltype((F))>, N>(E, F)
.
ranges::ends_with
's Returns misses difference castingSection: 26.6.17 [alg.ends.with] Status: WP Submitter: Hewill Kang Opened: 2024-05-17 Last modified: 2024-07-08
Priority: Not Prioritized
View all issues with WP status.
Discussion:
The Returns of the ranges version of ranges::ends_with
are specified as
ranges::equal(ranges::drop_view(ranges::ref_view(r1), N1 - N2), r2, ...)
which is not quite right
when N2
is an integer-class type and N1
is an integer type, because in this case
N1 - N2
will be an integer-class type which cannot be implicitly converted to the
difference_type
of r1
leading to the construction of drop_view
being ill-formed.
[2024-06-24; Reflector poll]
Set status to Tentatively Ready after five votes in favour during reflector poll.
[St. Louis 2024-06-29; Status changed: Voting → WP.]
Proposed resolution:
This wording is relative to N4981.
Modify 26.6.17 [alg.ends.with] as indicated:
template<input_range R1, input_range R2, class Pred = ranges::equal_to, class Proj1 = identity, class Proj2 = identity> requires (forward_range<R1> || sized_range<R1>) && (forward_range<R2> || sized_range<R2>) && indirectly_comparable<iterator_t<R1>, iterator_t<R2>, Pred, Proj1, Proj2> constexpr bool ranges::ends_with(R1&& r1, R2&& r2, Pred pred = {}, Proj1 proj1 = {}, Proj2 proj2 = {});-3- Let
-4- Returns:N1
beranges::distance(r1)
andN2
beranges::distance(r2)
.false
ifN1 < N2
, otherwiseranges::equal(ranges::drop_view(ranges::ref_view(r1), N1 - static_cast<decltype(N1)>(N2)), r2, pred, proj1, proj2)
basic_format_args
should not be default-constructibleSection: 28.5.8.3 [format.args] Status: WP Submitter: Hewill Kang Opened: 2024-05-17 Last modified: 2024-07-08
Priority: Not Prioritized
View all other issues in [format.args].
View all issues with WP status.
Discussion:
It's unclear why basic_format_args
originally provided a default constructor and its actual use cases,
all three major libraries declare them as default
, which allows value initialization of
basic_format_args
at compile time.
constexpr
specifier.
Additionally, the current wording only initializes the size_
member in the default constructor, which
may lead to undefined behavior when copying basic_format_args
as uninitialized data_
member
is copied. There is also an implementation divergence (demo):
#include <format>
constexpr std::format_args fmt_args; // only well-formed in MSVC-STL
One option is to add default member initializers for all members and default
the default constructor
to best match the status quo, which guarantees that basic_format_args
is constexpr
-able.
basic_format_args
has different implementation details in the three libraries,
its actual members may not be size_
and data_
. It is unnecessary to ensure that all
the internal members are initialized when default-constructed basic_format_args
, indicating that not
providing one is reasonable.
The proposed solution is to prefer the more aggressive one.
[2024-05-19; Daniel comments]
The here suggested proposal to remove the default constructor implicitly depends on the decision of
LWG 4061(i) to remove basic_format_context
's default constructor, since its usage
would require that the exposition-only member args_
of type basic_format_args<basic_format_context>
can be default-constructed as well.
[2024-06-24; Reflector poll]
Set status to Tentatively Ready after eight votes in favour during reflector poll.
[St. Louis 2024-06-29; Status changed: Voting → WP.]
Proposed resolution:
This wording is relative to N4981.
Modify 28.5.8.3 [format.args] as indicated:
[…]namespace std { template<class Context> class basic_format_args { size_t size_; // exposition only const basic_format_arg<Context>* data_; // exposition only public:basic_format_args() noexcept;template<class... Args> basic_format_args(const format-arg-store<Context, Args...>& store) noexcept; basic_format_arg<Context> get(size_t i) const noexcept; }; […] }basic_format_args() noexcept;
-2- Effects Initializessize_
with0
.
has-arrow
should required operator->()
to be const
-qualifiedSection: 25.5.2 [range.utility.helpers] Status: WP Submitter: Hewill Kang Opened: 2024-06-22 Last modified: 2024-11-28
Priority: Not Prioritized
View all issues with WP status.
Discussion:
The helper concept has-arrow
in 25.5.2 [range.utility.helpers] does not
require that I::operator->()
be const
-qualified, which is inconsistent with
the constraints on reverse_iterator
and common_iterator
's operator->()
as the latter two both require the underlying iterator has const operator->()
member.
has-arrow
so that
implicit expression variations (18.2 [concepts.equality])
prohibit silly games.
[St. Louis 2024-06-24; move to Ready]
[Wrocław 2024-11-23; Status changed: Voting → WP.]
Proposed resolution:
This wording is relative to N4981.
Modify 25.5.2 [range.utility.helpers] as indicated:
[…] template<class I> concept has-arrow = // exposition only input_iterator<I> && (is_pointer_v<I> || requires(const I i) { i.operator->(); }); […]
has_unique_object_representations<Incomplete[]>
Section: 21.3.5.4 [meta.unary.prop] Status: WP Submitter: Jonathan Wakely Opened: 2024-06-25 Last modified: 2024-11-28
Priority: Not Prioritized
View other active issues in [meta.unary.prop].
View all other issues in [meta.unary.prop].
View all issues with WP status.
Discussion:
The type completeness requirements for has_unique_object_representations
say:
T
shall be a complete type, cvvoid
, or an array of unknown bound.
This implies that the trait works for all arrays of unknown bound,
whether the element type is complete or not. That seems to be incorrect,
because has_unique_object_representations_v<Incomplete[]>
is required to have the same result as
has_unique_object_representations_v<Incomplete>
which is ill-formed if Incomplete
is an incomplete class type.
I think we need the element type to be complete to be able to give an answer.
Alternatively, if the intended result for an array of unknown bound is false
(maybe because there can be no objects of type T[]
, or because we can't
know that two objects declared as extern T a[];
and extern T b[];
have
the same number of elements?) then the condition for the trait needs to be
special-cased as false
for arrays of unknown bound.
The current spec is inconsistent, we can't allow arrays of unknown bound
and apply the current rules to determine the trait's result.
[2024-08-02; Reflector poll]
Set status to Tentatively Ready after nine votes in favour during reflector poll.
[Wrocław 2024-11-23; Status changed: Voting → WP.]
Proposed resolution:
This wording is relative to N4981.
Modify 21.3.5.4 [meta.unary.prop] as indicated:
Template Condition Preconditions … … … template<class T> struct has_unique_object_representations;
For an array type T
, the same result ashas_unique_object_representations_v<remove_all_extents_t<T>>
, otherwise see below.remove_all_extents_t<T>
shall be a complete typeT
,or cvvoid
, or an array of unknown bound.
[Drafting note: We could use
remove_extent_t<T>
to remove just the first array dimension, because only that first one can have an unknown bound. The proposed resolution usesremove_all_extents_t<T>
for consistency with the Condition column.]
generator::promise_type::yield_value(ranges::elements_of<R, Alloc>)
's nested generator
may be ill-formedSection: 25.8.5 [coro.generator.promise] Status: WP Submitter: Hewill Kang Opened: 2024-07-11 Last modified: 2024-11-28
Priority: Not Prioritized
View all other issues in [coro.generator.promise].
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Discussion:
The nested coroutine is specified to return generator<yielded, ranges::range_value_t<R>, Alloc>
which can be problematic as the value type of R
is really irrelevant to yielded
,
unnecessarily violating the generator
's Mandates (demo):
#include <generator>
#include <vector>
std::generator<std::span<int>> f() {
std::vector<int> v;
co_yield v; // ok
}
std::generator<std::span<int>> g() {
std::vector<std::vector<int>> v;
co_yield std::ranges::elements_of(v); // hard error
}
This proposed resolution is to change the second template parameter from range_value_t<R>
to void
since that type doesn't matter to us.
[2024-08-02; Reflector poll]
Set status to Tentatively Ready after five votes in favour during reflector poll.
[Wrocław 2024-11-23; Status changed: Voting → WP.]
Proposed resolution:
This wording is relative to N4986.
Modify 25.8.5 [coro.generator.promise] as indicated:
template<ranges::input_range R, class Alloc> requires convertible_to<ranges::range_reference_t<R>, yielded> auto yield_value(ranges::elements_of<R, Alloc> r);-13- Effects: Equivalent to:
[…]auto nested = [](allocator_arg_t, Alloc, ranges::iterator_t<R> i, ranges::sentinel_t<R> s) -> generator<yielded,ranges::range_value_t<R>void, Alloc> { for (; i != s; ++i) { co_yield static_cast<yielded>(*i); } }; return yield_value(ranges::elements_of(nested( allocator_arg, r.allocator, ranges::begin(r.range), ranges::end(r.range))));
zoned_time
with resolution coarser than secondsSection: 30.12 [time.format] Status: WP Submitter: Jonathan Wakely Opened: 2024-07-26 Last modified: 2024-11-28
Priority: Not Prioritized
View other active issues in [time.format].
View all other issues in [time.format].
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Discussion:
The
std::formatter<std::chrono::zoned_time<Duration, TimeZonePtr>>
specialization calls tp.get_local_time()
for the object it passes to its
base class' format
function. But get_local_time()
does not return a
local_time<Duration>
, it returns
local_time<common_type_t<Duration, seconds>>
.
The base class' format
function is only defined for
local_time<Duration>
.
That means this is ill-formed, even though the static assert passes:
using namespace std::chrono;
static_assert( std::formattable<zoned_time<minutes>, char> );
zoned_time<minutes> zt;
(void) std::format("{}", zt); // error: cannot convert local_time<seconds> to local_time<minutes>
Additionally, it's not specified what output you should get for:
std::format("{}", local_time_format(zt.get_local_time()));
30.12 [time.format] p7 says it's formatted as if by streaming to an
ostringstream
,
but there is no operator<<
for local-time-format-t
.
Presumably it should give the same result as operator<<
for
a zoned_time
, i.e. "{:L%F %T %Z}"
with padding adjustments etc.
The proposed resolution below has been implemented in libstdc++.
[2024-08-02; Reflector poll]
Set status to Tentatively Ready after seven votes in favour during reflector poll.
[Wrocław 2024-11-23; Status changed: Voting → WP.]
Proposed resolution:
This wording is relative to N4986.
Modify 30.12 [time.format] as indicated:
template<classDuration, class charT> struct formatter<chrono::local-time-format-t<Duration>, charT>;-17- Let
f
be alocale-time-format-t<Duration>
object passed toformatter::format
.-18- Remarks: If the chrono-specs is omitted, the result is equivalent to using
%F %T %Z
as the chrono-specs. If%Z
is used, it is replaced with*f.abbrev
iff.abbrev
is not a null pointer value. If%Z
is used andf.abbrev
is a null pointer value, an exception of typeformat_error
is thrown. If%z
(or a modified variant of%z
) is used, it is formatted with the value of*f.offset_sec
iff.offset_sec
is not a null pointer value. If%z
(or a modified variant of%z
) is used andf.offset_sec
is a null pointer value, then an exception of typeformat_error
is thrown.template<class Duration, class TimeZonePtr, class charT> struct formatter<chrono::zoned_time<Duration, TimeZonePtr>, charT> : formatter<chrono::local-time-format-t<common_type_t<Duration, seconds>>, charT> { template<class FormatContext> typename FormatContext::iterator format(const chrono::zoned_time<Duration, TimeZonePtr>& tp, FormatContext& ctx) const; };template<class FormatContext> typename FormatContext::iterator format(const chrono::zoned_time<Duration, TimeZonePtr>& tp, FormatContext& ctx) const;-19- Effects: Equivalent to:
sys_info info = tp.get_info(); return formatter<chrono::local-time-format-t<common_type_t<Duration, seconds>>, charT>:: format({tp.get_local_time(), &info.abbrev, &info.offset}, ctx);
Section: 17.3.2 [version.syn] Status: WP Submitter: Jiang An Opened: 2024-07-24 Last modified: 2024-11-28
Priority: 2
View all other issues in [version.syn].
View all issues with WP status.
Discussion:
Currently (N4986), it's a bit weird in 17.3.2 [version.syn] that some feature-test macros are not marked freestanding, despite the indicated features being fully freestanding. The freestanding status seems sometimes implicitly covered by "also in" headers that are mostly or all freestanding, but sometimes not.
I think it's more consistent to ensure feature-test macros for fully freestanding features are also freestanding.[2024-08-02; Reflector poll]
Set priority to 2 and set status to Tentatively Ready after seven votes in favour during reflector poll.
[Wrocław 2024-11-23; Status changed: Voting → WP.]
Proposed resolution:
This wording is relative to N4986.
Modify 17.3.2 [version.syn] as indicated:
[Drafting note:
<charconv>
is not fully freestanding, but all functions madeconstexpr
by P2291R3 are furtherly made freestanding by P2338R4. ]
[…] #define __cpp_lib_common_reference 202302L // freestanding, also in <type_traits> #define __cpp_lib_common_reference_wrapper 202302L // freestanding, also in <functional> […] #define __cpp_lib_constexpr_charconv 202207L // freestanding, also in <charconv> […] #define __cpp_lib_coroutine 201902L // freestanding, also in <coroutine> […] #define __cpp_lib_is_implicit_lifetime 202302L // freestanding, also in <type_traits> […] #define __cpp_lib_is_virtual_base_of 202406L // freestanding, also in <type_traits> […] #define __cpp_lib_is_within_lifetime 202306L // freestanding, also in <type_traits> […] #define __cpp_lib_mdspan 202406L // freestanding, also in <mdspan> […] #define __cpp_lib_ratio 202306L // freestanding, also in <ratio> […] #define __cpp_lib_span_initializer_list 202311L // freestanding, also in <span> […] #define __cpp_lib_submdspan 202403L // freestanding, also in <mdspan> […] #define __cpp_lib_to_array 201907L // freestanding, also in <array> […]
Section: 29.5.4.5 [rand.eng.philox] Status: WP Submitter: Ilya A. Burylov Opened: 2024-08-06 Last modified: 2024-11-28
Priority: 1
View other active issues in [rand.eng.philox].
View all other issues in [rand.eng.philox].
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Discussion:
The P2075R6 proposal introduced the Philox engine and described the algorithm closely following the original paper (further Random123sc11).
Matt Stephanson implemented P2075R6 and the 10000'th number did not match. Further investigation revealed several places in Random123sc11 algorithm description, which either deviate from the reference implementation written by Random123sc11 authors or loosely defined, which opens the way for different interpretations. All major implementations of the Philox algorithm (NumPy, Intel MKL, Nvidia cuRAND, etc.) match the reference implementation written by Random123sc11 authors and we propose to align wording with that. The rationale of proposed changes:Random123sc11 refers to the permutation step as "the inputs are permuted using the Threefish N-word P-box", thus the P2075R6 permutation table ([tab:rand.eng.philox.f]) is taken from Threefish algorithm. But the permutation for N=4 in this table does not match the reference implementations. It's worth noting that while Random123sc11 described the Philox algorithm for N=8 and N=16, there are no known reference implementations of it either provided by authors or implemented by other libraries. We proposed to drop N=8 and N=16 for now and update the permutation indices for N=4 to match the reference implementation. Note: the proposed resolution allows extending N > 4 cases in the future.
The original paper describes the S-box update for X values in terms of L'
and
R'
but does not clarify their ordering as part of the counter. In order to match Random123sc11
reference implementation the ordering should be swapped.
Philox alias templates should be updated, because the current ordering of constants matches the specific optimization in the reference Random123sc11 implementation and not the generic algorithm description.
All proposed modifications below are confirmed by:
Philox algorithm coauthor Mark Moraes who is planning to publish errata for the original Random123sc11 Philox paper.
Matt Stephanson, who originally found the mismatch in P2075R6
[2024-08-21; Reflector poll]
Set priority to 1 after reflector poll.
[2024-08-21; Move to Ready at LWG telecon]
[Wrocław 2024-11-23; Status changed: Voting → WP.]
Proposed resolution:
This wording is relative to N4988.
Modify 29.5.4.5 [rand.eng.philox], [tab:rand.eng.philox.f] as indicated (This effectively reduces 16 data columns to 4 data columns and 4 data rows to 2 data rows):
Table 101 — Values for the word permutation fn(j) [tab:rand.eng.philox.f] fn(j) j
0 1 2 3 456789101112131415n 2 0 1 4 2 01 30 23 1821476503160921361141510712314581
Modify 29.5.4.5 [rand.eng.philox] as indicated:
-4- […]
(4.1) — […]
(4.2) — The following computations are applied to the elements of the V sequence:
= mulhi
= mullomullo(,,w) xor xormulhi(,,w)xor xor-5- […]
-6- Mandates:
(6.1) — […]
(6.2) —
n == 2 || n == 4
is|| n == 8 || n == 16true
, and(6.3) — […]
(6.4) — […]
Modify 29.5.6 [rand.predef] as indicated:
using philox4x32 = philox_engine<uint_fast32_t, 32, 4, 10, 0xCD9E8D570xD2511F53, 0x9E3779B9, 0xD2511F530xCD9E8D57, 0xBB67AE85>;-11- Required behavior: The 10000th consecutive invocation a default-constructed object of type
philox4x32
produces the value1955073260
.using philox4x64 = philox_engine<uint_fast64_t, 64, 4, 10, 0xCA5A8263951211570xD2E7470EE14C6C93, 0x9E3779B97F4A7C15, 0xD2E7470EE14C6C930xCA5A826395121157, 0xBB67AE8584CAA73B>;-12- Required behavior: The 10000th consecutive invocation a default-constructed object of type
philox4x64
produces the value 3409172418970261260.
std::erase
for list
should specify return type as
bool
Section: 23.3.7.7 [forward.list.erasure], 23.3.11.6 [list.erasure] Status: WP Submitter: Hewill Kang Opened: 2024-08-07 Last modified: 2024-11-28
Priority: Not Prioritized
View all issues with WP status.
Discussion:
std::erase
for list
is specified to return
erase_if(c, [&](auto& elem) { return elem == value; })
.
However, the template parameter Predicate
of erase_if
only requires that the
type of decltype(pred(...))
satisfies boolean-testable
, i.e., the
return type of elem == value
is not necessarily bool
.
bool
to avoid some
pedantic cases (demo):
#include <list>
struct Bool {
Bool(const Bool&) = delete;
operator bool() const;
};
struct Int {
Bool& operator==(Int) const;
};
int main() {
std::list<Int> l;
std::erase(l, Int{}); // unnecessary hard error
}
[2024-08-21; Reflector poll]
Set status to Tentatively Ready after nine votes in favour during reflector poll.
[Wrocław 2024-11-23; Status changed: Voting → WP.]
Proposed resolution:
This wording is relative to N4988.
Modify 23.3.7.7 [forward.list.erasure] as indicated:
template<class T, class Allocator, class U = T> typename forward_list<T, Allocator>::size_type erase(forward_list<T, Allocator>& c, const U& value);-1- Effects: Equivalent to:
return erase_if(c, [&](const auto& elem) -> bool { return elem == value; });
Modify 23.3.11.6 [list.erasure] as indicated:
template<class T, class Allocator, class U = T> typename list<T, Allocator>::size_type erase(list<T, Allocator>& c, const U& value);-1- Effects: Equivalent to:
return erase_if(c, [&](const auto& elem) -> bool { return elem == value; });
Section: 22.9.2.1 [template.bitset.general], 23.3.14.1 [vector.bool.pspc] Status: WP Submitter: Jonathan Wakely Opened: 2024-08-21 Last modified: 2024-11-28
Priority: Not Prioritized
View all issues with WP status.
Discussion:
The standard shows a private default constructor for
bitset<N>::reference
but does not define its semantics, and nothing in the spec refers to it.
It was present in C++98, then in C++11 it got noexcept
added to it,
and in C++23 it was made constexpr
by P2417R2. That's quite
a lot of churn for an unusuable member function with no definition.
In libstdc++ it's declared as private, but never defined. In libc++ it doesn't exist at all. In MSVC it is private and defined (and presumably used somewhere). There's no reason for the standard to declare it. Implementers can define it as private if they want to, or not. The spec doesn't need to say anything for that to be true. We can also remove the friend declaration, because implementers know how to do that too.
I suspect it was added as private originally so that it didn't look like
reference
should have an implicitly-defined default constructor,
which would have been the case in previous standards with no other
constructors declared.
However, C++20 added reference(const reference&) = default;
which suppresses the implicit default constructor, so declaring the
default constructor as private is now unnecessary.
Jiang An pointed out in an editorial pull request that
vector<bool, Alloc>::reference
has exactly the same issue.
[2024-09-18; Reflector poll]
Set status to Tentatively Ready after eight votes in favour during reflector poll.
[Wrocław 2024-11-23; Status changed: Voting → WP.]
Proposed resolution:
This wording is relative to N4988.
Modify 22.9.2.1 [template.bitset.general] as indicated:
namespace std { template<size_t N> class bitset { public: // bit reference class reference {friend class bitset;constexpr reference() noexcept;public: constexpr reference(const reference&) = default; constexpr ~reference(); constexpr reference& operator=(bool x) noexcept; // for b[i] = x; constexpr reference& operator=(const reference&) noexcept; // for b[i] = b[j]; constexpr bool operator~() const noexcept; // flips the bit constexpr operator bool() const noexcept; // for x = b[i]; constexpr reference& flip() noexcept; // for b[i].flip(); };
Modify 23.3.14.1 [vector.bool.pspc], vector<bool, Allocator>
synopsis, as indicated:
namespace std { template<class Allocator> class vector<bool, Allocator> { public: // types […] // bit reference class reference {friend class vector;constexpr reference() noexcept;public: constexpr reference(const reference&) = default; constexpr ~reference(); constexpr operator bool() const noexcept; constexpr reference& operator=(bool x) noexcept; constexpr reference& operator=(const reference& x) noexcept; constexpr const reference& operator=(bool x) const noexcept; constexpr void flip() noexcept; // flips the bit };
Section: 22.5.3.1 [optional.optional.general], 22.6.3.1 [variant.variant.general], 22.8.6.1 [expected.object.general], 22.8.7.1 [expected.void.general] Status: WP Submitter: Jonathan Wakely Opened: 2024-08-22 Last modified: 2024-11-28
Priority: Not Prioritized
View all other issues in [optional.optional.general].
View all issues with WP status.
Discussion:
This issue was split out from issue 4015(i).
optional
, variant
and expected
all use similar wording to require
their contained value to be a subobject, rather than dynamically allocated
and referred to by a pointer, e.g.
When an instance ofoptional<T>
contains a value, it means that an object of typeT
, referred to as the optional object’s contained value, is allocated within the storage of the optional object. Implementations are not permitted to use additional storage, such as dynamic memory, to allocate its contained value.
During the LWG reviews of P2300 in St. Louis, concerns were raised about the form of this wording and whether it's normatively meaningful. Except for the special case of standard-layout class types, the standard has very few requirements on where or how storage for subobjects is allocated. The library should not be trying to dictate more than the language guarantees. It would be better to refer to wording from 6.7.2 [intro.object] such as subobject, provides storage, or nested within. Any of these terms would provide the desired properties, without using different (and possibly inconsistent) terminology.
Using an array of bytes to provide storage for the contained value would
make it tricky to meet the constexpr requirements of types like optional
.
This means in practice, the most restrictive of these terms, subobject,
is probably accurate and the only plausible implementation strategy.
However, I don't see any reason to outlaw other implementation strategies that
might be possible in future (say, with a constexpr type cast, or non-standard
compiler-specific instrinics).
For this reason, the proposed resolution below uses nested within,
which provides the desired guarantee without imposing additional restrictions
on implementations.
[2024-09-18; Reflector poll]
Set status to Tentatively Ready after seven votes in favour during reflector poll.
[Wrocław 2024-11-23; Status changed: Voting → WP.]
Proposed resolution:
This wording is relative to N4988.
Modify 22.5.3.1 [optional.optional.general] as indicated:
[Drafting note: This edit modifies the same paragraph as issue 4015(i), but that other issue intentionally doesn't touch the affected sentence here (except for removing the italics on "contained value"). The intention is that the merge conflict can be resolved in the obvious way: "An optional object's contained value is nested within (6.7.2 [intro.object]) the optional object."]
-1- Any instance of
optional<T>
at any given time either contains a value or does not contain a value. When an instance ofoptional<T>
contains a value, it means that an object of typeT
, referred to as the optional object's contained value, isallocated within the storage ofnested within (6.7.2 [intro.object]) the optional object.Implementations are not permitted to use additional storage, such as dynamic memory, to allocate its contained value.When an object of typeoptional<T>
is contextually converted tobool
, the conversion returnstrue
if the object contains a value; otherwise the conversion returnsfalse
.
Modify 22.6.3.1 [variant.variant.general] as indicated:
-1- Any instance of
variant
at any given time either holds a value of one of its alternative types or holds no value. When an instance ofvariant
holds a value of alternative typeT
, it means that a value of typeT
, referred to as thevariant
object's contained value, isallocated within the storage ofnested within (6.7.2 [intro.object]) thevariant
object.Implementations are not permitted to use additional storage, such as dynamic memory, to allocate the contained value.
Modify 22.8.6.1 [expected.object.general] as indicated:
-1- Any object of type
expected<T, E>
either contains a value of typeT
or a value of typeE
within its own storagenested within (6.7.2 [intro.object]) it.Implementations are not permitted to use additional storage, such as dynamic memory, to allocate the object of typeMemberT
or the object of typeE
.has_val
indicates whether theexpected<T, E>
object contains an object of typeT
.
Modify 22.8.7.1 [expected.void.general] as indicated:
-1- Any object of type
expected<T, E>
either represents a value of typeT
, or contains a value of typeE
within its own storagenested within (6.7.2 [intro.object]) it.Implementations are not permitted to use additional storage, such as dynamic memory, to allocate the object of typeMemberE
.has_val
indicates whether theexpected<T, E>
represents a value of typeT
.
format_parse_context::check_dynamic_spec
should require at least one typeSection: 28.5.6.6 [format.parse.ctx] Status: WP Submitter: Jonathan Wakely Opened: 2024-08-28 Last modified: 2024-11-28
Priority: Not Prioritized
View all other issues in [format.parse.ctx].
View all issues with WP status.
Discussion:
The Mandates: conditions for format_parse_context::check_dynamic_spec
are:
-14- Mandates: The types inTs...
are unique. Each type inTs...
is one ofbool
,char_type
,int
,unsigned int
,long long int
,unsigned long long int
,float
,double
,long double
,const char_type*
,basic_string_view<char_type>
, orconst void*
.
There seems to be no reason to allow Ts
to be an empty pack,
that's not useful. There is no valid arg-id value that can be passed to it
if the list of types is empty, since arg(n)
will never be one of the types
in an empty pack. So it's never a constant expression if the pack is empty.
[2024-09-18; Reflector poll]
Set status to Tentatively Ready after seven votes in favour during reflector poll.
[Wrocław 2024-11-23; Status changed: Voting → WP.]
Proposed resolution:
This wording is relative to N4988.
Modify 28.5.6.6 [format.parse.ctx] as indicated:
template<class... Ts>
constexpr void check_dynamic_spec(size_t id) noexcept;
-14- Mandates:
sizeof...(Ts)
≥ 1. The types inTs...
are unique. Each type inTs...
is one ofbool
,char_type
,int
,unsigned int
,long long int
,unsigned long long int
,float
,double
,long double
,const char_type*
,basic_string_view<char_type>
, orconst void*
.-15- Remarks: A call to this function is a core constant expression only if:
- (15.1) —
id < num_args_
istrue
and- (15.2) — the type of the corresponding format argument (after conversion to
basic_format_arg<Context>
) is one of the types inTs...
.
unique_ptr<T&, D>
Section: 20.3.1.3.1 [unique.ptr.single.general] Status: WP Submitter: Jonathan Wakely Opened: 2024-08-30 Last modified: 2024-11-28
Priority: Not Prioritized
View all other issues in [unique.ptr.single.general].
View all issues with WP status.
Discussion:
It seems that we currently allow nonsensical specializations of unique_ptr
such as unique_ptr<int&, D>
and unique_ptr<void()const, D>
(a custom deleter that defines D::pointer
is needed, because otherwise
the pointer
type would default to invalid types like
int&*
or void(*)()const
).
There seems to be no reason to support these "unique pointer to reference"
and "unique pointer to abominable function type" specializations,
or any specialization for a type that you couldn't form a raw pointer to.
Prior to C++17, the major library implementations rejected such specializations
as a side effect of the constraints for the
unique_ptr(auto_ptr<U>&&)
constructor
being defined in terms of is_convertible<U*, T*>
.
This meant that overload resolution for any constructor of unique_ptr
would attempt to form the type T*
and fail if that was invalid.
With the removal of auto_ptr
in C++17, that constructor was removed
and now unique_ptr<int&, D>
can be instantiated
(assuming any zombie definition of auto_ptr
is not enabled by the library).
This wasn't intentional, but just an accident caused by not explicitly
forbidding such types.
Discussion on the LWG reflector led to near-unanimous support for explicitly disallowing these specializations for non-pointable types.
[2024-11-13; Reflector poll]
Set status to Tentatively Ready after eight votes in favour during reflector poll.
[Wrocław 2024-11-23; Status changed: Voting → WP.]
Proposed resolution:
This wording is relative to N4988.
Modify 20.3.1.3.1 [unique.ptr.single.general] as indicated:
-?- A program that instantiates the definition of
unique_ptr<T, D>
is ill-formed ifT*
is an invalid type.
[Note: This prevents the intantiation of specializations such asunique_ptr<T&, D>
andunique_ptr<int() const, D>
. — end note]-1- The default type for the template parameter
D
isdefault_delete
. A client-supplied template argumentD
shall be a function object type (22.10 [function.objects]), lvalue reference to function, or lvalue reference to function object type for which, given a valued
of typeD
and a valueptr
of typeunique_ptr<T, D>::pointer
, the expressiond(ptr)
is valid and has the effect of disposing of the pointer as appropriate for that deleter.-2- If the deleter’s type
D
is not a reference type,D
shall meet the Cpp17Destructible requirements (Table 35).-3- If the qualified-id
remove_reference_t<D>::pointer
is valid and denotes a type (13.10.3 [temp.deduct]), thenunique_ptr<T, D>::pointer
shall be a synonym forremove_reference_t<D>::pointer
. Otherwiseunique_ptr<T, D>::pointer
shall be a synonym forelement_type*
. The typeunique_ptr<T, D>::pointer
shall meet the Cpp17NullablePointer requirements (Table 36).-4- [Example 1: Given an allocator type
X
(16.4.4.6.1 [allocator.requirements.general]) and lettingA
be a synonym forallocator_traits<X>
, the typesA::pointer
,A::const_pointer
,A::void_pointer
, andA::const_void_pointer
may be used asunique_ptr<T, D>::pointer
. — end example]
inplace_vector::emplace
Section: 23.2.4 [sequence.reqmts] Status: WP Submitter: Arthur O'Dwyer Opened: 2024-08-26 Last modified: 2024-11-28
Priority: Not Prioritized
View other active issues in [sequence.reqmts].
View all other issues in [sequence.reqmts].
View all issues with WP status.
Discussion:
Inserting into the middle of an inplace_vector
, just like inserting into the middle of a
vector
or deque
, requires that we construct the new element out-of-line, shift
down the trailing elements (Cpp17MoveAssignable), and then move-construct the new element
into place (Cpp17MoveInsertable). P0843R14 failed to make this change, but
it should have.
[2024-09-18; Reflector poll]
Set status to Tentatively Ready after nine votes in favour during reflector poll.
[Wrocław 2024-11-23; Status changed: Voting → WP.]
Proposed resolution:
This wording is relative to N4988.
Modify 23.2.4 [sequence.reqmts] as indicated:
a.emplace(p, args)-19- Result:
-20- Preconditions:iterator
.T
is Cpp17EmplaceConstructible intoX
fromargs
. Forvector
, inplace_vector, anddeque
,T
is also Cpp17MoveInsertable intoX
and Cpp17MoveAssignable. -21- Effects: Inserts an object of typeT
constructed withstd::forward<Args>(args)...
beforep
. [Note 1:args
can directly or indirectly refer to a value ina
. — end note] -22- Returns: An iterator that points to the new element constructed fromargs
intoa
.
unique_ptr::operator*
should not allow dangling referencesSection: 20.3.1.3.5 [unique.ptr.single.observers] Status: WP Submitter: Jonathan Wakely Opened: 2024-09-02 Last modified: 2024-11-28
Priority: Not Prioritized
View all other issues in [unique.ptr.single.observers].
View all issues with WP status.
Discussion:
If unique_ptr<T,D>::element_type*
and D::pointer
are not the same type, it's possible for operator*()
to return a dangling
reference that has undefined behaviour.
struct deleter {
using pointer = long*;
void operator()(pointer) const {}
};
long l = 0;
std::unique_ptr<const int, deleter> p(&l);
int i = *p; // undefined
We should make this case ill-formed.
[2024-09-18; Reflector poll]
Set status to Tentatively Ready after seven votes in favour during reflector poll.
[Wrocław 2024-11-23; Status changed: Voting → WP.]
Proposed resolution:
This wording is relative to N4988.
Modify 20.3.1.3.5 [unique.ptr.single.observers] as indicated:
constexpr add_lvalue_reference_t<T> operator*() const noexcept(noexcept(*declval<pointer>()));-?- Mandates:
reference_converts_from_temporary_v<add_lvalue_reference_t<T>, decltype(*declval<pointer>())>
isfalse
.-1- Preconditions:
get() != nullptr
istrue
.-2- Returns:
*get()
.
run_loop::run()
are too strictSection: 33.12.1.4 [exec.run.loop.members] Status: Resolved Submitter: Eric Niebler Opened: 2024-09-05 Last modified: 2025-02-07
Priority: Not Prioritized
View all issues with Resolved status.
Discussion:
From sender-receiver/issues #280:
The precondition onrun_loop::run()
is that the state
is starting.
Given that run_loop::finish()
puts the loop in the finishing state, the implication
is that one cannot call finish()
before calling run()
.
However, in most cases sync_wait
is calling finish()
on its run_loop
before calling run()
, violating this precondition. sync_wait
does the following:
run_loop loop; auto op = connect(sndr, sync_wait_receiver{&loop}); start(op); loop.run();
If sndr
completes synchronously, the sync_wait_receiver
will call finish()
on the loop before loop.run()
is called, violating run
's precondition.
[2025-02-07 Status changed: New → Resolved.]
Resolved by P3396R1.
Proposed resolution:
This wording is relative to N4988.
Modify 33.12.1.4 [exec.run.loop.members] as indicated:
void run();-5- Preconditions:
-6- Effects: Sets thestate
is starting or finishing.state
to running. Then, equivalent to:while (auto* op = pop-front()) { op->execute(); }-7- Remarks: When
state
changes, it does so without introducing data races.
philox_engine::max()
Section: 29.5.4.5 [rand.eng.philox] Status: WP Submitter: Ruslan Arutyunyan Opened: 2024-09-18 Last modified: 2024-11-28
Priority: Not Prioritized
View other active issues in [rand.eng.philox].
View all other issues in [rand.eng.philox].
View all issues with WP status.
Discussion:
There is a typo in philox_engine
wording that makes "-1" two times
instead of one for max()
method.
The reason for that typo is that the wording was originally inspired by
mersenne_twister_engine
but after getting feedback that what is written in
the philox_engine
synopsis is not C++ code, the authors introduced the
m variable (as in subtract_with_carry_engine
) but forgot to remove
"-1" in the m definition.
Note: after the proposed resolution below is applied the m variable
could be reused in other places: basically in all places where the mod 2^w
pattern appears (like subtract_with_carry_engine
does).
The authors don’t think it’s worth changing the rest of the wording to reuse
the m variable.
If somebody thinks otherwise, please provide such feedback.
[2024-10-02; Reflector poll]
Set status to Tentatively Ready after six votes in favour during reflector poll.
[Wrocław 2024-11-23; Status changed: Voting → WP.]
Proposed resolution:
This wording is relative to N4988.
Modify 29.5.4.5 [rand.eng.philox] as indicated:
-1- Aphilox_engine
random number engine produces unsigned integer random numbers in theclosedinterval [0, m]), where m = 2w− 1and the template parameter w defines the range of the produced numbers.
std::packaged_task
's constructor from a callable entity should consider decayingSection: 32.10.10.2 [futures.task.members] Status: WP Submitter: Jiang An Opened: 2024-09-18 Last modified: 2024-11-28
Priority: 3
View all other issues in [futures.task.members].
View all issues with WP status.
Discussion:
Currently, 32.10.10.2 [futures.task.members]/3 states:
Mandates:whereis_invocable_r_v<R, F&, ArgTypes...>
istrue
.
F&
can be a reference to a cv-qualified function object type.
However, in mainstream implementations (libc++, libstdc++, and MSVC STL),
the stored task object always has a cv-unqualified type,
and thus the cv-qualification is unrecognizable in operator()
.
Since 22.10.17.3.2 [func.wrap.func.con] uses a decayed type,
perhaps we should also so specify for std::packaged_task
.
[2024-10-02; Reflector poll]
Set priority to 3 after reflector poll.
"Fix preconditions, f
doesn't need to be invocable, we only invoke the copy."
Previous resolution [SUPERSEDED]:
This wording is relative to N4988.
Modify 32.10.10.2 [futures.task.members] as indicated:
-3- Mandates:
[...]is_invocable_r_v<R,
isFdecay_t<F>&, ArgTypes...>true
.-5- Effects: Constructs a new
packaged_task
object with a shared state and initializes the object's stored task of typedecay_t<F>
withstd::forward<F>(f)
.
[2024-10-02; Jonathan provides improved wording]
Drop preconditions as suggested on reflector.
[2024-10-02; LWG telecon]
Clarify that "of type decay_t<F>
"
is supposed to be specifying the type of the stored task.
[2024-10-09; LWG telecon: Move to Ready]
[Wrocław 2024-11-23; Status changed: Voting → WP.]
Proposed resolution:
This wording is relative to N4988.
Modify 32.10.10.2 [futures.task.members] as indicated:
template<class F> explicit packaged_task(F&& f);
-2- Constraints:
remove_cvref_t<F>
is not the same type aspackaged_task<R(ArgTypes...)>
.-3- Mandates:
is_invocable_r_v<R,
isFdecay_t<F>&, ArgTypes...>true
.
-4- Preconditions: Invoking a copy off
behaves the same as invokingf
.-5- Effects: Constructs a new
packaged_task
object with a stored task of typedecay_t<F>
and a shared state . Initializesand initializesthe object's stored task withstd::forward<F>(f)
.
Section: 17.12.6 [cmp.alg] Status: WP Submitter: Jiang An Opened: 2024-09-18 Last modified: 2024-11-28
Priority: Not Prioritized
View other active issues in [cmp.alg].
View all other issues in [cmp.alg].
View all issues with WP status.
Discussion:
In the resolution of LWG 3465(i),
F < E
was required to be well-formed and
implicitly convertible to bool
.
However, P2167R3 replaced the convertibility requirements
with just "each of decltype(E == F)
and decltype(E < F)
models boolean-testable
",
which rendered the type of F < E
underconstrained.
[2024-10-02; Reflector poll]
Set status to Tentatively Ready after seven votes in favour during reflector poll.
[Wrocław 2024-11-23; Status changed: Voting → WP.]
Proposed resolution:
This wording is relative to N4988.
Modify 17.12.6 [cmp.alg] as indicated:
(6.3) — Otherwise, if the expressionsE == F
,E < F
, andF < E
are all well-formed and each ofdecltype(E == F)
and,decltype(E < F)
, anddecltype(F < E)
modelsboolean-testable
,except thatE == F ? partial_ordering::equivalent : E < F ? partial_ordering::less : F < E ? partial_ordering::greater : partial_ordering::unordered
E
andF
are evaluated only once.
forward_list
modifiersSection: 23.3.7.5 [forward.list.modifiers] Status: WP Submitter: Jonathan Wakely Opened: 2024-10-05 Last modified: 2024-11-28
Priority: 3
View all other issues in [forward.list.modifiers].
View all issues with WP status.
Discussion:
The new std::list
members added by P1206R7,
insert_range(const_iterator, R&&)
,
prepend_range(R&&)
, and
append_range(R&&)
,
have the same exception safety guarantee as
std::list::insert(const_iterator, InputIterator, InputIterator)
, which is:
Remarks: Does not affect the validity of iterators and references. If an exception is thrown, there are no effects.
This guarantee was achieved for the new list
functions simply by placing
them in the same set of declarations as the existing insert
overloads,
at the start of 23.3.11.4 [list.modifiers].
However, the new std::forward_list
members,
insert_range_after(const_iterator, R&&)
and
prepend_range(R&&)
,
do not have the same guarantee as forward_list::insert_after
.
This looks like an omission caused by the fact that insert_after
's
exception safety guarantee is given in a separate paragraph at the start
of 23.3.7.5 [forward.list.modifiers]:
None of the overloads ofinsert_after
shall affect the validity of iterators and references, anderase_after
shall invalidate only iterators and references to the erased elements. If an exception is thrown duringinsert_after
there shall be no effect.
I think we should give similar guarantees for insert_range_after
and prepend_range
.
The change might also be appropriate for emplace_after
as well.
A "no effects" guarantee is already given for push_front
and emplace_front
in 23.2.2.2 [container.reqmts] p66, although that doesn't say anything
about iterator invalidation so we might want to add that to
23.3.7.5 [forward.list.modifiers] too.
For the functions that insert a single element, it's trivial to not modify
the list if allocating a new node of constructing the element throws.
The strong exception safety guarantee for the multi-element insertion functions
is easily achieved by inserting into a temporary forward_list
first,
then using splice_after
which is non-throwing.
[2024-10-09; Reflector poll]
Set priority to 3 after reflector poll.
It was suggested to change "If an exception is thrown by any of these member functions that insert elements there is no effect on the forward_list" to simply "If an exception is thrown by any of these member functions there is no effect on the forward_list"
Previous resolution [SUPERSEDED]:
This wording is relative to N4988.
Change 23.3.7.5 [forward.list.modifiers] as indicated:
None of theoverloads ofmember functions in this subclause that insert elements affect the validity of iterators and references, andinsert_after
shallerase_after
shall invalidateinvalidates only iterators and references to the erased elements. If an exception is thrownduringby any of these member functions thereinsert_after
shall beis no effect on theforward_list
.
[2024-10-09; LWG suggested improved wording]
The proposed resolution potentially mandates a change to resize
when
increasing the size, requiring implementations to "roll back" earlier
insertions if a later one throws, so that the size is left unchanged.
It appears that libstdc++ and MSVC already do this, libc++ does not.
[2024-10-09; LWG telecon: Move to Ready]
[Wrocław 2024-11-23; Status changed: Voting → WP.]
Proposed resolution:
This wording is relative to N4988.
Change 23.3.7.5 [forward.list.modifiers] as indicated:
None of the overloads ofThe member functions in this subclause do not affect the validity of iterators and references when inserting elements, and when erasing elements invalidate iterators and references to the erased elements only. If an exception is throwninsert_after
shall affect the validity of iterators and references, anderase_after
shall invalidate only iterators and references to the erased elements.duringby any of these member functions thereinsert_after
shall beis no effect on the container.
std::atomic<T>
's default constructor should be constrainedSection: 32.5.8.2 [atomics.types.operations] Status: WP Submitter: Giuseppe D'Angelo Opened: 2024-10-15 Last modified: 2024-11-28
Priority: Not Prioritized
View all other issues in [atomics.types.operations].
View all issues with WP status.
Discussion:
The current wording for std::atomic
's default constructor in
32.5.8.2 [atomics.types.operations] specifies:
constexpr atomic() noexcept(is_nothrow_default_constructible_v<T>);Mandates:
is_default_constructible_v<T>
istrue
.
This wording has been added by P0883R2 for C++20, which changed
std::atomic
's default constructor to always value-initialize. Before,
the behavior of this constructor was not well specified (this was LWG
issue 2334(i)).
std::atomic<T>
is always default constructible, even
when T
is not. For instance:
// not default constructible: struct NDC { NDC(int) {} }; static_assert(std::is_default_constructible<std::atomic<NDC>>); // OK
The above check is OK as per language rules, but this is user-hostile:
actually using std::atomic<NDC>
's default constructor results in an
error, despite the detection saying otherwise.
std::atomic<T>
already requires T
to be complete anyhow
(32.5.8.1 [atomics.types.generic.general] checks for various type properties
which require completeness) it would be more appropriate to use a
constraint instead, so that std::atomic<T>
is default constructible if
and only if T
also is.
[2024-11-13; Reflector poll]
Set status to Tentatively Ready after seven votes in favour during reflector poll.
[Wrocław 2024-11-23; Status changed: Voting → WP.]
Proposed resolution:
This wording is relative to N4993.
Modify 32.5.8.2 [atomics.types.operations] as indicated:
[Drafting note: There is implementation divergence at the moment; libstdc++ already implements the proposed resolution and has done so for a while.]
constexpr atomic() noexcept(is_nothrow_default_constructible_v<T>);-1- Constraints
-2- Effects: […]Mandates:is_default_constructible_v<T>
istrue
.
contiguous_iterator
should require to_address(I{})
Section: 24.3.4.14 [iterator.concept.contiguous] Status: WP Submitter: Casey Carter Opened: 2024-11-01 Last modified: 2024-11-28
Priority: Not Prioritized
View all other issues in [iterator.concept.contiguous].
View all issues with WP status.
Discussion:
The design intent of the contiguous_iterator
concept is that iterators can be converted
to pointers denoting the same sequence of elements. This enables a common range [i, j)
or counted range i + [0, n)
to be processed with extremely efficient low-level C
or assembly code that operates on [to_address(i), to_address(j))
(respectively
to_address(i) + [0, n)
).
A value-initialized iterator I{}
can be used to denote the empty ranges [I{}, I{})
and I{} + [0, 0)
. While the existing semantic requirements of contiguous_iterator
enable us
to convert both dereferenceable and past-the-end iterators with to_address
, converting
ranges involving value-initialized iterators to pointer ranges additionally needs
to_address(I{})
to be well-defined. Note that to_address
is already implicitly
equality-preserving for contiguous_iterator
arguments. Given this additional requirement
to_address(I{}) == to_address(I{})
and to_address(I{}) == to_address(I{)) + 0
both hold, so the two types of empty ranges involving value-initialized iterators convert
to empty pointer ranges as desired.
[2024-11-13; Reflector poll]
Set status to Tentatively Ready after eight votes in favour during reflector poll.
[Wrocław 2024-11-23; Status changed: Voting → WP.]
Proposed resolution:
This wording is relative to N4993.
Modify 24.3.4.14 [iterator.concept.contiguous] as indicated:
-1- The
contiguous_iterator
concept provides a guarantee that the denoted elements are stored contiguously in memory.template<class I> concept contiguous_iterator = random_access_iterator<I> && derived_from<ITER_CONCEPT(I), contiguous_iterator_tag> && is_lvalue_reference_v<iter_reference_t<I>> && same_as<iter_value_t<I>, remove_cvref_t<iter_reference_t<I>>> && requires(const I& i) { { to_address(i) } -> same_as<add_pointer_t<iter_reference_t<I>>>; };-2- Let
a
andb
be dereferenceable iterators andc
be a non-dereferenceable iterator of typeI
such thatb
is reachable froma
andc
is reachable fromb
, and letD
beiter_difference_t<I>
. The typeI
modelscontiguous_iterator
only if
(2.1) —
to_address(a) == addressof(*a)
,(2.2) —
to_address(b) == to_address(a) + D(b - a)
,(2.3) —
to_address(c) == to_address(a) + D(c - a)
,(2.?) —
to_address(I{})
is well-defined,(2.4) —
ranges::iter_move(a)
has the same type, value category, and effects asstd::move(*a)
, and(2.5) — if
ranges::iter_swap(a, b)
is well-formed, it has effects equivalent toranges::swap(*a, *b)
.
Section: 32.6.5.4.2 [thread.lock.unique.cons], 32.6.5.5.2 [thread.lock.shared.cons] Status: WP Submitter: Casey Carter Opened: 2024-11-13 Last modified: 2025-02-16
Priority: Not Prioritized
View all other issues in [thread.lock.unique.cons].
View all issues with WP status.
Discussion:
The postconditions in 32.6.5.4.2 [thread.lock.unique.cons] paragraph 19:
Postconditions:contradict themselves ifpm == u_p.pm
andowns == u_p.owns
(whereu_p
is the state ofu
just prior to this construction),u.pm == 0
andu.owns == false
.
*this
and the parameter u
refer to the same object.
(Presumably "this construction" means the assignment, and it is copy-pasta from
the move constructor postconditions.) Apparently
unique_lock
didn't get the memo that we require well-defined behavior
from self-move-assignment as of LWG 2839(i).
Also, the move assignment operator doesn't specify what it returns.
[2024-11-18; Casey expands the PR to cover shared_lock
]
shared_lock
has the same problems, and can be fixed in the same way.
[2025-02-07; Reflector poll]
Set status to Tentatively Ready after seven votes in favour during reflector poll.
"Should use parentheses not braces for the initializations." Jonathan volunteers to do that editorially after this gets approved.
[Hagenberg 2025-02-16; Status changed: Voting → WP.]
Proposed resolution:
This wording is relative to N4993.
Drafting Note: I've chosen to use the move-into-temporary-and-swap idiom here to keep things short and sweet. Since move construction, swap, and destruction are allnoexcept
, I've promoted move assignment from "Throws: Nothing" tonoexcept
as well. This is consistent with eliminating the implicit narrow contract condition that*this
andu
refer to distinct objects.
In the class synopsis in 32.6.5.4.1 [thread.lock.unique.general],
annotate the move assignment operator as noexcept
:
namespace std { template<class Mutex> class unique_lock { [...] unique_lock& operator=(unique_lock&& u) noexcept; [...] }; }
Modify 32.6.5.4.2 [thread.lock.unique.cons] as follows:
unique_lock& operator=(unique_lock&& u) noexcept;
-18- Effects:
IfEquivalent to:owns
callspm->unlock()
.unique_lock{std::move(u)}.swap(*this)
.-?- Returns:
*this
.
-19- Postconditions:pm == u_p.pm
andowns == u_p.owns
(whereu_p
is the state ofu
just prior to this construction),u.pm == 0
andu.owns == false
.
-20- [Note 1: With a recursive mutex it is possible for both*this
and u to own the same mutex before the assignment. In this case, *this will own the mutex after the assignment and u will not. — end note]
-21- Throws: Nothing.
Modify 32.6.5.5.2 [thread.lock.shared.cons] as follows:
shared_lock& operator=(shared_lock&& sl) noexcept;
-17- Effects:
IfEquivalent to:owns
callspm->unlock_shared()
.shared_lock{std::move(sl)}.swap(*this)
.-?- Returns:
*this
.
-18- Postconditions:pm == sl_p.pm
andowns == sl_p.owns
(wheresl_p
is the state ofsl
just prior to this assignment),sl.pm == nullptr
andsl.owns == false
.
get_env()
specified in terms of as_const()
but this doesn't work with rvalue sendersSection: 33.5.2 [exec.get.allocator], 33.5.3 [exec.get.stop.token], 33.5.4 [exec.get.env], 33.5.5 [exec.get.domain], 33.5.6 [exec.get.scheduler], 33.5.7 [exec.get.delegation.scheduler], 33.5.8 [exec.get.fwd.progress], 33.5.9 [exec.get.compl.sched] Status: WP Submitter: Lewis Baker Opened: 2024-11-10 Last modified: 2025-02-16
Priority: Not Prioritized
View all issues with WP status.
Discussion:
The current specification of std::execution::get_env()
defines get_env(o)
as as_const(o).get_env()
.
However, the as_const()
function has a deleted rvalue-taking overload, meaning that you cannot pass temporaries to it.
get_env()
which pass expressions which are either potentially rvalues
(e.g. in definition of connect(sndr, rcvr)
it uses the expression get_env(rcvr)
, but rcvr
could be,
and usually is, a prvalue) or always rvalues (e.g. scheduler
concept has the expression
get_env(schedule(std::forward<Sch>(sch)))
).
The intent here was that get_env()
is a function that takes as an argument a const T&
and thus
allows prvalues to bind to it. We basically just want to require that get_env()
finds a const-qualified
member-function. The use of as_const()
does not seem to mirror the semantics of a function with a
const T&
parameter, so I suggest we change it to something else that expresses the intent.
[2025-02-07; Reflector poll]
Set status to Tentatively Ready after five votes in favour during reflector poll.
This could use the "reified object" idea from 25.3 [range.access].
[Hagenberg 2025-02-16; Status changed: Voting → WP.]
Proposed resolution:
This wording is relative to N4993.
Add to the end of 33.1 [exec.general] as indicated:
-?- For a subexpression
expr
, letAS-CONST(expr)
be expression-equivalent to[](const auto& x) noexcept -> const auto& { return x; }(expr)
Modify 33.5.2 [exec.get.allocator] as indicated:
-1-
-2- The nameget_allocator
asks a queryable object for its associated allocator.get_allocator
denotes a query object. For a subexpressionenv
,get_allocator(env)
is expression-equivalent toMANDATE-NOTHROW(
.as_constAS-CONST(env).query(get_allocator))
Modify 33.5.3 [exec.get.stop.token] as indicated:
-2- The name
get_stop_token
denotes a query object. For a subexpressionenv
,get_stop_token(env)
is expression-equivalent to:
(2.1) —
MANDATE-NOTHROW(
if that expression is well-formed.as_constAS-CONST(env).query(get_stop_token))
Modify 33.5.4 [exec.get.env] as indicated:
-1-
execution::get_env
is a customization point object. For a subexpressiono
,execution::get_env(o)
is expression-equivalent to:
(1.1) —
MANDATE-NOTHROW(
if that expression is well-formed.as_constAS-CONST(o).get_env())
Modify 33.5.5 [exec.get.domain] as indicated:
-2- The name
get_domain
denotes a query object. For a subexpressionenv
,get_domain(env)
is expression-equivalent toMANDATE-NOTHROW(
.as_constAS-CONST(env).query(get_domain))
Modify 33.5.6 [exec.get.scheduler] as indicated:
-2- The name
get_scheduler
denotes a query object. For a subexpressionenv
,get_scheduler(env)
is expression-equivalent toMANDATE-NOTHROW(
.as_constAS-CONST(env).query(get_scheduler))
Modify 33.5.7 [exec.get.delegation.scheduler] as indicated:
-2- The name
get_delegation_scheduler
denotes a query object. For a subexpressionenv
,get_delegation_scheduler(env)
is expression-equivalent toMANDATE-NOTHROW(
.as_constAS-CONST(env).query(get_delegation_scheduler))
Modify 33.5.8 [exec.get.fwd.progress] as indicated:
-2- The name
get_forward_progress_guarantee
denotes a query object. For a subexpressionsch
, letSch
bedecltype((sch))
. IfSch
does not satisfyscheduler
,get_forward_progress_guarantee
is ill-formed. Otherwise,get_forward_progress_guarantee(sch)
is expression-equivalent to:
(2.1) —
MANDATE-NOTHROW(
if that expression is well-formed.as_constAS-CONST(sch).query(get_forward_progress_guarantee))
Modify 33.5.9 [exec.get.compl.sched] as indicated:
-2- The name
get_completion_scheduler
denotes a query object template. For a subexpressionq
, the expressionget_completion_scheduler<completion-tag>(q)
is ill-formed ifcompletion-tag
is not one ofset_value_t
,set_error_t
, orset_stopped_t
. Otherwise,get_completion_scheduler<completion-tag>(q)
is expression-equivalent toMANDATE-NOTHROW(as_constAS-CONST(q).query(get_completion_scheduler<completion-tag>))
Section: 26.6.15 [alg.search] Status: WP Submitter: Oscar Slotosch Opened: 2024-12-05 Last modified: 2025-02-16
Priority: Not Prioritized
View all other issues in [alg.search].
View all issues with WP status.
Discussion:
Originally reported as editorial request #7474:
During the qualification of the C++ STL Validas has pointed me to the following issue: The specification of 26.6.15 [alg.search] has a wrong range. Currently the range is "[first1, last1 - (last2 - first2))
" (exclusive) but should be
"[first1, last1 - (last2 - first2)]
" (inclusive). So please correct the closing ")" to "]".
Otherwise the last occurrence will not be found. We observed the issue in C++14 and C++17
and cppreference.com.
The implementations do the right thing and work correctly and find even the last occurrence.
For example in the list {1, 2, 3, 4, 5}
the pattern {4, 5}
should be found (obviously).
In the case the last element is not included it will not be found.
Demonstration on godbolt shows that the implementation
is working and finds the pattern.
[2024-12-07; Daniel comments and provides wording]
The mentioned wording issue is present in all previous standard versions, including the
SGI STL. It needs to be fixed in all search
variants specified in 26.6.15 [alg.search] (except for the form using a Searcher
).
[2025-02-07; Reflector poll]
Set status to Tentatively Ready after ten votes in favour during reflector poll.
[Hagenberg 2025-02-16; Status changed: Voting → WP.]
Proposed resolution:
This wording is relative to N4993.
Modify 26.6.15 [alg.search] as indicated:
template<class ForwardIterator1, class ForwardIterator2> constexpr ForwardIterator1 search(ForwardIterator1 first1, ForwardIterator1 last1, ForwardIterator2 first2, ForwardIterator2 last2); […] template<class ExecutionPolicy, class ForwardIterator1, class ForwardIterator2, class BinaryPredicate> ForwardIterator1 search(ExecutionPolicy&& exec, ForwardIterator1 first1, ForwardIterator1 last1, ForwardIterator2 first2, ForwardIterator2 last2, BinaryPredicate pred);-1- Returns: The first iterator
-2- […]i
in the range[first1, last1 - (last2 - first2)]
such that for every nonnegative integer)n
less thanlast2 - first2
the following corresponding conditions hold:*(i + n) == *(first2 + n), pred(*(i + n)
,*(first2 + n)) != false
. Returnsfirst1
if[first2, last2)
is empty, otherwise returnslast1
if no such iterator is found.template<forward_iterator I1, sentinel_for<I1> S1, forward_iterator I2, sentinel_for<I2> S2, class Pred = ranges::equal_to, class Proj1 = identity, class Proj2 = identity> requires indirectly_comparable<I1, I2, Pred, Proj1, Proj2> constexpr subrange<I1> ranges::search(I1 first1, S1 last1, I2 first2, S2 last2, Pred pred = {}, Proj1 proj1 = {}, Proj2 proj2 = {}); template<forward_range R1, forward_range R2, class Pred = ranges::equal_to, class Proj1 = identity, class Proj2 = identity> requires indirectly_comparable<iterator_t<R1>, iterator_t<R2>, Pred, Proj1, Proj2> constexpr borrowed_subrange_t<R1> ranges::search(R1&& r1, R2&& r2, Pred pred = {}, Proj1 proj1 = {}, Proj2 proj2 = {});-3- Returns:
(3.1) —
{i, i + (last2 - first2)}
, wherei
is the first iterator in the range[first1, last1 - (last2 - first2)]
such that for every non-negative integer)n
less thanlast2 - first2
the conditionbool(invoke(pred, invoke(proj1, *(i + n)), invoke(proj2, *(first2 + n))))is
true
.(3.2) — Returns
{last1, last1}
if no such iterator exists.-4- […]
template<class ForwardIterator, class Size, class T = iterator_traits<ForwardIterator>::value_type> constexpr ForwardIterator search_n(ForwardIterator first, ForwardIterator last, Size count, const T& value); […] template<class ExecutionPolicy, class ForwardIterator, class Size, class T = iterator_traits<ForwardIterator>::value_type, class BinaryPredicate> ForwardIterator search_n(ExecutionPolicy&& exec, ForwardIterator first, ForwardIterator last, Size count, const T& value, BinaryPredicate pred);-5- […]
-6- […] -7- Returns: The first iteratori
in the range[first, last-count]
such that for every non-negative integer)n
less thancount
the conditionE
istrue
. Returnslast
if no such iterator is found. -8- […]template<forward_iterator I, sentinel_for<I> S, class Pred = ranges::equal_to, class Proj = identity, class T = projected_value_t<I, Proj>> requires indirectly_comparable<I, const T*, Pred, Proj> constexpr subrange<I> ranges::search_n(I first, S last, iter_difference_t<I> count, const T& value, Pred pred = {}, Proj proj = {}); template<forward_range R, class Pred = ranges::equal_to, class Proj = identity, class T = projected_value_t<iterator_t<R>, Proj>> requires indirectly_comparable<iterator_t<R>, const T*, Pred, Proj> constexpr borrowed_subrange_t<R> ranges::search_n(R&& r, range_difference_t<R> count, const T& value, Pred pred = {}, Proj proj = {});-9- Returns:
-10- […]{i, i + count}
wherei
is the first iterator in the range[first, last - count]
such that for every non-negative integer)n
less thancount
, the following condition holds:invoke(pred, invoke(proj, *(i + n)), value)
. Returns{last, last}
if no such iterator is found.
regex_traits::transform_primary
mistakenly detects typeid
of a functionSection: 28.6.6 [re.traits] Status: WP Submitter: Jiang An Opened: 2024-12-18 Last modified: 2025-02-16
Priority: Not Prioritized
View all other issues in [re.traits].
View all issues with WP status.
Discussion:
28.6.6 [re.traits]/7 currently says typeid(use_facet<collate<charT>>) ==
typeid(collate_byname<charT>)
, which is always false
because
use_facet<collate<charT>>
is a function template specialization while
collate_byname<charT>
is a class template specialization. This looks like
misuse, and has been present in TR1 (N1836).
use_facet<collate<charT>>(getloc())
.
[2025-02-07; Reflector poll]
Set status to Tentatively Ready after seven votes in favour during reflector poll.
[Hagenberg 2025-02-16; Status changed: Voting → WP.]
Proposed resolution:
This wording is relative to N5001.
Modify 28.6.6 [re.traits] as indicated:
template<class ForwardIterator> string_type transform_primary(ForwardIterator first, ForwardIterator last) const;-7- Effects: If
typeid(use_facet<collate<charT>>(getloc())) == typeid(collate_byname<charT>)and the form of the sort key returned by
collate_byname<charT>::transform(first, last)
is known and can be converted into a primary sort key then returns that key, otherwise returns an empty string.
cache_latest_view
should be freestandingSection: 17.3.2 [version.syn], 25.2 [ranges.syn] Status: WP Submitter: Hewill Kang Opened: 2024-12-23 Last modified: 2025-02-16
Priority: Not Prioritized
View all other issues in [version.syn].
View all issues with WP status.
Discussion:
cache_latest_view
can be freestanding, but this never comes up in the discussion, which seems to be an oversight.
Previous resolution [SUPERSEDED]:
This wording is relative to N5001.
Modify 17.3.2 [version.syn] as indicated:
#define __cpp_lib_ranges_cache_latest 202411L // freestanding, also in <ranges>Modify 25.2 [ranges.syn] as indicated:
#include <compare> // see 17.12.1 [compare.syn] #include <initializer_list> // see 17.11.2 [initializer.list.syn] #include <iterator> // see 24.2 [iterator.synopsis] namespace std::ranges { […] // 25.7.34 [range.cache.latest], cache latest view template<input_range V> requires view<V> class cache_latest_view; // freestanding namespace views { inline constexpr unspecified cache_latest = unspecified; } // freestanding […] }
[2025-01-04; Tim Song suggests alternative wording]
While we are here, we can use the new convention from P2407 to dramatically simplify
<ranges>
. Most future additions to this header should have no problem being freestanding,
so that is the right default.
[2025-02-07; Reflector poll]
Set status to Tentatively Ready after nine votes in favour during reflector poll.
[Hagenberg 2025-02-16; Status changed: Voting → WP.]
Proposed resolution:
This wording is relative to N5001.
Modify 17.3.2 [version.syn] as indicated:
#define __cpp_lib_ranges_cache_latest 202411L // freestanding, also in <ranges>
Delete all "// freestanding" comments in 25.2 [ranges.syn], header <ranges>
synopsis, and then modify as indicated:
// mostly freestanding #include <compare> // see 17.12.1 [compare.syn] #include <initializer_list> // see 17.11.2 [initializer.list.syn] #include <iterator> // see 24.2 [iterator.synopsis] […] namespace std::ranges { // 25.5.6 [range.elementsof], class template elements_of template<range R, class Allocator = allocator<byte>> struct elements_of; // hosted […] // 25.6.6 [range.istream], istream view template<movable Val, class CharT, class Traits = char_traits<CharT>> requires see below class basic_istream_view; // hosted template<class Val> using istream_view = basic_istream_view<Val, char>; // hosted template<class Val> using wistream_view = basic_istream_view<Val, wchar_t>; // hosted namespace views { template<class T> constexpr unspecified istream = unspecified; // hosted } […] } […]
pow(complex<float>, int)
Section: 29.4.10 [cmplx.over] Status: WP Submitter: Tim Song Opened: 2025-01-10 Last modified: 2025-02-16
Priority: Not Prioritized
View all other issues in [cmplx.over].
View all issues with WP status.
Discussion:
Before C++20, 29.4.10 [cmplx.over] says that this produces a complex<double>
.
This was confirmed by LWG 844(i) and consistent with C99.
complex<common_type_t<float, int>>
,
which is complex<float>
. This is a breaking change that does not appear to have been
intentional.
[2025-02-07; Reflector poll]
Set status to Tentatively Ready after eight votes in favour during reflector poll.
[Hagenberg 2025-02-16; Status changed: Voting → WP.]
Proposed resolution:
This wording is relative to N5001.
Modify 29.4.10 [cmplx.over] as indicated:
-3- Function template
pow
has additional constexpr overloads sufficient to ensure, for a call with one argument of typecomplex<T1>
and the other argument of typeT2
orcomplex<T2>
, both arguments are effectively cast tocomplex<common_type_t<T1, T3
, whereT2>>T3
isdouble
ifT2
is an integer type andT2
otherwise. Ifcommon_type_t<T1, T3
is not well-formed, then the program is ill-formed.T2>
inplace_merge()
is incorrectSection: 26.8.6 [alg.merge] Status: WP Submitter: Stephen Howe Opened: 2025-01-22 Last modified: 2025-02-16
Priority: 4
View all other issues in [alg.merge].
View all issues with WP status.
Discussion:
For N5001, section 26.8.6 [alg.merge] p5, it says for merge()
Complexity (emphasis mine):
For the overloads with no
ExecutionPolicy
, at mostN - 1
comparisons and applications of each projection
For N5001, section 26.8.6 [alg.merge] p11, it says from inplace_merge()
Complexity (emphasis mine):
Complexity: Let
N = last - first
:
(11.1) — For the overloads with no
ExecutionPolicy
, and if enough additional memory is available, exactlyN - 1
comparisons.(11.2) — Otherwise,
𝒪(N log N)
comparisons.
This wording should be (emphasis mine)
Complexity: Let
N = last - first
:
(11.1) — For the overloads with no
ExecutionPolicy
, and if enough additional memory is available, at mostN - 1
comparisons.(11.2) — Otherwise,
𝒪(N log N)
comparisons.
Consider the 2 sequences in a std::vector
of int
s and assume that inplace_merge
has enough memory:
{ 1 }, { 2, 3, 4, 5, 6, 7 )
N
is 7
, 7 elements. So N - 1
is 6
inplace_merge()
the two sequences, the 1
is compared with 2
and 1
is output. But now the 1st
sequence is over, so the 2nd sequence is copied. Only 1 comparison is done, not 6.
[2025-01-25; Daniel comments]
The SGI STL archive correctly says "at most" as well.
[2025-02-07; Reflector poll]
Set status to Tentatively Ready after seven votes in favour during reflector poll. There were nine votes for P0 (Tentatively Ready), but also several for NAD Editorial, because 16.3.2.4 [structure.specifications]/7 explicitly says that all complexity requirements are upper bounds and it's conforming to do less work. The consensus was that this is still a clarifying improvement.
[Hagenberg 2025-02-16; Status changed: Voting → WP.]
Proposed resolution:
This wording is relative to N5001.
Modify 26.8.6 [alg.merge] as indicated:
template<class BidirectionalIterator> constexpr void inplace_merge(BidirectionalIterator first, BidirectionalIterator middle, BidirectionalIterator last); template<class ExecutionPolicy, class BidirectionalIterator> void inplace_merge(ExecutionPolicy&& exec, BidirectionalIterator first, BidirectionalIterator middle, BidirectionalIterator last); template<class BidirectionalIterator, class Compare> constexpr void inplace_merge(BidirectionalIterator first, BidirectionalIterator middle, BidirectionalIterator last, Compare comp); template<class ExecutionPolicy, class BidirectionalIterator, class Compare> void inplace_merge(ExecutionPolicy&& exec, BidirectionalIterator first, BidirectionalIterator middle, BidirectionalIterator last, Compare comp); template<bidirectional_iterator I, sentinel_for<I> S, class Comp = ranges::less, class Proj = identity> requires sortable<I, Comp, Proj> constexpr I ranges::inplace_merge(I first, I middle, S last, Comp comp = {}, Proj proj = {});-7- […]
-8- Preconditions: […] -9- Effects: Merges two sorted consecutive ranges[first, middle)
and[middle, last)
, putting the result of the merge into the range[first, last)
. The resulting range is sorted with respect tocomp
andproj
. -10- Returns:last
for the overload in namespaceranges
. -11- Complexity: LetN = last - first
:
(11.1) — For the overloads with no
ExecutionPolicy
, and if enough additional memory is available,exactlyat mostN - 1
comparisons.(11.2) — Otherwise,
𝒪(N log N)
comparisons.In either case, twice as many projections as comparisons.